Mixing flow equipment

So far we have treated two flow patterns, plug flow and mixed flow. These can give very different behavior (size of reactor, distribution of products). We like these flow patterns and in most cases we try to design equipment to approach one or the other because... 

First and foremost, the laminar mixing flow created in the reactive processing equipment, must reduce the striation thickness to a level where the diffusion characteristic time, tD = r2/ )AB, is small compared to the reaction characteristic time. Since the molecular diffusivities of low molecular weight components in polymeric melts (see Section 8.3) are very small and of the order of 10 6 cm2/s, the striation thickness must be reduced to the micron level in order to get a characteristic time t of the order of 1 s. Shear flow can accomplish this in reasonable mixing times because the striation thickness is inversely proportional to the total shear (see Section 7.3)... 

There have been several attempts to use the transition response method for the evaluation of the mixing performance of operations/equipment. In the transition response method for the flow system, a tracer is injected into the inlet and the change in its concentration at the outlet with time is measured. On the other hand, in the case of the batch system, the tracer is injected into some specific position, and the change in the spatial distribution of the concentration in the equipment with time is measured. In the following discussion, a method based on information entropy to evaluate the mixing operations/equipment on the basis of the transient response method is discussed. 

The correspondence of the conditions for the maximum and minimum values of H( t) with the practical mixing phenomena is considered. The condition under which H(t) takes the maximum value is realized when the perfect mixing flow is established in the equipment, that is, when the concentration of tracer in the equipment is perfectly homogeneous every time and the concentration of tracer at the outlet decreases exponentially. On the other hand, the condition under which H t) takes the minimum value is realized when a piston flow is established in the equipment, that is, when the tracer does not disperse in the... 

The above-described mixers are essentially low-viscosity devices. In many operations where the viscosity is high, when dealing with concentrated multiphase gas-liquid-solid binary or tertiary systems, or when liquid-to-solid phase transformation occurs during mixing, novel equipment designs are needed to intensify the heat/mass transfer processes. The multiphase fluids also represent an important class of materials that have microstructure developed during processing and subsequently frozen-in, ready for use as a product. To deliver certain desired functions, the control of microstructure in the product is important. This microstructure is developed in most cases by the interaction between the fluid flow and the fluid microstructure hence, uniformity of the flow field is important. 

In order physically to fulfill the one-mixing-stage condition, the early FIA titration systems did in fact incorporate a small mixing chamber (equipped with a magnetic stirrer) the volume of which (ca. 1 mL) dominated the flow design. This approach is illustrated in Fig. 4.56, which... 

In Chapter 2, the design of the so-called ideal reactors was discussed. The reactor ideahty was based on defined hydrodynamic behavior. We had assumedtwo flow patterns plug flow (piston type) where axial dispersion is excluded and completely mixed flow achieved in ideal stirred tank reactors. These flow patterns are often used for reactor design because the mass and heat balances are relatively simple to treat. But real equipment often deviates from that of the ideal flow pattern. In tubular reactors radial velocity and concentration profiles may develop in laminar flow. In turbulent flow, velocity fluctuations can lead to an axial dispersion. In catalytic packed bed reactors, irregular flow with the formation of channels may occur while stagnant fluid zones (dead zones) may develop in other parts of the reactor. Incompletely mixed zones and thus inhomogeneity can also be observed in CSTR, especially in the cases of viscous media. 

So far, we have assumed that the oxygen concentration in the cathode channel is constant. In this section, we will relax this assumption. Suppose that the cell is equipped with the single straight air channel let the axis z be directed from the channel inlet to the outlet (Figure 23.9). We will assume that the flow in the channel is a plug flow, that is, a well-mixed flow with a constant velocity v. The validity of this approximation is discussed in [3]. The oxygen mass conservation equation in the channel then reads... 

The Mixed Flowing Gas Test, ASTM B 827, is used extensively in the electronics industry. As the name implies, the test method uses a combination of various gases at controlled temperatures and humidities to produce the desired conditions. The MFG test requires equipment of a more complex nature than other corrosive gas tests. A schematic of a typical chamber is shown in Fig. 3b, as depicted in ASTM B 827. 

Oxidation Step. A review of mechanistic studies of partial oxidation of propylene has appeared (58). The oxidation process flow sheet (Fig. 2) shows equipment and typical operating conditions. The reactors are of the fixed-bed shell-and-tube type (about 3—5 mlong and 2.5 cm in diameter) with a molten salt coolant on the shell side. The tubes are packed with catalyst, a small amount of inert material at the top serving as a preheater section for the feed gases. Vaporized propylene is mixed with steam and ak and fed to the first-stage reactor. The feed composition is typically 5—7% propylene, 10—30%... 

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