Laboratory studies of binary salt CVD in combustion gas environments

Abstract

A flash-evaporation technique is used to obtain vapor deposition characteristics for the binary alkali sulfates K,SO, + Na,SO, at 1 atm above 1,100 K. This technique gives results of immediate engineering interest, such as dewpoint temperatures, condensate composition and rates of vapor deposition as well as useful data on the system's thermodynamic characteristics. It is concluded that alkali sulfate deposition and vaporization in combustion environments are inevitably influenced by chemical reactions such as hydroxide formation. It is also concluded that solution nonideality is important even for homologous alkali-salt mixtures.Predictions are made using convective-diffusion mass transfer theory, accounting for chemical reactions by means of effective volatilities, and assuming regular, nonideal condensate solutions. The predicted dewpoints, condensate compositions and deposition rates are quantitatively consistent with experimental observations. This approach, validated here, can be extended to more extreme conditions of engineering interest, including turbulent, high-temperature/ pressure systems. Baishen Liang, D. E. Rosner Department of Chemical Engineering High-Temperature Chemical ReactionEngineering Laboratory Yale University New Haven, CT 06520 IntroductionFor diverse reasons, increasing attention has been paid to chemical vapor deposition (CVD) in high-temperature flow systems in the past two decades (Powell et al., 1966;Stearns et al., 1983). For example, it was realized that the useful life of a highperformance combustion turbine engine in a marine environment, and hence the system economics, is often dictated by the deposition rate of corrosive trace inorganic salts (e.g., Na,S04) whose vapor precursors are inevitably present in the combustion products. As another technological driver, the CVD of thin solid films has become important to the semiconductor industry for the preparation of microelectronic chips. These and other technologies have stimulated a number of papers dealing with both experimental and theoretical aspects of CVD (Robinson et al., 1985;Rosner et al., 1979;Rosner and Nagarajan, 1985;Seshadri and Rosner, 1984); however, many of these papers have been confined to the deposition of a single-species condensate [e.g., pure Na2S04(I), pure Si(s), etc.]. A few authors have performed laboratory experiments on the deposition of multicomponent condensates (Stevens and Tidy, 1983), investigating the effects of other impurities in fuel, such as calcium or potassium, on the deposition rate of the main impurity, say sodium. For example, it was found that additional impurities could reduce the threshold concentration of Na-containing species for the existence of its stable condensate. Unfortunately, such findings have apparently not attracted enough attention nor stimulated much further work, perhaps partly due to the complexity of the thermodynamic system investigated and the high cost of the test techniques.Multicomponent condensate cases deserve much further study because of their practical im...

Calculation of soret‐shifted dew points by continuous mixture thermodynamics

“…Our previous laboratory and theoretical studies of hightemperature dew points in combustion systems containing condensable vapors were focused on corrosive alkali sulfates, with emphasis on the formation of binary nonideal liquid solutions such as Na 2 SO 4 ϩ K 2 SO 4 (see, for example, Rosner and Liang 3 and Liang and Rosner 4 ). In the present paper we return to this canonical problem, generalizing our procedures to now embrace a mainstream that contains a family of condensable species, each member of which is influenced by the temperature gradient prevailing near the actively cooled solid surface, that is, the site of multicomponent condensation if the temperature falls below the (Ludwig-Soret-shifted) dew point.…”

In Silico

“…Our previous laboratory and theoretical studies of high‐temperature dew points in combustion systems containing condensable vapors were focused on corrosive alkali sulfates, with emphasis on the formation of binary nonideal liquid solutions such as Na 2 SO 4 + K 2 SO 4 (see, for example, Rosner and Liang3 and Liang and Rosner4). In the present paper we return to this canonical problem, generalizing our procedures to now embrace a mainstream that contains a family of condensable species, each member of which is influenced by the temperature gradient prevailing near the actively cooled solid surface, that is, the site of multicomponent condensation if the temperature falls below the (Ludwig–Soret‐shifted) dew point.…”

Simultaneous condensation and reaction of metal compound vapors in porous solids

“…For diverse reasons, high-temperature interactions of metal compound vapors with solids have received considerable attention in recent years. For example, the useful life of turbines in combined cycle power generation from coal depends on the deposition and reaction rates of corrosive vapors of inorganic compounds such as Na2S04 on ceramic blades of the turbine (Liang and Rosner, 1987). Interactions of metal compound vapors with solids have also been considered in the studies on chemical vapor deposition and chemical vapor infiltration (Middleman, 1989; Wang and Spear, 1981).…”