WO2001003807A1

WO2001003807A1 - A process and an apparatus for spray drying a liquid to produce an agglomerated product

Info

Publication number: WO2001003807A1
Application number: PCT/DK1999/000400
Authority: WO
Inventors: Martin Andreas Grønlund SCHOLTEN; Ove Emil Hansen; Jens Mourits SØRENSEN
Original assignee: Niro A/S
Priority date: 1999-07-13
Filing date: 1999-07-13
Publication date: 2001-01-18
Also published as: AU4770099A

Abstract

In a process and an apparatus of spray drying an agglomeration is performed in a chamber having an internal fluidized bed and a plurality of filter elements to catch particles from the spent drying gases before exhaustion thereof, the agglomeration is substantially improved by releasing particles collected on said filter elements by means of intermittent reverse flow gas pulses individually controlled to each filter element or to each of small groups of filter elements each comprising only a few of the total number of filter elements.

Description

A process and an apparatus for spray drying a liquid to produce an agglomerated product

Field of the invention The present invention relates to spray drying technology applicable within a broad range of industries, e.g. the pharmaceutical, chemical, dairy and food industries, especially where an agglomerated product of even agglomerate size is desired. The term spray drying is herein applied in a broad sense covering e.g. also processes in which a prime aim is to agglomerate a solid already present as a particulate product, by dry injection of said product into a drying chamber wherein a liquid is atomized.

Background of the invention

Within the area of spray drying substantial improvements have been developed and implemented within the last many decades . A standard textbook on spray drying technology is Masters, Keath: Spray Drying Handbook, 5th edition, (Longman Scientific & Technical, 1991), incorporated herein by reference.

Modifications of the basical spray drying processes which are of special relevance in connection with the present invention include the application of an internal stationary fluidized bed in the bottom portion of the spray drying chamber as well as the arrangement of a filter in the very spray drying chamber to retain particles in the chamber, otherwise removed therefrom, entrained in the stream of spent drying gas exhausted from the chamber.

A process and an apparatus utilizing a fluidized bed in the bottom of the spray drying chamber is described in e.g. US 5,632,100 (Hansen) , incorporated herein by reference. By incorporating gas filters into the very drying chamber as suggested in WO 97/14288 (incorporated herein by reference) substantial advantages may be obtained, viz. a reduction of the requirements for particle collecting equipment, such as cyclones and filters for treating the spent drying gasses outside the drying chamber. Besides, improved conditions for particle agglomeration may be obtained as explained in said WO publication. A spray drying apparatus having such filters in the drying chamber and a fluidized layer in the bottom thereof is marketed by Niro A/S, Soeborg, DK and termed IFD™ (Integrated Filter Dryer) .

The main zones for agglomeration in a spray drying chamber are the cloud of droplets and wet particles at the vicinity of the atomizing device and the area near the surface of the portion of the chamber walls forming a truncated cone in the lower part of the drying chamber, of course provided that the chamber is of the well-known art having a conical lower portion. Agglomeration may also take place in the fluidized layer provided the product present therein has sufficient moisture content .

In an apparatus as the one depicted in Figs. 1-3 of the above-mentioned WO publication, the internal filters are arranged in such a way that when particles collected on the surface thereof are released, a large proportion of said particles will reach the agglomeration zones, as explained in details in said publica- tion.

Release of the collected particles from the surface of the filters is obtained by intermittent reverse flow gas pulses, either pressurized air or gas from the same source as used as drying gas in the spray drying chamber. When the filter comprises a plurality of filter elements, it is customary to subject all elements or a rather large group of such for a simultaneous reverse flow gas pulse. This causes a substantial sudden increase of the amount of fine particles introduced to the agglomeration zones.

In a continuous production, it has turned out that the uneven delivery of fine particles from the filters to the agglomeration zones imparts variations in the agglomeration process resulting in agglomerates having broader size distribution than desired and also having varying mechanical stability.

Summary of the invention

It is an object of the present invention to provide a process and an apparatus of the above described type in which a more even, nearly continuous flow of fine particles takes place from the filters to the agglomeration zones.

Other objects will appear from the following description.

