Enrico Drioli is a Professor Chairman of the Section on Membranes for the European Federation of Chemical Engineering. is research activities focus on membrane science and engineering. He is the recipient of numerous awards and is active in many international societies, scientific committees, editorial boards, and international advisory boards. Professor Drioli is currently Chairman of the European Federation of Chemical Engineering Section on Membranes. He is also the author of more than 600 scientific papers and 18 patents in the field of membrane science and technology.

Giuseppe Barbieri is a researcher at the Institute on Membrane Technology of the National Research Council of Italy (ITM-CNR). He has co-authored more than 50 papers in peer-reviewed journals, various chapters in books and numerous presentations at scientific conferences, workshops, and congresses in the field of membrane science and engineering.

Dr Barbieri is responsible for, or has participated in, numerous research and formation projects funded by the: European Union; Italian Ministry of Foreign Affairs; Italian Ministry of Education and Research; National Research Council of Italy, the Calabria Region, and various private companies. He is also a Professor at the University of Calabria Faculty of Science and an Invited Professor at the University of Strasburg School of Engineering. His particular interests lie in fuel processing and CO₂ separation/concentration, by means of membranes, for energy production from fossil and bio fuels.

Membrane Engineering for the Treatment of Gases

Volume 2: Gas-separation Problems Combined with Membrane Reactors

By Enrico Drioli Giuseppe Barbieri

The Royal Society of Chemistry

Copyright © 2011 Royal Society of Chemistry
All rights reserved.
ISBN: 978-1-84973-239-0

Contents

Volume 1,
Chapter 1 Multi-scale Molecular Modeling Approaches for Designing/ Selecting Polymers used for Developing Novel Membranes Elena Tocci and Pluton Pullumbi, 1,
Chapter 2 Simulation of Polymeric Membrane Systems for CO2 Capture Eric Favre, 29,
Chapter 3 Physical Aging of Membranes for Gas Separations B.W. Rowe, B.D. Freeman and D.R. Paul, 58,
Chapter 4 Recent High Performance Polymer Membranes for CO2 Separation S.H. Han and Y.M. Lee, 84,
Chapter 5 Design of Membrane Modules for Gas Separations M. Scholz, M. Wessling and J. Balster, 125,
Chapter 6 Gas/Vapor Permeation Applications in the Hydrocarbon-processing Industry Arnaud Baudot, 150,
Chapter 7 Membrane Gas Separation Processes for Post-combustion CO2 Capture Peter Michael Follmann, Christoph Bayer, Matthias Wessling and Thomas Melin, 196,
Chapter 8 Commercial Applications of Membranes in Gas Separations Pushpinder S. Puri, 215,
Chapter 9 Novel Hybrid Membrane/Pressure Swing Adsorption Processes for Gas Separation Applications Isabel A.A.C. Esteves and José P.B. Mota, 245,
Subject Index, 276,
Volume 2,
Chapter 10 Modeling of Membrane Reactors for Hydrogen Production and Purification F. Gallucci, M. van Sint Annaland and J.A.M. Kuipers, 1,
Chapter 11 Palladium-based Membranes in Hydrogen Production Rune Bredesen, Thijs A. Peters, Marit Stange, Nicla Vicinanza and Hilde J. Venvik, 40,
Chapter 12 Membrane Reactors in Hydrogen Production A. Brunetti, G. Barbieri and E. Drioli, 87,
Chapter 13 Palladium-based Selective Membranes for Hydrogen Production G. Iaquaniello, M. De Falco and A. Salladini, 110,
Chapter 14 Polarization and Inhibition by Carbon Monoxide in Palladium-based Membranes Giuseppe Barbieri, Alessio Caravella and Enrico Drioli, 137,
Chapter 15 Carbon Molecular Sieve Membranes for Gas Separation May-Britt Hägg and Xuezhong He, 162,
Chapter 16 Perovskite Membranes for High Temperature Oxygen Separation F. Liang and J. Caro, 192,
Chapter 17 Zeolite Membranes for Gas Separations C. Algieri, G. Barbieri and E. Drioli, 223,
Chapter 18 Engineering Aspects of MIEC Hollow Fiber Membranes for Oxygen Production X. Tan and K. Li, 253,
Chapter 19 New Metrics in Membrane Gas Separation A. Brunetti, G. Barbieri and E. Drioli, 279,
Subject Index, 302,

CHAPTER 1

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 of 1D non-isothermal models can show some limitations of PBMRs, especially in case of autothermal reforming (where an oxidation reaction is used to supply the energy required for the reforming).

