Elsevier

Chemical Engineering Science

Volume 231, 15 February 2021, 116273
Chemical Engineering Science

A novel molten tin reformer: Kinetics of oxygen dissolution in molten tin

https://doi.org/10.1016/j.ces.2020.116273Get rights and content

Highlights

  • Mass transfer of oxygen from bubbles to molten tin described by a logistic model.

  • LSM-YSZ/LSM reference electrode and YSZ potentiometric oxygen sensor demonstrated.

  • Temperature dependence of the solubility limit of oxygen in molten tin was defined.

  • The rate of oxygen dissolution in molten tin was controlled by chemical reaction.

  • Rate limiting step was the dissociation of SnO into molten tin and oxygen atom.

Abstract

The separation of oxygen atoms from a blend of oxygen and helium gases (10%O2-He) and its dissolution in molten tin, in a novel molten tin reformer, when the gas was bubbled through molten tin at 973–1123 K was investigated. A lanthanum strontium manganite double-layered reference electrode, air reference gas and yttria stabilized zirconia electrolyte potentiometric oxygen sensor was employed as an in situ online sensor for the measurement of oxygen concentration in molten tin. Solubility limit of oxygen, Gibbs energy change for tin oxide formation and oxygen dissolution kinetics were established. The entire course of oxygen dissolution in molten tin was described by a logistic model and the solubility of oxygen in molten tin in equilibrium with tin oxide in the temperature range 973–1123 K was ca. 0.019–0.107 atom%. The rate of oxygen dissolution was controlled by chemical reaction at the bubble|molten tin interface.

Introduction

A novel molten tin reformer for the conversion of methane to carbon monoxide and hydrogen (synthesis gas) is being investigated. Fig. 1 shows the schematic of this reformer; its 2-stage operation involves a first stage of oxygen dissolution in molten tin (reaction (1)) and a second stage of methane reaction with the dissolved oxygen at bubble|molten tin interfaces to produce synthesis gas (reaction (2)):First stage:O2(g)2OSnSecond stage:CH4(g)+[O]snCO(g)+2H2(g)

During the first stage of the operation of this reformer air bubbles are injected into molten tin and oxygen atoms are transferred from the air bubbles into molten tin, so it is essential to understand the dissolution of oxygen atoms in molten tin through bubble-molten tin interaction. The knowledge of oxygen solubility, its chemical reaction and kinetics of dissolution in molten tin are essential to the design and operation of this molten tin reformer.

The molten tin reformer requires an oxygen sensor to measure and monitor the concentration of dissolved oxygen atoms in the molten. Online oxygen sensors for in situ analysis are preferred over traditional sampling and chemical analysis for molten metals because they are easy to handle, give instant response, and can be computer controlled (Kurchania and Kale, 2000). Therefore, solid electrolyte potentiometric oxygen sensors are being developed for the measurement of oxygen concentration in molten metals (Kunstler et al., 2000, Kurchania and Kale, 2000, Courouau et al., 2002, Courouau, 2004, Fernández et al., 2002, Konys et al., 2001, Konys et al., 2004, Kurata et al., 2010, Laimböck and Beerkens, 2006, Lee et al., 2008). Yttria stabilized zirconia (YSZ) is the solid electrolyte that is mostly used because it has a high oxygen ion conductivity at temperatures higher than 623 K (Li, 2002) with thermal and chemical stability in both oxidizing and reducing atmospheres (Kunstler et al., 2000). The reference electrode is usually platinum with air as the reference gas (Konys et al., 2001, Konys et al., 2004) or a metal/metal oxide mix e.g. Sn/SnO2 (Kunstler et al., 2000), In/In2O3 (Fernández et al., 2002, Konys et al., 2001, Colominas et al., 2004), Bi/Bi2O3 (Konys et al., 2004, Kurata et al., 2010, Lee et al., 2008), Ni/NiO (Laimböck and Beerkens, 2006) and Cu/Cu2O (Kunstler et al., 2000). Mixed ionic-electronic conducting electrodes e.g. lanthanum strontium iron cobalt oxide (La0.6Sr0.4Fe0.8Co0.2) are also being investigated as reference electrodes for oxygen sensors (Alcock et al., 1992, Ramasamy et al., 2006). Strontium-doped lanthanum manganite (LSM) –La1-xSrxMnO3 is an established oxygen reduction catalyst which has been employed with YSZ electrolytes as solid oxide fuel cell cathodes (Abdalla et al., 2018, Jiang, 2008, Agbede et al., 2020a, Agbede et al., 2020b) so it presents an alternative to platinum which is more expensive. Hence, an LSM-YSZ/LSM double-layered reference electrode may be potentially employed in YSZ electrolyte potentiometric oxygen sensor for the measurement of the concentration of dissolved oxygen atoms in molten tin.

