Ammonium Nitrate by the Stengel Process J. J. DORSEY, JR.l Contnrerdol S d m b C-,
I
N THE summer of 1951, Commercial Solvents Corp. decided to embark on a $ZO,Mhl,Mhl expansion of its Sterlington, La., plant. This expansion was to include increased production of ammonia and methanol and new facilities for producing solid ammonium nitrate. Since a preliminary cost survey indicated that the Commercial Solvents ammonium nitrate process (8) should result in the lowest investment and production cost, it was necessary to obtain detailed design data from 8 pilot plant for a large plant. Tentatively, June 1952 was the deadline for obtaining enough information on the process to allow a detailed engineering comparison of the Commercial Solvents process with competitive processes and for at least a preliminary evaluation of the pilot plant product. The building, reaction equipment, and wster-cwled conveyor installation were completed hy January 1952 By March 1952, it was apparent that additiond handling equipment would be necessary. Accordingly, two small dryers, product screens, and several conveyors were procured. A coating drum waa fabricated, and a sewing machine for sewing filled bags was obtained. Throughout April, May, and June of 1952, the complete pilot plant wm operatad on a threeahift basis a t a production rate of about 300 pounds per hour. The pilot plant waa operated in order to obtain additional information and to train production personnel through May 1953. The required design information was obtained during 1952, to allow the construction of the plant to proceed during 1953. The production plant waa completed in August, 1953, and has been operating since that time (3). In Stengel Process, Control of Producl Moisture Is Independent of Climate
In this process, preheated ammonia and nitric acid react in a packet reactor, as described for the patented Stengel reaction system (8). The reactants are preheated ta a sufliciently higb temperature that the molten ammonium nitrate leaving the reaction system will be at the desired moisture level. This may be as low as 0.1%. From the reactor, the mixture of steam and ammonium nitrate passes to a cyclone-type separator where a separation of the molten ammonium nitrate and steam results. The steam is taken to a total condenser and sewered. The molten ammonium nitrate p " to ~ the stainless steel, water-bed conveyor where it solid3ies aa a continuous sheet. This sheet of ammonium nitrate is broken into about 5-mesh particles. This product is dried, cooled, and classilied into a -5 to +20 mesh mixture before paesing on to coatmg and bagging equipment. Provisions are incorporated for recovering fines from the screen and other possible loasea in the process. The Commercial Solvents solid ammonium nitrate process offem a simple method of producing a dry product without a prilling tower, crystalher, or dryers and coolers, thus d o r d i n g a process that is essentially independent of the weather. This is L
h n t addrar. O h Mathiason Chemical Corp., New Haven. Conn.
I a n m 1955
SMIngtOn, Lo
an important factor in the design of a solid ammonium nitrate plant, since many of them are located in the Sonth where humid weather conditions may seriously deet the output from the dryers of a conventional plant. A brief comparison of the major components of the several processes is given in Table I.
Table
I.
Comparison of Ammonium Nitrate Processes
0Lleration Preheatkg Neutralization EvapO?StiOn
b$$E;
,sation Flaking and aooling Grinding
Dniw
Cwling c1aasifioatlon Costing and bagging
commercis1 s01venta corp. (stengel) YeS YeS NO Yes
NO
NO Ye. Yea NO
NO
Ye* Yea
Prilling NO YW Yes NO
YB. NO NO NO
YW Yea
YeS Yea
OslolKryatal(6) NO
Yea YW NO NO YeS NO
NO
Yea NO Ye* Ye8
In conventional prilling ammonium nitrate processes (7), the control of the particle size from the prilling tower is a function of the properties of the solution of ammonium nitrate such m concentration and temperature, the type of spraying equipment, and the characteristics of the tower. The final product moisture level depends on the operation of two or three rotary vessels functioning &B dryers and/or a cooler. The atmospheric conditions, temperature, and humidity affect the capacity of the prilling tower and the dryers. In the Commeroial Solvents process the control of the product moisture o m be by the conditions maintained in the reaction system. This is a function of the preheating of the reactants and the operation of the stripper, which uses air to reduce further the product moisture to the d&d level. This operation readily lends i h l f to automatic control. Since the moisture content of the ammonium nitrate is established in the reaction system, the outside weather conditions have no effect on the product moisture. Pilot Plant Components A m Equipped for Automatic Control
The pilot plant waa located about one-half mile from the ammonia plant, the nearest major production unit. Steam, water, power, and air were brought to the area from the plant supplies. Ammonia and nitric acid were brought to the pilot plant by portable trailers which were filled a t available facilities in the production plant. The pilot plant building waa of prefabricated steel construction placed on a concrete slab. No special Boor was supplied and attack on the concrete was negligible except where hot nitrate frequently drained. The building was 20 feet wide by 48 feet long, and the reaction equipment was housed in a special barricade.
INDUSTRIAL AND ENGINEERING CHEMISTRY
u
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT
n
REACTOR SECTION
Non
-Condensables
I
I
I I I Ammoniq
G o ndenser
Vapor Scrubber
R e a c t or
Heater
NI-lqN03- 18.2 Ib./hr.
H20-274.51b./hr.
i
am
7
' '-
NH NO From T r a i l e r
S c r u b b e r Pum
616.2 Ib./hr.
H20-2755 ib/hr.
Ammonia -
7-
-0
Separator
Nitric A c i d P r eheater -
FINISHING SECTION 90lb./hr. O v e r s i z e R e c y c l e C o a t i n g Agent S c r e w F e e d e r
H 2 0 % 0.1- 0.4 A c i d i t y , 0.04 0.06 S c r e e n Analysis %
-
-5, i - 2 0
- 20 ,
-
72.0
14.2
N q N 0 3 - 6.18 tons/doy
T e m p e r a t u r e -OF.
-0
a Figure 1.
12
Pressure pressure
- inches
H2 0
Pounds per. sq.ini
Process Flow Sheet for Pilot Plant Production of Ammonium N i t r a t e
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
Vol. 47, No. 1
PILOT PLANT Ammonia Storage, Metering, and Preheating. The ammonia from a 1000-gallon trailer passed successively through a vaporizer, pressure controller, temperature controller, direct indicating Flowrator, superheating equipment, and into the reactor (Figure
1). The ammonia vaporizing and superheating equipment were of standard carbon steel construction and were designed to use available plant steam pressure of up t o 250 pounds per square inch gage. While some tests were conducted utilizing ammonia superheated to over 450' F. with electric heaters, this proved t o be unnecessary, and for most of the operation preheating to 300" F. was adequate. Nitric Acid Storage, Metering, and Preheating. The nitric acid from a 2000-gallon stainless steel trailer passed either directly t o the Ingersoll Rand Type s/4 MCS centrifugal pump or first through a scale tank. From the pump the nitric acid flowed through a direct indicating Flowrator and two acid heaters in series t o the reactor. Several types of nitric acid heaters were employed during the pilot plant operation. Generally, two heaters were employed in series, The first heater contained a number of coils of stainless steel tubing within a carbon steel jacket. This heater was operated a t less than 50 pounds per square inch gage steam pressure and preheated the nitric acid to 250" F., if necessary. Practically no corrosion of the tubing inside the heater resulted, but corrosion of the welds joining the two heaters was a continuing problem, although not serious enough to prevent operation of the pilot plant. The second heater initially employed to preheat the nitric acid above 260" F. was a Durco Series D4HF which is a standard Duriron heater. I t s principal disadvantages were its small size and low operating steam pressure limit of 75 pounds per square inch gage. This heatcr was replaced with a straight, jacketed section of 1-inch, Schedule 40, Type 304 stainless steel pipe. This type of heater lasted over 1800 hours. In this equipment corrosion also occurred primarily a t the zone adjacent to the weldof the top and bottom flanges. With this heater, acid preheat temperatures as high as 320' F. were used for short periods. Except for brief periods during the experimental program, it was not necessary to have an acid preheat temperature much over 290" F. It is entirely probable that some vaporization of nitric acid occurred at times. Control of the nitric acid preheat temperature was by manual adjustment of a steam pressure regulator for each acid heater to give the desired reactor gradient or temperature. Reactor, Separator, Stripper, and Scrubber. Two different types of operation of the pilot plant were explored-the operation leading t o the production of a n ammonium nitrate solution of 2 t o 3% water content which required a dryer and cooler for subsequent processing and, next, the operation to produce directly a product of 0.2 to 0.4% water, thus eliminating the need for a dryer or cooler. The major differences between these two types of operation were the nitric acid preheat level and the use of a stripping gas to dehydrate further the ammonium nitrate. The reactor was fabricated from a 16-gage stainless steel tube, 2.5 inches in outside diameter by 8 feet long. Provisions were made for entrance of the ammonia a t the side and nitric acid directly at the top. The reactor was packed with various sized packing to a packed height of up to 7 feet. A thermocouple well extended through the center of the packing from the bottom. Reactor temperatures were measured in this center thermowell. The separator, of conventional cyclone-type design, was fabricated at CSC's Sterlington shops. The separator was 8 inches in outside diameter by 24 inches high. The steam-ammonium nitrate mixture entered tangentially and the ammonium nitrate flowed by gravity to the stripper below. The steam and noncondensables left from the center discharge pipe and passed out to the scrubber. The stripper was fabricated from a piece of 3-inch, Schedule 40 January 1955
stainless steel pipe about 5 feet long. This stripper was packed with various types of packing t o various levels. The piping a t the bottom of the stripper allowed for entry of the heated stripping air, and the discharge of the molten ammonium nitrate to the Eastern Model D-11 centrifugal pump which pumped the molten ammonium nitrate to the water-cooled conveyor where it solidified. The vapor scrubber was fabricated from a section of 8-inch stainless steel pipe. The scrubber was designed to recover the ammonium nitrate and ammonia which escaped from the separator and t o cool the steam leaving the scrubber. Normally a packed height of 25 inches was employed. The scrubber a5sembly consisted of a circulating pump, a hot well controlled to a constant level by condensate addition, and a flowmeter in the pump discharge system. A side stream was taken off t o the acid charge tank for recovery. Also in the pilot plant reaction assembly was a surface condenser used t o condense all the steam from the vapor scrubber. This condensate was weighed and analyzed to provide a measure of the efficiency of the process. Water-cooled Conveyor. From the bottom of the stripper, the molten ammonium nitrate was pumped to the water-cooled conveyor where solidification and cooling ocurred. The watercooled, stainless steel conveyor was a standard 20-inch-wide belt with a cooling section 18 feet long. This conveyor belt was modified t o allow belt speed variations from 0 to 85 feet per minute. For maximum water economy and heat transfer, water was circulated through the two sections by a recycle pump until tht water temperature reached a specified maximum. The cooled sheet of ammonium nitrate was removed from the belt by a scraper and fed t o the product breaker and grinder which were located at the discharge end of the water-cooled conveyor. Drying, Cooling, and Product Coating. It was initially thought t h a t it might be necessary to have a t least one dryer and a cooler, as in most conventional ammonium nitrate plants. I n this phase of the pilot plant study, a General American Transportation flight-type dryer, 12 inches in diameter by 9 feet long, and a Link-Belt Model 207, Roto-louvre dryer were both evaluated. The installation was so designed that the product passed first through the flight-type dryer, and then into the Roto-louvre dryer, or into either dryer separately. The Roto-louvre dryer could be operated as a cooler, but generally sufficient cooling developed through the screens, and coating drum. The product coating was accomplished by adding coating agent and the classified ammonium nitrate to a rotating drum where the tumbling coated the product. The coating agent was fed t o the coating drum by a vibrating feeder or a screw feeder. The feeder was adjusted to discharge continuously a t a fixed rate to correspond to the known production rate. The product was bagged in conventional six-ply, moisture resistant bags for storage and field trials. All bags were sewn and overtaped. Grinding to Size and Product Classification. The grinding of ammonium nitrate to a product passed through 5 mesh and retained on 20 mesh screen with a minimum of fines was extensively studied. Many factors influence the characteristics of the ammonium nitrate with regard to the production of fines. Among these are temperature, sheet thickness, aging time, and moisture content. As part of this study of grinding ammonium nitrate the following types of grinders were considered: Corrugated roll, Several types of Several types of Several types of
roller mill hammer mills granulators so-called choppers
Generally, the best results were obtained with a hammer mill or granulator, both of which are used in the production plant. A vibrating screen was employed for classifying the product. The screen was 2 feet wide by 3 feet long. This screen was modi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
13
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT fied during later operation to incorporate an oversize w e e n , and the oversize mas recycled to the primary grinder. The screens were continuously blanketed with dry air to minimize blinding of the 20-mesh product screen. The fines from the screening operation were returned with the acid to the reaction system RS shown in Figure 1. Development Program Includes Investigation of Process Variables, Alternative Equipment
The program for pilot plant work was planned to obtain design information for those components of the plant for which the longest delivery time was indicated. Since it appeared that the major item in t,his regard was the number, type, and size of dryers and cooler, t,liisphase of the work was esplored first. Xext in importance n - ~ the s stainless steel, water-bed conveyors since the length and number of t,hese, and the rotary diyer and cooler, would dictate t o 3 large degree the size of the process building. Concurrent with these studies, n.ork was reaction system, grinding, coating, classification, bagging, and recovering of fines from the grinding system. Test8s on the small quantit>ieso i product available from the laboratory studie? hnvc indicated that the product should store and handle satisfactorily. Early in April and May of 1952 