Abstract:
Known static micromixers that work according to the principle of multilamination allow for a rapid mixing by diffusion. The invention provides a means for substantially increasing the throughput of known micromixers. To this end, the inventive micromixer for mixing two or more reactants comprises microstructures that define mixer cells. Each of said mixer cells is provided with a feeding chamber which adjoins at least two groups of digital channels. Said channels intermesh with the digital channels of the groups adjoining the feeding channels in a comb-like manner, thereby producing mixing zones]. Outlet ports are located above said mixing zones, said outlet ports extending perpendicularly to the digital channels and discharging the product. The inventive micromixer is especially useful for the large-scale production of mixtures, dispersions and emulsions.

Description:
FIELD OF THE INVENTION 
     The invention relates to a micromixer for mixing at least two reactants having penetrations for the supply of the reactants and discharge of the product, having at least one mixing plate with microstructures that define mixer cells, each of said mixer cells having a feeding chamber which adjoins at least one group of digital channels which intermesh in a comb-like manner with the digital channels of a group from the adjoining feeding chamber; and having a discharge plate arranged on the mixing plate, said discharge plate having an outlet port above each mixing zone, said outlet port extending perpendicularly to the digital channels. 
     BACKGROUND OF THE INVENTION 
     Although microfluid components were developed years ago for analytical applications, microengineering techniques have only recently been applied to the development of equipment for chemical synthesis, so-called microreactors. Principle components of such microreactors are mixers and heat exchangers. Conventional static micromixers work according to the principle of multilamination to ensure rapid mixing by diffusion. This is the only mixing mechanism that can be used with laminar flows in microchannels. The creation of alternating laminations by means of geometric parameters allows good mixing in the microscopic range. 
     The publication Int. Eng. Chem. Res. 1999, 38, 1075–1082, W. Ehrfeld et al., describes a generic micromixer. This single mixer comprises three components: a galvanically and X-ray lithographically structured plate having a mixing zone and two feeding chambers and a two-piece casing in which the plate is set. A means for the supply of reactant and the discharge of the product are provided in the upper section of the casing. 
     The single mixer has two mixer cells with a common mixing zone. The two fluid reactants are fed into the mixing chambers and split into partial flows in the digital channels. The partial flows of one reactant are not in contact with the partial flows of the other reactant—they are separated from one another by microwalls in the form of ribs. The two reactants first come in contact with one another in the port zone, which is above and perpendicular to the digital channels. The product is discharged from the casing through the ports. The pressure drop in the mixing zone is set by means of the port width. 
     A significant disadvantage of this single mixer is that its throughput is very limited. With a pressure drop of approx. 1.2 bar, throughput is only 0.8 l/h. Because of this low throughput, use of the single mixer for large-scale chemical production is limited. In an attempt to alleviate this problem, 10 single mixers were arranged in parallel in one casing, with the reactants supplied to the individual micromixers from a common source. The single mixers were arranged in a star configuration, with the supply line for one reactant in the center of the star and the supply lines for the other reactant running around the outside of the star (at the indicated locations). But this measure only resulted in increasing throughput from 0.8 l/h to approx. 3 l/h with a pressure drop of approx. 1.2 bar. 
     SUMMARY OF THE INVENTION 
     The object of the current invention is to provide a micromixer utilizing the same mixing principle as conventional micromixers but permitting significantly greater throughput at the same pressure drop. 
     This object is achieved by means of a micromixer for mixing two reactants having openings for the supply of the reactants and/or discharge of the product; microstructures that define at least one mixing plate with mixer cells, whereby each mixer cell has a feeding chamber which adjoins at least one group of digital channels, which digital channels intermesh comb-like with the digital channels of a group from the adjoining feeding chambers; and a discharge plate arranged on the first plate, said discharge plate having an outlet port above each mixing zone, said outlet port extending perpendicularly to the digital channels, characterized by the fact that each mixer cell has at least two mixing zones. 
