DK163714B

DK163714B - Atomisation drying apparatus

Info

Publication number: DK163714B
Application number: DK035391A
Authority: DK
Inventors: Mogens Andersen; Leo Staun Christensson; Otto Hulshof
Original assignee: Damrow Co Inc
Priority date: 1988-09-02
Filing date: 1991-02-28
Publication date: 1992-03-30
Also published as: DK35391D0; DK35391A; DK163714C

Description

iin

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Den foreliggende opfindelse angår et forstøvningstørreapparat til tørring af væsker, der indeholder faste stoffer i opløselig eller suspenderet form.The present invention relates to an atomizer dryer for drying liquids containing solids in soluble or suspended form.

5 I US påtentskrifterne nr. 3 615 723 og 3 741 273 er beskrevet forstøvningstørreapparater, der omfatter et tørretårn, forstøverorganer, der er placeret i den øvre del af tørrekammeret, til findeling af væsken, som skal tørres til fine dråber, organer for indføring af opvarmet 10 tørringsgas i den øvre del af tørrekammeret, rettet imod dråberne, en perforeret bevægelig overflade anbragt under den nedre ende af tørretårnet for opsamling af delvis tørrede partikler af fast materiale, organer for udsugning af gas med suspenderede delvis tørrede faste par-15 tikler fra tørretårnet gennem den perforerede bevægelige overflade, mens partiklerne eller en væsentlig del af partiklerne aflejres på den perforerede bevægelige overflade som et agglomereret porøst lag af materiale, og organer til sluttørring af det agglomererede porøse mate-20 rialelag, medens den perforerede overflade bevæges frem ad. Forstøvningsorganerne kan omfatte et antal nedadret-tede dyser til dannelse af væskedråber, som sammen med en laminær strøm af tørringsgas føres ned gennem kammeret. 1 stedet kan der anvendes et roterende hjul til forstøvning 25 af væsken. Tørringsgassen kan også anvendes som en turbulent gasstrøm, der passerer nedad eller cirkulerer i kammeret. De delvis tørrede partikler vil agglomerere og af lej res som et lag på den bevægende perforerede overflade, der kan være et transportbånd, bestående af tråd-30 net eller vævet filtermåtte.In U.S. Patent Nos. 3,615,723 and 3,741,273, atomizing dryers comprising a dryer tower, nebulizing means located in the upper portion of the drying chamber are disclosed for comminuting the liquid to be dried into fine droplets, means for introducing heated drying gas in the upper part of the drying chamber, directed against the drops, a perforated movable surface disposed below the lower end of the drying tower for collecting partially dried solid particles, means for extracting gas with suspended partially dried solid particles from drying tower through the perforated moving surface while depositing the particles or a substantial portion of the particles on the perforated moving surface as an agglomerated porous layer of material, and means for final drying the agglomerated porous material layer as the perforated surface moves forward. The atomizing means may comprise a plurality of downwardly directed nozzles for forming liquid droplets which, together with a laminar flow of drying gas, are passed down through the chamber. Instead, a rotating wheel can be used to atomize the liquid. The drying gas can also be used as a turbulent gas flow passing downward or circulating in the chamber. The partially dried particles will agglomerate and be deposited as a layer on the moving perforated surface which may be a conveyor belt consisting of wire mesh or woven filter mat.

Et tørreapparat af denne art er egnet til tørring af følsomme og let nedbrydelige materialer, såsom fødemidler og farmaceutika, fordi tørringstemperaturen er lav.A dryer of this kind is suitable for drying sensitive and easily degradable materials, such as food and pharmaceuticals, because the drying temperature is low.

For at få et sådant apparat til at køre tilfredsstillende er det en fordel at danne laget af partikler på trans- 35 2In order to make such an apparatus run satisfactorily, it is advantageous to form the layer of particles on the transporter.

