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Measuring rheological variations with time and depth in livestock waste slurries using rotational viscometry
Biological Wastes 19 (1987) 141-152
Measuring Rheological Variations with Time and Depth in Livestock Waste Slurries using Rotational Viscometry C. P. Schofield Waste Engineering Group, Farm Buildings Division, National Institute of Agricultural Engineering, Wrest Park, Silsoe, Bedford MK45 4HS, Great Britain (Received: 12 February 1986; accepted 16 June 1986)
ABSTRACT A commercial rotational viscometer mounted on a motorised stand was used to measure theological variations with depth, of separated pig-sluro', over a period of approximately 2Oh. Results showed that this equipment can measure changes in slurry characteristics with depth, caused by settling. However, it was not possible to measure true viscosities at known depths with standard viscometer spindles because of the slurry settling during the tests and because spindle end-effects were higher than their geometry predicted. It is concluded that any shear thinning or thickening effects which are indicated by rotational viscometers working in pig-slurries may actually be due to the settling of solids during the tests and that readings obtained from the viscometer are of more value as torque measurements for mixing power estimation than as viscosity measurements. Used in this unconventional mode the equipment is unlikely to achieve useful measurements in full-scale waste-storage tanks.
INTRODUCTION Methods for establishing the rheological properties of livestock waste slurries are being studied as part of the Livestock Waste Characterisation Project at NIAE (Schofield, 1983). One of the objectives is to identify reliable methods for determining how rheological properties vary with depth in unstirred slurry-storage containers (e.g. tanks, lagoons and treatment vessels). A review of the literature on the properties of livestock 141 Biological Wastes 0269-7483/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
142
c. P. Schofield
waste slurries (Schofield, 1984) showed that several researchers have examined the handling characteristics of homogenised slurries on a laboratory scale, but no work was found on variations of handling characteristics with depth through settled slurries, either on a laboratory scale or, more importantly, on full-scale systems. Recent work on mixing power requirements (Cumby et aL, 1985) stresses the importance of measuring rheological variations with depth as an indication of the effectiveness of mixing equipment. Slurry mixing can consume up to twice the energy needed for subsequent spreading of the slurry (Safley & Nye, 1982), so improvements in methods for assessing mixing requirements could yield substantial savings in energy. An 'on-farm' technique for measuring variations in slurry consistency with depth would provide valuable information for designers of mixing systems, pumps, pipelines and treatment plants, and could result in considerable financial savings and increased operating efficiency. The laboratory scale experiments reported here were undertaken as a first step towards designing an in-situ measuring device which would give information on slurry characteristics relevant to handling requirements, throughout the depth of the slurry. The objective was to find whether the technique could be used to obtain quantitative values of slurry viscosity, as well as to indicate significant changes in its properties with depth and time, and to establish whether the equipment and procedure could be developed to become of practical value.
METHODS The viscometer
A Brookfield Synchro-Lectric Model LVT rotational viscometer was used for the experiments. This instrument rotates a cylindrical or disc-shaped 'spindle' in a fluid and measures the torque necessary to overcome the viscous resistance to the induced movement. This is accomplished by driving the immersed spindle through a beryllium copper spring. The degree to which the spring is deflected is indicated by a pointer on the viscometer's dial and is proportional to the viscosity of the fluid for any given speed and spindle. Power is provided by a synchronous inductor type motor, driving through an eight-speed gear train, to provide a range of spindle speeds. The viscometer was mounted on a motorised Helipath stand, as shown in Fig. 1, so that it could be raised and lowered at a constant rate to enable readings to be taken over a range of depths in the sample slurry. To allow for this vertical movement an extension rod was
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143
Fig. 1. The Brookfield viscometer and Helipath stand. Spindle No. 1 is shown suspended on the extension rod.
used to connect the spindles to the viscometer. This gave a range of 212 mm travel between the upper and lower stops, but meant that the protective guard leg, normally fitted round the spindle, had to be removed. Two different spindles were used during the experiments. These were standard Brookfield spindles, type No. 1 LV cylindrical (18.84mm diameter × 65"1 m m long, and type No. 2 LV disc (18.72 m m diameter x 6.86 mm long). Conventionally, 'T' shaped spindles should be used in conjunction with the Helipath stand. These are designed for use in highly viscous materials, but do not give high enough readings on the viscometer scale when used in the relatively low viscosity pig-slurry, so the cylinder and disc spindles were used instead. To give readings which can be related directly to viscosity,
144
C. P. Schofield
these spindles should be used at a constant depth, so avoiding end-effects caused by motion through the slurry. As a result, the readings obtained in these experiments cannot always be related directly to viscosity (see 'Discussion'). It must be emphasised here that the Brookfield viscometer and Helipath stand were chosen for this work because they were readily available, and are widely accepted as reliable and accurate instruments when used conventionally. These experiments are outside their normal modes of operation, so the suitability or accuracy of these specific instruments when used conventionally is not in question.
