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Energy Savings and Higher Outputs for Cable Compounds with

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					                 Energy Savings and Higher Outputs for Cable Compounds
                        with the Newest Buss Kneader Technology
                                           Colin G. Richardson, Heini Gruetter
                                                        Buss AG
                                                    Basle, Switzerland
                     +41 61 8256848 · colin.richardson@busscorp.com, heini.gruetter@busscorp.com,


Abstract                                                                     A fundamentally new concept for the entire processing section
It is now some 60 years that the plastics industry has available             provides the basis for the improved performance and quality.
continuous compounding equipment. The reciprocating single                   Firstly, the ratio of outer to inner (root) diameter of the screw has
screw system of Buss is part of that history. The first Buss                 been increased, see Figure 1.
kneader for the continuous compounding of PVC was installed in
1950. In the intervening years developments and improvements
have been made but the operating system of the kneader has
remained essentially the same.
In 2001 a brand new concept of the kneading mechanism was
introduced into PVC compounding [1]. Now, similar
enhancements are being introduced into the new MX machine for
polyolefin compounds. It brings innovations to the fundamental
mixing mechanism permitting significantly higher outputs for
equivalent or better compound quality.
The advances in the reciprocating screw Kneader MX series
represent decisive gains with regard to applications, operating               Figure 1. Relationship Free Volume, Torque and Stroke
economics and investment outlays [2]. For the equivalent size
machine, up to 2.5 times increased outputs are foreseen. At the              As a result, the increased free volume gives a much larger
same time there are quantifiable gains in product quality. For               processing chamber. The transition to four-flight screw
example halogen-free flame-retardant cable compounds possess                 technology, however, is the major reason for the performance and
higher mechanical properties even at very high filler levels,                quality improvement. The ratio of stroke to outside diameter is
exhibit better values for flame retardancy and have improved                 increased, permitting a high screw pitch. In addition, there are
processing characteristics.                                                  greater opportunities with regard to design and optimization than
                                                                             with the three-flight screw: Flights with longer flanks improve the
The most important field of application for the MX Kneader is in             conveying characteristics, and the flight geometry can be used to
the production of high quality polyolefin based cable compounds.             influence the mixing action in specific ways in terms of both
These include flame retardant compounds, conductive and                      distributive and dispersive mixing. An open view of the mixing
semiconductive materials, reactive and grafted compounds, highly             section is shown in Figure 2.
filled formulations, black masterbatch and shear and temperature
sensitive applications.
Keywords: Flame retardant; zero halogen; semiconductive;
cable compounds; energy savings.

1. Introduction
The Buss Kneader is a reciprocating single-screw machine where
the screw simultaneously executes an axial oscillating motion
with every rotation. The screw has kneading flights and kneading
teeth or pins are fixed inside the barrel at locations corresponding
to the gaps between the kneading flights. The combination of
rotation and axial motion of the screw produces extensional flow
with highly dispersive mixing action between the kneading flights
and kneading teeth. The efficiency of the distributive mixing
results from the combination of the radial and longitudinal
actions. In the earlier MKS Kneader, each screw flight is divided
into three kneading segments. The latest, high performance,
development features four segments per revolution. This expands
the design possibilities in the processing section.                              Figure 2. Open barrel view showing pin and screw
                                                                                                     elements




