Note: Descriptions are shown in the official language in which they were submitted.
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A REFLECTOR FOR AN ACOUSTOPHORETIC DEVICE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims the benefit of U.S. Provisional
Patent
Application Serial No. 61/975,035, filed April 4, 2014, which is incorporated
herein by
reference in its entirety. This application is also a continuation-in-part of
U.S. Patent
Application Serial No. 14/026,413, filed on September 13, 2013, which claims
the
benefit of U.S. Provisional Patent Application Serial No. 61/708,641, filed on
October 2,
2012. U.S. Patent Application Serial No. 14/026,413 is also a continuation-in-
part of
U.S. Serial No. 13/844,754, filed March 15, 2013, which claimed the benefit of
U.S.
Provisional Patent Application Serial No. 61/611,159, filed March 15, 2012,
and of U.S.
Provisional Patent Application Serial No. 61/611,240, also filed March 15,
2012, and of
U.S. Provisional Patent Application Serial No. 61/754,792, filed January 21,
2013.
These applications are incorporated herein by reference in their entireties.
BACKGROUND
[0002] The ability to separate a particle/fluid mixture into its separate
components is
desirable in many applications. Acoustophoresis is the separation of particles
using
high intensity sound waves, and without the use of membranes or physical size
exclusion filters. It has been known that high intensity standing waves of
sound can
exert forces on particles in a fluid when there is a differential in both
density and/or
compressibility, otherwise known as the acoustic contrast factor. The pressure
profile in
a standing wave contains areas of local minimum pressure amplitudes at its
nodes and
local maxima at its anti-nodes. Depending on the density and compressibility
of the
particles, they will be trapped at the nodes or anti-nodes of the standing
wave. The
higher the frequency of the standing wave, the smaller the particles that can
be trapped
due the pressure of the standing wave.
[0003] Growth in the field of biotechnology has been due to many factors,
some of
which include the improvements in the equipment available for bioreactors.
Improvements in equipment have allowed for larger volumes and lower cost for
the
production of biologically derived materials such as monoclonal antibodies and
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recombinant proteins. One of the key components used in the manufacturing
processes of new biologically based pharmaceuticals is the bioreactor and the
ancillary
processes associated therewith.
[0004] A modern bioreactor is a very complicated piece of equipment. It
provides for,
among other parameters, the regulation of fluid flow rates, gas content,
temperature, pH
and oxygen content. All of these parameters can be tuned to allow the cell
culture to be
as efficient as possible of producing the desired biomolecules from the
bioreactor
process. One process for using a bioreactor is the perfusion process. The
perfusion
process is distinguished from the batch and fed-batch processes by its lower
capital
cost and higher throughput.
[0005] In the fed-batch process, a culture is seeded in a bioreactor. The
gradual
addition of a fresh volume of selected nutrients during the growth cycle is
used to
improve productivity and growth. The product, typically a monoclonal antibody
or a
recombinant protein, is recovered after the culture is harvested. Separating
the cells,
cell debris and other waste products from the desired product is currently
performed
using various types of filters for separation. Such filters are expensive and
become
clogged and non-functional as the bioreactor material is processed. A fed-
batch
bioreactor also has high start-up costs, and generally requires a large volume
to obtain
a cost-effective amount of product at the end of the growth cycle, and such
processes
include large amounts of non-productive downtime.
[0006] A perfusion bioreactor processes a continuous supply of fresh media
that is
fed into the bioreactor while growth-inhibiting byproducts are constantly
removed. The
nonproductive downtime can be reduced or eliminated with a perfusion
bioreactor
process. The cell densities achieved in perfusion culture (30-100 million
cells/mL) are
typically higher than for fed-batch modes (5-25 million cells/mL). However, a
perfusion
bioreactor requires a cell retention device to prevent escape of the culture
when
byproducts are being removed. These cell retention systems add a level of
complexity
to the perfusion process, requiring management, control, and maintenance for
successful operation. Operational issues such as malfunction or failure of the
cell
retention equipment has previously been a problem with perfusion bioreactors.
This has
limited their attractiveness in the past.
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[0007]
It would be desirable to provide means that can reduce the cost and effort of
using bioreactors and separating the desired products from the cells that make
them.
BRIEF DESCRIPTION
[0008]
The present disclosure relates, in various embodiments, to systems for
producing biomolecules such as recombinant proteins or monoclonal antibodies,
and to
processes for separating these desirable products from a cell culture in a
disposable or
non-disposable bioreactor system.
Generally, the bioreactor includes an
acoustophoretic device for producing multi-dimensional acoustic standing
waves, which
is located near an outlet port for the bioreactor. Such standing waves are
produced by
an ultrasonic transducer and a reflector. In the present disclosure, the
reflector is
formed from a thin material that is essentially acoustically transparent, such
as certain
plastic films, rather than a solid metal. The thin material provides a
constant pressure
boundary, also known as a free surface. In essence, these embodiments are
examples
of providing a pressure release surface, such as from a transparent layer of a
plastic
film.
[0009]
Disclosed in various embodiments are apparatuses that include a flow
chamber having at least one inlet and at least one outlet. At least one
ultrasonic
transducer is located on a wall of the flow chamber. The transducer includes a
piezoelectric material driven by a voltage signal to create a multi-
dimensional acoustic
standing wave in the flow chamber. A thin structure is located on the wall on
the
opposite side of the flow chamber from the at least one ultrasonic transducer.
The thin
structure provides a pressure release boundary that acts as a reflector.
[0010]
In particular embodiments, the thin structure is a plastic film. The plastic
film
can be made from a material selected from the group consisting of olefins,
polyurethanes, polyureas, polyesters, polystyrenes, polyamides, cellulosics,
ionomers,
polyvinyl chloride, polyvinyl butyral, polyvinylidene fluoride, polyvinylidene
chloride,
ethylene vinyl acetate, ethylene tetrafluoroethylene, polytetrafluoroethylene,
and
combinations thereof. More specifically, the plastic film can be a
polypropylene.
[0011]
The thin structure can be optically transparent. The thin structure may be
substantially flat. The thin structure may have a thickness that is 1/2 or
less of the
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wavelength relative to the frequency emitted by the at least one ultrasonic
transducer.
Generally, this thickness is in the range of 10 microns to 1 millimeter (mm).
[0012] The transducer may have a housing containing the piezoelectric
material.
