Distillation

Distillation separates  two  or more liquid components in  a  mixture using the

principle of relative volatility or boiling points. The greater the difference in relative

volatility the greater the nonlinearity  and  the easier it  is  to separate the mixture

using  distillation. The process involves production of vapour by  boiling the liquid

mixture in a  still and removal of  the vapour  from the still by condensation. Due  to

differences in relative volatility or boiling points, the vapour is rich  in  light

components and the liquid is rich in heavy components. 

Often a part of the condensate is returned (reflux)  back  to the still and is mixed 

with  the outgoing vapour. This allows further transfer of  lighter components to the 

vapour phase from the liquid phase and transfer of heavier components to the liquid 

phase from the  vapour  phase. Consequently, the vapour  stream becomes richer  in 

light components  and  the liquid stream becomes richer in heavy components. 

Different types  of  devices called plates, trays or packing  are  used  to bring the 

vapour and  liquid phases into intimate contact to enhance the mass transfer. 

Depending  on  the relative volatility  and  the separation  task  (i.e. purity  of  the 

separated components)  more  trays  (or more packing materials)  are  stacked  one 

above the other in a cylindrical shell to form a column. 

The distillation process can be carried out in continuous, batch or in semi-batch

(or  semi-continuous) mode. 

Continuous Distillation

Figure 1.1  shows a typical continuous distillation column. The liquid mixture (feed), 

which  is to be  separated into  its components,  is  fed to  the column at one or more 

points along the column. Liquid runs down the column  due  to  gravity while the 

vapour  runs up  the column. The vapour  is produced by  partial vaporisation of  the 

liquid reaching the bottom of  the column. The remaining liquid is withdrawn from 

the column as bottom product  rich in heavy  components. The vapour reaching the 

top  of  the column  is partially or fully condensed. Part  of  the condensed liquid is 

refluxed  to  the column while the remainder  is withdrawn as the distillate product. 

The  column section above the feed  tray  rectifies the vapour stream  with  light 

components and therefore is termed as rectlfying section. The column section below 

the feed tray strips heavy components from the vapour  stream to  the liquid stream 

and is termed as stripping section. 

The readers are directed to Smith (1963), Seader and Henley (1998), Perry et al.

(1997), McCabe  et al. (2001), Gani and  co-workers (1986a, 1986b, 1988, 2000), Perkins and co-workers (1996, 1999,2000, 2001) for detailed account of modelling design, operation, control and synthesis of continuous distillation processes. 

Batch Distillation

Batch  distillation is, perhaps  the  oldest operation  used  for  separation  of  liquid 

mixtures. For centuries and also today, batch distillation  is  widely used for  the 

production of  fine chemicals and specialised products such as alcoholic beverages, 

essential  oils,  perfume, pharmaceutical  and  petroleum products. It  is  the most 

frequent separation method in batch processes (Lucet et al., 1992). 

The essential features  of  a  conventional  batch  distillation  (CBD)  column 

(Figure 1.2) are:

• A bottom receiverheboiler which  is charged with the feed  to be processed and which provides the heat transfer surface. 
• A rectifying column (either a tray or packed column) superimposed on the  reboiler, coupled  with  either  a  total  condenser or  a  partial condenser  system. 
• A series of  product accumulator tanks connected to the product streams to collect the main and or the intermediate distillate fractions. 

Operation  of  such  a  column involves carrying out the fractionation until  a

desired amount has been distilled off. The overhead composition varies during the

operation  and  usually  a  number  of  cuts  are made.  Some  of  the cuts are desired 

products (main-cuts) while others are intermediate fractions (off-cuts) that can  be 

recycled to subsequent batches  to  obtain further separation.  A  residual bottom 

fraction may or may not be recovered as product (Mujtaba, 1989). 

Semi-batch (semi-continuous) Distillation

Figure 1.3 shows a  typical  semi-batch (semi-continuous) distillation column. The

operation of  such columns  is very similar  to  CBD columns except that  a  feed  is

introduced to  the  column  in  a continuous or  semi-continuous mode. This type of

column is suitable for extractive distillation, reactive distillation, etc. (Lang and co-

workers, 1994, 1995; Mujtaba, 1999). 

source: Batch Distillation Design and Operation, by I.M. Mujtaba
University of  Bradford,  UK