Commercial Alkylation Processes

Commercial Alkylation Processes

The H2SO4 processes to be covered include: effluent refrigeration, cascade (or au-to refrigeration), and time-tank processes.

Effluent refrigeration

The effluent refrigeration process of Stratco Inc., (H2SO4 catalyst) is employed in many refineries throughout the world.

The Stratco contactor reactor is a horizontal vessel with an impeller in one end and a tube bundle (or cooling coils) in the other. The hydrocarbon-acid suspension in the contactor is circulated through the outer shell and then passed through the tube bundle to remove the exothermic heat of alkylation and the energy provided by the impeller. Both the hydrocarbon feed mixture of isobutane and olefins and the acid feed are introduced to the eye of the impeller. The feed streams rapidly form a two-phase suspension which is maintained by the rapid circulation. Stratco contactors are usually sized to produce 2,000 b/d of alkylate. Changes were made in the contactor in the last few years to obtain improved quality alkylate and lower operating costs. Longer coils were included to obtain substantially more heat transfer surface. Improved positioning of the coils has resulted in 50% increases of the overall heat transfer coefficients. The contactor includes an improved pump/agitator system. The injection devices for introducing the hydrocarbon feeds and the acid into the contactor have been improved. The pump/agitator has been positioned below the center line in order to minimize partial settling of acid in the bottom of the contactor. Part of the two-phase suspension is continuously removed and sent to the acid settler (or decanter). The decanter was previously operated at residence times of about 1.0 hr to obtain complete separation of the two phases. Some undesired reactions, including conjunct polymer production, however, occurred. The decanter is currently operated with much lower residence time for the acid. Some hydrocarbons are recycled with the acid phase, but acid consumption is reduced and alkylate quality improved. The hydrocarbon phase, which is often at a temperature of about 10°C or less, is passed through a pressure-reducing valve. As a result, some of the more volatile hydrocarbons, and particularly isobutane, are vaporized and the temperature of the resulting liquid is decreased, often to about 0°C. This cold liquid is used as the coolant in the tube bundle of the contactor. The gas stream formed by partial flashing of the effluent hydrocarbon stream is mainly isobutane plus small amounts of propane and C5-C7 isoparaffins. This mixture is compressed and condensed using cooling water as the coolant. Propane is then removed by fractionation in the depropanizer. The bottom product of this column is partially flashed to produce a cold liquid which is recycled as part of the isobutane feedstreams to the contactor. The vapors from the last flashing step are once again compressed, condensed, and recycled. isobutane is hence both a refrigerant and a reactant. The liquid stream remaining after the flashing of the effluent hydrocarbon stream contains mostly isobutane and alkylate. It also contains a small amount of hydrocarbon-soluble sulfates such as di-sec-butyl and di-isopropyl sulfates. Three methods are employed to remove these sulfates. The liquid stream is washed first with fresh acid and then with a dilute caustic solution. During the acid washing, the sulfates react with H2SO4 to form acid-soluble sec-butyl acid sulfate and isopropyl acid sulfate. Acid washing results in slightly improved yields of alkylates and somewhat reduced acid consumption. Caustic washing is needed to remove the residual acid and sulfates from the hydrocarbon liquids.

The following method is also sometimes employed to remove hydrocarbon-soluble sulfates. The liquid hydrocarbon mixture is first washed with a fairly concentrated caustic solution and then with water. During the caustic wash, Na2S04 and a low-quality alkylate are formed. It is not easy to get intimate contact with the caustic solution, and sulfate removal is sometimes incomplete.

Water washing is needed to remove residual caustic. This technique is generally less preferred. The following sulfate removal technique is often preferred. The hydrocarbon liquids are first acid washed and then contacted with a bed of bauxite powder. The bauxite effectively adsorbs residual acid, sulfates, and water. A dry feed minimizes corrosion. This procedure has somewhat increased operating costs, but it is sometimes cost effective.

After the sulfates are removed from the hydrocarbon liquids, the resulting mixture is fed to the deisobutanizer (DIB). In larger plants, the DIB is generally a distillation column (and hence has a reflux condenser). In smaller plants, a stripping column is often used. The top product of the DIB is mainly isobutane which is cooled and fed as a liquid to the contactor. The stream is premixed with the olefin feed before entering the contactor. A side am rich in n-butane is generally withdrawn from the DIB. Steam is the heat source of the DIB. Better separations occur in distillation columns.