     In the inventive micromixer, the number of microstructures per surface area and thus the throughput is greatly increased. With the inventive micromixer, throughputs of several hundred l/h are achieved with a pressure drop of approx. 1.2 bar. This is many times greater than the throughput of the single mixer and single mixers connected in parallel. The plates of the micromixer can be manufactured of silicon wafers that are structured by means of deep plasma etching and connected to one another by means of anodic bonding, for example. The plates can be also be produced by structuring resist, e.g. lithographically or using a laser, with subsequent galvanic shaping. This is particularly well-suited for producing microstructures with large aspect ratios. 
     The reactants to be mixed in the micromixer can be any combination of gases, fluids, solutions or mixtures thereof. The micromixer is particularly well-suited for the manufacture of mixtures of two fluids or solutions, of fluid—fluid emulsions or gas-fluid dispersions. 
     The width of the individual digital channels is preferably between 5 and 150 μm and the height of the walls defining the digital channels is preferably between 50 μm and 2 mm. A group of digital channels comprises preferably 3 or more channels. Because of the required pressure drop, the width of the outlet ports is preferably between 10 μm to 1 mm and lesser than the height of the walls defining the digital channels. For complete mixing of the reactants, the width of the outlet ports in the discharge plate must be less than the overlap of the adjoining digital channels in the mixing zone. 
     There are two preferred embodiments of the inventive micromixer. Similar to the single mixer, the one embodiment has two feeding chambers, each of which has parallel main channels that intermesh in a comb-like manner, however. Branching off of each main channel are digital channels which likewise intermesh in a comb-like manner and form the mixing zones. This configuration increases both the number of mixing zones per mixer cell and the ratio of the surface area of the mixing zones relative to the total surface area of the mixer cell, and thus the throughput per surface are. The corresponding discharge plate has a multitude of parallel ports whose number is equal to the number of mixing zones. 
     Each feeding chamber preferably has two or more, and more preferably four or more main channels. The digital channels preferably branch off of both sides over the length of the main channels. 
     This micromixer makes it possible to integrate two or more such mixers into a micromixer system. This is done by arranging one micromixer over the other and configuring the ports for feeding the reactants and discharging the products such that the product of the one micromixer is fed to the other micromixer as the second reactant. This makes it possible to produce products whose reactions occur in two or more stages. 
     In the aforementioned variant, the micromixers are fluidically connected in serial, i.e. the product of one micromixer is fed to the next micromixer as one of the reactants. 
     In another embodiment, the micromixers are fluidically connected in parallel, i.e. all micromixers are supplied with the same reactant and the products are discharged together. 
     Both variants can be advantageously realized by stacking mixing plates and possibly additional supply and/or distribution plates. 
     In the second preferred embodiment of the inventive micromixer, mixing zones are arranged on all sides of the feeding chambers in the plane of the plates. Only those feeding chambers at the edge of the plane of the plates have mixing zones on only one or two sides. This increases the ratio of mixing zone area to mixer cell area and thus increases the throughput per surface area. The object of the invention is to arrange as many mixer cells as possible on the mixing plate. It is advantageous for the mixing plate to have 10 or more mixer cells per square centimeter. 
     It is advantageous if the feeding chambers are arranged according to the reactants in rows and/or columns in an alternating pattern. This further reduces the percentage of unutilized surface area. It is particularly advantageous if the feeding chambers are arranged in 4 or more rows and in 4 or more columns. 
     An optimal utilization of the surface area is achieved if the feeding chambers have either a rectangular or triangular outline, with squares or equilateral triangles preferred. The ports of the corresponding discharge plates are located along the edges of the squares or equilateral triangles, which are arranged so as to completely cover the discharge plate. 
     Two approaches have proven to be advantageous for the supply of reactants to the mixing plate. The first approach is to structure that side of the mixing plate facing away from the mixer cells. This creates a storage chamber for each reactant. Parallel channels lead out from each storage chamber and run beneath the feeding chambers. It is important that the channels for the two reactants intermesh in a comb-like manner so that the feeding chambers with one reactant are surrounded by the feeding chambers with the other reactant. Each channel has penetrations beneath the feeding chambers, which penetrations lead into the feeding chambers and through which the reactant can flow into the feeding chambers. It is not mandatory that the mixing plate be a monobody construction, one plate can be manufactured with the microstructures and another plate can have the feed structures, with both plates joined by means of anodic bonding, for example, to form a mixing plate. 
