Ice growth on the cooling surface in a jacketed and stirred eutectic freeze crystallizer of aqueous Na₂SO₄ solutions

Eutectic freeze crystallization (EFC) has been pronounced a promising separation technique to recover ice and salt simultaneously in an energy-efficient manner. Alike other freeze concentration methods, the accumulation of an ice layer, commonly known as ice-scaling, on the heat exchanger surface during operation thwarts the commercial application of EFC as the advantage of low energy requirement is outstripped by high investment cost, scaling up and operational complexities associated with the use of a scraper blade to remove the ice-scaling. Therefore, the aim of this research work is to investigate the ice-scaling phenomenon on a subcooled heat exchanger surface in the absence of mechanical scrapping. For a continuous EFC system, the influence of temperature driving force (ΔT) and the degree of agitation on the onset/induction time (t_ind) of ice-scaling are considered while keeping the other parameters, e.g., solution concentration (4 wt% Na₂SO₄ (aq) solution), the level of initial undercooling (0.28 °C) and the residence time (30 min) constant. The experimental results show that at a certain level of agitation, t_ind is inverse proportional to ΔT. At a low level of ΔT (i.e., <2.0 °C), the effect of the degree of agitation on t_ind declines. Contrary to this, at a high level of ΔT (>6.0 °C), the effect of the degree of agitation on t_ind becomes substantial. At the eutectic condition, the heat transfer coefficient of the solution (h_sol) in the crystallizer prevails over that of mass transfer (k_l). In the experimental setup, the overall heat transfer coefficient for a jacketed vessel type crystallizer is restricted by the heat transfer coefficient of the jacket side coolant (h_j). The power number (N_p) and pumping number (N_q) for a two-blade paddle type impeller and a crystallizer of specific dimensions and orientation are estimated by means of computational fluid dynamics (CFD) modeling and are used to calculate the specific power input (ε) and impeller pumping capacity (Q) at different levels of agitation. Furthermore, CFD simulations of heat transfer from the coolant to the bulk solution are performed to investigate the effect of the agitation level on solution cooling in the non-crystallizing condition.