17 Innovatory Conveying Systems
1
INTRODUCTION
The motivation to use dense phase conveying technology arises generally from a desire to convey at low velocity in order to avoid a range of operational problems; in particular, the problems of erosive wear of pipelines and fittings or the attrition of the conveyed material. However, the ability or otherwise of a material to be conveyed in a dense phase flow regime depends on the particle and bulk properties of the material to be conveyed. For materials that have natural dense phase performance in either of the major modes of dense phase flow, no special equipment is required. For these materials a standard pipeline and feeder may be used. In general, dense phase systems tend to use a blow tank to feed the conveying line since this device can operate over a very wide range of pressure conditions. For materials which do not exhibit natural dense phase capability, there is often a need to use specialized techniques and equipment to encourage the material to convey reliably in a dense phase mode of flow. Three basic approaches are used in order to condition the material in the conveying system. The first method involves a form of plug creating device at the feed point which aims to control the plug or slug formation in order to limit the size of plug which is initially fed into the conveying line. The second approach is to use an air addition system, commonly known as boosters, to inject additional
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
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Chapter 17
conveying air at various points along the pipeline in an attempt to ensure that the material in the pipeline is maintained in a fluidized condition. The third approach is to use an internal or external bypass line which aims to limit the maximum size of plug that will form in the conveying line. The exact design of these systems vary considerably, depending on the particular manufacturer, but they have all been used with varying degrees of success. There is, however, a distinct lack of detailed technical literature underpinning these systems, and hence the aim of this chapter is to approach these systems in a generic manner and to explain their operation as far as is possible from a technical point of view, rather than to review the systems of specific manufacturers. 2
PLUG CREATION SYSTEMS
In general, plug creation systems involve the use of a blow tank as a feeder in which the supplementary air supply is controlled in order to artificially create plugs of material of a given length. Typically, the supplementary air injection point is located in the conveying pipeline just downstream of the blow tank discharge valve. The exact positioning is important since if the injection point is located too far downstream, the pressure drop across the extruded flow in the discharge pipe will lead to unacceptably high pressure drops across the discharge pipe. This type of system was first developed following research undertaken by the Warren Spring Laboratory in the UK [ I ] in the early 1970's. A sketch of a typical system, as originally developed, with a bottom discharge blow tank feeding device, is shown in Figure 17.1. 2.1
Principle of Operation
The pulse-phase system consists of a pressure vessel feeding a conventional pipeline. The air supply to the blow tank is supplied both to the top of the vessel and to an aeration ring located around the conical section. The aeration ring provides fluidizing air which ensures that the material remains in a fluid-like state. This ensures that, for powdered products, the material flows in a reliable manner into the pipeline. The aerated state of the material also ensures that the material can be more easily split-up into plugs. At the start of the conveying line, an 'air knife' device is located. The air knife is essentially an annular device with a ring of small holes equally spaced around the pipeline. The air supply to the air knife is controlled to be either on or off using a timer and a solenoid valve. When the air knife is operating, a series of air cushions are created between the material plugs. The frequency of the solenoid switching will provide a degree of control over the plug length. Although this concept was originally created to handle fine powdered materials, the device has been used successfully for a wider range of materials including granular materials.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
487
Innovatory Systems
Air
110/240 v
Solenoid Valve
Air Knife
Figure 17.1 Schematic of the 'Pulse-Phase' System.
