In the attempt to improve the drying efficiency expressed in formula (17), page 129, the air outlet temperature To was in the two-stage drying (page 131) reduced to the limit where the powder with moisture contents of 5-7% became sticky and started to adhere to the chamber walls.

However, further improvements have been obtained by the introduction of a static fluid bed integrated in the conical part of the drying chamber. The secondary drying air is introduced into a plenum chamber below the perforated plate, through which the drying air is distributed. This type of dryer can be operated, so that the primary particles reach a moisture as high as 8-12%, corresponding to an air outlet temperature of 65-70ºC. This extreme utilization of the drying air means that for a given capacity the size of the plant can be reduced considerably.

Milk powders have always been regarded as difficult to fluidize. However, a specially designed and patented perforated plate, see Fig. 82 and page 131, provides an air/powder rotation in the same direction as that of the primary drying air. By means of this plate, and the correct combination of bed height and fluidization velocity, any milk-based product can now be fluidized in a static fluid bed.

Fig. 82  Perforated plate for directional air flow (BUBBLE PLATETM)

The static fluid bed is available in three configurations:

• Ring-formed fluid bed (Compact dryers)
• Circular fluid bed (MSD dryers)
• A combination of the above two (IFD dryers)

Compact Spray Dryer (CDI)

Multi-stage Spray Dryer (MSD)

RING-FORMED FLUID BED (COMPACT DRYERS)
The ring-formed back-mix bed is placed at the bottom of a conventional chamber cone round the exhaust duct placed in the centre. Thus there are no parts in the cone obstructing the air flow, and this together with the spouts from the fluidized powder layer keeps the chamber cone free from deposits even when handling sticky powders with high moisture contents. The cylindrical part of the chamber is kept clean by a wall-sweep system, where a small amount of air is introduced at high velocity tangentially into the cylindrical part of the chamber through specially designed air nozzles pointing in the same direction as the rotation of the primary drying air.

Due to the combined air/powder rotation and the resulting cyclone effect in the chamber, only a small amount of powder will be entrained in the exhaust air. The cyclone/CIP bag filter fraction is therefore low in this type of dryer, and so is the powder emission.

The powder is discharged continuously from the static fluid bed by overflowing an adjustable powder weir, thus maintaining a certain fluidized powder level.
Due to the low air outlet temperature, the drying economy is greatly improved compared with the conventional two-stage drying as shown in the table on the following page.

When the powder leaves the drying chamber it may be cooled in a pneumatic conveying system as described on page 115. See Fig. 83. The resulting powder will consist of single particles and have same or even better powder properties than those mentioned for two-stage drying on page 131.

Fig. 83  Compact spray dryer with pneumatic conveying system

For fat-containing products the cooling should be done in a vibrating fluid bed which is also used when agglomerated powders are produced. In this case the cyclone fraction is returned to the atomizer device for agglomeration. See Fig. 84.

Fig. 84  Compact spray dryer with Vibro-Fluidizer as agglomerator/instantizer, (CDI)

DRYING SYSTEM		Spray dryer with VF for two-stage drying	Spray dryer with ring- formed static fluid bed (Compact)	Spray dryer with circu- lar static fluid bed (MSD)
SPRAY DRYER
Inlet air temperature:	ºC	230	230	260
Drying air:	kg/h	31,500	31,500	31,500
Skim milk with 8.5% solids:	kg/h	19,800	24,000	31,300
Concentrate with 48% solids:	kg/h	3,510	4,250	5,540
Evaporation in chamber:	kg/h	1,720	2,010	2,620
Powder from chamber:
- 6% moisture:	kg/h	1,790	-	-
- 9% moisture:	kg/h	-	2,240	2,920
Fuel oil consumption:	kg/h	205	205	230
Power consumption:	kW	130	140	150
Energy consumption
Spray drying total:	Mcal	2,120	2,130	2,380
Energy/kg powder in chamber:	Kcal	1,184	950	820
FLUID BED		VF	SFB	SFB
Drying air:	kg/h	4,290	6,750	11,500
Inlet air temperature:	ºC	100	115	120
Evaporation in VF/SFB:	kg/h	45	125	165
Powder from fluid bed, 3.5% moisture:	kg/h	1,745	2,115	2,755
Steam consumption:	kg/h	167	290	400
Power consumption:	kW	20	25	35
Energy cons., total in fluid bed:	Mcal	115	195	265
DRYING TOTAL
Energy consumption total:	Mcal	2,235	2,325	2,645
Energy/kg powder total:	Kcal	1,280	1,038	960
Energy relation (see p. 61):	%	80	65	60
Dryer efficiency:		0.66	0.75	0.80

 

Fluidized static back-mix bed at base of the drying chamber

CIRCULAR FLUID BED (MSD DRYERS)
In order to improve the dryer efficiency even further without deposit problems a completely new spray dryer concept - named Multi-Stage Dryer MSD - has been designed.

The dryer operates with three drying stages, each adapted to the moisture content pre-vailing during the drying process. In the preliminary drying stage the concentrate is atomized by co-current nozzles placed in the hot drying air duct. The air enters the dryer vertically through the air disperser at high velocity, ensuring optimal mixing of the atomized droplets with the drying air. As discussed earlier the evaporation at this stage takes place instantaneously and during the passage vertically down through the specially designed drying chamber. The particles reach a moisture content of 6-15% all depending upon the type of product. At such high moisture content the powder will exhibit a high thermoplasticity and become very sticky. The high velocity air inlet is creating a venturi effect, the vacuum of which will suck surrounding air - with entrained fines particles - into the wet atomizer cloud. This will result in a "spontaneous secondary agglomeration", see page 153. The fluid bed is supplied with air at a sufficient velocity and temperature for the second stage drying. The drying air from the preliminary drying stage and the back-mix bed leaves the chamber from the top passing through a primary cyclone. The powder from this cyclone is led back to the back-mix bed and the air to a secondary cyclone for final separation.

