Patent Publication Number: US-8522793-B2

Title: Flash dryer for particulate materials

Description:
This application is a Continuation of co-pending PCT International Application No. PCT/JP02/12274 filed on Nov. 25, 2002, which designated the United States, and on which priority is claimed under 35 U.S.C. §120, the entire contents of which are hereby incorporated by reference. This application also claims priority of Application No. 2001-359617 and 2002-190447 filed in Japan on Nov. 26, 2001 and Jun. 28, 2002, respectively, under 35 U.S.C. §119. 
    
    
     TECHNICAL FIELD 
     The present invention relates to a flash dryer for transferring a particulate material with a heated dry gas flow, and drying the particulate material using the dry gas flow in the transferring process, more specifically to a flash dryer suitable for drying fillers for cigarettes. 
     BACKGROUND ART 
     Fillers for cigarettes include cut tobacco obtained by cutting raw materials, such as leaf tobacco from which main ribs are removed, the main ribs and the reconstructed tobacco, separately or by mixture. Alternatively, the fillers include cut tobacco subjected to an expanding process. Both kinds of cut tobacco have the given grading, that is, size. 
     In such a cut tobacco-forming process, cut tobacco is generally subjected to a liquid flaver-adding process, namely a flavoring process, so that the cut tobacco, which has undergone this process, has a high moisture content. Therefore, after the flavoring process, the cut tobacco needs to be dried to contain the desired moisture content before being fed to a cigarette making machine. The cut tobacco subjected to the expansion process contains not only a high moisture content but also an impregnant (liquid carbon dioxide). 
     Utilized in the cut tobacco-drying process in general are a cylinder dryer or a flash dryer. The flash dryer is capable of drying the cut tobacco in a short period of time, compared to the cylinder dryer, so that it has high drying processibility and is suitable for improving the productivity of cigarettes. 
     A flash dryer of this type generally comprises a gas flow path through which a dry gas flows, and also includes an air blower, a heater, a cut tobacco-receiving section and a cut tobacco-separating section that are each disposed in the gas flow path in order from the upstream side of the gas flow path. 
     The cut tobacco fed through the receiving section into the gas flow path is transferred from the receiving section toward the separating section with a dry gas flow and dried in this transferring process. After being dried, the cut tobacco is separated from the dry gas flow in the separating section and taken out of the separating section. 
     In cases where the cut tobacco is subjected to the drying process, the cut tobacco must be dried evenly. When the drying of the cut tobacco is uneven, for instance, if the cut tobacco is overdried, the cut tobacco generates an irritating odor and loses its flavor and taste. As a result, the quality of the cigarettes is also deteriorated. 
     Since the cut tobacco is dried in the transferring process as described, there needs to be enough length of the gas flow path from the receiving section to the separating section, namely a drying flow path, for subjecting the cut tobacco to the drying process. This forces the drying flow path to be long. Therefore, the drying flow path has at least one flection, which saves space for installation of the drying flow path. 
     If there is a flection in the drying flow path, however, the cut tobacco is prone to be fractured when passing the flection. Moreover, the cut tobacco is liable to remain in the flection, and such remaining makes the drying of the cut tobacco uneven. 
     It is said that smoke, which is generated from the cut tobacco during the burning of cigarettes, contains toxic components. Therefore, if the flash drying of the cut tobacco reduces the toxic components contained in the smoke, the flash dryer is more suitable for drying the cut tobacco. 
     DISCLOSURE OF THE INVENTION 
     An object of this invention is to provide a flash dryer capable of reducing fracture of a particulate material to be subjected to a drying process and drying the particulate material evenly. If the particulate material is cut tobacco for cigarettes, an object of the invention is to provide a flash dryer capable of reducing toxic components contained in smoke which is generated from the cut tobacco, in addition to the above capabilities. 
     To attain the above objects, the flash dryer according to the present invention comprises a gas flow path, air-blowing means for producing a one-way dry gas flow in the gas flow path, the dry gas flow having a given temperature, a feeding section disposed in the gas flow path and being capable of feeding a particulate material to be subjected to a drying process into the gas flow path by means of the dry gas flow, the particulate material being transferred with the dry gas flow and dried in the transferring process, and a separating section located in the gas flow path, downstream from the feeding section, and separating the dried particulate material from the dry gas flow to discharge the material from the gas flow path, wherein the gas flow path includes a drying duct for connecting the feeding section to the separating section and leading the particulate material fed from the feeding section toward the separating section with the dry gas flow, the drying duct curving upward in a convex shape. 
     With the above-described flash dryer, since there is no flection in the drying duct, the particulate material, which has been supplied from the receiving section into the dry gas flow in the gas flow path, easily flows through the drying duct with the gas flow without remaining in the drying duct and is guided to the separating section. Consequently, the fracture of the particulate material is reduced, and the particulate material is evenly dried. 
     Specifically, the drying duct may includes an upstream-side duct portion extending straight upward from the feeding section and having a given elevation angle with respect to a horizontal plane, and a downstream-side duct portion smoothly connected the upstream-side duct portion and the separating section, respectively, and being formed in a curve with a given curvature radius. In this case, the upstream-side duct portion has an elevation angle in a range of from 30° to 60°. 
     According to the drying duct, the particulate material supplied into the drying duct is blown up at a steep angle with the dry gas flow in the upstream-side duct portion. At this moment, the particulate material is dispersed well in the dry gas, which promotes the even drying of the particulate material. 
     The feeding section includes a venturi duct, which is connected to the drying duct and has a throat and a downstream portion linearly continuing to the upstream-side duct portion of the drying duct, and a rotary feeder for supplying the particulate material into the venturi duct at a feeding position which is defined immediately downstream of the throat. It is preferable that the venturi duct and the drying duct each have a rectangular flow-path cross section along the longitudinal direction thereof, and that the flow-path cross section of the venturi duct have width which is constant along the longitudinal direction thereof. 
     According to the above-described feeding section, since the width of the flow-path cross section of the venturi duct is constant along the longitudinal direction of the venturi duct, flux of the dry gas flow in the venturi duct is squeezed at the throat only heightwise, and the flux of the dry gas diverges toward the drying duct. Therefore, the dry gas flow does not form an eddy in the venturi duct, and the particulate material supplied into the venturi duct is dispersed well in the diverging dry gas immediately downstream of the throat and then directed to the drying duct without remaining. 
