Abstract:
The method relates to bringing about a mixed state of at least two fluids in a second part ( 34 ) of a line, which have flowed through a first part ( 32 ) of this line. This is attained in that a fractional flow of the fluids is withdrawn from the second part ( 34 ) of the line at a point ( 43 ), and this fractional flow is resupplied to the second part ( 34 ) of the line at a different point ( 45 ). A employment to avoid clogs in modules in a crossflow filtration system is described, along with apparatuses for performing the employment.

Description:
This application is a 371 of PCT/CH99/00294, filed on Jul. 02, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention relates to a method for bringing about a mixed state of at least two fluids in a second part of a line that have flowed through a first part of this line, to a use of the method in a crossflow filtration system, and to an apparatus for performing the method. 
     Known crossflow filtration systems are embodied as multipass systems, in which a plurality of filtration modules are disposed in a plurality of filtration routes that are supplied in parallel. Such filtration routes are known as passes. These passes are fed in parallel with the fluids to be filtered from a distributor line. Each filtration module in turn includes a number of membrane tubules that carry filtration membranes and are subjected in parallel to the fluids to be filtered. 
     FIG. 1 shows a diagram of one such known crossflow filtration system. It includes nine passes  1 . Each of which has four filtration modules  2 . In each pass  1 , the filtration modules  2  are connected in series with one another. The nine passes are supplied in parallel with fluids to be filtered through a distributor line  3 . In the filtration modules  2 , a fraction of the fluids is separated out in the form of permeate or filtrate, while the remaining fraction of the fluids is collected as retentate by a collecting line  4  and removed. The removal of the permeate is not shown here. 
     FIG. 1 shows two fluids  5  and  6  in the distributor line  3 , with a parting boundary  7  between them. The fluids  5  and  6  occur when the retentate  6 , after the conclusion of a filtration cycle, is positively displaced out of the filtration system by means of water. In the state shown, some of the filtration modules  2  and part of the collecting line  4  are already filled with water  5  as a consequence of the positive displacement, while others of the modules  2  and another part of the line  4  are still filled with retentate  6 . Under these circumstances, it is known that individual membrane tubules or entire filtration modules  2  repeatedly become clogged, because the high-viscosity retentate  6  is no longer positively displaced out of the remaining modules  2  if the water  5 , which has low viscosity, can flow out through some of the modules that have already been rinsed out. 
     If the pressure drop through the filtration modules  2  between the inlet  8  of the distributor line  3  and the outlet  9  of the collecting line  4  was still  5  bar, for example, before the water  5  was supplied, then in the state shown in FIG. 1, after rinsing out of the first pass  1 , the pressure drop is reduced to approximately  3  bar, and it drops further as further passes  1  are rinsed out. The reduced pressure drop slows down the flow speed in the modules  2  that are still filled with retentate  6 . A structural effect increases the high viscosity of the retentate  6  still further, until the flow comes to a stop. The remaining pressure drop of less than  3  bar is then no longer sufficient to positively displace the remaining retentate  6 . 
     Known apparatuses have abated this problem by means of symmetrical distributors and/or very slowly opening water valves while the retentate supply is still open. In the first case, the parting boundary  7  reaches all the passes  1  at the same time, while in the second case the difference in viscosity of the fluid mixtures reaching the passes  1  at the same time is decreased. 
     Known symmetrical distributors are designed for only a maximum of four passes, for reasons of space and expense, and are usually combined with static mixers. Water valves that open slowly require regulating devices to allow them to reach a sufficiently slow reduction in the viscosity of the retentate. Idle flow zones that nevertheless remain can still cause clogging in this case, however. 
     Experience shows that despite the known provisions described, clogging of modules cannot be avoided. The situation is especially problematic in filtration systems with distributors for up to two hundred membrane tubes, which is equivalent to a standard industrial-scale system with ten passes. 
     SUMMARY OF THE INVENTION 
     It is therefore the object of the invention to bring about a mixed state of at least two fluids in a line that makes it possible for module clogging of the kind described to be effectively prevented. 
