AU636405B2

AU636405B2 - Gas distributor and heater for spray drying

Info

Publication number: AU636405B2
Application number: AU65021/90A
Authority: AU
Inventors: Klaus Bro; Ove Hansen; Christian Schwartzbach
Original assignee: Niro AS
Current assignee: GEA Process Engineering AS
Priority date: 1989-09-26
Filing date: 1990-09-21
Publication date: 1993-04-29
Anticipated expiration: 2010-09-21
Also published as: JPH05504090A; AU6502190A; ZA907568B; FI921295A0; CA2026170A1; EP0493491A1; WO1991004776A1; FI921295A

Description

1 Gas distributor and heater for spray drying.

Technical Field The present invention is directed to an gas distributor and heating device for a spray dryer of the type including a feed liquid atomizer in a drying chamber, the device comprising a gas distributor having a gas inlet and means for distributing the gas from the inlet as a uniform flow of gas to a guide duct conveying said flow of gas from the gas distributor to an outlet providing a flow of said gas concentric with the atomizing means, Spray drying is the transformation of a feed material from a fluid state into a dried particulate form by spraying the feed material into a hot drying medium.

It is a one-step, continuous particle-processing operation involving drying. Background information relative to the field in which this invention finds application is provided in Spray Drying Handbook, third edition, K. Masters (John Wiley and Sons, New York).

The invention is applicable to spray drying, spray evaporation and spray reaction operations and processes which usually involve the introduction of a liquid feed material in the form of a solution or a suspension to an atomizing device which sprays small particles of the liquid feed material into a drying chamber where the droplets are dried or reacted and the resuiting desired particulate material is collected and removed in the form of a powder. A rotary atomizing wheel which provides a continuous spray of the liquid droplets is often used at the top of a tower-type drying chamber. In addition to the droplets of liquid feed material, a processing gas in the form of a temperature controlled gaseous medium is also introduced to the tower-type drying chamber to evaporate the moisture i SUBSTITUTE SHEET from the liquid droplets and thus provide the desired particulate material.

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zi WO 91/04776 PCT/DK90/00240 2 Drying chambers of this type utilizing a rotary atomizer wheel and heated air introduced at the top of a drying chamber have been widely used to dry consumable food products, such as milk, to produce chemical products such a kaolin clay, titanium dioxide and calcium carbonate, and for processing waste effluents.

Such processes are critically dependent upon the particle size of the material being dried, the temperature of the drying medium and the timely effective contact of each particle with the drying medium.

Rotary atomizer wheels or spray nozzles are generally used for spraying the feed material into the hot gas medium.

Various types of heating devices have been utilized to provide the hot gaseous medium. Gas heaters of the direct and indirect type have been utilized as well as steam, fuel oil, heat transfer fluids and electricity. Selection of a heat source for a particular process depends upon the product being spray dried as well as the availability, suitability and cost of the energy required in order to heat the gaseous medium. Open oil and gas fired burners may be utilized where products can withstand the high temperature generated by such heat sources as well as contact with the resulting products of combustion. Indirect heaters utilizing electrical heating elements, gas or fuel oil burners may also be utilized to provide a hot gaseous medium when it is necessary to protect the product being sprayed from contact with products of combustion.

The processing or drying gas medium is delivered from a source to a gas distributor or distribution means from which it is supplied to the interior of the spray drying chamber. The gas is typically heated while moving through the delivery system by means of a burner or other heat source located in the gas delivery system before the gas is delivered to the gas distributor. The CEME *EPLACEMENThHer

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WO 91/04776 PCT/DK90/00240 3 heated gas is then conveyed through insulated ducts to the gas distributor for introduction into the drying chamber and subsequent admixture with the feed spray.

Prior Art A typical process and apparatus for providing a hot drying gas around an atomizing device in a spray drying chamber is disclosed in US-A-3 621 902 and US-A-4 227 896. In those processes, the gaseous medium is heated by means of a device located in the gas delivery system remote from the spray drying chamber.

US-A-4 227 896 also discloses an efficient gas distributor having a spiral supply duct and a plurality of vanes which provide a gas flow of substantially constant velocity through a conical guide duct.

