Gradient temperature fluctuations

In the Godrej-Lurgi process, olefins are produced by dehydration of fatty alcohols on alumina in a continuous vapor-phase process. The reaction is carried out in a specially designed isothermal multitube reactor at a temperature of approximately 300°C and a pressure of 5—10 kPa (0.05—0.10 atm). As the reaction is endothermic, temperature is maintained by circulating externally heated molten salt solution around the reactor tubes. The reaction is sensitive to temperature fluctuations and gradients, hence the need to maintain an isothermal reaction regime. 

Model of ID dissipation spectrum from Pope [19] (line) and measured, noise-corrected spectrum of the square of the radial gradient of fluctuating temperature in a CH4/I-I2/N2 jet flame (Re = 15,200) (symbols). Each spectrum is normalized by its maximum value. The arrow indicates the 2% level, which corresponds to the normalized wavenumber k = 1 according to the model spectrum. (From Barlow, R.S., Proc. Combust. Inst., 31, 49,2007. With permission.)... 

Temperature gradients and local temperature fluctuations usually parameterized by t2 (Peimbert 1967) lead to a systematic bias when the electron temperature determined conventionally from [O m] X 4363/a 5007 is substituted into the expressions for effective recombination coefficients of hydrogen and helium. 

Refractive index detector (RID) 1 x 10 7g 1 x 104 Universal, dependent on refractive index difference with mobile phase Relatively insensitive to flow fluctuations, but sensitive to temperature fluctuations nondestructive, cannot be used with gradient elution solvents must be degassed to avoid bubble formations laser-based Rl detectors offer higher sensitivity... 

It must be noted that, even in the case of similar methods, some details in the procedures employed may lead to significantly different results. For example, the much larger oxygen gradient found by Peimbert et al. (1978) probably results from their using the temperature fluctuation scheme (with t2 =. 035). 

This is the most commonly used type of detector as it can be rather sensitive, has a wide linear range, is relatively unaffected by temperature fluctuations and is also suitable for gradient elution. It records compounds that absorb ultraviolet or visible light. Absorption takes place at a wavelength above 200 nm, provided that the molecule has at least ... 

Infrared observations of the effects of surface films on heat and gas transfer have been made in situ (Frew et al. 2004). Figure 10 (upper) shows the instrumented air-sea interaction catamaran LADAS operating within a banded surface slick during the CoOP97 study. Simultaneous infrared imaging and measurements of wave slope and surfactants outside and inside of the slick determined that, when the slick was entered, the surface temperature distribution shifted toward lower temperatures and the spatial scales of the temperature fluctuations at the sea surface increased (Fig. 10, lower), characteristic of reduced surface renewal and an attenuation of mixing in the aqueous boundary layer. The estimated temperature gradient increased from 0.13 Kelvin to 0.24 Kelvin and the net heat flux dropped from 77.2 Watt m 2 to 36.5 Watt m"2. The 80% increase in AT and a 50% decrease in the heat flux lead to a decrease in the estimated heat transfer velocity from 49.7 cm h"1 to 13.1 cm h"1. 

Gradient sublimation To ensure a high material yield in combination with a high selectivity, sublimation over an extended temperature gradient ( 500 K/m) is the preferred method if the sublimation temperature of the host material is known. Inside a glass tube separation across the temperature gradient takes place and, in most cases, the purified fraction occurs spatially well-separated from the contaminants. The efficiency can be improved in terms of yield and stability in the presence of an additional inert carrier gas, e.g. Ar or N2, which at low pressure (10 Torr) reduces the molecular mean free path and equilibrates temperature fluctuations across the glass tube. 

This UV detector is inexpensive, sensitive to normal flow and temperature fluctuations, and well suited to gradient elution. It is, however, a selective detector. Only sample molecules which absorb at 254 nm can be detected. 

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