Method and Fuel Composition for Catalytic Heater |
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| 申请号 | US13288516 | 申请日 | 2011-11-03 | 公开(公告)号 | US20120282556A1 | 公开(公告)日 | 2012-11-08 |
| 申请人 | Leighta Maureen Johnson; | 发明人 | Leighta Maureen Johnson; | ||||
| 摘要 | A method and fuel composition for pre-heating and/or operating a catalytic heater is described herein. In one aspect, there is provided a method of preheating a catalyst in a catalytic heater comprising the steps of: providing a preheat composition comprising greater than 60% by volume hydrogen and at least one fluid component selected from natural gas, propane, butane, refinery off gas, ethylene off gas, methanol, ethanol, butanol, liquefied biogas and mixtures thereof; and contacting the catalyst with the preheat composition for a period of time sufficient to raise the temperature of the catalyst to a one or more temperatures. In this or other embodiments, there is a fuel composition that comprises at least one component selected from natural gas, propane, butane and mixtures thereof and optionally hydrogen. | ||||||
| 权利要求 | What is claimed is: |
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| 说明书全文 | This patent application claims the benefit of provisional patent application U.S. Ser. No. 61,415,124, entitled “Method and Fuel Composition for Catalytic Heater” filed Nov. 18, 2010 and provisional patent application U.S. Ser. No. 61/422,780, entitled “Method and Fuel Composition for Catalytic Heater” filed Dec. 14, 2010, the disclosures of which are incorporated herein by reference. Described herein is a method and composition for pre-heating and/or heating a catalytic heater. Catalytic heaters are well suited for hazardous or other environments. These heaters use infrared heat to provide a uniform, low intensity, non-glowing heat. The catalytic heater produces infrared energy in the form of a long to medium wavelength energy which allows it to be readily absorbed by a wide range of materials. In a catalytic heater, infrared heat is produced when a gaseous fuel is brought into contact with a catalyst in the presence of air containing a normal level of oxygen. The gaseous fuels that are used to contact the catalyst include, but are not limited to, natural gas, propane, butane, and mixtures thereof. The chemical reaction, of oxidizing or burning the gaseous fuel is slowed to about 65% of normal resulting in a safe, flameless source of radiant heat. Besides heat, the byproducts of combustion are generally carbon dioxide and water vapor. Other byproducts such as carbon monoxide can be a result of inefficient combustion. Generally, the fuel is fed through the bottom of the catalytic heater and is dispersed through a gas distribution device at atmospheric pressure into contact with a high surface area layer such as a porous insulating layer or support. This layer contains the catalyst which may be, for example, platinum, palladium, or other metals and alloys thereof capable of promoting the oxidation of hydrocarbons within the fuel. Oxygen from the atmosphere enters the catalyst-containing layer and reacts with the gaseous fuel, promoted by the catalyst. This reaction releases the Btu content in the fuel in the form of radiant energy. The chemical reaction that occurs during the oxidation reduction process produces temperatures within the catalyst containing layer that range from about 500° F. to 1000° F. The actual temperature at the surface of the catalytic heater is dependent upon the rate at which the fuel gas is introduced to the catalyst, among other things. The surface of the heater is typically rectangular or circular and ranges from about 1 square foot (sq. ft.) to about 10 sq. ft. The volume of the gas delivered to the catalytic surface may range from about 2 to 6 cubic feet of gas per hour per square foot. Before a catalytic heater can be operated successfully, the heater and the catalyst must be preheated to a temperature at which the oxidation reaction with the fuel gas can be sustained by the heat of the reaction. A typical catalytic heater needs to be pre-heated to a temperature ranging from about 200° to about 400° F. Many catalytic heaters use an electrical resistance tubular heater to preheat the platinum catalyst before the fuel gas is introduced into the heater. However, some manufacturers use a flame pilot light that heats up the catalyst by impinging the surface with a flame. Another method consists of ducting hot air in close proximity to the catalyst, raising the temperature sufficiently to allow the oxidation reaction with the fuel gas to be sustained. These indirect methods, whereby the catalyst is preheated by preheating the heater surface, are slow and inefficient. The electrical method of preheating the catalyst may also require that the site where the catalytic heater device is to be installed has access to a high voltage power source. The catalytic heaters that are commercially available today display reasonably even or uniform distribution of temperature at the maximum rated input of 6 cubic feet of gas per hour per square foot. This will produce a reaction temperature on the heater surface of from about 750° F. to about 1000° F. However, when operating at the lower flow rates, that is, about 2 cubic feet of gas per hour per square foot, the temperature distribution across the heater surface will vary from about 200° F. to about 800° F. This poses many