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Influence of temperature on the flow properties of bulk solids
ARTICLE IN PRESS Chemical Engineering Science 65 (2010) 4007–4013
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Influence of temperature on the flow properties of bulk solids Marcus Ripp , Siegfried Ripperger Lehrstuhl Mechanische Verfahrenstechnik, TU Kaiserslautern, Germany
a r t i c l e in fo
abstract
Article history: Received 28 January 2010 Received in revised form 15 March 2010 Accepted 25 March 2010 Available online 31 March 2010
The flow behaviour of bulk solids depends upon a number of parameters. Shear tests are often used to determine the flow behaviour at room temperature. This paper aims to look at the influence of temperature on the flow behaviour of bulk solids. A newly developed ring shear test shows that the flow behaviour of some specific bulk solids depends upon the temperature. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Flow properties Bulk powders Bulk solids Influence of temperature Shear test Design of silos DEM
1. Introduction Bulk solids are found in nearly all industrial sectors. According to product the size of the particles ranges from the submicrometer region to several centimetres. Their flow behaviour depends upon a number of parameters (Ripp, 2009). Normally the flow parameters required for the design of silos are determined at room temperature. In the following it will be shown that the flow behaviour of some bulk solids depends upon the temperature as well. The influence of temperature on the flow behaviour of bulk solids was determined with a newly developed ring shear tester. In the literature this influence has been mentioned (Schulze, 2006; DIN 1055, part 6), however up to now no quantitative data have been presented for specific bulk solids.
2. Temperature dependence of the flow properties of bulk solids The flow properties of bulk solids have been characterised since Jenike (1964) in terms of the effective friction angle je and the wall friction angle jx. Furthermore the flow function i.e. the dependence of the compression strength sc on the consolidation stress s1 is often cited. This relationship considers the influence of a ‘‘pre-consolidation’’ on the compression strength of a bulk solid. The parameters named can be determined using the shear tester Corresponding author.
E-mail address: [email protected] (M. Ripp). 0009-2509/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2010.03.046
according to Jenike or the ring shear tester according to Schulze and the follow-on evaluation of the measured results. Up to now there is no shear tester on the market with which tempering of the bulk solids is possible, enabling the determination of these parameters as a function of temperature. In the following it will be shown that for temperature-dependent parameters and their determination in the wrong temperature region errors are to be expected in the design of silos. The causes for the alterations in the flow properties of a bulk solid in accordance with temperature are manifold. The temperature dependence of the bulk solid parameters can be due to: Changes to the crystal structureBulk solids having a crystal structure such as salts can change their crystal structure under the influence of pressure and temperature. Such changes can influence the flow behaviour as the rigidity form and surface properties of the particles can vary accordingly. Formation and alteration of adsorption layersThe type and formation of adsorption layers in the gaseous phase depend upon the combination of solid (adsorbent) and adsorbate (adsorbed material in the fluid), the structure of the adsorption surface (chemical structure, surface condition), as well as the temperature of the solid and the gaseous phase. A change in temperature brings about a change in the adsorption equilibrium. Such a change can alter the forces of adhesion and thus the flow behaviour. Formation of liquid bridgesLiquid bridges cause forces of adhesion between the particles as a result of the capillary pressure within the bridges and the peripheral force. According to the curvature of the phase boundary surfaces inside the liquid a capillary under-pressure prevails which attracts the particles to each other. The form of the liquid bridge and the magnitude of the
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force of attraction depend e.g. upon the wetting properties of the solid, the surface tension of the liquid, the distance between the particles, and the form of the particles, as well as the geometrical form of particle contact, which for non-spherical particles also depends upon the relative positions of the particles. The formation of liquid bridges can have different causes. With capillary condensation due to concave-shaped phase boundaries within the pore structure the vapour pressure is lowered. As a result depending upon the respective radius of curvature vapour already condenses under conditions for which condensation does not yet take place on a flat surface. This phenomenon depends upon the temperature, the humidity and the content of other vapours as well as the ambient pressure. Bulk solids with good solvent properties (e.g. urea, sugar, salt) liquid bridges can cause a partial transition of the solid to the liquid phase. This process is once again dependent upon the temperature and can have very different effects on the flow behaviour of the bulk solids. Natural products such as coffee or cocoa can contain up to 20% oil internally. With the pulverisation of these products oil can be released to the outside and form liquid bridges between the particles. Because of the temperature dependence of material values such as the viscosity of the oils, a change in temperature also affects the flow behaviour of the bulk solid. Other materials such as lactose or phosphate have water of hydration bound in their crystal structures. This water of hydration can escape from the crystal structure with a change in temperature. The temperature dependence of the flow behaviour for these materials is therefore to be expected. Mixtures of materials composed of different properties are also processed. In such a case it is possible that the softening point of a component is exceeded with a change in temperature, resulting in the formation of a liquid bridge in the molten material. This behaviour is observed e.g. with foodstuffs containing fat such as whey powder and instant soups. Formation of solid bridges Solid bridges are solid material linkages between particles which can transfer relatively large forces. Such linkages can e.g. occur when the material is stored at a temperature in the vicinity of the melting point. If a short-term temperature rise followed by a drop in temperature takes place during storage so-called sinter or fusion bridges are formed (e.g. with plastic). Often solid bridges already form during storage in a temperature range below the melting point for sufficiently long time of contact and compressive stress. Solid bridges can also arise due to the drying out of liquid bridges when material has been dissolved in the liquid bridge (e.g. urea, sugar, salt). Solid bridges are formed by the freezing of liquid bridges when a bulk solid is cooled down to a correspondingly low temperature. Such frozen liquid bridges are then considered solid bridges over which large forces can be transferred. Visco-plastic deformation at the contact points At the contact points of the individual particles in a bulk solid, visco-plastic deformation can take place according to changes in temperature. This entails an alteration of the surface structure and possible of the particle structure in turn resulting in changes to the forces of adhesion and friction between the particles. Such changes occur primarily with waxes or plastic granulates. This also influences the flow behaviour of the bulk solid.
