In the Vibro-Fluidizer the whole fluid bed is vibrating. The perforations of the plate are made so that the drying air is directed with the powder flow. To avoid that the perforated plate will vibrate with its own frequency, supporting is necessary. See Fig. 78.

In the spray dryer the outlet temperature is decreased resulting in an increase of the moisture content and a decrease of the particle temperature. The moist powder enters by gravity from the drying chamber into the Vibro-Fluidizer.

There is, however, a limit as to how far it is operationally possible to go, as the powder will become sticky with the increased moisture content despite the lower temperature, and the powder will form lumps and deposits will occur in the chamber.

 Usually a reduction in the outlet temperature of 10-15ºC may be achieved. This results in a much gentler drying especially during the critical drying stage (30% to 10% moisture), the shrinkage will continue, see Fig. 79, not interrupted by any case hardening thus reaching conditions close to the optimal drying example. The lower droplet temperature is achieved partly due to the lower surrounding temperature, but also due to the higher moisture content in the particles thus being closer to the wet bulb temperature. This has naturally a positive influence on the solubility properties of the final powder.

The decrease in the outlet temperature means a correspondingly higher capacity in the drying chamber due to the increased Δt. Very often the drying temperature and the solids content in the feed are increased thus increasing the efficiency of the dryer even more. This means, however, a necessary simultaneous increase in the outlet temperature, but the higher moisture content and lower particle temperature protect the particles, so that overheating and case hardening can be avoided.

Experimentalwise, the drying temperature has reached 250ºC or even 275ºC when drying skim milk, in which case the drying efficiency is as high as 0.75

When the chamber fraction reaches the base of the chamber it has higher moisture content and lower temperature than obtained from the conventional drying, as described previously. From the base of the chamber the powder should drop directly into the drying section of the Vibro-Fluidizer and be fluidized immediately. Any hold-up or conveying will cause the warm, moist and thermoplastic particles to stick together and lumps being hard to break will be formed. This has a direct negative influence on the drying efficiency in the Vibro-Fluidizer and part of the powder will leave the equipment with a too high moisture content, detrimental for the keeping quality of the powder.

Only the chamber fraction enters the Vibro-Fluidizer gravimetrically. The main cyclone fraction and the Vibro-Fluidizer cyclone (or CIP-able bag filter) fraction still have to be collected and conveyed to the Vibro-Fluidizer.

As the fines fractions consist of particles of a less mean diameter than particles from the chamber fractions, they will have a lower moisture content and thus do not require the same after-drying. Very often they are even dry enough, but despite that they are usually passed to the last third of the drying section of the Vibro-Fluidizer to make sure they become dried to the desired moisture content.

When the cyclone fractions are collected, the collection point cannot always be arranged directly above the Vibro-Fluidizer, so that it can be fed directly by gravity into the drying section. A pressure conveying system is therefore often installed to handle the powder. A pressure conveying system is very flexible in relation to where the powder is conveyed, as the conveying line is an ordinary 3 or 4 inch dairy pipe. The system consists of a low-volume/high-pressure air blower, a blow-through valve, al-lowing the air to pass through picking up the powder, and a conveying line, see Fig. 80. The amount of air is small in relation to the powder to be conveyed (only 1/5).

During production a small part of this powder becomes airborne again in the Vibro-Fluidizer and will leave with the air, collected in the cyclone and returned to the Vi-bro-Fluidizer again. During shut down of the plant it will therefore take some time before this recycling has stopped, if special arrangements are not foreseen.
The problem is for example handled by a change-over valve in the conveying pipe to another pipe passing the powder on to the very end of the Vibro-Fluidizer system, which is then emptied in a few minutes.

The powder is finally sifted and bagged off. As the powder may contain some primary agglomerates, it is recommended to use another pressure conveying system to a silo in order to obtain the maximum density.

It is a well-known, accepted fact that, when evaporating water from milk, the energy consumption/kg evaporation increases as the residual moisture approaches zero. See Fig. 81.

