Control evaporative crystallizers

Crystallization generally involves the evaporation and subsequent cooling of a solution to the point of supersaturation, whereupon the formation of crystals takes place. Modern technology often focus on the control of crystal size, since product demands frequently are rigorous in this regard. The process of crystallization is often conducted in evaporators. As in the evaporation of salt and in the recovery of salt and glycerin in soap manufacturing, salt separators are used to remove crystallized materials as rapidly as it settles. 

Virtanen, J. 1984. Automatic control of batch evaporative crystallization. In Industrial Crystallization 84. The Hague September 1984. Eds. S.J. Jancic and E.J. de Jong. Elsevier Science. 

Area 300 is controlled using a distributed control system (DCS). The DCS monitors and controls all aspects of the SCWO process, including the ignition system, the reactor pressure, the pressure drop across the transpiring wall, the reactor axial temperature profile, the effluent system, and the evaporation/crystallization system. Each of these control functions is accomplished using a network of pressure, flow, temperature, and analytical sensors linked to control valves through DCS control loops. The measurements of reactor pressure and the pressure differential across the reactor liner are especially important since they determine when shutdowns are needed. Reactor pressure and temperature measurements are important because they can indicate unstable operation that causes incomplete reaction. 

Figure 3.5 Controlled evaporation of crystallization drops in microbatch, (a) Trials incubated under a thin layer of oil that allows concentration via evaporation, thus leading to nudeation. (b) Arrest of evaporation/concentration by addition of oil to produce a thicker layer above the trials. Modified from Chayen and Saridakis (2002), Acta Cryst. D 58, 921-927, with permission from the lUCr.

Natural circulation evaporators like those shown on Figure 8.16 may be equipped for continuous salt removal and thus adapted to crystallization service. For large production rates, however, forced circulation types such as the DTB crystallizer of Figure 16.10(g), with some control of crystal size, are the most often used. The lower limit for economic continuous operation is l-4tons/day of crystals, and the upper limit in a single vessel is 100-300 tons/day, but units in parallel can be used for unlimited capacity. 

With flash evaporation of solvent from a sprayed mixture of solvent and brine the crystals have a Deq of 0.0024 inch. Although little control of crystal size is possible by the spray technique, the allowable lineal ice-production rate is 9 feet per hour with these crystals. This is completely satisfactory for an economical plant. 

On the other hand, if the shape of the particles can be controlled, self-assembly of these shaped particles can lead to crystal orientation of each particle in a self-assembled superlattice. For example, MnFe204 nanoparticles have been made in cube-like and polyhedron shapes, as shown in Fig. 6 [46]. Controlled evaporation of the carrier solvent from the hexane dispersion (about 2 mg/mL) of the particles led to MnFe204 nanoparticle superlattices. 

Numerous water types reflect weathering control these include Mg +-HCOj , Ca +-HCOj , Na+-HCOj", Mg2+-SO, Ca2+-SO, Na+-SO, Na" -Cl types (Table 3). The evaporation-crystallization control found in semi-arid and arid... 

These sources of variability all contribute to potentially severe scale-up problems with evaporative crystallization. Control of the distillation rate by control of the jacket temperature may require higher wall temperatures, thereby making supersaturation variation more severe. The decrease in bulk circulation and the increase in mixing time will further exacerbate this problem. In some cases, these problems can produce unacceptable results, requiring development of an alternative crystallization method. See Example 8-2 for a discussion of an application in which adequate PSD control could not be achieved. 

The unit operations are as applicable to many physical processes as to chemical ones. For example, the process used to manufacture common salt consists of the following sequence of the unit operations transportation of solids and liquids, transfer of heat, evaporation, crystallization, drying, and screening. No chemical reaction appears in these steps. On the other hand, the cracking of petroleum, with or without the aid of a catalyst, is a typical chemical reaction conducted on an enormous scale. Here the unit operations— transportation of fluids and solids, distillation, and various mechanical separations—are vital, and the cracking reaction could not be utilized without them. The chemical steps themselves are conducted by controlling the flow of material and energy to and from the reaction zone. 

It is desirable to set the heat flux in an evaporative crystallizer because, in essence, this sets the maximum supersaturation level within the equipment. While at any one condition, this can be done by controlling either pressure or steam flow, experience shows the best technique is to control the steam flow. (Not shown in Figure... 

Temperature. The process temperature of an evaporative crystallizer may be controlled by the absolute pressure in the vessel. Direct flow control of steam to an ejector, while economizing on steam utilization, is subject to pressure fluctuations due to disturbances in the steam supply pressure. Applying cascaded flow control would decrease the response time of the control loop. Flow control of a bleed gas or exhausted gas into the suction of the vacuum source is the most responsive and precise control option. Critically damped tuning of the pressure control loop should be implemented to prevent rapid temperature changes and high supersaturation generation from fast swings in pressure. 

A batch evaporative crystallizer (Figure 10.2) was used by Baliga (1970) to study the crystallization kinetics of potassium sulfate crystals. The crystallizer was equipped with a reflux condenser and a controlled distillate splitter so that the net solvent removal rate could be controlled closely. Heating of the crystallizer... 

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