GB2033259A - Mixture concentrator - Google Patents

Abstract

A concentrator (16) for removing part of a medium from a mixture of media, usable, for example, for increasing the amount of proppant in a formation-fracturing foam is disclosed. The concentrator (16) comprises an elongate vessel (80) and a rotor (86) rotatably mounted inside the vessel on a shaft (88). The vessel (80) has a mixture intake (70) at a forward end and a concentrated mixture discharge (76) and an extracted medium discharge (78) at the rearward end. The rotor comprises a cylindrical stabilizer (92) arranged to impart a spin to the incoming mixture and, downstream of the stabilizer, intake impeller means (94) which can comprise a plurality of annular intake impellers. The intake impeller means (94) remove a portion of one medium from the mixture in the annular channel (100) surrounding the rotor into an internal passageway inside the rotor (86) such that flow through the impeller means approximates a free vortex flow pattern. The removed medium passes along said passageway to discharge impeller means (96) and outlet (78). <IMAGE>

Description

SPECIFICATION Mixture concentrator This invention relates to a mixture concentrate for removing a medium from a mixture of media, such as, for example, for removing a liquid carrier from a slurry that may, for instance, be used in stimulating production of oil and or gas.

Various methods are well known for stimulating production of crude oil and natural gas from wells drilled in reservoirs of low permeability, and techniques on which particular emphasis has been placed are those involving hydraulic fracturing of such formations with various liquids, such as native crude oil, with or without propping agents, such as sand, suspended therein. In such techniques, the hydraulic pressure applied to low permeability formulations creates tensile stresses in the rock of the formation surrounding a well bore and these stresses cause splitting, parting or fracturing of the rock. The resulting fractures or channels are then extended by the injection under pressure of fluid containing a propping agent to be deposited in the fractures, the fluid typically having a concentration of propping agent of up to about 6 to 8 Ibs per US gallon (0.72 to 0.96 kg/l).When the pressure is released, the propping agent deposited in the fractures hold the fractures open, leaving wider channels for increased reservior fluid flow. The final width of the channels depends, inter alia, on the concentration of propping agent in the fluid, the greater concentration the greater the width. Such techniques are discussed in, for example, United States Patents Nos. 3664422,3561533, 3363691,3136361, 3842910,3245470 and 3138205.

A recent development in such techniques is the suggestion that foam be used for hydraulic fracturing. See, for example, United States Patent Nos. 3937283, an article by Bulien appearing in the July 22, 1974 issue of "Oilweek" and a paper by Blauer and Kohlhaas entitled "Formation Fracturing with Foam" SPE 5003. These documents disclose a process in which a foam composition is used to fracture formations, involving making a fracturing foam by bleding sand into jelled water and treating the slurry with a surfactant. The fluid pressure of the slurry is increased with a conventional pump and then a gas, such as nitrogen or carbon dioxide, is injected into the fluid, creating a high pressure foam that is then injected into a well to be treated. See also U.S. Patent No. 3980136.

The use of a foam as a fracturing fluid has a number of advantages. Foam has a low fluid loss and hence the fracture treatment is more efficient and, moreover, because larger-area fractures are created with the same treatment volume, formation damage is minimized because little fluid invades the formation. Reduction of fracture conductivity is also minimized. Theoretically, foam has a high capability for carrying and suspending proppant, e.g. sand, so that a greater amount of, e.g. sand will remain suspended in the fluid until the fracture starts to healg Because more proppant can be carried to the fracture, the ratio of the propped fracture area to the created fracture area theoretically approaches one.

In addition, the proppant does not settle quickly in the well bore during unlapped shut-downs during the treatment.

The foam has a high effective viscosity, permitting the creation of wider vertical fractures and horizontal fractures having greater area. The foam has a low friction loss which reduces the hydraulic horsepower necessary for injection and permits treatment of many wells down tubing. Because the foam has a low liquid content, the hydraulic horsepower necessary for injection is reduced which results in low hydrostatic head which, in turn, results in an unbalanced condition soon after opening the well thereby minimizing fluid entry and formation damage. Further, because there is substantially no fluid leakoff, a greater increase length of hydraulic fracture penetration into the formation is possible.

Experience has shown that the cost involved in using foam for moderately deep wells is less than or the same as conventional techniques.

Unfortunately, the use of foam produced by these prior art techniques has at least one major handicap, namely, the maximum proppant concentration obtainable in foam is quite low. For example, during the formation of foam using a method taught in U.S. Patent No. 3937283, a heavy gel is formed that typically has a maximum concentration of 6 to 8 Ibs. of proppant per gallon (0.72 to 0.96 kg/l) of gel. However, when the liquid is foamed, the gas expands the liquid to approximately four times the original volume of gel. The net result is that the sand-foam concentration is reduced to about 2 Ibs. per gallon (0.24 kg/l). As a result, the conventional foam process is not as useful to industry as it could be.

Most of the conventional methods of fracturing use sand as a propping agent because it is readily available, cheap, the particles can be easily graded and sized, it is schematically stable, has low interference with well activity, and when in crevices or fractures in rock is able to withstand the tremendous pressures from the overburden when the fluid pressure is relieved. However, sand is also a highly abrasive substance and it is consequently necessaryto use suitable, specially designed handling equipment. Rugged pumps and tanks are now abaiiable which can inject slurries with sand-fluid ratios as high as 9 Ibs. per gallon (1.08 kg/l). However this concentration of sand is insufficient to provide a foam fracturing fluid that has the desired concentration of sand or other propping agent.Because of the difficulty of pumping and otherwise handling large quantities and large flow rates of sand slurries, it is not possible to increase the initial sand concentration upstream of the pump to much greater than 8 Ibs.

per gallon (0.96 kg/i).

Thus, there is a great need and a large demand for apparatus that is capable of increasing the concentration of a medium in a mixture of media, e.g. a propping agent such as sand in a slurry, thus enabling production of fracturing foam with a higher concentration of propping agent that is obtainable with the known systems discussed above.

Hence, in one aspect the present invention provides a mixture concentrator for removing a medium from a mixture of media, comprising an elongate vessel having a mixture intake, a concentrated mixture outlet located downstream of said mixture intake, and a removed medium outlet; and an elongate rotor mounted for rotation within said vessel so as to define an annular channel between said vessel and said rotor, said rotor having an internal passageway and comprising intake impeller means, the arrangement being such that, on appropriate rotation of the rotor, a portion of the medium to be separated from mixture introduced to the annular channel via said mixture intake is drawn off by the intake impeller means into said internal passageway, such that flow through said impeller means approximates a free vortex flow pattern, and is converted to the removed medium outlet, the resulting concentrated mixtures in the annular channel exiting the vessel via the concentrated mixture outlet.

The present invention also provides, in a further aspect, a mixture concentrator for removing part of the liquid carrier of a mixture, comprising an elongate, substantially cylindrical vessel having a tangential mixture intake at an upstream part, a tangential concentrated mixture outlet at a downstream part, and a tangential carrier outlet; a shaft rotatably mounted and centrally located inside said vessel; shaft seals between said shaft and said vessel where said shaft extends therebeyond; an elongate rotor mounted for rotation with said shaft, said rotor and said vessel defining an annular channel therebetween, said rotor having an internal passageway extending from an upstream part to a downstream part thereof, and comprising an intake impeller means spaced downstream from said mixture intake, said intake impeller means being in fluid communication with said mixture intake and with one end portion of said passageway for drawing off carrier from said annular channel into said passageway, and carrier discharge means for discharging said removed carrier from the other end portion of said passageway to said carrier outlet, said annular channel providing a flow path for the mixture being concentrated from said mixture intake to said mixture outlet; and means located between said concentrated mixture outlet and said carrier outlet for limiting flow of said mixture to said carrier outlet.

In another aspect, the present invention provides a mixture concentrator for removing part of a medium from a mixture of media, comprising an elongate vessel having a mixture intake, a concentrated mixture outlet located downstream of said mixture intake, and a removed medium outlet; and an elongate rotor mounted for rotation with said vessel so as to define an annular channel between said vessel and said rotor, said rotor having an internal passageway and comprising intake impeller means, the arrangement being such that, on appropriate rotation of the rotor, a portion of the medium to be separated from mixture introduced to the annular channel via said mixture intake is drawn off by the intake impeller means into said internal passageway and is conveyed to the removed medium outlet, the resulting concentrated mixture in the annular channel exiting the vessel via the concentrated mixture outlet.

The concentrator of the present invention is suitable for increasing the density of and concentrating a slurry that may comprise highly abrasive substances, e.g. sand, and for producing a very dense fluid or fluid-like mixture. The concentrator is thus suitable for increasing the concentration of a propping agent, e.g. sand, in a slurry and may thus conveniently be used in a system for producing a fracturing foam fluid for hydraulic fracturing of a well to cause or extend fractures or otherwise stimulate flow, drainage and/or productivity of subterrean formations, such foam having higher concentrations of proppant than were previously achievable. Such a system can achieve a foam having up to twice the proppant concentration as that presently obtainable from conventional approaches.

However, a concentrator according to the present invention can also be used in other applications both to concentrate a slurry and to separate purer carrier from the bulk fluid, the use depending upon whether the desired output is the slurry or the carrier, respectively. In addition, the present concentrator could also find other uses in separating a medium from other mixtures of media, such as, for example, a liquid-liquid mixture, a liquid-gas mixture, a soid-gas mixture, and perhaps even a fluid-type solid-solid mixture.

