The primary difference between Segmented and Continuous flow is that in Segmented flow discrete reaction mixtures ( segments) are automatically prepared (metered and mixed) using an array of precision syringe pumps, the segment are then moved into an injection loop and then injected into a high pressure flow channel. Once in the flow channel the reaction segment is transported through the flow reactor using a high pressure HPLC style pump.
Segmented Flow Beneftis
- Use of more aggressive chemicals - strong acids and bases;
- More robust ability to handle the formation of precipitates;
- Flexibility of reaction scale - micrograms to hundreds of grams;
- Extreme reaction conditions - 150 Bar and 300 °C;
- Reaction Automation - preparation, conditions, analysis and collection.
In Accendo's flow systems, reaction chemicals are metered using ballast loops between the reagent reservoirs and the syringe pumps, which prevents exposure of pump components. By using an injection loop to introduce the reaction segment into the flow reactor, the Transport Pump components are also not exposed to any aggressive reagent.
Precipitation
Segmented flow is less prone to clogging of the flow channel due to precipitate formation in three ways:
- Large Flow Channels - 0.75 to 1.00 mm diameters, more robust to slurries;
- High Pressure - using a HPLC style pump keeps precipitates flowing;
- Reaction Segment - reaction volume is much less than the reactor volume so when precipitates form they do not form throughout the reactor often less than 25% of the sytem volume.
Segment volumes may be varied based upon the mode of operation. When screening and optimizing conditions segment volumes can be small (250 µL to 1.0 ml) in order to conserve starting material. When larger scale (grams to 100s grams) are required the flow systems are programmed to increase segment volumes and repeatedly make the same segment.
Extreme Conditions
Our flow systems enable high temperature and pressures (300 °C and 150 Bar) so exploring reactions that are are performed in a microwave are routinely conducted in the Conjure and Propel. It is not uncommon to use solvents such as ethanol and methanol at 250 °C.
Reaction Automation
Utilizing segmented flow better enables the integration of automation to prepare the reaction segments, control the flow conditions (incubation rate and temperature) and online analysis with LC/MS.
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