Apparatus for catalytic decomposition of nitrous oxide in a gas stream

Abstract

The invention relates to an apparatus (1) for catalytic decomposition of nitrous oxide in a gas stream derived from exhalation air from a patient. The apparatus (1) comprises an inlet arrangement (2) with a gas inlet (3) for the exhalation air, an outlet arrangement (11) with a gas outlet (12) for an outlet gas, and between these arrangements a through-flow decomposition chamber (9) containing a catalyst material. According to the invention the apparatus is provided with a nitrous oxide adsorption/desorption means (4) in the inlet arrangement (2) for level out variations in the concentration of nitrous oxide fed to the decomposition chamber (9).

Claims

1. An apparatus for catalytic decomposition of nitrous oxide in a gas stream derived from exhalation air from a patient, said apparatus comprises: a) an inlet arrangement with a gas inlet for the exhalation air, b) an outlet arrangement with a gas outlet for an outlet gas, and between these arrangements c) a through-flow decomposition chamber containing a catalyst material promoting the decomposition of nitrous oxide to N.sub.2 and O.sub.2, wherein the apparatus further comprises a control unit for controlling and monitoring the temperature and flow of the gas stream in the through-flow decomposition chamber and, in the flow direction of the gas stream from gas inlet and before the through-flow decomposition chamber, a fan, a flow meter, a heat exchanger arranged to transfer heat from the outlet gas to the exhalation air, and an electrical heater, and wherein the inlet arrangement comprises at least one nitrous oxide adsorption/desorption means comprising a container filled with activated carbon or zeolite for adsorbing/desorbing nitrous oxide to level out variations in the concentration of nitrous oxide fed to the decomposition chamber.

2. The apparatus according to claim 1, wherein in case the container is filled with activated carbon, the activated carbon is of the type bituminous coal based or coconut, having a surface area of about 500 to 2000 m.sup.2/g.

3. The apparatus according to claim 1, wherein said at least one nitrous oxide adsorption/desorption means is heated to increase the desorption rate of the nitrous oxide, if necessary.

4. The apparatus according to claim 1, wherein the electrical heater is arranged to heat only an incoming gas during the start-up phase of the apparatus.

5. The apparatus according to claim 1, wherein the apparatus is mobile and adapted to be directly connected to at least one patient.

Description

BRIEF DESCRIPTION OF DRAWINGS

(1) The invention will now be described by way of a non-limiting example with reference to the accompanying drawing, in which

(2) FIG. 1 is a schematic view of an apparatus for catalytic decomposition of exhaled nitrous oxide having an inlet arrangement provided with a nitrous oxide adsorption/desorption means according to the invention,

(3) FIG. 2 is a schematic view of the apparatus shown in FIG. 1 arranged for testing of the nitrous oxide adsorption/desorption means according to the invention,

(4) FIG. 3 is a diagram showing the temperature in the decomposition reactor with a nitrous oxide adsorption/desorption means (black line) and without a nitrous oxide adsorption/desorption means (dotted line),

(5) FIG. 4 is a diagram showing the fluctuation of nitrous oxide measured at the inlet of the catalytic reactor in case a concentration of 85 000 ppm of nitrous with stand-by phases with no nitrous oxide is passed to the inlet arrangement of the apparatus for decomposition of exhaled nitrous oxide, wherein the black line shows the concentration in case the inlet arrangement is provided with a nitrous oxide adsorption/desorption means according to the invention and the dotted line shows the concentration of nitrous oxide without the adsorption/desorption means according to the invention, and

(6) FIG. 5 is a diagram showing how long it takes for the adsorption/desorption means according to the invention to saturate when the flow of nitrous oxide is kept constant at 7 g/min (corresponding to a mass flow of about 85 000 ppm) up to about 16 minutes, where after this flow is zero and how long it takes for the adsorption/desorption means to be flushed.

DESCRIPTION OF PREFERRED EMBODIMENTS

(7) FIG. 1 show a schematic view of an apparatus 1 for catalytic decomposition of nitrous oxide contained in a gas mixture exhaled by a patient, the apparatus 1 preferably being in the form of a mobile unit to be used close to at least one patient receiving pain relief.

