Apparatus for decomposition of nitrous oxide

Abstract

An apparatus for the decomposition of nitrous oxide comprising: a buffered entrainment inlet 2; a variable output inlet fan 3; a decomposition chamber 4 containing a catalyst; a heater 5; a temperatur

Description

Nitrous oxide (N2O) is widely used in healthcare as an analgesic. N2O is not metabolized by the patient and the majority of administered N2O is exhaled. Due to its persistence in the upper atmosphere, N2O has a significant impact upon climate change with a warming potential approximately 300 times that of carbon dioxide. It is desirable to prevent the release of exhaled N2O to the atmosphere and several technologies have been established. One suitable technology relies upon a heated catalyst to convert N2O into its component nitrogen and oxygen molecules, thus removing the associated climate impact. The catalytic decomposition of N2O typically occurs at an elevated temperature circa 500~600°C and is a highly exothermic reaction. For optimum operational efficiencies it is desirable to operate a catalytic system under steady state conditions of flow, temperature, etc. However, the patient respiratory cycle generates a variable flow of N2O, being essentially zero during periods of inhalation and peaking during periods of exhalation. In addition, the patient typically is allowed to self-administer a mix of N2O-oxygen as required and thus the amount and flow of nitrous oxide can be highly variable. These variations in N2O flow create challenges for any decomposition apparatus including: i) maintaining the catalyst at operational temperature with minimal energy input during periods of low or zero N2O flow; ii) avoiding overheating from the highly exothermic reaction during periods of high N2O flow and iii) rapidly switching between these states multiple times per minute as required by the breathing cycle of the patient. The apparatus and method of the present invention solves these problems as following: In a first aspect of the present invention the apparatus includes a buffered entrainment inlet to smooth the input flow of N2O into the reactor system. The patient exhalate is drawn into the apparatus by the flow of air around the outlet of the patient exhalate pipe in a hydrodynamic process commonly referred to as entrainment. There is no physicalfluid connection between the apparatus and the patient thus the flow of N2O-air into the apparatus can be different from flow of exhalate from the patient. The inlet further comprises a buffer chamber which serves as a temporary reservoir to contain an exhaled breath allowing the apparatus to draw in the exhalate at a more regularized flow. Thus, the buffered entrainment inlet can provide the apparatus with a supply of inlet N2O-air that contains a smoothed flow of N2O when compared to the patient respiratory cycle of zero N2O / zero flow during inhalation and high N2O / high flow during exhalation. In a second aspect of the present invention the reactor system and enclosure are optimized for heat retention to maintain the decomposition catalyst at operational temperature during periods of zero or low N2O input. The reactor system utilizes a multistage heat exchanger surrounded by dual zone insulation. The insulation zone closest to the reactor system comprises a fibrous insulation. The second zone utilizes a metal insulator comprising a layer of air held between two layers of low thermal conductivity metal such as stainless steel. In a third aspect of the present invention the flow of N2O-air from the buffered entrainment inlet to the reactor system is varied using a variable output inlet fan to rapidly dissipate heat generated from the exothermic decomposition of N2O during peak inlet concentrations. Increasing the flow from the inlet into the apparatus reduces the temperature rise within the reactor system by a) forcing more ambient air through the reactor system which increases the portion of the reaction heat transferred external to the apparatus; b) drawing in more ambient air dilutes the effective inlet concentration of N2O which reduces heat generated per unit volume of air processed by the catalyst. These and other features of the present invention, as well as the methods of operation and functions of the related elements, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in various figures. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. List of Figures and Reference numerals. Figure 1, an embodiment of the apparatus of the present invention Figure 2, an embodiment of a buffered entrainment inlet Figure 3, an alternative embodiment of a buffered entrainment inlet Figure 4(a-d), example embodiments of a metal insulation means Figure 5, a flow diagram of an embodiment of the method of the present invention Reference numerals: 1 apparatus, 2 buffered entrainment inlet, 3 variable output inlet fan, 4 decomposition chamber, 5 heater, 6 temperature sensor, 7 heat exchanger, 8 flow sensor, 9 insulation means, 10 controller, 11 metal insulation means, 12 outlet, 13 patient exhalate pipe, 14 positioning means, 15 retaining means, 16 detection means, 17 flow straightening means, 18 flow retarding means, 19 jet diffusing means, 20 longitudinal axis (of inlet), 21 opening (of inlet), 22 buffer chamber, 23 outlet of patient exhalate pipe, 24 inlet volume, 25 metal sheet, 26 gap, T1 enclosed volume, 28 retarding means. Referring to figures 1 to 3, a buffered entrainment inlet 2 is characterized by two main aspects. Firstly, the inlet does not have a physical fluid connection to the patient exhalate pipe 13, rather the inlet operates by drawing exhalate into the apparatus using room air in a process known as entrainment. The movement of room air along the inlet induces a concurrent movement of exhalate using hydrodynamic processes including the inducement of turbulent flux. The effectiveness of the capture of exhalate into apparatus inlet air may be enhanced by maintaining the velocity of entraining room air in the immediate proximity of the output of the patient exhalate pipe 23. This enhances the hydrodynamic processes such as sheer mixing and induced vortices. This can be done through appropriate sizing of the inlet opening relative to the opening of patient exhalate pipe and a comparison of the relative flows of exhalate and entraining room air. Capture effectiveness may be enhanced by positioning the end of the patient exhalate pipe 23 substantially within the inlet. This prevents spillage of exhalate into general room air. Capture effectiveness may be enhanced by orientating the output of the patient exhalate pipe 23 in an essentially parallel direction to the flow of entraining room air. The patient exhalate pipe is typically a flexible plastic tube of diameter approximately 30mm. The cross-section area of the opening of the inlet 21 may be between 110% and 5000% of the crosssection area of the exhalate pipe. Preferably between 125% and 3000%. The end of the patient exhalate pipe may be positioned anywhere within the inlet. It is desirable to strike a balance in the positioning of the outlet between sufficient free gap to avoid unnecessary turbulence of the exhalate exiting into an overly confined volume and sufficient buffer volume 22 to retain the exhalate as outlined below. Preferablytheend of the patient exhalate pipe is positioned between 5% and 95% of the longitudinal axis 20 of the inlet. The longitudinal axis 20 of the inlet is defined as the longest axis of the inlet. Typically, the opening of the inlet is positioned away from the inlet fan 3. The opening of the inlet 21 and fan 3 may be at essentially opposite ends of the inlet to maximise the volume between. Optionally the inlet may include a positioning means 14 which helps the user correctly position the patient exhalate pipe with the inlet. The positioning means 14 may include a guide or guides to i) locate the pipe essentially central within the inlet; ii) straighten the portion of the pipe adjacent to the pipe outlet; iii) prevent the pipe being inserted beyond a set distance into the inlet; or a combination thereof. Optionally the inlet may include a pipe detection means 16 which helps the user correctly position the patient exhalate pipe within the inlet. For example the pipe detection means 16 provides the user feedback when the pipe has been inserted an appropriate distance into the inlet. The pipe may be detected bytechnologies established withintheartincludinga light beam; ultrasonics; physical switch ora combination thereof. The user feedback may use technologies established within the art including visual, audible, haptic feedback or a combination thereof. Optionally the present invention uses at least one light emitting diode (LED) to provide visual feedback. The multiple states for visual feedback include turning on, turning off, change in intensity, change in colour of the at least one LED and combinations thereof. Optionally user feedback is provided using a graphical user interface such as a touchscreen. Optionally the apparatus may include a retaining means 15 which reversibly holds the patient exhalate pipe in an essentially fixed position during use. The retaining means 15 may use technologies established within the art including a clamping mechanism, elastic strap or combinations thereof. Secondly, the inlet includes a buffer chamber 22 which serves as a reservoir to retain peak exhalation flow from the patient. The buffer chamber may be a distinct region within the overall inlet structure. The buffer chamber may be an integral part of the overall inlet structure. The volume of the buffer chamber may be defined as the portion of the total inlet volume contained within the distance from the outlet of the patient exhalate pipe 23 to the opening of the inlet 21. At peak exhalation rate the air inside the buffer chamber 22 is displaced by the exhalate, the inlet