224 research outputs found
Technologies for Optimisation and Control of Nucleation and Growth for New Generations of Industrial Crystallizers
No full textThe scope of this thesis is to study new methods for control of the most important phenomena in crystallization processes: nucleation and crystal growth. In order to achieve this, novel process equipment, which utilize alternative driving forces, were developed. It was proven that primary nucleation for solution crystallization can be controlled using laser irradiation. The relationship between creation, expansion and collapse of a vapor cavity induced by a laser pulse and the subsequent nucleation of crystals was both experimentally and theoretically investigated. Regarding crystal growth, it was shown that it can be controlled in an airlift crystallizer, in which, under the right conditions, growth can become the dominant crystallization mechanism, suppressing almost completely secondary nucleation. This air-mixed crystallizer enables the production of crystals with high quality and offers a large flexibility of the final crystal size. The research presented in this thesis brings new alternatives for control of nucleation and growth of the crystals and thus new methods for flexible process operation and enhanced product quality in industrial crystallization processes.Process & EnergyMechanical, Maritime and Materials Engineerin
Hydrothermal Synthesis and Characterization of 3R Polytypes of Mg-Al Layered Double Hydroxides
No full textLayered Double Hydroxides (LDH) is a unique group of clays that have an anionic exchange capability. This research explored the hydrothermal method as an alternative method to synthesize Mg-Al LDH. It is a simple and more environmentally friendly compared to the conventional method of co-precipitation. Furthermore, depending on the synthesis condition, two different polytypes, namely 3R1 and 3R2 can be synthesized. The first part of the research was focused on the optimization of the hydrothermal synthesis. Various pre-treatment techniques of the reactants were investigated. The use of a microwave system as an alternative energy source resulted in the formation of a unique donut-shaped crystal which provides enlargement of the specific surface area of the {hk0} faces, needed for adsorption application. The growth mechanism of such donut-like crystals is studied by AFM as well as by STEM-EDX. The interrelation of polytype 3R1 and 3R2 along with the chemical composition and structure of polytype 3R2 is addressed in the second part. The transition temperature is approximately at 110 ºC with 3R1 being stabile at lower temperatures and 3R2 at higher temperatures. Polytype 3R2 was also found to have more aluminum content compared to 3R1. The excess aluminium is the presence as tetrahedrally coordinated aluminate ion located in the interlayer as charge compensation. The apical oxygen of the aluminate is grafted onto the octahedral metal layer, inducing the formation of 3R2 stacking. This grafted structure might explain the reluctance of polytype 3R2 to be ion exchanged compared to 3R1.Process and EnergyMechanical, Maritime and Materials Engineerin
Monitoring and Characterization of Crystal Nucleation and Growth during Batch Crystallization
No full textBatch crystallization is commonly used in pharmaceutical, agrochemical, specialty and fine chemicals industry. The advantages of batch crystallization lie in its ease of operation and the relatively simple equipment that can be used. On the other hand a major disadvantage associated with it is the inconsistent and usually poor product quality. Quality of the crystalline product, which is defined in terms of the Crystal Size Distribution (CSD), purity, kind of solid state etc., is related to its performance when used as an ingredient during subsequent processes. Also the quality of the product from batch crystallization process has a strong influence on the efficiency of downstream operations like filtration and drying. Hence it is essential to reduce the batch-to-batch variations in the product quality. In this thesis three basic requirements for achieving consistent product quality have been identified. These requirements are a.) strong domain knowledge, b.) proper means of characterizing crystallization phenomena, c.) adequate process monitoring capabilities. The results presented in this thesis help in meeting the above requirements and are summarized below. a. Crystallization domain knowledge: Three important results related to the Metastable Zone Width (MSZW) have been obtained in this thesis which cannot be explained by the conventional understanding. It has been shown in this thesis that i. MSZW is not a deterministic property ii. MSZW is volume dependent iii. There exists a relationship between MSZWs measured at different volumes under similar conditions. The MSZW measurements at small volumes of 1 mL show large variations while the variations in the measurements reduce as the volume is increased. The extent of variations in the MSZW measurements at a particular volume changes from one model system to the other. The smallest measured MSZW at all volumes between 1 mililitre and 1 litre is the same. The dramatic deviation from the conventional understanding of the measured MSZWs is a result of inadequate understanding of the nucleation process. Conventionally, a multiple nuclei mechanism is assumed in which large number of nuclei are