This technology has been licensed to a number of vendor companies around the world who have also developed and refined the basic concept. Many systems have been operating around the world with solids loading ratios exceeding 300 in very short systems. Operating velocities have been reduced to values between about 300 and 600 ft/mm. 2.2
Stress State in Slugs During Feeding
Despite the use of 'pulse-phase' type systems, there is much evidence to suggest that the plug formation for many granular materials occurs quite naturally and that for coarse granular materials, with a high degree of permeability, no such conditioning is necessary. Hitt [2] found that for most free-flowing materials, no special conditioning was required at all, and that material plugs formed spontaneously and settled to a steady conveying condition during the steady state period of the blow tank cycle. Considerable discussion has also taken place regarding the stress state existing in the plug at the beginning of the pipeline and whether this is influenced by the method of feeding and/or any conditioning of the material that takes place at the feed point. Research undertaken by Li et al [3] suggests that when a full bore plug of material is formed at the feed point, the condition of the material during the plug formation does, in fact, influence the stress state in the slug and hence
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
488
Chapter 17
may influence the subsequent behavior of the material in the conveying line. Further work is required in this area to fully understand the operation of such systems. 3
AIR ADDITION SYSTEMS
Air addition along the length of the conveying pipeline is a method of conditioning the gas-solid mixture during conveying. Two approaches are generally used. The first involves the continuous addition of small quantities of air at regular intervals along the length of the pipeline. The second approach aims to prevent a pipeline blockage from occurring by injecting air into the pipeline at the point where a blockage is about to occur. The principle behind the first approach is to ensure that the material remains in a fluidized condition and hence can be conveyed in the fluidized mode of dense phase along the entire pipeline. In the second approach, air is only injected at the time and position it is required in order to prevent or clear a blockage. Usually, the control of this type of operation is based on a pressure signal. 3.1
Continuous Air Addition
The motivation to provide continuous air addition along the pipeline is generally an attempt to keep the material in the pipeline in an aerated state. In practice, this is very difficult to achieve and in general leads to velocities which are significantly higher than necessary. The most critical velocity in pneumatic conveying is the pick-up velocity or the velocity at the point where the material is fed into the pipeline. In most cases, and certainly for single bore pipelines, the velocity at the feed-point will be the lowest throughout the pipeline. Therefore, it is essential that the required minimum transport velocity is maintained at this point. In conventional systems, the air flow rate required for conveying is based on the m i n i m u m transport velocity for the material concerned. As the air expands along the pipeline, with the fall in static pressure, a natural consequence is for the air velocity to increase in proportion to the ratio of the static absolute pressures. Clearly the addition of air at various points along the conveying line will lead to a further increase in air velocity beyond that due to the expansion of the air. Clearly, this is not desirable with the potential for increased erosion and/or attrition, in addition to the higher specific energy requirements for conveying. Figure 17.2 shows the relationship between air velocity and pipeline length, both with and without air addition. The graph is based on a 4 inch single bore pipeline and an air supply to the start of the conveying pipeline of 216 fV/min of free air (scfrn). In both cases the air velocity at the pick-up point is 1500 ft/min and the conveying line pressure drop is 10 Ibf/irr. At this low velocity, it is clear that the mode of flow would be dense phase. In the case of no air addition, the air expands to an air velocity of 2500 ft/min at the end of the pipeline.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
489
Innovatory Systems
Pressure Drop = 101bf/in 2
10,000 With Air Addition
8,000 I
No Air Addition
6,000
£ I
'g
4,000
2,000
100
200
300
Length - feet Figure 17.2
Air velocity versus pipeline length with and without air addition.
In the air addition case, boosters are located every 10 feet along the entire length of the conveying pipeline and the air flow rate to each booster is 20 scfm. In this case, the air velocity reaches 9500 ft/min by the end of the pipeline. An important point to note is that in the case of a booster system of this type, the velocity at all points in the pipeline will be higher than in the case where the boosters are not used. It is clear that the use of continuously operating boosters makes velocity control in the pipeline very difficult. In the case illustrated above, it can be seen that even if the system operates in dense phase initially, the velocity at the end of the pipeline indicates that the system will be operating in dilute phase by that stage. Even if the air flow rates to each booster are halved, the exit velocity will still reach 6000 ft/min. A degree of control over the air velocity could be achieved by careful stepping of the pipeline at appropriate positions along the pipeline length. This is always a useful technique for controlling the air velocity, as considered in Chapter 9. A further consideration for continuous air injection systems is the level of gas flow rate required at each injection point to ensure reliable conveying. Clearly, the quantity of air will depend on the material being conveyed. Very little information is available in the literature to provide guidance on this point with most manufacturers of systems treating any information they have on this point as commercially confidential.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
490
3.2
Chapter 17
Non-continuous air addition systems
Many of the disadvantages of continuous air-addition systems can be overcome by using non-continuous systems. In general, these systems are aimed at either preventing a blockage from occurring, or breaking up a blockage that is in the process of forming. These systems generally consist of a main conveying pipeline and a parallel air only pipeline with a series of connections at some predetermined spacing. Associated with each of these 'boosters' is some form of pressure measurement. In general, the pressure measurement is aimed at determining when a significant pressure drop is detected between two boosters, which is assumed to indicate the location of a blockage, or a potential blockage. Once detected, the boosters inject air into the pipeline. The quantity of air injected, the injection period and the overall control of the injection vary from manufacturer to manufacturer and, once again, this information is generally considered to be commercially confidential. The action of the air injection under these circumstances has many similarities to the action of the bypass system which is considered in the next section. However, a critical difference between the two generic systems is that, in the case of the air addition systems, the air injected is additional to the air supply provided at the feed point to the system. 4
AIR BYPASS SYSTEMS
The air bypass system consists of two pipes; a main conveying pipeline and a second small bore pipeline which may be internal or external to the main pipeline. The small bore pipeline has openings into the main conveying pipeline at predetermined intervals which allow the conveying air to move between the two pipelines. A schematic of an internal bypass arrangement is given in Figure 17.3. Figure 17.4 shows a schematic of an external bypass arrangement.
