Refinery isobutane concentration

Isobutane concentration is generally expressed in terms of the isobutane-to-olefin ratio (I/O). This ratio is the most important process variable to control in terms of refinery alkylation productivity, yield, and quality of alkylate, as well as the add... 

The process variables temperature, acid strength, isobutane concentration, and mixing have to be carefully optimized in refinery alkylation to obtain high fuel quality. The optimum parameters differ for the H2SO4- and HF-catalyzed processes. 

The uses of propylene may be loosely categorized as refinery or chemical purpose. In the refinery, propylene occurs in varying concentrations in fuel-gas streams. As a refinery feedstock, propylene is alkylated by isobutane or dimerized to produce polymer gasoHne for gasoHne blending. Commercial chemical derivatives include polypropylene, acrylonitrile, propylene oxide, isopropyl alcohol, and others. In 1992, ca 64% of U.S. propylene suppHes were consumed in the production of chemicals (74). Polypropylene has been the largest consumer of propylene since the early 1970s and is likely to dominate propylene utilization for some time. 

Isomerization. Isomerization of any of the butylene isomers to increase supply of another isomer is not practiced commercially. However, their isomerization has been studied extensively because formation and isomerization accompany many refinery processes maximization of 2-butene content maximizes octane number when isobutane is alkylated with butene streams using HF as catalyst and isomerization of high concentrations of 1-butene to 2-butene in mixtures with isobutylene could simplify subsequent separations (22). One plant (Phillips) is now being operated for this latter purpose (23,24). The general topic of isomerization has been covered in detail (25—27). Isomer distribution at thermodynamic equiUbrium in the range 300—1000 Kis summarized in Table 4 (25). 

A typical feed to a commercial process is a refinery stream or a steam cracker B—B stream (a stream from which butadiene has been removed by extraction and isobutylene by chemical reaction). The B—B stream is a mixture of 1-butene, 2-butene, butane, and isobutane. This feed is extracted with 75—85% sulfuric acid at 35—50°C to yield butyl hydrogen sulfate. This ester is diluted with water and stripped with steam to yield the alcohol. Both 1-butene and 2-butene give j -butyl alcohol. The sulfuric acid is generally concentrated and recycled (109) (see Butyl alcohols). 

Natural gas butane is usually dominated by the straight chain or normal isomer. There is some isobutane present and in some fields, the concentration can be high, but rarely the dominant isomer. However, refinery catalytic cracking produces butane streams where the main isomer is branched, namely isobutane. The concentration is often 90% of the stream. 

With propylene, higher temperatures and more concentrated sulfuric acid are needed to promote the desired reactions with isobutane (and the formation of alkylate). In many refineries, rather substantial fractions of the isopropyl sulfates do not react. The sulfates that do not react decrease acidity and increase acid consumption. The diisopropyl sulfates, which dissolve in the hydrocarbon phase, increase the need for either caustic or acid washing. Improved operating conditions have recently been developed to promote more complete reactions of these sulfates in the reactor (and hence higher yields of alkylate) plus lower acid consumption (26). 

Isobutane is almost insoluble in the liquid acid catalysts used in refinery alkylation. Therefore, intense mixing is necessary to make sure that at least this low isobutene equilibrium concentration is maintained throughout the readion in the acid phase to allow the reaction to proceed properly. As the alkylation readion occurs at or near... 

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