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

vle phase equilibria rachford rice k values

6 replies to this topic
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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

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

Brand New Member

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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

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 !!

​Kind regards

### #5 breizh

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

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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 !!

​Kind 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

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