E N G I N E E R I N G AND PROCESS DEVELOPMENT (3) Higbie, R., Trans. Am. Inrt. Engrs., 31,365 (1935). International Critical Tables, Vol. 5, Kew York, 31cGraw-Hil1, 1929.
(4)
( 5 ) Kats, H. &I., 94.S~.thesis in Chemical Engineering, Univ. of
Cincinnati, 1950. (6) Kronig, R., and Brink, J. C., AppZ. Sci. Research, A2, 1942 (1950). (7) Licht, w., and Conway, J. B., IKD. ENG.CHExr., 42, 1151 (1950). (8) Perry, J. H., ed., “Chemical Engineers Handbook,” Sew York, McGraw-Hill, 1950.
(9) Shermood, T. K., Evans, J. E., and Longcor, J. 5’. A., Trans Am. Inst. Chem. Engrs., 35,597 (1939). (10) West, F. B., personal communication, Jan. 1, 1952. (11) West, F.B., Robinson, P. A., iUorganthaler, A. C., Beck, T. R , and MeGregor, D. K.. IND. ESG.CHmf., 4 3 , 234 (1981). (12) Wilke, C. R., Chem. Eng. Progr., 45, 218 (1949).
RECEIVED for review Xovember 19, 1951. ACCEPTED June 6, 1 9 3 . An abstract of the dissertation submitted by William F. Pansing t o the Graduate School of Arts a n d Sciences of the Unirersity of Cincinnati in partial fulfillment of the requirements for the degree of Ph.D., June 1931.
Cleaning Emulsion Polymerization Equipment Fouled by Synthetic Rubber latex J, S. NETTLETON, M. J. G. DAVIDSON,
AND
H. LEVERNE WILLIAMS
Process Engineering Deparfmenf and Resewch and Developmenf Division. Polymer Corp., ltd., Sarnia, Ont., Canada
R
EDUCED heat transfer coefficients can limit cold rubber pro-
surface. Another method tried Lvith limited success v a s to fill the vessel with hot water and 200 gallons of styrene, agitate for 16 duction rates because there is a lower limit for jacket temhours, drain, wash down the wall with a water jet from the manperature below which the latex freezes. It is therefore most imway, and finally scrape off the softened film before it dried. A pdrtant to keepinternal reactor surfaces clean and free of deposits. refinement of the manual scraping method involved working talc Deposits of rubber on the walls of reactors and equipment used dust into the coagulum which made the job easier when a soft, in the production and testing of synthetic rubbers are undesirable sticky coating was encountered. Refluxing benzene in the reacin other ways. The deposit not only represents loss of rubber but tor was tried without success, These methods were so laborious also becomes detached and contaminates rubber being produced. and inefficient that the more efficient cleaning method described Test equipment and sample lines are readily fouled by such in this paper lyas developed. material. Asymetrical ketones have been used in the laboratory as solPetroleum Naphtha-Cumene Hydroperoxide vents for cleaning laboratory polymerization and test equipment Solvent Dissolves Butadiene-Styrene Film ( 1 ) . The mechanism of the action was suggested b j Kinkler t o Two types of apparatus were used to determine the efficienry be degradation of the polymer to low molecular weight, soluble of solvents. One A as a percolator, illustrated in Figure 1, which fragments by the oxidizing action of peroxy compounds forme i could be immersed in a thermostatically controlled constant by exposure of these ketones to the atmosphere. Such peroy‘ temperature bath when temperacompounds form readily, and it ture higher than room temperawas further shown ( 2 ) t h a t the I / L. tures were desired. h slow stream dissolution of the polymer depends of air caused small portions of upon the presence of oxygen. Reflux, water-cooled solvent t o rise up the tube and The standard reactors in the condenser splash doim over a sample of polyPolymer Corp. plant are glass-lined, mer film removeJ froin the reactors. jacketed, pressure vessels of 3750 The sample of polymer film x a s U. S. gallons’ capacity, manuweighed before placing in the perfactured by the Pfaudler Co. Durcolator (usually 2 grams). h piece ing 8 years’ production of polymers of fine stainless steel or copper over a wide range of temperatures Polymer screen held the polymer suspended and pH levels, the glass linings above the liquid level. After the have become roughened or etched. Screen apparatus was started it ivasallored I n this condition rapid fouling t o operate for 24 or 48 hours. At occurs and frequent cleaning is Solvent the ~ i i of d this time the total volume necessary to maintain productivity. of solvent in the apparatus was Manual cleaning with plastic or measured and any solid material hardwood scrapers has been used Air 4 contained in it was allowed to settle. t o clean t h e r e a c t o r s . T h i s A 10-ml. aliquot of the clean liquid method took 8 t o 12 hours and as drawn off with B calibrated invariably left a thin, tightly adpipet, evaporated to dryness on a hering layer of rubber on most steam hot plate, and the concenof the reactor i d 1 which, besides reFigure 1. Air Lift Pump Circulator or Percolator tration of dissolved solids calcuducing heat trankfer, resultedin more for Dissolving Polymer Film lated. rapid fouling than the clean glass
