In continuous-flow photochemical synthesis, gas–liquid two-phase reactions have always been a challenge in process development.
In traditional batch reactors, bubbles vary in size and are randomly distributed, making the gas–liquid interfacial area difficult to quantify; at the same time, as reaction volume increases, light attenuation in the deeper regions of the reaction medium (limitations imposed by the Beer–Lambert law) causes reaction rates to drop sharply. This "black-box" state often leads to low reaction rates, more by-products, and poor linear scale-up.
How can this bottleneck be broken to achieve precise chemical control of gas–liquid two-phase systems? The answer lies in the special flow regime called segmented flow (Slug Flow / Taylor Flow).
In transparent photochemical reaction tubing, the ideal state is to present a clear, ordered flow structure: liquid slugs (liquid segments) and gas bubbles occupying the entire tube diameter alternating periodically.
This is the core characteristic of gas–liquid segmented flow. This flow regime is not only visually ordered but also represents a refined change in reaction mode: essentially, it converts traditional imprecise macroscopic reactions into precisely controllable "micro-unit reactions".
• Extremely narrow residence time distribution (RTD)
In batch reactors, part of the fluid may exit almost immediately while other portions remain for too long; this wide distribution leads to "overreaction" or "incomplete reaction." In segmented flow, gas bubbles divide the liquid into independent slug units that move at nearly identical speeds. This plug-flow–like behavior ensures that all reaction molecules experience the same "illumination history" and "residence time" in the reactor, thereby significantly improving product selectivity.
• Photon equity, higher photonic efficiency
The neatly arranged liquid-column micro-units are like "standing in line and sharing fruit." Under strong internal turbulence and vortices, each liquid-segment unit receives uniform illumination, achieving true "photon equity."
Core mechanism: features beyond traditional reactors
This flow design addresses problems that traditional batch and all-liquid reactors cannot reconcile in chemical engineering, particularly excelling in mass-transfer kinetics and optical utilization:
From a microscopic perspective, gas is supplied at fixed points and quantities and continuously diffuses into the liquid column.
• Huge contact interface increases rate: the alternating arrangement of gas and liquid columns creates a very large and highly consistent gas–liquid interface compared with batch reactors. This large contact area greatly enhances gas–liquid mass-transfer efficiency and can significantly increase reaction rates for mass-transfer–limited gas–liquid reactions.• Implementation of surface renewal theory: as the gas pushes the liquid slugs forward, wall friction induces strong internal recirculation vortices within each slug. These internal circulations continually bring fresh fluid from the slug core to the gas–liquid interface, greatly accelerating the surface renewal rate.
• Tunable turbulent mixing: by changing flow velocity and tube diameter, the system can tune the internal slug Reynolds number (from tens to thousands). This actively controlled mixing intensity, compared with passive mixing by an impeller, can increase the mass-transfer coefficient (kLa) by 1–2 orders of magnitude.
In segmented flow, a very thin liquid film (liquid film) exists between the bubble and the tube wall. For photochemistry, this film is especially significant: it sits in the region of strongest illumination and is extremely thin (typically micrometer scale), allowing light to penetrate virtually without obstruction. This makes the liquid-film region the highest-activity zone for reaction rates and effectively overcomes light-shielding effects in concentrated solutions.
Small-diameter tubing is naturally more tolerant of higher pressures, bringing twofold benefits:
1. Intrinsic safety: minimal liquid hold-up reduces operational risk.
2. Reaction intensification: high pressure compresses the gas; if the gas is a reactant (e.g., oxygen, hydrogen, carbon monoxide), elevated pressure significantly increases its equilibrium solubility in the liquid (Henry's law), directly raising the upper limit of reaction rates.
Application scenarios: Which reactions suit segmented flow?
Not all reactions require segmented flow, but for the following "tough" reaction types, the Pofeilai gas–liquid segmented-flow micro photoreactor can provide decisive advantages:
1. Gas–liquid photosensitized oxidation reactions (e.g., singlet oxygen reactions)
These reactions are typically limited by the solubility and mass-transfer rate of oxygen in the solvent. The high mass-transfer characteristics of segmented flow ensure continuous oxygen saturation while avoiding the gas waste associated with traditional sparging methods.
2. Hazardous reactions involving photo-generated gases or hydrogen
Using small-diameter, high-pressure-tolerant tubing and low liquid hold-up, photochemical chlorination, photo-hydrogenation, and similar reactions can be performed under safer conditions. For example, in chlorination using chlorine gas: chlorine rapidly dissolves at the two-phase interface and, upon photon absorption, forms chlorine radicals that react with substrates to form new radicals. The high mass transfer in segmented flow ensures continuous and rapid replenishment of chlorine into the liquid to sustain the radical-chain reaction, addressing issues in traditional reactors where insufficient dissolved gas leads to reaction interruption or side reactions.
3. High-concentration / deeply colored system reactions
Thin liquid films and strong internal turbulence solve the problem of light not penetrating the reaction bulk.
Engineering breakthrough: building a flexible reaction platform
Although theoretical segmented-flow models are ideal, achieving scale-up from lab to industrial level requires precise equipment support. The Pofeilai gas–liquid segmented-flow micro photoreactor is not just a flow-pattern generator but a process-development platform:
• Custom definition and innovation space
The system allows personalized definition using gas (whether as partitioning gas or reactant) and liquid according to kinetic characteristics. By adjusting the gas–liquid flow rate ratio, users can freely change liquid-slug length and bubble frequency to find optimal conditions for different reaction kinetics.
• Precise control and stable maintenance
The equipment can precisely control the microscopic flow field and prevent degradation into stratified or jetting flows, ensuring data reproducibility during long-term operation.
From "macroscopic mixing" to "micro-unit control," from "passive gas supply" to "active mass transfer," the Pofeilai gas–liquid segmented-flow micro photoreactor brings precise fluid-dynamic control into photochemistry, providing researchers with a high–mass-transfer, high-uniformity, intrinsically safe experimental platform that makes complex two-phase reactions clear, controllable, and efficient.
