Elliott Oxygen Pilot Plant IRVING ROBERTS, DAVID ARONSON, MACK ATCHESON', L. C. CLAITOR, J. L. COST, AND D. B. CRAWFORDP Elliott Company, Jeannette, Pa.
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HIS paper is one of B T h e 10-ton-per-day pilot plant which was e m t d to aa well as the carbon dioxide group which is being prove the Elliott tonnage oxygen process L described. A and scetylene in the air, are published on completion of flow sheet b presented giving temperatures, pressuns, frozen out in either of the air the d e v e l o p m e n t of t h e and Eows based on opemtiag data taken from the plant. exchangers, Cor D. I n FigElliott tonnage oxygen procThe flow sheet b used as the basis for a description of the ure 1, heat exchanger D is cycle; this is followed hy a description of theconstruetion o p e r a t i n g i n t h i s service. em. Other papers on the prooess describe the princiand operation of the plant. The results are discussed in Heat exchanger C is being plea of the cycle (a), the gas fernul of oleun-"p cycle perfomence, over-all plant heat derimed, so that it can be put b w heat exchangers (e), balance, and power consumption. into service when exchanger and the de& and economics D becomes f o u l e d . The of tonnage plants (4). deriming is acoomplished hy The pilot plant was designed as far en possible to be a prototype p&g part of the warm nitrogen from exchanger D through exof a tonnage oxygen plant, both in the choice of cycle conditions changer C; first, through the clean nitrogen pmsge, from the and in the deai@ of the proceas equipment. The plant was built warm end to the cold end; then through the fouled air passage, to prove the following points which had been estimated by calcufrom the cold end to &e warm end, where i t sublimes the deposits. lation and indicated by preliminary laboratory tests: This streamleaves the exchanger through F-10and valve 29. The remainder of the nitrogen product is throttled through valve 8. 1. That the cycle is operable, particularly as regards the When exchanger D becomes fouled, valves 11 and 29 are closed, clean-up system. 2. That carbon dioxide is removed from the air to an unand the nitroeen is switched throueh exchanger C bv " onenine . recedented degree, eliminating the need for periodic shutdowns valve 12 and ciosing valve 15. This-cwls exchanger C while the k r deriming. exchanger D. F~owsare maintained in this air stream 3, T~~ the explosion hazard due to accMulation of acetylene manner until the air stream has warmed exchanger D to approxiin the column relmiler is eliminated. 4. That the nitrogen circulation rates needed for refluxand mately 35' F., a t which time the air is switched to exchanger C by refrigeration. which together determine the major power resUireopening valves 6 and 13 and closing valves 9 and 16. A t the same ment, are in agrement with design calculations. time, valves 28 and 14 are opened, 80 that the waste nitmaen is switched through exchanger b to clean it in the manner described Desiep of the plant was begun in the fall of 1945,and the plant above for exchanger C. wen emted during 1946 and 1947 at the Jeannette, Pa., works of In an alternate method, used in initial operation of the plant, the Elliott Company. Various operating runs were made during all of the by-product nitrogen is used for clean up during part of . the period between September 1947 and April 1949. In the the cycle, instead of using part of the nitrogen during the entire last run,which consisted of 8 weeks of continuous operation, the period. This is accomplished in the cene of clean up of exchanger plant reached a s t e ~ d yproduction rate of about 10 tons of 95% C by opening valve 7 and closing the throttling valve 8 and valve oxygen per day. The flow sheet, Figure 1, gives temperatures, 29. After the exchanger is derimed, valve 7 is closed and valve preaeures, and flows based on operating data taken from the plant 8 ie opened. The exchanger remains idle during the remainder during this period. ofthe cycle. Deriming of exchanger D is similar to that deacribed DESCRIPTION OF THE CYCLE for exchanger C. By raising the temperature of the exchanger 35" F. before Because a complete description of the cycle has been given in up, thc vapor pressures of the carboir dioxide and water another paper (a),only a s u m u r y is given here, ~ l ~ t ilow v ~ l clean ~ deparita are increened, 90 that the deposita arc sublimed with a preasurr air is fractionated iu a distilling rolat low tempem smaller quantity of ga? than that which carries them in I(Figure I ) , with the necessary reflux and refrigeration aup plied by a nitrogen recirculation system. Oxygen is withdrawn Dl%SCRIPTIONOF PIWT PLANT as a vamr from the bottom of the volumn. and the nitropen is taken off en the overhead product. The rekgeration in the cold A plan view, showing the relative location of the major i t e m of oxygen and nitrogen stree&msis recovered by heat exchange with equipment, is given in Figure 2, and a 0.0625 scale model, built incoming air and with incoming high pressure nitrogen. before the plant waa erected,is shown in FIW 3. A photograph The intake air is fed to a Freon cooler and a ailica gel drier, of the plant, taken en it wen nearing completion, is shown in Figwhere the hulk of the water is removed. The remaioing water, ure 4,and a photograph of the completed plant is shown in Figure I Pleaent add-. El Paao Naturd Gul Compnny, EL PIYIO. Tar. 5. Thee photograph may be used in conjunction with the plan 1 Preaent add-. Amencon Machine and Foundrv Comoanv. BrookLvn. view to identify the various itema of equipment. N. Y.
