industrial chemistry for teachers D. E. HARMER
Process Radiation
The Dow Chemical Campony Midlond, Michigan 48640
T h e fundamental study of radiation chemistry presents a challenge for research in the fields of physical chemistry and mechanisms, since a great variety of reactive species are produced by the passage of radiation through matter. Several reviews of basic radiation chemistry have appeared in THE JOURNAL during the past decade (1-8). The primary task in translating such fundamental knowledge t o a practical industrial process is to learn how to choose systems and control conditions so that the initial species formed by irradiation can have the greatest desirable impact on the overall chemical process. The reactive species which are formed initially by irradiation are very expensive in comparison with normal chemical agents. However, the successful manipulation of a chemical system to multiply their effectiveness has led to several industrial processes which are now commercial realities. What kinds of processes are efficiently promoted by high energy radiation? A summary is presented in Figure 1. Table 1. Processes which are currently commercial polymer. Table 1.
Schematic representation of radiation induced crosslinking of a
Radiation Chemical Processes
Halogenation Addition to olefinic bondss Oxidation Sulfoxidation Polymerization" Graft copolymerization' Cmsslinkine of nolvmersa a
Process currently commercial
are indicated. From this list of most effective radiation-chemical processes, it is possible to generalize about the nature of chemical reactions which meet the criteria for sufficient multiplication of the effect of the high energy radiation in the system. Ifaterials of high molecular weight-polymersmake very efficient use of ionizing radiation because a small change in their chemical constitution can result in a large change in their overall physical properties. The ions and radicals produced in the early primary and secondary processes during irradiation lead to the breaking of chemical bonds of a molecule. Thus, we find that some high molecular weight materials such as methyl cellulose or polyethylene oxide are efficiently decreased in molecular weight. On the other hand, if scission of the high molecular weight chain does not occur immediately, it is possible for two free radical sites along different polymer chains to combine, with the formation of a new bond between the two chains. This process leads to a crosslinked material. Figures 1 and 616
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Figure 2. Schematic representation of a mode of rodiation.indused degradation of o polymer.
2 represent, schematically, the processes of crosslinking and scission, which may occur simultaneously, or almost exclusively, depending on the chemical structure of the polymer molecules. I n these figures the smaller circles attached to the main carbon backbone structure represent various chemical groups, depending on the given type of polymer. Polyethylene is a polymer in which crosslinking predominates over scission. Thus, it is possible to make products from polyethylene in which the radiation-produced crosslinks increase the heat and solvent stability by significant amounts. A good example of a commercial use for the crosslinking reaction is the packaging film branded "Cryovac" by its manufacturer, W. R. Grace Company. This polyethylene film was originally developed to allow vapors of certain packaged food products to pass through, while excluding dirt and con-
tamination (9). I t was known that polyethylene had the desired gas transmission properties, hut it was also known that the structure of polyethylene did not allow it to be softened, stretched, and cooled, without breaking. Radiation was employed to supply an amount of crosslinking sufficient to give the polyethylene a "memory" so that, following irradiation and orientation, it could be shrunk over the food article. The commercial process involves extrusion of low density polyethylene into a tape which is then crosslinked with accelerated electrons. Special electrical machines are used to generate these high-energy electrons (Fig. 3). Those used for the treatment of Cryovac
Figure 3. Polyethylene tope being irrodioted os it posses under window .of 2-meV eledron beom generator. Photo courtesy of W. R. Grace and Cornpony.
have an energy of 2 meV. Although chemical methods can, alternatively, be used for the production of crosslinked polyethylene, the use of radiation represented the best choice for this particular product. I t is interesting to note, as in this example, that radiation can be used to carry out chemical reactions on products even if they are in a finished form. Another way by which the radiation may be efficiently utilized is in the intiation of a chemical chain reaction. When such a reaction is possible, the species produced by irradiation are able to react with a molecule, which in turn activates yet another molecule, and so on until hundreds or thousands of molecules have reacted for each original radical or ion produced by the radiation. Vinyl polymerization is a typical chain reaction which is efficiently initiated by high energy radiation. At one time it was thought that only free radical chains could be promoted by radiation. I n more recent years, however, it has also been learned that ionic chain processes can be initiated by radiation and lead to high polymers. Various chemical reactions, notably halogenation, are also capable of being propagated by a chain mechanism. Figure 4 illustrates the use of radiation energy to bring about a hydrohromination reaction. The manufacture of ethyl bromide at The Dow Chemical Company is an example of a chemical chain reaction which is carried out commercially by the use of gamma radiation. I n 1963, it represented the first chemical reaction to be
RADIATION INiTlATlON : 7 n HBr
0,
P R O P A G A T I O N STEPS : C Y C L E I 1 L P E A I Z 10.000 TIMES 0 1 YORE
TERMINATION : Recomblnolion or r a d i c a l . D i $ p r o p o r t i o n o t i o n o f rodic.1. R e a c t i o n with i m p u r i t i e s Figure 4.
lnitiotion ond propagation of the hydrobrorninotion of ethylene.
