INDUSTRIAL AND ENGINEERING CHEMISTRY
654
(27) Hucks, W. X., Rubber Reserve Corp. communication. (28) Mugdan, IMartin, and Sixt, Johann (to Consortium fur Elektrochemische Industrie), U. S. Patent 2,101,868, 2,102,159 (Dee. 14, 1937); 2,108,829 (Feb. 22, 1938). (29) Ic'eal, A. M., and Verbanc, J. J . (to du Pont Co.), I b i d . 2,415,829 (Feb. 18, 1947). (30) Peterson, W. R. (to du Pont Co.), I b i d . , 2,174,527 (Oct. 3, 1939). (31) Pitcher, A. E., communication. (32) Reed, C. F. (to du Pont Co.), U. S.Patent 2,046,090 (June 30, 1936). (33) Rothrock, H. S. (to. du Pont C o . ) , Ibid., 2,282,827 (May 2, 1942). (34) Sohlack P. (to I. G. Farbenindustrie), Ibid., 2,241,321 (May 6, 1941). (35) Semon, W. L. (toB. F. Goodrich Co.), Ibid., 2,380,471,2,380,551 (July 31, 1945).
Vol. 40, No. 4
(36) Sixt, Johann, and Mugdan, Martin (to Tennessee Eastman Corp.), I b i d . 2,249,543 (July 15,1941). (37) Spanagel, E.W. (to du Pont Co.), I b i d . 2,163,636 (June 27, 1939). (38) Stewart, W. D. (to B. F. Goodrich Co.), Ibid., 2,380,473, 2,380,475, 2,380,476, 2,380,477, 2,380,710, 2,380,747, 2,380,905 (July 31, 1945). (39) Stewart, W. D., and Zwickel, B. M. G. ( t o B. F. Goodrich Co.), Ibid.,2,380,617 (July 31, 1945); 2,384,574 (Sept. 11, 1945). (40) Voss, Eisfeld, and Freudenberger, German Patent 664,337 (1933). (41) Wooldridge, D. E. (to Bell Telephone Laboratories), U. S. Patent 2,235,132 (-March 18, 1941). (42) Youker, M. A. (to du Pont C o . ) ,Ibid., 2,365,035 (Dec. 12, 1944). RECEIVED September 29, 1947.
Continuous Polymerization in Germany R. D. Dunlop' and F. E. Reese,
Monsanto Chemical ~ o m p a n ySpringfield, , ~
a
~
~
T h r e e common thermoplastic materials are produced by I. G. Farbenindustrie b y continuous polymerization methods. These include the mass polymerization of styrene and vinyl acetate, and the emulsion poll-merization of vinyl chloride. Information concerning these processes, which has been scattered through various publications of the Office of Technical Services, is summarized and discussed with emphasis on the equipment a d engineering features. The three processes are similar. In general the equipment has the form of tall slender autoclaves into which monomer is added a t the top and the pdymerized product drawn from the bottom. The increase in specific gravity as polymer is formed is utilized to obtain a separation of polymer from monomer. Material flows and heat removal are controlled to avoid turbulence. Each installation is designed to fit the characteristics of the material
being polymerized. Styrene is polymerized without a catalyst in aluminum equipment, and the polymerized product is extruded as a narrow band, cooled, and cut into a granular product. Vinyl acetate is handled in a manner similar to styrene with the addition of an organic peroxide catalyst. This process can be used for only a limited range of molecular weights of polyvinyl acetate; the process cannot be controlled to obtain low molecular weights, and the higher molecular wejghts do not have sufficient flow or mobility to be processed. Vinyl chloride is polymerized a t 5 to 6 atmospheres in glass enameled autoclaves. The aqueous component containing catalyst and emulsifier is charged continuously along with the vinyl chloride. The emulsion is formed and polymerization carried out as i t flows through the autoclave. The solid product is isolated by coagulation, drum drying, or spray drying.
B
Polymerization of vinyl type monomers is a highly exothermic reaction. All vinyl polymerizations are extremely sensitive reactions requiring very close control of the catalyst type and quantity, as well as precise temperature control, t o obtain uniform and useful finished products. A discussion of t h e mechanism of the vinyl polymerization with all the contingencies and problems which can arise in the development of industrial polymerization processes cannot be included in this paper. Undoubtedly, in developing the existing processes, the German technicians met and solved many problems. None of the Bmerican personnel who visited the German plants could devote the time that was required t o trace through the process development. Even if the time were available, the German records and the German personnel were so widely scattered that detailed information was almost'impossible t o accumulate. Of necessity, the material presented here is chiefly a description of the existing processes. The three most prominent thermoplastic synthetic polymers, polyvinyl chloride, polystyrene, and polyvinyl acetate are produced, at least in part, in I. G. operations by continuous polymcrization mcthods. Examples of three principal techniques used for pioducing vinyl polymers were included among the continuous proccsses in opelation: mass polymerization, a system in which polymerization is conducted in essentially pure monomer without dilution or dispersion of the monomer; solution polymerization, in which the monomer is diluted with B miscible, inert solvent; and aqueous emulsion polymerization, in which the monomer is emulsified or dispersed in water, are all
EFORE and during World War 11,Germany was the world's
largest producer of thermoplastic polymers. Although the products of this class covered a wide range of chemical types, the bulk of the materials were those obtained by the polymerization of vinyl type monomers because these monomers were obtained from the basic raw material-coal. Development work in Germany directed towards the production and application of these synthetic polymers started many years ago. It is not possible or pertinent t o trace all of the many methods of vinyl polymerization which were developed and utilized over the period of years. However, it is of particular interest, from the engineering and industrial chemical viewpoints, to examine the progress which was made in producing vinyl polymers by continuous methods. Considerable information concerning t h e German continuous .polymerization processes has been made available through reports issued by the Department of Commerce, Office of Technical Services (OTS). However, the detail is scattered through a number of reports. It has been the privilege of one of the authors not only t o have had the benefit of these reports, but also t o have visited Germany under the auspices of OTS, observed some of the processes, and discussed them with German personnel. As was all the chemical industry in Geiniany,.the production of high polymers was a virtual monopoly of I. G. Farbenindustrie A. G. (I. G.). This organization has done the only work in Germany directed toward continuous polymerization process development. 1
Present address, Monsanto Chemical Company, Texas City, Tex.
