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Improved distillation efficiency

Dividing wall column technology applied to a xylenes separation project delivered superior energy efficiency compared to a two-column arrangement.

NORIHITO SUZUKI, TonenGeneral Group
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Article Summary
As the global energy sector enters a new phase centred on energy reduction, dividing wall columns (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 columns. DWCs have produced tremendous savings in both capital and utility costs as compared to conventional distillation columns, and are now being used with advanced heat integration schemes.

DWCs are a novel form of distillation, featuring a vertical wall separating the column shell into two sections. The columns can produce high-purity products from multi-component fractions, as they eliminate the thermodynamic inefficiencies associated with regular columns operating in a traditional sequence. Additionally, DWCs have advantaged capital and operating costs, by eliminating the need for an extra column to separate the same number of products.

Mix-xylenes 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-column configuration, but taking into account the lack of plot 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 2016. 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-
column system.
Project requirements
The main objective of the facility was to produce 234400 t/y of mix-xylenes product from the heavy reformate feed. The unit was designed to recover 98.5% of the mix-xylenes (C8 aromatics) contained in the feed.

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 columns wherever possible.

The plant has been designed to process the feed and product compositions shown in Tables 1 and 2.

Different options explored Traditional two-column sequence
This configuration uses two columns in direct sequence to recover the mix-xylenes from the heavy reformate. The first column removes the toluene-rich stream and the second column separates the mix-xylenes from the C9 and heavier components by traditional distillation.

Additionally, the overhead from the toluene column and the mix-xylenes column could be heat integrated with the existing pre-cut and extract columns in the BEU. The heating duties for the new columns are to be provided through two fired heaters.

The two-column sequence is shown in Figure 2. The existing reformate light cut and reformate heavy cut columns are shown upstream of the two proposed new columns.

As Table 3 shows, the two-
column configuration requires substantially higher energy consumption than a single dividing wall column. This is primarily due to the lower thermodynamic efficiency associated with the traditional sequence, which involves remixing of the middle boiling components. In a three-
component separation, the middle boiling components are concentrated in the centre of the first column, but eventually this stream remixes with higher boiling components at the bottom before being separated again in the next column. Consequently, higher reboiling and cooling duties are required compared to a DWC design.

Dividing wall column configuration
Dividing wall column technology employs a single shell, fully thermally coupled distillation column 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 column into two zones. The first zone acts as a pre-fractionation column, 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 column pressure is elevated to have a higher overhead temperature. This high temperature vapour stream is used to provide the reboiling duties for the two upstream columns in the BEU, thereby providing more energy savings by effective heat integration within the existing and new columns.

The DWC configuration is shown in Figure 3. The existing reformate light cut and reformate heavy cut columns are shown upstream of the new column.
Results and discussion
Table 3 compares the results obtained for the same application in a two-column 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 columns.
• As compared to a two-column sequence, making the same high-purity products, DWCs provide major savings in operating and capital costs. A single column is required instead of two new columns, along with a single set of associated equipment.
• Efficient heat integration with the two upstream columns further eliminates the need for energy use in these columns, 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-
column system.
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