Speciality Chemicals Magazine SEP / OCT 2023 | Page 41

FLOW CHEMISTRY capabilities , give highly reproducible flow patterns and are relatively straightforward to implement and operate given the right flow controls . Dynamic flow reactors increase shear and turbulent mixing , and allow for precipitation reactions .

Flowid ’ s dynamic flow reactor , SpinPro , is a type of rotor-stator spinning disc reactor that is already used in multiple continuous manufacturing skids . These reactors comprise a range of readily scalable dynamic flow reactors , covering the entire range from the initial screening of chemistry in the lab , via pilot-scale optimisation studies to full-scale production units .

For many applications from research to production phase , there is a suitable dynamic reactor . Another advantage is that scaleup is straightforward . Over the past decade , dynamic reactors have transitioned from promising technology for process intensification to becoming a reliable and proven continuous manufacturing system .

A rotor-stator spinning disc reactor ( Figure 1 ) consists of a number of rotating discs ( rotors ), each inside a narrowly fitting encasing ( stator ). The disc is about the size of a DVD , rotating with a speed up to 4,000 rpm , while the typical rotor-stator distance is one millimetre . The very high rotational velocities lead to highly intense mixing , directly improving the mass transfer rate .

The large difference in velocity over the very narrow gap between the rotor and the stator leads to extremely fine dispersions of droplets and bubbles with a very high interfacial area , further enhancing mass transfer . 1 In comparison with other static flow reactors , the mixing intensity and level of turbulence do not depend on the flow rate , allowing an independent variation of both residence time and mass or heat transfer effects .

Successful applications of dynamic reactor technologies are found in highly exothermic reactions , such as nitrations and organolithium reactions . 2 , 3 The latter can even be performed under the formation of precipitates , as the rotor-stator spinning disc reactor is a dynamic type of flow reactor which does not suffer the drawback of clogging that is more common in static flow reactors . 4

Aside from being able to safely perform highly exothermic reactions , the dynamic reactor technology also offers major benefits for another type of reaction in chemical processing : multi-phase chemical reactions . These reactions are ubiquitous steps in many synthesis routes in the speciality chemicals and pharmaceutical industries .

In a multi-phase reaction , one or more molecules from different physical phases ( i . e . gaseous , organic , aqueous and / or solid ) need to come together to react . First , a reagent molecule in a gaseous state has to be transported from the bulk of the gas , through the gas-liquid interface towards the bulk of the liquid .

From there , the molecules of interest then have to diffuse further through one or several film layers surrounding droplets , bubbles or catalyst particles , depending on the type of chemistry . Only then , when all molecules have come into close contact and at the right place , can the actual reaction take place .

Each step in this sequence takes a certain amount of time and effort . The total production rate of the chemical reaction not only depends on the intrinsic kinetics , but it must be described as a sort of resistance in series .

Although the intrinsic reaction rate between the molecules can be very high when ( or if !) they are in contact , the total production rate often suffers from the time and effort required to get all the phases well dispersed and contacted . In a sense , overcoming the limitations related to mixing in this cascade of steps , brings total production rates much closer to the intrinsically fast reaction rates .

When comparing mass transfer rates between dynamic and batch reactors , it is evident that batch vessels suffer from drastically lower mass transfer rates , as gas-liquid mass transfer rates in rotor-stator spinning disc reactors are at least 40 times higher than in conventional reactors . 5 , 6 To reach an acceptable level of productivity ,

Table 1- Industrial cases with major advantages from going to flow

Gas Liquid ( s ) [ barg ] [° C ] [ rpm ] [ min ] [ ton / yr ]

More than 80 % reduction of energy cost and CO 2 emissions while doubling production capacity . Large reduction in hold-up of corrosive gas .

Large increase in production rate . The old process was cryogenic , while this was no longer necessary in the SpinPro , leading to a large cost reduction for energy .

Elimination of toxic gas hold-up and thus an intrinsically safe process . This allowed room to expand production capacity without problems with safety regulations .

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