Note: Descriptions are shown in the official language in which they were submitted.
CA 02250888 2002-04-02
TWO-STAGE SEPARATION PROCESS
Field of the Invention
The present invention is directed to a separation process for an oxidized
liquid effluent
mixture from a subcritical wet oxidation process and, more particularly, to a
two-stage
separation process for separating the oxidized liquid effluent mixture.
Background
Wet oxidation is well known for the treatment of aqueous wastewaters. The
process
I o generally involves heating a mixture of the wastewater and an oxygen-
containing gas to effect
oxidation of oxidizable substances contained in the wastewater. When air is
used as the
source of oxygen-containing gas, the wet oxidation process is generally
referred to as "wet air
oxidation."
Generally, subcritical wet oxidation systems include a wet oxidation chamber
followed
~ 5 by a single separator vessel, with a pressure control valve therebetween
to maintain the
oxidation chamber pressure. When the oxidation process is terminated, an
oxidized effluent
mixture traverses the pressure control valve to the separator vessel. The
pressure drop
between the wet oxidation system and the separator vessel causes the mixture
to separate into
an oxidized liquid effluent and a gaseous phase.
20 Low pressure separators generally operate at, or near, ambient pressure. If
a single low
pressure separator vessel follows the wet oxidation system, the oxidized
liquid effluent is
generally acceptable for downstream processes (such as for pumping, or for
storage in tanks).
However, the gaseous phase produced from a single low pressure separation
vessel is
disadvantageous because it is at a low pressure, and requires additional
equipment and energy
25 to transport to downstream processes. In some instances, the gaseous phase
may require
further treatment, and potentially the expenditure of significant amounts of
energy and
expense for transport to and through subsequent treatment processes. Moreover,
the gaseous
phase from a single low pressure separator vessel generally contains a
significant amount of
water vapor. The water vapor also adds expense to downstream treatment of the
gaseous
3o phase, for example, increasing the energy requirement in a high temperature
afterburner.
Condensation of the water vapor on the interior of equipment may also cause
corrosion to the
equipment that comes into contact with the gaseous phase.
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High pressure separators generally operate in the range of about 50 to 100
psig. If a
single high pressure separation vessel is used, the energy expenditure for
moving the gas
would be eliminated, as the gaseous phase would retain sufficient pressure to
provide the
motive force to be transported to another location. However, in this instance,
the oxidized
liquid effluent may not be suitable for discharge to downstream processes,
because the over
pressure may cause a significant amount of gases, including CO2, N~, and Oz,
to remain
dissolved in the oxidized liquid effluent. When such as oxidized liquid
effluent is stored in a
low pressure covered collection tank, for example, the dissolved gases
remaining therein will
come out of solution and collect within the tank, causing operational
problems. Several
~ o techniques associated with various high pressure or temperature processes,
including wet
oxidation, are reported below.
U.S. Patent No. 3,150,105 discloses a blow down tank to receive cooled
regenerated
carbon slurry from a wet oxidation reactor.
U.S. Patent No. 3,994,702 discloses a flooded sluicing chamber for ash removal
from
a pressurized gasification chamber.
U.S. Patent No. 4,620,563 discloses a blowdown pot with an inlet pressure
control
valve through which the pot receives unwanted residue e.g., ash from a high
pressure chemical
reactor.
U.S. Patent No. 5,011,114 discloses a pressure control valve with a valve seat
and
2o support assembly extending beyond the valve body to prevent erosion by the
blowdown slurry.
U.S. Patent No. 5,389,264 discloses a process for dissipating the energy of a
wet
oxidation mixture and preventing erosion of the phase separator vessel after
that stream
traverses a pressure control valve.
A need remains, however, for an improved process for providing an essentially
gas-
free oxidized liquid effluent with a gaseous phase with a reduced water vapor
concentration,
from a subcritical wet oxidation system. Lastly, there is a need to reduce or
eliminate the
energy requirement for transporting the gaseous phase to further treatment.
