WO2022009155A1

WO2022009155A1 - A control method for a tubular reactor

Info

Publication number: WO2022009155A1
Application number: PCT/IB2021/056163
Authority: WO
Inventors: Christos DIMOSTHENOUS; Peter Hofbauer; Siddhartha GADIRAJU; Matthew GINGERICH
Original assignee: ThermoLift Inc
Current assignee: ThermoLift Inc
Priority date: 2020-07-09
Filing date: 2021-07-08
Publication date: 2022-01-13
Anticipated expiration: 2023-01-09

Abstract

A tubular reactor is a combination combustor and heat exchanger. Combustion occurs adjacent to the tubes of the tubular reactor. A control method is disclosed that detects that combustion is occurring based on a temperature measurement. If not, a shutdown procedure or a restart is commanded. Additionally, the control method enters a shutdown procedure or a restart is commanded when temperature at the tubes exceeds a maximum temperature that would harm the tubular reactor.

Description

A Control Method for a Tubular Reactor

Field of Invention

[0001] The present disclosure relates to controlling a tubular reactor.

Background

[0002] A tubular reactor has been disclosed in PCT /IB2020/054910, which is commonly assigned. As combustion in the tubular reactor occurs adjacent to the tubes, it is important to keep the temperature of the tubes below their melting point of the tubes or even a temperature at which the tubes performance degrades to ensure longevity. Additionally, it is desirable to detect whether fuel and air provided to the tubular reactor continues to be reacting. If not, undesirable buildup of unburned fuel can occur.

Summary

[0003] Methods are disclosed herein to detect improper operating regimes and to take corrective action. A control method for a tubular reactor is disclosed that includes a starting sequence that includes: initiating flow of a fluid through the tubes of the tubular reactor, blowing air across the outside of the tubes of the tubular reactor for a predetermined pre-purge phase, providing gaseous fuel with the air after the predetermined pre-purge phase is complete, activating an ignitor disposed near the tubes of the tubular reactor, measuring temperature associated with the tubular reactor, stopping fuel flow when dT /dt is less than x, where T is temperature and t is time, and continuing to provide fuel flow while dT /dt is greater than x.

[0004] Temperature is measured by a thermocouple located coupled to one a plurality of tubes of the tubular reactor. The value of x is nonnegative.

[0005] The method further includes: an operating sequence following the starting sequence, which includes: continuing operation of control of air and fuel flow to the outside of the tubes of the tubular reactor and control of flow of the fluid through the tubes of the tubular reactor, continuing to monitor temperature, and proceeding to a shutdown procedure when T > Thigh. The shutdown procedure includes closing the fuel valve. [0006] The method may further include an operating sequence following the starting sequence: proceeding to a shutdown when T < Tiow.

[0007] The method further includes: proceeding to a shutdown when dT /dt < y. The shutdown includes closing the fuel valve. Alternatively, the method further includes: proceeding to a shutdown when dT /dt < y and T < Tiow. Tiow is a low end of a desired operating temperature range.

[0008] Shutdown includes: closing the fuel valve, blowing air flow onto the outside of the tubes of the tubular reactor for a predetermined post-purge phase, ceasing air flow to the outside of the tubes of the tubular reactor after the predetermined post-purge phase, and ceasing fluid flow through the tubes of the tubular reactor after the predetermined post-purge phase.

[0009] In some embodiments ambient air is provided to the tubular reactor from the environment through an inlet tube having a choke disposed therein and the starting sequence further comprises choking the air flow by setting the choke. The choke is opened up when the temperature is greater than Tiow.

[0010] The method may further include a restarting sequence, which includes: continuing to blow air flow onto the outside of the tubes of the tubular reactor, providing gaseous fuel with the air, activating the ignitor, measuring temperature, and stopping fuel flow when dT /dt is less than, and continuing to provide fuel flow while dT /dt is greater than x.

