Exploring the Steady Operation of a Continuous Pilot Plant for the Di‐Nitration Reaction

Abstract

Continuous‐flow synthesis of the selective herbicide pendimethalin was demonstrated in both a laboratory‐scale and a pilot‐scale reactor using only concentrated nitric acid as nitrating agent. The di‐nitration reaction follows second‐order kinetics where the reaction is first order with respect to both reactant and nitric acid. The pinched‐tube reactor was chosen for pilot‐scale reactor fabrication due to its excellent mixing and mass transfer characteristics compared to a straight‐tube reactor. The estimated mass transfer coefficient showed similar nature in the laboratory‐scale and the pilot‐scale pinched‐tube reactor, ensuring similar performance at the pilot scale. Di‐nitration in continuous flow, inline quenching, extraction, and phase separation are some of the salient features of the developed pilot plant. The importance of the start‐up time for achieving steady state in the flow system at the large scale is highlighted.

Design and scale-up of continuous di-nitration reaction using pinched tube flow reactor

“…Validated mass balance and energy balance equations were solved simultaneously to predict the reaction outcome for the proposed capacity and to select the suitable tube diameter for getting the desired heat transfer with predefined product quality. The model predictions showed that ¼-inch SS316 tube is sufficient for achieving the required temperature and product concentration [20]. Also, the model predictions showed that only 500 ml reactor volume was necessary for the desired throughput of 50 kg per day.…”

“…), residence time (2-10 min) and temperatures (40-60 °C). The detailed procedure for the measurement and analysis is mentioned in our previous publication [20]. It was found that the reaction follows second order kinetics (Table 1).…”

“…As discussed in our previous publication [20,21] we are using pinched tube reactor for scale-up due to its excellent mixing and mass transfer characteristics compared to straight tube. However, at the proposed scale to compare the mixing performance between normal tube and pinched tube the flow visualization study was conducted to check the flow inside the tubes.…”

“…13). A detailed analysis of the steady state operation is reported separately [20]. For operation of the scale up reactor, we recommend following the protocol that we had developed at the laboratory scale, where the reactor was initially filled with nitric acid and the temperature was allowed to reach at the desired reaction condition.…”

“…In a previous study [20] that we have reported using the same pilot plant the focus was to explore the effect of different parameters (temperature, pumping efficiency, control structure provided, dispersion and mass transfer etc.) during production stages on the time required to achieve the steady state of the process.…”