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Caustic Soda Production, Energy Efficiency, and Electrolyzers
ACS Energy Letters ( IF 22.0 ) Pub Date : 2021-09-15 , DOI: 10.1021/acsenergylett.1c01827 Amit Kumar 1 , Fengmin Du 1, 2 , John H. Lienhard 1
ACS Energy Letters ( IF 22.0 ) Pub Date : 2021-09-15 , DOI: 10.1021/acsenergylett.1c01827 Amit Kumar 1 , Fengmin Du 1, 2 , John H. Lienhard 1
Affiliation
Approximately 99.5% of caustic soda worldwide is produced through the traditional chlor-alkali process which simultaneously generates chlorine and hydrogen gas. The wider spectrum of caustic production technologies (Figure 1) includes the chlor-alkali membrane process, the chlor-alkali diaphragm process, bipolar membrane electrodialysis (EDBM), and direct electrosynthesis (DE). Both of the chlor-alkali processes produce H2 and Cl2 in addition to NaOH, while EDBM and DE produce HCl in addition to NaOH. Based on these underlying reactions, all of the methods can produce the same maximum amount of NaOH per kg of 7% w/w NaCl brine. Energy usage is one for the most important factors for caustic production and makes up a significant portion of the variable cost (Figure 2). The minimum energy required for each method can also be determined from the underlying chemical reaction.(4) EDBM and DE have lower minimum energy requirements (0.65–0.81 kWh/kg NaOH for the former;(4) and around 1.38 kWh/kg NaOH for the latter, estimated using the cell voltages in Figure 1) than the chlor-alkali processes (1.56–1.64 kWh/kg NaOH(4)). This indicates that if all methods had similar efficiencies, EDBM and DE would use less energy to produce NaOH than the chlor-alkali processes. In practice, the chlor-alkali membrane process consumes 2.10–2.15 kWhe/kg NaOH of electrical energy and 0.128–0.196 kWht/kg NaOH of thermal energy.(4) The chlor-alkali diaphragm process tends to use less thermal energy (0.038–0.047 kWht/kg) at the cost of slightly higher electrical energy usage (1.94–2.51 kWhe/kg NaOH). Considering only the electrical part (note that the thermal energy is of lower thermodynamic quality and also cheaper), the energy efficiency of conventional chlor-alkali processes therefore amounts to around 75%. As a more novel process, EDBM has been reported to consume in the range of 1.8–3.6 kWhe/kg NaOH of electrical energy (setups used in research),(5) thus having an energy efficiency of around 40%. Despite having the thermodynamic potential to require significantly less energy than the chlor-alkali process, at present, EDBM at best consumes only slightly less electrical energy than the chlor-alkali process and on average requires slightly more, although no heat energy is required. It becomes evident that more research is necessary to increase its efficiency and lower the realized energy consumption. U is the cell voltage (in V), Q the provided charge (in C), M the molar mass (40 kg/kmol for NaOH), and n the produced molar amount (in mol). Figure 1. Schematic diagram of a chlor-alkali membrane, a chlor-alkali diaphragm, a bipolar membrane electrodialysis, and a direct electrosynthesis process. The half-reactions and energy requirements are given (sources: chlor-alkali processes,(1) bipolar membrane electrodialysis,(2) direct electrosynthesis(3)). Key: A, anode; C, cathode; BP, bipolar membrane; NaOH, sodium hydroxide; HCl, hydrochloric acid. Figure 2. Theoretical and practical energy requirements of a chlor-alkali membrane, a chlor-alkali diaphragm, a bipolar membrane electrodialysis, and a direct electrosynthesis process. Theoretical numbers are taken from Thiel et al.(4) or estimated based on the theoretical voltage (for direct electrosynthesis). Practical numbers are taken from Thiel et al.(4) and Reig et al.