Membrane Suppliers

Membrane Characterization Membranes are always rated for flux and rejection. NaCl is always used as one measure of rejection, and for a veiy good RO membrane, it will be 99.7 percent or more. Nanofiltration membranes are also tested on a larger solute, commonly MgS04. Test results are veiy much a function of how the test is run, and membrane suppliers are usually specific on the test conditions. Salt concentration will be specified as some average of feed and exit concentration, but both are bulk values. Salt concentration at the membrane governs performance. Flux, pressure, membrane geome-tiy, and cross-flow velocity all influence polarization and the other variables shown in Fig. 22-63. 

In Table 4.3, a selection of inorganic membrane suppliers is given. Very little data are available on applications in catalysis so far. The membranes can be used for separation in the ultra- or nanofiltration range. 

In these experiments, the measured helium flux through the membrane was less than the flux predicted on the basis of the average bulk concentrations. Consequently, the helium permeability coefficients calculated from observed membrane flux and the bulk partial pressures are lower than the pure gas values obtained by the membrane supplier or independently by us. At the same time, observed nitrogen coefficients are higher than predicted. 

Microfiltration is used widely in the pharmaceutical industry to produce injectable drug solutions. Regulating agencies require rigid adherence to standard preparation procedures to ensure a consistent, safe, sterile product. Microfiltration removes particles but, more importantly, all viable bacteria, so a 0.22- xm-rated filter is usually used. Because the cost of validating membrane suppliers is substantial, users usually develop long-term relationships with individual suppliers. 

The interest in knowing the permeability of the membranes for a salty solution is to predict the fluxes which could be obtained for a real brackish water (total salinity near 6gL 1) without fouling. This parameter is not given by the membrane suppliers. 

Within the classification of seawater membranes, there are subsets of membrane that are rated for different performance. For example, Koch Membrane Systems offers a standard high rejection seawater membrane module plus a high-flow seawater element seawater membrane module. Table 4.5 lists the productivity and rejection for two seawater membrane types. Other membrane suppliers offer similar variety in seawater membranes. 

Validation of membrane characteristics normally requires specialized techniques that lie within the expertise and experience of the membrane suppliers. 

The design parameters that are needed to assess the performance of a specific membrane under given process conditions are a function of the material used to manufacture the membrane as well as the general arrangement of the equipment. The design parameters which are a function of the polymeric material are determined by the manufacturer of the membrane experimentally. They are not generally available and, therefore, a potential user must rely on a membrane supplier to evaluate and optimize a system to meet the needs of a specific application. 

Utilization of ceramic membranes for water purification started in the 1980s [66] with the implementation of small size membrane plants (5-100 mVh). Imeca in France was one of the first membrane suppliers entering this market with inorganic membranes. This membrane technology is now developing at a larger industrial scale with, for example, the ceramic membrane water purification systems designed... 

There exists a variety of applications for which gas-separation membranes have been applied along with their commercial membrane suppliers. The process engineer facing the task of designing a gas-separation process is well advised to consult with these suppliers to assist in his design. If the task involves common gas species, it is highly probable that the application has already been commercialized or is under consideration. This chapter provides an overview of some of these commercial applications. Also provided is an overview of considerations, limitations, and hurdles to commercialization of new applications for gas-separation membranes. Development of a new membrane for a new application is not for the faint of heart. 

The gas-separation characteristics have been measured and reported for the common gases for many polymers. Indeed, in the preceding chapter, materials have been identified and commercialized by membrane suppliers for many of the common separations. Having identified a gas-separation of interest, a thorough review of the available literature is in order This must include patent literature as many researchers have sought protection for membranes formed of proprietary polymers and their application in many specific gas-separations if not for membrane separations or devices as a whole. 

Commercial membrane suppliers have each selected and developed membranes with these factors plus a host of other considerations in mind, usually with a specific target application in mind. Sizeable investments in development and manufacturing capital are now in place. Thus as new gas-separation applications surface, membrane manufacturers first screen the separation on available products. Frequently, the new separation if fitted to the available products rather than the product to the application. Polymers more suited to the separation in terms of selectivity may indeed exist, however, the choices may be re-... 