.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1948
represented. The most important processes which have been reported are: mass polymerization of styrene a t the Ludwigshafen plant; aqueous emulsion polymerization of vinyl chloride a t the Ludwigshafen and Schopau plants ; mass polymerization of vinyl acetate at the Hoechst plant; and solution polymerization of vinyl acetate a t the Hoechst plant. The processes are given essentially in the order of their successful development and operation. That none of t h e specific chemical types was produced exclusively by continuous methods is shown by the production capacities of I. G. for the year 1944 (5). Totp,: l ’ u , $ ~ i i ~ t i n n Prodiurrion Cnriscity by
500
Continuous Nethod?. l l e t r i c Ton, Month 425
2300
800
800
150
C.paclry,
llerric Ton .\lonth Polystyrene Polyvinyl chloride Polyvinyl acetate
Production was limited for various reasons. That portion of the polystyrene produced by discontinuous methods was a newer modification produced in limited quantities by batchwise emulsion polymerization. I n the case of vinyl chloride a considerable production capacity had been installed before the continuous method was developed. A variety of polyvinyl acetate final products were produced but only a few of these were by continuous methods. Further technical limitations are discussed in the following sections where the specific processes are described. Evidently, the development of the continuous methods by I. G. started with a consideration of polystyrene some years ago, because a process for this material was nearly completed in 1933 ( l a ) . I n a sense the principal method developed for styrene set a general pattern or type of process that was followed for the other materials. This general method calls for the introduction of monomer or partially polymerized material at the top of a tall cylindrical vessel along with the necessary catalysts and other additives, and the removal of the polymer, polymer solution, or polymer dispersion from the bottom of the column or, in some cases, from a second column in series with the first. The method takes advantage of the increase in specific gravity of vinyl compounds during polymerization. The polymer, whether in mass, in solution, or dispersed as a n emulsion, will tend t o settle. Because all vinyl polymerizations are pronouncedly exothermic, the diameter of the towers is kept relatively’ small in order that the heat of reaction can be removed uniformly and the downward flow of: material allowed t o be constant without the occurrence of channeling or mixing. It was necessary t o develop methods for the removal of finished polymer from the polymerization equipment that were in keeping with the properties of the polymer and its ultimate use. The materials of construction were, of course, dictated by the properties of the specific polymerization systems, and by the availability in Germany of equipment and materials. The following sections describe the continuous vinyl polymerization methods which were in large scale commercial operation in Germany during 1944. With but one exception, production equipment was intact at t b e close of the war and operation has continued, at least in part, to the present time. STYRENE POLYMERIZATION
Two continuous polymerization methods were used by I. G. for the production of polystyrene (6, IO). The most extensively used was the tower method whereby at least SOY0 of the Gepman polystyrene was produced. The second method, a n exception t o the general procedure, and used t o only a limited extent, carried the polymerization only part way and then removed excess monomer by evaporation on a vacuum drum dryer.
655
TOWER METHOD
Equipment. The essentials of the equipment used in the tower process for polystyrene are shown diagrammatically in Figure 1. DESCRIPTION AND CHARACTERISTICS. Two vessels approximately 54 inches in diameter and 54 inches in height constructed of aluminum and having a capacity of 530 gallons are used with each tower. These vessels have internal pipe coils through which water is circulated for temperature control. The agitators rotating at 50 t o 60 revolutions per minute are aluminum, in the form of flat paddles approximately 28 X 28 inches, on a stainless steel shaft. The agitator shafts are sealed with rather conventional water-cooled stuffing boxes. A troublesome problem of polymer accumulation in the packing was eliminated by the introduction of a stream of nitrogen into the stuffing box. This nitrogen stream sweeps monomer vapor back into the kettle and also provides an ine& gas blanket in the kettle. The gas stream from the kettle is vented through a small water-cooled, pipe coil condenser. The monomer is added t o the kettle through a n indicating rotameter but the control of the monomer strram is manual.
NT
.SPEED EXTRUDE GRANULATED POLYSTYRENE
Figure 1.