problems particularly when the heaters are used for heating flat areas. The catalytic heaters develop hot and cold spots across the heating surface and produce an uneven heating profile to the object being heated. As a consequence, process control is very poor and efficiency is reduced. Another disadvantage of catalytic heaters is that some of the fuel gas is left unreacted by the catalyst and escapes through the heater into the atmosphere. This phenomenon is referred to as ‘methane slip’ for catalytic heaters that use natural gas as a fuel source and is expressed as a percentage of the input Btu/hr or a percentage of the total fuel flow that is supplied to the heater. A more general term is ‘fuel gas slip’ to take into account that catalytic heaters can use propane or butane as fuel in addition to, or in lieu of, natural gas. Device manufacturers have shown that commercial catalytic heaters exhibit methane slip rates or fuel gas slip rates as high as 25 percent. Typical operating levels are about 15 percent of the input Btu/hr rate. In addition to fuel-value loss, methane slip also increases the heater-start time and allows the high-GHG (Green-house gas) value methane to escape into the atmosphere. U.S. Pat. No. 6,932,593 (“the “593 patent”) provides a method of preheating a catalytic heater that is assembled with a sealed plenum chamber disposed below the catalytically active layer containing platinum and has a gas permeable wall portion facing toward the active layer and a gas inlet for introducing the fuel gas or hydrogen into the chamber. The '593 patent teaches that hydrogen gas will instantly oxidize with a platinum containing catalytically active layer at room temperature. According to the '593 patent, this reaction is generally so violent that a stream of 99% pure hydrogen impinging on this layer will create a temperature on the layer that causes the hydrogen stream to burn. To remedy this, the '593 patent teaches pre-heating its heater using a preheat gaseous mixture of hydrogen and nitrogen wherein the volume ratio of hydrogen to nitrogen is between about 60% hydrogen to about 40% nitrogen and about 40% hydrogen to about 60% nitrogen. There is a need in the art to eliminate an electrical heater and the associated capital costs associated with providing electrical power to the gas operated device for the preheating step. There is also a need in the art to improve efficiency by reducing the time that it takes to bring the heater to its steady state operating condition. There is a further need in the art to reduce the amount of fuel gas slip or methane slip that is emitted during the operation of the catalytic heater. There is a need to reduce the green-house-gas emissions from catalytic heaters. There is a need in the art to extend the operating life of the catalyst within the catalytic heater. The method and a preheat composition described herein for pre-heating and/or heating a catalytic heater comprising hydrogen satisfies one or more of the needs in the art. In one aspect, there is provided a method of preheating a catalyst in a catalytic heater comprising the steps of: providing a preheat composition comprising greater than 60% by volume hydrogen and at least one selected from natural gas, propane, butane, ethane, methane, carbon dioxide, and mixtures thereof; and contacting the catalyst with the preheat composition for a period of time sufficient to raise the temperature of the catalyst to a one or more preheat temperatures. In certain embodiments, the one or more preheat temperatures for the preheating step may range from about 175 to about 500° F. In another embodiment, there is provided a method of heating a catalytic heater comprising the steps of: providing a preheat composition comprising greater than 60% by volume hydrogen and at least one selected from natural gas, propane, butane, and mixtures thereof; contacting the catalyst with the fuel mixture for a period of time sufficient to raise the temperature of the catalyst to a one or more preheat temperatures; and introducing a fuel gas composition comprising at least one selected from natural gas, propane, butane and mixtures thereof and optionally hydrogen for a period of time sufficient to reach one or more operating temperatures. In one particular embodiment, the fuel gas composition comprises hydrogen. In this or another embodiment, the fuel gas composition may comprise at least 60% by volume or greater hydrogen. In another embodiment, the fuel gas composition comprises less than 60% by volume of hydrogen. The method and preheat composition described herein satisfies one or more of the needs in the art by using a preheat composition and/or fuel composition comprising 60% by volume or greater, 70% by volume or greater, 80% by volume or greater, 90% by volume or 95% by volume of hydrogen or greater in combination with one or more other components such as, but not limited to, natural gas, propane, butane, nitrogen, carbon dioxide, ethane, methane, refinery off gas, ethylene off gas, methanol, ethanol, butanol, liquefied biogas and mixtures thereof. The preheat compositions and fuel compositions described herein are comprised of one or more fluid components. The term “fluid” are used herein describes a component that is capable of flowing and includes, but is not limited to, a gas, a liquid, a sublimed solid, a vapor, a mist, or any combinations thereof. In certain embodiments, one or more of