It is important to develop understanding about how different scale factors affect cleaning and to assess whether the effects observed due to varying process parameters evaluated earlier in Sections 5.6 and 5.7, remain relevant. The effect of varying pipe length scale is assessed in this section. The length of the filled test section was varied between 0.3 m and 2 m for a 47.7 mm ID diameter pipe. The resulting turbidity profiles are given in Figure 5.15.
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Figure 5.15: ZEAL Pilot Plant: System to be cleaned, 47.7 mm diameter. Cleaning conditions 1.7 m s-1 based on clean tube and 40°C i) Optek turbidity profiles for different length scales, ii) Kentrak turbidity profiles for different length scales
Figure 5.15 shows the cleaning profiles from experiments conducted at a velocity of 1.7 m s-1 based on a clean tube velocity and a temperature of 40oC in a 47.7 mm ID pipe diameter system for various lengths. At all length scales the turbidity (FTU) profiles follow similar trends:
at t = 0 s until t = 6 s, zero readings were recorded as water in the pipework was passed through the system.
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At t ~ 6 s, the turbidity rapidly rose to a peak, as the initial core was displaced. The turbidity dropped to a primary minimum, at t ~ 25 s
This was followed by an increase at t ~ 80 s as the thin film cleaning progressed. A decline in turbidity measurement until an end-point was then seen.
The turbidity (ppm) measurement recorded a saturated reading at all length scales until ~ 200 s, when the readings rapidly dropped. The cleaning end-point for experimental set-up for lengths of 0.3, 1 m and 2 m was 211 ± 26 s, 169 ± 16 s and 213 ± 29 s respectively as seen in Figure 5.16.
Figure 5. 16: ZEAL Pilot plant data: System: 47.7 mm diameter, cleaning conditions for water at 40°C and 1.7 m s-1, velocity based on a clean tube. Cleaning times for test section pipes of length 0.3 m, 1 m and 2 m, errors based on three repeats of the experiment.
There is no significant difference in cleaning times for pipes of lengths between 0.3 m and 2m pipe filled with toothpaste. Initial impressions may suggest that this is counter intuitive, as more than six times the volume of paste must be removed when comparing the 0.3 m and 2 m systems. However, on further analysis, this result is strongly supported by the removal
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mechanism identified in Section 5.1, where two distinct removal phases are observed, namely a core cleaning phase followed by the thin film removal from the pipe wall.
The core removal phase is proportional to the fluid residence time, which is a function of pipe length. The residence time difference is 0.3 s to 2 s in a 1 m s-1 process, or less at higher flow rates, this is a very small fraction of the whole cleaning time. The total cleaning time is therefore dominated by the thin film removal, which takes > 50 times the core removal phase, and is not a strong function of pipe length in the range investigated here. To demonstrate that it is the thin film removal stage that is rate limiting an experiment with a pipe of length 2 m was conducted, where the test section comprised of two 1 m sections. At t = 1 s and towards the end of the experiment at t = 200 s, the experiment was stopped and the pipe opened to compare the thickness of the paste along the length of the pipeline. Results are shown in Figure 5.17.
Figure 5.17: ZEAL Pilot Plant, System is 47.7 mm ID diameter, 2 m length. Cleaning conditions at 40°C, for 1.7 m s-1 based on a clean tube velocity. Pictures taken on the pilot plant after 1 s and 200 s, experiment showing the amount of paste coating the pipe wall at different positions through the test section.
The thickness of paste coating the pipe wall after the core removal (1 s) was not significantly different along the length of the pipe as seen in the first row of Figure 5.17. After core removal, the subsequent cleaning was much slower. After 200 s there were small patches left
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along the length on the pipe wall in all cases, i.e. removal appears to happen uniformly along the length of the pipe rather than preferentially from one end. The cleaning time is thus controlled by the time to remove the thin surface layer.
To investigate if the measurement responses can distinguish between different volumes of paste, the integral of the turbidity measurement for the experiments has been calculated.
dt m eadings turbidityr dt readings turbidity Integral 1 (5.3)
The ratio for the Optek turbidity experiments was 0.83: 1: 1.56 for pipe lengths of 0.3 m: 1 m, 2 m and for the Kentrak turbidity meter for the same lengths the ratio was 0.5: 1: 1.4. Although an exact match with respect to number of total turbidity units is not achieved the trend is still evident. In the case of the Optek meter, the reading was saturated at turbidity > 50ppm and so the initial part of the experiment in all cases gave the same reading, as the saturation of the readings masked a more meaningful value of the integral.
The length of the pipe had limited impact on the cleaning times. The results from the experiments at different velocities and at lengths of 0.3 m and 1 m are shown in Figure 5.18 for different temperatures.
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Figure 5.18: ZEAL Pilot Plant: System: 47.7 mm ID diameter pipe and 0.3 m and 1 m pipe lengths. Cleaning conditions: i) 20°C, ii) 40°C and iii) 50°C,
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In all instances there was no difference observed between the 0.3 m and 1 m data within experimental error, as compiled in Figure 5.19.
Figure 5.19: ZEAL Pilot Plant: System: 47.7 mm diameter. Variety of lengths. Cleaning time as a function of flow rate for toothpaste removal at 20°C, 40°C and 50°C, varying test section length at fluid velocities of 1 m s-1, 1.3 m s-1, 1.5 m s-1, 1.7 m s-1 , 2.3 m s-1 and 2.9 m s-1, based on clean tube velocities,
Figure 5.19 shows data for all flows and velocities, i.e all the results reported earlier in the Chapter. The behaviours at each temperature are consistent across all length scales between 0.3 m and 2 m, at the temperatures and velocities studied.