The invention thus deals with a process for spray drying a liquid to produce an agglomerated product by atomizing the liquid into a drying chamber to form droplets, introducing a first stream of drying gas downward into the upper part of said chamber to partially dry the droplets, introducing a second stream of drying and fluidizing gas upward in the bottom portion of the chamber through a perforated plate to maintain a fluidized layer of particles on said plate, withdrawing a stream of spent drying gas comprising gas from said first and said second stream of gas from the chamber through a particle collecting filter comprising a plurality of filter elements, thereby retaining particles on the surfaces of said filter elements, releasing said retained particles by reverse flow gas pulses intermittently supplied to the filter elements, the thus released particles being subjected to agglom- eration in agglomeration promoting zones in the drying chamber, collected into the fluidized layer and withdrawn continously therefrom. The term continously being here used to distinguish the present process from essentially batch processes, but does not imply that no regulation of the fluidized layer may take place by controlling or even shortly interrupt the withdrawal.

According to the invention, this process is characterized in that the intermittent reverse flow gas pulses are individually controlled to each filter element, or the filter elements are divided into groups each comprising only a few of said plurality of filter elements, and the intermittent reverse flow gas pulses are individually controlled to each such group of elements . By this measure, simultaneous release of relatively large amounts of particles from the filter surfaces is avoided, and thus a more even introduction of particles to the agglomeration zones is obtained.

Also the fact that the gas used for reverse flow cleaning at each moment only results from a single or a few filter elements makes the introduction of said gas more even than in prior art processes where long rows of filter elements receive reverse flow pulses simultaneously. As described e.g. in WO 98/00001. The filters used in connection with the present invention are typically conventional bag-filters or less flexible filters as described in WO 97/14288.

Systems for reverse flow cleaning of individual filter elements in dust-filters in waste gas cleaning are known in the art, cf . DE 19701983 (Lubbers et al . ) . According to said German patent, a system in which the elements in external filter units are cleaned individually, is used to avoid disturbing effects on upstream treatment facilities. No beneficial effect of having a very even recovery of fine particles from the filter is described or obtained in connection with these external filters not directly combined with equipment for processing of said fine particles.

However, when a similar principle is used in the internal filters in a spray drier as explained, completely different and new improvements are obtained as exp1ained above .

The process comprises embodiments in which the liquid being spray dried contains a dissolved or otherwise dispersed solid which by the drying forms the particles which after agglomeration are the product of the process.

However, embodiments are also comprised in which a particulate solid is injected into the chamber for agglomeration promoting contact with the atomized liquid droplets or moist particles formed by partial drying thereof, in which case the solids contained in the liquid show adhesive properties when dried and/or the liquid comprises a solvent evoking stickiness of one or more components of the particulate solid.

The number of filter elements in an IFD TM apparatus may vary from a few to more than 400.

When a spray dryer is used having a smaller number of filter elements, e.g. 50 or less, it is preferred to control the reverse flow gas pulses individually to each filter element.

When the number of filter elements is large, e.g. above 50, it is possible to direct each reverse flow gas pulse to a group of filter elements. The number of filter elements in each group being typically from 2 to 5.

Preferably, the reverse flow gas pulses are controlled by a preset sequence-time system, but can, however, alternatively be controlled on the basis of signals obtained by direct or indirect monitoring the pressure drop over each filter element or over each of said groups of elements .

In the embodiments of the process according to the invention where more than one filter element receives reverse pulses simultaneously, the elements receiving such simultaneously pulses are preferably arranged spaced evenly to obtain an even distribution of the released particles.

The invention also deals with an apparatus for performing the above specified process, said apparatus having a drying chamber, an atomizing device in said chamber connected to a source for liquid to be spray dried, a drying gas disperser in the upper portion of the chamber, a perforated plate in the bottom portion of said chamber, a plenum below said plate for providing an upward flow of fluidizing and drying gas through the perforations in said plate, sufficient for maintaining a layer of fluidized particles on said plate, a plurality of gas filtering elements inside said chamber, through which filter elements spent drying gases are withdrawn, means for providing reverse flow gas pulses for releasing particles collected on the surface of said filter elements, and outlet means for recovering agglomerated particles from the fluidized layer existing on said plate by the operation of the plant .

According to the invention, the apparatus is characterized in comprising equipment for controlling the reverse flow gas pulses individually to each filter element or to each of a number of groups into which said plurality of filter elements is divided, each group comprising only a few of said plurality of filter elements .

