Multi-level modelling of the hydrodynamics in gas phase polymerisation reactors

The gas-solid fluidised bed process is the most versatile process for the production of polyolefins, because it allows the production of polyolefins with a broad distribution of both the product density and molecular weight. In spite of the excellent heat transfer characteristics of a fluidised bed, the production capacity is still limited by the rate of heat removal and temperature control in the polyolefin production process is a major point of attention. The rate of heat removal is largely determined by the large scale convection patterns prevailing in the gas phase olefin polymerisation reactor, induced by the in situ formed and growing gas bubbles. Moreover, the bubbles play an intricate role in the particle mixing and segregation behaviour of the fluidised bed. To improve the design, operation and scale-up of these gas-fluidised bed processes, a better understanding\ud and a quantitative description of these phenomena is required. Therefore, the main objective of this project was the development of fundamental hydrodynamic models to describe and study the hydrodynamic behaviour of the gas-solid fluidised bed reactor

Segregation in dense gas-fluidised beds: validation of a multi-fluid continuum model with non-intrusive digital image analysis measurements.

A non-intrusive digital image analyses technique is applied to study size driven segregation of a binary mixture of coloured glass beads in a bubbling gas-fluidised bed. Segregation rates and patterns obtained from experiments are compared to numerical simulations performed with a two-dimensional multi-fluid Eulerian model, that uses closure laws according to the kinetic theory of granular flow. It is demonstrated that prediction of segregation is a rather severe test case for fundamental hydrodynamic models, since bubble dynamics and momentum transfer between particles of different classes have to be modelled correctly. At all gas velocities segregation rates predicted by the multi-fluid model were much higher than those observed in experiments. At gas velocities higher than the minimum fluidisation velocity of the largest particles the model still predicts segregation, when it does not occur in experiments. It is concluded that the predicted intensity of bubbling is too low, since energy dissipation by particleparticle\ud interaction is still underestimated in the applied kinetic theory closure model

Heat transfer in a membrane assisted fluidised bed with immersed horizontal tubes

The effect of gas permeation through horizontally immersed membrane tubes on the heat transfer characteristics in a membrane assisted fluidised bed was investigated experimentally. Local time-averaged heat transfer coefficients from copper tubes arranged in a staggered formation with the membrane tubes to the fluidised bed were measured in a square bed (0.15 m x 0.15 m x 0.95 m) containing glass particles (75-110 ¿m) fluidised with air distributed via a porous plate, where the ratio of gas fed or removed through the membrane bundles and the porous plate distributor was varied. The experimental results revealed that high gas permeation rates through the membranes strongly decreased the heat transfer coefficient at high superficial gas velocities for tubes at the top of the tube bundle, which was attributed to the reduced mobility and heat capacity (higher dilution) of the emulsion phase. However, the effect of gas permeation was much less pronounced for tubes lower in the tube bundle because of the lower local dilution of the emulsion phase

Pyrolysis of brominated feedstock plastic in a fluidised bed reactor

Fire retarded high impact polystyrene has been pyrolysed using a fluidised bed reactor with a sand bed. The yield and composition of the products have been investigated in relation to fluidised bed temperature. The bromine distribution between the products and a detailed analysis of the oils using GC-FID/ECD, GC-MS, FT-ir, and size exclusion chromatography has been carried out. It was found that the majority of the bromine transfers to the pyrolysis oil and the antimony was detected in both the oil and the char. Oil made up over 89.9% of the pyrolysis products. Over 30% of the oil consisted of benzene, toluene, ethylbenzene, styrene and cumene. The pyrolysis gases were mainly hydrocarbons in the C1-C4 range but some HBr and Br2 was detected

Constructing Success in the Electric Power Industry: Flexibility and the Gas Turbine

This paper explains the success and failure of two technologies that generate electricity from fossil fuels. Both the Combined Cycle Gas Turbine (CCGT) and fluidised bed boiler burn fossil fuels more cleanly than more traditional technologies. Whereas the CCGT has been used for an increasing number of new power plants during the past fifteen years, the latter has struggled to attract attention outside a small-scale niche. The paper draws on economic and social constructivist approaches to technical change. It shows how a combination of economic, institutional and political factors can be used to explain success and failure. It also demonstrates the importance of technological flexibility for the long term development of the CCGT and its acceptance as the power industry's current technology of choice.technical change, flexibility, CCGT, fluidised bed boiler,

