Znd. Eng. Chem. Res. 1992,31, 561-568 Pharmacia LKB Biotechnology. Products Catalog; 1991. Rota,R.; Wankat, P. C. Intensification of Pressure Swing Adsorption Processes. AIChE J. 1990,36, 1299-1312. Rudge, S. R.; Ladisch, M. R. Process Considerations for Scale-up of Liquid Chromatography and Electrophoresis. In Separations, Recovery, and Purification in Biotechnology; ACS Symposium Series 314; American Chemical Society: Washington, DC, 1986; pp 122-152. SAS Institute Inc. User's Guide, 4th ed.; 1990; Vol. 2. Sherwood, T. K.; Pigford, R. L.; Wilke, C. R. Mass Transfer; McGraw-Hill: New York, 1975; section 6.9. Sternberg, J. C. Extracolumn Contributions to Chromatographic
561
Band Broadening. Adv. Chromatogr. 1966,2, 205-270. Stevenson, D. Size Exclusion Chromatography of Proteins Using Compressible Packings. M.S. Dissertation, Purdue Univesity, 1989. Wankat, P. C. Intensification of Sorption Process. I d . Eng. Chem. Res. 1987,26, 1579-1585. Wankat, P. C.; Koo, Y. M. Scaling Rules for Isocratic Elution Chromatography. AIChE J. 1988,34, 1006-1019. Received for review June 10, 1991 Revised manuscript received September 9, 1991 Accepted September 23, 1991
Batch Crystallization of Potassium Chloride by an Ammoniation Process Donepudi Jagadesh and Madhav R. Chivate Chemical Engineering Division, University Department of Chemical Technology (UDCT), Matunga, Bombay 400 019, India
Narayan S. Tavare* Department of Chemical Engineering, University of Manchester Institute of Science and Technology (UMIST), Manchester M60 1 8 0 , England
A process simulation technique for the production of potassium chloride from its aqueous solution by an ammoniation process in a batch crystallizer was developed. A laboratory-scale 1-L draft tube baffled crystallizer was used t o perform ammoniation experiments. Calculated time variations of ammonia and potassium chloride concentrations in the solution phase and population density c w e s determined from simulation algorithm were not entirely consistent with experimentally observed profiles. Potassium chloride can be crystallized commercially from treated sea bitterns containing chlorides of potassium, sodium, and calcium by ammoniation either at low temperature (-0 O C ) without preconcentration or at ambient temperature with preconcentration by evaporation of water.
Introduction Potassium chloride is one of the major plant growth stimulants and most commonly used form of potassium as a fertilizer. It is produced mostly from solid ores, viz., sylvinite (KC1-NaC1) and carnallite (KC1-MgC12*6H20). Sea bittern is relatively a small but an important source of potassium chloride as well. The production of potassium chloride through crystallization is one of the most important processes in industrial crystallization as it yields a better quality product (size,purity) than the conventional methods at high efficiency and low cost. The polythermal process of potassium chloride separation from the ternary system (KCl-NaCl-H,O) is temperature-concentration dependent requiring careful control in order to achieve reasonable efficiency. Fractional crystallization of concentrated sea bittern involving evaporation is an energyintensive and less efficient process. Many solutes can be precipitated from solution by adding a diluent to a system. For instance the presence of ammonia in aqueous solutions of inorganic salts influences their solubilities significantly. When ammonia is added to an aqueous solution containing potassium chloride and sodium chloride salts,the solubility of potassium chloride decreases markedly while that of sodium chloride changes slightly with an eventual slender increase at high ammonia concentration. A process suggested by Gaska et al. (1965) for selective crystallization of potassium chloride with ammonia addition appears less concentration and temperature dependent, less energy intensive, and more efficient than the conventional processes. The process offers economic advantages and operating flexibilities that are not possible in other processes. Ammonia can be recovered from the mother liquor by distillation for either recycle or neutralization to produce ammonium salts required in ferti0888-5885/92/2631-0561$03.00/0
lizers. When solid potassium chloride dissolves in water, it dissociates as potassium and chloride ions. The water molecule, being a dipole (dipole moment = 1.84 D)with high dielectric constant, induces a stronger dipole-ion interaction than ion-ion interaction resulting in a mobile hydration sheath around the cation by water dipoles. Ammonia is also good solvating agent with a fairly high dipole moment (1.46 D). When the concentration of ammonia in aqueous solution is increased, the activity of ammonia molecules reaches a point where they can displace water molecules in the solvation sheath around the cation with a degree of preference for a particular ion. The ionic radius of potassium ion (r = 0.133 nm) is larger than those of both sodium (r = 0.098 nm) and calcium ions ( r = 0.106 nm). Potassium ion tends to precipitate out preferentially as sodium and calcium ions are solvated by ammonia. This work was initiated with two objectives. The first purpose of this work was to study the process of crystallizing potassium chloride from its aqueous solution by adding ammonia as a diluent. The scope of this study was restricted to a process simulation analysis of crystallization of potassium chloride by ammoniation of its nearly saturated solution. The second purpose of this work was to investigate the process feasibility of recovering potassium chloride from aqueous solutions containing potassium chloride, sodium chloride, and calcium chloride in proportions usually found in processed sea bitterns, Le., the mother liquor left behind after crystallization of sodium chloride in solar ponds and treated with lime. A typical composition (in weight percent) of Indian sea bitterns having a specific gravity of 1.25 (29 Be') is sodium chloride 11.21, magnesium sulfate 5.89, potassium chloride 2.69, magnesium chloride 10.3, and magnesium bromide 0.17. 0 1992 American Chemical Society
562 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
