Extraction of Ginger Oil with Supercritical Carbon Dioxide

Feb 8, 1996 - Supercritical fluid extraction has received increasing attention in a variety of fields due to the following features: (a) supercritical...
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Ind. Eng. Chem. Res. 1996, 35, 607-612

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Extraction of Ginger Oil with Supercritical Carbon Dioxide: Experiments and Modeling Bhupesh C. Roy, Motonobu Goto,* and Tsutomu Hirose Department of Industrial Science, Graduate School of Science and Technology, Kumamoto University, Kumamoto 860, Japan

The extraction rate of oil from freeze-dried ginger root with supercritical carbon dioxide was measured as a function of CO2 flow rate, particle size, temperature, and pressure. The extraction curves were independent of flow rate in a plot of oil yield versus extraction time. This indicated that the extraction process is controlled by intraparticle diffusion within a particle of ginger root. The extraction rate increased as the particle size decreased due to a decrease in the diffusion path. In the case of temperature and pressure effect, a crossover effect was observed where the higher temperature favored the extraction at 24.5 MPa, while the lower temperature favored the extraction at 10.8 MPa. The shrinking core model was applied to analyze the experimental results, with the effective diffusivity and solubility as fitting parameters. The model successfully fitted the experimental data for larger particle size. Introduction

Table 1. Properties of Pure CO2 at Various Temperatures and Pressuresa

The extraction of soluble spice essences is an important operation in industrial food processing. One of the major spices with widespread uses in foods, beverage, and medicine is ginger (Purseglove et al., 1981; Govindarajan, 1982). The major pungent constituents of ginger are the gingerols, the most abundant of these being 6-gingerol, 1-(4′-hydroxy-3′-methoxyphenyl)-5-hydroxydecane-3-one (Connell and Sutherland, 1969; Connell, 1970). Extraction kinetics of 6-gingerol from ginger was studied using acetone (Spiro and Kandiah, 1989) and supercritical CO2 (Kandiah and Spiro, 1990) as solvent, and chromatographic analyses of gingerol compounds in ginger were studied by Chen et al. (1986). Supercritical fluid extraction has received increasing attention in a variety of fields due to the following features: (a) supercritical fluids provide high solubility and improved mass-transfer rates; (b) operation can be manipulated by changing the pressure or temperature. Carbon dioxide is used as the supercritical fluid mainly because it is a safe, noncombustible, inexpensive, odorless, colorless, tasteless, nontoxic, and readily available solvent. Its low viscosity enables it to penetrate the matrix to reach the material being extracted, and its low latent heat of evaporation and high volatility mean that it can be easily removed without leaving a solvent residue. By varying the temperature and pressure of the CO2 during extraction, the flavor or odor components can be selectively extracted. See Table 1 for the properties of CO2. The extraction of the flavoring materials from vegetable or fruit juices (Schultz and Randal, 1970) and many other natural products (Caragay, 1981; Moyler, 1984) with supercritical CO2 has been carried out. This method shows great potential in replacing conventional methods such as liquid solvent extraction and steam distillation (Meyer-Warnod, 1984). Natural materials may be regarded as a porous matrix containing solute components. The extraction process from porous solid materials by supercritical fluids usually includes dissolution of solid or liquid component into pore liquid, diffusion in porous materi* Author to whom correspondence is addressed. Fax: +81 96 342 3679. E-mail: [email protected].

