WO2015029065A2

WO2015029065A2 - Rotary dryer for drying bulk solids

Info

Publication number: WO2015029065A2
Application number: PCT/IN2014/000550
Authority: WO
Inventors: Parag Hanumanthappa Malode; Amlan Datta
Original assignee: Aditya Birla Science & Technology Company Limited
Priority date: 2013-08-28
Filing date: 2014-08-27
Publication date: 2015-03-05
Also published as: WO2015029065A3; IN2013MU02812A

Abstract

Rotary dryer for drying bulk solids. Each of the lifters (3) of the dryer comprises an angular member having a planar base (5) and a planar scooping lip (6) at an angle θ1 of 100 to 150° with respect to the planar base. The lifter also comprises a pair of end closing members (7, 7), one at each end of the angular member at an angle θ2 of 60 - 120° with respect to the respective end. Each of the lifters is mounted to the inner periphery of the drum of the dryer at the free edge (8) of the planar base at an angle θ3 of 50 - 90° with the inner periphery of the drum. The free edges (9) of the end closing members coincident with the inner periphery of the drum. The scooping lip of the lifter projects radially inwardly with respect to the drum (Fig 4).

Description

TITLE OF THE INVENTION

Rotary dryer for drying bulk solids

FIELD OF INVENTION

This invention relates to a rotary dryer for drying bulk solids.

BACKGROUND OF INVENTION

Bulk solids such as food grains, minerals, cement, specialty chemicals or pharmaceuticals are generally dried in rotary dryers. A rotary dryer comprises a rotary drum disposed for rotation about a horizontal axis and a carrier gas circulation system for the drum. Bulk solids are dried in the rotary drum by heat exchange between the bulk solids and the carrier gas being circulated in direct or indirect contact with the solids. Bulk solids and carrier gas are often at significantly different temperatures so as to ensure effective heat exchange therebetween and to dry the bulk solids. Air is a commonly used carrier gas and there are different types of rotary dryers depending upon the gas circulation system. The dryer can be direct contact type, indirect contact type, semi direct contact type or indirect cum direct contact type.

Rotary dryers are either with lifters or lifterless. Rotary dryers with lifters are generally used to invoke cascading and cataracting motions at low speeds of operation of the dryers so as to improve the mixing of the carrier gas and bulk solids and to improve drying of the bulk solids. Lifters are discrete elements extending along the inner periphery of the drum radially inwardly directed and are also called flights, lifting vanes, dispersion means, protruding vanes, vanes or lifting scoops. Lifters enable better agitation and mixing of the bulk solids with the carrier gas and profoundly affect the heat transfer between the carrier gas and bulk solids. During rotation of the drum about its horizontal axis each lifter carries a portion of bulk solids from the bed of solids at the bottom of the drum and discharges or throws the same gradually onto the bed of solids at the bottom of the drum until the lifters are emptied out. Drying efficiency of dryers with lifters depends on factors such as material holding capacity of the lifters, surface area of contact provided by the lifters for the material to be dried, quantity of material lifted and discharged by the lifters, and throw angle of the lifters ie the angle over which the lifters discharge the solids. The manner of discharging the solids by the lifters also significantly affects the resultant product properties. The manner of discharge of bulk solids is extremely important, especially in the case of friable bulk solids, for example pharmaceutical products such as granules or tablets to prevent, for instance, pellet disintegration or fines and dust generation and resultant pollution problems and loss of valuable material and weight and dosage variations. Weight and dosage variations are very consequential and critical in the case of pharmaceutical products. Rotary dryers with various lifter designs are known and reported. A commonly used lifter comprises an angular member having two planar faces at 135° with respect to each other. One of the planar faces of the angular member is fixed to the inner periphery of the drum with its entire flat surface in abutment with the inner surface of the drum and the other planar face of the angular member projects radially inwardly with respect to the drum. In such a lifter, the other planar face of the angular member with limited surface area, scoops in and holds the material to be dried. Another lifter comprises an angular member having two planar faces at right angle to each other. The angular member further comprises a right angled projection from one of the planar faces. The other planar face of the angular member is fixed to the inner periphery of the drum with its entire flat surface in abutment with the inner surface of the drum. The said one planar face of the angular member projects radially inwardly with respect to the drum. In such a lifter design, the said one planar face of the angular member with limited surface area, scoops in and holds the material to be dried.

