What will be the sand filter specification for LPG service?
Answers
|
01/03/2011
|
A:
|
Lindsay McRae, Pall Corporation, Lindsay_McRae@pall.com
First of all, we would not recommend sand filters for LPG service. Their size is generally quite large, their capital cost is expensive, maintenance difficult, and their performance is relatively poor compared to other options such as a cartridge filter or Liquid Liquid coalescers which are widely used for removal of solids and free water from LPG respectively. Some refiners also report sand contamination downstream of the sand filter as well so a guard filter is usually required to protect downstream equipment, reactors or sorbent beds from fouling.
Typical specification for LPG service for particulate removal would typically be 2um absolute or 10um absolute @ 99.98% efficiency. (absolute rated filter having Beta ratio of 5000 or 99.98% efficiency at the stated particle size). 20um absolute rated filters are also used on occasion by refiners. We would recommend a flux rate appropriate to the TSS level present. In LPG service the TSS level is generally quite low so filter disposable filter element use is not generally excessive. (typical filter element life is 6 to 24 months).
For water removal from LPG, we would recommend high efficiency Liquid Liquid coalescers. Typical specification for water removal is typically <15 ppmv free water downstream of the coalescer. (which is clear & bright at ambient temperature). Coalescer performance can be measured using a modified AquaGlo test.
Where the dispersed phase to be removed is water & caustic or water & amine, the horizontal PhaseSep coalescer should be specified due to chemical compatibility, and its ability to separate even very stable emulsions with an IFT (InterFacial Tension) down as low as 0.5 dynes/cm. It should be noted that sand filters and other bulk removal devices such as KO pots / mesh pads, & plate separators lose efficiency when the IFT is <20 dynes/cm which is often the case when caustic or amine are present also. (IFT often around 5 dynes/cm). There has been several reported cases of sand filters downstream of caustic treaters not performing and failing to meet C3=, C4= specifications. Therefore their use is not regarded any longer by many as 'best practice'.
|