Abstract:
A fluid distribution or fluid extraction structure for honeycomb-substrate based falling film reactors is provided, the structure comprising a one or two-piece non-porous honeycomb substrate having a plurality of cells extending in parallel in a common direction from a first end of the substrate to a second and divided by cell walls, and a plurality of lateral channels extending along a channel direction perpendicular to the common direction, the channels defined by the absence of cell walls or the breach of cell walls along the channel direction, the channels being closed or sealed to fluid passage in the common direction but open to the exterior of the structure through one or more ports in a side of the structure, the channels being in fluid communication with the plurality of cells via holes or slots extending through respective cell walls, the holes or slots having a width and a length, the width being equal to or less than the length, and the width at widest being less than 150 μm. Methods of fabrication are also disclosed.

Description:
PRIORITY 
       [0001]    This application claims priority to U.S. patent application Ser. No. 61/238301, filed Aug. 31, 2009, titled “FALLING-FILM REACTOR FLUID DISTRIBUTORS AND METHODS”. 
     
    
     BACKGROUND 
       [0002]    The disclosure relates to fluid distributors for falling film reactors and methods for forming them, and more particularly to fluid distributors adapted for use with or within honeycomb monolith substrate based falling film reactors and methods for forming them. 
         [0003]    Referring to  FIG. 1 , gas-liquid falling film reactors have been previously proposed by the present inventors and/or colleagues of the present inventors based on non-porous extruded honeycomb substrates  20  with selective end face machining and plugging. Such devices are disclosed in EP publication no. 2098285, assigned to the present assignee.  FIG. 1  shows a cross-sectional view of such a falling film monolith reactor  10 , with channels  24  closed by plugs  26  or plugging material  26  defining a heat exchange fluid path  28 , typically a serpentine path, and neighboring unplugged channels  22  dedicated to falling film reactions. Liquid reactant  21  applied on or near upper end face plugs forms a thin film  25  as it flows down the inner walls of adjacent unplugged channels  22 . Gas reactant  23  flows through the same unplugged channels, enabling a gas-liquid reaction to occur along the entire length of the channels  22 . The figure shows counter-current gas flow but co-current flow is also possible. Reactant fluid that collects at the bottom end face of the substrate can be removed by a variety off fluid guiding, wicking or drop formation methods, such as fluid collector  30 . 
         [0004]    A cross-section view of a falling film reactor assembly  100  with two stacked monolith substrates  20 A and  20 B is shown in  FIG. 2 . Liquid reactant  21  is supplied to a distribution zone  29  that forms a ring around the upper end face of the upper monolith substrate  20 A. This liquid reactant  21  flows around the distribution zone  29 , onto the end face of the monolith substrate  20 A, and then down interior channel sidewalls. A spacer monolith  36  is positioned between the two falling film reactor monolithic substrates  20 A,  20 B to improve reactant flooding performance. Counter-current gas reactant  23  enters at the bottom of the device and exits at the top. Reaction product liquid  21  is collected in a collection structure  30  at the bottom of the device (in this case, a ring-shaped collection structure  30  is used) and removed via one or more tubes  35  attached to the collection structure  30 . Monolith substrate temperature is controlled by introducing heat exchange fluid  37  through side-mounted ports  38 . Various o-ring seals  39 A and epoxy seals  39 B cooperate the collection structure  30  and with cylinders  39 C and an end plate  39 D, preferably of stainless steel, to complete the assembly. 
         [0005]    Rapid exothermic reactions within a falling film reactor can lead to explosions. The heat-exchange channels in the form of the closed channels  24  are positioned in close proximity to falling film reaction channels  22  to help prevent run-away thermal reactions. Some gas-liquid falling film reactors may be used with flammable liquid reactants and/or reaction products, while others may generate flammable or explosive chemical byproducts, liquid or gas. If combustion of these materials is initiated by a spark (via static electricity, for instance) a ripple effect may lead to rapid combustion throughout the entire reactor. Depending on how much heat is given off in the combustion reaction, an explosion may lead to destruction of the reactor and/or risk of injury. 
