Chemical Engineering

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This is the collection for the University of Waterloo's Department of Chemical Engineering.

Research outputs are organized by type (eg. Master Thesis, Article, Conference Paper).

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Now showing 1 - 12 of 12

Determination of Characteristic Transport Coefficients of Porous Media from Volumetric Images using the Diffuse Interface Method
(University of Waterloo, 2021-11-24) Aggarwal, Chahat; Gostick, Jeff; Abukhdeir, Nasser Mohieddin
Transport in engineered materials such as electrodes, membranes, filters, and natural materials such as rock, sand, soil can be modeled as transport in porous media. Direct Numerical Simulations (DNS) on volumetric images of porous media are commonly done using the Lattice Boltzmann Method (LBM), but this presents various challenges such as long the computational time required to reach steady-state, fixed grid coarseness, and limited availability of reliable LBM software, commercial or otherwise. Traditional finite element based methods require conformal meshes of porous domains that are able to accurately capture fluid/solid interfaces, but at the cost of significant computational complexity and user interaction in order to create the mesh. To address these challenges, this work presents the application of a diffuse-interface finite element method that approximates a phase-field from volumetric images of porous media without user interaction and enables the use of a simple structured grid/mesh for traditional finite element-based fluid mechanics methods. The presented diffuse interface method (DIM) is automated and non-iterative, enabling the direct calculation of three characteristic coefficients from input images: tortuosity, permeability, and inertial constant by simulating Fickian mass diffusion and single component incompressible Navier Stokes equation from low to high range of inlet velocity. Three different 2D test images with varying porosities are used to demonstrate the use of DIM. The method is compared to traditional FEM implementation using conformal meshes with respect to the agreement with the determination of the characteristic coefficients, numerical accuracy, and computational requirements (time). Different parameters affecting the accuracy of DIM were identified and ideal parameters were determined. At ideal parameters, the relative error in tortuosity less than 0.75%, the relative error in permeability less than 1%, and relative error in inertial constant less than 3% were achieved for all three images. Though, DIM was found to be slower than traditional FEM implementation calls for optimized solvers for fluid flow on structured meshes to speed up the DIM simulations. The developed method provides an automated approach for computing effective transport properties from volumetric images of porous media.
Development of a predictive model for the VAPEX process.
(University of Waterloo, 2019-01-11) Lowman, James; Ioannidis, Marios; Abukhdeir, Nasser Mohieddin
Heavy oil in-situ recovery in Canada is largely achieved through energy intensive and en- vironmentally detrimental steam assisted gravity drainage (SAGD). By exchanging steam for a vapour solvent, it has been shown experimentally that heavy oil recovery can be achieved with a significant reduction in both energy requirements and environmental im- pact. This project presents a model for predicting production rates for the vapour ex- traction (VAPEX) process developed using first principles. The governing equations are solved using the pseudo-spectral Chebyshev collocation method. Simulations replicating experimental results,a detailed derivation of the model, a discussion of model parameters, and numerical analysis of the solution are presented in this thesis.
