Ind. Eng. Chem. Res. 1993,32, 685-691
686
Separation of 2-Naphthol: Hydrotropy and Precipitation Paul 0. Raynaud-Lacroze and Narayan S. Tavare' Department of Chemical Engineering, University of Manchester Institute of Science and Technology (UMIST),Manchester M60 1 QD, England
A process of precipitating out selectively 2-naphthol from a commercial hydrotrope solution containing a mixture of 1-and 2-naphthols by dilution with water is investigated. Solubility data of 1- and 2-naphthols, determined in aqueous solutions of two hydrotropes (viz., sodium cumenesulfonate and sodium butyl monoglycol sulfate) by the weight disappearance methods at different hydrotrope concentrations and temperatures, are used t o construct a ternary diagram. Sodium cumenesulfonate appears promising for the separation of 2-naphthol for a typical industrial reactor product in crystalline form (with purity > 99 % ) at -80% recovery. Precipitation kinetics of 2-naphthol from its hydrotropic solution with sodium cumenesulfonate using water as a precipitant are studied in a laboratory scale agitated precipitator. The kinetics results obtained from experimental responses are correlated in terms of conventional power law expressions.
Introduction The term uhydrotropen was coined by Neuberg (1916). The phenomenon of h y d r o t r o p y was mainly used in detergents and drug solubilization until recently because of their solubilizing ability in aqueous solutions (Winsor, 1950; Booth and Evertson, 1948,1949,1950; Rath, 1965; Ueda, 1966a-c; Poochikian and Gradock, 1979; Saleh et al., 1983a,b). The process of solubilization of sparingly soluble hydrophobic compounds in the aqueous phase at a relatively high concentration level was exploited to enhance the rates of multiphase chemical reactions and separate selectively close boiling substances through extractive distillation and liquid-liquid extraction (Mckee, 1946;Janakiramanand Sharma, 1985;Pandit and Sharma, 1985;Gaikar and Sharma, 1986;Gaikar et al., 1988,1989). Hydrotropic substances are a class of chemical compounds that augment the solubility of otherwise insoluble organic or inorganic compounds in water. They themselves are freely soluble organic compounds and are effective at high hydrotrope concentrations in enhancing the aqueous solubility because of the possibility of molecular solution structures probably in the form of aggregates (Saleh and El-Khordagui, 1985; Balasubramanian, 1989; Srinivas et al., 1991; Sivakama Sundari et al., 1991). As hydrotropy is operative at high aqueous hydrotrope concentration, the solubilizate (or solute) will precipitate out on dilution with water (i.e., the original solvent) from most hydrotropic solutions. Tavare and Gaikar (1991) used a process of precipitating salicylicacid, from its nearly saturated aqueous sodium salicylate solution, by dilution with water to study the precipitation kinetics. In addition, dilute sulfuric acid was used to produce more salicylic acid by chemical reaction with sodium salicylate. Thus salicylic acid precipitated from its hydrotropic aqueous solution by dilution with water alone or reaction with dilute sulfuric acid (i.e., due to both chemical reaction and dilution effects). This type of processes may be used to recover the solute in crystalline form at an improved purity, and the remaining mother liquor may perhaps be used to concentrate the hydrotrope solution for recycle. A process feasibility of precipitating selectively o-chloronitrobenzene on dilution with the original solvent (i.e., water) from an aqueous hydrotropic solution of sodium butyl monoglycol sulfate (NaI3MGS) containing a mixture of o- and p-chloronitrobenzenes was reported by Geetha et al. (1991). Solubilities of both ortho and para isomers showed a rapid increase above the critical hydrotrope concentration. The enhancement in solubilityof o-chloronitrobenzenerelative 0888-588519312632-0685$04.00/0
to that at the critical hydrotrope concentration (-20fold) was however more than that ofp-chloronitrobenzene (- 8-fold). Different increases in solubilities of these isomers with hydrotrope concentrations were effectively exploited in separation of pure crystalline o-chloronitrobenzene at a reasonable recovery (-70% 1. The purpose of this article was to demonstrate the use of hydrotropes in a selective precipitation of 2-naphthol from its mixture with l-naphthol at a reasonable recovery. In this study two aqueous solutionscommercially available (viz., 40% sodium cumenesulfonate (NaCS) and 50% sodium butyl monoglycol sulfate (NaBMGS)) were used as hydrotropes for this separation. Large quantities of naphthols are produced for the manufacture of organic intermediates for