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An Overview of a Japanese National Project: Development of Technology for Energy-Saving Distillation through Internal Heat Exchange (Hidic)

Shuzo Ohe, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo, 162-8601, Japan

The separation of mixtures by distillation requires a great deal of heat energy. Compared to earlier operation, the amount of heat energy consumed in a large-scale chemical plant has been reduced, using energy-saving technology. However, the capability of that technology has reached its limits. A new concept and breakthrough in energy saving is offered by the HIDiC (heat integrated distillation column) principle. In the HIDiC system, the heat of the enriching section is transferred to the stripping section. The HIDiC system has a high capability for saving energy. The energy savings target for the project was 30%, but the actually demonstrated savings were 60% for the petrochemical plant and 40% for the air separation plant. Energy saving is a practice that should be pursued constantly by everyone. Nowadays, we are faced with significant energy problems. High oil prices are forcing a need for energy reduction. Global warming must be headed off by reducing CO2. Under such conditions, developing the HIDiC system is quite a timely project. In the conventional distillation system, the waste heat at the top of the enriching section is not put to use, making the heat efficiency comparatively low. The HIDiC system technology brings enormous energy savings. This is made possible through the HIDiC system's distinctive longitudinal partitioning of the distillation column into its enriching and stripping sections. In this approach pressure is applied to the vapor, compressing it and causing the temperature to rise, and the resultant heat is transferred to the stripping section.

Mah et al. presented the original idea of HIDiC as SRV (Secondary Reflux and Vaporization) distillation about thirty years ago. (1) Vapor from the stripping section is not fed to the enriching section directly. Instead, it is fed there after compression has elevated its temperature. (2) Heat transfer occurs from the vapor in each tray in the enriching section to the liquid in each tray in the stripping section. (3) The pressure at the enriching section is higher than that of the stripping section, so the pressure░░of the liquid from the enriching section is reduced and the liquid is then fed to the stripping section. Then the vapor rate in the enriching section decreases progressively as it approaches the top. The liquid rate in the enriching section increases progressively as it approaches the bottom of the section. Therefore, the slope of the operating line of the enriching section is greater than that in the conventional system, approaching the 45 degree line, with a consequent reduction in the number of theoretical plates. The slope of the operating line of the stripping section is less than that of the conventional system and it also approaches the 45 degree line, because the vapor rate increases and the liquid rate decreases. HIDiC makes the heat transfer from the enriching section to the stripping possible industrially by applying SRV and eliminating reflux at the top of the enriching section.

Although the basic research on HIDiC has been done, significant issues remain before profitability can be assured. Much additional funding is required for its development. As support from private enterprise cannot be expected, the project must be conducted by NEDO, an agency of the Japanese government. The HIDiC is evaluated highly as a chemical process in the international effort to combat global warming. In Japanese industry as a whole, 15% of all energy is expended in the chemical industry. In the chemical industry, separation by distillation expends 40% of total energy. For the basic study, a project was performed (1993-2000). The results are as follows. The energy saved was calculated as more than 30% over that of the same kind of plant operating with a conventional system. Moreover, the specifications were maintained for five hours without reflux, which, for ordinary distillation, is essential. The project focused on the tray, structured packing and plate fin column systems in distillation in the petrochemical and air separation industries. The term was four years, from April 2002 to March 2006 and the budget was 1,400,000,000 yen (about US $12,730,000 at that time). The project was broken down into five portions. Companies were selected that had reputations for expertise in research and development on the subject areas. Highly competent leaders were selected for each portion. The budget was distributed according to the objective of each portion. One third of the budget was allocated to the construction and operation of the pilot plant. Of the total budget, the project leader was allocated ten percent. This was used for developing computer programs for research and for performance simulation of the HIDiC, as the driving force of the project. The distributions for the three types of HIDiC distillation systems investigated ĘC shell and tube, tray and plate fin ĘC were also sufficient, taking into consideration the cost of making the test equipment. The four-year schedule was divided into two terms of two years each. The first term was used for basic study and the second term was used for pilot plant construction and testing to evaluate the project. Special care was given to designing a cost-effective and expedient project schedule. Between the two terms, evaluation of the competing systems was performed by an independent committee comprised of members from industries and academia. This evaluation showed that the principle of competition 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 ĘC specifically, development of the internal structure of the distillation column for large-scale application, to achieve 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 precludes the inherent complexity of significantly differing up and down flow rates. 4. Development of precise pressure control technology to enable practical application of the azeotropic mixture.

Continuous operation for demonstration was performed for binary and multi-component systems, using a test plant and a commercial scale pilot plant. At the start of the project, demonstration was performed using a heat exchanger type structured packing column. Subsequently, testing was also done using a tray column and a plate fin type distillation column. The initial results show the capability for 30% energy savings and continuous operation for 100 hours. Test equipment and test results The test equipment, which includes three 100-mm-dia tubes within a 318-mm-dia shell, was constructed by one of the project's member companies. The interiors of both the shell and the tubes are filled with structured packing in two vertical segments. The test equipment was operated at atmospheric pressure, so the benzene and toluene concentrations were set at differing ratios in order to create a difference in temperature between the sides of the tubes and the shell. The walls of the tubes were observed, using thermovision. The following results 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. Disappearance of liquid droplets that had been deposited on the wall of the descending reflux liquid was observed on the lower-temperature side. The presence of internal heat exchange seems to have no strong bearing on the performance of the distillation process. Subsequently, the height of the distillation section was extended to 1140 mm. Tests were then performed using various configurations, in which both the shell side and tube sides were filled with structured packings, random packings, or trays, and combinations thereof. The conclusion was that structured packing yields the best performance with the HIDiC system. Based on these results, A company constructed high-pressure test equipment. The equipment was composed of one 140-mm-dia tube and a 216-mm-dia tower with ten beds of structured packing inside both shell and tube. 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. Therefore, a mizetto plant was constructed in which the internal structure can be modified into various simplified structures in order to demonstrate the HIDiC operating principles. Different from 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. Specifically, investigation focused on determining the operational item most central to obtaining on-specification distillate and bottoms products to accommodate changes in the system. The structure of the HIDiC distillation column is much different from that of the conventional column. Accordingly, it is important to comprehend the HIDiC principle and create design, manufacturing, and operation methods at each stage. The considerable savings in energy will be a great stimulus toward approaching these tasks.