Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Title: ADIABATIC SEPARATOR
BACKGROUND OF THE INVENTION
(i) Field of the invention:
The present invention relates to an adiabatic separator for
the separation by condensation of a portion of a
homogeneous gas or of a condensible gas from a carrier gas,
more particularly for the separation of water vapour from
industrial flue gases.
This invention also relates to a process of such separation
and in particular to a process of removal of water vapour
from ~dehumidification of) exhaust-flue gases especially
where these latter also contain the pollutant sulphur
dioxide which may be simultaneously removed by its
dissolution in the condensed water, prior to discharge of
the flue gases into the atmosphere.
(ii) Description of the Prior Art:
The blades of steam turbines, used for instance in nuclear
power generators, suffer significant mechanical erosion
("pitting") problems owing to the impingement of condensed
water droplets on the fast rotating turbine blades. This
erosion requires that the turbine blades be replaced
annually at high cost. It would be desirable therefore to
supply such turbines with a "higher quality" steam, i.e.
steam at a slightly higher temperature than usual in order
to reduce the amount of droplets forming by condensation
within the turbines.
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Another branch of the power generation industry concerns
power boilers.
The majority of power boilers in North America are coal
fired units, in which ground coal is burned with air,
producing hot combustion ("flue") gases.
From the initial combustion temperature of approximately
3500 F, the flue gas loses its temperature towards the
boiler's exit and leaves the boiler proper at approximately
300 to 400~F. After leaving the boiler's periphery, the
flue gas enters a stack and is discharged directly to the
atmosphere.
These high temperatures allow considerable quantities of
air pollutants to be carried into the atmosphere with the
flue gas. The most prominent among all air pollutants are
solid particles of coal ash and gaseous sulphur dioxide.
Contamination of the atmosphere with sulphur dioxide has
proved to have devastating environmental effects as it is
directly responsible for so-called "acid rain".
Ever stringent air pollution control requirements enacted
by governments in recent years, have forced industries to
reduce the level of solid pollutants discharged into the
atmosphere. The technology that has been developed for
this purpose involves a so-called "scrubbing" process.
In such an arrangement, flue gases leaving the power
boiler pass through a special chamber called a "scrubber"
where large quantities of cold water are injected into the
flue gas stream causing coagulation of the solid ash
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particles. These particles subsequently collect in the
bottom of the scrubber and thus are effectively removed
from the gas stream. After being removed from the scrubber
the wet ash is then transported to a dump site for
permanent disposal.
The scrubbing process appears to be very effective in
removal of solid particles from the flue gas stream, but
has some undesirable side effects.
A significant portion of the cold water sprayed inside the
scrubber evaporates after coming in contact with the hot
flue gases. This effectively reduces the temperature of
the flue gases and leaves them saturated with moisture.
Upon leaving the stack the wet flue gas comes into direct
contact with cold ambient air causing instant precipitation
of the moisture resulting in a very environmentally
objectionable plume of white "smoke".
An additional drawback of the scrubbing process is its
inability to reduce the content of sulphur dioxide in the
gas stream. Instead the sulphur dioxide tends to remain
in gaseous form on leaving the scrubber. This is largely
because the temperature of the flue gas is still too high
for dissolution of sulphur dioxide in liquid water.
OBJECTS OF THE INVENTION
In the field of steam turbines, it is a first object to
reduce water droplet formation in steam turbine interiors
by raising the temperature of inlet steam at the expense of
a small portion of the same steam removed by prior
condensation.
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It is another object of the present invention to provide an
arrangement capable of raising the temperature of part of
the inlet steam before it enters the turbine by prior
removal by condensation of a small part of this steam.
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In the field of power boilers, it is an object of the
present invention to provide an arrangement capable of
eliminating undesirable side effects of the wet scrubbing
process without affecting performance of the scrubber
itself.
It is another object to provide an arrangement capable of
eliminating moisture from the flue gases prior to their
discharge into the atmosphere, without introducing an
additional cooling medium.
