【Haichuan Illustrated Chemical Equipment】Series Posts -- Rotary Extractor Tower: The “Rotating Magician” of Chemical Separation”

III. Uncovering the working principle: The waltz of liquid-liquid embrace and separation. Now that we understand its structure, let’s take a look at how it functions. The operation of a rotary disk extractor is usually counter-current. Suppose we have two liquids: one that is heavier (the heavy phase) and one that is lighter (the light phase). The heavy-phase liquid enters from the top of the tower and flows downward under the force of gravity ； The light-phase liquid enters from the bottom of the tower and flows upward under the effect of buoyancy. When these two liquids meet inside the tower, the action begins! Step 1: Shearing and dispersion (mixing zone) As the central axis rotates at high speed, it drives the rotor disk to rotate as well. Due to the viscosity of the liquid, the liquid near the turntable also rotates, resulting in strong centrifugal and shear forces. One of the liquid phases (usually the phase with lower flow rate or higher interfacial tension) is mercilessly torn apart and broken by the shear force, turning into countless tiny droplets (the dispersed phase) that are evenly dispersed within the other liquid phase (the continuous phase). The formation of tiny droplets means that the contact area between the two phases increases exponentially! At this moment, the target substance (solute) rapidly crosses the boundary from one phase and moves into another phase; this is what is known as \"mass transfer.\" Step 2: Circulation and coalescence (separation zone) – Due to the obstruction posed by the stator rings, the liquid cannot rotate chaotically. The mixed liquid is flung against the tower wall, and after encountering the stator ring, it splits into upper and lower streams, forming two vortices. In the corner areas near the tower wall and the stator ring, the turbulence of the fluid is significantly reduced. Here, those tiny droplets have the opportunity to collide with each other and recombine to become larger. Step 3: The droplets, whose inter-stage flow has increased, pass through the inner hole of the stator ring under the influence of gravity or buoyancy, entering the next \"small compartment\". The heavy phase continues to fall, while the light phase continues to rise. Upon entering the next compartment, they encounter the next rotor disk, where they are once again broken up, mixed, subjected to mass transfer, and coalesced... In this way, the liquid undergoes countless cycles of \"dispersion-coalescence-redispersion\" within the tower, enabling efficient counter-current multi-stage extraction. Ultimately, the extract loaded with solutes flows out from one end, while the cleaned residual liquid (raffinate) flows out from the other end

IV. Advantages and Disadvantages of the Rotary Disk Tower🌟 Its “highlight moments” (advantages): Extremely high mass transfer efficiency: Compared to packed towers or tray towers that lack mechanical agitation, the rotary disk tower can break liquid droplets into smaller pieces, thereby significantly improving mass transfer efficiency. Great operational flexibility: When dealing with different materials and processing volumes, operators only need to adjust the motor speed to change the size of the droplets and their residence time, thereby easily controlling the extraction efficiency. Relatively simple structure and low cost: although it has rotating components, there are no complex fillers inside, making maintenance and cleaning relatively easy. Strong anti-clogging capacity: Even when the liquid contains a small amount of solid suspended particles (such as sediment), the rotary drum tower can handle this situation easily, and it is less prone to clogging compared to packed towers. ⚠️ Its weaknesses (disadvantages): Axial backmixing cannot be ignored; as the liquid inside is vigorously stirred, some of it may mix in the opposite direction (liquid that should move upward moving downward instead), which slightly reduces the driving force for concentration and affects overall efficiency. It can lead to severe emulsification: if the two liquids are prone to emulsifying on their own, the high-speed shear from the rotary disk can turn them into a uniform mixture that can no longer be separated into layers. In this case, the rotation speed must not be too high, or the turntable tower should not be used at all. Mechanical maintenance is required: wherever there are rotating shafts, there are bearings and mechanical seals, and prolonged operation poses risks of wear and leakage, thus demanding high standards for sealing technology.

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