US 3576080 describes a rotary cooler having cylindrical shell mounted to rotate about its central axis. The interior of the shell is provided with parts assembled in groups to divide the interior into cells. Each of the cell defining assemblies includes a pair of wall structures and each wall structure has a radially inward projecting and axially extending surface forming a scoop which in end view is J- shaped having a back portion, a bottom portion and a lip portion defining a pocket therebetween. Each wall structure is arranged with the pocket on an end thereof projecting radially inward, and the back portion remote of the pocket is pivotally connected at longitudinally spaced locations to the inner periphery of the cooler shell by a pin parallel to the central axis of the cooler shell. The radially inner end of each wall structure is connected at longitudinally spaced locations to one adjacent wall structure by a hinge. Each hinge has a mid hinge pivot pin parallel to the central axis of the cooler shell. Thus each hinge has a pivot pin which is part of a cell defining assembly with two parallel pins at the inner periphery of the cooler shell and each such group of three pins provides pivotal and axially slidable connections that accommodate movement of the parts of their cell defining assembly relative to each other and relative to the shell upon thermal expansion of the parts without rupturing such parts.

US 3780447 describes a rotary dryer comprising radially extended flights generally near the material feed end of the dryer, and a dam assembly having a movable ring segment rotatable between a dam position, wherein the movable ring segment extends in a direction transverse to the general direction of material flow through the dryer, and a flight position, wherein the movable ring segment extends in a direction aligned with the general direction of material flow through the dryer.

US 4131418 describes an attached tube cooler for a rotary kiln for cooling solid particulate material discharged from the kiln. The cooler includes a plurality of cylindrical vessels attached to and circumferentially spaced around the discharge end of the kiln for rotation therewith. Each cooler tube has an inlet for solid particulate material communicating with the outlet of the kiln, an outlet for cooled solid particulate material, an inlet for cooling gas and an outlet for gas which communicates with the kiln so that gas heated in the cooler tube can be supplied as combustion gas to the kiln. Heat exchange fins are mounted inside the cooling tube at an angle such that the trailing surface of the fins in the direction of tube rotation forms an acute angle with the side wall of the cooler tube. A helical conveyor having a central opening therethrough advances material through the cooler tube.

US 4742622 describes an elongated drying drum disposed for rotation about its longitudinal axis. The inner surface of the drum is provided with a plurality of material distributing vanes in circumferential ly spaced relationship. The vanes are characterized by first surface extending generally transverse to a tangent to set body at the point of attachment and a second wedge-shaped surface intersecting the first surface. The wedge-shaped surface may also be provided with a lip extending circumferential ly therefrom. A plurality of interference structures are disposed from a central longitudinal support and provide means for distributing material gravitating from the outer peripheral vanes. A plurality of return flighting members are disposed along the inner surface of the drum between the material distributing vanes. These flighting members are disposed at an angle and conform generally to the curvature of the inner surface. US 4964226 describes a dryer drum for drying bulk material including a rotating drum having an inlet for material to be dried at one end and an outlet for dried material at the other end. A center shaft extends through the drum and vanes are mounted on the shaft circumferentially spaced from each other and axially spaced along the shaft. The vanes have a first panel extending substantially in a radial plane parallel to the axis of the shaft, a second panel attached to the first panel and facing in a downstream direction and a third panel attached to the second panel. The bulk material to be dried moves in sliding movement over the panels and is mechanically conveyed toward the dryer inlet.

US 5083382 describes a rotary drum dryer including a plurality of circumferentially spaced flights in each of a plurality of axially spaced flight sections. A radially inwardly directed dam is interposed between each flight section and serves as a pivotal mount for the flights of each section. The dams are provided with arcuate slots having a plurality of detent positions arranged about mounting holes for pins secured to the flights and forming the pivot axis therefor. The angular position of the flights is adjustable by locating bolts on the flights in selected detented positions in the arcuate tracks. The combination of pivotally mounted flights and dams affords uniform veiling across the drum interior and provides efficient heat transfer to the aggregate from the hot gases of combustion.

