Title of Invention

IMPROVED TUMBLING MIXERS AND ROTARY KILNS

Abstract

This invention relates to enhancement of mixing and heat transfer efficiency of particulates in tumbling mixers and rotary kilns, including rotary drums, V- blenders, rotary driers, bin blenders and double cone blenders operating as batch or continuous flow systems. The height of impeller is typically less than 10% of the depth of the bed material and the impeller is submerged in the flowing surface layer and thereby requires low torque. The impeller is found to substantially eliminate segregation and results in significant increase in the rate of mixing leading to improved heat transfer in kilns, faster blending in mixers and an improved homogeneous product. Best results are obtained for a reverse 'S shaped' impeller in shaft-impeller system rotated in the same direction as the tumbler. The design parameters and the position of the impeller, tumbler operating speeds, etc have been disclosed.

Full Text

FORM - 2 THE PATENTS ACT, 1970 (39 OF 1970) COMPLETE SPECIFICATION (See Section 10) TITLE OF INVENTION "IMPROVED TUMBLING MIXERS AND ROTARY KILNS." (a) INDIAN INSTITUTE OF TECHNOLOGY Bombay (b) having administrative office at Powai, Mumbai 400076, State of Maharashtra, India and (c) an autonomous educational Institute, and established in India under the Institutes of Technology Act 1961. The following specification particularly describes the nature of the invention and the manner in which it is to be performed. Field of the invention This invention relates to enhancement of mixing and heat transfer efficiency of particulates in tumbling mixers and rotary kilns, in batch and continuous flow systems. Background and prior art Rotary kilns generally comprise of a hollow cylinder that rotates the kiln about its axis. Inner circumference of the rotary kilns is refractory lined, whereas rotary drums are without inner lining surface. Kilns are installed by maintaining an inclination with the base so that feed materials flow from the upper input end to the lower discharge end with progress of cylinder rotation. In the rotary kilns particles are fed at the upstream end and are discharged from the downstream end. The feed materials are heated in a rotary kiln by hot gases or flame introduced into the kiln from the downstream end. Particles are filled typically 5 - 15 % of its volume. Higher filling reduces mixing efficiency. A tumbling mixer is a closed vessel rotating about its axis and the mixing occurs in response to the tumbling action of the ingredients as they are elevated and dropped in the drum. Examples are V-blender (Twin Shell), double cone blender, bin blender and horizontal/vertical drum. Industrial applications like, clinkering of cementitious materials, calcination of limestone, reduction of iron oxide, drying of cereal, calcination of petroleum coke, and the waste incineration are carried out in rotary kilns. Tumbling mixtures are commonly used for mixing chemicals, powdered metals, ceramics, pharmaceuticals, and food ingredients. In all types of rotary kilns and tumbling mixers the occurrence of segregation is the major drawback and ultimately it results in poor mixing. The material flow in a cross - section transverse to the axis of rotation in rotary kilns and tumbling mixers comprises a shallow layer of flowing particles at the surface while the remaining particles rotate as a fixed bed. The thickness of the layer is maximum at the mid point along the line formed by the intersection of the free surface with the transverse plane. Cylinder rotation aids mixing in the rotary kiln but the extent of mixing provided is not efficient. Specifically, smaller size particles or higher density particles accumulate and form the core at the center. In the high temperature rotary kilns, the particles on the surface of the bed are overexposed to the environment in the mixer while the particles in the interior of the bed are underexposed to the environment in the mixer. This causes the top surface particles to be over - burned or overheated, whereas the inside bed materials remain under - burned. The lack of uniform heating of the bed is due to insufficient agitation of the bed. Various methods are disclosed in prior art to improve mixing and heat transfer. Among these the use of lifters, flights, polygonal rods, polygonal shaped cylinders are in conventional art. Earlier methods focused primarily on the enhancement of heat transfer and did not adequately address the problem of segregation resulting in core formation within the bed. U.S. Pat. Nos. 1,065,597, 1,920,677, 2,190,271, 4,181,495, 4,136,965, 5,975,752, 1,544,504, 4,475,886, 3,445,099 disclose lifters attached on the