Kevin J. Fallon
Raytheon Engineers & Constructors Inc.
Cambridge, Mass.
Henry K.H. Wang
Shell U.K. Ltd.
Stanlow, England
Chaya R. Venkat
Mobil Research & Development Corp.
Princeton, N.J.
Utilizing inexpensive refinery offgas as an ethylene source, Shell U.K. Ltd.'s Stanlow, England, manufacturing complex has produced high yields of ethylbenzene (EB) for more than 3 years.
The Stanlow plant is the first of its kind worldwide to ethylate benzene by passing refinery offgas and benzene over a zeolite catalyst. The initial catalyst charge is still in the reactor.
This process is a rare example of fuel-grade material being used to replace a far more expensive chemical-grade feedstock. The operation enables Shell to produce lower-cost EB to supply its styrene plants in Europe.
In situations where the availability of polymer-grade ethylene is limited, the option of substituting refinery offgas becomes even more attractive. This is especially true when large fluid catalytic cracking (FCC) units are available.
BACKGROUND
Ethylbenzene is produced by alkylating benzene with ethylene in the presence of an acidic catalyst. Nearly all the world's ethylbenzene capacity uses polymer-grade ethylene produced in steam crackers.
EB is used almost exclusively as a feedstock to produce styrene monomer. Worldwide production of EB is expected to exceed 20 million metric tons/year (mty) by 1996.
Nearly all new EB capacity uses zeolite-based catalyst technology, although half of global production still is produced in older plants using aluminum chloride catalyst in a liquid-phase process. A small amount of EB capacity uses supported BF3 catalyst in a vapor-phase reactor.
But the ethylene source for EB production need not be limited to steam crackers.
Large quantities of ethylene are produced by a number of refining processes, such as FCC and delayed coking. The quantity of olefins produced in a typical large FCC unit (100,000 b/d) processing vacuum gas oil is summarized in Table 1(13051 bytes).
As demand for C3 and C4 olefins has risen in recent years, an increasing portion of demand has been satisfied by recovery of these components from refinery offgases, as a substitute for steam cracking sources. As demand for methyl tertiary butyl ether for gasoline blending has increased, FCC units and catalysts have been modified to produce increased yield of C4 olefins in particular.
Recovery, purification, and transportation of these heavier olefins at useful purity levels is not particularly difficult and can be accomplished at moderate pressures and without refrigeration. The much greater volatility of ethylene and the presence of light gases, however, render its similar recovery impractical, in spite of the large quantities of ethylene produced by these offgas sources.
Consequently, conventional processing recovers the portion of propylene and heavier olefins deemed economical, and the remaining constituents, including ethylene, are used as fuel gas. Most of the sulfur is removed in amine scrubbers to control furnace emissions and meet environmental requirements.
To recover the chemical value of ethylene, it is desirable to use it in "dilute" form (with the other offgas components), with minimal purification, and near the refinery. If offgas from a large FCC unit is used to feed a moderate-sized EB plant, the feedstock advantage is appreciable.
Table 2 (13044 bytes) shows that, based on stoichiometric yields, an annual feedstock savings of $15 million is possible for a commercial-scale plant the size of Shell Stanlow. Comparing two EB units having similar performance, the use of dilute ethylene becomes advantageous if the additional costs for pretreatment are not excessive.
Before the introduction of zeolite catalysts, available processes used Lewis acids to catalyze alkylation with polymer-grade ethylene. These processes were not well-suited for the use of dilute ethylene feeds.
The conventional liquid-phase process using aluminum chloride catalyst was unable to deal properly with a dilute gas feed and produced a waste water stream that was difficult to treat. The vapor-phase process used supported BF3 catalyst, but the catalyst proved difficult to regenerate and was volatile and corrosive, particularly in the presence of refinery offgas. For these reasons, other commercial uses of dilute ethylbenzene have been rare. The availability of zeolite-catalyzed vapor-phase technology from Mobil Research & Development Corp. and the former Badger Co. Inc. finally has made viable the use of refinery offgas for EB production.
Originally developed for use with polymer-grade ethylene and installed in more than 20 plants, this technology is adaptable to refinery offgas use because of the vapor-phase reactor condition. Of equal importance, however, the catalyst is not deactivated significantly by the contaminants found in refinery offgas.
STANLOW PROJECT
Mobil and Badger have offered this technology since its polymer-grade process was first available. It was more than 10 years later, however, before a manufacturer was willing to implement the refinery offgas option at commercial scale.
During this 10-year period, polymer-grade ethylene was readily available at a low price, which gave manufacturers little incentive to commercialize the first offgas-based plant and to accept the associated risks.
Interest in recovering the ethylene value from refinery offgases increased during the late 1980s as ethylene supplies became more restricted. This period also was marked by an increase in the size of new FCC units, as well as a trend toward increasing olefin yields from those units. Both of these trends led to the availability of more ethylene from refinery offgas.
