Prediction of multi component equilibrium isotherms for light hydrocarbons adsorption on 5A zeolite

doi:10.1016/j.fluid.2011.10.008

Fluid Phase Equilibria

Volume 313, 15 January 2012, Pages 165-170

https://doi.org/10.1016/j.fluid.2011.10.008 Get rights and content

Abstract

Multi component equilibrium isotherms for the adsorption of propane, n-butane, and i-butane mixture on 5A zeolite at 303 K were obtained experimentally and theoretically. The constant-volume method was used for measuring the experimental equilibria data. Both the extended Freundlich and modified extended Langmuir equations correlated the multi component experimental data fairly well. The selectivity of 5A zeolite to adsorb gases was also studied. The results of this study showed that n-butane was more selectively adsorbed than propane on 5A zeolite, while i-butane was not adsorbed on this zeolite at the studied temperature. Also, it was found that an increase in partial pressure of n-butane decreases the adsorbed amount of propane, while the increase of partial pressure of propane has no effect on adsorbed amount of n-butane.

Graphical abstract

Highlights

► Adsorption isotherms of propane, n-butane, and i-butane mixtures on 5A zeolite at 303 K were studied. ► The constant-volume method was used for measuring the experimental data. ► Extended Freundlich and modified extended Langmuir models correlated the data fairly well. ► 5A zeolite had more affinity with n-butane than propane. ► The inhibition of propane adsorption on 5A zeolite by n-butane was observed.

Introduction

Petroleum gases such as liquefied petroleum gas and bottom contents of a refinery depropanizer column are the common industrial source of light hydrocarbons such as propane, n-butane and i-butane. Due to rapid development in the field of petrochemicals, it became necessary to recover light hydrocarbons from such sources to be used as a fuel or petrochemical feed stocks. The adsorption separation processes have been largely employed to recover these components with high purities. In the design and optimization of adsorption processes basic equilibria data usually in the form of an adsorption isotherm are required [1].

An adsorption isotherm for a single gaseous adsorptive on a solid is the function which relates at constant temperature the amount of substance adsorbed at equilibrium to the pressure or concentration of the adsorptive in the gas phase. This isotherm is useful for indicating the affinity of an adsorbate for a particular adsorbent and to determine the adsorption capacity which is of paramount importance to the capital cost because it indicates the amount of adsorbent required, which also fixes the volume of the adsorber vessels. There are different types of adsorption isotherms as classified by the international union of pure and applied chemistry IUPAC, these isotherms can have very different shapes depending on the type of adsorbent, the type of adsorbate, and the intermolecular interactions between the gas and the surface [2].

There are three experimental methods generally used for measuring pure and multi component equilibrium adsorption isotherms, volumetric method, gravimetric method, and chromatographic method. The volumetric method is probably the best in terms of flexibility, decent accuracy, and low cost, as compared with the other methods [3].

Prediction of multi component adsorption isotherms is complicated and tedious. Therefore, many researchers have developed techniques for their estimating from pure component isotherms. The extended Langmuir isotherm, the modified extended Langmuir isotherm and the extended Freundlich isotherm are based on these techniques [4], [5].

The study of equilibrium isotherm for the adsorption of light hydrocarbons on different adsorbents has been an important subject for many investigators. Lu et al. [6] studied the adsorption isotherms of binary mixtures of n-butane and i-butane mixtures on MFI zeolites using Monte Carlo simulation. Zhu et al. [7] used a dual-site Langmuir isotherm to fit the experimental equilibrium data for adsorption of methane, ethane, propane, n-butane, and i-butane on silicalite-1. Granato et al. [8] and Sa Gomes et al. [9] studied the adsorption isotherms of propane, propylene and i-butane on 13X molecular sieve zeolite. The equilibrium adsorption isotherms were described by using the Darken equation and extended Toth model. Newalkar et al. [10] used the volumetric method to measure the adsorption isotherm of methane, ethane and propane on hexagonal mesoporous silica. They used both the Langmuir and Langmuir–Freundlich isotherms to fit the data. A simple model based on statistical associating fluid theory for variable range potentials was applied by Castro et al. [11] to simulate the experimental equilibrium isotherm data of ethane, ethylene and propane adsorption on activated carbon and silica gel. Walton et al. [12] and Esteves et al. [13] used the gravimetric method to evaluate adsorption isotherm of methane, ethane, propane and n-butane on activated carbon. Wender et al. [14] studied the adsorption isotherm of n-alkanes such as n-butane on NaY molecular sieve zeolite using the gravimetric method.