internal wall of the kiln to improve both heat transfer and mixing. As the materials are tumbled and mixed, new material surfaces are brought in contact with the hot combustion gases and hot lining surface. Flights are common in rotary driers. Examples of flights are disclosed in U.S. Pat. No. 4,189,300, 5,203,693, 3,423,222, and 3,799,735. Flights attached in various rows on the inner circumferential wall of the rotary drum improve the mixing along with improvement of gas solid contact. In the operation of both lifters and flights the material is repeatedly elevated and dropped like a shower during rotation of the drum resulting in improved mixing. However, repeated lifting and falling result in breakage of the particles thereby producing fines and dust. Dust formation not only increases the cost of installation of environmental protecting equipment but also results in loss of raw materials. Moreover, lifters lose their effectiveness with time. The continuous showering of the materials over the flights decreases the longevity of the flights. U.S. Pat. No. 3,135,504, 2,001,227 disclose chains, which improve, heat transfer and mixing. As the kiln continues to rotate the chain links move in the material causing stirring and agitation of the material and transfer heat thereto. Chains are normally fastened to the interior of the kiln at one or both ends. The disadvantage of the chains is that they wear out and need to be replaced. Also the links of the chains tend to become clogged or plugged with the material being treated. U.S. Pat. No. 4,014,643 discloses chain link that are attached at the end of lifters which to improve both heat transfer and mixing. U.S. Pat. No. 4,183,726 discloses a heat resistant rod having tapered structure of gradually enlarging transverse cross section, which freely rotates within the kiln and is utilized to aid in mixing the bed material. U.S. Pat. No. 4,753,019 discloses multi rods arranged in the inner periphery of the kiln, which increase drying, and heating of lime sludge. U.S. Pat. No. 3,318,538 discloses similar type of rods, for improved polymer mixing. Polygonal as well as sinusoidal shaped lining improve mixing and heat transfer of the material as it traverses forward through the kiln. Various shapes of polygonal cross sectional kiln are disclosed. Examples of such shapes of rotary cement kiln are described in U.S. Pat. No. 5,702,247, 5,460,518, 5,299,933, 5,616,023. U.S. Pat. No. 5,873,714 discloses a kiln having a sinusoidal shaped lining which increased mixing and agitation of the material in combination with heat efficiency. U.S. Pat. No. 5,097,773 discloses a rotary incinerator, which is octagonal in shape to incinerate solid waste. However, core formation continues to be a problem in both hexagonal rods and polygonal cylinders. Summary Of The Invention The main object of the present invention to provide improved tumbling mixers and rotary kilns for enhanced mixing and heat transfer of particulates in batch and continuous flow system. Another object of the invention is to provide tumbling mixers and rotary kilns capable of increase in rapid transverse mixing by eliminating core formation along the axis of the cylinder. Another object of the invention is to provide rotary kilns capable of increasing heat transfer between the particles and the hot combustion gases. It is yet another object of the invention to provide a method of increasing the capacity of the tumbling mixers and rotary kilns. It is another object of the invention to improve mixing and heat transfer in tumbling mixer and rotary kilns without increasing entrainment, lifting and showering of materials. It is another object of the invention to provide better gas solid contact and results uniform bed temperature. It is yet another object of the invention to enhance axial mixing using impellers in shaft - impeller systems whose cross sections vary along the rotational axis. It is another object of the invention to improve mixing to achieve efficient and uniform heat transfer to produce uniform products with substantially reduced formation of fines and dust. The impellers in shaft - impeller systems of the present invention improves mixing and heat transfer in tumbling mixers and rotary kilns thereby overcoming problems encountered in the prior art. In particular, the impeller in shaft - impeller system of the present invention results in homogeneity of the materials and temperature resulting in uniform end product. Thus in accordance with this invention plurality of shaft - impeller systems constructed of corrosion resistance high steel alloy and/ or heat stable materials such as ceramic, metal or metal clad ceramics attached on a rotating shaft