Abstract:
A method of starting up and shutting down a microchannel process is provided. Included are the steps of providing a first multi-planar process unit, preferably adapted to process an endothermic reaction, a second multi-planar process unit, preferably adapted to process an exothermic reaction, providing a containment vessel, the containment vessel containing at least a portion of the first, and preferably the second, process unit. In startup, the microchannel process is first checked for pressure integrity by pressurizing and checking the important components of the process for leaks. Subsequently, the process units are heated by introducing a dilute low-thermal energy density material, preferably to the second process unit, followed by the introduction of a dilute high-thermal energy density material, and adjusting the proportion of high-thermal energy density material as required. In shutdown, a purge material from the containment vessel is introduced into the first, and preferably the second, process unit.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to, and is a Continuation-in-Part of, application Ser. No. 10/774,298, filed Feb. 6, 2004, the contents of which, to the extent not inconsistent herewith, are incorporated herein by reference as if fully rewritten herein. 
    
    
     STATEMMENT REGARDING FEDERALLY-SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC 
     Not Applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is related to the control of microchannel processes, particularly microchannel processes which operate under generally high pressures and, optionally, generally high temperatures, and, more particularly, microchannel processes which comprise endothermic reactions such as steam methane reforming (SMR), and, optionally, exothermic reactions such as combustion. Control includes, particularly, methods of startup and shutdown of such processes. 
     2. Description of Related Art 
     Microchannel devices have demonstrated the capability of providing improved conversion of reactants to products as well as improved selectivity to desired products relative to undesired products and recent years have seen a significant increase in the application of microchannel processes to many unit operations. See, e.g., A. A. Rostami et al.,  Flow and Heat Transfer for Gas Flowing In Microchannels: A Review,  38 Heat and Mass Transfer 359-67 (2002) (applications in medicine, biotechnology, avionics, consumer electronics, telecommunications, metrology, and many others) and R. S. Wegeng et al.,  Compact Fuel Processors for Fuel Cell Powered Automobiles Based on Microchannel Technology , Fuel Cells Bulletin No. 28 (2002) (compact hydrogen generators for fuel cells). Microchannel processes utilize microchannel devices for carrying out unit operations that had previously been constrained to far larger equipment—often three to 1,000 times as large for comparable total throughput. Devices for microchannel processes, which microchannels contain features of at least one internal dimension of width or height of less than about 2 mm and preferably less than about 1 mm, have the potential to change unit operations in ways analogous to the changes that miniaturization has brought to computing technology. Microchannel processes can be used to advantage in small-scale operations, such as in vehicles or personal (portable) devices. 
     Importantly too, microchannel processes that can be economically mass-produced and connected together to accomplish large-scale operations are very desirable. For example, hydrogen gas is an important material in the operation of a petroleum refinery. The ability to economically generate hydrogen from a natural gas supply (i.e., methane) is important to such an operation and is typically effected, in part, via a reformer. In an SMR operation, for example, methane is catalytically reacted with water in the form of steam in the following reaction:
 
CH 4 +H 2 O→3H 2 +CO.
 
SMR being an endothermic reaction, a combustion reactor is often combined with the reformer to provide the necessary thermal energy. Notably, the reformer is operated at a temperature of about 650-1,000 deg. C. and a pressure of about 300 psig. Many microchannel devices utilized for unit operations such as SMR include a multi-planar design which then must operate in the high temperature and pressure regimes noted. Unlike a tubular reactor, a multi-planar device does not easily handle such pressures at the temperatures required.
