Simulation process flowsheet of ammonia synthesis

Aspen plus simulation process flowsheet developed with basic models present in the process simulator. Gibbs reactor, heat exchanger, isentropic compressor and flash separator are used for studying the process parameters and their effects on the conversion and yield for known input.

Simulation process flowsheet of ammonia synthesis using Aspen plus

Process description:

Simple model flowsheet shown here starts with synthesis gas. It is compressed in the isentropic compressor (Compr) and pressurized to 136.5 ATM the compressed gas passes through the heat exchanger to maintain the reactor feed temperature of 250oC. Hot gas converted to ammonia in a Gibbs model reactor, and the product gas temperature raised due to the exothermic reaction. Hot gases are passed to waste heat boiler to produce steam. The cooled gas is flashed into the separator to separate liquid ammonia and unreacted gases. Recycle stream from the splitter is fed back to the compressor and a portion of the unreacted gases are purged out.

This is model shows an overview how ammonia production is done, but in the industry, it is much more complex with heat exchange network and catalytic reaction. The only phase equilibrium model is considered here, but to simulate the process kinetic data is the need for further studies. The purpose of this simulation is to understand the Aspen plus its capability to solve the recycle stream values by iteration. An industrial feed composition is used in this flowsheet analysis.

 

Equipments representing the process flow sheet symbols

S.No Block names ASPEN models Equipment
1 FEEDER Stream Mixer Mixer
2 COMP Compr Compressor
3 HEATEX Heater Heat exchanger
4 REACTOR RGibbs Phase equilibrium reactor
5 WHB Heater Waste heat boiler
6 SEP Flash2 VLE flash drum
7 SPLITER FSplit Control splitter
Results from process flowsheet simulation:

Block $OLVER01 (Method: WEGSTEIN) has been defined to converge

> Loop $OLVER01 Method: WEGSTEIN     Iteration   10

# Converged             Max Err/Tol  -0.56211E+00

1 2 3 4 5 6 AMMONIA FEED PURGE RECYCLE
COMP HEATEX REACTOR WHB SEP SPLITER  — FEEDER  —  COMP
FEEDER COMP HEATEX REACTOR WHB SEP SEP SPLITER  SPLITER
VAPOR VAPOR VAPOR VAPOR MIXED VAPOR LIQUID VAPOR VAPOR VAPOR
Substream: MIXED
MoleFlow kmol/sec
HYDROGEN 23.85 28.22 28.22 4.96 4.96 4.86 0.10 23.85 0.49 4.37
NITROGEN 8.55 9.96 9.96 2.20 2.20 1.56 0.64 8.55 0.16 1.41
AMMONIA 0.00 0.16 0.16 15.67 15.67 0.18 15.49 0.00 0.02 0.16
CH4 0.36 0.60 0.60 0.60 0.60 0.26 0.33 0.36 0.03 0.24
ARGON 0.10 0.25 0.25 0.25 0.25 0.17 0.09 0.10 0.02 0.15
WATER 0 0 0 0 0 0 0 0 0 0
TotalFlow kmol/sec 32.86 39.18 39.18 23.68 23.68 7.03 16.65 32.86 0.70 6.32
TotalFlow kg/sec 297.41 358.08 358.08 358.08 358.08 67.41 290.68 297.41 6.74 60.67
TotalFlow cum/sec 33.51 13.09 12.32 7.78 2.04 1.21 0.47 33.51 0.12 1.09
Temperature K 313.15 555.7557 523.15 527.15 313.15 268.15 268.15 313.15 268.1484 268.1484
Pressure atm 25.20 136.50 136.50 131.66 134.56 127.79 127.79 25.20 127.79 127.79
Vapor Frac 1 1 1 1 0.341879 1 0 1 1 1
Liquid Frac 0 0 0 0 0.658122 0 1 0 0 0
Solid Frac 0 0 0 0 0 0 0 0 0 0
Enthalpy J/kmol -3.78E+05 6.26E+06 5.29E+06 -2.40E+07 -4.34E+07 -4.83E+06 -6.55E+07 -3.78E+05 -4.83E+06 -4.83E+06
Enthalpy  J/kg -41757.27 6.85E+05 5.79E+05 -1.59E+06 -2.87E+06 -5.03E+05 -3.75E+06 -41757.27 -5.03E+05 -5.03E+05
Enthalpy   Watt -1.24E+07 2.45E+08 2.07E+08 -5.68E+08 -1.03E+09 -3.39E+07 -1.09E+09 -1.24E+07 -3.39E+06 -3.05E+07
Entropy   J/kmol-K -20880.03 -18364.1 -20161.7 -79661.7 -1.29E+05 -41505.9 -1.84E+05 -20880.03 -41505.9 -41505.9
Entropy   J/kg-K -2306.946 -2009.48 -2206.18 -5267.08 -8509.6 -4325.86 -10522.07 -2306.946 -4325.86 -4325.86
Density  kmol/cum 0.98 2.99 3.18 3.04 11.63 5.81 35.41 0.98 5.81 5.81
Density   kg/cum 8.87 27.35 29.06 46.03 175.85 55.72 618.13 8.87 55.72 55.72
Average MW 9.05 9.14 9.14 15.12 15.12 9.59 17.46 9.05 9.59 9.59
Liq Vol 60F cum/sec 1.76 2.10 2.10 1.27 1.27 0.38 0.89 1.76 0.04 0.34
Process flowsheeting of ammonia preparation:
ammonia synthesis simulation process diagram in Aspen plus

Preliminary flowsheet of ammonia process simulation