IES61877B2 - Improvements in spray drying apparatus and process - Google Patents

Description

IMPROVEMENTS IN SPRAY DRYING APPARATUS AND PROCESS The present invention relates to spray drying operations and processes and in particular to an apparatus and method for enhancing the quality of recovered product in a spray drying process.

In the commercial manufacture of dried products i.e. 1Q skimmed milk powders, whole milk powders and whey powders, spray drying towers are generally used. Spray drying towers normally require large volumes (25,000 to 200,000m^) of hot air (160 - 240° C) to be passed through them to extract moisture from the product to be dried. The product 15 is fed into the drying tower utilising nozzle or spinning wheel atomisers to disperse the product into very fine droplets.

The temperature of the air decreases as it moves through the drying tower picking up moisture vapour from the product and supplying heat energy for the water evaporation This air is removed from the drying tower by extraction fans and is laden with moisture and valuable product fines.

The product fines are normally recovered from the air and this is generally done by cyclones, filters or wet scrubbers or by a combination of the above systems.

The present disclosure relates to the use of the cyclone 30 system for product fines recovery.

In the usual cyclone system and as shown in Figure 1, air emerging from the drying tower is drawn through the cyclone causing it to swirl within the cyclone in the directions shown by the arrows, thus throwing the particles of fine product to the outside wall of the cyclone from where the - 2 product falls down into the cone at the base of the cyclone leaving the air with greatly reduced product fines content to be sucked into the centre rising cone (thimble) of the cyclone, whence it exits from the system through an exhaust fan to the atmosphere.

The product fines drop out of the cyclone cone into a conveying system and are discharged from this system through a rotary seal valve. Finally, the fines are removed from the system via a lean phase pneumatic transport system.

Under the normal operating production mode of the spray drying system, the product moves freely through the cyclone system. However, during any period when the plant is shut down, the drier and cyclone structure together with the temperature gradients across the plant cause air to move out of the cyclones through the exhaust fan thus creating a relative negative pressure in the lower cone of the cyclone. This negative pressure in turns sucks in air from the lean phase pneumatic transport system through the rotary seal valves and conveyors back up into the cyclone, particularly when the valves have deteriorated due to wear and tear. This air can be at ambient temperature with high relative humidity. A skin of the product fines which normally accumulates on the inside surfaces of the drying system becomes exposed to this high relative humidity air and will absorb moisture from the air. Thus, the product skin becomes tacky and on subsequent production mode operations of the drier, the skin of product tends to increase in thickness. The longer the shutdown generally, and/or the more frequent the incidence of shutdown, the greater this effect. In addition, the cooling down of the conveying system when the drier is shutdown contributes to an increase in the relative humidity of the air in the cyclone and conveyor system leading again to moisture uptake by the skin of product in the system. The nett result of these factors is an accumulation of high moisture product in the system which can become lumpy. The lumpy material may subsequently break off and then discharge into the main product stream during operation of the plant.

Coupled with this, microbiological growth and development can take place in the high moisture product. Clearly, the presence of lumpy material and/or contamination in the product is wholly unacceptable.

An object of the present invention is to provide an improved apparatus and process for spray drying in which the build-up of a relatively high humidity product skin in the cyclone and conveyor during plant shutdown is reduced.

Accordingly, the present invention comprises a cyclone product fines recovery apparatus for use in a spray drying plant, the apparatus comprising a cyclone and a conveyor for transporting product fines recovered from the cyclone 20 to a downstream process area, characterised in that the conveyor includes closure means for substantially isolating the conveyor from the air circulating in the downstream process area and heating means for heating the conveyor when the plant is not operational.

The invention also provides a method of recovering product fines from a cyclone in a spray drying plant, comprising feeding product fines recovered from the cyclone to a downstream process area via a conveyor, characterised by 30 substantially isolating the conveyor from the air circulating in the downstream area and heating the conveyor when the plant is not operational thereby to avoid absorption of moisture by residual product fines within the conveyor.

The conveyor is preferably heated so that the air therein is retained at a temperature of between 25°C and 35°C, the < heating being by means of a jacket of warm fluid disposed about at least a portion of the conveyor. · Most preferably, the heating means comprises air heating means which provides a jacket of hot air about at least a portion of the conveyor. The heating means conveniently comprises a fan heater, for example one using at 2 or 3 kilowatts heater. Insulation is preferably provided about the conveyor to help retain the air temperature internally of the conveyor at between 25°C and 35°C. Most conveniently, the heated air is supplied to and from the conveyor by means of ducting.

Alternatively, the conveyor may be heated by means such as a jacket of steam or hot water about at least a portion of the conveyor or by direct electrical heating of the walls of the conveyor.

When the plant including the conveyor according to the invention is shut down, the conveyor is isolated from the downstream side, for example by the shutting off of valve means, and the fan heater which supplies the ducting is switched on. By heating and recirculating the air within the ducting, the heater serves to retain the air trapped within the conveyor at the chosen temperature, thereby preventing condensation of moisture from the air and avoiding the accumulation of high moisture product in the system.

The invention will now be more particularly described with reference to the accompanying drawings, which show by way of example only, one embodiment of apparatus according to the invention. In the drawings: Figure 1 is a schematic view of a prior art cyclone and conveyor system, showing the direction of flow of air and product within the cyclone and conveyor; Figure 2 is a plan view of the cyclone only of Figure 1; Figure 3 is a side elevational view of that section of a spray drying plant in accordance with the invention at which the cyclones feed into a pair of product conveyors and thence into a lean phase pneumatic transport system; Figure 4 is a front elevational view taken on line B-B of Figure 3; Figure 5 is a detailed view of the circled portion of Figure 4, showing the feed valve supplying heated air to the conveyor; and Figure 6 is a front view taken on line C-C of Figure 3.