     According to the invention, in a method of the kind referred to at the outset, this object is attained in that at at least one point of the second part of the line, at least a fractional flow is withdrawn from the line, and that this fractional flow is returned to the second part of the line again after a time lag. The method is preferably embodied such that the fractional flow is returned to the second part of the line at the same point where it was withdrawn or at a point located upstream thereof. 
     As an apparatus for mixing fluids in a line, at least one side line is preferably used, which connects at least two points of the line, which are spaced apart from one another in the direction of the line, to one another and recirculates a fractional flow from the downstream line point to at least one of the upstream line points. 
     A use of the method of claim  1  for avoiding clogs in modules in a crossflow filtration system, in which at least two filtration routes (passes) that include modules are supplied in parallel with the fluids to be filtered from a distributor line, is distinguished in that the second part of the line is used as the distributor line for the passes. 
     Further variants of the method as well as its use and the apparatus for performing it are defined by the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Exemplary embodiments of the invention are described in further detail in the ensuing description and shown in the drawing figures. Shown are: 
     FIG. 1, a diagram of a known crossflow filtration system; 
     FIG. 2, an apparatus according to the invention for mixing fluids in a line; 
     FIG. 3 a , a variant of the apparatus of FIG. 2 having an injector; 
     FIG. 3 b , a variant of the apparatus of FIG. 2 having a radial mixer; 
     FIG. 3 c , a variant of the apparatus of FIG. 2, having a line for distributing the mixed fluids; 
     FIG. 3 d , a variant of the apparatus of FIG. 2, in which a fractional flow from a line is resupplied to two points of the line; 
     FIG. 3 e , a variant of the apparatus of FIG. 3 d , having a line for distributing the mixed fluids; 
     FIG. 4, a diagram of a crossflow filtration system, in which the method of the invention for mixing fluids is employed; 
     FIG. 5, a variant of the filtration system of FIG. 4, having a device for interrupting a recirculated fractional flow; 
     FIG. 6, a variant of the filtration system of FIG. 4, with recirculation downstream of a retentate pump; 
     FIG. 7, a variant of the filtration system of FIG. 4, with recirculation via a batch tank; 
     FIG. 8, a variant of the filtration system of FIG. 4, with recirculation via a retentate line; 
     FIG. 9, a detail of a variant of the filtration system of FIG. 4, with recirculation via an injector; 
     FIG. 10, a detail of another variant of the filtration system of FIG. 4, having a vertically placed distributor and recirculation via an injector; 
     FIG. 11, a variant of the filtration system of FIG. 4, with recirculation upstream of a retentate pump; 
     FIG. 12, a detail of a variant of the filtration system of FIG. 4, having a side line and a distributor line integrated in one component; and 
     FIG. 13, a detail of a variant of the filtration system of FIG. 4, with one loop each for the distributor line and the collecting line. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 2 shows an apparatus according to the invention for mixing fluids in a line  13 . The fluids are supplied unmixed to the line  13  via a line  12  and are removed, mixed, from the line  13  via a line  14 . Two points  15 ,  16  in the line  13 , spaced apart from one another longitudinally, communicate with one another through a side line  17 . By means of a pump  18  in the side line  17 , a fractional flow from the downstream point  16  of the line is recirculated in a loop to the upstream point  15  of the line. 
     The mixing effect ensues in the apparatus of FIG. 2 when a plurality of fluids are located side by side in a cross section of the line  12 . This effect also occurs, however, especially when a plurality of fluids succeed one another longitudinally in the line  12 . Because of the recirculation of a fractional flow through the side line  17 , a first fluid is mixed into a second fluid, the latter following a parting boundary, until such time as the second fluid has positively displaced the first fluid entirely out of the side line  17 . The concentration of the second fluid upstream of the parting boundary will accordingly rise only slowly over a length that depends on the spacing of the line points  15 ,  16 . Only within this length is the mixing attained with the apparatus of FIG.  2 . 
     In FIG. 3 a , which shows a variant of the apparatus of FIG. 2, the reference numerals from FIG. 2 designate equivalent components. Instead of the pump  18 , in FIG. 3 a  at the upstream line point an injector  20  is provided. The injector directs a flow from its end to drive the surrounding fluid in the same direction and thereby aspirate or draw the fractional flow from the downstream line point into the upstream line point via the side line  17 ′. Various designs of such injectors are known per se, for example having a guide baffle, or in the form of a slotted injector. 
