Absorption - libvolume2.xyzlibvolume2.xyz/biotechnology/semester3/introductiontounit...absorption and decreasing the quantity of solvent required. ... Wetted wall Column:-Single tube

Absorption

Prepared by Dr.Nagwa El-Mansy

Cairo University

Faculty of Engineering

Chemical Engineering Department

Forth year

-References:

1-Coluson and Richerdson, Chemical Engineering vol ,

vol II , vol III.

2- Geancoplis, Principles of Unit Operation.

3- Mc-Cabe and Smith, Unit operations for Chemical

Engineering.

4- Traybal, Mass Transfer Operations.

5- Sherwood, Mass Transfer.

6-Perry’s , Chemical Engineering.

7- “Separation Process Principles”, 2nd ed, Seader et’al .

8- Site on Google search, Separation Processes.

Absorption-:Absorption

It is a gas-liquid mass transfer operation in which liquid solvent is contacted with gas mixture for differential dissolution of one or more components of gas and provide a solution of them in liquid.

-Uses of absorption:

1- Purification of gases (H₂S from HC’s).

2-Separation of gases (separation of dry gas [C₁,C₂]

from LNG [C₃,C₄].

3- Production of useful liquid product:-

HCL (g) + H₂O (liq) → HCL (liq)

2NO₂(g) + H₂O (liq) → HNO₃ + HNO2

SO₃ (g) + H₂O (liq) → H₂SO₄

Applications of absorption:

1- Hydrogen sulfide(H2S) is removed from hydrocarbon gases by washing with alkaline solution (Amines).2- Washing ethanol vapors from carbon dioxide from molasses fermentor tanks with water to remove ethanol.3- Acetone can be recovered from acetone-air mixture by passing the gas stream into water in which acetone is dissolved while air is passed out.

4-Carbon dioxide present in air is absorbed by sodium hydroxide (NaOH solution) in which chemical absorption takes place.

5- Nitrogen oxides are absorbed in water to give nitric acid.

6- Removal of ammonia coming from coke ovens by water

Choice Of Solvent For Gas Absorption

The factors to be considered are

High absorption power

Which means that gas solubility should be high in the

solvent, which results in increasing the rate of

absorption and decreasing the quantity of solvent

required.

Highly Selective

The selectivity of solvent must be high in which solvent

dissolve one and leave the others.

Easy to recover

Which means easily to be regenerated.

Low volatility

The solvent should have a low vapor pressure to

reduce loss of solvent in the gas leaving the

absorption column.

Small viscosity

Low viscosity is preferred for reasons of rapid

absorption rates, improving flooding characteristics

in packed column, low pressure drops on pumping

and good heat transfer characteristics.

Cost

The solvent should be inexpensive, so that losses

are not costly, and should be readily available.

Other properties

Non-toxic, Non-flammable, Non-corrosive,

Chemically stable, low freezing point

Absorption Equipments

(A) Plate Towers:-

1-Multistage contact.

2-High separation , high capacity.

3-Relatively large diameter.

4-Cooling is done by

providing the plate

with cooling coils.

5- High pressure drop.

6- Easy to be clean.

(B) Packed Columns

1-Differential contact.

2-Used for highly

corrosive materials.

3- Small diameters <70-80 cm

4-Not easy to clean.

5-Packing materials are

made from(ceramics ,

bricks, wood, gravels,

stones , steel ,……)

6-To increase surface area of contact between the two phases in

packed columns, make more than one section which increase

the performance of

the tower.

7- cooling is done by dividing

the column

To many sections

out side the column

(as seen in the opposite

Figure).

(C) Spray Column:-

1- Continuous contact.

2- Low pressure drop.

3- Low efficiency.

4- Low cost(empty).

5- Gas phase controlling.

6- Considered as one stage.

-(D) Wetted wall Column:

Single tube wetted wall column used

in labs for measuring mass transfer

coefficient.

-(E) Tubular Reactor:

1- used for highly

exothermic reactions.

2-for highly heats of

reactions.

3- proper for heat

transfer control.

4- low mass transfer

due to small surface

area of contact.

-Tube Absorber:-(F) U

1- Specially for highly corrosive

materials(HCL)

2- Small surface area of contact

between two phases.

4-Simple in construction.

5-Use any material of

construction( ceramic, cast iron,

silicon,……)to over come

corrosion problems.

6-Very difficult in casting and

welding.

(G)Centrifugal type of

-absorption:

1- Single stage absorber.