Calibration Although the results obtained from these experiments cannot be presented as measurements of actual viscosity for reasons given in the discussion below, the equipment was calibrated to compensate for the non-standard container, extension rod and lack of guard leg, to make comparisons by other researchers possible. Absolute calibration of Brookfield viscometers is based on immersion of the spindle to a precise depth in a Newtonian fluid, in a container of minimum diameter 83 ram, with the guard leg fitted (Technical Paper, 1984). The experiments described here were made at varying depths in a non-Newtonian fluid in a measuring cylinder of only 60 m m diameter, without a guard leg, so a new set of calibration factors was required to convert the dial readings from the viscometer into a standard format relevant to the equipment working in a Newtonian fluid. The procedure outlined by Brookfield (Technical Paper, 1982) was followed. A Newtonian fluid with a standard viscosity at 25°C of 99 × 10-3 Pa.s when measured in a standard container was used to check the calibration of the viscometer under standard conditions. By then transferring the fluid to the measuring cylinder to be used for subsequent tests, and fitting the extension rod to the viscometer, it was possible to obtain calibration factors (F) which were relevant to the new operating conditions over a range of depths. Results presented in Table 1 show there to be no significant variation in reading with depth, so a single value of F c o u l d be used for each spindle. The new calibration factor is given by the formula: F = q/x
where F = calibration factor; r/= viscosity of the material (Pa.s) and x = reading on viscometer scale. Applying this formula gives the F values shown in the last column of Table 1. These are used throughout this paper, and are relevant to both spindles when fully submersed in a Newtonian fluid in the columns used for
Rheology of livestock wastes
145
TABLE 1 Percentage Depth and Correction Factors for the Apparatus when Working in a Measuring Cylinder 60 mm Diameter and 348 mm Deep at 30 rpm. (Average of Five Repeat Runs)
Spindle No.
1 2
Run position
Top Bottom Top Bottom
Depth to lower spindle face
99 x l 0-3 Pa.s calibration fluid scale reading
(mm)
(%)
(x)
(SD)
126 338 107 319
36 97 31 92
46-2 46.4 12.0 12"2
0"45 0'55 0 0'45
Correction factor F (99 x I0- 3/x)
2" 14 8.18
the tests. The viscometer scale has 100 divisions, so readings can be taken to the nearest whole number, with corresponding reduced accuracy towards the lower end of the scale. Brookfield claims + 1% accuracy at full-scale, which is not disputed. The apparatus was set up so that the cylindrical spindle was submerged by 10mm when in the upper position, and had 10mm clearance from the bottom of the sample when in the lower position. This gave maximum vertical freedom of 212mm with the viscometer running up and down continuously between the upper and lower set points, each cycle taking 18"2 min. The depth of slurry in the test cylinder was 348 mm. Test procedure Fresh, separated pig slurry was used for the experiments. To obtain the sample, slurry was collected from a fattening house containing approximately 200 pigs of 25-45kg liveweight, fed on a commercial pelletted ration containing 18% crude protein, with ad libiturn access to water. The channels were agitated for at least 1 h, using a 2.5 kW centrifugal pump, prior to emptying, to ensure thorough mixing. The slurry was then passed through a brushed-screen, roller-press separator fitted with a 1-5 m m mesh to remove bristles and larger particles. A 200ml sub-sample was taken from this for analysis and found to contain 3.1% Total Solids (TS). After further mixing, three 1-1itre measuring cylinders were filled with the slurry, and labelled A, B and C. Cylinder A was immediately placed under the viscometer and the spindle positioned so tests could begin. Cylinders B and C were placed near to the apparatus, but left to settle undisturbed. The viscometer was calibrated and run at 30 rpm for all the tests. This rotational speed was judged to give a suitable wide range of viscosity
C. P. Schofield
146
readings from both spindles in the sample slurry. The temperature of the slurry remained between 12.5°C and 14°C throughout the test period. For simplicity, the start time for the experiment is called zero, and the following events counted from zero in minutes. The Helipath stand was started from the upper position, and readings were taken from the viscometer scale at 1-min intervals for one complete cycle of the spindle down and back up through the sample. Readings were taken using both spindles in cylinder A and then in cylinder B, as laid out in Table 2. Note that the apparatus was running up and down continuously through sample A between readings for the first 1200 min, whilst sample B was left undisturbed, and similarly in sample B for the remaining tests.
0
0% Curve A = Spindle-- 1 (depth to centre) C u r v e B = Spindle--1 (depth to lower face) Curve C = Spindqe= 2
20
~,
4o
Dark liquid
£
~
o
60
•
A
69%
8O
90%
1
Fibrous
95% 1 0~0n
I 50
I 100 Viscosity reading
I 150
I 200
~
~ : : : : "
Fig. 2. Comparison of viscosity readings from spindle No. 2 with those from the centre level and from the bottom face of spindle No. 1. The sketch shows settled zones observed in undisturbed slurry after settling for 1340min.
