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Unformatted text preview: DG 111013 31 Oct 2001 APPENDIX D EXAMPLE AIR STRIPPING BY PACKED COLUMN
. x L,
ai L y G,
ae G HTU HTU Z HTU x L,
ae L y ai G, G = 0 Figure D1. Random "dumped" packed tower. D1. Parameters.
NTU = number of transferunits HTU = heightof transfer unit[m] Z = NTU HTU packingdepth[m] kgmole 2 m sec kgmole G = molargas(air)flow perunitofstrippercrosssectionalarea m 2 sec kgmole xai = molefraction of contaminant a in liquid (water) influent kgmolewater L = molarliquid(water)flowperunitofstrippercross  sectionalarea D1 DG 111013 31 Oct 2001 xae = molefraction of contaminant a in liquid(water) effluent yai yae kgmole kgmolewater kgmole = molefraction of contaminant a ingas(air)influ ent kgmoleair kgmole = molefraction of contaminant a ingas(air)effluent kgmoleair ( xai  xae ) L = ( yae  yai ) G which is moles of contaminant a transferred from liquid to gas per unit of stripper crosssectional area per unit time (kg mole/s) xai  xae G y y = L ae ai kgmole air kgmole water which is the molar ration of gas (air) to liquid (water), and assumining uncontaminated influent air:
yai G L = 0 = xai  xae y ae where x ai and L are field measurements and x ae is imposed by ARAR, and pTe = total pressure of gas (air) effluent (atm) Pae = partial pressure of contaminant a in gas (air) effluent (atm). From Dalton's Law of partial pressures:
yae = pae p Te mole or atm mole atm p ae = y ae pTe [ atm] at equilibrium from Henry's Law:
p ae = H a x ai [ atm] substituting yields:
D2 DG 111013 31 Oct 2001 yae pTe yae = = H a x ai H a xa i p Te [ atm] [ atm] and from the material balance: ( xai  xae ) L = G Again substituting gives y ae mole mole ( xai  xae ) = G L H a x ai p Te xai  xae xai Ha = G L pTe The fraction of contaminant transferred from liquid (water) to gas (air) phase is:
Cai  Cae Cai = xai  xae xai where Cai = concentration of contaminant a in liquid (water) influent [g/L] Cae= concentration of contaminant a in liquid (water) effluent [g/L]. For convenience, the flows of water and air are measured volumetrically
Cai ( L) QL = Cae (L) QL + Cae (G ) QG and
Cai  Cae Cai = H 'a p Te QG Q L D3 DG 111013 31 Oct 2001 where pT e is measured as a fraction of the standard atmosphere (atm), H'a is the dimensionless Henry's constant Ha/C0 RT, actually (volume/volume), Qg/QL is reduced to common flow units [m3 /m3 ], and C0 is the molar density of water at 20C, 55.41 kg mole/m3 . The theoretical minimum, equilibrium, moles of gas required Gmin /L is calculated from the influent and effluent concentrations and the "dimensionless" Henry's constant (H'a).
Ru = 0.08205746 m 3 atm the universal gas constant kg  mole K At 1 atm and 20C the molar density of water is C0 , 55.41 kg mole/m3 . QG/QL [m3 /m3 ] is the airtowater ratio, ATW. yae = Ha x ai/pTe (mole/mole) Substituting gives
L ( xai  xae ) G = H a x ai p Te and rearranging yields
G min L = (x ai  x ae )p Te H a xa i which is the equilibrium molar ratio of gas (air) to liquid (water). D2. Develop the Design Basis. a. Characterize the influent conditions and effluent requirements, including RI/FS data + total organics + background inorganics and minimum water temperature. D4 DG 111013 31 Oct 2001 Table D1 Contaminants Contaminant Formula GMW* [g/gmole] 78.11 92.14 131.50 CAS Number Ha ** [atm/mole/mole] 309.2 353.1 506.1 Benzene Toluene Trichloroethylene (TCE) C6 H6 C6 H5 CH3 C2 HCl3 71432 108883 79016 *The [gram] molecular weight of the contaminant. ** Ha at 20C (296.13 K). b. Design the pumping system to maintain the flow. Use the real flow rate, not rounding up. Discharge head adjustments for the stripper are added to the TDH. The aggregate flow from the hydraulic barrier is 440 gpm (0.0278 m3 /s) in this example. c. Design the pretreatment system to prevent scale/slime from clogging the stripper (if water is high in hardness, iron or manganese). Table D2 Background Inorganic Concentrations
Ion CO2 mgL O GMW 44 96 35 61 100 10 40 0.3 10 0.05 23 40 56 24 55 Valence GEqW* 2 22 Anions 2 48 1 35 1 61 0 1 2 2 2 2 50 Cations 23 20 28 12 27 meq/L 0.00 1.25 1.52 0.49 TOTAL 0.00 0.43 2.00 0.01 0.82 0.00 TOTAL mg/L asCaCO3) 0.00 62.46 76.15 24.58 163.19 0.00 21.75 99.80 0.54 41.12 0.09 163.29 SO4 60 Cl 54 HCO3 30 CaCO3 Na Ca Fe Mg Mn * GEqW is the [gram] equivalent weight of the inorganic ion.