US-A-3 499 476 discloses a process for the production of particulate solids from a solution or suspension by nozzle spraying the feed material through a flame zone provided-by an annular burner at the top of a spray tower. In this process, the gaseous products of combustion are introduced directly into the drying chamber with the liquid particles of feed material.

GB-A-1 191 032 discloses apparatus providing a preliminary drying zone wherein feed material is sprayed through a nozzle surrounded by a burner and the gases of combustion intermingle with the liquid particles of feed material. A secondary drying agent is delivered around the spray nozzle and gas burner.

Still another spray drying apparatus utilizing an air heater provided in a' chamber located directly above a spray drying chamber is disclosed in US-A- 4 187, 617. In that apparatus air is delivered by means of a fan through a transition duct to a distribution chamber located, above the spray drying chamber. An air heater and a profile plate are provided in the distri- REPCEMENT8HEET 4 bution chamber together with a perforated diffuser.

Cold air is delivered past the profile plate and heater to ensure that the air is uniformly heated and then received through the perforations in the diffuser and ducted to the spray drying chamber. Due to the fact that the gas diffuser or distributor in that apparatus is provided between the air heater and the spray nozzle, the apparatus is limited to spray drying processes utilizing drying gas heated to only a moderately high gas temperature on the order of 550 C because of the diffuser and wire screens downstream of the air heater.

Conventional systems of producing hot processing or drying gas using either direct or indirect heating means in the form of a furnace or combustion device and then ducting or delivering the hot gas from the heating area into the drying chamber by conventional air distributors are known in the art. Such conventional systems are suited for spray drying processes requiring drying gas temperatures which are not in excess of about 550 C. However, such conventional systems are seldom used in spray drying operations requiring drying gas temperatures near or in excess of 1000 C since conventional systems would require the generation of substancial thermal energy remote from the point of use and extensive use of refractory materials in the delivery system between the heat source and a gas distributor adjacent the spray drying chamber.

Onc object of th present invention i to de a gas distributor, gas heater and atomizer fo high temperature drying system.

A further object of this in ion is to provide a spray drying apparatus ng a drying gas delivery system wherein thn aximum amount of gas distribution is accom 's ed before the gas is heated to the desired _temperature, the gas is then heated and conveyed Sdirectly to the spraying chamber.

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Z <\i A ztill further object is to provide a gac hsater for receiving gas from a cold gas distributo nd having an outlet exiting in the vicinity o e liquid droplets being discharged from an am zer device.

A further object is t rovide a hot processing or drying gas with its- aximum temperature attained adjacent an itom' ng device.

ill further object is to provide a gas heate for high temperature drying systems comprising a circularly disposed cold air distributor.-- The Invention The gas distributor and heating device initially mentioned is, according to the invention characterized by heating means located in the guide duct downstream of the distributing means and at such a distance from the atomizing means that the heat source does not directly contact the liquid particles sprayed therefrom.

By providing the heat source directly upstream of the point of use in the spray drying chamber there is permitted delivery of a cold gas by conventional conduits to a gas distributor adjacent a spray chamber and then heating the gas supplied from the gas distributor as it is conveyed to the spray chamber. The high temperature, hot drying gas is conveyed directly into the drying chamber which minimizes the refractory material required since the high temperature drying gas is exhausted directly from the area where the gas is heated into the drying chamber.

Thus the apparatus of the present invention is particularly suited for use in high temperature spray drying of materials such as iron oxide pigments, magnesium chloride, and titanium oxide, or industrial waste effluents such as an acid solution of iron sulfate and magnesium sulfate wherein drying gas or hot air having

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6 a temperature in the range of 1000 C or more is required.

Despite the high temperature the particulate material produced by the device according to the invention will not be adversely affected by the products of combustion because the heat source and the flame do not directly contact the liquid particles of feed material in the drying chamber.

The guide duct for conveying the flow of gas from the gas distributor is preferably provided in the form of an annular guide duct defined by two spaced surfaces and the outlet provides an annular flow of heated gas concentric with a liquid atomizer. The spaced surfaces may be two radially spaced cylindrical surfaces or conical surfaces and, in the case of an annular guide duct defined by two spaced conical surfaces, the outlet directs an annular flow of heated gas converging toward a centerline of the guide duct. The spaced conical surfaces may be convergent or divergent or a combination of the two to control the velocity of the gas at the outlet of the guide duct. Spaced surfaces comprising a cylindrical surface and a conical surface may also be utilized.

