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Packed Columns/Towers for Distillation and Gas Absorption

Packed Columns/Towers for Distillation and Gas Absorption

Packed columns are useful for distillation, especially whenever we have to carry out operations at low pressure (vacuum distillation) and whenever we are dealing with heat-sensitive materials.

Packed column distillation design

Packings are usually cheaper than plates for columns less than 600mm in diameter. A packed column consists of a cylindrical shell containing support plates and a liquid distributor. The cylindrical shell is filled with some sort of packing that rest on the support plate. The packing material offers a large interfacial area for mass transfer. The liquid distributor is designed for effective irrigation of the packings.

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Packed column for continuous distillation 

The ability of a given packing to affect the desired mass transfer between gas and liquid phase is usually expressed as the height equivalent to one theoretical plate (HETP). In plate columns wherein a process of enrichment is stage-wise the vapour leaving the plate is richer with respect to more volatile components than the vapour entering the plate by one equilibrium stage. In a packed column, the same enrichment of the vapour will occur at a certain height of packing and is termed as the height equivalent to one theoretical plate.

Packed Columns/Towers for Distillation and  Gas Absorption

Degree of Separation 

Thus in packed columns, one equilibrium step is represented by a certain height of the packed bed and the required height of packing for the desired degree of separation is given by: HETP x the Number of ideal plates required.

HETP can be estimated with the help of the following empirical equation. 

HETP = k1 • Gk2 • Dtk3 • Z1/3 • a •uL/pL

Where k1, k2, k3 are empirical constants for packing and are a function of the type and size the packings

G = Superficial gas mass velocity

Dt = Tower diameter

Z = Height of packing

a = Relative volatility

uL and uL are the viscosity and density of liquid respectively. 

Plate Efficiencies 

The relationship between the performance of theoretical/ideal and actual plates is expressed in terms of plate efficiency. The types of efficiency are:

1. Overall plate efficiency/overall column efficiency.  

2. Murphree plate efficiency and 

3. Point/local efficiency. 

Overall plate efficiency is the ratio of the number of ideal or theoretical plates required to produce a given separation in the entire column to the number of actual plates required to effect the same separation. 

If the overall efficiency is 60% and 12 ideal plates are called for, then the actual plates needed are 12/0.60=20.

Murphree plate efficiency

If applies to an individual plate in a column and is defined as the actual change in average composition accomplished by a given plate divided by the change in average composition if the vapour leaving the plate were in equilibrium with the liquid leaving the plate. 

Point efficiency is defined in the same manner as the Murphree plate efficiency but it is applied to a single location on a given plate. 

Packed columns/Towers for Absorption

Packed columns are most frequently used for gas absorption (and are used to a limited extent for distillation) wherein the liquid is dispersed in the form of film and the gas flows as a continuous phase. 

Packed column working principle

These are continuous contact equilibriums generally operated in a counter-current fashion. A packed column consists of a vertical cylindrical shell constructed out of metal, plastic, ceramic, etc. and filled with suitable packings which offer a large interfacial area for gas-liquid contact for mass transfer between the phases. A bed of the packing rests on a support plate which offers very low resistance to gas flow. 

Packed columns/Towers for Absorption

It is provided with a gas inlet and distributing space at the bottom, a liquid inlet and a liquid distributor at the top and gas-liquid outlets at the top and bottom. A liquid solvent is introduced from the top through the liquid distributor which irrigates/ wets the surface of packing uniformly, liquid trickles down the bed and finally, the liquid is enriched in in solute called a strong liquor (solute + solvent) leaves the bottom of the column. The liquid flow rate should be sufficient for good wetting of packing.

A solute-containing gas is introduced from the bottom of the tower and rises upward through the interstices/open spaces in the packing counter current to the flow of the liquid. The lean gas leaves the column from the top of the tower. In the case of tall columns/towers, liquid redistributors are used to redistribute liquid to avoid channelling of the same.

Advantages and disadvantages of packed columns

Advantages of packed columns

(i) Minimum structure

(ii) Low-pressure drop

(iii) Low liquid hold-up

(iv) Handle corrosive liquids and liquids that tend to foam

(v) Low initial investment and 

(vi) high liquid to gas ratios. 

Disadvantage of packed columns

(I) Relatively inflexible

(ii) Can not operate over a wide range of either vapour or liquid rates per unit cross-section

(iii) Distribution of liquid is difficult

(iv) Can not handle dirty fluids that tends deposit a sediment 

(v) Can not be used where large temperature changes are encountered.

>Azeotropic Distillation and Azeotrope explain 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane

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