literature Cited
Abegg, C. F., Stevens, J. D., Larson, hl. A,, A.Z.Ch.E. J . in press, I 1969).
Ain%,-XI R., Larsoii, hl. A,, Znd. Eng. Chem. Proc. Design Deoelop. l , 133 (1968).
Bransoni, 8. II., I)unning, W. J., hlillard, B., Discussions Faraday Soc. 5.83 11949).
Bransom, S.H., Brit. Chem. Eng. 5,838 (1960). Calming, T. F., Randolph, A. D., A.Z.Ch.E. J . 13,5 (1967). Cartier, li., Pindzola, l)., Bruins, P. F., Znd. Bng. Chem. 51, 1409 (1959).
Jackson, K. A., “Comments made at A.1.C;h.E. Advanced Seminar on Solid State Science in Ch.E,” Boston A.1.Ch.E. meeting, December 1964. IiaPvler, V., Znd. Eng. Chem. 44,1270 (1952). Murray, 1). C., Idarson, M. A,, A.Z.Ch.E. J . 11, 728 (1965). Nancollas, G. II., Purdie, N., Chem. SOC.Quart. Rev. (London) 18, l(l964).
Randolph, A. I)., Can. J . Chem. Eng., 42,280 (1064). Randolph, A. D., A.Z.Ch.E. J . 11,424 (1965). Randolph, A. D., Idarson,M. A., A.Z.Ch.E. J . 8, 639 (1962). Robinson, J. N., Roberts, J. E., Can. J . Chem. Eng. 35, 105 (1957)
Gibba, Willard, “Collected Works,” Vol. I, p. 94, Longmans, Green, New York, 1928. Hill, P. G., Witting, It, I)emetri, E. P., Trans. ASJIE, J . Heat Trans. 85,303 (November 1963). Hirth, J . P., Pound, G. hf., “Condensation and Evaporation,” “Progress in Materials Science,” Vol. 11, Prlacmillan, New York, 1963.
Ishii, T., Bull. ‘I’okUoInst.’I’echnol., No. 67 (1965).
Sakman,”W. C., A.Z.Ch.E. J . 2,107 (1956). Ting, H. II., McCabe, W. C., Znd. Eng. Chem, 26, 1201 (1934). RECEIVED for review June 18, 1968 A C C E r m o April 14, 1969 Work supported by the National Science Foiiridation, grant GK-1077. Presented at AIChE Symposium, New Orleans, La., March 1969.
Versatile Constant Transmission Photochemical Reactor W. B. lsaacson and S. 1. Ting 3M Co., St. Paul, ;Ifinn. 65101
A photochemical reactor has been designed which permits constant transmission of the light energy incident on the reactor wall by minimizing the effect of wall deposits through continual scraping of the reactor wall or interface between the light source and reacting solution. The reactor operates over a wide range of solution viscosities. It can be operated as a batch, continuous flow stirred tank, or wiped-film reactor and is highly suited for conducting photopolymerirationor reactions of difunctional molecules.
Tw;
1XiCRi:AsiINO u n L i I z . ~ T i o N of ultraviolet radiation as an initiator in many chemical processes such a3 halogenations, oxidations, cieconipositions, and polymerizations has created a dernalid for photocheniical reaction equipnicnt which can be utilized for producing pilot quantities of reacted products necessary to obtain complete physical and chemical characterization, supply product for developnieiit efforts, arid be utilized for obtaining reactor design and scale-np data. Quantitative aspects of photochemical reactor design have been the subject of many recent 1)ublications (Huff and Walker, 1962; Dolan el al., 1965; Chssano et al, 1968; Cassano and Smith, 1966; Harris and l h n o f f , 1965; Jacob and I)ranoff, 1966; Ziolkowaki et al., 1967). These investigations have generally been aimed at defining the photoreactor design parameters necessary for scale-up purposes and at determining reaction kinetics. The most cornmonly employed reactor and asihociated optical syihtem for laboratory itivestigation are the tubular reactor and ultritviolet light source situated a t the focal points of a11 ellipsoidal reflection chamber. Other types of photoreactors are hatch and continuous flow stirred tanks, falling film or filin, bubble-type, spray-type, and tubular. RIost of the chemical systems employed in published investigations 011 photoreactor design have been liquid and/or gas phase chlorination and simple decomposition reactions.
The use of ultraviolet light for the initiation of polymeriza tion reactions hiis created a need for a reactor system which cannot be satisfied by conventional photoreactors. Perfluorooxydipropionyl fluoride has been polymerized according to the following equation.