Bircumshaw and Preston (1936) investigated the kinetics of oxidation of molten tin at 673–1073 K by exposing the surface of tin melt to oxygen gas and reported that the rate of oxidation of molten tin could not be described by the parabolic law for oxide formation but appeared to be controlled by some factors other than the progressive increase in film thickness. Boggs et al. (1961) studied the rate of oxidation of pure tin at 423–493 K and several different oxygen pressures in the range 10−2 to 500 mmHg by means of a vacuum microbalance. They reported that for oxygen pressures below 1 mm, the oxidation rate increased continuously with time, and the rate-determining step appeared to be the dissociation of oxygen while for oxygen pressures 1 mm and above, the oxidation of tin appeared to proceed with time in three stages: in the initial stage, the rate was low but increased to a maximum; in the second stage it decreased and followed a direct logarithmic relationship; and finally for long times and thick films, the rate became erratic, either increasing rapidly or levelling off. In the review of Drouzy and Mascré (1969), two stages of oxidation of tin were identified, during the first stage the oxidation rate was very low at low temperatures whereas during the second stage at high temperatures, oxidation was rapid and the rate approached a constant value. A continuous film of oxide was initially formed on the tin melt which later lost its homogeneity and formed white tin dioxide and tin powder. Yuan et al. (1999) investigated the rapid oxidation of molten tin in pure oxygen or oxygen atmosphere diluted with argon at 873–1073 K by thermogravimetric analysis and concluded that there was no clear-cut rate determining process operative over the temperature and oxygen partial pressure ranges considered. More recently, Song and Wen (2009) investigated the oxidation of tin nano particles using a simultaneous thermogravimetric analysis and differential scanning calorimetry technique under both constant rates of heating and isothermal modes. They identified a two-stage oxidation process and showed by X-ray diffraction that the only oxide product was SnO at a temperature below 673 K, while SnO and SnO2 coexisted at the temperature range between 673 and 1173 K. In these studies the surface of molten tin was exposed to an overhead oxygen atmosphere and the reports suggest that the kinetics of oxygen dissolution in molten tin is not yet well understood. Besides, the kinetics of oxygen bubble-molten tin interaction at the elevated temperature at which the molten tin reformer would operate has not been reported. The successful implementation of this reformer requires the knowledge of the rate of transfer of oxygen atoms from bubbles to molten tin. Hence, the objectives of this study were to:

  • -

    demonstrate (and investigate the course of) oxygen dissolution in molten tin at 973–1123 K by transfer of oxygen atoms from oxygen-helium bubbles to molten tin which suggests the possibility of oxygen transfer from air bubbles to molten tin in the molten tin reformer.

  • -

    demonstrate the use of an LSM-YSZ/LSM double-layered reference electrode and YSZ electrolyte potentiometric oxygen sensor for the measurement of the concentration of dissolved oxygen atoms in molten tin.

  • -

    obtain the solubility limit of oxygen in molten tin in the temperature range 973–1123 K when a blend of oxygen and helium (10%O2-He) is bubbled through molten tin and define a correlation for the temperature dependence of the oxygen solubility in molten tin.

  • -

    define a correlation for the temperature dependence of the Gibbs energy change for the formation of the oxide of tin in equilibrium with oxygen at the solubility limit.

  • -

    define the kinetics of oxygen dissolution in molten tin by gas bubble – molten tin interaction.

Section snippets

Kinetic models for oxygen dissolution in molten tin

Ten percent oxygen in helium gas (10%O2-He) was used in this study. Figure S1 (supplementary data) shows that the diffusivities of oxygen molecules in helium and those of oxygen atoms in molten tin at 973–1173 K are of the order 10−4 and 10−9, respectively. Also, Figure S2a and Figure S2b (supplementary data) obtained from modelling studies conducted on a single 10%O2-He bubble rising through molten tin revealed that gas phase diffusion does not control the rate of oxygen dissolution in molten

Materials

Tin granules which had a maximum impurity of 0.05% antimony, 0.02% bismuth, 0.01% copper, 0.02% iron, and 0.05% lead were supplied by Sigma Aldrich. Solid electrolyte (10.5% yttria doped zirconia − 10.5% YSZ) tubes were obtained from McDanel Advanced Ceramic Technologies, LLC, Pennsylvania, USA, while lanthanum strontium manganite (LSM) - La0.80Sr0.20MnO3 – paste, LSM-YSZ (50 wt% La0.80Sr0.20Mn03 and 50 wt% (Y2O3)0.08(ZrO2)0.92,) paste, as well as the vehicle ink for diluting the pastes, were

Oxygen transfer from bubbles to molten tin and the oxygen sensor

Fig. 4a shows the time dependence of open circuit potential difference measured at 973 K by the YSZ electrolyte potentiometric oxygen sensor during the oxygen dissolution process; the oxygen sensor responded immediately and very well to changes in concentration of dissolved oxygen in the molten tin. The tin melt was well stirred by the gas bubbles hence a uniform oxygen concentration was achieved in the melt while the voltage read by the voltmeter was also stable. The open circuit potential

Conclusions

  • (a)

    Oxygen gas was successfully separated from its mixture with helium and dissolved in molten tin by bubbling a 10%O2-He blend through molten tin at 973–1123 K which implies that oxygen could be separated from air in the molten tin reformer by dissolution in molten tin. A logistic model suitably described the entire course of oxygen dissolution in molten tin.

  • (b)

    An LSM-YSZ/LSM double-layered reference electrode, air reference gas and YSZ electrolyte potentiometric oxygen sensor was successfully used

CRediT authorship contribution statement

Oluseye O. Agbede: Methodology, Investigation, Validation, Formal analysis, Writing - original draft, Project administration, Writing - review & editing. G.H. Kelsall: Methodology, Investigation, Supervision. K. Hellgardt: Conceptualization, Methodology, Investigation, Resources, Supervision, Project administration, Funding acquisition, Writing - review & editing.

Declaration of Competing Interest

None.

Acknowledgement

The authors thank the Petroleum Technology Development Fund, Nigeria for a studentship for Oluseye Agbede.

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