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Manufacture of ammonia from naptha and its process description.
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MANUFACTURE OF AMMONIA FROM SYNTHESIS GAS
BY
AMUTHAN.M
UNDER THEGUIDANCE OF
Dr.N.STALIN M.Tech PhD
ANNA UNIVERSITY TIRUCHIRAPPALLI
INTRODUCTION
• Synthetic ammonia (NH3) refers to ammonia that has been synthesized from natural gas.
• Natural gas molecules are reduced to carbon and hydrogen.
• The hydrogen is then purified and reacted with nitrogen to produce ammonia.
• Approximately 82 percent of the ammonia produced is used as fertilizer.
It is either directly as ammonia or indirectly after synthesis as urea, ammonium nitrate, and monoammonium or diammonium phosphates.
The remainder is used as raw material in the manufacture of polymeric resins, explosives, nitric acid, and other products.
USES
It is the starting material for the production of a various number of nitrogenous fertilizers like ammonium phosphates, ammonium sulfate, ammonium nitrate etc.
It is used directly or indirectly as the source for the production of hexamethylene diamine for the manufacture of nylon 6,6.
As a corrosion inhibitor in petroleum refineries
PROPERTIESMolecular Formula NH3
Molar mass 17.031 g/mol
AppearanceColourless gas with
strong pungent odour
Density0.73 kg/m3 @
1.013 bar at 15 °C
Melting point 195 K
Boiling point 240 K
Solubility in water42.8% @ 0 °C 34% @ 25 °C18% @ 50 °C
Acidity (pKa)32.5 @ −33 °C
Basicity (pKb) 4.75
GENERAL PROPERTIES
STABILITY Anhydrous ammonia is stable at
normal temperatures and pressures Decomposes at temperatures above
450°C. It is stored in steel containers with
pressure relief valves and with welded (not braised) joints
since Ammonia will corrode copper, brass and bronze materials
CONTD….
TOXICITY For humans, inhalation of ammonia
vapour causes irritation and corrosive damage to skin, eyes and respiratory tracts.
At very high levels, inhalation of ammonia vapour can be fatal.
When dissolved in water, elevated levels of ammonia are also toxic to a wide range of aquatic organisms
Braun Purifier process
• In the secondary reformer 1.5 times the stoichiometric quantity of air is used. This increases the heat load and reduces the radiant duty of the primary reformer to less than two thirds its usual duty.
• Excess nitrogen is removed by a cryogenic purification unit after methanation occurs.
• Owing to the high purity of synthesis gases, lower recycle gas flow, lower refrigeration duty and lower purge duty will suffice
Foster Wheeler AM2 process
• Instead of treating the whole feed in primary reformer, some bypass is fed to the secondary reformer.
• Excess air used is much higher than in the case of Braun purifier process.
• Absorption system provides refrigeration for ammonia recovery.
ICI Process
• Hydrocarbon feed is subjected to steam reforming in two stages to form oxides of Carbon, methane and hydrogen.
• In the secondary reformer air is mixed with the gases to get a N2 : H2 ratio of 1:3.
• Carbon monoxide is removed by shift conversion. Carbon dioxide is removed by absorption into MEA or K2CO3
SELECTION OF PROCESS
ICI process
Because…….
Waste heat can be recovered steam generated can be used to other
process plants. Less dependency on electricity. Capital cost is less.
Process Description\\]
SEQUENCE OF OPERATION
• Feedstock desulphurization
• Primary reforming
• Secondary reforming
• Shift conversion
• CO2 removal
• Methanation
• ammonia synthesis
DESULFURIZATION
Desulfurization is carried out to remove the sulfur content in the feed.
Sulfur is poisonous to catalyst and it corrodes the pipes.
The sulfur in the naphtha feed stock is converted to hydrogen sulfide
Cobalt – Molybdenum catalyst used at a temperature of about
673 K
Reaction:
• R-SH+H2 RH+H2 S
Primary Reforming
Desulfurized naphtha is mixed with steam in a tubular reforming furnace.
Steam to carbon ratio must be maintained between 3.5 and 4.5 to ensure that carbon deposition does not occur.
Reaction:
Heat + CH4 + H2O → 3H2 + CO
Secondary Reforming
The gases from the primary reformer are mixed with air and steam in the secondary reformer.
The remaining hydrocarbon are further sent to reformation and the overall yield of hydrogen is increased.
Reaction:
• O2 + 2H2 → 2H2O + heat
• heat + CH4 + H2O → 3H2 + CO
Shift Conversion
HIGH TEMP SHIFT CONVERSION
CO concentration is reduced to about 3% by volume by means of water gas shift reaction at a temperature of 593 to 693 K.
The reaction being exothermic the exit gases are at a higher temperature.
• Reactions:
• CO + H2O → CO2 + H2 + heat
LOW TEMP SHIFT CONVERSION
• The unconverted CO is converted to CO2 in the low temp shift conversion
• REACTION
• CO + H2O → CO2 + H2 + heat
CO2 removal
The converted CO2 by shift conversion is absorbed in Absorber.
Common absorbing medium
• Monoethanolamine
• Diethanolamine
The solvents are regenerated using a stripper column
Methanation
Carbon monoxide and Carbon dioxide will act as catalyst poisons in the synthesis loop.
So the unconverted CO & CO2 are converted as methane
The methane is removed as an inert gas
• Reactions:
• CO + 3H2 → CH4 + H2O
• CO2 + 4H2 → CH4 + 2H2O
Ammonia Synthesis
The H2 and N2 are mixed in the reactor to produce ammonia
The yield is about 17-19%
To increase the yield the ammonia is recycled. The system is called Ammonia Synthesis Loop.
Ammonia Synthesis
The reaction is a reversible one
The high pressure favours product formation
High temperature reduces the yield as it favours the decomposition reaction
Ammonia Separation
The product ammonia is removed
by using mechanical Refrigeration
(or) Distillation.
Typically Refrigeration is most economical.