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# AIR POLL CONTROL CEE 490

UW

GPA 3.91

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This 5 page Class Notes was uploaded by Mason Hackett on Wednesday September 9, 2015. The Class Notes belongs to CEE 490 at University of Washington taught by Michael Pilat in Fall. Since its upload, it has received 38 views. For similar materials see /class/191976/cee-490-university-of-washington in Civil and Environmental Engineering at University of Washington.

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Date Created: 09/09/15

Countercurrent Packed Tower for Absorption of Ammonia Gas Given Gas air flow upward of 4000 acfm at 68F or 528R and 1 atm pressure lniet gaseous cone of 6000 ppm NH3 dry basis Yin 0006 inlet water cone of 00006 mole fraction NH3 in water xin 00006 Need to define unit lbmole lbmole mole4536 18016lb Liquid flow rate 500 lb liquid 02 hr Moi Wt H20 ML18 lbllbmole ML t W Gaseous flow rate of 500 lb gas air I ft2 hr Mol Wt Gas air 29 lbllbmole atomic wt N 140067 atomic wt H 10079 MNH3 1793039 quot3 Assume Govt Emission Std is 1000ppm NH3 dry basis so yout 0001 ibmole lb Fm MG 20 a H Henry39s law constant from solubility data of NH3 in water at 20 C Ibmole b HOG from ammoniawater data of Fellinger for 15quot Raschig ring packing 0 Liquid molar flow rate Lm lb molesft2 hr Y39 1 0390 d Gaseous molar flow rate Gm lb molesft2 hr Xin 2 00006 e Xout via Mass Balance around the absorption tower f Number of gas phase mass transfer units NOG Yout 2 0001 g Height of packed section in towerZ HOG N06 h Tower Diameter ft Xout Unknown i Gas phase ov39erall mass transfer coefficient KGa lb molesit3 hr atm Pmta Tam Gas velocity m tower ftsec k Gas residence time in packed section of tower seconds T 2 523R L LIG LiquidGas flow rate ratio in gallons waterl ft3 1000 gas m Draw YX graph with equilibrium curve and tower operating line H atm n Ammonia colection efficiency mm g quot 750 a Cale Henry39s law constant H using equilibrium solubility data for NH3 in water at in 20 C range from Table 84 page 682 Cooper amp Alley 3rd Ed i 0 5 NHS 2 120 12 Pressure 3 182 182 4 249 WtNH3 lb PNH3 z mmHg PNH3 z torr 5 317 317 75 500 50 10 696 696 VH3 molesNHSinWater WtNHEi Xi I m Mole fraction NH3 In liquid XNH3 Totalmoksinwmer m Mole fraction NH3 in gas phase YNH3 Henry39s Law Constant H PNH3i XNH3i quotmale YNH3i mm WtNH3i 100 0016 0021 07614 1710 10m 0024 0031 07778 yNH3i lbmole lbmole H yNH3 0033 XNH3 0041 03063 1 XNH3i 0042 005 H 296 066 0 074 08 Assume H 07614 VmXNH3 because the Inlet 039 39 08942 liquid phase NH3 concentration Xin is low at 00006 0092 0096 09565 Henrys law constant H is in units of mole fraction NH3 in gas phase mole fraction NH3 in liquid phase a H 2 03614 AmmoniaAbsorpiionYmCd h Obtain height of gas phase mass transfer unit H0G 30 ft from graph shown below for 15quot Raschig rings L 500 lb liquid ftz hr G 500 lb air ftz hr pp 456 Noll 13 Figure 129 39 Column Packing Comparison for Ammonia and Water System 36 l i l t 1 1 quot G39 500 lbihr it2 28 r T3 39 Where I0 2 0 o 15 in Rasohig rings A l in Tellerettes 12 N 04 I l I l l 500 1 000 1 500 2000 L39 lbhr n2 Source Love 1951 ML 2 18016 gmmole i c Liquid flow rate in lbmole units L 2 Liquid flow rate in lb units L L 39 50011 hr ML 180161bibmoie 1 L Lm 2 m d Gaseous molar flow rate G 2 1 c Lm 277mm Gaseous mass flow rate G G 2 500vlbft in ft 1 1 3 G MG 29 3 mm MG 291blbmole 1 Gm M6 mole e The mass balance equation 1 cm 17 241 NH3 into tower NH3 out of tower Xin 00006 ft 391quot Yin Gm Xn Lm Yout Gm Xout Lm Xin was given as 00006 to can Gm soive for the unknown Xout Xout i rajYin Yout Xin H 07614 Note that liquid conc Xout 0003706 is in mole fraction units 8 Xout oposme not ppm in liquids ppm means parts per million by weight whereas in gases ppm means ppm by mole or by gaseous volume To graph the operating line using the liquid phase concentration X as the X axis variable we will need a linear equation relating the tower gas phase concentration Y in terms of the liquid phase concentration X Starting at Xin 00006 at the top of the tower and increasing X to Xout one can obtain the linear operating equation for Y Note that this equation represents the actual gas phase and liquid phase cone