2-Co-current contact

3-Used for highly viscous

liquids.

4-Used for foamy liquids.

5-Liquids are sprayed by

centrifugal force.

6-Provied good contact

between two phases.

7-Operating and initial cost

are very high.

Equilibrium Relations:-

Mass transfer between G/L depends

highly on the equilibrium between

G/L. Different gases and liquids yield

separate solubility curves , which

must be determined experimentally

for each system. If the equilibrium

pressure of a gas at a given liquid

concentration is high, as case (A) in

the opposite figure, the gas is said to

be relatively insoluble in liquid ,

while if its low, as for curve (B) , the

solubility is said to be high.

Effect of temperature on the equilibrium curve

Effect of temperature on the -equilibrium curve:

The solubility of any gas is influenced by the temperature. If the temperature of the system at equilibrium is raised , the solubility of a gas decreases . As shown in the opposite figure as temperature increases for the same solute (gas) the solubility decreases from (10-60)oC and the absorption power decreases .

Absorption process is usually accompanied by evolution of heat. So It is necessary to fit coolers to the absorber to keep its temperature sufficiently low.

Types of Equilibrium Relations :-

For dilute concentrations of many

gases the equilibrium relationship is

given by Henry’s law which relates

the partial pressure developed by a

dissolved solute(A) in a liquid

solvent (S) by the following

equation:-

PA = H xA

Where:-

H is Henry’s constant expressed as

kPa / mole fraction solute in liquid,

PA is the partial pressure of solute in

kPa,

xA is the mole fraction of the gas in

liquid phase

Henry’s law holds very well when the

partial pressure of the solute is less

than atmospheric. Above

atmospheric pressure , H may be

independent of the partial pressure.

The variation of H with temperature is

strongly nonlinear function.

For ideal systems Raoult’s law is valid:-

PA = PoA xA

Where

PA , is the partial pressure of solute .

PoA , is the vapor pressure of solute.

xA , is the mole fraction of the solute in

the liquid phase.

A A A

A A AT

T T

A A A A

AA

A B

P = H x

P H x = (by dividing each tearm by P )

P P

y = m* x (where x andy are mole fractions)

Conversion from mole fraction to mole ratio:-

nmole fraction of A in gas phase y = ,

n + n

Q

AA

A S

AA

B

AA

S

nmole fraction of A in liquid phase x =

n + n

n mole ratio of A in gas phase y = ,

n

nmole ratio of A in liquid phase x =

n

A A A

A A

A B A S

A SA B

A B B B A S S S

A A

A A

A A

A A A A

AA

A A

AA

A

P = H x

n m* n =

n + n n + n

m*n /nn /n =

n /n +n /n n /n +n /n

Y m* X =

Y +1 X +1

Y m*X =

Y +1-Y X +1- m*X

m*XY

X +1- m*X

or

m*X Y

X (1-

∴ =

=

Q

(an equilibrium relation at certain temperature and pressure) m*)+1

-Factors affecting absorption process:

-Choice of solvent flow rate :-A

-Usually given:

1- gas flow rate(Gin).

2- feed composition(yin).

3-solvent composition( xin ).

4-degree of separation= sharpness of

separation.

recovery= (Yin – Yout )/ Yin

in which we can calculate the outlet gas

composition.

Yout = Yin (1 – recovery)

Here we want to calculate proper

)insolvent rate(L

By using mole or mass ratios we must remove the amounts of

solute from gas and liquid flow rates as flows:-

Ginert = G’ = G in ( 1 – yin) . Where yin is feed mole fraction

L inert = L’ = L in ( 1 – x in). Where xin is solvent mole fraction

By making material balance on the absorber:

G’ Yin + L’ Xin = G’ Yout + L’ Xout

G’ ( Yin – Yout ) = L’ ( Xout – Xin)

L’ / G’ = (Yin – Yout) / (Xout – Xin) [ Operating line equation]

Operating line is a line between two points (Xin,Yout) and

(Xout, Yin) and has a slope - L’/G’

As the amount of solvent decreases (L’) the slope of the operating

line decreases and goes down and the number of stages increases.

-The effect liquid amount on the number of stages:

As the amount of liquid solvent decreases the driving force

decreases and the number of stages increases thus the tower

cost increases till operating line cuts or touch the equilibrium

curve at this point we reach pinch point which means no

separation.

But if we increase the amount

of liquid solvent ,the slope of

the operating line goes up

and the driving force

increases which means small

number of stages is

required( also small number

of transfer units). This

means that we must make

optimization for liquid

amount as shown in the

opposite figure.

