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PLANT DESIGN FOR THE PRODUCTION OF 400,000
METRIC TONNES OF NITRIC ACID PER ANNUM
FROM AIR OXIDATION OF AMMONIA GAS
BY
ANDREW OFOEDU
DEPARTMENT OF CHEMICAL ENGINEERING
FEDERAL UNIVERSITY OF TECHNOLOGY, OWERRI.
SEPTEMBER 2013
2
EXECUTIVE SUMMARY
This report describes the detailed design of a plant to produce 400000 tonnes of
nitric acid per year by Ostwald Process. The single pressure process was selected
as the most advantageous, having considered several factors one of which is
efficient energy management. The process begins with the vaporization of
ammonia at 1000 kPa and 35°C using process heat. Steam is then used to
superheat the ammonia up to about 80°C. Filtered air is compressed in an axial
compressor to a discharge pressure of about 740kPa and temperature of 155°C.
Part of the air is diverted for acid stripping. This preheated air and the ammonia
vapour are then mixed and passed through the platinum/rhodium catalyst gauze
in a converter for oxidation. The reaction gas flows through a series of heat
exchangers for recovery of energy as either high-pressure superheated steam, or
as shaft horsepower from the expansion of hot tail gas in the turbine. Considering
the proximity to market, sea port and source of raw materials, it was decided to
site the plant in Eleme, Rivers State. The plant’s estimated capital investment is
₦5.41 billion. The rate of return on investment is 26.25% and the payback period
is estimated to be 3 years and 7 months. Thus, the project is both technically and
economically feasible.
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TABLE OF CONTENTTitle page--------------------------------------------------------------------------------------- iExecutive Summary---------------------------------------------------------------------------iiTable of content-------------------------------------------------------------------------------iii
CHAPTER ONE 1.0 Introduction--------------------------------------------------------------------------11.3 Design justification-------------------------------------------------------------------31.4 Design Objectives---------------------------------------------------------------------4 CHAPTER TWO 2.0 Literature review------------------------------------------------------------------------52.1 History of Nitric acid production-------------------------------------------------------52.2 Ammonia oxidation chemistry----------------------------------------------------------82.3 Emission and Control-----------------------------------------------------------------------142.4 Structure and bonding---------------------------------------------------------------------152.5 Reactions-------------------------------------------------------------------------------------162.6 Uses---------------------------------------------------------------------------------------------192.7 Safety-------------------------------------------------------------------------------------------212.8 Pinch technology in modern plant------------------------------------------------------222.9 Plant Location ------------------------------------------------------------------------- 24 2.9.5 Plant layout----------------------------------------------------------------------------- -----292.9.6 Process routes for the production of nitric acid------------------------------------33
CHAPTER THREE
3.0 Material balance -----------------------------------------------------------------------42
3.1 Conservation of mass -----------------------------------------------------------------42
3.2 Methods of material balancing ----------------------------------------------------43
3.3 Materials balance assumptions-----------------------------------------------------44
3.4 Summary of material balance calculations--------------------------------------44
3.5 Material balance for each unit------------------------------------------------------44
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CHAPTER FOUR 4.0 Energy balance -------------------------------------------------------------------------534.1 Conservation of energy---------------------------------------------------------------544.2 Energy balance assumptions -------------------------------------------------------564.3 Summary for energy balances------------------------------------------------------56
CHAPTER FIVE5.0 Chemical Engineering design--------------------------------------------------------615.1 Process units of Nitric acid Production--------------------------------------------61
CHAPTER SIX
6.0 Equipment design and specification ----------------------------------------------66
6.1 Problem specification-----------------------------------------------------------------67
6.2 Analyzing the problem solution----------------------------------------------------68
6.3 Preliminary design-----------------------------------------------------------------------68
6.4 Material Selection-----------------------------------------------------------------------69
6.5 Design optimization---------------------------------------------------------------------69
6.6 Summary of design and equipment specification calculation---------------70
CHAPTER SEVEN
7.0 Process control and instrumentation---------------------------------------------73
7.1 Objective-----------------------------------------------------------------------------------73
7.2 Plant control instrumentation------------------------------------------------------74
7.3 Alarms and safety trips ---------------------------------------------------------------77
7.4 Lining, piping, valves and pumps --------------------------------------------------78
7.5 Pipe support-----------------------------------------------------------------------------81
CHAPTER EIGHT
8.0 Safety and environmental considerations---------------------------------------82
8.1 Safety------------------------------------------------------------------------------------82
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8.2 Hazard and Operability (HAZOP) study-------------------------------------------89
8.3 Environmental impact assessment-------------------------------------------------97
CHAPTER NINE
9.1 Overview---------------------------------------------------------------------------------103
9.2 Economic Consideration--------------------------------------------------------------103
9.3 Cost estimation---------------------------------------------------------------------------106
9.6 Economic analyses calculation------------------------------------------------------108
CHAPTER TEN
10.0 Start up and shut down procedure-----------------------------------------------113
10.1 Emergency shut down and emergency depressurization-------------------114
10.2 Notification----------------------------------------------------------------------------114
10.3 Record keeping -----------------------------------------------------------------------115
10.4 Startup operation--------------------------------------------------------------------116
CHAPTER ELEVEN
11.0 Conclusion/ Recommendation----------------------------------------------------118
11.1 Conclusion------------------------------------------------------------------------------118
11.2 Recommendation -------------------------------------------------------------------119
REFERENCES-----------------------------------------------------------------------------------120
APPENDIX I
Tables and Charts--------------------------------------------------------------------------------123
APPENDIX II
Material Balance Calculation------------------------------------------------------------------126
APPENDIX III
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Energy Balance Calculation------------------------------------------------------------------132
APPENDIX IV
Equipment Design Calculation----------------------------------------------------------------137
APPENDIX V
Equipment Costing Calculation---------------------------------------------------------------141
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CHAPTER ONEINTRODUCTION
1.1 BACKGROUND INFORMATION
Nitric acid is a strong acid and a powerful oxidizing agent with enormous
possibilities for applications in the chemical processing industry. It has
commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent,
catalyst and hydrolyzing agent. In relation to world production, approximately
65% of all nitric acid produced is used for the production of ammonium nitrate
(specifically for fertilizer manufacture).
Nitric acid is now produced commercially using the stepwise, catalytic oxidation
of ammonia with air, to obtain nitrogen monoxide and nitrogen dioxide. These
nitrogen oxides are subsequently absorbed in water to yield between 50% and
68% strength nitric acid by weight. For applications requiring higher strengths,
several methods of concentrating the acid are used.
The traditional methods are:
(a) Extractive distillation with dehydrating agents such as sulphuric acid or
magnesium nitrate;
(b) Reaction with additional nitrogen oxides.
The latter technique has the greatest application in industry.
The chemistry of ammonia oxidation is remarkably simple with only six main
reactions that need to be considered.
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1.1.1 PROPERTIES AND USES
Nitric acid is an oxidizing mineral acid with physical and chemical properties that
make it one of the most useful inorganic minerals. It is a colorless liquid at room
temperature and atmospheric pressure. It is soluble in water in all proportions
and there is a release of heat of solution upon dilution. Its high solubility in water
is the basis for the process methods used for commercial nitric acid manufacture.
It is a strong acid that almost completely ionizes when in dilute solution. It is also
a powerful oxidizing agent with the ability to passivate some metals such as iron
and aluminum. A compilation of many of the physical and chemical properties of
nitric acid are presented in the Appendix. Arguably the most important physical
property of nitric acid is its azeotropic point, this influences the techniques
associated with strong acid production. The constant-boiling mixture occurs at
121.9°C, for a concentration of 68.4%(wt) acid at atmospheric pressure.
Nitric acid has enormously diverse applications in the chemical industry. It has
commercial uses as a nitrating agent, oxidizing agent, solvent, activating agent,
catalyst and hydrolyzing agent. The most important use is undoubtedly in the
production of ammonium nitrate for the fertilizer and explosives industries, which
accounts for approximately 65% of the world production of nitric acid.
Nitric acid has a number of other industrial applications. It is used for pickling
stainless steels, steel refining, and in the manufacture of dyes, plastics and
synthetic fibers. Most of the methods used for the recovery of uranium, such as
ion exchange and solvent extraction, use nitric acid.
An important point is that for most uses concerned with chemical production, the
acid must be concentrated above its azeotropic point to greater than 95%(wt).
9
Conversely, the commercial manufacture of ammonium nitrate uses nitric acid
below its azeotropic point in the range 50 -65 %(wt.). If the stronger chemical
grade is to be produced, additional process equipment appropriate to super-
azeotropic distillation is required.
There is a potential health hazard when handling, and operating with, nitric acid.
Nitric acid is a corrosive liquid that penetrates and destroys the skin and internal
tissues. Contact can cause severe burns. The acid is a potential hazard, the various
nitrogen oxides present as product intermediates in the process are also toxic. An
assessment of the health risk must be fundamental to the design of any process.
Further consideration and recommendations for the operating health risk and
environmental impact of the plant are presented in the Appendix.
1.2 DESIGN JUSTIFICATION
At present, there is no Nitric acid plant in Nigeria. The little Nitric acid produced
mainly by fertilizer plants in the country is used up immediately by them to make
their fertilizer. This means that most of the all Nitric acid used in the country is
imported.
A Nitric acid plant sited in the country producing Nitric acid made available to the
Nigerian market will not only reduce importation of the acid but also encourage
fertilizer production, create job opportunities as well as develop the area in which
it is sited.
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1.3 DESIGN OBJECTIVES
To design a plant that will deliver 400000 metric tonnes of 60%(wt) Nitric
Acid per annum.
To determine the technical and economic feasibility of the plant.
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CHAPTER TWO
LITERATURE REVIEW
2.1 HISTORY OF NITRIC ACID PRODUCTION
Until the beginning of the 20th century, Nitric acid (HNO3), also known as aqua
fortis and spirit of niter was prepared commercially by reacting sulphuric acid
with either potassium nitrate (saltpetre) or with sodium nitrate (Chile saltpetre or
nitre). Up to four tonnes of the two ingredients were placed into large retorts and
heated over a furnace (Kirk 1996). The volatile product vapourized and was
collected for distillation. An acid of 93-95 %( wt) was produced (Gregory 1999).
In 1903 the electric-arc furnace superseded this primitive original technique. In
the arc process, nitric acid was produced directly from nitrogen and oxygen by
passing air through an electric-arc furnace (Ray 1990).
Gregory (1999, p.40) argues that ‘Although the process benefitted from an
inexhaustible supply of free feed material (air), the power consumption for the
arc furnace was cost prohibitive’
According to Ray (1989, p.8) Researchers returned to the oxidation of ammonia in
air, (recorded as early as 1798) in an effort to improve production economics. In
1901 Wilhelm Ostwald had first achieved the catalytic oxidation of ammonia over
a platinum catalyst. The gaseous nitrogen oxides produced could be easily cooled
and dissolved in water to produce a solution of nitric acid. This achievement
began the search for an economic process route.
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By 1908 the first commercial facility for production of nitric acid, using this new
catalytic oxidation process, was commissioned near Bochum in Germany (Ray et
al 1989). The Haber-Bosch ammonia synthesis process came into operation in
1913, leading to the continued development and assured future of the ammonia
oxidation process for the production of nitric acid. (Ray et al 1989)
During World War 1, the intense demand for explosives and synthetic dyestuffs
created an expansion of the nitric acid industry.
Many new plants were constructed, all of which employed the ammonia
oxidation process. This increased demand served as the impetus for several
breakthroughs in process technology.
These included:
(a) The development of chrome-steel alloys for tower construction, replacing the
heavy stoneware and acid-proof bricks. This enabled process pressures above
atmospheric levels to be used.
(b) The improved design of feed preheaters enabled higher process temperatures
to be attained. Higher temperatures improved the yields and capacities, and also
reduced equipment requirements (Ohrue et al 1999).
(c) Early developments in automatic process control improved process
performance and reduced labor requirements.
All of these factors helped to improve the process efficiency. The increasing
availability of ammonia reduced processing costs still further.
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In the late 1920’s the development of stainless steels enabled manufacturers to
use higher operating pressures. The increase in yield and lower capital
requirements easily justified the use of high pressure operation despite increased
ammonia consumption.
The introduction of higher pressure processes resulted in a divergence of
operating technique within the industry. The United States producers opted for a
high-pressure system, using a constant high pressure throughout the process. The
European manufacturers opted for a split-pressure system. This latter system
entails operating the ammonia oxidation section at atmospheric pressure, while
the absorption unit is operated at higher pressures, thus capitalizing on improved
absorption rates. (Harvin et al 1979)
Recent developments in the ammonia oxidation process have included efforts to
reduce catalyst losses in the process. Platinum recovery filters have been installed
at various stages in the process. (Ohrue et al 1999)
Gold/palladium gauze filter pads have been added on the exit side of the catalyst
bed, inside the reactor/converter units. These filters have reportedly ensured a
platinum recovery of 80% (Anon 1979). Another trend has been for the use of
additional filters in the downstream units. These filters are of alumino-silicate
construction.
Perhaps the greatest progress in nitric acid production technology has been in the
manufacture of strong nitric acid (>90% by weight). Advances in the areas of
super-azeotropic distillation and in high pressure absorption are most significant.
(Ohkubo et al 1999)
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Research work is continually being performed in an effort to reduce nitrogen
oxide emissions from nitric acid plants. The Humphreys and Glasgow/Bolme nitric
acid process is just one example of a new philosophy being applied to the
absorption systems of weak nitric acid plants (50-68% by weight). Nitrogen oxide
emissions have been reduced from 2000-5000 ppm to less than 1000 ppm (Ray et
al 1989).
For the production of stronger nitric acid, tail gases are now being treated by
selective or non-selective catalytic combustion systems. These innovative units
have reduced the nitrogen oxide emissions to below 400 ppm (Ray et al 1989).
2.2 AMMONIA OXIDATION CHEMISTRY
Notably, all commercial nitric acid production methods used today are centered
on the oxidation of ammonia. It is therefore appropriate to investigate the
chemistry of this process, in the knowledge that it is directly applicable to any of
the production processes available. (Chilton 1960)
The chemistry of the oxidation of ammonia is surprisingly simple. It begins with a
single pure compound, plus air and water, and ends with another pure compound
in aqueous solution, with essentially no by-products. The process may be
described by just six major reactions as shown as follows:
1.N H 3(g )+2O2→HN O3(aq)+H 2O(l)
2.4N H 3 (g)+5O2(g )→4NO(g)+6H 2O(l)
3. 2NO(g )+O2→2N O2(g )
4. 2NO2 (g)⇌N 2O4
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5. 3N2O 4+2H 2O(l )→4HN O3+2NO(g)
6. 3N O2(g)+H 2O(l)→2HN O3(aq)+N O( g)
Reaction 1 is the overall reaction for the process. This net result is achieved from
three separate, and distinct, chemical steps. The first is the oxidation of ammonia
to nitrogen monoxide (Reaction 2). The second is the further oxidation of nitrogen
monoxide to nitrogen dioxide (Reaction 3), then nitrogen dioxide to nitrogen
tetroxide (Reaction 4). The third and final stage involves the absorption of these
nitrogen-based oxides into water to form the nitric acid product (Reactions 5 and
6). In most commercial processes, each of these three stages is conducted in
separate process units. (Chilton 1960)
The first step in the process is the heterogeneous, highly exothermic, gas-phase
catalytic reaction of ammonia with oxygen (Reaction 2). The primary oxidation of
ammonia to nitric acid (over a catalyst gauze of 9:l platinum/rhodium alloy)
proceeds rapidly at process temperatures between 900-970°C. (Kent 1983)
The second step in the process involves two reactions (Reactions 3 and 4). These
are the oxidations of nitrogen monoxide to the dioxide and tetroxide forms. The
equilibrium mixture is loosely referred to as nitrogen peroxide. Both reactions are
homogenous, moderately exothermic, gas-phase catalytic reactions. All reactions
shown are highly exothermic. (Chilton 1960)
The third step in the process involves cooling the reaction gases below their dew
point, so that a liquid phase of weak nitric acid is formed. This step effectively
promotes the state of oxidation and dimerization (Reactions 3 and 4), and
16
removes water from the gas phase. This in turn increases the partial pressure of
the nitrogen peroxide component. (Chilton 1960)
Finally, nitric acid is formed by the reaction of dissolved nitrogen peroxide with
water (Reactions 5 and 6).
Nitric acid is produced by 2 methods. The first method utilizes oxidation,
condensation, and absorption to produce a weak nitric acid. Weak nitric acid can
have concentrations ranging from 30 to 70 percent nitric acid. The second
method combines dehydrating, bleaching, condensing, and absorption to produce
a high-strength nitric acid from a weak nitric acid. High-strength nitric acid
generally contains more than 90 percent nitric acid. The following text provides
more specific details for each of these processes. (Chilton 1960)
2.2.1 WEAK NITRIC ACID PRODUCTION
According to Ray(1989, Nearly all the nitric acid produced in the U. S. is
manufactured by the high-temperature catalytic oxidation of ammonia. This
process typically consists of 3 steps: (1) ammonia oxidation, (2) nitric oxide
oxidation, and (3) absorption. Each step corresponds to a distinct chemical
reaction.
1. AMMONIA OXIDATION
First, a 1:9 ammonia/air mixture is oxidized at a temperature of 1380 to 14700F as
it passes through a catalytic convertor, according to the following reaction:
4 N H 3+5O2→4NO+6H 2O
The most commonly used catalyst is made of 90 percent platinum and 10 percent
rhodium gauze constructed from squares of fine wire. Under these conditions, the
oxidation of ammonia to nitric oxide (NO) proceeds in an exothermic reaction
17
with a range of 93 to 98 percent yield. Oxidation temperatures can vary from
1380OF to 16500F. (Chilton 1960) Higher catalyst temperatures increase reaction
selectivity toward NO production. Lower catalyst temperatures tend to be more
selective toward less useful products: nitrogen (N2) and nitrous oxide (N2O).
