I n d . E n g . C h e m . Res. 1989, 28, 1236-1241
1236
Table VII. Typical Results of t h e U-Peeling Tests: Vega Mixtures (Peel S t r e n g t h i n N/m) time of antistripping agent, 0.2% by wt water curing, h none commercial A, B2 0 200 245 335 270 2 200 245 335 270 24 65 160 300 200
antistripping properties suitable for the actual application. Registry No. (TEPA)(CH,O) (copolymer), 87868- 1G-8; (TEPA)(CH20)(phenol) (copolymer), 27233-92-7; sulfolane, 126-33-0; toluene, 108-88-3.
Literature Cited Andersen, D. A.; Ducatz, E. L.; Petersen, J. C. The effect of antistrip additives on the properties of asphalt cement. Proc.-Assoc. Asphalt Paving Technol. 1982,51, 298-317. Blair, C. M., Jr.; Groves, W.; Lissant, K. J. Carbonate rock aggregate bonded with bitumen containing a polyalkylene polyaminoimidazoline. US Patent 2,812,339, Appl. Sept 2, 1957. Domaney, U‘. J. Stripping characteristics of paving grade asphalts
used in New Brunwick. Proc. Can. Techn. Asphalt Assoc. 1968, 13, 267-272. Ensly, E. K. A study of asphalt aggregate interaction and asphalt molecular interactions by microcalorimetric methods. Postulated interaction mechanism. J . Inst. Pet. 1973, 59, 279. Fromm, H. J. The mechanism of asphalt stripping from aggregate surfaces. Proc.-Assoc. Asphalt Paving Technol. 1974, 43, 191-196. Gianattasio, G. Additivi chimici utilizzati nella moderna industria delle pavimentazioni stradali. Strade Traffic0 1971, 208, 2-13. Giavarini, C.; Maura, G.; Rinaldi, G . Rivestimenti dell’acciaio con bitumi ossidati. Riv. Combust. 1972, 26, 313-317. Kalinonski, M. L.; Crews, L. T. Graf polymer-fortified bitumen additives. US Patent 2,812,339, Appl. Sept 2, 1957. Johnson, J. M. Bituminous composition having increased adhesion to mineral aggregate. US Patent 2,426,220, Appl. Sept 2, 1942. Plancher, H.; Petersen, J. C. Tertiary nitrogen heterocyclic material to reduce moisture-induced damage in asphalt-aggregate mixtures. US Patent 4,325,738, Appl. April 20, 1982. Plancher, H.; Holmes, S.; Petersen, J. C. Role of nitrogen compounds in reducing moisture-induced damage in bituminous pavements. Antek 1982, 2, 6-12.
Received for review August 10, 1988 Revised manuscript received February 23, 1989 Accepted April 12, 1989
Gas Chromatographic Study of the Evaporation from Films Composed of a Volatile Solvent plus a Nonvolatile, Nonpolymeric Liquid Reynaldo C. Castells,* M6nica
L. Casella, and Angel M. Nardillo
C I D E P I N T , 52 entre 121 y 122, 1900 L a Plata, Argentina, and Facultad d e Ciencias Exactas, Universidad Nacional de L a Plata, 1900 L a Plata, Argentina
The evaporation rates from films composed of n-octane + squalane and toluene + sulfolane were measured by a gas chromatographic method. Both the conditions at the gas/liquid interface and the transport of volatile solvent from within the liquid t o the interface determine the evaporation rate. The equations obtained by integrating Fick’s second law for a homogeneous film under the assumptions of constant diffusivity and constant film thickness fail t o interpret the experimental results. In principle, the films can be considered as heterogeneous, with a very thin surface layer where the volatile solvent has a very low diffusivity and an underlying liquid a t uniform composition.