It is a further object to provide an arrangement capable of
reducing the quantity of sulphur dioxide in the flue gas
stream, prior to its discharge into the atmosphere.
In seeking solutions to these goals, the present inventor
appreciated the advantages which would accrue using the
principle of kinetic cooling. This principle is well
exemplified by placing thumb and forefinger either side of
a metal tire valve through which air is escaping. After a
short period, the temperature of the valve drops as the
escaping air, forced as it is to pass through a restricted
orifice, travels at a much higher velocity through the
valve than either inside or outside the tire. The increased
velocity of the air means an increase in its kinetic
energy. Following the principle of conservation of energy,
this increase in kinetic energy of the air equals a loss of
its heat energy. This loss of heat energy is reflected by a
drop in temperature of the fast flowing gas. This, in
turn, cools the heat-conducting valve through which the air
lS passing.
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In his search for practical embodiments of this idea, it
became clear to the inventor that the same adiabatic
separator could be used not only for the phase separation by
condensation of a portion of a homogeneous gas but also for
the separation by condensation of a condensible gas from a
non-condensible carrier gas.
Therefore, it became a general object of the invention to
provide an adiabatic separator for the separation by
10 condensation of a portion of a homogeneous gas or of a
condensible gas such as water vapour from a heterogeneous
mixture of a carrier gas with the condensible gas.
Likewise, it is an object to provide for a process using
this separator, particularly for water removal.
SUMMARY OF THE INVENTION
In meeting these and other objects, the present invention
provides an adiabatic separator for the separation by
20 condensation of a portion of a homogeneous gas or of a
condensible gas from a carrier gas, said separator
comprising:
- an elongated vessel defining an interior space and having
a closeable condensate outlet, a gas inlet port and a gas
exit port;
- a plurality of heat-conducting tubes extending in said
interior space, each of said tubes having an inlet end
opening into said space at a point distant from said inlet
port to cause incoming gases to pass over said tube before
30 reaching said inlet ends, each of said tubes also having an
outlet end opening into said exit port, said exit port being
otherwise hermetic, each of said tubes further having a
substantial portion thereof within said vessel which is of
reduced internal cross-sectional area to ensure a higher
velocity for said gases within said tubes than over said
tubes along said portions of reduced cross-section area; and
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- means for providing a pressure difference between said
ports to force said gases into said vessel and through said
tubes.
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The invention also provides a process of separating a
portion of a condensible gas (preferably steam~ by passing
the gas through the separator mentioned above. This
results in a portion of higher temperature and a condensed
portion.
The invention also provides a process of separating a
condensible gas from a carrier gas by passing the mixture
of gases through a separator as above-mentioned.
The invention particularly provides a process of removing a
water soluble gas (e.g. S02) from a carrier gas carrying
both water vapour as well as the water soluble gas to be
removed. Again, this process comprises passing the gaseous
mixture through a separator as described above.
In particular, the invention provides a process for the
removal of a high proportion of the sulphur dioxide
contained in flue gases emanating from a power boiler. As
before, this process comprises passing the flue gases
through the separator described above, but in this
embodiment the water condensed from the flue gases in the
separator dissolves the sulphur dioxide to form sulphurous
acid removeable from the vessel via its condensate outlet.
In one embodiment, the present invention is an arrangement
similar in appearance to a conventional Tube and Shell type
heat exchanger. However, the distinguishing feature of the
present invention is its adiabatic function i.e. there is
no net heat transfer from the input gas to any other
cooling medium (e.g. cooling water) which would be the case
in a typical heat exchanger. In other words, the separator
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creates the cooling effect necessary for precipitation of
the condensible gas (e.g. water vapour) by accelerating
the very same gas rather than employing an external
cooling medium such as cooling water.
The present invention therefore has the advantage of not
requiring an external cooling medium.
Following once more the principle of conservation of
energy, the gas or gases leaving the vessel at the exit
port are at a higher temperature than those entering. This
facet is of great advantage for instance in the steam
turbine embodiment already discussed.
