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\begin_body

\begin_layout Title
Stock Tank Mixing Team
\end_layout

\begin_layout Author
Aliya Adili, Christine Curtis, Justin Kim, Xuying Wang
\end_layout

\begin_layout Part
Abstract
\end_layout

\begin_layout Standard
The Summer 2011 Stock Tank Mixing Team is tasked with improving the coagulant
 stock tank mixing process currently used in many AguaClara plants.
 Previous teams have determined the major differences in properties of Aluminum
 Sulfate (alum) and Polyaluminum Chloride (PACl) and experimented with the
 ‘double bucket,’ ‘constant upflow,’ ‘simple stirrer (stick),’ and ‘centrifugal
 pump’ designs.
 This team has made several modifications to the existing centrifugal pump
 design and performed mixing rate experiments to compare its efficiency
 to that of the previous model.
 In addition, we have examined the MathCAD program that calculates the energy
 input of the centrifugal pump for errors, begun development on a more efficient
 mixer based on the designs found in a bicycle, and constructed a standard
 curve for a hydrometer using concentrated sugar solution that can be utilized
 to create stirring guidelines for AguaClara plant operators in Honduras.
\end_layout

\begin_layout Part
Introduction
\end_layout

\begin_layout Standard
AguaClara plants rely on a source of coagulant solution for the flocculation
 of solids found in a given source water.
 This solution is typically water and either PACl or alum.
 Most of the AguaClara plants currently use alum coagulant but this coagulant
 is being gradually replaced with PACl.
 Although we used sugar instead of PACl, we believe that our experiments
 accurately represent what would occur with PACl solutions because we are
 analyzing concentration gradients and not the actual dissolving of granular
 particles.
 As PACl dissolves very readily in water, we feel that this is a safe assumption
 to make.
\end_layout

\begin_layout Standard
The current method used in AguaClara plants to create the solution is to
 simply stir the solid coagulant with water until all solid particles seem
 to dissolve.
 However, due to the unreliability and inaccuracy of this method, new ways
 of mixing the solution have been considered.
 It is one of the tasks of the Summer 2011 Stock Tank Mixing team to work
 with one of these considerations, the centrifugal pump, and to find ways
 of modifying it to maximize its efficiency.
 
\end_layout

\begin_layout Part
Centrifugal Mixer
\end_layout

\begin_layout Standard
One of the modifications to the centrifugal mixer (depicted in Figure 1)
 that we considered was the addition of impellers to the vertical axis of
 the pump to create horizontal mixing.
 Although the centrifugal pump is very successful in mixing vertical layers,
 it does very little to mix horizontally.
 Another modification we considered was the addition of baffles to the inner
 wall of the tank to disrupt any solid body rotation that may occur.
 Ultimately, we dedicated most of our time towards improving the seal between
 the rotation rod and the suction rod, allowing for more force to drive
 the concentrated solution from the bottom of the tank up the length of
 the rotation rod.
\end_layout

\begin_layout Standard
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	filename centrifugalpump2.png
	lyxscale 25
	scale 25

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\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Centrifugal pump mixer design.
 
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
We substituted clear 1 inch diameter PVC pipe for solid 0.75 inch diameter
 PVC in the rotation rod for improved visibility and shaved off the end
 of the PVC tube to allow it to freely rotate while also fitting more snugly
 into the suction rod, reducing head loss and thus improving suction.
 To hold the pipe more securely in place as we conducted our experiments,
 we glued an elastic ring to the bottom of the pipe, but we anticipate the
 need for rings made from plastic or other nonreactive material for use
 in the actual plant.
 While the previous design had a 90 degree elbow attached to the end of
 the spout, we were not able to find a 1 inch diameter elbow in the lab.
 In the interest of time, we conducted our experiments without any attachments
 to the end of the spout.
 In addition, future models of the centrifugal mixer should use the longest
 possible spout (close to the diameter of the tank) in order to maximize
 torque during rotation.
\end_layout

\begin_layout Standard
\begin_inset Float figure
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\begin_inset Graphics
	filename centrifugalpumpseal.jpg

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\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
The tighter seal between the suction rod and the rotation rod (left) contributed
 to the improved efficiency of the new pump (right).
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Section
Centrifugal Mixer Design
\end_layout

\begin_layout Standard
Experiments involving the rotation rate of the mixer are conducted in the
 following fashion:
\end_layout

\begin_layout Enumerate
The final desired concentration for the fully homogenized solution is decided
 (~360 g/L).
 
