CHE655 - Plant Design Project #6 Summer 2011
DESIGN OF AN AMMONIA PRODUCTION PROCESS
(Courtesy of the Department of Chemical Engineering at West Virginia University)
Introduction You work for a consulting company that has been hired to determine a profitable operating condition (but not necessarily the most profitable) for an ammonia synthesis loop at your client's facility. You are only to evaluate the ammonia synthesis portion of a much larger process that takes coal, converts it into a synthesis gas (carbon monoxide and hydrogen), adds nitrogen and removes the carbon monoxide, adjusts the composition and produces ammonia. Ammonia is one of the five most produced chemicals in the ...view middle of the document...
The reaction that occurs in the reactor is reversible 322 23 NHHN =+ (1) This is an equilibrium reaction, and the equilibrium constant over a wide range of temperatures is given by
⎡×= − T
K 806,11exp1029.3 12 (2)
In the reactor, 90% of the equilibrium conversion is obtained. Process Details Streams and Equipment Details Stream 1: syngas - at 200°C and 1000 kPa - contains 72 mol% H2, 24 mol% N2, and
4 mol% CH4 Stream 8: ammonia product - 50,000 tonne/y - a year is 8000 hours Stream 10: purge used as fuel-gas to furnace - may take credit for lower heating value Streams 9-11: unreacted syngas and ammonia not in the product stream are recycled -
the recycle split is a potential decision variable Equipment Information
Compressor (C-201) The compressor increases the pressure of the feed stream to the pressure of the reactor. The compressor may be assumed to be adiabatic. In that case, the compressor power sW& (kW) may be calculated as
⎜⎜⎝ ⎛= 1(kmol/s)000,20(kW)
out s P
PmW && (3)
where m& (kmol/s) is the total molar flowrate of Stream 1. Equation 3 includes the compressor efficiency. The cost of electricity to run the compressor is a utility cost. The compressor increases the temperature of the stream being compressed according to
out P P
where T is absolute temperature.
In general, the ratio of outlet to inlet pressure in a compressor is between 3 and 5. If a compression ratio greater than 5 is needed, compressors are usually staged with cooling in between the compressor stages, but not after the last stage. If you choose to do this, the compression ratio for each stage should be identical, and the "intercooling" should be to 50°C. The process-flow diagram should accurately represent the chosen compressor configuration.
Heat Exchanger (E-601) The reactor feed is cooled to T = 350°C using a cold utility. In any heat exchanger, the process stream may not be cooled below the temperature of the utility plus 10°C. The 10°C allowance is for design purposes. Reactor (R-601) This is an adiabatic reactor. It is essentially a large pipe packed with catalyst. The equilibrium conversion can be calculated based on a choice of the operating pressure and the outlet temperature. These are decision variables that you are expected to manipulate to find optimum values. The reactor may operate at pressures of 500 kPa ≤ P ≤ 20,000 kPa and at any temperature above 350°C. The actual conversion in the reactor is 90% of the equilibrium conversion. You will find the conversions to be low, requiring a large recycle stream. An alternative reactor configuration that can increase the conversion is to stage several adiabatic reactors with a heat exchanger...