     More specifically, the throat is defined in between a part of a bottom wall and a part of a top wall of the venturi duct, and the part of the top wall is formed in the shape of a substantial V in a longitudinal section thereof. It is desirable in this case that the bottom wall of the venturi duct have a downstream-side bottom portion having a substantial V-shape in a longitudinal section thereof at the downstream side of the throat. The downstream-side bottom portion defines a deep region that temporarily increases a cross-sectional area of the flow path of the venturi duct. Alternatively, the bottom wall of the venturi duct may extend straight. 
     According to the above-described venturi duct, the dry gas flow, which has passed through the throat, proceeds away from the feeding position, so that the particulate material can be smoothly supplied from the rotary feeder into the venturi duct. Since the cross-sectional area of the flow path of the venturi duct is widened downstream of the throat, the particulate material is dispersed well in the venturi duct. 
     If there is provided the deep region downstream of the throat, more favorable supplying and dispersion of the particulate material can be achieved. 
     Concerning the cross-sectional area of the flow path of the venturi duct, the increasing rate of the cross-sectional area of the flow path located downstream of the throat is limited to the range in which the dry gas flow is not detached from an inner wall of the venturi duct. The detachment of the dry gas flow creates an eddy in the dry gas flow in the venturi duct, and such an eddy causes remaining of the particulate material in the venturi duct. In the venturi duct of the present invention, however, there generates no eddy of the dry gas flow that causes the remaining of the particulate material. 
     The separating section is provided with a tangential separator having a horizontal axis, the tangential separator including a cylindrical separator housing and a rotary feeder. More specifically, the separator housing has an inlet located in a top portion of outer periphery of the separator housing, being open in a horizontal direction, and guides the particulate material with the dry gas flow from the drying duct, an outlet located in a bottom portion of the outer periphery of the separator housing, being open downward, and discharges the particulate material from the separator housing, an exhaust port formed in an end face of the separator housing, being open eccentrically with respect to the horizontal axis, and discharges the dry gas from the separator housing, and a pair of linear wall portions forming the bottom portion of the outer periphery of the separator housing and facing each other so as to converge toward the outlet. In this case, the rotary feeder is connected to the outlet of the separator housing and takes out the particulate material from the separator housing through the outlet. 
     According to the above-described separating section, the particulate material, which has flowed from the inlet of the separator housing into the housing with the dry gas flow, moves from the inner wall of the separator housing along one of the linear wall portions toward the outlet, while the dry gas flow in the separator housing is deflected in the direction to the exhaust port. More specifically, the dry gas flow that has transferred the particulate material to one of the linear wall portions is detached from the linear wall portion to collide with the other linear wall portion. Thereafter, the dry gas flow runs upward along the other linear wall portion, heading for the exhaust port. Thus, the particulate material is smoothly led from the first-mentioned linear wall portion to the outlet and taken out from the outlet through the rotary feeder without remaining in the separator housing. As a consequence, the particulate material passes through the drying duct and the tangential separator within a given time period so that the particulate material is subjected to the even drying process. 
     It is possible to increase or decrease the width of a portion of the drying duct, that is in the vicinity of the inlet. In this case, the velocity of the dry gas that flows into the tangential separator is changed, so that the particulate material is dispersed well in the tangential separator. 
     The separating section may further include chutes in plural tiers under the rotary feeder. These chutes are aligned in a vertical direction at given intervals, and the particulate material taken out from the rotary feeder passes through the chutes sequentially while drawing in outside air from between the chutes. Such drawing of outside air promotes cooling of the particulate material. 
     When the particulate material to be dried is cut tobacco for cigarettes, the dry gas may contain superheated steam. In this case, to bring the moisture content of the dried cut tobacco into the range of from 9 to 14 weight percent, it is preferable that the dry gas have a drying temperature in the range of from 160 to 260° C. and absolute humidity in the range of from 2.4 to 11.8 kg/kg. To bring the moisture content of the dried cut tobacco into the range of from 12 to 14 weight percent, it is desirable that the dry gas have a drying temperature in the range of from 160 to 190° C. and absolute humidity in the range of from 2.4 to 11.8 kg/kg. 
     If the cut tobacco is dried on the aforementioned drying conditions, the superheated steam in the dry gas flow reduces components, such as tobacco-specific nitrosamines, phenols, pyridine, quinoline, styrene, and aromatic amines, among components contained in a mainstream smoke of cigarettes. 
     On the other hand, when the cut tobacco impregnated with an impregnant, or liquid carbon dioxide, is subjected to the drying process as a particulate material, the dry gas is not particularly required to contain the superheated steam. If the dry gas contains the superheated steam, it is preferable that the dry gas have a drying temperature in the range of from 250 to 380° C. and absolute humidity in the range of from 2.4 to 11.8 kg/kg in order to bring the moisture content of the dried cut tobacco into the range of from 2 to 9 weight percent. On the contrary, if the dry gas contains no superheated steam, it is desirable that the dry gas have a drying temperature in the range of from 200 to 300° C. to bring the moisture content of the dried cut tobacco into the range of from 9 to 12 weight percent. 
     In addition, when the dry gas contains the superheated steam, it is preferable that the gas flow path form a circulation passage for the dry gas, and that the flash dryer further comprise exhaust means for discharging at least 10 percent of flow rate of the dry gas from the circulation passage. If part of the dry gas is discharged during the circulation of the dry gas in this manner, the dry gas flow running through the drying duct can contain fresh superheated steam, whereby the effect of reducing the above-mentioned components can be retained. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a view showing a schematic structure of a flash dryer; 
         FIG. 2  is a cross sectional view of a drying duct; 
         FIG. 3  is a cross sectional view of a receiving section according to one embodiment; 
         FIG. 4  is a longitudinal sectional view of a tangential separator; 
         FIG. 5  is a cross sectional view of a modified venturi duct; 
         FIG. 6  is a graph showing distribution of passing time of cut tobacco when the cut tobacco as a particulate material passes through the flash dryer; and 
         FIG. 7  is a graph showing a fracture degree of the cut tobacco with respect to the flow velocity of a dry gas in the drying duct. 
     
