CORRELATIONS FOR ESTIMATING THE DIFFUSIVITIES OF NITROUS OXIDE IN AQUEOUS SOLUTIONS OF DIETHANOLAMINE AND N-METHYLDIETHANOLAMINE AND THE SOLUTION DENSITIES AND VISCOSITIES

Experimentally measured diffusion coefficients of nitrous oxide in water, aqueous solutions of diethanolamine, aqueous solutions of N-methyldiethanolamine, and aqueous blends of diethanolamine and N-methyldiethanolamine have been used to arrive at a modified Stokes-Einstein correlation by using nonlinear least-squares filling. The average deviation of this correlation from the experimentally measured diffusion coefficients is about 13%. Comparisons are also made to the estimated diffusivities of nitrous oxide in the amine solutions which are obtained from the Wilke-Chang and Hayduk-Minhas correlations. The average deviations of the Wilke-Chang and Hayduk-Minhas correlations from the measured values are about 13% and 19%, respectively. Correlations were also presented for estimating the solution densities and viscosities. Predicted values for the solution densities and viscosities from these correlations deviate from experimentally measured values on average by 1.2% and 4.8% respectively.     

Hypoxia and vascular endothelial growth factor expression in human squamous cell carcinomas using pimonidazole as a hypoxia marker

Hypoxia in human tumors is associated with poor prognosis, but the molecular mechanisms underlying this association are poorly understood. One possibility is that hypoxia is linked to malignant progression through vascular endothelial growth factor (VEGF) induction and the associated angiogenesis and metastasis. The present clinical study measures hypoxia and VEGF expression on a cell-by-cell basis in human squamous cell carcinomas to test the hypothesis that hypoxia and VEGF protein expression are coupled in human tumors. Eighteen patients with invasive squamous cell carcinoma of the uterine cervix and head and neck have been investigated by a quantitative image analysis of immunostained sections from their tumors. The hypoxia marker pimonidazole was used to measure tumor hypoxia, and a commercially available antibody was used to measure VEGF protein expression. A quantitative immunohistochemical comparison of hypoxia and VEGF protein expression revealed no correlation between the two factors.     

Steady-state methanation kinetics over a Ni/Al2O3 catalyst

Extensive kinetic data for the methanation reaction over a Ni/Al₂O₃ catalyst were obtained in a specially designed gradientless reactor operating at steady state. The reactor pressure was 101.3 kPa, and three temperatures were used, namely, 503, 513 and 523 K. The following three-parameter phenomenological model based on a proposed Langmuir-Hinshelwood mechanism adequately describes the data: r = L² K₃ K₄^0.5 k₅ P^0.5H₂ Pco/ [1 + K₃ (k₅ / k₆)Pco + K₄^0.5 P^0.5H₂ ]² With dissociative adsorption of hydrogen and hydrogen-assisted dissociation of adsorbed carbon monoxide, the postulated mari is the CH surface group, and the rds is the hydrogenation of the surface CH group.     

On controlling an autothermal fixed-bed reactor at an unstable state—III: Model reduction and control strategies which avoid sstate estimation

A simplified approach for the control of fixed-bed reactors is discussed. Key elements include: (i) careful modeling of the reactor to obtain a lumped approximation with good internal structure (i.e. appropriate coupling between inputs and state variables through the different response modes); (ii) a major reduction in model size to an accurate, low-order form; (iii) derivation of a control algorithm for the low-order model; and (iv) direct application to the full-order system without the use of state estimation.The success of this approach is rooted in the choice of modeling techniques and of a model reduction procedure. The powerful new reduction method of Litz is evaluated here. Applications of the technique to a simulated unstable autothermal reactor are demonstrated for the case of modal control with incomplete state feedback. Results point to a high probability of success in subsequent experimental tests.     

CONCENTRATION FORCING IN AMMONIA SYNTHESIS: PLUG-FLOW EXPERIMENTS AT HIGH TEMPERATURE AND PRESSURE

Square-wave concentration forcing experiments have been performed for the ammonia synthesis reaction in an isothermal, fixed-bed, plug-flow reactor at a temperature and pressure approaching industrial conditions (703 K, 4.14Mpa). Using the optimal steady-state (OSS) production rate as a base for comparison, we have found that cycling between pure reactants (N₂, H₂) results in decreased time-averaged production rates as predicted by Wilson and Rinker (1982), while mixture cycling under appropriate conditions results in modest (but not insignificant) improvements over the OSS rate. We also have observed that the catalyst activity changes significantly under a variety of dynamic operating conditions.