 
     Although not exclusively, these process units are typically constructed by laminating multiple planar sheets together where some sheets comprise openings which cooperate with other sheets to form microchannels. See, e.g., Schmitt, “Method of Fabricating Multi-Channel Devices and Multi-Channel Devices Therefrom”, U.S. Pat. No. 6,851,171 and Mathias et al., “Multi-Stream Microchannel Device”, U.S. Pat. Pub. No. 2004/0031592 A1. In addition to the structural integrity issues raised by planar elements and laminations and the temperature and pressure issues noted above, thin walls to reduce weight and improve heat transfer add further complexity. This is even more evident during startup and shutdown (both normal and emergency and including shutdown and subsequent “hot startup”) when temperature and pressure dynamics can be most difficult to control and which have the potential to damage the device or create hazardous conditions when flammable or potentially explosive mixtures are present. Thus, excess pressure differentials and uneven heating and “hot spots” in the device must be avoided or minimized. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In accordance with the present invention, a method of starting up a microchannel process includes the steps of providing a first multi-planar process unit, providing a containment vessel at least partially containing the process unit, establishing a first containment vessel pressure within the containment vessel, sensing for leaks from the containment vessel, establishing a first process unit pressure within the first process unit, and sensing for leaks from the first process unit. 
     In a preferred embodiment, the method further includes the steps of providing a catalyst within the first process unit, providing a second multi-planar microchannel process unit, the second multi-planar microchannel process unit at least partially contained within the containment vessel, establishing a second containment vessel pressure within the containment vessel, checking for leaks from the containment vessel, initiating a first unit operation within the first process unit, initiating a second unit operation within the second process unit, and maintaining a differential between a containment vessel pressure and a first multi-planar microchannel process unit pressure, and, optionally, between the containment vessel pressure and a second multi-planar microchannel process unit pressure. More preferably, the first multi-planar microchannel process unit and the second multi-planar process unit are heated by introducing a stream to the second multi-planar process unit, the stream comprising a dilute low-thermal energy density material, decreasing the proportion of diluent in the stream (increasing the concentration of low-thermal energy density material), and replacing the low-thermal energy density material with a high-thermal energy density material. Thermal energy is transferred between the first process unit and the second process unit. Even more preferably, the first multi-planar microchannel process unit processes an SMR reaction and the second multi-planar microchannel process unit processes a combustion reaction. 
     In further accordance with the present invention, a method of starting up a microchannel process includes providing a microchannel process adapted to process a combustion operation, introducing a combustible compound and a diluent and an oxidizing compound to the process unit, initiating the combustion reaction, and decreasing the proportion of diluent. In a preferred embodiment, the method further includes providing a combustible compound comprising a low-thermal density material and replacing the low-thermal energy density material with a high-thermal energy density material. 
     In yet further accordance with the present invention, a method of shutting down a microchannel process includes providing a first microchannel process unit, discontinuing the flow of a first and a second reactant to the process unit, and introducing a fluid to the first process unit. In a preferred embodiment, the first microchannel process unit is at least partially contained within a containment vessel and the fluid is introduced from the containment vessel to the first process unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of an exemplary microchannel process according to the present invention. 
         FIGS. 2 and 2A  is a flow diagram of an exemplary microchannel process startup according to the present invention. 
         FIG. 3  is a flow diagram of an exemplary microchannel process emergency shutdown according to the present invention. 
         FIG. 4  is a flow diagram of an exemplary microchannel process high-temperature startup according to the present invention. 
         FIG. 5  is a flow diagram of an exemplary microchannel process medium-temperature startup according to the present invention. 
