Introduction
Crystallization is an extremely important separation unit operation,
which is used in the production of highly specialized fine solid
products1-3. Cooling
crystallization, one of classic crystallization process, is mostly
controlled by adding seed crystals within the metastable zone to induce
nucleation 4-7.
Successful artificial seeding operation depends on a lot of aspects,
seed size and amount, time point and place of addition, and experience
of the operator, etc. However, conventional cooling crystallization that
requires external seed adding procedure are suffered the risk of the
secondary nucleation, which will limit the crystal purity, morphology
and particle size
distribution11,12,
etc.
Recently, researchers are working to select and add seed by using
molecular sieves or additional physical fields (electric, magnetic,
etc.)8-11. However, none
of these methods can achieve the automate seeding with accurate
temperature control. In addition, nucleation induced via seeding
crystals and crystal growth kinetics in the crystallizer determine the
essential solid product quality and fundamentally influence further
downstream processing (solid-liquid separation, drying, etc.).
Decoupling the competition between nucleation and crystal growth from
space and time aspect simultaneously is the core concern for all the
researcher.
As a highly designable and environmentally friendly material, membrane
obtains a great development in many fields related to crystallization
process12-15. One of
the most impressive applications is the membrane served as a
heterogeneous nucleation interface to trigger the nucleation process16,17.
When the membrane unit concentrating the solution via selectively mass
transfer of the solvent, the supersaturated solution on the membrane
surface become nucleation and the formed particles can auto-detach from
the surface under proper dynamic force field18,19.
This finding is of importance for the accurate control of the mass
transfer related crystallization process (evaporative crystallization,
antisolvent crystallization, e.g.).
Inspiring transfer features of hollow fiber membrane also shed light on
the heat exchange and the relevant process control20. In addition, with
the high packing density, hollow fiber membrane module ensures the high
manufacture capacity for potential industrial applications21-23. The total heat
transfer coefficient of the hollow fiber membrane module can be as high
as 2000 W/(m2·K); the ratio between the heat transfer
coefficient of hollow fiber membrane heat exchangers verse the volume
was 2 to 15 times higher than that of commercial metal heat exchangers,
showing impressive application
advantages24-26.
Moreover, the potential advantage of hollow fiber membrane module on the
accurate heat exchange and temperature distribution is an interesting
topic. All the above properties of the hollow fiber membrane can
potentially benefit the cooling crystallization control via heterogenous
nucleation and high heat transfer efficiency, which had not fully
unfolded and in-depth investigated.
Thiourea, a fundamental chemicals in many industrial
fields27-31, was
commonly manufactured via cooling crystallization. Nowadays, high-purity
thiourea plays more and more irreplaceable role as the electrocatalytic
materials and battery
materials32,33,
which raised the urgent requirement on accurate nucleation and growth
control of its cooling crystallization
process34,35.
In this work, we proposed a new cooling crystallization control
mechanism to prepare high-purity thiourea crystals via introducing the
polymeric hollow fiber membrane module. The membrane module is
functioning as the key devices for inducing the nucleation and
self-seeding. Poly-tetrafluoroethylene (PTFE) and poly-ethersulfone
(PES) hollow fiber membrane were investigated on their thermal
conductivities and inducing nucleation properties. In this
membrane-assisted cooling crystallization (MACC), the surface induced
nucleation and accurate self-seeding will be validated from theoretical
and experimental aspects. The feasible accurate and automatic control
MACC process was then compared with the conventional cooling
crystallization (with seeding and none seeding) in terms of thiourea
crystal purity, morphology and crystal size distribution (CSD) to fully
reveal its advantages in nucleation and crystal growth control.