catalyzed by gamma radiation on an industrial scale. A review of its development serves to illustrate the path which may be followed by s radiation chemical reaction from idea to commercial reality. Shortly after radiation chemical research was instituted at The Dow Chemical Company, investigations were made to determine the efficiency of free radical addition reactions in the presence of gamma radiation. I t was found that rapid reaction rates were obtained when hydrogen bromide and ethylene were introduced into a variety of solvents in the presence of radiation (10). Under these conditions the gases produced a product which dissolved in the liquid phase. The scale of operation at this time was confined to a glass reactor of about 60 ml capacity. Success at this level led to further experiments in a reactor of liter capacity, provided with suitable heat transfer ability. Studies of the most significant reaction parameters were carried out with this type of apparatus. Although the process appeared efficient, the quest for an optimum process led to the investigation of an alternative method in which liquified ethylene and hydrogen hromide were irradiated at pressures as high as 1000 psi at -5'C. Data for the reaction utider the laboratory conditions, along with values derived from the literature, are presented in Table 2. Tclble 2.
Comparison of ~ o b o r o t o rYields ~ of Ethyl Bromide
Reaction Conditions Gas-phase, glass reaction vesselb 2-phase, gas-bquid, in glass, -2°C 152.5 Krads/hr Liquid, 'single phase, stainless steel reaction coil, -5"C, best case, 212 Krads/hr Liquid, smgle phase, in glass, -1fi5"C. 24 Krads/hrc
G-Value' -3 X 10' 3.9 X 10' 2 . 2 X l(r 10'
G is defined as the number of molecules which react per 100 electron volts absorbed by the system. It is also equal to 1.04 times the number of millimoles of product produced by one megarad of absorbed dose in one kilogram of reactant. * Reference (I1 ). Reference ( I d ) . a
Although radiation was shown to be a very efficient catalyst, it was still necessary to compare it with other more conventional methods of making ethyl hromide. After aperiod of careful consideration, it was determined that the process to be built should be based on gamma radiation as the initiator. The choice was based on the fact that this type of radiation does not leave catalyst fragments or residues in the system, is continuous in its production of free radicals, is low in mainteVolume 47, Number 9, September 1970
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Figure 5. facility.
Figure 6.
Flow-sheet for the ethyl bromide process.
Figure 7. dition.
Diogrmm of the ethyl bromide reaction yesel in operating con-
View of the loboratory prototype in operation with the gamma
nance, and can be operated at low cost. The two-phase gas-liquid system was chosen because the cost of equipment was low, even though the G-value (specific radiation yield, expressed as molecules reacted per 100 electron volts absorbed by the system) was not as high as in other alternate reaction conditions. Because the reaction represented a first-of-a-kind, it was deemed necessary to build a prototype of the system and operate it in the laboratory facilities. This prototype actually consisted of a 36" wedge-shaped segment of the envisioned full-scale reaction vessel (15). It is shown in operation in Figure 5. When all final operating conditions had been established, the commercial plant was built and has been used for production of ethyl bromide on a routine basis since its start-up, March 1, 1963. Figures 6, 7, and 8 illustrate this industrial radiation process. The use of high energy radiation to bring about chemical reactions is now seen to he an industrial reality. It can be expected that many other chemical processes will be most efficiently promoted by use of radiation from gamma-emitting isotopes or electron accelerating ""
tion route is found to he best, another example will be provided for the use of high energy radiation as a useful tool for industrial chemistry. Literature Cifed (1) H A M M ~W. L H., ~ .J. CHEM.EDUC..36, 246 11959). (2) G m c n m n . G . . J. Cxex. Eouc., 36, 262 (1959). (3) HAET. E.J., J. CIIEM.EDUO.. 36, 266 (1859). I41 I 3 u n r o ~ M., . J. CHEM.Eooc., 36, 273 (1959). 36, 279 (1959). (5) KUPPERMANN.A,, J. CHEM. EDUC., (6) Llvmas~on,R..J. CHEM.EDUC.,36, 340 (195'3). (7) T h r ~ o nE. , H..J. C x ~ n Eooc.. . 36, 396 (1958). (8) ALLEN.A. 0.. J. CHEM. EDCC.,45, 290 (1968). R., A N D LOEB,R. S., "Radioisotopes and (9) L ~ w n z ~ J. c 11.. ~ . MANOWITZ, Radiation," MeOrsw-Hill Book Company, N e v York. 1964. D..E..BEATS,J. 8.. P U M P E L ~ YC. , T., A N D WILIINBON.B. 110) H A R M ~ IT.. "Industrial Uses o f Large Radiation Sources," Vol. 11, International Atomic Energy Agency. Vienna, 1963, pp. 205-230. (11) An~s~noiia, D. A,, A N D SPINES,J. W. T., Con. J . Chem., 37, 1210 (1859). (12) A R M B T A O N ~D. . A,. A N D SPIN=., J. W. T.,Can. J . Chem.,37, 1 W 2 (1959). D. I:.. A N D BEAT.., J. 8.. Chem. Ener. Pwmcss, 60, No. 4. 113) HARMER. 33 (19641.
Figure 8. The chemical reoction unit of the Dow ethyl bromide rvdiation process in operating condition.
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