Styrene
4 Cosden’s BTX plant from wIiich i t gets its mixed-xylene feed streain
WHENR.
W. Grace brought Cosden Petroleum into the fold last February, it acquired a company with a process flowsheet like no other in the styrene industry. For one thing, it stretches from crude oil to finished polystyrene pellet, since Cosden has the only completely integrated styrene plant in the world. And where other flow diagrams show a reactor in which ethylene combines with benzene to form ethylbenzene, raw materials for styrene, Cosden’s shows a fractionator. Rather, it is a superfractionator, 600 feet of column in which Cosden pries ethylbenzene directly out of its mixed xylene stream. A neat trick, as the boiling point of ethylbenzene is only 3.9’ F. away from that of para-xylene, another component of the feed stream. T h e Cosden plant a t Big Spring, Tex., is laden with cost-saving features. I t has to be. With styrene heading for a two billion-pound market and with all but one other producer measuring their output in hundreds of million pounds, Cosden has to slash expenses to make its tiny-by comparison-plant competitive. Annual output a t Big Spring is 20 million pounds per year. By integrating the plant, the company minimizes interplant freight. Cosden polymerizes all of its monomer production, ships most of it bulk by rail or in designed trucks. Savings specially
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which these practices contribute allow Cosden to be competitive despite the freight disadvantage from West Texas. Focal point of the Big Spring plant is three towering columns which, together, add u p to 600 feet of fractionating magic. These give Cosden the privilege of foregoing the usual alyklation route to ethylbenzene. Instead, the company can recover ethylbenzene from a handy stream of mixed xylenes already available within the plant. Consequently, Cosden estimates that its ethylbenzene costs from 1 to 1.5 cents per pound less than that derived from ethylation of benzene. Nature, too, has given Cosden an added incentive for using fractionation to get its raw material for styrene. T h e West Texas crude oil which comes from fields surrounding Big Spring yields, upon processing, a mixed xylene stream with an unusually high ethylbenzene content-an attractive 28%. Normally, about 20% is par for the course and thermodynamic equilibrium content is only 10% (2). Cosden believes, hoivever, that fractionation is practical from any mixed xylene stream. Yet, technology had to give nature a helping hand. I t was not until catalytic reforming and Udex extraction came along that Cosden was able to take advantage of its good fortune. Reformate is extracted in a Udex unit
INDUSTRIAL AND ENGINEERING CHEMISTRY
with glycol to produce a n aromatic concentrate. Benzene and toluene are then removed in a BTX (benzenetoluene-xylene) plant, leaving a mixed xylene stream which is essentially made up of only the four isomers. T h e Cosden operation is a prime example of how the value of a product can be upgraded. As part of a n aromatic stream, ethylbenzene may be used as a gasoline blending component. I n this case, its value is about 17 cents per gallon, or 2.4 cents per pound. As a solvent, it is worth around 3.6 cents per pound. There is no established market for pure ethylbenzene because it is used almost exclusively as a captive raw material to make styrene: which sells for about 12.22 cents per pound. Cosden goes even further. Polymerizing the monomer yields a product with a 21.5 cent-per-pound price tag. The M i g h t y Towers
When Cosden decided to build its superfractionator, it was considered impractical to separate ethylbenzene from mixed xylenes because of the small difference in their boiling points (7). Some people in the styrene industry said that Cosden was building a 600foot monument to their ignorance. Despite this, Cosden teamed u p with Badger Manufacturing to design the
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C 0LL A 8 0RATlVE R E P0R T
Crude Oil to Polymer EARL V. ANDERSON, Associate Editor in collaboration with RENE BROWN and CAROL E. BELTON Cosden Petroleum Co., Big Spring, Tex.
