Design And Development of Secondary Oil Removal System for kW Class Indigenous Helium Refrigerator/Liquefier Plant

Abstract

The Helium Refrigerator/Liquefier (HRL) plant needs compression of helium gas, which is then, cooled and expanded to produce liquid helium at 4.5 K. To avoid high heating and to reduce the work requirement in the compression, oil of heavy molecule, which has high specific heat, is mixed with helium before compression. Before using this He gas for refrigeration, it is necessary to remove this oil impurity up to PPM (parts per million by volume) level from this mixture. For this, complete oil removal is performed in two stages: primary oil removal system (PORS) to remove bulk oil and secondary oil removal system (SORS) to remove oil up to PPM level. After bulk oil removal in the PORS, still significant oil is left in the helium stream which is removed first by a series of coalescers and then by an adsorber bed at 300 K and 14 bar. It is proposed to install 4 coalescer beds hydraulically connected in series. At the bottom of each coalescer bed (except the 4th coalescer), there is an automatic oil drainage system which drains oil back to the compressor once it fills, but selectively it restricts helium gas going back to the compressor system. At the outlet of the 4th coalescer, the oil impurity is about few PPM which then passes through the charcoal bed so that the amount of oil at the outlet of it is about 0.01 PPM by volume. The present study is related to the static adsorption capacity and adsorption kinetics for organic compounds on coconut shell based activated carbon using Dubinin-Radushkevich equation. The mass of adsorbent required for adsorbing impurity is estimated. Mass transfer zone is also discussed using Rosen and Axial Dispersion model. For reducing the heat loss/gain in the adsorber bed during regeneration as well as during normal operating conditions, a thermal insulation system is designed . Design of coalescer assembly for removal of fine oil droplets is carried out. Finally the helium gas passes through a filter to restrict the carbon dusts coming out of the charcoal bed. Design of all these components are discussed in this work for the compressor system of indigenous helium plant of equivalent cooling capacity of about 2 kW at 4.5 K.