Wachemo University

College of engineering and technology

Department of Chemical Engineering
Chemical engineering apparatus design

Lecture: Separation columns

Yodanos Abay (Msc.)
Introduction
Distillation and absorption are two most widely used mass transfer processes in
chemical industries.

Design of plate column for absorption and distillation involves many common steps
of calculation such as determination of number of theoretical plates, column
diameter, plate hydraulic design, etc.

In absorption process, a soluble component is absorbed in a liquid (called solvent)

from a gaseous mixture.

The separation in distillation is based on the relative volatility of the components.

Selection of column type: Plate or Packed
Packed towers (columns) are also used as the contacting devices for gas
absorption, liquid-liquid extraction and distillation.

The gaseous mixture is allowed to contact continuously with the liquid

counter-currently in a packed column.

The liquid flows downward over the packing surface, and the gaseous mixture
flows upward through the space in the packing.
Cont...
 The performance of the column strongly depends on the arrangement of the
packing to provide good liquid and gas contact throughout the packed bed.

 The solute gas is absorbed by the fresh solvent (liquid) entering at the top of
the tower where the lean gas leaves system.

 The liquid enriched with absorbed solute gas, leaves the column bottom
through the exit port.
Plate Vs Packed column (tower)
Plate column (tower) Packed column (tower)
The liquid and gas are  Gas-liquid contact is continuous in a
contacted in stage-wise packed column.
manner on the trays.
 Cont...

Plate towers exhibit larger pressure While, packed towers are not
drops and liquid holdup at higher gas appropriate for very low liquid flow
flow rate. rates.

Merit:

Packed column is the preferred choice

than a plate column to handle toxic and
flammable liquids due to lower liquid
holdup to keep the unit as small as
possible for the sake of safety.
Cont...
Plate columns are normally suitable for Packed towers are more suitable for
fouling liquids or laden with solids. foaming and corrosive services.

They are easier to clean and could handle

substantial temperature variation during
operation.

It is easier to make the provision for the

Difficult due packing inside.
installation of internal cooling coils or
withdrawal of side streams from a plate
column.
Plate contractors
Plate contractors/ towers are vertical cylindrical columns in which a
vertical stack of trays or plates are installed across the column height.

 The liquid enters at the top of the column and flows across the tray and
then through a down comer (cross-flow mode) to the next tray below.

The gas/vapor from the lower tray flows in the upward direction through
the opening/holes in the tray to form a gas-liquid dispersion.

In this way, the mass transfer between the phases (gas/vapor-liquid) takes
place across the tray and through the column in a stage-wise manner.
Tray or Plate column
 A tray column primarily consist of a vertical cylindrical shell and a set of ‘tower internals’ that
include

a. Tray or plates on which the gas-liquid contact occurs.

b. Arrangement for flow of the liquid from one tray to the lower one through the downcomer.

c. Inlet and outlet nozzles for the two phases.

cont.
1. The shell:

The shell is usually made of a metal or an alloy. Plastic shell are also used
sometimes.

The material is selected on the basis of corrosiveness of the fluids, temperature

and pressure conditions, and cost.

Tray tower of diameter less than 1 meter are rarely used. On the other extreme,
towers as big as 10 meter in diameter are known to be in use.
Definition of tray areas
Total tower cross-section area (𝑨𝑻): The empty tower inside cross-sectional area
without trays or downspouts.

Net area (𝑨𝑵) (also called free area):The total tower cross-sectional area (𝐴𝑇)minus
the area at the top of the down comer (𝐴𝐷𝑇). The net area symbolizes the smallest area
available for vapor flow in the inter-tray spacing.

Bubbling area or active area (𝑨𝑨): The total tower cross-sectional area minus sum of
the downcomer top area(𝐴𝐷𝑇) and downcomer seal area (𝐴𝐷𝐵)and any other non
perforated areas on the tray.

Hole area (𝑨𝒉): The total area of the perforations on the tray. The hole area is the
smallest area available for vapor/gas passage.
cont.
2. The Tray:

A tray has two major functions:

1. It allows the gas to flow through the holes or passages; the gas vigorously
bubbles through the liquid to from a ‘gas-liquid dispersion’. The tray holds
the dispersion on it.

2. The trays separate the column in a number of compartments each of which

constitutes a stage. Mass transfer between the phases occurs on a tray.
Therefore, the trays as a whole constitute the heart of a column.
Plate/tray types
Three principal types of cross-flow tray are used, classified according to
the method used to contact the vapor and liquid. These are:

1. Sieve plate (perforated plate)

2. Bubble-cap plates

3. Valve plates (floating cap plates)

1. Sieve plate (perforated plate)
Sieve trays are flat perforated plate in which vapour rises through small holes in tray
floor, & bubbles through liquid in fairly uniform manner.

They have comparable capacity as valve trays.

This is the simplest type of cross-flow plate.

Advantages:

Simple design, cost effective and versatile

Disadvantages:

limited vapor-liquid contact and

Limited efficient for some separation

Cont...
2. Bubble-cap plates
 A bubble cap consists of a riser (also called chimney) fixed to the tray through a hole and
a cap is mounted over the riser.