I
1896
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Vol. 45, No. 9
ENGINEERING AND PROCESS DEVELOPMENT
t
*
The other apparatus employed consisted of a three-neck, 1liter flask equipped with a reflux, water-cooled condenser and a stirrer, as illustrated in Figure 2. The flask was immersed in a thermostatically controlled constant temperature bath. The polymer ( 5 grams) and solvent (600 ml.) were placed in the flask. The stirrer was driven a t a relatively low speed. After 24 or 48 hours the amount of polymer dissolved was determined. A preliminary attempt was made to dissolve a few grams of polymer film taken from a reactor in which only butadienestyrene copolymers had been made. The samples of polymer were placed in beakers containing benzene, methyl amyl ketone, and carbon tetrachloride, respectively. After 24 hours a t room temperature considerable swelling of the polymer was noted. Benzene swelled the film the most but there was little evidence of solution of the polymer. A small amount of cumene hydroperoxide was added to each beaker, After another 24 hours the polymer changed very little in benzene or carbon tetrachloride. There was some evidence of disintegration of the polymer in the methyl amyl ketone-cumene hydroperoxide solution. The percolator brought the solvent into intimate contact.with air which, with the ketones, resulted in the formation of peroxides which degraded the gelled polymer. The results of tests with a number of solvents a t room temperature are shown in Table I. Two grams of polymer were used with 100 grams of the solvent.
In those experiments in which the polymer dissolved almost completely, a small amount of a black organic residue remained in the solvent. Since it was removable in this form it was not identified, but was assumed to be cross-linked polymer colored with insoluble impurities. The original plan was to spray the solvent over the sides of the reactor and recirculate continuously, either in the presence or absence of air. When it was found that petroleum naphtha (Varsol) was a suitable solvent it was decided to fill the reactors and agitate the contents. To simulate reactor conditions 5 grams of the film was placedin a 1000-ml. flask. To this was added 600 ml. of the solvent, usually containing 4% cumene hydroperoxide. The flask was immersed in a constant-temperature bath a t 175' F. and the contents stirred slowly for 24 hours, resulting in 80% dissolution of the film.
Dissolution of Butadiene-Styrene Film at Room Temperature
Table 1.
I-Liter flask Stirrer
(Percolator) Polymer Dissolved in 24 hours, % . 35 32 10 24 15 34 14 12 14 33
Solvent Methyl amyl ketone Methyl amyl ketone-Z% eumene hydroperoxide Benzene Benzene-2% cumene hydroperoxide Methyl iliobutyl ketone Methyl isobutyl ketone-2% cumene hydroperoxide Chlorobenzene Nitrobenzene Ethyl butyl ketone Ethyl butyl ketone-2% cumene hydroperoxide
-~ ~
-
~~
Solvent
Polymer
Figure 2.
Apparatus for Dissolving Polymer Film
~
All solvents dissolved a small portion of the film and the amount was increased by the presence of cumene hydroperoxide. Better results might be obtained if the rate of decomposition of the hydroperoxide were increased, either by the use of an activator such as diethylenetriamine or higher temperatures. The addition of a small amount of diethylenetriamine to the methyl amyl ketone-cumene hydroperoxide solvent resulted in dissolution of 69oJ, of the film in 48 hours. A solvent containing an amine would be undesirable because the hydroperoxide will continue to decompose during storage, even at low temperature (20" C . ) . Since decomposition is negligible a t this temperature in the absence of an amine, but can be readily controlled by raising the temperature, it was decided that a high temperature study offered the best approach to the probl~m. The results are shown in Table IT.