The piezoelectric material may be air backed, i.e. does not have a backing
layer. The
piezoelectric material may be a ceramic crystal.
[0013] In other embodiments, the piezoelectric material is backed by a
substantially
acoustically transparent material. The substantially acoustically transparent
material
may be balsa wood, cork, or a foam. The substantially acoustically transparent
material
can have a thickness of up to one inch. The substantially acoustically
transparent
material may be in the form of a lattice.
[0014] In some embodiments, the ultrasonic transducer may have a face that
contacts fluid within the flow chamber, the face being coated with a wear
layer
comprising chrome, electrolytic nickel, electroless nickel, p-xylylene, glassy
carbon, or
urethane.
[0015] The apparatus may further include an apparatus inlet that leads to
an annular
plenum, a contoured nozzle wall downstream of the apparatus inlet, a
collection duct
surrounded by the annular plenum, and a connecting duct joining the contoured
nozzle
wall to the flow chamber inlet.
[0016] The device can comprise a plurality of transducers that span the
width of the
flow chamber.
[0017] Also disclosed in various embodiments are methods of separating a
second
fluid or a particulate from a host fluid, comprising: flowing a mixture of the
host fluid and
the second fluid or particulate through an apparatus, the apparatus
comprising: a flow
chamber having at least one inlet and at least one outlet; at least one
ultrasonic
transducer located on a wall of the flow chamber, the transducer including a
piezoelectric material driven by a voltage signal to create a multi-
dimensional acoustic
standing wave in the flow chamber; and a thin structure located on the wall on
the
opposite side of the flow chamber from the at least one ultrasonic transducer,
the thin
structure providing a pressure release boundary that acts as a reflector; and
capturing
smaller particles of the second fluid or particulate in the multi-dimensional
acoustic
standing wave to separate the second fluid or particulate from the host fluid.
The
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secondary fluid or particles cluster or coalesce at specific points such that
gravity
separation eventually and continuously occurs. In other words, once the
clustering,
coalescing or clumping occurs, continuous gravity separation happens. A pulsed
voltage signal drives the at least one ultrasonic transducer.
[0018] The particulate may be Chinese hamster ovary (CHO) cells, NSO
hybridoma
cells, baby hamster kidney (BHK) cells, insect cells or human cells such as
stem cells
and T-cells. The mixture may be continuously flowed through the flow chamber.
The
standing wave may have an axial force and a lateral force, the lateral force
being at
least the same order of magnitude as the axial force.
[0019] Also disclosed in various embodiments are apparatuses that include a
flow
chamber having at least one inlet and at least one outlet. At least one
ultrasonic
transducer is located on a wall of the flow chamber. The transducer includes a
piezoelectric material driven by a voltage signal to create a multi-
dimensional acoustic
standing wave in the flow chamber. A thin structure is located on the wall on
the
opposite side of the flow chamber from the at least one ultrasonic transducer.
The thin
structure provides a pressure release boundary that acts as a reflector. The
apparatus
has an acoustic reflection coefficient of -0.1 to -1Ø
[0020] These and other non-limiting characteristics are more particularly
described
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] The following is a brief description of the drawings, which are
presented for
the purposes of illustrating the exemplary embodiments disclosed herein and
not for the
purposes of limiting the same.
[0022] Figure 1 is a schematic plan view of a flow chamber, illustrating
the thin
structure / reflector of the present disclosure.
[0023] Figure 2 is a schematic showing how the acoustic reflection
coefficient is
calculated for the device of Figure 1.
[0024] Figure 3A is a picture of an acoustophoretic separator having one
ultrasonic
transducer and a transparent thin plastic film acting as the reflector.
[0025] Figure 3B is a picture showing the thin plastic film reflector.
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[0026] Figure 4 is a cross-sectional view of an acoustophoretic separator
in which
the reflector of the present disclosure can be used.
[0027] Figure 5 is a cross-sectional diagram of a conventional ultrasonic
transducer.
[0028] Figure 6 is a cross-sectional diagram of an ultrasonic transducer of
the
present disclosure. An air gap is present within the transducer, and no
backing layer or
wear plate is present.
[0029] Figure 7 is a cross-sectional diagram of an ultrasonic transducer of
the
present disclosure. An air gap is present within the transducer, and a backing
layer and
wear plate are present.
[0030] Figure 8 is a graph of electrical impedance amplitude versus
frequency for a
square transducer driven at different frequencies.
[0031] Figure 9 illustrates the trapping line configurations for seven of
the peak
amplitudes of Figure 8 from the direction orthogonal to fluid flow.
[0032] Figure 10 is a graph showing the relationship of the acoustic
radiation force,
buoyancy force, and Stokes' drag force to particle size. The horizontal axis
is in microns
(pm) and the vertical axis is in Newtons (N).
[0033] Figure 11 is a picture of a test ultrasonic transducer having an
acoustically
transparent film cover.
DETAILED DESCRIPTION
[0034] The present disclosure may be understood more readily by reference
to the
following detailed description of desired embodiments and the examples
included
therein. In the following specification and the claims which follow, reference
will be
made to a number of terms which shall be defined to have the following
meanings.
[0035] Although specific terms are used in the following description for
the sake of
clarity, these terms are intended to refer only to the particular structure of
the
embodiments selected for illustration in the drawings, and are not intended to
define or
limit the scope of the disclosure. In the drawings and the following
description below, it
is to be understood that like numeric designations refer to components of like
function.
[0036] The singular forms "a," "an," and "the" include plural referents
unless the
context clearly dictates otherwise.
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[0037] The term "comprising" is used herein as requiring the presence of
the named
components/steps and allowing the presence of other components/steps. The term
"comprising" should be construed to include the term "consisting of", which
allows the
presence of only the named components/steps, along with any impurities that
might
result from the manufacture of the named components/steps.
[0038] Numerical values should be understood to include numerical values
which are
the same when reduced to the same number of significant figures and numerical
values
which differ from the stated value by less than the experimental error of
conventional
measurement technique of the type described in the present application to
determine the
value.
[0039] All ranges disclosed herein are inclusive of the recited endpoint
and
independently combinable (for example, the range of "from 2 grams to 10 grams"
is
inclusive of the endpoints, 2 grams and 10 grams, and all the intermediate
values).
[0040] The terms "substantially" and "about" can be used to include any
numerical
value that can vary without changing the basic function of that value. When
used with a
range, "substantially" and "about" also disclose the range defined by the
absolute values
of the two endpoints, e.g. "about 2 to about 4" also discloses the range "from
2 to 4."
