Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02524088 2005-10-21
MASS TRANSFER APPARATUS AND METHOD
FIELD OF THE INVENTION
The present invention relates generally to an apparatus and method for
dissolving a gas
into a liquid stream.
BACKGROUND OF THE INVENTION
The introduction of a gas into a liquid stream is common in many different
processing
operations. Accordingly, the present invention finds applications in a number
of fields such as
the treatment of waste and water streams, and disinfecting and clarifying
potable water and
other fluids. The present invention also finds applications in the food and
pharmaceutical
industry, as well as industries requiring products that require efficient mass
transfer of ozone,
air, or other gas for the purpose of flotation, clarification, and/or
disinfection. More specifically,
the introduction of a gas, such as ozone, air or oxygen, into a liquid stream
is common in many
disinfecting, treatment and clarifying processes. Very often, ozone is
introduced into drinking
water sources, ballast water, waste water streams and/or cooling water streams
so as to
disinfect, treat and/or clarify such liquids due to its superior disinfecting
effectiveness over other
gases, such as chlorine. Many different methods and techniques have been
designed to try and
improve the various disinfecting, treatment and clarifying process. When
dealing with ozone,
additional factors must be considered namely, the unstable nature of the gas
which tends to
result in higher equipment and operational costs as well as less compact
systems. Accordingly,
there is a desire to improve the techniques and methods used for introducing,
mixing, blending
and dissolving a gas into a liquid stream, especially for processes that
involve the use of ozone
gas.
Canadian Patent Application No. 2,301,583 (Separation Technologies Group PTY.
LTD.)
discloses a method and apparatus for mixing a first material and a second
material, wherein the
first material comprises a mixture of two or more dissimilar components that
are to be
separated. The '583 application discloses the use of a hydrocyclone to mix
different materials
together prior to their separation. The use of a hydrocyclone in the pre-
treatment of the
materials to be separated was found to improve the subsequent separation of
the materials.
CA 02524088 2005-10-21
The '583 application also discusses the benefits of introducing air or a gas
into the mixture of
materials to be separated prior to the mixture entering the hydrocyclone. More
specifically, the
aeration or gasification of the first material facilitates the separation of
the dissimilar
components in the first material as one of the dissimilar components is
entrained or otherwise
associated with the air or gas bubbles that are formed from mixing the first
and second
materials together. It is the formation of millions of tiny gas bubbles that
facilitates the
subsequent separation of materials as the bubbles entrain or suspend the solid
particles or
droplets, bringing them to the surface during the subsequent processing steps.
The '583
application does not disclose the complete dissolution of a gas within a
liquid, as it relies on the
formation of gas bubbles within the mixed stream to assist in subsequent
separation processes.
As well, the system is not necessarily well suited for the dissolution of
large amounts of ozone in
a liquid stream.
United States Patent No. 6,629,686 (Morse et al.) discloses a process and
system for
dissolving gas into a liquid at greater concentrations and saturations than
previous methods
known in the art. A hydrocyclone is used to introduce an intended gas into the
liquid stream to
be treated. The amount of gas dissolved in the liquid can be optimized by
adjusting various
parameters of the hydrocyclone, namely by altering the pressure of the
incoming liquid,
changing the aspect ratio of the inlet, and varying the diameter D and length
L of the barrel.
Upon exiting the hydrocyclone, the mixed liquid and gas stream enters a
diffusion chamber,
which converts the radial spin of energized liquid from the hydrocyclone into
laminar axial flow.
The diffusion chamber is disposed within a pressure chamber, which includes an
upper gas
region and a lower liquid region. The diffusion chamber is located in the
lower liquid region of
the pressure chamber so that only large bubbles of undissolved gas coalesce
and rise into the
gas region of the pressure chamber, while the dissolved gas and micro-size gas
bubbles that
are retained in the liquid flow with the liquid into the liquid region of the
pressure chamber. The
gas in the upper region of the pressure chamber is recycled back through the
system to the
hydrocyclone so that gas is not unnecessarily wasted, and the liquid and
dissolved gas mixture
can exit the pressure chamber and be held in a storage tank or can be passed
along to the next
process step in the system. While the '686 patent discloses the use of a
pressure chamber, the
pressure chamber does not serve as the primary treatment or disinfection
vessel. Furthermore,
the system does not achieve complete dissolution of the gas into the liquid as
it relies on the
creation of micro-bubbles to distribute the gas evenly through the liquid.
