Gas Transfer in Aeration Processes: Definitions and Operations

1. GAS TRANSFER

2. DEFINITION AND TERMS • Gas transfer  a physical phenomenon, by which gas molecules are exchanged between a liquid and a gas at a gas-liquid interface  (1) an increase of the concentration of the gas(es) in the liquid phase as long as this phase is not saturated with the gas under the given conditions of e.g. pressure, temperature (absorption of gas) (2) a decrease when the liquid phase is over saturated (desorption, precipitation or stripping of gas)

3. DEFINITION AND TERMS • Important natural phenomena of gas transfer  the reaeration of surface water: (1) the transfer of oxygen into surface water (2) release of oxygen produced by algal activities up to a concentration above the saturation concentration (3) release of taste and odor-producing substances (4) release of methane, hydrogen sulfide under anaerobic conditions of surface water or of the bottom deposits

4. ELEMENTS OF AERATION AND GAS TRANSFER OPERATIONS • Gas transfer occurs only through the gas-liquid interface  has to be carried out as to maximize the opportunity of interfacial contact between the two phases. • The engineering goal  to accomplish the gas transfer with a minimum expenditure of initial and operational cost (energy).

5. ELEMENTS OF AERATION AND GAS TRANSFER OPERATIONS • Four different types of aerators: • Gravity aerators (a) cascades  the available difference head is subdivided into several steps (b) inclined planes  eqipped with riffle plates to break up the sheet of water for surface renewal (c) vertical stacks  droplets fall and updrafts of air ascend in counter current flow

6. ELEMENTS -- CASCADES

7. ELEMENTS – INCLINED PLANES

8. ELEMENTS – VERTICAL STACKS

9. ELEMENTS – VERTICAL STACKS

10. ELEMENTS – VERTICAL STACKS

11. ELEMENTS – AMMONIA STRIPPING

12. ELEMENTS OF AERATION AND GAS TRANSFER OPERATIONS (2) Spray aerators  the water is sprayed in the form of fine droplets into the air  creating a large gas-liquid interface for gas transfer

13. ELEMENTS –SPRAY AERATORS

14. ELEMENTS OF AERATION AND GAS TRANSFER OPERATIONS (3) Air diffusers (bubble aeration)  air is injected into water (a) through orifices or nozzles in the air piping system (b) through spargers (c) through porous tubes, plates, boxes or domes  to produce bubbles of various size with different interfacial areas per m3 of air.

15. ELEMENTS –AIR DIFFUSERS

16. ELEMENTS –AIR DIFFUSERS (POROUS TUBES)

17. ELEMENTS –AIR DIFFUSERS

18. ELEMENTS –AIR DIFFUSERS

19. ELEMENTS –AIR DIFFUSERS

20. ELEMENTS OF AERATION AND GAS TRANSFER OPERATIONS (4) Mechanical aerators  create new gas-liquid interfaces by different means and constructions  two types of construction: (a) various construction of brushes  a horizontal revolving shaft with combs, blades or angles (b) turbine or cone aerators with vertical shaft

21. Boyle’s Law

22. Charles’ Law

23. Gay-Lussac’s Law

24. Ideal Gas Law The ideal gas law is a special form of an equation of state, i.e., an equation relating the variables that characterize a gas (pressure, volume, temperature, density, ….). The ideal gas law is applicable to low-density gases.

25. Absolute Zero and the Kelvin Scale The pressure-temperature relation leads to the design of a constant-volume gas thermometer. Extrapolation of measurements made using different gases leads to the concept of absolute zero, when the pressure (or volume) is zero.

26. Kinetic Theory: Applications • Kinetic theory investigates (on a molecular scale) topics such as: • Change of phase (evaporation; vapour pressure; latent heat) • Pressure • Change of shape and volume (elasticity; Hooke's law) • Transport phenomena (diffusion - transport of mass; viscosity - transport of momentum; electrical conduction - transport of electric charge; thermalconduction - transport of heat) • Thermal expansion • Surface energy and surface tension

28. Kinetic Theory of Gases: Basic Assumptions • The number of molecules is large, and the average separation between them is large compared with their dimensions. This means that the molecules occupy a negligible volume in the container. • The molecules obey Newton's laws of motion, but as a whole they move randomly. 'Randomly' means that any molecule can move equally in any direction. • The molecules undergo elastic collisions with each other and with the walls of the container. Thus, in the collisions both kinetic energy and momentum are constant. • The forces between molecules are negligible except during a collision. The forces between a molecule are short-range, so the molecules interact with each other only during a collision. • The gas is a pure substance. All molecules are identical.