In the preferred embodiments, the gas heater is one or a plurality of gas burners provided internally of an annular guide duct and extending circumferentially around the interior of the guide duct.

In a preferred embodiment there is means deflecting a portion of the gas supplied by the gas distribution means to bypass the gas burner and gases of combustion to provide a flow of gas insulating at least one of the guide duct surfaces from the temperature of the gas burner and the gases of combustion. Preferably, the deflecting means comprises annular guide plates located around the interior of the annular guide duct between the gas burner and each guide duct surface at SUBSTITUTE

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7 a distance from the guide surfaces, said guide plates extending from the gas burner to a position near the outlet of the guide duct.

In certain applications of the invention, it is also preferred that the gas burnr is disposed to emit a flame in the direction of the outlet and the gas burner is located a greater distance from the outlet than a length of the f,ame emitted by the burner so as to ensure that the gase, of combustion forming the flame are completely burned internally of the annular guide duct, In an alternate embodiment of the invention, wherein the gas dj,stribution means is provided in the form of a banjo shaped chamber having a circular portion and an arm extending radially relative to the circular portion and providing a supply inlet, the circular portion includes an annular outlet for supplying an annular flow of gas to the guide duct and at least one continuous circular perforated plate spaced between an interior surface of the circular portion and the annular outlet for distributing a uniform flow of gas from the supply inlet to the annular outlet, and the heating means comprises a plurality of gas burners arranged in a circular pattern for uniformly heating the uniform flow of gas supplied by said gas distributing means.

The gas distributor and heating device of the present invention may be utilized with atomizer wheels or spray nozzles. When used with an atomizer wheel, it is preferred that the guide duct provide an annular flow of heated gas concentric with the atomizer wheel.

In applications using spray nozzles, it is preferred that a plurality of spray nozzles be arrayed at the top of a spray chamber and the guide duct provide a flow of heated gas substantially centered with the top of the spray chamber and concentric with the surrounding array S. SUBST1TUTS

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of spray no7zles. The gas distributor and heating device may also be used with spray nozzles stpraying from below in a fountain configuration.

Brief Description of the Drawings The present invention and the advantages provided thereby will be more fully understood with reference to the following detailed description of the preferred embodiment taking in conjunction with the accompanying drawings.

Fig. 1 is an elevation view, partly broken away, showing a spray drying plant utilizing one embodiment of the present invention; WO 91/04776 PCT/DK90/00240 9 Fig. 2 is an enlarged sectional view showing the structure of a gas distributor and heating device of Fig. 1; Fig. 3 is a plan view showing the interior of the gas distributor of Fig. 2; Fig. 4 is a view similar to Fig. 2 showing an alternate guide duct for heating and conveying gas supplied by a gas distributor as shown by Figs. 1 and 3; Fig. 5 is a plan view similar to Fig. 3 showing an alternate gas distributor and heating device; Fig. 6 is a sectional view taken along line VI-VI of Fig. Fig. 7 is a plan view similar to Fig. 5 and showing a modification of the structure shown by Fig.

5; and Fig. 8 is a partial elevation view of an alternate embodiment of the spray drying apparatus of the present invention.

Preferred Embodiments With reference to the drawings wherein like reference numerals designate the same or like parts throughout, there is shown in Fig. 1 a spray drying plant designated by the reference numeral 10 which is particularly adapted for high temperature drying of ar industrial waste effluent such as an acid solution derived as a waste product of a netal treating facility.

The plant 10 is several stories high and includes a spray or drying chamber 12 and a penthouse 14 supported by means of a superstructure 11. The penthouse 14 provides an enclosure and work area for servicing a gas distributor 20 and a rotary atomizing wheel 30 provided adjacent a roof or top closure 15 of the spray drying chamber 12. The spray drying chamber 12 has a cylindrical outer wall which is substantially closed at the top SUBSTITUTE

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WO 91/04776 PC//DK9/00240 by the closure 15 and by a funnel or conical-shaped lower portion 18. An opening 16 is provided in the top closure 15 and a valve 19 is provided at the bottom of the lower portion 18 to permit removal of the particulate materials dried in the spray chamber 12. The spray chamber 12, opening 16 and the funnel-shaped lower portion 18 are substantially symmetrical about a vertical axis.

