Ethylene and Acetylene Plant

1

CHAPTER 1

1.0 INTRODUCTION

1.1 Relevance of work

Ethylene is one of the most important and largest volume petrochemicals in the world

today. It is used extensively as a chemical building block for the petrochemical industry.

The importance of ethylene results from the double bond in its molecular structure that

makes it reactive. Ethylene can be converted industrially into a variety of intermediate and

end products [41]. The major use of ethylene is conversion to low and high-density

polyethylene. Other significant uses of ethylene include chlorination to ethylene dichloride,

used in the manufacture of the polyvinyl chloride (PVC), oxidation to the ethylene oxide,

an intermediate in the manufacture of polyester fibres and films, and the conversion to ethyl

benzene, an intermediate in the manufacture of polystyrene [23].

Ethylene has become an important industrial intermediate and various technologies have

been utilized in ethylene production. Recently, ethylene has taken the place of acetylene in

virtually all large-scale chemical synthesis. However, acetylene itself is a by-product of

modern ethylene production [50].

More than 97% of ethylene around the world is produced by pyrolysis of hydrocarbons,

which is the thermal cracking of petrochemicals in the presence of steam. This process can

be described as the heating of a mixture of steam and hydrocarbon to the necessary

cracking temperature depending on the hydrocarbon used. This mixture is then fed to a

fired reactor or furnace and heated. As a result, the original saturated hydrocarbon cracks

into smaller unsaturated molecules. This process is extremely endothermic, and the product

2

must be cooled back to the original feed temperature upon leaving the reactor in order to

minimize secondary reactions. [2]

Chemical companies have a variety of options for feedstock as well as processes to produce

ethylene. Economics and environmental issues are the dominant factors considered in the

choice of feedstock and processes of ethylene production.

The focus in this report will be on the steam pyrolysis of hydrocarbons mainly ethane.

There are several reasons for this choice which include the cost of production, availability

of raw materials and the viability of process.

1.2 Objectives

The main objective of this project is to develop a simplified plant design for the production

of ethylene and acetylene which includes a thermal cracking section, quenching section, gas

compression/separation, ethylene purification, and an integrated refrigeration section.

The design is aimed at estimating the production of ethylene and acetylene using ethane as

a feedstock and also to determine the yield of ethylene and acetylene using the steam

pyrolysis process. The design is also aimed at determining the feasibility of the steam

pyrolysis process on an industrial scale.

3

CHAPTER 2

2.0 LITERATURE REVIEW

2.1 Chemistry of ethylene and acetylene

2.1.1 Ethylene

Ethylene (IUPAC name: ethene) is a gaseous organic compound with the formula C2H4. It

is the simplest alkene (older name: olefin from its oil-forming property). Ethylene has four

hydrogen atoms bound to a pair of carbon atoms that are connected by a double bond and

hence is classified as an unsaturated hydrocarbon. All six atoms that comprise ethylene are

coplanar. The H-C-H angle is 119, close to the 120 for ideal sp hybridized carbon. The

molecule is also relatively rigid: rotation about the C-C bond is a high energy process that

requires breaking the -bond. The -bond in the ethylene molecule is responsible for its

useful reactivity. [12]

Ethylene has a boiling point temperature of -103.7C, a melting point temperature of -

169.2C, and a flash point temperature of -136.1C. Physical properties of ethylene include:

It is colourless

It is flammable

It has a slightly sweet smell at normal condition, that is ambient temperature and

one atmosphere

2.1.2 Acetylene

Acetylene (IUPAC name: ethyne) with the chemical symbol C2H2 is a hydrocarbon

consisting of two hydrogen atoms and two carbon atoms. As an alkyne, acetylene

is unsaturated because its two carbon atoms are bonded together in a triple bond. The

4

carbon-carbon triple bond places all four atoms in the same straight line, with CCH bond

angles of 180. [16]

Acetylene is an extremely reactive hydrocarbon. It is moderately soluble in water or alcohol

and markedly soluble in acetone. Acetylene has a melting point temperature of -81.5C and

a boiling point temperature of -84C. Physical properties include:

It is a combustible gas

It has a distinctive odour

Acetylene is colourless

Once the gas is compressed, liquefied, mixed or heated with air, it becomes very explosive.

2.1.3 Uses of ethylene and acetylene

The major use of ethylene is conversion to low and high-density polyethylenes, which are

used in such applications such as construction, communications, packaging, and

manufacturing of industrial and domestic products.

Other significant uses of ethylene include chlorination to ethylene dichloride, used in the

manufacture of the polyvinyl chloride (PVC), oxidation to the ethylene oxide, an

intermediate in the manufacture of polyester fibers and films, and the conversion to

ethylbenzene, an intermediate in the manufacture of polystyrene [16]. In addition, ethylene

is also a major raw material to produce plastics, textiles, paper, solvents, dyes, food

additives, pesticides, and pharmaceuticals. [12]

Acetylene is used for the production of oxy-acetylene flame. The temperature of the flame

is above 3000oC. It is employed for cutting and welding of metals. Another common use of

acetylene is as a raw material for the production of various organic chemicals including

5

1,4-butanediol, which is widely used in the preparation of polyurethane and polyester

plastics. Acetylene is also used for artificial ripening of fruits.

2.2 Chemistry of the Ethylene Process

Ethylene, because of its double bond, is a highly reactive compound, which is converted to

multi-intermediates and end-products on a large scale industrially. The thermal cracking

process is the most interesting process to produce ethylene commercially. In general the

starting raw material for ethylene production by thermal cracking can be any kind of

hydrocarbon. In reality, the choice of starting material is narrowed by economical

considerations. [26]

As the molecular weight of the feedstock increases, the product complexity increases.

Because many reactions occur during thermal cracking, it is complicated to determine the

rate of the cracking and predict the distribution of the products. Yet, investigations have

confirmed that the primary reaction, which splits the original hydrocarbon, is unimolecular

and that conversion rates follow the first order kinetics for a wide range of molecular

weight and up to high conversion of the original reactant, if there is no distinct equilibrium

barrier [17].

2.3 Market survey

2.3.1 Global market

In the past ten years, ethylene demand and price have fluctuated based upon the economical

growth in the United Stated and the rest of the industrial world. [7]

Although many economic uncertainties surround the petrochemical industry, ethylene

production and consumption should grow because of continuing replacement of natural and

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inorganic materials with organic synthetics and the further development of radically new

synthesis materials [26].

Our target market is producers of polyethylene products, PVC, and the likes. Some of the

current global market prices of ethylene are as follows: $900/tonne in Asia, 970/tonne and

840/tonne in Europe. [9]

2.3.2 Local market

A research on the local market for ethylene did not produce significant values. However

there are industries in Ghana which can use ethylene and acetylene as raw materials or

intermediates. Our target market is the food industries, plastics industries, paints, ripening

of fruits, packaging, and for use in welding. Examples of such industries are Qualiplast,

Duraplast, Interplast, Blue Skies Ghana, and Ezzy Paints.

2.4 Feedstock

A variety of feedstock can be used in a steam pyrolysis process. The feedstock for an

ethylene plant could be methane, ethane, propane and heavier paraffin. With the

development of cracking technology, it can also be cracked from crude oil fractions:

naphtha, kerosene and gas oil. Sometimes, raffinates from aromatics extraction facilities

can also be used as feedstock. The choice of feedstock is a compromise of availability,

price and yield. In selecting a process for ethylene production, the most important factor is

the hydrocarbon feedstock. Although this is controlled by conditions like quantity, quality,

and economics, studies have shown that as the molecular weight of the feed hydrocarbon

increases, ethylene yield decreases.

2.4.1 Methane (CH4)

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Methane is the first member of the alkane series and is the main component of natural gas.

It is also a by-product in all gas streams from processing crude oils. It is a colourless,

odourless gas that is lighter than air. Methane is mainly used as a clean fuel gas. It is also

an important source for carbon black. Methane may be liquefied under very high pressures

and low temperatures. Liquefaction of natural gas (methane), allows its transportation to

long distances through cryogenic tankers. [17]. Methane, though an important and abundant

fuel, has not been an attractive raw material for ethylene production, because it is thermally

stable and has no carbon-carbon bonds. The carbon-hydrogen bond requires more energy to

break than the carbon-carbon bond. The C-H bond energy is 93.3 Kcal, whereas C-C

energy bond is 71.0 Kcal [6]. The net reaction for methane dehydrogenation is

2CH4 C2H4 + 2H2 (1)

2.4.2. Ethane (CH3-CH3)

It is the second member of the alkanes and is mainly recovered from natural gas liquids.

Ethane, like methane, is a colourless gas that is insoluble in water. After methane, ethane

has the second highest composition in natural gas. Ethane is separated most efficiently from

methane by liquefying it at cryogenic temperatures. Various refrigeration strategies exist,

but the most economical process presently in wide use employs turbo-expansion, and can

recover over 90% of the ethane in natural gas. [24] The principal use of ethane is in

chemical industry, mainly, in the production of ethylene by steam cracking. Ethane is

favoured for ethylene production because the steam cracking of ethane is fairly selective for

ethylene. Ethane may be cracked alone or as a mixture with propane. [2]

The net dehydrogenation reaction of ethane is

C2H6 C2H4 + H2 (2)

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2.4.3 Propane C3H8

Propane is normally a gas, but it is compressible to a liquid that is transportable.