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TABLE 2
Summary of Events during Tests Time (mins) 0
40 1 130 1 160 1 200
Event
Fill cylinders A, B and C with mixed slurry Put cylinders B and C to one side undisturbed Take readings: spindle 1 in cylinder A Take readings: spindle 1 in cylinder A Take readings: spindle I in cylinder A Replace spindle 1 with spindle 2 Take readings: spindle 2 in cylinder A Replace cylinder A with cylinder B Take readings: spindle 2 in cylinder B
1 300
Replace spindle 2with spindle 1 Take readings: spindle 1 in cylinder B
1 340
Record depths of sediment layers in cylinder C
Result
Fig. 3, curve D Fig. 3, curve E Fig. 3, curve B Fig. 2, curve C Fig. 4, curves F and G Fig. 4, curve H Fig. 2, diagram
Note: the apparatus was left running between readings.
To provide a visual check against the results from the viscometer, the third sample of the same slurry was allowed to settle undisturbed in cylinder C. The depths of the interfaces between the settled layers were measured for comparison (Fig. 2).
RESULTS A N D DISCUSSION The effects of settling and depth on viscometer reading All viscometer test results are presented graphically in Figs 2 to 4. The vertical axes represent the depth of the viscometer spindle in the test sample as a percentage of the sample depth. The horizontal axes represent the dial reading taken from the viscometer multiplied by the correction factors (F) obtained during calibration (Table 1) and called the Viscosity Reading. Figure 2 illustrates the need to relate Viscosity Readings to the depth of the bottom face of the spindles, and shows a sketch illustrating the depths of zones observed through the transparent walls of cylinder C, containing the undisturbed slurry after 1340min. Figure 3 shows the effect of time on Viscosity Readings at different depths and Fig. 4 illustrates the fragile nature of the 70-90% depth zone in the settled slurry. All curves (except
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148
0 Curve D = Spindle =1, 0 -10 minutes
Curve E = Spindle =1, 40 - 50 minutes Curve B = Spindle =1, 1130 - 1140 minutes 2O
4O
6O
8O
jB
10C
I
50
I
1O0
I
150
I
200
I
250
I
300
Viscosity reading
Fig. 3.
Viscosity readings related to the b o t t o m o f spindle No. 1 after 0, 40 and 1130 min
settling in column A.
curve G) are traced through points taken when the ~,iscometer was travelling downwards through the sample. Curve G represents readings taken on the upward pass. As the curves in Figs 2, 3 and 4 show, pig-slurry settles into defined layers of different viscosity. As a result, when the cylindrical spindle is passing from one layer to the next, it experiences proportionally different forces over its surface area. For a cylindricalspindle to give a true re~tding of viscosity, it must be working in a homogeneous fluid, so experiencing equal forces over its whole surface area. The readings taken from the viscometer when working in pig slurry cannot therefore be regarded as true viscosity values, although they can still be regarded as indicators of how viscosity changes with depth.
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149
o Curve F = Spindle = 2, 1200 - 121Q minutes Curve G = Spindle =2, 1210 • 1220 minutes Curve H = Spindle = 2 , 1300 - 1310 minutes
20
80 --
100
Fig. 4.
0
I 50
i 100
I i 150 200 Viscosity reading
t 250
I 300
Viscosity readings using spindle No. 2 in column B after settling undisturbed for 1200min.
Both the cylindrical and disc spindles are designed to work at constant depth in a fluid under test. Any vertical motion will cause changes in shear forces on the surfaces of the spindles which will affect the torque readings on the viscometer, thus giving false readings of viscosity. This is a second reason why results from these experiments are presented as Viscosity Readings, rather than actual viscosities. These changes in shear forces due to the vertical motion will be particularly apparent on the end of the spindle moving into the fluid. This is because the lower end of the spindle will not only be working in slurry of higher viscosity, due to settling, but will also be pushing down into the slurry so experiencing higher shear forces. As the cross-sectional areas of both types of spindle are practically the same, the only difference between them will be due to the forces over their lengths. These forces should be larger for the cylindrical spindle under ideal conditions. In Fig. 2, curve A
150
C. P. Schofield
represents Viscometer Readings plotted against the depth of the centre level of the cylindrical spindle, and curve B is the same curve adjusted to refer these readings to the depth of the bottom surface of the same spindle. Curve C represents readings using the disc spindle, also related to the depth of its lower face. As can be seen, curves B and C are almost identical. This confirms that the shear forces being measured by the viscometer are acting mainly on the lower faces of the spindles, so results from both spindles should be related to depths corresponding with their lower faces under these test conditions. This observation is further confirmed when curves B and C are compared with results from the visual settling test, also shown in Fig. 2. After 1340min of settling, the slurry was observed to form three distinct layers, with the interfaces being at 69% and 90% depths. These positions correspond closely to the turning points on curves B and C. A further interface at about the 95% depth position was less well defined, yet is in the same region as the highly viscous zone suggested by extending the lower ends of the measured curves. Curves D and E in Fig. 3 show that there is no recorded change in Viscosity Reading between 0% and 80% depth within the first 40min of settling. If this is accepted, and a well-mixed sample is obtained, then readings taken within these limits can be taken as representing the true viscosity of the mixed slurry. Researchers interested in measuring true viscosity should consider testing their slurries at time intervals to establish whether settling is taking place, and so needs to be accounted for. Figure 3 shows curves drawn through points recorded as the slurry settled over a long period of time. Curve D was obtained immediately after the