Mechanically or hydrodynamically, two or more materials are dispersed to each other to achieve a uniform degree of unit operation. Among them, fluid materials are involved in fluid power processes. The application of mixing in chemical production is very common, and its main purpose is: 1 to prepare various homogeneous mixtures, such as solutions, emulsions, suspensions and pastes, pastes or solid powder mixtures; 2 for some units Operation (such as extraction, adsorption, heat transfer, etc.) or chemical reaction processes provide good conditions. In the preparation of a homogeneous mixture, the mixing effect is measured by the degree of mixing of the mixture, ie the uniformity achieved. When accelerating physical or chemical processes, the mixing effect is usually measured by the total mass transfer coefficient, heat transfer coefficient, or reaction rate.
The mixed scale refers to the spatial extent in which the mixing phenomenon occurs. For the mixing of liquid systems, macro-mixing refers to the mixing of fluids on a macroscopic scale, caused by the macroscopic flow of the fluid, which involves mixing in the equipment due to shear forces due to bulk flow, turbulent pulsation and velocity gradients. Micro-mixing refers to the alternating arrangement or separation of two substances on a molecular scale, which can only be achieved by molecular diffusion. Different production processes have different requirements on the mixing scale: for example, the agitation of crude oil in a large oil tank of a refinery requires only uniform mixing on a macroscopic scale; the rapid reaction between the two fluids requires not only macroscopic uniformity but also microscopically. Mix quickly.
According to the phase of the mixture, the mixing method is as follows:
1 Blending: Two or more kinds of powdery solid materials are mixed by mechanical means. The machine used is called a blender, which is an empty cylinder that is asymmetrical to the rotary axis, and the material is tossed in the drum. The rotation speed of the cylinder and the material filling rate (the ratio of the bulk volume of the material to the total volume of the cylinder) are important operational parameters that affect the blending effect and can be determined by the test. The filling rate is generally about 30% to 50%, and the suitable rotation speed is determined by ω2R/g=0.25~0.8 (where ω is the angular velocity, R is the maximum radius of rotation of the cylinder, and g is the acceleration of gravity). Blending is often used for mixing various raw materials and starches, coloring of dyes or pigments, mixing of different types of synthetic resins, and the like.
2 Kneading: Mechanically mixing paste or high viscosity materials. The machine used is called a kneader. The machine usually has a pair of reverse-rotating Z- or S-type scrapers, which cut, extrude and fold the agglomerates to disperse the various components. During the kneading process, viscous friction or a concomitant chemical reaction causes the machine to heat up and needs to be cooled through the partition. Kneading is often used in the formulation of pharmaceutical dosage forms, food materials and electrode pastes.
3Kneading: It refers to the operation of mixing powdery granular compounding agent such as carbon black and sulfur into raw rubber. The machine used is a sealed mixer, also known as an internal mixer. The machine is equipped with a pair of cylindrical rolls with two or four spiral ribs on the roll. The two rolls are rotated in opposite directions at different speeds to strongly shear and divide the material. A cooling device is provided inside the machine to remove heat generated by friction of the material.
4 Stirring: One of the most common methods of dispersing liquid, gas or solid powder into a liquid.
5 jet mixing; the mixing of the energy of the working fluid itself during its flow. The working fluid is ejected at a high speed from a circular nozzle or a tapered nozzle to form a jet. Due to the turbulent pulsation at the junction of the jet and the surrounding fluid, the two fluids are mixed. Jet mixing is mainly used in the industry for mixing low-viscosity liquids in large containers. For example, adding a small amount of tetraethyl lead to a large gasoline tank is also used to prevent the deposition of suspended solids in the tank. At this point, the nozzle can be placed against the bottom of the tank. , sweep the sediment and let it float.
6 Pipe Mixing: The two fluids are merged by a three-way pipe, and then flow through a straight pipe to be mixed by turbulent flow. The addition of an orifice plate or a round-shaped baffle baffle in the tube can enhance the turbulence of the fluid and improve the mixing effect. This method is mainly used for the mixing of low viscosity liquids or gases. A static mixer is a pipe mixing device. A stationary dividing element is placed in the tube to divide and merge the flowing fluid multiple times. This mixer is not limited to turbulent operation, but is also suitable for the mixing of high viscosity liquids for laminar flow operations. A typical static mixer (see figure) is a plurality of twisted 180° spirals in the circular tube as components, and the left and right handed spirals are mounted one on the other. Each time the fluid flows through a spiral, it is divided into two pieces, flowing through n elements, and the fluid is divided and merged by 2n times. If there are 20 components in the tube, the number of times the fluid is divided will be more than 1 million times. Static mixers are commonly used in extraction and emulsion preparation.
A measure of the degree of uniformity of dispersion blending achieved by mixing different materials. The degree of mixing is related to the scale examined. In order to evaluate and compare different mixing methods and mixing equipment, an appropriate expression can be selected according to the specific situation.
The degree of mixing of the fluid system can be expressed in the following three ways:
1 The degree of uniformity is a measure of the uniformity of the overall size of the device. The average degree of uniformity I of a component is the arithmetic mean of the degree of blending of a component based on several samples in the same tank. It can be used to measure the overall mixing effect of the system, but must indicate the number of samples taken.
2 A measure of the size of dispersed micelles (eg, droplets, bubbles, or solid agglomerates) in a mixed-scale mixture. Within the range of separation scales, the substance is homogeneous or has only one substance. The smaller the separation scale, the more uniform the mixing. For immiscible systems, it is not possible to achieve molecular-scale separation scales.
3 A measure of the uniformity of the separation-strength miscible system on a small scale. During the mixing operation, mixture micelles of different sizes and concentrations are present in the mixture. The difference between the micelle concentration of these mixtures and the average concentration is the separation strength. The smaller the separation strength, the more adequate the material mixing and the more blurred the interface between the micelles. When the mixture has reached a desired state of mixing by molecular diffusion, that is, when the so-called molecular scale is uniform, the separation strength is zero.
In a mixing device, the overall flow of the fluid acts to increase the degree of mixing of the mixture, while turbulent pulsation reduces the separation and separation strength.
The degree of mixing of the solid system is expressed by the standard deviation of the content of a component in the sample taken.
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