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High efficiency coalescers for gas processing operations

High efficiency coalescers reduce running costs and shutdown time by removing contaminants in feed streams to each gas processing step

THOMAS H WINES, Pall Corporation
SAEID MOKHATAB, Gas processing consultant
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Article Summary
Many different contaminants exist within the boundaries of gas processing operations. Contaminants can enter a process through poor inlet separation, can be created in the process by corrosion of iron pipelines/vessels, or can be formed by reaction with impurities and chemical additives. They can also come from phase changes such as condensation from gases or cooling of hydrocarbon liquids. Contaminants can be in the form of solid particles, sticky gels, difficult to break emulsions or fine liquid aerosols. These contaminants, even in trace quantities, will negatively affect gas processing plant operations where maintenance cycles are shorter than they should be, operating costs are higher than necessary, and unscheduled shutdowns causing lost production are all too common. As such, understanding the nature of these contaminants can help in developing an appropriate mitigation strategy.

Many in the industry have a hard time selecting the appropriate separation technology due to the amount of different separation equipment and varied rating systems used. Selection of appropriate separation equipment needs to be balanced by separation performance requirements versus capital and operating expenses. However, high efficiency liquid-liquid and liquid-gas coalescers, which provide excellent contaminant removal, have found widespread acceptance in gas processing plants in recent years for a number of applications.

Coalescer technology
Coalescers are designed especially to separate either liquid aerosols from a gas stream or to break 
liquid-liquid emulsions. They can be constructed from fibres made of various materials, including glass, metal, polymers and fluoropolymers. The coalescer media can be configured as pleated sheets or as a depth type, but have in common a pore gradient that goes from smaller to larger sizes in the flow direction and an outer coarse sleeve material to complete the coalescing process. A schematic of the coalescing mechanism showing the growth or coalescence of droplets as they pass through the coalescer medium is shown in Figure 1. In either liquid-liquid coalescers or liquid-gas coalescers, the same mechanism prevails and the coalescing medium is structured to have small pores on the inlet that are adjusted to larger size pores near the outlet, to accommodate growing drops. The coalescer will also remove solids in the first layer, but in many cases, especially liquid systems, it is more economical to have a separate vessel with pre-filters to protect the coalescers. Cartridge coalescers have the advantage that they do not require the use of chemicals or electricity to achieve the separation.
High efficiency
liquid-liquid coalescersCartridge coalescers have varying performances according to the type of coalescer cartridge used. Due to disarming of glass fibre coalescers in the presence of surfactants, ‘conventional’ cartridge coalescers are usually restricted to emulsions featuring interfacial tensions (IFTs) of not less than 20 dyne/cm typically. Disarming involves the coating of the glass fibre media with surfactants that change their surface properties which leads to repelling small incoming drops and failure 
to coalesce.1

High efficiency coalescers are particularly suited to handle 
emulsions featuring low IFTs (<20 dyne/cm) to very low IFTs (<5 dyne/cm). High efficiency coalescers are constructed with specially formulated polymer media with enhanced surface properties that can influence the adsorption of droplets and the ultimate release of coalesced droplets. Cartridge coalescers can accommodate relatively high (up to 5% typically) and fluctuating inlet dispersed phase concentrations.

These coalescer systems can either be vertically or horizontally configured. The horizontal configuration (shown in Figure 2 as configured for removal of water from a hydrocarbon stream) is the most common one. In this configuration, the coalescer consists of a horizontal coalescer cartridge stage followed by a settling zone that relies on the density difference between the dispersed phase and the bulk phase to separate the coalesced droplets. The coalescer cartridges are mounted on a tube sheet (filter plate) and supported by tie-rods.

The vertical configuration uses the same coalescer stage, while the separation stage is achieved by another type of separator cartridge with hydrophobic barrier capabilities to allow a hydrocarbon stream to go through while retaining the coalesced aqueous droplets. A vertical configuration is not suitable for very IFT of 
<3 dyne/cm due to the inherently fragile nature of the coalesced droplets that make the separator cartridge become ineffective.2

Although the solids content in the bulk phase is usually in the ppm range, it may represent a solids loading of several kilograms per day when scaled up to the full flow rate of the installation, hence the importance of eliminating this solid contamination to protect downstream equipment. Besides, particle contamination tends to increase the stability of the liquid-liquid emulsion.

Cartridge coalescers, although designed for the separation of two liquid phases, have the ability to act as particle filters due to the fine pore structure of the coalescer medium and will plug over time. A frequent replacement of coalescer cartridges is not cost-effective due to the costs of change-out, maintenance and used cartridge disposal. So it is usually recommended that a separate pre-filter is installed upstream of the coalescer to remove particulate matter. The installation of a pre-filter extends the service life of the coalescer significantly and reduces particulate concentration to meet solids specifications. The removal rating of the pre-
filter should be selected according to the pore size structure of the coalescer medium, as well as according to the size distribution of the solid contaminants. The particulate pre-
filter increases the investment cost of the cartridge coalescer solution, but overall reduces the running costs of the coalescer assembly.

High efficiency liquid-gas coalescers
High efficiency liquid-gas coalescers are recent developments in the history of liquid-gas separation units. Figure 3 depicts a vertical, high efficiency liquid-gas coalescer system. Inlet gas with liquid aerosol contamination enters at the bottom of the housing into a first-stage knock-out section. Here any slugs or large droplets (>300 μm) 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 0.2-50 μm and after passing through the coalescer medium it is transformed into enlarged coalesced droplets ranging from 500-5000 μm. The advantage of flowing from the inside to the outside of the coalescer cartridge is that the gas velocity can be more easily adjusted in the annulus by selecting the optimum housing diameter to prevent re-
entrainment of coalesced droplets.3

The use of a surface treatment on the entire coalescer can greatly improve performance by lowering the surface energy and improving drainage.4 In most cases, the vertical high efficiency liquid-gas coalescer does not require a separate pre-filter. Typical service life of standalone vertical liquid-gas coalescers is 1-2 years. The liquids will have a washing effect on solids collecting on the media and the surface treatment promotes drainage, thereby extending service life. For special cases, such as gas coming from underground storage, a pre-
filter would be recommended where high solids loadings exist.

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