From catalytic reforming

Toluene disproportionation (TDP) is a catalytic process in which 2 moles of toluene are converted to 1 mole of xylene and 1 mole of benzene this process is discussed in greater detail herein. Although the mixed xylenes from TDP are generally more cosdy to produce than those from catalytic reformate or pyrolysis gasoline, thek principal advantage is that they are very pure and contain essentially no EB. 

Fig. 3. General scheme for producing benzene, PX, and OX from catalytic reforming.

Cyclic Hydrocarbons. The cyclic hydrocarbon intermediates are derived principally from petroleum and natural gas, though small amounts are derived from coal. Most cycHc intermediates are used in the manufacture of more advanced synthetic organic chemicals and finished products such as dyes, medicinal chemicals, elastomers, pesticides, and plastics and resins. Table 6 details the production and sales of cycHc intermediates in 1991. Benzene (qv) is the largest volume aromatic compound used in the chemical industry. It is extracted from catalytic reformates in refineries, and is produced by the dealkylation of toluene (qv) (see also BTX Processing). 

Benzene, toluene, and xylene are made mosdy from catalytic reforming of naphthas with units similar to those already discussed. As a gross mixture, these aromatics are the backbone of gasoline blending for high octane numbers. However, there are many chemicals derived from these same aromatics thus many aromatic petrochemicals have their beginning by selective extraction from naphtha or gas—oil reformate. Benzene and cyclohexane are responsible for products such as nylon and polyester fibers, polystyrene, epoxy resins (qv), phenolic resins (qv), and polyurethanes (see Fibers Styrene plastics Urethane POLYiffiRs). 

The main producers of benzene in Canada are the Nova Corp. of Alberta, Petro-Canada, Inc., and Shell Canada Ltd. These three companies have an armual capacity of 567,000 t. Most Canadian benzene is obtained from catalytic reformate, pyrolysis gasoline, and hydrodealkylation. Coal is not an important source of benzene in Canada. 

Benzene (CeHg) is the simplest aromatic hydrocarbon and by far the most widely used one. Before 1940, the main source of benzene and substituted benzene was coal tar. Currently, it is mainly obtained from catalytic reforming. Other sources are pyrolysis gasolines and coal liquids. 

The primary sources of toluene and xylenes are reformates from catalytic reforming units, gasoline from catcracking, and pyrolysis gasoline from steam reforming of naphtha and gas oils. As mentioned earlier, solvent extraction is used to separate these aromatics from the reformate mixture. 

The main use of naphtha in the petroleum industry is in gasoline production. Light naphtha is normally blended with reformed gasoline (from catalytic reforming units) to increase its volatility and to reduce the aromatic content of the product gasoline. 

Liquid solvents are used to extract either desirable or undesirable compounds from a liquid mixture. Solvent extraction processes use a liquid solvent that has a high solvolytic power for certain compounds in the feed mixture. For example, ethylene glycol has a greater affinity for aromatic hydrocarbons and extracts them preferentially from a reformate mixture (a liquid paraffinic and aromatic product from catalytic reforming). The raffinate, which is mainly paraffins, is freed from traces of ethylene glycol by distillation. Other solvents that could be used for this purpose are liquid sulfur dioxide and sulfolane (tetramethylene sulfone). 

Products from catalytic reformers (the reformate) is a mixture of aromatics, paraffins and cycloparaffins ranging from Ce-Cg. The mixture has a high octane rating due to presence of a high percentage of aromatics and branched paraffins. Extraction of the mixture with a suitable solvent produces an aromatic-rich extract, which is further fractionated to separate the BTX components. Extraction and extractive distillation of reformate have been reviewed by Gentray and Kumar. 

Ethylbenzene (EB) is a colorless aromatic liquid with a boiling point of 136.2°C, very close to that of p-xylene. This complicates separating it from the Cg aromatic equilibrium mixture obtained from catalytic reforming processes. (See Chapter 2 for separation of Cg aromatics). Ethylbenzene obtained from this source, however, is small compared to the synthetic route. 

Xylenes (dimethylbenzenes) are an aromatic mixture composed of three isomers (0-, m-, and p-xylene). They are normally obtained from catalytic reforming and cracking units with other Ce, C7, and Cg aromatics. Separating the aromatic mixture from the reformate is done by extraction-distillation and isomerization processes (Chapter 2). 

The Ce-Cg aromatic hydrocarbons—though present in crude oil—are generally so low in concentration that it is not technically or economically feasible to separate them. However, an aromatic-rich mixture can be obtained from catalytic reforming and cracking processes, which can be further extracted to obtain the required aromatics for petrochemical use. Liquefied petroleum gases (C3-C4) from natural gas and refinery gas streams can also be catalytically converted into a liquid hydrocarbon mixture rich in C6-C8 aromatics. 

Other major components in gasoline come from catalytic reforming, alkylation and the addition of an oxygenated octane booster, methyl tertiary-butyl ether (MTB E). 

Cortright, R. D. Davda, R. R. Dumesic, J. A., Hydrogen from catalytic reforming of biomass-derived hydrocarbons in liquid water. Nature 2002,418,964. 

Emissions from catalytic reforming (Figure 4.14) include fugitive emissions of volatile constituents in the feed and emissions from process heaters and boilers. As with all process heaters in the refinery, combustion of fossil fuels produces emissions of sulfur oxides, nitrogen oxides, carbon monoxide, particulate matter, and volatile hydrocarbons. 

The separation of organic mixtures into groups of components of similar chemical type was one of the earliest applications of solvent extraction. In this chapter the term solvent is used to define the extractant phase that may contain either an extractant in a diluent or an organic compound that can itself act as an extractant. Using this technique, a solvent that preferentially dissolves aromatic compounds can be used to remove aromatics from kerosene to produce a better quality fuel. In the same way, solvent extraction can be used to produce high-purity aromatic extracts from catalytic reformates, aromatics that are essentially raw materials in the production of products such as polystyrene, nylon, and Terylene. These features have made solvent extraction a standard technique in the oil-refining and petrochemical industries. The extraction of organic compounds, however, is not confined to these industries. Other examples in this chapter include the production of pharmaceuticals and environmental processes. 

The xylenes can be used as a mixture or separated into pure isomers, depending on the application. The mixture is obtained from catalytic reforming of naphtha and separated from benzene and toluene by distillation. 

The term mixed xylenes describes a mixture containing the three xylene isomers and usually EB. Commercial sources of mixed xylenes include catalytic refonuate. pyrolysis gasoline, toluene disproportionation product, and coke-oven light oil. Ethylbenzene is present in all of these sources except toluene disproportionation product. Catalytic reformate is the product obtained from catalytic reforming processes. 

Originally, extractive distillation was limited to two-component problems. However, recent developments in solvent technology enabled applications of this hybrid separation in multicomponent systems as well. An example of such application is the BTX process of the GTC Technology Corp., shown in Figure 6, in which extractive distillation replaced the conventional liquid-liquid extraction to separate aromatics from catalytic reformate or pyrolysis gasoline. This led to a ca. 25% lower capital cost and a ca. 15% decrease in energy consumption (170). Some other examples of existing and potential applications of the extractive distillations are listed in Table 6. 

---