Patent Publication Number: US-7223013-B2

Title: First in first out hydration tanks

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present invention is an improved hydration tank for use with Applicant&#39;s Gel Mixing System taught in U.S. patent application Ser. No. 10/426,742, now abandoned. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a first in first out hydration tank that prevents liquid flowing through the tank from stagnating in certain areas of the tank, thereby facilitating flow that is truly first in first out through the tank. 
     2. Description of the Related Art 
     In gel mixing systems, it is desirable to have first in first out hydration tanks so that gel mixtures flowing through the tanks have consistent and predictable residence time within each tank. Even if a traditional hydration tank has a defined flow circuit provided through the tank, such as the three tanks taught in FIG. 2 of Applicant&#39;s U.S. patent application Ser. No. 10/426,742, there can still be a problem when the fluid that is flowing through the tanks is a highly concentrated fracturing gel mixture. 
     The reason this is true is that fracturing gel made from guar is a non-Newtonian fluid. Newtonian fluids, such as water and oil, will flow whenever even a slight pressure is applied to the fluid. Non-Newtonian fluids, on the other hand, require that a certain threshold pressure be applied to them before they begin to flow. This is due to the yield point of the fluid. Thus, non-Newtonian fluids have a threshold pressure required to start them moving, and below which they may deform but will not move. This phenomenon is referred to as gel strength and is directly proportional to the force required to cause the fluid to start moving. 
     In the mixing system described in Applicant&#39;s U.S. patent application Ser. No. 10/426,742, a concentrated gel is prepared that can have significantly higher viscosity and gel strength than that of the final product. The concentrate allows greater hydration time in limited tank volume but has the problem of higher viscosities and gel strengths in the mixing and the hydration tanks. 
     If the fluid is not managed properly, parts of the tank will become gelled and motionless and will be difficult to get moving again. When gelation occurs, the objective of first in first out flow is defeated because the gelled fluid will remain in one place and the newly mixed fluids that enter the tank will bypass the gelled fluid. Thus, the tank is functionally smaller than its actual size since part of the fluid in the tank is not moving. 
     The present invention addresses this problem by stirring the fluid as it passes through the hydration tank, thereby preventing dead spots within the tank. By providing mixing that is normal to the nominal direction of flow, i.e. not forward or backward relative to the direction of flow through the tank and providing shear within virtually all of the volume, the mixing prevents the occurrence of dead spots or channeling within the flow path, while not moving some of the liquid towards the discharge port faster than other parts of the fluid volume, thereby insuring that all the fluid ends up with exactly the same residence time in the tank. By employing mixing that is normal to the flow of the liquid through the tank, all of the fluid flow paths through the tank move at a uniform velocity. 
     While the fluid is moving though the hydration tank, it is continuing to hydrate and thus continuingly increasing in viscosity. If the fluid does not keep moving uniformly through the hydration tank, it is possible that some parts of the fluid in the tank would develop greater viscosity due to slower velocity through the tank and therefore greater residence time. The slower moving volume within the tank will continue to develop higher viscosities which in turn tends to further slow its movement until eventually it could stop moving and become gelled. Once gelled, a much greater force is required to get the gel started moving again. 
     The present invention keeps all of the fluid moving at a uniform velocity so that there will not be areas with higher or lower viscosity at the same position within the flow path. Although viscosity will increase due to hydration from the entrance of the present tank to the exit, all fluid that is at the same position relative to the entrance and exit of the tank should have the same viscosity. 
     Still a further object of the present invention is an output from the tank that is uniform in its level of hydration. That is possible only if all of the liquid moves through the tank at the same velocity. 
     SUMMARY OF THE INVENTION 
     The present invention is a first in first out hydration tank that is provided with an interior rotating vessel located between the stationary wall of the hydration tank and the stationary wall of a central inlet tube provided in the center of the tank. The flow of liquid through the tank is downward inside the central inlet tube, then upward between the exterior surface of the inlet tube and an interior surface of the rotating vessel, then downward again between the exterior surface of the rotating vessel and the interior surface of the tank wall. 