It is derived from other petroleum products during oil or natural gas processing. Propane,

also known as liquefied petroleum gas (LPG), can be a mixture of propane with small

amounts of propylene, butane and butylenes. Propane is a by-product of natural gas and

petroleum refining. Propane is used as a feedstock for ethylene production. The production

of ethylene from propane is similar to the process of ethylene production from ethane. [2]

In the dehydrogenation of propane four initial reaction steps are conceivable when

producing ethylene and propylene; however, according to Sherwood [24, 25] and Martin

[19] the first two reactions are primary. The reactions are

C3H8 C2H4 + CH4 . (3)

C3H8 C2H6 + H2 ... (4)

2C3H8 C2H8 + 2CH4 . (5)

2C3H8 C2H6 + C3H6 + CH4 .. (6)

2.4.4 Naphtha

Naphtha, an important feedstock for ethylene production, is a collective of liquid

hydrocarbon intermediate oil refining products. It is a mixture of hydrocarbons in the

boiling point range of 30-200 C. For the naphtha cracker process, typical feedstock are

light naphthas (boiling range of 30-90 C), full range naphthas (30-200 C), and special cuts

(C6-C8 raffinates) [29]. Naphtha is obtained in petroleum refineries as one of the

intermediate products from the distillation of crude oil. The processing of light naphtha to

ethylene is similar to the ethane and propane processes.

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2.4.5 Kerosene

This is a distillate fraction heavier than naphtha, and is normally a product from distilling

crude oils under atmospheric pressures. It may also be obtained as a product from thermal

and catalytic cracking or hydrocracking units. Kerosene is usually a clear colourless liquid

which does not stop flowing except at very low temperature (normally below -30C).

However, kerosene containing high olefin and nitrogen contents may develop some colour

(pale yellow) after being produced. Currently, kerosene is mainly used to produce jet fuels,

after it is treated to adjust its burning quality and freezing point. Before the widespread use

of electricity, kerosene was extensively used to fuel lamps, and is still used for this purpose

in remote areas. It is also used as a fuel for heating purposes. [17]

2.4.6 Gas oil

Gas oil is a heavier petroleum fraction than kerosene. It can be obtained from the

atmospheric distillation of crude oils (atmospheric gas oil, AGO), from vacuum distillation

of topped crudes (vacuum gas oil, VGO), or from cracking and hydrocracking units.

Atmospheric gas oil has a relatively lower density and sulphur content than vacuum gas oil

produced from the same crude. The aromatic content of gas oils varies appreciably,

depending mainly on the crude type and the process to which it has been subjected. A

major use of gas oil is as a fuel for diesel engines. Another important use is as a feedstock

to cracking and hydrocracking units. Gases produced from these units are suitable sources

for light olefins and LPG. [17]

2.4.7 Natural Gas

As a feedstock, natural gas yields ethylene from its ethane or propane content and forms the

basis of a massive chemical industry. Large reserves exist in many regions of the world.

10

Much of the natural gas appears in regions that are remote from markets or pipe lines, and it

is called stranded gas, which is a natural gas field that has been discovered, but remains

unusable for either physical or economic reasons. Most of this gas is flared, re-circulated

back into oil reservoirs, or not produced. In addition, natural gas has a major disadvantage

in transportation. Because of the low density of natural gas, pipeline construction is very

expensive. [2]

2.4.8 Choice of feedstock

The choice of feed stock is an important economic decision as it influences other costs as

well. For the reasons following, the choice of feedstock for our steam pyrolysis is ethane.

Subject to availability, ethane is the best feedstock, as it has higher yield and selectivity of

ethylene than heavier feed stocks and its processing is relatively simple, involving lower

capital costs. Another reason for choosing ethane as feedstock is, ethylene plants based on

light hydrocarbons are much simpler and cheaper to build and operate than plants designed

to use heavy feedstock. The plant has to employ much greater control over the composition

of the final product once the heavier feedstocks are cracked and more variety of

components comes. The choice for a particular feedstock, together with processing

conditions (heat, pressure, steam dilution rate) will determine the yield of ethylene,

propylene and other co-products in steam cracking. Manufacturing plants fed with ethane

and propane can be constructed at much lower investment costs than naphtha crackers.

Table 2.1 shows how product yield varies with feedstock type. If ethane is used as

feedstock, almost no propylene, butadiene and aromatics are formed as by-products.

Our key suppliers of ethane would include Texas Gas Service, Alliance Pipeline, BP

Amoco Co., Chevron Texaco, Duke Energy Co., and Shell Oil.

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Table 2.1 Approximate material balance of pyrolysis with different feed stock

Products,% mass Gaseous feed Liquid feed

Ethane Propane butanes naphtha gas-oil

H2 and methane 13.0 28.0 24.0 26.0 23.0

Ethylene 80.0 45.0 37.0 30.0 25.0

Propylene 1.1 14.0 16.4 14.1 14.4

Butadiene 1.4 2.0 2.0 4.5 5.0

Butene mixture 1.6 1.0 6.4 8.0 6.0

C5+ 1.6 9.0 12.6 18.5 32.0

Ratio propylene/ethylene 0.003 0.3 0.5 0.4 0.6

Propylene content into C3

fraction

86.7 58.3 99.0 98.3 96.7

2.5 General Processes for Ethylene Production

Commercially ethylene is obtained by (1) thermal cracking of hydrocarbons such as ethane,

propane, butane, naphtha, kerosene, gas oil, crude oil, etc, (2) autothermic cracking (partial

oxidation) of the above hydrocarbons, (3) recovery from refinery off-gas, (4) recovery from

coke-oven gas, and (5) catalytic dehydration of ethyl alcohol or ethyl ether. Occasionally,

raffinates from aromatics extraction facilities are used as a supplementary raw material. Of

the five methods above, small quantities of ethylene are recovered from coke oven gas and

gases produced from crude oil directly [5] but this route to ethylene has for a variety of

technical and economic reasons, so far not gained commercial significance.

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The first step in ethylene production is thermal cracking of the hydrocarbon feedstock.

Thermal cracking of natural gas liquids (NGL) or crude oil fractions in the presence of

steam is still the dominant method for the production of ethylene. This thermal

decomposition results from adding heat to the feed to break its chemical bonds. The steam

does not enter directly into the reaction, but it enhances the product selectivity and reduces

coking in the furnace coils. The product of this thermal cracking process is a mixture of

hydrocarbons, which extends from hydrogen and methane to gasoline and gas oil [28].

Most current ethylene processes are basically similar to each other. Ethylene plants use

similar separation units.

In the following sections, each step of ethylene production will be discussed.

2.5.1 Thermal Cracking Section

The first section of ethylene production process is thermal cracking. Thermal cracking is

the heart of an ethylene plant. This section produces all the products of the plant, while

other sections serve to separate and purify the products. Additionally, this section has the

greatest effect upon the economics of the process. Various types of pyrolysis reactors have

been proposed and commercialized for the thermal cracker. These pyrolysis reactors

include (1) direct heating (2) indirect heating (3) autothermic cracking and others.

The direct heating process using fired tubular heater is the most common cracker in an

ethylene plant. In this process a variety of the hydrocarbon feedstock can be used ranging

from ethane to gas oil. Steam is added to the hydrocarbon feed for several reasons: (1)

reduce the partial pressure of hydrocarbon, (2) lower the residence time of the hydrocarbon,

and (3) decrease the rate of coke formation within the tubes by reaction of steam with

carbon to form carbon monoxide and hydrogen.

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Thermal cracking of hydrocarbons by indirect heating include the pebble bed reactors, the

fluidized bed reactors, and regenerative furnace. Even though construction costs seem to be

more expensive and the operation to be more complex, ethylene yield by indirect heating is

higher than that obtained in the fired tubular heaters. One of the advantages of the indirect

heating reactors is that crude oil and heavy fuel oil can be used as feedstock because the

coke by-product can be removed continuously or intermittently in the process [6].

The pyrolysis gas leaving the cracker usually has a temperature in the range of 375C to

500C in the case of naphtha pyrolysis and typically from 500C to 600C in the case of

gas oil pyrolysis. The outlet temperature depends upon the amount of the carbon deposits in

the transfer line exchanger [26]. Quenching of the conversion product or rapid temperature

reduction is important to prevent the decrease of ethylene yields caused by secondary

reactions. This is carried out either by transfer line exchangers or by injecting water and oil.

2.5.2 Gas Compression and Treatment Section

In addition to the thermal cracking section, the sections for removal of acid gases, drying of

the cracked gases, removal of acetylenic compounds, and purification of ethylene are also

very important, because an efficient ethylene plant is the result of the integration of these

process sections and because, in respect to cost, the thermal cracking section is only about

20-30% of the whole plant. In addition, the goal is to produce ethylene with high purity

above 99.9%.

Most ethylene processes call for compression of the pyrolysis gas leaving the quench tower.

Consequently, the cooled cracked gas leaving the water tower is compressed in four to five

stages. Plants based upon gaseous feedstock generally employ four stages, while many

naphtha-and gas oil-based plants employ five stages of pyrolysis gas compression. Between

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compression stages, the cracked gas is usually cooled in water-cooled exchangers. Water

and hydrocarbons condensed between stages are separated from the pyrolysis gas in inter-

stage separators.