The invention is now explained in terms of its use in
conjunction with flue gases emanating from a power boiler.
This is not to be taken as indicating that such is the only
use of the present invention whose scope is only limited by
the claims at the end of this disclosure.
The separator according to the invention may be attached
directly downstream (with respect to flue gas direction)
from a boiler furnace or there may initially be a wet
scrubber (as described above) for prior removal of solid
ash.
In the second case, moist flue gas leaving the wet scrubber
enters the adiabatic moisture separator at one end
(likenable to the "shell side" in the above discussed Tube
and Shell heat exchanger). Here, it is caused to pass over
the tubes and is thereby exposed to their cold surface.
Moisture contained in the flue gas condenses on the cold
surface of the tubes and is eventually collected at the
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bottom of the separator. The low temperature condensate
precipitating on the cold surface of the tubes creates a
functional environment for removal of sulphur dioxide,
which readily dissolves in the condensate forming low
concentration sulphurous acid.
Acidic condensate collected at the bottom of the separator
is removed from the system through the condensate outlet
specially provided for this purpose.
Upon reaching the opposite end of the adiabatic moisture
separator where the narrow tubes begin, the flue gases, by
now devoid of most of their moisture and sulphur dioxide
content, enter the internal passages of the tubes i.e. the
"tube side" in the above discussed analogy. Here, under
the influence of negative pressure (suction) applied at the
opposite end of tube passages, the dried flue gas is
accelerated to a high velocity with corresponding drop in
temperature. This drop in static temperature of the high
velocity gas is responsible for the low temperature of the
heat-conducting (preferably metallic) tube surface that
causes condensation of moisture in the gas entering the
separator on the "shell side".
After leaving the internal tube passages, the high velocity
gas undergoes diffusion (slowing down) and is then directed
to the stack and discharged to the atmosphere.
For the invention to operate, it is essential that the flow
of gas within the tubes be at a higher bulk velocity than
that flowing over the outside surface of the tubes, i.e.
between the vessel inlet and where the narrow tubes begin.
"Bulk" velocity means net velocity per unit volume.
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This velocity difference may be achieved in a number of
ways, for example by ensuring that the average cross-
sectional area of the tube interior is smaller than the
average area spacing between tubes. By "area spacing
between tubes" is meant the total perpendicular area
between central axes of adjacent tubes available for gas
flow.
Another way of achieving this difference in carrier gas
bulk velocity is the inclusion of partial baffles in the
separator vessel. These cause the carrier gas to follow a
sinusoidal path perpendicular to the tubes thus increasing
carrier gas contact with the tubes' exterior surfaces.
For water condensation, bulk velocity in the main vessel is
preferably about 10 times lower than in the tubes.
When the word "tubes" is used it is intended to include
pipes of any cross-sectional form. However, it is
preferred to use cylindrical tubes.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a simplified side view of one embodiment of the
invention showing only the details of the adiabatic
moisture separator arrangement applicable to its
performance;
Figure 2 is a simplified plan view of the same embodiment;
Figure 3 is a cross-sectional view of the same embodiment;
Figure 4 is a cross-sectional view of the same embodiment
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showing the end plate;
Figure 5 shows detail of a single tube including its
internal passages.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preferred embodiment is described in relation to water
vapour removal from flue gases. This does not restrict the
generality of the separator which may be used for any
suitable condensible gas borne by a carrier gas.
In Figure 1, wet flue gas enters the inlet port (2) of the
adiabatic separator vessel (1) driven by the pressure
differential P1-P2 existing between higher pressure (P1) of
the inlet port (2) and lower pressure (P2) of the exit
port (3), so that the inlet port (2) and the exit port (3)
are at the same end of the separator vessel (1).
Inside the separator vessel (1), the flue gas flows towards
the collecting chamber (4) located at the opposite end of
the separator vessel (1), guided sinusoidally by internal
baffles (5) (see also Figure 3) which also hold a bank of
straight tubes (6) in place. As a whole, the tubes (6) are
preferably disposed, equidistantly from each other, in a
cylindrical array (see Figures 3 and 4).