     The other approach for supplying the reactants to the mixing plate is to arrange two additional plates beneath the mixing plate. The one plate together with the mixing plate form a storage chamber for the one reactant and the other plate together with the first plate forms a storage chamber for the second reactant. The reactant in the storage chamber adjoining the mixing plate passed directly into the corresponding feeding chambers via penetrations in the mixing plate. The first additional plate is provided with penetrations for the supply of the other reactant, through which penetrations hollow bodies that also pass through penetrations in the mixing plate and empty into the corresponding feeding chambers are run. The second reactant flows from the second storage chamber into the feeding chambers through these hollow bodies. The flow resistance in these feeding chambers is particularly low so that the reactants are very evenly distributed between the individual feeding chambers. However, this requires more space than does the supply via structures in the back side of the mixing plate. 
     Of particular importance for the use of the micromixer as a microreactor is the integration of a heat exchanger in the micromixer, if necessary. The heat exchanger can be integrated into the micromixer in a variety of ways. The following solutions are preferred: for reactions or mixtures with little heat tone, it is sufficient to arrange hollow bodies on the discharge plate between the ports, through which hollow bodies a heating medium or coolant flows. 
     With greater heat tone, the discharge plate can be a two-piece construction in which two overlapping cover plates are arranged at some distance from one another to form a chamber that is filled with either a heating medium or coolant. To pass the product through the heating medium or coolant, flattened hollow bodies analogous to those in the example described above for the supply of reactant and having an outline corresponding to that of the ports are arranged in the ports of both parts of the discharge plate. This variant provides a particularly homogenous distribution of heat. In another embodiment, the discharge plate can be designed with sufficient thickness to include channels perpendicular to the ports, through which channels the heating medium or coolant can flow. 
     Under certain circumstances, it may be necessary to bring the reactants to a certain temperature. In these cases, it is advantageous to use a micromixer in which the reactants are supplied through storage chambers formed by two plates. To attemporate the reactants, an additional plate is inserted between the first plate and the mixing plate so that another storage chamber is formed between the mixing plate and this additional plate. The heating medium or coolant is fed into this storage chamber. Both reactants must be passed through the heat exchanger chamber in hollow bodies, e.g. tubes. The hollow bodies must be secured in the plates so as to obtain a tight seal. These can be welded, soldered/brazed, diffusion welded, pressed in or bent on. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  a first embodiment of a mixing plate 
         FIG. 1   b  a section from the mixing plate in  FIG. 1   a    
         FIG. 1   c  the discharge plate corresponding to the mixing plate in  FIG. 1   a    
         FIG. 2  a mixer system 
         FIG. 3   a  a second embodiment of a mixing plate and an associated discharge plate 
         FIG. 3   b  an area from  FIG. 3   a    
         FIG. 3   c  a section through the area in  FIG. 3   b  along the line A—A 
         FIG. 4  a third embodiment of a mixing plate 
         FIG. 5   a  a first embodiment of the reactant supply 
         FIG. 5   b  a section through  FIG. 5   a  along the line B—B 
         FIG. 6  a second embodiment of the reactant supply 
         FIG. 7  a first embodiment of an integrated heat exchanger 
         FIG. 8  a second embodiment of an integrated heat exchanger 
         FIG. 9  a third embodiment of an integrated heat exchanger 
         FIG. 10  a fourth embodiment of an integrated heat exchanger 
         FIG. 11  an exploded view of a micromixer 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows a first embodiment of a mixing plate  20 . This mixing plate  20  has two feeding chambers  33   a  and  33   b  for the reactants A,B. Both feeding chambers  33   a,b  branch into four primary channels  35   a,b . Microstructures  31  defining the mixing zones  32  between the main channels  35  for the reactants A and B are located on both sides along the main channels  35   a,b . The main channels  35   a,b  intermesh in a comb-like manner. The feeding chambers  33   a,b  together with the mixing zones  32  each form a mixer cell  30   a  and  30   b.    