D5 DG 111013 31 Oct 2001 d. Construct a contaminant material balance for the stripping system. Table D3 Removal Requirements Contaminant Concentration [g/L] Effluent Influent, Standard, Cai Cae 2500 750 1000 750 NA 10 100 100 Mole Fraction [mole/mole] Removal xai Requirement NA 98.7% 90.0% 86.7% NA 0.17330 0.19588 0.10294 xae NA 0.00231 0.01959 0.01373 Total VOCs Benzene Toluene Trichloroethylene (TCE) e. Assess the air pollution control requirements from the material balance and the regulations. D3. Determine the Column Diameter. a. Determine a preliminary stripper crosssectional area for the sustained pumping rate, 440 gpm (0.02776 m3 /s) using 45 gpm/ft2 (0.03056 m/s) for the stripper surface loading.
A= Q m2 s 3 45gpm 0.03056 m Q ft
2 32.72 Q s gpm m 2 ft m3 s = 0.0222(440gpm) 32.72 0.02776 s m gpm = 0.0222 Q ft
2 = 9.7778ft 2 ( 0.9084 m 2 ) b. Divide the are by the number of strippers.
D6 DG 111013 31 Oct 2001 a = = A # 9.7778 2 ft 2 0.9084m2 2 2 ft 0.4542m2 = 4.889 stripper stripper c. Divide a = (d2 /4) the unit area by , multiply by 4 and take the square root.
d d 4a 4 ( 4.889 ) d 6.22473 d 2.5ft ( 4 ( 0.4542 ) 0.5783 ) ( 0.762m) d. Bracket the calculated diameter with the nearest standard diameters. In this example, a 2.5ft (0.762 m) diameter column is standard for most manufacturers. The availability of standard metric sizes should be verified. D4. Find a Suitable Packing. a. Find packings in the diameter range of roughly 5 to 10% of the stripper diameter. The rule of thumb is 1 in. of packing diameter per 1 ft of tower diameter; 2.5 in. (0.0635 m) packing is not standard for most manufacturers. b. Reconsider the number of strippers if the packings and diameters don't correspond. Three 2ft diameter strippers with 2in. packing could be used in lieu of two 2.5 ftdiameter strippers. b. Find the area of the standard diameter strippers.
a = = d
2 4 4
2 ( 2.5ft 2 ) = 4.908ft ( 0.456m2 ) ( 0.762 m )2 4 D7 DG 111013 31 Oct 2001 d. Calculate the surface hydraulic loading Q/A and compare the loading with various packing manufacturers' recommendations. 0.01388 m 3 s = 2 A 4.908ft 0.456 m 2 220gpm gpm ft
2 QL VL = 44.82 ( per stripper 0.03044 m s ) e. Adjust the system configuration to get the hydraulics within the recommended range. D5. Calculate the Minimum Gas Flow. Determine Gmin and the critical contaminant from the following relationship:
QG min QL = ( Cai  Cae )
H'a Cai Table D4 Critical Contaminant For Pte = 1 atm and 20C (296.13 K) H'a = Ha/Co R T Contaminant Benzene Toluene Trichloroethylene (TCE) (C ai  C ae C ai )
0.2320 0.2649 0.3797 H'a 4.253 3.397 2.283 QG Q m m m m m m
3 3 3 3 3 3 min L 0.9867 0.9000 0.8667 Critical Contaminant (Benzene)
QG Q
min = 4.253 m m 3 3 (maximum) L D8 DG 111013 31 Oct 2001 D6. Calculate the Mass Transfer Rate. Use a model, if available, to confirm the results.
aw at 1 K LA 1.45 sc 0.75 s = 1 e = H NRe 0.01 NFr 0.05 Nwe 0.2 1 '
a + 1 K L aw K G aw where aw at KLA KL KG = = = = = wetted surface area of the packing(m2 /m3 ) total surface area of the packing (m3 /m2 ) overall mass transfer rate (m/s) liquid phase mass transfer rate (m/s) gas phase mass transfer rate (m/s). a. Calculate the dimensionless numbers (http://www.processassociates.com/process/dimen gives a comprehensive listing and definitions of dimensionless numbers).
