Patent Publication Number: US-9427716-B2

Title: Apparatus and method for introducing a first fluid into the flow path of a second fluid and use of such an apparatus

Description:
The present invention relates to an apparatus for mixing a first fluid into a flow path of a second fluid, said apparatus comprising:
         a chamber, which encloses the flow path and exhibits a first inlet for receiving the second fluid, a second inlet arranged downstream of the first inlet for receiving the first fluid, as well as an outlet arranged downstream of the second inlet for discharging a mixture of the first fluid and the second fluid, said flow path extending from the first inlet to the outlet and said second inlet opening into the flow path;   a throttle body, which is pivotally arranged inside the chamber for controlling the flow area of the flow path; and   pivoting means, for pivoting the throttle body for said controlling of the flow area.       

     The present invention also relates to a method for mixing a first fluid into a flow path of a second fluid, comprising the steps of:
         causing the second fluid to flow in a chamber from a first inlet to an outlet, said chamber enclosing said flow path;   supplying the first fluid to the flow path of the second fluid via a second inlet of the chamber, said second inlet being arranged downstream of the first inlet and upstream of the outlet; and   causing pivoting means to pivot a throttle body arranged in the flow path, to control the flow area of the flow path.       

     The present invention also relates to use of such an apparatus. 
     As used herein, fluid means a gas, a liquid, a steam, or a mixture of these. As used herein, the notion fluid is also meant to include a system consisting of a mixture of solid particles and a liquid or gas, where the mixture has fluid-like properties. One example of such a system is a suspension, e.g. a cellulose pulp suspension. 
     As used herein, introducing a first fluid into the flow path of second fluid means injection, mixing, dispersion or other admixing of the first fluid, which is also called the admixture fluid, into the flow path of the second fluid. 
     It is not unusual in industrial processes that fluids are mixed with each other. In e.g. the paper industry, it is not unusual that process chemicals, e.g. oxygen gas, chlorine dioxide or ozone, are introduced into a flow of pulp suspension. It is also common in this industry that steam is introduced into the flow of pulp suspension with the purpose of heating the pulp suspension. 
     When introducing a first fluid into the flow path of a second fluid, it is generally always desirable to obtain a mixing or dispersion of the first fluid which is as effective and uniform as possible. 
     A number of apparatuses for introducing a first fluid into the flow path of a second fluid are previously known. 
     SE 468 341 C discloses an apparatus for mixing a fluid, such as e.g. gases in the form of ozone, oxygen and chlorine, and liquids containing various active substances, e.g. chlorine dioxide, into the flow path of a suspension of a cellulosic fiber material. The apparatus has a funnel-shaped part and a cone-shaped, movable part arranged therein. Between the funnel-shaped part and the cone-shaped part, there is an adjustable gap through which the fiber suspension passes, wherein also the fluid is supplied in said gap. The apparatus comprises a pneumatic cylinder for displacing the cone-shaped part, to control the flow area of the gap. A control device continuously measures the pressure in the pulp suspension upstream and downstream of the gap, compares the recorded pressure difference with a predetermined set value, and controls the flow area of the gap via the cylinder, so that the predetermined set value is maintained. 
     SE 502 393 discloses another apparatus for mixing a first fluid, e.g. steam, oxygen, ozone or chlorine dioxide, into the flow path of a second fluid, e.g. a pulp suspension. The apparatus has a flow chamber for the second fluid. A double wedge-shaped restrictor member is arranged in this chamber, so that a gap for the second fluid is formed between the inside walls of the chamber and the wedge member, wherein also the first fluid is supplied in said gap. The restrictor member can be operated via cylinders for controlling the width of the gap, and thus also the flow through the apparatus. Pressure-sensing sensors are mounted upstream and downstream of the apparatus for measuring the pressure difference across the apparatus, and a control device is adapted to control the width of the gap, via the cylinders and the restrictor member, so that a predetermined set value of the pressure difference is maintained. 
     SE 514 543 discloses yet another apparatus for mixing a first fluid into a second fluid. The apparatus has a flow chamber for the second fluid, which can be a pulp suspension. Inlet passages for supplying the first fluid, which can be steam, open directly upstream of, or into the flow chamber. A throttle body is arranged in the flow chamber. The throttle body has a circle segment-shaped cross-section and is pivotally arranged by means of a cylinder relative to an opposite filling body, so that a gap having a controllable flow area is formed. By means of adjusting the size of the gap by pivoting the throttle body, the amount of suspension passing the steam inlet passages can be controlled, and thus the amount of steam in the pulp suspension can be controlled. 
