OXIDATION OF NUCLEAR-GRADE GRAPHITE BY NITRIC ACID AND OXYGEN JOSEPH B. F A R R E L L ' A N D PAUL A. HAAS Oak Ridge National Laboratory, Oak Ridge, Tenn.
The reaction of nuclear grade graphite with oxygen and nitric acid solution was studied at 2 7 5 " to 300" C. and 1000 to 1800 p.s.i.g. Oxygen was bubbled continuously through a reaction vessel containing nitric acid and graphite. Carbon dioxide and nitrogen were the only combustion products. Approximately 1 mole of nitric acid disappeared for every 30 moles of carbon consumed. Reaction rates were approximately proportional to acid normality up to 4.ON nitric acid concentration. With 2.ON acid at 1600 p.s.i.g. and 300" C., a rate constant of 0.23 hr.-I was achieved. Air could be used instead of oxygen without excessive sacrifice in rate. Corrosion in the Type 347 stainless steel reactor was excessive for 4 N acid. Runs made in a rocking autoclave gave much higher rates than the fixed-bed runs, either because of increased decrepitation of the graphite or improved contact between the graphite and the surrounding liquid.
N CERTAIN
types of fuel elements for nuclear reactors, uranium
I and thorium fuel particles (oxides or carbides) are dispersed in a graphite matrix. When the buildup of fission products has reached a point where the nuclear reaction is hampered, the fuel element must be replaced and the uranium and thorium recovered. One recovery process is based on oxidation a t high temperature with air, followed by dissolution of uranium and thorium in acid. This combustion process has certain drawbacks. The oxides produced are difficult to dissolve, particulate matter is produced, and some radioactive materials are vaporized. Another process has been proposed (7) in which the oxidation is carried out under oxygen pressure in an aqueous solution a t relatively low temperatures. The uranium and thorium oxides or carbides are converted to soluble nitrates. The relatively low temperature and high total pressure limit loss of volatile materials. The use of the liquid phase reduces the chance for carry-over of small particles. Preliminary studies in an autoclave demonstrated the feasibility of this process, and indicated that addition of nitric acid greatly accelerated the reaction rate. The purpose of the present study was to obtain more detailed information on the effect of variables on graphite consumption rate. Only reactor-grade graphite was investigated. Major variables investigated were temperature, oxygen partial pressure, and nitric acid concentration. Another phase of this work, the application of the process to graphite-based nuclear fuels, has been reported by Haas and Ferris ( 3 ) . Previous Work
A limited survey of the literature, covering the oxidation of graphite with nitric acid solution and oxygen, showed very little directly applicable information. Considerable information on oxidation of coal by various solutions is available (4). Most of this work was done to establish the structure of coal, or to recover various soluble products. An article by Savich and Howard (5), on the oxidation of coal by nitric acid and oxygen, yielded interesting information. Coal was oxidized in a rocking autoclave in acid solution under oxygen pressure. Present address, Manhattan College, Bronx, New York, N. Y .
The rate of production of COz increased linearly with acid concentration and with temperature for 4.5hT acid, and was independent of oxygen concentration over the range of 400 to 600 p.s.i.g. Corrosion of their stainless steel autoclave was substantial. Teletzke (6), in discussing the Zimmerman process, presented data showing that carbon in water suspensions is oxidized to carbon dioxide by bubbling air through the mixture. I t is presumed that no acid was present. Type of carbon, state of subdivision, and total air pressure were not given. Removal of carbon was reported to be very rapid above 200" C. Screening tests carried out by one of the authors (P. Haas) a t Oak Ridge National Laboratory in a rocking autoclave indicated that the oxidation of graphite by oxygen in the presence of water was greatly accelerated by addition of nitric acid. The work described here was carried out to obtain quantitative information on the effect of acid concentration, temperature, and oxygen partial pressure on the rate of oxidation of graphite. Experimental Apparatus
Fixed-Bed Unit. The apparatus constructed for these further studies was based on the simplest possible concept of the ultimate process. This is a reasonable position in view of the need for containment when dealing with radioactive materials. Figure 1 is a schematic representation of the unit. The reactor is a ll/*-inch, Schedule 160, stainless steel pipe. Reactor-grade graphite is charged to the pipe in a basket of stainless steel screen. Typically, about 170 grams of graphite, cut into '/d-inch cubes, i s charged. Aqueous nitric acid (ca. 420 ml.) is charged to the vessel through the top, although it can be pumped in if desired. There is no provision for circulating the acid or adding to the charge during a run. T o commence a run, oxygen pressure is applied through a bypass line to the top of the vessel. Heat is applied by means of resistance heating elements encasing the reactor. When operating temperature is achieved, pressure is adjusted to the desired value, and oxygen flow is started a t a known rate through the bottom inlet into the reactor. The oxygen bubbles through the nitric acid solution, which completely covers the graphite charge. The gases (oxygen and reaction-product gases) pass to the upper section of the reactor. This section is cooled by external cooling coils; cooling substantially reduces VOL.