Nitric oxide is considered to be a criteria pollutant and nitrous oxide is known to
be a global warming gas. The nitrogen dioxide/dimmer mixture then passes
through a waste heat boiler and a platinum filter. (Chilton 1960)
2. NITRIC OXIDE OXIDATION
The nitric oxide formed during the ammonia oxidation must be oxidized. The
process stream is passed through a cooler/condenser and cooled to 1000F or less
at pressures up to 116 pounds per square inch absolute (psia). The nitric oxide
reacts non-catalytically with residual oxygen to form nitrogen dioxide (NO2) and
its liquid dimmer, nitrogen tetra-oxide:
2NO2+O2→2N O2⇌ N2O 4
This slow, homogeneous reaction is highly temperature and pressure dependent.
Operating at low temperatures and high pressures promotes maximum
production of NO2 within a minimum reaction time (Kent 1983).
3. ABSORPTION
The final step introduces the nitrogen dioxide/dimmer mixture into an absorption
process after being cooled. The mixture is pumped into the bottom of the
absorption tower, while liquid dinitrogen tetra-oxide is added at a higher point.
De-ionized process water enters the top of the column. Both liquids flow
countercurrent to the nitrogen dioxide/dimmer gas mixture. Oxidation takes
place in the free space between the trays, while absorption occurs on the trays.
18
The absorption trays are usually sieve or bubble cap trays. The exothermic
reaction occurs as follows:
3N O2+H 2O→2HNO3+NO
A secondary air stream is introduced into the column to re-oxidize the NO that is
formed in Reaction 3. This secondary air also removes NO2 from the product acid.
An aqueous solution of 55 to 65 percent (typically) nitric acid is withdrawn from
the bottom of the tower. The acid concentration can vary from 30 to 70 percent
nitric acid. The acid concentration depends upon the temperature, pressure,
number of absorption stages, and concentration of nitrogen oxides entering the
absorber.
There are 2 basic types of systems used to produce weak nitric acid: single-stage
pressure process and dual-stage pressure process (Harvin et al 1979). In the past,
nitric acid plants have been operated at a single pressure, ranging from
atmospheric pressure to 14.7 to 203 psia. However, since Reaction 1 is favored by
low pressures and Reactions 2 and 3 are favored by higher pressures, newer
plants tend to operate a dual stage pressure system, incorporating a compressor
between the ammonia oxidizer and the condenser. The oxidation reaction is
carried out at pressures from slightly negative to about 58 psia, and the
absorption reactions are carried out at 116 to 203 psia. (Harvn et al 1979)
In the dual-stage pressure system, the nitric acid formed in the absorber
(bottoms) is usually sent to an external bleacher where air is used to remove
(bleach) any dissolved oxides of nitrogen. The bleacher gases are then
compressed and passed through the absorber. The absorber tail gas (distillate) is
sent to an entrainment separator for acid mist removal. Next, the tail gas is
reheated in the ammonia oxidation heat exchanger to approximately 3920F. The
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final step expands the gas in the power-recovery turbine. The thermal energy
produced in this turbine can be used to drive the compressor.
2.2.2 HIGH STRENGTH NITRIC ACID PRODUCTION
A high-strength nitric acid (98 to 99 percent concentration) can be obtained by
concentrating the weak nitric acid (30 to 70 percent concentration) using
extractive distillation. (Imai et al 1999) The weak nitric acid cannot be
concentrated by simple fractional distillation. The distillation must be carried out
in the presence of a dehydrating agent. Concentrated sulfuric acid (typically 60
percent sulfuric acid) is most commonly used for this purpose. The nitric acid
concentration process consists of feeding strong sulfuric acid and 55 to 65 percent
nitric acid to the top of a packed dehydrating column at approximately
atmospheric pressure. The acid mixture flow downward, countercurrent to
ascending vapors. Concentrated nitric acid leaves the top of the column as 99
percent vapor, containing a small amount of NO2 and oxygen (O2) resulting from
dissociation of nitric acid. The concentrated acid vapor leaves the column and
goes to a bleacher and a countercurrent condenser system to effect the
condensation of strong nitric acid and the separation of oxygen and oxides of
nitrogen (NO2) byproducts. (Ohkubo et al 1999) These byproducts then flow to an
absorption column where the nitric oxide mixes with auxiliary air to form NO2,
which is recovered as weak nitric acid. Inert and un-reacted gases are vented to
the atmosphere from the top of the absorption column. Emissions from this
process are relatively minor. A small absorber can be used to recover NO2. (Kirk et
al 1981)
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2.3 EMISSIONS AND CONTROL
Emissions from nitric acid manufacture consist primarily of NO, NO2 (which
account for visible emissions), trace amounts of HNO3 mist, and ammonia (NH3).
By far, the major source of nitrogen oxides (NO2) is the tail-gas from the acid
absorption tower. In general, the quantity of NO2 emissions is directly related to
the kinetics of the nitric acid formation reaction and absorption tower design. NO2
emissions can increase when there is (1) insufficient air supply to the oxidizer and
absorber, (2) low pressure, especially in the absorber, (3) high temperatures in
the cooler-condenser and absorber, (4) production of an excessively high-strength
product acid, (5) operation at high throughput rates, and (6) faulty equipment
such as compressors or pumps that lead to lower pressures and leaks, and
decrease plant efficiency. (Leray et al 1979)
Roudier (1979) states that the two most common techniques used to control
absorption tower tail gas emissions are extended absorption and catalytic
reduction. Extended absorption reduces NO2 emissions by increasing the
efficiency of the existing process absorption tower or incorporating an additional
absorption tower. An efficiency increase is achieved by increasing the number of
absorber trays, operating the absorber at higher pressures, or cooling the weak
acid liquid in the absorber. The existing tower can also be replaced with a single
tower of a larger diameter and/or additional trays.
In the catalytic reduction process (often termed catalytic oxidation or
incineration), tail gases from the absorption tower are heated to ignition
temperature, mixed with fuel (natural gas, hydrogen, propane, butane, naphtha,
carbon monoxide, or ammonia) and passed over a catalyst bed. In the presence of
the catalyst, the fuels are oxidized and the NO2 are reduced to N2. The extent of
21
reduction of NO2 and NO to N2 is a function of plant design, fuel type, operating
temperature and pressure. Space-velocity through the comparatively small
amounts of nitrogen oxides is also lost from acid concentrating plants. These
losses (mostly NO2) are from the condenser system, but the emissions are small
enough to be controlled easily by inexpensive absorbers. Acid mist emissions do
not occur from the tail-gas of a properly operated plant. The small amounts that
may be present in the absorber exit gas streams are removed by a separator or
collector prior to entering the catalytic reduction unit or expander. (Kent 1983)
The acid production system and storage tanks are the only significant sources of
visible emissions at most nitric acid plants. Emissions from acid storage tanks may
occur during tank filling.
2.4 STRUCTURE AND BONDING
Fig 2: Two major resonance representations of HNO3.
The molecule is planar. Two of the N-O bonds are equivalent and relatively short
(this can be explained by theories of resonance. The canonical forms show double
bond character in these two bonds, causing them to be shorter than typical N-O
bonds.), and the third N-O bond is elongated because the O is also attached to a
proton.
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2.5 REACTIONS
2.5.1 ACID-BASE PROPERTIES
Nitric acid is normally considered to be a strong acid at ambient temperatures.
The pKa value is usually reported as less than −1. This means that the nitric acid in
solution is fully dissociated except in extremely acidic solutions. The pKa value
rises to 1 at a temperature of 250 °C.
Nitric acid can act as a base with respect to an acid such as sulfuric acid.
HNO3 + 2H2SO4 NO2+ + H3O+ + 2HSO4
–
The nitronium ion, NO2+, is the active reagent in aromatic nitration reactions.
Since nitric acid has both acidic and basic properties it can undergo an
autoprotolysis reaction, similar to the self-ionization of water
2HNO3 NO2+ + NO3
– + H2O
2.5.2 REACTIONS WITH METALS
Nitric acid reacts with most metals but the details depend on the concentration of
the acid and the nature of the metal. Dilute nitric acid behaves as a typical acid in
its reaction with most metals. Magnesium, manganese and zinc liberate H2.
Others give the nitrogen oxides. (Ababio 2007)
Nitric acid can oxidize non-active metals such as copper and silver. With these
non-active or less electropositive metals the products depend on temperature
and the acid concentration. For example, copper reacts with dilute nitric acid at
23
ambient temperatures with a 3:8 stoichiometry to produce nitric oxide which may
react with atmospheric oxygen to give nitrogen dioxide.
3 Cu + 8 HNO3 → 3 Cu2+ + 2 NO + 4 H2O + 6 NO3-
With more concentrated nitric acid, nitrogen dioxide is produced directly in a
reaction with 1:4 stoichiometries.
Cu + 4 H+ + 2 NO3− → Cu2+ + 2 NO2 + 2 H2O
Upon reaction with nitric acid, most metals give the corresponding nitrates. Some
metalloids and metals give the oxides, for instance, Sn, As, Sb, Ti are oxidized into
SnO2, As2O5, Sb2O5 and TiO2 respectively.
Some precious metals, such as pure gold and platinum group metals do not react
with nitric acid, though pure gold does react with aqua regia, a mixture of
concentrated nitric acid and hydrochloric acid. However, some less noble metals
(Ag, Cu, ...) present in some gold alloys relatively poor in gold such as colored gold
can be easily oxidized and dissolved by nitric acid, leading to color changes of the
gold-alloy surface. Nitric acid is used as a cheap means in jewelry shops to quickly
spot low-gold alloys (< 14 carats) and to rapidly assess the gold purity.
Being a powerful oxidizing agent, nitric acid reacts violently with many non-
metallic compounds and the reactions may be explosive. Reaction takes place
with all metals except the noble metals series and certain alloys. As a general rule,
oxidizing reactions occur primarily with the concentrated acid, favoring the
formation of nitrogen dioxide (NO2). (Ababio 2007) However, the powerful
oxidizing properties of nitric acid are thermodynamic in nature, but sometimes its
24
oxidation reactions are rather kinetically non-favored. The presence of small
amounts of nitrous acid (HNO2) greatly enhances the rate of reaction.
Although chromium (Cr), iron (Fe) and aluminum (Al) readily dissolve in dilute
nitric acid, the concentrated acid forms a metal oxide layer that protects the bulk
of the metal from further oxidation. The formation of this protective layer is
called passivation. Typical passivation concentrations range from 20–50% by
volume (ASTM A967-05 2000). Metals which are passivated by concentrated nitric
acid are Iron, Cobalt, Chromium, Nickel, and Aluminum.
2.5.3 REACTIONS WITH NON-METALS
Being a powerful oxidizing acid, nitric acid reacts violently with many organic
materials and the reactions may be explosive. (Kent 1983)
Reaction with non-metallic elements, with the exceptions of nitrogen, oxygen,
noble gases, silicon and halogens, usually oxidizes them to their highest oxidation
states as acids with the formation of nitrogen dioxide for concentrated acid and
nitric oxide for dilute acid. (Ababio 2007)
C + 4 HNO3 → CO2 + 4 NO2 + 2 H2O
OR
3 C + 4 HNO3 → 3 CO2 + 4 NO + 2 H2O
Concentrated nitric acid oxidizes I2, P4 and S8 into HIO3, H3PO4 and H2SO4
respectively.
25
2.5.4 XANTHOPROTEIC TEST
Nitric acid reacts with proteins to form yellow nitrated products. This reaction is
known as the xanthoproteic reaction (Gregory 1999). This test is carried out by
adding concentrated nitric acid to the substance being tested, and then heating
the mixture. If proteins that contain amino acids with aromatic rings are present,
the mixture turns yellow. Upon adding a strong base such as liquid ammonia, the
color turns orange. These color changes are caused by nitrated aromatic rings in
the protein. Xanthoproteic acid is formed when the acid contacts epithelial cells
and is indicative of inadequate safety precautions when handling nitric acid
2.6 USES
2.6.1 NITRIC ACID IN A LABORATORY.
The main use of nitric acid is for the production of fertilizers. Nitric acid is
neutralized with ammonia to give ammonium nitrate. According to Gregory
(1999, p.408) this application consumes 75-80% of the 26M tons produced
annually. The other main applications are for the production of explosives, nylon
precursors, and specialty organic compounds.
2.6.2 PRECURSOR TO ORGANIC NITROGEN COMPOUNDS
In organic synthesis, industrial and otherwise, the nitro group is a versatile
functionality. Most derivatives of aniline are prepared via nitration of aromatic
compounds followed by reduction. Nitrations entail combining nitric and sulfuric
acids to generate the nitronium ion, which electrophilically reacts with aromatic
26
compounds such as benzene. (Gregory 1999) Many explosives, e.g. TNT, are
prepared in this way.
The precursor to nylon, adipic acid, is produced on a large scale by oxidation of
cyclohexanone and cyclohexanol with nitric acid.
1.6.3 ROCKET FUEL
Nitric acid has been used in various forms as the oxidizer in liquid-fueled rockets.
These forms include red fuming nitric acid, white fuming nitric acid, mixtures with
sulfuric acid, and these forms with HF inhibitor. IRFNA (inhibited red fuming nitric
acid) was one of 3 liquid fuel components for the BOMARC missile. (Gregory
1999)
2.6.4 ANALYTICAL REAGENT
In elemental analysis dilute nitric acid (0.5 to 5.0%) is used as a matrix compound
for determining metal traces in solutions. Ultrapure trace metal grade acid is
required for such determination, because small amounts of metal ions could
affect the result of the analysis. (Kirk 1981)
It is also typically used in the digestion process of turbid water samples, sludge
samples, solid samples as well as other types of unique samples which require
elemental analysis via flame atomic absorption spectroscopy. Typically these
digestions use a 50% solution of the purchased HNO3 mixed with deionized water.
In electrochemistry, nitric acid is used as a chemical doping agent for organic
semiconductors, and in purification processes for raw carbon nanotubes.
27
2.6.5 WOODWORKING
In a low concentration (approximately 10%), nitric acid is often used to artificially
age pine and maple. The color produced is a grey-gold very much like very old wax
or oil finished wood (wood finishing).
2.6.6 ETCHANT AND CLEANING AGENT
The corrosive effects of nitric acid are exploited for a number of specialty
applications, such as pickling stainless steel. A solution of nitric acid, water and
alcohol, Nital, is used for etching of metals to reveal the microstructure (Gregory
1999). Commercially available aqueous blends of 5–30% nitric acid and 15–40%
phosphoric acid are commonly used for cleaning food and dairy equipment
primarily to remove precipitated calcium and magnesium compounds (either
deposited from the process stream or resulting from the use of hard water during
production and cleaning). The phosphoric acid content helps to passivate ferrous
alloys against corrosion by the dilute nitric acid.(Anon 1979) Nitric acid can be
used as a spot test for alkaloids, giving a variety of colors depending on the
alkaloid.
2.7 SAFETY
Nitric acid is a strong acid and a powerful oxidizing agent. The major hazard posed
by it is chemical burns as it carries out acid hydrolysis with proteins (amide) and
fats (ester) which consequently decomposes living tissue (e.g. skin and flesh).
Concentrated nitric acid stains human skin yellow due to its reaction with the
28
keratin. These yellow stains turn orange when neutralized. Systemic effects are
unlikely, however, and the substance is not considered a carcinogen or mutagen.
The standard first aid treatment for acid spills on the skin is, as for other corrosive
agents, irrigation with large quantities of water. Washing is continued for at least
ten to fifteen minutes to cool the tissue surrounding the acid burn and to prevent
secondary damage. Contaminated clothing is removed immediately and the
underlying skin washed thoroughly. (Othmer et al 1981)
Being a strong oxidizing agent, reactions of nitric acid with compounds such as
cyanides, carbides, metallic powders can be explosive and those with many
organic compounds, such as turpentine, are violent and hypergolic (i.e. self-
igniting). Hence, it should be stored away from bases and organics.
2.8 PINCH TECHNOLOGY IN MODERN PLANTS
One of the most successful and generally useful techniques is that developed by
Bodo Linnhoff and other workers: pinch technology. The term derives from the
fact that in a plot of the system temperatures versus the heat transferred, a pinch
usually occurs between the hot stream and cold stream curves. (Sinnot 2005)
Pinch technology is a relatively modern engineering tool developed in the late
1970s and early 1980s. This new approach to evaluating the energy requirements
of a site quickly identified ways of improving the overall energy use. The name
“pinch technology” was applied because the technique identified the point or
points in the energy flow where restrictions applied and hence limited one’s
ability to reuse low grade energy.
29
The major difference between this new technology and the previous engineering
approaches was the formalized methodology involving the rigorous application of
thermodynamic principles. Pinch technology was initially adopted by major
chemical companies and petrochemical energy. Beet sugar was quite quick to
adopt it because of the industry’s energy profile and it is now being adopted by
the cane industry too. It has also been shown that the pinch represents a distinct
thermodynamic break in the system and that, for minimum energy requirements,
heat should not be transferred across the pinch, (Linnhoff et al 1983)
2.8.1 APPLICATIONS
Pinch technology is equally applicable to Greenfield project and refurbishments.
In either case, their objectives are to achieve:
1. Minimum energy consumption
2. Optimization of utilities
3. Minimum capital expenditure to achieve these
Minimizing energy consumption implies minimizing cooling water requirements
too because all of the energy used ultimately has to be rejected again in some low
grade form. ( Sinnot 2005)
The technology strength are its overall approach to process integration (rather
than optimizing a single station) and its blend of thermodynamics with
commercial requirements. It also takes into account the operational requirements
of the site and does reduce flexibility or availability.
30
2.9 PLANT LOCATION
Plant location refers to the choice of a region or the selection of a particular site
for settling up the business or a factory. However, the choice is made only after
considering alternative sites. It is a strategic decision that cannot be changed once
it is taken. Therefore, careful care must be taken before a decision is made on the
location of the plant site (Ray et al 1989).