N Re = a t N Fr = a t 1 VL L L Reynolds Number VL2
gc
2 Froude Number N We 1 V L = L a t gc S Weber Number NSc = gc L L DL m s
2 Schmidt Number = 9.807 gravitation constant b. Look up the properties of the liquid (water) at the minimum water temperature, T (Table D5). D9 DG 111013 31 Oct 2001 Table D5 Water at 20 C (293.16 K) s = 0.072764 N m = kg ms liquid density kg s
2 liquid surfacetension L = 0.0010042 L = 998.20 kg m
3 liquidviscosity c. Look up the properties of the critical contaminant, benzene, at the minimum water temperature, T,
DL = 8.91 10
10 m s 2 liquiddiffusivityof benzene at 20 C (296.13 K) d. Obtain data from product literature (Table D6). * Table D6 Packing Characteristics
d P = 0.0508 m a t = 157 m m
2 3 nominal diameter totalsurfacearea criticalsurfacetension for polyethylenepacking packing factor sc = 0.033 cf = 15 kg s
2 e. Liquid mass velocity is as follows. * Jaeger Tripacks 2in. (50.8 mm) plastic media. D10 DG 111013 31 Oct 2001 L = L QL kg liquid mass velocityat0.01388 m3 with a n o m i n a l columndiameterof 0 . 7 6 m 2 s A m s = 998.19 = 30.38 ( 0.01388 0.45599 ) kg m2 s f. Calculate the Reynolds Number, NRe.
N Re = VL L at = = = VL L a t L 0.3043 998.19 m m
2 3 (Reynolds Number) m s kg m
3 from Paragraph D4d 157 L = N Re = =
N Re =
0.1 0.0010042 kg ms 0.3043 998.19 157 0.0010042 192.7 1.692 g. Calculate the Froude Number, NFr.
N Fr = = =
N 0.05 FR at V L gc 2 (FroudeNumber)
2 157 ( 0.3043) 9.807 0.01483 = 1.234 h. Calculate the Weber Number, NWe. D11 DG 111013 31 Oct 2001 N We = = = 1 V L L a t g cs 2 (Weber Number) 2 1 ( 30.39) 998.19 9.807 0.072764 157 0.08094 0.6048 N We 0.2 = i. Calculate the wetted area of the packing, a w from the dimensionless relation:
aw at
0.1 0.75 sc 0.1 0.05 0.2 = 1  exp 1.45 N Re N Fr N We s ( ) N Re N Fr = 0.05 N We 0.2 = 1.692 1.234 0.6048 1.263 sc s 0.75 = = = ( 0.033 0.0728 ) 0.75 ( 0.45352 ) 0.75
0.553 j. Calculate the wetted surface area.
aw at = 1  exp [  1.45(0.553 1.263)] = 1  exp(  1.0125) = = at aw aw = = = 1  0.3633 63.67% 157 m2 m3
2 3 63.67% (157) 99.96 m m D12 DG 111013 31 Oct 2001 k. Calculate the liquid phase mass transfer coefficient, Onda KL from the following relationship: L KL L g c L 1 3 g L c 1 3 = VL L 2 L 3 0.0051 aw L L D L 3 998.19 ( 0.0010042 ) (9.8066 ) 1 0.5 (a d )
t p 0.4 = = =
2 3 (101,361)1 / 3
46.63 VL L a w L = ( 0.3043 998.19 99.96 0.0010042
2 ) 2 3 = = ( 302.7 ) 3
45.08 L 0.5 D L L = = = 0.5 0.0010042 10 ( 998.19 ) ( 8.91 10 ) (1129)
0.5 0.02976 ( at d P ) 0.4 = = = (157 0.0508) 0.4 ( 7.9756)
2.2946
V 3 0.0051 L L a w L 2 0.4 KL L g L c = = = L D L L 0.5 ( atd p ) KL KL 0.0051 45.08 0.02976 2.2946) 46.63 0.0003367 m s D13 DG 111013 31 Oct 2001 l. Calculate the gas phase mass transfer coefficient, Onda K G, using a stripping factor (R) between 2 and 5. Try R = 2.5 if air pollution control is required, R = 4.5 if it isn't.