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SUMMARY OF THE INVENTION
The present invention, however, provides an apparatus and method for more
effectively
dissolving a gas into a liquid stream. According to one aspect of the
invention there is provided
an apparatus for dissolving a gas into a liquid stream for the treatment,
disinfection and/or
clarification thereof. The apparatus comprises means for introducing a gas, at
atmospheric
pressure, into the liquid stream to create a mixed stream, and a pump having
an inlet for
receiving the mixed stream of liquid and gas, and an outlet for discharging
the mixed stream at
an elevated pressure. At least one hydrocyclone is connected downstream from
the pump
outlet for more thoroughly mixing and dissolving the gas into the mixed
stream, creating a more
intimately mixed stream, the at least one hydrocyclone including at least one
inlet for receiving
the pressurized mixed stream and having one outlet for discharging the more
intimately mixed
stream. A pressure retention vessel is connected downstream from the
hydrocyclone for
holding the intimately mixed stream at a predetermined pressure for a
predetermined time
period for effectively treating and/or disinfecting the intimately mixed
stream, thereby creating a
treated stream. The pressure retention vessel has an inlet for receiving the
intimately mixed
stream from the hydrocyclone, a first outlet for discharging the treated
intimately mixed stream,
and a second outlet for discharging any residual gas that has escaped from the
intimately mixed
stream. Pressure control means are provided in communication with the at least
one
hydrocyclone and the pressure retention vessel for adjusting the pressure of
the mixed and
intimately mixed streams to ensure effective dissolution of the gas within the
liquid stream.
According to another aspect of the invention there is provided a method for
dissolving a
gas into a liquid stream comprising the steps of (i) injecting a gas into a
liquid stream at
atmospheric pressure to create a mixed stream, (ii) pressurizing the mixed
stream to a
predetermined level, (iii) directing the mixed stream into a hydrocyclone to
create a more
intimately mixed stream, (iv) directing the intimately mixed stream from the
hydrocyclone to a
pressure retention vessel and holding the intimately mixed stream in the
pressure retention
vessel at a predetermined pressure for a predetermined time period to ensure
the proper
disinfection or treatment of thereof, thereby creating a treated stream.
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BREIF DESCRIPTION OF THE DRAWINGS
The present invention will be better understood with reference to the detailed
description
taken in combination with the drawings in which:
Figure 1 is a schematic view of the mass transfer apparatus according to a
preferred
embodiment of the invention;
Figure 2 is a partial cutaway view of the hydrocyclone of the mass transfer
apparatus of the
present invention;
Figure 3A is an elevation view of a spin inducer used in conjunction with the
hydrocyclone of
the mass transfer apparatus according to the present invention;
Figure 3B is another elevation view of the spin inducer as seen 90°
from the view shown in
Figure 3A;
Figures 3C-3D are respective top and bottom views of the spin inducer of
Figures 3A-3B;
Figure 4A is a is a cross-sectional view of a hydrocyclone liner used in the
hydrocyclone of
Figure 2;
Figure 4B is a side view of the spin inducer of Figures 3A-3D attached to the
hydrocyclone
liner;
Figure 4C is a top view of the hydrocyclone liner;
Figure 4D is a cross-sectional view of the configuration of Figure 4B showing
the flow
characteristics inside the spin inducer and hydrocyclone liner;
Figure 5 is a side view of the pressure retention vessel of the mass transfer
apparatus of the
present invention; and
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Figure 5A is a top view of the pressure retention vessel of Figure 5.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring to the drawings, there is shown in Figure 1 a schematic of the mass
transfer
apparatus 10 according to the present invention. The apparatus includes a
liquid feed line 12
located proximal to an untreated liquid source 11 and connected to the suction
side of a
progressive cavity or helical rotor pump 14. Liquid entering the liquid feed
line 12 is controlled
by a foot valve 16 located at the inlet end of the liquid feed line 12, the
foot valve 16 being in
communication with the untreated liquid source 11. A second valve 18 located
in the feed line
12 downstream from the foot valve 16 controls the amount of untreated liquid
entering the pump
14. When in operation, valve 18 is pinched, or partially closed, so as to
maintain a slight
vacuum at the pump inlet. A pressure indicator 20 is mounted at the inlet to
the pump 14, which
is designed to show the amount of vacuum present at the pump inlet or suction.
A gas feed line
22 is provided for introducing the intended gas, at atmospheric pressure, into
the liquid stream.
Ozone, air, oxygen or any other suitable gas, depending on the type of
process, may be used.
The apparatus 10 is particularly well-suited for use with ozone gas since the
gas is introduced at
atmospheric pressure. Due to the unstable nature of ozone gas, handling of the
gas becomes
more difficult when higher pressures are required; therefore the present
invention avoids any
such difficulties as the ozone is introduced at atmospheric pressure.