The terms "substantially" and "about" may refer to plus or minus 10% of the
indicated
number.
[0041] It should be noted that many of the terms used herein are relative
terms. For
example, the terms "upper" and "lower" are relative to each other in location,
i.e. an
upper component is located at a higher elevation than a lower component in a
given
orientation, but these terms can change if the device is flipped. The terms
"inlet" and
"outlet" are relative to a fluid flowing through them with respect to a given
structure, e.g.
a fluid flows through the inlet into the structure and flows through the
outlet out of the
structure. The terms "upstream" and "downstream" are relative to the direction
in which
a fluid flows through various components, i.e. the flow fluids through an
upstream
component prior to flowing through the downstream component. It should be
noted that
in a loop, a first component can be described as being both upstream of and
downstream of a second component.
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[0042] The terms "horizontal" and "vertical" are used to indicate direction
relative to
an absolute reference, i.e. ground level. However, these terms should not be
construed
to require structures to be absolutely parallel or absolutely perpendicular to
each other.
For example, a first vertical structure and a second vertical structure are
not necessarily
parallel to each other. The terms "upwards" and "downwards" are also relative
to an
absolute reference; an upwards flow is always against the gravity of the
earth.
[0043] The present application refers to "the same order of magnitude." Two
numbers are of the same order of magnitude if the quotient of the larger
number divided
by the smaller number is a value less than 10.
[0044] The acoustophoretic separation technology of the present disclosure
employs
ultrasonic acoustic standing waves to trap, i.e., hold stationary, particles
or a secondary
fluid in a host fluid stream. The particles or secondary fluid collect at the
nodes or anti-
nodes of the multi-dimensional acoustic standing wave, depending on the
particles' or
secondary fluid's acoustic contrast factor relative to the host fluid, forming
clusters that
eventually fall out of the multi-dimensional acoustic standing wave when the
clusters
have grown to a size large enough to overcome the holding force of the multi-
dimensional acoustic standing wave (e.g. by coalescence or agglomeration).
This is an
important distinction from previous approaches where particle trajectories
were merely
altered by the effect of the acoustic radiation force. The scattering of the
acoustic field
off the particles results in a three dimensional acoustic radiation force,
which acts as a
three-dimensional trapping field. The acoustic radiation force is proportional
to the
particle volume (e.g. the cube of the radius) when the particle is small
relative to the
wavelength. It is proportional to frequency and the acoustic contrast factor.
It also
scales with acoustic energy (e.g. the square of the acoustic pressure
amplitude). For
harmonic excitation, the sinusoidal spatial variation of the force is what
drives the
particles to the stable axial positions within the standing waves. When the
acoustic
radiation force exerted on the particles is stronger than the combined effect
of fluid drag
force and buoyancy and gravitational force, the particle is trapped within the
acoustic
standing wave field. This results in concentration, agglomeration and/or
coalescence of
the trapped particles. The strong lateral forces create rapid clustering of
particles.
Relatively large solids of one material can thus be separated from smaller
particles of a
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different material, the same material, and/or the host fluid through enhanced
gravitational separation.
[0045] One specific application for the acoustophoresis device is in the
processing of
bioreactor materials. It is important to be able to filter all of the cells
and cell debris from
the expressed materials that are in the fluid stream. The expressed materials
are
composed of biomolecules such as recombinant proteins or monoclonal
antibodies, and
are the desired product to be recovered. Through the use of acoustophoresis,
the
separation of the cells and cell debris is very efficient and leads to very
little loss of the
expressed materials. This is an improvement over current filtration processes
(depth
filtration, tangential flow filtration, and the like), which show limited
efficiencies at high
cell densities, so that the loss of the expressed materials in the filter beds
themselves
can be up to 5% of the materials produced by the bioreactor. The use of
mammalian
cell cultures including Chinese hamster ovary (CHO), NSO hybridoma cells, baby
hamster kidney (BHK) cells, and human cells has proven to be a very
efficacious way of
producing/expressing the recombinant proteins and monoclonal antibodies
required of
today's pharmaceuticals. The filtration of the mammalian cells and the
mammalian cell
debris through acoustophoresis aids in greatly increasing the yield of the
bioreactor.
The acoustophoresis process, through the use of multidimensional acoustic
waves, may
also be coupled with a standard filtration process upstream or downstream,
such as
depth filtration using diatomaceous earth, tangential flow filtration (TFF),
or other
physical filtration processes.
[0046] In this regard, the contrast factor is the difference between the
compressibility
and density of the particles and the fluid itself. These properties are
characteristic of the
particles and the fluid themselves. Most cell types present a higher density
and lower
compressibility than the medium in which they are suspended, so that the
acoustic
contrast factor between the cells and the medium has a positive value. As a
result, the
axial acoustic radiation force (ARF) drives the cells, with a positive
contrast factor, to the
pressure nodal planes, whereas cells or other particles with a negative
contrast factor
are driven to the pressure anti-nodal planes. The radial or lateral component
of the
acoustic radiation force trap the cells. The radial or lateral component of
the ARF is
larger than the combined effect of fluid drag force and gravitational force.
The radial or
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lateral component drives the cells/particles to planes where they can cluster
into larger
groups, which will then gravity separate from the fluid.
[0047] As the cells agglomerate at the nodes of the standing wave, there is
also a
physical scrubbing of the cell culture media that occurs whereby more cells
are trapped
as they come in contact with the cells that are already held within the
standing wave.
This generally separates the cells from the cell culture media. The expressed
biomolecules remain in the nutrient fluid stream (i.e. cell culture medium).
[0048] Desirably, the ultrasonic transducer(s) generate a three-dimensional
or multi-
dimensional acoustic standing wave in the fluid that exerts a lateral force on
the
suspended particles to accompany the axial force so as to increase the
particle trapping
and clumping capabilities of the standing wave. Typical results published in
literature
state that the lateral force is two orders of magnitude smaller than the axial
force. In
contrast, the technology disclosed in this application provides for a lateral
force to be
higher, up to the same order of magnitude as the axial force.