The aim of this investigation is to study experimentally and theoretically the multi component adsorption equilibrium isotherms of propane, n-butane, and i-butane mixtures on 5A zeolite by using the volumetric method to determine the adsorbed amount. The selectivity of 5A zeolite to adsorb gases was also studied.

Section snippets

Materials

Adsorbates: Three gaseous mixtures of propane, n-butane, and i-butane (supplied by Al-Durra Refinery) were used as adsorbates. The compositions of these mixtures are listed in Table 1.

Adsorbent: 5A zeolite (supplied by Lap orate Industries Ltd) of particle density 1263 g/l and an average particle size 0.11 cm³ was used as adsorbent.

Auxiliaries: Nitrogen (locally supplied) of purity greater than 99% was used as a carrier gas of gas–solid chromatography analyzer.

Apparatus

A schematic sketch of the apparatus

Mathematical model

Langmuir and Freundlich equations were used to correlate experimental equilibrium isotherms data for pure components. To correlate experimental equilibrium isotherms data for multi components, extended Langmuir, modified extended Langmuir, and extended Freundlich equations were used. These equations can be written as follows: $Langmuir q = \frac{q_{m} B P}{1 + B P}$ $Freundlich q = K P^{n}$ $Extended Langmuir x i = \frac{q_{i}}{\sum_{j = 1}^{n} q_{j}} = \frac{q_{m i} B_{i} P_{i}}{(1 + \sum_{j = 1}^{n} B_{j} P_{j}) \sum_{j = 1}^{n} q_{j}}$ $Modified extended Langmuir x i = \frac{q_{i}}{\sum_{j = 1}^{n} q_{j}} = \frac{q_{m i} B_{i i} P_{i}}{(1 + \sum_{j = 1}^{n} B_{i j} P_{j}) \sum_{j = 1}^{n} q_{j}}$ $Extended$

Multi component equilibrium isotherms

Equilibrium isotherm data for the adsorption of pure propane and n-butane on 5A zeolite were taken from Ruthven et al. [15]. In the present work these data were fitted with Langmuir and Freundlich equations. The calculated constants for the two isotherms equations along with R² values were presented in Table 2.

The experimental equilibrium data for adsorption of three multi component mixtures were fitted by extended Langmuir, modified extended Langmuir, and extended Freundlich equations. The

Conclusions

Experimental equilibrium isotherm data for multi component adsorption of propane, n-butane, and i-butane on 5A zeolite at 303 K, showed that n-butane was more strongly adsorbed than propane, while i-butane was not adsorbed at the studied temperature. Both extended Freundlich and modified extended Langmuir equations provide good predictions for multi component adsorption. The results showed that an increase in partial pressure of n-butane decreases the adsorbed amount of propane on 5A zeolite,

Acknowledgements

We gratefully acknowledge Universiti Kebangsaan Malaysia and Baghdad University for assist and support of this work.

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Prediction of multi component equilibrium isotherms for light hydrocarbons adsorption on 5A zeolite

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Materials

Apparatus

Mathematical model

Multi component equilibrium isotherms

Conclusions

Acknowledgements

Chem. Eng. Sci.

Chem. Eng. Sci.

Fluid Phase Equilibr.

Chem. Eng. Sci.

Micropor. Mesopor. Mater.

Physica A

Sep. Purif. Technol.

Fluid Phase Equilibr.

Chem. Eng. J.

Langmuir