Low-Velocity Pneumatic Conveying Technology for Plastic Pellets

P.W. Wypych, D.B. Hastie and J. Yi

Centre for Bulk Solids and Particulate Technologies Faculty of Engineering, University of Wollongong

Wollongong NSW 2522, Australia

The main aim of this paper is to present: the main features and characteristics of low-velocity dense-phase pneumatic conveying technology; the different methods of feeding, including important design and operating considerations, such as rotary valve venting; dense-phase conveying performance characteristics of plastic pellets, demonstrating possible variations and the effect of particle properties, pipeline configuration and process parameters; methods that can be used to control such dense-phase systems, especially where capacity variations are required; some results from the modelling of pressure drop and the dense-phase transport boundaries. INTRODUCTION “Dense-phase” pneumatic conveying is a generic term that generally refers to the non-suspension flow of particles. Many different modes of dense-phase have been developed to take advantage of the different properties and characteristics of bulk solids used in industry. For example [1-4]: 1. Fluidised dense-phase takes advantage of the good fluidisation and air retention properties of certain

powders (e.g. cement, fly ash, pulverised coal, lead dust, flour, carbon fines) and is considered the most reliable and efficient method of conveying.

2. Low-velocity slug-flow (LVSF) has been developed to allow friable and/or granular products with good permeability and low cohesion to be conveyed with extremely low levels of particle damage (e.g. sugar, grain, skim milk powder, peanuts, semolina, muesli, catalysts). In this mode of flow, a stationary layer of material is formed between the moving slugs.

3. Low-velocity plug-flow appears similar to slug-flow, but is suited more to cohesive powders (e.g. full-cream milk powder, drinking chocolate). The stationary layer of material that occurs in LVSF usually does not occur in plug-flow. Specialised equipment has been developed to promote consistent plug formation (e.g. air knife, oscillating distribution of air between the blow tank feeder and pipeline) [1].

4. By-pass dense-phase conveying is applied to a unique range of quickly deaerating materials (e.g. alumina, poly powder, coarse fly ash, atomised aluminium powder) that are otherwise “troublesome” in conventional (off-the-shelf) pipeline systems (e.g. severe plugging, pipe vibrations and/or pressure surges). Various by-pass technologies are available, such as external/internal by-pass and multi-point injection.

5. Extrusion flow is used for certain bulk solid “mixtures” that can be pneumatically “pumped” over relatively short distances. This mode of flow results in a pipeline completely “full” of material and is beneficial for certain products and applications (e.g. meat lumps or chopped fish chunks and gravy for canned pet food).

6. Single-slug conveying involves the transportation of a limited batch of material per conveying cycle. It is applied usually to coarse/heavy granular materials that are not suited to the above modes of flow (e.g. crushed coal, pet coke, blue metal, bone char) and relatively short conveying distances (e.g. up to 100 m). Materials suited to LVSF also can be conveyed in the single-slug mode. However, the latter is

considered relatively inefficient in terms of tonnage. For plastic pellets, the preferred mode of transport is LVSF. The main reason for selecting this mode of dense-phase is to avoid the many problems experienced during dilute-phase conveying (e.g. streamer/floss/dust generation, product deposition, cross contamination and poor quality control). The streamer/floss problem is exacerbated for: •

Freshly extruded material, where the granules still can be quite warm and “viscous” internally - such particles are more susceptible to damage/deformation caused by frictional heat generation (e.g. sliding around bends or along long straight sections of pipe) and impact/shear forces caused by internal ledges/protrusions (e.g. pipe misalignment);

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

•

Industries that employ vacuum conveying systems to unload the plastic pellets into their processes and overlook the relatively high conveying velocities and rates of velocity increase – compared with “equivalent” positive-pressure systems [5-6]. The main features of low-velocity pneumatic conveying of plastic pellets are listed below:

•

Plastic pellets display natural slugging ability [1] – hence, commercial slug-forming devices, such as air knives, choppers or oscillating air supply, are not needed.