US 5203693 describes a rotary drum dryer useful in the production of asphalt paving composition. The drum incorporates a burner which defines a combustion zone, and a dam and flight construction is mounted to the interior of the drum so that the flights surround the combustion zone. The dam is adapted to retain and raise the level of the aggregate flowing through the rotating drum so that the flights pass through the aggregate in the bottom of the drum, and the flights collect a portion of the aggregate on their top surfaces and carry the collected aggregate to an elevated discharge point, where the aggregate drops back to the bottom of the drum. The flights serve to shield the metal wall of the drum from the radiant heat energy of the burner flame which would otherwise result in its rapid deterioration, and they also serve to permit the aggregate to be effectively exposed to the radiant heat. The fact that more radiant heat energy is transferred to the aggregate, serves to further lower the temperature of the metal wall of the drum. Still further, the fact that the flights pass through the aggregate and are covered by the aggregate through a substantial portion of their circular path of travel, results in the flights being exposed to less radiant heat energy and thus subject to less deterioration.

US 5740617 describes a rotary drum dryer for drying a slurry into discrete solid particles. The dryer contains three long horizontal cylinders: an outer cylinder, a perforated cylinder inside the outer cylinder, and an inner cylinder inside the perforated cylinder. As the cylinders rotate, slurry in the outer cylinder is compressed through the perforated cylinder by the inner cylinder and deposited onto,_.the inner wall surface of the outer cylinder in discrete masses. The masses are dried as the cylinder rotates and then are removed by a scraper and conveyed outside the dryer for further treatment and/or packaging.

US 6143137 describes a rotary drum cooler for cooling particulate material such as coke particles having at least one cooling pocket which, in turn, includes a flexible vent pipe assembly which can slide in relation to the shell of the cooler. By making the vent pipe slidable, it can move in response to the expansion and contraction of the pocket within the shell. The vent pipe assembly is comprised of a tube which is connected to the pocket and which slidably extends through an aperture in the shell and a flexible means for sealingly connecting the outer end of the tube to the shell.

In our WO/201 1 /1 14351 A2 (claiming priority from our Indian patent application No 717/MUM/2010 filed on 18 March 2010), we have described a dispersion means for agitating a particulate mass resident in a rotating drum. The dispersion means comprises a plurality of perforated flights radially extending from the inner surface of the rotating drum which is rotated about an operatively longitudinal axis. The perforated flights help in increasing the throughput of the rotary drum and in reducing the time required for heat and mass transfer by increasing the contact surface of the perforated flights with the particulate mass to be dispersed by the dispersion means. The lifter designs discussed above suffer in general from drawbacks such as limited surface area for holding the material to be dried, limited surface area for contact with the material to be dried and small throw angles for the material being discharged by the lifters and/or are complex and complicated in construction and expensive. Therefore, inspite of the various known rotary dryers with lifter designs, there is still need and scope for rotary dryers with lifter designs which are simple in construction and economical and which have advantages such as enlarged surface area for holding the material to be dried, enlarged surface area for contact with the material to be dried and large throw angles and which reduce dust generation so as to prevent loss of valuable material and pollution problems and in the case of pharmaceuticals, in particular, to prevent weight and dosage variations which can be very consequential and detrimental.

DETAILED DESCRIPTION OF INVENTION

According to the invention there is provided a rotary dryer for drying bulk solids, comprising a rotary drum disposed for rotation about a horizontal axis and having a plurality of lifters located at the inner periphery thereof spaced apart from one another and a carrier gas circulation system for the drum, wherein each of the lifters comprises an angular member having a planar base and a planar scooping lip at an angle of 100 to 150° with respect to the planar base and a pair of end closing members, one at each end of the angular member at an angle of 60 - 120° with respect to the respective end and wherein each of the lifters is mounted to the inner periphery of the drum at the free edge of the planar base at an angle of 50 - 90° with the inner periphery of the drum, with the free edges of the end closing members coincident with the inner periphery of the drum and with the scooping lip projecting radially inwardly with respect to the drum. BRIEF DESCRIPTION OF. ACCOMPANYING SCHEMATIC DRAWINGS

Fig 1 is an elevation of the rotary drum of a rotary dryer for drying bulk solids according to an embodiment of the invention;

Fig 2 is a sectional view of the drum of Fig 1 ;

Fig 3 is an isometric view of the drum of Fig 1 showing only one lifter;

Figs 4 and 5 are isometric views of the lifter of Fig 3 from opposite sides thereof; Fig 6 is a sectional view of the lifter of Figs 4 and 5;

Fig 7 is a sectional view of the drum of Fig 1 showing the discharge pattern of bulk solids during rotation of the drum when the rotary dryer is in operation; Fig 8 is an elevation of the rotary drum of a rotary dryer for drying bulk solids according to another embodiment of the invention; and Fig 9 is an elevation of the rotary drum of a rotary dryer for drying bulk solids according to another embodiment of the invention;

DETAILED DESCRIPTION OF EMBODIMENTS OF INVENTION

The rotary drum 1A of a rotary dryer (not shown) for drying bulk solids 2 as illustrated in Figs 1 to 7 of the accompanying drawings comprises a single row of lifters 3 extending radially inwardly from the inner periphery of the drum and spaced around the inner periphery of the drum in radially equally spaced apart relationship with one another. The rotary drum is disposed for rotation about a horizontal axis 4 and includes a carrier gas circulation system (not shown). The rotary dryer may be direct contact type, indirect contact type, semi-direct contact type or indirect cum direct contact type. The various types of rotary dryers as stated above and a carrier gas circulation system for the dryers are all well known to a person skilled in the art. Therefore, further details of the dryers and the carrier gas circulation system for the dryers have not been illustrated and described as such are not necessary for understanding the invention.

Each of the lifters 3 comprises an angular member having a planar base 5 and a planar scooping lip 6 at an angle Θ1 of 100-150°, preferably an angle of 120 to 150° and still preferably an angle of 130° with respect to the planar base. Each of the lifters also comprises a pair of end closing members 7, 7, one at each end of the angular member at an angle Θ2 of 60 to 120°, preferably 70 to 1 10° and still preferably 90° with respect to the respective end. Each of the lifters is mounted to the inner periphery of the drum at the free edge 8 of the planar base at an angle Θ3 of 50 - 90°, preferably 55 to 90° and still preferably 60° with the free edges 9 of the end closing members coincident with the inner periphery of the drum and with the scooping lip projecting radially inwardly with respect to the drum.

According to a variation of the invention, each of the lifters comprises at least two compartments 10 and 1 1 formed by partitioning the angular member with a partition member 12 mounted across the angular member between the end closing members. The free edge 13 of the partition member is also coincident with the inner periphery of the drum. It is to be understood that the lifter may be partitioned into more than two compartments with corresponding number of partition members. However, partitioning of the lifter is optional. According to another variation of the invention, the width of the planar base is 0.3 to 3 times the width of the scooping lip. Preferably, the width of the planar base is 0.4 to 2 times the width of the scooping lip. Still preferably, the width of the planar base is 0.47 times the width of the scooping lip. The lifter comprising angular member and end closing members and optionally partition member(s) may be integrally formed or alternatively the components of the lifter may be fabricated separately and assembled together, preferably by welding.

During operation of the dryer, the drum rotates about its horizontal axis 4 and bulk solids 2 to be dried are fed into the drum from one end (feed end) thereof (not marked) and dried material is discharged from the drum at the other end (discharge end) thereof (not marked). While the drum rotates about its horizontal axis, the bulk solids getting collected at the bottom of the drum is scooped up by the scooping lips of the lifters into the large partially enclosed surface area defined by the angular member and end closing members of the lifters and discharged onto the bulk solids at the bottom of the drum over a large angle of rotation of the drum (throw angle) successively as illustrated in Fig 7 of the accompanying drawings. Carrier gas (not shown) being circulated heats up the lifters and bulk solids so as to dry the bulk solids and to remove evaporated moisture from bulk solids.

The rotary drum IB of a rotary dryer (not shown) as illustrated in Fig 8 of the accompanying drawings comprises a plurality of lifters 3 extending along the inner periphery of the drum longitudinally staggered along the inner periphery of the drum and radially equally spaced around the inner periphery of the drum. The rotary drum 1C of the rotary dryer (not shown) as illustrated in Fig 9 of the accompanying drawings comprises a plurality of lifters 3 extending along the inner periphery of the drum equally spaced along the inner periphery of the drum and radially equally spaced around the inner periphery of the drum. According to the invention, the partially enclosed surface area defined by the. angular member and end closing members _xof each of the lifters provides a large surface area to contain and retain large quantities of the bulk solids being scooped up by the scooping lips of the lifters. Because the lifter is mounted to the inner periphery of the drum at the free edge of the planar base at an angle of 50 - 90°, the entire angular member including the planar base is available for holding the material to be dried. The partially enclosed surface area is also increased if the planar base of the lifter is wider as compared to the planar scooping lip thereof. Large quantities of the bulk material are retained in the large partially enclosed area and are discharged from the partially enclosed area at a controlled rate over a large angle of rotation of the drum or over a large throw angle as illustrated in Fig 7. The large partially enclosed surface area of the lifters helps to retain large amounts of the bulk solids and to discharge the bulk solids over a large angle of rotation of the drum.

