I
G.M. BEGUN, J. S. DRURY, Chemistry Division,
and
E. F. JOSEPH
Oak Ridge National laboratory, O a k Ridge, Tenn.
Automatic Cascade for the Production of Nitrogen45 Scaling up the process is feasible, should the need arise for a fuel component in a nuclear reactor N r r R o G m - l j occurs with a natural abundance of 0.37 atom 70:and has an absorption cross section for thermal neutrons of 2.4 X 10-6 barn as compared with 1.86 barns for normal nitrogen. This difference makes nitrogen-1 5 potentially interesting as a fuel component in certain types of nuclear reactors. Preliminary estimates of the cost of producing high purity nitrogen-1 5 by several methods have been made a t Oak Ridge. The estimated costs varied from one to several thousand dollars per gram, depending on the process and the production rate. The cost (2) of producing 50 metric tons per year of 99$, nitrogen by gaseous centrifugation was estimated as $2000 per gram. Comparable costs (2) for other processes were: gaseous diffusion, $4.60; open reflux ion exchange, $4.00 (chemical costs only) ; gas-liquid chemical exchange, $1.00 to $2.50. Of the several gas-liquid chemical exchange processes that have been considered (2-4), the most attractive method seemed to be the Titrox system, which was developed by Spindel and Taylor (5, 6 ) a t Columbia University. The automatic cascade described was constructed and operated to evaluate the economic and technical feasibility of using this method to produce nitrogen-15 in large quantities. and to provide a basis for scaling up the process should a need arise. Chemical Basis of the Process
Isotopic fractionation in the cascade is achieved by a chemical exchange reaction between aqueous nitric acid and a gaseous mixiure of nitric oxide and nitrogen dioxide. A simplified statement of the enrichment reaction is given by the equation HN1403(aq.)
+ N150(g)
@
HN1603(aq.)
the required number of enriching stages between two exchange columns, one smaller in diameter than the other. The relative lengths of the columns were estimated by visually selecting, from a plot of minimum reflux ratio us. product enrichment, a point of taper which would minimize the combined holdup of nitrogen-15 in both columns. Such a plot is shown in Figure 1, where V is the interstage flow, D is the product withdrawal rate? and X , is the mole fraction of nitrogen-lj in the gas phase at stage n. Several variations of this preliminary choice then were calculated and the best point of taper, under the conditions specified, was determined to be that stage where the gas phase contained 0.06 mole fraction of nitrogen-1 5. Assuming a separation factor of 1.05, a minimum reflux ratio of approximately 5400 must be used a t the feed point of a Nitrox cascade to produce 99.37% nitrogen-15. In practice it is necessary to use a reflux ratio somewhat greater than the calculated minimum. In the calculations described here: a
and nitrogen dioxide by addition of gaseous sulfur dioxide. The sulfur dioxide is oxidized to sulfuric acid. The gas phase is refluxed by oxidizing the nitric oxide to nitrogen dioxide by addition of oxygen or air, then absorbing nitrogen dioxide in water to produce nitric acid. Conversion of the oxides of nitrogen to nitric acid in this way is a well-known commercial operation. In the cascade described. the stripping section above the feed point was replaced by a large reservoir of nitric acid; the waste oxides of nitrogen were oxidized with air. absorbed in water? and discarded. Design Considerations
The cascade was designed to produce 1 gram per day of 99.377, nitrogen-15. The average single-stage separation factor of the system was taken as 1.05, based on the use of 6:tl nitric acid as feed. To minimize the equilibrium time of the cascade, it was decided to taper the exchange section by dividing
The Nitrox system utilizes a chemical exchange reaction to c o n c e n t r a t e nitrogen- 15
VvASTE Yi4 ACUESLIS H 4 C 3
-
FEED AQUEOUSHN03
+ S140(g)
The single-stage isotopic fractionation factor, [N16/W4(aq .)/N15/N14(g)1, for this reaction varies from about 1.065 in 1 M nitric acid to 1.020 in 15M nitric acid (7). T o obtain more than a single stage of isotopic enrichment, the aqueous and gaseous streams must be refluxed. Nitric acid is converted to nitric oxide
GASEOUS SO2
I
H2SO,
VOL. 51, NO. 9
SEPTEMBER 1959
1035
reflux ratio about 10% greater than the calculated minimum was used. With this reflux ratio, 101 theoretical stages are required in the first column to enrich the feed material from normal abundance to 0.06 mole fraction of nitrogen15. Similarly, a reflux ratio of 375 and 207 theoretical stages were needed in the second column to upgrade the process stream from 0.06 to 0.9937 mole fraction of nitrogen-15. Because the product withdrawal rate and the reflux ratios of the cascade now were established, the diameters of the exchange columns were fixed by determining the flooding characteristics of the selected packing. From these considerations. a column 2 inches in diameter was chosen for the first section of the cascade. In a similar way the diameter of the second column was fixed as 0.5 inch. Estimates of the height equivalent of a theoretical stage were obtained from the manufacturer for the selected packing materials in 2-inch and 0.5-inch columns. From these data the required lengths of the 2-inch and 0.5-inch columns were calculated to be 19.' and 20.4 feet, respectively . From a consideration of the design flow rates and holdups. the time required for the two-stage cascade to approach isotopic equilibrium was calculated. It was estimated that not less than 5 days and not more than 10 days would be required to reach a steady state under total reflux conditions. The actual equilibrium time fell within this interval during each of the subsequent startup periods.