2.9.1 IDEAL PLANT LOCATION
An ideal plant location is one where the cost of the production is minimal, with a
large market availability, least risk involved and maximum gain obtainable. It is a
place of maximum net advantage or with lowest unit cost of production and
distribution. For achieving this objective, small and large scale entrepreneur can
make use of local analysis.
2.9.2 LOCAL ANALYSIS
Local analysis is a dynamic process where the entrepreneur analyses and
compares the feasibility of different sites with the aim of selecting the best site
for a given enterprise. It considers the following:
a. Demographic analysis: it involves the study of the population in the area in
terms of total number of people in the area, age composition, per capital
income, educational level and occupational structures etc.
b. Trade area analysis: it is an analysis of the geographic area that provides
continued clientele to the industry. It is advisable to also see the feasibility
of accessing the trade area from alternative sites. (Ray et al 1989)
31
c. Competitive analysis: it helps to judge the nature, location, size and quality
of competition in a given trade area.
d. Traffic analysis: this is done to have a rough idea about the number of
potential customers passing by the proposed site during the working hours
of the industry. The traffic analysis aims at judging the alternative sites in
terms of pedestrian and vehicular traffic passing by the site.
e. Site economics: alternative sites are evaluated in terms of establishments,
costs and operational costs under this. Cost of establishment of a plant is
basically cost incurred for permanent physical facilities but operation costs
are incurred for running the plant.
2.9.3 SELECTION CRITERIA
According to Ray (1989, p. 76) the important considerations for selecting a
suitable location are as follows:
I. Nature or climate conditions
II. Availability and nearness to the sources of raw materials
III. Transport costs: this should be considered both for obtaining raw
material and also distribution or marketing finished products to the
ultimate users.
IV. Close proximity to the anticipated market: the industry’s warehouse
should be located within the vicinity of densely populated areas.
V. Availability of infrastructural facilities such as developed industrial shed
or site, link roads, nearness to railway stations, airports or seaports,
32
availability of electricity, water, public utilities, civil amenities and means
of communication are important.
VI. Availability of skilled and non-skilled labor and technically qualified and
trained managers.
VII. Banking and financial institutions should be located nearby.
VIII. Safety and security should be given due consideration
IX. Government influences: tax relief, subsidies, liberation and other
positive policies of the government to support the start off of any
industry should be duly considered before any industry is set up. Also,
negative government influences like restrictions for setting up industries
in an area for reason of pollution control and decentralization of
industries should be considered.
X. Utility costs and availability.
2.9.4 SELECTION OF PLANT LOCATION FOR THE NITRIC ACID PLANT
There were three plant locations proposed. Each was evaluated and the final
decision based on maximum net advantage was made.
2.9.4.1 LOCATION ONE: AGBARA INDUSTRIAL ESTATE (OGUN STATE)
Advantages
1. Relatively cheap available land and labor cost.
2. Relatively close to market (Lagos Nylon and plastic market).
3. Relatively close to sea (Lagos Apapa) for import of raw material and export
of product if need be.
4. Availability of infrastructural facilities such as link roads, public utilities etc.
33
5. Availability of financial institution.
6. Relatively secure.
7. Availability of social amenities and means of communication.8. Disadvantages1. No local source of raw material nearby meaning all raw materials have to
be transported to the plant location.
2. The major roads that will be used for transportation (i.e form Apapa to
Agbara) are bad and one is prone to experience hold up on it.
3. Transport cost will be very high for both bringing in of raw material and
marketing finished product as the target market is Lagos and things are
known to be very expensive there.
4. The Nylon and plastic market in Lagos is not large enough to exhaust all
nitric acid produced by the plant.
5. Additional cost of providing water and electricity for the plant.
2.9.4.2 LOCATION TWO: ABA (ABIA STATE)
Advantages
1. Relatively cheap available land and labor cost.2. Availability of market (plastic and Nylon market)
3. Availability of financial institution.
4. Relatively secure.
5. Availability of social amenities and means of communication.
34
Disadvantages
1. Not close to source of raw material
2. Additional cost of providing water and electricity for the plant.3. Market available not enough to exhaust all nitric acid produced in the plant.
4. Lack of infrastructural facilities such as sea port, airport and railway stations
nearby.
2.9.4.3 LOCATION THREE: ELEME, PORT-HARCOURT (RIVERS STATE)
Advantages
1. Close to source of raw material: National Fertilizer Company of Nigeria
(NAFCON), an ammonia and fertilizer plant at Onne, Port-Harcourt, Rivers
State bought over by Notore started operation in Jan 2009. Their
production of ammonia per day of ammonia was 1,000MT as at 2009 of
anhydrous ammonia (more than enough raw material for our nitric acid
plant). Eleme Petrochemical located in Eleme, Port-harcourt, Rivers State is
also billed to come up with an ammonia plant in 2014 which will make
available to the Nigeria market 2300MT.
2. Availability of market in Port-Harcourt, closeness to sea for export of
product if necessary.
3. Availability of public utilities such as water, sea port, airport, etc.
4. Availability of both skilled and unskilled labor.
5. Availability of banking and financial institutions.
6. Availability of social amenities and means of communication.
7. Relatively secure.
35
Disadvantages
2. High cost of land
3. No regular power supply
2.9.5 PLANT LAYOUT
Having selected a suitable site for the chemical plant, it is possible and necessary
to make a preliminary decision regarding the layout of the plant equipment. (Ray
et al 1989) Although the equipment has not been designed in detail, preliminary
estimates of the physical size of each item should be available in the equipment
list. Any sizing differences between the initial and final estimates should not be
too excessive, and appropriate areas should be allowed around the plant items
when determining the layout.
A preliminary determination of the plant layout enables consideration of pipe
runs and pressure drops, access for maintenance and repair and in the event of
accidents and spills, and location of the control room and administrative offices.
The preliminary plant layout can also help to identify undesirable and unforeseen
problems with the preferred site, and may necessitate a revision of the site
selection. (Baasel 1989) The proposed plant layout must be considered early in
the design work, and in sufficient detail, to ensure economical construction and
efficient operation of the completed plant. The plant layout adopted also affects
the safe operation of the plant, and acceptance of the plant (and possibly any
subsequent modifications or extensions) by the community.
36
There are two schemes that can be adopted for determination of the plant layout.
(Buckhurst & Harker 1973) First, the ‘flow-through’ layout (or ‘flow-line’ pattern)
where plant items are arranged (sequentially) in the order in which they appear
on the process flow sheet. This type of arrangement usually minimizes pipe runs
and pressure drops (and is often adopted for small plants). Second, the
equipment is located on site in groupings of similar plant items, e.g. distillation
columns, separation stages, reactors and heat exchanger pre-heaters, etc. The
grouped pattern is often used for larger plants and has the advantages of easier
operation and maintenance, lower labor costs, minimizing transfer lines and
hence reducing the energy required to transfer materials. These two schemes
represent the extreme situations and in practice some compromise arrangement
is usually employed. The plant layout adopted depends upon whether a new
(‘grass roots’) plant is being designed or an extension/modification to an existing
plant. Space restrictions are the most common constraints; however, space
limitations are usually imposed even with new sites. Other factors to be
considered are:
(a) Siting of the control room, offices, etc., away from areas of high accident risk,
and upstream of the prevailing winds.
(b) Location of reactors, boilers, etc., away from chemical storage tanks.
(c) Storage tanks to be located for easy access, and a decision made as to whether
all tanks (for raw materials and product) should be located together or dispersed
around the site.
(d) Labor required for plant operation.
37
(e) Elevation of equipment.
(f) Requirements of specific plant items, e.g. pumps.
(g) Supply of utilities, e.g. electricity, water, steam, etc.
(h) Minimizing plant piping systems.
(i) Suitable access to equipment requiring regular maintenance or repair.
(j) Plant layout to facilitate easy clean-up operations and dispersion of chemicals
in the event of a spillage.
(k) Access to the plant in the event of an accident.
(1) Siting of equipment requiring cooling water close to rivers, estuaries, etc.
(m) Location of plant waste and water drainage systems (separate or combined?)
and treatment tanks.
(n) Adopting a plant layout that will act to contain any fires or explosions.
(o) Spacing between items of equipment (insurance companies specializing in the
insurance of chemical plants have specific recommendations for the distances
required between particular items of equipment).
The layout of plant equipment should aim to minimize:
(i) damage to persons and property due to fire or explosion;
(ii) Maintenance costs;
(iii) Number of plant personnel;
38
(iv) Operating costs; construction costs;
(v) Cost of plant expansion or modifications.
Some of these aims are conflicting, e.g. (i) and (iv), and compromises are usually
required when considering the plant layout to ensure that safety and economic
operation are both preserved. The final plant layout will depend upon the
measures for energy conservation within the plant and any subsequent
modifications, and the associated piping arrangements.
The process units and ancillary buildings are laid out in such a way to give the
most economical flow of materials and personnel around the site. Hazardous
processes are located a safe distance from other buildings. Consideration for
future expansion is also put in place. The ancillary buildings and service required
on the site include:
Administrative block
Laboratory
Storage for both raw materials and products
Maintenance workshop
Utilities (generator, steam boiler, transformer station)
Store for maintenance and operation supplies
Other amenities like car park, restaurant and clinic.
39
Fig 1.1 Expected plant layout.
2.9.6 PROCESS ROUTES FOR THE PRODUCTION OF NITRIC ACID
CHILE SALTPETRE/NITRATE PROCESS
Chile saltpetre is material which contains sodium nitrate NaNO3 with percentage
around 35-60%, and remaining percentage compounds with KNO3 and NaCl. This
raw material Chile saltpetre is concentrated by crystallization in pre-treatment of
ore to attain 95% NaNO3 and remaining KNO3 as feed raw material. (Kent 1983)
Sulphuric acid with 93% is mixed with the refined Chile saltpetre as per the ratio
required as per stoichiometry and sent into a retort which is made with cast iron
and the mixture is heated to 200oC with help of furnace flue gasses and coal fire.
40
Thus at this temperature, the following reaction is carried forward to produce
HNO3, nitric acid vapors.
NaNO3 + H2SO4 → NaHSO4 + HNO3
All hot vapors of nitric acid are sent to cool down in water circulated cooled silica
pipes, condensed HNO3 are collected in receiver which has material resistance to
nitric acid. Uncondensed gas which escapes from the collector is scrubbed with
cooled water in packed bed tower to collect nitric acid in dilute format. Liquid
sodium bi-sulphate is collected from the bottom outlet of the retort.
Advantage: it was one of the first methods used in the manufacture of nitric acid.
Disadvantage: source of raw material can be exhausted.
Fig 1.2: Manufacture of nitric acid from Chile Saltpetre.
41
BIRKELAND-EYDE PROCESS/ARC PROCESS
This process is based upon the oxidation of atmospheric nitrogen by atmospheric
oxygen to nitric oxide at very high temperature. An electric arc is used to provide
the high temperatures, and yields of up to 4% nitric oxide were obtained. ( Ohrue
1999)
N2 + O2 →2NO
The nitric oxide was cooled and oxidized by the remaining atmospheric oxygen to
nitrogen dioxide
2 NO + O2 →2NO2
This nitrogen dioxide is then dissolved in water to give dilute nitric acid.
3 NO2 + H2O → 2HNO3 + NO
Advantage: unlimited source of raw material (air)
Disadvantage: The process is very energy intensive and is only feasible when
electricity is available and cheap.
WINSCONSIN PROCESS/NITROGEN FIXATION PROCESS
Atmospheric oxygen and nitrogen are combined in a high temperature
regenerative furnace operating at about 2000oC. Nitric oxide is formed with a
yield of nearly 2%.
Advantage: it does not use electricity to provide the high temperature and
therefore does not have the disadvantage of the Birkeland-Eyed process.
42
Disadvantage: cannot compete favorably with the Ostwald process.
Another method of production of nitric acid via nitrogen fixation is the nuclear
nitrogen fixation route. This method directly combines oxygen and nitrogen.
Yields of nitrogen oxide of 5-15% have been reported by exposing air at 150 and
400oF to radiation from Uranium 235.
Advantage: gives a greater yield of nitrogen oxide than the Winsconsin process
Disadvantage: with this method comes all the disadvantages of nuclear reaction
(problem of managing the radiation which is harmful to living things)
OSTWALD PROCESS
In this process, anhydrous ammonia is oxidized to nitric oxide, in the presence of
platinum or rhodium gauge catalyst at high temperature of about 500K and a
pressure of 9bar. (Ray et al 1989)
4 NH3 (g) + 5 O2 (g) → 4 NO (g) + 6 H2O (g) (∆H= -905.2KJ)
Nitric acid is then reacted with oxygen in air to form nitrogen dioxide.
2 NO (g) + O2 (g) → 2NO2 (g) (∆H= -114KJ/mol)
This is subsequently absorbed in water to form nitric acid and nitric oxide
3 NO2 (g) + H2O (l) → 2 HNO3 (aq) + NO (g) (∆H= -117KJ/mol)
The nitric oxide is cycled back for re-oxidation. Alternately, if the last step is
carried out in air:
4 NO2 (g) + O2 (g) + 2H2O (l) → 4HNO3 (aq)
43
The aqueous HNO3 obtained can be concentrated by distillation up to about 68%
by mass.
There are 2 basic types of systems used to produce weak nitric acid:
Both processes follow the basic Ostwald process for the catalytic oxidation of
ammonia. In summary, this involves an oxidation stage whereby ammonia is
reacted with air in a catalytic converter at temperatures in the range of 850-
950°C. Reaction gases pass through a series of energy recovery stages before
entering an absorption column. The bottoms from the column are bleached of
dissolved nitrogen peroxide using air, and the resulting solution is the weak nitric
acid product (Roudier et al 1979).
The major difference between the two processes lies in the initial conversion
stage. The dual-pressure process employs a conversion stage operating in the
range l00-350kPa, and a reactor temperature of about 865°C. The single-pressure
process however operates the converter at 800-1100 kPa, with a reactor
temperature closer to 940°C. ( Harvin et al 1979)
1. Single-stage pressure process: in this case, the plant is operated at a single
pressure throughout.
44
Fig 1.3. Process flow diagram for single-stage pressure process.
Advantage:
Less expensive as less equipment’s are used.
The single-pressure process uses a higher ammonia conversion pressure.
This higher pressure provides advantages in terms of equipment design,
e.g. smaller converter dimensions and a single heat-exchanger-train layout.
( Leray et al 1979)
45
The higher temperature and the favorable pressure both increase the
energy recovery from the process.
Limited space availability may favor the single-pressure process
Disadvantage:
Less efficient as the overall process is favored by varying pressure.
Experimental work indicates that the rate loss of catalyst (without a catalyst
recovery system) is approximately three times more rapid at 973°C than at
866°C. This means that more catalyst is lost in the single-stage pressure
process ( Harvin et al 1979).
Absorber efficiency is reduced prompting the need for larger absorber
thereby increasing cost.
2. Dual-stage pressure process: here, the plant is operated at different
pressures and different stages.
Advantages:
The first reaction (catalytic conversion of anhydrous ammonia to nitric
oxide) is favored by lower pressure while the remaining reactions are
favored by higher pressures. This variation in pressure is achieved in dual-
stage pressure process. (Harvin et al 1979)
Capacities of 1130-1360 tonnes per day favor the dual-pressure process,
because of the possibility of absorption up to 1550 KPa.
Less catalyst is lost because of lower operating temperature
46
Fig 1.4. Process flow diagram for dual-stage pressure process.
47
The process selected for this design of nitric acid is single-stage pressure Ostwald
process because of its above mentioned advantages.
Fig1.5: Selected Process flow diagram for Nitric acid plant.
48
CHAPTER THREE
MATERIAL BALANCE
Material balance is one of the most important components of a process design.
Overall raw material of the entire process determines the qualities of raw
materials required and the products produced in the process.
Balance over individual process units determines the process stream flows and
their compositions and also the sizes of the various process equipment used in
the process.
Material balance on the plant used in the production of 400000 tonnes of Nitric
acid per year.
Mass flow rate = 400000 x 1000 kgyear = 50000
kghr
3.1 CONSERVATION OF MASS
For a steady state process, the accumulation term will be zero; but if a chemical
reaction takes place, particular chemical specie may be formed or consumed in
the process. When there is chemical reaction, the material balance equation is
given as,
Input + Generation = Output + Consumption
If there is no chemical reaction, the steady state balance reduces to;
Input = Output
49
A balance equation can be written for any identifiable specie present, elements or
compound; and for the total material.
3.2 METHODS OF MATERIAL BALANCE
There are two basic methods of material balance and they are;
(a) Algebraic Method
The algebraic method of material balancing is one of the simplest and most
common methods applied in balancing the materials that flow through a system.
It involves the systematic and sequential technique in indentifying some variable
sets which are related by some sets of linear or non-linear equations whose
solution depends on the resulting degree of freedom for the system. This degree
of freedom provides us with the limit of freedom for which we can set values for
some of the variable which is referred to as the design variables. A choice of
values for the design variables result in a corresponding value for the remaining
variables. The solutions to the equation set are obtained by the various method of
solution for simultaneous equations, most appreciably the methods of
substitution and elimination. The algebraic method is most efficient for simple
system but it may be inappropriate for complex systems involving large number
of units. The split fraction and method is recommended for such systems.
(b). Split Fraction Method
This method is based on the theory of recycle processes published by Magier
(1964). The method is based on the realization that the basic function of most
chemical processing units (Unit Operation) is to divide the inlet flow of a
50
component between two or more outlet streams. This method is ideal in carrying
out material balancing of complex of multi-unit plants.
3.3 MATERIALS BALANCE ASSUMPTIONS
The following assumptions were made during the material balance calculations:
1. The system is operating at steady state i.e. there is no accumulation of any
sort in the system.
2. There is negligible amount of inert in the process air.
3. Reasonably high conversion in the reactors.
4. Effect of side reactions is minimal.
3.4 SUMMARY OF MATERIAL BALANCE CALCULATIONS
From the steady state material balance equation, the flow rates of each stream
are calculated as follows.
