Choon Jim Lim

Professor

Relevant Degree Programs

 

Graduate Student Supervision

Doctoral Student Supervision (Jan 2008 - May 2019)
Biomass torrefaction in slot-rectangular spouted beds (2018)

Biomass, a nearly carbon-neutral energy resource, can reduce greenhouse gas emission and replace fossil fuels, but it is characterized by heterogeneity, high moisture content, low bulk density, low calorific value, pliability and hygroscopic nature, all of which challenge its utilization. Torrefaction, a thermal pretreatment method, is capable of modifying the physical-chemical properties of biomass and enhancing its calorific value. Slot-rectangular spouted beds (SRSBs) can effectively handle biomass particles and offer a promising way to overcome the scale-up challenge of conventional spouted beds.This study explored the potential application of SRSBs to biomass torrefaction. Solids mixing in a dual-compartment slot-rectangular spouted bed (DSRSB) was first studied to address the scale-up issue, while also providing fundamental information needed for the design and operation of a DSRSB reactor. SRSB and DSRSB reactors were developed for torrefying sawdust with the semi-batch operation. Hydrodynamics of SRSB and DSRSB, torrefaction performance, torrefied product properties and torrefied product pyrolysis were subsequently investigated. Temperature, biomass feed rate, sawdust particle size and oxygen concentration influenced torrefaction performance and torrefied product properties. Temperature was found to be the most important factor. Biomass torrefaction performed better in the DSRSB than in the SRSB. Higher temperature, lower biomass feed rate, larger sawdust particle size and greater oxygen concentration all led to increased weight loss and decreased energy yield of sawdust, and produced torrefied sawdust with higher HHV, greater atomic carbon content, lower atomic hydrogen and oxygen contents, less volatile matter, greater fixed carbon and less hemicellulose. Higher temperature and greater oxygen concentration were very helpful to produce more torrefied sawdust captured by a cyclone. Performance of oxidative torrefaction was similar to that of non-oxidative torrefaction. The effect of oxygen concentration was more significant at a higher temperature. Torrefied sawdust underwent size reduction during torrefaction, with smoother and cleaner surfaces compared to raw sawdust. The activation energy for non-oxidatively torrefied sawdust was higher than for oxidatively torrefied sawdust, which was in turn greater than that of raw sawdust. Sawdust particle size affected the pressure drop across the reactor.

View record

Drying and co-pelletization of microalgae with sawdust (2018)

Post-harvest handling of microalgae following mechanical dewatering is challenging due to the high moisture content of biomass (about 65-75% wet basis). Therefore, thermal drying is applied to decrease the moisture content to the safe value for storage, handling, and transportation. After drying, handling of dried microalgae powder is still difficult because of its low bulk density and possibility of blocking the flow of material inside handling equipment. Pelletization improves microalgal material characteristics by making high-density and homogenous pellets. The first goal of this research is to study the thin-layer drying mechanism of microalgae Chlorella at the temperature range of 40-140° C. The second goal is to study densification mechanism of pure and mixed microalgae with pine sawdust. The specific species used in this experimental investigation is Chlorella vulgaris. In the studied temperature range, microalgae drying from an initial moisture content of 65% wet basis occurred in the falling-rate period with no constant-rate phase. The results revealed that diffusion is the controlling mechanism in microalgae drying and all the water is entrapped in algal cells. This confirms the industrial experience that further mechanical dewatering to remove water is not effective. It was also understood that although the drying rate at 100-140° C is the highest, 60 and 80° C are the optimum drying temperatures to preserve microalgae surface color and chemical composition. Pelletization of pure Chlorella occurred in two distinct regions of particles’ rearrangement and particles’ deformation. However, there was no clear separation between the two regions when pure sawdust was pelletized. Adding microalgae Chlorella to sawdust resulted in a decrease in densification energy and improvement in pellets’ properties, i.e. higher durability, density, and heating value, lower porosity, moisture adsorption, and pellets’ expansion. The results indicated that adding microalgae to sawdust eliminates the need for high pelletization temperature and force. The temperature of 75° C and maximum force of 2500 N, which are considered as moderate conditions, were adequate for making pellets containing microalgae with desirable characteristics because of the presence of natural binders in microalgae. Working at a low/moderate temperature and force improves the process economics by decreasing energy consumption.