The understanding of the mechanism of solvent-castfilm formation is of great importance in several technological areas. Coating research and development laboratories have been very active in this field (see, for instance, Yoshida (1972), Newman and Nunn (1975), Kornum (19801, Ramsbotham (1980),Holten-Andersen and Hansen (1983), and the references cited therein). The vast majority of these studies have been performed by means of gravimetric techniques, making use of the Shell Thin Film Evaporometer (ANSI/ASTM, 1982),more sophisticated electrobalances, as the Evapocorder developed at the Chevron Research Company (Saary and Goff, 1973),or some other type of electrobalance installed within a wind tunnel (Eaton and Willeboordse, 1980). These instruments were designed for the measurement of total evaporation rates; they are not easily adapted for the measurement of reliable individual evaporation rates from solvent blends. Furthermore, the samples suffer an important evaporative cooling as a consequence of the relatively high flow rates employed (above 10 L/min) (Rocklin and Bonner, 1980), and the experimental results correspond to evaporation rates measured at uncertain temperatures. A method t o measure the evaporation rates of solvents 0888-5885/89/2628-1236$01.50/0
under rigorously controlled conditions has been developed in this laboratory (Castells and Casella, 1987a,b),using a gas chromatograph equipped with a flame ionization detector (FID) and an automatic gas sampling valve. A known quantity of the pure solvent or mixture under study is applied by means of a 100-pL microsyringe onto a filter paper disk (No. 31 Whatman Extra Thick, diameter 2 cm) placed within a thermostated glass cell. A t controlled temperature and flow rate, a nitrogen stream flows parallel to the paper plane, sweeping the vapors toward a sampling valve actuated at a constant frequency, thus injecting the contents of its loop into a chromatographic column where the vapors are separated before entering the FID. Both the evaporation rate of each component in the sample and the composition of the remaining liquid as functions of the time elapsed from the start of the run can be calculated by applying a material balance; the necessary information is the sample initial weight and composition, the nitrogen flow rate, the sampling frequency, and the area(s) of the peak(s) produced by each vapor injection. The FID sensitivity is several orders higher than that of the best electrobalance, thus enabling the measurement of the smaller evaporation rates occurring at low flow rates. As has been experimentally demonstrated in a former paper C 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1237 (Castells and Casella, 1987a), evaporative cooling is thus minimized, and the evaporation rates can be reliably referred to the working temperature. The method has been successfully used with pure solvents (Castells and Casella, 1987a) and with mixtures of volatile solvents (Castells and Casella, 1987b). Plots of evaporation rate against run time for pure solvents rapidly reach a constant height, remain at this plateau for almost all of the experiment, and drop to zero over the last few pulses. The value a t the plateau is considered to be the pure solvent evaporation rate, ul0, a t the temperature and gas flow rate of the experiment. In the present paper, this method is used to study the evaporation of the solvent from its mixtures with a nonvolatile, nonpolymeric substance; the conditions resemble those prevailing during the drying of a film very rich in plasticizer. It represents an intermediate stage toward future studies of drying processes in polymer films, where a larger dependence of diffusivities on concentration complicates the analysis of the results (Hansen, 1965, 1968, 1970). n-Octane + squalane and toluene + sulfolane were chosen as model systems for two reasons: (a) the availability of very good vapor-liquid equilibrium data over an extended range of composition; (b) because the former is representative of negative and the latter of positive deviations with respect to the ideal behavior.
Experimental Section Apparatus and Procedure. A detailed description of the evaporation cell, the chromatographic system, and the procedure employed was presented in earlier reports (Castells and Casella, 1987a,b). All the runs for the system toluene + sulfolane and six of the runs for the system n-octane + squalane were performed applying the samples onto the filter paper disk; in the remaining runs with the latter system, the samples were deposited in a cavity (1.5-cm diameter X 0.1-cm deep) machined in an aluminum plate. The aluminum plate was cleaned by the method proposed by Rocklin (1976). The pure solvent evaporation rate was measured immediately before each run to guarantee that the value used in the calculations referred to exactly the same conditions as those prevailing during the run with the mixture. All the experiments were done at 30 "C. Materials. Sulfolane was purchased from Merck for spectroscopy and squalane was purchased from Kodak; toluene and n-octane were Carlo Erba RPE grade. All the substances were used without further purification. Data Treatment. The valve loop volume was 1 cm3; therefore, if F (cm3/min) represents the nitrogen flow rate through the cell, At (min) the time interval between injections, and a the area under the peak generated by pulse i (injected (i - 1)At min after the run start), then aiF represents the peak area that the mass of solvent which evaporates during 1 min would produce if it were to be passed through the FID. Had all the solvent evaporated between t = 0 and t = ( j - 1 ) A t been accumulated and passed through the FID as a single pulse, the area under the peak thus generated would be A; = FAt[0.5~1+
J
CU~] i=2
The evaporation rate, uj, at the instant of injection of pulse j is given by wlFaj vi=-- AN
w1aj At[0.5~1+
N
CU~]
i=2
(2)
Table I. Summary of the Experiments Performed"
F
XI0
System: n-Octane
u10
+ Squalane; Evaporation from a Paper Disk
0.2923 0.3601 0.5371 0.7729 0.9132 0.3601
20.6 19.9 20.9 20.6 19.9 10.1
1.600 1.608 1.635 1.610 1.595 0.865
System: n-Octane + Squalane; Evaporation from a Metallic Surface 0.2664 0.2664 0.2664 0.2664 0.3601 0.3685 0.3685 0.3685 0.9086
System: Toluene
10.0 19.9 29.7 40.1 20.0 10.4 19.8 20.1 20.1
0.790 1.220 1.520 1.732 1.218 0.796 1.220 1.522 1.220
+ Sulfolane; Evaporation from a Paper Disk
0.3759 0.2963 0.1904
9.92 10.00 10.00
1.505 1.520 1.510
" x : = volatile component initial mole fraction; F = nitrogen flow rate (cm3/min); uIo = pure volatile component evaporation rate under identical conditions to those prevailing during the run with the mixture (mg/min).
