US3686373A - Urea-prill anti-caking process - Google Patents

Abstract

Hot urea prills can be made to resist caking by cooling them quickly under quiescent conditions. To accomplish this, the hot prills are collected in a cooling chamber and a flow of air at a temperature not exceeding about 130* F is directed through the prills so that the prill temperature is quickly reduced. Throughout the cooling step the prills are maintained in a quiescent state so that a glassy surface is maintained on each prill. Such surface helps reduce subsequent caking.

Description

United States Patent Griesheimer et al.

[ Aug. 22, 1972 22 Filed:

I [54] UREA-PRILL ANTI-CAKING PROCESS [72] Inventors: George T. Griesheimer, 496i Normandy Ave., Memphis, Tenn. 38117; Joe I. Chance, 4142 Fanon Ave., Memphis, Tenn. 38122 Feb. 9, 1970 [21] Appl. No.2 9,835

52 us. c1 ..264/14, 264/7 [51] Int. Cl. ..B01j 2/04 [58] Field of Search ..264/14, 7', 34/13, 20, 62

[56] References Cited UNITED STATES PATENTS 3,049,420 8/1962 Weiland ..264/14 3,130,225 4/1964 Friend ..264/14 2,887,724 5/1959 Bettes, Jr ..264/14 FOREIGN PATENTS OR APPLICATIONS 852,650 9/1970 Canada ..264/14 Primary Examiner-Robert F. White Assistant Examiner-J. R. Hall Attorney-Charles L. Harness and Kenneth E. Prince [57] ABSTRACT Hot urea prills can be made to resist caking by cooling them quickly under quiescent conditions. To accomplish this, the hot prills are collected in a cooling chamber and a flow of air at a temperature not exceeding about 130 F is directed through the prills so that the prill temperature is quickly reduced. Throughout the cooling step the prills are maintained in a quiescent state so that a glassy surface is maintained on each prill. Such surface helps reduce subsequent caking.

9 Claim, 7 Drawing Figures PATENTEDAUBZZ I972 SHEEI 1 0F 4 George 7. Gr/es/r e/mer Joe I. Chance INVENTORS BY w... L. H-

AT'TURNI'IY PATENTEDwszz I972 SHEET 2 0F 4 mm N mat NM,

Joe Chance INVENTORS ATTORNEY;

PATENTEUnuczz I972 AIR FLOW, THOUSAND STANDARO CUB/C F? MINUTE SHEET 3 UF 4 50 I 45 AIRFLOW Vs AIR Tam ms:

m ms UREA PRILL COOLER or Fla 3 ans/s: 4oq TN% UREA PRILL Pn/LLs m0 I60'F 35 PRILLS our-0 mo r NET HEAT nsuovzo 781,800 any/m. 3o

so so 00 AIR mm 0155, "F

George .7. anbshe/mer Joe 1 Chance @wa a L. Mam y INVENTORS PATENIEDwszz I972 SHEET 6 0F 4 b bbx INVENTORS George 7. Gneshe/mer SIVOUJIN W67 77/Ud I Joe Chance ATTORNEY UREA-PRILL ANTI-CAKING PROCESS This invention relates to the prevention of caking of urea prills.

Urea prills taken directly from the prill tower and bagged tend to cake badly on storage, and this tendency to caking is greatly exaggerated by the modern warehouse practice of stacking bags of prilled urea six or eight tiers high on pallets.

Even in short-term storage, bags of conventionally prepared urea prills frequently will be found with more than 90 percent of the urea solidly caked. Caking is an expensive nuisance, both for the farmer and for the chemical user. It is particularly annoying in connection with farm fertilizer spreaders, for few farmers have any equipment for breaking up solid chunks of urea. The chemical user finds it necessary to regrind badly caked urea, and must put up with dusting problems which frequently he is ill equipped to handle.

Normally, the prills, after falling to the bottom of the prilling tower, are elevated to a rotary cooler and inclined tubular device through which a current of air is forced as the prills roll down the incline to the cooler exit. To assure cooling, the tube is rotated, spilling quantities of prills through the air flow after the manner of a concrete mixer. 1

An examination of the product of this process will show that the prills are significantly abraded, for very fine discrete particles will be observed sticking to the surface of each prill, and the hard glassy surface which otherwise envelops the prill as it falls to the bottom of the prilling tower has been injured.

A reasonable explanation of the tendency of prills to cake is that the very fine powdered material which surrounds the prill absorbs atmospheric water or water vapor. The moisture subsequently dries out to indefinite degrees, causing the dissolved and re-dried urea to form a cement between particle and particle, which locks the whole into a rigid, stubborn cake.

