An experimental investigation of the swirling flow in a tall-form counter current spray dryer

This work studies the air flow in a large swirl counter-current dryer using sonic anemometry. Air velocity and turbulence fields are reported at isothermal conditions and in the absence of particles. In a tall-form unit the structure of the flow is largely influenced by the design of the exit. A contraction originates a central jet and suppresses the formation of recirculation zones despite the vortex acquires a high swirl intensity Ω (i.e. 1<Ω<2). Access to a full scale tower has permitted to: (a) identify asymmetries owed to the design of inlet and exhaust ducts, (b) present the first detailed turbulence data in production units, characterized by a highly anisotropic field and the axial decay of the turbulence kinetic energy, (c) study the flow stability, identifying the precession of the vortex core and oscillations at a constant Strouhal number and (d) study the impact that a rough wall has in the strength of the swirl. This work presents the first clear evidence of significant friction in spray dryers. The swirl intensity Ω decays exponentially in the dryer at a rate between 0.08 and 0.09, much higher than expected in pipe flow and independent of Re in the range 105-2.2{dot operator}105. Production dryers have a large characteristic wall roughness due the presence of deposits, which explains the stronger friction and the discrepancies found in the past between data at full scale or clean laboratory or pilot scale units. It is essential to address this phenomenon in current numerical models, which are validated on laboratory or pilot scale facilities and ignore the role of deposits, thus causing an overprediction of the tangential velocity above 30-40%

ansys fluent; CFD; PCRD; simulation; sago starch

Pengering pati sagu tipe pneumatic conveying ring dryer (PCRD) skala mini kapasitas 80 kg/hari telah diaplikasikan pada pengolahan pati sagu untuk memproduksi pati sagu kering. Kapasitas produksi dari mesin pengering PCRD tersebut, dapat ditingkatkan dengan memodifikasi pipa resirkulasi. Adapun bagian pipa yang dimodifikasi adalah diameter pipa venturi dan pipa vertikal upriser ditingkatkan. Diameter pipa vertikal upriser 2,5 kali lebih besar dari pipa u-bend dan pipa vertikal downcomer sehingga terbentuk buffer. Selain itu, dengan perbedaan diameter pipa resirkulasi tersebut dapat meningkatkan waktu tinggal (residence time) bahan. Tujuan penelitian ini adalah melakukan simulasi menggunakan ANSYS FLUENT R.15 untuk mengetahui profil suhu, kecepatan aliran udara, dan tekanan di dalam pipa pengering pati sagu tipe pneumatic conveying ring dryer (PCRD) kapasitas 1 ton per hari. Simulasi dilakukan dengan teknik Computational Fluid Dynamics (CFD) menggunakan paket perangkat lunak (software) ANSYS FLUENT R.15. Hasil simulasi menunjukkan bahwa suhu di sepanjang pipa mengalami penurunan sekitar 2oC (100oC menjadi 98oC) pada berbagai variasi kecepatan udara input dan variasi outlet pada kondisi batas. Begitupula dengan kecepatan udara pada ujung pipa outlet (pipa vertikal downcomer) meningkat (89 m/s sampai 166 m/s) karena adanya perbedaan diameter dengan pipa inlet. Tekanan pada pipa vertikal upriser lebih tinggi (8566,2 Pa sampai 26638,2 Pa) daripada tekanan pada pipa u-bend dan pipa vertikal downcomer. Hasil simulasi tersebut menunjukkan bahwa rancangan pipa resirkulasi sangat baik digunakan, sehingga dapat dilanjutkan untuk pembuatan pengering pati sagu tipe PCRD skala 1 ton per hari. Hal ini, sesuai dengan polasebaran suhu, kecepatan aliran udara, dan tekanan yang dihasilkan.The mini-scale pneumatic conveying ring dryer (PCRD) type sago starch dryer with a capacity of 80 kg / day has been applied to the processing of sago starch to produce dry sago starch. To increase the production capacity of the PCRD dryer, a recirculation pipe was modified. The modified pipe section is the venturi pipe diameter and the upriser vertical pipe increased. The diameter of the vertical upriser pipe is 2.5 times larger than that of the ubend pipe and the downcomer vertical pipe so that a buffer is formed. In addition, the difference in the diameter of the recirculation pipe can increase the residence time of the material. The purpose of this study was to simulate using ansys fluent to determine the temperature profile, air flow velocity, and pressure in the pneumatic conveying ring dryer (PCRD) type sago starch dryer pipe with a capacity of 1 ton per day. Simulations were carried out using the Computational Fluid Dynamics (CFD) technique using ansys fluent software package. The simulation results show that the temperature along the pipe has decreased by about 2oC at various variations of the input air velocity and variations in the outlet at the boundary conditions. Likewise, the air velocity at the end of the outlet pipe (vertical downcomer pipe) increases due to the difference in diameter with the inlet pipe. The pressure on the vertical upriser pipe is higher than the pressure on the ubend pipe and downcomer vertical pipe. The simulation results show that the recirculation pipe design is very well used so that it can be continued for the manufacture of PCRD-type sago starch dryer on a scale of 1 ton per day

