GB2167054A - Gas generator - Google Patents

Abstract

A process and apparatus for catalytic gas generation in which the liquid phase reactant is contained in a sealed reservoir and excess pressure resulting from volume change at the catalyst is fed back to the reservoir to nullify any pressure drop across the catalyst. The reactant supply to the catalyst may be automatically controlled according to the pressure of the gas produced. Specified example is that of supply of hydrogen peroxide from a reservoir to catalyst producing oxygen and water vapour. <IMAGE>

Description

SPECIFICATION Generation of gas This invention relates to the generation of gas. The invention is especially useful for applications wherein relatively minor quantities of gas are required in remote geographical locations.

In the practice of medicine and veterinary science, especially in the so-called developing countries, it is frequently necessary to have available a supply of oxygen. This gas is needed, albeit in relatively minor quantities, for example during surgical operations carried out in the field. Hitherto, however, although only minor quantities are required, the only source of supply has been high-pressure cylinders. These are of course inconvenient to handle in the field and are potentially hazardous. There is therefore a need for a safer and more convenient portable source of oxygen.

It is already known that oxygen may be produced by the catalytic decomposition of hydrogen peroxide solution. This reaction may be represented by the following equations: H2O2H2O + 02 (+98kJ/mole H202) H202 (35%; 1 litre) < H2O (0.94 litres) + 02(130 litres) Indeed, this is on its face a highly efficient potential way of generating oxygen, because 35% aqueous hydrogen peroxide solution is capable, as shown above, of producing about 130 times its own volume of gaseous oxygen. However, this method has never been exploited in practice because, inter alia, such a large volume differential between reactant and products occurring at the catalyst tends to create back pressure which prevents a continuing flow of hydrogen peroxide solution to the catalyst.

A further drawback to the generation of oxygen by catalytic decomposition of hydrogen peroxide has been the difficulty of making the supply responsive to demand. Even during "continuous" use, for example during a surgical operation, the demand is actually intermittent, corresponding with the respiration cycle. Hitherto it has not been possible to stop and re-start the catalytic reaction automatically according to the instantaneous demand of the user.

We have now found that the problem of back pressure may be overcome by providing a sealed system on the upstream (supply) side of the catalyst and a pressure feedback between the downstream (product) side of the catalyst and the hydrogen peroxide supply, thereby effectively reducing to zero any pressure drop across the catalyst.

Accordingly, the invention provides in one aspect a process for the generation of a gas by catalytic reaction of a liquid-phase reactant, wherein the reactant is supplied to the catalyst from a sealed reservoir, excess pressure at the downstream side of the catalyst being applied to the reservoir via feedback pressure means.

Preferably the catalytic reaction is a catalytic decomposition reaction on a discrete chemical reactant, optionally in solution, although the invention is also applicable to the generation of a gas by reaction between two or more chemical reactants in the liquid phase.

By "gas" is meant a product substantially in the vapour phase, whether below or above its critical temperature, possibly including minor quantities of entrained liquid as droplets or as a mist, for example of solvent. A phase change or substantial phase change at the catalyst is generally associated with a large volume change between reactant(s) and product(s) which tends to create back pressure. In the absence of pressurisation of the reservoir, the back pressure would interrupt or hinder the supply of reactant(s) to the catalyst. The use of feedback pressure according to the invention overcomes this disadvantage and avoids the need externally to pressurise the reservoir.Furthermore, since the invention effectively reduces to zero any pressure drop across the catalyst, the feedback pressure is at all times self-balancing and has no effect on the output pressure of the gaseous product, whereas an externallyapplied pressure would need to be externally controlled to overcome the back-pressure created at the catalyst without at the same time affecting the output pressure.

The process according to the invention is particularly applicable to the generation of oxygen by catalytic decomposition of hydrogen peroxide solution. In this reaction, and where the hydrogen peroxide solution is 35% in concentration, there is a volume differential of approximately 130 times between reactant and products.

In a preferred embodiment, therefore, the invention comprises a process for the generation of oxygen by catalytic decomposition of hydrogen peroxide in solution to produce a mixture containing oxygen and water vapour, wherein the hydrogen peroxide solution is supplied to the catalyst from a sealed reservoir and the decomposition products are passed on the downstream side of the catalyst to separation means for separating condensed water vapour from vapour-phase reaction products, feedback pressure being provided from the downstream side of the catalyst to the reservoir, whereby the pressure drop across the catalyst is reduced to a minimum and supply of hydrogen peroxide solution to the catalyst is continuously maintainable.