[0011] The tubular reactor has a centrally located diffuser through which the fuel and air are provided; and, the plurality of tubes is disposed radially from the diffuser. Each of the plurality of tubes have a portion that is linear. Such linear portions are mutually parallel and are disposed from immediately adjacent tubes by a predetermined distance.

[0012] A control method for a tubular reactor is disclosed that has an operating sequence that includes: providing flow of a fluid through a plurality of tubes of the tubular reactor, providing air and fuel flow to the outside of the tubes of the tubular reactor, monitoring temperature, T, in the tubular reactor, proceeding to a shutdown procedure when T > Thigh wherein the shutdown procedure includes discontinuing fuel provision wherein Thigh is a temperature at which degradation of the tubular reactor occurs. In some embodiments temperature is determined via a thermocouple coupled to one of a plurality of tubes of the tubular reactor

[0013] The method further includes a starting sequence that precedes the operating sequence. The starting sequence includes: initiating flow of the fluid through the tubes of the tubular reactor, blowing air across the outside of the tubes of the tubular reactor for a predetermined pre-purge phase, providing gaseous fuel with the air after the predetermined pre-purge phase, activating an ignitor disposed near the tubes of the tubular reactor, measuring temperature; stopping fuel flow when dT /dt is less than x, and continuing to provide fuel flow while dT /dt is greater than 0.

[0014] The operating sequence further includes stopping fuel flow when dT /dt < y and T < Tiow. Tiow is a low end of a desired operating temperature range.

[0015] The operating sequence further includes: ceasing air flow on the outside of the tubes after the fuel flow has stopped and ceasing fluid flow through the tubes after the fuel flow has stopped.

[0016] The operating sequence further includes: continuing to monitor temperature and stopping fuel flow when T > Thigh. Thigh is a temperature above which the tubular reactor degrades.

[0017] A control method is disclosed that has an operating sequence including: providing flow of a fluid through a plurality of tubes of the tubular reactor, providing air and fuel flow to the outside of the tubes of the tubular reactor, monitoring temperature via a thermocouple coupled to one tube of the tubular reactor, and proceeding to a shutdown when dT /dt < y and T < Tiow. Shutdown includes closing the fuel valve.

The operating sequence further includes stopping fuel flow when T > Thigh.

[0018] The operating sequence may further include stopping fuel flow when both dT /dt < y and T < Tiow.

[0019] The control method further includes a starting sequence that proceeds the operating sequence. The starting sequence includes: initiating flow of the fluid through the tubes of the tubular reactor, blowing air across the outside of the tubes of the tubular reactor for a predetermined pre-purge phase, providing gaseous fuel with the air after the predetermined pre-purge phase, activating an ignitor disposed near the tubes of the tubular reactor, measuring temperature, stopping fuel flow when dT /dt is less than x, continuing to provide fuel flow while dT /dt is greater than x. [0020] The tubular reactor has a centrally located diffuser through which the fuel and air are provided. The plurality of tubes are disposed radially from the diffuser.

Each of the plurality of tubes have a portion that is linear. Such linear portions are mutually parallel and are disposed from immediately adjacent tubes by a predetermined distance.

Brief Description of Drawings

[0021] Figures 1 is cross section of a compression-expansion heat pump having a tubular reactor;

[0022] Figure 2 is a cross section of a portion of a compression-expansion heat pump with a tubular reactor;

[0023] Figure 3 is a cross-section of the tubular reactor of Figure 2;

[0024] Figure 4 is an illustration of a single tube of the tubular reactor of Figure 2;

[0025] Figure 5 is an illustration of a cap that is placed over a U-shaped portion of the tubes;

[0026] Figure 6 is a cross-sectional illustration of the cap of Figure 5 placed over the U-shaped portion of the tubes; and

[0027] Figure 7 is a flowchart showing one embodiment of a method to control a tubular reactor.

