Nitrogen generator

Abstract

The invention relates to a nitrogen generator comprising a housing having two ends, an ignition device at one end of the housing and a gas outflow opening at the other end of the housing, a volume of filter at the outflow opening, a volume of solid propellant comprising sodium azide, a binder, a coolant and 1 to 10 wt% of iron (III) oxide. Between the ignition device and the volume of solid propellant there is an active layer. The active layer comprises 60 to 90 wt% of sodium azide, 1 to 15 wt% of a binder, 0.1 to 10 wt% of a coolant and 5 to 30 wt% of iron (III) oxide. The content of iron (III) oxide in the active layer is higher than the content of iron (III) oxide in the solid propellant.

Description

Technical Field

[0001] This invention relates to a nitrogen generator comprising a housing having two ends, an ignition device at one end of the housing and a gas outlet at the other end of the housing, a filter at the outlet, and a volume of solid propellant comprising sodium azide, a binder, a coolant, and iron(III) oxide present between the ignition device and the filter. Nitrogen gas is produced when the solid propellant burns. Background Technology

[0002] Such a gas generator is known from applicant WO2014 / 073370. This publication describes a nitrogen generator housed in a tubular casing. An ignition device, which can be a conventional pyrotechnic ignition device, is present at one end. Between these ignition devices and a filter (which can be a sand filter), a composition consisting of sodium azide, potassium silicate, iron(III) oxide, and lithium fluoride serves as a solid propellant or nitrogen-generating composition. When ignited at one end of the nitrogen-generating composition, nitrogen gas is formed, which flows through the unburned nitrogen-generating composition and the sand filter to an outlet opening. The relatively cool nitrogen gas is then released into the environment through said outlet opening. Such a nitrogen generator is advantageously used to protect electrical equipment, such as computer servers, from fire. This is because the relatively cool nitrogen gas will not damage other equipment or electrical components of the server near the fire.

[0003] US4203787 describes a sodium azide-based solid propellant, also known as a nitrogen gas generating composition. According to this disclosure, successful initiation of combustion of the sodium azide composition requires a sufficient amount of initiator to ensure that the thermal combustion products of the initiator come into contact with enough exposed sodium azide composition to ignite the self-sustaining flame leading edge. This disclosure describes a boron-potassium nitrate-lead azide initiator.

[0004] US4817828 describes a nitrogen generator comprising 61 wt% to 68 wt% sodium azide, 0 wt% to 5 wt% sodium nitrate, 0 wt% to 5 wt% bentonite, 23 wt% to 28 wt% iron oxide, 2 wt% to 6 wt% graphite fiber, and 1 wt% to 2 wt% fumed silica particles. The pyrotechnic ignition device is preferably composed of boron-potassium nitrate because it minimizes peak pressure, thereby preventing particle damage.

[0005] EP0619284 describes a nitrogen generator. The solid propellant consists of gamma-iron oxide and sodium azide in approximately stoichiometric ratios. More preferably, the composition contains between about 2 wt% and 40 wt% gamma-iron oxide and between about 71 wt% and 60 wt% sodium azide. A preferred ignition device may be the conventional ignition device described in US4902036. This ignition device includes a squib for igniting a booster package containing boron-potassium nitrate in an enclosed space. An electrical conductor delivers current to the squib, thereby igniting the squib and the booster package. The ignition of the rapidly combustible material provides the threshold energy required to ignite the nitrogen generating composition. Summary of the Invention

[0006] A problem with existing nitrogen generators is their operational reliability. Nitrogen generators, especially those installed for fire suppression, ideally should not fail upon ignition. However, in practice, a small percentage of installed nitrogen generators may fail to produce the optimal amount of nitrogen. This invention relates to a nitrogen generator that does not fail upon ignition or at least always produces a minimum amount of nitrogen.

[0007] This is achieved using a nitrogen generator. A nitrogen generator includes:

[0008] It has a housing at both ends, an ignition device at one end of the housing, and a gas outlet at the other end of the housing.

[0009] The volume of a filter at the outlet opening.

