ENGINEERING AND PROCESS DEVELOPMENT
Fixation of At mospheric Nitrogen in a Gas Heated Furnace WILLIAM G. HENDRICKSON'
AND
FARRINGTON DANIELS
Deportment o f Chemistry, University of Wisconsin, Madison 6, Wis. Y
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N A paper by Gilbert and Daniels (1) the principle of operation
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of the Wisconsin process for the fixation of atmospheric nitrogen is discussed. In this process, atmospheric air is heated above 2200' C. in a regenerative furnace containing two heat exchangers of refractory pebbles. At these temperatures the nitrogen and oxygen of the air combine to form about 2% of nitric oxide, the degree of conversion being determined by the gas composition and temperature. In its travel through the furnace, air is preheated in the first pebbre bed, then heated to reaction temperature by the addition of methane or other fluid fuel to the air in an intermediate combustion zone; the combustion gases are then cooled to nearly room temperature in the second pebble bed. The cooling must be accomplished with extreme rapidity20,000" C. per second or more-in order to prevent dissociation of the nitric oxide which is formed in the combustion zone. After the furnace is operated for a few minutes the flow of air is reversed and the second pebble bed becomes the preheating bed while the first bed, cooled by the incoming air, becomes the chilling bed. This alternation of the flow of air is continued indefinitely. One of the major problems in the early development of the process was to design structures and develop refractories that would be commercially operable a t these very high temperatures. At the time the work was initiated, 1550' to 1650' C. was considered to be the practical temperature limit of commercial furnaces operating in an oxidizing atmosphere. Thus, it became necessary to pioneer the development of furnaces and refractories which would be operable a t 600' C. above industrial practice. Gilbert and Daniels directed their major efforts toward the development of a furnace in which the two pebble beds were placed alongside each other. Above each bed there was an open combustion zone, with a horizontal mixing tube connecting the two beds. The entire refractory structure was made from a ramming mix containing 93 to 94% magnesium oxide, the best material available to the investigators a t that time. With this furnace, it was possible to maintain a 2000' to 2100' C. temperature level for only a few hours before the furnace became inoperable because of the formation of large shrinkage and settling cracks, primarily in the roof and the horizontal mixing tube. These cracks caused the enclosing iron shell to burn out quickly. Although many attempts were made to modify the design to compensate for the shrinking and settling, they were unsuccessful. It became apparent to the authors of this paper that it might not be possible to build a practical furnace of the open combustion zone design with the refractory material then commercially available; consequently, the program was redirected t o include the following: The development of a vertical furnace packed with large pieces of refractories and with macadamized pebbles of different sizes so that one pebble bed lies directly above the packed combusdion 1
Present address, Wisconsin Alumni Researoh Foundation, Madison, Wis.
December 1953
Figure 1. 1. 2. 3. 4. 5. 6. 7. 8. 9.
Furnace Design Showing Upward Air Flow
97%' 97% 97% 97%
magnesium oxide 1/4-inch pebbles magnesium oxide 2-inch spheres magnesium oxide 1/4-inch to 2-inch spheres magnesium oxide bricks Periciase insulation iron Gas pipe Inlet Water cooling locket Water-cooled peep site
chamber and the other lies directly below it, all being contained in a single shaft. The development of better refractories which can be used a t higher temperatures. Preheating and Chilling Beds Are Separated by Combustion Zone in Vertical-Type Furnace
I n carrying out the first phase of this program, the design shown in Figure 1 worked more satisfactorily than any of fifteen other modifications of the same general design that were built and tested. The unsupported hot roof and horizontal crossover tube rigidly attached to the roof had proved to be impractical in the earlier work and they were both eliminated in this design with its packed combustion zone. The refractories for the pebble beds and combustion zone will be described in a later section of this paper.
