Conversion hydrodealkylation

Mobil s High Temperature Isomerization (MHTI) process, which was introduced in 1981, uses Pt on an acidic ZSM-5 zeoHte catalyst to isomerize the xylenes and hydrodealkylate EB to benzene and ethane (126). This process is particularly suited for unextracted feeds containing Cg aHphatics, because this catalyst is capable of cracking them to light paraffins. Reaction occurs in the vapor phase to produce a PX concentration slightly higher than equiHbrium, ie, 102—104% of equiHbrium. EB conversion is about 40—65%, with xylene losses of about 2%. Reaction conditions ate temperature of 427—460°C, pressure of 1480—1825 kPa, WHSV of 10—12, and a H2/hydtocatbon molar ratio of 1.5—2 1. Compared to the MVPI process, the MHTI process has lower xylene losses and lower formation of heavy aromatics. 

Catalytic conversion processes include naphtha catalytic reforming, catalytic cracking, hydrocracking, hydrodealkylation, isomerization, alkylation, and polymerization. In these processes, one or more catalyst is used. A common factor among these processes is that most of the reactions are initiated hy an acid-type catalyst that promotes carhonium ion formation. 

To the dismay of toluene lovers, if there are any, the volume growth of benzene has overshadowed that of toluene, and toluenes major use is to make benzene in hydrodealkylation and toluene disproportionation units. About 50% of the toluene recovered in the United States is used this way. Conversion to para-xylene is also of growing importance. 

Hydrodealkylation, The process of removing a methyl or larger alkyl group as in the conversion of toluene (C6H5CH3) to benzene (CeHg). The alkyl group is replaced by a hydrogen atom. 

The converse reactions dealkylation and hydrodealkylation are practiced extensively to convert available feedstocks into other more desirable (marketable), products. Two such processes are (1) the conversion of toluene or xylene, or the higher-molecular-weight alkyl aromatic compounds, to benzene in the presence of hydrogen and a suitable presence of a dealkylation catalyst and (2) the conversion of toluene in the presence of hydrogen and a fixed bed catalyst to benzene plus mixed xylenes. 

The hydrodealkylation of side-chain aromatics to nonsubstituted parents is a major process in petrochemistry. A typical example is the conversion of toluene to benzene ... 

An alternate course, which was not pursued, would be to operate the reformer at less severe conditions where naphthene conversions are high but where isomerization and hydrocracking are less. The resultant reformate could be extracted with raffinate going to a naphtha cracker for olefin production while the extract is hydrodealkylated. 

With the possible exception of the SRC II straight-run naphthas, these Ce-Co reformates could be fed to a hydrodealkylator without first being extracted. However, it must be noted that hydrocracking, as evidenced by paraffin conversion, was not nearly as active as expected. In the event that a coal-derived naphtha contained a substantial portion of paraffin, particularly C6 paraffin, the aromatic content of the resultant reformate would be significantly less. 

The dehydrogenation of other hydrocarbons has also been studied in CMRs, generally with porous membranes. Conversions of ethane [47], propane [48], butane [49], and ethylbenzene [50] have been reported to be higher when membrane reactors were used. In the case of ethylbenzene dehydrogenation, the undesirable hydrodealkylation side reaction is slowed down due to the removal of H2, i.e. the membrane enables an increase in selectivity as well [50]. 

MFI zeolites seem to be the most efficient for EB dealkylation, in terms of activity, selectivity and stability. In the 70s, on metal-free MFI catalysts, EB was disproportionated into benzene and diethylbenzenes. As indicated above, with MFI catalysts, ethylbenzene disproportionation occurs through a deethylation-ethylation mechanism, with ethylene as desorbed intermediate. The addition of a metal (carried out early 80s) allows a rapid and irreversible conversion of ethylene into ethane with a consequent shift of ethylbenzene transformation from disproportionation to hydrodealkylation. The selectivity is highly sensitive to temperature that must be in the range 380°C-460°C to limit both alkylation and naphthene cracking. 

In the absence of catalyst, it occurs around 700 to 800< with once-through com er-sions of 20 to 30 per cent and overall yields not exceeding 50 to 60 molar per cent This low performance can be ascribed to the side reactions, especially hydrodealkylation to benzene and toluene, miscellaneous craddngs with the foimadon of coke or water gas, and the alkylation of the styrene formed to methylstyrene and the conversion of the by-products obtained. 

