(6a) Energy-Saving Distillation through Internal Heat Exchange (HΙDíC): Overview of a Japanese National Project

The separation of mixtures by distillation requires a great deal of heat energy. With energy-saving technologies, heat energy consumption has been reduced somewhat. However, that technology has reached its limits. The new HIDiC (heat integrated distillation column) principle offers a breakthrough in energy saving. With HIDiC, the heat of the enriching section is transferred to the stripping section. Though the pilot project's energy savings target was 30%, the actually demonstrated savings were 60% (petrochemical plant) and 40% (air separation plant). This is an indication of the significant benefits possible with the HIDiC system. In the conventional distillation system, the heat efficiency is comparatively low. This is because the waste heat at the top of the enriching section is not put to use. By contrast, HIDiC is able to make use of this heat, by promoting heat transfer from the enriching section to the stripping section. Pressure is applied to the vapor, compressing it and causing the temperature to rise, and the resultant heat is transferred to the stripping section. This is achieved through HIDiC's distinctive longitudinal partitioning of the distillation column into enriching and stripping sections. HIDiC adapts a principle which Mah et al. originally presented about thirty years ago as SRV (Secondary Reflux and Vaporization) distillation. HIDiC manifests this concept industrially to achieve heat transfer from the enriching section to the stripping section. It does this by applying SRV and eliminating reflux at the top of the enriching section. The basic research on HIDiC has been done. Before full profitability can be assured, significant issues must be addressed. Much additional funding may be required for its optimum development. In Japan, as support from private enterprise cannot be expected, the project must be conducted by NEDO, an agency of the Japanese government. (1) Vapor from the stripping section is fed to the enriching section only after compression has elevated its temperature. (2) The liquid from the bottom of the enriching section is thus at a higher pressure, so the pressure needs to be reduced before the liquid enters the top of the stripping section. (3) Heat from each tray in the enriching section is conducted directly to the corresponding tray in the stripping section, through the section walls. Then the vapor rate in the enriching section decreases progressively as the vapor approaches the top. Conversely, the liquid rate in the enriching section increases progressively as the liquid approaches the bottom of the section. Main achievements of the basic study (1993-2000) are as follows. 1. Calculated energy savings of more than 30% over the same kind of plant operating with a conventional system. 2. Maintaining of specifications for five hours without need for reflux (whereas, for ordinary distillation, reflux is essential). The study was conducted from April 2002 to March 2006 and the budget was 1,400,000,000 yen (about US $12,730,000 at that time). The four-year schedule was divided into two terms of two years each. The first term addressed basic study, determination of targets, and selection of project participants. The second term was used for pilot plant construction and testing to evaluate the project. It focused on the tray, structured packing and plate fin column systems in distillation in the petrochemical and air separation industries. The project was divided into five parts. Three of these parts involved research and design of systems for packings, trays, and plate fins. The fourth part was construction and operation of the pilot plant using the chosen system ? packings in a modified shell and tube configuration. The fifth part covered project leader tasks. Of the total budget, pilot plant construction and operation received one third. The project leader's allocation was used for developing computer programs for research and for performance simulation of the HIDiC, as the driving force of the project. Between the first and second terms of the project, evaluation of the competing systems was performed by an independent committee comprised of members from industries and academia. This evaluation confirmed that the idea of having company teams compete was promoting efficient use of budget and time. Topics needing R&D are as follows. 1. Optimization of the internal structure for scale-up to large-scale ? specifically, that of the distillation column for large-scale application ? for both homogeneous distribution of vapor and liquid flow and effective heat transfer through the wall. 2. Development of the technology for multi-component systems ternary and greater. 3. Design of a structure that avoids the complex problems involved when up and down flow rates differ significantly. 4. Development of precise pressure control technology to enable practical application of the azeotropic mixture.

The following observations and conclusions were obtained. The design of the structured packing had successfully conformed to the complex configuration. Generation of condensed liquid droplets of rising vapor was observed at the higher-temperature side. On the lower-temperature side, droplets of the descending reflux liquid that had been deposited on the wall were observed to have disappeared. The presence of internal heat exchange seems to have no strong bearing on the performance of the distillation process. Continuous operation for demonstration was performed for twelve-component systems, using a commercial scale pilot plant constructed in February 2005. The height of the distillation column was 27 m, with a diameter of 1.4 m. Tests were conducted with both the shell side and tube sides filled with structured packings. Inside the column were seven units of double-tube-type columns. Conventional distillation maintains the proper specifications by controlling the heat duty of the reboiler and the overhead condenser to match the feed conditions. As the innovative HIDiC is an entirely new concept in distillation, it was not immediately clear which operation should be regarded as the key point. And it is yet to be conclusively demonstrated that the internal structure is conducive to easy operation. Specifically, investigation focused on determining the operational item most central to obtaining on-specification distillate and bottoms products, to accommodate changes in the system. Unlike conventional systems, further development of the HIDiC operating system does not depend mainly on controlling the external reflux for the change of feed rate, feed composition and feed temperature. The structure of the HIDiC distillation column is much different from that of the conventional column. Accordingly, comprehension of the HIDiC principle is essential in order to develop the new design, manufacturing, and operation methods that will be required at each stage. A great stimulus toward approaching these tasks will be the outstanding savings in energy that can be achieved.