Diffuse Solid-Fluid Interface Method for Dispersed Multiphase Flows
(University of Waterloo, 2018-08-30) Treeratanaphitak, Tanyakarn; Abukhdeir, Nasser Mohieddin
Industrial chemical engineering processes such as bubble columns, reactors and separators involve multiphase flows of two or more fluids. In order to improve the design and operation of these processes, an understanding of their multiphase hydrodynamics is essential. An emergent tool in studying multiphase flow systems that is becoming readily accessible to researchers is computational fluid dynamics (CFD) simulation. CFD simulations of multiphase flow systems enable researchers to explore the effect of different combinations of operating conditions and designs on pressure drop, separation efficiency, and heat and mass transfer without the cost and safety issues incurred by experimental design and pilot studies. Consequently, CFD simulations are increasingly relevant for the design and optimization of chemical process equipment. The multiphase hydrodynamic model that is often used to study chemical engineering processes is the two-fluid (Euler-Euler) model. In this model, the fluids are treated as inter-penetrating continua and fluid phase fractions are used to describe the average spatial composition of the multiphase fluid. Generally, the physical boundaries (e.g. vessel walls, reactor internals, \textit{etc.}) in numerical simulations using the two-fluid model are defined by the mesh or grid, i.e. the mesh/grid boundaries correspond to an approximation of the physical boundaries of the system. The resulting conformal mesh/grid could potentially contain a large number of skewed elements, which is undesirable in numerical simulations. One approach to address this issue involves approximation of solid boundaries using a diffuse solid-fluid interface approximation. This approach allows for a structured mesh to be used while still capturing the desired solid-fluid boundaries. The diffuse-interface method also allows for the simulation of moving boundaries without the need for manipulation of the underlying mesh/grid or interpolation of boundary variables to the nearest node. This allows for the geometry of the domain of interest (i.e. process equipment) to be easily modified during the process of simulation-assisted design and optimization. In the two-fluid model, phase fractions are used to describe the composition of the mixture and are bounded quantities. Consequently, numerical solution methods used in simulations must preserve boundedness for accuracy and physical fidelity. Firstly, a phase-bounded numerical method for the two-fluid model is developed in which phase fraction inequality constraints are imposed through the use of an implicit variational nonlinear inequality solver. The numerical method is verified and compared to an established explicit numerical method. The effect of using separate phasic pressure fields as opposed to the commonly used single-pressure assumption is also found to be non-negligible in dilute dispersed flows (less than 3% gas fraction). Subsequently, the phase-bounded numerical method is extended to support a diffuse-interface method for the imposition of solid-fluid boundaries. The diffuse-interface is used to define physical boundaries and boundary conditions are imposed by blending conservation equations from the two-fluid model with the solid boundary condition. Simulations of two-dimensional channel flow and flow past a stationary cylinder are used to validate the diffuse-interface method. This is achieved by comparing the bubble plume width and time evolution of the overall gas hold-up from the diffuse-interface simulations with results obtained using boundary-conformal meshes. The results from the channel flow simulations are found to be in agreement with the boundary-conformal mesh solution when the interface width is sufficiently small. In the case of flow past a stationary cylinder, similar flow features are observed in both diffuse-interface and reference simulations.
Formation and Field-switching Dynamics of Nematic Droplets
(University of Waterloo, 2017-08-23) Fu, Fred; Abukhdeir, Nasser Mohieddin
Liquid crystals (LCs) refer to a class of materials which have anisotropic properties. They are used in many technological applications ranging from displays to biological sensors. One example of a category of technologically relevant LC applications is optical functional materials, which include polymer-dispersed liquid crystal (PDLC) films. In these films, non-deformable micron-scale LC droplets are dispersed in a solid polymer matrix. Application of an electric field through the thickness of a PDLC film results in "switching" between a transparent "on" state (field-on) and a translucent "off" state (field-off). Thus the main application of these films are as switchable windows. Key to this mechanism is the ability for external fields to reorient the direction LC molecules within the droplets. In this work, the LC phase formation and external electric field-switching dynamics of orientationally-ordered LC droplets are studied using the continuum Landau--de Gennes model. The model is able to capture phase transition and reorientation dynamics on device-relevant length and time scales when combined with numerical methods such as the finite element method. Formation dynamics correspond to transitioning from a high-temperature disordered liquid phase to an orientationally-ordered phase referred to as a nematic LC. Field-switching dynamics correspond to the imposition and release of an external (electric) field. Particular emphasis is placed on non-spherical droplets, which may form naturally or intentionally under controlled conditions in the manufacturing of PDLC films. The interactions between shape, LC/polymer interfacial effects, electric field strength, and other parameters are first investigated for capillary geometries using a simplified model, which is then followed by fully three-dimensional droplet simulations. Finally, simulation results predicting a symmetry-breaking phase transformation process for spherical droplet domain are presented. This observation is found to be predicted when using physically-realistic material parameters approximating the LC compound pentyl-cyanobiphenyl (5CB), but not for simulations with a typical simplification of nematic elasticity known as the single elastic constant approximation.