Summary of the Invention
3o The present invention is directed to a process for separating the discharge
from a
subcritical wet oxidation process. The process involves providing an oxidized
liquid effluent
mixture by reacting an aqueous waste stream containing oxidizable materials
with an oxidant
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at a first superatmospheric pressure. A first oxidized liquid effluent and a
first gaseous phase
are provided by subjecting the oxidized liquid effluent mixture to a first
pressure drop from
the first superatmospheric pressure to a second superatmospheric pressure. A
second oxidized
liquid effluent and a second gaseous phase are provided by subjecting the
first oxidized liquid
effluent to a second pressure drop from the second superatmospheric pressure
to essentially
atmospheric pressure. The pressure exerted on the first gaseous phase provides
energy for
transporting the first gaseous phase from a first location to a second
location.
In another aspect of the invention, the process involves separating the
discharge from a
subcritical wet oxidation process. The process involves discharging a cooled
wet oxidation
1 o mixture of liquid and gases from a subcritical wet oxidation system
operating at
superatmospheric pressure, through a first pressure control valve to a first
separator vessel
maintained at a superatmospheric pressure lower than the oxidation system, to
form a
superatmospheric first gaseous phase and a first oxidized liquid effluent
therein. The
superatmospheric first gaseous phase is discharged from the first separator
vessel to further
~ 5 treatment or to the environment. The first oxidized liquid effluent is
discharged from the first
separator vessel, through a first level control valve for controlling the
liquid level in the first
separator vessel, to a second separator vessel maintained at essentially
atmospheric pressure,
to form a second gaseous phase and an essentially degassed second oxidized
liquid effluent
therein. The second gaseous phase is discharged from the second separator
vessel, and
2o the essentially degassed second oxidized liquid effluent is discharged from
the second
separator vessel.
Brief Description of the Drawings
Preferred, non-limiting embodiments of the present invention will be described
by way
25 of example with reference to the accompanying drawings, in which:
FIG. 1 is a schematic illustration of a wet oxidation system including the
separation
system of the present invention.
Detailed Description of the Invention
3o The present invention is directed to an improved separation process for
treating the
oxidized effluent from a subcritical wet oxidation system to provide an
essentially gas-free
oxidized liquid effluent and a gaseous phase with a reduced water content. The
process also
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reduces or eliminates the energy requirement for transporting the gaseous
phase from a first
location to a second location for further treatment, to the environment, or to
waste.
"Sub-critical wet oxidation," as used herein, is used as it is known in the
art, which is
as a "moderate" treatment process performed at a temperature below the
critical temperature
of water, and at a superatmospheric pressure below the critical pressure of
water. The critical
temperature of water is 374 ° C and the critical pressure of water is
3193 psig (217 atm(g)).
Typical operating conditions for wet oxidation systems include temperatures
ranging from
about 150° C to about 350° C, and pressures of at least about
100 psig (6.8 atm(g)) up to
about 3,500 psig (238 atm(g)). "Gage pressure," is used herein conventionally,
meaning that
~ o the pressure values are with reference to atmospheric pressure.
The destruction of organics via subcritical wet oxidation processes takes
place through
oxidation/reduction reactions, typically providing oxidation efficiencies in
the range of about
50 to 80 percent. Therefore, discharges from subcritical wet oxidation
processes may require
further treatment to remove residual organics before they can be discharged to
the
I5 environment, or other downstream processes. As described previously, the
discharge from a
subcritical wet oxidation system is generally separated into an oxidized
liquid effluent and a
gaseous phase in a single separation vessel before being routed to further
treatment.
According to the process, a wet oxidized mixture is made to flow sequentially
through
two separator vessels, which provides an essentially gas-free oxidized liquid
effluent, and a
2o gaseous phase that has a reduced water vapor content, while reducing or
eliminating the
energy requirement for transporting the gaseous phase from a first location to
a second
location for further treatment, or to the environment. The reduced water vapor
content is a
result of the superatmospheric pressure used during first separation, which
prevents the water
from evaporating and going into the first gaseous phase. The term "reduced
water vapor
25 content," as used herein, means a reduced water vapor concentration,
relative to the gaseous
phase discharged from a system without a pressurized separator, and relative
to the second
gaseous phase discharged after the second separation which is performed at
essentially
atmospheric pressure. The reduction in water vapor content reduces the cost of
additional
downstream gaseous phase treatment, such as passage through a high temperature
afterburner.
