Toluene steam cracking

Pyrolysis gasoline is a by-product of the steam cracking of hydrocarbon feeds in ethylene crackers (see Ethylene). Pyrolysis gasoline typically contains about 50—70 wt % aromatics, of which roughly 50% is benzene, 30% is toluene, and 20% is mixed xylenes (which includes EB). 

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

Raw materials for obtaining benzene, which is needed for the production of alkylbenzenes, are pyrolysis gasoline, a byproduct of the ethylene production in the steam cracking process, and coke oven gas. Reforming gasoline contains only small amounts of benzene. Large amounts of benzene are further produced by hydrodealkylation of toluene, a surplus product in industry. 

When ethylene is reacted at 573 K in the presence of water in static conditions, oligomerization and conjunct polymerization give rise to paraffinic, olefinic and aromatic products (8). Nevertheless, the distribution of the aliphatics and aromatics is quite different from that of the steam-cracking products. In the former a great variety of products is formed they include propane, n-butane, isobutane and isopentane as aliphatics, and toluene, xylenes and ethylbenzene as aromatics (Figure 6B). 

In mice skin-painting studies, skin tumors were produced by steam-refined petroleum bitumens, an air-refined bitumen in toluene, two cracking residue bimmens, and a pooled mixmre of steam- and air-blown petroleum bitumens. In contrast, standard roofing petroleum asphalts produced no tumors. 

The main source of pX is the mixed xylenes cut, containing the four C8 aromatics including EB. The mixed xylenes are produced by catalytic reforming (82%), steam cracking (9%) and toluene disproportionation or toluene-heavy aromatics transalkylation (9%). The xylenes distribution is generally close to thermodynamic equilibrium values, while EB content depends on the origin of the cut (Table 9.1). 

On the w hole. these analyses show that gas oil steam cracking produces fewer light produas than the treatment of naphtha, and more heavy products which display a higher aromatics content. Hence the Cj- 200 C cut boosts the BTX (Benzene, Toluene, Xylenes) (majority benzene concentration. Similarly, fuel oil (fraction above 200 C) displays a more pronounced aromatic character. This feature makes it incompatible with straight-run distillation fuel oils. The mixture causes the deposition of asphaltenes and other... 

Metal granules also have been found in cokes formed or deposited on iron, cobalt, and nickel foils in experiments using methane, propane, propylene, and butadiene (7-10). Platelet-type coke, whose properties match those of graphite also was produced in some cases. Lahaye et al. (11) investigated the steam cracking of cyclohexane, toluene, and n-hexane over quartz, electrode graphite, and refractory steel. They report that heavy hydrocarbon species form in the gas phase, condense into liquid droplets which then strike the solid surface, and finally react on the solid surfaces to produce carbonaceous products. The liquid droplets wet and spread out on certain surfaces better than on others. 

Detol Houndry Process and Chemical Co. Toluene /xylenes, heavy catalytic cycle oil, aromatics from petroleum coking, steam cracking A process for production of benzene and naphthalene... 

Application Advanced Pygas Upgrading (APU) is a catalytic process technology developed by SK Corp. and is exclusively offered by Axens to convert pyrolysis (ex steam cracking) gasoline to a superior steam-cracker feed (LPG), and benzene, toluene and xylene (BTX) aromatics. 

Benzene is produced together with toluene and xylenes via two major routes. One source is the C5+ product cut of the steam-cracking process (see Section 6.6 for details). It typically contains 30-45% benzene, 20% toluene, and 5-10% xylenes. The second production route is the reforming process in refineries (see Section 6.9 for details) with a typical product composition of 5-8% benzene, 20-25% toluene, and 30% xylenes. In former times, the production of aromatic compounds from coking plants also played a significant role but today 98% of benzene production is based on crude oil. 

Figure 5.3.9 summarizes the most important technical applications of toluene. The fact that a very important reaction for toluene is its dealkylation to benzene (about 50% of industrial toluene use) indicates that more toluene is produced from steam cracking and reforming than industrially needed under typical market conditions. o-Xylene is mainly converted into phthalic add anhydride, m-xylene into iso-phthalic add, and p-xylene into terephthalic add. While phthalic add finds its most important use as a component in plasticizers, terephthalic add is a kqr monomer for the production of polyesters (see Section 6.13 for details). 

Steam cracking (or middle temperature pyrolysis ) converts alkanes and refinery cuts [e.g., ethane, light fuels (naphtha)] into a mixture of saturated and unsaturated hydrocarbons, with ethylene, propylene, butenes, butadiene, benzene, and toluene being the most valuable products. 

Steam cracking is the most important petrochemical process, producing the most relevant basic chemicals ethene, propene, benzene, and toluene from hydrocarbon feedstocks. 

Aromatics [benzene, toluene, and xylene (BTX)] are obtained from refinery and petrochemical light naphtha streams. Aromatics are produced in the reforming process and in steam cracking. Extraction or various extractive distillation processes are used to isolate and separate aromatics from the naphtha streams. Typical extraction processes are based on tetraethylene glycol, sulfolane, N,N -methylpyrolidene, or morpholine. They produce a mixture of aromatics that are subsequently separated by distillation, extractive distillation, or—in the case of xylene isomers—differential adsorption or fractional crystallization. 

When steam cracking or naphtha reforming produce an aromatics mixture short in benzene or o- and p-xylene, some interconversion is practiced. Toluene can be hydrodealkylated to benzene. Xylene can be isomerized to increase yields of o- and p-xylene. The analysis for aromatics thus falls into two general types to meet two different needs. Analysis for process optimization assists in obtaining the maximum product at the minimum unit cost. This involves analysis of feeds, products, and raffinate (purge) streams. These analyses must be tailored to the process and the plant streams involved. Generally, it is desirable to have one analytical procedure to apply to a variety of sample types. The final product specification analysis can also be used for process control. The ASTM standard... 

About 95% of toluene is obtained from the refinery catalytic reforming of naphtha feedstocks in what are called reformers. About 5% is obtained from pyrolysis gasoline from steam cracking of hydrocarbons associated with ethylene and propylene. 

More than 90% of today s petrochemicals are produced from refineiy products. Most are based on the use of C2-C4 olefins and aromatics finm hydrocarbon steam cracking units, which are even more closely linked to refineries. In North America, the feedstock for steam cracker units have generally been ethane, propane, or LPG. As a result, most of the propylene and aromatics have been provided by FCC units and catalytic reformers. In maity other parts of the world where naphtha feed has been more readily available, suppUes of propylene and aromatics have been produced directly by steam cracking. When necessary, the catalytic dehydrogenation of paraffins or dealkylation of toluene can balance the supply of olefins or benzene. In Table 7.2 some of the catalytic processes that convert olefins and benzene from a steam cracker into basic petrochemicals for the modem chemical industry are shown. 

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