dyes, drugs, perfumes, surfactants, and agrochemicals. The melting points of 1-and 2-naphthols are 96 and 122 "C and the boiling points are 288 and 295 "C,respectively. These two isomers form a simple eutectic at 73 "C and 38.3 mol % 2-naphthol. The scope of the study was confined not only to determining the relevant solubility data and assessing the selectivity and recovery of 2-naphthol but also to developing precipitation kinetic expressions from the experimental responses in terms of significant state variables. Such analytical studies are useful in gaining an understanding of the solubilization mechanism and identifying the efficacy of these hydrotropes in the separation of isomers. Generally conventional separation methods using properties based on molecular size are not suitable for this type of difficult separations. The specificity in solubilization in hydrotropic solution appears to be molecular structure dependent and can be conveniently exploited in selective precipitation of a component from isomeric mixtures.
Solubility Data
'
Solubility data of 1-and 2-naphthols were determined in aqueous solutions of two hydrotropes (viz., sodium cumenesulfonate (NaCS) and sodium butyl monoglycol sulfate (NaI3MGS))by the weight disappearance methods at different hydrotrope concentrations and temperatures. A t 25 OC the weight of the disappeared material in the solution during the equilibration step was evaluated as the difference between the initial weight and final residue to determine the solubility. In the equilibration step a known and excess amount of solubilizatewas equilibrated with the hydrotrope solution (usually about 20 mL) of
0 1993 American Chemical Society
686 Ind. Eng. Chem. Res., Vol. 32, No. 4,1993 T = 298
K
-
CRITICAL HYDROTROPE CONCENTRATION L
I
40
Q,
4-
1
NaCS
l
2
NaBMGS 20
I-NAPHTHOL NaBMGS
or
0
0 I ~
I-NAPHTHOL
2-NAPH TH 3L
2-NAPHTHOL
0
0
20
40
60
100
80
HYDROTROPE CONCENTRATION (g/lOOg water) Figure 1. Solubility data.
Table I. Polynomial Coefficients in Eq 1 NaCS coefficient ao
loa1 1o2az 104~ 106a4
1-naphthol 1.48 -3.48 3.26 -6.34 4.34
2-naphthol 0.95 -2.47 2.09 -3.54 1.97
NaJ3MGS
1-naphthol 2.80 -1.98 1.69 -1.90 0.74
2-naphthol 1.08 -0.59 0.89 -0.75 0.23
known concentration for more than at least 6 h in a magnetically stirred and jacketed vessel (100-mLcapacity) maintained at constant temperature of 25 "C by circulating water from a constant-temperature water bath. The temperature was controlled within -0.1 "C. The slurry was then filtered, the residue on the filter paper was dried and weighed, and the difference in weights between the initial charge and final residue estimated. The solubility data determined empirically by this method for pure components, viz., 1-and 2-naphthols, in two commercial hydrotrope solutions are depicted in Figure 1. These solubility data were adequately correlated, as represented by the solid curves in Figure 1, into a fourthorder polynomial as
+
The values of the critical or minimum hydrotrope concentration for sodium butyl monoglycol sulfate and sodium cumenesulfonate are about 0.8 moVL ( 20 g of NaBMGS/ 100 g of water) and 0.1 mol/L (-2 g of NaCS/100 g of water) and surface tension decreases gradually from 72 mN/m to limiting values of 37 and 43 mN/m, respectively. Solubilities of 1- and 2-naphthols in water at 25 "C are about 30 X lo4 and 75 X lo4 g/100 g of water, respectively. Both the hydrotropes augment the aqueous solubilities of both these isomers by 3-4 orders of magnitude. The relative enhancement in solubility, as determined by the ratio of solubility in neat hydrotrope to that at the critical hydrotrope concentration, for 2-naphthol is more than that of 1-naphthol for both these hydrotropes. The hydrotrope sodium cumenesulfonate appeared promising from both separation and recovery considerations and was employed in all the subsequent studies. The solubility variations with temperature of 1- and 2-naphthols in a neat (40% sodium cumenesulfonate) hydrotrope solution were determined over the range of temperature from 25 to 60 "C. The temperature at which the last crystal disappeared for a known solid concentration in a slow temperature rise (-0.1 OC/1200 s) of the slurry vessel near saturation point was used for the solubility measurements at higher temperatures. The activation energies in the Arrhenius-type solubility-temperature relation were about 25 kJ/mol for both these components over the temperature range 25-60 "C. The solubility of each pure component changed about three times over this temperature range, thus significantly influencing the solubilization process. Both dilution and thermal effects can therefore be exploited to improve the performance of this separation process.