The Cascade Apparatus and Operating Procedure. Nitric acid ( 7 M ) was metered from a 1000-gallon feed tank by a Lapp Pulsafeeder proportioning pump (Model CPS-1) into the top of column I, which consisted of 20 feet of glass pipe 2 inches in inside diameter packed with 0.05 X 0.10 X 0.10 inch stainless steel Heli-Pak (small, rectangular wire springs). Nitric acid from the bottom of column I flowed into refluxer I . This all-glass unit was 3 inches in inside diameter and 78 inches long. It was packed with '/*-inch Intalox saddles. An internal cooling coil 17/8 inches in diameter extended down 38 inches from the top. A cooling jacket 4 inches in outside diameter extended 36 inches down from the top. Below the cooling coils was a 34-inchlong, unjacketed section! in which the sulfuric acid was scrubbed by the incoming sulfur dioxide to remove dissolved oxides of nitrogen. Sulfur dioxide gas from a 1-ton cylinder passed through a pressure-regulating valve, a surge tank and a control system, and into the refluxer 72 inches from the top. The sulfuric acid produced in the refluxer flowed from a gravity leg through a neutralizing bed to the drain. The nitric oxide and nitrogen dioxide produced by the refluxer passed up the exchange column. The composition of the gas produced in the refluxer was controlled by regulating the concentration of the reacting nitric acid and the temperature of the reaction. The concentration of the nitric acid was adjusted by metering sufficient water into the
W
ln
d
01
I
a ln
a
0
Figure 1. Large reflux ratios are needed to produce 99.3770 nitrogen- 15
005
cz LL
0
z p
002
k u
a LL a LJ -1
001
0
s 2 0.005 I
_ -~L
__
I
0 002
I
I
0 ,PRODUCT WITHDRAWAL RATE
1 036
INDUSTRIAL AND ENGINEERING CHEMISTRY
refluxer with a microbellows pump to produce the desired ratio of nitric oxide to nitrogen dioxide in the gas phase. One sixteenth of the flow in column I was withdrawn, cooled, and pumped to the top of column 11. Column I1 consisted of 20 feet of glass pipe l / 2 inch in inside diameter packed with 0.035 X 0.070 X 0.070 inch stainless steel HeliPak. Refluxer I1 a t the bottom of the column was of all-glass construction and was 1 ' 1 2 inches in inside diameter and 30 inches long. A cooling tube 0.4 inch in diameter extended 8 inches down from the top. and a cooling jacket extended 7 inches from the top. The sulfur dioxide inlet was 25 inches from the top. The refluxer was packed with '/,-inch glass helices. The reaction interface was maintained 4 inches below the cooling jacket. Sitric oxide and nitrogen dioxide produced in the refluxer passed upward, out the top of column 11, and into the bottom of column I. Provision was made for withdrawal of gaseous samples and for removing product nitric acid periodically. Waste nitric oxide and nitrogen dioxide from column I entered refluxer I11 4 feet from the bottom. This refluxer consisted of 12 feet of glass pipe 6 inches in inside diameter. It was packed with '/2-inch Berl saddles. Air entered the bottom of the refluxer to convert nitric oxide to nitrogen dioxide, and water was metered into the top of the column. The nitric acid which formed flowed to the drain. T o produce highly enriched nitrogen15, it is necessary to operate the cascade continuously. Automatic operation and control of the apparatus are therefore advantageous. In operation, the nitric acid flow to column I was metered a t a constant rate. Sulfur dioxide flow to the refluxer was varied as needed to maintain the reaction interface a t a constant level in the refluxer. A definite color interface existed in the refluxer between the brown nitrogen dioxide color and the colorless sulfuric acid below. A spotlight which shone through the column or through a side protuberance and registered on a photocell on the opposite side of the refluxer was placed a t the desired operating level. The photocell output was amplified by a Densicron photometer, and the output was fed to a Brown recorder-controller. When the interface turned down, the controller operated a solenoid valve on a bypass sulfur dioxide line, which remained open until the interface color became light again. Each refluxer had its own control system. Spare lights went on automatically when a bulb burned out. By proper adjustment of the main and by-pass flows of sulfur dioxide, good control was obtained. To protect the stainless steel packing and other parts from attack by sulfuric acid and sulfur dioxide in the event of a power failure, a solenoid
