508
INDUSTRIAL AND ENGINEERING CHEMISTRY
outlet, or the like, is incorporated, be sure it is welded in such manner that none of the corroding substance can reach the backing steel. If a removable head is attached by steel bolts through a flange on a shell and the fit is not perfect, the steel bolt may be eaten out; rust may settle on the stainless surfaces and start electrolytic action. Thus, it is well to use a corrosion-resistant packing ring; in some cases a ring of solid stainless steel is inserted between the removable head and the flange, or stainless steel bolts may be preferred. There are so many places where continuity of cladding is essential that we cannot attempt to cover them in any detail or even indicate the main ones. The number of cases where this important precaution had not been observed is amazing.
Conclusions The precautions outlined are indicative rather than complete, There may be others that are known and observed by users and fabricators alike. I n any case, it is assumed that anyone using such material for the first time will seek the counsel of an accredited supplier. Certain precautions have to be observed with any material, especially a relatively new one. None of the precautions exceed reasonable commercial practice, and the fact that substantial quantities are being fabricated by all kinds of shops indicates that proper handling is neither impossible nor burdensome. In fact, many of the suggested precautions are self-evident and are mentioned merely to make this article more complete. Precise figures as t o the amount of stainless-clad steel now in service are not available, but as closely as we can estimate about 21,500 tons are giving satisfactory service. Yet those who produce stainless-clad and some of the larger users believe that this is a mere beginning. One of the producers spoke of utilizing clad for everything from “tin whistles t o battleships”. Perhaps there is some exaggeration in this broad coverage, but it is undoubtedly true that only a few of many applications have been developed. Whereas the present tonnage for the most part goes into the process industries, the
Vol. 33, No. 4
decorative, building, transportation fields, etc., are to be considered as well as places where stainless-clad may be used as a substitute for various materials other than solid stainless steel. The first and most obvious place to look for valid applications is where solid stainless steel is now used; by no means have all of these possibilities been exhausted. It is suggested that purchasing and engineering or research departments might, to their mutual advantage, cooperate more closely on stainless-clad steel. The purchasing men are attracted by the potential savings but often hesitate to involve themselves in negotiations with their engineers on design changes; the engineers, who are always busy, tend to resist a change in material where the unit is giving satisfactory service. It may mean some redesigning, but savings in material cost of as much as 45 per cent are not often obtainable. It has also been observed that some purchasers have resisted stainless-clad because of possible past difficulties in their own plants or because someone they knew had trouble; in most of these cases the engineers of the buyer and the consultants of the supplier may not have given the matter thorough consideration. Or the material, a t the time offered, may not have been comparable to that offered today. A good consultant for any of the accredited producers of stainless-clad can undoubtedly walk through a processing plant with an engineer and find a variety of potential applications; often the engineer may not have considered some of them. We do not mean to infer that stainless-clad is a cureall; nor is it true that every plant is sure to have a potential use. But its possibilities have not been exhausted nor has more than a good start been made except in a few isolated cases. It would seem self-evident that the plant which takes advantage of the savings made possible by using stainlessclad will have a competitive advantage on a cost basis. Those who make use of the material t o yield a better product, when substituting i t for something other than solid stainless steel, should also have a competitive advantage.
Rotarv Cooler for Ammoniated Fertilizers E. F. HARFORD AND F. G. KEENEN E. I. du Pont de Nemours & Company, Inc., Wilmington, Del.
which are strongly exothermic, are completed in a few seconds rather than days. These ammoniation reactions liberate heat in proportion t o the weight of ammonia added. Other slower reactions which may occur between the solid components of the mixture generate much less heat. Fertilizer manufacturers have long realized that to take full advantage of the rapid reactions a simple low-cost method of dissipating the heat developed would go a long way toward helping solve a number of problems.
Existing Cooling Operations
P
RIOR to 1928, practically all commercial fertilizers containing superphosphates were mixed and stored in bulk piles to complete the slow reactions between components. If this curing period was not completed, the fertilizer was probably in poor physical condition when received by the farmer. With the widespread use of the low-cost ammoniating solutions since 1928, some of these reactions, particularly those
For cooling solids, the process industries have used such equipment as water-jacketed screw conveyors, conveying screen coolers, and water-tube rotary coolers. Hot foundry sand has been cooled in a horizontal shell by controlled addition of water ( 3 ) . Calcined materials have been cooled in a rotary drum by transferring heat to the atmosphere through the shell or in a conveyor by spraying the material with water until the temperature reaches 212” F. None of these methods appeared adaptable to fertilizer cooling.