Previous studies (Gaska et al., 1965; Goodenough and Klopf, 1966; Bhatt and Chivate, 1986; Fernandez-Lozano and Wint, 1979,1982)have demonstrated the potential use of ammonia as a suitable diluent in separation and utilization of potassium chloride even from its undersaturated solutions containing sodium chloride. Low temperatures and high concentrations of ammonia in the mother liquor increase the recovery and purity of potassium chloride, yielding better performance than the conventional fractional crystallization process. The presence of magnesium ion would result in simultaneous precipitation of magnesium hydroxide by ammonia, and perhaps it should be removed by treatment with lime (Gadre et al., 1958) thus resulting in supernatant liquors containing potassium chloride, sodium chloride, and calcium chloride. In order to arrive at the most favorable conditions for the recovery of high-purity potassium chloride at a reasonable yield, a series of experiments was performed at different operating conditions with aqueous solution containing chlorides of potassium, sodium, and calcium in proportion usually found in treated sea bitterns. Crystallization of potassium chloride has received considerable attention in recent years, and some progress has been made in determining the applicable kinetic relations [see for continuous operations Genck and Larson (1969), Randolph et al. (1977,1981,1987),Qian et al. (1987,1989), and Paine and Rohani (1989); see for batch and crystallization characterization Jira-Arune and Laguerie (1979), Beer and Mersmann (1980),Glassner and Skurnik (1967), Sarig et al. (1977,1978,1979),Mersmann et al. (1979),and Mersmann (1984)l. None of these previous studies includes the use of ammonia as a diluent.
Process Description The present study attempts to simulate the process of ammoniation used to precipitate potassium chloride from its nearly saturated aqueous solution in a constant-volume, isothermal, batch dilution crystallizer, gaseous ammonia being added at constant flow rate throughout the batch run. Almost pure ammonia is added to the charge of nearly saturated solution of potassium chloride in a batch agitated vessel at a constant flow rate and temperature and absorbed completely because of its high solubility. The concentration profiles for ammonia and potassium chloride in solution may be described as dcNH3/dt
= RNH3
(1)
and dc/dt = -CY
(2)
RNH, = Q/VS
(3)
a = kvpc d/+/dt
(4)
where and the initial conditions being c", = 0 c = co at t = 0 (5) The variation of supersaturation with respect to potassium chloride with time during the period of a run may be expressed as -dAc = - - - dc* CY (6) dt dt where (7)
In addition to the concentration and supersaturation profiles, the population balance and moment equations with appropriate boundary conditions for a perfectly mixed, constant solvent capacity batch crystallizer in which crystal breakage and agglomeration are negligible and crystal growth is size independent are required to represent mathematically the potassium chloride batch crystallization configuration (Tavare, 1986, 1987).
Process Characterization In order to utilize the process description, the rate processes are necessary to be known. Previously established kinetic correlations for crystallization of potassium chloride from the literature are used. The nucleation rate [no./(s kg of water)] (Qian et al., 1987) as B = 1.98 x 1023G2.78MTO.6lu2.35 (8) was used for the present system. In this study the onset of nucleation process occurs by heterogeneous primary nucleation mechanism and once the solid crystalline surface is available the secondary nucleation process becomes significant in most of the remaining batch time. Equation 8 depicting the effective relative nucleation rate was derived from the comprehensive continuous MSMPR crystallizer studies where secondary nucleation mechanism was dominant. Jira-Arune and Laguerie (1979) derived the surface reaction controlled growth kinetics as G = k,Ac,2.19 (9) where k, = 2.9 X m/(s[(kg of KCl/kg of ~ a t e r ) ~ . ' ~ ] ) . In order to use eq 9, Ac, can be evaluated from eq 10 as Ac, = Ac/[l + (k,/k,)AcrP1] (10) by a trial and error procedure with a fixed number of iterations. In this trial and error procedure the initially assumed value of Ac, was matched with the calculated value of Ac,. The solid-liquid diffusional mass-transfer coefficient in the present case for a representative arithmetic average size of two extremes of the size distribution, D = 500 pm, was determined by the Frossling-type correPation presented by La1 et al. (1988). Thus eqs 8 and 9 were used to represent kinetic relations for the present simulation studies of ammoniation of nearly saturated potassium chloride solution. These kinetic correlations were obtained for the same system with different chemical compositions of feed solutions, operating modes, and crystallizer geometries. Process Responses In order to obtain process responses, a series of experiments was performed in a laboratory-scale l-L jacketed and agitated DTB (draft tube baffled) glass crystallizer (Figure 1). The glass lid was provided with four nozzles. The central nozzle was used to provide a liquid seal for an agitator. A mercury-in-glassthermometer, diluent gas inlet and outlet, and a blank were installed on the other nozzles. The whole crystallizer vessel was gastight and leak-proof during the operation. Four vertical stainless baffles were arranged mutually at 90" and mounted on the wall of the stainless steel draft tube. A six bladed disk turbine located through the central nozzle was used for agitation, the clearance between the bottom of the impeller and the base of the vessel being about 1 cm. The temperature of the crystallizer was maintained at constant value (25 "C) throughout the run by circulating water through the jacket at maximum possible flow rate. All the runs were performed at the same temperature (25 "C). In a typical run filtered solution of AnalaR grade potassium chloride, saturated at 25 "C (and having concen-
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 563
1
undissolved NH3 gas
T I T-
I
A 186
6
t
ID
W
00
All dlmonrlonr aro in mm
A
-
NH3 gas cylinder
B - Circulating fluid C - DTB crystallizer D - Acid solution
G - Glass tube M - Motor P - Thermometer pocket R - Speed regulator
S - Rotameter T - Thermometer V - Valve X - Turbine type impeiler (38 mm dia x 10 mm blade)
Figure 1. The crystallizer assembly.
