# Etiket Arşivleri: Absorption

# Absorption operations in packed column

# A packed column is a cylindrical vessel under pressure, filled with a suitable packing material between the internal section 1 and 2.

# The liquid flows down by gravity through the surface of the packing elements forming a thin film layer around it. The gas rises in counter-current mode with the liquid stream.

# The mass transfer takes place through the interface film present between the gas and the liquid.

# The column is now a continuous differential contacting devices. There is no distinguishable stage (like the plates in a tray column) present anymore.

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# Aerosol & Particulate Research Lab

# Gas Absorption

# Definition: transfer of a gaseous component (absorbate) from the gas phase to a liquid (absorbent) phase through a gas-liquid interface.

Q: What are the key parameters that affect the effectiveness?

Q: How can we improve absorption efficiency?

Mass transfer rate:

gas phase controlled absorption

liquid phase controlled absorption

Reading: Chap 13

Q: Does it matter if itâ€™s gas phase or liquid phase controlled?

# Gas Absorption Equipment

Mist

Eliminator

Liquid

Spray

Packing

Liquid outlet

Dirty gas in

Spray

nozzle

Clean gas out

Countercurrent packed tower

Spray tower

Mycock et al., 1995

Redistributor

Q: Limitations of a spray tower?

Q: Why redistributor?

Clean gas out

Dirty gas in

# Berl

Saddle

Intalox

Saddle

Raschig

Ring

Lessing

Ring

Pall

Ring

Tellerette

Three-bed cross flow packed tower

Liquid spray

Dry Cell

Packing

Mycock et al., 1995

Q: Criteria for good packing materials?

# Mass Balance

In = Out

Slope of Operating

Line = Lm/Gm

Lm: molar liquid flow rate

Gm: molar gas flow rate

x: mole fraction of solute in pure liquid

y: mole fraction of solute in inert gas

Known: ??

# Unknown: ??

(for a dilute system)

Gas in

Liquid out

Gas out

Liquid in

# Generally, actual liquid flow rates are specified at 25 to 100% greater than the required minimum.

G = 84.9 m3/min (= 3538 mole/min). Pure water is used to remove SO2 gas. The inlet gas contains 3% SO2 by volume. Henryâ€™s law constant is 42.7 (mole fraction of SO2 in air/mole fraction of SO2 in water). Determine the minimum water flow rate (in kg/min) to achieve 90% removal efficiency.

Q: How much is X2 if fresh water is used? What if a fraction of water is recycled?

# Channeling: the gas or liquid flow is much greater at some points than at others

Loading: the liquid flow is reduced due to the increased gas flow; liquid is held in the void space between packing

Flooding: the liquid stops flowing altogether and collects in the top of the column due to very high gas flow

Problems with high gas flow

Gas flow rate is 3538 mole/min and the minimum liquid flow rate is 2448 kg/min to remove SO2 gas. The operating liquid rate is 50% more than the minimum. The packing material selected is 2â€ ceramic Intalox Saddles. Find the tower diameter and pressure drop based on 75% of flooding velocity for the gas velocity. Properties of air:: molecular weight: 29 g/mole; density: 1.17Ã—10-3 g/cm3. Properties of water:: density: 1 g/cm3; viscosity: 0.8 cp.

# L: mass flow rate

of liquid

G: mass flow rate

of gas

Gâ€™: mass flux of gas

per cross sectional

area of column

F: Packing factor

ï†: specific gravity

of the scrubbing

liquid

ïL: liquid viscosity

(in cP; 0.8 for water)

(dimensionless)

# Mass Transfer

Two-Film Theory (microscopic view)

Cussler, â€œDiffusionâ€, Cambridge U. Press, 1991.

pG

CI

pI

CL

J: flux

k: mass transfer coefficient

(gas phase flux)

(liquid phase flux)

(overall flux)

# Macroscopic analysis of a packed tower

Mole balance on the solute over the

differential volume of tower

(equivalent concentration

to the bulk gas pressure)

(equivalent pressure to the

bulk concentration in liquid)

(overall liquid phase MT coefficient)

(overall gas phase MT coefficient)

# Mole balance on the solute in the gas only

NTU?

HTU?

a: packing area per volume

(tower height)

# Mass balance

Equilibrium

x1, y1*

x1, y1

xZ, yZ*

xZ, yZ

Alternative solution:

Assumptions for dilute/soluble systems?

# Pure amine

Lm = 0.46 gmole/s

0.04% CO2

1.27% CO2

Gm = 2.31

gmole/s

C* = 7.3% CO2 in amine

Q: A Packed tower using organic amine at 14 oC to absorb CO2. The entering gas contains 1.27% CO2 and is in equilibrium with a solution of amine containing 7.3% mole CO2. The gas leaves containing 0.04% CO2. The amine, flowing counter-currently, enters pure. Gas flow rate is 2.31 gmole/s and liquid flow rate is 0.46 gmole/s. The towerâ€™s cross-sectional area is 0.84 m2. KOGa = 9.34Ã—10-6 s-1atm-1cm-3. The pressure is 1 atm. Determine the tower height that can achieve this goal.

# Absorption of concentrated vapor

Mole balance on the controlled volume

Gas flux

Liquid flux

x1, y1

x1, y1*

xZ, yZ*

xZ, yZ

# Mole balance on the gas in a differential tower volume

# HTU (ft)

HTU

For a given packing material and pollutant, HTU does not change much.

# Summary

Transfer from gas phase to liquid phase; Gas phase or liquid phase controlled mass transfer.

Equipment: spray tower and packed tower.

Equilibrium line (Henryâ€™s law) and operating line (mass balance).

Design: (a) liquid flow rate by mass balance; (b) tower diameter by flooding condition; (c) tower height by mass transfer rate

Dilute and concentrated system

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# Absorption and Stripping of Dilute Mixtures

Absorption

Absorption and Stripping Equipment

Absorption in a single equilibrium stage

To approach the study of the absorption/stripping operation, a certain number of assumptions are needed in order to simplify the design and easily understand the basics concepts:

The equilibrium correlation

Linear Equilibrium

Typically in an absorption or stripping problem the solute, which has to be removed, is present in the liquid or in gas phase at very

low concentration (<1%). The Henry’s law is therefore used to represent the equilibrium correlation for a solute A between the gas and the liquid phase.

yA = m xA where m = H/Ptotal

In the x-y composition diagram the equilibrium correlation is represented as follows:

Non-linear Equilibrium

Of course in the range of concentration when Henry’s law can not be applied anymore (xA>1%), the relationship in between y and x is not linear but more in general a curve, expressed as:

yA = f(xA)

which in the same mole fraction composition diagram will give a curve:

Single stage

During the absorption operation,

the gas phase and the liquid phase must be in contact. Before considering the different possible configurations, we consider the thermodynamical aspects of this contact between phases.

The solute contained in G transfers to the liquid phase L. The concentration in the gas decreases while the concentration in the liquid increases. The pairs of points (concentration of solute in the gas and liquid phases) at each moment constitute the operating

line.

Staged operations: (a) single stage; (b) co-current; (c) countercurrent; (d) crosscurrent.

Graphical Method for Equilibrium Stage Trayed Towers

Graphical Method for Equilibrium Stage Trayed Towers

Graphical Method for Equilibrium Stage Trayed Towers

Minimum Absorbent Determination of the Equilibrium Number of Stages

Graphical Determination of N for Absorption

Graphical Determination of N for Stripping

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