    
    
     BEST MODE OF CARRYING OUT THE INVENTION 
       FIG. 1  schematically shows a flash dryer for use in a drying process of cut tobacco as a particulate material. 
     The flash dryer has a gas flow path  2 , in which a circulation fan  4  and a heater  6  are disposed in order. The circulation fan  4  sends gas such as air toward the heater  6 . The heater  6  heats the gas up to a given temperature, more specifically in the range of from 160 to 300° C., preferably from 180 to 260° C. 
     A steam-feeding pipe  8  extends from a portion of the gas flow path  2 , that is located in between the circulation fan  4  and the heater  6 . The steam-feeding pipe  8  is connected to a steam-feeding source. There is disposed a steam-feeding valve  10  in the steam-feeding pipe  8 . When the steam-feeding valve  10  is opened, steam is supplied from the steam-feeding source through the steam-feeding pipe  8  to the gas in the gas flow path  2 . This produces a dry gas flow that contains superheated steam in the gas flow path  2 . In this case, temperature of the dry gas flow is in the range of from 160 to 190° C., and absolute humidity thereof is in the range of from 2.4 to 11.8 kg/kg. 
     The gas flow path  2  has a horizontal duct  12 , which is disposed downstream of the heater  6 . The horizontal duct  12  is connected to a receiving section  14 , and the cut tobacco as a particulate material is fed from the receiving section  14  into the gas flow path  2 . 
     Extended from the receiving section  14  is a drying duct  16 , which is connected to a tangential separator  18  serving as a separating section. The drying duct  16  forms a part of the gas flow path  2 , or a drying flow path. 
     As is obvious from  FIG. 1 , the drying duct  16  curves upward in a convex shape viewed as a whole and smoothly connects the receiving section  14  to the tangential separator  18 . 
     Accordingly, the dry gas in the gas flow path  2  runs through the receiving section  14  into the drying duct  16 , and a velocity rate of the dry gas at this moment is in the range of from 13 to 40 m/s. 
     There is provided a return flow path  20  extending from an exhaust port of the tangential separator  18 , the return flow path  20  is connected to the circulation fan  4 . There is disposed a cyclone separator  22  in the return flow path  20 . 
     An exhaust pipe  24  branches from the gas flow path  2  and extends from between the circulation fan  4  and a steam-connecting pipe  8 . In the exhaust pipe  24 , there are disposed an exhaust control valve  26  and an exhaust fan  28  in order. The exhaust fan  28  extracts at least 10 percent of flow rate of the dry gas flow running through the gas flow path  2  to the exhaust pipe  24  and discharges the same. 
     The drying duct  16  has an upstream-side duct portion  16   a  and a downstream-side duct portion  16   b  with in a flowing direction of the dry gas flow. The upstream-side duct portion  16   a  is connected to the receiving section  14 , and the downstream-side duct portion  16   b  to the tangential separator  18 . 
     As illustrated in  FIG. 2 , the drying duct  16  has a rectangular flow-path cross section. The flow-path cross-sectional area may be either constant along the longitudinal direction of the drying duct  16  or varied. When the height and width of the flow-path cross section are indicated by H and W, respectively, the ratio of height H to width W, namely R(=H/W), is 1 or less. 
     The upstream-side duct portion  16   a  is a single upstream-side duct portion  16   b  which extends substantially straight in the upward sloping direction. More specifically, an angle between a horizontal plane and the upstream-side duct portion  16 , or elevation angle θ, is in the range of from 30 degrees to 60 degrees. The downstream-side duct portion  16   b  is a single downstream-side duct portion  16   b  which curves upward in a convex shape. Ends of the single downstream-side duct portion  16   b  are smoothly, or tangentially, and directly connected to an upper end of the upstream-side duct portion  16   a  and directly connected to a horizontally extending inlet  50  of the tangential separator  18 , respectively. The single downstream-side duct portion  16   b  has a curvature radius R in the range of from 6 to 20 m, and a length of the drying duct  16  from a starting end thereof to an outlet of the tangential separator  18  is in the range of from 8 to 15 m. The horizontally extending inlet  50  of the tangential separator  18  is located in a position at a higher level than a single downstream end of the upstream-side duct portion  16   a . In addition, the single downstream-side duct portion  16   b  of said duct portion  16  curves in a convex shape so that a curvature from the upstream end of the downstream-side duct portion  16   b  to the downstream end of the downstream-side duct portion  16   b  is limited to an angle that is substantially equal to the elevation angle θ. 
       FIG. 3  shows the receiving section  14  in detail. 
     The receiving section  14  comprises a venturi duct  30 , which connects the horizontal duct  12  to the drying duct  16  or the upstream-side duct portion  16   a . A flow-path cross section of the venturi duct  30  is rectangular as well as that of the drying duct  16 , and width of the flow-path cross section is constant in the flowing direction of the dry gas flow. 