Styrene Polymerization-Tower
Method
The gasket between the kettle cover and shell is a heavy felt paper, covered with a n Igamid (polyamide type polymer) paste. The packing around the agitator shaft is braided asbestos impregnated with talc. The tower is constructed of stainless clad steel (German type V2A) and is divided into six approximately equal sections. The diameter is 29.5 inches and the over-all length approximately 18.9 feet. The topmost section has only a n external jacket. The succeeding four sections are equipped with both internal coils and external jackets. The bottom section, which includes a coneshaped adapter, has an internal coil and external resistance type electrical heaters. The internal coils and jackets in each section are connected in series and can be supplied with either water or steam as needed, and each section can be individually controlled. The internal helical stainless steel coils have a radius of approximately 8 inches and in each individual coil there are about fourteen turns. The coils are centrally located. I n addition t o the opening for the introduction of monomer-polymer solution from the prepolymerization kettles, the cover of the tower has openings for a sight glass, nitrogen stream and vent. A short section, .approximately 5 feet, of the vent is iacketed t o serve as a reflux condenser. T o the bottom of the column is attached a small stainless steel screw extruder. This extruder has a screw diameter of approximately 2 inches and length of approximately 15 inches. The jacket is electrically heated. The screw is driven through a variable speed drive. The die or outlet of the extruder is designed t o discharge two strips approximately 1.25 X 0.25 inch. The discharge from the extruder fall8 on a continuous metal belt which serves as a conveyer and also as a cooler. Heat is removed from the product by contacting it with water-cooled rollers which also hold it in position on the belt. A simple breaker a t the end of the conveyer produces strips about 2 inches long which ?re collected in a hopper t h a t feeds into
656
INDUSTRIAL AND ENGINEERING CHEMISTRY
a n attrition mill which subdivides the product into its final granular condition. OPERATION.T o start operations, the prepolymerization kettles are charged with approximately 3100 pounds (360 gallons) of styrene, and are heated at 80 * 2" C. until the polymer content reaches 33 to 35r0. This requires a polymerization time oi 65 hours. At the end of this time, flow of prepolymerized sirup into the tower is started. At the start the temperrtures are maintained a t 100" C throughout the tower. As the column fills the tempwature of the lower sections are gradually increased. When t h e column is filled, after about 35 hours of flow, the temperatures have been adjusted to the normal operating conditions. The temperatures of the tower increase from top t o bottom. The external jacket of the uppermost section is maintained at 100" t o 110' C. The same temperatures are used for the external jacket and internal coil of the second section. The next two sections, the center sections, are maintained bath in jackets and coils a t 150" C. The jackets and the coils of the two bottom sections are controlled a t 180" C. The cone section at the bottom, which serves as the adapter t o the extruder, is heated t o approximately 200' C. The extruder cylinder is maintained a t about the same temperature as the cone. When operating in a steady state, freshly distilled styrene, free of inhibiter and without added catalyst, is pumped into each of the prepolymerization kettles a t a rate of 49 pounds per hour, or a total of 98 pounds per hour for one polymerizing unit. The propolymerizers are maintained a t 80" * 2" C. by means of circulating water at a temperature of 74" t o 78" C. These temperatures are recorded by instruments but are manually controlled. From the bottom of the prepolymerizers streams are discharged into the tower. These streams contain 33 t o 35% polymer and the rate of flow is the same as the rate a t which monomer is charged into the prepolymerizer. The flows from the prepolymerizers are manually adjusted so t h a t the liquid content in each of the vessels remains constant at 360 gallons. The polymer content of the material entering the polymerization tower is determined a t frequent intervals by means of a refractive index measurement. This is a simple test based on the linear relation between polymer content and refractive index. If this polymer content is not within prescribed limits, adjustments in temperature of the water t o the prepolymerizers are made. The liquid level in the polymerization tower is maintained a t about the bottom of the uppermost section by controlling manually the speed of the extruder screw which discharges the column. Capacity and Quality of Product. The capacity of each unit -that is, two polymerizers and one tower-is approximately 1 metric ton (2240 pound&)per day and this production can be maintained steadily over long periods of time if the units are properly operated and the quality of monomer well controlled. To be of proper quality, t h e monomer must contain 99.5% styrene and must be essentially free of divinyl benzene. The maximum allowable content of this latter impurity is given as 0.03%; the presence of a greater quantity results in the accumulation of insoluble polymer on the walls and particularly at the joints of the sections of the polymerization tower. The quality of the product by this method is good and coinpares favorably with the average quality of domestic polystyrene available in the United States market. It is reported t o have a n average molecular weight of 187,000 as determined by ultracentrifuge methods. Discussion. Although it is not possible t o trace completely the progress of the development work, some of the reasons for the particular design may be stated. Prepolymerizers were included because, in the early development, B tower alone was found t o be unsuitable as the polymerizing equipment. Several factors contributed t o this. It is considered necessary, in order t o obtain a product of proper molec-' ular weight, t h a t the early stages of the polymerization be carried out a t the relatively low Lemperature of 80' C. until an appreciable degree of polymerization is reached. Because the polymerization rate is low a t this temperature, an appreciable retention time is required; this necessitates a n excessive length of column. T o complete polymerization in a reasonable length of time, and also t o reduce the viscosity of fully or nearly completely polymerized material so t h a t it will flow through the equipment, it is necessary t h a t the temperature at t h e bottom of the column be raised t o 200' C. If pure monomer is intro-
Vol. 40, No. 4
duced into the tower, the differential temperature between the top and the bottom becomes large, and as a result, temperature control a t the top becomes difficult because of convection currents set up in the relatively low viscosity material in the upper sections. The prepolymerizer eliminated these difficulties. I n regard t o quality, 3375 polymerization a t 80" C. is considered satisfactory. Up t o a conversion of 35%, polymerization is easily controlled in the simple vessels which are used as prepolymerizers. At 35% polymer, the solutions a t 80" C. have nearly reached the top viscosity where satisfactory aEitation and maintenance of uniform temperature are still possible, and the material flom fairly readily. The prepolymerizers were divided into two units because a t one time these were the most vulnerable t o breakdown due to accumulation of polymer in the stuffing boxes. It was necessary t o shut down these units and clean and repair the stuffing boxes a t frequent intervals. With two units complete shutdown of the tower could be avoided, although running a t reduced rate required a lowering of the column temperatures. The introduction of a stream of nitrogen into the packing chamber has eliminated