I/EC
Equipment & Design
A Circulating System for Electron Irradiations . . . lets you irradiate liquids at con stant temperature and under pressure by W. S. Guentner and T. J. Hardwick, Gulf Research & Development Co.
I N RECENT YEARS, the use of high
voltage electron accelerators in ra diation chemistry investigations has increased considerably. As a result, many new technical problems have arisen which have not heretofore been present in x-ray or γ-ray irradiation techniques. Probably the most prom inent of these is the difficulty in controlling the temperature of sam ples irradiated by high energy elec trons. This problem is acute in the liquid phase, for the energy absorption may reach values of 100 watts per gram. T h e techniques of heat removal during a specific power input of this order of magnitude are, at present, poorly developed. A second problem is that of de signing irradiation vessels having windows sufficiently thin that satis factory electron penetration is achieved, yet of such construction that the system can withstand rea sonable internal pressures (10 to 20 atm.). A need exists for equipment which may be used in conjunction with an electron accelerator, in which liquids may be irradiated at constant tem perature and under pressure if desired. Since most radiation chem ical investigations at the present time are still in the laboratory stage, it is desirable to have provision for the sampling of liquid, for contin uous monitoring of the evolved gas and for accurate measurement of the energy absorbed. Such an a p paratus m a y be used for a wide variety of systems, and provision must be made for dismantling and 50 A
Comparison of the Advantages of Aluminum and Stainless Steel Cells Alurninum cells
Stainless steel cells
1. For a given cell wall thickness, aluminum absorbs only one third of the energy absorbed b y stainless steel.
1. Acids a n d alkalies may be used in the system.
2. Heat conductivity is much higher. 3. Errors due to reproduci bility of positioning are smaller.
cleaning with a minimum of in convenience. Equipment having these charac teristics has been developed in these laboratories and has been used suc cessfully for over 2l/s years. T h e electron accelerator currently used in this laboratory is a 3-mev., 1-ma. V a n de Graaff accelerator (Model K S , High Voltage Engi neering Corp., Burlington, Mass.). In this model electrons are acceler ated in a downward direction, striking materials placed beneath the scan ning tube. T h e irradiation cell and auxiliary equipment have been designed to use most conveniently the various features of this particular accelerator. Modifications for use with other types of accelerators and electron energies will be apparent. M a n y electron beam irradiations of liquids have been carried out by
INDUSTRIAL AND ENGINEERING CHEMISTRY
2. Use of stainless steel per mits all-welded construction. 3. At high temperatures ( > 1 0 0 ° C.) stainless steel is preferred to aluminum where experiments are made under pressure.
repeatedly passing small samples through the beam on a conveyor belt or rotating table. This technique is satisfactory for small ( < 100 ml.) samples where changes in certain specific chemical or physical proper ties are of interest, but it is unsuit able for larger unit volumes of sample or where gas evolution meas urements are required. T o achieve the controlled conditions desired in quantitative work, a recirculating system is necessary. T h e apparatus consisted of four units: a n irradiation cell, a degassifier, a heat exchanger, and a circu lating p u m p . All components, ex cept some irradiation cells, were stainless steel. Joints and elbows were welded together. Provision was m a d e to separate the various units by inclusion of pipe unions in lines interconnecting the units.
The cireulating loop.
Arrows indicate flow of constant temperature water from source placed 20 feet a w a y
T h e liquid to be irradiated (3.5 to 4.5 liters) flowed clockwise through the loop at about 5 gallons per minute. Constant temperature water from an external source passed through the center of the irradiation cell and through the heat exchanger. Irradiation Cell
T h e irradiation cell essentially had the shape of a condenser with a thin outer wall. T h e liquid to be irradiated passed through the outer annulus, while cooling water flowed through the central 1-inch pipe. T h e entry and exit ports of the annulus were set in tangentially to impart a spiral motion to the transient liquid. In our design, the annular volume was about 1350 ml.; the annular depth was about 1.6 cm. T h e accelerator was operated at a voltage which ensured that all electrons stopped short of the central pipe.
