Experiment 9: Increased Lime Concentration + Continued Pulsing

Overview

The purpose of this experiment was to maintain all the same conditions as Experiment 8 but to change the concentration of the stock solution. Experiment 8 failed because the lime pulses did not increase the pH sufficiently. Therefore, it was determined that a more concentrated solution was required. The amount of lime added to the solution was multiplied by 5, resulting in 622.5 grams of lime added to 15 liters of water--a concentration of 41.5 (g/L).

Procedure

An almost identical procedure was followed for this experiment as in the Experiment 8. A lime slurry of 100 grams of lime plus 1 liter of tap water was prepared for each apparatus. Then a more highly concentrated lime slurry of 622.5 grams of lime was prepared in 2 liters of tap water and then diluted further with 13 additional liters of water for the stock concentration.

Because the blender was broken, the large mixer had to be used to prepare all three lime slurries.

Results and Discussion

Approximately 120 hours later (5 days later), the stock solution was refilled with the same concentration (41.5 g/L). However, the blender had been repaired at this time and was used to prepare the slurry, instead of the mixer. At the same point in time (5 days from the beginning of the experiment) the flow rate for the peristaltic pump flowing to the 5 cm tube settler was increased from 40 mL/min to 50 mL/min in an attempt to raise the pH.   The team suspects that when the flow rate in the 5 cm tube settler apparatus is too low, the lime particles settle and create preferential flow paths which keeps the lime from interacting with the water.

Below in figures 1, 2 and 3 is the preliminary data from the experiment.  The 2.5 cm plate settler was relatively stable, achieving a pH of over 12 for well over a week!  The 5 cm plate settler, however, never reached a pH of 12 but rather oscillated around a value of approximately 11.5.  As time went on, the amplitude of the data's wave increased and the pH started regularly dipping below 10.                                          

                                                                                                                                           Figure 1 - Experiment 9
                                                             The above graph is hours 0 to 40 of an experiment pulsing in 41.5 g/L of lime slurry at 380 mL/min for 1 minute  every six hours.

                                                                                                                                            Figure 2 - Experiment 9
                                                               The above graph is hours 40 to 80 of an experiment pulsing in 41.5 g/L of lime slurry at 380 mL/min for 1 minute  every six hours.  
                                                                                                                                             Figure 3 - Experiment 9
                                                          The above graph is hours 80 to 120 of an experiment pulsing in 41.5 g/L of lime slurry at 380 mL/min for 1 minute  every six hours.

Experiment 9 ran for nearly 10 days before the team shut it down.  The performance of the 2.5 cm tube settler remained pretty consistent, with the pH remaining right around 12.  The 5 cm tube settler continued its wave-like motion, as seen in Figure 4.

 

                                                                                                                                    Figure 4 - Experiment 9, Day 9

This graph is zoomed in to display that the smaller apparatus does experience slight peaks with each lime pulse, and to not that the larger apparatus dips below a pH of 10 as the experiment continued.