The refinery flare gas can be considered one of the most challenging, but it can be handled with the right compressor.
So how can we compress flare gas from a refinery?
Several types of compressors are being used.
Centrifugal compressors should not be recommended in refinery flare gas applications because they cannot easily deal with wide swings of mole weight and are sensitive to any solids or liquids entrained by the gas stream.
Positive displacement machines are best suited for these conditions because they can handle easily wide changes in the composition of the gas. I know of five types of mechanical compressors being used, more of less successfully in flare gas applications:
- - Sliding vane compressors
- - Liquid ring compressors
- - Reciprocating piston compressors
- - Oil-flooded screw compressors
- - Oil-free screw compressors
Each of these technologies has its place and can be successful when applied correctly.
A few pros and cons of each of these types of machines:
Sliding vane compressors are simple compressors with a single shaft off-center in a cylindrical housing. They work best in low pressure clean gas applications. They are not very reliable when dust or liquid is ingested in the machine and prevent the vanes from sliding back-and-forth in the rotor slots. Jacket-cooling is required and oil needs to be injected continuously for lubricating the vanes that slide along the housing. They may require relatively frequent overhauls, but these are relatively easy to handle. Initial costs are low.
Like the sliding vane compressors, the liquid ring compressors are also simple machines. The principle of operation is very similar to that of sliding vane compressors with the exception that a ring of liquid, generally water, is used instead of vanes that slide radially. Liquid ring compressors are therefore well suited for wet gas, dirty gas applications. The gas maintains a low temperature during compression as the heat is absorbed by the liquid. These compressors are available in a variety of materials to handle a wide range of gases. As compressors, these machines are not very energy efficient, as it takes power to maintain the liquid ring in proper motion under all operating conditions. Moreover, the liquid will scrub and cool the gas. This requires the liquid to be treated, before being discharged, and also cooled, filtered if it is used in a closed loop. Of course, large amounts of water are not available everywhere and special measures may have to be taken in cold climate.
Reciprocating Piston compressors are well known and very efficient machines that can handle high compression ratios and high discharge pressures. Piston compressors exert alternating forces on supports and foundations and produce pulses in the gas stream that need to be attenuated in pulsation dampeners. Valves and piston rings are the components that are most sensitive to solid or liquid contaminants and do nor handle well gases that may polymerize in an accelerated manner.
Oil-flooded screw compressors are compact rotary positive displacement machines. They do not require heavy foundations. One rotor drives the other and oil is injected in the rotor chamber to lubricate the rotors, remove the heat of compression and seal the gap between the rotors and between rotors and housing. These machines are very simple, can reach in a single stage high compression ratios and achieve very good efficiency levels. The challenge is that the oil will scrub the gas of its contaminants and the oil may react with the gas. As a result, the oil may loose its lubricating properties, acid could form, and in some cases a gel like substances may develop. Therefore, precautions must be taken to avoid frequent oil changes, maintenance and repairs. An oil-flooded compressor therefore cannot be recommended in all applications.
The one machine that finds itself "at the intersection" of all the previously mentioned ones is the oil-free dry screw process gas compressor. Unfortunately, its initial costs may be the highest probably in the range of reciprocating compressor solution. However, the higher investment may be worth it because it combines the advantages of the above mentioned compressors without combining their individual disadvantages.
- The oil-free compressor is a rotary positive displacement machine that handles wide swings of mole weight and gas composition
- It does not require special foundations
- The process gas side of the machine is completely separated from the oil side by 4 internal seals. Therefore, the oil cannot be contaminated by the gas.
- The compressor can handle contaminants: solids and liquids and water or solvents can be injected intermittently or continually for cleaning and for cooling if required.
- The rotors are almost naturally self-cleaning, however, liquid injection may be required to remove substances that may stick to the rotors or the casing and accumulate in the inlet or discharge channels.
What is Aerzen's experience in this field?
Aerzen began supplying compressors for flare gas recovery to clients in Europe already some forty years ago. The machines for this application have been in the range between 500 cfm and 5000 cfm (15 to 150 m3/min) and discharge pressures from 30 to 190 psi (2 to 13 bar g) with the bulk of them around 90 to 100 psig (6 to 7 bar g). These machines are either single-stage or two-stage compressors with inter-cooling to achieve best efficiency. However, single stage machines are also used and may require constant water injection. The water flow requires varies of course with the gas, temperature, pressure and materials used. The amount of water is very small: in this particular application, typically less than 1 GPM / 1000 cfm of gas.
The oldest installations in North America are in Canada. In the early 1980s, several refineries, were equipped with Aerzen dry screw compressors for the compression of flare gas. These have been compressing 4000 cfm of gas to 95 psia.
Pierre Noack - President