While the Varsol-cumene hydroperoxide solvent appeared satisfactory for butadiene-styrene films, it was soon found that it did not as readily dissolve film from reactors used for the preparation of butadiene-acrylonitrile copolymers. Film from these reactors was therefore studied by the same procedure. The results for experiments with the film a t elevated temperatures are in Table 111. Of the various peroxides available the organic hydroperoxides used in the production of cold rubber were equally effective. Results are shown in Table IV. The dissolution of the butadiene-
Table 111.
Dissolution of Butadiene-Acrylonitrile Film at 140' to 175' F. (Percolator)
Table II.
Dissolution of Butadiene-Styrene Film at Elevated Temperatures (Percolator)
Polymer T ~ ~ ~Dissolved, . , -7?6 Solvent F. 24 Hours 48 Hours Methyl isobutyl ketone 170 BO .. Methyl isobutyl ketone-2% cumene hydroperoxide 170 100 .. Benzene 130 14 Benzene-2% cumene hydroperoxide 130 .. 64 Varsola-2% cumene hydroperoxide 176 85 .. Benzene-2 $6 cumene hydroperoxide 176 73 .. E Petroleum naphtha, flash point 105' F., boiling point 314' to 380a F. I
September 1953
.
Dissolved in 48 Hours, Solvent % 75% benzene721 % methyl isobutyl ketone-4% culnene 29 hydroperoxide Methyl ethyl ketone-4% cumene hydroperoxide 61a 75% Tiarsql-21% methyl ethyl ketone-4% cumene hy28 droperoxide Methyl isobutyl ketone-4% cumene hydroperoxide 73 71% o-dichlorobenzene-25% methyl isobutyl ketone-4% 64 cumene hydroperoxide 50% formamide-50% nitromethane-3% cumene hydro..b peroxide 75% benzene-21% methyl ethyl ketone-4% riimene hy56 droperoxide Methyl isobutyl ketone-4% cumene hydroperoxide 70 31 hours. Dissolved part of apparatus (corks and screen) b u t not all of rubber.
INDUSTRIAL AND ENGINEERING CHEMISTRY b
1897
ENGINEERING AND PROCESS DEVELOPMENT Table IV.
Dissolution of Butadiene-Acrilonitrile Film by Benzene-Peroxide Solvent at 155" F. (Stirred flask) Dissolved in 48 Hours, 5% 22 44 45 49 25 34
Peroxide None Cumene hydroperoxide p-Menthane hydroperoxide tert-Butyl hydroperoxide Hydrogen peroxide Benzoyl peroxide
Table
V.