A gas distributor 20, as best shown by Fig. 2, is mounted in the penthouse 14 with a conical-shaped guide duct 40 depending to a position substantially aligned with the top closure 15 and centered in the opening 16. The rotary atomizer wheel 30 is rotatably mounted in a housing 31 supported internally of the gas distributor 20 to project into the top cf the spray drying chamber 12.

Although the rotary atomizer wheel 30 is provided at the top of the spray drying chamber 12 in Fig.

1, the atomizer device may be one or more spray nozzles located at the top of the spray drying chamber or elsewhere in the spray drying chamber and spraying in the direction of the gas distributor A pressurized source of cold gas usually in the form of ambient air is provided to the gas distributor by means of a fan 2 and inlet or delivery duct 4 connected to the gas distributor 20. An outlet or exhauw.

port provided in the funnel shaped lower portion 12 at one side of the plant is connected by means of an exhaust duct or pipe 17 to a fines separator 6 which may be in the form of a cyclone separator. The fine particles are retained in the separator 6 where they may be periodically withdrawn and the gas is exhausted through a conduit 7 to an exhaust fan 8 where the gas may be exhausted to atmosphere or recycled by duct work to the fan 2. The fan 2 and inlet duct-4 comprise a delivery system for delivering cold processing gas to the

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WO 1/04776 PCT/DK90/0024( 11 gas distributor 20. The outlet duct 17, separator 6, exhaust duct 7 and exhaust fan 8 comprise means for withdrawing the processing gas frc ,he spray chamber 12.

The gas distributor 20 is of the type disclosed in US-A-4 227 896 and includes an inlet 21 opening to a spiral supply duct 22 which is internally opened to an annular transition zone 23. The transition zone 23 includes two succeeding sets of stationary guide vanes and 26. The vanes 25 are shaped to deflect the flow of gas from a purely tangential flow as indicated by the arrows adjacent the inlet 21 to a flow direction as shown by the arrows 27 in which the radial velocity component toward a center axis of the gas distributor 20 exceeds the tangential velocity component and the vanes 26 of the other set project into a space between the vanes 25 and extend substantially parallel to the direction which the gas flow has been deflected by the adjcent vane 25. The vanes 25 and 26 in the annular transition zone change the direction of flow of the gas supplied to the gas distributor 20 and direct the gas to a mouth 42 of an annular guide duct 40 while the vanes 26 also minimize turbulence in the flow. As best shown by Figs. 2 and 3, the internal dimensions or volume of the spiral duct 22 constantly decreases in the direction of gas flow from the inlet 21 around the annular transition zone so as to supply a substantially constant volume and velocity of gas moving from the spiral duct through the annul transition zone 23 to the mouth 42 of the annular guide duct The annular guide duct 40, as best shown by Fig.

2, is comprised of a hollow conical shaped insulating member 41 which may also provide support for the atomizer wheel drive housing 31. A conical shaped plate 24 is radially spaced from the insulating member 41 and the annular guide duct 40 is provided in the conically SUBSTITUTE SHEET WO 91/04776 PCr/ DK90/00240 12 shaped radial space between the insulating member 41 and the plate 24.

A ring shaped or annular gas burner 50 is also provided internally of the annular guide duct between the insulating member 41 and plate 24. As shown by Fig. 2, the gas burner 50 is disposed to emit the gases of combustion or direct the flame in the direction of the outlet 44 of the annular guide duct In the preferred embodiment, an annular plate 46 or series of plates are provided around the interior of the annular guide duct 40 between the gas burner 50 and the outer metal plate 24 to deflect a portion of the gas supplied by the gas distributor around the burner and serve as a heat shield between the burner and the outer plate 24. The plate 46 may be attached at 48 to the outer conical plate 24 and shields a portion of the gas flowing from the gas distributor from direct contact with the burner and the flame or gases of combustion emitted by the burner. The plate 46 thui provides an insulating flow of cold gas between the metal plate 24 providing one side of the annular guide duct and the temperature of the burner and the gases of combustion.

