I have an oil sample with 10 components and a gas mixture. For both of them we know the composition. We also know the pressure and temperature. For my calculation, first, I consider a molar ratio of oil to gas, for instance 2:1. Then I can calculate the initial (feed) composition {Zi}. And at the pressure and temperature of the system I do flash calculation and I calculate the equilibrium composition of vapor and liquid phase. When I change the molar ratio of oil to gas, for instance 1:2, at the constant pressure and temperature, the answers change! I can not understand this because If we consider our system at the temperature of the system, only one (specific) composition of liquid can produce the specified equilibrium pressure of the system. So the initial ratio should not affect the results of equilibrium calculations. But I do not understand why these results are affected by the initial ratio.
There is a mistake in your consideration: for a constant feed rate, temperature and pressure defines l/v ratio, determined according mass, energy and equilibrium. When you change the feed composition you are changing the entanpy of the feed, due this difference and thus affecting mass and energY and equilibrium balances
If the feed composition is different, then as per phase rule, for a hydrocarbon mixture of more than 2 components, at same temperature and pressure, the two phase vapour and liquid composition and fractions will be different
You simply changed the composition of the stream. Therefore, your results changed. Please let me know if you want me to write the equations. You increased lighter components and decreased heavier ones by increasing gas to oil ratio and thus everything changed.
Hope this helps and all the best,
You can see a magical trick in it. You take 50:50 mole percentage mixture of Methane and ethane and find out two phase composition at 130 K and 1 bar. Now you take a 75:25 mole percentage mixture of Methane and ethane and find two phase composition at 130 K and 1 bar. You will observe that the two phase compositions for these two binary mixtures at same temperature and pressure will be same. So what you are asking will be valid for binary mixtures but not for mixtures with more than two components.
You can see a magical trick in it. You take 50:50 mole percentage mixture of Methane and ethane and find out two phase composition at 130 K and 1 bar. Now you take a 75:25 mole percentage mixture of Methane and ethane and find two phase composition at 130 K and 1 bar. You will observe that the two phase compositions for these two binary mixtures at same temperature and pressure will be same. So what you are asking will be valid for binary mixtures but not for mixtures with more than two components.
Your proposed solution is correct but ONLY for binary mixtures. This is just a problem of phase rule.
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