Modeling Fixed Bed Membrane Reactors for Hydrogen Production through Steam Reforming Reactions: A Critical Analysis

Membrane reactors for hydrogen production have been extensively studied in the past years due to the interest in developing systems that are adequate for the decentralized production of high-purity hydrogen. Research in this field has been both experimental and theoretical. The aim of this work is two-fold. On the one hand, modeling work on membrane reactors that has been carried out in the past is presented and discussed, along with the constitutive equations used to describe the different phenomena characterizing the behavior of the system. On the other hand, an attempt is made to shed some light on the meaning and usefulness of models developed with different degrees of complexity. The motivation has been that, given the different ways and degrees in which transport models can be simplified, the process is not always straightforward and, in some cases, leads to conceptual inconsistencies that are not easily identifiable or identified

Progress on modeling and design of membrane reactors for hydrogen production

This paper presents an overview of recent research carried out by the authors on the development and analysis of mathematical models describing hydrogen production in membrane reactors. The case considered is that of methane steam reforming (SR) in a reactor with the typical double pipe configuration, in which a hydrogen-permeable membrane is present on the outer wall of the innermost tube. The model developed accounts for the rate of reaction, convective and dispersive transport in the axial and radial directions, and hydrogen permeation across the membrane. Density variations with pressure and gas composition have been accounted for, leading to a full coupling of mass and momentum transport. Different geometric aspect ratios have also been studied to assess the influence of catalyst volume on the overall performance of the system. The presence of two distinct transport regimes, in which hydrogen permeation is limited either by transport within the packed bed or permeation across the membrane, has been identified, along with the operating conditions that determine their range of existence. This has allowed the development of a simplified model, valid under the hypothesis that the reaction is fast compared to transport. In the permeation-controlled regime, the permeate flow rate and recovery may be found by solving a set of two PDEs, whereas an analytical solution is available for the transport-controlled regime. The main steps and observations that have brought to the development of the simplified model are presented, along with a guide to its implementatio

A tunable microfluidic device toiInvestigate the influence of fluid-dynamics on polymer nanoprecipitation

Polymer drug-embedding nanocapsules are attracting increasing attention as effective tools for the targeted delivery of pharmaceutical molecules on specific biological tissues. Besides, it is well established that an effective selectivity of the delivery dictates that the size of the carrier particles be accurately controlled, thus maintaining the size dispersion of the particle population as low as possible. To this end, microfluidics-assisted precipitation provides a promising alternative to the traditional processes in that the structure of the flow - ultimately controlling the particle size distribution - can be reliably predicted from the solution of Navier-Stokes equations in the laminar regime. Notwithstanding the great potential provided by microfluidics techniques, much about the interaction between fluid-dynamics and polymer transport and precipitation is yet to be understood. In this work, we investigate polymer precipitation in a simple cross-junction inflow-outflow microchannel, which has proven a viable benchmark to gain insight into the physics of nanoprecipitation in that the particle size distribution is sensitively dependent on the flow operating conditions. Specifically, previous experimental work by some of these authors proved that average particle size can vary by an order of magnitude for operating conditions where the solvent flow rate varies by a factor of three, while keeping the non-solvent flow rate constant. The scope of this work is to show that such sensitive dependence on operating conditions finds direct correspondence in the kinematic structure of the flow, which undergoes abrupt qualitative changes in the same range of operating conditions, provided a fully three-dimensional solution of the incompressible Navier-Stokes equation (thus retaining the inertial term in momentum balance) is afforded

How does radial convection influence the performance of membrane module for gas separation processes?

