Research On Energy Integration Of Cascaded Multivessel Batch Distillation Operation With Liquid Exchange

The cascaded multivessel batch distillation operation with liquid exchange(CMVBD)is proposed for the precise distillation process,which requires numerous theoretical trays.The key features of the CMVBD system are as follows:(1)the system is composed of two or more multivessel batch distillation columns;(2)each column runs under total reflux independently,and the cascaded operation is implemented by the exchange of the liquid holdup in the vessels;(3)the total pressure drop of the system equals to that of each column;(4)the system greatly reduces the adverse effects of back-mixing phenomenon on separation process.Due to high energy consumption of the CMVBD system,the vapor recompressed CMVBD system(VRCMVBD)is designed.Further,the CMVBD and VRCMVBD systems are studied by means of both numerical simulation and experimental investigation in this paper.The CMVBD system is developed based on multivessel batch distillation with constant total reflux operation(MVBD)and liquid series operation.At first,both simulated and experimental studies on the MVBD are carried out,and the results show that the MVBD has the advantages of high efficiency and short operation time compared to the regular constant reflux ratio mode.Moreover,the MVBD is more suitable for a binary mixture that is difficult to separate,such as separation of a mixture with low concentration of light component or having low relative volatility.Then on this basis,the mathematical model of the CMVBD system is established for simulating its transient behavior.The results demonstrate that the CMVBD system has significant potential for separation with high efficiency.The laboratory-scale CMVBD system is designed to verify the proposed operation strategy,and the ethanol-isopropanol mixture is separated in the experiment.The experimental results show good agreement with the simulated data,which indicates that the mathematical model is accurate and reliable and the CMVBD system is practical and feasible.The essence of the CMVBD process is the cascaded process of the MVBD.In order to reduce the energy consumption of the CMVBD system,the energy integration schemes of the MVBD,such as vapor recompressed MVBD(VRMVBD),internally heat integrated MVBD(IHIMVBD)and its intensified form(Int-IHIMVBD),are investigated and compared with each other by separating a binary mixture of ethanol and 1-propanol.The VRMVBD appears overwhelmingly superior to the other heat integration schemes in terms of simple structure,ease of design and operation,low energy consumption and considerable economic benefits.Therefore,the vapor recompression approach is introduced in CMVBD,which is referred to as VRCMVBD.The overhead vapor is compressed up to the necessary pressure for its condensation in the reboiler.The latent heat of compressed overhead vapor is utilized for vaporizing the reboiler liquid.This process indicates a decrease of energy requirements from external source.VRCMVBD accompanies a compressor that runs at a fixed as well as variable speed.The features of VRCMVBD are illustrated by separating the ethanol-isopropanol system.In conclusion,VRCMVBD appears overwhelmingly superior to CMVBD in terms of thermal efficiency and economic performance.Moreover,the variable speed VRCMVBD shows a better performance than the fixed speed VRCMVBD.