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ENGlNEERlNG AND PROCESS DEVELOPMENT As shown in Figure 1, there are two beds, 1, of magnesium oxide 28 inches high, composed of pebbles of 97% magnesium oxide inch in diameter. Between these beds is a combustion zone, 2, of 2-inch magnesia spheres, where the fuel gas (enriched water gas of 500 B.t.u. per cubic foot) burns with the preheated air. In the two pebble beds there is extremely rapid heat exchange between the pebbles and the gases passing through the bed. The large spheres in the combustion zone are necessary for mixing the gas and preheated air. The small pebbles are prevented from falling through and mixing with the large spheres by a macadamizing layer, 3, 14 inches deep of graded sizes (from to 2 inches). These hed?, 1, 2, and 3, are packed in layers into a vertical cylinder, 4, 36 inches in inside diameter and 11 feet high composed of 97% magnesium oxide bricks. This cylinder in turn is insulated with 2 feet of less pure magnesia, 5, and the whole furnace is enclosed in a gas-tight iron shell, 6. The gaseous fuel is introduced through the water-cooled burners,, 7, 8, surrounded by 1-inch insulating jackets of rammed magnesia. It way thought that it might be impractical to bring water-cooled tubes into a 2200 O C. zone, but it was found that this can be readily accomplished. The burners are made from l1/6-, and 2-inch carbon steel pipes, arranged concentrically, welded and connected so that water flows to the lip of the fuel tube through the central annulus and out through the outer annulus. The heat losses in the burners amount to about 750 B.t.u. per minute per burner a t normal operating temperatures. A water-cooled peep site, 9, with a window permits the reading of the temperature of the combustion zone with a ralibrated optical pyrometer, and it alqo permiti the 1 cnioval of gases for chemical analysii. One of the early problems with the furnace design shown was that of obtaining complete mixing of the fuel and air. It was found that mixing is incomplete with small pebbles and that it is necessary to use large pieces of refractories. I n many of the early furnaces which were tried, efforts mere made to promote better mixing by including refractory baffles in the combustion zone. The baffles, hou-evrr, shifted and disintegrated in use and became ineffective. I n the first vertical furnaces, two sets of burners were used-one in the lower pebble lied leading upward into the combustion zone, as shown in Figure 1. and another set in the upper pebble bed leading donxward. U-hen the lower bed acted as the preheating bed the fuel gas issued only from the lower set of burners and when the gases leaving the upper bed reached a temperature of 300" C. the flow of air was reversed to move downward and the upper bed became the preheating bed and the lower bed became the chilling bed. The gas was then turned off from the lower burners and turned on in the upper burners during this half cycle of operation. When the gas issuing from the lower bed reached 300" C., the air was changed to flow upward and the supply of fuel gas was again switched from the upper burners to the lower burners, This reversal of air flow and alternation of the lower and upper burners continued every few minutes as long as the furnace remained in operation. The injection of fuel gas through iipper burners was found to be impractical because the buoyancv of the flame caused the gas to rise into the top pebble bcd, in spite of the high air veIocity downward, with the result that large amounts of carbon were deposited in the upper pehhle bed. These carbon deposits plugged the bed, increasing the resistance to flow of air and, at periodic intervals when the cnrlion deposits did burn out, the heat was so intense that the pebbles 17 ere fused together. Accordingly, the upper w t of burners was removed and fuel was injected only from the lower hurners and only on the half cycle when the air was flov ing upward, as shown in Figure 1. On the other half cycle, when the air was flowing downward, no fuel gas x a s introduced. The large heat capacity of the upper pebble bed and combustion region allowed only a slight cooling of the furnace during the few minutes of this half cycle. The increased oxygen concentration, due to the absence of fuel gas, resulted in a higher per cent conversion into nitric oxide and partly offset the lower concentration due to the slightly lower temperature. By doubling the rate of fuel gas and using gas only on the "up" half cycle, the full production of nitric oxide was maintained without the difficulty of the carbon formation.
