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This article is going to cover off-gas compressor systems, why they are used, and how they are selected. Often when thinking about liquid ring pumps and thermocompressor ejectors, the first thing that comes to mind are towers and process vessels that operate under vacuum. Liquid ring pumps and ejectors are not only utilized in vacuum applications but can also be applied to applications that operate at a positive pressure. One of the more common of these applications is an off-gas compressor system.
Most processes in refineries and chemical plants have vent streams that can’t be discharged to atmosphere because they contain hydrocarbons or other potentially harmful vapors. Many of these systems are vented to either a flare system or furnace. Some facilities also have a sour gas or off-gas recovery system for the plant. Many of these non-atmospheric vent locations have long piping runs and operate at positive pressures. This results in a positive back pressure on the tail end of a process that needs to be overcome. Often a traditional ejector system can be designed to discharge to a slightly positive pressure without the need for a separate compressor system. More often, the off-gas stream operates at pressures that require a dedicated compressor system.
There are two primary types of off-gas compressor arrangements. One is a thermocompressor ejector system and the other is a liquid ring compressor skid. While there are other types of compressors that could be utilized for an off-gas service, thermocompressor ejectors and liquid ring pumps are ideal given that both are able to handle typical process load streams. Many other compressor types prefer a dry suction load, which is normally not present in these processes.
Which type of system is best for an off-gas service is going to be highly site dependent. One of the major differences between thermocompressor ejectors and liquid ring pumps is the type of utilities required. Thermocompressor ejectors traditionally use steam and have a downstream condenser that consumes cooling water. Liquid ring compressors use electricity but also require cooling and make up water, although at a much lower volumetric flow rate when compared to a thermocompressor ejector system. The availability of steam and cooling water are often driving factors in what type of system is selected. Another major factor is the initial capital cost. Thermocompressor ejector systems have a lower capital cost and much less instrumentation and controls when compared to a liquid ring compressor skid. Liquid ring compressor skids also will require more maintenance than thermocompressor ejector systems.
Liquid ring compressors are often utilized when steam or water supply is limited. This can be the case when a system is revamped. Revamps are common when the vent location of a system changes, like when moving from a flare header to a vapor recovery system header. Often in these instances the existing cooling water and steam infrastructure are limited in extra capacity. Liquid ring compressors are also popular in regions were access to cooling water and an additional heat load on the site’s cooling tower is not desirable. Liquid ring compressor systems are often installed with a redundancy to allow for maintenance to be performed online. Some common liquid ring compressor arrangements include two 100% capacity pumps or three 50% capacity pumps, with the backup pumps piped in as standby.
