Chaim Aharoni

Adsorption of sulfur dioxide by active carbon treated by nitric acid: I. Effect of the treatment on adsorption of SO2 and extractability of the acid formed

Abstract The process of adsorptive-catalytic cleaning of gas from sulfur dioxide using active carbon, treated by concentrated nitric acid, was studied. After oxidative treatment the acidity of the carbon increases and the basicity decreases. This results in an increase of the SO 2 adsorption and its oxidation to SO 3 , and in weaker retention of the sulfuric acid formed. This facilitates the removal of H 2 SO 4 by washing of the carbon and allows to obtain a more concentrated acid. The efficiency of the treated carbon is also higher in the process of SO 2 removal from a gas similar in composition to stack gases.

Carbonaceous deposits as catalysts for oxydehydrogenation of alkylbenzenes

Abstract The main industrial methods for the production of styrene and α-methylstyrene are the dehydrogenation of ethylbenzene and isopropylbenzene using mixed oxide catalysts: These reactions are endothermic and reversible, they take place at high temperatures (853–923 K), and their reversibility thermodynamically limits the yields of the products. Displacement of the equilibrium toward the formation of the vinylbenzenes is generally achieved by lowering the partial pressure of the reacting alkylbenzenes by diluting them in steam, heated to 973 K.

Adsorption of sulfur dioxide by active carbon treated by nitric acid: II. Effect of preheating on the adsorption properties

Abstract The adsorption and desorption of sulfur dioxide by active carbon, treated by nitric acid and followed by heating at 200–1000 °C, were studied. The HNO 3 treated carbon has a higher adsorption capacity for sulfur dioxide, than the untreated carbon, in spite of its lesser basicity. This may be the result of interaction of the SO 2 with the oxygenated surface groups of acidic character. Water promotes the adsorption of SO 2 . Heating results in a decrease of the carbon acidity because of decomposition of the surface oxygenated structures. It also leads to an increase of basicity resulting from the formation of basic sites. These changes of the carbon acid-base properties are particularly pronounced for the HNO 3 treated carbon due to higher concentration of the surface oxygenated groups. The increased basicity promotes adsorption of SO 2 and inhibits desorption by an acid-base mechanism different from the mechanism of adsorption on the carbon, which has not been preheated.

Adsorption of cyanogen chloride on impregnated active carbon

Abstract The adsorption of cyanogen chloride on impregnated carbon containing copper and chromium compounds and other additives is accompanied by the formation of carbon dioxide. Presorbed water enhances the formation of carbon dioxide and decreases the desorption of cyanogen chloride. Cyanogen chloride is adsorbed irreversibly because hydrolysis takes place at a rate higher than that of desorption.

Kinetics of adsorption: the S-shaped z-t plot

One characteristic property of the kinetics of adsorption is the fact that the plot of the reciprocal of the rate against the time is S-shaped. The initial part of this plot is convex and can be fitted by a power equation (i.e. with the amount adsorbed being proportional to a fractional power of the time), the final part is concave and can be fitted by a Langmuir-type equation (i.e. with the rate decreasing linearly with the amount adsorbed) and the intermediate region around the inflection point can be fitted by the Elovich equation (i.e. with the amount adsorbed increasing logarithmically with the time). In many cases the above regions are preceded by a region at which the rate is constant. The equation dθ/dt = A(1 – θ)/θ where θ is the fractional coverage, t is the time and a is a constant, is consistent with these kinetics. It corresponds to kinetics in which the activation energy increases logarithmically with the coverage. It can be derived on the basis of a statistical-rate theory or on the basis of a precursor-state theory. The diffusion equation q / q ∞ = 1 - ∑ n = 0 n = ∞ a n exp ( - b n tD / r 2 ) where q and qx are the quantities adsorbed at time t and t = ∞, an and bn are parameters determined by the integers n, D is the diffusion coefficient and r the length path, is also consistent with the above kinetics. Both equations have also been extended to heterogenous surfaces. Isotherms with constant initial rate are associated with non-activated adsorption and measurements at low pressures. The constant initial rate corresponds to a state at which the kinetics are determined by the rate of arrival of the gas molecules to the surface.

