Recycle isobutane alkylation

The process layout consists of two consecutive static mixers (Fig. 28). To the first mixer, the olefin feedstock is cofed with a recycled isobutane/alkylate stream. The stream coming out the first static mixer is then combined with the recycled IL-based composite catalyst and fed into the second static mixer where the alkylation reaction takes place at a reaction temperature around 15°C and a total pressure of 0.4 MPa. The reaction products are then sent to a settler where the composite catalyst is collected from the bottom, due to its higher density, and recycled. The supernatant is later split into a recycle (isobutane -I- alkylate) to the first static mixer upstream and a product effluent, which constitutes the incoming to the fractionation unit downstream. Total reaction time, considering residence times in the second static mixer and in the settler, is 10 min while the overall I/O ratio in the reactor is set to a value as high as 500. No details on catalyst regeneration or replacement have been disclosed (257). 

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. 

The fractionation section of the alkylation plant consists of a deisobutanizer, a debutanizer, and a rerun tower in series, and a depropanizer. The deisobutanizer overhead, which contains about 90% isobutane, recycles to the reactor. The deisobutanizer bottoms stream passes to the debutanizer, which removes normal butane diluent as an overhead stream. The debutanizer bottoms or raw alkylate stream then goes to a rerun tower for removal of the high boiling alkylate bottoms or trimers. The rerun overhead requires no further treatment to be satisfactory as an aviation gasoline blending stock. The depropanizer removes propane diluent from a slip-stream portion of the recycle isobutane stream to prevent propane build-up in the reaction system. 

Because of the relatively high normal butane tolerance in HF units, all of the recycle isobutane can be made from the effluent stream with little rectification. The purity of the recycle under these conditions is 75 to 85 % isobutane. Even though such a relatively impure external isobutane recycle can be used without appreciably lowering the quality of the alkylate, an increase in the internal isobutane-to-olefin ratio does not improve the quality as is the case with sulfuric acid catalyst. When the external isobutane recycle is charged to the first of a series of reaction zones and the olefinic feed is divided between the individual reaction zones, there is no apparent improvement in quality. This indicates that any increase in the isobutane-to-olefin ratio over and above the external ratio which may be... 

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. 

Case B - Maximize Profit, P and Minimize Isobutane Recycle, X2 Alkylate product with a higher octane number is better for blending with refinery products. Minimizing isobutane recycle helps to reduce fractionation and other costs associated with the recycle stream. 

A simplified flow diagram of a modern H2SO4 alkylation unit is shown in Eigure 1. Excess isobutane is suppHed as recycle to the reactor section to suppress polymerization and other undesirable side reactions. The isobutane is suppHed both by fractionation and by the return of flashed reactor effluent from the refrigeration cycle. 

Propane and light ends are rejected by touting a portion of the compressor discharge to the depropanizer column. The reactor effluent is treated prior to debutanization to remove residual esters by means of acid and alkaline water washes. The deisobutanizer is designed to provide a high purity isobutane stream for recycle to the reactor, a sidecut normal butane stream, and a low vapor pressure alkylate product. 

After reaction at 200 - 250 F and 350 psig the reactor effluent is stripped to remove recycle HCl. The stripper bottoms is cooled and caustic washed to remove remaining traces of HCl. The product can then be sent to the alkylation plant for fractionation or a tower provided in the isomerization plant for fractionation of isobutane and recycle of unconverted normal butane back to isomerization. 

The primary process variables affecting the economics of sulfuric acid alkylation are the reaction temperature, isobutane recycle rate, reactor space velocity, and spent acid strength. To control fresh acid makeup, spent acid could be monitored by continuously measuring its density, the flow rate, and its temperature. This can reduce the acid usage in alkyla-tion units. 

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... 

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... 

Impurities such as diolefins and mercaptans in the feed to the reactor affect adversely the quality of the alkylate produced. This is considered to be more critical in sulfuric acid than in HF catalyst units. Also, certain sulfuric unit designs seem to tolerate more normal butane than others. Whereas the isobutane content of the recycle to a hydrofluoric acid unit can operate at purities of around 75% without affecting product qualities appreciably, the isobutane content of the recycle stream to most sulfuric acid units should be 90 % or better. 

Both the contact efficiency of isobutane and olefin in the zones and the mixing efficiency of acid and hydrocarbon in these reactors are very important. The flow must be such that no isobutane recycle will bypass the reaction zone and no hydrocarbon will bypass the acid-hydrocarbon mixing zone. With this type of unit, a mixer driven by a 40-hp. motor is usually used in each of the zones. A reactor containing five such mixers can normally make 1500 bbl. per day of alkylate. The seven-zone reactors can make 3400 bbl. per day of alkylate. 

High isobutane recycle purity is not required on HF alkylation units as is required on many H2SO4 units because relatively high normal butane concentrations in the reaction zone do not appreciably affect the quality of the alkylate. Isobutane purities below 60% are usually avoided, however, since this purity definitely gives lower-quality alkylate and the cost of recycling the normal butane is considerable in heat requirements as well as fractionation equipment requirements. 

The first diagram of Figure 9.5 shows that the flow rate of the reactor-outlet stream is almost insensitive to variations of the recycle rate Fj. A similar picture is obtained when variations of the feed rate F0 are considered (not shown). The second diagram shows that, as expected, increasing the excess of isobutane by larger recycle has a beneficial effect on selectivity, and more alkylate is obtained. 

In the alkylation process, the main reaction involves the olefin and isobutane. In contrast, the secondary reactions consist of olefin polymerization or the reaction between the olefin and C8 paraffins. For this reason, a high concentration of isobutane in the reactor is necessary. In our design the isobutane olefin ratio is 7.3 1, while typical values are in the range 5 1 to 10 1 [7]. Note that the reactants are fed to the process in a nearly stoichiometric proportion, the high excess of isobutane being accomplished by recycling. 

---