View record

Heat and mass transfer in pulsed fluidized bed of biomass (2017)

Biomass is a promising energy source that has been considered in a variety of thermal conversion processes where fluidized beds with their exceptional heat and mass transfer rates, are often considered as potential candidates. However, the fluidization of biomass is held back by its cohesive nature. This work has demonstrated that pulsed gas flow in fluidized bed is highly effective in overcoming channeling, partial and complete defluidization, without the need for inert bed particles. Both heat transfer and mass transfer were investigated in a pulsed fluidized bed with 0.15 m by 0.10 m rectangular cross-section area, and a fluidized bed with a tapered bottom to improve reactor performance. Biomass used in this work included Douglas fir, pine and switchgrass. Batch drying test was selected as an indirect indicator of gas–solid contact, heat and mass transfer. Mass transfer was evaluated through batch drying tests, where better gas–solid contact and mass transfer was assessed through the water removal efficiency. An optimum operating condition was identified after analyzing the intricate relationship between pulsation frequency, gas flow rate and the hydrodynamics. A two-phase drying model that linked single-particle mass transfer to macroscopic hydrodynamics in fluidized bed was implemented to verify the effect of flow rate, temperature and biomass properties on drying and mass transfer. Good agreement was observed between the modelled effective diffusivity and experimental results. Bed-to-surface heat transfer coefficients of all three biomass species in two reactor geometries were measured at various operating conditions. The heat transfer coefficient was influenced greatly by the intensity and frequency of gas pulsation, where both particle convection and gas convection existed. A new heat transfer model was proposed to address the influence of gas pulsation. Modelling results showed good agreement with experimental data.

View record

Limestone as a sorbent for CO2 capture and its application in enhanced biomass gasification (2014)

Global greenhouse gas emissions continue to increase despite the knowledge that the rise in atmospheric concentrations will have devastating effects on climate and human lives. Carbon dioxide capture and storage can be a stop-gap measure to mitigate CO₂ emissions from existing fossil fuel facilities during their gradual replacement by low-carbon alternatives such as biomass. Calcium oxide-based CO₂ capture is a relatively mature technology, ready for implementation. Limestone CaO precursor is relatively low-cost and readily available. A thorough understanding of the CaO-CO₂ reaction and its reversibility over multiple cycles is required to aid in design, improve efficiency and reduce costs of industrial capture processes. A novel method of CaO cycling involving pressure swing is demonstrated which was found to give improved calcium utilization up to 16.1%, after 250 carbonation/calcination cycles. The kinetics of pressure swing cycling are examined, and a mechanism to describe the loss in calcium utilization resulting from cycling, is presented linking the morphological changes of sorbent particles to the decay in calcium utilization. Coupling CaO-based CO₂ capture and storage with energy production from biomass has the potential for energy production with negative CO₂ emissions.Biomass is a carbon neutral source of energy and through gasification can be converted in a variety of energy carriers. Biomass was steam-gasified in a semi-batch fashion in a fluidized bed of CaO, which absorbed CO₂ as it was produced, resulting in a 55% increase in hydrogen production and decreases in CO, CH₄, CO₂ and higher hydrocarbons of 63%, 16%, 47% and 4% respectively. Limestone enhanced gasification (L.E.G.) of biomass also increased carbon and hydrogen utilization efficiencies. Cycling of CaO between gasification/carbonation and calcination was conducted in a single reactor by switching the mass flows from biomass and steam to air, up to eight cycles. Syngas composition and gasification efficiency were only marginally affected by cycling, reducing H₂ concentration by less than 5%. The degree to which the sorbent was re-calcined had a greater impact on system operation. A simple equilibrium model is provided to predict syngas composition.

View record

Characterization of in-line mixing of pulp fibre suspensions based on electrical resistance tomography (2013)

In pulp bleaching processes, pre-distribution of chemicals in suspensions ahead of tower reactors is essential to ensure efficient lignin removal and optimal use of the chemicals. In-line mixers, combined with chemical injectors, are commonly used to achieve this goal. In spite of its importance, in-line mixing of pulp suspensions is not well understood. In this thesis, liquid distribution and gas dispersion were investigated downstream of in-line mixers, including jet and mechanical mixers, to provide better understanding and guidance for mixer design and process optimization. In the present work, non-intrusive electrical resistance tomography (ERT) was used to quantify mixing based on two novel mixing indices, derived from the standard deviation of image pixel values. This technique was also implemented as a real-time mixing assessment tool in industrial pulp bleaching, with success in monitoring mixing quality as a function of process operating conditions. Liquid jet mixing was found to depend strongly on the flow regime and jet penetration. For turbulent flow, the criteria for in-line jet mixing in water apply also to suspensions. When a suspension flows as a plug, mixing differs greatly from that in water, depending on the fibre network strength in the core of the pipe. With an impeller present, mixing improved substantially, primarily in the high-shear zone around the impeller, with rapid reflocculation downstream. Gas mixing depended on the flow regime and buoyancy in a complex manner. When buoyancy was not significant, impeller operation enhanced mixing since bubbles dispersed throughout the pipe cross-section, whereas without the impeller, the bubbles congregated near the wall due to robust fibre networks in the core of the pipe. For buoyancy-dominated flow, the impeller worsened mixing since it disrupted the fibre networks and delivered gas to the top of the pipe, whereas the networks caused liquid/pulp slugs to flow at the top for a tee alone.