\end_layout

\begin_layout Enumerate
For the desired tank volume, the mass of sugar needed for the desired concentrat
ion is calculated.
 
\end_layout

\begin_layout Enumerate
The volume of a sugar solution at its solubility limit is found that, when
 mixed with a volume of pure water, would result in the creation of a solution
 at the desired concentration and volume.
 
\end_layout

\begin_layout Enumerate
The tank is filled with pure water and the sugar solution at its solubility
 limit is carefully siphoned into the bottom of the tank using a plastic
 funnel and a length of PVC pipe.
 No mixing between the water layer and the sugar concentration is assumed.
\end_layout

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status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename centrifugalpumpexperiment.jpg

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
Improved centrifugal mixer assembled in a clear plastic 20 L tank with concentra
ted sugar solution at the bottom.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Subsection
Height Experiments
\end_layout

\begin_layout Standard
For our first set of experiments, we planned to mix 2 L of a 360 g/L red
 sugar solution into 18 L of pure water at rates of 15, 30, 45, and 60 rpm
 and to measure the height reached by the sugar solution in the clear PVC
 tube.
 These height measurements could then be used to calculate the energy required
 to fully mix the solution at each mixing rate.
 
\end_layout

\begin_layout Standard
We observed that at a rate of 15 rpm, the colored sugar mixture did not
 rise high enough in the tube to record a measurement, but that at rates
 of 30 rpm and 45 rpm, the sugar solution rose 9 cm and 20 cm respectively
 up the height of the tube.
 At 60 rpm, the sugar solution rose the full height of the tube and exited
 the spout.
 We recorded that a mixing time of 78 seconds at 60 rpm was required to
 fully mix the solution.
 However, some problems with our method of experimentation became apparent
 during and after experimentation.
 The heights recorded at 30 and 45 rpm were approximate, because the height
 of the solution rose and fell as significant amount with the natural variation
 in the motion created by our human mixers.
 In addition, the mixer was found to be still somewhat awkward to use, as
 the grip of the mixer was around eye level for Christine and Aliya, making
 it difficult to maintain a constant rate of mixing.
\end_layout

\begin_layout Subsection
Interface Experiments
\end_layout

\begin_layout Standard
Our second set of experiments took into account the problems we encountered
 earlier.
 For these experiments, we stirred the same 2 L of red 360 g/L sugar solution
 into 18 L of pure water at rates of 30, 45, 60, and 75 rpm.
 Justin, being the tallest member of the group, mixed the solution at hopefully
 a more constant and accurate rate.
 We measured the time required for the water-sugar interface to reach the
 bottom of the tank, which is summarized in Table 1 below at each of the
 different rates and depicted graphically in Figure 4.
\end_layout

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Rotating speed (rpm)
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Time (s), old pump
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Time (s), new pump
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\begin_inset Caption

\begin_layout Plain Layout
Comparison of average mixing times for old and new pumps.
 X: solution did not reach spout.
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Curve of mixing time as a function of rotating speed using data recorded
 in Table 1.
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\end_layout

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\begin_layout Standard
From this experiment, we concluded that feasible mixing rates for our system
 ranged from about 45-75 revolutions per minute with mixing times ranging
 from about 140-10 seconds for the improved mixer.
 We determined that a rotating speed of roughly 45 rpm represents a threshold
 rate for the mixer to function correctly; at this rate, the sugar solution
 just barely reaches the height of the spout, only exiting the spout some
 of the time.
 Therefore, the mixing times obtained for each mixer at 45 rpm are not reliable
 data points and our conclusion that the new mixer is a more efficient model
 due to its improved seal is still valid.
 
