Abstract:
A feeding device for feeding a chemical into a process liquid flowing through a process liquid flow duct, the feeding device includes: a feeding liquid duct having a discharge opening; a mixing liquid feed duct extending through the feeding liquid duct, a sidewall and a closed end wall, wherein the end wall extends beyond the discharge opening of the feeding liquid duct and extending into the process liquid flow duct; a mixing space adjacent the end wall and within the sidewall of the mixing liquid feed duct; a chemical feed duct extending through the mixing liquid feed duct and having a discharge opening proximate to the mixing space in the mixing liquid feed duct, and a mixture discharge opening in the side wall of the mixing liquid feed duct.

Description:
BACKGROUND 
     This application is the US national phase of international application PCT/FI2004/000586 filed 5 Oct. 2004 which designated the U.S. and claims benefit of Finnish Application No. 20031468 filed 8 Oct. 2003, the entire contents of both are hereby incorporated by reference. 
     The present invention is related to a method and apparatus for feeding chemical into a liquid flow. The method and apparatus of the invention are particularly well applicable to feeding of very small chemical volumes in precise amounts into large process liquid flows. 
     Naturally, there is practically an innumerable amount of prior art methods of feeding various chemicals into liquid flows. However, these methods may be divided into a few main categories as can be seen from the following. Firstly, it is quite possible to just let the liquid to be added flow freely into a second liquid without employing any special regulation or mixing means. This method of adding cannot be employed in situations where the mixing ratio or the uniformity of the mixing is important. Neither can it be employed in situations where the price of the chemical to be added is of significance. The next applicable method is to feed the chemical in a precise ratio to the liquid flow, whereby correct and economical dosage is obtained. However, even in this case one has to take into account that usually the dosage of the chemical is slightly excessive compared to the optimal dosage, because the mixing is known to be inadequate. The mixing may be improved, though, by feeding the chemical e.g. through a perforated wall of a flow channel, whereby at least the chemical to be mixed may be spread throughout the entire liquid flow. As the last example, a situation may be discussed, where the chemical is fed in a precise proportion either into the liquid flow upstream of the mixer or through the mixer itself. In that case, the efficiency of the mixing of the chemical into the liquid flow is totally dependent on the mixer design. 
     Finnish patent no. 108802 discusses as an essential case of mixing related to paper manufacture the mixing of a retention aid into fiber suspension flow flowing to the head box of a paper machine. In paper manufacture, retention chemicals are used especially in order to improve the retention of fines at the wire section of a paper machine. In the Finnish patent mentioned the mixing device is in fact a conical nozzle with an inlet for the retention chemical. The mixing device is functioning and efficient both in the mixing of retention chemicals and other chemicals in the short circulation of a paper machine and also in other applications in the paper and pulp industry. However, it has been noticed in connection with some applications that various solid substances carried with the feed and/or dilution liquid tend to accumulate in the device. In other words solid material tends accumulate in the device parts converging in the flow direction, which gradually harm the flow profile, the flow itself and in the end tend to clog the device. Fl patent application no. 20021350 describes a self-cleaning chemical feed nozzle. In other words when the nozzle starts to become clogged a change take place in its flow conditions which causes a reaction of the nozzle to open wider the cross-sectional flow area of the flow channel in which the solid material in question flows with the fiber suspension; as a result of this the solid particles attached to the channel can get loose from the nozzle and flow on. 
     In this kind of applications, i.e. feeding for example retention chemicals into a fiber suspension, the mixing devices and the nozzles described in the publications mentioned work well but in cases where only very small amounts of chemicals are needed in relation to the suspension flow to be fed, the operation of the these nozzles is not the best possible for example because they cannot guarantee an adequately homogenous mixing of the chemical into the process liquid flow because of the small volume of the chemical. 
