Hydrodynamics Of Trickle Bed Reactors: Measurements And Modeling

In this study we develop the computational and experimental tools to assist us in performance evaluation of trickle bed reactors: TBRs). The study focuses on experimental characterization of the flow distribution, and development of computational fluid dynamics: CFD) model of trickle flow. The experimental study has been performed to examine the quality of liquid phase distribution in a high pressure system. The results were provided in terms of distribution of the effluent liquid fluxes and cross-sectional liquid holdups. Their individual trends, but also their relation with respect to operating conditions was examined. Characterization of bed porosity distribution has been performed and used as the input to the computational model. The experimental study of the dependence of the extent of hysteresis on operating parameters in a high pressure TBR was performed. The extent of hysteresis was found uniquely determined by the pressure drop in the Levec prewetting mode. This fact and developed CFD model were then used to deduce conditions leading to operation with negligible hysteresis effects. Three-dimensional Eulerian CFD model is developed. Phase interaction closures are based on the film flow model, principles of statistical hydrodynamics and relative permeability concept. Model has been assessed against experimental data for liquid holdup, wetting efficiency and pressure drop hysteresis. Hydrodynamic Eulerian CFD model is then used together with species balance to examine the TBR performance for gas and liquid reactant limited systems. For each case a closed form approach of coupling bed and particle scale solution within CFD framework was presented

Effect of path length on valve tray columns: Experimental study

Experimental measurements of hydrodynamic and interfacial area parameters are carried out over two rectangular pilot scale valve tray columns. The effect of tray path length on extrapolation between the two columns is studied and phenomenological correlations for hydrodynamic and interfacial area are proposed. Correlations are compared both to literature and to industrial results showing good agreement and a significant improvement for the prediction of industrial conditions. Discrepancies preventing an accurate description of industrial trends are highlighted through comparison between typical emulsion height profiles on both columns

Thermochemical Conversion Processes for Solid Fuels and Renewable Energies: Volume II

The increasing share of renewable energy sources is drawing attention to a critical challenge. The availability of wind turbines and photovoltaic solar cells is limited and difficult to predict. They usually provide a fluctuating feed-in to the grid, so energy reserves, e.g., conventional thermal power plants or energy storage systems, are necessary to establish a balance between electricity supply and demand. Various solutions can be adopted to maintain the security of supply and improve the flexibility of the future power system, such as improving the efficiency of technical processes in areas such as thermal power plants, cement and metallurgy industries, the use of advanced thermochemical conversion technologies such as gasification, the expansion of high-voltage transmission infrastructure, the promoting of renewable energy sources, the employment of large-scale energy storage systems, and the use of highly flexible power generation units with carbon capture and utilisation, such as combined-cycle power plants. Given this background, this Special Issue contains fundamental scientific studies on the latest research progress in the development and optimisation of gasification processes, renewable energy source “solar energy”, synthesis of new hybrid nanocomposites and nanofluids, carbon capture, and energy storage systems. Special Issue Editors Falah Alobaid Jochen Ströhl

Experimental and modeling study of a cold-flow fluid catalytic cracking unit stripper