fan 3 continuously draws the exhalate from the buffer chamber mixed with ambient air into the apparatus, thus variations in the flow of exhalate can be dampened. Preferably the volume of the buffer chamber 22 is sufficient to retain all of the patient exhalate for a sufficient time period to allow the inlet fan to draw all exhalate into the apparatus. Thus, a suitable volume can be calculated using a combination of the inlet flow rate; the anticipated maximum patient expiratory volumes and breathing rate. Preferably the buffer volume 22 is in the range between 0.1 L and 30L. To enhance the retention of the bolus of exhaled air within the inlet and prevent spillage of exhalate out of the apparatus, the inlet may optionally include additional elements including: i) a flow straightening means 17; ii) a flow retarding means 18; iii) a jet diffusing means 19 or a combination thereof. A flow straightening means 17 may take the form of elongated elements essentially parallel to the desired direction of the flow. A flow straightening means 17 may take the form of a cellular structure such as honeycomb with the open axis essentially parallel to the desired direction of the flow. A flow retarding means 18 may take the form of a baffle or series of baffles. A flow retarding means 18 may have 5% to 50% open area relative to its total cross section. A flow retarding means 18 may be positioned essentially perpendicular to the desired direction of flow. A jet diffusing means 19 may take the form of a series of angled deflectors. A jet diffusing means 19 may take the form of concentric circles of deflectors. A jet diffusing means 19 may function to mix a flow of air with surrounding air. A jet diffusing means may function to reduce the velocity of a flow of air. A variable output inlet fan 3 is a means to move air into the apparatus. Typically, a centrifugal blower can be used, however other designs of fan may be suitable. Fan output may be altered by varying the speed of the rotor, angle of the vanes, sizing of diaphragmsand combinations thereof. Typically, rotor speed can be altered using a variation in input voltage, current or pulse width modulation (PWM). The decomposition chamber 4 is a section of the apparatus where the action of heat upon the nitrous oxide molecule converts it into the component oxygen and nitrogen molecules in an exothermic reaction. To lower the temperature required for conversion the decomposition chamber typically contains a catalyst. There are several examples of suitable catalyst provided in appropriate literature. Preferably the catalyst contains a transition metal or its salt. Typical metal salts include oxides and hydroxides although others may be used to generate the catalyst such as nitrates, sulphates, carbonates and hydrocarbonates. Several transition metals may be used, including copper, nickel, cobalt and palladium. The heater 5 uses electrical heating including resistive and inductive heating. Heating may be applied within the air path or external to the pipework or both. Resistive heaters comprising spiral wound Nichrome wire are particularly suitable for use within the air path. Ceramic heaters are particularly suitable for use external to the heated pipework. To maintain the catalyst at operational temperatures and prevent unnecessary loss of heat from the apparatus a heat exchanger 7 is desirable. The heat exchanger 7 can be parallel plate or shell and tube format ora combination of both. Tooptimize heat recovery within a compact apparatus volume more than one heat exchanger may be used in series. It is preferable to surround the heated components within the apparatus with an insulation means 9 to reduce heat loss. Insulation may be solid, fibrous, particulate or combinations thereof in nature. A mineral or ceramic fiber-based insulation may be suitable at operating temperatures. Preferably, the insulation means 9 includes a metal insulating means 11. Referring to figure 4, a metal insulation means 11 comprises at least two essentially parallel sheets of metal 25 of thickness in the range 0.1 mm to 5mm separated by a gap 26 of 0.5mm to 25mm. Optionally at least one of the metal sheets of the metal insulation means is comprised of a metal with a thermal conductivity <25 W / mK. Various grades of stainless steel may be suitable. Optionally two or more of the metal sheets of the metal insulation means are joined to form an essentially enclosed volume 27 within. The join may be formed by bending one or both of the essentially parallel metal sheets to close the gap. The join may use additional material to form the joint and may include welding. Optionally the metal insulation means 11 further comprises at least one retarding means 28 within the gap to restrict the movement of air molecules and thereby reduce heat transfer. Optionally the retarding means 28 is an aerogel or other material where the mean free path of the air molecules is less than the average