Detailed Description

[0028] As those of ordinary skill in the art will understand, various features of the embodiments illustrated and described with reference to any one of the Figures may be combined with features illustrated in one or more other Figures to produce alternative embodiments that are not explicitly illustrated or described. The combinations of features illustrated provide representative embodiments for typical applications. However, various combinations and modifications of the features consistent with the teachings of the present disclosure may be desired for particular applications or implementations. Those of ordinary skill in the art may recognize similar applications or implementations whether or not explicitly described or illustrated. [0029] One example of a thermodynamic apparatus in which a tubular reactor can be used, a compression-expansion heat pump 200, is shown in Figure 1. Heat pump 200 has a hot heat exchanger 202, a cylinder 204 in which a hot displacer 206 reciprocates and a cylinder 208 in which a cold displacer 210 reciprocates. Mechatronics actuators, in mechatronics section 220, are coupled to hot and cold displacers 206 and 210 and drive the displacers between ends of travel. Alow molecular weight gas, such as helium, is contained within cylinders 204 and 208 and inside tubes of hot heat exchanger 202. There is a hot chamber 276 delimited by dome 278, cylinder walls 280, and a top surface of displacer 206. There is also a warm-hot chamber, which is not visible in Figure 1 since displacer 206 is shown in its lower position in Figure 1. The warm-hot chamber is located between mechatronics section and displacer 206. A cold chamber 280 below cold displacer 210 is visible in Figure 1; although a cold- warm chamber is not visible due to displacer 210 being shown in its upper position. When displacers 206 and 210 are caused to reciprocate, the working gas moves among cold chamber 280, hot chamber 276, the warm-hot chamber, and the warm-cold chamber. The working gas accesses the various chambers by traveling through regenerators and/or heat exchangers located in an annular space located outside of cylinders 204 and 208. When hot displacer 206 moves upward toward hot heat exchanger 202, the working gas flows: from tubes of hot heat exchanger 202 into a regenerator 230; from regenerator 230 flow into a warm-hot heat exchanger 240; and from the warm-hot heat exchanger into the warm-hot chamber. When hot displacer 206 moves the other direction, flow is reversed compared to that described above. [0030] In regard to movement of cold displacer 210, working fluid moves between the volume within cylinder 208 below cold displacer 210 (away from mechatronics section 220] and a cold heat exchanger 260; between cold heat exchanger 260 and a cold regenerator 270; between cold regenerator 270 and a warm-cold heat exchanger 250; and between cold warm-cold heat exchanger 250 and the warm-cold chamber.

[0031] One of the fluids passing through heat exchangers 240, 250, and 260 is the working fluid. The other fluid in the present example is a liquid coolant In regard to warm-hot heat exchanger 240, coolant accesses passageways of warm-hot heat exchanger 240 through inlet 242 and exits through outlet 244. Similarly, passages of warm-cold heat exchanger 250 are coupled to an inlet 252 and an outlet 254; and passages of cold heat exchanger 260 are coupled to an inlet 262 and an outlet 264. [0032] Air alone or premixed air and fuel are provided to heat pump 200 via a blower 270. Premixed air and fuel are routed through a heat exchanger for preheating by exhaust gases leaving heat pump 200. It is a rather convoluted path that is not described here. The air and fuel are provided to a diffuser 272 through an inlet 274. [0033] As it was described above, when hot displacer 206 reciprocates, it causes the working gas, helium or other, to move through the tubes of the tubular reactor. Displacers 206 and 210 are controlled via a mechatronics controller in mechatronics section 220. This is only one non-limiting example of a system to cause reciprocation of displacers. Coils 610 act on armature 612, which is coupled to hot displacer 206. Coils 610 are coupled to a device driver 602 which is coupled to an electronic control unit 600. When upper coil 610 is provided current, it pulls upward on armature 612. When lower coil 610 is provided current, it pulls downward on armature 612. Coils 620 associated with cold displacer 210 act on an armature in a similar manner as that described with respect to hot displacer 206. ECU 600 is electronically coupled, directly or indirectly, with blower 270 to control air flow and to a fuel valve 634 which controls the amount of fuel from a fuel source 630 that is allowed to enter blower 270. The amount of air provided by blower 270 is controlled by blower 270 itself. In some embodiments, a valve or choke 644 is provided in air inlet pipe 640. In some embodiments, choke 644 is a two-position control: fully open and a partially-open position that decreases the airflow. In other embodiments, choke 644 is a fully-variable valve. To maintain a combustion temperature that does not melt tubes 202 of the hot heat exchanger, the ratio of fuel to air is at a lean ratio, i.e., excess air. Warmup of a cold heat pump can be speeded up if a richer mixture is provided to the hot heat exchanger for a brief period during warmup. To reduce the amount of air, choke 644 is closed to its partially-open position. When the temperature of tubes 202 nears a steady-state operational temperature, choke 644 returns to the full-open position.

