kilomentor | 29 December, 2010 05:48
Most synthetic reactions are second order or higher kinetically. Once initiated, they proceed most rapidly in the initial stage and then slow down as the starting materials are consumed and their concentration declines in a constant liquid reactor volume. As a consequence, the larger part of reaction time is spent waiting for a smaller part of the reacting to finish.
When such a reaction is exothermic, the largest part of the exotherm occurs in this early stage. It is for this reason that process chemists strenuously avoid having the full stoichiometric quantities of all the reactants together and then initiating the reaction (say by heating). This is a recipe for a disastrous runaway exothermic event. Instead, in the preferred approach, one reactant is added gradually to a mixture of the other essential chemicals at the reaction temperature. Operating this way, any unwanted exothermicity in excess of what can be balanced by cooling, can be choked off by slowing or stopping the addition.
The question being here considered is whether after the faster part of the reaction has passed can anything be done to accelerate the reaction in the later stages when the concentration of the rate controlling species have fallen? If the reaction is being conducted at the reflux temperature of a solvent, the reaction can in principle be accelerated by distilling out of the reaction part of the reaction solvent. Because the reaction temperature is the boiling temperature of the solvent, this distillation removes only solvent and does not change in any way the reaction conditions. Since removing solvent increases the concentration of starting materials, the rate of their consumption will increase and the point of effective disappearance of starting materials will arrive more quickly. If the volume for a bimolecular reaction is reduce in half, the rate of reaction will be increased by a factor of four.
There is a limit to how low the volume can be taken in a standard reactor. The volume should not be reduced below that which can be effectively stirred (the minimum stirrable volume).
Another advantage of concentrating the reaction mixture is that the volume at the point of maximum volume in the process may be lowered. This will allow a higher rate of production to be obtained. If your total volume at the point of maximum volume can be reduced in half (for the sake of simplicity of explanation) you would only need half as many repeats of the process step to transform the same amount of starting material.
A potential flaw with such a concentration procedure occurs if some essential element of the process is actually volatile with the solvent distillate and is removed. This would slow down or stop the reaction. Although some reaction ingredient may not be blown out of a reaction mixture distilling in the lab, distilling in the plant can have substantially different characteristics and one needs to be aware of the possible loss of even quite non-volatile materials in an aerosol.