Catalytic reforming of naphtha

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). 

Xylenes. The main appHcation of xylene isomers, primarily p- and 0-xylenes, is in the manufacture of plasticizers and polyester fibers and resins. Demands for xylene isomers and other aromatics such as benzene have steadily been increasing over the last two decades. The major source of xylenes is the catalytic reforming of naphtha and the pyrolysis of naphtha and gas oils. A significant amount of toluene and Cg aromatics, which have lower petrochemical value, is also produced by these processes. More valuable p- or 0-xylene isomers can be manufactured from these low value aromatics in a process complex consisting of transalkylation, eg, the Tatoray process and Mobil s toluene disproportionation (M lDP) and selective toluene disproportionation (MSTDP) processes isomerization, eg, the UOP Isomar process (88) and Mobil s high temperature isomerization (MHTI), low pressure isomerization (MLPI), and vapor-phase isomerization (MVPI) processes (89) and xylene isomer separation, eg, the UOP Parex process (90). 

Manufacture The xylenes are obtained with benzene (and toluene) from the catalytic reforming of naphtha and separated from the aromatic mixture by distillation. From the mixed isomers, the ortho- can be obtained by distillation because its boiling point is sufficiently different. The meta- and para- are separated by either selective adsorption or by crystallization. 

This reaction is endothermic and is favored by low pressure. In practice, however, the process is conducted at a pressure of 1-3 MPa (because of a concurrent hydrocracking reaction) and a temperature of 300-450°C using Pt-based catalysts [7]. The feedstock for the reforming process must be carefully purified from S- and N-compounds (below 1 ppm), which may use up a significant portion of hydrogen produced. The typical composition of the off-gas from the catalytic reforming of naphtha is as follows (vol%) H2—82, CH4—7, C2—5, C3—4, and C4—2 [7]. 

For many years benzene was made from coal tar even as late as 1949, when all of it was made by this old process. New processes began to take over in the 1950s, which were used for 50% of the benzene in 1959, 94% in 1972, 96% in 1980, and near 100% in the 1990s. These new processes consist of catalytic reforming of naphtha and hydrodealkylation of toluene in a 70 30 capacity ratio. 

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. 

The Platforming-Udex process for catalytic reforming of naphtha is also used for toluene. The feedstock should be rich in seven carbon naphthenes such as dimethylcyclopentanes, methylcyclohexane, and ethylcyclopentane... 

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. 

Endothermic reactions can also be run with interstage heating. An example we have considered previously is the catalytic reforming of naphtha in petroleum refining, which is strongly endothermic. These reactors are adiabatic packed beds or moving beds (more on these in the next chapter) in which the reactant is preheated before each reactor stage. 

In the catalytic reforming of naphthas there are a number of nonhydrocarbon materials which play an important part in the performance of the catalyst. Sulfur is a poison for the reforming catalyst. There appears to be evidence developing that the platinum-rhenium catalysts may be more sensitive to sulfur than the conventional catalysts. Effective pretreatment of the feed stock to maintain sulfur at low levels is desirable. 

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. 

Components that are produced in high yield by side reactions. Some examples include propylene, butylenes, and butadiene, all of which are byproducts of ethylene from steam cracking of naphtha feed. Orthoxylene and metaxylene are byproducts of paraxylene manufacture by catalytic reforming of naphtha. 

During the Second World War methylcyclohexane was transformed into toluene over molybdena/alumina catalysts. An attempt was made to use the same process for the catalytic reforming of naphtha in order to increase the octane number by carrying out isomerisation, cyclisation and dehydrogenation of... 

Derivation (1) Reaction of steam with natural gas (steam reforming) and subsequent purification (2) partial oxidation of hydrocarbons to carbon monoxide and interaction of carbon monoxide and steam (3) gasification of coal (see Note 1) (4) dissociation of ammonia (5) thermal or catalytic decomposition of hydrocarbon gases (6) catalytic reforming of naphtha (7) reaction of iron and steam (8) catalytic reaction of methanol and steam (9) electrolysis of water (see Note 2). In view of the importance of hydrogen as a major energy source of the future, development of the most promising of these methods may be expected. 

Edgar, M. Catalytic reforming of naphtha in petroleum refineries. In Applied Industrial Catalysis Leach, B., Ed. Academic Press New York, 1983 Vol. 1, 123. 

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