The present chapter is a part of presentation of conceptual and preliminary design review (PDR) of Dissolved Oxygen Extraction System (DOES). The chapter covers analysis of oxygen solubility in seawater and shows how seawater properties (temperature, salinity, pressure etc) influence on oxygen solubility and oxygen current concentration.
In the chapter we had collected information regarding to gases, in particular oxygen,
solubility in water and in seawater first of all.
Design of system using oxygen for underwater operations requires understanding of
physical land biochemical processes that governing by solubility of gases in water.
How much of gas can be dissolved in water? How does gas and water characteristics
affect on gas solubility? How is distributed dissolved gas in World Ocean? It is far from being full question list that have to be answered before beginning of DOES design.
We are interested in solubility of oxygen for depth up to 300 m. Gradients of seawater salinity (Halocline), temperature (Thermocline) and gradient of seawater density (Pycnocline) (as function of depth) are in the mention above range of depth.
Oxygen concentration in seawater is regulated by both physical and biogeochemical
processes. These processes provide mass balance of oxygen in seawater.
One of the processes taking part in dissolved oxygen mass balancing is so called “the carbon pump”. The “carbon pump” refers to the biologically produced flux of carbon out of the euphotic zone of the ocean. This process regulates to some extent the pCO2 content of the atmosphere and determines the O2 and nutrient content of the deep sea. The first step in the biological pump is the synthesis of both organic and inorganic carbon compounds by phytoplankton in the uppermost, sunlit layers of the ocean. Organic compounds in the form of sugars, carbohydrates, lipids, and proteins are synthesized during the process of photosynthesis:
CO2(gas) + H2O(liquid) + light → CH2O(solid) + O2(gas)
6CO2(gas) + 6H2O(liquid) + light → C6H12O6(aq) + 6O2(gas)
In this case, in the presence of light and available nutrients, plant material has formed from CO2 and H2O, and the carbon atom in CO2 has been rearranged into the carbon atom of CH2O and oxygen (O2).
On the other hand atmospheric gases penetrate into seawater by means of diffusivity. Oxygen solubility in seawater is controlled by Henry’s law.
It should be noted that atmospheric gases, excluding carbon dioxide, do not react with seawater. This fact gives possibility to apply successfully the Henry’s law.
The Henry’s Law had been used to define the equilibrium of oxygen between the fresh water surface and the atmosphere. Chemical equilibrium of a gas between gaseous and liquid phases can be represented as:
KeqO2 = [O2(aq)]/[O2(atm)]
Where: O2(aq)] – is dissolved oxygen concentration;
KeqO2 – is Equilibrium constant.
In according with the Henry’s law dissolved oxygen concentration [O2(aq)] can be
[O2(aq)] = KeqO2*pO2/R*T = KH*pO2
[O2(aq)] = βO2*pO2
Where: pO2 – is partial pressure of oxygen in atmosphere above water surface;
KH = KeqO2/R*T – is Henry Solubility coefficient;
βO2 = KH*22.414*T/273.15 – is Bunsen Solubility coefficient.
The Henry’s Law can not be used in the case of seawater directly. However, there are many different empirical investigations that suggest empirical equations modifying the Henry’s law and accommodating it to seawater features.
Oxygen solubility in seawater is often reported as empirical polynomial, in the following form:
ln(β) = a1 + a2*(1/T) + a3*ln(T) + S*(b1 + b2*T + b3*T2) (Weiss, 1970) or
ln[O2] = a1 + a2*(100/T) + a3*ln(T/100) + a4*(T/100) + S*[b1 + b2*(T/100) + b3*(T/100)2]
Where: β – is Bunsen Solubility coefficient
This equations are nothing more than a convenient polynomial functions that fits the water temperature and salinity as reasonably well dependence to given gas solubility.
In seawater chemistry this equation reported as Weiss’s empirical equation (1970).
Weiss’s semi‐empirical equation had been used to calculate dissolved oxygen concentration at equilibrium of oxygen between the seawater (ocean) and the atmosphere. It had been calculated the loss of dissolved oxygen in a water after oxygen leaved the ocean surface as well.
It should be noted that different states of atmosphere and seawater subject to latitude of World Ocean and season entail different solubility of atmospheric gases, including oxygen, in the different places of the World Ocean. Some maps showing horizontal and vertical distribution of dissolved oxygen in seawater are presented in the chapter.
DISSOLVED GASES IN SEAWATER
Atmospheric Gases Solubility in Sea Water
All of the atmospheric gases are found in solution in seawater. In addition to nitrogen
and oxygen, the most abundant gases in the air, carbon dioxide is present in large quantities in seawater, chiefly combined as carbonates and bicarbonates. As for rare gases, such as ammonia, argon, helium, and neon they had been founded in seawater as well. Hydrogen is undoubtedly present in minute quantities. In the absence of dissolved oxygen, hydrogen sulphide may be present, and it is possible that in stagnating water other products of putrefactive decomposition, such as methane, may occur .
Many different factors influence on solubility of gases in seawater.
We are interested in solubility of main atmospheric gases such as oxygen, nitrogen and carbon dioxide in seawater. However, first of all we are interested in oxygen.
What factors influence on solubility of the atmospheric gases in seawater?
- Physical characteristics of seawater [Salinity; Temperature; Pressure (depth)].
- Biological activity, which markedly affects the concentrations of oxygen and carbon dioxide.
- Currents and mixing processes, which tend to modify the effects of biological activity through mass movement and eddy diffusion.
It should be noted that different values of stated below parameters subject to latitude of World Ocean and season entail different solubility of atmospheric gases in these places.
|1.||NTRODUCTION. PUMP SELECTION SCOPE||4|
|2.||OXYGEN SOLUBILITY IN FRESH AND SEAWATER||27|
|2.1.||OXYGEN SOLUBILITY IN FRESH WATER||29|
|2.2.||OXYGEN SOLUBILITY IN SEAWATER||37|
|2.3.||OXYGEN MASS BALANCE||62|
|2.4.||DISOLVED OXYGEN CONCENTRATION MEASUREMENT||69|
|3.||NITROGEN SOLUBILITY IN FRESH AND SEAWATER||74|
|5.||ATMOSPHERIC GASES SOLUBILITY IN FRESH AND SEAWATER||80|
- Title – DISSOLVED OXYGEN CONCENTRATION IN SEAWATER.
- Classification – UNCLASSIFIED Author – Bakst A.
- Issue Data – 2016‐08 Updated – 2016‐08
- Pages (Slides) – 132 (including Cover and References);
- Cover – 2 slides;
- Tables – 29;
- Figures (pictures, graphs) – 48
- References – 30
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