Wall Adhesion and Constitutive Modelling of Strong Colloidal Gels

Wall adhesion effects during batch sedimentation of strongly flocculated colloidal gels are commonly assumed to be negligible. In this study in-situ measurements of colloidal gel rheology and solids volume fraction distribution suggest the contrary, where significant wall adhesion effects are observed in a 110mm diameter settling column. We develop and validate a mathematical model for the equilibrium stress state in the presence of wall adhesion under both viscoplastic and viscoelastic constitutive models. These formulations highlight fundamental issues regarding the constitutive modeling of colloidal gels, specifically the relative utility and validity of viscoplastic and viscoelastic rheological models under arbitrary tensorial loadings. The developed model is validated against experimental data, which points toward a novel method to estimate the shear and compressive yield strength of strongly flocculated colloidal gels from a series of equilibrium solids volume fraction profiles over various column widths.Comment: 37 pages, 12 figures, submitted to Journal of Rheolog

Characteristics of the Solid Volume Fraction Fluctuations in a CFB

In the paper, the fluctuation characteristics of the solids volume fraction in a CFB are evaluated from measurements and Eulerian-Eulerian CFD simulations. In both cases, similarly large fluctuations are observed in the intermediate voidage range whereas dense and dilute suspension regions are more uniform, as expected. The frequency distributions of solids volume fraction are classified to represent three different suspension density regimes: dilute, dense and intermediate “bimodal” regimes

A small scale regularly packed circulating fluidized bed. Part I: Hydrodynamics.

The present investigation is based on the idea of intensifying the gas¿solids contact in a circulating fluidized bed by introducing obstacles into it. Such obstacles may effectively suppress radial inhomogeneities in the solids flux and concentration, increase the dynamic solids hold-up, and break up solids clusters. This article (Part I) deals with the hydrodynamics (pressure drop and solids hold-up) investigated at ambient conditions, for cocurrent upward flow of air and microsize solid particles (FCC, 70 µm diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × 0.06 m2 and contains regularly-stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas¿solids contact. Slide-valves mounted above and below the packed section can be used to trap the solids inventory and determine the (dynamic) solids hold-up. Gas and solids mass fluxes have been varied in the range of 0.7 < Gg < 4.4 and O < Gs < 15 kg m-2s-2, respectively. Part II will report on the results of gas¿solids mass transfer measurements, which have been carried out in the same set-up at comparable experimental conditions. Results of this work show that: (i) the pressure gradient over the packed section increases linearly with increasing solids mass flux, but faster than linearly with increasing applied gas mass flux, (ii) the dynamic solids volume fraction can be described quite well by the correlation ß dyn = 0.0084 GsGg-1.22 for almost the entire range of applied gas and solids mass fluxes, (iii) the value for the solids friction factor derived for the gas flux range 0.7 < Gg < 3.7 kg (m-2s-1) varies from 1.4 to 2.5 and is linear with the solids volume fraction. These fs values are about 2 to 3 decades higher than those obtained from fs correlations derived for dilute-phase pneumatic conveying lines operated under the same experimental conditions

Gas-Solids Hydrodynamics in a CFB with 6 Cyclones and a Pang Leg

Solids volume fraction and particle velocity profiles were measured with a fiber optical probe in a cold circulating fluidized bed test rig with 6 parallel cyclones and a pant leg. Results in the pant leg zone, main bed zone and exit zone of the furnace are reported. The work also includes the influences of superficial gas velocity, secondary air rate and static bed height on the gas-solids hydrodynamics

การศึกษาพฤติกรรมไฮโดรไดนามิกส์และความสม่ำเสมอตามแนวรัศมีในปฏิกรณ์ฟลูอิไดซ์เบดชนิดไหลเวียนลงโดยการจำลองแบบซีเอฟดี