We have to use L/G > ( L/G)Min

(L/G) Opt =[ 1.2 to 2.5 ] (L/G) Min

-(B)Temperature:

In general absorption process is an exothermic process , it improves by lowering temperature. Thus we make good cooling for liquid solvent before entering the

Increasing column. -temperature results in:

1- equilibrium curve goes up and absorption power decreases.

Notice that for the same Y the separation increases with decreasing temperature as shown in the opposite Figure.

X3 < X2 < X1

2-For same (L/G), Number of stages (or NTU) increases , means

tall column and high cost ,which is bad conditions.

3-for same(L/G) , driving force decreases and separation

becomes difficult ,which is a bad conditions

In some cases even refrigeration is

economic, this happen when

losses in solvent is high ( to

minimize losses = economic).

Some times average slight

increase in temperature is

permissible and have +ve

-effect when:

1- Solvent has high viscosity.

2- Case of chemical reaction, in

which rate of absorption is

affected positively by increasing

temperature.

)MaxThe highest temperature (T

in the absorber can be found at

.the bottom of the column

-(C)Pressure:

As pressure increases absorption power increases

PA = HA xA( Henry’s law)

PA / PT = (HA / PT) xA

y A = m* xA

-Increasing pressure results in:

1- the equilibrium curve goes down which improves the absorption process.

Notice that for the same (Y) the separation increases with increasing pressure as shown in the opposite figure

X3 < X2 < X1

2-For same (L/G), Number of stages (or NTU) decreases ,means

short column and low cost which is good conditions

3-Driving force increases and separation becomes more easier

which means good separation.

-Physical vs chemical absorption:

There are 2 types of absorption processes: physical absorption and

chemical absorption, depending on whether there is any chemical

reaction between the solute and the solvent (absorbent).

When water and hydrocarbon oils are used as absorbents, no

significant chemical reactions occur between the absorbent and

the solute, and the process is commonly referred to as physical

absorption.

When aqueous sodium hydroxide (a strong base) is used as the

absorbent to dissolve an acid gas, absorption is accompanied by

a rapid and irreversible neutralization reaction in the liquid

phase and the process is referred to as chemical absorption or

reactive absorption.

More complex examples of chemical absorption are processes

for absorbing CO2 and H2S with aqueous solution of mono -

ethanolamine (MEA), di -ethanolamine (DEA), di-

ethyleneglycol (DEG) or tri-ethyleneglycol (TEG), where a

reversible chemical reaction takes place in the liquid phase.

Chemical reactions can increase:-

1- the rate of absorption.

2- increase the absorption capacity of the solvent.

3-increase selectivity to a certain components of the gas, and

convert a hazardous chemical to a safe compound.

-Physical absorption:

-Chemical absorption:

A solute of gas (A) is absorbed from a mixture by solvent liquid(B),

which combines with (A) according to the equaRon A + B→ AB.

As the gas approaches the liquid interface, it dissolves and

reacts at once with (B). The new product(AB),thus formed ,

diffuses towards the main body of the liquid.

The concentration of (B) at the interface falls; this results in

diffusion of (B) from the bulk of the liquid phase to the

interface. Since the chemical reaction is rapid,(B) is removed

very quickly, so that it is necessary for the gas (A) to diffuse

through part of the liquid film before meeting (B). There is a

zone of reaction between A and B which moves away from the

gas-liquid interface. The final position of this reaction zone will

be such that the rate of diffusion of (A) from the gas-liquid

interface is equal to the rate of diffusion of (B) from the main

body of the liquid.

, For instantaneous Eight distinct kinetic regimes are observed. , the reaction occurs only reaction or for rapid chemical reaction

in liquid film during the transportation of component(A). The concentration of (A) in the bulk of the liquid is zero(rate of reaction of A ((rA)=0), such as absorption of acid gas. These reactions are characterized by Hatta number (Ha) :-

Ha = max possible conversion in liquid film/max diffusion transport through the liquid film

= (K’ CBo δ2L /DA)> 3 where:- K’ = the reaction constant.

CBo = liquid concentration

δ2L = liquid film thickness

DA = diffusivity of solute A

occurs in for very slow chemical processes At the other extreme, the liquid bulk no reaction occurs in the film and mass transfer is used to keep the bulk concentration of component (A) close to the saturation value ( CA = CA*). These reactions are characterized by Ha<< 1 such as oxidation, hydrogenation.

