Solid-acid alkylation process development is at crucial stage

Pradip Rao
Consultant
San Jose, Calif.

Sorab R. Vatcha
Consultant
Mountain View, Calif.

The refining industry is seeking more environmentally acceptable and economical methods of producing reformulated gasoline (RFG).

Alkylate, the cleanest gasoline-blending stream produced in a refinery, is a prime blend stock for RFG production.

Alkylation with solid acid catalysts has potential environmental and safety advantages over conventional liquid-acid alkylation. This is especially true for the HF alkylation process.

Industry and academia have been researching and developing solid-acid catalysts for more than 25 years. Several processes have reached the pilot-plant stage, but none has been commercialized.

Difficult technical challenges must be overcome in the next few years to achieve a commercially successful solid-acid alkylation process. Substantial innovations in catalysts, catalyst regeneration, reactor design, and product separation will be required for solid catalyst processes to replace the incumbent processes, which are being improved continually.

U.S. alkylate production is expected to grow rapidly through the year 2000, then level out. If an economically and environmentally superior solid-catalyst alkylation process is not commercially available soon, the window of opportunity will close. As a result, new alkylation units will use sulfuric acid catalysts, and existing HF units will continue to operate or be retrofitted to use H₂SO₄.

Background

Alkylate constitutes about 13 vol % of the U.S. gasoline pool. It has a high octane number (typically 90-94 RON), low vapor pressure, and low aromatics and olefins contents. These properties make alkylate a valuable ingredient of the reformulated gasolines required by the 1990 Clean Air Act Amendments and the California Air Resources Board.

Alkylation units using HF and H₂SO₄ catalysts have been operated commercially since the 1940s, but growing concerns about their safety and environmental hazards are restricting their use.¹

At least four of the new solid-acid processes under development have reached the pilot-plant stage, but many technical barriers to commercialization remain. These barriers must be overcome in the next few years if a new solid-catalyst alkylation process is to be commercialized successfully.

A brief review of the commercial alkylation processes, including a discussion of the technical and commercial aspects of solid-catalyst alkylation, reveals the obstacles that must be overcome in order to successfully commercialize a solid-catalyst alkylation process. From these obstacles, the authors have devised a set of research and development (R&D) guidelines for achieving commercialization.

HF alkylation

About 60% of worldwide alkylate capacity is based on HF. Of the 115 HF alkylation units operating worldwide, about 60 are in the U.S. Of the 300,000 tons/year of HF produced in North America, 5% (15,000 tons/year) is used as alkylation catalyst.²

The main disadvantage of HF is that a release of HF (a gas at ambient conditions) creates a ground-hugging toxic vapor cloud that can be carried by the wind and injure many people in the plant and surrounding communities.

Proposed legislation in California (Assembly Bill 3276) would ban large-scale storage of HF and force HF users to switch to a less-hazardous substance.^{3 4}

Measures to improve the safety of HF include:

• Isolation valves, physical containment, and water sprays to mitigate the impact of an HF release.²

• Additives that reduce the volatility and cloud-forming tendency of HF by 60-90% (such additives have been developed and used by UOP and Texaco Inc., and by Mobil Corp. and Phillips Petroleum Co. ^{2 5-8}

• On site HF recovery and regeneration systems that reduce the hazards of HF transportation and handling, such as AlliedSignal Inc.'s Aqua tech separation technology.⁹

Retrofitting existing HF units to use H₂SO₄ is costly. The cost of converting the HF alkylation unit at Mobil's Torrance, Calif., refinery to use modified HF, for example, would cost an estimated $10 million, compared to $100 million to revamp the refinery to use H₂SO₄.⁷

Refiners that currently use HF probably will be permitted to continue using HF, thus avoiding the need to shut down their alkylation units or convert them to another acid catalyst at great expense. No new HF alkylation units, however, are likely to be constructed.

H₂SO₄ alkylation

Sulfuric acid is less hazardous than HF, but its consumption per unit of alkylate produced is greater. For a 10,000 b/d alkylate plant, the H₂SO₄ traffic is typically 100 tons/day. The acid consumption increases further with the C₃ and C₅ olefin content of the feed.

Sulfuric acid alkylation processes usually operate at about 10° C. and therefore require refrigeration. The HF processes operate at about 38° C. and use cooling water for temperature control. Cooling to subambient temperatures consumes more energy and involves higher cost per unit of delta-T than does heating.