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carry-over of liquid by condensing out HNO, and water. The gases pass through a water-cooled condenser, a small chamber for removal of entrained droplets, and finally through a throttling valve which is set to maintain the desired pressure in the system. The gases expand to atmospheric pressure across SAOR MPIE S this valve. Part of the gas is passed through a thermal OFF GAS conductivity analyzer, where the proportion of carbon dioxide CONTINUOUS in the oxygen stream isdetermined. Gas samplesare also taken ANALYZER for more complete analysis by mass spectrometry or gas chro(BY THERMAL CONDUCTIVITY) matography. After completion of a run, the remaining liquid is analyzed for nitric acid content, iron, chromium, and nickel.
COOLING WATER
RUPTURE DISC
CLAMSHELL JACKET
HEATERS
0 2 CYLINDER
IF=---
t LIQUID METERING PUMP
Figure 1.
Pressurized aqueous combustion reactor system
Material. 3 4 7 stainless steel Pipe schedule. 1 '/Z-inch Schedule 160 reactor body. 2000-p.s.i. ring joint flanges. ' / p and '/*-inch Schedule 8 0 connections to reactor. Other tubing and flttings rated 2000 p.s.i. a t room temperature
>
c 'L
-E
0.18
h
I
I PRESSURE: G A S FLOW:
I
1630 psig
28rtd cc/sec i
No analyses were made on the reactor grade graphite used in these tests. However, this material has virtually no impurities. For example, Currie, Hamister, and McPherson (2) note that total ash content is generally below 20 p.p.m. The information collected during a run allows calculation of both rate of reaction and mass of charge remaining at any time and complete material balances on major components. Reaction rate is determined from the flow rate of effluent gases and the concentration of carbon oxides in this gas stream. Total graphite consumption is also calculated from these data. Autoclave. The substantially different conditions between the fixed-bed unit and the autoclave in which the original exploratory work was done made it advisable to run comparative experiments in this equipment. The autoclave is a cylindrical vessel, rocked a t about 25 cycles per minute, which results in a high degree of movement of liquid in the vessel. Total volume is 13 liters. To make a n autoclave run, a graphite charge of about 80 grams is placed in a stainless steel basket, which is put into the autoclave. I n each case, 3 liters of aqueous nitric acid are added, which cover the charge for the greater part of the rocking cycle. The autoclave is sealed and the desired pressure of oxygen is applied. The rocking action is started, and the heat is turned on. Operating temperature is reached in about 2 hours. After the run has been carried on for the desired time, the heat is turned off, and at the same time the agitation is stopped. After the autoclave has cooled to room temperature, the autoclave is vented, and the effluent gas is sampled. The remaining solids are dried and weighed, and the solution is analyzed for nitric acid content, iron, chromium, and nickel.
I
0.06
Results and Discussion
I
Pressuret 1600 psig Gas Flow: 27std c c h e c
0.30 al
L
B
.-c
0.20
e
0
8
0.10
0 0
1.0
2.0
3.0
4.0
5.0
6.0
A c i d Normality
Figure 3. Effect of nitric acid concentration on maximum reaction rate for '/d-inch cubes 278
I&EC PROCESS DESIGN A N D DEVELOPMENT
Variation of Rate with Fraction Consumed. Early experiments in the fixed-bed reaction vessel established that reaction rate is a function of the fraction of the original charge consumed. Typically, reaction rate per unit mass of graphite actually present increases initially and then decreases as the graphite charge is consumed. The characteristic shape of a curve of reaction rate us. fraction of charge consumed is shown in Figure 2 . The increase in rate is attributed to an increase in surface area per unit mass, since the graphite cubes become roughened during the reaction and partially decrepitate. The decrease in rate results because nitric acid is being consumed and its concentration in the reaction vessel falls as a run proceeds. The decrease in rate is less steep when 2.ON acid is used, probably because the relative drop in acid concentration is less. I t is apparent then that the results obtained are not absolute, but are related to the conditions of the experiments. Interpretation of the results and comparison with other conditions have been obtained by determining for any given run the maximum rate from a curve of reaction rate per unit mass us. fraction of original charge consumed, and comparing these values. Effect of Acid Concentration and Temperature. The effects of temperature and acid normality were studied in a
0.22,
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2
I
I
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I
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0
0.16
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13-1 13-2 13-3
1600 1600 1600
12
1600
I
1
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9 =
0.10
0.08
0
0.2
0.4
0.6
0.8
I .o
FRACTION OF ORIGINAL CHARGE CONSUMED
Figure 4. comparison of rates with air and oxygen for I/d-inch cubes ai 275' C. with 2N " 0 3
two-level two-factor experiment. Pressure was held a t 1600 p.s.i.g. in these runs. Maximum rates of consumption of graphite (moles per hour per mole) are as follows:
275" C. 300" C.