pore size of the surrounding material. Such materials are known to greatly reduce thermal conductivity as they benefit from the Knudsen effect. A flow sensor 8 is any system capable of detecting the movement of air through the apparatus and providing electrical communication to the controller. Flow sensor 8 can be interpreted to mean a single sensor or multiple sensors. A flow sensor can measure airflow using a rotating vane anemometer, a moving vane meter, a hot-wire detector, a Karman vortex sensor, an electromechanical membrane sensor, MEMS technology or combinations thereof. The apparatus 1 typically uses additional electronic components well established in the art including temperature sensors 6, power supplies, microcontrollers 10 and a touchscreen user interface. Figure 5 shows a flow diagram of an embodiment of the method of the present invention. The operator positions the patient exhalate pipe 13 at the desired location within the buffered entrainment inlet 2. It is preferable to locate the outlet of the patient exhalate pipe 23 from 5% to 95% of the longitudinal axis 20 of the inlet 2. The apparatus is brought up to operational temperature via the heater 5. Once operational, the controller 10 varies the inlet fan 3 to control thetemperatureof the catalyst within the decomposition chamber 4 between 90% to 110% of the decomposition temperature of nitrous oxide in the presence of the catalyst. Optionally the controller maintains the temperature of the air exiting the apparatus via the outlet 12 to within 20°C of the inlet air temperature. Example embodiment. The outer cover of the apparatus is comprised of cut, folded and welded stainless steel sheet metal and of nominal dimensions 1000mm (h) x 500mm x 500mm sitting on 4 castors. The microcontroller is a Unitronics Samba PLC with integrated 3.5” touchscreen HMI. The inlet fan is Sanyo Denki San Ace B127 9BJ Series Blower. DC Power supplies, DIN rail connections, thermocouples, flow sensors and other electronic componentry are supplied from RS. The inlet is constructed from stainless steel and comprises a cylindrical pipe of diameter 120mm and length 500mm with approximate internal volume of 5.6litres. One end of the cylindrical inlet is closed and the inlet fan is positioned at this end. Centrally positioned within this outer pipe is a cylindrical pipe of diameter 40mm x 400mm which acts as the positioning means for the patient exhalate pipe. The central pipe terminates 100mm from the closed end of the inlet with a jet diffuser comprising three concentric angled discs. Three perforated metal discs spaced approximately equidistant support the central pipe within the outer pipe. The perforated metal discs have a series of 10mm holes to provide an open area approximately 10% of the total disc open cross section area; these act a baffle plates to retard the back flow of exhalate. At the open end of the cylinder there is a flow straightener of length 50mm comprised of a honeycomb of open cell hexagonals of side length 20mm. Positioned 125mm from the closed end of the inlet cylinder is a photoelectric barrel sensor supplied by Sick to detect the presence of the patient exhalate pipe. The decomposition chamber is comprised of a stainless steel pipe of diameter 150mm and wall thickness 4mm. The chamber is heated externally by three curved ceramic 250W heaters positioned equidistant around the circumference. Inside the chamber is 300g of a Nickel-Cobalt-Alumina catalyst as set out in Meloni, E. etal. Development of Innovative Structured Catalysts for the Catalytic Decomposition of N2O at Low Temperatures. Catalysts 2022, 12, 1405. The heat exchanger is a stainless steel shell and tube design with shell diameter 120mm, tube diameter 10mm and length 850mm. All heated pipework is constructed from stainless steel and insulated with 100mm of ProRox LF 970 technical insulation from Rockwool. The surface of the insulated heated pipework is separated from the main apparatus enclosure by a metal insulation means comprising two layers of 0.9mm stainless steel sheet held parallel with a 8mm gap between. The two sheets are deflected at the edges and welded to form an enclosed space therein. Between the metal sheets there is a retarding means comprising 50mm strips of ProRox insulation at 50mm intervals. Although the invention has been described in detail for the purpose of illustration based on what is considered to be the most practical and preferred embodiment, it is to be understood that such detail is solely for that purpose and that the invention is not limited to the disclosed embodiments, but, on the contrary, it is intended to cover modifications and equivalent arrangements that are within the scope of the appended claims. For example, it is to be understood that the present invention contemplates that, to the extent possible, one or more features of any embodiment can be combined with one or more features of any other embodiment.