View record

Evolution and stratification of off-gasses in stored wood pellets (2013)

Storage of wood pellets has resulted in several deathly accidents in connection with off-gassing and self-heating. The goal of the present study was to quantify off-gassing characteristics of white wood pellets when stored in an experimental silo. Wood pellets properties were characterized with respect to gas adsorption-desorption and spatial and temporal concentrations of off-gases and thermal conditions within the pilot storage were quantified. In the last part, the effectiveness of purging the silo in reducing off-gas concentration was evaluated. To assess the adsorption of off-gases by wood pellets in storage, Temperature Programmed Desorption was used. Highest CO₂ adsorption was seen by torrefied wood pellets while lowest uptake was showed to be for steam exploded pellets. Quantifying the uptake of CO was challenging due to chemical reaction and therefore strong bonds between the material and carbon monoxide.Studies on emission and stratification of off-gases showed higher emission factor compared to work done with white wood pellets in small scale. Some stratifications were observed for CO₂ and CH₄ over the first days of storage. However for CO the stratification was much clear and related to high uptake of CO by wood pellets over time. During the entire period of storage, maximum temperature in the silo was recorded on day 15 of storage (storage time was 63 days) at the elevation of 2.5 m (silo dimension was 1.2m diameter and 4.6m height). Measured temperature in the silo during 5.5 hour purging experiments with air at 18-18.5 °C, helped the temperature decrease in the lower parts and slightly middle parts of the silo after 200 minutes of purging. To evaluate the effectiveness of a purging system to sweep the off-gases from the experimental silo, multiple purging tests were done. Mixing experiments showed large deviations from plug flow and thus better mixing for all superficial velocities used. Predicted results showed the concentration model fitted best to the measured off-gas concentration at the bottom and in the middle of the silo while the model overestimated the exponential decay of the off-gases in the head-space of the silo.

View record

Self-heating and spontaneous combustion of wood pellets during storage (2013)

Self-heating of wood pellets is a major concern during long term storage. Internal temperatures rose to 57℃ in 10 days in a wood pellet silo of 21.9 m diameter in Fibreco Inc. (Vancouver, Canada) after pellets (about 20℃) were loaded into the silo. Self-heating could lead to serious accidental fires, causing enormous damage and danger to workers. In this study, the self-heating rate at different temperatures was experimentally determined, and the thermal properties were measured. for wood pellets produced in British Columbia, Canada. The factors such as moisture content, pellet age and environment temperature were investigated and their impacts on the self-heating process were analyzed. Moisture content has a significant effect on effective thermal conductivity and specific heat capacity of packed pellets, but has no effect on the self-heating at the temperature range of 30℃ to 50℃. Pellets age and environment temperature are two major factors impacting the self-heating and off-gassing process. The self-heating rate is significantly increased at higher a temperature and eventually will lead to a thermal runaway when the ambient temperature is high enough. Experimental results show that the critical ambient temperature for thermal runaway decreases as the reactor size increases. The reaction kinetics was studied at both low temperatures (30℃ to 50℃) and high temperatures (100℃ to 200℃) and kinetic parameters were extracted from experimental results and correlations were developed. Based on all measured properties data and kinetics data, a two-dimensional axi-symmetric self-heating model was developed to predict the self-heating process and thermal runaway in large wood pellet silo. The influences of cooling airflow rate, wall insulation, and dimension of the storage container, ambient temperature and wind condition were studied. The results show that air ventilation inside of the silo is a very effective approach for reducing self-heating and preventing thermal runaway at ambient temperatures lower than 330 K. The critical ambient temperature for a 21 m diameter silo can be as low as 36℃ in the absence of air ventilation. The current model can be used to safe guide the design and operation of large industrial wood pellets silos.

