摘要:

A supercritical drying process has been used to prepare MgO-Cr2O3, and MgO-Fe2O3powders from a mixture of magnesium acetate and the metallic acetylacetonates hydrolyzed in a methanolic solvent. The resultant aerogels have small particle sizes in the range of 10 to 20 nm and high surface areas in the range of 50 to 250 m2sol;g. After removal from the autoclave, characterization by X-ray diffraction (XRD) showed that the MgO-Cr2O3, was amorphous and that the MgO-Fe2O3was readily in the form of a spinel structure for magnesioferrite (MgFe2O4). Fourier transform infrared spectroscopy (FTIR) and thermogravimetric and differential Ihermal analyses (TG-DTG-DTA) showed a high level of retained surface methoxy and carboxylate species for the MgO-Cr2O3aerogel (lover 40% by weight). The MgO-Fe2O4, on the otherhand, had less than 10% organic matter. The differences observed between MgO-Cr2O3and MgO-Fe2O4have been attributed to the formation of Cr+6in the former case. After heating in air at 970 K. MgO-Cr2O3crystallized to the spinel structure MgCr2O4. The IR transmittance of pressed disks of MgFe204 aerogel showed behavior typical for a n-type semiconductor. The MgCr2O4, however, was a p-type semiconductor due to mainly Cr+6present on the surface. Lewis acid centers were prevalent on both materials. The acid strength was found to be higher on the MgFe2O4compare to the MgCr2O4

ISSN:0098-6445    

DOI:10.1080/00986449308936161

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Considerable amounts of ethanal may form in ethanol deep oxidation over platinum. The selectivity toward CO2increases at higher temperatures and at very low concentrations. An inhibiting effect of primarily formed aldehyde on ethanol conversion rate was observed; a power-law type equation rate is not applicable to describe the kinetics. The dependence of the selectivity on the contact time indicates that the gas-phase aldehyde may adsorb on the catalyst and further react to CO2. At lower temperature direct complete oxidation to CO2and partial oxidation occur as parallel competitive reactions; at higher temperature a series-parallel mechanism takes place

ISSN:0098-6445    

DOI:10.1080/00986449308936162

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Mixture effects are observed when ethanol is oxidized in mixture with benzene or elhenylbenzene (styrcne). Very lean mixtures (<2000ppmV) were tested over a commercial monolithic catalyst. In the mass transfer controlled regime higher conversions are obtained for ethanol in mixture with aromatic, due to the increased adiabatic temperature rise; but the ignition temperature increased. The hydrocarbon reaction rate is slightly affected, while ethanol oxidation (either complete or partial) is strongly inhibited, especially by elhenylbenzene. The selectivity of the reaction is modified, but the significant reduction in aldehyde emissions all over the temperature range is mainly due to the strong inhibition at low temperature and to the increased ignition temperature

ISSN:0098-6445    

DOI:10.1080/00986449308936163

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The simultaneous absorption of H2S and CO2has been studied both experimentally and theoretically. A model has been developed which predicts the absorption rates of H2S and CO2into a sodium carbonate solution. The absorption rates are calculated according to the two-film theory. In the liquid film, the finite rate of the CO2reaction was considered. Otherwise, in the liquid film as well as in the liquid bulk, equilibrium conditions for all reactions were assumed. Absorption experiments were performed on a packed column using a counter-flow strategy. In the experiments the influence of the initial carbonate concentration, the gas flow rate and the temperature on the removal efficiencies of H2S and CO2and the selectivity of H2S were investigated. It is desirable to absorb the H2S but not the CO2. The agreement between the absorption model and the experimental results from the absorber tower was satisfactory. The mass transfer coefficients were determined by fitting the experimental data to the model with respect to the H2S and CO2content in the outgoing gas. The H2S content was used to determine the gas side mass transfer coefficient and the CO2content was used to determine the liquid side mass transfer coefficient, The effective contact area of mass transfer was taken from published data. With a constant packing height, both the experiments and the model indicated that high carbonate concentration benefits the removal efficiency of H2S. Higher gas flow rate also benefits the selectivity for H2S. However, the removal efficiency will decrease. At higher temperatures the selectivity and the removal efficiency of H2S decreased. Under the conditions investigated, the absorption of H2S was essentially controlled by gas-side mass transfer and the absorption of CO2was controlled by liquid-side mass transfer