Effect of Cleaning on Heat Transfer Coefficients of Reactors
Av. Coefficient before Cleaning, B.t.u./ (Hr.)(Sq. Ft.)('F.) 40 28 24 21 25 32 30 24
Av. Coefficient after Cleaning, B.t.u./ (Hr.?(Sq. Ft.)(' F.? 47
Increase in Coefficient,
%
19 94 69 77 85 26 70 72
54
41 37 4B 40 50 42
acrylonitrile film in the stirred flask a t 155' F. in 48 hours was measured, using benzene as the solvent with the indicated peroxy compound. Cleaning Process Improves Heat Transfer, Resistance to Fouling, and Unit Productivity
The reactors were vented and hydrocarbon vapors removed and were then manually scraped to remove heavy accumulations of coagulum a t baffles and in the vapor space. Subsequent experience when preliminary scraping was not done showed t h a t inferior cleaning may result from slow degradation of thick film. A solution of Varsol containing 4% cumene hydroperoxide (which gave a practical dissolution rate a t maximum temperature of control equipment) was used to fill the reactor. A steam hose was used t o heat and maintain the water-filled jacket a t 170' F. A bleeder valve was left open a t the top of the vessel t o avoid any pressure build-up through liquid expansion. The mixture was agitated for 32 hours a t 170" F. The solution was cooled to room temperature and drained to a tank or transferred to another reactor by inert gas. ilfter this treatment the walls were found t o be practically free of rubber film and any small specks of rubber remaining were easily brushed off manually. The solution was adjusted t o 3.5 to 4% cumene hydroperoxide for each reactor cleaned and this strength was found t o be sufficienf. Higher temperatures would be desirable to speed up cleaning action. Equipment limitations prevented satisfactory operation above 170' F. Optimum exposure time was found to be 32 hours, although this varied from 24 to 40 hours, depending on the condition of the reactor before cleaning. Analysis of cleaning solution for total solids indicated that approximately 12 t o 15 pounds of rubber were removed per reactor, which, if assumed to be a uniform coating, would represent a film thickness of 0.010 to 0.015 inch, Cumene hydroperoxide usage averaged about 150 pounds per reactor. The increase in the average heat transfer coefficient for reactors on 55' F. cold latex ranged from 18 to 94%, as shown in Table V. Actual average values for individual reactors prior t o solvent cleaning ranged from 21 to 40 B.t.u. per (hour)(square foot)(' F.). This improved heat transfer, attributed to removal of the thin END
1898
OF
rubber film, allowed cold polymerization rates of 8% conversion per hour t o be adequately controlled a t 55' F. with minimum coolant temperatures of 15" F. Visual inspection showed that the cleaning method does not harm the glass lining of a reactor. Subsequent experience has shown t h a t the clean glass surfaces have resisted fouling much better than the film-coated wall. and manual precleaning after abnormal coagulum-forming batches is more efficient. ,Ilthough heat transfer coefficients have not been measured regularly since the reactors were cleaned about 2 years ago, most of the reactors are still sufficiently clean t h a t adequate temperature control can be maintained at full production rate. indicating that the heat transfer coefficient is adequate. Loss of temperature control indicates the need for recleaning, and recleaning has been necessary only in a few isolated instances following the production of unusually unstable, experimental materials. The increased productivity after use of this reactor cleaning technique is a strong economic incentive for its use. The used solvent can be returned t o a refinery for recovery of the petroleum naphtha. The only hazards of the process are those associated with solvents (flash points) and perhaps too rapid decomposition of the peroxide if the temperatures are too high, causing evolution of vapors. However, the results on films rich in bound acrylonitrile were disappointing, confirming laboratory tests. Because of its cost, a smaller amount of a solvent such as methyl isobutyl ketone would have to be used and would be sprayed on the walls and recirculated. Summary
The rate of solution of butadiene-styrene polymer films horn reactor walls is slow in solvents a t room temperature but may be increased by the presence of hydroperoxides such as cumene hydroperoxide which degrade high molecular weight polymers by oxidation. Some solvents form effective peroxides when exposed to the atmosphere. Methyl amyl ketone is best in this respect. .4t higher temperatures the rate of solution is sufficiently rapid t o enable practical use t o be made in cleaning reactors and other equipment. When a hydroperoxide such as cumene hydroperoxide is used the need for a peroxide-forming solvent is eliminated, and since petroleum naphtha is so inexpensive the reactors may be filled and the contents agitated for the desired time, up t o 48 hours, a t the necessary temperature, approximately 140' t o 160' F. The method has also been successfully used for cleaning valves, Mooney rotors, and other equipment coated viith a tough film of rubber and is particularly suitable for reactors containing obstructions such as cooling coils, baffles, and thermoaells. Films from reactors used in the preparation of butadieneacrylonitrile copolymers do not dissolve completely but do become swollen and easier to dislodge. Acknowledgment
The authors appreciate the courtesy of Polymer Corp., Ltd., for permission to publish this information. literature Cited (1) Williams, H. L., et al., report to Office of Rubber Reserve, Oct. 1 4 , 1947.
(2) Winkler, C. 9., private communication. RECEIVED for review April 13, 1953.
ACCEPTEDJune 15, 1953.
ENGINEERING AND PROCESS DEVELOPMENT SECTION
INDUSTRIAL AND ENGINEERING CHEMISTRY b
Vol. 45, No. 9