     The rotating vessel is provided with vanes that rotate in conjunction with the rotating vessel. The rotating vanes extend horizontally from both the inside and outside surfaces of the wall of the rotating vessel and interleaf with horizontally extending stationary vanes provided on both the interior surface of the wall of the tank and on the exterior surface of the wall of the central inlet tube. Together, the stationary and rotating vanes function to constantly mix the liquid in a direction that is normal to the direction of flow of the liquid as the liquid passes through the tank. This mixing creates a constant sheer action within the fluid as the fluid travels through the tank, thereby preventing gelation of the hydrating fluid. Thus, the tank achieves a true first in first out flow pattern through the tank and a consistent and predictable residence time of the liquid within each tank even at low flow rates through the tank. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a partially cut away view of a first in first out hydration tank constructed in accordance with a preferred embodiment of the present invention. 
         FIG. 2  is a partial view of the tank of  FIG. 1 , showing details of the roller bearings that stabilize the rotating vessel and also showing details of the float and valve to control the fluid level in the tank. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The Invention 
     Referring now to the drawings and initially to  FIG. 1 , there is illustrated a first in first out hydration tank  10  that is constructed in accordance with a preferred embodiment of the present invention. The tank  10  is provided internally with a rotating vessel  12  that is located between a stationary outside wall  14  of the hydration tank  10  and a stationary tube wall  16  of a central inlet tube  18  located centrally within the tank  10 . 
     The tank  10  is designed for receiving a liquid mixture consisting of previously combined gel and dilution water and for maintaining the mixture in a first in first out flow though the tank  10  while the mixture hydrates. The flow of liquid, as shown by the arrows in  FIGS. 1 and 2 , through the tank  10  is from the inlet  19  of the tank  10  downward inside the central inlet tube  18 , then reversing direction so that the liquid flows upward between the exterior surface  20  of the inlet tube  18  and an interior surface  22  of a side wall  23  of the rotating vessel  12 , then once again reversing direction so that the liquid again flows downward between the exterior surface  24  of the side wall  23  of the rotating vessel  12  and the interior surface  26  of the outside tank wall  14  where the liquid flows out of the tank via a bottom outlet  27 . 
     An air vent  25  is provided in the top of the tank  10  to allow air to escape the tank  10 . The air vent  25  is designed with a ball float  21  that is designed to float on the fluid level in the tank  10  when the tank  10  becomes full of liquid. When the fluid level reaches the ball float  21 , the ball float  21  moves upward, thereby closing the air vent  25  and allowing the tank  10  to continue to operate as a fluid filled or slightly pressurized tank until the fluid level again drops sufficiently to allow the ball float  21  to again move downward, thereby reopening the air vent  25 . By having a closed tank, a tank level system is not required. 
     The rotating vessel  12  is provided with vanes  28 , or alternately bars or rods on both the interior surface  22  and the exterior surface  24 . The rotating vanes  28  rotate in conjunction with rotation of the rotating vessel  12 . These rotating vanes  28  extend horizontally from the interior and exterior surfaces  22  and  24  of the wall  23  of the rotating vessel  12  and interleaf vertically with and are spaced apart from horizontally extending stationary vanes  30  provided on both the interior surface  26  of the outside wall  14  of the tank  10  and on the exterior surface  20  of the central inlet tube  18 . 
     Together, the stationary and rotating vanes  30  and  28  function to constantly mix the liquid in a direction that is normal, i.e. perpendicular or at right angles, to the direction of flow of the liquid as the liquid passes through the tank. Thus, by constantly mixing the liquid as it flow through the tank, the tank  10  achieves a true first in first out flow pattern through the tank  10  and a consistent and predictable residence time of the liquid within the tank  10 , even at low flow rates. 
     Although not illustrated, an alternate embodiment of the present invention can replace the stationary vanes  30  on the interior surface  26  and the exterior surface  20  with a coarse screen that covers the flow area but leaves radial slots so that the vessel  12  with its rotating bars  28  can be installed. 
     As illustrated in  FIG. 1 , the rotating vessel  12  is rotated within the tank wall  14  by means of a rotary motor  32 . The rotational speed of the vessel  12  should not be high. The rotary motor  32  is designed to provide enough shear to keep the fluid moving and not gelling, but does not spin at high speed like a washing machine. The rotary motor  32  is attached centrally at the bottom  34  of the tank  10  and is located exterior to the outside wall  14  of the tank  10 . The rotary motor  32  has a drive shaft  36  that extends through the outside wall  14  of the tank via a bearing  38  and seal  40  that are provided on the bottom  34  of the tank  10 . After passing through the bearing  38  and seal  40 , the drive shaft  36  attaches to the bottom  42  of the rotating vessel  12  where the rotating vessel  12  is rotatable supported from the bottom  34  of the outside wall  14  of the tank  10 . When the rotary motor  32  is activated, the rotating vessel  12  is turned or rotated. The rotary motor  32  is provided with a torque arm  44  that attaches to the rotary motor  32  and to an exterior surface  46  of the outside wall  14  of the tank  10  as a means of preventing the rotary motor  32  from turning relative to the tank  10 . 