Several companies, including Monsanto Corp. and Rhone-Poulenc, are developing on site facilities for recovering and regenerating spent H₂SO₄.⁹ Betz Process Chemicals and R. E. Davis Chemical Corp. have reported developing an additive that reduces H₂SO₄ consumption and increases alkylate yield and quality.¹⁰

Sulfuric acid has the highest production rate of all chemicals. About 47.7 million tons of sulfuric acid were produced in the U.S. in 1995.¹¹

Sulfuric acid is used in several major industrial applications. Alkylation consumes only about 3% of the total H₂SO₄ production, and H₂SO₄ alkylation plants are less strictly regulated than HF plants.¹² For this reason, transportation and use of sulfuric acid is not likely to be restricted.

Solid-acid catalysts

At least four solid-acid catalyzed processes for isobutane-olefin alkylation have been developed to the pilot plant stage (Table 1 [37534 bytes]). Other processes are in the R&D stage (Table 2 [64091 bytes]).

A few aromatic alkylation processes based on solid-acid catalysts are commercial or available for license, but isobutane-olefin alkylation has different requirements. In particular, the latter requires a catalyst of greater acid strength.

Reaction mechanism

The mechanism of isobutane-olefin alkylation, with either liquid or solid-acid catalysts, is shown in Fig. 1 [18644 bytes].

Isobutene reacts with an acid site to form the tertiary carbenium ion (tC₄+). The tC₄+ ion reacts with an olefin molecule (C₄=) to form a larger tertiary carbenium ion, tC₈+. The tC₈+ ion undergoes hydride transfer with isobutane (iC₄), releasing an iso-octane (alkylate) molecule (iC₈) and reproducing the tC₄+ ion.

Additional undesired reactions also occur. The tC₈+ can grow further to tCl2+, and so on, and produce higher alkylates via hydride exchange. The tertiary carbenium ions also can break down to the corresponding olefin and a proton if hydride transfer is inhibited, which usually happens at reduced acid strengths.

In the self-alkylation reaction, two molecules of isobutane are consumed per molecule of an olefin, and an isoalkane and normal paraffin are released. For example, propylene can react with two molecules of isobutane to produce iso-octane and propane.

This reaction wastes a molecule of isobutane per olefin molecule reacted. Additionally, when the alkylate molecules become very large, they tend to crack into a paraffin and an olefin.

When the olefin feed to alkylation contains propylene, pentenes, or amylenes, the reaction schemes are analogous to those in Fig. 1 [18644 bytes]. The cracked species of larger alkylates may also experience further chain growth and/or hydride transfer.

Ideally, the solid catalyst should exhibit high activity for the desired reactions while inhibiting the undesired reactions.

Activity, selectivity

To obtain high alkylate selectivity and efficient product separation, all commercial alkylation plants operate at nearly 100% conversion, and so should solid-catalyst processes. Almost complete conversion can be achieved by providing enough residence time in the reactor (typically 20-30 min in H₂SO₄ plants and 10-15 min in HF plants).

As a benchmark of solid-catalyst activity, an intrinsic first-order rate constant of 370 sec-1 at 100° C. was reported for an ultrastable H-Y (USHY) zeolite catalyst.¹³ For particle sizes of 91-231 mm, diffusion effects are severe at 100° C., but would be negligible at temperatures less than 0° C.

The hydride transfer mechanism is critical to alkylation.^{14 15} Loss of hydride-transfer activity increases the alkylate's molecular weight and reduces the yield based on olefin. In HF and H₂SO₄ processes, optimal amounts of "red oils" (conjunct polymeric by-products that are key to hydride transfer) are maintained by removing equilibrium acid and adding fresh acid makeup.

Solid catalysts also must promote hydride transfer. If the solid catalyst is a heterogeneous form of a commercial liquid acid, then the red oils, or their equivalent in the solid, are likely to foul the catalyst surface, resulting in deactivation. Both the solid catalyst and the catalyst regeneration system need to be well-designed to prevent fouling and prolong life.

Various types of solid catalysts, especially superacids and zeolites, have been researched for alkylating isobutane with olefins (Table 2 [64091 bytes]). Sulfated zirconia (SZ) exhibits a high initial activity for alkylation but deactivates rapidly with time on stream.^16-18

Catalyst activity needs to be maintained through proper catalyst design (especially controlled acid strength distribution) as well as through catalyst regeneration. SZ and zeolite-beta are about equally active for alkylation at 50° C., but SZ is more active and selective at 0° C. SZ, however, deactivates faster than zeolite-beta. To optimize activity and minimize deactivation, the alkylation reaction with SZ should be performed at low temperature.¹⁸

Among the zeolites, zeolite beta-has attracted interest because of its large pore diameters and tridirectional channels.^{18 19} Removal of the reaction products from the catalyst structure as fast as they are formed is believed to prevent the formation of larger molecules and fouling of the surface.

