hydrocarbon condensing 2

[kdashc]

is it true that if you have a hydrocarbon gas mixture and cool it down enough at constant pressure only certain gas components will start condensing (at their sat temp for their partial pressure) and not the entire gas mixture for a certain specific temperature?

i hope this makes sense.

 

[kdashc]

for example if i have 50% butane and 50% hexane (by volume) at 50psia and 200F the properties of hexane at these conditions are partial pressure of 25psi with a saturation pressure of 30.1psi and for butane partial press of 25psi and sat press of 193.7psi.

now if this gas mixture is flowing through a pipe with isobaric conditions and is cooled to 100F then the butane has a sat press of 51.8psi and the hexane has a sat press of 4.8psi.

Now since hexane has a partial press greater than sat press at 100F and 50psi then it will condense to a liquid untill the gas has a partial press equal to sat press.

And since this system is isobaric, butane's partial press will increase to 45.2psi since hexanes decrease. and now hexane's partial press is 4.8psi.

this leaves us with a final gas mixture composition of 9.6% of hexane and 90.4% of butane.

is my above reasoning correct???

 

[Compositepro]

Yes, your calculations are basically correct.

 

[dcasto]

Your next lesson will be with the Liquid / vapour ration refeered to as K values. The simple method is as you describe, but because nature is not simple, we need lab data to correct the equations.

tune in next time when we find out that a 50/50 liquid volume percent mixture of C4 and C6 is 100% liquid and that 50 gallons of C4 mixed 50 gallons of C6 doesn't make 100 gallons.

 

[25362]

K values are sometimes erroneously called vaporization equilibrium constants, but this is misleading since K is definitely not constant and depends not only on T and P but also on the composition of the mixture, meaning a parameter reflecting the interaction among molecules differing in size, type and quantity.

K values apply to non ideal solutions, and are the ratio of mole fractions in the vapor phase "y_i" to mol fractions in the liquid phase "x_i" for the component "i".
To get a better idea I suggest a visit to thread798-142881.

The ratio of K values, K_i÷K_j for two species "i" and "j", is called relative volatility ?_ij.

 

[siretb]

kdashc
Your calculations are correct, but are somewhat misleading.

with a 50/50 volume mixture at 50 psia the bubble point is 137°F. at 100°F you are below the bubble point, meaning that everything will condensate.
Yes the vapor fraction in equilibrium has about the composition that you indicate, but there is none of it.
At this pressure, as soon as your are below the temperature of 137°F, there is no vapor phase, in terms of quantity.

When you cool down from 200°F you will reach the so call dew point, at a temperature lower than 200°F but greater than 137°F, components will start condensing, and the first drops of condensate will have a compostion much richer in the heavy component (hexane) than the original feed mixture.

I hope this is clear to you.

 

[kdashc]

Thank you.