the components are in liquid form such as, for example, methanol, ethanol, butanol, and/or liquefied biogas. In the latter embodiments, the fuel is vaporized prior to the contact with the catalyst. As previously mentioned the method and preheat and/or fuel composition is used to pre-heat and/or operate catalytic heaters used in the art. Typical BTU outputs for catalytic heaters may range from about 1,500 to about 60,000 BTU. In one embodiment, the preheat composition comprises 70% by volume or greater of hydrogen and the balance is at least one selected from natural gas, propane, butane, nitrogen, carbon dioxide, and mixtures thereof. In this or other embodiments, the preheat composition is introduced into the catalytic heater in order to preheat the catalytic heater to one or more temperatures which include, but are not limited to, any one of the following preheat temperatures: 175, 200, 225, 250, 275, 300, 325, 350, 375, 400, 425, 450, 475, and 500° F. With regard to the foregoing, it is understood that any one of the preheat temperatures can serve as an endpoint to a range, such as, for example, 200 to 275° F. or 200 to 500° F. In certain embodiments, the flow rate of the preheat composition is introduced into the catalytic heater at a flow rate that ranges from about 2 to about 6 standard cubic feet per hour per square foot of heater surface. The preheat composition is introduced into a catalytic heater via one or more inlets within the device and contacts the layer which can be a porous and/or a high surface area support containing the catalyst. In one particular embodiment, the preheat composition comprising substantially pure hydrogen gas (e.g., 99% by volume or greater of hydrogen) in a catalytic heater produces a much more homogeneous heat distribution across the catalytic heater surface compared to mixtures of hydrogen containing less than 60% by volume of hydrogen and nitrogen described in the art. The preheat gas composition is introduced into catalytic heater in order for the heater to a reach certain temperature for a certain period of time. In certain embodiments, after the desired temperature is reached, the preheat gas composition is turned off and the fuel gas composition is introduced wherein the catalytic heater is run for a certain period of time at a certain temperature. Typical fuel compositions used to operate the catalytic heater are compositions comprising one or more of the following components: natural gas, propane, and/or butane. The time of a typical preheat step may range from about 1 minute to about 3 minutes. It is desirable to reduce the amount of time required for the preheat step. In one particular embodiment, the preheat composition comprises substantially pure hydrogen (e.g., 99% by volume or greater of hydrogen) to preheat the catalytic layer within the heater. In this embodiment, the more homogeneous heat distribution on the catalyst surface provides for a faster preheat than traditional preheat methods with a more efficient utilization of preheat gas. Additionally, the homogeneous heat distribution on the catalyst surface in this embodiment reduces the fuel slip or methane slip observed once natural gas fuel is initiated. Lower methane slip reduces greenhouse gas emissions, enhances the operational safety of the manufacturing process by lowering flammable gas concentrations in the exhaust and allows the end user to start a ‘cold spot sensitive’ production process sooner. In this regard, because the preheat composition is substantially pure hydrogen, the amount of methane that is released into the atmosphere as a unreacted byproduct or “methane slip” observed during startup of the catalytic heater, is reduced. The methane slip is calculated from the volume fraction of natural gas in the exhaust multiplied by the exhaust flow rate divided by the natural gas flow rate supplied to the catalytic heater when the exhaust concentration measurements were taken. In addition, the pure hydrogen preheat embodiment also has the advantage of using a simpler and less expensive gas supply system to implement than the use of preheat gas compositions comprising mixtures of nitrogen and hydrogen previously used in the art such as that described in the '593 patent. In certain embodiments, the methane slip or unreacted natural gas flow rate/natural gas supply flow rate after a period of time of heater operation of 8 minutes is about 50% or less using a natural gas flow rate of 8.7 standard cubic feet/ft2/hour. In other embodiments, the percentage of methane slip after a period of heater operation of 8 minutes is about 50% or less using a natural gas flow rate at flow rates lower than 8.7 standard cubic feet/ft2/hr. In certain embodiments, the fuel gas slip may change in proportion to the fuel gas flow rate until it reaches 100% as a maximum or the ‘normal’ fuel gas or methane slip observed for normal operation of the unit as a minimum. In a further embodiment of the method and composition described herein, the pre-heat composition may also be used in addition to, or in lieu of, the fuel composition. In this or other embodiments, the fuel gas composition is introduced into the catalytic heater after the preheat step has occurred and the heater is then operated and/or maintained at one or more temperatures which include, but are not limited to, any one of the following operating temperatures: 300, 325, 350, 375, 400, 425, 450, 475, 500, 525, 550, 