| Summary: | 碩士 === 國立臺灣大學 === 化學工程學研究所 === 106 === Extractive distillation is a common tool for separating close boiling and azeotrope mixtures. There are many papers in opening literature studying the separation of minimum boiling azeotrope. Thus, the long-term development of the configuration and concept about extractive distillation is based on the separation of minimum boiling azeotrope. There are two special points concerning the separation of maximum boiling azeotrope by extractive distillation. First, a distillation boundary will be formed between heavy entrainer and azeotropic composition. When the bottom composition of the extractive distillation column is located on the distillation boundary, the other high purity product cannot be obtained at the top of the entrainer recovery column. Therefore, for selecting the entrainers, it considers not only the ability to enhance the relative volatility, but also the feasible minimum entranier-to-feed ratio. Second, the azeotropic composition is close to the high boiling point component. Because the separation of heavy component from fresh feed is difficult, the stage from the entrainer feed location to the bottom of the column can be considered as the extractive section. If the light component is separated easily, high purity light component can be obtained at the top of the column without the need for entrainers.
For the process design and control of the separation of acetone and chloroform, when using N-methyl-2-pyrrolidone (NMP) as entrainer, the feasible minimum entranier-to-feed ratio is the lowest and the ability to enhance the relative volatility is greater than using dimethyl sulfoxide (DMSO) and ethylene glycol (EG) as entrainers. The total annual cost (TAC) can be separately reduced by 21 % and 26.2 %. Extractive distillation using NMP as entrainer with a feed-effluent heat exchanger (FEHE) is considered to be the most economic design. For the dynamic simulation of heat integrated flowsheet, comparison of single temperature control and dual temperature control, and closed-loop disturbance test was performed to confirm that despite the feed composition or flow rate disturbances, the control strategy proposed in this paper can get high purity product. As a result, a dual temperature control strategy is a preferred control option.
For the separation of phenol and cyclohexanone (CYC), triethylene glycol (TEG) is used as the entrainer in the extractive distillation. As a result, it is found that in TAC, the system with a cooler on the entrainer recycle flow is higher than that without a cooler. Because the light component is easily separated without the action of the entrainer, the high temperature entrainer feed will be closer to the bottom of column and further reduce the reboiler duty. This study also discusses the dynamic simulation of heat integrated system and finds control strategies for entrainer flow based on a dual temperature control strategy. Finally, fixing the ratio of entrainer flow to distillate of the entrainer recovery column can effectively eliminate feed composition and feed flow disturbance.
For the separation of N,N-dimethylacetamide(DMAC) and acetic acid(HAC), TEG and Tetraethylene glycol (TTG) serve as the candidate entrainers and each has advantages in enhancing relative volatility and the feasible minimum entranier-to-feed ratio. As a result, there is no difference in TAC of both entrainers in an extractive distillation without a cooler and the entrainer feed position is below the fresh feed . However, in the design process of heat integration, due to the large amount of entrainer in the TTG system, the fresh feed can obtain more heat under the heat exchange limit of heat exchanger. Finally, heat integrated process with TTG as entrainer is the most economic design process.
In summary, for maximum boiling azeotrope, the importance of entrainer selection for extractive distillation can be observed. In addition, because the heavy component is difficult to separate, when the light component can easily be separated out at column top, the high temperature entrainer can save energy more efficiently.
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