Modeling the reaction of gaseous HCl with CaO in fluidized bed

An integrated mathematical model is developed to evaluate the performance of the reaction of gaseous HCl and CaO in fluidized bed. The model considers initial pore size distribution of solid reactant, pore structure change and attrition caused by particles movement. Bethe network is used to describe the pore space topology, and the percolation theory is used to determine the accessible reaction surface area of the sorbent particles and the effective diffusion coefficient of gaseous HCl. This model prediction accounts for the diffusion of HCl in shrinking pore space as well as in product layer, and clearly demonstrates the increasing diffusion resistance and the isolation of partially reacted pores causing incomplete conversion of solid. The model shows excellent agreement with the experimental data.     

Main causes of refractory wear in the shaft of a blast furnace and means of improving the resistance

Conclusions During service, refractories in a blast furnace shaft are saturated by components of the batch, change their properties and phase composition, and acquire a zoned structure which reduces the resistance to the complex action of the physicochemical and thermomechanical destructive factors.The wear of the shaft is a maultistaged cyclic process, including the saturation of the refractories through the pores, joints, and cracks by liquid and gaseous components of the batch and the gaseous medium, the decomposition of mullite and the formation of new compounds and glass — mainly potassium phyllitic and nepheline compounds — solution and reduction of the new compounds, and abrasion of the reaction products by the batch materials.We recommended for the lining of the shaft shoulders and the bottom and upper parts of the hearth the use of dense kaolin refractories containing 41–42% Al₂O₃ and with an open porosity of 8–12%, the technology for which is being introduced at the Chasov-Yar Refractories Combine.Translated from Ogneupory, No. 4, pp. 41–45, April, 1980.     

Pore geometry influence on the deactivation behavior of Ni-based catalysts for simultaneous production of hydrogen and nanocarbon

Pore geometry of Ni-containing Mg–Al–O mixed oxide catalysts could be controlled by varying the Ni content in the synthesis. Low Ni content may lead to the catalysts having mesopores with shape cylinder and narrow pore size distribution; high Ni content results in the catalyst having pores with shape ink-bottle and a wide pore size distribution. In methane decomposition to produce hydrogen and nanocarbon, the 50 wt.% Ni/Mg–Al–O catalyst was rapidly deactivated after 2 h of reaction; however, the catalysts with 15 and 25 wt.% of Ni showed much longer lifetime, which can be explained by assuming a new modal related to pore geometry of the catalysts.     

Study of magnesite-chromite brick after service in a tunnel kiln roof

Conclusions During the service of magnesite-chromite brick in constant temperature conditions (1700°C) and an oxidizing gaseous atmosphere in the absence of corrosive agents in the furnace space (silicates, iron oxides), fusion melts developed in the brick migrate to the colder sections, forming dense silicate-iron subzone.The hot end of the brick is enriched with magnesia and consists of coarse grains of recrystallized periclase. The grains of chromite in these sections have a direct bond with the grains of periclase, as a result of which the thermal properties of the hot section on the whole are improved [4] despite the high porosity of the brick in the subzone.Investigation of the structure of the pores confirms the capilliary mechanism of migration of melts. Recrystallization of periclase in the subzones adjacent to the hot zone is accompanied not by sintering and shrinkage but by coalescence (coarsening) of the pores.Movement of the melts over the pore channels is accompanied by corrosion of the walls of the pores.Translated from Ogneupory, No. 8, pp. 36–41, August, 1966.     

Porosity and specific surface area of Roman cement pastes

Mercury porosimetry, water vapour and nitrogen adsorption were used to follow the hydration of Roman cements — belite cements calcined at low temperature. Generally, unimodal distribution of pore sizes was observed, with the threshold pore width decreasing considerably with increasing curing time. An open porous structure with the threshold pore diameter between 0.2 and 0.8 μm and the specific surface area not exceeding 20 m²/g was produced at early ages when quick growth of the C–A–H phases is observed. The surface area reached up to 120 m²/g and the threshold pore width shifted to around 0.02 μm when the subsequent formation of C–S–H gel filled the larger pores. Both mercury porosimetry and water vapour adsorption were found to be capable of following the progress of hydration of the Roman cements with high reliability at least for a comparative evaluation of historic Roman cement mortars and repair materials used in restoration projects.     