Cascade autorefrigeration

Cascade-type reactors are large horizontal drums. Generally, three to seven reactor stages are provided in one end of the reactor, and an acid settler (or decanter) is provided in the other end. Each stage is partially filled with liquid (an acid-hydrocarbon suspension), and a gas mixture is above the liquid. This process is widely used, and additional reactors have recently been installed in at least two refineries.

The isobutane and acid feedstreams are introduced to the first stage. Equal amounts of olefin are, however, added to each stage. Unreacted isobutane, alkylate, and acid flow over baffles (or weirs) from one stage to the next and eventually reach the acid settler. The temperature in each stage is controlled by vaporizing part of the more volatile hydrocarbons (mainly isobutane plus minor amounts or propane and C5-C7 isoparaf-fins). The impellers of a cascade reactor are positioned at the throat of venturi-shaped casings that open into cylindrical hollow housings. Several vertical tubes are positioned in the upper portion of the housings. The impeller produces an emulsion that is acid continuous, and the emulsion is pumped upward. The olefin feed to the stage is added just above the impeller and mixed into the emulsion. The emulsion is rapidly recirculated in and around the housing. The upper portion of the emulsion, and especially that outside the housing, is generally hydrocarbon continuous. Some of it is transferred to the next stage.

In a cascade reactor of different design, a turbine impeller is used, and the olefin feed is jetted into the eye of the impeller. The emulsion in this reactor presumably remains acid-continuous throughout the stage. Nevertheless, the average residence time of the acid phase in a stage is always greater than that of the hydrocarbon phase because of the density difference.

In a cascade reactor, the isobutane-olefin ratio in a five-stage cascade reactor have a combined ratio of 8:1, Stage 1 is about 35:1 and Stage 5 is 12:1.

This apparent advantage of a cascade reactor as compared to a Stratco contactor tends to be negated by the fact that the olefin feed is not premixed with isobutane. The olefin feeds tend to contact the acid phase first when the emulsions are acid-continuous at the injection points. Consequently, undesired reactions often occur that result in conjunct polymer formation plus polymerization reactions. The following conclusions apply to a cascade reactor:

  • The highest quality alkyl-ate is produced in Stage 1 and the poorest in Stage 5 because of the difference in isobutane-to-olefin ratios in the stages.
  • More acid-soluble hydro- carbons are produced in latter stages of the reactor.
  • Alkylate produced in the initial stages degrades to at (its a small extent in the final stages of the reactor.
  • The sec-butyl and isopropyl sulfates produced in the last stage of a cascade reactor react to a slightly lesser extent as compared to similar sulfates produced in a Stratco contactor. This conclusion is based on the shorter residence time in the last stage as compared to that of a Stratco contactor.

The remainder of the alkylation process using cascade reactors has a flowsheet very similar to the one for the effluent refrigeration process.

Time-tank process

Several refineries still operate time-tank processes, but apparently none have been built in the past 25 years. The following features are, however, typical:

  • Mixtures of isobutene and olefins are contacted with H2S04 in a fairly large cooled pipe near the entrance of a centrifugal pump. The acid and hydrocarbon phases then enter the pump which provides mixing and emulsification. Temperature rises because the exothermic reactions are moderate due to fairly high ratios of acid-to-hydrocarbon phases.
  • The emulsion enters a heat exchanger cooled by a refrigerant where the heat of reaction is removed.
  • The emulsion then is introduced to the middle section of a large vertical column called the reactor. Reactions occur here, but many have occurred before the mixture entered the reactor.

This vessel also acts as a decanter. Acid removed from the bottom of the reactor is recycled to the centrifugal pump.

  • The hydrocarbon steam from the top of the reactor is processed as follows: First, acid and dissolved are removed, and second, the hydrocarbons are separated in multiple distillation columns to recover un-reacted isobutane, alkylate, propane, and n-butane. The isobutane stream is recycled to the time-tank reactor.