3.5 MATERIAL BALANCE FOR EACH UNIT
Basis: 1hr
THE COMPRESSOR
1a 1a
Stream 1 Stream 2
Stream 3
51
Components Stream 1( kg/hr) Composition Stream 1a( kg/hr)
O2 49720. O2 49720.
N2 187080 N2 187080
Total 236750 Total 236750
THE MIXER
Stream 2 Stream 5
Stream 4
Components Stream 2(Kg/hr) Stream 4 (Kg/hr) Stream 5(Kg/hr)
52
O2 42760 - 42760
N2 160860 - 16086
NH3 - 13500 13500
H2O - 65 65
Total 203620 13565 217185
TOTAL 217185 217185
THE COVERTER
Stream 5 Stream 7
Stream 6
53
Composition Stream 5 Stream 6 Stream 7
O2 42760 - 11660
N2 160860 - 160860
NH3 13500 270 -
H2O 65 21060 -
NO - - 23320
HNO3 - - -
NO2 - - -
Total 217185 21330 195840
TOTAL 217185 217170
OXIDISATION VESSEL
Stream 7 Stream 8
Component Stream 7
(Kg/hr)
Stream 8
(Kg/hr)
O2 11660 -
N2 160860 160860
NH3 - -
H2O - -
NO 23320 1460
54
HNO3 - -
NO2 - 33530
Total 195840 195850
ABSORBER Stream 6 + Make -up water
Stream 9
Stream 8
Component Stream 6 +
make-up water
Stream 8
(Kg/hr)
Stream 9
(Kg/hr)
55
(Kg/hr)
O2 - - -
N2 - 160860 160860
NH3 270 - 270
H2O 28120 - 23830
NO - 1460 8600
HNO3 - 30000
NO2 - 33530 672
Total 28390 195850 224232
TOTAL 224240 224232
STRIPPER Stream 10
Stream 9
Stream 3 Stream 11
Component Stream 9 Stream 3 Stream 10 Stream 11
56
(Kg/hr) (Kg/hr) (Kg/hr) (Kg/hr)
O2 - 6960 4030 30000
N2 160860 26170 160860 -
NH3 270 - - -
H2O 23830 - 4260 20000
NO 8600 - 580 -
HNO3 30000 - - -
NO2 672 - 7260 -
Total 224232 33130 176990 50000
TOTAL 257362 226990
COMBUSTION CHAMBER
Stream 13
Stream 10
Stream 12
Component Stream 10
(Kg/hr)
Stream 12 Stream 13
(Kg/hr)
O2 4030 37370 2440
N2 160860 140580 301440
NH3 - - -
57
H2 - - 330
H2O 4260 - 26710
NO 580 - 410
HNO3 - - -
NO2 7260 - 7520
CH4 - - 860
C2H6 - - 50
CO2 - - 24480
Total 176990 177950 364240
TOTAL 354940 364240
PURIFICATION REACTOR
Stream 13 Stream 14
Component Stream 13 Stream 14
58
(Kg/hr) (Kg/hr)
O2 2440 2670
N2 301440 303790
NH3 - -
H2 330 170
H2O 26710 30230
NO 410 20
HNO3 - -
NO2 7520 380
CH4 860 -
C2H6 50 -
CO2 24480 27000
Total 364240 364260
Table 3.1: Summary of Material balance on each stream.
STREAMSCOMPONENT FLOW RATE (Kg/hr)
TOTALO2 N2 NH3 H2O NO HNO3 NO2 CH4 C2H6 CO2
1 49720 187080 - - - - - - - - 2367501a 49720 187080 - - - - - - - - 2367502 442760 160860 - - - - - - - - 2036203 6960 26170 - - - - - - - - 331304 - - 13500 65 - - - - - - 13565
59
5 42760 160860 13500 65 - - 217180
6 - - 27021060
- - - - - - 21330
7 11660 160860 - - 23320 - - - - - 1958408 - 160860 - - 1460 - 33530 - - - 195850
9 - 160860 27023830
860030000
670 - - - 224232
10 4030 160860 4260 580 - 7260 - - - 176990
11 - 30000 -20000
- - - - - - 50000
12 37370 140580 - - - - - - - - 177950
13 2440 301440 -26710
410 - 7520 860 5024480
364240
14 2670 303790 -30230
20 - 380 - -27000
364260
Table 3.2: Process matrix of the Nitric acid production process
EQUIPMENT ASSOCIATE STREAMSNUMBER NAME1 COMPRESSOR 1, -1a2 SPLITTER 1a, -2, -33 MIXER 2,4, -54 CONVERTER 5, -6, -75 OXIDISATION UNIT 7, -86 ABSORBER 6, 8, -97 STRIPPER 3, 9, -10, -118 COMBUSTION CHAMBER 10, 12, -139 PURIFICATION REACTOR 13, -14
CHAPTER FOUR
ENERGY BALANCE
60
The Energy balance gives the account of all the energy requirement of the process
which is based on the principle of conservation of energy. The principle states
that energy can either be create nor destroyed but can be transformed from one
form to another. Also energy can be transferred from one body to another.
If a plant uses more energy than its competitor, its product could be priced out of
the market. Accountability of the energy utilization of a process plant is
necessary in every design project.
The conservation of energy however differs from the mass in that energy can be
generated (or consumed) in a chemical process. Material can change form; new
molecular specie can be formed in a process unit and must be equal to the one
out at steady state. The same is not true for energy. The total enthalpy of the
outlet stream will not be equal to that of the inlet stream if energy is generated or
consumed in the processes, such as that due to heat of reaction.
Energy can exist in various forms: head, mechanical, electrical energy, and it is the
total energy that is conserved. In plant operation, an energy balance on the plant
will show the patterns of energy usage and suggest area for conservation and
saving.
4.1 CONSERVATION OF ENERGY
As for materials balance, a general equation can be written for energy balance;
W Q
Z1
Z2
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Energy out – Energy in + Generation – Consumption = Accumulation
This is a statement of the first law of thermodynamics. An energy balance can be
written for any process step. Chemical reactions will evolve energy (exothermic)
or consume energy (endothermic). For steady state processes, the accumulation
of both mass and energy will be zero (0).
Energy exists in many forms; the basic forms are listed below:
Potential Energy: This is due to position or height due to motion
Internal Energy: This is the energy associated with molecules and is dependent on
temperature.
Work: This is achieved when a force gets through a distance. Work done on a
system is positive while work done by a system is negative
Kinetic Energy: This is the energy due to motion.
For unit mass of material
U 1+P1V 1+U 12
g+Z1g+Q=U 2+P2V 2+
U 22
g+Z2g+W
Where, Q = Heat transferred across the system boundary
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W = Work done by the system
P1P2 = Pressure in Pressure Out
V1V2 = Volume in, Volume out
U1U2 = Velocity in, Velocity out
Z1Z2 = Height in, Height out
g = Acceleration due to gravity (9.81m/s2)
In chemical processes the kinetic energy factor (U 2
g ) and the Potential energy
factor (zg) are small and negligible and the relation between U and PV is
correlated in terms of enthalpy (H)
H = U + PV
H2 – H1 = Q – w
Also, the work term can be negligible in many chemical engineering systems.
Hence,
H2 - H1 = Q
4.2 ENERGY BALANCE ASSUMPTIONS
1. The process is at steady state
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2. No heat is lost from the vessel and from the pipe i.e. there is proper
lagging.
3. Effect of pressure on enthalpy is ignored .
4. Potential and kinetic energy changes are negligible.
4.3 SUMMARY OF ENERGY BALANCE
THE COMPRESSOR
Tin= 20°C Tout=155°C
TABLE 4.1: HEAT BALANCE AROUND COMPRESSOR.
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)N2 187030 0 187030 140.4O2 49720 87.56 49720 87.56
PROPERTIES QUANTITY/VALUEInlet Temperature( °C ) 20Outlet Temperature( °C ) 155 Heat duty( KJ/hr ) 26259012 Power and Actual Shaft work, repectively.(KJ/hr and KJ)
399515.49 and 475613.68
TABLE 4.2 HEAT BALANCE ABOUT THE AIR HEATER
For air component that passes through the air heater
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)
64
N2 187030 0 187030 80.79O2 49720 152.49 49720 152.49Inlet Temperature (oC) 155Outlet Temperature (oC) 200Heat Duty( KJ/hr ) 15107946.75
For nitrous gases recycled back to the air heater
PROPERTIES QUANTITY/VALUEInlet Temperature( °C ) 350Outlet Temperature( °C ) 200
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg) N2 160860 -161.10 16080 -161.10 NO 1460 0 1460 -155.11 NO2 33530 -196.5 3350 -196.5Heat Duty( KJ/hr ) -226460.6
TABLE 4.3 HEAT BALANCE AROUND THE CONVETER
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)NH3 13500 0 270 1902.99O2 42760 612.58 11660 612.58NO2 - - 23320 610.38N2 160860 693.63 160860 693.63H2O 65 1309.44 21060 1309.44Heat Duty( KJ/hr ) 20579273.83 KJ/hr
TABLE 4.4 HEAT BALANCE AROUND THE WASTE HEAT BOILER (Unit 9)
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg) O2 11660 -669.8 11660 -669.8 N2 160860 -719.25 160860 -719.25
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NO 23320 0 23320 -685.09PROPERTIES QUANTITY/VALUEInlet Temperature( °C ) 890Outlet Temperature( °C ) 250Heat Duty( KJ/hr ) -15976252Outlet Temperature of Steam (°C ) 410
TABLE 4.5 HEAT BALANCE AROUND THE OXIDIZING VESSEL
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg) O2 11660 100.27 - -N2 160860 107.80 160860 107.80NO 23320 0 1460 103.96NO2 - - 33530 131.00
PROPERTIES QUANTITY/VALUEInlet Temperature( °C ) 250Outlet Temperature( °C ) 350Heat Duty( KJ/hr ) 1240891.54
TABLE 4.6 HEAT BALANCE AROUND THE STACK GAS HEATER
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg) N2 160860 -52.909 160860 -52.909 NO 1460 0 1460 -719.25 NO2 33530 -65.50 33530 -65.50PROPERTIES QUANTITY/VALUE
Inlet Temperature( °C ) 150Outlet Temperature( °C ) 50Heat Duty( KJ/hr ) -149781.4Heat Duty of Steam( KJ/hr ) -74322.76 Temperature of Steam (°C ) 118.48
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TABLE 4.7 HEAT BALANCE AROUND THE ABSORPTION COLUMN
Component nin (Kg/hr) Hin (KJ/Kg) nout (KG/hr) Hout (KJ/Kg)NH3 270 0 270 576.48H2O 28120 117597.84 23830 99657.0N2 160860 167616.12 160860 167616.12NO 1460 1437.66 8600 8586.24HNO3 - - 30000 51600.0NO2 33530 43924.39 670 880.32
PROPERTIES QUANTITY/VALUEInlet Temperature( °C ) 50 Outlet Temperature( °C ) 54 Heat Duty( KJ/hr ) -53280.03
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HEAT BALANCE AROUND THE AMMONIA VAPORIZER, SUPERHEATER AND STRIPPER.
AMMONIA VAPORIZER
Heat Duty = 14978 KJ/hr
Outlet Temperature = -28.20 °C
THE AMMONIA SUPPERHEATER
Heat Duty = -1596252 KJ/hr
Outlet Temperature = 26.65 °C
THE STRIPPER
Heat Duty = -25873200 KJ/hr
Inlet Temperature = 250 °C
Outlet Temperature = 120°C
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CHAPTER FIVE
CHEMICAL ENGINEERING DESIGN
The equipment used in chemical process industries can be divided into two
classes: proprietary equipment such as pumps, centrifuge, etc which are designed
and manufactured by specialist firms; non-proprietary equipment which includes
the reactor, heat exchanger, evaporators, still, condensers and bleaching vessels.
The proprietary equipment will only be selected and specified while the non-
proprietary equipment will be designed as special, one-off, items for the
particular processes and purposes they are expected to serve.
The chemical Engineer’s part in the design of “non-proprietary” equipment is
usually limited to “selecting” and “sizing” the equipment. Same will be done in
this design work.
5.1 PROCESS UNITS OF NITRIC ACID PRODUCTION PLANT
The nitric acid process plant comprises:
1. Ion- Exchange Unit
This unit consists of series of packed beds containing various organic polymer
resins for the removal of unwanted divalent and monovalent ions. Used for the
generation of de-ionized water.
2. De-ionized water Cooler
Consist of finned fan-type cooler for cooling the circulating de-ionized water.
69
3. Air Compressor
Here air is compressed in two stages. The first-stage compression is a low-
pressure compression from atmospheric pressure up to 310 kPa. An axial
compressor is used which takes its shaft drive from a gas turbine. The second
compression utilizes a centrifugal-type compressor. The centrifugal compressor is
more efficient for the air flow-rate (36 000 kg/h) and outlet pressure (1090 kPa).
The centrifugal compressor takes its shaft drive from the expansion of tail gas.
Intermediate to the two compression stages is an intercooler which allows the air
temperature to be lowered from 180°C to 45°C, with a pressure loss of 10 kPa.
The temperature drop enables a more efficient second compression stage.
4. Ammonia Vaporizer
This unit consists of a shell and tube-type heat exchanger with two passes per
shell on the tube side. Operating pressure is 1240 kPa. The exchanger is made
from mild steel.
5. Ammonia Super-heater
It consists of a shell and tube-type heat exchanger of similar mechanical
construction to the ammonia vaporizer. It is constructed from mild steel.
6. Reactor
The reactor is a pressure vessel operating in the range 1050 kPa to 1100 kPa. The
bottom section of the reactor is jacketed. Air is preheated in this jacket prior to
mixing with ammonia. The bottom section of the reactor also contains a shell and
tube-type heat exchanger. This exchanger provides the final stage of tail-gas
70
preheating. Tail gas enters at 235°C and the reaction gases leave the exchanger
section of the reactor at 645°C.
7. Steam Super-heater
This unit superheats saturated steam from 250°C (and 4000kPa) to 380°C. The
product steam is of medium pressure and suitable quality for ‘in-house’
application and also for export. The super-heater cools the reaction gases from
the reactor exit temperature of 645°C to 595°C.
8. Waste-heat Boiler
A shell and tube-type exchanger required to heat pressurized (4000 kPa) hot
water from 117°C to a saturated vapour at 250°C. The waste-heat boiler cools
reaction gases from 595°C to 280°C.
9. Tail-gas Pre-heater
Also comprises of shell and tube-type exchanger. It takes reaction gases leaving
the platinum filter at about 315°C and 1020 kPa, and subsequently reduces their
temperature to 185°C. The cooling medium is tail gas. It enters at about 50°C and
leaves the tail-gas pre-heater at 235°C.
10. Cooler/Condenser
This unit condenses weak nitric acid from the gaseous mixture and cools the
remaining gases from an inlet temperature of 185°C to 60°C. The shell and tube-
type heat exchanger uses de-ionized water as its cooling medium.
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11. Oxidation Unit
The oxidation unit is an empty pressure vessel that takes input reaction gases and
blends in additional air from the bleaching column. The extra oxygen provided
enables further oxidation to occur and raises the gas mixture temperature to
140°C. At the top of the oxidation unit is a mist eliminator to prevent carry-over of
acid vapor by entrainment. At the bottom of the vessel is the weak-acid drain.
12. Secondary Cooler
The secondary cooler takes the exit gases from the oxidation unit at 140°C and
cools them down to 65°C, a suitable temperature for entry into the absorption
column. The cooling medium is circulating warm water from the warm-water
loop. The inlet temperature is 50°C and the exit temperature is about 80°C.
13. Absorber
The absorber is usually a sieve tray-type column. It has an operating pressure
around 990 kPa. A bursting disc is used for pressure relief. Each tray is provided
with cooling coils to allow the cooling of the absorption liquor. There are two
independent cooling circuits, each uses de-ionized water. The top section has an
inlet temperature of 7°C and an outlet temperature of 20°C. The bottom section
cooling loop has an inlet temperature of 20°C and an exit of 40°C. The use of two
cooling circuits provides greater flexibility in manipulating absorption conditions
in the column. The tail gas leaves the column at about 10°C. Weak acid from the
cooler/condenser is added to an appropriate tray midway up the column, and
make-up water at 7°C is added to the top tray. The acid drained from the bottom
of the column contains some dissolved nitrogen oxides.
72
14. Stripping Column
The bleaching column is a smaller sieve tray-type column. Impure acid runs down
the column from the top tray and air is bubbled up through the liquor to remove
dissolved nitrogen oxides. The acid from the base of the column is the final
desired 60% (wt.) product.
15. Storage Tank
Stores the supply of nitric acid produced from the process plant.
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CHAPTER SIX
EQUIPMENT DESIGN
The need to design process equipment may arise as a result of the desire to:
i. Modify an existing process equipment or
ii. Develop new equipment.
Modification of existing equipment may be required as a result of poor
performance or the need of scale up (or down). For example, increased market
success of a product may lead to increased production. It may be more
economical to increase the capacity of the existing equipment rather than add
another line of equipment. This is usually the case when operational cost costs
(man power, energy etc.) are high.
New equipment, on the other hand may be desired as a result of successful
laboratory research and pilot plant studies or as a result of satisfactory process
simulation using the computer.
In either situation (new or existing equipment), the actual design commences
with the assessment of the characteristics of the feed materials, the products and
the physical and chemical processes required to convert the raw material to
products. The overall satisfactory performance and reliability of the equipment
would depend on the following factors.
I. Optimum processing conditions
II. Appropriate materials of construction
III. Strength and rigidity of components
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IV. Satisfactory performance of mechanical part
V. Reliable methods of fabrication
VI. Ease of maintenance and repairs
VII. Ease of operation and control.
VIII. Safety requirements
IX. Environmental impact
The typical process equipment design procedure will involve:
1. Specifying the problem
2. Analyzing the probably solution
3. Preliminary design, applying chemical engineering process, principles and
theories of mechanics relevant to the problem.
