Flow Chemistry with DART-DM Flow Reactors
Advantages of flow chemistry
There are well-defined key advantages using flow
technologies as compared to standard batch chemistry methods:
- Improved mass transfer/mixing
- Extreme reaction conditions (high/low
temperature, high pressure)
- In-line
downstream processing
- Improved Safety (managing hazardous reagents and
intermediates)
Parameters in flow chemistry
Beyond the aforementioned advantages, running a reaction
under flow conditions requires knowledge of many reaction parameters (e.g.
stoichiometry, reaction time, concept of steady state, etc).
a.
Stoichiometry
Whilst under batch conditions the stoichiometry is set by
the molar ratio of the reagents used, in a flow process the ratio of parameters
such as flow rate and molarity is used to set the specific stoichiometry.
b.
Residence time as "reaction time"
In batch mode synthesis the reaction time is determined by
the time a vessel is stirred under fixed conditions, whereas the concept of
reaction time in a flow process is expressed by the residence time, i.e., the
time reagents spend in the reactor zone. Residence time is given by the ratio
of the reactor volume and the reaction flow rate (overall flow rate).
τ = V/q
Where τ is the variable corresponding to the residence time,
V is the volume of the system, and q is the flow rate for the system
c.
Flow rates
While in batch mode, the reaction kinetics are controlled
essentially by the reagent exposure time under the specified reactions
conditions, under flow conditions reactions kinetics are controlled by the flow
rates of the reagents streams. The flow rates of the reagents indeed will
influence the residence time of the reaction and have an impact on the outcome
of the transformation.
q = dV/dt
q is usually expressed
in units such as mL min-1
d.
Volume vs space (steady state)
When considering a batch reaction, the reagent and product
concentrations vary over the time, and mixing becomes a relevant aspect
(especially when increasing the scale of the reaction) in order to reduce
concentration gradients that affect the kinetics of a reaction. Under flow
conditions, each portion of the reactor is defined by specific concentrations
of the starting material(s) and product(s): in this sense, the reaction profile
within a flow reactor can be defined within space rather than time. A very
important parameter in flow chemistry is the steady state that defines a
condition where all the parameters are defined and remain unchanged (steady) at
a particular point in time.
e.
Mixing and mass transfer
Mixing in a flow process is highly advantageous, compared to
batch mode, as it is determined by diffusion within very small volumes of
reagents. A high degree of mixing translates into better reaction profiles.
Under flow conditions, indeed, mass transfer is considered very effective and
determines the specific and enhanced kinetics observed. There are specific
aspects of mixing that should be considered (e.g. axial vs. vortex mixing) and
are dependent on specific fluid behaviours, namely plug or laminar flow
patterns.
f.
Temperature control and heat transfer
The control of temperature in flow processes can be achieved
very accurately, due to the high surface area-to-volume ratio.
Accordingly, heat transfer can be very efficient although
this parameter depends on the specific aforementioned aspects of the fluid
behaviour. Indeed, depending on whether the flow is laminar or turbulent, heat
transfer can follow different patterns.
