Membranes for Membrane Reactors

Preparation, Optimization and Selection

Angelo Basile, Fausto Gallucci, Angelo Basile, Fausto Gallucci

• Publisher: Wiley
• Year: 2010
• Print ISBN: 9780470746523
• eBook ISBN: 9780470977576
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Industrial & Technical Chemistry

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

Chapter 1

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 Types of carbon membranes and transport mechanisms

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 [1], 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 [2–4] 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 [5–9]. 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 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 lattice vacancies in the microcrystallites and that pores are formed from imperfections in the packing between microcrystalline regions.

Figure 1.2 Structure of turbostratic graphite. This article was published in Handbook of Carbon, Graphite, Diamond, and Fullerenes. Vol. 3, Pierson, H., Graphite Structure and Properties, 48, Copyright (1993) with permission from Elsevier

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 (

). 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).

(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

.

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 [2–4], operate in this regime. SSF membranes can achieve high 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 C₃H₆/C₃H₈.

Figure 1.3 Gas permeance and selectivity of a CMS membrane derived from a polyimide hollow fibre measured at 25 °C

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

(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:

(1.3)

The exponential term is an energetic selectivity. For gas molecules that differ in both size, and shape, complex configurational 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 [11]. 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.

Figure 1.4 Changes in the weight and diameter of Kapton polyimide films as a function of the pyrolysis temperature (

= diameter; ⦿ = weight). 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, 3988–3994. Copyright (1997) with permission from American Chemical Society

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 [8]. 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 [12]. 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 [6]. 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-resol...

Table of contents

• Cover
• Title Page
• Copyright
• Contributors
• Glossary
• Introduction – A Review of Membrane Reactors
• Chapter 1: Microporous Carbon Membranes
• Chapter 2: Metallic Membranes by Wire Arc Spraying: Preparation, Characterisation and Applications
• Chapter 3: Inorganic Hollow Fibre Membranes for Chemical Reaction
• Chapter 4: Metallic Membranes Prepared by Cold Rolling and Diffusion Welding
• Chapter 5: Preparation and Synthesis of Mixed Ionic and Electronic Conducting Ceramic Membranes for Oxygen Permeation
• Chapter 6: Nanostructured Perovskites for the Fabrication of Thin Ceramic Membranes and Related Phenomena
• Chapter 7: Nanostructured Perovskites for the Fabrication of Thin Ceramic Membranes and Related Phenomena
• Chapter 8: Zeolite Membrane Reactors
• Chapter 9: Metal Supported and Laminated Pd-Based Membranes
• Chapter 10: PVD Techniques for Metallic Membrane Reactors
• Chapter 11: Membranes Prepared via Electroless Plating
• Chapter 12: Silica Membranes – Preparation by Chemical Vapour Deposition and Characteristics
• Chapter 13: Membranes Prepared via Molecular Layering Method
• Chapter 14: Solvated Metal Atoms in the Preparation of Catalytic Membranes
• Chapter 15: Electrophoretic Deposition for the Synthesis of Inorganic Membranes
• Chapter 16: Electrochemical Preparation of Nanoparticle Deposits: Application to Membranes and Catalysis
• Chapter 17: Electrochemical Preparation of Pd Seeds/Inorganic Multilayers on Structured Metallic Fibres
• Chapter 18: Membranes Prepared Via Spray Pyrolysis
• Chapter 19: Silica Membranes – Preparation and Characterisation of Nanocrystalline and Quasicrystalline Alloys by Planar Flow Casting for Metal Membranes
• Chapter 20: Silica Membranes – Preparation and Characterisation of Amorphous Alloy Membranes
• Chapter 21: Membranes Prepared Via Phase Inversion
• Chapter 22: Porous Flat Sheet, Hollow Fibre and Capsule Membranes by Phase Separation of Polymer Solutions
• Chapter 23: Porous Polymer Membranes by Manufacturing Technologies other than Phase Separation of Polymer Solutions
• Chapter 24: Palladium-Loaded Polymeric Membranes for Hydrogenation in Catalytic Membrane Reactors
• Chapter 25: Membrane Prepared via Plasma Modification
• Chapter 26: Enzyme-Immobilised Polymer Membranes for Chemical Reactions
• Final Remarks
• Color Plates
• Index