by Westinghouse Research Laboratories, December 1973. EPA 650/2-73-048a. (NTIS PB 231-162) Kozin, V. E., Baskakov, A . P.. Khim. Tekhnol. Top/. Masel, 12 ( 3 ) , 4 I$ il O C7\ UVl I
.
Yang, W.-C., Keairns. D:L.. AlChESymp. Ser.. 70, No. 141, 27 (1974). Zenz, F. A . . 1. Chem. E . Symp. Ser.. No. 30, 136 (1968).
Receiuedfor review July 8, 1974 Accepted February 10, 1975
Supplementam Material Available. Tables 4-6 will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X -148 mm, 2 4 X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order for $4.00 for photocopy or $2.50 for microfiche, referring to code number PROC-75-259.
Hydrodynamics of Vertical Liquid-Solids Transport Ronald J. Kopko, Paul Barton,* and Robert H. McCormick Departmenf of Chernrcal Engmeerrng, The Pennsylvanra State Unwersfty, University Park. Pennsylvania 76802
Hydraulic lifting of solids from a bed through a 2.4-in. X 13-ft vertical t u b e was investigated using 0.306-in. alumina pellets and 0.0734- and 0.131-in. iron shot with three different nozzle feeders. Each volume of lift water carried from 0.02 to 0.20 volume solids. T h e lift line void fraction ranged from 0.95 to 0.74, which encompasses the range of dilute phase to dense phase transport. Wall friction represented only a few percent of t h e total pressure drop of 0.7 to 1.0 psi per foot. T h e Richardson and Zaki’s correlation was extended from sedimentation and static fluidization modes to t h e fluidized transport mode. Pressure drops were predicted within 6% accuracy.
There is a need for accurate means of predicting pressure drops, jet pump capabilities, and energy requirements for the transfer of large solid particles in a relatively dense fluidized state using liquid carriers. The most obvious uses of such technology are in the mineral processing industries. Another potential large-scale application is for the design of direct contact heat exchangers for scaling and corrosive fluids. Exchangers of this type using parallel tower heaters and coolers operated continuously have been described by Barton and Fenske (1970) for application in desalination of sea water. Troublesome scale is attrited from the particles continuously. The exchangers use vertical lift lines for transport of the particles from the base of one tower to the top of the next in order that the solids can be recycled for continuous operation. Several technological uncertainties were encountered in the design of these fluidized particle heat exchangers. How dense could solids be transported in vertical lift lines using liquid jet slurry feeders? What would the pressure drop be in “dense phase” transport? How high a static pressure head could jet slurry pumps operate against? This work was undertaken to obtain experimental data and develop correlations for this application. The approach was to build a heat exchanger tower and to investigate its hydrodynamics while transporting several different particles with water as the carrier fluid. Theory The total pressure drop A P T for the vertical hydraulic transport of solid particles, outside of acceleration regions, can be expressed by A P T = AP,
+
APf
+
AP,,
+
AP,,
+ AP,,
(1)
The sum of A P s and A P f is the total static head pressure drop and is given by 264
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
(1 -
E)&
+
EPf
or (1 -
E’(&
- Pi) + Pf
(2)
The pressure drop due to solids-wall friction, Us,has , been studied experimentally by Newitt et al. (1961). They concluded that slurries composed of particles having terminal velocities in the transition and turbulent flow regions give frictional pressure drops only slightly higher than that for water alone, and then only at very low velocities. The frictional losses are the same at high velocities. Flash photographs showed that the particles are randomly distributed a t low velocities but tend to move toward the center of the pipe at higher velocities leaving an annulus of almost clear water at the wall. The presence of this annulus explains why the pressure gradient is much the same as that for the water alone. Consequently, pressure drop due to solids-wall friction, AP,,, can be assumed negligible compared to the total pressure drop. Pressure drop due to fluid-wall friction, APf,, can be estimated using the Fanning equation
In eq 3, uf is the velocity of the fluid in the lift line under transport conditions and can be calculated knowing the void fraction, t . The equation of Drew et al. (1932) can be used to calculate the friction factor for turbulent flow in smooth tubes to within *5% accuracy (in the absence of solids). Since it is reasonable to postulate that solids collision frequency is proportional to solids fraction, the pressure drop, Us. due , to solid-solid friction or collision is believed to be inconsequential at the high void fractions
