An Introduction to Membrane Science and Tecnology

Prefazione - Indice - Introduzione

Introduzione

The new Ashkelon SWRO plant will provide around 15% of Israel's domestic needs

The separation, concentration, and purification of molecular mixtures are major problems in the chemical industries. Efficient separation processes are also needed to obtain high-grade products in the food and pharmaceutical industries to supply communities and industry with high-quality water, and to remove or recover toxic or valuable components from industrial effluents. For this task a multitude of separation techniques such as distillation, precipitation, crystallization, extraction, adsorption, and ion-exchange are used today. More recently, these conventional separation methods have been supplemented by a family of processes that utilize semipermeable membranes as separation barriers.
Membranes and membrane processes were first introduced as an analytical tool in chemical and biomedical laboratories; they developed very rapidly into industrial products and methods with significant technical and commercial impact [Lonsdale, 1982; Ho et al., 1992; Osada et al., 1992; Zeman et al., 1996; Drioli et al., 2001; Bhattacharyya et al., 2003; Baker, 2004; Strathmann, 2004]. Today, membranes are used on a large scale to produce potable water from sea and brackish water, 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 in petrochemical processes. They are also key components in energy conversion and storage systems, in chemical reactors, in artificial organs, and in drug delivery devices.

Capillary membrane

The membranes used in the various applications differ widely in their structure, in their function and the way they are operated. However, all membranes have several features in common that make them particularly attractive tools for the separation of molecular mixtures. Most important is that the separation is performed by physical means at ambient temperature without chemically altering the constituents of a mixture. This is mandatory for applications in artificial organs and in many drug delivery systems as well as in the food and drug industry or in downstream processing of bioproducts where temperature-sensitive substances must often be handled. Furthermore, membrane properties can be tailored and adjusted to specific separation tasks, and membrane processes are often technically simpler and more energy efficient than conventional separation techniques and are equally well suited for large-scale continuous operations as for batch-wise treatment of very small quantities.
Although synthetic membranes are widely used as valuable scientific and technical tools in a modern industrialized society, they are not very well defined in terms of their structure and function. The most prominent association that many people have when thinking of a membrane resembles that of a filter, i.e. a device capable of separating various components from a mixture according to their size.
However, a membrane can be much more complex in both structure and function. A membrane may be solid or liquid, homogeneous or heterogeneous, isotropic or anisotropic in its structure. A membrane can be a fraction of a micrometer or several millimeters thick. Its electrical resistance can vary from millions of Ohm to a fraction of an Ohm.
Another characteristic property of a membrane is its permselectivity, which is determined by differences in the transport rates of various components in the membrane matrix. The permeability of a membrane is a measure of the rate at which a given component is transported
through the membrane under specific conditions of concentration, temperature, pressure, and/or electric field. The transport rate of a component through a membrane is determined by the structure of the membrane, by the size of the permeating component, by the chemical nature and the electrical charge of the membrane material and permeating components, and by the driving force, i.e. concentration, pressure or electrical potential gradient across the membrane. The transport of certain components through a membrane may be facilitated by certain chemical compounds, coupled to the transport of other components, or activated by a chemical reaction occurring in the membrane. These phenomena are referred to as facilitated, coupled, or active transport.

Protein crystals in membrane crystallizers

The versatility of membrane structures and functions makes a precise and complete definition of a membrane rather difficult. In the most general sense a membrane is a barrier that separates and/or contacts two different regions and controls the exchange of matter and energy between the regions. The membrane can be a selective or a contacting barrier. In the first case, it controls the exchange between the two regions adjacent to it in a very specific manner; in the second case, its function is mainly to contact the two regions between which the transport occurs.
We can distinguish between biological membranes, which are part of the living organism, and synthetic membranes that are man-made. Biological membranes carry out very complex and specific transport tasks in living organisms. They accomplish them quickly, efficiently, and with minimal energy expenditure, frequently using active transport.
Synthetic membranes are not nearly as complicated in their structure or function as biological membranes. They have only passive transport properties and are usually less selective and energy efficient. In general, however, they have significantly higher chemical and mechanical stability, especially at elevated temperature. The selectivity of synthetic membranes is determined by a porous structure according to their size or through a homogeneous structure according to the solute solubility and diffusivity. The permeability of the membrane for different components, however, is only one parameter determining the flux through the membrane. Just as important as the permeability is the driving force acting on the permeating components. Some driving forces such as concentration, pressure, or temperature gradients act equally on all components, in contrast to an electrical potential driving force, which is only effective with charged components. The use of different membrane structures and driving forces has resulted in a number of rather different membrane processes such as reverse osmosis, micro-, ultra- and nanofiltration, dialysis, electrodialysis, Donnan dialysis, pervaporation, gas separation, membrane contactors, membrane distillation, membrane-based solvent extraction, membrane reactors, etc. Even more heterogeneous than membrane structures and membrane processes are their practical applications. The large-scale industrial utilization of membranes began about 1970 with water desalination and purification to produce potable and high quality industrial water. Since then membranes have become a widely used tool in process engineering with significant technical and commercial impact. Today membrane processes are used in three main areas. The first area includes applications such as seawater desalination or wastewater purification. Here, the use of membranes is technically feasible, but there are other processes such as distillation and biological treatment with which membranes must compete on the basis of overall economy. The second area includes applications such as the production of ultra pure water or the separation of molecular mixtures in the food and drug industry. Here, alternative techniques are available, but membranes offer a clear technical and commercial advantage. The third area includes membrane applications in artificial organs and therapeutic systems. There is no reasonable alternative to membrane operations.
With the development of new membranes having better separation efficiency, new membrane processes such as membrane contactors and membrane reactors are becoming common unit operations in process engineering [Ho et al., 1992; Drioli et al., 1999; Marcano et al., 2002;
Klaassen et al., 2005]. The large-scale use of membranes is rapidly extending far beyond its present level.