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Heita
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Join Date: Mar 2007
Posts: 35

Water Dew Point Depression in Dehydration with TEG

07/22/2009 6:32 AM

Hi friends

I am studying the process units of a plant. There is something I don't understanding with the Dehydrating unit with TEG! This is what it says:

"The most common dehydration method used for natural gas is the absorption of water vapor in the liquid desiccant, Triethylene Glycol. The wet gas is brought into intimate contact with lean dry glycol in the tray or structured packing section of an absorber tower where water vapor is absorbed in the glycol thus depressing the water dew point."

Can somebody please explain how does the water dew point depress when water vapor is absorbed in the glycol?

Thanks
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Pathfinder Tags: dehydration Depression dew point TEG
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Milo
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#1

Re: Water Dew Point Depression in Dehydration with TEG

07/22/2009 4:22 PM

I'll try. The higher the dew point, the greater the "relative humidity" or water content." You have to understand that first. The more water there is in the gas , the more saturated, the closer the temperature will be to the gas temperature where the moisture can condense out..

When the gas is saturated with water, it doesn't need to have a low temperature to condense. Think of a cool drink on a humid day- the humidity in the air condesnes instantly. Even without ice.

On a dry day, when the air is not saturated, your cold drink doesn't have a puddle around it....and unless you load it with ice, you may not get much condensation.

Because the glycol removed the humidity from the gas, it is less saturated, and so it will take a much colder (lower) temperature to initiate condensation (dew).

In this case, the lower dew point temperature is the proxy measure for "how much moisture remains in the gas."

Hope that helps.

milo

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ky
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#2
In reply to #1

Re: Water Dew Point Depression in Dehydration with TEG

07/25/2009 1:33 AM

Well done Milo. GA, Ky.

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Heita
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#4
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Re: Water Dew Point Depression in Dehydration with TEG

07/26/2009 7:16 AM

Hi Milo

I get your point. But can you tell me why it says depression of water dew point? Isn't it the dew point of the gas that depresses by removing water via TEG?

Thanx
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DeltaT
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#3

Re: Water Dew Point Depression in Dehydration with TEG

07/26/2009 2:09 AM

By definition, dew point is the temperature at which water vapor can condense out of the air. When there is a low amount of water vapor present in the air (low humidity), the molecules are much scattered and the entropy change dS associated with condensing them (bringing them closer together from their originally random state) is a larger negative value. Hence, dG (Gibbs' energy change associated with the transformation) is higher and the process is less spontaneous. That's why we don't have dew on dry grasslands. Hence, T needs to be lowered. So, when Etglycol takes out humidity, dew pt. is lowered. dG=dH-TdS; dG<0 for spontaneity.
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Milo
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#5
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Re: Water Dew Point Depression in Dehydration with TEG

07/26/2009 10:16 AM

Nicely explained!

milo

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