Internally Heat-integrated Distillation Columns: a Review

The heat-integrated distillation column to be addressed in this paper is a special distillation column that involves internal heat integration between the whole rectifying and the whole stripping sections. An overview of the research on this process is presented in this work. It covers from the thermodynamic development and evaluations to the practical design and operation investigations for the process. Comparative studies against conventional distillation columns are introduced and the results obtained show distinctively the drastic advantages in energy efficiency of the process over its conventional counterparts. Some relevant issues of process design and operation are to be stressed and the results of the first of its kind bench-scale plant experimentation are given in great detail. The application of internal heat integration principle to other distillation-related processes is also discussed in depth. The prospective of the HIDiC and our future research work are then highlighted, finally. INTRODUCTION Distillation columns have been well known for its low energy efficiency. For effecting a separation heat has to be given at a high temperature in the reboiler and simply drawn off at a low temperature in the condenser. For improving its energy efficiency heat pump principle is often adopted, as an effective means, to reuse the rejected low-temperature heat [1], which is generally referred as heat pump assisted distillation column in the literature. Although it is a useful technique for energy savings, it suffers from some strict requirements imposed by mixtures to be separated. Since 1960s, internal heat integration between the rectifying and the stripping sections of a distillation column has gained significant incentives for improving energy efficiency of distillation processes. Freshwater might be the first person to advocate this technique [2]. Flower and Jackson further systematized the idea and clarified the advantages of this approach through numerical simulations based on the second law of thermodynamics [3]. In terms of the same principle, Mah and his coworkers [4,5] developed and worked with their own process called secondary reflux and vaporization distillation column, which actually included internal heat integration between part of the rectifying and part of the stripping sections. They established the general process configuration to approximate the theoretical model based on the second law of thermodynamics. However, they did not address the problem to which degree the internal heat integration should be adopted between the rectifying and the stripping sections of a distillation column. Takamatsu and Nakaiwa have continued the work on this subject both theoretically and experimentally since 1986 and confirmed firstly by large-scale experimental evaluations the high advantages of these kinds of heat-integrated distillation columns in binary close-boiling mixture separations over conventional distillation columns [6]. In 1995, they noticed that the degree of internal heat integration within a distillation column played a very important role in energy efficiency for a given separation. They proposed, therefore, to further extend the internal heat integration to the whole rectifying and the whole stripping sections and resulted in a sharply different process configuration from conventional distillation columns, which they called heat-integrated distillation column (HIDiC). They found further that the HIDiC was feasible for separations of binary close-boiling mixtures, just as other types of heat pump assisted distillation columns [7,8]. It is worth mentioning here that the HIDiC possesses several very attractive features and it is these features that stimulate us to pursue its realization in practical process engineering. These features include: (i) High energy efficiency. The highest degree of internal heat integration within the HIDiC generally offers itself higher energy efficiency than conventional distillation columns as well as other types of heatintegrated distillation columns, for instance, heat pump assisted distillation columns; (ii) Zero external reflux and reboil operation. Ever since the creation of distillation techniques, it has been the common practice to use condenser and reboiler to generate external reflux and reboil flows for distillation operation. For the HIDiC, the internal heat integration generates these two flows, instead, and thus neither of them is necessary. This may motivate new considerations on distillation process design and operation; and (iii) High potentials and effectiveness of internal heat integration techniques. Internal heat integration is a very efficient means to improve process energy efficiency and can find wide applications within distillation processes. As will be discussed later, it can even facilitate operation of batch distillation columns and pressure-swing distillation processes, which are used for the separation of pressuresensitive binary azeotropic mixtures. Moreover, internal heat integration is not limited only to a single distillation column. It can be considered between two distillation columns that may have no direct connections at all. Recently, the research on the HIDiC has aroused considerable interests and several research groups have been formed around the world. They have already begun their work on this subject with their emphasis ranging from process design [9,10], and process operations [11], to internal heat and mass transfer mechanism and internal structure arrangement [12,13], respectively. As we have been concentrating on this work for a quite long time, it seems to us that it is necessary to review its current development and predict its prospective. Therefore, the main objective of this paper is to give an in depth summation of our researches on the HIDiC. In the meantime, considerable emphasis has also been placed on the introduction of applications of internal heat integration principle to other distillation-related processes, for example, batch distillation columns and pressure-swing distillation processes, as it also represents a very important aspect of the HIDiC development. In this work a detailed overview of our researches on the HIDiC will be conducted, ranging from thermodynamic development and evaluations to the practical design and operation investigations for the process. Comparative studies against conventional distillation columns will be introduced and show distinctively the big advantages of the HIDiC over its conventional counterparts. Some relevant issues of process design and operation are to be stressed and the results of the first of its kind bench-scale plant experimentation will be given at full length. Some energy-efficient processes that make use of the internal heat integration principle, such as heatintegrated batch distillation columns and heat-integrated pressure-swing distillation columns, are also addressed in a straightforward manner. The prospective of the HIDiC and our future research work will be highlighted, followed by some concluding remarks in the last section of the work. THERMODYNAMIC ANALYSIS OF CONVENTIONAL DISTILLATION OPERATION Figure 1 shows a diagram of a conventional distillation column. In terms of the firstand secondlaws of thermodynamics, following equations can be derived. QREB QCOND + FHF DHD BHB = 0 (1) ∆S = QCOND /TCOND – QREB /TREB FSF + DSD + BSB ≥ 0 (2) Dissipation energy, WLoss, is the energy loss due to process irreversibility in the mass and heat transfer, pressure distribution and remixing within a distillation column. It is calculated as follows. WLoss = T0∆S = QREB(1 T0/TREB) QCOND(1 T0/TCOND) + F(HF -T0SF) D(HD T0SD ) B(HB T0SB) = QREB(1 T0/TREB) QCOND(1 T0/TCOND) Wmin (3) Here, Wmin is the minimum energy required by a certain separation and is determined by process operating conditions and product specifications. Wmin = (DHD +BHB FHF ) T0(DSD + BSB FSF) = ∆H T0∆S (4)