rged Combustion Equipm
I
W. I. WEISMAN Ozark-Mahoning
Co.,Tulsa, Okla.
Use of direct-fired evaporators is increasing owing to / Improved materials of construction / M o r e efficient fume scrubbers
WET-PROCESS
phosphoric acid produced from Florida phosphate rock is a heavy, viscous liquid. More often than not it contains an appreciable quantity of solids, depending on the weave of the filter cloth used, the type of filter which effected the separation of the weak acid from the accompanying gypsum, and whether or not the filtered acid has been clarified. Of course it is saturated with gypsum and carries substantial quantities of fluorosilicate salts, both 3f which precipitate out on concentration. It is well known that calcium sulfate and fluorosilicates scale out on heat transfer surfaces. On concentration of weak acid, fluorine compounds are driven off with the vapors and silica will deposit in the form of a hard, adhering scale. Usually, the final Concentration desired is around 54.3y0 of P205 or 75% of H3P04 and pure phosphoric acid of this strength has an atmospheric boiling point of about 275' F. Also the concentrated wet-process acid will contain from 3 to 5% of sulfuric acid which further elevates the boiling point, so that the atmospheric boiling point of such concentrated acid may be close to 300' F. As the acid is concentrated, it becomes more viscous. The hot boiling acid is very corrosive and difficult to handle because of the presence of sulfuric acid and fluorine-containing acids, such as hydrofluorosilicic acid and probably hydrofluoric acid.
Vacuum Evaporators All of the characteristics which have just been described combine to make wet-process phosphoric acid a difficult material to concentrate. Multiple effect evaporation definitely is out of consideration. Single-effect vacuum concentrators are being used with fairly good results. Operators report that frequent shutdowns to boil out scale are a necessity, although the Weber recirculation system claims to have minimized or almost eliminated trouble from this source (9). One of the older installations operating in the United
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States requires 11 single-effect vacuum evaporators and three operators per shift to Concentrate 115 tons of P?o5 per day. These have cast lead bodies and stainless steel tubes and tube sheets. More modern installations are reported to have Karbate tubes and rubberlined bodies with the heat exchanger body located outside of the evaporator body and utilizing vertical tubes with thermosyphon circulation ( 9 ) . The most recent published report states that an 8- to 12-hour wash period must follow 60 to 70 hours of operation ( 3 ) .
Why Submerged Combustion? Because of the serious scaling problem and the fact that if additional steam-generating facilities are needed, the cost of such an installation may be quite high, it was only natural that chemical engineers faced with the problem of designing phosphoric acid concentration facilities should give serious consideration to submerged combustion evaporators or other direct-fired equipment. In most instances where natural gas is availabIe as an industrial fuel, evaporation costs via submerged combustion should be considerably less than single-effect vacuum evaporators. .4t one location where a direct comparison could be made because both submerged combustion and vacuum evaporation were practiced on sodium sulfate solutions, the operating cost for the submerged combustion unit was slightly less than for a double-effect vacuum evaporator (7). Hence, depending on the economics at a particular location, the use of submerged combustion could be quite attractive. Ozark-Mahoning Co. made, what is perhaps, the first commercial installation of submerged combustion evaporation equipment in the United States, in the early 1930's, and for a number of years has been in the business of designing and building such equipment for sale to others (6, 70). In addition, this company has operated a submerged combustion concentrator at its plant in Tulsa. Okla.
INDUSTRIAL AND ENGINEERING CHEMISTRY
Submerged Combustion Applications
-4true submerged combustion burner burns the fuel under the surface of the liquid and discharges the products of combustion directly into the solution being treated. Since the time the first commercial installation was made at the Monahans, Texas, sodium sulfate plant of Ozark-Mahoning Go., now known as the Stengl Works, submerged combustion has been applied to a wide variety of processing problems (5). Several potash companies in the Carlsbad, N. M., area use submerged combustion for heating and concentrating various potash solutions. In addition to heating and evaporation applications submerged combustion has found uses in the controlled production of carbon dioxide and generation of inert gases, wherein the products of combustion are utilized rather than the heat released in the burner. Heat transfer efficiencies are high because the products of combustion are cooled to the same temperature as the liquid before being discharged and there is no transfer of heat through tube surfaces or other surfaces on which scale can form. Chemico Drum Concentrator Other direct-fired evaporation equipment may approach some of the advantages of submerged combustion for the same reason that heat transfer is direct from the products of combustion to the liquid. The Chemico drum concentrator (Chemical Construction Corp.) has been used for many years as an acid concentrator. In its modern form the burner is mounted on the evaporating tank above rhe liquid and the products of combustion are discharged through a downcomer tube beneath the surface of the liquid ( 9 ) . In general, this type of concentrator is subject to the same advantages and disadvantages of a submerged combustion burner, although it would seem that maintenance on the downcomer tube might be somewhat of
a problem and utilization of the heat released may not be quite as good.
