Esterification proceeds by a simple, continuous process in a reactive distillation column.
Alcohol vapour enters the base of the column and travels upwards, performing three functions:
- Firstly, it provides the necessary reactant for esterification.
- Secondly, it strips the reaction by-product water from the organic liquid, thus driving the equilibrium reaction virtually to completion. This is particularly important as the esterification reaction is reversible.
- Thirdly, it provides physical agitation at each reaction stage, ensuring good contact between reactants and catalyst.
The liquid acid reactant enters the top of the column and travels downwards, counter-current to the alcohol vapour.
The liquid travels via a series of proprietary design reaction trays which provide sufficient liquid residence time to ensure virtually complete equilibrium conversion to the ester product.
The solid catalyst remains on each reaction tray, eliminating the need for its downstream separation from the ester product stream.
Additionally, the catalyst can be replaced while the plant is online without any interruption of the process or loss of production.
Vapour leaving the top of the column contains the bulk of the excess alcohol and all the reaction water. These are subsequently separated, with the dry alcohol recycling to esterification.
A liquid ester stream, which also contains residual alcohol, exits the bottom of the esterification column for downstream processing or refining.
If no further reaction steps are required, the alcohol can be removed from the process stream to yield product-grade liquid ester.
In several Davy processes, however, the liquid ester proceeds to hydrogenolysis. In such cases, the residual alcohol can remain in the process stream as it does not interfere with the hydrogenolysis reaction.