tration -0.355 kg of KCl/kg of water) was charged into the crystallizer. When the desired temperature was attained, ammonia gas from a gas cylinder through a calibrated rotameter was bubbled into the solution at a predetermined constant flow rate. The outlet gas was scrubbed through a dilute acid solution in order to neutralize any ammonia traces passing through the gas outlet. At the end of run two samples of mother liquor free from crystals were withdrawn for ammonia and potassium chloride. The entire crystallizer suspension was then removed and filtered quickly, the product crystals were washed with acetone and air-dried, and a sieve analysis of product was carried out. The concentration of ammonia in the solution sample was determined by back-titration of excess hydrochloric acid with a standard sodium hydroxide solution with an accuracy of 1%. The other solution sample was used to determine potassium chloride concentration by evaporating the sample to dryness giving an estimated accuracy kg of KCl/kg of water. The of better than f2 X solubility relation of potassium chloride over the experimental range of ammonia concentration may be correlated empirically into a second-order polynomial in ammonia concentration from the literature results (Salun, 1960; Silcock, 1979) as
where c* is the solubility of potassium chloride at 25 "C (kg of KCl/kg of water) in the solution with ammonia concentration cNHa (kg of ammonoa/kg of water). The supersaturation at the end of the batch time was then determined by subtracting the solubility of potassium chloride at the estimated ammonia concentration from the actual solution concentration. The final product size distribution was evaluated by sieve analysis over the size range 53-1000 pm. A series of experiments was performed over the range of experimental conditions varying most significant process variables as reported in Table I. Experimental population density curves obtained from sieve analysis of product crystals and concentration profiles of
Table I. Ranges of Experimental Variables temperature, OC initial solute concn, c, kg of KCl/kg of water final solute concn, c, kg of KCl/kg of water initial diluent concn, cNH3,kg of NHB/kg of water final diluent concn, c"~, kg of NHs/kg of solution supersaturation, Ac, kg of KCl/kg of water inlet ammonia gas flow rates, Q, mL/s stirrer speed, N, rev/s run time, T x lo+, s
25 0.355 0.1-0.13 0.0 0.05-0.32 0.05 10-90 5-24 1.2-14.4
potassium chloride (solute) and ammonia (diluent) were used aa process responses in subsequent simulation studies. Simulation Responses In order to evaluate the simulated responses of the ammoniation process for potassium chloride crystallization from ita aqueous solution by added gaseous ammonia as a diluent, a set of eight differential equations describing the concentration and supersaturation profiles, moment equations, and crystal size variation and having appropriate boundary conditions was integrated simultaneously by the fourth-order RungeKutta method with an integration step length of 0.5 s. The initial stage of particle formation in an unseeded crystallizer is rather difficult to describe. Equation 8 describing the effective nucleation kinetics is not applicable without any magma concentration. In order to initiate the nucleation process, it was fictitiously assumed that very small seed loading (- 1 X lo-' kg) at a large size (-550 pm) for a short initial period (-200 s) was available. This assumption of very small seed loading a t large particle size provides an initiation of particle formation at small size and at the same time with a small amount of solid deposition on these seeds, thus providing negligible contribution to the final product crystals. Thus the whole depletion of solute results in newly formed particles. In the absence of an adequately correct description of the initial stage of the particle formation process, this simplified procedure provides an estimation of nucleation rate, perhaps this being generally lower than those predicted in the case of the primary nucleation mechanisms. Although both seed loading and size have an influence on the development of product size
564 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992
a L
0.3
m
2 $0.2
0.0. 0
ao I
rn
4Ooo Tlw, t
6ooo
L 2aoo
0
(ooo
Tlw, t(d
(0)
Figure 2. Ammonia concentration profiles.
distribution [see, for example, Chivate et al. (197911, the fictitious seeding in the present case has no influence on the product characteristics. The partial differential equation representing the population balance equation was solved by the modified method of numerical integration along the characteristics with a specified grid length of size (1 pm) following an algorithm similar to that developed previously in a reactive batch crystallizer (Tavare, 1986).