575, 600, 625, 650, 675, 700, 725, 750, 775, 800, 825, 850, 875, 900, 925, 950, 975, 1000, 1025, 1050, 1075, 1100, 1125, 1150, 1175, 1200, 1225, 1250, 1275 and 1300° F. With regard to the foregoing, it is understood that any one of the foregoing operating temperatures can serve as an endpoint to a range, such as, for example, 500 to 875° F. or 300 to 1300° F. As previously mentioned, typical fuel compositions used to operate the catalytic heater comprise one or more of the following components: natural gas, propane, and/or butane. The fuel composition is fed into the heater to maintain the heater at its required operating temperature. The fuel composition described herein can comprise at least 60% by volume hydrogen and at least one or more of the following components: natural gas, propane, and/or butane. In this embodiment, the fuel composition comprising at least 60% by volume can be run during the entire operation of the catalytic heater or alternatively during at least a portion of the operation of the catalytic heater. In other embodiments, the fuel composition can comprises less than 60% by volume of hydrogen and the balance one or more of the following components: natural gas, propane, and/or butane. In embodiments wherein hydrogen is also used in the fuel composition, once a hydrogen supply is onsite for the preheat step, it becomes convenient to use hydrogen and natural gas mixtures as the fuel composition when operating the catalytic heater at low heating rates, less than 2 cubic feet of gas per hour per square foot of heater surface. The hydrogen will impart improved surface temperature homogeneity to counteract the problems with hot spots encountered when operating the catalytic heater at low heating rates using natural gas, propane or butane as the fuel gas. In the embodiment depicted in In one particular embodiment, the preheat composition, fuel composition, or both preheat and fuel composition comprising hydrogen is introduced through the catalyst pad and is prevented from leaking out of the casing of the heater and combusting in air when the surface of the catalytic heater reaches its auto ignition temperature. In In the embodiment depicted in In certain embodiments such as those embodiments wherein the catalyst is operated proximal to a hydrogen generating process or device (e.g., a hydrogen effluent is liberated as a by-product of production), the catalytic heater can be operating using the hydrogen effluent. In one particular embodiment, the hydrogen effluent can be used as at least a portion of the preheat composition, fuel composition and or a combination thereof. hydrogen gas is used to operate a catalytic heater but where the hydrogen is expensive or relatively unavailable. The heater is pre-heated on a hydrogen-fuel gas mixture and then switches back to pure hydrogen or a greater concentration of hydrogen for operation. This scenario may arise when the heater is driving a hydrogen liberating reactor. In certain embodiments such as the embodiment depicted in The method and preheat gas composition described herein can provide a more homogeneous heat distribution in the catalytic heater than other preheat gas compositions known in the art as well as reduce the methane slip provided by the combination of hydrogen and fuel gas when the fuel gas is introduced into the heater during the transition from the pre-heat step to the sustained heating operation. In addition to providing a preheat gas composition and method for preheating a heater comprising a catalyst to a temperature sufficient to promote self sustained oxidation of natural gas, the preheat gas composition described herein may be substituted for the fuel gas during at least a portion of the heater's operation. This mode of operation using hydrogen and fuel gas mixtures as fuel may be advantageous for those embodiments wherein fuel flow rate to the heater is less than 3 cubic feet of gas per hour per square foot of heater surface. In this particular embodiment, it is believed that the addition of hydrogen aids in keeping the activated catalyst within the heater hot enough across the entire heater surface to self sustain methane oxidation by eliminating cold ‘islands’ (which promote methane slip) on the heater surface that form when pure methane is used as a fuel at relatively low flow fuel rates (e.g., less than 3 cubic feet of gas per hour per square foot of heater surface). The utility of the method and preheat gas composition described herein will now be illustrated by reference to the following non-limiting working examples wherein procedures and materials are solely representative of those which can be employed, and are not exclusive of those available and operative. The following examples illustrate the ability to use the method and preheat compositions described herein to preheat a typical catalytic heater. A series of tests were conducted whereby the catalytic heater was preheated using different preheat compositions described in Table 1. The tests were conducted using a catalytic heater from Vulcan Catalytic Systems to illustrate the reduced methane slip and the homogeneous surface temperature benefits of the preheat composition and method described herein. The catalytic heater was mounted in a vertical orientation at the center of a ventilated enclosure with a known constant ventilation rate for the purpose of the tests. The vertical mount facilitated capturing images of