unit. Badger also built the unit for Cosden, and it took only 13 months to complete it despite a two-month delay caused by a steel strike. The $3 million unit came on stream in February 1957. Instead of proving to be a n engineering fiasco, the superfractionator allowed Cosden to enter the styrene business at a time when several rubber companies scrapped announced plans to build styrene units. These companies would have made ethylbenzene by the conventional alkylation process. However, when price of styrene dropped 4 cents per pound, their plants suddenly became uneconomical. Cosden, on the
other hand, was able to meet the challenge. The problem of designing 600 feet of fractionating tower is not a n easy one. One 600-foot column was out of the question. Even two 300-foot columns would be impractical; they could sway so much a t the top that they could lose their liquid seal. This would decrease tray efficiencies and perhaps upset operations completely. Cosden and Badger decided upon three 200-foot columns, a more economical choice than four 150-foot columns. Besides the three fractionating columns, Cosden had to erect another 185-foot
Cosden’s giant superfractionators, which strip ethylbenzene out of a mixed xylene stream, tower over the rest of the plant
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_PROPOSED _____
I/EC
E T H Y L E N E FROM
I RESIDUE GAS
PLANT PROCESS SEW
I
MIXED XYLENES FROM B T X UNIT
I)
B
M E TA; ORT HO; PARAXYLENESTOSTORAGE SUP E R F RACT ION ATOR
TARS
SUSPENSION AGENT
STORAGE
REFRIGERATED STORAGE
0 LY ST Y RE N E ELLETS
BRINE COOLER
i
CENTRIFUGEWATER
LIQUOR
POLYMERIZATION REACTOR
L
TEST BINS 141
EXTRUDERS(31
WATER-COOLING TROUGHS ( 3 1
PELLETIZERS ( 3 )
Flowsheet for the manufacture of styrene, Cosden Petroleum Co., Big Spring, Tex.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
STORAGE SILOS ( 5 1
tower, a vacuum distillation unit to separate the unreacted ethylbenzene from styrene monomer. This is another Cosden innovation. Most other companies use several smaller towers to make the separation. Arranging these four columns, then, became a major headache. Refinery custom usually calls for lining columns of a unit in a row. ‘This would have meant three 200-foot columns and a 185-foot column on-line. Another combination which Cosden looked at was to arrange the fractionating columns in a triangle and build the vacuum distillation column separately. However, the final choice was to put the four columns in a quadrant. The four columns are joined laterally a t six platform levels, which gives them mutual support and helps them withstand 100 mile-per-hour winds which occur in West Texas. The feed stream can enter the fractionating column a t several different trays. Feed location extends over 100 trays, more than exists in the normal distillation column. However, the column is not particularly sensitive to feed tray location because of the exceptionally high reflux ratio and the fact that inventory of the column is about 1.5 times the daily charge. Time delay is great. I t may take a day or two after making an adjustment before the change is actually noticed. The large number of trays makes the pressure drop across the column sizable. Because of it, Cosden had to make sure that equilibrium data were good over a wide range of pressures. Another worry in the design stage: determine the effect of trace amounts of nonaromatics upon the separation. Presumably, if contamination concentrated in a particular fraction, it could form a n azeotrope, disrupt the separation, and make it difficult to get the desired purity of product. The three sections of the column are connected by vapor lines and liquid reflux lines. A vapor line goes from the top of the first, or feed, column to the bottom of the second column. Conversely, liquid reflux goes from the bottom of the second column to the top of the first. T h e third section, of course, ties into the second in the same way. Heat is supplied by a direct-fired reboiler. As the ethylbenzene overhead leaves the third column, it is condensed by air from a Combin-Aire cooler. By using air rather than water to condense the vapors, Cosden minimizes water fouling problems. Although it is not usually needed, Cosden has a water aftercooler in the line as a safety measure. The product ethylbenzene is over 99% pure, pure enough to make 99.6% styrene monomer. Actually, Cosden has obtained ethylbenzene with essentially 100% purity; no trace contaminants
Here Is a Look at the Styrene Monomer Scoreboard. , Company
Dow Chemical Monsanto Chemical Koppers Sinclair-Koppers Shell Chemical Foster Grant Union Carbide Chemicals Odessa Styrene Cosden Petroleum Total
..
