Membranes, the Finest Filtration
The notion that salt could be removed from seawater without a phase change took hold of Dr. Sourirajan's imagination in the late 1950's. The actual invention of the reverse osmosis (RO) membrane took place in his laboratory at UCLA around 1960. (Thus Sourirajan could be called not only the 'Father of Reverse Osmosis,' but the initiator of all crossflow membrane technology.) The full commercialization of RO and its sister crossflow technology, ultrafiltration (UF), occurred in the early 1970's. About a dozen U.S. companies and a couple in Europe and Japan found they had to educate their customer base on both the technical reality and the potential uses of their products. In the 1980's, crossflow membrane processes became well accepted in industry and medicine, and the inevitable industry shakeouts and price competition characteristic of a maturing technology followed. Today hundreds of manufacturing processes, waste treatment and water purification applications rely on crossflow membrane for cost-effective separations.
This paper overviews crossflow membrane technology and its current applications, highlights some recent innovations and speculates on emerging trends. The topic is very broad so will not be covered in detail here. Issues important to some users and potential users will be missed; especially regarding applications, which cannot be covered in less than a book. The reader is invited to read further in the large body of literature available on membrane, and contact the author with specific questions on applications.
Crossflow Membrane Technology Defined
Membrane filtration is the separation of the components of a pressurized fluid, effected by polymeric or inorganic membranes (generally man-made). The openings in the membrane material (pores) are so small that a significant fluid pressure is required to drive the liquid through them; the pressure required varies inversely with the size of the pores (basically classical orifice theory). There are now four commonly accepted categories or 'classes' of membrane, defined based on the size of the material they will remove from the carrier liquid. Moving from the smallest to largest pore size, these are Reverse Osmosis (RO), Nanofiltration (NF), Ultrafiltration (UF), and Microfiltration (MF).
These membranes have pores so small they will plug and blind off instantly, unless they are run in the crossflow mode. (Some MF applications are run in the traditional normal flow mode.) Unlike traditional filtration, all the influent does not pass through the media. Rather it is split into the permeate (filtrate) and concentrate streams, the latter of which flows parallel to the membrane. Hence the term 'crossflow.' (Somehow this process has also picked up the inaccurate term 'tangential flow,' which is not an accurate description of the geometry of the process.) Likewise, traditional filtration is often called by the clumsy terms 'dead-end' or 'straight-through' flow filtration.' The term 'normal-flow' would work better for two reasons: it is a geometric term which describes how the flow approaches the media, and it is what most people normally think of as filtration.
Briefly, crossflow membrane is a pressure driven process wherein a semipermeable media acts as a surface filter to split the feed stream into two effluents: a purified stream and a stream more concentrated in solutes too large to pass through the pores of the particular membrane. There are four basic classes of membrane, based on relative pore size.