•

Due to the extremely high levels of concentration that occur during transportation, the subsequent operating conditions depend quite strongly on the physical properties of the plastic pellets being conveyed. For example, changes in particle size and even particle shape (via different cutters) can result in significant changes in pressure drop and minimum transport behaviour. Hence, it is important that large-scale pneumatic conveying tests be carried out prior to the design or selection of such equipment, especially if relatively large capacities and/or conveying distances are required.

•

The average material transport velocity can be controlled and maintained easily in the range 0.25 to 5 m s-1 (depending on product properties and throughput requirements). Despite these low velocities, reasonable conveying rates still can be achieved due to the high volumetric concentration of product inside the pipe.

•

Due to material characteristics (e.g. permeability) and the relatively low velocities that are used, the conveying cycle can be stopped and restarted normally at any time.

•

Very little inter-particle movement occurs in the full-bore moving slugs. •

Efficient LVSF requires the proper distribution and control of air flow and pressure. •

Pipeline purging can be achieved by increasing the air flow under controlled conditions. •

The “traditional” acceleration length after the feed point is not needed, as indicated later in Figure 4. FEEDING METHODS The most common methods of feeding can be summarised into the following three categories: •

Single blow tank, Figure 1, where a batch of material is conveyed per cycle – this method of feeding is relatively inefficient due to the appreciable time needed to establish steady-state operation, especially for long conveying distances and/or large conveying rates. Note a discharge valve may not be needed for such feeders, as shown in Figure 1.

•

Tandem blow tanks, Figure 2, where two vessels in parallel are used to maintain an almost continuous feed of material into the pipeline. Note a piggy-back blow tank system (i.e. blow tanks in series) can be used to provide a truly continuous method of feeding, but the significant headroom needed for such a system usually precludes its selection.

•

High-pressure rotary valve (HPRV), Figures 3 and 4, which provides a continuous method of feeding at a greatly reduced headroom and capital cost (i.e. compared with tandem blow tanks). Such valves usually can be operated up to ≈350 kPag, but also can be custom-designed up to ≈500 kPag. Some important features include: anti-chopping inlet (Figure 5); deep feeding shoe (Figure 3) or slug-forming hopper (Figure 4); level probe (Figure 3) to avoid over-feeding and back-chopping problems; special axial (endplate) and/or radial (rotor tip) seals to minimise air leakage, especially at high operating pressures. Of the three previously described methods of feeding, the HPRV tends to be the mostly widely used in

industry. Furthermore, the “off-the-shelf” 350 kPag models tend to be used in most applications, where conveying distances are < 200 m and tonnages < 30 t h-1. In more demanding applications (e.g. 40 t h-1 over 400 m), where higher operating pressure are required, tandem blow tank feeders usually are selected. However, the new ≈500 kPag custom-designed valves also are being applied gradually to such systems.

One major disadvantage of the HPRV is air leakage, which: •

can reduce the filling efficiency of the rotor and hence, feed rate; •

can adversely affect the flow of material into the rotary valve; •

must be allowed for when sizing the air supply system (e.g. compressor, flow control); •

can be a significant proportion (e.g. up to 50%) of the air supplied to a LVSF system, especially for relatively small sizes of conveying pipeline, such as 80 and 100 mm NB;

•

usually “removes” a certain amount of material from the valve (due to particles being “caught” in the returning empty rotor pockets – caused by leakage and dynamic effects);

• is extremely difficult to estimate/measure (e.g. due to the complex dependency on product properties, operating conditions, valve clearance, number of pockets, rotor speed, etc).

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

Conveying Air

Top Air VentMaterial Inlet

Pipeline

Blow Tank

AerationAir

Figure 1 : Single (batch) blow tank feeder for low-velocity slug-flow.

