Turbo-expanders must-knows

Turbo-expanders are modern rotating devices that convert the pressure energy of a gas or vapor stream into mechanical work as the gas or vapor expands through the turbine. Turbo-expanders can be used as energy recovery devices or as a part of a thermodynamic cycle to provide low-temperature refrigeration. 

If chilling the gas or vapor stream is the main objective, the mechanical produced work is often considered a byproduct. If pressure reduction is the main objective, then heat recovery from the expanded gas is considered a beneficial byproduct. The most popular method of absorbing turbo-expander power is by means of a centrifugal compressor, or an electrical generator.

In the following series of articles, I will be focusing on cryogenic turbo-expanders used in the hydrocarbon processing industry.

Turbo-expanders for Hydrocarbon processing

In the Hydrocarbon processing industry, the term "Turbo-expander", is normally used to define an Expander/Compressor machine as a single unit. It consists of a radial inflow turbine, often controlled by variable-position inlet guide vanes(VIGV). typically expander wheels are designed as a 50% reaction turbine. This means that half of the static enthalpy drop occurs through the stator, or inlet guide vanes, and the other half through the rotor, or expander wheel. The turbine efficiencies achieved lies between 85% to 90%. The compression stage comprises a centrifugal compressor stage with a vaneless diffuser. The compression efficiency ranges between 70% and 80% at a wide range of head and flow.

Turbo-expander applications in the Hydrocarbon industry

Natural Gas Processing: NGL, LNG, Ethane Recovery, and Dew Point Control

Petrochemicals: Ethylene (Olefins), Carbon Monoxide, Hydrogen Recovery, and Refinery Off-Gas

Typical Turbo-expander Process

To recover valuable hydrocarbons like butane, propane…etc. from a natural gas stream, we need to control the environment i.e. pressure and temperature to change the state of matter from gas to liquid selectively. 

The typical turbo-expander process is described in the above figure. the high pressure pre-treated natural gas stream is precooled in the gas/gas heat exchanger. After dropping condensed liquids in the high-pressure separator, the precooled natural gas stream expands in either the expander wheel or the Joule-Thompson valve. This pressure drop allows for a further drop in temperature. After expansion, the gas/condensate mixture stream enters the low-pressure separator where the valuable NGLs are collected at the bottom and the lighter gaseous components leave at the top. Finally, the lean gas stream precools the inlet stream in the gas/gas heat exchanger before it is fed to the booster compressor to elevate its pressure.

Joule-Thompson Valve Vs. Turbo-expander

A J-T Valve produces isenthalpic expansion, enthalpy (H) remains constant and no work is available as a result of that expansion.  Unlike a J.T Valve, a Turboexpander converts the gas stream’s energy into refrigeration and work. Approximately 86% of the energy contained within the gas stream can be recovered using a turboexpander. From a typical system, this means recovering thousands of actual shaft horsepower.

“J-T Valve”, is a very simple device. It allows a pressure reduction, causing a temperature drop in a gas stream. However, while it is inexpensive, it is not efficient. No energy is recovered! The recovered work is the reason for the high efficiency of a Turbo-expander system.

From the above figure, it can be seen that for the same pressure ratio and inlet temperature. The turbo-expander process exit temperature is lower than the J-T valve process and hence the dew point is lower. this means that the turbo-expander process is more efficient in recovering heavier hydrocarbons as well.

To conclude, A Turbo-expander is a machine that produces refrigeration and recovers energy. this refrigeration effect is utilized by the natural gas processing industry in recovering precious NGLs and to control the pipeline gas quality. Turbo-expanders technology is the most economical way to recover heavier components from a gas stream.