     In FIG. 3 b , which shows an apparatus corresponding to FIG. 2, a radial mixer  22  is also inserted into the line  13 ′ for mixing purposes. The radial mixer  22  improves the mixing of the fractional flow from the side line  17  transversely to the axis of the line  13 ′. 
     FIG. 3 c  shows a variant of the apparatus of FIG. 2, in which the upstream point  15  of the line  13 ″ for mixing is supplied with a fractional flow via a side line  17 ″, which at the same time has six fractional outlets  23  for removing the mixed fluids. Accordingly, the line  14  for removal purposes of FIG. 2 is missing here, and as the fractional flow, the side line  17 ″ is supplied with the entire line flow reaching the downstream line point  16 . In the variant of FIG. 3 c , a recirculating pump  18 ′ is also inserted into the line  13 ″ for the sake of effective recirculation of the line flow via the side line  17 ″. 
     In the variant of the apparatus of FIG. 2 schematically shown in FIG. 3 d , it is made possible, by means of a mode of operation that is independent of the delivery of fluids via the line  12 , to proceed directly to a mixed state from a first fluid in the removal line  14  without a gradual transition. This is attained in that a fractional flow from the downstream line point  16  is supplied, via a first side line  17 , to a first upstream line point  15 ′ and, via a second side line  17 ′, to a second upstream line point  15 ″. As soon as a parting boundary between two fluids from the line  12  has moved into the region between the line points  15 ″ and  16 , the following process sequence occurs with this premixer: 
     two valves  80  and  81  upstream of the inlet and downstream of the outlet of the mixing line  13  are closed; 
     the recirculating pump  18  in the side line  17  is turned on; 
     by means of the circulation in the closed loops of the lines  13 ,  17 ,  17 ′ with different time lags, mixing occurs along the mixing line  13 . At the same time, by means of the radial mixer  22 , a transverse mixing occurs as well; 
     the valves  80  and  81  are opened; 
     a feed pump  33  in the supply line  12  is additionally connected, in order to remove the resultant mixture of fluids via the removal line  14 . 
     A divider  82  between the side lines  17 ,  17 ′ accordingly divides nonhomogeneities, arriving via line  17 , to a distance Δl between the line points  15 ′,  15 ″. Advantageously, the line length b between the divider  82  and the line point  15 ′ via  17 ′,  15 ″ is unequal to the line length a between the divider  82  and the line point  15 ′. Preferably, b=(n+½)a, where n is a natural number. To compensate for the different line lengths a and b, a throttle  83  is provided between the divider  82  and the line point  15 ′; with this throttle, the flows Qa through the throttle  83  and Qb via the line point  15 ″ are made approximately equal to one another; Qa≈Qb. 
     FIG. 3 e  shows a variant of the apparatus of FIG. 3 d , with a line for distributing the mixed fluids by way of four fractional outlets  23 . In a first operating mode,as long as the inlet valve  80  is closed, only a flow Q82 flows in the loop via the divider  82  and not via the fractional outlets  23 , because of the recirculating pump  18 ; the apparatus is operative as a pure premixer. In the second operating mode with an open inlet valve  80 , the flow Q80 through the inlet valve  80  is preferably adjusted to Q82≈½Q80. 
     FIG. 4 shows a diagram of a crossflow filtration system in which an apparatus for mixing fluids is employed. In a manner known per se, the system includes a container  30  for a media to be separated, which are supplied via a line  31 . Connected at the bottom to one outlet of the container  30  is a feed line  32  for the contents of the container, and a feed pump  33  is incorporated into this feed line. The pump  33  generates an outlet pressure of Δp=6 bar and a feed flow Q1. The feed line  32  is connected to a distributor line  34 , which distributes the media to be filtered over parallel filtration routes or passes  35 . The passes  35  lead to a collecting line  38 , by way of a filter unit  36  that excretes a filtrate or permeate via a line  37 . 
     The collecting line  38  carries a fraction, not excreted via the line  37 , oft he media to be filtered back into the container  30  in the form of retentate, via a retentate line  39  and a throttle element  40 . With the throttle element  40 , the pressure drop between the inlet  45  of the distributor line  34  and the outlet of the collecting line  38  can be adjusted. This pressure drop can be measured by a pressure sensor  41  for the inlet pressure p 1  and a pressure sensor  42  for the outlet pressure p 2 . 
