Dense phase- or dilute phase pneumatic conveying


Written by Teus Tuinenburg

Topic: Pneumatic Conveying

Date: September 11, 2008 11:16

The Netherlands

more information

In the bulk-online forum are several pneumatic conveying questions posed, using the descriptions dense phase conveying and dilute phase conveying.
It seemed then that there was not a general understanding about the definition of the two conveying regimes.
After the discussion on the Forum it became clear that the definition was related to the so called Zenz-diagram.
The Zenz diagram is widely accepted as a description of pneumatic conveying with explanatory properties.
Since the calculation of a Zenz diagram is now possible by an extensive computer program, it is also possible to investigate how the diagram is formed. 

The calculation approach is described in the Bulkblog article “Pneumatic Conveying, Performance and Calculations!”. By varying the air flow at constant capacity, the resulting partial pressure drops were calculated and combined into a table.
The summation of the partial pressure drops results in the total pressure drop of the system under the chosen conditions.
Dividing the calculated pressure drops by the total length, the pressure drop per meter is derived.

This procedure could also be differentiated to partial pressure drops over partial lengths.
Then it can be checked whether one part of the conveying pipeline is in f.i. dense phase, while another part of the conveying pipeline is dilute phase. This not executed for this article.

Zenz diagram 

The curve in the Zenz – diagram represents pneumatic conveying as the pressure drop per unit of length as a function of the air flow (or air velocity).

For this curve the solids flow rate and the pipeline are kept constant.

For a cement conveying pipe line, this curve is calculated. 

The calculation curves are given below:

cement

200

 

ton/hr

 

pipeline

12″

 

meter

 

 

pressure

 

SLR

Pumpvolume

pressure

/ meter

kWh/ton

mu

0,8

24745

134

0,86

55,68

0,9

20475

111

0,82

49,49

1,0

18577

100

0,83

44,54

1,1

17295

93

0,86

40,49

1,13

17048

92

0,87

39,53

1,2

16428

89

0,90

37,12

1,3

15794

85

0,95

34,26

1,4

15333

83

0,99

31,81

1,5

15040

81

1,05

29,69

1,6

14819

80

1,10

27,84

2,0

14612

79

1,37

22,27

2,1

14680

79

1,44

21,21

2,2

14750

80

1,51

20,25

2,3

14875

80

1,59

19,37

2,4

15013

81

1,67

18,56

2,5

15171

82

1,76

17,82

3,0

16175

87

2,22

14,85

3,5

17460

94

2,76

12,73

4,0

18844

102

3,37

11,14

4,5

20340

110

4,05

9,90

5,0

21900

118

4,81

8,91

5,5

23540

127

5,65

8,10

6,0

25260

137

6,57

7,42

 

From 0.8 m3/sec to 2.0 m3/sec, the pressure drop decreases.

This can be explained as the stronger influence of the decreasing loading ratio, opposed to the

weaker influence of the increasing velocity, which would increase the pressure drop per meter.

In addition, the residence time of the particles becomes shorter with increasing velocity and the required pressure drop for keeping the particles in suspension decreases.

From 2.0 m3/sec to 6.0 m3/sec, the pressure drop increases.

This can be explained as the weaker influence of the decreasing loading ratio and the decreasing pressure drop for keeping the particles in suspension, opposed to the stronger influence of the increasing velocity, which increases the pressure drop per meter.

The lowest pressure drop per meter occurs at 2.0 m3/sec.

Left of this point of the lowest pressure drop per meter, the pneumatic conveying is considered: dense phase and on the right of this point, the pneumatic conveying is considered: dilute phase.

As can be read from the calculation table, the loading ratio (mu) is higher on the left part of the curve than on the right part of the curve.

Regarding the energy consumption per ton conveyed, the lowest value occurs at 0.9 m3/sec.

This can be explained as follows:

The energy consumption per ton is depending on the required power for the air flow.
(solids flow rate is kept constant)

This required power is determined as a function of (pressure * flow ).

It appears that the minimum in pressure drop does not coincide with the lowest power demand of the air flow.

As soon as the decreasing airflow (causing lower power demand) is compensated by the increasing pressure drop, the lowest energy consumption per conveyed ton is reached. 

The calculation for an air flow of  0.8 m3/sec indicated the beginning of sedimentation in the pipeline, due to the velocities becoming too low.

