Inlet Partial Pressure Research Articles

The Effect of Water on the Activity and Selectivity for γ-Alumina Supported Cobalt Fischer–Tropsch Catalysts with Different Pore Sizes

The effect of water on the activity and selectivity for a series of γ-Al2O3 supported cobalt Fischer–Tropsch catalysts has been studied in an isothermal fixed-bed reactor at T = 483 K, P = 20 bar, and H2/CO = 2.1. The catalysts were produced applying incipient wetness impregnation and consisted of 20 wt.% cobalt and 0.5 wt.% rhenium deposited on γ-Al2O3 supports with different pore characteristics. For the narrow-pore catalysts, addition of water corresponding to an inlet partial pressure ratio of PH2O/PH2 = 0.4 reduced the reaction rates. In contrast, for a catalyst with larger pores the same water pressure increased the reaction rates. For all catalysts, water amounts equal to PH2O/PH2 = 0.7 at the reactor inlet suppressed the reaction rates and led to permanent deactivation. The addition of water increased the C5+ selectivity and decreased the CH4 selectivity for all catalysts. The pore characteristics seem to determine the effect of water on the rates.

First-principles based kinetic model for the hydrogenation of toluene

A fundamental kinetic model for the hydrogenation of toluene over platinum has been constructed, based on detailed first-principles density functional theory calculations for the hydrogenation of benzene over Pt(111). A Langmuir–Hinshelwood–Hougen–Watson (LHHW) kinetic model was derived based on an ab initio reaction path analysis [M. Saeys, M.-F. Reyniers, M. Neurock, G.B. Marin, J. Phys. Chem. B 109 (2005) 2064], enabling easy interpretation and evaluation of the parameters appearing in the rate equation. The activation energy, adsorption, and reaction enthalpies in the model were obtained from the first-principles calculations. The pre-exponential factors were calculated from statistical mechanics, using assumptions derived from the first-principles results. The coverage-dependent hydrogen chemisorption enthalpy was optimized to accurately model lab-scale experimental data for the gas phase hydrogenation of toluene over a 0.5 wt% Pt/ZSM-22 catalyst. The resulting hydrogen chemisorption enthalpy of − 54.0 ± 1.0 kJ / mol falls between the high and low coverage values, consistent with a simulated average hydrogen surface coverage of 61%. The LHHW model based on the ab initio calculations captures the main trends in the reaction rates. The reaction model predicts reaction orders for the inlet partial pressure of hydrogen and toluene ranging from 1.6 to 2.2 and from −0.4 to 0.7, respectively. This is in reasonable agreement with experimental values, which range from 1.3 to 1.8 and from −0.3 to 0.3, respectively.

Performance Measurements of Solid-Oxide Electrolysis Cells for Hydrogen Production

An experimental study has been completed to assess the performance of single solid-oxide electrolysis cells operating over a temperature range of 800 to 900°C. The experiments were performed over a range of steam inlet partial pressures (2.3–12.2 kPa), carrier gas flow rates (50–200 sccm), and current densities (−0.75–0.25A∕cm2) using single electrolyte-supported button cells of scandia-stabilized zirconia. Steam consumption rates associated with electrolysis were measured directly using inlet and outlet dew-point instrumentation. Cell operating potentials and cell current were varied using a programmable power supply and monitored continuously. Values of area-specific resistance and thermal efficiency are presented as a function of current density. Cell performance is shown to be continuous from the fuel-cell mode to the electrolysis mode of operation. The effects of steam starvation and thermal cycling on cell performance parameters are discussed.

Heterogeneous conditions in dissolved oxygen affect N-glycosylation but not productivity of a monoclonal antibody in hybridoma cultures.

It is known that heterogeneous conditions exist in large-scale animal cell cultures. However, little is known about how heterogeneities affect cells, productivities, and product quality. To study the effect of non-constant dissolved oxygen tension (DOT), hybridomas were subjected to sinusoidal DOT oscillations in a one-compartment scale-down simulator. Oscillations were forced by manipulating the inlet oxygen partial pressure through a feedback control algorithm in a 220-mL bioreactor maintained at a constant agitation. Such temporal DOT oscillations simulate spatial DOT gradients that can occur in large scales. Different oscillation periods, in the range of 800 to 12,800 s (axis of 7% (air saturation) and amplitude of 7%), were tested and compared to constant DOT (10%) control cultures. Oscillating DOT decreased maximum cell concentrations, cell growth rates, and viability indexes. Cultures at oscillating DOT had an increased glycolytic metabolism that was evidenced by a decrease in yield of cells on glucose and an increase in lactate yield. DOT gradients, even several orders of magnitude higher than those expected under practical large-scale conditions, did not significantly affect the maximum concentration of an IgG(1) monoclonal antibody (MAb). The glycosylation profile of the MAb produced at a constant DOT of 10% was similar to that reported in the literature. However, MAb produced under oscillating culture conditions had a higher amount of triantennary and sialylated glycans, which can interfere with effector functions of the antibody. It was shown that transient excursions of hybridomas to limiting DOT, as occurs in deficiently mixed large-scale bioreactors, is important to culture performance as the oscillation period, and thus the time cells spent at low DOT, affected cell growth, metabolism, and the glycosylation pattern of MAb. Such results underline the importance of monitoring protein characteristics for the development of large-scale processes.

