Turboexpander
The raw gas also contains various amounts of acid gases such as carbon dioxide (CO2), hydrogen sulfide (H2S) and mercaptans such as methanethiol (CH3SH) and ethanethiol (C2H5SH).
The extraction of the NGL often involves a turboexpander[7] and a low-temperature distillation column (called a demethanizer) as shown in the figure.
The bottom product from the demethanizer is also warmed in the cold box, as it cools the inlet gas, before it leaves the system as NGL.
The resulting high-pressure vapor flows to the turboexpander, where it undergoes an isentropic expansion and exits as a vapor–liquid mixture, which is then condensed into a liquid by heat exchange with the available cooling medium.
The system in the figure implements a Rankine cycle as it is used in fossil-fuel power plants, where water is the working fluid and the heat source is derived from the combustion of natural gas, fuel oil or coal used to generate high-pressure steam.
The combustion flue gas from the catalyst regenerator of a fluid catalytic cracker is at a temperature of about 715 °C and at a pressure of about 2.4 barg (240 kPa gauge).
As shown in the figure, expansion of the flue gas through a turboexpander provides sufficient power to drive the regenerator's combustion air compressor.
[14] The flue gas from the CO boiler is processed through an electrostatic precipitator (ESP) to remove residual particulate matter.
[2] In 1902, Georges Claude, a French engineer, successfully used a reciprocating expansion machine to liquefy air.
With an air pressure of only 40 bar (4 MPa), Claude achieved an almost isentropic expansion resulting in a lower temperature than had before been possible.
[2] The first turboexpanders seem to have been designed in about 1934 or 1935 by Guido Zerkowitz, an Italian engineer working for the German firm of Linde AG.
His first practical prototype was made of Monel metal, had an outside diameter of only 8 cm (3.1 in), operated at 40,000 revolutions per minute and expanded 1,000 cubic metres of air per hour.
[17] In 1983, San Diego Gas and Electric was among the first to install a turboexpander in a natural-gas letdown station for energy recovery.
Three main loading devices used in turboexpanders are centrifugal compressors, electrical generators or hydraulic brakes.
