The Effect of Condensible Impurities in the Working Fluid on the Performance of Cryogenic Regenerators

In many cryogenic regenerator applications the working fluid contains a small quantity of condensible impurity, which undergoes condensation/evaporation along its sublimation curve due to pressure cycling. The governing differential equations for such a system have been formulated in terms of conventional dimensionless parameters. Numerical solution of these equations is presented for several combinations of fluid flow parameters using nitrogen as the working fluid and carbon dioxide as the impurity. The model indicates that: (a) as long as there is no net accumulation of frost a higher pressure ratio has a negligible effect on the thermal performance of the regenerator; (b) a larger reduced period affects the thermal performance of the regenerator in the presence of a condensible impurity to a greater extent than in the absence of it; (c) the matrix temperature profile undergoes a much larger swing in the presence of a condensible component; and (d) since the exit impurity concentration is a function of reduced length, reduced period, and inlet composition, arbitrarily chosen regenerator parameters cannot be used to purify a warm process stream.