Hydrodealkylation Thermal

Toluene Hydrodeall lation. Benzene is produced from the hydrodemethylation of toluene under catalytic or thermal conditions. The main catalytic hydrodealkylation processes are Hydeal (UOP) and DETOL (Houdry) (49). Two widely used thermal processes are HD A (Arco and Hydrocarbon Research Institute) and THD (Gulf). These processes contribute 25—30% of the world s total benzene supply. 

Some of the principal Japanese producers of benzene are Mitsubishi Petrochemical Co., Ltd., Nippon Steel Chemical Co., Ltd., Sanyo Petrochemical Ltd., and Idemitsu Kosan Ltd. Until 1967, the main source of Japanese benzene was coal-based. Today, approximately 40—45% of benzene production in Japan is based on pyrolysis gasoline (74), about 40% catalytic reformate, and the remainder coke oven light oil and thermal hydrodealkylation. 

MHD [Mitsubishi hydrodealkylation] A thermal process for converting toluene to benzene. Developed by Mitsubishi Chemical. 

THDA [Thermal hydrodealkylation] A process for dealkylating alkyl benzenes to produce benzene. The by-product is mainly methane. Developed by UOP and licensed by that company. 

Pyrolysis gasoline gasoline produced by thermal cracking as a byproduct of ethylene manufacture. It is used as a source of benzene by the hydrodealkylation process. 

More toluene is formed than is needed in the catalytic reforming of naphtha. Benzene is always in tight supply. Table 8.7 shows the catalytic reformate production percentages of benzene, toluene, and xylene vs. the U.S. chemical demand. When the price is right it is economical to hydrodealkylate (add hydrogen, lose the methyl) toluene to benzene. This is best done on pure toluene, where the yield can be as high as 98.5%. The reaction can be promoted thermally or catalytically. As much as 30-50% of all benzene is made this way. 

For many years benzene (benzol) was made from coal tar, but new processes that consist of catalytic reforming of naphtha and hydrodealkylation of toluene are more appropriate. Benzene is a natural component of petroleum, but it cannot be separated from crude oil by simple distillation because of azeotrope formation with various other hydrocarbons. Recovery is more economical if the petroleum fraction is subjected to a thermal or catalytic process that increases the concentration of benzene. 

Toluene is the most abundant and lowest cost aromatic material. This is a reason to be the valuable raw material for the production of various chemical products. The main chemical use for toluene Is, in fact, production of benzene by dealkylation. The reaction can be carried out either thermally or catalytically using variety of catalysts including supported metals or metal oxides [1-6], amorphous silica-alumina [7] and zeolites [8-19]. The simplest toluene hydrodealkylation reaction is the reaction in which the methyl group is removed in the presence of the forming mainly methane CeHsCHs + H2 —> CeH0 + CH4... 

Additives. Eliminate catalysts, solvents, and adsorbents. Eor example, use thermal hydrodealkylation instead of catalyzed hydrodealkylation. 

Figure 3.1 presents a simplified Process Flow Diagram (PFD) for the thermal hydrodealkylation of toluene (THDA) by UOP technology (Mowry, 1986). Fresh and recycled toluene is mixed with hydrogen at the reaction loop pressure. After... 

Mowry, J. R., 1986, Thermal Hydrodealkylation (THDA) Process, in Handbook of Petroleum Refining Processes, editor R. A. Meyers, McGraw-Hill Mah, R. S. H., 1990, Chemical Process Structures and Information Flows, Butterworths Poling, B. E., J. M. Prausnitz, J. P. O Connell, 2001, The Properties of Gases Liquids, Fifth Edition, McGraw-Hill... 

Figure 6.6 presents data for gaseous, noncatalyzed reactions (pyrolysis, thermal cracking, oxidation, 10 species and 21 data) with typical heats of reaction of 120-150 MJ/kmol. In general, noncatalyzed reactions take about 1000 times longer than the same catalyzed reaction at the same reaction temperature. Data lying outside the factor-of-ten bands are benzene (-r) via hydrodealkylation and benzene (+) via hydrogenation of biphenyl (-r). 

The requirement for benzene is over 6Mt per annum in the U.S.A., somewhat lower in western Europe. Coal-tar operations make only modest contributions. In western Europe the main source is pyrolysis gasoline, whereas catalytic reforming (section 12.2.2.3) is the main source in the U.S.A., where the catalytic (clay) or thermal hydrodealkylation of toluene is also more important. 

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