Model of coke combustion and gasification in a mixture of reaction gases

A theoretical model is developed for combustion and gasification of porous cokes in a mixture of reaction gases (carbon diode, steam, hydrogen, oxygen). Processes of thermal and mass transfer are considered as are also the chemical reaction of substances in the gaseous phase and the kinetics of heterogeneous reactions inside the porous structure of the coke. The model makes it possible to determine the rate as well as the composition of the coke combustion and gasification products.Institute of Applied Mechanics, Russian Academy of Sciences, Moscow 117526. Translated from Fizika Goreniya i Vzyrva, Vol. 30, No. 2, pp. 25–33, March–April, 1994.     

Nonexponential Dynamics in Porous Glasses

The dielectric relaxation properties of porous glasses prepared from sodium borosilicate glasses are studied by dielectric spectroscopy over a wide range of frequencies (20 Hz–1 MHz) and temperatures (–100–300°C). The dielectric behavior reflecting the geometric disorder is analyzed within the models describing the non-Debye slowly damped dynamics. It is found that the dielectric response is very sensitive to microstructural and mesostructural features of the porous matrix and the properties of a material filling pores. The response contains information on the dynamics of water molecules in pores, which accounts for the interaction of these molecules with the pore surface.     

Calculation of the Cellular Structure of Detonation of Sprays in an H2—O2 System

On the basis of the mathematical model of a two–phase two–velocity medium, detonation of a cryogenic mixture (gaseous hydrogen—drops of liquid oxygen) was studied numerically. The dynamics of formation and the special features of the structure of the two–dimensional reaction zone of the detonation wave are discussed. The cellular structure of detonation is modeled for the first time for a cryogenic hydrogen—oxygen spray.     

Modeling of high-temperature syntheses in molds

On the basis of heterogeneous material mechanics a model is constructed for self-propagating high-temperature synthesis under the action of a constant load. The model considers the loosening effect of gas filtering in pores, sintering, and volumetric changes of the condensed phase during chemical reaction. Conditions are determined for compaction with liquid- and solid-phase reaction.Novokuznetsk. Translated from Fizika Goreniya i Vzryva, Vol. 29, No. 2, pp. 49–54, March–April, 1993.     

The mechanism of the deposition of pyrolytic carbons

The changes in crystallite-preferred orientation, distribution of 0.1–1.0-μm diameter pores, density, growth feature shape, and crystallite arrangement that occur with changes in the conditions of pyrolytic carbon deposition in a fluidized bed of particles are studied. Based on these changes a deposition mechanism is proposed wherein changes in the crystallite-preferred orientation and the pore distribution are controlled by the relative concentrations of two depositing components, which are solid particles and molecular species. Changes in the crystallite arrangement, and thereby the density, are controlled by changes in the reaction probability of the planar hydrocarbon molecules with the carbon surface.     

Methyl methacrylate polymerization in nanoporous confinement

The effect of nanoconfinement on the rate of isothermal polymerization of methyl methacrylate (MMA) polymerization is investigated using differential scanning calorimetry. Controlled pore glass (CPG) with pore diameters of 13, 50, and 111 nm are used for the confinement of the reaction. Both hydrophilic and hydrophobic pore surfaces are studied. The effective reaction rate and the apparent activation energy at low conversions, prior to autoacceleration, are unchanged in hydrophobic pores. On the other hand, in hydrophilic pores, the reaction rate increases by as much as a factor of 8 in the smallest 13 nm hydrophilic pores, and the effective activation energy decreases. For both pore surfaces, the time required to reach autoacceleration decreases with decreasing pore size, with the effect much more pronounced in the hydrophilic pores. The results are consistent with a model of nanoconfined free radical polymerization which accounts for suppressed termination due to a decrease in the diffusivity of nanoconfined chains.     

