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Research paper : Innovation in distillation processes (M. Nakaiwa et al.)−58−Synthesiology - English edition Vol.2 No.1 (2009) how to configure the equipment for “detuning” while keeping the effects in mind, and this is one process towards realization of PI.4 Equipment configurations for achieving “detuning”Interpretation of reversible distillation, an ideal process, is important for performing “detuning.” The reversible distillation column shown in Fig. 3 can be interpreted as the equipment involving the infinite number of heaters (reboilers) and coolers (condensers). If one column is divided into multiple (infinite) distillation columns of different heights, the configuration is represented in Fig. 4a). Moreover, by “detuning” to simplify the design to only 2 columns instead of an infinite number of columns, we have the configuration shown in Fig. 4b). Petlyuk et al. studied on this process in the 1960s, and thus this structure is generally called the Petlyuk column [11]. Further simplification along this consideration was carried out, and the resulting distillation process, developed by BASF of Germany, has been commercialized. In Japan, Kyowa Hakko Co., and Sumitomo Heavy Industries, Ltd. have developed this kind of commercial processes [12].Another example of “detuning” the reversible distillation from a different viewpoint is discussed next. As a typical characteristic, the concentration of low BP component is higher and the temperature is lower in the upper section of a distillation column. Using the principle of reverse Carnot cycle mentioned above, it becomes possible to take heat from the low-temperature source (rectifying section), to elevate the temperature, and then to supply it to the stripping section where heat is required. The strategy for “detuning” in this line is shown in Fig. 5a) [13]. In this process, the “integration” of distillation column with heat pump is achieved since the condenser plays the role of heat pump. The figure illustrates that the heat taken from the upper section is supplied to the lower section by elevating the temperature with each condenser. This process is close to that of reversible distillation, excluding the supply of work for condensation. Then, further “detuning” gives rise to the process shown in Fig. 5b) where the number of condensers is reduced to only one and the heat exchange operations are carried out at a single point each in the rectifying and stripping sections. The process shown in this figure is generally known as a vapor recompression column (VRC). The VRC is advantageous when the temperature difference between the top and bottom of the column is relatively small, and the enrichment of low-concentration ethanol is an example of commercialized use. In the VRC, the heat exchanges are executed only at the top and bottom of the column, where the temperature difference is the greatest in the apparatus. This makes the “simplification” of equipment much more than for the reversible distillation process, but the divergence from the ideal state is considerable, and thus only limited energy saving is achieved.5 HIDiC aiming toward more ideal conditionPetlyuk column and VRC process described in the preceding sections are two different ideas for energy-saving distillation, and correspond to the points B and C of Fig. 1. The basic structures of these apparatuses are completely different, but they were both derived from a common methodology of “detuning” from the ideal state. So, are there any other possibilities of “detuning”? The key point of “detuning” for the VRC was exchanging heat only at one location in the rectifying section, and another in the stripping section. The process would be able to approach closer to the ideal if the heat exchange were carried out in the multiple locations instead. However, this would call for the multiple condensers, which would make the strategy impractical. Therefore, to investigate a more realistic equipment configuration that allows for the heat exchange at the multiple locations, we once again turn our attention to the characteristics of a typical continuous distillation column (see Fig. 2). The internal temperature decreases with increasing height, due to the change in concentration of the solution in the continuous distillation column. The same characteristic is exhibited in a reversible distillation process: the cooling temperature b) Petlyuk Column・ ・a) Conceptual reversible distillation operation with multiple columnsFig. 4 Approach to reversible distillation operation with multiple columns.

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