The apparatus may also be provided with means for injecting a particulate solid into the drying chamber adjacent to the atomizing device.

In an embodiment of the apparatus having less than 50 filter elements, the reverse flow gas pulses are preferably controlled individually to each filter element.

In apparatuses having a larger number of filter elements, these may be divided into groups each comprising a few of the total number of elements, and means exist for directing each reverse flow gas pulse to one of said groups.

By such an embodiment, the number of filter elements in each group will typically be from 2 to 5, preferably spaced evenly.

Dependent on which system is utilized, the appar- atus can be provided with controlling equipment comprising probes for monitoring the pressure drop over each filter element or over each of the groups of elements, or the controlling equipment may be adjusted to provide a prefixed sequence of pulses to each filter or group of such.

The invention is not restricted to specific drying chamber designs, but most experiences have hitherto been obtained using apparatuses having a frusto-conical lower part and wherein the integrated filter elements are placed above the slanted walls of said part. By this arrangement the particles released from the filter elements fall directly down in the agglomeration zone near said walls.

The process and the apparatus according to the invention is further explained below with reference to the drawings .

Brief description of the drawings

Fig. 1 is a very schematic, vertical, partly sec- tional view of an apparatus according to the invention, and Fig. 2 is a schematical view of the same apparatus seen from above . Fig. 1 shows a drying chamber, which in the depicted embodiment, has an upper cylindrical portion 1 and a lower frusto-conical portion 2.

During the operation, a fluidized particle layer

3 exists in the bottom portion of the chamber, which fluidized layer receives drying and fluidization gas through conduit 4, via a plenum and a perforated plate

(not shown) .

In the upper portion of the drying chamber is a centrally positioned atomizer, shown as a pressure nozzle. Other types of nozzles or rotary atomizer wheels may be used. The atomizer, indicated as 5, receives a liquid feed through conduit 6.

In the optional embodiment shown on the drawings means 7 for introducing particulate solids debouches around the atomizer 5. Particulate solids are supplied to said means via conduit 8.

A total of six integrated filter elements 9a-f is arranged in the ceiling 10 of the drying chamber.

A drying gas disperser 11 receives drying gas through duct 12. Above the ceiling 10 of the drying chamber is a plenum 13 through which spent drying gases are withdrawn from the drying chamber via the filter elements 9a-f .

Above each filter element is a nozzle 14 for producing a controlled reverse blow down in the filter elements .

Each of said nozzles is for the sake of simplicity shown as connected to a separate source for pressurized gas 15. Normally, a common source for pressurized gas will be used.

The supply of pressurized gas from the sources 15 to the nozzles 14 is controlled by valve means 16a-f .

As indicated by the dotted lines, each of the valve means 16a-16f is controlled individually by the controller 17.

When operating the apparatus drying gas is provided through 12 and drying and fluidization gas through 4. The liquid to be spray dried is delivered through 6 and, optionally, dry particles are injected via 7 and 8.

During the drying process smaller particles as well as agglomerates of a plurality of particles are formed in the drying chamber.

A certain separation of larger agglomerates and particles and small agglomerates takes place since the larger agglomerates collect in the fluidized layer 3 in the bottom of the chamber, whereas a substantial part of individual particles and small agglomerates are entrained in the gases leaving the chamber through the filter elements 9a-f , and said particles collect on the outer surface of said filter elements. On the drawing this is especially shown for the filter element 9a.

At a time determined by the controller 17 one of the individual filter element receives a counter-blow or reverse-flow gas pulse thereby releasing the particles settled on the outer surface of the filter, as illustrated in connection with element 9d in Fig. 1.

As schematically shown, the thus released particles fall downwards in the area near the wall of the frusto-conical portion. It is established that this area is one of those in the drying chamber where an efficient agglomeration takes place.

Thus, the released particles and smaller agglomerates grow to larger agglomerates which collect in the fluidized layer from where a continuous product stream is withdrawn through conduit 18.

As it appears from the above specification, but not depicted on the drawings, the controller 17 can direct reverse pulses to a small group of filter elements simultaneously by means of one single valve or individual valves. If so, the elements in each such small group are preferably spaced evenly or symmetrically.

