Ind. Eng. Chem. Res. 1994,33, 1803-1808
1803
Theoretical and Experimental Study on Membrane Distillation in the Concentration of Orange Juice Vincenza Calabrb,' Bi Lin Jiao,t and Enrico Drioli Chemical Engineering Section, Department of Chemistry, University of Calabria, and CNR Institute on Membrane and Chemical Reactors, 87036 Arcauacata di Rende (CS), Italy
Membrane distillation is a relatively new membrane process in which two solutions a t different temperatures are separated by a microporous hydrophobic membrane. This process can be used to concentrate fruit juices. The transport phenomena involved in membrane distillation for the concentration of fruit juices are described. Heat and mass transfer balances have been formulated, accounting for the geometry and morphology of the membrane and the properties of juice solutions. An experimental study of the concentration of orange juice with a hydrophobic poly(viny1idene fluoride) flat membrane is also presented in this paper. The experiments mainly inspect the transient phenomena, the flux decay with respect to concentration increase, the effects of operating conditions on the permeate flux, and the membrane retention of orange juice compositions. The results show that membrane distillation is a potential and feasible concentration technique.
Introduction Actually, multistage vacuum evaporation is the most used technology in orange and lemon juice concentration; it implies however a loss of fresh juice flavors, cooked taste, and color degradation. The citrus industry has developed essence recovery and blending techniques, producing concentrates that are acceptable to consumers but still far from fresh juice. New techniques are of interest for improving product quality. Pressure-driven membrane concentration is the most promising alternative (Pauson et al., 1985;Drioli et al., 1988). Membrane processes such as microfiltration (MF), ultrafiltration (UF), and reverse osmosis (RO) have been widely used in the food industry, i.e., for the concentration of milk, tomato, instant coffee, and tea. Membrane processes, in general, are very attractive for their simplicity and flexibility. They operate molecular separations particularly compact and simple in their design and their scale-up. The RO process in the fruit juice concentration technique has been of interest to the citrus industry; it presents the advantages of a lower thermal damage to the product, reduction in energy consumption,and lower capital equipment costa (Benjamin and Albert, 1988; Gadea, 1987; Waker, 1990). However, the main disadvantage of RO is the final concentration of citrus juices that is generally limited by membranes and equipment to about 250-300 g/L with the most efficient flux and solute recovery, (Merson et al., 1980;Kimura and Nakao, 1987). By membrane processes and particularly by integrated membrane systems based also on new membrane operations such as membrane distillation (MD), it is possible to concentrate food solutions at higher values. MD in comparison with pressure-driven membrane processes has the advantage not to suffer strong limitation when high osmotic pressures are involved. An improved taste might be expected, with respect to the evaporation processes, due to the lower operating temperature and stresses. We have reached concentration levels up to 600 g/L and higher using membrane distillation with sugar solutions (Drioli et al., 1987). Experiments in juice concentration have been carried out in this paper, with the aim to analyze the potentiality * T o whom correspondence should be addressed at the University of Calabria. + Present address: Citrus Research Institute, Chinese Academy of Agriculture Sciences, 630712 Beibei, Chongqing, People's Republic of China
of MD in juice concentration by integrated membrane systems, considering the effect of viscosity and the necessity of juice pretreating. In some proposed systems the fruit juice can be treated in UF to remove pulp and pectic substances, responsible for the turbidity of the juice. The UF concentrate, rich in pectic substances, can be used as a source of pectins, while the UF permeate can be concentrated in RO at 200250 g/L. The RO concentrate, after a low heating at a maximum of 45 "C, can be further concentrated in MD. A similar process can be applied to other fruits. The possibility of producing concentrated juice also from kiwi is related to the preliminary reduction of the high pectin content of these fruits. UF is an ideal candidate for this scope (Jiao et al., 1992). The integration of these systems has been studied to evaluate the optimal operative parameters. An experimental analysis to investigate the potential capacity of membrane distillation in orange juice concentration has been carried out. The development of membrane distillation at the industrial application level requires the understanding of the complex simultaneousheat and mass transfer (Calabrb and Drioli, 1993). The process studied in this work is based on the use of microporous hydrophobic membranes in direct contact with the solutions having the feed at a temperature higher than the permeate. The phenomenon is related to the formation of a vapor phase at the warm solutionmembrane interface, to the transport of the vapor phase through the microporous membrane with a convective and/ or diffusive mechanism, and to its condensation at the cold membrane-solution interface. The driving force is the vapor pressure difference between the two solutionmembrane interfaces due to the existing temperature gradient. The most important heat and mass transfer resistances are located in the film near the membrane-solution interfaces and inside the membrane and can be correlated with the operating parameters and with the chemical and physical properties of the membrane and treated solutions, (Schofield et al., 1987; Drioli et al., 1987; Bandini et al., 1991; Calabrb and Drioli, 1993). The specific objective of this research was to study the effects of operative conditions (such as feed and permeate bulk temperature and their difference, flow rate, and feed concentration) on transport resistances, the driving force,
0888-588519412633-1803$04.50/0 0 1994 American Chemical Society
1804 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 Film worm side
,:-
Film cold side
Membrane
I
Bulk warm side
,Tfb
T
,P;b
T t m ,Ptm
F = l.064(
side
E
X A
6
Figure 1. Schematic heat and mass transport profiles in membrane distillation.