งานวิจัยนี้ศึกษาพฤติกรรมไฮโดรไดนามิกส์และความสม่ำเสมอตามแนวรัศมีในปฏิกรณ์ฟลูอิไดซ์เบดชนิดไหลเวียนลง (ปฏิกรณ์ดาวเนอร์) ด้วยการจำลองแบบปฏิกรณ์มีขนาดเส้นผ่านศูนย์กลาง 0.1 เมตร และสูง 9.3 เมตร แบบจำลองที่ใช้ในการทำนายพฤติกรรมการไหลคือแบบจำลองของไหลสองชนิด (Two-fluid Model) ร่วมกับทฤษฎีจลน์ของการไหลของอนุภาคแกรนูลาร์ผลการจำลองแบบแสดงให้เห็นว่า การกระจายของค่าสัดส่วนอนุภาคตามแนวรัศมีเป็นแบบ Core-annulus ค่าสัดส่วนอนุภาคมีค่าเพิ่มขึ้นเมื่อเพิ่มอัตราการไหลวนของอนุภาค (Gs) หรือลดความเร็วก๊าซป้อน (Ug) ทำให้ความสม่ำเสมอตามแนวรัศมีน้อยลง ส่วนความเร็วก๊าซและอนุภาคมีค่าเพิ่มขึ้นเมื่อเพิ่ม Ug แต่ไม่เปลี่ยนแปลงกับ Gs นอกจากนี้ยังพบว่าความสม่ำเสมอตามแนวรัศมีของความเร็วก๊าซและอนุภาคมีค่ามากขึ้นเมื่อลด Ugคำสำคัญ: ปฏิกรณ์ฟลูอิไดซ์เบดชนิดไหลเวียนลง พฤติกรรมไฮโดรไดนามิกส์ พลศาสตร์ของไหล เชิงคำนวณ การจำลองแบบThis research studies the hydrodynamics behavior and radial uniformity in a co-current down-flow circulating fluidized bed (downer reactor) by means of numerical simulation. A downer reactor with 0.1 m internal diameter and 9.3 m height was used. Two-fluid model based on kinetic theory of granular flow was adopted to predict the flow behavior in the system. The simulation results show that the radial distribution of solids volume fraction exhibits a core-annulus structure. The solids volume fraction increases with increasing of the solids circulation rate (Gs) or decreasing of gas velocity inlet (Ug). This leads to less radial uniformity of solids volume fraction. Gas and solids velocities increase with increasing of Ug but insignificantly change with Gs. In addition, the radial uniformity of gas and solids velocities increases with decreasing of Ug.Keywords: Co-current Down-flow Circulating Fluidized Bed, Hydrodynamics Behavior, Computational Fluid Dynamics, Simulatio

Simulation Studies of Gas-Solid in the Riser of a Circulating Fluidized Bed

A numerical parametric study was performed on the influence of various riser exit geometries on the hydrodynamics of gas-solid two-phase flow in the riser of a Circulating Fluidized Bed (CFB). A Eulerian continuum formulation was applied to both phases. A two fluid framework has been used to simulate fully developed gas-solid flows in vertical riser. A two dimensional Computational Fluid Dynamics (CFD) model of gas-particle flow in the CFB has been investigated using the code FLUENT. The turbulence was modeled by a k-e turbulence model in the gas phase. The simulations were done using the geometrical configuration of a CFB test rig at the Universiti Teknologi Malaysia (UTM). The CFB riser column has 265 mm (width), 72 mm (depth) and 2.7 m height. The riser is made up of interchangeable Plexiglas columns. The computational model was used to simulate the riser over a wide range of operating and design parameters. In addition, several numerical experiments were carried out to understand the influence of riser end effects, particle size, gas solid velocity and solid volume fraction on the simulated flow characteristics. The CFD model with a k-e turbulence model for the gas phase and a fixed particle viscosity in the solids phase showed good mixing behaviour. These results were found to be useful in further development of modeling of gas solid flow in the riser

A small scale regularly packed circulating fluidized bed. Part II: Mass transfer

The underlying objective of the present study is to increase gas¿solids contact in a circulating fluidized bed by the introduction of obstacles in the riser portion. The presence of such obstacles leads to suppression of radial inhomogeneities in the solids mass flux and concentration, and break-up of solids clusters. At ambient conditions, gas¿solids mass transfer was investigated for cocurrent upward flow of air and microsize solid particles (FCC, 70 ¿m diameter) over a regularly structured inert packing introduced into the riser part of a circulating fluidized bed unit. The packed section has a height of 0.48 m, a cross-sectional area of 0.06 × O.06 m2, and contains regularly stacked 0.01 m diameter Perspex bars as the obstacles meant to enhance the gas¿solids contact. Gas mass fluxes used were 1.4 and 2.7 kg m¿2 s¿1. Solids mass fluxes were varied in the range 0Gs 12 kg m¿2 s¿1. Experimental mass transfer data were obtained by applying the method of adsorption of naphthalene vapor on FCC particles. A conservative estimate of the apparent gas¿solids mass transfer coefficient kg* could be derived from the naphthalene vapor concentration profile along the packed section on the basis of a plug-flow-model interpretation, while assuming single-particle behaviour and neglecting intraparticle diffusion effects. Such kg* values appear to increase with increasing gas mass flux, but decrease with increasing solids mass flux (and consequently increasing solids volume fraction) probably due to the corresponding increase in particle shielding. Comparison of the present results with available literature data for similar solid materials suggests that the effect of the packing inserted into the CFB is significant: the Sherwood numbers derived from the present study are relatively high

