Gas 2025 Issue

Percentage of BTX in reformate, pygas, and COLO

Ranate (Nonaromatics)

Reformate

Pygas

COLO

Benzene Toluene Xylenes

30 19

70 12

19 24

ED column

Feed

Extract product to BTX fractionation

Table 1

Rich solvent

the 1960s when a US patent application was filed that described a complex heat integration scheme involving the extraction section as well as the fractionation section of an aromatics complex. 1 The use of xylene column vapours to reboil other distillation columns in a BTX separation scheme described in the patent application developed into a com- mon practice that is still used today. Heat integration of distillation columns is also widely practiced in the separation of benzene/toluene extract products. In these cases, toluene vapours from the toluene column overhead are used to reboil the benzene column. To maximise the efficiency of reboiler/condenser heat integra - tion, it is advantageous to have a large heat source (xylene column vapours for a C₆ to C₈ separation or toluene column vapours for a C₆ to C₇ separation) in comparison with the heat sinks. The relative amounts of BTX in typical refor - mate, pygas, and COLO aromatics sources shown in Table 1 indicate that reformate aromatic sources are better suited for distillation column heat integration schemes than pygas aromatics sources. In some cases it may be necessary to increase the pres- sure of the column that provides overhead vapours as a heat source to reboil upstream columns in order to ensure sufficient driving force for heat transfer. Operating the heat source column at a higher pressure will increase the reboiler duty required for the heat source column; however, in spite of this, a heat integration scheme will generally pro- vide a substantial net reduction in distillation column hot utility usage. In a recent heat integration revamp study for distillation of a benzene/toluene extract product, a refiner achieved a 40% net reduction in distillation column hot util- ity usage.² The application of dividing wall distillation to BTX sepa - rations is a fairly recent development. The energy savings that are attainable with a dividing wall column scheme can be disappointing in comparison with the net energy reduc- tion available in condenser/reboiler heat integrations. This arises from the fact that dividing wall distillation is most efficient when the final products are produced in compara - ble amounts. In the cases of both reformate and pygas feeds, an unbal- anced product slate is obtained. In reformate feeds, ben- zene product is deficient compared with toluene and xylene products, whereas with pygas, xylene product is defi - cient in comparison with benzene and toluene products. Considering the exceptional energy efficiency obtained through the use of column heat integration schemes, con- denser/reboiler heat integration should be considered in typical BTX applications. Another area that has been successfully explored for

Solvent recovery column

Lean solvent

Figure 2 ED process

that would improve the quality of aromatics produced from COLOs. In the mid-1960s, Uhde developed an extractive distillation process that became known as the Morphylane process for recovering aromatics from feeds with high aro- matics concentrations. A schematic representation of a typical ED process is shown in Figure 2 . Lean solvent enters the ED column near the top of the column, and the feed mixture is introduced near the middle of the column. Extraction and distillation take place simultaneously in the ED column. The presence of a polar solvent increases the relative volatility of nonar- omatics, which are recovered from the top of the column. As the polar solvent works its way down the column, the solvent becomes enriched with aromatic compounds. The rich solvent is sent from the bottom of the ED column to a solvent recovery column, where an aromatics extract prod- uct is separated from lean solvent. For many years, ED column aromatics extraction units were favoured principally for feeds with high aromatics concentrations, and LLE aromatics extraction units were favoured in all other applications. ED technology was also originally considered best suited for extracting a single carbon number aromatic feed (for example, extraction of a C₆ cut). Over the past few decades, the use of ED tech - nology has become widespread in grassroots facilities and expansions of aromatics complexes for processing a wide range of aromatics feed concentrations. The most common feeds for new ED units are C₆-C₇ reformate or pygas cuts. The C 8 + portions of the reformate or pygas products are commonly prefractionated and directed either to gasoline blending or to an xylene loop for paraxylene production. Energy optimisation Over time, designers of aromatics complexes have devel- oped advances to improve the energy efficiency of fraction - ating aromatic extract products. Advances in the energy efficiency of fractionating BTX mixtures can be divided into three categories: heat integration of distillation columns; dividing wall distillation; and thermally coupled distillation columns. Development of heat integration schemes for BTX dis - tillation columns appears to have begun in earnest during

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