U.S. patent number RE33,524 [Application Number 07/240,203] was granted by the patent office on 1991-01-22 for particle separation.
This patent grant is currently assigned to National Research Development Corporation. Invention is credited to Cornelius J. Schram.
United States Patent |
RE33,524 |
Schram |
January 22, 1991 |
Particle separation
Abstract
The separation of different types of particulate matter in a
carrier liquid is obtained by using an ultrasonic standing wave and
relying on the different acoustic responses of the different
particle types. By varying the acoustic energy propagation
cyclically a more effective separation rate can be obtained, with a
more readily attracted particle type being subjected to a further
discrimination step in each cycle. The cyclical energy variation
may be in the intensity of the standing wave, e.g. using
suppression means, and/or the velocity of the standing wave
relative to the liquid medium, e.g. using phase control means.
Inventors: |
Schram; Cornelius J. (Bedford,
GB) |
Assignee: |
National Research Development
Corporation (London, GB2)
|
Family
ID: |
10563492 |
Appl.
No.: |
07/240,203 |
Filed: |
September 2, 1988 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
751951 |
Jul 5, 1985 |
04673512 |
Jun 16, 1987 |
|
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Foreign Application Priority Data
Current U.S.
Class: |
210/748.05;
204/157.15; 209/1; 435/261; 436/177 |
Current CPC
Class: |
B01D
15/3866 (20130101); B01D 21/283 (20130101); B01D
43/00 (20130101); B01J 19/10 (20130101); B03B
5/00 (20130101); C12N 13/00 (20130101); C12M
47/02 (20130101); G01N 30/02 (20130101); G01N
30/02 (20130101); B01D 2015/389 (20130101); Y10T
436/25375 (20150115) |
Current International
Class: |
B01D
43/00 (20060101); B01J 19/10 (20060101); B01D
15/08 (20060101); B03B 5/00 (20060101); C12N
13/00 (20060101); G01N 30/02 (20060101); G01N
30/00 (20060101); B01D 017/06 (); C02F
001/36 () |
Field of
Search: |
;55/15,277 ;204/157.15
;210/635,636,748,198.2 ;209/1 ;435/261 ;436/177 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0147032 |
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Jul 1985 |
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EP |
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836640 |
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Mar 1952 |
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DE |
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1442610 |
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Apr 1969 |
|
DE |
|
3218488 |
|
Nov 1983 |
|
DE |
|
821419 |
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Dec 1937 |
|
FR |
|
828204 |
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May 1938 |
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FR |
|
2098498 |
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Nov 1982 |
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GB |
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Primary Examiner: Wyse; Tom
Attorney, Agent or Firm: Cushman, Darby & Cushman
Claims
I claim:
1. In a method of separating different types of particles in which
an ultrasonic standing wave having an axis of propagation, and a
series of nodes transverse to the axis is propagated in a liquid
medium and there is relative motion between the medium and the
standing wave, the different types of particles being differently
influenced by the acoustic energy of the standing wave and/or the
Stokes or drag forces of the fluid medium, the improvement
comprising cyclically varying the energy propagation of said
standing wave .Iadd.by moving the standing wave at a cyclically
varying rate .Iaddend.so that the different particle types are
caused to move at .[.cyclically varying.]. .Iadd.different
.Iaddend.rates in the direction of the axis of propagation and
thereby progressively separating said different particle types
while .[.remaining.]. .Iadd.they remain .Iaddend.suspended in the
liquid medium.
2. A method according to claim 1 wherein substantially
instantaneous phase changes are introduced between two acoustic
energy outputs which interact to produce the moving standing wave,
thereby superimposing additional intermittent movements onto the
substantially continuous movement of the standing wave.
3. A method according to claim 1 wherein the standing wave is
subjected to cycles of velocity variation in each of which the
velocity is varied unidirectionally between a maximum and a minimum
rate of movement for the standing wave.
4. In a method of separating different types of particles in which
an ultrasonic standing wave having an axis of propagation, and a
series of nodes transverse to the axis is propagated in a liquid
medium and there is relative motion between the medium and the
standing wave, the different types of particles being differently
influenced by the acoustic energy of the standing wave and/or the
Stokes or drag forces of the fluid medium, the improvement
comprising cyclically varying the energy propagation by varying the
intensity of said standing wave so that the different particle
types are caused to move at different rates in the direction of the
axis of propagation .Iadd.and thereby progressively separating said
.Iaddend.different particle types while .[.remaining.]. .Iadd.they
remain .Iaddend.suspended in the liquid medium.