mixed sample was placed in column A. As can be seen, there is a clear increase in Viscosity Reading towards the bottom of the curve, even though these points were read within 10 min of the start. Within 40 min the settling is very apparent at greater than 90% depth (curve E). After l l 3 0 m i n , a further settling zone increasing in viscosity between 65% and 85% depth has developed, and there is a distinct drop in Viscosity Readings between 0% and 65% depth. This is due to fine particles falling slowly from the upper levels, and gently compacting towards the lower levels. Hence the drop in viscosity near the top, and gradual increase with depth. These observations can be related to results which would have been recorded had the spindle remained at one level in the cylinder over this period of time. If the spindle had been stationary anywhere above the 60% depth line, a decrease in viscosity would have been observed, and, neglecting the effect of settling, the slurry could have been classified as shear thinning (thixotropic). However, if the spindle had been below the 70% depth line, then a rise of viscosity with time would have suggested that the
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151
slurry was shear thickening (rheopectic)! Although researchers are likely to account for this, conditions where a slurry might effectively display this behaviour do occur in practice. Take, for example, a pump at the bottom of a stirred tank. If the slurry is completely mixed then the pump could be designed to work in slurry with a Viscosity Reading of under, say, 50. If however, the tank was inefficiently stirred, so allowing settling, or if the pump was started before all sediment had been re-suspended by a mixer, then it would have to handle slurry with a Viscosity Reading far higher than 50, requiring much more power or possibly a completely different type of pump. When spindle 2 was passed down through cylinder B after the slurry had settled for 1200 min, very distinct changes in Viscosity Reading with depth were recorded as shown by curve F (Fig. 4). These compare very well with the observed levels of cylinder C (Fig. 2). Curve G is drawn through points taken on the upward pass recorded immediately after curve F, and shows how the Viscosity Readings through the 70-90% zone drop due to the disturbance caused by a single pass of the spindle. Curve H, taken 80 min later using spindle 1, shows how the viscosity of this layer has been reduced by the spindle continuously passing through, and is very similar to curve B of sample A. This suggests that the layer at 70-90% depth is very fragile and makes measuring its true viscosity very difficult. It would certainly be unreliable to take a sample from this depth in a farm-scale store containing settled slurry and hope to measure its viscosity and relate this back to the undisturbed slurry. Even the gentle movement of the spindle passing through once is shown to cause a 14% drop in reading, so sampling would inevitably destroy its structure. Looking on the practical side, though, this layer should be very easy to re-suspend by mixing, and pose little problem to either pumping or mixing equipment. The changes of Viscosity Reading with depth shown in curve F can be related to the power requirements for mixing or pumping equipment working at those depths, but as these Viscosity Readings do not represent true viscosities, actual power requirements cannot be calculated. As already stated, a pump or mixer working in the upper layer of a tank or store would not be affected by settling, but it could experience serious over-loading or blocking on start-up if positioned near the bottom. The middle zone, between 70% and 90% depth, presents a rather different problem. The rapid destruction of the structure of this layer means that, provided the pump or mixer can overcome the initial loading, the resistance will quickly drop, and so the power requirement will fall. Knowledge of these properties could help reduce the power input and size of mixing and pumping equipment. If the mixer was designed to start working in the upper layer and be lowered gradually through the slurry, the
152
C. P. Schofield
progressive mixing and reduction in viscosity of the settled slurry as the mixer moves down into it will mean that less power is required. Similarly, if a pump is started in the upper layer then lowered slowly to the bottom, then its starting and running power requirements will be less. The equipment described here worked satisfactorily under the ideal conditions which were carefully prepared and maintained in the laboratory. However, measurements of torque, which the viscometer must read, are very precise. The inconsistencies of slurry properties in farm-scale slurry stores, and the much harsher environment on the farm, lead to the conclusion that it is unlikely that this equipment could be developed for evaluating slurry properties in full-scale stores. It is felt that a device which is capable of indicating changes in properties of stored slurries with depth on the farm would be a valuable tool to designers and users of handling equipment, and work is continuing at NIAE on development of an instrument for this purpose.
REFERENCES Cumby, T. R., Wood, P. R. & Shaw, M. (1985). Slurry mixing with impellers. Agricultural Waste Utilization and Management, Proceedings of the Fifth International Symposium on Agricultural Wastes, Chicago, December 1985, American Society of Agricultural Engineers, St Joseph, Michigan, 694-704. Safley, L. J. & Nye, J. C. (1982). Energy data for manure handling equipment. Trans. of the ASAE, 25(4), 850-8. Schofield, C. P. (1983). Livestock waste characterisation for the design of handling systems. Project Paper, PMP/WE/83/014075/P National Institute of Agricultural Engineering, Silsoe, Bedfordshire (Unpublished). Schofield, C. P. (1984). A review of the handling characteristics of agricultural slurries. Journal of Agricultural Engineering Research, 30(2), 101-9. Technical Paper AR-2 (1982). Solutions to sticky problems. Brookfield Engineering Laboratories, Stoughton, Massachusetts, USA. Technical Paper AR-89 (1984). More solutions to sticky problems. Brookfield Engineering Laboratories, Stoughton, Massachusetts, USA.
