Packed Columns for Distillation

 Packed columns are useful for distillation especially whenever we have to carry out operation at low pressure (vacuum distillation) and whenever we are dealing with heat sensitive materials. Packings are usually cheaper than plates for columns less than 600 mm diameter. 

 A packed column consists of a cylindrical shell containing a support plate and a liquid distributor. The cylindrical shell is filled with some sort of packings that rest on the support plate. The packing material offers large interfacial area for mass transfer. The liquid distributor is designed for effective irrigation of the packings. 

 The ability of a given packing to effect the desired mass transfer between gas and liquid phases is usually expressed (in an empirical form) 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 the more volatile component than the vapour entering the plate by one equilibrium stage. In packed column, the same enrichment of the vapour will occur in a certain height of packing and is termed as the height equivalent to one theoretical plate. Thus, in packed columns, one equilibrium step is represented by a certain height of packed bed and the required height of packing for a specified degree of separation is given by : HETP × number of ideal plates required. 

Fig.: Packed column for continuous distillation 

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

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

 G – Superfacial gas mass velocity

 Dt – Tower diameter

 Z – Height of packing

  α – Relative volatility  

µL and ρL 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 plate 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 (stages) 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 :

 It applies to a 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 applies to a single location on a given plate.