Membrane units are usually supplied as a complete assembly requiring minimal field erection, save connections to process and utility piping and instrumentation systems. Fabricators and OEMs working closely with the membrane suppliers have developed membrane-system packages that address a wide variety of application and industry code requirements. The nearly 20 years of experience for most membrane supphers has afforded many membrane-module, system-design and cost-reduction innovations to be incorporated into these packages. New applications wiU most certainly rely heavily on these experiences. 

Industrial gas sales - Many membrane suppliers are owned by or affiliated with industrial gas companies. They use onsite nitrogen generators to supply nitrogen to meet customer demand from baseload to total site nitrogen requirements, frequently supplemented with hquid nitrogen supply or backup. 

Numerous membrane materials with selectivities and permeabilities far in excess of those used by the industrial membrane suppliers have been reported in the academic and patent literature. Unfortunately, when evaluated under real-world conditions (high-pressure gas containing heavy hydrocarbons, aromatics and water), these membranes are seldom able to match their laboratory performance. Having said this, cellulose acetate, the current industry workhorse... 

Membrane supplier Memcor (US Filter/ Siemens) Memcor (USFilter/ Siemens) PaU Norit Aquasource Zenon... 

Unfortunately, all the properties required for these calculations are not available in the open literature for the membranes that are used commercially. Therefore, some simplified descriptions and assumptions are employed here to show the trends expected of a membrane under given operating conditions. One can refine these calculations with more precise data, when made available by the membrane suppliers. 

The exact specifications depend on the membrane, cell design, and operating conditions [125-127], The brine specifications recommended by membrane suppliers are shown in Table 4.8.8, and Table 4.8.9 describes the impurities and possible mechanisms of membrane damage. The reader is referred to the Appendix, where the effects of impurities are summarized along with the recommended analytical methods suggested by the membrane manufacturers. References [128-131] provide a discussion on brine treatment costs vs membrane costs. 

Membrane Technology Developments. The three major membrane suppliers are DuPont, Asahi Glass, and Asahi Kasei. All these manufacturers are constantly striving to improve membranes to achieve low energy consumption, long life, and insensitivity to impurities and upsets in operation. 

Finally, there are the ubiquitous intangibles such as corporate agreements or deals between the chlor-alkali producer and the cell technology supplier or its parent organization, the membrane supplier, and the anode vendor. This is sometimes a reality, in which case the cell technology comparison may become moot. 

Figure 6.10 shows the anolyte balance. The sulfate flow into and out of the electrolyzers is an arbitrary but reasonable number chosen to suit a typical membrane supplier s specification. The balance ignores any flow of sulfate out of the anolyte. Sulfate in some form is known to penetrate the membrane (therefore the need to limit its concentration). The rate of penetration is so small, however, that the assumption of zero flux is the basis for one method of estimating current efficiency [3]. The chlorate flows are in the same category as the sulfate flows. There is a small difference between the flows out and in, representing the rate of chlorate production in the cells. This amount would be removed by purging or by deliberate destruction in the brine process. 

Section 6.5 mentioned that the transport coefficients are much lower in KOH service than in NaOH production. KOH production therefore requires higher recirculation pumping rates. In NaOH service, the water transport coefficient is higher, but it depends on a number of variables, not least the type of membrane. Membrane suppliers are the best source of information on water transport coefficients. In the extreme case of a reduction from 4.0 to 2.0, the required brine feed rate increases by about 25% and that of 30.3% caustic more than doubles. 

Membrane suppliers will place restrictions on barium concentration in the treated brine in order to prevent damage to their products. The anodes also can be a problem. Barium sulfate can deposit on the anode coating and eventually destroy it, and so the brine must also conform to the specification set by the anode supplier. 

While none of the membrane suppliers has provided technical details on the composition or structure of these membranes, we can expect interest to remain high and to bring about more developments. 

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