Melt crystallization continuous plants

Back mixing, slurry transport, moving parts in the equipment, and scaling problems are the biggest difficulties in continuously running plants. Therefore, a truly ideal continuous countercurrent melt crystallizer that is flexible enough to handle a variety of substances with high efficiency is not, unfortunately, found to date. Due to problems, the semibatch-type of processes dominates in most of the recently built plants. 

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. 

The basics of melt crystallization are provided in Chapter 15. Here, the concepts of plants and/or existing and commercially available equipment are shown. As mentioned in Chapter 15, the concepts of plants can be divided into two lines of technology solid layer crystallization and suspension crystallization. Furthermore, in industrial applications these two techniques are split into continuous and batchwise as well as into static and dynamic (stagnant or flowing melt) operating modes. A detailed overview of the different techniques and apparatuses in solid layer as well as suspension crystallization is provided in the Sections 17.1.1 and 17.1.2. 

The number of apphcations of melt crystallization (solid layer as well as suspension) is continuously growing. This development is supported by the fact that in the future a pardigm shift is arising in terms of the design of chemical, pharmaceutical, and food processes, which means away from single plants toward to the so-called hybrid processes. Hybrid processes mean a combination of several separation techniques, for example, distillation and crystallization, in order to enhance the throughput, the heat and mass transfer, and the reaction rates. Hybrid processes are a subset of the so-called process intensification techniques. Process intensification paves the way... 

For an earlier introduction to column crystallization vis-a-vis the variety of equipment and patents, see Albertins et al. (1967). Ulrich (1993) provides a more modern version of melt crystallization devices, plants and processes. For mathematical modeling and optimization of multistage crystallization processes, see Gilbert (1991). A more Involved model of continuous countercurrent contacting with axial dispersion in melt crystallization has been illustrated in Hemy and Moyers (1984). 

Naphthalenol. 2-Naphthol or p-naphthol or 2-hydroxynaphthalene/7i3 -/5 -i7 melts at 122°C and boils at 295°C, and forms colorless crystals of characteristic, phenoHc odor which darken on exposure to air or light. 2-Naphthol [135-19-3] is manufactured by fusion of sodium 2-naphthalenesulfonate with sodium hydroxide at ca 325°C, acidification of the drowned fusion mass which is quenched ia water, isolation and water-washing of the 2-naphthalenol, and vacuum distillation and flaking of the product. A continuous process of this type has been patented (69). The high sulfate content ia the primary effluent from 2-naphthol production is greatiy reduced ia modem production plants by the recovery of sodium sulfate. 

Were any of the early pioneers in fat fractionation able to visit a modern dry fractionation plant they would still recognize the basic process. Melted fat is crystallized batchwise in large tanks, and then the crystals are filtered or separated from the remaining liquid oil in some way. It has always been assumed that batch processes are inefficient in their use of resources compared with continuous processes. As will be seen in the next sections, improvements have been sought in two main areas first, the final elimination of entrainment, and second the key to continuous fractionation from the melt. A selection of the most recent attempts to achieve these aims follows. 

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