Prayon Direct-Fired Concentrator The Prayon evaporator (Engineering and Industrial Corp. S.A., Luxembourg) is direct-fired unit which has been used in phosphoric acid plants in Europe and elsewhere. Products of combustion from a furnace are tempered with secondary air and flow upward through an empty tower countercurrent to drops of phosphoric acid which are distributed at the top of the tower. A patented, rotating distributor creates large drops of acid and it is claimed that these prevent formation of acid mist. A high rate of recirculation of acid is maintained through the tower to prevent build-up of solids and the rotating distributor provides a self-cleaning feature. A complete unit, however, includes a knockout drum, a spray tower for cooling, and two Venturi scrubbers such as the Schutte and Koerting type to clean up the gases before discharge. The Prayon concentrator usually is fired with heavy fuel oil and certainly merits consideration in areas where natural gas is not available and fuel oil is the cheapest fuel. As would be expected, the fuel economy is not as good as for a submerged combustion unit. To con-
Right. Submerged phosphoric acid concentrator o f the type installed a t the Ozark Mahoning plant at Tulsa, Okla.
centrate 100 tons of P206 per day from 30 to 55% of P206 requires 16 tons of fuel oil which is equivalent to a gross heat input of about 25,000,000 B.t.u. per hour. A submerged combustion concentrator of the same capacity requires 20,000,000 B.t.u. per hour. Power requirements also are considerably higher for the Prayon unit, about 265 kw. connected load for this size unit against 150 kw. for a submerged combustion unit. The additional power is required to move the large quantity of secondary air through the system, which secondary air is used to cool the combustion products to 700' C. before entering the tower. Also pumping the high circulating load requires power. An interesting feature of the Prayon concentrator is that the acid is effectively defluorinated on concentration. For example, it is reported that a weak phosphoric acid containing 1.9570 of F will be reduced to 0.22% of F on concentration. In contrast, a similar product from submerged combustion would contain around 0.80% F. ( 2 ) . The only unit of this type installed in the United States is a t the Tuscola, Ill., plant of U. S. Industrial Chemicals Co., Division of National Distillers and Chemical Corp. No information has been released on its performance.
APORATINO
Drawbacks of Submerged Combustion In the past, thrre have been two objections to the use of submerged combustion concentration for phosphoric acid. In some of the early units, materials of construction presented quite a problem, but a t the present time these problems have been solved very well. The other problem is that the burner produces a fume of phosphoric acid particles which are carried out of the evaporator with the products of combustion and steam and which are rather difficult to scrub out. Because of these reasons the first submerged combustion phosphoric acid concentrator built in the United States was abandoned a number of years after its installation. This unit was manufactured by OzarkMahoning Co. and installed at the plant of Southern Acid and Sulfur Co. at Pasadena, Texas (now part of Olin Mathieson Chemical Corp.), in 1945. I n that initial installation, the submerged burner was fabricated from a stainless steel alloy and high maintenance costs were incurred because frequent burner replacement and repair were required. Design and construction of the evaporator vessel also were far from perfection and required a good deal of maintenance. No scrubbing
TANK
t o w e r left. Evaporator tank, air blower, and piping for phosphoric acid concentrator
T
t o w e r right. Dip pipe scrubber, which has proved satisfactory when using spent alkylation acid
VOL. 53, NO. 8
SEPTEMBER 1961
709
equipment was included with this unit and the fume losses were considered to be high. Two other direct-fired concentrator installations which were made since that time also have suffered a similar fate. One of these was reported to be a Chemic0 concentrator and the other a submerged combustion unit manufactured by Submerged Combustion Co. of America. Materials
t CONCENTRATED ACID TO STORAGE
,-Prayon
HOSPHORIC ACID
of Construction
The materials of construction problem in the present day Ozark submerged combustion evaporator has been solved very adequately by utilizing carbon and graphite as the only material in contact with the boiling acid. A schematic sketch is shown of the phosphoric acid concentrator in operation at the Tulsa, Okla., plant. The burner is fabricated from graphite electrode stock and there are no metals or alloys in contact with the acid. The tank comprises a steel shell, lined with a neoprene membrane followed by one course of acid-proof" shale brick for insulation and an inside course of carbon brick in contact with the acid. The carbon brick lining is extended all the way to the top of the tank and to the top of the stack elbow which is an integral part of the tank. This type of construction is completely acid-proof and although it represents a substantial part of the cost of installation, has demonstrated that it can resist successfully the very corrosive conditions existing in the evaporator and provide for an unusually long life.
instrumentation
GERTICAL SUBMERGED PUMP
The Prayon concentrator i s a cylindrical, vertical t o w e r with no packing. Tower shell i s special acid resisting silica-alumina brick protected by o 2l, 2inch layer of carbon bricks. This is surrounded by a lead casing supported by a steel structure which forms the tower framework and also supports the roof and the distributor with its drive. Tower r o o f is acid-resistant concrete. Acid is distributed into the t o w e r from the t o p in thin jets o r trickles o f liquid o v e r the whole section of the t o w e r . M o v i n g parts of the distributor which come in contact with the acid a r e stainless steel. Stationary parts a r e synthetic acid-resistant material. The t o w e r bottom acts as a reservoir for the acid. Liquid collected here flows through an hydraulic seal into a sump outside the tower. The agitator i n the sump is special stainless steel and the submerged pump i s HV9 and HRSM stainless steel. A mixture of air and fuel oil combustion gases enters the l o w e r part of the t o w e r a t about 700' C. and outlet gases escape through ducts in the t o w e r roof a t 100' t o 150' C These gases carry a w a y most o f the fluorine in the w e a k a c i d
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INDUSTRIAL AND ENGINEERING CHEMISTRY
The unit at Tulsa is fully instrumented so that operation takes only a fraction of one man's time. The panel board is located with the panel board for the phosphoric acid plant and one operator and helper per shift can run the complete acid plant, with the exception of one man to unload phosphate rock on the day shift. Control of the unit is based on the fact that the submerged conibustion boiling point for 54% Pz05 wetprocess acid is about 260' F. This bailing point is somewhat under the normal atmospheric boiling point because the products of combustion contribute partial pressure to the s)-stem. Boiling point provides a rather good measure of concentration of phosphoric acid, although variations in the amount of sulfuric acid and other impurities may introduce small deviations. A thermocouple in the stack, utilizing a Hastelloy C shield, which is the only metal part in the evaporator body, measures the vapor temperature which is the same as the liquid temperature, and by means of a temperature controller (Leeds and Northrup) automatically controls the amount of combustion air
SUBMERGED COMBUSTION EQUIPMENT going to the burner. In turn a ratio controller (Foxboro) maintains the flow of gas to the burner in the proper ratio to the air. Feed of weak acid to the evaporator is measured by a magnetic flowmeter (Foxboro). The rate of feed may be set at any value within the operating range of the burner (60 to 100% of rated capacity) and the automatic temperature control system takes care of process variations so that the product is delivered at a fairly constant strength.
Safety Features Occasionally, questions arise concerning safety hazards with submerged combustion units and much has been made about the advantages or disadvantages of premixing air and gas as is done in the Ozark burner, or mixing the air and gas a t the burner itself. I t is the author's considered opinion that with the various safety devices available on this equipment, dangers in an Ozark submerged combustion evaporator are no more and quite possibly less than with a standard boiler or any other modern combustion equipment of comparable heat release. The safety control system is built around a Fireye unit (Combustion Control Division, Electronics Corp. of America) which scans the flame from the top of the burner and in the event of flame failure for any reason, closes a magnetic valve in the gas line to prevent further flow of gas into the system. The Fireye also controls a push-button programming type of ignition pattern so that if for any reason the burner does not ignite as expected, the gas flow is cut off and air continues to flow to purge the tank, stack, scrubbers, etc., of combustible mixture before the ignition procedure can be repeated. A further precaution that i.; provided by the safety control system is the prevention of ignition of the burner in the event no liquid is in the tank. A pressuresensing element on the air line to the burner is used as an indication of liquid level in the tank and an alarm is sounded for both high level and low level. In the event of extremely low level, the system functions to close the magnetic valve in the gas line and cut off the flame, or to prevent ignition. Unusually high level is undesirable from an operating standpoint and this system also provides the operator with a warning of such a condition. Further additional safety features are included in the instrumentation in the event that very unusual circumstances would prevent the proper functioning of the previously mentioned controls. A thermocouple is imbedded in the burner wall and if the temperature indicated here exceeds a predetermined limit, the flame is cut off. Likewise another thermocouple in the
stack will institute the flame cut-off procedure.
Cooling Concentrated Acid At Tulsa, the concentrated phosphoric acid is stored in steel tanks with neoprene linings. The acid leaves the concentrator a t a temperature of 260' F. which is excessive for this type of lining. Hence the acid overflows into a smaller vessel where air is blown through it to cool it down to about 165' to 175' F. which is an acceptable temperature range for the linings.