Figure 17.3
Schematic of internal bypass arrangement.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
Innovatory Systems
Figure 17.4
491
Schematic of external bypass arrangement.
The work of Barton [4] is probably the most recent work dealing specifically with bypass systems. The work was predominantly a global approach to the problem which involved a direct comparison between two pipelines of the same geometry; one containing an internal bypass pipeline and the other without. A range of bypass lines were used, including copper pipelines with varying flute (or hole) spacing, as well as a totally porous pipeline constructed from a permeable polymer. The conveyed material was alumina, and various grades were used in the test program. The test pipeline was 160 ft long with 6 x 90° bends each having a bend diameter to pipe bore ratio of 6:1. The pipeline was two inch nominal bore and the majority of the pipeline was in the horizontal plane, with only 12 ft being vertical. This pipeline could be operated as a conventional pipeline with no bypass line. Alternatively, an internal bypass pipeline could be inserted. Various designs of bypass pipeline were tested during the course of this work. To establish the effect of the bypass pipeline, Barton used a macro approach to pipeline testing, whereby the performance of various bypass systems were compared directly with a conventional pipeline of the same geometry. The general effect of the internal bypass was two-fold; firstly, conveying at lower minimum conveying velocities was achieved when the bypass line was fitted; and secondly, the stability of the material flow was significantly enhanced. However, as expected, the conveying rate for a given air flow rate and pressure drop was reduced, largely due to the reduction in flow channel cross-sectional area due to the presence of the internal bypass pipeline. In the case of the sandy grade of Alumina, the m i n i m u m superficial gas conveying velocity in the conventional pipeline was about 2000 ft/min. In the case where a bypass line was installed, minimum conveying velocities were as low as 220 ft/min. However, whereas maximum conveying rates of about 37,000 Ib/h were achieved with a conveying line pressure drop of 45 lbf/in 2 in the conventional pipeline, maximum rates of about 28,000 Ib/h were achieved under similar conveying conditions where the bypass line was installed. This is due to the reduction
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
492
Chapter 17
in cross-sectional area of the main pipeline when the internal bypass line is installed. 4.1
Analysis of the Operation of a Bypass Line
The primary function of the bypass pipeline is to prevent blockages of the main conveying pipeline from occurring. The bypass line achieves this in two ways. Firstly, the bypass line provides an alternative route for the air to flow when a blockage occurs. Secondly, the provision of an alternative route for the air to flow prevents a build up of pressure behind the blockage, which reduces further compaction. The total air (or conveying gas) flow rate in the system is the sum of the gas flows in the main pipeline and in the bypass line. The ratio of these flow rates will depend on the comparative resistance in each of the two flow channels. Under steady state conditions, the pressure profiles in each of the two flow channels will be the same. Clearly, if an increase in the flow resistance occurs in the main pipeline, a greater flow rate will occur in the bypass line in order to balance the pressures. It is clear, therefore, that the design of such a system must be based largely on the resistance of the bypass pipeline. The most likely reason for an increase in pressure in the main pipeline will be due to the formation of a plug or slug of material. Hence, the pressure required to move a plug of material in the main pipeline is of critical importance to the design of bypass systems. Barton [4] approached the problem by first attempting to establish the relationship between the length of the material plug and the pressure required to move the slug. An example of the results reported in Barton's thesis is given in Figure 17.5 for alumina. Knowing the relationship between slug length and the pressure required to move the slug, Barton chose a critical slug length which corresponded to the maximum pressure available to move the slug. The bypass pipe diameter and the flute spacing was then selected to balance the resistance across a slug of critical length. To establish the relationship between the slug length and the pressure required to move the slug, Barton carried out experimental trials and compared the results with the relationship developed by Konrad [5] given in equation I : Ap L
D
D
' "°
'"'
- - - - - -
D ( 1 )
A comparison between the calculated values and the measured values of pressure drop against plug length for sand are also given in Figure 17.5.
Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.
493
Innovatory Systems
uu -
CL
o
o 5 ^c o " 20QJ •*->-< 3
LO [/]
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L-
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Konrad Model
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• Barton's
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Test Data II
10 20 Plug Length - ft
^
(b)
" Barton's
Test Data
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Konrad Model
10 20 Plug Length - ft
30
Figure 17.5 Pressure drop versus plug Once the relationship between the plug length and the pressure required to move the plug is determined, a decision can be made regarding the maximum desired plug length that will be allowed to occur in the pipeline. This decision would be made based on the system pressure available with some margin of safety. Barton's analysis focuses on determining the diameter of bypass pipeline required to ensure that a plug never exceeds the critical plug length. The analysis is based on a constant mass flow rate of gas supplied to the pipeline system such that: (2)
or m
(3)
m
