摘要:

AbstractThe quality of large semiconductor crystals grown from the melt for use in electronic and optoelectronic devices is strongly influenced by the intricate coupling of heat and mass transfer and melt flow in growth systems. This paper reviews the present state of understanding of these processes starting from the simplest descriptions of solidification processes to detailed numerical calculations needed for quantitative modeling of processing with solidification. Descriptions of models for the vertical Bridgman‐Stockbarger and Czochralski crystal growth techniques are included as examples of the level of understanding of industrially important method

ISSN:0001-1541    

DOI:10.1002/aic.690340602

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractManufacture of optical waveguide preforms by modified chemical vapor deposition (MCVD) is theoretically investigated. For the first time, a model accounting for the concurrent heat transfer, chemical kinetics, and silica aerosol dynamics during lightguide preform fabrication by MCVD is presented. Silica particles are formed by high‐temperature oxidation of SiCl4, grow by coagulation, and deposit to the preform walls by thermophoresis and Brownian diffusion. Assuming first‐order SiCl4oxidation and approximating the aerosol size distribution by a log normal function throughout the process, five partial differential equations describe this process. The emphasis of this study is on the achievement of high process yield (deposition efficiency of MCVD).Process conditions for operation in the particle transport‐limited and reaction‐limited regimes are quantitatively identified. Operation in the former regime results in complete oxidation of the inlet SiCl4but only about half of the product SiO2particles deposit to the tube wall. Operation in the latter regime results in limited oxidation of inlet SiCl4(about 40%) but almost all product SiO2particles deposit to the tube wall. This study shows that high process yields and deposition rate can be achieved in lightguide preform manufacture by MCVD by combining operation in the reaction‐limited regime with recycling of the exit gases from the pre

ISSN:0001-1541    

DOI:10.1002/aic.690340603

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractCO2absorption rates into diethanolamine (DEA)‐promoted carbonate solutions were measured over a wide range of carbonate conversions and temperatures. Analysis of the data showed that the CO2‐DEA reaction rate was controlled by the rate of formation of zwitterion intermediate for carbonate conversions below 30%, and was controlled by the rate of abstraction of proton from the zwitterion intermediate for carbonate conversions greater than 30%. Hydroxyl ion, free amine, and carbonate ion were identified to act as bases to abstract the proton from the zwitterion. The modeling of the CO2absorption rates and the effects of carbonate conversion and amine concentration on the CO2‐DEA reaction kinetics were investi

ISSN:0001-1541    

DOI:10.1002/aic.690340604

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractMixing time and power consumption have been measured in baffled, paddle‐agitated cylindrical tanks with single‐ and double‐impellers, and mixing efficiency has been defined by a reciprocal of the product of the mixing time and power consumption. The mixing efficiency and mixing time of double‐impeller agitation have been compared with those of single‐impeller agitation, and they have been related, through a circulation time, to the flow patterns obtained from the measurements of mean axial and radial v

ISSN:0001-1541    

DOI:10.1002/aic.690340605

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractCoherent flow‐oriented structures, with a dimensionless spacing of λ+≈ 100, observed close to a wall in turbulent flows are believed to control the production of turbulence. A 2 1/2 D computer model developed by Nikolaides is used to explain the dimensionless spacing of these structures. The calculations suggest that the net production of turbulence in the viscous wall region is sensitive to changes in λ+in the neighborhood of λ+≈ 100. They indicate that the net production is negative for λ+<85 and that for λ+= 93 the net production is large enough to supply the turbulent energy dissipated in the