From this calculation, it can be concluded that a pneumatic conveying design for the lowest possible energy demand, is also a design, using the lowest possible air flow (or velocity).

The lowest possible velocities are also favorable for particle degradation and component’s wear.

 

Contribution of partial pressure drops to the total pressure drop

To investigate the physical background of the shape of the Zenz diagram, a cement pressure conveying installation is assumed and calculated, whereby the partial pressure drops are noticed.

The installation is described by:

Horizontal conveying length              =         71        m

Vertical conveying length                  =         28        m

Number of bends                               =         2

Pipe diameter                                     =         243      mm (10”)

Capacity basis for Zenz diagram        =         200      tons/hr

The compressor airflow is varied from 0.5 m3/sec to 3.0 m3/sec

The calculation results are presented in the following table.

Compressor flow in m3/sec

0,50

0,55

0,60

0,65

0,70

Pressure drop mbar/meter

 

 

 

 

 

intake

0,10

0,10

0,10

0,10

0,10

acceleration

0,62

1,03

1,27

1,42

1,61

product

14,33

12,26

10,70

9,58

9,63

elevation

5,73

5,00

4,45

4,01

3,65

suspension

21,46

16,93

14,10

12,06

10,35

gas

0,12

0,13

0,13

0,14

0,17

filter

0,02

0,03

0,03

0,04

0,04

 

 

 

 

 

 

total dp

42,39

35,48

30,78

27,35

25,55

kWh/ton

0,90

0,86

0,84

0,83

0,85

SLR

97,90

87,60

79,30

72,60

67,00

 

Sedimentation

 

 

 

 

Sub turbulent flow

Turbulent flow

 

Compressor flow in m3/sec

0,75

0,80

0,85

0,90

0,95

Pressure drop mbar/meter

 

 

 

 

 

intake

0,10

0,10

0,10

0,10

0,10

acceleration

2,65

2,76

2,86

2,95

3,04

product

8,72

8,98

9,16

9,27

9,34

elevation

3,35

3,09

2,86

2,67

2,49

suspension

8,95

7,80

6,89

6,14

5,52

gas

0,19

0,22

0,25

0,29

0,33

filter

0,02

0,06

0,06

0,07

0,08

 

 

 

 

 

 

total dp

23,98

23,01

22,18

21,49

20,90

kWh/ton

0,87

0,89

0,92

0,96

0,99

SLR

62,20

58,20

54,30

51,40

48,60

 

No sedimentation

 

 

 

Turbulent flow

 

 

 

 

Compressor flow in m3/sec

1,00

1,25

1,50

2,00

2,10

Pressure drop mbar/meter

 

 

 

 

 

intake

0,10

0,10

0,10

0,10

0,10

acceleration

3,12

3,55

4,01

4,96

5,16

product

9,37

9,11

8,53

7,22

6,96

elevation

2,34

1,79

1,45

1,06

1,01

suspension

4,90

3,33

2,45

1,59

1,49

gas

0,37

0,61

0,91

1,66

1,84

filter

0,09

0,14

0,20

0,35

0,39

 

 

 

 

 

 

total dp

20,29

18,64

17,65

16,95

16,95

kWh/ton

1,02

1,20

1,39

1,80

1,89

SLR

46,10

36,60

30,30

22,60

21,50

 

No sedimentation

 

 

 

Turbulent flow

 

 

 

 

Compressor flow in m3/sec

2,20

2,30

2,40

2,50

2,60

Pressure drop mbar/meter

 

 

 

 

 

intake

0,10

0,10

0,10

0,10

0,10

acceleration

5,35

5,55

5,75

5,94

6,14

product

6,72

6,48

6,25

6,03

5,82

elevation

0,96

0,92

0,88

0,85

0,82

suspension

1,40

1,33

1,26

1,20

1,14

gas

2,02

2,20

2,39

2,59

2,79

filter

0,43

0,46

0,50

0,55

0,59

 

 

 

 

 

 

total dp

16,98

17,05

17,14

17,26

17,40

kWh/ton

1,99

2,08

2,18

2,29

2,39

SLR

20,60

19,70

18,80

18,10

17,40

 

No sedimentation

 

 

 

Turbulent flow

 

 

 

 

Compressor flow in m3/sec

2,70

2,80

2,90

3,00

Pressure drop mbar/meter

 

 

 

 

intake

0,10

0,10

0,10

0,10

acceleration

6,33

6,52

6,71

6,90

product

5,62

5,42

5,24

5,06

elevation

0,79

0,77

0,74

0,72

suspension

1,10

1,05

1,01

0,98

gas

3,00

3,21

3,42

3,63

filter

0,63

0,68

0,72

0,77

 

 

 

 

 

total dp

17,56

17,75

17,95

18,17

kWh/ton

2,50

2,61

2,73

2,85

SLR

16,80

16,20

15,60

15,10

 

No sedimentation

 

 

Turbulent flow

 

 

The above calculation results are showed in the following graph.

