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How Do I Calculate Equilibrium Ratios (K Values) For The Isothermal Pt

vle phase equilibria rachford rice k values

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#1 Ahmad Abbasi

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Posted 10 July 2019 - 02:51 AM

Hi everyone

 

For calculating the vapour fraction and phase compositions of a hydrocarbon mixture at given Pressure, Temperature and overall compositions, the rachford-rice equation needs to be solved. 

 
Because of the absence of phase composition (x-y) information in the beginning, we need starting values for the equilibrium K Ratios, (since the K values are composition dependent) for initialising the rachford rice solution procedure. Upon subsequent computation of phase composition, K values are reevaluated using the new information and the procedure is repeated until convergence.
I know about the raoult's approximation for K values (vapor pressure/System pressure) and the wilson correlation.
But they don't seem to be very helpful, because it often happens that for the initial estimated values of the K ratios, no feasible solution of the vapour fraction exists, which should ideally be between 0 and 1.
So I would like for any leads which would help me set good initial values for K ratios to ensure robust evaluation of the vapour fraction


#2 breizh

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Posted 10 July 2019 - 03:19 AM   Best Answer

Hi ,

Let you consider the excel sheet attached where you can find several options to support your work .

hope this is helping you and others .

Breizh 

Attached Files



#3 Ahmad Abbasi

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Posted 10 July 2019 - 11:53 AM

Hi ,

Let you consider the excel sheet attached where you can find several options to support your work .

hope this is helping you and others .

Breizh 

Dear Breizh

 

I had a long and detailed look at your spreadsheet and compared it to what DWSIM outputs. Then I compared it to my code. It's a pretty good spreadsheet, only thing is it will not incorporate the new phase compositions to evaluate the new K values.

 

Anyway, it does what I wanted it to: give me a good way to initialise the rachford rice method and do one iteration with a reasonable output of vapour fraction (ie 0<= V<=1).

 

Thanks. It helped me validate my code.



#4 HARAB

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Posted 02 February 2021 - 05:48 AM

I would like to know about convergence pressure Pk ..assume that we have reservoir fluid composition (Pres, Tres, Zi are  Known) , and we would like to estimate Pb pressure (unknown), by using Rachford Rice equation(RR eq.) when we want to calculate the K values  we need the value of Pk, Is there any equation to calculate it direct with high accuracy  ,any example done by spreadsheet !!

 

‚ÄčKind regards 



#5 breizh

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Posted 02 February 2021 - 06:51 AM

Hi,

To get a meaningful answer I encourage you to define the variables included in your post ( Pb , Pk) .

Good luck

Breizh  



#6 SilverShaded

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Posted 04 February 2021 - 05:34 AM

I would like to know about convergence pressure Pk ..assume that we have reservoir fluid composition (Pres, Tres, Zi are  Known) , and we would like to estimate Pb pressure (unknown), by using Rachford Rice equation(RR eq.) when we want to calculate the K values  we need the value of Pk, Is there any equation to calculate it direct with high accuracy  ,any example done by spreadsheet !!

 

‚ÄčKind regards 

 

Typically you use rachford rice to calcuate the amount of vaporisation at a known temperature and pressure, you iterate around until the K values converge. (if your doing it rigorously).

 

To find the Pressue at a specific level of vaporisation you have another outer convergence loop, whereby you adjust P untl you get the bubble point/dew point or whatever other criteria your trying to meet.


Edited by SilverShaded, 04 February 2021 - 05:34 AM.


#7 breizh

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Posted 04 February 2021 - 06:51 AM

Hi ,

You may consider the excel sheet attached , in particular the methodology used 

 

You may also consider this resource  from internet :

https://www.questcon...oftware/canary/

Good luck

Breizh 

Attached Files






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