Separation isobutane alkylation

The general treatment of the hydrocarbon stream leaving the alkylation reactor is similar in all processes. First, the acid and hydrocarbon phases have to be separated in a settler. The hydrocarbon stream is fractionated in one or more columns to separate the alkylate from recycle isobutane as well as from propane, n-butane, and (sometimes) isopentane. Because HF processes operate at higher isobutane/alkene ratios than H2S04 processes, they require larger separation units. All hydrocarbon streams have to be treated to remove impurity acids and esters. 

Products are fractionated in the deisobutanizer and debutanizer to separate isobutane and butane from alkylate HF-produced alkylate is fractionated in a similar manner... 

The emulsion leaving the reactor enters a settler. Residence times there often average up to 60 min to permit separation of the two liquid phases. Most of the acid phase is recycled to the reactor, being injected near the eye of the impeller. The hydrocarbon phase collects at the top of the decanter it contains unreacted isobutane, alkylate mixture, often some light n-paraffins, plus small amounts of di-isoalkyl sulfates. The sulfates must be removed to prevent corrosion problems in the distillation columns. Caustic washes are often employed to separate the sulfates they result in destruction of the sulfates. Acid washes have the advantage that most of the sulfates eventually react to reform sulfuric acid, which is reused, and to produce additional alkylate product. 

Two to four distillation columns are usually required to separate the liquid hydrocarbon product stream that contains unreacted isobutane, alkylate mixture, n-butane, and propane. The major column is designated as the deisobutanizer (DIB) column. Often this column separates the isobutane as the overhead stream, the alkylate as the bottom stream, and a n-butane rich sidestream. In many plants, the feed isobutane is also fed to the DIB to remove most of the n-butane. A second column is generally needed to remove propane from the isobutane. Sometimes a third column is provided to purify further the n-butane sidestream and to recover more isobutane. In an alternate arrangement, the bottom stream of the DIB column is a mixture of alkylate and n-butane. This mixture is then separated in another column. 

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). 

At the heart of the UOP HF alkylation unit is a vertical reactor-heat exchanger, shown in Fig. 14. The isobutane-alkene mixture enters the shell of the reactor through several nozzles, and HF enters at the bottom of the reactor. The reaction heat is removed by cooling water, which flows through cooling coils inside the reactor. After phase separation in the settler, the acid is recycled to the reactor. The hydrocarbon phase together with a slipstream of used acid and makeup isobutane is sent to the isostripper , where the alkylate product, n-butane, and isobutane are separated. The isobutane is recycled to the reactor. During normal... 

Besides ethylene and propylene, the steam cracking of naphtha and catalytic cracking in the refinery produce appreciable amounts of C4 compounds. This C4 stream includes butane, isobutane, 1-butene (butylene), cis- and trans-2-hutene, isobutene (isobutylene), and butadiene. The C4 hydrocarbons can be used to alkylate gasoline. Of these, only butadiene and isobutylene appear in the top 50 chemicals as separate pure chemicals. The other C4 hydrocarbons have specific uses but are not as important as butadiene and isobutylene. A typical composition of a C4 stream from steam cracking of naphtha is given in Table 8.3. 

The mixture leaving the reaction zone is in the form of a hydrocarbon-acid emulsion and passes to an acid settler for separation of acid and hydrocarbon phases. This acid settler is usually a separate vessel from the reactor itself, although it is an integral part of one type of system. The hydrocarbon-free acid from the acid settler recirculates to the reactor. The hydrocarbon layer, which consists of alkylate, excess isobutane, and the inert diluents introduced with the feed, receives a caustic treatment and goes to the fractionating section of the plant. Caustic treatment is necessary at this stage of the process to neutralize acidic components, such as sulfur dioxide, which are formed in small quantities by catalyst degeneration. 

Although not a separate process, isomerization plays an important role in pretreatment of the alkene feed in isoalkane-alkene alkylation to improve performance and alkylate quality.269-273 The FCC C4 alkene cut (used in alkylation with isobutane) is usually hydrogenated to transform 1,3-butadiene to butylenes since it causes increased acid consumption. An additional benefit is brought about by concurrent 1-butene to 2-butene hydroisomerization. Since 2-butenes are the ideal feedstock in HF alkylation, an optimum isomerization conversion of 70-80% is recommended.273... 

One possible arrangement for a hydrofluoric acid alkylation unit is shown schematically in Fig. 1. Feedstocks are pretreated, mainly to remove sulfur compounds. The hydrocarbons and acid are intimately contacted in the reactor to form an emulsion, within which the reaction occurs. The reaction is exothermic and temperature must be controlled by cooling water. After reaction, the emulsion is allowed to separate in a settler, the hydrocarbon phase rising to the top. The acid phase is recycled. Hydrocarbons from the settler pass to a fractionator which produces an overhead stream rich in isobutane. The isobutane is recycled to the reactor. The alkylate is the bottom product of tile fraetionater (isostripper). If the olefin teed contains propylene and propane, some of the isoshipper overhead goes to a depropanizer where propane is separated as an overhead... 

As can be seen from Table I, only the Airlift catalytic cracking unit has sufficient isobutane to react with all of the butylenes. The isobutane requirement for alkylation is about 1.15 vol. for every volume of butylene, as indicated in Table II. Besides this quantity required for the reaction, there is also an additional quantity of isobutane which is lost from the alkylation fractionation section. The latter can be from 2 to 25% of the normal butane leaving the unit, depending upon the separation efficiency in the deisobutanizer. 

The trimethylpentanes are easily produced by alkylating isobutane with isobutylene, but unfortunately, the content of isobutylene produced by catalytic cracking is only about one-third of the total butylenes in the C4 stream, the remaining butylenes being butylene-1 and butylene-2. Although most of the butylene-2 tends to form trimethylpentanes, the butylene-1 must be isomerized to butylene-2, either in the alkylation reaction or in a separate previous reaction, before it will form trimethylpentane. If not isomerized, the butylene-1 when alkylated forms the much lower-octane material, dimethylhexane. 

Chapter 9 Isobutane/Butene Alkylation illustrates in detail the integration of design and plantwide control. Special attention is paid to the reaction/separation/... 

As shown in Fig. 18.23, dry liquid feed containing olefins and isobutane is charged to a combined reactor-settler. In this example, the reactor uses the principle of a differential gravity head to circulate through a cooler before contacting a highly dispersed hydrocarbon feed in the reactor pipe. The hydrocarbon phase, generated in the settler, is sent to a fractionator, which separates LPG-quality propane, isobutane recycle, n-butane, and alkylate products. A small amount of dissolved catalyst is also removed from the propane product by a small stripper tower. 

HF is intimately contacted with isobutane and mixed with light olefins (ethylenes, propenes, etc.) under pressure at 40-45°C to produce branch chain fuel which has very high octane value. HF, being only slightly soluble in hydrocarbons, is easily separated, recycled and regenerated. The alkylate is water washed and dried. The consumption of hydrofluoric acid per barrel of alkylate varies from 0.09 to 0.23 kg. 

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