0888-5885/96/2635-0607$12.00/0

pressure [MPa] 24.5 17.6 10.8

a

temp [K]

F [kg/m3]

µ × 105 [kg/m‚s]

313 328 343 313 328 343 313 328 343

879.5 806.2 730.0 814.6 712.5 599.1 676.1 397.4 283.6

8.15 6.85 5.67 6.99 5.39 4.09 4.69 2.74 2.40

Key: F ) density; µ ) viscosity.

als, and external mass transfer around the solid particle. A diffusion model was used to explain the extraction rate of caffeine from coffee beans by Brunner (1984). The extraction behavior of adsorbed solute from an adsorbent such as activated carbon was explained by intraparticle and external mass transfer with linear or nonlinear adsorption-desorption kinetics (Srinivasan et al., 1990). Extraction of essential oil from peppermint leaves was analyzed by a model involving adsorption phenomena (Goto et al., 1993). The extraction behavior of oil from tomato seeds was studied, and the extraction rate was analyzed by the models containing the masstransfer process both in the solvent phase and in the solid phase (Roy et al., 1994). The objective of this paper is to study the extraction rate of oil from freeze-dried ginger with supercritical CO2 as a function of solvent flow rate, particle size, temperature, and pressure. The shrinking core model is employed to analyze the extraction data by using the effective diffusivity and solubility as fitting parameters. Experimental Section Experimental Apparatus. The experimental apparatus is shown in Figure 1. Oil was extracted with supercritical carbon dioxide in a continuous-flow extractor. Liquid carbon dioxide from a cylinder with a siphon attachment is passed through a cooling bath of about 273 K and compressed to the operating pressure by a HPLC pump (Jasco, PU-980). Compressed carbon dioxide flows into the heat exchanger and the extraction column placed in a constant-temperature bath that maintains the operating temperature within (0.1 K. © 1996 American Chemical Society

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Figure 1. Schematic of the experimental setup. Table 2. Flow Rate Effect on Parametersa pressure [MPa]

temp [K]

Q × 105 [kg/s]

u × 104 [m/s]

τ [s]

Re

kf × 104 [m/s]

24.5

313

2.2 4.4 6.3

5.7 10.7 16.1

88 47 31

16 30 46

0.31 0.43 0.55

DAB ) 4.25 × 10-9 m2/s and dp ) 2.56 × 10-3 m. Key: Q ) mass flow rate of supercritical CO2; u ) superficial velocity; τ ) resistance time; Re ) Reynolds number; kf ) film mass-transfer coefficient. a

The temperature is monitored continuously by means of a thermocouple. The extraction column is 50 mm long × 7.5 mm i.d. The effluent from the column is passed through a high-pressure UV/vis spectrophotometer (Jasco, 780-UV), which measures the absorbance of the effluent. The exit fluid from the extractor is expanded to ambient pressure by a back-pressure regulator (Jasco, PC 880-81), and extracted solute is collected in an empty tube. The pressure in the extractor is maintained by the back-pressure regulator. The pressure in the system is monitored at the pump and at the back-pressure regulator. Since the pressure at these points did not give a meaningful difference, that is, there were no pressure drops within the heat exchanger and the extractor, the pressure monitored at the pump was considered as the operating pressure. Experimental Procedure. Matured ginger root (local product) was washed, sliced, freeze-dried, and sieved into several grades of particle size. The initial oil content was 3.8 wt %. About 0.02-0.025 g of sample was placed between glass beads in the extractor for the uniform distribution of solvent flow. Then the extractor was placed into the constant-temperature bath. After the pressure and fluid flow rate reached the desired values, a six-port valve was turned on so that carbon dioxide was passed through the extraction column to start the extraction. At the beginning of a run, the highest flow rate (10 mL/ min at the pump condition) was used to reach the desired pressure quickly to reduce the initial lag time which depends on the operating flow rate. The effect of flow rate was studied at 313 K and at 24.5 MPa for an average particle size of 2.56 mm (7-8 mesh). The flow rates of CO2 employed were (2.2-6.3) × 10-5 kg/s, and the density of CO2 at this condition was 879.5 kg/m3. Accurate flow rates were measured at the exit by a dry gas meter. Flow rate effects are given in Table 2. For studying the effect of particle size, extractions were conducted at a temperature of 313 K, at a pressure of 24.5 MPa, for three particle sizes 2.56, 0.68, and 0.35 mm in average diameter, and at a flow rate of 4.4 × 10-5 kg/s. The effect of temperature and pressure was studied for particle size 0.35 mm and for flow rate 2.2 × 10-5 kg/s. The temperatures and pressures employed were 313-343 K and 10.8-24.5 MPa, respectively. The