Hydrogen sulphide and carbon dioxide are removed from pyrolysis gas between the third

stage and fourth stage of the compression system. This location is optimum because the

actual gas volume has been reduced significantly in the first three stages of compression

while acidic components are still present in the gas stream [26]. Acid gas produced in

thermal cracking must be removed before the first major fractionation step. In removing

acid gases such as carbon dioxide and hydrogen sulphide, non-regenerative caustic washing

followed by water washing is employed in the most of the existing plants and proves to be

most economic. The pyrolysis gas leaving the caustic scrubber contains less than 1 ppm of

acid gases and hence assures that the final products of the plant will meet specification in

this respect.

Compressed cracked gas usually is dried to reduce the moisture content to 1 ppm or less

and avoid problems with freezing and hydrate formation in downstream low temperature

equipment. In drying the cracked gases, alumina, silica gel, and molecular sieves are used

commercially. Among them, molecular sieves seem to have an economic advantage over

conventional desiccants because of their higher desiccant activities and lower regeneration

temperatures [17]. Recovery of acetylene and removal of acetylenic materials from the

process gas is very important in manufacturing polymer-grade ethylene.

2.5.3 Recovery and Purification Section

After the cracked gases have been quenched, compressed, freed of the acid gases, and

dried, they generally contain hydrogen and light hydrocarbons in the C1-C6 range.

Depending upon the cracking method employed, carbon monoxide and nitrogen also may

15

be present. Low temperature straight fractionation, absorption, and selective adsorption are

three different methods to recover and purify ethylene. The aim of this section is to separate

ethylene and acetylene from hydrogen and methane fractions, ethane and propane fractions,

and heavier hydrocarbons. Commercial separation processes operate at four ranges of

pressure: 450-600 psia, 100-150 psia, 70-90 psia, and 30-40 psia. The most popular is the

450-600 psia because it offers an attractive combination of purity, recovery, efficiency, and

investment for large ethylene plants. [41]

In ethylene purification section, demethanized process streams are introduced to the de-

ethanizer in most cases. The de-ethanizer is a simple tower refrigerated by propane or

propylene to make reflux. The net overhead from the de-ethanizer flows to an ethylene-

ethane separator. This is the second most costly separation step in an ethylene plant because

the volatility is low and a large amount of reflux is required.

2.5.4 The Refrigeration Section

The separation of pyrolysis gas through condensation and fractionation at cryogenic

temperatures requires external refrigeration and is an important part of the ethylene system.

An ethylene refrigerator has two or three stages for a total of between five and seven stages

for the entire refrigeration cascade. Reflux ratios in the columns are selected carefully to

avoid large refrigeration consumption [47].

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CHAPTER 3

3.0 PROCESS SELECTION AND DESCRIPTION

The different processes in ethylene production include steam pyrolysis, catalytic pyrolysis,

recovery from fluid catalytic cracking off gas, autothermic and fluidized-bed cracking, and

membrane reactor.

3.1 Steam pyrolysis

The most commonly used process is steam pyrolysis of hydrocarbons. The feedstock,

mixed with dilution steam, enters the cracking section and is pyrolysised by heat into small

components. The pyrolysis gas enters the quench section and is cooled there to some

controlled temperature. Water enters the water quench tower, a part of quench section,

cooling down the high temperature pyrolysis gas and becoming steam. That steam, called

dilution steam, mixes with the feedstock before entering the pyrolysis section to decrease

the partial pressure of the cracked gases and slow coke formation. Finally, the pyrolysis

gas goes into the separation section to be separated into a variety of desired final products.

[33]

Figure 3.1 A simplified ethylene plant diagram sheet. [33]

QUENCH

SECTION CRACKING

SECTION

SEPARATION

SECTION Final

product Feedstock

Steam

Pyrolysis

gas

Water

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3.2 Catalytic pyrolysis

A catalytic pyrolysis process for production of ethylene from heavy hydrocarbons,

comprises heavy hydrocarbons that are contacted with a pillared inter layered clay

molecular sieve or other high silica zeolite containing catalysts in a riser or down flow

transfer line reactor in the presence of steam. It is catalytically pyrolysed at a temperature

of 650 C to 750 C and a pressure of 0.15 to 0.4 MPa for a contact time of 0.2 to 5

seconds. The weight ratio of catalyst to feedstock ranges from 15:1 to 40:1 and the weight

ratio of steam to feedstock is about 0.3:1 to 1:1.

Catalytic pyrolysis combines catalytic cracking and steam pyrolysis and has the advantages

of both catalytic cracking and steam pyrolysis. It can raise the yields of light olefins,

expand the flexibility of products distribution, and simultaneously lower reaction

temperature and decrease energy consumption for the whole system; so it has broad

application prospect. The raw material is usually crude oil.

Considering that the feed that is used in catalytic pyrolysis is crude oil, this process is not

exactly feasible in Ghana for the production of ethylene and acetylene. This is due to the

shortage in supply of Ghanas crude oil. Furthermore, the crude oil which is imported

mainly from Nigeria and Equatorial Guinea is chiefly refined to produced petroleum,

diesel, kerosene, etc. which is highly useful on the market. Furthermore, this process

requires the use of catalysts in large quantities, and which would require frequent plant shut

down in case of short catalyst life.

3.3 Autothermic and fluidized bed cracking

Most of the autothermic cracking processes produce acetylene as a main product and

ethylene as a by-product. Most of these processes operate at atmospheric pressure, and

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hydrocarbon feedstock, air, or oxygen and fuels are preheated to about 593C to reduce the

oxygen consumption and increase the yield. The process of ethylene production by

autothermic cracking is based upon the thermal cracking of crude oil using fluidized beds.

[2]

Fluidized bed reactors are relatively new tools in the chemical engineering field developed

for the oil and petrochemical industries. Here catalysts are used to reduce petroleum to

simpler compounds through cracking. Today fluidized bed reactors are still used to produce

gasoline and other fuels, along with many other chemicals. Many industrially produced

polymers are made using FBR technology, such as rubber, vinyl chloride, polyethylene,

and styrene. A major advantage of this process is the ability to operate the reactor in a

continuous state. However because of the expansion of the materials in the reactor, a larger

vessel is often required, which increases the cost of production. Again, the fluid-like

behaviour of fine solid particles within the bed eventually results in the wear and tear of the

reactor vessel. This requires expensive maintenance which adds to the cost of production.

3.4 Membrane reactor

Membrane reactors may be used in either batch or continuous mode, and allow the easy

separation of the enzyme from the product. Due to the ease with which membrane reactor

systems may be established, they are often used for production on a small scale (g to kg),

especially where a multi-enzyme pathway or co-enzyme regeneration is needed.

Membrane reactors combine reaction with separation to increase conversion. One of the

products of a given reaction is removed from the reactor through the membrane, forcing the

equilibrium of the reaction "to the right" (according to Le Chatelier's Principle), so that

more of that product is produced. Membrane reactors are most commonly used when a

19

reaction involves some form of catalyst. [8] There are two main types of these membrane

reactors (1) the inert membrane reactor and (2) the catalytic membrane reactor.

The inert membrane reactor allows catalyst pellets to flow with the reactants on the feed

side (usually the inside of the membrane). In this kind of membrane reactor, the membrane

does not participate in the reaction directly; it simply acts as a barrier to the reactants and

some products. [14]

Catalytic Ceramic Membrane is a system for the dehydrogenation of ethane to produce

ethylene and hydrogen through the use of a catalytic ceramic membrane having selective

permeability, thus permitting the separation of hydrogen from the reaction zone which

causes further dehydrogenation of ethane. The catalytic ceramic membrane tube is enclosed

within an alloy tube of suitable composition to permit heating to the temperature range of

300 to 650 C. The reactor is connected to a recovery system which permits separation of

pure ethylene and unconverted ethane. A steady stream of H2O or argon continuously

sweeps away the H2 coming out through the selective membrane, thereby further

facilitating the conversion process. [14]

The membrane reactors major advantage is its combination of reaction and separation to

produce a good amount of conversion and yield. However, the membranes (ceramic and

metallic) are poor in mechanical strength and need to be replaced at regular intervals.

Another major disadvantage is the cost of the membranes and its low resistance to harsh

environments. Also, the membrane reactors are usually used for production on small scale

(g to kg). [11] Considering the amount of ethylene and acetylene we want to produce,

which runs into thousands of tonnes, this process is not recommended.

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3.5 Fluidized catalytic cracking

Fluidized catalytic cracking (FCC) is an important process in oil refineries. It upgrades

heavy hydrocarbons to lighter more valuable products by cracking, and is the major

producer of gasoline in refineries. FCC Units present challenging multivariable control

problems.

The heavy molecule cracking process occurs in a riser tubular reactor, at high temperatures,

building up fuel gas, LPG, cracked naphtha (gasoline), light cycle oil, decanted oil, and

coke. The coke deposits on the spent catalyst surface, causing its deactivation. The catalytic

activity is re-established by coke combustion in a fluidized bed reactor, dominated

regenerator. The system riser-regenerator is called the converter. Steam lifts the heated

regenerated catalyst to be combined with the oil in the riser so that the oil-catalyst mixture

rises in an ascending dispersed stream to the separator. The control valve manipulates the

quantity of hot regenerated catalyst from the standpipe to the "riser" in order to maintain a

predetermined outlet riser temperature. On the top of the separator, the catalyst particles are

separated from vapour products by cyclones. The stream transfers the reaction products

overhead to the products recovery section. The standpipe transfers spent catalyst

continuously from the separator to the regenerator by a control valve.

3.6 Choice of Process

Based on the comparisons above, steam pyrolysis shall be used in this project. Steam

pyrolysis is one of the most important processes of petrochemistry. The main advantage of

this process compared with other processes is that it is quite flexible in terms of feed stock.

