ANC CONTROL

EXPERIMENT 2: Testing Carbonates Hypothesis

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

The team used the same procedure described on materials and methods. It is important to note that 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.

Discussion

Improvements 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.