April, 1941
INDUSTRIAL AND ENGINEERING CHEMISTRY
I n fertilizer plants, rehandling the warm fertilizers from the mixer or storage pile through screening units, disintegrating mills, elevators, etc., has been tried. Also, merely moving the material from one storage pile to another by crane buckets, or spreading the goods in thin layers by mixing into alternate storage areas on alternate days are present practices. Rasping has been used. This consists in dropping the mixture into a revolving fan or cage-type disintegrator, thus throwing it through the air into the storage heaps. Inefficient air to material contact and insufficient air volume or time of contact inherent in these practices result in limited cooling. Two methods that have been proposed in the patent literature are a vertical aerator and a horizontal cooler (a).
Preliminary Cooling Attempts Evaluation of the cooling problem began with a test in a fertilizer factory with a centrifugal fan installed to blow air through an elevator conveying the warm fertilizer from the mixer to the storage pile. The volume of air was sufficient for adequate cooling, but since the air-to-material contact was so poor and the time was short, a temperature drop of only 0" F. occurred. Some experiments were made also with a cascade-type cooler made of corrugated iron flights, set at a 38" slope and supported by an open wooden framework. The total height was approximately 18 feet, which allowed the material to reach the bottom in 5 seconds. The temperature reduction of 12" F. obtained was very slight, considering the fact that the difference between air and fertilizer temperatures was 110" F. during the tests. Data were taken on the cooling obtained in pneumatic conveyors. Even though the fertilizer was held in the air stream only a fraction of a second, the air-to-material contact was so good that an adequate temperature drop was achieved. The disadvantages of this method include relatively high airhandling costs, inefficient use of air which amounts to 15 cubic feet per pound of fertilizer handled, a particle grinding or pulverizing effect in the high-velocity air stream, and lack of positive conveying action if the air stream is loaded or a plug occurs in the line.
809
Advantages resulting from cooling ammoniated fertilizers are substantial reduction in reversion of available phosphate, improvement of physical characteristics such as reduced caking and dustiness, lower handling costs obtained through increased equipment capacities, and quicker curing. The discovery that a large portion of the heat in an ammoniated fertilizer mixture could be removed quicldy by vaporizing a portion of the water inherently present, and the necessity of bringing about intimate contact between each fertilizer particle and the air, directed attention toward the rotary dryer type of equipment. Pilot-plant and full-scale operations have demonstrated that such a rotary cooler need be only one third the size indicated by rotary dryer design equations. This reduces the cost of such equipment to a feasible level for incorporation in fertilizer mixing and ammoniating operations. A horizontal rotary cooler 5 feet in diameter and 20 feet in length, using 6 cubic feet of air per pound of fertilizer, should drop the temperature of 30 tons per hour by 40' F. Operating costs have been estimated under 2 cents per ton of fertilizer, exclusive of fixed charges.
