Page 132 - Compression Machinery for Oil and Gas

Centrifugal Compressors Chapter 3 119

FIG. 3.81 Installation of a large propane compressor with three sidestreams. (Courtesy of Omatick
and Carpenter, LNG Industry, September 2014, Figure 6.)

Control systems can be particularly difficult for this arrangement. The aero-
dynamics between each nozzle is known as a section and this compressor has
four sections. Each section must be monitored and controlled. Flow monitoring
is critical and is performed by at least four flowmeters: one for each section. A
fifth flowmeter would be costly, but would enable confirmation of the flows
thus determining calibration errors and drift, and this can be quite valuable
in maximizing the range. Each stage has an antisurge controller with each con-
trolling one of four recycle valves.

Hydrogen Recycle
Hydrogen (H 2 ) recycle—hydrogen recycle compressors are used in a variety of
refinery processes including catalytic reforming (platforming), hydrocracking,
and hydrotreating. While these different processes have unique operating pres-
sures, temperatures, and catalysts, many basic elements are common. A simple
hydrotreating process is shown schematically in Fig. 3.82. This process is used
for fuel desulfurization.
The process starts with liquid feedstock sent to a heater by a charge pump.
The liquid is heated and combined with hydrogen-rich recycle gas coming from
the recycle gas compressor then fed to a reactor. Within the reactor, in the pres-
ence of a catalyst, sulfur compounds are decomposed to form a hydrocarbon and
hydrogen sulfide. Olefins and other unsaturated hydrocarbons are saturated by
the additional hydrogen, producing stable hydrocarbons. Hydrogen will further
react with nitrogen, oxygen, and chlorine impurities in the feed to form