Pattern formation in pulsed gas-solid fluidized beds - The role of granular solid mechanics

Under certain conditions, gas-solid fluidized beds are known to develop a structured flow of bubbles when exposed to periodically pulsating air flows. In quasi-two-dimensional beds, periodically rising bubbles form a triangular tessellation in the vertical plane. Bubble nucleation sites at the distributor plate alternate during each cycle. This pattern sets an excellent benchmark for fundamental studies of fluidization. Notably, most common Eulerian descriptions of granular flow do not yet capture this interplay between solid mechanics and fluid-solid momentum exchange, which we show to be instrumental to the dynamic rearrangement of bubbles in a pulsed bed. We report the first successful CFD simulations of structured bubble flows in a deep, quasi-2D geometry using a Eulerian-Lagrangian CFD-DEM framework. Numerical results are in quantitative agreement with experiments. The simulated dynamics reveal that the patterns emerge from the transition of the granular collective behavior between solid-like and fluid-like, which is an outcome of dynamical coupling between gas and particles. The simulated results point out the essential role of solid frictional stresses on inducing and maintaining the formation of bubble patterns. This underscores the value of investigating pulsation-induced patterns as a prime manifestation of the mesoscopic physics underpinning fluidization, and highlights the direction for improving current practices

Effect of curing conditions and harvesting stage of maturity on Ethiopian onion bulb drying properties

The study was conducted to investigate the impact of curing conditions and harvesting stageson the drying quality of onion bulbs. The onion bulbs (Bombay Red cultivar) were harvested at three harvesting stages (early, optimum, and late maturity) and cured at three different temperatures (30, 40 and 50 oC) and relative humidity (30, 50 and 70%). The results revealed that curing temperature, RH, and maturity stage had significant effects on all measuredattributesexcept total soluble solids

STUDIES ON GRANULATION, DRYING AND TRIBOCHARGING BEHAVIOUR OF PHARMACEUTICAL POWDERS IN A FLUIDIZED BED DRYER

Wet granulation and drying are two crucial unit operations in the pharmaceutical industry. The two operations can be conducted in one piece of equipment, namely a fluidized bed. Fluidized beds have the advantages of excellent mixing, large contact area, and superior heat and mass transfer. Drying is typically operated at bubbling and turbulent regions. Despite the wide applications of fluidized beds in wet granulation and drying, there still remain challenges. Observation and measuring the dynamic granulation process are challenging with conventional experimental methods due to the opaque nature of pharmaceutical powders and the complex interaction between powders and liquid taking place in a short period of time. In this study, the dynamic granulation process was, for the first time, captured with synchrotron-based X-ray imaging techniques. The dynamic interaction between the pharmaceutical powders and the liquid binder was captured by high-resolution X-ray images. Results show that pharmaceutical powder properties, including particle size, hydrophilicity, and morphology, have significant influences on the dynamic granulation process and the final granular product. After wet granulation, the presence of high moisture content within pharmaceutical granules results in considerable cohesiveness. Agglomeration, channeling, defluidization, caused by the strong inter-particle forces, pose significant challenges to fluidization and drying, particularly at the beginning of the drying process. In this work, the drying performance of pharmaceutical granules was investigated in a pulsation-assisted fluidized bed dryer. It was found that pulsed airflow is effective in eliminating channeling and enhancing the drying rate at higher superficial gas velocity. Lower pulsation frequency is more favoured to improve the drying rate. Two typical drying stages were observed during the drying process, the constant rate period and the falling rate period. During the constant rate period, energy efficiency is between 60% to 45% for the drying process. The energy efficiency falls to 10% during the falling rate period. Nine thin-layer drying models were examined to predict the drying curve of the pharmaceutical granules. It was found that the Midilli and Kucuk model provided the best agreement between the experimental results and the predicted values. The pharmaceutical granules can be easily charged because of repeated collision and separation between particles and between particles and wall. The tribocharging behaviour of the pharmaceutical granules in a conventional FBD and a PFBD was investigated by varying operating conditions such as superficial gas velocity, inlet air temperature, pulsation frequency, and pulsed air ratio. It was found that the specific charge of the pharmaceutical granules remained lower than 0.2 µC/kg during the constant rate period. When the moisture content was reduced to a critical moisture content, namely 10%, the specific charge increased sharply regardless of the superficial gas velocity and inlet air temperature. Then, the increase in the specific charge continued before it reached an equilibrium value during the falling rate period. The equilibrium specific charge is influenced by the superficial gas velocity and pulsation frequency. Higher superficial gas velocity and lower pulsed frequency resulted in a higher specific charge. When the superficial gas velocity is low, there is no noticeable difference between the conventional FBD and the PFBD at different pulsation frequencies. The inlet air temperature and pulsed air ratio did not show an impact on the equilibrium specific charge value

