Single - and multi-component liquid phase adsorption measurements by headspace chromatography

SINGLE - AND MULTI-COMPONENT LIQUID PHASE ADSORPTION MEASUREMENTS BY HEADSPACE CHROMATOGRAPHY

A.E.B.Torres¹, S.B.Neves², J.C.N. Abreu², C.L.Cavalcante Jr.^1,* and D.M.Ruthven³

¹Universidade Federal do Ceará, Departamento de Engenharia Química, Grupo de Pesquisas

em Separações por Adsorção, Campus do Pici, Bl. 710, 60455-760, Fortaleza - CE, Brazil.

²Universidade de Campinas, Faculdade de Engenharia Química, Campinas - SP, Brazil.

³University of Maine, Department of Chemical Engineering, Orono - ME, USA.

(Received: October 4, 1999 ; Accepted: December 5, 2000)

Abstract - This short communication presents a new technique for evaluation of adsorption equilibrium in the liquid phase using headspace chromatography. The technique may be used in two different modes: single-component equilibrium (the "finite bath" mode) and the multicomponent selectivity mode. Illustrative experimental results are presented for xylenes adsorption on Y zeolite pellets.

Keywords: Adsorption, xylenes, headspace chromatography.

INTRODUCTION

Although many important adsorption separation processes are carried out industrially in the liquid phase, the published literature contains comparatively few reports of the experimental measurement of liquid phase adsorption equilibria. Those studies that have been published generally utilize the "finite bath" method, in which the adsorbed phase composition, at equilibrium, is deduced by mass balance from the change in liquid phase composition in a stirred vessel containing known quantities of liquid and solid adsorbent (see for example Santacesaria et al., 1982; Cavalcante Jr. and Gubulin, 1990). This method, however, becomes unreliable in certain composition ranges since the mass balance calculation can involve only a small change in liquid phase composition.

An alternative approach involving the use of a chromatographic headspace analyser has proved a useful alternative to the finite bath method. Although the headspace technique was first introduced more than fifteen years ago (Hulme, 1981), it does not appear to have been reported in the open literature. In this note we report two variants of the headspace method which are suitable for the measurement of single-component isotherms and binary (or multicomponent) separation factors in liquid systems.

EXPERIMENTAL METHOD AND RESULTS

Headspace chromatography involves measuring the composition of the vapor phase at equilibrium with a condensed phase in a closed sample vial. This is usually accomplished in a headspace analyser, (e.g., HP 7694), an accessory to a gas chromatograph which allows a number of such vials to be loaded and analysed automatically. In the present study the chromatograph was a HP-5890 fitted with FID and a capillary column (OV-17), 50 m length, 0.32 mm i.d..

Finite Bath Mode

Known quantities of adsorbent together with known quantities of liquid of a known composition are added to a series of headspace vials and allowed to equilibrate at the required temperature. The vials are then transferred to the headspace analyser and the vapor phase composition (i.e., the partial pressures of the components) are measured. Using VLE data, the composition of the corresponding liquid phase may be determined, and by straightforward mass balance, the composition of the adsorbed phase is found just as in a traditional finite bath experiment. This approach was used to study the adsorption of p-xylene from n-decane in a Ba-exchanged Y zeolite adsorbent. The results are compared in Figure 1 with measurements for the same system by the traditional finite bath method. Since p-xylene is adsorbed much more strongly than n-decane, the results of these measurements may be regarded as yielding an approximate single-component isotherm for p-xylene. There appears to be reasonable agreement between the headspace and finite bath data (Figure 1a), except at low concentrations where the finite bath method is subject to considerable error (Figure 1b).

The main advantage of the headspace method over the traditional finite bath technique is that it allows the use of a much smaller quantity of liquid and therefore a higher ratio of solid to liquid, thereby increasing the magnitude of the change in concentration and hence the accuracy of the method.

Selectivity Measurements

The measurement of adsorbent selectivity for liquid phase adsorption by the headspace technique depends, in essence, on the large differences in molar volumes between the vapor phase and the condensed phases (liquid or adsorbed). Consequently, if liquid sorbate is added drop by drop to a well regenerated sample of adsorbent in a closed system, virtually all the liquid will be imbibed by the solid until the saturation limit has just been exceeded. It follows that, up to that point, the adsorbed phase, at equilibrium, will have essentially the same composition as the original liquid added (since the vapor phase density is negligible as compared to the adsorbed phase density). In contrast, the composition of the vapor phase (i.e., the equilibrium partial pressure of each component) will not depend on the composition of the liquid added, but rather it will directly reflect how strong each component is adsorbed. Once the saturation limit is exceeded the system will contain free liquid sorbate and the partial pressures will then be governed by vapor-liquid equilibrium.

