Adiabatic evaporative 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. 

When the WBT drops to the switch point, the process bypassing has reached 100 percent. This switch point WBT is at or above 5°C (41 F). As the ambient temperature drops further, adiabatic evaporative cooling continues to be used, to lower the dry-bulb temperature... 

Fig. 1.52 Adiabatic evaporative cooling. Temperature d and partial pressure pA of the water in the air...

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. 

An early report of adiabatic evaporative cooling (Takegami, 1993) considered the purification of caprolactam (melting point 69 ""C) in the presence of water... 

Based on differences in melting points and Hquid-phase solubilities four modes of operation possible drown-out, isothermal evaporation, adiabatic evaporation, and cooling (choice depends on stream characteristics). 

Order of preference adiabatic evaporation > isothermal evaporation > cooling >drown-out. 

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

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. 

Note that during the cooling step the maximum amount of evaporation which is permitted by the material balance is 50 lb for the step shown. In an evaporative-cooling step, however, the actual evaporation which results from adiabatic cooling is more than this. Therefore, water must be added back to prevent the NaCl concentration from rising too high otherwise, coprecipitation of NaCl will occur. 

The patents quoted in Table 20 give rather broad temperature ranges. An explanation for this feature is the polymerization mode which is used for the large-scale polymerization of Nd-BR. To the best of our knowledge, adiabatic rather than isothermic modes are used. In the adiabatic mode polymerization heat is neither removed by external nor by evaporation cooling. Therefore,... 

We will look at evaporative cooling as an application of film theory. A solid, adiabatic, insulated wall is covered by a film of water, over which unsaturated humid air flows, as shown in Fig. 1.52. 

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... 

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