[0010] A volume of solid propellant comprising 70 wt% to 90 wt% sodium azide, 1 wt% to 15 wt% binder, 0.1 wt% to 20 wt% coolant, and 1 wt% to 10 wt% iron(III) oxide, present between the ignition device and the volume.

[0011] An active layer exists between the ignition device and the volume of solid propellant, and the active layer comprises 60 wt% to 90 wt% sodium azide, 1 wt% to 15 wt% binder, 0.1 wt% to 10 wt% coolant and 5 wt% to 30 wt% iron(III) oxide, wherein the content of iron(III) oxide in the active layer is higher than the content of iron(III) oxide in the solid propellant.

[0012] The applicant has now discovered that a more reliable nitrogen generator is obtained when an active layer comprising a relatively high iron oxide content is used. Furthermore, this results in faster nitrogen production. A further advantage is that an active layer comprising sodium azide will also produce nitrogen, and thus increase the volume of nitrogen that can be produced by a single nitrogen generator according to the invention.

[0013] This invention is particularly applicable to gas generators with an ignition device comprising an ignition tube and an enhancer package. Suitably, the enhancer package comprises boron-potassium nitrate (KBNO3). It has been found that propagation after startup is enhanced when an active layer is present according to the invention. This enhancement has been shown to improve reliability with the same amount of KBNO3, and even with a reduction in the amount of KBNO3. The nitrogen generator according to the invention can release nitrogen faster than nitrogen generators without an active layer and with a higher content of boron-potassium nitrate. This feature is relevant in applications requiring a rapid response to threats. The need to use less boron-potassium nitrate is advantageous because it improves safety during production and enhances product safety. A further advantage is that using less boron-potassium nitrate increases the purity of the released nitrogen.

[0014] One problem with the aforementioned systems involving ignition tubes and enhancer packages is that special safety measures must be taken during the assembly of the nitrogen generator due to its explosive nature. Another disadvantage is that a special compartment must exist within the nitrogen generator for the ignition tube and enhancer package. For this reason, it is preferable that the ignition device includes an electrically heated initiator, such as a glow plug. An electrically heated initiator is advantageous because it eliminates the need for a combination of the ignition tube and the boron-potassium nitrate enhancer package. This simplifies and enhances the safety of nitrogen generator assembly. Furthermore, the elimination of a special compartment for the boron-potassium nitrate, including the enhancer package, simplifies the design. An additional advantage of not needing to use boron-potassium nitrate is that, when using the nitrogen generator, no or significantly less nitrogen oxides are formed. A further advantage is that the electrically heated initiator does not cause gas surges or shock waves within the nitrogen generator that could damage it. Such gas surges or shock waves can occur when the ignition tube and enhancer package are used as the ignition device. Another advantage is that the combination of the electrically heated initiator and active layer provides a more reliable nitrogen generator for operation at lower ambient temperatures, such as -25°C.

[0015] An electrically heated initiator can be a glow plug, such as those known for self-ignition in diesel engines. One glow plug is sufficient for many applications. For high-reliability applications, such as aerospace applications, having two glow plugs to achieve a dual (and therefore redundant) system can be advantageous. Glow plugs are typically designed to be temporarily energized for short periods, preferably by resistance heating, to a preselected temperature in the range of 200°C to 1200°C. A glow plug preferably includes a heating element assembly. This heating element assembly includes an advantageously integral sheath and a heating element. The sheath has a relatively thin, typically annular wall defining a blind orifice, in which the heating element is positioned and adapted to dissipate heat, and the heating element assembly preferably includes heat transfer means adapted to transfer heat from the heating element to the sheath.

[0016] A glow plug may have a conventional heating element, preferably at or near its tip, which comprises a metal coil or wire enclosed in a heat-resistant metal or ceramic sheath.

[0017] The heating element preferably comprises at least one or more metal wires or coils, which preferably have high resistance such that at least one of the one or more metal wires or coils heats rapidly when current passes through it, thereby rapidly heating the sheath surrounding it. The heating element may further include an insulator to protect the heating element from direct contact with the sheath. This insulator can be formed of any suitable material, preferably a ceramic material.

[0018] One or more heating wires or heating coils may preferably be protected from direct contact with the sheath by being encapsulated in an insulator, thereby the heating assembly including the heating wires or heating coils and the insulator is encapsulated in the sheath.

