che572 chapter 7 gas cyclone

8/19/2019 CHE572 Chapter 7 Gas Cyclone

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7.0 Separation of Particles from a Gas: GasCyclones

- Cyclone : Generic name given to severaldifferent devices that have the commonattribute of utilizing centrifugal force toseparate to separate particulate from a gas orliquid flow stream.

-

Application : Dust removal devices usedwithin industrye.g:- In fluidized bed - entrains of fine particles

must be removed from the gas and returnedto the bed before the gas can be dischargedor sent to the next stage in the process.

- In the combustion of solid fuels, fineparticles of fuel ash become suspended inthe combustion gases and must be removedbefore the gases can be discharged to theenvironment.


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- Advantages of Cyclone- Low capital cost- Ability to operate at high temperature

- Low maintenance requirement – no movingparts

- Disadvantages of Cyclone- Low efficiencies (especially for very fine

particles)- High operating costs (due to pressure drop)

- Cyclone Efficiency (from research study)- Greater than 98% (particles larger than

5 µm)

- 90% (particles larger than 15-20 µm)- Not suitable for large proportion of particles

less than 10 micron- Most efficient for particles size of 2 µm

7.1 How Cyclones Work

a. Reverse-Flow Cyclone

- Inlet gas enters near the top of the cyclonetangentially into the cylindrical section and astrong vortex is thus created inside thecyclone body (Figure.1)


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Figure 1: Reverse Flow Cyclone Separator

- Centrifugal force and inertia cause theparticles to move outward, collide with theouter wall and then, slide downward to thebottom of the device


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- Near the bottom of the cyclone, the gasreverses its downward spiral and movesupward in a smaller inner spiral

- The cleaned gas exits from the top while theparticles exit from the bottom of the cyclone

b. Straight-Flow Cyclone

- The straight-flow cyclone operates using thesame principle as the reverse-flow cycloneonly the gas maintains the same direction of flow while spinning without reversedirection

7.2 Flow Characteristics

- Rotational flow in the forced vortex within thecyclone body gives rise to a radial pressuregradient

- Static pressure drop = Pressure gradient +frictional pressure losses at the gas inlet

and outlet and losses due to changes in flowdirection


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- This static pressure drop, measuredbetween the inlet and the gas outlet, isusually proportional to the square of gas

flow rate through the cyclone

- Resistance coefficient, the Euler number Eu,

relates the cyclone pressure drop ∆P to a

characteristic velocity, v where ρf is the gasdensity:

)2 / ( 2v

p Eu

f ρ

∆= (1)

- Characteristic velocity, v:2 / 4 D q v π = (2)

Where q is the gas flowrate and D is the

cyclone inside diameter

7.3 Efficiency of Separation

a. Total Efficiency and Grade Efficiency

- Total material balance: M = Mf + Mc (3)

• Mc (known as the coarse product)

• Mf (known as the fine product), solids massflow rate leaving with the gas


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- ‘Component’ material balance for eachparticle size x (assuming no breakage orgrowth or particles within the cyclone);

• Component:

M(dF/dx) = Mf(dFf /dx) + Mc(dFc /dx) (4)

where, dF/dx, dFf/dx and dFc/dx are thedifferential frequency size distributions by mass (i.e. mass fraction of size x) for thefeed, fine product and coarse product

respectively. F, Ff and Fc are thecumulative frequency size distributions by mass (mass fraction less than size x) forthe feed, fine product and coarse productrespectively.

- The total efficiency of separation of particlesfrom gas, ET, is defined as the fraction of thetotal feed which appears in the coarse productcollected, i.e.

• ET = Mc /M (5)


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Figure 2: Typical Grade Efficiency Curves for Gas-Parti


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- The grade efficiency which the cyclonecollects particles of a certain size is definedas;

• feedinxsizeofsolidsofmass

productcoarseinxsizeofsolidsofmassG(x) =

Using the notation for size distributiondescribed above,

M(dF/dx)

/dx)(dFM G(x) cc= (7)

• Combining with Equation (5), we find anexpression linking grade efficiency with totalefficiency of separation:

) / (

) / ()(

dx dF

dx dF E x G c T = (8)

- From Equations (3) to (5),

/dx))(dFE-(1+ /dx)(dFE=(dF/dx)fTcT

(9)

• In cumulative form this becomes;

F = ETFc + (1- ET)Ff (10)

b. Cyclone Grade Efficiency in Practice

- In practice the cyclone does not achieve sucha sharp cut-off as predicted by the theoreticalanalysis


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Figure 3: Theoretical and actual grade efficiency

Actual

xcritical


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- The grade efficiency curve for gas cyclones isusually S-shaped (Figure 3)

- For such a curve, the particle size for whichthe grade efficiency is 50%, x50 is often usedas a single number measurement of theefficiency of the cyclone.

• x50 has a 50% probability of appearing in thecoarse product. This also means that, in a

large population of particles, 50% of theparticles of this size will appear in the coarseproduct.

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

• The concept of x50 cut size is useful where

the efficiency of a cyclone is to beexpressed as a single number independentof the feed solid size distribution, such as inscale-up calculation.

7.4 Scale-up of Cyclones

- Based on a dimensionless group, the Stokesnumber, which characterizes the separationperformance of a family of geometricallysimilar cyclones.