Capacity, Million of Pounds Present By 1961 B y 1962
Location
Freeport, Tex. ; Midland, Mich. Texas City, Tex. Kobuta, Pa. Houston, Tex. Los Angeles, Calif. Baton Rouge, La. Institute, W. Va. Odessa, Tex. Big Spring, Tex.
550
800
800
360
460
174
174
560 174
0
70
70
135 110 108 40
135 150
135 150 108 50 60 2107
2 0 1497
108 50
60 2007
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Cosden passes air through water in this Combin-Aire cooler, uses the wet air to condense overhead streams in the plant VOL. 52, NO. 7
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Xylenes on the Side
Cosden dehydrogenates ethylbenzene to styrene in this reactor
could be found by the usual laboratory methods. However, this is the exception rather than the rule. Normally, overhead contains some para-xylene. Ethylbenzene produced by fractionation has proved to be a n excellent raw material for the styrene monomer plant. Since they went on stream in 1957, the ethylene dehydrogenation reactor
Stairway to Profit
20
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Styrene Monomer
n
z 3 0 a
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l-
As ethylbenzene moves along Cosden’s crude oil-to-polystyrene chain, it steadily becomes more valuable. Here is how it increases its worth
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and styrene monomer finishing section have operated almost continuously without major turnaround. Fouling problems have been nonexistent at Big Spring. Cosden hasn’t proved it, but it feels that there may be some advantage in using ethylbenzene which comes from fractionation rather than that obtained by alkylation. Bottoms from the superfractionator is a mixture of metu-, ortho-, and paraxylenes. Cosden pumps these to storage, then uses them as feed to produce 99O7, ortho-xylene.
Compared to the fractionating columns which yield ethylbenzene, the dehydrogenation reactor is a midget. The reactor is made of a special alloy steel and is insulated. as conversion takes place at about 1100 to 1200’ F. and low pressures. Cosden dehydrogenates ethylbenzene to styrene over Shell 105 Catalyst (iron oxide promoted with potassium carbonate and chromium oxide). Cosden takes advantage of the high heat content of styrene monomer leaving the reactor to preheat the ethylbenzene entering it. Ethylbenzene entering the reactor picks up some heat by passing through a heat exchanger countercurrently to the reactor effluent. A superheater then brings the incoming feed stream up to reaction temperature. Another Combin-Aire cooler brings
INDUSTRIAL AND ENGINEERING CHEMISTRY
Ethylbenzene isn’t the only isomer in the mixed xylene stream which Cosden upgrades. The company now recovers the three other xylenes and is considering a new unit to improve the purity of its meta-xylene. When ethylbenzene is produced in the superfractionator, the bottoms is a mixture of meta-, ortho-, and para-xylene. Cosden stores this material, then uses it as feed for the fractionator in a “blockedout” operation-that is, it discontinues feeding in the mixed xylene stream to make ethylbenzene. Operating “blocked-out,” the result is a bottoms product which is 99% purity ortho-xylene, probably the highest purity ortho made. For this separation the 600 feet of column are a classical example of a n infinite number of trays. Additional trays would not improve the separation. Cosden produces 10 million pounds per year of ortho-xylene. T h e company has recently taken steps to augment its own ortho production. Cities Service will make 120 d o n pounds per year of 95% ortho at its Lake Charles, La., refinery. Cosden will market the entire amount overseas through its exclusive agent, Fallek Chemical of New York. Overhead from the “blocked-out” operation, essentially meta- a n d paraxylene, goes to the paraxylene plant which Cosden operates for Phillips Petroleum. There, para is recovered by fractional crystallization. Mother liquor from the para-xylene plant is 80y0 metaxylene. However, Cosden is planning a 2.5 to 3 millionpound semiworks plant to upgrade it to 95y0 purity. About the proposed plant, the company says only that it will be a nonconventional process.