International Wire & Cable Symposium                                   222                                     Proceedings of the 57th IWCS
Consequent to the new design features the MX kneaders can be                  dynamic insulation and serves to temperature-condition the inner
operated at speeds up to 750rpm. As a result significantly higher             surfaces of the barrel. This conditioning is necessary to ensure the
outputs are possible for equivalent machine sizes.                            desired material flow characteristics.
                                                                              Only by gaining control of the temperature rise in the processing
2. Process Section                                                            section has it become possible to increase the speed in the MX
2.1 Functional steps in processing                                            Kneader, to as high as 750 rpm, and simultaneously maintain the
Within the processing section, Figure 3, five functional steps are            necessary temperature limits.
defined:                                                                      A single-flight discharge screw is directly flanged to the MX
•   Feeding of raw materials; polymers, additives, fillers                    kneader. The discharge screw builds up the pressure for melt
•   Melting with dissipative introduction of energy                           filtration, as required, and pelletizing and is equipped with an
•   Downstream addition of fillers                                            additional venting/degassing port. For certain applications a gear
•   Homogenizing and conveying with little or no energy                       pump can also be attached for pressure build-up as an alternative
    introduction                                                              to the discharge screw.
•   Degassing, removal of entrained air, moisture, volatiles
                                                                              3. Applications and examples
                                                                              3.1 Halogen free flame retardant (HFFR)
                                                                              compounds
                                                                              Flame retardant compounds are specially designed to withstand
                                                                              fire and to reduce burning. The nature of the filler in these type of
                                                                              materials is non halogen and without acidic gas emission.
                                                                              In order to achieve different levels of flame retardancy the filler
                                                                              level will be increased in the formulation. Many of these
                                                                              formulations will contain up to 65% of reactive mineral filler. Of
                                                                              the mineral fillers available to formulators the most commonly
                                                                              used are aluminium trihydrate (ATH) and magnesium hydroxide.
                                                                              ATH, which is much the most widely used, adds a further demand
                                                                              on the compounding process in that it decomposes and releases
                                                                              the quenching water vapour at approximately 190degC. The
                                                                              compounding and mixing process cannot therefore go above this
                                                                              limiting temperature.
                 Figure 3. Processing section
                                                                              In order to properly introduce and disperse such high filler levels
For each of these steps, the interaction of kneading flight
                                                                              the Buss machine is able to use a split feed technique. A
geometry and kneading pin has been optimized with the aid of
                                                                              proportion of the filler is added with the polymer and the
mathematical models and subsequently confirmed in laboratory
                                                                              remaining portion downstream. The flow sheet of this type of
trials. In this way, and using this knowledge, it was possible to
                                                                              compounding operation is shown in Figure 4 below.
lower the overall specific energy input – indeed it is between 15%
to 25% less in the new development – whilst simultaneously                     Polymer     Additives   Filler 1       Filler 2    Degassing Unit
improving the mixing action.                                                                              M              M




2.2 Geometry and kneader processing interaction
The standard processing section of an MX Kneader has an L/D
ratio of 15:1 (L = length of processing section, D = screw
diameter) and is divided into three barrel segments. For especially
complicated compounds or when the process requires a longer                                       M               M

residence time, the processing section can be lengthened by an                           ISX 1            Open           ISX 2
                                                                                                          Vent
additional 7D to an overall L/D = 22:1. The first barrel zone is
used to feed in polymers, additives and, where necessary, a
portion of the fillers. The second barrel zone with its rear venting            M
is used to add remaining fillers to the now-molten material. The
resultant cooling of the melt, from the new filler addition,                                           MX 77 - 15 l/d
contributes significantly to keeping the stock temperature to a
                                                                                     Figure 4. Schematic compounding line HFFR
minimum and the following melt homogenizing step takes place
without any significant additional energy input. A degassing port             The oscillating screw mechanism means that very little pressure is
is located in the last zone, shortly before the end of the processing         generated along the length of the mixing chamber. The gravity
chamber. It is possible to meter in liquid components at most                 feed of filler downstream is achieved simply by placing a
positions along the whole length of the kneader.                              restriction ring in the chamber just prior to the inlet.
The energy required for melting and mixing is provided almost                 The improvements achieved with the modified geometry of the
entirely by the screw as shear energy. The temperatures of the                processing section can be illustrated in the following diagram
barrel halves of the MX Kneader are controlled using tempered                 showing melt temperature and throughput versus screw speed
liquid medium i.e. water or oil. The heating acts primarily as a              (Figure 5). In spite of the much higher screw speeds in the MX
                                                                              kneader, the melt temperature increases with screw speed much




