Promotion of CO2 mass transfer in carbonate solutions

A general, physico-chemical analysis of mass transfer rate promotion in the system CO₂-potassium carbonate-water-promoter is presented. Different possible mechanisms of promoter action including homogeneous catalysis, “shuttle” mechanism and surface reactions are discussed and classified. A unified picture of promoter chemistry is presented, showing that differences between inorganic and organic promoters are predominently quantitative, not qualitative.The “shuttle” mechanism is analyzed for absorption. The analysis leads to the prediction that the CO₂ mass transfer rate may be influenced by the liquid hold up. This is related to the fact that, although the reaction in the interface region may be fast enough to enhance the mass transfer rate, the different reaction in the bulk may not be fast enough to maintain chemical equlibrium. This complex type of chemical absorption process has not been considered previously in the literature.     

The stripping of CO2 from amine-promoted potash solutions at 100°C

The rate of desorption of CO₂ from 30% potash solution at 100°C under controlled conditions, with and without the addition of small concentrations of alkanolamines, has been measured and interpreted. Diethanolamine was the most effective of the amines in increasing the rate of desorption. Various aspects of the cycle of absorption and stripping in industrial purification plants are discussed in the light of the findings.     

Reaction of CO2 with ethanolamines: kinetics from gas-absorption

CO₂ at very low pressure was absorbed in a stirred cell into aqueous solutions of MEA and DEA. The rates of absorption were determined from the fall in pressure. The results were analysed on the assumption (for which experimental support is given) that the solubilities and diffusivities Of CO₂ in these solutions bear a constant proportionality to those of N₂O, which have been measured. The results for MEA at 25°C agree very closely with those of Hikita et al.[1], determined by a rapid-mixing thermal method. The results for DEA appear to support a reaction mechanism proposed by Danckwerts[13], the reaction tending to be second-order in amine at low concentrations and first-order at high concentrations.     

On the mechanism of enzyme-catalysed gas—liquid reactions: Absorption of CO2, into buffer solutions containing carbonic anhydrase

The gas absorption with chemical reaction data of Lee and Tsao [1, 2] (i.e. oxidation of glucose solutions containing glucose oxidase or platinum deposited on activated carbon and absorption of carbondioxide into buffer solutions in the presence of carbonic anhydrase) were reexamined and various reasons for their apparent inconsistency with the classical gas absorption models were suggested.The rate of absorption of carbondioxide into 0.5 gmol/l Na₂CO₃ + 0.5 gmol/l NaHCO₂ in the presence of carbonic anhydrase was measured in a stirred cell and also in a laminar jet and a wetted wall column aparatus. Enhancement in the absorption rate due to enzyme was observed in all three absorbers and the results were found to be compatible with classical mass transfer models (such as Higbie's and Danckwerts' models). On the contrary to Tsao[2], who proposed a two-zone model, which assumes a surface-excess of the enzyme at the gas—liquid interface, these results suggest that the carbonic anhydrase distribution in the liquid is probably uniform.     

Chemical kinetics of the reaction of CO2 with amines in pseudo m–nth order conditions in polar and viscous organic solutions

The specific rate of absorption for five gas-liquid systems: CO₂-monoethanolamine (MEA) in ethanol solution, CO₂-diethanolamine (DEA) in ethanol, CO₂-MEA in ethylenglycol, CO₂-DEA in ethylenglycol and CO₂-cyclohexylamine (CHA) in ethylenglycol was measured in a laboratory wetted wall contactor. Reactions were found to be first order with respect to CO₂ and second order with respect to amine for the first four ones and first order with respect to amine for the last one. The values of the specific rate of absorption are such that these reactions may be used to determine interfacial areas and mass transfer coefficients in gas-liquid reactors working with polar and viscous organic liquids.     

Reforming of C6 hydrocarbons on a PtAl2O3 catalyst

Isomerization, dehydrocyclization and hydrocracking of C₆ hydrocarbons on a commercial reforming PtAl₂O₃ catalyst were studied in a tubular reactor. The temperature was varied from 420 to 500°C, the pressure from 1.6 to 16 bar, the molar H₂/hydrocarbon inlet ratios from 1.5 to 20. Reaction rate equations of the Hougen—Watson type, corresponding to a bifunctional mechanism, were found to describe the experimental data. Parameter estimates for the reforming reactions were obtained by application of a generalized least square criterion to the calculated and observed production rates of the gas phase components. Electrobalance experiments showed that both the reforming reaction rates and the coking rate decrease exponentially with coke content. Multiplying the reaction rates with one common exponential deactivation function allowed the prediction of the observed conversions by one single set of kinetic parameters. The coking rate equation was derived by fitting the final coke content profiles obtained in the tubular reactor. The main contribution to coke formation was attributed to Me-cyclopentadienes.     

