Temperature, variable

Porous solids are not good conductors of heat, so reactions with appreciable heats of reaction can develop significant gradients of temperature and reaction rate along the pore, as well as over an external film. Enhancement of a rate by temperature can counteract the effect of falling concentration. Exothermic reaction rates in pores, as a consequence, can be much greater than at the surface condition. Another peculiarity that can arise with adiabatic reactions is multiple steady states. 

For slab geometry as an example, the material and energy balances over a pore are 

Elimination of rc followed by integration gives the linear relation between temperature and concentration in the pore, 

The maximum temperature difference will develop when C = 0, that is, 

Problems P7.06.03 and P7.06.04 are numerical examples, Usually a temperature drop through an external film is much greater than over a pore. 

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A connnon approach has been to measure the equilibrium constant, K, for these reactions as a fiinction of temperature with the use of a variable temperature high pressure ion source (see section (Bl.7.2)1. The ion concentrations are approximated by their abundance in the mass spectrum, while the neutral concentrations are known from the sample mlet pressure. A van t Hoff plot of In K versus /T should yield a straight Ime with slope equal to the reaction enthalpy (figure B1.7.11). Combining the PA with a value for basicityG at one temperature yields a value for A.S for the half-reaction involving addition of a proton to a species. While quadnipoles have been tire instruments of choice for many of these studies, other mass spectrometers can act as suitable detectors [19, 20]. 

Szule]ko J E and McMahon T B 1991 A pulsed electron beam, variable temperature, high pressure mass... 

Lyerla J R, Yannoni C S and Fyfe C A 1982 Chemioal applioations of variable-temperature CPMAS NMR speotrosoopy in solids Accounts Chem. Res. 15 208-16... 

I still did not have suitable low-temperature instrumentation of my own to carry out the low-temperature NMR studies, but Martin Saunders at Yale did. Thus our samples now traveled the Massachusetts Turnpike from Boston to New Haven, where with Marty we were able to study solutions of the norbornyl cation at increasingly lower temperatures using his home-built variable-temperature NMR instrumentation housed in the basement of the old Yale chemistry building. We... 

Temperature and pressure are not considered as primary operating variables temperature is set sufficiendy high to achieve rapid mass-transfer rates, and pressure is sufficiendy high to avoid vaporization. In Hquid-phase operation, as contrasted to vapor-phase operation, the required bed temperature bears no relation to the boiling range of the feed, an advantage when heat-sensitive stocks are being treated. 

In the context of chemometrics, optimization refers to the use of estimated parameters to control and optimize the outcome of experiments. Given a model that relates input variables to the output of a system, it is possible to find the set of inputs that optimizes the output. The system to be optimized may pertain to any type of analytical process, such as increasing resolution in hplc separations, increasing sensitivity in atomic emission spectrometry by controlling fuel and oxidant flow rates (14), or even in industrial processes, to optimize yield of a reaction as a function of input variables, temperature, pressure, and reactant concentration. The outputs ate the dependent variables, usually quantities such as instmment response, yield of a reaction, and resolution, and the input, or independent, variables are typically quantities like instmment settings, reaction conditions, or experimental media. 

Normally when a small change is made in the condition of a reactor, only a comparatively small change in the response occurs. Such a system is uniquely stable. In some cases, a small positive perturbation can result in an abrupt change to one steady state, and a small negative perturbation to a different steady condition. Such multiplicities occur most commonly in variable temperature CSTRs. Also, there are cases where a process occurring in a porous catalyst may have more than one effectiveness at the same Thiele number and thermal balance. Some isothermal systems likewise can have multiplicities, for instance, CSTRs with rate equations that have a maximum, as in Example (d) following. 

VARIABLE TEMPERATURE FTIR SPECTROSCOPY IN THE STUDIES OF SURFACE PHENOMENA... 

The lecture deals with the advantages of IR spectroscopy at low or variable temperatures in the studies of molecule-surface interactions, lateral interactions between the adsorbed molecules and catalysis. 

Spectroscopy at variable temperatures enables us to reveal linkage isomerism of adsorption, when certain molecule fonu with the same site two or more complexes with different geometry and chemical properties. The most studied so far is the case of CO in zeolites, when besides the usual C-bonded complexes with the cations or OH-groups, energetically unfavorable O-bonded complexes ai e formed. 

Three different conformations are possible for monomeric chalcogen diimides (Eig. 10.1). Variable-temperature NMR spectra indicate that the cis,trans isomer of S(NR)2 is most stable in solution for small organic groups (R = Me, Bu). With bulkier organic substituents, small amounts of the trans,trans isomer exist in equilibrium with the cis,cis isomer. " The cis,cis isomer is observed in solutions of certain sulfur diimides with... 

The solvated sulfenamides [Li2( BuNSC6H4Me-4)2(THF)n] (n = 2,4) have dimeric structures with a central Li2N2 ring. The coordination mode is determined by the extent of solvation of the Li" ions monosolvation allows for rj -N,S coordination whereas disolvation restricts the coordination mode to // -M Variable temperature NMR studies indicated that a dynamic exchange between these two structural types occurs in THF solution (Scheme 10.10). The dihapto coordination mode is observed exclusively in transition-metal complexes and the... 

Simple examples of diazoalkylideneamine-l,2,3-triazole equilibria have been demonstrated for a series of l,2,3-triazolo[l,5-a]pyrimidines by variable-temperature NMR [74JCS(CC)671]. Tautomers A, B, and C interconvert rapidly at elevated temperatures the energy barrier for these ring-opening-ring closure processes was found to be AG = 76 kJ mol (for = H, Me R = CONH2) (Scheme 111). 

Definitive proof of the structure of porphine in the solid state awaits a variable-temperature crystallographic (X-ray or neutron diffraction) study the analysis of the anisotropic displacement factors (ADP) should disclose any rotational motion or its absence as well as determine the positions of the inner hydrogens. A search in the September 1998 version of the Cambridge Structural Database [CSD (91MI187)] showed that the only structures of porphine (codename PORPIN) were obtained in 1965 and 1972. 

Compound 233 was prepared from 144 and 234, and its intramolecular ligand reorganization was investigated by variable temperature NMR spectroscopy (80JA5073). 

The tautomeric behavior of six-membered azaphospha heterocycles 136 has been studied in solution by variable-temperature NMR [82JCS(D)1549]. 

The phase behavior of polybibenzoates has been investigated mainly by DSC, variable-temperature x-ray diffraction, and optical microscopy. However, only the first two techniques are useful in the case of polymers with the high-molecular weights required for materials with good mechanical properties and, in such cases, revealing textures are not usually observed by optical microscopy. 

The structural environments of cations absorbed onto clays Cs variable-temperature MAS NMR spectroscopic... 

An endothermic animal generates its own body temperature, while an ectothermic animal does not. In general, endothermic animals have constant body temperatures that are typically greater than that of the surrounding environment, while ectothermic animals have variable temperatures. Ectotherms rely on behavioral temperature regulation—a snake will move from sun to shade until it finds a suitable microclimate that is close to its optimal body temperature. When exposed to direct sunlight, an ectotherm can increase its body temperature as much as 1°C (32.8°F) per minute. 

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