International Wire & Cable Symposium                                    223                                       Proceedings of the 57th IWCS
more slowly (~3.5°C/100 rpm) than in the MKS Kneader (in                     structure. The process therefore requires introduction and wetting
excess of 20°C/100 rpm).                                                     of the carbon agglomerates, mixing and dispersing to provide a
                                                                             homogeneous, highly smooth finished product. All this whilst
                                                                             respecting strict upper temperature limits according to the type of
                                                                             base polymer used in the matrix. Dispersion is critical since this is
                                                                             the paramount feature for interfacial integrity between the
                                                                             different layers [3].
                                                                             The challenge now lies in incorporating the high level of carbon
                                                                             black combined with effective mixing. The energy input must be
                                                                             carefully controlled since the carbon black chains are sensitive
                                                                             and extremely prone to mechanical scission. Applied shear forces
                                                                             must be kept under control or electrical properties will be
                                                                             adversely affected due to shear breakdown of the carbon black.
                                                                             Figure 6 illustrates the negative result of over mixing.
                                                                                         Conductivity




Figure 5. Temperature and throughput with screw speed
In practice a generic nonhalogen flame retardant formulation
containing 60% of ATH has been compounded on both the 3
flight kneader and the 4 flight MX development. Comparative
processing data is shown below in Table 1 where the higher screw
                                                                                                                              Optimum
speeds and output rates are noted along with the measured                                                                     mixing time
temperatures.
            Table 1. Processing results for NHFR                                                                                   Mixing time

 Kneader       Output   Screw      Temp       ΔT7       ΔT (°C)                          Figure 6. Conductivity with mixing time
  type         (kg/h)   speed      at T7      (°C)         /                 Experiments were run using similar formulations in both the 3
                        (rpm)      (°C)                 100 rpm              flight and 4 flight kneader machines. The information shown in
MKS 70-         180      250        186       ---         21.0               Figure 7 shows the temperature profile of the compounding
  14                                                                         operation in the two systems.
 MX 77-         500      750        171       -15         3.6                                                                                             240



  14
                                                                                                                                                          230

                                                                                                                                                          220

                                                                                                                                                          210

                                                                                                                                                          200

                                                                                                                                                          190




The corresponding energy requirements of the two kneading
                                                                                                                                                          180

                                                                                                                                                          170




                                                                                                                                                                Temperatur °C
                                                                                          24 15:45

systems are shown in Table 2.
                                                                                                                                                          160

                                                                                          27 16:20                                                        150

                                                                                                                                                          140
                                                                                          29 16:45

   Table 2. Comparison of energy data for nonhalogen                                      30 16:55
                                                                                                                                                          130

                                                                                                                                                          120


                     formulations                                                         2 10:00

                                                                                          3 10:15
                                                                                                                                                          110

                                                                                                                                                          100



Kneader type                   Spec. energy            Saving                             6 11:50
                                                                                                                                                          90

                                                                                                                                                          80



                                (kWh/kg)                 (%)                        70              60   50   40
                                                                                                               Pin Position
                                                                                                                              30      20         10   0
                                                                                                                                                          70




MKS 70-14                         0.140              Ref base line
MX 77-14                          0.122                   13                    Figure 7. Temperature profiles 3 flight versus 4 flight
                                                                                                  mixing system
3.2 Semiconductive formulations                                              The 3 flight technology has always used downstream feeding of
Cable performance and cable integrity is dependent on all of the             the carbon black. In this system the polymer matrix is first melted
individual components that go to make up a cable. But                        and the carbon black introduced into the molten polymer via a 2nd
specifically, and never more so as ever high voltage cables are              and 3rd inlet. As a result the machine has been of 20l/d length.
produced from synthetic polymers, on the quality of the extrudate
                                                                             In the 4 flight system a proportion of the carbon black is now
layers. These extrudate layers, predominantly applied in a single
                                                                             introduced along with the polymer base and the remainder
(triple extrusion) pass, are in intimate contact with each other and
                                                                             downstream. The 4 flight offers more effective mixing and better
rely on zero interfacial imperfections for cable integrity.
                                                                             distribution earlier in the machine. Vertical feeding ensures free
Semiconducting cable compounds pose a special challenge. The                 flow of the black into the mixing section without cramming and
layers are incorporated to eliminate electric field deformation and          the risk of compacting. As a result a shorter mixing section of
to provide a uniform electric field within the insulating material.          15l/d is now possible.
To fulfil this function, semiconductive compounds usually
                                                                             The processing data of a 37% carbon black filled formulation is
contain 30 to 40 wt.% conductive carbon black. During
                                                                             shown in Table 3 for the two systems.
compounding, the mixing equipment must disperse the carbon
black agglomerates, but not destroy the individual carbon black




