Vapor-Liquid Equilibria for Water+Hydrochloric Acid+Magnesium Chloride and Water+Hydrochloric Acid+Calcium Chloride Systems at Atmospheric Pressure

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1 Chnese J. Chem. Eng., 4() (006) RESEARCH OES Vapor-Lqud Equlbra for Water+Hydrochlorc Acd+Magnesum Chlorde and Water+Hydrochlorc Acd+Calcum Chlorde Systems at Atmospherc Pressure ZHAG Yng( 张颖 ) and ZHOU Rongq( 周荣琪 )* Department of Chemcal Engneerng, snghua Unversty, Bejng 00084, Chna Abstract Vapor-lqud equlbra for water+hydrochlorc acd+magnesum chlorde and water+hydrochlorc acd+calcum chlorde systems at atmospherc pressure were measured usng a Othmer-type equlbrum stll. he epermental data are correlated usng a modfed Messner s method. Satsfactory agreements are obtaned between the epermental and the calculated results. Keywords hydrochlorc acd, magnesum chlorde, calcum chlorde, vapor-lqud equlbra IRODUCIO Aqueous soluton of a volatle strong electrolyte, such as HCl-H O system, has a mamum azeotrope so that separaton by dstllaton s dffcult. he addton of salts n ths system s an effectve method for changng the relatve volatlty and modfyng the azeotropc ponts, and the recovery of waste hydrochlorc acd by etractve dstllaton usng magnesum chlorde or calcum chlorde has receved consderable ndustral attenton []. Vapor-Lqud equlbra (VLE) data for H O+HCl +MgCl and H O+HCl+CaCl systems have been reported by Sako [] wth MgCl mole fracton of and CaCl of However, more concentrated salt solutons are often used to get greater salt effect n the separaton process. In ths artcle, VLE for H O+HCl+MgCl system wth MgCl mole fracton of and H O+HCl+CaCl system wth CaCl mole fracton of at atmospherc pressure were measured. he epermental results are correlated wth a modfed Messner s method [3]. EXPERIMEAL. Materals MgCl and CaCl (mnmum purty 98%) and HCl (mamum mpurty 0.% ) were purchased from Bejng Chemcal Reagents Co, all the reagents were used wthout further purfcaton. Dstllated water was used throughout the epermental work.. Apparatus and procedure he Othmer-type equlbrum stll [] was used to obtan VLE data. he pressure was controlled at (0.3±0.)kPa through the buffer tank by manual operaton and the equlbrum temperature was measured wthn ± 0.K by a standard mercury thermometer. After crculaton for about 4h, the equlbrum temperature was measured and samples of the lqud and vapor phases were wthdrawn from the equlbrum stll and the vapor phase samplng cell to determne the compostons of both phases. HCl n both phases was analyzed by acd-base ttraton wth 00mol m -3 sodum hydrode soluton. MgCl and CaCl were ttrated wth 0mol m -3 EDA (Ethylene damne tetraacetc acd, mnmum purtes 98%) soluton. he accuraces of analyss were wthn mole fracton. o check the consstency of the epermental apparatus, the VLE for the H O+HCl system was measured under atmospherc pressure and was n good agreement wth the lterature []. 3 RESULS AD DISCUSSIO VLE data for H O ()+HCl ()+MgCl (3) and H O ()+HCl ()+CaCl (3) systems at 0.3kPa are gven n ables, where, s the lqud mole fracton of component, y s the vapor mole fracton of component, and s the equlbrum temperature. he VLE for the volatle strong electrolyte s v v y p = γ H () t ± ± Receved , accepted * o whom correspondence should be addressed.

2 Vapor-Lqud Equlbra for H O+HCl+MgCl and H O+HCl+CaCl Systems at Atmospherc Pressure 77 able Epermental VLE data for H O ()+HCl ()+ MgCl (3) system at 0.3kPa 3, K y able Epermental VLE data for H O ()+HCl ()+ CaCl (3)system at 0.3kPa 3, K y where ± s the lqud phase mean onc mole fracton, γ ± s the mean onc actvty coeffcent, and H s Henry s law constant of the electrolyte. he VLE for the solvent s S t ap y p = () where p S s the vapor pressure of pure solvent at the system temperature, and a s the lqud phase actvty of the solvent whch s normalzed for the pure solvent at the system temperature, he γ ± n Eq.() s calculated by a modfed Messner s method. he Messner s method [4] lead to large errors n calculaton at hgh temperature and onc strength, and three-suff Margules terms are added by Sako [3] to descrbe the short-range nteractons between the chemcal speces for onc strengths hgher than the upper lmt of applcablty of the Messner s method. (Append ) he actvty of water s calculated from the Gbbs-Duhem equaton. (Append ) he Henry s law constant n Eq.() and Margules constants n Eq.(A), whch are to be determned from the epermental data, are assumed to be functons only of temperature. h0 ln H = + hln + h + h3 (3) A0 A = + Aln + A + A3 (4) B0 B = + Bln + B + B3 (5) Chnese J. Ch. E. 4() 76 (006)

