Rex B. Thorpe

The prediction of particle cluster properties in the near wall region of a vertical riser (200157)

Abstract Correlations for predicting the properties of clusters of particles travelling near the riser wall are presented. The correlations were developed from experimental data published in the literature on vertical risers ranging from laboratory to industrial scale. Expressions are presented for predicting the size, shape, density, wall film coverage and velocity of particle clusters. These expressions should prove useful in the development of heat transfer and process models for gas–solid riser flow.

Environmental & economic life cycle assessment of current & future sewage sludge to energy technologies.

The UK Water Industry currently generates approximately 800GWh pa of electrical energy from sewage sludge. Traditionally energy recovery from sewage sludge features Anaerobic Digestion (AD) with biogas utilisation in combined heat and power (CHP) systems. However, the industry is evolving and a number of developments that extract more energy from sludge are either being implemented or are nearing full scale demonstration. This study compared five technology configurations: 1 - conventional AD with CHP, 2 - Thermal Hydrolysis Process (THP) AD with CHP, 3 - THP AD with bio-methane grid injection, 4 - THP AD with CHP followed by drying of digested sludge for solid fuel production, 5 - THP AD followed by drying, pyrolysis of the digested sludge and use of the both the biogas and the pyrolysis gas in a CHP. The economic and environmental Life Cycle Assessment (LCA) found that both the post AD drying options performed well but the option used to create a solid fuel to displace coal (configuration 4) was the most sustainable solution economically and environmentally, closely followed by the pyrolysis configuration (5). Application of THP improves the financial and environmental performance compared with conventional AD. Producing bio-methane for grid injection (configuration 3) is attractive financially but has the worst environmental impact of all the scenarios, suggesting that the current UK financial incentive policy for bio-methane is not driving best environmental practice. It is clear that new and improving processes and technologies are enabling significant opportunities for further energy recovery from sludge; LCA provides tools for determining the best overall options for particular situations and allows innovation resources and investment to be focused accordingly.

Particle residence time distributions in circulating fluidised beds

Abstract This paper gives experimental measurements of the particle residence time distribution (RTD) made in the riser of a square cross section, cold model, circulating fluidised bed, using the fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002a) 127). This technique depends upon all particles having phosphorescent properties. A small proportion of the particles become tracers when activated by a flash of light at the riser entry; the concentration of these phosphorescent particles can subsequently be detected by a photomultiplier. The influence of the solids circulation rate and superficial gas velocity on the RTD were investigated. The results presented are novel because (i) the experiments were performed in a system with closed boundaries and hence give the true residence time distribution in the riser and (ii) the measurement of the tracer concentration is exceedingly fast. The majority of previous studies have measured the RTD in risers with open boundaries, giving an erroneous measure of the RTD. Analysis of the results suggests that using pressure measurements in a riser to infer the solids inventory leads to erroneous estimates of the mean residence time. In particular, the results cast doubt on the assumption that friction and acceleration effects can be neglected when inferring the axial solids concentration profile from riser pressure measurements. An assessment of particle RTD models is also given. A stochastic particle RTD model was coupled to a riser hydrodynamic model incorporating the four main hydrodynamic regions observed in a fast-fluidised bed riser namely (i) the entrance region, (ii) a transition region, (iii) a core-annulus region and (iv) an exit region. This model successfully predicts the experimental residence time distributions.

The influence of the riser exit on the particle residence time distribution in a circulating fluidised bed riser

Abstract This paper reports measurements of the influence of riser exit geometry upon the particle residence time distribution in the riser of a square cross section, cold model, circulating fluidised bed. The bed is operated within the fast fluidisation regime. The fast response particle RTD technique developed by Harris et al. (Chem. Eng. J. 89 (2002) 127–142) was used to measure the residence time distribution. The geometry of the riser exit is shown to have a modest but consistent influence upon the particle RTD; the influence of operating conditions, i.e. superficial gas velocity and solids flux is more significant. Increasing the refluxing effect of the riser exit increases the mean, variance and breakthrough time and decreases the coefficient of variation of the residence time distribution. Changes in reflux do not have a systematic effect upon the skewness of the RTD.

A novel method for measuring the residence time distribution in short time scale particulate systems

Abstract A novel, non-intrusive method is described for measuring the particle residence time distribution (RTD) in a system with a short mean residence time. The method uses phosphorescent tracer particles activated by a high intensity pulse of light at the inlet. Tracer is detected using a light sensitive photomultiplier unit. Appropriate boundary conditions are maintained by using an annular feeder fluidised bed at the entrance boundary and an inline jet mixer installed at the exit boundary. This well defined arrangement of experimental boundary conditions represents a significant advance in the measurement of unbiased particle RTDs in these systems. The method was developed for measuring the particle RTD in a circulating fluidised bed (CFB) riser, but is applicable to other particle–fluid systems where a fast response measurement is required.

