(202f) The Separation of Multi-Glycols with Divided-Wall Column
AIChE Annual Meeting
2019
2019 AIChE Annual Meeting
Separations Division
Distillation: Recent Developments in Advanced Techniques and Process Intensification
Monday, November 11, 2019 - 5:35pm to 5:59pm
Ethanediol (EG), propanediol (PG) and butanediol (BD) are all important platform compounds. They can be produced by catalytic hydrocracking of sorbitol obtained from cereal enzymolysis and often obtained as multi-glycol mixtures. It is difficult to separate these diols due to the similarity in molecular structures and properties, which limited their industrial production. Many methods have been studied to separate mixtures of glycols in literatures, such as azeotropic distillation, adsorption, extraction, etc. Still they have some specific weakness and wait for further study.
In this paper, distillation has been considered for the separation and purification of mixtures comprised of (1,2-PG, 1,3-BD, and 1,4-BD) due to its technical maturity and ease to scale up. A Divided-wall column (DWC), which is well known as low energy consumption and equipment cost due to its process intensification, has been studied systemically and compared with the normal distillation sequence process.
First, the residue curve map (RCM) of the ternary system was calculated and there are no distillation boundaries in the ternary system, so the separation can be accomplished by distillations.
Second, two-column distillation sequence process was simulated with the UNIQUAC-HOC equation. The Hayden-O’Connell equation was taken in considering the non-ideality of the vapor phase, and the UNIQUAC model for the liquid phase. The optimized conditions and product specifications were determined for each column in the two-column distillation sequence process.
Third, the DWC was constructed in Aspen Plus, with which the detailed design variables and operation conditions were obtained. The initial design of the DWC was completed using the three-column method based on the Fenske-Underwood-Gilliland-Kirkbride equation. The DWC process was initially designed via the single-factor testing by sensitivity analysis taking the purity of products, capital cost, and heat duty in both the condenser and the reboiler into consideration. The appropriate conditions of the DWC process which includes number of main tower trays, number of prefractionator trays, reflux ration (r mole basis), feed stage, distillate rate (mole/s), side rate (mole/s) and location of side stream were obtained by sensitivity analysis at 1atm.
Then, the DWC configuration was compared with the conventional two-column distillation sequence in terms of the purity of products, energy consumption and total annual cost (TAC). Here, the column diameters were estimated with the function of Tray Sizing in Aspen Plus, and the tray space was taken as 0.61 m. A payback period of 3 years was assumed. The TAC was calculated with the method proposed by Luyben. The following results were obtained: The purity of 1,3-BD in mass could be increased from 90% to 96% with DWC configuration, that of 1,4-BD from 98% to 99%. Approximate 30.0% energy savings and 28.4% TAC savings could be achieved. Some parameters, such as the reflux ratio r, the internal liquid flow to the prefractionator l and the internal vapor flow to the prefractionator v, had great effects on the purity of products. They were optimized with the response surface methodology (RSM) on the basis of the Box-Behnken design (BBD) in terms of the purity of products and decrease energy requirements after preliminary DWC design. The results showed that heat duty in reboiler was mainly determined by the reflux ratio r. The energy consumption increased rapidly with the increase in the reflux ratio. The influences of l and v on the heat duty were small compared to that of r. The three parameters r, l and v had significant effects on the concentration of product of 1,3-BD. They were set as 6.0, 0.133 mole/s, 0.506 mole/s, respectively, by taking the purity of products and the energy requirements into consideration. The energy requirements and TAC of DWC configuration could be furtherly reduced after optimization with the RSM and showed much more benefits in terms of energy saving and hence cost reduction. The purity of products would be improved with energy saving up to 33.5% and TAC reduction by 32.0% with DWC configuration in contrast to the normal two-column distillation sequence process with the same feed.
According to the above, it is feasible to employ DWC for the separation of multi-component glycols. DWC can be a promising technology for the production of bio-based glycols on large scale due to the big energy saving and thus TAC reduction.
In this paper, distillation has been considered for the separation and purification of mixtures comprised of (1,2-PG, 1,3-BD, and 1,4-BD) due to its technical maturity and ease to scale up. A Divided-wall column (DWC), which is well known as low energy consumption and equipment cost due to its process intensification, has been studied systemically and compared with the normal distillation sequence process.
First, the residue curve map (RCM) of the ternary system was calculated and there are no distillation boundaries in the ternary system, so the separation can be accomplished by distillations.
Second, two-column distillation sequence process was simulated with the UNIQUAC-HOC equation. The Hayden-O’Connell equation was taken in considering the non-ideality of the vapor phase, and the UNIQUAC model for the liquid phase. The optimized conditions and product specifications were determined for each column in the two-column distillation sequence process.
Third, the DWC was constructed in Aspen Plus, with which the detailed design variables and operation conditions were obtained. The initial design of the DWC was completed using the three-column method based on the Fenske-Underwood-Gilliland-Kirkbride equation. The DWC process was initially designed via the single-factor testing by sensitivity analysis taking the purity of products, capital cost, and heat duty in both the condenser and the reboiler into consideration. The appropriate conditions of the DWC process which includes number of main tower trays, number of prefractionator trays, reflux ration (r mole basis), feed stage, distillate rate (mole/s), side rate (mole/s) and location of side stream were obtained by sensitivity analysis at 1atm.
Then, the DWC configuration was compared with the conventional two-column distillation sequence in terms of the purity of products, energy consumption and total annual cost (TAC). Here, the column diameters were estimated with the function of Tray Sizing in Aspen Plus, and the tray space was taken as 0.61 m. A payback period of 3 years was assumed. The TAC was calculated with the method proposed by Luyben. The following results were obtained: The purity of 1,3-BD in mass could be increased from 90% to 96% with DWC configuration, that of 1,4-BD from 98% to 99%. Approximate 30.0% energy savings and 28.4% TAC savings could be achieved. Some parameters, such as the reflux ratio r, the internal liquid flow to the prefractionator l and the internal vapor flow to the prefractionator v, had great effects on the purity of products. They were optimized with the response surface methodology (RSM) on the basis of the Box-Behnken design (BBD) in terms of the purity of products and decrease energy requirements after preliminary DWC design. The results showed that heat duty in reboiler was mainly determined by the reflux ratio r. The energy consumption increased rapidly with the increase in the reflux ratio. The influences of l and v on the heat duty were small compared to that of r. The three parameters r, l and v had significant effects on the concentration of product of 1,3-BD. They were set as 6.0, 0.133 mole/s, 0.506 mole/s, respectively, by taking the purity of products and the energy requirements into consideration. The energy requirements and TAC of DWC configuration could be furtherly reduced after optimization with the RSM and showed much more benefits in terms of energy saving and hence cost reduction. The purity of products would be improved with energy saving up to 33.5% and TAC reduction by 32.0% with DWC configuration in contrast to the normal two-column distillation sequence process with the same feed.
According to the above, it is feasible to employ DWC for the separation of multi-component glycols. DWC can be a promising technology for the production of bio-based glycols on large scale due to the big energy saving and thus TAC reduction.