Investigation, mathematical modelling and simulation of the crystallisation process in a circulation crystalliser
Advancing the chemical engineering fundamentals
Crystallization (T2-9)
Keywords: attrition mechanism, growth mechanism, crystallization, mathematical model, ammonium sulphate
The industrial crystallisation of well soluble substances is often dominated by secondary nucleation, in particular contact nucleation, and crystal growth. In different crystallisers types the attrition is mainly caused by the crystals impact onto stirrer blades, pumps, etc. The ultimate objective is to quantitatively describe a crystallization process, which is governed by the formation and growth of attrition fragments.
For the investigation of attrition and growth mechanisms a number of experiments have been carried out with the crystallizing system ammonium sulphate in the lab-scale. They have been carried out attrition experiments for different seed crystals in size under variation of the volumetric flow and, as a result, under variation of the impingement velocity of the crystals, and crystallization experiments under variation of volumetric flow, nozzle diameter and feed rate. All experiments have been lead in a continuously operated 5.85-liter-forced circulation crystalliser (FC-crystalliser), which uses a particularly gentle pump to circulate the suspension at minimal attrition of crystals. The attrition behavior of crystals has been investigated by imposing to them additional mechanical stress with the help of an impingement device implemented in the slurry loop. This device consists of a nozzle and an orthogonal positioned cylinder. The formation of attrition fragments was preceded by crystals collides with this cylinder.
The attrition process and the crystallization process were quantitatively described. While mathematical modelling each of the processes was subdivided in separate zones. The process only with crystals attrition was subdivided in only one zone – the attrition. The crystallization process with crystals attrition and their subsequent growth was subdivided in mixing, cooling, attrition, growth (crystallizer) and sampling zones. Mass, energy and population balances have been presented for each zone. Necessary model parameters have been determined in the process of experimenting. Equation of particle number conservation for the attrition zone, growth rate equation and growth parameters of the ammonium sulphates crystals have been selected in the process of calculations. For the calculation of these processes a Delphi 7.0 program has been developed. Median crystals size, supersatutation and particle sizes distribution have been calculated for all experiments conditions and compared with the experimental findings.
The mathematical model of the industrial crystallization process in a FC-crystalliser has been designed, because the purpose of the modeling is an opportunity to predict the median crystals size and the particle size distribution. Calculations for different frequency of an axial pumps propeller have been done. With the help of these calculations optimum conditions of the process have been selected for a product of certain quality.
This work was supported by grants and PRO3 e. V. (Pro3 process engineering expertise network).
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Presented Wednesday 19, 15:20 to 15:40, in session Crystallization (T2-9).