Stochastic modelling of the particle residence time distribution in circulating fluidised bed risers

Abstract A set of stochastic mathematical models have been developed to simulate the residence time distribution of solids in the riser of a circulating fluidised bed. The models simulate the motion of single particles moving up and down the riser using a Markov chain. Two models are presented in detail: (i) a core-annulus solids interchange model, and (ii) a four zone model that follows from the fast fluidised bed hydrodynamic profile. Both discrete and continuous time versions are presented. Each model incorporates different sections to account for the different flow regimes that exist within a riser. Simulations are linked to actual experimental conditions using local particle transfer rates between each model section. Simulations are able to predict the influence of changes in the solids flux, riser height and riser exit geometry. The influence of core-annulus solids transfer is also investigated. Comparison with a range of experimental data is presented.

Viscosity changes in hyaluronic acid: irradiation and rheological studies.

Hyaluronic acid (HA) is a significant component of the extracellular matrix (ECM), particular interest being shown herein in synovial fluid. The present study aims to investigate the degrading effects of X-ray radiation on HA at radiotherapy doses. Measurements of viscosity and shear stresses on HA solutions have been made at different shear rates using various types of viscometer for different concentrations in the range 0.01-1% w/v of HA. The HA has been subjected to doses of 6 MV photon radiation ranging from 0 to 20 Gy, the major emphasis being on doses below 5 Gy. It is found that there is a dose-dependent relationship between viscosity and shear rate, viscosity reducing with radiation dose, this being related to polymer scissions via the action of radiation-induced free radicals. The dependency appears to become weaker at higher concentrations, possibly due to the contribution to viscosity from polymer entanglement becoming dominant over that from mean molecular weight. Present results, for HA solutions in the concentration range 0.01% to 1% w/v, show reduced viscosity with dose over the range 0-4 Gy, the latter covering the dose regime of interest in fractionated radiotherapy. The work also shows agreement with previous Raman microspectrometry findings by others, the possible bond alterations being defined by comparison with available published data.

Incipient motion of a small particle in the viscous boundary layer at a pipe wall

A force balance is derived for a hemispherical particle in the viscous boundary layer at the wall of a horizontal pipe conveying Newtonian fluid; the hemisphere, of radius much less than that of the pipe, rests on the bottom with its flat face against the wall. The drag on the hemisphere is calculated from the creeping flow field of Price (Q. J. Mech. Appl. Math. Pt. 1 (1993)). This yields a prediction of the maximum velocity gradient at the wall for equilibrium, with limiting friction between the hemisphere and the wall. It is shown that the flow field of Price predicts a zero lift force but the validity of this, for actual flows, is questioned. Use of a hemisphere formulates a relevant well-posed problem, capable of mathematical solution. However, the flow field around real particles, e.g. sand, is complex, because of their irregular shapes, but the hemisphere work gives a qualitative indication of the behaviour of irregular particles. For turbulent flow in a pipe it is pertinent to consider a particle wholly within the viscous sub-layer, because it is isolated from significant turbulence and therefore hard to move; for such flow, the theory gives Eq. (21) to predict the critical pipe velocity, vC, for incipient motion of the hemisphere. For laminar flow, the wall shear rate is readily obtained from the parabolic velocity profile leading to Eq. (26) for vC. The flow field of Price (and therefore the force acting on the hemisphere) is valid only for creeping flow (i.e. very low particle Reynolds number). Modifications to the force balance are tentatively suggested to account for inertial components to the drag force. The predictions of critical velocity are tested against our data for the incipient motion of small hemispheres at pipe walls in hydraulic conveying as well as new and previously published data for both hydraulic and pneumatic conveying. The new method of predicting incipient motion works well for both the pneumatic and hydraulic conveying of hemispheres and sand shaped particles but it overpredicts the critical velocity for more rounded particles. The dependency of critical velocity on particle shape is under-researched.

A Comparison of Gasification with Pyrolysis for the Recycling of Plastic Containing Wastes

In the present investigation, pyrolysis and gasification, two widely used thermochemical processes, are compared as potential chemical recycling methods for MWP and plastic rich MSW in terms of products of high value and their end uses. High temperature pyrolysis results in a wide spectrum of products which also contain monomers of C2-C4 range such as ethylene and 1,3-butadiene. Recovery of monomers from their isomers and other products is difficult and energy-intensive. Gasification breaks solid wastes into simple molecules (mainly CO & H2) which subsequently can be converted to value added liquid chemicals (namely alcohols) by a catalytic synthesis processes. Synthetic alcohol then can be converted to the desired petrochemical precursors. After reviewing different aspects of both pyrolysis and gasification, recycling through gasification is chosen as the preferred route for project SPORT as syngas product can be converted into several key petrochemical products in high yield.

Characterisation of the annular film thickness in circulating fluidised-bed risers

A new correlation has been developped for predicting the wall film thickness in risers operating in the fast fluidisation regime. The correlation has been developed from published solids flux and particle velocity measurements made in circulating fluidised-bed risers.