This sounds like a flash calculation more than a distillation calculation and is directly performed by thermodynamic servers made available via CAPE-OPEN.

Implementing such an algorithm in a stable and efficient manner from scratch is not straight forward and has been the subject of many publications in literature. If you are restricted to two phases (vapor and liquid) and you are sure to be between the dew and the bubble point (so that you are sure that both exist) it becomes a lot easier. There are literally dozens of algorithms that do this.

Phase equilibrium conditions at specified T and P are such that

- all fugacities of all compounds in the two phases are equal
- the vapor compositions add up to unity
- the liquid compositions add up to unity
- the amount of any compounds in the vapor added to the amount of the same compound in the liquid must equal the overall amount of the compound (compound balances, aka phase summation equation)

You can use simply a Rachford-Rice approach (https://en.wikipedia.org/wiki/Flash_evaporation), or the approach sketched by Michael Michelsen, "The isothermal flash problem, part II, phase split calculation" (Fluid Phase Equilibria 9 (1982), pp 21-40). If you are sure you need to track over a constant temperature line between a dew point and bubble point, a continuation method comes to mind.

But you will still need to have the (log) fugacity coefficients of all compounds for the vapor and the liquid phase.

Best wishes,

Jasper.


On 8/1/2013 09:51, Dang, Christophe wrote:
Hello,
the following message was sent to me, so I post it back on the list so it can find the right audience.

Regards

%%%%%%%%%% Original message %%%%%%%%%%

The variation of flash vapor/liquid composition for various temperatures chosen 
between the dew point and bubble point temperature of the liquid feed.

-----Original Message-----
From: "Dang, Christophe" <christophe.d...@sidel.com>
Date: Wed, 31 Jul 2013 18:06:43
To: sandy_...@yahoo.com<sandy_...@yahoo.com>; International users mailing list for 
Scilab.<users@lists.scilab.org>
Subject: RE: [Scilab-users] (no subject)

Hello,

De la part de Eze-Okoli Ifeoma Sandra
Envoyé : mardi 30 juillet 2013 17:36

Good day, i would like to know how to design a multi-component flash
distillation system using scilab
This seems to be a physical/chemical engineering problem.

Scilab performs calculations, but you decide which calculations.

So, first decide what you have to calculate and how, and then we might be able 
to help you.

Best regards

--
Christophe Dang Ngoc Chan
Mechanical calculation engineer

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