Agglomeration in counter-current spray drying towers. Part A: Particle growth and the effect of nozzle height

Agglomeration of particles and droplets is critical to the operation of spray dryers, however it remains relatively unexplored. This paper studies the effect of the nozzle height on product properties, wall deposits and dryer conditions in a counter-current spray drying tower of detergent with a swirling air flow. The process efficiency is driven by changes in particle agglomeration. To interpret the results and facilitate the study of swirl towers, it is useful to subdivide these units according to the sources of growth in (a) spray region(s), (b) concentrated near-wall region(s) and (c) wall deposits. The particles formed are very heterogeneous and show a size-dependent composition. In this case, particle properties are driven by the separation of solid and liquid phases during atomization and the formation of a heterogeneous set of droplets. Agglomeration serves to homogenise the product and create a distinct source of porosity. The capacity and energy consumption of the dryer are also determined by the evolution of the particle size, as fine powder is elutriated from the tower top and coarse particles are removed from the product. When the nozzle is moved to lower positions in the tower the increased temperature near the spray suppresses agglomeration, however the residence time is shortened and ultimately it leads to creation of wet, coarse granules. An optimum location is found high enough to maintain the drying efficiency but sufficiently far from the top exit to minimise the loss of fine particles. In this way, a capacity ratio (i.e. product vs spray dried powder) C &gt; 90% can be obtained and energy efficiency maximised

Hydrodynamic Simulation of Cyclone Separators

Cyclone separators are commonly used for separating dispersed solid particles from gas phase. These devices have simple construction; are relatively inexpensive to fabricate and operate with moderate pressure losses. Therefore, they are widely used in many engineering processes such as dryers, reactors, advanced coal utilization such as pressurized and circulating fluidized bed combustion and particularly for removal of catalyst from gases in petroleum refinery such as in fluid catalytic cracker (FCC). Despite its simple operation, the fluid dynamics and flow structures in a cyclone separator are very complex. The driving force for particle separation in a cyclone separator is the strong swirling turbulent flow. The gas and the solid particles enter through a tangential inlet at the upper part of the cyclone. The tangential inlet produces a swirling motion of gas, which pushes the particles to the cyclone wall and then both phases swirl down over the cyclone wall. The solid particles leave the cyclone through a duct at the base of the apex of the inverted cone while the gas swirls upward in the middle of the cone and leaves the cyclone from the vortex finder. The swirling motion provides a centrifugal force to the particles while turbulence disperses the particles in the gas phase which increases the possibility of the particle entrainment. Therefore, the performance of a cyclone separator is determined by the turbulence characteristics and particle-particle interaction.Full Tex

Experimental and Numerical Investigations of Skim Milk Powder Stickiness and Deposition Mechanisms