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portbåndet som et åbent struktureret, stabilt produkt, der har en ensartet tykkelse i tværretningen, hvilket nødvendiggør en jævn fordeling af partiklerne, når de af-lejres. Dette er et mål, der er vanskeligt at opnå, især 5 når man anvender store eller cylinderformede tørrekamre, i hvilket tilfælde laget i princippet (når man anvender en laminær gasstrøm) vil have den største tykkelse i midten og aftagende tykkelse mod kanten af transportbåndet, selv om sidstnævnte dækker hele bunden af kammeret. Selv 10 om adskillige forstøvningsdyser er ensartet fordelt i den øvre del af tørretårnet, således at de delvis tørrede partikler bliver ensartet fordelt i tørringsgassen og suspensionen danner en laminær nedadrettet strøm, vil tykkelsen af det aflejrede lag alligevel variere.the door belt as an open structured, stable product having a uniform thickness in the transverse direction, which necessitates an even distribution of the particles as they are deposited. This is a goal that is difficult to achieve, especially when using large or cylindrical drying chambers, in which case the layer, in principle (when using a laminar gas stream) will have the greatest thickness in the middle and decreasing thickness towards the edge of the conveyor belt. although the latter covers the entire bottom of the chamber. Although several atomizing nozzles are uniformly distributed in the upper part of the drying tower, so that the partially dried particles are uniformly distributed in the drying gas and the suspension forms a laminar downward flow, the thickness of the deposited layer will nevertheless vary.

1515

Hvis en roterende skive, anbragt i den centrale øvre del af tørretårnet anvendes som forstøvningsorgan, vil partiklerne slynges ud imod kammervæggene, således at et tykkere lag vil opbygges i de ydre zoner af transportbån-20 det ind i den centrale zone, og dannelsen af en agglome-reret ensartet pulverstruktur opnås ikke.If a rotating disk, located in the central upper part of the dryer tower is used as a nebulizer, the particles will be thrown out against the chamber walls such that a thicker layer will build up in the outer zones of the conveyor belt into the central zone and the formation of a agglomerated uniform powder structure is not obtained.

For at overvinde denne ulempe og opnå lag af partikler med ensartet tykkelse på transportbåndet har det været 25 foreslået at tilvejebringe en turbulent hvirvlende zone mellem kammeret og transportbåndet, jvf. WO-A-89/09372.In order to overcome this disadvantage and to obtain layers of particles of uniform thickness on the conveyor belt, it has been proposed to provide a turbulent swirling zone between the chamber and the conveyor belt, cf. WO-A-89/09372.

Dette forslag løser mere eller mindre problemet med at opnå en i tværretningen ensartet tykkelse af det aflejrede lag af partikler på transportbåndet. Det er dog en 30 ulempe, at partikler som følge af den forholdsvis høje gashastighed, og på grund af indført supplementær procesluft af lej res til et kompakt lag, der vil give en høj modstand mod gasindtrængning og resultere i en formindsket tørrehastighed samt forøge risikoen for blokering af 35 måtten og den understøttende, bevægende, perforerede overflade.This proposal more or less solves the problem of obtaining a uniform thickness in the transverse direction of the deposited layer of particles on the conveyor belt. However, it is a disadvantage that particles due to the relatively high gas velocity, and due to introduced supplementary process air of bearing, are stored in a compact layer which will provide high resistance to gas penetration and result in a reduced drying rate and increase the risk of blocking the 35 mat and the supporting, moving, perforated surface.

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33

Det er et formål med den foreliggende opfindelse at undgå de ovennævnte ulemper, at opnå et lag af ensartet tykkelse og høj porøsitet på transportbåndet eller en tilsvarende perforeret overflade og at forøge tørreappara-5 tets alsidighed.It is an object of the present invention to avoid the aforementioned disadvantages, to obtain a layer of uniform thickness and high porosity on the conveyor belt or a corresponding perforated surface, and to increase the versatility of the dryer.

Apparatet ifølge opfindelsen omfatter et tørretårn, hvis nedre del er udstyret med eller forbundet til et hvirvel-kammer, der omfatter organer til indstilling af mængden 10 og/eller fordelingen af gas, som suges gennem den perforerede overflade. Overfladen er fortrinsvis kontinuert eller intermitterende bevægelig langs bunden af tørretårnet. Den perforerede overflade kan også være et trådnet eller en filtermåtte, der dækker bunden af tørretårnet.The apparatus according to the invention comprises a drying tower, the lower part of which is equipped with or connected to a vortex chamber comprising means for adjusting the amount of 10 and / or the distribution of gas sucked through the perforated surface. The surface is preferably continuous or intermittently movable along the bottom of the dryer. The perforated surface may also be a wire mesh or filter mat covering the bottom of the dryer.