Irrespective of the use for which the concentrator of the invention is employed, the terms "mixture" and medium are used very broadly and the term "slurry" is also used in a broad sense to mean fluid mixtures which are generally composed of solids and liquids, the liquids serving as the carrier for the solids. Such fluids are non-Newtonian in the sense that the consistency of the fluid is a function of shear stress as well as of temperature and pressure. It is believed that, as thus defined, the slurries which can be concentrated by the concentrator of the present invention can include those non Newtonian fluids whose properties are independent of shear duration (e.g., Bingham plastic fluids such as sewage sludge, pseudoplastic materials such as suspensions of paper pulp, and dilatent materials such as quicksand), and those whose properties are dependent upon duration of shear (e.g., thixotropic fluids such as drilling muds and rheoptic materials such as gypsum suspensions in water).

Preferred embodiments of the invention will now be described, by way of example only, with reference to accompanying drawings, in which; Figure 1 is a schematic diagram of a system for creating and injecting foam into a well; Figure 2 is a schematic diagram of an alternative embodiment of a system for creating and injecting foam into a well; Figure 3 is a schematic diagram of a third embodiment of a system for creating and injecting foam into a well; Figure 4 is a schematic diagram of a separator or concentrator in accordance with the invention; Figure 5 is an enlarged scale front elevational view of an intake impeller on the rotor of the concentrator depicted in Figure 4; Figure 6 is a cross-sectional view taken along line 6-6 of Figure 5;; Figure 7 is an enlarged scale front elevational view of a discharge impeller on the rotor of the concentrator depicted in Figure 4; Figure 8 is a cross-sectional view taken along line 8-8 of Figure 7; Figure 9 is an enlarged scale side elevational view in cross section of the concentrator depicted schematically in Figure 4, but with the inlets and outlets rotated so as to make them all visible in the view and with some parts omitted for clarity; Figure 10 is an end elevational view of the concentrator depicted in Figure 9, with the inlets and the outlets shown in their actual positions; Figure 11 is a cross-sectional view taken along line 1111 in Figure 9, but with the rotor omitted for clarity; Figure 12 is a cross-sectional view taken along line 12-12 in Figure 11;; Figure 13 is a schematic diagram of an alternative embodiment of a concentrator according to the invention; Figure 14 is an enlarged scale side elevational view in cross-section of the concentrator depicted in Figure 13, but with the inlets and outlets rotated so as to make them all visible in the view and with some parts omitted for clarity; Figure 1 5 is an end elevational view of the concentrator depicted in Figure 1 3, with the inlets and the outlets shown in their actual positions; Figure 1 6 is a side elevational view in cross-section of a further alternative embodiment of a concentrator according to the present invention, but with the inlets and outlets rotated so as to make then all visible in the view and with some parts omitted for clarity;; Figure 1 7 is a perspective view of one inlet impeller disk suitable for use in either of the concentrators depicted in Figures 9 and 14 and also suitable for use with other such disks as a replacement for the impeller means of the embodiment of Figure 1 6; Figure 1 8 is a side elevational view of inlet impeller means suitable for use in the embodiment of Figure 16; Figure 19 is a schematic cross-sectional view, rotated somewhat, taken along line 1 9-1 9 of Figure 18; and Figure 1 9a shows part of Figure 19 on an enlarged scale.

Referring to the drawings, Figure 1, shows schematically a system for producing foam and injecting the same into a well in order to fracture,the surrounding formation. a tank 10 stores a foamable carrier which may be a liquid, a gel, a colloidal suspension, or the like. In the present example, the carrier comprises water thickened with a guar gum at a concentration of 2 to 2.5 Ibs per US gallons (0.24 to 0.30 30 kg per 1001) of water. The water-guar gum solution forms a gel, the viscosity of which depends on the rate of shear. The gel is classified as a non-Newtonian fluid with a plastic viscosity ranging from 10 to 30 centipoise.

The carrier is passed by means of suction from tank 10 to a blender 12 where suitable proppant is added, producing a slurry. The presently preferred proppant material is 40 to 60 screen sand. However, other suitable materials can also be used as proppants, including, for example particles of glass, plastics materials, and metals. The particles of proppant can be of any shape, e.g. spherical, ellipsoidal, or irregular. The concentration of the sand (or other proppant in the resulting slurry) is typically in the range 6 to 8 Ibs. of sand per U.S. gallon (0.72 to 0.96 kg/l) of carrier.

The slurry produced in the blender 12 is then fed to a high pressure injection pump 14 that may be of conventional construction and that raised the pressure of the slurry to at least the required well head pressure.

The pressurized slurry is then fed into the input of a separator or concentrator 16 which increases the proppant-slurry ration by "dehydrating" the slurry. Concentrator 16 segregates a fraction of the carrier and discharge it via a pressure reduction system 1 8 to a settling tank 20 which is at atmospheric pressure and from which it can be sent to a storage tank (not shown). The remaining fraction of the input to concentrator 16 is discharge therefrom as a concentrated slurry. As an example, the slurry from the discharge of concentrator 16 can have from 12 to 15 Ibs. of proppant per U.S. gallon (1.44 to 1.80 kg/l) of carrier.

The pressure reduction system 18 comprises a throttling valve system which must reduce the pressure of the carrier from a high level of from 2,000 to 6,000 psig (1.38 to 4.14 x 107 N/m2) to near atmospheric pressure. Although this large amount of pressure drop can present problems in valve wear and noise control, conventional commercial equipment is available for performing this function. An automatic, two stage valve system is disclosed in Figure 1 and comprises a first reduction valve 22 and a second reduction valve 24 with an intermediate, manually operated, throttling valve 26. It is preferable that the reduction and throttling valves have ceramic chokes. The pressure of first reduction valve 22 is automaticily controlled by signals generated by an upstream differential pressure transmitter 28 and a downstream differential pressure transmitter 30.An electrical output signal is generated by upstream transmitter 28 and is sent through a bias relay to a three-mode flow controller 32 which also receives directly the electrical output signal from transmitter 30. An electrical signal from flow controller 32 operates a valve positioner 34 which in turn is mechanically coupled to the first reduction valve 22.

In addition, valve positioner 34 generates an electrical signal which is sent to a booster relay 36 which in turn is electrically coupled to first reduction valve 22 for providing a more rapid initial response of the valve.

The second reduction valve 24 requires a much simpler control system. a differential pressure transmitter 38 generates an electrical signal which is coupled to a three-mode pressure controller 40.

Pressure controller 40 generates a valve control signal which is fed to a valve positioner 42 that in turn is both electrically and mechanically coupled to the second reduction valve 24 for adjusting the positioning thereof. Various pressure indicators 44 and a flow recorder 46 are used to monitor the operation of the pressure reduction system 18.

In order to reduce the surface tension of the concentrated slurry discharged from concentrator 16, a suitable surfactant is injected at a mixing junction 48, using a conventional, high pressure injection pump 50. The particular surfactant or surface active foaming aqent utilized will, of course, depend upon the carrier and the type and character of the formation. Examples of suitable conventional surfactants are disclosed in U.S. Patent No. 3937283 referred to herinabove.

At a point downstream of surfactant mixing junction 48, a second mixing junction 52 is used for the introduction to the concentrated slurry of a foam-generating gas. Suitable gases for this purpose include nitrogen, carbon dioxide, air hydrocarbon gases, and the "inert" gases such as argon, helium, krypton and xenon. a large amount of gas at high pressure must be used in order to produce foam, which is then introduced into a well 54. The resulting foam is injected into the well at a rate approximately 4 to 5 times that at which fluid is supplied to the pump 14.

In an alternative embodiment of a foam injection system, shown in Figure 2, the high pressure slurry output from pump 14 is divided into two parts. A first part of the slurry is fed to a concentrator or separator 56, where a portion of the carrier is separated from the slurry and is discharged through a throttling valve 58 back to tank 10. The foaming gas, such as nitrogen, may be introduced into separator 56. The second part of the slurry bypasses separator 56 and is sent to a mixing junction 60 where surfactant is added. The surfactant-slurry mixture is then combined at a mixing junction 62 with the concentrated slurry-nitrogen mixture discharged from separator 56. Additional nitrogen is added at mixing junction 62 to yield foam having desired properties which is then piped into the well.The proppant concentration in the concentrated slurry is regulated by controlling the carrier recycling rate using throttling valve 58.

A third embodiment of a foam injection system is shown in Figure 3 wherein a hydraulic motor 64 is used to depressurize the recycled carrier. A manually operated control valve 66 can be used to control the concentration of the proppant in the slurry discharged from separate 56. One way of reducing the power requirement is to arrange for hydraulic motor 64 to control a second injection pump 68 which is in parallel with the principal injection pump 14. The system disclosed in Figure 3 is similar to that disclosed in Figure 2 in all other respects.

It is noted that in the systems of Figures 1,2 and 3, the surfactant is added in the high pressure part of the system. However, if the injection pump and the concentrator are sufficiently gas free, the surfactant can alternatively be added in the low pressure part of such systems. The only advantage of adding the surfactant on the high pressure side of the system downstream of the concentrator or separator is that foaming is prevented in the injection pump or in the concentrator or separator.

Moreover, a high pressure injection pump must be used to inject the surfactant on the high pressure side, adding to the cost of the system With reference now to Figures 4 to 8, a concentrator or separator according to the present invention and suitable for use in the aforedescribed formation fracturing system is shown at 16.

Concentrator 16 receives a high pressure slurry at an inlet or intake 70 located at a forward end thereof, the slurry being supplied from a slurry storage tank 10 through a high pressure injection pump 14. If desired, a mixing line 72 can be connected through a valve 74 to the discharge of pump 14 for supplying a small quantity of slurry back to tank 10 to ensure that the proppant does not settle out of the carrier.

Concentrated slurry is discharged from the concentrator 1 6 at a slurry outlet 76 and substantially proppant-free carrier is discharged from concentrator 16 at a carrier outlet 78.