[0049] For three-dimensional acoustic fields, Gor'kov's formulation can be
used to
calculate the acoustic radiation force Fac applicable to any sound field. The
primary
acoustic radiation force Fac is defined as a function of a field potential U,
FA= ¨V(U),
where the field potential U is defined as
U = Vo ¨
(P2) i, 3pf(u2) f ¨
J 2
2pf C f2 ji 4
- ,
and f1 and f2 are the monopole and dipole contributions defined by
1 f2 = 2(A ¨ 1)
f1 =1
Ao-2 ' 2A + 1 '
where p is the acoustic pressure, u is the fluid particle velocity, A is the
ratio of cell
density pp to fluid density pf, a is the ratio of cell sound speed cp to fluid
sound speed cf,
V0 is the volume of the cell, and < > indicates time averaging over the period
of the
wave. Gor'kov's formulation applies to particles smaller than the wavelength.
For larger
particle sizes, Ilinskii provides equations for calculating the 3D acoustic
radiation forces
for any particle size. See Ilinskii, Acoustic Radiation Force on a Sphere in
Tissue, The
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Journal of the Acoustical Society of America, 132, 3, 1954 (2012), which is
incorporated
herein by reference.
[0050] Perturbation of the piezoelectric crystal in an ultrasonic
transducer in a
multimode fashion allows for generation of a multidimensional acoustic
standing wave.
A piezoelectric crystal can be specifically designed to deform in a multimode
fashion at
designed frequencies, allowing for generation of a multi-dimensional acoustic
standing
wave. The multi-dimensional acoustic standing wave may be generated by
distinct
modes of the piezoelectric crystal such as the 3x3 mode that would generate
multidimensional acoustic standing waves. A multitude of multidimensional
acoustic
standing waves may also be generated by allowing the piezoelectric crystal to
vibrate
through many different mode shapes. Thus, the crystal would excite multiple
modes
such as a Ox0 mode (i.e. a piston mode) to a 1x1, 2x2, 1x3, 3x1, 3x3, and
other higher
order modes and then cycle back through the lower modes of the crystal (not
necessarily in straight order). This switching or dithering of the crystal
between modes
allows for various multidimensional wave shapes, along with a single piston
mode
shape to be generated over a designated time.
[0051] It is also possible to drive multiple ultrasonic transducers with
arbitrary
phasing. In other words, the multiple transducers may work to separate
materials in a
fluid stream while being out of phase with each other. Alternatively, a single
ultrasonic
transducer that has been divided into an ordered array may also be operated
such that
some components of the array will be out of phase with other components of the
array.
[0052] It may be necessary, at times, due to acoustic streaming, to
modulate the
frequency or voltage amplitude of the standing wave. This may be done by
amplitude
modulation and/or by frequency modulation. The duty cycle of the propagation
of the
standing wave may also be utilized to achieve certain results for trapping of
materials. In
other words, the acoustic beam may be turned on and shut off at different
frequencies
to achieve desired results.
[0053] The lateral force of the total acoustic radiation force (ARF)
generated by the
ultrasonic transducers of the present disclosure is significant and is
sufficient to
overcome the fluid drag force at high linear velocities up to 1 cm/s and
beyond. For
example, linear velocities through the devices of the present disclosure can
be a
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minimum of 4 cm/min for separation of cells/particles, and can be as high as 1
cm/sec
for separation of oil/water phases. Flow rates can be a minimum of 25 mL/min,
and can
range as high as 40 mL/min to 1000 mL/min, or even higher. This is true for
batch
reactors, fed-batch bioreactors and perfusion bioreactors.
[0054] The present disclosure relates to acoustophoretic devices and
structures
which can make such devices more economical and also provide the opportunity
to
enhance the range of applications in which they can be used. In this regard,
Figure 1 is
a plan (top) view of a flow chamber 128. An ultrasonic transducer 130 is
present on one
wall of the flow chamber, and a reflector 132 is present on the wall opposite
the
transducer. Fluid flow is in/out of the plane of the figure.
[0055] Reflectors are typically made from a solid material, such as a steel
or
aluminum plate. While a metal plate provides good reflection, it also adds
weight to the
flow chamber 128. In the present disclosure, the reflector 132 is a thin
structure that
can provide a pressure release boundary. A pressure release boundary occurs
when
the acoustic pressure is zero at the interface.
[0056] As illustrated here in Figure 1, the thin structure 132 has a
substantially flat
profile relative to the chamber 128. The thin structure separates the fluid
138 inside the
flow chamber 128 from the medium (typically air) 139 on the exterior of the
flow
chamber 128. In operation, the ultrasonic propagating wave 134 (illustrated as
dotted
lines) is generated by the ultrasonic transducer 130 will reflect off the
boundary 137
created at the reflector/air interface. In other words, the wavelength of the
standing
wave will pass through the material of the reflector, and then reflect off the
boundary
137. Thus, the thin structure 132 should be made from an acoustically
transparent
material, i.e. will not impede the ultrasonic wave or have very low impedance.
It is
noted that the acoustic wave actually reflects off the air, i.e. at the
interface of the thin
structure and the air. For purposes of this disclosure, the term "reflector"
can be used to
refer to the structural component that separates the interior of the flow
chamber from
the exterior of the flow chamber and provides the interface with the air.
However, for
example, in particular embodiments, the transducer may be vertically oriented
with the
multi-dimensional acoustic standing wave propagating upwards into the fluid
from the
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transducer. In this case, the fluid¨air boundary will be the free surface
providing a
pressure release boundary, with no other physical structure necessary.
[0057] In specific embodiments, the thin structure has a thickness that is
1/2 or less
of the wavelength of the ultrasonic transducer that it is being used with, and
in more
particular embodiments is at most 1/20 or at most 1/50 of the wavelength.
Generally,
this means the thin structure has a thickness of 10 microns to 1 millimeter.
[0058] In specific embodiments, the thin structure that provides the
pressure release
boundary is an acoustically transparent film, such as a plastic film. The
plastic film is
typically stretched within a frame. The plastic film can be transparent,
thereby allowing
visualization of the interior of the flow chamber 128. The plastic film can be
made of a
material selected from the group consisting of olefins, polyurethanes,
polyureas,
polyesters, polystyrenes, polyamides, cellulosics, ionomers, polyvinyl
chloride, polyvinyl
butyral, polyvinylidene fluoride, polyvinylidene chloride, ethylene vinyl
acetate, ethylene
tetrafluoroethylene, polytetrafluoroethylene, and combinations thereof.