The gas feed line 22 connects with the liquid feed line 12 at junction 23 to
create a
mixed stream 24 of liquid and gas which then enters the pump 14. A gas flow
meter (or
rotameter) 25 and needle valve 26 are used to control and provide a visual
reading of the
amount of gas that is being introduced into the liquid stream. Progressive
cavity/helical rotor
pumps are able to accept the mixed stream 24 with the entrained vapours/gas
without
detrimental cavitation, which is what makes this type of pump ideal for use in
the subject
apparatus. Once the gas has been introduced into the liquid stream and the
mixed stream 24
enters pump 14, the liquid and gas are pressurized to between about 80-150
psig depending on
the type of gas and liquid stream involved in the process. When ozone is the
gas being used,
for instance in a water treatment process, the mixed untreated water ozone
stream is
pressurized to about 150 psig. This pressure has been found to be optimal for
ozone, as much
more ozone can be dissolved into the liquid at this pressure, thereby
increasing its effectiveness
CA 02524088 2005-10-21
as a disinfectant. Conventional mass transfer systems have been unable to
achieve the same
level of dissolution of ozone into the liquid stream.
If high-pressure gas (i.e. more than 150 psig) other than ozone is being
introduced into
the liquid stream, an alternate set-up can be used where a gas feed line 22'
connects with the
liquid stream on the discharge side of the pump 14 (as opposed to the suction
side of the pump
14) at junction 23' to create mixed steam 24' on the discharge side of the
pump 14. Once the
mixed stream 24 (24') has been created and is pressurized to the desired
level, the mixed
stream 24 (24') enters a shearing hydrocyclone 28 where the gas is further
sheared and
dissolved and therefore is more completely mixed with the liquid. Once again,
this system
proves advantageous when using ozone as the gas, since the ozone is completely
dissolved in
the liquid rather than being diffused or bobbled into the liquid, as is common
with many
conventional mass transfer systems. Complete dissolution of the ozone gas into
the gas is
preferable as it provides the most complete contact with the liquid for more
effective
treatment/disinfection thereof.
As shown in Figure 2, the hydrocyclone 28 comprises an outer housing vessel 54
that is
divided into two sections by mounting plate 56. The hydrocyclone vessel 54 can
have one or
more tangential inlet ports 30, which may be equipped with ramps to initially
induce a rotational
flow at the head of the hydrocyclone 28. The vessel 54 contains one or more
hydrocyclone
liners 58, depending on the desired flows and pressures of the system. Not
only can one or
more hydrocyclone liners 58 be enclosed in one vessel 54, but more than one
vessel containing
a number of hydrocyclone liners can be used depending on the size and
economics of the
apparatus.
A spin inducer 60 (see Figures 3A-3D) is also housed within the vessel 54 and
is
attached to the upper portion of the hydrocyclone liner 58. The spin inducer
60 includes one or
more inlet openings 62 in communication with the one or more tangential inlet
ports 30 of the
vessel 54. As the mixed stream of liquid and gas enters the hydrocyclone 28
through the one or
more tangential inlet ports 30, it is directed towards the openings 62 of the
spin inducer 60,
which force the mixed stream 24 (24') of liquid and gas to travel in a
circular motion. According
to one embodiment, the spin inducer 60 is secured to the hydrocyclone liner 58
by means of a
flexible lip 64 (Figure 3B) located on the bottom rim of the spin inducer 60
which mates with a
corresponding lip 65 (Figure 4A) on the hydrocyclone liner 58, when the spin
inducer 60 is made
of a flexible material such as polyurethane. Alternate materials for both the
spin inducer 60 and
the hydrocyclone liner 58 include various grades of stainless steel. If the
material being used
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for the spin inducer 60 is of a rigid nature, such as steel or ultra high
molecular weight
polyethylene, the spin inducer 60 is preferably threaded to the hydrocyclone
liner 58. The
hydrocyclone liner 58 with the spin inducer 60 attached thereto is shown in
Figure 4B.
From the spin inducer 60, the liquid and gas mixed stream 24 enters the neck
of the
hydrocyclone liner 58. The reducing internal diameter of the hydrocyclone
liner 58 (see Figures
4A and 4C) causes the gas and liquid mixed stream 24 (24') to accelerate to
the single outlet 32
of the hydrocyclone 28. The typical flow pattern created by the hydrocyclone
liner 58 is shown
in Figure 4D. The cyclonic action of the entire feed stream (i.e. the liquid
and gas mixed stream
24) as it enters the hydrocyclone 28 promotes instantaneous, intimate contact
between the
liquid and the gas. As the mixture accelerates, any gas bubbles are sheared,
then dissolved,
and are dispersed evenly throughout the liquid forming a homogeneous, stable,
aerated and
blended product stream or more intimately mixed/dissolved stream 34. With no
other exit or
outlet provided in the hydrocyclone 28 for the less dense, entrained gas to
escape, the gas
follows the liquid to the only outlet 32 provided which ensures the thoroughly
mixed/dissolved
and blended product stream 34 at the outlet 32 of the hydrocyclone 28.
