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Chapter: Aquaculture Engineering - Aeration and Oxygenation

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Theory of oxygenation

Theory of oxygenation: 1. Increasing the equilibrium concentration, 2. Addition under pressure, 3. Gas transfer velocity.

Theory of oxygenation

When water is oxygenated pure oxygen gas is added; by this the saturation of oxygen in the water can be raised above, the equilibrium level of 100%. The following prossesses are included in oxygena-tion (cf. aeration): (1) increase of the equilibrium concentration, (2) increase of the gas transfer veloc-ity, (3) addition under higher pressure.

 

Increasing the equilibrium concentration

Henry’s law can be used to describe what is happening:

 

pi=Hxi

 

where:

 

pi=partial pressure of gas i H =Henry’s constant

xi=amount of gas i in the liquid (mg/l).

 

xis the value of interest here. The oxygen contentof air is 20% compared to a theoretical value of 100% for pure oxygen. The partial pressure (pi) when having pure oxygen atmosphere is therefore almost five times as high as when using an atmos-phere of air. Since Henry’s constant does not vary almost five times as much oxygen can be dissolved in the water by oxygenating in a pure atmosphere than when doing it in air. An example of this is the packed column aerator where the atmosphere inside is pure oxygen.

 

Gas transfer velocity

The same equation as for aeration can be used to describe the gas transfer:

 

dc/dt=KLA(C*C0)

where:

 

dc/dt= change in concentration per unit time, i.e.

 

velocity of gas transfer

KLA   diffusion coefficient

 

C* equilibruim concentration of the gas in the

 

liquid

C0  concentration of gas in the liquid at the start

 

point.

The equilibrium concentration C* for oxygen dis-solved in water standing in a pure oxygen atmos-phere will be higher than for water in normal atmosphere (see Section 8.3.1, Henry’s law). There-fore the velocity of gas transfer into the water is greater, because the difference CC is larger. Less time is then needed to increase the concen-tration of oxygen.

 

Addition under pressure

When using pure oxygen gas it is possible to increase the pressure and thereby increase the amount dissolved in the water. As distinct from adding air under pressure, there is no possibility for supersaturation of nitrogen which is toxic. Here the Dalton and Henry–Dalton laws can be used to describe what is happening:

 

pi=pt y

 

where:

 

pi=partial pressure of gas i

pt=total pressure of a mixture of gases

y mol fraction of gas i (mol gas i/mol total gases).

 

By increasing the total pressure in the oxygen gas above the water surface, the partial pressure also increases.

 

When combining the Henry and Dalton laws the following is obtained:

 

pt y =Hxi

 

where:

 

y mol gas i

 

pt=total gas pressure H =Henry’s constant

xi=amount of gas i in the liquid.

 

The oxygen concentrations in the liquid (xi) is of interest and will be increased by increasing the total pressure (pt) of the oxygen gas.

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