[0059] Figure 2 is a schematic explaining the operation of the thin
structure that
provides the pressure release boundary. The flow chamber 128 is depicted, as
is the
transducer 130 and the thin structure 132. During operation, the flow chamber
is filled
with a fluid, typically water, that has an acoustic impedance Z1, which is the
product of
the density of the fluid and the speed of sound in the fluid. When the thin
structure is
very thin, its acoustic impedance can be ignored. The medium 139 outside of
the flow
chamber (typically air) also has an acoustic impedance Z2. As illustrated on
the right-
hand side, the fluid inside the chamber and the medium outside the chamber
result in a
system having an acoustic reflection coefficient R that is determined
according to the
formula:
Z2 ¨ Z1
R= ________________________________________
Z2 + Z1
[0060] The acoustic impedance is measured in Rayls (1 Rayl = 1 kg/m2/sec).
As an
example of the efficacy of the thin structure, the acoustic impedance of air
at 0 C is 428
Rayls, and the acoustic impedance of fresh water is 1.48 million Rayls. Thus,
the
system would have an acoustic reflection coefficient of -0.999. This indicates
that most
of the acoustic energy will be reflected with a 180 degree phase change.
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[0061] Pictures showing an acoustophoretic particle separator 100 using an
acoustically transparent film as a reflector are shown in Figure 3A and Figure
3B.
Referring first to Figure 3A, a multi-component liquid stream (e.g. water or
other fluid)
enters the inlet 104 and separated fluid exits at the opposite end via outlet
106. It should
be noted that this liquid stream is usually under pressure when flowing
through the
separator. The particle separator 100 has a longitudinal flow channel 108 that
carries
the multi-component liquid stream past an ultrasonic transducer 112 and the
acoustically transparent film 114, which is located on the wall opposite the
transducer.
As seen here, a thin plastic film was used as the interface between the air
and the fluid
within the flow chamber. Figure 3B is a picture of the plastic film during
operation of
the device.
[0062]
Figure 4 is a cross-sectional view of an acoustophoretic separation apparatus
in which the thin structure reflector of the present disclosure (e.g. a thin
plastic film) can
be used. This is a figure of a 4" by 2.5" flow cross sectional area
intermediate scale
apparatus 124 for separating a host fluid from a buoyant fluid or particulate.
The
acoustic path length is 4". The apparatus is shown here in an orientation
where the flow
direction is downwards, which is used for separating less-dense particles from
the host
fluid. However, the apparatus may be essentially turned upside down to allow
separation of particles which are heavier than the host fluid. Instead of a
buoyant force
in an upward direction, the weight of the agglomerated particles due to
gravity pulls
them downward. It should be noted that this embodiment is depicted as having
an
orientation in which fluid flows vertically. However, it is also contemplated
that fluid flow
may be in a horizontal direction, or at an angle.
[0063]
A particle-containing fluid enters the apparatus through inlets 126 into an
annular plenum 131. The annular plenum has an annular inner diameter and an
annular outer diameter.
Two inlets are visible in this illustration, though it is
contemplated that any number of inlets may be provided as desired. In
particular
embodiments, four inlets are used. The inlets are radially opposed and
oriented.
[0064]
A contoured nozzle wall 129 reduces the outer diameter of the flow path in a
manner that generates higher velocities near the wall region and reduces
turbulence,
producing near plug flow as the fluid velocity profile develops, i.e. the
fluid is
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accelerated downward in the direction of the centerline with little to no
circumferential
motion component and low flow turbulence. This generates a chamber flow
profile that
is optimum for acoustic separation and particle collection. The fluid passes
through
connecting duct 127 and into a flow/separation chamber 128. The contoured
nozzle wall
129 also adds a radial motion component to the suspended particles, moving the
particles closer to the centerline of the apparatus and generating more
collisions with
rising, buoyant agglomerated particles. This radial motion will allow for
optimum
scrubbing of the particles from the fluid in the connecting duct 127 prior to
reaching the
separation chamber. The contoured nozzle wall 129 directs the fluid in a
manner that
generates large scale vortices at the entrance of the collection duct 133 to
also enhance
particle collection. Generally, the flow area of the device 124 is designed to
be
continually decreasing from the annular plenum 131 to the separation chamber
128 to
assure low turbulence and eddy formation for better particle separation,
agglomeration,
and collection. The nozzle wall has a wide end and a narrow end. The term
scrubbing
is used to describe the process of particle/droplet agglomeration,
aggregation, clumping
or coalescing, that occurs when a larger particle/droplet travels in a
direction opposite to
the fluid flow and collides with smaller particles, in effect scrubbing the
smaller particles
out of the suspension.
[0065] The flow/separation chamber 128 includes a transducer array 130 and
reflector 132 on opposite sides of the chamber. The reflector can be the thin
film-air
interface described above in Figure 1, with one side of the film exposed to
the fluid
within the flow chamber and the other side of the film exposed to the air
outside of the
flow chamber. In use, standing waves 134 are created between the transducer
array
130 and thin film-air interface 132. These standing waves can be used to
agglomerate
particles, and this orientation is used to agglomerate particles that are
buoyant (e.g. oil).
Fluid, containing residual particles, then exits through flow outlet 135.
[0066] As the buoyant particles agglomerate, they eventually overcome the
combined effect of the fluid flow drag forces and acoustic radiation force,
and their
buoyant force 136 is sufficient to cause the buoyant particles to rise
upwards. In this
regard, a collection duct 133 is surrounded by the annular plenum 131. The
larger
particles will pass through this duct and into a collection chamber 140. This
collection
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chamber can also be part of an outlet duct. The collection duct and the flow
outlet are
on opposite ends of the apparatus.
[0067] It should be noted that the buoyant particles formed in the
separation
chamber 128 subsequently pass through the connecting duct 127 and the nozzle
wall
129. This causes the incoming flow from the annular plenum to flow over the
rising
agglomerated particles due to the inward radial motion imparted by the nozzle
wall.
This allows the rising particles to also trap smaller particles in the
incoming flow,
increasing scrubbing effectiveness. The length of the connecting duct 127 and
the
contoured nozzle wall 129 thus increase scrubbing effectiveness. Especially
high
effectiveness is found for particles with a size of 0.1 microns to 20 microns,
where
efficiency is very low for conventional methods.
