A thermodynamic model for the adsorption of heavy hydrocarbons on silica alumina gel in natural gas processing

IntroductionBeside light hydrocarbons natural gas includes hydrocarbons of higher molecular weight, which are valuable raw materials for chemical industry. A common method for hydrocarbon recovery is fixed bed adsorption in... [ view full abstract ]

Introduction

Beside light hydrocarbons natural gas includes hydrocarbons of higher molecular weight, which are valuable raw materials for chemical industry. A common method for hydrocarbon recovery is fixed bed adsorption in cyclic temperature swing adsorption processes (TSA). To optimize the design of these processes a deep knowledge about the thermodynamics of ad- and desorption is necessary. Therefore the University of Duisburg-Essen and the adsorbent manufacturer BASF investigate systematically ad- and desorption mechanisms for common and newly designed adsorbents.

Experimental and theoretical methods

The experiments are performed in a fixed bed adsorption unit with process temperatures between 25 and 300 °C. Adsorption isotherms are calculated from outlet concentrations by mass balances. The isosteric heat of adsorption is derived by the Clausius-Clapyeron approach.

Results and discussion

In the talk results for the adsorption of n-hexane, cyclohexane and benzene on different silica gels will be presented. By comparing hydrocarbons with different molecular properties and functional groups we obtain information on adsorption mechanisms and strength of interactions. E.g. the aromatic component benzene shows the highest equilibrium capacity and the strongest interactions with the surface of silica alumina gels, due to the strong electrostatic interactions and the parallel stacking of benzene inside the pore system.

We evolved an equilibrium model of adsorption using the adsorption potential and the Freundlich equation to get a global description of a temperature dependent field of isotherms on heterogeneous surfaces. Using this model it is also possible to estimate the adsorption enthalpy as a function of loading and temperature.