Contamination in gas compression: causes, effects, and solutions
Identifying and quantifying contaminants including solids, dissolved species, and liquids is essential in the design of a more reliable compression system.
DAVID B ENGEL and SCOTT N WILLIAMS
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Gas compression systems are a vital yet delicate part of any gas plant. There are many different types of compressors, but all work by imparting energy to the gas and reducing the available volume, thereby increasing pressure. This process creates a drastic change in process conditions for the gas stream, and even small amounts of contamination can create significant challenges in the compression system or downstream. Only a small number of compression systems have the necessary means to adequately remove contaminants in the gas stream, so the need for accurate gas testing and high efficiency separation systems for compressor protection is greater than ever.
Contamination in gas compression
Compressor failures and downstream problems can be and are caused by a variety of mechanisms. Dissolved contaminants in the gaseous phase or in water entrained in the gas can precipitate in the system during compression, leading to fouling and corrosion, reduced throughput, and eventually failure. Suspended solids can cause similar effects. One of the most common and difficult challenges in gas compression however is dealing with lubrication oils and additives injected within the system.
Lubrication oils typically contain a high percentage of a base oil and a low percentage of chemical additives designed for various functions. These additives impart reduced metal-metal friction and wear, viscosity modifications, resistance to corrosion, oxidation, ageing, and traces of contamination among others. Most chemical additives in lubrication oil packages have surfactant properties causing a number of downstream problems in processing units. Base oil, usually a heavy hydrocarbon, can also in some cases cause detrimental effects downstream. These are often related to agglomeration with solid particles in the gas stream, forming larger residues and causing deposition and fouling in many gas lines and downstream equipment.
To illustrate this point, Figure 1 shows the change in surface tension of pure water when contacted with lubrication oil. A mixture of 95% water and 5% of a standard lubrication oil is contacted in a small vial and shaken for one minute. Water is then separated and analysed. The decrease in surface tension of pure water changed from 72 mN/m to 46 mN/m. This is a clear indication of the surfactant properties of water-soluble chemical additives present in lubrication oil. The decrease in surface tension leads to an increase in entrained water and dissolved contaminants downstream as separation equipment loses liquid removal efficiency. Different lubrication oils will have different additive packages and will show somewhat different results. However, lowering of the surface tension in pure water is a consistent observation.
Compression systems are so vital because without the system pressure created by compressors, the process or sometimes the entire plant cannot operate. Compression systems in oil refinery fluid catalytic cracking (FCC) units are often responsible for more than 40% of a refinery’s capacity, and failure can even lead to complete refinery shutdowns, causing tens of millions of dollars in lost revenue. Natural gas compressor stations are responsible for the transportation of raw and processed natural gas, and supply the pressure needed for gas plant operations. Adequate compression system protection is thus an extremely important aspect of successful plant operation, as compressor failure and also contamination breakthrough has enormous consequences.
The first step in compressor protection is in understanding the nature of contamination in the stream, and gas testing is a vital piece of any troubleshooting protocol. Feed contamination can play a major role in compression system challenges and is often cited as the main cause of process problems. Plant operators and engineers often overlook the injection of lubrication oils and additives in the compressor itself and neglect to identify that source of contaminant ingress. In cases where lube oils and additives are of concern, downstream effects such as foaming and fouling are often observed, and testing of the gas stream at the outlet of the compressor should be performed.
Liquid contamination in gas streams is a complex challenge for a number of process units downstream. This is especially of concern in amine units and glycol units where lubrication oil ingress often causes solvent foaming, severely limiting the processes. The effect is also extended to metering stations, leading to erroneous lower readings. Therefore lubrication oils and chemical additives, in addition to heavy hydrocarbons, water, and other contaminants, must be identified and quantified before any removal solution can be developed. Testing for liquids in gas streams is performed quantitatively using a gas-liquid super coalescer (GASCO) test system (see Figure 2). A slip stream of the gas flows across the GASCO test unit where aerosols are coalesced and liquids are drained from the internal element into a sight glass. These liquids are quantified, sampled, and further analysed for their composition and concentration. Solids are removed from the element media for characterisation purposes only.
In any case where feed contamination is present, it is always advisable to locate the source. Oftentimes a capital investment can be avoided by identifying and correcting issues upstream. In gas compression, liquid contamination from injected lube oils and additives is always present, and source removal is not an option, so process protection is a must.
Separation of liquid contaminants in gas streams is usually carried out using demisters (also known as knock-out drums) equipped with a metal coalescing pad element or vane pack installed near or at the outlet of the vessel. Demister systems are typically vertical in orientation, but they are only adequate for removing large diameter contaminant droplet sizes. These separators were originally designed for bulk liquids and slug removal and are not designed for solids separation (usually done by a wet scrubber or a particle filter) with the exception of cyclonic systems that can remove large solid particles and some larger liquid droplets. Only a small number of compression systems have the necessary means to adequately separate the lubrication oil liquids in the gas stream caused by injection at the compressor itself.
As far as contamination in gas streams is concerned, the most prevalent and difficult contaminants to separate are sub-micron liquid aerosols, finely divided liquid droplets with diameters ranging from less than 0.1 micron to a few hundred microns. Droplet sizes below 1.0 micron are the most difficult to remove due to the absence of a specific separation mechanism that yields high removal efficiency. The typical aerosol distribution in gas streams is primarily in the sub- micron range. Larger droplets tend not to be as persistent as they are likely to be separated by gravity but can shatter due to the shear forces surrounding the droplet surface with certain deficient vessel design features. When large droplets shatter, progressively smaller droplets are created until the distribution is stabilised by the balance of energy distribution, gravitational settling, and shear.
Other devices such as mesh pads, vane packs, and cyclones are ineffective because they are not able to capture the small and most penetrating sub-micron aerosols. Vane packs are especially ineffective when dealing with sub-micron liquid aerosols since the small droplets do not have enough momentum to contact the vane surface properly. Interfacial layers in many vane packs and some mesh pads are one cause of inefficiencies and companies have mitigated this by using different designs (double and single pockets). Their efficiencies can be enhanced somewhat for larger liquid droplets, low liquid loadings, and gas velocities within certain limits. Mesh pads suffer similar inefficiencies and are prone to particle fouling, but their removal rate is somewhat better due to the higher surface area. Today, the technology of choice for high efficiency removal of sub-micron aerosols in gas streams is built around specially formulated microfibre media.
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