7.6 Simulation of the Propan-1-ol Dehydration Process via Azeotropic Distillation in
7.6.3 Process Flowsheet and Simulation Sequence
The azeotropic distillation process involves a two stage separation; the separation of the two azeotropic components in the presents of the solvent and the recovery of the solvent. The flowsheet configuration will vary such that the best possible results are obtained. The final process flow diagram and Aspen flowsheet resulting in the highest recovery of 1-propanol are given in Figures 7-50 and 7-51 respectively. With reference to Figure 7-51, the simulations sequence starts by feeding the water rich mixture (60 molar % water) in the dehydration column at stage 12. The separation requires anhydrous propan-1-ol, thus, the separation is to pass the azeotropic composition in order to rich pure propan-1-ol. The DIPE solvent is fed on stage 18 since it is a light boiling entrainer and allows for formation of a heterogeneous azeotropic mixture at the top of the column and a binary mixture of the water + propan-1-ol stream with t small % of solvent at the bottom. The top stream is cooled resulting in the formation of two phase mixture DIST1 which separates into a solvent rich phase (organic phase) and water rich phase in the first decanter (DECANT1). The equimolar stream at the bottom of azeotropic column has a composition passed the azeotrope (59 mole% H2O) is sent to the first recovery column (RCOL1) and results in highly pure propan-1-ol bottom stream (PROD1, 96.44%
(wt/wt)). The propan-1-ol fraction recovery in this stream is 34.2% of the initial feed and is relatively low. The top stream of the recovery column DIST2 consisting of water, propan-1-ol and DIPE is combined in the first decanter with stream DIST1. The first decanter allows for
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removal of 79.4 % of feed water in the aqueous phase. The organic phase from the decanter is made of mostly the DIPE solvent and propan-1-ol and the rest being water is separated in the second recovery column (RCOL2), which produces a bottom product of 99.72 % (wt/wt) propan- 1-ol with fractional recovery of 57.4%. The top product DIST3 consist mainly of DIPE and the remainder of water, with small traces of propan-1-ol is sent to the second decanter (DECANT2) where the rest of process water is removed. The organic phase in the second decanter consists of 96.22 % (wt/wt) solvent with the rest being propan-1-ol. The solvent recovered is 99.36 % of the initial solvent fed to the process, and is returned to the azeotropic column where it is combined with the makeup stream which is used at the start of the process simulation. It is important to note that the purge stream shown in Figure 7-51 is used to aid the convergence of the process simulation when closing the aspen loop by slowly increasing the split ratio of the recycle stream towards stream 9. This is crucial because sending the whole recycle stream in the azeotropic column in Aspen results in disruption of the simulation and severe errors, thus failure to convergence. This method is mostly adapted by process simulators to quickly converge the simulated Aspen process. The simulated column results are given in Table 7-15 and the process stream results are given in appendix F. Figures 7-52 to 7-54 shows the Molar composition profile inside the distillation column.
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Figure 7-50: The propan-1-ol dehydration process via azeotropic distillation of water + propan-1ol using diisopropyl ether as an entrainer.
Stage 1
Water rich feed at stage 16 (Propan-1-ol)
---~zeotropi Column
Solvent (DIPE)
Pump PC-02
Reboiler
Two phase distillate Water+ DIPE rich
Steam
Condensate
Stage 22
L _ _ _ _ _ _ [ _ _ _ _ _ _ _ ~ Recovery
Column Propan-1-ol rich
One
Solvent Recycle
Condenser
Reboiler
Recovery t---~ Column
Two Two phase distillate
Water+ DIPE rich
Steam
1
Condensate
Product one (Propan-1-ol)
Condenser
Organic phase
Reboiler
Water
Steam
Condensate
Product two (Propan-1-ol)
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Figure 7-51: Aspen Plus flowsheet of the propan-1-ol dehydration process via the azeotropic distillation of water + propan-1-ol using diisopropyl ether as an entrainer.
FEED SOLRICH
DIST1
4
PROD1 DIST2 MAKEUP
9
SRECYCLE PURGE
H2O1
ACOLUMN
RCOL1 B4
SPLIT
DECANT1
RCOL2
DECANT2
ORGPHASE
DIST3
H2O
PROD2
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Table 7-15: Column specifications for the propan-1-ol dehydration process.
Parameter Value
Azeotropic column(Redifrac)
Separating agent molar flow (kmol.hr-1) 45.32
Distillate molar flow (kmol.hr-1) 60.01
Molar composition of propan-1-ol in the bottoms 0.490 Mass fraction of propan-1-ol in the bottoms 0.761
Temperature of the process feed (K) 338.15
Temperature of the entrainer (K) 333.15
Molar reflux ratio in the column 0.95
Number of theoretical stages in the column 22
Process feed stage 16
Pressure in the column (kPa) 130
Entrainer feed stage 18
Solvent to feed ratio (S/F) 0.45
First recovery column (Redifrac)
Number of theoretical stages in the column 10
Pressure in the column (kPa) 120
Feed stage of first bottoms 5
Molar reflux ratio in the column 1.5
Distillate molar flow (kmol.hr-1) 70
Second recovery column (Redifrac)
Number of theoretical stages in the column 10
Pressure in the column (kPa) 100
Feed stage of organic phase mxiture 5
Molar reflux ratio in the column 5
Distillate molar flow (kmol.hr-1) 56
Net energy consumption (sum of reboiler and condenser duties)
Energy consumption azeotropic column (kJ/s) 245.3
Energy consumption of the first recovery column (kJ/s) -7.63 Energy consumption of the second recovery columns (kJ/s) 228.5
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Figure 7-52: Composition profile in the azeotropic column.
Figure 7-53: Composition profile in the first recovery column.