Figure 2. Schematic drawing of the shrinking core model.

absorbance of the effluent was monitored continuously with the spectrophotometer at a wavelength of 270 nm. The absorbance was converted to the concentration in the effluent solvent by the following relation:

∫0tA dt

C ) FMA/Q

(1)

Mathematical Model This model describes the situation of the irreversible desorption followed by diffusion in the porous solid through the pores. When the mass-transfer rate of the solute in the nonextracted inner part is much slower than that in the outer part where most of the solute has been extracted, or the solute concentration is much higher than the solubility of the solute in the solvent phase, a sharp boundary may exit between the outer and inner regions. A core of the inner region shrinks with the progress of the extraction. These situations described in Figure 2 can be modeled by the shrinking core model. The following assumptions have been made to derive fundamental equations. The solvent flows axially with an interstitial velocity v through a packed bed in a cylindrical extractor of height L. Pure solvent enters the bed. The process is isothermal. Considering axial dispersion, the material balance on the bulk fluid phase in the extractor is:

∂C ∂C ∂2C 1 -  3kf +v ) DL 2 (C - Ci(R)) ∂t ∂z  R ∂z

(2)

Time variation of the solid phase concentration (average oil concentration in a particle) is equated with the rate of mass transfer of the solute within the external film surrounding the particle.

∂q j ∂t

)

3kf (C - Ci(R)) R

(3)

The diffusion in the outer region is given by

( )

De ∂ 2 ∂Ci r )0 ∂r r2 ∂r

(4)

Solid phase solute exists within the core.

q j q0

)

() rc R

3

(5)

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Boundary conditions are given as follows. At the core boundary the concentration in the fluid phase is at its saturation value.

Ci ) Csat

at r ) rc

(6)

Diffusion flux at the outer surface of a particle is equal to the mass transfer through the external film.

(

De

{

)

∂Ci ∂r

r)R

) kf(C - Ci(R))

Initial conditions are given as follows.

vC - DL

∂C ) 0 at z ) 0 ∂z

∂C )0 ∂z C)0 Ci ) Ci,0 rc ) R

at z ) L at t ) 0 at t ) 0 at t ) 0

(7)

}

(8)

The following dimensionless groups are defined to derive dimensionless formulas of the fundamental equations.

x ) C/Csat

xi ) Ci/Csat

ξ ) r/R

a ) vR2/DeL

Z ) z/L

θ ) (De/R2)t

yj ) qj /q0

Pe ) Lv/DL

x0 ) Ci,0/Csat Figure 3. (a) Effect of CO2 flow rate on the extraction yield of ginger oil at 313 K and 24.5 MPa versus extraction time. (b) Effect of CO2 flow rate on the extraction yield of ginger oil at 313 K and 24.5 MPa versus CO2 flowed.

b ) Csat/q0

Bi ) kfR/De

After several manipulations of dimensionless equations, variations of dimensionless concentration and dimensionless core radius are given by:

3Bi(x - 1) ∂x a ∂2x 1 -  ∂x (9) +a ) ∂θ ∂Z Pe ∂Z2  1 - Bi(1 - 1/ξc) ∂ξc bBi(x - 1) ) 2 ∂θ ξ [1 - Bi(1 - 1/ξ )]

(10)

yj ) ξc3

(11)

c

{

c

Boundary and initial conditions are

x-

1 ∂x ) 0 at Z ) 0 Pe ∂Z

∂x )0 ∂Z x)0 xi ) x0 ξc ) 1

at Z ) 1 at θ ) 0 at θ ) 0 at θ ) 0

}

Figure 4. Effect of particle size on the extraction yield of ginger oil at 313 K and 24.5 MPa.