[0019] The sheath can be formed from a preselected material chosen and configured to minimize failures of the heating element assembly due to thermal stress, oxidation, and / or corrosion, and to avoid any stability or performance problems caused by its proximity to the solid propellant. In particular, the sheath should not contain any heavy metals such as copper, lead, iron, nickel, silver, and mercury, which may evaporate or otherwise migrate, forming explosive and / or toxic heavy metal azides or other undesirable compounds in the gas generator. The advantage of this is that conventional metal coils or wires incorporating heavy metal alloys can be used, which would otherwise be unsuitable due to the risk of direct contact with the propellant charge.

[0020] The heating element of the glow plug has a metal coil or wire connected to a power source. A preferred power source can provide a voltage between 10 and 30 volts and a current between 5 and 20 amperes to the glow plug for at least 5-20 seconds, causing the glow plug sheath to rapidly heat to a steady-state operating temperature of at least 200°C, more preferably at least 250°C, more preferably at least 500°C, even more preferably at least 750°C, and preferably between 900°C and 1200°C, and even more preferably between 900°C and 1000°C. A particularly preferred power source is a battery that can provide at least 12 volts and at least 10 amperes to the glow plug for at least 10 seconds and can be attached to a gas generator. The actual temperature to be achieved is preferably at or above the self-decomposition temperature of the active layer. The temperature should be sufficient to ignite the active layer.

[0021] When the metal coil of the glow plug's heating element is energized, the coil heats up due to its resistance, causing its sheath to heat up until it glows. By applying only partial power to the glow plug, its sheath can be heated significantly below its operating temperature, and in this case, used to heat the solid propellant surrounding the generator, thus preparing the propellant for ignition. When full electrical power is supplied to the coil of the glow plug's heating element, its sheath will begin to glow, significantly heating the solid propellant surrounding the generator. When portions of the active layer surrounding the glow plug's glowing sheath reach their decomposition temperature, these portions of the active layer will begin to ignite, causing other portions of the active layer further away from the glow plug's sheath to burn.

[0022] Preferably, the active layer portion, particularly the sodium azide propellant of said layer, can be activated within 10 seconds, more preferably within 5 seconds, after the coil of the heating element of the glow plug has been fully electrically powered. After the coil of the heating element of the glow plug has been fully electrically powered, the solid propellant layer itself can be activated within 20 to 90 seconds, more preferably within 30 to 60 seconds.

[0023] Preferably, once the glow plug is started, it reaches the desired temperature in less than 5 seconds, more preferably less than 3 seconds, and even more preferably less than 2 seconds. The amount of electricity that must be supplied to the glow plug according to the invention to initiate the decomposition of the active layer will depend on the composition and physical appearance of the layer. Suitably, at least 30 watts, preferably about 50 to 100 watts, should generally be sufficient to initiate the controlled, self-sustaining decomposition of the active layer.

[0024] The heating element of the glow plug is preferably positioned at the center of the gas generator housing. The heating element of the glow plug is preferably encapsulated by an active layer. More specifically, the sheath surrounding one or more metal wires or coils with high resistance is at least partially encapsulated by the active layer. This allows for the simple use of commercially available glow plugs.

[0025] Solid propellants and active layers may include binders and coolants as known in the art and described in the applicant’s previously mentioned WO2014 / 073370.

[0026] The adhesives included in the solid propellant and the active layer can be the same or different, preferably the same. The adhesive can be polytetraazole, preferably an alkaline non-organic adhesive material, more preferably an alkali metal silicate, such as potassium silicate (K₂SiO₃). Preferably, the adhesive is potassium silicate (K₂SiO₃) for both the solid propellant and the active layer.