placed inside the tumbling mixers and rotary kilns without hindering one another in a transverse plane so that the impellers are submerged in the flowing layer, wherein at least one shaft - impeller system is positioned at the mid point along the line formed by the intersection of the free surface with the transverse plane, where the layer thickness is maximum. The rotational speed and the direction of rotation of the impellers in shaft -impeller systems are independently controlled, wherein the shaft - impeller system is rotated in the same direction of cylinder rotation at a rotational speed more than 60 times the speed of mixer/kiln. Further the height of the impellers in shaft - impeller systems when installed with an inclination with respect to the horizontal axis optionally decreases towards the discharge end, wherein the height of the impeller is in the range of 0.06H to 0.12H, and varies along the length of the shaft depending on H, where H is the depth of the bed material in rotary kilns or tumbling mixers with circular cross - section. In case of rotary kilns or tumbling mixers of noncircular cross - section H is the time average depth of the bed material at any transverse cross - section. The impeller cross-section perpendicular to the shaft axis is of any shape, preferably rectangular, rhombus, "S shaped", "reverse S shaped", double edged Y shaped, "T shaped", Z shaped", "reverse Z shaped" or "X shaped", and the impeller may comprise several segments of different size and shape fitted to the shaft. The above and other objects and novel features of the invention will become clear from the detailed description and non-limiting examples. Detailed Description of the invention Description of the drawings The drawings illustrate specific embodiment of the invention and variations may be made within the meaning of this invention. Figure 1: Schematic of reverse "S" shaped impeller in shaft - impeller system. Figure 2: Variation of fraction of smaller particles (f) along the scale radial distances (r/R) in a mixture of 1 and 2 mm steel balls for different sizes of impeller rotated at 200 rpm as indicated in the legend. Cylinder is rotated at 3 rpm. Figure 3: Photographs show the transverse mixing of red colour glass beads within colourless glass beads of same size and density for a cylinder with and without a rotating impeller at the center. (a) No impeller at the center; and (b) a rotating reverse "S shaped" impeller at the center and the impeller is rotated at 120 rpm in the same direction of cylinder rotation. Figure 4: Photographs show the azimuthal mixing of red colour glass beads within colourless glass beads of same size and density for a cylinder with and without a rotating impeller at the center. (a) No impeller at the center; and (b) a rotating reverse "S shaped" impeller at the center and the impeller is rotated at 120 rpm in the same direction of cylinder rotation. Figure 5: Variation of fraction of smaller particles (f) along the scale radial distances (r/R) in a mixture of 1 and 2 mm steel balls for various cylinder shapes as indicated in the legend. Cylinder is rotated at 3 rpm. The preferred embodiment described in detail is directed towards an impeller that achieves homogeneous mixing of the bed materials and temperature to produce substantially uniform end product. FIG.1 presents the cross section perpendicular to the shaft axis of a "reverse S shaped" impeller in shaft -impeller system as a preferred embodiment of the impeller of this invention, wherein rotation of the mixer/kiln and the impeller is in the anticlockwise direction. Also both the mixer/kiln and the impeller rotating in the clockwise direction is an equivalent system. The impeller has two segments an upper segment (10) and lower segment (12). Both the segments are attached on a rotating shaft (14). The height of the impeller is in the range of 0.06H to 0.12H, where H is the depth of the bed material in rotary kilns or tumbling mixers with circular cross - section. The edges of the impeller (10, 12) are curved. The curvature angles of all the faces (10, 11, 12, 13) are equal. The "angle of curvature" means the angle formed by the "inner" concave surface with the horizontal drawn at the middle of the top or bottom segments of the impeller. The angle made by a tangent (AB) to the impeller surface near the tip and a tangent (CD) near the impeller mid point is in the range of 15° to 40°. The ratio of impeller length (L) to impeller width (W) at the center of the impeller is in the range of about 3 to about 4. The impeller (10) is placed in the tumbling mixer or rotary kilns so that the shaft (14) of the impeller is submerged on the top surface of the flowing