Recent advances in the internally heat-integrated distillation columns (HIDiC)

Distillation is the most widely used but the most energy-consuming separation process in the chemical industry. The internally Heat-Integrated Distillation Columns (HIDiC) are one of the promising alternatives of the conventional distillation processes to reduce the energy consumptions in chemical processes. The HIDiC has a similar structure to a heat exchanger. The rectifying section is contacted with the stripping section through walls in order to allow heat exchange between these two sections. Thus the residual heats in the column can be utilized in itself. In Japan, a national project on the research and the development of the HIDiC has been conducted, and now it is in an important phase before its commercialization. For the analysis of the energy consumption of the HIDiC, simulation studies have been carried out. The simulation results show that an energy saving of 50 % is expected for the HIDiC when it is applied to the separation of hydrocarbons mixtures. On the other hand, developments of internals and the analysis of their hydrodynamic characteristics by experiments have been also addressed. Here we report such recent advances in the HIDiC. Introduction Distillation is said to be the most matured and frequently used technology of separation processes in many literature on chemical engineering. Adding a large amount of thermal energy as a separation agent, products of a chemical process can be highly purified and fractionated in the distillation process. In addition, the production rates of a distillation column are usually higher than the other separation instruments. Therefore, the energy consumption of a distillation process is generally much bigger than the other separation methods. So far, many modifications of distillation columns have been proposed and utilized in the chemical industries. Among such new developments of distillation processes, an internally Heat-Integrated Distillation Column (HIDiC) is one of the promising alternatives. The early concept of the HIDiC was proposed by Haselden with examples of gas separations (Haselden, 1958). In the concept, the compressed feeds or products from a distillation column can be utilized as an energy source for the distillation columns and it leads to a reduction of the energy consumption of the process. The idea was reintroduced in 1970’s as a Secondary Reflux and Vaporization (SRV) method by Mah et al. (1977), and the general configuration of the today’s HIDiC was established. At that time, the Japanese government was paying a lot of efforts to overcome the two energy crisis in 1970’s, since almost all energy resources have been imported from other countries. Therefore, there were large demands on the technology for energy savings from all industries in Japan. Since 1980’s, researches on distillation columns with the SRV method have been carried out under the name of the HIDiC by Takamatsu and Nakaiwa (Nakaiwa et al., 2003). They further analyzed and modified the original arrangement of the SRV method. From 1990’s, a national project on the HIDiC was conducted. In the project, the structure, the performance, the dynamic response and the control strategy for the HIDiC were investigated by both experiments and simulations. With an example of benzene-toluene separation, an energy saving of 30 % in average was achieved by a pilot HIDiC. It was operated even without any condensers, i.e., at zero reflux condition for more than 100 hours. Since then, the commercialization of the HIDiC becomes the next big target. From 2002, a new national project on the HIDiC has been started to industrialize the HIDiC as a key technology for the energy savings of chemical industries. In this paper, the recent advances of the HIDiC are reported for simulation results of multicomponent separation examples, and developments and evaluations of the internals. Among the advances, the simulation results are mainly shown here. In addition, the HIDiC project in the other countries are also introduced. The structure of the HIDiC Figure 1 shows the schematic diagram of a typical configuration of the HIDiCs. As well as the conventional distillation column, the HIDiC has a rectifying and a stripping sections, a reboiler, and a condenser. In addition, a compressor and a throttling valve are equipped for the HIDiC, and either a preheater or a valve cooler, or both may be accompanied with them. By the compressor, the pressure of the vapor flowing up from the top of the stripping section is elevated, and the pressure of the liquid flowing down from the rectifying section is lowered by the throttling valve. If the temperatures for the rectifying and the stripping sections are different from each other and these two sections are contacted through a dividing wall, the temperature difference between the two sections bears a driving force for the heat transfer. Furthermore, when the pressure for the rectifying section is higher enough than the one for the stripping section, the one-way heat transfer from the rectifying section to the other can be expected. This decreases the required reboiler heat duty and the vapor and liquid flow rates inside the column, and enhances the degree of the energy utilization in a distillation column. Thus the HIDiC can realize the reduction of the energy consumption of the distillation process. Figure 1. A schematic diagram of the HIDiC. The model for the HIDiC The model for the HIDiC is described in the paper by Mah et al. (Mah et al., 1977), and summarized by Nakaiwa with examples of binary separation problems (Nakaiwa, 1988). Since it is based on the tridiagonal matrix method proposed by Wang and Henke (Wang and Henke, 1966), here the difference between the original one and the model for the HIDiC is briefly explained. In order to represent the amount of the heat exchange between the rectifying and the stripping sections, the overall heat transfer coefficient, U, and the heat exchange area, A, are introduced. If the HIDiC is assumed to be a tray column, the amount of the heat exchange between the coupled stages, Qex, is expressed as follows Qex,j = UjAj (Tj – Tj+Nhex) (1) where Nhex is the number of the heat exchange stages, and the subscript j denotes the j th stage in the rectifying section. Treating Qex,j as a duty for a side cooler or a side reboiler, the tridiagonal matrix for the mass and the energy balances is solved. If one takes A = 0, Qex,j becomes also zero and the terms for the heat exchange in the tridiagonal matrix disappear. Thus the Nakaiwa’s model for the HIDiC can also handle the conventional column without Compressor