Many particulate processes are preferably implemented in circulating fluidized beds (CFB) over traditional low-velocity fluidization to take advantage of the many benefits of circulating systems. Fluid catalytic cracking (FCC) is one of the most successfully applied processes in CFB technology, with more than 350 FCC units in operation worldwide. Despite its extensive use, an understanding of the complex behaviour of these units is incomplete. A theoretical and experimental evaluation of the fluidization behaviour was conducted in the CFB riser, standpipe, and stripper. Initially, an extension of the existing CFB in the Fluidization Laboratory of Saskatchewan was designed. The experimental program conducted in this study included an examination of the solids flow behaviour in the riser, interstitial gas velocity in the downcomer, and stripping efficiency measurements. The hydrodynamic behaviour of the stripper was modeled using Multiphase Flow with Interphase eXchanges (MFIX) CFD code. The solids flow behaviour in the bottom zone of a high-density riser was investigated by measuring the local upwards and downwards solids flux. Solids circulation rates between 125 and 243 kg/(m²·s) were evaluated at a constant riser superficial gas velocity of 5.3 m/s. The effect of the riser superficial gas velocity of the local upflow at the riser centerline was also conducted at a solids circulation rate of 187 kg/(m²·s). The results show that there is little variation in the local net solids flux at radial locations between 0.00 ≤ r/R≤ 0.87. The results indicate that a sharp regime change from a typical parabolic solids flux profile to this more radially uniform solids flux profile occurs at a gas velocity between 4.8 and 4.9 m/s. To quantify stripping efficiency, the underflow of an injected tracer into the standpipe must be known. Quantification of the underflow into the standpipe requires knowledge of two main variables: the interstitial gas velocity and the tracer gas concentration profiles in the standpipe. Stripping efficiency was determined for stripper solids circulation rates of 44, 60, and 74 kg/(m²·s) and gas velocities of 0.1, 0.2, and 0.3 m/s. For most conditions studied, the interstitial gas velocity profile was found to be flat for both fluidized and packed bed flow. The stripping efficiency was found to be sensitive to the operating conditions. The highest efficiency is attained at low solids circulation rates and high stripping gas velocities. In the numeric study, stripper hydrodynamics were examined for similar operating conditions as those used in the experimental program. Due to an improved radial distribution of gas and decreasing bubble rise velocity, mass transfer is deemed most intense as bubbles crest above the baffles into the interspace between disc and donut baffles. Stripping efficiency is thought to improve with increasing gas velocity due to an increased bubbling frequency. Stripping efficiency is thought to decrease with increasing solids circulation rates due to a lower emulsion-cloud gas interchange coefficient and a decreased residence time of the emulsion in the stripper

Hydrodynamic and RTD of Sectionalized Bubble Column

The sectionalization of conventional bubble columns to tray partitioned bubble column using perforated trays has been used to investigate the effect of tray hole diameter, tray open area, superficial gas velocity, gas sparger design, and liquid phase properties on gas holdup, residence time distribution (RTD), and overall liquid-phase backmixing. The erected column is sectionalized into three stages using two perforated plates of different holes diameter and open free area. Overall gas holdup is measured experimentally by bed expansion technique. Liquid backmixing, mixing time and axial dispersion model (ADM) is determined using tracer response experiments. In general, it seems that the partitioned trays are significantly increases the overall gas holdup. Tray holes diameter and superficial gas velocity are found to be the most important factors on gas holdup. Axial mixing of the liquid phase is numerously reduced by the presence of partitioned trays. Comparison of the results with the published data of other authors indicates good agreement which enforced the reliability and confidentiality of computational procedure to be used for design and scale-up purposes

Tar Destruction in a Coandă Tar Cracker

Increasing the utilisation of bioenergy systems has the potential to become a vital component in the struggle to maintain and fulfil global energy demands. In particular, biomass gasification can offer a solution to the ‘Energy Trilemma’, and provide an affordable, reliable and carbon neutral technology. The limiting factor hampering the progression of biomass gasification power plants is tar. Tars formed during the thermal breakdown of biomass, condense and foul downstream equipment, causing reliability issues and damaging energy conversion equipment, such as engines and turbines. Treating tar through partial oxidation offers tar destruction without waste and soot, as well as maintaining the heating value of the tar in the producer gas. Coandă burners which are fuelled by more conventional fuels have been proven to operate close to, and below, stoichiometric conditions; as such, these devices were prime for further investigation. The main objective of this research project was to develop a small-scale system which utilises a novel Coandă burner for tar destruction. An experimental rig consisting of a wood pellet pyrolyser, which produced a gas loaded with tar, and a Coandă tar cracker, was designed, constructed and operated in order to determine the effectiveness of the process, with respect to tar reduction. The principal experimental program was divided into two phases, so that comparisons of the tar composition, before and after treatment, could be formed. In the first experimental phase, wood pellets were pyrolysed at a range of temperatures between 500 and 800ºC. The pyrolysis products (gas, tar and char) were analysed. As the pyrolysis temperature increased from 500 to 800ºC there was a decrease in the yield of gravimetric tar in the sampled gas from 78.59 to 16.55 g/Nm3. In the second phase the tarry gas was treated by the Coandă tar cracker. The Coandă tar cracker was shown to be effective at significantly reducing the tar content in the product gas. The yield of key tar components in the treated gas was reduced for all tested pyrolysis temperatures. For example; when the pyrolysis temperature was 800ºC; the yields of benzene, toluene and naphthalene were reduced by over 90% and the gravimetric tar yield by 88%. The success of the tar cracker can be attributed to the high flame temperature (>1000ºC) and the addition of oxygen which leads to the production of a greater proportion of radicals in the flame which initiate tar destruction reactions