Natural gas is not ready-to-use as it comes out of the ground – it needs treatment and purification. As a result, natural gas processing is very filtration intensive – and while it may seem intuitive that gas-phase filters are used, liquid filters are also extremely important in gas processing. Specifically, the amine sweetening process and the glycol dehydration processes use lots of liquid filters. There are other applications for filters as well. In this blog we will explore the use of filters in gas processing, as well as some basic terms that one speaking to a plant operations , maintenance, or engineering person might hear.
Natural gas production is a fascinating topic that has been in the news for a lot of reasons. It might be useful to digress from filtration for a moment to provide some bullet points about the broader issues surrounding natural gas.
Some Reasons why Natural Gas is “In the News”
- Natural gas is more environmentally friendly than coal for generating electricity.
- New techniques for drilling and producing gas – especially, horizontal drilling (or more accurately, directional drilling) and fracking – have greatly increased supplies of natural gas, making gas very affordable – especially in North America.
- Fracking, one of the methods often used in production of natural gas, has many people concerned for environmental and health reasons – for example, because of the potential for accidental pollution of drinking water supplies – despite precautions taken by drillers.
- The United States has vast reserves of natural gas. In fact, the USA is estimated to be the world’s largest producer of oil and natural gas – ahead of both Russia and Saudi Arabia. (See graphic.)
- As Russian troops move into the Ukraine Crimea (at the writing of this blog), and the world decides how to react, analysts are assessing the importance of Russian natural gas supplies to Europe.
We have hit upon some hi-lights of the geo-political aspects of natural gas – so let’s move on to filtration!
Treatment of Natural Gas
We said earlier that natural gas is not quite ready for use when it is produced – that is because freshly-produced gas generally has contaminating gasses, plus liquid and solid contaminants in it. Examples of contaminants are liquid hydrocarbons, acid gasses such as C02 and H2S, water / moisture, and solids such as iron sulfide particles.
Amine Sweetening and Glycol Dehydration
Two key fluids that are filtered in gas plants are amine and glycol. Amine removes acid gas, while glycol removes water / moisture. Gas plants can be stand-alone entities, and oil refineries often have gas processing plants within them. Other types of operations can have amine, glycol or similar processes too. A filtration person visiting a gas plant, refinery, or other area where natural gas is produced, processed, or transmitted would do well to ask about maintenance issues that might be prevented through filtration – including those related to amine and glycol.
Two general processing concepts are the “contactor” and the “stripper.” A contactor uses a type of tower – for example, amine or glycol will flow down the tower while gas flows up, in a counter-current mechanism. In a stripper there is also a tower (or column), a counter current mechanism, heat – (which might be provided by steam), and pressure loss – and they are used in concert to strip out the contaminant from the amine or glycol. Thus, the contacting fluid (such as amine or glycol) first goes through a contactor tower where it removes the targeted contaminating gas and becomes “rich”, then proceeds to the stripper column where it is regenerated, becoming “lean” for repeated use.
Contactor towers often have “trays” with “bubble caps” in them – these are means to maximize the contact area and contact time between the gas and adsorbing fluid. Other examples of devices that increase contact area and contact time in contactors and strippers are “weirs” (lips on a series of trays that create a series of waterfalls or cascades within the tower), and “packing” materials such as Pall rings or Dixon rings (simple, roughly golf-ball size structures with cutouts, folds and holes, that are loaded into a tower or column.)
Fluids to be stripped often must be heated in “reboilers”, but then cooled on the way back to the contactor in a “cross exchanger.” Fluids on their way to the stripper from the contactor also get pre-heated in the cross exchanger.
“Foam Outs” – Why Does Filtration Matter?
One of the types of problems often discussed in a gas plant is the infamous “foam out” – or foaming of amine. Foaming can also occur with glycol. A little bit of frothing in the contactor is important to maximize contact between the fluid and the gas – but too much foaming reduces effectiveness of the process, causes carry over fluid losses, actually contaminates the gas instead of purifying it, and causes maintenance problems downstream. Foaming is generally attributed to the presence of liquid hydrocarbons which get into the amine or glycol, and there combine to behave as soapy, bubble-causing surfactants. To prevent the liquid hydrocarbons from getting into the process in the first place, gas plants use a series of steps, including “knock out pots”, demister packs, and liquid-gas coalescing filters on the gas steam. Somehow, some hydrocarbons still get into gas plant fluids (such as amine) and behave as surfactants, so carbon filter beds are used to remove the surfactants. And despite this, some foaming still occurs, so antifoam chemicals are used. Despite this, foam can still form, and solid particles (such as those comprised of iron sulfide) stabilize the foam, increasing the chance of an upset. Liquid filtration is critical for removal of foam-stabilizing particles.
Delta Pure Filtration manufactures filters that remove particles that can stabilize foam, foul heat exchange surfaces, and plug bubble caps and other equipment. DW Series String Wound Filters are economical depth filters available in a range of materials with quick lead times, and DMB Series melt blown polypropylene filters have built in pre-filtration sections to provide reliable filtration and longer onstream life.
Removing small submicron particles in an amine or glycol stream with tighter filters helps foam to “break” – this prevents upsets. On the other hand, operations personnel like low differential pressure and infrequent filter change-outs, so this might encourage the maintenance folks to use 5 or 10 micron rated filters instead of 0.5 micron rated filters. Anti-foam, carbon, and filter suppliers may sometimes have different opinions on what causes upsets and may provide slightly different advice. One might say “your amine filter is too tight and is actually coalescing the hydrocarbons and causing the foaming problem” and another might say “your filters are too coarse and you need to remove the small foam-stabilizing particles.” Delta Pure Filtration, working with our knowledgeable distributors and partners, stands ready to help the gas plant operations folks with tight, coarse, and in-between filtration!
The “Saw the Filter in Half” Test
One of the tools used to assess the performance of a filter in an amine or glycol stream is the hand saw. Most filters are white when they are new, and contaminants in glycol and amine streams tend to turn the filters black. To see how a used filter has performed, maintenance folks will sometimes saw the filter in half and observe the cross section. If the filter is mostly white on the inside, and black only on the outside, the filter is too “tight” and a filter with a coarser micron rating might be selected instead. If the filter is entirely black on the inside, then it is likely contaminants are getting all the way through, and a tighter micron rating might be selected. If there is just a tinge of white at the inner diameter, but most of the cross section is black, then the filter is said to be “just right” according to this analysis method.
Sometimes filter salespeople will use this technique to help their customers select and confirm the correct grade of filter. Both string wound filters and melt blown filters are capable of showing the “just right” annulus of white at the inner filter diameter – if the correct micron rating is selected – and this is one of many reasons why string wound filters remain so popular, despite efforts of “melt-blown-only” advocates.
Delta Pure Filtration makes a variety of filters used in hydrocarbon production and processing. For amine and glycol solutions our DW Series and DMB Series filters remove particles that stabilize foam and cause maintenance headaches. Delta Pure also provides carbon filters and dual-stage (sediment/carbon) filters. Contact Delta Pure Filtration for assistance in finding a solution to your next filtration challenge.