ERT 313BIOSEPARATION ENGINEERING

ABSORPTION

Prepared by:Miss Hairul Nazirah Abdul Halim

Topic Outline

• Introduction• Gas – Liquid Equilibrium• Mass Transfer between Phases• Unit operation for Absorption:

a) Packed towerb) Plate Column

Introduction

• Absorption – occur when the two contacting phases are a gas and a liquid.

• In gas absorption, solutes are absorbed from the gas phase into the liquid phase.

• Absorption does not destroy the gases, it simply transfers the contaminated gas from the gaseous state to the liquid state.

• Example: 1. Absorbing SO2 from the flue gases by absorption in alkaline solutions2. Hydrogenation of edible oils in food industry

- hydrogen gas is bubbled into oil and absorbed.3. Absorbing dimethyl sulfide from the food processing industry

• The reverse of absorption is called stripping or desorption.

Gas-Liquid Equilibrium

• Consider the SO2-air-water system.

• An amount of gaseous SO2, air and water are put in a closed container and shaken repeatedly at a given temperature until equilibrium is reached.

• Samples of the gas and liquid are analyzed to determine the partial pressure pA of SO2 in the gas and mol fraction xA in the liquid.

• The equilibrium plot is shown in Figure 10.2-1.

• The data for some common gases with water are given in Appendix A.3 (Geankoplis, Transport Process and Separation Process Principles, 4th ed., Prentice Hall)

• The equilibrium relation between pA in the gas phase and xA can be expressed by a straight line Henry’s Law equation at low concentration:

pA = H xA

Where H = Henry’s law constant (mol frac gas/ mol frac liquid)

Mass Transfer Between PhasesTwo-Film Theory• In absorption, solute from gas phase must diffuse into liquid

phase.• The rate of diffusion in both phases affect the overall rate of mass

transfer.• Assumption in Two-Film Theory:

a) equilibrium is assumed at the interfaceb) the resistance to mass transfer in the two phases are

added to get an overall resistance.• Nomenclature:

ky = mass-transfer coefficient in gas phase

kx = mass-transfer coefficient in liquid phase

Ky = Overall mass-transfer coefficient in gas phase

Kx = Overall mass-transfer coefficient in liquid phasea = interfacial area per unit volume

• The rate of absorption per unit volume of packed column is given by any of the following equations:

where y and x refer to the mole fraction of the component being absorbed.

• The overall coefficient:

• Where m = the local slope of the equilibrium curve.

• In Eq. (18.12),

= the resistance of mass transfer in the gas film.

= the resistance of mass transfer in the liquid film

In Eq. (18.12):

The liquid film resistance control the rate of absorption • when kya = kxa and m > 1.0.• This means that any change in kxa has a nearly proportional effect on

both Kya and Kxa on the rate of absorption,• whereas a change in kya has little effect.

The gas film resistance control the rate of absorption • when kya = kxa and m << 1.0 (very small)• Solubility of the gas is very high • Such as absorption of HCl in water and absorption of NH3 in water

Unit Operation 1: PACKED TOWER• A common apparatus used in gas absorption is the packed tower.

as shown in Figure 18.1

• The device consist of:

a) cylindrical column or tower

b) gas inlet and distributing space at the bottom

c) liquid inlet and distributor at the top

d) gas & liquid outlets at the top & bottom, respectively

e) tower packing – supported mass of inert solid shapes

PACKED TOWER

• The liquid inlet - pure solvent or weak liquor

- is distributed over the top of packing by the distributor

- uniformly wets the surfaces of the packings

• The distributor - is a set of perforated pipes (Fig. 18.1)

- a spray nozzles in a large towers

• The gas inlet - enter the distributing space below the packing - flow upward in the packing

countercurrent to the

flow of the liquid

PACKINGS• The packing - provides a large area of contact between the liquid

and gas

- encourage intimates contact between the phases

• Common tower packings is shown in Figure 18.2.

• 3 principal types: dumped packings, stacked packings and structured/ordered packings.

• Made from: plastic, metal or ceramic

Structured Packing

Ceramic Intalox Saddle Packing

Pressure Drop & Limiting Flow rates• Figure 18.4 shows typical data for the pressure drop in a packed

tower.

• Pressure drop is due to fluid friction

• The graph is plotted on logarithmic coordinates for ΔP (inches H20/ft packing) versus the gas flow rate, Gy (lb/ft2.h)

Loading & Flooding Point

• Point K is the loading point

• Point L is the flooding point for the given liquid flow.

• Loading point is a point where liquid hold up starts to increase and caused a change in the slope of the pressure drop

• Flooding point is a point where the gas velocity will result in all packing to be wetted at this point, the contact area between gas and liquid will be maximized.

Unit operation 2: PLATE COLUMN

• Plate Column absorbers distribute a contacting liquid over plates situated one above the other.

• The contacting liquid flows downward through the column from one plate to the other in a stepwise fashion.

• The inlet gas rises through each plate through openings in the plate and comes into contact with the liquid.

• Usually, a layer of foam and froth is formed above each plate resulting from the mixing of liquid and gas.

• The gas not absorbed rises through the foam layer to the next plate for another stage of absorption.

• Plate column absorbers result in a high removal efficiency since there are multiple stages of contact between liquid and gas.

