15 replies to this topic

[Sulaiman Idrees]

Dear Members,

 

Can anybody tell me that is it possible to control the dew point of Natural gas through injection of some chemical..? 

 

Thank you in advance.

 

[PaoloPemi]

the dew point represents the condition of incipient liquid formation in an initially gaseous mixture,

if , in a natural gas mixture, you add heavy components, at given pressure the dew point temperature will increase,

if you remove heavy components at given pressure the dew point temperature will decrease,

non hydrocarbon components as water, glycols, salts etc. have their own fugacities and will change the equilibrium conditions,

 

Unfortunately it is not easy to predict the inlfuence of each component because it interacts with the other components in mixture.

 

in those cases I prefer to generate phase envelopes for different compositions and observe the variations,

see for example

 

http://www.prode.com...aseenvelope.htm

[Art Montemayor]

Sulaiman:

 

Paolo is addressing the Hydrocarbon dewpoint in natural gas - the temperature at which the first hydrocarbon (above methane) starts to condense within the mixture.

 

Are you - by any chance - inquiring about the WATER dewpoint being controlled by the addition of a glycol?   If so, you should be specific and state which dewpoint you are referring to.

[Himanshu Sharma]

Dear All !

 

May be i am off the track here,but the confusion amongst HC dew point and Water Dew point generally prevails as most of the commercial simulators provide Water Dew Point if you are calculating dew point of a HC Mixture.(for HC mixtures i have encountered ,water tends to condense first )

 

Situation may  get out of hand if you are calculating dew point of an acid flue gas for the prupose of heat recovery (and get water dew point indeed ) !

 

Reagrds

Himanshu

Edited by Himanshu Sharma, 06 September 2013 - 07:57 AM.

[PaoloPemi]

You are correct.

 

There are many different definitions.  However I prefer to consider only one dew point, which by definition it is the point of incipient condensation for mixtures of hydrocarbons + water.  If water begins to condense before hydrocarbons, the water dew point coincides with the mixture dew point,  Additionally, water condensate can form

(depending on the conditions) a separate phase in a liquid-liquid equilibria.

 

Of course, in natural gas processing plants the point where water begins to condense has importance for the different effects it can produce - as on corrosion etc. and we can call this point different names if that gives us a better understanding of the process.  

 

Anyway, as discussed in previous post, you can calculate vapor-liquid, liquid-liquid etc. equilibrium lines with a phase envelope utility that will show you where the different phases (if more than one) begin to condense.

[curious_cat]

You are correct.

 

There are many different definitions.  However I prefer to consider only one dew point, which by definition it is the point of incipient condensation for mixtures of hydrocarbons + water.  If water begins to condense before hydrocarbons, the water dew point coincides with the mixture dew point,  Additionally, water condensate can form

(depending on the conditions) a separate phase in a liquid-liquid equilibria.

 

I like your one dew point approach but the "water dew point" term confuses me. What exactly does that mean?

 

Say, you had a benzene + toluene + xylene mixture of a given composition entirely in gaseus phase. This has a unique dew point. But does it make any sense to refer to benzene dew point or xylene dew point seperately? 

 

Or in a water + EtOH + MeOH gaseous mixture are there two distinct "dew point" & "water dew point" terms? I think not. 

 

What exactly does "water dew point" mean in a natural gas context?  

[PaoloPemi]

usually it means the point where the water begins to condense,

that point could be different from the dew point of mixture (see above)

it is questionable whether to call it water dew point or give a different name...

[curious_cat]

usually it means the point where the water begins to condense,

that point could be different from the dew point of mixture (see above)

 

I find that hard to understand. If the water in a sample begins to condense at a certain point, that is the dewpoint of the sample you started with. 

