Week # 11

MR Chapters 9 & 10

• Tutorial #11• MR #9.1, 10.1.

• To be discussed on April 11, 2018.

• By either volunteer or class list.

MARTIN RHODES (2008)

Introduction to Particle

Technology, 2nd Edition.

Publisher John Wiley & Son,

Chichester, West Sussex,

England.

Gas Cyclones

• During processing and handling of particulate solids, separation of particles from suspension in a gas may be required

• Generally, particles larger than about 100 mm can be separated by gravity settling

• For particles less than 100 mm, more energy intensive methods such as filtration, wet scrubbing and electrostatic precipitation must be used

• Gas cyclones are best suited as primary separation devices and for relatively coarse particles

• Electrostatic precipitator or fabric filter may be used downstream to remove very fine particles

• Gas cyclones are generally not suitable for separation involving suspensions with a large proportion of particles less than 10 mm

• Most common type of cyclone is known as reverse flow cyclone separator

• Inlet gas is brought tangentially into the cylindrical section

• A strong vortex is created inside the cyclone body

• Particles in the gas are subjected to centrifugal forces which move them radially outwards, against the inward flow of gas and towards the inside surface of the cyclone

• Direction of vortex flow reverses near the bottom of the cylindrical section

• Gas leaves the cyclone via the outlet in the top

• Solids at the wall are pushed downwards by the outer vortex and out of the solids exit

• Gravity has been shown to have little effect on the operation of the cyclone

Flow Characteristics

• Rotational flow in the forced vortex within the cyclone body gives rise to a radial pressure gradient

• This pressure gradient, combined with the frictional pressure losses at the gas inlet and outlet and losses due to changes in flow direction, make up the total pressure drop

• Pressure drop, measured between the inlet and gas outlet, is usually proportional to the square of gas flow rate through the cyclone

• A resistance coefficient, the Euler number Eu, relates the cyclone pressure drop Dp to a characteristic velocity:

• Where rf is the gas density

• The characteristic velocity v can be defined based on the cross-section of the cylindrical body of the cyclone

• Where q is the gas flow rate and D is the cyclone inside diameter

• The Euler number represents the ratio of pressure forces to the inertial forces acting on a fluid element

• Value is practically constant for a given cyclone geometry, independent of the cyclone body diameter

Efficiency of Separation

• Consider a cyclone to which solids mass flow rate is M, mass flow discharged from the solids exit orifice is Mc

(known as the coarse product) and solids mass flow rate leaving with the gas is Mf (known as the fine product)

• Total material balance on the solids may be written:

• Component material balance for each particle size x (assuming no breakage or growth of particles within the cyclone):

• Where dF/dx, dFf/dx and dFc/dx are the differential frequency size distributions by mass (mass fraction of size x) for the feed, fine product and coarse product respectively

• Total efficiency of separation of particles from gas, ET, is defined as the fraction of the total feed which appears in the coarse product collected

• The efficiency with which the cyclone collects particles of a certain size is described by the grade efficiency, G(x):

• Using the notation for size distribution described above:

• Combine to find an expression linking grade efficiency with total efficiency of separation

• Above equation relates size distribution of feed, coarse product and fine product

• In cumulative form, this becomes

• Consider a reverse flow cyclone with a cylindrical section of radius R

• Particles entering the cyclone with the gas stream are forced into circular motion

• The net flow of gas is radially inwards towards the central gas outlet

• The forces acting on a particle following a circular path are drag, buoyancy and centrifugal force

• The balance between these forces determines the equilibrium orbit adopted by the particle

• Drag force is caused by the inward flow of gas part the particle and acts radially inwards

• Consider a particle of diameter x and density rp following an orbit of radius r in a gas of density rf and viscosity m

• Let the tangential velocity of the particle be U and the radial inward velocity of the gas be Ur

• If we assume that Stokes’ law applies under these conditions then the drag force is given by:

• The centrifugal and buoyancy forces acting on the particle moving with a tangential velocity component U

at radius r are,

• Under the action of these forces, the particle moves inwards or outwards until the forces are balanced and the particle assumes its equilibrium orbit

• A relationship between U and the radius r for the vortex in a cyclone is needed

• For a rotating solid body, U = rw, where w is the angular velocity and for a free vortex Ur = constant

• For the confined vortex inside the cyclone body, it has been found experimentally that the following holds approximately:

