ANC CONTROL

Experiment 1: Minimum amount of lime input in two reactors

Introduction

In the first experiment of Spring 2010 team, the goal was to evaluate the lime feeder's performance with respect to its effluent pH with a minimum lime input based on theoretical calculations. This is the mass of lime which would dissolve in 12 hours given the flow rate and the calcium hydroxide solubility constant (equation 1.1). In equation 1.2, the theoretical lime requirement is based on what could dissolve in twelve hours, and didn't take into account other factors which limit how much of the available lime will actually dissolve.

Procedure

The team basically used the same procedure described in the materials and methods section, although the lime is now fed through a vertical tube.

To make the experiment comparable to that carried out by the Fall 2009 team (Experiment 3, Trial 4), the Spring 2010 team used the same flow rate, which was 40mL/min in both reactors, and a lime mass of 8gm based on the solubility calculations (figure 1.1). One important change was that the lime in this experiment was fed dry, not mixed with water as a slurry. This may affect the particle size distribution, which is discussed in the hypotheses section. The following was used to determine the amount of mass necessary for a 12-hour run with a given flow rate.  K _sp is the lime solubility constant, MW is the molecular weight of lime, Q is the volumetric flow rate, and [OH ^- ] in the second equation is the concentration of hydroxide in a saturated lime solution (pH ~12.6).

Equation 1.1:

Equation 1.2:

                                                                           Figure 1. Theoretical lime required as a function of flow rate

Results and Discussion

From the experimental data shown in figure 1.2 we can see that the small reactor (A1) failed to reach saturation and the large reactor (A2) produced saturated effluent only very briefly. In contrast to the experiments run in Fall 2009, the lime in this experiment was added as dry powder instead of mixed with water in the form of a slurry. This likely has an effect on particle size, as the large particles are never broken up if the lime is never stirred with water. A discussion of particle size can be found in the hypotheses section. The results of this run also suggest that the new large reactor performs better than the small one. For this particle size and this amount of lime, the team observed that the flow rate of 40mL/min is too low to get the best suspension; the large reactor, in particular, could have handled a much higher flow rate. However, we believe that the poor results were a result of not having enough lime at small enough particle size to provide the contact time necessary to produce saturated effluent.


                                                                                          Figure 2. Experiment result of 1 trial