of the electron beam is not generally of concern. A certain degree of loss of electron energy in the irradiation cell walls is permissible if the effective energy of electrons absorbed in the liquid can readily be measured. With any irradiation cell the voltage reading on the accelerator controls is related to the energy absorbed by the circulating liquid, and this relationship may be determined experimentally. Three cells were used, two aluminum and one stainless steel. Each cell required a separate calibration of the energy absorbed in the liquid as a function of machine voltage. Plumbing arrangements for each cell were such that all cou'd be
readily interchanged at the appropriate union connections. In some cells a drain valve was installed at the inlet end. In practice, the unit was placed in such a manner that the irradiation cell was parallel to and centered beneath the scanning tube window. Adjustments were made in the height to give W i n c h clearance between the cell and the scanning tube base. Alignment by eye was quite adequate for obtaining reproducible results on radiolysis. Degassifier
Liquid entered the degassifier from below and was sprayed through a perforated disk against the wall,
Irradiation cell
Many irradiation cells for use with electron accelerators have been described in which a thin window is bolted onto a larger tube. Such devices have two limitations: When enough scattered radiation strikes gasketing material, serious deterioration is caused from either radiationinduced changes or from heat; and the pressure limitation of such apparatus is small, and ruptures under pressure are particularly hazardous to the accelerator window. In an experimental radiation program, some inefficiency in the use VOL. 52, NO. 11 ·
NOVEMBER 1960
51
A
EQUIPMENT AND DESIGN bringing the gas a n d liquid into equilibrium. T h e evolved gas passed out to the atmosphere through a condenser which was maintained at any desired temperature. I n gen eral, this condenser was kept at 0° G. in order that none of the evolved gases would condense at room temperature. T h e rate of gas evolution a n d / o r total amount were measured by conventional means. Arrangements were made to sample the gas periodically with out interfering with the total flow. T o purge the system initially, the liquid was circulated and the purge gas admitted through the inlet port on the degassifier and out through the condenser. When opera tion above atmospheric pressure was desired, the condenser was replaced by a pressure gage, which was read from the control room during irradiation by means of closed circuit television. A second function of the degassi fier was to act as a holdup volume when large amounts of liquid were to be irradiated. Although a mini m u m volume of 3.5 liters is required for the operation of the loop, there is no upper limit, for degassifiers of any size may be used. I n several experiments a 10-gallon degassifier was installed for the radiolysis of 7-gallon quantities of liquid. Heat Exchanger A stainless steel Ross heat ex changer (Model 300-8, SSCF, Ameri can Radiator & Standard Sanitary Corp., Buffalo 13, Ν. Υ.) was used to maintain the temperature of the irradiated liquid at that of the constant temperature water. As the heat exchanger was the lowest part of the system, a drain valve was inserted in its base. This was also used as a sampling port during interruptions in the irradiation. Circulating Pump A glandless turbine p u m p (Chemp u m p Series E) was used to circulate the liquid through the loop.
I n many cases it was necessary to raise the system initially to the desired temperature, then cool when irradiation began. T h e cooling coils h a d sufficient capacity to remove all heat arising from the irradiation, yet this cooling would be overcome by the 10-kw. heater. Water was circulated from the bath through the irradiation system by a p u m p mounted on the water bath. Provision was made to bypass the irradiation equipment while the bath was brought to the desired temperature. I n operation, with an electron energy input of 2800 watts and with water circulating in the loop, the temperature rise across the irradiation cell was 2° C. Collection of Electron Current T h e circulating unit was mounted rigidly on a n aluminum plate 3 / 8 X 30 X 48 inches. This plate sat on a small movable table, the top of which traveled horizontally in one direction a n d vertically by means of screw adjustments. Elec trical insulation from the table was provided by four large rubber stop pers inserted between the plate and the table top. T h e beam current was collected from the aluminum plate which is a common ground for all loop com ponents. A problem exists, how ever, in the possibility of scattered electrons grounding on some parts of the apparatus a n d being collected along with those electrons absorbed in solution. T o overcome this, a crude Faraday cage was placed about the irradiation cell and accelerator window. This cage was insulated from the aluminum base plate and was grounded separately, bypassing the accelerator controls. T h e ac celerator was operated and stabilized on those electrons collected in the irradiation cell. In our arrangement, electrons grounded through the Fara day cage or reflected onto the scan ning tube were not part of the operating beam current. Electrical Connections