View record

Coupling dehydrogenation of ethylbenzene with hydrogenation of nitrobenzene in an autothermal catalytic membrane reactor (2011)

Dehydrogenation of ethylbenzene and hydrogenation of nitrobenzene form an interesting pair of reactions to be coupled in a catalytic membrane reactor. The former is reversible and thermodynamically limited, supplying hydrogen with a net endothermality, while the latter is irreversible and exothermic, consuming hydrogen to produce aniline. In this work, coupling of these two reactions is simulated in a catalytic fixed bed membrane reactor where hydrogen produced on the dehydrogenation side is transferred through hydrogen membranes to the hydrogenation side where it reacts to produce aniline. Heat generated on the hydrogenation side is transferred to the dehydrogenation side, where it is utilized by the endothermic dehydrogenation reaction to improve the styrene yield. A pseudo-homogeneous model for the coupled reactor based on the concept of fixed bed reactors accounting for both the diffusion of hydrogen and transfer of heat is first developed. The effects of the operating and design parameters considered on the production of styrene and aniline show conflicting behaviour, i.e. improving the yield of styrene results in decreased production of aniline. Consequently, the cocurrent configuration of the coupled reactor was optimized within constraints so that it can be operated effectively to produce ~98% styrene as a one limiting option or ~80% aniline at the other extreme. The intraparticle diffusion resistance, a major limitation in fixed bed reactors, is evaluated by developing a heterogeneous reactor model based on Fickian diffusion and the dusty gas model for both isothermal and non-isothermal catalyst pellets. Both heterogeneous models predict a significant reduction in yield and conversion relative to the pseudo-homogeneous model, indicating the importance of heterogeneity. This reduction is generally less severe for the dusty gas model than for Fickian diffusion. The mean square deviation and absolute deviation along the reactor are calculated for all models relative to the heterogeneous reactor model with dusty gas for non-isothermal catalyst pellets, considered to be the most rigorous model tested. Assuming isothermality causes larger deviations than assuming Fickian diffusion. The deviations in the predictions of the homogenous model and the heterogeneous models from those of the dusty gas model for non-isothermal pellets are ~6% and ~11%, respectively.

View record

Chemical looping combustion : cold model hydrodynamics and modeling of methane combustion (2010)

A novel interconnected fluidized bed (IFB) reactor with a bypass line for chemical looping combustion (CLC) has been developed to overcome the problem of short residence time of oxygen carrier in the air reactor. A comprehensive hydrodynamic study was carried out on the cold-flow model of the proposed reactor. Detailed mapping of the operating conditions for the reactor system was studied. Pressure transducers were applied to investigate the pressure loops and the cross-sectional average solids hold-up along the air reactor. Solids circulation flux between the two reactors was measured using butterfly valves by estimating the time interval for collecting a given volume of solids. Helium was used as gas tracer for gas leakage measurement. The experiments examined the gas leakage from air reactor to fuel reactor, from fuel reactor to air reactor, from loop-seals to fuel reactor and from fuel reactor to the cyclone. For scaling consideration, the cold-flow reactor was operated with fluidizing gas mixture of helium and air to simulate the hydrodynamics of the hot unit. The effect of density ratio of solids to gas on the solids circulation flux, pressure loops and voidage distribution along the air reactor was investigated. The connection between cold unit and hot unit is achieved by applying a scaling law. It can be stated that the cold-flow model operated with fluidizing gas mixture of 96 vol% helium and 4 vol% air can be used to simulate the hydrodynamics of an atmospheric CLC hot unit. A comprehensive model for the investigation of the reactor is introduced by combining fluidization properties and a particle population balance for calculation of the bed particle conversion, considering the chemical reaction of a single particle. The dimensionless parameters, Mrfuel and Mrair, which represent the mass ratio of input oxidized-particles to the input fuel in unit time for the fuel reactor and the mass ratio of reduced-particles to the input oxygen in unit time for the air reactor, respectively, are introduced. The model shows that Mrfuel should be more than 50 for achieving fuel conversion of 90% in the fuel reactor and Mrair should be more than 60 for achieving oxygen conversion of 85% in the air reactor. A procedure for optimizing the performance of the atmospheric CLC reactor is developed. The modeling analysis indicated that the optimum operating condition of an atmospheric CLC reactor hot unit should be chosen as follows: fuel capacity is 80 kW, Ua0=6.6 m/s, Uf0= 0.076 m/s, UA1=4Umf, UA2=1Umf, and the temperature in air reactor is 1223 K and in fuel reactor is 1173 K.