Capacities The Tulsa installation is relatively small and the unit has a rated heat output of 10,000,000 B.t.u.'s per hour. At present, two 27,000,000 B.t.u. per hour units are under construction for the new wet-process phosphoric acid plant of Electric Reduction Co. at Port Maitland, Ontario. Sulfuric acid concentrators of somewhat similar construction have been built with individual graphite burners capable of releasing 30,000,000 B.t.u.'s per hour.
Fume Scrubbing Systems Several references have been made above to scrubbing systems and to phosphoric acid fumes in the off gases from the concentrator. This one unfavorable factor has been the biggest deterrent to more widespread use of submerged combustion for concentrating phosphoric acid because as much as 5% of the PZOSvalue in the feed may be carried out in the stack. The ideal solution to the problem would be to alter the burner or the products of combustion in some way to avoid the formation of fume and mist. About 8 years ago, efforts along these lines were quite successful with pilot plant equipment in the Ozark-Mahoning Co. laboratories; it was impossible to translate these results into large size commercial units. Hence, since that time efforts have been
directed toward scrubbing the gases. There are available on the market several types of scrubbers which, in commercial service. have been doing a satisfactory scrubbing job and recovering the P106 in such a form that it may be returned to the concentrator or to the phosphoric acid plant itself for economical over-all recoveries. These are : 0 Electrostatic precipitators
Pease-Anthony Venturi scrubbers Schutte and Koerting Venturi scrubbers 0 Doyle scrubbers 0
0
Each of these has some advantages and disadvantages. The electrostatic precipitator apparently can do a very efficient job and would seem to have very low maintenance costs (4). The biggest drawback is the high capital investment. The recovered P206 is a t such a strength that it can be fed back to the concentrator and may be relatively free of fluorine. Of course, the electrostatic precipitator would have to be followed by a fairly efficient wet scrubber for complete removal of the fluorine. The Pease-Anthony Venturi scrubber requires quite a high power consumption. I t seems that no matter how it is operated much of the fluorine will be caught along with the PzOS. A recent U. S. Patent suggests that using weak phosphoric acid as the scrubbing fluid in the Pease-Anthony, the amount of fluorine being dissolved here would be considerably less than if water were used ( 7 ) . Test data quoted in the patent show that a considerable amount of fluorine is picked u p along with the PzO5. Schutte and Koerting Venturi scrubbers were installed for two 20,000,000 B.t.u. per hour Ozark concentrators a t the Phillips Chemical Co. plant a t Pasadena, Tex., in 1952. These did a very effective job of removing PZOS and fluorine from the stack gases and the plant had a very clean discharge stack. Unfortunately, the design was such that it was impossible to recover the Pa05
Three-stage Schutte and Koerting Venturi scrubbing system and duct for carrying submerged combustion gases to c e n t r a l scrubber
VOL. 53, NO. 9
SEPTEMBER 1961
71 1
-International
Minerals-
Individual evaporator burner built by Internotional Minerals and Chemical Corp. shown suspended above vessel i n IMC’s Bonnie, Flo., plant
in the scrubbing waters. TVhen the Ozark-Mahoning Co. plant at Tulsa was built in 1955 it was decided to put in a three-stage Schutte and Koerting Venturi scrubbing system with provision for recovering P205 in the scrubbing water by returning it to the leach system in the acid plant. At the outset. it was feared that the return of fluorine to the system would cause scaling problems in the phosphoric acid plant and blinding of filter cloths. Operation proved that this was not a serious problem and a good portion of the fluorine left the system with the gypsum. It is true, however, that the fluorine content of the product acid increased under these conditions and filter cloths do blind in a shorter time. This plant was started on virgin sulfuric acid and the neoprene-lined scrubbers gave a fairly good life, although the first stage unit was exposed to rather high temperatures. At a later date, the plant began utilizing spent alkylation acid from a petroleum refinery and the presence of small quantities of hydrocarbons greatly increased the corrosive nature of the acid. These hydrocarbons also caused swelling and decomposition of neoprene linings and this was particularly severe in the scrubbing system as it seemed that much of the oil was carried off with the vapors from the concentrator. As a result, elastomer linings failed rather rapidly and maintenance on these scrubbers was quite high. Power costs for pumping the scrubbing liquid also
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was an important item. It was decided to replace the Venturi scrubbers with equipment OF simpler design that would eliminate, if possible, pumps and fans and simplify materials of construction problems. The result was a scrubber of the Ozark-Mahoning Co.‘s design called a dip pipe scrubber. In this unit, the products of combustion pass through a duct which is submerged beneath the body of scrubbing liquid. Although this single unit is not as efficient a scrubbing device as the three-stage Schutte and Koerting units, it does meet the other requirements which were previous1)outlined. The discharge from this dip pipe scrubber contains an appreciable amount of P206 and is exhausted through a central scrubbing tower which handles all of the off gases from the ammonium phosphate fertilizer plant. Inasmuch as the gases from the fertilizer plant may contain a little free ammonia, the acid in the gases from the concentrator are neutralized and an excellent recovery is made. ‘The scrubber water containing the ammonium phosphate in solution is returned to the fertilizer plant. Doyle scrubbers have been listed above as being recommended for this service because there are certain similarities between the Doyle and the dip pipe scrubber the Ozark-Ivfahoning Co. is using. The Doyle scrubber is different in several ways and may be expected to be somewhat more efficient than the dip pipe scrubber. I t also meets the above requirements of simplicity and minimum of moving parts. Reinforced polyester and epoxy resin plastics are being used for ductwork and scrubber parts. Some mechanical difficulties have been encountered with these plastics, but it appears that most of these are being overcome by proper design. The dip pipe itself is Hastelloy C. Of course carbon brick-lined ductwork would be best suited for the points where severest corrosive conditions exist.