c* = a, alCN + a2CN2 + agCN3 + a4cN (1) where c* is the solubility (expressed in g/100 g of water) of a compound at CN,the concentration of hydrotrope (g/ 100 g of water). The resulting polynomial constants from a least-squares fitting of experimental data are reported in Table I. Most hydrotrope molecules appear to self-aggregate in aqueous solution to form organized assemblies in a stacklike fashion and solubilize the solute by a similar associative mechanism above a minimum hydrotrope concentration. Above the minimum hydrotrope concentration the solubilization rises markedly and levels off to a plateau resulting in a sigmoidal solubility-hydrotrope concentration curve (Balasubramanianet al., 1989;Tavare and Gaikar, 1991; Srinivas et al., 1991). The critical hydrotrope concentration appears to be the characteristic of a hydrotrope as it is the same for many solubilizates.
Ternary Diagram The phase diagram for the present system can be conveniently depicted by a ternary diagram with each apex represented by one of three components, viz., 1-naphthol, 2-naphthol, and water. Solubility data determined by the above techniques over the ranges of hydrotrope concentrations and temperatures for pure components were used to construct a ternary diagram. In addition solubility data were obtained for mixtures of 1-and 2-naphthols of known compositions by the crystal disappearance method. The hydrotrope concentration at which the last crystal disappeared from a slurry with a given composition of two crystalline components in a very slow addition of neat (40% sodium cumene sulfonate) hydrotrope near saturation point was used to determine the solubility. The entire range of hydrotrope concentration can be covered for a fixed solid-phase composition of 1-and 2-naphthols by changing the initial charge to the equilibration vessel. The cross plots of solubility versus composition of a component at different hydrotrope concentration levels were used to construct seven hydrotrope isoplethal (i.e., the same hydrotrope concentration) curves. For the sake of clarity only the enlarged water apex section of the ternary diagram is included in Figure 2, the triangular section depicting two equilibrium isotherms and seven hydrotrope isoplethal curves.
Separation of Naphthols The feasibility of separation for two different mixtures of naphthols with commercial hydrotrope (40% sodium cumenesulfonate) solution was explored. In an industrial manufacturing process of 2-naphthol the product usually consists of 85 wt 5% 2-naphthol and 15 wt % 1-naphthol
Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 687 WATER
B8
2-NAPHTHOL
38.3
F y r e Z . Ternaryphasediagram: the 1-naphthol-2-naphthollnater system(allcomponentconeentrationsinmo1%I. EN = Concentration of NaCS, g/lW g of water.
