Apr-2025

Advances in distillation processes for BTX aromatics production

A review of the most important innovations in the use of distillation processes to recover and separate aromatics products.

David Kockler
Consultant

Article Summary

The use of aromatics in chemical synthesis in the chemical industry predates World War II. Since the earliest days of aromatics production, benzene, toluene, and xylene (BTX) have been the most important aromatic compounds used as feedstocks in the chemical industry. Over the past century, several key innovations have created an aromatics production industry yielding BTX products from refinery, petrochemical, and, to a lesser extent, coke oven sources.

The first industrial-scale production of aromatics took place in coal carbonisation processes used to produce coke and coal tar. A byproduct of coal carbonisation processes known as coke oven light oil (COLO) contains high concentrations of aromatics, particularly benzene. After World War II, the worldwide demand for benzene exceeded the available production capacity of benzene from coke manufacturing.

Increased demand for benzene led to the development of processes to recover aromatics from new aromatics sources that would be capable of meeting the steadily increasing demand for benzene and other aromatics. Advances in separation processes used to recover aromatics from these different sources largely followed the emergence of new aromatics sources.

Early efforts
The earliest processes for recovering and separating aromatics from COLOs were fairly primitive and produced BTX products of substantially lower purity than those produced in modern aromatics complexes.

Fractional distillation was used to make a crude separation between aromatic and nonaromatic compounds. Nonaromatics were separated out of the COLO mixtures by removing several nonaromatic-rich slop cuts from a series of distillation columns. Additional removal of impurities, principally unsaturated and sulphur-containing compounds, was accomplished by washing the product with concentrated sulphuric acid.

The distillation processes for recovering BTX from COLOs provided the earliest indication of the limitations of fractional distillation for separating close boiling nonaromatic and aromatic compounds. The COLO distillation systems produced poor yields of BTX since the removal of nonaromatics-rich slop cuts by distillation made substantial losses of aromatics unavoidable. Product quality was also poor compared with modern aromatics product specifications, particularly with respect to nonaromatic impurities. Nonaromatic impurities in early BTX products ranged from 1.0 wt% for benzene products to 4.0 wt% for xylene products.

A sharp increase in demand for aromatics in the chemical industry after World War II led to a search for new sources of aromatics. Eventually this search led to the emergence of new sources of aromatics in the petroleum and petrochemical industries. The development of catalytic reforming in the petroleum industry resulted in a quantum leap forward in the expansion of aromatics production capacity and led to major breakthroughs in downstream separation processes. In the petrochemical industry, pygas produced from steam cracking also became a major source of aromatics.

LLE technology with extractive distillation
Shortly after the development of catalytic reforming, several large players in the petrochemical industry independently began investigating the possibility of using liquid-liquid extraction (LLE) to recover aromatics from reformate. The first LLE process to be commercialised was the glycol-based Udex process developed by Dow and licensed by UOP. The proprietary Udex process was introduced in the 1950s and was followed shortly thereafter by the emergence of other competing LLE processes.

In the early 1960s, Royal Dutch Shell commercialised an LLE process known as Sulfolane. This process introduced a new extraction solvent, which would eventually play a leading role in LLE and extractive distillation (ED)-based technologies because of the outstanding selectivity and solvent capacity of the Sulfolane solvent.

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 levels 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 hydrocarbon 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 distillation 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 recovery column. The extract product is then sent to a fractionation 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 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 recycled light aromatics displace heavy nonaromatics present in the extract phase because of the higher solubility of light nonaromatics in the solvent. The displaced heavy nonaromatics enter the hydrocarbon phase in the extractor and are recovered as raffinate. The displacement of heavy nonaromatics 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 distillation 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 recovery of BTX products. First-generation LLE-based aromatics 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.