     The venturi duct  30  has a throat  32 . When the dry gas passes through the throat  32 , the flow velocity of the dry gas is increased. More specifically, the flow velocity of the dry gas passing through the throat  32  is higher than that of the dry gas running through the drying duct  16 . 
     The throat  32  is formed by caving a part of a top wall of the venturi duct  30  and includes an upstream-side top portion  34  and a downstream-side top portion  36 . The top portions  34  and  36  form a substantial V-shape in a longitudinal section of the venturi duct  30 . That is, the upstream-side top portion  34  is slanted toward a bottom wall of the venturi duct  30 , whereas the downstream-side top portion  36  is inclined in a direction getting away from the bottom wall side of the venturi duct  30  to extend to the drying duct  16 . 
     The bottom wall of the venturi duct  30  includes an upstream-side bottom portion  31  and a downstream-side bottom portion  33 . The upstream-side bottom portion  31  extends straight from the horizontal duct  12  to the throat  32 , that is, a location where the flow-path cross-sectional area of the venturi duct  30  is minimum. Oblique angles α 1  and α 2  formed by the top portions  34  and  36  with respect to the upstream-side bottom portion  31  are each in the range of from 2° to 20°. It is preferable that the oblique angle α 1  be larger than the oblique angle α 2 , whereby the flow-path cross-sectional area of the venturi duct  30  is sharply decreased up to the throat  32  and then gradually increased from the throat  32 . 
     The downstream-side bottom portion  33  of the venturi duct  30  is formed in the shape of a substantial V in the longitudinal section of the venturi duct  30 . In other words, the downstream-side bottom portion  33  has a deep region  38  downstream of the throat  32 . Therefore, after being decreased temporarily at the throat  32 , the flow-path cross-sectional area of the venturi duct  32  is increased by degrees toward the deep region  38  located downstream of the throat  32  and gradually decreased from the deep region  38  toward the drying duct  16 . 
     The downstream-side bottom portion  33  has an inclined surface  39  extending from the throat  32  to the deep region  38 . An oblique angle β formed by the inclined surface  39  with respect to the upstream-side bottom portion  31  is identical to the oblique angle α 1  of the upstream-side top portion  34 . Accordingly, the inclined surface  39  and the upstream-side top portion  34  are parallel to each other. This means that the dry gas flow, which has passed through the throat  32 , proceeds without being detached from the inclined face  39 . That is, concerning the flow-path cross-sectional area of the venturi duct  30 , an increasing rate of the cross-sectional area of the flow path downstream from the throat  32  is so set as not to detach the dry gas flow from the bottom wall of the venturi duct  30 . 
     The downstream-side top portion  36  of the venturi duct  30  has the same elevation angle as the upstream-side duct portion  16   a  of the drying duct  16 . 
     Additionally, the horizontal duct  12  may have either a rectangular flow-path cross section as well as the venturi duct  30  or a circular flow-path cross section. 
     There is formed a feed port  40  being open in the downstream-side top portion  36  of the venturi duct  30 , and the feed port  40  is located immediately downstream of the throat  32 . An outlet of the rotary feeder  42  is directly connected to the feed port  40 , and an inlet of the rotary feeder  42  is connected to a feeding line  44  of cut tobacco. 
     The rotary feeder  42  includes a cylindrical casing and a rotor rotatably located in the casing, the rotor having a plurality of pockets  46  on an outer peripheral surface thereof. The pockets  46  are arranged at regular intervals in a circumferential direction of the rotor. When the rotor isrotated, one of the pockets  46  is connected to the inlet of the rotary feeder  42  or the housing thereof. At this moment, the pocket  46  receives cut tobacco from the feeding line  44 . Thereafter, the received cut tobacco is transferred with the rotation of the rotor toward the outlet of the housing along with the pocket  46 . When the pocket  46  coincides with the outlet, the cut tobacco in the pocket  46  is supplied into the venturi duct  30  through the feed port  44 . 
     The rotor of the rotary feeder  42  is rotated counterclockwise direction as viewed in  FIG. 3 . Accordingly, when each of the pockets  46  passes through the outlet of the housing, a transferring direction of the pocket  46  coincides with a flowing direction of the dry gas flow in the venturi duct  30 . 
     In this case, the cut tobacco fed to the rotary feeder  42  is to be subjected to an expansion process by means of flash drying, and has a high moisture content. Specifically, the moisture content of the cut tobacco is adjusted to the range of from 17 to 35 weight percent, preferably from 18 to 25 weight percent. 
       FIG. 4  shows the tangential separator  18 . 
