Dairy Process Engineering

Lesson 17. MECHANISMS OF FLUIDIZATION, CHARACTERISTICS OF GAS FLUIDIZATION SYSTEMS, MINIMUM POROSITY, BED WEIGHT, PRESSURE DROP IN FLUIDIZED BED

Module 3. Fluidization

Lesson 17
MECHANISM OF FLUIDIZATION, CHARACTERISTICS OF GAS FLUIDIZATION SYSTEM, MINIMUM POROSITY, BED WEIGHT, PRESSURE DROP IN FLUIDIZED BED

Most powder-like products produced by spray drying or grinding are dusty, exhibit poor flow characteristics and are difficult to rehydrate. It is well known, however, that agglomeration in most instances will improve the redispersion characteristics of a powder. Added benefits of agglomerated powders are that they exhibit improved flow ability and are non-dusting. All of these characteristics account for their increased demand in recent years. Depending on the application or industry where the process is being used, agglomeration is sometimes referred to as granulation or instantizing.

Fig. 17.1 Fluidization regimes

Fluidization of fixed bed takes place in distinct stages of air velocity and trans bed pressure drop. Initially, the air cannot escape the bed, and the bed remains stagnant. As the pressure drop across the bed keeps increasing, the bed slowly expands in its height. After a certain peak, the air finds paths to escape and there is a slight dip in the pressure drop. At this stage we can call it as the beginning of the fludization of the bed, but the flow is smooth and the particles are not showing much movement. On further increase in the velocity and flow rate of air, the bed expands further and the particles are separate from each neighbouring particle and there significant turbulence among the particles and air flow. Any further increase in the air velocity, will make the particles to be carried away by the air and passes into the region of Pneumatic transport.

Mechanism of Fluidization

Fluidized bed drying is generally used in combination with spray and roller drying, as a final drying procedure to bring the moisture of the powder down to the level required for storage stability. Fluidized bed drying is an integral part of instantization. It is also used for production of lactose and granular powders. The fluidized bed method is not only used for drying but also used for cooling any material which can be made to flow.

The powdered products are made to flow through a permeable support on which material rests, when gas is passed. The packing of the bed of material will become less dense. i.e. the bed will start to expand when a certain velocity is reached. At still greater velocities the particles in the bed will begin to be in turbulent motion. All particles of the product are thoroughly mixed in the turbulent layer and they dry therefore at a uniform rate. A further increase in the stream of gas finally leads to a velocity at which the particles float, which occurs when the upward force equals the force of gravity acting on the particles. Beyond this velocity it is impossible to maintain a fluidized bed and the particles are carried away in the gas stream (pneumatic conveying). A fluidized bed therefore maintained at air velocities which lie between those at which the bed starts to expand and those at which the particles float. The velocity at which the bed starts to expand is reached when the pressure drop on passing through the bed equals the force of gravity acting on the whole mass of the bed. shows perforated plate for directional air flow, (Fig. 17.2) shows animation of fluidized bed dryer and (Fig. 17.3) shows animation of vibro fluidized bed dryer.

Force of gravity acting on the bed:

ΔP = H_{0 . g} (ρ_k - ρ_A) (1- ψ₀)

Where,

H₀ = depth of the bed of material at rest

ρ_k = density of the individual particles

ρ_A = density of the air

ψ₀ = porocity of the bed at rest (usually ψ₀ = 0.4)

The pressure drop of the gas on passing through the bed is:

ΔP = 64 (μ₁/ ψ²) { (1/Re) + (ξ_∞/64)} (H₀/d*) (ρ_{A/2) V}²

μ₁ = factor due to increase in length of the path, μ_{1= 0.75 for beds}

Re = Reynold’s number, composed of d* and Vέ = v/έ₀

ξ = resistance coefficient for turbulent flow

_V = velocity of gas based on the cross section of empty tower

d* = the hydraulic diameter of the pores

The above equation is valid for laminar, transition and turbulent flow. Conbining the two equations yields the velocity at which the bed starts to expand.

Types of Fluidized Bed Dryer

Particles of a moist product can be put into turbulent motion using only air provided their surfaces are not stuck together by moisture or thermoplasticity. In such a case fluidized bed drying can be carried out in a relatively simple installation.

If particles stick together initially, stable fluidized beds can be produced by using mechanical stirrer to break up particle agglomerates. Another possibility of forced mixing, which at the same time, also provides forced transport is the much employed vibration fluidized bed drier. The dryer is based on a vibrating conveyor trough which can be up to 10 m long and up to 1 m wide. Eccentric motors produce the vibration of the drier which may be supported on helical or leaf springs. The direction of the vibration can be changed by rotating the motor and the amplitude by changing its eccentricity.

To ensure a uniform distribution of air over the whole area of the air permeable base plate the resistance offered by the plate to the air flow should be in the range of 1000 to 2000 Pa, depending on the type of product and the depth of the bed. The perforations occupy usually between 1.5 and 5 % of the area of the plate, and the depth of the bed, which can be adjusted by means of an overflow weir, is usually between 50 and 300 mm. The vibration fluidized bed has the advantage of forced conveyance in a given direction, fluidization even at low air velocities and a residence time of the product which can be at least partially controlled.