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A flow reactor is a chemical process where raw materials are added to a reaction vessel, typically a series of tubes, to create products continuously. This is different from a batch process, where all the materials are added and reacted, then removed and processed. Flow reactions continue as long as raw materials are added at one end, and can be used for both liquids and gases.
The design of a flow reactor is typically a series of tubes inside a temperature-controlled shell. Heat or coolant can be added to the shell to provide temperature control of the reaction inside the tubes. The reactor can be constructed from metals, plastics or composites as needed to prevent the raw materials from attacking it.
Tube designs for a flow reactor can vary greatly depending on the desired reactions. Rarely are the tubes empty, because mixing will not occur in empty tubes; the materials will remain separated inside the tubes and not react. Coils of smaller tubes, small shapes called packing, or internal barriers called baffles are all used to mix the reactants, or raw materials, together.
Placing coiled tubing inside a flow reactor can help with mixing or heat transfer. Coils add distance for the chemicals to travel in the reaction section, providing more time for the chemical process to occur. Heating or cooling liquids can also be inside the coils, with the reactants outside, for better temperature control. Tube size, which changes the internal diameter or cross-section, can be varied to change reactant flow rates.
Tubes can be packed with various materials based on what reactions are needed. Some chemicals require a catalyst, a material that accelerates reactions without being used up in the process. Catalysts can be added to ceramic glass beads or other materials and packed in the tubes. Non-catalyst packing can also be used to help mix the reactants, which is often needed if heat or cooling is applied on the outside of the tubes. Without mixing, the material closest to the tube walls will be too hot or cold, which will affect product quality.
Baffles can vary greatly in design, but all help mixing by creating vortices, or swirls of turbulence inside the tube. They can be layers of mesh installed in the tubes, or grooves machined into the tube walls. Catalysts can also be coated onto baffle surfaces, providing reaction control in addition to mixing.
Flow reactor design also takes into account the reaction rate of the chemicals. The movement of chemicals through all tubes must be the same, or the finished product quality may be different from each tube. Designing the flow rates to gain plug flow conditions ensures a consistent product. Plug flow is a characteristic of tube design and flow rate control where the time reactants spend in the reactor is the same regardless of which tube is observed.