Membranes for Membrane Reactors: Preparation, Optimization and Selection - Hardcover

Hardcover

ISBN 10: 0470746521 ISBN 13: 9780470746523

Publisher: Wiley, 2011

Synopsis

A membrane reactor is a device for simultaneously performing a reaction and a membrane-based separation in the same physical device. Therefore, the membrane not only plays the role of a separator, but also takes place in the reaction itself.

This text covers, in detail, the preparation and characterisation of all types of membranes used in membranes reactors. Each membrane synthesis process used by membranologists is explained by well known scientists in their specific research field.

The book opens with an exhaustive review and introduction to membrane reactors, introducing the recent advances in this field. The following chapters concern the preparation of both organic and inorganic, and in both cases, a deep analysis of all the techniques used to prepare membrane are presented and discussed. A brief historical introduction for each technique is also included, followed by a complete description of the technique as well as the main results presented in the international specialized literature. In order to give to the reader a summary look to the overall work, a conclusive chapter is included for collecting all the information presented in the previous chapters.

Key features:

• Fills a gap in the market for a scientific book describing the preparation and characterization of all the kind of membranes used in membrane reactors
• Discusses an important topic - there is increasing emphasis on membranes in general, due to their use as energy efficient separation tools and the ‘green’ chemistry opportunities they offer
• Includes a review about membrane reactors, several chapters concerning the preparation organic, inorganic, dense, porous, and composite membranes and a conclusion with a comparison among the different membrane preparation techniques

About this title

About the Author

Angelo Basile
Institute on Membrane Technology, ITM-CNR c/o University of Calabria, Rende (CS), Italy

Fausto Gallucci
Faculty of Chemical Engineering and Chemistry, Eindhoven University of Technology, Eindhoven, The Netherlands

From the Back Cover

A membrane reactor is a device for simultaneously performing a reaction and a membrane-based separation in the same physical device. Therefore, the membrane not only plays the role of a separator, but also takes place in the reaction itself. They can be used in a wide range of applications, ranging from in-vivo reactions, to high temperature gas phase reactions.

The core of the membrane reactor is the membrane, which can be either organic (polymeric) or inorganic (ceramic, metal). Each application needs a specific membrane (type, geometry) and each membrane needs an appropriate preparation method. This text covers the preparation and characterization of all types membranes used in membrane reactors.

The book opens with an exhaustive review and introduction to membrane reactors and membrane bioreactors, introducing the different types of reactors and their applications. The rest of the book is divided into two parts – inorganic and organic – and contains chapters devoted to the preparation methods of the different membranes.

Intended for PhD students, chemical engineers, environmental engineers, materials science experts, biologists, and researchers, Membranes for Membrane Reactors is an ideal resource for anyone investigating membrane reactors.

The core of the membrane reactor is the membrane, which can be either organic (polymeric) or inorganic (ceramic, metal). Each application needs a specific membrane (type, geometry) and each membrane needs an appropriate preparation method. This text covers the preparation and characterization of all types membranes used in membrane reactors.

The book opens with an exhaustive review and introduction to membrane reactors and membrane bioreactors, introducing the different types of reactors and their applications. The rest of the book is divided into two parts – inorganic and organic – and contains chapters devoted to the preparation methods of the different membranes.

Intended for PhD students, chemical engineers, environmental engineers, materials science experts, biologists, and researchers, Membranes for Membrane Reactors is an ideal resource for anyone investigating membrane reactors. 

Excerpt. © Reprinted by permission. All rights reserved.

Membranes for Membrane Reactors

Preparation, Optimization and Selection

John Wiley & Sons

Copyright © 2011 John Wiley & Sons, Ltd
All right reserved.
ISBN: 978-0-470-74652-3

Chapter One

Microporous Carbon Membranes

Miki Yoshimune and Kenji Haraya

Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Japan

1.1 Introduction

There is growing interest in the development of microporous inorganic membranes made of zeolites, silica, carbon, or similar materials, whose separation mechanisms are controlled mainly by the molecular sieving effect. Such inorganic membranes are capable of achieving excellent separation efficiencies and, unlike conventional polymeric membranes, can function at high temperatures or in harsh environments. Carbon membranes have the greatest potential among these inorganic membranes because of the relative ease with which they can be produced and their resulting low cost.