0
0
ii
I/
uv
0
0
11
I1
n-FCCF2CFsOCF2CF2CF+ FC(CF2CF20CE’,CF2) .CF $.
00 (TL
/I I1
- 1) FCCF
The necessity of photolyzing a relatively viscous solution with removal of volatile by-products from difunctional reactants which have a tendency to undergo undesirable branching reactions has led t o the design of a new photoreactor which has utility for liquid phase chemical reactions. Design features provide: Constant light intensity or transmission. Rlinimal effect of wall deposits by continual scraping of the reactor wall or interface between the light source and the reacting solution. Operation over a wide range of viscosities. Frequently, experimental work is carried out in the early stages in the laboratory by methods which provide little VOL. 9 NO. 1 FEBRUARY 1970
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Figure 1. Wiped-film photochemical reactor in selective photodimerization of difunctional materials
information for scale-up of the reactor system. Our reactor has filled the needs for experimental investigation and small sample production, and provided basic data for scale-up. Reactor Development
Among the experimental problems associated with photochemical reactor operation is the coniplication introduced by the deposition of an opaque layer of polymeric material or other reaction products on the reactor walls transmitting the effective radiation. The decrease in reaction rate with time due to reduction in intensity has been reported for both gas (Cassano and Smith, 1966; Ziolkowski et al., 1967) and liquid phase (Dolan et al., 1965; Harris and Dranoff, 1965) reactions, even a t low concentrations and conversions. It is thus impossible to carry out some reactions beyond low conversions and valuable material in the form of highly branched polymer could be lost. No quantitative method is yet available to account for the variation in light intensity and reaction rate due to wall deposit. Though the use of low concentrations and conversions will help to avoid these difficulties, this does not solve the design and scale-up problem. It is, therefore, evident that to eliminate the effects of wall deposits, it is necessary to renew the relatively stagnant layer of photolyzate continually at the inner transmitting surface. The system described below is capable of this fuxiction. The basic wiped-surface reaetor (Figure 1) has given consistent results in repeated photopolymerization runs over a period of months. It consists of a 2-inch i.d. X 10-inch quartz cylinder (wall thickness 2 mm) joined to a 2-inch standard 172
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borosilicate glass flanged pipe joint by means of a quartzborosilicate glass graded seal. A 316 SS 1/4-inch plate serves as the cover plate with a Teflon gasket providing the required seal. The effective capacity of the reactor-Le., the quartz portion-is 500 cc. The openings required for introducing the reactant and the escape of the reaction by-product are provided in the borosilicate glass section of the reactor. T o avoid using a weak graded seal in the reactor drain line, a ball and socket joint was used. The wiper system includes a 1/8-hp variable speed motor a wiper-holder, and Teflon wiper blades. The wiper-holder consists of three equally spaced vertical slots, made from 316 SS sheet, which hold the grooved 1/4 X 1/4 X 8 inch Teflon wiper blades which are spring-loaded against the reactor surface to provide an efficient scraping action. The cover plate of the reactor also supports the variable speed drive. The seal between the wiper shaft and the cover plate is formed with a magnetic seal. The speed of the wiper can be controlled with a Variac. The ultraviolet radiation is provided by Hanovia mercury vapor lamps. The reactor can be placed a t the common focus of a dual ellipsoidal reflector with the two lamps positioned a t the remaining foci, the reflecting surface being highly reflective Alzak-treated aluminum (Figure 1). The lamp-reactor distance is 4 inches. The lamp could be cooled by means of water-cooled quartz immersion walls manufactured by Hanovia, or with air in thin cylindrical quartz envelopes. Alternatively, the dual ellipsoidal reflector could be replaced by two small ellipsoidal reflectors concentrating the light in the region just beyond the inner surface of the reactor wall. This arrangement allows the lamps to be brought to about 3/4 inch of the reactor. It is doubtful whether the reflecting ellipsoidal surface could focus the available radiation very efficiently, because the size of the lamp represents a considerable departure from a line source. However, measurements made with a thermopile a t 50 nim showed that the small reflectors doubled the intensity of available ultraviolet radiation. The uniformity of intensity within the reacting space was not checked. The optical efficiency of such systems was estimated to about 11% (Cassano and Smith, 1966). Discussion