where w1 is the mass of volatile solvent initially applied in the cell and N is the number of the first pulse producing a nondetectable peak. The mass, Rj, of volatile solvent remaining in the cell in the same moment is Rj =
W1[1
- (A;/AN)I
(3)
and its mole fraction in the liquid mixture can be calculated by means of (4)
where w2 represents the mass of the nonvolatile component and MI and M2 are the molecular weights of the volatile and nonvolatile components, respectively. The concentrations, cj (mol/cm3), of the volatile component can be calculated from eq 4 by assuming that the mixture excess volumes are negligible. Activity coefficients of n-octane in squalane were calculated by combining a configurational contribution (calculated by means of the well-known Flory-Huggins expression) and a thermal contribution obtained through Ashworth's (1973) experimental values of the interaction parameter and of its dependence on concentration. The NRTL equation, with the values of the parameters obtained by Ashcroft et al. (1979), was used to calculate the activity coefficients of toluene in its mixtures with sulfolane. The experiments carried out have been listed in Table I. The time interval between pulses was 5 min; between 35 and 60 pulses were needed to evaporate all of the volatile component, depending on the initial composition of the mixtures and on the nitrogen flow rate. Differences between the evaporation rates measured from paper substrate and from metal substrate (for the same n-octane + squalane initial composition) can be mostly attributed to differences in the corresponding evaporation surface areas.
Results and Discussion Our liquid films can be idealized as plane sheets bounded by two parallel planes; one of the planes ( z = 0) contacts an aluminum surface, and the other ( z = I ) is
1238 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989
swept by an inert gas stream that flows parallel to it. The initial composition is uniform; heat flows to the film through both of it faces, and since evaporation occurs a t relatively low rates, the process can be considered as isothermal (Castells and Casella, 1987a). Two extreme conditions can be imagined for the evaporation from a film of these characteristics: (I) The process is controlled exclusively by the conditions at the liquid/gas interface. Diffusion within the condensed phase is fast enough to relax any concentration gradient resulting from the evaporation. It is accepted that drying of polymer films during the initial or “wet” phase occurs under these conditions (Hansen, 1965,1968,1970). The simplest assumption is to consider that the flux normal to the interface, J , (mg/(cm2.s), is proportional to the difference between the true volatile solute activity at the interface, a,, and the activity, a,, that would be in equilibrium with the vapor at a remote point (Crank, 1956): J , = ul/S = d ( a , - a,) (5) where u1 is the rate of evaporation, S is the area of the liquid/gas interface, and CY‘ is a constant of proportionality. The cell is swept by pure nitrogen; therefore, a, = 0. On the other hand, since the condensed phase is homogeneous, the value of the activity of the volatile component will be the same at all its points, Le., a, = a, = ylxl, where y1 represents the rational activity coefficient. If activity coefficients are defined according to the symmetrical or Raoult’s law reference system, and if it is assumed that eq 5 is obeyed over all the compositions range, then it can be written that u1 = a’Sal = uloal = ulOylrl (6)
