Vapor alkali transport

Iron-based ammonia synthesis catalysts of enhanced activity alkali promoter, e.g., sodium, added to the finished catalyst by vapor-phase transport. T.A. Gens (Indianapolis Center for Advanced Research). US 4235749 (1980). 

Rb, Cs) can be obtained using vapor-transport techniques [115]. Thereby, a weighed amoimt of Cjq is treated with a large excess of the alkali metal at 225 °C under vacuum. This procedure leads to the saturation-doped products MgC g. To these fullerides a specific amount of Cjq is added to give the desired stoichiometry. The process of the formation of is completed after a subsequent annealing... 

This reaction is highly exothermic. If the heat of the reaction is not conducted thru the walls of a closed container at a rate capable of maintaining an equilibrium temperature, an increase in pressure results with an increase in reaction rate, leading to explosive conditions. Acid salts, such as stannic chloride and zinc chloride, and bases, such as alkali metal hydroxides, either solid or in aqueous solution, and tertiary amines are all effective catalysts. It is, therefore, imperative that the concentration of such contaminants be kept at a minimum when transporting or storing sizeable quantities of ethylene oxide Accdg to Hess Tilton (Ref 16), a 90% decompn takes place if 100% vapor of EtnO in a closed container is. initiated with MF. There is no upper limit of EtnO in air (the previously reported value of 80% was in error), but the lower expl limit is 3% (Ref 17, p 87)... 

Behavior in Fire Vapors are heavier than air and may travel considerable distances to a source of ignition and flash back. Fires are difficult to control because of recognition Ignition Temperature (deg. F) 446 Electrical Hazard. Not pertinent Burning Rate 4.4 mm/rnin. Chemical Reactivity Reactivity with Water No reaction Reactivity with Common Materials No reactions Stability During Transport Stable Neutralizing Agents for Acids and Caustics Not pertinent Polymerization May occur in the presence of heat, acids or alkalis Inhibitor of Polymerization Not pertinent. 

Thermionic conversion is a technology that needs, and can immediately use, research on high temperature properties of alkali metals. Electron transport properties of alkali vapors and characteristics of atomic clusters are particularly Important. Improved understanding in these areas could lead to performance improvements that would more than double the output power density and efficiency of cesium ignited mode thermionic converters. 

Alkali Vapor Transport in Coal Conversion and Combustion Systems... 

Even a minor amount of alkali vapor transport can be significant, as revealed by the turbine tolerance level of 0.02 ppm alkali needed for corrosion control in pressurized fluidized bed combustors (12). If we consider only the alkali halide content of the dolomite component, this tolerance level would require an alkali-scrubbing efficiency of better than 99.9999 percent for PFBC. Even if corrosion (alkali) resistant materials were available, uncontrolled alkali vapor transport would still lead to unmanageable deposits on cool downstream components. For instance, under typical coal gasifier conditions, a species partial pressure as low as 10 atm would lead to vapor transport and deposition in metric ton quantities on an annual basis. 

The need for a basic understanding of alkali vapor transport in fossil energy systems can be appreciated when we consider the diversity of conditions such as temperature, pressure, chemical composition, and time scale, present in existing and developing fossil fuel technologies. Table 1 summarizes some typical process conditions. 

HASTiE ET AL. Alkali Vapor Transport Coal Mineral Characteristics... 

Alkali vapor transport and deposition is a well-known, though poorly understood, factor in the corrosion or fouling of alloys and ceramics, both in established and developing technologies. Problem areas include oil-fired glass melting operations (21), blast furnaces, boilers, turbines, coal gasification ( ), MHD (23, 24, 25) and coal-fired pressurized fluidized beds (26). 

Despite the incomplete state of a thermodynamic data base and limited mechanistic insight, several attempts to model alkali vapor transport in reactive atmospheres have been made. The increased sophistication of modeling efforts in recent years is demonstrated by the following examples ... 

In complex combustion systems, alkali vapor transport can occur as the metal or as molecular species, such as NaCl, (NaCl)2, NaOH, (NaOH)2, Na2S04, NaSO (x = 2,3), NaPO (x = 2,3) and,... 

Future studies should be pursued under controlled doping conditions and in atmospheres containing CO2 and O2. The known synergistic effect of CO2 on 02-solubility in silicate melts at very high gas pressures has, in fact, been interpreted in terms of Na2C03 formation in solution. Effects of this type could significantly enhance alkali vapor transport in practical combustion systems. 

Background. In the previous sections, we have considered alkali vapor transport from condensed phase systems in the absence of external influences, such as reactive gases. However, some of the component gases of combustion systems, such as H2O, HCl, SO2, O2, CO, and H2, can be expected to significantly modify alkali vapor transport through mass action effects or formation of new molecular species. Some representative cases are considered as follows. 

We believe that a similar water vapor solubility enhancement of alkali vapor transport is possible in soda-lime-silica glass systems, and work is in progress to verify this. Some of the disparities between various glass vaporization studies may well result from variations in water content and, hence, alkali activities. The common explanation for water vapor enhanced alkali vapor transport over silicates has revolved around formation of volatile NaOH (77) and KOH (53) species. However, no direct test for the presence of these species has been made, and the possibility of water vapor enhancement of atomic Na and K transport exists in these systems. 

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