GB2155452A

GB2155452A - Active adsorption media for oxygen and water vapour

Info

Publication number: GB2155452A
Application number: GB08328662A
Authority: GB
Inventors: John Thomas Mccullins
Original assignee: Individual
Current assignee: Individual
Priority date: 1983-10-26
Filing date: 1983-10-26
Publication date: 1985-09-25
Also published as: GB8328662D0

Abstract

The active medium is produced by first absorbing a solution of a salt of the transition metals chromium, manganese, vanadium or molybdenum, in their higher oxidation states, onto the pore structure of silica gel, alumina or pumice stone. After this the material is reduced, at a temperature within the range 250 DEG C to 400 DEG C, in a stream of hydrogen, carbon monoxide or coal gas. The active medium strongly absorbs oxygen and water vapour from other fluids, especially gases, and in doing so passes through various colour changes. The spent medium can be activated or regenerated by heating it in a reducing gas stream within the previously stipulated temperature range.

Description

SPECIFICATION Active adsorption media for purification purposes There are many requirements in industry for supplies of processing gases which are essentially free, or practically free, from oxygen and water vapour.

As the microprocessor manufacturing sector increases its processing of pure and doped semiconductor metals the requirement for dry, oxygen-free gases increases as well.

Various measures are described for the removal of impurities of this type (for example see British Patent Number 1246483) but there are disadvantages in the processes described in that it is necessary, in all cases, to first oxidise the silica supported metal salts to a high valency state by the use of oxygen gas at relatively high temperatures (in the range 300 to 600 C) before chemical reduction is attempted. This reduction produces the active adsorption medium for the desired purification process and invariably a colour change takes place in the surface of the medium during reduction and again when oxygen or water vapour is adsorbed.

It is important that the active medium be readily produced and that it be active at ambient temperatures so that no pre-heating of an adsorption bed is necessary.

Also it is desirable that the moisture, which is adsorbed in use, can be driven off during regeneration of the medium and that regeneration be accomplished by the use of a reducing gas such as hydrogen or carbonmonoxide at about 2500C to 400 C.

The present invention provides a range of materials which comply with these criteria and accordingly there will be described a series of substances which will remove homogeneously dissolved impurities, especially oxygen and water vapour, from a fluid by chemisorption.

In the invention herein described there is provided a range of active adsorption media which consist of what I believe to be low oxidisation states of transition type metals supported on a high temperature resistant mass such as silica or aluminium oxide.

The active adsorption material, in all cases, can be produced directly from the treated silica or aluminium oxide base by a single heat treatment process which involves only one step and that is one of reduction.

It has been found that if a metal salt is selected which is already in a high state of oxidisation and if this is loaded onto a support of either silica or aluminium oxide then the desired active adsorption medium can be obtained by a simple reduction process at about 4000C by the use of hydrogen or carbon monoxide. In selecting such metal salts, for example, those of chromium, manganese, vanadium and molybdenum it is important to select an oxidisation state which is unstable at the temperature of the reduction. In the examples cited the salts are chromates or dichromates, permanganates, metavanadates and molybdates and it is beneficial, though not essential, if the associated cation is also thermolabile for example ammonium.

It is believed that the method of preparation, which is described in this invention, is capable of giving the active adsorption states of the selected transition elements because the decomposition, which occurs during the reduction process, is vigorous and bond disruptive. In the process the peroxy salts would be especially reactive in oxidising the reducing gas.

The process of making an oxygen or water gettering agent using the novelty of this invention is best illustrated by means of examples thus.

Example 1 Silica gel of particle size 0.2 mm to 0.5 mm (Merck 7733) was impregnated with an aqueous solution of ammonium dichromate and the damp mass was dried at 105 C to give a free flowing fine granule with a chromium content of about 1.4% by weight. When this mixture was heated in a stream of hydrogen at 400 C a greeniblue product was obtained which changed to a black colour on exposure to air. Chromatogram B illustrates the efficiency of removal of oxygen from an air/helium mixture.

Example 2 Silica gel was impregnated, as in Example 1, but this time potassium permanganate was used in place of the chromium salt. The gel was loaded with manganese to the extent of about 1.4% by weight. On decomposing this mixture at 3500C in a stream of hydrogen for about 15 to 20 minutes a pale yellow mass was obtained which changed to brown on exposure to air. The chromatogram C indicates how successful it is in removing oxygen from a mixture of air and helium.

Example 3 The silica gel loading technique as described in Example 1 was used to load the Merck 7733 grade to the extent of 1.4% vanadium using ammonium metavanadate as the water soluble salt. This mass was reduced at 400 C in a mixture of hydrogen and a product was obtained which changed from black to brown when exposed to air but experimental work on the gas chromatograph revealed that it did not completely remove oxygen from air/helium mixture as was the case in the previous examples.

However on repeating the operation with gels containing 2.8% by weight of vanadium the mass completely removed oxygen from the gas mixture and this is indicated by chromatogram D.

Example 4 Silica gel, as before, was impregnated with ammonium molybdate so as to contain about 1.4% by weight of molybdenum and the resulting mass was heated to 4000C in a stream of hydrogen gas. The active product was a brown/black mass which changed to white on exposure to air. Its effectiveness in removing oxygen from an air/helium mix ture is indicated by chromatogram E.

It is important to note that all this reactivity with oxygen in each case occurs at room temperature and there is no requirement to heat the mass when it is used in a purifying capsule in a gas purifying train. In the chromatograms the gas peaks to the left of S represent the mixture entering the adsorption medium whilst that to the right represents the exit gas.

In addition the products of all the examples can be regenerated, after use, by reheating in the same reducing gas stream at temperatures below 400 C.

This means that insitu regeneration is possible in gas purification trains since even in the limiting case the required temperatures are below the maximum working temperature of borosilicate glasses.

In addition to the above described use of the active adsorption medium in connection with gas purification it is interesting to note that the compounds react with water and some (that of Example f) exhibit sharp colour changes; so much so that it is possible to use them to visually indicate when a normally anhydrous organic liquid, or solid, reaches a particular moisture content. In the case of ethanol for example the mass of Example 1 only turns colour when the moisture content reaches 95%.

Claims

1. A process for removing oxygen and water vapour from a fluid mixture, especially a gas, by providing a compound selected from the higher oxidation states of the transition metals which have been reduced at an elevated temperature in an atmosphere of a reducing gas such as hydrogen or carbon monoxide.

2. A process according to claim 1 in which the compound provided is a salt of chromium, manganese, vanadium or molybdenium in a high oxidation state and which has been reduced at an elevated temperature in a stream of a reducing gas such as hydrogen or carbon monoxide.

3. A process according to claims 1 and 2 in which the provided compound is produced from a higher oxidation salt of chromium, manganese, vanadium or molybdenium by the reduction of the salt in a stream of a reducing gas after the said salt has been absorbed on a heat resistant, inert support such as silica gel or alumina.

4. A process according to claims 1, 2 and 3 in which the compound, in its higher oxidation state as an adsorbate on silica or alumina, is reduced in a gas stream at a temperature in the range of 250 C to 400cm.

5. A process according to claims 1, 2, 3 and 4 in which the reducing agent, used to make the provided compound, is hydrogen, carbon monoxide, methane or coal gas.

6. A process according to any of the above claims in which the inert support for the transition metal compounds can be any synthetic or natural refractory such as silica gel, aluminium oxide or pumice stone.

7. A process according to any of the above claims in which the provided compound, when spent, can be re-activated by heating at 2500C to 4000C in an atmosphere of any of the previously claimed reducing gases.