Batch to Continuous

Continuous processing is considered attractive as it can offer several advantages over batch operation, namely, reduced capital costs, reduced operating costs and reduced plant footprint. Despite claims that 'most molecules can be produced continuously', there are in fact a number of crucial technical considerations that dictate whether a process is fundamentally suitable for continuous operation. Those associated with the reaction stage alone include:

• Reaction rate: the kinetic rate constants for the majority of batch reactions are generally in the range of 10^-3 to 10^-5s^-1 (although a relatively small number are much faster). Such rates would result in an excessively large continuous reactor in order to achieve the required levels of chemical conversion. One solution for fairly slow reactions is to use an oscillatory flow reactor though even this may offer little advantage if rate constants are much below 10^-3s^-1

• Exotherm: If the reaction rate is sufficiently fast there still may be a problem if the reaction is exothermic. Even for what would normally be considered a relatively mild exotherm (ΔHr < 10^-4kJ/kgMol) a high rate of heat release may occur in the first part of a plug flow reactor

• Reaction engineering advantage: Semi-batch operation enables an important additional degree of freedom in reaction engineering terms to optimising chemical yields. This means of optimisation is lost to continuous plug flow operation (tubes and oscillatory flow reactors) unless provision is made to, for example, add reagents along the length of the reactor. Continuous flow reactors may also be problematic due to the high degree of backmixing of fresh reactants with products

• Reaction-specific issues: Certain reactions are generally thermally neutral (negligible exotherm), chemically reversible and their reaction rates may be relatively slow unless operated at high temperature. Thus some of the problems above may be further exacerbated by the need to drive off products whilst retaining sufficient reagent in the system so as not to reduce the reaction rate to an unacceptably low level.

Fast Model Development

Our experience has enabled many clients to determine whether their chemistry is a match for continuous reactor technology.

Using our range of chemistry, modelling expertise and technologies, we can quickly (often within a few days):

• Evaluate the suitability of a process for continuous processing using non- specialist equipment

• Where necessary, organise the key experiments to provide the necessary data

• Give a good indication of the effects on yield, selectivity, thermal safety of the change for a range of continuous reactor types

We have examined a range of systems, including complex systems involving simultaneous reactions and phase separations.

A representative case study is shown here.