3o The improved separation process of the present invention is shown with
reference to
FIG. 1. A wet oxidized mixture at a first superatmospheric pressure,
preferably from a
subcritical wet oxidation system, flows from the wet oxidation system (not
illustrated), into a
CA 02250888 2002-04-02
conduit 10, and through a pressure control valve 12. As the mixture traverses
the pressure
control valve 12, it is subjected to a first pressure drop to a second
superatmospheric pressure.
The mixture flows through another conduit 14 and into a first separator vessel
16, where the
partially depressurized mixture separates into a first oxidized liquid
effluent and a first
gaseous phase. A demister device 24, which is known in the art, may be used to
assist in
phase separation within the first separation vessel 16. The pressure within
the first separator
vessel 16 is maintained at a relatively low superatmospheric pressure by a
pressure controller
18, which operates a pressure control valve 20. Valve 20 controls the flow of
the first gaseous
phase through outlet conduit 22. The first gaseous phase exits near the top of
the first
1 o separator vessel 16. A gas/liquid scrubber 21 may be located between the
first separator
vessel 16 and the pressure control valve 20 to condition the first gaseous
phase stream flowing
through the outlet conduit 22 prior to traversing the pressure control valve
20.
Preferably, the mixture is partially depressurized in the first separator
vessel 16 to at
least about 10 psig (0.68 atm(g)), although higher pressures may be used, or
required, in some
instances. The partially depressurized first gaseous phase retains sufficient
motive force to be
transported to any downstream location, including passage through all
necessary control
valves. Also in the first separation vessel 16, the first oxidized liquid
effluent is maintained at
a desired level by a level controller 26, which operates a level control valve
28. Valve 28
controls the flow of the first oxidized liquid effluent through outlet conduit
30, where the first
oxidized liquid effluent exits the first separator vessel 16. In the first
separator vessel 16, the
majority of the noncondensible gases from the mixture enters the first gaseous
phase.
However, a portion of noncondensible gases may remain dissolved in the first
oxidized liquid
effluent, while it is contained in the first separator vessel 16, which is
maintained at a
superatmospheric pressure of at least about 10 psig (0.68 atm(g)).
As more of the oxidized effluent mixture from the wet oxidation system enters
the first
separator vessel 16, the pressure and liquid level in the first separator
vessel 16 rises. The
pressure controller 18 maintains the desired pressure of at least about 10
psig (0.68 atm(g)) in
the first separator vessel 16 by modulating the flow of the first gaseous
phase through the
conduit 22 using the pressure control valve 20. As the liquid level rises,
which is sensed by
3o the level controller 26, a portion of the first oxidized liquid effluent,
at the second
superatmospheric pressure, flows through the exit conduit 30, through the
level control valve
28, and through a conduit 32, to a second separator vessel 34.
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The pressure in the second separator vessel is maintained at essentially
atmospheric
pressure. As the first oxidized liquid effluent traverses the pressure control
valve 20, it is
subjected to a second pressure drop to essentially atmospheric pressure.
Therefore, as the first
oxidized liquid effluent flows into the second separator vessel 34, the
majority of dissolved
gases remaining therein come out of solution, and are transferred to a second
gaseous phase.
The second gaseous phase may be combined with the first gaseous phase, or it
may exit the
second separator vessel 34 via a vent conduit 36. Preferably, a demister
device 38 may be
used to retain fine droplets of liquid in the second separator vessel 34. The
liquid level in the
second separator vessel 34 is maintained at a desired level by a second level
controller 40,
1o which operates a second level control valve 42. Valve 42 controls the flow
of the second
oxidized liquid effluent through conduit 44, where it exits the second
separator vessel 34. A
centrifugal pump 43 may be connected to outlet conduit 44, ahead of the level
control valve
42, to assist in removing liquid from the second separator vessel 34.
As previously discussed, the majority of gases contained in the wet oxidation
mixture
separate into the first gaseous phase in the first separator vessel 16.
