Results of Cacl2-Nacl-Kcl Crystallization Experiments

Results of CaCl2-NaCl-KCl Crystallization Experiments

The solution as shipped contains 47% CaCl2, 1.3% KCl and 0.8% NaCl. A plot of the cooling crystallization of this solution is given in Figure II.1 This solution can be solubilized by heating to 90-100C but we did not do this in our experiments due to heater limitations. We heated to only 60C and as a result some solids, almost completely KCL were left in the bottom of the drum and our feed solution is altered to be CaCl2 47.331wt%, KCl 0.604wt%, and NaCl 0.806wt%. A plot of the crystallization from this solution is given in Figure II.2. Cooling this feed solution, at temperatures above 30C the CaCl2 is soluble and both the KCl and NaCl crystallize out of solution in roughly equal proportions. A cooling crystallization to purify the CaCl2 is required by Dow for this crystallizer. The solubility of the system is shown in Figure II.2. Here we see that as the temperature is cooled from 60C to 32C, as we have done in our experiments, increasing amounts of KCl and NaCl crystallize out of solution with the KCl being roughly equivalent to NaCl at 32C.

A short explanation of this work follows. A full listing of the thesis is given at \\che-2551- 38.che.utah.edu\Faculty$\00028368\public_html\CrystallizationW eb\Choi_Dissertation.pdf. Figure II.1 Dominant solids crystallized out of a solution containing 47% CaCl2, 1.3% KCl and 0.8% NaCl initially heated to 100C and cooled to various temperatures. Figure II.2 Dominant solids crystallized out of a solution containing CaCl2 47.331wt%, KCl 0.604wt%, and NaCl 0.806wt% initially heated to 100 C and cooled to various temperatures.

The results of our experiments show that indeed we see increasing amounts of crystals as the solution is cooled as shown in Figure II.3 (SEM), but that the crystals are all most completely NaCl as shown in Figure II.4 (EDAX of SEM) and Figure II.5 (ICP of reactor outlet supernatant as a function of time). The EDAX results suggest that they are 1% by weight CaCl2 and less than 1% KCl. The ICP results show a Major decrease in the NaCl concentration, nearly zero decrease in the KCl concentration and a slight decrease in the CaCl2 concentration verifying the EDAX results. These experimental results are in disagreement with the OLI predictions of the dominant solids.

The crystals produced at steady state are single crystals of 40 microns aggregated in to larger particles as shown in Figure II.6. However, there is a fraction of smaller spherical particle early on in the crystallization process as shown in Figure II.7. As the crystallization precedes these spherical octahedral crystals also shown in Figure II.7 replace particles. This suggests that there is some special kinetic phenomenon taking place in our experiments that arrives at a steady state but that may not be predicted by equilibrium calculations. As a result, I am wary of applying the OLI research tool to this work and expecting the model to validate the results.

Analyzing Figure II.8 we also see that there are no crystals of very small size as was the case with the adipic acid crystallization. This is due to aggregation sweeping up all the small particles into aggregates shown in Figure II.5. Thus, to model this system, a model that includes nucleation (secondary), growth, aggregation and breakage is needed.

Figure II.3 SEM of particles produced by crystallization in a baffled 1.3 liter stirred tank with a Ruston Impeller operating at 400 rpm. A solution of CaCl2 47.331wt%, KCl 0.604wt%, and NaCl 0.806wt% at 60 C was fed to the tank and cooled to 32 C in the tank to induce crystallization. Figure II.4 EDAX of particles shown in SEM. The crystals are predominantly NaCl with a trace of CaCl2 and KCl.

Figure II.5 ICP of Supernatant of crystallizer output versus time in number of residence times of 54 min. The feed solution is CaCl2 47.331wt%, KCl 0.604wt%, and NaCl 0.806wt% at 60 C. These results do not agree with Dow Analysis!!! The K concentration is not correct due to an analytical problem with ICP.

Figure II.6 Blowup of particles shown in Figure II.2 showing the crystal habit and particle size distribution. Agglomerated particles and broken particles and/or nuclei are also observed. Figure II.7 Particles produced at an early stage of the crystallization before the crystallizer has reached steady state. Initially the particles are all spherical and later there is an increasing amount of octahedral crystals produced.

Figure II.8 Particle size distribution (PSD) of particles produced by crystallization in a baffled 1.3 liter stirred tank with a Ruston Impeller operating at 400 rpm. A solution of CaCl2 47.331wt%, KCl 0.604wt%, and NaCl 0.806wt% at 60 C was fed to the tank and cooled to 32 C in the tank to induce crystallization. The different curves correspond to the PSD at different multiples of the mean residence time including 3x (01), 5x (02-01) and 6x (03- 01). Only at 5x and 6x is the crystallizer at steady state.