Modeling of freezing phenomena induced by chemical reactions

Mar 12, 2009 - 16:00

Mar 12, 2009 - 17:00

Creativity Hall (Room 118) Chemical Engineering

Seminars

K. S. Gandhi Department of Chemical Engineering Indian Institute of Science Bangalore

Spatial variations in the concentration of a reactive solute in solution are often encountered in a catalyst particle and this leads to variation in the freezing point of the solution. Depending on the operating temperature this can result in freezing of the solvent on a portion of catalyst rendering that part of the active area ineffective. Freezing can occur by formation of a sharp front or a mush that separates the solid and fluid phases. In function the electrodes of a battery are very similar to a porous catalyst and we will analyse the performance of a lead-acid battery at low temperatures that includes the freezing phenomena. Discharge periods of lead-acid batteries are reduced significantly at sub-zero centigrade temperatures. The reduction is more than what can be expected due to decreased rates of various processes caused by a lowering of temperature and occurs despite the fact that active materials are available for discharge. The major cause for this is the freezing of the electrolyte. The concentration of acid decreases during discharge with a consequent increase in the freezing temperature. Battery freezes when the discharge temperature falls below the freezing temperature. A mathematical model is developed for describing this phenomena. It is argued that freezing begins from the mid-planes of electrodes and proceeds towards the reservoir in-between. Ionic conduction stops when one of the electrodes freezes fully and the time taken to reach that point namely the discharge period is calculated. The predictions of the model compare well with observations made at low current density (C/5) and at – 20 ◦C and – 40 ◦C. At higher current densities diffusional resistances become important and formation of mush further degrades the performance. We also discuss effects of temperature in a model system.