1N " 0 3 0.08 hr.-I 0.13 hr.-l
2N "03 0.15 hr. -I 0 . 2 3 hr.-l
The 25' C. increase in temperature increases the rate by a factor of 1.5 at both levels of acidity; the increase in acid strength from 1.ON to 2.ON increases the rate by a factor of 1.9 at both temperatures. These same data have been plotted in Figure 3. Also shown on this curve is the reaction rate obtained with water (no reaction), and an additional point with 4.ON acid at slightly different conditions. These data show that, in the range investigated, the reaction rate is approximately proportional to the acid normality. Other Variables. Other runs established the effect of using air instead of oxygen, the rate of corrosion in the reaction vessel, and the consumption of nitric acid. The results are presented below. Reaction rate per unit mass of graphite present with oxygen is compared with the reaction rate with air in Figure 4. I n this experiment, oxygen was first fed to the reactor, and then air was substituted for the oxygen. After a period of time, the air was shut off and oxygen flow resumed. The reaction rate is seen to be about 25y0 lower when air is substituted for oxygen. The relatively minor influence of a fivefold drop in partial pressure of oxygen is significant, and indicates that oxygen transfer into the liquid does not seriously limit the reaction rate. The dotted line on the figure was obtained in another oxygen run, and shows good reproducibility. Corrosion rates in these tests were erratic but could be roughly correlated with acid concentration. Relative corrosion rates were about 1:10:100 for 1.0, 2.0, and 4.ON nitric acid. The approximate absolute magnitude is 0.05 inch per year for 2.ON acid. No examination was made for pitting or intergranular attack. Consumption of nitric acid, measured from the drop in H + concentration, ranged from 0.02 to 0.10 mole of nitric acid per mole of graphite consumed, and had a median value of 0.04 mole per mole. No correlation was achieved but there may be a relation between graphite consumption and the degree
of oxygen saturation of the nitric acid at the beginning of and during a run. The nitric acid is consumed by reaction with the vessel walls and by entering the oxidation reaction with the graphite. As anticipated, material balances revealed a substantial disappearance of nitrate. The final form of this nitrate is of interest. Extensive analytical work on all solid, liquid, and gaseous material leaving the reactor showed no compounds or oxides of nitrogen except nitrate in the solution. The nitrate apparently is converted to nitrogen. This conclusion could not be checked experimentally, since the high pressure cylinder oxygen fed to the unit contained some nitrogen. The small amount of nitrogen produced in the reaction could not be accurately estimated by the difference between inlet and outlet analyses. Support for the conclusion that the nitrate is eventually converted to nitrogen is given by Savich and Howard ( 5 ) ,who noted formation of nitrogen when coal was oxidized with nitric acid and oxygen; this they attributed to reduction of nitric oxides. A single run made with nitric acid and with nitrogen gas instead of oxygen showed substantial consumption of graphite. The reacted gases contain carbon dioxide, oxides of nitrogen, and nitrogen. The presence of oxygen is apparently not necessary for at least some degree of reaction to take place. Analysis of the liquid collected in the entrainment removal chamber indicated that nitric acid was being manufactured in the cool upper section of the reactor. This liquid was essentially all aqueous nitric acid. Its concentration was invariably higher than the average acid concentration in the reactor and in some cases was actually higher than the original acid concentration. This appears not to be simply vaporization of nitric acid with subsequent condensation, since in this concentration range (less than 2N), the volatility of nitric acid is less than that of water. A likely explanation is that oxides of nitrogen formed in localized reducing regions below the liquid surface are reoxidized to nitric acid in the upper part of the reactor. Above the liquid surface, the gases rapidly cool, and evaporated nitric acid and water vapor condense. As a consequence, oxygen partial pressure increases. Residence time is high (ca. 10 minutes). These conditions are good for nitric acid formation. The nature of the reactions occurring in the reaction vessel now can be discussed with some degree of confidence. I n the liquid phase surrounding the graphite particles, graphite is VOL. 6