When the powder has reached a certain moisture content it is discharged via a rotary valve into a Vibro-Fluidizer for the final drying and subsequent cooling. The dry-ing/cooling air from the Vibro-Fluidizer is passed through a cyclone separating the powder contained in the air. The fine powder is returned back to the atomization device, the chamber cone (static bed), or the Vibro-Fluidizer. On today's modern dryers the cyclone(s) are replaced by a CIP-able bag filter.

The powder exhibits a coarse powder structure originating from the "spontaneous secondary agglomeration" in the atomizer cloud, where there is a continuous supply of dry fines particles that will stick to the semi-dry particles thus creating the agglomera-tion. But the agglomeration is further enhanced, when the spray gets in contact with the fluidized powder in the static fluid bed. See Fig. 85.

Fig. 85  Multi-stage spray dryer (MSD)

It has been possible to operate this plant at a very high primary drying air temperature (220-275C) and an extremely short residence time, still maintaining a good solubility of the powder. The physical dimensions of this type of plant are small, and thus the requirements to the size of the building are limited. This together with the improved drying economy (10-15% less compared to conventional two-stage drying), due to the high primary drying air temperature, makes it a very attractive solution especially for agglomerated products.

Spray Drying Plant with Integrated Filters and Fluid Beds (IFD)

The Integrated Filter Dryer design (patented), see Fig. 85a, is based on proven spray dryer unit operations, such as:

• Feed system with concentrate preheating, filtration, homogenization, and high-pressure pumps. All equipment as used in conventional spray dryers.
• Atomization using either pressure nozzles or rotary atomization. Pressure nozzles are used mainly for fat containing products and products with high protein content, and rotary atomization for all kind of products in general and concentrates with crystals in particular.
• Drying air filtration, heating, and distribution using an air disperser suitable for rotating or vertical air streams.
• Drying chamber designed to ensure hygienic operation conditions and to maintain lowest possible heat loss by means of e.g. dismountable insulation panels featuring air-filled sandwich panels, see page 75.
• Integrated fluid bed designed as a combined back-mix bed for the drying and plug-flow bed for the cooling. The fluid bed is a fully welded construction with no hollow spaces. Between the back-mix bed and the surrounding plug-flow bed there is an air gap to avoid heat transmission. The new patented GEA Niro BUBBLE PLATE™ is used. See page 119.

Fig. 85a  Integrated Filter Dryer

The dryer exhaust air system is new and though the idea is revolutionary, it is still based on the same principles as applied in Niro's SANICIP™  CIP-able Bag Filter. The fines collection system operates with particulate filters integrated in the drying chamber. The filter bags are supported on stainless steel cages mounted in the ceiling around the circumference of the drying chamber. These filter elements operate with blow-back air cleaning systems similar to those used in the SANICIP™. See page 111.

The bags are purged one by one, or four by four, by compressed air blown into the bag using the reverse jet air nozzles, see Fig. 85b. This gives a regular and frequent release of powder into the integrated fluid bed.

Fig. 85b  Reverse jet air nozzle

Integrated fluid bed

The selection of filter material follows procedures already known from the SANICIP™ bag filter and with the same air-to-cloth ratio.

The reverse air jet nozzle has a dual function: During operation it is used for purging, and during CIP it is used for the wet cleaning of the bags from the inside towards the dirty out-side. Clean water is injected through the reverse air jet nozzle and atomized by compressed air into the inside of the bag and pressed out on the dirty side. This patented feature is extremely important, as it is otherwise difficult, if not impossible, to extract this entrained powder from the outside only.

To clean the underside of the chamber ceiling close to the bags, a specially designed nozzle is used, also with a dual-function: During the drying operation the nozzle is purged with air to keep the area free of deposits and during cleaning it is used as a normal CIP nozzle. The clean air plenum is cleaned using a standard CIP nozzle.

Advantages of the IFD™ Plant

IFD^TM chamber with special insulation panels

Product:

• Higher yield of first-grade powder. In traditional dryers, with cyclones and bag filters, the bag filter fraction is considered a second-grade product accounting for approximately 1% of the production.
• Less mechanical handling of products in ducts, cyclones and bag filter and no external recycling of fines is necessary, since the air flow pattern inside the dryer ensures an optimal primary and secondary agglomeration.
• Improved product quality, as the IFD™ plant can operate at low exhaust air temperature compared to a conventional spray dryer. This possibility means you can obtain a higher drying capacity per kg of drying air.

Safety:

• Simpler safety protection installations, as the drying process takes place in one vessel only.
• Fewer components to protect.
• Lower maintenance costs

Projecting:

• Simpler plant layout
• Reduced building size
• Simpler supporting structure

Environmental aspects:

• Reduced possibility of powder leakage into working area
• Easier cleaning operations as surface area in contact with the product is reduced
• Less effluent discharge during CIP
• Less powder emission, down to 10-20 mg/Nm3
• Reduced energy consumption, saving up to 15% of the power consumption
• Reduced noise level, due to reduced pressure drop over the exhaust air system
  

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