Transalkylation of Methylamines: Kinetics and Industrial Simulation

A kinetic model for the transalkylation of methylamines over amorphous silica/alumina is presented. The experimental data obtained at temperatures ranging from 623 to 683 K and pressures ranging from 0.2 to 2 MPa were described adequately. Transalkylation reactions occur between ammonia/methylamine adsorbed on an acid site and ammonia/methylamine adsorbed on a neighboring base site. In all reaction paths, the surface reaction between adsorbed species was assumed to be rate determining. At all pressures, the fraction of free acid sites as calculated by the model is very low. The fraction of free base sites is low at high pressures only. Models derived from other mechanisms were rejected based on a statistical analysis, mechanistic considerations, and physicochemical interpretation of the parameters. An industrial transalkylation reactor was simulated. To enhance the rate of dimethylamine formation, a mixture with an excess of monomethylamine and a deficit of ammonia and trimethylamine is recommended as the inlet composition for a transalkylation reactor. Decreasing the inlet partial pressure of ammonia results in a reduced amount of catalyst required to obtain the equilibrium concentration of dimethylamine.

Kinetics of the CO+N 2O reaction over noble metals : II. Rh/Al 2O 3 and Pt–Rh/Al 2O 3

The subsequent reduction of N 2O by CO is an important subreaction in the overall CO+NO reaction. This study reports a kinetic investigation of the single CO+N 2O reaction on Rh/Al 2O 3, and on Pt–Rh/Al 2O 3, between 264 and 310 °C with inlet partial pressures in the range 3.4–13.8×10 −3 and 1.5–5.9×10 −3 atm respectively for CO and N 2O. Our mechanism proposal differs from those proposed earlier in the literature mainly by the dissociation step of chemisorbed N 2O species involving a nearest neighbor vacant site. The difference in the selectivity behavior toward the transformation of NO into N 2O on Pt and Rh has been related to competitive adsorptions between NO and N 2O. A comparison of the values of the equilibrium constant for the adsorption of N 2O with those earlier calculated for NO from the kinetic study of the CO+NO reaction on similar catalysts [J. Catal. 175 (1998) 194] shows that, on Pt, this competition is in favor of N 2O, which can readsorb and dissociate at low temperatures and low NO conversions. In contrast, the strongest NO adsorption on Rh inhibits the subsequent CO+N 2O reaction. In the second part of this study, the selected mechanism has been tentatively validated in a wider range of temperatures and conversions. The adsorbate coverage dependencies of the strength of the metal–adsorbate bond has been accounted for to model temperature-programmed conversion curves.

A Nonlinear Observer Design for Fuel Cell Hydrogen Estimation

We present an observer design to estimate the partial pressure of hydrogen in the anode channel of a fuel cell. A precise knowledge of this pressure is of importance to ensure reliable and efficient operation of the fuel cell power system. Our design makes use of a monotonic nonlinear growth property of the voltage output on hydrogen partial pressures at the inlet and at the exit of the channel. By treating the slowly varying inlet partial pressure as an unknown parameter, an adaptive observer is developed that employs a nonlinear voltage injection term. We then study the robustness of this observer against variations in the inlet partial pressure, and analyze its sensitivity to other modeling errors. We also prove a robustness property of the observer against the parameter estimation error, which means that it can be implemented with alternative parameter estimator designs.

