Abstract

Benzene (B), toluene (T), and the xylenes (X) are the lowest molecular-weight aromatic hydrocarbon homologues. They are each very large-scale chemical feedstocks. Since they are often produced together in the same process, they can be considered as a group, ie, BTX. However, BTX as such is not an article of commerce. It is either an important component of a crude mixture such as reformate or pyrolysis gasoline, or it is separated and purified into its individual components. This article mainly discusses the processes for making those crude mixtures. Since World War II, the production of BTX has been intimately connected with the production of gasoline. BTX constitutes part of an important gasoline component called reformate, which is highly valued for gasoline because it has a very high octane rating, the results of a high concentration of aromatic compounds, all of which have very high octane values. Any BTX needed for chemical use is separated from the reformate stream before it is blended into the gasoline pool. Although at a given refinery the total volume of gasoline production usually dwarfs the BTX volume and may have a higher priority, BTX production is often important enough to support its own reforming facilities and should not be considered simply as a gasoline by-product. This independence from gasoline may be even further emphasized in the future because of restrictions on the allowed level of BTX in gasoline and because new BTX processes may utilize light feeds or natural gas. However, despite possible dislocations in supply, the availability and price of BTX for chemical uses probably will not be greatly affected by the change in gasoline composition. The principal chemical uses of BTX are (for benzene) polystyrene, ABS and phenolic resins, methacrylates, and nylon-G and nylon-G, G; polyurethanes (for toluene); and plasticizers and polyester resins and fibers (xylenes). Reforming, as currently practiced, is a platinum-catalyzed high temperature vapor-phase process which converts a relatively nonaromatic C-6–C-12 hydrocarbon mixture (naphtha) to an aromatic product called reformate. A typical reformate contains BTX in the proportions of 19:49:32, respectively. In response to environmental pressure, it is probable that many U.S. refiners will reduce the proportion of benzene in their reformate. The choice of reforming process depends on the product desired, plant size, and capital availability. If BTX is to be only a coproduct, the refiner might select a semiregenerative process. For high BTX yields a swing reactor or continuous regeneration process might be the choice because BTX yields are highest at high severity and low pressure. A large amount of BTX is obtained as a by-product of ethylene manufacture. The amount produced strongly depends on the feed to the ethylene plant. In one variant of the high temperature pyrolysis of methane, methane reacts in an electric arc. At higher temperatures or with a catalyst and added hydrogen, BTX is produced with fairly high selectivity. Only about 1% of the U.S. supply of BTX comes from coal pyrolysis. Outside the United States, coal pyrolysis is more important as a source of BTX. The complexity of separating and purifying the individual BTX components from crude BTX products depends on the amount of nonaromatic impurities present. If the amount is small enough, simple distillation can suffice. If not, distillation alone is not sufficient. The most important extraction processes use either sulfolane or glycols as the polar extraction solvent. In addition to extraction, various downstream operations are often carried out on the BTX product to produce products in proportions to fit the market demand. Benzene and p-xylene are generally in higher demand than toluene, o-xylene, and m-xylene. These are produced from raw BTX by reactive conversion processes and purification processes. BTX processing has come under steadily increasing pressure to reduce emissions and workplace exposures. Reductions in the permissible levels of both benzene and total aromatics (BTX) in gasoline have been legislated. The main focus is on benzene because it is considerably more toxic than the others and is classified as a known carcinogen.