this difficulty. The German personnel state that the prepolymerization could be carried out in one vessel of larger size. The tower was divided into six individual sections t o simplify construction, and also for convenience in disassembling and cleaning the equipment preliminary t o making any repairs which might be necessary t o the internal heating coils. The internal heating coils were included because it was found that satisfactory temperature control could not be achieved with the jackets alone. At some elevations in t h e tower, heat of polymerization is being removed, whereas at others, heat is being imparted t o the material in the column. I n either case, t h e heat conductivity of the material is extremely poor and a considerable temperature gradient between the center and the walls can result. The internal coils were introduced t o lessen t h e distance through which heat must flow and to ensure a uniform temperature a t each elevation. This uniformity of temperature is essential because the viscosity of the material is extremely temperature-sensitive. Consequently, temperature irregularities result in channeling of the product in the column. I n addition, local overheating increases the p r o p o ~ i o nof lowmolecular-weight polymer. The temperatures a t which the column was operated were chosen empirically a s the optimum from the viewpoint of both capacity and quality. The I. G. technicians visualize or speculate that, in spite of the rise in temperature as the material approaches the bottom of the tower, both viscosity and specific gravity increase progressively from top t o the bottom. All in all the process operates satisfactorily. The labor requirements a t approximately 10 man-hours per ton of product are moderate. The product is well protected throughout and contamination is at a minimum. There are no troublesome material-handling problems. The packaging of finished product is the only operation in which there is manual handling. DRUM DRYER METHOD
For the details of the drum dryer method, the second and lesser used method for the production of polystyrene, we are dependent entirely upon the description given by the German personnel. The building in which the only two completed units were located was entirely destroyed by bombing. At present the equipment is completely buried in a heap of rubble. The drum dryer technique is a mass polymerization method. I n this case the polymerization is not carried t o completion, for after partial polymerization, the excess monomer is removed. Equipment. The equipment described by the German personnel is shown in Figure 2.
-
April 1948 STYRENE
INDUSTRIAL AND ENGINEERING CHEMISTRY
0 a
WATER
BRINE
r4
VACUUM JET
10-15
Figure 2.
Styrene Polymerization-Drum
Dryer Method
The prepolymerization vessels are similar in design and construction t o those used with the towers. The dryer rolls are approximately 20 inches in diameter by 48 inches long and are chrome plated. Steam at approximately 200 pounds per square inch gage is supplied t o the rolls which revolve a t 1.5 t o 2 r.p.m. The pressure in the dryer chamber is maintained a t 10to 15mm. absolute pressure by means of a three-stage, condensing steam ejector. The polymer is scraped from the rolls with brass knives and falls into small collection carts in the dryer chamber. Locks are provided t o allow periodic removal of the product, which, after cooling, is broken and ground. Condensing equipment is included t o recover the monomer. A water-cooled condenser of 16 square feet and a brine-cold unit of 8 square feet are used in series. OPERATION.The prepolymerizers are operated in the same manner as for the towers. These produce 98 pounds per hour of a solution containing 33 t o 35% polymer: The prepolymerized solution or sirup is charged t o the nip of the dryer rolls which are set to distribute a film on each roll approxima.tely 0.060 inch thick. The knives are adjusted t o scrape the rolls clean. The monomer is condensed and recycled t o the prepolymerization vessels.
65 inches in diameter and has a capacity of 3600 gallons. The second has the same over-all length but is only 33 inches in diameter and has a capacity of 925 gallons. Both the vessels are equipped with agitators, of very simple blade design, driven with variable-speed motors so that the rotational speed can be varied up t o a maximum of about 50 r.p.m. The polymerizers are designed for a n operating pressure of 150 ounds per square inch. In the cover of the larger autocrave, openings are provided for the introduction of the polymerizing components-water, vinyl chloride, emulsifier, catalyst, etc. I n the bottom of the autoclave there is a large manhole opening provided, in addition t o the liquid outlet, for access for cleaning and the removal of lumps. The large polymerizer is cooled by a circulating brine system. The temperature of this system can be controlled a t any temperature from -20' C. up t o the polymerizer temperature by regulating the amount of cold brine (-20" C.) which is introduced into the circulating systems. The smaller vessel is cooled by passing a stream of cold water through the jacket. The stuffing boxes and the agitators are water cooled, and are packed with braided asbestos impregnated with talc. A small, amount of lubricating oil is introduced into the packing gland. The gaskets for this polymerizer are made of blue asbestos impregnated with Buna N. Deionized water, catalyst solution, and emulsifier solution prepared batchwise are fed through indicating rotameters into a combination mixing and consrant head tank from which the overflow is pumped to the polymerizing vessels. Flow rate of these streams is controlled manually. Storage vessels are constructed of steel and are lined with unplasticized polyvinyl chloride sheeting. The piping is of aluminum. The vinyl chloride is pumped into the polymerizer by means of a steel, multistage centrifugal pump. The pump operates against a constant head. The main stream circulates back to the monomer storage vessel, and the stream t o the polymerizer is drawn off through a control valve. The vinyl chloride is handled in steel PlPlng. From the polymerizer, the emulsion is discharged into one of two large tanks. These are used intermittently in order that the entire system can be kept under pressure. Actually, these vessels are of the same design as the larger polymerizing vessel. This was a matter of chance because the tanks are remnants of other polymerizers which were destroyed by bombing. The design of these receivers is not of any great importance, except t h a t the must stand the operating pressure and must be equipped w i d agitators. Three methods for the recovery of solid polymer from the emulsion are used in different locations in Germany: continuous drum drying (16); continuous spray drying (IS); and coagulation, centrifuging, and tray drying as an intermittent operation Y). The first two are most commonly employed and the use of eit er results in an almost completely continuous process. OPERATION. T o start operation, the first polymeriser is charged with 1530 gallons of water, 380 gallons of emulsifier solution, and 25.5 gallons of 1% potassium persulfate solution. The polymerizer is evacuated t o 29-inch vacuum, and 1450 gallons of
Capacity and Quality of Product. Since approximately 65% of the monomer is recycled, the capacity of one of these units is less than that of a tower unit. A daily production of 750 t o 800 pounds is obtained. The quality of the product differs slightly from that obtained from the towers. The average molecular weight, estimated t o be 370,000 by ultracentrifuge methods, is higher than that obtained in the tower. The molecular weight distribution curve is probably not as wide. These differences are reflected in a slightly greater strength and higher heat distortion temperature for the material from this process. The flow properties (when it is used for delicate exVINYL CHLORIDE trusion) were also said to be more uniform. I
.