[0027] The coolant included in the solid propellant and the active layer can be the same or different. A preferred coolant is an inorganic salt with a heat capacity of at least 1400 J / K / kg, determined at 600 K, to provide sufficient cooling. The coolant also functions as a slag modifier, which helps retain the slag in place after the gas generator has been running. The heat capacity of the coolant is preferably at least 1900 J / K / kg. The coolant should be inert so that it does not decompose at the reaction temperature at which the gas is generated or react with other components in the generator. The coolant is preferably one or more compounds selected from LiF, Li3N3, Li2SO4, and Li2SiO3, or NaCl, NaF, KF, CaF2, Li2B2O4, and Li2B4O7. Given the excellent combined performance of lithium compounds as slag modifiers, lithium compounds are preferred, and LiF is most preferred.

[0028] The solid propellant comprises 1 wt% to 10 wt% iron(III) oxide, preferably 1 wt% to 5 wt% iron(III) oxide, and even more preferably 1 wt% to 4 wt% iron(III) oxide.

[0029] Solid propellant is suitably present in the form of extrusions, or more preferably in the form of tablets. The extrusions or tablets preferably have uniform size and shape. This ensures that these extrusions or tablets will have uniform packing with well-defined void spaces. The presence of void spaces is preferred because it provides a flow path for nitrogen, which is initially generated by the ignited active layer and subsequently by the solid propellant, and flows through these void spaces between the unreacted solid propellant extrusions or tablets to an outflow opening at the other end of the casing. Preferably, the solid propellant extrusions or tablets are present in a volume of solid propellant having a volume of 25 mm. 3 Up to 1000mm 3 Between, more preferably within 50mm 3 Up to 500mm 3 Between. The void space between extrudates or tablets in the solid propellant volume is suitably between 20% and 75% (vol / vol) of the total volume occupied by the solid propellant in the casing, more preferably between 40% and 60% (vol / vol).

[0030] The tablets are preferably bound together. This can be achieved by adding an interparticle binder (such as an aqueous solution of K2SiO3, preferably with a K2SiO3 content between 10 wt% and 30 wt%) to the solid propellant tablets when the tablets are placed inside the housing of the nitrogen generator. Bounding the tablets together is advantageous because it results in improved packaging that can maintain its structure over time (i.e., retain the relevant void space) and avoid the formation of packaging defects such as short cuts and dead zones.

[0031] The active layer can exist as a uniform powder layer or as stacked particles, such as extrusions or tablets. For example, the active layer can be a cylindrical tablet with a diameter just large enough to allow it to be placed inside a tubular shell.

[0032] Preferably, the active layer exists in the form of a uniform powder layer. Even more preferably, this uniform powder active layer is preferably combined with a volume of solid propellant, which exists in tablet form. A very preferred embodiment is the combination of this uniform powder active layer with a volume of solid propellant in tablet form, wherein the ignition device includes a glow plug. Even more preferably, a channel exists through the active layer, which fluidly connects the volume of solid propellant to the side of the active layer where the ignition device also exists. Such a channel may suitably have a maximum cross-sectional dimension, such as diameter, between 5 mm and 20 mm.

[0033] The active layer comprises 60 wt% to 90 wt% sodium azide, 1 wt% to 15 wt% binder, 0.1 wt% to 10 wt% coolant, and 5 wt% to 30 wt% iron(III) oxide.

[0034] The volume ratio of the active layer to the solid propellant is between 5:95 and 30:70 (vol / vol). These volumes include the space within the shell occupied by the total active layer and total solid propellant, including any void spaces, not the space occupied by individual tablets.

[0035] The filter can be any intermediate material that allows nitrogen gas to pass through and has a heat capacity to reduce the temperature of the nitrogen gas. Suitable filters are described in the previously cited WO2014 / 073370 and can be activated carbon, sand, zeolite, or metal. The volume ratio of the filter to the solid propellant is between 20:80 and 60:40 (vol / vol), more preferably between 30:70 and 60:40 (vol / vol).

[0036] The shell can have any shape. Preferably, the shell is elongated, allowing the combustion front of the solid propellant to move from one end to the end including the outflow opening. The cross-section can have any shape. The preferred shape is tubular, as they provide optimal strength per mass of the shell.

[0037] The present invention will be illustrated by the following non-limiting examples. Detailed Implementation

[0038] Example 1

[0039] The tube comprises a housing with two ends, an ignition device at one end of the housing, and a gas outlet at the other end of the housing. The tube is filled with a layer of solid propellant LE tablets having the composition described in Table 1. The volume of the solid propellant LE tablets is 117 mm³. 3 / sheet. The void space in this layer is approximately 49% (vol / vol). The length of this layer is 168 mm.