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Figure 1.
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-. 1
Flow Sheet for Production of Oxygen, Elliott Pilot Plant
The air blower takes in air through a wreen filter and through a leakage of cold nitrogen. A sintered metal filter is inst:tllcd upstream from each expander to remove fine solid p:trticles weight flow control meter and valve. The air blower is a gearwhich might erode the nozzle ring. One of the expanders is driven, 10,000-r.p.m., single stage centrifugal blower, powered by a 2300-volt induction motor. The blower is followed by a finned shown in Figure 6. Exchangers A , B, C, and D are of the interrupted strip fin type tube aftercooler. described elsewhere (6). A section of this construction is shown The air drying system, which consists of a Freon cooler and a in Figure 7. The exchangers are counterflow units of brass plates silica gel drier, is designed to dry the air to a dew point of -40’ and copper fins for two-fluid service, so arranged that the two gas F. The Freon unit uses a conventional 7.5-hp. conipressor which streams are in alternate passages. supplies Freon to two finned tube cooling coils mounted in a steel cabinet. Air is cooled to 40 F. in the coils, and condensed water Exchanger G and the reflux cooler are of finned tube construction, in which the high pressure fluid flows in the tubes in a multiis trapped out of the bottom of the cabinet. The silica gel drier consists of two 4-foot diameter vessels, one of which is in operation pass arrangement, whereas the low pressure gas makes one crossflow pass over the finned tubes. The copper heat eschmgcr while the other is being regenerated. A screen filter after the drying unit removes any silica gel dust which may have been cores are contained in sta.inless steel casings. The accumulator is a stainless steel tank, 30 inches in diameter picked up by the air streams. From the filter, the air passes to filled with 660 pounds of silica gel. The accumulator adsorbs the the switching valve manifold preceding the clean-up exchangers. last trace of acetylene and carbon dioxide in the air feed, and The other large item of warm equipment is the nitrogen compressor, a two-stage Elliott-Lysholni compressor (8) with two parallel units in the low pressure stage and one unit in the high pressure stage. This unit is an oil-free, positive displacement machine with a capacity of 1000 cubic feet per minute, driven by a 2300volt induction motor. The intercooler and aftercooler are of finned tube construction. Two expanders are provided; both of these are used during srart-up of the plant, but only one is required for normal operation. Each machine is a radial flow type turbine ( 7 ) , designed especially for use with this pilot LYSHCLM COMPRESSOR plant. The turbine wheel is of cast aluminum construction, 2.5 inches in diameter, and runs a t 60,000 r.p.m. The power produced is dissipated in an oil brake mounted on the same shaft as the expander wheel. A labyrinth seal is used on the shaft between the main casing and the bearing REFLUX CQ+.FR‘ L X C H A N W R A (ASOVC) housing, and clean, dry nitrogen taken from the compressor discharge is introFigure 2. Plan View of Elliott Oxygen Pilot Plant Showing Location of Major duced into the seal in order t o eliminate Items of Eyuipiixent
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The box is packed with loose Perlite insulation. The equipment is located in the cold box so as to allow a minimum insulation thickness of 9 inchee between the equipment and the cold box wall. .A small stream of dry nitrogen from the discharge of the compressor is bled into the box continuously, producing an outward leakage to prevent moisture from diffusing into the insulation. OPERATION OF PILOT PLANT
Figure 3.
Model of Pilot Plant
damps out temperature fluctuations caused by switching of the exchangers. The distillation column, of copper construction, has nine 26inch diameter bubble trays spaced 12 inches apart above the feed, and ten 19-inch bubble trays spaced 6 inches apart in the lower section. Construction of the trays is conventional, using 3-inch bubble caps, and with the addition of entrainment separators on each tray. The copper reboiler is a vertical tube evaporator with nitrogen condensing on the outside of the tubes' and oxygen boiling inside the tubes. For warm service, Schedule 40 steel piping and standard cast iron valves are used. The warm control valves are diaphragm operated, and the warm clean-up switching valves are bellows operated. For cold service, brass pipe and bronze fittings are used. Threaded joints of the cold piping are screwed together and silversoldered. Flanged joints are gasketed with 0.0625-inch Vellumoid, and bolted with stainless steel bolts. To take care of contraction of the cold equipment, expansion joints are placed in the piping a t required points. All cold valves are made of brasa, are completely enclosed in the cold box, and are pneumatically operated by metal bellows, using clean, dry nitrogen from the discharge of the compressor. The valve stems are sealed with metal bellows, eliminating the leakage and sticking problems of the conventional stuffing box. The automatic controls of the pilot plant are indicated on the flow sheet, Figure 1. Most of the instruments are shown in the panel board photograph, Figure 8. Flows, temperatures, and pressures are measured a t all points indicated on the flow sheet in order to determine the performance of each item of equipment. The recording flowmeters were calibrated in position with an A.S.M.E. standard orifice. The cold box is a steel framework covered with aluminum panels and sealed a t the seams with cemented neoprene strips.