[0068] The design here provides an optimized velocity profile with low flow
turbulence at the inlet to the flow chamber 128, a scrubbing length before the
flow
chamber to enhance particle agglomeration and/or coalescence before acoustic
separation, and the use of the collection vortices to aid particle removal at
the collection
duct 133.
[0069] The transducer setup of the present disclosure creates a three
dimensional
pressure field which includes standing waves perpendicular to the fluid flow.
The
pressure gradients are large enough to generate acoustophoretic forces
orthogonal to
the standing wave direction (i.e., the acoustophoretic forces are parallel to
the fluid flow
direction) which are of the same order of magnitude as the acoustophoretic
forces in the
wave direction. This permits enhanced particle trapping, clumping and
collection in the
flow chamber and along well-defined trapping lines, as opposed to merely
trapping
particles in collection planes as in conventional devices. The particles have
significant
time to move to nodes or anti-nodes of the standing waves, generating regions
where
the particles can concentrate, agglomerate, and/or coalesce, and then gravity
separate.
[0070] In some embodiments, the fluid flow has a Reynolds number of up to
1500,
i.e. laminar flow is occurring. For practical application in industry, the
Reynolds number
is usually from 10 to 1500 for the flow through the system. The particle
movement
relative to the fluid motion generates a particle Reynolds number much less
than 1.0 for
that particle. The Reynolds number represents the ratio of inertial flow
effects to
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viscous effects in a given flow field. For Reynolds numbers below 1.0, viscous
forces
are dominant in the flow field. This results in significant damping where
shear forces are
predominant throughout the flow. This flow where viscous forces are dominant
is called
Stokes flow. Wall contouring and streamlining have very little importance
under such
conditions. This is associated with the flow of very viscous fluids or the
flow in very tiny
passages, like MEMS devices.
[0071] The large annular plenum is followed by an inlet wall nozzle that
accelerates
and directs the fluid inward toward the centerline as shown in Figure 4. The
wall
contour will have a large effect on the profile. The area convergence
increases the flow
average velocity, but it is the wall contour that determines the velocity
profile. The
nozzle wall contour will be a flow streamline, and is designed with a small
radius of
curvature in the separator.
[0072] It may be helpful now to describe the ultrasonic transducer(s) used
in the
acoustophoretic filtering device in more detail. Figure 5 is a cross-sectional
diagram of
a conventional ultrasonic transducer. This transducer has a wear plate 50 at a
bottom
end, epoxy layer 52, ceramic crystal 54 (made of, e.g. PZT), an epoxy layer
56, and a
backing layer 58. On either side of the ceramic crystal, there is an
electrode: a positive
electrode 61 and a negative electrode 63. The epoxy layer 56 attaches backing
layer 58
to the crystal 54. The entire assembly is contained in a housing 60 which may
be made
out of, for example, aluminum. An electrical adapter 62 provides connection
for wires to
pass through the housing and connect to leads (not shown) which attach to the
crystal
54. Typically, backing layers are designed to add damping and to create a
broadband
transducer with uniform displacement across a wide range of frequency and are
designed to suppress excitation at particular vibrational eigen-modes. Wear
plates are
usually designed as impedance transformers to better match the characteristic
impedance of the medium into which the transducer radiates.
[0073] Figure 6 is a cross-sectional view of an ultrasonic transducer 81 of
the
present disclosure. Transducer 81 is shaped as a disc or a plate, and has an
aluminum
housing 82. The piezoelectric crystal is a mass of perovskite ceramic
crystals, each
consisting of a small, tetravalent metal ion, usually titanium or zirconium,
in a lattice of
larger, divalent metal ions, usually lead or barium, and 02- ions. As an
example, a PZT
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(lead zirconate titanate) crystal 86 defines the bottom end of the transducer,
and is
exposed from the exterior of the housing. The crystal is supported on its
perimeter by a
small elastic layer 98, e.g. silicone or similar material, located between the
crystal and
the housing. Put another way, no wear layer is present.
[0074] Screws 88 attach an aluminum top plate 82a of the housing to the
body 82b
of the housing via threads. The top plate includes a connector 84 for powering
the
transducer. The top surface of the PZT crystal 86 is connected to a positive
electrode
90 and a negative electrode 92, which are separated by an insulating material
94. The
electrodes can be made from any conductive material, such as silver or nickel.
Electrical power is provided to the PZT crystal 86 through the electrodes on
the crystal.
Note that the crystal 86 has no backing layer or epoxy layer. Put another way,
there is
an air gap 87 in the transducer between aluminum top plate 82a and the crystal
86 (i.e.
the air gap is completely empty). A minimal backing 58 and/or wear plate 50
may be
provided in some embodiments, as seen in Figure 7.
[0075] The transducer design can affect performance of the system. A
typical
transducer is a layered structure with the ceramic crystal bonded to a backing
layer and
a wear plate. Because the transducer is loaded with the high mechanical
impedance
presented by the standing wave, the traditional design guidelines for wear
plates, e.g.,
half wavelength thickness for standing wave applications or quarter wavelength
thickness for radiation applications, and manufacturing methods may not be
appropriate. Rather, in one embodiment of the present disclosure the
transducers,
there is no wear plate or backing, allowing the crystal to vibrate in one of
its eigen modes
with a high Q-factor. The vibrating ceramic crystal disk or plate is directly
exposed to the
fluid flowing through the flow chamber.
[0076] Removing the backing (e.g. making the crystal air backed) also
permits the
ceramic crystal to vibrate at higher order modes of vibration with little
damping (e.g.
higher order modal displacement). In a transducer having a crystal with a
backing, the
crystal vibrates with a more uniform displacement, like a piston. Removing the
backing
allows the crystal to vibrate in a non-uniform displacement mode. The higher
order the
mode shape of the crystal, the more nodal lines the crystal has. The higher
order modal
displacement of the crystal creates more trapping lines, although the
correlation of
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trapping line to node is not necessarily one to one, and driving the crystal
at a higher
frequency will not necessarily produce more trapping lines.
[0077] In some embodiments, the crystal may have a backing that minimally
affects
the Q-factor of the crystal. The backing may be made of a substantially
acoustically
transparent material such as balsa wood, foam, or cork which allows the
crystal to
vibrate in a higher order mode shape and maintains a high Q-factor while still
providing
some mechanical support for the crystal. The backing layer may be a solid, or
may be a
lattice having holes through the layer, such that the lattice follows the
nodes of the
vibrating crystal in a particular higher order vibration mode, providing
support at node
locations while allowing the rest of the crystal to vibrate freely. The goal
of the lattice
work or acoustically transparent material is to provide support without
lowering the Q-
factor of the crystal or interfering with the excitation of a particular mode
shape.