(11) The process is controlled exclusively by diffusion within the condensed phase. The activity of the volatile solvent at the surface reaches equilibrium with the gas instantaneously; since the cell is swept by pure nitrogen, a, falls immediately to zero. The rate of evaporation shall depend exclusively on the capability of the diffusion process within the liquid film to furnish the volatile component at the surface (from where it immediately evaporates) and shall be independent of the gas flow rate. It is accepted that drying of polymer films during the second or “dry” phase occurs under these conditions. When the extreme possibilities I and I1 are contrasted against the experimental evidence gathered in this laboratory, the following are found: (a) A strong dependence of the evaporation rates on the gas flow rate has been detected for all the studied systems; exclusively diffusive control can thus be discarded. (b) Experimental plots of evaporation rates against time for mixtures of two or three volatile solvents (Castellsand Casella, 1987b) are coincident with the curves computed by means of eq 6 with activity coefficients calculated by the UNIFAC method (Fredenslund et al., 1977; Gmehling et al., 1982) and a repetitive procedure similar to that employed by Walsham and Edwards (1971) and by Rocklin and Bonner (1980). The coincidence exists both for the mixture total evaporation rates and for the individual evaporation rates of the components. Evaporation from totally volatile mixtures seems to occur by a process of type I and is properly described by eq 6. (c) For the systems studied in the present paper, the evaporation rate increases, although not linearly, with the activity of the volatile solvent in the condensed phase; with the only exception being the first two or three points obtained with samples initially very rich in volatile solvent, the experimental evaporation rate a t a given activity is notoriously lower than the value calculated with eq 6.
Figure 1. Evaporation rate, u (mg/min), against mean activity, til, for five mixtures of n-octane + squalane of different initial composition. Nitrogen flow rate: 20 mL/min. Evaporation from paper disk. n-Octane initial mole fractions: 0.9132 (m); 0.7729 (+); 0.5371 (0); 0.3601 ( 0 ) ;0.2927 (0). Dashed line: u:d, = I.6OOd1.
Figure 2. Evaporation rate, u (mg/min), against mean activity, ti1, for four mixtures of n-octane + squalane of different initial composition. Nitrogen flow rate: 20 mL/min. Evaporation from metallic 0.3685 (m); 0.3605 plate. n-Octane initial mole fraction: 0.9086 (0); (+I; 0.2664 ( 0 ) .Dashed line: u:dl = 1.220d1.
Points belonging to experiments performed a t the same nitrogen flow rate with mixtures of different initial composition fall on a common curve in plots of evaporation rate against activity. These trends are shown in Figures 1-3; the influence of the nitrogen flow rate on the drying of a given mixture of n-octane + squalane is depicted in Figure 4. The values of activity employed in these plots, al,are those corresponding to mole fractions calculated by means of a material balance (eq 4);if the evaporation process is diffusion limited, a concentration gradient shall
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1239 component in its upper layers, both components leave the mixture simultaneously (although at different rates) in systems of type b, and composition changes in the liquid phase occur at a lower velocity. Second, nonvolatile solvents in systems of type c shall be constituted by relatively large or associated molecules; diffusion coefficients of a volatile solvent in these environments can be 2 or 3 orders of magnitude lower than in its mixtures with another volatile solvent. Diffusion in mixtures of type c is neither rapid enough to relax concentration gradients within the liquid nor so slow as to allow the surface to reach equilibrium with the vapor. An attempt to interpret the process may be done by considering one-dimensional diffusion between the parallel planes at z = 0 and at z = 1 and integrating the corresponding expression of Fick's second law,
0
0.5
1.0
51
Figure 3. Evaporation rate, u (mg/min), against mean activity, a,, for three mixtures of toluene + sulfolane of different initial composition. Nitrogen flow rate: 10 mL/min. Evaporation from paper 0.2963 (+I; 0.1904 disk. Toluene initial mole fractions: 0.3759 (0); (a). Dashed line: ul0& = 1.620ti1.
