GB2260086A

GB2260086A - Removal of hydrocarbon from gas; ozone production

Info

Publication number: GB2260086A
Application number: GB9121094A
Authority: GB
Inventors: Michael Ernest Garrett
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1991-10-04
Filing date: 1991-10-04
Publication date: 1993-04-07
Anticipated expiration: 2011-10-04
Also published as: GB9121094D0; JPH05220319A; GB2260086B; ZA927128B

Abstract

A gas, e.g. air or oxygen, is compressed and after compression is heated and passed through a vessel 8 containing a catalytic material which oxidises any hydrocarbon gas impurities present to carbon dioxide and water. The hydrocarbon free gas is then passed through a bed of molecular sieve to remove nitrogen and carbon dioxide and a desiccant to remove water in pressure swing adsorption vessels 16, 18 before being passed to an ozoniser for the production of ozone. <IMAGE>

Description

APPARATUS FOR REMOVING HIDROCARBON IMPURITIES FROM GASES The present invention relates to an apparatus for and method of removing hydrocarbon gas impurities from a gas containing said hydrocarbon gas impurities. More particularly, the present invention relates to a method of removing hydrocarbon gas impurities from air or oxygen which, after purification, is suitable for use as a feeder gas for an ozoniser for the production of ozone.

It is known to produce ozone directly from the oxygen present in air by means of a silent electric discharge or corona. The power efficiency of this known method of production is not good and the concentration of ozone which can be produced is in the region of between 3% and 4%. Even this result cannot be achieved unless the air is rigorously dried to a dew point of better than -5O0C.

It is known to increase the efficiency of this known method of production by using oxygen rather than air as the feeder gas for the ozoniser. When oxygen is used yields are higher and higher concentrations in the order of 5% to 8% can also be produced. The oxygen feeder gas for ozonisers can be provided direct from a cryogenic separation plant, a non-cryogenic separation plant, e.g. a pressure swing adsorption plant, liquid oxygen or compressed oxygen. However, as with air it is important to ensure that the oxygen is thoroughly dry.

In most sources of ozoniser feeder gas there is often a trace of hydrocarbon gases, such as methane, and although this is usually only one or two parts per million; in an ozoniser it is converted to carbon dioxide and water vapour thereby lowering the dew point and in practice reducing the efficiency of the ozoniser by up to 10%.

It is an aim of the present invention to provide a feeder gas for an ozoniser, either air or oxygen, which is both substantially dry and free from hydrocarbon gas impurities.

According to one aspect of the present invention, a method of removing hydrocarbon gas impurities from a gas comprises the steps of:a) compressing the gas; b) heating the gas and subsequently passing the gas over a catalytic material which oxidises the hydrocarbon gas impurities to carbon dioxide and water/water vapour; c) cooling the hydrocarbon free gas; and d) removing water/water vapour from the hydrocarbon free gas.

According to a further aspect of the present invention, an apparatus for removing hydrocarbon gas impurities and water/water vapour from a gas comprises a compressor, conduit means connected to the outlet of the compressor for the passage of compressed gas through a heater and hence to a vessel containing catalytic material, the catalytical material being capable of oxidising hydrocarbon gas impurities, means for cooling the hydrocarbon free gas and further means for removing water/water vapour from the hydrocarbon free gas.

An embodiment of the invention will now be described, by way of example, reference being made to the Figure of the accompanying diagrammatic drawing which is a schematic diagram of an apparatus for removing hydrocarbon gas impurities from air prior to its use as a feeder gas for an ozoniser.

As shown, the outlet of a compressor 1 (preferably an oil-free compressor) is connected by conduit means 2 to a vessel 8. The vessel 8 contains a catalytic material such as an extrudate of precious metal on tin oxide which oxidises any hydrocarbons present to carbon dioxide and water.

Between the compressor 1 and the vessel 8, there is located in the conduit means 2 a heat exchanger 4 and a heater 6. The conduit means 2 effectively forms one leg of the heat exchanger 4.

Further conduit means 10 passes from the outlet of the vessel 8 back through the heat exchanger 4 and effectively forms a second leg of the heat exchanger, through an aftercooler 12 to a pressure swing adsorption apparatus 14.

The pressure swing adsorption apparatus 14 comprises two pressure vessels 16, 18 each containing a bed of adsorbent material and a desiccant in the form of an alumina layer in a manner known per se. The adsorbent material preferentially adsorbs nitrogen and carbon dioxide relative to oxygen such that an oxygen enriched product gas exits from the pressure swing adsorption apparatus 14 again in a manner known per se.