[0078] Placing the crystal in direct contact with the fluid also
contributes to the high
Q-factor by avoiding the dampening and energy absorption effects of the epoxy
layer
and the wear plate. Other embodiments may have wear plates or a wear surface
to
prevent the PZT, which contains lead, contacting the host fluid. This may be
desirable
in, for example, biological applications such as separating blood. Such
applications
might use a wear layer such as chrome, electrolytic nickel, or electroless
nickel or
glassy carbon. Chemical vapor deposition could also be used to apply a layer
of poly(p-
xylylene) (e.g. Parylene) or other polymer. Organic and biocompatible coatings
such as
silicone or polyurethane are also usable as a wear surface.
[0079] In the present systems, the system is operated at a voltage such
that the
particles are trapped in the ultrasonic standing wave. The particles are
collected in well-
defined trapping lines, separated by half a wavelength. Within each nodal
plane, the
particles are trapped in the minima of the acoustic radiation potential. The
axial
component of the acoustic radiation force drives the particles, with a
positive contrast
factor, to the pressure nodal planes, whereas particles with a negative
contrast factor
are driven to the pressure anti-nodal planes. The radial or lateral component
of the
acoustic radiation force is the force that traps, clumps, and gravity
separates the
particles. In systems using typical transducers, the radial or lateral
component of the
acoustic radiation force is typically several orders of magnitude smaller than
the axial
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component of the acoustic radiation force. On the contrary, the lateral force
in separator
1 can be significant, on the same order of magnitude as the axial force
component, and
is sufficient to overcome the fluid drag force at linear velocities of up to 1
cm/s. As
discussed above, the lateral force can be increased by driving the transducer
in higher
order mode shapes, as opposed to a form of vibration where the crystal
effectively
moves as a piston having a uniform displacement. The types of waves thus
generated
can be characterized as composite waves, with displacement profiles that are
similar to
leaky symmetric (also referred to as compressional or extensional) Lamb waves.
The
waves are leaky because they radiate into the water layer, which result in the
generation of the acoustic standing waves in the water layer. Symmetric Lamb
waves
have displacement profiles that are symmetric with respect to the neutral axis
of the
piezoelectric element, which causes multiple standing waves to be generated in
a 3-D
space. These higher order modes of vibration can include modes (1,1), (1,2),
(2,1),
(2,2), (2, 3), or (m, n), where m and n are 1 or greater. The acoustic
pressure is
proportional to the driving voltage of the transducer. The electrical power is
proportional
to the square of the voltage.
[0080] In some embodiments, the pulsed voltage signal driving the
transducer can
have a sinusoidal, square, sawtooth, or triangle waveform; and have a
frequency of 500
kHz to 10 MHz. The pulsed voltage signal can be driven with pulse width
modulation,
which produces any desired waveform. The pulsed voltage signal can also have
amplitude or frequency modulation start/stop capability to eliminate
streaming.
[0081] The size, shape, and thickness of the transducer determine the
transducer
displacement at different frequencies of excitation, which in turn affects
separation
efficiency. Typically, the transducer is operated at frequencies near the
thickness
resonance frequency (half wavelength). Gradients in transducer displacement
typically
result in more places for particles to be trapped. Higher order modal
displacements
generate three-dimensional acoustic standing waves with strong gradients in
the
acoustic field in all directions, thereby creating equally strong acoustic
radiation forces
in all directions, leading to multiple trapping lines, where the number of
trapping lines
correlate with the particular mode shape of the transducer.
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[0082]
To investigate the effect of the transducer displacement profile on acoustic
trapping force and separation efficiencies, an experiment was repeated ten
times using
a 1"x1" square transducer, with all conditions identical except for the
excitation
frequency. Ten consecutive acoustic resonance frequencies, indicated by
circled
numbers 1-9 and letter A on Figure 8, were used as excitation frequencies. The
conditions were experiment duration of 30 min, a 1000 ppm oil concentration of
approximately 5-micron SAE-30 oil droplets, a flow rate of 500 ml/min, and an
applied
power of 20W. Oil droplets were used because oil is denser than water, and can
be
separated from water using acoustophoresis.
[0083]
Figure 9 shows the measured electrical impedance amplitude of the
transducer as a function of frequency in the vicinity of the 2.2 MHz
transducer
resonance when operated in a water column containing oil droplets. The minima
in the
transducer electrical impedance correspond to acoustic resonances of the water
column
and represent potential frequencies for operation. Numerical modeling has
indicated
that the transducer displacement profile varies significantly at these
acoustic resonance
frequencies, and thereby directly affects the acoustic standing wave and
resulting
trapping force. Since the transducer operates near its thickness resonance,
the
displacements of the electrode surfaces are essentially out of phase. The
typical
displacement of the transducer electrodes is not uniform and varies depending
on
frequency of excitation. As an example, at one frequency of excitation with a
single line
of trapped oil droplets, the displacement has a single maximum in the middle
of the
electrode and minima near the transducer edges. At another excitation
frequency, the
transducer profile has multiple maxima leading to multiple trapped lines of
oil droplets.
Higher order transducer displacement patterns result in higher trapping forces
and
multiple stable trapping lines for the captured oil droplets.
[0084]
As the oil-water emulsion passed by the transducer, the trapping lines of oil
droplets were observed and characterized.
The characterization involved the
observation and pattern of the number of trapping lines across the fluid
channel, as
shown in Figure 9, for seven of the ten resonance frequencies identified in
Figure 8.
Different displacement profiles of the transducer can produce different (more)
trapping
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lines in the standing waves, with more gradients in displacement profile
generally
creating higher trapping forces and more trapping lines.
[0085] The transducer(s) is/are used to create a pressure field that
generates forces
of the same order of magnitude both orthogonal to the standing wave direction
and in
the standing wave direction. When the forces are roughly the same order of
magnitude,
particles of size 0.1 microns to 300 microns will be moved more effectively
towards
regions of agglomeration ("trapping lines"). Because of the equally large
gradients in the
orthogonal acoustophoretic force component, there are "hot spots" or particle
collection
regions that are not located in the regular locations in the standing wave
direction
between the transducer 130 and the reflector 132. Hot spots are located at the
minima
of acoustic radiation potential. Such hot spots represent particle collection
locations.