5. A method according to claim 3 wherein the standing wave is
periodically suppressed and re-established.
6. A method according to claim 5 wherein said suppression and
re-establishment of the standing wave is performed without
disturbing the phase continuity of electrical driving signals
producing the acoustic energy. .Iadd.7. In a method of separating
different types of particles in which an ultrasonic standing wave
having an axis of propagation, and a series of nodes transverse to
the axis is propagated in a liquid medium and there is relative
motion between the medium and the standing wave, the different
types of particles being differently influenced by the acoustic
energy of the standing wave and/or the stokes or drag forces of the
fluid medium, the improvement comprising cyclically varying the
energy propagation of said standing wave so that the different
particle types are caused to move at different cyclically varying
rates in the direction of the axis of propagation and thereby
progressively separating said different particle types while they
remain suspended in the liquid medium. .Iaddend. .Iadd.8. A method
as in claim 7 whereby said cyclically varying energy propagation
step comprises the further step of causing the standing wave to
move at a cyclically varying rate. .Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to the separation of different types of
particulate matter in a liquid medium using an ultrasonic standing
wave propagated through the medium. It relates particularly,
although not exclusively, to a method and a means for
chromatography.
In one of its main aspects, the invention is concerned with the
separation of biological particles, which term is used here to
include a range of particulate matter from macromolecules--e.g.
globular proteins--through viruses, bacteria and yeasts, to tissue
cells--e.g. plant cells, animal cells and aggregates--but it can
also be employed on many finely divided inorganic and organic
materials, including siliceous minerals such as clays.
In chemical chromatography, the isolation of chemical components
from a mixture for their identification is achieved relying on very
small quantities of any one component. That even very complex
mixtures can be represented safely by a small sample, so that the
column is not overloaded, is due to the uniformity of any given
molecular species, individual molecules differing only in features
such as isomeric form and isotopic composition to which the
separation process is quite insensitive.
It has been proposed in U.S. Pat. No. 4,280,823 to provide a
chromatographic column to analyse a sample of red blood cells which
is entrained in a gas flow through the column while an ultrasonic
transducer at one end of the column directs its output onto a
reflector at the opposite end, its frequency and its distance from
the reflector being so matched that a standing wave is produced by
the interaction of the emitted and reflected waves. It is desired
in that disclosure how the nodes of the standing wave can function
in the same way as a series of filter plates of a chemical
chromatograph to promote separation of the constituents of the
sample as it moves along the column.
However, biological particles such as cells are much less uniform,
individual members of a group differing in size, age, metabolic
state and so forth. Moreover, many of these variations within a
group are those to which an acoustic separation is acutely
sensitive. There are difficulties therefore in applying
chromatographic methods using ultrasonic energy to the analysis of
large populations of particles and to the detection of fine
distinctions of various groups by having each represented by
adequate cell populations.
The method disclosed in U.S. Pat. No. 4,280,823 would have at best
a limited utility, because to obtain substantial and sufficiently
complete separation of any mixed group of biological particles a
very large column length is dictated. But apart from the bulk and
cost resulting from any substantial increase in size, there is a
limit to the maximum column length that can be employed, owing to
the attenuation of an ultrasonic wave that occurs with distance and
that restricts the length over which the incident and reflected
wave energies are sufficiently well matched to form a predominating
standing wave. It may be mentioned here that, apart from this major
problem, the method disclosed in U.S. Pat. No. 4,280,823 has
further disadvantages because of the difficulty of handling
biological particles in a gaseous environment, in particular as
regards difficulty of control and prevention of damage to or
transformation of the particles.
The problem of separating large populations of particles,
particularly biological particles may be even more severe if
acoustic energy methods are to be employed for a bulk separation
process rather than simply the analysis of a very small sample.
It may be expected, for example, that problems would be encountered
if an apparatus such as is described in GB No. 2 089 498A were to
be used for the separation of large quantities of particles in a
mixed population. In that apparatus a flow of liquid in a conduit
passes through a zone in which ultrasonic transducers at opposite
sides of the conduit are driven with a controlled phase angle
between their driving signals so as to establish a standing wave
pattern that moves across the conduit, along the common axis of
propagation of the two transducers. Particles carried along by the
flow through the conduit enter the standing wave transverse to its
axis and the acoustic energy is effective only over a very short
distance along the length of the conduit. The extent to which
particles can be differently displaced along the standing wave is
correspondingly severely limited. This limitation, coupled with the
difficulties of achieving the separation of groups of non-uniform
particles discussed above, means that the apparatus described in GB
No. 2 089 498A would have no application to the separation of
biological particles.
