Heat and mass transfer in honeycomb catalysts—II

Heat and mass gas-to-solid coefficients associated with the vaporisation of water and some hydrocarbons from the surface of a porous, monolithic structure were experimentally established. The mass transfer results are correlated using Reynolds and Schmidt number and the length-to-diameter ratio. The data for heat transfer are correlated on the Nu-Re(d/L) plot. The experimental results can be used for design of afterburner reactors utilizing monolithic structures.     

Heat and mass transfer in monolithic honeycomb catalysts—I.

The body of information presented in this paper is directed to engineers and scientists concerned with control of automobile emissions and exhaust gases from some industrial processes. The differential equations describing heat and mass transfer in a monolithic honeycomb catalyst are developed. Following transport mechanism is considered: convective heat and mass transfer in the holes of the structure, longitudinal thermal conductivity of the honeycomb support and gas-to-solid heat and mass transfer. The magnitudes of governing parameters for monolithic modules in use are discussed. Two methods for the numerical solution of a system of coupled, nonlinear ordinary differential equations with split boundary conditions are proposed. The first method-shooting procedure can be used only for problems with low values of Peclet number. For high values of Peclet number finite-difference approach along with the Newton-Raphson algorithm is suggested. It is shown that two stable steady states exist in certain regions of operation of a particular monolithic structure. The all-metal monolithic supports are more prone to multiplicity of steady states than the ceramic ones. For ceramic supports the two-phase piston-flow model is sufficiently accurate.     

Effect of fluid dispersion coefficients on particle-to-fluid mass transfer coefficients in packed beds: Correlation of sherwood numbers

Gas-phase and liquid-phase mass transfer data published in the literature are corrected for the axial fluid dispersion coefficient values proposed by WThe corrected Sherwood numbers in the range of Reynolds number from about 3 to 10,000 are correlated by Sh = 2 + 1.1 Sc^1/3Re^0.6.     

Heat transfer between gas fluidized beds of solid particles and the surfaces of immersed heat exchanger elements,part I

After some general remarks about fluidization, and a section on the hydrodynamic behaviour of fluidized beds, the mechanisms of heat transfer between the surfaces of heat exchanger elements and gas—solid fluidized beds are discussed in detail. A theoretical model, presented some years ago, is slightly modified and further developed to improve its applicability within a wide range of variables. The model makes use of some of the basic concepts of molecular kinetic theory as applied to solid particles in a fluidized bed. A complete derivation as well as all the parameters required to apply the model equations are given.     

Kinetics,poisoning and mass transfer effects in liquid-phase hydrogenations of phenolic compounds over a palladium catalyst

The influence of poisoning and mass transport on the performance of a suspended palladium catalyst for the liquid-phase hydrogenation of o-cresol h for the various transport and reaction steps occuring in the three-phase slurry system and these were combined into an overall rate expression. From th analysed either by an experimental approach or by calculation.The hydrogenation in the absence of poison is described by a Langmuir—Hinshelwood adsorption model with adsorption of hydrogen and substrate on diffe present in the feed, the activity loss can be described by an extended model of the same type; experimental results show that the poison is very strong between poison and substrate on the remaining catalyst surface. The selectivity is not affected by increasing poison concentration.     

Adsorption of phosgene and chloroform by activated and impregnated carbons

Equilibrium adsorption and desorption isotherm data at room temperature have been obtained for phosgene and chloroform vapors on BPL grade activated carbon, ASC whetlerite and four ASB impregnated carbons. Isotherm data were plotted in the form of the Dubinin-Polanyi equation. Experimental affinity coefficients (β_ex) for phosgene with chloroform as a reference, were calculated for all the carbons except ASC whetlerite from the slopes (k) of the Dubinin-Polanyi plot, and were compared with the theoretical affinity coefficients (β_th) in order to assess the adsorption capabilities of different adsorbents. In the case of phosgene on ASC whetlerite carbon, significant chemisorption takes place along with physical adsorption and the resulting isotherm shows non-linear behavior. Attempts were made to separate the chemisorption contribution from the total adsorption and thus assess β_ex for the physical adsorption contribution.     

