Purification of aqueous electrolyte solutions by air-cooled natural freezing
Hasan, Mehdi (2016-10-19)
Väitöskirja
Hasan, Mehdi
19.10.2016
Lappeenranta University of Technology
Acta Universitatis Lappeenrantaensis
Julkaisun pysyvä osoite on
https://urn.fi/URN:ISBN:978-952-335-005-2
https://urn.fi/URN:ISBN:978-952-335-005-2
Tiivistelmä
Freeze crystallization is a particular type of a purification method where the solvent
freezes out, which constricts the volume of the solution, leaving thus behind a more
concentrated solution. In the case of freezing an aqueous solution, water is the solvent
which crystallizes and can be separated from the concentrated solution by the virtue of
buoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces ice
crystals that do not contain any of the impurities present in the original solution. As the
process continues, the original solution becomes more concentrated and the freezing
temperature declines progressively.
Freezing point depression (FPD) is of vital importance in characterising the freezing
behaviour of any solution. Due to this necessity, a new calculation method to predict FPD
is presented in this work. In this method, designated ion-interaction parameters for the
Pitzer model are extracted from reliable FPD data found in the literature, other than
calorimetric data. The extracted parameters from FPD data are capable of predicting the
freezing point more accurately than those resulted from the calorimetric data. The
calculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes.
Impurities in excess of the maximum recommended limits must be removed from
wastewater prior to discharge because of their persistent bio-accumulative and
detrimental nature. Natural freezing is suggested in the present work as a purification
technique to treat huge volumes of wastewater in a sustainable and energy-efficient
manner. The efficiency of freeze crystallization in the purification of wastewater by
imitating natural freezing in a developed winter simulation with the provision of altering
winter conditions is scrutinized in this thesis. Hence, natural freezing is simulated
experimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4
solutions to assess the feasibility of such a technique to be used to purify wastewaters
containing electrolytes. This work presents a series of data in similitude of natural
freezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrations
and freezing conditions. The influence of solution concentration and different freezing
conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distribution
coefficient (K) of the solute between ice and the solution. The experimental results
demonstrate clearly that high purity ice can be obtained from slow freezing of the solution
with the concentration typically found in industrial wastewater.
During freeze crystallization, the diffusion of impurities from the solid-liquid interface to
the bulk of the solution, along with the growth mechanism of the solid phase play an
important role in determining the purity of the ice layer. Therefore, a calculation method
is introduced to estimate the concentration of the solution at the advancing ice–solution
interface in terms of the limiting distribution coefficient (K*) from experimental K values
at different growth conditions. The heat transfer -controlled growth rate of the ice limited
by the free convective heat transfer coefficient of air (hair) rather than the thermal
conductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) was
found to prevail over the mass transfer of rejected solute molecules from the ice–solution interface to the bulk solution of experimental interest. A simplified and robust model is developed to estimate the thickness and growth rate of the ice layer formed from solutions at different freezing conditions, and the model is validated with experimental results. In addition, inclusion formation within the ice matrix during freezing is investigated for various solution concentrations, both macroscopically and microscopically.
freezes out, which constricts the volume of the solution, leaving thus behind a more
concentrated solution. In the case of freezing an aqueous solution, water is the solvent
which crystallizes and can be separated from the concentrated solution by the virtue of
buoyancy. In an ideal situation, freeze crystallization of an aqueous solution produces ice
crystals that do not contain any of the impurities present in the original solution. As the
process continues, the original solution becomes more concentrated and the freezing
temperature declines progressively.
Freezing point depression (FPD) is of vital importance in characterising the freezing
behaviour of any solution. Due to this necessity, a new calculation method to predict FPD
is presented in this work. In this method, designated ion-interaction parameters for the
Pitzer model are extracted from reliable FPD data found in the literature, other than
calorimetric data. The extracted parameters from FPD data are capable of predicting the
freezing point more accurately than those resulted from the calorimetric data. The
calculation method is exemplified for numerous 1-1 and 1-2 types of electrolytes.
Impurities in excess of the maximum recommended limits must be removed from
wastewater prior to discharge because of their persistent bio-accumulative and
detrimental nature. Natural freezing is suggested in the present work as a purification
technique to treat huge volumes of wastewater in a sustainable and energy-efficient
manner. The efficiency of freeze crystallization in the purification of wastewater by
imitating natural freezing in a developed winter simulation with the provision of altering
winter conditions is scrutinized in this thesis. Hence, natural freezing is simulated
experimentally for ice crystallization from unsaturated aqueous Na2SO4 and NiSO4
solutions to assess the feasibility of such a technique to be used to purify wastewaters
containing electrolytes. This work presents a series of data in similitude of natural
freezing of water from aqueous Na2SO4 and NiSO4 solutions in various concentrations
and freezing conditions. The influence of solution concentration and different freezing
conditions, such as ambient temperature, freezing time and freezing rate, on the efficiency of the purification process is investigated by analysing the effective distribution
coefficient (K) of the solute between ice and the solution. The experimental results
demonstrate clearly that high purity ice can be obtained from slow freezing of the solution
with the concentration typically found in industrial wastewater.
During freeze crystallization, the diffusion of impurities from the solid-liquid interface to
the bulk of the solution, along with the growth mechanism of the solid phase play an
important role in determining the purity of the ice layer. Therefore, a calculation method
is introduced to estimate the concentration of the solution at the advancing ice–solution
interface in terms of the limiting distribution coefficient (K*) from experimental K values
at different growth conditions. The heat transfer -controlled growth rate of the ice limited
by the free convective heat transfer coefficient of air (hair) rather than the thermal
conductivity of the ice (kice) and the heat transfer coefficient of the solution (hsol) was
found to prevail over the mass transfer of rejected solute molecules from the ice–solution interface to the bulk solution of experimental interest. A simplified and robust model is developed to estimate the thickness and growth rate of the ice layer formed from solutions at different freezing conditions, and the model is validated with experimental results. In addition, inclusion formation within the ice matrix during freezing is investigated for various solution concentrations, both macroscopically and microscopically.
Kokoelmat
- Väitöskirjat [1037]