plant having a rotary drum type of granulator. Potassium chloride can be added during granulation to give threecomponent grades, such as 38-1 3-0, 34-17-0, 26-1 3-1 3, 29-29-0, and 19-19-19. For a minimum of equipment the granulation plant should be adjacent to a urea plant. The urea can be supplied as a concentrated solution or as a solid. If the solutions are concentrated to 96 to 98y0 urea, the recycle requirements will be significantly less than for solutions containing about 85% urea. The use of solid urea further decreases the recycle requirement. However, additional equipment such as a crystallizer or prilling tower is required to produce the solid form and the product is not as homogeneous. T h e ammonia in the off-gas from the decomposer in the urea plant or gaseous or liquid anhydrous ammonia can be used to ammoniate the phosphoric acid. Products made with wet-process acid granulate and store much better than those made with straight thermal-process acid. However, satisfactory products can be made with ~ supplied as thermal-process acid, if about 5% of the P z O is phosphate rock reacted with the acid or 25% is supplied as wet-process acid. The presence of urea makes drying difficult because low
moisture contents are required for satisfactory storage and high gas temperatures cannot be used without melting the product. The design of the dryer is critical. Products require drying to about 1% moisture and coating with 2% of a n appropriate conditioner for satisfactory storage in bags. T h e products are less hygroscopic than ammonium nitrate or urea but considerably more hygroscopic than diammonium phosphate ; controlled humidity would be desirable for bulk storage. T h e biuret content of urea-ammonium phosphate products can be maintained a t a satisfactorily low level by the use of conventional equipment with short retention time for evaporation of the solution. literature Cited
Chenoweth, G. E., Chem. Eng. Progr. 54, No. 4, 55-8 (1958). Newman, E. L., Hull, L. H., Farm Chem. 128, 48-9 (June 1965). Strelzoff, S., Cook, L. H., Advan. Petrol. Chem. Rejrning 10, 315-406 11965). Young, R. D., Hicks, G. C., Davis, C. H., J . Agr. Food Chem. 10, 442-7 (1962). RECEIVED for review January 16, 1967 ACCEPTEDAugust 18, 1967 Division of Fertilizer and Soil Chemistry, 152nd Meeting, ACS, New York, N. Y . , September 1966.
CRYSTALLIZATION OF CALCIUM SULFATE FROM PHOSPHORIC ACID A. B. A M I N ' A N D M. A. LARSON Dejartment of Chemical Engineering and and Enginetring Research Institute, Iowa State University of Science and Technology, Ames, Iowa
The kinetics of the crystallization of calcium sulfate from phosphoric acid was studied using a laboratory continuous crystallizer. The apparatus was operated so that an unclassified suspension was achieved and an unclassified product was obtained. Nucleation and growth rates were determined from an analysis of crystal size distribution. Both reagent grade and wet process phosphoric acid were used. Nucleation rates were lower and growth rates were higher under conditions which produced hemihydrate crystals rather than gypsum cyrstals. When reagent grade raw materials were used, nucleation rates were generally higher and growth rates generally lower than when the raw materials were phosphate rock and wet process phosphoric acid. Phosphoric acid concentration had little effect on the kinetics of nucleation and growth, but increased suspension density increased the particle size.
N THE
production of phosphoric acid by the wet process, a n
I important but troublesome step is the separation of calcium
sulfate from the product acid by crystallization. T h e conditions under which crystallization takes place and the type of crystal produced generally determine the amount of phosphate lost with the calcium sulfate, and largely determine the speed and efficiency of the subsequent filtration step. Depending upon the conditions during crystallization, appreciable amounts of phosphate may be incorporated in the crystal lattice and, because of undesirable crystal habit or small size, appreciable quantities of phosphate may not be washed free from the cake during filtration. Both phenomena result i n lost phosphate ; therefore, the crystallization step must be carried out to produce the best possible crystalline product. T h e crystallization step should be designed and operated Present address, American Cyanamid Co., Princeton, N. J.