     The mixing plate  20  also has recesses  14  by means of which the individual plates comprising the micromixer are screwed together. 
       FIG. 1   b  is an enlarged view of that section of  FIG. 1   a  marked with a broken line. One can see that digital channels  34  branch off from the main channels  35   a,b . These digital channels are separated from one another by microstructures in the form of thin walls  36   a . These walls  36   a  are corrugated and meandering to increase their mechanical stability. This is necessary as these walls  36   a  and also the digital channels  34  are only approximately 40 μm wide. In contrast, the digital channels  34  are approximately 300 μm long. One also can see that wider walls  36   b  separate the main channels  35   a,b  and also the feeding chambers  33   a,b  from one another. The reactants A and B first come into contact with one another as they pass through a port in a discharge plate  21  (cf.  FIG. 1   c ) arranged above and extending perpendicularly to the digital channels  34  and over the entire mixing zone  32 . The port is approximately 80 μm wide. 
       FIG. 1   c  shows the discharge plate  21  corresponding to the mixing plate in  FIG. 1   a . The ports  37  are arranged in a series of parallel curves. The number of ports corresponds to the number of mixing zones in the mixing plate  20  below and the ports are arranged such that they extend perpendicularly to the digital channels and over the entire length of the respective mixing zones  32 . Furthermore, the discharge plate  21  also has recesses  14  for bolting the micromixer together as well as penetrations  12  through which the reactants or also the product can flow. 
       FIG. 2  shows a mixer system. This mixer system is created by arranging two micromixers per the example in  FIGS. 1   a–c  one above the other. At the bottom is a cover plate  26   a  with recesses  14  for screws and two penetrations  12   a,b  through which the reactants A and B are supplied. (Reactant A through the front right penetration  12   a  and reactant B through the rear left penetration  12   b ). The direction of flow is from bottom to top. 
     Above that is a first mixing plate  20   a . In addition to the two feeding chambers  33   a  and  33   b , the main channels  35  and the mixing zones  32 , the mixing plate  20   a  has two penetrations  12   c,d  arranged in two diagonally-opposed corners of the mixing plate  20   a . The feeding chambers  33   a  and  33   b  are designed such that they are connected to the penetrations  12   a,b  in the bottom cover plate  26   a , through which penetrations the reactants A,B are fed into to the feeding chambers  33   a,b.    
     Above the first mixing plate  20   a  is a first discharge plate  21   a  with ports  37 . The intermediate product C, which is a product of A and B, is discharged through these ports  37 , collected in a collecting chamber  39  formed by plates  27   a  and  25  arranged above the discharge plate  21   a , and directed to the penetration  12   c.    
     The intermediate product C is fed via the intermediate plate  25  into the feeding chamber  33   c  of the second mixing plate  20   b  as reactant for the second reaction. Reactant D is fed into the second feeding chamber  33   d  of the mixing plate  20   b  via penetrations  12   d  that form a channel through the front left edge area of the mixer system. Another discharge plate  21   b  and a collecting plate  27   b  are arranged above the second mixing plate  20   b.    
     The collecting plate  27   b  together with the top cover plate  26   b  forms a collecting chamber  39  for the product of C and D, which is directed to the upper right front penetration  12   a ′ and can exit via the top cover plate  26   b.    
     Reactant D is passed through the front left penetrations  12   d ; reactant B is passed through the rear left penetrations  12   b ; the intermediate product C is passed through the rear right penetrations  12   c ; reactant A is passed through the lower front right penetrations  12   a  and the end product is passed through the upper right front penetrations  12   a ′. The intermediate plate  25  forms the boundary between the bottom micromixer  10   a  and the top micromixer  10   b.    
