Overview

Contents

1.1 Membrane Technology - What is it?
1.2 The Development of Membrane Technology
1.3 The Driving Forces of Separation
1.4 Purification, Concentration, Fractionation
1.5 Performance Limits
1.6 Membrane Structures
1.7 Quality, Productivity, and Life

1.1 Membrane Technology - What is it?

Membrane technology is devoted to the separation of the minutiae of particles ranging from bacteria to atoms. To some people the concern is simply the removal of this detrious. To others the recovery of the inhabitants of this sub-microscopic kingdom is the essential goal. In size its constituents span some 4 orders of magnitude, and they are dominated by colloidal/molecular forces, rather than by the gravitational forces of their larger brethren. The various inlet and outlet streams can be all liquids, all gases or combinations. Not surprisingly membrane technology is not one technology but many technologies with one common aspect; the use of a membrane which separates two streams enabling materials to be selectively transported across it. As might be expected there is plenty of commonality between these various membrane processes, but, equally, the diversity and range of applications mean that there are significant differences. In recognition of these differences a classification of membrane processes has developed.

Of the various membrane technologies, the class of membrane filtration is the largest and most diverse. One of the commonest questions is where does conventional filtration end and membrane filtration begin. In a similar vein where does ultrafiltration take over from microfiltration. To answer this sort of question can be likened to defining where does the desert end and arable land begin; the two are clearly different but there is obviously some arbitrariness in defining the boundary. Nevertheless, a semantic definition provides a quick and expedient guide as to what to expect. However, to focus too heavily on the boundary is to miss the point. Customers are not interested in whether something lies on one side or other of a boundary but on what that something can do for them. The purpose of a classification is to convey the potential use.

Membrane technology is generally regarded as addressing the separation needs of sub-micron particles. Selectivity comes through the interaction between the membrane and the surrounding phases. Two factors contribute to selectivity, the partitioning of molecules and or particles between the membrane and the surrounding phase, and the relative diffusion rates of these materials once in the membrane. It is invariably the product of these two factors which contributes to the overall selectivity of the membrane.

One feature that is common to many membrane processes, though not to all, is cross-flow. Cross-flow involves moving fluid tangentially across the membrane surface (see figure 1.1) as well as normal to it. The benefit is that particles/solutes that would otherwise accumulate at the membrane surface are moved along, achieving a steady-state distribution of particles or solutes at the interface, rather than the continually developing one that is seen in conventional filtration. The consequence of cross-flow is that in continuous operation the flux through the membrane tends to a constant while in conventional filtration the flux continues to fall. If higher fluxes are desired then higher cross-flows are required.

The benefits of cross-flow do not come without a penalty, which is the energy required to move the fluid across the surface. Fortunately, the additional cost is small compared to that required in conventional filtration to push the fluid through a filter cake. A key factor in this effect is the ratio of the cross-flow to the permeate flow. Not surprisingly, this ratio is a key aspect underlying the design of membrane elements, and selecting optimal operating conditions.

Another consequence of cross-flow is that the system is basically designed to remove only a small proportion of the feed. Thus a feature of most membrane plants is how to design systems to overcome this limitation (see Chapter 8).

In the last few years the boundary between conventional filtration and membrane filtration has been further blurred with the development of hybrid processes. These processes allows some-build up of material at the membrane surface but then the material is dislodged by passing water or air back through the membrane. By repeating this process at frequent intervals (circa 15 min) a reasonable flux through the membrane can be maintained. In this way the deposits on the surface have limited effect and the membrane remains the controlling factor.

1.2 The Development of Membrane Technology

Membrane technology grew out of a 19th century endeavour to investigate a kingdom of particles too small to be seen. With no way of seeing these sub-microscopic constituents, membranes proved to be a useful tool to probe these invisible components. The resulting exploration that ensued provided key ingredients in the development of molecular theory of matter, which burst onto the scene at the start of the 20th century. In contrast it took nearly a 100 years to engineer membranes from a scientific tool to an industrial tool.

The Early Years - A Scientific Tool

A significant contributor in these early years was Thomas Graham, a Scottish chemical physicist and Master of the Mint. In 1861 he discovered that substances like salt and sugar rapidly passed through parchment, whereas material like gum arabic and gelatin would not pass. Materials that permeated he called crystalloids, since these materials could easily be crystallised. Those materials which did not pass, typified by glues, which at the time he believed did not crystallise, he called colloids after the Greek word for glue (Kolla). Graham showed how colloidal material could be purified from crystalloid contamination by putting the colloid in a porous container which was then placed in running water. The crystalloids pass through and the colloids remain. This process he called dialysis and the transport through - osmosis.

Thomas Graham made another important contribution as a result of studying the diffusion of gases through flat rubber membranes. In explaining his results he regarded the rubber as a liquid in which the gas dissolves and then diffuses due to a concentration gradient....