Small-scale Rotary Cooler Trials
FIGURE 1. EXPERIMENTAL ROTARY COOLER
The initial experiments on a small laboratory rotary dryer results in an unexpected degree of cooling, accompanied by the evaporation of 2.5 per cent water. This indicated that water vaporization played an important role in the fertilizer cooling operation, and led to the idea that cooling in a rotary dryer type of apparatus might be far more efficient than was previously believed. Additional design data and further substantiation of the water vaporization effect on cooling were obtained from operation of a rotary horizontal cooler, 20 inches in diameter and 8 feet long (Figure l), which had a capacity of about 1 ton per hour. As Table I shows, good performance was obtained even with 80-85' F. air a t high relative humidities. The most significant developments from these large-scale trials were the confirmation of the large amount of heat (50-60 per cent) removed by vaporizing some of the water normally present in the fertilizer, and the rapidity with which this occurred. The
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
510
importance of the moisture vaporization factor in this cooling process is illustrated by the amount of water vaporized at corresponding temperature drops when half the
dryer experience as well as recent observations on the cooler unit. An optimum range appears between 200 and 300 linear feet per minute. MOISTURE VAPORIZATION. Experience with coolers has shown a moisture vaporization of 0.5 to 1.5 per cent from material entering a t 130" to 150" F. The necessity for a sufficient volume of air t o carry away this vaporized moisture is a major consideration in design. The moisture content of fertilizers averages 5 to 7 per cent, with extreme limits of 3 to 10 per cent. This moisture is a t the temperature of the material (130-150" F.) and has a relatively high vapor pressure. As the air passes through the cooler, it is heated and water is vaporized from the solid material which raises the humidity. Since a t normal temperatures many fertilizer mixtures become hygroscopic around 65 per cent relative humidity, and a t even lower relative humidities a t higher temperatures, the exit air from the cooler will always be below these relative humidities. Figure 2 is a modified Carrier humidity chart showing dry-bulb temperature plotted against humidity in unit weight of water per unit weight-of dry air for 65 and 100 per cent relative humidity. The volume of air per unit weight material can be calculated from the following equation and the humidity chart:
FIQTJRE 2. MODIFI~D CARRIER HVMIDITY CHART
heat is removed by water vaporization, and the specific heat of material is taken a t 0.25 : Total Material Temp. Drop, F. 20 30 40 60 60
where 17 M HI
(Half Total Heat Lost) 0.25 0.38
0.60
Consequently, the volume of air required to remove the heat was reduced in proportion, and the cooler had to be only about one third as large as indicated by dryer design equations.
Ammoniated superphosphate Ammoniated triple superphosphate Single-strength complete fertilizer Double-strength complete fertilizer
0.24 0.28 0.23
air volume, cu. ft./lb. of material at inlet tempera-
ture and normal barometer moisture evaporated from material, yo humidity of inlet air, Ib. waterjlb. dry air humidity of exit air, lb. waterjlb. dry air
Under normal conditions, a t least 4 cubic feet of air per pound of material are required. T o keep the equipment as small as possible, not over 6 or 7 cubic feet of air per pound of material should be used. Performance data have shown that a t least half'tbe heat is removed by water vaporization. The volume of air required t o do this is sufficient t o remove the remainder of the heat by its own heat capacity.
0.63 0.75
In the design of horizontal rotary coolers for ammoniated fertilizers where the air flow is countercurrent to material flow, the following conditions, assumptions, and variables must be considered: INLETAND EXITMATERIAL TEMPERATCRE. The ammoniated fertilizer temperature may be from 120" to 200" F., d e pending upon the amount of ammoniating solution added; the usual range is 130" to 150" F. The exit material should he cooled t o a t least 110" F. and preferably below 100" F. AIR TEMPERATURES. Since most of the fertilizer is mixed between November and March, air temperatures probably would average 60" F. Winter operations in the North would encounter air of 32" F. or lower, and late spring in the South possibly 80" F. This factor is not so significant as might be supposed. It is largely counterbalanced by the greater moisture-carrying capacities of warmer air which tend to increase the importance of the vaporization factor. SPECIFICHEATS. The quantity of heat to be removed is dependent upon specific heat capacity of the material. Hardesty and Ross (1) determined the specific heats of ordinary fertilizer materials to be as follows:
=
= = H1 =
"/o Water Removed by Vaporization
Design Data and Equations for Rotary Coolers
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I
A=COOLER CROSS SECTIONAL AREA IN SQ.FT. PER TON FERTILIZER PER HR
FIGURE 3. RELATIONBETWEEN COOLER DIMENSIONS AND CAPACITY