       FIG. 3   a  shows an additional embodiment of a mixing plate  20  and the corresponding discharge plate  21 . The feeding chambers  33  are square and have mixing zones  32  on each of the four sides. A mixer cell  30  comprises a feeding chamber  33  and four mixing zones  32 . Each of the feeding chambers  33  has its own penetration  12  for the supply of a reactant. The feeding chambers  33  are arranged equidistant from one another in rows  60  and columns  61  so that a feeding chamber  33   a  for one reactant is always surrounded by four feeding chambers  33   b  for the other reactant. This arrangement of feeding chambers  33  in a regular grid is reflected in the arrangement of the ports  37  in a discharge plate  21  above the mixing plate  20 . The ports  37  run along the edges of squares and form a regular box pattern. 
       FIG. 3   b  shows an enlarged view of that area of  FIG. 3   a  indicated by a broken line. The digital channels  34  that make up the mixing zones  32  can be seen more clearly in this enlarged excerpt. 
     The section through the area shown in  FIG. 3   b  along the line IIIc—IIIc is shown in  FIG. 3   c . The penetrations  12  through which the reactants are fed into the feeding chambers  33  can be clearly seen here. The digital channels  34  arranged around the feeding chamber and the walls  36  defining these digital channels  34  can also be seen. 
     The individual digital channels  34  are between 5 and 150 μm wide and the walls  36  defining the digital channels are between 50 μm and 2 mm high. Because of the pressure drop required, the width of the outlet ports  37  is preferably less than the height of the walls  36  defining the digital channels  34 . Furthermore, the width of the outlet ports  37  in the discharge plate  21  must be less than the overlap between adjacent digital channels  34  in the mixing zone  32  to achieve complete mixing of the reactants. 
     A modification of the embodiment described above is shown in  FIG. 4 . Here the feeding chambers  33  are triangular with three sides of equal length. Once again there is a mixing zone  32  on all sides of the feeding chamber  33 , and the feeding chambers  33  themselves are arranged at the corners of even, adjacent hexagons. The feeding chamber  33   a  for one reactant is surrounded by three feeding chambers  33   b  for the other reactant. 
       FIG. 5   a  shows a first example for the reactant supply. This is a structured plate attached to the back of the mixing plate, e.g. by means of anodic bonding. A storage chamber  57   a  for reactant A and a storage chamber  57   b  for reactant B are found on two opposing sides of the plate. Channels  56   a,b  lead out from these storage chambers  57   a,b . These channels  56   a,b , intermesh in a comb-like manner. They run beneath the feeding chamber of the mixing chamber and are parallel to one another. Penetrations  12   a,b  aligned with corresponding penetrations  12   a,b  of the mixing plate lead away from each channel  56   a,b . The penetrations  12   a,b  connect the feeding chambers of the mixing plates to the channels  56   a,b  and thus also to the storage chambers  57   a,b . The reactants A,B are supplied to the feeding chambers via this connection. These structures can be produced using deep plasma etching of silicon, for example. 
     The section along the line Vb—Vb is shown in  FIG. 5   b . The channels  56   a,b  are shown again. Only the penetrations  12   a  can be seen due to the orientation of the section. 
     Another embodiment of the reactant supply is shown in  FIG. 6 . All that is shown of the mixing plate  20  are the penetrations  12   a,b . Below the mixing plate  20  is the first additional plate  22 , below which a second additional plate  23  is arranged. The three plates  20 , 22 , 23  are arranged parallel to and at some distance from one another so that a storage chamber  57   a  for reactant A is formed between the mixing plate  20  and the first additional plate  22 , and the first additional plate  22  and the second additional plate  23  form a storage chamber  57   b  for reactant B. Reactant A is supplied to the feeding chambers of the mixing plate  20  through the penetrations  12   a , which directly connect the feeding chambers of the mixing plate  20  and the storage chamber  57   a  for reactant A. In contrast, reactant B must be passed through the storage chamber  57   a . The first additional plate  22  is therefore provided with recesses arranged below the penetrations  12   b  of the mixing plate  20 . Hollow bodies in the form of tubes  58  are passed through the penetrations  12   b  and the recesses in the first additional plate  22 . These tubes  58  form the connection between the storage chamber  47   b  and the feeding chambers for reactant B. 
