Packed Tower Absorber

8/13/2019 Packed Tower Absorber

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Packed Scrubber Design

Basics

Gas-Liquid Equilibrium

Operating Line Tower Diameter and Pressure Drop

Mass Transfer and Tower Height


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In absorption processes, gaseous

contaminants are removed from a gasstream by transferring them to a liquid.

That liquid is usually water or an aqueous

solution that contains chemicals selected

to react with the absorbed contaminants.

Most systems operate with collection

efficiencies of 70 percent to over 99

percent

Packed bed systems are typically used for

highly water soluble gases such as HCl orHF or NH3

Spray towers are usually used on power

plants due to their lower pressure drop

Absorption Basics


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The most important factor affecting collection

efficiency is the solubility of the contaminantin the liquid.

Gases are more soluble in cold liquids than

in hot liquids and less soluble in liquids with

low pH.

The liquid surface area and the time

available for diffusion of the gaseous

contaminants into the liquid are also

important factors affecting performance

Solubility


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There are many equipment designs for contacting theliquid with the contaminated gas stream. The most

common ones are:

spray towers, tray towers,

packed beds, and

spray dryers.

Types of Absorbers


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Basics





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Basics





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Two film theory


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For a highly soluble gas such as HCl or HF in

water, m = 0. Then Nog simplifies to:

1

2

1ln ln

1O G

YN

Y

In this case, to achieve 99% removal:

1ln 4 .6

0 . 01

O GN


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(g/mole)gaspollutantofweightmolecularM

(atm)pressuresystemtotalP

)/m(minterfaceliquid-gasofareaeffectivea

Pa)-m-(moles/hphasegasonbasedtcoefficientransfer-massoverallK

)m-(g/hvelocitymasslsuperficiagasG'

(m)resistancephasegasoverallonbasedunittransferaofheightH

(m)unittransferaofheightHTU

))(P)(M(aK

G'HHTU

G

32

e

2

OG

2

OG

GeOG

OG

where

Height of a transfer unit


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D d P f


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There are several operating problems that can occur in absorption systems.

The most common of these include the following:

Inadequate liquid flow

Low inlet liquid pH

Poor gas-liquid contact

Inadequate chemical feed rate

Excessive liquid temperature

Plugged beds or mist eliminators

Corrosion

Decreased Performance

D d P f


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To determine if an adsorption system is working properly,

field personnel should observe if possible:

Outlet Gas Stream Contaminant Measurement

not likely available

Temperature Difference between the Gas Inlet and

Outlet,

Liquid Flow Rate and Liquid Pressures.

Pressure Drop, and Stack Opacity

Decreased Performance

Packed Tower Absorber

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