The large surface area of the lifters provides large surface area for contact with the bulk solids and heat exchange between the solids and the contact surface of the lifters. Due to retention of large quantities of the bulk material in the large partially enclosed area in contact therewith for long periods, the residence time of the bulk material in the drum is increased and heat exchange by conduction between the enlarged surface area provided by the lifters and the bulk solids is improved. Because of this, the drying efficiency of the dryer is improved and thermal energy available for drying is optimally utilized. The surface area for contact and heat exchange for the bulk solids is further increased with the partition member(s) to further improve the drying efficiency of the dryer and utilization of thermal energy. Due to controlled discharge of the bulk material over a large angular path also drying of the bulk solids is improved. Furthermore, dust generation is reduced. This is particularly advantageous in the case of friable materials such as pharmaceuticals where weight and dosage variations are very consequential and critical. Due to reduction in dust generation, pollution problems are reduced and loss of valuable material is reduced so as to achieve substantial cost benefits. The lifters are also simple and easy to fabricate and are very cost effective.

The following experimental examples are illustrative of the invention but not limitative of the scope thereof:

Example 1

Comparative simulation studies were carried out using dryers comprising conventional angular lifters and lifters according to the invention and using Discrete Element Method (DEM). Details of lifters used for the studies were as given in the following Table 1 :

Table 1

Lifters Dimensions

One planar face width 19 mm

Other planar face width 19 mm

Conventional angular lifters Thickness 3 mm

Angle between planar 135°

faces

Width of planar base 19 mm

Width of planar scooping 9 mm

lip

Lifters of invention Thickness 2 mm

Θ1 130°

Θ2 90°

Θ3 60° Each simulation was carried out with a rotary dryer comprising a single row of either conventional lifters or lifters according to the invention located centrally along the length of the drum of the dryer. Length of each lifter along dryer axis was 50 mm. A fixed quantity of material was fed to the drum and allowed to settle at the bottom of the drum. The drum was then rotated about its horizontal axis at a fixed speed of rotation for a fixed number of drum rotations. Operational details used for the simulation studies were as given in the following Table 2:

Table 2

Three benchmark evaluation parameters were formed on the basis of the simulation studies to assess lifter performance as follows :

Parameter 1. Lifter holdup capacity which reflects the maximum amount of material an individual lifter can hold

Parameter 2. Lifter throw angle which reflects the angle at which the material is deposited by the lifter onto the surface of the bed of material at the bottom of the drum during the rotational movement of the drum with the material

Parameter 3. Lifter effect on material fines (material breakage) which determines the change in fines generation or particle breakage(s). Simulation study results were as given in the following Table 3

Table 3

It is seen from the simulation results that lifter of the invention showed improved lifter hold-up capacity and lifter throw angle. The lifter effect on material attrition or fine generation remained unchanged which is significant in that in the case of the invention the fine generation did not increase while achieving the other benefits. While drying bulk friable solids, it is normally not desirable to improve drying efficiency at the cost of product quality.

Example 2

Actual comparative experimental studies were carried out using conventional angular lifters and lifters according to the invention in an inclined horizontal dryer, in which the drum was lined with lifters as shown in Fig 8 of the accompanying drawings. The dryer was indirect contact type with arrangement for circulating hot air outside the drum and for pulling out evaporated moist air from inside the drum. Length of each lifter along dryer axis was 75 mm. Details of lifters used were as shown in the following Table 4. Table 4

Operational details of the dryer were as shown in the following Table 5:

Table 5

In each experimental run, wet material was continuously fed into the dryer at the higher feed end and dried product was collected from the lower discharge end. The dry product discharge was monitored for a stable discharge rate followed by 0 regular measurements of the moisture content of feed and product throughout the duration of the experiment. The experimental results were as shown in the following Table 6 Table 6

It is evident from the above experimental results that dryer with the lifters of the invention showed an additional 9% moisture removal over the dryer with conventional lifter along with lower energy consumption, as evident from the 22% improvement in hot air utilization. It is seen that 22% less air is consumed in drying the same amount of material with the lifters of the invention than the conventional lifters. It is also evident from the above experimental results that the efficacy of the lifters of the invention lies in pushing 29% more material through the dryer yet removing an additional 9% moisture. This 22% improvement in specific energy utilization translates to an 2% improvement in dryer thermal efficiency. A reduction in fine generation is also achieved as is observed in the experimental results.

