PECOFacet PEACH Saturated Depth Coalescer
By Robert McIlvane
The market to remove liquids from oil and gas is close to $2 billion per year and includes the range of devices to remove both small and large droplets as well as water in emulsified solutions. Mercury removal from natural gas is also a significant market opportunity for the filtration and separation industry. Water and other liquids need to be removed from natural and synthetic gas. Common applications are:
- Gas processing plants
- Pipeline compressor stations
- LNG liquefaction/Regas plants
Natural gas processing consists of separating hydrocarbons and fluids from the pure natural gas to produce “pipeline quality” dry natural gas to be fed to gas transportation systems. High efficiency cartridge Liquid/Gas (L/G) coalescers are used for vapor removal from the gas stream during processing and transportation. Typically, they are used downstream from other separation equipment.
Gravity Separator – also known as knock-out drum, droplet removal >300╬╝m; for bulk separation or first stage scrubber. The force used to separate solids and liquids from gas is gravity.
Centrifugal Separator– also known as cyclone separator, when properly sized can remove droplets down to 8-10╬╝m.
Mist Eliminators – there are three types of mist eliminators:
- Vane, also known as baffle, chevron or plate type, closely spaced blades arranged to provide zigzag gas flow paths, using inertial impaction for droplet removal >10╬╝m; sturdier than mesh pads and impose less pressure drop, sometimes used in combination with mesh pads.
- Mesh pad-most typical structure is knitted mesh pad in tightly packed layers; media composites of plastics or glass coalesce droplets > 1.0╬╝m, media of metal or plastic wire for droplets > 5.0╬╝m; large vessel required since operated at low velocity.
- Fiber mist eliminators are in cylindrical form (candles) or flat panels and capable of separating particles > 0.5╬╝m.
- Coalescer cartridges are capable of high efficiency on particles > 0.1╬╝m. The usual configuration is vertical. Gas travels upward and flows from inside to outside of the cartridge, where submicron droplets coalesce into larger droplets, then drain down. L/G cartridge coalescers can operate at peak performance at reduced flow rates (i.e., during partial shutdowns).
Different stages of separation are required. A knock-out drum will remove the larger droplets, followed downstream by a mist eliminator and/or a high efficiency cartridge coalescer. The different stages can be in separate vessels or in one vessel.
A high-efficiency vertical liquid/gas coalescer such as Pall’s product is designed for inlet gas with liquid aerosol contamination entering at the bottom of the housing into a first-stage knock-out section. Here any slugs or large droplets are removed by gravitational settling. The gas then travels upward through a tube sheet and flows radially from the inside of the cartridges through the coalescer medium to the annulus. The inlet aerosol distribution ranges from 0.1 to 300╬╝m, and after passing through the coalescer medium, is transformed into enlarged coalesced droplets ranging from 0.5 to 2.2╬╝m.
Liquid-liquid coalescers can be segmented into stationary and mobile. In the stationary segment, the major players, accounting for approximately 30 percent of the revenues, are Pall, PECOFacet, ParkerVelcon, Jonell, Pentair, Faudi Aviation, Kaydon, Vokes and Sulzer.
Major companies supplying coalescing filters in the mobile sector include Mann+Hummel, Mahle, Donaldson and Racor.
Two of the largest media companies are Lydall Filtration/Separation, Inc. and Hollingsworth and Vose.
Lydall offers their LyPore┬« Unity™ liquid/liquid and liquid/gas coalescing media grades for efficient separation of both water from other liquids and oil and water from air streams. All grades are constructed with borosilicate microfiber glass that offers the highest level of coalescence at the lowest pressure drop. Lydall’s fluoropolymer oil and water repellency treatment processes ensure exacting separation of target compounds and long element life. LyPore Unity grades can be pleated or wrapped and are available in a wide range of efficiencies, repellency levels, and binders.
Hollingsworth & Vose (H&V) manufactures a comprehensive line of coalescer media solutions for applications that require gas-liquid and liquid-liquid separation. H&V offers a choice of fiberglass, cellulose and synthetic media combined with specialized organic binders. H&V microfiber glass media fibers are naturally oleophobic, so oil droplets adhere but do not swell them. The cellulose coalescer media is an economical alternative to glass. For applications requiring additional structural integrity, these media are also available with lamination.
Mercury is present in many natural gas streams. While mercury levels can vary greatly, even low levels must be removed to avoid damage to downstream equipment. It is desirable to analyze mercury removal adsorbents for two specific types of gas streams: wet gas and condensates.
The need for mercury removal adsorbents is primarily a function of the amount of natural gas produced and the average concentration of mercury in the gas. However, the concentration of mercury in natural gas varies greatly from region to region, from a low of 0.02 g/Nm3 in the Gulf of Mexico to 300 to 400 g/Nm3 in Southeast Asia. Consequently, the largest natural gas producing regions are not necessarily the largest markets for mercury removal products.
Current methods for removing mercury from natural gas and condensate use fixed beds of mercury removal materials. There are three types of materials: sulfur impregnated activated carbon, metal sulfides or oxides on an alumina carrier, and silver impregnated molecular sieves. Carbon-based sorbents are the most commonly used, but the trend is towards a greater use of metal sulfides in order to remove higher concentrations of mercury and for occupational safety and environmental reasons.
Molecular Sieves: Regenerative mercury removal is usually practiced simultaneously with another regenerative adsorption application such as drying. Since nearly all cryogenic gas processing plants use molecular sieve dehydrators, the mercury removal function can easily be added to the dehydrator by replacing some of the molecular sieve with a silver-containing mercury removal adsorbent. When the adsorbent is heated to the normal dehydrator regeneration temperature, the mercury is released from the silver and it leaves with the spent regeneration gas. However, the spent regeneration gas may require some secondary mercury removal treatment.
Activated carbon: The most common mercury removal technology uses elemental sulfur dispersed within a porous carrier such as activated carbon granules or pellets. The mercury reacts with the sulfur to form mercuric sulfide, which stays on the sorbent. As mercury accumulates on the sorbent material, the pressure drop in the vessel will increase. Once it reaches a predetermined level, usually after several years, the vessel will be taken off line and the sorbent replaced.
Activated carbon sorbents can be used in water-containing and dry natural gas streams. However, elemental sulfur is highly soluble in liquid hydrocarbons. The presence of liquid hydrocarbons in a gas stream will “wash off” the sulfur and reduce the sorbent’s capacity for mercury.
Ionic liquids have been the subject of a number of research projects related to CO2 capture from power plant flue gases (for carbon capture and sequestration). Ionic liquids have also been studied as a means of replacing chemical solvents (such as MEA and MDEA) to remove CO2 from natural gas streams. The Queens University Ionic Liquids Laboratories (QUILL) and Petronas have developed an ionic liquid tailored to mercury removal from natural gas and condensates. The substance has been tested at two gas terminals operated by Petronas in Malaysia since November 2011. In March 2014, Petronas signed a licensing agreement with Clariant, a Switzerland-based specialty chemical company, to begin marketing the technology.
The underlying research suggests that the Clariant HycaPure Hg has a lifetime of up to three times that of commercial alternatives (such as sulfur-impregnated activated carbon). This could represent a substantial saving for mercury removal units, which each cost around $180,000 per fill. Downstream equipment is also protected during mercury level fluctuations by the technology’s rapid mercury capture kinetics.
Since this technology is relatively new it will be desirable to closely track its success not only in stand-alone removal devices but also to learn whether this technology can be combined with particulate removal.