Fast pyrolysis of halogenated plastics recovered from waste computers

The disposal of waste computers is an issue that is gaining increasing interest around the world. In this paper, results from the fast pyrolysis in a fluidized bed reactor of three different waste computer monitor casings composed of mainly acrylonitrile-butadiene-styrene (ABS) copolymer and two different waste computer body casings composed of mostly poly(vinyl chloride) (PVC) type polymers are presented. Preliminary characterization of the waste plastics was investigated using coupled thermogravimetric analysis-Fourier transform infrared spectrometry (TGA-FT-IR). The results showed that the plastics decomposed in two stages. For the ABS-containing monitor casings, aromatic and aliphatic material were released in the first and second stages. The PVC-containing computer body casing samples showed a first-stage evolution of HCl and a second stage evolution of aromatic and aliphatic material and further HCl. In addition, each of the five plastics was fast-pyrolyzed in a laboratory-scale fluidized bed reactor at 500 °C. The fluidized bed pyrolysis led to the conversion of most of the plastics to pyrolysis oil, although the two PVC computer body cases produced large quantities of HCl. The pyrolysis oils were characterized by GC-MS and it was found that they were chemically very heterogeneous and contained a wide range of aliphatic, aromatic, halogenated, oxygenated, and nitrogenated compounds

Three-dimensional modelling on the hydrodynamics of a circulating fluidised bed

The rapid depletion of oil and the environmentalimpact of combustion has motivated the search for cleancombustion technologies. Fluidised bed combustion (FBC)technology works by suspending a fuel over a fast air inletwhilst sustaining the required temperatures. Using biomassor a mixture of coal/biomass as the fuel, FBC provides alow-carbon combustion technology whilst operating at lowtemperatures. Understanding the hydrodynamic processes influidised beds is essential as the flow behaviours causing heatdistributions and mixing determine the combustion processes.The inlet velocities and different particle sizes influence theflow behaviour significantly, particularly on the transitionfrom bubbling to fast fluidising regimes. Computationalmodelling has shown great advancement in its predictive capabilityand reliability over recent years. Whilst 3D modellingis preferred over 2D modelling, the majority of studies use2D models for multiphase models due to computational costconsideration. In this paper, two-fluid modelling (TFM) isused to model a 3D circulating fluidised bed (CFB) initiallyfocussing on fluid catalytic cracker (FCC) particles. Thetransition from bubbling to fast fluidisation over a rangeof velocities is explored, whilst the effects on the bubblediameter, particle distributions and bed expansion for differentparticle properties including particle sizes are compared. Dragmodels are also compared to study the effects of particleclustering at the meso-scale

Recycling of carbon fibre composites

A clear case for carbon fibre recovery and reuse exists on environmental grounds due to the high cost and energy use of virgin fibre production. On a specific energy basis, carbon fibres can be recovered at around 10% of the energy required to manufacture virgin fibres but the scale of the recovery process can make a large difference to overall cost effectiveness. This study will describe the technical and economic challenges associated with the recycling of carbon fibres, the state of the art in recycling technologies and the re-use of fibres in high performance composites

Comparison of Eulerian hydrodynamic models with non-intrusive X-ray measurements in pressurized dense gas-fluidised beds

The objective of much fluidisation research has been the prediction of industrial beds, which are often large and operated at high pressure and temperature. This may be achieved through the use of Eulerian two-fluid models. Two such models have been compared with experimental results of the structure of a jet in a bubbling fluidised bed. In the first two-fluid model the particles are treated as a Newtonian fluid with a constant viscosity; in the second, kinetic theory for granular flow is used to describe the rheological properties of the particulate phase. The experiments were examined non-intrusively using X-ray equipment. The comparison of the experiments with the models revealed significant and systematic differences. These are described and the implications they have for the modelling and scaleup of bubbling fluidised beds are discussed