[0034] In Figure 2, a portion of a thermal-compression heat pump 140 is shown. An upper portion of heat pump 140 has a displacer 90 disposed within a cylinder 88. Displacer 90 is coupled to a mechatronics system (not illustrated in Figure 2], analogous to the system described in Figure 1, that commands displacer 90 to reciprocate within cylinder 88. The volume of working gas, such as helium or hydrogen, in a hot chamber 84 changes as a result of the movement of displacer 90. When the working gas is pushed out of hot chamber 84, the working gas is pushed into orifices 94 that pass through a dome 96. Orifices 94 are coupled to tubes, each having a first connector section 142 coupled to a first linear portion 150 coupled to a U-shaped portion 158 coupled to a second linear portion 154 coupled to a second connector section 144. Second connector section 144 fluidly couples to a regenerator 92 that is located between a housing 86 and cylinder 88. The space between housing 86 and cylinder 88 is an annulus. Regenerator 92 is annular.

[0035] At the center of the tubes is a diffuser 68 to which premixed fuel and air are provided. Diffuser 68 is a cylinder with a plurality of small holes on the outer surface. The diffuser causes the fuel and air to be distributed uniformly to the first linear portion of the tubes 150.

[0036] A cross-section of Figure 2, as indicated by 3-3, is shown in Figure 3. The cross-section in Figure 3 is through the entire heat pump 140 (of Figure 2], not just the cross section of Figure 2. Diffuser 68 is in the center (center of combustor at 49] At a displacement 60, from the surface of diffuser 68, is a first linear portion of a first plurality of tubes 50. First linear portions 50 are mutually parallel and are displaced from each other centerline to centerline by a distance 58. A gap 59 is the edge to edge distance. Gap 59 is less than or equal to a predetermined gap to avoid flashback. Air and fuel from diffuser 68 travels toward first linear portions 50. It is preferred that there is no combustion occurring between diffuser 68 and first linear portions 50; instead, it is desired for oxidation of the fuel with the air to occur near first linear portions 50. Thus, gap 59 is less than the predetermined gap so that the combustion or oxidation of the fuel and air does not propagate from the side of linear portions 50 that is remote from diffuser 68 toward diffuser 68.

[0037] First linear portions of a second plurality of tubes 52 is show in Figure 3. Referring to Figure 2, first linear portion 152 of the second plurality of tubes are coupled via a U-shaped portion 158 to second linear portion 154 of the second plurality of tubes. The second plurality of tubes is displaced farther from diffuser 68 than first plurality of tubes (including portions 150, 154, and 158] Now referring to Figure 3, first linear portions of the second plurality of tubes 52 are viewed in cross section. First linear portions of the second plurality of tubes 54 are mutually parallel. Centerlines of the second plurality of tubes and are displaced from an outer surface of diffuser 68 by a displacement 62. Second linear portions of the first plurality of tubes 54 and second linear portions of the second plurality of tubes 56 are interspersed at the same distance from diffuser 68. Also shown in Figure 3 is an ignitor 70. A tip of ignitor 70 is positioned between first linear portions of the second plurality of tubes 52 and the second linear portions of the first and second plurality of tubes 54, 56. Such position of ignitor 70 is one non-limiting example.