ISSN:0098-6445    

DOI:10.1080/00986449308936164

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Some aspects of the interfacial phenomena that occur during liquid infiltration of porous media are discussed. Experimental data are presented on the critical speed at which air bubbles are trapped when a horizontal liquid surface passes (from below) across an isolated cylindrical rod or filament whose axis is horizontal. The results are correlated over a wide range of liquid viscosities and surface tensions. We observe that bubble entrapment occurs when the Capillary number exceeds a value defined by $ where γ is the so-called Property number, defined by $ The coefficientsAandmdepend upon the wetting characteristics of the liquid with respect to the solid (filament). A simple physical model is proposed which is in qualitative agreement with the observations.

ISSN:0098-6445    

DOI:10.1080/00986449308936165

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Branched or fractal-like extended surfaces are commonly found in natural systems and in engineered systems such as heterogeneous catalysts to enhance the rates of heat or mass transfer, or chemical reaction. Simple models can be derived to simulate the effectiveness and efficiency of these structures and provide some speculation concerning transport limitations affecting the evolutionary design of natural branching structures. Though the surface area of branching structures increases with each successive generation, their efficiency and effectiveness quickly approach asymptotic values. The transport limitations inherent in the structures render subsequent branching ineffective. Such limitations, characterized by environmentally driven, external transport resistances, may affect branching patterns found in plant root systems, mammalian motoneurons, and animal circulatory systems

ISSN:0098-6445    

DOI:10.1080/00986449308936166

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Branched or fractal-like extended surfaces are commonly found in natural systems and are incorporated in engineered systems to enhance the rate of heat or mass transfer, or lo enhance the rate of chemical reaction. Many of these systems are required to respond to periodic inputs or to produce periodic outputs. Simple models can be derived to simulate the transport effectiveness and efficiency of these engineered structures and thereby provide some speculation concerning the evolutionary design and environmental control of natural branching systems

ISSN:0098-6445    

DOI:10.1080/00986449308936167

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In general, physical processes are usually nonlinear and control system design based on the linearization technique cannot control the process well for a wide range of operation. Use of the variable transformation method may not always solve the problem. In this paper, a fuzzy adaptive controller is proposed to control the nonlinear process. The CSTR control problem has also been considered. The results are compared with the method of nonlinear model predictive control (NMPC) with constrained and unconstrained control variables. A fuzzy model-following control system scheme is also proposed. The results show that the proposed controller is a feasible control structure for a nonlinear or parameter-variations process control.

ISSN:0098-6445    

DOI:10.1080/00986449308936168

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The invasion percolation algorithm is used to simulate two-fluid immiscible displacement of a wetting fluid by a non-wetting fluid in various porous media represented by two-dimensional and three-dimensional networks of interconnected capillaries. Trapping of the displaced fluid occurs, thereby creating isolated islands. The effects of the thickness of the porous medium on the island size distribution are studied for capillary displacements for the case in which buoyancy effects are negligible. It was found in a previous study that the number of islands of size s scales approximately as s~" in two-dimensional porous media, where a is a function of the fluid viscosity ratio. The present work reveals that there is a cross-over behavior between the two-dimensional and the three-dimensional problems.

ISSN:0098-6445    

DOI:10.1080/00986449308936169

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In this paper, a number of modifications are instituted in implementing the quadrature method for solving chemical engineering problems with semi-infinite domains and/or steep gradients. This improvement in the curve-fitting ability of differential quadratures is achieved by adopting trial functions of forms other than the polynomials. Formal criteria are first developed (and proved) for the selection of proper function forms. If the trial functions are restricted to the products of polynomials and some auxiliary functions, explicit formulae are derived to facilitate the calculation of the corresponding modified quadrature coefficients. If, in addition, the grid points are chosen to be the zeros of an orthogonal polynomial, e.g. Jacobi, Laguerre and Hermite, further simplifications can be realized to promote the efficiency and accuracy of the computation procedure. The modified differential quadratures have been applied to various example problems. From the data we have collected so far, it can be concluded that the proposed approach yields more accurate results in regions where most of the variations in the dependent variables occur and tends to lose its edge at locations where negligible changes can be detected in the numerical solutions.

ISSN:0098-6445    

DOI:10.1080/00986449308936170

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor