Gas 2025 Issue

processes developed a decade after the commercialisation of LLE. In LLE processes, light nonaromatics are recycled from the extractive stripper back to the extractor. The recy- cled light aromatics displace heavy nonaromatics present in the extract phase because of the higher solubility of light nonaromatics in the solvent. The displaced heavy nonaro- matics enter the hydrocarbon phase in the extractor and are recovered as raffinate. The displacement of heavy non - aromatics from the extract phase ensures that the extract phase leaving the extractor is essentially free of heavy nonaromatics. By contrast, in ED processes, the extraction and distilla - tion of the feed mixture are integrated into a single step. As a result, a portion of the heavy nonaromatics found in the feed to the ED column migrates to the bottom of the column and remains in the extract product as it leaves the ED column. Aromatics recovery breakthrough Newly developed LLE processes for recovering aromatics proved to be a breakthrough in terms of purity and recov- ery of BTX products. First-generation LLE-based aromat- ics extraction units produced aromatic products with less than 0.1 wt% nonaromatics. This represents an order of magnitude improvement in comparison with the fractional distillation processes originally developed for recovering aromatics from COLOs. In modern LLE unit designs, it is not unusual for benzene products to contain less than 0.01 wt% nonaromatics. The recovery of BTX products also improved considerably from the early days of aromatics production from COLOs. First-generation LLE-based aromatics extraction units typ- ically achieved 99.9 wt% benzene, 99.0 wt% toluene, and 97.0 wt% xylene recoveries. Current LLE aromatics extrac- tion units are able to recover 99.9 wt% benzene, 99.8 wt% toluene and 98.0 wt% xylene from reformate and pygas feeds. Newly developed LLE-based aromatics extraction pro- cesses did not benefit the coke-producing industries. COLOs proved to be an unsuitable feed for new LLE pro- cesses because COLOs contain significantly higher concen - trations of aromatics (85 wt% and higher) than reformate feeds. LLE processes are not well suited for extracting aro- matics from feeds with high concentrations of aromatics because a hydrocarbon phase does not form in the extractor when the feed is contacted with a polar solvent. Instead, a single liquid phase containing the polar solvent, aromatics, and nonaromatics is formed, and no separation between aromatics and nonaromatics is obtained in the extractor. Development of ED technology The poor suitability of LLE processes for extracting aro- matics from COLOs led to another major advance in the use of distillation to recover aromatics. As a result of the competition created by high purity aromatics produced in LLE processes, the coke manufacturing industry faced the prospect of being unable to market low purity aromatics recovered from COLOs. The German steel and coke indus- tries approached Uhde with a request to develop a solution

Water

Ranate (Nonaromatics)

Water wash

Extract product to BTX fractionation

Extractor

Solvent recovery column

Light nonaromatics

Feed

Steam

Rich solvent

Extractive stripper

Lean solvent

Figure 1 LLE with extractive distillation

All LLE processes developed to recover aromatics from reformate share several key features. The most important commonality among these processes is the combination of an extraction step with a separate extractive distillation step to produce an extract product with extremely low lev- els of nonaromatic contaminants. The introduction of an extractive distillation step in LLE processes represented a pivotal advance in the development of distillation processes to recover aromatics products. A simplified process flow diagram of a typical Sulfolane LLE process is shown in Figure 1 . The feed mixture is sent to an extractor, where the feed is contacted with a polar solvent. Aromatics are extracted into the solvent (extract phase), and nonaromatics remain in a separate hydrocar- bon phase, which is removed at the top of the extractor. The extract phase contains nearly all of the aromatics that are present in the feed. The high concentration of aromatics in the extract phase removed from the extractor increases the solubility of nonaromatics in the extract phase. This results in the carryover of a small amount of nonaromatics in the extract phase. Effective combinations The extract phase is sent to an extractive stripper to remove the nonaromatics for recycle back to the extractor. The nonaromatics are easily separated by extractive distilla- tion from the aromatics and solvent because the solvent increases the relative volatility of the nonaromatics. After the extractive distillation step, the solvent is separated from the extract in a solvent recovery column. The lean solvent from the bottom of the solvent recovery column is recycled back to the extractor, and the high-purity extract product is recovered as an overhead product from the solvent recov- ery column. The extract product is then sent to a fractiona- tion section to separate the extract product into individual aromatic products by direct sequence distillation. The combination of LLE and extractive distillation proved to be particularly effective at producing high-purity xylene products from C₆-C₈ feed mixtures compared with ED

Gas 2025