In addition steam pyrolysis is the best economical solution to produce ethylene and

acetylene, because other methods are more expensive.

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3.7 Process description (steam pyrolysis)

The fresh feed of ethane is combined with the recycled ethane from the ethylene column

and charged to the pyrolysis furnace. Dilution steam is added to ethane before it enters the

cracking furnace, to reduce the partial pressure of ethane and lower the residence time of

ethane in the high temperature zone, which decrease the rate of coke formation within the

tubes. The mixture of ethane and steam is preheated in the convection section of the

furnace, and the ethane cracks in vertical tubes within a residence time of 0.1 to 0.5 s. The

cracked gas leaves the furnace at 800 C and 8.0107kPa and is quickly cooled to 340 C in

the transfer line exchangers to preserve gas composition. It generates 370C steam at a

pressure of 16690kPa. The gas is then further quenched in quench towers by direct contact

with water where the gases leave as overhead to the compressor and the quench water is

separated and recycled.

The cracked gases are then compressed in four stages. Acid gases such as carbon dioxide,

are removed after the third stage of compression. The effluent gas leaves the compression

section at 42C and 3500kPa from which it is dried and cooled in a series of heat

exchangers. It is then passed to a de-methanizer where methane and hydrogen is separated

as overhead. The net bottom stream of the demethanizer is charged to the de-ethanizer.

The overhead vapour from the de-ethanizer is partially condensed by heat exchange and

propylene refrigerant. The bottom stream leaving the de-ethanizer contains C3+

hydrocarbons (mostly propane) that are stored in C3+ storage tanks. The net overhead from

the de-ethanizer is the ethylene-ethane stream with traces of acetylene. This stream is then

fed to the C2splitter to separate ethylene and acetylene from ethane. The ethane leaves as

bottoms product and is recycled back to the furnace. The overhead vapour (ethylene-

acetylene mixture) is forwarded to an acetylene absorber where acetone is used as the

22

extracting solvent. The gas is then finally sent to the ethylene column where high purity

polymer grade ethylene is recovered as product. The bottoms product containing mainly

acetylene is sent to the acetylene stripper where acetylene is recovered by further

separation.

3.8 Capacity

Our plant is likely to have a capacity of 100,000 tonnes per year of ethylene and about 550

tonnes per year of acetylene. This is because we are operating in a continuous process. Our

main product is ethylene. Acetylene is only a by-product, which is recovered in our quest to

produce 99.95% polymer-grade ethylene to meet market demands. The percentage yield for

acetylene is about 0.2% when using ethane.

23

CHAPTER 4

4.0 MATERIAL AND ENERGY BALANCES

The general material balance equation is

( Material out) = ( Material in ) + ( Material generation ) (Material consumption )-

( Material accumulation)

Assumptions:

1. Steady state, no accumulation

2. All masses are calculated on hourly basis

The quantity of ethylene produced per annum = 13888.89kg = 100000tonnes

Plant attainment is 300 days to allow for downtime for maintenances.

The calculations that resulted in the charts shown in this chapter are represented in the

appendix A

24

4.1 SUMMARY OF MATERIAL BALANCES

4.1.1 FURNACE

Fresh ethane

Component mass

flowrate

(kg/hr)

Mass,

%

Ethane 29189.55 100

TOTAL 29189.55 100

Temperature: 70C

Flue gases

Component mass

flowrate

(kg/hr)

Mass,

%

CO2 44 13.3

O2 9.6 2.9

N2 242.2 73

H2O(v) 36 10.8

TOTAL 331.85 100

Temperature: 250C

Material Steam 1

Component mass

flowrate

(kg/hr)

mass,

%

H2O 5259.6 100

TOTAL 5259.6 100

Temperature: 180 C

Material stream 2

Component mass

flowrate

(kg/hr)

mass (%)

Methane 1062.4992 3.08

Ethane 10216.35 29.59

Propane 2643.5992 7.66

H2O 5201.3132 15.06

Hydrogen 1018.2284 2.95

Acetylene 82.2172 0.24

Ethylene 14166.656 41.03

CO2 139.1368 0.40

TOTAL 34530.00 100.00

Temperature: 840.8 C

Pressure : 107kPa

Fuel/air

Component mass

flowrate

(kg/hr)

Mass,

%

Methane 1544 5.48

Air 25890.0922 94.52

TOTAL 27390.5322 100

Temperature: 25 C

Pressure: 101.325kPa

Pyrolysis furnace

25

4.1.2 TRANSFER LINE EXCHANGER

Material stream 2

Component mass

flowrate

(kg/hr)

mass (%)

Methane 1062.4992 3.08

Ethane 10216.35 29.59

Propane 2643.5992 7.66

H2O 5201.3132 15.06

Hydrogen 1018.2284 2.95


Ethylene 14166.656 41.03

CO2 139.1368 0.40

TOTAL 34530.00 100.00


Pressure : 101kPa

Cooling water 1

Component Mass

flowrate

(Kg/hr)

Mass %

H2O 10620 100

TOTAL 10620 100

Temperature: 25C

Pressure: 4177 kPa

Material stream 3

Component mass

flowrate

(kg/hr)

mass

(%)

Methane 1062.4992 3.08

Ethane 10216.35 29.59

Propane 2643.5992 7.66

H2O 5201.3132 15.06

Hydrogen 1018.2284 2.95


Ethylene 14166.656 41.03

CO2 139.1368 0.40

TOTAL 34530.00 100.00

Temperature: 350 C

Pressure : 150kPa

Steam at 500 C

Component Mass

flowrate

(Kg/hr)

Mass %

H2O 10620 100

TOTAL 10620 100

Temperature: 500C

Pressure: 16690 kPa

Transfer-line exchanger

26

4.1.3 QUENCH TOWER

Material stream 3

Component mass

flowrate

(kg/hr)

mass

(%)

Methane 1062.4992 3.08

Ethane 10216.35 29.59

Propane 2643.5992 7.66

H2O 5201.3132 15.06

Hydrogen 1018.2284 2.95


Ethylene 14166.656 41.03

CO2 139.1368 0.40

TOTAL 34530.00 100.00

Temperature: 350 C

Pressure : 150 kPa

Material stream 4

Component mass

flowrate

(kg/hr)

mass

fraction

(%)

Methane 1062.4992 3.62

Ethane 10216.35 34.83

Propane 2643.5992 9.01

H2O 5.25402 0.02

Hydrogen 1018.2284 3.47


Ethylene 14166.656 48.29

CO2 139.1368 0.47

TOTAL 29333.940

8

100

Temperature: 34 C

Pressure : 1930 kPa

Cooling water 2

Component Mass

flowrate

(Kg/hr)

Mass %

H2O 26864.82 1.0

TOTAL 26864.82 1.0

Temperature: 30C

Pressure: 4177 kPa

Water out

Component mass

flowrate

(kg/hr)

mass

fraction

H2O 141737.094 100

TOTAL 141737.094 100

Temperature: 80 C

Pressure : 6987 kPa

Quench tower

27

4.1.4 CAUSTIC TOWER

Material stream 5

Component mass

flowrate

(kg/hr)

mass

fraction

(%)

Methane 1062.4992 3.62

Ethane 10216.35 34.83

Propane 2643.5992 9.01

H2O 5.25402 0.02

Hydrogen 1018.2284 3.47


Ethylene 14166.656 48.29

CO2 139.1368 0.47

TOTAL 29333.9408 100

Temperature: 35 C

Pressure : 3500 kPa

Caustic solution

Component Mass

flowrate

(Kg/hr)

Mass

fraction

NaOH(aq) 45068.4 100

TOTAL 45068.4 100

Temperature: 30C

Pressure: 101.325 kPa

Material stream 6

Component mass

flowrate

(kg/hr)

Mass, %

Methane 1062.4992 3.64

Ethane 10216.35 34.99

Propane 2643.5992 9.06

H2O 5.25402 0.02

Hydrogen 1018.2284 3.49


Ethylene 14166.656 48.52

CO2 trace 0.00

TOTAL 29194.70971 100

Temperature: 38 C

Pressure : 3500 kPa

Spent caustic solution

Compon

ent

Mass

flowrate

(Kg/hr)

Mass %

Na2CO3 9631.954 20

H2O 38520.498 80

TOTAL 48152.452 100

Temperature: 40C


Caustic tower

28

4.1.5 SPRAY TOWER

Material stream 7

Component mass

flowrate

(kg/hr)

mass

(%)

Methane 1062.4992 3.64

Ethane 10216.35 34.99

Propane 2643.5992 9.06

H2O 5.25402 0.02

Hydrogen 1018.2284 3.49


Ethylene 14166.656 48.52

CO2 trace 0.00

TOTAL 29194.70971 100

Temperature: 40 C

Pressure : 3500 kPa

Cooling water 3

Component mass

flowrate

(kg/hr)

mass %

H2O 87.9234 100

TOTAL 87.9234 100

Temperature: 21.5 C

Pressure : 101 kPa

Material stream 8

Component mass

flowrate

(kg/hr)

mass

(%)