The feature of cooling one or both of the walls of the annular guide duct by means of an insulating flow of cold gas is an important feature particularly in extremely high temperature drying applications of the present invention. For examplc., in a process for spray drying industrial waste effluents, it may be desirable to convey gas heated to a temperature in the range of about 700 to 1200 C in order to dry the desired material. The temperature of a flame issuing from a burner such as the burner 50 shown by Fig. 2 may be as high as 2,000 C. Such high temperatures significantly limit the choice of materials which may be utilized in a structure adjacent such a high temperature

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WO 91/04776 PCT/DK90/00240 13 flame. Although a number of ceramic and refractory materials are capable of withstanding such high temperatures, some of the materials are not capable of providing long term service in such a high temperature environment and such materials are both heavy and difficult to work with and the most temperature resistent materials are relatively expensive. Thus, although ceramic and refractory materials may be used for flame and high temperature resistance, it is of course desirable to have other devices and arrangements whereby more conventional high temperature steel plates may be suitably formed and used in such environments. This is significant since most high temperature steel and other metal plate materials cannot withstand temperatu, greater than about 550 C, and therefore special arrangements or devices must be provided if one wishes to obtain the low cost and fabrication advantages offered by such materials. The heat shield guide plates or deflector plates 46 which provide an insulating flow of cool gas between the burner 50 and the outer conical plate 24 of the annular duct 40 provide such an advantageous arrangement.

The temperature of the flame /i gases of combustion emitted from the burner 50 with 2Q% surplus 0 air is on the order of about 1700 C. By appropriately spacing and positioning the guide plates 46 in the annular guide duct 40 to bypass 40% of the air supplied by the gas distributor 20 around the burner 50, the average gas temperature after mixing in the vicinity of 0 the outlet 44 will be on the order of 1200 C with the advantage that the insulating air flowing between the guide plate 46 and the outer conical plate 24 of the annular guide duct 40 will have served to thermally insulate the outer conical plate 24 from the high temperature of the flame and. also provide a, cooling effect to the guide plate 46 as the air flows past or through

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WO 91/04776 PCT/DK90/00240 14 that plate. If 50% of the air is bypassed around the burner 50 and flame, the gas temperature after mizing will be on the order of about 1000 C and if 68% of the air is bypassed, the temperature of the gases after mixing will be about 715 C. Thus, substantial amounts of the air supplied by the gas distributor 20 may be bypassed around the burner 50 for cooling purposes and still provide a gas mixture at the outlet 44 of the annular duct 40 that has a temperature in the range of between about 715 C and 1200 C.

The length of the flame or gases of combustion to be emitted from a burner such a the burner 50 shown in Fig. 2 can be calculated or determined by experimentation. In the embodiment shown in Fig. 2, the burner 50 is positioned in the annular outlet 40, a distance d from the outlet 44 of the annular guide duct which is greater than the flame length 1. This in turn ensures that the droplets of feed solution being dispersed by the atomizer wheel 30 will only be contacted by hot gas conveyed through the annular guide duct 40 and never have direct contact with the flame.

In other embodiments, the burner 50 may be positioned in the annular outlet 40, a distance d from the outlet 44 which is only 1/2 to 2/3 the flame length to permit the flame tip; the maximum temperature of the flame, to project out of the guide duct 40 without impinging on the spray issuing from the atomizing device.

In the preferred embodiment, a plurality of directional vanes 49 are also spaced circumferentially around the annular guide duct 40. The directional vanes 49 may be fixed in place or adjustable to direct the flow of gas conveyed through the annular guide duct in a desired direction. That is, after the gas has been uniformly distributed and supplied to the mouth 42 of the annular guide duct 40, it may be desirable to chan-

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WO 91/04776 PCT/DK90/00240 ge the direction of gas flow so as to not only converge toward but rotate around the atomizer wheel 30. Directional vanes such the vanes 49 would be located in the flow of gas downstream from the vanes 25 or such other means as may be utilized for uniformly distributing the gas delivered to the annular guide duct 40 and upstream of the gas burner 50 so as not to interfere with the uniform distribution of the gas flow on the one hand and to avoid exposure to the high temperature of the gas burner and flame on the other hand.