b
-
1
HNG)
HNO)
HO + NO*
EXCHANGE COLUMN 11
+i-
A U T O M A T I C CASCADE
,
, I
2-tn 10 I
2041
EXCHANGE COLUMN 1
,REFLUXES
I
7 A,
,3"PHCTOCELcCOC.IYGWATEQ
7
lAhK
An automatic cascade i s used to produce high purity nitrogen-1 5 on a large scale
valve was placed in the sulfur dioxide supply line. A time-delay circuit shut off the pumps and the sulfur dioxide supply when a power failure lasted for more than 30 seconds. At the bottom of column I, a deflecting cup diverted a small portion of the acid, tvhich then \vas metered to the top of column 11. .4 micro-Lapp Pulsafeeder proportioning pump (Model LS-10) \yas used as the metering pump. T o avoid vapor lock of this pump by gas contained in the equilibrium liquid, it was necessary to precool the entering stream through the use of a small refrigeration unit. At the bottom of column 11, a swinging funnel which was deflected periodically by a n electromagnet was used to withdraiv product nitric acid. Stainless steel tubing was used for vents, and for all sulfur dioxide and nitric acid lines. All flange gaskets were made of Teflon or Kel-F. Drain lines were composed of polyethylene tubing. Both nitric acid and sulfur dioxide were filtered through micrometallic stainless steel filters. Spare nitric acid pumps were provided to ensure continuity of operation.
of 7.M acid in column I and 200 ml. per hour in column 11. \Vater was pumped to refluxer I a t the rate of 350 ml. per hour. Operation {vas continued for 28 days. Figure 2 sho\\-s the isotopic gradient obserized during this period. T h e drop in isotopic concentration after 5 da)-s of operation \\-as due to loss of product from refluser I1 during the night. \\-hen the sulfur dioxide pressure in the system \\-as reduced excessively by atmospheric cooling of the sulfur dioxide reservoir. T-his difficulty was removed later by keeping the sulfur dioxide cylinders in a thermostated room. Product removal \\-as started after 9 day-s of operation and continued until the final shutdo\\-n. T h e rate of production of high purity nitrogen-1 5 was approximately 1.2 grams per day. Operation of the system was stopped because of erratic performance of refluxer 11. I n runs 3 and 4 the concentration of the sulfuric acid produced was The average nitrogen about 9.0M. loss in the discarded sulfuric acid ran from 7 to 14 p.p.m. of the feed nitrogen, which was considered satisfactorily low for the production of 99%; nitrogen-15.
T o increase the throughput oC the system, the feed nitric acid concentration was increased in run 4 to 8 . 7 M a t a rate of 7 liters per hour. \rater was pumped to refluxer I a t the rate of 1700 ml. per hour. The data from run 4 are summarized in the table and Figure 3. The analysis of ivaste effiuent for nitrogen \vas discontinued after 64 days of operation, because no appreciable quantity of nitrogen was passing through the refluxer. T h e product averaged 11 to 12.64 nitric acid. The drop in assay after 77 days of operation was due to SO2 pressure fluctuations, which caused shifting of the isotopic inventory. T h e drop in assay after 88 days of operation was due to the accidental removal of too much product. The run was terminated after 103 days of continuous operation, because ol' a major power failure which cut off all power to the building for several hours. T h e time for the system to attain the 95% nitrogen-15 level was 6 to 7 days. 'There was, however, a slow increase in the concentration of nitrogen-1.5 in column I, which apparently was still going on after 103 days of operation.