Results and Discussion Concentration Profiles. A reasonable agreement is depicted in Figure 2 between the calculated (solid line by smooth curve fitting) and observed (data points) ammonia concentration profiles for two runs, each run being a series of four experiments performed under otherwise similar conditions for different lengths of period at a constant inlet ammonia gas flow rate. The assumption of complete ammonia gas absorption was used. The liquid-side masstransfer coefficient, kL., determined from a correlation reported by Hassan and Robinson (1980) for the conditions as in run 1,was -0.05 s-l resulting in a half-life for this fast physical absorption process of -15 s. Variations of potassium chloride concentration, solubility, and supersaturation with time for these two runs are shown in Figure 3, all these curves being the result of smooth curve fitting through the calculated values. Also included in Figure 3 are the observed potassium chloride concentrations in the solution phase at the end of the series of experiments performed for different periods under otherwise similar conditions. Solution-phase ammonia concentration increases with time (Figure 2) due to addition of ammonia at a constant flow rate. Consequently, solubility of potassium chloride (as described by eq l l ) decreases as shown by dotted curves (Figure 3). Supersaturation increases rapidly, both crystal nucleation and growth commence, and their rates increase markedly with supersaturation resulting in rapid decay in potassium chloride concentration in the solution phase. Thus supersaturation passes through a maximum and remains at a low level for most of the run time adjusting generation and depletion rates (Tavare and Chivate, 1980). Potassium chloride concentration remains constant initially as no crystallization occurs. With an increase in supersaturation rapid decay in concentration is apparent, and later a gradual decay in potassium chloride concentration in the solution phase
am
6ooo
Figure 3. Potassium chloride concentration and supersaturation profiles. 1
xlC
Run1
A
---
--
A
t.lW
T
t.=
A 0
t.Y#
0
A T
0
100
Cry.t.1
ea
600
4#)
O I Z . ,
1wO
L ' p
Figure 4. Population density plots (run 1).
results at low level of supersaturation. Earlier and sharper supersaturation peak and faster decay in concentration for experiments in run 1 performed at higher ammonia flow rate (Q = 35 m L / s ) are apparent in Figure 3 in comparison to those for run 2 (Q = 14 mL/s). Observed potassium chloride concentration in the solution phase at the end of experiments in both runs appeared to follow reasonably the calculated concentration profiles. Table I1 also depick the comparison between observed and calculated values from the computational algorithm. Crystal Size Distribution. A comparison between the calculated population density curves from the simulations corresponding to the experimental conditions and that determined from the sieve analysis of the product crystals obtained during runs 1 and 2 is shown in Figures 4 and 5, respectively. The calculated population density curves shown in all these figures are the results of smooth curve fitting through numerical values of population densities derived from the algorithm for a given experiment and pass through maximum reflecting supersaturation variation. With an increase in the length of experimental period, the
Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 565 Table 11. Comparison between Calculated and Observed Results at the End of Experiments C N H ~ 'g/g
runlexpt l i 1 ii 1 iii 1 iv 2i 2 ii 2 iii 2 iv 3i 3 ii 3 iii 3 iv 4i
4 ii 4 iii 4 iv
8, mL/s
WT,
35.0 35.0 35.0 35.0 14.0 14.0 14.0 14.0 21.0 35.0 56.0 84.0 35.0 35.0 35.0 35.0
1.44 2.52 5.76 8.64 1.2 2.4 3.6 5.4 14.4 8.64 5.44 3.60 8.64 8.64 8.64 8.64
N , Hz 11.7 11.7 11.7 11.7 10.0 10.0 10.0 10.0 11.7 11.7 11.7 11.7 5.0 11.7 18.3 23.3
obs 0.067 0.108 0.205 0.312 0.023 0.043 0.050 0.071 0.309 0.312 0.311 0.310 0.312 0.312 0.312 0.312
calc 0.059 0.100 0.214 0.300 0.020 0.039 0.058 0.086 0.306 0.308 0.309 0.308 0.308 0.308 0.308 0.308
cob, g/g
cederg/g
0.307 0.272 0.212 0.142 0.285 0.317 0.295 0.284 0.133 0.132 0.132 0.132 0.132 0.130 0.132 0.131
0.307 0.271 0.186 0.137 0.355 0.326 0.306 0.282 0.127 0.130 0.130 0.131 0.130 0.129 0.129 0.129
I
Lpm,pm 166 182 209 220 91.8 220 236 251 236 212 192 178 361 212 167 148
I
xto*
Run2
CV,,
%
CV,,
35.9 32.9 28.4 26.9 29.2 38.3 36.0 33.9 28.5 29.7 31.0 32.2 35.1 29.7 27.9 27.2
%
42.2 43.3 67.4 67.9 36.1 29.2 25.1 32.6 68.3 67.9 66.8 65.5 58.1 67.9 76.1 73.3
Run3
xtP
L
I
L,,pm 147 250 267 259 301 288 291 238 270 259 252 245 362 259 255 234
x l d'
B xto*
3)XlO"
c xtd
c xto"
h 1
h
k
iiX1@
-s
xtd
G xld
*r
d
B
n
xld
0
200
400
xtd
xtd
600
mo
lo00
CryotoL oizo, LCp)
0
200
400 600 mo G v o t a l 01zo.L U -
Io00
1
Figure 5. Population density plots (run 2).
