Journal article

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


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Publication Details

Author list: Hasan M, Filimonov R, Chivavava J, Sorvari J, Louhi-Kultanen M, Lewis AE.

Publisher: Elsevier

Publication year: 2017

Journal: Separation and Purification Technology

Journal name: SEPARATION AND PURIFICATION TECHNOLOGY

Volume number: 175

Start page: 512

End page: 526

Total number of pages: 15

ISSN: 1383-5866


Abstract

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 (tind) 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, tindis inverse proportional to ΔT. At a low level of ΔT (i.e., <2.0 °C), the effect of the degree of agitation on tinddeclines. Contrary to this, at a high level of ΔT (>6.0 °C), the effect of the degree of agitation on tindbecomes substantial. At the eutectic condition, the heat transfer coefficient of the solution (hsol) in the crystallizer prevails over that of mass transfer (kl).
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 (hj). The power number (Np) and pumping number (Nq)
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.


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Last updated on 2018-14-09 at 15:49