and, consequently, the permeate flux. The main aim of this paper is the analysis of heat and mass transfer phenomena in juice concentration via MD; experiments have been limited preliminarly at low concentrated juice (250-300 g/L), using flat membranes. Finally, the rejection and solute retention of juice components in the membrane distillation have been measured and analyzed to verify the possibility to introduce membrane distillation in the productive cycle of juice processing.
Theoretical Analysis of the Membrane Distillation Process When a microporous hydrophobic membrane separates two aqueous solutions a t different temperatures, a selective mass transfer across the membrane occurs: the process takes place at atmospheric pressure and at temperatures that may be much lower than the boiling point of the solutions. In Figure 1a schematic description of heat and mass transfer phenomena involved in MD is presented. The hydrophobic properties of the polymeric material prevent bulk transport of the liquid phase across the membrane. A maximum critical pore size exists at which the liquid penetrates the microporous hydrophobic phase, and for a given pore size rp,a critical penetration pressure P, can be defined by the Laplace equation:
P, = (
2 cos ~ WrP
(1)
Water vapor can be transported across the pores of the membrane from the warm side, condensing at the cold surface. When a vapor phase is maintained in the deaerated membrane pores, the hydrophobicity of the membrane prevents the transport of the liquid phase across the microporous membrane. The transport of water vapor can be described in terms of gas permeation through microporous structures: the resistance imposed by the membrane structure, in deaerated systems, can be described by either the Poiseuille flow model or the Knudsen diffusion model, introducing the effective diffusion coefficient through isotropic porous materials as De, = &/x
3). If the mean free path is much greater than the pore size, the Knudsen diffusion is the dominant mechanism (eq 4).
z)(m) MW
0.5APmo
(4)
These laws are correct for deaerated systems, without coupling effects between the Knudsen and Poiseuille mechanisms. In the citrus industry, the deaeration of the juice is generally not effected. This means that in the analysis of real juice these laws will be consequently modified (Bird et al., 1960; Sarti and Gostoli, 1986). In deaerated systems a further effect will be considered: both the pore size and free path of the molecules are influenced by the solute interaction with the membrane and by the variation of the membrane or solution temperature. In this case it is necessary to control the operative conditions and consequentlythe mass transport mechanism. Furthermore, when both the mechanisms are present, or in the case of a large pore distribution with pore sizes higher and lower than the free molecular path, a hindrance coefficient can be introduced to correct the flux. The factors affecting vapor flux in deaerated systems are heat and mass transfer resistances, membrane thermal permeability, and the amount of solute in the feed. The most important heat and mass transfer resistances are located in the film near the membrane-solution interfaces and inside the membrane and can be correlated with the operating parameters and with the chemical and physical properties of the membrane and of the treated solutions. The feed solution concentration Cf mainly influences the vapor pressure PO and film transfer coefficient hf. The correct driving force in concentrated solutions becomes (Sarti and Gostoli, 1986)
APmD= P,,"(1-
Xf,m)
- Pp,mo
(5)
In the experiments with orangejuice, the concentration was not higher than 300 g/L; the contribution of the molar fraction (about 0.03)to the vapor pressure was considered negligible. Convective heat transfer coefficients in the film near the membrane, hfand h,, and the overall heat transfer coefficient in the membrane, H,can be used to evaluate the thermal flux across the membrane using a series-parallel resistance model: qf = hf(Tf,b
-
Tfpl)
(6)
(8)
with qf = qm = qp = 4. Heat transfer in the pores of the membrane occurs by two mechanisms, latent heat transfer accompanying vapor flux and heat transfer by conduction across the membrane, as expressed by the following:
(2)
