COLEGE OF ENGINEERING INDUSTRY PROGRAM TEE EFFECT OF GAMMA RADIATION ON SOME CHEMICAL REACTIONS OF POSSIBLE INDUSTRIAL IMPORTANCE Leigh C. Anderson Bruce G. Bray Joseph J. Martin This paper has been accepted by the International Conference on Peaceful Uses of Atomic Energy, Geneva, Switzerland August 8-20, 1955. July, 1955 IP-118 University of Michigan Ann Arbor

This work was done under AEC contract No. AT(11-1) - 162, Engineering Research Institute Project No. 1943-4,

THE EFFECT OF GAMMA RADIATION ON SOME CHEMICAL REACTIONS OF POSSIBLE INDUSTRIAL IMPORTANCE Leigh C. Anderson, Bruce G. Bray, Joseph J. Martin University of Michigan For the past four years this laboratory has been investigating chemical reactions promoted by gamma radiation. The work has been carried out as part of a broad program to find uses for the large amounts of radioactive fission products that are expected to be made available from the operation of nuclear reactors. However, because development programs for packaging and shipping fission product sources have not been completed, the studies have been conducted with artificial cobalt-60. Two cobalt-60 sources of gamma radiation have been employed in this investigation. The first source is an aluminum-clad hollow cylinder of cobalt about 3.5 cm. in diameter and 35 cm. long. It was activated by neutron radiation in the pile of the Brookhaven National Laboratory and was given a nominal rating of 1000 curies. Later calibration indicated its true radiation strength to be about one-third this value. The cylinder of cobalt was mounted in a heavy lead container so that objects to be radiated were simply placed in the center of the cylinder. This permitted only small volumes to be treated at any one time. A second cobalt-60 source was installed which made provision for radiation of large volumes of material. This source consists of 100 aluminum-clad rods of cobalt about 0.6 cm. in diameter and 25 cm. long. These rods are mounted in two close concentric circles in an aluminum rack of about 15 cm. internal diameter. The whole unit is stored under water about 5 meters deep. When it is to be used, the unit is raised by remote control into a chamber where the various materials to be radiated have been stacked. The radiation chamber is constructed with concrete walls about 1.2 meters thick. The cobalt rods were activated in the pile at Chalk River, Canada, and were nominally rated at 10,000 curies; though, as with the other source, calibration indicated the radiation strength to be about one-third this amount. In selecting chemical reactions to be studied in the presence of gamma radiation, emphasis has been placed on those which might have commercial applications. When a reaction was found to be promoted only minutely by radiation, it was discarded in favor of other reactions that proceeded vigorously. Two classes of chemical reactions that result in appreciable yields of products in the presence of gamma radiation are (1) polymerizations, and (2) chlorinations. In the former category this paper presents information on the polymerization of ethylene. In the latter category the chlorination of a series of aromatic compounds is considered. Also, the economics of a plant to chlorinate benzene are presented and a

comparison is made between the use of ultraviolet and. gamma radiation. I. POLYMERIZATION OF ETHYIENE The gas-phase polymerization of ethylene has been studied in the presence of al ha radiation, ultraviolet radiation2 12, and gamma radiation9)13 4.15, The data presented here were taken in a continuation of the work of Lewisl3l, and cover a pressure range of 50 to 100 atm. and temperatures from 6 to 220~ C. (a) Experimental Work The high pressure stainless steel reaction vessel of Lewis was used in all runs. This reactor, shown in Figure 1, could be maintained at any desired high temperature by a set of electrical resistance heaters which fitted around the outside. A uniform procedure of loading and cleaning was followed in each test. The reactor was sealed at the flanges and pressure tested with nitrogen at 1000 psi. for 12 hours. The nitrogen was vented and the reactor evacuated, after which ethylene gas was charged at room temperature. The loaded vessel was placed in the radiation chamber and controlled to the desired temperature before raising the ten-kilocurie cobalt-60 source to initiate the reaction. Runs in the one-kilocurie source could be made only at room temperature as there was no room for the electrical heaters on the outside of the reactor. During the runs, pressure and temperature measurements were made at various intervals. At the conclusion of the run the reactor was removed from the radiation field and the excess ethylene was vented. The flanges were opened -nd the polymer product was removed mechanically and weighed. The reactor was cleaned by washiing, ith boiling xylene, acetone, and distilled water. The ethylene was about 97% pure with a gas analysis showing it to contain 1.7% N2, 0.8% C5H12, 0.37% C02, 0.05% CO, and 0.02% 02. The amount of ethylene charged to the reactor was determined from the measured temperature, pressure, and volume by means of the thermodynamic properties of ethylene2. The radiation received by the reactor during a run was calculated by the dosimetry techniques of Lewis, et.il 14.1 (b) Product Yields A sumary of the reaction conditions and the polymer yields is given in Table I. White, curdy, solid polymers were formed at reaction temperatures from 6 to 1300 C. A white wax formed at about 160~ C. and above this the product was an opaque liquid that separated into two layers after standing awhile. The top layer was colorless and the bottom layer contained a finely divided solid. The yield of polymer is presented as the A valuel3, gm. moles of ethylene reacted/ (metric ton of ethylene charged) x (megarep of gamma radiation). The rep is based on the absorption of 93 ergs per gram. From the scatter of the data it is difficult to determine the effect 2