KG = ( at DG ) G 5.23 a t G 0.7 G 1  2.0 3 D ( at dp ) G G m. Look up the properties of the gas (air) at the minimum water temperature, T (Table D7). Table D7 Air at 20 C (293.16 K) and 1 atm G G = = 1.773 10 5 kg ms 1.2046 kg 3 m gasdensity gasviscosity n. Look up the properties of the critical contaminant, benzene, at the minimum water temperature, T.
DG = 9.37 10
6 m
2 s gasdiffusivity(benzeneinairat20 C,1atm) o. Calculate the gas flow rate from the relationship:
QG
min QL = = ( Cai  Cae )
H 'aCai 4.253fromTable D4 0.03044 m s VL VG = = = min 4.2635 0.03044 0.1297
m s D14 DG 111013 31 Oct 2001 R = VG = VG = =
G = = = 3.5 R VG min 3.5 0.1297 0.4531
VG G 0.4531 0.5458 m kg 1.2046 3 s m kg s m2 m s p. See Table D6 for packing characteristics, at and dp . G 0.7 = a t G = = 0.7 0.5458 157 1.773 10 5 (196.06 )0.7
40.24
1 Gas phase Reynolds number G G DG 1 3 = = = 1.773 10 5 3 1.2046 9.37 10 6 (1.571)3
1.162
1 Gas phase Schmidt number (a d )
t p  2.0 = = = (157 0.0508 ) 2.0 ( 7.976)
2.0 0.01572
6 at DG = = 157 9.37 10 0.001471 m s D15 DG 111013 31 Oct 2001 ( at DG )
KG KG = = = = 5.23 40.24 1.162 0.157 3.846 3.853 0.00147 0.005658 m s q. Calculate the overall mass transfer coefficient, Onda KLA.
1 K LA = = = = K LA HTU = = = = 1 1 + H 'a KG a w K L aw 1 1 + 0.2320 0.005658 99.96 0.003367 99.96 7.622 + 29.71 37.33 0.02679 s VL K LA 0.03044 0.02679 1.136m
1 r. Determine NTU for the selected R.
R = =
= G G min H 'a PTe
3.5 G L NTU = ( ) xae R ln R 1 R xai ( R 1) + 1 D16 DG 111013 31 Oct 2001 NTU = 3.5 ln 3.5  1 750 ( 3.5  1) + 1 10 3.5 3.5 ln ( 75 2.5 ) + 1 = R 2.5 = 1.4 ln ( ( 187.5 + 1 3.5 ) ) = = = = Z = = = A W A W = 1.4 ln ( )
188.5 3.5 1.4 ln 53.86 1.4 3.99 5.88 NTU HTU 5.88 3.07 17.13m
m3 air s m3 0.02776 water s 0.4132 = 14.89 s. Calculate the system headlosses, including the packing, the stripper inlet, and the exit losses. Size equipment, including blowers and pumps. Verify that blower discharge pressure is less than the value that would cause flooding. D7. Complete the Design. a. The following drawings are required. (1) Site plans. (2) Profiles. (3) Layout drawings.
D17 DG 111013 31 Oct 2001 (4) Details. b. Design Analysis should be done in accordance with ER 1110345700, Design Analysis, Drawings, and Specifications, containing the following: (1) Narrative. (2) Documentation. (3) Description. (4) Calculations. (5) Computer print out with documentation. c. Specifications should be done in accordance with ER 111018155, and the following United Facilities Guide Specifications 02150 Piping; OffGas. 02521 Water Wells. 11212 Pumps Water Vertical Turbine. 11215 Fans/Blowers/Pumps OffGas. 11220 Precipitation/Coagulation/Flocculation Water Treatment. 11242 Chemical Feed Systems. 11378 Thermal (Catalytic) Oxidation Systems. 13405 Process Control. 15200 Pipelines, Liquid Process Piping. d. Cost Estimate should be done in accordance with ER 111031301, Cost Engineering Policy Requirements for Hazardous, Toxic Radioactive Waste (HTRW) Remedial Action Cost Estimate. e. Draft O&M manual should include cleaning procedures, as well as the O&M of the mechanical equipment. D18 ...
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