CENG 4710 Environmental Control. Air Stripping

Transcription

1 Air Stripping a mass transfer process passing air through water useful for removing low concentration (<200 mg/l) volatile organic compounds (VOCs) using packed towers, tray towers, spray systems, diffused aeration, or mechanical aeration the reverse of absorption 1 m = 3.28 ft 1 ft = m 1 US gallon = liters 1 UK gallon = liters 1 US gallon per minute (GPM) = m 3 /h 1 m 3 /h = GPM Density of water at 0 o C = 1000 kg/m 3 = 62.4 lb/ft 3 39

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3 Since the concentration in air stripping is low, Henry s law can be applied to describe the equilibrium between the gas and liquid phases, i.e. A H' C where A is the concentration in air, C is the concentration in water, and H is the dimensionless Henry s law constant. If the stripping tower is assumed ideal, the effluent air is in equilibrium with the inlet water, i.e. A H' Cin Furthermore, if the influent air contains no contaminant (Ain = 0) and the effluent water is free of contaminant (C = 0, 100% efficiency), the mass balance equation is QwCin QA( H' Cin ) Qw QAH' or H' ( QA / Qw) 1 The expression R=H (QA/ QW) is called the stripping factor. R > 1 stripping R = 1 equilibrium R < 1 absorption 41

4 Stripping Theory The transfer of a volatile organic compound from water to air follows the two-film theory covering mass transfer from: bulk liquid to liquid film liquid film to air film air film to bulk film An overall mass transfer coefficient, KLa (s -1 ) can be used to describe the transfer rate of contaminant from water to air. For design purpose, KLa should be determined experimentally. However, for dilute solutions, Sherwood and Holloway equation may be used: where K L a D L 1n L 305 DL 0.5 DL = liquid diffusion coefficient (m 2 /s), L = liquid mass loading rate (kg/m 2 s) = viscosity of water ( Pa s at 20 o C) = density of water (998.2 kg/m 3 at 20 o C), n = constants from Table 9-1 (p. 450) 42

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6 DL may be estimated using the Wilke-Chang method: T DL (cm 2 / s) 0. 6 Vm where T = temperature (K) = viscosity of water (centipoises, cp) Vm = molar volume of contaminant (cm 3 /mol) (Table 3-4, p. 97) 44

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8 Design equation Consider a section of the stripping Tower with a crosssectional surface area B, and a differential thickness dz, the mass transferred per unit volume of the tower is Q dc Q dc 3 M w w (kmol/s m ) where BdZ B dz Qw = liquid flow rate (m 3 /s) C = contaminant concentration (kmol/ m 3 ) B = surface area (m 2 ) Z = depth in column (m) DC/dZ = concentration gradient (kmol/ m 4 ) This mass transfer should be the same as that transferred across the air/water interface: M K a C L C eq 46

9 where Ceq = concentration in water in equilibrium with the air at a specified point. C - Ceq = the degree to which the system is of equilibrium Since the concentration is low in air stripping, Ceq = A/H where A = concentration in air (kmol/ m 3 ) H = dimensionless Henry s constant Hence, Qw dc KLaC Ceq B dz The liquid flow rate (Qw) can be replaced by the liquid molar loading rate (L) (kmol/s m 2 ): Qw L B M w where Mw = molar density of water = 1000 (kg/m 3 )/18 (kg/kmol) So, dz L M = 55.6 kmol/ m 3 = 3.47 lb mol/ft 3 M w w dc dz L K L dc a C C K L eq a C C eq 47

10 The required column height is L C dc Z in C M wkla C Ceq The first term is independent of C, and is called the height of a transfer unit (HTU): L Q HTU w M wkla BK La The integration part is dimensionless, which is designated as the number of transfer unit (NTU): NTU C C in dc C C eq Hence, Z = HTU x NTU Ceq may be determined from the mass balance from the bottom of the column up to the differential section: QAA Ain QwC C and A = Ceq H if Ain = 0, QACeqH' QwC -C Q C C C C w Ceq QAH ' R where R = stripping factor = H (QA /Qw). 48

11 NTU R R 1 C C C C in in R ( C ln R 1 dc C C C R d[( R 1) C C ( R 1) C C in / C C C in ] )( R 1) 1 R RC RdC C C This equation can only be used if the inlet air has no contaminant (Ain =0). Example 9-2 Preliminary design of air stripping column. A ground water supply has been contaminated with ethylbenzene. The maximum level of ethylbenzene in the ground water is 1 mg/l and this must be reduced to 35 g/l using an air stripping column. KLa = s -1 Qw = 7.13 L/s T = 20 o C H = 6.44 x 10-3 atm m 3 /gmol Select: Column diameter = 0.61 m Air-to-water ratio (QA/Qw) = 20 Determine: Liquid loading rate (L) Stripping factor (R) HTU, NTU, height of packing in column 49

12 Solution: H = H/RT = /( ) = Liquid loading rate: Cross-sectional area of column = R 2 = (0.61/2) 2 = m 2 mass rate = 1.0 kg/l x 7.13 L/s = 7.13 kg/s mass loading = 7.13/0.292 = 24.4 kg/(s m 2 ) L = (24.4 kg/(s m 2 ))(1000 g/kg)(1/18 mol/g) = 1360 mol/s m 2 2. Stripping factor R=H (QA/Qw) = 0.27x20 = Height of transfer unit: L 1360 HTU 1.53 m M wkla Number of transfer units: R ( C / C )( R 1) 1 NTU ln in R -1 R 5.4 (1000/35)(5.4 1) 1 ln transfer units 5. Height of packing in column Z = NTU x HTU = (3.88)(1.53) = 5.93 m = 17.7 ft Take a 10% safety factor, the column length used should be Z = = ft 50

13 Design Consideration Stripping tower: diameter = m height = 1-15 m QA/QW > 5 R = 2-10 or higher Flooding: as the air flow in a tower is increased, it will ultimately hold back the free downward flow of water. Channelling: this occurs when water flows down the tower wall rather than through the packing, use distribution plates at every 5 diameters to avoid this. Pressure drop: to avoid flooding, this should be N/m 2 m packing height (= inch H2O/ft) 51

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15 Example 9-3 Use the data in example 9-2 to determine the pressure drop through the tower and examine the impact on effluent quality of varying the air-to-water ratio (A/W) and the column height. Solution 53

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