The operating procedure, which was used for the plant in the final form given in Figure 1, was developed after n numbcr of changes had been made in equipment and instrumentation , However, before discussing some of the difficulties and changes made in the original plant; this operating procedure will be presented to provide background for the reader. Prior to cooling the plant, moisture is removed from the equipment by circulating air through the drying system and through tho plant, with the air blower and the Lysholm compressor, until dew point measurements shorn that no more moisture is picked up by the air from the equipment. Following this, the steady on-stream condition is attained in three steps: cooling down, making liquid, and normal operation. For the cooling down period, air in the nitrogen compressorexpander circuit is recirculated, while enough air is brought in
Figure 4. Partially Completed Pilot Plant
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Figure 5.
Completed Pilot Plant
from the air blower t o make up the leakage in the system. Because the make-up flow is considerably be lo^ the surge limit of the blower, the system is arranged to pass a much larger air flow. 811 of the air from the blower first passes through the dehumidification system and valve 9. At this point the air stream is divided, and the small make-up flow passes through the air side of exchanger D,valve 16, and the accumulator, and mixes with the air in the column. The excess air passes through valve 10, control valve 5, and start-up valve 2, and returns to the blower inlet. Control valve 5 is adjusted to maintain 18.3 pounds per square inch absolute at the top of the column to ensure that the nitrogen compressor inlet is above atmospheric pressure. Both expanders are operated with air from the nitrogen compressor flowing through exchanger B, through the expanders to the top of the column. The air is then split into two streams: one passes through the nitrogen side of clean-up exchanger D and back to the compressor through valve 19; the other passes down through the column, out through exchanger A , and returns to the compressor through start-up valve 27. When the temper3ture a t the warm end of exchanger A starts to fall, some of the cold air is diverted through the reflux cooler, exchanger G, and exchanger B , by opening valve 21 Valve 22 is opened to give balanced flow in exchanger A to prevent loss of refrigeration. Valve 17 is opened allowing cold, high pressure air to flow through the reboiler. The return of cold air through exchanger B lowers the inlet temperature of the expander, which in turn lowers the discharge temperature of the expander, so that progressively colder gas is circulated, and the equipment is cooled to a lower tempera-
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ture. Periodically, the clean-up exchangers are switched so that both unitsare cooled down. When the compressed, recirculated air is cooled to about -225' F., it reaches its dew point with respect to carbon dioxide, which can precipitate and block the nozzle ring of the expander. To avoid this effect, the carbon dioxide in the make-up air is adsorbed in the accumulator, but to do this, the accumulator must be cooled to about -200" F. to become effective. This is accomplished by passing a large flow of cold air through the accumulator a t the time that the expander inlet temperature reaches -220" F. The additional air flow is taken through the accumulator through the column, out through exchanger A , through valve 25, and back to the air blower. The necessary flow for balancing the clean-up exchanger is brought back to the nitrogen compressor through exchanger D and valve 19. When the temperature of the air leaving the accumulator reaches -200" F., valve 25 is closed, so that again only the make-up for leakage passes through the clean-up exchanger. Cooling down continues, and within a short time the plant is making liquid, The first formation of liquid occurs by condensation in the high pressure side of the reflux cooler, from which this liquid is throttled into the column. Here it is evaporated, and when the column is sufficiently cold, thc liquid drains into the reboiler and evaporates while high pressure gas is condensed around the tubes. This greatly increases the condensation rate, which reduces the gas flow available to the expanders. T o overcome this effect, reflux valve 17 is throttled, limiting the flow of liquid and maintaining the expander inlet pressure a t 75 pounds per square inch absolute or higher. When the reboiler is almost full of liquid the plant is ready for operation, and the air weight flow controller F-1 is set a t about 70 pound moles per hour. The clean-up cycle controller is switched into operation, placing valves 6 through 16 and valves 28 and 29 under automatic control. At this time, valves 4 and 25 are opened, and valves 2 and 27 are closed. One expander is then shut down and reflux valve 17 is throttled to give a flow of about 80 pound moles per hour, as indicated by F-8. The controller for valve 18 is put in operation, throttling the flow to the expander as the liquid level on the oxygen side of the reboiler increases, thus maintaining the refrigeration produced by the expander a t the proper level. During this period the excess f l o of ~ the positive displacement compressor returns to the compressor inlet through valve 24. Oxygen is now being produced. T o obtain the proper purity, the draw-off rate of oxygen is adjusted by manually operating valve 23 until approximately the proper purity is obtained, at which time the purity controller is put in operation. This controller uses the temperature diffcrence between the reboiler and the fifth plate of the column as an indication of the oxygen purity. The principle of this device may be seen in Figure 9, which shows the teniperature distribution throughout the column during normal operation. A small change in the purity of the product has only a slight effect on the temperature in the reboiler but i i accompanied by changes in concentrations in the body of thr column. The largest change in composition, and therefore in temperature, occurs a t about the fourth theoretical tray (fifth actual tray) where the concentration gradient is the greatest As the purity increases, the temperature difference decreases, and valve 23 is opened. The resultant increase in oxygen draw-off
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Figure 6.
