S.J.R. Simons

Investigation of accelerated carbonation for the stabilisation of MSW incinerator ashes and the sequestration of CO2

Accelerated carbonation has been used for the treatment of contaminated soils and hazardous wastes, giving reaction products that can cause rapid hardening and the production of granulated or monolithic materials. This technology provides a route to sustainable waste management and it generates a viable remedy to the problems of a decreasing number of landfill sites in the UK, global warming (due to greenhouse gas emissions) and the depletion of natural aggregate resources, such as sand and gravel. The application of accelerated carbonation (termed Accelerated Carbonation Technology or ACT) to sequester CO2 in fresh ashes from municipal solid waste (MSW) incinerator/combined heat and power plants is presented. The purpose of this paper is to evaluate the influence of fundamental parameters affecting the diffusivity and reactivity of CO2 (i.e. particle size, the reaction time and the water content) on the extent and quality of carbonation. In addition, the major physical and chemical changes in air pollution control (APC) residues and bottom ashes (BA) after carbonation are evaluated, as are the optimum reaction conditions, and the physical and chemical changes induced by accelerated carbonation are presented and discussed.

Hardness of moist agglomerates in relation to interparticle friction, granule liquid content and nature

Abstract Wet agglomerates deform plastically until they break through crack propagation. On the particulate level, liquid bridges are responsible for the strength of the wet agglomerate as they hold the particles together. The experiments reported in this paper identify the role of liquid surface tension, bridge Laplace pressure and liquid viscosity, which, in combination, explain the axial strength of pendular liquid bridges. Different situations exist depending on the degree the liquid wets the particles, and on the saturation of the agglomerate mass. A parabolic approximation can be used to describe the shapes adopted by pendular liquid bridges. On the wet agglomerate level, the hardness is related to three factors: the liquid binder surface tension and viscosity and the interparticle friction. A simple model is developed in this paper, based on the powder and liquid binder properties, which shows that the forces due to interparticle friction are generally predominant in wet agglomerates made from non-spherical particles. Although mechanical interlocking is not predicted, this model yields accurate prediction of wet agglomerate hardness independently measured on wet masses of varying composition. This theoretical hardness could prove an interesting tool for wet granulation research and technology.

Modelling of agglomerating systems: from spheres to fractals

The modelling of agglomerating systems is demonstrated by focusing on processes where liquid binder provides the dominant adhesion force between particles. Current theories, developed from the classic pendular bridge force expressions, are still restricted to using empirical parameters to apply the pair-wise forces to multi-particle agglomerates. However, the use of contact mechanics, coupled with such models, is proving to yield useful information on the deformation and breakage behaviour of agglomerates, which in turn will apply to overall growth behaviour. Advances are also being made in computer simulations which can incorporate these theories. Recent advances in the concepts of fractal analysis to provide quantitative information on the openness of agglomerate structures (from which qualitative information on the growth mechanisms can be inferred) are described. Here, too, the use of computational techniques, such as image analysis, is enhancing our ability to make comprehensive measurements of agglomeration phenomena, from which new models can be developed to improve the design and operation of process equipment.

Kinetic study of accelerated carbonation of municipal solid waste incinerator air pollution control residues for sequestration of flue gas CO2

It is known that accelerated carbonation technology can stabilise municipal solid waste incinerator air pollution control (APC) residues through encapsulation of hazardous components and cementation by carbonate precipitation. The aim of this work was to investigate the possibility of sequestering flue gas CO2 in APC residues with a view to reducing greenhouse gas emissions. The fundamental parameters affecting the carbonation process have been studied. An adverse effect of the CO2 concentration was observed and the optimum water-to-solid ratio and temperature were 0.3 and 20–30 °C, respectively. The reaction consisted of two stages. Initially, the reaction rate was controlled by the movement of the carbonation interface and the activation energy at this stage was 14.84 kJ mol−1; as the reaction proceeds, the rate controlling regime switched to gas diffusion through product layer control, and the activation energy was calculated to be 30.17 kJ mol−1. The openness of the pores in the solid is the key to carbonation efficiency. 10–12% (w/w) of CO2 can be trapped in APC residues during the carbonation process if flue gas is used.

A microscale investigation of liquid bridges in the spherical agglomeration process

Abstract Spherical agglomeration is an industrial process traditionally used to separate or recover fine solids dispersed in a liquid suspension through the addition of a second immiscible liquid (binder) which presents an affinity for the solids and is capable of forming small liquid bridges that hold the particles together. Under appropriate physico-chemical conditions, the desired particles can be selectively agglomerated and removed from the slurry. More recently, the spherical agglomeration technique has been used for the manufacture of high value products, such as crystalline pharmaceutical drugs, and is attracting increasing attention in the bioprocessing area. However, spherical agglomeration has yet to reach widespread commercialisation beyond the minerals industry. This is despite the simplicity of the process, the low cost of installation required and the possibility of agglomerating particles down to a few microns in size and is probably due to the lack in understanding of the controlling mechanisms involved. In this paper, a microscale approach to investigate the mechanisms that lead to spherical agglomeration is presented. The geometry and the strength of liquid bridges formed between pairs of particles with diameters in the range 80–130 μm, submerged in a second liquid, are analysed and compared with values predicted by theory.