110
= 0
80
! B I W L
40
the original volume of the neat hydrotrope solution) was addedslowlyforprecipitation. Theslurrywas keptstirring and allowed to equilibrate a t 25 "C in all the experiments. The slurry was then filtered and the residue was dried, weighed, and analyzed on a gas chromatograph. The recovery, R, defined as the ratio of weight of solid 2-naphtholto that of the total 2-naphthol initiallypresent in the solution phase and the purity, P,being the ratio of weight of solid 2-naphthol to the total weight of the solids, were determined. Typical results of recovery and purity for two compositions used in the precipitation experiments are reported in Figure 3. Also included in Figure 2 are the two tie lines joining locations of feed and the equilibrium product in these two separation experiments. Clearly in the case of the eutectic composition the product crystals precipitating from the hydrotropic solution have nearly the same purity as the starting solidphase mixture used for preparing the initial charge. The hydrotrope (40% sodium cumenesulfonate) solution is not successful in this case in improving the purity of the eutectic composition mixture. The intermolecular interactions existing between 1-and 2-naphtholsat the eutectic composition are perhaps stronger than those that can develop between naphthol and hydrotrope molecules in the solubilization process. When hydrotrope assemblies in the solution phase are broken down by dilution with water, none of the naphthol molecules has perhaps a particular interaction with this hydrotrope and therefore they precipitate out at the same composition. The initial solution in the case of the crude reactor product composition was nearly saturated a t 55 "C, and concentration of 2-naphthol was much higher than that at 25 OC due to the increased solubilization at higher temperature. Supersaturation with respect to 2-naphthol was generated as a result of reduction in solubility due to both dilution with water and cooling, and its selective precipitation with reasonable recovery was accomplished in this composition range. Both temperature and hydrotrope concentration appear to he important variables influencing the separation performance. Thus 2-napthol from typical industrial reaction product (15wt % l-naphtholand 85 wt % 2-naphthol) was precipitated selectively by both solvent addition and cooling effects in crystalline form (with purity > 99%) a t -80% recovery).
Precipitation Kinetics
0 0
80 80 PERCENTME of ?-NAPHTHOL
10
40
100
Figure 3. Binary phase diagram for the 1-naphthol-2-naphthol system.
and requires further purification. The binary phase diagram for the 1-naphthol-2-naphthol system as shown inFigure3indicatesthattheisomersformasimpleeutectic at 38.3 mol % 2-naphthol (and 73 "C). Compositions corresponding to crude 2-naphthol from an industrial reactor and eutectic point were used in the separation experiments. Equilibrium precipitation experiments for separation were performed in a small (-100 mL) jacketed and magnetically stirred vessel maintained at 25 "C for the eutectic Composition and at 55 OC for the industrial crude product composition. The temperature was maintained at the constant value by circulating water through the jacket from a constant-temperature water bath. In each experiment a slightly undersaturated solution corresponding to the desired composition and temperature for these two mixtures was initially prepared in the precipitation vessels and a known amount of water (usually twice
Precipitation kinetics of the process of precipitating soluhilizate out from its aqueous hydrotropic solution by adding water a t a controlled rate was investigated in an isothermal batch crystallizer. For the sake of simplicity only one isomer, viz., 2-naphthol, precipitated from its nearlysaturatedsolution withneat 40% NaCS hydrotrope as a result of the reduction in solubility due to changes in aqueous hydrotrope concentration. Experimental responses of transient population density and solution phase concentration data from a solvent capacity varying batch crystallizer were used. The method of s-plane was used to determine both the growth and nucleation rates from a pair of experimentaltransient population density curves obtained from a perfectly mixed and solvent capacity varying precipitator (Tavare and Garside, 1986, 1987; Tavare and Gaikar, 1991). In this method a plot of time rate of change of the Laplace transformed population density, defined on the basis of total solvent capacity, against the product of the Laplace transform variable and the average Laplace transformedpopulation density, based on total solvent capacity over an optimal range of the Laplace transform variable, should yield a straight line
688 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 Pmoutml wdltirr nl( 1 waterboth 2 preclpitant etock eoiutlon 3 perloteltic pump 4 constont head tank 5 needle VOIH 13 rotameter 7 capillary tube
P m y W o r d 8 proclpltator 9 electrlc motor 10 dlgltal rpm meter 11 thermometer 12 aampllng tube 13 waterbath
5 P 0
0
h
*
RUN 6
0
RUN6
v
RUN 7
t
s
-I
5a Figure 4. Experimental setup for precipitation kinetics: semibatch precipitation unit.