born together in a very short time interval. However in this thesis evidence is presented for a mechanism in which only a single nucleus is formed initially in a supersaturated solution which grows into a single crystal. After growth to a certain size, this single crystal undergoes extensive secondary nucleation which results into multiple crystalline fragments. The newly postulated mechanism is called the Single Nucleus Mechanism. All the crystals produced in an unseeded batch crystallization therefore originate from a single primary nucleus by secondary nucleation. This indicates that during an unseeded industrial batch crystallization process, there will be different generations of crystals present. Hence, in order to achieve crystals with desirable quality, control strategies must be focused on controlling both primary and secondary nucleation. b. Crystallization characterization: In this thesis novel methods to characterize crystal nucleation, growth and MSZW have been developed. The characterization of crystal nucleation and MSZW is done with the help of a stochastic model developed based on the Single Nucleus Mechanism. The stochastic model indicates that the nucleation rate is several orders of magnitude smaller than that postulated by the Classical Nucleation Theory. The low nucleation rate leads to the stochastic MSZWs. Unlike the conventional population balance model which shows that the MSZW is independent of volume, the stochastic model indicates that the MSZW is a function of volume. The stochastic model also enables scale dependent study of the MSZW. The characterization of crystal growth is performed by the combination of information from both the concentration measurement sensor and the crystal size distribution (CSD) measurement sensor. It is shown that by combining of the concentration and CSD measurements a better parameter estimation and better process description could be achieved. c. Crystallization process monitoring: In this thesis in situ measurement of several process variables has been successfully demonstrated not only at lab scale but also at industrial scale. A comparison has been performed between two spectroscopy based techniques viz. attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and Fourier transform near infrared spectroscopy (FT-NIR) for in situ concentration monitoring during crystallization at lab scale. Based on the comparison, ATR-FTIR is found to be more accurate than FT-NIR for different model systems. In spite of accurate concentration monitoring at lab scale, the concentration monitoring with ATR-FTIR leads to biased measurements at industrial scale due to the differences in the curvature of fiber optics. To facilitate the in situ concentration measurements in industrial environment, two calibration procedures have been investigated which circumvent problems associated with calibration transfer from lab to industrial scale. In the first procedure data from a cheap ultrasound based concentration probe is combined with the spectra from ATR-FTIR spectroscope. It is shown that this combination of data enables a rapid calibration of ATR-FTIR at industrial scale. In the second procedure, multiple Process Analytical Technology (PAT) tools that were arranged in a measurement skid were calibrated simultaneously at industrial scale. The skid configuration of the PAT tools allows for the combination of the calibration procedure with process characterization. The monitoring of the process at industrial scale with multiple sensors brings new process insights which can lead to better process control and optimization strategies. The results presented in this thesis will enable achievement of consistent product quality by facilitating efficient process and equipment design, process development, and process control.Process and EnergyMechanical, Maritime and Materials Engineerin
Intensified Protein Structuring for more sustainable foods: Development of the up-scaled Couette Cell for the production of meat replacers
No full textTo meet the increasing need for protein-rich food of an ever growing population, plant-based proteins are being utilized in meat products as replacements for animal-based proteins. Legumes such as soy can serve as an alternative protein source, by featuring both high protein content (36%) and protein functionality (gelation). Nowadays various meat replacement products are commercially available and thus more and more customers are willing to switch their diet to a vegetable-based one. Currently, the most efficient technology for the production of meat replacers is extrusion cooking and new methods of protein structuring (Shear Cell and Couette Cell) have only recently been introduced. These two new technologies were developed based on the principle of applying simple shear flow and heat in the protein mixture. Initially, a device called the Shear Cell was developed featuring a cone-cone design that could structure soy-based mixtures in meat-like products. However, since the Shear Cell design is limited to lab use only, a new technology was developed and presented in this thesis. The Couette Cell concept, which is based on the concentric cylinder principle, has been studied, since it allows for further upscaling at industrially relevant production volumes. The research starts with a proof of concept study by using the lab scaled Couette Cell, which features a volume of 0.14 L and a shearing zone gap