Crystallization is an alternative separation and purification technique to
conventional chromatography for proteins due to specific characteristics, such as good
scalability, low cost, and the attainable stability of the crystalline products. However,
protein crystallization often suffers from long batch time due to the slow crystallization
kinetics. Furthermore, protein crystallization is conventionally conducted in the batch
mode of operation, which has potential disadvantages compared to continuous
operation. These challenges of protein crystallization are addressed in this thesis by
employing process intensification principles. The work presented in this thesis will focus
on two typical process intensification domains, i.e., the space and time domains
involving equipment structuriza...[
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Crystallization is an alternative separation and purification technique to
conventional chromatography for proteins due to specific characteristics, such as good
scalability, low cost, and the attainable stability of the crystalline products. However,
protein crystallization often suffers from long batch time due to the slow crystallization
kinetics. Furthermore, protein crystallization is conventionally conducted in the batch
mode of operation, which has potential disadvantages compared to continuous
operation. These challenges of protein crystallization are addressed in this thesis by
employing process intensification principles. The work presented in this thesis will focus
on two typical process intensification domains, i.e., the space and time domains
involving equipment structurization and continuous operation. 3D printing is used as a
tool to achieve process intensification through structurization, because of its ability for
rapid fabrication of customized process equipment with complex geometries. Novel
crystallizer types and process configurations are investigated with the aid of 3D printing
as the fabrication technique.
First, a workflow for the fabrication of crystallizers with 3D printing is presented.
The workflow aids in the selection of appropriate 3D printing techniques and suitable
materials for the fabrication of custom-designed crystallizer. Furthermore, the designs for
the crystallizers used in this thesis are developed based on models or heuristics. These
designs can be a platform or template for other crystallizers with geometric similarities
and can also be the start of a library of crystallizers that can be customized and fabricated
as required. 3D printing has found limited applications in the fabrication of equipment for separation processes, including crystallization, despite the extensive utilization in,
for example, reaction engineering. The workflow aims to facilitate the adoption of 3D
printing as a tool to achieve process intensification of crystallization processes through
structurization.
Second, the performance of a pneumatically agitated airlift crystallizer (ALC) is
characterized and compared to a mechanically agitated stirred tank crystallizer (STC)
for the crystallization of the model protein lysozyme. The desupersaturation profile is
substantially shorter in the ALC compared to STC for all tested conditions, which can
be an advantage for processes with slower crystallizer kinetics. The shorter
desupersaturation profile in ALC may be attributed to the gas-liquid surface of the
bubbles lowering the free energy barrier for nucleation. Furthermore, larger, and less
agglomerated crystals are obtained from the ALC. Additionally, the biological activity
of the product indicates that the functionality of the protein is preserved during the
process in the ALC. Protein crystallization in an ALC has not previously been reported.
Process intensification is achieved in this work through structurization by the adoption
of an ALC for protein crystallization resulting in higher throughput, along with larger
and unagglomerated crystals with high activity.
Third, the 3D printed ALC and STC are characterized for continuous crystallization of
lysozyme with a focus on the attainable solid-state forms from a mixed-suspension mixed-product-removal (MSMPR) crystallizer. The metastable needle-like form of lysozyme can
be obtained from continuous crystallization in MSMPR crystallizers for conditions at
which the stable tetragonal form is obtained from conventional batch crystallization. The
needle-like crystals appear after nucleation of the tetragonal form. An earlier transition
from the tetragonal form to the needle-like form is observed in ALC compared to STC,
which is possibly caused by reduced attrition and the presence of a gas-liquid interface
in the ALC. Furthermore, a long residence time and high precipitant concentration favor
the formation of the needle-like crystals. This work presents the first experimental study
on protein crystallization in an MSMPR crystallizer, which reveals a dramatic influence
of the mode of operation on the attainable solid-state form of a protein.
Finally, three 3D printed tubular crystallizers, a standard Kenics static mixer
(sKTC), a novel gapped Kenics static mixer (gKTC), and a hollow tube (HTC), are
designed and characterized numerically and experimentally. This work aims for process intensification through structurization by the adoption of a novel design for the static
mixer in a tubular crystallizer. Numerical results indicate a shorter mixing length can
be achieved in the sKTC and the gKTC compared to the HTC and, therefore, reduced
supersaturation gradients can be expected when applying the former crystallizers. The
residence time distribution (RTD) from the sKTC and gKTC is similar and closer to
plug flow compared to the HTC. A lower shear rate is present in the gKTC compared to
the sKTC, which can result in a lower secondary nucleation rate in the former.
Experimental characterization of the crystallizers shows that both the sKTC and the
gKTC can avoid segregation of the crystals due to gravity at relatively low flow rates
compared to the HTC, while a substantially lower pressure drop is observed in the
gKTC compared to the sKTC for the same flow rate. The crystallizers are characterized
for seeded crystallization of lysozyme, which indicates that the continuous re-orientation
of flow in the sKTC can promote settling of smaller crystals on the walls through
shear-induced migration of larger crystals towards the center of the tubular crystallizer.
This study shows that the novel gKTC outperforms the HTC in terms of mixing length,
RTD, and solid suspension and the sKTC in terms of pressure drop, solids suspension,
and lower shear while achieving the same RTD and mixing length, which has important
implications for practical applications of continuous protein crystallization.
Process intensification is achieved through structurization by employing the airlift
crystallizer and the static mixer-based tubular crystallizer and continuous operation by
MSMPR crystallizer and plug flow crystallizer. The work presented in this thesis can
provide a basis for the development of innovative concepts for process intensification
through structurization and continuous operation. These concepts include an integrated
multi-stage airlift crystallizer and a hybrid static mixer-based crystallizer. The workflow
developed in this work can aid in the fabrication of these innovative crystallizers with a
custom-designed geometry.
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