     Apparatuses comprising a rotating part, which ensures that the fluid is mixed into the pulp suspension, are also used for mixing a fluid into a pulp suspension. If the fluid is steam, a problem associated with these apparatuses is that the rotation leads to large pressure fluctuations, which create local zones of very low pressure into which the steam tends to flow. Accordingly, there is a risk of accumulation of steam in these zones, with steam implosions as a result. Another problem with these devices is that they are relatively energy intensive and that they require relatively much maintenance. 
     One problem with the previously known systems mentioned above is that they have difficulty in producing an effective mixing or dispersion of the admixture fluid in case of sudden variations in the flow of the fluid in which the mixing is to occur. This is especially a problem when heating a liquid or a suspension by means of steam, where the steam is injected or mixed into the liquid or suspension and is then allowed to condense. 
     The objective of such an injection is to admix, i.e. to mix and disperse, the added steam, and simultaneously to keep the liquid or suspension in such a movement, that a slow and continuous condensation of the steam occurs. If the mixing or dispersion is not sufficient, there is a risk of steam bubbles forming in the liquid or suspension, wherein said steam bubbles may subsequently implode. In case of sudden flow rate fluctuations of the liquid or suspension, there is a risk of the mixing of the steam becoming insufficient, wherein there is a risk of intermittent steam implosions occurring. These steam implosions cause pressure shocks in the liquid or suspension, which in their turn may propagate to machine supports, apparatuses and other process equipment and cause knocks and vibrations, which can be so powerful that mechanical damage results. This is especially a problem when a large amount of steam is added to a cellulose pulp suspension and especially to a cellulose pulp suspension of medium consistency. As used herein, a pulp suspension of medium consistency means a pulp suspension having a dry solids content in the range of approx. 8-14%. 
     Accordingly, there is a general need for an apparatus and a method which enable an effective mixing and dispersion of a first fluid into the flow path of a second fluid even if the flow rate of the second fluid fluctuates. There is especially a need for an apparatus and a method which enable an effective mixing and dispersion of steam into a liquid or suspension, e.g. a cellulose pulp suspension, even if the flow rate thereof fluctuates. 
     It is thus the main object of the present invention to solve the above-mentioned problems and to produce an apparatus and a method which enable a simple and effective mixing of a first fluid into the flow path of a second fluid, even if the flow rate of the second fluid fluctuates. 
     Another object of the present invention is to produce an apparatus and a method which enable an effective mixing of steam into a liquid or a suspension, e.g. a cellulose pulp suspension, even if the flow rate of the liquid or suspension fluctuates. 
     Still another object of the present invention is to produce an apparatus and a method which enable mixing of steam into a liquid or a suspension, e.g. a cellulose pulp suspension, without the occurrence of powerful steam implosions. 
     Another object of the present invention is to produce an apparatus and a method which are capable of effectively mitigating the negative effect of steam implosions, should they still occur. 
     Yet another object of the present invention is to produce an apparatus and a method which enable an effective mixing of a process chemical in gaseous form, e.g. oxygen gas, chlorine gas and ozone, or in liquid form, e.g. a pH-adjusting liquid, chlorine dioxide or other treatment liquid, into the flow path of a second fluid, e.g. process liquor or a cellulose pulp suspension, even if the flow rate of the second fluid fluctuates. 
     Still another object of the present invention is to produce an apparatus and a method which prevent clogging of the inlet openings for the first fluid and the flow path of the second fluid. 
     The above-mentioned and other objects are achieved by means of an apparatus according to the invention, which is characterized in that the pivoting means is adapted to pivot the throttle body so that the flow area decreases with a decreasing flow rate of the second fluid and increases with an increasing flow rate of the second fluid, in order to maintain the flow velocity of the second fluid at the second inlet within a predetermined range. 
     The above-mentioned and other objects are also achieved by means of a method according to the invention, which is characterized in the step that the pivoting means is caused to pivot the throttle body so that the flow area decreases with a decreasing flow rate of the second fluid and increases with an increasing flow rate of the second fluid, in order to maintain the flow velocity of the second fluid at the second inlet within a predetermined range. 
     Accordingly, by means of the invention, an effective mixing of fluids is achieved even if the flow rate of the fluid in which the mixing is to occur fluctuates. In case the admixture fluid is steam, the further advantage relative to the prior art that the occurrence of steam implosions can be eliminated, or at least substantially reduced, is obtained. This, in its turn, means that failures of the apparatus and associated process equipment (piping etc.) causing shutdowns are avoided. Furthermore, the heavy and over-dimensioned foundations and/or piping that were previously needed to handle the mechanical stresses caused by the steam implosions are no longer needed. 
    
    
     
       In the following, the invention will be described more closely with reference to accompanying drawings. 
         FIG. 1  shows a perspective view of a first, preferred embodiment of an apparatus according to the invention. 
         FIG. 2  shows the apparatus of  FIG. 1  in a side view. 
         FIG. 3  shows the apparatus of  FIG. 1  in a top view. 
         FIG. 4  shows the apparatus of  FIG. 1  in a view from behind. 
         FIG. 5  shows the apparatus of  FIG. 1  in a side view, in cross-section, wherein a control unit of the apparatus is shown in greater detail. 
         FIG. 6  shows an axle and a flap of the control unit in cross-section. 
         FIG. 7  shows an embodiment where the apparatus comprises a second conduit. 
         FIGS. 8-10  show an embodiment of a flap according to  FIG. 6 . 
     
    
    
     The embodiment of the invention that will be described in the following is intended to be used in a process plant for mixing a first fluid, in the form of steam, into the flow path of a second fluid, in the form of a cellulose pulp suspension, wherein the hot steam is intended for heating the pulp suspension to a desired temperature, e.g. to a temperature that is suitable for a subsequent bleaching step. It will be appreciated, however, that the principle of the invention can be used for mixing other fluids, such as gases, e.g. oxygen gas, chlorine gas or ozone, or liquids, e.g. pH-adjusting liquids, chlorine dioxide or other treatment liquid, into a pulp suspension. It will also be appreciated that the second fluid may be of another type than a pulp suspension, e.g. process liquor. 
     The apparatus comprises a substantially parallelepipedic housing  1 , for receiving a pulp suspension from a first conduit  2  located upstream, as well as for discharging the pulp suspension into a second conduit  3  located downstream. The apparatus further comprises a supply means  4  for supplying steam to the flow of pulp suspension. The apparatus further comprises a control unit  5 , which ensures that there is a suitable flow velocity in the pulp suspension when supplying the steam, in order to avoid the occurrence of steam implosions. Accordingly, the control unit  5  ensures that the flow velocity of the pulp suspension exceeds a certain predetermined minimum value when supplying the steam. 
     The housing  1  is delimited externally by an upper delimiting surface, constituted by a roof portion  6 , lateral delimiting surfaces, constituted by side walls  7  and  8  and by a short side wall  9  located upstream and a short side wall  10  located downstream, and a lower delimiting surface, constituted by a base portion  11 . 
     Internally, the housing  1  comprises a substantially parallelepipedic chamber  12 , which is approx. 500-700 mm long, approx. 200-250 mm wide, and approx. 150-300 mm high. The chamber  12  exhibits a circular first inlet  13  for receiving the pulp suspension from the first conduit  2  disposed upstream, and a rectangular outlet  14  for discharging the pulp suspension into the second conduit  3  disposed downstream. The first inlet  13  is formed by an opening in the short side wall  9  located upstream and has a diameter of approx. 80-200 mm. Accordingly, the inlet  13  has an area that is smaller than the cross-sectional area of the chamber  12 . The rectangular outlet  14  is substantially equally large as the cross-sectional area of the chamber  12 . 
     Accordingly, the chamber  12  encloses a flow passage  44  for the pulp suspension, said flow passage  44  extending from the first inlet  13  to the outlet  14 . 
     Furthermore, the chamber  12  exhibits an elongated second inlet  15  for receiving the pressurized, hot steam from the supply means  4 , said inlet  15  opening into the flow passage  44 . The inlet  15  is arranged in the roof portion  6  of the housing and is located approx. 100-150 mm from the outlet  14  of the chamber. The supply means  4  connects to the second inlet  15  from the top side of the roof portion  6 . The second inlet  15  is arranged with its longitudinal direction transversely to the chamber  12  and the flow passage  44 , i.e. transversely to the flow direction of the pulp suspension, and extends across substantially the entire width of the flow passage  44 . In other words, the second inlet  15  has a length that is substantially equal to the width of the chamber  14 . The width of the inlet  15 , i.e. its extension in the longitudinal direction of the chamber  14 , is approx. 25-70 mm. 