A series of headspace vials is prepared, each containing the same weight of regenerated adsorbent with successively increasing quantities of liquid sorbate. These are allowed to equilibrate, and the composition of the vapor phase is then analysed and plotted against the quantity of liquid sorbate added. In the present example the sorbate liquid was an equimolar mixture of o-xylene and p-xylene with BaY zeolite as the adsorbent. The results of the headspace measurements are shown in Figure 2.

For this system vapor phase measurements (Ruthven and Goddard, 1986) have shown that adsorbent selectivity develops only as the saturation limit is approached. In conformity with this observation the headspace measurements show a decreasing relative equilibrium vapor pressure for the more strongly adsorbed species (p-xylene) and an increasing relative pressure for the less strongly adsorbed species as saturation loading is approached. The maximum and minimum in these plots represent the moment at which the intracrystalline pores have just become saturated with sorbate with no free liquid remaining. Beyond that moment the vapor pressures decline towards the values corresponding to the saturated liquid.

The results in Figure 2 show that the micropore capacity is about 13wt% (for pelleted adsorbent). This is substantially lower than the capacity of KY zeolite (22wt% crystal basis) reported by Hulme et al. (1991), presumably reflecting both the lower intrinsic capacity of BaY crystals compared with KY and perhaps the presence of 20wt% inert binder in the pellets. At the maximum point the ratio of vapor phase mole fractions y_PX/y_OX is approximately 0.18/0.82, while the ratio of the adsorbed phase mole fractions (z_PX/z_OX) is 1.0, corresponding to the composition of the original liquid feed. To convert the ratio of vapor phase mole fractions to the corresponding liquid mole fractions requires vapor liquid equilibrium data. At 60^oC the ratio of saturation vapor pressures p_PX/p_OX is 0.068/0.054. Since p-xylene and o-xylene form an essentially ideal system (Raoult’s Law), we may write

and the adsorbed phase/liquid phase selectivity is therefore given by

This is close to the values reported in the UOP patent literature for BaY zeolite –(see for example Stine and Broughton ,1969 and Neuzil, 1975). The extension of this approach to a multicomponent mixture is obvious.

CONCLUSIONS

The headspace method provides a simple and useful technique for measuring both single-component isotherms and separation factors for binary (or multicomponent) liquid mixtures. This approach is therefore useful for screening of potential adsorbents as well as for investigating the variation in selectivity in relation to composition.

ACKNOWLEDGEMENTS

The authors wish to acknowledge the support received from CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico), BN (Banco do Nordeste) and Copene Petroquímica do Nordeste S/A.

NOMENCLATURE

q adsorbed phase concentration (g-mol/g) c liquid phase concentration (g-mol/ml) OX o-xylene p saturation vapor pressures (atm) PX p-xylene VLE vapor-liquid equilibrium x_i mole fraction of component i in liquid phase y_i mole fraction of component i in vapor phase z_i mole fraction of component i in adsorbed phase a_i/j separation factor º (z_i/z_j)/(x_i/x_j)

• Cavalcante Jr., C.L. and Gubulin, J.C., Liquid Phase Selective Adsorption of Xylenes, I. Chem. E. Symp. Series, No. 118, 83-94 (1990).
• Hulme, R. to D.M. Ruthven, Personal Communication (1981).
• Hulme, R., Rosensweig, R.E. and Ruthven, D.M. Binary and Ternary Equilibria for C₈ Aromatics on K-Y Faujasite, Ind. Eng. Chem. Res., Vol. 30, 752-760 (1991).
• Neuzil, R.W., U.S. Patent No. 3,997,620 (Oct. 28, 1975) to UOP.
• Ruthven, D.M. and Goddard, M., Sorption and Diffusion of C₈ Aromatic Hydrocarbons in Faujasite Type Zeolites. I. Equilibrium Isotherms and Separation Factors, Zeolites, 6, 275-282 (1986).
• Santacesaria, E., Morbidelli, M., Danise, P., Mercenari, M. and Carrŕ, S., Separation of Xylenes on Y Zeolites. 1. Determination of the Adsorption Equilibrium Parameters, Selectivities and Mass Transfer Coefficients through Finite Bath Experiments, Ind. Eng. Chem. Prod. Res. Dev., 21 , 3, 440-445 (1982).
• Stine, L.O. and Broughton, D.B., U.S. Patent No. 3,636,121 (Nov. 7, 1969) to UOP.

*

To whom correspondence should be addressed

Publication Dates

• Publication in this collection
  25 May 2001
• Date of issue
  Mar 2001

History

• Received
  04 Oct 1999
• Accepted
  05 Dec 2000