Mixing process ratio control

The mixing processes are controlled by the thermodynamic and rheological properties of components [18], as well as by the composition and the process variables (temperature, pressure, stress field, residence time, etc.). The HDPE/LDPE viscosity ratios at different temperatures and different shear stress level are shown in Table 3 and Figure 10. 

Flow ratio control is essential in processes such as fuel-air mixing, blending, and reactor feed systems. In a two-stream process, for example, each stream will have its own controller, but the signal from the primary controller will go to a ratio control device which adjusts the set point of the other controller. Figure 3.17(a) is an example. Construction of the ratioing device may be an adjustable mechanical linkage or may be entirely pneumatic or electronic. In other two-stream operations, the flow rate of the secondary stream may be controlled by some property of the combined stream, temperature in the case of fuel-air systems or composition or some physical property indicative of the proportions of the two streams. 

Figure 2.50 shows a combination of cascade and ratio controls of blending fluids A and B and sending the mix into a blend tank. In this process, the... 

Table III gives the change in various mixing parameter ratios for 4 different scaleup calculations. What this shows is that we cannot control the ratio of every individual fluid mechanics parameter. It also indicates that it is better to use correlating parameters to find out how they change on scaleup, rather than searching for a constant scaleup parameter for every conceivable mixing process. There is no assurance that there is a constant scaleup parameter for any given process job we are studying. Therefore, pilot plant studies should be directed toward establishing a controlling factor in the process, so that its control on scaleup can be more closely monitored.

Figure 15.52 shows the application of ratio control to the effluent pH for a wastewater neutralization process applied in a mixing tank. This controller can effectively handle wastewater feed flow rate changes when the chemical makeup of the wastewater remains relatively constant. Small changes in the chemical makeup of the wastewater can usually be handled by the feedback controller, which adjusts the reagent-to-wastewater ratio to maintain the specified effluent pH. 

Flow controllers set the rates of both streams, one being under flow-ratio control. In principle, either caustic soda or dilution water can be the master stream, with the other following it to maintain the ratio. Blending is controlled by a feedforward system, ultimately reset by the product concentration or density. Feedback from caustic concentration measurement (usually by density) could be used for final adjustment, but the concentration of the hypochlorite solution is the more important variable. The simple flow-ratio controller mentioned here can be replaced by a multi-stream version that allows use of other streams in addition to the principal 50% NaOH and dilution water. A cooler downstream of the mixing point removes the heat of dilution. The standard design is a titanium plate exchanger, which can also provide turbulence to complete the mixing process. Chlorine joins the diluted caustic in the reactor. Its rate of addition is controlled by an oxidation-reduction potential (ORP) instrument. The reaction mass recirculates from a collection tank around the system to reduce the increase of temperature across the reactor and to promote turbulence. The net production is removed from the tank, normally under level control. 

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