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EXPERIMENT 2: Testing Carbonates Hypothesis
INTRODUCTION
Based on ANC Control team Fall 2009 future challenges, it was suggested to develop tests with Tap water and distilled water in order to test carbonates influence over the apparatus performance. The ANC Control Team Spring 2010 ran two identical experiments: one with tap water, one with distilled water. Results from the two experiments will be compared and analyzed.
Introduction
In order to test the calcium carbonate hypothesis developed by the Fall 2009 team, the Spring 2010 team ran a second experiment to compare the performance of the reactors using distilled water versus tap water. There should be no carbonates present in the distilled water, so if the formation of calcium carbonate precipitate inhibiting dissolution is what is limiting the reactor's performance, the use of distilled water should produce better results. The two runs in this experiment were identical except for the water type so that that variable could be isolated.
Procedure
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The team used the same procedure described on materials and methods. It is important to note that The lime was added as dry powder.
According to the calculations based on lime solubility constant; this experiment required 20g of lime and is projected to last for 12 hours, the maximum or theoretical pH will be 12.6, with a flow rate of 120mL/min.
RESULTS
Although a pH of 12 was calculated to last for 12 hours; during the experiment, the pH attained was 12 units or above for 40 minutes for the A2 apparatus run with DI water and about 86 minutes for the A2 apparatus run with tap water. The A1 apparatus never reached a pH of 12. The following graph shows pH behavior over time.
It is shown that A2 requires a larger time to increase pH, which is due to the difference in design. A2 has a larger retention time. On the other hand, pH in A1 decreases faster than pH in A2, which could be explained on the same basis of design differences. Capture velocity in A1 is faster and it is possible that small particles of lime are getting out at a high rate while they can stay and react for a larger period of time in A2.
To have a better understanding of what is happening during the run, the team plotted OH fraction versus time fraction. Theoretical pH was established as 12.6, making the theoretical pOH 1.4, and the theoretical time was 12 hours. Results are shown in the following graph.
As shown, OH concentration in A2 was highly above the theoretical estimation having values up to 12.8; while A1 never reached a pH of 12, having its maximum at 11.68. On the other hand, the experiment lasted only 15% of the expected time. OH concentration is a good indicator of design performance since the main objective of ANC team is to regulate the neutralizing capacity of Honduran water, it is projected to establish an optimal base concentration that will allow dealing with the addition of chemicals with acidic concentration in other stages of the purification process.
CONCLUSIONS
For the up-flow velocity of 3.6 mm/sec, it was calculated that 20g of lime were required for the effluent to be saturated for 12 hours. The amount of lime was calculated as explained in Experiment 1., proportions were kept.
Results
Figure 1. shows effluent pH over time. The green line represents the 2.5cm diameter tube settler, and the 5cm diameter tube settler is represented by yellow and orange lines. We can see that none of the runs were anywhere close to reaching the goal of twelve hours with a saturated effluent. Both tap water and distilled water runs with the 5cm diameter tube settler failed within an hour and a half, while the run with the 2.5 cm diameter tube settler never produced any saturated effluent. There is certainly no performance improvement running the reactor with distilled water.
Figure 1. Effluent pH vs time in hours. Comparison of performance between tap water and distilled water.
Figure 2. shows the fraction of saturated hydroxide concentration versus the fraction of time the run is theoretically calculated to last. This is simply a clearer way to display the reactor's performance over the course of the experiment relative to our goal, which would be to have a straight line at an OH fraction of one. In this case, Figure 2. illustrates what are likely measurement errors: a hydroxide concentration nearly double the saturation value in one run and a failure to reach saturation in what should be an identical trial.
Figure 2. Fraction of saturated hydroxide vs Fraction of time
Discussion
The similarity in performance between the 5 cm diameter tube settler run with tap water and the same apparatus run with distilled water indicates that calcium carbonate precipitate may not be the main issue. Failure with distilled water suggests that there must be another issue. See the hypotheses page for further information.
Even with the results, one experiment is not enough to reject the carbonates hypothesis. Other factors could affect such as coating of the pH probe with lime particles. Further experiments are required to compare resultsImprovements in the design have worked very well; it is possible to detect an increase in retention time, a decrease in capture velocity and a well mixing in the bottom of the A2 apparatus. With a high flow rate, A1 shows a well mixing in the bottom but capture velocity is very high and it is possible that lime particles escape through the effluent.
The similarity in the performance between the A2 apparatus run with tap water and the A2 apparatus run with distilled water indicates that carbonates may not be an issue. Failure with distilled water indicates that there must be another issue - perhaps a kinetics issue relating to the dissolution of lime particles. This will require further research with a fluidized bed.