         FIG. 6  is a flow diagram of an exemplary microchannel process low-temperature startup according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following reference indicators are provided as an aid to an understanding of the figures:
       10  microchannel process     20  containment vessel     30  first microchannel process unit     40  second microchannel process unit     50  containment vessel inlet     60  containment vessel vent     70  first microchannel process unit inlet     80  first microchannel process unit outlet     90  first process unit inlet to second microchannel process unit     100  second process unit inlet to second microchannel process unit     110  second microchannel process unit outlet     150  process unit—process unit heat transfer   A first reactant material   B first catalyst activation material   C second reactant material   D pressurizing material   E third reactant material   F fourth reactant material   G vented/flared material   H first products material   I second products material   J purge material   

     Reference to  FIG. 1 , an exemplary microchannel process  10 , will assist in an understanding of the invention. At least one microchannel process unit  30 ,  40  of a design and construction suitable for the unit operation(s) of interest is at least partially contained within a containment vessel  20 . While two microchannel process units  30 ,  40  are shown, one or a large plurality of process units  30 ,  40  may be included. For example, it is known in the microchannel art to construct a device which embodies hundreds or even thousands of individual reactors, oftentimes in a configuration which interleaves a plurality of unit operations. In this way, for example, a unit which processes an endothermic reaction may be placed in close contact with a unit which processes a combustion or exothermic reaction. More particularly, the first microchannel process unit  30  comprises a catalyzed endothermic SMR reaction and the optional second microchannel process unit  40  comprises an optionally catalyzed exothermic combustion operation. For optimal performance, the first microchannel process unit  30  and the second microchannel process unit  40  are placed in close proximity to promote the transfer of thermal energy  150  from one unit to another. 
     Piping and stream flows include a containment vessel inlet  50  via which pressurizing material D may be introduced into the containment vessel  20 . The pressurizing material D may comprise any suitable material and generally a readily-available inert gas such as nitrogen is used. Depending upon the operation, however, steam or non-combustible or non-oxidizing material may be suitable as may reactive material. Compressor(s) (not shown) for boosting the pressure of the pressurizing material D may also be utilized. Finally, the pressurizing material D may be introduced to other regions of the microchannel process  10  via, for example, a first microchannel process inlet  70  to the first microchannel process unit  30  and the first inlet to the second microchannel process unit  90  to the second microchannel process unit  40 . 
     Piping exiting the containment vessel  20  may include, nominally, a vent  60  through which vented or flared material G may flow. Purge material J may also exit the containment vessel  20  and be directed to the first microchannel process unit  30  through the first microchannel process inlet  70  and/or the second microchannel process unit  40  (via the first inlet to the second microchannel process unit  90  shown). 
     Piping and stream flow to and from the first microchannel process unit  30  will be suitable for the unit operation desired. As shown in  FIG. 1 , for example, in addition to the pressurization material D, a first reactant material A and a second reactant material C may be introduced into the first microchannel process unit  30  via the first microchannel process unit inlet  70 . In the exemplary SMR operation, the first reactant material A would comprise steam and the second reactant material C natural gas or other mixture than contains methane. Optionally, a first catalyst activation material B, combined with, for example, nitrogen from pressurization material D, may be introduced into the first microchannel process unit  30  via the first microchannel process unit inlet  70 . In the exemplary SMR operation utilizing a Group VIII-, e.g., nickel-, or preferably, a rhodium-based catalyst, the first catalyst activation material B comprises hydrogen. Provision may also be made for introducing a material J from the containment vessel  20 , via the first microchannel inlet  70 , to the first microchannel process unit  30  for, for example, purging. In the case of a shutdown, and particularly an emergency shutdown, the availability of an inventory of hot purge material J, is highly desirable. Damage from thermal shock may be minimized or avoided by purging with a purging material J that is at, or nearly at, the temperature of the at least one microchannel process unit  30 ,  40 . Finally, first products material H may be withdrawn from the first microchannel process unit  30  via a first microchannel process unit outlet  80 . 
     So, too, piping and stream flow to and from the optional (or additional) second microchannel process unit  40  will be suitable for the unit operation desired. As shown in  FIG. 1 , for example, in addition to the pressurization material D, a third reactant material E may be introduced into the second microchannel process unit  40  via the first inlet to the second microchannel process unit  90  and a fourth reactant material F via the second inlet to the second microchannel process unit  100 . In the exemplary SMR operation, the second microchannel process unit  40  processes a combustion reaction unit operation. The third reactant material E comprises combustible fuel such as natural gas and the fourth reactant material F comprises an oxidizer such as air. As with the first microchannel process unit  30 , provision may be made for introducing the material J from the containment vessel  20 . Finally, a second product material I may be withdrawn via the second microchannel process unit outlet  110 . 