Numerical Study on Spout Elevation of a Gas-Particle Spout Fluidized Bed in Microwave-Vacuum Dryer

The dynamic characteristics of gas-particle spout fluidized bed in a pulsed spouted microwave-vacuum drying system (PSMVD) were investigated. The spout fluidization process in a pseudo-2-D spout fluidized bed was simulated by computational fluid dynamics (CFD) using the inviscid two-fluid theory method (TFM) based on the kinetic theory of granular flow. The dynamic characteristics of the spout fluidized bed and the effect of spout elevation on the particle movement were revealed, which could be used to improve the uniformity of particle mixing and microwave heating. The mathematical model demonstrated that the spout fluidization process includes isolated, merged and transitional jets and the fluidization at a specific spout gas velocity has a start-up stage and a quasi-steady fluidization stage. The spout velocity was an important factor controlling particle status in the spout fluidized bed and a critical velocity was identified for effect transition of the flow pattern. There was an approximately linear correlation between the jet penetration depth and the spout velocity. When the spout gas velocity increased up to the critical velocity region, the pressure drop tended to convert from negative pressure to positive pressure

Ultrasound-assisted surface engineering of pharmaceutical powders

Effective processing of powdered particles can facilitate powder handling and result in better drug product performance, which is of great importance in the pharmaceutical industry where the majority of active pharmaceutical ingredients (APIs) are delivered as solid dosage forms. The purpose of this work was to develop a new ultrasound-assisted method for particle surface modification and thin-coating of pharmaceutical powders. The ultrasound was used to produce an aqueous mist with or without a coating agent. By using the proposed technique, it was possible to decrease the interparticular interactions and improve rheological properties of poorly-flowing water-soluble powders by aqueous smoothing of the rough surfaces of irregular particles. In turn, hydrophilic polymer thin-coating of a hydrophobic substance diminished the triboelectrostatic charge transfer and improved the flowability of highly cohesive powder. To determine the coating efficiency of the technique, the bioactive molecule β-galactosidase was layered onto the surface of powdered lactose particles. Enzyme-treated materials were analysed by assaying the quantity of the reaction product generated during enzymatic cleavage of the milk sugar. A near-linear increase in the thickness of the drug layer was obtained during progressive treatment. Using the enzyme coating procedure, it was confirmed that the ultrasound-assisted technique is suitable for processing labile protein materials. In addition, this pre-treatment of milk sugar could be used to improve utilization of lactose-containing formulations for populations suffering from severe lactose intolerance. Furthermore, the applicability of the thin-coating technique for improving homogeneity of low-dose solid dosage forms was shown. The carrier particles coated with API gave rise to uniform distribution of the drug within the powder. The mixture remained homogeneous during further tabletting, whereas the reference physical powder mixture was subject to segregation. In conclusion, ultrasound-assisted surface engineering of pharmaceutical powders can be effective technology for improving formulation and performance of solid dosage forms such as dry powder inhalers (DPI) and direct compression products.Jauhepartikkelien pintakäsittelyn avulla voidaan parantaa lääkevalmistuksessa