Energy Balances and Numerical Methods Design Project Ammonia Production Your assignment is to continue to evaluate the feasibility of a process to produce 50,000 tonne/y of ammonia from syngas. A suggested process flow diagram (PFD) is shown in Figure 1. You should use this as a starting point. Your assignment is to develop a “best” case, where “best” is dependent upon economic considerations, i.e., EAOC. In reporting your best case, clearly indicate any modifications to the PFD and state the operating conditions for the modified process and the corresponding EAOC. Chemical Reaction Syngas is available from a pipeline at 1000 kPa and 200°C. It is compressed, mixed with a recycle stream, and heated or cooled to 350°C to be fed to the reactor. The reactor operates adiabatically. The reactor effluent is cooled, the pressure is reduced by a valve, and the stream partially condensed, producing an ammonia-rich stream. The ammonia liquid product is in Stream 8. Some of Stream 9 is recycled and some is purged. Depending on the pressure of the flash separator, V-601, the recycle stream may need to be compressed up the pressure of Stream 2. The reaction that occurs in the reactor is reversible 32223 NHHN=+ (1) This is an equilibrium reaction, and the equilibrium constant over a wide range of temperatures is given by ⎥⎦⎤⎢⎣⎡×=−TK806,11exp1029.312 (2) In the reactor, 90% of the equilibrium conversion is obtained.23Process 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 (kW) may be calculated as sW& ⎥⎥⎦⎤⎢⎢⎣⎡−⎟⎟⎠⎞⎜⎜⎝⎛= 1(kmol/s)000,20(kW)286.0inoutsPPmW&& (3) where (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 m& 286.0⎟⎟⎠⎞⎜⎜⎝⎛=inoutinoutPPTT (4) 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.4Heat 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 as you will learn next year. 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 between the stages to reduce the inlet temperature to each subsequent reactor. The number of reactor stages is determined by the economics. The temperature of the “intercooled” stream is a potential decision variable. The process-flow diagram should represent the chosen reactor configuration. Heat Exchanger (E-602) and Vessel (V-601) This heat exchanger cools and partially condenses the reactor effluent to a temperature that condenses ammonia. The subsequent valve reduces the pressure to the desired pressure for the separator. Equation 4 is used to determine the outlet temperature of the valve for a chosen pressure. This vessel allows the vapor and liquid produced in E-602 to be separated. The vapor exits in the top stream, and the liquid exits in the bottom stream. Stream 9 contains all of the light gases in Stream 7 plus some ammonia. Stream 8 contains only ammonia, and the ammonia split must be calculated for the chosen temperature and pressure.. E-602, the valve, and V-601 may all be treated together for computational purposes as a flash operation at the chosen temperature and pressure. The temperature and pressure of this flash are potential decision variables. The appropriate utility must be used in E-602, and the appropriate utility depends on the temperature chosen for the separation. 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 as you will learn next year. Compressor (C-602) The compressor increases the pressure of the recycle stream to the pressure of the stream with which it is mixed. The compressor may be assumed to be adiabatic. In that case, the compressor power (kW) may be calculated as sW& ⎥⎥⎦⎤⎢⎢⎣⎡−⎟⎟⎠⎞⎜⎜⎝⎛= 1(kmol/s)000,10(kW)286.0inoutsPPmW&& (5)5where (kmol/s) is the total molar flowrate of Stream 11. 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 Equation 4. m& 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