(5) No study has yet reported on the practical energy requirements of direct electrosynthesis. No study has yet reported on the practical energy requirements of DE, necessitating further research in this area. Lower theoretical energy consumption is expected for DE than the conventional chlor-alkali process, yet the theoretical energy use is not as low as in the EDBM process as a result of the relatively higher amount of water splitting taking place. A practically attractive feature of DE could be its lower number of electrolyte chambers and membranes (in addition, the absence of BP), which potentially reduces energy consumption by lowering ohmic resistances compared to EDBM processes. At the same time, the attractiveness of the DE process will also increase if H2 is additionally a desired side product. We now direct our discussion toward the energy efficiencies of the above-mentioned caustic production processes. Table 1 compares the specific energy consumption based on theoretical and practical performance. Generally, causes of energy inefficiency (additional energy consumption relative to the thermodynamic limit) can roughly be subdivided into the following two categories:
中文翻译:
烧碱生产、能源效率和电解槽
全世界大约 99.5% 的烧碱是通过传统的氯碱工艺生产的,该工艺同时产生氯气和氢气。更广泛的苛性碱生产技术(图 1)包括氯碱膜工艺、氯碱隔膜工艺、双极膜电渗析 (EDBM) 和直接电合成 (DE)。两种氯碱工艺都会产生 H 2和 Cl 2除NaOH外,EDBM和DE除NaOH外还产生HCl。基于这些潜在的反应,所有的方法都可以产生相同的最大量的 NaOH/kg 7% w/w NaCl 盐水。能源使用是碱生产的最重要因素之一,占可变成本的很大一部分(图 2)。每种方法所需的最低能量也可以从潜在的化学反应中确定。(4) EDBM 和 DE 的最低能量要求较低(前者为 0.65–0.81 kWh/kg NaOH;(4) 约 1.38 kWh/kg NaOH对于后者,使用图 1 中的电池电压估计)而不是氯碱工艺(1.56–1.64 kWh/kg NaOH(4))。这表明,如果所有方法都具有相似的效率,则与氯碱工艺相比,EDBM 和 DE 将使用更少的能源来生产 NaOH。在实践中,e /kg NaOH 电能和 0.128–0.196 kWh t /kg NaOH 热能。 (4) 氯碱隔膜工艺倾向于使用较少的热能(0.038–0.047 kWh t /kg),但成本略高电能使用量(1.94–2.51 kWh e /kg NaOH)。仅考虑电气部分(请注意,热能具有较低的热力学质量且更便宜),因此传统氯碱工艺的能效约为 75%。作为一种更新颖的工艺,据报道 EDBM 的消耗范围为 1.8-3.6 kWh e/kg NaOH 电能(研究中使用的设置),(5) 因此具有大约 40% 的能源效率。尽管具有比氯碱工艺所需能源少得多的热力学潜力,但目前,EDBM 最多只消耗比氯碱工艺略少的电能,平均需要略多一些,尽管不需要热能。很明显,需要更多的研究来提高其效率并降低实现的能耗。 U是电池电压(V),Q是提供的电荷(C),M是摩尔质量(NaOH 为 40 kg/kmol),n产生的摩尔量(以摩尔计)。图 1. 氯碱膜、氯碱隔膜、双极膜电渗析和直接电合成工艺示意图。给出了半反应和能量需求(来源:氯碱工艺,(1)双极膜电渗析,(2)直接电合成(3))。关键:A,阳极;C、阴极;BP,双极膜;NaOH、氢氧化钠;盐酸,盐酸。图 2. 氯碱膜、氯碱隔膜、双极膜电渗析和直接电合成过程的理论和实际能量需求。理论数字取自 Thiel 等人 (4) 或根据理论电压估算(用于直接电合成)。实际数字取自 Thiel 等人(4)和 Reig 等人。(5) 目前还没有关于直接电合成的实际能量需求的研究报告。目前还没有研究报告 DE 的实际能源需求,需要在该领域进一步研究。与传统的氯碱工艺相比,DE 的理论能耗预计较低,但由于发生相对较高的水分解量,因此理论能耗不如 EDBM 工艺低。DE 的一个实际吸引人的特征可能是其电解质室和膜的数量较少(此外,没有 BP),与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 目前还没有研究报告 DE 的实际能源需求,需要在该领域进一步研究。与传统的氯碱工艺相比,DE 的理论能耗预计较低,但由于发生相对较高的水分解量,因此理论能耗不如 EDBM 工艺低。DE 的一个实际吸引人的特征可能是其电解质室和膜的数量较少(此外,没有 BP),与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 目前还没有研究报告 DE 的实际能源需求,需要在该领域进一步研究。与传统的氯碱工艺相比,DE 的理论能耗预计较低,但由于发生相对较高的水分解量,因此理论能耗不如 EDBM 工艺低。DE 的一个实际吸引人的特征可能是其电解质室和膜的数量较少(此外,没有 BP),与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 与传统的氯碱工艺相比,DE 的理论能耗预计较低,但由于发生相对较高的水分解量,因此理论能耗不如 EDBM 工艺低。DE 的一个实际吸引人的特征可能是其电解质室和膜的数量较少(此外,没有 BP),与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 与传统的氯碱工艺相比,DE 的理论能耗预计较低,但由于发生相对较高的水分解量,因此理论能耗不如 EDBM 工艺低。DE 的一个实际吸引人的特征可能是其电解质室和膜的数量较少(此外,没有 BP),与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加 与 EDBM 工艺相比,这可能通过降低欧姆电阻来降低能耗。同时,如果H,DE过程的吸引力也会增加2另外是期望的副产物。我们现在将讨论指向上述苛性碱生产过程的能源效率。表 1 比较了基于理论和实际性能的特定能耗。一般来说,造成能源效率低下的原因(相对于热力学极限的额外能源消耗)大致可细分为以下两类:
更新日期:2021-10-08
- Voltage efficiency: additional voltage needed beyond the thermodynamic driving force Umin (which is the difference of the thermodynamic equilibrium potentials on both half-cell reactions). Additional voltage results from various sources of overpotential (∑ηkin) as well as ohmic losses in the electrolysis cell (∑ηohm). Further losses may also be incurred, e.g., by limitations on reactant mass transport. In the case of the EDBM process, one of the major voltage losses stems from the activation barrier of water splitting (to proton and hydroxyl ions) within the bipolar membrane.(2) For DE processes, it may be expected that the major inefficiency will result from the oxygen evolution reaction (OER) on the anode, which is well known to be a kinetically strongly hindered reaction in acid media.
