Segregation, scale

Mixing between mutual-soluble liquids is usually carried out by the action of certain external forces, e.g. agitation. Mixing occurs at various scales and the segregation scale varies with time. An idealized model of the mixing between a black liquid and a white liquid is illustrated in Fig. It). I. 

The last three steps control the small-scale or micromixing in the reactor. The characteristic time constant for the third step, that is for reduction in segregation scale (inertial-convective mixing), is (Corrsin, 1964 Baldyga, 1989)... 

Finally, if the sampling volume is larger than the smallest segregation scale, then it is difficult to determine whether the two fluids are mixed or not within the measurement resolution. 

It can be seen that, provided that the segregation scale is large (this notion will be clarified further), stretching is the controlling process for mixing and mechanical dissipation is an interesting tool to decrease the mixing time. 

Diffusion and convection are then competitive processes, but according to the shear rate value, for large segregation scales, the diffusion process is slow compared with convection and mixing is almost controlled by stretching. At fine segregation scales, diffusion becomes the controlling step. 

As explained, the main drawback of passive tracer (physical) methods arises if the sampling volume is larger than the smallest segregation scales. Under these circumstances, it is impossible to determine whether the two fluids are mixed or not within the measurement resolution. Several authors [39] have pointed out that the problem of finite sampling volume can be solved by using a fast and irreversible chemical reaction of the type A + B P. If dilute reactant is added to one stream and B to the other, then the amount of chemical product formed is equal to the amount of molecular scale mixing between the two streams at the reaction stoichiometric ratio. This is the reason why chemical methods have been developed. 

Since the diffusion coefficient D is unknown exactly, the discussion is limited to the qualitative interpretation of the experimental results. The D of PPO is supposed to be considerably small because PPO has much higher Tg than SPS. Considering that the growth rate of SPS is fairly fast, the S will be a small value, and hence the segregation of PPO occurs between the lamellae. Also, with increasing PPO content, the diffusivity of a PPO molecule decreases, so that 5 becomes smaller. In this case, the lamellar thickness is supposed to be controlled by the segregation scale, that is, the smaller the scale, in other words, the slower the diffusion of PPO, the smaller the lamellar thickness <4>. 

Table 18.3 shows the structural parameters of SPS/PPO blends as a function of the intrinsic viscosity [iiJppo. The long period L is independent of [t ]ppo, while the thickness of the crystalline phase <4> tends to decrease with increasing [tiJppo. This phenomenon can be explained by the segregation scale 8 of PPO molecules given by Equation 18.3. The higher the [t)]ppo, the smaller the diffusivity of PPO, which causes a smaller 8 and hence a smaller lamellar thickness. 

If the chain ends are segregated on the surface at the start of the welding, then the same general scaling law applies but with different values of r and s for some of the number properties. The average properties remain unaffected [1]. 

Continuous releases of concentrated HjS streams must be segregated in a separate flare system to limit the extent of fouling and plugging problems. Releases of HjS such as diversion of sour gas product to flares during shutdown or upset of a downstream sulfur recovery unit are considered to be continuous, but safety valve releases are not included in this category. However, if a special HjS flare system is provided for continuous releases, the concentrated HjS safety valve releases should be tied into it rather than into the regular flare system. Due to the nature of HjS one should plan on frequent inspection and flushing of HjS flares to remove scale and corrosion products. 

Scale of segregation A measure of the average distance between clumps of the same component in a mixture. 

Wang, Y.-D. and Mann, R., 1992. Partial segregation in stirred batch reactors effect of scale-up on the yield of a pair of competing reactions. Transactions of the Institution of Chemical Engineers, 70, 282-290. 

An important parameter characterizing the laminar regime is the intersegment spacing over which the species must diffuse to be able to react. For kinetic control, as opposed to diffusion control of the reaction, this spacing must not exceed a certain scale of segregation given by [53J ... 

Like sulphur, phosphorus appears to have little effect on the overall scaling of iron alloys in air. It may, however, play a role in suppressing breakaway oxidation in carbon steels in CO/CO2 environments. Donati and Garaud " found that the tendency for breakaway was lower over ferrite, where P segregates. To confirm this, the authors doped pure Fe with P and found that... 

---