F.30 I m l / m l n
F: 19 8
under the assumptions that both the volatile component diffusion coefficient, D , and the film thickness, 1, are constant. The boundary conditions are as follows: BC1. There is no mass transfer through the metal/ liquid interface; therefore,
ac/az
=o
z
=o
(8)
BC2. In order to take into account mass-transfer resistance in the gas phase, the flux normal to the liquidlgas interface may be expressed either by Fick's first law or by an equation similar to eq 5:
ml/min
where a is a mass-transfer coefficient (Crank, 1956, Carslaw and Jaeger, 1959). Since c, = 0, the condition may be written as
ac/az F:
IO
+ hc = o
z=i
(10)
L ml / m n
with h = a / D . The initial condition is
c=co
t=O
(11)
The assumption of constant D and 1, indispensable to obtain an analytical solution for the system of eq 7,8, 10, and 11, is open to severe criticism. Khatir et al. (1986), however, have demonstrated that toluene sorption and evaporation from rubber sheets is well described by diffusion with a constant diffusion coefficient. The method of separation of variables affords the following well-known general solution for eq 7: Figure 4. Evaporation rate, u (mg/min), against mean activity, dl, at different nitrogen flow rates for a mixture of n-octane + squalane. n-Octane initial mole fraction: 0.3685. Evaporation from metallic plate.
build up over the film thickness, the compositions calculated by the material balance shall represent average values, and the surface concentrations shall be smaller than the average values. These trends could account for the observed differences between the experimental evaporation rates and the results obtained with eq 6. The evaporation from mixtures containing one nonvolatile component demands for a model combining characteristics both of I and 11. The differences between the behaviors described under b and c can result as a combination of two factors. First, while only one component evaporates from mixtures of type c, leaving behind a solution enriched in the nonvolatile
m
c(z,t) = C ( A , cos (A,z) n=l
+ B, sin (Xnz))e-X-*Dt (12)
where A,, B,, and A, are determined by the initial and boundary conditions. BC1 demands that B, = 0, and using BC2, it is possible to write A, sin (AJ) = h cos (AJ)
(13)
or its equivalent PntgPn = L
(14)
with 0, = h,l and L = h l = a1ID. Positive roots of eq 14 are real and lie within the interval [ ( n- l ) ~ (n,- 1/2)?r]; Carslaw and Jaeger (1959) have tabulated their values for n between 1and 6. Expressions for A , may be obtained in a similar way to that in which coefficients in the Fourier series are found: by the initial condition,
1240 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989
(15)
The orthogonality of the integrals on the right-hand side of eq 15 has been demonstrated by Carslaw and Jaeger; therefore, eq 15 may be reduced to 1
co&
cos (X,z)
1
dz = A , S cos2 (X,z) 0
dz
”’1 . 5
I
1.0-
Integrating and using eq 13, it is found that
and the dependence on concentration of z and t may be written as
0
Starting with eq 18, it is possible to obtain expressions for the evaporation rate, u, and for the mean concentration at time t , F:
10
20
40
30
Figure 5. Evaporation rate, u (mg/min), against mean concentration, El (mol/mL), for a mixture of n-octane + squalane. n-Octane initial mole fraction: 0.9086. Nitrogen flow rate: 20 mL/min. Evaporation from metallic plate. f ( z ) , and the concentration drops from C to c, across
Both series appearing in eq 19 and 20 are strongly convergent for moderate and large times. To a very good approximation, only the first term is retained in each of them, and by combining the resulting expressions, it is possible to write u =
as-cPLI 2
Equation 21 states an apparently linear relationship between u and F. Actually, for a given weight of nonvolatile solvent, the film shall be thicker (and, consequently, L shall be larger) at higher volatile solvent concentrations; since p12(L is a rapidly decreasing function of L, plots of u against E should be concave toward the F axis. Experimental plots confirm this prediction (see Figure 5); the effect is more notorious in runs starting with mixtures rich in volatile solvent, because more important changes in film thickness occur in those instances. This model is unable to explain how, in runs performed at the same nitrogen flow rate but starting with different volatile solvent concentrations, equal values of evaporation rates are reached for equal mean concentrations (see Figures 1-3). If x I o and xIo’ represent the volatile solvent initial mole fraction in two different runs, it is easily demonstrated that the corresponding f i thicknesses 1 and l’shall be related by 111’ = (1- xl0)/(1 - xlO’)when the mean concentration is equal in both films. Differences in initial concentrations should result in different evaporation rates at the same mean concentration. As a matter of fact, no homogeneous film model is able to explain both the concavity of the u against c plots and the coincidence of such plots for runs performed at the same flow rate but with different initial compositions. An explanation for these experimental facts may be attempted by assuming that a surface layer with a thickness s 1. The existence of a surface skin having properties different from those of the underlying layer was proposed by Crank and Park (1951) for the drying of polymer films and was latter adopted by several authors (Sletmoe (1970) and Kornum (1980), for instance). Our experimental results seem to indicate that this behavior is not limited to polymer films and that the understanding of the drying process in films constituted by substances of relatively low molecules weight, such as squalane or sulfolane, could demand such a model.
