Sponsored by AsyntReviewed by Louis CastelJun 10 2024
Transitioning from batch reactions to flow chemistry is not as complicated as it seems. This article will outline how to describe the parameters around reactions and reactors in flow so researchers can better understand the flow process and accurately report their experiments as they would with batch.
The Process Diagram
A process diagram illustrates the interconnections between the various elements of a flow setup using online and in-line measurements. Yet such a diagram serves another purpose: it is more than simply highlighting the connections; it is a starting point for describing the conditions under which the reaction occurs, specifically focusing on the reactor itself.
A detailed description would describe the physical environment of the reaction. For instance, if the reaction is occurring in a tubular reactor, then the characteristics of the reactor (pipe diameter and pipe length) and the characteristics of the fluid (composition, viscosity, density, and flow rate) should be described.
This, in combination with a record of the temperature, will enable replication of the process. It also makes it easy to describe the conditions more fundamentally—that is, the local environment under which the reaction is taking place. It is also possible to depict the global characteristics of the reactor as well as the information to support a kinetic understanding, which provides a framework to initiate the scale-up processes.
Reaction and Reactor Parameters
Both reaction and reactor parameters should be described in as much detail as possible. Feed compositions, temperature, and pressure are key descriptions of the setup. When describing temperature, it is recommended that the temperature most representative of the reaction is measured rather than the temperature of the reactor’s exterior.
Furthermore, in flow reactions, the flow rates of the feed materials should be carefully noted. For multiphasic systems, the start-up procedure can influence which phase is continuous and which is dispersed.
Other parameters, including stirring speed or conditions associated with light (photochemistry) or electricity (electrochemistry), may also need to be reported with respect to the reactor.
Finally, the reactor's volume, geometry, and mixing elements should be described. Naturally, additional measurements (such as pH and composition) may be taken within the reaction to aid in determining the reaction itself.
The flow rate of material is more than adequate to describe the hydrodynamics of single-phase reactions.
For liquid-liquid reactions, how the reactor is started, besides the flow of material, determines the state within the reactor. For a liquid-liquid system, it is possible to have an organic in-water phase or a water-in-organic phase. What is left is due to the relative flow rates, the reactor's start-up conditions (the continuous phase is usually filled first), and mixing conditions. Moreover, surfactants can influence this behavior.
For gas-liquid reactions, the gas will preferably be small bubbles within the liquid, offering good mass transport. The gas will take up a certain reactor volume and the liquid should occupy the remainder. Knowing the volume fraction makes it possible to identify the residence times of the two phases.
For solid-liquid reactions, particle quantity in the reactor is crucial. For instance, to achieve a well-mixed reactor, it is desirable to ensure the particles remain well suspended. Yet, if particles can collect in dead regions in the flow, there may be pockets of solids. Good mixing eliminates this issue.
Collecting all of this data is not always possible, but taking the basic steps to carefully report the set-up and how the reaction runs together with visual observations will allow further study, which should support scaling-up.
Residence Time
In a batch reaction, the time of reaction is recorded. Similarly, in a flow reactor, the mean time the fluid spends in the reactor should be noted—this is known as the mean residence time. Increasing the flow rate decreases the mean residence time for a particular reactor volume.
When thinking of the flow of a discrete collection of packets of fluid, depending on the reactor’s flow paths, some packets may pass through the reactor at faster rates than others. This phenomenon is referred to as the residence time distribution (RTD), which will depend on the reactor design.
The infographic below demonstrates the RTD for an idealized reactor (the plug flow reactor) and two real reactors - the laminar flow tubular reactor and a 5-stage cascade-CSTR (representative of the fReactor-Classic).
The y-axis refers to the percentage of time the flow spends in the reactor in relation to the time selected on the x-axis. The x-axis is shown as multiples of the mean residence time—less than 1 results in a time quicker than the mean residence time, and anything greater than 1 indicates a time slower than the mean residence time.
Basics of Residence Time Distributions
Plug Flow Reactor
Image Credit: Asynt
Description of Reactor Behavior
This is an imaginary reactor with perfect behaviour! With a plug flow reactor, each packet of fluid takes exactly the same time to travel through the reactor.
The term plug flow reactor is often used when laminar flow pipe reactor or tubular reactor would be a more precise definition.
Laminar Tubular Reactor
Image Credit: Asynt
Description of Reactor Behavior
The hydrodynamics of flow within a pipe means the flow in the centre of the pipe moves fastest, and the flow near the pipe walls moves slowest. Packets near the centre spend the least amount of time in the reactor and those nearest the edge the longest time.
C-CSTR 1 Stage
Image Credit: Asynt
Description of Reactor Behavior
A single stage continuous CSTR, whilst having active mixing and good multiphasic behaviour, has a broad desidence time distribution - some fluid exits quickly whilst some spends a long time in the reactor. A cascade of CSTRs, such as the 5 stage fReactor-Classic, narrows the RTD whilst retaining good multiphasic performance.
C-CSTR 5 Stage (fReactor-Classic)
Image Credit: Asynt
Description of Reactor Behavior
Connecting CSTRs in series enormously improves the reactor's residence time distribution. The fraction of fluid exiting quickly or slowly is minimised, and a cascade of CSTRs, such as the fReactor-Classic, which incorporates 5 stages, can bring both good multiphasic performance and a narrow RTD.
Mixing and Micro-Mixing in Reactors
Mixing produces a reaction environment with uniform conditions, such as balanced temperature, concentration, and phase fractions. Macro-mixing typically occurs on the length scale of the vessel; for instance, the blend time, where the time to reach 95% homogeneity, is a macro-mixing property.
In flow chemistry, where the rapid kinetics of the reaction can play a part due to the rate enhancements caused by the high temperatures, the micro-mixing, which is at a scale toward the molecular, can also be extremely important as it controls the rate at which the molecules are combined for a quickened reaction step.
For some reactions, poor micro-mixing simply results in a slower reaction (for instance, a neutralization reaction), while for others, the mixing rate can have a considerable impact on the conversion and selectivity of the reaction—the reaction is not only influenced by the molecules but also by the local environment in which it occurs.
In multiphasic systems, micromixing directly influences the rate of heat and mass transfer across the phase boundaries, which can also have a major impact on the reaction. In the absence of good micromixing practices, transfer can be slow.
To assess the rate of micromixing, competitive multi-step reactions are employed—for instance, the Villermaux/Dushman reaction, where a quick neutralization reaction that dominates under the right mixing contends with a parallel but slower redox reaction. The latter produces iodine, which enables detection using spectrophotometry. Evaluating the transfer between two phases in multiphasic systems can indicate the micromixing process.
Reactors with active mixing, where energy is introduced externally (for instance, fReactor-Classic, which has a physical stirrer bar), or those with passive mixing, where energy is transmitted through the flow (for instance, a static mixer), will generate a much-improved mix environment than flows dominated by diffusion alone, as is the case in a laminar pipe flow.
Notes:
- Several assumptions exist about the fluid and flow within the residence time distribution section.
- The laminar pipe flow reactor assumes the fluid is Newtonian, which leads to a parabolic flow profile. This analysis neglects the effect of diffusion—it is a purely convective model.
- For the continuous CSR models, the reactor is assumed to be perfectly mixed. If researchers dropped fluid into the reactor, it would instantly be dispersed, giving a uniform concentration throughout. The mixing is instantaneous, and there are no dead zones or bypass zones (regions of fluid the main flow does not access) within these models.
This information has been sourced, reviewed and adapted from materials provided by Asynt.
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