Conventional cracking

A larger feed fraction that does not vaporize under conventional cracking conditions. For this reason resid in FCC is sometimes defined as the fraction of the feed boiling above an effective "cutpoint" of the flash in the bottom of the FCC riser. 

Another approach for overcoming the problems posed by conventional cracking catalysts has been disclosed recently by Reverse et al. [101]. In this case, direct cracking is performed by using as catalyst a molten bed of pure metal or a metal mixture (mainly lead, zinc, tin) at a temperature of 460-550°C wherein the waste polymer is loaded inside the reactor at a certain depth. The authors point out that the products are indeed a result of the combination of both thermal and catalytic cracking. The catalyst composition may also include some acidic component such as metal silicates, metal carbonates and their mixtures. The process can be applied to pure and mixed polymers (PE, PET, PP, PVC), as well as to the plastic fraction of municipal solid wastes. 

However, it should be pointed out that conventional cracking conditions are not suitable for such feed stocks. Conventional cracking conditions lead to lower ethylene yields and to increases in fuel oil... 

In the course of commercial operations> extensive yield data were collected on various liquid feedstock operations over the full short residence time range and varying hydrocarbon partial pressure. Yields were determined in the field by a special sample conditioning and analysis system ( ) operating on the furnace effluent. The results of these studies substantiated the laboratory and pilot plant findings and confirmed that ethylene yields with the Millisecond Furnace can be increased by 10-20 wt % relative to those obtained with conventional cracking. Similar increases were achieved in the yields of other valuable co-products while the yield of methane was significantly reduced in all cases. 

A kinetic model of the cracking process, covering both conventional cracking and the ACR range, must include all the features of the decomposition reactions we have mentioned, plus the equilibrium effects, plus the kinetic and equilibrium relations among the liquid products. It is obviously highly complex. A detailed model is not necessary, however, to point towards the... 

If we fix, for example, the ethylene/propylene ratio, however, we fix severity and - as with conventional cracking - the other product yields from a given feedstock are now determined. Exceptions to this are minor dependencies - especially of acetylene - on the partial pressure variables. 

For each of the infinite number of enthalpy regimes, there is an optimum time corresponding to an economically optimum product mix. This optimum can be calculated either from actual rate data such as shown in Figures 6-8, or from a kinetic model of the process. It obviously will depend on the value of and demand for the various products. This ability to choose both enthalpy input and residence time gives us a degree of control not possible in conventional cracking furnaces. 

Figures 9 and 10 show yields of major products as functions of severity, here measured by the CH4/C3H0 molar ratio. Both time and temperature were varied to vary severity. For comparison we show yields by conventional cracking at about 1/1 by weight steam dilution, the yields shown were obtained by cracking a feedstock with a hydrogen/carbon atom ratio (H/C) of 1.89. This would correspond to a light mid-east crude oil, hydrogenated to reduce sulfur, or to a wide range distillate (naphtha through vacuum gas oil) from the same crude or to an atmospheric gas oil from one of these crudes. Since crude oils cannot be cracked in conventional plant furnaces, the data shown correspond to a high-quality atmospheric gas oil.

Acetylene becomes a much more important coproduct than it is in conventional cracking. It is indeed important enough to justify revival of acetylene-based processes now falling into disuse. We therefore tend to think in terms of ethylene plus acetylene yields. Figure 12 shows how this summary yield depends on feed-... 

Conventional cracking catalysts operate at a temperature of - 500-600°C. The difficult step is the generating carbenium ions from alkanes. This occurs through a reaction sequence that initially proceeds with formation of carbonium ions ... 

This process has no waste, complete atom economy, is nontoxic, occurs at a much lower temperature than conventional cracking, and is catalytic, fulfilling all the requirements... 

At about the same time that news of the Ziegler discovery was released, the Phillips Petroleum Company in the United States atmounced that it had developed a medium-pressure, catalytic process (500 psig) to produce a high-density, crystalline polyethylene. The process was discovered when traces of ethylene in a flue gas had polymerized over conventional cracking catalysts. The Phillips catalyst contained chromic oxide supported on silica. The Standard Oil Company of Indiana (later Amoco) also introduced a medium pressure process using a catalyst comprising molybdenum oxide supported on carbon or alumina, but it did not enjoy the success of the Ziegler or Phillips processes and was only operated in three full-scale plants. ... 

The cracking reaction may be described in a general way as a first-order reaction if the decomposition is limited as in conventional cracking operations to a low conversion per pass (20 to 25 per cent) ... 

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