We have discovered that if prilled urea, immediately upon its exit from the prilling tower, is run to a device where the prills may rest in a substantially quiescent state until they have reached a low temperature at least as low as 130 F. and preferably even room temperature, when the prills are taken from such a cooling device, their surfaces remain glassy and unabraded, and are relatively free of urea dust. Such prills resist caking much better than-do the prills which are passed through a rotary drum type cooler. In fact, conventional liquid anticaking agents (as hereinafter discussed) can be added to these quick-cooled prills in conventional rotary mixing equipment, and negligible abrasion will result. Preferably, of course, the liquid additives are added to the prills while minimizing prill-toprill contact.

We have found it preferable, and have determined that very effective non-caking conditions can be established if, in addition to the quick-cooling provided by this invention, the prills, as they fall into, through, or out of the cooler, are coated with a thin, possibly only a mono-molecular, film of a liquid which is strongly adsorbed on the urea prill surface. The precise action of the treating fluid in greatly reducing the tendency of the prills to cake is not completely understood, for essentially hydrophobic liquids, slightly soluble liquids, and liquids which are infinitely soluble in water, have been found to be effective anticaking sprays for our quick-cooled prills.

After the prills have been cooled by the process of this invention and then optionally sprayed or coated with a liquid as described, the prills may be bagged, handled in normal manners both in freight transport and in warehousing, and, particularly in warehousing, can be compacted with considerable weight. When bags of urea treated in this manner are opened, instead of the hard lump or lumps which otherwise would be found, a very substantial amount of the contents of the bag will be free-flowing prills. The lumps which do exist are not the hard and stubborn concretions which previously have been experienced, but rather are aggregations of loosely adherent particles which break up with little mechanical effort.

It is the object of the present invention to eliminate, so far as possible, caking and maintain the free-flowing characteristics which originally were possessed by the prills at the moment of their manufacture. The process by which we achieve superior results is illustrated in the accompanying drawings wherein:

FIG. LA is a diagrammatic representation of a cooling method, showing the batch (or static) cooler in the prill-receiving position.

FIG. l-B shows the same cooler with cover and air tubes attached.

FIG. l-C shows the batch cooler discharging the cooled urea prills.

FIG. 2 is an elevated view of the static cooler with part of its exterior cut away to show a portion of the retaining screen.

FIG. 3 is a flow diagram of a continuous or semistatic urea prill cooler system.

FIG. 4 is a graph showing air flow versus air temperature rise in a moving urea prill bed such as that of FIG. 3.

FIG. 5 is a graph showing terminal velocity of urea prills.

Turning now to FIG. l-A, the static cooler 10 receives prills which have been lifted from the bottom of the prilling tower 11 to said cooler by an elevator belt 12. The prills fall into the cooler and accumulate on a support screen 13 until their level reaches the top of the sight glass (shown at 23, FIG. 2) or other suitable level. The elevator belt is then shut off or the flow directed to an empty cooler (not shown).

Turning to FIG. l-B, air tubes 14 and 15 are attached to air ducts 16 and 17 respectively. Air, suitably at 60-85 F. is forced into the cooler through 14 and exits through 15.

Air cooling is continued until the temperature of the prills falls at least down or to 130 F., and preferably to a temperature in the range of -l 10 F. When the prills reach this lower temperature, (requiring 10 minutes to 1 hour), air flow through the side arm 14 is cut off, the tubes are disconnected, and the static cooler is up-ended by being rotated on its trunions, 22 through half a revolution.

When the cooler is upside down, as in FIG. l-C, the prills flow out of the pipe 17, optionally on to a sizing screen (not shown) and then pass along any desired form of transport apparatus 18, to a conventional bagging device (not shown).

In FIG. 2, the batch cooler is shown generally at 10. It is equipped with a separable conically shaped cover 19 which has duct 17 for the attachment of an air tube. (Such attachment is shown in FIG. L3.) The cover is removed when the cooler is receiving prills and is replaced when cooling air is passed through the cooler. The cover 19 is conveniently fastened to the cooler by means of nuts and bolts (not shown). After the cooler is filled with prills to the desired level, the cover is fastened on and cooling air is passed through the cooler. After the cooling step, the cooled prills are discharged. For this step, the air tubes are disconnected and the cooler is inverted, turning on trunions 22 which rest in cooler support 21.