     The tangential separator  18  comprises a cylindrical separator housing  48 , which has a horizontal axis and an inlet  50 . The inlet  50  is located in the top portion of an outer periphery of the separator housing  48  and protrudes in a tangential direction to the outer periphery of the separator housing  48 , that is, in the horizontal direction. The inlet  50  is smoothly connected to a downstream end of the downstream-side duct portion  16   b  of the drying duct  16 . Therefore, the inlet  50  also has a rectangular flow-path cross section, and the thickness of the separator housing  48  along the horizontal axis is identical to the width of the drying duct  16 . 
     As is clear from  FIG. 4 , the downstream end of the downstream-side duct portion  16   b  has a bottom inclined upward in a direction to the inlet  50 . 
     The separator housing  48  further has an outlet  52 , which is located in the bottom portion of the outer periphery of the separator housing  48 . The outlet  52  is directly connected to an inlet of a rotary feeder  54  which is similar to the rotary feeder  42 . 
     The separator housing  48  includes a peripheral wall including a guide wall  56  in the shape of a circular arc, that extends from the inlet  50  to the outlet  52 , and a guide wall  58  in the shape of a circular arc, that extends from the outlet  52  to the inlet  50  in an inflow direction of the dry gas flow from the inlet  50 . The guide walls  56  and  58  have linear wall portions  60  and  62  in lower portions thereof, respectively. The linear wall portions  60  and  62  are located away from each other in the rotating direction of the rotary feeder  54  and extend toward the outlet  52  convergently. As is apparent from  FIG. 4 , the outlet  52  has an axis inclined at a given angle γ (for example, γ=0° to 30°) with respect to a vertical plane. Therefore, the rotary feeder  54  is also connected to the outlet  52  while being inclined. 
     One end wall of the separator housing  48  has an exhaust port  64 , which is connected to the return pipe  20 . As is obvious from  FIG. 4 , the exhaust port  64  is located closer to the guide wall  58  than to the guide wall  56  and closer to the inlet  50  than to the outlet  52 . The separator housing  48  may be provided with the exhaust port  64  in each of the end walls thereof. In this case, both the exhaust openings  64  are each connected to the return pipe  20 . 
     As illustrated in  FIG. 1 , a plurality of chutes  66  are arranged in a line in the vertical direction under the outlet of the rotary feeder  54 . Each of the chutes  66  has a hopper-like upper end, and there is assured a given space between two vertically adjacent chutes  66 . 
     Operation of the flash dryer will be described below. 
     Once the dry gas flow is led into the venturi duct  30 , the dry gas flow is directed upward in the venturi duct  30 . At this moment, flux of the dry gas flow is squeezed toward the throat  32 , and the dry gas flow then passes through the throat  32  while the flow velocity thereof is increased. 
     As mentioned before, the venturi duct  30  has the flow-path cross section that is constant along the longitudinal direction of the venturi duct  30 , and the deep region  38  located downstream of the throat  32  temporarily increases the flow-path cross section of the venturi duct  30 . In other words, the upstream-side top portion  34  of the throat  32  and the inclined face  39  forming the deep region  38  are parallel to each other. Therefore, the dry gas flow that has passed through the throat  32 , as shown by arrow X in  FIG. 3 , proceeds largely to the deep region  38  and is thereafter returned from the deep region  38  toward the center of the venturi duct  30  to be led to the drying duct  16 . 
     Accordingly, after passing through the throat  32 , the dry gas flow proceeds away from the feed port  40 , so that the dry gas flow never hinders the supply of the cut tobacco from the feed port  40  into the venturi duct  30 . As a consequence, the cut tobacco is easily fed into the venturi duct  30 . 
     The flow path of the venturi duct  30  located downstream from the throat  32  is not bent at the deep region  38 . Therefore, the cut tobacco supplied from the feed port  40  does not remain in the deep region  38 . After being dispersed well in the deep region  38 , the cut tobacco is returned to the center of the venturi duct  30 , which prevents the cut tobacco from being led to the drying duct  16  in masses. 
     Moreover, the venturi duct  30  includes a downstream portion having an elevation angle θ identical to the elevation angle of the drying duct  16 , and this generates a sharp rise of the dry gas flow in the venturi duct  30 . Such an ascendent flow of the dry gas further promotes the dispersion of the cut tobacco. 
     Thereafter, the cut tobacco is led along with the dry gas flow from the venturi duct  30  into the drying duct  16 . The upstream-side duct portion  16   a  of the drying duct  16  is formed in a straight line, whereas the downstream duct portion  16   b  is formed in the shape of a moderate circular arc, so that the drying duct  16  has no flection. Thus, the cut tobacco smoothly runs through the drying duct  16  along with the dry gas flow as it is evenly dispersed in the drying duct  16 . This means that the cut tobacco is led to the tangential separator  18  without remaining in the drying duct  16 , and that the time required for the cut tobacco to pass through the drying duct  16  is substantially constant. 