Methane 1062.4992 3.64

Ethane 10216.35 34.99

Propane 2643.5992 9.06

Hydrogen 1018.2284 3.49


Ethylene 14166.656 48.52

CO2 trace 0.00

TOTAL 29194.70971 100

Temperature: 40C

Pressure : 3500 kPa

Water out

Component mass

flowrate

(kg/hr)

mass %

H2O 87.9234 100

TOTAL 87.9234 100

Temperature: 80 C

Pressure : 101kPa

Spray tower

29

4.1.6 DEMETHANIZER

Material stream 9

Component mass

flowrate

(kg/hr)

mass %

Methane 1062.4992 3.97

Ethane 10216.35 34.28

Propane 2643.5992 9.06

Hydrogen 1018.2284 3.60


Ethylene 14166.656 49.00

TOTAL 28910.6902 100

Temperature: -120.0 C

Pressure : 2000 kPa

Material stream 10

Component mass

flowrate

(kg/hr)

mass

%

Methane 1062.4992 48

Hydrogen 1018.2284 46

Ethylene 141.29108 6

TOTAL 2267.6624 100

Temperature: -127 C

Pressure : 3200 kPa

Material stream 11

Component mass

flowrate

(kg/hr)

mass %

Methane 11.448 0.04

Ethane 9909.4343 37.20

Propane 2621.0494 9.84


Ethylene 14024.7251 52.63

TOTAL 26643.4 100

Temperature: 6.05 C

Pressure : 2600 kPa

Demethanizer

30

4.1.7 DE-ETHANIZER

Material stream 11

Component mass

flowrate

(kg/hr)

mass %

Methane 11.448 0.04

Ethane 9909.4343 37.20

Propane 2621.0494 9.84


Ethylene 14024.7251 52.63

TOTAL 26643.4 100

Temperature: 6.05 C

Pressure : 3006 kPa

Material stream 12

Component mass

flowrate

(kg/hr)

mass

%

Methane 5.048 0.02

Ethane 9909.435 41.4


Ethylene 13954.6 58.28

TOTAL 23945.82 100


Pressure : 3000 kPa

Propane product

Component mass

flowrate

(kg/hr)

mass %

Ethylene 110.31 0.04

Propane 2623.79 0.96

TOTAL 2734.1 100


Pressure : 3000 kPa

De-ethanizer

31

4.1.8 C2 SPLITTER

Material stream 12

Component mass

flowrate

(kg/hr)

Mass %

Methane 5.048 0.02

Ethane 9909.435 41.4


Ethylene 13954.6 58.28

TOTAL 23945.82 100


Pressure : 3080 kPa

Material stream 13

Component mass

flowrate

(kg/hr)

mass %

Ethylene 13919.217 0.55

Acetylene 76.7416 99.45

TOTAL 13995.96 100


Pressure : 3200 kPa

Material stream 14

Component mass

flowrate

(kg/hr)

mass %

Methane 10.6528 99.53

Ethane 9909.435 0.11

Ethylene 35.385 0.36

TOTAL 9955.47 100


Pressure : 2000kPa

C2 splitter

32

4.1.9 ACETYLENE ABSORBER

Material stream 14

Component mass

flowrate

(kg/hr)

mass %

Ethylene 13919.217 99.45


TOTAL 13995.96 100

Temperature: -10 C

Pressure : 3200 kPa

Material stream 15

Component mass

flowrate

(kg/hr)

mass %


Ethylene 13900 99.98

TOTAL 13901.535 100

Temperature: 20 C

Pressure: 1722.44kPa

Material stream 17

Component mass

flowrate

(kg/hr)

mass %

Acetone 127.201 100

TOTAL 127.201 100

Temperature: 35 C


Material stream 16

Component mass

flowrate

(kg/hr)

mass %

Acetylene 75.2066 37.16

Acetone 127.201 62.84

TOTAL 202.4076 100

Temperature: 30 C

Pressure : 1000 kPa

Acetylene absorber

33

4.1.10 ACETYLENE STRIPPER

Material stream 16

Component mass

flowrate

(kg/hr)

mass %

Acetylene 75.2066 37.16

Acetone 127.201 62.84

TOTAL 202.4076 100


Pressure : 2138 kPa

Material stream 17

Component mass

flowrate

(kg/hr)

mass %

Acetone 127.201 100

Acetylene

TOTAL 127.201 100


Pressure: 40.09 kPa

Material stream 18

Component mass

flowrate

(kg/hr)

mass %

Acetylene 79.8872 100

Acetone

TOTAL 79.8872 100


Pressure : 6000 kPa

Acetylene stripper

34

4.1.11 ETHYLENE COLUMN

Material stream 15

Component mass

flowrate

(kg/hr)

mass %


Ethylene 13900 99.98

TOTAL 13901.535 100

Temperature: 20 C

Material stream 20

Component mass

flowrate

(kg/hr)

mass %

Ethylene 13888.89 99.9


TOTAL 13888.9 100

Temperature: -10 C

Pressure: 2500 kPa

Material stream 19

Component mass

flowrate

(kg/hr)

mass %


Ethylene 11.1 87.51

TOTAL 12.68 100

Temperature: -30 C

Pressure : 2500 kPa

Ethylene Column

35

4.2 ENERGY BALANCES

4.2.1 FURNACE

Fresh ethane (stream 1)

Component Enthalpy

(kJ)

Ethane 2445111.3

TOTAL 2445111.3

Temperature: 70 C

Steam (stream 2)

Component enthalpy

(kJ)

H2O 1551719.1

TOTAL 1551719

Temperature: 180 C

Component Enthalpy

(kJ)

Methane 3101484.4

Ethane 26843935.6

Propane 6885500.9

H2O 8968631.9

Hydrogen 11856702.8

Acetylene 154495.7

Ethylene 31518168.1

CO2 140909.5

TOTAL 89469828.9

Pyrolysis furnace

36

4.2.2 QUENCH TOWER

Component Enthaply

(kJ)

Methane -196337745.9

Ethane 82223346.4

Propane 6876775.4

H2O 1589144.4

Hydrogen 221078167.1

Acetylene 13753328.5

Ethylene 9533474.4

CO2 75212.34

TOTAL 138791702.6

Componen

t

Enthalpy

(kJ)

Methane -44726466.4

Ethane 82218800.9

Propane 6877143

H2O 6977.6

Hydrogen 22107265.9

Acetylene 654218

Ethylene 96107084

CO2 4167.8

TOTAL 138791702.6

Cooling water (stream 9)

Component Enthalpy

H2O 187605993

TOTAL 187605993

Temperature: 30C

Pressure: 4.177 kPa

Material stream 8

Component Enthalpy

H2O 18176207.5

TOTAL 18176207.5

Temperature: 80 C

Pressure : 6987 kPa

Quench tower

37

4.2.3 CAUSTIC TOWER

Component Enthalpy

(kJ)

Methane -44726466.4

Ethane 82218800.9

Propane 6877143

H2O 6977.6

Hydrogen 22107265.9

Acetylene 654218

Ethylene 96107084

CO2 4167.8

TOTAL 138791702.6

Component Enthalpy

(kJ)

Methane -350957.8

Ethane -6953247.8

Propane 96371.2

H2O 711.3

Hydrogen 20846495.5

Acetylene -66039.6

Ethylene 597934.1

CO2 5414.2

TOTAL 13578747

Caustic soda

Component Enthalpy (kJ)

NaOH 159261400

TOTAL 159261400

Temperature: 30C

Pressure: 4.177 kPa

Spent caustic solution

Component Enthalpy

(kJ)

Na2CO3(aq) 166306735.6

TOTAL 166306735.6

Temperature: 40C

Caustic tower

38

4.2.4 DEMETHANIZER

Component Enthalpy

(kJ)

Methane -1058204.1

Ethane 5382174.6

Propane -863333.2

Hydrogen -6524474.9

Acetylene -5711589.7

Ethylene 8242969.9

TOTAL

Component Enthalpy

(kJ)

Methane 1094052.6

Hydrogen -1009895.2

TOTAL 84157.4

Component Enthalpy

(kJ)

Ethane -704981.6

Methane 420434.0

Propane -130952.4

Acetylene -7726.2

Ethylene 8086124.9

TOTAL 7662898.7

Demethanizer

39

4.2.5 DE-ETHANIZER

Component Enthalpy

(kJ)

Methane 4650.7

Ethane -706674.8

Propane -131409.9

Acetylene -6957.5

Ethylene -1145519.4

TOTAL -1985910.9

Component Enthalpy

(kJ)

Methane 10154.4

Ethane -1980235.4

Acetylene -19722.6

Ethylene -3058549.5

TOTAL -5048353.1

Component Enthalpy

(kJ)

Propane 110680.1

Ethylene 5306.8

TOTAL 115986.9

De-ethanizer

40

4.2.6 C2 SPLITTER

Component Enthalpy

(kJ)

Methane -4909.6

Ethane -2026150.9

Acetylene 36772.2

Ethylene -2986782.9

TOTAL -4981071.2

Component Enthalpy

(kJ)

Acetylene 30135.8

Ethylene -2318046.8

TOTAL -2287911

Component Enthalpy

(kJ)

Methane -2323.5

Ethane 3174817.8

TOTAL 3172494.3

C2 splitter

41

4.2.7 ETHYLENE COLUMN

Component Enthalpy

(kJ)

Acetylene -36.7686

Ethylene -317733.97

TOTAL -317770.7386

Component Enthalpy

(kJ)

Ethylene -430.4059

Acetylene -155.096

TOTAL -585.5019

Component Enthalpy (kJ)

Acetylene -76.4246

Ethylene -2836982.474

TOTAL -2837058.903

Ethylene column

42

CHAPTER 5

5.0 EQUIPMENT SPECIFICATIONS

Specifications for all processing equipment based on the operating conditions and flow

rates of the input and output streams among others form a major part in plant design.