Performance of Cascade
In the initial operation of the cascade, difficulty was experienced with refluxers I and I1 due to channeling and poor heat exchange characteristics. After several refluxer designs were tested, the refluxers described were installed and run 3 was begun. T h e nitric acid flow during this run was 4.5 liters per hour
The Nitric Acid Refluxers Convert SO2 to Dilute HzS04
Refluxer I Waste H I S O I , moles/liter Dissolved SOZ,mole/liter Nitrogen loss, p.p.m. of feed
Run 3
Run 4
8.60 0.38
8.36 0.43 11
13
VOL. 51, NO. 9
Refluxer -!I Run 3 Run 4 9.20 0.26 14
10.16 0.39 7
SEPTEMBER 1959
1037
8 -
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,
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PRODuC-
R E N C ? O L 5.‘ARTEC
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2
4
E
8
Figure 2.
IC
$2
44 46 TIME i d w s )
48
2C
21
LC
26
28
3 39
Build-up of nitrogen-15 during run 3
Approximately 1.2 grams of high purity nitrogen-1 5 were produced per day
The only explanation for this increase is that there was a slow increase in the efficiency of the packing, which led to a shorter stage height in the large column as time went on. This could have been caused by a cleaning action of the solution, or by a slight etching of the packing. As the system was not allowed to come to equilibrium a t total reflux but only with product removal, the maximum value possible for nitrogen-1 5 enrichment was not attained. The stage height was less than 3.1 inches for the large column and less than 1.4 inches for the small column. Conclusions
T h e Nitrox system is a n attractive method for the concentration of nitrogen isotopes. Dependable, automatic oper-
ation of all the cascade components was demonstrated over a long period of time. The only critical element involved in the scale-up of the cascadt. \vas the design of the product reflusers: these cannot be successfull>-scaled up as a simple function of any single parameter, but must be completel>- redesigned for each ne\v size. The major problem to be solved is effective removal of the large quantities of heat evolved in the very small volume occupied by the reaction interface. If product 1cflusei.s more than 5 or 6 inches in diameter must be used, the material of construction becomes a n important problem. For smaller refluxers, glass serves very \veil. Holvever, for large refluxers glass is impractical and few metals ivithstand the drastic corrosive conditions in the S 0 2 - H S 0 3
I ..
reaction zone Porcelain or ceramiclined metals appear to be kdsible materials of construction for larger refluxers. if adequate heat transfer properties can be obtained. With properlv designed refluxers, the loss of product nitrogen in the sulfuric acid waste stream is negligiblx small. It appears unlikely thdt these losses \$ill become important. even in plants c o n t i n i n g very large refluxers. The only other apparent major scale-up problem is a n economic one In the large-scale production of nitrogen15, the favorable economic position of the h-itrox process is dependent upon credit received from the sale of byproduct nitric and sulfuric acids More than 50 tons of low grade sulfuric acid is formed in the production of each pound of 95y0nitrogen-15 It does not appear economically feasible to upqrade this sulfuric acid to commercial standards. For a plant producing 100 pounds per day of 95% nitrogen-15, economically favorable disposition of more than 5000 tons per day of low-grade sulfuric acid lvould be required. If no credit Ivere obtained for this sulfuric acid. ics disposition Lvould become a n expense and the economic advantage ol the Nitrox process would be lost. The technical feasibility of producing large amounts of nitrogen-13 by the Nitrox process seems \vel1 established. However, the economic feasibility depends on the size of the plant and the market for sulfuric acid in the community in irhich the plant is located. literature Cited
(1) Brown, L. L., Begun, G. XI,,“Xitrogen Isotopic Fractionation between Nitric Acid and the Oxides of Nitrogen,” J . Chem. Phys. 30, 1206-9 (1959). (2) Hayford, D. A,, Johnson, W. S . , Levin, S. A., Shacter, J., Von Halle, E., “Feasibility of Large-Scale Nitrogen-15 Production for Nuclear Reactors,” Oak Ridge Gaseous Diffusion Plant Rept. K-1232 (Aug. 17, 1955). (3) Klima, B. B., Ward, \V. T.. Chem. Eng. Progr. 52,381-7 (1956). (4) Klima, B. B., LVard, 1.V. T., It‘iethaup. R. R., Drury, J. S., Oak Ridge National Laboratory Rept. ORNL-CF-55-1-22 (Jan. 14, 1955). (5) Spindel, LV., Taylor, T. I., J . Chem. P h y ~ 23, . 981-2 (1955). 16) Ibid., 24,626-7 (1956).
RECEIVED for review December 13, 1958 ACCEPTED May 14, 1959 Based on work performed for the L. S. Atomic Energy Commission by Union Carbide Corp.
Correction Under New Books [IND.ESG. CHEM. 51, No. 4, 9 5 A (1959)] the correct title of the book by Maurice G. Larian should be “Fundamentals of Chemical Engineering Operations.” 1038
INDUSTRIAL AND ENGINEERING CHEMISTRY