The particle gun method and Computational Fluid Dynamics (CFD) modelling was used to study stickiness and deposition mechanisms of skim milk powder in an impingement jet hitting a stainless steel plate. The particular focus was on the effect of jet velocity and particle size distribution on deposition. Low jet velocities of 10.3, 14.8 and 19.4 m/s were studied at fine particle size levels of 30, 51, and 61 mm, using a jet-plate height to jet diameter ratio of 4. For skim milk powder with a bulk particle size (d(0.5) = 61 μm), lowering the air velocity from 19.4 m/s to 10.3 m/s increased the level of deposition and decreased the point at which deposition first occur as measured by the temperature difference between the glass transition temperature (Tg) of amorphous lactose and the air jet temperature of the particle gun. This point is called (T-Tg)critical. The critical point decreased from 39.0 °C to 18.6 °C as the velocity decreased from 19.4 to 10.3 m/s and the (T-Tg)critical obtained at the lower velocity is in closer agreement with previously reported fluid bed rig results. The (T-Tg)critical point and level of deposition was also found to be highly dependent on particle size. Increasing the average particle size from 30 μm to 61μm increased the (T-Tg)critical from 8.2 °C to 18.6 °C and 14.8 °C to 39.0 °C for jet velocities of 10.3 m/s and 19.4 m/s respectively. Levels of particle deposition also dramatically decreased for both velocity ranges. Ring shaped deposit morphologies were observed with increasing particle stickiness. Beyond (T-Tg)critical powder deposits formed at the periphery of the plate creating a large round clear zone which decreased until a striped deposit ring formed and finally deposits formed only at the centre. Particles were observed to bounce radially from the centre of the plate before sticking. Milk powder deposits are therefore governed by the kinetic energy of the impinging particle in addition to particle surface stickiness. The particle gun method was modelled using Fluent CFD software as an adhesion phenomenon arising from the particle-surface contact dynamics of a particle laden impingement jet contacting a vertical collection plate. The development of a wall boundary condition for specifying the particle-surface interaction has been the focus. A particle is captured by the wall if the impinging kinetic energy is below the prescribed critical normal kinetic energy; otherwise the particle rebounds with reduced kinetic energy. The model was developed through the User Define Function option of Fluent. The CFD model confirmed that particles rebound radially from the collection plate several times before sticking. Circular deposit morphology results from such modelled contact dynamics which are similar to the observed experimental deposits. The level of deposition predicted by CFD increased with increasing levels of critical normal kinetic energy, in the same way experimental deposits increased with increasing particle stickiness. The current model did not considered the contribution from the tangential velocity component to particle stick/rebound behaviour, but it is expected the tangential velocity may also play a significant role and should be included in future CFD models. It is recommended that the particle-surface interaction needs to be studied in more detail, preferably with imaging systems such as Particle Image Velocimetry (PIV), so that individual particle trajectory and deposition behaviour can be followed and analysed

Thermochemical conversion of biomass in a Swirling Fluidized Bed: a design procedure and numerical simulation

Process intensification of biomass conversion as a route to a low-carbon manufacturing industry pursues novel solutions able to achieve safe, cost-efficient, energy-efficient, and environment-friendly processes. Implementation of process intensification in gas-solid operations enhances mass, heat, and momentum transfer rates, while develops multifunctional equipment to increase production capacity per size of installation. The Swirling Fluidized Bed reactor is a gas-solid contacting device that replaces the Earth's gravitational field with a centrifugal field generating a centrifugal bed that achieves more uniform beds, higher transfer rates, and shorter processing times than conventional fluidized beds. However, there is a gap of research in two points: a binary-phase numerical simulation to study both gas and solid hydrodynamics, and the constructive design of the swirling fluidized bed reactor related to expected operating conditions. In the present work, a design procedure of swirling fluidized beds for thermochemical conversion of biomass is proposed. The study of the swirling fluidized bed reactor comprises three stages: a systematic literature review, a numerical simulation of the reactor, and the development of the design procedure. The simulation gives insight of the SFB reactor operation useful for the decision making during early stages of design. Thermochemical and mechanical models together with technical procedures are used for the reactor design. The proposed design shows good agreement with an operational reactor used for rice husk combustion.MaestríaMagister en Ingeniería Mecánic

