Orientation (Part - 2) | Chemical Technology - Chemical Engineering PDF Download

Figure 1.1: Process flow sheet for ammonia manufacture using Haber’s process.

1.6 Illustration of quickly learning process technology: Ammonia manufacture using Haber’s process (Figure 1.1) 

a) Reaction: The Haber process combines nitrogen from the air with hydrogen derived mainly from natural gas (methane) into ammonia. The reaction is reversible and the production of ammonia is exothermic. Nitrogen and hydrogen will not react under normal circumstances. Special conditions are required for them to react together at a decent rate forming a decent yield of ammonia. These conditions are T = 400°C; P = 200 atm; Iron catalyst with KOH promoter.

b) Raw materials: H₂ from synthesis gas, N₂ from synthesis gas/air liquefaction process

c) Process technology: Illustrated in Figure 1.1

d) Unit processes: Feed Guard Converter; Main Reactor

e) Unit operations in the technology: Condensation/Gas Liquefaction; Separation; Refrigeration; Centrifugal Re-circulator f) Striking feature: Conversions are low (8 - 30 % per pass) and hence large recycle flow rates exist.

g) Functional role of various processes

a. Feed guard converter:

» CO and CO₂ conversion to CH₄ and removal of traces of H₂O, H₂S, P and Arsenic.

» These compounds could interfere with the main haber’s reaction as well as poison the catalyst.

b. Main reactor: 

» Cold reactants enter reactor from reactor bottom and outer periphery to absorb heat generated in the reversible reaction.

» Carbon steel used for thick wall pressure vessel and internal tubes

» Gas phase reaction at 500 – 550° C and 100 – 200 atms.

» Pre-heated gas flows through the tube inside with porous iron catalyst at reaction conditions.

» Catalyst removed from the converters is re-fused in an electric furnace

c. Condenser: 

» Complete liquefaction not possible due to vapor liquid equilibrium between NH₃ in vapor and liquid phases.

» Cooling fluid: Chilled water

» Incoming stream has: Gaseous NH₃, un reacted N₂ and H₂, impurity gases like CO, CO₂, CH₄

» Product stream has: Liquefied NH₃, Vapor phase NH₃in equilibrium with liquid, non-condensable gases such as N₂ and H₂

» System pressure is high therefore NH₃ bound to be present in higher concentrations in the vapor.

d. Separator: 

» Working principle: Density difference between vapor and liquid

» Liquefied NH₃ is separated from the un-reacted gases (NH₃ still present in the vapors).

e. Refrigeration:

» Why?: NH₃ available in vapors needs to be condensed. Therefore, refrigeration is required for gaseous product emanating from the separator.

» The condensed NH₃ emanates at -15°C.

» After refrigeration, the un-reacted N₂ and H₂ are recycled to the reactor.

f. Centrifugal pump: 

» A centrifugal pump adjusts the pressure of the stream from the separator to the pressure of the feed entering the reactor

» A purge stream exists to facilitate the removal of constitutes such as Argon.

g. Striking feature: 

All units such as Condenser, Separator, Refrigerator operate at high pressure. This is because loosing pressure is not at all beneficial as the unreacted reactants (corresponding to large quantity in this case due to low conversion in the reversible reaction) need to be supplied back to the reactor at the reactor inlet pressure conditions.

1.7. Technical questions:

1. How is Le-chartlier principle used in the process? 

i) Only reactants and not product (Ammonia) are fed to the reactor. According to Lechartlier principle, higher yields are obtained at high pressures. Exothermic reaction implies that the temperature cannot be too high. ii) Ammonia removal from vapor phase emanating from the condenser is according to Le-chartlier principle, which indicates that the presence of ammonia in the feed can reduce conversions and hence yield.

2. Why is purge stream taken out after centrifugal pump. If purge stream is taken before centrifugal pump, then pumping cost would reduce ?

i) The purge stream consisting of Argon impurity would go to pressure swing adsorption process for Argon recovery. Therefore, it will be advantageous to have as much high pressure as possible for the purge stream and hence it is desired to take the purge stream after the centrifugal pump.

3. How can ammonia whose boiling point is -33°C can get liquefied at a temperature of -15°C using ammonia refrigerant ?

i) Ammonia’s boiling point is -33°C at 1 atm pressure. At higher pressures, the boiling point of ammonia will increase. Therefore, at the system pressure (which is close to 200 atm.), the boiling point of ammonia would be much higher and hence liquefaction is possible at -15°C. ii) The ammonia refrigerant is at 1 atm pressure and hence can be a liquid upto a temperature of -33°C. The stream undergoing refrigeration is at 200 atm. And therefore liquefies at about -15°C.