Cracking yields

M. Dente, E. Ran2i, and S. Barendregt, "Adiabatic Cracking Yields Theoretically Predicted," paper presented stHIChE Meeting, New Orleans, July 1981. 

The radicals may further crack, yielding an olefin and a new free radical. Cracking usually occurs at a bond beta to the carbon carrying the unpaired electron. 

The condensed cycloparaffin rings crack yielding n-butyl groups, containing, in part, the 13C label in a terminal CHg position (signal at 14.0). 

For n-decane isomerization, when a good balance between the metal phase and the acidic phase of the catalysts is reached, the isomerization and cracking yield curves of the catalysts are a unique function of the conversion, meaning that these yields do not depends on the porosity nor the acidity of large pore materials. Formation of the most bulky isomers, such as 4-propylheptane and 3-ethyl-3-methylheptane was favored in mesoporous solids (figure 1). Criteria based on the formation of these particular isomers are linked with mesoporosity and could be useful to discriminate between zeolites catalysts with and without mesopores. 

In the previous section, we showed that resid feed takes more advantages of HCO recycling. In this section, we look at the effect of the boiling point range of recycle stream on the cracking yields. In Section 1.3.4, the effect of conversion level will be discussed. The data presented in these two sections are from resid feed. 

Fisher, I. P. Effect of Feedstock Variability on Catalytic Cracking Yields. Applied Catalysis 65 (1990) 189-210. 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

Figure 3 shows Arrhenius plot of cracking yield on various catalysts. The activities of mesoporous silica catalysts lay between AMS and zeolites. Among mesoporous silica catalysts, FSM-16 exhibited the highest activity and MCM-41P... 

Laboratory scale PQC evaluation studies are usually conducted in fixed bed reactors such as MAT, the results from which can provide a reliable and rapid means of ranking catalyst performance [4]. Depending upon the conditions employed, the effect of added ZSM-5 can also be predicted [5] and can give the same trends as those experienced in commercial reactors. For example, the effect of 2.5 wt% addition of ZSM-5 on gas oil cracking yields with Quantum 2000 is described in Table 1. In this example, a 4% reduction in gasoline yield occurs, predominantly from 105°C+ material. The L.P.G. composition indicates an enhancement of propene, butenes, and iso-butane, in agreement with commercial results and, furthermore, the relative increase in the individual butenes are similar to those reported by Schipper et al [1]. 

Figure 4. Catalytic cracking yields from hydrotreated feeds. Shale oil arm Arabian Light vacuum gas oil at 975°F (O), shale oil containing 870 ppm N (0), shale oil containing 385 ppm N.

KTI Propylene/isobutylene Butanes (field) Thermal steam cracking yields 79 mol% of products for MTBE units NA NA... 

Table 2.3 Cracking Yields from Liquid Feedstock...

In this case, the endothermicities of Reactions A-3 and A-4 are not equal since Reaction A-3 produces an allyl ladical. Therefore, one would expect Reaction A-3 to be somewhat faster than Reaction A—4. If Radicals I and II are present in equal concentrations, then the yield of nonene from the hydrogen-abstraction path is between one and two times the yield of octene. The nonene in excess of this is formed by the retro-ene reaction. This defines a range of importance for the molecular path and by difference the contribution of the hydrogen abstraction path. This procedure is illustrated for the dodecene cracking yields from column two of Table II. 

Cracking yields are highly influenced by cracking parameters thus, better yields require better cracking conditions. Main cracking parameters are hydrocarbon partial pressure, residence time, and temperature. 

The catalysts (50 mg) were heated up to 673 K in He at a rate of 6 K/min and pretreated at this temperature in situ either in He or in a mixture of 5% HgS in H2 for 1 h. Thiophene HDS test reactions were carried out at 673 K and atmospheric pressure in a flow reactor system (30 cm min flow of 3% thiophene in Hg). Thiophene and the products were detected by GC. The conversion is the fraction of thiophene converted to coke and gaseous products, the yield is the fraction of thiophene converted to gaseous products. The catalytic properties were characterized by activities in C-S hydrogenolysis without C-C bond breaking (yield - (Ci+Cj+Cg products)), cracking (yield - C4 products), and coking (conversion - yield). The catalytic conversions determined after 5 minutes time on stream are discussed here, because all samples deactivated fast due to coke formation. 

Since cracking processes make relatively more normally gaseous products than found in the usual crude distiller, there is usually a substantial gas compression section also. This gas compression section presents an opportunity to keep the column pressure low, perhaps only 5 or lOpsi gage, in turn allowing lower reactor and catalyst regenerator pressures, which in turn improve cracking yields and reduce regeneration air supply horsepower. 

There is good agreement between the conversion, isomerisation and cracking yield calculations (full lines) and the experimental data (dots), Fig. 30. 

Weight of paraffins with less than 20 carbon atoms—approaching a cracking yield. 

Heptaldehyde can be hydrogenated to heptanol or oxidized to heptanoic acid, which can be marketed. The ester cracking yield is up to 75 molar per cent of methyl undecenoate. 

This mechanism explains the composition of the gas produced in the cracking of 1-pentene. According to this scheme alkyl radicals are formed, some of which correspond to the starting olefins and some with a greater number of carbon atoms. This makes it possible to explain some experimental results which cannot be explained by the Voevodsky mechanism. For example, isobutene cracking yields a large amount of ethylene (6, 9, 10), and tracer methods proved that some of the ethylene resulted directly from isobutene (9). On decomposition, the radical... 

Fig.6 Product analysis for SG (24h) (A) Product distribution vs. Conversion (B) methyl-nonane distribution (C) Cracked product distribution at 50% cracking yield.

From Figs. 10-2d and e, it can be calculated that at 400°C in the case of a larger paraffin, e.g., hexane there is a spontaneous tendency to decompose or crack, yielding a smaller paraffin and smaller olefin, e.g., propane and propylene. At 100°C, such a reaction does not tend to occur. Therefore, if a relatively large olefin were to be hydrogenated to a paraffin of the same molecular weight, apparently it would be desirable to carry out such a reduction at lower temperatures for two pertinent reasons (1) the equilibrium yield is better, (2) decomposition or side reactions are less likely to be troublesome. Because of this preferred lower-temperature operation, catalysts must be available to operate at these lower temperatures. Such hydrogenation catalysts are known, as was pointed out elsewhere in this chapter. 

With the variety of feedstocks involved, there is a need to optimize cracking conditions on the basis of an analysis of the feedstock. For refinery distillate cuts, characterization is a difficult analytical problem. A hydrocarbon type analysis, as discussed in Section 13.3.4, can be correlated with cracking yields. Another technique was described by Greco (112) in which microscale pyrolysis was used to simulate cracking. A tube-type pyrolyzer with a quartz... 

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