Concentrator 1 6 is composed of a substantially horizontally extending, substantially cylindrical vessel 80 having forward and rearward hydraulic seal chambers 82 and 84, respectively. A rotor 86 is rotatably mounted inside vessel 80 on a shaft 88 that is journalled inside seal chambers 82 and 84. A high torque developing motor 90 is connected to shaft 88 for rotating rotor 86 at operating speeds which can be'from 1600 to 2000 r.p.m. Rotor 86 in turn comprises a substantially cylindrical stabilizer 92 at the forward end thereof, an intake impeller section 94 downstream of stabilizer 92, a discharge impeller section 96 spaced downstream from intake impeller section 94 and an internal passageway 98 in fluid communication with the intake impeller section 94 and the discharge impeller section 96.Rotor 86 and the circular walls of vessel 80 define an annular concentric channel 100, through which the concentrated slurry is accumulated and into which slurry outlet 76 is tangentially connected.

Stabilizer 92 has two basic functions: to impart a spin to the incoming slurry through frictional contact; and to dampen torsional vibrations of the rotor and shaft by the frictional torque exerted by the rotor. Stabilizer 92 preferably has a rubber cover to resist abrasion and a plurality of longitudinal or axial grooves therein to increase the surface area and to improve traction. The spin induced by stabilizer 92 develops sufficient centrifugal force on the proppant in the introduced slurry to initiate the proppant separation process before the slurry reaches the intake impeller section 94. This provides a boundary layer of carrier largely free of proppant and helps to protect the intake impeller section from contact with an excessive quantity of the possibly abrasive proppant.

Intake impeller section 94 consists of a plurality (for example 1 5) of individual impellers 102 which are conveniently of the configuration illustrated in Figures 5 and 6. Each impeller 102 has a plurality of axially extending orifices 104 spaced about the outer part of the circumference thereof. The orifices 104 receive stabilizing rods (not shown) which extend through all of the individual impellers and thereby permit the impellers 102 to rotate together. a large, central orifice 106 in impeller 102 permits the impellers to be mounted onto shaft 88. a plurality of inner orifices 108 shaped as segments of a circle and circumferentially spaced about central orifice 106 extend completely through impeller 102 and provide an axial fluid course or channel for the separated carrier.The size of the inner orifices 108 must be such that the required volume of effluent carrier flow can occur at reasonable fluid velocities.

A plurality of spaced apart vanes 110, preferably equal in number to the number of inner orifices 108, spiral outwardly from the outer periphery of inner orifices 10 in a direction opposite the direction of rotation opposite the direction of rotation, indicated by arrow 11 2, to the outer periphery of impeller 102. The vanes 110 define a plurality of shallow (e.g., 1/4 inch (6.4 mm)), outwardly spiralling intake channels 114 for permitting a fraction of the intake slurry to be admitted therein. The intake channels 114 are arranged to spiral in a direction opposite to the direction of the rotation of impeller 102, so as to discourage the admission of proppant thereto.The intake channels should not be so small that they are liable to be come plugged with fine or coarse proppant, and, moreover, they should have a sufficiently large cross sectional area to guide the slurry effectively. The total cross sectional area of all of intake channels 114 in all of impellers 102 should also be appropriately sized for the rated flow therethrough and hence the flow through concentrator 16. The flow through intake channels 11 4 is indicated by dashed line 116.

Finally, a plurality of spokes 118 emanate radially outwardly from the central orifice 106 to the vanes 110 and function not only as supporting members for impeller 102 but also for providing increase fluid traction and the torque supplied to the fluid contributes to the rotational strength of the fluid vortex system.

The discharge impeller section 96 similarly comprises a plurality (eg 14 or 1 5) of individual impellers 120 of the configuration shown in Figure 7 and 8. Each impeller 120 is composed of the same elements as an intake impeller 102, albeit difficult in number and shape, and the various elements of impeller 120 are thus denoted by the primed number that is used to denote the corresponding element of the intake impeller 102. Discharge impeller 120 rotates in the direction shown by arrow 122 and causes an outwards fluid flow, as shown by arrow 124. Discharge impeller 120 also differs from intake impeller 102 in that the former has a larger diameter so as to provide the required discharge head.For example, typical diameters of intake and discharge impellers 102 and 1 20 are 4 1/2 inches (114 mm) and 5 inches (127 mm), respectively, each with a nominal thickness of 1/2 inch (12.7 mm) and each with fluid passages of channels of 1/4 inch (6.4 mm) in depth. The impellerfaces can be molded from epoxy with the mating faces of adjacent impellers ground true.

It may also be desirable to locate a small propellor at the upstream end of the intake impeller section 94 so as to provide axial thrust. In addition, a rubber annular restrictor 1 26 can be located immediately downstream of the slurry outlet 76 so as to form an orifice and thereby restrict axial flow to a small annular space around rotor 86. By restricting the axial flow area beyond slurry outlet 76, the wash of fluid from discharge impellers 96 should be sufficient to check the axial momentum of proppant and therby limit the amount of proppant discharge with the carrier.

A plurality of perforated deflector fins 128 are mounted between intake impeller section 94 and discharge impeller section 96 to impart additional spin to the concentrated slurry at the discharger and to check the axial momentum of the proppant. An axial vane 130, pivotally mounted with a shaft extending through the wall of vessel 80, is located in the annular space just downstream of slurry discharge or outlet 76 and is useful for controlling the volume of carrier washing past restrictor 126 from discharge impeller section 96.

Concentrator 16 also has a valved recycle line 1 32 leading from a tangentially disposed exit located opposite to the discharge impeller section 96 at one end of the vessel 80 and entering the vessel upstream of stabilizer 92 at the other end. Recycle line 1 32 provides a means for washing axial channel 100 at an increased flow rate td prevent plugging of the channel with solids.

Concentrator 16 may be used in a horizontal orientation, or in appropriate cases, may be inclined, extending vertically or at any intermediate orientation. For this purpose, the concentrator may be equipped with, for example, adjustable legs at the motor 90 end thereof so that it can be inclined at the forward end.

Exemplary specifications of concentrator 16 when used in a formation fracturing system can include a working pressure of 5,000 psig (3.45 x 1 07N/m2), a maximum input slurry rate of 210 gpm (795 litres/min) having an input concentration of 4 to 8 Ibs. per gallon (0.48 to 0.96 kg/l). An output concentration of proppant in the concentrated slurry can be expected to be greater than 12 Ibs. per gallon (1.44 kg/l).

In operation, in a formation fracturing system, slurry is fed into concentrator 16 via inlet 70, which preferably is tangentially connected to vessel 80. The pressure inside concentrator 1 6 is maintained at an appropriate level by regulating the input and output fluid flows. A fraction of the input slurry is induced to flow radially inwardly, entering intake impellers 102 of section 94 through intake channels 114, and the fluid within the rotor is induced to flow axially along and through inner orifices 108 towards discharge impeller section 96. Fluid flows out of the discharge impellers 120 of section 96 through discharge channels 114' into a rearward part of annular channel 100. As the fluid is flowing through inner orifices 108, any proppant remaining in the fluid is forced radially outwardly back into the axial channel or annulus 100.Slurry at a concentration increased by the withdrawal of carrier flows axially along axial channel 100 toward slurry outlet 76. Deflector fins 1 28 increase the angular velocity of the concentrated slurry in the vicinity of slurry outlet 76 and particles of the proppant impinging on the deflector fins lose axial momentum. Flexible restrictor 126 also serves as a guide to deflect proppant toward slurry outlet 76.

A part of the flow from discharge impeller section 96 is removed as carrier effluent, a further part is recycled in recycle line 132, and the residual flow from discharge impeller section 96 washes back to slurry outlet 76 where it is discharged with the concentrated slurry. The purpose of the residual flow is to limit flow of the slurry past restrictor 126 and out of carrier outlet 78, but this residual flow should, of course, be as little as possible so that there is minimal dilution of the concentrated slurry.

Now that a particular embodiment of a concentrator according to the present invention has been described, the general theory behind the construction of the present invention will now be discussed.

Referring to Figure 19a, a fluid particle is depicted moving along a streamline at position, with velocity relative to theimpeller rotating at a constant rotational speed w. The flow rate, q, through the impeller is constant The effective depth of the fluid passages is b and the radial thickness of the impeller is r2-r1. the particle is subjected to a radial velocity u, a tangential velocity v, a velocity relative to the rotating impeller v" and an acceleration a. The fluid particles is located at an angular position 0, has a mass m and is subjected to a torque T (in foot-pounds). Those symbols having an overline represent the vector quantity and those without the overline represent the scalar quantity.

The classical equation in mechanics for the acceleration of the particle with respect to a fixed, non-rotating reference frame (e.g. the stationary vessel) is given as follows, realizing that impeller rotation w is constant and hence angular acceleration is zero and that W .T= 0:

where the first term is the acceleration of the particle with respect to the rotating reference frame, the second term is the centripetal acceleration and the final term is the Coriolis acceleration.

Using the D'Alembert's principle concerning inertia moments and equation (1), the torque on the particle is given as follows:

where the first term is the torque due to acceleration with respect to the rotating reference frame (i.e., the impeller) and the second term is the torque romponent due to Coriolis acceleration, and where the cross product of the radius r with the centripetal acceleration is zero.Equation (2) can be rewritten using polar coordinates, integrating over all annular elements between rq and r2, using the following relationships: q (3) 2rrrb (4) dm = 2nrbpdr (p being the density), as follows:

where the first term depends on blade curvature and the second term represents the torque due to Coriolis acceleration. for example, if

k being a constant, the first term, denoted T1, of equation (5) becomes:

(7) 6 = 1/4 k(r2 - r2i) + c in r2 r1 where c is a constant of integration.

To calculate the radial power, p, the following relationship is used: p = F.u; where T is the force.