Notice that chemical reaction affect the equilibrium curve.

-Effect of temperature on the absorption tower:

Many absorbers and strippers deal with dilute gas mixtures and liquids, in these cases it’s assumed that the operation is isothermal. But actually absorption operations are usually exothermic, and when large quantities of solute gas are absorbed to form concentrated solutions, the temperature effects cannot be ignored. If by absorption the temperature of the liquid is raised to a considerable extent, the equilibrium solubility of the solute will be appreciably reduced. Cooling must be done to over come the increase in liquid temperature.

Consider the tray tower shown in the figure. If Qc is the heat removed per unit time from the tower by any means.

-:Enthalpy balance

-adiabatic operation:-For non

G ( Yin – Yout) q = L cp ( Tout - Tin ) + Qc .

.For adiabatic operation

G (Yin –Yout) q = L cp (Tout -Tin )

Cp = specific heat for pure liquid.

q = heat of absorption (J/mole).

By studying the adiabatic operation there

will be some assumptions must be

considered:-

1-No heat is removed inside the tower,

Qc= zero.

2-All the amount of heat due to absorption

increase the liquid temperature only.

3-No evaporation in the liquid solvent

(no losses).

To estimate the temperatures inside the

absorber, the heat balance equation to

compute the temperature of the liquid

leaving each plate from the top to the

bottom ,is as shown in the opposite figure:-

-):1Section (

L ( X1 - Xin) q = L Cpliq (Tout 1 -Tin )

X1 – Xin = [Cp liq /q ] ( Tout 1 - Tin )

-):2Section (

L ( X2 – Xin ) q = L Cp liq ( Tout 2 – T in )

X2 – X in = [Cp liq / q] ( Tout 2 – T in )

After calculating X1 , X2 , X3 ,……., we must

plot new equilibrium curve differ than

the case of isothermal absorption.

-component absorption:-Multi

-(A) Graphical method:

The procedures for multicomponent absorption are identical for binary mixtures. Instead of having a single equilibrium curve and operating line, there are now an equilibrium curve and operating line for each absorbed component of the gas. Gas flow rate(G) and liquid flow rate(L) are approximately constant through the column. The operating line is located with the point (xin,yout) and the slope (L/G) for the key component(the component which has more data), and because the feed composition is known we can locate the terminal point of the operating line (xout,yin). Now the number of stages required for specified recovery can be determined by stepping-off stages from the other end . Exactly the same number of stages are available for the other components. Also the operating lines must have the same slope. Thus we can calculate the recovery for each entering component.

Equilibrium relations may be based on the mole fraction

-(B) Analytical method:

Kremser equation represents an

analytical solution to a classical

separation problem of N ideal

equilibrium stages concerned with

countercurrent gas and liquid flow.

The equilibrium and operating

relations are assumed to be linear.

keyBy using the data of the

and by calculating the component

absorption factor (A=L/m*G) we can

calculate number of stages from the

following chart. After calculating N

we can calculate the recovery for

each other component.

-Stripping (desorption):

Stripping is the opposite of

absorption and involves the

removal of dissolved gases in

liquid by stripping agent.

-Purpose of stripping:

1- recover the dissolved solute.

2- recover the solvent.

3-to recover both solute and

solvent.

Usually absorption is followed

by stripping or desorption.

The most commonly used

stripping agent is steam.

-Good stripping agent must be:

1- easily condensed.

2- easily separated from the material stripped.

-Equilibrium relations:

(As absorption)

The following points must be -:taken into consideration

1-Operating line is under the equilibrium curve.

2- slope of the operating line

= L/G= (Y’in-Y’out / X’out-X’in )

3- As (L/G) decreases, G increases , operating line goes down, driving force increases, N decreases and NTU decreases.

4-As (L/G) increases, G decreases, operating line goes up, driving force decreases, N increases and NTU increases, till we reach pinch point.

-Special types of absorbers:

) Absorber with reboiler 1(

)(combined absorber/stripper

When wet gas( C2H6/ C3H8/C4 H10) is

contacted with oil solvent it dissolve

small amount of C2H6.

C2H6 can be concentrated in the gas

stream leaving the column by heating

the rich liquid oil stream to strip out

C2H6.

)Absorption with two 2(

solvents

Recovery of highly volatile solvent

e.g. recovery of C5 from C3 and C4.

Solvent should have low vapor

pressure to minimize losses in it’s

amount. A second less volatile

solvent (Kerosene)can be used to

recover the evaporated portion

of the first solvent (Benzene).