The above described batch static cooler outlines the invention in its simplest aspect. In large scale commercial practice, essentially the same concept is applied on a continuous basis, with prills continuously entering and leaving the cooler, meanwhile presenting a bed of prills to a cooling air stream. The air stream is recooled and recycled. This embodiment is set forth diagrammatically in FIG. 3 and is described below.

The main piece of equipment used in operating this invention continuously is the urea cooler shown generally at 30. This cooler is about 8 feet in diameter by 8 feet long (straight side). The cooler is covered by a dome, 31 which contains an entrance port for the urea prill feeder 32. The urea prill feeder is a tube equipped at its exit end 33 (within the cooler 30) with a hinged gate (not shown) which is pushed open by the fall of prills down the tube, but which closes shut by its own weight at other times to prevent the escape of vapors back up the feeding tube 33 during the operation. The cooler 30 is equipped with a motor-driven paddle 34, which is a sensing device. This paddle turns at a rate of about 5 rpm, and during the feeding operation, when the level of urea prills rises to meet the paddle, the torque on the paddle increases, thereby actuating electrically the motor-driven star valve 35 at the bottom of the static cooler, to increase its delivery of cooled prills.

In one embodiment of this invention the star valve 35 controls the entrance of the urea prills to the cooler 30 in the following manner: when the drag on the paddle 34 is zero, the star valve will not move; when the paddle drag is normal, the star valve will move at a normal rate; and, when the paddle drag is maximum, the star valve will move at a rate in excess of the capacity of the feed belt supplying urea prills. Other variations, include an on-ofl operation with the star valve capacity larger than the belt feed rate or a high level and low level sensing element to either stop the feed belt or star valve as required.

Preferably, the depth of prills in the cooler 30 is kept at about 3.5 feet. Under the preferred conditions, the air velocity through this bed will be about 2.74 feet per second, which permits the fall into the bed of all prills except those which have a diameter below 140 microns. Since prills smaller than 140 microns occur only in very small amounts in normal prilling operations (about 1 part per 1,000), such undersized material is simply swept around in air recycled to the air cooler 36. A bed of this size for the apparatus stated will provide a prill volume of about l73 cubic feet. Air at about 17.0 psia from an air cooler 36 at 70 F. and at percent relative humidity (i.e., 37,900 lb/hr dry air admixed with 606.4 lb/hr water vapor) is transported through a pipe 37 to the prill cooler 30 via an air inlet port 38, and an air sparger 39, which preferably discharges downward to avoid plugging its air holes with fine urea particles. The cool air passes through the prill bed and is recovered at substantially atmospheric pressure on the top side of the bed, with a lowering in pressure of 3.17 psi. Under equilibrium conditions, average residence time of the prills in the cooler is about 15 minutes. When the operation reaches equilibrium, the prills leave the cooler 30 at about 100 F. The cooling air leaves the cooler at about F. and with a relative humidity of 6.6 percent, through the air exit port 40 at 8,260 standard cubic feet/minute and is transported to an air blower 41 at atmospheric pressure. The air blower has a capacity of 382 brake horsepower. It raises the temperature to 215 F. and the pressure to 21.07psia. The air blower employed in this invention has the effect of compressing the air, thus causing the air to exhibit a rise in temperature.

After passing through a butterfly control valve 42, which controls the flow of air, which in turn controls the temperature of the prills exiting the cooler, the air passes into the bottom chamber of air cooler 36, which has a 6-foot diameter, is 30 feet high (straight side) and contains 625 cubic feet of 2-inch raschig rings (a packing of 22 feet). The air is cooled by circulating 100 gpm of 25 percent urea solution downward through the packed bed air cooler, countercurrent to the air being cooled. The air passes upward through the packing 48, through a demister 49, and is recycled to the cooler 30 via line 37. The urea solution enters the cooler at an inlet 43 at 60 F. and exits via outlet 44 at 97 F. whence it is pumped via pump 45 to urea chiller 46, which is refrigerated by means of an ammonia refrigeration unit (not shown). The injection system 47 provides optional urea coating agent which, when used, discharges into air line 37 and enters the cooler 30 at the air inlet port 38.

FIG. 4 shows the air flow versus air temperature rise in a moving bed prill cooler such as cooler 30 shown in FIG. 3. This curve is based on the heat necessary to be removed to cool the 400 T/D of urea prills at the given conditions. In this case the net heat removed is 781,800 Btu per hour, and the design air flow rate is set for an 80 F. air temperature rise.