         [0006]    Propagation of combustion flame fronts through frame barrier structures can be prevented as long as the size of flame barrier internal passageways does not exceed a maximum value. Flame barriers can be formed using fine mesh metal screens or inorganic or metallic materials with maximum open porosity on the order of 75-150 um. With reference to  FIG. 3 , the present inventors and/or their colleagues have previously described flame barrier screens  84  that may be applied to each monolith substrate end face to prevent flame propagation. 
         [0007]    A challenge with use of this type of flame barrier screen  84  is introduction of liquid reactants  21 A into the falling film reaction channel  22  without wetting the flame barrier screen  84 . The concern is that if the flame barrier screen  84  becomes excessively wetted by liquid reactants  21  as they enter the reaction channel  22 , a liquid barrier may under certain conditions form across the screen  84 . This liquid barrier may hamper the formation of a uniformly thick falling film in the reaction channel  22 . The same challenge exists at the lower end face of the monolith substrate where gas-liquid separation takes place. If liquid reaction product  21 B contacts the flame barrier screen  84  the presence of the liquid  21 B on the screen  84  may interfere with the uniform flow of gas reactants  23  through the reaction channels  22 . 
       SUMMARY 
       [0008]    One embodiment is a fluid distribution or fluid extraction structure for honeycomb-substrate based falling film reactors, the structure comprising a one or two-piece non-porous honeycomb substrate having a plurality of cells extending in parallel in a common direction from a first end of the substrate to a second and divided by cell walls, and a plurality of channels extending along a channel direction perpendicular to the common direction, the channels defined by the absence of cell walls or the breach of cell walls along the channel direction, the channels being closed or sealed to fluid passage in the common direction but open to the exterior of the structure through one or more ports in a side of the structure, the channels being in fluid communication with the plurality of cells via holes or slots extending through respective cell walls, the holes or slots having a width and a length, the width being equal to or less than the length, and the width at widest being less than 150 μm. 
         [0009]    A further embodiment includes a method of forming a fluid distribution or fluid extraction structure, the method comprising providing a honeycomb substrate; breaching selected walls of the honeycomb substrate so as to form one or more channels perpendicular to the direction of the cells of the honeycomb substrate; forming slots or holes through sidewalls of the one or more channels; sealing above and below at least a portion of the slots or holes such that the one or more channels become one or more internal channels accessible through the slots or holes; and providing access to the one or more internal channels from the exterior of the substrate. The slots or holes have a width and a length, the width being equal to or less than the length, and the width at widest being less than 150 μm. 
         [0010]    Additional features and advantages will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the embodiments as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
         [0011]    It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understanding the nature and character of the claims. The accompanying drawings are included to provide a further understanding, and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and operation of the various embodiments. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIGS. 1-3  are a cross-sectional views embodiments of a honeycomb-substrate based falling film reactor or reactor assembly previously proposed by the present inventors and/or their colleagues; 
           [0013]      FIG. 4  is one alternative embodiment of fluid distributors useful for a honeycomb-substrate based falling-film reactor; 
           [0014]      FIGS. 5A-5C  are perspective schematic views showing certain steps in the formation of a fluid distributor useful for a honeycomb-substrate based falling-film reactor; 
           [0015]      FIGS. 6A-6B  are perspective schematic views showing certain alternative steps in the formation of a fluid distributor useful for a honeycomb-substrate based falling-film reactor; 
           [0016]      FIG. 7  is a diagrammatic cross-sectional view of a honeycomb substrate based falling film reactor assembly including fluid distributors prepared as in  FIG. 5  or  6 . 
           [0017]      FIG. 8  is a diagrammatic cross-sectional view of an alternative embodiment of the honeycomb substrate based falling film reactor assembly including fluid distributors of  FIG. 7 . 