Claims

258Claims:

1. An apparatus for the decomposition of nitrous oxide comprising: a buffered entrainment inlet, a variable output inlet fan; a decomposition chamber containing a catalyst; a heater; a temperature sensor; a heat exchanger; a flow sensor; an insulation means; and a controller characterized in that:a. the volume of the buffered entrainment inlet is in the range of 0.1 litres to 30 litres;b. the cross section area of the opening of the buffered entrainment inlet is in the range of 780mm2 to 35,340mm2.

2. An apparatus according to claim one further comprising a positioning means to locate the patient exhalate pipe within the buffered entrainment inlet to a distance of 5% to 95% of the longitudinal axis.

3. An apparatus according to any preceding claim wherein the insulation means includes a metal insulation means comprising at least two essentially parallel sheets of metal of thickness in the range 0.1mm to 5mm separated by a gap of 0.5mm to 25mm.

4. An apparatus according to any preceding claim wherein at least one of the metal sheets of the metal insulation means has a thermal conductivity less than 25 W / mK.

5. An apparatus according to any preceding claim wherein at least one of the metal sheets of the metal insulation means is comprised of stainless steel.

6. An apparatus according to any preceding claim wherein two or more of the metal sheets of the metal insulation means are joined to form an essentially enclosed volume within.

7. An apparatus according to any preceding claim wherein the metal insulation means further comprises at least one retarding means within the gap to restrict the movement of air molecules.

8. An apparatus according to claim 7 wherein the retarding means is an aerogel.

9. An apparatus according to any preceding claim further comprising a retaining means to removably hold the patient exhalate pipe within the buffered entrainment inlet.

10. An apparatus according to any preceding claim wherein the buffered entrainment inlet further comprises a patient exhalate pipe detection means.

11. An apparatus according to any preceding claim wherein the buffered entrainment inlet comprises at least one of the following: a flow straightening means; a flow retarding means; a jet diffusing means.

12. An apparatus according to any preceding claim wherein the catalyst is comprised of between 0.01% and 10% of at least one of the following: a transition metal; a transition metal oxide; a transition metal hydroxide.

13. A method of decomposing nitrous oxide by using an apparatus according to any of the preceding claims comprising the steps of:a. positioning the outlet of a patient exhalate pipe within a buffered entrainment inlet to a distance between 5% to 95% of the longitudinal axis;b. heating a catalyst to operational temperature;c. varying the inlet airflow to maintain the catalyst within 90% and 110% of the decomposition temperature of nitrous oxide in the presence of the catalyst.

14. A method of decomposing nitrous oxide according to claim 13 further comprising the steps of:d. maintaining the temperature difference between the inlet air temperature and outlet air temperature to less than 20°C.

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