4. Selecting appropriate materials of construction.
5. Evaluating and optimizing the design, the possible application of computer
aided design (CAD) system like HYSIS, Aspen Plus etc
6. Preparing the drawings and specifications
6.1. PROBLEM SPECIFICATION
The specification of the problem is the key stone in the quest to design an
equipment to meet the needs of the customer. Specification of a problem may
include:
1. The quantity of material to be processed in a given time such as the
proposed capacity of the equipment.
2. The physical and chemical properties of the product.
75
Constraints such as:
a. Availability and cost of materials of construction
b. Availability and cost energy, water, oil etc.
c. Budget for production
d. Availability and cost of manpower with relevant skill for fabrication
e. Space to be occupied by the equipment
f. Environmental issues
g. Safety issues
h. Number of working days in the year
i. Ergonomics
6.2. ANALYZING THE PROBLEM SOLUTION
A thorough analysis will reduce the list for example if the equipment is to be used
for small scale processing. All the constraint listed above will need to be
considered.
6.3. PRELIMINARY DESIGN: APPLYING CHEMICAL ENGINEERING
PROCESS PRINCIPLE AND THEORIES OF MECHANICS.
Probably the most important expression in the design of process equipment is
that of mass and energy balance which may be expressed in general term as;
Input + generation – output – consumption = accumulation
This expression is found in various forms in thermodynamics, fluid mechanics,
transport phenomena, heat transfer, separation process and other subject areas.
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It is simply an expression of indestructibility of matter and energy. This expression
applies to all raw materials, intermediate and product.
6.4 MATERIAL SELECTION
Materials are critical in the design of process equipment. Materials must be
selected to take care of possible corrosion problems. Materials of construction
should also possess adequate mechanical properties to withstand tensile,
compressive, shear and impact stresses.
Stainless steel of various grades finds wide application in process equipment
design especially for parts in contact with raw materials and product. Glass,
plastic and rubber lined vessels are also used are also used when materials tend
to react with steel. Steel of various carbon contents are used for compounds such
as shaft, springs and gears and for support structure.
6.5 DESIGN OPTIMIZATION
The calculation process in the design of equipment may require simple arithmetic,
algebraic, differential calculus or integral calculus. In many cases an exact solution
may not be feasible thus necessitating the use of various approximation
techniques such as graphical or numerical methods.
In many cases also, only some parts of the equipment are designed on the basis of
analytical calculations. Practical conditions are used to determine the
specifications of the remaining part. It is thus not unusual to have several feasible
solutions. There is thus the need to select the best solution. The ultimate goal is
to minimize cost or maximize profit.
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In chemical process industries, equipment used are classified into two;
Proprietary equipment such as pumps, centrifuges which are designed and
manufactured by a specialist firm.
Non- proprietary equipment such as reactors, heat exchangers, condenser,
bleaching vessels etc are designed as specially requested.
6.6 SUMMARY OF THE DESIGN AND SPECIFICATION OF EQUIPMENT
CALCULATION.
In designing and specifying of equipment for chemical industries, the variables
/parameters involved namely; pressure, temperature, density, volume, area,
diameter, height, heat duties, heat capacities etc must be carefully calculated.
This gives the designer exact data for fabrication and manufacturing. For the
production of Nitric Acid; the following equipment are designed and specified;
Nitric Acid storage tank, Ammonia storage tank, Absorber, Converter, Oxidation
vessel, heat exchangers.
FOR REACTORS
The operating intensity is given for the reactors=11296.324kg/m2/24hrs
=11296.324kg/m2/day
Equipment Mass of reactant (kg/h)
Area(m2) Diameter(m)
Converter 13500 28.68 6.04Oxidation Vessel 23320 49.55 7.94Absorber 33530 71.24 9.52
The stripper column has 10 plates
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FOR STORAGE TANKS
Equipment Type Nitric Acid storage tankShape Cylindrical Nature InsulatedMaterial of Construction Stainless SteelCapacity 50000000kg/hrVolume(m3) 23.8Diameter(m) 4.6Height(m) 13.9
Equipment Type Ammonia Storage tank Shape Cylindrical Nature Insulated
Material of Construction Stainless Steel Capacity 13565kg/hrVolume(m3) 66.8Diameter(m) 6.5 Height (m) 19.6
FOR HEAT EXCHANGERS
Using the formulae; Q=AUDTm
A= Q/UdTm
Where
Q= Heat Duty of the heat exchanger(KW)
A= area(m2)
U= Overall Heat Transfer Coefficient(KW/m2)( This is assumed for all)
79
DTm=Log Mean Temperature Difference(Celsius)
Using a countercurrent flow; DTM= DT1−¿DT2/ ln(DT1 /DT2)
DT1 = Thin-TcoutDT2 = Thout –TCinEquipment Q (KJ/hr) Thin Thout TCout TcoutWaste Heat Boiler (1) 15976252 890 250 30 410Air Heater 350 200 150 250
Stack Gas Heater
74323 200 150 30 118.5
Waste Heat Boiler(2)
149781.4 208 50 150 32
NH3 Super Heater
4437.85 410 330 26.6 28.2
NH3 Vaporizer
149781.4 208 167.2 28.2 33.4
Table 6.1: Table showing the heat transfer area of some equipment
Equipment Q(KW/S) Area(M2)Stack Gas Heater 20.65 2Waste Heat Boiler(1) 4437.85 130.5Waste Heat Boiler(2) 41.61 11.83NH3 Super Heater 4437.85 127.6NH3 Vaporizer 41.61 1.87Air Heater 4133.7 588.7
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CHAPTER SEVEN
PROCESS CONTROL AND INSTRUMENTATION
Instruments are provided to monitor the key process variables during plant
operation. They may be incorporated in automatic control loops, or used for the
manual monitoring of the process operation. They may also be part of an
automatic computer data logging system. Instruments monitoring critical process
variables will be fitted with automatic alarms to alert the operators to critical and
hazardous situations.
It is desirable that the process variable to be monitored be measured directly;
often, however, this is impractical and some dependent variable, that is easier to
measure, is monitored in its place.
7.1 OBJECTIVES
The primary objectives of the designer when specifying instrumentation and
control schemes are:
1. Safe plant operation:
(a) To keep the process variables within known safe operating limits.
(b) To detect dangerous situations as they develop and to provide alarms and
automatic shut-down systems.
(c) To provide interlocks and alarms to prevent dangerous operating procedures.
2. Production rate: To achieve the design product output.
3. Product quality: To maintain the product composition within the specified
quality standards.
4. Cost: To operate at the lowest production cost, commensurate with the other
objectives.
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These are not separate objectives and must be considered together. The order in
which they are listed is not meant to imply the precedence of any objective over
another, other than that of putting safety first. Product quality, production rate
and the cost of production will be dependent on sales requirements. For example,
it may be a better strategy to produce a better-quality product at a higher cost.
In a typical chemical processing plant these objectives are achieved by a
combination of automatic control, manual monitoring and laboratory analysis.
7.2 PLANT CONTROL CONFIGURATION
The plant will be designed for manned operation and will be linked to the
adjacent fertilizer manufacturing plant. Certain configurations will be put in place
to monitor some key parameters of the plant.
The acid plant process control will be embedded in the plant DCS. The
instruments of the individual process units will be terminated in junction boxes
located at the unit’s skid limits. From here these instruments will be connected to
instrument cabinets in the auxiliary room and integrated in the PAS.
The plant safety instrument system (SIS) will be independent of the PAS. There
will be a link between the PAS and the SIS for data monitoring/logging and
maintenance/operational override control purposes. Fire and gas monitoring will
also be a dedicated module integrated in the safeguarding system.
The process control schemes of some vital units are discussed as follows:
Absorption column
The process control scheme for the absorption column is presented in fig It was
designed from the recommendations presented in the HAZOP analysis.
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It features ratio control on the make-up water stream. The signals from flow
transmitters on this line and on the gas input line are fed to the ratio controller,
whereby the make-up water stream is adjusted.
Other control features include a pressure controller on the tail-gas outlet stream
so that the column absorption pressure can be maintained at the design
operating value of 950 kPa. A temperature transmitter on the tail-gas outlet
stream provides the signal for control of the overall cooling-water flow rate. This
is the temperature which is most useful in determining good absorption. The
cooling circuit itself is fed from a common line (on which the overall flow rate is
controlled). Small block valves on each of the tray cooling-coil feed lines enable
flow rate regulation to each of the coils. These valves feature a removable top
whereby a magnetic flow meter may be inserted to read the flow rate. The valves
need only be set initially and then periodically adjusted manually.
There is no automatic control on the flow rate of the gas inlet stream or weak-
acid condensate stream, since both of these flows are predetermined by feed
flow rates earlier in the process. Isolation valves and provision for spectacle blinds
are included to enable the column to be isolated during shutdown periods.
The product-acid solution is withdrawn from the column using a level control
valve on this line. The liquid level in the base of the column must be maintained
slightly above the level of the plate downcomer to prevent incoming gas from by-
passing the sieve plates.
All controllers suggested for the absorption column feature HIGH and LOW alarms
for good control.
The final safety requirement is a relief line with a relief valve protected by a
bursting disc.
83
Air heater
The process control scheme suggested for the air heater is shown in Fig. This flow
scheme features a control valve on the compressed air inlet line. A temperature
controller taking its signal from the heater outlet line ensures the flow is
regulated to maintain the heater temperature of 250°C. Air pressure is controlled
prior to entry into the unit and is kept constant at 7.3 atm.
A pressure indicator on both inlet and outlet steam lines enables this parameter
to be adequately monitored.
The nitrogen oxide reaction gas stream cannot be directly controlled from the air
heater. Instead the flow rate, temperature and pressure are predetermined by
the reactor feed conditions.
Both inlet and outlet lines possess isolation valves for plant shutdown. These lines
would be blanked before any platinum recovery work was attempted on the
heater. Inlet and outlet lines also feature temperature indicators, consistent with
the policy of constant monitoring of this parameter throughout the process.
Ammonia Vaporiser and Superheater
Pressure indicator and controller will be installed to maintain ammonia vapor at
7.3 atm. Temperature indicator and controller is required to ensure that the
outlet temperature of 250°C is achieved in the superheater. The control scheme is
shown in the figure below.
Ammonia Converter
Temperature control system is needed within the converter to ensure that the
temperature in the converter does not drop below the reaction temperature of
890-900°C, to avoid loss of heat.
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7.3 ALARMS, SAFETY TRIPS AND INTERLOCKS
Alarm systems need to be installed in specific areas to alert operators of serious,
and potentially hazardous, deviations in process conditions. Key instruments are
fitted with switches and relays to operate audible and visual alarms on the control
panels and annunciator panels. Where delay or lack of response, by the operator
is likely to lead to the rapid development of a hazardous situation, the instrument
would be fitted with a trip system to take action automatically to avert the
hazard; such as shutting down pumps, closing valves, operating emergency
systems.
The basic components of an automatic trip system are:
1. A sensor to monitor the control variable and provide an output signal when a
preset value is exceeded (the instrument).
2. A link to transfer the signal to the actuator, usually consisting of a system of
pneumatic or electric relays.
3. An actuator to carry out the required action; close or open a valve, switch off a
motor.
The high-temperature alarm operates a solenoid valve, releasing the air on the
pneumatic activator, closing the valve on high temperature.
7.3.1 INTERLOCKS
Where it is necessary to follow a fixed sequence of operations for example, during
a plant start-up and shut-down, or in batch operations interlocks are included to
prevent operators departing from the required sequence. They may be
incorporated in the control system design, as pneumatic or electric relays, or may
be mechanical interlocks. Various proprietary special lock and key systems are
available.
85
Table 7.1: Letter Code for Instruments Symbols
Property
measured
First letter Indicating only Controlling only
Flow – rate F FI FC
Level L LI LC
Pressure P PI PC
Temperature T TI TC
Humidity H HI HC
I - Indicator C - Controller
L - Level T - Temperature
F - Flow rate P - Pressure
H - Humidity
(Source: Sinnott, R.R 1999).
7.4 LINING, PIPING, VALVES AND PUMPS
In Fig.7.1, which is the piping and instrument diagrams, there are various
mechanical component introduced in the plant to obtain maximum efficiency
some of which includes, flanges, valves, piping lines, blinds, gaskets and so on.
7.4.1 VALVES
The valves used for chemical process plant can be divided into two broad classes,
depending on their primary function:
Shut-off valves (block valves), whose purpose is to close off the flow.
Control valves, both manual and automatic, used to regulate flow.
86
The table below shows some of the valves used in the P and I diagram (figure 5),
their symbols, and functions.
Table 7.2: Types of Valves and Symbol Used In PID
NAME SYMBOL FUNCTIONS
Used to control flow in lines.
Fitted on sensitive lines and are either pneumatically or digitally controlled.
Fitted in lines of relatively high pressure or velocity
Used for control of gas or vapour flows
7.4.2 JOINTS
Control Valves
Automatic Valves
Check Valves
Butterfly Valves
87
There are various joints used in fig 3.0 either as flow reducers, or to aid the
carrying property of pipe. And effective transport of fluids in the piping flow.
Below is a table of the various elbows and joints used in the P and I diagram:
Table 7.3: Joints
JOINTS AND
ELBOWS
SYMBOLS FUNCTIONS
EQUAL ‘T” REDUCER
JOINT
90o T – CONNECTOR
ELBOW
LONG – RADIUS
ELBOW
Used to reduce a flow line
into three equal lines
Used in joining a running line
to a flow line.
Used in channeling lines also
reduces flow speed.
Used in channeling lines in
pipe support.
Used in branching lines.
Used in reducing pressure
88
45o LATERAL
REDUCER
flow.
7.5 PIPE SUPPORT
The Design of a plant’s P and I is not complete without the use of supports. Pipe
supports in plant piping helps in reducing cost and number of pump required to
maintain line flow parameter and safety of personnel through operation zone.
Below is some major type of support:
I – BEAM Support to carry pipe lines
H – BEAM Support above 2m
U – CHANNEL
PLATES TO ALIGN VALVES
SHOES TO HOIST PIPE INTO PROPER ORIENTATION
CHAPTER 8
SAFETY AND ENVIRONMENTAL CONSIDERATIONS
8.1 SAFETY
Safety is the condition of being protected against any danger. Every organization
89
CHAPTER 8
SAFETY AND ENVIRONMENTAL CONSIDERATIONS
8.1 SAFETY
Safety is the condition of being protected against any danger. Every organization
90
The term “engineering safety” covers the provision in the design of control
systems alarms, trips, pressure relief devices, automatic shutdown system and
duplication of key equipment, firefighting equipment and service; personnel
protect equipment and so on.
8.1.1 SAFETY OF THE ENVIRONMENT
There are several hazards associated with industrial process. These hazards need
to be prevented and kept in check in order to protect the environment.
Environment in this context refers to the immediate surroundings around the
plant. For the safety of the environment to be ensured, the following points
should be noted and applied;
1. Flaring of gases should be done minimally.
2. The level of toxicity of effluent should be monitored regularly and kept in
check.
3. Storage tanks should be situated in areas away from vehicle traffic.
4. The control room should be attended to at all times to ensure that there is
an immediate response if an alarm is triggered.
5. There should be a way of informing the community around the facility if
there is danger that might affect them e.g Fire. An alarm is suggested, and
this should be tested regularly.
6. Protect pipe racks and cable trays from fire.
7. Fire-fighting system must be provided within the complex. This consist:
Fire water pipe network throughout the facility supported by necessary
hydrants. Hoses should be permanently placed near these hydrants.
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The system should have a suitable water pump. It is advised that there be
at least 2 pumps. One big one to fill the lines or pump large volume of
water into it (when the water is being depleted very fast like in a case of
fire) and a smaller jockey pump to maintain the pressure in the line. It
would start more frequently than the big pump.
It is suggested that the system should have its own separate standby
generator.
Fire entry suits and other protective clothing, compressed air breathing
apparatus and fire blankets should be made available in every building.
8.1.2 SAFETY OF THE PERSONNEL
The personnel of a company refers to the operators and staff of that company
who ensure that the production process move on smoothly. Their safety can be
ensured in the following was.
1. Provision of personal protective equipment (PPE) and ensuring that they
are properly used.
2. Pipes and equipment that contain very hot liquids for example the heater
must be lagged to make sure it does not cause injury to personnel.
3. All chemicals in the plant must be properly label with its chemical hazard
identification chart and their Material Safety Data Sheet (MSDS) must be
available and updated regularly also.
4. First aid kits must be provided in all buildings.
5. Emergency means of transportation must be provided in case of any
accident.
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6. Emergency exit doors must be provided and these exits clearly marked for
all to see.
7. Cleanliness of the facility must be ensured at all times to avoid
unnecessary risk or accident.
8. Smoking should be avoided in process area.
9. Fire extinguishers must be made available at strategic points within the
facility.
10. All ladders must have hand rails and personnel encouraged to use them
whenever climbing.
11. Safety signs and symbols should be placed at hazardous area.
8.1.3 SAFETY OF THE PLANT AND EQUIPMENT
A plant includes any machinery, equipment (including scaffold), appliance,
implement or tool and any associated computer or fitting used in the production
process of a material or substance. There are different types of risks associated
with using different types of equipment. To mitigate this risk, the following must
be done:
1. As Nitric acid is known to be highly corrosive, regular pigging of the
pipelines with both smart and scraper pigs should be conduct so as to
check this.
2. Corrosion protection is achieved by the well proven use of suitable
austenitic stainless steel where condensation can occur and by regular
monitoring of the conditions.
3. Regular maintenance of equipment should be carried out.
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4. Check to ensure that equipment and machineries meet health and safety
standards before it is purchased.
REACTOR
Reactor is a vessel in which chemical transformation takes place. The converter,
oxidizing unit and absorber are the reactors in this design. The catalytic reactor is
designed to give a uniform distribution of the air/ammonia mixture over the
catalyst gauzes. Maintenance of the catalyst operating temperature is very
important for the NO yield. This is achieved by adjusting the air/ammonia ratio
and ensuring that the lower explosive limit for ammonia in air is not exceeded.
The following safety steps should be followed in the design and operation of the
reactor.
