Continuous Polymerization J
of Cold Rubber R. W. LAUNDRIE, E. E. ROWLAND, A. D. SNYDER, W. K. TAFT, AND G. J. TIGER] Government Laboratories, University of Akron, Akron, Ohio
F o r the continuous polymerization of butadiene and styrene a t 41' F., process conditions not encountered in the manufacture of GR-S continuously a t 117" F. are involved. This paper describes briefly the reaction vessels, charging procedures, cooling of feed stock, and the refrigeration equipment employed for the continuous emulsion polymerization of cold rubber a t the pilot plant of the Government Laboratories, University of Akron, as the agent for the Reconstruction Finance Corporation, Office of Rubber Reserve. The pilot plant unit has been satisfactory in that good correlation has been established with subsequent full scale plant production runs.
T
HE development of batchwise reactors in continuous poly-
merization systems for the preparation of synthetic rubber was initiated in 1942 by the engineers of the Goodyear Tire and Rubber Company under the sponsorship of the Office of Rubber Reserve. The continuous process for production of G R S and GR-5-10 polymers (at about 117' F.) has already been described ( 4 , 7 ) . The batchwise preparation of similar polymers at about 41' F. ( 2 , 3, 9, 1%) as well as by a continuous pilot plant process (11)has also been reported. I n this paper, the pilot plant equip1
Present address, Gladstone, N. J.
rnent utilized for low temperature continuous polymerization a t the Government Laboratories, University of Akron, is disoussed. Investigations conducted in this unit have shown good agreement with subsequent full scale plant operations conducted in the continuous polymerization system operated by the B. F. Goodrich Chemical Company for the Office of Rubber Reserve. REACTORS AND PIPING
The polymerization reaction is conducted in a series of twelve 20-gallop glass-lined vessels constructed of welded carbon steel for a working pressure of 125 pounds per square inch gage. Several of the reactors are shown in Figures 1, 2, and 3. Each reactor FS jacketed for cooling or heating the reactants. The demountable tops of the reactors are constructed of No. 304 stainless steel with openings for charging, for pressuring and evacuating, for the agitator, for thermometer wells, and for cleaning. A thermal-resistant sight glass and safety d i s h are provided. To prevent the loss of gaseous reactants along the agitator shafts, special Dura-metallic seals are used. These operate on the principle of a metallic ring rotating on a carbon block. The metallic ring is seated on the carbon block by maintaining any desired oil pressure from 0 to 200 pounds per square inch in the chamber containing the seal.
Figure 2.
Figure 1. Reactors and Feed-Stock Pumps 1439
Bank of Six Reactors
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installed and operated successfully witliin the range of 200 to 850 r.p.m. The charge is pumped, as described later, through a precooler and into the bottom of the first reactor. The latex then flows from top to bottom of succeeding reactors through the conventional 1-inch internal diameter stainless steel interconnecting lines. A manifold system of the same size pipe is used to by-pass any reactor. The latex is discharged through a single port proportioning-type valve to the stripper. This valve is pneumatically operated to maintain a back pressure in the reactor system which can be varied from 20 to 80 pounds per square inch gage. The effective reaction space of the unit, including the interconnecting lines and the feed-stock cooler, is about 287 gallons. CONTROL OF TEMPERATURE
1 Figure 3.
Single 20-Gallon Reactor
For emulsion polymerizations at about 120" F. agitation was accomplished by means of 6-inch diameter stainless ste6l Brumagim-type impellers (Figure 3). At 41" F., 6-inch diameter turbine-type agitators have proved successful. The agitators for each set of three reactors were originally driven by a 7.5-hp. motor through a variable speed drive a t 150 to 800 r.p.m. Individually controlled oil drives (Figures 3 and 4) have recently been
To remove the exothermic heat of reaction, a forced-circulatiori brine cooling system is provided for each bank of three reactors (Figure 5). The t'hermobulb for the recorder-controller can be put' in any of the three reactors. To conserve refrigeration and t,o maintain uniform brine temperatures, both the refrigeration unit system and the polymerization reactor system, except the tops of the reactors, are insulated with rock vool. The refrigeration unit employed for the emulsion polymerization of butadiene and styrene a t 41" F. consists of a Freon 12 unit ( I S ) with a capacity of approximately 30 tons a t 32" F., which has been increased to a capacity of approximately 50 tons at, 32" F. by the substitution of Freon 22 and the installation of a 50-lip. motor. The Freon 22 serves as the primary refrigerant in the system, and aqueous calcium chloride (specific gravity, 1.28) inhibited with sodium chromate, is circulated as the coolant through the jackets of the reactors. The calcium chloride brine was select,ed because of its low cost, the small losses encountered in transferring, and the desirable viscosity cliaracteristics of the brine a t low temperatures. The compressed Freon is condensed and cooled in a combination condenser-cooler storage tank. From the stora,ge tank, the Freon 22 expands into a horizontal, tubular heat exchanger with an effective heat-exchanging surface area of 325 square feet. The brine to be cooled is circulated around the tubes a t a rabe of about 40 gallons per minute. The liquid refrigerant is metered
WRA-SEAL OIL RETURN
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Figure 4.