A comparison of the mean free path of the gas and the mean pore size of the structure permits the dominant transport mechanism to be individuated (Schofield et al., 1990). If the mean free path of the vapor is much less than the pore size, then the dominant flux is viscous (eq
Convective heat transfer coefficients hfand hpshould be calculated by Nusselt correlations (Bird et al., 1960) or by experimental data. In the latter, the overall convective heat transfer coefficient h may be calculated as l / h = l/hf + l/hp
(10)
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1805 Introducing the overall heat transfer coefficient U as
U=
1
llh, + 1/H + l/h,
PVDF Mernbrane
(11)
the correlation between the measured thermal difference in the bulk of the solutions ATb and the real driving force ATmmay be formulated as a function of heat transfer coefficients:
The Clausius-Clapeyron law permits the driving force
AP," to be correlated with the measured temperature and vapor pressure differences in the bulk, ATb and @bo (Sarti and Gostoli, 1985). The simplified ClausiusClapeyron law has been used to formulate the relationship between ATmand Urno:
Figure 2. Schematic of the membrane distillation laboratory plant. I
A
0
-
I
-
c. = ioe g/i Tp,b = 20 OC W = W
= 5.0 kg/min
rl
- 2 h
with i = b or m. The thermal driving force reduction on the membrane, ATJATb, represents the temperature polarization coefficient, indicated as TPC, and can be assumed equal to the vapor pressure driving force reduction, as follows:
"s\ 31
v
W = W
= 2.5 kg/min
c*
0 ,
I
I
Consequently the mass flux in the pores can be estimated as
where
with dominant viscous flux and KM = 1.064(
E)(m) MW
OS5
with dominant Knudsen flux. In the case of orange juice, the most important physical parameters are the viscosity and density of the solution, strongly dependent on the temperature and concentration. These two parameters characterize the Prandtl number in the heat transfer coefficient evaluation. Consequently it is possible to separate their effects on the permeate flux: in the case of two different feed concentrations, Cfl and Cn,or different feed rates W Iand w2, it is possible to evaluate the ratio of the permeate flux as
Experimental Section The experimental analysis has been carried out by studying all the effects to predict the best operative conditions for reaching high concentration with low flux reduction. Materials and Methods. Source of Juice and Preparation of Feed Juice Samples. A commercial orange juice 540 g/L concentrate, pasteurized, obtained from Panagrum, SPA, Italy, was used to prepare singlestrength samples. It was stored frozen a t -3 "C before the experiment.
Results and Discussion Effect of Operative Parameters on the Permeate Flux. The functionality of permeate flux versus feed temperature is shown in Figure 3; the flux gradually increases with an increase of the feed juice temperature for each flow rate a t a constant inlet temperature of the cooling water. The increase in feed temperature means an increase of all transport parameters, with an improvement of the driving force (Drioli et d., 1992).
1806 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1
3-
I
various TI,b n 0
I
I
I
I
0.8 I
C. = 108 g/l
I
I
I
- T,,b
I
I
I
= W ,= 5.0 kg/min h = 0.82 kW/mg K
W
-
I
-4
h
2-
m
"R\
/
rn
$1 -
45
- 25 C'
35
- 20
v
h
A
0 , 1
1
OC
I
I
I
I
2
3
4
5
WI= W
h = 0.25 kW/mP K
(kg/min)
Figure 4. Permeate mass flux v8 feed and permeate flow rates at different temperature conditions. Table 1. Heat Transfer Coefficients and the Knudsen Diffusion Coefficient as Functions of the Membrane Temwrature
22.5 25.0 27.5 30.0 32.5 35.0
1.697 1.699 1.683 1.676 1.669 1.662
865 957 1060 1173 1299 1435
194 199 203 206 210 213
422 443 463 484 504 523