The materials going into the reactor must be purified. This is done to
remove impurity that will affect the reactor.
The reaction condition i.e. temperature, pressure etc must be monitored
closely.
The reactor should be cleaned accordingly during periodic maintenance.
COMPRESSOR
While operating the compressor, the following precautions should be observed.
The air must be dried properly to avoid water entering into the compressor
which could damage it.
The air must be filtered properly to avoid foreign particle from entering into
the compressor.
The proper operating pressure should be maintained at all times.
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Coolant must be checked regularly and topped.
In case of shut down, the shut down and start up procedures should be
strictly adhered to.
HEAT EXCHANGERS
For heat exchangers to work effectively and safely, the following must be
implemented.
Ensure that the heat exchanger is pressure tested as designed.
Ensure that it is cleaned periodically and faults and leakage rectified if
found. Inspect after cleaning before coupling back.
The water must be purified especially the one going into the boiler to
remove chlorine because ignition occurs when chlorine is passed into
ammonia forming nitrogen and hydrochloric acid and if chlorine is present
in excess, then a highly explosive nitrogen trichloride (NCl3) is formed. As
we cannot guarantee 100% conversion of ammonia in the converted and
no leak in the boiler, the water should be purified.
PUMPS
For the pumps to work effectively and safely, the process operators should ensure
that;
The right operating temperature and pressure must be used.
There should be a minimum flow line that helps to maintain a certain rate
of flow in the pump preventing it from going into cavitation.
Lube oil to gland bearings is available, bottle filled and ready. The lube oil
should be changed at regular intervals.
The vents should be properly cleaned and capped
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There should be no particle or foreign body in both the suction and
discharge lines.
TANKS
Nitric acid is normally stored in flat bottomed, roofed tanks, made from low
carbon austenitic stainless steel, installed in areas provided with suitable
containment facilities. The acid level in the tank is monitored by means of a level
indicator. A vent to the atmosphere allows the escape of gas which comes from
liquid movement and thermal effects. It is normal to earth the tanks. For the life
span of the tanks to be preserved, the following should be done:
The tank should be cleaned regularly
Periodically, the tanks should be checked for corrosion and the affected
part could be painted.
The right operating conditions for the tanks must be maintained.
8.1.4 GENERAL SAFETY PRECAUTION
Generally, precaution is taken to prevent accidents or hazards (a potential
danger). Hazards can either be intrinsic or extrinsic. Intrinsic hazards are naturally
occurring hazards caused by wind, earthquake, lighting and water. The effects of
intrinsic hazards include leakage in pipes, explosion of pipes, collapse of
production buildings, rupture of welded joints etc. the following precautions can
be taken to mitigate the dangers posed by intrinsic hazards:
Lightning rods should be installed at strategic points.
Drainages should be wide and deep enough, and should also be channeled
properly to prevent flooding with the facility.
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Explosive substances have to be stored in a cool dry place away from
sunlight and ultraviolet rays.
The floor around the industry should be properly cast around with concrete
or other weather resistant covering and the road networks properly tarred
Extrinsic hazards are man-made hazards caused by man due to carelessness.They
may include dropping of an oil filter on the floor, igniting flame close to a
flammable substance, not knowing how to operate a machine or equipment,
ignoring procedure for starting or using equipment, using wrong tools. They can
be prevented by the following measures:
Personnel coming into the facility for the first time must be given a proper
safety orientation on the do’s and don’ts of the company policy on safety
and regular safety talks on safety and maintenance of plant must be
conducted.
Regular training of personnel on safety issues should also take place.
Good housekeeping practice by all employees should be encouraged.
8.2 HAZARD OPERABILITY (HAZOP) STUDY
Hazard and operability study sometimes simply referred to as operability
Studies, provide a systematic and critical examination of the operability of a
process. They indicate potential hazards due to deviations from the intended
design conditions.
8.2.1 BASIC PRINCIPLES
A formal operability study is the systematic study of the design, vessel by
vessel, and line by line, using “guide words” to help generate thought about
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the way deviations from the intended operating conditions can cause
hazardous situations.
The following words are also used in a special way, and have the precise
meanings given below:
• Intention: the intention defines how the particular part of the process was
intended to operate; the intention of the designer.
• Deviations: these are departures from the designer’s intention which are
detected by the systematic application of the guide words.
• Causes: reasons why, and how, the deviations could occur. Only if a
deviation can be shown to have a realistic cause is it treated as meaningful.
• Consequences: the results that follow from the occurrence of a meaningful
deviation.
• Hazards: consequences that can cause damage (loss) or injury.
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Table 8.1 HAZOP STUDY: Weak-acid condensate stream
Deviation Possible consequences Consequence Action RequiredNo Flow 1.Pump Failure
2. Valve fails shut.
3. Line fracture
Deficient quality product and high NOx tail gas emission levels.As for IValve overheats.As for I and 2.
a) Install LOW LEVEL ALARM on LIC at the base of the absorption column.Covered by a).b) Install kick-back on pumpsCovered by a) and b).c) Regular inspection and patrolling of weak-acid transfer lines and seals.
More Flow 4. Higher humidity in feed air. Higher make but weaker product acid.
d) Install a HIGH LEVEL ALARM on LIC at the base of the absorption column
More Temperature 5. high feed rate causing larger heat of reaction
Possible higher NOx emission. See Table 8.6.
More Pressure 6. Isolation valve is closed in error while pump running.
7. Thermal expansion on the isolation.
Lines subject to full delivery pressure
Possible line fracture or flange leakage
Covered by b)f) Perhaps worthwhile installing a pressure gauge upstream of the delivery pumpg) Provide thermal expansion relief in theValve section.
Less Flow 8. Flange leakage or valve stub blanked but leaking
Decreased absorption.Lower product make
Covered by a), c), and d).
Less Temperature 9. Reaction gas temperature in oxidation unit lower.
Increased dissolved NOx concentrations in product acid.
See Table 8.6.
Maintenance 10. Equipment failure, flange leak, catalyst changeover in reactor, etc.
Process stops. Ensure all pipes and fittings are constructed of the right materials and arc stress relieved.
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Table 8.2 HAZOP study: Make-up water feed stream.
Deviation Possible causes consequences Action RequiredNo Flow 1.Pump Failure
2. Valve fails shut.
3. Line fracture
Deficient quality product and high NOx tail gas emission levels.As for IValve overheats.As for I and 2.
a) Install LOW LEVEL ALARM on LIC at the base of the absorption column.Covered by a).b) Install kick-back on pumpsCovered by a) and b).c) Regular inspection and patrolling of weak-acid transfer lines and seals.
More Flow 4. Control valve fails open Dilute acid product is formed d) Install a HIGH LEVEL ALARM onLIC at the base of the absorption column
More Temperature 5. Higher feed temperature to the Refrigeration unit. 6. Failure in the refrigeration unit.
Possible higher NOx emission due to lower absorption. As for 5.
See Table 8.6
e) Ensure refrigeration unit is wellmaintained with adequate control
More Pressure 7. Isolation valve is closed in error whilst pump running.
8. Thermal expansion in the isolation valve section (fire).
Line subject to full delivery pressure.
Possible line fracture orflange leakage
Covered by b).IJ Perhaps worthwhile installing a pressure gauge upstream of the delivery pump.g) Provide thermal expansion relief in the valve section.
Less flow 9. Flange leakage or valve stub blanket but leaking
Decreased absorption.Higher operating cost in lost water
Covered by a),c) and d)
Less Temperature I0. Reaction gas temperature in oxidation unit lower.
Increased dissolved NOx concentrations in product acid.
See Table 8.6
Maintenance 11. Equipment failure, flange leak, catalyst changeover in reactor, etc .
Process Stops Ensure all pipes and fittings are constructed of the right materials and are stress relieved.
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Table 8.3. HAZOP study: Gas-inlet stream
Deviation Possible causes consequences Action RequiredNo Flow 1.Flow stopped upstream
2. Line blockage or the isolation valve shut in error.
3.Line fracture
No absorption in column. Entire process stops as tail-gas Row stops.
As for I. Pressure buildup in pipe and secondary As for I. Gases escape into the surroundings.
a) Ensure liquid feeds to absorber and other process unit shut down.b) Install LOW FLOW ALARM on FIC.Covered by a) and b).c) Install kick-back on upstream pumps and ensure pressure relief system is adequated) Ensure regular patrolling of feed transfer lines.e) Plant emergency shutdown procedures
More Flow 4. Increased feed Possible reduction in absorptionEfficiency.May cause flooding.
F) Ratio control on the liquid feed streams should be sufficient.g) Install HIGH LEVEL ALARM on the FIC.
More Pressure 5.Flooding
6. Isolation valve accidently closed7. Thermal expansion in isolation.
Unit subject to high pressure, bursting discs may rupture, tail gas release.As for 2.Line fracture or flange leakage.
Covered by c).h) Ensure correct sizing on pressure relief system.Covered by b) and c). i) Provide for thermal expansion relief in the design of the isolation valve section
More Temperature 8. Insufficient cooling Decreased absorption, higher pollution.
j) Ensure accurate temperature control on the internal cooling circuit.
Less flow 9. Leaking inlet range As for 3. Covered by b), and d)Less Temperature 10. Overcooling. Increased dissolved gasses in acid. Covered by j)H i g h NOx composition 11. Improved yield from reactor. Higher tail-gas emission levels
possible.k) Manually increase make-up water Composition flow rate.
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Table 8.4. HAZOP study: Gas – Outlet stream.
Deviation Possible causes consequences Action RequiredNo flow I. No inlet gas flow.
2. Flooding in column.
3. PCV fails shut, line blockage or isolation valve closed in error.
4. Line fracture or flange failure
No tail gas for expansion.Pressure build up in column and lineAs for I.
As for 2.
As for 1.
See Table 8.1.a) Install LOW LEVEL ALARM on PIC.b) Install pressure relief valve with bursting disc.Covered by b).c) Install HIGH LEVEL ALARM on PIC.Covered by a)d) Institute regular inspection of all transfer lines.
More flow 5. Increased gas feed at inlet.
6. Decreased NOx absorption
Transfer line subject to higher pressures. As for 5. Tail-gas emission levels up.
Covered by b) and c).Covered by b) and c).d) Look to altering make-up water feed rate in response.
More Temperature 7. Higher feed gas or liquid inlet temperature.
Decreased absorption and higher NOx emission.
e) Install HIGH LEVEL ALARM on TIC
More pressure. 8. All of 5, 6, and/or 7.9. Thermal expansion in isolation valve section (fire).10. PCV fails shut or isolation valve shut in error.
As for 5, 6, and 7.Line fracture or flange leakage.
As for 3
Covered by b) and c).f) Install thermal expansion relief in isolation valve section.Covered by b) and c).
Less flow 11. Leaking flange or valve stub not blanked and leaking.12. Flooding.
Less tail gas for expansion and release of NOx to the environment.
Covered by a) and d).
Liquid Carryover 13. The entrainment device ineffective.
Condensation is down, steam lines (corrosion).
Replace entrainment device.
Maintenance 14. Equipment failure, flange leak, catalyst changeover in reactor, etc .
Process stops. Ensure all pipes and fittings areconstructed of the rightmaterials and are stress relieved
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Table 8.5 HAZOP study: Liquid-outlet stream.
Deviation Possible causes consequences Action RequiredNo flow. I. No liquid inlet from either make-up
water or acid condensate.2. Flooding in column.
3. LCV fails shut.
4. Line fracture.4. Line fracture.
See Tables 9.6 and 9.7
Increase in column pressure.Liquid level in column increases adding to flooding problems.
Discharge of acid into the surroundings.Loss of feed to the stripping column
a) Covered by control and alarms specified in Tables 8.1, 8.2 and 8.3.Covered by a).b) Install HIGH LEVEL ALARM onLIC.c) Regular patrolling and inspection of transfer lines.d) Install suitable alarms to strippingcolumn to indicate loss of flow
More flow 5. LCV fails open. Gas begins to bypass the plates causing higher NOx emissions.
e) Install LOW LEVEL ALARM on LIC.
More temperature 6. Higher inlet temperatures Less dissolved NOx in acid but higher NOx tail-gas emissions
Covered in Tables 8.1, 8.2 and 8.3.
More pressure 7. LCV fails shut or isolation valve close in error.
Line subject to full surge or delivery pressure.
Covered by a)
Less flow 8. Leaking flange or valve stub not blanked and leakage.
Loss to surrounding. Covered by d) and e)
Less temperature 9. Lower inlet stream temperatures or over capacity from cooling circuit.
Higher concentrations of dissolved NOx in product acid.
See Tables 8.1, 8.3 and 8.4.
NOx dissolved 10. Lower steam temperature. Higher downstream operating costs.Same as 9.
See Tables 8.1,8.3 and 8.4
maintenance 11. Equipment failure, flange leak, catalyst changeover i n reactor, etc.
Process stops Ensure all pipes and fittings are constructed of the right materials and are stress relieved
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Table 8.6: HAZOP study: cooling-water circuit.
Deviation Possible causes consequences Action RequiredNo Flow 1.Pump Failure
2. Valve fails shut.
3. Line fracture
High emissions of NOx in tail gas.
As for 1Valve overheats.As for 1 and 2
a) Install HIGH LEVEL ALARM on TIC on the tail-gas outlet line to indicate high emissions.Covered by a).b) Install kick-back on pumps.Covered by a) and b).c) Regular inspection and patrolling ofcooling-water circuit lines and associated
More flow 4. Control valve fails open. Product acid is at lower temperature therefore, higher dissolved NOx.
d) Install a LOW LEVEL ALARM on TIC on the tail gas outlet line
More temperature 5. Higher feed temperature to the refrigeration unit.
Possible higher NOx emissions due to lower absorption.As for 5.
Covered by a)e) Ensure refrigeration unit is wellmaintained with adequate control
More pressure 7. Isolation valve is closed in error while pump running.8. Thermal expansion in the isolation valve section (fire).
Lines subject to full delivery pressure. Possible line fracture or flange leakage.
Covered by b).f) Perhaps worthwhile installing a pressure gauge upstream of the delivery pump. g) Provide thermal expansion relief in the valved section
Less flow 9. Flange leakage orvalve stub blanked but leaking
Decreased absorption. Low quality product and high emissions.
Covered by a), c), and d).
Less Temperature 10. Higher duty from refrigeration unit.
Increased dissolved NOx concentration in product acid.
Covered by a), c), and d).
Maintenance 11. Equipment failure, flange leak, catalyst changeover in reactor , etc
Process stops Ensure all pipes and fittingsare constructed of the rightmaterials and are stress relieved
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8.3 ENVIRONMENTAL IMPACT ASSESSMENT (E.I.A)
This is the assessment of the possible positive and negative impact that a
proposed project may have in the environment, together consisting of the
environmental, social and economic aspect. It is a systematic process of
identification, prediction evaluation, mitigating and presentation of possible
consequences on the environment of proposed actions at a stage in decision
making process so that environmental damage can be minimized or avoided.
8.3.1 WHAT EIA DOES
• Describes the project or operation
• Describes the environment that will be affected
• Predicts the impact on the environment
• Adopts options, techniques and controls to reduce negative impact.
• Monitors the project or operation to ensure that identified key impact is
minimized.
8.3.2 GOAL
• To ensure that decision makers consider the ensuing environmental
impacts when deciding whether to proceed with a project or not.
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8.3.3 BENEFITS OF ENVIRONMENTAL IMPACT ASSESSMENT
• May be prerequisite for permit approval by government or international
agencies.
• Required by financiers of the proposed projects
• Help to prevent environmental problem, risk or costly-time working
liabilities.
• Boosts Proponent Company’s image.
• Repose confident /assurance in Proponent Company.
8.3.4 ENVIRONMENTAL IMPACT ASSESMENT (EIA) OF A NITRIC ACID PLANT.
Negative impact
The major negative impact of a nitric acid plant is NOx emissions of the tail gas
from the absorption tower especially during start up and shut down before the
plant stabilizes. Others include:
Risk of fire/explosion hazard.
Reduced soil and marine water quality.
Increase in water and electricity demand.
Noise pollution.
Positive impact
Provide skilled and unskilled job opportunities.
Make available nitric acid in the country, thereby encouraging the production
of fertilizer as it is a major chemical used in its production.
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Bring development to the area where the plant is sited.
8.3.5 NOx EMISSION FROM NITRIC ACID PRODUCTION.
Nitric acid production is one of the larger chemical industry sources of NO. Unlike
NOx found in combustion flue gas, NOx from nitric acid production is part of the
process stream and is recoverable with some economic value. Vent gas containing
NOx is released to the atmosphere when the gas becomes too impure to recycle
or too low in concentration for recovery to be economically practical.
The chemical reactions for each of the nitric acid production process steps
demonstrate that NOx must first be created before nitric acid can be produced.
The first reaction,
4NH3 + 5O2⇌4NO + 6H2O + heat Eq. 1
Shows NO forming from the reaction of NH and air. The NO is then oxidized in the
second step,
2NO + O2⇌ 2NO2 + heat Eq. 2
Producing NO2. The NO2 is subsequently absorbed in water to produce nitric acid.
However, as the absorption reaction,
3NO2(g) + H2O(l)⇌2HNO3 (aq) + NO(g) + heat, Eq. 3
Shows, one mole of NO is produced for every three moles of NO2absorbed,
making complete absorption of the NOx impossible. The unabsorbed NOx, if not
controlled, is emitted in the absorber tail gas.
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8.3.6 FACTORS AFFECTING NOx EMISSION LEVELS.
Re-oxidation of NO into NO2 is a very slow reaction. As more air is added,
the reaction becomes increasingly slower as the reactants become diluted
with excess nitrogen.
Increased temperatures due to exothermic absorption tend to reverses eq.
3 producing more NO2.
Low temperature (less than 380C [1000F]) is a key factor forhigh absorption
efficiency but is also one that is difficult and expensive to control.
Completion of the absorption process which reduces NOx emission is aided
by increased pressure (800 to 1,400 kPa)
Increasing acid strength beyond design specification typically increases the
NOx emission rate.
