Ammonia Is Delivered by Tank Car to Distributors and by Truck Directly to Farmer Storage
~~
WILL H. SHEARON, JR. Associate Editor
in collaboration with
THE
H. L. THOMPSON Mississippi Chemical Corp., Y a z o o C i t y , Miss.
World War I1 brought an expansion of ammonia production that permitted fulfillment of plans and dreams of agricultural and industrial users. These wartime demands necessitated construction of nitrogen facilities later turned over to ammonia production for agricultural uses. Yet, in spite of the fact that new government plants for the production of nitrogen of all types plus increases in private plants had increased the total productive capacity of the country to more than 1,000,000tons of nitrogen per year (three times the prewar capacity), the demand for nitrogen continued apace following the war, until signs of a shortage began to be evidenced. Increases in population, in living standards, in fertilizer exports, and in extent of soil depletion all had inexorably increased demands for agricultural nitrogen. Mississippi was affected by all these factors, but soil depletion had one of the most telling effects on nitrogen demand. The southern states have always been a t a disadvantage because of the fertility extracted by King Ootton and the system emphasizing annual cash returns a t the expense of soil capital. Even greater were the losses caused by erosion of uncovered fields during winter months, leaching of plant nutrients from the soil, and destruction of soil humus. Further, nitrogen cannot be stored in the soil as can phosphorus, potash, and other fertilizer materials, and the Mississippi Delta area, although relatively rich in minerals, has always suffered from nitrogen deficiency. Despite the fact that there were three nearby plants in adjoining states producing fertilizer nitrogen, people interested in Mississippi agriculture, spurred on by the signs of a possible shortage, began to look into the economics of constructing nitro-
state of Mississippi is one of the leading states in the use of nitrogen as fertilizer and uses more nitrogen per acre as anhydrous ammonia on its farm land than does any other state in the Union. For the fiscal year 1950-51 nitrogen use was 94,963 tons; 1951-52 use is expected to be greater than 145,000 tons, This fact does not a t all imply that Mississippi’s soil is the poorest, although without question a portion of the nitrogen is necessary because of excessive depletion throughout the years. The more important-and probably most overlooked-point is that the greatest fertilizer-consuming area is the Delta, whose mineral-rich soil requires little else but nitrogen. Hence Mississippi farmers concentrate their attention, fertilizer-wise, on nitrogen products. The purpose of this article is to describe this country’s first fanner-owned nitrogen fertilizer plant, the Mississippi Chemical Corp. a t Yazoo City, and the factors which were involved in its construction. Heavily entwined with considerations of marketing and use in the immediate area in which it is located, it is an excellent example of what is being done to bring industry to the people of a long industrially backward state, Anhydrous ammonia was first used directly as a source of agricultural nitrogen almost 20 years ago when the Shell Chemical Co. began experimenting with it in California as a cheap source of nitrogen for plant growth (8). This ammonia was produced by cracking natural gas t o produce the hydrogen necessary for combination with nitrogen. Mississippi soon became interested, and Andrews and associates (1) at Mississippi State College began application in the early 1940’swith results more encouraging than anyone had dared hope for. 254
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INDUSTRIAL AND ENGINEERING CHEMISTRY
gen-producing facilities in their own state. At fi ation did not seem at all favorable. From the investment standpoint older producers who had built their plants in prewar days and even the newer producers who had acquired (in good condition and at a fraction of their original cost) government plants based on natural gas an a raw material, were well situated. Thcse latter plants, with a typical annual capacity of about 125,000 tons of fixed nitrogen had cost their buyers about $90 per annual ton of nitrogen capacity. Against this, it was estimated by some t h a t a new investor would have t o invest $200 t o $360 per annual ton of nitrogen for even t h e smallest plant, similar in design and approaching t h e large scale plants in economy of operation. Cope ( 4 ) discusses in detail ammonia production and cost patterns. Not only were investment costs a disadvantage, but raw m a t e rials and labor costs were also discouraging. Natural gas was the new and now almost the only feasible source of raw material and fuel; location of suitable sources a t reasonable long term contract prices was a difficult problem. Further, a new producer, particularly if independent and without heavy financial backing, could not construct a plant large enough t o spread costs of operating and maintenance t o result in low unit costs. Fixed costs alone could result in an operating cost disadvantage of $20 to $30 per ton of nitrogen produced. That these factors coupled with an uncertain market and t h e relatively low margin of profit on nitrogen items were operative is shown in the fact t h a t no new private investment had been made since 1940 in facilities for direct production of ammonia, u p until the time of current amortization program. However, investment by farmera in such a plant would largely remove the last two deterrents. If a farmer could base his investment on his own needs for nitrogen and then tie his investment to a n ensured supply of fertilizer, marketability would no Iongei be a question. Further, where such a plant might not be a desirable investment to the private investor, who could benefit only to the extent of the spread between manufacturing costs and sales price, this spread is only the beginning of a farmer’s benefit from fertilizer in time of shortage, when a dollar’s worth of ammonia can produce several dollar’s worth of additional crops. It has been estimated t h a t 10 pounds of nitrogen (at 49 cents from ammonia or 87 cents from ammonium sulfate or nitrate) applied to t h e soil will produce a n additional 4 bushels of corn ($7.60) or 8 bushels of oats ($7.20). Nevertheless, the farmer would still have t o overcome, a t least in part, t h e handicaps of high investment and high operating costs before he could reasonably be assured t h a t he might not in some low market era find himself unable t o buy nitrogen from his own plant. Since in Mississippi he would be starting with one small advantage, proximity t o the consuming market, t h e technical problem was t h a t of reducing the handicaps to a point where delivered costs t o Mississippi, counting the freight advantage, would be close t o t h a t of established competitive plants. ORGANIZATION OF THE MISSlSSIPPI CHEMICAL CORP.
The very active Mississippi Farm Bureau Federation (with a background of experience in dealing with farmers as groups,
through insurance companies it had sponsored) conducted a ninecounty survey in 1947 t o indicate the possibilities of a stock subscription program, The results were encouraging, and in the late Fall its representatives appeared before the Mississippi Agricultural and Industrial Board with a formal request for a study of initial investment and operating costs for a synthetic ammonia plant t o produce 150 tons of ammonia per day, with facilities for converting one third of the production into ammonium nitrate. This study resulted in an estimate of $10,073,000 for the ammonia plant construction, including auxiliaries a n d utilities sufficient t o supply also a nitric acid and an ammonium nitrate plant, and an operating cost of $30 t o $40 per ton of ammonia.
255
n per day plant had for a number of years been considered as t h e minimum economical size for a n ammonia plant, but no work had ever been reported showing just exactly where t h e break-even point occurred. This was a very pertinent question, since as small a plant as was economically possible was required for the cooperative venture. Sometime after the Mississippi Farm Bureau survey was made, a preliminary analysis made by the collaborating author of this articleindicated that a 125-ton-per-day plant could be constructed, t o operate economically, for about $5,650,000. On the basis of this estimate another outside engineering firm was employed t o make a more comprehensive study of three plant sizes-60, 120, and 180 tons per day. Table I shows t h e results of this study, with a relatively small unit cost advantage accruing to the 180-ton plant as opposed t o the 120-ton. The fact t h a t it was economically feasible t o construct a plant for production of as little as 120 tons per day was considered equally as important as the sczle down on other factors, such as operating costs. Estimated construction cost for the 120-ton plant was $4,962,000, and t h e decision was made t o go ahead with the project.
TABLE I. RELATION OF ESTIMATED c05T” TO CAPACITY OF ANHYDROUS AMMONIA PLANTS
a
Size, Tons/Day Investment Coet 60 $3,088,700 120 4,961,700 180 7,099,400 Costa related to early 1949 conditions.
Operating Cost/ Ton of An+:donis $38.34 31.23 29.03
Plant Location A promotional campaign was inaugurated by the Mississippi Farm Bureau Federation, through a 26-person committee incorporated as the “Committee on Nitrogen Plant for Mississippi, Inc.” With the State Marketing Commission authorized by the legislature t o spend $50,000 in helping t h e committee carry out its aims, the campaign was begun. Two hundred twenty Mississippi banks entered into an escrow agreement t o take the initial deposits of potential stockholders’ money, and when i t became apparent t h a t sufficient funds would be raised for constructing the plant, the Mississippi Chemical Corp. was formed in November 1948. A total of $4,250,000 was subscribed by Mississippi farmers (including a small amount in Alabama) and a n additional $3,349,000 was obtained from t h e Reconstruction Finance Corporation. Thus no private out-of-area investment capital was involved. One of the primary purposes of the enterprise, other than increasing the availability of nitrogen fertilizers t o the Mississippi farmer, was t o give him those materials a t the minimum delivered cost. T h e corporatiQn did not anticipate underselling the market but rather selling a t an average market price, without disadvantage t o the farmer because of his location. This was an important factor in site selection. Thirteen cities in Mississippi were considered as plant site locations, and calculations were made on the total annual delivery cost from each of these cities t o the state’s 82 counties. Figure 1 shows these cities, with principal highways and gas lines of the state. Product tonnage that would be shipped t o these counties could be estimated with reasonable accuracy on t h e basis of stockholdings within the individual counties, since the stockholders were t o receive patronage rights in proportion to the amount of stock held. Transportation costs were estimated by multiplying the highway mileage from each of the prospective plant sites t o each of the 82 county seats by the number of tons of ammonia or ammonium nitrate expected t o go into the respective counties and by a n average cost per ton mile for each of these products. Estimates made by the Mississippi Agricultural and Industrial Board in its
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preliminary survey were used for these latter figures. Estimated a n n u a l . highway transportation costs from two of the thirteen prospective plant sites were checked with rail rates, and agreement between the two methods was found sufficient to justify using highway mileage figures for determining the order of magnitude of differences between the various plant sites. Results of the analysis indicated that the optimum location from the standpoint of transportation would be close to Greenwood, but there was only a small difference between Greenwood, Greenville, and Yazoo City in this respect, and delivery costs from Vicksburg would also have been only a few per cent above those a t Greenwood. T h e water supply, generally artesian wells, is good over most of the state and did not influence site selection. Cost estimates for generating necessary power a t the plant were made on several different bases. This step would have realized a saving of approximately $2 per ton in ammonia cost, but the substantial investment involved (gas engine drive equipment, with spare compressor and engine, would have added about $500,000 t o investment costs) would have largely negated this saving, and it was particularly important that the plant be placed in operation with the minimum cash outlay. The decision to purchase power, then, eliminated this factor from location considerations, as the Mississippi Power and Light Co. serves almost all the cities proposed, and its rates are the same in all areas that it serves. Practically all communities in the Figure 1. Principal Highways and Gas Lines of Mississippi area have poor soil from the construction standpoint, and extra foundation work would be reProcess Selection quired, so that this, too, was not regarded as a factor. The important consideration, therefore, was that of natural There are almost as many ammonia processes as there are gas, the primary raw material. Firm commitments on a continplants in the United States, if all the various modifications are uing gas supply could not be obtained either a t Greenwood or considered. Shreve (11) presents a very concise table comparing Greenville. The price of gas a t Vicksburg was attractive, but the important characteristics of each of the main systems in use the reserves were not considered as good as those available at today. All these, however, depend on the fundamental’ammonia Yazoo City. Further, conditions for floating a bond issue synthesis reaction Nz 3H2 + 2NH8, which requires pressure, under Mississippi’s “Balance Agriculture with Industry” prohigh temperature, and a catalyst. I n the reaction there is a gram, were highly favorable in Yazoo County, and a low interest liberation of 13,000 calories per gram-mole of ammonia formed. rate was expected on the bonds. Yazoo County ad valorem Principal differences arise in method of hydrogen production tax also was low. Highways in the area are good, and Yazoo and in operating pressures and ammonia recovery techniques City is situated on the Illinois Central Railroad’s main freight (6, 7). Prior to World War I1 the predominant portion of hydroline from Chicago to New Orleans. The choice, then, was in gen for synthetic ammonia production was made from water gas, favor of Yazoo City. One minor disadvantage to the site purusing coke and steam as raw materials. Most of the plants built chased was the possibility of flooding of the main industrial area in America during the ’last generation were modifications of during creek floods of short duration. T o overcome the flash what is termed the American process, worked out by the Fixed flood hazard, a complete levee system was installed, affording Nitrogen Research Laboratory set up after World War I by the several feet of protection above the highest recorded flood level.