solids or initiated by certain temperatures as well. In each case the flow behaviour of bulk solids changes. This influence was determined with a newly developed ring shear tester. 3.1. Design of the shear cell The ring shear tester according to Schulze is a recognised instrument for the measurement of the flow properties of bulk solids. As a rule it is used up to a particle size of ca. 2 mm at room temperature. For the investigation of the temperature dependence of the flow behaviour for bulk solids a shear cell was developed on the basis of the ring shear tester according to Schulze. The ‘‘internal friction’’ and the ‘‘wall friction’’ can be determined in the temperature range 80to220 1C. A detailed description of the shear tester is given in Ripp (2009). For the new tester the shear cell is arranged on a glass fibre– reinforced teflon bottom. It is driven by the already existing spur gear. As the tie rods are intended to connect the lid of the shear cell horizontally to the shear force transducers, the transducers were also offset upwards. A calibration of the entire shear tester was then performed in accordance with the instructions in the manual for the ring shear tester. During the course of development of the temperaturecontrolled shear cell for the determination of ‘‘internal friction’’, the entire shear cell (vertical walls and bottom) was provided with a double casing through which a heating or a cooling medium can flow. Inside this double casings are conductive plates which ensure the homogeneous distribution of the cooling and heating media and prevent the occurrence of short-circuits. In this way a virtually uniform temperature is achieved over the walls of the shear cell. The shear cell is connected by tubing to a cooling and heating thermostat. The supply tube is joined by a screw connection to the lower-lying connection in order that air can escape via the upper-lying return flow. The entire shear cell is made of stainless steel (material no. 1.4571) (see Fig. 1). A similarly constructed shear cell was also developed for the determination of wall friction (see Ripp, 2009). For the determination of wall friction in this case a wall sample is placed in the shear cell. This cell as well by contrast with other cells according to Schulze has a double bottom through which heating or cooling media can flow.
3. Temperature-controlled ring shear tester The flow behaviour of some bulk solids depends upon the temperature. It could be an effect of time consolidation of bulk
Fig. 1. Temperature-controlled shear cell for the determination of ‘‘internal friction’’.
ARTICLE IN PRESS M. Ripp, S. Ripperger / Chemical Engineering Science 65 (2010) 4007–4013
For the shear cells of a ring shear tester there are fitting the lids of the shear cells. These shear cell lids have a cross-beam joined via the tie rods to the force transducers. On the lower side of the lid webs to prevent the sliding of the bulk solid directly against the lid. For the reduction of heating or cooling losses a new shear cell lid was designed. This lid is isolated and can be equipped with additional heating. The weight of the electrical heating and the two cables to the temperature control are added to the weight of the lid in the software. Fig. 2 illustrates the lid of the shear cell with electrical heating. For measurements at high temperatures a high-temperature heating circulator driven by thermal oil is connected to the shear cell. For temperature control at low temperatures a lowtemperature thermostat is used. The measurement of the bulk solid temperature for the determination of the ‘‘internal friction’’ is made with a thermocouple in the shear zone beneath the lid of the shear cell. For the determination of the ‘‘wall friction’’ the temperature of the wall material sample is measured. 3.2. Test procedure with the temperature-controlled shear cell The tests performed with the temperature-controlled shear cells were analogous to those performed at room temperature. The heating and cooling of the samples takes place directly in the shear cell. Bulk solids with poor thermal conductivity are pretempered in an external tempering cabinet. The measurements with the temperature-controlled shear cells are started when the required reference temperature is reached. As the shear cells are permanently connected via two tubes the bulk solid mass is determined as the difference between a definite sample mass and the remainder which is not filled. In this way it is no longer necessary to weigh the filled shear cell. 3.3. Validation of the temperature-controlled shear cells
Temperature curve and temperature distribution For the determination of the temperature curve for the shear cell the cell for measuring the inner friction was filled with a
Fig. 2. Electrically heated shear cell lid.
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barium sulphate powder. The thermostat was set to 200 1C and the temperatures were measured on the bottom of the shear cell and underneath the lid of the shear cell. At every level four positions in intervals of 901 were measured at the centre of the shear ring. Measurement point 1 was at the inlet for the tempering liquid and measurement point 4 at the outlet. Fig. 3 shows the rise in temperature within the shear cell over the time after switching on the thermostat. The heated lid of the shear cell (Fig. 2) was placed over the filled shear cell and subjected to a load of 2.0 kg. This normal load ensures a defined pressure on the lid of the shear cell. The temperature rise in the flow zone underneath the lid of the shear cell and on the wall material is plotted in Fig. 3. The mean values of the four temperature measurements are plotted as a point. Based on the scattering of the values the radial temperature distribution can be derived. It can be seen that a virtually uniform temperature distribution is obtained. Preliminary investigations have shown that with additional heating the required reference temperature can be set faster over the entire cross-section of the shear cell. During the investigations a thermostat setting of 200 1C and a shear cell lid temperature of 200 1C were used. The results also show that the temperature profile over the perimeter of the shear cell is very uniform following the heat-up phase (20–30 min). Analogous investigations of the temperature distribution were performed with the determination of wall friction. The temperature measurement locations were under the bulk solid (barium sulphate powder) and on the surface of the wall. As the height of the bulk powder in this shear cell was only 10–12 mm measurements were made only on the surface of the wall. The temperature of the thermostat was set to 200 1C. The result shows that in this case with this experimental arrangement a wall temperature of ca. 170 1C can be obtained. This can be explained by the fact that the diameter of the wall shear cell was modelled according to Schulze whereby the outer diameter of the shear cell is larger than that of the lid of the shear cell. Heat is lost from the free uncovered surface of the bulk solid sample. Furthermore a non-insulated lid of the shear cell was used. The required temperature of the wall sample was set by increasing the thermostat temperature and using an insulated or temperaturecontrolled shear cell lid. Comparison of temperature-controlled and non-temperature-controlled shear cells For the validation of the new shear cell five different bulk materials and different wall materials were investigated at room temperature with the temperature-controlled and the nontemperature-controlled ring shear cell. A total of 193 shear tests
Fig. 3. Mean temperature in the shear zone underneath the heated shear cell lid (200 1C) and on the bottom of the shear cell at 200 1C heating temperature.