           [0018]      FIGS. 9 and 10  are perspective schematic views showing certain additional alternative steps in the formation of a fluid distributor useful for a honeycomb-substrate based falling-film reactor; 
           [0019]      FIG. 11  is a close-up perspective view of a portion of an endface of an extruded substrate useful in the context of the present invention; 
           [0020]      FIG. 12  is a diagrammatic cross-sectional view of an alternative embodiment of the honeycomb substrate based falling film reactor assembly including fluid distributors of the type of  FIGS. 9  or  10  and  11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0021]    The following description provides details of some embodiments of the present invention. Like features will generally be referred to with the same or similar reference characters across all of the figures herein. 
         [0022]      FIG. 4  shows the present inventors and/or their colleagues have developed porous monolith substrates  20 A,  20 B that can be integrated with a non-porous falling film monolith substrate  20  to provide fluid distribution and liquid reaction product collection. One porous monolith substrate  20 A is mounted on the upper end face of the non-porous monolith substrate  20  with its axial internal cells  41  aligned with the non-porous substrate falling film reaction channels  22 . A flame barrier screen  84  is positioned on top of the porous monolith substrate  20 A to prevent unwanted flame propagation between reaction channels  22 . A similar substrate  20 B is employed on the lower face of the substrate  20 . 
         [0023]    Liquid reactant  21 A flows into the porous monolith substrate  20 A through lateral internal channels  46  defined in part by non-porous plugs  44 . The fluid is fed to channels  46  via an internal or external fluid manifold (not shown in the cross section of the figure). The liquid reactant  21 A flows through the porous walls of the monolith substrate  20 A, forms a thin film on the sidewalls of the axial internal channels  41 , and then flows downward into the non-porous monolith substrate falling film reaction channel  22 . While this type of fluid distributor has many advantages, a potential challenge in this approach is that cells of the porous monolith substrate  20 A must be well-aligned to cells of the nonporous monolith substrate  20 . Since monolith substrate cells sometimes experience distortion in extrusion and/or sintering it may be difficult to make cells in two different monolith substrates  20 A,  20  line up with each other. 
         [0024]    The present disclosure accordingly focuses on improved honeycomb-extrusion based falling film reactor fluid distribution and collection structures, particularly those having improved registration or fit with an associated reactor, and low-cost fabrication methods for providing such structures. Throughout this document references made to fluid distributors at the top of a monolith-substrate-based falling film reactor will also be assumed to apply to fluid collectors at the bottom of the substrate. These structures can be formed using non-porous monolithic substrates mated with other non-porous falling film monolith substrates, or, in an alternative embodiment, can be integrated into the same substrate that houses the reaction channels. In both cases non-porous plugs are desirably used to confine fluids within the distribution structures. Improved fluid distribution channels and flame barriers can also be integrated into these structures, as will be shown below. 
         [0025]    Reference will now be made in detail to the accompanying drawings which illustrate certain instances of the methods and devices described generally herein. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. One embodiment of a falling film reactor with fluid distributors is shown in  FIG. 7 , and is designated generally throughout by the reference numeral  10 .  FIGS. 5A-5C  and  6 A- 6 B show various alternative methods of providing fluid distributors for the reactor  10  of  FIG. 7 . 
         [0026]    To substantially avoid difficulties in aligning cells on mated fluid distributor and falling film reactor substrates  20 A and  20 , the substrates  20 A and  20  can be fabricated from adjacent portions of a single extruded log. To maintain alignment during shrinkage that normally occurs during sintering both substrates are then sintered in identical conditions so that they are both non-porous. As another option, the full desired length of reactor plus fluid distributor(s) may be sintered as one piece, and then sawed apart. The following describes various techniques for incorporating fluid distribution and flame barrier structures into the resulting non-porous distributor structures. 
