Modular Multi-Scale Reactor for Ionic Liquid Production - Development, Realisation and Characterisation

Synthesis of ionic liquids often uses reaction paths which at least in parts can benefit from transferring current batch processing to continuous processing in microstructured reactors. In the case of the alkylation of methyl imidazole by diethyl sulphate there are different driving forces: the reaction is highly exothermic and no solvent is used. Therefore the large heat release can result in hot spot formation connected with the formation of side products and unwanted colouring of the product if heat transfer is insufficient. Microreactor application allows also process intensification and reduction of chemical hold-up which is safety relevant.

The corresponding lab-scale investigations demonstrating the general potential of continuous microreactor processing in this specific case partly have been reported earlier. [1-2] Lab-scale throughput amounted up to 0.45 kg/h. The lab-scale experimental set-up has seen several development phases. The initial first lab-scale experiments have been done using a micromixer connected to a 1/8 inch tube section and have shown that controlled continuous production in the minutes range is principally feasible. A better temperature profile was observed for a multi-scale design of the tube section: a 1/16 inch tube section for the first reaction phase followed by a 1/8 inch tube section for completion of the reaction. Further improvements were reached as well as better scalability when the 1/16 inch tube section has been replaced by a microstructured plate heat exchanger. Generally, experimentation covered also the aspect of working with different temperature zones. To achieve improved temperature control in the feed section, the first microstructured heat exchanger was replaced by one with two-temperature zones.

The decisive following step was to bring this processing concept to pilot or rather production scale which is in the range of 100 kg/d, i.e. a roughly tenfold throughput increase. A modular and multi-scale reactor concept was taken. The reactor consists of a sequence of a micromixer, a modular constructed microstructured reactor with integrated heat exchange function which can be operated at two or more different temperatures and a multi-tubular reactor to complete the reaction. The higher throughput was achieved by an increased number of plates. Details (e.g. concerning connectivity issues) of the step from the last lab set-up to the production scale concept will be described and discussed. This reactor set-up approach additionally picks up the aspect of flexibility and better cleanability enabled by the modular, flangeable microstructured reactor part and by a switchable multi-tubular reactor.

The results of basic characterisation of the set-up including pressure drop, residence time, residence time distribution, heat exchange, pressure stability and mixing quality will be discussed. For the characterisation of the set-up the methods suggested in the context of the EU-project IMPULSE for structured equipment are applied.[4] Standardised equipment characterisation is seen as a step towards future simplified equipment selection for chemical processes in general.[4]

 

[1]        Renken, A. et al., Chemical Engineering and Processing 46 (2007) 840-845.

[2]        Löb, P. et al., Chimica Oggi (Chemistry Today) 24, 2 (2006) 46-50.

[4]        Double, J., Proceedings CHISA Conference 2008.