Synthesis of soft shell poly(styrene) colloids for filtration experiments

Separating a solid from a liquid is an important unit operation in
many different industries e.g. mining, chemical, pharmaceutical
and food industries. Solid liquid separation can roughly be
divided into three groups. 1) Separation by gravity forces e.g.
sedimentation, centrifugation, 2) Separation by evaporation of the
liquid e.g. drying and 3) separation by pressure forces e.g.
vacuum filtration, belt presses, pressure filtration. In this
Ph.D. thesis only dead-end pressure filtration dewatering is
considered. In pressure filtration dewatering the pressure is
forcing the liquid through a filter medium whereupon the solid is
deposited and whereby the liquid is separated from the solids.

Mathematical models have been developed to describe and predict
the solid liquid separation process during pressure filtration.
These mathematical models are mainly developed from experiments
performed on inorganic solids (e.g. titanium oxide, clay,
bentonite). It is often possible to apply these model in
predicting and scaling up the filtration dewatering process for
inorganic materials based on fundamental material parameters (e.g.
particle size, density) and/or based on laboratory experiments. It
has, however, been hard to find examples in literature where the
mathematical models have been used to predict or scaling up the
filtration dewatering process for organic materials (e.g.
activated sludge). The reason for this might be due to the complex
structures found in organic materials and a lack of knowledge in
how these structures influence the filtration dewatering
characteristic of organic materials. One of the structures that
have been identified to affect the filtration dewatering behavior
of activated sludge is the amount of extracellular polymer
substance (EPS). EPS is a water swollen material and it has been
shown that the more EPS the larger resistance to filtration.

It is difficult to access the influence of water swollen material
(as EPS) on the filtration dewatering of activated sludge by
performing filtration dewatering experiments on activated sludge.
A reason for this is that activated sludge is a physical and
chemical complex material that is biological active. It is
therefore difficult to extract the effect from one single material
parameter as the effect from the amount or type of water swollen
material from filtration dewatering experiments performed on
activated sludge. Substituting the complex organic material with a
simple non-living synthetic material in which it is possible to
vary one specific physio-chemical parameter might be a method to
obtain a better understanding of the processes and mechanisms
occurring during filtration dewatering of organic materials. In
the present study model colloids consisting of a solid
poly(styrene) (PS) core with a water swollen shell have been
employed in investigating the effect from varying amounts and type
of water swollen material on filtration dewatering properties.
Three series of model material have been used in this
investigation 1) poly(styrene-co-acrylic acid) core-shell
colloids with varying thickness of the poly(acrylic acid) (PAA)
shell. 2) poly(styrene-co-acrylic acid) core-shell
colloids with varying diameter of the PS core and 3)
poly(styrene-co-N-isopropylacrylamide)
core-shell colloids with varying thickness of the
poly(N-isopropylacrylamide) (NIPAM) shell. The first
series has been chosen because it has been argued in literature
that the amount of charges in the water swollen material has an
influence in the filtration dewatering properties of activated
sludge. The second series has been chosen to investigate the
effect of varying core size of the core-shell colloids on the
filtration dewatering behavior. The third and final series has
been chosen to investigate the effect from a non-ionic water
swollen material.

The model colloids from series one and two were synthesized by a
free-radicale surfactant-free emulsion co-polymerization process.
The produced colloids were characterized by titration, dynamic
light scattering (DLS), scanning electron microscopy (SEM) and
transmission electron microscopy. Results showed that it is
possible to synthesize core-shell colloids with increasing
thickness of PAA shell on a PS core by increasing the amount of
acrylic acid monomer added to the synthesis. Further it was shown
that it is possible to increase the PS core diameter by increasing
the ionic strength of the synthesis solution. The model colloids
from series three were synthesized by a two step free-radical
surfactant-free emulsion polymerization (SFEP) process. The first
step was a homo-SFEP of styrene and during the reaction NIPAM
monomer were added. The resulting colloids were analyzed by DLS
and SEM. Results showed that the resulting polymer had a core of
PS and a shell of poly(NIPAM).

The produced model materials were applied in filtration dewatering
experiments. The filtration dewatering experiments were made in a
piston cylinder based dead-end filtration cell with an applied
pressure in the range from 2 to 6 bar. Results from the filtration
dewatering experiments showed that the colloids were random close
packing in the filter cake after filtration dewatering independent
of colloids size, type and amount of shell material and applied
pressure. Results further showed that increasing the amount of
water swollen polymer on the surface of the model colloids gave an
increase in the average specific resistance to filtration
(α). The results also showed that this increase in
α is depending on the type (ionic or non-ionic) water
swollen shell material. It was also seen that changing the core
diameter has a relative small effect on α compared to
increasing the amount of water swollen shell material. Results
also showed that a significant amount of the water expelled during
the consolidation phase is expelled during secondary
consolidation. Based on the results obtained from the filtration
dewatering of the model colloids a conceptional model for the
interaction between the colloids in the filter cake during
filtration dewatering has been proposed.

This investigation showed that the amount of water swollen
material has a large impact on the filtration behavior and a
comparison between the results obtained from filtration dewatering
of series one and three showed that there is a significant effect
from the type of water swollen material. These results showed that
it is possible to apply synthetic model colloids as a tool to
access the effect from specific physio-chemical properties on
filtration dewatering of organic material.