Conveying Air

Top Air Material Inlet

Blow Tank 1

Aeration Air

Top Air VentMaterial Inlet

Pipeline

Blow Tank 2

AerationAir

Full-Bore Discharge Valves

Figure 2 : Tandem blow tank feeding system for low-velocity slug-flow.

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

Figure 3 : High-pressure rotary valve feeder.

Figure 4 : High-pressure rotary valve with slug-forming hopper.

Figure 5 : Anti-chopping inlet for Waeschle rotary valve feeder.

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

For these reasons, particular care must be given to the design and installation of the HPRV and an

appropriate method of venting. Body venting (Figure 6), which removes the air leaking up from the “empty” side of the valve, is quite popular. However, air also leaks up through the “product” side of the valve (see Figure 6). A more effective method that can remove the majority of air leakage is to install a small hopper and vent pipe above the HPRV, as shown in Figure 7. An adjustable or fixed baffle-plate also usually is installed inside the vent hopper to encourage the separation of product and air (and minimise the carry-over of material into the vent pipe) – the end of a baffle-plate shaft can be seen at the top of the vent hopper in Figure 7. One advantage of using an adjustable baffle-plate is the option of throttling the flow of material into the HPRV. Note the on-off valve shown in Figure 7 is used to avoid air leaking from “stationary” rotary valves in a multi-feeder system. Another venting method, which combines the advantages of body and hopper venting is shown in Figure 8. Note the tangential connection is designed to separate out centrifugally any material escaping up through the body vent pipe.

Vent

Figure 6 : Rotary valve body venting.

Once the venting method is selected, the vent pipe then must be designed/selected to suit the material and HPRV (e.g. matching size of vent pipe to air leakage to avoid deposition/blockage; minimising use of bends; maximising use of vertical sections – so that material can run back down into the valve when the conveying system is turned off). Most vent pipes are directed back to the top of the feed bin or a stand-alone filter. However, in some cases, a long vent sock (supported by a wire cage) is attached directly to the vent pipe. A potential problem with the latter is the likelihood of material building up inside the vent sock, causing over-pressurisation and bursting of the sock. Hence, the former option of venting back to the top of the feed bin is preferred. CONVEYING CHARACTERISTICS A typical set of pneumatic conveying characteristics (PCC) is provided in Figure 9, which shows the variation of total pipeline pressure drop (∆pt) with respect to air mass flow rate (mf) and constant solids mass flow rate (ms) in the dense-phase LVSF regime. Note the dilute-phase (suspension-flow) regime and the pressure minimum curve (PMC), which tends to indicate the onset of saltation under dilute-phase conditions, are included for comparative purposes.

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

Figure 7 : High-pressure rotary valve feeder with vent hopper.

Vent Pipe

Vent Pipe

Vent Hopper

Tangential Connection

Figure 8 : Combined body and hopper venting.

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

Air Mass Flow Rate

Dilute

DenseUnstable

or Blockage

Tota

l Pip

elin

e Pr

essu

re D

rop

Solids Mass Flow Rate

PMCUnstable

B

A

C

ms1 ms2 ms3

Figure 9 : Typical pneumatic conveying characteristics of plastic pellets.

Despite the popularity and widespread use of dense-phase conveying, the accurate prediction of PCC

(especially the LVSF boundaries) still is not possible from first principles. Also, the effects of pipe wall material, particle properties and pipe diameter have not been investigated properly as yet. Hence, many researchers and/or designers have adopted the approach of determining this information on a small-scale rig and then scaling up this information to the actual system. For example, refer to Figures 10 and 11, which show the predicted (scaled-up) PCC of two different plastic pellets that were tested in a pilot-plant and had to be conveyed through the same stepped-diameter pipeline. These two PCC also demonstrate the common approach of using a constant pick-up air velocity (Vfi) to define each LVSF boundary (in this case, Vfi = 3 and 7 m s-1). Note due to changing production requirements, each material had to be conveyed according to: •

