Microreactors performance increasement

Novel Chemistry - Tailoring Protocols to Increase Microreactor Performance... 

When the reaction was performed in the microreactor, the maximum conversion of 97.0 % was attained when the flow rate of Boc-AMP solution was 9 ml/min and the molar equivalents of KOH to Boc-AMP was 13 as shown in Fig. 1. Optimum operating conditions were obtained from a statistical method by using factorial design [6]. The yield decreased over the KOH equivalency of 13 in Fig. 1, since the phase separation between the t-Boc20 and the aqueous phase was observed due to the increased water content with increasing KOH equivalency. As the heat transfer performance of the microreactor was greatly improved compared with conventional reactors, higher reaction temperature could be admissible. 

The catalytic experiments were performed at the stationnary state and at atmospheric pressure, in a gas flow microreactor. The gas composition (NO, CO, O2, C3H, CO2 and H2O diluted with He) is representative of the composition of exhaust gases. The analysis, performed by gas chromatography (TCD detector for CO2, N2O, O2, N2, CO and flame ionisation detector for C3H6) and by on line IR spectrometry (NO and NO2) has been previously described (1). A small amount of the sample (10 mg diluted with 40 mg of inactive a AI2O3 ) was used in order to prevent mass and heat transfer limitations, at least at low conversion. The hourly space velocity varied between 120 000 and 220 000 h T The reaction was studied at increasing and decreasing temperatures (2 K/min) between 423 and 773 K. The redox character of the feedstream is defined by the number "s" equal to 2[02]+[N0] / [C0]+9[C3H6]. ... 

In this way, the authors have proven several significant advantages of the reactions performed in a microreactor shorter reaction times, improved atom efficiency, excellent product yields and purities, efficient catalyst recycling and the increased safety of the reaction, thanks to the closed system which prevents the release of the cyanide. 

In a second and possibly alternative stage of the kinetic investigation, laboratory experiments are performed over the same catalyst as for the microreactor tests, but now in the form of small monolith samples with volumes of few cubic centimeter. Flow rates, as well as catalyst size, are thus typically increased about by a factor of 100 with respect to the microreactor kinetic runs. This experimental scale provides data either for intermediate validation of the intrinsic kinetics from stage one, or directly for kinetic parameter estimation if runs over catalyst powders are omitted. 

In order to produce oxime 199 in greater quantities, the authors subsequently evaluated the use of DMF as the reaction solvent due to the increased solubility of the nitrite precursor 277 (36 mM). In conjunction with two serially connected microreactors, each containing 16 microchannels [1,000 pm (wide) x 500 pm (deep) x 1.0 m (length)] and eight black lights, the photochemical synthesis was performed continuously for 20h at a flow rate of 250 pi min 1 (residence time = 32 min). After an off-line aqueous extraction and silica gel column, 3.1 g of the oxime 199 was obtained equating to an isolated yield of 60% and successfully demonstrating the ability to use photochemical synthesis for the scalable preparation of pharmaceutically relevant compounds. 

The increased interfacial area in the microreactor led to an increased pressure drop. The energy dissipation factor, the power unit per reactor volume, of the microreactor process was thus higher (sv = 2-5 kW/m3) than that of the laboratory trickle-bed reactors (sv = 0.01-0.2 kW/m3) [277]. This is, however, outperformed by the still larger gain in mass transfer so that the net performance of the microreactor is better. 

The Novartis Institute for BioMedical Research in Basel, Switzerland, and the University of Hull, UK, performed the diastereoselective alkylation of metal-stabilized enolates using a pressure-driven microreactor at — 100°C, whereby increased conversions and diastereoselectivity were observed compared to the batch process [20]. 

To test for fouling, a 24 h run of a pilot-scale microreactor for azo pigment production was performed using a diazo suspension [65]. At the end of this period, the pressure loss of the microreactor increased exponentially. Special means were developed to prevent clogging and instable operation. By partial removal of the deposits, the pressure loss was brought back to normal. 

Such an approach was used in a previous study by Duvenhage, et al. in 1994 [5]. These authors performed a series of deactivation studies on catalysts which were first used in an industrial fixed bed reactor at SASOL, then removed, under nitrogen, from sections along the bed, and transferred to microreactors for further study. Their results [5] showed that the activity lost at the front of a fixed bed is caused by S poisoning. However, downstream into the reactor, they observed a clear correlation between the loss in activity and an increase in magnetite concentration. Further, they observed that in the region of highest activity, the catalyst contained... 

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