Measuring Rheological Variations with Time and Depth in Livestock Waste Slurries using Rotational Viscometry C. P. Schofield Waste Engineering Group, Farm Buildings Division, National Institute of Agricultural Engineering, Wrest Park, Silsoe, Bedford MK45 4HS, Great Britain (Received: 12 February 1986; accepted 16 June 1986)
ABSTRACT A commercial rotational viscometer mounted on a motorised stand was used to measure theological variations with depth, of separated pig-sluro', over a period of approximately 2Oh. Results showed that this equipment can measure changes in slurry characteristics with depth, caused by settling. However, it was not possible to measure true viscosities at known depths with standard viscometer spindles because of the slurry settling during the tests and because spindle end-effects were higher than their geometry predicted. It is concluded that any shear thinning or thickening effects which are indicated by rotational viscometers working in pig-slurries may actually be due to the settling of solids during the tests and that readings obtained from the viscometer are of more value as torque measurements for mixing power estimation than as viscosity measurements. Used in this unconventional mode the equipment is unlikely to achieve useful measurements in full-scale waste-storage tanks.
INTRODUCTION Methods for establishing the rheological properties of livestock waste slurries are being studied as part of the Livestock Waste Characterisation Project at NIAE (Schofield, 1983). One of the objectives is to identify reliable methods for determining how rheological properties vary with depth in unstirred slurry-storage containers (e.g. tanks, lagoons and treatment vessels). A review of the literature on the properties of livestock 141 Biological Wastes 0269-7483/87/$03.50 © Elsevier Applied Science Publishers Ltd, England, 1987. Printed in Great Britain
142
c. P. Schofield
waste slurries (Schofield, 1984) showed that several researchers have examined the handling characteristics of homogenised slurries on a laboratory scale, but no work was found on variations of handling characteristics with depth through settled slurries, either on a laboratory scale or, more importantly, on full-scale systems. Recent work on mixing power requirements (Cumby et aL, 1985) stresses the importance of measuring rheological variations with depth as an indication of the effectiveness of mixing equipment. Slurry mixing can consume up to twice the energy needed for subsequent spreading of the slurry (Safley & Nye, 1982), so improvements in methods for assessing mixing requirements could yield substantial savings in energy. An 'on-farm' technique for measuring variations in slurry consistency with depth would provide valuable information for designers of mixing systems, pumps, pipelines and treatment plants, and could result in considerable financial savings and increased operating efficiency. The laboratory scale experiments reported here were undertaken as a first step towards designing an in-situ measuring device which would give information on slurry characteristics relevant to handling requirements, throughout the depth of the slurry. The objective was to find whether the technique could be used to obtain quantitative values of slurry viscosity, as well as to indicate significant changes in its properties with depth and time, and to establish whether the equipment and procedure could be developed to become of practical value.
METHODS The viscometer
A Brookfield Synchro-Lectric Model LVT rotational viscometer was used for the experiments. This instrument rotates a cylindrical or disc-shaped 'spindle' in a fluid and measures the torque necessary to overcome the viscous resistance to the induced movement. This is accomplished by driving the immersed spindle through a beryllium copper spring. The degree to which the spring is deflected is indicated by a pointer on the viscometer's dial and is proportional to the viscosity of the fluid for any given speed and spindle. Power is provided by a synchronous inductor type motor, driving through an eight-speed gear train, to provide a range of spindle speeds. The viscometer was mounted on a motorised Helipath stand, as shown in Fig. 1, so that it could be raised and lowered at a constant rate to enable readings to be taken over a range of depths in the sample slurry. To allow for this vertical movement an extension rod was
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143
Fig. 1. The Brookfield viscometer and Helipath stand. Spindle No. 1 is shown suspended on the extension rod.
used to connect the spindles to the viscometer. This gave a range of 212 mm travel between the upper and lower stops, but meant that the protective guard leg, normally fitted round the spindle, had to be removed. Two different spindles were used during the experiments. These were standard Brookfield spindles, type No. 1 LV cylindrical (18.84mm diameter × 65"1 m m long, and type No. 2 LV disc (18.72 m m diameter x 6.86 mm long). Conventionally, 'T' shaped spindles should be used in conjunction with the Helipath stand. These are designed for use in highly viscous materials, but do not give high enough readings on the viscometer scale when used in the relatively low viscosity pig-slurry, so the cylinder and disc spindles were used instead. To give readings which can be related directly to viscosity,
144
C. P. Schofield
these spindles should be used at a constant depth, so avoiding end-effects caused by motion through the slurry. As a result, the readings obtained in these experiments cannot always be related directly to viscosity (see 'Discussion'). It must be emphasised here that the Brookfield viscometer and Helipath stand were chosen for this work because they were readily available, and are widely accepted as reliable and accurate instruments when used conventionally. These experiments are outside their normal modes of operation, so the suitability or accuracy of these specific instruments when used conventionally is not in question.