of temperature on the yield since largest yields were at 39 and 217~ C. TABLE I. PRODUCTS FROM THE IRRADIATION OF ETHYLENE Run Reaction Reaction Total Radi- Radiation Polymer Tensile Temp. Pressure ation, Dose Yield A* State Strength ~C. Ave.psia Megarep. Psi 839 6 755 4.20 1340 Solid 3050 833 8 985 5.46 1630 Solid 3075 800 13 1145 4.10 1530 Solid 3350 801 13 875 5.28 2270 Solid 3100 807 39 1380 4.47 3040 Solid 2520 808 88 1325 1.90 490 Solid 812 90 1300 5.85 433 Solid 845 132 1700 4.05 955 Solid 712 160 1490 2.76 945 Wax 848 190 1125 2.88 1300 Liquid 814 217 1100 3.77 4030 Liquid 813 220 1175 4.86 1400 Liquid *^ =.. _..,gmi moles reacted (metric ton charged) (megarep) (c) Evaluation of the Polymer Products Melting points of the solid and waxy products were measured on a melting point bar3. Melting points of the liquids were observed in small tubes of the polymer which were allowed to warm up after being frozen in a dry ice bath. Because each product is a composite of polymers of varying molecular weights, a range of melting points was found. The points at which melting could first be detected have been plotted as a function of the reaction remperature in Figure 2. Solid products produced in appreciable quantity were pressed in a one-compartment mold at 1500 psi and 1500 C. for 5 minutes. Specimens were punched from the molded polymer and placed in an Instron Tensile Test Machine. The measured tensile strengths are presented in Table I. Densities of the solids were determined by the Archimedes principle while those of the liquids were measured by weighing in a calibrated pipette. Figure 3 shows the densities as a function of the reaction temperature. Crystallinities or the degree to which the molecules are arranged in parallel positions were determined from the densitiesll and these are also shown in Figure 3. Melt viscosities of solid products were obtained in a parallel plate plastometer at about 137~ C. Viscosities of liquid products were determined in a modified Ostwald pipette. Molecular weights were estimated4 3

from the viscosities and are shown in Figure 2 as a function of reaction temperature. The molecular weights are not considered very accurate, but the general trend is indicated. TABLE II. CHLORINATION OF AROMATIC COMPOUNDS WITH G AlA RADIATION Run Aromatic Temp. Dose C12 add. C1 sub. Average Average Chlorin- Reaction Rate moles/ moles/ G* G*,subated ~C. Kilo- liter liter addition stitution rep/hr 1 30% Benzene 20 61.0 35.20 --- 23400 -- 2 20% Benzene 20 61.0 32.60 --- 252,000 3 10% Benzene 20 61.0 33.30 --- 90,000 4 30% Benzene -10 610 13.70 --- 90,000 5 10% Benzene -10 61.0 32.70 --- 57,000 6 Toluene 20 14.0 17.42 7.54 523,000 226,000 7 Toluene - 5 14.0 14.12;.59 900,000 420,000 8 Toluene 35 14o.0 1035 6.12 812,000 480,000 9 Toluene 20 None 3.23.85 - 10 Xylene 20 13.7 5.90 3.48 462,000 272,000 11 Xylene - 5 13.7 7-45 2.94 448,000 177,000 12 Xylene 20 None 3.20 2.63 13 Mesitylene 20 13.7 4.20 15.10 210,000 827,000 14 Ethyl Benzene 20 13.7 1.81.47 137,000 35,000 15 Ethyl Benzene 20 None 1.61.13 - * G molecule reacted based on Chlorine 100 ev absorbed From the above results it is clear that a wide variety of polymers of ethylene may be produced in the radiation reaction. The yields are sufficiently high to consider the process feasible for commercial applications. At this stage economic studies are in order to determine the possible competitive position of polyethylenes produced by radiation processing with those produced in conventional processes. II. CHLORINATION OF SOME AROMATIC HYDROCARBONS In general chlorination of an aromatic hydrocarbon may result in three possible reactions: (1) substitution of chlorine for hydrogen in the benzene ring, (2) substitution in an alkyl group attached to the ring, and (3) addition of chlorine to the ring. Friedel-Craft catalysts promote substitution in the ring; high temperature in the absence of catalyst favors substitution in the alkyl group; and radiation appears to favor addition to the ring. Ultraviolet-56, roentgen raysl7, alpha radiation1, and gazmma radiation7'8 have been employed to pronmote the addition reaction. In this laboratory Harmner7 initiated a study of the chlorination of benzene and 4

