...
The design of the water outlet was changed in order to ensure an adequate pH measurement (Figure 2). The pH probe was installed in a vertical tube and making a curved outlet was added for the effluent. With these changes, the team could obtain an accurate pH measure.
...
The mass of lime added was 17.89 gm. With this amount of lime, the upflow velocity chosen was 10.71cm/min, because the lime remained suspended and it was assumed the apparatus would run for a longer time according to the results observed in Experiment 1. The initial pH reading was 12 and stayed that way for 2 hours. It gradually decreased to 8.85 (saturation pH of calcium carbonate) by the 24th hour.
One of the possible explanations about regarding the decreasing decrease of pH at a constant rate during the trial , was that the upflow velocity was not enough to mix the lime and keep the dissolution. As a probe, it was increased to 16 cm/min. When the this change was made, the probe registered an increase to a pH of 9.4.
After the change in flow rate was made, the pH measure showed the same behavior, decreasing constantly. When a pH of 8.5 was reached, the experiment was terminated, despite the fact that there was still a suspended bed of lime.
Other possible explanations were that the calculations were not precise about the lime concentration required for the desired pH; or that the vertical column didn't allow a continuous lime mixing and an important amount was going out of the apparatus. It will be the a challenge for the next experiment.
Figure - 1 Apparatus for experiment 2, using a pH probe
...
- The arrangement of the pH probe was not stable so the pH data was not accurate.
- The simple column design was not adequate in giving any range in terms of flow rates that could fulfill the objectives of maintaining a good suspension and maintaining a stable pH for 24 hours on the effluent.
- The vertical column (Volume= 0.211 L) was not large enough to store the required amount of lime to run the experiment for 24 hours.
...