(8) Said apparatus 1 comprises, as seen in the flow direction of the gas mixture to be treated, an inlet arrangement 2 comprising a gas inlet 3 for an inlet gas being a mixture of oxygen containing gas and nitrous oxide, at least one adsorption/desorption means 4 for selective adsorption/desorption of nitrous oxide, a fan 5, a flow meter 6, a heat exchanger 7, and a heater 8. The apparatus 1 further comprises a nitrous oxide decomposition reactor 9 provided with at least one temperature sensor 10, an outlet arrangement 11 comprising at least a gas outlet 12, and a control unit 13 for controlling and monitoring the temperature and gas flow in the nitrous oxide decomposition reactor and possibly for heating the adsorption/desorption means 4. The control unit 13 is connected (not shown) in a suitable way to the apparatus 1.

(9) The apparatus 1 for decomposition of nitrous oxide may also comprise at least one gas analyzer, (not shown), preferably of IR type, for measuring the concentration of nitrous oxide feed to the nitrous oxide decomposition reactor 9.

(10) The adsorption/desorption means 4 for selective adsorption/desorption of nitrous oxide is preferably in the form of at least one container 4 having gas inlet means and gas outlet means (not shown) and is filled with a suitable material that is able to adsorb/desorb preferably only nitrous oxide. In one embodiment, the container 4 is filled with activated carbon, preferably of the type bituminous coal based or coconut, having a surface area of about 500 to 2000 m.sup.2/g. The container 4 may be filled with other material(s) or a mixture of materials suitable for adsorption/desorption of nitrous oxide, such as zeolite(s).

(11) Since nitrous oxide has the property regarding the activated carbon that it is physically adsorbed (but not chemically) to the activated carbon/zeolite surface, this property is also reversible, i.e. the activated carbon easily release the nitrous oxide when the concentration in the gas mixture containing nitrous oxide is decreased. This means in practice that nitrous oxide is first adsorbed onto the activated carbon as long as the concentration is maintained and the activated carbon micro pores are available, i.e. they are not filled with nitrous oxide. This process is stopped when the activated carbon micro pores are filled. Furthermore, when the concentration of nitrous oxide decreases, the activated carbon will release the nitrous oxide into the environment and the concentration goes gradually down with respect to the nitrous oxide in the gas. Eventually the carbon will release all of its adsorbed nitrous oxide.

(12) This means the activated carbon/zeolite is thus available to take up new nitrous oxide since the micro pores have been emptied.

(13) Note further that the actual release procedure, i.e. the speed at which the nitrous oxide is released from the activated carbon micro pores and comes out in the gas phase again depends on the diffusion constant as well as the temperature of the activated carbon.

(14) To increase the rate of desorption of nitrous oxide the container 4 or at least the activated carbon may be heated, if necessary.

(15) Thus, the adsorption/desorption of the nitrous oxide of the inlet gas should be made in such a way that the concentration of the nitrous oxide in said gas to be treated after the adsorption/desorption means 4 should provide a self-sustaining decomposition reaction in the decomposition reactor 9. Thus, the concentration of nitrous oxide passed to the decomposition reactor 9 is leveled out since only the nitrous oxide part of the exhalation air is stored which means that during periods of time when the inlet gas is rich in nitrous oxide, the nitrous oxide will be stored in adsorption/desorption means 4, and when the inlet gas is lean in nitrous oxide, the nitrous oxide will be released from adsorption/desorption means 4. By this storage/release function of the adsorption/desorption means 4 it is possible to reduce/level out the concentration of nitrous oxide fed to the decomposition reactor 9 to avoid overheating/hot spots of the catalyst and destruction of the same. In the same way, since the nitrous oxide will be released/desorbed under quite some time when no or only a low concentration of nitrous oxide is present in the inlet gas it is not necessary to, during stand-by phases of the apparatus, pass heated gas substantially free of nitrous oxide through the nitrous oxide decomposition reactor for heating the same to the reaction temperature necessary.

(16) Below, with reference to FIG. 2, a test apparatus 1 is described for determining the function and capacity of the adsorption/desorption means 4. In the test the adsorption/desorption means 4 in the form of a container 4 was filled with activated carbon of coconut type.

(17) Equipment

(18) As seen in FIG. 2, the test apparatus 1 of essentially the same type as described above with reference to FIG. 1, was used. The test apparatus 1 was further provided with gas analyzer 14 for measuring the nitrous oxide concentration and of type IR having a measuring range of 0 to 100 000 ppm and a mass flow controller 15 for nitrous oxide for determining the amount of nitrous oxide fed into the test apparatus. The mass flow of nitrous oxide can be between 0.5-100 g/min.

(19) The adsorption/desorption means 4 comprised activated carbon of coconut type, having a surface area of about 1200 to 1500 m.sup.2/g. The activated carbon container could hold about 3 litres of activated carbon.

(20) The flow meter 6 was provided for the control of gas flow in the test apparatus 1.

(21) The temperature sensor 10 was provided in the catalyst bed and was of type thermocouple.

(22) The catalyst was a catalyst for the decomposition of nitrous oxide into N.sub.2 and O.sub.2. The catalyst consisted of about 0.1-2% palladium on an aluminium carrier.

(23) The catalyst bed could accommodate up to 2 litres catalyst material.

(24) The fan 5 was speed-controlled and the gas flow may vary between 2-6 m.sup.3/h.

(25) Pure nitrous oxide from a gas bottle 16 was mixed with oxygen to simulate exhalation air coming from a patient in receipt of pain relief and fed to the inlet arrangement 2 of the test apparatus 1 either before the adsorption/desorption means 4 (line II) or after the adsorption/desorption means 4 (line I).

(26) Experimental Data

(27) Air gas flow: 2.6 Nm.sup.3/h

(28) Supplied amount of N.sub.2O: 7 g/min

(29) Reactor temperature 400-450° C.

(30) Gas concentration N.sub.2O: 85 000 ppm

(31) Run time: 10-30 minutes

(32) In the experiments below the adsorption/desorption means 4 for the nitrous oxide is called activated carbon means.

(33) The experiments were designed to simulate real conditions in connection with the uptake of expiratory air from the patient during typical pain relieving procedures. In these cases, nitrous oxide was used for simulating pain relief of a patient. For example, in such a procedure, the patient normally inhales a gas mixture consisting of 50% nitrous oxide and 50% oxygen. A normal nitrous oxide consumption regarding this procedure is about 10-15 l/min of this gas mixture. This corresponds to about 10-15 grams of pure nitrous oxide per minute. Due to technical reasons, the maximum limit was set to 7 gram/min.

(34) A normal treatment of a patient is about 15-30 minutes.

(35) During the treatment, the patient normally receives nitrous oxide gas mixture for about 2-5 minutes via a breathing mask. After receiving the nitrous oxide gas mixture, the patient is breathing room air for about 1-2 minutes after which the patient again inhales nitrous oxide gas mixture through the mask. This whole process takes about 15-30 minutes depending on the extent of the procedure itself.

(36) In this mask, the patient breathes in and out. Upon inhalation, the patient receives the nitrous oxide gas mixture and upon exhalation, the patient breathes out the nitrous oxide gas mixture. The exhalation air is sucked up in a hose connected to the mask and further transported by a fan to e.g. a mobile nitrous oxide destruction unit where the nitrous oxide gas is destructed.

(37) It was found that during these conditions, the concentration of nitrous oxide in the inlet arrangement of the test apparatus 1 was about 50 000-150 000 ppm depending on the technical conditions.

(38) The experiments disclosed herein were performed in order to stabilize the reactor temperature and minimize the risk of hot spots in the catalyst. This is accomplished by utilizing the properties of the activated carbon to adsorb nitrous oxide, i.e. to take up nitrous oxide for later release when the concentration of nitrous oxide is reduced in the gas stream.

(39) From the conducted tests, it is concluded that the activated carbon can take up about 2 percent by weight of nitrous oxide, i.e. 1 kilo of activated carbon may adsorb about 20 grams of nitrous oxide.