Hydrogenation kinetics of toluene on Pt/ZSM-22

Kinetic experiments on the hydrogenation of toluene were performed on 0.5 wt.% Pt/ZSM-22 at temperatures in the range 423–498 K, H 2 inlet partial pressures of 100–300 kPa and toluene inlet partial pressures of 10–60 kPa. Construction of a kinetic model was based on a critical evaluation of available literature data on the hydrogenation of aromatic components together with physicochemical studies on the interaction of aromatic components and related hydrogenated products with metal surfaces as well as on quantumchemical calculations. This lead to a general kinetic model, analogous to the Horiuti Polanyi mechanism for ethylene hydrogenation, with the first four H atom addition steps not in quasi-equilibrium. Chemisorption of H 2 and toluene was assumed to occur on identical sites. No dehydrogenated surface species was taken into account. The preexponential factors were calculated using transition state theory. A model with equal surface reaction rate coefficients for the H addition steps was selected as the best model. The estimated toluene and H 2 chemisorption enthalpies amounted to −70 and −42 kJ mol −1. An activation energy in the range of 40–50 kJ mol −1 was found. Under typical reaction conditions, 60% of the surface is covered by toluene and 20% by H atoms. The remaining 20% are free. Negligible amounts of partially hydrogenated species were found to be present on the catalyst surface.

Influence of coke formation on the conversion of hydrocarbons: II. i-Butene on HY-zeolites

The influence of coke formation on the conversion of i-butene was investigated on a series of HY-zeolites differing by the number of framework and extra framework Al (EFAL) atoms per unit cell. The reactions were carried out at 723 K, an inlet partial pressure of i-butene of 1 kPa and a total pressure of 105 kPa, in a recycle electro-balance reactor, operating gradientlessly under high conversion, with on-line gas chromatographic analysis of the effluent. Coke formation not only leads to pore blockage but also to reactions involving coke molecules that cause changes in the activity and the selectivity of the catalysts. Even in the presence of important pore blockage, the occurrence of these reactions can result in an increased activity of the catalyst. In the absence of EFAL, the conversion rate is found to decrease with coke content for H +/u.c.<29, while for H +/u.c.≥29, the conversion rate of i-butene initially increases with increasing coke content until pore blockage becomes dominant. Up to 2 wt.% of coke, and in the absence of EFAL, the influence of coke formation on the formation rate of n-butenes is positive for catalysts with H +/u.c.≥25, while it is negative for H +/u.c.<25. On all catalysts, the rate of the dimerization-β-scission path decreases with coke content. Except for the zeolite with H +/u.c.=6, the rate of hydride transfer and of coke formation are positively influenced by coke formation. The rate of hydride transfer parallels the rate of coke formation. To explain the influence of coke formation on the reaction rates, a reaction mechanism involving coke molecules is proposed.

Isothermal “light-off” during catalytic N2O decomposition over Fe/ZSM-5

The catalytic decomposition of N2O over Fe/ZSM-5 was studied at pressures up to 1 atm. As the partial pressure of N2O in the feed to a packed bed reactor was varied, the catalytic activity was observed to abruptly increase at inlet partial pressures above some critical value. This abrupt increase in activity was not due to the reaction exotherm, but is believed to be a kinetic phenomenon. In similar experiments where the temperature was varied the activity did not jump to a higher level. The dual levels of activity were observed for several Fe/ZSM-5 catalysts that spanned a range of SiO2/Al2O3 ratios from 30 to 280, irrespective of whether the iron was introduced into the zeolite by ion exchange or by sublimation of iron chloride. When the inlet partial pressure of N2O to the packed bed reactor was sufficiently high to cause the catalyst to operate in the high activity regime, the high activity state was sustained throughout the catalyst bed, even though the N2O partial pressure in the latter part of the bed had dropped below the critical level. Microkinetic modeling shows that this kind of behavior is possible in an isothermal catalytic system. The microkinetic model includes two redox cycles. In one cycle the cation oxidation state alternates between Fe2+ and Fe3+. In the second, more active redox cycle there is alternation between a surface nitrite and nitrate. The former redox cycle predominates at low partial pressures and the latter at high N2O partial pressures.

Kinetics of ethane aromatization over a gallium-doped H-ZSM-5 catalyst

The kinetics of the catalytic ethane aromatization over Ga-(3 wt.-%) H-ZSM-5 ( Si/Al=15) were investigated in a catalytic fixed-bed micro reactor under atmospheric pressure. Reaction temperatures were 873, 898, and 923 K; inlet partial pressures ranged from 0.1 to 0.8 bar for ethane and from 0.1 to 0.7 bar for hydrogen. The aromatization reaction was described by a 4-step reaction scheme, comprising the formation of aromatic hydrocarbons via the subsequent formation of ethylene and higher alkenes, and the formation of methane as a side product of cracking. The kinetic model takes into account the bifunctionality of the catalyst by assuming gallium sites for the dehydrogenation of ethane and acid sites for alkene interconversion and aromatics formation. The latter proceeds with the formation of surface hydrogen on the zeolite, which can only desorb after a back-spillover step to the gallium sites.