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being oxidized by nitric acid, forming carbon dioxide and oxides of nitrogen. Some of the oxides of nitrogen are reduced to nitrogen. Direct consumption of oxygen by reaction with graphite may take place but its magnitude is unknown. I n the upper part of the reactor, cooler temperatures, high residence time, and the increasing partial pressure of oxygen as nitric acid and water vapor condense out, cause the oxides of nitrogen to be reconverted to nitric acid. Comparison with Autoclave. The conditions a t which the autoclave runs were made, and the reaction rates obtained, are given in Table I. These runs parallel reasonably closely the two-level, two-factor experiment in the fixed bed which has been discussed previously (Figure 3). The reaction rates in Table I have been calculated in the following fashion. Instantaneous rate is presumed to be proportional to the mass of graphite present: -dM/dO
= kM
0
k
0.00
Y
2
0.60
0: E
.5
0.40
d u
F
0.20
3 0 0
(1)
1.0
2.0
3.0
Acid Normality
where
M
i
mass of graphite present = time of reaction = reaction rate per unit mass of reactant =
Integrating,
where subscripts i and f refer to initial and final conditions. The value of k is the reaction rate per unit mass of reactant (hr.-l), and is based on the assumption that rate does not change markedly as the graphite is consumed. The reaction rates shown in Table I have been plotted in Figure 5 with the reaction rate obtained with plain water included. The results differ strikingly from those obtained in the fixed bed. A somewhat higher rate might be expected with the autoclave. The nitric acid concentration does not fall significantly during a run, because the ratio of nitric acid to graphite in the charge to the autoclave is high, and the oxygen partial pressure is higher. Both of these factors should increase the rate; however, the difference in rate is about fivefold on the average, which is far higher than expected from these differences. A reasonable explanation for the markedly higher rates in the autoclave as compared with the fixed bed is that the rate of reaction in the fixed bed is being limited by inadequate mass transfer between the liquid and solid surface. This explanation is supported by the fact noted earlier (Figure 4) that large changes in oxygen partial pressure did not greatly affect the reaction rate. However, a factor which cannot be overlooked is the possible acceleration of the decrepitation of the graphite by the motion of the autoclave. Greatly increased surface could cause the large changes in rate noted.
Figure 5. Effect of nitric acid concentration on average reaction rate for '/cinch cubes in autoclave
The difference in dependence of the reaction rate on acid normality between the fixed bed and the autoclave is evident from examination of Figures 3 and 5 . Instead of a direct proportionality between normality and rate, the results in the autoclave show a lower effect on reaction rate at higher acid concentrations. At 275' C., a maximum rate has virtually been reached with 2.ON acid. These curves probably represent a practical maximum which could be reached in a fixed bed if conditions were optimized. Conclusions
The results of the experiments in the fixed bed indicate that a range of conditions exists where reaction rates are reasonable, chemical consumption is moderate, and corrosion is not excessive. Optimum conditions appear to be at 300' C. with 2 N acid under 1600-p.s.i.g. pressure. Corrosion is excessive at higher acid concentrations. Substitution of air for oxygen causes some decrease in rate, although the reduction is not large. The substantially higher rates achieved in the autoclave experiments indicate that a fixed bed arrangement in which the liquid surrounding the graphite is agitated only by the oxygen or air input stream is not the optimum arrangement, although it is certainly the least complex. Improved design of the fixed bed, to prevent possible bypassing by gas, might be of value. Recirculation of the liquid, stirring, or pulsing appears to be the best means of increasing the rate, but this advantage must be weighed against the increased complexity in the process. Acknowledgment
Table 1.
Oxidation of Graphite in 13-liter Autoclave Using Oxygen and Nitric Acid
A-2 A-3 A-4 A-6 Graphite particle size, 1/4 1/4 1/4 inch 1/4 Initial "01 normality 1 .O 1.o 2.0 2.0 Temperature, C. 275 300 275 300 Total pressure during run, p.s.1.g. 1770 2160 1740 2190 Average reaction rate, hr . 0.34 0.65 0.40 0.96 Run
The Oak Ridge Institute of Nuclear Studies assisted by providing a summer research participation appointment for one of the authors. The assistance of D. A. McWhirter, who performed much of the experimental work, is gratefully acknowledged.
O
literature Cited (1) Culler, F. L., Oak Ridge National Laboratory, personal
communication, December 1963.