Zeolites such as ZSM-3, ZSM-4, ZSM-18, and ZSM-20 have shown alkylation activity.¹⁵ The alkylation performance of ZSM-4 is enhanced in the presence of a Lewis acid such as BF3.

The large-pore zeolites MCM-22, MCM-36, and MCM-49 are more selective than REY in isobutane alkylation with 2-butene.²⁰ The ideal zeolite catalyst for alkylation should have a large pore structure, strong acidity, and a high hydride transfer rate, and it should not promote the oligomerization of olefins.²⁰

Alkylation catalysts can comprise combinations of different materials-for example: a Lewis acid (BF3, BC13, SbF5, AlCl3), a nonzeolitic solid inorganic oxide (silica, alumina), a large-pore crystalline molecular sieve (zeolites, pillared clays, aluminophosphates), and a macroreticular cation-exchange resin (Amberlysts, Amberlites).²¹

A directly fluorided ion-exchange resin is described in an AlliedSignal patent.²² A Brnsted-acid-treated, transition-metal oxide catalyst, developed by Hydrocarbon Technologies Inc., operates at ambient pressure and temperatures as high as 170° C., exhibits high conversion and selectivity, and retains its activity for as long as 72 hr.²³

I/O ratio

A simple model of alkylation consists of two competing reactions involving isobutane (I) and olefin (O):

I + O'Alkylate (de sir able) (1) 
O + O ' Polymer (undesirable) (2) 

The desirable reaction is first-order in olefin, while the undesirable reaction is second-order in olefin. Therefore, to obtain a high selectivity to alkylate, a high ratio of isobutane to olefin (I/O) must be maintained in the reactor by diluting the feed significantly with isobutane and operating in a backmixed mode at conversions approaching 100%. This requirement is unavoidable for solid-catalyst processes as well.

In commercial alkylation processes, the feed I/O ratio is typically 5-8 for the H₂SO₄ process and 10-14 for the HF process. In both processes, the deisobutanizer (DIB) distillation column is one of the most expensive pieces of equipment in the plant.

The HF process requires a larger DIB column than the H₂SO₄ process. Therefore, HF plants could be converted to H₂SO₄ without changing the DIB column or reducing capacity. Conversion to H₂SO₄ is an attractive opportunity for many HF process operators and a serious competitor to the new solid-catalyst processes.

A low I/O ratio reduces alkylate octane and yield, but also reduces operating costs. Solid catalysts that can provide high yields of high-quality alkylate with low-I/O feeds are desirable.

Pore-diffusion effects in the pore structures of certain solid catalysts (such as zeolites) could be exploited to obtain a high I/O within the pores, even with low-I/O feeds. This unique technique for optimizing I/O ratios at the catalyst surface is not available with liquid acids.

Feedstock effects

A solid catalyst should be selective to high-octane alkylate components with various olefin feedstocks. Olefin feeds comprise mainly 1-butene, 2-butene, and isobutene, but refiners are increasingly using pentenes and propylene as well.

Branched pentenes (amylenes) and isobutene can be effectively converted to, respectively, tertiary amyl methyl ether (TAME) and methyl tertiary butyl ether (MTBE) if the refiner has the requisite plants; for the other olefins, however, the only fuel outlet is alkylate.

H₂SO₄ and HF respond differently to olefins. For example, isobutene produces a lower-octane alkylate at a higher acid consumption with H₂SO₄ than with HF.

In the H₂SO₄ process, 1-butene and 2-butene are indistinguishable and both produce a high-quality alkylate. HF, on the other hand, works well with 2-butene but not with 1-butene.

The acids also differ in their tendency to promote hydrogen transfer with propylene and pentenes. Solid catalysts are also expected to be olefin-sensitive; hence, commercial solid catalysts should be customized to specific feedstocks, like FCC and hydrotreating catalysts are.

Catalyst life

As the alkylation reaction progresses, paraffins (iCn) and possibly olefins (Cn=) of increasing molecular weight are inevitably formed (Fig. 1 [18644 bytes]). At some threshold molecular weight, these large molecules become too heavy to dissolve into the hydrocarbon reaction mixture and remain on the catalyst surface, thereby fouling the active sites and deactivating the catalyst. For this reason, catalyst regeneration is critical and must be addressed in catalyst development.

A throwaway catalyst, no matter how cheap, would be unacceptable because of the solid waste disposal problem.