Incremental machine learning-based accelerator for computational fluid dynamics simulations
(University of Waterloo, 2023-10-16) Mokbel, Sajeda; Abukhdeir, Nasser Mohieddin
The simulation of physicochemical processes with computational methods is key for engineering design, with applications in a variety of industries, ranging from pharmaceuticals to aerodynamics. Despite its importance and widespread use, significant challenges related to the accuracy and computational complexity of these simulations remain prominent. These systems are governed by non-linear transport equations with physical and chemical processes occurring at different spatiotemporal scales in complex geometries. This leads to problems which are computationally expensive and often infeasible to solve. As such, reducing the computational complexity of multiphysics problems without compromising on accuracy is a central goal in the engineering community. Recently, machine learning has proven to be a promising direction towards this goal. The availability of data from both multiphysics experiments and simulations have led to high-performing neural networks capable of accelerating traditional methods for solving multiphysics problems. Despite these hopeful results, there still exists a gap between machine learning and its optimal application in a realistic engineering design process. This work aims to bridge that gap through two main approaches. The first approach is by developing a framework which hosts neural network training and existing computational multiphysics software in a unified framework. The second approach is to incrementally determine optimal neural network parameters by running computational multiphysics problems and neural network training in parallel. This has shown to reduce data collection and training time while increasing the speedup of multiphysics simulations over increments.
Iterative Coupled Shell/Tube Simulation of Waste Heat Boilers using Computational Multiphysics
(University of Waterloo, 2019-08-02) Guiguer, Victor; Abukhdeir, Nasser Mohieddin
Removal of sulphur from fossil fuels is important in order to avoid the emission of sulphur oxides into the atmosphere, exposure to which has negative health and environ- mental effects. Sulphur is removed from refinery petrochemical products via the Claus process which contains a waste heat boiler (WHB). These WHBs are exposed to extreme temperatures and corrosive conditions, yet they are expected to operate continuously for years at a time. Typically WHBs have been designed using empirical correlations and heuristics, but more recently using process and multiphysics simulation. In this work a proof of concept for the numerical simulation of a WHB and its protective insulation is demonstrated. Continuum multiphysics models for both shell and tube side of a WHB are developed. An iterative coupling method for the determination of steady-state numerical solution of these models is then used to simulate a sub-region of a typical WHB. Simulation results for the tube-side of the WHB predict both the temperature profile and nature of the turbulent energy transport in the inlet region, highlighting complex flow profiles. Simulations of the shell-side of the WHB predict the multiphase convective boiling behaviour in the bulk (far from wall effects). Finally, preliminary results of the coupled shell/tube configurations are presented and compared to previous results.
Modelling and Performance of a Hydrogel-Based Photobioreactor
(University of Waterloo, 2024-07-05) Rasmussen, Nicholas; Ward, Valerie; Abukhdeir, Nasser Mohieddin
This work is motivated by the need for in situ food production with respect to future space activities due to the technical and economic in-feasibility of long-term earth-based resupply. The unique size constraints of space have prevented conventional food systems from demonstrating feasibility. Owing to their high growth rates and phototropic activity, microalgae are a promising candidate to meet the caloric and respiratory needs of astronauts as part of a biological life support systems (BLSS). However, the gravity dependence and size of transitional photobioreactors poses a challenged to their utilization in space. As such, a solid-state hydrogel-based photobioreactor (hPBR) is proposed to achieve inherent phase separation allowing for extra-terrestial use. Initially proposed for the Canadian Space Agency (CSA) Deep Space Food Challenge (DSFC) (Design A), this design was further iterated to improve productivity and reactor performance (Design B). Using Chlorella vulgaris, Design B achieved a biomass productivity of 2.4 and 3.2 g m−2d−1 when using physically (pPVA) and chemically (cPVA) crosslinked poly(vinyl) alcohol (PVA) respectively with a water demand of 0.44 kg g−1 biomass. Over 23 days of growth, the lipid content increased from 18.9% to 56.6% and 13.8% to 43.2% for pPVA and cPVA respectively, and the chlorophyll content also decreased. However, cell viability remained high at over 97% and surface coverage analysis showed good coverage within a few days. Observations made with the prototype suggested mass transport limitations were impacting growth, and that poor humidity control led to the hydrogels drying out. To this end, a continuum model of the hydrogel was proposed to better understand mass transfer and to inform future design iterations. Hydrogels are two phase systems where the polymer is fixed due to crosslinking leading to a moving boundary with changes in water content. The proposed model did not require any parameter fitting as values were determined with independent experiments. The model enabled the prediction of the transient material response to changing relative humidity. This helped to explain why humidity control was critical in maintaining cell viability. Humidity impacted the water content of the gel’s surface which needed to be high enough to support algae growth. Using the steady-state solution to the model, the solute transport through the system was also modelled. The solute profile suggested that nutrient concentrations throughout the hydrogel were similar to that in the media tank. This suggests nutrient supply was not the cause of the diminishing biomass quality and that other factors such as photo-inhibition, and mechanical stresses from solid-state cultivation may be issues to address in future work.