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were performed. In all shear tests the shear cells were newly filled before measuring. The results of measurement show that both shear cells give nearly the same results for the determination of the effective friction angle je, both for relatively large particles (polyester granulates) and for fine powders (BaSO4 and limestone powder). The maximum deviation of the mean wall friction angle between the standard shear cell and the temperature-controlled shear cell at room temperature was found to be 2.4 K for the materials used. The greatest deviation within the individual repeated measurements with the standard shear cell was 3.6 K. The deviations between the two shear cells are therefore within the range of fluctuation of the measurement system. For this method of measurement the results show good agreement.
θ
Fig. 5. Dependence of the cone angle y for mass flow on temperature.
4. Temperature dependence of ‘‘internal friction’’ and ‘‘wall friction’’ for different bulk materials The temperature influence on the flow behaviour was investigated for seven bulk materials. The results of the investigations are presented and discussed in the following. 4.1. Polyester granulate During production polyester granulates are condensed in bulk material reactors at temperatures of 180–200 1C and freed of liquid residues. The reactors can have diameters of up to 4 m and heights of up to 30 m. The reactors are filled with granulate and with gravity-induced flow-through and hot gas counter-flow fed in. In order to obtain a narrow molecular weight distribution it is necessary to maintain a narrow residence time distribution of the granulate in the reactor. The reactors must therefore be designed around mass flow. As the reactors are operated near the melting point the measurement of the flow parameters must take place at the high reactor temperature. Measurement with the temperature-controlled shear cell indicated no significant temperature influence on the inner friction values. The influence of temperature on the wall friction angle for the granulate on a stainless steel wall sample (material number 1.4571) is shown in Fig. 4. The effect of this change is well documented for the example of the maximum cone angle y according to Jenike (1964) for which mass flow is ensured. The cone angle y is the angle between the cone of the discharge hopper and the vertical (Fig. 5). The results of the design for a rotationally symmetrical stainless steel silo (1.4571) are given in Fig. 5 for different temperatures. The results show that the cone angle for the bulk material reactor at 180 1C may not be greater than 321 in order to ensure
Wall friction angle ϕx (°), 1.4571
20 18 16 14 12 10 8 6 4 2 0
0
25
50
75
100
125
150
175
200
Temperature (°C) Fig. 4. Wall friction angle jx for polyester granulate as a function of temperature for stainless steel 1.4571 wall material.
Fig. 6. Flow function of a spray-dried lactose powder.
mass flow over the entire system. A conventional measurement with a shear cell at room temperature would give larger angles which could result in funnel flow in the reactor.
4.2. Lactose Lactose powder or powder containing lactose (e.g. whey powder) tends to form clumps at excessively high humidity. This can be attributed to the structure of lactose. In a spray-dried powder lactose is predominantly amorphous. With increasing humidity and temperature in an added process (agglomeration) the crystalline fraction increases. Amorphous is virtually the opposite of crystalline and means that the polymer molecules are arranged in an irregular manner. The water of hydration bound in the crystal structures. The powder becomes less hygroscopic improving the flow behaviour. The crystalline state of lactose alone does not sufficiently explain the flow behaviour and has to be measured in a temperature-controlled shear cell. Fig. 6 shows the flow functions for spray-dried lactose for different temperatures. The graphs indicate that in this case the flow behaviour does not change over the temperature range 20–80 1C. At a temperature of 120 1C on the other hand a pronounced change in the compressive strength can be measured. Fig. 7 shows the flow functions for a spray-dried powder which is subsequently agglomerated (information from the manufacturer). Although this powder is coarser-grained, its flow behaviour at 50 1C is poorer. This can be seen by the high compressive strength at a temperature of 50 1C. At a temperature greater than 80 1C the flow behaviour improves. This behaviour may indicate the escape of water of hydration which forms liquid bridges and thus increases the force of adhesion between the individual particles. This water of hydration can escape from the crystal structure with a change in temperature. At higher temperatures the water evaporates and the flow behaviour of the lactose powder improves.
ARTICLE IN PRESS M. Ripp, S. Ripperger / Chemical Engineering Science 65 (2010) 4007–4013
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that this must be considered for the silo design. So it is possible to design a silo with a flat cone. The costs of the silos will be decreased. 4.4. Polyphosphates
Fig. 7. Flow function of a powder with agglomerated lactose.
Polyphosphates are used in many industrial sectors in the form of powders or granulates. They incorporate water of hydration in their crystalline structures which is released at higher temperatures. A comparison of Figs. 10 and 11 shows that the wall friction angle jx becomes greater with increasing temperature resulting in poorer flow behaviour in a silo. According to information from the manufacturer calcium phosphate releases the water of hydration only at higher temperatures than sodium phosphate. The results indicate that sodium phosphate already reaches a wall friction angle jx of 301 at a temperature of 80 1C while calcium phosphate only reaches this value at a temperature of 100 1C. The results show that the design of a silo for such products must consider the process temperatures. 4.5. Cocoa powder For the production of cocoa powder a part of the fat is pressed out of the cocoa mass. The press cake remaining behind in the press is pulverised in a primary crusher and then ground in an impact crusher. The fine product then separated with pneumatic sifters is stored as cocoa powder in silos. According to the pressing strength a fat content of 10–12% or 20–22% results in the cocoa powder. The cocoa butter in the powder melts in the temperature range of 28–36 1C (Belitz and Grosch, 1992). Due to this property an alteration
Fig. 8. Flow function for ground coffee at 25 and 50 1C.
Fig. 10. Wall friction angle jx as a function of temperature for sodium phosphate for stainless steel 1.4571 wall material. Fig. 9. Wall friction angle jx for ground coffee as a function of temperature for stainless steel 1.4571 wall material.
