Evaluation of previous experiments with the new system
Objective
This set of experiments attempts to replicate the grain size research performed with the previous experimental setup. Additionally, the effectiveness of each of the components in the setup will be assessed with respect to theoretical expectations. Theoretically, it is expected that the aerator under 2 atm of pressure will be able to supersaturate the water with 18 ml/L of dissolved gas. With the current aerator, a major assumption made is that the dissolved gas concentration equilibrates with the pressure in the aerator, sending water with 18 ml/L of dissolved gas through the sand filter.
General Procedure
For the two experiments listed below, the same procedure was used with varying sand grain sizes. Sand 40 (0.49 mm - 0.57 mm) and Sand 30 (0.59 mm - 0.84 mm) were used for experiments one and two, respectively.
(Are you referring now to how to run an experiment? Perhaps create this as a link to another page as guidance) In Process Controller, configure the system so that the aerator air pressure is maintained at roughly 100 kPa. Fill the sand column with 60 cm of Sand 40 and adjust the flow rate on the pump forcing water through the sand filter to establish a bed expansion of 50%. For the first experiment, manual measurements of flow rate were performed by unhooking the influent water tube into the sand filter and allowing the influent to fill a large graduated cylinder over the course of a minute. In order to minimize changes made to the system, flow rate measurements for the second experiment were taken at the system effluent tube. The flow rates were roughly 225 ml/min and 485 ml/min for Sand 40 and Sand 30, respectively.
Run the Process Controller method file, given here, on the "On" state. The "On" state regulates the air pressure in the aerator by releasing small amounts of air through a valve when the system exceeds the maximum aerator air pressure of 102 kPa. The water level in the aerator is controlled in a similar manner; however, the water wasting valve is also subject to a duty cycle in which the valve will open for a set period of time and close for a set period of time. If the "on" condition for the wasting valve is not met (that is, if the water level does not exceed the regulated height), the wasting valve will remain closed.
The water entering the aerator and leaving is maintained at a constant rate throughout the experiment via manually controlled pumps. The water is allowed to flow through the sand column, where bubbles can form. When bubbles grow large enough in the filter, they can float up to the top and out through a tube into the bubble collector. Throughout the duration of the experiment, the bubble collector goes through cycles of emptying and refilling. Initially, an air valve at the top of the bubble collector opens and the water effluent valve located at the bottom of the bubble collector closes, allowing the collector to fill like a sitting column of water. Once a maximum height is reached, the air valve shuts off and the water valve opens, resulting in a partial vacuum at the top of the collector. This suspends the column of water in the bubble collector. As bubbles enter the collector, gas in the bubbles fills the partial vacuum, allowing the water column to slowly drain from the collector. Once the minimum water level in the collector is reached, the apparatus refills and the cycle begins again.
For each emptying period of the bubble collector, data is collected through Process Controller. Analysis of the data collected can be quantified as a gas removal rate by considering the cross sectional area of the collector and the flow rate through the system. Please see the results and discussion for each experiment by clicking the links below.
Results and Discussion
Experiments 1 and 2 - Replicates of the Previous Fluidized Bed Experiments
- Experiment 1 was performed shortly after the installation of the new setup. Sand 40 was used with the purpose of replicating previous grain size results and assessing the overall functionality of the setup.
- Experiment 2 was performed after making modifications to the system to account for the issues found in the Experiment 1. Sand 30 was used with the purpose of testing the functionality again and replicating previous results.
- When it was confirmed that the new aerator was not saturating water as much as possible, concentrations of dissolved oxygen were measured at various points in the experimental apparatus.
General Conclusions
The experimental results for gas removal rate using Sand 40 and Sand 30 gave consistent results. This data has been essential in confirming that the apparatus is performing reliably. Yet the results deviate from the theoretical model, which predicts the theoretical bubble formation potential to be around 18mL/L for water that has been previously exposed to 1 atm gage pressure at temperature of 25 C. The model calculates the theoretical bubble formation potential as a function of the air pressure that the water equilibrated with prior to returning to atmospheric pressure. For further analysis, we measured the dissolved oxygen concentration at the effluents from the aerator, sand filter, and bubble collector. Both the model and DO measurements have been instrumental in pointing out several improvements in design of the current apparatus.
Consequently, the following design modifications have been advised to make:
The unexpected results come from measuring dissolved oxygen concentrations at various points in the system. The results indicate that high pressure in the sand filter causing more bubbles to dissolve into the water. Fixing this problem requires some major rethinking of the way the system works, especially in improving aerator efficiency and pressure maintenance in the sand filter. We will replace the large aeration stone in the aerator with four smaller ones to try to improve gas dissolution in the aerator.