4 replies to this topic

[scottishchemenger]

Hello,

 

I need help estimating the amount of moles vapor in a KOH/water solution at 30 wt% KOH, 80 degrees C and 1 bar.

 

I have found Antoine coefficients for KOH solutions at the specified temp and concentration.

 

Do you think this with the modified raoult's law with activity coefficients would suffice for making a good estimation?

 

Thanks,

 

ScottishChemEnger

[breizh]

Hi,

Consider this handbook to support your work , you will find process data.

Good luck 

Breizh 

[MrShorty]

I'm not sure I understand exactly what you are wanting to calculate. In my applications, I have always assumed that total pressure (which is what I assume your Antoine equation would represent) would be the same as water's partial pressure and assume that the partial pressure of KOH is 0/insignificant. The only reason I could see for bringing Raoult's law/activity coefficients into this is if I didn't want to assume KOH's partial pressure was 0.

 

In sum, the way I would usually calculate something like this would be to use the Antoine equation to calculate the partial pressure (Pi) of H2O at 80 C, assume Pi(KOH)=0, since the total system pressure is 1 atm, I assume there are inert gases present, so I would ask myself if it is worth the effort to estimate how much gas is dissolved and how the dissolved gas will impact the partial pressure of water (at 1 bar, I would tend to be lazy and assume any inert gases are insoluble enough that they have no significant impact on the Pi of H2O).

 

It all depends on exactly what assumptions you want to bring to this calculation.

[scottishchemenger]

Thank you Breizh, that document will be useful to me.

 

Mr Shorty, 

 

First of all, I appreciate your response.

 

The mixture is contained in an electrolyser, where I have no inert gases present. There may be some dissolved oxygen that comes from the anode side of the electrolyser, but this will be about 0.5%vol and I am assuming this has no effect. I could assume KOH partial pressure to be 0, but from NEL Hydrogens design schematic of their electrolyser, they have a scrubber to remove 'residual' electrolyte and I need to try and access how much KOH passes through. My colleague is designing the scrubber and we has estimated values but for the design I am aiming to be as accurate as possible.

[MrShorty]

It seems to me that, if there are really no inert gases present, then either the 80 C must be in error or the 1 bar must be in error, or the system is not at equilibrium or something. If you really only have a binary system of KOH and water, then a pressure of 1 bar seems too high, or a temperature of 80 C seems too low.

 

The beginning point of calculating the amount of KOH in the vapor will be the vapor pressure of KOH. Using DIPPR's equation for liquid KOH (melting point 679 K). I get values around 10^-16 mmHg. Assuming nothing else, this means the maximum vapor concentration I would expect for KOH would be 10-16/740 or 10^0-16/200 (depending on which total pressure we use, Briezh's link suggest a vapor pressure of 200 mmHg at 175 F/80 C) which is a concentration on the order of 10^-18 mole fraction. Adding in that the KOH is diluted in water and the overall activity coefficient of KOH should be less than 1, and the actual concentration should be less. Of course, who knows how much error there is in the long extrapolation of the vapor pressure, and I really don't know what the activity coefficient of KOH is, how high do we think the concentration of KOH could be? 10^-10 mole fraction? 10^-5 mole fraction?

 

For the most accurate number, we would need to choose a reasonable VLE/thermo model, put together the needed inputs and parameters, then run the calculation. I don't know what VLE model you like for this. Have you thought about your desired VLE model?

 

Are we certain that the scrubber is there to catch KOH in the vapor phase? Could it be there to capture some form of entrained liquid (misting, atomization, splashing, plant disruption, other method that allows the liquid phase to become entrained in the vapor phase, but not actual vapor)? KOH losses due to entrained liquid (if such is possible) would almost certainly be more significant than losses due to vaporization of the KOH. Did the designer even consider the actual concentration of KOH in the vapor when installing the scrubber? It could be that the designer, knowing that the KOH vapor concentration could not be exactly 0, simply put the scrubber in there to catch the tiny amount of KOH in the vapor without really thinking about just how much KOH would be present in the vapor stream?

 

A lot of talk that didn't say anything, but maybe something there will suggest a next step for us.