15 Under driften kan nettet eller filtermåtten bevæge sig, når laget af agglomererede partikler har nået en ønsket tykkelse. Ved denne udførelsesform kan apparatet i stedet være udstyret med organer for intermitterende fjernelse af pulveret, der er aflejret på nettet eller filtermåt-20 ten.During operation, the mesh or filter mat may move when the layer of agglomerated particles has reached a desired thickness. In this embodiment, the apparatus may instead be provided with means for intermittently removing the powder deposited on the web or filter mat.

Tårnet er fortrinsvis en aflang nedefter åben cylinder.The tower is preferably an elongated downwardly open cylinder.

Ifølge en foretrukken udførelsesform består hvirvelkammeret af en kappe, der omgiver den nedre ende af tørre-25 tårnet og står i forbindelse med tårnet, idet kammeret er tilsluttet udsugningsorganer til fjernelse af en fraktion af tørringsgassen fra den nedre ende af tørretårnet og eventuelt også har indblæsningsorganer for indføring af en gas i den nedre ende af tørretårnet på visse steder.According to a preferred embodiment, the vortex chamber consists of a sheath surrounding the lower end of the drying tower and communicating with the tower, the chamber being connected to suction means for removing a fraction of the drying gas from the lower end of the drying tower and optionally also having blow-in means. for introducing a gas at the lower end of the dryer at certain locations.

30 Dette arrangement gør det muligt at indstille strømmen af luft, der rettes imod transportbåndet, så der opbygges et løst og porøst lag med foretrukken ensartet fordeling.This arrangement makes it possible to adjust the flow of air directed towards the conveyor belt, so that a loose and porous layer is formed with preferred uniform distribution.

På grund af den høje porøsitet vil partikellaget give mindre modstand mod gennemtrængning af luft, således at 35 sluttørringstrinnet vil forløbe lettere, ensartet og hurtigere. Det er også muligt at indstille fordelingen af luften på en sådan måde, at der opnås en ensartet tykkel-Due to the high porosity, the particle layer will provide less resistance to air penetration, so that the final drying step will be easier, uniform and faster. It is also possible to adjust the distribution of air in such a way that a uniform thickness is obtained.

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4 se.4 se.

Ved indblæsning af sekundær gas gennem åbninger i kammervæggen, er det endvidere muligt at opnå en ønsket styring 5 af vægtemperaturen på kammervæggens inderside, hvilket medfører store fordele, hvor der er tale om tørring af temperaturfølsomme produkter, hygroskopiske produkter eller hvor der tørres ved konditioner tæt ved dugpunktet.Furthermore, by supplying secondary gas through openings in the chamber wall, it is possible to obtain a desired control 5 of the wall temperature on the inside of the chamber wall, which results in great advantages in terms of drying temperature-sensitive products, hygroscopic products or where drying at conditions close at the dew point.

Den indblæste mængde gasart kan i visse processituationer 10 udgøre en mindre mængde af den totale tørreluft og kan være indeholdt i den luftmængde, der suges ud via kapperummet, medens den i andre tilfælde kan medføre et luftoverskud, der delvis afsuges gennem den perforerede overflade under produktmåtten.The blown amount of gas may, in certain process situations 10, constitute a minor amount of the total drying air and may be contained in the amount of air sucked out through the jacket space, while in other cases it may cause an excess of air which is partially sucked through the perforated surface below the product mat. .

1515

Sekundær gas kan også indføres i kammeret fra oven, f.eks. gennem koncentriske kanaler.Secondary gas may also be introduced into the chamber from above, e.g. through concentric channels.

Medens organerne for styring eller indstilling af mængden 20 og/eller fordelingen af den gas, der suges igennem den bevægelige perforerede overflade, normalt består af udsugningsorganer alene, kan det være hensigtsmæssig, på visse positioner, også at tilføre indblæsningsorganer, hvis det er passende for at balancere trykforskellen 25 mellem de to sider af den bevægelige perforerede overflade og eventuelt for fjernelse af partikler fra de nævnte positioner.While the means for controlling or adjusting the amount of 20 and / or the distribution of the gas sucked through the movable perforated surface usually consists of suction means alone, it may be convenient, at certain positions, to supply blowing means if appropriate for balancing the pressure difference 25 between the two sides of the movable perforated surface and optionally for removing particles from said positions.