 The gas flows up through the riser, directed downward by the cap through the annular
space between riser and cap. Finally, the gas is dispersed into the liquid.
Cont...
• Advantages:

excellent vapor-liquid contact

Suitable for high capacity operation

• Disadvantages:

higher cost and complexity

Challenging maintenance
3. Valve plates (floating cap plates)

They are essentially sieve plates with large-diameter holes covered by

movable flaps, which lift as the vapour flow increases.

As the area for vapour flow varies with the flow-rate, valve plates can operate
efficiently at lower flow-rates than sieve plates: the valves closing at low
vapour rates.
Cont.

advantages:
 Excellent liquid/ vapour contacting.

 Higher capacity.

 Higher flexibility than sieve trays.

 Disadvantage:

 Higher cost and complexity

 Maintenance of moving part

Cont...
Table: Comparison of three types of cross-flow trays
Cont.
3. Weir:

The function of a weir is to maintain a desired liquid level on the tray.

Typical weir height is between 2 to 4 inch. Low weirs are frequently used in
low pressure column.

 The higher the liquid level, the higher the tray pressure drop. Higher liquid
level also imply more liquid hold up on the tray, which may be undesirable if
the liquid is toxic or hazardous.

The weir length may vary from 60 to 80% of the tower diameter.
Cont.
4. Downcomer:

Downcomer are used to guide liquid flow from an upper tray to a lower tray.

The liquid, along with some dispersed gas or vapour bubbles, overflows the weir and
enters the downcomer.

The Downcomer must provide sufficient residence time for gas-liquid disengagement.

The ‘clear liquid’ velocity in the downcomer normally ranges between 0.3 to 0.5 ft/s.

The downcomer plate may be straight or inclined.

Cont.
4. Nozzle:
 A tower for contacting a liquid and a vapour should be provided with a few nozzles for feed
entry, entry of reflux at the top and of the reboiler vapour return at the bottom, and for product
withdrawal from the tower.
PLATE HYDRAULIC DESIGN

The basic requirements of a plate contacting stage are that it should:

Provide good vapor-liquid contact.

Provide sufficient liquid hold-up for good mass transfer (high efficiency).

Have sufficient area and spacing to keep the entrainment and pressure drop
within acceptable limits.

Have sufficient downcomer area for the liquid to flow freely from plate to
plate.
Flooding: Entrainment, Tray Spacing, Column Height,
and Column Diameter
 Determining the number of trays is only the first step in designing a tray column.

 The height is based on the tray spacing. The diameter is based on a concept known as
flooding, which can be caused by excessive entrainment. Additionally, the tray spacing affects
flooding.

1. Entrainment

 Entrainment is the situation where the upward-flowing vapor carries liquid from the tray

below to the tray above.

 Effectively, this results in a mixing of liquids at different compositions, negating or reducing

the separation that has occurred. Flooding can be caused by excessive entrainment.
2. Flooding
The column diameter is based on flooding.

Flooding in the column occurs when the upward vapor velocity is too large, the
drag force on the liquid exceeds gravity, and the liquid does not fall through the
column.

Excessive liquid buildup inside the column leads to column flooding condition.

The nature of flooding depends on the column operating pressure and the
liquid to vapor flow ratio.
Cont.
It may be downcomer backup, spray entrainment or froth entrainment type
flooding.

Higher tray pressure drop due to excessive vapor flow rates holds up the liquid
in the downcomer, increases the liquid level on the plate and leads to
downcomer flooding situation.

 The flooding condition fixes the upper limit of vapour velocity.

Tray spacing
The tray spacing determines the column height.

 Lower tray spacing is desirable to minimize construction cost by

checking against the column performance criteria.

The suggested tray spacing (𝑇𝑡) with column diameter is appended below.

Tray spacing :
Operating range
Satisfactory operation will only be achieved over a limited range of vapour and
liquid flow rates.

A typical performance diagram for a sieve plate is shown below.

1. Flooding

The upper limit to vapour flow is set by the condition of flooding.

At flooding there is a sharp drop in plate efficiency and increase in pressure
drop.
Cont...
Cont...
Flooding is caused by either the excessive carry over of liquid to the next plate
by entrainment, or by liquid backing-up in the downcomers.

2. Weeping

The lower limit of the vapour flow is set by the condition of weeping.

Weeping occurs when the vapour flow is insufficient to maintain a level of liquid
on the plate.

3.Coning

Coning“ occurs at low liquid rates, and is the term given to the condition where
the vapour pushes the liquid back from the holes and jets upward, with poor
liquid contact.
Plate-design Procedure
1. Calculate the maximum and minimum vapor and liquid flow rates for the turndown
ratio required.

2. Collect, or estimate, the system physical properties.

3. Select a trial plate spacing

4. Estimate the column diameter, based on flooding considerations .

5. Decide the liquid flow arrangement

6. Make a trial plate layout: downcomer area, active area, hole area, hole size, weir height

7. Check the weeping rate, if unsatisfactory return to step 6.

8. Check the plate pressure drop, if too high return to step 6.

Cont.
9. Check downcomer backup, if too high return to step 6 or 3.