Multiphysics Modelling of Liquid Crystal Based Adaptive Lenses
(University of Waterloo, 2020-09-30) Vasile, Alexandru Andrei; Abukhdeir, Nasser Mohieddin
Conventional lenses are limited by their fixed shaped and optical properties. Liquid crystal adaptive lenses (LCLAs) are a promising candidate to move beyond these limitations thanks their tuneable optical properties. A difficulty of working with LCs is that their properties are the result of an experimentally un-observable structure. Thankfully, modelling is capable of providing insight into this structure. Unfortunately, progress has been hamstrung by an over-reliance on experimentation. Further, what little modelling is being done usually involves simplified models and/or close-source software packages. This work uses a general model for thermotropic nematic liquid crystals based on Landau-de Gennes theory to study the texture of liquid crystal adaptive lenses. The most general version of this model was used, without the common simplifications such as: hard anchoring, neglecting elastic constants, or geometric symmetry. In order to find the equilibrium state for the nematic model, the Euler-Lagrange equation for the total free-energy is set to zero. This form is converted into a transient PDE in order to capture the dynamics of the system, and to evolve the texture towards its equilibrium state. The nematic model is coupled with a model for the electric field within the cell, and the two are solved simultaneously. This is accomplished by using the method of lines for temporal discretization and the finite element method for spatial discretization. The validity of the implemented model was first verified by modelling two important LCD configurations: the TN cell and the IPS cells. The TN cell was modelled with the electrodes off and with them on. In both cases the correct equilibrium texture was obtained. Modelling light propagation with cross-polarization microscopy produced the correct results, a bright cell when the electrodes were off and a dark one when they were on. Next, the IPS cell was also modelled. Again, the correct equilibrium result was obtained; a twisted texture was when the electrodes are turned off and an untwisted texture when the electrodes are turned on. Modelling light propagation resulted in the correct dark state when the electrodes were off andthe correct bright state when they were on. Having successful produced the expected texture and cross-polarization microscopy results, the model was applied to a LCAL. The literature review of this work identified a wide range of potential liquid crystal adaptive lenses. The final design was chosen using three criteria: 1) availability of published results, 2) modelling requirements, and 3) ease of manufacture. Based on these criteria, a design called the HMD cell was chosen. When modelled, the resulting texture and cross-polarization microscopy did not agree with previously published results. An investigation into the cause of these discrepancy was performed, but the cause has not yet been identified.
