1

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 35 : Design Approach to Dilute Phase Pneumatic

Conveying

2

Design Approach to Dilute Phase Pneumatic Conveying

3

1. Introduction

• For many years gases have been used successfully in industry to transport a wide range of particulate solids - from wheat flour to wheat grain and plastic chips to coal.

• The pneumatic transport of particulate solids is broadly classified into two flow regimes: dilute (or lean) phase and dense phase

5

Dilute phase transport –

•high gas velocities (greater than 20 m/s), •low solids concentrations (less than 1% by volume) •low pressure drops per unit length of transport line (typically less than 5 mbar/m). •limited to short route, continuous transport of solids at rates of less than 10 tonnes/hour •the only system capable of operation under negative pressure. •solid particles behave as individuals, fully suspended in the gas, and fluid-particle forces dominate.

6

Fig. 1: Dilute phase transport

7

dense phase transport

•low gas velocities (1-5 m/s), •high solids concentrations (greater than 30% by volume) •high pressure drops per unit length of pipe (typically greater than 20 mbar/m). •particles are not fully suspended and there is much interaction between the particles.

the boundary• the boundary between dilute phase flow and

dense phase flow is not clear cut, and there are as yet no universally accepted definitions of dense phase and dilute phase transport. [see defination from Konrad (1986)]

• In this chapter the "choking velocity” will be used to mark the boundary in vertical pipelines and the "saltation velocity” will be used to mark the boundary in horizontal pipelines.

9

-- 2: The choking velocity in vertical transport –

Fig. 3. Phase diagram for dilute phase vertical transport, showing the general relationship between pressure gradient p / L and gas velocity for a vertical transport line.

•In the region DE as velocity solids concentration rapidly, and a point is reached when the gas can no longer entrain all the solids.

•At this point a flowing, a slugging fluidized bed is formed in the transport line. -known as "choking" and is usually attended by large pressure fluctuations

•UCH is defined as the lowest velocity at which this dilute phase transport line can be operated at the solids feed rate G1.

10

Punwani (1976) :

(Note that the constant is dimensional and that S.I. units must be used). Equation 1 represents the solids velocity at choking and includes the assumption that the slip velocity USLIP is equal to UT (see section 4 below for definition of slip velocity). Equations 1 and 2 must be solved simultaneously by trial and error to give CH and UCH.

(Eq. 1 and Eq. 2) where

CH is the voidage in the pipe at the choking velocity UCH p is the particle density   f is the gas density  G is the mass flux of solids (= Mp / A) UT is the free fall or terminal velocity, of a single particle in the gas (Note that the constant is dimensional and that S.I. units must be used).

11

-- 3: The saltation velocity in horizontal transport --

Fig. 4: Phase diagram for horizontal pneumatic transport

At point D the gas velocity is insufficient to maintain the solids in suspension and the solids begin to settle out in the bottom of the pipe. The gas velocity at which this occurs is termed the saltation velocity.

12

The correlation of Rizk (1973),

Xs,g is the solids loading = [mass flowrate of solids] / [mass flowrate of gas] Mp is the mass flowrate of solids USALT is the superficial gas velocity at saltation       where (The units are S.I.)

D is the pipe diameter x is the particle size

13

The total pressure drop across a length of transport line has in general six components:       1. pressure drop due to gas acceleration       2. pressure drop due to particle acceleration       3. pressure drop due to gas-to-pipe friction       4. pressure drop related to solid-to-pipe friction       5. pressure drop due to the static head of the solids       6. pressure drop due to the static head of the gas

4. Pressure drop

14

Steps in Dilute Phase Conveying Design

1. Material and Gas properties (density, size shape, viscosity, etc.)

2. Specify desired conveying rate3. Estimate pipe diameter (with a little practice you get a

feel for what works)4. Calculate saltation velocity5. Check loading (mass solids/mass gas)

• If > 10 to 15 then need larger diameter pipe• If < 1 then need smaller diameter pipe

15

Steps Continued

6. Calculate pressure drop• May require iteration• If too large, may need to gradually increase pipe sizes

(telescoping)• Do not let velocity drop below saltation velocity

7. Size blower

16

Components sum to total pressure drop

Filter

Blower Hopper

Other Components

CycloneDust collector

FilterValves

1. Blower, silencer, inlet filter2. Acceleration3. Horizontal pipe section4. Pipe bends5. Vertical pipe section

1.

2. 3.

4.