Figure 6. Population density plots (run 3): effect of ammonia inlet gas flow rate.
nuclei population density decreases and thus the curve starts at a lower value and the peak moves at larger size with time. The difference between these curves is also the reflection of supersaturation curves; initially the changes in supersaturation are high and result in relatively large separation. The observed population density data do not show this characteristic of maximum in most runs. Generally uncertainties associated with the measurements in most conventional size analysis techniques appear to be large at both the ends of distributions, resulting in apparent scatter in observed population data typically shown in Figures 4 and 5. There is a reasonable trend in a movement of population density with time over the size range (100-600pm) in most runs. The calculated population density curves determined from the present simulation appear in reasonable agreement with the observed data. Experiments performed in run 2 at low ammonia gas inlet flow rate (Q = 14 m L / s ) except at a low value of time ( t = 1200 s) seem to yield better accuracy between calculated and observed product size distributions than those performed in run 1 at Q = 35 mL/s, although the trend in variation in population density curves with time k consistent in those of run 1. Both visual and microscopic observations tended to indicate small percentages of agglomerates in product crystals. The statistics of both the calculated and observed product size distribution as reported in Table I1 show a similar trend with time, both
the mean (L,) and coefficient of variation (CV,) of the observed product size distribution being higher than those (Lpm,CV,) of the resulting product size distribution from the present simulations, respectively. Effects of Gas Flow Rate and Stirrer Speed. The final results from two series of experiments depicting the effect of most significant variables, viz., ammonia gas inlet flow rate and stirrer speed, are also included in Table 11. Both the final ammonia and potassium chloride concentrations in the solution phase show satisfactory closure between observed and calculated values. Both the mean size and coefficient of variation determined from the sieve analysis of the product crystals are higher than those of the calculated crystal size distribution using the present numerical algorithm. With an increase in both inlet ammonia gas flow rate and stirrer speed, the mean product size decreases. Coefficient of variation, though small, shows different variations; CV, increases with ammonia inlet gas flow rate and decreases with stirrer speed while CV, decreases with ammonia inlet gas flow rate and increases with stirrer speed. Comparison between the calculated population density curves from simulation for the conditions of experiments in runs 3 and 4 and that measured during the same series of experiments is shown in Figures 6 and 7, respectively. With increases in both ammonia inlet gas flow rate and stirrer speed, calculated population density as represented by smooth curves
566 Ind. Eng. Chem. Res., Vol. 31, No. 2, 1992 Ammonium ohloridr rddrd. w t U
.
2xld'
h 1
1
I
6
4
2
0 100 I
1
I
I
1
I I
I
I
I
8o
xlo.
xld
lo
t
>;
.5 n
C
I
xld
I.
1
'
40-
./ 0
Figure 7. Population density plots (run 4): effect of stirrer speed.
through the numerical values obtained from the present algorithm show earlier and sharper peak thus resulting in smaller mean size, the effect of stirrer speed being significant (see Table I1 and Figures 6 and 7). Again the observed crystal size distributions do not show such a peak and have apparent experimental scatter but are in reasonable agreement with the calculated size distributions over some part of the size range. In most cases at large size the trend in movement of both calculated and observed size distributions with the parameter (viz.,gas flow rate or stirrer speed) appears the same. Similar observations have been reported previously in batch potash alum crystallization and semibatch silica precipitation studies (Tavare and Garside, 1986, 1987). For potash alum crystahation nucleation rates determined from batch and continuous crystalhers were comparable for similar values of significant variables. The presence of ammonia in crystallizing solution would have some influence on the kinetic rates, but changes in the solubility characteristics of potassium chloride were significant.
Process Feasibility The second set of crystallization experiments was carried out in a DTB crystallizer as above (Figure 1). In some cases with a high concentration of ammonia in the mother liquor, an autoclave crystallizer of similar capacity was used. A charge of 500 g of filtered feed solution usually containing 20% solids by weight (having composition on dry solid basis of sodium chloride 43.07%, potassium chloride 11.09%, and calcium chloride 45.84%) was used in all the experiments. When the desired initial conditions were attained for a given experiments, ammonia gas was bubbled into the solution through a rotameter at a constant flow rate (Q 35 mL/s). During the gas addition and equilibration period the crystallizer contents were agitated by a six bladed disk turbine impeller at a constant stirrer speed ( N 11.7 rev/& At the end of a run with an equilibration time of 2 h, mother liquor samples free from crystals were withdrawn for analysis. The product slurry was filtered, and the crystals were dried, weighed, and analyzed. Analysis of salt mixtures was carried out by flame photometry and atomic absorption spectrophotometry. Four important variables, viz., ammonia concentration, temperature, addition of salt like ammonium chloride, and concentration of feed by evaporation, were varied in the crystallization experiments. Variation of
-
N
:
A",
0
l
0
20
40
80
,
\'O 0
60
Ammonia rddrd, w t U Wrtrr rvrporrtrd, w t %
Figure 8. Crystal purity and yield: effect of final ammonia concentration, addition of ammonium chloride, and evaporation of water for preconcentration. Table 111. Summary of Crystallization Experiments temp, cNHgf, purity, yield, no. feed" O C wt% % wt% i A 25 10-50 ii A 0 38.7 83 9.9 iii A with 2% NHJ1 0 30.4 93 16.7 iv A with 30% evapn 25 35.3 88 21.5 v B 25 26.5 89 40.9
"Solution A: 20% solid (43.07% NaCl, 11.09% KCl, 45.84% CaClzon dry solid basis). Solution B saturated with respect to all three salts.