FIBRE-OPTIC PROBE FOR THE SIMULTANEOUS MEASUREMENT OF GASEOUS SPECIES COMPOSITION AND SOLIDS VOLUME FRACTION

A novel infrared fibre-optic probe was developed to measure quantitatively and simultaneously solids volume fraction (1-E) and gaseous species composition (Yi) in a gas/solid fluidized bed. The fibre-optic probe was used with a FT-IR spectrometer to perform real-time and in-situ measurements of absorbance in the fluidized bed. The effect of (1-E) and Yi on the absorbance spectra were additive and could be independently calibrated. To calibrate the probe, fuel mole fractions and (1-E) were varied between 1.8 - 10.1 mol% and 0 - 0.45, respectively. A proof of concept for a novel application in fluidized beds was completed: the fibre-optic probe was used to measure the molar fraction of a tracer gas inside the emulsion and bubble phases during gas tracer experiments

Non-invasive velocity and volume fraction profile measurement in multiphase flows

Multiphase flow is the simultaneous flow of two or more phases, in direct contact, and is important in the oil industry, e.g. in production wells, in sub-sea pipelines and during the drilling of wells. The behaviour of the flow will depend on the properties of the constituent phases, the flow velocities and volume fractions of the phases and the geometry of the system. In solids-in-liquid flows, measurement of the local solids volume fraction distribution and the local axial solids velocity distribution in the flow cross section is important for many reasons including health and safety and economic reasons, particularly in oil well drilling operations. However upward inclined solidsliquid flows which are frequently encountered during oil well drilling operations are not well understood. Inclined solids-liquid flows result in non- uniform profiles of the solids volume fraction and axial solids velocity in the flow cross- section. In order to measure the solids volumetric flow rate in these situations it is necessary to measure the distributions of the local solids volume fraction and the local axial solids velocity and then to integrate the product of these local properties in the flow cross section. This thesis describes the development of a non-intrusive Impedance Cross-Correlation (ICC) device to measure the local solids volume fraction distribution and the local solids axial solids velocity distribution in upward inclined solids-water flows in which these distributions are highly non-uniform. The ICC device comprises a non-conductive pipe section of 80mm internal diameter fitted with two arrays of electrodes, denoted „array A‟ and „array B‟, separated by an axial distance of 50mm. At each array, eight electrodes are equispaced over the internal circumference of the pipe. A control system consisting of a microcontroller and analogue switches is used such that, for arrays A and B, any of the eight electrodes can be configured as an "excitation electrode" (V+), a "virtual earth measurement electrode" (Ve) or an "earth electrode" (E) thus enabling the local mixture conductance in different regions of the flow cross-section to be measured and thereby allowing the local solids volume fraction in each region to be deduced. The conductance signals from arrays A and B are also cross-correlated to yield the local solids axial velocity in the regions of flow under interrogation. A number of experiments were carried out in solids-in-water flows in a flow loop with an 80 mm inner diameter, 1.68m long Perspex test section which was inclined at three different inclination angle to the vertical ( o 0 , o 15 and o 30 ). The obtained results show good quantitative agreement with previous work carried out using intrusive local probes. Integration of the flow profiles in the cross section also yielded excellent quantitative agreement with reference measurements of the mean solids volume fraction, the mean solids velocity and the solids volumetric flow rate. Furthermore, this study also showed good qualitative agreement with high speed film of the flow. It is believed that the method of velocity and volume fraction profile measurement described in this thesis is much simpler to implement, more accurate and less expensive than the currently very popular technique of dual-plane Electrical Resistance Tomography (ERT). Finally, the thesis describes a mathematical model for predicting the axial velocity distribution of inclined solids-water flows using the solids volume fraction profiles measured by the ICC device. Good agreement was obtained between the predicted velocity profiles and the velocity profiles measured using the ICC device.EThOS - Electronic Theses Online ServiceGBUnited Kingdo