     SUMMARY 
     In order to solve, among other things, the problem described above, a new type of a chemical feeding device has been developed the structure of which is very favorable in feeding small chemical amounts into a liquid flow. The feeding device according to the invention includes a thin pipe-like duct disposed preferably inside the feeding device/nozzle so that the desired amount, in this case as small an amount as possible, of chemical can be mixed evenly into the process liquid flow. The pipe-like duct feeding the chemical supplies the chemical into a special nozzle of the feeding device which is preferably designed to have a kind of an isolated mixing space where the chemical and mixing liquid supplied to the feeding device through an inlet of its own are mixed and from which they only after this mixing are fed through openings in the mixing space at first into the feeding liquid and after that aided by the feeding liquid mentioned to the flowing process liquid. The mixing and the dilution of the chemical to a chemical solution before it is fed to the process liquid flow pipe ensure uniform mixing of the chemical into the process liquid. As a result of this, the volume of the chemical to be fed into the feeding device can be of the order of even less than half a percent of the rest of the liquids supplied into the feeding device, which are the mixing liquid and the feeding liquid supplying the mixing liquid and the chemical into the liquid flow. If desired, several feeding devices according to the invention instead of one, may be disposed in connection with the process liquid flow duct. 
     The structure of the feeding device according to the invention, more precisely expressed the isolated mixing space provided at the end of the mixing liquid feed pipe, improves the mixing of the chemical also in another way. When hitting the wall of the isolated mixing space the liquid chemical is “dispersed” evenly to the whole interior of the isolated mixing space of the nozzle and is mixed and diluted more homogenously into the mixing liquid. In addition to this structure the feeding device can further include a kind of an additional counter piece which, when disposed in the middle of the mouth of the pipe-like duct feeding the chemical, further improves the mixing to the other liquids to be fed and further to the liquid flow to be fed. 
     The chemical can be fed into the feeding device according to the invention without separate dilution, in other words the dilution takes place with the mixing liquid in the isolated mixing space of the feeding device. This solution dispenses among other things with the need to use separate dilution vessels, reduces the consumption of fresh water and thus reduces the operation and maintenance costs. On the other hand, it is possible also to dilute the chemical before it is supplied to the feeding device if so desired. 
     The feeding device according to the invention may be used for example in the feeding of chemicals, such as for example TiO 2 , optical brighteners, paper dyes and silicates, into the flowing process liquid, only to mention a few chemicals. Thus the feeding device according to the invention is applicable in all processes into which the chemicals mentioned must be supplied, In particular when the amount of the chemical is very little compared with the total flow of suspension flowing to the process. As advantageous examples, only, of the processes may be mentioned for example fiber suspension flows of paper mills, thickening processes of various sludges, recycling fiber processes, bleaching processes and in general processes where feeding of chemical in particular in very small amounts into filtrate, fiber suspension, sludge or the like is necessary. 
     The mixing device according to the invention allows using as the feeding liquid with which the chemical is supplied into the process liquid, for example into fiber suspension, the same fiber suspension into which the chemical is to be fed. Of course also more dilute suspensions, various filtrates or corresponding or mere fresh water can be used as the feeding liquid in the feeding device of the publication. The mixing liquid may also be any liquid from the process itself of fresh water. Thus all the liquid obtained from another process stage that can be used in the feeding of the chemical, saves at the same time fresh water and thus for example reduces the consumption of fresh water of the mills. 
     Other characteristic features of the method and the feeding device of the invention are disclosed in the appended patent claims. 
    
    
     
       DESCRIPTION OF DRAWINGS 
       In the following, the method and the apparatus according to the invention are disclosed in more detail with reference to the appended figures, where 
         FIG. 1  illustrates a prior art chemical feeding apparatus; 
         FIG. 2  illustrates an other prior art chemical feeding apparatus; and 