käytettävien kiinteiden lääke- ja apuaineiden käsiteltävyyttä, prosessoitavuutta ja vaikutusta lopputuottessa/ elimistössä. Tämän väitöskirjatutkimuksen tavoitteena oli kehittää uusi ultraääntä hyväksikäyttävä nesteen sumutusmenetelmä, jonka avulla olisi mahdollisuutta muokata/pinnoittaa erilaisia jauhemaisia lääke- ja apuaineita siten, että niiden jatkoprosessoitavuus ja viime kädessä myös lääkevalmisteen laatu paranisivat. Tutkimuksessa ultraääntä on käytetty muodostamaan vesi-höyrysumua joko päällystysaineen kanssa tai ilman sitä. Väitöskirjatyössä kehitetyn uuden jauheenkäsittelytekniikan avulla oli mahdollista muokata huonosti valuvien ja karkeapintaisten jauhepartikkelien (mm. tiamiinihydrokloridi ja laktoosi) pintoja suoraan veden avulla ja vähentää niiden välistä kontaktipinta-alaa. Tämä puolestaan vähensi jauhepartikkelien välistä vuorovaikutusta (koheesiota) ja paransi ko. jauhemassan valuvuutta. Lisäksi menetelmän avulla voitiin päällystää suoraan hienojakoista lääkeainetta (ibuprofeenia) hydroxypropyylimetyylselluloosa (HPMC) -polymeerin muodostamalla ohuella kalvolla. Tuloksena lääkeaineen jauhepartikkeleiden valuvuus parani. Yllä mainitulla ultraääniavusteisella sumutustekniikalla onnistuttiin pinnoittamaan myös laktoosijauhetta maitosokeria pilkkovalla bioaktiivisella β-galaktosidaasientsyymillä. Laktoosin nanopäällystyminen ja tutkitun ultraääniavusteisen tekniikan tehokkuus selvitettiin ensyymi-tuotteen konsentraatiomääritysten avulla. Ensyymipinnoituksella vahvistettiin, että työssä kehitetty menetelmä sopii myös labiilin proteinimateriaalin käsittelemiseksi. Lisäksi tämän tyyppisellä maitosokerin esikäsittelyllä voitaisiin lisätä laktoosia sisältävien lääkevalmisteiden käyttöä laktoosi-intoleransista kärsivillä ihmisillä. Ultraääniavusteista tekniikkaa käytettiin myös lääkeapuaineen jauhepartikkeleiden päällystämi-seen parantamaan pieniannoksisten tablettivalmisteiden annostarkkuutta. Tabletit valmistettiin mikrokiteisesta selluloosasta, jonka partikkelit oli ennen puristusprosessia päällystetty malliaineen (riboflaviininatriumfosfaatti) vesiliuoksella. Käsittelyn tuloksena tablettien paino ja annoksen jakelutarkkuus oli selkeästi parempi kuin fysikaalisesta binääriseoksesta puristetun referenssi-tabletin vastaavat laatuominaisuudet. Yhteenvetona jauheiden pintojen hallittu muokkaus ultraääniavusteisesti antaa lupaavan mahdollisuuden jatkossa parantaa kiinteiden lääkemuotojen (kuten esimerkiksi jauheinhalaatio-valmisteiden ja suorapuristeisten tablettien) prosessoitavuutta ja vaikutusta elimistössä

A Study of Particle-laden Flows from Meso and Micro-scale Perspectives

Particle-laden flows are investigated numerically from a meso-scale perspective using Computational Fluid Dynamics coupled with Discrete Element Method (CFD-DEM) and from a micro-scale perspective using Particle Resolved Direct Numerical Simulation (PR-DNS). For the former, the dynamics of a pseudo-2D pulsed fluidized bed (PFB) consisting of 400,000 to 800,000 particles was investigated (Chapters 2 and 3). The focus is on the capabilities of CFD-DEM to (1) reproduce pattern formation in these systems and (2) further the understanding of the dynamics of PFB\u27s as a function of pulsation parameters. In Chapter 4, a two-spheres system is investigated with a recently implemented PR-DNS code, using the Basilisk open source framework. A high-resolution study is performed to investigate the flow field structures and their relation to experienced hydrodynamic forces by the spheres under the influence of a wall. In Appendix A, some verification and validation cases are reported with both the aforementioned codes, presenting capabilities that can be further explored in future work

