PDF Publication Title:
Text from PDF Page: 012
In a membrane cell the anode and cathode are separated by a water-impermeable, ion-conducting membrane (see Figure 6-7). In this process the brine solution flows through the anode compartment and chlorine gas is generated at the anode. Sodium ions migrate through the membrane to the cathode compartment, where sodium hydroxide solution is flowing. Water hydrolizes at the cathode and releases hydrogen gas and hydroxide ions. The combination of sodium and hydroxide ions produces sodium hydroxide which reaches a concentration of about 30 to33 percent before leaving the cell. The membrane preferentially passes positive sodium ions from the anode chamber to the cathode chamber. Negatively charged chloride and hydroxide ions are primarily rejected by the membrane. As a result, the sodium hydroxide solution typically contains less than 100 ppm NaCl. The depleted brine leaves the anode compartment and is resaturated with salt for reuse in the membrane cell. Stainless steel or nickel is typically used as a cathode in the membrane cell. The cathodes are also often coated with a catalyst (nickel-sulfur, nickel-aluminum, nickel-nickel oxide, platinum group metals) to increase surface area and reduce the hydrogen evolution potential. Anodes are similar to those used in both diaphragm and mercury cells (ruthenium and titanium oxide on titanium). The membrane’s material and design are critical to cell operation. Membranes must maintain stability after exposure to both chlorine and strong caustic solution. They require low electrical resistance and must allow the transport of sodium ions but not chloride ions. Membrane materials currently in use are fluorinated polymers with pendant functional groups that make them ion-selective. The advantage of membrane cells is the relatively pure sodium hydroxide solution produced and the lower electricity requirements than either diaphragm or mercury cells. In addition, membrane cells do not require the use of toxic materials (e.g., asbestos, mercury). Disadvantages of the membrane cell include the need for processing of the chlorine gas to remove oxygen and water vapor, and for moderate evaporation to increase the concentration of the caustic solution. Another disadvantage is that the brine entering a membrane cell must be of very high purity to prevent contamination of the membrane, which requires costly purification of the brine prior to electrolysis. The membrane separator in these cells is expensive and easily damaged, and has a shorter lifetime than diaphragm and mercury separators. 6.1.2 Brine Production Brine Production and Purification Are Required for All Cells Regardless of the type of cell employed, a suitable brine must be prepared prior to entering the cell. Different levels of purification are required for the various cells, and some require removal of metals or other impurities. Salt is obtained from the mining of natural deposits or from seawater (via solar evaporation), and contains impurities that must be removed before it can be used in the electrolytic cell. Impurities include primarily calcium, magnesium, barium, iron, aluminum, sulfates, and trace metals. Impurities can negatively affect the electrolytic cell by precipitating out and blocking or damaging membrane or diaphragm materials. Impurities can also poison the catalytic coating on the anode and cathode. In the case of the mercury cell, some trace metals (e.g., vanadium) can reduce efficiency and cause the production of potentially dangerous amounts of hydrogen gas. Impurities can also lead to the production of chlorinated compounds, a situation that can negatively impact cell performance (EPA 1995a). The first phase of brine preparation includes the dissolution of sea salt and rock salt in a water and dilute brine mix (see Figure 6-8). The amount of material dissolved is a function of brine concentration, residence time, and temperature. In all cases the brine-solid salt mixture is allowed to reach equilibrium so that a saturated solution of 186PDF Image | The Chlor-Alkali Industry
PDF Search Title:
The Chlor-Alkali IndustryOriginal File Name Searched:
profile_chap6.pdfDIY PDF Search: Google It | Yahoo | Bing
Salgenx Redox Flow Battery Technology: Power up your energy storage game with Salgenx Salt Water Battery. With its advanced technology, the flow battery provides reliable, scalable, and sustainable energy storage for utility-scale projects. Upgrade to a Salgenx flow battery today and take control of your energy future.
CONTACT TEL: 608-238-6001 Email: greg@salgenx.com (Standard Web Page)