In some cases where an FCC unit is adjacent to a steam cracker, a refiner can recover ethylene value by compressing the offgas and feeding it to the cracker purification train. This is only possible, however, when the purification train is oversized relative to the cracking furnace throughput.
Shell's situation at Stanlow provided strong incentives to feed the FCC offgas to an EB unit. The FCC unit was world-scale and could process long residue at high severity, thereby producing sufficient ethylene to feed a moderate-sized EB unit (I 60,000 mty). Benzene also was available at the site.
As a major European producer of styrene monomer, Shell also could make captive use of EB production by feeding it to its styrene monomer/propylene oxide (SMPO) complex at Moerdijk, The Netherlands. This was desirable because, historically, nearly every SM producer has had a captive supply of EB; therefore, the merchant market has been limited.
Shell selected the technology in late 1988. The basic design package was executed in Badger's engineering office in The Hague. Fluor Daniel performed the detailed engineering and construction.
Construction began in November 1989. The plant was started up successfully in May 1991.
Shell efficiently prepared for the commissioning by extensively training operators and providing an experienced and dedicated start-up team of technical operations and maintenance personnel.2 Mobil and Badger also provided specialists to assist the commissioning and monitor the performance test run.
On-spec EB was produced 3 days after ethylene was introduced to the reactor. The performance test run was initiated 10 days later and passed on the first trial.
EB TECHNOLOGY
In this EB process, superheated ethylene and benzene vapors are passed over a heterogeneous catalyst. Both polymer-grade and dilute ethylene feed are suitable for use with the same Mobil ZSM-5 catalyst.
All acid zeolite catalysts have in common the ability to facilitate a number of hydrocarbon conversion reactions, such as alkylation, polymerization, cracking, isomerization, and transalkylation.
Activated natural clays and binary oxides, such as silica-alumina, were the first solid-acid catalysts to be used. Due to their amorphous nature, however, they are difficult to characterize and their optimization in commercial use has remained very much an empirical art.
Unlike natural clays, zeolites lend themselves very well to catalyst design. This is particularly true of synthetic zeolites, which can be tailored to provide a degree of molecular shape selectivity that had not been possible before their introduction.
In any heterogeneous catalytic system, it is necessary to control three important performance criteria:
- Activity for the desired chemistry
- Selectivity toward the desired products
- Stability of operation.
Mobil developed the ZSM-5 family of catalysts in the early 1970s and achieved precise control of all three of these parameters for the first time.
The Mobil/Badger ethylbenzene process using a ZSM-5-based catalyst was the first commercial demonstration of an aromatic alkylation process using a synthetic zeolite catalyst.
Catalyst design know-how makes it possible to control activity very precisely in ZSM-5-based catalysts by controlling the population of acid sites in the zeolite framework, their distribution through the crystalline matrix, and their acid strength. Control of the crystal size ind tortuosity of the pore geometry allows shape-selective stearic control of the complex chemistry taking place inside the pores.
While the pore dimensions of ZSM-5 easily allow the diffusion of feed and product molecules, the formation of large coke-precursor molecules is drastically reduced because of size constraints. This means the catalyst ages very slowly and very little feed is lost to the formation of heavy materials.
ZSM-5 catalysts also exhibit excellent thermal and hydrothermal stability, allowing easy regeneration of coked catalyst by simply burning off the hydrocarbon deposits. Over the years, optimization of the catalyst has led to a very stable catalyst that provides long cycle lengths and high product yield.
The characteristics of this catalyst are beneficial when processing FCC offgas, which, during upset conditions, can contain high levels of amines or sulfur components. These species at high concentrations act as temporary catalyst poisons, but they can be removed easily by catalyst regeneration, or even desorbed by passing specification feed over the catalyst.
Regeneration is facile and fast because the catalyst quantity is small. In addition, the operating temperature is sufficiently high to limit sorption of basic species onto the acid sites.
PROCESS DESCRIPTION
A schematic of the plant, on separate sides illustrating gas pretreatment and the core EB plant, is shown in Fig. 1(45786 bytes).
PRETREATMENT
The pretreatment step, which includes some proprietary Shell technology, is comprised of sweetening, followed by removal of heavy materials. The sweetening entails diisopropanolamine addition to remove sulfur compounds from the raw cracker offgas feeding the EB unit.
The gas composition after sweetening is shown in Table 3(12376 bytes).
Removal of heavy olefins (primarily propylene), which would otherwise alkylate benzene to form una,anted by-products, is then accomplished in a refrigerated de-ethanizer unit. Propylene is removed to a concentration of less than 50 ppm(vol) in the overhead offgas.
It is not necessary to remove ethane and lighter compounds, which are inert in the alkylation reactor. Molecular sieve dryers are placed upstream of the de-ethanizer. These dryers are required to dehydrate the feed gas to prevent formation of hydrate in the refrigerated sections.
