ANC CONTROL

Spring 2010 Research

Experiment 5 Overloading the reactors with different up-flow velocities in each one

Overview

The results of the fourth experiment showed that decreasing the up-flow velocity was a good way to increase the efficiency of the reactors, but 0.9 mm/sec might be too low for the 5cm tube settler. In this experiment, all conditions of the fourth experiment remained the same; the only change is the up-flow velocity in the 5 cm tube settler that was increased to 1.8 mm/sec. It was observed that fewer lime particles were reaching the diagonal column in the 5cm tube settler in comparison to the 2.5cm tube settler. The team hypothesized that it was due to the concentration of lime in the middle section of the reactor and the larger apparatus needed a higher velocity to facilitate particles flowing in the slanting tube. The team wants to test if the efficiency of the 5cm tube settler will increase with a higher flow rate and if the clogging into the elbow can be prevented by implementing this measure.

Procedure

100 grams of lime were mixed with tap water and stirred to form a slurry. This was fed through the vertical column. The flow rate was 30ml/min for the 2.5 cm tube settler and 60mL/min for the 5cm tube settler. All the other procedures were the same as described on materials and methods.

Results and Discussion

As the figure shows, the 5 cm tube settler never reached a pH above 12 units. It is difficult to know the reason for this. A lot of lime remains inside the elbow and might cause clogging. It creates preferential paths to lime particles and reduces the available space inside the diagonal column. By increasing up-flow velocity in the 5cm tube settler, it is possible that the contact time of lime in the fluidized bed was reduced and the space between particles increases. If particles don't have enough time to dissolve, they will flow with the effluent without contributing to increase alkalinity.

The 2.5 cm tube settler had a better performance, as seen in Figure 1, staying above a pH of 12 for 14.6 hours and after approximately five more hours, it increased again to above 12 for 2 hours. The fluctuations in the graph near the 30 hour point are due to washing the pH probe and switching pH probes between both reactors to determine if there was a problem with the readings. Readings above 12.6 are observed again, thus it is strongly probable that there is a problem with the pH probes or with the data acquisition system. It is difficult to accurately measure pH when the solution is flowing.

After analyzing the results the team now considers that by increasing up-flow velocity the amount of lime in the reactor can decrease and the performance will get worse. Therefore, the team is now considering the effects of lime particle size. Because the slurry and mixing method is not exact and reproducible, it is possible that all the experiments have been done with different particle size distributions. Big particles remain trapped in the middle part of the elbow and it is difficult to clean the reactors after each experiment. For future experiments a blender or a coffee grinder will be required in order to ensure a homogeneous distribution of particles size. In addition, the team asked for new pH probes and signal conditioning boxes.


                                                                                                                                    Figure 1 - Experiment 5

                                The purpose of this experiment was to keep the same conditions as Experiment 4 but to double the upflow velocity in the 5 cm tube settler apparatus.