design of packed columns for absorption and distillation processes_prelecture slids

Download Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

Post on 07-Jul-2018

222 views

Category:

Documents

0 download

Embed Size (px)

TRANSCRIPT

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    1/100

    ENSC3019/CHPR8503: Week 6 Design of Packed Columns

    1

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    2/100

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

    2

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    3/100

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

    diffusion problems.

    3

    ( ) 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 ……

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    4/100

    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

    4

    ( ) 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

    = − −

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    5/100

    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.

    5

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    6/100

    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

    6

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    7/100

    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 (

    )

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    8/100

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    9/100

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    10/100

    Packed columns – random packings

    10

    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/

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    11/100

    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/

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    12/100

    Tray columns

    V-grid www.sulzerchem.com

    Sieve tray

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    13/100

    High performance trays eg. Shell calming section tray

    www.sulzerchem.com

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    14/100 14

    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:

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    15/100 15

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    16/100

    Column internals – process design

    16

    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

  • 8/18/2019 Design of Packed Columns for Absorption and Distillation Processes_prelecture Slids

    17/100

    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