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Control for Cryogenic Turboexpanders

Enhanced Control Techniques for Cryogenic Turboexpander Recompressors.

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Turboexpander compression trains are commonly used in petrochemical processes for extracting the heavier hydrocarbons from natural gas. These heavier hydrocarbon gases include but are not limited to ethane, propane, butane and pentane and are commonly called natural gas liquids or NGL. Figure 1 below shows a typical gas processing plant design. In this example, the expander is used to cool the process gas stream before entering a distillation column or deethanizer where it is used as reflux for the distillation. Gas from the top of the column is routed through a cold box before being compressed by the compressor portion of the turboexpander compression train.

The turboexpander and recompressor parts of the unit operate on the same shaft. A J-T (Joule-Thompson) valve is used to bypass flow around the turboexpander when the expander is offline or is unable to meet the flow demands of the process. The J-T valve is also used for startup of the train. The recompressor uses a recycle valve for antisurge protection as well as startup and shutdown of the unit. There is a bypass line around the recompressor to divert flow when the turboexpander train is offline and the process is operating in J-T mode. See Figure 2 below.