Columns isobutane alkylation

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. 

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. 

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. 

It may be assumed that the recycle is pure isobutane and that propane, alkylate, and n-butane are completely recovered as pure products in the columns. Propane and n-butane do not react. 

Alkylation of Isobutane with propylene or 2-methylbutene-2 resulted In products containing about 20% of Isooctanes (Table III, columns 2 and 6). Similarly to the sulfuric acid alkylation, C0 paraffins formation may be explained by self-alkylation of Isobutane. 

Dimerization presumably takes place on the transition metal-containing sites, and alkylation on the acidic sites of zeolltic surface. The sodium form of zeolite exchanged with transition metal cations Is capable of dimerization (and further polymerization), but does not practically exhibit alkylating capacity. This explains the composition of the product obtained from ethylene and Isobutane over this catalyst (Table V, column 3). 

Ca cation Introduction leads to the catalyst containing both kinds of catalytic sites, and the ethylene-isobutane Interaction over this catalyst proceeds through dimerization to the alkylation step, yielding high quality alkylate (Table V, column 4). The alkylate yield was 120X. The reaction does not occur unless transition metal cations are present In the catalyst even though the latter may contain acidic sites (CaY for example). It Is understood that no butenes are formed In this case, and ethylene does not Interact directly with Isobutane under the conditions of this experiment. 

The alkylation reaction of isobutane with a mixture of C4 linear olefins was carried out in liquid phase at temperatures between 25 and 80°C, and at 30 Kg/cm, in a fixed-bed reactor. The space velocity was WHSV = 1 ft referred to the olefins. The isobutane is premixed with the olefins. The molar ratio used in this study was 15. The C4 olefins fraction contains 38% 1-butene, 22% trans-butene, 14% cis-2-butene and 26 % isobutene. In order to analyze the products coming out of the reactor, a ten-loop valve was used to collect the sample to be analyzed after the mn. Products are analyzed by GC, using a 100 m squalane column. Prior to the reaction, catalysts were pretreated in-situ, heating up to 250 C in an air stream. 

Alkylation of isobutane with 1-butene was carried out in a fixed bed down flow stainless steel tubular microreactor. The experiments were carried out in the gas phase at 1 atm total pressure, isobutane/l-butene molar ratio of 14 and 1-butene space velocity 1.0 h". Premixed isobutane and 1-butene (>99% purity, Matheson) was fed from a gas cylinder. A high paraffin to olefin ratio was choosen to reduce the chance of olefin dimerization. The catalysts (300 mg, 60-80 pm particle size) were activated in the reactor by calcining in air at 450°C for 4 hours. Air flow was then replaced by nitrogen and the catalyst temperature was lowered to the desired reaction temperature. The feed gas and the products were analyzed by a on-line gas chromatograph equipped with a CP-Sil PONA capillary column (length 50 m, film thickness 0.25 pm). 

Reaction pressure was maintained with a dome-loaded back-pressure regulator (Circle Seal Controls). All heated zones were controlled and monitored with a Camile 2500 data acquisition system (Camile Products). Products were analyzed online by gas chromatography with an HP 5890 II GC, equipped with an FID, and a DB-Petro 100 m column (J W Scientific), operated at 35° C for 30 min, ramped at 1.5°/min to 100° C, 5°/min to 250° C for 15 min. An alkylate reference standard (Supelco) allowed identification of the trimethylpentanes (TMP) and dimethylhexanes (DMH). The combined mass of TMP and DMH is referred to hereafter as the alkylate product . As discussed elsewhere [19], propane, an impurity in the isobutane feed, was used as an internal standard for butene conversion calculations. Since isomerization from 1-butene to 2-butene isomers is rapid over acidic catalysts, reported conversion is for all butene isomers to C5 and higher products. Isobutylene formation was not observed under any conditions. 

It is also possible to suppress undesirable chemical reactions, such as in the alkylation of isobutane by butene in the manufacture of isooctane. In the presence of butene, isooctane can undergo further alkylation, thus reducing the selectivity. Use of distillative reaction removes isooctane continuously from the column, thus enhancing the selectivity. 

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