As the global energy sector

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1 Improved distillation efficiency Dividing wall technology applied to a xylenes separation project delivered superior energy efficiency compared to a two- arrangement MANISH BHARGAVA, ROOMI KALITA and JOSEPH GENTRY GTC Technology NORIHITO SUZUKI TonenGeneral Group As the global energy sector enters a new phase centred on energy reduction, dividing wall s (DWC) are providing a unique way to meet the current challenges facing the refining and petrochemical industries. The concept of a dividing wall is not new; in fact, it has been utilised in the chemicals industry for some time. However, over the past decade, this concept has gained momentum following successful revamps of existing distillation s. DWCs have produced tremendous savings in both capital and utility costs as compared to conventional distillation s, and are now being used with advanced heat integration schemes. DWCs are a novel form of distillation, featuring a vertical wall separating the shell into two sections. The s can produce high-purity products from multi-component fractions, as they eliminate the thermodynamic inefficiencies associated with regular s operating in a traditional sequence. Additionally, DWCs have Figure 1 New DWC facility for mixxylenes recovery at TonenGeneral Group refinery, Chiba, Japan advantaged capital and operating costs, by eliminating the need for an extra to separate the same number of products. from reformate The TonenGeneral Group recently applied a DWC process at its refinery in Chiba, Japan. The goal of the Chiba Aromatics Recovery Project was to recover mix-xylenes from a full-range reformate feed. The original benzene extraction unit (BEU), operational since 1999, consisted of pre-cut and extraction sections for the extraction of benzene from the reformate produced by a catalytic reforming unit. The unit separated the feed into a light reformate, heavy reformate and a benzene-rich stream. The crude benzene was fed to the extraction section of the BEU. The light and heavy reformate were sent to the tankage offsite as mogas blendstocks. The heavy reformate contained a substantial amount of mix-xylenes along with toluene, which could be recovered as separate products. With the decrease in the demand for transportation fuels, the Japanese energy industry shifted its focus to the petrochemicals business. TonenGeneral followed this trend by building a mix-xylenes recovery facility in Chiba with an investment of 5 billion ($42 million). The company initially considered a grassroots two- configuration, but taking into account the lack of plot PTQ Q

2 space in the refinery as well as the favourable project economics provided through heat integration, DWC technology showed substantial improvement as compared to the other options (see Figure 1). TonenGeneral successfully executed this project with the start-up and performance guarantee test completed in April The project economics proved the lower energy consumption and recovery of pure mix-xylenes as product with the DWC design as compared to a traditional two system. Project requirements The main objective of the facility was to produce t/y of mix-xylenes product from the heavy reformate feed. The unit was designed to recover 98.5% of the mix-xylenes (C 8 aromatics) contained in the feed. Components Units Total mass rate, kg/hr Components, wt% Non-aromatics Ethyl benzene and heavier 15.2 Table 1 Feed composition Additionally, due to plot space constraints in the Chiba refinery, the amount of additional equipment for the new facility had to be kept to a minimum. TonenGeneral expressed a desire to further reduce the operating and capital costs by application of heat integration with existing s wherever possible. The plant has been designed to process the feed and product compositions shown in Tables 1 and 2. Components Specifications Remarks C 8 aromatics, wt% 98.5 Minimum Ethyl benzene, wt% 18.0 Maximum Non-aromatics, wt% 1.0 Maximum aromatics, wt% 0.3 Maximum Table 2 Product specifications Product cpecifications Two- sequence DWC configuration product, kg/hr C 8 aromatics, wt% Non-aromatics,wt% Ethyl benzene, wt% aromatics, wt% Reboiler duties, MMkcal/hr Condenser duties, MMkcal/hr Operating cost savings, % - 20 Capital cost, $ million Table 3 Comparison between a two- sequence and a DWC configuration Different options explored Traditional two- sequence This configuration uses two s in direct sequence to recover the mix-xylenes from the heavy reformate. The first removes the toluene-rich stream and the second separates the mix-xylenes from the and heavier components by traditional distillation. Additionally, the from the toluene and the mix-xylenes could be heat integrated with the existing pre-cut and extract s in the BEU. The heating duties for the new s are to be provided through two fired heaters. The two- sequence is shown in Figure 2. The existing reformate light cut and reformate heavy cut s are shown upstream of the two proposed new s. As Table 3 shows, the two configuration requires substantially higher energy consumption than a single dividing wall. This is primarily due to the lower thermodynamic efficiency associated with the traditional sequence, which involves remixing of the middle boiling components. In a threecomponent separation, the middle boiling components are concentrated in the centre of the first, but eventually this stream remixes with higher boiling components at the bottom before being separated again in the next. Consequently, higher reboiling and cooling duties are required compared to a DWC design. 2 PTQ Q