Neglecting frictional force, there are four radial acceleration components which act on a particle moving within the impeller and these determine the inertial forces acting on the particle. These are: 1) where the cross sectional area for flow increases with the radius; 2) where the fluid particle has an angular velocity with respect to the rotating impeller reference frame thereby yielding a centripetal acceleration; 3) the component associated with a Coriolis acceleration: and 4) the component due to acceleration of the rotating reference frame (ie, the centripetal acceleration).In general, these radial powers (which should be multiplied by 550 for horsepower), can be calculated by the following equations which are derivable using the foregoing considerations:

Note that equations (8) and (11) are general, but equations (9) and (10) are dependent on the blade characteristic. Thus, for example, using the same blade characteristic used above to obtain equations (6) and (7) (and which resulted in an impeller similar to that shown in Figure 5), the following equations result: and

The preliminary specification for impeller size and numbers, in the case of impellers such as those depicted in Figure 6, 7 and 1 7, are determined as mentioned above with respect to factors such as volume of effluent flow, rotational speed, and structural integrity, to name a few.The two integration constants used for the disk impeller depicted in Figures 5 and 6 where the inner radius was 1.8 inches (45.7 mm) and the outer radius was 2.25 inches (57.2mm) are: k = 414.40/in2 (0.640-mm2); and c = 1,049.00. With these figures plus using the number of water courses per impeller and the angular displacement of each water course, a general purpose digital computer can be easily programmed to obtain the angular displacement and radii for the impeller. Having established the curvature parameters for the intake impeller, if discharge impellers are used, those parameters can be similarly obtained. From here, the torque due to water course curvature can be established using equation (6) and the Coriolis torque can be established using the following equation: T2=pqw (r22-r21).

Similarly, the power required for radial thrust in the pump system and the torque input can be calculated.

Underlying the above calculations is the principle that the impeller of a separator according to the present invention is preferably designed so that a free vortex condition is achieved. In a free vortex condition, the velocity (V) of a particle at any particular radius (R) is inversely proportional to that radius, i.e.,; K (14) R where K is a constant or proportionality. Therefore, the centrifugal force on the particle is: mK2 (15) F =##; "m" being the mass of the particle. Thus, the cantrifugal force on a particle in a free vortex incresses rapidly by the inverse cube of the radius as the particle moves towards the centre of the vortex. On the other hand, in a forced vortex the velocity of the particle at any radius is proportional to that radius and the centrifugal force on that particle is also proportional to the radius.Thus, a free vortex is obviously a much more desirable condition for separating liquids from a slurry as a guar carrier from a well fracturing fluid. Such a consideration also leads to the conclusion that the liquid carrier of the slurry should be drawn off from the centre of a centrifugal machine and the slurry should enter the machine at the perimeter of the stationary vessel, contrary to the conventional type of centrifugal separator.

An inlet impeller means designed in accordance with the foregoing calculations results in the liquid carrier passing through the impeller so as to increase the velocity thereof tangential to the impeller. This results in the fluid following a path through the impeller which approximates a spiral path. Thus, as shown in Figure 5, an angle 0 between a line drawn through the centre of intake impeller 102 and a line drawn through the centre of each passage should be as large as possible. Because of structural considerations, the angle 8 in Figure 5 is approximately 600. Also, the intake channel 114 decreases in cross sectional area from the outer inlet thereof from annular channel 1 00 to the inner outlet thereof to inner orifices 108 of impeller 102. This intake channel design forces the carrier liquid of the slurry to increase its velocity as the liquid progresses through the impeller. The velocity of the carrier liquid is also increased as a result of the spiral or curvature of intake channel 114 being in an angular direction opposite the normal forward rotational direction of the rotor because the liquid must have a greater angular velocity than the rotor in order to enter the inlet plenum of intake channel 114.

In other impeller designs according to the present invention, such as those shown in Figures 1 7, 1 8 and 19, the intake channel length is rather small compared to the overall impeller radius (which in these embodiments is also coupled with a relatively small impeller Ar, Where (Ar=r2-r1)). From experimentation, it was found that the carrier flow increased as the channel length decreased so long as the carrier flow had a tangential velocity component. In other words, carrier flow increased with decreasing channel length until there was a direct radial flow path from the outer perimeter of the impeller to the inner channel thereof.

Referring now to Figures 9 and 10 of the drawings, the concentrator of Figure 4 is illustrated in much greater detail. It is to be noted that Figures 9 and 10 are engineering-type drawings drawn approximately to scale.

Vessel 80 is a substantially cylindrical vessel having a normal six inch (152 mm) diameter at the forward part thereof, denoted 134, and at the downstream, rearward end part, denoted 136. At each end of vessel 80 are integral flange portions 138 and 140, respectively located at the forward and rearward ends thereof, which are provided around the periphery thereof with a plurality of drilled bolt holes 142 so that shaft seal 144 can be rigidly mounted thereat, only the shaft seal 144 at the forward end of the vessel being shown in the figure. Shaft seal 144 is of conventional design and will not be described further.

Adjacent to rear end part 1 36 is an enlarged portion of vessel 80 which has a nominal diameter of 8 inches (203 mm) and is connected to the 6 inch (1 52 mm) diameter portions of vessel 80 through two similar 8 inch (203 mm) by 6 inch (1 52 mm) by 0.188 reducers 148 and 149. slurry inlet or intake 70, slurry outlet, 76, and carrier outlet 78 are tangentially arranged around vessel 80 in the direction of rotor rotation, shown by arrow 150 (see Figure 10). Similarly, an inlet 152 and an outlet 154 for recycle line 132 are tangentially arranged on vessel 80 in the direction of normal rotor rotation. Nominal internal dimensions of these connections to vessel 80 are as foilows: slurry inlet 70, slurry outlet 76, carrier outlet 78, and recycle inlet 1 is 1 inch (25.4 mm).In one embodiment of vessel 80, the axial length between the ends of flange portions 138 and 140 is 56.65 inches (1.44 mm). The wall thickness and the material from which vessel 80 is made are not critical provided they are such that the vessel will withstand the nominal working pressures with which concentrator 16 is used.

Concentrator 16 also comprises shaft 88, motor 90 (Figure 4) for rotating said shaft, elongate rotor 86 and an annular orifice defining member 156 (equivalent to restrictor 126 of Figure 4) located between slurry outlet 76 and carrier outlet 78 for limiting flow of the concentrated slurry outlet 76 and into carrier outlet 78. In this connection it should be noted that the location of recycle outlet 1 54 between carrier outlet 78 and slurry outlet 76 and on the upstream side of reducer helps to prevent slurry carry-over into carrier outlet 78 by removing any proppant which travels beyond orifice defining member 156 and recycling it to the upstream end of concentrator 16. In this regard, reducer 149 also acts as a means for limiting the slurry carry-over, the reducer causing an increase in velocity of the fluid flowing downstream therebeyond.Any proppant backing up at reducer 149 will tend to be caught in the wash of the carrier exiting discharge impeller 96 and forced into recycle outlet 1 54, as mentioned above. Finally, as a result of appropriate selection of operating parameters, wash from discharge impeller 96 should flow upstream past orifice defining member 1 56 and out slurry discharge 76, thereby further limiting slurry carry-over.

Shaft 88 is made of solid steel without any indentations or cut away areas, so as to provide better balance, and extends the entire length of vessel 80, protruding therefrom at each end thereof through shaft seals 144, as mentioned above. Rotor 86 consists of a plurality of individual parts that are pressed and bolted together and rigidly mounted to shaft 88 at each end with two similar split tapered bushing assemblies 158 and 220 which are press fitted onto shaft 88. In this manner, keyways in shaft 88 are not required, thereby permitting an easier balancing of shaft 88. Bushing assembly 158 is of conventional construction and comprised a split taper bushing 160, a key 162 for attaching the two halves of bushing 160, and a steel hub 164 that is keyed with a key 166 to bushing 160.A hub protected 168, composed of an abrasion resistant rubber cover 170 of, for example,6065 DUROMETER rubber which is bonded to a steel washer 172 having a 0.06 inch (1.5 mm) nominal thickness, serves to protect bushing 158 and the rotor components to which they are attached from the highly abrasive action of the slurry.

As mentioned above, rotor 86 comprises the following elements going from the upstream end to the downstream end thereof: stabilizer 92; intake impeller section 94; a rotor housing 174 which defines internal passageway 98; and discharge impeller section 96. Each of these components is bolted to the adjacent component, and the end components, namely stabilizer 92 and discharge impeller section 96, are bolted to the hub of the adjacent bushing assembly with bolts 176 or torque rods 178.

Stabilizer 92 preferably comprises a hollow steel cylinder 180 having a 1/2 inch (12.7 mm) thick rubber sleeve of 60-65 DUROMETER rubber, for example, force4itted around cylinder 180 and resiliently held in place, and a forward and a rearward hub plate 184 and 186, respectively. Rubber sleeve 182 can also be bonded onto cylinder 1 80 and has a plurality of 0.1 75 inch (4.4 mm) deep by 0.275 inch (7.0 mm) wide grooves cut therein. Cylinder 1 80 is forcefitted onto, and can be welded to, hub plates 184 and 186. Forward hub plate 184 has a larger diameter than stabilizer 92 and is provided with a plurality of orifices around the perimeter thereof so it can be bolted to the hub 1 64 of the adjacent bushing assembly 158 with bolts 176, as mentioned above. Rearward hub plate 186 has a diameter equal to the inner diameter of cylinder 180 and is in turn bolted with bolts 188 to forward impeller section 94.