Expander
rate returns the oxygen purity, and therefore the control temperature difference, to the proper value. Valves 20 and 22 are set to give the balance on exchanger B which will produce the proper expander inlet temperature-that is, the inlet temperature which will produce a slightly superheated discharge. T o obtain maximum output of the plant, all the available high pressure nitrogen is utilized as reflux and expander feed. The reflux rate is increased by adjusting valve 17 until valve 24 closes. The air rate is then increased to correspond to the quantity of reflux entering the column. As the insulation in the plant continues to cool, the expander flow decreases, making more reflux available, so that the air rate may be increased gradually. Finally, after about 36 hours from the start of operation, the plant achieves its maximum production rate. Under the steady conditions which follow, the automatic control instruments maintain the operation without manual adjustments, other than regeneration of the silica gel driers once every 8 hours. Most of the difficulties encountered on initial operation of the plant were of a minor nature and were overcome easily. The more troublesome defects in the original plant and the measures taken to correct them are discussed. One early difficulty related to the control of the liquid levels and reflux flow in the column. I n the column reboiler, the oxygen liquid level is maintained constant by automatic control of the expander nitrogen flow. Hence, the level of liquid nitrogen determines the amount of heat transfer surface exposed and available for nitrogen condensation. For a given nitrogen flow, the heat load in the reboiler is constant, so that the amount of exposed surface determines the temperature difference between the boiling oxygen and the condensing nitrogen. As the boiling temperature of the oxygen is fixed by the column pressure, the nitrogen condensation temperature and, hence, its pressure are determined by the liquid nitrogen level. I n the original plant, it was intended to use this effect for control of the plant pressure. A liquid level controller was installed on the nitrogen side of the reboiler to operate the reflux throttle valve, 17, to maintain a chosen liquid level. Thus, to obtain a given plant pressure, the proper liquid level control point would be found and set on the instrument panel. This method of control was found to be unsatisfactory from the start. Operation was characterized by cyclic fluctuations of 5 to
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10 pounds per square inch in nitrogen condensation pressure, accompanied by appreciable fluctuations in both liquid oxygen and nitrogen levels. It was thought that this effect was caused by inability of the liquid level controller, which was a differential pressure controller operating from the liquid head, to follow rapid variations in liquid level due to surging of liquid in the pressure tap lines. Therefore, the instrument was disconnected, and the reflux throttle valve instrument line was put on manual control. However, even with hand Operation, similar fluctuations in the plant pressure were observed. As a next step, the reflux valve, 17, was rebuilt to give more favorable throttling characteristics, by changing the shape of the plug from its original conical form to a parabolic one and by increasing the stiffness of the spring to give a wider working range of motor pressure. Operation with the rebuilt valve was much smoother, but cycling was still occasionally observed. Finally, this residual cycling was eliminated by increasing the size of the liquid reflux line upstream from the valve from 0.5- to 1-inch copper tubing. It i s thought that this last improvement was made by eliminating the possibility of flashing upstream from the valve, due to the higher press&e a t the valve inlet resulting from the decreased pressure drop in the line. Another control problem which appeared in the early runs was that of flow balance in the operating clean-up exchanger. I n the original plant, valve 19 was manually operated to balance the nitrogen flow against the air flow in exchangers C or D. The criterion used for attaining this balance was observation of the temperature at the mid-point of the exchanger, which is very sensitive to flow unbalance-for example, a 1% change in flow balance results in approximately a 20" F. change in mtd-point tbmperature. This method proved to be workable, but the frequent adjustments required too much of the operator's attention. Therefore, the automatic mid-point temperature controller shown in Figure 1 was installed, and this proved to be entirely satisfactory, maintaining the mid-point temperature constant within 2 F. and requiring no attention. A purely mechanical difficulty which resulted in shutdown of
Figure 7.