         [0027]      FIGS. 5A-5C , are perspective views of certain steps in the preparation fluid distributors for honeycomb-based falling film reactors. Initially a honeycomb substrate  40  is provided, such as by forming via extrusion or other suitable means, and then desirably kept in the green state through the steps shown in  FIGS. 5A and 5B , although these steps may also be performed after final firing or sintering. The substrate  40  has multiple channels  86  extending through the substrate  40  from a first end  80  to a second end  82  thereof and is non-porous, or at least non-porous after final firing or sintering. Methods and materials for producing such bodies are known in the art of ceramic honeycomb extrusion. Suitable materials can include, but are not limited to, cordierite, aluminum titanate, silicon carbide, alumina, and so forth. 
         [0028]    The substrate  40  is preferably of relatively thin but uniform thickness in the direction of the channels from the first end  80  to the second end  82 . For example, the substrate may be in the range of 3-15 mm thick, more preferably about 5-8 mm thick. A green extruded substrate may be relatively easily sawn to a size in this range, for example. 
         [0029]    Desirably (but not necessarily in every instance) while the substrate  40  is still in the green state, selected cell walls  45 , in this case those positioned between cells of the odd numbered rows  43 , are breached so as to join selected ones of channels  86  so as to produce one or more open lateral passages  42  extending in a direction crossways to the direction of the channels. Breaching may be performed, for example, by removing the walls by machining them away, as shown in  FIG. 5B . Machining may be performed in any suitable manner, such as wire saw cutting, laser cutting, water jetting, or the like. Alternatively, breaching may be performed by drilling holes  200  through the row, as shown in  FIG. 6A . Removing walls as in  FIG. 5B  can allow for complex patterns, but drilling as in  FIG. 6A  may be preferred for ease of execution, if the depth of drilling required is not too deep. In either case, selected ones of the channels  86  are thus joined by the breached walls, so as to produce one or more open lateral passages  42  extending in a direction crossways to the direction of the channels, as shown in  FIGS. 5B and 6A . In the embodiments shown in  FIGS. 5A-5C  and  6 A- 6 B, the lateral passages  42  are formed in the odd numbered rows  43 . Machining can be used remove cell walls completely, as shown in  FIG. 5B , or may only remove walls to a significant degree, such as 60-80%, leaving shortened walls in place (not shown) if needed to help preserve the stability of the extruded substrate  40 , or for any other desirable reason. 
         [0030]    Either before or after breaching, microchannels  70  are machined through the sidewalls  49  that divide the lateral passages  42  from the axial internal cells or channels  41 . This machining may be performed by a laser L with the extruded substrate  40  in the green state or in the sintered state. The beam size and motion of the laser L are selected such that the width W of the microchannels  70  is not greater than 150 micrometers, desirably not greater than 100 micrometers, and most desirably, for some applications, not greater than 50 micrometers. 
         [0031]    As depicted generally by the alignment of the laser L in  FIG. 5A , the laser machining of microchannels  79  may be carried out from the side of the substrate  40 , and may open microchannels through all of the walls laterally across the honeycomb structure (with microchannels inside the honeycomb not visible in perspective view of  FIG. 5 ). The outermost microchannels, such as those visible in  FIGS. 5A and 5B , are later filled so that no microchannel access to the exterior side  90  of the substrate  40  remains, as in  FIG. 5C . As depicted generally in  FIG. 6A , the microchannels  70  need not be round, but may be oblong as shown. Also as a further alternative, the microchannels  70  do not have to be machined by a laser from the side of the substrate  40 . They can also be formed, particularly if oblong, by a steep-angle laser beam tilted roughly as shown by the (optional) position of laser L in  FIG. 6A . Thus in this optional embodiment the outside wall  90  is never machined so subsequent plugging is not required, although a larger number of laser cuts is required, since multiple dividing walls  45  are not machined at once. 
         [0032]    Where the microchannels are not round, but have a length (greatest dimension) and a width (lesser dimension), the largest width should be no more than 150 micrometers, desirably not greater than 100 micrometers, and most desirably, for some applications, not greater than 50 micrometers. 
