SAJJAD KHUDHUR ABBASCeo , Founder & Head of SHacademyChemical Engineering , Al-Muthanna University, IraqOil & Gas Safety and Health Professional – OSHACADEMYTrainer of Trainers (TOT) - Canadian Center of Human Development

Episode 42 : Gas Solid Separation

INTRODUCTION

The process may be interpreted to mean both degassing of solids and dedusting of the solids.

3 phases may be distinguished in any gas cleaning process, i.e;• transport of particles onto a surface (separation)• collection of separated particles from the separation

surface into discharge hoppers (or particle fixation)• disposal of the collected particles from the gas cleaning

equipment

All phases are equally important as the failure of any of the phases will result in the failure of the separation process

.

In the 1st. phase, forces are applied to the particles in order to bring them to a collecting surfaces. The principles of particles separation are usually classified according to the nature of the force involved. These may be:1.External forces due to fields of acceleration which are external to the gaseous suspension, such as gravity, electrostatic or magnetic forces2.Internal forces due to fields of effects which take place within the suspension itself. E.g. intertial or centrifugal forces, diffusion, coagulation, electrostatic effect  Gas cleaning equipment often combines 2 or more of the above mentioned principles in one unit.

The most common classification of g-c equipment- is divided into 4 groups, i.e.

1. aero-mechanical dry separators in which gravity and/or inertial effects prevail. This group includes yclone, settling chambers, inertials separators, dual vortex separators and fan collectors (or mechanical cyclones)

2. aero-mechanical wet separators (scrubbers) which make use of diffusional and inertial effect

3. electrostatic precipitators which depend on electrostatic and gravity forces

4. filters which use inertial and diffusional effects

General characteristics of equipment

The factors affecting the choice of gas cleaning equipment. For any particular application are;

1. flowrate-pressure drop relationship2. efficiency3. economic criteria4. suitability for different conditions (the nature of both solids and ga)5. solids concentration,6. method of disposal7. reliability

Flowrate-pressure drop relationship

Most gas-cleaning devices have fixed relationship between static pres. drop and gas flowrate, depending on the configuration of the equipt.

Most frequently, the relationship is expressed is expressed as: 

P =(1/2).Eu.v2g (1)

where:P is static pressure dropEu is the Euler number, and is a resistance coeff. (analogous to CD) which may be

function of Re, other operational variables such as the feed concentration of solids, etc.v is characteristic velocity = Q/A where A is characteristic area in the separator ( e.g cross-section

of the cylindrical body in cyclone)g is gas density

 One exception in the application of eq. 1 is in air filters, where pressure drop is also a function of the amount of dust deposited on the filter.

Efficiency

It is an important criteria since it governs the degree of cleaning. It is best expressed as gred efficiency. G(d) increases from zero for ultrafine particles to 100 for coarse particles.

Figure 6.1 shows typical gred efficiency curves for dry sep (D), wet sep. (W), electrostatic and filter

Economic criteria

It is consist of the capital and running costs. Capital cost is normally expressed per 1000m3 of cleaned gas per hour. It may be further split into the cost of the construction material, cost of labour, erection, design, etc.

The running cost include cost of power, maintenance, water, etc.

Suitability for different conditions

Thera are a number of other factors such as gas temperature and humidity, the cohesiveness and abrasiveness of th dust, reliaility, limits in dust concentrations, etc, which may exert an overiding influence on the final choice

Gas Solid Separation: Cyclone

Definition of cyclone separators

A cyclone separator is an equipment for the removal from air streams of particles above 10 micrometer in diameter. The equipment is a settling chamber in the form of a vertical cylinder, so arranged that the particle laden air spirals round the cylinder to create centrifugal forces which throw the particles to the outside walls (Learle, 1966 )

Advantages/disadvantages

The advantages of cyclone and all aero-mechanical dry separators include: Simple design Low capital cost Suitability for higher temperatures Low energy consumption Product is dry ReliabilityDisadvantages:Their relatively low efficiency for very fine particles which leads to their

frequent role as a pre-cleaner

Cyclone types most commonly used

Cyclone separators can be classified according to either their geometrical configuration in (tangential inlet axial discharge, tangential inlet peripheral discharge, axial inlet and discharge, and axial inlet peripheral discharge, Figures a-d below, respectively)

or their efficiency in (( high efficient (98-99%), moderate efficient ( 70- 80%),and low efficient ( 50%)) (Othmer,1978) and (Storch et al.,1979).