- Current efficiency: additional current/charge required beyond the stoichiometry dictated by Faraday’s law, e.g., due to side reactions, shunt currents, or membrane crossover. For a typical chlor-alkali process, current inefficiencies are mainly caused by OER as a side reaction and chlorine dissolution in the anode solution (both of which reduce chlorine yield), as well as crossover of OH– ions (reduces caustic yield).(6) In the case of DE, OER is the desired reaction on the anode, and chlorine evolution reaction (ClER) is the main competing reaction, which contributes to current inefficiency.(3)
中文翻译:
烧碱生产、能源效率和电解槽
全世界大约 99.5% 的烧碱是通过传统的氯碱工艺生产的,该工艺同时产生氯气和氢气。更广泛的苛性碱生产技术(图 1)包括氯碱膜工艺、氯碱隔膜工艺、双极膜电渗析 (EDBM) 和直接电合成 (DE)。两种氯碱工艺都会产生 H 2和 Cl 2除NaOH外,EDBM和DE除NaOH外还产生HCl。基于这些潜在的反应,所有的方法都可以产生相同的最大量的 NaOH/kg 7% w/w NaCl 盐水。能源使用是碱生产的最重要因素之一,占可变成本的很大一部分(图 2)。每种方法所需的最低能量也可以从潜在的化学反应中确定。(4) EDBM 和 DE 的最低能量要求较低(前者为 0.65–0.81 kWh/kg NaOH;(4) 约 1.38 kWh/kg NaOH对于后者,使用图 1 中的电池电压估计)而不是氯碱工艺(1.56–1.64 kWh/kg NaOH(4))。这表明,如果所有方法都具有相似的效率,则与氯碱工艺相比,EDBM 和 DE 将使用更少的能源来生产 NaOH。在实践中,e /kg NaOH 电能和 0.128–0.196 kWh t /kg NaOH 热能。 (4) 氯碱隔膜工艺倾向于使用较少的热能(0.038–0.047 kWh t /kg),但成本略高电能使用量(1.94–2.51 kWh e /kg NaOH)。仅考虑电气部分(请注意,热能具有较低的热力学质量且更便宜),因此传统氯碱工艺的能效约为 75%。作为一种更新颖的工艺,据报道 EDBM 的消耗范围为 1.8-3.6 kWh e/kg NaOH 电能(研究中使用的设置),(5) 因此具有大约 40% 的能源效率。尽管具有比氯碱工艺所需能源少得多的热力学潜力,但目前,EDBM 最多只消耗比氯碱工艺略少的电能,平均需要略多一些,尽管不需要热能。很明显,需要更多的研究来提高其效率并降低实现的能耗。
- 电压效率:超出热力学驱动力U min(这是两个半电池反应的热力学平衡电位之差)所需的额外电压。额外电压由各种过电位源 (∑η kin ) 以及电解槽中的欧姆损耗 (∑η ohm)。还可能招致进一步的损失,例如,由于反应物质量传输的限制。在 EDBM 过程的情况下,主要电压损失之一源于双极膜内水分解(至质子和氢氧根离子)的活化屏障。 (2) 对于 DE 过程,可能会出现主要的低效率由阳极上的析氧反应 (OER) 产生,众所周知,这是在酸性介质中的动力学强受阻反应。
- 电流效率:超出法拉第定律规定的化学计量所需的额外电流/电荷,例如,由于副反应、分流电流或膜交叉。对于典型的氯碱工艺,电流效率低下主要是由 OER 作为副反应和阳极溶液中的氯溶解(两者都会降低氯产率)以及 OH -离子的交叉(降低苛性碱产率)引起的。 6) 在 DE 的情况下,OER 是阳极上所需的反应,析氯反应 (ClER) 是主要的竞争反应,导致电流效率低下。 (3)