L
VqNT
-r
VINYL CHLORIDE POLYMERlZATlON
All polyvinyl chloride produced by I. G. was obtained by emulsion polymerization. Originally, this had been conducted batchwise in a large installation at Bitterfeld (8, 13). More recently the continuous metbod described here was developed and installed at Schopau (3, 7 ) and Ludwigshafen (7, 16). It is rather evident that the development of this process was encouraged by the successful operation of the styrene process.
Equipment. Equipment used for the continuous polymerization of vinyl chloride in emulsion is shown diagrammatically in Figure 3. Two polymerizing vessels connected in series are used. Both are glass lined and jacketed for heating and cooling. The f i s t of the two kettles is the larger. It is approximately 23 feet long and
657
VfNT
i _
ROTAMETERS
WATER^
1
CATALYST SOL EMULSIFIER SOL,-
POLYVINYL CHLORIDE
1 /I /I
WAOULATION BRINE TO REFRIG.
GLASS
CEMTRl
LINED
-
1
GLASS LINE0
WATER
I
WATER
TRAY
DRIER
Figure 3. Polyvinyl Chloride
POLYVINYL CHLORIDE
658
INDUSTRIAL AND ENGINEERING CHEMISTRY
vinyl chloride ale pumped in slowly. Warm water is circulated through the jacket of the polymerizer and as the desiied polvmerizing temperature is approached, the water is cooled ana, finally, when polymerization is underway, refrigerated brine is circulated t o maintdin the polymerizer a t the proper temperature. After several hours' polymerization, the specific gravity of the emulsion sampled from the bottom is measured and when a specific gravity of 1.024 is reached, a n addition of 97.5 gallons of vinyl chloride is made. Again, when the density reaches 1.032, another 97.5-gallon quantity is added. These additions of vinyl chloride are continued a t specific gravity check points of 1.040, 1.048, 1.056, 1.064, and 1.072. When the specific gravity reaches 1.084, continuous streams of vinyl chloride and other ingredienbs are started. At this point the first polymerizer is nearly full to operating level, and the flow t o the second polymerizer is started. The flow t o the polymerizer, when operating a t a steady state is: Material Water deionized Emulsifier (10% solution of synthetic eniulsifiei) Potassium persulfate (1% solution) Vinyl chloride
Quantity per Hour, Gal, 41
.
14 5 1 43 4
The liquid level in the two polymerizers is controlled manuallv by adjusting the flow from each. The level is held at the top df the agitator blades. The fluctuation in level can be as much as several feet. However, contact of the emulsion with the agitator packing must be avoided t o prevent contamination with grease. The pressure in the first autoclave (about 5 atmospheres) is somewhat greater than the pressure in the second because the polymerization is 88% complete when the emulsion leaches thc second vessel. I n turn, the pressure in the storage vessels is less than that in the second vessel. The polymerization, when the emulsion reaches the storage vessels, is Yay0 complete. I t is also possible t o cool the emulsion in the storage vessels slightly and increase the pressure differential. The course of the polymerization is followed by checking the specific gravity of the emulsion leaving each of the polymerizing vessels. If the polymerization is proceeding normally, the specific gravity of the emulsion, after excess monomer has evaporated, will be constant within narrow limits. If t h r rate of pollmerimtion has slowed down and polymerization is not complete, the specific gravity will fall. The specific gravity a t each control point depends upon the recipe of ingredients charged. For the quantities given above, the fiwres are 1.086 and 1.120 for the first and second vessels, respectyvely. The temperature conditions within the polymerizer also serve as a check on the rate of polymerization. When the rate of polymerization drops off, the intcrnal temperature falls and approaches that of the jacket. The molecular weight of the final pioduct is controlled by regulating the temperature in the polymerizcr. The molecular weight decreases as the temperature increases. The tcmperatures vary from 38" t o 50" C. depending upon the molecular weight of the product desired. On leaving the second polymerization vessel, the emulsion enters a storage tank. When a tank is full the excess monomer is vented t o the atmosphere and a solution of either sodium carbona t e or disodium phosphate is added and mixed with the emulsion. These materials serve t o stabilize the product against thermal decomposition. It has been reaorted that some aolvmerization units a t Scho. " VINYL ACETATE
PROPIONALDEHYDE COLD
CONDENSER
WATER
I l POLYVINYL ACETATE
Figure 4.
