Poor mixing

Many similar incidents have occurred when a stirrer or circulating pump stopped. For example, an acidic waste stream in a tank was neutralized with chalk slurry. The operator realized that the liquid going to drain was too acidic. Looking around, he then found that the stirrer had stopped. He switched it on again. The acid and chalk, which had formed two separate layers, reacted violently, and the gas produced blew the bolted lid off the tank. 

No accident has a simple cause. In this case a contributory factor was the inexperience of the operator, who had to ask someone to show him how the quench system worked. It had not been used for several years. Emergency equipment is usually used infrequently, and without regular training, people forget (or never learn) how to use it [6]. 

At the time of the explosion, the wooden stagings were being replaced by iron ones, but the work was going slowly, as the materials needed were rationed and the Munitions Works Board had classified the change as desirable but not absolutely necessary. 

The reactor was also used for other processes. The cooler was not adequate for one of them, so an additional coil was fitted. To make room for it, the stiirer was removed and replaced by a turbine agitator, located higher up the vessel. At a later date, when the process that needed the extra cooling was no longer used, the extra coil, which had corroded, was removed. This lowered the level in the reactor. 

This incident and the previous one show how easy it is to reduce mixing inadvertently and how serious the results can be. 

If the batch of material is incorrectly mixed, there will be zones of material that will have either too much or too little curative. This will result in a number of potential problems, such as  

Very poor mixing will be evident by areas of uncured prepolymer appearing as wet sticky zones. Mixing containers should be cured off and checked for any wetness on the edges between the container and the cured polyurethane. 

The mixing technique should be such that all the surfaces of the vessel and mixing utensils are well scraped off and that the curative is well mixed in. For smaller batches, a spatula-type instrument is normally used. Poor mixing is evident by the formation of striations in the mix. These are lines of different color or intensity that are evident in both the unpigmented and pigmented mixtures. The hand mixing should be such that the color or clarity of the mix is completely even. For large batches, an electric mixer may be used, but in this case care must be taken that air is not sucked down and whipped into the mix. 

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Hypolimnion Poorly mixed 8 1 o v 10 aiioxie Decomposers (cold-water species when... 

Fig. 5. Typical thermal stratification of a lake, reservoir, or poorly mixed estuary in summer which, because of density differences, estabUshes discrete 2ones...

Pressure and residence time have relatively Htde effect on reaction selectivity, at least within the ranges normally encountered. Poor mixing and excessive residence time result in increased carbonization of the reactor. 

Control of the polymorphic forms in cocoa butter is further compHcated by the presence of other fats such as milk fat. The fat in a chocolate can be likened to the mortar between the bricks in a mason s wall. The soHd particles in a weU-conched chocolate bed down better than the soHds in a coarsely refined and poorly mixed one (30). 

Heterogeneous Mixtures (Zabetakis, Flammability Characteristics of Combustible Gases and Vapors, Bulletin 627, Bureau of Mines, 1965.) Heterogeneous (poorly mixed) gas phase mixtures can lead to fires that normally would be totally unexpec ted. It is important... 

Could poor mixing or inefficient distribution of reactants and heat sources result in undesirable side reactions, hot spots, runaway reactions, fouling, etc ... 

ThemodelweconstructedinFigureScanbethoughtofasametaphorforoneunitwith incomplete mixing. Rather imagine that in this poorly mixed unit some of the fluid goes... 

At least three factors must be considered when using Eq. (10.132). The first is that the room air is seldom well mixed the second is that the emission rate is usually unknown and the third is that measurement of is often very difficult, especially in spaces where a mixture of mechanical and natural ventilation is used. If the room air is poorly mixed, a sampling strategy is needed in order to determine the room air concentration (C,). There exist different strategies for this, and two are as follows ... 

Through ventilation Ventilation that takes place through a space due to wind forces, normally resulting in poor mixing of the room air due to short circuiting. 

Models of seeondary kinetie proeesses were eonsidered in detail Chapters 5 and 6 whilst aeeounting for poor mixing effeets by use of CFD-based methods were deseribed and illustrated in Chapters 2 and 8. Sueh CFD eodes ean be linked via eompartmental and mixing models, or eoupled direetly with the population balanee if the preeipitation proeesses are partieularly fast in eom-parison to the mixing proeess. 

Figure 19.2 illustrates a burner at a fixed firing rate. In practice, many burners will have a varying firing rate with inferior performance at turndown, mainly because of poor mixing caused by reduced kinetic energy of both air and gas. To allow for this, the control system must provide for an increasing excess air with turndown. 

Hie problem associated with poor mixing in a large vessel was identified as low dissolved oxygen in the aerated vessel. The mixing time has been correlated with turbulent flow. In... 

FIGURE 20.15 Phase images of samples of triblock copol3mier styrene and ethylene-butylene (SEES) filled with clay particles (5 wt%) with (a) poor mixing and exfoliation and (b) fine distribution of clay layers. 

Good Mixing cnly fcr Bateh Feeding Poor Mixing fcr Ccntinuous Feeding... 

Results of calculations for Sc = 1000 are given in Table 5.4-23. The total stream of B added was divided into 20 portions (N = 20). Each CPU time was less than 5 s at an integration step size of lO ". The formation of the bisazo product is promoted by poor mixing (low values of E, and, consequently, high values of Da) and high volume ratio of reagent solutions Va/Vb-... 

Fig. 36. The spatial variation of agglomerate sizes in simulations of erosion in the journal bearing flow. Initially there are 10000 agglomerates consisting of 400-500 aggregates. The grey scale represents the number of aggregates in the agglomerates, (a) Poorly mixed flow after four periods, (b) Well-mixed flow after one period (Hansen, el al. 1998).

As previously discussed, we expect the scaling to hold if the polydisper-sity, P, remains constant with respect to time. For the well-mixed system the polydispersity reaches about 2 when the average cluster size is approximately 10 particles, and statistically fluctuates about 2 until the mean field approximation and the scaling break down, when the number of clusters remaining in the system is about 100 or so. The polydispersity of the size distribution in the poorly mixed system never reaches a steady value. The ratio which is constant if the scaling holds and mass is conserved,... 

Note that only one system, the one corresponding to constant capture radius clusters in chaotic flows, behaves as expected via mean field predictions. In general, the average cluster size grows fastest in the well-mixed system. However, in some cases the average cluster size in the regular flow grows faster than in the poorly mixed system. 

The fractal nature of the structures is also of interest. Because of the wide range of flow in the journal bearing, a distribution of fractal clusters is produced. When the area fraction of clusters is 0.02, the median fractal dimension of the clusters is dependent on the flow, similar to the study by Danielson et al. (1991). The median fractal dimension of clusters formed in the well-mixed system is 1.47, whereas the median fractal dimension of clusters formed in the poorly mixed case is 1.55. Furthermore, the range of fractal dimensions is higher in the well-mixed case. 

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