Upon exposure to the cold outer surface (7) of the tubes
(6), the moisture contained in the flue gas flowing inside
the vessel (1) precipitates and trickles down to the bottom
of the vessel (1), whose floor preferably slopes towards
the condensate outlet (12). The condensate (13) collected
in the bottom part of the separator vessel (1) generally
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follows the flow direction of the flue gas and eventually
may be discharged from the vessel (1) through the
condensate outlet (12).
Upon reaching the collecting chamber (4), the single stream
of flue gas devoid of most of its moisture content and
sulphur dioxide divides into a multitude of streams (Figure
2), prior to entering internal passages (8, 9, 10) of the
tubes (6) referring now to Figure 5. Each tube (6)
preferably has an inlet end (8) of gradually reducing
diameter, an outlet end (10) of gradually increasing
diameter and a central portion (9) of constant but reduced
interior diameter. Low velocity gas (V1) enters the inlet
end (8) of the tube (6) rapidly accelerating to velocity
v2~ A maximum velocity (V3) is attained in the central
portion (9) of the tube (6), with a corresponding drop in
static temperature as a result of partial conversion of the
flue gas' internal energy into kinetic energy.
The internal surface (14) of the tube (6) being directly
exposed to high velocity (V3) low temperature flue gas,
cools down. By the process of thermal conduction, the
outer surface (7) of the tubes (6) cools down as well,
providing the necessary "heat sink" for the fresh
quantities of moist flue gas entering the inlet port (2) of
the condenser vessel (1).
Towards the end of the central portion (9) of each tube(6),
a high velocity gas (V3) enters the diffuser section (10)
of the tube (6), rapidly decelerating i.e. losing velocity
(V4) to reach its final velocity (V5).
Upon reaching the outlet port (3), dried flue gas is
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subsequently discharged from the separator to the stack,
and eventually to the atmosphere.
An internal hermetic baffle (11) separating the inlet and
exit ports (2 and 3 - Figure 1) and through which pass the
tubes, is preferably circular in shape (Figure 4) to
prevent detrimental leakage directly between the two ports.
EXAMPLE
The separator and a process of using it according to the
invention is hereafter described in relation to the
physical conditions at various points in the separator.
Inlet (flue gas) condition:
- Temperature: 140 to 175~ (average: about 150 F)
- Pressure: Atmospheric (14.7 psia)~ Humidity: 10 to 15% (by weight) i.e. close to saturation
point
- Velocity: approx. 50 ft/sec (typical)~ Sulphur content: 0.1 to 0.35% depending on sulphur
(as SO2) content of coal.
"Shell Side": (inside vessel, outside tubes)
- Bulk velocity: 50 to lOOft/sec (average: about
75ft/sec)
- Temperature: 150 F droping to approx. 120 F
- Pressure: Atmospheric or slightly below.
Collecting chamber:
- Velocity: (marginal)
- Temperature: 110~F to 130~F (average: about 120 F)
- Pressure: Atmospheric or slightly less
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- Humidity: 5 to 7% by weight; or 50% relative
- Sulphur content: 0.025 to 0.09% by weight
(as SO2)
- Expected efficiency of SO2 removal: 75%
"Tube Side" (inside tubes):
- Velocity: 750 to lOOOft/sec (approaching sonic velocity)
- Temperature 65~F to 90~F (average: about 70~F)
- Pressure: 8.5 psia (6.2 psi. vacuum)
- depending on pressure drop
- Humidity and sulphur content: As in collecting chamber
Exit Port:
- Velocity: 50ft/sec (typical)
- Temperature: 200 F to 220~F
- Humidity: 5 to 7% by weight or 25% relative
- Pressure: 11.7 psia (3 psi. vacuum)
- Sulphur content: as in collecting chamber
While there have been shown and described what are at
present believed to be the preferred embodiments of the
invention, it will be obvious to those skilled in the art
that various changes and modifications may be made to them
without departing from the scope of the invention as
defined by the appended claims.
