Solvent extraction of permanganates (Na, K) by 18-crown-6 ether from water into 1,2-dichloroethane: elucidation of an extraction equilibrium based on component equilibria.
Ion-pair formation constants (K(MLA) mol(-1) dm(3)) of Na(+)- and K(+)-18-crown-6 ether (18C6) complexes with MnO(4)(-) in water (w) were determined potentiometrically at 25 degrees C. Simultaneously, extraction constants (K(ex) mol(-2) dm(6)) of the permanganates with 18C6 from w into 1,2-dichloroethane at 25 degrees C were obtained from the spectrophotometric determination of distribution ratios of the permanganates. These K(ex) values were divided into K(MLA) and other three component equilibrium constants and thereby extraction-selectivity and -ability were discussed in comparison with corresponding metal picrate-18C6 extraction systems reported before.
Solvent Extraction of Alkali Metal (Li-Cs) Picrates with 18-Crown-6 into Various Diluents. Elucidation of Fundamental Equilibria which Govern the Extraction-Ability and -Selectivity
The actual constants of the overall extraction equilibrium, the distribution for various organic solvents having low dielectric constants, and the aqueous ion-pair formation (KMLA) of 18-crown-6 (18C6)-alkali metal (Li-Cs) picrate 1:1:1 complexes were determined at 25°C; the partition constants of 18C6 were also measured at 25°C. The log KMLA values are 2.53±0.21 for Li, 3.29±0.23 for Na, 4.76±0.27 for K, 4.62±0.36 for Rb, and 4.49±0.36 for Cs. The relatively large difference in the log KMLA value between light (Li, Na) and heavy alkali metals (K, Rb, Cs) reflects the difference in the structure of the 18C6-alkali metal ion 1:1 complex between light and heavy alkali metals. Almost all of the partition behavior of 18C6 and its 1:1:1 complexes with alkali metal picrates can be explained by regular solution theory. The molar volumes and solubility parameters of 18C6 and the complexes were determined. For every diluent, the extraction-selectivity order of 18C6 is K > Rb > Cs > Na > Li. The extraction-ability and -selectivity of 18C6 for the alkali metal ion were completely elucidated in terms of the four fundamental equilibria.
Evaluation of CuAl2O4 as an Oxygen Carrier in Chemical-Looping Combustion
The chemical-looping combustion (CLC) process is a novel solution for efficient combustion with intrinsic separation of carbon dioxide. The process uses a metal oxide as an oxygen carrier to transfer oxygen from an air to a fuel reactor where the fuel, or gasification products of the fuel, reacts with the solid oxygen carrier. In this work, copper(II) aluminate (CuAl2O4) was assessed as a potential oxygen carrier using methane as fuel. The carrier particles were produced by freeze–granulation and calcined at 1050 °C for a duration of 6 h. The chemical-looping characteristics were evaluated in a laboratory-scale fluidized-bed reactor in the temperature range of 900–950 °C during 45 alternating redox cycles. The oxygen carrier exhibited reproducible and stable reactivity behavior in this temperature range. Neither agglomeration nor defluidization was noticed in any of the cycles carried out at 900–925 °C. However, after reactivity tests at 950 °C, soft agglomeration and particle fragmentation were observed. Systematic phase analysis of the Cu–Al–O system during the redox cycle was carried out as a function of duration of reduction and oxygen concentration during the oxidation period. It was found that the CuAl2O4 is reduced to copper(I) aluminate (CuAlO2; delafossite), Cu2O, and elemental Cu. The CuAlO2 phase is characterized by slow kinetics for oxidation into CuO and CuAl2O4. Despite this kinetic limitation, complete conversion of methane with reproducible reactivity of the oxygen carrier is achieved. Thus, CuAl2O4 could be a potential oxygen carrier for chemical-looping combustion.