layer. The impeller (10) is placed at the center of the flowing layer. More specifically, the impeller (10) is placed in the flowing surface where layer thickness is " maximum. The material of construction of the impeller may be selected from any heat resistant material such as ferrous or non-ferrous metals or refractory, ceramics, etc so as to withstand the environment in which it is to be used. If the agitator is used in a high temperature tumbling mixer then the impeller is to be coated with a refractory material, for example, magnesia, alumina, alumina-silica and the like. In one of the embodiments the impellers are of varying cross section along the axial length so as to increase the local axial mixing. Such impellers may be spiral, screw, or an impeller that has alternate clockwise and anti¬clockwise twists. Experiments are performed for a binary mixture of 1 and 2 mm spherical steel balls in a cylinder rotated at various speed of rotations for various size of reverse "S shaped" impeller rotated at various speed of rotations in the same direction of cylinder rotation. The cylinder is half filled. Experiments performed are with and without an impeller under identical conditions. A desired quantity of sample is collected from various radial locations of the cylinder after completion of 150 number of cylinder rotations. The weight fraction profile for smaller particles are then obtained and the mixing index (M) value is calculated by using the expression as given in equation 1. The quality of mixing is measured with these two quantities, i.e., segregation index (I) and mixing index(M). Segregation index is a measure of relative proportion of smaller particles within the core in any arbitrary segregation (Wsa) to complete segregation (Wss) and smaller the value, better will be the mixing. Mathematically, where, Rs is the radius of the core in the limit of complete segregation, and f(r) is the radial weight fraction profile of the smaller particles. The radius of the core is , where φs is the weight fraction of small particles in the system. For complete segregation and for perfect mixing Example 1: Sizes of impeller Experiments are carried out for impellers rotated at 200 rpm in the same direction of cylinder rotation and the cylinder is rotated at 3 rpm. The impellers are placed at the center of the cylinder. The profiles are shown in FIG. 2 and index values are indicated on the table 1. Surprisingly, the profile for 50 mm size impeller is nearly flat. This indicates that the core is vanished from the center. The value of mixing index in this case is 1, which corresponds to well mix state. Though the impeller size of 80 mm was reduced the core size yet it does not eliminate the entire core at 200 rpm. Results in table 1 show the improvement of mixing in the transverse plane. This implies that heat transfer in the cross-section will be enhanced. Table 1: Mixing Index values for various sizes of Reverse S shaped impeller rotated at 200 rpm in the same direction of cylinder rotation are indicated. Cylinder is rotated at 3 rpm in the clockwise direction. Similar experiments are performed for other sizes of impeller placed at the center of the cylinder and the impellers are rotated at 180 rpm in the same direction of cylinder rotation and the cylinder is rotated at 3 rpm. Index values are shown in the table 2. Mixing index value for a given impeller speed with increasing the size of the agitator decreases. Results show that the of quality of mixing in transverse plane is best for a 50 mm impeller and decreases with increase of impeller size for the given impeller speed. Impeller type Impeller size (mm) Impeller speed (rpm) Cylinder speed (rpm) Mixing Index (M) Nil Nil Nil 3 0.26 Reverse S shaped 50 180 3 0.84 Reverse S shaped 60 180 3 0.74 Reverse S shaped 80 180 3 0.62 Table 2: Mixing Index values for various sizes of Reverse S shaped impeller rotated at 180 rpm in the same direction of cylinder rotation are indicated. Cylinder is rotated at 3 rpm in the clockwise direction. Experiments are also performed in a cylinder rotated at 3 rpm for various size of Reverse S shaped impeller rotated at 120 rpm in the same direction of cylinder rotation are shown in the table 3. The mixing index value is higher for 50 mm size Reverse S shaped impeller and lower for 30 mm size impeller. Results show the improvement of transverse mixing with increase of size of the impeller at 120 rpm. Impeller type Impeller size (mm) Impeller speed (rpm) Cylinder speed (rpm) Mixing Index (M) Nil Nil Nil 3 0.52 Reverse S shaped 30 120 3 0.62 Reverse S shaped 40 120 3 0.70 Reverse S shaped 50 120 3 0.76 Table 3: Mixing Index values for various sizes of Reverse S shaped impeller