The major considerations under equipment specification are:

1. Identification of the equipment

2. Function of the equipment

3. Basic design data

4. Material of construction

Information and data used are from Stanley M. Walas (1999), R K Sinnott (1999), Perry

and Green (1999). [49, 43]

5.1 Equipment list

5.1.1 Pyrolysis furnace

Duty: To crack the ethane feedstock into smaller hydrocarbons under carefully controlled

temperature to yield the optimum amount of ethylene and acetylene.

Type or description: cylindrical

Height: 15.16m

Operating temperature: 1200C

Heat duty: 23476.165KW

Material of construction: Stainless steel (SS 310) and Refractory brick

43

5.1.2 Heat exchanger

Duty: Immediately quenches the cracked gases to a lower temperature to stop further

undesired reactions and coke formation

Type or description: Transfer-line exchanger

Temperature: 700C

Quantity: 1

Material of construction: stainless steel

5.1.3 Quench tower

Duty: to further cool cracked gas and condense water vapour present by direct contact with

water

Type or description: packed tower

Operating temperature: 350C to 34C

Pressure: 150kPa

Height: 18m

Material of construction: austenitic stainless steel type 304

5.1.4 Gas compressor

Duty: increases the pressure of the gas to liquefy it for the distillation and separation

processes.

Type: centrifugal compressor

Output pressure: 3500kPa

44

Number: 4-stage

Material of construction: Carbon steel

5.1.5 Caustic tower

Duty: To remove the acid gases CO2, from the ethylene gas stream

Type: packed tower


Height of tower: 6.846m

Tower diameter: 1.9155m

Vessel volume: 19.729m3

Material of construction: carbon steel

5.1.6 Spray tower

Duty: To dry the gas stream of water (vapour) before cooling it for distillation.

Type: spray tower



5.1.7 Chilling train.

Duty: the gas is cooled and in turn is liquefied for distillation

Type: series of heat exchangers

Number: 3

Operating temperature: -120C

45


5.1.8 De-methanizer

Duty: To separate and remove methane and hydrogen from the ethylene gas stream

Temperature: 120C

Pressure: 30bar

Height: 15m

Diameter: 1.4m


5.1.9 De-ethanizer

Duty: To separate the C2s and C3s

Temperature: 6C

Pressure: 32bar

Height: 25 m

Diameter: 1.5 m

Number: 1


5.1.10 C2-splitter

Duty: To separate or split the C2 into ethylene and acetylene as overhead and ethane as

bottom stream to be recycled back to the furnace.

46

Type: continuous type tray column

Operating temperature: -24.95C

Feed pressure: 30bar

Height: 14m

Diameter: 1.7 m


5.1.11 Ethylene Column

Duty: To recover and obtain our final ethylene product.

Type: tray tower

Temperature: 20C

Height: 78.2 m

Diameter: 3.6 m


5.1.12 Acetylene Absorber

Duty: To separate the ethylene as overhead into the ethylene still and acetylene as bottoms

to the acetylene stripper.

Type: packed tower

Temperature: 21C

Height: 19.09m

47

Diameter: 3.248m


5.1.13 Acetylene Stripper

Duty: To strip the acetylene from the extracting solvent used in the acetylene absorption

column.

Type: packed tower

Temperature: 20C


5.2 SPECIFICATION OF STORAGE TANKS

Tanks are typically filled to 80% of capacity to function safely. [43]

See appendix C for detailed calculations

5.2.1 Ethane storage tank

Duty: To temporarily store the ethane feedstock before cracking.

Type or description: cylindrical vertical tank on concrete support.

Capacity: 7,427,160.362gal (US)

Internal diameter of ethane storage tank =

Length of ethane storage tank = 50.1667 in = 1.274m

Thickness of tank = 25mm


48

5.2.2 Propane storage tank

Duty: To store propane produced from the cracking process temporarily.

Description: Vertical cylindrical tank with flat base and conical roof [43]

Capacity: 3,537,431gal (US) per week

Internal diameter =

Length = 42.376in = 1.076m

Thickness of tank = 28.18mm


5.2.3 Acetylene storage tank

Duty: To temporarily store acetylene produced

Type or description: cylindrical vertical tanks with flat base on concrete foundation.

Capacity: 3,374,331.513 gal (US) per week

Internal diameter =

Length = 42.05 in =1.068 m



5.2.4 Ethylene storage tank

Duty: To store our polymer-grade ethylene produced for 7 days.

Type or description: flat bottomed vertical cylindrical tank on concrete foundation.

Capacity: 6,361,192.115 gal (US) per week

49

Internal diameter =

Length = 48in = 1.22m



5.3 PIPE SPECIFICATION

The most common means of transporting fluid is the pipeline. Every pipe is a long,

cylindrical, completely enclosed conduit used to transport gas, liquid, or both from one

point to another. Sizing of pipes for fluid flow in a given plant does not only depend on the

fluids physical properties, but also to some extent, on the sound economic factors. In most

engineering practices under this heading, the criterion used is the optimum diameter which

is the diameter of the pipe that gives the least total cost for annual pumping charges. The

design parameters considered are:

1) The nominal size

2) Schedule number

3) Material of construction

4) Wall thickness

Approximately, Schedule numberS

1000

Where P = Internal pressure, psig.

S = Allowable working stress in psi.

The optimum diameter is first of all estimated based on the fluid density, capacity and

viscosity depending on the nature of the fluid.

50

According to Sinnott, the optimum diameter of a stainless steel pipe is given as:

d,optimum = 230G 0.52-0.37

where, G = Mass flow rate in kg/s and = density in kg/m3

5.3.1 SAMPLE CALCULATION FOR PIPE SPECIFICATION

Pipe Location: from acetylene absorber to acetylene stripper

Mass flow rate = 13574.7079 kg/hr =3.85 Kg/s

Density of gas = 1.73 kgm-3

The optimum pipe diameter for turbulent flow using stainless steel pipe is given as:

dopt = 260G0.52-0.37 .. (1) [43]

Where: G = mass flow rate of feed

= density of slurry

It implies, dopt = 260(3.77075)0.52

(1.73) -0.37

= 428mm, 16.85 in

From the above calculation, a 428mm (16.85in) pipe diameter can be used.

Reynolds number, d

G

4Re (2) [43]

Where G is mass flow rate.

428.0001.077075.34

Re

51

Re = 11217

Re is greater than 4000 and hence flow is turbulent.

From Wallas (1990) Table A5

For optimum pipe diameter of 16.85 in

Nominal size = 16 in

Pipe schedule number = 30s

Outer diameter (do) = 16.00 in

Inner diameter (di) = 15.25 in

2222 1405.0423.044

mmD

The normal fluid velocity is given by:

. (3) [44]

1471.02686.8

77075.3

msu

Also Maximum design fluid velocity is assumed to be given by the correlation;

Maximum design fluid uu 2max 2.1 .. (4) [44]

12max 67824.0471.02.1 msu

52

5.3.1.1 Line Equivalent length

The pressure loss through the bends and check valves can be included in the line pressure-

loss calculations as an equivalent length of pipe. Assuming all the bends to be 90 elbows

of standard radius, and the isolation valves as plug-type valves.

Elbow equivalent length = 30D....................................... (9) [44]

= 30 x 0.428 m

= 12.84 m

Plug-valve equivalent length = 18D................................ (10) [44]

= 18 x0.428 m

= 7.704 m

Entry losses (at maximum design velocity) are calculated from the equation:

Entry loss = KPau

4.02

678.073.1

2

22

max

53

Table 5.1 Summary of pipe line specifications for our ethylene plant

Locatio

n

Optim

um

diamet

er

(mm)

Nomi

nal

Size(i

n)

Sched

ule

Numb

er

Material

of

Construct

ion

Outer

diamete

r

(in)

Inner

diamet

er

(in)

Cross-

sectio

nal

area(i

n2)

Nor

mal

fluid

veloc

ity

(m/s)

Maxim

um

design

fluid

velocit

y (m/s)

From

Furnace

to TLE 655.53 25.83 20

Stainless

steel 24 23.25 0.3375 2.297 3.3082

From

TLE to

quench

tower 423.24 16.66 40

Stainless

steel 16.00 15.25 0.2277 2.713 3.906

From

quench

tower

to

compre

ssor

733.00

28.87

20

Stainless

steel

29

28.80

0.422

1.464

2.108

From

compre

ssor to

caustic

tower

467.6

18.40

40

Stainless

steel

18.00

17.25

0.252

2.452

3.53

From

caustic

tower

to dryer

737.96 29.05 20

Stainless

steel

29.20 28.70 0.425 1.453 2.094

From

dryer to

cooler

729.29 28.65 20

Stainless

steel

28.70 28.40 0.421 1.434 2.077

From

cooler

to de-

methani

zer

721.28

28.40

20

Stainless

steel

28.40

28.20

0.418

1.413

2.035

From

de-

methani

zer to

de-

ethaniz

er

696.56

27.42

30

Stainless

steel

27.45

27.00

0.403

1.353

1.95

From

de-

ethaniz

er to C2

splitter 659 25.95 30

Stainless

steel 26 25.50 0.3806 1.43 1.206

54

From

C2

splitter

to

acetyle

ne

absorbe

r

495.15

19.5

30

Stainless

steel

19.5

0

19.25

0.193

1.89

2.73

From

acetyle

ne

absorbe

r to

acetyle

ne

stripper

428

16.85

30

Stainless

steel

16.6

0

16.00

0.1405

0.471

0.678

From

acetyle

ne

absorbe

r to

ethylen

e still

491 19.33 30

Stainless

steel

19.3

0

19.00 0.190 0.64 0.920

From

ethylen

e still to

storage

tank

44.95

17.70

30

Stainless

steel

17.7

0

17.00

0.0152

4.00

5.76

5.4 PUMP SELECTION

Centrifugal pumps will be used throughout the process. These pumps are characterised by

their specific speed which is a dimensionless variable. Different types of pumps have

different efficiency envelopes according to their specific speed. Pump selection is made

based on the flow rate and the head required, together with other process considerations,

such as corrosion or the presence of solids in the fluid. The pressure developed by a

centrifugal pumps depend on:

Fluid density

Diameter of the pump impeller

The rotational speed of the impeller

55

Volumetric flow rate through the pump

5.4.1 PUMP SPECIFICATION

Sample Calculation

Location: Between acetylene absorber and ethylene still

Volumetric flow rate = 836.934 m3s

-1

From the above pipe specification,

Optimum pipe diameter = 16.66in

Nominal size = 16 in

Pipe schedule number = 30s

Outer diameter (do) = 16 in

Inner diameter (di) = 15.25 in

Velocity of fluid in the pipe = 0.678ms-1

Reynolds number of fluid = 11350

5.4.1.1 Power requirement

Total pump head, scdc HHH

g

VZ

g

P

g

VZ

g

PH scsc

scdcdc

dc

22

22

Therefore,

g

VVZ

g

PPH scdcscdc

2

22

56

Where Pdc = Discharge pressure, 405 KPa

Psc = Suction pressure, 40 KPa

The suction velocity, 1873.20034.0

0097688.0 msVsc

The discharge velocity, 1284.3000033.0

0001084.0 msVdc

The total discharge head,

g

VZ

g

PH dcdc

dcdc

2

2

The total suction head,

g

VZ

g

PH dcdc

dcdc

2

2

But the Total pump head, scdc HHH

g

VZ

g

P

g

VZ

g

PH scsc

scdcdc

dc

22

22

Therefore,

g

VVZ

g

PPH scdcscdc

2

22

Where, mZZZ scdc 2

that is the height difference at the centre line of the pump between the suction and

discharge pipe. [44]

KPaPP scdc 365

1411.0 msVV scdc

57

= 790kg/m3; g = 9.81m/s2

Hence, mH 11.4981.92

411.02

81.9790

365000

Useful power, gQHPuseful

Q = 0.0097688; H = 49.11m ; = 790kg/m3;

g = 9.81m/s2

Hence, WPuseful 433.371811.4900977.081.9790

The value of specific speed represents the ratio of the pump flow rate to the head at the

speed corresponding to the maximum efficiency point. It depends primarily on the design

of the pump and impeller. The specific speed can be used to avoid cavitations or to select

the most economical pump for a given system layout.

The value of specific speed can be calculated from the relation;

4

3

H

QNNs .. (11)

Where N is in rpm (1750rev/min), Q in gpm (586.2gpm), and H in feet (15.41ft).

903.936

122.161

2.5861750

4

3sN

Specific speeds for centrifugal pumps usually lie the range 900-15000 but values above

12000rpm are considered impractical .Since the calculated value lies within the range it

suggest that the calculated value is correct.

58

5.4.1.2 Net positive suction head

NPSH is the absolute pressure at the pump inlet expressed in feet of liquid, plus velocity

head, minus the vapour pressure of the fluid at pumping temperature, and corrected to the

elevation of the pump centreline in the case of horizontal pumps or to the entrance to the

first-stage impeller for vertical. Thus if NPSH is zero or less, the liquid can vaporise. The

NPSH increases as the pump capacity increases. Hence it is important to consider the range

of flow requirement during the pump selection time.

Net positive suction headg

PP vapi

(12)

Where Pi = absolute static pressure at the pump inlet, N/m2

Pvap = Vapour pressure, N/m2 = 0.1233 x 10

5 Pa (Rogers and Mayhew)

satmi gHPP (13)

Patm = atmospheric pressure, N/m2

= density of pulp

Hs = Suction head

Inserting values into equation (12), it implies,

Net positive suction head m444.3481.9790

12330589.279272

59

Table 5.2 Summary of pump specification

Pump location Qty Power

requirement

(w)

Net positive

suction head

(m)

Specific

speed

(r/min)

Efficiency

From TLE to Quench

Tower

1 1637 21 293 65

For pumping caustic

solution into the

caustic tower

1 1142.82 15.095 192 60

For pumping acetone

into the acetylene

absorber

1 3718.433 34.44 862 72

60

CHAPTER 6

6.0 DESIGN OF A FURNACE

6.1 Problem statement

To design a cracking furnace to crack ethane feedstock to yield ethylene and acetylene as

products. Furnace to operate at thermal efficiency of 85 %.

Figure 6.1 A schematic diagram of a typical industrial furnace

6.2 CHEMICAL ENGINEERING DESIGN

Air

Fuel

Stack

Stack damper

Convection section

sesection

Radiant section

Cracked gas

Ethane feed

Stea

m

61

6.2.1 SCOPE OF DESIGN

Design constraints

Total energy absorbed

Total Energy absorbed

Heat flux across the cracking coils

Heat transfer coefficient across tube

Pressure drop across tubes

Stack height

Fuel and air requirement

6.2.2 Design constraints

Furnace geometry cylindrical

Tube diameter OD = 0.168275 m (6.625 in.)

Center-to-center spacing =0.3048 m (12 in.)

Tube thickness =0.00762 m (0.3 in.)

Diameter of the radiant section = 5 m

Number of tubes in the radiant section = 30

Number of tubes in the convective section = 16

Tube length = 10.7 m

62

Height of the radiant section = 11.5 m

Methane use as fuel

Excess air 10%

A single row tube alignment

6.2.3 Total energy absorbed

6.2.3.1 Reactions in the furnace:

C2H6 C2H4 + H2 Reaction 1

2C2H6 C3H8 + CH4 Reaction 2

C3H8 C2H2 + CH4 + H2 Reaction 3

C + 2H2O CO2 + 2H2 Reaction 4

Since there are a series of reaction in the furnace the four

63

Reference: CO2, C2H2, C2H4, C3H8, CH4, H2O, C2H6, H2 at 25 oC 1 atm

64

Table 6.1 Enthalpy table

Substance Nin 103 Hin KJ/mol

Nout 103

mol/hr

Hin KJ/mol

CO2 - - 3.555 H3

C2H2 - - 3.199 H4

C2H4 - - 505.869 H5

C3H8 - - 60.79 H6

CH4 - - 66.477 H7

H2O 291.896 H1 291.265 H8

C2H6 972.985 H2 340.564 H9

H2 - - 509.068 H10

Estimation of the enthalpy of the inlet stream

,

,

,

,

,

65

Total heat absorbed = Heat for preheating the feed + Heat absorbed for cracking of feed

Assume a furnace efficiency of 85 %

Duty of the furnace is 20.862 MW

6.2.4 Energy absorbed

Assume 70 % of the total heat absorbed in used for the cracking of Feed stock the

remaining is used for preheating of the feed stock.

6.2.5 Heat flux in the radiant coils

66

6.2

[2]

Where: N tube is number of tubes, Do is Outer diameter of tubes, L tube is the length of the

tube

6.2.6 Heat lost to the surroundings

The heat lost to the surroundings is in the range of 0.02 to 0.03 as a fraction of the total

released heat [Wallas, 1948].Since Q lost is an allowance and for this design we can set it to

be equal to 0.02.

6.2.7 Heat lost is the stack gas

6.2.8 TEMPERATURE PROFILE IN THE FURNACE

6.2.8.1 Temperature of the process fluid leaving the convective section

The stream entering the radiation section has absorbed 30 % of the total heat absorbed.

Qabsorbed in the convective tubes = Hout Hin

67

Where Qabsorbed in the convective tubes = heat absorbed in the convection section (MJ/hr)

Hout = Enthalpy of the stream entering the radiant section (KJ/mol)

Hin= Enthalpy of the feedstock (KJ/mol)

Qabsorbed in the convective tubes

=

103+0.688105 +0.76041012 2)

T = 312 C

6.2.8.2 Temperature of flue gas entering the convective section

By rule of thumb the temperature of the flue gas entering the convective section should be

150 C above the process temperature. This mean the temperature of the gas is 990.8 C.

6.2.8.3 Temperature of flue gas entering the stack section

(Waals 1990,pg. 214)

Where Ts temperature of flue gas leaving the convection section oF

a = 0.22048 - 0.35027z + 0.92344z2, b = 0.016086 + 0.29393z - 048139z

2

Where z = fraction excess air =0.1

Therefore substitute into the equation above: a= 0.1946874, b=0.0406653

Now solving for Ts,

68

6.2.9 STACK DESIGN

Where:

P = the suction available from a natural draft system, Pa

C = 0.0342

a = atmospheric pressure in Pa, h = height of the stack (m).,Ti=inlet temperature in K

To =ambient temperature in K (25 oC)

Setting the P = the suction available from a natural draft system to 400 Pa which is in the

acceptable range [2]

=80.04 m

=80.04 m

6.2.10 PROCESS SIDE HEAT TRANSFER

6.2.10.1 Process-side heat transfer

.. .. 6.14 [2]

Neglecting the viscosity correction factor

69

Where Nu = Nusselt number =

, Re =Reynolds number =

,

Pr =Prandtl number =

, hi= inside coefficient (W/m

2 oC), di= tube inside diameter (m),

ut = fluid velocity(m/s), kf = fluid thermal conductivity(W/moC), Gt = mass velocity, mass

flow per unit area(kg/m2s), = fluid viscosity at the bulk fluid temperature (Ns/m

2),

w= fluid viscosity at the tube wall temperature (Ns/m2),

Cp = fluid specific heat, heat capacity, J/kgoC.