• Plate columns have certain advantages over packed

bed towers:

a) plate columns can handle high gas flow rates accompanied by a low liquid flowrate with little chance of flooding.

b) little chance for channeling inside of a plate column compared to a packed bed tower.

c) sediment build-up often can be easily removed in plate column absorbers (packed bed towers are harder to clean).

• However, plate columns are usually more expensive than packed bed towers.

• The advantages of plate columns are usually not justified in small operations where a packed bed tower will suffice.

Principles of Absorption

• Material Balances:

a) Packed Column

b) Plate Column

• Limiting gas-liquid ratio

• Rate of absorption

• Calculation of tower height

• Number of transfer unit

Material Balances for Packed Column

L = molal flow rate of the liquid phase V = molal flow rate of the gas phasex = liquid phase concentrationy = gas phase concentration

Material balances for the portion of the column above an arbitrary section (dashed line)

• Total material balance:

• Material balance on component A

Overall material equations• Total material balance:

• Material balance on component A:

• Rearrange Eq. (18.3) gives operating-line equation:

• The operating line can be plotted on an arithmetic graph along with the equilibrium curve as shown in Fig. 18.10.

• The operating line must lie above the equilibrium line in order for absorption to take place.

Absorption in Plate Column

• Besides packed tower, gas absorption can be carried out in a column equipped with sieve trays or other types of plates.

• Plate column is used instead of packed column because:

a) to avoid the problem of liquid distribution in a large diameter tower

b) to decrease the uncertainty in scaleup

Plate Column

Material Balances for Plate Column

• A general stage in the system is the nth stage, which is number n counting from the entrance of the L phase.

yn+1 = mole fraction of component A in the V phase leaving stage n + 1.

Ln = molal flow rate of the L phase leaving the nth stage.

Material balances for the portion of the column above an arbitrary section (dashed line)

• Total material balance:

• Material balance on component A

Overall material balance equations• Total material balance:

• Material balance on component A:

Graphical Methods for Two-Component Systems

• It is possible to solve many mass transfer problems graphically for system containing only two components.

• The operating line equation for the plate column can be rearranged from Eq. (20.2) as below:

• The operating line is a plot of the points xn and yn + 1 for all the stages.

• The equilibrium line is a plot of equilibrium values of xe and ye.• The equilibrium data is found by experiment, by thermodynamic

calculations or from published sources.• The position of the operating line relative to the equilibrium line

determines the direction of mass transfer and how many stages are required for a given separation.

Ideal Contact Stages

• The ideal stage is a standard to which an actual stage may be compared.

• In an ideal stage, the V phase leaving the stage is in equilibrium with the L phase leaving the same stage.

• For example, if plate n in Fig. 20.3 is an ideal stage, concentrations xn and yn are coordinates of a point on the curve of xe and ye showing the equilibrium between the phases.

Determination of the number of Ideal Stages

• A simple method of determining the number of ideal stages when there are only two components in each phase is a graphical construction using the operating-line diagram.

• Figure 20.5 shows the operating line and the equilibrium curve for a typical gas absorber.

FIGURE 20.5 Operating-line diagram for gas absorber

EXAMPLE 20.1. (McCabe et al., 2005)

By means of a plate column, acetone is absorbed from its

mixture with air in a nonvolatile absorption oil. The entering

gas contains 30 mole percent acetone, and the entering oil is

acetone-free. Of the acetone in the air 97 percent is to be

absorbed, and the concentrated liquor at the bottom of the tower

is to contain 10 mole percent acetone. The equilibrium

relationship is ye = 1.9xe. Plot the operating line and determine

the number of ideal stages.

SOLUTION

Calculation of Tower Height

Fig. 18.12Diagram of packed absorption tower

• Consider the packed column shown in Figure 18.12.

• The cross section is S, the differential volume in height is S dZ.

• The amount absorbed in section dZ is –V dy, which equals the absorption rate times the differential volume:

• Rearrange for integration:

• The equation for column height (ZT) can be written as follows:

Number of Transfer Units (NTU)

• The integral part in Eq. (18.16) is called the number of transfer units NTU (NOy) =

• The other part of Eq. (18.16) has the units of length and is called the height of a transfer unit (HTU) HOy:

• Hence,

• The number of transfer units is somewhat like the number of ideal stages (theoretical plates).

• The NTU = ideal stage if the operating line and equilibrium line are straight and parallel as in Fig. 18.13 a.

•For straight operating and equilibrium lines:

•Where:

•The corresponding equation based on the liquid phase:

bbb

aaa

a

b

abL

yyy

yyy

yy

yyy

ln

EXAMPLE 18.3 (McCabe).

A gas stream containing 3.0 percent A is passed through a packed column to remove 99 percent of the A by absorption in water. The absorber will operate at 25°C and 1 atm, and the gas and liquid rates are to be 20 mol/h.ft2 and 100 mol/h.ft2 respectively. Mass-transfer coefficients and equilibrium data are given below:

y* = 3.l x at 25°C

kxa = 60 mol/h.ft3. unit mol fraction

kya = 15 mol/h.ft3. unit mol fraction

(a) Find NOy, HOy, and ZT, assuming isothermal operation and neglecting changes in gas and liquid flow rates. What percent of the total resistance is in the gas phase?

(b) Calculate ZT, using NOx and HOx.

SOLUTION