the heater surface. The catalytic heater inlet was connected to a flow skid that is capable of metering a supply of hydrogen at a certain flow rate or a fuel composition containing one or more of the following components: hydrogen, nitrogen and natural gas at a specified flow rate and composition. During the testing, the combination of hydrogen, natural gas or optionally nitrogen comprised the preheat composition and natural gas was used as the fuel composition. The total flow rate of the gases supplied to the catalytic heater is the same for all tests with the exception of Example 5. Example 5 had a flow rate of 17.4 cubic feet of gas per hour per square foot of heater surface, whereas the balance of the tests had a flow rate of 8.7 cubic feet of gas per hour per square foot of heater surface. A thermocouple (referred to herein as TC7) was contacted with the catalyst support material on the face of the heater and the temperature was recorded at intervals during the test. The temperature of TC7 was recorded during certain time intervals and the results of these intervals are provided in Table 1. The catalytic heater surface was allowed to cool to less than 105° F. on TC7 between tests. A combustible gas sensor continuously measured the combustible gas concentration in the ventilated enclosure exhaust. The combustible gas composition was recorded at various time intervals during the test. The methane slip for each test was calculated from the combustible gas concentration measured by the exhaust gas sensor and the exhaust flow rate within the ventilated enclosure during the test. The results of the natural gas slip flow rate/natural gas supply flow rate (%) are summarized in Table 1. An Infrared (IR) camera was trained on the top section of the heater and photos of the surface temperature distribution at the catalytic heater surface were taken at recorded intervals during the test. The IR images show the temperature distribution on the catalytic heater surface by assigning specific colors to specific temperatures. The temperature scale is adjusted automatically by the camera and is shown at the top right of each image. The maximum temperature that the IR camera can measure is 680° F. The results of a visual inspection of the IR images are reported in Table 2. The rectangular shaped heater surface was divided into four quadrants: Quadrant A (left top), Quadrant B (right top), Quadrant C (left bottom), and Quadrant D (right bottom). The IR images for each Quadrant were reviewed to determine the temperature of the surface in each quadrant. Zones of like temperature were identified and the surface coverage of each temperature zone within the Quadrant was estimated to be certain percentage which added to a total of 100%. A homogeneous temperature distribution over the catalytic heater surface is typified by large fractions, >30%, of surface that differ in temperature by between 200-350° F. and do not exceed the maximum temperature measured by the camera (>680° F.). A ‘warm region’ is a region of catalytic heater surface that has a temperature cooler than a ‘hot spot’ and warmer than 120° F. A ‘hot spot’ is a region with less than 50% surface coverage that has a temperature at least 400° F. higher than the lowest temperature on the balance of the heater surface. Hot spots are typically characterized by regions >680° F. except in cases where the preheat gas composition does a poor job of heating the catalytic heater surface. As previously mentioned, Table 1 provides the results of the thermocouple readings of the surface of the catalytic heater and the methane gas slip over the course of time. Table 2 provides an analysis of the IR readings for each quadrant of the surface of the catalytic heater over the course of time. In Comparative Example 1, the catalytic heater preheats the composition composed of 40.5% hydrogen, 34.5% nitrogen and 25% natural gas is supplied to the catalytic heater for two minutes. The hydrogen and nitrogen gas supply is shut off and the natural gas flow is increased to maintain a constant flow of gas through the heater. As Table 1 indicates, fuel gas flow over regions of the catalytic heater surface from a time interval from 2 to 3 minutes actually cooled the surface of the catalyst as indicated by the drop in temperature of TC7 from 245° F. to 224° F. This is supported by the IR data provided in Table 2. It is believed that at low preheat gas hydrogen concentrations, such as utilized in Comparative Example 1, the nitrogen and/or natural gas in the preheat gas act as diluents. The effect of a diluent in the preheat gas is to produce an inhomogeneous heat distribution across the catalytic heater surface, creating regions that are not warm enough to promote oxidation of fuel gas, leading to surface cooling once fuel gas begins to flow over those sections of the heater surface. Comparative Example 2 uses a 55% hydrogen/45% nitrogen gas composition, which was described in the '593 patent as a suitable mixture for catalytic heater preheating. In Comparative Example 2, the catalytic heater preheat gas composition was supplied to the catalytic heater for two minutes. The test was discontinued after two minutes because the bulk of the heater surface remained at room temperature, as observed in the IR images of the heater surface during the preheat step. The methane slip for this test was not measured, but the IR image results provided in Table 2 indicate