Adsorption and desorption kinetics of cesium in an organic matter-rich soil saturated with different cations

Samples of soils made homoionic with K, Na, or Ca were reacted with solutions containing Cs ions, and the quantities of Cs sorbed and the rates of exchange were measured. The samples were then reacted with solutions containing K, Na, or Ca, and the quantities of Cs desorbed and the rates of exchange were again measured. Samples made homoionic with Na had a greater ion exchange capacity than samples made homoionic with K, and, in both cases, the ion exchange capacity increased with the organic matter content of the soil. For samples pretreated with Ca, the ion exchange capacity is not related in a simple way to the organic matter content. The kinetics were assessed by plotting the rate of exchange vs. the time and vs. the quantity exchanged. A first-order equation was obeyed during most of the run in Cs desorption experiments and during a limited part of the run in Cs adsorption experiments. An increase in the rate of Cs exchange was observed at the beginning of the experiments especially for Cs adsorption. This increase is presumably due to an increase of the ionic strength of the liquid phase during the exchange process.

Effect of subsurface penetration on the kinetics of chemisorption

Abstract Expressions describing the kinetics of chemisorption of gases by metallic surfaces are derived, assuming that the sorbate penetrates into a narrow subsurface selvedge adjacent to the surface. It is also assumed that m the selvedge, the energy of sorption decreases with depth and the transport of the sorbate is governed by diffusional laws. The model leads to plots of the sticking coefficient against the quantity adsorbed, in which the sticking coefficient is high and constant at low coverage but drops sharply at some higher coverage with a slope that decreases gradually. The initial part corresponds to a range in which the kinetics are determined by the rate of arrival of the sorbate from the gas phase to the solid phase, and the final part to a range in which the diffusional process is rate determining. The process may appear to be non-activated even when diffusion is activated. The coverage at which the sharp decrease of the sticking coefficient occurs is small when the pressure is high, when the selvedge is deep or indistinguishable from the bulk and when diffusion is slow.

Silver supported on zeolite as catalyst for the oxidation of olefins

Abstract The preparation of Ag-X and Ag,Ca-X zeolites, and their properties as catalysts for the oxidation of cyclohexene by air, were studied. The catalysts were prepared by ion-exchange of zeolite 13X, and subsequent reduction with hydrogen. The oxidation of cyclohexene was performed in a tubular reactor, at 270 °C and atmospheric pressure and the conversion was 6–8%. The products obtained, identified by gas chromatography and infrared spectroscopy, were cyclohexene oxide, 2-cyclohexene-1-one, 1-cyclopentene-1-carboxaldehyde, and 1,6-hexanedial. Only traces of CO 2 (0.2%) were detected. The selectivity of the catalysts was affected by altering the calcium content; thus, Ag-X showed rather high selectivity for the epoxide, while in the presence of Ca the main product was 1-cyclopentene-1-carboxaldehyde.

Kinetics and mechanism of catalytic hydrodesulfurization of gas oil: adsorption and hydrogenation of the sulfur compounds

Abstract Sulfur compounds contained in gas oil were adsorbed on hydrodesulfurization catalysts containing Ni and Mo, or one of these metals, at temperatures between 420 and 650 K and pressures of 20 MPa. In runs performed with a limited amount of gas oil and excess adsorbent during 4 h, the extent of sulfur removal increases with the temperature and is determined by the rate of adsorption of the slowly adsorbed species. Some of the compounds adsorbed at low temperatures are desorbed at higher temperatures without decomposition and formation of H 2 S, but when the temperature is raised further they are desorbed with the formation of H 2 S. The nickel in the NiMo catalyst does not promote the adsorption of the sulfur compounds but it inhibits cracking and other hydrocarbon reactions.

The Adsorption of Chloropicrin by Carbon Filters: Characterisation of the Breakthrough Curves

Chloropicrin at concentrations of naiiograms per liter in air eluted through carbon filters was determined by gas chromatography with electron capture detection. For constant inlet concentration and constant flow, the breakthrough curve, exit concentration versus time, is exponential, and the concentration at any time is proportional to the total amount of gas eluted. The breakthrough times defined according to a given exit concentration correspond to those defined according to an amount only for curves with the same proportionality constant, obtained at similar conditions.