5.

blower

filtervertbendhorizacc

P

PPPPPP

17

Acceleration• Gas (outside of blower) and

particles must accelerate up to operating velocity– Conveying gas velocity– Gas density– Particle velocity– Pipe cross sectional area– Conveying rate (mass/time) = – Solids loading =

g

p

c

gg

c

ps

c

gg

solidsagasaaccel

VV

gV

AgVw

gV

PPP

212

22

2

gV

g

pV

A

swAVpp

g

s

ww

18

Must relate Vp and Vg

• Vg – Vp = slip velocity• Empirical relation by

Hinkle (1954)– dp = particle diameter

(meters)– Particle density in

kg/m3

• In English units– dp in feet– density in lbm/ft3

503004401 ... ppg

p dVV

503012301 ... ppg

p dVV

19

Bends

• Major source of wear/attrition• Pressure loss associated with re-acceleration of gas and

solids• Bends usually specified by a ratio of the bend radius to

pipe diameter R/D• Typical in conveying R/D = 6 to 12

RD

20

Bends

• Chambers and Marcus (1986)

• R/D >= 6, B = 0.5• R/D = 4, B = 0.75• R/D = 2, B = 1.5

c

ggbend g

VBP

21

2

g

s

ww

21

Horizontal Pipe

c

ggzhoriz g

VDLfP

24

2• Pressure loss

associated with frictional losses by gas and solids

• L = length of horizontal section

• = solids friction factor

•Use experimental data if available

z

g

s

ww

22

Solids Friction Factor

• For dp > 500 microns

• For dp < 500 microns

• where

10

250860300820.

....

psz d

DFrFr

10

25013012.

...

psz d

DFrFr

gDV

Fr g2

gDUFr t

s

2

tU = TerminalVelocity

g

s

ww

23

Vertical• Pressure loss accounts for frictional losses

plus static head (gravity)

cc

ggzvert g

gzgV

DLfP 0

2

24

pp

s

pg

VAw

1

10 Bulk density

Void fraction of gas phase(recall for fluidized beds voidage is typically 0.4 to 0.7)

24

Top Ten Tips for Reliable Design & Operation ofPneumatic Conveying Systems

Lyn Bates, Shrikant Dhodapkar, George Klinzing

1. Allow a ‘reasonable’ horizontal conveying length, (15 to 20 pipe diameters), before the first bend to allow the bulk material to accelerate. Use of flexible hose at the pickup should be kept to minimum. Excessive length of flexible hose, often in the form of a coil, is the worst pickup pipe configuration. Inability to

provide proper configuration at the pickup will resultin plugging condition at gas velocities higher than saltation velocity.

2. Avoid conveying line layout with bends or elbows placed back to back. This will inevitably causes excessive pressure drop and premature line plugging.

25

3. Consider stepping conveying lines (increasing the pipe diameter) to prevent excessive velocity at the end of the line. Be sure to maintain the minimum required Froude number at the step location, otherwise the material will salt out of the suspension. Properly stepped systems

result in more efficient systems with lower degradation and wear. Using ISO pipes or

tubes allows for more choices in diameters.

4. Conveying lines should not be routed like utility (compressed gas / steam) lines which follow

the contours of a building. Minimizing the number of bends or directional change results inhigher capacity, lower degradation, less erosive wear and more reliable flow.

26

5. Electrostatics effects in conveying systems can be minimized by increasing the relative

humidity of the conveying air to more than 70%.

6. ‘More air’ can be ‘Less transfer capacity’ in dilute phase systems. Larger solids and gas

frictional losses caused by higher gas velocities can absorb more energy than the extra inputof energy. There is an optimum gas flow rate for a given lean phase flow system.

27

7. Install sufficient ports or couplings in the systems for pressure measurement during troubleshooting.Pressure measurement is a convenient method to measure the pulse of a conveying system. Take some time to generate the base line data for an existing conveying system. It comes in handy for future troubleshooting.

8. Proper venting of rotary airlock/feeder in a positive pressure system is critical for reliable

operation. Air leakage due to clearances and returning pockets can result in reduced flow or

unsteady feed rate. The leakage can be vented in a number of ways. The vent system shouldbe designed much akin to a conveying system with sufficient gas flow and minimal bends.

28

9. Product damage and wear at bends is very material dependent. Blind Tee's usually have much

merit, but cause a greater pressure drop than long radius bends. Mitered elbows can be

a good compromise in some cases.

