Steam sintering

As shown in Fig. 17, the number of protons from a series of silica-alumina samples with differing BET surface areas is a nearly linear function of surface area, indicating that essentially all of the protons are situated in the surface phase of the solid. The samples corresponding to the data of Fig. 17 were prepared by steam sintering of the above-mentioned 425 meterVgram sample at various temperatures above 500° so as to reduce the surface area to the values given and then soaked in water for 4 hours, oven-dried at 110° and evacuated at 500° for 16 hours. A plot somewhat similar to Fig. 17 has been obtained for silica gel. The slope of the best straight line repre-... 

In Fig. 5 the striking similarity of the isotherms for a virgin DA-5 and a steam-treated DA-5 is apparent. The pore volume and area of the steam-sintered sample are far below those of the virgin, but the isotherm contours are almost identical. A small increase in pore radius is observed. Thus the presence of steam during sintering does not alter significantly the pore structure of the silica-magnesia catalysts studied. It should also be noted here that the isotherms for the DA-5 catalyst... 

This represents a threefold decrease in area, the same decrease as in the vacuum sintering example above. However, in this case of steam sintering, the pore volume does not change and, consequently, there is a threefold increase in pore width. 

A relatively recent catalyst preparation, Houdry Porous Beads, is compared with the virgin and steam-sintered TCC Beads in Fig. 11. 

In Fig. 13 is illustrated the sharp contrast between the isotherms for the virgin and for the steam-treated Aerocat. The steam sintering treatment reduces the area from 700 to 85 sq. m./g., a decrease of almost 90%, whereas the pore volume has fallen only from 0.580 to 0.504 cc./g., a loss of approximately 13%. The average pore radius increases from 16.6 to 119 A., a sevenfold increase. The isotherm plots indicate that the pore size distribution is quite narrow in both cases with steep portions... 

The sintering curves, in which surface area is plotted against temperature of treatment, are presented in Fig. 15. These curves also demonstrate the wide difference between vacuum and steam sintering of Aerocat. The area value of 85 sq. m./g. obtained in steam at 621°C. (1150°F.) corresponds approximately to a vacuum sintering value in the... 

In preliminary steam sintering experiments with Davison silica gel it appears that sintering is accelerated considerably by the presence of steam at atmospheric pressure. Furthermore, there is a small but significant increase in pore radius. The small hysteresis loop that almost disappears on vacuum sintering is apparently maintained or perhaps slightly broadened on sintering in steam. Detailed data must await the completion of the steam series. 

Sintering of catalysts 1, 2, and 3 in the presence of steam (12 hrs., 60 p.s.i.g., 566°) results in a loss of catalytic activity and a decrease in surface area. The relative decrease of catalytic activity upon steaming is more pronounced in catalyst 1 than in catalysts 2 and 3. The surface area stability in the presence of steam of catalysts 2 and 3 is even more pronounced than their stability of catalytic activity. Thus, it appears that the tendency of these two catalysts to form gamma-alumina is associated with their steam stability. It can be postulated that the stability of catalytic activity of these two catalysts is due to a hydrothermal reaction of uncombined alumina with silica to generate new catalyst activity during the steam sintering. Roy and Osborn (6) have shown that mullite is the product of the hydrothermal treatment of silica-alumina systems at 550°. 

The above problems can be avoided by the application of an active compound onto a thermally and chemically stable support material. Since desulfurization has to be performed on a large scale, the absorbent must be cheap. Expensive supports or active materials are not acceptable. The only generally used support that is inert towards coal gas and SO2 is silica. However, silica is not therm ly stable at high temperatures. Especially in the presence of steam sintering proceeds rapidly and even volatilization may occur. Until now a supported absorbent appropriate for high-temperature desulfurization is therefore not available. Therefore our research was aimed at the development of an absorbent that fulfills all the specifications for employment in hot-gas clean up. 

Method B. Place 125 g. (106 -5 ml.) of diethyl phthalate and 25 g. of molecular sodium (sodium sand see Section 11,50,6) in a 500 ml. round-bottomed flask fitted with a reflux condenser and dropping funnel. Heat the flask on a steam bath and add a mixture of 122 5 g. (136 ml.) of dry ethyl acetate and 2 5 ml. of absolute ethanol over a period of 90 minutes. Continue the heating for 6 hours, cool and add 50 ml. of ether. Filter the sodium salt (VI) on a sintered glass funnel and wash it with the minimum volume of ether. Dissolve the sodium salt (96 g.) in 1400 ml. of hot water in a 3-htre beaker, cool the solution to 70°, stir vigorously and add 100 ml. of sulphuric acid (3 parts of concentrated acid to 1 part of... 

Boric acid (boracic acid) [10043-35-3] M 61.8, m 171 , pK 9.23. Crystd three times from H2O (3mL/g) between 100° and 0°, after filtering through sintered glass. Dried to constant weight over metaboric acid in a desiccator. It is steam volatile. After 2 recrystns of ACS grade it had Ag at 0.2 ppm. 

Steam forms a protective white film at temperatures up to about 250°C, but above this temperature steam can, under some conditions, react with aluminium progressively to form aluminium oxide and hydrogen. Sintered aluminium powder (S. A.P.) has relatively good resistance to steam at 500°C, but at about 300°C an addition of 1% nickel to the S.A.P. is needed to prevent rapid disintegration. 

The activity loss measured here is caused by recrystallizations. This was demonstrated by using scanning electron microscopy to determine nickel crystallite size in the same catalyst samples. These tests revealed that the catalyst used in demonstration plants has only a slight tendency to recrystallize or sinter after steam formation and loss of starting activity. 

Sufficient thermal stability against sintering, structural change or volatilization inside the reaction environment (e.g. when steam is a byproduct of the reaction). 

Figure 8.13. Rate of methanol synthesis of a Cu/Zn0/Al203 catalyst in a plug flow reactor as a function of time on stream. The catalyst was operated at 494 K and 63 bar in a gas steam of 5 % CO, 5 % COj, 88% H2, and 2% N2. Note the steady decrease in reactivity, which is ascribed to sintering ofthe copper particles. The CO2 was removed from the reactants for 4 h after 168 h. After reintroduction the catalyst displays a restored...

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