The operation of the high temperature, waste effluent spray drying plant shown by Figs. 1-3 will now be briefly described. A source of pressurized ambient air is provided by the compressor 2 and inlet 4 to the gas distributor 20 where it is uniformly distributed around and supplied with a substantially uniform velocity to the mouth 42 of the annular guide duct 40. The annular guide duct 40 serves as means for conveying the flow of gas from the gas distributor 20 to the outlet 44 in a flow that converges toward the atomizer wheel The gas burner 50 is located internally of the annular guide duct 40 between the gas distributor 20 and the outlet 44 and heats the gas conveyed through the annular guide duct 40 to a predetermined temperature suitable for drying droplets of a feed solution dispersed by the atomizer wheel 30. The industrial waste effluent in the form of a liquid feed solution is conveyed by conventional means to the atomizer wheel 30 where it is dispersed in the form of small droplets and is distributed by centrifugal force radially across the top of the spray chamber 12 thereby forming an atomization zone radially outward of the atomizer wheel The high temperature gas conveyed by the annular duct converges toward the atomizer wheel 30 and evaporates the liquid content of the droplets permitting the solids to fall to the funnel shaped lower portion 18 REPLACEMENT SHEET WO 91/04776 7PCf/ )K90/0)t0240 16 where they may be removed. The drying gas is withdrawn through the outlet or exhaust conduit 17 and delivered to a separator 6 where fine particles retained in the drying gas may be recovered. The gas is then withdrawn from the separator 6 through the conduit 7 by exhaust fan 8 where the gas may be exhausted to atmosphere or returned partly as source gas to the fan 2. In the event that other gases may be released from the droplets of feed solution and mixed with the drying gas, it may of course be desirable to provide a scrubber or other device to separate the additional gases released during the drying process from the drying gas before the drying gas is exhausted to atmosphere or returned to the compressor.

Although the operation of the spray drying plant as shown in Fig. 1 has been described in terms of providing a source of cold gas or air to the gas distributor 20, it should of course be understood that the drying gas provided to the gas distributor 20 may be warm or preheated and then the burner 50 serves to increase the temperature of the warm or preheated source gas to the desired temperature.

Figure 4 illustrates an alternate annular guide duct 52 which may be utilized in place of the annular guide duct 40 shown by Figs. 1-3. In Figure 4, a gas distributor 20 similar to that shown by Fig. 3 distributes a uniform flow of gas to an annular guide duct 52 provided by two radially spaced cylindrical surfaces in the form of the interfacing surfaces of metal plates 53 and 54. A uniform and constant velocity flow of gas is supplied by the gas distributor 22 by means of the vanes 25 and 26 to a mouth 51 of the annular guide duct 52. Directional vanes 49 are provided in the annular guide duct 52 upstream of a pair of annular burners and 56. In this embcdiment, a pair of annular shield or guide plates 57 and 58 are provided internally of the

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WO 91/04776 PCT/DK90/00240 17 annular guide duct 52 to shield the metal plates 53 and 54 from the high temperature of the burners 55 and 56 and the flames issuing therefrom. The guide plate 57 is provided between the burner 55 and a surface of the metal plate 53 and the guide plate 58 is provided between the burner 56 and a surface of the metal plate 54 to provide an insulating flow of cold air between the deflector plates 57 and 58 and the metal plates 53 and 54, respectively, and thereby insulate the metal plates 53 and 54 from the high temperature of the burners and 56 and the flames issuing therefrom.

The annular guide duct 52 of Fig. 4 would operate in the waste effluent spray drying plant shown by Fig. 1 in much the same manner as the annular guide duct 40 shown by Fig. 2. The major difference is that the annular guide duct 52 of Fig. 4 would convey a flow of gas in the form of an annular column of air into the atomization zone concentric with the atomizer wheel rather than providing a flow of drying gas converging toward the atomizer wheel Figs. 5 and 6 illustrate an alternate gas distributor and heating device which could be utilized in the high temperature spraying plant of Fig. 1 in place of the gas distributor 20 and annular guide duct 40. In Figs. 5 and 6 there is shown a banjo shaped gas distributor 60 having a generally circular portion 61 and a radially extending arm 62 providing a processing gas inlet 63 similar to the inlet 21 of the gas distributor 20. A plurality of vanes 64 are provided at the inlet 63 to distribute the processing gas around the interior of the circular portion 61. A pair of circular perforated plates 65 and 66 are provided internally and concentric with the circular portion of the gas distributor 60 to diffuse and provide a gas flow of substantially uniform velocity in the direction of a mouth 67 of an annular guide duct 68. A plurality