4.3. Ground coffee Intermediate steps in the production of ready to market coffee are grinding and interim storage in silos. During grinding the powder is heated to more than 50 1C. As coffee and in particular ground coffee changes over a period of time particular attention must be given to mass flow within the silo. Figs. 8 and 9 show the flow functions determined for ground coffee at 25 and 50 1C as well as the temperature dependence of the wall friction angle jx. As wall material polished stainless steel plating (material number 1.4571) was used. The flow behaviour of the coffee powder investigated is better at 50 1C than at room temperature. The results show that the flow properties of ground coffee clearly depend upon the temperature and
Fig. 11. Wall friction angle jx as a function of temperature for calcium phosphate for stainless steel 1.4571 wall material.
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of the flow behaviour for cocoa powder can be expected from a temperature of ca. 20 1C. For this reason the flow behaviour of two cocoa powders was investigated with the temperature-controlled ring shear cell over the temperature range from 20 to 50 1C. Fig. 12 compares the wall friction angle jx as a function of temperature for a cocoa powder with 10–12% fat (10/12) and with 20–22% fat (20/22). The results of the measurements show that cocoa (20/22) with the higher fat content flows better than cocoa powder (10/12) on a stainless steel surface in the temperature range 20–40 1C. The reason for this effect can lie in a ‘‘lubricating’’ effect of the still solid fat fraction. With an increase in temperature the flow behaviour of both samples on the wall material worsens. This can be related to the melting of the fat fraction. It is possible that the cocoa oil build liquid bridges. The forces between the particles and the wall material are rising. The flow behaviour is worsening. The cocoa powder (10/12) has a low fat content. The forces of the liquid bridges decrease in this case because the viscosity of the oil near 50 1C is low and the bridges could degrade easily. In the cocoa powder (20/22) with the higher fat content the fat escapes and it is building liquid bridges over 50 1C with the effect of worsening the flow behaviour by rising temperature.
Fig. 13. Wall friction angle jx as a function of temperature of a bedding compound granulate for a polished stainless steel 1.4571 wall sample.
Temperature T
Cone angle θ
20 °C
40 °
30 °C
40 °
4.6. Bedding compound
40 °C
39 °
The intermediate space between the electrically conductive leads is as a rule filled out with a relatively soft plastic. This bedding compound is manufactured and sold in granulate form. It has the property of already becoming soft at relatively low temperatures. With silo vehicles above all in summer the possibility exists that the bedding compound is heated by solar radiation which can render unloading impossible. Knowledge of the product-specific limit temperature at which transport in a silo vehicle becomes problematical is therefore important. For this purpose the wall friction angle jx for a bedding compound was measured and is shown as a function of temperature in Fig. 13. On the basis of these measured results a silo design according to Jenike (1964) was executed. For a mass flow in a cylindrical silo the cone angles summarised in Fig. 14 were determined. The results show that up to a temperature of 40 1C mass flow in a cylindrical silo with a cone angle of 401 from the vertical can be expected. For temperatures above 40 1C the flow behaviour of the granulate is altered and the cone angle must be reduced in order to ensure mass flow.
50 °C
24 °
θ
Fig. 14. Temperature dependence of the cone angle y for a cylindrical stainless steel silo.
4.7. Dry ice Dry ice is the designation for solid carbon dioxide (CO2). It sublimates in the atmosphere at 78.48 1C and is used for example as a coolant in block form or as fragments or more recently in pellet form for the blasting treatment of surfaces. Blasting treatment with dry ice pellets yields a gentle and residue-free surface cleaning. The dry ice is blasted with a velocity of ca. 150 m/s onto the surface to be cleaned as a result of which the dirt, etc. becomes brittle and is blasted off the surface. The required dry ice pellets are stored in containers and fed via tubing to the blasting device. Here a uniform flow of pellets must be ensured, therefore no bridges or dead zones may form in the containers. Even in this case a steady mass flow must be obtained. In this connection with the temperature-controlled shear cell the wall friction angle jx for dry ice on a stainless (electropolished) steel surface and a plastic surface was determined. The dry ice pellets used had a diameter of 3 mm and a length of up to ca. 23 mm. The following wall friction angles were measured at a temperature of 78 1C: Stainless steel ðelectro-polishedÞ : jx ¼ 343 Plastic compound material : jx ¼ 273 The results show that due to the smaller wall friction angle jx a container made of the investigated plastic compound material allows better discharge flow behaviour of the pellets than a stainless steel container.
5. Summary Fig. 12. Wall friction angle jx as a function of temperature for cocoa powder samples with 10–12% fat and 20–22% fat for a polished stainless steel 1.4571 wall sample.
Today the onset of flow in bulk solids can be determined well with shear testers operated at room temperature. The influence of
ARTICLE IN PRESS M. Ripp, S. Ripperger / Chemical Engineering Science 65 (2010) 4007–4013
temperature on the flow behaviour of bulk solids has up to now been largely ignored. For the investigation of the influence of temperature on the flow behaviour of bulk solids the shear cells of a ring shear tester were newly developed in order to enable the determination of the flow parameters in the temperature range of 80 to 220 1C. The results of the measurements on certain model bulk solids show that their flow behaviour is influenced by temperature. With the temperature-controlled shear cells such alterations can be measured.
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References Belitz, H.-D., Grosch, W., 1992. Lehrbuch der Lebensmittelchemie, fourth ed. Springer-Verlag, Berlin, Heidelberg. DIN 1055, Part 6, 2005. Einwirkung auf Tragwerke—Part 6: Einwirkungen auf Silos ¨ ¨ und Flussigkeitsbeh alter. Jenike, A.W., 1964. Storage and Flow of Solids, Bulletin no. 123. University of Utah, Engineering Experiment Station, Salt Lake City. ¨ Ripp, M., 2009. Kraftwirkungen auf Einbauten in Schuttgutsilos unter Beachtung ¨ von Temperatureinflussen. Dissertation, TU Kaiserslautern. ¨ uter. ¨ Schulze, D., 2006. Pulver und Schuttg Springer-Verlag, Berlin, Heidelberg.