crystal purity and yield with these most significant variables is shown in Figure 8, and a summary of an illustrative experimental run for several strategies is reported in Table 111. The crystal purity was defined as the ratio of weight of potassium chloride to the weight of product crystals (i.e., wt % KC1 in product crystals), and the yield was defined as the ratio of weight of potassium chloride in the product crystals to the total weight of potassium chloride present in feed solution (usually expressed in wt %). In crystallization experiments with ammoniation of feed solution at 25 "C, practically no enrichment of potassium chloride over up to 50% ammonia concentration in liquor was realized while at 0 OC reasonable purity of potassium crystals was obtained. When ammonium chloride was added to feed solution (up to 5% wt), both purity and yield appeared to improve. Alternatively, instead of cooling the crystallizer, the feed solution was concentrated before ammoniation by evaporating water. In all the experiments -35 wt % ammonia in concentrated mother liquor to 25 "C was achieved at the end of a run. Improvements in both purity and yield were achieved with an increase in percent of water evaporation from feed solution. The cubic crystal habit of potassium chloride did not change when crystallized in the ammoniation process even with treated bitterns. Some twinning was observed especially when a low speed of stirrer was employed. Addition of ammonium chloride to the feed solution not only improved the purity and yield of potassium chloride but also modified the morphology from cubes to platelets. The percentage of agglomerates in product crystals from treated sea bitterns was generally smaller than that from nearly saturated
Ind. Eng. Chem. Res., Vol. 31, No. 2,1992 667 potassium chloride solution. Thus experimental results indicated that reasonable purity of potassium chloride from feed solution could be obtained by two different methods. In the first method the feed solution should be first cooled to 0 "C followed by ammoniation up to about 40 wt % ammonia concentration; perhaps addition of about 2 wt % ammonium chloride to feed solution would have a beneficial effect on both purity and yield. In the second method the feed solution should be first concentrated by evaporation of about 55% by wt of the original solvent and the concentrated solution ammoniated to about 35 w t % ammonia concentration at 25 "C. Although both these routes employing an ammoniation step yield a reasonable purity of potassium chloride (-go%), economic considerations are required to arrive at optimal conditions for any commercial application. Gaska et al. (1965) in their study of separation of potassium chloride from sylvinite clearly pointed out the economic advantages and operating flexibilities of ammoniation processes. Recovery of ammonia by conventional distillation for recycle or its proper utilization in suitable salts was important from economic considerations. Preliminary assessment indicated that this process of recovering potassium chloride from Indian sea bitterns by an ammoniation route deserved serious consideration (Bhatt, 1985; Jagadesh and Chivate, 1988), and further detailed study is required to demonstrate the economic feasibility of several routes of the process.
Conclusions The present study demonstrates the feasibility of the ammoniation process to crystallize potassium chloride from aqueous solution having a composition similar to that of treated sea bitterns with a reasonable purity and recovery. Crystallization of potassium chloride by ammoniation of ita aqueous solution was studied in a 1-L laboratory-scale batch crystallizer. A numerical algorithm to evaluate the concentration profiles of potassium chloride and ammonia in the solution phase and the transient crystal size distribution of product in a batch dilution crystallizer was developed. Calculated results of both concentration profiles and product crystal size distributions were not entirely consistent with experimental observations, the agreement between these being similar to those observed in previous studies (Tavare, 1986, 1987). Nomenclature B = nucleation rate (eq 8), no./(s kg of water) c = solute solution, kg of salt/kg of water cNHa= concentration of ammonia in solution, kg of ammonia/kg of water, kg of ammonia/kg of solution c* = saturation of salt, kg of salt/kg of water Ac = concentration driving force (c - c*), kg of &/kg of water Ac, = interfacial driving force (ci - c*), kg of salt/kg of water CV = coefficient of variation on weight basis, % CV, = coefficient of variation of observed product crystal size distribution, % CV = coefficient of variation of calculated product size Astribution, % D, = mean particle diameter, m G = overall linear growth rate (eq 9), m/s i = relative kinetic order, i.e., exponent of G in eq 8 k, = surface shape factor (=3.675) kd = diffusional mass-transfer coefficient, m/s kL. = liquid-side mass-transfer coefficient, m/s k, = rate coefficient for surface reaction step (eq 9), m/[s[(kg of salt/kg of k, = volume shape factor (=0.525) L = crystal size, m, pm
L,,
= weight mean product size of calculated distribution (=p4/113),
Pm
L, = weight mean size of observed product crystals, pm MT = magma density, kg of crystal/kg of water n = population density, no./(m kg of water) no = nuclei population density, no./(m kg of water) N = stirrer speed, rev/s Q = inlet volumetric flow rate of ammonia, mL/s R" = ammonia addition rate to crystallizer suspension, kg of ammonia/(s kg of water) S = solvent capacity of crystallizer, kg of water t = time, s T = temperature "C, K u = volume of 1 kg of ammonia at inlet conditions, mL/kg Greek Symbols = solid deposition rate, kg of salt/(s kg of water) p = density of solution, kg m3 pc = crystal density, kg/m pj = jth moment of crystal size distribution, no. of crystals mi/ kg of water 7 = run time, s w = tip speed of impeller, m/s CY
I
Subscripts c = crystal f = final j = index variable NH3 = ammonia p = product pm = mean product r = surface reaction w = weight wm = weight mean 0 = initial Wstry No. KC1,7447-40-7;NH,,7664-41-7; NaC1,7647-14-5; CaC12, 10043-52-4.