It is an object of the present invention to provide a method in
which the separation of particles types having different acoustic
properties can be more effectively performed.
SUMMARY OF THE INVENTION
According to one aspect of the invention there is provided a method
of separating different type of particles in which an ultrasonic
standing wave is propagated in a liquid medium and there is
relative motion between the medium and the standing wave, the
different types of particles being differently influenced by the
acoustic energy of the standing wave and/or the Stokes or drag
forces of the liquid medium, characterised in that the acoustic
energy propagation is varied cyclically, whereby the different
particle types are caused to move at different rates with respect
to the standing wave and are thereby progressively separated.
The method may be performed by varying the intensity of the
standing wave to effect said cyclical variation of the acoustic
energy propagation. In one particular way of putting this method
into effect, the standing wave is alternatively suppressed and
reestablished without disturbing the phase continuity of electrical
driving signals producing the acoustic energy.
In another method of effecting said variation of the ultrasonic
wave propagation, the standing wave is caused to move at a varying
rate. It is also possible to combine such cyclic variations of the
standing wave intensity and velocity.
According to another aspect of the invention, there is provided
apparatus for the separation of different types of particles in a
liquid medium, comprising means for propagating an ultrasonic
standing wave in the medium and for generating a relative movement
between the medium and the standing wave, the apparatus further
comprising means for varying cyclically the acoustic energy
propagation in order that different types of particles having
different responses to the acoustic energy of the standing wave
and/or the Stokes or drag forces generated by relative movement
between the particles and the liquid medium are caused to move at
different rates with respect to the standing wave and are thereby
progressively separated.
The invention will be described by way of example with reference to
the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 illustrate schematically the separation of different
types of particles in a liquid medium through which a periodically
suppressed standing wave is propagated,
FIG. 3 is a schematic illustration of an apparatus for performing
the separation process described in FIGS. 1 and 2,
FIG. 4 illustrates schematically the separation of different types
of particles in a liquid medium through which a standing wave is
propagated with variable rate of movement,
FIG. 5 is a fragmentary illustration of a modification of the
apparatus of FIG. 4 to operate the separation process of FIG.
4,
FIG. 6 is another schematic illustration of the separation of
different types of particles employing phase changes in the
propagation of the standing wave to produce stepped displacements,
and
FIG. 7 is a further fragmentary illustration of another
modification of the apparatus of FIG. 3 to operate a continuous
separation process.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates the propagation of an ultrasonic standing wave
having a wavelength of 0.7 mm, and thus an internodal distance of
0.35 mm at a constant velocity of one internodal distance per
second. (Such as internodal distance corresponds, for instance,
with a 2 MHz wave in water at room temperature). This propagation
in the liquid medium occurs in such a manner that there is uniform
relative movement along the axis of propagation of the standing
wave between the standing wave and the liquid. Distance in mm along
the axis of propagation is plotted against a time base (t) in
seconds and the graph represents the moving nodes (full lines) and
antinodes (broken lines) having a velocity of 0.35 mm/sec relative
to the liquid. FIG. 1 also shows interruption of the standing wave
in a 1 second cycle with the wave being propagated for 0.7 sec and
then suppressed for 0.3 sec, but the movement between the standing
wave and the liquid corresponds to that of a continuously
propagated standing wave with uniform relative motion, giving a 1
second internodal period (wavelength divided by relative
velocity).
If a mixed population of particles suspended in the liquid is
subjected to the acoustic energy of the standing wave, at a given
relative velocity between the wave and the liquid/any particles
uninfluenced by the acoustic energy will remain static, some that
are only weakly influenced will oscillate about a mean position as
each node passes, while others more strongly influenced will move
with the nodes. (It should be mentioned here that the factors
determining whether any given particle type tends to be attracted
to the nodes or the antinodes are unclear, but this lack of
theoretical understanding is not material to the present invention
and where the context permits the term "nodes" can be read to
include both nodes and antinodes).