Intake impeller section 94 comprises a plurality of individual impeller disks which can be of the type shown in Figure 5 or of the type to be described hereinbelow and shown in Figure 17. As mentioned above, the number of impeller disks used depends upon the flow criteria of concentrator 1 6 and the individual flow through each impeller disk. The stack of impeller disks are held together by a forward hub plate 190 and a rearward hub plate 192 and a plurality of torque rods 194, threaded at eact end, which extend through each impeller disk and the two hub plates and are retained therein by internally threaded, epoxy covered caps 196. As mentioned above, forward hub plate 190 is bolted, with bolts 188, to rearward hub plate 1 86 of stabilizer 92. Rearward hub plate 1 92 is, in turn, forcefitted onto, and welded if desired, to rotor housing 1 74.

Rotor housing 1 74 serves two functions: to provide a sealed, internal fluid passageway from intake impeller section 94 to discharge impeller section 96, and to maintain spinning the concentrated slurry traveling through annular channel 100 so that the slurry is directed into and discharged through slurry outlet 76.Rotor housing 1 is composed of two cylindrical shells, a smaller forward shell 198, the forward end of which is force-fitted onto an annular rim 200 of rearward hub plate 1 92 (and can be welded thereto) and a larger rearward shell 202 which is provided with a rearward end flange 204 having a plurality of orifices around the periphery thereof so that it can be bolted to discharge impeller 96. The rearward part of shell 1 98 is telescopically received by the forward part of shell 202 and is bolted thereto with bolts 206.Shell 202 has a tapered rearward portion 208 and a cylindrical forward portion 210 which join together and provide a radial ridge 212. A plurality of annular rubber disks 214 interspersed with a plurality of rubber spacers 21 6 are stacked between ridge 212 and a snap ring 218 retained within a groove provided therefore in cylindrical portion 210. Disks 214 eliminate any axial flow along the surface or rotor 86 in the vicinity of the inlet to slurry outlet 76. If desired, disks 214 can have axial orifices therethrough to ensure that a rotational spin is given to the concentrated slurry.

Exemplary dimensions for disks 214 are a diameter of 6 1/2 inches (165.1 mm) and a thickness of 0.25 inches (6.4 mm).as seen in Figure 9, the forwardmost disk 214 is axially located on rotor 86 such that the periphery thereof will almost be in contact with the wall of reducer 148. Obviously, rear hub plate 1 92 and flange 204 of shell 202 have been provided with internal orifices sufficiently sized so as not to interfere with the flow of the separated carrier.

Discharge impeller section 96 is similar to intake impellers section 94 and is composed of a plurality of disk impellers which are stacked between flange 204 and a hub 220' of assembly 220 that BE similar to hub 1 64 of bushing assembly 1 58. Hub 220' has a plurality of orifices around the periphery thereof for receiving torque rods 1 78, which can be identical to torque rods 1 94 and serve the same purpose. Likewise, bolts or torque rods 178 have threaded caps 196 on the ends thereof. Alternatively, caps 1 96 can be force fitted and welded or glued onto the ends of the torque rods.

Referring now to Figure 11 and 12, a preferred construction sf otoriice-de,ining member or restrictor 1 56 is illustrated in greater detail. Member 1 56 is composed of four over-lapping, annular segment vanes 222 which are bolted together with bolts 224 in the manner shown in Figure 12. Each vane 222 is mounted in vessel 80 in the following manner. A 1/2 inch (12.7 mm) orifice 226, drilled into the side of vessel 80, receives one end of a one-quarter inch (6.4 mm) bolt 228, the other end of which is securely fastened to vane 222, for example, by being welded thereto. A 1/2 inch (12.7 mm) washer and nut combination, shown at 230, securely and rigidly mounts vane 222 to vessel 80. Water tight integrity of vessel 80 is maintained and access to nut and washer 230 is obtained with a capping assembly 232.Capping assembly 232 comprises a one-inch (25.4 mm) thread-0-Iet 234 welded at one end to vessel 80 over orifice 226, a threaded, one inch (25.4 mm) close nipple 236 threaded into thread-0-let 234 at one end, and a cap 238 threaded onto the other end of nipple 236.

Alternatively, orifice defining member 156 can comprise four vanes which do not overlap and are not connected together. In either case, each vane 222 preferably made of steel that can withstand the abrasive nature of the slurry and which has a thickness so that its dimensional integrity will be maintained at the high pressures and the flow rates which exist in cocentrator 1 6.

As shown in Figure 9, vessel 80 has at least one flagged opening 240 and an access cover 242 removably mounted thereto so as to provide access to intake impeller 94. Obviously, other access openings or hand holes can be provided in vessel 80 as desired.

Further, it is to be noted that the dimensions mentioned above regarding concentrator 16 are only exemplary and would be varied depending upon the particular operational requirements for concentrator 16.

A second embodiment of a concentrator 300 according to the present invention is illustrated in Figures 13, 14 and 1 5. Concentrator 300 is very similar in principle to concentrator 1 6 of Figure 9 and similarly comprises a substantially horizontally extending vessel 380 and an elongate rotor 386 mounted on a shaft 388 that is rotatably mounted in the vessel 380. Rotor 386 comprises a stabilizer 392, an intake impeller section 394, a discharge impeller section 396 and a hollow housing 374 for providing an internal passageway 398 between intake impeller section 394 and discharge impeller section 396. Vessel 380 also has a mixture inlet 370, a concentrated mixture outlet 376, and a separated medium outlet 378. Similarly, concentrator 300 has a recycle inlet 352 and a recycle outlet 354.The vessel 380 similarly has a forward part 334 and a rearward part 336 both with a nominal 6 inch (152.4 mm) diameter and an enlarged portion 346 having a nominal diameter of 8 inches (203.2 mm) and joined to the forward part 334 and rearward part 336 with respective reducers 348 and 349.

Also, the length of concentrator 300 is the same as the length of concentrator 1 6 depicted in Figure 9.

There are, however, significant differences between the concentrator 300 illustrated in figure 14 and concentrator 16 illustrated in Figure 9.

Firstly, there are various differences in the impellers. By increasing the water depth in the individual discharge impellers to 0.55 inches (14.0 mm) from 0.25 inches (6.4 mm), fewer discharge impellers are required and only five are depicted in discharge impeller 396. In contrast, discharge impeller section 96 of concentrator 1 6 uses nineteen discharge impeller disks. Also, the outer and inner diameter of the individual impeller disks of concentrator 300 have been increased so that a greater head can be developed. Similarly, the individual intake impeller disks of intake impeller section 394 have been modified by increasing the water course depth to 0.5 inches (12.7 mm) from 0.25 inches (6.4 mm) and fewer impeller disks are used, only 10 being used in intake impeller 394.

Three further major changes have also been made to concentrator 300. These are the addition of a second concentrated slurry discharge 302 upstream of slurry inlet 352, the repositioning of the rotor components with respect to vessel 380, and the addition of an outlet plenum 304 to concentrated slurry outlet 376.

The outlet plenum 304 extends along approximately one-third of the length of impeller section 394 and thus provides a large low-pressure area in which the slurry can accumulate. As shown in Figure 15, plenum 304 enters vessel 380 radially rather than tangentially as is the case with concentrated slurry outlet 76 of concentrator 1 6. In addition, discharge impeller section 396 is now positioned almost entirely in front of recycle outlet 354 so that the backwash of carrier upstream past orifice-defining member 356 can be maintained and sufficient pressure can be developed to ensure the positive recycle flow through recycle outlet 354.

The second slurry discharge 302 provides a very highly concentrated slurry at low flow rates. Second slurry discharge 302 enters vessel 380 tangentially, but in a direction opposed to the rotation of rotor 386, as shown in Figure 1 5 by arrow 306. It is believed that highly concentrated slurry tends to creep upstream from slurry inlet 370 and to reside in the low flow, low turbulence area upstream of the forward end of rotor 386. With the addition of second slurry outlet 302, controlled operation of the flow rates therefrom (up to flow rates of 25 gpm (94.6 litres per min)) improves machine operation by lowering the residual proppant concentrations in the fluid effluent and also significantly reduces the amount of rotor torque.

Referring now to Figure 13, concentrator 300 is shown in a laboratory set-up that is similar to that illustrated in Figure 4. Therefore, corresponding elements have been identified by corresponding reference numbers and are not described further.

Concentrator 300 is provided with two pneumatic control systems,~3 T0 andes t2, that automatically control the operation of cbncentrator 300 by maintaining system pressure inside vessel 380, by controlling the primary concentrated slurry flow out of concentrator 300 and by controlling the secondary slurry flow out of concentrator 300. The fluid carrier flow is manually regulated with a throttle valve 314 to a preselected value and set according to the flow measured by a flow meter 316.

Pressure controller system 310 comprises a conventional, 3 inch (76.2 mm) diameter butterfly valve 318 with a conventional pneumatic actuator 320 operated by a valve positioner 322. A conventional recording controller 324 which has proportional and integral action, such as commercially available Foxboro Model 40, is connected to valve positioner 322. Valve 318 is spring loaded to close, but is prevented from crossing completely with a high pressure limit relay on the controller output line.

This is necessary so that concentrated slurry will continue to be flushed through the primary slurry discharge even if the vessel operating pressure should inadvertently fall below the set point pressure. Vessel pressure is preferably monitored upstream of orifice defining member 356 so that valve 318 will principally be controller by the concentrated slurry pressure inside vessel 380.

Secondary slurry discharge -302 is controlled with another butterfly valve 326 and pneumatic actuator 328 and valve positioner 330 combination connected to a two-mode controller 331, such as the aforementioned Foxboro Model 40. Controller 331 receives a signal from a differential pressure transmitter 332 connected across an orifice 333 in the secondary slurry discharge line downstream of valve 326. Controller 312 uses oil to fill the diaphragm chamber of differential pressure transmitter 332.