     Consequently, when passing through the drying duct  16 , the cut tobacco, which is evenly dispersed in the drying duct  16 , is brought into satisfactory contact to the dry gas flow at a whole surface thereof, and moreover the time required to pass through the drying duct  16  is substantially constant, so that the cut tobacco can be evenly dried in the drying duct  16 . As a result, the cut tobacco is prevented from being overdried or deficiently dried, which enables an even drying process of the cut tobacco and avoids a deterioration in flavor and taste of the cut tobacco. 
     Since the flow-path cross-sectional area of the drying duct  16  is constant along the longitudinal direction of the drying duct  16  as described above, when the cut tobacco passes through the drying duct  16 , collision of the cut tobacco against an inner wall of the drying duct  16  is suppressed. Therefore, even if the particulate material to be dried is cut tobacco that is relatively prone to be fractured, the cut tobacco is prevented from being fractured, and the quality of the drying-processed cut tobacco is improved. In addition, the cut tobacco expanded by the drying process and the cut tobacco obtained by cutting reconstructed tobacco sheets are especially liable to be fractured. 
     Thereafter, the dried cut tobacco is led with the dry gas flow into the inlet  50  of the tangential separator  18 . Since the inlet  50  tangentially protrudes from the outer periphery of the separator housing  48 , the cut tobacco can flow into the separator housing  48  through the inlet  50  without difficulty. That is, the cut tobacco flows toward the outlet  52  while being smoothly guided along the guide wall  56  as shown by arrows Y in  FIG. 4 . As a result, the cut tobacco never collides violently against the guide wall  56  of the separator housing  48 . 
     Air is discharged from the separator housing  48  through the exhaust port  64 . The discharge of air produces in the separator housing  48  a spiral flow shown by dashed lines Z in  FIG. 4  in cooperation with the dry gas flow that flows in from the inlet  50 , and the spiral flow advances toward the exhaust port  64 . Such a spiral flow acts to separate the dry gas flow, which tends to run along the guide wall  56 , from the guide wall  56 . The dry gas flow then collides with the linear wall portion  62  continuing to the outlet  52  and proceeds toward the exhaust port  64 . 
     Once the cut tobacco that runs along the guide wall  56  reaches the linear wall portion  60  continuing to the outlet  52 , the cut tobacco is substantially detached from the dry gas flow. Thereafter, the cut tobacco smoothly flows downward while being guided along the linear wall portion  60 , and is discharged from the outlet  52  through the rotary feeder  54 . Accordingly, the cut tobacco does not remain in the separator housing  48 , and the time required for the cut tobacco to pass through the tangential separator  18  becomes constant, thereby averting the overheating of the cut tobacco in the tangential separator  18 . 
     Consequently, the time required for the cut tobacco that has been fed by the feeding section  14  to be discharged from the tangential separator  18 , namely a total drying time of the cut tobacco, becomes constant, thereby securing the even drying process of the cut tobacco. 
     Specifically, in the case of the flash dryer, the total drying time of the cut tobacco is in the range of from 0.5 to 1.8 sec. This means that the cut tobacco does not remain in the flash dryer and that the overheating of the cut tobacco is prevented. 
     The moisture content of the cut tobacco discharged from the tangential separator  18  is in the range of from 9 to 14 weight percent, preferably from 12 to 14 weight percent. The cut tobacco is rapidly reduced in moisture content. 
     When the cut tobacco is quickly dried in the above manner, the moisture contained in the cut tobacco is rapidly vaporized. Such vaporization of the moisture curls the cut tobacco, which makes the dried cut tobacco into so-called curling cut tobacco. Such curling cut tobacco has a high expansion volume, so that it is possible to reduce a filling density of the cut tobacco in a cigarette. 
     The cut tobacco discharged from the outlet of the rotary feeder  54  falls, sequentially passing the chutes  66  formed in tiers. At this moment, the falling of the cut tobacco draws outside air from between the two vertically adjacent chutes  66  into the chute  66  at a lower side, so that the cut tobacco is satisfactorily cooled by the outside air, which prevents a deterioration in flavor and taste of the cut tobacco. 
     The dry gas flow in the separator housing  48  is discharged from the exhaust port  64  and passes through the cyclone separator  22 . At this moment, the cyclone separator  22  removes fine dust of the cut tobacco and the like from the dry gas flow. 
     Using the cut tobacco dried by the flash dryer, object cigarettes A, B and C were produced. At the same time, using the cut tobacco dried by a general cylinder dryer, comparative cigarettes corresponding to the object cigarettes A, B and C were produced. Contents of components contained in a mainstream smoke produced by these cigarettes were measured, and a result of comparison as to the contents of some components was obtained as shown in TABLE 1. The result of the comparison shown in TABLE 1 indicates a decreasing rate of contents of components contained in smoke of the object cigarettes based on the respective comparative cigarettes. 
     