W
n
3
o
v)
Table 11. Experimental Conditions
run no.
8 @is)
T (OC)
1
0.033 0.054 0.095 0.065 0.060 0.053 0.056
25 25 25 35 55 25 25
2 3 4 5 6 7
N (rev/min) 350 350 350 350 350 520 670
with slope equal to the negative average growth rate and the intercept the nucleation rate based on the total capacity of solvent. The growth and nucleation rates represent the average value over the time interval, and so all other state variables incorporated in the kinetic correlations should correspond to an average time. All the results of the nucleation and growth rates obtained from experimental runs can be used to correlate empirical power law kinetic expressions in terms of significant state variables.
Experimental Section
A series of precipitation experiments were performed in a small 0.5-L jacketed and agitated glass vessel fitted with four full baffles and a flat glass lid having four entry holes to house impeller, thermometer, water inlet, and sampling port. The schematic diagram of the experimental setup is shown in Figure 4. A 3.5-cm-diameter stainless steel six blade turbine impeller was mounted at the central axis with a clearance of about 1cm from the bottom. The temperature within the crystallizer was maintained at a constant desired value by circulating constant-temperature water from the water bath through the jacket. The precipitant, i.e., water, maintained at the same constant temperature as the crystallizer in a water bath was pumped via a peristaltic pump to a constant head tank and then added continuously at a predecided rate into the crystallizer via a rotameter from the constant head tank. Seven experimental runs as shown in Table I1 were performed to study the influence of three important variables, viz., water addition rate (0.033-0.095 g/s), temperature (25-55 "C), and stirrer speed (5.83-11.16 Hz). In a typical run the crystallizer was initially used to prepare a nearly ( 90-95 % 1 saturated solution of 2-naphtho1 in a 60-mL neat solution of commercial (40% NaCS) hydrotrope at a desired operating temperature. When a clear solution at the working temperature was achieved and stirrer speed was adjusted to a desired value, the precipitant addition was started at a predecided rate to begin the run. Small suspension samples of known volume (-3 mL) were taken at 10-min intervals by means of a
-
-1
Y 0
2
4
6
TIME x 1 0 - 3 ( ~ )
Figure 5. Supersaturation profiles.
syringe fitted with a suction tube from the Crystallizer, all the samples being taken from the same sampling point located at the same level as the impeller. The slurry samples were filtered quickly by a small Biichner filter. The filtrate was used for 2-naphthol concentration analysis while crystals retained on the filter paper were dispersed in the standard electrolyte (Isoton supplied by Coulter Electronics) for crystal size analysis. After the crystallizer was run for a predecided run time ranging from 60 to 120 min, the entire contents were removed and filtered and the product crystals were air-dried to observe the crystal habit by an optical microscope. The concentration of 2-naphthol in the solution phase was determined by iodometry (Fierz-David, 1949). Supersaturation at any time was determined by subtracting the solubility, estimated at the hydrotrope concentration calculated on the basis of mass balance, from the measured concentration. The product crystal size distribution was determined using a multichannel Coulter counter (Model TAII with population count accessory) fitted with a 280pm-diameter orifice tube, measurements being made in the size range 8-120 pm. As only crystals retained on the filter paper were resuspended in Isoton, further precipitation of 2-naphthol from the hydrotropic solution due to dilution and thereby an addition of new nuclei in the size range were avoided.
Results and Discussion During the first three runs both the temperature and the stirrer speed were kept constant at 25 OC and 5.83 Hz, respectively, and the precipitant addition rate varied from 0.033 to 0.095 gls. The operating temperature was varied in the next two runs while the stirrer speed was changed in the last two runs. Measured supersaturation profiles and crystal size distribution data for the final sample are reported in Figures 5 and 6, respectively. Water addition rate has a significant influence on supersaturation profiles and consequently on the product crystal size distributions. For a given water addition rate the solution gets saturated first and then supersaturated as a result of lowering the solute solubility due to reduction in hydrotrope concen-
Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993 689
P
0
X
+
t
3000
A
t
3600 8
0
t
X
t
0
t
. . .