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• The Stokes number, Stk is defined as

D

v x Stk

p

µ 18

2

50

50 = (11)

where µ = gas viscosity

ρp = solids density

v = characteristic velocity

D = diameter of the cyclone body

- For large industries cyclone and forsuspensions of concentration less than about5g/m3, the stokes and Euler numbers areconstant for a given cyclone geometry.(geometric proportion relative to cyclonediameter, D) as in Figure 4


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Figure 4 : Geometries for two commoncyclones

A B C E J L K N

H.E 4.0 2.5 1.5 0.375 0.5 0.2 0.5 0.5

H.R 4.0 2.5 1.5 0.575 0.875 0.375 0.75 0.75

High EfficiencyStairmand CycloneStk50 = 1.4x10

-4

Eu = 320

High FlowrateStairmand CycloneStk50 = 6x10

-3 Eu = 46


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- Perry (1984) gives the grade efficiencyexpression:

]) / (1[

) / (2

50

2

50

x x

x x

efficiency grade += (12)

for a reverse flow cyclone with the geometry:

A B C E J K N

4.0 2.0 2.0 0.25 0.625 0.5 0.5

This expression gives rise to the gradeefficiency curve as shown in Figure 5 for an x50

cut size of 5 µm.


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Figure 5: Grade efficiency curve described by equatio

size x50

= 5 µm


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7.5 Range of Operation

-

Total efficiency of separation and pressuredrop vary with gas flow rate.

- Theory predicts that efficiency increases withincreasing of gas flow rate.

- Referring to Figure 6;• In practice; ET curves falls at high flowrates,

due to re-entrainment of separate solidsincreases with increased turbulence at highvelocity

• Optimum operation is between point A and B

• Position of point B changes slightly fordifferent dust

• Correctly designed and operated cycloneshould operate at pressure drops withinrecommended range (approximately 500to 1500 Pa)


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• Within this range ET increases with ∆P

• Above the top limit, ET not increases withincreasing ∆P and it will decline due to re-entrainment of dust from dust outlet (so?Energy will be wasted to operate the cycloneabove the limit)

• At ∆P below the bottom limit, cyclone will onlyact as settling chamber (why? Low ET due tolow velocity which are not capable ofgenerating stable vortex)


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Figure 6 : Total separation efficiency and pressure droflowrate through a reverse flow cyclone


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7.6 SOME PRACTICAL DESIGN AND OPERATION DETAILS

The following practical considerations for design

and operation of reverse flow gas cyclones listedby Svarovsky (1986).

7.6.1 Effect of Dust Loading on Efficiency

-

High dust loadings (above about 5 g/m

3

leadto higher total separation efficiencies due toparticle enlargement through agglomeration ofparticles (caused, for example, by the effect ofhumidity).

7.6.2 Cyclone Types

- Divided into two main groups:

a. high efficiency designs (e.g. Stairmand HE)

b. high rate designs (e.g. Stairmand HR).

- High efficiency cyclones:

a. give high recoveries

b. characterized by relatively small inlet andgas outlet orifices.


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- High rate designs:

a. low total efficienciesb. but low resistance to flow: a unit of a given

size will give much higher gas capacity thana high efficiency design

c. large inlets and gas outlets

d. shorter.

7.6.3 Abrasion

- important aspect of cyclone performance

-

affected by the way cyclones are installed,operated, material construction and design.

- two critical zones for abrasion:

i. in the cylindrical part just beyond the inletopening

ii. in the conical part near the dust discharge.


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7.6.4 Attrition of Solids

-

break-up of solids

- usually occurs in recirculating systems (suchas fluidized beds where cyclones are used toreturn the carry-over material back to thebeds)

7.6.5 Blockages

- usually caused by overloading of the solidsoutlet orifice

- cyclone cone rapidly fills up with dust, the

pressure drop increases and efficiency fallsdramatically.

- blockages arise due to mechanical defects inthe cyclone body bumps on the cyclone cone,

protruding welds or gasket) or changes inchemical or physical properties of the solids(e.g. condensation of water vapour from thegas onto the surface of particles).


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7.6.6 Discharge Hoppers and Diplegs

-

If the cyclone operates under vacuum, anyinward leakages of air at the discharge endcause particles to be re-entrained and willleads to decrease in separation efficiency.

- If the cyclone is under pressure, outwardleakages may cause a slight increase inseparation efficiency, but also results in lossof product and pollution of the localenvironment.

- therefore, it is best to keep the solidsdischarge as gas-tight as possible.

- in fluidized beds with internal cyclones,‘diplegs’ are used to return the collectedentrained particles into the fluidized bed.


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7.6.7 Cyclones in Series

-

to increase recovery.

- the primary cyclone would be of medium orlow efficiency design and the secondary andsubsequent cyclones of progressively moreefficient design or smaller diameter.

7.6.8 Cyclones in Parallel

- for a given cyclone geometry and operatingpressure drop, x50 decreases with decreasingcyclone size

- the size a single cyclone is determined bythat gas flowrate

- for large gas flowrates - the resulting cyclonelarge, the x50 cut size is unacceptably high.

- split the gas flow into several smallercyclones operating in parallel. Thus, both the


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operating pressure drop and x50 cut sizerequirements can be achieved.

7.7 General Design Procedure

1. Seleact either H.E or H.R design

2. Obtain an estimate of particle sizedistribution of the solids in the stream to be

treated3. Estimate the number of cyclones needed

4. Calculate the cyclone diameter

5. Calculate the scale-up factor

6. Calculate the cyclone performance andoverall efficiency. If unsatisfactory, try a

smaller diameter7. Calculate the cyclone recommended

pressure drop.

che572 chapter 7 gas cyclone

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