COOLER DIMENSIONS.Having established the volume of air likely to be required, the diameter is defined by fixing the linear velocity of the air flow. The relation between cooler diameter and length is a comparatively simple function:
0.27 AL0.56
The mean specific heat of air is 0.24. AIR VELOCITY.This should be kept below 400 linear feet per minute to avoid excessive dust losses, on the basis of rotary
=
K
where A = cross-section area, sq. ft./ton fertilizer/hr. L = length, f t . K = constant depending on air velocity
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INDUSTRIAL AND ENGINEERING CHEMISTRY
April, 1941
TABLE I. SUMMARY OF COALING TRULSIN PILOPPLANT ROTARY FERTILIZER Temp., Material In Out
175 150 110
110 102 60
Air Velocity % relative in shell, Cu. ft./lb. humidity ft./min. material Inlet Exit
F. Air Out
In 85 82 57
190 160 270
124 116 78
14 13 22
75 74 40
32 40 48
Heat Balance, I). T. U./Hr. Heat t o Heat water to air vapor
Fertilizer Throu hput, ?6 water lb./fr. In out
1800 I900 1600
6.5 6.63 6.16
TABLE 11. SUMMARY OF FULL-SCALETESTS ON 4 PER CENT NITROGEN-%
PER
Heat removed
5.5 5.62 5.24
32,800 24,600 18,400
CENT Pa05-12
Air Temp., O F . Material I I1 Out In
Air
Out
Velocity in shel! Cu. ft.[lb. ft./m;n. material
18000 19:OOO 14,700
CENT KaO GRADE
Heat Balance, I). T. U./Hr. Heat to Heat Heat water removed t o air vapor
relative Inlet
PER
15,100 11,100 11,500
Fertilizer Throughput, % ' water Exit tons/hr. In out
Winter Conditions
149
94
26
108
280
4.3
48
70
21.0
5.81
4.74
625,000
248,000
460,000
3.75
2.72
524,000
145,000
398,000
Summer Conditions
169
119
80
128
280
4.6
72
46
Values for K have been found to be: Air Velocity Linear Ft./M:n. 200 300
From the humidity chart (Figure 2) the humidity of the inlet air is 0.003 pound of water per pound of dry air and of the exit air, 0.014 pound water per pound dry air. Volume of air per unit weight of material is:
K 4.7 3.5 2.2
400
19.4
v=
This function is shown graphically in Figure 3 for air velocities of 200, 300, and 400 linear feet per minute.
0.50 ,,6 (o.014
- o.oos)
=
6 cu. ft. air/lb. material
The total air volume is: (6 X 60,000)/60 = 6000 cu. ft./min.
Example of Design Calculations
A typical example of design calculations for the cooler required to handle 30 tons fertilizer per hour is as follows: Assume that the air enters the cooler at 40' F. and 65 per cent relative humidity; that 0.50 per cent water is vaporized, based on the fertilizer weight; that the air leaves the cooler at 80" F. and 65 per cent relative humidity; that a n air velocity of 300 linear feet per minute is used; that material enters the cooler a t 150" F.; and that material leaves the cooler a t 1IO" F.
The cooler diameter under the designated air flow conditions is:
?rD2
6000
800-4 D
= 5
ft.
The cooler length (K being 3.5) is: 3.5 =
L
L 0 . 6 6
x
7r(5)2
30 X 4 =
20 f t .
Installation and Operation of Factory Unit
7 I
p1
1 I \/
DUST
COLLECTOR
A- A
Tf
COOLER
SHELL
FIGURE 4. DIAGRAM OF FACTORY INSTALLATION
Figure 4 illustrates the manner in which a factory instaPation was assembled. The cooler took the material discharged from the mixer, cooled it, and returned it t o an elevator from where it passed to the stock pile. The air drawn countercurrent to material flow through the cooler was led into a cyclone dust collector and then out through the exhaust fan. The dust carried out in the air stream and caught by the collector depends considerably upon the specific nature of the fertilizer mixture, but a good average seems to be around 2 per cent of the total throughput at air velocities of 200-300 feet per minute. The mixing rate a t this factory had averaged 15 to 17 tons per hour, and it was estimated that the temperature of the material at this rate could be reduced from 150-160' F. to below 110' F. in the 4 X 25 foot shell. Actually performance was somewhat better than anticipated, and cooling below 100" F. was obtained even a t rates as high as 25 tons per hour in winter. Much of this increased throughput was due to the free-flowing nature of the cooled material and increased ease of handling in the elevator. The equipment and the material throughput had been estimated so that 6 cubic feet of air
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INDUSTRIAL AND ENGINEERING CHEMISTRY
would be used per pound of fertilizer. The increased throughput due to greater ease of operation reduced this t o 4.04.5 cubic feet of air per pound of material. Experience has shown in the full-scale cooler that the relative humidity of the air in the dryer is usually 65 per cent until the air temperature reaches 100-110" F., and then gradually decreases to 40 or 50 per cent a t 130" F. Table I1 contains data typical of the cooler operation for summer and winter conditions. In summer, with the cooler operated beyond estimated capacity, 72 per cent of the heat was removed by vaporizing water. If it had been feasible
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to increase the air volume while maintaining the same material throughput, then a lower material temperature would have been reached, and a larger percentage of the heat would have been transferred to the air.
Li terature Cited (1) H a r d e s t y a n d Ross, IND. ENG.CHEM.,29, 1283 (1937). (2) Saokett, U. 9. P a t e n t s 2,028,413 (1936); 2,174,896 2,174,897 (1937). (3) Smith, I b i d . , 2,188,798 (1940).
(1936);
PRESENTED before the Division of Fertilizer Chemistry a t the 100th Meeting of the American Chemical Society, Detroit, Mioh.
Urea-Formaldehyde Film-Forming Compositions' Air-Drying Films by Acid Catalysis T. S. HODGINS AND A. G. HOVEY Reichhold Chemicals, Inc., Detroit, Mich.
HE hardening effect of facturers of refrigerators, autoUrea-formaldehyde film-forming compoacidic catalysts on hydromobiles, bicycles, toys, and other sitions, hitherto used in high-baking important industrial articles, phylic urea-formaldehyde enamel vehicles, are now made to air-dry particularly when used with the condensation products was early by special acidic accelerators which are nondrying and semidrying alkyd recognized (W,21). It has often soluble in organic solvents, including resins. The new resins make been stated in the literature that possible not only greater harda pH below 7.0 tends to promote hydrocarbons. These accelerators consist ness without sacrifice in color, instability of an aqueous ureaprincipally of inorganic acid esters of alcobut also many new types of enformaldehyde solution and to hols; the latter contain at least 4 carbon amel such as the polychromatic cause gelation (22,24). Calcium atoms in length i n the alcoholic radical, finish described by Hicks ( 3 ) . chloride as a catalyst for heat which imparts solubility in organic solTo obtain conversion a t lower hardening aqueous urea-formaltemperatures than the presdehyde plywood adhesives was vents, and yet since the acid components ent industrial baking schedules, described by Ludwig (16) and are only partially esterified, a low pH of there appear t o be two prinby others. The authors (6, 7) the vehicle system may be maintained cipal lines of attack-the use of and others (19, 87, 28) have which renders gelation possible in a relaacidic catalysts and modification elaborated upon the effect of low tively short time. By the use of these by melamine (2,4,6-triaminop H by the introduction of acidic (18) l,3,5-triazine). Pollak materials, but until recently this acidic accelerators, not only can baking and Ripper (23) early described work has been mostly confined schedules be shortened and the baking the uses of small quantities of to the aqueous type of ureatemperatures lowered, but also air-drying melamine in urea-formaldehyde formaldehyde condensationproditself can be effected. condensation products. Reucts. Ludwig (16) and others cent discoveries (IO, 26) have (go), in preparing urea-formalshown that melamine modificadehyde-butanol resins, used a tion of urea-formaldehyde-butanol resins not only gives better volatile organic acid (formic) 'as the catalyst during the heat resistance, but also promotes faster setting and lower processing, but made certain that it was removed by volabaking schedules. The use of acidic accelerators with ureatilization before the final product was packaged in an effort formaldehydc-butanol rcsins may be considered as more to improve stability of the resin solution; thus, any catalytic desirable than their use with melamine-modified urea-formaction was stopped. aldehyde resins in lowering the baking temperature and in I n the past three years the butanol-etherified urea-formaleffecting air-drying, because of the stability characteristics. dehyde resins have achieved vast industrial acceptance Where cost and package stability are not major factors, it is (1, 4, 6, 11, 12, 17, 35). These resins have been widely probable that some combination of both lines of attack may adopted as part of the vehicle of baking enamels by manuproduce the ultimate results. 1 For previous papers in this series, see literature citations 4, 6, and 11
T
