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PRINCIPLES AND OPERATION OF GLYCOL DEHYDRATION UNIT
DESCRIPTION OF PROCESS AND EQUIPMENT
In the contactor, the up-flowing gas gives up water vapour to the glycol flowing down from the top tray. At the tower top, the dry gas passes through mist extractor elements and then leaves the contactor top to go on to other processes.
The mist extractor coalesces fine particles of liquid into large droplets which fall back into the glycol passing down the tower. In this way, glycol carry-over with the gas stream is minimised. See Figure: 26.
Referring to Figure: 48, the wet natural gas enters the bottom of the glycol contactor tower and rises through the column where it is brought into contact with the lean glycol flowing downwards across bubble cap trays.
Figure: 26 - Demister Pad Operation
The absorption of water vapour during the process, gradually dilutes (weakens) the glycol. The rich (dilute) solution collects in the bottom of the glycol contactor tower from where it is discharged to the glycol regeneration unit by way of a level control system.
At the regeneration unit, the wet glycol flows first to the glycol flash tank where a pressure drop takes place causing the dissolved gases to leave the glycol as it passes into the flash tank. (This is similar to opening a bottle of Pepsi for example. As the cap is removed, the gas bubbles out of the liquid).
Flash tank pressure is controlled by a PCV in the gas line. (The released gas may be piped to a flare or fuel system or may be passed into the still column). The rich glycol leaves the bottom of the flash tank under level control and passes through a 'Reflux coil' placed in the top of the still column. (This will be explained later).
After the reflux coil, the glycol is filtered and then passed through a double-pipe heat exchanger to be heated by the regenerated 'lean' glycol leaving the unit reboiler. (This in turn, cools the regenerated (lean) glycol).
The rich glycol now enters the still column at the top tray (below the reflux coil), and flows down across the bubble-cap trays.
THE REFLUX COIL
The cool, rich glycol as it passes through the reflux coil, picks up heat from the hot, rising gases passing up the tower from the reboiler. These hot vapours consist of water vapour (steam), entrained gases and glycol vapour.
The exchange of heat between the liquid in the reflux coil and the rising hot vapours causes the glycol vapour to condense and drop back down the still column.
Some water vapour will also condense but, as it drops back, it is re-vaporised on the top trays of the column. These liquids dropping down from the reflux coil form the internal reflux in the tower thus controlling the tower top temperature and therefore the final separation process. Above the reflux coil, the uncondensed vapour consisting of water vapour and entrained gases pass from the tower top to atmospheric vent stack.
Improper operation of the still column - excess vapour flow, fouling of the reflux coil, low flow rate of rich glycol through the coil .. etc, will result in glycol vapour remaining uncondensed and escaping to atmosphere causing glycol losses.
As the glycol flows down the tower across the contacting devices, the absorbed water is stripped out by hot rising vapours from the reboiler.
The glycol, as it collects in the still column bottom section, is now partially regenerated and is referred to as 'Semi-lean' glycol. It is then passed into the reboiler for final water removal.
Generally, the reboiler is of the 'Fire-tube' type and contains a 'weir' which ensures that the fire tube is completely immersed in glycol.
(In systems that have separate reboiler and still column, the weir also maintains the level in the still column bottom).
In the reboiler, the glycol flows over the weir and enters a stripping section containing Raschig rings or other contacting devices.
(Extra stripping action may be provided by an injection of dry stripping gas into the reboiler stripping section).
On leaving the reboiler, the lean glycol passes through the glycol/glycol exchanger into the glycol accumulator (or storage drum) from where the circulation pumps take suction and discharge the glycol back to the contactor via the glycol cooler, to complete the circuit.
(In some small field units, the still column may be a packed type and is usually an integral part of the reboiler).
(See Figures: 27 & 28).
Figure: 27 -Complete Glycol Unit -Absorption & Regeneration
Figure: 28 - Package type Glycol Unit as used in Field Locations
Figure: 29. Shows the 'Double-pipe, Glycol/glycol exchanger, in more detail.
Figure: 29 - Double-pipe Glycol/Glycol Exchanger
Figure: 30 - Types of Packing Used in Some Still Columns