Sample of Heat Exchanger Core
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the plant was the oceur~enceof a leak in the nitrogen contpressor intercooler. The circulating high pressure nitrogen stream must be kept free of condensable impurities, since no provision is made for cleaning exchangers A and B. I t is for this reason that the oil-free Lysholm type machine was used for nitrogen compression, rather than a reciprocating type. On the occasion that the leak developed, the pressure drops in exchangers A and B were observed to increase rapidly. This forced a plant shutdown in order to warm exchangers A and B for removal of condensed water, as well as to repair the leak in the intercooler. Also, because the high pressure nitrogen is used to operate the cold valves, the water vapor in the nitrogen caused plugging of the instrument lines connecting to some of the valves. These blockages were cleared either by warming the appropriate lines, or by pumping on the lines with a varuiim pump
Figiiro 0. Tnstrument Panel
Khile this difficulty did not reappear in opeiation of the pilot plant, it indicates that future plants should be designed with the intercooler water pressure lower than the intcrstage nitrogen pressure. However, reducing the mater pressure in itself would not constitute a completc solution to the problem. If a leak were present in either the intcicooier 01 the aftercooler, and the compressor were shut doan, xvater could still collect in the nitrogen sides of these coolers and be cairied into exchangers A and B on start-up of the plant. Theiefoie, drain valves should l x provided on the nitrogen sides of thew coolers to allow the presence of water to be checked before starting the plant. The major difficulty encountered in the pilot plant was refrigcration losses due to physical leakage of cold gas and to heat leak through the insulation. A heat balance on the over-all plant, of the type given later in this paper, will not distinguish between these two types of refrigeration losses, since the result obtained is an apparent heat leak, representing merely the difference between the refrigeration supplied and the losses due to warm end temperature difference. The problem of refrigeration losses will be discussed in connection with the three major runs of the pilot plant, which were made in addition to several starts and stops of relativelj- short duration. I n the first run of the plant, i t was apparent that the iefrigeration loss problem was a severe one, since the apparent heat leak was
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found t o be about 20,000 B.t.u. per hour (about 2.5 times the expected value), and the cold box walls showed large areas covered with ice. The largest iced area was adjacent to the cold end of exchanger B and was believed to be caused, a t least in part, by leakage of cold high pressure nitrogen from the bolted joint connection to this passage of the exchanger. All the original gas-togas exchangers supplied for the plant used bolted connections, which could be tightened with varying degrees of succcss; some of these are now known to have been underdesigned ( 5 ) . The cold high pressure connection of exchanger B proved to be most troublesome and was never fully corrected with the original heat exchanger design. I n this first run, the plant was insulated with bulk slag wool. It was later thought that the insulation was not packed sufficiently uniformly. During this run, which was of 3 week's duration, the plant reached a steady production rate of about 5 tons per day of 95% oxygen. This low rate was caused partly by the excessive refrigeration losses and partly by the necessity to operate both expanders because of inability to raise the nitrogen pressure above 67 pounds per square inch absolute due to the cycling effect mentioned above. For the second run, the suspected flanges were reconditioned, and, in some cases, reinforced with back up strips. The plant was insulated with a 6-inch thickness of glass wool insulation applied to each item of cold equipment and with bulk slag wool in the remaining space in the cold box. The apparent heat leak was found to be about 18,000 B.t.u. per hour, representing a negligible improvement over the prcvious figure. The iccd area of the cold box wall adjacent to exchanger B was as large as in the first run. However, the steady oxygen production rate achieved in this run was about 8 tons per day; this improvement resulted primarily from elimination of the cycling effect, which allowed the plant pressure to be increased, and froin installation of a larger nozzle ring in the operating expander t o provide a larger quantity of refrigerhtion. This run lasted for 9 weeks. I n preparation for the third and last run, heat exchanger B was replaced with a new exchanger design ( 6 ) , in which the major improvement is the use of soldered connections instead of bolted flanges. All of the old insulation was removed, and the cold box was poured full of loose perlite powder. The plant in this condition met, a t last, the predictions of the original design. The apparent heat leak mas found to be about 8000 B.t.u. per hour. and the only ice spots on the cold box walls were some small once close to the expanders where the insulation is thin. The steady oxygen production rate was slightly over 10 tons per day, which improvement can be attributed entirely to the reduction in apparent heat leak. However, because the corrective measures were taken simultaneously, it is impossible to state accurately how much of the improvement can be attributed to the elimination of physical leaks and how much to the better insulation. The 36-hour start-up time mentioned in the operating instructions was obtained only in this last run. The previous runs showed start-up periods of 3 to 5 days. The difference is due to the low density of the perlite insulation (about 4 pounds per cubic foot), giving a considerable reduction in initial refrigeration load when compared with the heavier mineral wool types of insulation. CLEZY-UP CYCLE PERFOlt\J LUCK
