Physical Treatment Air Stripping (Section 9 – 1)
Volatility • Tendency to move from solution to gas phase • Function of: • Vapor pressure (VP) • Molecular weight (MW) • Henry’s constant (H) • Solubility (S) • etc.
Equipment • Spray systems • Aeration in contact tanks • Tray towers • Packed towers
Design of Air Stripping Column Parameters • Chemical properties • Range of influent flow rates, temperatures, and concentrations • Range of air flow rates and temperatures • Operation as continuous or batch • Packing material
Design, in General • Tower diameterfunction of design flow rate • Tower height function of required contaminant removal
Assumptions: Plug flow Henry’s Law applies Influent air contaminant free Liquid and air volumes constant Depth of Packing Design Equations
Depth of Packing • L = liquid loading rate (m3/m2/s) • KLa = overall mass transfer rate constant (s-1) • R = stripping factor • C = concentration
Stripping Factor (R) • Process: mass balance on contaminant • Initial assumptions: • Previous • Plus • dilute solution • no accumulation • no reactions • 100% efficient
Example: Removal Efficiency Calculate the removal efficiency for an air stripper with the following characteristics. • Z = 12.2 m • QW = 0.28 m3/s • H’ = 0.2315 • QA = 5.66 m3/s • KLa = 0.0125 s-1 • D = 4.3 m
Activity – Team Ethylbenzene needs to be removed from a wastewater. The maximum level in the wastewater is 1 mg/L. The effluent limit is 35 g/L. Determine the height of an air stripping column. The following data is available: • KLa = 0.016 s-1 • QW = 7.13 L/s • T = 25 oC • D = 0.61 m • QA/QW = 20 • T = 25 oC
KLa: Two-Film Theory CL CI PI PG Bulk Liquid Liquid Film Air Film Bulk Air
KLa: Transfer Rate • KLa (s-1) • KL = liquid mass transfer coefficient (m/s) • a = area-to-volume ratio of the packing (m2/m3) • Determination: • experimentally • Sherwood-Holloway equation • Onda correlations
KLa: Column Test • System • Small diameter column • Packing material • Blower • Pump • Contaminated water • Test • Range of liquid loading rates • Range of air-to-water ratios
Column Test Continued • Determining KLa • Plot sample (packing) depth vs. NTU (which varies based on Ce/Ci) • Slope = 1/HTU • KLa = L/HTU
Sherwood-Holloway Equation • L = liquid mass loading rate (kg/m2/s) • = liquid viscosity (1.002 x 10-3 Pa-s at 20 oC • = water density (998.2 kg/m3 at 20 oC) • , n = constants (next slide) • DL = liquid diffusion coefficient (m2/s) • Wilke-Chang method • B T/
DL: Wilke-Change Method • DL = liquid diffusion coefficient (cm2/s) • T = temperature (K) • = water viscosity (0.89 cP at 25 oC) • V = contaminant molal volume (cm3/mol)
Onda Correlations • Accounts for gas-phase and liquid-phase resistance • Better for slightly soluble gases • No empirical constants
Gas Pressure Drop • Physical parameter: describes resistance blower must overcome in the tower • Function of: • gas flow rate • water flow rate • size and type of packing • air-to-water ratio • Found from gas pressure drop curve
Example: Pressure Drop Figure Determine the air and liquid loading rates for a column test to remove TCE. The stripping factor is 5 when 51-mm Intalox saddles are used at a pressure drop of 100 N/m2/m. The influent concentration is 230 g/L and the effluent concentration is 5 g/L. The temperature is 20oC.
Preliminary Design • Determine height of packing • Z = (HTU) (NTU) • Zdesign = Z (SF) • Determine pressure drop and impact on effluent quality by varying air-to-water ratio (QA/QW) and the packing height (Z)
Activity – Team Determine the dimensions of a full-scale air stripping tower to remove toluene from a waste stream if the flow rate is 3000 m3/d, the initial toluene concentration is 230 g/L, and the design effluent concentration is 1 g/L. Assume that the temperature of the system is 20 0C. A pilot study using a 30-cm diameter column, 25-mm Raschig rings, a stripping factor of 4, and a pressure drop of 200 N/m2/m generated the following data. Depth (m) [Toluene] (g/L) 0 230 2 52 4 21 6 6 8 1.5
Design Procedure • Select packing material. Higher KLa and lower pressure drop produce most efficient design. • Select air-to-water ratio and calculate stripping factor or select stripping factor and calculate operating air-to-water ratio. • Calculate air flow rate based on selected gas pressure drop and pressure drop curve.
Design Procedure Continued • Determine liquid loading rate from air-to-water ratio. • Conduct pilot studies using gas and liquid loading rates. Develop NTU data from Ce/Ci, and calculate KLa. • Determine tower height and diameter. • Repeat using matrix of stripping factors.
Discharged Air • Recover and reuse chemical • Direct discharge • Treatment
Common Design Deficiencies • Poor efficiency due to low volatility • Poor effluent quality due to insufficient packing height/no. of trays • Poor design due to inadequate equilibrium data and/or characterization data • Inadequate controls for monitoring • Heavy entrainment due to no mist eliminator • Not sheltered so difficult to maintain in inclement weather • Lines freeze during winter shutdowns due to no drains or insulation
More Design Deficiencies • Tray Towers • Inadequate tray seals • Heavy foaming • Trays corroded • Packed Towers • Inadequate packing wetness due to poor loading and/or inadequate redistribution • No means to recycle effluent to adjust influent flow • Plugging due to heavy solids or tar in feed • Inadequate blower capacity
Physical Treatment Steam Stripping (Section 9 – 3)
Steam Stripping Design • Strippability of organics • Separation of organic phase from steam in decanter • Fouling
Rules of Thumb • Strippability • Any priority pollutant analyzed by direct injection on a gas chromatograph • Any compound with boiling point < 150 oC and H > 0.0001 atm-m3/mol • Separate phase formation • At least one compound with low solubility • Operating parameters • SS < 2% • Operating pressures as low as possible
Mixture A 37 mg/L methanol 194 mg/L ethanol 114 mg/L n-butanol Mixture B 37 mg/L methanol 194 mg/L ethanol 114 mg/L n-butanol 110 mg/L toluene 14 mg/L xylene Example – Feasibility Analysis
Common Design Deficiencies • High packing breakage due to thermal stresses • Heavy fouling due to influent characteristics & elevated temperature • Inadequate steam capacity • No control for steam flow • Dilute overhead product due to inadequate enriching section • Inadequate decanter to separate immiscible phase