in the absorption tower one could take a sample of the tower gases amp liquids and measure the concentrations L Y 31143 Xin vi Yout im AmmoniaAbsorptinn7mcd Am moniaAbsorption7 mod If Number of Gas Phase Mass Transfer Units or NOG 1 prom 1am Because it is difficult to put an asterisk to denote the equilibrium cone for Y or Y in Mathcad let Ystarin be the gas phase mole fraction cone of NH3 in equilibrium with the liquid water at the gaseous inlet bottom of this tower and let Ystarout be the gas phase cone of NH3 in equilibrium with the liquid Ystarin 2 HXout water at the gaseous outlet top of tower Using nomenclature of Yin You Xm Xout Ystarin and Ystarout should help to reduce confusion as to where these concentrations occur and make it easier to label diagrams and tower illustrations out Ystarin 000282 Ystarout 2 H Xin Yin Yout Yin Ystarin NOG ln Ystarout 000046 Yin Ystariu Yout Ystamut Yout Ystarout Nos 2 3352 there are 3352 gas phase mass transfer units Note that the above equation for N06 assumes a straight equilibrium line and a straight operating line which is an OK assumption for the absorption of dilute gas concentrations and air pollutants are usually in the dilute concentration range lg Height of Packed Tower Section Z HOG NOG Z HOGNOG The height of the 15 inch Raschig ring packed section in the absorption tower 10057 ft h Tower Diameter T 523R 12 13tm I 29g RG 2 0082054 literaim M F moleK 1 gas density pg PMg m0 6 3 39 RGvT A 4ooort3min 1 rea 2 pg 0075217lbft G Pg Am 2 36104 2 I Area4jo395 Diameter 1 1 ii Gas Phase Overall Mass Transfer Coefficient KGa h Tower Diameter 678 ft Gm Gm 17241lbmoleft 21quot KGA 39 Gra hs of the measureth 1 versus 1 uid or 39 p G q 1100 HOG3ft szm gas ow rates are provuded by equipment manufacturers for various packing types and sizes L liq Ij Gas Velocity quot quot in packed tower Velocity lt4000ft3min1gtArea1 0 Gas Velocity 1857 ftsec k Gas quot 4 time in packed 39 quot tower section I M 1n ResidenceTime 2 Velocity 1L Liquid to gas flow rate ratio LIG in gallons1000 of gas I 1 L834 lbj LG 2 ga LIG 9019 gallons water G pgo1 I per 1000 ft3 gas AmmoniaAbsorption7 mcd m GraEh of YX eguilibrium curve and Mass Balance Ogerating Line j 0 7 0005 Ystar Gas phase equilibrium NH3 conc 0006 in mole fraction units The 8 X values shown in the matrix to the right are for the horizontal axis 001 Yout 0001 xi 10006 in the graph and span the range from 002 Yin 0006 Xout 0003700 X39quot m X quot X 2 0025 Now to put the operating tine equation OpSlope I Ein H 0761 0030 on an YX graph Gm Ystarj H39Xj 0035 YYj 2 OpSloper OpSlope 15096803 003706 YYj OpSlopeXj Xin You YY7 0006 X7 0003706 The operating line equation YY 160968 X is upper dash dot line shown in below graph The difference between the operating line amp the lower solid equilibrium equation Ystar H X line shows the NH3 concentration gradient driving force the concentration gradient y y causes the NH3 to move from the gas into the liquid NH3 Equilibrium amp Operating Line 1m Y 006 r3 0006 L I In I 2 Xout 003706 2 3 Operating Line 2 3 YY160988X 1 L2 YStar0004 f E I Z YY I x r o aquot 73 0002 Equiiibrium Line 5 Ystar H X gt Yout 001 I X n 0006 I o 394 0 5 3910 0001 00015 0002 00025 0003 00035 X X mol fraction NH3 in liquid Yin 0006 xout 0003706 Govt Emission Std is 1000ppm NH3 dry basis so Yout 0001 Yout 0001 Xin 00006 Remember the operating line repreSents the actual concentrations of NH3 in the gas and liquid inside the tower and thus these are measurable parameters Yout and Xi are at the top of the tower and Yin and Xout are at the bottom of the tower for countercurrent ow see illustration on next page Yin Yout 100 CollectionEff 2 Yin Cnil e on a AmmoniaAbsorption7 mcd Illustration below shows the ammonia absorption inlet and outlet concentrations for a countercurrent absorption tower 0 vi oooe quot l r Y I e 12 0 riving Jr Force X xOut 0003706 Y gas phase pollutant concentrations mole fraction X liquid phase pollutant concentrations mole fraction Pilat Comment I prefer to use Yin Yum Xin and Xout rather than Y1 Y2 X1 and X2 because I think it is too easy to forget whether the subscripts 2 and 1 indicate the top or bottom of an absorption tower AmmoniaAbsorntion7Amcd

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