ANC CONTROL

TRIAL 1. TAP WATER

INTRODUCTION

The first trial on the sloped-column lime feeder consisted of using tap water. The flow rate for this experiment 120mL/min which was calculated based on an upflow velocity of 4.42mm/s, as found in experiments 1 and 2. The amount of lime for this experiment was 80 grams, which was calculated in volumetric proportion with the experiment 2.
The objectives for this trial were:

• To check if the sloped column lime feeder model works.
• Determine if there are any preliminary technical difficulties within the model and/or process and make improvements for them for future experiments.
• Find the maximum time for which the pH at the effluent remains at 12.

PROCEDURE

The experiment consisted of the injection of tap water with a flow rate of 120mL/min (i.e. an up flow velocity of 4.421mm/s ) regulated with a peristaltic pump being operated by the process controller software. At the effluent end (top of the sloped column)a pH probe was connected to collect periodic (every 5 seconds) data of the effluent pH. The Process Controller files can be found in the Materials and Methods section.

RESULTS

The apparatus worked well with the established upflow velocity. The movement of particles was clearly visible and in accordance with the hypothesized movements, i.e. a fluidized bed up to the middle portion of the lime feeder and a relatively clear effluent towards the top end of the (sloping) column which acted like a tube settler. At the top of the sloped column, the water looked clear and free from visible particles but according to the pH sensor measurements, it was saturated with base (i.e. pH 12), which was in accordance with the main objectives. A great amount of lime remained captured in the middle of the apparatus, although it was held in suspension. This signifies that the upflow velocity was high enough to give good suspension but the sloped column took care that the extra lime that would have been flushed out had it not been there. This was a desirable behavior because more lime was still in the apparatus, donating hydroxides to the solution.

However, the pH remained stable at 12 just for 20 minutes. It was necessary to calibrate the pH probe again, to clean the apparatus and to review the calculations in order to determine the reason for the abnormal pH behavior. Other reasons for this behavior included the fact that since the lime was poured into the narrow column in a powder-form, it resulted in the clogging and pressurizing of the column. Subsequent flushing (using higher flow rates to force the lime to unclog) caused loss of lime. More importantly, during this period of time, data was not recorded since the lime feeder had to be overturned manually to get the solidified, stuck-up lime to dissolve.

Figure 1. pH vs time

CONCLUSIONS

The design seemed to work as expected and the movement of the particles was desirable. However there were some difficulties faced in the running of this trial.

As the lime was poured into the apparatus in powder form with very little water inside, this resulted in the solidifying of the lime sludge and caused initial pressurizing within the column. It also took up time to clear the clog especially since the apparatus was extremely long and unusually shaped. To overcome this difficulty it was decided that henceforth for all future experiments, lime will first be made into a slurry form and poured into the apparatus.
Secondly, since the experiment lasted only 20 min, it was necessary to run at least one more trial using tap water using a different (larger) amount of lime.

Hence the next step was to run a second trial using tap water, the same upflow velocity but a larger amount of lime.