Isobutane cracking

Poisoning studies carried out by several groups have shown that the equivalents of poison needed to quench the catalytic activity of the dealuminated Y-type zeolites are much less than the number of Alf atoms. Beyerlein et al. (9) reported that residual sodium cations extensively decreased the isobutane cracking activity of steam-dealuminated Y-type zeolites. From their results it was concluded that only one-third of the Alf atoms were associated with strong acidity throughout the Si/Al > 5 composition domain. 

D. L. Trimm in Fundamental Aspects of the Formation and Gasification of Coke in L. F. Albright, B. L. Crynes, W. H. Corcoran (eds.), Pyrolysis Theory and Industrial Practice , Academic Press, New York, 1983 L. F. Albright, B. L. Crynes, W. H. Corcoran (eds.), Pyrolysis Theory and Industrial Practice , Academic Press, New York, 1983 T. J. Ford, Ind. Eng. Chem. Fundam., 25, 240, 1986 Coastal Isobutane Cracking Process developed by Foster Wheeler P. B. Venuto and E. T. Habib, Fluid Catalytic Cracking with Zeolite Catalysts , Marcel Dekker, New York, 1979... 

Thus, the polymer molecule is made up of two (or more) molecules of propylene plus one molecule of isobutane. Cracking or scission of C q... 

Using excess amount of porous silica grains as the Si-source in ZSM-5 synthesis, crystals were grown inside the pores of amorphous silica. The ZSM-5 crystals in the mesopores were very small (0 02-0.035 pm) those in the macropores were somewhat larger (0.5-2 pm), whereas the ones on the outside surface of the particles were between 3 and 5 pm. The composite catalyst was more active in isobutane cracking than conventional H-ZSM-5 [67]. 

The primary products produced in isobutane cracking are propylene and isobutylene. Propane, ethylene, and ethane and methane are produced in subsequent secondary reactions as well as significant amounts of liquid products (dripolenes). 

More reliable are atmospheric pressure pyrolysis data of Marek and Neuhaus (36), who did account for liquid products. Marek and Neuhaus data show no significant selectivity differences between 1112°F (+) and 1202°F (V) (see Figures 3,4, and 5). Hepp (25) has patented an isobutane cracking process operating at 12Q0°-1350°F, 130-150 psig. 

Consider the case of 50% isobutane conversion. This is a magic number, for which product from an isobutane cracker is equimolar in olefins and isobutane (except for a minor total selectivity correction). This mixture could be fed directly to an alkylation unit. Total propylene plus isobutylene weight percent selectivity at atmospheric pressure is 65 wt %. The price spread of isobutane at approximately 1.89/bbl to alkylate value olefins at 4.62/bbl or better (these prices are equivalent to 2.20 cents/lb isobutylene and 2.53 cents/lb propylene) suggests that isobutane cracking may be significant in the near future by alleviating olefin demands for alkylate, in effect, releasing more propylene for chemical consumption. 

More to the point of this study is that propylene selectivity at atmospheric pressure is almost constant at approximately 38 mole % across the entire isobutane conversion range (Figure 4). Isobutane cracking could be used to produce chemical propylene directly, and the justification to do this instead of producing propylene for alkylate is only a function of price. The price that propylene would have to command to justify its chemical sale, rather than alkylate use, is basically its gasoline value, which is the 3.0 cents/lb figure developed earlier in this paper. 

It is recommended that refiners scrutinize isobutane cracking economics to see if chemical propylene manufacture by this route is justified for their particular situation. In all probability, some significant portion of chemical propylene needs can and will be met by special situation isobutane cracking. 

The author thanks the Esso Research and Engineering Corp. for their support of propane dehydrogenation experimentation at Worcester Polytechnic Institute. The isobutane cracking study was funded by Chem Systems, Inc. the assistance and expertise obtained from Bronek Dutkiewicz, Bert Struth, and Martin Sherwin of Chem Systems and from John Johns on technical and economic points is deeply appreciated. 

Calculated concentration of radicals, molecules, and conversion along an isothermal reactor for isobutane cracking T = 775°C Po=lA atm abs 8 = 0.4 kg of steam/kg of butane. From Sundaram and Froment [1978]. 

The models developed here account for unmeasurable intermediates such as adsorbed ions or free radicals. Microkinetic analysis, pioneered by Dumesic and cowokers"", is an example of this approach. It quantifies catalytic reactions in terms of the kinetics of elementary surface reactions. This is done by estimating the gas-phase rate constants from transition state theory and adjusting these constants for surface reactions. For instance, isobutane cracking over zeolite Y-based FCC catalysts has 21 reversible steps defined by 60 kinetic parameters." The rate constants are estimated from transition state theory... 

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