Mininum gas velocity for glycol injection
Mininum gas velocity for glycol injection
(OP)
Which is the minimum gas velocity required in natural gas exchangers (gas-gas/chiller) to provide adequate glycol distribution and good hydrate inhibition?
Is it velocity the correct parameter or others may be used
(e.g. ro.v2)?
Thanks
Is it velocity the correct parameter or others may be used
(e.g. ro.v2)?
Thanks





RE: Mininum gas velocity for glycol injection
Normally, natural gas is dehydrated at the production site (offshore or onshore) using TEG dehydration prior to sending it to downstream equipments for further processing.
Methanol injection is done to protect the piping from the gas well to the gas processing facilities. Glycol is seldom used or heard of for this function.
Perhaps, you may want to consider building a gas dehydration facility to remove most of the water prior to sending it for further processing.
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RE: Mininum gas velocity for glycol injection
Adequate glycol injection is usually accomplished by ensuring you have proper dP across the glycol spray nozzles and the design of the spray nozzles themselves to distribute the glycol across the face of the exchanger inlet tubesheet. Gas velocity is not usually intended to redistribute poor glycol distribution.
The gas flow and the temperature drop across the exchanger will give you how much water will condense. Simulators or the Hammerschmidt formula will give you the required 'rich' glycol concentration coming out of the exchanger. A mass balance around the exchanger on the glycol side will give you the required flow rate based on the lean glycol concentration your regeneration system will give you (typically about 80% for this type of a system I believe). A significant safety factor is commonly applied to make sure you have enough glycol to prevent hydrates.
RE: Mininum gas velocity for glycol injection
I agree with you that glycol distribution is acomplished by good Dp in spray nozzles and correct spray design (e.g spray angle)
I want to know if a minimum gas velocity/ro.v2 inside the tubes is required to avoid glycol settling (in the tubes).
What about a maximum gas velocity/ro.v2?
thanks
RE: Mininum gas velocity for glycol injection
RE: Mininum gas velocity for glycol injection
Jim Maddocks, P.Eng.
jmaddocks@gasliquids.com
RE: Mininum gas velocity for glycol injection
I developed an Excel spreadsheet for designing EG injection systems. I used it to redesign the EG injection systems in 4 of our facilities with 100% success. We successfully lowered EG injection rates from 5-10gpm to 2-3gpm.
RE: Mininum gas velocity for glycol injection
As a result the velocity should be high enough to ensure a good, even distribution of liquid onto and down the metal surfaces and low enough to prevent the glycol film from remaining in the gas phase or being re-entrained.
RE: Mininum gas velocity for glycol injection
RE: Mininum gas velocity for glycol injection
And here's a few rules of thumb...
(1) Try to put about 80% of the glycol that you inject at the front end of the exchanger train. If there is a front end split, divide the amount injected evenly between each exchanger.
(2) When you select a nozzle try to get a very finely atomized pattern. Spend some time and calculate the distance the nozzle needs to be from the tube sheet so that the tube sheet is fully covered by the spray pattern. Try for about 100 psid (the nozzle mfg shuld be able to give you cone angle and should want to know what the nozzle delta P will be). Don't overlook downstream pressure drop in the delta P calc. By that I mean exchanger pressure drop. If you have exchangers in series the downstream exchanger will operate at some pressure that is lower than the upstream exchanger. Simple yes but easily overlooked.
(3) When you install the nozzle try to install it in such a way that it is easily removed. Nozzles wear out. Also, install a block valve before the nozzle piping enters the vessel and then a vent valve downstream of the block. If the nozzle plugs up you can sometimes shut it in and open the vent valve to allow gas to flow backward and unplug the nozzle. Sometimes that works and sometimes it doesn't.
(4) Pay attention the the rich return stream composition. Specifically it's freeze point. Try optimize the lean injection rate to result in a return stream that is at the very lowest point of the curve.
(5) Don't over circulate!
(6) Pay attention to the residence time of the 3-phase separator boot. I like to have no less than 45 minutes. Doing so will minimize the amount of hydrocarbon entrained in the return steam. This will cut down on reboiler load and stack emissions to some degree.
(7) Don't over Cicrculate! :D
RE: Mininum gas velocity for glycol injection
Years ago I worked for an E&C firm. A customer had purchased some heat exchangers and wanted us to build a gas plant with them (a long time before PSM). The design inlet gas rate of the gas plant was 15 MMscfd. The exchangers came out of a 50 MMscfd plant. We tried to get the customer to at least let us plug some tubes in the exchanger (to increase tube side velocity) but they would not allow it. To keep the story short, we had 2 MMscfd for startup, it was a 950 psig design and we had 700 psig. Glycol stacked in the system, and freezing problems occured. The customer drilled the nozzles out and tripled the glycol injection rate which worsened the problems. Eventually the gas rate came up and we put new nozzles in. Once that happened the problems all but went away. Years later I came to work for that company (still here) and completely revamped the glycol injection system in 5 of our plants. The first thing I did was to cut the circ rate drasticly and do a rigorous analysis of the injection nozzles and their installation. Glycol losses dropped significantly. Freeze-ups stopped occuring. The reboiler firing rate dropped significantly. All in all things were better.