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WO091/04776 61"CT/DK90/00240 18 of vanes such as the vanes 25, 26 of Fig. 2 may also be utilized instead of the perforated plates 65 and 66 to distribute the drying gas.

A plurality of gas burners 69 are provided internally of the gas dispersing perforated plates 65 and 66 in a circumferential array surrounding the mouth 67 of the annular guide duct 68. The perforated plates and 66 serve to supply a gas flow of substantially uniform velocity toward the mouth 67 of the annular guide duct 68. The circumferential array of gas burners 69 serve to uniformly heat the gas flow of substantially uniform velocity vented through the circularly disposed perforated air distribution plates 65 and 66. As shown by Fig. 6, the high temperature flow of gas heated by the burners 69 is conveyed between spaced conical surfaces provided internally of an annular guide duct 68 formed of a refractory material. The annular guide duct 68 conveys the flow of gas from the gas distributor to an outlet 71 where the flow of heated gas is directed in a flow of gas converging toward the atomizing wheel A gas distributor 70 is shown in Fig. 7 which is a modification of the gas distributor 60. In Fig. 7 a plurality of gas burners 72 are arrayed in a different circular pattern between the perforated plates 65, 66 and the mouth 67 of the annular guide duct 68. In both embodiments, that is, the continuous circumferential array provided by the hexagonal arrangement of the burners 69 in Fig. 5 and the burners 72 which are spaced but still provide a circumferential array of heating means, the flow of gas having a substantially equal velocity venting through the perforated plates 65 and1 66 is uniformly heated before it arrives at the mouth 67 of the annular guide duct 68 which conveys the gas to the outlet 71 concentric with the atomizer wheel Fig. 8 illustrates a spray chamber 80 which is distinctly different from the spray chamber 12 shown by

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WO 91/04776 PCX1/DK90/00240 19 Fig. 1. In Fig. 8 the spray chamber 80 is comprised of a cylindrical wall 81 closed by a top member 82 which converges toward the central axis of the spray chamber and has an opening 83 provided therein. The gas distributor is a cylindrical chamber 84 located adjacent to and above the top member 82 and includes perforated plates 85 and 86 for supplying a flow of gas having a substantially uniform velocity. A plurality of burners 88 are provided internally of a tubular or cylindrical refractory member 89 conveying the flow of gas from the gas distributor 84 to the spray chamber 80. A plurality of spray nozzles 90 are equally spaced around the top member 82. The spray nozzles 90 are distinctly different from the atomizer wheels utilized in the other embodiments and a relatively large number of spray nozzles 90 perhaps as many as 20 to 40 spray nozzles are equally distributed around the top member 82 and the droplets of feed solution material are sprayed internally toward a vertical axis of the spray chamber 80. The processing or drying gas flow supplied by the distributor 84 and heated by the gas burners 88 is directed downwardly in a column having a substantially circular cross section concentric with the plurality of spray nozzles 90 arrayed around the top member 82.

The present invention thus provides an improved gas distributor and heating device which not only facilitates the use of high temperature processing or drying gases; that is, drying gas having a temperature on the order of 715 C to 1200 C which was heretofore not economically feasible; but also provides a novel arrangement and structure for heating processing or drying gas while conveying the same directly into a spray drying chamber which provides distinct advantages over prior art systems wherein the processing or drying gas was heated at a location remote from the spray drying chamber in the system delivering the processing

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WO 91/04776 PCT/DK90/00240 or drying gas to a gas distributor. That is, the present invention not only facilitates the use of a drying gas having a temperature substantially greater than that heretofore considered economically feasible, but it also provides a novel arrangement wherein processing or drying gas may be heated after the gas has passed through a distributor for providing an annular flow of gas having a substantially uniform velocity as that gas is being conveyed directly to a spray chamber. The apparatus of the present invention thus offers significant advantages over the prior art practice of heating the processing or drying gas in a delivery system before the gas is provided to a gas distributor and may be also utilized to provide more cost effective spray drying at conventional temperature ranges.

