E. Meirovitch

Solar test of an integrated sodium reflux heat pipe receiver/reactor for thermochemical energy transport

Abstract A chemical reactor for carbon dioxide reforming of methane was integrated into a sodium reflux heat pipe receiver and tested in the solar furnace of the Weizmann Institute of Science, Rehovot, Israel. The receiver/reactor was a heat pipe with seven tubes inside an evacuated metal box containing sodium. The catalyst, 0.5 wt% Rh on alumina, filled two of the tubes with the front surface of the box serving as the solar absorber. In operation, concentrated sunlight heated the front plate and vaporized sodium from a wire mesh wick attached to the other side. Sodium vapor condensed on the reactor tubes, releasing latent heat and returning to the wick by gravity. The receiver system performed satisfactorily in many tests under varying flow conditions. The maximum power absorbed was 7.5 kW at temperatures above 800°C. The feasibility of operating a heat pipe receiver/reactor under solar conditions was proven, and the advantages of reflux devices confirmed.

Chemical reactions in a solar furnace 2: Direct heating of a vertical reactor in an insulated receiver. Experiments and computer simulations

The performance of a solar chemical heat pipe was studied using CO2 reforming of methane as the endothermic reaction. A directly heated vertical reactor, packed with a rhodium catalyst was used. The solar tests were carried out in the Schaeffer solar furnace of the Weizmann Institute of Science. The power absorbed was up to 6.3 KW, the maximal flow rates of the gases reached 11,000 1/h, and the methane conversions reached 85%. A computer model was developed to simulate the process. Agreement of the calculations with the experimental results was quite satisfactory.

Theoretical modeling of a directly heated solar-driven chemical reactor

Abstract A theoretical formulation for calculating the performances of a solar-driven catalytic chemical reactor was developed. It accounts for the spatial distribution of the deposition of primary energy within the receiver, the heat transfer into the catalytic bed and the thermochemical endothermic reaction, chemical composition and flow distribution within the reactor. The theory set forth was applied to analyze results obtained in a solar furnace with a directly heated U-shaped tubular reactor, wherein catalytic carbon dioxide reforming of methane occurred. We find that the receiver/reactor assembly acts as a self-regulating system. Beyond a fractional catalytic bed length of 0.14, solar energy can be converted primarily into chemical enthalpy. The fluid temperature gradient monitors the heat balance by adjusting the overall rate of conversion to the rate at which energy is being transferred through the reactor walls. Under certain circumstances, the process may be heat-transfer limited or controlled by chemical thermodynamics. A good fit between theory and experiment and accountability of all the intricate details in the various calculated performances of the receiver/reactor system support the theoretical model set forth in this study. We offer it as a tool for simulating future experimental results and for designing solar-driven reactors.

Magnetic resonance relaxation of 1H and 17O in aqueous solutions of concanavalin A

Abstract We have studied the nuclear magnetic resonance relaxation times of 1 H and 17 O nuclei in aqueous solutions of Mn 2+ complexes of concanavalin A. The results indicate that the Mn 2+ is coordinated to one exchangeable water molecule. The mean residence time, τ h , of this water molecule at the Mn 2+ is approximately 1 · 10 −7 s in Mn 2+ -concanavalin A and is ten times as long in Mn 2+ , Ca 2+ -concanavalin A. No changes occur on binding of α-methyl glucoside, or on raising the pH to a value where molecular aggregation occurs. It is possible that on occupation of the Ca 2+ -binding site, the immediate electronic environment of the Mn 2+ alters and thus affects the mean residence time of the water molecule. It is also possible that the increase in τ h is brought about by restrictions imposed on the exchange of water molecules between bulk solvent and the Mn 2+ .

Dynamic NMR in liquid crystalline solvents—ring inversion of s‐trioxane

The proton magnetic resonance spectrum of s‐trioxane in nematic liquid crystalline solvents was studied over a wide range of temperatures. The spectra exhibit striking line shape variations with temperature which are attributed to the ring inversion of s‐trioxane. At high temperatures (≳100 °C) the spectra correspond to an average Hamiltonian due to the two fast interconverting chair forms of the molecule. As the temperature is lowered the resonance lines undergo selective broadening due to incomplete averaging of the two conformers. In order to obtain quantitative kinetic results the line shape theory for dynamic NMR in anisotropic liquids is formulated and is used to calculate theoretical spectra for s‐trioxane undergoing ring inversion. Comparison of the experimental and theoretical spectra yields kinetic parameters for the reaction in the temperature range at which the liquid crystalline solutions were studied, i.e., −20 to +110 °C. The proton NMR of s‐trioxane was also studied in the isotropic solven...

The structure of the smectic phases of terephthal-bis-(butylaniline) studied by electron spin resonance spectroscopy

Electron spin resonance (E.S.R.) measurements at X-band (3·3 kG) of the spin probe 17β-hydroxy-4,4′-dimethylspiro[5α-androstane-3,2′-oxazolidin]-3′-yloxyl dissolved in the smectic phases (A, C and B) of the mesogen 4,4′-terephthal-bis(butylaniline) (TBBA) are reported. Two types of samples were studied in the dependence on temperature, sample orientation and strength of the magnetic field: (i) glass plate sandwiches with less than 0·1 mm of the mesogen, and (ii) 4 mm i.d. cylindrical glass tube. The sandwich samples were prepared as monodomains with the smectic layers parallel to the glass plates. In these samples the orientation of the layers and of the director is fixed and cannot be reoriented even in a magnetic field of 21 kG. From the angular dependence of the spectrum, the tilt angle, the order parameter and their temperature dependence were determined. In the cylindrical samples the original orientation of the smectic layers is preserved, and up to about 3 kG the directors orientation with respect...

Magnetic instabilities of smectic C liquid crystals studied by electron spin resonance spectroscopy

Single crystals of smectic C samples in a magnetic field will deform so as to minimize the sum of their magnetic and elastic energies. This deformation was studied in the smectic C phase of di-n-heptyloxyazoxybenzene using E.S.R. spectra of a spin probe dissolved in the mesogen. The samples consisted of slabs 20 µm to 200 µm thick, between glass surfaces, and their spectra were studied as function of the orientation and intensity of the magnetic field. Thin samples behaved as single crystals under rotation but the thicker samples exhibited elastic deformation. The results are quantitatively interpreted in term of a recent theory of the magnetic instabilities of smectic C [1], and the elastic constant for reorientation of the director is derived. The behaviour of multidomain samples of smectic C in magnetic fields is also discussed.

An 2H N.M.R. study of solid methyl parathione-d6 and parathione-d6

2H N.M.R. spectra from polycrystalline powders of methyl parathione (MP) and parathione (P) deuterated at the methyl position were studied between 120 and 304 K. To interpret the spectra we used a primarily qualitative rationale, aided by lineshape simulations based on various models for molecular structure and dynamics. We find that all the methyl groups spin rapidly about their internal symmetry axis. Onset of discrete jumps executed by the methyl groups is observed to occur at 275 K with MP-d6 and at 160 K with P-d6. The former process is interpreted in terms of methyl groups exchanging with the lone pair of the oxygen, whereas the latter reflects predominantly isomerization about the O-CH2 bond. The temperature-dependence of the 2H N.M.R. spectra is interpreted to reflect geometric alterations and a differential in the flexibility of the two methyl groups of a given molecule.