[0040] Between the solid propellant layer and the gas outlet is a 90 mm layer of sand. On its opposite side, there is a 12 mm active layer of TL powder with the composition shown in Table 1. Within this layer, there is a small axial channel with a diameter of approximately 12 mm connecting the ignition device and the solid propellant LE tablet layer. In the active layer, the ignition tube and the enhancer package containing KBNO3 particles serve as the ignition device.

[0041] Table 1

[0042]

[0043] Before igniting the active layer, the tube was kept at ambient temperature for several hours, and the pressure, temperature, and time of nitrogen discharge from the gas generator were measured. The time to the highest pressure at the gas outlet was measured. The times to discharge 75 wt% (T75) and 95 wt% (T95) of the theoretically possible amount of nitrogen were measured. The results are shown in Table 2.

[0044] Example 2

[0045] Repeat Example 1, except that HE tablets were used instead of LE tablets. The results are shown in Table 2.

[0046] Example 3

[0047] Example 2 was repeated, except that there were no axial channels in the TL powder layer. The results are shown in Table 2.

[0048] Comparative Experiment A

[0049] Example 2 was repeated, except that the active layer of TL powder was absent. An ignition tube and a booster pack containing KBNO3 particles were used as the ignition device for direct contact with the solid propellant HE tablet. The results are shown in Table 2.

[0050] Table 2

[0051]

[0052] The results in Table 2 show that when an active layer is present, more nitrogen is generated and emitted in a shorter time compared to when such an active layer is not present.

[0053] Example 4

[0054] Example 2 was repeated three times under ambient conditions, and the results are shown in Table 3 (Examples 4a, 4b, and 4c). In this table, conversion is also expressed as wt%. Conversion is the percentage of nitrogen produced compared to the theoretically maximum possible nitrogen that can be produced from available NaN3 in HE tablets and TL powder.

[0055] Comparative Experiment B

[0056] Experiment A was repeated three times. In all experiments, propagation failed, resulting in very low conversion rates. The results are shown in Table 3, B1, B2, B3, and B4.

[0057] Table 3

[0058] 4a yes 5.4 7,6 99.6％ 4b yes 21.8 30,6 97.3％ 4c yes 16.6 20,9 99.3％ B1 no Dissemination failure Dissemination failure 2.4％ B2 no Dissemination failure Dissemination failure 1.4％ B3 no Dissemination failure Dissemination failure 1.9％ B4 no Dissemination failure Dissemination failure >99.9％

[0059] The results in Table 3 show that the hydrogen generator with the active layer has better repeatability and reliability compared to the results obtained from the nitrogen generator without this active layer.

[0060] Example 5

[0061] Repeat Example 1, except that the tube was kept at -25°C for several hours before the active layer was ignited. The results are shown in Table 4.

[0062] Comparative Experiment C

[0063] Experiment A was repeated, except that the tube was kept at -25°C for several hours before ignition. The results are shown in Table 4.

[0064] Example 6

[0065] The experiment was repeated as shown in Figure 1, except that the tube was kept at 65°C (+65°C) for several hours before the active layer was ignited. The results are shown in Table 4.

[0066] Comparative Experiment D

[0067] Experiment A was repeated, except that the tube was kept at 65°C (+65°C) for several hours before ignition. The results are shown in Table 4.

[0068] Table 4

[0069]

[0070]

[0071] Comparison of the results of Example 5 and Experiment C with those of Example 6 and Experiment D shows that the presence of the active layer leads to faster nitrogen production at extreme low and high temperatures, as indicated by the lower T75 and T95.

[0072] All generators in Examples 4-6 and Experiment BD were allowed to be cooled for operation under controlled conditions. Gas generators with active layers were observed to exhibit better conversion rates radially, which was thought to contribute to the desired more reliable propagation in the axial direction.