(12)

These differential equations coupled with boundary and initial conditions were solved numerically by Crank Nicholson’s method. Results and Discussion Flow Rate Effect. The extracted material was a golden-brown oily liquid, and the yield was about 3.8% w/w dry weight basis. The effect of CO2 flow rate is shown in Figure 3a,b. Figure 3a is a plot of yield versus time, and Figure 3b is a plot of yield versus the total weight of CO2 that has been passed through the extrac-

tor reduced by the weight of the original sample. The yield is defined by the weight extracted divided by the weight of the original sample. In Figure 3a all curves lie on a single line whereas the extraction curve raised steeper for the smaller flow rate in Figure 3b. This suggests that intraparticle diffusion resistance is dominant in this extraction process. Particle Size Effect. Figure 4 is a plot of yield versus the total weight of CO2 that has been passed through the extractor reduced by the weight of the original sample, showing the effect of particle size on the extraction rate. The extraction rate increased with decreasing the particle size because the intraparticle diffusion resistance is smaller for smaller particle size due to the shorter diffusion path. In the case of the largest particle size (2.56 mm), intraparticle diffusion resistance offers a significant effect. Effect of Temperature. Figures 5-7 show the effect of temperature at 24.5, 17.6, and 10.8 MPa,

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Figure 5. Effect of temperature on the extraction yield of ginger oil at 24.5 MPa.

Figure 6. Effect of temperature on the extraction yield of ginger oil at 17.6 MPa.

Figure 7. Effect of temperature on the extraction yield of ginger oil at 10.8 MPa.

respectively. It is evident from these figures that the extraction rate increased with an increase in temperature at 24.5 MPa and decreased with an increase in temperature at 10.8 MPa. However, there is no such effect at 17.6 MPa, where all the curves lie almost on a single line. Therefore, the effect of temperature reversed around 17.6 MPa. This crossover effect has been observed in solubility (Goto et al., 1993; King and Bott, 1993). This behavior was explained by King and Bott (1993) by the competing effects of the reduction in solvent density and the increase in solute vapor pressure with an increase in temperature. At lower pressure, the change of solvent density is more effective than the solute vapor pressure, as extraction rate increases with a decrease in temperature. However, the extraction rate does not increase any more with solvent density and starts to decrease. At 24.5 MPa, the extraction rate is dependent on the solute vapor pressure and it increased with an increase in temperature.

Figure 8. Effect of pressure on the extraction yield of ginger oil at 313 K.

Figure 9. Effect of pressure on the extraction yield of ginger oil at 343 K.

Effect of Pressure. Figure 8 shows the effect of pressure at 313 K. At constant temperature, the density of the solvent increased with an increase in pressure but the vapor pressure of the solute decreased with an increase in pressure. At elevated pressure, the magnitude of such a density change becomes smaller (Marentis, 1988) and the solute vapor pressure change becomes more effective and can easily overcome the effect of solvent density change on the extraction rate. Due to this fact, Figure 8 reveals that at 313 K the lower pressure favored the extraction rate where the vapor pressure of the solute is dominant in this extraction process. Figure 9 shows the effect of pressure at the temperature of 343 K. Elevation of pressure showed a positive effect on the extraction. At this temperature the increase in solvent density with pressure overcame the relatively small change of solute vapor pressure, resulting in favored extraction at higher pressure. This crossover property due to the solubility is controlled by a balance between solvent density and solute vapor pressure changes with the change of pressure and temperature. The same crossover effect of solubility was observed for menthol where the highest solubility was observed around 15 MPa at 313 K (Goto et al., 1993). However, the decrease in solubility with pressure up to 30 MPa was not observed at higher temperature. Estimation of Properties for Theoretical Analysis. The density of CO2 at experimental conditions was estimated from Adachi’s correlation (Malanowski and Anderko, 1992). The viscosity of CO2 at higher pressures was calculated by the residual viscosity correlation of Stiel and Thodos (1964) using the low-pressure gas viscosity estimated by the Golubev (1959) method. These estimated values are shown in Table 1. The