rotated at 120 rpm in the same direction of cylinder rotation are indicated. Cylinder is rotated at 3 rpm in the clockwise direction. Example 2: Speed of rotation of the impeller Experiments are performed for 50 mm size Reverse S shaped" impeller rotated at various speeds of rotations in the same direction of cylinder rotation and the impeller is place at the center of the flowing layer. The profiles show that as the speed of rotation of the "Reverse S shaped" impeller is increased the mixing is gradually increased. The best result is obtained when the impeller is rotated at 200 rpm. Index values are shown in the table 4. Mixing index value is increased with speed of rotation of the impeller. Results show the improvement of transverse mixing with increase the speed of rotation of the impeller. Impeller type Impeller size (mm) Impeller speed (rpm) Cylinder speed (rpm) Mixing Index (M) Nil Nil Nil 3 0.52 Reverse S shaped 50 80 3 0.68 Reverse S shaped 50 120 3 0.70 Reverse S shaped 50 160 3 0.74 Reverse S shaped 50 180 3 0.84 Reverse S shaped 50 200 3 1 Table 4: Mixing index values for 50 mm size "Reverse S shaped" impeller rotated at various speed of rotation in the same direction of cylinder rotation. Cylinder is rotated at 3 rpm in the clockwise direction. Example 3: Speed of rotation of the cylinder Experiments are carried out in a cylinder with and without a rotating impeller at the center for cylinder and the cylinder is rotated at 3, 6, and 12 rpm, respectively. Profiles show there is a striking improvement of mixing at low cylinder rotation. Index values are shown in the table 5. The value of mixing index increases with decreasing speed of rotation of the cylinder. Substantially uniform mixing is achieved when cylinder is rotated at 3 rpm. Results show the decrease of quality of mixing in the transverse direction with increase of speed of rotation of the cylinder for a given impeller speed. Impeller type Impeller size (mm) Impeller speed (rpm) Cylinder speed (rpm) Mixing Index (M) Nil Nil Nil 3 0.52 Nil Nil Nil 6 0.42 Nil Nil Nil 12 0.26 Reverse S shaped 50 200 3 1 Reverse S shaped 50 200 6 0".70 Reverse S shaped 50 200 12 0.64 Table 5: Mixing index values with and without a 50 mm size "Reverse S shaped" impeller rotated at 200 rpm in the same direction of cylinder rotation for various speed of rotation of the cylinder as indicated. Cylinder is rotated in the clockwise direction. Example 4: Improvement of transverse mixing Experiments are performed in a half filled horizontal cylinder (ID 320 mm and thickness 10 mm) for same size and same density but different colour spherical glass beads of size 2 mm. Red coloured glass beads are kept at the center of the cylinder and rest of the cylinder is filled with colourless glass beads. Two sets of experiments are performed separately. In the first case there is no rotating impeller at the center but in the second case " Reverse S shaped " impeller is kept at the center of the cylinder. In each case cylinder is rotated at 2 rpm in the clockwise direction. Photographs are taken after the entire the bed material passes once through the fluidized layer. We call this as "one pass". In the case 2 " Reverse S shaped " impeller is rotated at 120 rpm in the same direction of cylinder rotation. Photographs are also taken after the completion of each pass. The photographs in FIG. 3 show that after four mixer revolutions the cylinder with no impeller, complete mixing occurs only after 34 revolutions where as the cylinder with a rotating impeller complete mixing is obtained only after 4 revolutions. The impeller of this invention significantly improves the transverse mixing efficiency. Example 5: Improvement of azimuthal mixing Experiments are carried out in the same cylinder as used in transverse mixing experiments. Glass beads of size 2 mm are used in the experiments. In these experiments the red glass beads are placed in a triangular pattern as compared to semicircular pattern in transverse mixing. Each case the cylinder is rotated at 2 rpm in the clockwise direction. In the second case "Reverse S shaped" impeller is rotated at 120 rpm in the same direction of cylinder rotation. Photographs are taken after completion of each pass. The photographs in FIG. 4 show a comparison of mixing with and without a rotating impeller after 27 mixer revolutions. The cylinder with no impeller complete mixing occurs only after 57 revolutions where as the cylinder with a rotating impeller complete mixing is obtained only after 27 revolutions. Further mixing in the transverse direction significantly faster than the mixing in the azimuthal direction. Example 6: Circular geometry The concentration profiles obtained for a binary