Adsorption isotherms for thiourea at the mercury-solution interface

Double layer parameters for the adsorption of thiourea on mercury were obtained at constant charge and at constant potential on the electrode. In both cases the adsorption isotherm was found to be congruent on the electrical variable. The virial adsorption isotherm is quite adequate to describe the adsorption of thiourea but a different virial coefficient is needed in each case. The study of this simultaneous congruence led to the conclusion that when the adsorption of neutral molecules is considered in terms of the two electrical variables a much more convenient picture of the behaviour of the interface is obtained.     

Adsorption of phloroglucinol at the mercury/aqueous solution interface

The adsorption of phloroglucinol on mercury electrodes has been studied from 0.5 M NaClO₄ aqueous solution by means of capacitance and electrocapillary methods. Capacitance curves show a condensation type behaviour. Congruence Tests indicate simultaneous congruence at negative surface charges. Frumkin isotherm is obeyed both at constant charge and contant potential. The interaction parameter obtained through Frumkin fit corresponds to a repulsive interaction. A new isotherm based on Free Volume Theory was tried. A model for the structure of the double layer in presence of phloroglucinol has been advanced, based on these results and the available crystallographic data of phloroglucinol and its dihydrate.     

Heat transfer in laminar flow of non-Newtonian fluids

The main objective of this work is the consideration of local heat transfer coefficients for non-Newtonian power-law pseudoplastic liquid in laminar flow in circular conduits. The wall boundary conditions chosen are cases involving uniformly constant heat flux and step change in heat flux.Analytical solutions are developed for the wall temperature profile and compared with experimental data. Additionally, the experimental data have been correlated for comparison with existing relationships, hitherto not verified adequately. The limits of experimental data are:

     

Modeling of chemical reactors—XXX: Steady-state analysis of combined axial and gas-to-solid heat and mass transfer in a tubular adiabatic reactor

The topic of interest in this paper is the behaviour of a dispersion model which embraces external gas-to-solid heat and mass transfer. Two methods of solution of transport equations-shooting technique and parameter mapping method are suggested. The effect of governing parameters on the occurrence of multiple steady states is investigated. It is shown that multiple steady states can be caused by two factors: axial dispersion and external gas-to-solid heat and mass transfer. If the interphase heat and mass transfer is responsible for the multiple steady states then an infinite number of steady states may exist. Under certain conditions hot spot temperatures may be expected in an adiabatic reactor.     

Longitudinal dispersion of solid particles in fluid beds with horizontal baffles

The axial pressure profiles, allowable gas velocities and temperature distributions are measured for the fluidization of air—FCC cracking catalyst systems in 12- and 19-cm-diam. eight-stage fluid beds equipped with seven horizontal baffles. From these measurements, gas bubble holdup, apparent longitudinal dispersion and intermixing velocity of solid particles through the baffles are studied as functions of baffle design. It is shown that the gas bubble holdup increases, the operational range of gas flow decreases and the flow pattern of solid particles approaches plug flow with decreasing free area of baffles.     

Adsorption of HCN and H2O vapor mixtures by activated and impregnated carbons

Adsorption isotherms for HCN-H₂O vapor mixtures have been determined on BPL grade activated carbon and on ASC whetlerite (impregnated) carbon. The results show that the adsorption of HCN by carbons which are already in equilibrium with high levels of water vapor is significant. Adsorption of HCN strongly dominates when HCN and H₂O are simultaneously adsorbed from the vapor phase. Adsorbed HCN discourages additional adsorption of water vapor.     

X-ray crystal structure of the dye 2-bromo-4-cyano-4′-N,N-diethylaminoazobenzene

The crystal structure of 2-bromo-4-cyano-4′-N,N-diethylaminoazobenzene has been determined from X-ray diffraction data: C₁₇H₁₇N₄Br, mol. wt = 357·1. Triclinic, Pī (No. 2), α = 13·162(5) Å, b = 7·516(3) Å, c = 8·496(4) Å, α = 101·63(4)°, β = 95·79(4)°, γ = 91·49(4)°, V = 818·10 ų, Z = 2, D_c = 1·45 g cm^?3, F(000) = 378, λ(Mo_Kz) = 0·7107 Å, μ(Mo_Kα) = 26·70 cm^?1. The structure was solved by direct methods and refined by full-matrix least-squares to R = 0·053 for 2081 independent reflexions. The molecule possesses an essentially planar azobenzene skeleton. The effects of substituents on the geometry of the azo group are discussed. Significant molecular parameters are: NN, 1·264(6) Å; ₁BrC, 1·904(5) Å; mean NC, 1·410(7) Å; NNC, 115·7(2)° and 113·0(2)°; NCC (cis relative to NN), 125·4(3)° and 123·1(2)°; CC(Br)C, 123·0(2)°.     