Plastic Pellets A: 10 ≤ ms ≤ 27 t h-1; •

Plastic pellets B: 10 ≤ ms ≤ 24 t h-1; •

Maximum conveying pressure ≈ 280 kPag. From Figures 10 and 11, it was found that the one conveying air flow would not be able to cope with the

proposed variations in throughput and product grade. Hence, it was necessary to develop a “clever” control system (using sonic nozzles) according to the following strategy: mf = 0.30 kg s-1 for 10 ≤ ms < 16 t h-1; and mf = 0.38 kg s-1 for ms > 16 t h-1. This case study demonstrates: the relative sensitivity of LVSF with respect to particle/bulk properties; the relatively narrow operating range of LVSF (compared with dilute-phase); and the importance of determining accurate PCC over the full range of possible operating conditions. Modelling Some progress has been made [7-10] in the fundamental modelling of LVSF, but the pressure drop predictions still are inaccurate and the reliable estimation of dense-phase boundaries has not been achieved to date. For these reasons, investigations have been carried out recently [11] on various horizontal pipes (to avoid bend and vertical flow effects): D = 60.3 and 98.4 mm and L = 21 m. This effort has resulted in the development of a new theoretical model [12] to predict pressure drop and the LVSF Boundary B. To date, good agreement has been obtained between the experimental data and predictions – examples are presented in Figures 12 and 13. Note in Figure 12: each experimental data point represents a successful operating condition; the “blank” region located between Boundaries B and C depicts the unstable zone. Some other findings obtained from this work are provided below: •

It was necessary to develop new bench-scale testers for stress transmission coefficient and particle-wall friction (using actual sections of conveying pipeline) [11];

•

Boundary B was found to comprise: an upper section (high solids flow over slowly moving bed) and lower section (low solids flow over stationary layer), where it was difficult to maintain good slugging;

•

Boundary B cannot be defined accurately using the “popular” constant Vfi approach, Figures 10 and 11; •

Boundary A was found to be more of a feed-limitation boundary (in this case, a HPRV); •

A capacity-limitation boundary was found to occur in the upper region of the LVSF regime (dependent on the “maximum” slugging ability of the material).

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

0

100

200

300

400

500

0.15 0.25 0.35 0.45 0.55 0.65

Air Mass Flow Rate (kg/s)

Tota

l Pip

elin

e Pr

essu

re D

rop

(kPa

)Vfi = 3 m/s

Vfi = 7 m/s

Poly Pellets A

10

16

27ms (t/h)

Figure 10 : Predicted PCC of Plastic Pellets A (D = 175/200/230/250mm and L = 181 m).

0

100

200

300

400

500

0.15 0.25 0.35 0.45 0.55 0.65

Air Mass Flow Rate (kg/s)

Tota

l Pip

elin

e Pr

essu

re D

rop

(kPa

)

Vfi = 3 m/s

Vfi = 7 m/s

Poly Pellets B

10

1620

ms (t/h) 2530

Figure 11 : Predicted PCC of Plastic Pellets B (D = 175/200/230/250mm and L = 181 m).

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

0

10

20

30

40

50

0 0.04 0.08 0.12 0.16 0.2

Air Mass Flow Rate (kg/s)

Pres

sure

Dro

p (K

Pa)

Length of pipeline=21 mPipe diameter=98 mmParticle d iameter=4.5 mm

Particle dens ity=897 kg/m3

Bulk dens ity=546 kg/m3

fw=0.2fp=0.5Kw=0.75

Model predictionExperiment data

ms=1.82; 1.46; 1.14; 0.77; 0.38 kg/s

D

B

Figure 12 : Comparison of experimental results with Boundary B model predictions for plastic pellets, D = 98.4 mm ID and L = 21 m stainless steel horizontal pipeline [11-12].