Ifølge en foretrukken udførelsesform for opfindelsen kan 30 organerne for styring af mængden og/eller fordelingen af gassen, der senere skal suges gennem den bevægelige perforerede overflade, omfatte flere mindre åbninger anbragt ved omkredsen af hvirvelkammeret og forbundet til individuelle indstillelige udsugnings- og/eller indblæsningsor-35 ganer. Indstillingen af luftstrømmen individuelt ved forskellige positioner gør det muligt at sikre en meget ensartet tykkelse af partikellaget på transportbåndet.According to a preferred embodiment of the invention, the means for controlling the amount and / or distribution of the gas to be subsequently sucked through the movable perforated surface may comprise several smaller apertures located at the circumference of the vortex chamber and connected to individual adjustable suction and / or blow-in tubes. -35 whole. The adjustment of the air flow individually at different positions makes it possible to ensure a very uniform thickness of the particle layer on the conveyor belt.

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55

Hvirvelkonstruktionen kan hensigtsmæssigt være et overgangsstykke, hvis nedre ende har et rektangulært tværsnit imod transportbåndet og i hovedsagen samme bredde som transportbåndet.The vortex assembly may conveniently be a transition piece, the lower end of which has a rectangular cross-section towards the conveyor belt and substantially the same width as the conveyor belt.

55

Ifølge en anden foretrukken udførelsesform for opfindelsen er hvirvelkonstruktionen forbundet til et kammer, der omgiver transportbåndet og består af flere afdelinger placeret under båndet, idet hver afdeling har organer for 10 individuel indstilling af trykdifferencen over den pågældende sektion af transportbåndet. Dette arrangement har til formål yderligere at sikre en ensartet tykkelse i tværretningen af laget af partikler, som aflejres på transportbåndet.According to another preferred embodiment of the invention, the vortex assembly is connected to a chamber surrounding the conveyor belt and consists of several compartments located below the belt, each compartment having means for individually adjusting the pressure difference over that section of the conveyor belt. The purpose of this arrangement is to further ensure a uniform thickness in the transverse direction of the layer of particles deposited on the conveyor belt.

1515

Opfindelsen skal i det efterfølgende illustreres nærmere under henvisning til tegningen, hvor fig. 1 viser en foretrukken udførelsesform for apparatet 20 ifølge opfindelsen, fig. 2 er en sektion af hvirvelkonstruktionen langs linie a-a, og 25 fig. 3 er en sektion af hvirvelkonstruktionen, udformet som en overgangsenhed med et rektangulært tværsnit imod transportbåndet.The invention will be further illustrated in the following with reference to the drawing, in which 1 shows a preferred embodiment of the apparatus 20 according to the invention; FIG. 2 is a section of the vortex assembly along line a-a, and FIG. 3 is a section of the vortex assembly, formed as a transition unit having a rectangular cross-section against the conveyor belt.

fig. 4 viser et tværsnit af en hvirvelkonstruktion med 30 organer til indblæsning af sekundær luft i den nedre del af et tørringskammer, og fig. 5 illustrerer en udførelsesform, hvor sekundær gas indføres ovenfra gennem koncentriske kanaler langs 35 kammervæggen.FIG. 4 is a cross-sectional view of a vortex assembly having 30 means for supplying secondary air to the lower portion of a drying chamber; and FIG. 5 illustrates an embodiment in which secondary gas is introduced from above through concentric channels along the chamber wall.

