Steady-state and dynamic modeling of a chlor-alkali cell with oxygen depolarized cathode

Abstract

Chlor-alkali electrolysis, the electrolytic splitting of NaCl solutions, is an energy intensive process. The most modern variant, the membrane electrolysis process, has been continuously improved over the last decades. Nevertheless, the average energy demand with the current state of the art of this technology is 2292 kWh/tCl2 at 6 kA/m2. Consequently, any reduction of the electrical energy demand in chlor-alkali electrolysis would be highly desirable for both economic and environmental reasons. Replacing hydrogen evolving reaction by oxygen reduction reaction can reduce the energy demand by approximately 30%. In this work mathematical models for steady and dynamic operation of an industrial scale chlor-alkali electrolysis cell with ODC are developed. The steady state model predicts the distributions of temperature, concentration, current density, and overpotential as a function of height. At an industrially relevant current density of 4 kA/m2 neither current density nor overpotentials exhibit strong variations along the cell height. Main reason for this behaviour is the uniformity of temperature distributions in the solid compartments of the cell (anode, membrane, ODC) which can be explained by efficient heat transfer between the electrodes and the electrolyte streams. This is especially true for the caustic solution, through which most of the irreversible heat released in the cell is removed. However, the temperature of the oxygen stream increases slowly along the height. Due to the initially low temperatures and the low water content of the inlet oxygen stream, the gas phase takes up considerable net amounts of water vapor. Nevertheless, the oxygen partial pressure at the electrochemically active regions of the ODC remains high allowing for efficient operation of the cathode. Operating cell at higher current requires better heat management as the heat production is performance of the electrode significantly. The dynamic model also predicts the performance of the electrode under the ripple voltage. However, Current Interruption (CI) measurements have been used to validate this model. Due to ripples, hysteresis appears even at low frequency of 100 Hz. As frequency increases the amplitudes of current oscillation reduce. No significant difference in the hysteresis can be seen after lowering the oxygen partial pressure down to 75%.%%%%CAhblsotrr-aaclkt:a li electrolysis, the electrolytic splitting of NaCl solutions, is an energy intensive process. The most modern variant, the membrane electrolysis process, has been continuously improved over the last decades. Nevertheless, the average energy demand with the current state of the art of this technology is 2292 kWh/tCl2 at 6 kA/m2. Consequently, any reduction of the electrical energy demand in chlor-alkali electrolysis would be highly desirable for both economic and environmental reasons. Replacing hydrogen evolving reaction by oxygen reduction reaction can reduce the energy demand by approximately 30%. In this work mathematical models for…