[0086] Figure 10 is a log-log graph (logarithmic y-axis, logarithmic x-
axis) that shows
the scaling of the acoustic radiation force, fluid drag force, and buoyancy
force with
particle radius. Calculations are done for a typical SAE-30 oil droplet used
in
experiments. The buoyancy force is a particle volume dependent force, and is
therefore
negligible for particle sizes on the order of micron, but grows, and becomes
significant
for particle sizes on the order of hundreds of microns. The fluid drag force
scales
linearly with fluid velocity, and therefore typically exceeds the buoyancy
force for micron
sized particles, but is negligible for larger sized particles on the order of
hundreds of
microns. The acoustic radiation force scaling acts differently. When the
particle size is
small, the acoustic trapping force scales with the volume of the particle.
Eventually,
when the particle size grows, the acoustic radiation force no longer increases
with the
cube of the particle radius, and will rapidly vanish at a certain critical
particle size. For
further increases of particle size, the radiation force increases again in
magnitude but
with opposite phase (not shown in the graph). This pattern repeats for
increasing
particle sizes.
[0087] Initially, when a suspension is flowing through the system with
primarily small
micron sized particles, it is necessary for the acoustic radiation force to
balance the
combined effect of fluid drag force and buoyancy force for a particle to be
trapped in the
standing wave. In Figure 10 this happens for a particle size of about 3.5
micron,
labeled as Rd. The graph then indicates that all larger particles will be
trapped as well.
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Therefore, when small particles are trapped in the standing wave, particles
coalescence/clumping/aggregation/agglomeration takes place, resulting in
continuous
growth of effective particle size. As the particle size grows, the acoustic
radiation force
reflects off the particle, such that large particles will cause the acoustic
radiation force to
decrease. Particle size growth continues until the buoyancy / gravity force
becomes
dominant, which is indicated by a second critical particle size, Rc2, at which
size the
particles will rise or sink, depending on their relative density with respect
to the host
fluid. Thus, Figure 10 explains how small particles can be trapped
continuously in a
standing wave, grow into larger particles or clumps, and then eventually will
rise or
settle out because of increased buoyancy / gravity force.
[0088] In biological applications, it is contemplated that all of the parts
of the system
(e.g. the reaction vessel, tubing leading to and from the bioreactor, the
temperature-
regulating jacket, etc.) can be separated from each other and be disposable.
The
frequency of the transducers may also be varied to obtain optimal
effectiveness for a
given power.
[0089] The following examples are provided to illustrate the apparatuses,
components, and methods of the present disclosure. The examples are merely
illustrative and are not intended to limit the disclosure to the materials,
conditions, or
process parameters set forth therein.
EXAMPLES
EXAMPLE 1
[0090] A polyolefin heat shrink film having a thickness of 0.60 mills
(15.24 microns)
was used as the acoustically transparent film to form a fluid-air interface,
and was
sandwiched in place using an empty transducer housing. This thickness is 1/50
of a
wavelength when the transducer is operated at a frequency of 2.2 MHz. Figure
3A is a
picture of the test device.
[0091] Figure 3B is a picture of the plastic film-air interface reflector
during
operation. The operation of a 5x5 trapping line mode can be seen through the
plastic
film, which is also optically transparent. The white trapping lines are
visible through the
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plastic film. The overall efficiency of the apparatus dropped only 3% compared
to using
a steel reflector, which was within the range of measurement error.
EXAMPLE 2
[0092] Acoustically transparent thin films 170 were attached to the face of
the
piezoelectric crystal (dimensions 1 inch by 1 inch) 172 of the ultrasonic
transducer. Two
different plastic thin films were used, one about 60 microns thick and one
about 350
microns thick. A thin layer of ultrasonic transmission gel 174 was used to
ensure there
were no air pockets between the thin film and the crystal face. Figure 11 is a
picture of
the square transducer and a diagram of the resulting structure.
[0093] Three types of reflectors were tested: a steel reflector, a thin
plastic film
reflector about 60 microns thick (R-ATF), and a thin plastic film reflector
about 350
microns thick (R-TBC). Three different types of piezoelectric crystals were
used: a
crystal with the plastic thin film cover about 60 microns thick (C-ATF); a
crystal with the
plastic thin film cover about 350 microns thick (C-TBC); and an uncoated gamma
sterilized crystal (UC).
[0094] These crystal / reflector combinations were tested to determine the
effect on
separation of a 3% yeast feed having 200 million cells/mL and starting
turbidity as
indicated. The feed flow rate was 30 ml/min, the concentrate output was 5
mL/min, and
the permeate output was 25/mL/min. The power to the crystals was 7-11 watts,
unless
otherwise noted, and the frequency was 2.2455 MHz. The 350-micron-thick film
was
about one-half the thickness of the wavelength at this frequency.
[0095] After 30 minutes, the concentrate, permeate, and retentate were
measured.
The concentrate was the portion exiting the device that contained the
concentrated
yeast, along with some fluid. The permeate was the filtered portion exiting
the device,
which was mostly liquid with a much lower concentration of yeast. The
retentate was
the remaining substance left in the device after operation.
[0096] The results are provided in the following Table 1.
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Table 1.
Starting Turbidity Permeate Concentrate Retentate
Turbidity Reduction Turbidity Turbidity Turbidity
Reflector Crystal (NTU) (%) (NTU) (NTU) (NTU)
Steel UC 5400 97 164 24000 7760
R-ATF UC (8 watts) 5690 95 309 23440 8210
UC (11
R-ATF watts) 5520 91 308 22480 9530
Steel C-ATF 5130 98 134 24520 6600
Steel C-TBC 5420 91 450 28160 8070
R-TBC UC 5730 91 432 29480 8190
C-TBC (10-
R-TBC 11 watts) 5840 88 660 24120 7500
C-TBC (19-
R-TBC 20 watts) 5690 93 379 31080 8700
[0097] As seen here, the turbidity was heavily reduced in the permeate and
heavily
increased in the concentrate, indicating the efficiency of the system.
[0098] The present disclosure has been described with reference to
exemplary
embodiments. Obviously, modifications and alterations will occur to others
upon
reading and understanding the preceding detailed description. It is intended
that the
present disclosure be construed as including all such modifications and
alterations
insofar as they come within the scope of the appended claims or the
equivalents
thereof.
