absorption and stripping - mass transfer

Absorption and Stripping in Packed Column

Megha Rajasekhar

Mass Transfer Self Study – Phase 1

Absorption and Stripping

• Absorption – removal of a solute gas from a mixture of gases by extracting into a liquid solvent.

• Stripping is the reverse process of absorption, also known as desorption

• It is removal of a solute gas from a liquid

Comparison between plate and packed columns

• Packed columns do not hold much liquid and hence can be used for absorption of toxic and flammable liquids, keep size of the unit small.

• Plate columns have are used for solvents which contain large amounts of solids.

• It easier to install cooling systems in plate columns.

Absorption

• Counter current flow. Gas containing the solute enters from the bottom and flows out at the top. Solvent liquid, enters at the top and flows out the bottom. • Solvent is chosen such that the solute gas is preferentially soluble in the solvent rather than the gas mixture.•Two film theory can be used to explain diffusion from gas to liquid phase.

Selection of solvent

• Gas Solubility- High solubility of a gas in the solvent is preferred and it should not dissolve carrier gas. If chemical reaction takes place between solute and solvent, rate of absorption is extremely high. But the reaction should be reversible to recover solvent during desorption.

• Volatility- Low volatility or low vapor pressure of the solvent enhances the adsorption operation as solvent loss with carrier gas is very small.

• Viscosity- Greater amount of power is required for high viscous solvent and flooding is also caused at lower liquid and gas flow rates.

Selection of solvent

• Corrosiveness- Non-corrosive or less corrosive solvent.• Cost- The solvent should be cheap.• Toxicity and Hazard- The solvent should be non-toxic,

nonflammable, non-hazardous and should be chemically stable.

For stripping, an inert gas must be selected. Steam is usually used for this purpose as, it is easy to condense and separate from solute gas at the outlet.

Mass Balance (Single component)

yyY

1x

xX

1

Mole ratios

Amount of gas Vs

YVyVVs

1

)1(

Amount of solvent Ls

)1()1(

XLxLLs

tsssts YVXLYVXL Mass balance

t

s

st

s

s XVLYX

VLY

Self study - Phase I 9

Mass Balance


Packed Column Height

Considering a section of the column and applying mass balance,Gasin = Gasout + gastransferred

If S is the cross sectional area of the column and G, N are the molar fluxes of gas entering and gas getting diffused,

zSaNzzYGSzYGS )()(

a- Interfacial area of contact



dzdYGaN

Also, N can be given by

*YYKN y Ky is the overall mass transfer coefficient for gas phase



*YYaKdzdYG y

*0

2

1YYdY

aKGdzH

H y

y y

Therefore, height of the column can be found by integrating

NTUHTUH *



Y* can be replaced by the equation of the equilibrium curve and hence, NTU can be evaluated to find the height of the column.

Height of the column can also be evaluated by HETP (Height Equivalent to a Theoretical Plate), which is an empirically determined value, specific to a column.

Applications

Absorption– Removal of gases like H2S from hydrocarbons using

amines as solvent.– Washing of ammonia from ammonia-air mixture

using water as solvent.– Gas purification by removal of CO2.

Applications

Desorption– Removal of volatile organic solvents from

groundwater.– Removal of TBT(tributyltin) from harbour soils. – Removal of sulphides and ammonia from ‘sour

water’

absorption and stripping - mass transfer

Engineering