For its Natsyn pilot plant site, Goodyear chose a location next to its SBR pilot plant. Natsyn facilities are shown in foreground and SBR unit in back. The company’s latex-producing facilitiss, with rated capacity of 27,500 tons a year, are also nearby
Natsyn Pilot Plant
operation -since- December 1957 and is capable of producing on a tonnage basis. Polyisoprene, the man-made material which closely resembles natural rubber, h a been widely publicized as the answer to any possible shortage of the treegrown product. Goodyear is a latecomer to the field compared to several others, but once under way has moved rapidly. I n December 1956, Goodyear management was satisfied with the soundness of laboratory work on the polyisoprene process and gave the goahead for a pilot plant. Just one year later the plant was in operation.
Two.500-gallon stainless steel reactors handle the polymerization. At present, one at left is used for reaction and other as a hokling tank for finished cement. Above each unit i s a 10-hp. motor which drives the turbine-type mixer within the reactor
This plant has gone a long way t between the research la tion. A more efficient
rd bridging the gap commercial producs rrfethod is the next step
VOL. 51, NO. 1
JANUARY 1959
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A chromatograph i s used to check analysis of isoprene monomer and the hydrocarbon solvent. This laboratory, adjacent to the plant area, is also used to check water content of raw materials and to analyze final product
Polyisoprene, which can replace or be used as an extender of natural rubber, depends on a stereospecific catalyst which will produce a high czs-1,4-polymer. Such a polymer can be made with Ziegler-type or lithium-derived catalysts. The Natsyn pilot plant is designed to use either type, but Ziegler is the one Goodyear prefers a t the moment. The process seems disarmingly simple. Isoprene monomer is mixed with a hydrocarbon solvent, polymerized a t mod-
The two tanks behind ihe operator are used to weigh out exact amount of isoprene and solvent. After weighing, constant flow valves (being adjusted) meter the two raw materials as they are mixed before being fed into reactor
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INDUSTRIAL AND ENGINEERING CHEMISTRY
erate temperature and pressure, and devolatilized. But: as demonstrated by the maze of equipment and instrumentation in the Goodyear plant, the process is nowhere this simple. Process troubles start at the very beginning-the raw materials. Extreme purity is required. Oxygen and certain unsaturated compounds in the monomer or solvent poison the catalyst and even minute traces of water will upset the reaction. As polymerization proceeds
toward completion, high viscosity of the cement, as the mixture of plyisoprene rubber and solvent is called, presents agitation and heat transfer problems. Then, too, solvent must be stripped off in a closed system which will give good recovery of solvent. Throughout the process. conditions must be controlled to give optimum polymer structure and molecular weight. Goodyear buys petroleum-derived isoprene monomer for its pilot plant operation. Goal for a commercjal Natsyn plant would be to have high-purity isoprene available for use immediatelv after removal of inhibitor. Currentl,, Goodyear distills isoprene before use to remove polymerization inhibitor and to reduce the quantity of impurities. Solvent also is distilled before use. A4n\ straight-chain hydrocarbon can be used as the solvent and the Natsyn unit has operated successfully on several different materials. One of those prefrrred is n-pentane, as it has a boiling point very close to isoprene. Isoprene and solvent are passed through a silica gel or alumina dehldratjng unit as final purification before polymerization. The usual procedure is to weigh out the right amount of isoprene and solvent, mix them by simply running the two lines together at a T-joint, and pump the mixture over the dehydrating bed. As the key point of the process is the accuracy with which raw materials are measured, weigh tanks are used rather than any type of volumetric measuring such as would be used in a full-scale plant. The pilot plant has separate tanks for monomer
N A T S Y N PILOT P L A N T and solvent. Dehydrating equipment is located adjacent to the that chances of picking u p drying are cut to the minimu Polymerization begins as soon as the catalyst is added. The reaction is exothermic, and heat is removed by circulating water or brine in the jacket of the reactor. Goodyear finds 50' C. a good average reaction temperature. Lower temperatures give polymers of higher molecular weight, but this advantage is offset, engineers believe, by increased catalyst consum slower reaction rates. Shortly after the reaction next processing problem crops up. At about 7y0 solids, the solution turns into a heavy viscous cement which, although thixotropic in nature, is still difficult to cool. The first few runs in the Natsyn pilot plant were stopped at about 15% solids because cosity and heat transfer With addiiional experience year can operate u p to 25y0solids and hopes to obtain a n even higher figure in the future. A way to measure the cement's actual viscosity during polymerization has not yet been found. Some indication of the nature of the problem is given by a laboratory test of a sample which had l5y0solids and a viscosity of 100,000 C.P.S. When a t rest, a cement of approximately 40% solids has a consistency somewhat like that of a soft rubber ball. I n the pilot plant reactor, cement is agitated by a turbine-type mixer drive by a 10-hp. motor. Because of the high viscosity, jacket cooling works better than cooling coils within the vessel. When the reaction is complete, the thick cement is transferred by pump to a holding tank, where catalyst deactiliator and antioxidant are added. The next processing task is to remove and .recover the solvent. A drum dryer or .similar piece of equipment can be used ,for this job. Because maximum solvent recovery is important, the Natsyn pilot plant makes use of a n extruder which :gives a tight, closed system. Cement is preheated in the holding tank and ,in the line to the extruder as it is trans,ferred by a variable-speed gear pump which also meters the flow. Remaining heat needed to strip off the solvent is added in the extruder itself. Solvent is taken off in stages, the last of which is under vacuum. Vaporized solvent then is condensed and distilled, and is ready for re-use. Product from the extruder contains less than 1% volatile material. I t is either .cut into spaghettilike strands or chopped into shorter pellets and then baled into 50-pound packages. The Natsyn pilot plant is located in
.