Studies on MnO2—III. The kinetics and the mechanism for the catalytic decomposition of H2O2 over different crystalline modifications of MnO2

The catalytic decomposition of aqueous H₂O₂ solution over MnO₂ of various crystalline modifications has been studied in the temperature and the pH ranges of 20–35°C and 3–14 respectively. The active site in the pH range of 3–8 is considered to be Mn⁴⁺ and in the range 8–14 is Mn³⁺ ion. Catalytic activity attains maximum at pH of about 11.5–12.0 and then decreases with further increase in pH. The experimental rate data agrees satisfactorily with the proposed mechanism based on the above considerations. Catalytic activity has been correlated with some physico-chemical characteristics of the samples such as surface oxygen, surface OH groups, lattice parameters and in somewhat complex manner with electrochemical or discharge capacity of MnO₂. Although the discharge capacity increases with increase in catalytic activity for some samples (mostly γ-MnO₂) a number of samples belonging to α, β and δ-variety show sharp deviation from the above behaviour. This is explained from the state of occurrence of MnOOH in the host lattice.     

The electrode properties of polycrystalline SnO2 containing up to 10% Sb or Ru oxides

Thin tin dioxide films containing up to ca. 10% Sb or Ru oxides were prepared by the hydrolysis of SnCl₄/SbCl₅ or SnCl₄/RuO₄ vapour mixtures at 420°C, on conducting substrates. The distribution of electronic states in the band gap of the semiconducting materials was determined by UPS (ultraviolet photoelectron spectroscopy). The films were then used as electrodes and the electrode kinetics of the redox-systems Fe^{2 + /3+} and Ce^{3 + /4+} were determined. By correlating the UPS and the electrochemical results, a model for the charge-transfer mechanisms is suggested in which electrons can pass the narrow semiconductor space charge barrier by mediation of the deep donor states in the band gap being introduced by the dopants.     

Studies on MnO2—II. Relationship between physicochemical properties and electrochemical activity of some synthetic MnO2 of different crystallographic forms

The electrochemical activity of a number of synthetic MnO₂ samples of various crystalline modifications has been determined by discharging a thin pellet containing 0.150 g MnO₂ and 0.015 g acetylene black in both NH₄ClZnCl₂ (pH 4.1) and 9M KOH medium. An attempt has been made to find correlations between depolarizing capacity (time to reach 0.6 V) and various physico-chemical properties; such as, density, surface area, lattice parameters, surface OH groups, surface oxygen, magnetic susceptibility etc. Whereas depolarizing capacity for γ-MnO₂ increases with increase in both true and packing (powder) density, capacity tends to decrease with increase in surface area. For other modifications of MnO₂ there exist direct relationships between depolarizing capacity and the various properties of the samples mentioned above. The relationships between lattice parameters and depolarizing capacity indicate that samples with an extended ab plane in general possess higher activity. It has been observed that surface OH groups and surface oxygen when normalised to unit weight are not only important properties in active MnO₂ but are also interrelated.     

Importance of surface free radicals in an aqueous chlorination reaction (Indan-ClO2) promoted by granular activated carbon

Indan reacted with chlorine dioxide in the presence of ICI HD 3000 granular activated carbon (GAC), as well as other GAC's, to yield some chlorine-containing products. However, when the GAC was pretreated with 2,6-di-t-butyl-p-cresol (BHT) or diazomethane, reagents which rapidly react with or destroy free radicals, chlorinated products were not formed. Reagents which selectively destroy carboxyl (methanolic HCl or BF₃) or phenolic hydroxyl (acetic anhydride) groups did not significantly affect the chlorination reaction. A GAC (Calgon F 400) which was less effective in producing the chlorinated derivatives could be converted to a more active form by oxidizing the carbon with ClO₂. There was little or no correlation between the ash content of a given GAC and its ability to promote chlorination, suggesting that the importance of metal ions in the reaction scheme is probably negligible.     

Ionic mass transfer rate of CuSO4 in electrodialysis

Ionic mass transfer rate of copper ion in the ion exchange membrane electrodialysis at limiting current density has been studied using a planar flow electrodialyzer consisted of a cation exchange membrane and an anion exchange membrane. Their effective area is 4 × 5 cm². The effects of flow rate, viscosity of electrodialysate solution and thickness of electrodialyzer to the ionic mass transfer rate have been studied. An empirical correlation equation (Nu)_exp. = 4.57 × Re^0.333 Sc^0.307 (de/L)^O.33 is obtained. It fitted well with the theoretical performance equation (NU)_theor.= 3.70 × (Re · Sc · de/L), which is derived from the Nernst-Planck Equation based on the assumption that mass transfer in the concentration boundary layer in the desalting compartment is controlling.     