Calibration Although the results obtained from these experiments cannot be presented as measurements of actual viscosity for reasons given in the discussion below, the equipment was calibrated to compensate for the non-standard container, extension rod and lack of guard leg, to make comparisons by other researchers possible. Absolute calibration of Brookfield viscometers is based on immersion of the spindle to a precise depth in a Newtonian fluid, in a container of minimum diameter 83 ram, with the guard leg fitted (Technical Paper, 1984). The experiments described here were made at varying depths in a non-Newtonian fluid in a measuring cylinder of only 60 m m diameter, without a guard leg, so a new set of calibration factors was required to convert the dial readings from the viscometer into a standard format relevant to the equipment working in a Newtonian fluid. The procedure outlined by Brookfield (Technical Paper, 1982) was followed. A Newtonian fluid with a standard viscosity at 25°C of 99 × 10-3 Pa.s when measured in a standard container was used to check the calibration of the viscometer under standard conditions. By then transferring the fluid to the measuring cylinder to be used for subsequent tests, and fitting the extension rod to the viscometer, it was possible to obtain calibration factors (F) which were relevant to the new operating conditions over a range of depths. Results presented in Table 1 show there to be no significant variation in reading with depth, so a single value of F c o u l d be used for each spindle. The new calibration factor is given by the formula: F = q/x
where F = calibration factor; r/= viscosity of the material (Pa.s) and x = reading on viscometer scale. Applying this formula gives the F values shown in the last column of Table 1. These are used throughout this paper, and are relevant to both spindles when fully submersed in a Newtonian fluid in the columns used for
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145
TABLE 1 Percentage Depth and Correction Factors for the Apparatus when Working in a Measuring Cylinder 60 mm Diameter and 348 mm Deep at 30 rpm. (Average of Five Repeat Runs)
Spindle No.
1 2
Run position
Top Bottom Top Bottom
Depth to lower spindle face
99 x l 0-3 Pa.s calibration fluid scale reading
(mm)
(%)
(x)
(SD)
126 338 107 319
36 97 31 92
46-2 46.4 12.0 12"2
0"45 0'55 0 0'45
Correction factor F (99 x I0- 3/x)
2" 14 8.18
the tests. The viscometer scale has 100 divisions, so readings can be taken to the nearest whole number, with corresponding reduced accuracy towards the lower end of the scale. Brookfield claims + 1% accuracy at full-scale, which is not disputed. The apparatus was set up so that the cylindrical spindle was submerged by 10mm when in the upper position, and had 10mm clearance from the bottom of the sample when in the lower position. This gave maximum vertical freedom of 212mm with the viscometer running up and down continuously between the upper and lower set points, each cycle taking 18"2 min. The depth of slurry in the test cylinder was 348 mm. Test procedure Fresh, separated pig slurry was used for the experiments. To obtain the sample, slurry was collected from a fattening house containing approximately 200 pigs of 25-45kg liveweight, fed on a commercial pelletted ration containing 18% crude protein, with ad libiturn access to water. The channels were agitated for at least 1 h, using a 2.5 kW centrifugal pump, prior to emptying, to ensure thorough mixing. The slurry was then passed through a brushed-screen, roller-press separator fitted with a 1-5 m m mesh to remove bristles and larger particles. A 200ml sub-sample was taken from this for analysis and found to contain 3.1% Total Solids (TS). After further mixing, three 1-1itre measuring cylinders were filled with the slurry, and labelled A, B and C. Cylinder A was immediately placed under the viscometer and the spindle positioned so tests could begin. Cylinders B and C were placed near to the apparatus, but left to settle undisturbed. The viscometer was calibrated and run at 30 rpm for all the tests. This rotational speed was judged to give a suitable wide range of viscosity
C. P. Schofield
146
readings from both spindles in the sample slurry. The temperature of the slurry remained between 12.5°C and 14°C throughout the test period. For simplicity, the start time for the experiment is called zero, and the following events counted from zero in minutes. The Helipath stand was started from the upper position, and readings were taken from the viscometer scale at 1-min intervals for one complete cycle of the spindle down and back up through the sample. Readings were taken using both spindles in cylinder A and then in cylinder B, as laid out in Table 2. Note that the apparatus was running up and down continuously through sample A between readings for the first 1200 min, whilst sample B was left undisturbed, and similarly in sample B for the remaining tests.
0
0% Curve A = Spindle-- 1 (depth to centre) C u r v e B = Spindle--1 (depth to lower face) Curve C = Spindqe= 2
20
~,
4o
Dark liquid
£
~
o
60
•
A
69%
8O
90%
1
Fibrous
95% 1 0~0n
I 50
I 100 Viscosity reading
I 150
I 200
~
~ : : : : "
Fig. 2. Comparison of viscosity readings from spindle No. 2 with those from the centre level and from the bottom face of spindle No. 1. The sketch shows settled zones observed in undisturbed slurry after settling for 1340min.