It has already been shown that the efficiency of the spray drying can be influenced by the inlet and outlet temperatures.

While the steam consumption is 0.10-0.20 kg/kg evaporated water in the evaporator, it is 2.0-2.5 kg/kg evaporated water in a conventional one-stage dryer, or in other words 20 times as high as in the evaporator. Attempts are therefore always made to increase the solids content from the evaporator. This means that the evaporator will remove more water at a low energy consumption. It will of course lead to a slight increase of the energy consumption/kg evaporated water in the spray dryer, but the total energy requirement will be less.

The above mentioned steam consumption/kg evaporated water is an average figure, as the steam consumption at the beginning is very low increasing towards the end of the drying. A calculation shows that a powder with 3.5% moisture required 1,595 Kcal/kg powder, while for a powder with 6% moisture it is only 1,250 Kcal/kg powder. In other words this last evaporation corresponds to about 23 kg steam/kg evaporation.

The table on the next page illustrates the calculation. The first row of figures shows the conditions when operating the plant under ordinary conditions with pneumatic cooling and conveying of the powder leaving the drying chamber and cyclones.

The next row of figures shows the conditions when running the plant as a two-stage drying unit, in which the powder is dried from 6% moisture to 3.5% moisture in a Vibro-Fluidizer. The third row of figures shows the two-stage drying with high inlet temperature.

DRYING SYSTEM		Spray dryer with pneumatic conveying	System Spray dryer with VibroFluidizer	Spray dryer operated at high temp. with VibroFluidizer
SPRAY DRYER
Inlet air temperature:	ºC	200	200	230
Drying air:	kg/h	31,500	31,500	31,500
Skim milk with 8.5% solids:	kg/h	12,950	16,150	19,800
Concentrate with 48% solids:	kg/h	2,290	2,860	3,510
Evaporation in chamber:	kg/h	1,150	1,400	1,720
Powder from chamber:
- 6% moisture:	kg/h	-	1,460	1,790
- 3.5% moisture:	kg/h	1,140	-	-
Fuel oil consumption:	kg/h	175	175	205
Power consumption:	kW	120	125	130
Energy consumption
Spray drying total:	Mcal	1,818	1,823	2,120
Energy/kg powder in chamber:	Kcal	1,595*	1,250*	1,184
VIBRO-FLUIDIZER
Drying air:	kg/h		3,430	4,290
Inlet air temperature:	ºC		100	100
Evaporation in VF:	kg/h		40	45
Powder from VF, 3.5% moisture:	kg/h		1,420	1,745
Steam consumption:	kg/h		135	167
Power consumption:	kW		20	22
Energy cons., total in VF:	Mcal		95	115
DRYING TOTAL
Energy consumption total:	Mcal	1,818	1,918	2,235
Energy/kg powder total:	Kcal	1,595	1,350	1,280
Energy relation:	%	100	85	80
Dryer efficiency:		0.54	0.62	0.66

By using the figures marked with *) we obtain:

1,595 - 1,250  =  345 Kcal/kg powder

The evaporation per kg powder is:  0.025 kg (6% - 3.5% = 2.5%)

The energy consumption per kg evaporation is then:

345/0.025 = 13.800 Kcal/kg
 
corresponding to 23 kg steam/kg evaporated water.

In the Vibro-Fluidizer the steam consumption is in average about 4 kg steam/kg evaporation, all naturally depending on the amount of air and temperature that can be used. Even if the steam consumption is about twice as high in the Vibro-Fluidizer as in the spray dryer, it is still (due to the special design allowing a residence time as long as 8-10 min. compared to 20-25 sec. in the spray dryer) far below the energy requirement, if the same water should have been evaporated in the spray dryer. At the same time it should be remembered that the plant will get higher capacity, produce a better product, the powder emission will be lower, and on top of that the plant will be very flexible.

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