size of 5 mm, between the two cylinders (Chapter 2). Applying simple shear and heat at varying process conditions (temperature, time and rotation rate) to a soy-based mixture, has yielded anisotropic structures that resembled meat. In particular, fibrous structures were favoured at temperatures between 90 and 100 °C. The fibrous products with the highest anisotropy indices were further examined and characterized with a set of complementary techniques (Chapter 3). With light microscopy we could observe structure formation over the visible surfaces of the specimens and by using a stain we could distinguish between the different ingredients. According to the texture analysis results, the anisotropy indices of the obtained meat replacer and raw meat (beef) are comparable. We introduced the use of neutron refraction method by utilizing spin-echo small angle neutron scattering (SESANS) to provide a look inside the bulk of the anisotropic meat replacer. It was therefore possible to quantify the number of fibre layers and the orientation distribution of the fibres present inside the specimens. The calculated fibre thickness was in line with the observations obtained with scanning electron microscopy (SEM). Since the Couette Cell concept proved successful and enabled scalable operation, we developed a new up-scaled Couette Cell, which can treat 7 L per batch, 50 times more than the lab-scaled Couette Cell. The detailed design of the up-scaled Couette Cell is discussed in Chapter 4. The up-scaled device allows for production of fibrous meat replacers at industrially relevant scales and opens the possibility of commercial production in an emerging market. The device is comprised of two concentric cylinders with the inner cylinder rotating while both are being heated by means of steam. The unique characteristic feature of the up-scaled Couette Cell is its 30 mm gap size, which is 6 times more than the lab-scaled counterpart. Finally, a parametric study was used to find the optimum process conditions between the process time and rotation rate while maintaining a constant temperature (Chapter 5). This study yielded highly fibrous structures with a characteristic 30 mm thickness, which emulates meat accurately. The Couette Cell concept and the flexibility in its design allow production of meat replacers at proportions currently not available. Additionally, no barriers were found for further upscaling this concept by preferably designing a continuous process.Process and EnergyMechanical, Maritime and Materials Engineerin
Microwave effects in heterogeneous catalysis: Application to gas-solid reactions for hydrogen production
No full textDue to the quest for more efficient production processes both from the energy and selectivity point of view, microwave irradiation has attracted significant scientific attention over the last three decades, as an alternative means of chemical activation. Over this period, striking process benefits, such as higher conversions and selectivities and/or a shorter reaction times, compared to the respective conventionally heated processes have been reported. The aim of this work is to investigate the influence of microwave energy on heterogeneous gas-solid catalytic reactions. As example process, the steam reforming of methanol and the water-gas shift reaction were selected. In a first step, the interaction of microwaves with different catalysts was investigated in a non-reactive environment, followed by investigation of the microwave effects on the reactions themselves. Comparison of the microwave- and electrically heated processes was performed in terms of conversion, selectivity and energy efficiency of the reactor. Contrary to other works in the literature, a two-dimensional temperature map along the centre plane of the reactor was recorded with both heating modes. The study of interaction of microwaves with the solid state catalysts revealed that the heating rate, the maximum temperature at constant power, and the heat distribution inside the catalyst bed strongly depend on the catalytic support morphology, the metal loading and the particle size of the catalyst. Moreover, the experiments proved that even small catalytic samples (~2g) experience non-uniform heat distribution inside their volume when exposed to a well-defined, mono-mode type of microwave field. These temperature gradients, although sometimes being severe, are undetectable by the commonly employed in microwave chemistry infra-red temperature sensors. These types of sensors are often built-in in the microwave applicator and serve as benchmark for the power control unit, which adjusts the power delivered into the microwave cavity. Therefore, a fibre optic based direct temperature measurement was selected as more accurate method in further stages of the research. The investigation of methanol steam reforming reaction was performed with employment of two catalysts - PdZnO/Al2O3 and CuZnO/Al2O3 – at an average reaction temperature ranging between 190oC – 250oC and 170oC – 230oC, respectively. In order to account for possible temperature gradients occurring across the catalytic bed, multi-point temperature mapping was implemented. The experiments revealed that at corresponding thermal conditions, the feed conversion in the microwave-heated process is significantly higher than in the electrically-heated process, regardless of the employed catalyst. However, the product distribution remained