     A side line  44  is connected to one end  43  of the distributor line  34  and recirculates a fractional flow from the end  43  to the inlet  45  of the distributor line  34 . If separate fractions of the media to be filtered reach the inlet  45  of the distributor line  34  in succession via the feed line  32 , then they are delivered in a mixed state to the passes  35 , and the problems described in conjunction with FIG. 1 do not arise. A second feed pump  46  and a throttle element  47  are incorporated in the side line  44 , as FIG. 4 shows. The pump  46  generates an outlet pressure of Δp=0.2 bar and a feed flow which amounts to 50% to 100% of the flow Q1 of the first pump  33 . 
     As a consequence, a flow is obtained in the distributor line  34  that amounts to from 150% to 200% of the flow Q1. As a result, any remaining differences in concentration of the media to be filtered reach all the passes  35  with a slight time lag and can never have an effect for very long at individual passes. Such differences in concentration occur especially whenever, after the conclusion of a filtration cycle, rinsing water for positively displacing high-viscosity retentate residues via an outlet valve  49  is supplied via a valve  48  of the container  30 . 
     FIG. 5 shows a variant of a detail of the filtration system of FIG.  4 . FIG. 5 schematically shows the distributor line  34  with the filtration routes (passes)  35 , the collecting line  38 , and the side line  44 . As the device for interrupting the recirculated fractional flow, the side line  44  of FIG. 5 has a blocking valve  50  on its lower end, upstream of the inlet  45  of the distributor line  34 , and a drain valve  51  preceding the blocking valve. The valves  50 ,  51  are used for a final rinsing out of the lines  34 ,  44  with water or an alkaline lye as a rinsing agent in cleaning. Retentate that according to FIG. 4 has circulated in the retentate loop  30 ,  32 ,  34 ,  36 ,  38 ,  39  during one filtration cycle is thus rinsed out. 
     FIG. 6 shows a diagram of a variant of the filtration system of FIG.  4 . The recirculation of the fractional flow from the distributor line  34  is done here at a point  55  directly downstream (on the pressure side) of the retentate feed pump  33 , in contrast to the inlet  45  of the distributor line  34  as in FIG.  4 . The advantage of this kind of preliminary circuit resides in better longitudinal mixing in the line between the points  45  and  55 , since the distance between these points, in systems embodied for practical use, is relatively long and has a plurality of elbows. 
     In a further variant of the filtration system of FIG. 4, FIG. 7 schematically shows a recirculation of the fractional flow from the end  43  of the distributor line  34  via a line  44 ′ and the batch tank  30 . This dispenses with the second feed pump  46  of FIG. 4. A throughput regulating valve  47 ′ or a fixedly set baffle is inserted into the line  44 ′ here in such a way that approximately 50% of the retentate flow, supplied via the feed line  32 , is returned directly to the batch tank  30 . This variant has the advantage of dispensing with the second feed pump  46  of FIG.  4 . However, the energy consumption is greater, since approximately 50% of the energy of the feed flow from the line  32 , at approximately 6 bar, goes unused for the filtration in the filter unit  36  (FIG.  4 ). The throughput regulating valve  47 ′ is controlled via two sensors  56 ,  56 ′ for the throughput. 
     FIG. 8 shows a variant of the filtration system of FIG. 4 that again functions without the second feed pump  46  of FIG.  4 . The recirculation of the media to be filtered from the distributor line  34  in this case is done by feeding at a point  60  into the retentate line  39  from the collecting line  38 . A throttle  61  for pressure adaptation is also inserted between the distributor line  34  and the point  60 . Once again, an additional burden on the feed pump  33  arises from the flow through the throttle  61 , which while being somewhat less than in the variant of FIG. 7 is still relatively high. 