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The preferred furnace design which was developed in these studies (Figure 1) requires approximately 2 square feet of pebble bed cross section per ton nitric acid (100%) capacity and one fuel tube per 3 square feet of pebble bed area. It wax found that with the air-fuel ratios used, a minimum combustion zone depth of 5 feet is required. Thus a furnace having the dimensions shown would handle some 1500 standard cubic feet per minute of air to produce about 2.4 pounds of nitric oxide per minute as 2% by volume of the stJack gases. Approximately 20 standard cuhic feet per minute of natural gas would be required as fuel. Firing, Grinding, and Refiring Produces Dense, High Purity Magnesium Oxide Refractory
The second phase of the development program was directed to the improvement of refractory materials. The best material available to the authors a t the time the work was initiated was a ramming mix containing about 93% magnesium oxide (known as periclase). This material was not good enough. The only suitable refractories n-orth considering were magnesium oxide, calcium oxide, and zirconium oxide. Of these, limestone (which converted to calcium oxide) was found to be unsatisfactory, for shrinkage and cracking were so serious that it could not be used. It was also susceptible to deterioration under any reducing conditions. Zirconium oxide was prohibitively costly a t that time. Therefore, the principal effort in the refractory development program was concerned with higher purity magnesium oxide materials. Although pure magnesia refractories were not commercially available, the authors believed that it would be possible to manufacture a dense refractory shape from high purity magnesium oxide raw materials. These materials were of low density, but it was believed that the required high density could be achieved if the grain were proceqsed at high temperatures and pressures. I n the f i s t attempts to manufacture a refractory shape out of high purity magnesium oxide, 2-inch spheres were processed in a hydraulic press operated a t 3800 pounds per square inch. After a preliminary firing to 1500' C., these spheres were used for combustion zone packing in a vertical furnace similar to that shown in Figure 1. After the furnace had been operated at 2100" C. for 3 days, the spheres %-ere removed and examined. Although they had shrunk from 2 to 1.8 inches in diameter, they now possessed 90% of theoretical density and had excellent mechanical strength. From these results it was concluded that it should be possible to make a high density, high purity brick by using a high purity grain, processing it a t a high pressure and temperature, and then regrinding this denser material and making it into refractory shapes. This pretreatment was designed to make refractories which are so dense that they will not shrink materially in use in the furnaces. Subskquently, the Harbison-Kalker Co. cooperated in manufacturing such bricks out of +97% magnesium oxide grain supplied by the Westvaco Chlorine Products Corp. (now a division of the Food Machinery and Chemical Corp.). The grain used had the following composition:
Ignition loss SiOp Fe202 A1203 CaO MgO
Composition, Wt. % ' 0.1 0.s 0.2 0.3 1.5 97.1
In order to make standard brick shapes, the Harbison-Walker Co. first made dobies of the grain. These dobies were pressed into regular brick shapes and fired to about 1500' C. I n the firing process the dobies shrank and cracked very badly. They
INDUSTRIAL AND ENGINEERING CHEMISTRY
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ENGINEERING AND PROCESS DEVELOPMENT run, the samples taken from the stack were of the same nitric oxide concentration as those withdrawn from the combustion zone by means of the water-cooled peep site tube. The nitric oxide was chilled so quickly by passage into this water-cooled tube leading frbm the combustion zone that i t didn't have time to decompose, and the chemical analysis of the gases gave the true concentration of nitric oxide in the high temperature region. After 90 hours of operation, however, the furnaces usually deteriorated and the nitric oxide concentration in the stack gases fell off, as shown in Table I, although a high concentration was maintained in the combustion zone.
' 0
20
40
60 TIME
Figure 2.