unaffected. Comparison of the reactor energy efficiency demonstrated that the MW-assisted process exhibits higher reactor energy efficiency than the corresponding electrically heated process for both catalysts and over the range of the studied reaction temperatures. This entails that a given conversion can be achieved with lower net heat input to the reactor under the microwave heating mode and thus indirectly confirms the selective microwave heating principle (microscale hot spot formation). Pre-conditioning of the catalyst in the presence of the microwave field prior to the reaction did not affect the reaction performance. The catalyst surface investigation showed no difference in the morphology of the catalyst used either between the microwave and the conventionally heated process, or between the preconditioned and the non-preconditioned samples. Consequently, specific non-thermal microwave effects were excluded as justification for the enhancement of the reactor performance. In order to confirm the thermal nature of the microwave effects observed in the methanol steam reforming reaction, a mildly exothermic process, a water-gas shift reaction, was investigated at the latest stage of the research. Contrary to steam reforming, the water-gas shift reaction did not exhibit significant enhancement neither in terms of conversion nor in terms of reactor energy efficiency. This is because a significant part of the net heat input to the reactor comes from the heat of reaction; therefore, the heat input from microwave irradiation and the associated local overheating of active sites diminishes. Consequently, the microwave effect is not pronounced. Based on the experimental experience obtained and the theoretical knowledge regarding the shortcomings of the available microwave types of equipment, an alternative reactor design, based on travelling microwave fields, is proposed for application to heterogeneous gas-solid catalytic reactions. The new concept may enable uniform spatial heating, improved electromagnetic energy utilization and electromagnetic field spatial localization (i.e. on the catalytic reactor walls).Process and EnergyMechanical, Maritime and Materials Engineerin
Microwave Enhanced Reactive Distillation
No full textThe application of electromagnetic irradiation in form of microwaves (MW) has gathered the attention of the scientific community in recent years. MW used as an alternative energy source for chemical syntheses (microwave chemistry) can provide clear advantages over conventional heating methods in terms of reaction time, yield and selectivity. Several applications using this technology have been proven effective in diverse scientific fields. In this thesis, the scope of microwave chemistry was further expanded to a reactive distillation (RD) process with the primary objective to evaluate its use in view of possible process intensification (PI). The ultimate goal was to conceptually address the novel concept of a MW enhanced RD process (MWeRD) based on demonstrated effects in partial processes namely; molecular separation and chemical reaction. The thesis is divided in four main parts, each of them covering different aspects of the research. Part I, comprises Chapters 1, 2 and 3, giving the introductory guideline and the basic data neededTo proof the concept, the synthesis of n-propyl propionate was chosen as case system. The thermo-physical data required to accurately address RD design and operation, and the dielectric properties relevant for MW dielectric heating were experimentally determined. The thermodynamic behavior of the system was accurately predicted using a fitted UNIQUAC-HOC model, while experimental reaction kinetics data were used to fit parameters of a pseudo-homogenous model. Both models were used to build the residue curve maps, needed to determine process feasibility. Experiments performed in a conventionally heated pilot-scale column (DN-50) equipped with two types of structured packings (Sulzer BX and Katapak-SP 11) are reported. In addition, a non-equilibrium stage model (NEQ model) for the column was implemented. Model predictions were compared to experimental results showing good accuracy. Theoretical investigations of the most important operating parameters (total feed, molar feed ratio, reflux ratio and heat duty) and their effect on the overall process performance are presented. The fundamental research performed with MW was divided in two parts. First, the influence of MW on molecular separation of the binary mixtures composing the quaternary case system is discussed based on experimental results. Four binary pairs were studied showing, in some cases, an enhanced separation of the components. Then, the effects of MW radiation on the case reaction were studied comparing reaction conditions under MW and conventional heating using different homogenous and heterogeneous catalysts. From all the catalysts tested, Zn triflate proved to be more effective under microwave heating producing 40% more ester compared to the conventionally heated experiment. Finally, the general benefits and barriers of the technology integration are discussed based on the results of the MW enhanced reaction and separation. The novel concept of a MWeRD process is presented, giving recommendations for further research in terms of hardware, operating conditions and up-scalability of the process.Process & EnergyMechanical, Maritime and Materials Engineerin