     The base portion  11  exhibits an elongated recess  16 , which extends transversely to the longitudinal direction of the chamber  12  close to the first inlet  13 , and each of the side walls  7  and  8  exhibit a respective crescent-shaped opening  17 , which connects to the recess  16  at the ends thereof. A tubular cover  41  is fixedly disposed in these recesses  16 ,  17 , as is evident from  FIGS. 5 and 6 . The cover  41  has a length that exceeds the width of the housing  1 , for which reason the cover projects outwardly on both sides of the housing  1 , as is evident from  FIG. 1 . The lower portion  45  of the cover  41  protrudes below the chamber  12  and from the base portion  11  of the housing  1 . As is most clearly evident from  FIG. 6 , the central portion of the upper part of the cover  41  has been cut out, so that no part of the cover  41  projects into the chamber  12 . Furthermore, this cut-out makes the axial space of the cover  41  accessible from the chamber  12  via the recess  16 . 
     Two removable stoppers  45  and  56  are arranged in the base portion  11  of the housing  1  and in the cover  41 . The stoppers  45 ,  56  enable rinsing of the housing  1  and the cover  41  in case of so-called plugging, i.e. that the pulp suspension clogs the housing  1  and the cover  41 . 
     The supply means  4 , for supplying the pressurized, hot steam to the chamber  12  and the flow passage  44  via the second inlet  15 , comprises a pipe flange  19  that connects to a steam conduit (not shown) for feeding pressurized steam to the supply means  4 . Furthermore, the supply means  4  comprises a pipe part  20 , which exhibits a first end  21  and a second end  22 . The first end  21  connects to the pipe flange  19  and the second end  21  connects to an elongated valve  23  of the supply means  4 . The second end  22  is compressed, as is evident from  FIG. 1 , making the pipe opening of the second end  22  elongated. The valve  23  connects to the second inlet  15  of the chamber  12  via a screw joint  24 . The valve  23  comprises, on the one hand, a pivotal valve spindle  25 , exhibiting an elongated longitudinal gap  26  for passage of the steam, and, on the other hand, a valve spindle housing  27 , enclosing the valve spindle  25 . By turning the valve spindle  25 , the valve  23  can be adjusted to a fully open position, to a fully closed position, or to a desired position therebetween. The gap  26  extends across the entire length of the second inlet  15 . The position of the valve spindle  25  is controlled by a control means  28 , which is disposed on the valve spindle housing  27  at one end of the valve spindle  25 . 
     The distance between the valve spindle  25  and the orifice of the inlet  15  is relatively short, approx. 20-50 mm. This, together with the simple geometry of the outlet, ensures that any pulp suspension, which may have accumulated in the inlet during an interruption of the steam supply, easily can be pushed out by the steam when the steam supply is resumed, which provides for good operating reliability. 
     The control unit  5  comprises a throttle body in the form of a flap or lip  29 , a pivotal axle  37 , two lever arms  48  and  49 , and pivoting means in the form of two pneumatic cylinders  50  and  51 . 
     The axle  37  is pivotally arranged inside the axial space of the cover  41  by means of self-lubricating bearings  43 , as is evident from  FIG. 6 . The axle  37  is longer than the cover  41  and exhibits axle journals  46 ,  47 , projecting outwardly through end plates  42 , which are arranged at the ends of the cover  41 . Accordingly, the outer portions of the axle journals  46  and  47  constitute projecting ends of the axle  37 . 
     The flap  29  is arranged inside the chamber  12  and has the shape of a substantially rectangular plate, having a thickness of approx. 25-35 mm. The flap  29  exhibits a top side  30 , facing away from the base portion  11  of the housing, a bottom side  31 , facing toward the base portion  11  of the housing, two parallel long sides  32 ,  33 , facing toward the side walls of the housing, a first end  34  or short side  34  located upstream, and second end  35  or short side  35  located downstream. 
