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Refinery Reactors
Published in James G. Speight, Refinery Feedstocks, 2020
The traditional and perhaps most common way to deal with mass transport limitations is the stirred-tank reactor in which a two-phase slurry of particles in suspension is stirred using an agitator. This generates a convective flow in the reactor which increases the relative transport of the liquid phase and solid particles, bringing reactants together at a higher rate than if left only to diffusion. The agitator also improves the mixing time of the reactor, being the time taken to disperse any concentration gradients in the liquid bulk that arise due to the local generation or removal of chemical species at the reaction sites. However, the stirred-tank reactor may cause physical damage to the solid phase as the particles collide with the agitator and each other. These collisions often lead to grinding of the particles, producing fines that are hard to remove and limit the ability to reuse the solid material. For most applications, the stirred-tank reactor introduces a need for filtration after the reaction is completed.
Physical Processes
Published in Ralph L. Stephenson, James B. Blackburn, The Industrial Wastewater Systems Handbook, 2018
Ralph L. Stephenson, James B. Blackburn
The most common type of mixer is the turbine agitator type. A turbine-agitator is a mechanical device that produces motion in a fluid through the rotary action of impellers. The turbine impeller consists of fixed-angle blades attached to a hub driven by the agitator shaft, which in turn is driven by a motor or other power providing device.
Bead Mill Disruption
Published in Juan A. Asenjo, Separation Processes in Biotechnology, 2020
Horst Schütte, Maria-Regina Kula
The agitator has to accomplish the energy transfer to the grinding beads; therefore, its design is highly important for the grinding efficiency. The different types of agitator available can be classified into three groups: (1) disk agitators, (2) pin agitators, and (3) rotor-stator systems. Disk agitators are characterized by a number of disks fitted to the central shaft in a centric or eccentric position. Disks are constructed from metal or plastic (e.g., polyurethan) and may be solid, perforated, slitted, or sectioned. Centric as well as eccentric disk agitators induce centrifugal and possibly centripetal acceleration of the grinding beads at high agitator speed (Figs. 1 and 2). With a pin agitator the beads are activated by short pegs that are fastened to the wall of a large-diameter, hollow shaft serving as a central axis. The activation of the beads is further enhanced by rows of stationary pegs that are radially fastened to the inner wall of the grinding chamber. The velocity of the agitator peg at its base is nearly equal to the velocity at the tip. This is in contrast to the large difference in velocity of a mill with a small-diameter shaft. The third group of agitators are rotor-stator systems. The grinding zone is defined by the smooth cylindrical or conical walls of the stator and the rotor. Through the reduction of the grinding chamber to the narrow gap, a very high energy density in the grinding zone is achieved. Different types of agitators are favored for special applications; for the disruption of microbial cells the first type, with disks, is mainly used. Glass beads are quickly destroyed by pin agitators in slurries of low viscosity such as microbial suspensions. Rotor-stator as well as pin agitator mills are usually applied to process highly viscous media.
Study of mixing and discharging of dry particles in a truck mixer
Published in Particulate Science and Technology, 2020
Yuanqiang Tan, Rong Deng, Hao Zhang, Shengqiang Jiang
Mixing of particulate materials is a common process involved in the industry for making productions of fresh concrete, pharmaceuticals, mineral, plastics, household products (such as detergents) and food and so on. The mixing process can be performed using a variety of devices through mechanical stirring, in which an agitator is usually used to generate motion for mixing the mixture to achieve uniformity. In construction engineering, the truck mixer is commonly used to deliver the well mixed and homogeneous fresh concrete to the construction site. During the transportation process, the truck mixer needs to further mix the fresh concrete to keep a good uniformity. When the concrete mixture is discharged from the truck mixer, its properties depend on the mixing process and discharging procedure used. Therefore, reasonable motion parameters should be chosen to achieve a high discharging homogeneity.