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ENSC3019/CHPR8503: Week 6 Design of Packed Columns

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Dr Kevin Li Kevin.li@uwa.edu.au

Recommended reading: McCabeet al. , Unit Operations of Chemical Engineering,Chapter 21 Treybal, R. E. Mass Transfer Operations, 3rd Edn. McGraw-Hill 1955,Chapter 9 Coulson, J. M. and Richardson, J. F. Chemical Engineering, Volume 6: Particle Technology and Separation Processes, 5th Edn. Butterworth-Heinemann 2002

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Mass Transfer (MT) across phase interface: two- resistance model

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Gas film Liquid filmBulk gas Bulk liquid y A,G

y A,i

x A,i

x A,L

distance

Resistances to diffusion of A: (i) in the gas phase film (ii) in the liquid phase film

At the interface: assume local equilibrium between y A and x A, no resistance to MT across the interface

( ), , A y A g A i N k y y= − ( ), , A x A i A L N k x x= − 1 yk ∝ 1 xk ∝

see McCABE et al. p547; BENÍTEZ p165

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Mass-transfer coefficients: an engineering concept that allows us to simplify complex

diffusion problems.

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( ) A y i N k y y= −

Flux (mole/m2/s) Coefficient

Driving force (concentration

difference) = ×

Since concentration could be defined in different ways, a variety of coefficients can be defined:

• k y , k x, K y , K x ……

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Summary of general forms of MT Rates for two-phase films

ky is local MTC for gas phase y i is mole fraction (of component A) in gas at the gas- liquid interface , y is bulk vapour composition

kx is local MTC for liquid phase xi is mole fraction (of component A) in liquid at the gas-liquid interface, x is bulk liquid composition

K y is overall MTC for gas phase y* is composition of vapour that would be in equilibrium with the bulk liquid of composition x

K x is overall MTC for liquid phase x* is composition of vapour that would be in equilibrium with the bulk vapour of composition y

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( ) A y i N k y y= −

( ) A x i N k x x= −

( )* A x N K x x= − ( )* A y N K y y= −

MTC=mass transfer coefficient. Subscripts A, and G, L dropped here for simplicity.

See McCabe et al. page 547-548. Or if you’re keen for more discussion look at Treybal’s Chapter 5..

m’ is local slope of equilibrium curve

i.e.

1 1 '

y y x

m

K k k

= + ( ) ( )

*' i im y y x x

= − −

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Tutorial 1 Equilibrium for component A between air and water is described by Henry’s law y*=4x . The local mass transfer coefficients are k x =2 mol m -2s-1 and ky =1 mol m -2s-1 .

(1) What is the overall mass transfer coefficient for gas phase? (2) Evaluate the flux of A between phases at a point in a

column where bulk compositions are 0.08 mole fraction inthe gas and 0.01 mole fraction in the liquid.

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6.1 Packed columns for absorption

Dr Kevin Li Kevin.li@uwa.edu.au

Consultation hours 15:00-17:00Thursdays 2.49A in Civil & Mech Eng building

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Equipment for gas-liquid absorption Need intimate contact between the immiscible phases to achieve mass transfer (MT) between phases. Flux N A

rate of transfer per unit area of gas-liquid interface

Engineering MT equipment focuses on increasing

the interfacial area for transfer (

)

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Main equipment types

Packed columnsRandom (let to fall randomly into column during installation) Structured (engineering for lower Δ P, higher cost )

Tray columns - liquid levels on each tray

Gas sparging tanks

http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html

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Column internals

9 GREEN, D. W. & PERRY, R. H. (eds.) ( 2008). Perry's chemical engineers' handbook, New

York: McGra w-Hill.

Packing material, plus

Liquid inlet systems

Liquid & vapour distributors

Liquid collecting devices Packing supports

Good info at manufacturer www.sulzechemtech.com

http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html http://www.sulzechemtech.com/ http://www.sulzechemtech.com/ http://www.co2crc.com.au/imagelibrary2/vid_absorp_desorp.html

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Packed columns – random packings

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Metal pall rings

Raschig rings

VSP Inner arc ring

see more images at

www.tower-packing.com

http://www.tower-packing.com/ http://www.tower-packing.com/ http://www.tower-packing.com/ http://www.tower-packing.com/

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Structured packings www.sulzerchemtech.com

Mellapak TM

www.sulzerchemtech.com

Grids

http://www.sulzerchemtech.com/ http://www.sulzerchemtech.com/ http://www.sulzerchemtech.com/ http://www.sulzerchemtech.com/

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Tray columns

V-grid www.sulzerchem.com

Sieve tray

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High performance trays eg. Shell calming section tray

www.sulzerchem.com

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1. Tray columns can be designed to handle a wider range of liquid and gas flow rates. Packed columns are not suitable for very low liquid rates.

2. The efficiency and performance of a tray column can be more accurately predicted.

3. Easier to make provisions for withdrawal side streams in plate columns.

4. Fouling & cleaning: can install manholes on trays. However, may be easier to replace packing when fouled.

Plate columns can be designed with more assurance - some doubt that good liquid distribution can be maintained in a packed column.

It is easier to provided cooling or heating in a plate column – coils

directly on plates.

Coulson and Richardson Vol 6. list some of the factors which influence choice of trays or packing in a column:

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Trays/Plate columns vs. Packed columns 5. For corrosive liquids a packed column will be cheaper

than a plate column (due to materials).

6. The liquid hold-up is lower in a packed column. Important

if amount toxic or flammable liquid needs to be keep lowfor safety.

7. Packed columns are more suitable for foaming systems

8. The pressure drop per equilibrium stage can be lower for packed columns.

9. Packing cheaper for small columns, d < 0.6 m

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Column internals – process design

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Process design or process tech support to operation needs to consider:

Type of contacting device

Number equilibrium stages Height of packing required

Pressure drop

Fouling Corrosion and other materials issues

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MT Rate,r A, for absorption per unit volume of packed column

k ya is local MTC for gas phase on unit volume basis y i is mole fraction (of component A) in gas at the gas- liquid interface , y is bulk vapour composition

k xa is local MTC for liquid phase on unit volume basis xi is mole fraction (of component A) in liquid at the gas-liquid interface, x is bulk liquid composition

K ya is overall MTC for gas phase on unit volume basis y* is composition of vapour that would be in equilibrium with the bulk liquid of composition x

K xa is overall MTC for liquid phase on unit volume basis x* is composition of vapour that would be in equilibrium with the bulk va