     The flap  29  has its first end  34  fixedly connected to the pivotal axle  37  by means of bolts  36  and extends, through the recess  16  in the base portion  11 , downstream in the flow direction of the pulp suspension. The second end  35  of the flap  29  is free, and its connection to the top side  30  is chamfered, as is evident from  FIG. 5 . The flap  29  has a length that is approx. 300-450 mm, i.e. slightly longer than the height of the chamber  12  and slightly shorter than the length of the chamber  12 , so that its free end  35 , located downstream, is substantially aligned with the second inlet  15 . 
     The lever arms  48 ,  49  are fixedly disposed on the free ends of the axle journals  46 ,  47  of the axle  37 , at right angles to the longitudinal direction of the axle  37 . Accordingly, the lever arms  48 ,  49  rotate together with the axle  37  and the flap  29 , when these are turned. The respective lever arm  48 ,  49  abuts against one of said pneumatic cylinders  50 ,  51 . In the shown embodiment, these pneumatic cylinders are constituted by piston rod-free bellows cylinders  50 ,  51 , which exhibit end plates  52 ,  53  abutting against the lever arms  48 ,  49 . In the shown embodiment, the respective bellows cylinder  50 ,  51  is fixedly disposed on a respective side wall  7 ,  8  of the housing  1 . 
     The flap  29  is pivotable between a lower end position, where the bottom side  31  of the flap abuts against the base portion  11  of the chamber  12 , and an upper end position, where the free end  35  of the flap  29  abuts against the roof portion  6  of the chamber  12 . The flap  29  has a width that is substantially equal to the width of the chamber  12 . Accordingly, when using the apparatus, the pulp suspension is forced to pass over the top side  30  of the flap  29 . Thus, the upper end position of the flap  29  constitutes a fully closed position, where the flow passage  44  is fully closed, and the lower end position of the flap  29  constitutes a fully open position, where the flow passage  44  is fully open. Accordingly, when the flap  29  is located between its end positions, the flap  29  forms a constriction in the flow passage  44 , where the flow area of the flow passage decreases continuously from the end  34  of the flap  29  located upstream to the free end  35  thereof located downstream. Immediately downstream of the flap  29 , i.e. directly downstream of its free end  35 , the flow area of the flow passage  44  suddenly increases to its initial value, i.e. to the same value as directly upstream of the flap  29 . The inlet  15  opens near the free end  35  of the flap  29 , and the steam is thus supplied in the region where the cross-section of the flow passage  44  suddenly increases, which is advantageous for the mixing and dispersion of the steam into the pulp suspension. 
     While the pulp suspension passes over the flap  29 , the pulp suspension exerts a torque about the axle  37  on the flap  29 , which tends to push the flap  29  down, i.e. to pivot the flap  29  clockwise about the axle  37  in  FIG. 5 . Accordingly, the top side  30  of the flap  29  constitutes a guiding or diverting surface, which diverts the direction of flow of the flow path  44 , with which surface the pulp suspension interacts to produce said downward torque. The bellows cylinders  50 ,  51 , in their turn, are pressurized to a predetermined pressure. When they are compressed, they exert a torque on the flap  29 , via the lever arms  48 ,  49  and the axle  37 , which strives to push the flap up, i.e. to pivot the flap  29  anti-clockwise about the axle  37  in  FIG. 5 . 
     At a constant flow rate of the pulp suspension, the flap  29  adjusts itself to an equilibrium position, where the torque that the flow of pulp suspension exerts on the flap  29  is balanced by the torque that the bellows cylinders  50 ,  51  exert on the flap  29 . In other words, the bellows cylinders  50 ,  51  are adapted to continuously exert a torque on the flap  29 , which balances the torque that the pulp suspension exerts on the flap  29  at every flow rate of the pulp suspension. 
     If the flow rate of the pulp suspension increases, the flap  29  is pushed down, so that the smallest flow area of the flow passage  44 , i.e. its flow area at the end  35 , increases. If the flow rate of the pulp suspension stabilizes at this new, higher level, the flap  29  adjusts itself to a new equilibrium position, where the flow area of the flow passage  44  at the end  35  is larger than in the previous equilibrium position. If the flow rate of the pulp suspension decreases, the flap  29  is pushed up by the bellows cylinders  50 ,  51 , so that the flow area of the flow passage  44  at the end  35  decreases. If the flow rate of the pulp suspension stabilizes at this new, lower level, the flap  29  thus adjusts itself to a new equilibrium position, where the flow area of the flow passage  44  at the end  35  is smaller than in the previous equilibrium position. Accordingly, an increasing flow rate of the pulp suspension causes the flow area of the flow passage at the end  35  to increase, and a decreasing flow rate causes the flow area to decrease. 