The pressure drop values are added on to each other.

Thereby, the contribution of each partial pressure drop at any airflow becomes visible.

The upper graph, representing the total pressure drop in mbar/meter is the Zenz curve.

 

From the graph, the following information can be taken:

 

  1. Filter pressure drop increases with increasing air volume
  2. Gas pressure drop increases with increasing air volume
  3. Suspension pressure drop decreases with increasing air volume
  4. Elevation pressure drop decreases with increasing air volume
  5. Product pressure drop:
  • In case of sedimentation the product pressure drop decreases with increasing airflow.
  • Above sedimentation, the product pressure drop firstly increases with increasing airflow, due to the stronger influence of the velocity increase opposed to the influence of the SLR.
  • Above the airflow, whereby the influence of the decreasing SLR is stronger than the influence of the increasing velocity, the product pressure drop decreases with increasing airflow.
  • The acceleration pressure drop increases with increasing airflow.
6. The pressure drop for the material intake stays constant
7. The summation of the partial pressure drops for each airflow generates the Zenz curve.
8. The lowest point in the Zenz curve, where the pressure drop per meter is minimum, is the boundary between dense- and dilute phase conveying.
Left of this point, the pneumatic conveying is considered dense phase conveying.
In dense phase conveying the solid Loading Ratio (SLR) is higher than in dilute phase conveying.
As the absolute SLR is depending on the conveying length, the absolute SLR in it self is not an indication about the conveying regime being dense or dilute.
9. At lower airflows, sedimentation starts because of too low air velocities and at even lower velocities, the airflow changes from turbulent to non turbulent.
In that region the pneumatic conveying circumstances vary significantly with a small change in airflow.
Airflow changes can also occur through air surges, caused by material flow irregularities.
Induced pressure waves forces the pneumatic conveying conditions from f.i  asub turbulent flow into a turbulent flow and combined with changes in sedimentation, the pneumatic conveying behavior can become unstable, which is often observed in practice.
When the graph is build up in the reverse order (from intake to filter), discontinuities show up in the curves for acceleration and product resistance.

The remarkable discontinuities in the graph are caused by the introduction of criteria for sedimentation and turbulence into the calculation algorithm.
10. The lowest energy consumption occurs at the point where the turbulence changes from turbulent to non turbulent.
Sedimentation is then already present.
The lowest pressure drop per meter is therefore not the most energy efficient conveying design.
This is because the energy is the multiplication of pressure and airflow.
Although the pressure is low, the airflow has increased more, resulting in a higher energy consumption.

 

In general an installation should be designed in dense phase in order to have the benefit of low energy consumption per ton. Although in dense phase, the installation is not necessarily optimal, regarding energy efficiency.

In case the design is in the region where sedimentation and turbulence changes are occurring and the material conveying properties are difficult under that regime, then additional measures have to be taken to ensure controlled “unstable” conveying or redesign to a stable conveying region.

Also irregular feeding can induce unstable behavior.

The magnitude in which these phenomena are occurring depends on installation time constants and -time responses to changes.

Whether an installation is operating in the dense phase mode or the dilute phase mode can only be concluded after calculations for different air flows have been executed.

Judging whether a conveying installation is in dense- or dilute phase just by the Solid Loading  Ratio is useless, because a long pipe line (low SLR) can be in dense phase and a short pipe line (high SLR) van be in dilute phase.

These examples show that dense- and dilute phase pneumatic conveying are 2 regions by definition, but belong to the same pneumatic conveying technology.

In theoretical discussion, the interpretation can be very useful, but in practice it is not so important, because a properly designed installation should always be the installation with the lowest energy demand and still meeting the desired performance.

This could be a dense phase installation but a dilute installation as well.

The interpretation of dense- and dilute phase conveying should now be clear.

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