International Wire & Cable Symposium                                   224                                            Proceedings of the 57th IWCS
                                           Table 3. Processing results for semiconductive                                                                              If we can assume that a typical compounding plant is not
                                                            compounds                                                                                                  dissimilar then the specific energy requirements of the
Kneader                                                Output                         Screw         Temp                  ΔT7                  ΔT (°C)                 compounding process are by far and away the largest cost item.
 type                                                  (kg/h)                         speed         at T7                 (°C)                    /                    Using the two applications shown above the specific energy
                                                                                      (rpm)         (°C)                                       100 rpm                 savings were 13% in flame retardant compounds and a huge 31%
 MKS                                                       180                         450           212                   ---                   20                    in semiconductive compounds between the former technology and
 70-20                                                                                                                                                                 the new development. Taking the semiconductive example the
MX 77-                                                     500                         750               209                -3                         7               figures are:
 14                                                                                                                                                                    3 flight kneader:               0.250 kWh/kg (0.113 KWh/lb)
                                                                                                                                                                       4 flight kneader:               0.172 kWh/kg      (0.078 KWh/lb)
The corresponding energy data of the two systems follows in
Table 4                                                                                                                                                                Energy saving:        0.078 kWh/kg       (0.035 KWh/lb)

                                           Table 4. Comparison of power consumption for                                                                                The US average industrial retail price of electricity varies widely
                                                    semiconductive formulations                                                                                        across the country but taking a value of 10cts per kilowatt hour
                                                                                                                                                                       and an operating time of 7000 hours / year the annual savings for
Kneader type                                                                             Spec. energy                                Saving                            the compounder are given in Table 5 for two nominal output
                                                                                          (kWh/kg)                                     (%)                             rates.
MKS 70-20                                                                                   0.250                                  Ref base line
MX 77-14                                                                                    0.172                                       31                             Table 5. Annual energy savings from 4 flight MX kneader
                                                                                                                                                                             technology for semiconductive formulations
Here to the MX kneader met all expectations in production trials.
The surface quality of the compound extrudate is better than                                                                                                                        Output                               Savings
previously, with no loss in conductivity. This indicates that the                                                                                                                 (lbs/hour)                                ($)
carbon black is dispersed without any damage to its structure.
                                                                                                                                                                                     2500                                 61,250
The very important control of temperature is best demonstrated in                                                                                                                    5000                                122,500
Figure 8. The curves illustrate output and temperature with screw
speed. For every 100rpm increase in screw speed the temperature
increase for the 4 flight technology is 7°C in fact much lower than
                                                                                                                                                                       5. Discussion and conclusions
                                                                                                                                                                       The mixing achieved in the MX Kneaders produces enhanced
for the 3 flight system with 20°C per 100rpm. . Residence time
                                                                                                                                                                       product quality, with respect to dispersion, surface finish,
also plays a role here.
                                                                                                                                                                       mechanical and performance characteristics, and, importantly,
                                     600                                                                                                                               processing characteristics.
                                                      MX 77-14 G output

                                                      MKS 70-20 G output
                                                                                                                                                                       What has also been illustrated in the applications so far tested is that
                                     500              MX 77-14 spec. e                                                    MX 77-14                                     the energy requirements in the mixing process are lower, even
                                                                                                                                                                       considerably lower, using the 4 flight technology. This is a
 T = °C, G = kg/h, spec. e = Wh/kg




                                                      MKS 70-20 spec. e

                                                      MX 77-14 Temperature
                                     400              MKS 70-20 Temperature                                                                                            significant result.
                                                      Linear (MKS 70-20 Temperature)