     It will be appreciated that this controlling of the flow area compensates for the decrease and increase, respectively, in the flow velocity of the pulp suspension that results from a decrease and an increase, respectively, of its flow rate. If e.g. the flow rate of the pulp suspension decreases, also the flow velocity of the pulp suspension in the region upstream of the flap  29  decreases, since the flow area in this region is unchanged. However, due to the decreasing pressure of the pulp suspension on the flap  29  in this situation, the flap is pivoted  29  upward and the flow area at the flap  29  decreases. This, in its turn, implies that the flow velocity of the pulp suspension at the end  35  is maintained at substantially the same level as before the flow rate decrease. If the flow rate of the pulp suspension increases, an adjustment is effected in the other direction, i.e. due to the increasing pressure of the pulp suspension on the flap  29 , the flap  29  is pushed down, the flow area above the flap  29  increases, and the flow velocity of the pulp suspension at the end  35  is maintained substantially at the same level as before the flow rate increase. Accordingly, the flap  29  acts as a throttle body, which controls the flow area of the flow passage  44  while being actuated by the cylinders  50 ,  51 , so that the flow velocity of the pulp suspension is maintained within a desired range. Accordingly, the control unit  5  ensures that a decrease of the flow rate of the pulp suspension does not lead to a situation, where the flow velocity of the pulp suspension at the steam supply position falls below a level where the mixing of the steam risks becoming so inadequate that there is a risk of damaging steam implosions occurring. 
     In addition to the fact that the bellows cylinders  50 ,  51  abut against the lever arms  48 ,  49  with a pushing force, the bellows cylinders  50 ,  51  also dampen any pressure waves which may occur in the pulp suspension, e.g. when the pulp suspension passes over the flap  29 , or if damaging steam implosions still occur. Accordingly, the bellow cylinders  50 ,  51  also constitute spring or damping means. 
     Accordingly, the flap  29  adjusts itself to an equilibrium position, where the flow of pulp suspension imposes a pushing force on the flap  29 , which is balanced by the force from the bellows cylinders  50 ,  51 . Thus, the flap  29  is self-adjusting and its actual angle relative to the base portion  11  is dependent on the magnitude of the pulp flow. The predetermined flow velocity range can be set by adjusting the abutting force of the bellows cylinders  50 ,  51  against the lever arms  48 ,  49 , whereby the desired equilibrium position can be set. By increasing the abutting force of the bellows cylinders  50 ,  51  against the lever arms  48 ,  49 , the axle  37  is rotated so that the flap  29  is pushed up to a new equilibrium position. This implies that the cross-sectional area above the flap decreases, which causes the flow velocity of the pulp suspension at the second inlet  15  to increase. 
     Accordingly, the apparatus is self-adjusting in that the control unit  5  ensures that the flow velocity of the pulp flow at the second inlet  15  is always sufficiently high to avoid, or at least reduce the occurrence of steam implosions. The control unit  5  also ensures that an increase of the flow rate of the pulp suspension does not lead to an undesirably high flow resistance across the apparatus. 
     It will be appreciated that the minimum allowable flow velocity of the pulp suspension at the steam supply position is dependent on a number of factors, e.g. the concentration of the pulp suspension, the steam flow rate, i.e. the amount of steam supplied, etc. As an example of a suitable flow velocity range when supplying steam to a pulp suspension, it may be mentioned that, when mixing steam at a flow rate of approx. 2-20 kg/s into a pulp suspension of medium consistency, the flow velocity of the pulp suspension at the free end  35  should be within the range of approx. 30-35 m/s, if the embodiment shown in the figures is used. 