       
         
           
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 OBJ. 
                 OBJ. 
                 OBJ. 
               
               
                 COMPONENTS 
                 DETAILED 
                 CIGARETTE 
                 CIGARETTE 
                 CIGARETTE 
               
               
                 IN SMOKE 
                 CLASSIFICATION 
                 A (%) 
                 B (%) 
                 C (%) 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
            
               
                 Tobacco-specific 
                 NNN 
                 −2.20 
                 16.20 
                 12.00 
               
               
                 Nitrosamines 
                 NAT 
                 3.30 
                 7.60 
                 3.90 
               
               
                   
                 NAB 
                 22.40 
                 12.10 
                 — 
               
               
                   
                 NNK 
                 22.40 
                 1.20 
                 7.40 
               
               
                 Phenols 
                 Hydroquinone 
                 10.50 
                 10.90 
                 9.80 
               
               
                   
                 Resorcinol 
                 8.60 
                 9.80 
                 5.50 
               
               
                   
                 Catechol 
                 12.00 
                 9.50 
                 8.20 
               
               
                   
                 Phenol 
                 17.20 
                 16.70 
                 10.90 
               
               
                   
                 m-Cresol + p-Cresol 
                 13.50 
                 15.20 
                 10.70 
               
               
                   
                 o-Cresol 
                 10.40 
                 16.10 
                 11.20 
               
               
                   
                 Pyridine 
                 5.00 
                 4.50 
                 3.00 
               
               
                   
                 Quinoline 
                 13.00 
                 8.20 
                 7.20 
               
               
                   
                 Styrene 
                 0.00 
                 0.60 
                 0.50 
               
               
                 Aromatic amines 
                 1-Aminonaphthalene 
                 9.80 
                 10.30 
                 18.00 
               
               
                   
                 2-Aminonaphthalene 
                 8.40 
                 8.30 
                 18.30 
               
               
                   
                 3-Aminobiphenyl 
                 8.00 
                 8.00 
                 13.90 
               
               
                   
                 4-Aminobiphenyl 
                 6.70 
                 8.20 
                 11.80 
               
               
                   
               
            
           
         
       
     