the first product from t,he pilot piant was set aside under controlled conditions for stomge stabilit Aftel all of the above phase e well along, the possibility of eliniinat ing rotary clryirig equipment came under intensive study, and it was deterniined that the dryer and cooler could be omitted in the commercial installation. During hIay and June of 1953 the pilot plant as operated to train production supervisors and operat,ors. Operation of the reactor system t o produce molten a,mnionium nitrate of about 0.3% moist'ure content resulted in a higher ammonium nitrate temperature to the water-cooled conveyor. The addition of ammonium nitrate fines to the moiten ammonium nitrate was found desirable, since otherwise the higher t,emperatures put additional coo!ing loa,d on the stainless steel, watercooled conveyor. If necessary, the belt could be operated to compensate for t,lie higher temperature, although the uniformity of t.he sheet leaving the conveyor was not as good. Also, Kith low moisture it was desirable to cool the product leaving the water-cooled convpgor to a. lo-xer tempemtiire sirice the product, , the bag. passed very quickly LO t'he screens, the co:tting d r ~ i mand When the tIyo dryere were eliminated from the pilot plant building, t.he scrmis vere modified t,o allow the installation of an oversize screen and t,lie recycle of the oversize material to the primary grinder. The pilot plant was operaied during most of the first 6 months of 1953 without a dryer or cooler. During this period it was possible t o obtain product moisture contents oonsistently less than 0.3%. Any amount of fines greater than the quantity that could be returned to the fines-dissolving system could be proportioned to the nitric acid by means of anot>herscrew conveyor and passed again through the reaction system. At the same time, the vapor scrubber operation concentrated any fog loss from the system to between 50 and 90% animonium nitrate; this concentrated materia! was continually returned to the nitric acid for recycle through the reaction system. A stainless steeI suyface condenser condensed the water Iraving the vapor scrubber. The weight and analysis of this water gave an accurate picture of the losses and these data were necessary for material balance calculatisns. The molten amnionium nitrate was sampled as soon as it reached the Sandvik belts. Changes based on the acidity and moisture content of t,hese samples could be made t o bring this phase of the operation t o the desired conditions. A calibration was availahle for each reactant flowmeter as a general means of controlling the ratio of ammonia t o nitric acid. 14
In addition, the condensate from the total condenser located after the vapor scrubber was passed through a pH cell, thus allowing the continuous measurement of the p H of the condensate after the recovery system In operation the fine control of a slight excess of ammonia could best be done by referring to the pIS recorder. Start-up of Reaction Section. Normally, the finishing section was started first since there was no point in Lhe process stream where the product collected until i t reached the bag. The coating drum, screene, conveyors, grinder, and water-cooled conveyor were started in that order. The reactant preheaters were adjusted to the desired steam pressure. The reactor, separator, stripper, and connecting lines were steamed a t atmospheric pressure. The ammonia flow n-as started first and then the acid flow was begun at a low rate until a rise in temperature in the reactor as observed. M t c r this, the acid flow could be brought, rapidly to the desired level. I s soon as the ternperat,ures of the molten ammonium nitrate leached certain values, it mas possible t o switch the flow froni the vapor scrubber sump t o the watercooled conveyor. 'rhe temperature at which this switch was made depended on whet~lieror not a dryer was used. After start,-up, a level T T ~ Sdeveloped in the scrubber sump and the recovered ammonium nitrate soluthri was cireulat>edthrough the scrubber. When t,he concentration of the solution in the vapor scrubber reached a definite value, the solution was continuously removed to the acid charge tank for return through the reactor. The level was maintained constant in the vapor scrubber sump to control the concentration. To reduce entrainment losses from the vapor scrubber, a,n entrainm.ent separator was installed. This proved highly beneficial since at times the scrubber tended to carry over and this aininonium nitrate solution was removed from the gas strrain and retnrned to the vapor xruhher sump. %'hen stripping air vtis ured, the stripper rvas started as soon as the molten ammonium nitrate temperature reached about 400" F. The stripping air also passed through the separator and vapor scrubber. Water-Cooled Conveyor Operation. Depcnding on the production rate and manner of operation, the make-up cold \-rater was sent to the circulating -xater system of t'he water-bed conveyor, or added only io the first section. It is important to solidify the molten