[0038] In some embodiments, a ring 72 is provided that is reflecting on the inner surface. The reflective surface causes radiant energy from tubes 50, 52, 54, and 56 to be reflected onto those same tubes to reduce heat losses from the system.

[0039] Referring to Figure 2, some of the tubes have a lower U-shaped portion 168 than the rest of the tubes that have a U-shaped portion 158. In the embodiment in Figure 2, the ignitor (not shown] is inserted from the top and extends below the level of U-shaped portions 158. Oxidation of the fuel occurs in the volume between first linear portions 150 and second linear portions 154, more of it occurs next to first linear portions 150 and 152, which causes heat transfer from the oxidizing gases at elevated temperature to the linear portions and the U-shaped portions of the tubes to be more effective than oxidation at other locations.

[0040] A single tube of the first plurality of tubes is shown in Figure 4. First connector portion 142 is fluidly coupled to first linear portion 150, which is fluidly coupled to U-shaped portion 158, which is fluidly coupled to second linear portion 154, which is coupled to second connector portion 144.

[0041] Combustion is quenched when heat transfer from the combustion zone, e.g., into a solid surface is such that the flame fails to propagate. The quench distance can be determined, for example, by determining the maximum distance that two plates can be displaced from each other which does not allow a flame to propagate therethrough. In the present example, tubes have a gap therebetween which prevents flame propagation. The quench distance depends on the fuel type and the mixture concentration with air. (If the oxidizer is not air, quench distance also depends on the oxidizer composition.] In some embodiments where a range of mixture concentrations and/or fuel types is contemplated, the gap between adjacent tubes is selected for the most demanding condition anticipated in practice.

[0042] Depending on the performance goals in designing a heat pump system of other device into which the tubular reactor is employed, the flow of helium, or other low-molecular weight gas, through the tubes is determined. Based on the fluid flow rate, the maximum gap, and the additional considerations that the pressure drop through the tubes shouldn’t be excessive and the typical wall thickness of tubes, the number of tubes can be determined. In the embodiment in Figure 3, two rows of tubes are used to provide sufficient flow cross-sectional area for flowing the helium. In other examples, it is possible that one ring of tubes is sufficient. And in even other examples, more than two rings of tubes are used.

[0043] For each tube in Figure 3, two orifices are formed in dome 96 to accommodate first and second connector sections 142 and 144. A high concentration of orifices in dome 96 weakens the dome. In Figure 3, first and second connector sections 142 and 144 are bent so that the orifices in dome 96 are less weakening than if arranged close together.

[0044] As described above, to prevent flashback from the space beyond tubes 150 of Figure 2 toward diffuser 68, a consistent gap 59, as seen in Figure 3, prevents such flashback when the gap is sufficiently narrow, i.e., the gap quenches the reaction.

In some applications, temperature variations due to warmup, cooldown, and operational range of output can cause the tubes to deflect slightly. To avoid the deflection to becoming more than can be tolerated to prevent flashback, a fixture is applied to maintain the proper gap between adjacent tubes. Such a cap 300 is shown in Figure 5. Cap 300 has a covering portion 302 that sits atop the U-shaped portion of the tubes. Covering portion 302 is an annulus with an outer edge 310 and an inner edge 312. Inner edge 312 couples to a cylindrical portion 304. Cylindrical portion 304 has an inner surface that is substantially smooth. An outer surface 306 of cylindrical portion 304 has a plurality of notches formed therein. First linear portions 150 of the first plurality of tubes snap into the notches, in one embodiment, with a slight interference fit. A notch is provided for each of first linear portions 150. Also shown in Figure 5 is a cutout 314 of covering portion 302 to accommodate insertion of an ignitor (not shown]