Variation of the pore structure of coal chars during gasification

The variation of the pore structure of several coal chars during gasification in air and carbon dioxide was studied by argon adsorption at 87 K and CO₂ adsorption at 273 K. It is found that the surface area and volume of the small pores (<10 Å) do not change with carbon conversion when the coal char is gasified in air, while those of the larger pores (10-20 Å, 20-50 Å, 50-2500 Å) increase with increase of carbon conversion. However in CO₂ gasification, all the pores in different size ranges increase in surface area and volume with increase of carbon conversion. Simultaneously, the reaction rate normalized by the surface area of the pores >10 Å for air gasification is constant over a wide range of conversion (>20%), while for CO₂ gasification similar results are obtained using the total surface area. However, in the early stages of gasification (<20%) the normalized reaction rate is much higher than that in the later stage of gasification, due to existence of more inaccessible pores in the beginning of gasification. The inaccessibility of the micropores to adsorption at low and ambient temperatures is confirmed by the measurement of the helium density of the coal chars. The random pore model can fit the experimental data well and the fitted structural parameters match those obtained by physical gas adsorption for coal chars without closed pores.     

Investigating the porous structure of reaction sintered silicon nitride by the standard contact porometric method

Conclusions A study was made of the change in the porous structure of blanks containing powdered silicon, with subsequently increasing reaction-sintering time in a nitrogen atmosphere. It is shown that in the initial stages of reaction-sintering, the reduction in the porosity occurs on account of the reduction in the sizes of the large pores and the growth of the fine ones. In the following stages the porosity of the ceramic is reduced on account of the growth of the pores of predominant size.On the specimens containing the coarse-grained filler and finely dispersed silicon we noted the formation, during reaction sintering, of two types of pore, one of which is determined by the silicon nitride matrix, and the other by the development of microgaps between the matrix and the grains of filler as a result of marked differences in the coefficients of thermal compression of the grains of corundum and the silicon nitride matrix forming during the reaction sintering.It is shown that the ÉKP method can be used to evaluate the porous structure of ceramics.Translated from Ogneupory, No. 1, pp. 32–35, January, 1986.     

The consequences of different temperatures on pore structure development in carbon

Particles of a porous, degassed Wyoming coal were reacted with carbon dioxide under surface-rate-limited conditions using a differential reactor. Different temperatures were used, and as the reaction progressed, extensive pore-structure measurements were made. This enabled the pore development to be followed at the different temperatures. Several characteristics of the burning were notable. The intrinsic surface rates (i.e., the reaction rates per unit surface unaffected by diffusion) in the macropores differed from those in the intermediate pores, and in turn the surface rates in the intermediate pores differed from those in the micropores. The intrinsic burning rates of the different sizes of pores relative to one another changed with temperature. The intrinsic burning rates of the different sizes of pores relative to one another also changed with burnoff. These three characteristics indicate that the parameters in the rate equation (pre-exponential factors and activation energies) changed with both pore size and extent of reaction. In the macropore system (pore radius >10 nm), the material constricting the pores appeared to burn faster than the material composing the bodies of the pores. In the intermediate pore system (1 nm < pore radius < 10 nm), there was some evidence of single-crystal burning to produce pores of the same size as the carbon crystallites. The last two of these characteristics have been observed before, but the first three appear to have gone unreported.     

Raney-nickel activated H2-cathodes Part I: Modelling the current/voltage behaviour of flat Raney-nickel coated microporous electrodes

The current voltage behaviour of hydrogen evolving electrodes covered with a closed, smooth layer of Raney-nickel has been treated theoretically. Since the pores of Raney-nickel typically are from 1 to 10 nm wide, they are able, according to the Young-Laplace equation, to keep evolved hydrogen in solution up to concentrations of the order of 0.1 mol dm^–3 which correspond to very high effective pressures of the order of 100 MPa. High concentrations of dissolved hydrogen cause substantial concentration-polarisation in the pores even at pore depths of several tenths of micrometres. The consequence is a limited effective pore length and catalyst utilization, respectively. This results in current voltage curves with steadily increasing Tafel slopes, which reach a limiting value of 140 mV dec^–1 (-0.5) at 90°C at current densities well above 1000 mA cm^–2.     

