Gas 2025 Issue

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

T he use of aromatics in chemical synthesis in the chem- ical industry predates World War II. Since the earliest days of aromatics production, benzene, toluene, and xylene (BTX) have been the most important aromatic com- pounds 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 con- centrations 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 aro- matics 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 sepa- ration 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 impu- rities, principally unsaturated and sulphur-containing com- pounds, 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 aro- matic 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 petrochemi- cal 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. The development of catalytic reforming in the petroleum industry resulted in a quantum leap forward in the expansion of aromatics production capacity 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.

Gas 2025