Coupled transports of heat and mass at the surface of and inside silicalite

Abstract

 

The objective of this thesis was to gain a better understanding about the transport of gas and heat across a membrane and to shed light on the coupling effects between heat and mass transfer.

 

As a model system, we have chosen a silicalite-1 membrane with

 

n-butane as representative of an organic molecule to be transported. A silicalite is a pure siliceous zeolite with periodic arrangements of cages and channels of nanometer dimensions. It exhibits specific properties of adsorption and catalysis and, for these reasons, it is ideally suited to a number of different industrial applications, e.g. for catalytic cracking and for separation processes.

The dynamic behaviour of the molecules entering a membrane and inside its micropores plays an essential role in determining its catalytic and separating properties. The diffusive and adsorptive properties of alkanes in zeolites have been the focus of numerous experimental and theoretical studies. Whereas the equilibrium data in the literature are consistent, the dynamic data, i.e. diffusivities, show variation of several orders of magnitude depending on the experimental technique that is used. Although several possible explanations have been suggested, the origin of this discrepancy still remains unclear.

In this work molecular dynamics (MD) simulations have been used to study the simultaneous transport of gas and heat into and across a silicalite membrane. These simulations allow to follow the time evolution of the system and are ideally suited to get insight into the microscopic mechanisms of the processes involved. With MD simulations both equilibrium as well as the dynamical behaviour of a system can be studied. For these reasons MD simulations were used to obtain the equilibrium properties, i.e. the adsorption isotherms, as well as the transport properties of the silicalite-butane system. Both transport in the zeolite pores and through the external surfaces were investigated. In some of the studies the transport processes were described by non-equilibrium thermodynamics (NET) and specific non-equilibrium molecular dynamics (NEMD) algorithms were developed for the purpose. NET is a new theory in the context of zeolite transport. A purpose has therefore also been to test the usefulness and viability of this theory. Gradients of temperature and butane concentration were created in the crystal to get insight in the thermal diffusion process, i.e. mass transport induced by thermal forces, and across the external surface, in order to obtain the surface transport resistivities. According to our knowledge, this is the first time that a solid surface is studied by NEMD simulations in the framework of NET. Therefore, there are not any other reports on coefficients for heat and mass transport of the surface or inside the silicalite. This work attempts to use NEMD to provide a first set of such coefficients.