Contents lists available at ScienceDirect
Chemical Engineering Science journal homepage: www.elsevier.com/locate/ces
Influence of temperature on the flow properties of bulk solids Marcus Ripp , Siegfried Ripperger Lehrstuhl Mechanische Verfahrenstechnik, TU Kaiserslautern, Germany
a r t i c l e in fo
abstract
Article history: Received 28 January 2010 Received in revised form 15 March 2010 Accepted 25 March 2010 Available online 31 March 2010
The flow behaviour of bulk solids depends upon a number of parameters. Shear tests are often used to determine the flow behaviour at room temperature. This paper aims to look at the influence of temperature on the flow behaviour of bulk solids. A newly developed ring shear test shows that the flow behaviour of some specific bulk solids depends upon the temperature. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Flow properties Bulk powders Bulk solids Influence of temperature Shear test Design of silos DEM
1. Introduction Bulk solids are found in nearly all industrial sectors. According to product the size of the particles ranges from the submicrometer region to several centimetres. Their flow behaviour depends upon a number of parameters (Ripp, 2009). Normally the flow parameters required for the design of silos are determined at room temperature. In the following it will be shown that the flow behaviour of some bulk solids depends upon the temperature as well. The influence of temperature on the flow behaviour of bulk solids was determined with a newly developed ring shear tester. In the literature this influence has been mentioned (Schulze, 2006; DIN 1055, part 6), however up to now no quantitative data have been presented for specific bulk solids.
2. Temperature dependence of the flow properties of bulk solids The flow properties of bulk solids have been characterised since Jenike (1964) in terms of the effective friction angle je and the wall friction angle jx. Furthermore the flow function i.e. the dependence of the compression strength sc on the consolidation stress s1 is often cited. This relationship considers the influence of a ‘‘pre-consolidation’’ on the compression strength of a bulk solid. The parameters named can be determined using the shear tester Corresponding author.
E-mail address: [email protected] (M. Ripp). 0009-2509/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2010.03.046
according to Jenike or the ring shear tester according to Schulze and the follow-on evaluation of the measured results. Up to now there is no shear tester on the market with which tempering of the bulk solids is possible, enabling the determination of these parameters as a function of temperature. In the following it will be shown that for temperature-dependent parameters and their determination in the wrong temperature region errors are to be expected in the design of silos. The causes for the alterations in the flow properties of a bulk solid in accordance with temperature are manifold. The temperature dependence of the bulk solid parameters can be due to: Changes to the crystal structureBulk solids having a crystal structure such as salts can change their crystal structure under the influence of pressure and temperature. Such changes can influence the flow behaviour as the rigidity form and surface properties of the particles can vary accordingly. Formation and alteration of adsorption layersThe type and formation of adsorption layers in the gaseous phase depend upon the combination of solid (adsorbent) and adsorbate (adsorbed material in the fluid), the structure of the adsorption surface (chemical structure, surface condition), as well as the temperature of the solid and the gaseous phase. A change in temperature brings about a change in the adsorption equilibrium. Such a change can alter the forces of adhesion and thus the flow behaviour. Formation of liquid bridgesLiquid bridges cause forces of adhesion between the particles as a result of the capillary pressure within the bridges and the peripheral force. According to the curvature of the phase boundary surfaces inside the liquid a capillary under-pressure prevails which attracts the particles to each other. The form of the liquid bridge and the magnitude of the
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force of attraction depend e.g. upon the wetting properties of the solid, the surface tension of the liquid, the distance between the particles, and the form of the particles, as well as the geometrical form of particle contact, which for non-spherical particles also depends upon the relative positions of the particles. The formation of liquid bridges can have different causes. With capillary condensation due to concave-shaped phase boundaries within the pore structure the vapour pressure is lowered. As a result depending upon the respective radius of curvature vapour already condenses under conditions for which condensation does not yet take place on a flat surface. This phenomenon depends upon the temperature, the humidity and the content of other vapours as well as the ambient pressure. Bulk solids with good solvent properties (e.g. urea, sugar, salt) liquid bridges can cause a partial transition of the solid to the liquid phase. This process is once again dependent upon the temperature and can have very different effects on the flow behaviour of the bulk solids. Natural products such as coffee or cocoa can contain up to 20% oil internally. With the pulverisation of these products oil can be released to the outside and form liquid bridges between the particles. Because of the temperature dependence of material values such as the viscosity of the oils, a change in temperature also affects the flow behaviour of the bulk solid. Other materials such as lactose or phosphate have water of hydration bound in their crystal structures. This water of hydration can escape from the crystal structure with a change in temperature. The temperature dependence of the flow behaviour for these materials is therefore to be expected. Mixtures of materials composed of different properties are also processed. In such a case it is possible that the softening point of a component is exceeded with a change in temperature, resulting in the formation of a liquid bridge in the molten material. This behaviour is observed e.g. with foodstuffs containing fat such as whey powder and instant soups. Formation of solid bridges Solid bridges are solid material linkages between particles which can transfer relatively large forces. Such linkages can e.g. occur when the material is stored at a temperature in the vicinity of the melting point. If a short-term temperature rise followed by a drop in temperature takes place during storage so-called sinter or fusion bridges are formed (e.g. with plastic). Often solid bridges already form during storage in a temperature range below the melting point for sufficiently long time of contact and compressive stress. Solid bridges can also arise due to the drying out of liquid bridges when material has been dissolved in the liquid bridge (e.g. urea, sugar, salt). Solid bridges are formed by the freezing of liquid bridges when a bulk solid is cooled down to a correspondingly low temperature. Such frozen liquid bridges are then considered solid bridges over which large forces can be transferred. Visco-plastic deformation at the contact points At the contact points of the individual particles in a bulk solid, visco-plastic deformation can take place according to changes in temperature. This entails an alteration of the surface structure and possible of the particle structure in turn resulting in changes to the forces of adhesion and friction between the particles. Such changes occur primarily with waxes or plastic granulates. This also influences the flow behaviour of the bulk solid.