Effect of the Kinetics of Evaporation of Solvent on the Performance Characteristics of Ultrafiltration Membranes Made of Secondary Cellulose Acetate

Two methods are proposed for dry-wet formation of membranes from SCA for ultrafiltration of cheese whey: dry formation followed by modification in aqueous medium and spontaneous gelation on a calender followed by modification in aqueous medium, which significantly differ in the solvent evaporation rate (12 h and 3–5 min). It was shown that the solvent evaporation rate affects formation of the pore structure of the membranes. In formation with the spontaneous gelation method on a calender, membranes with a less developed pore structure and better permeability for water and cheese whey are obtained in comparison to membranes formed by the dry method. The pore structure of the membranes can be regulated with NaHCO₃ filler. Membranes from 5–7% solutions of SA at a 3–4 wt. % NaHCO₃ content are the most efficient.__________Translated from Khimicheskie Volokna, No. 2, pp. 58–61, March–April, 2005.     

Some observations on the variation of tortuosity with Thiele modulus and pore size distribution

Stochastic pore networks have been used to interpret the effects of pore structure on simultaneous diffusion and reaction in a porous catalyst. A stochastic pore network is a square network of pores, each pore segment having a radius assigned at random from any stipulated pore size distribution. The equations for diffusion and reaction within the network are solved rigorously for first order irreversible kinetics. The model is used to predict effectiveness factors and tortuosities for a hypothetical catalyst having one of four pore size distributions. The analysis predicts that tortuosity will vary with the Thiele modulus, the strength of this variation depending on the pore size distribution. It is further observed that the concentrations experienced by pores in a network are markedly different to those predicted by the parallel bundle model. This is relevant to the question of selectivity in complex reaction schemes.     

Microstructure and phase composition of the crust formed on the surface of the lining of a rotating kiln in firing of mullite-corundum chamotte

Conclusions Samples of the crust formed on the lining of a rotating kiln in firing of mullite-corundum chamotte were investigated. The crust consists of a mullite-glass mass and grains of corundum. Depending upon the medium in the furnace it has different structures, phase compositions, and colors.The white crust is less sintered and has a porous and loose structure. The size of the pores is up to 2.4×1.36 mm. The structure of the black crust is monolithic porous with a pore size up to 1.2×0.47 mm. The rust-colored crust has a coarse porous structure and a pore size of up to 1.7×0.9 mm.The crust is nonuniform in phase composition. The greatest mullite content is in the white crust and the least in the rust-colored.Translated from Ogneupory, No. 8, pp. 58–60, August, 1985.     

A microporosity study of Villamayor Sandstone (Salamanca, Spain)

In the clay fraction (<2 μm) of the Villamayor Sandstone (Salamanca, Spain), pores with a diameter close to 3 nm are predominant. These pores are in the mesoporosity range (2–40 nm) and micropores are absent. The clay fraction is composed mainly of mica and chlorites with small amounts of quartz and feldspars; very little palygorskite or smectite is present. Specific surface area values range from 62 to 78 m² g⁻¹. On heating in vacuo at 470 K no significant change in the pore size distribution was observed, but the specific surface area decreased by ca. 10%, probably due to sintering of the particles on loss of water and surface hydroxyl groups. Treatment with humid nitric oxide for 20 h at this same temperature decreased the contribution by these pores but increased that from pores in the range 5–15 nm diameter. No change was observed, however, in the X-ray diffraction diagram, indicating that changes upon nitric oxide treatment affect isolated, non-periodical portions of the crystals.     

Penetrable ceramic materials based on clays with a controllable porous structure

The effect of isometric and fiber organic and inorganic additives such as chamotte, carbon black, hydrophilic graphite, glass fiber, mullite-siliceous fiber, carboxymethyl cellulose, and cellulose introduced jointly and separately on the penetrability, maximum pore size, size distribution of penetrable pores, and mechanical strength of ceramic materials based on low-melting clay is investigated. It is shown that the pore structure can be controlled by simultaneous introduction of additives of different natures. The materials obtained have a maximum pore size of 1.2–2.9 µm, a gas penetrability of 0.150 µm², and satisfactory mechanical strength. The materials can be used as ceramic filters.Translated from Ogneupory, No. 4, pp. 14 – 17, April, 1996.