Klemenz Kromm, development engineer at Saltigo, shares practical examples of combining batch and continuous operations throughout development.

Continuous reactors can, depending on reactor geometry, offer efficient heat removal and controlled mass transfer conditions through high surface-area-to-volume ratios and low hold-up volumes. This is particularly advantageous for highly exothermic, mixing-sensitive or residence-timesensitive reactions, especially when unstable intermediates are involved.

Local temperature and concentration gradients typical of conventional batch processes can be reduced, side reactions minimised, and selectivity improved. However, adapted safety concepts and equipment design remain essential.

These technical advantages do not imply that every process step should be operated continuously. The following examples illustrate two different approaches: a hybrid route that combines batch and continuous steps, and one that integrates a continuous reaction with downstream separation.

Hybrid route

In practice, hybrid process routes can be preferable when individual reaction stages impose different process requirements. One example is a twostep route to a functionalised aromatic intermediate. A cryogenic organometallic step is initially performed in batch mode to generate the sensitive intermediate selectively, followed by direct transfer into a continuous carbonylation step.

The continuous gas-liquid reaction enables precise dosing of the reaction gas, short residence times and reproducible selectivity. Downstream continuous operations can facilitate extended campaigns without batchrelated interruptions, for example by combining phase separation in mixersettlers with continuous filtration of an insoluble by-product.

A planning study indicated that, under defined throughput and annual operating-time assumptions, the annual production capacity of multiple 600 – 1,000 L batch reactors could be matched by two 20 L continuous stirred tank reactor( CSTR) autoclaves. At pilot scale, the route was successfully established as a hybrid process. For commercial operation, the hybrid configuration can be retained; alternatively, a fully continuous implementation of both the low-temperature metalation and carbonylation may be feasible.

This example demonstrates that neither full batch operation nor fully

continuous operation needs to be the starting assumption. Instead, the operating mode can be selected step by step, according to the technical requirements of the individual reaction and separation stages. Where both reaction and downstream processing are suitable for continuous operation, a more extensively integrated production concept may be possible.

Integrated reaction & separation

The case of 2-( 2-hydroxyethyl) pyridine( HEP) illustrates the potential to integrate a continuous reaction with downstream processing. High-temperature reactions are a relevant application area for continuous processing, particularly where heat management and residence-time control are decisive.

The continuous synthesis of HEP is one example of Saltigo’ s process development( Figure 1). HEP is an important intermediate in the production of Saltidin *, the active ingredient used in insect repellent formulations. The reaction is based on the conversion of 2-picoline with formaldehyde under elevated temperature and pressure conditions.

The project combined conventional batch development with the assessment of a continuous manufacturing route. A tubular reactor system was selected to enable precise adjustment of temperature and residence time under controlled thermal conditions.

40 SPECIALITY CHEMICALS MAGAZINE ESTABLISHED 1981