       FIG. 7  shows a first embodiment for the integration of a heat exchanger in the micromixer. In this example, the product is heated or cooled by means of hollow bodies in the form of tubes  41  that are arranged on the discharge plate  21  between the ports  37  and extend over the entire length of the discharge plate  21 . A coolant or heating medium is passed through these tubes  41 . 
       FIG. 8  shows a second embodiment for the integration of a heat exchanger. In this embodiment, the product is again heated or cooled. The discharge plate comprises two individual plates  21   a  and  21   b . These are arranged parallel to and at some distance from one another to form a chamber  40  for holding a heating medium or coolant. Both individual plates  21   a  and  21   b  are provided with discharge ports  37 . The product is transported from one side of the cover plate [sic]  21  to the other by means of flattened hollow bodies  41   a  arranged in the ports to form a connection from one side of the discharge plate  21  to the other. 
       FIG. 9  shows a third embodiment for heating or cooling the medium. The discharge plate  21  is again a two-piece construction, with an upper slotted plate  21   a  and a very much thicker lower slotted plate  21   b . In addition to the ports  37 , the lower, thicker plate  21   b  also has open-ended slots  42  for holding the coolant or heating medium which extend perpendicular to the ports  37  for the product. It is advantageous if a material with good thermoconducting properties is used for the manufacture of the lower plate  21   b.    
     In some cases, it can also be desirable to preheat or cool the reactants. An embodiment enabling this is shown in  FIG. 10 . This is a micromixer with storage chambers  57   a,b  for the supply of reactants comprising two additional plates  22 ,  23 . A third additional plate  24  is arranged between the first additional plate  22  and the mixing plate  20 . This creates an additional chamber  40  between the mixing plate  20  and the third additional plate  24 , in which chamber  40  a heating medium or coolant is found. Because both reactants A and B must be passed through this heat exchanger chamber  40  en route to the feeding chambers  33   a,b , in the mixing plate  20 , the third additional plate  24  has recesses arranged beneath the penetrations  12   a,b  of the mixing plate  20 . Through these recesses pass hollow bodies in the form of tubes  58   a,b  which empty into the penetrations  12   a,b  of the mixing plate  20  and are connected at the other end to either storage chamber  57   a  or storage chamber  57   b . The reactants A,B are evenly attemporated as they pass from the respective storage chamber  57   a,b  to the feeding chamber  33   a,b  for the respective reactant A,B. 
       FIG. 11  is an exploded view of a micromixer. This micromixer  10  comprises a casing  11  having two penetrations  12  for each reactant A,B. The casing  11  also has recesses  14  for seating screws  13 . At the bottom of the casing is an intermediate plate  25  with two penetrations  12  for each reactant. A mixing plate  20  structured on both sides is arranged above the intermediate plate  25 . The bottom of the mixing plate is provided with microstructures for the supply of the reactants (cf.  FIG. 5   a ). Mixer cells with square feeding chambers are arranged on the top of the mixer plate  20 . A slotted discharge plate  21  is arranged above the mixing plate  20 . Above the discharge plate is a cover plate  26  having an opening  12  for the product. The cover plate  26  also has recesses  14  for seating screws  13 . These screws  13  are used to securely screw the micromixer  10  together. 
     With a micromixer  10  of this type configured with approximately 1500 feeding chambers per mixing plate, a surface area of 45×45 mm and a volumetric flow of 700 l/h with a pressure drop of approximately 1 bar can be achieved. 
     REFERENCE NUMBERS 
     
         
           10  micromixer 
           11  casing 
           12  penetration 
           13  screw 
           14  recess 
           20  mixing plate 
           21  discharge plate 
           22  first additional plate 
           23  second additional plate 
           24  third additional plate 
           25  intermediate plate 
           26  cover plate 
           27  collecting plate 
           30  mixer cell 
           31  microstructure 
           32  mixing zone 
           33  feeding chamber 
           34  digital channel 
           35  main channel 
           36  wall 
           37  outlet port 
           39  collecting chamber 
           40  heat exchanger chamber 
           41  hollow body 
           41   a  flattened hollow body 
           42  heat exchanger slot 
           56  channel 
           57  storage chamber 
           58  hollow body 
           60  row 
           61  column