22.3.3 Technical questions

1.  Why are benzoic acid crystals again subjected to water wash ?

Ans: The benzoic acid crystals could have some water soluble impurities despite having water wash. This is because of the fact that in real life, infinite separation factors don't exist and components do get distributed in both phases. Therefore, an additional water wash facilitates the removal of these impurities.

2.  Why phenol enters both bottom and top product ?

Ans: The trick in the operation of the second oxidation reactor is to maintain temperature and pressure such that benzoic acid leaves the unit as vapour. This way, benzoic acid can be easily fractionated and sent back to the reactor. However, this has a limit as well i.e., phenol gets evenly distributed between the vapour and liquid phases.

3.  Is there any opportunity for energy integration in the process ?

Ans: Yes, the vapours from the reactor can be heat integrated with the reboilers in the toluene column or any other reboilers in the other two columns.

4.  What exactly happens in the column that is fed with phenol + water mixture, given the fact that phenol forms an azeotrope with water ?

Ans: At atmospheric pressure, phenol forms an azeotrope with water at about 9.4 wt % phenol and 90.6 wt % water. Therefore, crude phenol column shall produce a water rich stream at the top which is the azeotrope and the pure phenol product as the bottom product. From the VLE data (not shown here), it is also apparent that from about 10 wt % phenol to about 90 % phenol, the relative volatility value is pretty low and after 90% phenol in the feed solution, the relative volatility increases significantly.

Therefore, it makes sense now to understand that crude phenol fed to the first tower splits into azeotrope at the top (with 9.4 wt % phenol) and a purer phenol product at the bottom. Eventually, the phenol + water stream entering the second unit along with the azeotrope composition is improved substantially in its composition to move away from the azeotropic composition (with more phenol in water) and hence, it should again tend to produce a purer phenol product and a waste water product at the top. The waste water product could be again an azeotrope of a different class.

Again, if we alter the pressures of these columns, the VLE data (not shown here) will reduce the phenol composition in the wastewater stream and that's what would be probably considered to extract maximum phenol using the distillation technology.

Alternatively, extractive distillation process can be used in which toluene can be used as a solvent to alter the relative volatility. Probably, this process is more common presently in the industry than the process outlined in this lecture.

 

References:

1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008

2. Kirk R. E., Othmer D. F., Encyclopedia of Chemical Technology, John Wiley and Sons, 1999-2012