1. Fixed and Fluidized Beds

2. Extended Damköhler Equations
Transfer operations:
-Absorption
-Adsorption
-Drying
-Heat exchange
-Distillation
-Extraction
-Evaporation
-Crystallization
-…etc.

3. Transfer flow (Jt) Jt = ε . A . ∆Г 3-ways to achive this:
ε = transfer coefficient
A = surface
∆Г = driving force
Aim of production: to make profit time is money
shorter operating times more money
3-ways to achive this:
Increase of ε (selection of proper material): i.e. NH3 dissolves better in water than in organic solvents.
Increase of A (ω): Bigger apparatus ⇒ bigger suurface (but higher equipment cost)
Increase of ∆Г (gradient): limited by the operation conditions

4. Increased surface : Packed towers
Tarus saddle
Pall Ring
Rings (Raschig,etc)

5. Packed Columns
In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material.
Packed beds can be used in a chemical reactor, a distillation process, or a scrubber, but packed beds have also been used to store heat in chemical plants.
The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured packing.
The purpose of a packed bed is typically to improve contact between two phases in a chemical or similar process.
In this case, hot gases are allowed to escape through a vessel that is packed with a refractory material until the packing is hot. Air or other cool gas is then fed back to the plant through the hot bed, thereby pre-heating the air or gas feed.
The gas liquid contact in a packed bed column is continuous, not stage-wise, as in a plate column.
The liquid flows down the column over the packing surface and the gas or vapor, counter-currently, up the column. Some gas-absorption columns are co-current
The performance of a packed column is very dependent on the maintenance of good liquid and gas distribution throughout the packed bed.

6. Representation of a Packed Column
Packing material
Packing Height (Z)

7. Components of a Packed Column

9. Advantages of Trayed Columns
Plate columns can handle a wider range of liquid and gas flow-rates
than packed columns.
Packed columns are not suitable for very low liquid rates.
The efficiency of a plate can be predicted with more certainty than the equivalent term for packing (HETP or HTU).
Plate columns can be designed with more assurance than packed columns. There is always some doubt that good liquid distribution can be maintained throughout a packed column under all operating conditions, particularly in large columns.
It is easier to make cooling in a plate column; coils can be installed on
the plates.
It is easier to have withdrawal of side-streams from plate columns.
If the liquid causes fouling, or contains solids, it is easier to provide cleaning in a plate column; manways can be installed on the plates. With small diameter columns it may be cheaper to use packing and replace the packing when it becomes fouled.

10. Advantages of Packed Columns
For corrosive liquids, a packed column will usually be cheaper
than the equivalent plate column.
The liquid hold-up is lower in a packed column than a plate column. This can be important when the inventory of toxic or flammable liquids needs to be kept as small as possible for safety reasons.
Packed columns are more suitable for handling foaming
systems.
The pressure drop can be lower for packing than plates; and packing should be considered for vacuum columns.
Packing should always be considered for small diameter columns, say less than 0.6 m, where plates would be difficult to install, and expensive.

11. Packing Materials
Ceramic: superior wettability, corrosion resistance at elevated temperature, bad strength
Metal: superior strength & good wettability
Plastic: inexpensive, good strength but may
have poor wettability at low liquid rate

12. Packing - Basic Requirements
chemically inert to the fluids
strong but without excessive weight
contain adequate passages (void volume) for both streams without excessive liquid hold-up or pressure drop
provide good contact between the liquid and the gas.
reasonable in cost
Types of packing
Random Packing
Structured Packing

13. Reference:
Seader and Henley

14. Structured packing materials

15. Characteristics of Packing11

16. 12

17. Random packing 2.Lessing ring
Random packings are simply dumped into the tower during installation and allowed to fall at random. 1) Raschig rings: Diameter ranges from 6 to 100 mm. Made of chemical stoneware or porcelain (Not used for alkali & acids), carbon (Except strongly oxidizing atmospheres), metals or plastics (deteriorates with certain organic solvents & oxygen bearing gases at elevated temperature)
2.Lessing ring

18. 3) Berl Saddle 4) Intalox saddle 5) Tellerette
(Chemical stoneware or plastics) 6 to 75 mm diameter
(Chemical stoneware or plastics) 6 to 75 mm diameter
(Plastics & metals)

19. 6) Pall ring
The Pall ring attempts to increase the useful aspects of packing, by giving an increased number of edges to disrupt flow, whilst also reducing the volume taken up by the ring packing medium itself. Rather than using a solid-walled tube, the Pall ring resembles an open basket structure of thin bars. These form both a tube and also a radial structure of cross bars . Pall rings may be injection moulded of plastics, moulded of ceramics or press-formed from metal sheet. In order to prevent the breakage of ceramic or carbon packing , the tower may first be filled with water to reduce the velocity of falling object.

20. 1.4.2.2 Regular or Structured Packing
Advantage of low pressure drop for gas side flow and greater fluid flow but on the other side requires more cost for installation. Stacked Raschig rings are economically practical in very large size only.

21. Wood grids or hurdles are inexpensive and frequently used where large void volume is required. Woven wire screen rolled as a fabric into cylinders provide a large interfacial surface for contacted liquid and gas , and very low pressure drop.

22. Tower shell
These may be of wood , metal , chemical stoneware, acid proof brick , glass , plastic , glass-plastic lined metal or other material depending upon the corrosion condition. For ease of construction and strength they are circular in cross section.

