Example Mixer

The feed flow is used for adjustment of the throughput, consequently the number of controlled variables exceeds the number of manipulated variables by one. One possibility is to leave one controlled variable free, but it is better to make the level or temperature control a slave controller of a quality controller by means of a cascade control stmeture (see chapter 33). The choice is determined by the variable with the highest impact on quality, either the temperatme or the residence time. 

For the polymerization reactor shown in Fig. 3.9, quality is represented by the molecular weight distribution of the polymer that is produced. In the separation train this cannot be affected anymore. For most chemical reactors, however, maximum conversion and selectivity are in some cases more important rather than a specific final quality. 

A mixer is a simple example that demonstrates the principle of interacting balances. Behavior is determined by two differential equations one total mass balance and one component balance. If the level is constant by means of an ideal level controller, the number of balances reduces to one. This wdl be demonstrated in a subsequent chapter. This chapter limits itself to the environmental model. 

The goal of process operation, shown in Fig. 3.11, is to mix two substances. Temperature effects, for example heat of mixing, can be ignored. 

Goal of the study is to investigate the behavior of the eoneentration to ehanges in the throughput and mixing ratio. The process is considered to be ideally mixed, i.e. it is assumed that the components can be well mixed and that the mixing time is relatively short compared to the residence time. Hence, the level of detail which will be observed is in the order of the magnitude of the residence time. 

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In a typical multiple contaminant problem the maximum amount of water that can be used, while still obeying any concentration constraints, is determined by a limiting component and there is generally contaminant mass added for each contaminant present. In this problem, contaminant mass is only added to the water for one contaminant, namely the residue left from the specific product in a mixer. This then makes the limiting component in each mixer the component that leaves residue in the mixer. For example mixer 1 has shampoo as the limiting contaminant, since this is the only component which leaves a residue in the mixer. The maximum amount of water for each mixer is given for each mixer in Table 6.10. 

The main point here is that in this particular example, mixer horsepower and capital cost can effect tremendous changes in productivity because of their low cost in terms of the total cost. 

Figure C3.1.1. The basic elements of a time-resolved spectral measurement. A pump source perturbs tlie sample and initiates changes to be studied. Lasers, capacitive-discharge Joule heaters and rapid reagent mixers are some examples of pump sources. The probe and detector monitor spectroscopic changes associated with absorjDtion, fluorescence, Raman scattering or any otlier spectral approach tliat can distinguish the initial, intennediate and final...

Convective heat transfer is classified as forced convection and natural (or free) convection. The former results from the forced flow of fluid caused by an external means such as a pump, fan, blower, agitator, mixer, etc. In the natural convection, flow is caused by density difference resulting from a temperature gradient within the fluid. An example of the principle of natural convection is illustrated by a heated vertical plate in quiescent air. 

When choosing the scale-up method, changes in other flow/power parameters and their impact on the process result must be considered. Figure 11 shows changes in important parameters for different scale-up bases. For example, scale-up based on same tip speed maintains the T / Ubut decreases P/ Uby 80%. T / Uis almost always increased on scale-up. Scale-up based on the same P/ Umeans a reduction in mixer speed by 66%, which also... 

Fig. 36. Examples of specially designed mixers (a) draft tube circulator, (b) airlift with draft tube, (c) Fluidics vortex mixer, and (d) mixer emulsifier.

Correlations of nucleation rates with crystallizer variables have been developed for a variety of systems. Although the correlations are empirical, a mechanistic hypothesis regarding nucleation can be helpful in selecting operating variables for inclusion in the model. Two examples are (/) the effect of slurry circulation rate on nucleation has been used to develop a correlation for nucleation rate based on the tip speed of the impeller (16) and (2) the scaleup of nucleation kinetics for sodium chloride crystalliza tion provided an analysis of the role of mixing and mixer characteristics in contact nucleation (17). Pubhshed kinetic correlations have been reviewed through about 1979 (18). In a later section on population balances, simple power-law expressions are used to correlate nucleation rate data and describe the effect of nucleation on crystal size distribution. 

High-Speed Mixers High-speed mixers include continuous-shaft mixers and batch high-speed mixers. Continuous-shaft mixers have blades or pins rotating at high speed on a central shaft. Both horizontal and vertical shaft designs are available. Examples include the vertical Schugi mixer (Fig. 20-84) and the horizontal pin or peg... 

For non-New tonian fluids, viscosity data are very important. Every impeller has an average fluid shear rate related to speed. For example, foi a flat blade turbine impeller, the average impeller zone fluid shear rate is 11 times the operating speed. The most exact method to obtain the viscosity is by using a standard mixing tank and impeller as a viscosimeter. By measuring the pow er response on a small scale mixer, the viscosity at shear rates similar to that in the full scale unit is obtained. 

An example of liquid/liquid mixing is emulsion polymerization, where droplet size can be the most important parameter influencing product quality. Particle size is determined by impeller tip speed. If coalescence is prevented and the system stability is satisfactory, this will determine the ultimate particle size. However, if the dispersion being produced in the mixer is used as an intermediate step to carry out a liquid/liquid extraction and the emulsion must be settled out again, a dynamic dispersion is produced. Maximum shear stress by the impeller then determines the average shear rate and the overall average particle size in the mixer. 

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