Ethanol Production Simulation using Aspen Plus
Objective
The objective of this process is to optimize this model of an ethylene to ethanol production process and recover 98% of the product ethanol in the final finishing column using non-ideal thermodynamic models.
Process Overview
- First Mixer
- Reactor
- Flash drum
- Absorber
- Splitter
- Second mixer
- Dewatering column
- Deethering column
- Finishing column
M = Methane
EL = Ethylene
PL = Polyethylene
DEE = Diethyl Ether
EA = Ethanol
IPA = Isopropyl Alcohol
W = Water
First Mixer
Thermodynamic Method: NRTL
- polar non-electrolytic liquid/liquid equilibrium
- 4 streams going into the initial mixer
- MU01 is initial feed stream containing M, EL, PL
- MU02 contains water
- MU51 is recycling stream from upper recycle
- MU81 is recycling stream from lower recycle
Reactors
Thermodynamic Method: PSRK
Operating Conditions: 590 K and 69 bar
- RStoic and REquil are used to represent one reactor
- RStoic defines the reaction stoichiometry
- Limiting reagents are EL and PL
- Fractional conversion for these components are defined to be 0.07 and 0.007
- REquil specifies the reaction equilibrium process
- Operating conditions are predetermined based on the reaction chemistry of the process
Reaction 1: EL + W –> EA
Reaction 2: 2EA <–> DEE + W
Reaction 3: PL + W –> IPA
Flash Drum
Thermodynamic Method: PSRK
Pressure: 68 bar
Vapor fraction: 0.69
- Specified vapor fraction to ensure that enough water is being sent to the lower recycling
- Dewatering column requires sufficient water to achieve the desired separation efficiently
Absorber
Thermodynamic Method: PSRK
Pressure: 40 bar
Temperature: 314 K
Total Stages: 2
Design Specification:
– 99.5% of EA should be recovered in the lower stream
- Sufficient pressure needed to help separation of heavier components from the lighter components
- Less stages to prevent stages from drying up and decrease capital cost
- Extra feed water fed into the column at 150 mol/sec to help separation of EA to satisfy the design specification above
Splitter
Thermodynamic Method: PSRK
Split Fraction: 0.005
- Splitter in the upper recycle purges a fraction of the recycling stream
- Prevent inert species from building up in the process
- Small split fraction due to valuable reusable reactants (EL, PL etc..) still in recycling stream
Second Mixer
Thermodynamic Method: PSRK
Dewatering Column
Thermodynamic Method: PSRK
Reflux Ratio: 4.1
Distillate to Feed Ratio: 0.17
Total Stages: 5
Design Specification:
– 99.5 % of the inlet EA should be recovered in the distillate
– 90% of inlet W should be removed in the bottoms
- High reflux and small distillate to feed ratio increases contact between vapor/liquid phases to assist with separation
- Advantage: less stages reduces capital cost
- Disadvantage: operational cost increases with increasing reflux ratio
Deethering Column
Thermodynamic Method: PSRK
Reflux Ratio: 3.5
Distillate to Feed Ratio: 0.69
Total Stages: 19
Design Specification:
– 99.5% of inlet EA should be in the bottoms going into the finishing column
– 99.5% of inlet DEE should be removed in
- Reflux and distillate to feed ratio are greater than the dewatering column but still sufficient to achieve separation
- higher number of stages to increase contact between vapor and liquid phase
Finishing Column
Thermodynamic Method: UNIFAC
Reflux Ratio: 2.5
Distillate to Feed Ratio: 0.61
Total Stages: 12
- Limitations on separating azeotropes in the system
- IPA and EA cannot be separated using distillation column
Conclusion and Improvements
- Around 98% of ethanol recovered in finishing column
- Reduce EL lose from splitter by adding a separation unit before splitter to separate more EL from waste product
- Possibly adding recycling stream from dewatering column back into mixer 1 or into the pump to reduce wastewater
- Improve separation of IPA from EA by using Azeotropic distillation through the use of an entrainer to form a new azeotrope with IPA at a lower boiling point