Simulation-based Design of Bioreactors Using Computational Multiphysics
(University of Waterloo, 2021-01-28) Entezari, Kimia; Abukhdeir, Nasser Mohieddin
The Covid-19 pandemic highlighted the importance of quickly scaling up the production of vaccines and other pharmaceutical products. These products are typically made within bioreactors: vessels that carry out bioreactions involving microorganisms or biochemical substances derived from microorganisms. The design, construction, and evaluation of bioreactors for large-scale production, however, is costly and time-consuming. Many builds are often needed to resolve issues such as poor mixing and inhomogeneous nutrient transfer. Nevertheless, computational methods can be used to identify and resolve these limitations early-on in the design process. This is why understanding the flow characteristics inside a bioreactor through computational fluid dynamics (CFD) can save time, money, and lives. Bioreactors contain three phases: 1) a continuous liquid medium which is the host for cells to feed and grow, 2) a dispersed solid phase which is the microorganism particles inside the tank, and 3) a dispersed gas phase which includes the air or oxygen bubbles for microorganisms aspiration. Due to the complexity of solving a three-phase flow problem, most bioreactor multi-phase simulations in the literature neglect the dispersed microorganism phase and its effects entirely–thus assuming two phases only. In this research project, a hybrid model is developed that captures the effects of all three phases. The model first approximates the liquid and solid phase as a single “mixture” using the drift-flux model. Subsequently, the Euler-Euler method is used to simulate the resulting mixture with the added dispersed gas. This allows the simulation of bioreactors and other bioprocesses with the computational complexity of the two-phase simulation while capturing all three phases. The “mixture” portion of the model was simulated inside a stirred tank bioreactor. Its results were then validated by comparing them to empirical evidence in the literature. Two parameters were chosen for this validation: 1) the hindered settling velocity of the solid phase in the absence of impeller motion, and 2) the computed power number of the impeller. The validation showed an overestimation of the hindered settling velocity and an underestimation of the impeller power number.
Simulation-based Design of In-Plane Switching Liquid Crystalline Display Pixels
(University of Waterloo, 2016-08-25) Mitra, Anindya; Abukhdeir, Nasser Mohieddin
Liquid crystal displays (LCDs) constitute an important class of modern display tech- nologies. Their light-weight nature, coupled with their favourable power consumption char- acteristics make them useful in applications ranging from large area projection displays to small electronic devices such as digital watches and calculators. Despite being the market leader in the display industry, traditional configurations of LCDs suffer from serious drawbacks such as having a very narrow viewing cone. Newer configurations of LCDs, however, employ the in-plane switching (IPS) mode and its deriva- tives. These provide a much wider viewing cone with lower degradation of image quality as one moves off the central axis. IPS pixels have a unique configuration as they contain the electrodes on only one side of the domain. The electrodes are arranged in an interdigitated pattern and produce an electric field that varies periodically in space parallel to the substrates and decays exponentially in space along the through-plane direction. The highly non-homogeneous nature of the electric field makes the simulation of the electric field within an IPS domain more challenging as a minimum of two dimensions is needed to model the electric field with sufficient accuracy, in contrast to the electric field in the twisted nematic (TN) mode that may be modelled in only one dimension. Traditional approaches have employed an iterative technique wherein the Gauss law equations are solved for a pre-determined director configuration and the electric field thus obtained is employed to calculate the new director configuration over the domain. The iterations are continued till convergence is attained. Our method involves calculating the electric field by means of a semi-analytical expres- sion for an electric field produced by interdigitated electrodes and using this expression to calculate the domain configuration. This methodology is advantageous in terms of computational time and effort as it gives a possible way to do away with the back and forth iterations involving the dynamic equations and the Gauss’ law equations. In this work, we attempt to look at dynamic characteristics of the liquid crystalline domain in an IPS-LCD. Metrics were evolved to quantify the deformation in the domain. Finally, these metrics were used to examine the dependence of the equilibrium orientation on the domain thickness, electrode width, electrode spacing and electric voltage applied. The results show good match with the trends that can be expected from theoretical considerations. The variation of the domain deformation characteristics with the change in the geometric and physical parameters is along expected lines. For instance, increasing the voltage results in the domain getting deformed to a much greater extent and the defor- mation to penetrate deep within the domain. A greater pixel depth with the same values of the other parameters results in more of the domain staying undeformed as the electric field only penetrates upto a fixed distance into the domain. Increase in the electrode spac- ing was not found to make a significant contribution to the deformation while increasing the width of the electrodes increases the area affected by the electric field and thus, this increases the overall deformation. To conclude, the framework provided here is a valid first step in evolving a complete software package to model deformation characteristics of an LCD pixel. The code is flexible enough to accommodate different LCD configurations and thus, may be used to model a variety of other LCD configurations also. A parallel development of an optics code using a matrix based method may be used to model the propagation of light through the domain and this may be added very easily on top of the existing framework to create a complete package for analysing the electro-optical properties of the LCD.