usually found during transport, This hypothesis is limited to uniform diameter and uniform density spherical particles. With the above constraints, eq 1becomes
the slip velocity, usl, is a predominant function of the physical properties of the liquid, the particle diameter, the distances between the particles, and the drag force, usl is the same regardless of the type of fluidized system. Therefore, eq 8 and 10 can be equated with the result us(sedimentation) = u,,(batch fluidization) =
In order to use eq 4 for the prediction of solids transport pressure drop, the lift line void fraction, t , must be known. Of the previous workers who have incorporated the void fraction in a total pressure drop equation, almost all either measured it experimentally or erroneously assumed that it was the same as the volumetric fluid fraction discharged. The former procedure is of design interest when it is shown that c is correlative with definable system variables. The procedure in this work for calculation'of the liftline void fraction is limited to heterogeneous particulate solids transport systems. Particulate or dispersive solids transport is the rule in liquid systems, although for small, high density solids the phenomenon of aggregative solids fluidization has been observed (Wilhelm and Kwauk, 1948). Solids greater than 50 in diameter, when transported in water, fall into the particulate transport category (Condolios and Chapus, 1963). Our technique is based on the Richardson and Zaki (1954) equation, which they originally formulated for sedimentation and batch fluidization. In their equation
14,
(11)
The term uc is in essence a superficial slip velocity, and for the general case ?4,
= EU,I
=
E(Uf - Us)
(12)
Combining eq 5 and 12 to eliminate u c , gives ?liE"
=
E(Uf
- I(,)
(13)
By inserting the following substitutions
(14) (15)
the final working equation for determining the void fraction, t , in terms of known parameters is obtained.
The term ui was defined for batch fluidization by Richardson and Zaki (1954) as an adjusted terminal velocity
(5) uc is defined as the falling velocity of a sedimentary suspension relative to a fixed plane, or the empty-tube water velocity during batch fluidization. To attempt application of eq 5 to vertical solids transport requires a broader definition of the term uc. This term can be shown to be directly related to the liquid-solid slip velocity, uSl,which is fundamental to all modes of fluidization and is defined by the equation
where uf and us are the actual fluid and particle velocities, respectively. In a sedimenting fluidized system, where the solids velocity is us, the upward velocity of the displaced fluid relative to the walls is
thereby providing a correction for wall effects. Orr (1966) had also suggested that the Richardson and Zaki equation (eq 5 ) be applied to vertical solids transport. However, the equation presented by Orr
is inaccurate. This equation, which relates the void fraction in the lift line to measurable or specified variables, is derived from material balances for the flow of solids and fluid within the lift line, together with eq 6. The result is that
(1 -
E) =
//,(1
- E)$€
Ed?/,
-
Ell,,
(19)
O m erred at this point by assuming incorrectly that /(,I
=
/I,
= /ltEn
(20)
since only the fractional cross-sectional area t is available for flow. The liquid-solids slip velocity is then given as
rather than the correct result in eq 12. Furthermore, Orr assumed that Ui in eq 5 was equal to ut, thereby neglecting any wall effect as presented in eq 17. Making the correct substitutions in eq 19 gives
For a batch fluidized system, the net solids velocity is zero and the fluid velocity is
(21)
llf
ll0f
=E
(9)
where uof is the superficial fluid velocity. The solids-liquid slip velocity is therefore I(, 1
=
& lf
E
(10)
In all fluidized systems the drag on a constituent particle is equal to its buoyant weight in the liquid, and since
which should be used in place of eq 18. Equation 21 is equivalent to eq 16, since both were derived ultimately from the same fundamental equations. While both equations require a trial-and-error solution for t , eq 16 is preferred since it is less unwieldy and requires only the volumetric flow rates of liquid and solids. In calculating ui, the viscosity of water was obtained from the equation of Bingham (1922) and the density of water was obtained from the equation of Keenan and Keyes (1937). Two differently defined particle diameters, Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3. 1975
265
I W A C T PLATE7
DISCHARGE 2 I / 2"
-
NPT
-*
SECTION
IO- I /4"
Figure 1 . Schematic diagram of the vertical solids transport unit.
D,, were tested in the correlations. One particle diameter was a weighted-geometric-mean based on a sieve analysis, and the other was that of the equivalent-volume sphere.
