Gas distribution system for venturi scrubbers and absorbers

Gas and liquid contact apparatus – Contact devices – Liquid spray

Reexamination Certificate

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C261SDIG003, C096S323000, C096S348000, C096S360000, C096S368000

Reexamination Certificate

active

06808166

ABSTRACT:

FIELD OF INVENTION
This invention relates generally to apparatus for removing contaminants from gaseous streams, and more specifically relates to venturi scrubbers.
BACKGROUND OF THE INVENTION
One well known type of device for removing contaminants from a gaseous effluent stream (such as flue gas) is a venturi scrubber. In such device the effluent gas is flowed through a venturi tube having a narrow throat portion. As the gas moves through the throat it is accelerated to a high velocity. A scrubbing liquid or slurry is added to the venturi, usually at the throat, and enters the gas flow. The scrubbing droplets used are generally much larger than the contaminant particles or contaminant gases to be collected and, as a consequence, accelerate at a different rate through the venturi. The differential acceleration causes interactions between the scrubbing droplets and the contaminants, such that the contaminant particles or gases are collected by the droplets. The scrubbing droplets are then removed from the effluent stream which is thereby cleansed.
The inventors' experience in operations at typical power plants indicates that severe gas maldistribution can exist in certain types of venturi scrubbers and that this contributes to a number of serious equipment problems. The symptoms are slurry carryover and buildup in the scrubber mist eliminators, induced draft fans, duct work, and second stage venturi absorbers that may be currently only used for mist elimination. In such systems liquid can even escape a second venturi mist eliminator and be re-entrained into the flue gas exiting the chimney, resulting in liquid droplets falling from the plume (termed “stack rain”).
The reasons for the above difficulties can be understood by reference to
FIG. 1
which is a schematic block diagram depicting a prior art venturi scrubber flow system for sulfur dioxide and fly ash removal to which the present invention is applicable.
FIG. 1
may be considered simultaneously with the prior art showing of
FIG. 2
which depicts the venturi scrubber vessel in greater detail. The system
10
shown in
FIG. 1
includes a scrubbing vessel
12
which is of a venturi-type known in the prior art. Exhaust flue gas
14
from a furnace
16
is provided to the inlet
18
of vessel
12
. After being contacted with a slurry reagent which is sprayed via spray head
20
at the top of vessel
12
and by spray head
22
at an intermediate point, the flow of flue gas and spray droplets proceeds about a plumb bob body
24
and is then converged and enters the constricted venturi passage
26
where increased contact between the scrubbing slurry and flue gas is enabled. The slurry reagent
28
, as for example an aqueous slurry of lime or a limestone, is fed into a slurry reservoir
30
at the bottom of vessel
12
. The slurry level
32
in reservoir
30
is maintained at a relatively constant point within the bottom of the vessel. The slurry having contacted the flue gas descends to this underlying reservoir
30
where it is collected. The collected slurry is refreshed by reagent
28
and makeup water is added as needed—e.g., from wash water used for mist eliminator
50
. The slurry is recirculated via the recirculation pump
34
to the spray heads
20
and
22
. The solids are recovered from the slurry via the output line
36
which proceeds to a conventional thickener
38
. A valve
35
opens to admit slurry from the reservoir to thickener
38
upon the solids in the reservoir exceeding apreset point. (Controls for valve
35
are not shown.) The thickener underflow proceeds via line
40
to a disposal or collection point.
It is seen that the vessel
12
is provided with an external wall
42
, and an internal wall
44
which converges in the direction of gas stream flow and then defines a boundary for the constricted venturi passage
26
by its downwardly extending portion
46
. Portion
46
terminates well above the slurry level
32
. Thus it is seen that the flow of the gases and entrained slurry to be scrubbed, as depicted by the large arrows
53
in
FIG. 2
, proceeds downwardly through venturi passage
26
and then is turned about through approximately 180° beneath the wall
46
and above the slurry level
32
, and enters the annular space
48
defined between the aforementioned internal and external walls
46
and
42
. The gases then proceed upwardly in annular space
48
and impact and pass through a conventional mist eliminator
50
. An outlet duct
52
intersects annular space
48
at one side thereof (at the right in the sense of the drawing) and allows the gases proceeding above the mist eliminator
50
to exit and proceed via line
54
to stack
56
for discharge.
The two curves designated “current operation” in
FIG. 5
depict a typical flow velocity distribution in a prior art venturi scrubber vessel of the type shown in
FIGS. 1 and 2
, and illustrates the gas maldistribution problem occurring for the velocities near the exterior walls of the annular space
48
. The dotted curve represents results for a computerized fluid dynamics (“CFD”) model for a plane 1 foot below mist eliminator
50
. The curve defined by triangles depicts measured values at a plane about
5
feet below the mist eliminator
50
. The parallel lines in the middle of the graph represent the downwardly extending input channel, i.e., the venturi passage
26
. As seen from this graph, the flow velocity of the mist-entraining gases is vastly different in the annular space
48
depending upon whether one is considering the upward velocity of the gases adjacent internal wall
46
or the velocity as one approaches the external wall
42
of the vessel. In fact it will be noted that adjacent wall
46
the gases actually have a negative velocity indicative of eddying and swirling. Even though in the design considered the average velocity is only of the order 10 fps, the regions of high velocity typically exceed the 15 fps design velocity of the mist eliminator used in the present system, which results in slurry penetration through the mist eliminator and into downstream equipment. It is this marked nonuniformity in velocities in the annular space
48
which the present invention is intended to remedy.
SUMMARY OF INVENTION
In accordance with the present invention, a venturi scrubber is provided for scrubbing a gas stream to remove undesired gaseous components, which includes a scrubber vessel having an external wall, an upper inlet for admitting the gas stream, and a reservoir at the bottom of the vessel for collecting the scrubbing liquid sprayed into the vessel. An internal wall extends from the upper part of the external wall to define an axial converging passage for converging the gas stream from the inlet, and an adjoined downwardly extending restricted venturi passage receives the downwardly flowing gas stream from the converging passage. The internal wall abounding the venturi passage terminates above the liquid level at the reservoir. An annular space is thereby defined between the internal and external walls. An outlet duct for the scrubbed gas stream intersects the annular space at its upper reaches. The gas stream flowing downwardly through the converging passage and venturi passage is turned about the bottom of said internal wall to enter the annular space and proceeds therein upwardly and exits the vessel through the outlet duct. Spray means are provided for spraying a scrubbing liquid or slurry into the gas stream, and an annular mist eliminator is mounted in the annular space at an axial point below the outlet duct.
Pursuant to a first aspect of the invention a gas diverter means is mounted in the annular space below the mist eliminator to divert the gas stream which is turned upwardly into the annular space away from the external wall and toward the axis of the vessel, whereby the velocity of the upward flow through the annular space to the mist eliminator is rendered more uniform. The gas diverter is preferably an annular ring the transverse cross-section of which is adjacent to but slightly spaced from the external wall,

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