toluene in the presence of gamma radiation. This has been extended to include xylene, mesitylene, and ethyl benzene. (a) Experimental Work Figure 4 is a flowsheet of the equipment developed by Harmer for carrying out chlorinations in the presence of gamma radiation. The glass bulb is the reactor which holds the aromatic liquid to be chlorinated. This was equipped with inlet and outlet tubes for passage of gases, a steel cooling jacket, glass baffles to promote mixing and a thermocouple well. Constant temperature was maintained by circulating cold methanol through the cooling jacket. The temperature of the methanol was controlled by a thermocouple potentiometer which governed the periods of operation of heaters and cooling coils. The reactor unit was placed in a portable hood in the ten kilocurie radiation chamber and connected to the cooling and gas analysis equipment by glass tubing that passed through ports in the concrete shielding wall. In starting a run nitrogen was passed through the system to sweep out all air. Then the chlorine flow was started and the cobalt-60 source was raised into the radiation chamber. The reaction time was measured from the moment the source was raised to the midpoint of the period during which a sample of gas was collected. Radiation dosages were determined by the time and dosimetry calibrations. Rates of reaction between chlorine and the aromatic hydrocarbons were determined by chlorine balance. The amount of chlorine into the system was measured with a rotameter. Excess chlorine was passed through the reactor at all times to maintain saturated conditions. At certain intervals samples of the exit gas were absorbed in a solution of 0.2 N. sodium arsenite and 3.0 N potassium hydroxide. The amount of free chlorine was determined by titrating the excess arsenite with ceric sulfate and total chloride ion was measured by the Volhard method. In this way the amount of substitution and addition could be determined. (b) Results of Chlorinations Benzene, Toluene, xylene and mesitylene reacted vigorously with chlorine in the presence of gamma radiation. Addition of six chlorine atoms to the ring and alkyl substitution appeared to be the primary reactions. The ratio of addition to substitution was different from that obtained in chlorination reactions without radiation. In general this ratio appeared to be increased by radiation and decreased by increasing the temperature. The radiation yield is reported as G'value (molecules chlorine reacted per 100 e. v. of radiation absorbed) in Table II. For some reason radiation had little effect on the reaction with ethyl benzene. Whether this is due to inherent inactivity or to inhibiting impurities is not known. 5

Harmer9 demonstrated the inhibiting properties of small amounts of benzyl chloride and oxygen in the chlorination of benzene and toluene. TABLE III. COST ESTIMATE FOR PRODUCING 454 kg/day OF GAMMA ISOMER OF BENZENE HEXACHLORIDE WITH FOUR RADIATION SOURCES Gross Radiation Source CesiumFission Gross Gross 137 Product Fission Fission 0.5-yr. Product Product 1 - yr. 2 - yr. Half-life, years, 0.2 0.3 1.1 33 Total Curies Reqd.-5 years. 84,000 57,000 19,000 5,000 Selected High Cost/Curie, $. QOQ05 0.10 0.25 5.00 Radiation Cost 5 yr., $. 4,200 5,700 4,800 25,000 Initial Investment in Radiation Source. 250 500 1,250 25,000 Radiation Chamber Cost, $. 22,000 22,000 22,000 22,000 Process Equipment Cost, $. 49,000 49,000 49,000 49,000 Total Investment, Including Radiation. 71,250 71,500 72,250 96,000 Annual Charges on Investment at 80%, Depreciation, Taxes, Interest, Maintenance, Etc. 57,000 57,200 57,800 76,800 Annual Radiation Replacement Cost. 800 1,000 700 0 Salaries of Workers, $/yr. 21,000 21,000 21,000 21,000 Chlorine & Benzene, $/yr. 109,000 109,000 109,000 109,000 Utilities, $/yr. 3,000 3,000 3,000 3,000 Sales and Marketing, $/yr. 8000 8,000 8 000 8,000 Total Annual Cost, $. 199,000 199,000 200,000 218,000 Annual Production, 290 Working Days, kg. Gamma Isomer 131,500 131 31,500 131500,500 Cost/kg. Gamma Isomer, in 12% Mixture, $. 1.51 1.51 1.52 1.66 Radiation chlorination of benzene takes place so quickly that temperature control is difficult. Also the production of the solid addition product tends to plug up the reactor unless the benzene is diluted with carbon tetrachloride. Tolun xene, xyene and mesitylene do not react quite so fast and may be charged to the reactor in a pure condition. However, in all cases reaction under gama radiation takes place so rapidly as to make it very attractive for commercial application. This is particularly true where the addition product might be desired, for radiation favors the 6

