Mixing performance

The term macromixing refers to the overall mixing performance in a reactor. It is usually described by the residence time distribution (RTD). Originally introduced by Danckwerts (1958), this concept is based on a macroscopic lumped population balance. A fluid element is followed from the time at which it enters the reactor (Lagrangian viewpoint - observer moves with the fluid). The probability that the fluid element will leave the reactor after a residence time t is expressed as the RTD function. This function characterises the scale of mixedness in a reactor. 

The application of draft tubes as related to various mixing operations is showm in Figures 5-231 and 5-24A-5-241. The draft tubes are basically a tube or shell around the shaft of the mixer including the usual axial impeller, which allows a special or top-to-bottom fixed flow pattern to be set up in the fluid system. The size and location of the tube are related to both the mechanical and mixing performance characteristics as well as peculiar problems of the system. Usually they are used to ensure a mixing flow pattern that cannot or w ill not develop in the system. Weber gives the followdng points for draft tubes [23] ... 

Figure 5-34. Typical vessel baffles to improve mixing performance. Adapted/modified by permission, Casto, L. V., Chem. Engr., Jan. 1, 1972, p. 97 [30].

The proper placement of the impeller for specific applications is necessary for good mixing performance therefore, a thorough discussion with a mixing company specialist is useful. 

Mixing performance, 306 Blending, 324 Emulsions, 324 Extraction, 324 Gas-liquid contacting, 324 Gas-liquid dispersion, 325 Liquid-liquid dispersion, 325, 326 Mixing vortex, 311 Motionless mixing, see static mixing National Fire Protection Association, 399 Net positive suction head, 160-194 Available from system, 160, 188, 189,... 

A reaction is to be carried out in an agitated vessel. Pilot-plant experiments were performed under fully turbulent conditions in a tank 0.6 m in diameter, fitted with baffles and provided with a flat-bladed turbine. It was found that the satisfactory mixing was obtained at a rotor speed of 4 Hz, when the power consumption was 0.15 kW and the Reynolds number 160,000. What should be the rotor speed in order to retain the same mixing performance if the linear scale of the equipment is increased 6 times What will be the power consumption and the Reynolds number ... 

For instance, in order to characterize the mixing performance of any transparent reactor, the reaction of discoloration of an iodine solution with sodium thiosulfate could be used according to the following reaction scheme ... 

For instance, the mixing performances of different channel designs for the DeanHex reactor have been investigated in our lab, and the respective operating conditions are given in Table 12.6, while Figure 12.7 shows the mixing time versus the Reynolds number. 

This study investigates the hydrodynamic behaviour of an aimular bubble column reactor with continuous liquid and gas flow using an Eulerian-Eulerian computational fluid dynamics approach. The residence time distribution is completed using a numerical scalar technique which compares favourably to the corresponding experimental data. It is shown that liquid mixing performance and residence time are strong functions of flowrate and direction. 

Compared to the Category Test, SSP and SRT results show a relatively mixed performance profile. This profile is indicative of temporal lobe impairment and may explain the idiosyncratic character of PCP-induced brain dysfunction. Other HRB subtest data suggest that parietal lobe-mediated functions are less influenced by PCP abuse, since approximately 30 percent of this sample had error-free performances on a test sensitive to finger agnosia. 

One of the complications in stirred tank flows is the presence of macroinstabilities (i.e., low-frequency mean flow variations) that may affect the mixing performance. Various authors have distinguished between various types and investigated their occurrence and their frequencies under varying operating conditions and with several types of vessels and impellers (Yianneskis et al., 1987 Haam et al., 1992 Myers et al., 1997 Hasal et al., 2000 Nikiforaki et al., 2002). 

In a research and development laboratory at the Dow Chemical Company in Midland, Michigan, rotational viscometry experiments on various dilutions of a test fluid, such as corn syrup, can generate the required data. Once various challenges are overcome, such as obtaining a uniform and constant temperature throughout the fluid and dealing with unusual physical behaviors of the test fluid, accurate viscosity measurements can be made and the project to optimize mixing performance can move forward. 

If the desired yield target cannot be achieved then an antisolvent system can be selected using the same techniques. The antisolvent should be fully miscible with the primary solvent and have a low solubility for the solute. It should be noted that the addition of an antisolvent to reduce solubility and generate supersaturation may introduce scale up issues, caused by the differences in micro-mixing performance between the laboratory and manufacturing plant. 

A considerable amount of literature exists on mixing mechanisms, processes, and devices. Most of the devices developed utilize fundamental principles to provide mixing to a polymer system. Some devices, however, have been placed on the market with good intention but provide lower mixing performances. The number of devices on the market is considerably more than what could be covered here. Instead only the most-used devices and their performances will be covered in this chapter. The reader is directed to other sources for detailed mixing mechanisms, devices, and applications [1, 2]. 

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