• If we also assume uniform flow of gas towards the central outlet,

• Combining,

• Where r is the radius of the equilibrium orbit for a particle of diameter x

• If we assume that all particles with an equilibrium orbit radius greater

than or equal to the cyclone body radius will be collected,

• Then substituting r = R, we derive the expression below for the

critical particle diameter for separation, xcrit:

• Values of the radial and tangential velocity components at the

cyclone wall, UR and UR, may be found from a knowledge of

cyclone geometry and gas flow rate

• This analysis predicts an ideal grade efficiency curve

• All particles of diameter xcrit and greater are collected

• All particles of size less than xcrit are not collected

• In practice, gas velocity fluctuations and particle-particle interactions result in some particles larger than xcrit being lost and some particles smaller than xcrit being collected

• Consequently, the cyclone does not achieve such a sharp cut-off as predicted by the theoretical analysis

• Grade efficiency curve for gas cyclones is usually S-shaped

• Particle size for which the grade efficiency is 50%, x50, is often used as a single number measurement of the efficiency of the cyclone

• x50 is also known as the equiprobable size since it is that size of particle which as a 50% probability of appearing in the coarse product

• In a large population, 50% of the particles of this size will appear in the coarse product

• x50 is sometimes simply referred to as the cut size of the cyclone

Scale-Up of Cyclones

• Scale-up of cyclones is based on a dimensionless group, the Stokes number

• Characterizes the separation performance of a family of geometrically similar cyclones

• Stokes number Stk50 is defined as:

• Where m is gas viscosity, rp is solids density, v is the characteristic velocity and D is the diameter of the cyclone body

• Physical significance of the Stokes number is that it is a ratio of the centrifugal force (less buoyancy) to the drag force, both acting on a particle of size x50

• For large industrial cyclones the Stokes number, like the Euler number, is independent of Reynolds number

Range of Operation

• For a particular cyclone and inlet particle concentration, total efficiency of separation and pressure drop vary with gas flow rate as follows:

• Theory predicts that efficiency increases with increasing gas flow rate

• In practice, total efficiency curve falls away at high flow rates because re-entrainment of separated solids increases with increased turbulence at high velocities

• Optimum operation is achieved somewhere between points A and B, where maximum total separation efficiency is achieved with reasonable pressure loss

• Position of point B changes only slightly for different dusts

• Correctly designed and operated cyclones should operate at pressure drops within a recommended range

• For most cyclone designs operated at ambient conditions, this is between 500 to 1500 Pa

• Within this range, total separation efficiency ET increases with applied pressure drop

• Above the top limit the total efficiency no longer increases with increasing pressure drop and it may actually decline due to re-entrainment of dust from the dust outlet orifice

• It is therefore wasteful of energy to operate cyclones above the limit

Storage and Flow of Powders

• In perfect mass flow, all the

powder in a silo is in motion

whenever any of it is draw from

the outlet

• The flowing channel coincides

with the walls of the silo

• Core flow occurs when the

powder flows towards the

outlet in a channel formed

within the powder itself

• Regions of powder lower down

in the hopper are stagnant until

the hopper is almost empty

• In mass flow, motion of powder is uniform and steady state can be closely approximated

• Bulk density of the discharged powder is constant and practically independent of silo height

• Stresses are generally low throughout the mass of solids, giving low compaction of the powder

• No stagnant regions in the mass flow hopper

• Risk of product degradation is small compared with core flow

• First-in-first-out flow pattern of mass flow hopper ensures narrow range of residence times for solids in the silo

• Segregation of particles according to size is less of a problem

• Friction between moving solids and hopper walls results in erosion of the wall, which gives rise to contamination of the solids by material of the hopper wall

• For conical hoppers, the slope angle required to ensure

mass flow depends on the powder-powder friction and

the powder-wall friction

• A hopper which gives mass flow with one powder may

give core flow with another

• In general, powders develop strength under the action of

compacting stresses

• The greater the compacting stress, the greater the

strength developed

• Gravity flow of a solid in a channel will take place

provided the strength developed by the solids under the

action of consolidating pressures is insufficient to

support an obstruction to flow

• An arch occurs when the strength developed by the

solids is greater than the stresses acting within the

surface of the arch

• The hopper flow factor, ff, relates the stress developed in

a particulate solid with the compacting stress acting in a

particular hopper

• A high value of ff means low flowability since high sC

means greater compaction

• A low value of sD means more chance of an arch forming

• Hopper flow factor depends on:

• Nature of the solid, nature of the wall material, slope of

the hopper wall

• Suppose that the yield stress (stress which causes flow) of

the powder in the exposed surface of the arch is sy

• This stress is known as the unconfined yield stress of the

powder

• If stresses developed in the powder forming the arch are

greater than the unconfined yield stress, flow will occur:

• This criterion may be rewritten as:

• The unconfined yield stress, sy, of the solids varies with

compacting stress, sC

• This relationship is called the powder flow function and is a

function only of the powder properties

• The limiting condition for flow is:

• This may be plotted on the same axes as the powder flow function to reveal the conditions under which flow will occur for this powder in the hopper

• The limiting condition gives a straight line of slope 1/ff

• Where the powder has a yield stress greater than sC/ff, no flow occurs

• Where the powder has a yield stress less than sC/ff, flow occurs

• For powder flow function (b), there is a critical condition where unconfined yield stress, sy, is equal to stress developed in the powder, sC/ff

• This gives rise to a critical value of stress, scrit, which is the critical stress developed in the surface of the arch

• The stress developed in the arch increases with the span of the arch and the weight of solids in the arch

• Stress developed in the arch is related to the size of the hopper outlet, B, and the bulk density, rB, of the material:

• Where H() is a factor determined by the slope of the hopper wall and g is the acceleration due to gravity

• An approximate expression for H() for conical hoppers is:

• The following are required for design for ensuring mass flow from a conical hopper:

• (1) relationship between strength of powder in the arch, sy (unconfined yield stress) with compacting stress acting on the powder, sC

• (2) variation of hopper flow factor, ff, with: nature of the powder (characterized by effective angle of internal friction, d), nature of the hopper wall (characterized by angle of wall friction, FW), slope of hopper wall (characterized by semi-included angle of conical section, or angle between sloping hopper wall and vertical)

• Hopper flow factor is therefore a function of powder properties and hopper properties

• Knowing the hopper flow factor and powder flow function, critical stress in the arch can be determined and minimum size of outlet found corresponding to this stress

• Mohr’s circle represents the possible combinations of normal and shear stresses acting on any plane in a body under stress

• Mohr’s circle construction gives the unconfined yield stress, sy

and compacting stress sC

• Experiments have demonstrated that for an element of powder flowing in a hopper:

• This property of bulk solids is expressed by the relationship:

• Where d is the effective angle of internal friction of the solid

• To examine the variation of stress exerted on the base of a bin with increasing depth of powder

• Assume that powder is non-cohesive

• Consider a slice of thickness DH at a depth H below the surface of the powder

• Downward force is

• Where D is the bin diameter and sv is the stress acting on the top surface of the slice

• Assuming stress increases with depth, reaction of powder below the slice acts upwards and is

• The net upward force on the slice is then

• If the stress exerted on the wall by the powder in the slice is sh and the wall friction is tan FW, then the friction force (upwards) on the slice is

• The gravitational force on the slice is

• Where rB is the bulk density of the powder, assumed to be constant throughout the powder (independent of depth)

• If the slice is in equilibrium the upward and downward forces are equal

• If we assume that horizontal stress is proportional to vertical stress and does not vary with depth,

• As DH tends to zero,

• Integrating,

• If in general, the stress acting on the surface of the powder is svo (at H = 0) the result is

• If there is no force acting on the free surface of the powder, svo = 0

• When H is very small

• Equivalent to the static pressure at a depth H in fluid density rB

• When H is large,

• And so vertical stress developed becomes independent of depth of powder above

• Contrary to intuition, force exerted by a bed of powder becomes independent of depth if the bed is deep enough

• Hence most of the weight of the powder is supported by the walls of the bin

• In practice, the stress becomes independent of depth (and independent of any load applied to the powder surface) beyond a depth of about 4D

• Rate of discharge of powder from an orifice at the base of a bin is found to be independent of depth of powder unless the bin is nearly empty

• Observation for a static powder that pressure exerted by the powder is independent of depth for large depths is also true for a dynamic system

• Confirms that fluid flow theory cannot be applied to the flow of a powder

• For flow through an orifice in the flat-based cylinder, experiment shows that:

• Where a is a correction factor dependent on particle size

• For cohesionless coarse particles free falling over a distance h, their velocity neglecting drag and interaction, will be

• If these particles are flowing at a bulk density rB through a circular orifice of diameter B, then the theoretical mass flow rate will be: