Experiment 7: Pulsing in a Stock Solution of Lime
Overview
The purpose of this experiment was to test pulsing lime into the two apparatuses. Because both apparatuses fail after a shorter time than desired, it was hypothesized that the lime initially added to the apparatus (100 grams) eventually reached a point where it would no longer properly dissolve and raise the pH. Thus, more lime was needed as the experiment progressed. However, adding 100 grams each time would severely overwhelm the system. The following calculations were performed to determine the appropriate amount of lime that needed to be pulsed in every 6 hours:
From this graph one can see that the amount of lime that must be added for a flow rate of 30 mL/min is approximately 2.365 grams and for a flow rate of 40 mL/min, 3.154 grams of lime must be added.
The group determined that it would be best to pump in the stock solution at the highest possible flow rate, which is 380 mL/min.
Procedure
For the apparatuses, the same procedure was followed as outlined in the materials and methods page. Two 100 gram samples of lime slurry were prepared by blending them in the blender on the "liquefy" setting. These were then fed into each apparatus via the vertical column. The water used was tap water and A1 had a flow rate of 30 mL/min and A2 had a flow rate of 40 mL/min.
A third peristaltic pump was added to the system to provide a pump from the stock solution of lime slurry to the tubing that flows to each apparatus. The tubing for the A1 pump and the A2 pump met the tubing leaving the third peristaltic pump with as little tubing in between as possible. This was done in order to minimize the amount of lime that might be able to settle out in the tubing. There was tubing attached to the stock concentration bucket (approximately 5 gallons) and then split to run through two separate pump heads on the third peristaltic pump (Figure 1). Because the pump had to run at only one speed (380 mL/min to add the lime slurry as quickly as possible) there had to be two different tubing sizes running through the two pump heads in order to accommodate for the difference in pulse volume needed for A1 and A2. The tubing sizes were determined to be 17 for A1 and 18 for A2. Calculations can be seen here: 
INSERT MATHCAD CALCULATIONS HERE
The concentration needed in the bucket was determined to be 124.5 grams of lime dissolved in 15 liters of water. This came from the following calculation:
INSERT MATHCAD CALCULATIONS HERE
For this experiment, the lime slurry in the bucket was not blended using the blender but rather using the mixer. The mixer stayed in the bucket throughout the course of the experiment and Process Controller was configured to turn the mixer on for two minutes before the pump was configured to run. Thus, the stock solution would mix for two minutes and then be pumped for one minute at 380 mL/min. This repeated every six hours.
Results and Discussion
As the first pulsing experiment, the apparatuses behaved approximately as expected. However, neither apparatus ever reached saturation (pH = 12.6) which could have been due to a number of things, including on-going difficulties with the pH probes.
Apparatus A1 took a while to reach equilibrium but eventually reached a pH of slightly greater than 12. A1 was fed the 100 grams of lime slurry at approximately 12:15 pm. It then leveled off at a pH of slightly less than 10 until 6:19 pm, the time of the first scheduled lime pulse. A1's pH then shot up to approximately 11.5, where it remained until the second lime pulse when it finally reached a pH of slightly greater than 12. After the third pulse around 6:15 am, the pH level started wavering and dropping below 12. By the time of the fourth pulse around 12:15 pm (24 hours after the experiment began) the pH was steadily dropping and continued in this trend, with the occasional spike every six hours with the lime pulse.
Why do the first couple pulses behave differently, i.e. no spike in the graph?
• The 100 gram lime slurry that you're adding has to settle into equilibrium position
• Particles really fine, settle slowly
A2 performed similarly to A1, except that it reached a level pH of approximately 11.5 much more quickly. It started failing sooner than A1, at around 6:00 am, only 18 hours after the experiment began.
Figure 2 - Experiment 7