\end_layout

\begin_layout Standard
We then calculated flow rate through the new pump for each mixing rate while
 assuming that total flow volume in the centrifugal pump was 20L for the
 whole mixing process, and we compared our calculated flow rate with the
 flow rate predicted according to the prediction file developed by the previous
 research team as shown in Table 2 below: 
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Time (s)
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Calculated flow rate (L/s)
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Predicted flow rate (L/s)
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2.8
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\begin_inset Caption

\begin_layout Plain Layout
Flow rate as a function of mixing rate for the new mixer.
 X: solution did not reach spout.
\end_layout

\end_inset


\end_layout

\end_inset


\end_layout

\begin_layout Standard
The disparities seen in our calculated flow rates and the flow rates calculated
 using previous teams' equations led our team to take a closer look at the
 files provided by previous teams.
 To use MathCAD to calculate and evaluate the centrifugal mixing systems,
 previous teams created the ‘worst case scenario’ for a given target concentrati
on.
 Simply put, the worst case scenario is the case that would require the
 most energy input to achieve full homogenization.
 However, the program assumes that the dimensions of the mixer are ideal
 (for example, it assumes that the length of the spout rod is equal to the
 diameter of the tank in order to maximize torque) and neglects to account
 for some significant sources of head loss: the seal between the suction
 rod and the rotation rod and the larger dimensions of the clear PVC pipe.
 For these reasons, our modifications and, we anticipate, future improvements
 to the mixer may not be reflected in the MathCAD program as written.
\end_layout

\begin_layout Part
Bicycle Mixer
\end_layout

\begin_layout Standard
One of the models we conceived for an improved water mixing pump implements
 the designs found in a bicycle.
 Since an important component of AguaClara's design philosophy is that plants
 must operate without the use of electricity, mechanical energy transferred
 by pedaling is an excellent and efficient source of energy; legs provide
 more power and are easier for operation than hands.
 Power (kW) can be calculated via the formula
\end_layout

\begin_layout Standard
\begin_inset Formula 
\begin{equation}
P=\text{τ × rpm × (2π radians)/(60 sec × 1000 W)}
\end{equation}

\end_inset

where rpm is the mixing rate in the tank in revolutions per minute and 
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τ is torque in Newton-meters.
\end_layout

\begin_layout Standard
Our first design used the rotation created by the pedaling to operate a
 system of sprockets, similar to a pulley system.
 Attached to the axis of the submerged sprocket would be impellers to provide
 both horizontal and vertical mixing, and rotation of the axis of the sprocket
 about the vertical axis would serve to further maximize the variety of
 mixing.
 
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename bikemixer1.jpg
	lyxscale 80
	scale 80

\end_inset


\begin_inset Caption

\begin_layout Plain Layout
Preliminary design for a bicycle impeller mixer (as viewed from above).
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
Under this design, the bicycle mixer would need to be built on a platform
 above the surface of the tank.
 This platform would need to be large enough and sturdy enough to ensure
 operator safety and ease of access.
 In addition, the shape of the tank opening would need to be taken into
 account; a bottle-like opening as opposed to an wide-mouthed opening spanning
 the diameter of the tank would require that the impeller part of the mixer
 remain in the tank.
 This would make installation and maintenance difficult and limit the materials
 used to construct the mixer to plastic or a nonreactive metal such as aluminum.
 Finding a bicycle chain made of these materials would also be a challenge.
 In light of the complications associated with this design, we developed
 another version of the bicycle mixer shown in Figure 6.
 The mixer would be located at ground level and make use of a pulley system
 attached to the wall of the plant building.
 Our design was influenced by the models for the Bicycle Rope Water Pump
 developed by the Guatemalan-based organization Maya Pedal, which builds
 a variety of 
\begin_inset Quotes eld
\end_inset

bicimáquinas
\begin_inset Quotes erd
\end_inset

, or pedal-powered machines, one of which is shown in Figure 7 below.
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename bikemixer2.jpg
	lyxscale 87
	scale 87