Figures (Othmer, 1978.) [112]Figure a. Tangential inlet, axial discharge.Figure b. Tanential inlet, peripheral dischargeFigure c. Axial inlet, axial dischargeFigure d. Axial inlet, peripheral discharge

Operating principles of cyclone separators

Although there are four commonly used cyclone separators, their operating principles based on that of the conventional cyclone, are very similar

In the conventional cyclone, the gas enters a cylinder tangentially, where it spins in a vortex as it proceeds down the cylinder.

A cone section causes the vortex diameter to decrease until the gas reverses on itself and spins up the center to the outlet pipe or vortex finder.

A cone causes flow reversal to occur sooner and makes the cyclone more compact.

Dust particles are centrifuged toward the wall and collected by inertial impingement.

The collected dust flows down in the gas boundary layer to the cone apex where it is discharged through an air lock or into a dust hopper serving one or more parallel cyclones (Othmer, 1978 ) .

Although conventional cyclones can be built to larger diameter, they are commonly 600 to 915 mm in diameter.

Operating principles of cyclone separators

Applications of cyclone separators

Cyclones can be used for separating particles from liquids as well as from gases and they can also be used for separating liquid droplets from gases (Learle,1966 ).

The first cyclones used for dust separation probably were built about 1885 by the Knickerboker Company ( U.S.Pat.325,521) (Othmer, 1978 ).

In industries such as food industries, cyclones are used for removing the dry product from the air.

In synthetic detergent production, fast reactor cyclones are used in separating a cracking catalyst from vaporized reaction products (Coker, 1993 ).

Cyclones are used for classification as for example, in the degritting of kaolin clay where sand is removed from the crude clay suspension before finer classification in a conveyor discharge centrifuge and final product recovery in a disk centrifuge.

Applications of cyclone separators

Key parameters of cyclone separators

The most important parameters of a cyclone as for any separating device are its collecting efficiency and the pressure drop across the unit. (Storch ,et al.,1979).

The collecting efficiency of a cyclone is defined as its ability to ability to capture and retain dust particlescapture and retain dust particles

whereas the pressure drop is the amount of power that the unit needs to do so.

Pressure Drop

Factors that contribute to cyclone pressure drop (static pressure differential across the cyclone):

1. Gas expansion as it enters the cyclone2. Formation of vortex3. Wall friction 4. regain of rotational kinetic energy as pressure energy

2

21 vEuP f

Pressure Drop

EU is a resistance coeff., the Euler no.

f is the gas density

is the characteristic velocity

2/4 DQv Q is the gas flow rate

D is the cyclone inside diameter

Efficiency Is defined as the fraction of particles of acertain size that

are collected by the cyclone. It increases with:1. Increasing particle diameter and density2. gas inlet velocity3. cyclone diameter4. cyclone length5. Drawing some of the gas from the cyclome through the

dust exit6. Wetting the cyclone walls

Typical cyclone fractional efficiency curve

The particle size for which the grade efficiency is 50%, d50 is often used as a single number measurement of the efficiency of the ccyclone.

d50 –the cut size of the cyclone (or other separation device)

Scale up

The scale up of cyclones is based on a dimensionless group, the stokes number, which characterized the separation performance of a family of geometrically similar cyclones.

DxStk p

18

250

5050

12Stk

Eu

Geometries , Euler and Stokes numbers for 2 common cyclones

Cyclone type

A B C E J L K N 

Eu Stk50

 

Stairmand, HE

4.0

2.5

1.5

0.375

0.50 0.2 0.5 0.5 320 1.4 x 10-4

Stairmand, HR

4.0

2.5

1.5

0.575

0.875 0.375 0.75

0.75

46 6 x 10-3

Correctly designed n operated cyclone should operate at pressure drop within a recommended range ( at ambient conditions), is between 50-150 mm of water gauge (WG) (500-1500Pa)

Data necessary for cyclone design

Particle size distribution Inlet dust loading (g/m3) Particle density (kg/m3) Gas flowrate (m3/h) Gas temperature (oC) Special condition of corrosivity, abrasiveness, fluctuation in

gas flowrate.etc.

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