Similarlythe permeate flux increasestoo, with a decrease of the cooling water temperature, as shown in Figure 4, where the correlation with the feed rates w fand wp is also shown. The reduction of the flow rate implies the reduction of the Reynolds number and, consequently, the reduction of the transport coefficients. The permeate flux increases by increasing the crossflow rate for each different operating temperature; the additional shearing forces generated a t the high flow rate in fact reduce the fouling phenomena due to the deposition of particulates such as pectin and cellulose on the membrane surface. Moreover, a lower cross-flowvelocity causes a lower Reynolds number, thus hindering the heat transfer from the bulk of the solution to and from the membrane surface with an increase of the temperature polarization phenomena (Drioli et al., 1992). Mass Transfer Analysis. The proposed mass transfer mechanism was suggested to be Knudsen diffusion as a consequence of the pore size, d,= 0.22 pm, and the mean free path, equal to 0.3 pm. The Knudsen coefficient was calculated by eq 17 (assuming the tortuosity factor as a parameter to estimate), in the range of T, from 20 to 40 "C. The behaviors of the overall membrane heat transfer coefficient H and of the overall heat transfer coefficient the values are summarized Uwere found as function of Tm; in Table 1. The temperature polarization coefficient was calculated for each Tm value. Its behavior is shown in Figure 5,at different feed rates. It is possible to describe the different effects of the membrane temperature on the heat transfer coefficients H and U and mass transfer coefficient KM. The resistance in the film near the membrane increases the temperature profile decay dramatically, which means a TPC close to 0.2 at a low feed flow rate. The increase in the feed flow rate allows the module performance to be improved as a consequence of a reduced resistance in the films of about 50 5%. To calculate the theoretical distillate flux, it is necessary to estimate the tortuosity factor and to correct the diffusion coefficient. In Figure 6 the correlation between experimental and theoretical flux is shown, as a function of different tortuosity factor values at different feed flow
2.0
-
2
"a\ 3 '1.0 1 Fil
0.0
y 0
F-.
I
I
1
2
(kg/m' s) 010'
6.0 Knudsen Diffusion model
p 4.0 "33.0 B
1
1
0
= x =
OOCOO X 0
1
00000
2
Few, (kg/m2
3 s)
4
5
0.89 1
i
6
*loS
Figure 6. Predicted distillate mass flux vs experimental maaa flux, assuming differenttortuosityfactorvalues: (a,top) feed and permeate flow rate equal to 2.5 kg/min, (b, bottom) feed and permeate flow rate equal to 5.0 kg/min.
rates. By the least-squares method the best correlation was found for x equal to 1.2. In Figure 7 the comparison between the experimental and predicted fluxes, calculated on the basis of eq 15, assuming a tortuosity factor equal to 1.2,is shown. The correlation between the permeate flux and vapor pressure difference is practically linear in this range of operating conditions. The factors of correlation between the experimental data and theoretical predictions have been calculated. They are 0.996 and 0.950, respectively, for 2.5 and 5 kg/min feed mass rates.
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1807 4 ,
I
I
I
I
0.0
4.0
2.0
APb.
I
I
8.0
I
1
I
1 .o
8.0
I
2.0
Cr /Cree
(kPa)
(/I
I
3.0
Figure 7. Permeate mass flux vs bulk vapor pressure difference: comparison between experimental resulta and predicted values from eq 15.
Figure 8. Permeate mass flux decay expressed as the ratio between actual and initial mass fluxes vs the concentration factor expreesed as the ratio between actual and initial feed concentrations.
Effects of Operating Parameters on Flux Decay. The flux decay as a consequence of the feed concentration increase has been observed and can be attributed to the reduction of the driving force due to the decrease of the vapor pressure of the feed solution and to the exponential increase of the viscosity of the juice solution. As a consequence, it is possible to find an optimum feed temperature value which by reducing initial driving force reduces the flux decay too (Calabrb et al., 1992). Referring to eq 14, it is possible to observe that the ratio of TPC is a function of the kinetic viscosity:
Table 2. Retention of Orange Juice Constituents in Membrane Distillation Concentration Experiments, G/C, = 3.0 permeate concn (g/L) constituent Cf.o= 105 g/L Cf= 315 g/L solute recovery (%) glucose O.OO0 O.OO0 100.00 fructose o.OO0 O.OO0 100.00 saccharose 100.00 0.000 0.000 malic acid 100.00 0.000 O.OO0 Na 0.002 0.003 99.95 K 0.002 0.003 99.99 0.025 0.027 99.97 MI3 O.OO0 O.OO0 100.00 PO4
TPC
&=
fl{-)
Consequently, the ratio between mass fluxes can be expressed as
Plotting both terms of this equation as functions of the feed concentration in Figure 8, it is possible to conclude that the flux decay can be attributed to the increase of the kinetic viscosity v; on the contrary, the influence of the vapor pressure decrease is negligible. Therefore, the form of the functional f1 is
f1W =y-l and finally the flux decay can be evaluated as
which permits the flux decay to be estimated by evaluating the variation of vapor pressure and viscosity as a function of the concentration factor Cf/C,,, when all other parameters are constant. Effect of MD on the Composition of Orange Juice. It was observed that the PVDF membrane used in the experiment has very good retention of orange juice compoundssuch as soluble solids,sugars,and organicacids, with rejection of sugars and organic acids equal to 100% (Drioli et al., 19921, see Table 2. The retention of sugars and organic acids in the concentration of fruit juices is important due to the sweetness, sugar/acid ratio, and their overall organoleptic concentration. The color and flavor of concentrated juice were satisfactory. No formal test by experts, however, has been made. Effect of UF Pretreatment on the Permeate Flux of Orange Juice. As shown in Figure 8 the mass flux
decay can be attributed to an increase of viscosity as a consequence of a rich amount of pulp and pectins of the concentrated orange juice. The reduction in the pectin content can reduce the viscosity of the solution also at a high concentration level (Todisco et al., 1992). Further experimental study with integrated ultrafiltration and membrane distillation units has been carried out, to compare the performance of the juice concentration in the presence or in the absence of a pretreatment. Commercial module ROMICON HF43 PM50 with polysulfone hollow fiber with a molecular cutoff of about 50000 has been used in the ultrafiltration system. Polypropylene hollow fibers from ENKA Inc. with 0.1 m2 and 0.2-pm pore size were used in an integrated ultrafiltration and membrane distillation system. The ultrafiltration test permits pulp and pectin to be removed and a retentate concentrated 3.5 times and a permeate juice at low viscosity level to be obtained. The permeate was the feed in membrane distillation: as shown in Figure 9, an increase of the permeate flux in MD was observed without flux decay when the concentration was increased from 100 to 400 g/L.
Conclusions Membrane distillation is a new membrane process that can be used in the food industry for concentration or separation technology. Because it can be carried out at atmospheric pressure and at a temperature that can be much lower than the boiling point of the solution, the MD process can be used to concentrate solutes sensitive to high temperature, also at high osmotic pressure. In this paper, the study of the suitability of MD in the concentration of orange juice has been carried out. The effects of operating conditions on the permeate flux are especially described and discussed in this work to find some useful correlations. The results show that the MD process has a significant potential role as a concentration process for
1808 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 Greek Symbols
.
Ultrafiltered Juice
"0 0.6 3
-
-
Un-Ultrafiltered Juice
y = surface tension in the capillary 6 = membrane thickness (m)
A = difference c = membrane porosity (76 ) 6 = contact angle (dimensionless) p = viscosity (Pa s) Y = kinematic viscosity (m2/s) p
Lr
W, = 5.0 kg/min 0.0
pores (Pa m)
1
Wp = 3.5 kg/min I
= density (kg/m3)
x = membrane tortuosity (dimensionless) I
Literature Cited
citrus juices. However, more research needs to be done to improve the membrane and membrane system, to optimize the operating conditions, and to determine the influence of MD on the citrus juice compositions in detail. During the course of our experiments no significant change in membrane wettability was observed. This phenomenon might however occur, particularly on longterm and high-concentration runs. A standard cleaning procedure with NaOH and HC1has been used for restoring the initial permeability of the membrane. The possibility of controlling the flux decay due to the increase of pectins and suspended solids in the feed by preultrdiltering the juice has been evidenced. Acknowledgment We wish to thank the Panagrum SPA for supplying the juices. B.L.J.acknowledges