AbsorptionReading: Chap. 13

Definition Equipment Packing materials Design considerations: Mass balance High gas flow Mass flow

Concentrated systems HTU and NTU

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DefinitionTransfer of a gaseous component (absorbate) from the gas phase to a liquid (absorbent) phase through a gas-liquid interface.Q: What are the key parameters that affect the effectiveness? Q: How can we improve absorption efficiency?

Mass transfer rate:

gas phase controlled absorption liquid phase controlled absorption

Q: Does it matter if its gas phase or liquid phase controlled?11/21/08 Aerosol & Particulate Research Lab 2

EquipmentSpray towerClean gas out Clean gas out Countercurrent

packed towerMist Eliminator Liquid Spray Packing

Spray nozzle

Dirty gas in

Dirty gas in

Q: Limitations of a spray tower?11/21/08

Redistributor Q: Why redistributor?Aerosol & Particulate Research Lab

Liquid outletMycock et al., 1995 3

Three-bed cross flow packed tower

Liquid spray

Dry Cell

Packing

Berl Saddle11/21/08

Intalox Saddle

Raschig Ring

Lessing Ring

Pall Ring

TelleretteMycock et al., 1995 4

Q: Criteria for good packing materials?Aerosol & Particulate Research Lab

Design considerations: What are known? What are we looking for?11/21/08 Aerosol & Particulate Research Lab 5

Mass BalanceIn = OutLiquid in

Gas out

Gas in

Liquid out

Gm1 + Lm 2 = Gm 2 + Lm1Gm ( y1 y2 ) = Lm ( x1 x2 )(for a dilute system)

Lm: molar liquid flow rate Gm: molar gas flow rate x: mole fraction of solute in pure liquid y: mole fraction of solute in inert gas11/21/08 Aerosol & Particulate Research Lab

Slope of Operating Line = Lm/Gm

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Dirty air

Clean air Clean water Dirty water7

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Generally, actual liquid flow rates are specified at 25 to 100% greater than the required minimum.

Q: How much is X2 if fresh water is used? What if a fraction of water is recycled?

G = 84.9 m3/min (= 3538 mole/min). Pure water is used to remove SO2 gas. The inlet gas contains 3% SO2 by volume. Henrys law constant is 42.7 (mole fraction of SO2 in air/mole fraction of SO2 in water). Determine the minimum water flow rate (in kg/min) to achieve 90% removal efficiency.11/21/08 Aerosol & Particulate Research Lab 8

Problems with high gas flow Channeling: the gas or liquid flow is much greater at some points than at others Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the void space between packing Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow Gas flow rate is 3538 mole/min and the minimum liquid flow rate is 2448 kg/min to remove SO2 gas. The operating liquid rate is 50% more than the minimum. The packing material selected is 2 ceramic Intalox Saddles. Find the tower diameter and pressure drop based on 75% of flooding velocity for the gas velocity. Properties of air:: molecular weight: 29 g/mole; density: 1.1710-3 g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp.11/21/08 Aerosol & Particulate Research Lab 9

(G ' ) 2 F 0.1 L G L gL: mass flow rate of liquid G: mass flow rate of gas G: mass flux of gas per cross sectional area of column F: Packing factor : specific gravity of the scrubbing liquid L: liquid viscosity (in cP; 0.8 for water)11/21/08

L G

G (dimensionless) L10

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Mass Transfer J ( = M / A ) = k ( Ci C )J: flux k: mass transfer coefficient( mass ) area time

rate of mass Flux = transferred

/ interfacial = k concentration area difference

Two-Film Theory (microscopic view)

CI

J = k G ( pG pI )(gas phase flux) (liquid phase flux)

J =k L ( C I C L )

CL pG pI

pI = HC I

1 ( pG HCL ) J= 1 / kG + H / k L Cussler, Diffusion, Cambridge U. Press, 1991.(overall flux)Aerosol & Particulate Research Lab 11

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1 pG K OL = C* = J = K OL ( C* C L ) H (overall liquid phase MT coefficient) 1 / k L + 1 / k G H (equivalent concentration 1 to the bulk gas pressure) = K OG ( pG p* ) K OG = p* 1 / kG + H / k L (equivalent = HC L the (overall gas phase MT coefficient) pressure to2bulk concentration in liquid)

Macroscopic analysis of a packed tower Mole balance on the solute over the differential volume of tower

accumulation = flow of solute in of solute minus flow out

dy dx 0 = G 'm + L 'm dz dz

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G 'm x = x1 + ( y y1 ) L 'mAerosol & Particulate Research Lab

Lm: molar flux of liquid Gm: molar flux of gas

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Mole balance on the solute in the gas only

solute = solute flow in solute lost accumulation minus flow out by absorption

dy 0 = G 'm K OG aP ( y y*) dz Z y G 'm dy Z = dz = 0 K OG aP y ( y y *) (tower height)1 Z

a: packing area per volume

y* = Hx

y1 Hx1 1 Z = ln y Hx K OG aP (1 / G 'm H / L'm ) Z Z y1 Hx1 G 'm 1 = ln y Hx K OG aP (1 HG 'm / L'm ) Z Z 1HTU? NTU?Aerosol & Particulate Research Lab 13

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L'm ( x x1 ) y = y1 + G 'mEquilibrium

Mass balance

x1, y1

y* = Hxy1 G 'm dy Z= K OG aP y Z ( y y *)

x 1, y 1* xZ, yZ

Alternative solution:

xZ, yZ*

G 'm y1 y z Z= ; K OG aP y LM11/21/08

y LM

(y y )(y =* 1

Assumptions for dilute/soluble systems?

y* 1 z z * y1 y1 ln y y* z z

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Pure amine Lm = 0.46 gmole/s

0.04% CO2

Q: A Packed tower using organic amine at 14 oC to absorb CO2. The entering gas contains 1.27% CO2 and is in equilibrium with a solution of amine containing 7.3% mole CO2. The gas leaves containing 0.04% CO2. The amine, flowing counter-currently, enters pure. Gas flow rate is 2.31 gmole/s and liquid flow rate is 0.46 gmole/s. The towers cross-sectional area is 0.84 m2. KOGa = 9.3410-6 s-1atm-1cm-3. The pressure is 1 atm. Determine the tower height that can achieve this goal.

1.27% CO2 Gm = 2.31 gmole/s C* = 7.3% CO2 in amine

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Absorption of concentrated vaporMole balance on the controlled volume

d d 0 = (G 'm y ) + ( L'm x) dz dz 1 G 'm = G 'm 0 1 y

x 1, y 1x1 , y1 *

Gas flux

Liquid flux 1 L 'm = L ' m 0 1 x

xZ, yZ

y1 L'm 0 x x1 1 y + G' 1 x 1 x 1 m0 1 y= y1 L'm 0 x x1 1+ 1 y + G' 1 x 1 x 1 m0 1 11/21/08 Aerosol & Particulate Research Lab

xZ, yZ*

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Mole balance on the gas in a differential tower volume

G 'm 0 dy 0= K OG aP ( y y*) 2 (1 y ) dzZ =Z 0

G 'm 0 y1 dy dz = yZ (1 y) 2 ( y y *) = HTU NTU K OG aP

G 'm0 HTU = K OG aP

NTU =

y1

yZ

dy 2 (1 y ) ( y y*)

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HTU (ft)

HTU

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For a given packing material and pollutant, HTU does not change much. Aerosol & Particulate Research Lab

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Quick Reflection

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