 

Unless,  by convention the "dew point of the mixture" means  "dew point of a  corresponding  hypothetical mixture on a water free basis"

[Padmakar Katre]

the dew point represents the condition of incipient liquid formation in an initially gaseous mixture,

if , in a natural gas mixture, you add heavy components, at given pressure the dew point temperature will increase,

if you remove heavy components at given pressure the dew point temperature will decrease,

non hydrocarbon components as water, glycols, salts etc. have their own fugacities and will change the equilibrium conditions,

 

Unfortunately it is not easy to predict the inlfuence of each component because it interacts with the other components in mixture.

 

in those cases I prefer to generate phase envelopes for different compositions and observe the variations,

see for example

 

http://www.prode.com...aseenvelope.htm

Hello.

I think the effect will be reverse, the dew point temperature will decrease if you add heavy components. Bubble point temperature will go up after addition of heavies. This is basic science, on heating high volatile will start vaporizing first and then low volatile will follow whereas on cooling heavies will condense first and then lighters.

[curious_cat]

I think the effect will be reverse, the dew point temperature will decrease if you add heavy components. 

 

Nope. I think you are wrong and what @Paolo wrote earlier is correct. 

 

" you add heavy components, at given pressure the dew point temperature will increase,"

 

If you remain unconvinced I can try to explain my logic. 

Edited by curious_cat, 19 September 2013 - 12:49 AM.

[Padmakar Katre]

Hello,

Good to know someone challenging my basics. Please go ahead and explain me how it works with an example of VLE.

Please note that I am presuming the gas flow conditions are same i.e. P,T and if you add heavies in the mixture you will have some condensations at the same temperature, so my reply is in context of the thread. In general I am also of the same opinion that the dew point temperature will increase as you add heavy components in the gas mixture.

 

What we do in dew point depression unit in gas transmission systems is we remove heavies to control the dew point i.e. by any mean if gas encounters a lower temperature at any point in transmission network, there should not be any condensation.

Edited by Padmakar, 19 September 2013 - 01:23 AM.

[curious_cat]

Hello,

Good to know someone challenging my basics. Please go ahead and explain me how it works with an example of VLE.

Please note that I am presuming the gas flow conditions are same i.e. P,T and if you add heavies in the mixture you will have some condensations at the same temperature, so my reply is in context of the thread. In general I am also of the same opinion that the dew point temperature will increase as you add heavy components in the gas mixture.

 

What we do in dew point depression unit in gas transmission systems is we remove heavies to control the dew point i.e. by any mean if gas encounters a lower temperature at any point in transmission network, there should not be any condensation.

 

I am now confused by what exactly you mean. If you say that (a) "dew point temperature will increase as you add heavy components in the gas mixture." then we are all in agreement. 

 

OTOH if you go with your previous statement that ( b ) "the dew point temperature will decrease if you add heavy components. " then we are not in agreement. 

 

Obviously both (a) and ( b ) cannot be simultaneously true? Unless you think this specific example is an exception to your more general rule? 

 

I agree with the rest of what you write. 

Edited by curious_cat, 19 September 2013 - 02:03 AM.

[Padmakar Katre]

Hello,

Statement "b" is in context of the thread i.e. addition of heavy components will not help to solve the condensation issue, as addition of heavies will make the gas to condense at given constant P & T. If I understand the thread correctly, Sulaiman is asking about addition of some chemicals to solve the issue of dew point and in reply by PaoloPemi suggested to add heavy HC. This will not work as your transmission gas P,T is fixed. As soon as you add any heavy molecule, the gas phase will result in two phase mixture; hence I commented that this will not work or not a right way.

 

I am sure Sulaiman's question has nothing to do with VLE, but he has concern of condensation in gas transmission network at constant P & T.

Edited by Padmakar, 19 September 2013 - 02:41 AM.

[PaoloPemi]

Padmakar,

with all the due respect, your comment

"Sulaiman is asking about addition of some chemicals to solve the issue of dew point and in reply by PaoloPemi suggested to add heavy HC"

is not true, please kindly quote,

thanks,

Paolo

[Padmakar Katre]

Dear Paolo,

It's my mistake, I misunderstood your comments.

[curious_cat]

I think we are all on the same page now!