solids or initiated by certain temperatures as well. In each case the flow behaviour of bulk solids changes. This influence was determined with a newly developed ring shear tester. 3.1. Design of the shear cell The ring shear tester according to Schulze is a recognised instrument for the measurement of the flow properties of bulk solids. As a rule it is used up to a particle size of ca. 2 mm at room temperature. For the investigation of the temperature dependence of the flow behaviour for bulk solids a shear cell was developed on the basis of the ring shear tester according to Schulze. The ‘‘internal friction’’ and the ‘‘wall friction’’ can be determined in the temperature range 80to220 1C. A detailed description of the shear tester is given in Ripp (2009). For the new tester the shear cell is arranged on a glass fibre– reinforced teflon bottom. It is driven by the already existing spur gear. As the tie rods are intended to connect the lid of the shear cell horizontally to the shear force transducers, the transducers were also offset upwards. A calibration of the entire shear tester was then performed in accordance with the instructions in the manual for the ring shear tester. During the course of development of the temperaturecontrolled shear cell for the determination of ‘‘internal friction’’, the entire shear cell (vertical walls and bottom) was provided with a double casing through which a heating or a cooling medium can flow. Inside this double casings are conductive plates which ensure the homogeneous distribution of the cooling and heating media and prevent the occurrence of short-circuits. In this way a virtually uniform temperature is achieved over the walls of the shear cell. The shear cell is connected by tubing to a cooling and heating thermostat. The supply tube is joined by a screw connection to the lower-lying connection in order that air can escape via the upper-lying return flow. The entire shear cell is made of stainless steel (material no. 1.4571) (see Fig. 1). A similarly constructed shear cell was also developed for the determination of wall friction (see Ripp, 2009). For the determination of wall friction in this case a wall sample is placed in the shear cell. This cell as well by contrast with other cells according to Schulze has a double bottom through which heating or cooling media can flow.
3. Temperature-controlled ring shear tester The flow behaviour of some bulk solids depends upon the temperature. It could be an effect of time consolidation of bulk
Fig. 1. Temperature-controlled shear cell for the determination of ‘‘internal friction’’.
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For the shear cells of a ring shear tester there are fitting the lids of the shear cells. These shear cell lids have a cross-beam joined via the tie rods to the force transducers. On the lower side of the lid webs to prevent the sliding of the bulk solid directly against the lid. For the reduction of heating or cooling losses a new shear cell lid was designed. This lid is isolated and can be equipped with additional heating. The weight of the electrical heating and the two cables to the temperature control are added to the weight of the lid in the software. Fig. 2 illustrates the lid of the shear cell with electrical heating. For measurements at high temperatures a high-temperature heating circulator driven by thermal oil is connected to the shear cell. For temperature control at low temperatures a lowtemperature thermostat is used. The measurement of the bulk solid temperature for the determination of the ‘‘internal friction’’ is made with a thermocouple in the shear zone beneath the lid of the shear cell. For the determination of the ‘‘wall friction’’ the temperature of the wall material sample is measured. 3.2. Test procedure with the temperature-controlled shear cell The tests performed with the temperature-controlled shear cells were analogous to those performed at room temperature. The heating and cooling of the samples takes place directly in the shear cell. Bulk solids with poor thermal conductivity are pretempered in an external tempering cabinet. The measurements with the temperature-controlled shear cells are started when the required reference temperature is reached. As the shear cells are permanently connected via two tubes the bulk solid mass is determined as the difference between a definite sample mass and the remainder which is not filled. In this way it is no longer necessary to weigh the filled shear cell. 3.3. Validation of the temperature-controlled shear cells
Temperature curve and temperature distribution For the determination of the temperature curve for the shear cell the cell for measuring the inner friction was filled with a
Fig. 2. Electrically heated shear cell lid.
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barium sulphate powder. The thermostat was set to 200 1C and the temperatures were measured on the bottom of the shear cell and underneath the lid of the shear cell. At every level four positions in intervals of 901 were measured at the centre of the shear ring. Measurement point 1 was at the inlet for the tempering liquid and measurement point 4 at the outlet. Fig. 3 shows the rise in temperature within the shear cell over the time after switching on the thermostat. The heated lid of the shear cell (Fig. 2) was placed over the filled shear cell and subjected to a load of 2.0 kg. This normal load ensures a defined pressure on the lid of the shear cell. The temperature rise in the flow zone underneath the lid of the shear cell and on the wall material is plotted in Fig. 3. The mean values of the four temperature measurements are plotted as a point. Based on the scattering of the values the radial temperature distribution can be derived. It can be seen that a virtually uniform temperature distribution is obtained. Preliminary investigations have shown that with additional heating the required reference temperature can be set faster over the entire cross-section of the shear cell. During the investigations a thermostat setting of 200 1C and a shear cell lid temperature of 200 1C were used. The results also show that the temperature profile over the perimeter of the shear cell is very uniform following the heat-up phase (20–30 min). Analogous investigations of the temperature distribution were performed with the determination of wall friction. The temperature measurement locations were under the bulk solid (barium sulphate powder) and on the surface of the wall. As the height of the bulk powder in this shear cell was only 10–12 mm measurements were made only on the surface of the wall. The temperature of the thermostat was set to 200 1C. The result shows that in this case with this experimental arrangement a wall temperature of ca. 170 1C can be obtained. This can be explained by the fact that the diameter of the wall shear cell was modelled according to Schulze whereby the outer diameter of the shear cell is larger than that of the lid of the shear cell. Heat is lost from the free uncovered surface of the bulk solid sample. Furthermore a non-insulated lid of the shear cell was used. The required temperature of the wall sample was set by increasing the thermostat temperature and using an insulated or temperaturecontrolled shear cell lid. Comparison of temperature-controlled and non-temperature-controlled shear cells For the validation of the new shear cell five different bulk materials and different wall materials were investigated at room temperature with the temperature-controlled and the nontemperature-controlled ring shear cell. A total of 193 shear tests
Fig. 3. Mean temperature in the shear zone underneath the heated shear cell lid (200 1C) and on the bottom of the shear cell at 200 1C heating temperature.
