4 replies to this topic

[peaston]

Hi there. I am currently trying to carry out a Henry's law calculation and having some bother.


I am using the equation from "Introduction to Chemical Engineering Thermodynamics" by Smith Vas Ness and Abbot.

y_i.P = x_iHi

Where yi is the mole fraction in the vapour phase
P is the pressure in atmospheres
xi is the mole fraction in the liquid phase and
Hi is the henrys law constant / bar

I am needing to use this for carbon dioxide dissolving in water and solve for xi.

The textbook states that at 25degC and atmospheric pressure, Hi-1670 H/bar.

In order to work xi first, yi must be computed:

y2=P₂^sat/P

I don't know where to obtain a value for P₂^sat for carbon dioxide, anywhere i look seems irrelevant.

With y₂ you can solve for y₁=1-y<sub>2


Any help in obtaining the saturation vapour pressure for carbon dioxide would be greatly appreciated!!! Thank you.</sub>

[Shivshankar]

<sub>Peaston,

http://en.wikipedia.org/wiki/Carbon_dioxide_data</sub>

At T=25Deg, Psat=6401Kpa.


Regards
Shivshankar

Edited by Shivshankar, 07 May 2012 - 10:11 AM.

[clarenceyue]

Hi Peaston,

Firstly, I would like to make a comment that it would be best if you told us that water is component (1) and CO₂ is component (2)

I think the equation you propose to use for CO₂ is:

y_i = P_i^sat/P

This is incorrect especially for a sparingly soluble compound such as the system considered here. I believe what you really meant was that this equation is applicable to water. Since CO₂ is only sparingly in the liquid phase, a good first approximation is to assume that the mole fraction of water in the liquid phase is = 1.0.

Using this assumption as the basis, you can work out what is the vapour phase mole fraction of CO2 and then back calculate the liquid phase mole fraction. Using this value, you can repeat your calculation to check your initial assumption that the mole fraction of water is indeed very close to 1.0.

I hope this helps.

[bhatt_ms]

Hi Peaston,
the following relation wl help u to find mole fraction of CO2 in gas, provided the mixture is binary, and you know the saturation pr of both the pure components at 25 C.

Pa- saturation pressure(SP) of CO2
Pb- SP of other gas B
Pt- Total pr of system, which i assume is 101.3kPa.

ya is mol frac of CO2 in gas.

ya = (Pt-Pb)/(Pa-Pb)

this is derived from the fact that, sum of partial pressures of gases equals the total pressure of the system(Pt).

Eg, in a mixture of H2O & CO2,

VP of H2O at 25C ~ 3 kPa
VP of CO2 at 25 = 6401 kPa

Hence a saturated mixture of CO2 + H2O at 25C & 101.3 kPa will have 0.015 mole frac of CO2.

Mehul

[S.AHMAD]

Dear Peaston
1. You need to do iteration
2. Data required: Total system pressure (P), vapor pressure of water and Henry's constant at the temperature of interest.
3. Iteration steps
Step 1: Assume x, the mole fraction of CO2 in water.
Step 2: Calculate partial pressure of water ppw = (1-x) x vapor pressure
Step 3: Calculate partial pressure of CO2 = total pressure P - partial pressure of water
Step 4: Calculate x from the Henry's constant equation.
Step 5: Is assumed x equal calculated x? if yes stop iteration, else repeat step 1 using the calculated x.
4. The above steps can be formulated into a single formula:
P - (1-x)VPwater = xH

Edited by S.AHMAD, 10 May 2012 - 10:02 PM.