An experimental study of heat transfer in pulsed fluidised beds via infrared thermography

This work investigates the heat transfer process in a quasi-2D pulsed fluidised bed, created by applying a pulsating gas flow around the state of minimum fluidisation. Heat transfer characteristics are quantified using infrared thermography (IRT). Digital detail enhancement and image restoration are employed to enhance image quality, minimise noise, and facilitate identification of local temperature fields. Under conditions close to minimum fluidisation, an oscillatory flow allows for the mixing of the solid phase, providing up to a 19% increase in the average overall heat transfer coefficient compared to a constant flow condition, while also leading to more homogeneous local mixing. Analysis of the local temperature fields suggests that bubble patterns formed by an oscillating gas flow lead to the creation of compartments, akin to Rayleigh-Bénard convection cells. The oscillatory flow regulates spatial and temporal temperature distributions in the solid phase and enhances powder mixing without the need to increase the gas flow rate. These findings highlight some advantages of using oscillating, dynamically structured fluidised beds to intensify gas–solid operations operating at a low gas throughput

Dynamically structured fluidization: Oscillating the gas flow and other opportunities to intensify gas-solid fluidized bed operation

Various approaches to structure gas-solid fluidized beds are reviewed, followed by detailed discussion on the use of gas pulsation to induce dynamic structuring. Granular media are dissipative systems, which develop complex spatiotemporal patterns when excited by an oscillating energy source. Here, we discuss how such perturbations initiate surface patterns and how these could propagate into a macroscopically organized flow. We call this dynamically structured fluidization. Vibrated shallow granular layers form ordered surface waves. The hydrodynamics of pulsed gas-fluidized layers are related, but more complex: Under appropriate conditions, surface waves transition into a three-dimensionally ordered bubbling flow. This occurs in much deeper granular beds than under vibration, indicating distinct physics. In this dynamically structured state, bubbles organize into a scalable sub-harmonic, triangular lattice that is highly predictable and responsive to changes in oscillation parameters, allowing for an unprecedented level of control. Structured bubbling is observed only under sufficiently dense conditions; thus, a dynamically structured fluidized bed sits between fixed and fluidized beds, offering opportunities for process intensification, due to less macromixing than traditional fluidization, but a higher level of control through micromixing. This informs new intensified designs for processes that are highly exothermic, involve particle formation, thermally sensitive or high-value materials

A review of solar thermal energy storage in beds of particles: Packed and fluidized beds

This review summarizes different solar thermal energy storage techniques from a particle technology perspective, including sensible, latent and thermochemical techniques for low- and high-temperature applications that use particles as the storage medium in the thermal energy storage system. The focus is on applications, experimental results, modeling and future trends. This review describes two different particle technologies used to store thermal energy: packed and fluidized beds. The advantages and disadvantages of both technologies are reviewed throughout different studies found in the literature for various thermal energy storage systems. Packed beds have the main advantage of thermal stratification, which increases the efficiency of solar collectors in low-temperature sensible energy storage systems and augments the exergy content in the bed. Moreover, they have been proven to be suitable as dual-media thermocline storage systems for CSP plants. In contrast, the high mixing rates of fluidized beds makes them suitable for the rapid distribution of concentrated solar energy in particle receiver CSP systems. In addition, their high heat and mass transfer rates, compared with those of packed beds, make them the preferred particle technology for thermochemical energy storage applications. This review also notes that it is important to find new materials with an appropriate size and density that can be properly used in a fluidized bed. Additionally, more specific research efforts are necessary to improve the understanding of the behavior of these materials during the fluidization process and over a high number of charging/discharging cycles