A study of chlor- alkali cell configurations: a mini review

Chlor- alkali electrolysis (CA) is used to produce chlorine, hydrogen gas and caustic soda with high purity (>۹۹.۹۹۹%). Nevertheless, this electrochemical process consumes a large amount of electrical energy. The three primary electrolytic processes for producing products are the diaphragm cell process, the mercury cell process, and the membrane cell process. In membrane process, elements such as cell components and the arrangement of components in the cell are decisive factors in the amount of energy consumption, which is investigated in this study [۱]. The purpose of this study is to investigate to incorporate some configuration strategies to mitigate the energy consumption of the cell and as a result increase the efficiency simultaneously. To avoid the drastic problems for three-chamber electrolyzers (standard design), a design called zero- gap configuration has been considered which is adapted from fuel cell technology. In a zero-gap cell, the cathode is in close contact with the ion exchange membrane. Water or oxygen is fed into the cell chamber and caustic soda is collected from it. Electrolysis cells that contain ion selective membranes are manufactured in three basic designs: gap cell, solid polymer electrolyte (SPE) and zero-gap cell. Zero-gap electrolysis was first proposed in ۱۹۶۷ by Costa et al. using electrodes on both sides of an ion exchange membrane. New cell designs including the use of porous electrodes and the use of fuel cell electrodes that are placed directly on the membrane have been studied. In the chlor-alkali process, the zero-gap cell design works by compressing two porous electrodes (mainly based on mesh and nickel foam) on both sides of a sodium ion exchange membrane, which eliminates the gap between the two electrodes compared to the standard arrangement, so It significantly reduces the contribution of the ohmic resistance of the electrolyte between the two electrodes. Chloral-alkali process cells are used in both advanced and normal ways in the industry. In the advanced process, the gas permeation layer provides an electrical connection from the porous electrode to the membrane, while simultaneously providing the possibility of feeding the electrolyte solution and removing gaseous products. It should be noted that care must be taken in this arrangement to avoid deformation of the membrane when pressure is applied. The zero-gap cell design creates a gap between two electrodes equal to the thickness of the membrane, which isless than ۰.۵ mm, which is a much smaller distance compared to the standard design (the gap is more than ۲ mm). Therefore, it significantly reduces the contribution of the ohmic resistance of the electrolyte between the two electrodes.[۲]The prominent parameters which required for the construction of a zero-gap cell in the chloralkali process are as follows: ۱. Membrane thickness ۲. conductivity of the membrane ۳. Cross-sectional area of the membrane ۴. Cross-sectional area of the electrodes per unit area of the membrane ۵. Information on the conductivity of the catalyst for electrodes ۶. Current density used ۶. Caustic soda flow efficiency ۷. Chlorine gas flow efficiency ۸. Product concentration ۹. Examining relationships and formulas for cell modeling such as the amount of sodium passing through the membrane from the anodic side to the cathodic side based on changes in caustic soda concentration ۱۰. Membrane potential as a variation of caustic soda and salt concentration ۱۱. The amount of sodium ion transfer from the membrane .[۴]While researchers have been developing zero-gap cells for the chloralkali process for years, the biggest challenge that stands in the way of this process is the high energy consumption, so that the theoretical voltage of the process is ۴.۴ V, but the lowest voltage recorded is ۸.۶ V has been reported in recent studies on a laboratory scale. [۶].As it can be seen, according to the studies conducted in the field of energy consumption, the obtained results indicate that:• Cell voltage increases with increasing pH.• Cell voltage decreases with increasing temperature.• Cell voltage has a direct relationship with current density and increases with its increase.• The concentration and speed of the incoming feed does not depend on the cell voltage, but it should be noted that the purity of the products also changes with the changes in the concentration.