styrene monomer down to ambient temperature before it enters a surge drum where water is separated, and hydrogen, methane, carbon dioxide, and a little ethane are vented. I n the dehydrogenation reaction, about 90% of the ethylbenzene is converted to styrene. At the same time, some de-ethylizing to benzene and demethylizing to toluene occurs. But only small amounts, about 2 to 47,, are present in the exit stream and these are easily fractionated from the monomer in a 50-foot tower. The bottoms from this tower, essentially styrene monomer and unreacted ethylbenzene, go to the fourth of the skyscraper towers, the 185-foot vacuum distillation tower. It is a carbon-steel column, containing low pressure-drop
trays. The vacuum is maintained by a steam jet ejector. T h e overhead stream from the 185foot fractionator is ethylbenzene recycle. The bottoms, styrene monomer free of ethylbenzene. feeds continuously to a 40-foot column, which draws off small amounts of tars and polymers at the bottom. These bottoms are collected and held in a batch pot, from which Cosden periodically recovers some additional styrene monomer. Cosden finds that continuous finishing is economical, although many companies still hold to batch operation. The purified styrene monomer leaves the finishing tower and goes to storage M here the monomer is continuously circulated through a brine cooler while in storage to maintain it at 60" F.
Styrene Production
Polymer Plant
Threading the NeedleI
1.
290L
I
292.0
ORTHO-XYLENE
I
i
z
To ease its ethylbenzene out of a mixed xylene stream, Cosden has to do a spectacular fractionation job. Boilingpoint range of the four-component stream is only 14.9" F. and the boiling point of para-xylene is a scant 3.9" F. away
The next link in Cosden's process chain is its polymerization plant, which was designed and built by Blaw-Knox Chemical Plants Division. Of the several methods for polymerizing styrenesuspension, mass, emulsion, and solution-Cosden chose the suspension system, chiefly because of the excellent quality control lvhich it offers. Polymerization takes place in a glasslined, agitated reactor. Cosden has four of them, 2600-gallon tanks, which it operates on a staggered schedule. Ingredients besides the monomer are water, catalyst, and suspension agent. Clays, insoluble sulfates, and phosphates are often used as suspension agents but Cosden isn't saying exactly what it uses. The reactor is first charged with warm water. Then follows liquid styrene monomer. Suspension agent and catalyst may be added with the styrene or separately.
0 60{
0 5 1 L0 I -1 . -- 1 . 19%
52
54
56
56
60
Source, Stanford Research institute
Exact operating conditions are also confidential but polymerization usually takes place a t about atmospheric pressure and below the boiling point of water. In any event, the result is a slurry of polystyrene beads, about 40 to 60 mesh, in water. 'The desired end product also dictates reaction time, which ranges from 6 to 20 hours. Polymerization rate is the variable which has the greatest effect on the molecular weight of the polymer. When polymerization is complete, the slurry goes to hold, or wash, tanks to free the reactor for another load. There are two wash tanks, Lvhich are also glasslined and of the same capacity as the reactors. While the slurry is in these tanks, undisclosed chemicals are added to it which make it easier to remove the suspension agent later on. The slurry is also agitated while in the wash tanks. Then it is dumped into a continuous Bird Centrifuge, where about 95% of the water is removed. Centrifugation takes place in three steps. First, the mother liquor is thrown off. Polystyrene beads then receive a water wash and, finally, the remaining water is thrown out in the last step. Water content of the beads leaving the
4 Cosden's polystyrene reactor room. In these glass-lined reactors polystyrene beads are formed in a water suspension VOL. 52,
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enclosed in a pressurized building. Cosden maintains a slight positive pressure with air that has been waterscrubbed and filtered.
