An Introduction to Membrane Science and Tecnology

Prefazione - Indice - Introduzione

Prefazione

Membranes and membrane processes are not a recent invention. They are part of our daily life and exist as long as life exists. The preparation of synthetic membranes and their utilization on a large industrial scale, however, are a more recent development which has rapidly gained a substantial importance due to the large number of practical applications.
Today, membranes are used to produce potable water from the sea, to clean industrial effluents and recover valuable constituents, to concentrate, purify, or fractionate macromolecular mixtures in the food and drug industries, and to separate gases and vapors. They are also key components in energy conversion systems, and in artificial organs and drug delivery devices. The membranes used in the various applications differ widely in their structure and function and the way they are operated in the various membrane processes. It is, therefore, difficult to obtain a reasonably comprehensive and complete overview of the entire field of membranes and membrane processes including their applications which is extremely fragmented and covered in the literature by a large number of publications in different scientific journals and in several excellent books focusing more on certain aspects of membrane science such as theoretical treatment of membrane functions, engineering consideration of membrane process design, or membrane preparation and large scale production.

Cross section of an asymmetric membrane

The purpose of this book is to provide a short but reasonably comprehensive introduction to the membrane science for students and interested persons with an engineering or scientific background to gain a basic understanding of membranes and membrane processes in various applications and their present and future technical relevance and economic impact. The book is concentrated on the discussion of selected fundamental and application related aspects.
Following a short general introduction and definition of terms used in the description of membrane structures and properties some fundamental thermodynamic and mathematical relations necessary for an understanding of the membrane functions in the various processes and their applications are discussed. In the next chapter of the book the basic principles of the more relevant practically utilized membrane processes are described in some detail and their technical and commercial advantages as well as their limitations are pointed out. New and emerging membrane processes are more briefly treated and their potential applications are indicated.
The design of membrane processes and the construction of hardware components for various applications are discussed in the following chapter which also contains membrane process cost assessments and general process optimization strategies. This is followed by a chapter on the discussion of other engineering considerations such as mass transfer in membrane modules, the causes of concentration polarization and membrane fouling and their consequences for the module design and a proper operation of a membrane process in a certain application.
In the next chapter the preparation and characterization of porous symmetric, asymmetric and composite membranes made from polymers or inorganic materials to be used in the different membrane processes and applications are described. The preparation of ion-exchange membranes and supported and unsupported liquid membranes containing specific carrier components and other special property membranes is also discussed.
The final chapter is dedicated to the practical application of membranes and membrane processes. In selective examples the application of the mature membrane processes such as reverse osmosis, ultra- and microfiltration or dialysis and electrodialysis in water desalination and purification and in the chemical industry or food and drug production are described and energy requirements and process costs of a given plant capacity are estimated. The application of more membranes in new and emerging processes such as controlled release of drugs in medical therapy, in artificial organs and membrane reactors or membrane conversion systems is also discussed in selected examples. However, from the large number of applications only very few have been discussed or even mentioned. A more extensive treatise of all present and future possible applications of membranes and membrane processes is far beyond the scope of this book and further reading of the relevant publications on this subject is recommended.
A great deal of the literature on the practical application of membranes originated in the United States where units such as gallons, pounds, inches, mils, or pounds per square inch are widely in engineering practice. In Europe and most other countries, however, metric units, i.e. meter, second and kilogram are used. To facilitate the understanding of the membrane related literature an appendix is added which contains a number of tables with commonly used constants and the conversion of the different units.

An outlook for future membrane developments
Membrane operations in the last years have shown their potentialities in the rationalization of production systems. Their intrinsic characteristics of efficiency, operational simplicity and flexibility, relatively high selectivity and permeability for the transport of specific components, low energy requirements, good stability under a wide spectrum of operating conditions, environment compatibility, easy control and scale-up have been confirmed in a large variety of applications and operations, as molecular separation, fractionation, concentrations, purifications, clarifications, emulsifications, crystallization, etc., in both liquid and gas phases and in a wide spectrum of operating parameters such as pH, temperature, pressure, etc.
Some of the largest plants in the world for sea water desalination are already based on membrane engineering. The Red-Sea/Dead-Sea desalination project, under discussion today, is based for example on RO with a productivity of 27m3/s of permeate. Membrane operations are
practically the dominant technology in desalination and they will confirm this role in the next decades.

Passive membrane transport

A similar situation in part exists in the treatment of gas streams, where for example the non-cryogenic nitrogen production and hydrogen purification are already present at industrial level. The development of new polymeric or inorganic membranes characterized by a high permeability and selectivity for CO2 might offer a solution to the problem of CO2 capture and purification, significantly impacting with the strategy for a sustainable industrial growth. The possibility of having the membrane systems also as tools for a better design of chemical transformation is becoming attracting and realistic. For biological applications, synthetic membranes provide an ideal support to catalyst immobilization due to their available surface area per unit volume. In addition, membrane bioreactors are particularly attractive in terms of eco-compatibility because they do not require additives, are able to operate at moderate temperature and pressure, and to reduce the formation of by-products. Potential applications have been and will be at the origin of important developments in various technology sectors, mainly concerning induction of microrganisms to produce specific enzymes, techniques of enzymes purification, overall design of efficient productive cycles.
Development of catalytic membrane reactors for high temperature applications became realistic only in recent years with the development of high temperature resistant membranes. Most of these reactors use inorganic membranes that can be dense or porous, inert or catalytically active. No large scale industrial applications have been reported so far, because of a relatively high prize of membrane units. However, current and future advances in the material engineering might significantly reverse this trend. Besides the huge progresses in the last years, membrane engineering is probably still at its infancy. Process intensification is the most interesting strategy offered today for realizing a sustainable industrial growth, compatible with a desirable high quality of our life. Membrane engineering in its various aspects, molecular separations, membrane reactors, membrane contactors, is quite consistent with practically all the requirements for making this strategy a reality.
Membrane based artificial organs such as the artificial kidney are a standard part of modern biochemical engineering and medicine. New hybrid artificial organs such as the artificial liver and artificial pancreas are expected to become more and more equally utilized in a relatively short period of time and new organs such as the artificial retina, or the artificial brain are attracting the interest of the new generation of membranologists.
Also traditional areas such as encapsulation and packaging will be substantially modified and innovated with the transfer of more basic understanding of transport phenomena and membrane phenomena in general in these sectors. The redesigning of overall industrial productions such as the petrochemical plants as integrated membrane systems might become real in few years from now. Contributions of membrane technologies to the life in space and in other planets are already in progress in various laboratories around the world. Membrane contactors in their various configurations and operations (emulsifiers, crystallizers, strippers, scrubbers, etc.) will make the opportunities of integrated membrane systems for an industrial sustainable growth more realistic.
The possibility of developing new nanostructured materials with specific configurations and morphology is offering powerful tools for the preparation of membranes with controlled selectivity and permeability higher than the membranes existing today. Membranes characterized by highly selective transport mechanisms as the perovskite studied for oxygen separation from air, or the palladium for H2 purification are suggesting the use of molecular dynamic studies for identifying new structures characterized by similar selectivity towards a larger spectrum of chemical species. Biological membranes reproduce themselves continuously, controlling important physiological processes, where fouling e.g. does not represent a problem as in artificial systems. The mechanisms which generate our memory or the function of our brain are other important membrane phenomena.
The role that membrane science and membrane engineering play in our life, justifies growing efforts in the education of young generations of researchers, engineers and technicians on their basic properties and on their possible applications. This book has been written with the scope of contributing to these efforts.

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