to satisfy the following rather obvious criteria. T h e crystal growth rate should be a t a maximum consistent with good crystal formation, the nucleation rate should be a t a minimum, the crystal form should not be such that phosphate ions are incorporated i n the lattice, and the crystal form and habit should be such that filtration and cake washing rate are maximized. T h e relative kinetic rates of growth and nucleation determine the particle size distribution. T h e respective maximum growth rate and minimum nucleation rate sought would be those giving the largest particles in the minimum holding time. A part of a n investigation to find the optimum operating conditions consistent with these requirements requires the measurement of growth and nucleation rates and the determination of the effects of the operating conditions on these rates under conditions experienced in practice. This paper illustrates how such growth and nucleation kinetic data can VOL. 7
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JANUARY 1968
133
be obtained and gives some kinetic expressions useful in studying the process. T h e study utilized experimental and data analysis techniques previously described by several investi, Murray, 1964; Randolph, 1965; gators (Bransom et ~ l . 1949; Randolph and Larson, 1962; Timm, 1965).
tion density, no. A typical plot of a hemihydrate crystal product is shown in Figure 2. Using the definition of population density
Experimental Apparatus and Procedure
and growth rate,
A continuous, mixed suspension, mixed product removal (CMSLMPR) crystallizer was used to carry out the experiments. The complete system is shown in Figure 1. The crystallizer was a round-bottomed glass vessel, equipped with a n agitator and baffle, submerged in a constant temperature bath. Normal operating capacity was 1 liter. T h e vessel was fed by two Mec-0-Matic diaphragm metering pumps and the product slurry was removed by a vacuum liquid-level controller. The two feed streams consisted of phosphoric acid solutions of sulfuric acid and monocalcium phosphate, respectively. Both reagent and plant grades were used. The concentration and proportions of the feed stream were adjusted to achieve the desired concentration and suspension conditions in the crystallizer. T o obtain data. the crystallizer was operated long enough to achieve steady state, a t which time a sample of the slurry was taken. T h e slurry was quickly filtered, washed with water, then washed with absolute alcohol, and dried. The size distribution was obtained with a Coulter counter. The subsequent analysis was based on the size distribution obtained. For this analysis the size distributions pvere expressed in terms of particle population density, n, as defined in Equation 1.
dN dL
(3)
dL dt
(4)
n=-
r=as L approaches zero
Equation 5 shows that the nucleation rate in number per unit time, is the product of the growth rate and the intercept of the semilog plot. Using the above analysis, crystal product size distributions obtained as a result of various operating conditions can be used to determine the nucleation and growth rates. Growth rates and homogeneous nucleation rates may be expressed as functions of supersaturation, s, as follows:
A S n = lim AL+O AL
(7) d'\'O
Data Analysis
For a C M S M P R crystallizer it has been shown, for a number of systems (Murray, 1964; Randolph and Larson, 1962; Timm, 1965), that the size distribution obtained in terms of the population density is given by
n = no exp (--L/rT)
(2)
where no is the population density of nuclei, L is a characteristic dimension of the crystal, r is the linear growth rate, and T is the holding time. Growth rate is independent of particle size when size distribution data can be represented by Equation 2. This implies that McCabe's AL law applies (McCabe, 1929). A semilog plot of data represented by Equation 2 , therefore, will give a straight line with the slope related by holding time, T , to the growth rate, r. T h e intercept is the nuclei popula-
Eliminating supersaturation and - using Equations 5, 6, and dt 7 gives
(8)
no = k l h(r)
Equation 8 is the essential relationship relating nucleation kinetics and growth kinetics and determines the possible size distribution which can be achieved. For a number of systems (Bransom et ~ l . 1949; , Timm, 1965) over a reasonably large supersaturation range, Equation 8 can be expressed as a simple power function. no =
kl ri-1
(9)
Equation 9 results from the assumption thatj(s) = s andg(s) = Si.
Assuming homogeneous nucleation and using Equation 9 and a mass balance, growth rates obtained under conditions
r
-To
Vacuum
Baffles Controlled Heater
Figure 1.
Experimental apparatus Constant
Tern Ea t h
134
l&EC
PROCESS DESIGN A N D DEVELOPMENT
,000~
30001
110-
h\
L
1000
g 100L
a"
Slope = -00166 T = 3 7 5 minutes r 96.6
n
90-
v)
e 5 00-
i
hr
Temp 7 0 - C
100
1000
I
-
600)
c
0
30 -
f
3
e 60-
10-
0
'/
1'
?