80 100 (HOURS)
120
I40
Operating Data Run A
were then crushed and the dense granulated material was again pressed into bricks and fired to about 1500" C. This time the shrinkage was negligible. A vertical furnace similar in design to Figure 1 was built from these Harbison-Walker bricks and operated a t 2100' to 2200" C. for a few days. When, a t the end of this period, the furnace was dismantled, the bricks were found to be in nearly perfect condition. The bricks had only shrunk about 2% during this runthus only insignificant shrinkage cracks were formed. The development of this new refractory had a major impact upon the nitrogen fixation program; with these materials i t was possible to have greater freedom in furnace design. Volatility of Magnesium Oxide Prevents Long Continued, High Temperature Operation
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The vertical furnace made of the improved refractory remained operable for about a week a t 2100" t o 2200' C. The cause of furnace deterioration this time was not shrinking and cracking of the walls (as had been the case with the lower quality refractories), but the deterioration of the refractories a t the top of the burners. The flame impinging upon these refractories reduced and volatilized the magnesium oxide, transporting it to the cold pebble beds where it condensed, filling the interstices among the pebbles. Large caves were burned out at the tips of the burners. The magnesium oxide lost there plugged the pebble beds, with resulting high pressure drop. These plugged beds finally shrank and cracked so that the gases by-passed their normal course, causing a loss in preheat and chilling efficiencies, This shortcircuiting of the pebble bed heat exchangers, in addition to leakages of gases through the small cracks in the refractory walls, brought about a decrease in yields because the air containing 1.7% nitric oxide from the furnace became diluted with ordinary air which did not go through the pebble beds. Several furnaces were tested and in most of them a temperature of over 2000" C. was achieved for many hours. I n a few cases a temperature over 2100' C. was obtained and, in one case, a temperature of 2300' C. was maintained for about 18 hours before the refractory failed. The lines in Figure 2 give the data of a typical run chosen from among the more than ten different sets of operating data to show high nitric oxide concentration, All the data were obtained from a vertical furnace 18 inches in inside diameter of the general design shown in Figure 1. The dotted line at the top of Figure 2 shows the high temperatures achieved during another run. The other data for this run are not shown. For a time, early in a December 1953
T o show that the chilling bed of pebbles was still effective, samples of gas were withdrawn from a point in the upper pebble bed 2 inches below the top. The analyses of these gases were practically the same as those samples taken simultaneously from the combustion zone (as shown in Table I). These gases had been sufficiently cooled to preclude any further decomposition and they had not had a chance to become mixed with air which had by-passed the hi h temperature combustion zone. The plugging of t i e pebble beds with condensed magnesium oxide vapor is indicated in Figure 2 by the increase in the pressure drop through the furnace from about 5 cm. of mercury to 18 cm.; and the further deterioration of the furnace after 70 hours of operation is indicated by the decrease in the concentration of nitric oxide in the exhaust stack gases, as shown in Figure 2 and in Table I. As the pebble beds became plugged and cracks developed in the furnace walls, more of the incoming air was forced to by-pass the preheating bed, combustion zone, and chilling bed and this air, containing little or no nitric oxide, became mixed in the stack chamber with the gases which had gone through the major part of the furnace and had acquired their full concentration of nitric oxide, corregponding to the analysis of the gases taken from the combustion zone and from the upper pebble bed. The low concentration of nitric oxide in the exhaust was not due to an failure of the chilling bed, but to a dilution of the gas with air wcich had short-circuited the furnace. Table 1.
Concentration of Nitric Oxide after Deterioration of Furnace
Combustion sone Exhauststackgases 2 inches below top of yperpebblebed
Nitric Oxide Concn., % 1 59 1 52 1 71 1 34 0 64 0 60 0 52 0 34 1 61
1 49
1 70
1 31
When this deterioration of the furnace became serious, the operation was discontinued and the furnace was cooled and repacked. Improvements in furnace design have now been effected which prevent this deterioration. Even without any changes the furnace described in t h k aper is capable of long continued operation at 1900" to 2000 but these temperatures are too low for the practical production of nitric acid. Although measurements of rates of flow, volume of furnace, temperature, and concentration of nitric oxide obtained in this investigation contribute to the knowledge of the kinetics of the formation and decomposition of nitric oxide, more accurate data obtained in this laboratory on a smaller scale with a special electric furnace will be reported shortly.
b,
Acknowledgment
The authors are pleased t o acknowledge very substantial support from the Wisconsin Alumni Research Foundation and from the Research Committee of the Graduate School of the University of Wisconsin. They are greatly indebted to the Department of Mining and Metallurgy, University of Wisconsin, for making space available for this work and generously providing laboratory and shop facilities, and t o the War Production Board for their interest and for aid in obtaining materials and men. Literature Cited (1) Gilbert, N., and Daniels, F., IND. ENG.CHEM., 40, 1719 (1948). R E C ~ I Y Efor D review April 14, 1953. ACCEPTED August 20, 1953. Further details of this research are contained in the P h . D . thesis of William G. Hendrickaon, filed in the library of the University of Wisconsin in 1946.
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