Comparison of the TCD and E-TCD sequences based on atmospheric distillation
Atmospheric distillation processes which consisted of TCD (scenario 1) and E-TCD (scenario 2) sequences were developed and optimized firstly. Based on the construction and optimization strategies described in “Strategies” section, energy demands of the heating and cooling streams of the TCD and E-TCD sequences are illustrated in Fig. 3a and b, respectively. 95 wt% of ethanol (stream 6), 99.7 wt% of acetone (stream 4) and completely dehydrated butanol (100 wt%, stream 10) were obtained in both scenarios (detail stream composition and flows rate of different types of sequences are shown in Additional file 1: Table S1).
Data shown in Fig. 3 refer to the heating and cooling energy demand of distillation columns in two scenarios. As there were no differences between the upstream columns, the energy demands for the upfront two columns (beer and acetone columns) were also similar. Nevertheless, the results were quite different in TCD and E-TCD sequences in the downstream columns (ethanol, butanol and water columns). The lower energy requirement of the ethanol column in scenario 2 might be caused by the fact that a higher ethanol concentration in the bottom stream of the ethanol column is possible due to the recirculation loop. The stream which consisted of the distillate of butanol and water columns was mixed with the bottom outlet of the acetone column in scenario 2 (Fig. 2b). Thus, the actual flow rate that inlets into ethanol column in scenario 2 was 433.8 kg/h (the sum of flow rates from bottom outlet of acetone column and distillate of water column), which was higher than that of the case in scenario 1 (383.8 kg/h, only from the bottom outlet of acetone column). Nevertheless, the ethanol product flow rate in scenario 2 was not increased, which was maintained at 18.62 kg/h. Hence, the ethanol concentration of the bottom outlet of ethanol column in scenario 2 was higher than that in scenario 1 (2.42 wt% in scenario 2 vs. < 200 ppm in scenario 1), which could significantly reduce the energy requirement in ethanol column in scenario 2 (2.39 MJ/kg compared to 4.56 MJ/kg in scenario 1, see Fig. 3). Correspondingly, the reflux ratio of the ethanol column in scenario 2 (~ 25) was far lower than that in the scenario 1 (~ 57) when above 95 wt% of ethanol was reached (Additional file 1: Fig. S2).
Because of the lower concentrations of butanol in the organic phase from decanter (streams 9 in Additional file 1: Table S1) and the aqueous phase from decanter (streams 12 in Additional file 1: Table S1) in scenario 2 (81.66 wt% for the organic phase of the decanter and 4.6 wt% of the aqueous phase of the decanter), the heating and cooling energy requirement of the water and butanol columns was higher in the E-TCD sequence (1.99 MJ/kg and 0.73 MJ/kg for heating, and − 1.44 MJ/kg and − 0.38 MJ/kg for cooling in butanol and water columns, see Fig. 3b) in comparison to the conventional TCD sequence (1.8 MJ/kg and 0.56 MJ/kg for heating, and − 1.28 MJ/kg and − 0.24 MJ/kg for cooling in butanol and water columns, see Fig. 3a). Fortunately, the low butanol concentrations in both organic phase and aqueous phase in scenario 2 were mainly caused by the participation of the higher ratio of ethanol, which is the light component exist in the butanol–ethanol–water mixture [26]. Water fractions, the heavy component distributed in the mixture, did not increase significantly. Thus, the energy demand in water and butanol columns in scenario 2 were only slightly higher compared to the energy demand of the two columns in scenario 1.
Energy consumption in ethanol column was the decisive factor of the overall energy cost during the atmospheric distillations processes. Even though the energy cost for butanol and water columns was higher, the overall energy requirement for heating and cooling the streams in scenario 2 was far lower than that in the scenario 1. As a result, the energy demand of 13.42 MJ/kg and − 10.75 MJ/kg for heating and cooling, respectively, was consumed in scenario 2, which were only 88.1% and 85.1% of that compared to the energy demand in scenario 1. Therefore, the E-TCD sequence enables energy savings for the subsequent distillation separation ABE mixture after pervaporation. More importantly, as the ethanol (contained in the distillate of water and butanol columns) was recycled into the ethanol column in the E-TCD sequence, no ethanol accumulated in the TCD sequence (see Additional file 1: Table S1, streams 8 and 11). Hence, the E-TCD sequence showed a better controllability in contrast to the conventional TCD sequence. As it was suggested in previous report [19], the stable E-TCD sequence also enjoys the advantage of making the distillation system more cost effective.
In our previous work, the application of heat-exchange system could significantly decrease the energy requirement in the conventional TCD sequence based on distillation process [13]. To further decrease the energy demands in the two scenarios, heat-exchange system was established and optimized. Energy requirements for ABE separation based on TCD and E-TCD sequences were also compared after the heat exchange. Generally, 12 heat exchangers (HEs) were connected with the streams in both scenarios. The minimum temperature difference for heat exchange was set at 15 °C. Key parameters and the heat-exchange strategies are carried out in Fig. 4. Besides, grand composites curves and the basic structure of the heat-exchange system are shown in Additional file 1: Fig. S3. As can be seen, under the optimized conditions, heat exchanges were mainly carried out in the streams 1, 9 and 12 in both scenarios, which were not in line with our previous works that applied the two-stage gas stripping–pervaporation process [13]. For the case of scenario 1, after heat exchange, the temperature of stream 1 gradually increased from 25 to 40.7 °C (after HE1), 62.77 °C (after HE4), 77.8 °C (after HE7) and 82.9 °C (after HE10), respectively. Similarly, the stream 9 which was fed into the butanol column was sequentially heated by HE2, HE5, HE8 and HE11, and its temperature finally reached 91 °C. Correspondingly, stream 12 (85.35 °C) was fed into the water column after being heated by HE3, HE6, HE9 and HE12. In contrast, in the case of scenario 2, the temperatures of stream 1, 9 and 12 were increased from 25 °C, 20 °C and 20 °C, to 84.25 °C (after HE1, HE4, HE7 and HE10), 91.6 °C (after HE2, HE5, HE8 and HE11) and 86 °C (after HE3, HE6, HE9 and HE12), respectively.