       FIG. 7  shows an embodiment of the apparatus that is especially advantageous when supplying steam to a pulp suspension. In this embodiment, the apparatus comprises the second conduit  3  disposed downstream of the chamber  12 . The outlet  14  of the chamber  12  connects to the inlet of the second conduit  3  by means of a pipe flange  55 , which is fitted to a pipe flange of the second conduit  3 . The flow area or cross-sectional area of the second conduit  3  is larger than the cross-sectional area of the outlet  14 . In a preferred embodiment, the cross-sectional area of the second conduit  3  is at least 50% larger than the cross-sectional area of the outlet  14 . The area increase between the cross-sectional area of the outlet  14  and the cross-sectional area of the second conduit  3  occurs suddenly, in a single step. The length of the second conduit  3  is advantageously from two times all the way up to ten times the diameter, or any other equivalent cross-sectional dimension of the second conduit  3 . 
     Since the cross-sectional area of the second conduit  3  is larger than the cross-sectional area of the outlet  14 , the pulp suspension will decelerate after the outlet  14  in those regions of the second conduit  3  which are located radially outside the outlet  14 , and be retained against the inside surface of the second conduit  3 . Therefore, a volume of fibers will be built up successively by stagnant pulp, along the inside shell surface of the second conduit  3 , which can absorb pressure waves in the pulp suspension which may occur due to any steam implosions. The pulp suspension in the middle of the second conduit  3 , on the other hand, will continue at a high velocity through the second conduit  3  to a subsequent third conduit  54 , which has a diameter that is substantially smaller than the diameter of the second conduit  3 . 
       FIGS. 8-10  show one embodiment of the flap  29  in greater detail. As is evident from the figures, each long side  32 ,  33  exhibits an angled portion  60  and a planar portion  61 , wherein the planar portion  61  is adapted to slidably interact with an opposite side wall  7 ,  8  of the chamber  12 , with a good fit, to prevent the pulp suspension from passing between the long sides  32 ,  33  of the flap  29  and the side walls  7 ,  8  of the chamber. 
     However, the fit must be such that the pivoting movement of the flap  29  is not impeded by frictional forces between the portions  61  and the side walls  7 ,  8  of the chamber  12 . 
     The chamfer of the free end  35  is also clearly evident from  FIGS. 8-10 . According to one embodiment of the flap  29 , this chamfer exhibits a planar surface  62  forming an angle of approx. 30 degrees with the top side  30  of the flap  29 . 
     According to one embodiment of the flap  29 , which is particularly preferred when the apparatus is to be used for mixing a gas, e.g. ozone, into a pulp suspension, the flap  29  exhibits turbulence-generating means, which are adapted to promote the formation of turbulence in the pulp suspension when it leaves the free end  35  of the flap  29 . In the shown embodiment, these turbulence-generating means have the shape of elongated, parallel grooves or recesses  63 , which are disposed at the free end  35  of the flap  29  and which extend in the longitudinal direction of the flap  29  from directly upstream of the planar surface  62 , and which open into this surface  62 . Each groove  63  exhibits a U-shaped cross-section and is approx. 80-100 mm long, approx. 5-15 mm wide and approx. 10-20 mm deep. The grooves are arranged at a mutual distance of approx. 15-25 mm and each has a bottom forming an angle of approx. 10 degrees with the top side  30  of the flap  29 . During operation, a first group of partial flows of the pulp suspension will be guided in the grooves  63 , whereas a second group of partial flows will be guided along the surface  62  between the grooves  63 . At the mouths of the grooves  63 , these partial flows will intersect and mix, resulting in the formation of turbulence. 
     In the foregoing, the invention has been described based on a specific embodiment. It will be appreciated, however, that further embodiments and variants are possible within the scope of the following claims. With reference to the above-described embodiment, for example another type of pneumatic cylinder can be used, e.g. cylinders of piston rod-type. It will also be appreciated that another pushing means can be used, e.g. a piston rod cylinder, a spring-loaded cylinder, or a mechanical spring, e.g. a torsion spring. 
     It will also be appreciated that the apparatus can be provided with other elements or means than the above-described grooves  63 , said other elements or means contributing to increasing the turbulence in the pulp suspension at the outlet  15 , which in its turn increases the mixing and dispersion of the supplied fluid. 
     It will also be appreciated that the apparatus does not necessarily have to comprise a conduit in accordance with the second conduit  3  of  FIG. 7 . The apparatus can of course be fitted to conduits of any dimensions whatsoever which transport pulp suspensions. 
     It will also be appreciated that the throttle body may have a different design than the above-described flap  29 . The throttle body can e.g. be wedge-shaped.