CEE 452/652

Week 9, Lecture 2 –Absorption

Dr. Dave DuBoisDivision of Atmospheric Sciences,

Desert Research Institute

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Today’s topics

• Today’s topic: chapter 13 on absorption• Cover odor control on Tuesday, Oct 30

– Also have review session next class Oct 30• Midterm is Nov 1 (next Thursday)

– Cumulative, information up to Oct 30 lecture is fair game

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Major Techniques for Capture/Elimination of Gas

Pollutants• Oxidation to form nontoxic compounds• Chemical reduction to form nontoxic

compounds• Adsorption onto solid surfaces

Absorption into liquids• Biological oxidation to form nontoxic

compounds• Condensation of vapors to form liquids

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The Absorption Process• The transfer of material from a gas (absorbate) to

a liquid (absorbent)• Transfer is based on the preferential solubility of a

gaseous component in the liquid• Also known as “scrubbing” or “washing”• Examples include removal and recovery of NH3 in

fertilizer manufacturing• Control of SO2 from combustion source• Control of odorous gases from rendering plants

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Mass Transfer in Absorption

diffusion

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Types of Absorber Control Equipment

• Packed bed tower absorbers• Spray tower absorbers• Tray tower absorbers• Venturi Absorbers• Ejector Absorbers• Biofiltration Bed Absorbers

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Gas Absorption Equipment

• Packed bed absorbers – most common

• Counter-current flow tower configuration

• Gas flow enters bottom of tower and flows upward

Gas in

Gas out

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Gas Absorption Equipment

• Another counter-current flow tower configuration

Gas exit

Gas in

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Cross-Flow Scrubber

Gas Flow in

Gas Flow out

Concentration gradients exist in two directions in the liquid- from top to bottom and front to rear

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Packed Bed Abs. Applications

• Suited to applications where high gas removal efficiency is required

• Exhaust gas is relatively free from particulate matter

• Control of SO2 and HCl in sulfuric acid and hydrochloric acid production

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Packing Elements

Lessing ring

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Packing Elements• Packing material provides a large surface

area for mass transfer• Packing elements made of plastic

(polyethylene, polypropylene, polyvinylchloride), ceramic or metal

• Sizes range from 1 to 4 inches each• Design depends on corrosiveness of gas,

scrubbing liquid, size of absorber, static pressure drop and cost

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Packing Elements

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Spray Tower Absorber• Simplest device used for absorption• Consists of open vessel and a set of liquid spray

nozzles to distribute scrubbing liquid (absorbent)• Limited efficiency because of limited contact

between gas and spray droplets– Used when gases are extremely soluble in absorbent– Chemical reaction in liquid could cause clogging

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Gas Absorption Equipment

Co-current Spray TowerScrubber

Full Cone NozzleSpray Tower

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Tray Tower Absorber

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Packed Tower Design

Diameter and height of the bed can be estimated for this design

Use generalized flooding and pressure drop correlation graph

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Simplified Design of Packed Absorber

1. calculate value of

In the Generalized Sherwood flooding and pressure drop correlation graph

5.0

⎟⎟⎠

⎞⎜⎜⎝

⎛=

L

g

GLabcissa

ρρ

L = mass flow rate of liquidG = mass flow rate of gasρg = gas densityρL = liquid density

19L

G

GL

ρρ

gFG

LG

L

ρρμΦ 1.02)'(

L = mass flow rate of liquid

G = mass flow rate of gas

G’ = mass flux of gas per cross sectional area of column

F = Packing factorΦ = specific gravity

of the scrubbing liquid

μL = liquid viscosity(in cP; 0.8 for water)

(dimensionless)

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Simplified Design of Packed Absorber

2. calculate flooding pressure drop

Fp = packing factor (dimensionless)

7.0115.0 pflood FP =Δ

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Simplified Design of Packed Absorber

3. use graph to find ordinate at the flooding pressure drop, ΔP

4. And find gas flow rate, G’

gFG

LG

L

ρρμΦ 1.02)'(

G’ = mass flux of gas per cross sectional area of column (lb/ft2-s)F = Packing factorΦ = specific gravity of the scrubbing liquidμL = liquid viscosity (lb/ft-s)g = gravitational accelerationρg = gas densityρL = liquid density

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Simplified Design of Packed Absorber

5. Calculate actual gas flow rate per unit area as a fraction of the gas flow rate at flooding

Goperating = G’ fWhere

Goperating = actual flow rate per unit area (lb/ft3-s)f = coefficient (0.75)

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Simplified Design of Packed Absorber

6. Calculate packed bed diameter on the actual gas flow rate per unit area in the system

areaunitperrateflowGasrateflowgasTotalAreaTower =

πAreaTowerDiameterTower ⋅

=4

AreaTowerDiameterTower 13.1=

Note correction

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• 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

Problems with high gas flow