6.2.10.1.1 In the convection tubes

=

6.2.10.1.2 For the radiative tubes

70

6.2.11 PRESSURE DROP

... 6.15 [2]

Where P = tube-side pressure drop, KPa,

Np= number of tube-side passes, Ut = tube-side velocity, m/s, L = length of one tube, m

jf = Friction factor

6.2.11.1 Pressure drop in the radiative tubes:

At Reynolds number of 5.872 , jf = 2 10-3

[2], w = 0.123810-3

, Np=30, L=11m

= 26.152 KPa

6.2.11.2 Pressure drop in the Convective tubes:

At Reynolds number of 2.446 , jf = 1.7 10-3

[2], w=1.69810-3

, Np=3, L=11m

=5.743KPa

6.2.12 FUEL CONSUMPTION AND REQUIRED AIR FLOW RATE

6.2.12.1 Fuel consumption

Using methane as the fuel

Q released=W fuel LHVfuel ( Mullinger et al.2008)

71

Where Q released= total heat released MJ / hr

W fuel = Fuel flow rate (Kg/hr), LHVfuel= Low heating value of fuel (CH4) (MJ/Kg)

LHVfuel=50.055 MJ/Kg (21520 Btu/Ibm) (Waals 1990,pg. 216 )

6.2.12.2 The flow rate of air to be required:

CH4 + 2(O2 + 3.76N2) CO2 + 2H2O + 7.52N2

72

Table 6.2 chemical engineering design summary of pyrolysis furnace

DESIGN PARAMETERS VALUES UNITS

Heat Duty 20.862 MW

Temperature in/out 70/840.8 o C

Pressure drop 5.737 KPa

Fuel consumption rate 0.417 Kg/s

Air required flow rate 7.192 Kg Air /s

Excess air required 10 %

Feedstock flow rate 3.858 Kg/s

Steam required 1.461 Kg/s

Outlet process flow rate 9.588 Kg/s

Operating pressure 107 KPa

73

6.3 MECHANICAL ENGINEERING DESIGN

6.3.1 Design Pressure

For vessels under internal pressure, the design pressure is normally 5 to 10 percent above

the normal operating pressure (Sinnott, 2005).The internal pressure in the furnace is related

to the hydrostatic head, atmospheric pressure and the pressure drop by:

Design Pressure (Pi) =hydrostatic pressure + atmospheric pressure = gh + (101325 - P),

Pa

Where = density of the flue gas =3.896 Kg/m3

g = acceleration due to gravity=9.81m/s2

h = height of furnace =15.16 m

P= negligible

H conv. =the height of the convective section = 3.66 m

H rad. = the height of the radiative section = 11.5 m

Design Pressure (Pi) = (3.8969.81 (3.66+11.5) + (101325)

= 101.904 KPa

10 % of the design pressure =1.1101.904

=112.095 KPa

74

6.3.2 Minimum Thickness of Cylindrical shell

For a cylindrical shell, the minimum thickness required to resist internal pressure is given

as:

Where Pi is the internal pressure = 112.095 N/m2

Di is the internal diameter = 6 m

F is design stress,

Typical design stress for stainless steel at 3500C is 100000 N/m

2 (Sinnott, 2005).

Allowing for a corrosion allowance of 0.002m, the minimum thickness is 0.005365m.

6.3.3 Design Temperature

The design temperature at which the design stress is evaluated should be taken as the

maximum working temperature of the material (Sinnott, 2005). The design temperature is

4000C (523.15K)

6.3.4 Materials of Construction

Stainless steels are the most frequently used corrosion resistant materials in the chemical

industry. Type 304 stainless steel (the so called 18/8 stainless steel) is the most generally

used stainless steel. If the equipment is being deigned to operate at high temperatures,

materials that retain their strength must be selected. The stainless steels are superior in this

75

respect to plain carbon steel. Stainless steel is to be used for this design (Coulson et al,

Volume 6).

6.3.5 STRESS ANALYSIS

The main sources of loads to consider are:

The internal Pressure

The total longitudinal and circumferential stresses due to internal pressure are given as:

Longitudinal stress, Lt

DP ii

2

Circumferential stress, ht

DP ii

4

KPa267.62681005365.02

0.6095.112

KPah 634.31340005365.04

0.6092.112

3.6 DEAD WEIGHT OF THE FURNACE

6.3.6.1 Weight of the refractory

The density of high alumina refractory bricks is given as 2579kg/m3 (Rotary kiln transport

phenomena and transport processes)

Where,

76

R= external radius of refractory shell, r = internal radius of refractory shell

6.3.6.2 Weight of steel shell

Density of steel is given as 8027Kg/m3.

Where,

R= external radius of steel shell, r = internal radius of steel shell

6.3.6.3 Weight of the content in tube

Total weight of the fluid in the tubes

6.3.6.3.1 Volume of convection section

Mass of content:

77

6.3.6.3.2 Volume of radiant section

6.3.6.4 Total dead weight

6.3.6.5 Choice of support

The support will be so strong enough to with stand the weight exerted by the furnace

Table 6.3 Summary of mechanical engineering design for pyrolysis furnace

PARAMETER VALUE

Design Temperature 400 oC

Design Pressure 112.095 KPa

Minimum thickness of shell 0.005365 m

Longitudinal stress 62681.267 KPa

Circumferencial stress 31340.634 KPa

Total force exerted on the surpport 1827.722 kN

78

CHAPTER 7

7.0 DESIGN OF HEAT EXCHANGER

7.1 PROBLEM STATEMENT

To design a heat exchanger to cool cracked gases at a flow rate of and

to cooled gases at using cooling water at to .

7.2 PARAMETERS TO CALCULATE

1) Heat transfer area

2) Bundle diameter

3) Bundle clearance

4) Heat transfer coefficient

5) Overall heat transfer coefficient

6) Tube side and Shell side fouling resistances

7) Pressure drops

7.2.1 CHEMICAL ENGINEERING CALCULATIONS

The fundamental heat transfer equation is given by,

79

The log mean temperature difference, for countercurrent flow is given by:

Where equations (5) and (6) are the dimensionless temperature ratios of the correction

factor.

80

7.2.2 Exchanger type and dimensions:

The graph of FT against S at various R values on page 9 of Perrys Chemical Engineers,

Section 11, 8th

Edition gives a corresponding

Hence the chosen Heat Exchanger is 2-4 Shell-and-Tube Heat Exchangers.

7.2.3 Heat Load

81

Overall Heat Balance gives,

7.2.4 Overall coefficient:

7.2.5 Heat transfer area:

82

7.2.6 Layout and tube size:

Using a split-ring floating head exchanger. Neither fluid is corrosive, so plain carbon steel

can be used for the shell and tubes.

From the tubing characteristics as given in Perrys, I chose the following dimensions of the

tube.

1-inch Outer Diameter (O.D) tubes with 1.25-inch Triangular Pitch, 16 BWG

Length of tube = 6m (standard length)

7.2.7 Number of tubes

83

7.2.8 Bundle and shell diameter

For a split ring floating head exchanger,

7.2.9 Tube-side heat transfer coefficient calculations:

84

The Reynolds (Re) and Prandtls number (Pr) of the cracked gas at the tube side is given

by,

85

Hence the Nusselt Number (Nu) is thus calculated as,

7.2.10 Shell-side heat transfer coefficient calculations:

86

Choose a baffle spacing (Lb) of 100 mm.

The shell side linear velocity is appreciable since it falls in the standard range 0.3 1.0 m/s.

Equivalent Diameter (De) of the triangular pitch is given by,

87

The Reynolds (Re) and Prandtls number (Pr) of the cooling water at the shell side is given

by,

88

7.2.11 Overall coefficient:

Using carbon-steel for the tube and shell side because neither fluid is corrosive and the

temperature is very high.

Thermal Conductivity of carbon-steel (KW) = 55

Taking the fouling coefficients of cracked gas = 0.00030

Taking the fouling coefficients of water = 0.00090

The overall coefficient is the reciprocal of the overall resistance to heat transfer, which is

the sum of several individual resistances.

The other parameter above were defined previously, hence

+ + +

89

Since the calculated U = is above the assumed value of

Hence the design with the above parameters is accepted.

7.2.12 Tube Side Pressure Drop Calculations:

The total pressure drop at tube side is given by the equation,

Where Np = number of tube passes

L Length of tube

For (friction factor at tube side)

The total tube side pressure drop is less than 70 kPa, hence within specification.

7.2.13 Shell Side Pressure Drop Calculations:

The shell side pressure drop is also related by the equation,

90

Where De = equivalent diameter of shell

Lb = baffle spacing

As this pressure drop on the shell side is less than 70 kPa, the design is acceptable from the

pressure drop point of view.

7.2.14 SUMMARY OF PROCESS DESIGN FOR HEAT EXCHANGER

Heat transfer area

Tube side coefficient

Shell side coefficient

91

Overall transfer coefficient, assumed

Overall transfer coefficient, re

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Ethylene and Acetylene Plant