A running production from a major dye and chemical company proved to be problematic. Solvent losses during work-up resulted not only in increased costs, but also presented an environmental concern.
Initial investigations showed a serious process safety problem in addition to the ecological problem, which required immediate shutdown of the entire production. A quick solution to this problem became essential as product could no longer be delivered to customers. A reaction calorimeter was used in an attempt to develop an improved process in a shorter time.
The reaction, a Friedel-Crafts acylation, is started in the melt at 65 °C. The reaction mixture is then kept for one hour each at 70 °C and 90 °C. After cooling to 65°C, the reaction mass is dissolved in chlorobenzene for subsequent processing.
Process Safety Problems
The viscosity of the melt may increase to the extent that the reaction heat will mostly be accumulated in case of a stirrer breakdown. Severe decomposition sets in at temperatures above 200 °C. A thermal potential of over 500 kJ/kg is present in the reaction mass. Since it is virtually impossible to dissipate the heat if there is a malfunction, a thermal explosion is anticipated within hours. As there are no legitimate emergency solutions, production had to be stopped as soon as these facts became known.
Chlorobenzene was replaced by a solvent that has a higher boiling point and lower solubility in water. To reduce the viscosity of the reaction mass, the reaction is now run, right from the start, in solution and not in the melt. Due to the reproducible performance of the RC1 reaction calorimeter, few experiments were needed for optimization and testing of the new procedure. The quality of the product was checked by HPLC in a duplicate RC1 experiment with the following results:
Exp. 1 Exp. 2
Desired Product 97.8% 98.3%
Undesired Isomers <0.1% <0.1%
Sum of Elutable Unknown Substances <0.1% <0.1%
Due to precise and comprehensive process data, it was possible to restart operations with the modified procedure after an interruption of only one month. The company was then able to resume deliveries within a short time.
Without the process modification:
- The same task would have taken at least six months
- A significant number of lab experiments would have been necessary
- A pilot study would have been required for process safety and process optimization
- The product shortage would have resulted in considerable commercial losses
Because no pilot study was needed, considerable savings in time and money were achieved. Even if malfunction occurs, the safety of the process is now assured as the viscosity of the reaction mass is considerably lower.