All modern FCC units contain absorber/strippers followed by fractionation. These units recover most of the propylene from the offgas--typically 90%. Installation of the pretreatment plant increases that recovery to virtually 100% because the de-ethanizer bottoms can be recycled to the propylene recovery unit.
EB UNIT
Pretreated dilute ethylene is mixed with excess benzene vapor in the reactor. The reactor effluent is distilled overhead in the benzene recovery system, then recycled to the reactor.
The uncondensed offgas leaves the overhead of this system and is sent to an absorber for recovery of benzene, and the offgas is returned to the refinery. Process residue from the back end of the process is an acceptable absorbing medium.
This absorber is the only portion of the EB unit operation not required when processing polymer-grade ethylene. Consequently, the cost of the EB unit is not substantially greater when using refinery offgas as an ethylene source.
After benzene removal, EB product is recovered by fractionation in the EB recovery column. The heavy materials from the column bottom consist primarily of diethylbenzene (DEB) and lesser amounts of triethylbenzene, which are known collectively as polyethylbenzene or PEB. The bottoms are distilled to recover PEB overhead.
This stream is recycled to the reactor, where DEB is transalkylated with benzene to recover additional EB.
The net residue can be used as a fuel.
UNIT PERFORMANCE
The plant performance easily met the guarantees offered for the process. The plant has operated trouble-free since 1991.
The initial catalyst charge has been in the plant for more than 3 years, with no loss of ethylene conversion. The catalyst has been regenerated at 3 to 6-month intervals to recover minor losses in ethylene conversion.
Plant performance with respect to product purity and yield is very similar to polymer-grade plants of the same flow configuration. It is important, however, to note that the Shell Stanlow unit does not contain a separate transalkylation reactor, as is the practice in newer Mobil/Badger units. Use of a transalkylator substantially improves yields and product purity.
Typical EB composition is given in Table 4 (9590 bytes). Much of the toluene impurity originates in the feed benzene. Cumene, xylenes, and styrene are reaction by-products.
The cumene level is controlled by the design of the EB are controlled by reactor conditions.
The purity shown in Table 4 (9590 bytes) is indistinguishable from that produced in a polymer-grade plant. (The amount of xylenes in new polymer-grade plants, however, is reduced about 50%.)
In Table 5 (15473 bytes), the raw material yields and yield losses for the Stanlow unit are tabulated in the left-hand column. Typical results for plants of similar design operating on polymer-grade ethylene are shown in the right-hand column.
Both sets of data assume freshly regenerated catalyst. Again, note that the yield losses are about 50% less in new polymer-grade units with transalkylators.
Performance with polymer-grade feed is only slightly better than with FCC offgas. The primary difference is slightly reduced ethylene conversion of FCC offgas-about 99.1% compared to 99.6% for polymer-grade units.
This difference is attributable to the reduced ethylene partial pressure in the reactor, which slows the mass transfer of ethylene to the active catalyst sites.
The effect is small because, in both cases, the ethylene is substantially diluted in benzene.
The residue that is produced also is slightly greater with operation on FCC offgas.
This is most likely caused by subtle effects resulting from trace impurities in the FCC offgas, the result being increased formation of residue precursors.
Utility consumptions are not tabulated, but they are not significantly different for the two cases, with the exception of the pretreatment utilities. Utilities required for the pre-treatment process are Very site specific and depend on refinery offgas pressure and olefin composition, among other factors.
The authors' companies do not permit disclosure of capital costs until more extensive data for dilute ethylene technology are demonstrated.
Some general considerations are:
- For a plant the size of Stanlow, the raw materials savings attributed to using refinery offgas are about $15 million/year.
- The extra utilities associated with pretreatment also are insignificant relative to the raw materials savings.
- Because of favorable economies of scale, dilute ethylene is most favored when large supplies are available.
SUITABILITY
Manufacture of EB from refinery offgas has been demonstrated as a viable means of utilizing low-cost ethylene. The process is most attractive to refiners operating large crackers and who have a secure arrangement for product offtake.
This may become less important if a larger EB merchant market should develop.
It also is advantageous to produce benzene on site. For large refiners, EB manufacture ultimately may prove to be an outlet for benzene, as less of it enters the gasoline pool and extraction units are expanded.
REFERENCES
1. Dwyer, F.G., and Lewis, P.J., "Ethylbenzene unit operates well on dilute ethylene," OGJ, Sept. 26, 1977, P. 55.
2. Want, H.K.H., "Plant Commissioning: A Manufacturer's Account," paper for University of Leeds, Department of Chemical Engine(!ring, 1993.
3. Fallon, K.J., and Ram, S., "The Mobil/Badger Ethylbenzene Process Optimum Results Using Vapor Phase Technology," Journal of Japan Aromatic Industry Association Inc., Vol. 45, 1993.
