EXPERIENCE IN PHYSICAL SEPARATION DEVICES FOR ETHYLENE UNITS

Siang Hua Lee

Titan Petrochemicals (M) Sdn Bhd

 wahlooi@yahoo.com

Prepared for

15th Symposium of Malaysian Chemical Engineers

Johor Bahru, Malaysia

12th September 2001

 

Abstract 

As a naphtha cracker, we are used to several types of physical separation units especially at the quenching section and hot fractionation section. There are four types of physical separation devices currently apply in Titan crackers, i.e., liquid-liquid separation, particulate removal (filtration), 3-phase separation and gas-liquid separation.

Liquid-liquid separation

There are two types of liquid-liquid separation device used here, the first is using chevron baffle in water quench tower to separate RPG from quench water. This operation is based on “gravity separation” where the two immiscible liquid phases separate within the vessel by the differences in density of the liquids. The second type is applying “coalescing separation” in the oil-water coalescer to separate dissolved hydrocarbon in quench water. This is where small particles of one liquid phase must be removed from a large quantity of another liquid phase.

Filtration

Particulate removal is most common in both crackers. The application is only to separate solid particles from a fluid by passage through a porous medium. The most commonly used filter is metal perforations with wire mesh lining. All pumps had strainer at the suction piping to remove potential particles in the liquid. This is to protect the pumps casing and impeller from erosion. The design of filter can be T-type, Y-type, basket types or duplex basket depends on piping routing or cleaning frequency. Besides that, we have also cartridge filter made of fiber.

Gas-liquid Separation

The commonly used gas-liquid separator here is wire mesh mist extractor. Wire mesh pad is located at the top of drums or vessels as entrainment separator for the removal of very small liquid droplets. The pad is generally horizontal with the gas and entrained liquid passing vertically upwards. This we can effectively avoid caustic carry over from caustic scrubber and eliminate liquid droplets carry over to cracked gas compressor.

3-phase Separation

This apply when we have water, liquid hydrocarbon and hydrocarbon gases in the inlet stream. Two types of device used here, the first is vertical vessel with two compartment where oil will overflow from one compartment to another. The second is horizontal vessel with wire mesh pad as coalescer to separate oil and water. Water will than collected in a boot at the bottom part as a lower liquid barrel.

Conclusion

The theory for physical separation is very straightforward and the operation is easy and effective. The only major consideration for physical separation in cracker is monitoring of pressure drop and change out clogged cartridge or clean up strainer when pressure drop increases. The jobs seems to be easy, but if it does not carried out, a clogged filter will effect the down stream equipment severely. 

 

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