Referring back to the Figure 1, the more intimately mixed/dissolved stream 34
of
completely dissolved gas and liquid exits the hydrocyclone 28 via outlet 32
and is directed
toward a pressure retention vessel 36. The intimately mixed/dissolved stream
34 remains in the
pressure retention vessel 26 for a pre-determined period of time required for
the proper
disinfection or treatment of the intimately mixed/dissolved stream 34 to
create a treated stream
44. The pressure within the pressure retention vessel 36 is maintained at a
predetermined level
to ensure that the gas remains completely dissolved in the liquid, and is not
permitted to
escape. This provides for more effective disinfection and/or treatment of the
intimately
mixed/dissolved stream 34 as there is more complete contact between the gas
and the liquid to
be treated. This is particularly true in the case of ozone. As well, the gas -
liquid (e.g. ozone -
liquid) contact time required in the present system is significantly reduced
due to the complete
dissolution of the gas within the liquid which, therefore, decreases the
overall "treatment time".
Furthermore, various sizes of pressure retention vessels may be used which
allows for more
complete usage of the gas. In the case of ozone gas, the more complete usage
of the gas
reduces ozone generation capacities for any given treatment or disinfection
operation.
The pressure across the hydrocyclone 28 and the pressure retention vessel 36
is
controlled by a back pressure control valve 37 located downstream of the
pressure retention
vessel 36. The back pressure control valve 37 can be hand controlled,
controlled by a
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programmable logic controller (PLC), or controlled by a conventional pressure
control loop. A
pressure indicator 38 is provided at the inlet to the hydrocyclone 28, which
provides a reading of
the pressure of the mixed stream 24 (24') as it enters the hydrocyclone 28. A
second pressure
indicator 40 is located downstream of both the hydrocyclone 28 and the
pressure retention
vessel 36 which shows the pressure at the outlet 32 of the hydrocyclone 28 as
well as the
pressure within the pressure retention vessel 36.
As shown in Figures 5 and 5A, the pressure retention vessel 36 includes an
inlet 66 for
receiving the intimately mixed/dissolved stream 34, and has two outlets 68,
70. The first outlet
68 is for the disinfected/treated intimately mixed/dissolved stream or treated
stream 44 and the
second outlet 70 provides a means for evacuating any residual gas that may
have escaped from
the liquid or accumulated in the pressure retention vessel 36. The gas is
evacuated through the
second outlet 70, and can then be recycled through a vapour-return line 42 to
the inlet or
suction side of the pump 14, so that no gas is wasted. As is shown more
clearly in Figure 5, the
first outlet 68 extends into the pressure retention vessel 36 so that it is in
contact with the liquid
in the vessel. This ensures that only the liquid, treated stream 44 exits
through the first outlet
68.
Once the disinfection/treatment period is complete, the treated stream 44 can
be
directed to a storage tank or can be put through additional processing steps.
It is only once the
disinfection/treatment period is complete that the pressure downstream of the
pressure retention
vessel is reduced, thereby allowing any remaining vapours to be released in
micro-bubbles,
which promotes additional contact between the liquid and the gas. If the
treated stream 44 is
going through additional processing steps, the micro-bubbles that are released
as the pressure
is reduced not only serve to promote further contact between the liquid and
the gas, but also
serve to facilitate additional processing steps. For instance, the treated
stream 44 can be
directed from the pressure retention vessel 36 and fed into a dissolved air
flotation system 46
(shown in dotted lines in Figure 1 ) for further treatment where the micro-
bubbles act as a gas
supply for the additional processing steps. The dissolved air flotation system
46 produces a
purified stream 47. When the gas being used is ozone, the purified stream 47
from the
dissolved air flotation system then passes through a degassing vessel 48. In
the degassing
vessel 48, any residual ozone gas is separated out of the stream 47 and is
directed to an ozone
destruct chamber 49 for a final treatment before being released from the ozone
destruct
chamber as air 50. The purified stream 47 exits the degassing vessel 48 as a
disinfected, clean
effluent stream 52, in accordance with practices known in the art.
Alternatively, the treated
CA 02524088 2005-10-21
stream 44 from the pressure retention vessel can pass directly to the
degassing vessel 48 and
ozone destruct chamber 49. As well, a portion of the disinfected/treated
intimately mixed
stream 44 can also be recycled back into the liquid feed line 12 via a liquid
return line 54 as it
exits pressure retention vessel 36.
While the present invention has been described with respect to certain
preferred
embodiments, it will be understood by persons skilled in the art that
variations or modifications
can be made without departing from the scope of the invention as described
herein.
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