     In TABLE 1, NNN represents nitrosonornicotine, NAT nitrosoanatabine, NAB nitrosoanabasine, and NNK 4-N-nitrosomethylamino-1-3-pyridyl-1-butanone. 
     The cut tobacco of the object cigarettes A, B and C is processed with the above-described flash dryer on the following drying conditions. 
     Temperature of the dry gas flow: 160-190° C. 
     Velocity of the dry gas flow: 17 m/s 
     Absolute humidity of the dry gas flow: 5.6 kg/kg 
     Exhaust ratio of flow rate of the dry gas flow: 50% 
     Moisture content of the cut tobacco before drying: 20 wt % 
     Moisture content of the dried cut tobacco: 13 wt % 
     Feeding flow rate of the cut tobacco before drying: 80 kg/h 
     The cut tobacco of the object cigarettes A and C includes plural kinds of fillers, and these fillers are subjected to the drying process in a lump. The cut tobacco of the object cigarette B also includes plural kinds of fillers, and the fillers are individually subjected to the drying process. More specifically, the object cigarettes A and B are “Mild Seven” (trademark), and the object cigarette C is “Hi-lite” (trademark). 
     The cut tobacco of the comparative cigarettes is subjected to the drying process using a general cylinder dryer. Drying conditions of the cylinder dryer are as follows: 
     Heating temperature of the cylinder wall: 120° C. 
     Temperature of the heated air: 60° C. 
     Absolute humidity of the heated air: 0.1 kg/kg or less 
     Exhaust rate of the heated air: 20% 
     As is obvious from TABLE 1, the cut tobacco of the object cigarettes A, B and C is substantially reduced in contents of the components, such as tobacco-specific nitrosamines, phenols, pyridine, quinoline, styrene, and aromatic amines, that are contained in the mainstream smoke, compared to the cut tobacco of the comparative cigarettes. One possible reason for this is that the cut tobacco is dried not by the heated air but by the dry gas flow. 
     It is possible to further reduce the moisture content of the dried cut tobacco to 9 weight percent by raising the temperature of the dry gas up to 260° C. 
     A solid line in  FIG. 6  shows distribution of time required for the cut tobacco that has been fed from the receiving section  14  to be discharged from the tangential separator  18 , that is, distribution of time required for the cut tobacco to pass through the flash dryer of the present embodiment. Moreover, a dashed line and a double-dashed line in  FIG. 6  indicate the distribution of time for the cut tobacco to pass through the conventional flash dryer. 
     As is apparent from  FIG. 6 , in the case of the flash dryer of the embodiment, variation of passing time of the cut tobacco ranges within ±0.2 sec. This proves that the cut tobacco is evenly dried. The conventional flash dryer having a characteristic shown by the dashed line includes a C-shaped drying duct, and the conventional one having a characteristic shown by the double-dashed line includes an S-shaped drying duct. 
     Furthermore,  FIG. 7  shows a fracture rate of the cut tobacco with respect to the flow velocity of the dry gas in the drying duct. In this case, the fracture rate of the cut tobacco is indicated by deviation between an initial grain diameter (1.9 mm) of the cut tobacco fed from the receiving section  14  and a grain diameter of the cut tobacco discharged from the tangential separator  18 . As is evident from  FIG. 7 , according to the flash dryer of the embodiment, even if the flow velocity of the dry gas is increased, the grain diameter deviation of the cut tobacco is not much increased. On the contrary, in the case of the conventional flash dryer, the greater the flow velocity of the dry gas is, the larger the grain diameter deviation of the cut tobacco is. 
     The present invention is not limited to the above-described embodiment, but may be modified in various ways. 
     For instance, the feeding section  14  illustrated in  FIG. 5 , or the venturi duct  30 , does not have the deep region  38 , but has the bottom wall that linearly extends. Also in this case, since the dry gas flow that has passed through the throat  32  is so directed as to be separated away from the feed port  40 , the supply of the cut tobacco from the feed port  40  into the venturi duct  30  is performed without difficulty. Moreover, even if the deep region  38  is not present, the flow-path cross-sectional area of the venturi duct  30  located downstream from the throat  32  is gradually increased toward the drying duct  16 , so that the cut tobacco is satisfactorily dispersed. 
     Furthermore, the flash dryer of the present invention can be applied to the drying process of cut tobacco impregnated with liquid carbon dioxide as an impregnant. 
     Regarding the specification of the flash dryer in this particular case, only distinctions to the specification of the aforementioned flash dryer will be listed below. 
     Temperature of the dry gas (including the superheated steam): 160-400° C., preferably 250-380° C. 
     Oblique angle β: 0° 
     Moisture content of the dried cut tobacco: 2-9 wt %, preferably 2-7 wt % 
     When the dry gas contains no superheated steam, it is desirable that the dry gas have a temperature in the range of from 200 to 300° C. In this case, the moisture content of the dried cut tobacco is adjusted to be in the range of from 9 to 12 weight percent. 
     Additionally, the flash dryer is applicable not only to the drying of cut tobacco but also to that of many different particulate materials as well. Therefore, modification may be made in specific size, shape and the like of the drying duct  16 , tangential separator  18 , venturi duct  30 , etc., according to particulate materials to be dried.