TRAPHE3- Gas Absorption

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MASS TRANSFER

Gas Absorption

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Absorption and Stripping

• Absorption (or scrubbing) is the removal of a component (the solute or absorbate) from a gas stream via uptake by a

nonvolatile liquid (the solvent or absorbent).

• Stripping (or desorption) is the removal of a component from a liquid stream via vaporization and uptake by an insoluble gas stream.

• Thus, absorption and stripping are opposite unit operations, and are often used together as a cycle.

• Both absorption and stripping can be operated as equilibrium stage processes using trayed columns or, more commonly, using packed columns.

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Absorber/Stripper Cycle

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Absorption operations

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Absorption Systems

Absorption Systems –

–

– Physical

–

Physical

• Examples:

CO₂ and water Acetylene and acetic acid

CO and water NH₃ and acetone

H₂S and water Ethane and carbon disulfide

NH₃ and water N₂ and methyl acetate

NO₂ and water NO and ethanol

• Physical absorption relies on the solubility of a particular gas in a liquid.

• This solubility is often quite low; consequently, a relatively

large amount of liquid solvent is needed to obtain the required separation.

• This liquid solvent containing the solute is typically regenerated by heating or stripping to drive the solute back out.

• Because of the low solubility and large solvent amounts

required in physical absorption, chemical absorption is also used…

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Absorption Systems

Absorption Systems –

–

– Chemical

Chemical

• Chemical absorption relies on reaction of a particular gas with a reagent in a liquid.

• Examples:

– CO₂ / H₂S and aqueous ethanolamines – CO₂/ H₂S and aqueous hydroxides – CO and aqueous Cu ammonium salt – SO₂ and aqueous dimethyl aniline – HCN and aqueous NaOH

– HCl / HF and aqueos NaOH

• This absorption can often be quite high; consequently, a

smaller amount of liquid solvent/reagent is needed to obtain the required separation.

• However, the reagent may be relatively expensive, and it is often desirable to regenerate when possible.

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Absorption and Stripping Assumptions

Assumptions:

– The carrier gas(inert gas) is insoluble (or it has a

very low solubility), e.g, N

₂

or Ar in water.

– The solvent is nonvolatile (or it has a low vapor

pressure), e.g., water in air at low temperatures.

– The system is isothermal. e.g., the effects of heat of

solution or reaction are low or there is a cooling or

heating system in the column.

– The system is isobaric.

– The concentration of the solute is low, say <10% –

this is the limit for the use of Henry’s Law,

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Equilibrium Data for Ammonia

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Equilibrium Data for SO

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Material Balance in an Absorption Tray Tower

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Material Balance Diagram for Plate Column

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Absorption: Design considerations

y x (mole fraction of A in L) (mole fraction of A in V) x_a x_b y_a y_b equilibrium line x*_b 1 +

−

n e

y

The driving force for mass transfer

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Operating and equilibrium lines: (a) for gas absorption (b) fo Operating and equilibrium lines: (a) for gas absorption (b) foOperating and equilibrium lines: (a) for gas absorption (b) fo Operating and equilibrium lines: (a) for gas absorption (b) for r r r desorption

desorptiondesorption desorption

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No. of Theoretical Stages in Plate Absorbers

Graphical Solution – Mc Cabe-Thiele Method

(a) Using mole fractions (x, y)

To determine the theoretical number of stages, plot

the equilibrium diagram (EC) and the operating line

(OL). From the point (

x

₀

,y

₁

)

or (

x

_a

, y

_a

)

draw triangular

steps using alternately the OL and EC until the point

(

x

_N

, y

_N+1

) or (

x

_b

, y

_b

) is reached or overrun.

The

theoretical no. of stages is equivalent to the no. of

triangles formed.

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McCabe

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McCabe

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Absorption Factor Method

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Concentration Differences

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No. of Theoretical Stages in Plate Absorbers

Analytical Solution

– If OL and EC are both linear use

Absorption Factor or Kremser Equation

(a) based on the gas phase (V phase)

* a * b a b * a a * b b * a a * b b y y y y ln y y y y ln A ln y y y y ln N − −− − − −− − − −− − − −− − = = = = − − − − − − − − = = = =

where : _{N = no. of theoretical stages or plates}

* a * b a b y y y y mV L A − − − − − − − − = = = = = = = = A = absorption factor

y* = equilibrium concentration corresponding to x y* = equilibrium concentration corresponding to xy* = equilibrium concentration corresponding to x y* = equilibrium concentration corresponding to x

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No. of Theoretical Stages in Plate Absorbers

Analytical Solution

– If OL and EC are both linear use

Absorption Factor or Kremser Equation

(a) based on the liquid phase (L phase)

* b * a b a * b b * a a * b b * a a x x x x ln x x x x ln S ln x x x x ln N − − − − − − − − − − − − − − − − = == = − − − − − − − − = = = =

where : _{N = no. of theoretical stages or plates}

* b * a b a x x x x L mV A L S − − − − − − − − = == = = == = = = = = S= stripping factor

x* = equilibrium concentration corresponding to y x* = equilibrium concentration corresponding to yx* = equilibrium concentration corresponding to y x* = equilibrium concentration corresponding to y

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Minimum Absorbent Rate

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Limiting Liquid to Gas Ratio

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Minimum Liquid to Gas Ratio for Absorption

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Minimum Liquid to Gas Ratio for Absorption

• For a given gas flow, a reduction in liquid flow decreases the slope of the operating line

• As the upper end of the operating line (OL) is shifted in the direction of the equilibrium line the concentration of the strong liquor, x_b or x_n increases.