During all operation of the pilot plant, a 4-hour on-stream period was used for each clean-up exchanger. This choice was made on the basis of the increase in pressure drop in the air passage of the exchanger and allowed a reasonable margin before the pressure drop would become prohibitive. For the air flow shown in Figure 1, a period of 2.2 minutes was allorved to obtain the 35" F. warm-up of the fouled exchanger. I n planning the first pilot plant operation, the qumtity of uitrogen flow required for clean up was uncertain. Therefore the system was designed to utilize the entire by-product nitrogeri
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stream for clean up, with provision for reducing the amount of time during which the nitrogen was allowed to flow. Using this method, i t was found that a period of 45 minutes was adequate for complete removal of the deposits. This type of operation is shown in the upper chart of Figure 10, which is a typical record of the pressure drop in the air passage of each exchanger. This chart gives pressure drop in inches of water, with the nero at the center of the scale rather than at the inside edge. A reading toward the outside of the chart shows pressure drop for flow in air-cooling service, whereas a reading toward the inside of the chart shows pressure drop with nitrogen gas flowing in the reverse direction-that is, during clean up. Pressure drop records of both exchangers are shown on the same chart, and a single cycle for one exchanger has been subdivided in Figure 10 to show the various periods. These are. 1. FOULING. The exchanger goes into service at 12:55 A X with a pressure drop of 24 inches of water. The pressure drop increases during the following period to 33 inches of water at 4:50 A.M., at which time the warm-up is started. 2. WARM-UP, During the following 2.2-minute period when only air is flowing in the exchanger, the pressure drop increased rapidly to 38 inches of water. This increase is caused by the 35 O F increase in temperature throughout the exchanger. When this period is completed, the air is switched to the other exchanger. 3. CLEANUP. Nitrogen begins flowing in the reverse direction through the fouled passage with an initial pressure drop of 24 inches of water, As the deposits are removed, the pressure drop decreases to about 19 inches of water. Superimposed on this effectis a slight increase in pressure drop due to the gradual warming of the cold end of the exchanger by the clean-up flow during this period. This rise becomes noticeable during the last 15 minutes of the clean-up period, which ends a t 5:37 A.M. The clean-up nitrogen flow is then stopped. 4. IDLE. The exchanger remains idle until about 8:50 A.M., at which time nitrogen is switched into the nitrogen passage of this exchanger. 5. COOLING,During this period of 2.2 minutes, the exchanger is being cooled about 35' F. by the cold nitrogen stream. Because there is no flow in the air passage during this period, no change is seen on the press6-e drop chart for this exchanger. However, this period coincides with the period of warm-up of the other exchanger. After this cooling period, air is switched into the exchanger, putting i t back into operation to repeat the entire cycle. Use of the above method of clean-up operation served the major purpose of establishing the total quantity of nitrogen required for clean up when the heat exchanger is fouled for a 4-hour period, This same quantity of nitrogen can also be passed through the exchanger by using 45/240 = 19% of the by-product nitrogen flow for the full 4-hour period available. As was later found, this second method will clean the exchanger successfully, and there are several advantages in favor of this type of operation. At the lower nitrogen flow rate, the performance of the extended surface type of exchanger is such that the warm end temperature difference during clean up is reduced from about 9.1 O to about 4.6 O F . , thus decreasing the clean-up refrigeration loss by about 50%. Furthermore, the pressure drop in the clean-up circuit a t the lower nitrogen rate is reduced by a factor of about 15, thus decreasing the head and power required for the air blower. For these reasons the reduced flow method was put into operation about halfway through this work and was used without difficulty until the project was completed. Actually, there is an advantage in the reduced flow method in so far as clean up itself is concerned. The nitrogen stneam which passes over the fouled surface is always lower in temperature than tbe metal itsclf. With the reduction in clean-up flow, this difference in temperature is reduced from 4.55' to 2.3" F., resulting in a higher equilibrium content of impurities and in more efficient clean up. Although this advantage would have allowed a further
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reduction of the clean-up nitrogen flow, this was not done, as the saving in clean-up loss and pressure drop would have been small. The pressure drop record for this method, using 19% of the byproduct nitrogen for clean up, also is shown in Figure 10. The chief differences from the upper chart, are the pressure drop during the clean-up period is very small and the clean-up period lasts for almost 4 hours.
Figure 9.