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l&EC PROCESS DESIGN A N D DEVELOPMENT
(2) Currie, L. M., Hamister, V. C., McPherson, H. G., “Progress in Nuclear Energy,” Series IV, “Technology and Engineering,” R. Hurst, and S. McLain, Eds., pp. 65-107, McGraw-Hill, New York, 1956. (3) Haas, P. A., Ferris, L. M., IND.ENG.CHEM.PROCESS DESIGN DEVELOP. 5.234-8 (1966). ( 4 ) Howard, H. C., “Chemistry of Coal Utilization,” H. H. Lowry, Ed., pp. 346-76, Wiley, New York, 1945. (5) Savich, S. R., Howard, H. C., Znd. Eng. Chem. 44, 1409-11 (1952).
(6) Teletzke, G. H., “Wet Air Oxidation,” 56th Annual Meeting, A.I.Ch.E., Houston, Tex., December 1963. RECEIVED for review February 7, 1966 ACCEPTEDJanuary 5, 1967 Division of Industrial and Engineering Chemistry, 150th Meeting, ACS, Atlantic City, N. I., September 1965. Work sponsored by the U. S. Atomic En( vy Commission under contract with the Union Carbide Corp.
EVALUATION OF TEMPERATURE PULSE CHARACTERISTICS AND PULSE TESTING FOR THERMAL DYNAMIC ANALYSIS CLARENCE I. LEWIS, JR.1, DUANE F. BRULEY, AND DANIEL H. H U N T Clemson University, Clemson, S. C. Experimental temperature pulses were generated and studied in terms of normalized frequency content to evaluate optimum pulse characteristics for the determination of reliable frequency response data. All results indicate that total pulse duration is the best criterion for predicting pulse quality with respect to valid data reduction capability. A wetted-wall column was forced using temperature pulses. The pulse-testing data were compared with previously determined direct frequency forced results and mathematical model solutions for laminar and turbulent air flow conditions. The results illustrate the validity of pulse testing but also demonstrate some difficulties and limitations of temperature pulsing for thermal dynamic data recovery.
uLsE-testing techniques were used for the experimental
p thermal dynamic analysis of a wetted-wall column operati n g a s an adiabatic humidifier. The investigation is an extension of previous studies (7) carried out on the same physical system and concerned with the formulation and solution of the system’s mathematical models, in the frequency domain, for both turbulent and laminar air flow conditions. The models were verified by experimental testing, using direct frequency forcing techniques and plotting theoretical and experimental frequency response curves in the form of Bode plots. Although the mathematical models and experimental data agreed well, the experimental frequency range was fixed by physical limitations of the uniquely designed mechanical generator used for producing a harmonic temperature variation a t the inlet to the column. With the apparatus, as described (7), it was difficult to produce a reasonable sinusoidal forcing function at frequencies above 2.5 or below 0.03 radian per second. This frequency range was not sufficient to describe the column dynamics completely, as the maximum obtainable frequency is just slightly greater than the break frequency of the system. To extend the previous investigation the present study was initiated to accomplish essentially three purposes: (1) compare pulse-testing results with those from direct frequency forcing and mathematical formulation for an actual physical system ; (2) extend the frequency range to characterize the thermal dynamics of the wetted-wall column better; and (3) study the effect of arbitrary pulse characteristics in predicting optimum pulse shapes for obtaining the system dynamic response. Present address, Enjay Chemical Co., Baton Rouge, La.
The air-water process investigated is representative of a distributed parameter system involving simultaneous heat and mass transfer. A wetted-wall column, used as the process, provided a definite transfer area and allowed the development of essentially reproducible air and water velocity profiles. Adiabatic humidification conditions, together with the large heat capacity and transfer rates in the liquid phase, relative to those in the gas phase, ensured a constant liquid and interfacial temperature in the column. Hence the analysis of the system was simplified to essentially a one-phase flow problem involving only the air-vapor phase. Frequency Response from Pulse Testing
Pulse testing is a transient response method giving data which can be mathematically reduced to yield frequency response information in terms of magnitude ratio and phase shift. This technique involves disturbing a system, initially operating at steady state, in a pulselike manner and simultaneously recording the pulse traces of the input and output variables. The pulse traces are then reduced to determine the system frequency response. Several authors (2, 4 ) present the mathematics of the pulse-testing method. Based on the mathematics, a digital computer program employing Filon’s (3) method for numerical integration was written for calculating magnitude ratio, phase shift, and normalized frequency content from experimental pulse data. To debug and check the program validity, simple first- and second-order systems were simulated and pulsed on the analog computer. The pulse traces were recorded and then reduced using the digital computer program. The theoretical system VOL. 6
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