Types of Crystallisers

A further important factor is the crystal retention time t, which is related to the hnear crystal growth rate G (Eq. (9.4)) and the mean crystal size L using the population balance theory (Eq. (9.5)). 

Longer retention times are necessary if larger crystals are desired. But in practice also the opposite can be observed. The reason is the mechanical attrition rate G which works against the kinetic growth rate The effective linear crystal growth 

Because attrition is intensified with increasing crystal size there always will be a critical size, where the effective crystal growth rate becomes zero (Eq. (9.8)). Therefore, under certain conditions a maximum achievable crystal size exists and longer retention times may lead to smaller crystal sizes. 

with rising temperature the conciitions for crystalliation get improved. Besides the specific energy input to a crystalliser system also the energy level takes influence to the nucleation rate. The specific energy input e by impeller pumps can be defined as 

p denotes the specific density of the suspension or solution g, the acceleration due to gravity H, the delivery head r, the efficiency and the filled 

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Crystallisation equipment can be classified by the method used to obtain supersaturation of the liquor, and also by the method used to suspend the growing crystals. Supersaturation is obtained by cooling or evaporation. There are four basic types of crystalliser these are described briefly below. 

Typical applications of the main types of crystalliser are summarised in Table 10.10 (see page 440) see also Larson (1978). 

Most of the early studies concerned with molecular fractionation dealt with samples having a broad molar mass distribution. The crystallisation of binary mixtures of sharp fractions was studied to a lesser degree. The crystallisation of binary mixtures of linear polyethylene sharp fractions in the molar mass range from 1000 to 20,000 g mol1 depended upon the cooling rate, and two types of crystallisation were observed [155] ... 

Later work on binary linear polyethylene blends reported by Rego Lopez and Gedde [115], Rego Lopez et al. [156] and Conde Brana et al. [157] provided a somewhat different view. The blends studied were based on the combination of a low molar mass linear polyethylene (Mw=2500 g mol-1 MwIMn= 1.1) with one of a series of higher molar mass linear polyethylenes (11,000Different types of crystallisation were observed in the binary linear polyethylene blends [156] ... 

It is clear that the extension must be maintained until the melt has solidified by crystallisation. For spherulitic crystallisation this would require residence times of 0.5-50 s (0.5 s for PE and 50 s for PETP), which excludes normal spherulitic crystallisation for most polymers, since only fractions of seconds are available in practice. It is therefore fortunate that another type of crystallisation, that of the microfibrillar crystallite, is the dominant mode. This is due to the fact that under tension threadlike nucleation is favoured. 

In principle, all ribbon processes have the characteristics that almost all silicon supplied into the process is converted into wafers. There is also no wafer cutting from a block, although in dependence upon the production method separation of larger sheets into wafers may be needed. Depending upon the type of crystallisation, silicon ribbon growth processes are either relatively slow processes, which can be run on rather low-cost equipment or high-speed processes operated with rather complex machinery. 

This type of crystallisation was first suggested by Storks in 1938. He made films of gutta percha 27 nm thick by evaporation from solution. Electron diffraction showed that the films were composed of large crystallites with the chain axes normal to the plane of the film. The only possibility was that the chains folded back and forth upon themselves, so that adjacent segments were parallel and in crystal register. 

Crystallisers designed according to these rules will certainly come to function. Of course, these principles can be found in the well-known basic types of crystallisers (Figure 9.5a-c). 

Types of crystallisers, therefore, differ from each other mainly in design and position (whether operated in clear liquor or in suspension) of the impeller pump (compare Figure 9.5). It can be summarized as follows ... 

Instead of the FC-type crystalliser any other type of crystalliser can be installed. Also multiple-effect units can be taken. Or, instead of using steam for heating it could be decided for mechanical or thermal vapour re-compression. In this example the vapours from the (last) crystalliser are condensed in a... 

However, providing the materials that can be made reproducibly, these problems do not prevent the application of the principles of polymer physics to the analysis of crystallisation in these systems. It is interesting to note that this type of crystallisation, observable on the DSC, will also occur in commercial products, such as confectionery chew-like materials with a porous aerated structure. With this industrial aspect in mind, it is relevant to develop methods to measiue crystallisation rates. Methods for isothermal and non-isothermal analysis exist to do this. These methods are considerably easier in fats where there are no difficult-to-control variables, such as the level of plasticiser/water. Nevertheless this has been attempted in sucrose using the approach of Chan et al. [31, 32]. Rates were obtained for experiments on isothermal crystallisation of amorphous sucrose [33] and crystallisation exotherms were measured from DSC curves scanned at different rates. Shift factors were then calculated for both isothermal and non-isothermal measurements and are plotted in Figure 9.9. 

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