Many industrial catalysts, such as FCC and hydrotreating catalysts, are usually regenerated by combusting the coke deposited on them. Solid-catalyst alkylation processes would probably operate below 100° C., while catalyst regeneration via combustion starts at about 510° C.

Oxidative regeneration in situ would subject the reactor vessels to large temperature swings. Therefore, ex situ regeneration techniques, such as solvent extraction and oxidative calcination are being developed.^{24 25}

In Haldor Tops e's solid-catalyst process, a liquid acid supported on a solid can be washed, recovered, and reused.^{26 27} A built-in mechanism for catalyst regeneration, such as enabling cracking activity only when the molecules reach a size of C₁₆ or larger, could extend the catalyst life in the reactor substantially.

Resistance to poisons

A catalyst's resistance to potential poisons in the feed is important. The presence of butadiene, methanol, MTBE, and TAME in the feed can substantially increase acid consumption by neutralizing acid sites.

In HF and H₂SO₄ alkylation, the adverse effects of trace components in the feed can be overcome by the addition of makeup acid. With a solid catalyst, however, these basic poisons could become permanently attached to the surface and destroy active sites. The susceptibility of solid catalysts to these poisons needs to be accurately estimated and addressed.

Feed pretreatment by adsorption would require periodic regeneration of the adsorbent bed and proper disposal of the adsorbed species, thus increasing both capital and operating costs. These characteristics are undesirable in a new technology that is intended to be more environmentally compatible than existing technology.

Butadiene in the feed can be selectively hydrogenated, as is done in current alkylation plants. But solid catalysts also will need to resist acetonitrile and mercaptans.

Reactors and processes

Solid catalysts are most often used in slurry, fixed-bed, or fluidized-bed reactors. The reaction systems involve a variety of features, including heat removal and product separation.

Slurry reactors

In slurry reactors, the catalyst activity can be maintained at the desired level by addition of fresh catalyst and withdrawal of spent catalyst. The spent catalyst can then be regenerated off-line and returned to the reactor.

Thorough mixing of isobutane and olefin is necessary but difficult to accomplish by agitation. The point of feed introduction is where the olefin concentration will be highest.

Solid-liquid separation is typically achieved by filtration. The appropriate slurry concentration in the reactor depends on the ability of the filter system to separate the slurry without plugging, as well as on catalyst activity, mass transfer, mixing effectiveness, and other factors.

The solids need to be attrition-resistant to maintain their integrity, although too much attrition resistance could cause erosion of the agitation equipment, pumps, piping, and vessels. Because slurry processes are difficult to scale up and operate, they are not favored in the refining industry. Ultrasonics and hydrodynamic cavitation might be useful for achieving thorough mixing without mechanical agitation.

Fixed-bed reactors

In conventionally operated fixed-bed reactors, the catalyst bed eventually becomes inactive and needs to be regenerated. Multiple swing reactors would enable uninterrupted product output.

Alkylation feedstocks often come from fluid catalytic cracking (FCC) units, which have turnaround times of 3-5 years. A fixed-bed alkylation unit with intermittent regeneration would need to have swing reactors if the life of the catalyst is less than the FCC turnaround time, but additional reactors increase costs. The on stream time for each swing reactor would need to be at least 6 months-the longer the better.

Fixed-bed reactors with continuous catalyst regeneration can stay on-line indefinitely, and might be adaptable to isobutane-olefin alkylation. The catalyst would have to be sufficiently attrition-resistant to withstand the rigors of such operations. Increasing attrition resistance usually lowers catalyst surface area and activity, however, because of heat treatment and the addition of binders.

A fixed-bed, plug-flow reactor requires a high degree of recycle, which leads to a high pressure drop and requires crush-resistant catalyst pellets. The fresh feed has to be well-mixed at the inlet so that the olefin concentration is sufficiently low at the point where the mixed feed reaches the catalyst bed. In such cases, the use of a radial-flow reactor or a honeycomb (monolithic) catalyst might reduce the pressure drop to a more acceptable range.

Three alternative reactor designs are shown in Fig. 2 [21508 bytes]. For the reaction model represented by Reactions 1 and 2 (described previously), a distributed feed reactor (DFR) (Fig. 2c) provides the highest selectivity.

The Mobil patent on catalytic distillation describes a distributed feed of olefin to the catalyst bed.²¹ The Kellogg cascade process for sulfuric acid alkylation, in a series of stirred-tank reactors, approximates the DFR.²⁸

DFRs with a permeable, porous wall have been developed by SRI International and Pacific Northwest Laboratory.^29-31 Further R&D of DFRs for solid-catalyst alkylation is warranted.