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66

Det i fig. 1 viste apparat omfatter et lodret cylindrisk tørrekammer 1, forstøvningsorganer 2, anbragt i den øvre del af tørretårnet, indføringsrør 3, 3 for opvarmet tørringsgas. Hvirvelkonstruktionen 4 omgiver den nedre ende 5 af tørretårnet. Det indre 5 af hvirvelkammeret står i forbindelse med det indre 6 af den nedre ende af tørretårnet. Tørringsluften med suspenderede dråber passerer nedad, illustreret ved hjælp af pile 7. Luft fra hvirvelkonstruktionen 4 suges ud gennem rør 8, 8, strømmen ind-10 stilles ved hjælp af ventiler 9, 9 således, at en del af tørringsgassen passerer fra den nedre ende af tørrings-kammeret til hvirvelkonstruktionen i retning af pilene 10, 10 og ud gennem rørene 8, 8. Transportbåndet 12 er fuldstændig indeholdt i kammeret 13, og et antal afde-15 linger 14, 15, 16, forbundet til individuelt indstillelige udsugningsorganer 17, 18, 19, anbragt i kammeret 13 under transportbåndet 12. Det indre af hvirvelkammeret er opdelt i flere afdelinger 20, 21, 22, 23, 24, 25, 26 og 27, hvilket ses tydeligere på fig. 2, idet afdelingerne 20 er adskilte ved skillevægge 11.The FIG. 1, a vertical cylindrical drying chamber 1, atomizing means 2, located in the upper part of the drying tower, feed tube 3, 3 for heated drying gas. The vortex assembly 4 surrounds the lower end 5 of the dryer tower. The interior 5 of the vortex chamber communicates with the interior 6 of the lower end of the dryer. The drying air with suspended droplets passes downwards, illustrated by arrows 7. Air from the vortex structure 4 is sucked out through pipes 8, 8, the flow is adjusted by means of valves 9, 9 so that part of the drying gas passes from the lower end. of the drying chamber of the vortex assembly in the direction of arrows 10, 10 and out through the tubes 8, 8. The conveyor belt 12 is completely contained within the chamber 13, and a plurality of compartments 14, 15, 16, connected to individually adjustable suction means 17, 18, 19, located in the chamber 13 below the conveyor belt 12. The interior of the vortex chamber is divided into several compartments 20, 21, 22, 23, 24, 25, 26 and 27, as can be seen more clearly in FIG. 2, the compartments 20 being separated by partitions 11.

Transportbåndet 12 passerer gennem to afdelinger 30, 31, forbundet med udsugningsorganer 32, 33 og til indføringsrør for supplementær tørregas. I disse afdelinger bliver 25 det agglomererede lag 36 af partiklerne sluttørret. Endvidere vil transportbåndet med det tørrede lag af partikler passere endnu et kammer 37, hvortil der er sluttet udsugningsorganer 38 og et indføringsrør 39 for afkølingsgas.The conveyor belt 12 passes through two compartments 30, 31, connected to suction means 32, 33 and to the supply pipe for supplementary dry gas. In these compartments, the agglomerated layer 36 of the particles is finally dried. Furthermore, the conveyor belt with the dried layer of particles will pass another chamber 37 to which suction means 38 and cooling gas feed tube 39 are connected.

3030

Laget af tørrede partikler brydes op ved enden af transportbåndet og opsamles gennem en tragtformet udføringsåbning 40.The layer of dried particles is broken up at the end of the conveyor belt and collected through a funnel-shaped aperture 40.

35 Fig. 3 viser en sektion af en hvirvelkonstruktion udformet som et overgangsstykke med et rektangulært tværsnit 41 tilsluttet transportbåndet 12. HvirvelkonstruktionenFIG. 3 shows a section of a vortex structure formed as a transition piece with a rectangular cross-section 41 connected to the conveyor belt 12. The vortex structure

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7 er opdelt i flere afdelinger 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52 og 53, forbundet til (ikke viste) organer til regulering af trykket i hver afdeling individuelt.7 is divided into several compartments 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52 and 53, connected to (not shown) means for regulating the pressure in each compartment individually.

5 Sekundær luft indeholdende gasformige, væskeformige eller pulveragtige materialer eller additiver, såsom aromastoffer eller antisammenbagningsmaterialer etc. kan indføres gennem en eller flere af afdelingerne til materialet på båndet. Gasformige additiver kan også tilsættes gennem en 10 eller flere af afdelingerne 14, 15 under transportbåndet.Secondary air containing gaseous, liquid or powdery materials or additives such as flavors or anti-caking materials, etc. may be introduced through one or more compartments of the material on the tape. Gaseous additives may also be added through one or more of the compartments 14, 15 below the conveyor belt.

Den i fig. 4 viste hvirvelkonstruktion omfatter en kappe 411, som omslutter den nedre del af et tørrekammer 406. Tørrekammerets væg 401 er udstyret med et antal åbninger 15 420, 421, 322, 423, hvorigennem en sekundær gas kan ind føres i kammeret 406 i retning af pilene 430, 431, 432, 433 fra kappen 411. Derved kan indføres sekundær tørringsgas, som hindrer aflejring af tørrede partikler langs kammervæggen og samtidig sørger for en· mere ensar-20 tet fordeling af de tørrede partikler, der aflejres på en perforeret bund 436, der kan være stationær eller være et bevægeligt transportbånd.The FIG. 4 shows a swirl structure comprising a sheath 411 which encloses the lower part of a drying chamber 406. The wall 401 of the drying chamber is provided with a number of openings 15 420, 421, 322, 423 through which a secondary gas can be introduced into the chamber 406 in the direction of the arrows. 430, 431, 432, 433 from the jacket 411. Thereby, secondary drying gas can be introduced which prevents the deposition of dried particles along the chamber wall and at the same time provides for a more uniform distribution of the dried particles deposited on a perforated bottom 436. there may be stationary or a movable conveyor belt.