H2SO4 process comparison

Of the three processes employing H2SO4 as catalyst, the effluent refrigeration process and the cascade process are generally considered to be superior to the time-tank process. One refiner who uses the time-tank process claims to produce a better quality alkylate and to consume less acid as compared to the cascade process. Slightly poorer performance compared to the effluent refrigeration and cascade processes results from higher energy costs for the time-tank process. All unreacted isobutane must be separated from the alkylate by distillation, and because the refrigeration method uses more energy.

Unless major improvements are made in the design and operation of time-tank reactors, additional plants will likely not be built in the future.

One major company has made a direct comparison between its cascade and effluent refrigeration units. The company attempted to use identical feeds and operating conditions in making this comparison. Details of the specific reactors used and the operating conditions employed are not known, but better alkylate quality and less acid consumption occurred in the effluent refrigeration process.

A recently built effluent refrigeration process uses C4 olefins. The alkylates produced in a series of test runs had 96.4-97.3 RON and 93.6-94.2 MON. Acid consumption was less than 0.3 lb acid/gal alkylate.

Computer models have been developed by various companies for cascade and effluent refrigeration processes. The reactor used in cascade process had modified internals and resulted in acid consumption values of 0.2 lb H2S04/gal of alkylate. Recent Stratco pilot plant results confirm that such low acid consumption occurs at preferred operating conditions and with preferred olefins. In many cases, removal of undesired impurities from the olefins is cost-effective.

The size and energy demands of both the compressors and DIB differ substantially for the effluent refrigeration and cascade processes. The compressor in each process serves two purposes: compresses and recycles the isobutane that is flashed in order to maintain the desired reaction temperature, and compresses the isobutane that is flashed as refrigerant in order to cool the feed hydrocarbons to the desired reaction temperature.

The two processes were compared at conditions that are considered preferred in both cases. The following operating conditions were assumed for each process: reaction temperature, 43° F.; alkylate yield, 1.753 bbl of true alkylate/bbl olefin; 68% isobutane in reactor effluent; refrigerant condensation temperature, 100° F.; and compressor efficiency, 75%. The temperature difference for heat transfer in the contactor was set at 11.5° F.

In summary, the compressor demands for the effluent refrigeration process are larger, more isobutane is vaporized, and a larger pressure increase occurs in the compressor.

Steam requirements in the DIB column of the cascade process are significantly higher because more isobutane is separated. The combined energy costs for the compressor and DIB column are most identical for the effluent refrigeration and cascade processes.

Calculations indicate that contactors which result in smaller temperature differences for heat transfer result in smaller energy requirements for the compressor and larger ones for the DIB tower. Substantial energy reductions occur for the compressors of both processes if the isobutane is condensed at lower pressures (and hence at lower temperatures). Such condensation can be realized if the temperature of the cooling water is relatively low as sometimes occurs in winter. The ratio of isobutane to olefins used also affects the comparison.

Improved operations in a cascade reactor would be expected if the isobutane and olefins were premixed before being introduced into the first stage, the olefins were introduced into the hydrocarbon phase in the reactor or into the hydrocarbon-continuous emulsion sometimes present, and jetting the olefin feed into the emulsion in order to provide better mixing.

Methods of H2SO4 addition

When a large alkylation plant is built, two or more reactors are generally needed. The reactors are frequently arranged with the acid being fed in series flow from one reactor to the next. In such a case, the acidity of the acid decreases from one reactor to the next. In one example, fresh acid is continuously introduced in the first of three cascade reactors, and used acid is continuously withdrawn from the third reactor. RONs for the alkylates a 95.4, 96.6, and 96.0 for Reactors 1, 2, and 3, respectively. With more than five reactors, combinations of series and parallel flows of acid are sometimes preferred. The objective is to employ acids with preferred compositions in as many reactors as possible.

For a plant using a single reactor, two methods can be used for adding fresh acid (and for removing the used acid). The new acid can be added continuously (and the old acid can be removed continuously).

Alternatively, a given batch of acid can be recycled until the acidity has dropped to a given end point and then most of the acid is replaced. The latter technique usually produces a better average quality alkylate.

Reference: L. F. Albright: Alkylation will be key process in reformulated gasoline era, Oil & Gas Journal, 77, 79-92, 1990.