Extra ventilation needed when equipment is opened for cleaning is provided by flexible hoses which are connected to a central exhaust fan. Here, an employee is shown draining the tank used for solvent
Akron adjacent to Goodyear's SBR pilot plant. Equipment is housed in a neat one-story-high bay building with operating platforms where required. Floor space, including that occupied by laboratory and warehouse areas, totals 4800 square feet. Focal point of the plant is two 500gallon stainless steel reactors. They are identicil and can be used interchangeably. . One usually is used for the reaction'itself, whereas the other now serves as a Polding tank. Other equipment includes two smaller reactors, each holding 26 gallons, three distillation columns used for isoprene, solvent, and recycle solvent, a glasslined tank for acid washing of solvent,
the extruder, baler, and weigh and storage tanks for isoprene, solvent, and catalyst. New ideas and changes to process conditions are tested in the small reactors and in this way good ideas can be seleeted a t relatively low cost. The 500-gallon reactors are probably somewhat hrger than would be required just to gather data. Extra size is important, however, because the plant must turn out enough Natsyn for development of its use in truck tires. Standard types of equipment are used throughout the plant and most are stainless steel or stainless steel-lined. A commercial plant probably would not require such heavy use of stainless steel, VOL. 61, NO. 1
JANUARY 1959
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To withdraw samples from the reactor without losing any of the volatile components, Goodyear uses a hypodermic syringe. Samples like this are taken every 2 hours during each run
but in the pilot plant it is useful to remove any changes of process troubles due to corrosion. Handling of isoprene and solvent presents no unusual hazards. T h e two separate ingredients of the catalyst, however, must be handled in a safe manner. Triisobutylaluminum will burn spontaneously in air. T o mix the catalyst, the right amount of each component is weighed from storage cvlinders under pressure into small bombs and then premixed in solvent before being added to the reactor. Solvent and isoprene are checked for water content before and after charging to the reactor. If materials already in the reactor are found to have too much water, they are pumped out and dehydrated before catalyst is fed in to start the reaction. A laboratory for control tests is located next to the plant area and isoprene and solvent are analyzed by gas chromatography. During polymerization, the cement is checked every 2 hours for solids content. Samples are taken lvith a hypodermic syringe. a technique Goodyear first used on styrene-butadiene rubber. This provides a convenient way to extract a sample from the reactor and weigh it without losing any of the volatile liquid. When these samples show that the solids content of the cement has reached the desired level, polymerization is considered complete and the batch is discharged.
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Ziegler-type catalyst used for polymerization burns spontaneously in air. Material from storage cylinder at right i s weighed into pressure bomb which will be used to feed catalyst into reactor. In case of fire, aluminum shade at the top automatically drops to seal off where catalyst i s handled and thus confines flames
Before the product is pumped to the extruder, molecular weight and viscosity tests are made. After drying and extruding, the final product is checked for residual volatiles, Mooney viscosity, gel content, and dilute solution viscosity. Goodyear has added an interesting feature to the instrumentation in its pilot plant. SVherever the same control function is required in more than one spot-for instance, the measuring and controlling of jacket temperature in the three distillation columns-Goodyear uses different brands of instruments. The company’s experience with performance of the various makes of instruments under the specific conditions of the pilot plant will determine what kind of instruments will be installed in a commercial plant of the future. During the first year of operation. fairly \vide differences have shown up in the Xvay similar instruments perform. Recording-controlling instruments are located on the three stills, the reactors, and the extruder. Isoprene and solvent stills have instruments to control steam flow, vapor pressure and temperature, jacket temperature, and internal temperature. The recycle still has a liquid level control instead of the steam flow control. On the reactors, instruments control the internal and jacket temperatures, whereas the extruder has an instrument to check vacuum on the final stage of solvent take off.
INDUSTRIAL AND ENGINEERING CHEMISTRY
At intervals, the reactors are cleaned of the gel that forms on the sides when a particular batch gets out of hand. I n theory, it is possible to operate the reactors indefinitely without extensive cleaning. This is a goal G.oodyear eventually hopes to reach. I n practice, however, the reactors must be flushed out occasionally with a solvent to remove gelled material. I n addition to the time required for this operation, it is one more chance for water to enter the system. Gocdyear finds that most cases of water in the reactor appear after one of these cleanings. The need to clean a reactor is dictated by a loss in heat transfer efficiency. h-ormally, the Natsyn pilot plant runs 24 hours a day, 5 or 6 days a week. The staff includes four full time technical people and three others who work for the unit on a part-time basis. Three laboratory technicians, two nontechnical operators Fer shift: and maintenance men as required round out the staff. So far: this pilot plant has gone a long way toward the eventual goal of bridging the gap between research laboratory and full scale commercial production. Eut there is still much to be learned. Just as manufacture of SBR went from a batch to a continuous process, polyisoprene no doubt will be made some day by a more efficient continuous method. And this is the next step for the Goodyear pilot plant.