Particles of two different types A and B are shown in FIG. 1 at the
beginning of the separation process both attached to a node of the
standing wave and thus moving relative to the liquid with the
standing wave, but the response to the acoustic forces and
therefore the strength of attachment to the node is greater for
type A than for type B. When the standing wave is suppressed (as
first occurs at 0.2 sec as shown in FIG. 1) the particles are left
static in the liquid. Very small particles e.g. of the order of
microns, have very little inertia and in a liquid medium both types
A and B will stop moving virtually immediately the wave is
suppressed.
When the standing wave is re-established at 0.5 sec, with the nodes
displaced a distance proportional to the period of interruption,
particles A and B are now positioned between the node on which they
were originally held and the following antinode. An A type
particle, being acted on more strongly by the acoustic forces, will
move towards the original node at a speed faster than the node is
itself moving relative to the liquid and will thus quickly at
reattached to the node. A B type particle will also move towards
the original node but is less strongly attracted, to the extent
that its velocity is less than the relative velocity between the
standing wave and the liquid. The particle B thus soon finds itself
at the follow antinode, where the attraction forces of the original
node and its following node cancel each other out, and the particle
then quickly comes under the influence of the approaching following
node to move towards it. The particle B soon attached itself to the
following node, so that the two particle types are now separated by
an internodal distance. The standing wave is again suppressed and
the cycle repeated, whereupon the particle A attaches again to the
original node and the particle B falls back a further internodal
distance. As this process is continued, the particles become
separated by as many internodal distances as there are
interruptions in the propagation of the standing wave. In general,
the interruption cycle time will be of the same order as the
internodal period, so that particles A and B will be presented with
a large number of successive opportunities to increase their
separation over a relatively small time period.
In the system shown in FIG. 1 the progress of the nodes can be
related to a standing wave moving with uniform velocity. For any
given internodal period, the period during which the standing wave
is suppressed determines the position relative to the wave pattern
that the particles occupy when the wave is re-established. Clearly,
it is necessary for the period of suppression to have an interval
significantly less than half the internodal period in order that
the particle A finds itself between a node and the following
antinode (it can also comprise any integral number of internodal
periods, but there will not be any advantage generally in so
extending the period), so that it may be advanced with the standing
wave while the particle B falls back to a following node. With the
suppression period limited to a fraction of the internodal period,
those particles that have been carried forwards on a node will find
themselves starting again near that node; the particles should not
be so close to the original node, however, that the attraction
forces on a particle B are strong enough to draw it also towards
that node.
As regards the period of propagation of the standing wave in each
cycle, the simplest method maintains the standing wave long enough
to ensure that both types of particle attach themselves to spaced
nodes. The conditions in each interruption of the standing wave can
thus be relatively precisely repeated. It will be noted, however,
that since the particle A reattaches itself to the original node
before the particle B falls back to the succeeding node, the period
of propagation can be shortened to a time sufficient to allow that
reattachment of the particle A, leaving the particle B somewhere in
the region of the following antinode.
This has particular significance in handling high concentrations of
particles in which groups are often formed, and in other conditions
in which the cycle should be repeated as frequently as possible to
achieve optimum separation. For example, it can be expected that
some particles of one group will be entrained by concentrations of
particles of the other group, or if the process is carried out in a
standing wave which is not entirely uniform in energy density, with
the minor lateral disturbances that will always be present this
process allows continual redistribution of particles with a final
degree of separation truly reflecting the average conditions in the
column.
FIG. 2 illustrates a process in which the conditions are generally
the same as in FIG. 1, except that the propagation period has been
reduced to 0.35 sec, giving an 0.65 sec cycle. At the end of the
first cycle a group A.sub.1 of particles enriched with particle A
is reattached to the original node, while a group B.sub.1 of
particles enriched with particle B lies in the region of the
following antinode. When the wave is re-established in the second
cycle, group A.sub.1 is again in the same position relative to the
nodal array as in the first cycle and another B-rich fraction
B.sub.11 from that group falls back to the following antinode, so
that the group A.sub.2 attaching itself to the original node in the
third cycle has a further reduced content of group B particles,
while the group B.sub.1 is exposed to no selection process in this
second cycle and merely joins the following node. In the third
cycle, the group A.sub.2 has more B type particles removed in group
B12 leaving a further purified group A.sub.3 of A particles to
reattach itself to the original node in the following cycle. In the
third cycle the group B.sub.1 is also subjected to another
separation process since it starts the cycle in a corresponding
position to that of the original mixture in the first cycle, and an
A-rich fraction A.sub.11 is drawn from it, leaving a purer group
B.sub.2 of B particles.
