Oil-Free Screw Compressors in Ethylene Storage Boil Off Gas Service

Major Ethylene production facilities utilize large atmospheric tanks to store liquid Ethylene at its cryogenic saturation temperature of -155 °F. Depending on the ambient conditions and operating scenarios, the Ethylene tends to evaporate at various rates. In order to recover the vaporized ethylene, the gas is routed through a liquefaction process. In this process, the boil off gas compression units capture this evaporating gas and recompress it. The gas is liquefied by passing through a condensing chiller and expansion valve and is then recycled back to the tank. The compressor used in this service should maintain the pressure ratio in various suction volumetric flow rates. Thus, traditionally positive displacement compressors, mostly reciprocating machines, have been used for this purpose as they can maintain the pressure ratio under varying suction flow rates. Alternatively, oil-free screw compressors can be used in this service as a positive displacement machine with much higher reliability and proven record of much less required maintenance. This paper will describe which circumstances, i.e. flow rates and pressures, favor the use of oil-free screw compressors. Technical details will be given that describe the features of this compressor type.