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were performed. In all shear tests the shear cells were newly filled before measuring. The results of measurement show that both shear cells give nearly the same results for the determination of the effective friction angle je, both for relatively large particles (polyester granulates) and for fine powders (BaSO4 and limestone powder). The maximum deviation of the mean wall friction angle between the standard shear cell and the temperature-controlled shear cell at room temperature was found to be 2.4 K for the materials used. The greatest deviation within the individual repeated measurements with the standard shear cell was 3.6 K. The deviations between the two shear cells are therefore within the range of fluctuation of the measurement system. For this method of measurement the results show good agreement.
θ
Fig. 5. Dependence of the cone angle y for mass flow on temperature.
4. Temperature dependence of ‘‘internal friction’’ and ‘‘wall friction’’ for different bulk materials The temperature influence on the flow behaviour was investigated for seven bulk materials. The results of the investigations are presented and discussed in the following. 4.1. Polyester granulate During production polyester granulates are condensed in bulk material reactors at temperatures of 180–200 1C and freed of liquid residues. The reactors can have diameters of up to 4 m and heights of up to 30 m. The reactors are filled with granulate and with gravity-induced flow-through and hot gas counter-flow fed in. In order to obtain a narrow molecular weight distribution it is necessary to maintain a narrow residence time distribution of the granulate in the reactor. The reactors must therefore be designed around mass flow. As the reactors are operated near the melting point the measurement of the flow parameters must take place at the high reactor temperature. Measurement with the temperature-controlled shear cell indicated no significant temperature influence on the inner friction values. The influence of temperature on the wall friction angle for the granulate on a stainless steel wall sample (material number 1.4571) is shown in Fig. 4. The effect of this change is well documented for the example of the maximum cone angle y according to Jenike (1964) for which mass flow is ensured. The cone angle y is the angle between the cone of the discharge hopper and the vertical (Fig. 5). The results of the design for a rotationally symmetrical stainless steel silo (1.4571) are given in Fig. 5 for different temperatures. The results show that the cone angle for the bulk material reactor at 180 1C may not be greater than 321 in order to ensure
Wall friction angle ϕx (°), 1.4571
20 18 16 14 12 10 8 6 4 2 0
0
25
50
75
100
125
150
175
200
Temperature (°C) Fig. 4. Wall friction angle jx for polyester granulate as a function of temperature for stainless steel 1.4571 wall material.
Fig. 6. Flow function of a spray-dried lactose powder.
mass flow over the entire system. A conventional measurement with a shear cell at room temperature would give larger angles which could result in funnel flow in the reactor.
4.2. Lactose Lactose powder or powder containing lactose (e.g. whey powder) tends to form clumps at excessively high humidity. This can be attributed to the structure of lactose. In a spray-dried powder lactose is predominantly amorphous. With increasing humidity and temperature in an added process (agglomeration) the crystalline fraction increases. Amorphous is virtually the opposite of crystalline and means that the polymer molecules are arranged in an irregular manner. The water of hydration bound in the crystal structures. The powder becomes less hygroscopic improving the flow behaviour. The crystalline state of lactose alone does not sufficiently explain the flow behaviour and has to be measured in a temperature-controlled shear cell. Fig. 6 shows the flow functions for spray-dried lactose for different temperatures. The graphs indicate that in this case the flow behaviour does not change over the temperature range 20–80 1C. At a temperature of 120 1C on the other hand a pronounced change in the compressive strength can be measured. Fig. 7 shows the flow functions for a spray-dried powder which is subsequently agglomerated (information from the manufacturer). Although this powder is coarser-grained, its flow behaviour at 50 1C is poorer. This can be seen by the high compressive strength at a temperature of 50 1C. At a temperature greater than 80 1C the flow behaviour improves. This behaviour may indicate the escape of water of hydration which forms liquid bridges and thus increases the force of adhesion between the individual particles. This water of hydration can escape from the crystal structure with a change in temperature. At higher temperatures the water evaporates and the flow behaviour of the lactose powder improves.
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that this must be considered for the silo design. So it is possible to design a silo with a flat cone. The costs of the silos will be decreased. 4.4. Polyphosphates
Fig. 7. Flow function of a powder with agglomerated lactose.
Polyphosphates are used in many industrial sectors in the form of powders or granulates. They incorporate water of hydration in their crystalline structures which is released at higher temperatures. A comparison of Figs. 10 and 11 shows that the wall friction angle jx becomes greater with increasing temperature resulting in poorer flow behaviour in a silo. According to information from the manufacturer calcium phosphate releases the water of hydration only at higher temperatures than sodium phosphate. The results indicate that sodium phosphate already reaches a wall friction angle jx of 301 at a temperature of 80 1C while calcium phosphate only reaches this value at a temperature of 100 1C. The results show that the design of a silo for such products must consider the process temperatures. 4.5. Cocoa powder For the production of cocoa powder a part of the fat is pressed out of the cocoa mass. The press cake remaining behind in the press is pulverised in a primary crusher and then ground in an impact crusher. The fine product then separated with pneumatic sifters is stored as cocoa powder in silos. According to the pressing strength a fat content of 10–12% or 20–22% results in the cocoa powder. The cocoa butter in the powder melts in the temperature range of 28–36 1C (Belitz and Grosch, 1992). Due to this property an alteration
Fig. 8. Flow function for ground coffee at 25 and 50 1C.
Fig. 10. Wall friction angle jx as a function of temperature for sodium phosphate for stainless steel 1.4571 wall material. Fig. 9. Wall friction angle jx for ground coffee as a function of temperature for stainless steel 1.4571 wall material.