Figure 1.1 shows the general types of carbon membranes together with a classification of their gas transport mechanisms into various categories, such as molecular sieving, surface diffusion, Knudsen diffusion, and viscous flow (VS), together with the ranges of pore sizes that correspond to each particular mechanism. Microporous carbon membranes can be categorised into two types: (i) carbon molecular sieve (CMS) membranes (Figure 1.1a) and (ii) nanoporous carbon membranes (Figure 1.1b). CMS membranes, first prepared by Koresh and Soffer, have micropores with diameters of approximately 0.3–0.5 nm, and they are characterised by high selectivities in gas separations as a result of the selective permeation of smaller gas molecules. Nanoporous carbon membranes were designed by Rao and Sircar as selective surface flow (SSF) membranes, and have larger micropores (0.5–0.7 nm) than CMS membranes.

Because separations using microporous carbon membranes have attracted consistently high levels of research interest, they are the subject of a number of excellent reviews and books. This chapter presents an overview of recent researches on microporous carbon membranes and explores their possible applications in membrane reactors. Section 1.3 reviews and discusses the Membranes for Membrane Reactors: Preparation, Optimization and Selection, Edited by Angelo Basile and Fausto Gallucci factors that control the preparation of high-performance microporous carbon membranes. Trends in mixed-matrix carbon membranes prepared from polymeric precursors that incorporate inorganic materials such as metals, metal oxides, or zeolites are discussed in Section 1.3.10. These incorporation methods can also be used to prepare catalytic membranes for use in membrane reactors; such membranes are discussed in Section 1.5.

1.2 Transport Mechanisms in Carbon Membranes

The microporous carbon membranes that are used for gas separation usually have a turbostratic structure [10] in which layer planes of graphite-like microcrystallites are randomly stacked. Figure 1.2 shows that there are latticevacancies in the microcrystallites and that pores are formed from imperfections in the packing between microcrystalline regions.

The mechanism of gas transport through porous carbon membranes is essentially the same as that in other inorganic porous membranes. When the pore diameter (d_p) is greater than the mean free path of the gas molecule (λ), intermolecular collisions predominate and the transport of gas molecules through porous membranes under a pressure or a concentration gradient corresponds to viscous flow and is nonselective.

When d_p is smaller than λ, collisions between the gas molecules and the pore walls predominate so that the transport of gas molecules is controlled by the thermal mean velocity of the gas molecules (v = [square root of 8RT/πM]). In the case of a capillary pore with a diameter of d_p, the diffusion of the gas can be described by Equation (1.1).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1.1)

Here, D_k is the Knudsen diffusion coefficient, R is the universal gas constant, T is the absolute temperature, and M is the molecular weight of the penetrant gas. On the basis of Knudsen diffusion, the selectivity (i.e., the ideal separation factor) of a gas pair A–B is given by the expression [square root of M_B/M_A.

When the temperature is within the range where adsorption of gas molecules on the pore walls becomes important, transport of the gas molecules along the surface (surface diffusion) occurs in combination with Knudsen flow. The effects of surface diffusion increase with decreasing d_p and they produce selectivity in the flow as a result of selective adsorption. Selective surface flow (SSF) membranes, as named by Rao and Sircar, operate in this regime. SSF membranes can achievehigh performances in separations of gas mixtures consisting of a readily adsorbed species and a component that is not readily adsorbed, such as mixtures of hydrocarbons with hydrogen. If penetrants are condensable, such as vapours, the condensates can completely fill the pores resulting in capillary condensation that blocks the permeation of noncondensable components. This mechanism has been observed in other inorganic porous membranes, but has not yet been reported in carbon membranes.

When d_p is of a similar size to that of a gas molecule (0.5 nm or less), selective transport as a result of a molecular sieving effect can be observed. Smaller molecules pass readily through the pores, whereas the passage of larger molecules is obstructed or highly restricted. Microporous carbon membranes in this regime are usually known as carbon molecular sieve (CMS) membranes. Typical examples of the permeances of various gases through a CMS membrane are plotted in Figure 1.3 as a function of the size of the gas molecule. This figure shows that the membrane is not only effective in separating mixtures of gases of different molecular sizes, such as H₂/CH₄, H₂/C₃H₈, He/N₂, or N₂/SF₆, but also in separating gases of similar molecular sizes, such as O₂/N₂, CO₂/CH₄, CO₂/N₂, or C₂H₂/C₂H₂.