FIG. 5 shows the terminal velocity in feet per second versus urea prill diameter in microns. This curve will give an indication of the prill size that will carry over in an air stream of a given velocity. FIG. 5 is actually a combination of two graphs, one depicting Stokes Law and the other an intermediate law. The curve shows both laws plotted on log log scale. It has been found that Stokes Law is not applicable when the prills diameter is greater than about 200 microns. It has also been found that when the prill size is greater than 400 microns, the terminal velocity of the prills in feet/second q l 0 415 g 0.5 where 1) equals the diameter g equals the specific gravity of the prills, in this instance 1.335. The area from 200 to 400 microns represents a transition zone where neither law is in all cases, entirely accurate. However, the curves have been joined and it has been found that there is not more than about a 12.5 percent variation from numbers indicated by the curve. For a velocity of 2.74 ft./sec. in the prill cooler, all particles smaller than 140 microns will be carried overhead.

As we have stated, anticaking characteristics of prills cooled by the process of this invention, although good, are further improved by the use of conventional anticaking agents, particularly liquid agents. The agents I herein discussed are particularly useful when applied at amounts in the range of 0.00250.5 percent based on the urea. Among liquids, it has been found that n-octylamine is one of the best for optional use in this invention.

n-Octylamine is commercially available as an amine mixture which comprises approximately 92 percent noctylarnine. At the temperature of application, usually 68-70 F., its viscosity is suitable for application either in a rotary mixer or by a pressure spray system. It is preferably applied to the prills at a level of 0.01-0.2 percent by weight.

Other particularly advantageous treating liquids are the monoethers of polyethylene-polypropyleneglycol, which are available commercially as the so-called U- con fluids. The fluids are more and more hydrophobic as the proportion of the polypropylene moiety is increased. Ucon fluids prooved effective in extremely small proportions (0.0025 to 0.05 of 1 percent).

Another class of fluids contributing anticaking characteristics to the prills is ethyleneglycol monobutyl ether, which is effective in ranges of from 0.01 to 0.05 of 1 percent by weight of the urea.

Upper limits for the weight of additives are not imposed for chemical reasons, but are based on economics. The small increase in freedom from caking which is brought about by more than the amounts stated does not appear to warrant the cost of the extra treating agent involved.

EXAMPLES The static cooling unit described above in connection with FIGS. l-A, l-B, l-C, and 2 consisted of a inch X 20 inch X 38 inch plywood box with flanged connections at both top and bottom for air introduction and removal. Two 50-mesh screens were mounted internally to support the prill charge. A forced air mover provided the necessary movement of air through the urea prill bed. Connections were provided for temperature and relative humidity instrumentation and for pressure differential measurements across the urea prill bed.

A hundred pounds of urea prill product exiting a commercial prill tower and having a temperature of about l50-l60 F. was charged into the static cooling unit. The inlet air temperature was adjusted to range from 73 F. to 130 F. Time required to cool prills to set temperature (see Table I below) was approximately 13 minutes.

After reaching the desired temperature, 50 pounds of urea prills were packaged into standard 50-pound shipping bags. The test material was placed on a test platform in the warehouse and weighted with 1,000 pounds of concrete blocks for 7 days. The weight was then removed, and the bag carefully placed on 4-mesh screen mounted on a shaking device (a Gump Shaker). One side and both ends are opened and the contents through the screen at this point is discarded.

The Hardness Index is the time in seconds required for the lumped material to be abraded through the 4- mesh screen by operating the Gump Shaker at rpm. All material so abraded through the screen is collected and weighed. The weight is expressed as percent Lump (on the starting weight of 50 pounds).

Thus, there is an inherent correlation in percent Lump and Hardness Index, and it will be obvious that, in a qualitative way, the greater the caking, the greater the percent lumps, and the greater the Hardness Index. All three characteristics are substantially lowered by use of the static cooler of this invention.

Fifty pounds of urea prills from a standard commercial rotary drum cooler were packaged into standard 50-pound shipping bags and subjected to standard warehouse caking and hardness tests. (Examples 1 and 2 in Table I below.) The resulting difference in caking tendency and hardness of cake in prills from the commercial cooler and prills from the static cooler of this invention (Examples 3 and 4 is shown in Table I.

As already mentioned, further improvement in anticaking is obtained by adding certain liquids to the urea prills cooled in accordance with this invention. These liquids can be added to such prills in any conventional liquid-solid mixing apparatus. A continuous r0- tary mixer is suitable. For small batches, even a concrete mixer can be used. Results of runs using these liquids, are given below. The additives were used at 0.01 percent level.

NOTE:

UCON LB is polyethylene-polypropylene glycol monoether, available commercially from Union Car bide Corp.

