Solid Layer Processes

Limitations are to be found mainly in four points. First, the limited surface area of the cooled surface and thereafter the surface area of the solid layer, since the surface area is a limiting factor for mass transfer processes. This is a weak point in solid layer processes compared to suspension processes. 

Second, solid layer formation by the product on the heat transfer surface requires either an increase in temperature driving force to maintain the same growth rate (therefore production rate) or leads to a reduction in capacity with increasing thickness. 

it is a limiting point that the product sticks like an encrustation on the cooled surfaces of the apparatus and has to be remolten in order to be discharged. This requires not only additional energy for melting the crystal coat but also a partial heating up of the whole apparatus. On the other hand, this remelting makes it necessary to have batch processes. 

Finally, in some cases it can also be a disadvantage for the product to leave the apparatus in liquid form and to be solidified again. The last point as well as the third, could well be avoided someday if it becomes possible to build continuously operating processes in one plant for solid layer melt crystallization processes. 

Mass transfer processes are governed by the driving force difference in the chemical potentials, the physical proportions (mass transfer coefficient) of the substance, and the surface area (the interface between the phases to be separated). This is known from the basic transport equations of heat and mass transfer. A large surface area, therefore, favors separation processes. A suspension with a distribution of mainly small particles would feature a high interface area. There is, however, a limitation to the size of the disperse solid phase. This is due to the necessary liquid-solid separation at the end of the process, on the one hand, and the necessity of the disperse phase to move in different directions as the main flow direction of the continuous phase so that a maximum of the driving potential between the two phases can be maintained, on the other hand. 

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Crystal growth rates within crystallization processes from solutions in most cases are in the range 10 -10 m/s. Growth rates in melt crystallization are quite often in the range of about 10 m/s and in extreme cases in some solid layer processes as high as 10 m/s. 

Figure 7.6 is a flow diagram showing all liquid streams and all operational steps in a solid-layer process. 

In the group of suspension processes, there are the jacket cooled and the directly (e.g., by inert gases) cooled processes. The Amoco process, on the one hand, and the Maruzen or Chevron process, on the other hand, are, according to Ransley (1984), representatives for such processes. Other developments are scrapes crystallizers according to the Humble-Oil, the Krupp-Harpen (see Ritzer 1973), or the Hoechst AG (DE-PS 1969) process which are used for the production of p-nitrochlorobenzene. These scraped surface crystallizers are in fact solid layer processes, but in the handling of the products—crystals in suspension—they have to be treated like suspension processes. A number of processes that feature a combination of a scraped crystallizer and suspension techniques will be discussed. 

Figure 7.21 Bremband-process, continuous solid layer process.

Suspension melt crystallization crystals and melt same temperature, design on degree of supersaturation, separation of crystals from melt depends on density difference in countercurrent operation. Scraped surface crystallizer. Section 4.6. The suspension methods have slower rates of crystal growth compared with the solid layer processes. 

Figure 15.3 Principle flow diagram of solid layer processes (according to Ref. [4]).

Suspension crystallization is capable of producing very pure crystals mostly in a continuous operating mode, which is an advantage compared to the most batch solid layer crystallization processes. Another positive feature compared to solid layer crystallization is the better purification per process step and hence a less number of process steps usually with respect to crystallization. Therefore, suspension crystallization plants need in principle less energy compared to solid layer processes. Whether the investment costs of such plants are smaller as well depends on the complexity of the moving parts in suspension plant concepts compared to solid layer concepts (no moving parts, except pumps). 

Granulation of urea [13] is a complex process that has to be controlled by experienced process operators in order to avoid critical shutdown situations. The parameters most often used for monitoring granulation processes are measured by classical univariate sensors, such as temperature, pressure and flow. However, these standard process measurements carry only little or no relevant information, or are only indirectly related to, for example particle size, clogging of the reactor, or the accumulation of a solids layer on the bottom plate. The response from these sensors often comes with quite a substantial delay time. 

Vife have extensively studied (10) the photoinduced catalytic system (Figure 2) which is based upon the photochemical generation of cyanide ions from various cyanometa I lates, especially from octa-cyanomolybdate(IV) aod octacyanotungstate(IV) ions. Upon photolysis both compounds form cyanide ions with relatively high quantum yields. Cyanide Sons may act as a catalyst for the dimerizat ion of appropriate heterocyclic carb-2-aldehydes to enediols. Since this photocatalytic system has proved to operate also in solid layers, it may be used for an unconventional photographic process (11,12). 

Figure 10.15. Evolution process of wavelike solids flow structure inferred from the solids concentration measurements and visual observations (from Jiang et al., 1993) (a) Particle interaction yielding a thin, dense, wavy solids layer in the wall region (b) Solids accumulation and descent of enlarged solids layers (c) Shaping as a bluff solids layer (d) Solids layer bursting (e) Solids sweeping to the core region.

Let us begin an analysis of the process of formation of chemical compounds in heterogeneous systems with the simplest case of growth of a solid layer between elementary substances A and B which form, according to the equilibrium phase diagram of the A-B binary system, only one chemical compound ApBq, p and q being positive numbers (Fig. 1.1). The substances A and B are considered to be solid at reaction temperature 7, and mutually insoluble. 

The idea about the summation of the times of consecutive steps of the examined solid-state process is of primary importance for understanding the peculiarities of multiphase growth of compound layers in binary heterogeneous systems. Moreover, even in the case of formation of a single compound layer, this idea makes it possible to reveal a few aspects of reaction... 

The economy of melt crystallization processes depends on the product purity, which is normally increased by an additional cleaning step. The application of gases under pressure is investigated to show possibilities of product quality improvement. Experimental devices for the determination of the freezing curve under gas pressure and for a solid layer crystallization process are shown. The influence of gas and pressure in respect to the freezing curve are explained on the basis of two binary mixtures (trioxane/water and para-/meta-dichlorobenzene) under CO2- and N2- pressure are presented. Furthermore the results of solid layer crystallization experiments with naphthalene/biphenyl and para-/meta-dichlorobenzene mixtures are shown. 

Modern technological developments and many fields of pure and applied research depend on the quantitative information about the spatial element distribution in thin solid layers and thin-film systems. For example, without the use of thin films the experimental studies on the physics of semiconductor are very difficult. Similarly the diffusion processes in solids, sandwich-like thin films structures in microelectronics, anti-reflecting or selectively transparent optical films, catalysts, coatings, composites - all rely on material properties on an atomic scale. The development of these new materials as well as the understanding of the basic physical and chemical properties that determine their specific characters are not possible without the knowledge of their compositional structure, in particular in the interface regions. 

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