that this gas mixture is not capable of effectively preheating the catalytic heater. The entire heater surface remained at room temperature, except for two small spots that reached a temperature near 300° F. (see Table 2). Such a tiny area of surface near the threshold temperature required for natural gas oxidation, (greater than 200° F.) with the remainder of the surface at room temperature, is insufficient to effectively preheat the catalytic heater. The lack of effectiveness of the preheat conditions in Comparative Example 1 are accentuated by the improved, but still highly ineffective preheat conditions in Comparative Example 2. Table 2 provides a description of the IR camera photos of the catalytic heater surface for Comparative Example 1 as a function of time. After one minute using a preheat gas composition of 40.5% hydrogen, 34.5% nitrogen and 25% natural gas, there are points on the surface of the catalytic heater that have reached 423° F. The 12% surface area of quadrant C at 423° F. corresponds to only 3% of the total surface in view of the IR camera. A significant proportion of surface in Quadrants C and D is at temperatures between 160 and 300° F. However, the bulk of the total surface, 64%, is below 120° F. After two minutes of preheat gas flow and one minute of natural gas flow, well defined hot spots are visible on the surface. However, these hot spots, or regions of surface at 655° F. in Quadrants C and D, correspond to only 5% of the total surface area of the heater. The bulk of the total surface, 63%, remains below 100° F. Significantly, the warm regions of the heater surface that had initially reached temperatures between 160 and 300° F. have actually cooled down after being exposed to gas flow for an additional two minutes. The warm regions are now at a temperature ranging from about 140 to 180° F. The total surface coverage of these warm regions is unchanged. After two minutes of preheat gas flow and three minutes of fuel gas flow, the hot spots marginally increase in size from 5% to 16% of the total surface area and the temperature of the hot spots rise to >680° F. Still, nearly three quarters of the total surface, 72%, remains less than 110° F. after three minutes of fuel gas or 100% natural gas flow. Further evidence of the ineffectiveness of this preheat treatment can be found by looking at the temperature of the ‘warm’ regions at five minutes total elapsed time. The warm regions experience further cooling to below 120° F. Referring to Table 1, Temperature measurements by the thermocouple TC7 capture an initial surface cooling (after 3 minutes) followed by a temperature increase (after 5 minutes). TC7 was located near an expanding hot spot. Since the expansion of the hot spots is not rapid enough to prevent the warm spots from cooling further, the surface of the catalytic heater cannot reach a level that will support fuel gas oxidation. The failure of the preheat method is corroborated by the methane slip values provided in Table 1. The methane slip for Comparative Example 1 is 100% after five minutes total elapsed time. It is clear from the data for both Comparative Example 1 and Comparative Example 2, that neither preheat gas composition provides for an effective and sustainable preheating step. The results for the preheat gas compositions used in Comparative Examples 1 and 2 illustrate that both nitrogen and natural gas can act as diluents to the active gas hydrogen. The use of gas diluents with hydrogen during catalytic heater preheating promotes heterogeneous temperature distribution across the catalyst surface, cooling of warm regions on the catalytic heater surface and methane slip. It is believed that if the amount of hydrogen diluent is minimized and the diluent is fuel gas, the deleterious effects of the diluent can be neutralized sufficiently to successfully preheat the catalytic heater. The composition of the optimum preheat gas will depend upon a variety of factors, including but not limited to, fuel cost, hydrogen cost, heating value of the heater, whether the system needs to be retrofitted to utilize hydrogen gas mixtures for preheating, the sensitivity of the process using the heat to hot spots, and/or the details of the process control system. Therefore, it is useful to describe the following preferred embodiments of this invention in terms of a more general parameter, the maximum fuel gas slip, or the maximum methane slip since natural gas is used as a fuel. In Example 1, the catalytic heater preheats gas composed of 62% hydrogen and 38% natural gas is supplied to the catalytic heater for two minutes. The hydrogen gas supply is shut off and the natural gas flow is increased to maintain a constant flow of gas through the heater. In Example 2, the catalytic heater preheat gas composed of 100% hydrogen is supplied to the catalytic heater for two minutes. The hydrogen gas supply is shut off and the natural gas supply is turned on at a flow rate that maintains a constant flow of gas through the heater. In Example 3, the catalytic heater preheat gas composed of 100% hydrogen is supplied to the catalytic heater for three minutes. The hydrogen gas supply is shut off and the natural gas supply is turned on at a flow rate that maintains a constant flow of gas through the heater. In Example 4, the catalytic heater preheat gas composed of 100% hydrogen is supplied to the catalytic heater for four minutes. The hydrogen gas supply is then shut off and the natural gas supply is turned on at a flow rate that maintains a constant flow of gas through the heater. In Example 5, the catalytic heater preheat gas composed of 100% hydrogen is supplied to the catalytic heater for one minute at twice the flow rate of the previous tests. The hydrogen gas supply is then shut off and the natural gas supply is turned on such that the flow rate of natural gas is identical to the flow rate of fuel gas through the heater in Examples 1 through 4. Table 2 provides the description of IR images in each quadrant of the catalytic heater surface. The surface of the catalytic heater in Example 1 after two minutes of preheat gas flow, with the preheat gas composed of 62% hydrogen and 38% natural gas, exhibits hot spots covering 19% of the total surface at temperatures >680° F. The remaining 81% of the surface is warm, at temperatures between 150 and 250° F. Unlike Comparative Examples 1 and 2 described above, the temperature of the warm surface does not cool down with this preheat gas composition, but remains constant after a total elapsed time of three minutes, including one minute of natural gas flow. The total surface coverage of the warm regions decreases from 81% to 56% and hot spot total surface coverage increases from 19% to 44% as the warm regions are converted to hot spots. After a total elapsed time of four minutes, the total surface coverage of the warm regions decreases from 56% to 37.5% and the hot spot total surface coverage increases from 44% to 62.5%. This pattern continues until the entire surface reaches the maximum temperature that the IR camera can measure at a total elapsed time of nine minutes. Referring to Table 1, the temperature measurements from TC7 as a function of time also capture the gradual rise in surface temperature from 512 to 782° F. over the course of the test from 0 seconds to 9 minutes. The methane slip, also given in Table 1, goes through a maximum of 72% at five minutes preheat time and decreases slowly from that value to 56% and 61% after 7 and 9 minutes. In contrast to Example 1, the surface temperature of the catalytic heater is very homogeneous after one minute of preheat gas flow in Example 2, with the preheat gas composed of 100% hydrogen. The catalytic heater surface in Example 2 can be described by four different warm zones: a zone with temperatures between 110 and 160° F. covering 39% of the total surface; a zone with temperatures between 210 and 290° F. covering 11% of the total surface; a zone with temperatures between 180 and 340° F. covering 43% of the total surface; and a zone with temperatures between 400 and 508° F. covering 7% of the total surface (calculated from the data obtained from the IR camera provided in Table 2 wherein the percentages add up to 100%). The temperature homogeneity of the surface is preserved after two minutes of preheat gas flow in Example 2. The temperature of each of the four warm zones has increased and the temperature distribution has shifted to higher overall surface temperatures after two minutes of preheat gas flow. The four zones are comprised of the following: a zone with temperatures between 140 and 200° F. covering 23% of the total surface; a zone with temperatures between 230 and 290° F. covering 35% of the total surface; a zone with temperatures between 330 and 400° F. covering 28% of the total surface; and a zone with temperatures between 460 and 589° F. covering 14% of the total surface (calculated from the data obtained from the IR camera provided in Table 2 wherein the percentages add up to 100%). Hot spots appear after the catalytic heater surface is exposed to 2 minutes preheat gas and one minute fuel gas (natural gas). The temperature of the warm zones continue to increase after the preheat gas flow is shut off and the fuel gas flow is turned on, similar to what was observed in Example 1. The surface area of the hot spots at temperatures >680° F. is 15%. Two warm zones remain, one with temperatures between 160 and 280° F. covering 37% of the total surface and another with temperatures between 240 and 370° F. covering 48% of the total surface. After three minutes of fuel gas flow, the hot spots on the catalytic heater surface expand rapidly and a single warm zone remains. The hot spot total surface coverage increases from 15% to 70% and the total surface coverage of the warm zone between temperatures of 240 and 370° F. is a mere 30%. At longer durations of natural gas flow, the total surface coverage of the warm region decreases as the warm regions are converted into hot spots, until the entire heater surface is hot, after a total elapsed time of eight minutes. Again, in contrast to Example 1, the 100% hydrogen preheat gas composition used in Example 2 preheats the catalytic heater surface more rapidly with a more homogeneous temperature distribution. Referring to Table 1, the more homogeneous temperature distribution on the heater surface serves to substantially reduce the methane slip for Example 2 to only 50% after a total elapsed time of eight minutes. Example 3 is similar to Example 2 except that the 100% hydrogen preheat gas was turned on for three minutes instead of two minutes. Similar to Example 2, the surface of the catalytic heater in Example 3 was very homogeneous while the preheat gas composition was flowing and exhibited no hot spots (see Table 2). The surface can again be divided into four different