Literature Cited Beer, W. F.; Mersmann, A. B. Influence of Power Input on Crystal Size in Cooled Draft-Tube Crystallizers. Ger. Chem. Eng. 1980, 3, 333-341. Bhatt, V. Y. Studies in Batch Crystallization. M.Chem. Eng.Thesis, University of Bombay, Bombay, 1985. Bhatt, V. Y.; Chivate, M. R. Separation of KCl and NaCl in a Batch Dilution Crystallizer. Indian Chem. Eng. 1986,B (l),Tll-T13. Chivate, M. R.; Palwe, B. G.; Tavare, N. S. Effect of Seed Concentration in a Batch Dilution Crystallizer. Chem. Eng. Commun. 1979,3, 127-133. Fernandez-Lozano,J. A.; Wint, A. Production of Potassium Sulphate by an Ammoniation Process. Chem. Eng. 1979, 349, 688-690. Fernandez-Lozano,J. A.; Wint, A. Double Decomposition of Gypsum and Potassium Chloride Catalyzed by- Aqueous Ammonia. Chem. Eng. J. 1982, 23, 53-61. Gadre. G. T.: Bhavnamrv. H. M.:Rao. A. V. Potassium Chloride from Sea Bitterns: -11 Recovej of Potassium Chloride, Magnesium Sulphate and Potassium Sulphate. J. Sci. Znd. Res. (India) 1958,17A, 376-378. Gaska, R. A.; Goodenough, R. D.; Stuart, G. A. Ammonia as a Solvent. Chern. Eng. Prog. 1965,61 (l), 139-144. Genck, W. J.; Larson, M. A. Temperature Effects of Growth and Nucleation Rates in Mixed Suspension Crystallization. AZChE Symp. Ser. 1969, 68 (No. 121), 57-66. Glassner, A.; Skurnik, S. Growth of Potassium Chloride Crystals from Aqueous Solutions I The effect of Lead Chloride. J. Chem. Phys. 1967,47, 3687-3689. Goodenough,R. D.; Klopf, A. H. Selective Precipitation of Potassium Chloride by Addition of Ammonia. US. Patent, 3,279897 1966; Chern. Abstr. 1967,66, 20657~. Haesan, I. T. M.; Robinson, C. W. Mass Transfer Coefficients in Mechanically Agitated Gas-Aqueous Electrolyte Dispersions. Can. J. Chem. Eng. 1980,58, 198-205. Jagadesh, D.; Chivate, M. R. Separation of Potassium Chloride from Sea Bitterns. Proceedings of Seminar on Fertilizer Industry: Process Problems and Perspectives; B. H. U., Varanasi, Ed.; 1988; Vol. I, pp 115-117.
Znd. Eng. Chem. Res. 1992,31, 568-574
568
Jira-Arune, N.; Laguerie, C. Croiseance des Cristaux de Chlorure de Potassium en Couche Fluidise’e: Comparaison avec la Dissolution. Chem. Eng. J. 1979,18,47-57. Lal, P.; Kumar, S.; Upadhyay, S. N.; Upadhya, Y.D. Solid-Liquid Mass Transfer in Agitated Newtonian and Non-Newtonian Fluids. Ind. Eng. Chem. Res. 1988,27,1246-1259. Mersmann, A. B. Design and Scale-up of Crystallizers. Znt. Chem. Eng. 1984,24,401-418. Mersmann, A. B.; Beer, W. F.; Seifert, D. Current State of Crystallizer Design. Ger. Chem. Eng. 1979,2,1-13. Paine, K.; Rohani, S. Determination of Potash Crystallization Kinetics in a Laboratory-Scale Continuous Crystallizer. Can. J . Chem. Eng. 1989,67,584-589. Qian, R. Y.; Chen, Z. D.; Ni, H. G.; Fan, Z. Z.; Cai, F. D. Crystallization Kinetics of Potassium Chloride from Brine and Scale-up Criterion. AZChE J. 1987,33 (lo),1690-1697. Qian, R. Y.;Fang, X.S.; Wang, Z. K. Supersaturation and Crystallization Kinetics of Potassium Chloride. Znd. Eng. Chem. Res. 1989,B (6),844-850. Randolph, A. D.; Beckman, J. R.; Kraljevich, Z. I. Crystal Size Distribution Dynamics in a Classified Crystallizer: Part I. Experimental and Theoretical Study of Cycling in a Potassium Chloride Crystallizer. AZChE J. 1977,23 (4),500-510. Randolph, A. D.; White, E. T.; Low, C. D. On-Line Measurement of Fine-Crystal Response to Crystallizer Disturbances. Znd. Eng. Chem. Process Des. Deu. 1981,20(3),496-503. Randolph, A. D.; Chen, L.; Tavana, A. Feedback Control of CSD in a KCl Crystallizer with a Fines Dissolver. AZChE J . 1987,33(4), 583-59 1.