Constant Temperature Water T h e source of constant tempera ture water was a 20-gallon water bath equipped with a 10-kw. heater, a thermostat, and cooling coils connected to a tap water source. 52 A
T h e normal operation of the accelerator required a continuous collection of the electrical charge absorbed in the loop. Low im pedance electrical leaks to ground seriously interfere with accelerator
INDUSTRIAL AND ENGINEERING CHEMISTRY
operation. I n our equipment there are several sources of such leaks. 1. T h e cooling water may con duct electricity slightly, particularly when ionic-type rust inhibitors are used. This was overcome by using about 20 feet of connecting rubber hose between the bath and the loop. Such a length of hose allowed the water bath to be placed con veniently out of the way. 2. Single-phase motors must be used. 3. Thermocouples must be in sulated electrically from the system. I n practice, a layer of tape was wrapped about a pipe and a short sleeve of copper was soldered around it. Copper-constantan thermo couples were connected to this sleeve and the voltage recorded in the control room. Although the response was not particularly fast, such a delay was of little import in a dynamic system. T h e thermo couples served a second purpose— that of warning in case of trouble with the loop. Whenever the flow of liquid in the loop decreased or stopped, the first indication was an abnormal temperature rise across the irradiation cell. However, in two and one-half years of operation, there has never been any malfunc tion of the apparatus serious enough to cause damage. 4. It was found desirable to ground all equipment not connected electrically to the loop. A buildup of charge from scattered electrons or from faulty wiring in equipment may result in erroneous measurements of the electron beam. Measurement of Absorbed Energy Quantitative measurements were m a d e of the energy absorbed in the irradiated liquid for a known total electron charge and operating volt age. T h e total charge is the product of current a n d time, a n d was meas ured by a current integrator (El dorado Electronics Co., Berkeley, Calif., Model CI-100). T h e operat ing voltage was that registered on the accelerator controls, and was probably within 5 % of the absolute value. T h e amount of energy absorbed in a circulating liquid was measured by irradiating a 0.1 M solution of sodium formate as a chemical do simeter [Hardwick, T . J., Guentner, W. S., J. Pkys. Chem. 63, 896 (1959)]. Operating at a fixed voltage and constant total charge, the following results were obtained:
EQUIPMENT AND DESIGN.
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Figure 1 . Effective voltage is adsorbed in the liquid as a function of operating voltage for a stainless steel and an aluminum irradiation cell
• T h e reproducibility of the total energy absorbed as measured by chemical change was ± 1 % in the case of aluminum cells, ± 3 % for stainless steel cells. • Varying the scan width at constant current between 5 and 14
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energy absorbed as measured by the chemical dosimeter total electron charge
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inches had no measurable effect on the total energy absorption. • Varying the current from 50 to 900 μΆ. showed no effect on the total energy absorption. Effect of Voltage Variation
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Absorption of electron energy occurs in the cell walls. As the voltage of the accelerated electrons varies, so docs the energy absorbed in the circulating liquid. M a n y side effects of voltage are found to a minor degree—e.g., more air scatter, resulting in a broader beam across the cell circumference; more re flection and energy loss in the cell wall at the lower voltages; and more electrons "lost" to the col lection system at lower voltages. All these effects may be amalgam ated by defining an effective or mean energy of the electrons absorbed in the liquid system. This effective energy is related to the operating
Circle No. 21 on Readers' Service Card
54 A
voltage in a manner which may be determined experimentally. Experiments were made using the sodium formate dosimeter in which the effective voltage (Ve) was found by the relationship :
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Data were obtained over the operating voltage range of 2 to 3 mev. T h e results for an aluminum cell and a stainless steel cell are shown in Figure 1, in which the effective voltage, Ve, is plotted against the operating voltage V0. Results for other cells are similar. T h e energy absorbed by the slow ing of electrons in the cell walls is 11 kev per '/moo inch for aluminum; the corresponding value for steel is 29 kev. per Viooc inch. In practice these values appear low because of scatter of electrons in the metal.
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