Good maintenance practices and careful control ofoperations play
important roles in reducing emissions of NOx.
8.3.7 CONTROL TECHNIQUES FOR NOx EMISSIONS FROM NITRIC ACID
PRODUCTION.
Extended absorption
Extended absorption reduces NO emissions by increasing absorption efficiency
and is achieved by either installing a single large tower, extending the height of an
existing absorption tower, or by adding a second tower in series with the existing
tower. Increasing the volume and the number of trays in the absorber results in
moreNOx being recovered as nitric acid (1-1.5% more acid) and reduced emission
levels.
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Non-selective catalytic reduction (NSCR).
Nonselective catalytic reduction uses a fuel and a catalystto
1. Consume free oxygen in the absorber tail gas.
2. Convert NO2 to NO for decolorizing the tail gas.
3. Reduce NO to elemental nitrogen.
The process is called nonselective because the fuel first depletes all the oxygen
present in the tail gas and then removes the NOx. It can be operated at any
temperature, heat used to operate it can be recovered and it can achieve higher
NOx reduction than extended absorption but it is expensive due to the cost of
fuel.
Selective catalytic reduction (SCR)
Selective catalytic reduction uses a catalyst and ammonia in the presence of
oxygen to reduce NOx to elemental nitrogen. The process is called selective
because the ammonia preferentially reacts with NOx in the absorber tail gas. The
following sections discuss SCR used as a NO control technique for nitric acid
plants. Proper operation of the process requires close control of the tail gas
temperature.
Chilled Absorption.
Chilled absorption provides additional cooling to the absorption tower. This
process is frequently used in addition to other control techniques such as
extended absorption. The principal advantage of chilled absorption is improved
absorber efficiency due to lower absorption temperature. However, chilled
absorption by itself typically cannot reduce NOx emissions to the level that any of
the three primary control techniques can achieve.
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8.3.8 ENVIRONMENTAL MANAGEMENT PLANT
The following are actions taken to mitigate the negative impacts of the plant sited
above.
1. Use extended expander and chilled absorption to increase absorber
efficiency and thus reduce NOx emission.
2. Site the plant far away from residential area so as to reduce noise pollution
effect and risk of fire.
3. Update on site emergency response plan.
4. Have a generator to provide the electrical needs of the plant and own
water supply system.
5. Monitor stack gas emissions.
6. Test liquid waste to make sure the level of chemical in it is acceptable
before discharging to sea.
CHAPTER 9
ECONOMIC ANALYSIS
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9.1 OVERVIEW
Chemical processes have been harnessed to transform resources, and raw
materials into more useful and hence more valuable products to improve the
living standards of people. This principle is at the core of chemical engineering,
and there industries have matured over the last 100 years, and have been very
successful at creating wealth. The means of establishing which products to make
and how to optimize the process required for the manufacture have been based
on economic principles. Approaches to accounting for the risks to the economic
value of projects are also considered to ensure that they deliver the expected
benefits.
9.2 ECONOMIC CONSIDERATION
The following are considered under economic evaluation;
1. Cost and Assets Accounting: This provides a survey of accounting
procedures for the analysis of cost and profits as used for industrial
applications.
2. Cost Estimation: This provides information regarding the estimation of
fixed capital cost and also recurrent operating expenditure.
3. Interest and Investment costs: This discusses the concept and calculation
of interest, i.e payment as compensation for the use of borrowed capital.
4. Taxes and Insurance: Taxes represent a significant payment from a
company’s earnings and although insurance rates are only a small fraction
of annual expenditure cover for a plant is essential.
111
5. Depreciation: This is the measure of the decrease in value of an item, with
respect to time and can be considered as a cost incurred for the use of the
equipment.
6. Profitability, Alternatives, Investments and Replacements: The
profitability of an investment is a measure of the amount of profit
generated. It is important to assess the profitability accurately and also the
profit that could be obtained from alternative investments.
10.4 TYPES OF COST
X.4.1 INDIRECT COST
1. Design and engineering cost; which cover the cost of design and the cost of
“engineering” the plant: purchasing, procurement and construction
supervision. Typically 20% to 30% of the direct capital cost.
2. Contractor’s fees: if a contractor is employed his fees (profit) would be
added to the total capital cost and would range from 5% to 10% of the
direct cost
3. Contingency allowance: this is an allowance built into the capital cost
estimate to cover for unforeseen circumstances (labor disputes, design
errors, adverse weather) typically 5% to 10% of the direct cost.
Table 9.1: typical factors for estimation of project fixed capital cost
MAJOR EQUIPMENT , TOTA; PURCHASE PCE
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COST (F1 TO F9)
Equipment erection 0.40
Piping 0.70
Instrumentation 0.20
Electrical 0.10
Building process 0.15
Utilities 0.50
Storage 0.15
Site development 0.05
Ancillary buildings 0.15
9.4.2 Total physical plant cost (PPC)
PPC = PCE (1 + ∑factors ) = PCE × 3.4
Ranging from f10 to f12
Design and engineering 0.30
Contractors fee 0.05
contingency 0.10
Fixed capital = PPC (1 + f10 + f11 + f12) = PPC × 1.45
9.4.3 OPERATION COSTS
An estimate of the operating costs, the cost of producing the product, is needed
to judge the viability of a project and to make choices between possible
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alternative processing schemes. These costs can be estimated from the flow
sheet, which gives the raw material and service requirements, and the capital cost
estimate
The cost of producing a chemical product will include the items listed below. They
are divided into two groups.
1. Fixed capital cost: Costs that do not vary with production rate. These are
the bills that have to be paid whatever the quantity produced
2. Variable operating cost: Costs that are dependent on the amount of
product produced.
9.4.4 FIXED COSTS
1. Maintenance (labor and materials)
2. Operation labor
3. Laboratory costs
4. Supervision
5. Plant overhead
6. Capital charges
9.5 COST ESTIMATION
9.5.1 THE RATIO METHOD
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The ratio method is a simple technique whereby known capital cost data for an
existing chemical plant are adjusted to provide a cost estimate for the desired
plant capacity. This method is also able to update figures to account for
inflationary effects of past years. Finally the capital cost figure is adjusted for
exchange rate differences between countries .The method is centered around the
use of key cost estimation indices such as the CE plant cost index and the Marshall
and Stevens (M&S)index.
Ratio method calculations;
Cost of Designed plant¿Cost of previous plant ( capacity of designedCapacity of previous plant
)n
Cost of1200 tonsday
=Costof 280 tons /day×( 1200280
)0.6
cost of 280 tons /day=¿ $60million
Therefore;
cost of 1200/day=$60million ×( 1200280
)0.6
= $143.66 million
= ₦22.7 billion
9.5.2 STEP COUNTING METHOD
Step counting estimating methods provide a way of making a quick order of
magnitude estimate, of the capital cost of a proposed project. The technique is
based on the premise that the capital cost is determined by a number of
significant processing steps in the overall process. Factors are usually included
to allow for the capacity, and complexity of the process: material of
construction, yield, operating pressure and temperature.
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step counting method calculations;
C=14000NQ0.615
Where Q=plant capacity, tonne per year.
N=number of functional units
C=capital cost.
Q=400,000tons/yr
N=13
C=14000×13×(400,000)0.615
=$507millionequivalent ¿79.8billionnaira.
9.5.3 FACTORIAL METHOD
Capital cost estimates for chemical process plants, are often based on an estimate
of the purchase cost of the major equipment items, required for the process, the
other costs being estimated as the factors of the equipment cost .The accuracy of
this type of estimate will depend on which stage the design has reached at the
time.
ECONOMIC ANALYSIS CALCULATIONS
ESTIMATION OF FIXED CAPITAL COST
Rough Estimate
116
Cf = Fl Ce
Ce = ₦ 824.43 million
Cf = 4.7 × 828.43 = ₦ 3.89 billion
Detailed Estimate
PPC = Ce ( 1 + ∑factors )
∑factors = 3.3
PPC = 824.43 (1 + 3.3) = ₦3.55 billion
Total Fixed Capital = 3.55 ( 1 + 0.45) = ₦ 5.15 billion
Working capital = 5% × 5.15 = ₦258 million
Total Capital Investment Cost = Total Fixed Capital + Working Capital
Total Capital Investment Cost = 5.15 + 0.258 = ₦ 5.41billion
OPERATING COST ESTIMATION
Variable Cost ₦ million
1) Raw materials 0.25
2) Miscellaneous 0.24
3) Utilities Cost
Cooling water Negligible
Steam Negligible
117
Power 0.4
4) Shipping $ Packaging 1.15
Total 2.04
FIXED COST
₦ Million
1) Maintenance 1.21
2) Operating Labor 7.26
3) Laboratory Cost 2.18
4) Plant Overhead 3.63
5) Insurance 387.15
6) Royalties Not applicable
Total 401.43
Annual Total Operating Cost = 2.04 + 401.43 = ₦ 403.47 million
Annual Operating Cost Rounded = ₦ 403.5 million
ANNUAL PROFIT CALCULATION
Total Expenses = operating cost + capital finance cost
Total fixed capital investment = ₦ 5.15 billion
118
Working capital = ₦ 258 million
Total capital cost = ₦ 5.41 billion
(Assuming 5% compound interest rate annually and 2 years investment)
Total interest = p (1 + r )n
= 5.41 (1 + 0.05)2
= ₦ 5.96 billion
Operating cost for 2 years = 2 × 0.403 = ₦ 806 million
Total Expenses = ₦ (0.806 + 5.96) billion
= ₦ 6.77 billion
Cost of Nitric acid per ton = ₦ 10500
Annual total cost of Nitric acid = 7100 × 400,000
= ₦ 4.2 billion
Total cost of Product for 2 years = ₦8.4 billion
Annual total cost of steam =
10500 kg 8000 hours ₦ 7.5
hour 1 year 454kg
119
Annual total cost of steam = ₦ 1.39 million
Total income before tax = ₦ (5.68 billion + 2 × 0.00139 billion)
= ₦ 8.4 billion
Total income after tax (based on 2.5% tax) = ₦ (8.4 – 0.025 × 8.4) billion
= ₦ 8.19 billion
Profit after tax = ₦ (8.19 -6.77) billion = ₦1.42 billion
Payback period (no interest) = Depreciable FCI / Total profit
= 5.15 / 1.42 = 3.63 years
Therefore, payback period = 3years 8 months rounded.
BREAK EVEN ANALYSIS
On the assumption that market price of nitric acid will remain constant for a
reasonable length of time. The breakdown period for the plant will simply be the
inverse of the rate return on the investment
∴ Break even time (yrs) = Totalcapital investment
annualnet profit
¿ 5.411.42
=3.81 yrs
Break even time= 3yrs 10 months.
RATE OF RETURN ON INVESTMENT
R O R = yearly profit / total initial investment × 100%
120
= 1.42/5.41 × 100/1 = 26.25%
CHAPTER TEN
STARTUP AND SHUTDOWN PROCEDURES
121
Shutdown is that period of time during which a boiler, gas turbine, process
heater or nitric acid production unit is allowed to cool from its normal
temperature range to a cold or ambient temperature.
The shutdown philosophy is based on the nitric acid plant process control and
safeguarding philosophy reference and adapted to suit the developments in the
design. High nitric acid supply availability is of paramount importance. The level of
safeguarding reflects the need for the plant to operate safely whilst ensuring
maximum availability.
Shutdowns of the main process will be avoided as much as possible within the
constraints of safer operation. Additional time is given to the operator to correct
process upsets by intentionally accepting cascading events. This in turn will result
in fewer disruptions in the process
For all separators, low low liquid level will cause the corresponding liquid outlet
SDV (Shut down Valves) to close rather than generating an OSD (Operational Shut
Down)
On high liquid level and high pressure in the main nitrous gas stream, gas
flow is stopped by closing the inlet shutdown valves. This is to avoid liquid
carry over to the absorption column and stripper.
Gas compressor unit.
In the absorption column, a high liquid level and a high pressure will close
the corresponding inlet SOV.
Trips in the off air compressor package will stop the compressor and the air
flow will be directed to the flare.
122
10.1 EMERGENCY SHUTDOWN (ESD) AND EMERGENCY
DEPRESSURIZATION (EDP)
Emergency Shutdown and Depressurization of pressurized vessels and piping is
the acknowledged way to reduce the likelihood of escalation from accidental
hydrocarbon release incidents.
An ESD will be automatically initiated on confirmed low instrument air pressure
and manually initiated on confirmed gas detection via ESD push button. The aim
of an ESD is to bring the plant to a safe condition by;
1. Isolate the plant from the flow lines, stopping all hydrocarbons containing
streams from coming in and going out of the plant.
2. Depressurization the plant.
3. Starting down the fired heaters
10.2 NOTIFICATION
Prior notification of scheduled shutdowns and scheduled start-ups following
scheduled shutdowns shall be made in a timely manner and form. Shutdowns and
start-ups must be scheduled in pairs with scheduled dates for each. Notification
of scheduled start-ups and shutdowns is required only if an exemption from the
emissions limit is required. This notification shall contain the following
information:
1. Dates and times of the scheduled start-up and shutdown and its duration,
and
2. Any other process variable that is appropriate as determined.
123
10.3 RECORD KEEPING
Records shall be maintained and kept on-site and made available for two years
indicating hour-by-hour firing rates, flue gas temperatures, NOx emissions and
such process variables that are appropriate.
Once all of these equipment checks are performed, the complete unit is
disassembled, all parts and bearings are rechecked and oiled, the lubrication
system is drained and flushed, and the train is re-assembled. A time-consuming
aspect of the drive train checkout involves plotting of the unit’s surge curves.
Once the unit is operational, the air compressor can be used to blow out
downstream air and stream lines.
Other equipment debugging procedures are performed according to individual
“punch lists” and are summarized as follows:
Liquid piping and coded vessels: Pressure tested with water at maximum
working pressure.
Gas lines: cannot be checked until plant is operating
Relief valves: bench tested with required pressure—if serious problems
exist, they are sent out for repairs.
Heat exchanger: flushed with water or a cleaning solution.
Waste Heat Boiler: undergoes a hydrostatic check followed by pre-
treatment with chemicals to prevent corrosion due to oxygen or water prior to
plant start-up. A final procedure before production starts consists of filling the
boiler with water and warming with steam to prevent shock to the system
Absorber column: shipped to the plant as a complete package and can be
of either a bubble cap or sieve tray arrangement. The column is prepared by
124
flushing with water to clean and check flow and level indicating instruments.
Sieve tray columns are more sensitive to gas versus liquid flow and may
require 1hr to seal properly whereas a bubble cap unit may take about 20mins
Instrumentation: Cannot be installed until all other equipment is in place. A
critical component is ammonia/air ratio control system which must be
accurately calibrated to read concentrations of about 9-11 Percent ammonia in
air.
10.4 STARTUP OPERATIONS
Once all equipment is installed and thoroughly checked for proper mechanical
operation (this may take from 2 to 6 months), the plant is ready to undergo
initiation of nitric acid production. Preliminary startup operations consist of the
following steps:
1. Startup of air compressor system
2. Initiation of water flow to absorber tower
3. Platinum gauze lit by hydrogen torch to initiate burning of ammonia (flame
is self-sustaining)
4. Ammonia flow is begun
Within 2 to 3 weeks of this initial startup, the plant is ready for a test or
demonstration run. Test runs usually last 3, 7, or 14 days depending on the
contract. During this time, the plant must achieve its peak efficiency, of maximum
design rate, and meet all applicable emission regulations. A violation of any of
these conditions or other equipment mal-functions results in a cessation of the
test run. The conclusion of a successful test run results in the “legal acceptance”
of the plant from the contractor.
125
The best point in time to define plant startup is when the ammonia flows to the
converter is initiated. Barring no usual problems, the completion of a successful
test run and the achievement of maximum production rate should be about one
month or less from this starting point. An important point with respect to nitric
acid facilities is that the summer months are the most critical for proper operation
due to cooling requirements for the exothermic reaction involved. For this reason,
most new plants try to come online during the hotter periods when a successful
test run would be most meaningful. Because of the requirement for performance
testing within 180 days of startup, it is conceivable that testing could be required
during the cooler months when a plant would find it easiest to meet applicable
emission limitations. In this instance, regulatory agencies might want to conduct
testing as soon after startup as possible, consider postponement of tests until the
following summer, or consider winter testing and subsequent summer testing.
CHAPTER 11
CONCLUSION AND RECOMMENDATION
11.1 CONCLUSION
From the design procedures followed and results obtained, it can be concluded
that a plant can be set up to produce 400,000 tonnes of Nitric acid per annum
from ammonia oxidation. The excess steam generated in the process can be
gathered and sold to increase the total income to be realized from sale of
126
products. Also, the exhaust gases from the turbine is reduced to the lowest
minimum (<1000ppm). This is to reduce the NOx emission from the plant which is
in line with the Federal Environmental Protection Agency (FEPA) regulations. The
produced acid will be sold mainly to fertilizer manufacturing plants and oil
servicing companies in Nigeria, and can be exported as well.
Finally, an economic evaluation of the plant showed that the rate of return on
investment is about 26.25% and the payback time is about 3years and 7 months.
Therefore the project can be said to be economically feasible.
11.2 RECOMMENDATION
Additional control schemes should be put in place to ensure very low nitrous oxide emission; this will contribute to the global objective in reducing environmental degradation. There should be considerations for a two stage air compression to supplement the fluctuations in air requirement due to the anticipated increase in the demand of nitric acid. There should be provisions for preventive maintenance, as this will help to reduce frequent shutdowns due to repairs. It is also anticipated that this plant will be part of a larger chemical complex. Ammonia will be produced by steam reforming of natural gas. The nitric acid plant will take a portion of the ammonia product, and nitric acid and ammonia will then be used to produce ammonium nitrate.
127
REFERENCES
Ababio, O.Y. 2005, New General Chemistry, Africana- Fep Publishers, Sydney.
Aneke, L. E. 2009, Principles of chemical engineering process design,
De-adroit innovation, Enugu. Anon, A. 1979 ‘Nitric Acid rolls on’ Chemical Engineering 29 June, pp. 24-25.