         FIG. 3  illustrates chemical feeding apparatus according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates schematically a mixing device according to a preferred embodiment of Fl patent no. 108802. The mixing apparatus  34  according to  FIG. 1  is, in fact, a nozzle comprising preferably an essentially conical casing  50 , flanges  52  and  54  arranged into it and preferably, but not necessarily, placed at its opposite ends, and a conduit  56  for the retention chemical. The mixing apparatus  34  is connected by a flange  52  to a dilution medium pipe  44  and by a flange  54  to a fiber suspension flow duct. In the arrangement according to the figure, the casing  50  of the mixing apparatus  34  is converging from the flange  52  towards the flange  54 , an opening  58  of the mixing apparatus being located inside the flange  54 . The purpose of the conical form of the casing  50  is to accelerate the medium flow in the mixing apparatus  34  so that the velocity of the jet discharging from the mixing apparatus  34  into the fiber suspension flow is at least five times the velocity of the fiber suspension flow. In the embodiment according to the figure, the retention chemical feeding conduit  56  is preferably tangential in order to ensure that retention chemical discharging through the opening  58  of the mixing apparatus  34  into the fiber suspension flow is distributed homogeneously at least on the whole periphery of the opening  58 . Inside the mixing apparatus  34  there is a hollow member  60  arranged centrally inside the mixing apparatus  34 , into which member the retention chemical is guided from the conduit  56 . In other words, the conduit  56  pierces the conical wall  50  of the mixing apparatus  34  and further leads via the annular space between the cone  50  and the member  60  into the member  60 , at the same time preferably carrying the member  60  in its place. The member  60  is pierces in the axial direction by a hole  62 , into which mixing liquid is brought via a valve  164  and a duct  162 ; thus the mixing liquid can flow from inside the chemical flow to the fiber suspension flow duct. The retention chemical flow guided tangentially into member  60  turns in the form of a spiral flow towards the opening  58  of the mixing apparatus, where there is at the lower end of the member  60  (according to the figure) an annular opening  64  for the retention chemical, wherefrom the retention chemical is discharged as a fan-shaped jet into the fiber suspension together with the feed liquid discharging from outside the opening  64  and the mixing liquid discharging from the inside of the opening  64  through the hole  62 . The figure clearly shows that the retention chemical is not in any contact with the mixing liquid before it is discharged through the opening  64  into the fiber suspension flow duct. 
       FIG. 2  illustrates another prior art feed nozzle  34 . It comprises, starting from the bottom of the figure, in other words from the liquid flow duct  70 , an essentially cylindrical nozzle casing  80  the end of which facing the liquid flow duct has a conically converging portion  82 . The converging portion  82  ends at a central feed opening  84 , which continues to the direction of the flow duct  70  in apparatus  86  for attaching the feed nozzle  34  to the liquid flow duct  70 . The side wall of the nozzle casing  80 , preferably the cylindrical portion thereof, has been provided with an opening  88  communicating with a feed liquid duct connection  144  for supplying feed liquid into the mixing nozzle  34 . The end of the nozzle casing  80  opposite the flow duct  70  has been provided with a round central opening  90  and with a pressure medium cylinder  92  forming an extension of the nozzle casing  80  while the other end of the pressure medium cylinder  92  is the end  94  of the nozzle casing located opposite the flow duct. At the opposite end of the pressure medium cylinder  92  there is an end plate  96  with a central round opening  98  like in the upper end of the nozzle casing  80 . 
     Both chemical and mixing liquid feed apparatus  100  extend to the nozzle casing  80  from above through the openings  98  and  90  in the ends  96  and  94  mentioned above. These feed apparatus include among other things a chemical feed duct  102  which has a flow connection with the chemical conduit  56  and a mixing liquid feed duct  104  which in turn communicates with a mixing liquid feed conduit  162 , which in this embodiment is located centrally inside the chemical feed duct  102 ; the feed ducts  102  and  104  being attached to each other at the upper end. The chemical feed duct  102  is preferably cylindrical for the most of its length as in this embodiment it functions at the same time as a piston rod of the pressure medium cylinder  92 . A piston disc  106  sealed relative to the pressure medium cylinder  92  and secured to the outer surface of the chemical feed duct  102  has been provided to serve as the piston itself. Naturally both the ends  94  and  96  of the pressure medium cylinder  92  have been provided with suitable sealing to ensure the operation of the cylinder. 
     The chemical feed duct  102  has at the lower end of it, in other words at the end facing the fiber suspension flow duct  70  and extending inside the nozzle casing  80 , a conical converging portion  108  which is essentially located at the conical portion  82  of the nozzle casing  80  and the coning angle is of the same order as that of the conical converging portion  82  of the nozzle casing  80 . The mixing liquid feed duct  104  in turn runs centrally inside the chemical feed duct  102  and extends to a distance outside the conical converging portion  108  of the chemical feed duct  102 . The figure further illustrates how the chemical feed duct  102  continues as a cylindrical nozzle duct  110  after the converging portion  108  so that a narrow slot is created between the wall of the mixing liquid feed duct  104  and the nozzle duct  110 ; in the slot the velocity of the chemical flow is increased to the required level for feeding to the fiber suspension flow. 