Snooker, not pinball
No full textIn 2011, Andrzej Stankiewicz, professor of process intensification (3mE Faculty), received a grant of 2.3 million euros from the European Research Council to conduct research into the improvement of chemical reactors ‘at molecular level’. Last September, Stankiewicz’s ‘Perfect Reactors Lab’ opened its doors. One type of microreactor (1) is already known: a glass plate through which molecules can flow through micro- or nano-channels. But how can one perfect a microreactor and how does it work?Process EnergyMechanical, Maritime and Materials Engineerin
Crystal Nucleation and Polymorph Control: Self-association, Template Nucleation, Liquid?Liquid, phase Separation
No full textCrystallization is an essential step in many processes in chemical industries, ranging from bulk chemicals to special products. It is a separation and purification technique that results in a solid particulate product, which is generally preferred in the pharmaceutical industry. The crystal product quality is determined by the specific crystal form (polymorph) crystallized, and by the crystal size, morphology and purity. It depends heavily on the process conditions under which crystal nucleation occurs. During the crystal nucleation event, parameters that are essential for the product quality, in particular the polymorph formed, are not very well established. The nucleation event is still poorly understood and is therefore difficult to control and optimize. Crystal nucleation sets the initial crystal size distribution at the start of unseeded batch crystallization processes and is the first and most important step in this process (Chapter 1). A fundamental understanding of crystal nucleation is needed for the rigorous control and prediction of the crystalline product quality of any crystallization processes on industrial scale. Also the lead compounds in pharmaceutical industry become more and more complex. As a result crystallization research becomes increasingly fundamental while model compounds have shifted from bulk chemicals to high added-value chemicals. A fundamental understanding of molecular processes during crystallization is becoming increasingly important not in the least when applied on an industrial scale. In this thesis we improve the knowledge and understanding of crystal nucleation of organic compounds from solution. The research starts with comparing two newly developed methods to measure heterogeneous nucleation kinetics by determining crystal nucleation rates in stirred solutions (Chapter 2). Both methods make use of the stochastic nature of crystal nucleation by determining and analysing the variation in nucleation kinetic measurements. The values of the kinetic parameter (A) obtained in the present thesis are low compared to the theoretical values. This could be due to both a lower than expected attachment frequency of building units to the nucleus and a lower than expected concentration of active nucleation sites (heterogeneous particles) in the solution. It was further identified that the single nucleus mechanism, in which all crystals in the suspension originate from the same parent single crystal, might occur more generally than is currently recognized, even in larger volumes. In this thesis we used concomitant polymorphism as a tool to validate this single nucleus mechanism (Chapter 3). The single nucleus mechanism has important implications for the control of industrial crystallization processes of polymorphic compounds. In terms of crystal size distribution, control can be obtained by controlling the secondary rather than the primary nucleation event for which completely different control procedures are needed. In terms of polymorphism, the control can be achieved by controlling the primary nucleation event that leads to the single crystal, which in turn defines the crystal form of the secondary nuclei. One of the major challenges the pharmaceutical industry is faced in production, where often during cooling crystallization the product separates not as crystals but as a viscous liquid. This phenomenon is referred to as oiling out or liquid-liquid phase separation (LLPS). The effect of LLPS on the crystallization of 4-hydroxyacetophenon (4HAP) in water, water-ethanol mixtures and ethyl acetate solutions were shown in Chapter 4. For HAP, the LLPS is a stable region above the saturation temperature of 52 °C, 36 °C and 30 °C of 4HAP in water, water-ethanol (90-10 wt%) and water-ethanol (80-20 wt%) mixtures, respectively. Cooling crystallization experiments always resulted into mixtures of polymorphic fractions if the LLPS preceded 4HAP crystallization. The results suggest that the crystallization behavior is strongly influenced by the presence of this LLPS. Due to the LLPS the nucleation may proceed on the droplet surface and the single nucleus mechanism does not hold anymore. The crystallization within the LLPS region seems to lead to agglomeration of the particles. One of the causes for the low kinetic parameter A of the nucleation rate equation was identified in Chapter 3 to be the building units that attach to the nucleus and thus determine the attachment frequency. The effect of these solution-building units or associates in solution was investigated in Chapter 5. The model compound used was isonicotinamide, which has an amide group that can form both homosynthons and heterosynthons by self-association. We show that, in a