4 replies to this topic

[north]

Hi, my question is basicly this:

In a mixture of molecules (say H2, CH4 and C2H4), do the components all have their own partial pressure? And, if the former is true, do they affect eachothers boiling points?

I had some trouble formulating my question, and it might seem very simple, so I will post the background for it to make it easier to understand:
I work as an apprentice operator at an ethylene cracking plant. In our plant we have a few different distillation columns. In one of them, we inject steam into the column to reduce the partial pressure of the hydrocarbon feed. The reduced (partial) pressure decreases the boiling points (and increases the relative volatility i presume) of the hydrocarbon fractions (pyrolysis oil and gasoline) we want to separate. However, none of our other columns have any mention of the different molecules affecting the boiling points. Although I'm sure all the components have their own partial pressures, right? Why would this apply for a column with steam, pyrolysis oil and pyrolysis gasoline and not for a column with for example hydrogen, CH4, C2H2, C2H4 etc.?

I have tried asking our engineers, googling and searching in our archives but I can't find an answer, so this is my "last resort". I hope this forum is not only for Engineering students.

Appreciate all answers.

[latexman]

Each component has it's own partial pressure. This is a result of it's molecular motion (rotation, translation, vibration, etc.) at the temperature it is at.

Components do affect each other. Some have a strong affect, and some have a weak affect. This is a result of molecular interactions (hydrogen bonding, polarity, dimerization, etc.)

[rana680]

Yes everyone will have its own partial pressure and all will give some effect to the boiling point of others.

In case of the steam distillation the pressure in the column will be P_T=P₁+P₂

I am attaching a pic which shows vapor pressures of two immiscible compounds and their mixture as functions of the temperature.

In the figure pure A has a lower boiling point than pure B. It can also be observed from the figure that when A and B are mixed, the boiling point of the mixture (the temperature at which the total vapor pressure equals 760 Torr) is lower than the boiling points of both A and B. In steam distillation one of the components is always water, and thus the boiling point is always less than 100ºC.

Since a substance boils when the vapor pressure equals the external pressure i.e atmospheric pressure in our case , some extra vapor pressure is provided to the system by the vaporized steam.

Attached Files

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[north]

First of all, thank you for clarifying. Your figure was very helpful and easy to understand.

Yes everyone will have its own partial pressure and all will give some effect to the boiling point of others.

In case of the steam distillation the pressure in the column will be PT=P1+P2

Won't this be the case in columns without steam aswell? I'm assuming Dalton's law applies in all our columns (the highest pressure in our columns is 30 barg). That means that separating a light component that is a small mole fraction in the column is easier due to its low partial pressure (at the feed entry), and gets increasingly difficult as it progresses upwards in the column (gaining higher mole fraction, higher partial pressure and is more inclined to condensate while the opposite happenens to the heavier fraction). Is this one of the reasons that the VLE line (for example in McCabe-Thiele diagrams) gets closer to the x=y-line (diagonal line) when reaching a higher mole fraction, or is this unrelated or neglectable?

Sorry if I am way off base here.

[MrShorty]

In a mixture of molecules (say H2, CH4 and C2H4), do the components all have their own partial pressure?

Yes, each component has its own partial pressure.

And, if the former is true, do they affect eachothers boiling points?

It depends on what you mean by "each others boiling point." One compound does not change the pure component properties of another -- those are fixed. Adding a compound to a mixture will change the overall boiling point of the mixture (more precisely, we would use the term "bubble point" for a mixture), and will effect the partial pressure of other compounds in the mixture. To fix ideas, let's look at the simplest expression for this phenomenon: Raoult's law

Pi=xi*Pi0 (Dalton's law: Pbub=sum(Pi)) (Aside: For supercritical components like H2 and CH4, we might use Henry's law, which is essentially the same, only substituting the Henry's constant Hi for Pi0)

From these equations, we can see that the addition of a new compound to the mixture will affect the bubble point pressure depending on it's vapor pressure and the amount in the liquid phase. (It's easier to talk about bubble point pressures because we can solve the equations explicitly for pressure. Talking about bubble point temperatures is also possible, but it is difficult to impossible to solve the equations explicitly for temperature)

Raoult's law does not account for any intermolecular interactions. As mentioned above, those interactions are real, and can be quantified using the activity coefficient gi

Pi=gi*xi*Pi0

So now, changes in the composition of one component not only effect the composition x, these changes may also change g. In cases like a minimum boiling azeotrope as described by Rana680, the activity coefficients are large enough that the bubble point temperature of the mixture is lower than the boiling point of either pure compound.

I don't know if you have seen this before. If you have, I apologize for repeating it. If you haven't, then this should be a good introduction to VLE calculations.