The Future
Cosden’s polystyrene plant. Building a t left houses reactors, centrifuge, dryers, extruders, and pelletizers. Five large storage silos are on the right. Smaller building in right foreground contains bulk-loading facilities. In the center background, three 200-foot columns separate ethylbenzene from a mixed xylene stream
centrifuge varies from 1 to 5%. All water and mother liquor go to waste. Remaining moisture is removed in a hot air rotary dryer. Cosden keeps bead temperature in the dryer below 185’ F. Temperatures beyond this affect the polymer’s heat distortion properties. The dryer will handle about 4500 pounds per hour, but Cosden usually operates it between 3000 to 4000 pounds per hour. The dried polystyrene beads follow a fairly conventional route to the final storage bins. From the dryer, they go to hold tanks where product quality is carefully determined. From there they feed through a hopper into an extruder. Movement is either by pneumatic tubes or by gravity. The extruded polymer travels through cooling troughs to choppers which transform the polystyrene strands into pellets. The pellets then again move pneumatically to test bins where a final quality check is made before transferring to storage silos to await shipment. After the storage silos, Cosden again digresses from the conventional. Ex-
cept for very small amounts, the plant’s 20 million-pound output leaves in bulk shipments either by rail or truck. T h e trucks, specially designed by SproutWaldren, carry u p to 33,000 pounds of polymer in five aluminum compartments. They are equipped to pneumatically transfer their cargo directly to the customer’s bulk storage. Customers within a 1000-mile radius of Big Spring are usually served by truck. Beyond this, trains take over. Bulk shipments, Cosden feels, eliminate multiple handling in bags and drums. What’s more, it saves substantially in handling costs at each end of the line, compared to bag shipments. Although Cosden was the first to introduce this method of handling polystyrene in the summer of 1958, it is no longer unique with them. Several primary producers of thermoplastic resins now use bulk handling facilities. Contamination is a problem a t Big Spring because of the dust storms common to West Texas. To guard against them, the entire polystyrene plant, including the bulk loading terminal, is
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Every curve in the styrene industry is on the rise. Monomer production last year jumped 28y0 to 1.56 billion pounds, paralleling output of polystyrene and other styrene resins, which rose 20% to about 920 million pounds in 1959. These materials should be leaving reactors a t a better than one billion-pound clip this year. Another product with a big appetite for styrene monomer is styrene-butadiene (SBR) rubber. It will account for 495 million pounds of the monomer this year and, in 1965, when 1.3 million long tons of SBR rubber is expected to be produced, this outlet will consume approximately 560 million pounds of styrene monomer. In the face of this bright outlook, monomer producers are rushing to add new capacity to their plants. Present capacity now stands just below 1.5 billion pounds. But this should increase to more than 2 billion pounds in two years, a hefty 40% reinforcement. Cosden is swimming along with the current. I t will add 40 million pounds to its annual output, due on stream by the end of 1960. When the plant is completed, it will represent another first for Cosden in the styrene field. I t now gets all of the ethylbenzene that is available from its 45,000 barrels-per-day crude throughput. So, in order to get ethylbenzene for its additional 40 million pounds of styrene, Cosden will use the first commercial Alkar unit in the U.S.A. Alkar is a Universal Oil Products’ process which catalytically reacts light olefins with benzene to yield alkyl aromatics. Ethylene will be recovered from residue gas of a cat cracking unit; benzene will come from Cosden’s own BTX unit. To Cosden, ethylbenzene from the Alkar process represents a compromise between that which it gets from fractionation and that which comes from conventional alkylation. Alkar doesn’t require high purity ethylene. And with a captive source of benzene, Cosden feels that it will still obtain its ethylbenzene a t a n attractive price. The proposed output will call for additional reactors, which Cosden plans to install. Some of the new monomer production is committed to sale; the remainder will be polymerized.
literature Cited (1) Haines, Harry W., Powers, J. M.,
This specially designed truck carries 33,000 pounds of polystyrene in five aluminum compartments. It has a self-contained pneumatic discharge system
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INDUSTRIAL AND ENGINEERING CHEMISTRY
Bennett, R. B., IND. ENG. CHEM.47, 1097 (1955). (2) Kj:k, Raymond E., Othmer, Donald F., Encyclopedia of Chemical Technology,” 15, p. 189, Interscience Publishers, Inc., New York, N. Y., 1956.
