The emerging process of CO2 capture and sequestration will likely require large volumes of caustic. The fossil fuel demand and carbon footprint of transporting liquid caustic is self-defeating, and hence, there is a need for energy-efficient, on-site caustic production for carbon capture projects. Caustic production is dominated by the well-established “chlor-alkali” processes. This process requires highly concentrated (~25 wt.%) and pure (>99.5 wt.%) NaCl feed brines, uses high-cost ion-exchange membranes and high operating temperatures (90 C), and generates a highly-concentrated (>33%) caustic stream that can be further concentrated using thermal evaporation. This highly concentrated caustic is then shipped to customers, where it is typically diluted to the required level. We have developed a flow-through membrane/cathode electrolysis process that produces a caustic solution (pH 10.22-12.26) at a specific energy consumption (SEC) of 1.71 kWhe/kg NaOH at room temperature using a 3.5% NaCl solution as feed, while achieving pure H2 generation without the use of ion exchange membranes. We demonstrate that the SEC is strongly dependent on the flow rate through the cathode, reaching a minimum at a high rate of 1,200 L/m2/hr. Electrochemical impedance spectroscopy, confocal microscopy, and finite element modeling show that the SEC is lowered through a combination of enhanced mass transport (of H+ and OH- ions) to and from the cathode surface and H2 gas stripping, both facilitated by the high flow rates. This technology offers the opportunity for the on-site production of dilute caustic streams (potentially from softened seawater) at a significantly reduced energy cost (compared to conventional chlor-alkali processes that consume >2.1 kWhe/kg NaOH).

中文翻译：

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使用加压流通阴极高效生产碱液和氢气


新兴的 CO2 捕获和封存过程可能需要大量的碱液。运输液体碱液的化石燃料需求和碳足迹会弄巧成拙，因此，碳捕获项目需要节能的现场碱液生产。苛性碱生产以成熟的“氯碱”工艺为主。该工艺需要高浓度 （~25 wt.%） 和纯 （>99.5 wt.%） 的 NaCl 进料盐水，使用高成本的离子交换膜和高工作温度 （90 C），并产生高浓度 （>33%） 的苛性碱液流，可以使用热蒸发进一步浓缩。然后将这种高度浓缩的苛性碱液运送给客户，在那里通常会将其稀释到所需的水平。我们开发了一种流通膜/阴极电解工艺，该工艺使用 3.5% NaCl 溶液作为原料，在室温下产生比能耗 （SEC） 为 1.71 kWhe/kg NaOH 的苛性碱溶液 （pH 10.22-12.26），同时在不使用离子交换膜的情况下实现纯 H2 生成。我们证明 SEC 在很大程度上取决于通过阴极的流速，在 1,200 L/m2/hr 的高速率下达到最小值。电化学阻抗谱、共聚焦显微镜和有限元建模表明，通过增强与阴极表面之间的质量传输（H+ 和 OH- 离子）和 H2 气体剥离相结合，SEC 会降低，这两者都是由高流速促成的。该技术为现场生产稀碱液（可能来自软化海水）提供了机会，且能源成本显著降低（与消耗 >2.1 kWhe/kg NaOH 的传统氯碱工艺相比）。