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TABLE 2
Summary of Events during Tests Time (mins) 0
40 1 130 1 160 1 200
Event
Fill cylinders A, B and C with mixed slurry Put cylinders B and C to one side undisturbed Take readings: spindle 1 in cylinder A Take readings: spindle 1 in cylinder A Take readings: spindle I in cylinder A Replace spindle 1 with spindle 2 Take readings: spindle 2 in cylinder A Replace cylinder A with cylinder B Take readings: spindle 2 in cylinder B
1 300
Replace spindle 2with spindle 1 Take readings: spindle 1 in cylinder B
1 340
Record depths of sediment layers in cylinder C
Result
Fig. 3, curve D Fig. 3, curve E Fig. 3, curve B Fig. 2, curve C Fig. 4, curves F and G Fig. 4, curve H Fig. 2, diagram
Note: the apparatus was left running between readings.
To provide a visual check against the results from the viscometer, the third sample of the same slurry was allowed to settle undisturbed in cylinder C. The depths of the interfaces between the settled layers were measured for comparison (Fig. 2).
RESULTS A N D DISCUSSION The effects of settling and depth on viscometer reading All viscometer test results are presented graphically in Figs 2 to 4. The vertical axes represent the depth of the viscometer spindle in the test sample as a percentage of the sample depth. The horizontal axes represent the dial reading taken from the viscometer multiplied by the correction factors (F) obtained during calibration (Table 1) and called the Viscosity Reading. Figure 2 illustrates the need to relate Viscosity Readings to the depth of the bottom face of the spindles, and shows a sketch illustrating the depths of zones observed through the transparent walls of cylinder C, containing the undisturbed slurry after 1340min. Figure 3 shows the effect of time on Viscosity Readings at different depths and Fig. 4 illustrates the fragile nature of the 70-90% depth zone in the settled slurry. All curves (except
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0 Curve D = Spindle =1, 0 -10 minutes
Curve E = Spindle =1, 40 - 50 minutes Curve B = Spindle =1, 1130 - 1140 minutes 2O
4O
6O
8O
jB
10C
I
50
I
1O0
I
150
I
200
I
250
I
300
Viscosity reading
Fig. 3.
Viscosity readings related to the b o t t o m o f spindle No. 1 after 0, 40 and 1130 min
settling in column A.
curve G) are traced through points taken when the ~,iscometer was travelling downwards through the sample. Curve G represents readings taken on the upward pass. As the curves in Figs 2, 3 and 4 show, pig-slurry settles into defined layers of different viscosity. As a result, when the cylindrical spindle is passing from one layer to the next, it experiences proportionally different forces over its surface area. For a cylindricalspindle to give a true re~tding of viscosity, it must be working in a homogeneous fluid, so experiencing equal forces over its whole surface area. The readings taken from the viscometer when working in pig slurry cannot therefore be regarded as true viscosity values, although they can still be regarded as indicators of how viscosity changes with depth.
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o Curve F = Spindle = 2, 1200 - 121Q minutes Curve G = Spindle =2, 1210 • 1220 minutes Curve H = Spindle = 2 , 1300 - 1310 minutes
20
80 --
100
Fig. 4.
0
I 50
i 100
I i 150 200 Viscosity reading
t 250
I 300
Viscosity readings using spindle No. 2 in column B after settling undisturbed for 1200min.
Both the cylindrical and disc spindles are designed to work at constant depth in a fluid under test. Any vertical motion will cause changes in shear forces on the surfaces of the spindles which will affect the torque readings on the viscometer, thus giving false readings of viscosity. This is a second reason why results from these experiments are presented as Viscosity Readings, rather than actual viscosities. These changes in shear forces due to the vertical motion will be particularly apparent on the end of the spindle moving into the fluid. This is because the lower end of the spindle will not only be working in slurry of higher viscosity, due to settling, but will also be pushing down into the slurry so experiencing higher shear forces. As the cross-sectional areas of both types of spindle are practically the same, the only difference between them will be due to the forces over their lengths. These forces should be larger for the cylindrical spindle under ideal conditions. In Fig. 2, curve A
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represents Viscometer Readings plotted against the depth of the centre level of the cylindrical spindle, and curve B is the same curve adjusted to refer these readings to the depth of the bottom surface of the same spindle. Curve C represents readings using the disc spindle, also related to the depth of its lower face. As can be seen, curves B and C are almost identical. This confirms that the shear forces being measured by the viscometer are acting mainly on the lower faces of the spindles, so results from both spindles should be related to depths corresponding with their lower faces under these test conditions. This observation is further confirmed when curves B and C are compared with results from the visual settling test, also shown in Fig. 2. After 1340min of settling, the slurry was observed to form three distinct layers, with the interfaces being at 69% and 90% depths. These positions correspond closely to the turning points on curves B and C. A further interface at about the 95% depth position was less well defined, yet is in the same region as the highly viscous zone suggested by extending the lower ends of the measured curves. Curves D and E in Fig. 3 show that there is no recorded change in Viscosity Reading between 0% and 80% depth within the first 40min of settling. If this is accepted, and a well-mixed sample is obtained, then readings taken within these limits can be taken as representing the true viscosity of the mixed slurry. Researchers interested in measuring true viscosity should consider testing their slurries at time intervals to establish whether settling is taking place, and so needs to be accounted for. Figure 3 shows curves drawn through points recorded as the slurry settled over a long period of time. Curve D was obtained immediately after the