• The maximum possible liquor concentration and the minimum liquid rate (L_min or L’_min) are obtained when the operating line just touches the equilibrium diagram

• At L_min or L’_min an infinite no. of plates or an infinitely deep packed section is necessary. At this condition the concentration difference for mass transfer becomes zero at the bottom of the tower

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Minimum Liquid to Gas Ratio for Absorption

• In any actual tower the liquid rate must be greater than this maximum to achieve the specified change in gas composition

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Problem # 1

Problem # 1 –

–

– Plate Absorber

–

Plate Absorber

A tray tower is absorbing ethyl alcohol from an inert gas stream A tray tower is absorbing ethyl alcohol from an inert gas streamA tray tower is absorbing ethyl alcohol from an inert gas stream A tray tower is absorbing ethyl alcohol from an inert gas stream using pure water at 303 K and 101.3

using pure water at 303 K and 101.3 using pure water at 303 K and 101.3

using pure water at 303 K and 101.3 kPakPakPakPa. The inert gas stream . The inert gas stream . The inert gas stream . The inert gas stream flow rate is 100.0 kg/h and it contains 2.2 mol% alcohol. It is flow rate is 100.0 kg/h and it contains 2.2 mol% alcohol. It is flow rate is 100.0 kg/h and it contains 2.2 mol% alcohol. It is flow rate is 100.0 kg/h and it contains 2.2 mol% alcohol. It is desired to recover 90% of the alcohol. The equilibrium desired to recover 90% of the alcohol. The equilibrium desired to recover 90% of the alcohol. The equilibrium desired to recover 90% of the alcohol. The equilibrium relationship is y =

relationship is y = relationship is y =

relationship is y = mxmxmxmx = 0.68x for the dilute stream. Calculate = 0.68x for the dilute stream. Calculate = 0.68x for the dilute stream. Calculate = 0.68x for the dilute stream. Calculate the number of trays graphically and analytically for an

the number of trays graphically and analytically for an the number of trays graphically and analytically for an

the number of trays graphically and analytically for an operaingoperaingoperaingoperaing flow rate of 1.3 times the minimum.

flow rate of 1.3 times the minimum.flow rate of 1.3 times the minimum. flow rate of 1.3 times the minimum.

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Problem # 2

Problem # 2 –

–

– Plate Absorber

–

Plate Absorber

A mixture of 5% butane and 95% air is fed to a sieve-plate absorber containing eight ideal plates. The absorbing liquid is a heavy, nonvolatile oil having a molecular weight of 250 and a specific gravity of 0.90. The absorption takes place at 1 atm, and 15o_{C. The butane is to be recovered to the}

extent of 95%. The vapor pressure of butane at 15o_C

is 1.92, and liquid butane has a density of 580 kg/m3

at 15o_{C. (a) Calculate the cubic meters of fresh}

absorbing oil per cubic meter of butane recovered. (b) Repeat, on the assumption that the total pressure is 3 atm and all other factors remain constant. Assume that Raoult’s Law and Dalton’s Law apply

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Problem # 3

Problem # 3 ---- Plate Absorber

Plate Absorber

A relatively nonvolatile hydrocarbon oil contains 4.0 mol% propane and is being stripped by direct superheated steam in a stripping tray tower to reduce the propane content to 0.2%. The temperature is held constant at 422 K by internal heating in the tower at 2.026 x 105 _{Pa pressure. A total of 11.42 kgmole direct}

steam is used for 300 kgmol of total entering liquid. The vapor-liquid equilibria can be represented by y = 25x, where y is mole fraction propane in the steam and x is the mole fraction propane in the oil. Steam can be considered an inert gas and will not condense. Plot the operating and equilibrium and determine the number of theoretical trays needed.

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Notes on Absorbers

• Note that the operating line for an absorber is above the equilibrium curve. For a given solute concentration in the liquid, the solute concentration in the gas is always greater than the equilibrium value, which provides the driving force for the separation.

• The solute is transferred from the gas to the liquid in absorption. In

distillation, we plotted the more volatile component, which was transferred from the liquid to the gas. In distillation, if we had plotted the less volatile component, which was transferred from the gas to the liquid, the OL would also lie above the equilibrium curve.

• Also note that the OL is linear. This results because of the form of the operating line where L’/V’ is a constant. L’ and V’ are based upon the nonvolatile solvent and insoluble carrier gas, respectively, which do not change.

• If we had used mole fractions and total gas and liquid rates, the OL would be curved because the total gas and liquid rates would change since we are

removing the solute from the gas and absorbing it into the liquid.

• One could use mole fractions and the total gas and liquid streams in our calculations only if the solute is in low concentrations, say < 1%, in most systems. Don’t confuse this requirement with that for the use of Henry’s Law, which requires low solute concentrations, < 10%, to be valid.

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Final Notes on Strippers

• We use the same assumptions and mole ratio methods that we used for absorbers for the design of strippers.

• The OL will be the same as that used for absorbers.

• The difference, compared to an absorber, is that the equilibrium curve will be below the operating line.

• This is analogous to the stripping section of a distillation column. • Just as we stepped up from the bottom of a distillation column’s

stripping section, we step up from the bottom of the stripper.

• Thus, one uses the same McCabe-Thiele algorithm method that we used for stepping up in distillation.

• In the algorithm, the equilibrium relationship is expressed as YEq = YEq(XEq), and the operating line is expressed in the form YOL = XOL(YOL).

• Otherwise, the design approach for strippers is the same as that for absorbers.