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Jul-2012

Removing fines from gas scrubbing liquids

Thin cake candle filter technology can be applied to a range of refining and gas processing applications to remove fines from circulating scrubbing liquids

Barry Perlmutter
BHS-Filtration
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Article Summary
Treating acid gas streams at refineries, gas plants, cogeneration facilities and chemical manufacturing plants is becoming more important as stricter environmental rules are being developed. Further, removing CO2 for carbon sequestration is also being adopted as a control measure for greenhouse gas. One critical aspect of these processes is the solid-liquid filtration technology that is used to keep the recirculating fluid free of solid contaminant fines.

The need for filtration systems in scrubbing units has been well documented for keeping downstream equipment functioning properly and ensuring maximum scrubbing/removal efficiency. This article discusses thin-cake candle filter technology that is used to remove trace amounts of solid contaminant fines from the recirculating fluid streams in amine, glycol and water scrubbers. These contaminants originate from various sources and are generally less than 1 micron in size, which makes their removal very difficult. Candle filter technology, and the process of thin-cake building, is a new approach for the highly efficient and cost-effective removal of fines. 

This article begins with a definition of the problem and a discussion of the bench-top laboratory tests that are conducted for problem analysis, technology selection and scale-up. The tests include pressure, filter media, temperature and viscosity concerns, precoat material and similar process parameters. The candle filter technology is examined for its ability to filter and dry the contaminants to meet the standards for solids disposal. Cake washing is also discussed, as, in some cases, soluble or insoluble components or hazardous solvents must be removed from the solid cake prior to disposal. The article concludes with a review of typical case histories and installation details.  

Sulphur dioxide (SO2) and carbon dioxide (CO2) removal systems are installed in many types of applications such as fluid catalytic cracking (FCC) and coker units at refineries, coal-fired power plants, acid plant tail gas, spent acid recovery plants, smelters, pulp mills, natural gas, power generation, cogeneration, chemical process plants, as well as process vent streams in sulphur plants. While the plant location and type of removal process may vary, there is a need to remove the fine particulate matter from the recirculating liquid streams for efficient scrubbing operations. The particulate matter can be catalyst fines and other contaminants in the feedstock. It is difficult or impossible to remove particles of this size in settling tanks, hydrocyclones or centrifuges, so the particles must be removed by filtration. The use of thin-cake candle filter technology has been proven to be a cost-effective and reliable approach to removing the contaminants, recovering the scrubbing liquids and drying the cake for easy landfill disposal.

Defining the problem
Various catalyst and carbonised particles are carried into the gas and are captured by the scrubbing fluid. The flow rate of the scrubbing fluid can be as high as 200 m3/h, so the fluid must be regenerated and reused. The fine particles are less than one micron and will accumulate in the scrubbing system as well as foul the ion-exchange resins that are used to remove the heat-
stabilised salts (HSS). The scrubbing fluid can be amines, glycols, water or some other proprietary fluid. This article discusses specifically the removal of fines from the scrubbing fluids and provides four illustrative case histories.

Technology of clarification and recovery of slurries
Candle filters are installed for clarification and recovery applications from liquids with a low solids content. They provide clean fluids to 0.5 micron, with either a dried cake (no free liquids) or a concentrated slurry.

Description and operation of the  candle filter
The BHS candle filter provides for thin-cake pressure filtration, cake washing, drying, reslurry and automatic discharge in an enclosed pressure vessel. Candle filter systems are available for up to several hundred square metres of filter area.

Filter candles and media
The filter candles (see Figure 1) consist of three components: a single-piece dip pipe for filtrates and gas, a perforated core with outer support tie rods and a filter sock media. The filtrate pipe is the full length of the candle and ensures high liquid flow as well as maximum distribution of the gas during cake discharge. The perforated core can be a synthetic material, stainless steel or alloy and is designed for the full pressure of the vessel. The outer support rods provide for an annular space between the media and the core for a low pressure drop operation and efficient gas expansion of the filter sock for cake discharge. Finally, the synthetic filter media has a removal efficiency to less than 0.5 microns.

Filter vessel and candle registers for dry cake discharge
The vessel is constructed of 
stainless steel, alloy or carbon steel lined. Within the vessel are candle registers. Each candle is connected to a register with a positive seal to prevent bypass. Each register may contain from one to 20 candles depending upon the filter size. The registers convey the liquid filtrate as well as the pressure gas for filter media expansion. Each register is controlled with automated valves to ensure an optimum flow in both directions. Figure 2 illustrates the candle filter vessel.

Automatic process cycles
•  Filling The slurry feed enters the bottom of the filter vessel
•  Filtration The slurry is pumped under pressure into the vessel. Cake deposits on the outside of the candle; the separated filtrate flows through the filtrate pipe and the registers. This process continues to a maximum pressure drop, the maximum cake thickness or the minimum flow
•  Washing Displacement washing or recirculation washing
•  Drying Blowing gas, steam or shock drying
•  Cake discharge Gas flows through the register pipes and down the filtrate pipe. The filter media gently expands, allowing for cake discharge.
Alternatively, the cake can be discharged as a slurry via a tubesheet design (see Figure 3).
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