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binary diffusivity DAB of 6-gingerol-CO2 at supercritical condition was estimated by the method of Takahashi (Reid et al., 1987) based on low-pressure values from the Fuller equation (Reid et al., 1987). The estimated binary diffusivity was 4.25 × 10-9 m2/s at 24.5 MPa and 313 K. The value of the porosity, β, calculated by (1 Fp/Fs), was 0.81, where Fp and Fs are the densities of the solid matrix with and without void volume of the solid matrix, respectively. The densities Fp and Fs were 112 and 611 kg/m3, respectively. The bed void fraction  was about 0.90. The external mass-transfer coefficient, kf, was estimated from the Wakao-Kaguei correlation (1985) as shown in Table 2. Analyses of the Extraction Curves by the Model. The binary diffusivity and external mass-transfer coefficient were estimated by correlations. Unknown parameters are, thus, intraparticle diffusivity, De, and solubility of the solute in solvent, Csat. The shrinking core model was fit to the experimental data with intraparticle diffusivity and solubility as fitting parameters. As an initial guess value for intraparticle diffusivity (De), β2DAB (Wakao and Smith, 1962) was used. These parameters optimized for the experimental data at various flow rates as shown in Figure 3 are the intraparticle diffusivity, De ) 2.50 × 10-10 m2/s, and solubility, Csat ) 1.23 kg/m3 (0.001 38 kg/kg CO2). The predicted De value was an order of magnitude smaller than the initial guess value and larger by 3 orders of magnitude than the value obtained by Reverchon et al. (1993) for the extraction from some herbaceous leaves. Their values were, however, some orders of magnitude smaller than those obtained by Brunner (1984) for rapeseed oil extraction. Reverchon et al. (1993) explained that these differences could be related to the different mass-transfer resistance offered by the different kinds of plant materials due to different types of cell structure and different mechanisms of solute extraction. The effective diffusivity obtained here was about 2 times larger than that for the extraction of ginger with acetone (Spiro and Kandiah, 1989). Since natural materials have cellular structure, diffusion across the cell walls may be more restricted than that for porous materials such as catalyst or adsorbent where continuous pores exist in a particle. Diffusion resistance may also depend on the nature of the solute such as molecular size, hydrophilic property, etc. The solubility, Csat, obtained was similar to that of caraway essential oil extraction (Sovova et al., 1994). Comparing with literature, these two optimized values in this work are reasonable. The broken lines in Figures 3a,b and 4 indicate the predicted curves. In Figure 3a,b, calculated curves agreed well with experimental curves. The initial part of the curves was significantly influenced by the solubility, and the later part was almost controlled by the diffusivity. Figure 4 shows the comparison results between experimental and predicted curves for the different particle sizes. For larger particle size, the model satisfactorily described the extraction curve, but in the case of smaller particle sizes, only the initial part was fitted well. In the case of smaller particle sizes, particles of ginger root were not spherical and tended to agglomerate, resulting in channeling in the bed and nonhomogeneous distribution of the solute in the bed within the extractor. In addition to these nonidealities, the shrinking core model may not be suitable for smaller particle sizes

