6 replies to this topic

Why is ethanol injected into gas compressors?

[Narayanan.u]

Why is ethanol injected into gas compressors?

Hi,

Probably to ensure no trace of moisture enters the compressor. Am i right?


Narayanan

[Art Montemayor]

Narayanan:

Why is it that you guess that ethanol (or any other alcohol) would prevent water from entering a gas compressor?

What gives ethanol the ability to bar water from flowing into a compressor?

I think what you mean to say is that ethanol might prevent the formation of methane or other hydrocarbon hydrates. Is that correct? However, if that were so, the water would still exist in the system - but not as part of a hydrate crystal.

[gvdlans]

Normally methanol or glycol are used as hydrate inhibitors, not ethanol.

[Art Montemayor]

You are absolutely right Guido.

However, some engineers who have a lot of ethanol around (like those in Petrobras - Brasil) like to use ethanol instead due to the local economy. Ethanol also inhibits hydrate formation - and less of it is wasted in the vapor phase since its vapor pressure is lower than that of methanol.

I'm still curious about the basis for the question raised. I wouldn't inject any thing other than the process gas into the suction of a centrifugal compressor if I could help it. However, there are some strange applications out there......

I hope the O.P. responds with some detailed data and explanation(s) for the question.

[Andrei]

Everybody is right; an alcohol injection will inhibit the hydrate formation.
A hydrate is a physical combination of water and organic molecules with a crystalline structure resembling ice, but different than ice.
An essential condition for a hydrate formation is to have the gas at or below the water dew point, or saturation condition. Hydrates do not form if water is vaporized.
The alcohol, having a higher volatility will decrease the water dew point of the gas. In other words the hydrate will form at lower temperatures than your normal operating.
Water is still present in the gas stream but in vapor form, and is maintained in vapor phase by partial pressure effect of the vaporized alcohol.
In most of the areas methanol or glycols are used as hydrate inhibitors. Ethanol I would expect to have a lower dosage need, since has a higher volatility.

[Zauberberg]

Interesting topic. According to many experiments and field experience, hydrates can form even in the case when water content of a gas is below saturation point at given pressure and temperature, and definitely they can form if only water vapor is present in the system.

Hydrate formation kinetics is also an important thing to be considered since reaction is highly time-dependent. A perfect example is the use of Twister technology for gas dew-pointing and dehydration: many applications worldwide are showing that hydrates cannot be formed even at conditions which are well below hydrate formation point - if gas velocity approaches sonic velocity, like it is the case in Twister tubes.

There are many publications about hydrates available on the internet, Gas Liquids Engineering website is one of good starting points. Still, what is not clar in this thread is the purpose of ethanol injection in compressor suction. Like Art said, injecting liquid in compressor suction - unless one is 100% sure that liquid will vaporize before it reaches compressor internals - is potential hazard and very dangerous situation.