Supersaturation generation

The main driving force of a salting-out precipitation process is the supersaturation generated by the different solubilities of the solute in the solvent and antisolvent. The one-shot mode generally means that the necessary amount of the antisolvent is added in one portion to the initial solution (either saturated or undersaturated) as it is schematically shown in Fig. 2. In our cases the aqueous solution was added to the antisolvent. As a result of the antisolvent addition the original solubility of the substance will change until the new equilibrium solubility, ceq has been reached. To characterize the supersaturation driving force in a precipitation system the so called initial supersaturation,. S, is used ... 

Fig. 21 Cocrystallization of molecular complex A B from the supersaturation generated by the dissolution of solid reactants, A and B, in a microphase of solvent. (Reproduced from Ref. l.)...

Experimental isolation of task Supersaturation Generation with membranes... 

For solvent systems with a window of operating temperature, proper selection of the method of supersaturation generation (e.g., cooling and antisolvent addition) and mode of crystallization (e.g., batch vs. semicontinuous) can also affect the overall crystal growth rate. In many instances in which solvent or impurity rejection becomes critical, adequate mixing to avoid local high supersaturation can be critical. Examples 9-2 and 10-4 illustrate two cases of rejection of impurities and residual solvent. These examples show how various means are applied to overcome these complications. 

Lindberg, M. and A.C. Rasmuson (2000). Supersaturation generation at the feed point in reaction crystallization of a molecular compound. Chem. Eng. Sci. 55, 1735-1746. 

The removal of the metastable supersaturation is a slow process. A large amount of crystal surface is required to allow for the large number of random collisions necessary to remove the supersaturation generated during the cycle. The proper orientation of both the molecules in solution and the molecule on the crystal surface is required for deposition, and the increased complexity of the molecule increases the number of collisions required for proper orientation. 

If the supersaturation generated during the cycle is not completely removed, the level of supersaturation attained during the following cycle is increased. This increase from cycle to cycle will continue until the supersaturation level of the solution exceeds the metastable region and enters the labile region, where spontaneous nucleation occurs. The occurrence of spontaneous nucleation means loss of control of crystal size. 

Once initial nucleation has been achieved successfully, the control of secondary nucleation becomes important. Since crystal growth is a surface phenomenon, each nuclei formed is available to absorb the supersaturation generated by the cycle. This means that only one nuclei is to be formed for each single crystal removed if a constant crystal size is to be maintained. 

Due to the nature of long chain organic molecules, deposition on the crystal surface is more difficult and time consuming than for most inorganic chemicals. This must be taken into consideration and additional time allowed between cycles so that the supersaturation can be relieved. Another effective method for handling this potential problem is to limit the supersaturation generated during each cycle. 

Temperature. The process temperature of an evaporative crystallizer may be controlled by the absolute pressure in the vessel. Direct flow control of steam to an ejector, while economizing on steam utilization, is subject to pressure fluctuations due to disturbances in the steam supply pressure. Applying cascaded flow control would decrease the response time of the control loop. Flow control of a bleed gas or exhausted gas into the suction of the vacuum source is the most responsive and precise control option. Critically damped tuning of the pressure control loop should be implemented to prevent rapid temperature changes and high supersaturation generation from fast swings in pressure. 

Precipitation is an operation known for producing small crystals that are difficult to filter and dry. Batch precipitation is usually carried out in the form of a semibatch process, i.e., one or two reactants are continuously added to the reactor. The control of the particle size distribution in a precipitation process is very complicated because of the high level of supersaturation generated by the fast reaction. According to Mersmann [16], the important factors for sparingly soluble systems in isothermal precipitation are ... 

The solubility product principle can only be strictly applied to equilibrium conditions, although it has often been used to explain such precipitations as those encountered in qualitative analysis by the traditional wet-test methods. However, these sudden precipitations do not take place under anything like equilibrium conditions and the fact that reasonably successful predictions can usually be made is mainly due to the enormous excess ionic concentrations (supersaturations) generated compared with those required by the corresponding solubility products. Errors of magnitude of 10 —10 per cent have been estimated (Lewin, 1960) for such calculations and these clearly swamp other variations such as neglect of solute activity coefficients, complex ion formation, etc. 

TABLE 7 Supersaturation Needed to Cause Bubble Nucleation (Mostly Heterogeneous) in Water or Aqueous Solutions. Supersaturations Generated by Nonelectrochemical Means... 

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