[0045] A portion of a tubular reactor is shown in cross section in Figure 6 where notches of cap 302 are engaged with first linear portions 320 of the first plurality of tubes. The first plurality of tubes includes linear portion 320, linear portion 320” and U-shaped portion 320’ that fluidly couples 320 with 320”. Only a portion of notch 308 is visible in Figure 6 because first portion 320 is engaged with notch 308 over most of the length of notch 308. Shorter tubes are shown on the left-hand side of Figure 6. A tube has a first linear portion 330, a second linear portion 330", and a U-shaped portion 330’ that couples linear portions 330 and 330” together. Another tube that is displaced from the centerline 336 further than the tube including 330, 330’, and 330” has a first linear portion 332 fluidly coupled to a U-shaped portion 332’. A second linear portion that fluidly couples to U-shaped portion 332’ is barely visible as it is behind second linear portion 330”. The cutout for the ignitor is not visible in cap 300 in the view in Figure 6. However, it is located above U-shaped portions 330’ and 332’.

[0046] Continuing to refer to Figure 6, a thermocouple 350 is shown on U-shaped portion 324’ of a tube that also includes straight sections 324 and 324”. Another thermocouple 352 is shown on the U-shaped portion 330’ of a tube that also includes straight sections 330 and 330”. In some embodiments the bead of thermocouples 350 and 352 are welded to portions 324’ and 330’, respectively. In other embodiments, a cup or other holder is provided for the bead of thermocouples 350 and 352 to hold them against the tubes. Two thermocouples are not required. Figure 6 merely shows two optional locations. Any other suitable locations] can be used including in the gas in the vicinity of the tubes. Thermocouples 350 and 352 provide signals to electronic control unit 360. Also, shown in Figure 6 is an ignitor 356.

[0047] A method for controlling the tubular reactor begins with a start in 500 is shown in Figure 7. Flow through the tubes of the tubular reactor is initiated in block 502. In the case of a thermal-compression heat pump, the displacers within the heat pump are actuated to reciprocate to cause flow of a working gas to shuttle back and forth through the tubes. In block 504, the combustion zone is purged with air. The combustion zone and surrounding volume are purged particularly of fuel so that later when ignition is initiated, a known fuel and air mixture is provided. The combustion zone and surrounding volume is the volume outside the tubes of the tubular reactor.

The purge can be accomplished for a particular period of time or based on an amount of air that flows through the zone.

[0048] In block 506 of Figure 7, the fuel valve (634 of Figure 1] is opened to allow fuel to flow along with the air provided by the blower. The fuel valve is infinitely variable between fully closed and it fully open position. The fuel valve is controlled to obtain a particular stoichiometry (fuel-to-air ratio] in the tubular reactor. Also in block 506, the ignitor is activated to ignite the fuel-air mixture. In embodiments that include a choke (644 of Figure 1], the choke is set to the partially- closed position, which reduces the cross-sectional area for air to flow into tubular reactor. The choke could be an infinitely variable valve or a two-stage valve, meaning either being in the wide-open or the partially-closed position. If ignition is successful, temperature increases. In block 510, successful ignition is assessed by evaluating whether dT/dt, the time rate of change of temperature, exceeds x. In some applications, x is zero, meaning while dT /dt is positive, it indicates that the combustion is occurring. In other applications, x may be a slightly negative number. In other applications, x is a positive with the expectation that temperature is always increasing during a warmup phase. If a negative result in block 510, control passes to block 550 for a shutdown procedure that is discussed below. If a positive result in block 510, control passes to block 514 in which it is determined whether temperature has exceeded Tiow, the lower end of the desired operating range for temperature. If a negative result in block 514, control passes to block 510 to continue to monitor that temperature continues to rise to bring the tubular reactor into its desired operating range. If a positive result in block 514, control passes to block 530. Startup of the tubular reactor is completed when control passes to block 530.