Oil Flow for Drive Motors and Reactor Seals
July 19m
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INDUSTRIAL AND ENGINEERING CHEMISTRY HEAT EXCHANGERS
CMlTROL VPLVrs
COOLING BRINE SYSTEM FLOW OF BRINE IS COVNTER TO T M T W YLTERIALS WHGH FLOW TNiOUGH RE&CTORS IN NUMERICAL ORDER
Figure 5.
Flow of Secondary Kefrigerant
tlirough an expansion valve and expanded in the tubes a t a pressure dependent on the temperature of the effluent brine. After it leaves the heat exchanger, the expanded refrigerant enters a n economizer where t h e liquid refrigerant is cooled further before it enters the expansion valve. The savings realized with the economizer are not as important as its value in preventing vapor lock or flashing in the liquid refrigerant line upstream from the expansion valve. For safety, pressure operated cutouts are installed on the compressor; these devices stop the compressor motor a t zero suction pressure or a t a discharge pressure of 210 pounds per square inch. During the early operation of the polymerization unit, the amount and temperature of the brine were automatically controlled for each bank of three reactors by passage through a steam heated exchanger. Since the refrigeration unit was not operated efficiently and economically by this method and at various times the requirements exceeded the capacity of the unit, changes as shown in Figure 5 were made so that the temperature of the coolant circulating through the jackets was automatically controlled by the entry of cold brine into the reactor cooling system. Fresh brine is delivered from the refrigeration unit t o the reactor circulating system. The introduction of fresh brine is regulated by a proportioning valve in the fresh brine feed line for each bank of three reactors. When cooling liquid is not required by the reactors, the pressure in the line a t the discharge side of the brine feed pump on the refrigeration unit rises and opens a back-pressure valve which allows the brine to return directly to the storage tank. Since only one control instrument is used for each bank of three reactors, the flow of brine through the jackets of each bank must be high t o obtain uniform temperatures. A centrifugal pump with a capacity of about 30 gallons per minute circulates brine through the four banks of three reactors each.
CHARGING PROCEDURES
Prior to charging the ingredients, the effective reaotion spaces are purged with butadiene vapors. The discharge valve in the effluent line is then set t o release a t 100 pounds per square inch gage, and the system is filled with a 1%aqueous solution of the emulsifier. After the emulsifier solution has attained the desired operating temperature, the polymerization unit is tested for pressure and temperature and, if necessary, the control instruments are adjusted. The instrument for control of the discharge pressure is then set at any desired point between 20 and 80 pounds per square inch gage, and the pumps are started. The monomer phase, consisting of a blend of the butadiene, styrene (except that withheld for dilution of the catalyst, if the catalyst is t o be added with a separate charging pump), and the modifier, is charged from a 300-gallon weigh tank (Figure 6 ) with a diaphragm proportioning pump (IO)-maximum capacity, 32 gallons per hour and minimum capacity, 0.32 gallon per hourinto a header where the monomer phase contacts the aqueoui phase (Figure 1). The mixture is passed through a turbine mixing pump (capacity, 10 gallons per minute) (6) into a feedstock cooler and subsequently into the bottom of the first reactor. General views of this equipment are shown in Figures 1 and 6. Originally the precooler consisted of about 55 feet of jacketed 1-inch pipe. Improved heat transfer and flexibility have been gained by the installation of four helical heat exchangers ( 6 ) one of which was placed in the recycle line of the turbine miving pump. The emulsifier phase, which consists of the water (cxcept that reserved for the activator solution), the emulsifier, and the electrolyte, is charged in the same manner as the monomer phase. At various times, the activator solution has been mixed with the emulsifier or the catalyst has been solubilized in the emulsifier phase and charged as such.
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W e n not ch:~igcdwil~lillic eiiiuluifiw p h s e tlic activator soluIs piiiiiped directly by rrieari: of piston purrips (3/s-incli l ~ o r t i . .i/ana!)le stroke) ( 8 ) . \\7rcri vh:rrged iiitlcperitlt.nt,ly, tlre c:tl:tl i:, pumped as a 3 to 107, sulutiorr in The calt:tlyst and/or tht: wtivator, under p i ~ ( > , ? s(8:iii i i ~ ~:rlw , 111: ohwged iiitcrinittently 011 :ivoluinc1,ric: basis. T!rc shortstoppirig apcntu, trans, are addcd t o t.11~c:ffiiir:i bo,ck-pr HSSUrC valvo iiim
STRIPI'ERS VOW 1
