Separation & Separators - Types of Separator - Liquid / Liquid Separators PDF Print E-mail
Written by Norrie   
Tuesday, 26 January 2010 08:23
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This type of separator is referred to as a 'Coalescer' and is used to separate two immiscible liquids like hydrocarbon and water emulsions. Tiny water droplets entrained in the hydrocarbon liquid would take a long time to separate out in a conventional separator.

The Coalescer vessel contains 'Filter' type elements, generally made of fibre-glass. As the mixed liquids pass through the elements, the heavier (more dense) water droplets are slowed down and stick to the fibre-glass surfaces of the elements where, as more droplets collide with them, they coalesce into larger drops and fall to the bottom of the vessel and flow into a 'Boot' in the vessel bottom.

The lighter hydrocarbon liquid rises to, and leaves by the top, of the Coalescer.

The Coalescer is operated 'Liquid Full' and, should any gases be released during the process, they are vented to flare or fuel system by automatic 'Vent-trap' systems in order to maintain the liquid-filled state of the vessel. (If gases were allowed to build up in the vessel, the liquid level in the vessel would be forced down by the gas. This would gradually decrease the efficiency of the Coalescer operation). (See Figures : 18 & 18A)

Figure 18: Coalescer

Figure 18a: Coalescer with Controls

LOW PRESSURE SEPARATION (Recovery of Naphtha-rich Gases)

After the 1st - Stage (High Pressure - HP) and 2nd - Stage (Medium Pressure - MP) separators (GOSPS), the liquids, (oil & water) still contain some heavy solution gases rich in Naphtha compounds – Propane, Butane & heavier.

The liquids are piped to further separation units to recover this heavy gas.

The first unit is called a 'Degassing Boot' where the liquids are decreased to a Low Pressure (LP) causing most of the gas to be released from the liquid and piped to a compressor station.

The liquids leaving the degassing boot, is finally passed via an Oil Boot into a 'Surge Tank' where the pressure is decreased to just above atmospheric causing most of the last traces of gas to leave the liquid as Very Low Pressure (VLP) gas that is then piped to a small compressor where its pressure is increased to that of the LP boot gas and added to it.

The total gas stream is then compressed further, cooled and the resulting condensate (C3 + Naphtha) is separated, metered and put to other processes. The lighter gases in the surge tank, the oil and water are also separated. (A demulsifying agent is added to the liquids upstream of the

degassing system to speed up the separation of the water from the oil).

The water is then pumped to a de-oiling station and drained away to a disposal pit.

The oil is metered and pumped to storage for distribution.

The following diagrams and picture show such a degassing system.

(Figures: 19 & 20, & photo)


Figure: 19 - Degassing System

Figure: 20

Last Updated on Wednesday, 24 February 2010 19:58