0

10

20

30

40

50

0 10 20 30 40 50

Experimental Pressure Drop (KPa)

Pred

icte

d Pr

essu

re D

rop

(Kpa

) Length of pipeline=21 mPipe diameter=98 mmParticle diameter=4.7 mm

Particle dens ity=897 kg/m3

Bulk dens ity=546 kg/m3

fw=0.2fp=0.5Kw=0.75

Figure 13 : Comparison of experimental results with pressure drop model predictions for plastic pellets, D = 98.4 mm ID and L = 21 m stainless steel horizontal pipeline, based on [11-12].

Unstable

Dense

Dilute

C

6th World Congress of Chemical Engineering Melbourne, Australia, 23-27 September 2001

CONCLUDING REMARKS Plastic pellets display natural slugging ability and hence, are generally good candidates for dense-phase low-velocity slug-flow. However, due to the extremely high levels of concentration that occur during transportation, the subsequent operating conditions depend quite strongly on the physical properties of the material being conveyed.

High-pressure rotary valve feeders are being used increasingly for low-velocity slug-flow systems. Due to air leakage effects, particular care must be given to the design and installation of the rotary feeder, an appropriate method of venting and the interface between the feeder and the pipeline.

The common approach of using a constant pick-up velocity to define the low-velocity slug-flow boundary is inaccurate. This boundary is dependent on particle/bulk properties, solids loading and air flow, and is represented well by the new model described in this paper.

ACKNOWLEDGEMENTS The authors would like to thank the International Fine Particle Research Institute, Inc, for their continued funding, without which the PCC research presented in this paper would not have been possible. REFERENCES 1. Wypych, P.W. and Hauser, G., Design considerations for low-velocity conveying systems & pipelines,

Pneumatech 4, Int Conf on Pneumatic Conveying Technology, Glasgow, Scotland, 1990, Proc, pp 241-260.

2. Wypych, P.W., Arnold, P.C. and Armitage, W.R., Developing new methods for the pneumatic transport of bulk solids through pipelines, Chemeca 88, Sydney, 1988, Proc, pp 652-656.

3. Pan, R., Mi, B. and Wypych, P.W., Pneumatic conveying characteristics of fine & granular bulk solids, KONA Powder and Particle, No 12, 1994, pp 77-85.

4. Wypych, P.W. and Arnold, P.C., Plug-phase pneumatic transportation of bulk solids and the importance of blow tank air injection, Powder Handling & Processing, Vol 1, No 3, 1989, pp 271-275.

5. Wypych, P.W., The problem with dilute-phase pneumatic conveying, The 3rd Israeli Conf for Conveying and Handling of Particulate Solids, 2000, The Dead Sea, Israel, Proc, pp 10.45-10.58.

6. Stoess, H.A., Pneumatic Conveying, 2nd Edition, John Wiley & Sons, New York, USA, 1983. 7. Konrad, K., Harrison, D., Nedderman, R.M. and Davidson, J.F., Prediction of the pressure drop for

horizontal dense phase pneumatic conveying of particles, Pneumotransport 5, London, UK, 1980, Proc, pp 225-244.

8. Legel, D. and Schwedes, J., Investigation of pneumatic conveying of plugs of cohesionless bulk solids in horizontal pipes, Bulk Solids Handling, Vol 4, No 2, 1984, pp 399-405.

9. Mi, B. and Wypych, P.W., Pressure drop prediction in low-velocity pneumatic conveying, Powder Technology, Vol 81, No 2, 1994, pp 125-137.

10. Pan, R. and Wypych, P.W., Pressure drop and slug velocity in low-velocity pneumatic conveying of bulk solids, Powder Technology, Vol 94, 1997, pp 123-132.

11. Wypych, P.W., Prediction of optimal operating conditions for dense-phase pneumatic conveying, Annual Progress Report for IFPRI, Inc, Dec 2000, Centre for Bulk Solids & Particulate Technologies, 19 pp.

12. Yi, J. and Wypych, P.W., Modelling the unstable boundary for low-velocity slug-flow conveying, The 3rd Israeli Conf for Conveying and Handling of Particulate Solids, 2000, The Dead Sea, Israel, Proc, pp 10.11-10.17.