The effect of vanadium oxidation states on the conversion of hydrogen sulphide to elemental sulphur
The formation of toxic hydrogen sulphide (H₂S) gas as a by-product is unavoidable during production of fuels and other chemicals from coal, crude oil, and natural gas. There are different processes available to recover H₂S of which the Stretford™ process is one of them. The concentration of vanadium (V⁵⁺) plays a crucial role in the Stretford™ process during H₂S absorption and conversion to elemental sulphur. However, the effect of vanadium oxidation states (V⁴⁺ and V⁵⁺) on hydrogen sulphide ions (HS⁻) conversion to sulphur product and its quality has not been systematically studied hence the purpose of this study was to understand these effects. A series of experiments was conducted using sodium ammonium vanadate (SAV) and vanadyl sulphate (VOSO₄) as sources of V⁵⁺ and V⁴⁺ respectively. The analytical techniques used to determine the quality of elemental sulphur were X-ray powder Diffraction Spectroscopy (XRD), Differential Scanning Calorimetry (DSC), Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES), gravimetric and Particle Size Distribution (PSD) analyses. The first set of experiments was conducted at variable V⁵⁺ concentrations with impurities (Na₂SO₄ and NaSCN) added to the synthetic Stretford™ solutions to determine the best V⁵⁺ concentration to achieve maximum conversion of HS⁻ to sulphur of good quality. The H₂S absorption and sulphur production were found to increase as the concentration of V⁵⁺ increased. However, the elemental sulphur particle sizes were finer and the purity decreased as the V⁵⁺ concentration increased. The elemental sulphur particle sizes and purity were affected by the impurities in the technical SAV that was used as a source of V⁵⁺. Some of these impurities in SAV include Na, NH₃, K, Al, Si, Mg, and Fe. The impurities in SAV interfered with the sulphur sol nucleation and Ostwald ripening process. The nucleation and Ostwald ripening process facilitates the formation of coarse elemental sulphur. However, the elemental sulphur formed during the experimental work was crystalline and melted at approximately 122°C, which was comparable to 120.6°C reported in the literature. A second set of experiments was conducted at best V⁵⁺ concentration of 0.029 mol.dm⁻³ to investigate the effect of impurities (Na₂SO₄ and NaSCN) on H₂S conversion to elemental sulphur. The sulphur production was slightly higher without impurities added in synthetic Stretford™ solution. The elemental sulphur particle sizes slightly shifted to the coarser side but the thermal behavior, crystallinity, and colour were comparable. The elemental sulphur produced without impurities was also of high purity compared to sulphur produced with impurities. The best concentration of V⁵⁺ , which is based on the experimental results obtained, is 0.029 mol.dm⁻³. This concentration of V⁵⁺ was also selected for the experiments at variable V⁵⁺ to V⁴⁺ molar ratios without the addition of sodium salt of 2,7 Anthraquinone Di-Sulfonic Acid (Na₂[ADA]) to the Stretford™ aqueous solution; vanadium re-oxidation catalyst to the simulated StretfordTM liquor. The H₂S conversion to elemental sulphur reduced as the V⁵⁺ to V⁴⁺ molar ratios decreased. There was high loss of total vanadium concentration (5 - 27%) due to the precipitation of V⁴⁺ and the free OH⁻ from the simulated Stretford™ liquor-containing NaOH in the form of (VO(OH)₂). The loss of total vanadium due to precipitation decreased as the V⁵⁺ to V⁴⁺ molar ratios decreased. The HS⁻ conversion to elemental sulphur decreased as the V⁵⁺ to V⁴⁺ molar ratios decreased. The elemental sulphur particle sizes were coarse and comparable to elemental sulphur particle sizes produced at the V⁵⁺ concentration of 0.029 mol.dm⁻³. Elemental sulphur was crystalline and melted at temperature similar to sulphur produced at V⁵⁺ concentration of 0.029 mol.dm⁻³ with Na₂[ADA] added in solution. In addition, the elemental sulphur product was filtered with difficulty from the sulphur slurry due to precipitation of V⁴⁺ ions which react with free hydroxides to form a dark colloidal precipitate (VO(OH)₂). The formation of this precipitate resulted in sticking and blinding the filter paper during filtration of the sulphur slurry. The dominant V⁵⁺ species in all solutions in this study were the dimer, tetramer, and pentamer. V⁵⁺ ions in the solutions were qualified by ⁵¹V NMR using the NMR literature data to predict dimer, tetramer, and pentamer in these solutions. A thermodynamic model was developed using the OLI studio analyzer. This model showed that V⁵⁺ is stable in both acidic and alkaline conditions. However, vanadium in its reduced oxidation state (V⁴⁺) is only stable in acidic conditions. Traditionally, the Stretford™ process is operated in alkaline conditions and this explains why V⁴⁺ precipitated at higher pH (8.2 - 9.5). Hydrogen peroxide which was released from Na₂[ADA] oxidizes V⁴⁺ to V⁵⁺ in solutions. The statistical models confirmed the experimental findings that H₂S absorption and elemental sulphur production were proportional to the concentrations of V⁵⁺ in all experiments in this study. Therefore, the experiments that use Stretford™ aqueous solution should be operated at the best V⁵⁺ to HS- molar ratios (between 4.8 and 7.3 in stoichiometry) with Na₂[ADA] to oxidizes V⁴⁺ to V⁵⁺ in the aqueous solutions. This will subsequently result in the significant production efficiency of the elemental sulphur. The oxidation of almost all of V⁴⁺ to V⁵⁺ can minimise the sodium based salts and V⁴⁺ precipitations. The low stoichiometry V⁵⁺ to HS⁻ molar ratios (below 1) results in high hydrogen sulphide emission. The Stretford™ aqueous solution which does not contain Na₂[ADA] which can release hydrogen peroxide cannot oxidize V⁴⁺ to V⁵⁺ and eventually V⁴⁺ precipitates in the form of VO(OH)₂. The sodium based salts can co-precipitate together with VO(OH)₂, while the elemental sulphur particles formed can settle at the bottom of the simulated Stretford™ aqueous solution. Generally, the precipitation of V⁴⁺ requires an increase in make-up vanadium to achieve best H₂S absorption.
- Engineering