6-
I
L Figure 2.
,
,
I
60
30
70
120
1
r = KlM;f3
and the nuclei population densities are related to the suspension density by i-1
K ~ M I ~
(1 1)
where K1 and K;. are proportionality constants dependent on the other conditions of the crystallization. If one assumes heterogeneous nucleation-that is, nucleation rates dependent on the amount of solids present in suspension-one can use the following equation instead of Equation 7 (Wolff, 1965).
dS0 - = k.,'M3sL dt Using Equation 1 2 with j = 1, equations for heterogeneous nucleation corresponding to Equations 10 and 11 are
no
constant
(13)
K2M
(14)
=
Equations 10, 11, 13, and 14 can be used to detexmine the existence or absence of nucleation phenomena dependent on the amount of solids phase in suspension. Experimental Results
Effect of Temperature. T o determine the relative growth rates of gypsum and hemihydrate and the effect of temperature on the kinetics, a number of runs were made a t different temconcentration of 42%. peratures and a t a phosphoric acid PBOS T h e results are shown in Figure 3. T h e numbers by each point indicate the water content of the crystal. Experiments were made for a reagent grade system and a commercial grade system. As can be seen from the plot, the growth rate is higher a t higher temperature, and the nucleation rate is relatively lower. Hemihydrate has a higher relative growth rate and a lower relative nucleation rate than does gypsum. T h e four points from the impure system were made a t the same suspension density. T h e pure system runs also were made a t a
L , microns
'C
Figure 3. Temperature growth and nucleation
giving different suspension densities, M , but with all other conditions the same, are related to the suspension density by
=
C
3
Temperature
Typical size distribution
71
ox
I
microns
no =
\ 'il kr.
30
E 70-
A
\
effect
on
Figure 4. Residence time effect on size distribution
uniform suspension density, but a t a different level than the impure runs. This plot shows that large crystals Mill be obtained if calcium sulfate is crystallized in the hemihydrate form. Effect of Phosphoric Acid Concentration. Phosphoric acid concentration had a nonmeasurable effect on growth and nucleation so long as the concentration was kept within the region in which a single compound was formed-Le., gypsum or hemihydrate. Effect of Supersaturation and Residence Time. A number of experiments were conducted a t 70' C. and 42y0 P ~ O S concentration at different residence times under conditions giving a small excess of calcium ion. At these conditions, hemihydrate was the primary crystal produced. Similar experiments can be made under conditions which produce g)psum crystals. Because of the gro\\th and nucleation kinetics of gypsum however, longer residence time and more precise condition control would be necessary to obtain the desired crystal form. T h e net result of different residence times was the generation of different supersaturations and consequently different growth and nucleation rates. These runs Mere made with both reagent grade materials and commercial wet process acid. T h e size distribution data Ivere treated as previously described and for each experiment the growth rate and nuclei population density were determined. A comparison of two typical runs, made a t 12- and 45-minute holding times, respectively, is shown in Figure 4. A larger sized product is indicated by the line with the least slope. This corresponds to the long holding time and consequent low supersaturation. This size enhancement is primarily due to the fact that nucleation rate decreased to a greater relative degree than growth rate when holding time was increased from 12 to 45 minutes. In all cases, straight lines represented the data well. indicating that the assumptions made in the original derivation were justified. T h e nuclei population density was plotted against the corresponding growth rate for a number of runs. These plots are shown in Figures 5 and 6, for reagent grade and commercial grade material, respectively. A simple power model can be used to relate the two rates over a rather wide range of holding times. T h e kinetic order is nearly the same for both the pure VOL. 7
NO. 1
JANUARY 1968
135
600-
300-
N I
2 X
ot100-
.
60-
2,
+ .-
E
30-
0)
n C
.-O
10-
)O
Growth Rote - r microns/ hr.
