br Principle of operation The feed is

Principle of operation The feed, is dissolved in the recovered solvent P with the addition of a small amount of fresh solvent P (Fig. 1). The amount of P depends on the loss of solvent with concentrated solution W. Energy required for dissolution in the process can be supplied by solvent P coming from evaporator E. Solution F of KNO3 is fed to crystallizer Cr where it is cooled to temperature t which will change with the concentration of KNO3 in the solution (Fig. 2). As a result of crystallization, magma S + L is produced which is fed to separator SP (Fig. 1). Mother liquor L at temperature t, after its BAY 87-2243 from crystals S, is fed into evaporator E where most of the solvent is recovered, leaving concentrated solution W. Vapors P of the solvent are compressed from pressure p1 to pressure p2 raising the enthalpy of the vapors from hv1 to hv2. The compressed vapors are then fed into the heating chamber of evaporator E where upon condensation boils the mother liquor L. The condensate P with enthalpy h3 and the concentrated solution W which is rich in impurities exit the evaporator. Pressure of the condensate P drops from p3 to p4 on passing through expansion valve EV. Recovered solvent P from the evaporator E is hot and if its heat content is higher than the heat needed in dissolution stage then it is necessary to cool to the required temperature t2. For this purpose, the hot solvent is fed to heat exchanger H1 to pre-heat mother liquor to temperature t. The temperature of mother liquor can further be raised to t by passing it through heat exchanger H2 which is heated by hot concentrated solution W from the evaporator. Heating mother liquor L before feeding to the evaporator, can significantly reduce the amount of heat needed in the evaporator.
Results and discussion Calculated results for elevation of boiling point of the solvent are presented in Table 1: With increase in concentration of KNO3, boiling point of the solution rises. This directly affects the amount of vapors generated in the evaporator and subsequently flow rate of the recovered solvent (Fig. 3(a)). More solvent will be recovered in the evaporator from a more concentrated feed solution to the crystallizer than a less concentrated feed solution to the crystallizer. Gazagnes et al. (2007) established that with increase in NaCl concentration, the performance of a desalination unit diminished because less water vaporized at the membrane surface. The amount of exiting concentrated solution, however, increases with elevation of boiling point as less water evaporates from mother liquor with increase in its concentration (Fig. 3(b)). Recycled solvent P supplies the required heat for dissolution in stage D and as a result, reduces demand for additional heat at this stage. Mathematical analysis has shown that concentration x depends primarily on temperature t2 (Fig. 4). This is because the recovered solvent supplies heat of dissolution of the fresh solid. Thus, with increasing x, temperature t2 at which recovered solvent is supplied to stage D increases. A low fractionation temperature leads to a lower t2 than a high fractionation temperature. t2, also increases with concentration x of mother liquor and x of solution (Fig. 5). This is due to the elevation of boiling point of the solvent. Another important factor influencing the process of recrystallization of KNO3, is energy consumption at the recovery of solvent. In this case, thermal energy consumption QE in the evaporator, depends on parameters of dissolution of solids and subsequent crystallization. It was found that QE is mainly dependent on the concentration x of mother liquor and its temperature t (Fig. 6(a) and (b)). On increasing concentration x there is a reduction in the amount of heat QE per unit mass of the solution F since generation of vapors P decreases and mass flow rate of concentrated solution W increases. An increase in temperature t through preheating with recovered solvent P and waste stream W also leads to a reduction of QE.