addition reaction. Chlorination of many other compounds may also be accelerated by gaa radiation. III. ECONOMICS OF RADIATION CHEMICAL PROCESSING The laboratory studies on the polymerization of ethylene and the chlorination of aromatic hydrocarbons have demonstrated the feasibility of promoting chemical reactions with gamma radiation. It is of interest to examine the costs of such radiation processes. The chlorination of benzene is well suited to cost estimation and comparison because the reaction is presently conducted on a commercial scale using ultraviolet radiation. The addition product of benzene and chlorine is technically known as 1, 2, 3) 4, 5, 6 hexachlorocyclohexane, but it is usually referred to as benzene hexachloride. The gamma isomer of this compound is a powerful insecticidel0'21 which is sold under various trade names. In the usual radiation reaction this isomer constitutes about 12% of the total addition product. The present commercial process utilizes small quartz equipment necessary for transmission of the ultraviolet. The following calculations are made for the gamma radiation process using a single larger reaction vessel made of nickel alloy. Costs are estimated for four different gamma radiation sources derived from fission products. (a) Design of a Chlorination Process Using Gama Radiation In a typical batch run at 20~ C. and 62 kilorep/hr. of radiation, Harmer7 obtained 63.6 gn. of benzene hexachloride in 0.333 hr. from a charge of 20 ml. of benzene and 80 ml. of carbon tetrachloride. The theoretical yield is 65.6 ia. based on the benzene density of 0.88 gi./ml. Neglecting the actual increase in volume of the reactor contents, the benzene concentration decreased from 20 to 0.6% by volume. Assuming the reaction is first order with respect to benzene concentration C, the rate is dC/d9= - kC, with k being the reaction velocity constant and e the time., Integrating this from the beginning to the end of the batch reaction gives ln(20/0.6) - k(0.333), or k - 10.6 hr1.As a basis it will be assumed a plant is to be designed to produce 454 kg./day of the gamma isomer. This corresponds to a production of 541 gin. moles/hr. of mixed isomers. A cylindrical agitated flow reactor is to be employed with a uniform benzene concentration of 80%. The gamma source is to be placed in a center hole according to the scheme shown in Figure 5. The source is to be of such strength that the mean reaction velocity constant is 1.0, so the reaction rate is (1.0)(0.8) ml. benzene/hr. -ml. solution. This is equivalent to 9.02 gin mole benzene/hr, -liter solution. Since a gm. mole of benzene makes a ga. mole of product, the volume of the reactor is 541/9.02 or 60 liters. This volume may be realized in a cylinder 50 cm. high and 50 cm. in diameter, with a hole 20cm. in 7