En lignende effekt kan opnås ved den på fig. 5 viste ud-25 førelsesform. Figuren udgør et tværsnit af den øverste del af et forstøvningsanlæg, omfattende et kammer 501 med en kammervæg 502. En række cylindriske skillerum rager ned foroven til dannelse af kanaler 505, 506, 507, som udmunder i forskellig højde i kammeret. En forstøverenhed 30 503 med et forstøverhjul er placeret centralt foroven i kammeret. Gasstrømningen for tørringsluft er vist ved pilen 504. Gennem kanalerne 505, 506, 507 kan indføres varierende mængder af sekundær tørringsluft, vist ved pilene 508, 509, 510. Ved regulering af mængderne af tør-35 ringsluft i kanalerne kan opnås en bedre fordeling af det tørrede materiale, ligesom man undgår aflejring af materiale langs kammervæggen.A similar effect can be obtained from the one shown in FIG. 5. The figure is a cross-section of the upper part of a nebulizer, comprising a chamber 501 with a chamber wall 502. A series of cylindrical partitions extend downward to form ducts 505, 506, 507 which open at different heights in the chamber. An atomizer unit 30 503 with an atomizer wheel is located centrally at the top of the chamber. The gas flow for drying air is shown by arrow 504. Through the ducts 505, 506, 507 varying amounts of secondary drying air can be introduced, shown by arrows 508, 509, 510. By controlling the amounts of drying air in the ducts a better distribution of the dried material, as well as avoiding the deposition of material along the chamber wall.

Claims

A spray drying apparatus for drying liquids containing 5 solids in dissolved or suspended form and consisting of a drying tower (1) with a lower opening (6), 10. a nebulizer (2) located in the upper part of the drying bar, to atomizing the liquid to be dried into fine droplets, means (3) for introducing a heated drying gas 15 into the upper part of the dryer, directed against the droplets, a movable, perforated surface (12) disposed below the opening at the lower end of the dryer. collecting partially dried solid particles, 20 means (17, 18, 19) for extracting gas with suspended partially dried solid particles from the drying tower through the perforated surface, while the particles or a substantial portion of the particles settle on the perforated surface as an agglomerated porous layer of material, and means (32,33,38) for final drying of the agglomerated porous layer of material, characterized in that the lower part of the dryer tower (1) is provided with or connected to a vortex structure (4) comprising a chamber (11) surrounding the lower end of the dryer tower (1) and communicating with the tower, which vortex chamber is associated with suction means (17,18,19) for removing a fraction of the drying gas from the lower end of the drying tower (1) and, if desired, also with blowing means (420,421, -422,423) for introducing a gas into the the lower end of the tower at certain positions, the means (17,18,19) for extraction and the optional means (420,421,422,423) for blow-in being individually adjustable.

Apparatus according to claim 1, characterized in that the drying tower (1) is an elongated lower open cylinder.

Apparatus according to claim 1, characterized in that the means (17, 18, 19) for adjusting the amount and distribution of the gas sucked through the movable perforated surface comprise several smaller openings (20-27) arranged at the circumference of the vortex structure. . 15

Apparatus according to claim 1, wherein the perforated movable surface is a conveyor belt (12), characterized in that the vortex structure comprises a transition unit, the lower end of which has a rectangular cross-section in the vicinity of the conveyor belt (12) and in it. substantially the same width as the conveyor belt.

Apparatus according to claim 4, characterized in that openings at the two sides perpendicular to the direction of movement 25 of the conveyor belt (12) are connected to the suction means (17, 18, 19) and that the openings or at least some of the openings at the two sides parallel to the direction of movement are associated with the supply means (420,421,422,423). 30

Apparatus according to claim 4, characterized in that the vortex structure is connected to a chamber (13) surrounding the conveyor belt and comprises several compartments (14,15,16) arranged under the conveyor belt (12), each compartment being provided with means. for individually adjusting the pressure difference across the connected section of the conveyor belt.