Temperature Distribution in Column
In normal operation, it was found that the dehumidification system produced a dew point of -80" F. or lower. Also, by measurement of pressure drops in each half of the exchangers, it was found that carbon dioxide removal was the controlling factor in the original determination of the 45-minute period. To determine the effect of higher dew points on operation of the clean-up exchangers, experiments were run in which a controlled quantity of air was by-passed around the silica gel driers. For resultani water contents equivalent to a -40" F. dew point, i t was found that water removal was the controlling factor, and that complete clean up, using 19% of the waste nitrogen, could be obtained by increasing the warm-up time to give a 39 F. rise instead of 35 O F. The over-all effectiveness of the clean-up operation is shown by the fact that in the last two runs, of 9 weeks and 8 weeks of continuous operation, respectively, shutting d o m of the plant was voluntary and not for any process reasons. At the end of these runs, heat exchangers A , B,and G showed no increase in pressure drop, indicating that the circulating nitrogen remained entirely free of water and carbon dioxide. This w w also indicated by the facts that the reflux throttle valve, 17, and the expander nozzle ring showed no indication of plugging and that the reboiler showed no decrease in heat transfer coefficient. The clean-up exchcngere also showed no evidence of a permanent increase in pressure drop, indicating perfect deriming in each clean-up cycle. The liquid oxygen remained clear and free of suspended matter, indicating the absence of significant quantities of carbon dioxide. Because of analytical difficulties, no attempt was made to measure the extremely small concentrations of carbon dioxide in the air stream either entering or leaving the accumulator. There is little doubt, however, that the accumulator does adsorb carbon dioxide vapor from the air and does filt,er out any solid carbon dioxide which may have been carried through the exchangers physically. This accumulation of carbon dioxide could in time saturate or plug the silica gel in the accumulator, and in any plant designed for long-time continuous production, proviaion should be made for periodic regeneration of the aCCUMUhtOr without shutting down the plant. This could easily be accomplished by warming the accumulator while by-passing the air stream directly into the column. For this service, i t is felt that the provision of duplicate accumulators would be unnecessary.
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Figure 10. Pressure Drop in Clean-up Exchangers (Top)
Full flow method; (bottom) reduced flow method
Vol. 41, No. 11 since the quantity of impurities carried into the column during the regeneration period would be small and would later be carried out with the oxygen product between regeneration periods. In view of the observations made of the pilot plant a t the end of the 8- and 9-week runs, it is believed that, with provision for accumulator regeneration, such an oxygen plant will operate for an indefinite period without the n e c e s s i t y f o r shutdown for deriming. There is a long history of explosions in the column reboilers of oxygen plants due to the presence of acetylene in the atmosphere (I,3 ) . Although the exact cause of the explosions is not known, it is apparently true that the explosions occur only when solid acetylene is present in the liquid oxygen. The concentration of acetylene in the air has been stated t o be 0.01 to 0.1 p.p.m. in the neighborhood of a number of oxygen plants and to be as much as 2 t o 3 p.p.m. in the neighborhood of acetylene installations (I). When acetylene enters the column with the air, it is carried down into the liquid oxygen in the reboiler. According to Burbo (f), the solubility of solid acetylene in liquid oxygen is about 6 p.p.m., and the concentration in the ox$gen vapor above a saturated solution a t -293" F. is about 0.3 p.p.m. As the volume of oxygen product leaving the column is about 20% of the volume of air entering the column, a saturated solution will not be formed unless the concentration of acetylene in the air is 0.3/5 = 0.06 p.p.m. or greater. Below this concentration, the solution will be unsaturated, and above this concentration, solid acetylene will precipitate and continue to accumulate. By cooling the air to a temperature of -307" F., as in the present pilot plant, the equilibrium concentration of acetylene in the air is reduced to about 0.02 p.p.m. leaving the clean-up exchangers, as calculated from an extrapolation of the vapor prcssure data of Burbo. Thus, if this air stream were allowed to pass directly into the column, the oxygen product would reach a maximum concentration of 0.1 p.p.m. of acetylene, and assuming Henry's law, the liquid
,
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
November 1949
oxygen would be one third saturated. Actually, the presence of the silica gel accumulator tends to remove the residual acetylene from the air because of its high adsorptive capacity a t this low temperature. TABLEI. HEATLEAK. Xxnnnder refrigeration .u./lb. mole X 65.3 lb. moles/hr.
.
-
B.t.u./Hr. 20,900
!hangera 6.96 X 124.1 lb. moles/hr. = 8,660 Clean-ut, loss = 740" Exchange; A (12.5-2.1)'' F.b X 6.96 X 26.05 lb. moles/hr. = 1,890 Exchanger B (4.6-2.l)O F . b X 6.96 X 130 lb. moles/hr. = 1,750 Total 18,Orn Heat leak 20,900 13,030 = 7,870 The clean-up loss is the sum of two items: during the 2.2-minute warm-up period the loss, averaged over 4 hours, is estimated as 140 B.t.u. per hour: durinb: the 4-hour clean-up period, the warm end loss is given by: 4.6O F. X 6-96 X 18.6 lb. moles/hr. = 600 B.t.u./hr. b 2.1" F. = net Joule-Thomson effect a t 70° F.