Acknowledgment The author? thank the Consejo Nacional de Investigaciones Cientificas y TBcnicas 1CONICET) and the Comisi6n de Investigaciones Cientificas de la Provincia de Buenos Aires (CICPBA) for the financial support of this work. Registry No. n-Octane, 111-659; squalane, 111-01-3; sulfolane, 126-33-0; toluene, 108-88-3.
Literature Cited ANSI/ASTM Evaporation Rates of Volatile Liquids. In Annual Book of ASTM Standards; American Society for Testing and Materials: Philadelphia, PA, 1982; Part 27, pp 761-771, Standard Test Method D3539-76. Ashcroft, S. J.; Clayton, A. D.; Shearn, R. B. Isothermal VaporLiquid Equilibria for the Systems Toluene-n-Heptane, ToluenePropan-2-01, Toluene-Sulfolane, and Propan-2-01-Sulfolane. J.
Ind. Eng. Chem. Res 1989,28, 1241-1245 Chem. Eng. Data 1979,24,195-199. Ashworth, A. J. Activity Coefficients of C5 to C8 Hydrocarbons in Squalane and Dinonyl Phthalate a t 30 OC Determined by a Vacuum Microbalance Technique. J. Chem. SOC.,Faraday Trans. 1 1973,69,459-466. Carslaw, H. S.; Jaeger, J. C. Conduction o f H e a t i n Solids, 2nd ed.; Oxford University Press: London, 1959. Castells, R. C.; Casella, M. L. Solvent Evaporation Rates Measured by Gas Chromatography. Prog. Org. Coat. 1987a,15,73-81. Castells, R. C.; Casella, M. L. Evaporation Rates of Solvent Blends Measured by Gas Chromatography. J. Chromatogr. 1987b,402, 65-72. Crank, J. T h e Mathematics of Diffusion; Oxford University Press: London, 1956. Crank, J.; Park, G. S. Diffusion in High Polymers: some Anomalies and their Significance. Trans. Faraday SOC.1951,47,1072-1084. Eaton, R. F.; Willeboordse, F. G. Evaporation Behavior of Organic Cosolvents in Water-Borne Formulations. J.Coat. Technol. 1980, 52(660),63-70. Fredenslund, A,; Gmehling, J.; Rasmussen, P. Vapor-Liquid Equilibria using UNIFAC; Elsevier, Amsterdam, 1977. Gmehling, J.; Rasmussen, P.; Fredenslund, A. Vapor-Liquid Equilibria by UNIFAC Group Contribution. Revision and Extension. Ind. Eng. Chem. Process Des. Dev. 1982,21,118-127. Hansen, C. M. The Free Volume Interpretation of Plasticizing Effectiveness and Diffusion of Solvents and Plasticizers in High Polymers. Off. Digest Fed. SOC.Paint Technol. 1965,37(480), 57-77. Hansen, C. M. A. Mathematical Description of Film Drying by Solvent Evaporation. J. Oil Colour Chem. Assoc. 1968,51,27-43.
1241
Hansen, C. M. Polymer Coatings. Concepts of Solvent Evaporation Phenomena. Ind. Eng. Chem. Prod. Res. Dev. 1970,9,282-286. Holten-Andersen, J.; Hansen, C. M. Solvent and Water Evaporation from Coatings. Prog. Org. Coat. 1983,11, 219-240. Khatir, Y.;Bouzon, J.; Vergnaud, J. M. Liquid Sorption by Rubber Sheets and Evaporation: Models and Experiments. Polym. Test. 1986,6, 253-265. Kornum, L. 0. Evaporation and Water Dilutable Coatings. J. Oil Colour Chem. Assoc. 1980,63,103-122. Newman, D. J.; Nunn, C. J. Solvent Retention in Organic Coatings. Prog. Org. Coat. 1975,3,221-243. Ramsbotham, J. Solvent Formulation for Surface Coatings. Prog. Org. Coat. 1980,8,113-141. Rocklin, A. L. Evaporation Phenomena: Precise Comparison of Solvent Evaporation Rates from Different Substrates. J. Coat. Technol. 1976,48(622),45-57. Rocklin, A. L.; Bonner, D. C. A Computer Method for Predicting Evaporation of Multicomponent Aqueous Solvent Blends at any Humidity. J. Coat. Technol. 1980,52(670),27-36. Saary, Z.;Goff, P. L. New Instrument to Measure Solvent Evaporation. J. Paint Technol. 1973,45,45-55. Sletmoe, G. M. The Calculation of Mixed Hydrocarbon-Oxigenated Solvent Evaporation. J.Paint Technol. 1970,42(543),246-259. Walsham, J. G.; Edwards, G. D. A Model of Evaporation from Solvent Blends. J. Paint Technol. 1971,43(554),64-70. Yoshida, T.Solvent Evaporation from Paint Films. B o g . Org. Coat. 1972,1, 73-90.