     In the exemplary SMR operation, the first microchannel process unit  30  effects an endothermic reformation reaction unit operation and comprises at least one microchannel (not shown) and, optionally, a suitable catalyst (not shown). The second microchannel process unit  40  effects an exothermic combustion reaction unit operation by combining the third reactant material E (e.g., natural gas, hydrogen, or other suitable fuel) with the fourth reactant material F (e.g., air or other suitable oxidizer). Process unit—process unit heat transfer  150  enables the heat generated in the second microchannel process unit  40  to be utilized in the first microchannel process unit  30 . The first product material H comprises a typical yield of 75 percent hydrogen, 15 percent carbon monoxide, and ten percent carbon dioxide. The second product material I comprises combustion products. 
     Cold startup of the microchannel process  10  begins ( FIGS. 2 and 2A ) with the standard preparation steps of inspecting all utilities, control equipment, and valve alignment followed by a full system inspection. (Exemplary plant parameters reflect a first microchannel process unit  30  performing SMR and a second microchannel process unit  40  performing combustion.) The containment vessel  20  is then pressurized with pressurizing material D, preferably with an inert such as nitrogen, and preferably to a standard plant supply pressure of, for example 90 psig. (While exemplary pressures of above-atmospheric are shown and discussed, it will be appreciated by those skilled in the art, that sub-atmospheric pressures may also be considered within the scope and spirit of the invention.) After checking for leaks from the containment vessel  20  using traditional methods (e.g., loss of pressure in the containment vessel  20  or pressure gain in either the first or second microchannel process unit  30 ,  40 ), the containment vessel  20  is next pressurized with pressurizing material D to a pressure higher than nominal working pressure, for example to 400 psig. The containment vessel  20  is again checked for leaks using traditional methods. 
     Next, the first microchannel process unit  30  is pressurized with pressurizing material D, preferably to a standard plant supply pressure. Preferably, the pressure in the containment vessel  20  is maintained at the higher (above nominal working pressure) pressure. Checks are made for leaks, particularly to the second microchannel process unit  40 . The first microchannel process unit  30  is then pressurized with pressurizing material D to a pressure higher than nominal working pressure, for example to 400 psig, the pressure in the containment vessel is reduced to a minimal value, for example 10 psig, and the first microchannel process unit  30  checked for leaks into the containment vessel  20 . Note that this reverse pressure differential may be tolerated by the first microchannel process unit  30  at lower temperatures. At higher temperatures, in the 650-1,000 deg. C. range, such a differential may not be tolerated in a microchannel unit. The first microchannel process unit  30  is then depressurized to a minimal value, for example 10 psig, through the first microchannel process unit outlet  80 . If the pressurizing step for the first microchannel process unit  30  utilizes an inert such as nitrogen, the depressurizing step acts as a purge and reduces the oxygen content in the first microchannel process unit  30 , depending upon the pressures utilized, from  21  percent to just over one percent. The same effect can be had in the containment vessel  20 . Optionally, if required, the pressurization/depressurization steps may be repeated until an acceptable level of oxygen is achieved. The optional second microchannel process unit  40  is similarly pressured checked and purged as required. Following completion of the pressure checks, the containment vessel  20  is pressurized with pressurizing material D to its operating pressure of, for example 300 psig. 
     In the exemplary catalyzed SMR operation, for example, the catalyst may require an activation step. Suitable materials, for example pressurizing material D in the case of nitrogen and first catalyst activation material B (hydrogen, e.g.), are flowed over the catalyst in the first microchannel process unit  30 . Concurrently, the catalyst may be heated at a controlled rate, preferably, in the case of an SMR, of 50 deg. C. per hour. Upon reaching a pre-established temperature, preferably about 300 deg. C., the catalyst is held at that temperature for a suitable length of time, preferably one hour. In the case of nitrogen and hydrogen, the hydrogen level is preferably controlled at or below ten percent. 