                                     300
                                                      Linear (MX 77-14 Temperature)
                                                                                                                                                                       As a result of the new concept the MX Kneaders can be operated
                                                                                                                                                                       at speeds up to 750 rpm. Compounding can already be made in
                                     200       y = 0.0692x + 156.85
                                                                                                                                                                       the Buss kneader at lower temperatures than in other types of
                                                                                                                                                                       compounding equipment and the temperature increases in the
                                     100       y = 0.1954x + 129.03                             MKS 70-20                                                              MX, at the faster screw speeds, are even lower than in the
                                                                                                                MKS spec. energy average = 176 Wh/kg
                                                                                                                MX spec. energy average = 231 Wh/kg
                                                                                                                                                                       previous MKS series. Consequently, throughputs up to 2.5 times
                                       0
                                                                                                                                                                       higher than previously are attainable for equivalent kneader
                                           0            100                200          300        400         500           600              700          800
                                                                                                                                                                       machine sizes.
                                                                                              n kneader rpm
Figure 8. Temperature and output with screw speed for 3                                                                                                                The MX development is the fruit of theoretical and technical
              flight and 4 flight systems                                                                                                                              design. It has been supported by detailed practical tests and a still
                                                                                                                                                                       ongoing experimental program over a number of applications.
4. Energy needs and savings
Rising energy costs, not to mention energy supplies are subjects of                                                                                                    6. Acknowledgments
topical concern. Energy use in plastics processing [4] is a                                                                                                            The authors acknowledge with special thanks the contributions from
combination of base load i.e. fixed energy required irrespective of                                                                                                    colleagues Hansueli Siegenthaler and Rico Trachsel.
whether production is taking place or not, and process load which
is the variable load which relates directly to process machinery. In
the cited reference (an injection moulding factory) it is calculated                                                                                                   7. References
that of all the energy required in the factory some 77% is used in                                                                                                     [1] C. G. Richardson “Quantec Advances in Compounding
the plastics processing. The remainder is for the offices, general                                                                                                         Technology” in Wire & Cable Technology International, p58,
services and some downstream functions e.g. assembly.                                                                                                                      Vol 34 (2006).




International Wire & Cable Symposium                                                                                                                             225                                      Proceedings of the 57th IWCS
[2] H. Gruetter, R. Trachsel, H-U Siegenthaler, “New Generation                               Heini Grütter began his early career as an
    of Buss Kneader for Cable Compounds” in Kunststoffe                                       apprentice draughtsman at Dätwyler AG in
    International, p. 207 Vol 9, (2007)                                                       Altdorf, Switzerland. After completing his
[3] C. G. Richardson “Compounding of Semiconductive                                           apprenticeship    he    studied  mechanical
    Compounds for High Voltage Cables” Jicable 07, Int.                                       engineering at the Engineering School in
    Conference on Insulated Power Cables, Versailles, France                                  Luzern graduating in 1969.
    (2007).                                                                                   From 1969 to 1974 he worked in the
                                                                         engineering department at Hoffman la Roche in Basel responsible
[4] R. Kent, “Cutting Energy Consumption by 30%” in Plastics             for standardisation and purchase of pumps, tanks and drying units.
    Engineering, p 27, June (2008).                                      In 1974 he joined Buss AG in Basel as process engineer in the
                                                                         research and development department working with various
                                                                         engineering thermoplastics. Since 1978 he has been manager of
8. Authors                                                               the process engineering group involved in a wide range of plastic
                                                                         compounding applications. For the last 15 years he has
                                                                         specialised in wire and cable applications including
                            Colin Richardson has 30 years of
                                                                         responsibilities for world wide plant commissioning.
                            experience in the wire and cable
                            industry     covering      materials
                            development, compounding and
                            compounding machinery. He joined             Address
                            Buss AG in 1993 where he is                  Buss AG
                            Technical Marketing Manager. Prior           10 Hohenrainstrasse
                            to this he held positions in Europe          4133 Pratteln
                            and the USA in the former Wire &             Switzerland
                            Cable Businesses of Union Carbide
                            and BP Chemicals.




International Wire & Cable Symposium                               226                                    Proceedings of the 57th IWCS

				
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