     FIG. 9, similarly to FIG. 3 a , shows a detail of a variant of the filtration system of FIG. 4, with recirculation via an injector  20 . The feed line  32 , distributor line  34 , side line  44 , filtration routes (passes)  35 , and collecting line  38  are identified by the same numerals as in FIG.  4 . The distributor line  34  and the collecting line  38  are advantageously horizontal, for the sake of uniform distribution of sludge deposits when there are interruptions in production in the system. If only a vertical disposition is possible, then advantageously this is selected to be as shown in FIG.  10 . 
     Still another variant of the filtration system of FIG. 4, of which FIG. 10 shows a detail, employs recirculation via an injector  20 . In contrast to the variant of FIG. 4, however, here the point  55 ′ at which the retentate is recirculated by the side line  44  from the end  43 ′ of the distributor line  34  into the feed line  32 , is placed as far upstream as possible from the inlet  45 ′ of the line  34 . Sludge deposits  65 , which can occur in the side line  44  during interruptions in operation of the filtration system, are thus better mixed in again if the side line  44  is located at the bottom and there is a flow from top to bottom through the distributor line  34 . The flow toward the passes  35  is also more uniform than in the variant of FIG.  5 . 
     A variant of the filtration system of FIG. 4, with a recirculation of a fractional flow of the media to be filtered via a side line  44 ″ into the feed line  32  at a point  15 ′ upstream of retentate pump  33  is schematically shown in FIG.  11 . For adaptation of the pressure, a throttle valve  47 ″ is inserted into the side line  44 ″. This variant of the filtration system offers an especially good longitudinal mixing in the event of deposits of solids, such as activated charcoal, bentonite, and so forth, after interruptions in operation. 
     FIG. 12 shows a detail of a variant of the filtration system of FIG. 4, with a side line  44  and distributor line  34 ′ integrated in one component. These lines  44 ,  34 ′ can be made by incorporating a partition  68  into a line of circular, oval or rectangular cross section. Thus advantageously one end  69  of the feed line  32  can be designed as an injector, which returns the flow from the side line  44  back to the distributor line  34 ′. 
     A detail of a variant of the filtration system of FIG. 4 for adaptation of the pressure drop via the individual passes  35  is schematically shown in FIG.  13 . Here in addition to the preliminary circuit  44 ,  46  for the distributor line  34 , the collecting line  38  is also provided with a subsequent circuit  70 . The feed pump  46  in the preliminary circuit and a feed pump  71  in the subsequent circuit assure that in both loops, as shown in FIG. 13, the passes located at the bottom are reached first by the flows, and the passes located at the top are reached last. By dimensioning the feed pumps  46 ,  71  and the lines  34 ,  44 ,  38 ,  70  appropriately, it is attained that the pressure drop Δp over each individual one of the passes  35  is equal and approximately constant. In addition, the cross sections of the lines  34 ,  38  are reduced inside each loop, as FIG. 13 shows, so that nonhomogeneities of the media to be filtered reach the individual passes with the least possible time lag, because of the greater flow speeds. 
     For one skilled in the art, many variants in the use of the method and the apparatus for mixing fluids in a line will suggest themselves, none of them departing from the scope of the appended claims. The apparatuses are suitable for all areas of crossflow filtration and all types of membrane, such as hollow fibers, capillaries, wrapped modules, tube modules, and organic and inorganic membranes in RO, NF, UF, MF, and screen filtration. The fluids to be mixed include inorganic and organic products to be separated, with or without solid components. Filtration systems that include these apparatuses can operate discontinuously or continuously. 
     The distributor lines  34 —see FIG.  4 —are advantageously arranged horizontally or vertically, depending on the type of products. Operating the preliminary circuits  44 ,  46  for recirculating the fractional flows during the entire running time of the systems provides increased reliability in the event of fluctuations in viscosity of the products. Operation only immediately before and during a positive displacement of product out of the system leads to an energy savings, although relatively slight. Instead of the modular passes  35  described in conjunction with FIG. 4, individual membrane tubes are also suitable for these filtration routes; their number can be equivalent to the number of passes, up to approximately two hundred. 
     While conventional filtration modules of the type described in conjunction with FIG. 1 include approximately twenty filtration tubes, industrial-scale modules with up to about two hundred membrane tubes are also already known. Often, a system includes only one such industrial-scale module, with which the collecting and distributor lines can then be integrated along with the preliminary circuit, to form an expanded modular unit in accordance with FIG.  4 .