Filtration in Oil and Gas: A Focus on Droplet Removal from Natural Gas Streams

July/August 2015 | Volume 34 No. 4

By Karen Vacura, Mcilvaine Company

A number of different filter types are used in the oil and gas industry, from macro to micro filtration. The combined 2015 revenues for filters in this industry will be $3 billion, with Asia and the Americas accounting for the majority of the market.

These totals include a variety of filters: coalescers, liquid cartridges, liquid macro-filtration (automatic backwash and bag filters, filter presses, drum & disk filters, and granular media filters), sedimentation/centrifugation (sand filters, clarifiers, centrifuges, cyclones and dissolved air flotation devices), reverse osmosis filtration, and micro- and ultra-filtration.

Snap shot of filter revenues for the oil and gas industry for the current year.

Snap shot of filter revenues for the oil and gas industry for the current year.

 

Various segments of this large market will be analyzed in future International Filtration News articles; however, the present scope for this article will focus on removal of droplets from natural gas streams. The revenue for one segment of this market, small droplet removal using liquid/gas high efficiency coalescer cartridge filters, is over $200 million per year.

Overview

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.

Liquid aerosols are formed by condensation, atomization, and re-entrainment from upstream separation equipment. Condensation and atomization usually form fine aerosols smaller than one micron in diameter. High efficiency cartridge liquid/gas coalescers are used for vapor removal from the gas stream during processing and transportation. Typically they are used downstream from other separation equipment such as mist eliminators (vane, mesh pads) that remove larger droplets.

The worldwide revenue for liquid/gas high efficiency coalescer cartridge filters is over $200 million and projected to grow over the next 5 years to $217 million. The United States has a 30 percent share of revenue at $61 million based on capacity reports. Russia is the second leading gas producer, with an estimated $29 million in revenue for this market.

Technology

More than one stage of separation is required. Several technologies are available to remove liquids and solids from gases: gravity separators, centrifugal separators, mist eliminators and liquid/gas cartridge coalescers. A mechanical separator uses a series of filters or dividers to induce water molecules to move from a mixture to collect together. Electrostatic coalescers use a weak electric charge to increase the droplet size for removal, and are usually seen in liquid/liquid coalescers.

Different technologies are used for different droplet size removal, from sprays greater than 10 microns to mists less than 10 microns to aerosols 1 micron or less.

Gravity Separators – also known as “knock out drums,” are used for bulk separation or a first stage scrubber. Knock out drums will remove larger droplets (>300 m), 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. The force used to separate solids and liquids from gas is gravity.

Centrifugal Separator- also known as “cyclone separator,” can remove droplets down to 8-10 m.

Mist Eliminators – Mist eliminators have lower efficiency at the submicron size, thus allowing some vapor droplets to escape. There are three types of mist eliminators:

Vane, also known as “baffle, chevron or plate” type, are closely spaced blades arranged to provide zigzag gas flow paths, using inertial impaction for droplet removal >10 m. Vane mist eliminators are sturdier than mesh pads and impose less pressure drop, and are also sometimes used in combination with mesh pads

Mesh pads, with the most typical structure being a knitted pad in tightly packed layers, use media composites of plastics or glass that coalesce droplets > 1.0 m. Media of metal or plastic wire are used for droplets > 5.0 m. A large vessel is required and is operated at low velocity.

Fiber bed/candle types use fine fibers (cellulose, glass, or plastic) packed into a mat with thickness of a few inches. These fiber mist eliminators are in cylindrical form (candles) or flat panels and offer > 0.5 m removal. These mist eliminators remove sub-micron liquid aerosols from gas streams, but have higher pressure drops than cartridges. Fiber bed filters have a large contacting surface and are used in applications including acid mist removal and oil mists created in various processes.

Coalescer cartridges offer high removal efficiencies down to > 0.1 m. Usual configuration is vertical, where gas travels upward and flows from inside to outside of cartridge, where submicron droplets coalesce into larger droplets, then drain down. Liquid from gas cartridge coalescers can operate at peak performance at reduced flow rates (i.e., during partial shutdowns).

