Heat Effects in a Crystallization Process

At equilibrium the heat of crystallization is equal and opposite in sign to the heat of solution. Using the heat of solution at infinite dilution as equal but opposite in sign to the heat of crystallization is the equivalent, however, of neglecting the heat of dilution. With many chemicals the heat of dilution is small in comparison with the heat of crystallization and the approximation is justified. However, there are some exceptions. 

Relatively large heat effects are usually found in the crystallization of hydrated salts, such as Glauber s salt or epsom salt. In 

The heat effects in a crystallization process can be computed by two methods  

A heat balance can be made wherein the individual heat effects, such as the sensible heat required for heating or cooling the solution, the heat of crystallization of the product formed, and the latent heat of vaporization for liquid vaporized can be combined into an equation for the total heat effects. 

An enthalpy balance can be made in which the total enthalpy of all leaving streams minus the total enthalpy of all entering streams is equal to the heat absorbed from external sources by the process. 

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The advantage of the enthalpy-concentration diagram method is that both heat and mass effects are taken into account simultaneously. This method has only limited use, however, because of the difficulty in obtaining enthalpy-concentration data that have been published for only a few systems. In using either method, however, it is necessary to make a corresponding mass balance since the heat effects in a crystallization process are related to the quantities of solids produced through the heat of crystallization. 

Probably the most satisfactory method of calculating heat effects during a crystallization process is to use the enthalpy-concentration chart for the solution and the various solid phases which are present for the system. However, only a few such charts are available, including the following systems calcium chloride-water.,(HI), magnesium sulfate-water (P2), and ferrous sulfate-water (K2). When such a chart is available, the following procedure is used. The enthalpy if, of the entering solution at the initial temperature is read off the chart, where//, is kJ (btu) for the total feed. The enthalpy i/j of the final mixture of crystals and mother liquor at the final temperature is also read off the chart. If some evaporation occurs, the enthalpy Hy of the water vapor is obtained from the steam tables. Then the total heat absorbed q in kJ is... 

The actual yield may be obtained from algebraic calculations or trial-and-error calculations when the heat effects in the process and any resultant evaporation are used to correct the initial assumptions on calculated yield. When calculations are made by hand, it is generally preferable to use the trial-and-error system, since it permits easy adjustments for relatively small deviations found in practice, such as the addition of wash water, or instrument and purge water additions. The following calculations are typical of an evaporative crystallizer precipitating a hydrated salt. II SI units are desired, kilograms = pounds X 0.454 K = (°F -I- 459.7)/I.8. 

Moreover the effectiveness of inoculants influence can be assessed on the basis of the hypothesis presented in the paper 0ura, 1968). This hypothesis was developed at the assumption that the fundamental physical factors affecting on the crystallization process are the amount of give up heat in the crystallization process on the interphase boundary of liquid - solid and the rate of give up heat of crystallization. After analyzing the results of own researches, the author proposed to determine the index (a), which characterizes the type of inoculant. 

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