14.4.3 Technical questions
1. Why compressor is used before partial condenser ?
Ans: The compressor increases the pressure of the system which is beneficial to increase the boiling points of the mixtures. Note that the boiling points of chloromethanes are -97.7, -97.6, -63.5 and -22.6°C for CH3Cl, CH2Cl2, CHCl3 and CCl4 respectively. On the other hand, the boiling point is -161.6°C. For these boiling point mixtures, when the system pressure is increased substantially, the boiling points of the compounds increase and could reach close to those of the cooling water (20 – 30°C). Cooling water is required in the partial condenser and if it is not used, a refrigerant needs to be used which requires an additional refrigeration plant. Therefore, the system pressure is increased.
2. Why water is removed using the dryer ?
Ans: Water enters the vapour system due in the absorption column where solvent loss to the vapour will be a common feature. Water molecule can react with the highly active intermediate chloromethanes to form oxychlorides, which are highly undesired.
3. Will there be any difficulty in separation by increasing boiling points of the chloromethanes in the distillation sequences ?
Ans: Definitely yes. This is because the relative volatility of compounds atleast slightly increases with reducing pressure and vice versa. But due to cooling water criteria in the distillation sequences also, there is no other way economical than doing distillation at higher pressure.
4. Since the boiling point of CH3Cl and CH2Cl2 are very close, what do you expect for the production of CH3Cl from the first column ?
Ans: It is indeed difficult to separate CH3Cl and CH2Cl2 and therefore, good number of separation trays be used. Or structured packing be used to reduce the height of the first column.
5. When the reactions are highly exothermic, why is the feed pre-heated ?
Ans: Irrespective of the reactions being exothermic or endothermic, the reaction rate always increases with temperature for non-equilibrium reactions. Therefore, feed is pre-heated to the desired temperature so as to fastly convert the reactants to products.
References:
1. Dryden C. E., Outlines of Chemical Technology, East-West Press, 2008
2. Shreve R. N. , Austin G. T., Shreve's Chemical process industries, McGraw – Hill, 1984