Because diffusion is an activated process in both CMS and polymeric membranes, the diffusion coefficient (D) can be expressed by an Arrhenius-type relationship:

D = D₀ exp(–E_D/RT 1:2

Here, E_D is the energy of activation required for a gas molecule to execute a diffusive jump from one cavity to another, and D₀ is the temperature-independent pre-exponential term. The diffusion selectivity of A–B gas molecules can be expressed as follows:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] 1:3

The exponential term is an energetic selectivity. For gas molecules that differ in both size, and shape, complexconfigurational effects related to factors affecting D₀ for the components A and B can occur. These configurational selectivity contributions to the D_A/D_B ratio are often referred to as the entropic selectivity. The excellent selectivity observed in CMS membranes is the result of a favourable contribution from this factor, which is generally lacking in conventional polymeric membranes.

1.3 Methods for the Preparation of Microporous Carbon Membranes

1.3.1 General Preparation and Characterisation

Microporous carbon membranes are generally formed by pyrolysing polymeric precursor membranes. Pyrolysis (or carbonisation) is the process whereby the precursor membrane is heated to a pyrolysis temperature in the range 500–1000 °C under a controlled atmosphere, such as a vacuum or an inert gas (N₂, He, or Ar), at a specific heating rate and then held at the pyrolysis temperature for a sufficiently long thermal soak time. Gaseous decomposition products are evolved during the pyrolysis of the polymeric precursor, resulting in formation of micropores in the membrane; this is accompanied by a considerable loss in weight and dimensional shrinkage. Figure 1.4 shows a typical example in which a weight loss of up to 40% and shrinkage by up to 25% were observed during pyrolysis of circular films of a polyimide. The pyrolysed membranes are sometimes post-treated by chemical vapour deposition (CVD) or by activation processes to improve their performance.

The greatest interest in the resulting carbon membranes is in evaluating the possibilities for their use as separation membranes. For this reason, the permeabilities of gases or vapours through the membranes are usually measured by using a permeation test apparatus. In some cases, pervaporation tests are also performed to test the separation performances for organic solutions such as water–ethanol or benzene–cyclohexane. Gas permeability or permeance through flawless carbon membranes depends mainly on the size of the gas molecules, as shown in Figure 1.3, so that the relationship can be considered as an index of the pore size distribution.

The microstructures of carbon membranes are generally investigated by several analytical techniques; these include gas adsorption measurements, wide angle X-ray diffraction (WAXD), high-resolution transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). Gas adsorption measurements using N₂, CO₂, or hydro-carbons as sorbing gases provide information on the pore size, the pore size distribution, and the specific surface area. WAXD is used to evaluate the degree of packing of the microporous carbon structures, whose interlayer distance (i.e., the average d spacing value) is calculated by using the Bragg equation, d = nλ/2 sin θ, where d is the d-spacing, θ the diffraction angle, λ the wavelength of the X-ray radiation, and n is an integral number (1, 2, 3, etc). The average d spacing does not indicate the essential pore dimensions, but is believed to provide a measure of the length of the diffusion pathway for gas molecules through the carbon membranes. These properties relating to the pore structure are strongly dependent on the pyrolysis conditions and the nature of the polymeric precursors. The dependence of the pore size distribution on the pyrolysis temperatures is illustrated in Figure 1.5, where the average pore diameters decreased from about 0.45 to about 0.35 on increasing the pyrolysis temperature from 873 to 1273 K.

High-resolution TEM can give information on the pore structure in the form of a visual image. An example is shown in Figure 1.6, where the black regions represent the carbon matrix. These pictures show that the membrane pyrolysed at the higher temperature (1273 K) developed layer planes of graphite-like microcrystallites.

It is also important to measure other parameters related to the course of the pyrolysis reaction. TGA is used to determine the decomposition temperature, and it can also be used to study the effects of the atmosphere on weight losses from the membrane during pyrolysis. Simultaneous Figure 1.5 Pore size distributions for Kapton CMS pyrolysed at various temperatures (873, 1073 and 1273 K). Limiting micropore volume (W0) is plotted against the kinetic diameters of sorbate probe molecules. Reprinted from Journal of Physical Chemistry B, Suda, H., Haraya, K., Gas permeation through micropores of carbon molecular sieve membranes derived from kapton polyimide. Vol 101, studies of off gases by means of mass spectrometry (MS) provide information on the chemical groups that are decomposed at high temperatures. FTIR, X-ray photoelectron spectroscopy (XPS), and elemental analyses of the precursor and of carbonised membranes pyrolysed at various temperatures are very helpful in providing an understanding of the changes in chemical structure that occur during pyrolysis.