The nature of the catalyst is immaterial to the effectiveness of the invention and may comprise one or more metals, metal oxides or compounds, or mixtures of two or more of these, capable of catalysing the reaction. The catalyst may take any convenient physical from and may be supported or unsupported and held in a suitable container. For the decomposition of hydrogen peroxide solution, palladium supported on alumina granules held in a container having means for the inflow and outflow of reactants and products respectively may conveniently be used.

Preferably, in order to restrict the maximum output flow of the product, a flow limiter is inserted between the reservoir and the catalyst.

The gaseous reaction products are conveniently passed from the catalyst to a separator from which the feedback pressure line is taken to the reservoir. Preferably the reservoir is equipped with a pressure relief valve to enable any excessive pressures to be vented to atmosphere.

Conveniently, in the decomposition of hydrogen peroxide solution to give oxygen, the flow limiter is arranged to restrict the maximum output flow to 5 1 minor1 of oxygen and the pressure relief valve is set at approx. 85kPa (12 psi).

The catalytic reaction is generally exothermic and the gaseous reaction products are therefore generally warm or hot. They are separated in the separator from any entrained liquid and primary condensate, which accummulates as a liquid phase fraction and may be removed via a drain valve. The remaining gaseous fraction may be used as such, optionally after being passed through a flow meter, but is preferably passed through a condenser to cool the vapour and condense water or other solvent vapour present in the reaction products to provide secondary condensate which may be returned to the separator A dryer may also be incorporated downstream of the condenser to remove any residual water or other solvent vapour.

The condenser is preferably air cooled and may be fan-assisted, operation of the fan conveniently being automatically controlled either by a temperature-responsive device or by a timing device.

Where, as in surgical operations, a mixture of oxygen and air is required, oxygen produced according to the invention may be mixed with air according to known methods.

The process according to the invention provides a ready and convenient supply of gas, for example oxygen, without the need for high pressure storage cylinders. The process requires no power or other external services although, if a fan-assisted condenser and/or a digital flowmeter is used, power may be supplied either from a mains supply or from a battery which may be rechargeable.

The invention also provides apparatus for the generation of a gas by catalytic reaction of a liquid-phase reactant, the apparatus comprising a reservoir sealed to atmosphere for holding liquid phase reactant, the reservoir being in communication with a catalyst, feedback pressure means being provided between the downstream side of the catalyst and the reservoir.

In situations where the gas demand is actually intermittent within a period of overall continuous use, it is desirable to stop and re-start the reaction according to the instantaneous demand of the user. We have found that the process according to the invention can be rendered demand-responsive by means of automatic control regulator means which interrupts reactant supply to the catalyst when the pressure of the gaseous product exceeds a predetermined value during periods of low or zero demand. The automatic control regulator means may comprise a pressure switch in conjunction with a solenoid valve or a mechanically-operated pressure control regulator.

Accordingly, the invention further provides a process for the generation of a gas by catalytic reaction of a liquid-phase reactant, as hereinbefore defined, wherein the supply of reactant to the catalyst is automatically isolated when the pressure of gas exceeds a pre-determined value and is automatically restored when the said pressure falls below the said value.

The invention also includes apparatus for the generation of a gas by catalytic reaction of a liquid-phase reactant, wherein the apparatus includes in the line for supplying reactant to the catalyst a control regulator means responsive to the pressure of the gaseous product. Preferably the regulator means comprises a pressure switch responsible to the gaseous product in conjunction with a solenoid valve in the reactant supply line.

Embodiments of the invention will now be described by way of example with reference to the accompanying drawing (Figure 1), which is a flow sheet diagram illustrating apparatus for the generation of oxygen and incorporating an automatic control regulator means.

