Continuous evaporative crystallization

In the United States boric acid is produced by United States Borax Chemical Corp. in a 103,000 2 3 ric ton per year plant by reacting cmshed kernite ore with sulfuric acid. Coarse gangue is removed in rake classifiers and fine gangue is removed in thickeners. Boric acid is crystallised from strong hquor, nearly saturated in sodium sulfate, in continuous evaporative crystallizers, and the crystals are washed in a multistage countercurrent wash circuit. 

The simulation of a continuous, evaporative, crystallizer is described. Four methods to solve the nonlinear partial differential equation which describes the population dynamics, are compared with respect to their applicability, accuracy, efficiency and robustness. The method of lines transforms the partial differential equation into a set of ordinary differential equations. The Lax-Wendroff technique uses a finite difference approximation, to estimate both the derivative with respect to time and size. The remaining two are based on the method of characteristics. It can be concluded that the method of characteristics with a fixed time grid, the Lax-Wendroff technique and the transformation method, give satisfactory results in most of the applications. However, each of the methods has its o%m particular draw-back. The relevance of the major problems encountered are dicussed and it is concluded that the best method to be used depends very much on the application. 

In this section the model for a continuous evaporative crystallizer is discussed. The crystallizer is of the draft tube baffled (DTB) type and is equiped with a fines removal system consisting of a large annular zone on the outside of the crystallizer (see Figure 1). In order to vary the dissolved fines flow without changing the cut-size of the fines removal system, the flow in the annular zone is kept constant and the flow in the dissolving system is varied by changing the recycle flow rate. The model assumptions are ... 

In the proposed continuous evaporative crystallization approach, a fixed amount of sohd compound obtained from a batch crystallization was slurried in a crystallizer in a solvent mixture of ethanol and n-butyl acetate. (n-Butyl acetate replaced acetone as the antisolvent.) n-Butyl acetate has a higher boiling point than ethanol. Therefore, as described below, ethanol can be readily removed by evaporation. 

The number of inputs which are available for controlling crystallisation processes is limited. Possible Inputs for a continuous evaporative crystallisation process are, crystalliser temperature, residence time and rate of evaporation. These Inputs affect the crystal size distribution (CSD) through overall changes in the nucleatlon rate, the number of new crystals per unit time, and the growth rate, the increase in linear size per unit time, and therefore do not discriminate directly with respect to size. Moreover, it has been observed that, for a 970 litre continuous crystalliser, the effect of the residence time and the production rate is limited. Size classification, on the other hand, does allow direct manipulation of the CSD. 

In most commercial processes, borax is obtained from lake brines, tincal and colemanite. The primary salt constituents of brine are sodium chloride, sodium sulfate, sodium carbonate and potassium chloride. The percent composition of borax as Na2B40 in brine is generally in the range 1.5 to 1.6%. Borax is separated from these salts by various physical and chemical processes. The brine solution (mixed with mother liquor) is subject to evaporation and crystahzation for the continuous removal of NaCl, Na2C03 and Na2S04, respectively. The hot liquor consists of concentrated solution of potassium salts and borate components of the brine. The insoluble solid particles are filtered out and the liquor is cooled rapidly in continuous vacuum crystallizers under controlled conditions of temperatures and concentrations to crystallize KCl. Cystallization of borax along with KCl from the concentrated liquor must not occur at this stage. KCl is separated from the hquor by filtration. Bicarbonate then is added to the liquor to prevent any formation of sodium... 

Ostwald in 1897 is credited with introducing the concept of supersaturation and extending it to metastable and labile areas (Mullin 1961). The metastable area occurs in the first stages of supersaturation produced by cooling a saturated solution or by continued evaporation beyond the saturation point. Crystallization does not occur readily in this supersaturation range. The labile area is found at higher levels of supersaturation, where crystallization occurs readily. 

Material in the evaporator feed tank goes to an evaporator/ crystallizer. The SCWO product contains about 4 weight percent salt. The evaporator system can handle about 125 percent of the 11,122 lbs/hr of caustic-treated feed from each SCWO reactor. It operates continuously, producing 24,870 lbs/hr of condensate and 574 lbs/hr of solids. 

Evaporated solution is then pumped into the stirred continually operating crystallizers where it is cooled from 90° to 25 °C. All impurities and salt crystals are then allowed to separate in settling vessels. The clear solution is afterwards pumped into storage tanks, where a small amount of sodium hypochlorite is added in order to decolorize the lye. 

A stream of a saturated brine suspension of crystals is continuously withdrawn from each evaporator-crystallizer, and the salt crystals separated on a continuous rotary filter with return of the brine to the evaporator. Much of the salt may be marketed in moist condition, or it may be passed through a drier moving countercurrently to heated air to give vacuum salt of typically 99.8-99.9% purity [21]. 

Although widely practiced for production of industrial chemicals, continuous evaporation for crystallization is rarely if ever used in pharmaceutical operations. Although continuous operation has the advantages of using massive seeding and increased control of supersaturation and the crystal surface area, the throughput necessary for its application is rarely, if ever, achieved for final bulk drug substances. In addition, continuous operation to achieve the conditions for crystallization (as discussed above for resolution of optical isomers) is often not... 

Evaporative crystallization can be carried out in a wide variety of equipment for both semibatch and continuous operation. Discussions of these systems may be found in several references including Myerson (2002, chapter 10) and Mullin (2001, chapter 7). They are utilized in some areas of the chemical industry that require the high production rates that can be achieved with these specialized designs. 

In the crystallization of the sodium salt of a drug candidate, extremely fine needles were generated. In addition, changes in crystal morphology and crystallinity were noted after filtration and drying. In order to avoid these and to grow better crystals, an alternate continuous evaporative crystaUization approach with heavy seeding was evaluated. 

In direct-contact refrigeration crystallizers, the refrigerant is mixed with the magma circulated within the crystallizer body where it absorbs heat and is vaporized. Refrigerant vapor leaves the surface of the crystallizer similar to water vapor in a conventional evaporative crystallizer. The refrigerant vapor must be compressed, condensed, and then recirculated to the crystallizer to maintain the continuous operating conditions. Refrigerants chosen must be relatively insoluble in the solutions processed and have the necessary thermodynamic characteristics to minimize compressor horsepower. 

In continuous adiabatic evaporative crystallizers, the temperature distribution in the vessel is a function of the operating level. (The pumping rate also affects the distribution but is not subject to change during normal operation.) The surface temperature is set... 

When the evaporation reaches E the droplets consist entirely of the highly viscous liquid crystal and the emulsion is now transferred to a suspension of almost solid particles (although by definition it is an LC/O emulsion). Continued evaporation takes place from flie liquid-crystalline particles to point F, when the surfactant liquid G begins to form inside the liquid-crystalline particles. At G all flie liquid crystal is changed to the surfactant liquid and wifii die last water removed an emulsion of the surfactant liquid with 16% triglyceride-in-oil [14% (by weight) surfactant liquid-in-84% oil] is the final state. Owing to the extremely low vapor pressure of the oil, this is the final state for applications. 

The overall design is similar to that used in salt crystallization and is suitable either for batchwise or continuous operation. In the former case the still contents need to be kept in a form that can easily be discharged. For continuous evaporation the vapour can be fed to the column after passing through a combination of flash vessel and disentrainer. Provided that the latter function is effective, clean side streams can be taken from the column below the feed point. 

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