Figure 5. Nucleation kinetics for reagent grade materials
t 100
IO1 IO
I
30
60
100
I
300
I
and 3.760% a t 70' C. and 42% PzOa, to produce hemihydrate. T h e resulting size distributions are reported in Figure 7 . As the population density is based on 1 gram of crystal product, a n increase in particle size results from a n increase i n suspension density. T h e two growth rates obtained are consistent with Equation 9, and if the nuclei population densities are converted to a total product basis, the results are also consistent with Equation 10. T h e indication from this analysis is that the predominant source of nuclei is homogeneous nucleation, and the nuclei generation rate is not dependent o n solids present, but only on supersaturation. This result is i n contradiction with the authors' previous view that heterogeneous nucleation is the principal source of new particles in continuous, mixed-suspension crystallization processes. Evidently, it is not possible to make a general statement in this regard for all systems. Each system of interest must therefore be studied thoroughly to determine the relative importance of the two nucleation phenomena. Discussion
T h e experimental technique described here has a number of important advantages as a method for determining nucleation and growth rates of calcium sulfate i n phosphoric acid. T h e most important is that the technique permits these kinetic rates to be measured under steady-state conditions and under conditions nearly identical to those experienced in a n industrial crystallizer. Second, the very low supersaturation existent under such conditions need not be directly measured. Finally, the two kinetic rates may be measured simultaneously, and the effect of changing conditions on the size distribution can be readily determined. A precise physical interpretation of supersaturation is not possible. I n the wet process, the sulfate ion concentration may be more or less than the calcium concentration, depending on the relative feed rates. T h e exact equilibrium concentration of calcium sulfate would be determined by the solubility product. T h e authors assumed that the calcium sulfate concentration in excess of that determined by the solubility product is the proper supersaturation to use and that it constituted the driving force for both nucleation and growth.
600 I 00
Growth Rate - r Microns / hr.
Figure 6. Nucleation kinetics for plant grade materials
and the impure materials, being 2.8 and 2.6, respectively. Power law models obtained in this way will, of course, not apply when the level of supersaturation is close to the metastable region. T h e authors feel, however, that in most process applications, operating conditions are such that the supersaturation level is considerably above the metastable limit. Effect of Suspension Density. Generally, in practice, the suspension density has a n important effect on the size of the crystals obtained. From the foregoing analysis, one would conclude that this would be true if the prime source of new crystals were nuclei formed homogeneously. However, if the presence of solids gives rise to generation of new particles by attrition or by heterogeneous effects, the effect would not be pronounced. For example, if new particle formation was proportional to crystal mass present, no improvement in size distribution could be obtained by increasing the suspension density. This is clearly shown by Equation 13. Experiments were run a t two suspension densities, 1.525 136
I h E C PROCESS DESIGN A N D DEVELOPMENT
Pi
100
\Y
I oc
0.21 0
I
I
I
I
I
50
60
90
120
150
L , microns
Figure 7. Suspension density effect on size distribution
Under these assumptions, the actual magnitude of the supersaturation may be eliminated from the analysis and the size distribution related to the growth and nucleation kinetics. T h e important consideration in size distribution consideration is the relationship between nucleation and growth rates, not the absolute rates. In this analysis, the crystallizer was assumed to be perfectly mixed. This condition can be closely approximated in small laboratory equipment, but in large production units mixing conditions are usually considerably removed from the ideal. Consequently, high local supersaturations may exist in the neighborhood of the feed points, giving rise to abnormally high nucleation rates. Under such conditions, the above kinetic model for nucleation rate may give a nucleation rate lower than that observed. These preliminary results show that the crystallization of calcium sulfate as hemihydrate has several advantages. T h e most important one is the fact that the hemihydrate has a much higher growth rate and a much lower relative nucleation rate than does gypsum. T h e combined effect of these desirable properties is that a larger average crystal size can be obtained in a shorter holding time. I n addition, a more concentrated acid can be produced, which reduces the concentration required to produce product acid. A more suitable habit is formed and is less prone to modification by impurities or concentration effects. I n the work discussed here, the hemihydrate was in such a form that it was easily filtered and washed free of phosphate. Hydration of the hemihydrate in the filter was not a problem, although in a commercial application this may well be troublesome. Apparently, because of the crystallization advantage of producing hemihydrate, a process producing this crystal product would have considerable advantage in the wet acid industry.