ammonium nitrate as rapidly as possible to keep the sheet of ammonium nit,rate as uniform as possible. When the complete pilot plant was operated over extended periods without shutdon-ns, the fines from below the vibrating screens were continuously returned to a fines dissolving pot d i i c h cont,ained an agitator. A thermocouple was located in the exit line overflowing from the dissolving pot to the water-lxd conveyor. The amount of fines returned to the dissolving pot was regulated to control the temperature of the molten ammonium nitrate to t,he vater-bed conveyor to perhaps 30" F. above the solidifying point of the ammonium nitrate. Returning the fines to the production system in this manner resulted in cooling of the ammonium nitrate and disposing of the fines. This reduc'ed the water requirements to the mater-bed conveyor. The control of the cooling through the water-bed convog'or was based on the temperature of the solidified ammonium nitrate pabesing over a Weston thermometer. TVhiIe this temperature was not necessarily the true ieniperature of the ammonium nitrate, it did give a sufficiently reliable tempcrature for coiltrolling belt operation. Grinding, Drying, Cooling, Coating, and Bagging. From the water-bed conveyor t h e product passed to the grinder which was located directlyat theend of the belt. For most of the pilot platlt work a n Art.hur Coltoii single rotor granulator was used. Various screen sizes were tried as various sized products were made in order to determine the ei7e most suitable. When the pilot plant was operated with the two dryers, the material was conveged t o
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 1
PILOT PLANT
January 1955
INDUSTRIAL A N D ENGINEERING CHEMISTRY
15
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT the top of the flight-type dryer. After passing through this flighttype dryer, the product discharged into a Koto-louvre dryer direct'ly below. From the Roto-louvre dryer the product discharged onto another conveyor which carried the product to the screen. The screen cooled and also classified the product to a plus 20 mesh size. From the screen the product, passed into the coating drum where the proper amount of coating agent was applied. The product discharged directly from the coat,ing drum into the bag, which was brought to the sewing machine and sewed. The operation of the flight-type dryer was tried with both counter-current and concurrent flow of air and product. The counter-current flow of air and product resulted in the best drying, although some difficulty was experienced with a carry-over of the fines from the product into the air discharge duct from the dryer. In general, the drying conditions for the predryer were controlled to keep the ammonium nitrate at about the same temperature as it entered. Since it, was simple to control the amount of cooling on the water-bed conveyor, it was possible for t h e product to reach the predryer at a t,emperature anywhere between 130" and 190" F. The temperature of the inlet air and the quantity of air were varied to give t,he desired product moisture content and product temperature at the discharge. Drying condit,ions and, of course, capacities vere different when the t,wo dryers Lvere used in series and when either dryer was used alone. In general, if the temperature of the ammonium nitrat,e was increased as it' went through the drycr, very little moisture reduction resulted. Most of the loss of moisture resulted if the temperature of the ammonium nitrate was decreased in passing through the dryer. The Roto-louvre dryer employed \vas one of the smaller size production dryers. Again it was possible to control the exit product temperature and to sdme est,ent t'he moisture content, by varying the air rate, air inlet temperatiire, speed of rotation of the dryer, bed level, and retention time. I t was found early in the course of the investigat,ion that under humid conditions there was a t,enclency for thc vibrating screens to blind. To minimize this blinding it was necessary to introduce a small amount of conditioned air below the screens. This air passed up through the screens and out through the top where the product entered. Very little difficulty was experienced with screen operation while conditioned air \vas employed, and the air helped to cool the product. It was difficult to obtain a uniformly coated product in the pilot plant. A screw-type feeder arid a vibrating feeder were tried for feeding coating agent to the coating drum at a steady rate. Both of these types of feeders tended to stop feeding or to feed erratically, primarily because t,he coating agent Tyould not, flow steadily dawn to the feeder mechanisms. Since the amount of coating agent used was only a few pounds per hour, it was possible to install the feeder on platform scales; thus the amount of coating agent fed could be watched and recorded on an hourly basis. 11-hilethis served to indicate the amount of coat,ing agent used, it did not necessarily ensure that, the discharge of coating agent was continuous. For selected storage stability tests, material was prepared by charging 97 pounds of ammonium nitrate a,nd 3 pounds of coating agent to a drum and rolling the drum across the floor until thorough mixing resulted. In this manner it was possible t o obtain data regarding the effect,ivenese of vm.ious types of coating agents and various concentration levels of coating agent. Abnormal Reactor Operating Conditions Are Intenlionally Included in Study
The decomposition of ammonium nitrate has been studied extensively by many investigators. These investigations have been sunimarized in many excellent bulletins issued by govern16
ment agencies ( 1 , 2, 4, 6). l l a n y equations can be written for decomposition of ammonium nitrate-for example,
-
XH&Os
SHaS03
PIIHIXOB
+ HSOj NzO + 2x20 N?$. ' / z 02 + 2H20 KHS
(1)
(2)
(3)
In the Stengel reactor equilibrium in Equation 1 is approached from the right. At t,he normal operating temperatures of the Commercial Solvents pilot plant, equilibrium is such that the exit gases from the reactor contain comparatively small a,niounts of ammonia and acid vapor, and these combine as the gases are cooled and are largely recovered in the vapor scrubber. Under normal operating conditions there should be even less decomposition according to the reaction of Equation 2. Since the reaction of Equation 3 proceeds at still higher temperatures, there can be litt'le ammonium nitrate decomposition by t'his mechanism. Based on operation of the pilot plant and commercial plant to date, normal operat,ing temperatures after the separation of the ammonium nitrate and st,eamwill be less than 400" F., and it is believed unnecessary t,oexceed a temperature of 430" F. Considering that about a minute elapses between the time t,he ammonium nit,rate leaves the separator until it reaches the individual water cooled belts where cooling is very rapid, it is apparent that, there can be little decomposition of the molten ammonium nitrate beyond the reactor. Khile temperatures higher than 460" F. may occur at times in the reactor itself, the rcsidence time is normally only a small fraction of a second and thus too short for significant deconiposition. Under normal conditions of operation, Bemperaturea in the reactor above 460' F. r$.ould not occur. During the laboratory and pilot plant stages of this investigation, abnormal reactor operating conditions were intentionally included on occasion; for example, temperatures well above 600" F., extreme ratios of reactants, and reactants contaminated v,-ith oil and other materials known to sensitize ammonium nitrate were studied. However, during all of the laboratory and pilot plant operat,ions no uncontrollable conditions developed. Yields €ram this process depend primarily on the mechanical operation beyond the reaction system and on the efficiency of the vapor scrubber recovery system. Yields across the reaction system of 98% and better have been realized. Essentially the only nonrecoverable material is that which decomposes. A thermodynamic analysis of this process will be presented in a subsequent papcr. Pilot Plant Production Is Sufficient to Allow Product Evaluation
During the pilot plant development of the process it was also necessary to obtain information about desirable product characteristics. It was known that a low product moisture was desirable. It was expect,ed t,hat ahout, 3yo coating agent would required. The major difference bet,ween the new product and available fertilizer-grade ammonium nitrate was the shape and size of the particles. Prills and crystal ammonium nitrate are smooth of surface and rounded in shape. The Commercial Solvents process product is irregular in shape tending toward cubic, and can be sized as desired. Products of several sizes were prepared for storage, field spreading, and mixing tests in comparisons with existing products. The larger sized Conimercial Solvents process particles showed the least tendency to clog conventional-type spreaders, but smaller particles wcre more readily distributed in mixed fertilizers. A - 5 to +20 mesh product T ~ selected S as the most satisfact,ory product for all users. In April and hlay of 1952 some of the first pilot plant production was put aside for storage stabi1it)y evaluation. Some of this mRt,erial is still in storage and is still satisfactory. Some of
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 47, No. 1
ENGINEERING, DESIGN, AND PROCESS DEVELOPMENT this early pilot plant production and later production was also tested by Mississippi State College, and Louisiana State University. Satisfactory storage stability has been demonstrated with products made with all of the process variables discussed. Summary
Extensive pilot plant work preceeded the commercial development of a new process (the Stengel process) for manufacturing ammonium nitrate. This process is now in commercial operation at Commercial Solvents Corp., Sterlington, La. The process offers significant investment savings and lower operating costs through reduced personnel and utilities requirements. Since the control of the product moisture is in the reaction equipment, the process is essentially independent of climatic conditions. Acknowledgment