         [0033]    Either before or after machining microchannels  70 , the lateral passages  42  are plugged at the top and bottom thereof with a non-porous plugging material  44 , as shown in  FIGS. 5C and 6B . The plugs  26  or plugging material  26  may be positioned level with the top and bottom ends  80  and  82  of the substrate  40 , and have plugging depth set relative to each other such that enclosed lateral passages  46  are formed between the respective opposing walls of the substrate  40  and the respective upper and lower plugs  44  within the (formerly open) lateral passages  42 . As mentioned the substrate  40  is desirably an extruded green substrate, and as such may be plugged before sintering using green plugs, or after sintering using post-sinter-CTE matched organic plugs or inorganic epoxy plugs. Cells above falling film channels may optionally be plugged with porous plug material  88  or porous plugs  88  (shown in  FIG. 7 , but not in  FIGS. 5 and 6 ) to also serve as a flame barrier. After the non-porous fluid distributor is plugged it is aligned and attached to the upper surface of the falling film reactor. The resulting reactor is shown in diagrammatic cross section in  FIG. 7 . Reactant liquid  21 A flows from lateral internal channels  46  in the substrate  40 A through machined microchannels  70  into the reaction channels or open cells  22  of the main monolith substrate  20 . Product liquid  21 B is removed in similar fashion by substrate  40 B by means of overpressure in the cells  22  or partial vacuum in the lateral internal channels of substrate  40 B. 
         [0034]    As mentioned above, a non-porous substrate fluid distributor may also be integrated with a falling film reactor substrate in one extruded substrate. The laser machining process for fabricating non-porous fluid distributor sidewall microchannels can also be applied to the falling film substrate. In this case the separate distributor substrate ( 40 A) is eliminated and all processing takes place on the central substrate of the falling film substrate  40 ,  20 . As with the previous example a laser is directed at the non-porous substrate sidewall from the side, above or below to form one or more microchannels of the preferred size(s) mentioned above so as both pass fluid and prevent flame propagation.  FIG. 8  shows two sets of non-porous plugs applied above and below fluid distribution channels within the falling film substrate. The upper non-porous plugs  44  can be applied directly via a plug masking process. The lower non-porous plugs  51  can be fabricated by inserting an injection needle into the respective channel and completely filling a portion of the channel with plug material. 
         [0035]    This approach has the advantage that the fluid distributor and collector are integrated into the falling film substrate. Therefore it eliminates the step of joining any fluid distributor and collector substrates to the falling film substrate. The main challenge is that fabrication of the deep non-porous plugs involves a plug injection process that is most likely carried out serially over each end face. In a production-grade process plug injection could be performed more rapidly by providing multiple injectors so plugs can be injected at multiple locations on the substrate end face simultaneously. 
         [0036]    In the previous non-porous fluid distributor approach microchannels were formed by directly a laser through selected walls of the falling film substrate. A similar microchannel structure for fluid distribution can be created by joining a separate distributor substrate with a falling film substrate as shown and described below with respect to  FIGS. 9-12 . In the approach shown in  FIGS. 9-12 , the fluid distribution channel and flame barrier are formed by the union of the distributor substrate  40 A and falling film substrate  40 . First a fluid distributor similar to that in  FIG. 5C  is prepared, but without the lower plugs, resulting in the structure shown in  FIG. 9 . Alternatively, a fluid distributor similar to that in  FIG. 6B  may be prepared, but again without the lower plugs, resulting in the structure shown in  FIG. 10 . 
         [0037]    To create the microchannels  70  required for fluid transport from fluid distributor channels  46  to the falling film channels  22 , narrow slots or trenches  71  are selectively machined at the distributor substrate/falling film substrate interface on the distributor substrate and/or falling film substrate, as shown in the magnified partial perspective view of  FIG. 11 .  FIG. 11  shows an example of narrow slots  71  selectively formed on a portion of an end face of a distributor substrate  40  or of a reactor substrate  20 . The narrow slots  71  can be mechanically machined via a precision dicing saw or formed via laser ablation. In both cases slots that are 50-150 um wide can be formed in the substrate walls. Experiments show that green substrate material is relatively easy to machine via mechanical sawing or laser ablation. Precision microstructures formed using these techniques are well-preserved during sintering. Sintered ceramic can also be machined, if not quite as easily. 