23. Packing support
Every packed bed will need a support. Two critical factors to be considered in the design of a packing support are: It must physically retain and support the packed bed under operating conditions in the column including but not limited to packing type and size, design temperature, bed depth, operating liquid holdup, material of construction, corrosion allowance, material build up in the bed and surge conditions. It must have a high percentage of free area to allow unrestricted counter current flow of down coming liquid and upward flowing vapour .

24. A bar grid which we have seen in above can be used but the support which have different passage way for liquid as well as gases can be used. It may be made up of metal , expanded metal ,ceramic , plastic etc.

25. Liquid distribution
Function: Uniformly distribute the liquid on the surfaces of packings. Spray Nozzles Ring of perforated pipe in small towers
Shower nozzle type
Overflow pipes
Annular tubes with multi-holes

26. Dry packing is ineffective for mass transfer
Dry packing is ineffective for mass transfer . Therefore it is required to wet the packing . The importance of adequate distribution of liquid is shown in fig.
Liquid redistributor
Function: Reducing the non-uniform distribution of liquid, and reducing the wall flow.
To maintain the uniform contact between the liquid and gas throughout the tower , it is provided at various length interval of tower depending upon the length and diameter of tower . e.g weir trough liquid redistributor.

27. Packing restainers
These are necessary when gas velocities are high and they are generally desirable to guard against lifting of packing during a sudden gas surge . heavy screens or bars may be used . For heavy ceramic packing , heavy bar plates resting freely on the top of the packing may be used . for plastics and other light weight packings , the restrainer is attached to tower shell.

28. Entrainment eliminator
Function: Eliminating the entrained liquid drops in the gas stream at the outlet. During high gas velocity the gas may carry away liquid droplets . To remove the liquid droplets from outgoing gas mist eliminator is provided above the liquid inlet.

29. Goals Describe forces that act on a bed of particles.
Describe how pressure drop and bed height (or void fraction) vary with fluid velocity.
Apply basic equations to compute pressure drop across the bed, the bed height and the diameter of the bed.
List advantages and disadvantages of fluidized beds.

30. Flow Through a Bed of Particles

31. Response to Superficial Flow
Low Velocity
Fluid does not impart enough drag to overcome gravity and particles do not move. Fixed Bed.
High Velocity
At high enough velocities fluid drag plus buoyancy overcomes the gravity force and the bed expands. Fluidized Bed.
p for Increasing u0
Until onset of fluidization p increases, then becomes constant.
Bed Length for Increasing u0
L is constant until onset of fluidization and then begins to increase.

32. Response to Superficial Velocities

33. Fixed Bed How do we calculate the pressure drop across a fixed bed?
Start with the MEB:
For pipe flow we determined:

34. Pressure Drop For now make the following assumptions:
Horizontal Bed (or small L) Gravity not important.
Particles pack uniformly giving rise to continuous flow channels
Bed can be modeled as bundle of small pipes.
Flow is laminar (f = 16/Re).

35. Laminar Flow ? ?
What are the proper velocity and diameter?

36. Velocity For a unit length of bed: Lb S = Volume of Bed
e Lb S = Volume Available for Flow
For a unit length of bed:
Mass
Balance

37. Diameter Multiply by L/L
Since this is not true pipe flow must use hydraulic radius.
Multiply by L/L

38. Diameter as is the ratio of particle surface area to volume.
The denominator above is then the particle volume multiplied by as or the particle surface area.
For a sphere:

39. Laminar Flow
In actuality the above equation does not account for the tortuous path through the bed and DL is much longer. Experimental data show that a numerical constant of 150 should replace the 72.
Blake-Kozeny equation. Assumes e < 0.5 and Rep < 10.

40. Turbulent Flow
One cannot use the Hagen-Poiseuille approximation when flow is turbulent. After substituting in Dh and velocity correction
Experimentally:
Burke-Plummer Equation

41. Intermediate Flow Ergun Equation
Note: equation can be used with gases using average gas density between inlet and outlet.

42. Fixed Bed “Friction Factor”

43. Irregular Shapes
To increase surface area and liquid solid contact, many particles are often of irregular shape. In that case the particle is treated as a sphere by introducing a factor called sphericity Fs which allows calculation of an equivalent diameter.
Where Dp is the diameter of a sphere of the same volume as the particle

44. Example: Cube
What is diameter of sphere of volume a3?

45. Sphericity
Note entries for cubes and cylinders. For convenience, some just calculate a nominal (average) diameter and assign a sphericity of unity.
For greatest contact area want lower sphericity.

46. Adsorbent Mesh Sizes
6 X 8 Mesh dp = ( ) / 2 = in ( ft)

47. Irregular Shapes
So the final Ergun equation is:

48. Example
A packed bed is composed of cubes 0.02 m on a side. The bulk density
of the packed bed, with air, is 980 kg/m3. The density of the solid cubes is
1500 kg/m3.
Calculate the void fraction (e) of the bed.
Calculate the effective diameter (Dp) where Dp is the diameter of a sphere
having the equivalent volume.
Determine the sphericity of the cubes.
Estimate the water flow rate (m3/sec) required for minimum fluidization of the
solid using water at 38 C and a tower diameter of 1.0 m.

51. LHS
RHS Term No. 1

52. RHS Term No. 2
Final Equation