8 replies to this topic

[JKM]

Dear All,

 

I need to estimate the vapor composition of the vapors coming from the methanol tank. 

 

Operating data are as below:

Op Pressure: 10 mmAq(almost Atmospheric Tank)=101.423kPa

Op Temp(max.): 40 degC

 

Tank is provided with N2 blanketing to keep the positive pressure in the tank. Vapor coming from the tank shall be fed to the Vent Gas Canister and then finally to the ATM.

 

In the vapor space it will be Nitrogen and Methanol Vapor. I would like to know the method to determine the composition of the vapor going to the Vent Canister. The Canister replacement frequency will depend on this composition.

 

My approach is as below:

Considering the Dalton law, I made this equation:

X*MeOH Vapor Pressure @ 40 deg C+ (1-X)*N2 inlet pressure to tank= 101.423

 

Where X= mole fraction of methanol in the vapor

Vapor Pressure of Methanol @ 40 degC=32.011 kPa

 

The problem is we have two variable (N2 Press. and Mole fraction) but have only one equation. So there are many possibilities.

Kindly let me know if my approach is correct or not. What will be 2nd equation to determine the both variables(mole fraction, N2 Pressure) correctly.  Also if there is any other method to determine the composition then let me know as well.

 

Regards

JKM

Edited by JKM, 26 January 2018 - 12:33 AM.

[breizh]

Hi ,

You forget the equilibrium relation between component in liquid and vapor phase .

If I'm not mistaken you should get :  Y methanol ( Vapor) = ( Pv methanol ) / P total    = ( 32.01 ) / 101.42

 

Hope this helps

Breizh

Edited by breizh, 26 January 2018 - 02:19 AM.

[JKM]

Hi Breizh,

 

Thanks a lot for your reply. However your equation seems not correct.

 

First it seems you are assuming only methanol in the vapor phase. No nitrogen is considered.

 

Your equation can rearranged as follows:

 

P total = Pv Methanol/(1-Y methanol vapor)

Which does not take into consideration of Nitrogen.

 

 

As per Dalton's law, Total Pressure should be:

 

P total= Y * Methanol Vapor pressure + (1-Y) * Nitrogen Pressure

 

However in above equation as I mentioned in my query, both Y and Nitrogen Pressure are unknown

 

Regards

JKM

[breizh]

hi ,

Do You agree that Pt =PN2+PCh3OH and  PCH3OH = XCH3OH *Pv CH3OH with XCH3OH =1 ?

then PCH3OH = Pv CH3OH thus YCH3OH = Pv CH3OH /Pt

 

Breizh

[JKM]

I would like to know why you are taking XCH3OH=1? This implies that in vapor phase there is no Nitrogen. However that is not the case.

This is true when there is no blanketing however there is blanketing so in vapor phase there shall also be nitrogen. So mole fraction can not be 1.

 

Do you agree?

[MrShorty]

Considering the Dalton law, I made this equation:

X*MeOH Vapor Pressure @ 40 deg C+ (1-X)*N2 inlet pressure to tank= 101.423

Is that truly Dalton's law, or have you combined Dalton's law with Raoult's law? Because this looks to me like the combination of Raoult's law/Henry's law with Dalton's law where X is not the vapor mole fraction, but the liquid mole fraction of MeOH.

 

Dalton's law: P(MeOH)+P(N2)=P(total) where P(i) is the partial pressure of i

Raoult's law: P(MeOH)=X(MeOH)*P0(MeOH) where X(MeOH) is the liquid mole fraction MeOH and P0(MeOH) is the vapor pressure of pure MeOH

Henry's law: P(N2)=X(N2)*H(N2)=(1-X(MeOH))*H(N2) where H(N2) is the Henry's constant of N2 in MeOH.

 

I expect that N2 is only sparingly soluble in MeOH (especially at these low pressures), so I would not be surprised if one chose to assume X(MeOH)=1 X(N2)=0 and reduce this to P(total)=P(N2)+P0(MeOH), but I don't know if you are wanting to make that assumption or not. I think this is what Briezh is doing when he assume X(MeOH) is 1 -- he is meaning X as liquid mole fraction and assuming no N2 dissolved in the liquid.

[breizh]

Hi ,

Right MrShorty .

Breizh

[JKM]

Dear Shorty and Breizh,

 

Thanks a lot for your explanation.

 

JKM

[Technical Bard]

Is the nitrogen flowing through the tank?  If so, you should also consider the mass transfer limitations at the surface.  Often, using the laws defined here, or a process simulator, will assume saturation in the vapour phase of the liquid component, and it is often the case if the nitrogen is flowing through that saturation is never achieved because mass transfer is limiting.  Trying using the equations for evaporation from a pond and diffusion coefficients (Kawamura and Mackay, 1985; Mackay and Matsugu, 1973) and Graham's Law to adjust from the equations that are generally made for water.