(40) Results

(41) Below are the results of different experiments. Experiment 1 concerns injection of nitrous oxide gas after the activated carbon means. This means that the nitrous oxide gas does not pass through the activated carbon means. Experiment 2 concerns injection of nitrous oxide gas before the activated carbon means. In the latter case, the nitrous oxide gas is first taken up by the activated carbon means before being fed to the catalyst, i.e. the activated carbon means is acting as the adsorption/desorption means 4.

(42) The experiments 1 and 2 show that the temperature of the catalyst increases with time. This is natural since this high level of nitrous oxide also generates heat which is detectable. Note that in these experiments, no additional supply of heat was added after the nitrous oxide was introduced into the reactor.

(43) In FIG. 2 experiment 1 named “Without means” is shown by dotted line and experiment 2 named “With means” is shown by continuous line in which the nitrous oxide containing gas is fed via the activated carbon means.

(44) The experiments 1 and 2 show that the activated carbon retains nitrous oxide long enough to hold the concentration of the gas down in the catalyst and thus hold the reactor temperature in the catalyst down. The difference between experiments 1 and 2 is about 60° C. under these conditions after 12 minutes.

(45) The experiment disclosed in FIG. 3 was conducted to see the activated carbon's ability to reduce and level out the concentration of nitrous oxide in a simulated case of a normal processing procedure for pain relief. The experiment was conducted by letting the incoming nitrous oxide first pass through the activated carbon means before entering the catalyst. Nitrous oxide was fed for about 3 minutes and then turned off for about 2 minutes. The air flow through the catalyst was kept constant at about 2.6 Nm.sup.3/h all the time. Power supply to the heater was turned off after initial heating of the decomposition reactor to the necessary reaction temperature. The whole test lasted about 25 minutes.

(46) The experiment shows that when the no activated carbon means is used the concentration of nitrous oxide fed to the catalyst is 85 000 ppm for 3 minutes and then down to zero. Thereafter, the level rises back to 85 000 ppm after a break of about 2 minutes.

(47) When the activated carbon means is used the concentration of nitrous oxide is slowly rising in the catalytic reactor/apparatus.

(48) Note that the nitrous oxide is supplied in the same way as above, i.e., alternately in the range of 3 min on and 2 min off.

(49) When the activated carbon means is used, the level of nitrous oxide is kept at a comfortable level during the entire process time and the gas fed to the catalytic reactor is never free from nitrous oxide. The latter also means that the temperature of the catalytic reactor can be maintained and no additional supply of energy in the form of heated air substantially of nitrous oxide has to be fed to the reactor. The activated carbon means makes sure to maintain a high enough level of nitrous oxide throughout the treatment process to sustain an exothermic reaction.

(50) Conclusions

(51) Advantages of an activated carbon means are that the concentration of nitrous oxide rises slowly in the system and is significantly lower than without the activated carbon means. This gives a lower temperature rise in the reactor.

(52) Overall, this eliminates the possibility of hot spots in the catalyst bed.

(53) Malfunctions due to overheating have thereby significantly been reduced.

(54) Another advantage is that the process is self-sustained with respect to energy since the level of nitrous oxide is smoothed out compared to without an activated carbon means.

(55) FIG. 5 shows an experiment in which the flow of nitrous oxide was kept constant at 7 g/min up to about 16 minutes. This corresponds to a concentration of about 85 000 ppm. After this, the flow of nitrous oxide is zero. Shown by dotted line.

(56) Throughout the process, the electric heater is turned off, i.e. no heat was added.

(57) In the same figure is shows how long it takes for the activated carbon means to saturate and how long it takes for the same to be flushed when concentration of nitrous oxide is zero. Shown by continuous black line.

(58) As shown, it takes about 18-19 minutes until the concentration of nitrous oxide is down to about 5 000 ppm after the activated carbon means has been saturated and the incoming gas to the activated carbon means contains no nitrous oxide.

(59) For technical reasons the resolution of the measuring was about 3 000-2 000 ppm. For that reason, the curve does not evolve smoothly but more stepwise. In reality, the concentration of nitrous oxide from the activated carbon means reduces “smoother”, i.e. more curve like.