Design, Experimentation, and Modeling of a Novel Continuous Biodrying Process

RÉSUMÉ La production massive de boue provenant de l’industrie des pâtes et papiers rend la gestion efficace de cette boue un problème grandissant et critique pour l’industrie des pâtes et papiers. Ceci est dû aux hauts coûts d’enfouissement et de transport, ainsi que des cadres de normalisation complexes pour les options d’épandage et de compostage. Les défis concernant le séchage mécanique des boues se sont aggravés dans plusieurs usines dus à la progression de la récupération interne de fibre complémentée par une augmentation de la production de boue secondaires, ce qui résulte en une boue mixte avec une haute proportion de matière biologique, qui est difficilement asséchée mécaniquement. Dans cette thèse, un réacteur de bio-séchage en continu novateur a été conçu et développé pour sécher la boue mixte d’usine de pâtes et papiers à un niveau de solide permettant la combustion sécuritaire et économique pour la récupération d’énergie dans une chaudière à combustion de biomasse. En plus de l’aération forcée, le procédé de séchage est amplifié par la génération de chaleur biologique provenant de l’activité microbienne des bactéries mésophiles et thermophiles naturellement présentes dans la boue. Ceci rend le procédé de bio-séchage plus attrayant comparativement à des technologies de séchages conventionnelles, puisque le réacteur est un procédé auto-chauffant. Le réacteur est divisé en quatre compartiments nominaux, sur le vertical, et la boue bouge de haut vers le bas du réacteur. Les temps de résidence étudiés ont été de 4-8 jours, ce qui est 2-3 fois plus court que les temps de résidence avec le réacteur de bioséchage en mode batch et ce pour des siccités finales de la boue semblables. Une analyse des variables du procédé a été performée pour déterminer les variables clés du procédé de bio-séchage en continu. Les paramètres ayant été étudiés sont : la biomasse en terme du pH et du ratio C/N, le temps de résidence, le recyclage de la boue bio-séchée et le profile de l’humidité relative à la sortie. Deux de ces paramètres ont été reconnus comme ayant le plus d’impact sur le bio-séchage en continu : le type de biomasse en terme de nutriments (ratio C/N), une variable incontrôlable, et le profile d’humidité relative à la sortie, une variable contrôlable. Le type de biomasse est fixé par choix, donc l’influence du profil d’humidité relative à la sortie a été étudiée sur la performance générale et celle en deux dimensions du réacteur de bio-séchage en continu. La meilleure efficacité, avec le bio-séchage, a été obtenue avec un profil d’humidité relative à la sortie qui contrôle l’évaporation d’eau interstitielle et libre à la température de bulbe humide pour le 1er et 2e compartiments du réacteur et l’évaporation d’eau liée et de surface à la température de bulbe sèche dans le 3e et 4e compartiments.----------ABSTRACT Massive production of sludge in the pulp and paper industry has made the effective sludge management increasingly a critical issue for the industry due to high landfill and transportation costs, and complex regulatory frameworks for options such as sludge landspreading and composting. Sludge dewatering challenges are exacerbated at many mills due to improved inplant fiber recovery coupled with increased production of secondary sludge, leading to a mixed sludge with a high proportion of biological matter which is difficult to dewater. In this thesis, a novel continuous biodrying reactor was designed and developed for drying pulp and paper mixed sludge to economic dry solids level so that the dried sludge can be economically and safely combusted in a biomass boiler for energy recovery. In all experimental runs the economic dry solids level was achieved, proving the process successful. In the biodrying process, in addition to the forced aeration, the drying rates are enhanced by biological heat generated through the microbial activity of mesophilic and thermophilic microorganisms naturally present in the porous matrix of mixed sludge. This makes the biodrying process more attractive compared to the conventional drying techniques because the reactor is a self-heating process. The reactor is divided into four nominal compartments and the mixed sludge dries as it moves downward in the reactor. The residence times were 4-8 days, which are 2-3 times shorter than the residence times achieved in a batch biodrying reactor previously studied by our research group for mixed sludge drying. A process variable analysis was performed to determine the key variable(s) in the continuous biodrying reactor. Several variables were investigated, namely: type of biomass feed, pH of biomass, nutrition level (C/N ratio), residence times, recycle ratio of biodried sludge, and outlet relative humidity profile along the reactor height. The key variables that were identified in the continuous biodrying reactor were the type of biomass feed and the outlet relative humidity profiles. The biomass feed is mill specific and since one mill was studied for this study, the nutrition level of the biomass feed was found adequate for the microbial activity, and hence the type of biomass is a fixed parameter. The influence of outlet relative humidity profile was investigated on the overall performance and the complexity index of the continuous biodrying reactor. The best biodrying efficiency was achieved at an outlet relative humidity profile which controls the removal of unbound water at the wet-bulb temperature in the 1st and 2nd compartments of the reactor, and the removal of bound water at the dry-bulb temperature in the 3rd and 4th compartments