Gibbs energy of formation of Na2WO4 and Na3WO4 in liquid sodium

Bhat, N.P.; Borgstedt, H.U.

doi:10.1016/s0022-3115(96)00715-5

Cited by 5 publications

(4 citation statements)

References 10 publications

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“…This indicates that the value for Δ H f (Na 2 WO 4 ) can at the most be no more than 25 kJ mol -1 more negative than the NBS Tables value. As a result, the occurrence of this reaction appears to be incompatible with the more recently determined smaller values. , …”

Section: Resultsmentioning

confidence: 58%

“…The NBS Tables recommend a value of −1548.9(±8.4) kJ mol -1 at 298 K, that is supported by an empirical correlation analysis and an emf cell measurement . More recent measurements based on the reaction of liquid sodium with WO 3 derive values of the free energies of formation of Na 2 WO 4 that imply a greater stability and a heat of formation that is 40−90 kJ mol -1 more negative. , Very recently, the thermodynamic properties of Na 2 W 2 O 7 (s) have been measured and are anchored by its heat of formation. The latter is the accepted value 75 and is quoted with an accuracy of 3.3 kJ mol -1 .…”

Section: Resultsmentioning

confidence: 87%

“…As a result, the occurrence of this reaction appears to be incompatible with the more recently determined smaller values. 78,79 Equilibrium calculations for the flame deposits confirm the greater stability of Na 2 W 2 O 7 over Na 2 WO 4 when the ratio of W/Na increases above 0.5. This is reflected in the Raman spectra of many of the flame deposits when it is somewhat difficult to observe the lines of Na 2 WO 4 and the spectra of Na 2 W 2 O 7 and Na 2 W 4 O 13 are most commonly encountered.…”

Section: High-temperature Corrosion Mitigation Methodsmentioning

confidence: 83%

“…77 More recent measurements based on the reaction of liquid sodium with WO 3 derive values of the free energies of formation of Na 2 WO 4 that imply a greater stability and a heat of formation that is 40-90 kJ mol -1 more negative. 78,79 Very recently, ) 1:1 in the flame and then with Na/S/W ) 0.67:0.67:1 for a length of time such that overall the ratio of Na/S/W ) 1:1:1. φ ) 0.9, C3H8/O2/N2 (0.9:5:14). (b) On a platinum-clad probe at 825 K. Collected initially with Na/W ) 2:1 and 75 ppm SO2 and then similarly continued but without the tungsten for the same length of time, φ ) 0.9 (0.9:5:16).…”

Section: High-temperature Corrosion Mitigation Methodsmentioning

confidence: 99%

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A New Method to Minimize High-Temperature Corrosion Resulting from Alkali Sulfate and Chloride Deposition in Combustion Systems. I. Tungsten Salts

Schofield

2002

Energy Fuels

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Based on a recently ascertained understanding of the parameters that control flame generated deposition of Na2SO4, NaCl, or Na2CO3 onto cooled surfaces immersed in flames, a new process has been developed to inhibit their formation. It is seen that no alkali sulfate is formed if tungsten salts are added to a flame containing an alkali (sodium or potassium), sulfur, and chlorine, when the tungsten-to-alkali gas-phase ratio is larger than about 2-fold on an atomic basis. Instead, the alkali exhibits a greater affinity for the tungsten and benign alkali polytungstates are deposited. The presence of chlorine appears to have a negligible effect on these systems. This modified behavior is further confirmed in experiments in which an Na2SO4 deposit is initially formed on a probe and then is seen to be fully converted by the addition of sufficient tungsten to the flame and overlaid by a similar tungstate growth. For sodium, the preferred polytungstates formed are Na2W2O7 and Na2W4O13, explaining to some degree the required W/Na addition ratios. Deposition appears to reflect the relative condensed phase thermodynamic stabilities of these salts and follows the order Na2W4O13 > Na2W2O7 > Na2WO4 > Na2SO4 > NaCl > Na2CO3 . In the case of potassium, behavior is very similar and the dominant polytungstate produced is K2W6O19, with some contribution from K2W3O10 . Conversions can occur in the direction of greater stability but are irreversible. The method is insensitive to fuel type, equivalence ratio, or general flame parameters. Deposits have been acquired on stainless steel and platinum clad probes operating in the temperature range 600 to 900 K. Analysis has utilized Fourier transform Raman spectroscopy and inductively coupled plasma atomic emission spectroscopy. An examination of potential interferences for the tungsten addition also has been made. This concerns whether any other element might have a greater affinity for the tungsten over that of the alkali. Calcium, strontium, and barium appear to be the most likely to fall into this potential category having well-defined and stable tungstates. Initial experiments with and without calcium addition do, in fact, show an interference with evidence of CaWO4 formation and a need for enhanced additions of tungsten. The process, outlined in detail herein, appears to be the first viable active solution for preventing Na2SO4- and NaCl-induced corrosion in a variety of combustion systems. Coupled to present techniques for reducing burned gas alkali concentrations, the method appears to be economically feasible.

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Section: Resultsmentioning

confidence: 58%