Adiabatic evaporative crystallization

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... 

Solution crystalli tion (adiabatic evaporation (vacuum cooling))... 

If the crystallizer is not of the evaporative type but rehes only on adiabatic evaporative cooling to achieve the yield, the heating element is omitted. The feed is admitted into the circulating hue after withdrawal of the slurry, at a point sufficiently below the free-hquid surface to prevent flashing during the mixing process. 

The scale of operation often has an overriding importance on the selection of the equipment because of the means used for heat transfer. For very small-scale crystallization work it is common to use radiation. The capacity of such equipment varies from a few liters up to several hundreds of liters per day (of solution cooled). For operation on scales up to several thousand liters per day, it is possible to use tanks with water-cooled coils and an agitator. For large-scale applications where the quantity of solution is thousands of liters per day, it is almost universal practice to use vacuum evaporation to remove the solvent this is true whether the solution is cooled by adiabatic evaporation or in equipment where crystallization occurs because of isothermal evaporation. 

In the adiabatic evaporative growth type crystallizer shown, the pressure is controlled by recirculating exhausted noncondensables to a vacuum pump suction, and the condensate is returned to the crystallizer body. Returning the condensate maintains a dilution of the impurities in the feed and provides a means of controlling the product purity by preventing crystallization of the impurities. It also lowers the concentration of impurities in the mother liquor adhering to the crystal mass in downstream separation operations. 

Since supersaturation is the important prerequisite of crystallization, crystallizers can be conveniently classified according to the primary methods by which supersaturation is brought about or released by (1) supersaturation by cooling, (2) supersaturation by evaporation of solvent, (3) supersaturation by adiabatic evaporation (cooling plus evaporation by vacuum), (4) circulation of solution over crystal bed to release supersaturation produced by one of the above methods, and (5) salting out. 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

Evaporation combined with adiabatic cooling, such as in vacuum crystallizers... 

In the special case of adiabatic vacuum cooling, the quantity of solvent evaporated is fixed by the crystallizer heat balance. Thas. ft, is a fixed ratio—referred to as which can be determined from the following... 

For shell and tube condensation Related topics evaporation Section 4.1, distillation Section 4.2. Prefer condensation outside horizontal tubes use vertical tubes when condensing immiscible liquids to subcool the condensate. Assume pressure drop of 0.5 of the pressure drop calculated for the vapor at the inlet conditions. Baffle spacing is 0.2 to 1 times the shell diameter with the baffle window about 25 %. Limit pressure drop for steam to 7 kPa on the shell side. U = 0.5-0.85 kW/m °C. For shell and tube boiling approach temperature < 25 °C to ensure nucleate boiling. Related topics evaporation Section 4.1, distillation. Section 4.2, solution crystallization Section 4.6 and reactors PFTR non-adiabatic. Sections 6.8 and 6.12. 

Draft tube baffte DTB or draft tube, DT (either adiabatic cooling or evaporative type also called flash growth or Pachuca) MSMPR. Mother liquor is pumped up a vertical central draft tube liquor overflow and flows down the atmulus. Limits the amount of supersaturation created per pass past the heating surface to 1 °C and therefore limits the nucleation rate to very low values. Operates with a suspension of soUds that is 25-50% apparent settled volume. Used where solute solubility is temperature independent or moderately dependent and where excess nudeation makes it difficult to achieve crystals in the size 0.6-2 mm. Minimum crystallization buildup on walls no places with dose dearances (as in Oslo). 

Operational Charaaeristics The feed stream saturation temperature is higher than that maintiuned in the crystallizer body thus, adiabatic cooling occurs. Also, external heat is added to evaporate solvent and precipitate additional solute. 

Major AppUcatim Used for adiabatic cooling and medium evaporation loads also adaptable for cooling crystallization. 

In crystallizers producing supersaturation by cooling the substances must have a solubility curve that decreases markedly with temperature. This occurs for many substances, and this method is commonly used. When the solubility curve changes little with temperature, such as for common salt, evaporation of the solvent to produce supersaturation is often used. Sometimes evaporation with some cooling will also be used. In the method of cooling adiabatically in a vacuum, a hot solution is introduced into a vacuum, where the solvent flashes or evaporates and the solution is cooled adiabatically. This method to produce supersaturation is the most important one for large-scale production. 

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