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

VARIOUS MANUFACTURING PROCESSES

Braun Purifier process

Foster Wheeler AM2 process

ICI process

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.

FLOW SHEET

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.