ISSN:0001-1541    

DOI:10.1002/aic.690340606

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractParticle dispersion in a two‐dimensional mixing layer is analyzed numerically by calculating the particle trajectories in a free shear layer simulated by discrete vortices. Important global and local flow quantities reported in experimental measurements are reasonably simulated.The particle dispersion results demonstrate that the extent of particle dispersion depends strongly on the Stokes numberSt, the ratio of particle aerodynamic response time to the characteristic time of the mixing layer flow. Particles with relatively smallStvalues are dispersed at approximately the fluid dispersion rate. Particles with largeStvalues are dispersed at a rate that is less than the fluid rate. Particles with intermediate values ofStmay get flung outside of the vortex structures in the mixing layer and therefore get dispersed at a higher rate than the fluid. This result is in agreement with some previous experiments in plane and axisymmetric jets. It is also found that a higher dispersion rate is associated with the particles introduced to the flow from the low‐speed side and from near the middle of the mixing la

ISSN:0001-1541    

DOI:10.1002/aic.690340607

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractModern equation‐oriented simulators solve systems of equations and procedures using Newton's method or its variants. Partial derivatives of the output variables of procedures with respect to their input variables are required and are usually approximated by perturbation or quasi‐Newton methods. Here it is shown how this partial derivative information can be computed exactly for procedures in general, with particular applications to flash and multicomponent distillation procedures. Results indicate that 1. physical property procedures should provide analytical derivatives of properties in addition to point values; 2. commonly used procedures for vapor‐liquid equilibria calculation can easily be modified, and should be, to also return output/input gradients; and 3. the use of exact derivatives at all levels results in significant improvements in efficiency over present me

ISSN:0001-1541    

DOI:10.1002/aic.690340608

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractTearing and simultaneous correction techniques coupled with an odd‐even cyclic reduction parallel algorithm are used to carry out calculations of multicomponent separation problems. The old‐even algorithm implements effectively on most parallel processors. No noticeable differences are found when numerical results are compared with those obtained using serial algorithms. The results for three example problems are presented to illustrate the method. Calculations were carried out using single precision arithme

ISSN:0001-1541    

DOI:10.1002/aic.690340609

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractThe partial internal wetting of spherical catalyst particles exposed to saturated vapor is investigated here by considering the interplay of capillary condensation phenomena and thermal effects for a first‐order irreversible reaction. It is shown that for certain very narrow ranges of parameters steady states can exist in which the macropores are vapor‐filled while micropores in an inner core, or an outer shell, are liquid‐filled. Under certain conditions even two such steady states are found, although only one of them can be stable. In addition to these there exist states in which all the micropores are liquid‐filled or those in which the entire particle is interna

ISSN:0001-1541    

DOI:10.1002/aic.690340610

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY

摘要:

AbstractIn this study the impact of the transport processes and the degree of wetting on the overall selectivity of a single catalytic pellet is examined for the commercially important consecutive‐parallel networkA(g)+vBB(l)→vR1R(l), A(g)+vR2R(l)→P(l). The one‐dimensional formulation applies to catalysts in which the active component is concentrated in a thin porous surface shell coating an impermeable support. Model modifications provide an approximate description of uniformly active catalysts. The kinetics are assumed to be first and zero order with respect to the volatile (A) and nonvolatile (B, R) components, respectively, as encountered in many olefin hydrogenations. The common literature assumption of volatile reactant limiting reactions is relaxed by accounting for nonvolatile reactant depletion. The depletion can significantly reduce the desired product (R) selectivity below its intrinsic value attained under fully wetted, no‐depletion conditions. A model employing the single limiting reactant assumption cannot predict such selectivity variations. The selectivity is shown to be a complex function of the wetting efficiency, the stoichiometry, and the interacting reactions and mass transport processes. In many cases the intrinsic selectivity is attained for wetting efficiencies exceeding a critical value. Increases in the wetting above this value can reduce significantly the production rate ofRbecause of resistance toAsupply through the liquid film. Thus, an optimal range of wetting efficiencies exists for which the selectivity is equal to its intrinsic value and overall rates are high. The significance of the main findings with regard to trickle‐bed reactor performance ar

ISSN:0001-1541    

DOI:10.1002/aic.690340611

出版商:American Institute of Chemical Engineers    

年代:1988

数据来源: WILEY