As connected, as shown in Figure 13, secondary slurry controller 312 provides a density-sensitive flow controller.

in each of the concentrators depicted in Figure 9 and 1 4, slurry carryover into the liquid carrier was limited solely by non-sealing means, namely through the use of an orifice, backflow of carrier (which naturally reduces the proppant concentration), through vessel configuration and concentrated slurry flow patterns, and through other mechanical means, such as disks 214. However, it has been found that a more efficient separation of the carrier flow and the concentrated slurry flow can be achieved through the use of rotor sealing means.

Figure 1 6 thus illustrates a further concentrator 400 in accordance with the invention, incorporating such sealing means 414. Concentrator 400 is generally similar in construction to concentrators 16 and 300 and comprises a substantially cylindrical, horizontally extending vessel 480, a rotor 486 mounted onto a shaft 488 which in turn is rotatably mounted for rotation inside vessel 480, such as in a conventional journal bearing 402. In contrast to the rotors of concentrators 1 6 and 300, however, rotor 486, only comprises a stabilizer 492, an intake impeller section 494 and a rotor housing 474, and does not include a discharge impeller section. Instead, there is simply an end cup 404 which has a plurality of orifices in the end thereof for permitting the separated carrier to be discharged from internal passageway 498 of rotor 486.

A split bushing assembly 458 is used to mount the forward end of rotor 486 to shaft 488, but the rearward end of rotor 486 is keyed to shaft 488 and a conventional collar lock 406 with a radial tightening screw 408 is used to prevent axial movement of rotor 486 with respect to 488.

Stabilizer 492 is similar to stabilizer 92 and stabilizer 392 and therefore need not be described in greater detail. On the other hand, intake impeller section 494 can either be similar to intake impeller sections 394 or 94, or can be of a new, squirrel cage type described hereinbelow.

Vessel 480 has a nominal 8 inch (203.2 mm) diameter throughout and has a much thicker wall so that concentrator 400 can withstand pressures of up to 5,000 psi (3.45 x 107 N/M2mm). Vessel 480 has a standard mixture intake 470, concentrated mixture outlet 476 and separated medium outlet 478, all of which are tangentially connected to the interior of vessel 480. However, vessel 480 has neither a recycle line nor a secondary mixture discharge because neither is needed. In addition, as should be evident from Figure 1 6, the interior of vessel 480 has a simple cylindrical shape without reduced or enlarged portions. Obviously, the cylindrical shape greatly simplifies the ease and cost of manufacture.

Vessel 480 is also provided with two instrument taps 410 and 412, one provided on each side of a sealing means 414. Instrument taps 410 and 412 can be used for example, to monitor the vessel pressure on either side of sealing means 414 so that a failure in the sealing means can easily be detected. In order to ensure positive rotation of sealing means 414, keyways 416 are provided in shaft 488, as noted above.

As a result of eliminating a recycle line and the necessity for backwash through an orifice-defining member, it should be apparent that concentrator 400 has a greater efficiency than that of concentrators 16 and 300.

If the efficiency of the concentrator is defined as follows: A (16) E=(1----)x100, where C E = the efficiency in percent A = the weight of sand carryover in the effluent per unit of time C = the weight of sand flow in the machine per unit of time; then the efficiency of concentrator 1 6 or concentrator 300 ranges from 65% to 85% at carrier fluid flow rates of 60 to 80 gpm (227 to 303 litres per min). It is noted that higher efficiencies were obtained occasionally at lower flow rates.

In contrast, however, the operating efficiency of concentrator 400 has an approximate range of 94% to 100% at flow rates of 50 to 100 gpm (189 to 378 litres per min). On the other hand, because of the friction of the rotor sealing means 414, concentrator 400 required up to twice as much power and had up to twice as much rotor torque as compared with a concentrator similar to concentrator 1 6 or 300.

It is noted that the seal used with concentrator 400 in which the above efficiency figures and power and rotor torque values were obtained has a packing seal gland equipped with a latern ring to permit injection of a flushing medium. Unfortunately, a seal of this type had an approximate lifetime of only about 8 hours.

In this connection, it should be noted that a conventional gland seal is denoted a radial seal because the sealing surfaces are in the radial direction and extend axially. On the other hand, sealing means 414 depicted in Figure 16 is denoted an axial seal because the seal is in the axial direction and the sealing elements extend radially.

The axial sealing means 414 depicted in Figure 1 6 was found to be a far superior seal both because of longevity of the seal in the very abrasive environment of the liquid treated, eg sand slurry, and because the seal works effectively even when there is vibration and lateral movement of shaft 488.

With ordinary pump type packing such as in a radial seal, it is necessary to have a bearing located close to the packing in order to prevent lateral movement of the shaft because otherwise the lateral freedom of movement would quickly damage the packing. As mentioned above, rotor 486 can rotate at a very high speed, up to 2400 rpm, in a very dense environment and any unequal loading of slurry could easily cause excessive vibrations and shaft movement, similar to those resulting in an improperly loaded washing machine. An axial sealing means using a resilient sealing member has been found to be extremely effective in the environment of concentrator 400, and is described in greater detail hereinbelow.

Referring to Figure 16, housing 474 is of similar construction to housing 174 in the embodiment of Figure 9 and comprises a forward shell 41 8 telescopically received within a rearward shell 420 and rigidly mounted thereto with countersunk bolts 422. The forward end of forward shell 418 is integrally attached to a hub 424 which also serves as an end plate for intake impeller 494. The rearward end of rearward shell 420 comprises an integral end cap 404. As mentioned above, both hub 424 and end cap 404 (i.e., the forward and rearward end members of forward and rearward shells 418 and 420, respectively) are keyed to shaft 488 to prevent relative movement therebetween.

Sealing means 414 comprises a stationary member 428 and a rotatable member 426 concentrically mounted around housing 474 and rigidly secured thereto with bolts 422. The stationary member 428 is rigidly mounted, for example with bolts 430, to an annular boss 432 which, in turn, is integrally mounted to the inside of vessel 480 for example by being welded thereto. Stationary member 428 comprises an annular steel disk having an inner opening which is slightly larger than the external diameter of rearward shell 420 so as to permit relative movement therebetween.

Rotatable seal member 426 comprises a steel ring 434 which has a plurality of radial orifices 436 for receiving bolts 422 which rigidly mount rotating member 426 to housing 474 of rotor 486.

Rotatable seal member 426 further comprises a thick annular ring 438 made of natural rubber and rigidly bonded at the upstream face thereof to steel ring 434. The opposing face of rubber ring 438 abuts the forward face of stationary member 428 in sealing relationship therewith. Although rotatable member 426 is tightly fitted around housing 474, an O-ring sealing means can also be used if appropriate, to prevent flow between these two rotating elements.

A mechanical seal of such construction, using natural rubber against a steel face, has a longer life and a more reliable life. In addition, by having the stationary member 428 located rearwardly of the rotatable member 426, sealing means 414 is readily accessible maintenance from the rearward end of vessel 480 and can be easily dismantled for mgintenance. The axial loading of the sealing faces of sealing means 414 is set in a conventional thrust bearing (not shown) and therefore the working fluid has no function in setting of the seal. A preferred loading on the sealing faces is 25 psi (1.72 x 105 N/m2). By appropriate selection of the resiliency of rubber ring 438, a 25 psi (1.72 x 105 N/m2) loading will result in approximately 1/16 of an inch (1.59 mm) compression of rubber ring 438.

As mentioned above, intake impeller section 94, 394, or 494 can be composed of a plurality of individual disk impellers such as impeller 102 depicted in Figures 5, and 6. However, a preferred type of intake impeller from which such sections may be composed is depicted at 500 in Figure 17. The illustrated impeller 500 comprises a disk with shorter vanes or lands 502 and three longer vanes or lands 504 interspersed therebetween extending arcuately along an outer peripheral portion 506 of one planar surface of the disk. A respective axial bore extends through each of the longer lands 504 and through the adjacent portion of the disk for receiving respective torque rods (not shown) as mentioned above.The impeller disk includes a central orifice 508 for mounting impeller 500 on the concentrator shaft, and also three flow channels 510 each having the shape of an annular segment concentrically spaced around central orifice 508, the channels 510 being separated by intervening spokes 512.

The outer radial surface 514 of lands 502 and 504 form segments of a cylinder and together define sections of the circumference of a circular cylinder. Peripheral portion 506 extends radially outwardly beyond these radial surfaces 504 so as to provide a flange 51 6 between lands of adjacent impellers 500. (This is shown in impeller section 394 in Figure 14.( Flanges 51 6 appear to be effective to reduce the turbulence between adjacent disk impellers 500 and thus to improve linear flow into each disk impeller 500. In addition, flanges 51 6 prevent the formation of a large boundary surface surrounding rotor 386 and prevent axial migration of the mixture being treated along impeller section 394. Finally, there are apparently some eddy currents set up by the rotation of rotor 386 which rotate in a longitudinal direction at 900 to the rotation of rotor 386.Flanges 51 6 tend to reduce these eddy currents and the turbulence generated thereby.

With reference now to Figures 18 and 19, there is illustrated a cylindrical impeller 600, similar to a "squirrel cage", provided with a plurality of axially extending, parallel slots 602 separated by vanes 608.

For the sake of clarity, each of the edges of the slot 602 in Figure 1 8 is labelled with a distinctive lower case letter and the equivalent edge is similarly labeled in figure 1 9.

Each slot 602 is cut through the wall of impeller 600 in two steps. In a first step, a longitudinal slot which enters the interior of impeller 600 tangentially to the inner surface of the wall thereof is cut out, forming a tangential face 606 of one vane 608 and an opposing face 610 of an adjacent vane. If the direction of rotation of impeller 600 is as shown by arrow 604 in Figure 19, the tangential faces 606 are the trailing faces of the vanes, 608 formed between adjacent slots 602, and faces 610 are the leading faces of the vanes 608.

For each face,606, the angle 0 between the outer radius, rut, to the outer edge 612 of face 606 and the inner radius, r2 to the inner edge of the face is given by the following equation: r2 (17) 0 = sin~' (-) r1 where r, is the outer radius and r2 is the inner radius of the wall of impeller 600.

The maximum thickness (t) of slot 602 is determined by the requirement that any fluid entering slot 602 must have a tangential velocity component. This requirement is satisfied so long as there is overlap in a radial direction between the leading face 610 of one vane 608 and the trailing face 606 of the adjacent vane 608. This thickness t is given by the following equation: r2 (18) t = (r,r2).

r, In order to provide each slot 602 with an outer plenum or inlet 614 that is larger than the inner portion thereof that constitutes an outlet 61 6 leading to the interior of the impeller, a portion of the outer region of the leading face of each vane 608 is removed in a second step, for example by grinding.

This then produces a slot having a cross-sectional shape that approximates an outward spiral in a direction opposite the direction of rotation.

An exemplary cylindrical impeller 600 has the following dimensions: an inner radius, r2 of 1.803 inches (45.8 mm) and an outer radius, r1, of 2.25 inches (57.2 mm) which results in a wall thickness of 0.447 inches (11.4 mm). This results in an angle 0 of 53 degrees, 15 minutes. The slots 602 are each formed by first cutting a tangential slot having a width of 0.375 inches (9.5 mm) and then grinding away from faces 610 a section of impeller wall having a substantially triangular cross section defined by one edge of the tangential cut and a line perpendicular to a radius of the cylinder.Preferably, the material is ground away so as to leave the leading of vane 608 with a thickness of 0.125 inches (3.2 mm) so as to maintain the structural integrity of impeller 600. Because impeller 600 has to be used at extremely high pressures and rotates at relatively high speeds, it is preferably made from a stainless steel pipe having an FDR 11.

The use of a cylindrical impeller has several advantages, including the following: it reduces turbulence, provides a continuous longitudinal slot for removing carrier from the mixture, and a cylindrical impeller can be of smaller length than a comparable series of disk impellers used to provide the same flow. Theoretically, the larger the slots of a cylindrical impeller, the lower will be the velocity of fluid entering the impeller, but more likely that longitudinal turbulence will be decreased. In any event, it is desirable to have laminar flow through impeller 600 and a flow pattern that approximates a free vortex.

Numerous tests have been conducted using prototype concentrators according to the present invention. Some of this data has been accumulated into three Tables, Table 1 providing information from a concentrator similar to the one depicted in Figure 1 6, but having disk impellers of the type depicted in Figure 17, and Tables II and Ill for a concentrator of the type depicted in Figure 1 6 using the cylindrical impeller depicted in Figures 1 8 and 1 9.In the Tables, S represents the rate of intake of slurry to the concentrator; S, represents the concentration of sand in the slurry fed to the concentrator; SO represents the concentration of sand in slurry removed from the concentrator; CO represents the concentration of sand in the carrier removed from the concentrator and is indicated as being either zero (a) a trace (T) or between zero and a trace (O-T); C represents the rate at which the carrier was removed from the concentrator; and S-C represents the rate at which slurry was removed from the concentrator, calculated from the slurry rate in and carrier rate out, assuming that total flow into the concentrator equals the toatal flow out.

It is to be noted that there was initially considerable difficulty in obtaining representative slurry and fluid samples. However, efforts were made to secure samples from the entire cross section of the various flow streams. The flow rates were calculated based on orifice meter readings using flow tables with corrections for slurry density. Because of the abrasive nature of the slurry, erosion of the orifice plate in the orifice meter occured and the orifide plate had to be replaced from time to time. Fluid effluent flow rates were measured by a Turban water having a digital readout. However, indicated flows had a high rate of error, up to as much as 40%. Finally, it should be noted that the information has been arranged in the Tables according to rotor rpm and that some of the figures are outside an acceptable deviation.Also, the inlet pressure of the slurry was relatively low (about 40 psi (2.76 x 105 N/m2)) compared to operating pressures of 5,000 psi (3.45 x 107 N/m2) but is believed that higher pressures should not have any significant effect on the results. It is also possible that the composition of the slurry used in the tests (sand in a guar gum carrier) varied between the tests in the different Tables and that this affected the concentration ratios.

A comparison of Table I and Table II shows that the concentration ratio (which is defined as the concentration of the sand in the concentrated slurry being discharged divided by the concentration of stand in the slurry being fed to the concentrator) increased approximately linearly with variation flow.

However, the use of a cylindrical impeller permits a much greater carrier output flow which in turn results in a greater concentration of sand in the concentrated slurry being discharged. The one other observation which can be made from the Tables is that the higher the rotational speed, the greater the concentration ratio. In other words, a desired output concentration of sand in the concentrated slurry can be achieved within certain limits by adjusting the rotor rotational speed.

It is to be noted that the present invention has been described with respect to particular embodiments having exemplary orientations, dimensions and shapes, but the scope of the invention is not meant to be limited thereby. Furthermore, the present invention has been described for use in concentrating a slurry mixture of a solid medium and a liquid medium (a non-Newtonian fluid).

Obviously, the present invention can also be considered as relating to a separator for removing one medium from a mixture of media and can be used for separating any suitable types of media. TABLE I (DISK IMPELLERS) (Slurry Inlet Pressure 37 to 44 psig (2.55 x 105 to 3.03 . 105 N/m)) Run No.Rotor RPM Slurry Rate In Slurry Rate Out Carrier Concentration Concentration (GPM) (LPM) (S) (GPM) (LPM) (S-C) Rate Out of sand (ML Ratio (GPM)(LPM) of sand/L) (S0/S1) (C) S1 S0 S0 5 890 200(757) 150(568) 50(189) 165 2200-T 1.33 4 1080 175(662) 103(390) 72(272) 165 2800-T 1.70 3 1280 171(647) 76(288) 95(359) 165 3700-T 2.24 1 1505 180(682) 90(341) 90(341) 165 330 0 2.00 2 1525 187(708) 77(291) 110(417) 165 400 0-T 2.42 6 2005 239(904) 80(303) 159(601) 165 490 0-T 2.97 7 2205 236(893) 73(276) 163(276) 163 530 0-T 3.21 TABLE II (CYLINDRICAL IMPELLERS) (Slurry Inlet Pressure 44 to 49 psig (3.03 x 105to 3.38 x 105 N/m2)) Carrier Concentration Rate Out of sand (ML Concentration Run Rotor Slurry Rate In Slurry Rate Out (GPM) (LPM) ofsand/L) Ratio No.RPM (GPM) (LPM)(S) (GPM) (LPM) (S-C) (C) SW SO CO SJS, 10 1140 202 (765) 87 (329) 115(436) 235545 T 2.34 9 1145 172(651) 92 (348) 80 (303) 235 440 0 1.87 15 1160 153 (579) 63 (238) 90 (341) 165 400 O-T 2.42 8 1160 188 (712) 63(238) 125(474) 165490 T 2.97 13 1180 180 (682) 80 (303) loo (379) 165 310 OT 1.88 14 1180 187 (708) 77(291) 110(417) 165400 T 2.42 21 1180 221(836) 91(344) 130(492) 165400 OT 2.42 16 1300 201 (761) 71(269) 130(492) 165465 T 2.82 17 1350 201(761) 71(269) 130(492) 165465 T 2.82 6 1400 182(689) 70 (265) 112(424) 165 430 0-T 2.61 7 1420 177(670) 67 (254) 110(416) 165 435 0 2.64 18 1620 194(734) 58(220) 136(514) 165 550 O-T 3.33 5 1640 178(674) 68 (257) 110(417) 165 430 O-T 2.61 20 1800 215(814) 55(208) 160(606) 165645 T 3.91 11 1845 216(818) 72(273) 144(545) 235 705 0-T 3.0 19 1850 205(776) 55(208) 150(568) 165 610 O-T 3.70 1 1955 144(545) 32(121) 112(424) 165 690 OT 4.18 2 2200 156(590) 36 (136) 120(454) 165705 T 4.27 12 2240 231(874) 60(227) 171(647) 210805 T - 3.83 3 2400 166(628) 34(129) 132(499) 165800 T 4.85 4 2500 187 (708) 37 (140) 150 (568) 165 840 0 5.09 23 2600 202 (765) 42 (159) 160(606) 165790 T 4.79 22 2640 214(810) 44(167) 170(643) 165800 T 4.85 TABLE Ill (CYLINDRICAL IMPELLER) (Slurry Inlet Pressure about 40 psig (2.75 x 105N/m2g)) Carrier Concentration Run Rotor Slurry Rate In Slurry Rate Out Rate Out of sand (L Concentration No. RPM (GPM) (LPM) (S) (GPM) (LPM) (S-C) (GPM) (LPM) of sand/L) Ratio (C) S, SO SO (SO/S,) 1 1225 160(606) so (189) 110(747) 0.16 0.5 0 3.125 2 1600 170(643) 35(132) 135(511) 0.160.780 4.875

Claims (38)

1. A mixture concentrator for removing a medium from a mixture of media, comprising an elongate vessel having a mixture intake, a concentrated mixture outlet located downstream of said mixture intake, and a removed medium outlet; and an elongate rotor mounted for rotation within said vessel so as to define an annular channel between said vessel and said rotor, said rotor having an internal passageway and comprising intake impeller means, the arrangement being such that, on appropriate rotation of the rotor, a portion of the medium to be separated from mixture introduced to the annular channel via said mixture intake is drawn off by the intake impeller means into said internal passageway, such that flow through said impeller means approximates a free vortex flow pattern, and is conveyed to the removed medium outlet, the resulting concentrated mixture in the annular channel exiting the vessel via the concentrated mixture outlet.