View record

Fluidized bed membrane reactor for steam reforming of higher hydrocarbons (2010)

With growing demand for hydrogen in the industrial and energy sectors, research on novel hydrogen production processes is gaining importance. Fluctuations in price and availability of different hydrocarbons emphasize the need to diversify feedstock options beyond natural gas, the major source for hydrogen. Traditional steam reformers for making hydrogen from hydrocarbons suffer from low catalyst effectiveness factors, poor heat transfer and limited hydrogen yield due to thermodynamic equilibrium constraints. A fluidized bed membrane reactor (FBMR) was designed, fabricated, installed with close attention to safety and operated with methane, propane and heptane as feedstocks at average bed temperatures up to 550°C and pressures up to 800 kPa. When operated without membranes, near-equilibrium conditions were achieved inside the reactor with fluidized catalyst due to the fast reforming reactions. Installation hydrogen permselective Pd₇₇Ag₂₃ membrane panels inside the reactor to extract pure hydrogen shifted the reaction towards complete conversion of the hydrocarbons, including methane, the key intermediate when propane and heptane were the feed hydrocarbons. Reforming of higher hydrocarbons was found to be limited by the reversibility of the steam reforming of this methane. To assess the performance due to hydrogen in situ withdrawal, experiments were conducted with one and six membrane panels along the reactor. The results demonstrated that the FBMR could produce pure hydrogen from higher hydrocarbon feedstocks at moderate operating temperatures of 475-550°C.A two-phase fluidized bed reactor model was developed, with gas assumed to be in plug flow in both the bubble and dense phases. Diffusional mass transfer, as well as bulk convective flow between the phases, was incorporated to account for concentrations changing due to reactions predominantly in the dense phase, and due to increased molar flow due to reaction. Membranes withdraw hydrogen from both the dense and bubble phases.These studies show that an FBMR can provide compact reactor system with favourable hydrogen yield, and high purity. The model predicted feedstock flexibility capabilities achieved by the experiments, with the higher hydrocarbon feedstock rapidly producing methane and the non-permeate mixture approaching chemical equilibrium.

View record

Comprehensive modelling and its application to simulation of fluidized-bed reactors for efficient production of hydrogen and other hydrocarbon processes (2009)

A generalized comprehensive model, coupled with experimentation in a pilot reactor, is developed to simulate the performance of fluidized-bed catalytic reactors. The model characterizes multiple phases and regions (low-density phase, high-density phase, staged membranes, freeboard region) in different geometries. It accounts for conventional and balancing interphase transfer, catalytic reaction, solid sorption, change in molar/volumetric flow, temperature and pressure profiles, anisotropic dispersion, hydrodynamic regime variation, catalyst deactivation, energy options, feed distribution along the reactor, selective membranes, fluidization hydrodynamics and dynamic behaviour. It also allows for seamless introduction of features and/or simplifications depending on the system of interest. The literature is comprehensively analyzed, reviewing the most important models proposed since 1952. A systematic algorithm for formulating chemical/biochemical reaction engineering problems is developed for systems of different complexity. Simulations are conducted for specific processes including: 1) steam methane reforming (SMR) for production of ultra-pure hydrogen, 2) oxychlorination of ethylene to ethylene dichloride, 3) partial oxidation of n-butane to maleic anhydride, and 4) partial oxidation of naphthalene to phthalic anhydride. Special emphasis is dedicated to steam reforming in fluidized-bed membrane reactors comparing their performance under bubbling, turbulent and fast fluidization regimes in a variety of configurations. Bubbling regime simulations predict somewhat less hydrogen production due to the effects of conventional and balancing interphase mass transfer. Overall reactor performance is predicted to be best under turbulent fluidization operation. A concomitant experimental program was performed to collect detailed experimental data in a novel pilot scale prototype reactor operated under SMR and auto-thermal reforming (ATR) conditions, without and with membranes of different areas under diverse operating conditions. Hydrogen permeate purity of up to 99.995+% as well as a pure-H₂-to-methanes yield of 2.07 were achieved with only half of the full complement of membrane panels active. A permeate-H₂-to reactor methane feed molar ratio >3 was achieved when all of the membrane panels were installed. The reactor model is tested with no adjustable parameters by comparing predictions against axially distributed concentration in the pilot reactor, leading to reasonable agreement and better understanding of a variety of phenomena.

View record

 
 

If this is your researcher profile you can log in to the Faculty & Staff portal to update your details and provide recruitment preferences.