Reduction of taconite concentrates in a cyclone reactor

A cyclone reactor system for the partial reduction and melting of taconite concentrate fines was designed and operated. A nontransferred arc plasma torch was employed as a heat source. Taconite fines, carbon monoxide and carbon dioxide were fed axially into the reactor, while the plasma gas was introduced tangentially into the cyclone. The average reactor temperature was maintained above 1673°K (1400°C), and reduction experiments were performed under various conditions. The influence of the following parameters on the reduction of taconite was investigated: carbon monoxide to carbon dioxide inlet feed ratio, carbon monoxide inlet partial pressure and average reactor temperature. The interaction of the graphite lining with carbon dioxide and taconite were also studied.

Influence of temperature gradients on partial pressures in a low-pressure chemical-vapor-deposition reactor

Measurements and calculations of the influence of temperature gradients on the partial pressures of the gas species in a cold-wall chemical-vapor-deposition reactor are presented. The experiments were performed at low pressures (300–500 Pa total pressure) and gas mixtures consisting of hydrogen, nitrogen, and tetrafluoromethane. The partial pressures were determined by Raman spectroscopy. The Soret effect (or thermal diffusion) has a large influence on the partial pressures of heavy gases in the vicinity of the heated wafer. In some cases a decrease in partial pressure of 20% compared to the inlet partial pressures was observed. Numerical calculations were performed to predict the behavior of the gas mixture. For mixtures under investigation the gas temperatures as well as the changes in partial pressures due to the Soret effect were predicted correctly.

Separation of Dilute CO2-CH4Mixtures by Adsorption on Activated Carbon

Abstract The adsorption separation process of diluted gas mixtures containing carbon dioxide and methane on activated carbon has been investigated. Experimental breakthrough curves for both the adsorption and desorption processes have been obtained using mixtures of one or two adsorbable components and one nonadsorbable. A parametric study was conducted to determine the influence of temperature, inlet partial pressure, and superficial velocity on the performance of the adsorption process. The separation factor for the activated carbon was determined to be 3–3.5 at 40°C and 2.7–2.8 at 60°C. The experimental curves have been calculated using a mathematical model based on the approximations of local equilibrium and isothermal conditions. The system of differential equations was solved numerically using a Crank-Nicolson finite difference discretization scheme. The resulting nonlinear system of equations was solved iteratively using the Gauss-Seidel method. The parameters in the model have been estimated either by empirical correlations or using experimental data. The model simulations for both the adsorption and desorption processes are generally in good agreement with experimental data. The evident deviations that resulted in a few cases can be attributed to deficiencies in the Langmuir model itself.

Photo-assisted growth of ZnTe by metalorganic chemical vapour deposition

The metalorganic chemical vapour deposition (MOCVD) photo-assisted growth of ZnTe using a xenon lamp has been performed at atmospheric pressure under hydrogen as a carrier gas. Epitaxial growth was achieved on (100) GaAs, (100) GaSb and (100) ZnTe substrates with diethylzinc (DEZn) and diethyltellurium (DETe) as precursors. We have studied the growth rate as a function of the growth temperature, the partial pressure of precursors, the inlet partial pressure ratio R = VI/II, the light intensity and the energy of the irradiating photons. A growth rate enhancement has been observed for illuminated layers grown on GaAs and ZnTe substrates in comparison with those grown without illumination. We have not observed any measurable enhancement for layers grown onto (100) GaSb. The growth rate as a function of the light intensity increases for intensities higher than 20 mW / cm 2 and saturates for P > 120 mW / cm 2. We relate the growth rate enhancement to irradiating photons with an energy higher than the band gap of ZnTe at the growth temperature.

Mass-spectrometric study of thermal decomposition of diethylzinc and diethyltellurium

A mass-spectrometric study of the thermal decomposition of Group II and VI alkyls involved in ZnTe growth by metal-organic chemical vapour deposition (MOCVD) is reported. The pyrolysis of diethylzinc (Et2Zn) and diethyltellurium (Et2Te) alone and together were followed with the change of the inlet partial pressure ratio (R= Te/Zn = 1 and 5). Under He and H2, the thermal decomposition of Et2Zn organometallic species leads to the formation of n-C4H10, C2H4 and C2H6. The pyrolysis of Et2Te produces ethene in excess of other hydrocarbons. For co-decomposition in an He atmosphere, pyrolysis curves as a function of temperature, with R= 1 show that the temperature corresponding to the half-decomposition ratio (T50) of Et2Te is lowered by 25 °C relative to that of Et2Te alone. For R= 5, the T50 of Et2Zn is increased by 30 °C relative to that of the precursor alone. The kinetics of decomposition of Et2Zn and Et2Te alone showed no thermal hysteresis, whereas a strong hysteresis loop between heating and cooling of the susceptor was recorded during the co-decomposition. The dependence of these effects on R and temperature are discussed in relation to surface reactions.

