Membranes processes

Membrane process. Adapted from Hamann and Vielstich (2005). 

The mercury process needs the most electrical energy, but no steam is required to concentrate the caustic solution. The overall primary energy needed (if a power plant efficiency of 40% is assumed) for the mercury and diaphragm process is about the same, whereas the membrane process is more efficient (Table 6.19.6). Today, 54% of all chlor-alkali plants are membrane plants (Table 6.19.7) in the last 20 years, all new plants utilize this technology (Behr, Agar, and Joerissen, 2010). 

The following electrochemical processes take place during water electrolysis  

Left-hand electrode (anode) H2O (RedL) O.5O2 (Oxl) + 2H + (Oxl) + 2e Overall cell reaction H2O (RedL) H2(RedR) + O.5O2 (Oxl) 

The electrolysis voltage depends on the activities and partial pressures of the reactants, which may differ from the standard values of 1 and 1.013 bar. By the Nernst equation [Eq. (6.19.14), we obtain (with aH o = 1, H2 Pm/Po Oj Po IPo - 

In the membrane process the cathode and anode chambers are separated by a water-impermeable ion-conducting membrane (see Fig, 1.7-11). 

The membrane has to be stable under electrolysis conditions i.e. high salt concentrations, high pH-jump between anode and cathode chambers and to the strong oxidizing agents chlorine and hypochlorite. 

These demands are fulfilled by membranes with a perfluorinated polyethene main chain with side-chains with sulfonic acid and/or carboxylic acid groups as produced by DuPont and Asahi Glass. 

Multilayer membranes are also used, which have, for example, thin sulfonamide layers on the cathode side. 

Operation of membrane cells The same processes take place on the anodes and cathodes as in diaphragm cells. Activated titanium is used for the anodes and stainle.ss steel or nickel is preferred for the cathodes. No water transport takes place in the absence of current, but upon application of current solvation-water is transported by the current-carrying Na ions as they travel from the anode chamber to the cathode chamber. 

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Reverse osmosis is a high-pressure membrane separation process (20 to 100 bar) which can be used to reject dissolved inorganic salt or heavy metals. The concentrated waste material produced by membrane process should be recycled if possible but might require further treatment or disposal. 

Membrane filtration Membrane module Membrane permeability Membrane process Membrane processes Membrane reactor Membrane roofing Membranes... 

Early demand for chlorine centered on textile bleaching, and chlorine generated through the electrolytic decomposition of salt (NaCl) sufficed. Sodium hydroxide was produced by the lime—soda reaction, using sodium carbonate readily available from the Solvay process. Increased demand for chlorine for PVC manufacture led to the production of chlorine and sodium hydroxide as coproducts. Solution mining of salt and the avadabiHty of asbestos resulted in the dominance of the diaphragm process in North America, whereas soHd salt and mercury avadabiHty led to the dominance of the mercury process in Europe. Japan imported its salt in soHd form and, until the development of the membrane process, also favored the mercury ceU for production. 

Electrolytic Cell Operating Characteristics. Currently the greatest volume of chlorine production is by the diaphragm ceU process, foUowed by that of the mercury ceU and then the membrane ceU. However, because of the ecological and economic advantages of the membrane process over the other systems, membrane ceUs are currently favored for new production facHities. The basic characteristics of the three ceU processes are shown in Eigure 5. 

Removal of brine contaminants accounts for a significant portion of overall chlor—alkali production cost, especially for the membrane process. Moreover, part or all of the depleted brine from mercury and membrane cells must first be dechlorinated to recover the dissolved chlorine and to prevent corrosion during further processing. In a typical membrane plant, HCl is added to Hberate chlorine, then a vacuum is appHed to recover it. A reducing agent such as sodium sulfite is added to remove the final traces because chlorine would adversely react with the ion-exchange resins used later in the process. Dechlorinated brine is then resaturated with soHd salt for further use. 

Alkaline Chloride Electrolysis by the Membrane Process, Uhde GmbH, Dortmund, Germany, 1989. 

R. Rautenbach and R. Albrecht, Membrane Processes, John Wiley Sons, Inc., New York, 1989. 

L. CeciUe and J. C. Toussaint, Future Industrial Prospects of Membrane Processes, Elsevier AppHed Science, London, 1989. 

The fourth fully developed membrane process is electrodialysis, in which charged membranes are used to separate ions from aqueous solutions under the driving force of an electrical potential difference. The process utilizes an electrodialysis stack, built on the plate-and-frame principle, containing several hundred individual cells formed by a pair of anion- and cation-exchange membranes. The principal current appHcation of electrodialysis is the desalting of brackish groundwater. However, industrial use of the process in the food industry, for example to deionize cheese whey, is growing, as is its use in poUution-control appHcations. 

Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

FoUowiag Monsanto s success, several companies produced membrane systems to treat natural gas streams, particularly the separation of carbon dioxide from methane. The goal is to produce a stream containing less than 2% carbon dioxide to be sent to the national pipeline and a permeate enriched ia carbon dioxide to be flared or reinjected into the ground. CeUulose acetate is the most widely used membrane material for this separation, but because its carbon dioxide—methane selectivity is only 15—20, two-stage systems are often required to achieve a sufficient separation. The membrane process is generally best suited to relatively small streams, but the economics have slowly improved over the years and more than 100 natural gas treatment plants have been installed. 

G. Belfort, ed.. Synthetic Membrane Processes, Aca demic Press, Inc., Orlando, Fla., 1984. 

The three streams and associated variables of the RO membrane process are shown in Figure 2b the feed the product stream, called the permeate and the concentrated reject stream, called the concentrate or retentate. The water flow through the membrane is reported in terms of water flux, J. ... 

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

R. Kioman and D. Nutiai, Proceedings of the 1990 International Congress on Membranes and Membrane Processes, Chicago, lU., 1990. 

Electrodialysis. Electro dialytic membrane process technology is used extensively in Japan to produce granulated—evaporated salt. Filtered seawater is concentrated by membrane electro dialysis and evaporated in multiple-effect evaporators. Seawater can be concentrated to a product brine concentration of 200 g/L at a power consumption of 150 kWh/1 of NaCl (8). Improvements in membrane technology have reduced the power consumption and energy costs so that a high value-added product such as table salt can be produced economically by electro dialysis. However, industrial-grade salt produced in this manner caimot compete economically with the large quantities of low cost solar salt imported into Japan from Austraha and Mexico. 

H. Z. Friedlander and L. M. Litz in M. Bier, ed.. Membrane Processes in Industry and biomedicine. Plenum Press, New York, 1971. 

Equipment Available for Membrane Processes, Bulletin No. 115, International Dairy Eederation, Bmssels, p. 1979 (available through Library of Congress or... 

The individual membrane filtration processes are defined chiefly by pore size although there is some overlap. The smallest membrane pore size is used in reverse osmosis (0.0005—0.002 microns), followed by nanofiltration (0.001—0.01 microns), ultrafHtration (0.002—0.1 microns), and microfiltration (0.1—1.0 microns). Electro dialysis uses electric current to transport ionic species across a membrane. Micro- and ultrafHtration rely on pore size for material separation, reverse osmosis on pore size and diffusion, and electro dialysis on diffusion. Separation efficiency does not reach 100% for any of these membrane processes. For example, when used to desalinate—soften water for industrial processes, the concentrated salt stream (reject) from reverse osmosis can be 20% of the total flow. These concentrated, yet stiH dilute streams, may require additional treatment or special disposal methods. 

While the ambient-temperature operation of membrane processes reduces scaling, membranes are much more susceptible not only to minute amounts of scaling or even dirt, but also to the presence of certain salts and other compounds that reduce their ability to separate salt from water. To reduce corrosion, scaling, and other problems, the water to be desalted is pretreated. The pretreatment consists of filtration, and may include removal of air (deaeration), removal of CO2 (decarbonation), and selective removal of scale-forming salts (softening). It also includes the addition of chemicals that allow operation without scale deposition, or which retard scale deposition or cause the precipitation of scale which does not adhere to soHd surfaces, and that prevent foam formation during the desalination process. 

Common membrane processes include ultrafiltration (UF), reverse osmosis (RO), electro dialysis (ED), and electro dialysis reversal (EDR). These processes (with the exception of UF) remove most ions RO and UF systems also provide efficient removal of nonionized organics and particulates. Because UF membrane porosity is too large for ion rejection, the UF process is used to remove contaminants, such as oil and grease, and suspended soHds. 

When a potential is appHed across the ceU, the sodum and other cations are transported across the membrane to the catholyte compartment. Sodium hydroxide is formed in the catholyte compartment, because of the rise in pH caused by the reduction of water. Any polyvalent cations are precipitated and removed. The purified NaOH may be combined with the sodium bicarbonate from the sodium dichromate process to produce soda ash for the roasting operation. In the anolyte compartment, the pH falls because of the oxidation of water. The increase in acidity results in the formation of chromic acid. When an appropriate concentration of the acid is obtained, the Hquid from the anolyte is sent to the crystallizer, the crystals are removed, and the mother Hquor is recycled to the anolyte compartment of the ceU. The electrolysis is not allowed to completely convert sodium dichromate to chromic acid (76). Patents have been granted for more electrolytic membrane processes for chromic acid and dichromates manufacture (86). 

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