10. Decide plate layout details: calming zones, unperforated areas. Check hole pitch, if
unsatisfactory return to step 6.

11. Recalculate the percentage flooding based on chosen column diameter.

12. Check entrainment, if too high return to step 4.

13. Optimize design: repeat steps 3 to 12 to find smallest diameter and plate spacing
acceptable (lowest cost).

14. Finalize design: draw up the plate specification and sketch the layout
1. Plate Areas
• The following area terms are used in the plate design procedure:
2. Diameter
• The flooding condition fixes the upper limit of vapor velocity.

• A high vapor velocity is needed for high plate efficiencies, and the velocity will normally
be between 70% to 90% of that which would cause flooding. For design, a value of 80%
to 85% of the flooding velocity should be used.

• The flooding velocity can be estimated from the correlation given by Fair (1961):
Cont.
Cont...

The design gas velocities (𝑈v) is generally 80-85% of 𝑈𝑛𝑓 for non-foaming
liquids and 75% or less for foaming liquids subject to acceptable entrainment
and plate pressure drop.
Liquid-flow arrangement
The choice of plate type (reverse, single pass or multiple pass) will depend on
the liquid flow-rate and column diameter.

An initial selection can be made using Figure 11.28, which has been adapted
from a similar figure given by Huang and Hodson (1958).
Entrainment
Entrainment can be estimated from the correlation given by Fair (1961),

Figure 17.36, which gives the fractional entrainment ψ (kg/kg gross liquid
flow) as a function of the liquid-vapor factor FLV, with the percentage
approach to flooding as a parameter.

The percentage flooding is given by

Cont...
• As a rough guide the upper limit of ψ can be taken as 0.1; below this figure the
effect on efficiency will be small. The optimum design value may be above this
figure; see Fair (1963)
Figure 17.36. Entrainment correlation for sieve plates
Weep point
• The lower limit of the operating range occurs when liquid leakage through
the plate holes becomes excessive.

• This is known as the weep point.

• The vapour velocity at the weep point is the minimum value for stable
operation.

• The hole area must be chosen so that at the lowest operating rate the vapour
flow velocity is still well above the weep point.
Cont...
The minimum design vapour velocity is given by:
Cont...
Weir liquid crest
The height of the liquid crest over the weir can be estimated using the Francis
weir formula.
Weir dimensions
Weir height

• The height of the weir determines the volume of liquid on the plate and is an
important factor in determining the plate efficiency.

• A high weir will increase the plate efficiency but at the expense of a higher plate
pressure drop.

• For columns operating above atmospheric pressure the weir heights will normally be
between 40 mm to 90 mm (1.5 to 3.5 in.); 40 to 50 mm is recommended.

• For vacuum operation lower weir heights are used to reduce the pressure drop; 6 to
12 mm (0.25 to 0.5 in.) is recommended.
Inlet weirs
Inlet weirs, or recessed pans, are sometimes used to improve the distribution of liquid
across the plate; but are seldom needed with segmental downcorners.

Weir length

With segmental downcorners the length of the weir fixes the area of the downcomer.
The chord length will normally be between 0.6 to 0.85 of the column diameter. A good
initial value to use is 0.77, equivalent to a downcomer area of 12 per cent.

The relationship between weir length and downcomer area is given in Figure 11.31.

For double-pass plates the width of the central downcomer is normally 200-250 mm (8-
10 in.).
Cont...
Hole size
The hole sizes used vary from 2.5 to 12 mm; 5 mm is the preferred size.

 Larger holes are occasionally used for fouling systems. Typical plate
thicknesses used are: 5 mm (3/16 in.) for carbon steel, and 3 mm (12 gauge) for
stainless steel.

When punched plates are used they should be installed with the direction of
punching upward.
Hole pitch
• The hole pitch (distance between the hole centres) should not be less than 2.0
hole diameters, and the normal range will be 2.5 to 4.0 diameters.

• Within this range the pitch can be selected to give the number of active holes
required for the total hole area specified.

• Square and equilateral triangular patterns are used; triangular is preferred.

• The total hole area as a fraction of the perforated area Ap is given by the
following expression, for an equilateral triangular pitch:
Cont...
Rate pressure drop
It is convenient to express the pressure drops in terms of millimeters of
liquid. In pressure units:
Dry plate drop
The pressure drop through the dry plate can be estimated using expressions
derived for flow through orifices.

where the orifice coefficient Co is a function of the plate thickness, hole

diameter, and the hole to perforated area ratio.

Co can be obtained from Figure 11.34;

Residual head
Methods have been proposed for estimating the residual head as a function of
liquid surface tension, froth density and froth height.

However, as this correction term is small the use of an elaborate method for its
estimation is not justified, and the simple equation proposed by Hunt et al.
(1955) can be used:
Total pressure drop
The total plate drop is given by:

If the hydraulic gradient is significant, half its value is added to the
clear liquid height.
Cont.
• Reading assignment

Perforated area

Design of down comer

Design of packed column