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Improved bicycle impeller mixer design (side view).
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename BiciBomba.jpg
	lyxscale 50

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
MayaPedal bicycle rope water pump.
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
Materials needed for this design include either a rope or cog/timing belt,
 6 wheels (8cm in diameter) made of a nonreactive material, axial impellers,
 PVC pipe (or steel for use in the real plant) to construct the pulley system
 and for bike supports, and large right-angled steel supports for mounting
 the pulleys on the wall.
 The dimensions of the tank will be needed to calculate the size of impellers.
 Modification of this design will most likely be necessary and many design
 details will need to be decided upon prior to testing, as drilling into
 the wall of the lab in order to attach the pulley system is not feasible.
\end_layout

\begin_layout Part
Hydrometer
\end_layout

\begin_layout Standard
To address the goal of creating stirring guidelines for AguaClara plant
 operators in Honduras, we developed a concentration profile using a hydrometer
 to measure the specific gravity of a sugar solution, as shown in Figure
 8 below:
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename concentrationVdensity.jpg

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Standard curve of hydrometer.
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Standard
The high coefficient of determination indicates the reliability of our data.
 In addition, we used the hydrometer to measure the relative density of
 the mixed solution directly after each trial for the interface experiments
 (see Figure 9).
 We found the measured relative density of the solution after mixing to
 be 1.012, very close to theoretical value of 1.0114 calculated from standard
 curve (assuming that the solution was fully mixed after we stopped timing).
 This indicates that the hydrometer can be used to tell the mixing condition
 of water and sugar.
 
\end_layout

\begin_layout Standard
\begin_inset Float figure
wide false
sideways false
status open

\begin_layout Plain Layout
\begin_inset Graphics
	filename hydrometer.jpg

\end_inset


\end_layout

\begin_layout Plain Layout
\begin_inset Caption

\begin_layout Plain Layout
Obtaining measurements with the hydrometer.
\end_layout

\end_inset


\end_layout

\begin_layout Plain Layout

\end_layout

\end_inset


\end_layout

\begin_layout Part
Future Work
\end_layout

\begin_layout Enumerate
Further improve the centrifugal pump mixing system by adding baffles or
 impellers to improve mixing efficiency.
 Determine if the system can be scaled up for the 55 gallon drum.
\end_layout

\begin_layout Enumerate
Modify the existing MathCAD file or develop a new file to more accurately
 reflect impacts of head loss on the energy input required to achieve complete
 mixing of the system.
\end_layout

\begin_layout Enumerate
Develop specific guidelines for stock tank mixing operators using the sugar
 concentration profile.
 These guidelines may specify a required mixing time or a final solution
 concentration to be achieved.
\end_layout

\begin_layout Enumerate
Research the possibility of constructing a less expensive, more durable
 hydrometer.
 This homemade hydrometer should be properly calibrated and marked with
 the desired final concentration of the mixed solution.
 Ideally, it should be easier to read than the hydrometer supplied by the
 AguaClara lab that we used in our experiments, on which the difference
 in specific gravity between fully separated and fully mixed solutions is
 represented by a distance of only about 3 mm.
 An instrument with larger demarcations would yield more accurate results
 with less effort on behalf of the operator.
\end_layout

\begin_layout Enumerate
Further develop the bicycle impeller mixer.
\end_layout

\begin_layout Bibliography
\labelwidthstring References
\begin_inset CommandInset bibitem
LatexCommand bibitem
key "key-1"

\end_inset


\begin_inset Quotes eld
\end_inset

Bicycle Machines.
\begin_inset Quotes erd
\end_inset

 MayaPedal Guatemala.
 2010.
 <http://www.mayapedal.org/machines.html>.
\end_layout

\end_body
\end_document