Experimental Equipment A schematic diagram of the equipment used in this investigation is presented in Figure 1. Details can be found in the original reference (Kopko, 1969). The overall height of the fluidized heat exchanger module is 20 f t . The solids transport system includes a lift line 2.4 in. in diameter and 13 f t high, a selection of slurry feeder nozzles, a 220 gpm pump, and solids and water storage facilities. The solids flow is not restrained a t the top. Flow rates, pressures, and temperatures are monitored a t appropriate locations. Corrosion-inhibited water from the lower holding tank is strained, pumped, and then metered into the base or pot section of the unit through a main jet and four side jets positioned a t 90" to each other. Particulate solids in the tower are then mixed and entrained by the water and are caused to flow up the lift line. Residual water which does not enter the lift line flows up the annular section and out into a weir, where, after passing through screens, it is returned to the lower holding tank. The solids-water mixture flowing up the lift line is discharged into a flow diverter containing an impaction plate. The flow of the mixture could be observed through an acrylic plastic tubular section in the lift line, near the discharge point. Because of the swivel joint provided in the lift line, the flow diverter could be turned 180" from that shown in Figure 1, thereby allowing continuous recycle of the solids back into the tower and the attainment of steady-state conditions to be achieved. In the diverter position shown, the mixture is discharged down a chute and into a basket where the solids are retained and then weighed, making an adjustment for residual occluded water. The water which passes through the basket then flows into a calibrated holding tank. The nozzle system shown in Figure 1 consists of a 1.7-in. main jet, four 0.8-in. side jets, and a 4.0 X 2.4-in. slurry nozzle. Two other types of slurry feeding systems tested are shown in Figures 2 and 3. These are liquid jet pumps or eductors, one enclosed and one open type. The operating conditions varied included water temperature ( a threefold change in viscosity), particle diameter 266
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
MOTIVE FLUID INLET,, I -1/2 NPT
Figure 2. Liquid jet slurry pump (enclosed type). DISCHARGE
INLET, 1-1/2 NPT
Figure 3. Liquid jet slurry pump (open type)
Table I . Summarv of Solids ProDerties Chilled iron
Chilled iron ~
Weighted g e o m e t r i c mean particle d i a m e t e r . in. D i a m e t e r of e q u i v a lent volume s p h e r e , in. Particle density, l b / c u ft L o o s e bed void fraction
Alumina pellets ~
~-
0.0734
0.131
0.306
0.0751
0.140
0.303
473.6 0.409
472.0 0.414
229.6 0.428
Table 11. Exoerimental and Calculated Data for Vertical Transoort of solids with Water Calcd A P / L , Ib,/ft3
CD
VOl.
fract. water disT, "F charged
Actual velocity, ft/sec Calcd E
Water Solids Buoyant
Waterwall Solids Water fric Static Static tion Total
Exptl Ib,/ft3
Davies and (NRo)n Exptl Robinson
146.7 81.1 114.8 114.3 126.1 109.5 88.8
134.3 85.0 100.1 93.0 120.9 108.1 86.1
665
1.49
1190 819 1180 466 1280 1280
0.667
168.5
1090 2020 2310 1620 1540 2670 1430
2.39
2.5
0.760 0.880
0.75
133.2 154.8
154.8 89.3 97.0 137.7 105.8 131.2 151.9
1.86
0.85 1.4 1.2 1.5 1.9
96.1 72.0 80.7 93.3 94.7 93.2 95.2
99.3 73.5 81.2 94.3 96.1 95.4 94.8
4030 6840 4010 2780 2200
2.24 1.04 1.17 1.79 2.07 1.96 2.05
2.0 1.6 1.4 1.7 1.6 1.9 1.7
WL,
~~
15 90 80
104 45 108 91 61 68 84 I1
61 120 76
102 115 80 64 55 117 57
4.97 3.25 2.11 6.64
0.846 0.971 0.959 0.906 0.885 0.919 0.984
0.800
0.880 0.968
0.976 0.815 0.944 0.907 0.834
0.742 0.940 0.914 0.848 0.882 0.828 0.781
2.52 4.41 2.80 8.94 3.49 3.55
0.871 0.973 0.982 0.870 0.861 0.867 0.860
0.803 0.945 0.892 0.842 0.820 0.826 0.820
