We can calculate the mass balance of a flash tank using our thermodynamic graphics! We can use what we call: the lever rule! We will explain what is the lever rule through a simple example.
In this example we are dealing with a Toluene-bencene mixture in order to make it simple to understand. F stands for Feed, with Za and Zb fractions of toluene and bencene. V stands for vapor with Ya and Yb fractions of toluene and bencene respectively and L stands for liquid with its respective molar fractions.
We now perform the overall mass balance:
And now the species balance:
Substituting “nv” with the equation for the overall mass balance we obtain the following equations, which will also be solved fo nf:
Therefore, when measuring the distance between the molar fractions of each component we can obtain the ratio of entrance and liquid outlet moles. This is what we call: the lever rule
References
Course Genie. (2010) Chemical interactions. The lever rule. Retrieved on November 22nd 2016 from http://www.chm.bris.ac.uk/~chdms/Teaching/Chemical_Interactions/page_25.htm
Course Genie. (2010) Chemical interactions. The lever rule. Retrieved on November 22nd 2016 from http://www.chm.bris.ac.uk/~chdms/Teaching/Chemical_Interactions/page_25.htm