where the cell size is almost the same as the particle size. Therefore, experimental data for smaller particle size could not be simulated with the shrinking core model. Conclusions Oil from freeze-dried ginger root was extracted by using supercritical carbon dioxide as a solvent to measure the effects of solvent flow rate, particle size, temperature, and pressure on the extraction rate. The experimental data were described by the shrinking core model. The intraparticle effective diffusivity, De, and the solubility, Csat, were optimized to provide the best fit of the data for the effect of flow rate and for the effect of particle size. Intraparticle diffusion resistance was dominant in this extraction process. A crossover effect was observed for the effect of temperature and pressure. Acknowledgment This work was partly supported by a Grant-in-Aid for Scientific Research (No. 04238106) from the Ministry of Education, Science and Culture, Japan. Nomenclature A ) absorbance C ) solute concentration in CO2 in the bed void volume, kg/m3 Ci ) solute concentration in the pore volume of solid, kg/ m3 Csat ) saturation concentration of solute in fluid phase, kg/ m3 DAB ) binary diffusivity of solute and solvent, m2/s De ) effective binary diffusivity, m2/s DL ) axial dispersion coefficient, m2/s dp ) particle diameter, m kf ) film mass-transfer coefficient, m/s L ) bed length, m M ) mass extracted, kg q ) solid phase concentration, kg/m3 q j ) average solid phase concentration, kg/m3 q0 ) initial solid phase concentration, kg/m3 Q ) mass flow rate of supercritical CO2, kg/s r ) radial coordinate in particle, m rc ) radius of the core, m R ) radius of the solid particle, m t ) time, s u ) superficial velocity, m/s v ) interstitial velocity of solvent in bed, m/s z ) axial distance, m Greek Letters R ) void fraction in bed β ) porosity of the solid µ ) viscosity of CO2, kg/m‚s F ) density of CO2, kg/m3 τ ) residence time, s Dimensionless Groups a ) dimensionless interstitial velocity (vR2/DeL) b ) Csat/q0 Bi ) Biot number (kfR/De) Pe ) Peclet number (Lv/DL) Re ) Reynolds number (2RuF/µ) Sc ) Schmidt number (µ/FDAB) x ) dimensionless concentration in the external fluid (C/ Csat) xi ) dimensionless concentration in the pore of the particle (Ci/Csat) θ ) dimensionless time ((De/R2)t)

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ξ ) dimensionless radial coordinate in the particle (r/R) ξc ) dimensionless core radius in the particle (rc/R)

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Moyler, D. A. Carbon Dioxide Extracted Ingredients for Fragrances. Perfum. Flavor. 1984, 9, 109. Purseglove, J. W.; Brown, E. G.; Green, C. L.; Robins, S. R. J. Spices; Longman: London, 1991; p 447. Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties of Gases and Liquids, 4th ed.; McGraw-Hill: New York, 1987; p 587. Reverchon, E.; Donsi, G.; Osseo, L. S. Modeling of Supercritical Fluid Extraction from Herbaceous Matrices. Ind. Eng. Chem. Res. 1993, 32, 2721. Roy, B. C.; Goto, M.; Hirose, T.; Navaro, O.; Hortacsu, O. Extraction Rates of Oil from Tomato Seeds with Supercritical Carbon Dioxide. J. Chem. Eng. Jpn. 1994, 27, 401. Schultz, E. G.; Randall, J. N. Liquid Carbon Dioxide for Selective Aroma Extraction. Food Technol. 1970, 24, 94. Sovova, H.; Komers, R.; Kucera, J.; Jez, J. Supercritical Carbon Dioxide Extraction of Caraway Essential Oil. Chem. Eng. Sci. 1994, 49, 2499. Spiro, M.; Kandiah, M. Extraction of Ginger Rhizome: Kinetic Studies with Acetone. Int. J. Food Sci. Technol. 1989, 24, 589. Srinivasan, M. P.; Smith, J. M.; McCoy, B. J. Supercritical Fluid Desorption from Activated Carbon. Chem. Eng. Sci. 1990, 45, 1885. Stiel, L. I.; Thodos, G. The Viscosity of Polar Substances in the Dense Gaseous and Liquid Regions. AIChE J. 1964, 10, 275. Wakao, N.; Smith, J. M. Diffusion in Catalyst Pellets. Chem. Eng. Sci. 1962, 17, 825. Wakao, N.; Kaguei, S. Heat and Mass Transfer in Packed Beds; Gordon & Breach: New York, 1985.

Received for review June 14, 1995 Revised manuscript received October 17, 1995 Accepted November 7, 1995X IE950357P

X Abstract published in Advance ACS Abstracts, January 1, 1996.