Gas compression technology comes in many flavors including centrifugal, reciprocating, screw, and scroll. In a Hydrocarbon Engineering article, Playing it Cool, Emerson’s Mark McCormick shared how shale gas processing has increased the need for rotary screw compressors required in process refrigeration applications.
Natural gas is composed of many components including heavier hydrocarbons such as propane, butane, etc. Mark notes two reasons for recovering natural gas liquids (NGLs) from the natural gas:
First, NGL recovery is done to meet pipeline specifications for hydrocarbon dew point for any natural gas entering the system (to avoid forming unsafe liquids during transport). Second, NGL recovery enables the customer to capture a profitable source of revenue. These high BTU liquids often have greater value as a separate value stream in liquid form, rather than as part of the natural gas being sold.
Heavier gaseous hydrocarbons: ethane (C2H6), propane (C3H8), normal butane (n-C4H10), isobutane (i-C4H10), pentanes and even higher molecular weight hydrocarbons.
Mechanical refrigeration of the gas is used to recover these NGLs:
Mechanical (or external) refrigeration involves a refrigerant to chill the gas to low temperatures for condensing NGLs… The most common refrigerant is propane, but other commercial refrigerants or ammonia are sometimes used.
These refrigerants can cool down to their atmospheric boiling points in a range between -40 to -30 degC. The control challenge is to control the natural gas dew point and NGL recovery rate over a wide range of process chilling loads:
…for example, pressure, temperature, flow rate, and inlet gas compositions…
Vapor recovery units are needed the NGLs held in storage tanks to capture any emissions from the tanks. Mark explains that tradition vapor recovery units:
…have been maintenance intensive, resulting in uneconomical operating costs and are often abandoned.
Reciprocating and screw compressors are both positive displacement-type compressors. Mark compares these compressors along with centrifugal compressors in this refrigeration-type application.
Centrifugal compressors have historically been used. They are:
…nearly pulsation and vibration free and usually oil free. However, centrifugal compressors do not respond well to changing gas conditions because of limited capacity control.
…gained popularity because its efficiency gains over centrifugals and ability to manage load changes better. …have lower reliability and frequent maintenance requirements though, often requiring two stages to reach higher pressures.
…include lower applied cost, application flexibility, higher reliability and lower required maintenance… One potential issue is the risk of process gas or refrigerant becoming entrained in the compressor oil system. This oil dilution condition can cause premature wear, even catastrophic failure of the compressor bearings.
To address this possibility, some screw compressors have isolated and dedicated bearing lubrication systems. Mark highlighted a single, screw-type oil-flooded screw compressor that:
….shares the twin oil flooded screw design benefits, but also has the unique capability of being able to deliver much higher pressures.
The compression process is symmetrical with no net radial or axial forces exerted on the main screw or drive shaft. This results in:
…lower mechanical loading during the compression process.
Here’s a 15:57 Youtube video, Vilter Single Screw Compressor Operation that shows how a single screw compressor works.
Rapid expansion of gas processing refrigeration applications is tied to the production growth of wet shale gas. This continues to drive demand for new screw equipment including screw compressor based refrigeration systems for processing shale gas for the downstream markets.