Vol.2 No.1 2009

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Research paper : Innovation in distillation processes (M. Nakaiwa et al.)−59−Synthesiology - English edition Vol.2 No.1 (2009) in the rectifying section is always lower than the heating temperature in the stripping section. Therefore, the heat taken from the rectifying section cannot be supplied directly to the stripping section. In Fig. 5, the condenser (heat pump) is used to elevate the temperature to solve this problem.So, how can we achieve the heat transfer in the multiple locations simply? In Fig. 5a), all the cooling points in the rectifying section have a lower temperature than all the heating points in the stripping section. If the temperature of the rectifying section is higher than that of the stripping section, the heat from the cooling points can be supplied directly to the heating points. Would it then be possible to elevate the temperature of the rectifying section? In the separation via distillation, the equilibrium relation of liquid-gas is used. Increasing the pressure results in a higher equilibrium temperature, and we can achieve it by increasing the pressure in the rectifying section to above that in the stripping section. This requires an equipment configuration that the rectifying and the stripping sections are divided, and the pressure of rectifying section are increased until the heat from each cooling point in the rectifying section can be transferred to the corresponding heating point in the stripping section. Increasing the pressure in the rectifying section can be achieved by pressurizing the steam from the stripping section. Only one compressor is required to accomplish this task. Once the temperature of the rectifying section exceeds that of the stripping section, various methods can be considered for the heat transfer. The simplest one is to arrange the direct contact between the rectifying and the stripping sections. The HIDiC aims to achieve the high efficiency by closely approximating the reversible distillation process, with the addition of compression [14] (see Fig. 6). We consider that this is also a type of “detuning,” and that it corresponds to point A in Fig. 1. The HIDiC also can be regarded as an “integration” process of distillation, heat transfer, and heat pump. A better energy-saving performance can be achieved in the HIDiC for a system of liquid mixture that is difficult to separate in a conventional distillation column because the temperature difference between the top and bottom of the column is small. In the propylene/propane mixture that the difference of BP is small, our evaluation shows that the HIDiC can separate the mixture using only 1/10 of the energy consumption in a conventional distillation column.Table 1 briefly summarizes the characteristics of the HIDiC, VRC, and Petlyuk column. The “Initial cost” in the table is the construction cost of a distillation column. The difference in total construction cost of distillation process is not as significant as that in construction cost of the column, because additional costs of plumbing and measuring instruments are required. “Applicability” corresponds to the number of industrially relevant solution systems to be processed. The HIDiC is particularly suitable for the solution systems where the temperature gradient in the bottom-to-top direction of column is relatively uniform. The key products for the petrochemical industry such as the mixture solutions of benzene/toluene/xylene and purification of crude cyclopentane are classified in this category. On the other hand, Petlyuk column is suitable for removing impurities of low concentration while VRC is for separating solutions with small BP difference. A conventional distillation column can be applicable in a wider range of solution systems, and the initial cost is generally favourable. The HIDiC realizes the concept of reversible distillation operation more faithfully, and has a wider range of application with achieving higher energy efficiency compared against Petlyuk column. It has also more advantage of energy efficiency since there is great improvement on the problem that the large temperature increase via compression is required in the VRC. In the HIDiC, the increase in energy efficiency of more than 20 % is expected over the Petlyuk column and the VRC, though it would depend on the separation specifications.Professor Richard Mah of Northwestern University in the USA published the basic concept of the HIDiC in the 1970s for the air separation by cryogenic distillation [15]. The waste heat from the rectifying section can be reused internally at the stripping section by controlling the pressure. The amount of energy consumption can be reduced significantly (about 30%).Rectifying section (high pressure)Stripping section(atmospheric pressure)Heat transferProduct BProduct AFeedHeat-integrated distillation column(HIDiC)Most of the heat supplied by the reboiler is discarded as waste heat in the rectifying section.Stripping section(atmospheric pressure)Rectifying sectionProduct BReboilerFeedProduct ACondenserConventional distillation columnTo top of columnOne compressorStrippingsectionRectifyingsectionb) Vapor recompression processMultiple compressorsFeeda) Conceptual process with multiple compressorsFig. 5 Approach to reversible distillation operation with multiple compressors.Fig. 6 Internally heat-integrated distillation column (HIDiC).

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