Other Successful Instaliations Today there are other direct-fired evaporators which also appear to be giving satisfactory performance like the Tulsa unit. I t is reported that VirginiaCarolina Chemical Corp. at Nichols. Fla., is using a Chemic0 concentrator and doing a very effective scrubbing job with an electrostatic precipitator (4). Another very successfill and quite unusual installation is that of International Minerals and Chemical Corp. at the Bonnie, Fla., plant (8). Here International has developed and installed submerged combustion equipment of their own design utilizing Bunker C fuel oil which represents qu:te an engineering achievement. This is a multiple installation with a capacity of burning 800 gallons of oil per hour to evaporate 90,000 pounds of water. This represents a
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
heat output of about 120,000,000 B.t.u.‘s per hour and the thermal efficiency compares favorably to that attained in gas burning units. The burners are constructed of type 316 stainless steel and Ni-o-nel. The evaporator tank is concrete with membrane lining, acid and carbon bricks. International reports that the exhaust gases are scrubbed very effectively for recovery of PZOB and fluorine by a Pease-Anthony Venturi scrubber followed by a fluorine recovery spray tower and a final scrubber condenser. This represents one of the largest submerged combustion phosphoric acid concentrators in the country.
Superphosphoric Acid The Tennessee Valley Authority has produced superphosphoric acid in what is called a modified submerged combustion unit. This performance has been duplicated in the Ozark-Mahoning laboratories, with an Ozark submerged combustion pilot plant, and the company is prepared to offer the equipment for commercial size units, in this application, on the basis of the favorable pilot plant results. The author feels that the status of development on submerged combustion concentration of wet-process phosphoric acid has now reached the point where the operator may select such equipment with full confidence in the results to be obtained. Whether or not submerged combustion is a logical choice is dependent on the availability and cost of fuel. steam; and other local considerations at the plant site in question. ‘The author thinks that more and more phosphoric acid plants will be equipped with submerged combustion concentrators.
Literature Cited (1) Atkin, S.:Prince, S. S., U. S, Patent
2,905,535 (September 22, 1959.)
(2) Beetz. P., Engineering and Industrial
Corp.. Luxembourg, private communication. December 1960. (3) Bennett, Richard C., Chemical Processing 23 (iYo. 7),122 (1960). (4) Chem.Eng. 64 (No. 5), 144 (1957). (5) Cronan. C. S.,Zbid., 63 (No. Z), 163 11956) \ - - - -, ’
(6) Douglass, E. W., Anderson, C . O., Chem. t 3 Met. Eng. 48 (No. 5 ) , 135--37 (1 041 \ \-’
‘-1.
(7) Dudley, J. W.. American Viscose Corp.. Philadelphia, Pa., private communication. June 1951. (8) Tuttle, R. E., International Minerals and Chemical Corp., Skokie, Ill., private communication, May 1960. (9) Waggaman, Wm. H., “Phosphoric Acid, Phosphates and Phosphatic Fertilizer.” 2nd. Ed., p. 201, Reinhold, New York, 1952. (10) Weisman, Wm. I., Anderson, R. C., Mining Eng. 5 (No. 7), 711-15 (1953).
RECEIVED for review September 2, 1960 ACCEPTEDMarch 20, 1961 Division of Fertilizer and Soil Chemistry, 138th Meeting, ACS, New York, September 1960.