Simulation-based Design of Temperature-responsive Nematic Elastomers
(University of Waterloo, 2017-06-16) Neufeld, Ryan Alexander Epp; Abukhdeir, Nasser Mohieddin; Zhao, Boxin
Liquid crystal elastomers (LCEs) are a class of polymer networks which involve the incorporation of liquid crystal (LC) molecules into their polymer backbone or side chain. This results in anisotropy in their mechanical, optical, and electromagnetic properties similar to those exhibited by traditional LC materials. Their mechanical properties are highly coupled to the internal state of LC order, which can result in large mechanical deformations as LC order changes. This can occur in response to a variety of external stimuli such as changes in temperature, exposure to light, and application of external fields. The interplay between LC order and mechanical properties makes LCEs a highly promising class of functional materials and subsequently, they have been the subject of much research over the past several decades. However, developing an application of LCEs remains difficult in that their mechanical response is both complex and coupled to the state of liquid crystal order prior to cross-linking. Their physics are sufficiently complicated that in most cases, the use of pen-and-paper analysis is precluded. Additionally, the LCE fabrication process is complex and expensive, making trial-and-error experimental design methods unsuitable. This motivates the development and use of simulation-based methods to augment traditional experimental design methods. The two main contributors to the complexity of the design of LCE applications are the choice and imposition of liquid crystal order, or "texture", prior to cross-linking. In this work, simulation-based methods are developed and partially validated for use in applications-focused design of temperature-responsive nematic LCEs. These methods enable the simulation of LCEs of macroscopic size and of non-trivial geometry through the use of continuum mechanics and suitable numerical methods (the finite element method). LC texture is an input parameter in the presented method, allowing many choices of texture to be explored at low cost given that the textures are physically accessible. In addition to methods development and validation results, proof-of-concept simulation-based design studies were performed for two types of LCE-based actuators that are of current interest in the field: grippers and hinge mechanisms. Finally, preliminary results are presented resulting from the integration of nematic texture dynamics simulation (pre-cross-linking) and LCE mechanical simulations (post-cross-linking) which address the two main sources of complexity in the design process of LCE functional materials.
A Topology-Based Method for the Compartmentalization of Multiphysics Flows
(University of Waterloo, 2020-09-28) Donnelly, Thomas Richard; Abukhdeir, Nasser Mohieddin; Pritzker, Mark
Continuum simulations of multiphysics processes are costly due to the coupling of transport phenomena. Compartment modelling decouples hydrodynamics from other transport phenomena, offering a low-cost simulation alternative for applications such as design screening. While different methods will produce compartment models with different accuracy levels, no rigorous compartmentalization approaches currently exist. In this work, a compartmentalization algorithm is proposed that identifies distinct flow modes from an analysis of the topology of a fluid velocity field. This topological analysis is based on an analogy between modes of fluid flow and deformation modes that have been identified for the molecular alignment of liquid crystals. A velocity alignment vector is defined and used to compute the deformation modes splay, twist, and bend for a velocity field. This topologically-informed compartmentalization algorithm is developed through its application to a test case of steady single-phase laminar flow through a cylindrical vessel with a step increase in cross-sectional area. This case is observed to exhibit unidirectional, recirculatory, and diverging flow based on a continuum simulation. Local alignment deformation, defined as the sum of splay, twist, and bend for a velocity field, is computed and thresholded to segment the domain into compartments dominated by each of the distinct flow modes. These compartments are incorporated into a compartment model, which is validated against the continuum simulation through a comparison of their residence time distributions (RTDs). The compartment model RTD is shown to deviate from the continuum simulation in terms of having lower mean residence time, higher variance, and higher skewness. The deviation between the compartment model and continuum simulation is attributed to the approximation of unidirectional compartments as well-mixed. The compartment model is modified to approximate each unidirectional compartment as a series of ideal continuous stirred-tank reactors (CSTRs), which improves the variance and skewness of the RTD but does not increase the mean residence time. As a final modification to the compartment model considered, an adjustment to the thresholding used for the compartment model is shown to have an insignificant effect on the RTD. While additional work is required to improve the accuracy of the compartment modelling approach proposed, compartment models based on velocity topology offer a promising approach for multiphysics simulation.

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