Gas 2025 Issue

in plants by incorporating ED technology in existing aro- matics complexes. The incorporation of ED columns into LLE-based aromatics complexes creates a ‘hybrid’ design that best utilises the strengths of each of the two technol- ogies. Many options exist for integrating ED columns into LLE-based aromatics complexes, and it is beyond the scope of this article to discuss the various options. Compared with the extraction section of aromatics com- plexes, fewer options exist to debottleneck the BTX fraction- ation section. The most obvious opportunity for revamping a series of BTX distillation columns is the replacement of the original column internals with new high-capacity inter- nals. Until recently, once the column internals have been upgraded, replacement of the entire series of distillation columns has been the only option available to provide addi- tional distillation capacity. BTX distillation systems can now be debottlenecked with the addition of a single new column using a process called product pair distillation (PPD), which was developed for the purpose of increasing the throughput of a series of three or more distillation columns that separate products by direct sequence distillation. Figure 3 is an example of a PPD con- figuration for revamping a BTX distillation system. The existing benzene column and toluene column are converted to prefractionator columns, while the xylene column continues to separate xylene product from C₉+ heavy aromatics. The former benzene column produces a benzene/toluene intermediate product pair, and the former toluene column produces a toluene/xylene intermediate product pair. Both of these intermediate product mixtures are sent to a new main column with a side draw, where the intermediate product pairs are separated into benzene, toluene, and a first xylene product. The xylene product from the xylene column represents a second xylene product and is combined with the first xylene product from the new main column. Optimised reflux-to-feed ratios An increased throughput can be obtained in the existing BTX distillation system because the reflux-to-feed ratio is reduced in columns converted to prefractionation service in a PPD configuration. In the former benzene and tolu - ene columns, reduced reflux-to-feed ratios are obtained because separations performed in the prefractionator ser- vice are not as sharp as in these columns’ original designs. In the former xylene column, the sharpness of separation (and reflux-to-feed ratio) is unchanged from the original design. However, additional feed capacity is possible in the xylene column because a high percentage of the xylene product in the BTX extract feed is removed in the former toluene column. The compositions of the intermediate product pairs produced in the former benzene and toluene columns are important because they permit the final products recovered in the side draw column to meet the desired product spec- ifications. The benzene/toluene distillate mixture produced in the former benzene column is virtually free of xylene and contains essentially all of the benzene present in the extract feed. This permits a toluene/xylene distillate mixture that is

Benzene product

Main column (new)

B/T product pair

Toluene product

T/X product pair

Xylene product #1

Xylene product #2

DeT/X

Combined Xylene product

BTX extract to PPD conguration

Xylene column

DeB/T

C+

Figure 3 PPD distillation scheme for BTX separation

innovative distillation design applied to aromatics recov- ery is the area of thermally coupled distillation columns. Dividing wall columns represent a particular subset of thermally coupled distillation columns, but many different configurations of thermally coupled distillation columns are possible. ThyssenKrupp Uhde has introduced a thermally coupled single-column design that integrates an ED col- umn with a solvent stripper. The thermally coupled design has demonstrated energy savings ranging from 16% for reformate feeds to 20% for pygas feeds. 3 An increased throughput can be obtained in the existing BTX distillation system because the reflux-to-feed ratio is reduced in columns converted to prefractionation service in a PPD configuration Hybrid debottlenecking options The existence of many different aromatics complex config - urations can complicate a discussion of the tools and tech- niques used to debottleneck aromatics complexes. A focus on low complexity aromatics complexes that extract and separate extract products to produce BTX products in the fractionation section is warranted to simplify a discussion of debottlenecking options. Aromatics extraction units have a multitude of debot- tlenecking options. Older LLE extraction units that are still using glycol solvents can achieve significantly higher capacities through a combination of switching to higher capacity/higher selectivity solvents and upgrades to extrac- tor and stripper internals. Further increases in extraction capacity can be obtained

Gas 2025