mixture of 1 and 2 mm steel balls rotated at 3 rpm in various types of cylinder geometry as indicated in the legend is shown in FIG. 5. Experiments are performed in two-cylinder geometry i.e., namely, circular and regular pentagon as used in U.S. Pat. No. 5,702,247 (Schoof), 5,460,518 (Mosci), and 5,097,773 (Freeman). Profiles show that there is no improvement of mixing for regular pentagon as compared to circular geometry indicating that cylinder geometry does not play a major role in reduction of core formation as compared to that obtained by the rotating impeller of this invention. This improved mixing and "substantially reduced core formation" in the mixers / rotary kilns provides for uniform exposure of the particles to the heat from the combustion gases and to facilitates interparticle heat transfer, thereby yielding homogeneity in reaction and end products. We claim: 1. Improved tumbling mixers and rotary kilns for enhanced mixing and heat transfer of particulates in batch and continuous flow systems reducing segregation, resulting in homogeneity in end products, and increasing capacity comprising: A. a mixer/kiln having feed entry means and the product discharge means; B. one or plurality of shaft - impeller systems; wherein the shaft - impeller systems are positioned without hindering one another in a transverse plane so that the impellers are submerged below the surface of the flowing layer of the rotating bed material; at feast one shaft-impeller system is positioned at the mid point along the line formed by the intersection of the free surface with the said transverse plane, where the layer thickness is maximum. 2. Improved tumbling mixers and rotary kilns as claimed in claims 1 wherein the height of the impeller is in the range of 0.06H to 0.12H and varies along the length of the shaft depending on H, where H is the depth of the bed material in rotary kilns or tumbling mixers with circular cross - section. 3. Improved tumbling mixers and rotary kilns as claimed in claims 1-2, wherein the height of the impeller for rotary kilns or tumbling mixers of noncircular cross - section is in the range of 0.06H to 0.12H and varies along the length of the shaft depending on H, where H is the time average depth of the bed material at any transverse cross - section. 4. Improved tumbling mixers and rotary kilns as claimed in claims 1-3, wherein the impeller cross-section perpendicular to the shaft axis is of any shape, preferably rectangular, rhombus, "S shaped", "reverse S shaped", double edged Y shaped, "T shaped", "Z shaped", "reverse Z shaped" or "X shaped". 5. Improved tumbling mixers and rotary kilns as claimed in claim 4 wherein the shaft - impeller system is single or multi segmented along the axis of the shaft. 6. Improved tumbling mixers and rotary kilns as claimed in claims 1-2 wherein the rotational speed and the direction of rotation of the impellers are independently controlled. 7. Improved tumbling mixers and rotary kilns as claimed in claims 1, 4 wherein rotation of the mixer/kiln and the impeller is in the anticlockwise direction. 8. Improved tumbling mixers and rotary kilns as claimed in claims 1,4 wherein rotation of the mixer/kiln and the impeller is in the clockwise direction. 9. Improved tumbling mixers and rotary kilns as claimed in claims 1,4,7,8 wherein the angle between a tangent to the impeller surface near the tip and a tangent near the impeller mid point of the "reversed S" or "S" shaped impeller in the shaft - impeller system is in the range of 15 degree to 40 degree. 10. Improved tumbling mixers and rotary kilns as claimed in claim 1 wherein the impeller is rotated in the same direction of mixer/kiln at rotational speed more than 60 times the speed of mixer/kiln. Date: 14/08/2003 Dr.TRABUDDHA GANGULI Agent on behalf of the applicant

Documents:

809-mum-2003-abstract(13-05-2005).doc

809-mum-2003-abstract(13-05-2005).pdf

809-mum-2003-cancelled pages(13-05-2005).pdf

809-mum-2003-claims(granted)-(13-05-2005).doc

809-mum-2003-claims(granted)-(13-05-2005).pdf

809-mum-2003-correspondence(ipo)-(22-01-2008).pdf

809-mum-2003-correspondence1(03-05-2005).pdf

809-mum-2003-correspondence2(18-03-2004).pdf

809-mum-2003-drawing(13-05-2005).pdf

809-mum-2003-form 1(13-08-2003).pdf

809-mum-2003-form 1(14-08-2003).pdf

809-mum-2003-form 19(17-03-2004).pdf

809-mum-2003-form 19(18-03-2004).pdf

809-mum-2003-form 2(granted)-(13-05-2005).doc

809-mum-2003-form 2(granted)-(13-05-2005).pdf

809-mum-2003-form 26(08-08-2006).pdf

809-mum-2003-form 26(13-08-2003).pdf

809-mum-2003-form 3(13-08-2003).pdf

809-mum-2003-form 5(13-08-2003).pdf

809-mum-2003-form 5(18-03-2004).pdf

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