3. AERATION, ABSORPTION AND STRIPPING HENRY’S LAW Gas Transfer Applications • Transfer of gas across a gas-liquid interface can be accomplished by bubbles or by creating large surfaces (interfaces). • The following are some common applications of gas transfer in treatment process. Gas Transfer Applications 1. Oxygen transfer to biological processes. 2. Stripping of volatile toxic organics (solvents). 3. CO2 exchange as it relates to pH control. 4. Ammonia removal by stripping. 5. Odor removal – volatile sulfur compounds 6. Chlorination, ozonation for disinfection and odor control. The materials of interest are soluble in water and volatile (i.e. they exert a significant vapor pressure). Equilibrium and Solubility: For such materials (soluble and volatile), there is an equilibrium established between the liquid phase and the gaseous phase if there is enough time allowed and if the environmental conditions are held constant. This equilibrium is usually modeled, for dilute solutions, as Henry's Law. Pressure and Solubility of Gases The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law). C is the concentration (M) of the dissolved gas C = kP P is the pressure of the gas over the solution k is a constant for each gas (mol/L•atm) that depends only on temperature low P high P low c high c 12.5 In general saturation goes up as T goes down and as TDS goes down. There are various forms of Henry's law because gas phase and liquid phase concentrations can be defined by different terms as shown in the following figure. Consider a closed gas-liquid system. PT, Pi, yi Gas phase CT, Ci, xi Liquid phase , where Pi = partial pressure of componentn i (atm) P = total pressure of system = Pi T i1 P y = mole fraction of component i in the gas phase = i i P C = conc. of component i in the liquid phase (mole/L3) i n C CT = total concentration = i i1 Ci x = mole fraction of component i in the liquid phase = i C Form of H Liquid phase Gas phase equation conc. conc. 3 atm m 3 mol/m atm PHC mol 3 3 dimensionless mol/m mol/m CHCg c l atm mol/mol atm PHXa dimensionless mol/mol mol/mol yi K H X i Some typical “H” values: gas Ha (atm) 4 O2 4 x10 He 12 x104 4 CH4 3.76 x10 4 CO2 0.14 x10 4 H2S 4.83 x10 Volatile Hc H compound Carbon 1.24 0.0304 tetrachloride Chloroform 0.15 0.00367 Vinyl chloride 1.14 0.0278 Henry's constants for various compounds are reported in a variety of forms so it's necessary to know how to convert between these forms. Here is a sample calculation to show how to convert between various forms of "H" Look at conversion between Hc (dimensionless) and H(atm-m3/mol). Conversions PHCl CHCg c l P V n R T (ideal gas law) R = 0.0821 atm-L/mol-oK = 0.0821 x 10-3 atm-m3/mol-oK @ 25oC T = 273 + 25 = 298oK n P Cg Hc Cl C V l H P V n R T nPHP c V RT H H 1 c H RT H 1 c H RT @ 25oC 11 40.82 RT atm m3 (0.0822 103 ) (298K) mol K H c 40.82 H Hc H Carbon 1.24 0.0304 tetrachloride H c 40.82 H Hc 40.82 0.0304 1.241 Conversions y Cg K i H H x c i Cl C P i g i C iL yi xi CPg, T T C LT C i g y C C KHi gT, . LT, HcC xCii L g, T CLT, Conversions nP C TT gT VRT. CC LT water g1063 cm 1 kg kg d 1 1000 water cm31 m 3 1000 g m 3 kg1 mole mole C 1000 5.56 104 water m318 10 3 kg m 3 Conversions CLT, KHHc . CgT, 5.56 1044 5.56 10 RT KHH.. H cP c T PT RT At 1 atm PT 4 KHRTHc (5.56 10 ) P P H H C a iL xi C x iL i C LT C P LT H a HPa H 4 Ci C L iL 5.56 10 C LT atm Ex: Oxygen HO 0.75 @ 20 C 2 mole/ m3 H a) H c RT 103L 0.75atm / mole / m3 3 H 1m H c atm. L RT 0.082 (20 273)oK mole.o K.