20-07-2012, 03:34 PM
MEMBRANE SEPARATION SYSTEMS
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Introduction
MEMBRANE PROCESSES
Seven major membrane processes are discussed in this report. They are listed
in Table 3-1. There are four developed processes, microfiltration (MF), ultrafiltration
(UF), reverse osmosis (RO), and electrodialysis (ED). These are all well established
and the market is served by a number of experienced companies. The first three
processes are related filtration techniques, in which a solution containing dissolved
or suspended solutes is forced through a membrane filter. The solvent passes through
the membrane; the solutes are retained.
Introduction
There are two developing processes: gas separation with polymer membranes
and pervaporation. Gas separation with membranes is the more developed of the
two techniques. At least 20 companies worldwide offer industrial, membrane-based
gas separation systems for a variety of applications. Two companies currently offer
industrial pervaporation systems. The potential for each process to capture a
significant slice of the separations market is large. In gas separation, a mixed gas
feed at an elevated pressure is passed across the surface of a membrane that is
selectively permeable to one component of the feed. The membrane separation process
produces a permeate enriched in the more permeable species and a residue enriched
in the less permeable species. The process is illustrated in Figure 3-3. Major current
applications are the separation of hydrogen from nitrogen, argon and methane in
ammonia plants, the production of nitrogen from air and the separation of carbon
dioxide from methane in natural gas operations. Gas separation is an area of
considerable current research interest and it is expected that the number of
applications will expand rapidly over the next few years.
JAPANESE GOVERNMENT SUPPORTED MEMBRANE RESEARCH
Although the dates of origin of the membrane industries in Japan and the U.S.
differ by about 20 years, in many ways the experiences of the two countries are
similar. The Japanese government continues to support a large research effort in
membranes that began in the 1970s. A number of programs will begin to expire in
the early 1990s, but will undoubtedly be replaced by others, although their size may
decrease and their focus change. A reduction in government support would reflect
the current size and status of the Japanese membrane industry.
Ministry of Education
Academic research is sponsored by general grants to faculty, and by specific
research programs with relevance to the membrane field. Ministry of Education
programs are said to be primarily for the training of students, with little regard for
the utility of the research in the near term. Pervaporation membrane research has
been a particularly active area recently. The general level of this support is estimated
at $2 million annually.