31. SOLUBILITY OF GASES • The solubility of gases in water (and also in other liquids) depends upon: (1) the nature of the gas generally expressed by a gas specific coefficient  the distribution coefficient, kD (2) the concentration of the respective gas in the gaseous phase  related to the partial pressure of the respective gas in the gas phase (3) the temperature of the water (4) impurities contained in the water

32. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • The higher the gas concentration in the gaseous phase  the greater will be the saturation concentration in the liquid phase • The relation between the saturation concentration cs (g/m3) and the gas concentration in the gas phase cg (g/m3): cs = kD . cg

33. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • The molar gas concentration in the gas phase (according to the universal gas law): (n/V) = p / (RT) (moles/m3) • Hence the corresponding mass concentration cg is obtained by multiplication with the molecular weight (MW) of the gas: cg = (p . MW)/ (RT) (g/m3)

34. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • The combination yields: cs = (kD . MW . p)/ (RT) • Henry’s law is generally written as: cs = kH . p • The relation between distribution coefficient kD and Henry’s constant: kH = (kD . MW)/ (RT)

35. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • Bunsen absorption coefficient, kb how much gas volume (m3), reduced to standard temperature (0oC) and pressure (101,3 kPa), can be absorbed per unit volume (m3) of water at a partial pressure of pO = 101,3 kPa of the gas in the gas phase : cs (m3STP gas/m3 water) = kb

36. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • And any other partial pressure p: cs = kb . (p/p0) (m3STP/m3) • Since 1 m3STP contains p0/R.T0 moles of gas and a mass of gas equal to MW. p0/R.T0 : cs = (kb . MW)/(R.T0 ) p (g/m3)

37. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • The relation between kD and kb: kb = kD T0/T • The interrelationship between the three coefficients: kD = kH .R.T/MW = kb .T/T0

38. INFLUENCE OF THE GAS CONCENTRATION ON SOLUBILITY • In the practice of aeration the gas phase will always be saturated with water vapor exerting a certain partial pressure pw the partial pressure p of the other gases are reduced  p’ = p . (P – pw)/P

39. INFLUENCE OF TEMPERATURE ON SOLUBILITY • Gases dissolved in water  accompanied by liberation of heat H • Le Chatelier principle  increase of temperature results in a decrease of solubility  van’t Hoff’s equation: [d(ln kD)/dT] = H/(RT2) where R = universal gas constant T = absolute temperature K H = change of heat content accompanying by the absorp- tion of 1 mole of gas (J/mole)

40. INFLUENCE OF TEMPERATURE ON SOLUBILITY • By integrating between the limits T1 and T2: ln[(kD)2/(kD)1]= (H/R)(T2-T1)/(T1.T2) • The product T1 .T2 does not change significantly within the temperature range encountered in gas transfer operations: (kD)2= (kD)1. econst (T2 – T1)

41. INFLUENCE OF IMPURITIES ON SOLUBILITY • Other constituent that may be contained in water influence the solubility of gases  expressed by an activity coefficient  : cs = (kD/).cg • For pure water  = 1   generally increases as the concentration of substances dissolved in water rises  lowering the solubility

42. INFLUENCE OF IMPURITIES ON SOLUBILITY • The influence of concentration of impurities cimp on the activity coefficient: for non-electrolytes log  = f . Cimp for electrolytes log  = f . I where f = a constant depending on the matter dissolved in water I = ionic strength of electrolyte

43. DIFFUSION • The phenomenon of diffusion  the tendency any substance the spread uniformly throughout the space available to it  in environmental engineering  diffusion phenomena the liquid phase in gas transfer operations

44. DIFFUSION • For a quiescent body of water of unlimited depth contacting the gas by an area of A  the rate of mass transfer dM/dt as a consequence of diffusion of the gas molecules in the liquid phase  Fick’s Law (dM/dt) = -D.A (dc/dx) (g/s) where D = coefficient of molecular diffusion (m2/s) x = the distance from the interfacial area A dc/dx = concentration gradient

45. DIFFUSION

46. DIFFUSION

47. DIFFUSION

48. DIFFUSION • The total amount of gas M (g) that has been absorbed through the surface area A during the time t  independent of x  under conditions of unlimited depth of water body

49. DIFFUSION • If the depth is not too small  the time of diffusion is not too long  diffusion is very slow process and only very little gas is brought into deeper layers of the water body:

50. THE CONCEPT OF GAS TRANSFER COEFFICIENTS