Sufficient pressure is
maintained in the first gaseous phase to provide the motive force to transport
the first gaseous
phase to further treatment, or waste, without the expenditure of additional
energy. Moreover,
smaller vessels and piping may be used when handling pressurized gases,
providing an
economic benefit for the process. Additionally, as a result of the
superatmospheric pressure of
2o the first separation, the first gaseous phase has a reduced water vapor
concentration relative to
the gaseous phase discharged from a system without a pressurized separator,
and relative to
the second gaseous phase discharged from the second separator of the present
process. The
reduced water vapor concentration the cost of additional downstream gaseous
phase treatment,
such as passage through a high temperature afterburner. Likewise, corrosion of
the gaseous
2s phase handling equipment in contact with water condensed from the gaseous
phases is
minimized by low humidity gases. Thus, the first superatmospheric separation
improves the
handling and further treatment of the first gaseous phase.
After discharging the first oxidized liquid effluent to the second separator
vessel 34,
operating at essentially atmospheric pressure, the majority of any remaining
dissolved gases in
3o the first oxidized liquid effluent come out of solution, and are
transferred to the second
gaseous phase. Therefore, after the second separation, the second oxidized
liquid effluent is
essentially free of dissolved gases. The second oxidized liquid effluent exits
from the second
CA 02250888 2002-04-02
7 _
separator vessel 34 via the exit conduit 44.
As stated previously, the majority of noncondensible gases are separated into
the first
gaseous phase, and only the gases remaining dissolved in the first oxidized
liquid effluent are
liberated within the second separator vessel 34. Therefore, the volume of the
second gaseous
phase generated in the second separator vessel 34 is generally quite small in
relation to the
volume of the first gaseous phase. Moreover, as the second oxidized liquid
effluent is at a low
pressure, problems associated with dissolved gases coming out of solution from
the second, or
final, oxidized liquid effluent in downstream treatment or storage are
avoided. Finally, the
first gaseous phase does not require the expenditure of additionally energy
for transport.
In another aspect of the invention, the pH of the first or second oxidized
effluent may
be adjusted to a preselected range. Generally, it is desirable to adjust the
final oxidized
effluent to a pH at or near neutral prior to discharging to a downstream
process. However, in
some instances it may be desirable to discharge an extremely acidic or basic
final oxidized
liquid effluent to a downstream process. The pH adjusting steps will
accommodate either of
the previous situations. Accordingly, an appropriate pH adjusting substance,
either acidic or
basic, may be added to the first oxidized liquid effluent after it has passed
from the first
separator vessel 16 through the level control valve 28. The pH adjusting
substance, stored in a
tank 50, may be added to the first oxidized liquid effluent in the conduit 32
from a conduit 52
supplied from a pump 54 drawing from the tank 50. The amount of pH adjusting
substance
2o added is controlled by a pH controller 56 which monitors pH downstream of
the addition
point, such as within the second separator vessel 34.
For example, the first oxidized liquid effluent from the first separator
vessel 16 may
have a basic pH. Since further biological treatment requires a near neutral
pH, the pH must be
adjusted beforehand. The pH controller 56 adjusts the addition of acidic
material to maintain
the pH within the second separator vessel in a pH range of about 6 to about 8.
In this instance,
an acidic solution such as sulfuric or hydrochloric acid, may be added to the
first oxidized
liquid effluent in conduit 32, to neutralize the basic components to produce a
second oxidized
liquid effluent with a pH in the desired range, suitable for discharge.
Similarly, an acidic first
oxidized liquid effluent may require the addition of a solution of basic
substance, such as
3o caustic soda or metal carbonate, to neutralize the liquid to a pH suitable
for discharge. Any
addition of material to adjust the pH at this stage may result in the
generation of additional
gases, such as CO2, in the second separator vessel 34.
CA 02250888 2002-04-02
g
In an alternative embodiment of the invention, the pH adjusting substance may
be
added to the wet oxidation mixture after it traverses the first pressure
control valve 12, as it
flows through the inlet conduit 14 to the first separator vessel 16. The pH
adjusting substance
may be added, for example, from a conduit 58, to which the pH adjusting
substance is
supplied from the pump 54, drawing from the tank Sd, to produce a first
oxidized liquid
effluent in the first separator vessel 16 with a pH in the preselected range.
Any addition of
material to adjust the pH at this stage may also result in the generation of
additional gases,
which may enter the first gaseous phase.
While the invention has been particularly shown and described with reference
to a
1 o preferred embodiment thereof, it will be understood by those skilled in
the art that various
changes in form and details may be made therein without departing from the
spirit and scope
of the invention.
