(512g) Application Of Dynamic Regulation Profiles Of Vacuum Pressure In Batch Crystallization Of Cane Sugar

The crystallization as industrial process is important due to the great variety of materials that are commercialized in crystalline form. The product quality in a crystallization process is related to the crystal size distribution (CSD), the crystal morphology, the polymorphic effect and the purity degree. The main causes to control the product quality in batch crystallizers focus in the study of both phenomenological and process operation factors. The crystallization depends on thermodynamic and kinetic factors: nucleation rate, growth rate and production-reduction term (births and death rates of crystals), that are governed by variables like the local and average supersaturation levels, the mixed degree, impurities, additives, pH, dissolvent type, temperature and pressure of operation. It has been considered that both the crystal size distribution and the formed crystal mass (FCM) are important factors to the process and defines the final product characteristic. In this work, the dynamic regulation profiles of vacuum pressure implemented by means of a supervisory control system and data acquisition (SCADA) are introduced like novel operation technique in batch crystallization. This new operation technique represents an alternative for the manipulation of the average supersaturation, avoids agglomerate of formed crystalline product and eliminates the use of unsaturated liquor like means of dissolution in the batch process operation. This work has the objective of analyzing both the DTC and MCF behavior in the batch crystallization of cane sugar due to the application of dynamic regulation profiles of vacuum pressure.

Like a part of the methodological process, a strategy of dynamic regulation profiles of vacuum pressure was developed, which has a very important effect on the cooling by adiabatic evaporation into the batch process. The experimentation was carried out dissolving 9235.98 g of cane sugar and 2922.68 g of water into a stirred batch crystallizer, until reaching 75.96 °Brix, corresponding to the supersaturation temperature of 70 °C. It was included 6.33 g of crystal seed with average size of D(4,3) 195.30 µm (% volume) and standard deviation of S(4,3) 19.52 µm, during the first minute of process, approximately to 68 °C. A constant evaporation took place during the first 30 minutes of the batch, benefiting both the stabilization and the seeded crystal growth. Then, a dynamic regulation profile of vacuum pressure (called as maximum frequency, linear and cubic) was applied during 60 minutes, causing an abrupt cooling by adiabatic evaporation, which took to an increase of both crystal size and the formed mass. The final batch time was 90 minutes, with samplings every 15 minutes to gets experimental information of the process variables.

For the FCM analysis, the evaporated water amount was calculated and it depends on the dynamic regulation profile of vacuum pressure to follow during the crystallization, relating the amount of initial water of process and the obtained one by the temperature difference from 40 to 70 to ºC. Later, the evaporated water fraction that retires every 15 minutes was determined. The sugar remnant amount is obtained by means of the relation of evaporated water and of the solution concentration based on the sampling times. By overall mass balance and based on the sugar remnant amount and the initial one, the FCM is obtained. This FCM is multiplied by a compensation of 2% (industrial parameter) corresponding to the sugar amount that is lost when unloading the crystallized solution, and at the centrifuging time (wastage). For the experimental concentration calculation, the ºBrix obtained in each sampling are used, relating them to the sugar grams by each 100 milliliter of water. The supersaturation calculation (S) is obtained by means of the concentrations difference (theoretical and experimental).

An electronic microscopy technique was used to make the particles analysis, having like support the software of images acquisition system called IMAQ Vision Builder by National Instruments, with the purpose of capturing the crystal image in every sampling time. Later, CSD was determined (diameters average and the standard deviation) on the basis of the distribution moments: % number, % length, % surface and % volume, by means of the DTC Adq-Im software by Instituto Tecnológico de Orizaba, which receives like input data the crystals diameters obtained by IMAQ system in pixels.

The results corresponding to the ?maximum frequency? dynamic regulation profile of vacuum pressure produce 1,128.70 g of evaporated water, a FCM of 4,910.38 g at the final batch time. With the °Brix measurement it was able to quantify and to give pursuit to the exhaustion of the sugar concentration in the batch available solution, from 3.1601 to 2.4364 g sugar/g water; this favored the formed crystal mass. At 15 minutes of process operation, it reached a supersaturation value of 0.038552 to finish with a decreasing behavior until 0.008635, as well as the relative supersaturation that it initiated in 0.012808 and it finished in 0.003557. This indicates that available supersaturation in the system has been exhausted in its majority. The error percentage between the experimental and theoretical FCM (overall mass balance) was of 0.922%, at the final batch time. For both ?cubic? and ?linear? dynamic regulation profiles of vacuum pressure with respect to the described profile as ?maximum frequency?, it determines that they produce minors amounts of evaporated water (717.65 g, and 842.41 g, respectively) and minor FCM at the final batch time (3,841.01 g and 4,155.68 g, respectively), because the adiabatic cooling in both cases are slower than in the ?maximum frequency? dynamic regulation profile of vacuum pressure. The error percentage between the experimental and theoretical FCM for the ?cubic? and ?linear? dynamic regulation profiles of vacuum pressure was of 2.17% and 0.31%, respectively, at the final batch time.

The results of CSD analysis for the ?maximum frequency? dynamic regulation profile of vacuum pressure produce a average crystal size of D(4,3) 732.72 µm (% volume) with standard deviation of S(4,3) 78.04 µm, being observed an almost linear growth of crystals during the process with variability in their size by hydrodynamic effects (agitation). The error between the base run and its repetition was of 6.07%. For the ?cubic? and ?linear? dynamic regulation profiles of vacuum pressure, it is obtained a D(4,3) of 567.79 and 585.18 μm with standard deviation of S(4,3) 64.07 and 69.44 μm, respectively. The error between the base runs and their repetitions were of 3.05% and 3.37%, respectively.

According to both CSD and FCM analysis, it is concluded that with the ?maximum frequency? dynamic regulation profile of vacuum pressure, it is obtained the biggest crystal size distribution (D(4,3) of 732,72403 µm and S(4,3) of 78,042381 µm) and maximum amount of formed crystal mass (4.910,3820 g) with respect to the described profiles as ?cubic ? and ?linear?, contributing to the exhaustion of the processing molasses.