3 Ref. Light Cut Light reformate feed Light Cut Ref. heavy cut From toluene Benzene Heavy Cut From mix-xylenes Clay treater Heavy reformate + cut Figure 2 Two- sequence along with existing BEU; this was rejected in favour of a DWC scheme Dividing wall configuration Dividing wall technology employs a single shell, fully thermally coupled distillation to separate mixtures of three or more components into high purity products. The arrangement consists of a vertical wall, which separates the middle of the into two zones. The first zone acts as a pre-fractionation, while the other zone makes the pure component fractionation. The low and heavy boiling components can be separated efficiently, while a concentrated middle cut is obtained with comparatively low reboiling energy consumption. For this case, the pressure is elevated to have a higher temperature. This high temperature vapour stream is used to provide the reboiling duties for the two upstream s in the BEU, thereby providing more energy savings by effective heat integration within the existing and new s. The DWC configuration is shown in Figure 3. The existing reformate light cut and reformate heavy cut s are shown upstream of the new. Results and discussion Table 3 compares the results obtained for the same application in a two- system with that of a DWC. They are summarised as follows: For this particular application, DWC provides a significant reduction in heating and cooling duties as compared to the conventional sequence of distillation s. As compared to a two- sequence, making the same high-purity products, DWCs provide major savings in operating and capital costs. A single is required instead of two new s, along with a single set of associated equipment. Efficient heat integration with the two upstream s further eliminates the need for energy use in these s, thereby decreasing the operating costs further. DWC is a profitable alternative for improving energy efficiency in grassroots applications, where the conventional approach is a two system. Design highlights of DWC DWC technology provides PTQ Q

4 Light reformate feed Light Cut Ref. Light/Heavy Cut From GT-DWC Benzene Heavy Cut From GT-DWC Clay treater Heavy reformate + cut Figure 3 Grassroots DWC along with existing BEU Figure 4 Liquid splitting arrangement flexibility in operation by a proprietary liquid splitting arrangement (see Figure 4). In this application, a combination of external and internal splitting devices is utilised on both sides of the dividing wall. This enables smooth operation of the in the Figure 5 The off-centre wall within the event of a drastic change in feed composition. The mechanism, facilitated via gravity, eliminates the need for an additional pump or drum. Additionally, the diameter of the external splitting piping is kept low, thereby allowing easy operation and use of cost-efficient low pressure drop control valves. This particular DWC design uses an off-centre wall, which separates the zones in the (see Figure 5). The position of the wall is such that the vapour-liquid flow within the is optimised to ensure more efficient separation of the middle boiling components, while at the same time reducing the heating and reflux duties. This article is based on a presentation to Crambeth Allen Publishing s 4th Refining India Conference, New Delhi, India, September Manish Bhargava is Licensing Manager Advanced Distillation Systems for GTC Technology US, LLC, headquartered in Houston, Texas. With over 15 years of experience in the process industry, prior to joining GTC he worked at KBR as the Principal Technical Professional, and with DCM Shriram Consolidated Limited as a Process Engineer, and holds a master s 4 PTQ Q

5 degree in chemical engineering from Illinois Institute of Technology. Roomi Kalita is Licensing Process Engineer Advanced Distillation Systems for GTC Technology, US, LLC. With over five years of experience in the process industry, she worked previously for Indian Oil Corporation Ltd, India and Fluor Corporation in the refining and petrochemicals areas, and holds a BS in chemical engineering from National Institute of Technology, Trichy, India, and a master s degree in chemical engineering from Carnegie Mellon University. Joseph C Gentry, PE is Vice President Technology, R&D, and Engineering, for GTC Technology US, LLC, headquartered in Houston, Texas. He has 35 years of industry experience in olefins and aromatics and is the inventor of several patented separations technologies and has specialised in the application of these for the petrochemicals industry. Norihito Suzuki is Staff Engineer of the technical department for TonenGeneral Sekiyu K.K. in Chiba, Japan. With 14 years of experience in refinery process engineering in the TonenGeneral Group, he worked on the Chiba Aromatics Recovery Project as a process designer and an initial start-up engineer, and holds a master s degree in earth and planetary sciences from Tokyo Institute of Technology. PTQ Q