Porosity, particle systems

This treatment of capillary bonds assumes a two-particle system with no bridge interference. As humidity increases the area of the particle surface covered by the bridge increases and the independence of neighbouring liquid bridges becomes a function of porosity and of absolute particle size. 

B. Designing Porosity Inside of Particle Systems and Thin Films. 527... 

M Propster, J Szekely. The porosity of systems consisting of layers of different particles. Powder Technol 17 123 138, 1977. 

Resuspension and scour of noncohesive sediments are well understood as functions of particle diameter, and reasonable estimates of resuspension rates can be determined for noncohesive sediments with information about the system hydraulics and physical properties of the sediment. However, widely applicable relationships predicting cohesive sediment erosion have not yet been developed. Quantifying resuspension of cohesive sediments usually requires development of site-specific data and experimentation. Cohesive sediment resuspension has been observed to depend on sediment bulk density (or porosity), particle size, surface and porewater chemistry, algal colonization, bioturbation and gas formation within the sediments, in addition to bottom shear velocity. 

Suspension polymerization of VDE in water are batch processes in autoclaves designed to limit scale formation (91). Most systems operate from 30 to 100°C and are initiated with monomer-soluble organic free-radical initiators such as diisopropyl peroxydicarbonate (92—96), tert-huty peroxypivalate (97), or / fZ-amyl peroxypivalate (98). Usually water-soluble polymers, eg, cellulose derivatives or poly(vinyl alcohol), are used as suspending agents to reduce coalescence of polymer particles. Organic solvents that may act as a reaction accelerator or chain-transfer agent are often employed. The reactor product is a slurry of suspended polymer particles, usually spheres of 30—100 pm in diameter they are separated from the water phase thoroughly washed and dried. Size and internal stmcture of beads, ie, porosity, and dispersant residues affect how the resin performs in appHcations. 

Over the past years considerable attention has been paid to the dispersing system since this controls the porosity of the particle. This is important both to ensure quick removal of vinyl chloride monomer after polymerisation and also to achieve easy processing and dry blendable polymers. Amongst materials quoted as protective colloids are vinyl acetate-maleic anhydride copolymers, fatty acid esters of glycerol, ethylene glycol and pentaerythritol, and, more recently, mixed cellulose ethers and partially hydrolysed polyfvinyl acetate). Much recent emphasis has been on mixed systems. 

The terms filter and strainer are in common use and may lubricant systems contain both. The word strainer is often associated with the removal of large particles, and though it is true that in the majority of cases a strainer is in fact employed to remove coarse particles, the fundamental difference between it and a filter is not one of porosity but purely one of geometry. In a strainer, the liquid passes through in a straight line, but in a filter, a far more devious route is followed. 

The small particles are reported to be very harmful for human health [98]. To remove particulate emissions from diesel engines, diesel particulate filters (DPF) are used. Filter systems can be metallic and ceramic with a large number of parallel channels. In applications to passenger cars, only ceramic filters are used. The channels in the filter are alternatively open and closed. Consequently, the exhaust gas is forced to flow through the porous walls of the honeycomb structure. The solid particles are deposited in the pores. Depending on the porosity of the filter material, these filters can attain filtration efficiencies up to 97%. The soot deposits in the particulate filter induce a steady rise in flow resistance. For this reason, the particulate filter must be regenerated at certain intervals, which can be achieved in the passive or active process [46]. 

For suspensions of fine particles, or systems containing a significant amount of fines, the suspending fluid can be considered to be homogeneous, with the density and viscosity modified by the presence of the fines. These properties depend primarily on the solids loading of the suspension, which may be described in terms of either the porosity or void fraction (s) or, more commonly, the volume fraction of solids, buoyant force on the particles is due to the difference in density between the solid (ps) and the surrounding suspension (p ), which is... 

According to the packing geometry, the systems present different porosity and specific surface. The final characteristics of the dried gel are determined by the physicochemical conditions at every step of the preparation the size of primary particles at the moment of aggregate, the concentration of particles in solution, the pH, salt concentration, temperature, and time of aging or other treatment in the wet state, mechanical forces present during drying, the temperature, pH, pressure, salt... 

Methods for synthesizing highly porous microspheres were investigated, and surface area measurements were used to confirm the porous nature of the samples [19]. A high surface area was measured and was compared with the calculated surface area value. The measured value was 35 times that of a nonporous particle, indicating the extensive porosity of the spheres. The surface area was also used to explain the drug release mechanisms in the pores of these systems. 

Textural mesoporosity is a feature that is quite frequently found in materials consisting of particles with sizes on the nanometer scale. For such materials, the voids in between the particles form a quasi-pore system. The dimensions of the voids are in the nanometer range. However, the particles themselves are typically dense bodies without an intrinsic porosity. This type of material is quite frequently found in catalysis, e.g., oxidic catalyst supports, but will not be dealt with in the present chapter. Here, we will learn that some materials possess a structural porosity with pore sizes in the mesopore range (2 to 50 nm). The pore sizes of these materials are tunable and the pore size distribution of a given material is typically uniform and very narrow. The dimensions of the pores and the easy control of their pore sizes make these materials very promising candidates for catalytic applications. The present chapter will describe these rather novel classes of mesoporous silica and carbon materials, and discuss their structural and catalytic properties. 

It is essential that all PSs are multiphase. The easiest case to handle is the biphase system consisting of a condensed phase (solid) and a void inside porous particles or between consolidated ensembles of nonporous or porous particles. The void occupies a part of the volume, s, which is referred to as porosity. The other part of a PS volume is equal to ri=(l -e), and is termed density of packing. It is filled with the condensed phase (see Section 9.4). Generally, PSs can include various condensed phases of different structure, including combinations of solid(s) and liquid(s). 

The catalyst activity depends not only on the chemical composition but also on the diffusion properties of the catalyst material and on the size and shape of the catalyst pellets because transport limitations through the gas boundary layer around the pellets and through the porous material reduce the overall reaction rate. The influence of gas film restrictions, which depends on the pellet size and gas velocity, is usually low in sulphuric acid converters. The effective diffusivity in the catalyst depends on the porosity, the pore size distribution, and the tortuosity of the pore system. It may be improved in the design of the carrier by e.g. increasing the porosity or the pore size, but usually such improvements will also lead to a reduction of mechanical strength. The effect of transport restrictions is normally expressed as an effectiveness factor q defined as the ratio between observed reaction rate for a catalyst pellet and the intrinsic reaction rate, i.e. the hypothetical reaction rate if bulk or surface conditions (temperature, pressure, concentrations) prevailed throughout the pellet [11], For particles with the same intrinsic reaction rate and the same pore system, the surface effectiveness factor only depends on an equivalent particle diameter given by... 

Mechanistic rate laws assume that only chemical kinetics is operational and transport phenomena are not occurring. Consequently, it is difficult to determine mechanistic rate laws for most solid phase systems due to the heterogeneity of the solid phase system caused by different particle sizes, porosities, and types of retention sites. 

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

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