Salun, A. Measere de solubilitie’s dans le biriaire KCl-NH3-H20. Bull. SOC. Chim. Fr. Part ZZ, 1960,1106-1114. Sarig, S.; Glasner, A.; Epstein, J. A.; Eidelman, N. Growth of Potassium Chloride Crystals. J . Cryst. Growth 1977,39,255-266. Sarig, S.;Eidelman, N.; Glasner, A.; Epstein, J. A. The Effect of Supersaturation on the Crystal Characteristics of Potassium Chloride. J. Appl. Chem. Biotechnol. 1978,28,663-667. Sarig, S.; Leci, C. L.; Eidelman, N. Agglomeration of Potassium Chloride Crystals from Supersaturated Aqueous Solutions. J . Cryst. Growth 1979,47,365-372. Silcock, H. L. Solubilities of Inorganic and Organic Compounds; Pergamon Press: Oxford, 1979;Vol. 3,Part 2,pp 61-63. Tavare, N. S.A Reactive Batch Crystallizer. J . Sep. Process Technol. 1986,7,25-30. Tavare, N. S.Batch Crystallizers: a Review. Chem. Eng. Commun. 1987,61,259-318. Tavare, N. S.;Chivate, M. R. CSD Analysis from a Batch Dilution Crystallizer. J. Chem. Eng. Jpn. 1980,13,371-379. Tavare, N. S.; Garside, J. Simultaneous Estimation of Crystal Nucleation and Growth Kinetics from Batch Experiments. Chem. Eng. Res. Des. 1986,64,109-118. Tavare, N. S.; Garside, J. Reactive Precipitation in a Semi-batch Crystallizer. In Recent Trends in Chemical Reaction Engineering; Kulkarni, B. D., Mashelkar, R. A., Sharma, M. M., Eds.; Wiley Eastern: New Delhi, 1987;Vol. 11, pp 272-281.
Received for review June 3, 1991 Revised manuscript received September 11,1991 Accepted September 27, 1991
In Situ Monitoring of Extraction and Separation Behavior of Lipids with Supercritical Carbon Dioxide Yutaka Ikuehima,* Norio Saito, Kiyotaka Hatakeda, and Shota Ito Government Industrial Research Institute, Tohoku, Nigatake 4-chome, Miyagino-ku, Sendai 983,Japan
Masahiko Arai Institute for Chemical Reaction Science, Tohoku University, Katahira, Aoba-ku, Sendai 980,Japan
Kunio Arai Department of Biochemistry and Chemical Engineering, Tohoku University, Aramaki, Aoba-ku, Sendai 980, Japan
On-line supercritical fluid extraction system (SFE) with Fourier transform infrared spectroscopy (FTIR) has been developed. Effluent species can be analyzed by FTIR a t the same high pressures as for the extraction. An on-line supercritical fluid chromatography system (SFC) has also been developed by modifying the SFE system with adding a separation column in the line between the extractor and the FTIR. These systems were used for the measurements of extraction yields of lipids such as higher fatty acids and DL-a-tocopherol and in situ observation of the separation behavior of their mixtures in supercritical carbon dioxide (SC-COJ. The extraction yield was well correlated with a parameter derived from the solubility parameter concept. The separation efficiency was represented by a model introducing several aspects of solvation power of SC-C02as well as interaction forces of solute with stationary (silica gel or silver nitrate coated silica gel) and mobile (SC-COJ phases. In the fields of food and pharmaceuticals, supercritical carbon dioxide (SC-C0.J extraction is noted to be a useful method for the extraction without denaturation of valuable materials such as docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), linoleic acid, linolenic acid, vitamins, and others contained in natural resources (Yamaguchi and Murakami, 1986; Arai and Saito, 1986; Taniguchi et al., 1988). The utility of these compounds was described elsewhere (Hirano et al., 1980; Sanders and Younger, 1981; Yamamoto et al., 1987). When we (Ikushima et al., 1988, 1989a) used SC-C02 for the selective
extraction from a mixture of higher fatty acids, preferred extraction and separation could be attained at near-ambient temperatures by adding entrainers and/or using a separation column attached after the extraction chamber. In this method, the usage of a silver nitrate coated silica gel as the column packing was found to well separate a mixture of higher fatty acids with different degrees of unsaturation (Ikushima et al., 1988). This method enabled a large portion of DHA contained in squid oil to be extracted selectively in a concentration over 90 w t % (Ikushima et al., 1989b). However, it is difficult to make an
0888-588519212631-0568$03.00/0 0 1992 American Chemical Society