As also explained above, the controller 17 may deliver signals to the valves 16a-16f in a pre-deter- mined sequence, frequency, and duration, the frequency and duration of the pulses being individually or equally set. The controller can also react on signals indicating the pressure drop over each single element or each small group of elements.

Claims

P A T E N T C L A I M S

1. A process for spray drying a liquid to produce an agglomerated product by atomizing the liquid into a drying chamber to form droplets, introducing a first stream of drying gas downward into the upper part of said chamber to partially dry the droplets, introducing a second stream of drying and fluidizing gas upward in the bottom portion of the chamber through a perforated plate to maintain a fluidized layer of particles on said plate, withdrawing a stream of spent drying gas comprising gas from said first and said second stream of gas from the chamber through a particle collecting filter comprising a plurality of filter elements thereby retaining particles on the surface of said filter elements, releasing said retained particles by reverse flow gas pulses intermittently supplied to the filter elements, the thus released particles being subjected to agglomeration in agglomeration promoting zones in the drying chamber, transferred into the fluidized layer and continously withdrawn therefrom, c h a r a c t e r i z e d in individually controlling the intermittent reverse flow gas pulses to each filter element, or to each of a number of groups into which said plurality of filter elements is divided, the number of elements in each group being only a few of said plurality.

2. The process of claim 1, c h a r a c t e r i z e d in that the liquid being atomized contains a dissolved or otherwise dispersed solid.

3. The process of claim 1, c h a r a c t e r i z e d in that a particulate solid is injected into the chamber for agglomeration promoting contact with the atomized liquid droplets or moist particles formed by partial drying thereof, and in that solids contained in the liquid show adhesive properties when dried and/or the liquid comprises a solvent evoking stickiness of one or more components of the particulate solid.

4. The process of anyone of the claims 1-3, c h a r a c t e r i z e d in that the reverse flow gas pulses are controlled individually to each filter element .

5. The process of anyone of the claims 1-3, c h a r a c t e r i z e d in that the number of filter elements in each group is from 2 to 5.

6. The process of anyone of the preceding claims, c h a r a c t e r i z e d in that the reverse flow gas pulses are controlled in a fixed sequence, frequency and with a fixed duration to each filter element or to each of said groups of elements, the frequency and duration of the pulses being individually or equally set .

7. The process of anyone of the claims 1-5, c h a r a c t e r i z e d in that the reverse flow gas pulses are controlled on the basis of signals obtained by monitoring the pressure drop over each filter element or over each of said groups of elements.

8. An apparatus for performing the process defined in claim 1, having a drying chamber, an atomizing device (5) in said chamber connected to a source for liquid to be spray dried, a drying gas disperser (11) in the upper portion of the chamber, a perforated plate in the bottom portion of said chamber, a plenum below said plate for providing an upward flow of fluidizing and drying gas through the perforations of said plate, sufficient for maintaining a layer (3) of fluidized particles on said plate, a plurality of gas filtering elements (9a-f) inside said chamber, through which filter elements spent drying gases are withdrawn, means (14,15) for providing reverse flow gas pulses for releasing particles collected on the surface of said filter elements, and outlet means (18) for continous recovering agglomerated particles from the fluidized layer existing on said plate by the operation of the plant, c h a r a c t e r i z e d in comprising equipment (16a-f, 17) for controlling the reverse flow gas pulses individually to each filter element (9a-f) or to each of a number of groups into which said plurality of filter elements is divided, each group comprising only a few of said plurality of filter elements.

9. An apparatus according to claim 8, wherein said plurality of elements is divided into groups, c h a r a c t e r i z e d in that the elements of each group is arranged evenly spaced.

10. An apparatus according to claim 8 or 9, c h a r a c t e r i z e d in having means (7,8) for injecting a particulate solid into the drying chamber adjacent to the atomizing device (5) .

11. An apparatus according to claim 8 or 10 having less than 50 filter elements, c h a r a c t e r i z e d in having means (16a-f, 17) for controlling the reverse flow gas pulses individually to each filter element .

12. An apparatus according to anyone of the claims 8-11, c h a r a c t e r i z e d in having a lower portion (2) in the form of a truncated cone, the internal filter elements (9a-f) being arranged vertically above the walls of said truncated cone.