warm zones. The additional minute of preheat hydrogen allows each of the warm zones on the catalytic heater in Example 3 to reach a higher temperature range than the warm zone temperature ranges achieved with only two minutes of preheat gas flow. The maximum temperature detected by the IR camera on the catalytic heater surface rises from 589° F. in Example 2 to 659° F. in Example 3. Hot spots appear on the catalytic heater surface and the warm regions coalesce into a single zone once the preheat gas is turned off and the natural gas flow is turned on. The same behavior was observed for Example 2. The higher warm region temperatures translate into higher surface coverage of hot spots after three minutes of fuel flow which for Example 3 was 91% as compared to 70% total surface coverage of hot spots for Example 2. The catalytic heater surface is hot after eight minutes for both Example 2 and Example 3. Although preheat conditions for both Example 2 and 3 achieve a hot surface in the same amount of total elapsed time, the accelerated expansion of the hot spots on the catalytic heater surface as a result of the preheat conditions used in Example 3 translate to a reduction in methane slip. Referring to Table 1, the methane slip in Example 3 reaches 50% after a total elapsed time of only five minutes whereas the methane slip in Example 2 reaches 50% after a total elapsed time of eight minutes. Furthermore, the pattern of methane slip observed for Examples 1 and 2 no longer holds for Example 3 in that the methane slip does not proceed through a maximum value of 72% prior to falling to the lower value. This means that the cumulative methane slip for the preheat step for Example 3 is much lower than the cumulative methane slip for Examples 1 and 2. If there are process, safety or emissions advantages to minimizing methane slip, the preheat conditions used in Example 3 would be preferred. Example 4 is similar to Examples 2 and 3 except that the 100% hydrogen preheat gas was turned on for four minutes. Just as in Examples 2 and 3, the surface of the catalytic heater was very homogeneous in Example 4 while preheat gas is flowing, exhibiting no hot spots. The surface can again be divided into four different warm zones, which collapse to a single warm zone after the preheat gas is turned off and the fuel gas is turned on. This time, the additional minute of preheat hydrogen has little effect on the catalytic heater surface temperatures. The maximum temperature detected by the IR camera on the catalytic heater surface at the end of the preheat gas treatment remains unchanged at 659° F. in both Example 3 and Example 4. The temperature of the warm region after two minutes fuel gas flow is also the same for both Example 3 and Example 4, or ranges from about 250 to about 330° F. There is a slight cooling observed after two minutes of fuel gas flow for one of the warm regions, with a temperature range between 370 and 430° F. The catalytic heater surface is hot after eight minutes for Examples 2, 3 and 4. Methane slip is not reduced by increasing the time that the preheat gas is flowing, since the methane slip profile measured in Example 4 is similar to that achieved in Example 3. The preheat treatment used in Example 4 achieves the desired results, but at higher cost since larger amounts of hydrogen are required. Example 5 was conducted with one minute of 100% hydrogen preheat gas flowing at twice the flow rate of the other examples. The temperature profile on the surface when the preheat gas flow is turned off is the hottest of all the preheat conditions tested. The maximum temperature detected by the IR camera is >680° F. There is a single warm zone with a temperature range of 530-590° F. However, when the fuel gas flow is turned on for one minute, the entire surface of the catalytic heater cools. The maximum temperature detected by the IR camera is reduced from >680° F. to 631° F. The warm zone cools from 530-590° F. to 360-380° F. After two minutes of fuel gas flow, hot spots appear and the warm zone cools further. The hot spots continue to expand until 95% of the surface of the catalytic heater is >680° F. at a total elapsed time of 9 minutes. The methane slip profile for Example 5 is consistent with the temperature profile of the catalytic heater surface for this test. The methane slip goes through a maximum value of 78% and decreases to a value of 56% after a total elapsed time of eight minutes. Example 5 is an effective preheat method, but it produces higher methane slip. The surface temperature homogeneity imparted by using hydrogen as a preheat gas could be used to reduce the amount of catalyst required per square foot of heater surface without reducing the maximum heat output that can be supplied by the device. Since the catalyst is the largest contributor to the capital cost of a catalytic heater, this represents a significant savings on the cost to manufacture the device. It is believed that in certain embodiments, the method of operating a catalytic heater using preheat gas compositions and/or fuel gas compositions comprising hydrogen can reduce the amount of catalyst required per cubic foot of catalytic heater to be at least 5% lower than the amount of catalyst required for generating the same heat release as measured in BTU per hour per cubic foot of heater surface using indirect preheating and/or heating methods. |
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