4.3. Ground coffee Intermediate steps in the production of ready to market coffee are grinding and interim storage in silos. During grinding the powder is heated to more than 50 1C. As coffee and in particular ground coffee changes over a period of time particular attention must be given to mass flow within the silo. Figs. 8 and 9 show the flow functions determined for ground coffee at 25 and 50 1C as well as the temperature dependence of the wall friction angle jx. As wall material polished stainless steel plating (material number 1.4571) was used. The flow behaviour of the coffee powder investigated is better at 50 1C than at room temperature. The results show that the flow properties of ground coffee clearly depend upon the temperature and
Fig. 11. Wall friction angle jx as a function of temperature for calcium phosphate for stainless steel 1.4571 wall material.
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of the flow behaviour for cocoa powder can be expected from a temperature of ca. 20 1C. For this reason the flow behaviour of two cocoa powders was investigated with the temperature-controlled ring shear cell over the temperature range from 20 to 50 1C. Fig. 12 compares the wall friction angle jx as a function of temperature for a cocoa powder with 10–12% fat (10/12) and with 20–22% fat (20/22). The results of the measurements show that cocoa (20/22) with the higher fat content flows better than cocoa powder (10/12) on a stainless steel surface in the temperature range 20–40 1C. The reason for this effect can lie in a ‘‘lubricating’’ effect of the still solid fat fraction. With an increase in temperature the flow behaviour of both samples on the wall material worsens. This can be related to the melting of the fat fraction. It is possible that the cocoa oil build liquid bridges. The forces between the particles and the wall material are rising. The flow behaviour is worsening. The cocoa powder (10/12) has a low fat content. The forces of the liquid bridges decrease in this case because the viscosity of the oil near 50 1C is low and the bridges could degrade easily. In the cocoa powder (20/22) with the higher fat content the fat escapes and it is building liquid bridges over 50 1C with the effect of worsening the flow behaviour by rising temperature.
Fig. 13. Wall friction angle jx as a function of temperature of a bedding compound granulate for a polished stainless steel 1.4571 wall sample.
Temperature T
Cone angle θ
20 °C
40 °
30 °C
40 °
4.6. Bedding compound
40 °C
39 °
The intermediate space between the electrically conductive leads is as a rule filled out with a relatively soft plastic. This bedding compound is manufactured and sold in granulate form. It has the property of already becoming soft at relatively low temperatures. With silo vehicles above all in summer the possibility exists that the bedding compound is heated by solar radiation which can render unloading impossible. Knowledge of the product-specific limit temperature at which transport in a silo vehicle becomes problematical is therefore important. For this purpose the wall friction angle jx for a bedding compound was measured and is shown as a function of temperature in Fig. 13. On the basis of these measured results a silo design according to Jenike (1964) was executed. For a mass flow in a cylindrical silo the cone angles summarised in Fig. 14 were determined. The results show that up to a temperature of 40 1C mass flow in a cylindrical silo with a cone angle of 401 from the vertical can be expected. For temperatures above 40 1C the flow behaviour of the granulate is altered and the cone angle must be reduced in order to ensure mass flow.
50 °C
24 °
θ
Fig. 14. Temperature dependence of the cone angle y for a cylindrical stainless steel silo.
4.7. Dry ice Dry ice is the designation for solid carbon dioxide (CO2). It sublimates in the atmosphere at 78.48 1C and is used for example as a coolant in block form or as fragments or more recently in pellet form for the blasting treatment of surfaces. Blasting treatment with dry ice pellets yields a gentle and residue-free surface cleaning. The dry ice is blasted with a velocity of ca. 150 m/s onto the surface to be cleaned as a result of which the dirt, etc. becomes brittle and is blasted off the surface. The required dry ice pellets are stored in containers and fed via tubing to the blasting device. Here a uniform flow of pellets must be ensured, therefore no bridges or dead zones may form in the containers. Even in this case a steady mass flow must be obtained. In this connection with the temperature-controlled shear cell the wall friction angle jx for dry ice on a stainless (electropolished) steel surface and a plastic surface was determined. The dry ice pellets used had a diameter of 3 mm and a length of up to ca. 23 mm. The following wall friction angles were measured at a temperature of 78 1C: Stainless steel ðelectro-polishedÞ : jx ¼ 343 Plastic compound material : jx ¼ 273 The results show that due to the smaller wall friction angle jx a container made of the investigated plastic compound material allows better discharge flow behaviour of the pellets than a stainless steel container.
5. Summary Fig. 12. Wall friction angle jx as a function of temperature for cocoa powder samples with 10–12% fat and 20–22% fat for a polished stainless steel 1.4571 wall sample.
Today the onset of flow in bulk solids can be determined well with shear testers operated at room temperature. The influence of
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temperature on the flow behaviour of bulk solids has up to now been largely ignored. For the investigation of the influence of temperature on the flow behaviour of bulk solids the shear cells of a ring shear tester were newly developed in order to enable the determination of the flow parameters in the temperature range of 80 to 220 1C. The results of the measurements on certain model bulk solids show that their flow behaviour is influenced by temperature. With the temperature-controlled shear cells such alterations can be measured.
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References Belitz, H.-D., Grosch, W., 1992. Lehrbuch der Lebensmittelchemie, fourth ed. Springer-Verlag, Berlin, Heidelberg. DIN 1055, Part 6, 2005. Einwirkung auf Tragwerke—Part 6: Einwirkungen auf Silos ¨ ¨ und Flussigkeitsbeh alter. Jenike, A.W., 1964. Storage and Flow of Solids, Bulletin no. 123. University of Utah, Engineering Experiment Station, Salt Lake City. ¨ Ripp, M., 2009. Kraftwirkungen auf Einbauten in Schuttgutsilos unter Beachtung ¨ von Temperatureinflussen. Dissertation, TU Kaiserslautern. ¨ uter. ¨ Schulze, D., 2006. Pulver und Schuttg Springer-Verlag, Berlin, Heidelberg.