controlled and reproducible way, specific solvents lead to specific polymorphic forms of isonicotinamide. We argue on the basis of Raman and FTIR spectroscopy that the hydrogen bonding (self-association) in solution kinetically drives the nucleation towards a specific form. The self-association in solution reflects the crystal structure of the obtained polymorph. The method based on self-association of molecules in solution may help in reproducible production of polymorphs. In chapter 6 we propose a polymorph screening method based on the identification of crystal building units using Raman, FTIR and NMR techniques. We demonstrated this new approach by relating the structural outcome of the crystallization process of Isonicotinamide (INA), Nicotinamide (NA), Picolinamide (PA), Carbamazepine (CBZ) and Diprophylline (DPL) to the association and self-association processes in solutions, which are largely influenced by the hydrogen bonding capacity of the solvent. The screening method based on the identification of crystal building units may help to discover new polymorphs. The self-association method offers the ability to identify solvents or solvent mixtures that promote or avoid the presence of specific building units and in this way control the building unit towards polymorphs having specific structural features. As identified in chapter 3, another cause for the low kinetic factor in nucleation is the heterogeneous particle. Nothing is known about the actual concentration and functionality of these heterogeneous particles while they tremendously affect nucleation behavior. We therefore investigated the interplay between self-associates in solution and well-defined heterogeneous template surface by studying the crystallization behavior of isonicotinamide (INA) and 2,6-dihydroxy benzoic acid (DHB) (Chapter 7). Well-defined templates were prepared by making Self-Assembled Monolayers (SAM) onto a gold surface. The self-association of INA and DHB were investigated using spectroscopic techniques. Raman spectroscopy of the crystal-template surface after template crystallization suggests that molecular interactions between INA or DHB associates and the SAM are responsible for the formation of specific polymorphs. XRPD helped in the identification of the crystal orientation on the template surface further verifying the importance of solute interactions with the functionalized template surface. The systematic analysis of the association processes in solutions and the interplay with well-defined templates is beneficial in the development of polymorph discovery and preparation methods as well as control over crystallization processes. Industrially, the thesis results may not only help to discover new polymorphs but can also help in reproducible industrial production of polymorphs. On Industrial scale seeding approaches using only a single crystal can lead to the avoidance of primary nucleation and thus control over the polymorph obtained. The combination of well-defined template surfaces and the self-association method can be used as a screening method in the early drug discovery and development phase but also define robust conditions for industrial crystallization of polymorphs. This will not only help to discover and reproducibly prepare polymorphs, but a more comprehensive screening can be performed at reduced cost. Industries can implement the results to improve crystal product qualities and can also discover and optimize the quality of new crystal products by incorporating the methods in the development process. Scientifically, this thesis opened the route towards a thorough study of heterogeneous nucleation of polymorphic compounds taking into account self-association, template effects and relative stability of polymorphs. Such a study would result in an accurate molecular interpretation of crystal nucleation and would finally enable the validation of heterogeneous nucleation theories. As analytical techniques become more and more powerful, finding new and better ways to powerful insights in the crystal nucleation research become easier. Utilization of these principles and tools not only allow studying crystal nucleation, but also allows the understanding of nucleation processes to a new level. Molecular simulations are still needed to bridge the gap between solution chemistry and crystal nucleation rate analysis to come to a molecular interpretation of crystal nucleation of organic compounds. The new experimental approaches described in this thesis will boost the existing methods for polymorph prediction and in particular for predicting the conditions for polymorph formation.Process and EnergyMechanical, Maritime and Materials Engineerin
Fixed bed reactors of non-spherical pellets: Importance of heterogeneities and inadequacy of azimuthal averaging