     Prior to actual startup of the first microchannel process unit  30 , the first microchannel process unit  30  is purged as required. To startup the first microchannel process unit  30 , the first reactant material A is introduced. In the case of the exemplary SMR operation, the first reactant material A comprises steam. When the flow of the first reactant material A is established and any desired pressure or temperature levels achieved, the second reactant material C is introduced into the first microchannel process unit  30 . (In the case of SMR, the desired temperature level would be about 300 deg. C. Also, in the case of SMR, the second reactant material C comprises a methane-based material such as natural gas.) Since SMR is an endothermic reaction, the conversion of steam and methane is self-limiting without a heat source. Beginning an endothermic reaction in the first microchannel process unit  30  and then initiating an exothermic reaction in the second microchannel process unit  40  reduces the chances of a “hot spot” in the microchannel process  10  as the material in the first microchannel process unit  30  acts as a heat sink. Note that with a catalyzed SMR process and activated catalyst, excessive exposure to steam can at least partially deactivate the catalyst. Thus, if the addition of the second reactant material C (e.g., natural gas for SMR) is delayed, material should be added with the first reactant material A (e.g., catalyst activation material B (hydrogen) to steam for SMR). 
     To startup the optional second microchannel process unit  40 , the fourth reactant material F is introduced. (Air or other oxidizer in an SMR case with a combustor.) The third reactant material E (e.g., hydrogen) is then introduced. Optionally, and preferably for an SMR, either the third reactant material E, the fourth reactant material F, or both are initially diluted with, for example, a pressurizing material D such as nitrogen. This provides the benefit of controlled heating of the first microchannel process unit  30 . The temperature of the first microchannel process unit  30  is allowed, in a controlled manner (e.g., 50 deg. C./hour), to rise to operating conditions (e.g., 850 deg. C.). As thermal control may be critical in the microchannel environment, temperature increases are closely controlled. For example, introducing a high-thermal density material (e.g., methane) at the outset, may cause hot spots to form which may damage the integrity of the process units  30 ,  40 . To overcome this problem, a low-thermal density material (e.g., hydrogen), preferably a dilute low-thermal density material, may be introduced as the third reactant material E. In the exemplary SMR case, the third reactant material comprises five percent hydrogen. As the temperature increases, the proportion of hydrogen is increased to, e.g., 15 percent. This allows a modest and easily-controlled temperature rise. Preferably, and at an appropriate time in the startup process, a high-thermal density material (e.g., methane), preferably a dilute high-thermal density material is introduced in place of the low-thermal density material. Again, in the exemplary SMR case, the third reactant material comprises five percent methane. As the temperature increases, the proportion of methane is increased and operating conditions established as required. 
     Importantly, presence of the containment vessel  20  operating as a thermal blanket over, for example, the second process unit  40 , enables improved thermal control. Where the second process unit  40  comprises a catalyzed combustion unit operation, the catalyst may be required to function at a designed temperature of 700-900 deg. C. but may be required to be active at a much lower temperature for startup. Thus, the ability to raise the temperature of the second process unit  40  from ambient to, for example, 300 deg. C., enables greater catalyst design flexibility. Filling the containment vessel  20  with, for example, superheated steam, can achieve such temperatures. Additionally, during normal operation the pressurizing material D contained within the containment vessel  20  may function to reduce heat loss from the first process unit  30  and the optional second process unit  40  to ambient could be reduced, thus reducing any temperature gradient within the process units  30 ,  40  resulting in potentially improved overall performance. For example, with an exemplary endothermic reaction such as SMR, lower temperatures in peripheral microchannels could cause lower conversion in those channels. 