Although specific embodiments of the invention and several modifications of a gas distributor and an annular guide duct have been disclosed, the present invention is not to be construed as limited to the particular embodiments and forms disclosed herein, since the foregoing description is to be regarded as illustrative rather than restrictive and it should be understood that modifications and variations in details of construction may be made without departing from the spirit and scope of the invention as defined by the claims appended hereto.

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Claims

1. A gas distributor and heating device ;or a spray dryer of the type including a feed liquid atomizer in a drying chamber, the device comprising a gas distributor having a gas inlet and means for distributing the gas from the inlet as a uniform flow of gas to a guide duct conveying said flow of gas from said gas distributor to an outlet providing a flow of said gas concentric with said atomizer, including heating means located in said guide duct downstream of said distributing means and at such a distance from said atomizer that the heating mePns does not directly contact the liquid particles sprayed therefrom.

2. The device according to claim 1, wherein said guide duct is an annular duct defined by two spaced cylindrical surfaces and said outlet provides an annular flow of heated gas concentric with said atomizer, wherein said heating means is located in and extends circumferentially around said annular guide duct.

3. The device according to claim 1, wherein said guide duct is an annular duct defined by two spaced conical surfaces and said outlet provides an annular flow of heated gas converging towards the centerline of said guide duct, and wherein S said heating means is located in and extends circumferentially around said annular S: guide duct.

4. The device according to claim 2 or claim 3, wherein said heating means is S comprised of at least one gas burner provided internally of said annular guide duct S and extending circumferentially around the interior of said guide duct.

The device according to claim 4, including means deflecting a portion of the i, I -22- gas supplied by said gas distribution means to bypass said gas burner and gases of combustion to provide a flow of gas insulating at least one of the guide duct surfaces from the temperature of said gas burner and the gases of combustion.

6. The device according to claim 5, wherein said deflecting means comprises annular guide plates located around the interior of said annular guide duct between said gas burner and each guide duct surface at a distance from said guide surfaces, said guide plates extending from said gas burner to a position near the outlet of said guide duct.

7. The devrice according to any one of claims 4 to 6, at least one gas burner is disposed to emit a flamrne in the direction of said outlet and said gas burner is located at a greater distance from said outlet than a length of said flame so as to ensure that the gases of combustion are completely burned internally of said annular guide duct.

8. The device according to any one of claims 4 to 6, wherein at least one gas burner is disposed to emit a flame in the direction of said outlet and the gas burner is located a distance from said outlet which is less than a length of said flame to locate a maximum temperature of the flame upstream of said atomizer. i

9. The device according to any one of the preceding claims, wherein said gas distribution means is comprised of a banjo shaped chamber having a circular portion and an arm extending radially relative to said circular portion and providing a supply inlet, wherein said circular portion includes an annular outlet for supplying an annular flow of gas to said conveying means and at least one *,N 23 continuous circular perforated plate or set of distributor vanes spaced between an interior surface of said circular portion and said annular outlet for distributing a uniform flow of gas from said supply inlet to said annular outlet, an3 said heating means comprise a plurality of gas burners arranged in a circular pattern for uniformly heating the uniform flow of gas supplied by said gas distributing means.

A spray dryer for drying of a feed solution, comprising a drying chamber having a vertical axis and a drying space concentric with said vertical axis, a gas delivery system for providing a continuous flow of gas to said drying sp ice, means for removing powder particles from said chamber, means for withdrawing gas from said chamber and at least one atomizing means which forms and introduces dispersed droplets of feed solution in said drying space, wherein said spray dryer comprises a device as claimed in one of the preceding claims.

11. A gas distributor and heating device for a spray dryer, substantially as herein described with reference to the accompanying drawings.

12. A spray dryer for drying of a feed solution, as claimed in claim i i substantially as herein described with reference to the accompanying drawings. DATED this 19th day of February 1993. SNIRO A/S iBy their Patent Attorneys: CALLINAN LAWRIE