Dehydration tower inlet gas temperature. 1

[I2P]

Presently we are maintaining 50 deg C temp of natural gas at inlet of our dehydration column. The design temp. is 55 degC.Till date we were keeping it 50 to be on the safer side for better water removal through TEG. I would like to know the impact on TEG regeneration system, lean and rich TEG concentrations of increasing the gas inlet temp from 50 to 55 degC.
As I understand it, the extent of water removed by the lean TEG will decrease as relative to 50 deg C at 55 deg C the absorption will decrease.But this would also mean that the rich TEG would also be less loaded with water which would translate into lower reboiler heating requirements and consequently lower fuel gas(fired heater type) consumption.So long as the water content of the sales gas is within the specifications I think operating the coulumn at 55 deg C seems to be ok, since it can lower TEG reboiler heat duty and entail fuel gas savings.
Could anyone please validate this scenario, or share his experience of dehydration operations. To share with all, the water content specs is 112 Kg/mmscm, and at 50 deg C inlwet gas temp, the water content observed in the sales gas is 30-35 Kg/mmscm.
Thanks in advance

 

[Chemeng1122]

General practice for TEG dehydration is to maintain a ~5C temperature difference between the inelt gas and the lean glycol. The reason for this is to prevent any condensation of hydrocarbons from the vapor phase into the liquid phase in the contactor. So this means that with 55 C gas, your lean TEG should be about 60C.

Depending on how well you are regenerating the TEG, you may have a hard time getting a low enoughj dewpoint. With a 98.5% lean TEG, you can expect about 45C of dewpoint depression (give or take). If you can get a 99.5% TEG, usually with stripping gas, you can see 60C of dewpoint depression. If you have saturated gas at 55C, the lean temp being 60C, with a 98.5% TEG, the dewpoint of the sales gas would be in the ballpark of 15C, with a 99.5% TEG, the sales dewpoint could be 0C.

Based on the water contents you gave, it does not appear that you are dealing with saturated inlet gas, so the above may or may not apply.

As long as you can meet sales gas specs with the 55C inlet temps, you can run at these conditions safely. I have seen in the past, operations compensating for higher temperatures by increasing circ rates, which consumes more energy than low circ rates with low temperatures.

I dont understand your comments about the TEG being less loaded at the different temperatures. As long as the inlet gas water content, TEG circ rate, and degree of regeneration is the same, then you will see identical loadings of the rich glycol.

If you really want to save on fuel gas requiremetns in the reboiler, I suggest you optimize circ rates in the system.

Andrew Lechelt
Technical Support Engineer
Quadra Chemicals

http://www.quadrachemicals.com

 

[I2P]

Andrew,
Thanks for your reply. For your information our inlet gas is saturated. Checking with Mc Ketta curves it reveals the saturation dew point temperature to be 45 deg C, at a pressure of 60-65 bar and inlet water content of 90 lb/mmscf.
Regarding my commnet on higher loading of TEG what I meant was that since absorption is a function of temperature, wouldnt operating at 50 and 55 deg C make a difference to the water absorbed by TEG for the same pressure? I think it would absorb less water at 55 deg C than it would at 50 degC. As far as methane absorption goes, it definitely must be significant enough to warrant studies on minimising methane loss along with BTEX losses. I have personally read a presentation on the STAR program of EPA of USA, wherein they have touched upon this methane loss minimisation as a part of regeneration unit optimisation
regards

 

[jcaiken]

The comment about the equilibrium water absorption of glycol being a function of temperature is correct.

However, cooling the saturated gas to 50°C gives a water partial pressure of about 0.1234 BarA as opposed to 0.1574 BarA at 55°C. Only cooling the TEG to 55°C is equivalent to an increase in the water content of 25%, with a similar effect on the reboiler duty (ignoring the additional TEG circulation required for the increased water load).

This is far more significant than the effect of the temperature on the TEG/water equilibrium on the operation.