2. A concentrator according to ciaim 1, wherein said impeller means comprises a plurality of intake channels, each intake channel extending arcuately outwardly in the radial direction opposite to the direction of rotation of said rotor.

3. A concentrator according to claim 2, wherein each intake channel has a relatively larger outer inlet opening and a relatively smaller inner outlet opening.

4. A concentrator according to claim 3, wherein each impeller intake channel decreases in crosssectional area from the other inlet thereof that communicates with said annular channel to the inner outlet thereof that communicates with said rotor passageway.

5. A concentrator according to claim 2, 3 or4, wherein each impeller intake channel is defined by a leading face and a trailing face with respect to the direction of rotor rotation, and wherein the angle between a radius and the leading face of said channel at the outer inlet thereto is approximately 600.

6. A concentrator according to any one of claims 2 to 5, wherein said impeller means comprises a plurality of spaced apart, axially extending lands located at an outer annular portion of said impeller means, the spacing therebetween defining said intake channels, the outer trailing edge of one land with respect to the direction of rotor rotation overlapping the inner leading edge of the land adjacent thereto in the direction opposite to said direction of rotor rotation such that fluid entering said intake channel must have a rotation velocity greater than said rotor and must have a tangential velocity component.

7. A concentrator according to claim 6, wherein said edges of said adjacent lands just barely overlap.

8. A concentrator according to any one of the preceding claims, wherein said intake impeller means comprises a plurality of impeller disks stacked together in substantially fluid tight relationship and means for causing all of said impellers to rotate together, said disks having fluid openings in an external perimeter thereof.

9. A concentrator according to claim 8, wherein said disks comprise a fast-setting, substantially non-shrinking polymer casting resin.

10. A concentrator according to any one of the preceding claims, wherein said intake impeller means is axially spaced from said mixture intake.

11. A concentrator according to claim 10, wherein said concentrated mixture outlet is spaced upstream of said removed medium outlet, and further comprising means located between said concentrated mixture outlet and said removed medium outlet for limiting flow of said concentrated mixture into said removed medium outlet.

12. A mixture concentrator for removing part of the liquid carrier of a mixture, comprising an elongate, substantially cylindrical vessel having a tangential mixture intake at an upstream part, a tangential concentrated mixture outlet at a downstream part, and a tangential carrier outlet; a shaft rotatably mounted and centrally located inside said vessel; shaft seals between said shaft and said vessel where said shaft extends therebeyond; an elongate rotor mounted for rotation with said shaft, said rotor and said vessel defining an annular channel therebetween, said rotor having an internal passageway extending from an upstream part to a downstream part thereof, and comprising an intake impeller means spaced downstream from said mixture intake, said intake impeller means being in fluid communication with said mixture intake and with one end portion of said passageway for drawing off carrier from said annular channel into said passageway, and carrier discharge means for discharging said removed carrier from the other end portion of said passageway to said carrier outlet, said annular channel providing a flow path for the mixture being concentrated from said mixture intake to said mixture outlet; and means located between said concentrated mixture outlet and said carrier outlet for limiting flow of said mixture to said carrier outlet.

13. A concentrator according to claim 12, and further comprising a tangential second concentrated mixture outlet located upstream from said impeller means.

14. A concentrator according to claim 12 or 13, and further comprising means for automatically controlling the amount of concentrated mixture through said concentrated mixture outlet so as to maintain vessel pressure within a predetermined range.

15. A concent, ator according to claim 14, and further comprising a tangential second concentrated mixture outlet located upstream from said impeller means and means for automatically controlling the amount of flow through said second concentrated mixture outlet.

1 6. A mixture concentrator for removing part of a medium from a mixture of media, comprising an elongate vessel having a mixture intake, a concentrated mixture outlet located downstream of said mixture intake, and a removed medium outlet; and an elongate rotor mounted for rotation within said vessel so as to define an annular channel between said vessel and said rotor, said rotor having an internal passageway and comprising intake impeller means the arrangement being such that, on appropriate rotation of the rotor, a portion of the medium to be separated from mixture introduced to the annular channel via said mixture intake is drawn off by the intake impeller means into said internal passageway and is conveyed to the removed medium outlet, the resulting concentrated mixture in the annular channel exiting the vessel via the concentrated mixture outlet.

1 7. A concentrator according to any one of the preceding claims, wherein said rotor further comprises stabilizer means at the upstream end thereof and located opposite said mixture intake for imparting a spin to incoming mixture.

18. A concentrator according to claim 1 7, wherein said stabilizer means is a substantially cylindrical member with a plurality of axial grooves in the surface thereof.

19. A concentrator according to any of the preceding claims, wherein said intake impeller means is spaced axially downstream of said mixture intake.

20. A concentrator according to claim 1 6, wherein said intake impeller means comprises a plurality of individual impellers stacked together in substantially fluid tight relationship, and means for causing all of said impellers to rotate together.

21. A concentrator according to claim 20, wherein said rotor further comprises a shaft capable of being driven by motor means and onto which said intake impellers are rigidly mounted; and wherein each said intake impeller comprises a plurality of inner orifices which extend axially through said impeller to provide a part of said rotor pflssageway, and a plurality of spaced apart and overlapping vanes which extend spirally outwardly from the outer periphery of said inner orifices in a direction opposite said direction of rotor rotation, said vanes defining a plurality of relatively shallow, intake channels that extend spirally outwardly and provide communication between said inner orifices and said concentric channel and that permit a fraction of the medium to be admitted therethrough.

22. A concentrator according to any one of the preceding claims, wherein said rotor further comprises discharge means.

23. A concentrator according to claim 22, wherein said discharge means comprises a plurality of individual discharge impellers stacked together in substantially fluid tight relationship and means for causing all of said discharge impellers to rotate together.

24. A concentrator according to any one of the preceding claims, wherein said concentrated mixture outlet is located upstream of said discharge means and said removed medium outlet is located proximate rotor discharge means; and said concentrator further comprises means located between said mixture outlet and said removed medium outlet for limiting backflow of said removed medium from said discharge means to said concentrated mixture discharge and for limiting flow of the concentrated mixture into said removed medium outlet.

25. A concentrator according to claim 24, wherein said limiting means comprise an annular restrictor located proximate to said concentrated medium discharge, the inner radius of which is of such size as to restrict axial flow to a small annular space around said rotor.

26. A concentrator according to claim 25, wherein said limiting means further comprise a plurality of perforated deflector fins mounted on said rotor between said intake impeller means and said discharge means

27. A concentrator according to any one of the preceding claims, and further including a recycle line connected in communication between the downstream and upstream ends of said vessel so as to provide a means for recirculating a portion of the removed medium for washing said annular channel.

28. A concentrator for concentrating a mixture of a proppant and a medium, comprising a substantially horizontally extending vessel having a mixture intake at an upstream end, and a concentrated mixture outlet and a removed medium outlet at the downstream end; a rotor rotatably mounted inside said vessel and defining an annular channel between said vessel and said rotor, said rotor comprising a substantially cylindrical stabilizer at the upstream end thereof for imparting a spin to incoming slurry, a plurality of intake impellers downstream of said stabilizer, a plurality of discharge impellers spaced downstream from said intake impellers, and an internal passageway through said intake impellers, said discharge impellers and that part of said rotor therebetween; and motor means for rotating said rotor.

29. A concentrator according to claim 28, wherein said rotor further comprises a shaft driven by said motor means and onto which said intake impellers and said discharge impellers are rigidly mounted; and wherein said intake impellers each comprise a plurality of inner orifices which extend axially through said impeller to provide a part of said pasasageway, and a plurality of spaced apart and overlapping vanes which extend spirally outwardly from the outer periphery of said inner orifices in a direction opposite the direction of rotation of said rotor, said vanes thereby defining a plurality of relatively shallow, intake channels which extend spirally outwardly in fluid communication between said inner orifices and said concentric channel and which permit a fraction of the carrier to be admitted therethrough.

30. A concentrator according to claim 29, wherein said concentrated slurry outlet is located upstream of said discharge impellers and said carrier outlet is located proximate said discharge impellers, said concentrator further comprising means located between said mixture outlet and said removed medium outlet for limiting backflow of said removed medium from said discharge impellers upstream to said concentrated mixture discharging and for limiting downstream flow of the concentrated mixture past said concentrated mixture outlet.

31. A concentrator according to claim 30, and further including a removed recycle line connected in fluid communication between the downstream and upstream ends of said vessel so as to provide a means for recirculating a portion of the removed medium for washing said concentric channel.

32. A separator for concentrating a slurry of a proppant and a carrier, comprising a substantially horizontally extending vessel having a slurry intake at a forward end and a concentrated slurry discharge and a carrier discharge at the rearward end; a rotor rotatably mounted inside said vessel and defining an annular channel between said vessel and said rotor, said rotor comprising a substantially cylindrical stabilizer at the forward end thereof for imparting a spin to the incoming slurry, a plurality of discharge impellers spaced downstream from said intake impellers, and an internal passageway through said intake impellers, said discharge impellers and that part of said rotor therebetween; and motor means for rotating said rotor.

33. A mixture concentrator substantially as herein described with reference to, and as shown in, Figures 4 to 12 of the accompanying drawings.

34. A mixture concentrator substantially as herein described with reference to, and as shown in, Figures 13 to 15 of the accompanying drawings.

35. A mixture concentrator substantially as herein described with reference to, and as shown in, Figure 1 6 of the accompanying drawings.

36. A concentrator according to claim 33,34 or 35, modified substantially as herein described with reference to, and as shown in, Figure 17 of the accompanying drawings.

37. A concentrator according to claim 33, 34 or 35, modified substantially as herein described with reference to, and as shown in, Figures 18, 19 and 19a of the accompanying drawings.

38. Any novel feature or novel combination of features disclosed herein.