IWNET12
IWNET12
Coupled heat and mass transfer during crystallization
Marcos Rodriguez Pascual1, Fatma Elif Genceli Güner2 and Signe Kjelstrup22,3
1 University of Cape Town, Department of Chemical Engineering - Crystallization and Precipitation Research
Unit, Cape Town, South Africa
2 Technical University of Delft, Faculty Mechanical, Maritime and Materials Engineering - Process and
Energy Department, Delft, Netherlands
3 Norwegian University of Science and Technology, Department of Chemistry - Faculty of Natural Science and
Technology, Trondheim, Norway
Abstract
Crystallization takes place and is a key factor in a large variety of industrial and chemical processes such as
pharmaceutical production, food processing, or petrochemical production. Control of crystal growth mechanisms
is essential in order to obtain the desired product purity and quality. Crystallization is a pure thermodynamic
process where coupled heat and mass transfer depend on the local supersaturation, i.e. the responsible driving
force for crystal growth. Supersaturation depends on the local temperature and concentration. Therefore,
in order for the crystal surface to develop, the surrounding liquid interface must be supersaturated. In the
case of exothermal crystallization these conditions mean lower temperature and higher concentration than the
equilibrium temperature and concentration values. An exothermal crystallization process occurs because the
system changes to a lower energy state liberating this excess of energy to the surroundings. This heat release
during crystallization creates an increase of temperature on the local front of the growing crystal aecting the
local supersaturation, density and viscosity, thus crystal growth and heat diusion. On the other hand, these
variables are coupled to the mass transfer, as these changes directly aect mass diusion and therefore the local
concentration gradients. Detailed knowledge of these coupling eects and their impact on development of the
interface is necessary to control growth rates, impurity uptake, morphology and crystal structure and defects.
To achieve such knowledge, in situ measurements of the developing crystal solid-liquid interface were under-
taken. In order to obtain the local temperature eld uctuations surrounding a growing crystal, Liquid Crystal
Thermometry was used and simultaneous image data was acquired, documenting the growth rate of the crys-
tal in a temperature gradient. The ux-force relations were described using non-equilibrium thermodynamic
theory for the heterogeneous system, and the corresponding transport coecients were determined (diusion
coecients and heat of transfer).
The experimental setup consisted of a cubic quartz cell with a Thermochromic Liquid Crystal sheet in the
middle. On the squared bottom of the cell was a square, at heat exchanger surface and on the top of the cell
two orices which allow introduction or extraction of solution and possible volume expansion. The whole cell
was thermally insulated from the exterior by a double wall. The gap was under vacuum or lled by argon gas.
The Thermochromic Liquid Crystal sheet was illuminated by a cold white light source and pictured with a 3
Mega pixel digital color camera. Images were post-processed to obtain temperature elds and corresponding
growth rates. Experiments are reported on crystallization of ice in dierent aqueous solution concentrations
keeping the same heat exchanger surface temperature. The results showed a direct relation between dierent
crystal growth rates, morphology and structure of ice and development and thickness of the thermal boundary
layer interface surrounding the crystal growing surface. This can be explained by coupling of mass and heat
uxes.
E-mail: marcos.rodriguezpascual@uct.ac.za

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