Melt crystallization devices

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). 

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. 

The Sulzer MWB process (Fischer, Jancic and Saxer, 1984) is a melt crystallizer that operates basically by crystallization on a cold surface, but with features which allow it to operate effectively as a multistage separation device. Consequently, it can be used to purify solid solution as well as eutectic systems. 

Samples were melt pressed in a vacuum laboratory hot press (Carver Press, Model C) at 160°C for 30 min. The molded films were then allowed to cool to room temperature under vacuum. A dual temperature chamber for the melt crystallization experiments consists of two large thermal chambers maintained at the melt temperature (Ti = 160°C) and the crystallization temperature (Ts = 81°C, 83°C, 86°C, 89°C, 92°C or 96°C). After 5-10 min at Ti, the copper sample cell was transferred rapidly ( 2 s) to the other chamber by means of a metal rod connected to a pneumatic device. A detailed description of the arrangement of the sample and of the two detectors used to measure WAXS and SAXS simultaneously has been provided previously [32]. Each polymer sample within the copper cell was 1.5 mm thick and 7 mm in diameter and was contained between two 25 im thick Kapton films. The actual sample temperature during crystallization (T2) and melting (Ti) was monitored by means of a thermocouple inserted into the sample cell. The crystallization temperature was usually reached 120 s after transfer without overshooting. Under isothermal conditions the fluctuations in the sample temperature are less than 0.5°C. Unless stated otherwise, all references to time are times elapsed after transferring the sample to the crystaUization chamber. 

System type (3) Two different immiscible phases flow countercurrendy in the device/column different immiscible phases are generated from one feed input stream into the column by the application/withdrawal of thermal energy. Examples include distillation (vapor-liquid system), melt crystallization (solid-liquid system). 

In the following part of this section, we provide simple mathematical descriptions of a few common features of two-phase/two-region countercurrent devices, specifically some general considerations on equations of change, operating lines and multicomponent separation capability. Sections 8.1.2, 8.1.3, 8.1.4, 8.1.5 and 8.1.6 cover two-phase systems of gas-Uquid absorption, distillation, solvent extraction, melt crystallization and adsorption/SMB. Sections 8.1.7, 8.1.8 and 8.1.9 consider the countercurrent membrane processes of dialysis (and electrodialysis), liquid membrane separation and gas permeation. Tbe subsequent sections cover very briefly the processes in gas centrifuge and thermal diffusion. 

For solids which melt above 100° and are stable at this temperature, drying may be carried out in a steam oven. The crystals from the Buchner funnel should then be placed on a clock glass or in an open dish. The substance may sometimes be dried in the Buchner funnel itself by utilising the device illustrated in Fig. 77, <33, 1. An ordinary Pyrex funnel is inverted over the Buchner funnel and the neck of the funnel heated by means of a broad flame (alternatively, the funnel may be heated by a closely-fltting electric heating mantle) if gentle suction is applied to the Alter flask, hot (or warm) air will pass over the crystalline solid. 

Silicon is prepared commercially by heating silica and carbon in an electric furnace, using carbon electrodes. Several other methods can be used for preparing the element. Amorphous silicon can be prepared as a brown powder, which can be easily melted or vaporized. The Gzochralski process is commonly used to produce single crystals of silicon used for solid-state or semiconductor devices. Hyperpure silicon can be prepared by the thermal decomposition of ultra-pure trichlorosilane in a hydrogen atmosphere, and by a vacuum float zone process. 

Where substances are sufficiently stable, removal of solvent from recrystallised materials presents no problems. The crystals, after filtering at the pump (and perhaps air-drying by suction), are heated in an oven above the boiling point of the solvent (but below this melting point of the crystals), followed by cooling in a desiccator. Where this treatment is inadvisable, it is still often possible to heat to a lower temperature under reduced pressure, for example in an Abderhalden pistol. This device consists of a small chamber which is heated externally by the vapour of a boiling solvent. Inside this chamber, which can be evacuated by a water pump or some other vacuum pump, is... 

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