Polyvinyl Acetate Mass Polymerization
Vol. 40, No. 4
pau were constructed with only one polymerizing tower. The discharge of this tower was 87 to 88% polymerized. In these units excess monomer was allowed t o evaporate by passing the emulsion continuously through an open spiral trough contained in a tank. To ensure removal of the monomer, this evaporating unit was maintained under partial vacuum. The next step in the production is the isolation of the solid polymers. The two methods most commonly used can be classified as direct drying. The contained water is evaporated on either a drum dryer or in a spray dryer. The third method involves coagulation by the addition of alum, partial dewatering on a centrifuge, and drying in a continuous tray dryer. These techniques can be considered as aftertreatment applied to the finished polymer and the details should not be included here. Quality. It is difficult to evaluate the quality of the polyvinyl chloride obtained and attempts to compare I. G. products with materials produced in the U. S. may be misleading unless many factors other than processing methods are considered. For many applications in Germany the products appear adequate. A number of grades of polyvinyl chloride are produced by I. G. These variations in t,ype are obtained in the equipment discussed without major alteration of engineering design or operating technique. The ma,in difference between various grades is in average molecular weight and typc and amount of stabilizer. I n addition, some change in electrical properties is achieved when hydrogen peroxide is substituted for potassium persulfate as the catalyst. The hydrogen peroxide yields materials 'of greater electrical resistivity because the product is free of combined sulfuric acid. The physical form of the product obtained by spray drying is the basis for one specific product type. The small uniform spherical particles obtained by spray drying the emulsion are of importance in a t lenst one method of applying and using polyv'nyl chloride. This method is known as the paste technique (9. I O ) . Discussion. It is of interest to consider the conditions which the I. G. technicians visualize as cxisting within the polymerizer. Cnfortunately there x-ere essentially no quantitative data to support these speculations. The I. G. chemists state that the main controlling factor, in regard t o capacity, is the rate a t which heat of polymerization can be removed from the equipment. This is not entirely a matt,er of poor heat transfer coefficient nor is it solely a problem of obtaining sufficient temperature differential. The heat of polymerization must be removed, but the t,emperature of the emulsion must be maintained as uniform as possible and excessive turbulence must be avoided. The Germans point out that attempts either to increase the rate of feed or use a cat,alyst system by which rate of polymerization is increased, result in a loss of temperature control. Attempts t o regain temperature control by either increasing rate of agitation or temperature differential result in excessive turbulence. -4delicate balance is established and maintained within the system which limits allowable temperature differential between contents and jacket and also limits the rate a t which heat can be liberated. I n the control of temperature arid maintenance of flow the agitators play an important and delicate role. The agitators are designed t o give a gentle swirling motion to the emulsion. Some agitation is necessaiy t o form the emulsion, but if excessive, may result in coagulation. In so far as heat conditions are concerned, the agitation, in addition to imparting motion which will improve heat transfer, also sets up an internal recyc1:ng which is necessary to obtain a uniform temperature. The following experimental cvidence was cited to give an idea of the extent of this internal recycling. Blue dye was introduced with the monomer. The blue appeared in the product discharging from the first polymerization vessel in less than 20 hours and could be detected for a duration of more than 100 hours. Theoretically, the blue
.
April 1948
should have appeared in 28 hours and should have been sustained for only a short period. Although this process operated fairly successfully, it required the close and almost continual supervision of a technician very familiar with the mechanism of polymerization and, in particular, the details of this specific system. Rates of polymerization are affected by many extremely small factors-the presence of ions in the water and the presence of oxygen and minute impurities in the monomer or other ingredients. It is not pertinent or possible t o discuss all the contingencies which can be encountered in a complex and delicate polymerization system of this type. To anticipate or diagnose difficulties that are encountered requires experience and skill. The accumulation of lumps of polymer and coating of polymer on the walls requires occasional shutdown and cleaning. The periodicity of these shutdowns varies. The average period of continued operation is a matter of weeks and, in some instances, extends into months.
ET
VINYL ACETATE POLYMERIZATION
Only a small portion of the polyvinyl acetate produced in Germany is obtained by continuous polymerization (1,4 , 8 , 1 1 , 1 4 ) . Some polymer sohtions and some solid, mass Polymer are SO produced, but the results are not entirely satisfactory. The largest portion of polyvinyl acetate utilized in Germany is in the form of aqueous emulsions and dispersions. Attempts to convert the batch processes for these systems t o continuous operation were discouraged by accumulation of a l a w of solid polymer on the walls of polymerizing vessels. This difficulty which had not been eliminated from the batch processes was accentuated in continuous operation and prevented accurate temperature control. Obviously the two operating Processes are of fairly recent origin and were patterned after the more completely developed and successful methods for polystyrene and polyvinyl chloride. CONTINUOUS M A S S POLYMERIZATION OF VINYL ACETATE
Continuous mass polymerization has been applied with partial success t o only a narrow molecular-weight range of polyvinyl acetate. The very ~ow-mo~ecu~a~-weight materials are too fluid a t the polymerization temperature and turbulent conditions result in the polymerization tower. The high-molecular-weight materials do not have sufficient mobility or flow t o travel through the equipment unless a pressure considerably greater than atmospheric is used. The range of molecular weights which be produced is 30,000 t o 60,000. Equipment. In general, the equi ment shown in Figure 4 resembles that used for polystyrene g y the tower method. The polymerization tower is constructed of aluminum. It consists of a jacketed cylindrical section 23.5 inches internal diameter by 11 feet-6 inches in height in which a torpedo shaped heater is centrally located in the bottom of the tower. This torpedo has a diameter of 15 inches and is approximately 8 feet long. The lower end of the column is fitted with a nozzle which discharges the finished product as a band onto an endless steel belt where it is cooled by a blast of air. On top of the column is an enlarged section 39 inches in diameter by 31 inches in height equipped with an agitator. This allows release of vapor and ensures mixing of monomer returning from reflux condenser with the incoming stream of fresh monomer and catalyst. The equipment used for preparing the Solution of monomer and catalyst IS all constructed of aluminum. The tanks used for preparing and storing these solutions are equipped with internal coils for coaling and maintaining the temperature of the contents. The monomer-catalyst solution is metered into the polymerizing tower by means of a motor driven, stainless steel, reciprocating pump of adjustable stroke. OPERATION.The feed for the unit is prepared by dissolving the catalyst (benzoyl peroxide) in a portion of the vinyl acetate and allowing the solution t o settle so that the water contained in the catalyst can be separated from the solution. Diring this operation and subsequent storage before feeding t o the poly'
659
INDUSTRIAL AND ENGINEERING CHEMISTRY CONDENSERS
Figure 5.