Referring to the Figures, product collected in the cyclones 1 falls through the base thereof into a conveyor 2a or 2b and thence is fed to the lean phase pneumatic transport system 3. Each conveyor 2a, 2b has three cyclones 1 feeding into it and the conveyors 2a, 2b are parallel to one another. Legs 8 support the conveyors above the ground.

With reference to Figures 1 and 2, product fines suspended in air derived from an upstream drying tower (not shown) are circulated within the cyclone 1 in the direction of the arrows and collect on the walls of the lower cyclone cone 11, whence they fall downwards into the conveyor 2a, 2b. The air, now substantially devoid of product fines, rises into the cyclone rising cone or thimble 12 and thence is expelled to atmosphere. < As shown in Figures 3 to 6, the conveyors 2a, 2b are ♦ provided with insulated ducts 21a, 21b, linked by piping 22a,22b to an air fan and heater 23. The ducts 21a, 21b and piping 22a, 22b are all insulated. The insulated ducts 21a, 21b are fabricated of stainless steel and are welded to the conveyors 2a or 2b, each conveyor having two ducts 21a and two ducts 21b welded to it.

As shown in Figure 3, hot air, heated by the fan heater 23, is circulated via the piping 22a to the ducts 21a and via piping 22b to the ducts 21b. The hot air is fed back from the ducts to the fan heater 23 for reheating and recirculation via pipe 22c. The directions of the air supply and return are shown by the arrows in Figure 3. Adjoining pairs of ducts 21a, 21b are interconnected by hosing 24. The circuit from fan heater 23 through piping 22a, 22b, on through the ducts 21a, 21b and hosing 24 and returning to the fan heater 23 via pipe 22c is substantially closed and insulated. An air inlet filter 25 is incorporated into the circuit to provide for replacement of any air lost from the circuit.

Hot air supplied by the fan heater 23 is introduced into the air space between the ducts 21a, 21b and the conveyors 2a, 2b through the ball valves 26. As shown more clearly in Figure 7, the air supply to the duct 21a is passed via the ball valve 26 to the space 27. A ball valve 26 is provided for each conveyor at one end of each set of ducts 21a, 21b and these four valves serve to balance the air flow through the eight insulated air ducts 21a, 21b. Thus, considering ducts 21a, two of these are on conveyor 2a and two are on conveyor 2b and the air supply to these two pairs of ducts is balanced by a pair of ball valves 26, one valve to each pair of ducts. Similarly, the four ducts 21b are supplied on a pair of valves 26.

All of the fan heater 23, piping 22 and ducts 21 are insulated to prevent loss of heat from the system.

Referring to Figure 6, product gathered on the conveyors 2a, 2b is transported to the ends of these conveyors by q q agitation of the sloped conveyors on their springs 9 and then falls down through the blow-through rotary seal valves 4a,4b into the lean phase pneumatic transport system 3. A pair of butterfly valves 5a, 5b installed in the transport system 3, one to the upstream side and the other to the 15 downstream side of the valves 4a, 4b enable the conveyors to be isolated from the transport system 3 to prevent air from the transport system 3 passing back into the conveyor and cyclone system. These butterfly valves close automatically when the plant is shut down. Thus, when the 20 plant is not in use, the conveyor system is substantially isolated from the lean phase transport system and is heated to inhibit moisture being absorbed by the product fines.

The temperature of the air circulating in the ducts is kept between about 25°C and 35°C to keep the product fines dry.

Under these conditions of low humidity, microbial growth is not favoured.

As the entire conveyor air-jacketing system is insulated, energy use in retaining the conveyor warm is relatively 30 low.

Thus, the apparatus and process according to the invention provide an efficient means for retaining the residual product fines in the conveyor system dry during plant shut down.

Means of heating the conveyor other than the hot air jacket described herein will suggest themselves to the skilled person. For example, the hot jacket may be a hot water or steam jacket or the conveyor may be heated directly by electrical elements incorporated in or about it, or by other suitable means. The heating may be provided about the entirety of the conveyor, or about only a portion of it. In any case, it is advantageous from the point of view of energy efficiency always to provide insulation to reduce heat lost from the conveyor and/or its heating system.

It will of course be appreciated that the invention is not limited to the specific details described herein, which are given by way of example only, and that various modifications and alterations are possible within the scope of the invention as defined in the appended claims.

Claims (5)

CLAIMS :

1. A cyclone product fines recovery apparatus for use in a spray drying plant, the apparatus comprising a cyclone and a conveyor for transporting product fines recovered from the cyclone to a downstream process area, characterised in that the conveyor includes closure means for substantially isolating the conveyor from the air circulating in the downstream process area and heating means for heating the conveyor when the plant is not operational.

2. A method of recovering product fines from a cyclone in a spray drying plant, comprising feeding product fines recovered from the cyclone to a downstream process area via 15 a conveyor, characterised by substantially isolating the conveyor from the air circulating in the downstream area and heating the conveyor when the plant is not operational thereby to avoid absorption of moisture by residual product fines within the conveyor.

3. The invention of Claim 1 or Claim 2, in which the conveyor is heated by means of a jacket of warm fluid disposed about at least a portion of the conveyor. 25

4. The invention of Claim 3, in which the conveyor is heated to between 25°C and 35°C when the plant is not operational.

5. The invention according to any preceding claim, 3Q substantially as herein described with reference to Figures 3 to 6 of the accompanying drawings.