An alternative presentation of the design parameters for mass flow hoppers

The procedure for determining the critical hopper outlet dimension and wall slope for mass flow hoppers by the Jenike method is well established and documented. However, existing presentations relating the bulk solid flow properties of effective angle of internal friction and the kinematic angle of wall friction with the hopper wall slope and flow factor for mass flow are often inconvenient for manual use and present difficulties when included in design programs suitable for microcomputers.This paper details an alternative presentation of the original Jenike flow factor charts. These alternative charts have been abbreviated to display only the critical design values in the border region between mass flow and funnel flow. The charts eliminate the need for imprecise parameter interpolations by displaying the required design parameters in the form of contours of constant wall slope and critical flow factor as a function of the effective angle of internal friction and the kinematic angle of wall friction.An illustrative example is provided to demonstrate the inherent advantages of this presentation for the evaluation of mass flow hopper geometries.     

Lateral mixing of solids in gas—solid fluidized beds with continuous flow of solids

Lateral mixing of solids in a gas—solid fluidized bed with continuous flow of solids can be adequately expressed by the dispersion model. An expression for estimating the lateral dispersion coefficient in such a bed is proposed.     

Electrochemical behaviour of nickel in nitric acid

The electrochemical investigations of nickel in a wide range of concentrations (1–14.6 M) of nitric acid were studied by potentiostatic, potentiodynamic and coulostatic method and by weight-loss measurements. The potentiostatic current-potential curves of nickel in nitric acid solutions can be classified into three categories, according to the proportion of the concentration of the acid. An increase of nitric acid concentration increases the influence of the cathodic reactions in the systems. The importance of cathodic reactions has been investigated by eliminating HNO₂ by additions of p-nitro aniline in the electrolyte. The coulostatic polarization has given distinct repassivation curves, indicating an intermediate state of nickel electrode between active and passive state. In the first part of repassivating process the potential value is more positive than the stationary potential of spontaneous passivation of nickel electrode in concentrated nitric acid. The return of the stationary potential of the passivity is a slow, time-dependent process. According to the experimental results and data given in the literature, it may be possible that in the beginning of the passivation process on nickel Ni(NO₃)₂ × 4 H₂O forms which changes to NiO₂ and than transforms slowly to NiO.     

Acidization—II. The dissolution of calcite in hydrochloric acid

The dissolution of calcite in hydrochloric acid was studied with the aid of a rotating disk system at 800 psig in the temperature range ?15·6–25°C. At 25°C the dissolution process is mass transfer limited even at high disk rotation speeds whereas at ?15·6°C both mass transfer and surface reaction rates limit the dissolution rate. The multicomponent coupled ionic diffusive fluxes of reactants and products were defined by using the gradient of the electrochemical potentials as driving forces for the diffusion. The activity coefficients used in calculating the multicomponent diffusivities of the diffusing species were estimated by Harned's rule. The concentration profiles of the ions in the boundary layer were then determined by numerically integrating the system of coupled convective diffusion equations. The effects of variable density, viscosity, and high mass fluxes on the fluid velocity in the boundary layer were taken into account. The rate of the surface reaction was found to be proportional to the 0·63 power of the surface hydrochloric acid concentration. Analysis of the experiments suggests that the absorption of hydrogen ion (described by a Freundlich adsorption isotherm) on the solid calcite surface and subsequent reaction of the adsorbed hydrogen ion with the solid calcite matrix is the reaction mechanism.     

Electrochemical reduction of 4-(2′-thienyl)quinazoline in acid solutions

The electrochemical behaviour of 4(2′-thienyl)quinazoline (4-TQ) was studied in acid solutions of varying ionic strengths using polarography, cyclic voltammetry and controlled-potential electrolysis. 4-TQ is reduced in two single electron steps to give the dihydro-4-TQ product. The two polarographic waves were found to be diffusion-controlled, the first wave being reversible and the second wave irreversible. On the basis of the results obtained, a mechanism is suggested for 4-TQ reduction in which a dimerization reaction goes in parallel with the electron-transfer step.