The author wishes to express sincere appreciation to Commercia1 Solvents for permission to present this information. Credit is due t o all Commercial Solvents personnel who were associated
with this project, in particular t o W. 0. Bell, Jr., J. D. Kramer, A. P. Miller, L. A. Stengel, and R. S. Egly. Literature Cited
Burns, J. J., and associates, U. S. Bur. Mines, Rept. Invest. 4944. Elliot, Martin A , Ibid., 4244. Hester, A. S., Dorsey, J. J., and Kaufman, J. T., IND.ENG. CHEM.,46,622(1954). Grant, It. L., and Scott, G. S., U. S.Bur. Mines, Infor. Ciro. 7463,June 1948. AIiller, Phillip, and Saeman, W. C., Chcm. Eng. Progr., 43, 667 (1947). Rous, W.H., and associatee, U. S.Dept. Agr., Tech. Bull. 912, J u n e 1946. Shearon, W.H.,Jr., and Dmwoody, W. B., IND. ENG.CHEM., 45,496 (1953). Stengel, L. A. (to Commercial Solvents Corp.), U. S. Patent 2,568,901 (September 25, 1951). ACCEPTED October 29, 1964. RscBIvEn for review July 30, 1954. presented at the Regional Meeting of the American Institute of Chemical Engineers, Washington, D. C., March 1954.
Thermal Calculations for Sugar Process Engineers HOWARD E. HIGBIE' Deparfmenf of Chemirfry, University o f Piffsburgh, Piffsburgh, Pa.
T
HERMAL properties of gas mixtures have been frequently tabulated as the enthalpies of the individual components (9). Then the total enthalpy or heat content of the mixture is computed by summing, for all components, the product of the enthalpy of the pure component times the fraction of the component present. This procedure is justified x h e n the heat of mixing is negligible; otherwise, the heat contents of the mixtures are not additive. The volume, enthalpy, and free energy changes on forming liquid solutions from their components are not negligible in general, hence workers in the field of theory of solutions have expressed the solution properties in terms of partial quantities (6)-for example, partial molal volume, relative partial molal heat content, and chemical potential. These quantities are defined to be additive so that the specific volume of a solution, for example, is the sum of the products of the mole fractions times the partial molal volumes of the components. The partial quantities are intensive properties of the solution and depend on the composition of the solution as well as on the other variables which determine the value of the total property. The partial enthalpies of water and sucrose in solution provide a convenient basis for engineering calculations of the thermal effects attending changes in these solutions. These changes, which occur in the sugar refining and processing industries, may involve the gain or loss of either component from the solution. A tabulation of the partial enthalpies of water and sucrose is presented in Tables I and 11, for the concentration range from 0 t o 65 weight % sucrose and the temperature range from 32" to 200" F. An enthalpy table for crystalline sucrose has been prepared to cover the same temperature range (Table IV) and an enthalpy table for water vapor over this temperature range (Table 111) has been included for convenience in making computa1
tions. In the following sections a method of preparing these tables is outlined and methods of using the tables for the calculation of the heat effects attending several types of changes are described. The justification for presenting the thermal properties of sucrose solutions as two tables of partial enthalpies rather than aa a single table of the total enthalpy or of the apparent enthalpy can be seen in the relationships between the heat effects and solution changes. The thermal equations can be stated by inspection and the calculations are straightforward. The solution partial enthalpies can be used in conjunction with existing enthalpy tabulations-e.g., the steam tables. A recent communication from Lyle points out that he derived an empirical equation for the enthalpy of sucrose solutions in terms of concentration and temperature (6). Although it wm written without the benefit of more recent heat of dilution and solution data, the equation agrees well with the present tabulation. The Lyle equation does not express the partial enthalpies explicitly so that the calculation of the heat effects accompanying solution, precipitation, and vaporization processes is less convenient than with t h e present tabluation. Development Is Based on Relationships between Partial Quantities and Measurable Quantities
The partial enthalpies of Tables I and I1 as well as the partial specific heats used in their evaluation are defined similarly to the well-known partial molal properties of solutions (6, page 33) except that they have been put on a unit weight rather than a unit mole basis. Thus, the partial enthalpies of water and sucrose in solution are defined by
Present address, 7813 Maple Ridge Rd , Bethesda 14, Md.
January 1955
INDUSTRIAL AND ENGINEERING CHEMISTRY
17