mixed sample was placed in column A. As can be seen, there is a clear increase in Viscosity Reading towards the bottom of the curve, even though these points were read within 10 min of the start. Within 40 min the settling is very apparent at greater than 90% depth (curve E). After l l 3 0 m i n , a further settling zone increasing in viscosity between 65% and 85% depth has developed, and there is a distinct drop in Viscosity Readings between 0% and 65% depth. This is due to fine particles falling slowly from the upper levels, and gently compacting towards the lower levels. Hence the drop in viscosity near the top, and gradual increase with depth. These observations can be related to results which would have been recorded had the spindle remained at one level in the cylinder over this period of time. If the spindle had been stationary anywhere above the 60% depth line, a decrease in viscosity would have been observed, and, neglecting the effect of settling, the slurry could have been classified as shear thinning (thixotropic). However, if the spindle had been below the 70% depth line, then a rise of viscosity with time would have suggested that the
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slurry was shear thickening (rheopectic)! Although researchers are likely to account for this, conditions where a slurry might effectively display this behaviour do occur in practice. Take, for example, a pump at the bottom of a stirred tank. If the slurry is completely mixed then the pump could be designed to work in slurry with a Viscosity Reading of under, say, 50. If however, the tank was inefficiently stirred, so allowing settling, or if the pump was started before all sediment had been re-suspended by a mixer, then it would have to handle slurry with a Viscosity Reading far higher than 50, requiring much more power or possibly a completely different type of pump. When spindle 2 was passed down through cylinder B after the slurry had settled for 1200 min, very distinct changes in Viscosity Reading with depth were recorded as shown by curve F (Fig. 4). These compare very well with the observed levels of cylinder C (Fig. 2). Curve G is drawn through points taken on the upward pass recorded immediately after curve F, and shows how the Viscosity Readings through the 70-90% zone drop due to the disturbance caused by a single pass of the spindle. Curve H, taken 80 min later using spindle 1, shows how the viscosity of this layer has been reduced by the spindle continuously passing through, and is very similar to curve B of sample A. This suggests that the layer at 70-90% depth is very fragile and makes measuring its true viscosity very difficult. It would certainly be unreliable to take a sample from this depth in a farm-scale store containing settled slurry and hope to measure its viscosity and relate this back to the undisturbed slurry. Even the gentle movement of the spindle passing through once is shown to cause a 14% drop in reading, so sampling would inevitably destroy its structure. Looking on the practical side, though, this layer should be very easy to re-suspend by mixing, and pose little problem to either pumping or mixing equipment. The changes of Viscosity Reading with depth shown in curve F can be related to the power requirements for mixing or pumping equipment working at those depths, but as these Viscosity Readings do not represent true viscosities, actual power requirements cannot be calculated. As already stated, a pump or mixer working in the upper layer of a tank or store would not be affected by settling, but it could experience serious over-loading or blocking on start-up if positioned near the bottom. The middle zone, between 70% and 90% depth, presents a rather different problem. The rapid destruction of the structure of this layer means that, provided the pump or mixer can overcome the initial loading, the resistance will quickly drop, and so the power requirement will fall. Knowledge of these properties could help reduce the power input and size of mixing and pumping equipment. If the mixer was designed to start working in the upper layer and be lowered gradually through the slurry, the
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progressive mixing and reduction in viscosity of the settled slurry as the mixer moves down into it will mean that less power is required. Similarly, if a pump is started in the upper layer then lowered slowly to the bottom, then its starting and running power requirements will be less. The equipment described here worked satisfactorily under the ideal conditions which were carefully prepared and maintained in the laboratory. However, measurements of torque, which the viscometer must read, are very precise. The inconsistencies of slurry properties in farm-scale slurry stores, and the much harsher environment on the farm, lead to the conclusion that it is unlikely that this equipment could be developed for evaluating slurry properties in full-scale stores. It is felt that a device which is capable of indicating changes in properties of stored slurries with depth on the farm would be a valuable tool to designers and users of handling equipment, and work is continuing at NIAE on development of an instrument for this purpose.
REFERENCES Cumby, T. R., Wood, P. R. & Shaw, M. (1985). Slurry mixing with impellers. Agricultural Waste Utilization and Management, Proceedings of the Fifth International Symposium on Agricultural Wastes, Chicago, December 1985, American Society of Agricultural Engineers, St Joseph, Michigan, 694-704. Safley, L. J. & Nye, J. C. (1982). Energy data for manure handling equipment. Trans. of the ASAE, 25(4), 850-8. Schofield, C. P. (1983). Livestock waste characterisation for the design of handling systems. Project Paper, PMP/WE/83/014075/P National Institute of Agricultural Engineering, Silsoe, Bedfordshire (Unpublished). Schofield, C. P. (1984). A review of the handling characteristics of agricultural slurries. Journal of Agricultural Engineering Research, 30(2), 101-9. Technical Paper AR-2 (1982). Solutions to sticky problems. Brookfield Engineering Laboratories, Stoughton, Massachusetts, USA. Technical Paper AR-89 (1984). More solutions to sticky problems. Brookfield Engineering Laboratories, Stoughton, Massachusetts, USA.