3.59 3.38 1.93 1.42 4.84 5.44 5.64
0.942 0.813 0.881 0.853 0.890 0.936
6.08
5.42 2.23
5.67
3.62 1.55 0.64 5.09 4.61 3.84 0.54
0.0734-in. Diameter Iron 82.3 94.8 49.8 2.1 23.9 21.1 58.5 1.0 52.2 60.1 54.3 0.5 49.0 56.4 54.6 3.3 60.4 69.5 53.2 3.4 45.3 52.1 55.1 2.3 26.3 30.2 58.1 0.5
7.09 1.53 1.15 4.02
0.131-in. Diameter Iron 105.7 121.7 46.2 0.1 24.6 28.3 58.6 1.8 35.2 40.6 56.8 0.8 62.3 71.8 52.8 3.2 55.0 1.2 48.3 55.6 70.6 81.0 51.1 1.0 89.7 103.5 48.6 2.7
2.16
0.306-in. Diameier Alumina 1.1 49.8 33.0 45.2
1.59 0.29 5.90 3.39 3.95 4.18
9.2' 18.1 26.4 30.1 29.2 30.1
0.99 2.27 0.75
12.7 24.8 36.3 41.3 40.0 41.2
58.4 55.5 52.5 51.2 51.0 51.2
0.7 0.4 4.5 2.2 2.3 2.8
88.7
98.2 130.8 111.8
5060
2300
0.913 0.935 1.10 0.883 0.696
1.25 1.03 1.40
2.1 0.9 1.4 1.1 1.6
1.2 0.95
Table I. Photographs of samples of the solids are shown in Figure 4. The particles are almost spherical and have narrow size ranges. Lift Line Hydrodynamics
Figure 4. Samples of solids investigated
(0.073-0.306 in.), particle density (3.68-7.60 g/cm3), water flow rates (30-145 gpm), and jet pressures and flow distributions. A summary of the solids properties is given in
Pressure drops, temperature, and volumetric input and discharge flow rates were measured in 335 runs in the fluidized heat exchanger module using the three different particle types and three different feed nozzles. Void fraction, Reynolds number, water and solid velocities, and pressure drops were calculated for each run. The results are reported in Appendix I (available as supplementary material). A summary of the results of some of the tests encompassing the complete ranges of operating conditions encountered is included in Table 11. The range of terminal particle Reynolds number was 789 to 10,600, which corresponds to the turbulent flow regime. The range of calculated terminal velocity, ut, was 1.97 to 2.65 ft/sec, while the range of corrected terminal velocity, ui, was a hit lower a t 1.84 to 2.34 ft/sec. The volume fraction water in the discharge ranged from 0.83 to 0.98; that is, each volume of water lifted from 0.02 to 0.20 volume solids. On a weight basis, the specific loading varied from 0.07 to 1.51 pounds of solids per pound ofwater. The void fraction in the lift line was calculated from eq 16, using the Richardson and Zaki (1954) value of n equal to 2.39 for turhulent flow. The values ranged from 0.74 to 0.95, which encompasses the range of dense phase transport to dilute phase transport. Note in Table 11 how much less the lift line void fraction is than the fraction fluid discharged. Using the calculated void fraction, the range of actual water velocities was calculated to he 1.93 to 8.94 ft/sec Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3. 1975 267
Table 111. Nozzle Performance
Type Nozzle
-
D D fin. 0.131 (iron)
Solids PresSolids trans - sure at Pres Water transported bottom sure at Lift line rate up ported per of solids bottom Vol. fract. water through per water power recycle of lift water rate, bed, input, input, lb/ tower, line, discharged gal/min gal/min Ib/gal (min)(hp) psig Psig 0.849 0.933
0.970
Water jet (enclosed type) Water jet (open type)
0.306 (alumina) 0.0734 (iron) 0.0734 (iron)
0.864 0.851 0.939 0.93 1 0.944
and the range of solids velocities was 0.29 to 7.09 ft/sec. The range of the calculated slip velocities is 1.53 to 2.14 ft/sec for the 0.131-in. diameter particles, which can be compared to 2.33 ft/sec for the corrected terminal velocity and 2.64 ft/sec for the uncorrected terminal velocity. For the other particle sizes, the comparisons are similar. The applicability of the Richardson and Zaki equation was checked by comparing the pressure drop calculated with eq 4 to the experimental values. The results can be compared in Appendix I, Table 11, and Figure 5. The average % error resulting was +6.3, +1.5, and -1.3 for the 0.0734-in. iron, 0.131-in. iron, and 0.306-in. alumina pellets, respectively. The particle diameter used to calculate the particle Reynolds number in the data reported here is the weighted-geometric-mean diameter based on a screen analysis. We prefer this to the equivalent-volume-sphere diameter because the solids will tend to orient themselves with the smallest diameter perpendicular to the flow. In Table I, the geometric-mean diameter is generally the smaller of the two. The 70 error in calculated pressure drop with the equivalent-sphere diameter was +6.5, +2.1, and -1.4% for the 0.0734, 0.131, and 0.306-in. particles, respectively, which is not too much different than with the geometric-mean diameter. In Table 11, it is noted that the solids static head and the water static head contribute most to the total pressure drop of 0.7 to 1.0 psi/ft. The water-wall friction is a t most several percent of the total. The buoyant pressure drop listed in Table I1 is not used as part of the total, but is tabulated to indicate the magnitude of the particle-fluid friction drop. It is somewhat less than the weight of the solids in the bed, the difference being due to the work done in displacing the liquid volume occupied by the solids. The correlation of Zenz (1957) and Davies and Robinson (1960) was used to check the drag coefficients obtained experimentally in this work. The experimental values were calculated from physical properties, void fractions, and slip velocities. These are compared to those calculated from physical properties and void fraction in Table 11. The agreement is satisfactory, with some scatter no doubt coming from difficulty in interpolating readings in their graphical correlation. Liquid J e t P u m p Performance Selected runs made with the three nozzles investigated in this work are compared in Table 111. Details of all the runs can be found in the original reference (Kopko, 1969). The water which emanates from the main jet and the side 268
Ind. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
78.7 64.2 55.6 77.8 59.9 31.0 58.3 56.0
54.9 35.2 27.1 68.0 0.0 0.0 69.1 67.6
6.63 2.93 1.32 2.57 11.1 4.09 2.14 1.70
7 84 4 84 266 469 325 305 158
C
0 0734 inch IRON 0 I3 I inch IRON
A
0 306 inch ALUMINA
0
I
14.5 10.4 8.5 7.4 6.3 7.3 6.5 6.5
I
I
I
80 IO0 120 EXPERIMENTAL AP, lb, / ( f t 1 2 ( f t )
13.1 9.6 8.2 8.8 12.2 9.7 9.2 8.9
I
I40
160
Figure 5 . Comparison of calculated and experimental pressure drop for vertical transport of solids with water.
jets is distributed between gainful lifting of the solids and in maintaining the pressure balance in the apparatus by flowing up through the solids in the feed storage bed. With the enclosed water jet pump no water was sidetracked in this way; in fact, make-up water flowed down through the storage bed with the solids. This jet pump performed better than the other slurry feeders in both specific loading and energy requirements. It can pump against a head of a t least 6 psi; we were not able to test its maximum capability because of the design of our apparatus. The lowest pumping energy requirements correspond to the highest solids loading in the water discharge. Thus, the ability to operate in the dense phase regime is a desirable goal. In conclusion, this work has provided design data and correlations for hydraulic transport of solids from beds of solids into and through vertical pipes. Of particular note, the Richardson and Zaki correlation for sedimentation and static fluidization has been extended to the fluidized transport mode. Acknowledgment The authors gratefully acknowledge the financial support of R. J . Kopko by the National Aeronautics and
Space Administration, Exxon Research and Engineering Company, and the donors of the Petroleum Research Fund, administered by the American Chemical Society.
