Modeling of Membrane Reactors for Hydrogen Production and Purification

10.1 Introduction

Membrane reactors (often multiphase reactors) integrate a catalytic reaction (generally reforming reactions in case of hydrogen production) and a separation through a membrane in a single unit. This combination of process steps results in a high degree of process integration/intensification.

When compared with a conventional configuration in which a reactor is followed by a separation unit, the use of membrane reactors can bring various potential advantages such as reduced capital costs (due to the reduction in the number of process units), improved yields and selectivities (due to the shift in the reaction equilibrium in case of selective removal of one of the products) and reduced downstream separation costs (the separation is integrated).

The success of membrane reactors for hydrogen production is basically associated with (i) the advances in membrane production methods; and (ii) the design of innovative reactor concepts, which allow the integration of separation and energy exchange, the reduction of mass and heat transfer resistances and simplification of the housing of the membranes.

As a result of continuous improvements in membrane science, ultra-thin, highly permeable and highly selective H2 membranes have recently become available, which has triggered the development of novel, improved membrane reactor concepts for the production of ultra-pure hydrogen.

Different types of membrane reactors for hydrogen production have been proposed in the literature. Most of the previous work has been performed in packed bed membrane reactors (PBMRs); however, there is an increasing interest in novel configurations such as fluidized bed membrane reactors (FBMRs) and membrane micro-reactors (MMRs), especially because better heat management and decreased mass transfer limitations can be obtained in these novel reactor configurations.

The aim of this chapter is to show the design features of different types of membrane reactors (ranging from packed bed to fluidized bed reactors), the simulation of which is an important step towards the scale-up and industrial exploitation of membrane reactors.

The packed bed membrane reactor configuration is the first and most studied configuration for hydrogen production in membrane reactors. In fact, the first studies on membrane reactors mainly focussed on the effect of the hydrogen permeation through the membranes on the performance (in terms of con- version) of the reaction system. Thus, it was relatively straightforward to compare (both experimentally and theoretically) the performance of two packed bed reactors in one of which the tubular wall was replaced by a membrane.

Packed bed membrane reactors have been used for producing hydrogen via reforming of methane, reforming of alcohols, autothermal reforming, partial oxidation of methane, water gas shift, etc.

In the packed bed the catalyst is in a fixed configuration and in contact with a hydrogen permselective membrane. The most used packed bed configuration is the tubular one where the catalyst may be packed either inside the membrane tube or in the shell side, while the permeation stream is collected on the other side of the membrane (in case of hydrogen selective membranes) or one reactant is fed at the other side of the membrane (in case of oxygen selective membrane).

The models used for PBMRs are essentially the same models available for fixed bed reactors in which the permeation through the membrane is added as a source/sink term in one-dimensional (1D) models and incorporated in the boundary conditions at the membrane wall in two-dimensional (2D) models.

In this chapter both 1D and 2D models (pseudo-homogeneous and heterogeneous) will be described and applied to hydrogen production for a standard reaction system (methane reforming).

One-dimensional models have been extensively used in the literature to simulate membrane reactors and to compare the reactor performance with the conventional systems (without membranes). This comparison has been, so far, fair enough because thick membranes (i.e. low flux membranes) were generally considered in those works. Even 40–100 mm thick self-supported membranes have been considered. At these conditions (unfortunately too far away to be considered interesting for industrial exploitation) the flux through the membrane is generally the limiting step in the reactor performance, and external mass transfer limitations (bed-to-wall mass transfer limitations, also known as concentration polarization) can be neglected and 1D models can well describe the laboratory scale experimental results. Moreover, often steam reforming reactions carried out in a small reactor placed in a furnace, have been considered. In these cases even a 1D isothermal model was sufficient to model the system. The application...