(256g) Transfer of a Two-Step Azo Dye Synthesis from Batch to Continuous Flow: Reactor Design and Scale-up

Azo dyes account for over 60% of all dyes used today. They are commonly employed in the photometric determination of various metal ions in waste water and medical applications, as well as in the textile and fine chemical industries. The synthesis of azo dyes involves the creation of diazonium salts as an intermediate. These salts are highly reactive, thermally unstable, and potentially explosive. Due to the hazardous nature of diazonium salt generation and its exothermic properties, careful handling and precise temperature control are essential.

Flow chemistry helps to reduce the risks of handling diazonium salts by lowering the reaction volume, allowing in situ quenching of the intermediate and improving heat transfers. Additionally, continuous flow systems enable real-time process control, providing precise adjustments to reaction conditions (e.g. temperature and pH).

We propose a two-step flow setup for the continuous synthesis of a chromotropic acid-based azo dye. The first stage features a tubular reactor with an integrated 3D-printed static mixer. The necessary temperature for the diazotization reaction is adjusted via an external thermostat. In the second stage, the reaction takes place in a 3D-printed continuous stirred tank reactor (CSTR). The jacket of the CSTR enables accurate temperature control using an external thermostat, while stirring is accomplished with a magnetic stirrer. Additionally, the reactor design includes a pH probe mounted at the top of the CSTR to measure the pH value in-line. A control loop adjusts the flow rate of the base to ensure optimal reaction conditions.

In conclusion, the combination of modern process control and 3D printing with continuous flow chemistry minimizes the risks, enables optimal reaction conditions for the synthesis of azo dyes and builds the basis for scale-up.