Referring to Figure 1, apparatus for the generation of oxygen by catalytic decomposition of hydrogen peroxide solution is shown in flow sheet form. in the Figure, a reservoir for holding 35% hydrogen peroxide solution is shown at 11. The reservoir has a filler cap 12 which is pressure sealed. Hydrogen peroxide solution is gravityfed through automatic control regulator 13, which is a solenoid valve responsive to oxygen pressure through pressure switch 14, then through flow limiter 15 to catalyst chamber 16. The flow limiter is set to limit the maximum flowrate of hydrogen peroxide solution to that which will provide an oxygen flow of approx. 5 1.minor1. The catalyst chamber is a stainless steel vessel containing catalyst pellets of diameter approx. 3.5 mm (1/8 inch). On the downstream side of the catalyst, gaseous reaction products (oxygen and water vapour) are passed to separator 17. Feedback pressure line 18 connects the separator with the reservoir via pressure relief valve 19 set at approx. 85kPa (l2psi). From separator 17, the oxygen and water vapour are passed via line 20 to fan-cooled condenser 21, condensed water vapour being returned to separator 17 via line 22. Cooled oxygen is then available for use, optionally via flowmeter 23, as required, for example mixed with air using known mixing apparatus (not shown). The fan and thermal switch for control thereof may be powered by a 12 volt d.c. source or from mains source, optionally via transformer.The pressure switch 14 is set to close the solenoid valve 13 when the oxygen pressure exceeds approx. 20kPa (3psi), and to re-open the valve automatically when the oxygen pressure falls below this figure.

In use, the valve of regulator 13 is initially open since at this stage there is no oxygen pressure. Hydrogen peroxide solution flows through the valve and flow limiter 15 to the catalyst chamber 16 where the catalyst decomposition reaction starts spontaneously. The excessive volume of products compared with reactants results in a large volume expansion at the catalyst which is accommodated by feedback pressure line 18, resulting in pressure equilibration across the catalyst so that continuity of supply of hydrogen peroxide solution to the catalyst under force of gravity is assured.

As the catalytic reaction is exothermic, the reaction products are hot and hence are passed through condenser 20 to cool them down and to condense water vapour for return to the separator. The condenser is fan-cooled, the fan (not shown) being arranged to start operating when the temperature of the reaction products exceeds a predetermined value.

As long as the valve of regulator 13 is open, hydrogen peroxide solution continues to flow to the catalyst. On closure of the valve, when the oxygen pressure exceeds approximately 20 kPa, the hydrogen peroxide supply to the catalyst is isolated thus immediately stopping the reaction. On automatic opening of the valve when oxygen pressure falls due to demand, supply of hydrogen peroxide is restored and generation of oxygen immediately re-starts.

We have found that the process and apparatus according to the invention, when used for generation of oxygen by catalytic decomposition of hydrogen peroxide solution, provides continuous production of oxygen of minimum 99% purity for one hour at outputs of up to 5 I/min from 3 1 of 35% hydrogen peroxide in the reservoir, which can readily be replenished.

Claims (8)

1. A process for the generation of a gas by catalytic reaction of a liquid-phase reactant, wherein the reactant is supplied to the catalyst from a sealed reservoir, excess pressure at the downstream side of the catalyst being applied to the reservoir via feedback pressure means.

2. A process according to claim 1, wherein the gas comprises oxygen and water vapour and the reactant comprises hydrogen peroxide solution, the reaction products being passed on the downstream side of the catalyst to separation means for separating condensed water vapour from gaseous reaction products, feedback pressure being provided from the downstream side of the catalyst to the reservoir, whereby the pressure drop across the catalyst is reduced to a minimum and supply of hydrogen peroxide solution to the catalyst is continuously maintainable.

3. A process according to claim 1 or claim 2, wherein the supply of reactant to the catalyst is automatically isolated when the pressure of product gas exceeds a pre-determined value and is automatically restored when the said pressure falls below the value.

4. Apparatus for the generation of a gas by catalytic reaction of a liquid-phase reactant, the apparatus comprising a reservoir sealed to atmosphere for holding liquid phase reactant, the reservoir being in communication with a catalyst, feedback pressure means being provided between the downstream side of the catalyst and the reservoir.

5. Apparatus according to claim 4, wherein the apparatus includes in the supply line for supplying reactant to the catalyst a control regulator means responsive to the pressure of the gaseous product.

6. Apparatus according to claim 5, wherein the control regulator means comprises a pressure switch responsive to the gaseous product in conjunction with a solenoid valve in the reactant supply line.

7. A process as herein described with reference to and as illustrated in the accompanying drawing.

8. Apparatus as herein described with reference to and as illustrated in the accompanying drawing.