Process gas is a hydrocarbon gas which contains benzene, butadiene, and other hydrocarbon compounds. For health and environmental reasons, it is important that this gas not be allowed to escape to the atmosphere. In Assignee's olefins units, a process gas compressor (PGC) is used to compress pyrolysis furnace effluent (process gas) from an inlet pressure of 8 psi to an outlet pressure of 570 psi. The PGC may be, for example, a turbine-driven, 3-case, 4-stage machine. Each compressor case is split horizontally to allow access for maintenance. A closed- loop lubrication system provides lubrication to the turbine and compressor bearings.
The PGC does, however, permit some leakage of process gas to the atmosphere. This leakage occurs because of a combination of factors. Over time and exposure to heat and stress, the mating surfaces of the top and bottom halves of each compressor case may have become non-coplanar resulting in process gas leakage across the splitline. This situation, along with the design of the compressor case, makes it difficult to properly torque all of the case bolts. Further, the case bore and rotor may no longer be perfectly concentric because of distortion from years of thermal and pressure cycles. With the compressor case in this condition, process gas will leak past the compressor seals and into the bearing housings of each compressor case. Inside each bearing housing, the leaking process gas is entrained in the lube oil from the closed-loop lubrication system. After exiting each bearing housing, this entrained mixture of lube oil and process gas is returned to the lube oil reservoir where the lube oil begins its recirculation through the closed-loop lubrication system. Inside the reservoir, the lube oil and process gas separate into liquid and gas phases. Prior to the invention, the process gas escaped to the atmosphere through atmospheric vents in the top of the lube oil reservoir.
Further, as a result of the process gas leakage, it became necessary to utilize a lube oil reclaimer to remove benzene, butadiene, and other contaminants from lube oil exposed to process gas. (Process gas-contaminated lube oil is commonly referred to as "sour lube oil".) The lube oil reclaimer uses a small vacuum flasher to vaporize contaminates and is well known by those skilled in the art of reclaiming contaminated lube oil. Prior to the invention, these vapors were also vented to atmosphere.