Graphite trifluoromethylsulfate C12+SO3CF3−

Graphite trifluoromethylsulfate C₁₂SO₃CF₃, a new binary graphite salt is formed by the irreversible solvolysis of C₈SO₃F in a large excess of trifluoromethylsulfuric acid. The salt is identified as a stage one intercalation compound with a C₀ value of 8.12 Å and characterised by microanalysis, epr and ¹⁹F NMR spectroscopy. Both Raman spectra, in the back scattering configuration, and IR spectra, in transmission as well as reflection geometries, are used to support an ionic formulation as C₁₂⁺SO₃CF₃^?.     

Simultaneous absorption of two gases reacting between themselves in a liquid—II: Approximate solutions

An attempt has been made to analyse the hitherto existing approximate solutions of nonlinear differential equations representing a mass balance of two gases which react between themselves in an inert liquid. New approximate analytical solutions of the problem have been obtained and compared with the numerical and the existing approximate solutions in order to show the best approximations and the most convenient mathematical formulas for calculating the concentration profiles and the enhancement factors.     

Simultaneous absorption of two gases reacting between themselves in a liquid—I exact solutions

Simultaneous absorption and irreversible second order chemical reaction of two gases in an inert liquid have been considered. The nonlinear differential equations describing the absorption process have been solved analytically for particular cases by the use of the Weierstrass elliptic function and Airy functions. It is impossible to obtain a general analytical solution and therefore the accurate numerical solution for the diffusion-reaction process based on the film theory has been developed. The approximate formula for the enhancement factor E_A, in the range of large values of the dimensionless parameter M is given.     

The effect of electric fields on the absorption of pure sulfur dioxide by water drops

The effect of an electric field on the absorption of sulfur dioxide by pure water drops is examined experimentally. Field strength is varied up to 8.0 kV/cm, and nozzle Reynolds number is varied between 26 and 78. Drop surface charge density, size, shape, shape oscillation, acceleration and total sulfur dioxide uptake are measured directly. Significant reduction in drop size and increase in acceleration with increasing field strength are the major effects observed, while alteration in drop shape and oscillation are slight. Calculated average mass transfer coefficients increase with both field strength and nozzle velocity, but total sulfur dioxide uptake decreases with field strength due to reductions in drop-gas contact time. The results suggest that the design of scrubbers with electric fields concentrated in the region of drop formation but absent elsewhere show the greatest promise for improved efficiency.     

Hydrogenation of conjugated dienes over ZrO2 by H2 and cyclohexadiene

Hydrogenation of 1,3-butadiene and 2-methyl-1,3-butadiene and equilibration reaction of H₂D₂ were carried out over ZrO₂ using H₂ and Cyclohexadiene as hydrogen sources. Reaction rates were measured by changing the activation temperature of the catalyst. While the hydrogenation with H₂ and H₂D₂ equilibration reaction gave an optimum activity at 600 °C, another optimum was obtained for transfer hydrogenation at 800 °C. Hence it is concluded that the sites responsible for the transfer hydrogenation are not the same as those which catalyze hydrogenation with H₂ and H₂D₂ equilibration. Product distributions in n-butenes and methylated butenes were compared on ZrO₂, ThO₂, La₂O₃, and MgO and were also compared in direct hydrogenation with H₂ and transfer hydrogenation with Cyclohexadiene. Assuming an ionic intermediate, selectivity changes in the monoolefins produced over different catalysts and by different hydrogen sources were interpreted in terms of the variation of the anionic character of the intermediate and the shift of anionic to neutral or cationic intermediate, respectively.     

The oxidation of Cu(I) by oxygen gas in concentrated NaCl solutions: A kinetic study

The oxidation of Cu(I) by oxygen gas in concentrated NaCl solutions was studied in a standard stirred reactor. Each test was carried out at constant pH and under stable hydrodynamic conditions. The effect of stirring speed, solution volume, temperature, pH, oxygen partial pressure, chloride and copper ion concentration of the solution were evaluated.The oxidation of Cu(I) followed linear kinetics up to 85% of the copper ions in the cupric state. The experimental results were interpreted by using the film theory of mass transfer with chemical reaction. In the linear range, the chemical reaction was found to be first order with respect to oxygen concentration. The kinetic constant is given. The flux equation was specific of a fast reaction regime and the overall reaction rate was independent of the mass transfer coefficient k_L up to a k_L-value of about 10^?2 cm/sec. A parallel study of the oxidation of SO₃^2? by oxygen gas was carried out in the same reactor. The effects of stirring speed and solution volume were similar for both systems.