10. Minimum conveying velocity is a function of conveying rate. Make sure that the gas velocity

at the pickup is greater than the saltation velocity at the highest solids flow rate. Safety

margin must be allowed for non-optimal line configuration at the pickup (short acceleration

length, bends etc.)

29

Top Ten Tips for Dense Phase Pneumatic Conveying

1. The term “dense phase” is often misused in practice. For example, many so-called “dense-phase” systems are found to be operating in dilute-phase (or suspension flow).Also, many researchers and designers define “dense-phase” as a flow mode with solidsloading greater than 10 or 15. Many different types of dense-phase have been developed over the past few decades to take advantage of certain product properties (e.g. air retention, deaeration, permeability, cohesion, particle size distribution). In most cases, “densephase”can simply be considered as some form of non-suspension flow that occurs at some time at any section along the pipeline. Using solids loading as an indicator of flow mode can be misleading (e.g. solids loading is a mass concentration parameter that depends on the mass or density of the particles; some dilute phase systems are operating at a solid loading greater than 40 and some dense phase systems less than 10). Ensure that the choice of a flow mode (and system) is based on the product properties (rather than an imprecise definition or misleading solids loading), and that the selected or supplied flow mode is confirmed during commissioning.

30

2. Coarse/granular materials that can be conveyed in dense phase (viz. low-velocity slugflow) exhibit an unstable operating zone in between (high-velocity) dilute phase and (low velocity) dense phase conveying. The dense phase regime is bound by a high-velocity (unstable zone) boundary and a low-velocity (blockage) boundary. Make sure the operating point (gas flow rate, solids flow rate) falls well within these bounds at all locations in the system and for all pipeline configurations (if applicable).

3. The minimum and maximum conveying rates in a process must be defined upfront. Compared with dilute phase, the dense phase regime can be more limiting and sensitive to variations in air flow and/or conveying rate. For some materials, a reduction in solids flow rate can shift the operating point into the unstable zone, thereby causing severe instability (line vibrations and pressure spikes).

4. Bench top characterization tests (wall friction, permeability, deaeration, etc.) are helpful for preliminary screening of a material’s suitability for dense phase conveying. Dense phase conveying performance can be quite sensitive to variations in material property (particle size, size distribution, shape, density, moisture, cohesion, etc.). It is highly recommended that pilot scale or full scale testing be conducted on representative material, especially for new or different products where no prior experience is available.

31

5. Using a conventional or “off-the-shelf” pipeline, not all materials can be conveyed reliably in dense phase. Some materials can be conveyed in single or multi slug/plug mode, some in fluidized moving bed type flow, while others can only be conveyed in dilute phase. Not selecting the right flow mode for a particular material or the right operating condition for a given flow mode can result in excessive pressure spikes, system shutdown, unstable vibrations and/or pipeline blockages. For materials that do not have a natural tendency for conventional dense phase conveying, consider specialized systems with controlled and regulated gas injection or bypass pipeline technology. Also, ensure proper dense phase flow actually is achieved during commissioning.

32

6. Feeder gas leakage can be a significant fraction (up to 50%) of total gas consumption. The gas leakage at the feeder (esp. rotary valves) must be considered in design calculations and compensated appropriately. Ensure proper venting at the feeder to avoidfeeding problems that may result from the gas blowback.

7. Use a gas flow control system for multi-product and multi-destination systems to ensure that the operating point is maintained within the stable operating zone. Also, ensurethat the gas flow control system provides a constant gas mass flow rate for the full range of operating pressures and pressure fluctuations. Numerous control logic schemes are available from various vendors or can be designed by reputable consultants.

8. In dense phase systems, gas expansion can be significant between feed point and destination. This will result in a corresponding increase in gas velocity, and a possible transition from dense to dilute phase flow along the conveying pipeline. For high pressure drop systems (7 psi or 50 kPa and higher), consider stepping the line diameter to reduce the velocity and maintain dense phase conditions.

33

9. The motion of slugs and stresses generated within the conveying line during directional changes (bends or diverter valves) results in significantly higher stresses on pipe supports as compared to dilute phase systems. It is essential to work closely with an experienced vendor to design and install proper pipeline supports to prevent excessive deflection and line movement, and reduce the prospect of fatigue failure.

10. To purge a dense phase line clean, a controlled increase in gas velocity may be required. A proper purge control sequence may need to be designed and tested to avoid unnecessary product degradation and/or pipeline blockage. The dust collector must be designed to handle the peak gas flow rates.

34

Thanks for Watching Please follow me / SAJJAD KHUDHUR ABBAS