3 78 Chnese J. Ch. E. (Vol. 4, o.) he parameters for the H O ()+HCl () system and H O ()+salt (3) system, H, A, A 3, B and B 3 were determned from the vapor pressure data of the electrolyte solutons [5] by the mnmzaton of the followng objectve functon. F = [( pep pcal) / pep] (6) and for the H O ()+salt (3) system, B 3 =0, as the system s not so complcated. he values of these parameters and mean absolute devatons n vapor pressures are reported n ables 3 6. he other parameters for the ternary systems, A 3, B 3 and Q 3, were determned from the ternary VLE data n ables by the mnmzaton of the followng objectve functon F = [( p p ) / p ] + ( y y ) (7) ep cal ep ep cal the temperature dependence of the three parameters s neglected because the temperature range of the data used s narrow. he values of these parameters and the devaton δp/p and δy are reported n ables 7 8. he results ndcate a good agreement between the able 3 Parameters of Henry s law constants System h 0 h h h 3 H O+HCl able 4 Parameters of A for bnary system System A 0 A A A 3 H O+HCl H O+MgCl H O+CaCl able 5 Parameters of B for bnary systems System B 0 B B B 3 H O+HCl able 6 Mean absolute devatons for bnary systems System Range of, K Range of δp/p 0 H O+HCl H O+MgCl H O+CaCl δ / = Σ ( )/. ote: pp pep pcal pep able 7 Parameters for ternary systems System A 3 B 3 Q 3 H O+HCl+MgCl H O+HCl+CaCl able 8 Mean absolute devatons for ternary systems System H O+HCl+MgCl H O+HCl+CaCl Present Sako [] Present Sako [] Range of onc Strength, mol kg δp/p δy δ / = Σ ( )/ ; ote: pp p p p ep cal ep δ y = Σ y y.,ep,cal Aprl, 006

4 Vapor-Lqud Equlbra for H O+HCl+MgCl and H O+HCl+CaCl Systems at Atmospherc Pressure 79 epermental and calculated data. Although the data reported by Sako [] could not be drectly compared wth those reported n ths work because of the dfference n epermental condtons, we were able to use the model and parameters n ths artcle to reproduce the VLE data at condtons reported by these authors. he devatons n able 8 ndcate that the results are also n good agreement wth the epermental data. When MgCl or CaCl s added n the hydrochlorc acd soluton, the concentraton of the chlorc on, the chemcal potental of HCl, and the pressure of HCl ncreases n the vapor phase. And accordng to the hydraton theory, the actvty of solvent decreases sgnfcantly due to a fracton of the water beng bound n the hydraton shells of the ons, and ths bound water of dmnshed actvty s less avalable as solvent for the ons, so actvty of the ons s rased. As a result, HCl becomes more volatle and water becomes less volatle when the salt s added. OMECLAURE A, B bnary Margules constant a actvty H Henry s law constant, kpa I onc strength, mol kg - p equlbrum pressure, kpa p t total pressure, kpa Q ternary Margules constant equlbrum temperature, K lqud phase mole fracton ± mean onc mole fracton n lqud phase y vapor phase mole fracton α constant calculated by Eq.(A4) γ onc actvty coeffcent of component on mole fracton bass γ± mean onc actvty coeffcent on mole fracton bass ν number of moles of on produced by dssocaton of one mole of electrolyte Superscrpts S saturated Subscrpts cal calculated results ep epermental results component (=,,3) ± mean onc REFERECES Furter, W.F., Salt effect n dstllaton: A lterature revew Ⅱ, the Canadan J. Chem. Eng., 55, 9 39(977) Sako,., Hakuta,., Yoshtome, H., Salt effect on vapor-lqud equlbrum for volatle strong electrolyte-water systems, J. Chem.Eng. Jpn., 7, (984) 3 Sako,., Hakuta,., Yoshtome, H., Vapor-lqud equlbra of strong electrolyte aqueous solutons, Flud Phase Equlbra, 38, 63 8(987) 4 Joseph F.Z., Handbook of aqueous electrolyte thermodynamcs: heory & applcaton, AIChE, ew York (986) 5 Sako,., Hakuta,., Yoshtome, H., Vapor pressures of bnary (H O-HCl, H O-MgCl and H O-CaCl ) and ternary aqueous (H O-MgCl -CaCl ) solutons, J. Chem. Eng. Data, 30, 4 8(985) 6 eruya, K., Hosako, S., akano,., akamor, I., Estmaton of water actvtes n multcomponent electrolyte solutons, J. Chem. Eng. Jpn., 9, 5(976) APPEDIX he actvty coeffcent of the electrolyte s calculated by combnng the Messner and Margules equatons ν ν ± ± (MEI) (MAR) ln γ = ln γ + ln γ, =, 3 (A) where γ ± ( MEI) s the mean onc actvty coeffcent of electrolyte determned by Messner s equaton [4], and γ ( MAR ) s calculated by Margules s equaton. For the bnary system ( ) ( ) ν ν 3 ± γ ± (MEI) A ± B ± ± ln γ = ln (A) For the ternary system ν ln γ = α ( ) + α ( ) + ± (MAR) 3 3± ± 3 ± 3± ± α ± ( ± ) α ± ± ( ± ) α3 ± 3± α3 ± 3± + [0.5( α3 + + α3 + α+ α3+ α3) Q3] ( ) α (A3) ± 3± ± ± 3± where Q 3 s the ternary Margules constant. and α j n the above equaton are related to the bnary Margules constants α = A + B, α = A + 0.5B α3 = A3 + B3, α3 = A B3 α = A + B, α = A + 0.5B (A4) ± s the lqud phase mean onc mole fracton, whch s defned by Chnese J. Ch. E. 4() 76 (006)

5 80 Chnese J. Ch. E. (Vol. 4, o.) /( + ν ) ± = Σ (A5) ν+ ν / ν ± + = ( ), =,3 (A6) APPEDIX he actvty of water s calculated from the Gbbs-Duhem equaton, for the bnary system + ( ν ) γ ± ln a = ln v d ln γ γ ± = ± (A7) and for the ternary system, the actvty of water s calculated by a smplfed epresson [6] ln a = I ln a (A8) k ( k ) It k where I t s the total onc strength n the ternary system, I k s the onc strength of electrolyte k n the ternary mture, and a (k) s the actvty of water n the bnary soluton contanng electrolyte k at the same temperature and onc strength as n the ternary soluton. Aprl, 006