Located in the conduit means 2 between the compressor 1 and the heat exchanger 4 is a conventional knock-out pot 30. Similarly located in the conduit means 10 between the aftercooler 12 and the pressure swing adsorption apparatus 14 there is arranged another conventional knock-out pot 32.

In use, air is compressed by compressor 1 and the compressed air leaving the compressor 1, by the act of compression, is supersaturated with water and very hot. Any liquid water in the air leaving the compressor is removed by means of the conventional knock-out pot 30. The compressed air then passes through the heat exchanger 4 and heater 6 where the temperature of the compressed air is raised to approximately 300 C. At this temperature the compressed air enters the vessel 8 and passes over the catalytic material which oxidises any hydrocarbons present in the air to carbon dioxide and water vapour.

The wet but hydrocarbon free air now passes down the conduit means 10 through the heat exchanger 4 and is cooled against the incoming air to approximately the same temperature at which it left the compressor 1. The hydrocarbon free air then enters the aftercooler 12 where much of its water content is condensed; the liquid water being removed by the conventional knock-out pot 32. The air then passes through the pressure swing absorption apparatus (PSA apparatus) where the alumina layer removes water vapour and the molecular sieve material preferentially removes nitrogen and carbon dioxide thereby leaving a dry, oxygen enriched gas free from hydrocarbon gases to exit the PSA apparatus 14 via line 20 for use as a feeder gas in an ozoniser.

It will be apparent that the same method of removing hydrocarbon gas impurities can be used, for example, in the case of oxygen gas from a liquid storage. The liquid gas would be the feed gas for the compressor 1 and the purified oxygen passing through the conduit means 10 and through the knock-out pot 32 would be directed immediately to the ozoniser rather than first pass through a pressure swing adsorption apparatus.

Claims

1. A method of removing hydrocarbon gas impurities and water/water vapour from a gas comprising the steps of: a) compressing the gas; b) heating the gas and subsequently passing the gas over a catalytic material which oxidises the hydrocarbon gas impurities to carbon dioxide and water/water vapour; c) cooling the hydrocarbon free gas; and d) removing water/water vapour from the hydrocarbon free gas.

2. A method as claimed in Claim 1, in which the gas is air the hydrocarbon free air being passed through a bed of molecular sieve adsorbent which preferentially adsorbs nitrogen and carbon dioxide relative to the oxygen and a desiccant for the removal of water vapour, the dry, hydrocarbon free oxygen leaving the bed of molecular sieve adsorbent being passed to an ozoniser for the production of ozone.

3. A method as claimed in Claim 1, in which the gas is oxygen.

4. A method as claimed in any one of Claims 1 to 3, in which the gas is heated to substantially 3000C.

5. A method as claimed in any one of Claims 1 to 4, in which the catalytic material is an extrudate of precious metal on tin oxide.

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6. A method as claimed in any one of Claims 1 to 5, in which the gas passes through a first leg of a heat exchanger prior to its passage over the catalytic material and the hydrocarbon free gas passes through a second leg of the heat exchanger to be cooled by the gas passing through the first leg of the heat exchanger.

7. An apparatus for removing hydrocarbon gas impurities and water/water vapour from a gas comprising a compressor, conduit means connected to the outlet of the compressor for the passage of compressed gas through a heater and hence to a vessel containing catalytic material, the catalytical material being capable of oxidising hydrocarbon gas impurities, means for cooling the hydrocarbon free gas and further means for removing water/water vapour from the hydrocarbon free gas.

8. An apparatus as claimed in Claim 7, in which a knock-out pot is provided between the compressor and the heater for removing liquid water from the compressed gas.

9. An apparatus as claimed in Claim 7 or 8, in which prior to its passage through the vessel the compressed gas passes through one leg of a heat exchanger.

10. An apparatus as claimed in Claim 9, in which further conduit means extends from the outlet of the vessel and forms a second leg of the heat exchanger.

11. An apparatus as claimed in Claim 10, in which the further conduit means extends from the vessel towards a pressure swing adsorption apparatus.

12. A method of removing hydrocarbon gas impurities from a gas substantially as hereinbefore described.

13. An apparatus for removing hydrocarbon gas impurities from a gas arranged and adapted to operate substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.