(139e) Determination of Temperature Profile within Continuous Micromixer-Tube Reactor Used for the Exothermic Additon of Dimethyl Amine to Acrylonitrile

The addition of secondary amines to a,b unsaturated carbonyl compounds and nitriles, such as acrylic acids and their esters or acrylonitrile, is a common synthesis path in organic chemistry and a variant of the Michael-addition. Using a batch procedure these reactions give good yields, in some cases even exceeding 85%, but at long reaction times up to 24 hours. The latter, however, is not caused by the intrinsic kinetics of the reaction, but relates in many cases to their great exothermicity. In order to enable safe removal of the heat created by the reaction, it is necessary to add the a,b unsaturated compound quite slowly to the amine. Microstructured reactors of suited scale enable to profit from the fast kinetics of the reaction, while still ensuring efficient heat removal and avoiding thermal overshooting.[1] Therefore, the process has been shifted to a continuous one using a micromixer-tube reactor embedded into a thermostated bath. Systematic investigations were performed for different systems using as amine either dimethyl amine, diethyl amine, piperidine and either acrylonitrile or arylic acid ethyl ester as a,b unsaturated carbonyl compound.[2] The set-up allowed a direct contacting of the reactants under controllable conditions. Determination of yield as function of temperature and total flow rate (residence/reaction time) for the different combination possibilities allowed to compile the reactivity row of the amines and the a,b unsaturated carbonyl compounds (piperidine > dimethyl amine > diethyl amine; acrylonitrile > acrylic acid ester). As expected the microreactor processing also lead to higher space-time yields. The exothermic addition of aqueous dimethyl amine to acrylonitrile was selected for further studies focusing on the temperature profile within the micromixer-tube reactor. Therefore, the micromixer-tube reactor was modified by integration of seven temperature sensing elements distributed over the length of the tube and inserted via mini-T-junctions placed in between the tube sections. The aim of the performed investigations was to obtain some generic relationships between heat release with connected temperature profile (hot spot formation) and reactor/process engineering data. Total flow rate was varied from 0.3 to 3.4 l/h and the reaction was either performed at room temperature (additionally without tempering by the water bath) and at 60°C. Hot spot formation up to 100°C was observed especially for the higher flow rates in the case of using 1/8 inch tube. Furthermore, it was found that the higher the flow rate is, the more downstream the hot spot is detected and the broader the temperature profile. Based on the temperature profiles it was possible to explain rather unexpected results, e.g. higher yields for higher flow rates (i.e. reduced residence and reaction time), as consequence of hot spot formation. The experimental set-up is flexible with regard to exchange of tube sections. For another reaction example, it was possible to demonstrate that the reaction temperature could be kept almost isothermal when exchanging the first two 1/8 inch tubes by 1/16 inch tubes.[3] The obtained results by the determination of temperature profile assist the design of future adopted microreactors.

[1] Hessel, V., Löb, P., Löwe, H., Development of reactors to enable chemistry rather than subduing chemistry around the reactor: potentials of microstructured reactors for organic synthesis, Curr. Org. Chem. 9, 8 (2005) 765-787. [2] Löwe, H., Hessel, V., Löb, P., Hubbard, S., Addition of Secondary Amines to a,b unsaturated Carbonyl Compounds and Nitriles by Using Microstructured Reactors, submitted to Org. Proc. Res. Dev. [3] Hessel, V., Löb, P., Löwe, H., Uerdingen, M., Fine Chemical and Initial Ionic-liquid Syntheses in Microstructured Reactors; to be presented during ACS Symposium Atlanta, 2006.