8
4200 8 4800
8
6400 8
0 0
+ x
x
0
&A +0 X
0
+
0
X
+
B
X
10
20
0
40
CllYsTAL a Z t
w
180
(/ld
80
100
140
(pm) Figure 7. Population density data (run 2, T = 25 O C , Q = 0.064g/s). CRYSTAL SIZE
Figure 6. Final differential population size distributions.
tration on dilution. Both particle formation and their subsequent growth deplete the level of supersaturation. Supersaturation therefore passes through a maximum. At high water addition rate supersaturation passes through a sharper and earlier peak resulting in a large number of small-size crystals, while at low addition rate it passes through a wider and later peak yielding a small number of large-size crystals. All these observations are consistent with the previous observations (Tavare and Chivate, 1980; Tavare and Gaikar, 1991). At higher temperature a crystalline product with large size and small number of crystals was obtained perhaps due to higher crystal growth rates that resulted. At high stirrer speed a large number of small-size crystals resulted due to its stronger influence on nucleation rate relative to crystal growth. In all the runs as the precipitant addition at a fixed predecided rate proceeded, the initial charge of undersaturated solution became saturated and then supersaturated prior to an initiation of precipitation. The appearance time as defined by the period of time elapsed between the saturation point and the visual observation of the first crystal in the crystallizer was measured. It decreased with an increase in water addition rate, temperature, and stirrer speed and ranged from 500 to 1150 s over the set of experimental conditions. Optical microscopic observations indicated that most product crystals had well-defined prismatic shape. From the precipitation process point of view the precipitator should be operated at high temperature and low water addition rate with gentle agitation in order to obtain a large product crystal size and facilitate their downstream separation. Kinetic Correlations
Crystal size distribution data obtained from the Coulter counter measurements, some results from run 2 being shown in Figure 7, were used to calculate the crystal growth and nucleation rates by the method of s-plane analysis. Growth and nucleation rates obtained from all the experimental runs (46 observations from 7 runs) using the successive pairs of population density data sets were correlated by power law kinetic expressions as
G = 25A~'.'N-l.~ exp(l3/RT)
(2)
B = 5 X 104G1.1N1.2 exp(25/RT) (3) where G is the overall linear growth (pm/min), B the nucleation rate (no./(min cm3 of slurry)), and R the universal gas constant (8.314 X kJ/(mol K)). Note that the stirrer speed (N, rev/min), temperature (T, K), and supersaturation (Ac, g/100 g water) were significant variables influencing the rate processes during this experimental program. The graphical representations of these correlations depicted in Figure 8 show reasonable quality for precipitation kinetics (multiple correlation coefficient for linear regression r = 0.7 for the growth rate correlation (eq 2) and 0.9 for the relative nucleation correlation (eq 3)). The scatter in growth rate correlations is usually large when such an approach is employed (Tavare and Garside, 1986,1987;Tavare and Gaikar, 1991). The other conventionalterm in eq 3, i.e., magma concentration, was not significant over the range encountered in the present study. The relative nucleation kinetic correlation (eq3) (in terms of G) showeda better fit than the nucleation correlation (in terms of Ac) from this set of experimental results. Such a kinetic representation for the nucleation process has been used in many kinetic studies employing a continuous MSMPR (mixed suspension mixed product removal) crystallizer technique. The correlation for the relative nucleation kinetics not only eliminates the use of unreliable measurements of supersaturation but also neglects the influence of segregation effects present in fast precipitation systems. A possibility of segregation does exist in precipitators operating in a continuous MSMPR, a semibatch, or a capacity varying batch crystallizer mode. In the present case the segregation effects can be minimum at high stirrer speed and low water addition rate in a small volume of the crystallizer. All the exponents in both these kinetic correlations were reasonable. Both growth and relative nucleation kinetic orders (i.e., exponent of Ac in eq 2 and that of G in eq 3) were estimated with low values of the standard error at 95% confidencelevel in parameter estimates (0.09 in both these orders). The negative values of the apparent activation energies in both the correlations (eqs 2 and 3)
690 Ind. Eng. Chem. Res., Vol. 32, No. 4, 1993
0.6
-
-0.6
-
-1.6
-
/A% I
-%
Q
c?