The lifter hold up (Parameter 1) and throw angle (Parameter 2) together affect the residence time of material within the lifters and hence the residence time within the dryer which significantly controls the drying kinetics. The longer retention results from the larger lifter surface area available for contact with the material and hence higher heat transfer. The lifter surface area is governed by the lifter design geometry. Thus the invention results in significant gains in terms of capacity while drying the additional capacity with a gain in drying kinetics as well as controlling the attrition of material particles during drying. The lifters of the invention also has the advantage of comparatively less thickness and consequential cost benefit. The end closing members and partition member(s), if any strengthen the lifters when mounted in the drum and improve the functionality of the lifters.

Claims

CLAIMS:

1. A rotary dryer for drying bulk solids, comprising a rotary drum disposed for rotation about a horizontal axis and having a plurality of lifters located at the inner periphery thereof spaced apart from one another and a carrier gas circulation system for the drum, wherein each of the lifters comprises an angular member having a planar base and a planar scooping lip at an angle of 100 to 150° with respect to the planar base and a pair of end closing members, one at each end of the angular member at an angle of 60 - 120° with respect to the respective end and wherein each of the lifters is mounted to the inner periphery of the drum at the free edge of the planar base at an angle of 50 - 90° with the inner periphery of the drum, with the free edges of the end closing members coincident with the inner periphery of the drum and with the scooping lip projecting radially inwardly with respect to the drum.

2. The rotary dryer as claimed in claim 1, wherein each of the lifters comprises at least two compartments formed by partitioning the angular member with a partition member mounted across the angular member between the end closing members, the free edge of the partition member corresponding to the free edges of the end closing members of the lifter also being coincident with the inner periphery of the drum at an angle of 50 - 90°.

3. The rotary dryer as claimed in claim 1 , wherein the angular member comprises a planar base and a planar scooping lip at an angle of 120 to 150° with respect to the planar base.

4. The rotary dryer as claimed in claim 3, wherein the angular member comprises a planar base and a planar scooping lip at an angle of 130° with respect to the planar base.

5. The rotary dryer as claimed in claim 1 , wherein the end closing members are mounted at the ends of the angular member at an angle of 70 to 1 10° with respect to the respective end of the angular member.

6. The rotary dryer as claimed in claim 5, wherein the end closing members are mounted at the ends of the angular member at an angle of 90° with respect to the respective end of the angular member.

7. The rotary dryer as claimed in claim 1 , wherein the angular member is mounted to the inner periphery of the drum at the free edge of the planar base at an angle of 55 to 90 °.

8. The rotary dryer as claimed in claim 7, wherein the angular member is mounted to the inner periphery of the drum at the free edge of the planar base at an angle of 60 °.

9. The rotary dryer as claimed in claim 1 , wherein the width of the planar base is 0.3 to 3 times the width of the scooping lip.

10. The rotary dryer as claimed in claim 9, wherein the width of the planar base is 0.4 to 2 times the width of the scooping lip.

1 1. The rotary dryer as claimed in claim 10, wherein the width of the planar base is 0.47 times the width of the scooping lip.

12. The rotary dryer as claimed in claim 1 or 2, which is indirect contact type.

13. The rotary dryer as claimed in claim 1 or 2, which is direct contact type.

14. The rotary device as claimed in claim 1 or 2, which is semi-direct contact type.

15. The rotary dryer as claimed in claim 1 or 2, which is indirect cum direct contact type.

16. The rotary dryer as claimed in claim 1 or 2, which comprises a single row of lifters spaced around the inner periphery of the drum in radially equally spaced apart relationship with one another.

17. The rotary dryer as claimed in claim 1 or 2, which comprises a plurality of lifters extending along the inner periphery of the drum longitudinally staggered along the inner periphery of the drum and radially equally spaced around the inner periphery of the drum.

18. The rotary dryer as claimed in claim 1 or 2, which comprises a plurality of lifters extending along the inner periphery of the drum equally spaced along the inner periphery of the drum and radially equally spaced around the inner periphery of the drum.