diameter for the readiation source and cooling coils circling the inside of the walls. The reaction velocity constant is assumed to vary directly with radiation intensity I (Hamer ) showed the chlorination of toluene to vary as \T)which is conservative for intensities less than that of the laboratory experiment. Thus, k - mI, or k - 0.171 I from the fact that k = 10.6 at I 62. For rough estimation assume the cylindrical reactor is the equivalent of a 25 cm. radius sphere with a point source of radiation at the center. There will be a spherical hole of 10 cm. radius at the center to account for the volume occupied by the actual source. If the radiation intensity on the surface of the hole at 10 cm. from the center is 50 kilorep/hr., and if the intensity decreases according to the inverse square law and the exponential absorption law, the average k throughout the reactor volume may be calculated as kCv 0.1710(50)(lo)2e0 eo 6(.88)(rlo)(.r2)dr 1.0 e10 r2(4/3)-r (25) - (10) Here o.064 is the gamma absorption coefficient and 0.88 is the density of the benzene. In order to obtain an intensity of 50 kilorep/hr. at 10 cm. from the center for a fission-product source, about 1250 curies would be required. This is multi-plied by a factor of four to account for selfabsorption, absorption by the wall and the agitater between the source and the benzene, and losses of radiation from the cylinder through the hole. Therefore, 5000 curies will be taken as a conservative estimate. The radiation chamber housing the reactor will be made 3 meters' square to provide adequate working space. To insure that the intensity of radiation is less than 0.5 millirep/hr, outside this chamber with a 5000curie source, the concrete walls must be 85 cm. thickl9. The other units in the system are calculated by conventional means. The process is straightforwardwith the benzene being saturated with chlorine before entering the reactor, and excess chlorine bubbling through the reactor to keep the contents saturated. The product from the reactor is flashed to separate out the solid benzene hexachloride. The benzene is condensed and returned to the chlorine saturator. Refrigerated coolant is required to maintain the reactor feed and contents at 20~ C. (b) Cost Estimations The costs of fission-product sources are not yet available. Therefore estimations are made for a range of values that seem reasonable. Because of varying half-life of different sources, it will be assumed that charges are made approximately according to the radiant enery delivered. Since the energy E of a radioactive source is given by SIoeA d I(1 - e e> )/3., at large values of time 6 the energy varies inversely as the disintegration constant A. But X is the reciprocal of the halflife, so the energy varies directly as the half-life. The costs will 8

therefore be selected in proportion to the half-life. The cost of long half-life sources is to include shipping. Short half-life sources would involve excessive shipping costs and are assumed to be used in the vicinity where they are produced. If the source intensity is not to fall below say 80% of its initial strength, short half-life materials will require frequent replacement. These factors have been considered in Table III, where the costs of producing the mixed isomers of benzene hexachloride are given as a function of the type of radiation source. In estimating the total radiation required, the source is to be composed of five sections. Whenever the intensity falls off 20%, one section is replaced with fresh radioactive material. In the case of cesium-137, it is apparent that no replacements are necessary. In fact, at the end of a five-year period, the cesium activity is still more than 90% of its initial value. In selecting costs for radiations sources, ten-fold ranges were estimated for each case, but only the highest value is given in the table. It is noted that the variation in costs of radiation actually has only a small effect on the product cost. The costs of benzene and chlorine are taken from the latest trade journals. These also give the cost of commercial benzene hexachloride as $1.90 to $3.00 per kg. of gamma isomer in 12% mixture with the other isomers. The gamma radiation process evidently can compete with the ultraviolet process for making this compound. Of course, the true comparison between the two should be made on the basis of the comparative costs of the reactors and radiation sources, since all other equipment is the same. What this comparison does show, however, is that gamma radiation processing is not out of line with other processing methods and that it should be given serious consideration by industry. 9

Figure 1. High-Pressure Polymerization Reactor with Fittings AQo z 8, eop 50050 w J- II I I -O 20 4060 80 00 120 40 60 I80 O 20.2 0 0 60 O 100 0 40 10 2 220 REACTION TEMP C REACTION TEMP *C Figure 2. Melting Point and Figure 3. Density and CrystalMolecular Weight linity of Polyethylene of Polyethylene 10

ANALYTICAL E R ABSORIP N T DOUBLE THROWl W E I AYPELLETS TAIL GAS BOTTLES 1.!! t I"" I 11 1 J fJJ IHJ CARBO TETRACHLDRIDE- INLET MANOMETERS I l CHLOROFORM-DRY ICE VENT MI LAt L < = nn ug RII L_ F| r NaOH-SOLUTIONS SURGE BOTTLE__ _E _ > I~ I1 | — II GRORCREYC GLASS JOINTS C6 H6 SATD. WITH C T2 SOURCE RUIN P M-~TM I X ER REACTORHOUSE EXCESS SHIELLS' STEAM AND/ORKET GLASS |AC6H TH ERE Figure 4. Sohemsatie Diagraop of Laboratory E rPilpaent for AromatBo Cblorinations RECYCLE C Hs HCra | A REFRIGERATION STORAGE /SSTEM,SAOD. WITH. C STOURCE IN MIXER I -- REACTOR [ ~ k ~,,,,,,COLD BRINE 1 EXCESS n^'T x Cl12 RECYCLE C 6 H 6 J^ "^ ^ _____iCONDENSOR MIXER~~~~ T REACTOR HOUSED |^ _ IN SHIELDED CR Hr JCHAMBER 6 Hr. ^ HOT WATER RADIATION H20 + Cr__ HC, C16 ZX —1 /r SOURCE IN WELL TO SEPARATIONS CYL STORNDGENSOR SAWELL OF H6 FOR SOURCE STORAGE Be~sgl~b~f~.OaHE R C11H

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