Crystallizers discharge control

In the forced-circulation-type crystallizer (Fig. 19-43) primary control over particle size is exercised by the designer in selecting the circulating system and volume of the body. From the operating standpoint there is little that can be done to an existing unit other than supply external seed, classify the discharge crystals, or control the slurry... 

These operations may he conducted separately or in combination in an effort to separate a solid from its solvent. The product from an evaporation is a concentrated solution, whereas a crystallizer discharges a slurry of crystals in a saturated solution. These two operations may not be technically classified as mass transfer, in that no equilibrium exists between the composition of the two phases-the vapor leaving an evap orator and the crystals in the crystallizer are both essentially pure. Yet the control of both these operations is heavily dependent on the material balance. 

Figure 7.4 Microcomputer programming of a hatch cooling crystallizer. A, crystallization vessel, B, control heater, C, control cooler. surrounding the draft-tube), D, contact thermometer, E, discharge plug and conical baffle), F, recorder, G, relay, H, temperature programmer, I, cooling water pump, J, cooling water reservoir, K, water inflow L, water outflow after Jones and Mullin, 1974)...

Luo et al. [1,153] used a slurry containing ultra-fine diamond (UFD) powders to polish the surface of HDD sliders. The powders are from 3 nm to 18 nm in diameter and 90 % around 5 nm. They are crystal and sphere-like [154]. The pH value of the slurry is kept in the range from 6.0 to 7.5 in order to avoid the corrosion of read-write heads, especially pole areas. A surface-active agent is added into the slurry to decrease the surface tension of the slurry to 22.5 Dyn/cm, and make it spread on the polish plate equably. An anti-electrostatic solvent is also added to the slurry to avoid the magnetoresistance (MR) head being destroyed by electrostatic discharge. The anion concentration of the slurry is strictly controlled in ppb level so as to avoid the erosion of magnetic heads as shown in Table 5. The concentration of UFDs in the slurry is 0.4 wt %. 

The brine solution may be enriched with NaCl or KHT to increase its conductivity and acidified to the same pH of the wine under treatment. It is recirculated between the concentrating compartments of the ED stack and another tank equipped with conductivity and pH automatic controls so as to avoid precipitation of potassium bitartrate onto the membranes by diluting the brine with deionized water and/or discharging more or less aliquots of the brine itself when its conductivity reaches 70-80% of its saturation value (Goncalves et al., 2003). In this way, a waste effluent in the range 10-20% of the wine volume treated is to be disposed of or used to recover KHT crystals (Nasr-Allah and Audinos, 1994). 

Hexamine is manufactured from anhydrous ammonia (NH3) and a 45% solution of methanol-free formaldehyde (HCH=0). These raw materials, plus recycle mother liquor, are charged continuously at carefully controlled rates to a high-velocity reactor, since the reaction is exothermic. The reactor effluent is discharged into a vacuum evaporator that also serves to crystallize the product, and the hexamine crystals are washed, dried, and screened. Typically, the yield of hexamine is on the order of 96%. 

Ocular Effects. Ocular effects can occur as a result of direct contact of eyes with chromium compounds. These include corneal vesication in a man who got a drop or a crystal of potassium dichromate in his eye (Thomson 1903) and congestion of the conjunctiva, discharge, corneal scar, and burns in chromate production workers as a result of accidental splashes (PHS 1953). Higher incidences of subjective complaints of eye irritation were reported by housewives who lived near a chromium slag construction site than by controls (Greater Tokyo Bureau of Hygiene 1989). 

Various temperature-sensitive materials may be processed in this type dryer because of short material retention times and proper temperature control. Typical drying times are on the order of a few seconds. Flash dryers are most useful for moist, powdery, granular, and crystallized materials, including feeds that are wet and that are discharged from filtration equipment (Christiansen and Sardo, 2001). Because of the rapid drying process, they are often used to remove surface water, but they are not suitable for diffusion-controlled drying. Particle size of the product material is small, usually less than 500 microns, and the most suitable feed is that which can be fried, rather than a sticky material. 

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