Ind. Eng. Chem. Res. 2009, 48, 983–987
983
Effects of Organic Impurities on Chloralkali Membrane Electrolyzer Performance James M. Silva,* Grigorii L. Soloveichik, and Donald Novak† General Electric Global Research Center, 1 Research Circle, Niskayuna, New York 12309
Laboratory chloralkali membrane electrolyzer tests showed a dramatic voltage increase and mild catholyte foaming when low levels of chloromethyl triethylammonium chloride (CTACl), a quaternary ammonium salt, were present in the feed brine. Current efficiency was not measurably affected by CTACl. In contrast, laboratory membrane electrolyzers showed no voltage sensitivity to sodium gluconate, bisphenol A, or triethylamine, contaminants that are often present in interfacial polycarbonate plant byproduct brine. The voltage increase and onset of catholyte foaming were rapid when the feed was switched from ultrapure brine to CTACl-containing brine, requiring about 3 h to achieve a steady state. Both effects were completely reversible, but the system required about 20 h to return to baseline voltage after the feed was switched back to ultrapure brine. The cell voltage was remarkably sensitive to CTACl: 8 ppm CTACl yielded a 200 mV voltage increase vs ultrapure brine. Cyclic voltammetric measurements with CTACl-spiked brine showed no effect of CTACl on anode or cathode overpotentials. At steady state, 87% of the feed chloromethyl triethylammonium ion (CTA+) is recovered either in the electrolyzer catholyte as the hydroxide, CTAOH (56%), or in the depleted brine as CTACl (31%), which demonstrates that CTA+ is rather stable toward chloralkali conditions. It is concluded that the increased cell voltage is caused by chloromethyl triethylammonium ions adsorbing onto membrane ion exchange sites, which reduces the population of sites for sodium ion transport, and that catholyte foaming is caused by the presence of CTAOH in the catholyte. An adsorbent screening study showed that various carbons, including Ambersorb 572, are effective for CTACl removal from brine. A laboratory electrolyzer fed with Ambersorb 572 treated plant brine showed normal voltage and no catholyte foaming. Introduction Since its discovery in 1953, bisphenol A (BPA) polycarbonate (PC)1,2 has undergone exceptionally strong growth in both volume and breadth of applications.3 Currently, 90% of worldwide polycarbonate capacity utilizes interfacial process technology, in which sodium hydroxide and chlorine are consumed and sodium chloride is a byproduct. The overall reaction stoichiometry for interfacial PC synthesis is shown in Scheme 1. The current worldwide interfacial PC capacity of 2.2 million metric tons4 represents an annual chlorine rate of 0.61 million metric tons and an annual sodium chloride byproduct rate of 1.0 million metric tons. Closing the brine loop by reconverting byproduct sodium chloride to chlorine and sodium hydroxide is thus attractive from both environmental and economic perspectives. Asbestos diaphragm chloralkali electrolyzers have been successfully used to convert this byproduct brine to chlorine,5 but asbestos poses environmental issues and is subject to limited availability. The demonstrated superiority of membrane electrolyzers over asbestos diaphragm electrolyzers with respect to power consumption, chlorine hydrogen levels, separator lifetime, product purity, and process simplicity makes it very attractive to consider membrane electrolyzers for this recycle application.6 This study investigates the effects of organic species found in an interfacial polycarbonate byproduct brine stream on membrane electrolyzer performance. The importance of brine purity on membrane chloralkali electrolyzer performance is well-known. Several reviews describe the impact of various inorganic contaminants on electrolyzer voltage and current efficiency.7-9 Membrane electrolyzers require considerably cleaner brine than diaphragm electrolyzers. For example, membrane electrolyzers require hardness levels of less than 20 ppb (calcium plus magnesium), * To whom correspondence should be addressed. E-mail: silva@ crd.ge.com. † De Nora Tech Corp. (formerly Eltech Systems Corp.), Chardon, OH 44024. Current address: Altair Nanotechnologies Inc., Reno, NV.
whereas diaphragm electrolyzers require hardness levels of less than 4 ppm calcium and 0.5 ppm magnesium.5 Inorganic impurities can affect the membrane itself or the electrodes. For example, low-solubility metal hydroxides such as Ni(OH)2 and Mg(OH)2 may precipitate within the membrane’s ion-conducting channels, thus limiting sodium ion flow and increasing voltage. Ca(OH)2 precipitates in the membrane near the cathode surface, roughening the electrode surface and reducing current efficiency. BaSO4 can precipitate on the anode and raise the overvoltage.7 Brine Purification Polycarbonate synthesis byproduct brine contains a variety of organic impurities, for example, residual solvent, such as methylene chloride, residual monomer (BPA), and amine catalyst, typically triethylamine (TEA).10-12 The brine stream may also contain quaternary ammonium salts, which arise from reactions between aliphatic amines and aliphatic chlorinated hydrocarbons. For example, methylene chloride reacts with TEA to yield chloromethyl triethylammonium chloride (CTACl),13 which is highly watersoluble. Both TEA and CTACl are potential sources of nitrogen trichloride in chloralkali cells, and must therefore be carefully monitored and controlled.14 Finally, the brine stream may also contain chelating agents such as sodium gluconate. Table 1 shows the crude and purified brine compositions. The crude brine was sampled from an interfacial polycarbonate synthesis plant. This brine was first purified by primary treatment, which includes filtration, air-stripping to remove residual solvent and amine, and adsorbent treatment (XAD-7) to remove residual monomer.15 For secondary treatment, the brine underwent two stages of ion exchange to achieve membrane brine specification levels of hardness and heavy metals. In the first stage, the brine was treated under acidic conditions by an iminodiacetic acid functionalized chelating ion exchange resin (IDA resin) to remove heavy metals such as iron.16 In the second stage, the brine was treated under alkaline
10.1021/ie071184u CCC: $40.75 2009 American Chemical Society Published on Web 12/05/2008
984 Ind. Eng. Chem. Res., Vol. 48, No. 2, 2009 Scheme 1. Polycarbonate Synthesis Stoichiometry
Table 1. Brine Compositions (300 g/L NaCl)a component
crude brine
test brine
Ca Mg Sr Ba Al Ni Cr Fe SiO2 TOC CTACl sodium gluconate Na2SO4
0.6 0.2 0.3 10.8 27 8 80 160
11 ppb 7 ppb
