Alkylation Plants

In conventional alkylation operations, 98 wt. %, sulfuric acid is used as the catalyst, although some processes use HF.The spent alkylation acid, withdrawn as 88-92% acid, is not consumed in the chemical sense, but is diluted by carbonaceous material and small amounts of water. Acid reconditioning is usually completed in a separate plant. The range in makeup acid requirement and in octane quality varies with plant design, with type of feedstock, and with alkylate product rate. A wide variety of feedstocks can be processed through alkylation plants, as both low and high boiling olefins can be alkylated. 

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. 

Washing light hydrocarbons with water is a common refinery practice. It finds application on the feed to catalytic polymerization plants. It is used to remove any entrained caustic from the mercaptan removal facilities as well as any other impurities such as amines which tend to poison the polymerization catalyst. Another use for water wash is in alkylation plants to remove salts from streams, where heating would tend to deposit them out and plug up heat exchanger surfaces. Water washing can be carried out in a mixer- settler, or in a tower if more intimate contacting is necessary. 

ConvEx A process for converting an HF-catalyzed alkylation plant to one using sulfuric acid. Developed by Stratco. 

A company has two alkylate plants, Ax and A2, from which a given product is distributed to customers Cv C2, and C3. The transportation costs are given as follows ... 

Isobutylene is the most chemically reactive of the butylene isopiers. If the objective is just to get the isobutylene out of the C4 stream, it can be removed by reaction with methanol (CH3OH) to make MTBE (methyl tertiary butyl ether), by reaction with water to make TBA (tertiary butyl alcohol), by polymerization, or by solvent extraction. After that, butene-1 can be removed by selective adsorption or by distillation. That leaves the butene-2 components, together with iso- and normal butane, which are generally used as feed to an alkylation plant. 

The isobutylene in the C4 stream generally ends up in one of four places a refinery alkylation plant (covered further below), an MTBE plant, a polymerization process, or in gasoline. The first three are methods of removing isobutylene from the C4 stream the fourth is the default—the isobutylene just follows the other butanes and butylenes to the gasoline blending pool. 

In the chapter on olefms plants, in the section on propylene, a route to making propylene involved butene-2. In this process, called metathesis, ethylene and butene-1 are passed over a catalyst, and the atoms do a musical chair routine. When the music stops, the result is propylene. The conversion of ethylene to propylene is an attraction when the growth rate of ethylene demand is not keeping up with propylene. Then the olefins plants produce an unbalanced product slate, and producers wish they had an on-purpose propylene scheme instead of just a coproduct process. The ethylene/butene-2 metathesis process is attractive as long as the supply of butylenes holds out. Refineries are big consumers of these olefins in their alkylation plants, and so metathesis process has, in effect, to buy butylene stream away from the gasoline blending pool. 

The butanes are used as gasoline-blending components. Normal butane is sometimes an olefins plant feed. Isobutane is used in refinery alkylation plants with propylene or butylene to make alkylate, a high-octane gasolineblending component. 

The C4 s can be recovered in numerous ways to malce petrochemical feeds. Sometimes everything but the butadiene ends up in an alkylation plant that makes gasoline blend stock. 

If they re in the chemical business and the price of butylenes is sufficiently lower than butadiene, which it often is, it makes sense to build a dehydrogenation plant to make butadiene from butylenes. But if they are a petroleum refiner and have no petrochemical business, then they want to get the butadiene out of the C4 stream before it goes to the alkylation plant. Butadiene will cause the process.to foam up, lowering conversion and making a general mess. 

Safety in the Operation of Hydrogen Fluoride Alkylation Plants. Universal Oil Products Co., Booklet No. 252. 

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. 

Commercial operation of sulfuric acid alkylation plants has been relatively trouble-free and shutdowns for reasons other than normal inspection and maintenance are seldom required. Many plants operate on a one-year regular shutdown and high on-stream efficiencies are obtained. In spite of the fact that a strong acid is used as catalyst, corro-... 

Figure 2 presents a schematic flow diagram for a typical hydrofluoric acid alkylation plant. Since processing with hydrofluoric acid is very similar to processing with sulfuric acid, this discussion will point out only the significant differences in the two processes. 

Divinylbenzene. This is a specialty monomer used primarily to make cross-linked polystyrene resins. The largest use of divinylbenzene (DVB) is in ion-exchange resins for domestic and industrial water softening, Ion-exchange resins are also used as solid acid catalysts for certain reactions, such as esterification. Divinylbenzene is manufactured by dehydrogenation of diethylbenzene, which is an internal product in the alkylation plant for ethylbenzene production,... 

An important part of this chapter will be devoted to robustness. The design will ensure that feasibility is preserved when operation and design parameters change or are uncertain. Thus, we will take into account the fact that alkylation plants are often built as onstream units, being required to deal with large changes of the butene feed stream. In addition, we will be aware of large errors of the kinetic data. 

In alkylation plants, the large volumes of LPG that are stored should be taken into account by the training procedures. A key element of managing the hazard is preventing the release. Additionally, the facilities should be protected against the impact of fire. 

In this study, the acid concentration was not explicitly taken into account. At concentrations below 85%, olefins react preferentially with olefins, to produce polymers. Due to their unsaturated nature, these polymers are soluble in sulfuric acid, which leads to a further concentration decrease. Additionally, polymers are oxidized, while sulfuric acid is reduced to water and sulfur dioxide. These positive-feedback effects are known as acid runaway [17]. Acid runaway is a major concern in alkylation plants, as the acid strength can no longer be controlled and the olefin feed must be cut out. 

Heavy gas-oil is fed to an FCC unit, where the primary product is gasoline. The light olefin containing the three- and four-carbon molecule cut is fed to an alkylation plant. 

One way to reduce the amount of magnesium chloride is to use a caustic injection after the desalter. The injection amount is guided by the amount of salt in the overhead condensate. Spent caustic washes can be used if they do not contain contaminants that cause increased corrosion or plugging. For example, a spent alkylation plant caustic would add sulfur dioxide, which can react with hydrogen sulfide to cause sulfur plugging. The following steps are required to avoid problems with a caustic injection system ... 

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