13 replies to this topic

[Mayank Joshi]

Hi all !!
 I had a query , I have a process gas which has water vapour, the composition of gas is known the pressure of the gas is known , How do I calculate the dew point of the gas?Process Gas pressure is 21 Kg/cm2 gauge.

Composition:- vol basis

 

h2   78%

c02   15.4%

co    3.8%

ch4    2.8%

total condensate in process gas -8 m3/hr

Process gas dry basis flow rate- 34000 nm3/hr

 

I want to know at what temperature the moisture inside the gas will start condensing?

 

[srfish]

I believe one needs to know the temperature where 8 m3/hr condenses before the dewpoint temperature can be calculated.

[marchem]

as commented by PaoloPemi on another thread if you wish to calculate the phase equilibria for a certain component (in your case water) then you need to define the amount of that component in your mixture, you can calculate the dew point of your composition (H2 78% c02 15.4% CO 3.8% CH4 2.8%) at 21 Kg/cmq without water, but to include water you need to define the amount.

[Mayank Joshi]

By process condensate i mean water , the water in the gas is 8 m3/hr

[Mayank Joshi]

the 8 m3/hr water vapour is a constituent of the process gas ie the composition of the 21 kg process gas is :

h2   78%

c02   15.4%

co    3.8%

ch4    2.8%

h20 vapour - 8 m3/hr

 

the gas flow rate after condensation of water vapour (dry basis) is  34000 nm3/hr

[Pilesar]

Determine the partial pressure of the water vapor (mole fraction times total pressure). Find the partial pressure in a saturated steam table and read the corresponding temperature.

[Santoshp9]

Dear Mayank,

What Pilesar explained is right,You determine partial pressure of water vapor in your process gas by mole fraction & check with steam table the corresponding temp.

[jrtailor09]

Dear mayank Joshi,

 

Do u have hysys software ?

 

If yes then add said composition & temperature , pressure & flow rate.

 

You will get HC dew point in stream properties.

 

I have a excel file to calculate Acid dew point by 2 different method.

 

Regards,

Jatin

 

 

[Art Montemayor]

Jatin:

 

I believe the OP finally realized the error of his communications and stated, "By process condensate i mean water".

 

Therefore, he clearly is trying to calculate the water dew point at the process conditions and not the hydrocarbon (HC) dew point.  Bad communications cause a lot of confusion and extremely long threads.

[Mayank Joshi]

Thanks santoshp !!! 

[Santoshp9]

Dear Jatin,

I highly appreciated if you share your calculation sheet of acid dew point.

 

Regards,

Santosh

[Ajay S. Satpute]

Hi,

 

Did anyone get the answer?

 

I got -9 oC from hysys. Just need to cross check.

 

Regards.

 

Ajay

Attached Files

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[marchem]

Art is correct,
in your first post you say

Composition: vol basis
h2 78%
c02 15.4%
co 3.8%
ch4 2.8%
total condensate in process gas 8 m3/hr
Process gas dry basis flow rate 34000 nm3/hr

in the secon post

the 8 m3/hr water vapour is a constituent of the process gas ie the composition of the 21 kg process gas is :
h2 78%
c02 15.4%
co 3.8%
ch4 2.8%
h20 vapour 8 m3/hr
the gas flow rate after condensation of water vapour (dry basis) is 34000 nm3/hr

now in the first case the 8 m3/hr are of condensate (liquid phase) and in the second it's H20 vapour 8 m3/hr

in the cecond case you have the volume flows of all components at std. conditions and can calculate the fractions,
while in first case you can convert the 8m3/h of water in gas volume (at normal conditions) and calculate the fractions of each component in the mixture.

Of course for calculating the dew point you can utilize a simulator,
for example with Excel I utilize Prode Properties (you can download a free version for non commercial applications from Prode site)

However the basic steps for calcuting the dew point given a mixture with composition W are

estimate a initial temperature

calculate the fugacities of vapor Fv and liquid Fl phases (in a ideal model Roault's and Dalton's laws apply and Fv is the pressure in the system and Fl vapor pressure of liquid, for a real gas you should use a EOS or equivalent system)

calculate K values Ki=Fli/Fvi

for the dew point solve SUM(Wi/Ki) = 1
you may utilize the Excel solver to find the final temperature


for water dew point (when water forms a separate phase) for not much non-ideal mixtures you may follow the method proposed by Pilesar
"Determine the partial pressure of the water vapor (mole fraction times total pressure).
Find the partial pressure in a saturated steam table and read the corresponding temperature"

the limit of this method (as the ideal model described above) is that doen't consider the interactions of water with other components in the mixture.


a useful alternative to calculate individual dew points is the phase envelope which you can draw easily in Excel,
see

http://www.cheresour...-in-excel-2010/

with the phase envelope given a pressure and temperature you can easily identify the condition liquid, vapor or liquid + vapor and even identify if two or more liquid or solid phases can form.

Edited by marchem, 08 April 2013 - 03:51 AM.

[Mayank Joshi]

hi all 
I am sorry for creating the confusion here , my thread name was 

Calculating The Water Dew Point Of Process Gas

 

but in the description i wrote "How do I calculate the dew point of the gas?" although i specifically mentioned in the end "I want to know at what temperature the moisture inside the gas will start condensing?"

I had calculated the partial pressure of water vapour , and i was square rooting it twice ,( this gives pretty accurate results) i just wanted to know if there is a different formula for it.

 

Thanks Marchem for your detail post.