     The top  48  of the rotating vessel  12  is stabilized by several roller bearings  50  that are either attached to the exterior surface  20  of the central inlet tube  18 , as illustrated in  FIG. 1 , or alternately attached to the interior surface  26  of the outside wall  12 , as illustrated in  FIG. 2 . The roller bearings  50  are provided at several locations around the tank  10  and they engage in rolling fashion a lip  51  provided on the top  48  of rotating vessel  12 , as best illustrated in  FIG. 2 , in order to hold the rotating vessel  12  is a stable upright posture as the vessel  12  rotates within the tank  10 . 
     Instead of providing the tank with an air vent  25 , as illustrated in  FIG. 2 , the tank  10  can alternately be provided with a float  52  for regulating flow of liquid into the tank  10  so the liquid level within the tank  10  does not exceed a predetermined level. The float  52  movably attaches to the exterior surface  20  of the central inlet tube  18  so that the float  52  rises and falls relative to the central inlet tube  18  in conjunction with rise and fall of the liquid level in the tank  10 . A float rod  54  attaches on one end  56  to the float  52  and on an opposite end  58  to a downwardly directed shield  60  so that the float rod  54  raises the shield  60  as the float  52  rises and lowers the shield  60  as the float  52  falls. The shield  60  is secured to a valve sleeve  62  that closely engages and encircles a lower end  64  of the tube wall  16  of the central inlet tube  18 . The lower end  64  of the central inlet tube  18  is provided with valve openings  66  that extend through the tube wall  16  so that the valve sleeve  62  serves to open up or close off flow of liquid through the valve openings  66  in response to the lowering and raising of the float  52 , respectively. Liquid must flow through the valve openings  66  in order to flow from out of the lower end  64  of the central inlet tube  18 . As illustrated by the arrows in  FIG. 2 , once the liquid has passed through the valve openings  66 , the shield  60  forces the liquid to flow downward until it encounters a bottom drain valve seal  68 , and then from there it flows upward, as previously described. 
     The valve sleeve  62  is provided with a stop  70  that reversibly engages a closed bottom end  72  of the central inlet tube  18  to limit the upward movement of the valve sleeve  62  beyond its fully closed position relative to the valve openings  66 . 
     Regardless of whether the tank  10  is provided with an air vent  25  or the float  52  for level control, the tank  10  is provided with the bottom drain valve seal  68  as a means of draining the rotating vessel  12 . The bottom drain valve seal  68  is operated by a cylinder  74  that is located on the top  76  of the tank  10  and exterior to the tank  10 . The cylinder  74  connects to the bottom drain valve seal  68  via a cylinder shaft  78 . The bottom drain valve seal  68  closes against bottom openings  80  provided in the rotating vessel  12 . The purpose of the bottom openings  80  is to provide a means of draining the rotating vessel  12  when desired. To drain the rotating vessel  12 , the cylinder  74  is activated to lift and disengage the bottom drain valve seal  68  from the bottom openings  80 . Likewise, to once again close the bottom openings  80 , the cylinder  74  is reversed to lower the bottom drain valve seal  68  into a sealed engagement with the bottom openings  80 . 
     Although the tank  10  has been described as having a rotating vessel  12  located internally, the invention is not so limited. The invention can alternately be practiced by employing a stationary inner vessel and a system of rotating stirring elements located within the tank so that the stirring elements agitate in a direction that is normal to the direction of flow of the liquid through the tank. 
     While the invention has been described with a certain degree of particularity, it is manifest that many changes may be made in the details of construction and the arrangement of components without departing from the spirit and scope of this disclosure. It is understood that the invention is not limited to the embodiments set forth herein for the purposes of exemplification, but is to be limited only by the scope of the attached claim or claims, including the full range of equivalency to which each element thereof is entitled.