+
IND
February 1952
u ST R I A L
A N D E N G I N E E R I NG
federal government. I n following pages this will be identified by the term “conventional process.” This employs pressure of 200 t o 300 atmospheres, synthesis temperature of 500’ C., and doubly promoted iron catalyst. Six of the ten government ammonia plants built in World War I1 were designed to use this process, and five produced the hydrogen for the reaction by catalytic reaction of methane with steam a t high temperatures. A new and relatively untried method of ammonia production is that employing controlled partial combustion of natural gas with substantially pure oxygen. This requires a separate plant for producing the oxygen. In selecting the proper processes and most suitable layout t o provide the most efficient production with least capital investment and operating cost, (a) simplicity and compactness in plant layout were emphasized, ( b ) outside types of construction were specified t o take advantage of the mild climate in cutting building costs, and ( c ) where feasible, service and utility operations were integrated with process operations. Since power was t o be purchased rather than generated, particular attention was paid t o conservation of heat and energy. Process and equipment controls were largely centralized t o reduce supervisory duties. It should be noted that no part of the program, however, was based on the use of inferior or substandard equipment, materials, or safety factors, t h a t it was not converted from a wartime defense plant, and that all equipment was purchased new. Particularly in the gas synthesis process, Mississippi Chemical examined most existing processes and modifications, reviewed each process step for technical performance, capital investment, and operating expense, selected the optimum process step for a given purpose, and recombined the steps. No step was selected
m I.
#
PROCESS EOUIPMENT
1. PROCESS
$ 3,734,300
874,600
53SW
1,192,800
1,054,800
1,106,900
sqsoo
(INCLUMOIN 182)
885,600
Bs5,SoO
295,300
518.800
223,500
252900
518,600
ZS5,7QO
232800
203,600
so,aoo
‘1.05S.oOO
EWIPMENT ERECTION
4. STEAM FACILITIES
S. M
2#sS,700
AND SITE PREPARATION
6. WATER SYSTEM (INCLUDINB FIRE
275 Y.
250 225
PROTECTION I
7. POWER SYSTEM
40
~,400,40400
5,234,700
TOTAL
3, Iss,mo
COMPARISON OF [INVESTMENT COSTS
-
M.C.C. COST CONVENTIONAL ESTIMATE
0
SAVINGS (Y BASED ON CONVENTIONAL ESTIMATE1
COST TRENDS DURING PROCUREMENT AND CONSTRUCTION I
2
s
4
5
6
7
ITEM
Figure 2.
257
that had not been fully proved in large scale operation, yet none was included where previous use seemed t o have been based on tradition or convenience alone, and functions were combined in a single piece of equipment where feasible. The conventional process, with its great improvement over older methods, has generally be,en considered the ultimate in ammonia plant design. Because of this and the fact t h a t most ammonia plants have been built under conditions where capital outlay was not the most significant factor, such critical attention t o design considerations had not been considered necessary. Mississippi Chemical’s plant contains no startling innovations but is of peculiar interest because of the improvements in design and layout and combinations of process steps and equipment t h a t have not been used before. I n order t o appreciate the value of the attention given t o compact and centralized construction and careful evaluation of techniques in relation t o reduction of costfi, Figure 2 should be carefully exa,mined. This compares the actual costs of the Mississippi Chemical’s plant with a n estimate typical of several presented t o the sponsors before the present program was made available. This particular estimate appeared t o be the most reliable prior t o adoption of the final program and compares very closely in investment per unit of capacity with wartime construction costs a t much larger ammonia plants. It is t o be noted t h a t the conventional estimate was determined by using an SO% prorata of estimates for the 150-ton-per-day plant, except on engineering and construction fee which was used in full, and that estimates for contingencies, miscellaneous fees, and engineering and construction costs were prorated over the seven items shown for both types of estimates. It is also pertinent t o note that the costs shown were achieved in spite of a
CONVENTIONAL ESTIMATE ’ DIFFERENCE
M.G.C. COST
2, M O C E S S AND AUXILIARY WILMNOS
cH E M IST R Y
Cost Comparison of Conventional and Mississippi Chemical’s Ammonia Production Processes
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INDUSTRIAL AND ENGINEERING CHEMISTRY
substantial rise in construction and general business costs during the period of purchasing and building, as shown very strikingly in the accompanying graph. The largest single item of saving was on steam plant construction, where by integrating steam generation with process functions a separate steam plant was avoided. Per annual ton of production capacity, plant investment costs for Mississippi Chemical were approximately $145 on nitrogen basis and $120 based on ammonia, as contrasted with figures of $235 and $195, respectively, for the conventional process. Construction Contract
The same financial considerations that demanded close attention to design also demanded careful consideration of the type construction contract selected. A straight cost plus or cost plus a fixed fee contract was regarded as too risky because such contracts offer no incentive for cost reduction by the contractor, nor do they provide for adjustment of incorrect estimates originally used or establish a basis for negotiation by the contractor. A lump-sum, “lock-and-key” contract, has some apparent advantages but also some drawbacks: The biggest advantage was considered to be a finalized cost, but weighed against that was the large premium demanded by the contractor for the risk taken: this would have been an additional $1,000,000 to $1,500,000 above estimated construction costs. A further disadvantage in lump sum contracts, where a completely new organization is concerned, is the development and supervision of the exact specifications ordinarily required as a basis for such a contract. Mississippi Chemical specified a modified type of contract which, although more complicated to administer than either the lump sum or straight cost plus contract, called for the contractor t80 pay 20% of the excess if his estimate was exceeded or to receive 20% of the reduction if the cost was less than the estimate. Both a maximum penalty and a maximum bonus were prescribed, with the penalty twice the bonus. Engineering and purchasing decisions of the contractor were subject to approval by Mississippi Chemical before executibn. Capacity and performance guarantees also were included in the contract. AMMONIA SYNTHESIS AT YAZOO CITY, MISS., PLANT
As a result of the considerations mentioned above, the ammonia synthesis technique chosen was that of Claude, principally characterized by’ operating pressures near 1000 atmospheres. T h e first Claude units had been installed in America in 1927 and, with modifications from the Casale process, are now owned entirely by D u Pont a t Belle, W. Va. Hercules Powder Co. selected this process for its entry into the manufacture of ammonia in California in 1940 and operated similar equipment a t Louisiana, Mo., during the war. Part of this latter equipment is now used for other purposes and the remainder was moved to Houston, Tex., where it is privately operated as the San Jacinto Chemical Corp. The government plant at Morgantown, W. Va., was designed and built by D u Pont using that company’s own modification of the high pressure process. Hydrogen Production
Ammonia synthesis catalyst requires gas that is exceptionally pure with respect to oxygen, sulfur, phosphorus, and particularly carbon monoxide, any of which would act as a catalyst poison. Production of this pure hydrogen is by the standard method-catalytic reforming of methane with steam a t high temperature (IO),but a number of departures in equipment and techniques were utilized a t the plant being described. These are highlighted in Table I1 and Figure 3.
259
The methane reforming reactions are: CHa HzO +GO 3Hz CH4 2H20 +COz f 4H2
+ +
+
(1) (2)
All the wartime plants except the one now in operation near Houston, Tex., used a two-stage reforming system. However, Mississippi Chemical Corp. chose the single-stage reformer developed by Hercules Powder Co. and used by Hercules in its California plant. This reformer and subsequent operations in hydrogen production are similar in general aspects to a unit previously described in some detail in this journal (IO). Multiple burners up and down the single reformer afford close temperature control and enable achievement of a sufficiently high temperature to obtain complete reforming of the methane in one step. More fuel is required than in the primary reformer of a two-stage system, but none of the reformed gas is burned. It is difficult to obtain measurements of total fuel consumption of the single- and two-stage systems under sufficiently comparable conditions, but it is likely that any difference between them is small. In the present case the single-stage system was offered for less initial cost and was expected to require less operating attention. Mississippi Chemical has two reformer furnaces each containing alloy steel tubes filled with a nickel-base catalyst on a diatomaceous earth carrier. Superheated steam and natural gas in the ratio of 2:l are introduced into the kibes, together with flue gas from the boilers and/or tail gas from the nitric acid plant as a source of nitrogen. The natural gas used is approximately 920/, methane, with 3% additional mixed hydrocarbons, a gross heating value of 1028 B.t.u. per cubic foot, and maximum sulfur content of 20 grains per 100 cubic feet. This sulfur is removed by activated carbon before the gas enters t h e reformer. Boiler flue gae is approximately 86% nitrogen and nitric acid tail gas %yo nitrogen. Approximately 30% of the total natural gas is used as fuel in the reformer furnace and 25% to fire the boilers. Over-all steam supply is centralized and simplified in comparison with conventional plants, which not only have central steam plants but also waste heat boilers in conjunction with both reforming and carbon monoxide conversion. Mississippi Chemical generates all steam in combination boilers, using hot waste gas from the reformer furnace t o supplement direct gas firing. Labor is reduced by locating the unit in the hydrogen area where regular operators can also take care of the boilers.
TABLE 11. COMPARISON OF AMMONIA PRODUCTION TECHNIQUE Step Steam-gas ratio Nitrogen source Reformers Steam generation Heat recovery from reformed gas Gas purification
cos
Mississippi Chemical Corp. Synthesis Gas Production 2:l Acid plant tail gas; flue gas Single-stage Combination waste heat
Conventional
3:5:1 Air Two-stage Two waste heat boilers One waste heat boiler and preheating Interstage
Amine absorption Water scrubbing Methanation Copper liquor Ammonia Synthesis Pressure 1000 atmospheres 200 to 350 atmospheres Two in series with recircu- Single with recirculation Converters lation Make-up addition Ahead of ammonia con- Ahead of ammonia condenser coils, seoond condenser coils verter Space velocity cu. A converter, 48,000 9000 to 11,000 ft./cu. f t . catalyst B converter 30,000 Space-time yields A converter: 410 48 fb. NHs/hr./cu.’ft, B converter, 245 catalyst Promoted iron oxide Catalyst Promoted iron oxide Typical catalyst life 2000 to 2600 hours 2 t o 5 years Ammonia MeJlanical Cooling water con de n 8 a ti on refrigeration Inerts purge Immediately ahead of Between heat exchanger and entry into conmake-up gas addition verter Continuous Laboratory control 1 shift per 24 hours CO removal
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Recovery of latent as well as sensible heat from the hot reformed gas is not feasible in a waste heat boiler but can partly be accomplished in a heat exchanger and water heater-saturator setup. Such equipment is standard in ammonia plants using coke as a primary raw material but has not been used before where natural gas is the raw material. One 12-foot diameter carbon steel vessel 65 feet high serves both reformers. It consists of two separated sections as a unit, the upper section packed with 15 feet of
compression stage. This method requires handling large volumes of liquid under pressure] complicated compressor design and operation] and increased compressor investment costs. Therefore Mississippi Chemical chose to carry out its compression a t one point, after all purification except final carbon monoxide removal. Synthesis gas production from hydrocarbons is a continuous process, and therefore an atmospheric type holder for surge storage of purified gas is not necessary. Without increasing- reformer furnace feed pressure beyond values already in use, it was found possible with proper piping and equipment design to minimize pressure drop through the gas plant and to supply partly “precompressed” gas (9 pounds per square inch pressure) to the compressors, resulting in substantial savings in compressor investment and operating power. This, in conjunction with the prescrubbing of carbon dioxide and use of a slightly higher average compression ratio per stage, permitted a t t a i n m e n t of 15,000 pounds per square inch pressure in the same number of stages (six) required t o reach 5000 pounds per square inch in conventional wartime plants, On all piping and vessels ( 6 E ) requiring insulation, aluminum sheeting has been used instead of the more conventional galvanized sheeting or paper coverings, with or without paint. Hydrous calcium silicate insulation (4E) on vessels is covered with 0.032-inch sheeting] and piping with 0.020-inch (SE). This application of aluminum] developed following World War I1 when galvanized stocks were low and Figure 4. Water Heater-Saturator and Associated Equipment aluminum plentiful, has proved quite Aluminum sheeting is used as covering for hydrous calcium silicate insulation useful both from the standpoint of p r o t e c t i o n a n d appearance. It is corrosion-resistant except in strongly 3-inch Raschig rings and the lower with 25 feet. The lower alkaline atmospheres, requires no oaintintr. and its cost is about tower serves as the saturator and the upper as the water heater. equal to that of the usual 28-gage galvanized covering before Figure 4 is a view of the heater-saturator equipment. Gas painting. effluent from the converters enters the bottom of the upper tower, Removal of the last traces of carbon monoxide is accomplished flows countercurrently to quenching water, heats the quenching in the Mississippi Chemical plant by methanation. General water to nearly boiling, and passes out the top of the tower to the aspects of this technique and its advantages for reducing carbon gas cooler. The hot water passes down into the lower section monoxide content to a n inconsequential percentage are discussed where it meets the reformer effluent gas coming from the tube in the description of the vegetable oil plant already referred to, side of the heat exchangers. This gas is thus saturated with which was one of the first such plants to employ this technique steam for the water gas shift reaction which takes place in the at low pressures. Methanation is simply the reverse of the converters] passes out the top of the bottom section, through the first reforming reaction, catalytically converting carbon monoxide shell side of the exchangers, and on to the two converters. Here to methane. It replaces the more conventional but expensive the carbon monoxide reacts with steam, in the presence of an and complicated copper liquor system, in which there is no loss iron-chromium conversion catalyst, t o form carbon dioxide and of hydrogen as in methanation] but there is required substantial additional hydrogen. The converter gas is cooled, and the carbon additional equipment, operating, labor, and chemical control dioxide is absorbed by amine solutions (10, $E) rather than by costs. Since it is actually one of the two principal characteristics water scrubbing under pressure as practiced in the wartime plants. of the Claude process, it is more properly considered as a part of Although some of the wartime plants have changed t o amine the actual ammonia synthesis. solutions, they have retained the pressures under which water Synthesis scrubbing was carried out. While such practice enables absorption of more carbon dioxide per unit volume of amine solution Until the construction of the Yazoo City plant, there had not and requires less steam for regeneration, corrosion can be aggraappeared any significant discussion in U. S. literature of the current vated, and contamination of carbon dioxide (if recovered) is Claude process and its modifications. A comprehensive demore likely. Conventional wartime plants employed interscription of the operation of this particular portion of the Misstage purification, compressing the gas through three stages sissippi Chemical plant has been presented elsewhere (IS), and before any purification, absorbing most of the carbon dioxide, discussion here will be limited t o a summary of this operation and then compressing through two more stages before final removal information considered pertinent hut not included in the article mentioned, of carbon monoxide and dioxide, and following with the final -I
February 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
The iuethanntion step employed :st Mississippi Cheinical differ8 from the basic technique s1read.y mcnticnod ( I O ) in that it is esrriod out at synthesis pr~isures(zpprosirnately 1000 atmospheres) instead of atmospheric pressure and that it uses an iron eatnlynt simihr to the synthesis catalyst, rather than nickel. Approximately half 2~8much catalyst is used for metharmtion as nthesis, aiid it,s life is twice ns long. Mealranation at synpresenre results in reduction of carbon morioaide in t,he synthesis gas to :is low as O . O O O l ~ oas opposed to approxirnatoly 0.01%when carried out st atmospheric pressure. I n plant practice nicthanstor effluent gus w i scldom analyzed for carbon monoxide content,, on the asvuinpt.ion that methsnrrtion is complete :is long as proper operat.ing temperature range (730" to 930' F.) is muiirttLined, Because the syntliesis reaction is reversible, the rate of COIXvenion t,o nniirionia approsches aero as the ammonia content npproeehes equilibrium, making equilibrium by a pingle exposurn of the ga8 to reaction conditions not feasible. Because of the imperfect lipproacli and incomplete conversion at cquilibriurn, itmmonia must be removed from the freshly react.ed gas and the unreacted gas returned to B reaction zone. Conventional low pressuro plants use one converter, with recirculation, nherewa Mississippi Chemicd einpioys two convertors in series and also recirculates the gas stream. Series arrangement instead of parallel requires R lower capacity circulating pump; by-pass vdvea allow the conversion load to he varied hetwoen the converters; and flow t.hrough t.he converters can he reversed t.o lengthen over-all cat;tlyst>life. The two eanveiters are carbon stecl cylinders (110, 25.75 feet inside diameter uml 16.67 feet high, with slrellv 10.375 inches thick. A reinovahie basket, or cartridge, fits inside the shell and contains heat exchange t,ubes holding spprosirnately 16.5 cubic feet of catalyst per corrvertw, electric heating N ~ I R for startup and temperature control, and the catdy8t. This catalyst is B carefully purified magnetic iron oxide, doubly promoted with ~ m dpercentages l of aluminum and potasium osirfes. Synthesis gas enters at the bottom of t.he converters, is preheated by passing around the converter t.ubea, passes through the cetdyst in the tubes, and ammonia. boaring gas is discharged aut the top of the converters. 'Temperature of the c l i t d y 8 t is controlled by n by-pa= stream of oold gas introduced into the hottom of the converter shell through a 8eparat.e opening. The convertors, along with tho methanator, are housed in a concrete structure which function8 ns a supporter and h a heavily reinforced sails, 6 to 12 inches thick, seperating them fronr the rest of the syritliosis plant and from each other. Heat exchange within the converter is such that the only h a t which appears in tho exit gas as sensible hest is the reaction heat. The gas, precooled on exit from the converter tops, is cooled further for ammoniu Condensation, then passed through an ammonia separator from which liquid ammonia goes to a receiver and storage arid the uncondensed gas continues in the circulatory system. Yynthcsis gas enters the circulatory system irnmodistely following the precooler at the second converter, joining the recycle strmui rontaiiiing amruonia and a predet,ermind BCCUIUUlation of iucrts. The emmorria is scpsrated out, and thc sbream passes irrirnediately to the first canvorter (temperature 900' to 1150" F. dopending on catalyst activity), which normally carries about 60% of the conversion lond. From this convcrter, after removal of tho :immonia formed, the g i i pas~esthrough the recirculator which restores t,he 500- to 1000-pound pressure drop, is coded, passrs through a water arid oil separator, and thence to the second converter and through its precooler to the ammonia condenser ment.ioned at the heginning of the cycle. During this one pass approximately 45% of the hydrogen and riitrogen in the synt,hesis gm has been convericd to ammonia. Recovery of ammonia in the separator following the first conv o r t m v in ohm,+ P O 4 n n r l
ill,,+ F . 4 1 ~ . . . & ~
'hn
~nn-n-l PI0
+.
,907
261
Imvirrg tho residual concentration of smnortia in the recycled ga.? streurn at 4 to 6%. Tho preeoolers iollowing the inetliamator and tho coirvertora consist of 6Cbfoot coil8 of pipe (six for the mcthanator and twelve each for thc converters) in open rectangular m t e r tanks of 675gallon capacity for the coiivertera and 5W-gallon for the methanator. The tanks are on roller6 in a circular track, so bhat. the entire gas out,let nnd precooling unit can bo diseonnecteri and
Figure 5. Water-Tank Ammonia Precooler Is Built on Track to Allow Easy Removal of Converter'l'op for Catalyst Basket Replacement
swung away from (.Ire coiwemion vessel when cartridge change is required. One of thc coolers is shoan in Figure 5. Gas outlet tulm from the convwsion v e ~ s e kand the piping bo the precoolem are an alloy of 60% nickel, 12% eiiromium, and 2.5% tungsten. Arumonia condensers consist of banks of return bend concentric piping wilh water flowing in the outside tube. All high pressure piping is two sizes, depending on the gas flow to be handled: 1.375 inches inside diameter by 3 inches outside and 0.8125 by 1.81 25. Airmionin separators and the eight water-oil separators are high pressure vessels, each constructed from a single ~ t r e lforging (5E). General opinion in industry lies been that, while embon steel is satisfactory for equipment, and piping in conventional plants, slloys must bo used in Claude-type planta. Current cxperienoes have shown that carbon stcel is quite satishctory in Claude units. Since a small amount of rnethsne w a introduced ~ in the methanation step and it is not possible to elimiuste cornpletely other inert gases from the syatam, these inerta tend to accumulate. This causes n reduction in ammonia at equilibrium and purging of inert8 is required. The nialn purge is controlled on the hasip of total system pressure and the line comes off the circulatory line j u t after the precooler at the second converter and immediately preceding t,Jre point of introduction of fresh synthesis gas. At this point the r h o of niarogen and Bydrogeii t,o inerts is at a minimum. Some purge gas is also taken off as top Row from each of the two separators. The purse gas is condrnscd _I._.^"L^il.
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262
INDUSTRIAL AND ENGINEERING CHEMISTRY
drous ammonia. For this reason, the ratio of ammonia to inerts is not critical. The purge gas, containing about 10% of the original nitrogen and hydrogen in the synthesis gas, normally analyzes (on an ammonia-free basis) about 20% methane, 19% nitrogen, 58% hydrogen, and 3% argon. It is available in quantities of 50,000 to 70,000 cubic feet per hour and is suitable as boiler fuel in the synthesis gas plant. The objective of using high pressure is not only for high conversion per pass; only about one seventh as much catalyst and converter volume as the conventional process uses is necessary a t 1000 atmospheres to produce the s8me quantity of ammonia, and about one fifth as much gas must be pumped and piped on the basis of actual volume a t synthesis conditions. One significant disadvantage of the Claude process is the short catalyst life, usually about 3 months. The importance of this disadvantage is minimized, however, by the fact that only a small load of catalyst is carried because of the relatively small size of the Claude converters. Further, the design of the converter to allow quick removal of the catalyst cartridge and replacement by a previously recharged spare means that changeover and return to normal operation can be accomplished in about one S-hour shift. Also production loss may be saved after operators become skilled in that maintenance work required on other parts of the plant may be scheduled by them to dovetail with catalyst changing. Products Storage and Distribution
Production of ammonia for agricultural use always involves the significant problem of storage. An ammonia plant operates 12 months of the year; the peak season of ammonia usage b y farmers in the southern states occupies only about 3 months. Actually, about 7 months' storage is required and since capital cost of ammonia storage is almost equivalent, per ton, to the capital investment required for ammonia manufacture the best answer is storage by the farmer-user/or sale by the plant to industry during the farmer's off semons. Mississippi Chemical was established, of course, t o provide the Mississippi farmer with a dependable source of nitrogen; therefore the company is encouraging storage installations by its farmer stockholders. T o foster this, the company purchases 6000-gallon tanks and sells them to the farmer a t cost. For plant storage, the same facilities are used for storing anhydrous ammonia whether it is intended for sale or for use in the nitric acid and ammonium nitrate units. Industry in general employs two types of anhydrous ammonia storage: (1) pressure vessels capable of withstanding any reasonable outside temperature and (2) refrigerated Hortonspheres. Costs for pressure storage are usually figured a t about $160 per ton of anhydrous ammonia. For the refrigerated Hortonspheres, Mississippi Chemical estimated its costs would be approximately $120 per ton, and g o chose this method of storage, There are two of these 48foot diameter vessels, capacity 100 tons each: design pressurk is 75 pounds per square inch and operating pressure 55 pounds, a t a temperature of 38" F. The spheres are constructed of 0.9% inch carbon steel covered with two layers of 2-inch cork, followed by chicken wire, a coat of mastic, and finally aluminum paint. Mississippi Chemical Corp. is organized into 22 districts, each district corresponding to approximately $200,000 worth of stock holdings. Stockholders in each district elect one director; the board of directors names an executive committee which meets monthly. Stockholders are entitled t o purchase ammonia in proportion to the amount of their holdings, one ton of ammonia per 20 shares, and the organization of the corporation is such that the board of directors can also declare a patronage refund or dividend. Ammonia is sometimes delivered directly to the farmerstockholder; seven 5000-gallon tank trucks are operated by the corporation for this delivery.
Vd. 44, No. 2
Utilities and Labor
Water is obtained from two sources: cool water (66" F.) comes from a shallow (160 to 170 feet) stratum. This water is high in dissolved impurities; especially iron, and therefore cannot be exposed to air, if it subsequently has t o travel through metal equipment. It is used where the temperature is critical, such as in final gas cooling before compression in the synthesis plant. It is also used in ammonia cooling because the coolers are totally enclosed. Two 1500-gallon per minute wells (one spare) are available for this purpose. Where temperature is not critical, 78" F. water is used. This water, which has low hardness and low dissolved solids, is obtained from three 1500-gallon deep wells, and one 500-gallon deep well is available as a spare. All power is purchased. It is a general practice among industrial plants to take power at 13,000 volts or less, but since the Mississippi PoR-er Company's high line crosses near the plant site, it was possible to contract for the power a t a reduced rate by taking it a t 115,000 volts. It is then stepped down to 4160 volts through a 12,000 kv.a. outdoor metal-clad substation consisting of two 6000/7500 power transformers throat-connected to the switchgear (main5 and four magnetic air-type feeder breakers). Motors of 200 hp. and larger are 4160 volts and areline-started through indoor metal-clad switchgear essentially a duplicate of the outdoor equipment. Ivhcre possible motor control is grouped by use of standard control centers; otherwise standard combination control is employed, with enclosure t o meet location requirements. Motors of 150 hp. and lower are 440 volt. Small motors and auxiliary power units are fed from small power centers located near the equipment. Where central control is not feasible, oil-immersed starters are generally used. Total installed motor load is approximately 10,000 hp. (7700 synchronous), with estimated plant power demand of 7200 kw. a t 95% power factor. All electrical equipment is standard design and has extra varnish as added protection against moisture, weak acids, and alkalies. Built-in heaters prevent condensation during periods of shut down. Emergency power is supplied from a 400-kw. geared turbine generator connecting to a 4160-volt feeder backfeeding into the complete electrical system. Selection of this unit wa8 primarily on the basis of low installed cost and speed with which it can be put on the line (full load within 15 seconds, depending on the operator). The unit will supply 500 kw. for periods longer than estimated necessary to clear process lines. Under extreme disaster conditions i t can be used to supply lighting and pumping requirements for extended periods. Since high pressure is the dominant operating characteristic of this plant, the compressors deserve mention in some detail. Like several other units of the plant they are arranged in duplicate trains, each compressor train consisting of a primary and a booster machine. All units have 14-inch stroke with frame and running gear parts duplicated throughout, Primary units are each powered by a 2250-hp,, 300-r.p.m. engine-type synchronous motor. Each primary compressor has two first-stage cylinders and one each for second, third, and fourth stages, all double-acting. Intake gas is a t 9 pounds per square inch pressure and discharge from the fourth stage is a t 3500 pounds. Compression to 15,000 pounds is accomplished in the two-stage boosters each of which has two sets of cylinders operating in parallel. These consist of a single-acting fifth stage and a single-acting sixth stage plunger arranged t o compress on opposite strokes. The high pressure cylinders were segregated on separate units because most of the compressor down time is caused by maintenance on the high pressure end, and these machines were specifically designed for that service and for minimum and easy maintenance. With pressures as high as are necessary in the Claude process, it is desirable to operate a t reduced speed, and segregation also permits operating the low stages a t full rated speed.
INDUSTRIAL A N D ENGINEERING CHEMISTRY
February 1952
Cspneity control ia designed to perrnit operation down to 25% of plant capacity. This is accomplished by a combination of manually operated clearance pooketx in the firat-stage cylinderr plu8 throttling of the intake pressure to the primary compresaom. Mimissippi Chemical expected ~ o m edifficulty in recruiting operating labor of a sufficiently high celiber because of the reliitive isoletion of the community. Thio prohlem, except in t h r field of highly skilled maintenance personnel, haa been sntisfactorily solved. Skilled niaintenance craftsmen have proved particularly turd to find and competition with construction jotis has twcn grerLt,. Superintendents in the various plant ~ ~ O have /md roru;iilerable previous experience. Appioxiinrtbely hulf o f the shift suyjervirom are experienced in the various operations which they supervisc; thc other half are recently gmdunt,erl engineers. Plant opelators s e r e loedly rocruitcd from tlro high school gradunte level and were employed W days prior to the expected on-stream date. Intensive clmroom instruction was given them in operating tcchniques. In addition Mississippi Chemical entered into an arrangement with eight cheniical conipaniea, having unit operations similar to some of ita own, where each of the operatom was given on-the-job instruction uu an operu1,ingunit. Operating
Probahiy tlre most important item in Mimisippi Chemical's reduction of operating costa w&9 tlro cut aehioved in operating and mairitennrico permiinel requirements. Compitrison of actual employment figures with t,hoso estimated for a conventional plant are:
Nis.iwippi Chemical Corp. 46
Adminintrative, clerical Ammonia plant Muinteasnee and service (inclrzdes nitrir x i d and ammonium nit,rateplants)
24 41
Conventional 61 80 65
Estimate for the mnventional plant included n u pwmrmcl for chemical oontral, engineering services, or shift aupeivision, nddition uf which would increhsl totals given. Breakdown on comparative cmts (three major items orrly) per ton of ammonia produced is:
Natural gas nnd puwer Salaries and sages Pixed costs 011 capital
Mississippi Cilemid Corp. $12.50 4.95 13.50 $30.95 ~
Convcntiorial
5 9.90 13.40 23.40 .~
$46.70
Current estimates show ttiat N; cost per ton of ammonia in existing oonventional plants of $30 to $37 may he considered a proper range today. With a freight advantage of S5.00 for Mississippi Chemical on ammonia delivered in the state, a permissible range of $35 to 942 pcr tori of ammonia would he allowed for comperitive snfety of the Yazw, City operation. It is appnrent, thco, that thc origirml objective has been achieved. FC'TI!RE PROSPECTS
Agrieultural prsetiax farvoiing use of fertilizers are spreading ( d ) , rind the increm in the demand for nitrogenous fertilizersappearseven greater than for thoot.her major plant nutrients. Since animoniii and amnioriium nitrate productioli itre inextricably tied togpthw, no discussion of the future of mrmonia as a fertil i a m sotITFp a.,,,,ld ha onmnlda & + h n d ----:.,-+:--I AI_.
U F
263
264
INDUSTRIAL AND ENGINEERING CHEMISTRY
ammonium nitrate situation. More than 1,000,000 tons of ammonium nitrate were produced for the fertilizer trade in 1950 ( 2 ) , a new all time record and an increase of 17.5% over 1949. Yet, a recommendation was recently made to a House subcommittee by the Production and Marketing Administration that this country still needs an additional 500,000 tons of nitrogen fertilizer capacity annually. A dramatic illustration of the general increase in demand for all plant nutrients is obtained from statistics (6) that show how the Corn Belt states of the North Central Unikd States have just become seriously interested in fertilizers since the start of World War 11. Agricultural experts do not expect a reversal of attitudes in this region even in a future depression, although a general reduction of fertilizer purchases countrywide may be anticipated whenever general farm income declines. While the basic demand for fertilizer nitrogen looks promising, a vigorous debate can be started as to the preferred form in which the nitrogen will be supplied when a buyer’s market returns, as anticipated, in the near future. The coincidence that ammonium nitrate is both fertilizer and munition has meant that a tremendous plant capacity for this form now exists in the United States. Availability of ammonium nitrate when the farmer was crying for fertilizer nitrogen has firmly and widely established its acceptability within the span of a few years (9). Nevertheless, some commercial fertilizer manufacturers apparently believe that ammonium sulfate will win out in a competitive market. This is true in spite of the lower current price of ammonium nitrate per unit of nitrogen and the fact that ammonium sulfate eventually has a harmful acidifying effect on most soils whereas ammonium nitrate is much better in this respect. With everything else equal, time should favor the more concentrated nitrogen compounds, and as the most concentrated solid nitrogen carrier now in the economic range of volume *fertilizer distribution, ammonium nitrate is in the most favorable position. Anhydrous ammonia is both the most concentrated and the least expensive form of nitrogen that can be applied t o the soil. Tremendous interest in the direct application of ammonia has developed in the past few years, and the product has been found very promisingin some applications when added in irrigation water and when applied directly to the soil. However, present-day qualified opinion seems to be that the special equipment and the handling problems will discourage its use on all but the largest farming enterprises. It is possible that farmer cooperatives or local fertilizer dealers eventually will develop a service for applying ammonia to the soil for a fee. One such instance has been reported (J), in which the application fee was $ N O per acre. A somewhat comparable development for solid fertilizers is under way by the Southern States Farmer Cooperatives; bulk fertilizer is distributed to the fields directly from trucks a t a total cost about equal to the price of the fertilizer, f.0.b. warehouse in bags. Projecting a little beyond the horizon, new commercial fertilizers o? even higher concentration are entirely possible, especially materials such as ammonium phosphate, potassium metaphosphate, and nitric acid superphosphate, which contain more than one plant nutrient. TVA ( 1 2 ) has carried a project for diammonium phosphate (21% nitrogen, 54% phosphorus pentoxide) production from phosphoric acid and ammonia through pilot plant stage, and it is interesting to know that the ammonium nitrate crystallization plant there can, the Authority estimates, be converted readily to a 100-ton-per-day pnoduction unit for diammonium phosphate. Food Machinery and Chemical Corp. has developed a process for direct thermal fixation of atmospheric nitrogen and is installing a unit a t Lawrence, Kan., for the Army Ordnance Department. This process was investigated earlier at the University of Wisconsin and as an Office of Production Research and De-
Vol. 44, No. 2
velopment project by TVA. Problems involving refractories capable of withstanding thermal shock at the process temperature above 2200” C. were found to be the principal technical difficulty to contend with in these investigations. Such a development might make obsolete present-day nitric acid processes and perhaps even thb synthetic ammonia method from which fixed nitrogen products are now largely derived. The thermal fixation process in itself will not be very likely to bring a radical change, however, in the kind of nitrogen fertilizers that might be offered to the farmer of the future. The problems of storing anhydrous ammonia for agricultural use have already been mentioned. Mississippi Chemical Corp. has some ideas for improvement in this area. To the high costs of storage either in pressure vessels or refrigerated Hortonspheres must be added inspection and maintenance costs. Mississippi Chemical is working from the angle of storing ammonia as an aqueous solution (25% to 30%) in conventional mild steel tanks and subsequently distilling as required. Estimates a t current equipment costs are $60 per annual ton for storage and $1.50 to $2.00 per ton stripping costs. ACKNOWLEDGMENT
The authors gratefully acknowledge the cooperation and help of the Girdler Corp., designers and constructors of the Mississippi Chemical Corp., the Societt? L’Air Liquide and its companion company Soci6t6 Chimique de la Grande Paroisse, and the Mississippi Academy of Science, which permitted free use of material presented before its 1951 annual meeting. Appreciation is also expressed to the following individuals whose advice during preparation of the manuscript was very helpful: W. B. Dunwoody, chief engineer, Mississippi Chemical Corp. ; J. D. Gordon, chief engineer, The Girdler Corp.; Charles W. Gibbs, IngersollRand; and J. H. Bennett, Westinghouse Electric Co. LITERATURE CITED
Andrews, W. B., et al., “Ammonia as a Source of Nitrogen,” Mississippi Agricultural Experiment Station, State College, Miss., Bull. 448 (1947). Chem. Eng. News, 29, 824 (1951); 30, 206 (1952). Comnt. Fertilizer, 76, No. 3 , 30 (1948). Cope, W. C., Chem. Inds., 64, No. 6, 920-5; 65, No. 1, 52-6 (1949). Fertilizer Rev.. 23. No. 3. 13-16 (19481. Miller, A. M:, and Junkins, J. -N., Chem. & Met. Engr., 50, NO.11,119-25 (1943). Mitchell, Guy S., Petroleum Refiner, 25, 245-57 (1946). Sawyer, F. G., Chem. Eng. News, 26,3258-60 (1948). Soholl. Walter, Mehrina. - A. L., and Wallace. Hilda M.. Ibid.. 26,986-90 (1948). Shearon, Will H., Jr., Seestrom, H. E., and Hughes, J. P., IND. ENG.CHEM.,42,1266-78 (1950). Shreve, R. Norris, “Chemical Process Industries,” New York, McGraw-Hill Book Co., 1945. Thompson, H. L., el al., IND.ENG.CHEM.,41, 485-94 (1949): 42,2176-82 (1950). Thompson, H. L., presented before American Institute of Chemical Engineers, regional meeting, Kansas City (May 13-16,1951). Processing Equipment (1E) Chemical Engineering Catalog, New York, Reinhold Publish-
ing Corp., 1948-9, Bethlehem Foundry and Machine Corp., shells for high pressure reaction vessels. (2E) The Girdler Corp., Louisville, Ky., Cat. G18-2-146, Girdler hydrogen units. (3E) The Industrial Insulators, Inc., 1121 Rothwell, Houston, Tex., 25 and 35 1/z hard mill finish aluminum sheeting. (4E) Owens-Illinois Glass Co., Kaylo Division, Toledo 1, Ohio, heat insulating blocks. (5Ej Penn Tool and Forge Inc., 1015 N. Front, Philadelphia, Pa., high pressure forgings for separators. (6E) Wyatt Metal and Boiler Works, Houston, Tex., low pressure reaction vessels. RECEIVED November 27, 1951.