Nomenclature
proportionality constant, temperature dependent proportionality constant for growth proportionality constant, homogeneous nucleation proportionality constant, heterogeneous nucleation proportionality constant, relating growth and suspension density proportionality constant, relating nucleation population density and suspension density crystal size suspension density, mass of crystals per unit volume population density number of crystals linear growth rate supersaturation time
SUPERSCRIPTS i
=
j
= order of suspension density effect = nuclei
o
order of nucleation
literature Cited
Bransom, S. H., Dunning, W.J., Millard, B., Discussions Faraday SOC.5 , 83-95 (1949). McCabe, W. L., Ind. Eng. Chem. 21, 112-19 (1929). Murray, D. C., “Size Distribution Dynamics in Continuous Crystallization,” unpublished Ph.D. thesis, Iowa State University of Science anh Technology, Ames, Iowa, 1964. Randolph, A. D., A.I.Ch.E. J . 11, 424-30 (1965). Randolph, A. D., Larson, M. A,, A.1.Ch.E. J . 8 , 639-45 (1962). Timm, D. C., “Crystal Size Distribution Dynamics,” unpublished Ph.D. thesis, Iowa State University of Science and Technology, Ames, Iowa, 1965. Wolff, P. R., “Suspension Density Transients in a Mixed Suspension Crystallizer,” unpublished M.S. thesis, Iowa State University of Science and Technology, Ames, Iowa, 1965 RECEIVED for review February 20, 1967 ACCEPTED September 1, 1967 152nd Meeting, ACS, New York, N. Y., September 1966.
HYDROGEN CYANIDE PRODUCED FROM COAL AND AMMONIA G. E. J O H N S O N , W. A. D E C K E R , A. J . F O R N E Y , A N D J . H. F I E L D
Pittsburgh Coal Research Center, U . S.Bureau of Mines, Pittsburgh, Pa. 15213
cyanide has been one of the country’s strongest U . S. production increased from 174,000,000 in 1960 to 350,000,000 pounds in 1964, a 100% increase over the 4-year period. T h e growth of production of hydrogen cyanide has been directly related to the expansion in production of synthetic textiles from acrylonitrile. Relative growth and production of hydrogen cyanide and acrylonitrile are as follows: YDROGEN
51 growth petrochemicals in recent years.
Production of Hydrogen Cyanide and Acrylonitrile Hydrogen Acrylonitrile, Cyanide,a Million Lb. Million Lb. Year
22!Ib 174 1960 21 1 250b 1961 360b 266 1962 455b 293 1963 593b 1964 350 371 (6 months)C 1965 a U . S. Department of Commerce ( 1 9 6 6 ) . b U. S. T a r t f f Commission (1961, 7962, 7963, 1964, 1965). c
U . S. Business and Defense Services Administration ( 1 9 6 5 ) .
About 50% of the total output of hydrogen cyanide goes into the production of acrylonitrile; most of the remainder is used in production of adiponitrile and methyl methacrylate (Fugate, 1962). However, in recent years acrylonitrile and adiponitrile are being produced by processes which generate hydrogen cyanide as a by-product (Chemical W e e k , 1966). ‘The bulk of acrylonitrile is used in production of acrylic fiber (Orlon, Acrilan, Dynel, Zefran, etc.) ; a smaller amount in production of nitrile rubber, the adiponitrile in manufacture of nylon. T h e manufacture of sodium cyanide utilizes about 7% of hydrogen cyanide production. T h e remainder goes to a large number of relatively small uses, including ferrocyanides, acrylates, ethyl lactate, lactic acid, chelating agents, optical laundry bleaches, and pharmaceuticals. T h e Andrussow process, the major commercial process used to make hydrogen cyanide, involves the reaction of methane, ammonia, and air over a platinum catalyst a t 1000° to 1200’ C. (Sherwood, 1959). T h e platinum catalyst is usually alloyed with rhodium (10 to 20%). Conversion by the Andrussow process in a single pass is limited to about 69y0 of the ammonia (about 75% with gas VOL. 7
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1968
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