[0073] Comparative Experiment E

[0074] A small cold gas generator with a 58-gram N2 production capacity was started using a standard glow plug. This small cold gas generator consisted of HE tablets (Table 1) and a porosity similar to the previous example. The decomposition reaction started 20 seconds after the glow plug was activated and reached complete decomposition 10 seconds after startup (or 30 seconds after the glow plug was activated).

[0075] Comparative Experiment F

[0076] The second small cold gas generator was started using a fast high-T glow plug, similar to the apparatus used in Experiment E. The decomposition reaction was now initiated 7 seconds after the glow plug was activated, and complete decomposition was achieved again 10 seconds after activation (or 17 seconds after glow plug activation).

[0077] Example 7

[0078] In a small generator similar to the apparatus used in Experiment F, 10 wt% of HE tablet particles were replaced with a high-energy top layer composed of TL powder (Table 1). The decomposition reaction now initiates 3 seconds after the glow plug is activated and reaches complete decomposition 4 seconds after activation (or 7 seconds after glow plug activation). Example 7 shows that the presence of the active layer results in more than twice the rapid initiation and complete decomposition compared to a gas generator initiated by a glow plug and without such an active layer.

Claims

1. A nitrogen generator, comprising: It has a housing at both ends, an ignition device at one end of the housing, and a gas outlet at the other end of the housing. A filter with a volume ratio of 20:80 to 60:40 (vol / vol) at the outlet opening. A solid propellant of volume comprising 70 wt% to 90 wt% sodium azide, 1 wt% to 15 wt% binder, 0.1 wt% to 20 wt% coolant, and 1 wt% to 10 wt% iron(III) oxide, wherein the solid propellant is present between the ignition device and the volume filter. 3 and 500mm 3 The solid propellant exists in tablet form, and the void space between the tablets in the solid propellant volume is between 40% and 60% (vol / vol) of the total volume occupied by the solid propellant in the casing. An active layer exists between the ignition device and the volume of solid propellant, and the active layer comprises 60 wt% to 90 wt% sodium azide, 1 wt% to 15 wt% binder, 0.1 wt% to 10 wt% coolant, and 5 wt% to 30 wt% iron(III) oxide, wherein the content of iron(III) oxide in the active layer is at least twice the content of iron(III) oxide in the solid propellant. There is a channel through the active layer that fluidly connects the volume of solid propellant to the side of the active layer where the ignition device is located.

2. The nitrogen generator according to claim 1, wherein, The solid propellant comprises 1 wt% to 4 wt% iron(III) oxide.

3. The nitrogen generator according to claim 1, wherein, The content of iron(III) oxide in the active layer is at least three times that in the solid propellant.

4. The nitrogen generator according to claim 1, wherein the ignition device comprises an ignition tube and an enhancer package.

5. The nitrogen generator according to claim 4, wherein the enhancer includes KBNO3.

6. The nitrogen generator according to claim 1, wherein the ignition device comprises a glow plug.

7. The nitrogen generator according to claim 6, wherein, The glow plug has a heating element that is encapsulated by the active layer.

8. The nitrogen generator according to claim 1, wherein the solid propellant and / or the binder in the active layer comprises a non-organic binder material.

9. The nitrogen generator according to claim 8, wherein the solid propellant and / or the binder in the active layer comprises an alkaline non-organic binder material.

10. The nitrogen generator according to claim 9, wherein the binder is potassium silicate (K2SiO3).

11. The nitrogen generator according to claim 1, wherein the coolant is LiF.

12. The nitrogen generator according to claim 1, wherein the filter is a sand filter.

13. The nitrogen generator according to claim 1, wherein the tablets are bonded together.

14. The nitrogen generator according to claim 1, wherein, The active layer is a uniform powder layer, and the ignition device includes a glow plug.

15. The nitrogen generator according to claim 1, wherein the volume ratio of the active layer to the solid propellant is between 5:95 and 30:70 (vol / vol).

16. The nitrogen generator according to claim 1, wherein the volume ratio of the filter to the solid propellant is between 30:70 and 60:40 (vol / vol).

17. The nitrogen generator according to claim 1, wherein, The shell is a tubular shell.

18. The nitrogen generator according to claim 1, wherein, The channel has a maximum cross-sectional size between 5mm and 20mm.

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