Refined and extended investigation of microreactor based ionic liquid synthesis for further process intensification
Special Symposium - EPIC-1: European Process Intensification Conference - 1
EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-3)
Keywords: microreactor, process intensification, novel chemistry, ionic liquids
Ionic liquids have gained an enormous interest over the past years because of their unique properties allowing them to replace traditional organic solvents in chemical reactions and as extraction media.[1] The respective synthesis often uses reaction paths which comprise at least one feature which render reactions tailored for use in microstructured reactors – large heat releases which may result in impurity formation, if heat transfer is insufficient. Microstructured reactors by their superb transfer properties principally allow overcoming such limits.
As reported already earlier [2], we have investigated in the framework of the EU project IMPULSE the limitations of conventional batch processing and the principal potential of continuous microreactor based operation for the highly exothermic alkylation of methyl imidazole by diethyl sulphate. Initial experiments in lab-scale batch vessel demonstrate impressively the occurrence of hot spot for the one-step dosing of the reactants and therewith connected the fast change of the reaction mixture from a clear colourless to a dark black-brownish solution. Improvement is obtained via a multi-step dosing leading to an only slightly yellowish product. The obvious disadvantage is a long reaction time. The process was then transferred successfully in lab-scale into a microreactor based continuous operation. The set-up thereby consisted basically of a micromixer for contacting the undiluted reactants followed by a Teflon tube (7.5 ml inner volume) as residence time section, both embedded in a thermostat bath. Based on measurements of the temperature profile over the length of the reactor, the tube dimension was finally fixed to 1/16´´ for the first part of the tube section and 1/8´´ for the following part. A temperature controlled operation sufficient for the avoidance of colouring of the product is possible up to a total flow rate of 0.2 l/h. Compared to the conventional batch processing reactor volume is reduced down to 7.5 ml and reaction time also from the hour range to a few minutes. Once more microreactors have proven therewith as enabling tool for new process routes (“novel chemistry”, [3]).
The set-up has now been further improved by switching from Teflon to stainless steel as tubing material leading due to the better heat transfer capability of stainless steel to even better temperature control. Also the robust set-up allows now to investigate other ionic liquid syntheses (e.g. the alkylation of imidazole using ethyl bromide) requiring working under pressure because of reaction temperatures above the boiling point of the alkylating agent.
Based on the results obtained so far an adapted reactor concept targeting at higher, production throughput (about 4 – 8 l/h) has been developed in which a microstructured heat exchanger forms the reactor part which has to handle the largest heat release. The throughput capacity of the latter will be adjusted by internal numbering-up [4]. A first prototype of the micro heat exchanger has been realized for a total flow rate of 0.4 l/h and integrated to a complete set-up. The principal functionality was proven and the set-up is now in operation for long time testing by the industrial partner.
Supplementary, calorimetric and kinetic measurements were performed for the alkylation of methyl imidazole by diethyl sulphate. In conjuction with the experimentally derived temperature profiles in the tube section of the above mentioned set-ups and characterisation of heat transfer properties of the equipment used, these will be used to perform simulation using the software Berkeley-Madonna aiming at the exploration of even more intensified operation regimes e.g. by different temperature zones. Decomposition studies performed with the reactants and products thereby give the maximum temperature limit.
[1] Wagner, M., Chimica Oggi (Chemistry Today) 22, 6 (2004) 17-19.
[2] Löb, P., Hessel, V., Krtschil, U., Löwe, H., Chimica Oggi (Chemistry Today)
24, 2 (2006) 46-50.
[3] Hessel, V., Löb, P., Löwe, H., Current Organic Chemistry 9 (2005) 765-787.
[4] Schenk, R., Hessel, V., Hofmann, C., Kiss, J., Löwe, H., Ziogas, A., Chemical
Engineering Jorunal 101, 1 (2004) 421-429.
Presented Thursday 20, 10:10 to 10:30, in session EPIC-1: Intensified Hydrodynamics & Structured Environments (IHSE-3).