On the other hand, with ultra-thin (high permeation flux) membranes, which have recently become available, it has been experimentally shown that the extent of bed-to-wall mass transfer limitations in case of hydrogen purification/ production become prominent, which greatly influences the reactor performance. When these limitations prevail, the hypotheses behind the 1D model are no longer valid and more sophisticated 2D models need to be used.

In this chapter it will be shown how 2D models can be used to predict the extent of external mass transfer limitations and their effect on the reactor performance. Also the effect of a radial porosity profile (important where the ratio of the tube diameter over the particle diameter is smaller than about 10) can be included in the model.

The main drawbacks of the packed bed membrane reactors are related to the temperature profiles occurring in these reactors (with possible hot spot formation) which are detrimental for the membrane stability, the bed-to-wall mass transfer limitations and, to some extent, also the intra-particle mass transfer limitations because relatively large particle sizes are often applied to prevent large pressure drops. All these detrimental phenomena can be circumvented by using a fluidized bed membrane reactor. In this case the membranes are immersed in a fluidized bed of small catalytic particles. The fluidization regime results in a virtually uniform temperature throughout the reactor even in highly exothermic reactions. At the same time the bed-to-wall mass transfer limitations are strongly reduced while the small particle size also results in negligible intra-particle mass transfer limitations. Possible bubble-to-emulsion mass transfer limitations can be reduced by optimal positioning of the membranes in the fluidized bed. In this chapter the fluidized bed membrane reactors will be simulated with a two-phase phenomenological model. The extent of bubble-to-emulsion mass transfer limitations will also be discussed. The results in terms of recovery and conversion in fluidized bed membrane reactors will be compared with the results of packed bed membrane reactors. Finally, the multi-scale approach to gas–solid fluidized beds will be shortly introduced to indicate how the modeling can be improved in the future.

10.2 Limit Conversion in Membrane Reactors

Membrane reactors are often used to circumvent the equilibrium constraints which limit the conversion in conventional reactor systems. Although a membrane reactor is, in principle, able to give higher conversion than a traditional system, it is a good exercise to first compute the limit conversion attainable in a membrane reactor before proceeding in a more detailed simulation and experimentation of such a system. From simplified calculations it can be found that this limit conversion exists only in some cases, being 100% in the other cases.

Let us now consider the main differences between a traditional system and the corresponding membrane system: whatever the reactor type considered (packed bed, stirred tank, fluidized bed, etc.) the difference between the two systems is the selective permeation through the membrane of one or more species which occurs in the membrane system.

We can use this knowledge to compute the limit conversion in a membrane reactor by simply adding the equilibrium of permeating species through the membrane to the condition of chemical equilibrium of the traditional system. This means that, the pseudo-equilibrium state of the membrane system is achieved if the two following conditions are simultaneously satisfied:

• Chemical equilibrium inside the reaction zone (as for the traditional system)

• Partial pressures equilibrium between the reaction zone and permeation zone (valid for the membrane reactor).

If a non-isothermal system is considered, the energy balance has to be satisfied along with the above-mentioned conditions. The term pseudo-equilibrium is used here to indicate the limit conversion and to compare it with the equilibrium conversion of a traditional system. Since the membrane reactor is an open system, it is indeed not fully correct to use the term “equilibrium”.

By using this definition of pseudo-equilibrium, it is straightforward that the limit conversion of a membrane system can be as high as 100% in particular cases. Let us consider two similar cases. The first is a system where the total pressure at the permeation side is zero and only (one or more) products can permeate membrane, while the second case occurs when, at the permeation side, an infinite amount of inert (and not permeating) gas is used and only products can permeate through the membrane.