     In the normal state the feed nozzle is in the operation position illustrated in  FIG. 2 ; thus both the nozzle duct  110  of the chemical feed duct  102  and the openings  122  in the mixing liquid feed duct  104  are located outside the nozzle casing  80  essentially to the level of the wall of the fiber suspension flow duct. In the flushing position the pressure medium supplied to the pressure medium cylinder  92  via the opening  116  moves the chemical and mixing liquid feed apparatus  100  by means of the piston disc  106  upwards so that the distance between the conical portions  82  and  108  increases and the end  118  of the mixing liquid feed duct  104  rises so high that the feed liquid flow flushes all impurities or solid particles via the opening  84  from between the conical portions to the fiber suspension flow duct. After a certain time, preferably the flushing time is about 1-6 seconds, pressure medium is fed to the cylinder from the opening  120  in the opposite end of the pressure medium cylinder  92 , and the piston disc  106  presses the chemical and mixing liquid feed apparatus  100  back to the operation position. The function described above is guided either by pressure, the pressure difference or volume flow of the feed liquid. 
       FIG. 3  illustrates a preferred embodiment of the feed apparatus, i.e. feed nozzle  34  of the present invention. It comprises, starting from the bottom of the figure, in other words from the liquid flow duct  70 , an essentially cylindrical nozzle casing  80  the end of which facing the liquid flow duct has a conically converging portion  82 . The converging portion  82  ends at a central feed opening  84 , which continues to the direction of the flow duct  70  in apparatus  74  and  76  for attaching the feed nozzle  34  to the liquid flow duct  70 . The side wall of the nozzle casing  80 , preferably the cylindrical portion thereof, has been provided with an opening  88  communicating via a duct  144  and a valve  42  with the feed liquid feed duct for supplying feed liquid into the feed nozzle  34 . 
     A mixing liquid feed duct  142  forms together with a chemical feed duct  162  the cylindrical upper portion of the feed apparatus  34 . Both the feed ducts  142  and  162  extend inside the nozzle casing  80  up to the liquid flow duct  70 . The location of the end of the feed ducts is adjustable in relation to the liquid flow duct  70  so that the end of the ducts extends preferably inside the flow duct. The end of the nozzle casing  80  opposite the flow duct  70  is provided with an end part  94  having a round central opening  90  for the feed duct  142 . The upper portion formed by the feed duct  142  is provided with a flange  136  and a movable screw/nut connection  138  or a corresponding member by means of which the upper portion (feed duct  142 ) and the lower portion (nozzle casing  80 ) of the feed apparatus  34  are attached to each other. In addition to feature that the parts  136  and  138  secure the upper and the lower portions to each other the adjustable screw  138  may be used for adjusting the position of the mixing liquid  142  and the chemical feed duct  162  of the feeding device  34  in relation to the liquid flow duct  70 . The adjustability of the feed device  34  and the structure of securing means  74  and  76  allow the use of the feed device  34  with process liquid ducts  70  of various thicknesses, in other words the device can be secured to these ducts. 
     The side wall of the feed duct  142 , preferably the cylindrical portion thereof, at a location outside the end parts  94  and  136 , the nozzle casing  80  and the feed liquid feed opening  88  as seen from the flow duct  70 , has been provided with an opening  56  for the mixing liquid to be fed to the feed device  34 . The feed opening  56  communicates via a mixing liquid conduit  146 , which in this embodiment is preferably tangential in relation to the feed device  34 , and an adjustable valve  44  with the mixing liquid feed pipe for supplying mixing liquid into the feed device  34 . 
     The chemical feed duct  162 , which is preferably a very thin pipe for feeding small chemical volumes, extends in this embodiment of the invention to the feed device  34  from above. The feed duct  162  is also In this embodiment bent at a location above the feed device  34  to the same direction as the connections  144  and  146  for feed and mixing liquids. The volume of the chemical to be fed may be adjusted for example with a valve  46  located in the chemical feed duct  162 . The chemical feed duct  162  has been secured to an elongate outer end  22  of the feed device  34  with a securing means  20 . The feed duct  162  communicates with the mixing liquid feed duct  142  by being located in this embodiment centrally inside the mixing liquid feed duct  142  and extending close to the special nozzle part  150  of the feed duct  142  which nozzle part in turn is adjustable to extend inside the process liquid flow duct  70 . 