g = fluid viscosity, lb/(ft)(sec)
Nomenclature A = cross-sectional area of tube, ft2 CD = drag coefficient, dimensionless D , = particle diameter, f t Dt = tube diameter, ft f = Fanning friction factor, dimensionless gc = unit conversion factor, 32.17 (€t)(lb)/(lbf)(sec)2 L = tube length, ft n = constant defined bv ea 5 (NR,), = Dppf(uf - u,)t/pf = particle Reynolds number, dimensionless = Dppfut/pf = particle terminal Reynolds number, dimensionless A P f = pressure drop due to static head of fluid, lbf/ft2 APf, = pressure drop due to fluid-wall friction, lbf/ftZ AP, = pressure drop due to static head of solids, lbf/ft2 = pressure drop due to solids-solids friction, lbf/ft2 SP,, = pressure drop due to solids-wall friction, lbf/ft A P I = total pressure drop, lbf/ft2 q f = volumetric flow rate of fluid, ft3/sec q5 = volumetric flow rate of solids, ft3/sec uc = superficial slip velocity, ft/sec uf = fluid velocity, ft/sec u1 = adjusted terminal velocity defined by eq 17, ft/sec urns = mean suspension velocity, ft/sec uof = superficial fluid velocity, ft/sec uoi = superficial solids velocity, ft/sec us = solids velocity, ft/sec us] = (fluid-solids) slip velocity, ft/sec ut = particle terminal velocity, ft/sec Greek Letters A = differenceoperator c = void (fluid) fraction in lift line, dimensionless (1- c ) = solids fraction in lift line, dimensionless t d = delivered water fraction, dimensionless
Literature Cited
I
pf ps
= fluid density, lb/ft3 = solids density, lb/ft3
Barton, P., Fenske, M . R . , Ind. Eng. Chem.. Process Des. Dev.. 9, 18 (1970)
Bingham, "Fluidity and Plasticity," McGraw-Hill, New York. N . Y . , 1922. Condolios, E . . Chapus, E. E., Chem. Eng.. 70 ( 1 3 ) . 93 (1963). Davies, G., Robinson, D. B., Can. J , Chem. Eng., 38 ( 6 ) , 175 (1960). Drew, T . E.,Koo. E. C . , McAdarns. W . H.. Trans. AICh€, 28, 5 6 (1932) Keenan: J. H., Keyes. F . G . , "Thermodynamic Properties of Steam," Wiiey. New Y o r k , N . Y . , 1937. Kopko. R. J.. P1.D. Thesis, The Pennsylvania State University, University Park, Pa.. 1969. Newitt, D . M . , Richardson, J. F . , Gliddon, B. J.. Trans Inst. Chem. Eng , 39,93 (1961). Orr, Ciyde, "Particulate Technology," Macmiilan, New York, N . Y . , 1966 Richardson, J. F . , Zaki, W. N . , Trans. lnst. Chem Eng.. 32,36 (1954) Wilhelm, R. H., Kwauk, M . . Chem Eng. Prog.. 4 4 , 201 (1948) Zenz, F. A . . Pet. Refiner, 36 ( B ) , 147 (1957)
Receiued f o r revieu August 23, 1974 Accepted February 20,1975 Microfilm of "Hydrodynamics of Cocurrent Countergravity Solids Transport for Liquid-Fluidized Heat Exchangers," a Doctoral Dissertation by Ronald J . Kopko, may he obtained, free of charge for specified periods, on interlibrary loan from Pattee Library, The Pennsylvania State University, University Park, P a . 16802. A purchase of the microfilm can be made through University Microfilms, Inc., Ann Arbor, Mich.
Supplementary Material Available. Experimental results (Appendix I) will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 24X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D.C. 20036. Remit check or money order $4.50 for photocopy 01 $2.50 for microfiche, referring t o code number PROC-75-264.
Pilot-Plant Production of Urea-Ammonium Sulfate Gordon C. Hicks and John M. Stinson* Tennessee Valley Authority. Muscle Shoals. Alabama 35660
Urea-ammonium sulfates (6- to 12-mesh size) of high-nitrogen content (43-30% N) and containing readily available sulfur (1-13% S) were produced in pilot-plant facilities from concentrated urea solutions (99+"/0) and by-product ammonium sulfate crystals. Granulation was by processing fluid mixtures (275-300°F) of the raw materials in a pan granulator and by prilling in oil. The feed to the pan was introduced in the form of a spray: granulation temperatures were controlled with recycle. In oil prilling, the fluid mixture was fed to a prilling cup, the prills were quenched in a lightweight mineral oil (8O-10O0F), and deoiled (1-2% oil) by centrifuging. No drying steps were required in these processes. Results of exploratory tests of air prilling also are described. All of the products had greater strength than air-prilled urea and should be more suitable for use in bulk blending because of the larger size. Granulation in a pugmill was not considered satisfactory.
Sulfur is an essential secondary plant nutrient, required for healthy growth of all crops. In the past, cultivated crops generally have received sufficient sulfur either from native soil content, from sulfur content of rain, or from the collateral sulfur content of fertilizers. The sulfur con-
tents of the fertilizers were mainly from ammonium sulfate, which was used chiefly for its nitrogen content, and ordinary superphosphate, which was used chiefly for its phosphorus content; both of these materials contain abundant sulfur in available sulfate form. In recent years, howInd. Eng. Chem., Process Des. Dev., Vol. 14, No. 3, 1975
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