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 graph (Figure 1) was referenced to determine the appropriate amount of lime that needed to be pulsed in every 6 hours.  The mass of lime was calculated in terms of various flow rates by determining the concentration of lime (g/m^3) and multiplying it by the flow rate (m^3/hr) and the time (6 hr).
                                     
                                                                                                                                                                Figure 1

                                                                            The above graph displays the mass of lime needed to saturate the system given a specific flow rate.

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.  In determining flow rates, the team learned from experience that when the larger apparatus had too slow of a flow rate the particle settled in the tube settler and this created preferential flow paths through the top half of the settler.  Thus, a slightly higher flow rate was chosen for the 5 cm tube settler apparatus.

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.  The group determined that pumping in the lime solution at the highest rate possible for only one minute would not create too much of a disturbance in the overall flow rate of the apparatuses.  Thus, 380 milliliters of stock solution are pumped in every six hours.  One challenge to this set up is that there is only one pump and one concentration for the stock solution.  However, each apparatus requires a different mass of lime to be pulsed in, according to Figure 1 above.  To solve this problem, the higher concentration of stock solution was made according to the calculations for the 5 cm tube settler, which resulted in a concentration of 8.3 grams of lime per liter. 

The team then installed two pump heads on the third peristaltic pump and used two different tubing sizes to regulate the flow to each apparatus.  Referencing the information about peristaltic pumps  it was determined that the 2.5 cm tube settler should have tubing size 17 while the 5 cm tube settler has tubing size 18.  This allowed for each apparatus to receive the proper lime mass with each pulse while still running off the same pump.

Using a 5 gallon bucket, the team calculated the volume of the bucket and the lime mass needed (taken from Figure 1).  By multiplying the concentration of lime needed by the volume of the bucket, it was calculated that 124.5 grams of lime, dissolved in 15 liters of water, can run for approximately 40 pulses, or 240 hours!  The set up can be seen in Figure 2 below.
                                                                                                                                                         Figure 2
                                       The laboratory set up of a bucket of stock solution connected to a third peristaltic pump that pulses in a specified concentration of lime slurry.

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 the 2.5 cm tube settler had a flow rate of 30 mL/min and the 5 cm tube settler 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 leaving the pumps connecting the raw water to the two apparatuses 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 3).

                                                                                                                                                             Figure 3

                                                                       This picture depicts the third peristaltic pump with two different pump heads, one for each apparatus with

                                                                                      size 17 tubing for the 2.5 cm tube settler and size 18 tubing for 5 cm tube settler.

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.

The 2.5 cm tube settler apparatus took a while to reach equilibrium but eventually reached a pH of slightly greater than 12. It 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. It'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

The first couple of pulses do not create spikes in the graph.  This may be due to the fact that the 100 gram lime slurry takes time to settle into equilibrium.  It could also be caused by the very fine lime particles that settle slowly.

The 5 cm tube settler apparatus performed similarly, except that it reached a level pH of approximately 11.5 much more quickly. It started failing sooner than the smaller apparatus, at around 6:00 am, only 18 hours after the experiment began.

                                                                                                                                                Figure 4 - Experiment 7
                              This purpose of this experiment was to develop a set up in which stock solution of lime is automatically pumped into the two apparatuses every six hours.