Deaeration tank theory 1

[Lasse Andersen]

Hey . I have not been able to find the needed information elsewhere so I will try here.

I wish to calculate the concentration of oxygen in seawater leaving a deaerator operating at a known temperature and pressure (less than 1 atm). If I can calculate the partial pressure of oxygen I can calculate the O2 concentration with Henry's law and correct it to apply for seawater with Sechenov constants. I think I can calculate the partial pressure like this using Dalton and Raoult's law.
(1) Ptot(T) = PO2 + PN2 + Pseawater,vap.(T)
(2) PN2 = 79/21 PO2

Is eq. 2 true for any pressure and temperature as it (almost) is for 1 atm? I can't prove it, but it seems "correct" for some reason..

Other methods of calculating this are welcome. I have also thought of making a full flash calculation, but it seems like too much work for this problem.

 

[Compositepro]

This has to be a student question. Deaeration is generally a non-equilibrium constant flow process. You seem to de talking about pulling vacuum on a pot of water. But, in any case, a Google search quickly shows your assumptions wrong:

Oxygen (O2) and nitrogen (N2) are the two most abundant gases in the Earth’s atmosphere and have generally similar physical properties, yet O2 is twice as soluble in water as N2, a feature that may have physiological and other biological consequences. Furthermore, examination of 47 other solvents shows that the mole-fraction solubility of O2 always exceeds that of N2, with O2/N2 solubility ratios ranging from 1.20 in n-dodecane to 2.31 in nitromethane. The greater solubility of O2 is especially puzzling since the molecular polarizability of N2, a feature normally associated with higher solubility, is greater than that of O2. Several theoretical and empirical approaches are explored in an effort to understand this observation: (1) molecular structure–property relationships, (2) thermodynamic analysis, (3) scaled particle theory, (4) ideal solution theory, and (5) quantum-chemical calculations. Speculations on the causes are offered.

 

[Lasse Andersen]

Thank you for your time and help. I really appreciate it. I am aware that it will probably never reach equilibrium, but my client cannot afford me setting up a hefty model. He asked me to make a graph showing him the potential O2 concentrations the deaerator can achieve if it works perfectly (like pulling vacuum on a pot of water ). I will make sure to let him know that he cannot fully reach the concentrations in the graph at the given pressure and temperature due to flow, dispersion etc. I'm sure I don't know all the reasons. This is my first time working with deaeration.

Regarding the different solubilities of O2 and N2, which is evident by the different Henry's law constants, does it change the ratio of partial pressures at different temperatures and pressures (ie. my original question) assuming equilibrium?

 

[Compositepro]

When the vapor pressure of water is greater than the total pressure in the tank, the water boils and eventually there will be no air left dissolved in the water. You can use a vacuum pump to decrease the pressure till the water boils or you can increase the temperature till the water boils. That is all that is important in a deaerator. Remember that the pressure increases with depth in water, so mixing of the water is also a minor factor. If the water is not boiling, it would be silly to call it a dearator.

 

[georgeverghese]

Eqn 2 would only be valid in an open system - ie open to free air. But this would not be valid in a seawater de aerator since water vapor would have driven out all the free N2 that was there initially before startup. Henry Law constants for O2 and N2 in plain water are found in tables 2-138 to 2-141 in Perry Chem Engg Handbook. Note that H in these tables is expressed in units of atm pressure. I dont have these corrections for H in seawater.

Answers you get from theory are never reached in actual practice in your bog standard offshore de aerators operating at partial vacuum. Operators are heavily reliant on O2 scavenger chemical dosing to reduce dissolved O2 concentration. Main reason for failure to reach equilibrium value is due to insufficient vapor traffic rising up the column to aerate the trays, which is a small quantity of almost all water vapor. Tray also get plugged with algal / microbial growth. Instruments used for measuring dissolved O2 in in the desired ppb range the field are also unreliable for many reasons. Hence why oil reservoirs running on water reinjection for EOR all begin producing H2S and / or get plugged up with algal/ microbial growth after some years. Shell developed a process scheme called MINOX to improve operations, and this depends on a high purity nitrogen steam recirculating in a closed loop to provide the additional vapor traffic in the de aerator. Spent N2 is regenerated by combustion of stripped O2 in a special oxidation reactor loaded with platinum ( or palladium??).

 

[Lasse Andersen]

Thanks for the help