Numerical Analysis of Dry Nitrogen-Water Chilling Unit of an Air Separation Unit

Abstract

The free atmospheric air is sucked by a multi-stage air compressor through a filter and compressed to a certain working pressure. The compressed air then passes through the cascade cooler and then to the molecular sieve battery where the moisture and carbon dioxide are removed from the process air. The main objective is to develop an effective pre-cooling unit so that the production time reduces ultimately reducing the total power consumption of the plant. The compressed feed air fed into the air purification unit at low temperatures results in greater molecular heat exchange by the molecular sieves inside the purification unit. The cooler unit consists of copper tubes bundled and immersed in one-third volume of water in a mild steel chamber. The dry nitrogen led from bottom of the cooler. The idea of cascading the dry nitrogen-water cooler postulated in this study mainly to eliminate the use of Freon cooler for air separation unit (Agrawal, and Thorogood Production of Medium Pressure Nitrogen by Cryogenic Air Separation. Gas Separation & Purification Vol. 5 - Issue 4 (1991) 203–209.). The numerical simulation has been conducted on single copper coil bundle and a series of copper coil bundles with multi-phase flow, temperature contours reported. The observed result indicate the optimum case of mass flow rate, pressure, and temperature, where compressed feed air temperature reduced to 330 K in single copper coil bundle and 279 K in a series of copper coil bundles or two-stage cascade cooler which is the most effective respectively.