A two-dimensional axial-symmetric isothermal model, based on full coupling between mass and momentum transport, has been developed to describe the separation of a binary gaseous mixture in a packed bed membrane module. Steady-state conditions have been studied. The gaseous mixture to be separated enters an annular gap between two co-axial cylinders. The inner wall of the outer cylinder is impermeable to both components, whereas a membrane, with infinite selectivity towards one of the components, is supported onto the outer wall of the inner cylinder. A radial flux of the permeating components is therefore present. The main focus was on the determination of the influence of radial convection on the performance of the separator, which has been analysed in terms of three dimensionless groups. Different transport regimes could be identified, corresponding to different values of the dimensionless groups. The impact of radial convection has been assessed by comparing model predictions with those of a fully uncoupled one-dimensional model. A discrepancy up to 20% of the recovery has been observed in industrially relevant ranges of the parameters

Modeling Fixed Bed Membrane Reactors for Hydrogen Production through Steam Reforming Reactions: A Critical Analysis

Membrane reactors for hydrogen production have been extensively studied in the past years due to the interest in developing systems that are adequate for the decentralized production of high-purity hydrogen. Research in this field has been both experimental and theoretical. The aim of this work is two-fold. On the one hand, modeling work on membrane reactors that has been carried out in the past is presented and discussed, along with the constitutive equations used to describe the different phenomena characterizing the behavior of the system. On the other hand, an attempt is made to shed some light on the meaning and usefulness of models developed with different degrees of complexity. The motivation has been that, given the different ways and degrees in which transport models can be simplified, the process is not always straightforward and, in some cases, leads to conceptual inconsistencies that are not easily identifiable or identified

Numerical analysis of the performance of membrane reactors for NH3 decomposition

An analysis of the performance of a membrane reactor for the production of pure hydrogen through ammonia decomposition is presented here. The system is numerically studied under a wide range of operating conditions to identify those most favorable for the production of pure hydrogen. The underlying idea is that, in the case of ammonia decomposition, a membrane reactor allows to operate at low temperature not only because the selective removal of hydrogen shifts the equilibrium of the reactor towards the products, but also because the reaction rate is enhanced by the removal of hydrogen, which at low temperatures inhibits the kinetics of ammonia decompositio

Identification of a reaction boundary layer in membrane reactors for hydrogen production

Integrated membrane reactors for the production of pure hydrogen at low temperatures are currently attracting much interest. In this work, we developed a model of a membrane reactor for low temperature methane steam reforming, which was solved numerically using COMSOL Multiphysics. The system studied consists of an annular reactor, packed with a catalyst. A hydrogen permeable membrane is supported on the inner wall of the reactor, allowing the selective removal of hydrogen as it is being produced through the steam reforming reaction. The behavior of the reactor may be described as the result of a competition between the steam reforming reaction, which tends to bring the system towards equilibrium, and hydrogen permeation, which moves the system away from equilibrium conditions. The presence of a region of the reactor, close to the wall on which the membrane is supported, in correspondence of which the system is not capable of reaching equilibrium, was noticed. The extent of this region, which we will refer to as a reaction boundary layer, depends on the operating conditions and on reactor geometry. The latter quantity may be defined as a function of the ratios between the length of the reactor and its inner radius, L/R1, and the ratio between the outer and inner radii, R2/R1. The effects of the main operating parameters on the thickness of the reaction boundary layer has been studied, along with the influence of the ratio R2/R1. This procedure has allowed an optimization of reactor design when considering a fixed membrane area and feed flow rate

An enhanced Sherwood number to model the hydrogen transport in membrane steam reformers

It is well known that membrane reactors are inherently two-dimensional systems in which species concentrations vary as a consequence of both the reaction and permeation across the membrane, which occurs in the direction perpendicular to that of the main gas flow. Recently, an expression for an enhanced Sherwood number was developed to describe the hydrogen concentration gradients arising in methane steam-reforming membrane reactors as a consequence of the combined effect of hydrogen production, dispersion, and permeation. Here, the analysis is developed in further detail with the aim of (i) assessing the validity of the simplifying assumptions made when developing the 1D model and (ii) identifying the operating conditions under which it is possible to employ the 1D model with the enhanced Sherwood number