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The first trial on the sloped-column lime feeder consisted in of using tap water. The flow rate for this experiment ( 120mL/min ) which was calculated based on the an upflow velocity (of 4.42mm/s) , as found for 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:
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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. (Where is your process control file? How did you regulate flow rate in process control? You should have a separate materials and methods section)The Process Controller files can be found in the Materials and Methodssection.
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) of the 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 did not fall out with the effluent water but remained inside the columnthat would have been flushed out had it not been there. This was a desirable behavior because the 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
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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 out the clogging clog especially since the apparatus was extremely long and difficult to deal withunusually 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 for only 20 min, it was necessary to run at least one more trial using tap water using a different (larger) amount of lime.
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