the financial support of the CNR during his stay in Italy. Nomeaclat ure A = membrane area (m2) C = concentration (g/L) dp = pore diameter (m) D = diffusivity (m2/s) Deft = diffusivity in the pore (m2/s) F = mass flux [(kg/(m2s)l h = heat transfer coefficient in the film [(J/s)/(m2K)l H = membrane heat transfer coefficient [(J/s)/(m2K)] k = conductivity coefficient [(J/s)/(mK)1 K M = membrane characteristic coefficient [(kg/(m2s))/Pal MW = molecular weight (kg/kmol) P = pressure (Pa) Po = vapor pressure (Pa) q = heat flux [(J/s)/m21 rp = pore radius (m) R = ideal gas constant [(J/(kmol K)] T = temperature ("C or K) U = overall heat transfer coefficient [(J/s)/(m2K)1 w = flow mass rate (kg/min) x = molar fraction (dimensionless) A",, = heat of vaporization (J/kg) TPC = temperature polarization coefficient (dimensionless) Subscripts
b = bulk f = feed
m = membrane o = initial value p = permeate v = vapor
Bandini, S.; Gostoli, C.; Sarti, G. C. Role of heat and mass transfer in membrane distillation. Proceedings ofthe TwelfthZnternational Symposium on Desalination and Water reuse MALTA; 1991; VOl. 3, p 91. Benjamin, G. M.; Albert,G. I. Concentration of orangejuice by reverse osmosis. J.Food Process Eng. 1988,10, 217. Bird, R. B.;Stewart, W. E.; Lighfoot, E. N. Transport Phenomena, John Wiley & Sons: New York, 1960. Calabrb, V.; Drioli, E. Heat and Mass Transfer in Membrane Distillation. Proceedings of the First Conference on Chemical and Process Engineering, Firenze, 1993; p 501. Calabrb, V.; Drioli, E.; Jiao, B. L. Concentration of Orange Juice by Membrane Distillation in Integrated Membrane Systems.Recents Progres en Genie des Procedes, Membrane Preparation, Fouling, Emerging Processes, Proceedings of Euromembrane '92,Paris, 1992;Vol. 6,No. 22,p 463. Drioli, E.; Calabrb,V.; Wu, Y. Membrane Distillation in the treatment of aqueous solutions. J . Membr. Sci. 1987,33, 277. Drioli, E.; Calabrb,V.; Molinari,R.; de CindioB. An ExergeticAnalysis of Tomato Juice Concentration by Membrane Processes. In Preconcentration and drying offood materials; Bruin, s.,Ed., Eleevier Science: Amsterdam, 1988; p 103. Drioli, E.; Jiao, B. L.; Calabrb, V. The Preliminary Study on the Concentration of Orange Juice by Membrane Distillation. Proc. Znt. SOC.Citric. 1992,in press. Gadea, A. Reverse Osmosis of Orange Juice. Proceedings of the Zntermtional Fruit Juice Congress, Orlando, 1987;p 223. Jiao, B. L.;Calabrb, V.; Drioli, E. Concentration of Orange and Kiwi Juice by Integrated Ultrafiltration and Membrane Distillation. Proceedings of ZMSTEC 92,Sydney, 1992;Vol. B2-1, p 92. Kimura, S.; Nakao, S. I. Transport Phenomena in Membrane Distillation. J . Membr. Sci. 1987,33, 285. Merson, R. L.; Paredes, G.; Hosaka, D. B. Concentrating Fruit Juice by Reverse Osmosis. Ultrafiltration membranes and applications; Plenum Publishing: New York, 1980; p 405. Pauson, D. J.; Wilson, R. L; Spatz, D. D. Reverse Osmosis and Ultrdiltration applied to the processing of Fruit Juice. Reverse Osmosis and Ultrafiltration;ACS Symposium Series; American Chemical Society: Washington, DC, 1985;Vol. 281,p 325. Sarti, G. C.; Gostoli, C. Low Energy Cost Desalination Process using Hydrophobic Membranes. Desalination 1985,56,277. Sarti, G. C.; Gostoli, C. Use of Hydrophobic Membranes in Thermal Separation of Liquid Mixtures: Theory and Experiments. In Membranes and Membrane Processes; Drioli, E., Nakagaki, M., Eds.; Plenum Publishing: New York, 1986;p 349. Schofield, R. W., Fane, A. G., Fell, C. J. D. Heat and Mass Transfer in Membrane Distillation. J . Membr. Sci. 1987,33, 299. Schofield,R. W.; Fane, A. G.; Fell, C. J. D. Gas and Vapour transport through Microporous Membranes. J.Membr. Sci. 1990,53,159. Todisco, S.;Calabrb, V.; Iorio, G. Pectin Hydrolysis in an Enzyme Membrane Reactor: a Theoretical and Experimental Analysis. Recents Progres en Genie des Procedes, Membrane Preparation, Fouling, Emerging Processes, Proceedings of Euromembrane '92, Paris, 1992;Vol. 6,No. 22,p 411. Waker, J. B. Membrane Processes for the Production of Superior Quality Fruit Juice Concentrates. Proceedings of ZCOMW, Chicago, 1990 p 283. Received for review July 23, 1993 Reuised manuscript received February 23,1994 Accepted March 10, 19948 0 Abstract published in Advance A C S Abstracts, April 15, 1994.