     In this embodiment of the invention the mixing liquid feed duct  142  has at the lower end of it, in other words at the end facing the fiber suspension flow duct  70  and extending inside the nozzle casing  80 , a conical converging portion  148  which is essentially located at the conical portion  82  of the nozzle casing  80  and its coning angle is of the same order as that of the conical converging portion  82  of the nozzle casing  80 . The conical converging portion  148  of the mixing liquid feed duct  142  does not extend quite to the lower end of converging portion  82  for the feed liquid but the feed duct continues preferably as a cylindrical duct  116  inside the feed opening  84  whereby the cross-sectional flow area between these parts reduces in the flow direction caused an increasing in the flow velocity of the feed liquid. The flow velocity of the mixture of the chemical to be fed into the process liquid flowing in the process liquid flow duct  70  and the feed liquid is at the feed moment at least five times the speed of the process liquid flow. 
     The cylindrical duct  116  at the lower end of the mixing chemical feed duct  142  ends at the nozzle part  150  which provides the mixing space  154  isolated from the feed liquid and the flowing process liquid required for the chemical mixing and from which the chemical solution (a mixture of chemical and mixing liquid) is at first fed via openings  152  to the feed liquid flow and further by means of the feed liquid in an even flow to the liquid flow duct  70 . The isolated mixing space  154  in the nozzle part  150  is formed for example of a cup-like “closed” end  156  of the mixing liquid flow duct  142  and of the openings  152  provided at its sides. The closed end  156  is impervious to the flow of liquid. The openings  152  have been provided in the wall of the flow duct  142  above the mixing space  154  of the nozzle part  150 . Via the openings  152  the mixing liquid and the chemicals mixed into it are discharged practically in a radial fan-like flow to the feed liquid. The openings  152  may have a round, angular or for example slot-like configuration only to mention a few examples. The thin pipe-like chemical feed duct  162  extends to the end  156  of the nozzle part  150 , preferably past the openings  152 . This embodiment guarantees a good chemical mixing result as the chemical jet hits the end of the nozzle part  150  and is from there dispersed evenly to the entire mixing liquid volume and further via openings  152  to the liquid flow duct  70 . The mixing and the dilution of the chemical thus take place before the feeding to the process liquid by means of the feed liquid. This ensures that precise chemical amounts are mixed into the whole cross-sectional flow area of the process liquid. According to another preferred embodiment of the invention a kind of an additional, for example conical, counter piece has been provided, if necessary, in the end of the chemical feed duct  162  quite in the center of it whereby, when hitting it, the chemical jet is dispersed and mixed even more efficiently. Another alternative is to design the end cup  156  of the duct  142  so that it divides the chemical flow coming from the duct  162  evenly to different sides of the duct  162  for example by providing the bottom of the end cup at a central position relative to the duct  162  with a conical or corresponding bulge converging towards the duct. 
     Preferably the nozzle part  150  of the mixing liquid flow duct  142  and the mixing space therein are located inside the process liquid flow duct  70  or at least in the close vicinity of the inner surface of the flow duct  70  mentioned so that the mixing of the chemical to the mixing liquid takes place 0.5 seconds, at the most, before the chemical solution is mixed with the process liquid. Compared with the situation illustrated in  FIG. 3 , where the openings  152  are located just inside the wall of the process liquid flow duct  70  (illustrated schematically), the openings  152  may be located at the annular feed opening  84  for feed liquid, thus inside the duct portion  76 . 
     The function of the feed liquid discharging from the opening  84  of the feed device  34  is to give the chemical solution the required velocity which feeds the chemical solution efficiently across the whole cross-sectional flow area of the liquid flow duct  70 . The feed liquid hits mainly axially the chemical solution jet discharging from the openings  152  in an almost radial direction, increasing the velocity of the chemical and improving the mixing with the process liquid flowing in the flow duct  70 . The direction and penetration of the chemical jet are adjusted by adjusting the feed device  34  with the screw  138  and the feed pressure with valves  42 ,  44  and  46 . 
     As may be seen from the above, a feeding device of a new type for feeding and mixing various chemicals in small, precisely predetermined amounts to process liquid flows has been developed. It should also be noted that although the above description generally discusses the use of the feed nozzle according to the invention particularly in connection with applications in wood processing industry the invention may be applied anywhere where chemicals need to be fed and mixed into a medium flow evenly and in precise amounts. Thus, the field of application and the scope of protection of the invention are defined by the appended patent claims, only.