ANC CONTROL

EXPERIMENT 2: Testing the lime feeder performance using a pH probe

INTRODUCTION

The new apparatus was equipped with a pH probe whose values could be logged into the computer via the Process Controller. In addition, Process Controller was utilized to automate the experiment variables. With those additions, the team measured the pH of the effluent, looking for the optimal flow rate that could maintain a pH of 12 pH units for 24 hours.

RESULTS

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Improvements made, included a change in the design of the water outlet to ensure an adequate pH measurement (Figure 2). Also the pH probe was installed in a vertical tube and a curved outlet was added for the effluent. With these changes, an accurate pH measure could be obtained.

DESCRIPTION

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.

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Figure - 1 Apparatus for experiment 2, using a pH probe

The initial pH reading was 12 and stayed that way for 2 hours. It gradually decreased to 8.85 (close to the pH of a solution that is fully saturated calcium bicarbonate) by the 24th hour.

One of the possible explanations regarding the 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 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 a challenge for the next experiment.

RESULTS

1. With an upflow velocity of 10.71cmWith a flow rate of 40mL/min, the pH dropped from 12 to 11 after about 3.5 hours.
2. After increasing the flow rate at 60mL/min and keeping the upflow velocity of 16cm/min (in order to enhance the lime mixing) and keeping all other parameters constant, no significant changes were noted.
   Figure 2 shows the pH measures during the experiment.The results of the experiment are on an attached excel file. Excel Spreadsheet
   

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   Figure 2. Time vs pH
   
   

   CONCLUSIONS

   Several problems were diagnosed from the above experimental results. They are as follows:

1. The arrangement of the pH probe was not stable so the pH data was not accurate.
2. 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.
3. 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.

These problems prompted for a modification in the design of the column leading to the next stage of experiments.

By the end of the experiment, changes were made in the design of the water outlet in order to ensure an adequate pH measure (following Professor Monroe's suggestions). It included putting the pH probe in a vertical tube and making a curved outlet for the effluent. With these changes, the apparatus could get a more stable pH measure. A figure explaining this description is shown below.

The team also worked on the calculation of the relationship between the ratio of flow rate at the lime feeder and at the plant with the alkalinity provided by the effluent lime. That information was useful to edit MathCAD calculations and to determine the changes in the Ph at the plant with respect to changes in this flow rate ratio (fraction of water flow rate entering the lime feeder from the entrance tankTo solve the above difficulties, a new model for the lime feeder, consisting of a sloped column above a vertical column was considered and several trials were experimented on this new model. These trials are detailed in the next experiment (Experiment 3).