This work proposes one novel process modified from Patraşcu et al. for separating biobutanol from the acetone/butanol/ethanol fermentation. Therein, the effect of existing acetone on the liquid-liquid equilibrium of butanol and water is verified and included in the design of separation sequence. It is found that the updated thermodynamic models lead to unsatisfactory separation of butanol/water by the decanter due to the remaining-acetone feed of the decanter. Large amount of butanol/water accumulated in the organic/aqueous phase. Eventually, reboiler duty in columns for separating butanol and water are severely high. Besides, amounts of acetone in the organic phase is only recycled to the decanter and in the aqueous phase are required to go through many columns before purification. The high circulation rate and the long journey of acetone imply high reboiler duty in the distillation sequences. Thus, several novel modifications are proposed for efficient separation. The first design removes acetone at first to simplifying the subsequent separation. The total reboiler duty decreases 33% from 12,159 kW to 8,055 kW. Also, TAC decreases by 33% from 3,671 to 2,459 kUSD/year. The membrane is applied secondly to break ethanol/water azeotrope and to improve ethanol purity. The penalty of 3% TAC (2,516 kUSD/year) can increase ethanol purity from 93.7 wt% to targeted 99.5wt%. However, the duty of the condenser and cooler are still high and the feed temperature is not high enough in this case to waste more energy consumption.
Adequate heat integration is proceeded thirdly to reduce external energy consumption.
The implementation of heat integration can reduces total reboiler duty by 27% (5,855 from 8,075 kW) and TAC by 20% (1,998 from 2,516 kUSD/year). Apart from improving feed temperature in the distillation, there is other possibility that one condenser of column supplies heat to other reboiler of the column. The condenser of CB is suitable to supply heat to reboiler in other low-temperature columns. Ultimately, optimal column stacking CB-CW-CA can decrease 31% external heat supply (4,040 from 5,855 kW) in the expense of higher TAC. However, capital cost is rising a lot since heat exchanger area is larger than original reboiler duty area in the distillation. TAC decreases 12% (1,768 from 1,998 kUSD/year). Remarkably, the energy requirement for all product purification (including butanol, acetone and ethanol) of column stacking CB-CW-CA process is only 0.88 MJ/kg sugar, compared to updated Patraşcu process (2.63 MJ/kg sugar). In addition, net energy generation of biobutanol (9.71 MJ/kg sugar) in column stacking is more than that of bioethanol (9.29 MJ/kg sugar) in Kiss reference7. This modified process can promote the economic value of biobutanol and other products.
In dynamic simulation, effective plant-wide control for the Proposed Design-2 – Membrane Recovery of Ethanol is proposed. Temperature/pressure correlation function is used to adjust Tsetpoint of one stage in CW, which can improve water deviation in a disturbance. Besides, temperature/feed flowrate function is set to adjust heater
temperature before membrane so that ethanol in the retentate can recover desired value since the membrane cannot be directly controlled. The results show that product offsets will not exceed 0.05% in throughput and feed composition disturbances. Also, effective plant-wide control for Proposed Design-3 – Heat Integration is then proposed and studied.
Similarly, temperature/pressure and feed flowrate/pressure correlations are applied in Columns B and W and heater before membrane to enhance butanol, water and ethanol results in a disturbance. Product deviations will not exceed 0.05% in throughput and feed composition disturbances. Plant-wide control for Proposed Design-4 – column stacking is finally studied. However, column stacking process is required to be modified slightly since compositions of the distillation like Column-W cannot be controlled without reflux ratio or reboiler duty. Part of reboiler duty is essential to be retained so that reboiler duty can control stage temperature. Also, three functions are set to improve all purity in this case. Product offsets will not exceed 0.05% in throughput and feed composition disturbances.
To sum up, all cases are acceptable from the view point of control performance, and proper use of temperature control can replace composition control. Under the same good control in dynamic simulation, column stacking CB-CW-CA is the most optimal result due to the least total annual cost.
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Appendix
A-1. Total annual cost information
Table A-1. Total annual cost information29, 30
A-2. Pervaporation Model
The membrane is applied in this section, so this research introduces its fundamental properties and some membrane limitations in advance. Aspen Custom Modeler of Membrane is according to Dai et al.30 and Luyben31. A membrane is divided into three same size lumps. Below are the material and energy balance equations.
The total mass balance at the steady-state:
dM𝑅
dt = 0 = 𝐹𝑅,𝑛−1− 𝐹𝑅,𝑛− 𝐹𝑃,𝑛 (1) The component mass balance for species i:
MRdZR,n,i
dt = 𝐹𝑅,𝑛−1𝑍𝑅,𝑛−1,𝑖− 𝐹𝑅,𝑛𝑍𝑅,𝑛,𝑖− 𝐹𝑃,𝑛𝑍𝑃,𝑛,𝑖 (2) The energy difference between the inlet flow and outlet flow:
MRdhR,n
dt = 𝐹𝑅,𝑛−1ℎ𝑅,𝑛−1− 𝐹𝑅,𝑛ℎ𝑅,𝑛− 𝐹𝑃,𝑛𝐻𝑃,𝑛 (3)
Where MR is the mass holdup and assumes constant. FR,n is the mass flow rate of retentate from cell n. FP,n is the mass flow rate of permeate from cell n. ZR,n,i or ZP,n,i is the mass fraction of retentate or permeate for species i from cell n. hR is the mass enthalpy of
retentate from cell n. HP,n is the mass enthalpy of permeate from cell n.
The permeation rate of water and other species i equations:
Jw = kaexp (−𝐸𝐷
𝑅𝑇 ) [exp(𝑘𝑏𝑤𝑤,𝑓) − 1] (4) Ji = kaexp (−𝐸𝐷
𝑅𝑇) exp(𝑘𝑏𝑤𝑤,𝑓) (5)
The above parameters are shown in Table A-2. Butanol and Acetone can be directly
neglected since a membrane is unfavorable for the organic compounds. Also, membrane operating limitations are shown on Table A-3.
Table A-2. Membrane relative parameters32
Table A-3. Membrane operating limitations of PERVAP 220130 Component Ka (kg/h/m2) ED (kJ/mol) Kb (kg/h/m2)
H2O 1.19 × 107 49.96 2.17
EtOH 4.25 × 105 54.41 8.10
Manufacturer: Sulzer ChemTech
Maximum temperature (oC) 105
Maximum water content in the feed <50
Organic acids <50
pH 2-7