A high-efficiency vertical liquid/gas coalescer can be designed for inlet gas with liquid aerosol contamination that enters at the bottom of the housing into a first-stage knockout 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 mm.

Cartridges are most advantageous for smaller applications, used for inlet aerosol concentrations of a few thousand ppmw (parts per million weight) and are placed downstream of other bulk removal separators.

Coalescers, typically manufactured as either pads or cartridge filters, are designed to take small droplets in a gas stream and grow them into large drops to allow separation by gravity. This is accomplished with pore gradient of medium, with inlet of fine pore sizes that increase in size with flow direction.

Table 1. Natural gas production forecast

Table 1. Natural gas production forecast

Media

Media for high efficiency cartridge coalescers is typically glass fibers for higher efficiency, treated with oleophobic/hydrophobic finish. Two of the largest media companies are Lydall and Hollingsworth & Vose.

Lydall Filtration/Separation, Inc. offers 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 micro-fiberglass 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. For applications requiring additional structural integrity, these media are also available with lamination. Micro-fiberglass provides superior coalescence of the smallest contamination droplets. Cellulose media are rugged and economical. Carded nonwovens are suitable as coarse layers and protective wraps.

Applications

Liquid from gas coalescers can be used upstream for equipment protection or downstream for product recovery. Specific applications in the gas processing industry include:

Well Head Hydrate Inhibition: Removing aerosols directly at the wellhead, is an effective way to prevent issues downstream and reduce chemical injection (i.e., glycol) costs.

Amine contactor protection/carryover recovery: Amines absorb sulfur compounds from natural gas to produce what is known as sweet gas. The solution can be reused repeatedly. Contaminants can cause foaming, corrosion and plugging of equipment. Also, amine carried over into the sweet gas can result in fouling of downstream equipment including compressors and burners, and product loss. Before entering the absorber, the gas is passed through a system of separators and mist eliminator pads, and if aerosols are determined to be present, high efficiency coalescing filters are used. L/G coalescers can also be installed downstream for product recovery.

Glycol contactor protection/ carryover recovery: Glycol injection systems remove water and other solids from the gas stream. Excessive amounts of liquid and solid contaminants present in the process stream can cause fouling and foaming problems. Coalescers installed upstream will protect contactors, installed downstream of the glycol dehydration system will protect the molecular sieve bed from glycol carryover.

Molecular sieve protection: Molecular sieves are used to dehydrate gas. When water, hydrocarbon and solid aerosols are present, the molecular sieves can reach maximum capacity and lead to frequent regeneration.

Compressor protection/lube oil recovery: Compressors at gas and LNG processing facilities require contaminant free gas to avoid failure and frequent costly maintenance. Also, pipeline compressor stations need filtered gas – even though it left the gas plant “clean and dry” it can pick up particulates and liquids along the way. Recovery of lube oil downstream of a compressor is the primary reason for installing a coalescer on the outlet of a compressor.

Low and ultra-low NOx burner protection: Burners at refineries are used for furnaces, cracking, gas turbines, etc. The advanced low and ultra-low NOx burners to meet environmental regulations in the United States have finer orifices and are more prone to fouling than the older designs, which can cause unacceptable increases in NOx emissions and maintenance costs. High efficiency coalescers can be installed directly upstream.

Natural Gas Processing Outlook

Global gas demand is projected to grow 2% on average between 2014 and 2020, according to the International Energy Association’s Medium-Term Gas Market Report, 2015. Lower prices will feed a pick-up in global natural gas demand over the next five years following the marked slowdown in 2013 and 2014.

Table 1 from the EIA Annual Energy Outlook 2013 shows U.S. natural gas production steadily increasing through 2020.

The U.S. gas production will continue to grow at a higher rate than some other more expensive alternatives such as subsea extraction. The movement from coal to gas fired power generation is another positive trend. The combination insures a growing market for filters in the oil and gas industry in the U.S. The growing demand for energy around the world also insures growth in the filter market elsewhere.