By the effect of heat exchange, the energy requirement of beer column, butanol column and water column were decreased. Energy requirements in both scenarios were decreased slightly. The energy demands of 12.27 MJ/kg and 10.12 MJ/kg were achieved in scenario 1 and scenario 2, respectively, which were 13.1% and 15.5% lower than that of the conventional processes without heat exchange (Fig. 5). Therefore, after heat exchange, energy requirement for the E-TCD sequence based on distillation (scenario 2) was still lower than that of the TCD sequence-based process (scenario 1), and the scenario 2 was more sensitive to heat integration for showing a relatively higher energy decreasing rate.
The effect of column condenser pressures on the distillation performances and the improvement of the processes
Adjusting the pressure level of distillation columns showed advantages in further decreasing energy requirement in alcohols separation processes [32,33,35]. By applying VDP, the reflux ratios of several columns were decreased, and the heat exchange network was also intensified in VDP.
In this section, VDP was applied for ABE separation based on E-TCD process. Figure 6 shows the effect of condenser pressures on the reflux ratios in output streams. In comparison to the beer, butanol and water columns, the reflux ratios of acetone and ethanol columns were more sensitive to the condenser pressures. To generate the acceptable acetone product in distillate, the reflux ratio of acetone column was gradually increased from 2 in 50 kPa to 15 in 120 kPa. By contrast, the reflux ratio of ethanol column did not change until the condenser pressure increased to 90 kPa. After that, the reflux ratio was significantly increased with the increase of condenser pressure, and finally reached 200 when the condenser pressure was 120 kPa. Hence, acetone and ethanol columns, the more sensitive ones, were selected to decrease the pressures.
The effect of the condenser pressures of acetone and ethanol columns on the distillate temperatures were evaluated. The distillate temperatures were decreased with the decrease of condenser pressures. 40 °C was considered to be the lowest temperature that can meet the needs of condensation (based on the minimum temperature for heat exchange of 15 °C). The suitable condenser pressures of acetone and ethanol columns were 57 kPa and 18 kPa, respectively (details are also shown in Additional file 1: Fig. S4).
After decreasing the acetone and ethanol column pressures to 57 kPa and 18 kPa, the effect of reflux ratios on the distillate acetone and ethanol concentrations was further investigated. The TCD (scenario 3) and E-TCD (scenario 4) sequences based on VDP were compared (details see Additional file 1: Fig. S5). After decreasing the condenser pressures of acetone and ethanol columns, the optimized reflux ratio for acceptable purities of solvents was decreased sharply in both of the TCD and E-TCD sequences Therefore, the energy consumption might be also decreased. More specifically, the optimized reflux ratio in acetone column was decreased from 5.8 to 2.4, while the optimized reflux ratio in ethanol columns was only 48 and 18 in the sequences of scenario 3 and scenario 4 after increasing/decreasing columns pressure, respectively.
After optimizing the condenser pressure of acetone and ethanol columns, key parameters of the water and butanol columns were subsequently determined by changing the distillate of butanol and water column following the iterative strategy shown in Fig. 2 (The stream flow rates showed in Additional file 1: Table S2). Based on the specific conditions for VDP, the key parameters of TCD and E-TCD sequences are conducted in Fig. 7. As it is illustrated, the condenser pressure of beer column was increased to make the process of transferring the heat easier. Accordingly, the pressures of the acetone and ethanol columns were decreased while the condenser pressures of butanol and water columns remained in the atmospheric pressure. It showed that the heat requirements for the acetone and the subsequent ethanol, butanol and water columns in the TCD and E-TCD sequences all decreased after adjusting the columns pressures. The overall energy requirements in scenario 3 and scenario 4 were 11.53 MJ/kg and 10.03 MJ/kg (Fig. 9), respectively, which were 81.66% and 83.72% compared with the energy requirement in scenario 1 and scenario 2 without applying VDP. Compared with other columns, the energy requirement for ABE concentration in beer columns occupied 43.54% (for scenario 3) and 50.05% (for scenario 4) of the overall heating cost. Additionally, although the heat requirement of the water and butanol columns in scenario 4 was a little higher than that of scenario 3, the sharp reduction of the energy cost in ethanol column also resulted in a lower overall energy demand in scenario 4.
Figure 8 shows the heat-exchange system for the VDP. Details of the grid diagram and grand composite curve were given in Additional file 1: Fig. S6. Compared to the VDP before heat integration, the energy requirement sharply decreased in both scenarios. Only 7.17 MJ/kg and 5.3 MJ/kg heat were consumed for ABE distillation separation from the permeate of in situ pervaporation separation in scenario 3 and scenario 4, respectively (Fig. 9). Under these conditions, 37.81% and 47.16% of energy could be saved after heat exchanges. Remarkably, it showed that no additional energy was required for heating the acetone and ethanol columns in scenario 4, and all the heat requirements were provided by the hotter streams. For the scenario 3, the bottom of acetone column was also warmed by the overhead product of beer column. It is also noteworthy that the number of heat exchangers can be, thus, reduced in scenarios 3 and 4 based on VDP (total 7 heat exchangers, see Fig. 8) compared with conventional distillations in scenarios 1 and 2 (total 12 heat exchangers, see Fig. 3).