By varying the propagation period, the suppression period and cycle
time in relation to the internodal period there is scope for
adjusting the degree of discrimination and the rate of working
required. It is possible, for example, to shorten the cycle still
further than is described in FIG. 2, since will be possible to
ensure continuing separation when the two groups are separated by
less than half an internodal distance in a period of propagation.
In particular it will not normally be necessary to ensure that the
more strongly influenced group reattaches to the original node
before the standing wave is suppressed. By keeping the cycle time
to the minimum practical period possible, the process can be highly
selective because of the very large number of separation stages
that can be contained over a very short distance in the liquid.
FIG. 3 illustrates schematically an apparatus in which the
processes of FIGS. 1 and 2 may be performed. A liquid-filled column
2 has a standing wave propagated in it by opposed ultrasonic
transducers 4 at opposite ends of the column. Opposite ends of the
column are immersed in liquid baths 6, 8, but are sealed from the
contents of the baths by end plugs 10 transparent to the ultrasonic
energy, and the transducers 4 are disposed in the liquid of the
baths, aligned with each other so that the axis of propagation of
ultrasonic energy from each is coaxial with the central axis of the
column. The transducers are driven from an oscillator, 12, having a
power supply 12a, through respective amplifiers 14. A phase control
unit 16 between the oscillator 12 and one amplifier produces a
relative phase shift between the outputs of the two transducers so
that the standing wave resulting from the interference of the two
coaxially propagated ultrasonic outputs from the transducers is
caused to move along the column in a direction and at a rate
determined by the phase control. A power supply 18 for the two
amplifiers 14 is controlled by switching means 20 so that the
energisation of both transducers can be switched on and off jointly
to produce the cyclic suppression of the standing wave already
described.
The column has inlet and outlet ports 22, 24 for a carrier liquid
adjacent opposite ends of the liquid-filled space between the
plugs. Sample injection ports 26, 28 are disposed between the
carrier liquid ports one adjacent each port.
In the one mode of operation, a continuous slow flow of liquid is
established between the liquid ports 22, 24 and a mixed sample of
two particle types is injected into the column through the port 26
adjacent to the liquid inlet port 22. The standing wave is caused
to move in the direction from the inlet port 22 to the outlet port
24 and, as described with reference to FIGS. 1 and 2, the types of
particles are progressively separated and spaced apart as they
travel towards the opposite end of the column. By closing the
liquid outlet port 24 and opening the adjacent sample port 28 when
the separated group of the first type of particle approaches the
further end of the column, the different groups of particles can be
collected sequentially as they arrive at the sample port over
different intervals of time.
Even more simply, by relying solely on the motion generated by the
standing wave, it is possible to use a liquid column with only one
entry port at one end and a pair of opposed exit ports at the other
end. The entry port is utilised to inject a sample into a column
and a flushing liquid flow between the two ports at the opposite
end of the column will sweep out one group of particles that have
been moved to that end by the moving wave, the rest of the
particles remaining at the entry end uninfluenced by the acoustic
energy. By choice of different ultrasonic frequencies and/or energy
intensities different fractions can be separated from a
mixture.
A further process according to the invention is illustrated in FIG.
4, in which the standing wave is caused to move through the liquid
medium at a variable rate. In this example, a cyclical series of
stepped phase changes are introduced between the two opposed
transducers, the figure showing a 1 sec. cycle with ten stepped
stages, the wave velocity (v) being indicated on the left-hand
vertical scale in mm/minute from a maximum wave velocity of 95
mm/min to a minimum of 5 mm/min in 0.1 sec steps. The number of
steps can be increased and in the extreme case the linear variation
indicated by the broken line in FIG. 4 illustrates a continuous
linear rate of change from 100 mm/min to 0 over the 1 sec cycle.
The total distance travelled by a node of the standing wave pattern
is plotted in curve DA against the vertical scale of distance (d)
in mm indicated on the right-hand side of the figure; thus over one
complete cycle the travel distance totals 0.83 mm.