128
Boland, D. & Linnhoff, B. 1979 ‘The preliminary design of networks for heat exchangers by systematic methods’ Chemical Engineering, London 22 April, pp. 25-27.
Brown, K. J. 1989 Process integration initiative (review of the process integration initiatives funded under the Energy Efficiency R&D Program), Energy Technology Support Unit, Harwell Laboratory, Didcot United Kingdom, pp. 221-236.
Canon, B.W 1998 Safety and health in workplace, Nostrand Rein hold, New York, pp.201-203.
Cheremisinoff, N. P. 2000, Chemical process equipment, Butterworth Heineman, New Delhi.
Chilton T.H. ‘The manufacture of nitric acid by oxidation of Ammonia : the Du pont pressure process’ Chemical Engineering Progress, Monograph Series Vol. 56, AIChe, New York.
Coulson J. M. & Richardson J. F. 2004 Coulson & Richardson’s Chemical engineering, 6th Ed. Vol. 1, Elseiver publishers London.
Durilla, M. 2009, NOx and NO2 control in nitric acid plants, Queens Publishing House, U.S.A.
Felder R. & Rousseau R. 2000, Elementary principles of chemical processes, 3rd Ed. John Wiley & sons, New York.
Gregory T.C 1999, Uses and Applications of chemicals and related materials, Reinhold Publishing, New York.
129
Harvin R.L, Leray D.G & Roudier L.R 1979, ‘Single pressure or dual pressure nitric acid: an objective comparison’, Ammonia Plant Safety, Vol. 21, pp.173-183, AIChe, New York.
Himmelblau, D. M. 2003, Basic principles and calculations in chemical engineering, 6th Ed. Prentice Hall, India.
House, F. F. 1969 ‘Engineers guide to plant layout’ Chemical Engineering, NY 76 July 28 p.120.
Kent J.A, 1983, Reigel’s Handbook of Industrial Chemistry, Van Nostrand Ranhold Publishing, New York.
Kirk B.E & Othmer D.F (Eds) 1981, Encyclopedia of Chemical Technology 3rd Ed. Vol.15 Wiley-Interscience, New York, pp.853-871.
Linnhoff, B, Dunford, H & Smith, R 1983, Heat integration of distillation columns into overall processes, Chem. Eng. Sc., 38(8), pp. 1175-1188.
Martyn, S.R. & David, W. J. 1989, Chemical engineering design: a case study approach, Bell and Bain Ltd, Glasgow.
Max, S.P, Klus, D. T. & Ronald, E.W 2003, Plant design and economics for chemical engineers; 5th Ed., McGraw-Hill, New York.
Ohrue T., Ohkubo K. & Imai O. 1999, Technological improvements in strong nitric acid process, Vol. 21 pp.164-170, AIChe, New York.
Perry R. H., Green D. W. & Maloney J. O 2008, Perry’s Chemical Engineers’ Handbook, 8th Ed. McGraw-Hill, New York.
130
Sinnot, R.K 2005, Chemical engineering design, 4th Ed., Butterworth-Heinemann, London.
131
APPENDIX I
TABLES AND CHARTS
Table A.1: Conversion factors for some common SI units
132
Table A.2: Typical Overall Coefficient
133
Table A.3: Typical Design stress for Plates
Figure A.1: Temperature correction factor: for one Shell; two or more even tube passes Heat exchange
134
APPENDIX II
MATERIAL BALANCE CALCULATION
Basis: 1hour
4000000tons HNO3 1 year
1 year 8000 hours
=50tonsHNO3 solutions/hour
ABSORBER AND STRIPPER
3NO2+H2O ⇌2HNO3+NO
50tons HNO3 solution 0.6 tons HNO3
1 ton HNO3 solution
=
30 tons HNO3 Produced
30 tons HNO3 3 tons moles
NO2
1 ton mole
HNO3
46 tons NO2 100 tons NO2
fed
2 ton moles
HNO3
63 tons HNO3 1 ton mole
NO2
98 tons NO2
converted
=33.528 tons NO2 fed
135
1 ton mole NO 30 tonsHNO3 1 ton mole HNO3 30 tons NO
2 ton moles HNO3 63 tons HNO3 1 ton mole NO
= 7.143 tons NO Produced
OXIDISING UNIT
2NO+O2⇌2NO2
33.528 tons NO2
fed
1 ton mole O2 1 ton mole NO2 32 tons O2
2 ton moles NO2 46 tons NO2 1 ton mole O2
=11.662 tons O2 converted
11.662 tons O2 1 ton mole O2 2 tons moles NO 30 tons NO
32 tons O2 1 ton mole O2 1 ton mole NO
=21.866 tons NO converted
11.662 tons O2 1 ton mole O2 2 tons moles NO
fed
30 tons NO fed
32 tons O2 1 ton mole O2 fed 1 ton mole NO fed
=21.866 tons NO fed
CONVERTER
4NH3+5O2⇌4NO+6H2O
23.197 tons NO 5 tons moles O2 1 ton mole NO 32 tons O2
136
converted converted
4 tons moles NO 30 tons NO 1 ton mole O2
converted
=30.929 tons O2 converted
Quantity of O2 fed to the converted= (11.662+30.929) =42.591 tons O2 fed.
23.197 tons NO 4 tons moles NH3 1 ton mole NO 17 tons NH3
4 tons moles NO 30 tons NO 1 ton mole NH3
=13.145 tons NH3 converted.
13.145 tons NH3 1 ton NH3 fed
0.98 ton NH3 converted
=13.413 tons NH3 fed.
Quantity of NH3 leaving converter= (13.413-13.145) =0.268 tons NH3.
23.197 tons NO 6 tons moles H2O 1 ton mole NO 18 tons H2O
4 tons moles NO 30 tons NO 1 ton mole H2O
=20.877 tons H2O Produced.
0.005 tons H2O 13.413 tons NH3
0.995 tons NH3
0.0679
Total Quantity of H2O leaving Converter= (20.877+0.0679) = 20.95 tons.
Make up H2O is added to ensure efficient chemosorption.
Quantity of makeup water= 7.059
137
Quantity of H2O fed to Absorber= (7.059+21.582) = 28.641 tons of H2O.
30 tons HNO3 1 ton mole HNO3 1 ton mole H2O 18 tons H2O
63 tons HNO3 2 tons moles HNO3 1 ton mole H2O
=4.286 tons H2O Required.
=0.64 tons O2
O2 left to react=6.32 tons O2 {6.96 – 0.64}
Assume 50% conversion of NO
Amount of NO reacted = 0.5 × 8.6 =4.3tons NO
0.27 tons NH3 5 mols O2 1 mol NH3 32 tons O2
4 mols NH3 17 tons NH3 1 mol NH3
4.3 tons NO 1 mol O2 1 mol NO 32 tons O2
2 mol NO 30 tons NO 1 mol O2
=2.29tons O2 reacted
Amount of leaving stripper: 6.32 – 2.29 = 4.03 tons O2
Amount of NO left unreacted: 8.6 – 4.3 = 4.3 tons
=0.48 tons NO
Total amount of NO leaving stripper = 4.3 + 0.48 =4.78 tons NO
138
0.27ton NH3 6 mols H2O 1 mol NH3 18 tons H2O
4 mols NH3 17 tons NH3 1 mol H2O
=0.43 tons H2O
Total amount of water vapor leaving the stripper = 23.83 + 0.43 -20 =4.26 tons
NB: Amount ofH2O in HNO3 solution =20 tons.
4.3 tons NO 2 mols NO2 1 mol NO 46 tons NO2
2 mols NO 30 tons NO 1 mol NO2
=6.59 tons NO2
Total NO2 leaving stripper =6.59 + 0.672 = 7.262 tons NO2
COMBUSTION CHAMBER AND PURIFICATION REACTOR.
COMBUSTION CHAMBER
CH4 + 2O2 CO2 + 2H2O
2C2H6 + 7O2 4CO2 + 6H2O
2H2 + O2 2H2O
2NO + O2 2NO2
Assume 10 tonnes of natural gas supplied to combustion chamber with
composition in wt %
CH4:85.7, C2H6: 4.8, N2: 3.2, H2: 6.3
8.75 tons CH4 2 mols O2 1 mol CH4 32 tons O2
1 mol CH4 16 tons CH4 1 mol O2
=34.28 tons O2 required
8.57 tons CH4 1 mol CO2 1 mol CH4 44 tons CO2
139
1.02 1 mol CH4 16 tons CH4 1 mol CO2
=23.10 tons CO2
8.57 tons CH4 2 mols H2O 1 mol CH4 18 tons H2O
1.02 1 mol CH4 16 tons CH4 1 mol H2O
=18.9 tons H2O
0.48 tons C2H6 7 mols O2 1 mol C2H6 32 tons 02
2 mols C2H6 30 tons C2H6 1 mol O2
=17.92 tons O2
0.48 tons C2H6 4 mols CO2 1 mol C2H6 44 tons CO2
1.02 2 mols C2H6 30 tons C2H6 1 mol CO2
=1.38 tons CO2
0.48 tons C2H6 6 mols H2O 1 mol C2H6 18 tons H2O
1.02 2 mols C2H6 30 tons C2H6 1 mol H2O
=0.847 tons H2O
0.3 tons H2 2 mols H2O 1 mol H2 18 tons H2O
2 mol H2 2 tons H2 1 mol H2O
2.7 tons H2O
N/B: Assume 50% conversion of H2, 90% conversion of CH4 and C2H6,30%
conversion of NO
Amount of CH4 leaving combustion chamber = 8.57(0.1) = 0.857 tons
Amount of C2H6 leaving combustion chamber = 0.48(0.1) =0.048 tons
APPENDIX III
ENERGY BALANCE CALCULATION
Unit 3: THE COMPRESSOR
140
Tin= 20°C Tout=155°C
Heat, Q = n∆H = ∫T 1
T 2
CpdT
Components involved N2 an O2
Specific heat capacities;
N2 = 1.04 KJ/KgK and O2 = 0.6486 KJ/KgK
Enthalpy, H;
H1= 1.04(428−293) = 140.4 KJ/Kg
H2 = 0.64886(428− 293) = 87.56KJ/Kg
H3 = 0.6486 (428 – 293) = 87. 56KJ/Kg
Q= Heat output from the Compressor;
Q = n∆H = ∑ nHOut −∑ nHIn
187030(140.4) + 49720 (87.56) −49720 (87.56)
Q = 2625901 KJ/hr
Let q = Volumetric flowrate of air
q = Fair// Dair
Fair = Flow rate of air, Dair = Density of air
q = 236750 Kg/m3/1.178Kg/m3 = 200976.23 m3/hr
Theoretical Power of the Compressor= P1Q1ln (P1/P2)
= 1 ×200976.23 ln (7.3/ 1) = 399515.49 KJ/hr
Actual Shaft Work required = Theoretical power
Efficiency
= 399515.49/ 0.84 = 475613.68KJ
141
APPPENDIX IV
EQUIPMENT DESIGN CALCULATION
STACK GAS HEATER
Q = 74322.76KJ/h, Th in =2000C, Th out = 1500C, Tc in = 300C,Tc out =118.480C, U =0.102Kw/m2
Converting Q = 74322.76KJ/h to KJ/s
74322.76/3600 =20.6452KJ/h
Tm= (Th¿−Tcout )−(Thout−Tc ¿)
ln [Th¿−Tc outThout−Tc¿
]
(200−118.48 )−(150−30)
ln [(200−118.48)
(150−30)]
81.52−120
ln81.52120
38.480.387
∆T m=99.43oC
A= QU ∆T m
20.64520.102×99.53
A = 2.03m2
142
AMMONIA VAPORIZER
Q=149781.4KJhr
,∈KJ /sec=149781.43600
=41.606KJ /S
Tc¿=−33.4oC
Tcout=¿−28.19 9oC ¿
Th¿=208
Thout=167.2∆T m=(Th¿−Tcout)−(Thout−Tc¿)
ln [Th¿−Tc outThout−Tc¿
]
(208+28.199)−(167.2+33.4)
ln [ 208+28.199167.2+33.4
]
236.199−200.6
ln [236.199200.6
]
35.599ln 1.177
=35.5990.163
=218.4oC
U = 0.102KW/m2
A= QU ∆Tm
= 41.6060.102×218.4
=1.868m2
WASTE HEAT BOILER 1
∆T m=(Th¿−Tc out)−(Thout−Tc¿)
ln [Th¿−Tc outThout−Tc¿
]
143
(890−410)−(250−30)
ln [ 890−410250−30
]= 480−220
ln480220
∆Tm=333.76
A= QU ∆Tm
= 4437.80.102×333.76
=4437.834.04
=130.73m2
WASTE HEAT BOILER 2
∆Tm=(208−150)−(50−32)
ln [ 208−15050−32
]=58−18
ln5818
= 40ln 3.2
=34.48
A= QU ∆Tm
Q = 149781.4KJ/hr, converting to KJ/s
= 41.61KJ/S
41.610.102×34.48
=11.83m2
NH3 SUPERHEATER.
∆T m=(Th¿−Tc out)−(Thout−Tc¿)
ln [Th¿−Tc outThout−Tc¿
]
∆Tm=(410−26.65)−(330−28.2)
ln [ 410−26.65330−28.2
]= 81.55ln 1.2702
=340.95oC
Q = 4437.85KJ/S.
U = 0.102KW/m2
144
A= 4437.850.102×340.95
=127.6m2
AIR HEATER
Q = 14881486.75KJ/hr = 4133.7KJ/S.
∆Tm=(350−250)−(200−155)
ln [ 350−250200−155
]= 100−45
ln (10045
)= 55ln2.22
= 550.799
=68.8 4oC
A= QU ∆Tm
=4133.77.02
=588.7m2
145
APPENDIX V
EQUIPMENT COSTING CALCULATION
IN 1998
Cost∈Am2=cost of 500m2×( A m2
500m2)0.6
IN 2013
Cost∈2013=cost∈1998×( 2013 index1998 index
)
Index in 1998 = 390
Index in 2013 = 683.6
AMMONIA VAPORIZER
Cost of 500m2 = N1.84 million
A = 1.87m2
Cost∈1998=N1.84million×(1.87500
)0.6
=N 0.06million
Cost∈2013=N 0.06million×( 683.6390
)
N 0.11 million.
WASTE HEAT BOILER 1
146
IN 1998
Cost∈Am2=cost of 500m2×( A m2
500m2)0.6
IN 2013
Cost∈2013=cost∈1998×( 2013 index1998 index
)
Index in 1998 = 390
Index in 2013 = 683.6
Cost of 500m2 = N1.84 million
WASTE HEATER BOILER 2
In 1998
Cost∈Am2=N 1.84million×( 11.8500
)0.6
N 1.84million×(0.0236)0.6
cost of Am2=N 1.84million×0.106N 0.19million
In 2013
cost of 2013=N 0.19million× 683.6390
=N 0.33million
AMMONIA SUPERHEATER
A = 128m2
Cost in 1998
147
Cost of 128m2=N 1.84million×( 128500
)0.6
=N 0.8214million
Cost in 2013
cost of 2013=N 0.8124million× 683.6390
=N 1.424million
WASTE HEAT BOILER 1
A = 130.5m2
In 1998
Cost∈Am2=cost of 500m2×( A m2
500m2)0.6
Cost of 130.5m2=N 1.84million×( 130.5500
)0.6
=N 0.8219mi llion
cost of 2013=N 0.8219million× 683.6390
=N 1.44million
FOR AIR HEATER
IN 1998
Cost∈Am2=cost of 500m2×( A m2
500m2)0.6
IN 2013
Cost∈2013=cost∈1998×( 2013 index1998 index
)
Index in 1998 = 390
Index in 2013 = 683.6
Cost of 500m2 = N1.84 million
Cost of Am2=N 1.84million×( 588.7500
)0.6
=N 2.1million
148
cost of 2013=N 2.1million× 683.6390
=N 3.68million
STACK GAS HEATER
1998
Cost∈Am2=N 1.84million×( 2.03500
)0.6
=0.068million
cost of 2013=N 0.068million × 683.6390
=N 0.119million
CONVERTER
Cost index 2013 = 680.1
Cost index 1990 = 390
Volume = 600 gallon
Cost for 600 gallon in 1990 = 17000
Cost of 1million gallon∈1990=17000×(1000000600
)0.6
=$1.457 million
Cost∈2013=$ 1.457( 680390
)=$2.541million
COMPRESSOR
In 1990
Cost of eq1 = cost of eq2(cap1cap2
)0.6
Cost of eq1 = $2100 ×( 7294447.8
)0.6
Cost of eq1 = $ 11203.4
149
Cost in 2013 = cost in 1990 ×( cost index2013cost index1990
)
cost in 2013 = 11203.4 (928.1756.3
)
=$ 13748.4
= # 2.2 million
Cost for nitric acid storage tank
Given volume of the tank at 1990 = 12 × 1606 gallons
Cost at 1990 = $170000
Cost index at 2013 = 683.6
Cost index at 1990 =395
Volume of storage tank =28.3m3 to liters =23800l
For 5000 gallons
Cost at 1990 = 170000( 5000
12×106)0.6
1637.96 = $2830.56
= #447228
Cost in 2013 = 1593.39( 683.6395
¿
= $2830.56
= N447228.00
For year 2006
Cost of equipment 1 = cost of equipment 2( capacity of equipment1capacity of equipment2
)
150
For year 2006 cost∈2006cost∈2013 = CPE∈2006
CPE∈2013
RATIO METHOD
Cost of designed plant = cost of previous plant (capacity of designed plantcapacity of previous plant
)n
Cost of 1200tons per day =cost of 280 per day( 1200280
)0.6
= $60 million( 1200280 )0.6 = $144 million
=#23 billion
STEP COUNTING METHOD
C = 14000 N Q0.615
N = 13(number of functional units)
Q = 400000 tons/yr (capacity of plant)
C = 14000 ×13×(400000)0.615= $507 million
= # 79.8 billion
