Crystallizers circulating liquid

Figure 8.16. Some types of evaporators, (a) Horizontal tube, (b) Calandria type, (c) Thermocompressor evaporator, (d) Long tube vertical, (e) Falling film, (f) Forced circulation evaporator-crystallizer, (g) Three types of Oslo/Krystal circulating liquid evaporator-crystallizers.

Example 16.5. Teflon heat transfer tubes that are thin enough to flex under the influence of circulating liquid cause a continual descaling that maintains good heat transfer consistently, 20-65 Btu/(hr)(sqft)(°F). Circulating types such as Figures (d) and (e) of ten are operated in batch mode, the former under vacuum if needed. High labor costs keep application of batch crystallizers to small or specialty production. 

In order to avoid an accumulation of iron compounds (max 50 mg. Fe per litre) a oertain portion of the circulating liquid is led into spacious precipitating tanks where it is neutralized by gaseous ammonia. The precipitated ferric hydroxide is allowed to settle and the clear solution is siphoned and boiled down. The separated crystals of pure ammonium sulphate are centrifuged and recycled into the electrolytic process. 

Although a crystallizer may be operating continuously at steady state with respect to a small portion of the circulating liquid, over-all the unit... 

Circulating-liquid evaporator-crystallizer. In a combination evaporator-crystallizer shown in Fig. 12.1 l-3a, supersaturation is generated by evaporation. The circulating liquid is drawn by the screw pump down inside the tube side of the condensing steam heater. The heated liquid then flows into the vapor space, where flash evaporation occurs giving some supersaturation. The vapor leaving is condensed. The supersaturated liquid... 

Figure 12.11-3. Types of crystallizers (a) circulating-liquid evaporator-crystallizer, (b) circulating-magma vacuum crystallizer.

The pH and the inorganic salt concentration in the circulating liquid are kept to the pre-set value by means of pH control and inorganic strength (10% salts w/w) control (dilution water feed system). These controls are veiy important in order to keep the alkalinity at the required pH value (10-12) and to avoid crystallization of the inorganic salt from solution. 

Crystallizers with Fines Removal In Example 3, the product was from a forced-circulation crystallizer of the MSMPR type. In many cases, the product produced by such machines is too small for commercial use therefore, a separation baffle is added within the crystallizer to permit the removal of unwanted fine crystalline material from the magma, thereby controlling the population density in the machine so as to produce a coarser ciystal product. When this is done, the product sample plots on a graph of In n versus L as shown in hne P, Fig. 18-62. The line of steepest ope, line F, represents the particle-size distribution of the fine material, and samples which show this distribution can be taken from the liquid leaving the fines-separation baffle. The product crystals have a slope of lower value, and typically there should be little or no material present smaller than Lj, the size which the baffle is designed to separate. The effective nucleation rate for the product material is the intersection of the extension of line P to zero size. 

Major evaporator designs include forced-circulation, long-tube vertical (both rising and falling film), and calandria-type evaporators. The economics of a particular process will dictate the evaporator style and model best suited to a particular application. Forced-circulation and calandria evaporators are required for processes where crystals are formed. These evaporators are designed to keep crystals suspended in solution to prevent scaling of the equipment. Long-tube vertical evaporators are used to concentrate a liquid that does not have solids present. 

Evaporators employ heat to concentrate solutions or to recover dissolved solids by precipitating them from saturated solutions. They are reboilers with special provisions for separating liquid and vapor phases and for removal of solids when they are precipitated or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be adequate in some applications, especially if enough freeboard is provided. Some of the many specialized types of evaporators that are in use are represented on Figure 8.16. The tubes may be horizontal or vertical, long or short the liquid may be outside or inside the tubes, circulation may be natural or forced with pumps or propellers. 

It may polymerise with violence on melting at 86°C [1], The concentrated aqueous solutions of commerce will crystallise in cool weather. The usual industrial technique for softening raw materials, applying a steam hose to the bottom of the container, to which crystals have fallen, will create domains of molten, unstabilised, monomer, initiating polymerisation of the whole, then rupturing containers through steam pressure. Rather, the liquid at the top should be warmed and circulated. Photoinitiation seems involved, black plastic containers give less problem than translucent ones. Best do not store below 15°C [2],... 

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