1.3.2 Classification of Carbon Membranes

Carbon membranes can be grouped into two categories: (i) unsupported or freestanding carbon membranes and (ii) composite or supported carbon membranes. Unsupported membranes are generally produced as flat films, capillary tubes, or hollow fibres, whereas supported membranes are generally in the form of flat sheets or tubes. Many polymers, such as cellulose derivatives, polyacrylonitrile (PAN), polyimides, phenolic resins, poly(furfuryl alcohol) (PFA), poly(vinylidene chloride), and poly(phenylene oxide) (PPO), have been used as precursors for the production of carbon membranes. To prepare microporous carbon membranes that show a high performance in gas separations, it is important to select an appropriate polymeric precursor and to optimise the conditions for its pyrolysis. In the following sections, factors that control the preparation of carbon membranes are reviewed and discussed.

1.3.3 The Pyrolysis Process

In inert or vacuum atmospheres, the heat treatment of polymers can be separated into three processes: (i) annealing at 100–400 °C, (ii) intermediate heating at 400–500 °C, and (iii) pyrolysis to form carbon at 500–1000 °C. The pyrolysis process is governed by several parameters such as the heating rate, the final pyrolysis temperature, the thermal soak time, and the pyrolysis atmosphere.

1.3.3.1 Pyrolysis Temperature

The pyrolysis temperature is generally chosen to be above the decomposition point of the polymer but below the graphitisation temperature (500–1000 °C). Koresh and Soffer studied the effects of the carbonisation temperature by preparing membranes at 800 and 950 °C, and found that membranes pyrolysed at the higher temperature exhibited lower permeabilities but higher permselectivities. In a study on hollow fibre CMS membranes derived from 6FDA/ BPDADAM polyimide precursors (abbreviations for dianhydrides and diamines of polyimides are listed in Table 1.1), Geiszler and Koros found that increasing the final pyrolysis temperature from 500 to 800 °C decreased the permeability but increased the permselectivity. Suda and Haraya reported that CMS membranes from pyrolysed Kapton (PMDA–ODA polyimide) showed a decrease in gas permeabilities but an increase in permselectivities on increasing the final pyrolysis temperature in the range 600–1000 °C. Similar trends in the relationship between permeabilities, permselectivities, and the pyrolysis temperature have been reported for P84 (BTDA–TDI/MDI) polyimide CMS membranes, Matrimid (BTDA–DAPI) polyimide CMS membranes, BTDA–ODA polyimide CMS membranes, and PFA CMS membranes. As a general rule, an increase in the pyrolysis temperature reduces the permeability of CMS membranes but increases their selectivity [5]. High pyrolysis temperatures produce increased crystallinity, increased density, and lower average interplanar spacings in CMS membranes.

Several researchers have made detailed studies on the effects of the pyrolysis temperature and have reported exact results. Increasing the pyrolysis temperature from 500 °C results in an increase in the gas permeability of the carbonised membranes by one or two order of magnitude, with a maximum at around 650–750 °C. On further increasing the pyrolysis temperature, the resulting carbon membranes become less permeable. In some cases, maxima in the permselectivities are also observed. Hayashi et al. reported that BPDA–ODA polyimide CMS membranes pyrolysed at 550–700 °C showed maximal permeabilities, whereas those pyrolysed at 800 °C exhibited peak He/N₂ selectivity. Kusuki et al. found that a BPDA-aromatic diamine polyimide hollow fibre CMS membrane pyrolysed at 650 °C displayed maximal permeability to H₂, whereas one pyrolysed at 850 °C exhibited peak H₂/CH₄ selectivity. Centeno and Fuertes [69] observed a peak in the He permeability of a phenolic resin-based CMS membrane carbonised at around 700 °C. In studies on 6FDA/BPDA-DDBT copolyimide hollow fibre CMS membranes, Yoshino et al. observed maximum permeabilities for membranes pyrolysed at 550 °C, whereas the peak permselectivities occurred at 650 ° C. Yoshimune et al. found that CMS membranes based on PPO and PPO derivatives pyrolysed at 650 °C exhibited maximal permeabilities, whereas peak permselectivities were observed at different pyrolysis temperatures.

(Continues...)

Excerpted from Membranes for Membrane Reactors Copyright © 2011 by John Wiley & Sons, Ltd. Excerpted by permission of John Wiley & Sons. 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.

Edition details

• Publisher: Wiley
• Publication date: 2011
• Language: English
• ISBN 10: 0470746521
• ISBN 13: 9780470746523
• Binding: Hardcover
• Edition number: 1
• Number of pages: 640
• Editor: Basile Angelo, Gallucci Fausto