ARMEEN 8D is an amine mixture, 92 percent noctylamine, 3 percent hexylamine, and 5 percent decylamine, available commercially from Armour Chemical Co.

EGWE is ethylene glycol monobutyl ether.

As the terms are used herein, static cooling refers to a batch cooling operation in which the prills do not move while they are being cooled; and semi-static cooling or continuous cooling refers to a cooling operation in which cooling air is passed through a constant-depth bed of prills, which bed is discharging prills below at substantially the same rate as it receives prills from above. Both types of cooling, as applied in this invention operate in the same way, with minimal abrasion to the prill surface.

'In certain preferred embodiments of this invention the hot urea prills are received into the cooling chamber from the prill tower at a temperature in the range of about 150-160 F. Cooling air is passed through the prill bed at velocities sufficient to cool the prills, or at least the exiting prills, down to a temperature within the range of about 70-130 F., and more preferably down to a temperature in the range of about 70-8O F. This cooling is accomplished by using a stream of air having a temperature of 130 F. or below, e.g., down to 35 F. or even lower, and more preferably a temperature below about 6080 F. In general, the lower the temperatures of the cooling air, the better,

but of course it should not chill the prills to the point where they will condense moisture from the ambient air. The residence time of the prills within the cooler can be 10-60 minutes, but is preferably about -30 minutes.

What is claimed is:

1. In a process for preparing urea prills of reduced caking in a heated prilling tower which provides hot solid prills and then cooling said prills, that improvement which comprises collecting the hot solid prills in a closed chamber cooler and directing a flow of air at a temperature not exceeding about 130 F. through the prills at an air velocity such that the prill temperature is reduced to below about 130 F. within about l0-60 minutes, and throughout the duration of cooling maintaining the collected prills in a substantially quiescent state, whereby an unabraded glassy surface is main- I tained on each prill and subsequent caking is reduced.

2. The process according to claim 1, in which the closed chamber cooler is a static cooler in which the prills do not move while they are being cooled.

3. The process according to claim 1, in which the temperature of the air passed into the cooler is in the range not exceeding about 6080 F. and the temperature of the prills entering the cooler is about 150-160 F. and leaving the cooler does not exceed about -130bL F.

4. The process according to claim 1, in which the cooling chamber is a continuous cooler in which the prills are constantly being received by said cooler and discharged by said cooler at the same rate, without substantial abrasion.

5. The process according to claim 4, in which the cooling air enters the cooler at about 70 F. and leaves it about F.

6. The process of claim 1, in which the cooled prills are treated with 0.0025-0.5 percent of a liquid anticaking additive selected from the group consisting of polyethylene-polypropylene glycol monoether, n-octylamine, and ethylene glycol monobutyl ether to coat the prills with said additive, to reduce caking further.

7. The process of claim 6, wherein the prills are treated with 0.01 to 0.2 percent of n-octylamine.

8. The process of claim 6, wherein the prills are treated with 0.0025 percent to 0.05 percent of the monoether of mixed polyethylene-polypropyleneglycols.

9. The process of claim 6, wherein the prills are treated with 0.01 to 0.5 percent of ethylene glycol monobutyl ether.

Claims (8)

1. 2. The process according to claim 1, in which the closed chamber cooler is a static cooler in which the Prills do not move while they are being cooled.
2. 3. The process according to claim 1, in which the temperature of the air passed into the cooler is in the range not exceeding about 60*-80* F. and the temperature of the prills entering the cooler is about 150*-160* F. and leaving the cooler does not exceed about 70*-130* F.
3. 4. The process according to claim 1, in which the cooling chamber is a continuous cooler in which the prills are constantly being received by said cooler and discharged by said cooler at the same rate, without substantial abrasion.
4. 5. The process according to claim 4, in which the cooling air enters the cooler at about 70* F. and leaves it about 150* F.
5. 6. The process of claim 1, in which the cooled prills are treated with 0.0025-0.5 percent of a liquid anticaking additive selected from the group consisting of polyethylene-polypropylene glycol monoether, n-octylamine, and ethylene glycol monobutyl ether to coat the prills with said additive, to reduce caking further.
6. 7. The process of claim 6, wherein the prills are treated with 0.01 to 0.2 percent of n-octylamine.
7. 8. The process of claim 6, wherein the prills are treated with 0.0025 percent to 0.05 percent of the monoether of mixed polyethylene-polypropylene-glycols.
8. 9. The process of claim 6, wherein the prills are treated with 0.01 to 0.5 percent of ethylene glycol monobutyl ether.

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