Step-change Experiments Research Articles

Methane oxidative coupling: II. A study of lithium-titania-catalyzed reactions of methane

Methane conversion to higher hydrocarbons via oxidative coupling was studied on a series of lithium-doped titania catalysts by cofeeding methane and oxygen. The degree of lithium promotion was studied by varying the lithium loading from 0 to 31.7 wt% lithium on the rutile crystal structure of titania. Catalytic results indicate that lithium-doped titania catalysts are effective for oxidative coupling of methane with hydrocarbon product selectivities ranging from 20 to 85%. Increasing the lithium loading reduces the combustion activity of the catalyst, improves hydrocarbon selectivity, and increases space-time yields. Product yield reached a maximum for the 16.2-wt% loading; this catalyst had methane conversions of ca. 15% with hydrocarbon selectivities of ca. 75%. The 16.2-wt% lithium-titania catalyst was studied under various operating conditions and compared favorably with other catalytic results that have been reported. The observed activation energy of the 16.2% lithium-titania catalyst was dependent on the reactant partial pressure and ranged from 25.6 to 37.6 kcal/mole. X-ray diffraction, X-ray photoelectron spectroscopy, infrared analysis, and differential thermal analysis results of the catalysts indicate that increasing the degree of lithium promotion lowers the surface area and the surface concentration of oxygen, and creates different phases at elevated temperatures. Routes for surface-catalyzed reactions coupled with gas-phase reactions were studied by transient pulse and step-change experiments. The transient experiments suggest that adsorbed oxygen is responsible for the C 2 hydrocarbon activity of the lithium-titania catalyst and that lattice oxygen activates methane nonselectively.

Thermal study of the receiver of a focusing solar collector using infrared thermography

In the present paper a three-dimensional model of the receiver of a THEK-type parabolic dish is proposed. This model is validated by step change experiments, using infrared thermography. Experimental results and numerical calculations show a good agreement. The three-dimensional model provides calculations of the temperature gradients in the receiver; examples are given for a solar flux step change and a flow rate step change. Such calculations of the thermal operating conditions of the receiver can be very useful for aging studies of the working fluid or of the absorbing coatings and also for mechanical resistance studies of the tubes.

Packed‐ and fluidized‐bed biofilm reactor performance for anaerobic wastewater treatment

Anaerobic degradation performance of a laboratory-scale packed-bed reactor (PBR) was compared with two fluidized-bed biofilm reactors (FBRs) on molasses and whey feeds. The reactors were operated under constant pH (7) and temperature (35 degrees C) conditions and were well mixed with high recirculation rates. The measured variables were chemical oxygen demand (COD), individual organic acids, gas composition, and gas rates. As carrier, sand of 0.3-0.5 mm diameter was used in the FBR, and porous clay spheres of 6 mm diameter were used in the PBR. Startup of the PBR was achieved with 1-5 day residence times. Start-up of the FBR was only successful if liquid residence times were held low at 2-3 h. COD degradations of 86% with molasses (90% was biodegradable) were reached in both the FBR and PBR at 6 h residence time and loadings of 10 g COD/L day. At higher loadings the FBR gave the best performance; even at 40-45 g COD/L day, with 6 h residence times, 70% COD was degraded. The PBR could not be operated above 20 g COD/L day without clogging. A comparison of the reaction rates show that the PBR and FBR per formed similarly at low concentrations in the reactors up to 1 g COD/L, while above 3 g COD/L the rates were 17.4 g COD/L day for the PBR and 38.4 g COD/L day for the FBR. This difference is probably due to diffusion limitations and a less active biomass content of the PBR compared with the fluidized bed.The results of dynamic step change experiments, in which residence times and feed concentrations were changed hanged at constant loading, demonstrated the rapid response of the reactors. Thus, the response times for an increase in gas rate or an increase in organic acids due to an increase in feed concentration were less than 1 day and could be explained by substrate limitation. Other slower responses were observed in which the reactor culture adapted over periods of 5-10 days; these were apparently growth related. An increase in loading of over 100% always resulted in large increases inorganic acids, especially acetic and propionic, as well as large increases in the CO(2) gas content. In general, the CO(2) content of the gas was very low, due to the large amount of dissolved CO(2) that exited with the liquid phase at low residence times. The performance of the FBR with whey was comparable to its performance with molasses, and switching of molasses to whey feed resulted in immediate good performance without adaptation.

Rates of Heat Exchange in Largemouth Bass: Experiment and Model

A mathematical model of body-core temperature change in fish was derived by modifying Newton's law of cooling to include an initial time lag in temperature adjustment. This model was tested with data from largemouth bass (Micropterus salmoides) subjected to step changes in ambient temperature and to more complex ambient regimes. Nonlinear least squares was used to fit model parameters k (min⁻ⁱ) and L (initial lag time in minutes) to time series temperature data from step-change experiments. Temperature change halftimes ($t_{1/2}$, in minutes) were calculated from k and L. Significant differences (P < .05) were found in these parameters between warming and cooling conditions and between live and dead fish. Statistically significant regressions were developed relating k and $t_{1/2}$ to weight and L to length. Estimates of k and L from the step-change experiments were used with a computer solution of the model to stimulate body temperature response to continuously varying ambient regimes. These simulations ...