Hexane, catalytic cracking

This concept was used for the study of the deactivation of n-hexane catalytic cracking on a US Y zeolite catalyst. The interpretation of the flow patterns in the recycle reactor, necessary for the quantification of the degree of mixing, was based upon tracer experiments. 

Residues containing high levels of heavy metals are not suitable for catalytic cracking units. These feedstocks may be subjected to a demetallization process to reduce their metal contents. For example, the metal content of vacuum residues could be substantially reduced by using a selective organic solvent such as pentane or hexane, which separates the residue into an oil (with a low metal and asphaltene content) and asphalt (with high metal content). Demetallized oils could be processed by direct hydrocatalysis. 

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

The effect of coke on the rate of 2-Me-pentane formation, a primary product of the catalytic cracking of n-hexane, will be dealt with in detail to illustrate the methodology. 

The catalytic cracking of n-hexane was earned out in a continuous flow reaction system over 0.5 g of electrically heated Ni/Al203 catalyst at 733K and 1 atm. The total flow rate of hexane and nitrogen was controlled at 60 ml per minute. I he amount of carbon deposited on the used catalysts was determined by chemical analysis. The eilluent gas was analyzed by gas chromatography (HP 5890 A) using a flame ionization detector. 

So far these processes have been modeled in terms of lumps. In catalytic cracking the 3-lump -and the 10-lump model [Nace et al, 1971 Jacob et al, 1976] are still widely used although the lumps are based on boiling ranges rather than on chemical nature. These models contain in general only one deactivation function of an empirical nature for the reactions of the various lumps, b their study of the catalytic cracking of n-hexane on a US-Y-zeolite in an electrobalance with recycle Beimaert et al, [1994] derived an empirical deactivation function of the type (2) for the various reactions, but with different a-values, as illustrated in Table 2 for the isomerizations. 

Figure 2. Molecular reaction scheme for the catalytic cracking of n-hexane.

Fig. 5.1. Chromatograms of products of catalytic cracking (A) without reactor and (B) with reactor. Sorbent, 11% quinoline on refractory brick temperature, 25 C column length, 10.5 m. Peaks 1 = propane 2 = propylene 3 = isobutane 4 = n-butane 5 = isobutene 6 = butene-1 7 = rmns-butene-2 8 = cis-butene-2 9 = isopentane 10 = 3-methylbutene-l 11 = n-pentane 12 = pentene-1 13 = 2,2-dimethylbutene 14 = 2-methylbutene-l 15 = tnms-pentene-2 16 = cfsi)entene-2 17 = 2-methyl-butene-2 18 = 2,3-dimethylbutane 19 = 2-methylpentane 20 = 3-methylpentane 21 = 3-methylpen-tene-1 22 = 4-methylpentene-l 23 = c -4-methylpentene-2 24 = cyclopentane 25 = 2,3-dimethyl-butene-1 26 = fmns-4-methylpentene-2 27 = w-hexane 28 = cyclopentene 29 = 2-methylpentene-l 30 = hexene-1 31 = 2,4-dimethylpentane 32 = cis-hexene-3 33 = tnms-hexene-3 34 = 2-ethylbu-tene-1 35 = trans-hexene-2 36 = methylcyclopentane 37 = cis-methylpentene-2 38 = 2-methylpen-tene-2 39 = pisns-3-methylpentene-2 40 = methylcyclopentene-4 41 = 4-methylcyclopentene 42 = cw-3-methylpentene-2 43 = 2,3-dimethylpentane 44 = 2-methylheptane 45 = 2,3-dimethylbutene-2 46 = methylheptane 47 = cyclohexane 48 = C, olefin. Reprinted with permission from ref. 1.

Because zeolites can also be manufactured with various proportions of aluminate, a catalyst can be tailored to meet the exact requirement of the process. It is calculated that the medium-pore zeolite ZSM-5 (a), operating at 454 °C and lOOtorr (1.3 X 10" Pa) pressure of hexane, can crack more than 37 molecules per active site per minute. At 538 °C the turnover rises to over 300 molecules per minute per active site. Other catalytic processes - toluene disproportionation, xylene isomerization, and methanol conversion (see later) - operate even faster, with hexane isomerization showing a turnover of as much as 4 x 10 per minute per active site. This indicates that rates of catalytic reactions with zeolites equal or exceed rates for enzyme catalysis. 

One of the most interesting problems in the catalytic cracking of paraffins is the behavior of structural isomers. The work of Good, Voge, and Greensfelder (6) is of particular interest in this respect since it deals with the catalytic cracking of the five isomeric hexanes. The results indicate that, with one exception, there is essentially no isomerization of... 

Groten W.A., Wojciechowski, B.W. and Hunter, B.K. (1990), Coke and deactivation. II- Formation of coke and minor products in the catalytic cracking of n-hexane on USHY zeolite, J. Catal. 125, 311-324. 

There are many other unitary operations which are used by organic chemistry plants to manufacture synthetic solvents. These include alkoxylation (ethylene glycol), halogenation (1,1,1-trichloethane), catalytic cracking (hexane), pyrolysis (acetone and xylene), hydrodealkylation (xylene), nitration (nitrobenzene), hydrogenation (n-butanol, 1,6-hexanediol), oxidation (1,6-hexanediol), esterification (1,6-hexanediol), and many more. 

Propylene monomer is produced by catalytic cracking of petroleum fractions or the steam cracking of hydrocarbons during the production of ethylene. Conventional processes in liquid phase and in slurry use stirred reactors and a diluent such as naphtha, hexane, or heptane. The reaction takes place typically at a temperature of about 60-80°C and at 0.5-1.5 MPa, and the final product is obtained as a solid suspension of polypropylene in the liquid phase. Isolation of the resin requires a separation step (such as centrifugation) followed by washing the resin free of residual diluent and drying. 

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