Modeling and Optimization of the Step Coverage of Tungsten LPCVD in Trenches and Contact Holes

A model is presented to calculate the step coverage of blanket tungsten low pressure chemical vapor deposition (W‐LPCVD) from tungsten hexafluoride . The model can calculate tungsten growth in trenches and circular contact holes, in the case of the reduction by , , or both. The step coverage model predictions have been verified experimentally by scanning electron microscopy (SEM). We found that the predictions of the step coverage model for the reduction of are very accurate, if the partial pressures of the reactants at the inlet of the trench or contact hole are known. To get these reactant inlet partial pressures, we used a reactor model which calculates the surface partial pressures of all the reactants. These calculated surface partial pressures are used as input for our step coverage model. In this study we showed that thermodiffusion plays a very important role in the actual surface partial pressure. In the case where was present in the gas mixture trends are predicted very well but the absolute values predicted by the step coverage model are too high. The partial pressure of , which is a by‐product of the reduction reaction, may be very high inside trenches or contact holes, especially just before closing of the trench or contact hole. We found no influence of the calculated partial pressure on the step coverage. Differences between step coverage in trenches and contact holes, as predicted by the step coverage model, were found to agree with the experiments. It is shown that the combination of the step coverage and reactor model is very useful in the optimization towards high step coverage, high throughput, and low flow. We found a perfect step coverage (no void formation) in a 2 μm wide and 10 μm deep trench using an average flow of only 35 sccm, at a growth rate of 150 nm/min. In general, it is shown that the reduction of by offers no advantages over the reduction by as far as step coverage is concerned.

Oxidative coupling of methane over La 2O 3CaO catalysts Effect of bulk and surface properties on catalytic performance

The oxidative coupling reaction of methane to higher hydrocarbons over lanthanum-calcium-oxide catalysts was studied in a microcatalytic fixed-bed reactor in the temperature range from 973 to 1013 K applying inlet partial pressures of methane and oxygen of 93 and 7 kPa, respectively. Lanthanum to calcium atom ratios of the catalyst were varied as follows: 0/100, 1/99, 5/95, 10/90, 14/86, 20/80, 27/73, 30/70, 50/50, 70/30, and 100/0. Maximum C 2+ selectivities of 76% and C 2+ yields of 9% were achieved (1013 K). The bulk-phase and surface compositions of the catalysts were determined by XRD and XPS, respectively. A relative measure of surface basicity and acidity was derived from the product distribution of the isopropyl alcohol conversion used as a probe reaction. The catalytic performance of the differently composed materials is affected by their surface composition and not by their bulk composition. Maximum selectivity is achieved with a surface content of lanthanum between 60 and 90%. It is tentatively suggested that a catalyst which is primarily basic in nature requires acidic sites also for achieving high C 2+ selectivity.

Engine power simulation for transonic flow-through nacelles

Conclusions Heterogeneous nucleation is seen to begin at partial pressures of about 10 times the wall partial pressure. Homogeneous nucleation is seen to begin at partial pressures on the order of 10 times the wall partial pressure. Initial gas temperature has only a weak effect on the onset of snow as long as the wall temperatures are substantially below this inlet temperature. Snow may be avoided by. staying below the onset curves, lowering the inlet partial pressures of the water vapor, or increasing the wall temperatures. These calculations and curves apply to any uniform temperature water cold trap. These curves can even be used to predict the humidity required for the thin natural convection boundary layer flowing down the outside of a glass of ice water to be visible due to small droplet formation (fog) in this boundary layer.

Diffusion of oxygen and carbon dioxide through blood flowing in a channel

The numerical solutions of diffusion equations have been obtained for the cases of oxygen and carbon dioxide diffusing through blood flowing between two porous parallel planes. It is assumed that at the entrance to the channel the concentration profiles are uniform and the velocity profile is fully developed. The rheological characteristics of blood are described by the Casson equation. The computations have been made employing the explicit finite-difference forward-marching procedure. The results have been obtained for a wide range of yield numbers, inlet partial pressures, pH, membrane resistances and haemoglobin concentrations.