     Shutdown, particularly an emergency shutdown ( FIG. 3 ) which is performed in a short timeframe, must be accomplished not only quickly, but safely and with consideration to the process units  30 ,  40  and any catalysts employed. Initially, the flow of a reactant material to the first process unit  30  is reduced and stopped. In the SMR case, for example, this is the second reactant material C (e.g., natural gas). The flow of the first reactant material A to the first process unit  30  is also reduced and stopped. As will be appreciated by those skilled in the art, these steps may be done in either order or simultaneously. To avoid off-spec reactant material, the first products material H may be diverted (not shown) to a containment vessel vent  80 . The optional second process unit  40  may be shutdown similarly by terminating flow of the third reactant material E to the second process unit  40 . Preferably, the second reactant material C is purged from the first process inlet  70  with purge material J from the containment vessel  20 . Additionally, the third reactant material E is likewise purged from the first process inlet  70 . Finally, the process units  30 ,  40  are purged with purge material J from the containment vessel  20 . This process provides an important thermal management benefit because it utilizes hot material from the containment vessel  20  which may supplied in sufficient quantities in a short timeframe. Again, as will be appreciated by those skilled in the art, these purge steps may be performed in varying orders and to varying feed lines to meet the unit operations specifics of the process. 
     Depending upon the conditions, particularly the thermal conditions, existing at the time of startup or restart, the startup process may be abbreviated. For example, the catalyst may be hot enough to work well (e.g., a palladium combustion catalyst would work above 500 deg. C. for combustion of methane, but would not have sufficient activity to ignite methane combustion below 400 deg. C.) without supplemental heating. Importantly, the process units  30 ,  40  may be hot enough to proceed with high temperature operation without creating undesirable hot spots. 
     Turning now to  FIG. 4 , if the process units  30 ,  40  are above a threshold temperature (e.g., 500 deg. C. in an SMR operation), the first reactant material A and second reactant material C may be introduced to the first microchannel process unit  30  forthwith. If the second process unit  40  processes a combustion unit operation, it is generally desirable to purge any combustibles (e.g., hydrogen or methane) from the first process unit inlet to the second microchannel process unit  90  and the second microchannel process unit  40 . Preferably, inert material such as the pressurizing material D (e.g., nitrogen) may be used. In the hot startup case shown in  FIG. 4 , high-thermal density fuel such as methane (third reactant E) may be introduced, albeit preferably in a dilute stream, to the second microchannel process  40 . Flow of the third reactant E may then be increased to effect an increase in temperature of the first microchannel process unit  30  as required. 
       FIG. 5  shows a mid-range temperature situation. In the exemplary SMR operation, the range is 300-500 deg. C. As shown,  FIGS. 4 and 5  are identical. In the exemplary SMR case, however, there would be a difference. Where the hot startup case of  FIG. 4  utilizes a third reactant E comprising dilute high-thermal density fuel (e.g., methane), the mid-range temperature case preferably uses a third reactant E comprising dilute low-thermal density fuel (e.g., hydrogen) initially, which is then gradually converted to high-thermal density fuel (e.g., methane) as required. 
     Finally,  FIG. 6  shows a low-temperature situation. In the exemplary SMR operation, this is below 300 deg. C. and the startup procedure mimics the “Initial (Cold) Startup” illustrated in  FIGS. 2 and 2A  without the pressurization checks. 
     Following from the above description and invention summaries, it will be appreciated by those skilled in the art that, while the processes and methods described herein and illustrated constitute exemplary embodiments of the present invention, the invention is not limited to those precise embodiments and that changes and modifications may be made thereto without departing from the scope of the invention as defined by the claims. Likewise, it is to be understood that the invention is defined by the claims and it is not intended that any limitations or elements describing the exemplary embodiments set forth herein are to be incorporated into the claims unless explicitly recited in the claims themselves. Finally, it is to be understood that it is not necessary to meet any or all of the recited advantages or objects of the invention disclosed herein in order to fall within the scope of any claim, since the invention is defined by the claims and since inherent and/or unforeseen advantages of the present invention may exist even though they may not have been explicitly discussed herein.