No full textDespite the substantial simplicities inherent in pseudo-continuum models of fixed bed reactors, there is a continued interest in the use of such models for predicting fluid flow and transport scalars. In this paper, we aim to quantitatively address the inadequacy of 2D pseudo-continuum models for narrow-tube fixed beds. We show this by comparing with spatially resolved 3D results obtained by a robust and integrated numerical workflow, consisting of a sequential Rigid Body Dynamics and Computational Fluid Dynamics (RBD-CFD) approach. The RBD is founded on a physics-based hard-body packing algorithm, recently proposed by the authors (Moghaddam, E.M., Foumeny, E.A., Stankiewicz, A.I., Padding, J.T., 2018. A Rigid Body Dynamics Algorithm for Modelling Random Packing Structures of Non-Spherical and Non-Convex Pellets. Ind. Eng. Chem. Res. 57, 14988–15007), which offers a rigorous method to handle resting contacts between particles. The methodology is benchmarked for simulations of flow fields in all flow regimes, for 5 ≤ Rep ≤ 3,000, in random packings of spheres and cylinders with tube-to-pellet diameter ratios, N, between 2.29 and 6.1. The CFD results reveal a remarkable influence of local structure on the velocity distribution at the pellet scale, particularly in low-N packings, where the spatial heterogeneity of the structure is very strong along the bed axis. It is also demonstrated that azimuthal averaging of the 3D velocity field over the bed volume, which has been considered as an advancement over plug flow idealization in classical pseudo-continuum models, cannot reflect the role of vortex regions emerging in the wake of the pellets, and leads to underestimation of the local velocity values by more than 400% of the inlet velocity.Large Scale Energy StorageIntensified Reaction and Separation SystemsComplex Fluid Processin
Application of microwave heating to a polyesterification plant
No full textUtilizing microwave irradiation, a fundamentally different method of the energy transfer, to the chemical process units can potentially be advantageous compared to the conventional heating, inter alia due to the selective nature of interaction of the microwaves with the matter. This doctoral dissertation addresses some of the aspects associated with the use of the microwave technology in a polyesterification plant. In this context, application of the microwave irradiation to a model polyestrerification reaction, using different types of the microwave applicators was studied. Influence of the microwave irradiation as an alternative form of heat for two selected separation techniques, which can potentially be used in a downstream recovery section in the polyesterification process (for the water purification and recovery unreacted dihydroxy alcohols), namely pervaporation and adsorption-desorption processes, were studied. The microwave-activated processes were compared with the conventionally heated ones focusing on possible reduction in the production time and energy consumption and increase in product quality. Study related to heating of the individual reagents of the polyesterification process in a multimode microwave oven showed that the heating time is several times shorter than with the conventional heating at the expense of a higher electric energy consumption. The results obtained for a model reaction system of adipic acid and neopentyl glycol under the conventional and microwave heating using the multimode microwave cavity did not show significant differences in terms of conversion and the end-product properties. Optimized (lower) usage of the microwave power during the process and effective scale up possibility compared to the multimode cavity were presented by applying the Internal Transmission Line (INTLI) microwave reactor to the studied reactor system (polyesterification reaction of maleic anhydride, phthalic anhydride and 1,2-propylene glycol). The INTLI allows for irradiation of the liquid phase from the inside of the reactor, thereby enabling better coupling of the microwave energy with the liquid mixture. For the selected process conditions, the total energy consumption of the INTLI reactor was up to a factor two lower. Comparison of desorption kinetics and desorption efficiencies for a model polar and non-polar molecules from 13X molecular sieves for two techniques, the microwave swing regeneration (MSR) and the temperature swing regeneration (TSR) showed that microwaves can help to overcome the heat transfer limitations by direct heating of the adsorbent. Significantly faster desorption processes were enabled under the microwave heating with the enhancement being more pronounced in the event of the polar compound. The MSR process runs faster even when the adsorbent temperature is quite lower than the gas temperature in TSR. It was verified that the microwaves do not affect the adsorption capacity of the molecular sieves after several consecutive adsorption-desorption cycles. Membrane pervaporation for dewatering of water/ethanol mixtures, using a hydrophilic membrane, were conducted under the microwave and conventional heating in a multimode microwave oven and a convection oven, respectively. Observations were made that at the conditions with higher water content in the feed, the water flux through the membrane was higher under the conventional heating. On contrary, with lower water in the feed, the opposite trend was found; the water flux through the membrane was higher under the microwave heating. Enhancement in the permeate flux under microwaves, compared to conventional heating, at higher ethanol concentrations in the feed can be explained on the basis of stronger coupling of microwaves with ethanol than water. Contrary to water, the dielectric loss of ethanol increases with increasing temperature; therefore, microwave dissipation is preponderant in the hot areas and can easily lead to local heating and the spatial temperature gradients.Process and EnergyMechanical, Maritime and Materials Engineerin
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