         [0038]    Once narrow slots  71  are selectively micromachined into the distributor and/or falling film substrates, porous plugs  88  and non-porous plugs  44 ,  51  are applied to the distributor as shown in  FIG. 12 . Non-porous plugs  51  are also selectively applied to the falling film substrate. These non-porous plugs prevent leakage of heat exchange from the falling film substrate, and also guide fluid within the distributor after assembly. 
         [0039]    Next the distributor substrate  40 A is mounted on the falling film substrate  40 , aligned and then attached using chemically-resistant adhesive or pressure via an externally applied clamping approach. The narrow slots  71  form through-holes or microchannels  70  that are no more than 50-150 um wide. The small channel size enables fluid transport to the falling film channels while preventing flame propagation. 
         [0040]    In an alternative approach the separate distributor substrate can be eliminated if the depth of the machined slots can be made to exceed the typical plugging depth. The resulting structure appears similar to the one shown in  FIG. 8 , but with a machined slot that extends from the end face of the substrate to the location where the micromachined microchannel  70  is shown in the figure. The plug material will only plug portions of the slot that are close to the substrate end face, leaving the portions of the slot closer to the center of the falling film substrate unplugged for fluid transport. This also requires double plugging, where the fluid distribution channels are defined by an upper and lower plug. Experimental 
         [0041]    Laser ablation of narrow trenches in green alumina substrate end face walls has been demonstrated under a variety of laser conditions. In one experiment a 6 mm thick slice sample from a 2″ diameter green 200/12 alumina substrate was mounted on a laser translation stage. A scanning laser beam system above the sample directed a focused laser beam downward upon the exposed edges of substrate channel walls. When operating, the laser beam is scanned along a linear path one or more times at a user-defined velocity. 
         [0042]    In another laser experiment trenches as narrow as ˜30 um were fabricated in alumina using a Lumera Picosecond laser (355 nm wavelength, ˜20 um spot using 100 mm F-Theta lens, 100 kHz repetition rate, 10 cm/sec sweep speed). Laser cutting produced very clean cuts with no evidence of thermal damage. 
         [0043]    The methods and/or devices disclosed herein are generally useful in performing any process that involves mixing, separation, extraction, crystallization, precipitation, or otherwise processing fluids or mixtures of fluids, including multiphase mixtures of fluids—and including fluids or mixtures of fluids including multiphase mixtures of fluids that also contain solids—within a microstructure. The processing may include a physical process, a chemical reaction defined as a process that results in the interconversion of organic, inorganic, or both organic and inorganic species, a biochemical process, or any other form of processing. The following non-limiting list of reactions may be performed with the disclosed methods and/or devices: oxidation; reduction; substitution; elimination; addition; ligand exchange; metal exchange; and ion exchange. More specifically, reactions of any of the following non-limiting list may be performed with the disclosed methods and/or devices: polymerisation; alkylation; dealkylation; nitration; peroxidation; sulfoxidation; epoxidation; ammoxidation; hydrogenation; dehydrogenation; organometallic reactions; precious metal chemistry/homogeneous catalyst reactions; carbonylation; thiocarbonylation; alkoxylation; halogenation; dehydrohalogenation; dehalogenation; hydro formylation; carboxylation; decarboxylation; amination; arylation; peptide coupling; aldol condensation; cyclocondensation; dehydrocyclization; esterification; amidation; heterocyclic synthesis; dehydration; alcoholysis; hydrolysis; ammonolysis; etherification; enzymatic synthesis; ketalization; saponification; isomerisation; quatemization; formylation; phase transfer reactions; silylations; nitrile synthesis; phosphorylation; ozonolysis; azide chemistry; metathesis; hydrosilylation; coupling reactions; and enzymatic reactions. 
         [0044]    It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the invention.