(8a) 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the 1/16 inch tube section has been replaced by a microstructured plate heat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.