Fluidized-bed reactors

An FCC-type reactor system for alkylation was recently patented by UOP.³² Both reactor and regenerator operate continuously. Makeup catalyst can be added, and spent catalyst can be withdrawn, as needed to maintain catalyst activity.

Temperature control

The alkylation reaction is exothermic, releasing 84 kJ/mole (20 kcal/mole) of olefin reacted to alkylate. Hence, a suitable means of heat removal and temperature control must be provided.

It is preferable that a new process operate at or above ambient temperature and achieve satisfactory gasoline quality; subambient temperature operation entails higher cooling costs.

Product separation

Operation at nearly complete conversion is necessary because olefins in the reactor effluent complicate downstream product separation. The separation in the DIB column is difficult and expensive because the C₄ components present are close-boiling (Fig. 3 [22851 bytes]).³³

If there is propylene in the feed and some propane is formed by hydrogen transfer, the required propane-propylene separation also is difficult.

A more economical and energy-efficient separation system than the traditional DIB column would improve any type of alkylation system (solid or liquid catalyst). Extractive distillation, selective adsorption, membranes, and industrial-scale chromatography should be investigated to replace or augment distillation.³³

Reactive distillation

Catalytic or reactive distillation combines the functions of reaction and separation in one vessel. Catalytic distillation is used commercially in the production of MTBE and cumene, and it has been proposed for many other reactions, including isobutane-olefin alkylation.²¹

Chemical Research & Licensing (CR&L), which has been developing a solid-acid slurry alkylation process jointly with Chevron, has considerable experience with catalytic distillation in other applications.^34-38 Catalytic distillation may be appropriate or advantageous under the following conditions:

• The main reaction is equilibrium-limited, so that continuous removal of the product as it is formed increases conversion.

• The reaction is exothermic and the heat of reaction is of comparable magnitude to the heat of vaporization so that the system is nearly autothermic.

• The operating temperature and pressure ranges of reaction and distillation roughly coincide.

• The reaction is fast enough to be completed in the catalyst bed but not so fast as to occur in only a small portion of the bed (otherwise the heat would be liberated unevenly in the column).

• By-products (polymers) that tend to plug the column are not formed.

• The catalyst can be regenerated without interrupting product output.

Because isobutane-olefin alkylation does not fulfill all of the above conditions, catalytic distillation does not appear to be well suited to this application.

Business issues

The refining industry is conservative and capital-intensive. For any new process to displace existing commercial processes, the new process must demonstrate overwhelming technical, economic, safety, and environmental superiority over the incumbent processes.

The ongoing efforts of H₂SO₄ and HF process licensers' and refiners' to make their units safer and more environmentally acceptable create stiff competition for new solid-catalyst processes.

If HF alkylation plants are forced by regulation to shut down and a satisfactory solid-acid process is not available, refiners will have to switch to the sulfuric acid process. Refiners who are seeking alternatives to HF or who plan to increase alkylation capacity will make these decisions in the near term.

U.S. alkylation capacity is just over 1 million b/d and constitutes 75% of world alkylation capacity, which increased at about 4% in 1995 (Table 3 [6893 bytes]).³⁹ U.S. alkylation capacity utilization is tight, at 90% or more.

Because the window of opportunity is closing, a new solid-catalyst process needs to be better now than the HF process. Later on it will need to have shutdown economics relative to those of the sulfuric acid process, because refiners are unlikely to replace an operating sulfuric acid process with a new process that has similar economics.

R&D guidelines

The ideal solid-catalyst alkylation process would have the following characteristics, which can serve as a goal and benchmark for research and development and competitive evaluation:

• The new process should be simple and easy to operate. A fixed-bed reactor is preferable to a slurry reactor in this respect.

• A new process that is intended to ameliorate the safety and environmental problems of current processes should not introduce new problems of this type.

• In a retrofit situation, the new process should utilize existing equipment as much as possible, and the new equipment should fit in the available space.

• The overall process should have turnaround times long enough to match those of the FCC units. This calls for standby reactors and continuous (or nearly continuous) catalyst regeneration.

• The process should not require esoteric feedstock cleanup technology that would increase costs.

• The yield and quality of gasoline should be better than that of the incumbent processes.

• The catalyst should be selective with all C₃ to C₅ olefins in the feed.

• The process should accept feedstocks of I/O ratio less than or equal to that used in sulfuric acid processes.

• The process should operate at or above ambient temperature.

• The product separation system should be more economical and energy-efficient than the traditional DIB column.

• The process should be more economical, environmentally compatible, and safer than sulfuric acid processes.

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The Authors