[0049] In block 530, control of the fuel, air (blower], and flow of the working gas through the tubes occurs via a control strategy separate from this disclosure. Also, in the embodiments with a choke, the choke is moved to the fully open position. The control strategy may be based on user demand and signals from sensors within the thermodynamic apparatus and may use typical PID control or any suitable control method. The method shown in Figure 7 relates to detecting undesired operational conditions with respect to operating the tubular reactor and appropriate response to such conditions.

[0050] If combustion were to cease, control in block 530 is unable to process this situation. To protect against such a scenario, control passes to block 532 in which it is determined whether temperature has dropped below the lower end of the desired operating range (T < Tiow] and that the time rate of change of temperature (dT /dt] is less than y, where y is a negative number. When the time rate of change in temperature is so low, it indicates that combustion has ceased. If both are true, this indicates lack of combustion and control passes to block 550, the shutdown procedure. In embodiments in which a choke has been partially closed, the choke is opened in block 550. [0051] If a negative result in block 532, control passes to block 536 to determine if temperature is exceeding the higher end of the desired operating range, i.e., is T greater than Thigh. If a negative result in block 536, normal control resumes in block 530. A positive result in block 536 indicates an overtemperature that can lead to system failure, in which case control passes to block 550, the start of the shutdown procedure. In a suitable alternative, blocks 532 and 536 order is reversed. If neither block 532 or 536 yield positive results, i.e., results indicating an undesirable operating condition, control passes back to block 530 in which the normal control algorithm is in place. Periodically, block 530 passes control through blocks 532 and 536 to check for error conditions.

[0052] In block 550, the fuel valve is closed. If the choke is still closed, e.g., if control passes directly from block 510 to block 550, then the choke is opened fully in block 550. Control passes from block 550 to block 552 in which the flowthrough the combustion zone is continued for a desired period of time or until a certain amount of air has flowed through the combustion zone to purge combustibles. Control passes to block 554 in which it is determined whether a restart is desired or a complete shutdown. If a complete shutdown, control passes to block 556 in which the blower is turned off and the flow of the working fluid through the tubes of the tubular reactor is ceased; and, control ends in 558. If a positive result in 554, meaning a restart is desired, control passes to block 560. Recalling that the combustion is still flowing through the combustion zone, in block 560, the flow continues until the temperature is below Trestart. That is, flow is provided through the combustion zone until the temperature of the tubes is below Trestart, a safe temperature at which to restart as shown in block 560. When block 560 is satisfied, control passes to block 506 in which the fuel valve is opened and the ignitor is activated. Trestart is lower than Tiow.

[0053] A setpoint temperature, Tset, for the tubular reactor is a temperature enough lower than a temperature in which the system overheats, potentially causing melting or deformation of the tubes. It is often desirable for the Tset to be near Thigh to obtain high system efficiency. There is an acceptable temperature operating range between Tiow and Thigh, with the understanding that it is difficult to constrain the system to remain at a single temperature, particularly as conditions vary, such as user demand for heat or boundary conditions such as ambient temperature, to name a couple of nonlimiting examples. As described above, the operating range is related to Thigh, a temperature at which the system is in or close to a system overheat situation in which the tubular reactor is damaged or degraded. In some applications, it may be desirable to operate in a different operating temperature range, which may vary as a function of the operating condition. In such situations, Tiow, Tset, and Thigh may be expressed as functions of variables.

[0054] While the best mode has been described in detail with respect to particular embodiments, those familiar with the art will recognize various alternative designs and embodiments within the scope of the following claims. While various embodiments may have been described as providing advantages or being preferred over other embodiments with respect to one or more desired characteristics, as one skilled in the art is aware, one or more characteristics may be compromised to achieve desired system attributes, which depend on the specific application and implementation. These attributes include, but are not limited to: cost, strength, durability, life cycle cost, marketability, appearance, packaging, size, serviceability, weight, manufacturability, ease of assembly, etc. The embodiments described herein that are characterized as less desirable than other embodiments or prior art implementations with respect to one or more characteristics are not outside the scope of the disclosure and may be desirable for particular applications.

Claims

We claim:

1.A control method for a tubular reactor, comprising: a starting sequence, comprising: initiating flow of a fluid through the tubes of the tubular reactor; blowing air across the outside of the tubes of the tubular reactor for a predetermined pre-purge phase; providing gaseous fuel with the air after the predetermined pre-purge phase is complete; activating an ignitor disposed near the tubes of the tubular reactor; measuring temperature associated with the tubular reactor; stopping fuel flow when dT /dt is less than x, where T is temperature and t is time; and continuing to provide fuel flow while dT /dt is greater than x.

2. The control method of claim 1 wherein temperature is measured by a thermocouple coupled to one of a plurality of tubes of the tubular reactor; and x is nonnegative.

3. The control method of claim 1, further comprising: an operating sequence following the starting sequence: continuing operation of control of air and fuel flow to the outside of the tubes of the tubular reactor and control of flow of the fluid through the tubes of the tubular reactor; continuing to monitor temperature; and proceeding to a shutdown procedure when T > Thigh wherein the shutdown procedure includes closing the fuel valve.

4. The control method of claim 1, further comprising: an operating sequence following the starting sequence: continuing operation of control of air and fuel flow to the outside of the tubes of the tubular reactor and control of flow of the fluid through the tubes of the tubular reactor; continuing to monitor temperature; and proceeding to a shutdown when T < Tiow wherein shutdown includes closing the fuel valve.

5. The control method of claim 1, further comprising: an operating sequence following the starting sequence: continuing operation of control of air and fuel flow to the outside of the tubes of the tubular reactor and control of flow of the fluid through the tubes of the tubular reactor; continuing to monitor temperature; and proceeding to a shutdown when dT /dt < y wherein the shutdown includes closing the fuel valve.

6. The control method of claim 1, further comprising: an operating sequence following the starting sequence: continuing operation of control of air and fuel flow to the outside of the tubes of the tubular reactor and control of flow of the fluid through the tubes of the tubular reactor; continuing to monitor temperature; and proceeding to a shutdown when dT /dt < y and T < Tiow wherein shutdown includes closing the fuel valve; and Tiow is a low end of a desired operating temperature range.

7. The control method of claim 6 wherein the shutdown further comprises: blowing air flow onto the outside of the tubes of the tubular reactor for a predetermined post-purge phase; ceasing air flow to the outside of the tubes of the tubular reactor after the predetermined post-purge phase; and ceasing fluid flow through the tubes of the tubular reactor after the predetermined post-purge phase.

8. The control method of claim 6, further comprising: a restarting sequence following the shutdown, the restarting sequence comprising: continuing to blow air onto the outside of the tubes of the tubular reactor; providing gaseous fuel with the air; activating the ignitor; measuring temperature; stopping fuel flow when dT /dt is less than x, where T is temperature and t is time; and continuing to provide fuel flow while dT /dt is greater than x.

9. The control method of claim 1 wherein: the tubular reactor has a centrally located diffuser through which the fuel and air are provided and the plurality of tubes are disposed radially from the diffuser; each of the plurality of tubes have a portion that is linear; and such linear portions are mutually parallel and are disposed from immediately adjacent tubes by a predetermined distance.

10. The control method of claim 1 wherein: ambient air is provided to the tubular reactor from the environment through an inlet tube having a choke disposed therein; the starting sequence further comprises choking the air flow by setting the choke.

11. The control method of claim 3, wherein the operating sequence further comprises: proceeding to a shutdown procedure when T > T_high wherein the shutdown procedure includes discontinuing fuel provision wherein T_high is a temperature at which degradation of the tubular reactor occurs.