Comparative Analysis of Different Nitrogen Pre-Cooling Configurations in Helium Liquefaction

Liquid Nitrogen is used for precooling in Helium liquefier/refrigerator not only to permit a faster cool down to 80 K, but also to increase the cooling power and/or the rate of liquid Helium production. Transferring cooling power of liquid Nitrogen to the Helium stream primarily involves heat exchangers which offer several choices in terms of configurations and flow arrangements. The decision to adopt one option over another should be based on some practical comparative criteria. A designer has to fix parameters such as number of heat exchangers, number of streams and surface-area of exchangers, flow divisions etc. as they play important roles in the process of heat transfer. An appropriate configuration for a particular Helium plant demands maximum performance. There are also other objectives such as minimization of pressure drop, sizes of heat exchangers and amount of liquid Nitrogen consumed. The optimum configuration would be the one which optimally satisfies all the objectives mentioned. In this work, some potential configurations for Nitrogen precooling have been identified from the literature. These configurations are connected to a Collins cycle. Comparative studies are performed keeping all the parameters of Collins cycle the same and also keeping liquid Nitrogen consumption the same. Parametric studies are performed on each configuration using the process modeling package Aspen HYSYS® V7.3 to understand the sensitivity of each on the overall plant performance. Results generated have been applied to arrive at the optimum configuration for precooling in a selected Helium liquefier/refrigerator plant.