Received for review August 4, 1988 Accepted February 28,1989
Mass Transfer of Liquid Cumene in ZSM-5 Zeolites Using a Novel Volumetric Apparatus Vasant R. Choudhary,* Ajit S. Mamman, and Vikram S. Nayak Chemical Engineering Division, N a t i o n a l Chemical Laboratory, P u n e 411 008, I n d i a
Mass transfer of cumene in ZSM-5 zeolites from the liquid phase a t 283-328 K has been studied using a novel volumetric apparatus. The influence of temperature and zeolite parameters such as cation type (viz., H ' , Na', and NH,'), dehydroxylation, and poisoning of the strong acid sites (by pyridine) of the zeolite on the mass transfer has been investigated. The mass transfer of cumene in ZSM-5 and the activation energy of diffusion are found to be strongly affected by the above zeolite parameters. The influence of the parameters on the mass-transport rate is explained by considering the changes in the effective channel diameter, particularly at channel intersections, and the interaction of diffusion species with cations in the zeolite. Intracrystalline diffusion plays a very vital role in ZSM-5 zeolites in deciding the product distribution in catalytic processes (Weisz, 1980) and also in adsorption-separation processes in which the adsorption is kinetically controlled (Dessau, 1980). Most of the previous studies on diffusion in ZSM-5 dealt with gaseous compounds (Choudhary and Srinivasan, 1986a). In the present investigation, sorptionlmass transfer of pure liquid in ZSM-5 using cumene as a model sorbate has been investigated. The influence of the type and/or degree of cation exchange, calcination temperature, and presence of foreign compounds (or poison molecules) in the zeolite channels on the mass-transport rate of cumene has been studied. The sorption kinetic data were obtained by using a novel volumetric apparatus which is similar in principle to that used earlier by Satterfield and Cheng (1971) but less cumbersome and very easy to operate.
Experimental Section The unit cell compositions of the ZSM-5 zeolites (Si/Al = 31.1; average crystal size = 2.08 pm) used in this study 0888-5885/89/2628-1241$01.50/0
are
as
follows:
for
NH4-ZSM-5,
( N H 4 ) 2 , g ~ N a ~ . ~ l A 1 ~ . ~ g S i 9 3 . ~ 1 0 ~ gfor 2.nH H-ZSM-5, 20;
H2.9sNao.olA12.99Sig3.0101~~~nH20; for HeNa-ZSM-5, ~ 1 , 3 ~ ~ ~ l , 6 ~ ~ ~ Z . 9 9 ~ i 9 3 . 0 ~ ~ 1 9 ~ ' ~ ~ ~ ~ ~
The crystal size distribution of the above zeolites is narrow. The zeolite crystals were pressed binder-free and crushed to the required particle size. The acidity distribution on the H-ZSM-5 and H-NaZSM-5 zeolites (Table I) has been determined by the chemisorption and stepwise thermal desorption of pyridine from 473 to 673 K, using GC techniques (Choudhary and Nayak, 1982; Choudhary, 1983). The preparation and characterization of the zeolites were given earlier (Nayak and Choudhary, 1982). Satterfield and Cheng (1971) used a glass apparatus for investigating the kinetics of single-component sorption of organic liquids in type Y zeolites. Their apparatus is, however, quite cumbersome and required fusing and breaking of the glass capillary for every sorption experiment. We have developed a very simple and easy to operate apparatus, which does not require fusing and 0 1989 American Chemical Society