The initial reaction rates for hydrodealkylation, hydrocracking and condensation were determined as slope of the dependence of the conversion on reciprocal space velocity at 1/WHSV=0. The obtained rates as a function of the amount of strong acid sites are given in Figure 1. 

Primary steam reforming Secondary steam reforming Carbon monoxide conversion Carbon monoxide methanation Ammonia synthesis Sulfuric acid synthesis Methanol synthesis Oxo synthesis Ethylene oxide Ethylene dichloride Vinylacetate Butadiene Maleic anhydride Phthalic anhydride Cyclohexane Styrene Hydrodealkylation Catalytic reforming Isomerization Polymerization (Hydro)desulfurization Hydrocracking... 

All the Mo-zeolites were tested using the toluene conversion reaction. In most cases the carrier gas was hydrogen, then the conversion of toluene mainly consisted of disproportionation, hydrodealkylation and hydrocracking. 

Laboratory data indicate that the reactions proceed irreversibly without a catalyst at temperatures in the range of 1,200-1,270°F with approximately 75 mol% of the toluene converted to benzene and approximately 2 mol% of the benzene produced in the hydrodealkylation reaction converted to biphenyl. Since the reactions occur in series in a single processing unit, just a single reaction operation is positioned in the flowsheet, as shown in Figure 4.16. The plant capacity is based on the conversion of 274.2 Ibmol/hr of toluene, or approximately 200 MMlb/yr, assuming operation 330 days per year. 

Toluene is converted to benzene by hydrodealkylation. Typically, a 75% conversion is used in the reactor, which necessitates the recovery and recycle of unreacted toluene. In addition, a side reaction occurs that produces a small amount of a biphenyl byproduct, which is separated from the toluene. A hydrodealkylation process is being designed that includes a distillation column for separating toluene from biphenyl. The feed to the column is 3.4 Ibmoiyhr of benzene, 84.6 IbmoVhr of toluene, and 5.1 Ibmol/hr of biphenyl at 264°F and 37.1 psia. The distillate is to contain 99.5% of the toluene and 2% of the biphenyl. If the column operates at a bottoms pressure of 38.2 psia, determine the bottoms temperature and select a suitable heat source for the reboiler. Steam is available at pressures of 60, 160, and 445 psig. The barometer reads 14 psia. 

Make an order-of-magnitude estimate of the total capital investment, as of the year 2001 (MS = 1,110), to produce benzene according to the toluene hydrodealkylation process shown in Figure 5.13. Assume an overall conversion of toluene to benzene of 95% and 330 days of operation per year. Also, assume the makeup gas enters at the desired pressure and a clay adsorption treater must be added to the flow sheet after the stabilizer. The treater removes contaminants that would prevent the benzene product from meeting specifications. In addition, in order for the reactor to handle the high temperature, it must have a brick lining on the inside, so take a material factor of Fm = 15. Otherwise, aU major equipment is constructed of carbon steel. The plant will be constructed outdoors with major additions to existing facilities. 

When striving for high reactor conversions, it may be necessary to consider the reverse reaction even when the reaction is considered to be irreversible. This is the case for the hydrodealkylation of toluene. A rate equation for the reverse reaction can be derived from the rate equation for the forward reaction, given by Eq. (8.2), by assuming that the two rate equations are consistent with the chemical-reaction equilibrium constant. Assume that the gas reacting mixture is ideal at the high temperature of the reaction. Then, the chemical equilibrium constant can be expressed in terms of concentrations and equated to the ratio of the rate constants by ... 

The overall conversion tells us what fraction of the toluene in the feed to the process (Stream 1) is converted to products. For the hydrodealkylation process, it is seen that this fraction is high (99.3%). This high overall conversion is typical for chemical processes and shows that unreacted raw materials are not being lost from the process. 

The equilibrium conversion for the hydrodealkylation reaction remained high in spite of the high temperature. Although there is no real problem with using the elevated temperature in the reactor, it cannot be justified from a thermodynamic point of view. 

High-Pressure Concern (see Table 6.21. From the reaction stoichiometry, we see that there are equal numbers of reactant and product moles in the hydrodealkylation reaction. For this case, there is no effect of pressure on equilibrium conversion. From a thermodynamic point of view there is no reason for the high pressure in the reactor. 

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