Unlike the previous reactor of Bryce-Smith et al. (1967), which employed rotating pads of fused silica fibers and could be used only as a stirred-tank reactor, the system described is capable of operating either as a wiped-surface stirred-tank reactor or a wiped-film reactor. The slanted grooves cut in the Teflon wiper blades promote the downward flow of the thin film. Teflon wiper blades are considerably more durable than silica fibers and are, hence, capable of giving continued trouble-free service. When used as a stirred-tank reactor, the surface was free from wall deposits except in wall irregularities beyond the reach of the wiper blades. I n a conventional tubular or stirredtank reactor wall deposits became a serious problem a t reactant concentration of 10 weight %. A more selective reaction-e.g., the dimerization of a difunctional reactant-could be achieved by operating the reactor as a wiped-film reactor such that the reaction mixture could be exposed only for short durations as agitated thin film falling down the reactor wall. I n this application the reactor effluent was returned into a still pot from which the unreacted monomer was redistilled with the inert diluent into the reactor; the reaction could be monitored by VPC or vapor temperature. We have operated the reactor at liquid flow rates of up to 35 cc per minute. The apparatus for a batch operation of this nature is shown in Figure 1. The results of a typical run (Figure 2) indicate a constant rate of dimer production over extended periods of continuous operation. The operation of the photochemical reactor at two
toot-
Conversion vs Tme , Batch Dimerization with Recycle
gOL c
PI
80
a
70
2 X 450 Watt UV L m p s c
0
i
I-
>
"
100 150 200 250 300 REACTION TIME (hrs) Figure 2. Effect of radiation on conversion in batch dimerization with recycle
0
50
attain finer control of temperature a cold finger, which the wiper-holder is designed t o accommodate, could be introduced within the reactor. Alternatively, the reactor contents could be circulated through an external coil. T o control the spectral distribution of the transmitted radiation, a n external jacket, through which the chemical filter solution could be circulated, may be incorporated. This jacket could then also serve as a cooling jacket. It is not difficult t o envision a continuous reactor system assembled from individual units similar to the one described above. The poor mechanical strength of quartz could undoubtedly impose an upper limit to the size of such a reactor. We have, however, operated successfully a 4-inch-diameter, 3-liter-capacity unit as a stirred-tank reactor with a Rayonet photochemical reactor supplying the ultraviolet radiation. Further kinetic studies using this reactor are being carried out in this laboratory. Acknowledgment
The authors are indebted to the Air Force Rocket Propulsion Laboratory, Research and Technology Division, Edwards, Calif., and 3M Co. for permission to publish this paper. References
Bryce-Smith, D., Frost, J. A., Gilbert, A., Nature
213, 1121
(1967).
levels of light intensity shows that the reaction rate is proportional t o the effective ultraviolet radiation. Our experience has shown that in a falling-film annular reactor described by Cohen et al. (1967) even a small extent of wall reaction would lead to occlusion of the surface transmitting the effective radiation. With the advantage of constant transmission and the versatility of operation as either a stirred-tank or wiped-film unit, this reactor should be well suited to kinetic studies. Also, it is felt that through the efficient action of the wiper system, the mixing in this reactor is as close to perfect as an industrial unit can approach. Further modifications could be made to the basic reactor design described above. For our present purpose two air blowers were found adequate for temperature control; to
Cassano, A. E., Matsuura, T., Smith, J. hI., IND.ENQ.CHEM. FUNDAMENTALS 7,655 (1968). Cassano, A. E., Silverston, P. L., Smith, J. M., Ind. Eng. Chem. 59 (11, 19 (1967).
Cassano, A. E., Smith, J. hI.,A.Z.Ch.E.J. 12 (6), 1124 (1966). Cohen, J. D., hfijovie, M. V., Newman, G. A., Pitts, E., Chem. Znd. 1967, 1079.
Doede, C. hl., Walker, C. A., Chem. Eng. 62 (2), 159 (1955). Dolan, W. J., Dimon, C. A., Dranoff, J. S., A.I.Ch.E. J . 11 (6), 1000 (1965).
Harris, P. R., Dranoff, J. S., A.I.Ch.E. J . 11 (3), 497 (1965). Huff, E. J., Walker, 0. A., A.Z.Ch.E. J . 8 (2), 193 (1962). Jacob, S. hl., Dranoff, J. S., Chem. Eng. Progr. Symp. Ser. 62 (68), 47 (1966).
Ziolkowski, D., Cassano, A. E., Smith, J. M., A.I.Ch.E. J .
13 (j),
1025 (1967).
RECEIVED for review December 13, 1968 ACCEPTED November 5, 1969 Work sponsored by Air Force Rocket Propulsion Laboratory,
Research and Technology Division, Edwards, Calif.
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