Full Papers Prediction of Vapor-Liquid Equilibrium Inside Capillary Porous Plates By Fahmi Abu Al-Rub* Capillarydistillationisarelativelynewseparationprocesswhichusescapillaryporousmediafortheseparationofmixtures.It utilizes the solid-liquid interfacial forces to alter the vapor-liquid equilibrium (VLE) of mixtures inside the capillary porous plates.PreviousstudieshaveshownthatcapillaryporousplatescouldsignificantlyaltertheVLEofbinarymixtures.Itwasshown thatthemainfactorsaffectingthealterationofVLEofbinarymixturesarethepolarizationoftheliquidsandtheporousplates as well as the polarization difference between the mixture components. In this study, the VLE data for the mixtures inside capillary porous plates have been predicted using the Wilson model and the theory of Abu Al-Rub and Datta on VLE inside capillary porous media. The validity of the developed model has been assessed by comparing predicted VLE results of some binary mixtures inside capillary porous plates with experimental VLE data. The comparison showed that the model developedcouldpredicttheVLEofthestudiedsystemsaccurately. 1Introduction Separation of liquid mixtures using capillary distillation process has proven itself as a good alternative for the separationofmixtures,especiallythosewithazeotropicpoints [1±12]. Capillary distillation depends on altering the vapor- liquid equilibrium (VLE) of mixtures by altering the intermolecular forces amongst the components of these mixtures utilizing the surface forces of the solid and the consequentintermolecularinteractionsbetweenthesolidand the liquids. The solid-liquid intermolecular interactions are expected to create a layer next to the solid with different physico-chemical properties from those in the bulk phases [1,2]. Interface curvature is another factor that can alter the VLEofmixturesinporousmedia. AbuAl-RubandDatta[1]havetheorizedthattheratioof vaporpressureinsidecapillaryporousplate, P s i ,tothenormal one, P s i;o ,isgivenby 1) ln P s i P s i;o  !  C E i  D ~ H E ivl RT  2r il ~ V il rRT (1) where C i E º C i ±C i,o . C i is the constant of integration in the Clapeyronequation,ln P i s = C i ± DH ~ ivl /RT ,whereas C i,o isthe constant in the equation corresponding to the case in the absence of external fields, i.e., the original Clapeyron equation, ln P i,o s = C i ± DH ~ ivl /RT . DH ~ ivl E º DH ~ ivl ± DH ~ ivl,o is theexcessenthalpyduetotheexternalforcefieldcreatedby the solid-liquid intermolecular interactions. Previous studies by Yeh et al. [10±12], Abu Al-Rub and Datta [1±5,8], Abu Al-Rub [6] and Abu Al-Rub et al. [7] showed that the interface curvature effect is negligible for pore radii greater than 0.1 lm. Abu Al-Rub and Datta [1,2] analyzed the VLE of pure components and mixtures inside capillary porous media using a molecular thermodynamics approach. They concluded that the main factor affecting the alteration of VLE in porous media is the intermolecular interactionsbetweentheliquidsandsolid.Moreover,AbuAl- Rub et al. [7] and also Abu Al-Rub and Datta [5] conducted experimental studies on separation of different liquid mixtures having different polarity and including some azeotropic mixtures using different capillary plates. The experimentalresultsshowedthatpolarcapillaryporousplates could enhance the separation of binary mixtures with a polarity difference. Yeh et al. [10] incorporated the theory of corresponding states and the dependence of the surface tension on temperature as given by the van der Waals relation to empirically correlate the vapor pressure in porous media to the surface tension. Their model gave satisfactory results in predictingtheVLEofbinarymixturesinporousmediaonlyat relativelylowconcentrationsofthehighlypolarliquids,where mixtures can be assumed to behave ideally. At higher concentrations their model fails to predict the VLE of mixtures because they did not account for the effect of the solid-liquidinteractionsinchangingtheactivitycoefficientsof theliquids. The objective of this study is to formulate an approach to predict the VLE data within capillary porous media. This approachwillbebasedonincorporatingthetheoryofAbuAl- Chem. Eng. Technol. 25 (2002) 12, Ó 2002 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 0930-7516/02/1212-1171 $ 17.50+.50/0 1171 ± [*] Fahmi Abu Al-Rub, Chemical and Petroleum Engineering Department, United Arab Emirates University, P.O. Box 17555, Al-Ain, United Arab Emirates (e-mail: f.rub@uaeu.ac.ae). 1) Listofsymbolsattheendofthepaper. 0930-7516/02/1212-1171 $ 17.50+.50/0