-
-
During the operation of the pilot plant, frequent analyses of the reboiler liquid oxygen for acetylene were made. The method of analysis consisted of evaporation of a sample through a trap surrounded by liquid oxygen a t -297" F. The trapped acetylene was then treated t o form copper acetylide, which was determined colorimetrically. This method was used satisfactorily to check the literature data on the solubility of acetylene. The concentration of acetylene in a sample which will just escape detection by this method is calculated t o be 0.1 p.p.m. No positive test for acetylene was ever obtained in any of the reboiler liquid oxygen samples. This indicates that the concentration of acetylene in the air entering the column waa less than 0.06 X 0.1/6 = 0.001 p.p.m. For analysis of gaseous air for acetylene, the method used was similar t o that given above, except that the sample was passed a t reduced pressure for several hours through a trap surrounded by liquid nitrogen a t -321" F. The concentration of acetylene which will just escape detection in this case is calculated to be about 0.005 p.p.m. Analysis of the air leaving the accumulator failed to give a positive test, indicating that the concentration is less than 0.005 p.p.m., which is to be expected from the result calculated from the liquid oxygen tests given above. Analysis of the air entering the accumulator showed positive results, usually about 0.01 p.p.m. of acetylene. Attempts to measure the acetylene content of the raw air entering the plant were unsuccessful because the accumulation of solid water and carbon dioxide in the trap prevented passing through a large enough sample. The results show that the accumulator reduces the acetylene concentration of the air by adsorption to 0.001 p.p.m. or less. Thus, in a continuously operating plant, with provision for accumulator regeneration, it is to be expected that no detectable quantity of acetylene will reach the liquid oxygen in the reboiler. PLANT PERFORMANCE
I n the typical operating condition shown in Figure 1, the pilot plant produces slightly over 10 tons per day of 95% oxygen. The purity of the products is given in the following table. Only oxygen contents were measured; argon contents were estimated from tray-by-tray column calculations. Oxygen Product
95.0
2.9 2.1
Nitrogen Product 1.3 0.4 98.3
These compositions are equivalent to a recovery in the product of about 95% of the oxygen in the entering air. Of the nitrogen by-product, 81% is produced clean and dry, and the remaining 19% contains all the water and carbon dioxide brought in with the air.
2669
The refrigeration supplied by the expander is that required to overcome the effects of warm end temperature difference and heat leak. I n an operating plant, heat leak is not measured directly but is determined by difference. For the conditions of Figure 1, the heat leak is given in Table I. This heat leak figure is estimated to have an accuracy of f2070, due to uncertainties in measurement of expander efficiency and warm end temperature differences. Based on the various runs, the calculated adiabatic efficiency of the expander ranged between 70 and 75%, the scatter being caused primarily by thermocouple errors. The power consumption of the pilot plant was not measured, but it may be calculated from the conditions of Figure 1, using estimated adiabatic efficiencies of the machines. This is given in the Table 11, in which the requirement for other utilities is also included. The power consumption given above would be misleading if extrapolated linearly to large scale plants. As the design capacity of an oxygen plant is increased above 10 tons per day, the relative importance of heat leak decreases markedly. Thus, the proportion of nitrogen compressed for expander flow decreases, reducing the power consumption per ton of oxygen produced. Furthermore, in larger plants, the efficiencies of the machines will be more favorable. Actually, in any proposed installation, the choices of the values of the process variables, such as warm end temperature difference, compressor discharge pressure, number of column trays, and oxygen recovery, are largely a question of economics, and these values will have major effects on the power requirement. A discussion of these factors, and estimates of power requirements of the Elliott system in full scale plants are given in another paper ( 4 ) .
TABLE 11. POWER CONSUMPTION
stea_m, 176 Item Nitrogen compressor and coolers Air blower and aftercooler Freon cooler Air dryersa Oil pumps and coolers Expander a b
'
Cooling Water, Gal./Min. 50
ddiabatic HP. 124
Estimated Shaft Hp. 190
19
38
10
.. ..
7
2 12 14 3 91
.. ..
5
3 -8.2b 243 Utilities are averaged over the 24-hour day. Not recovered and not included in total.
-
Lb./Sq. In. Gage, Lb./Hr.
iio
.. .. 120
ACKNOWLEDGMENT
The writers wish t o thank E. S. Dennison and R. B. Smith for their continued advice and encouragement throughout this development. They wish to express particular appreciation to the test mechanics, Don Kealey, Art Welsh, and Tom Newmeyer, for their aid and ingenuity in assembling and operating the pilot plant. Thanks are also due t o J. R. Meyer and R. R. Resnick for their extensive aid in operating the plant. LITERATURE CITED
(1) Burbo, P.S., J. Tech. Phys. (U.S.S.R.), 13, 116-22 (1943). (2) Crawford, D. B.,unpublished. (3) Pollitzer, F., 2.angm. Chem., 36, 262-6 (1923). (4) Roberts, I., presented before Am. Inst. Chem. Engrs., Tulsa, Okla. (May 10, 1949). (5) Roberts, I., Paper No. 49-A-2, t o be presented before National Meeting of Am. SOC.Mech. Engrs., New York, N. Y. (December 1949). (6) Simpelaar, C., and Aronson, D., Ibid. (December 1949). (7) Ywertringen, J. S.,Chem. Eng. Progress, 43, 83-90 (1947). (8) Wilson, W. A., and Crooker, J. W., Mech. Eng., 66, 514-18 (1948).
RBCEIVBD July 14, 1949;