To understand how variation of the standing wave velocity brings
about separation of particle types that are differently influenced
by the standing wave, it will be understood from the earlier
examples that the progressive movement of any particle with the
movement of the standing wave will have a limiting velocity
depending on the strength of response of the particle to the
standing wave, since that response must be used to overcome the
Stokes forces on the particle. If the velocity range in the regime
illustrated in FIG. 4 is matched to the responses of the different
types of particles in a sample, only the most strongly influenced
particles are entrained by the wave at its highest velocity
relative to the carrier liquid medium and will move at the same
mean speed as that of the standing wave. Less strongly influenced
particles will only be entrained by the standing wave when its
velocity falls below some critical value less than the maximum
velocity. FIG. 4 illustrates curves DA, DB and DC of the distance
travelled by three different particles, A, B and C with different
responses to the standing wave such that the critical or
transitional velocity for particle A is 110 mm/min, that for
particle B is 60 mm/min and for particle C is 30 mm/min. particle
A, having a critical velocity greater than the maximum velocity of
the wave remains attached to a node throughout and moves with the
node a distance of 0.83 mm in one cycle. Particle B is unable to be
entrained by the standing wave until the wave velocity falls below
60 mm/min, at 0.4 sec into the cycle, and its movement over the
remaining part of the cycle totals 0.3 mm. particle C can similarly
only be entrained after 0.7 sec travels only 0.075 mm by the end of
the cycle.
Continuing repetition of the cycle progressively improves the
separation into groups and increases the spacing between separated
groups of particles. The efficiency of the process is relatively
independent of the cycle frequency; although shorter cycle times
are preferred it may be found that at frequencies of the order of 4
MHz a cycle time substantially shorter than one second cannot
provide sufficiently long periods of energisation to displace
particles significantly towards a node.
FIG. 5 illustrates in block diagram form the modified driving
circuit for the transducers to produce the variable velocity
pattern shown in FIG. 4.
The transducers may be set up with a liquid column in the same way
as is shown in FIG. 3. The oscillator 12 now drives one of the two
amplifiers 14 through a phase lock unit 32, capable of providing a
chosen phase difference between its input and output, and a phase
shift control 34 that varies that chosen phase difference in
accordance with a desired wave velocity profile. Further details of
such a method of control of the standing wave appear in co-pending
application Ser. No. 751,952 by Michael W. B. Lock filed
simultaneously herewith, that contents of which are incorporated
herein by reference.
By use of a phase shift system it is also possible to establish a
regime in which, in place of the periods of wave suppression shown
in FIGS. 1 and 2 there is a more or less instantaneous change of
phase giving an equivalent displacement of the nodes in a
substantially shorter cycle time. This is illustrated in FIG. 6. It
may be required in such a regime to allow for the inertia of the
particles, although this is small, if relatively abrupt and large
changes of force are imposed on them.
It has already been mentioned that the invention is not only
applicable to chromatography, and an example of its use in a
continuous separation process will now be given with reference to
FIG. 7, which shows a modified liquid column 42 that can replace
the column 2 of FIG. 3. The means for generating an ultrasonic
standing wave are not illustrated, but a variable intensity wave of
the character described with reference to FIG. 4, can be employed,
using the means described with reference to FIG. 5.
The column 42 has a series of ports 44, 46, 48, 50, 52 spaced along
its length between the end plugs 10. The ports 44, 46 are connected
to a circulatory conduit 54 through which liquid is drawn by a pump
56 so that liquid flows through the column from port 46 to port 44.
Liquid is also pumped into the column through port 50 by a further
pump 58 to exit through ports 48 and 52. The particulate matter to
be separated is introduced into the circulatory conduit 54 through
a port 60. The pumping rates are such that the liquid velocity is
greater from port 46 to port 44 than it is from port 50 to port 48,
while there is a low velocity flow in the opposite direction from
port 46 to port 48.
At the region opposite the port 46 particles sufficiently
influenced by the standing wave are picked up by the nodes which
move towards the port 50 as the relative velocity between the
standing wave and the liquid changes from the relatively high value
between the ports 46 and 44 to a relatively low value between the
ports 46 and 48. Above the port 48, there is a counterflow of
liquid so that its velocity relative to the standing wave increases
again, but it is not so great as to cause all the particles to be
shed from the standing wave. Thus, particles so strongly attached
to the nodes as to resist the Stokes forces will continue up the
column with the standing wave but the remainder will leave the
column with the flow through the port 48. That group of particles
continuing upwards past the port 48 is removed from the column by
the flow through the port 52.
In this method of operation two distinct counterflow systems are
established in two successive portions of the column, so that of
the particles drawn off from the circulating flow, two separate
groups are formed. It will be clear from this example that the
apparatus may include further liquid inlet and/or outlet ports
along the length of the column to establish a series of different
velocity regimes, thereby to increase the number of fractions into
which a mixed group of particles is separated in a continuous
process.
* * * * *