In some cases, steam is either not available, or it forms unacceptable byproducts when in contact with process gases. In those situations, it is possible to use some other gas or other vapor to motivate an ejector. The theory of a gas or other vapor motivated ejector is identical in principle to a steam ejector. The Mollier diagram for steam is well established, and the thermodynamics of a steam jet ejector an be shown quite well on this diagram. Mollier diagrams for other gases and vapors are not always available. The design of gas or other vapor motivated ejectors is done on a more theoretical basis, knowing such things as molecular weight, heat capacity, heat capacity ratio, compressibility factor, etc.
Compressed air motivated ejectors are used in many situations, and often can compete successfully with steam motivated units. Single stage units can provide design suction pressures as low as 125-150 torr. Two-stage non-condensing units can provide design suction pressures as low as 20-25 torr. They are not as efficient as steam motivated ejectors in terms of lbs/hr of motive gas per lb/hr of suction gas.
A typical application for an air motivated ejector would be in the pumping of dry hydrogen chloride gas. With an air motivated ejector, one could use a steel unit. With a steam motivated unit, one would have to use a more exotic construction such as graphite. (The motive steam inherently makes the load wet, in this case forming hydrochloric acid vapors which would rapidly attack a steel unit.)
Vapor powered ejectors are used when the vapor is compatible with the gases being pumped. If the partial pressure of the motive vapor is low enough, it may be possible to condense between even high vacuum V and W-stage ejectors. This makes the system more efficient and the amount of motive vapor less than if these ejectors had been non-condensing. Frequently the motive vapor is at a low pressure (15 psig) and is generated by a steam heated shell and tube reboiler. A liquid ring vacuum pump sealed with the same vapor in its liquid state is usually used to replace the Z-stage ejector, or Y and Z-stage ejectors.
Croll Reynolds has designed units motivated by such vapors as methanol, ethanol, benzene, toluene, xylene, cumene, ethylene glycol and propylene glycol. An increasing number of users in the plastics industry are specifying ethylene glycol motivated units, be-cause the solids carried over from the process are glycol soluble. This keeps the ejectors from plugging and allows for much longer service. A typical system operates at 0.1 torr. It consists of three ethylene glycol motivated ejectors with ethylene glycol cooled barometric condensers between them followed by an ethylene glycol sealed two-stage liquid ring vacuum pump.
Gas operated ejectors are also beginning to find more and more use as compressors in the petrochemical industry. High pressure natural gas at 500-600 psig is used to boost well head hydrocarbon fugitive emissions form 0 psig to about 100 psig. At this pressure, the combined gas stream can be put back into a low pressure pipeline for use elsewhere, rather than letting the hydrocarbons be released to the atmosphere.