W
0
0 -I
/
-1
B(no/rnin cm3 slurry)
I
/I
-a
n
0
m
4-
W
0 0 -I
Figure 8. Growth and relative nucleation kinetic correlations.
and also the exponent of stirrer speed in the growth rate correlation (eq 2) and high values of the standard error at 95% confidence level in parameter estimates (0.5 in eq 2 and 0.4 in eq 3 for stirrer speed) might be in part due to small variations of temperature and stirrer speed used in this study. The apparent activation energiesfor the growth and relative nucleation processes were about -15 and -25 kJ/mol pointing toward a small temperature influence over the small temperature range encountered. The temperature influence on the solubilization process was however beneficial in augmenting the solute capacity and thus reducing the amount of hydrotrope solution required for a given separation duty. The precise mechanistic implications are rather difficult to attribute to these exponents derived from the limited range of variables employed in this study. Conclusions The use of hydrotropes in a selective precipitation of 2-naphthol from its mixture with 1-naphthol a t a reasonable recovery was investigated. Solubility data of 1-and 2-naphthols were determined in aqueous solutions of two hydrotropes (viz., sodium cumenesulfonate and sodium butyl monoglycol sulfate) by the weight disappearance methods at different hydrotrope concentrations and temperatures. Sodium cumenesulfonate appeared promising for the separation of 2-naphthol from typical industrial mixtures (15 wt % 1-naphthol and 85 wt % 2-naphthol) as 2-naphthol was precipitated selectively by both solvent addition and cooling effects in crystalline form (with purity > 99%) at -80% recovery. It was
however unsuccessful in the separation of a mixture of the eutectic composition. The ternary equilibrium phase diagram for the 1-naphthol-2-naphthol-water system was constructed. Precipitation kinetics of 2-naphthol from its solution with commercial (sodium cumenesulfonate) hydrotrope were investigated in a laboratory scale agitatedvessel using water as a precipitant. The method of s-plane analysis was used to deduce simultaneously growth and nucleation rates from the measured transient population density data in a solvent capacity varying batch crystallizer. Growth and relative kinetic expressions in terms of significant variables were correlated from the experimental results. The kinetic orders of about 0.7 and unity and the apparent activation energies of about -15 and -25 kJ/mol were observed for the growth and relative nucleation processes, respectively. Nomenclature ai = coefficients in polynomials (eq 1) B = nucleation rate (no./(cm3of slurry min)) c = concentration of solute (g/100 g of water) c* = equilibrium concentration (g of solute/100 g of water) Ac = concentration driving force (g of solute/100 g water) CN = concentration of hydrotrope (g of hydrotropeI100 g of water) CN, = critical hydrotrope concentration (g of hydrotrope/100 g of water) F = initial composition of 2-naphthol in the charge, wt % G = overall linear growth rate (pm/min) i = relative kinetic order, i.e., exponent of G in eq 3 L = crystal size (pm) n = population density (no./(pm cm3 of slurry)) N = stirrer speed (rev/min, Hz) P = purity of 2-naphthol, i.e., ratio of weight of 2-naphthol in product to the total solids weight (% ) Q = precipitant (water) addition rate (g/s) r = mgtiple correlation coefficient from a linear regression analysis R = universal gas constant (4.314 X kJ/(mol K)) R = recovery of 2-naphthol, Le., ratio of weight of solid 2-naphthol recovered to total weight of 2-naphthol in original solution (wt %) s = Laplace transform variable with respect to size (m-l) t = time (min, s) T = temperature ("C, K)
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Received for review June 16, 1992 Revised manuscript receiued December 15, 1992 Accepted December 29,1992