Polyvinyl Acetate Solution Polymerization
merizadon tower, it is kssential that the monomer-catalyst solution be maintained a t 25" C. or less. The concentrated catalyst solution is then mixed with the remainder of the vinyl acetate and other ingredients t o produce a solution of the following formula: vin 1 acetate 3140 pounds, benzoyl peroxide (100%) 27 pounds, anJpropionaldehyde 11.8 pounds. (This formula produces a material with a molecular weight of approximately 40,000. T o increase molecular weight, the benzoyl peroxide and propionaldehyde are reduced, and t o decrease molecular weight, more catalyst ingredients are used.) To start operations, with an empty polymerizer, about 45 pounds of monomer-catalyst solution are charged t o the tower, and hot water a t about 75" C. is circulated in the jacket and torPedo-shaPed inner heater* The flow of monomer-catalYst started at a rate of 45 pounds per hour This rate is increased by 22 pounds per hour up to a rate of 180 pbunds per hour. When the unit is three quarters full (152 gallons), the discharge nozzles are partially opened. The opening is increased as the column fills until the discharge rate equals the charge rate. The liquid level is maintained a t the top of the agitator in the enlarged upper section of the tower (approximately 205 gallons). The temperature of the circulating water is adjusted in the range 75" t o 82' C. so that there is a small reflux. When in operating condition, both the discharge and feed are increased to between 200 and 225 pounds per hour. The retention time in the tower is approximately 8 hours. Capacity and Product Quality. The capacity of the unit described is 50 metric tons Per month* A smaller unit with % Capacity of 30 tons Per month was in operation before this one was constructed. The quality is said to be entirely comparable to that obtained by a batch process. The molecular weight control is good and the monomer content in the polymer is maintained at less than 0.5% Discussion. The process operates satisfactorily but requires occasional shutdown t o remove a less mobile material which gradually collects on the walls of the tower. This can usually be removed by boiling with a good solvent-thy1 acetate-but if allowed t o remain too long tends t o become insoluble. A deficiency of the procedure is its nonversatility. Only one or twomembers of a seriesof varyingmolecular weight can be produced. At least four other mass polymerized products were produced by batch methods. CONTINUOUS MASS POLYMERIZATION O F VINYLACETATE I N SOLUTION
Solutions of polyvinyl acetate in ethyl acetate were obtained continuously, although the volume was only a small part of the total polyvinyl acetate production. Equipment. The towers used for polymerization as shown in Figure 5 are enameled steel, or glass lined. Both are 23.5 inches in diameter and a p roximately 21 feet in height. The first column in the series {as an enlarged top section 35 inches in diameter and 51 inches in height which was included t o allow area
INDUSTRIAL AND ENGINEERING CHEMISTRY
660
for release of vapor. The first column is also equipped with a n agitator of stainless steel which consists of a series of paddles on alternate sides of the agitator shaft. The shaft revolves at 40 r.p.m. The second column has no agitator. Both columns are jacketed and heated with circulating hot water. They are equipped with tubular reflux condensers of 195 square feet and 16 square feet of cooling area, constructed entirely of aluminum. Mixing equipment for preparing solutions of vinyl acetatepropioneldehyde, and ethyl acetate-peroxide catalyst is of aluminum. The two streams of ingredients are metered and delivered t o the column by reciprocating, stainless steel, proportioning pumps. On leaving the second column, the solution flows into two 750-gallon afterpolymerization tanks which are glass lined and equipped with aritators and jackets. The temperature is maintained in these tanks by circulating hot water. Solvent can be added in these tanks t o adjust concentration. OPERATION. Polyvinyl acetate of several molecular weights is produced by this procedure. The general procedure is the same for each with the following changes in formulation and flow rate: German designation Approximate mol. wt. Formulation Vinyl acetate, lb. Ethyl acetate lb. PropionaldehGde, ib. Benzyl peroxide (100’%), lb.
Mowilith 15 14,000
Mouilith 20 20,000
Mowilith 30 32,000
1.725 715
1,725
1,725 715
~
Production rate, lb./hour
77.5 78.5 324
715 17.3 32.8 224
8.87 3.45 168
The propionaldehyde is dissolved in the monomer. and the benzoyl peroxide is dissolved in the ethyl acetate. The w t e r which is contained in the benzoyl peroxide is allowed t o settle and is separated from the solution. The solutions are fed separately through the proportioning pumps in the ratio of 70 parts monomer solution t o 30 parts of solvent-catalyst solution. To an empty polymerizer a charging rate of about two thirds the final rate is used a t the start. When the polymeriaers are about one half full and polymerization is underway, the flow is increased t o the final rate. The jacket temperature is maintained at 80” C. in both towers by means of circulating hot water. The temperature in the towers is the temperature a t which the solutions reflux, approximately 80’ C. The reflux stream in the first tower is heavy, whereas t h a t in the second is relatively small. As the solutions leave the second tower the monomer content is 5 t o 8%. This is reduced to less than 2% by maintaining the solutions in the afterpolymerization tanks a t SO” C. for several hours.
