Absorbents

Details Absorbents Absorbents

1996-10-08

1999-04-27

Langel, Wayne

Chemistry of inorganic compounds

Modifying or removing component of normally gaseous mixture

423240S, 502415, B01J 2000, C01B 700

Patent

active

058978453

DESCRIPTION:

BRIEF SUMMARY
This invention relates to absorbents, and to a process for their manufacture, in particular to absorbents suitable for removing contaminant acidic gases such as hydrogen chloride from gas streams.
Such absorbents are typically employed as a bed through which the gas stream to be treated is continuously passed: the contaminant hydrogen chloride is absorbed by the absorbent so that the effluent gas has a relatively low hydrogen chloride content. After a period of time, the absorbent becomes loaded with absorbed hydrogen chloride until the absorbent bed is unable to reduce the hydrogen chloride content to an acceptable level: typically it is desired to have an effluent gas containing less than a specified amount, e.g 0.1 ppm by volume of hydrogen chloride. When the effluent gas contains an unacceptable proportion of hydrogen chloride, "break-through" is said to have occurred. It is normally found that, when break-through has occurred, the chloride content of the bed is somewhat less than the theoretical maximum: thus while samples taken from bed inlet region may have a chlorine content equal to or near the theoretical maximum, samples taken from the bed outlet region are liable to have a chlorine content significantly below the theoretical maximum.
One type of known absorbent for removing contaminant acidic gases such as hydrogen chloride from gas streams comprises alumina granules that have been impregnated with sodium hydroxide or carbonate and then calcined at a high temperature, at, for example above 500.degree. C. Calcination at such temperatures causes at least some of the sodium hydroxide or carbonate to react with the alumina to form sodium aluminate.
Such absorbent granules, in which some of the sodium is present as sodium aluminate as a result of the reaction between the sodium carbonate and alumina, typically have a total sodium content (expressed as sodium oxide, Na.sub.2 O) of the order of 12-14% by weight as measured on a sample that has been ignited at 900.degree. C. These absorbents exhibit acceptable activity for the absorption of hydrogen chloride at relatively high temperatures, for example of the order of 350.degree. C., and "break-through" (giving a product gas containing more than 0.1 ppm by volume of hydrogen chloride) typically occurs when the absorbent has a total chlorine content of about 15-17% by weight. However the activity of such absorbents at low temperatures, eg below about 150.degree. C., is limited, and the chlorine content of the bed when break-through occurs may be relatively low. For example, at ambient temperature, break-through (0.1 ppm HCl) may occur when the bed has an average chlorine content of only about 8-9% by weight.
It has been proposed in GB 1433762 to employ as an absorbent for acidic gases, activated alumina granules of a high surface area that has been impregnated with a solution of sodium carbonate, followed by evaporation of the water and heating at 120.degree. C. In PCT publication WO 93/22049 it has been proposed to make similar absorbents by impregnating alumina with an alkali metal salt of an organic acid such as acetic acid followed by drying and calcination at 300-550.degree. C. This latter reference suggests that absorbent granules could be made by an agglomeration technique involving feeding alumina powder to a rotating pan and spraying an aqueous solution of the alkali metal compound onto the alumina powder in the rotating pan. By this technique it is suggested that the amount of alkali that could be incorporated is such that, expressed as alkali metal oxide, the alkali content of the absorbent is up to 15% by weight of the alumina. By such an impregnation method it is not practical to produce absorbents containing more than about 15% by weight alkali, expressed as alkali metal oxide.
By utilising an alternative production process, we have found that it is possible to achieve absorbents having a much higher alkali content, and hence a greater chloride capacity. Such absorbents have significantly improved low temperature performance.
Accordingly t

REFERENCES:
patent: 3557025 (1971-01-01), Emerson et al.
patent: 5316998 (1994-05-01), Lee et al.
patent: 5505926 (1996-04-01), Lee et al.

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