Rupture energy and wetting behavior of pendular liquid bridges in relation to the spherical agglomeration process

A novel micro force balance (MFB) is used to investigate the rupture energy of a silicon oil liquid bridge formed in water between two glass particles of either the same or dissimilar surface energy. Rupture energies are integrated from force curves and compared with the models proposed by Simons et al. (Chem. Eng. Sci. 49 (1994) 2331) and Pitois et al. (Eur. Phys. J. B 23 (2001) 79). The latter showed slightly better agreement to the experimental data. Glass ballotini ( approximately 100 microm diameter) are either silanized, in order to increase their wettability toward the oil binder, or kept untreated. Results showed how the interaction between the binder and the particle influences the geometry, the capillary pressure, the force, and the rupture energy of the liquid bridge. Higher values of force and liquid bridge energy were measured between particles characterized by higher interaction (silanized-silanized configuration). A thermodynamic approach to the evaluation of the energy stored in a liquid bridge is also proposed. The mechanical work done to stretch apart the liquid bridge is evaluated as the difference of internal and hysteresis energy between the initial and the rupture configuration of the bridge. This approach showed good agreement with the experimental data only for liquid bridges formed between silanized and untreated glass particles.

Modelling of binder-induced agglomeration

From the analysis of pendular liquid bridge forces between spherical particles, a model has been developed to predict bridge rupture energies. Whilst this model has been shown to predict the correct trends in certain stages of agglomeration, it is limited by the assumptions of zero contact angle, toroidal bridge geometry, spherical particles and quasi-static rupture. The work described here is aimed at extending the model to more generally applicable conditions using direct measurement of bridge rupture energies between particles down to 3 mu m in diameter under different physiochemical conditions. Initial results show that the spreading coefficient of the binder to the particle has a marked effect on the dynamic behaviour of the bridge itself and its subsequent geometry at equilibrium, i.e. for non-zero contact angles theoretical equations tend to over-predict the force of adhesion, whilst fur zero contact angles the force of adhesion agrees with that predicted using the Laplace equations for constant curvature. On the other hand, the corrected expression for maximum separation distance shows good agreement with experimental results.

Predicting granule behaviour through micro-mechanistic investigations

Abstract A novel micro-manipulator device has been developed to observe and measure, directly, the behaviour of binder liquid bridges between pairs of solid particles. The objective has been to develop the fundamental understanding of the role of the liquid and solid properties in the growth and consolidation of granules, from the initial contact between the liquid and particles to the resultant multi-particle bodies. On the particle level, it is the liquid bridges that are responsible for the strength of “wet” agglomerates, since they hold the particles together. In this paper, results of experiments will be reported that identify the role of liquid surface tension, bridge Laplace pressure, liquid viscosity and, hence, wetting behaviour, in the axial strength of the bridges. In particular, the differences in bridge shape when particles of different surface properties come together (i.e. in mineral mixtures) provides a crucial insight into whether granules will grow successfully or not. A parabolic approach to describing the shapes adopted by the liquid bridges, from which parameters such as resistance to deformation can be calculated, will be shown. From the theoretical behaviour of individual bridges, determined through the direct experimental observations, a simple model has been constructed, which relates granule porosity, liquid content and the physicochemical properties of the materials to the agglomerate hardness. Some experimental measurements using spherical particles and powders commonly granulated in the pharmaceutical industry, such as lactose, will be compared to the model predictions and the role of interparticle friction and liquid surface tension and viscosity will be shown quantitatively.

Chapter 27 Liquid bridges in granules

Publisher Summary In many industries, granulation processes are often applied to mixtures of powders in which the component solids do not exhibit the same surface properties. Large discrepancies in surface energy create problems during granulation because powders can be selectively wet at the expense of others. In the pharmaceutical industry, most drugs have a low surface energy and therefore are poorly wet by common granulation liquids. However, the behavior of pendular liquid bridges when the liquid-to-solid contact angle is large has not been extensively covered in the literature. The majority of models dealing with pendular liquid bridges either assume zero, small, or fixed contact angles. When the wetting hysteresis is important, the solid–liquid interfaces at the boundaries of the liquid bridge can remain constant with particle separation until a critical separation is reached, at which point the bridge liquid recedes from one particle surface and the corresponding solid–liquid interface reduces. The reduction of the solid–liquid interfacial area with particle separation is accentuated by low wetting hysteresis that drastically alters the shape of pendular liquid bridges, their rupture distance, and post-rupture liquid volume distribution on the solid particles. In this chapter, the current theory on liquid bridges between pairs of particles is presented, followed by a detailed review of the work on the modeling of liquid bridges and the relationship between the micro- and macro-scale granule behavior, developed from direct measurements and observations of liquid bridges among smooth spheres and between real pharmaceutical powders and binders.

Sequestration of carbon dioxide in artificial aggregates: Determining the potential for fluidised-bed processing

Publisher Summary Accelerated Carbonation Technology (ACT) is a controlled accelerated version of the naturally occurring carbonation process that affects a wide variety of calcium and magnesium based material on exposure to atmospheric carbon dioxide. It is a new method for both carbon dioxide sequestration and upgrading of waste materials, through a single process. ACT has the advantage of carbonating materials in minutes, rather than days or even weeks, rendering certain waste materials non-hazardous and giving them engineering properties. The particulate nature of the raw material and the nature of the operations that have to be conducted, point to the suitability of fluidization as the unit operation for ACT, provided the material properties are compatible. The work reported in the chapter investigates the suitability of a three-phase fluidized bed system to conduct the process. Experiments on a laboratory scale fluidized bed indicate that defluidization of the Air Pollution Control residues is a crucial factor. Utilizing an aeratable carrier solid has been seen as a possible method to achieve satisfactory fluidization behavior. The optimal phase to phase ratios to operate the fluidization have been elucidated and reported in the chapter.