Passively Actuated Vibration to Decrease Membrane Fouling Rate for Centrifugal Reverse Osmosis
Abstract
A novel method of shaking a CRO membrane was designed. This method, called the Plug and
Pipe method, was simulated to determine how well it would work in a full CRO system. This
was done by a series of simulations. First, the geometry was meshed and a CFX simulation was
performed. This would be used to find the water velocity at discrete points along the radial direction
of the orifice. These discrete velocities were then put into a curve fitting program, MATLAB’s
cftool which provided a velocity distribution across the orifice. This velocity distribution was
then integrated and to find the flow requirements and water jet force, Fvjet. Fvjet was found to
depend in large part on the orifice size, while other variations, such as Pipe length and diameter,
had little to no effect. Fvjet varied from 27.13 N to 94.64 N, based on orifice size. This water jet
force was then included in a dynamic model, which was created to describe the nonlinear equation
of motion of the shaker. This dynamic model had two parts. The first was the script into which the
characteristics of the system were put, called osc_model, while the second was the function which
held the nonlinear forcing functions for the water jet and wall interactions. The simulated shaker
was designed based on a test size membrane of 28 kg. It was determined that the shaker would
be capable of providing the proper dynamic characteristics based on previous work, which were a
1mm amplitude of vibration at 20 Hz.
Another model was created to show the amount of mass that could be shaken by a full shaker
based on the orifice size and wall stiffness. The orifice diameter was varied from 0.16 in to 0.5 in,
while the wall stiffness varied from 10^5N=m to 10^8N=m. It was found that, as stiffness approached
10^8 N=m, the amount of mass that could be shaken began to plateau, meaning that this was the
value at which the wall began to act rigidly for this system. The flow requirements were provided
for each orifice size as well, and it was determined that any orifice diameter above 0.25 in would
have excessive flow requirements and would limit a CRO rig’s ability to purify water.
An experimental rig was built based on the specifications set out in the simulations as well. A
high pressure gear pump was used to bring the water up to osmotic pressure, about 800 psi. This
high pressure water went into the shaker. It was intended that once the shaker showed vibration,
adjustments would be made to achieve the 1 mm and 20 Hz vibration set by previous work. Upon
final installation, however, there were misalignment problems between the shaft and the linear
bearings, which prevented the shaker from vibrating at all.
Citation
Martinez, Matthew Aaron (2018). Passively Actuated Vibration to Decrease Membrane Fouling Rate for Centrifugal Reverse Osmosis. Master's thesis, Texas A & M University. Available electronically from https : / /hdl .handle .net /1969 .1 /174056.