Discussion. This process is somewhat of a n exception t o the pattern set by the polystyrene and polyvinyl chloride ton-er systems, In this case the towers can be considered as prepolymerizers feeding the vessels which are characterized as afterpolymerization tanks, but in which the polymerization is actually completed. The towers operate continuously while the finishing operation is intermittent. This process operated fairly satisfactorily with one outstanding deficiency. The towers require frequent boiling out with solvent t o remove a less soluble polymer t h a t forms and collects on the walls. The accumulation of this polymer increases as the molecular weight increases. The cleanings must be rather frequent because the material which collects becomes increasingly insoluble with time. Glass-lined equiprnent was chosen for this service because the insoluble polymer collection is less if the surface of the polymerizer is smooth. GENERAL DISCUSSION
The I. G. methods were ingenious and the developments were in most respects complete t o the point of operable, reliable prooesses. Considering the time during which the processes were developed and the magnitude of the production, the operating equipment wag on a smaller scale than was expected by one familiar with U. S production methods. For example, before bombing raids reduced the size of the plant, the polymerization of styrene by the tower method was carried out in fourteen identical units, t w h having a capacity of one ton per day. These may have d l bet n installed a t t h e same time, but it is doubtful if this was thtl case because in the years just preceding the war plastics uwge was increasing not only in Germany but throughout the world in markets supplied by I. G.
Vol. 40, No. 4
The detailed design and construction of individual pieces of equipment as well as their installation were exceptionally good and displayed the outstanding mechanical skill of German craftsmen. There was no crowding of equipment. The buildings were generally large and roomy and of outstandingly sturdy construction. The fourteen polystyrene tower units were housed in a steel and reinforced concrete building six stories tall and over 200 feet long by approximately 40 feet wide. These few specific operations should not be considered representative of the entire German chemical industry. Hovtever, there was essentially no instrumentation of the processes observed. Temperatures were generally recorded by instruments, but valve and regulator adjustments necessary t o maintain uniform conditions mere usually carried out manually. Maintenance of liquid levels and rates of flow was dependent upon close attention from the operators. I n some cases narning signals were used t o indicate when temperature conditions were outside of prescribed limits. I n summary, the processes described here reflected the ingenuity of the German chemists, and the equipment displayed outstanding craftsmanship, but in terms of material and manpower, the producing facilitics represented a considerable investment by United States standards The processes did not include the efficiencies and refinements which are expected in well engineered processes in t b c chemical industry of the Knited States. I
A C K \ O W LEDGR’IENT
The authors are deeply indebted to many people who cannot be individually identified but who are, or who were, members of the staffs of Office of Technical Services, U. S. Department of Commerce; Field Information Agency, Technical Office of Military Government United States in occupied Germany; as well a s the staffs of other military agencies which preceded these in the accumulation of technical information in Germany. They are also. appreciative of the facilities provided by the Monsanto Chemical Company for the preparation of this manuscript and are indebted to the members of that organization who willingly assisted in the preparation. LJTERATURE CITED
(1) Baum,
S.J., and Dunlop, R. D., “Polymerization of Vinyl Ace-
tate in Germany,” FIAT Final Report 1102, U. s. Dept. of Commerce. OTS PB 78741. (2) Boundy, R. H., and Hasche, R. L., “Manufacture of Polyvinyl Chloride, I. G. Farbeninduutrie, Bitterfeld,” I b i d . , OTS PB 514.
(3) I b i d . , “Manufacture of Vinyl Chloride and Polyvinyl Chloride, I. G. Farbenindustrie, Schopau;” OTS PB 403. (4) I b i d . , “Report on Visit t o I. G. Farbenindustrie a t Hoechst/ Main,” OTS P B 1771, 1849, 23741. ( 5 ) DeBell, J. M., Goggin, W. C., and Gloor, W. E., “German Plastics Practice,” pp. 13, 40, Springfield, Mass., DeBell and Richardson, 1946. (6) I b i d . , pp. 26-33. (7) I b i d . , pp. 63-66. (8) I b i d . , pp. 103-111. (9) I b i d . , pp. 374, 415-25. (10) Holbach. E. IT.. “Miscellaneous Chemical Processes and Plastic8 Machinery,” FIAT Final Repoit 724, U. S. Dept. of Commerce, OTS PB 34808. (11) Kline, G. >I.,et al., “Investigation of German Plastics Plants,” Pt. 2, Ibid.,O T S P B 25642. (12) Office of Technical Services, U. S. Dept. of Commerce, “Remarks Relating to Continuous Polvmeiieation,” 10th Meeting of Plastics Commission, I. G. Farbenindustrie, Ludwigshafeni OTS P B 40835. (13)’Raine, €1. C., “Manufacture and Fabrication of Polyvinyl
Chloride, Bitterfeld Area,” Brit. Intelligence Objectives SubCommittee Final Report No. 999, London, H.M. Stationery Office, 1946. (14) Richardson, R. E., Kern, J. G., Murray, R. L., and Sudhoff, 1%. W., “I. G. Farbenindustrie, Hoechst/Main,” U. S.Dept. of Commerce, O T S P B 218, pp. 50-54. (15),Smith, Arthur F., “Polyvinyl Chloride Production at Burghausen and Ludmigshafen,” FIAT Final Report 862, Ibid.. OTS P B 44674. RECEIVED September 29, 1949