In both cases, the partial pressure of the products in the permeation side is always equal to (or close to) zero, so that the second condition (equilibrium of the partial pressures) is reached only if the reaction conversion and products permeation is complete: limit conversion is 100%.

Let us now consider a simple reaction in a membrane reactor where only a product is permeating through the membrane (and where the pressure in the permeate side is different than zero). For this calculation consider the typical steam reforming of methane which is an equilibrium reaction producing hydrogen, carried out in a Pd-based dense membrane which allows only the product hydrogen to permeate and leave the reaction zone.

For simplicity we will consider only one reaction as:

CH4 + H2O [??] CO + 3H2 (10.1)

Applying the conditions for the limit conversion results in the following:

• Local chemical equilibrium inside the reaction zone

• Hydrogen permeation equilibrium through the membrane (partial pres- sure of hydrogen is the same in the two zones of the membrane reactor).

The first condition can be written by using the stoichiometric method with the ideal gas law as:

[MATHEMATICAL EXPRESSION OMITTED] (10.2)

The second condition leads to the following equality:

PH2,reac = PH2,perm (10.3)

Let us solve this system with initial concentrations CH4/H2O = 1/3, no initial products and a permeation pressure equal to 1 bar. The calculation show the results reported in Figure 10.1.

From the figure it can be seen that the membrane reactor is able to overcome the equilibrium conversion of a conventional system; however, this calculation also shows that the membrane reactor has a limit conversion which is well below 100% for a wide range of temperatures and pressures. For instance, by working at temperatures lower than 900 K and 20 bar it will be impossible to reach 100% conversion. This implies that it is not worth installing more membrane area or thinner membranes in a real reactor when the conversion has reached the limit conversion.

The same calculation can be done for different reaction systems, with more reactions taking place simultaneously and with more compounds permeating the membrane, and it is irrespective of the membrane reactor considered. Once the limit conversion for the reactor has been computed, the detailed simulation of the membrane reactor can be carried out by using the models described in the following sections.

10.3 Packed Bed Membrane Reactors

A packed bed membrane reactor is an assembly of usually uniformly sized catalytic particles, which are randomly arranged and firmly held in position within a vessel or tube. A permeable membrane (generally tubular) is immersed within the particles or represents the tube wall of the fixed bed. The PBMR could look, for example, like a tube-in-shell or a multi-tubular reactor. Zooming in on the reaction zone the different phenomena occurring in the reactor can be described as follows:

• The reactants are transported first from the bulk of the fluid to the catalyst surface.

• The reactants permeate through the pores of the catalyst.

• The reactants adsorb on the surface of the pores.

• The chemical transformation takes place.

• The formed products desorb from the surface.

• They are transported back into fluid bulk.

• The desired product is transported from the bulk to the membrane surface.

• The product is transported through the membrane and separated from the reaction zone.

It has to be noted that the last phenomenon is, in general, a combination of different contributions (elementary steps) depending on the type of membrane (porous or dense, organic or inorganic, etc.) and it is often represented by a phenomenological permeation equation (see below).

Along with these general steps, the convection of the bulk fluid is tied in with heat and mass dispersion. Dispersion effects are largely caused by the complex flow patterns in the reactor induced by the presence of the packing, by trans- port phenomena like molecular diffusion, thermal conduction in fluid and solid phases and radiation and by the presence of the membrane itself.

Last, but not least, chemical reactions are generally accompanied with heat generation or consumption to be taken into account when modeling the process.

The above-mentioned phenomena make an exact description of a packed bed membrane reactor either impossible or lead to very complex mathematical problems. The more detailed the mathematical model, the more parameters it will contain.

Most of the elementary steps described above can hardly be individually and independently investigated and for this reason the more detailed models suffer from a lack of accurate parameter estimations.

Therefore, the description of PBMRs is often carried out via simplified models capturing the most crucial and salient features of the problem at hand. The best model is selected on the basis of the properties of the particular system under consideration, the features of the system one is interested in and the availability of the parameters included in the model.

(Continues…)Excerpted from Membrane Engineering for the Treatment of Gases by Enrico Drioli Giuseppe Barbieri. Copyright © 2011 Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.