Nitration Reactions in Microchannels Using a Retention Mixing Unit
Nitrations of aromatics are common but potentially dangerous biphasic liquid-liquid reactions, due to their exothermal nature. The reaction kinetics depends strongly on the mass transfer between the two phases, i.e. the diffusion limits caused by the interfacial area of slugs or droplets. In the case of a slug flow reactor and laminar flow conditions, the interfacial area is defined by the capillary diameter, but can be slightly enhanced at high flow rates when the slugs are disrupted into non-regular droplets. To achieve high conversion rates, the interfacial area has to be maximized by applying a suitable micro mixer to establish a dispersion flow. A glass wool packed column was used as a reactor (a so-called retention mixing unit) to stabilize the initial droplet size along the whole reactor length.
As an example, the nitration of 1,3-dichlorobenzene (1) to 1,4-dichloro-2,5-dinitrobenzene (3) was investigated.
For identical conditions of temperature (130 °C, fixed) and reaction time, the conversion of 2 to 3 was significantly improved by a dispersion flow (squares, red) compared to the common slug flow experiment (triangles, black). At high flow rates (short residence times) and slug flow conditions, the reaction kinetic is increased due to irregular flow patterns with undefined droplet sizes, i.e. it depends on the geometric parameter. For a dispersion flow, the reaction rate becomes time dependent and droplet size invariant. The necessary ratio of nitric acid / organic substrate could be significantly decreased to 6 : 1, a step forward to process intensification even for long time commonly established chemical reactions.