Retrofit of a center inlet type scrubber with absorption/gas distribution tray to improve gas-liquid contact in the absorption zone

An improvement in a center inlet type scrubber that retrofit the scrubber with a perforated tray made up of rectangular or partially rectangular sections supported on beam supports in the annulus of the scrubber. The supports can be modified to form turning vanes or additional turning vanes can be employed. One or more perforated trays may be installed in the annulus to help correct the problem.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates in general to gas scrubbers, and in 
particular to an improvement for center inlet type scrubbers. 
2. Description of the Related Art 
Several types of gas scrubbers are known in the FGD (flue gas 
deulfurization) industry. A detailed discussion of FGD construction and 
use can be found in STEAM , 40th Edition, Babcock & Wilcox Company, 
Chapter 35. Center inlet type scrubbers have a unique design as shown in 
FIG. 1. The scrubber shown in FIG. 1 consists of two concentric 
structures. The outer structure contains spray headers 12 delivering the 
scrubbing solution and may contain one or two stages of mist elimination 
devices 30. In some cases, one or both stages of the mist elimination 
device 30, are located in the outlet flue 14. The inner center structure 
consists of the inlet flue 16, a venturi section 18 used to quench the gas 
and separate ash particles from the gas stream, and a downcomer 20 to 
deliver the gas downwardly under the level of the sprays. A plumb bob 22 
is used to control and accelerate the entering gas to insure maximum 
particulate separation. 
The entering gas is quenched in the venturi section 18 located above the 
downcomer. The gas flow rate and efficiency of particulate separation is 
controlled by the plumb bob position and the pressure drop across the 
plumb bob 22. In the inlet downcomer 20, the cleaned and quenched gas 
travels downwardly with the quenching/scrubbing solution. At the bottom of 
the downcomer 28, the quenching/ash collecting solution drops into an 
integral tank 24 or a device directing this stream into a separate tank. 
The gas makes a 180.degree. turn and travels upwardly in a countercurrent 
direction towards the flow of the spray headers 12. In the outer annulus 
26 of the scrubber, several levels of sprays are located midway between 
the bottom 28 of the downcomer and the absorber outlet 14. Each spray 
header contains one or more nozzles that sprays the scrubbing solution or 
slurry which is normally alkaline counter to the gas flow. The scrubbed 
gas exits through the scrubber outlet 14 located at one side of the outer 
annulus 26. Mist is removed from the cleaned gas in one or more stages of 
mist elimination device 30. 
The mist elimination device(s) may be located horizontally in the vertical 
section of the outer annulus 26, or may be located vertically in the 
scrubber outlet 14. The configuration of the scrubber, the 180.degree. 
turn at the bottom of the downcomer, and the location of the gas outlet 
promote gas maldistribution in the outer annulus 26. A typical gas 
velocity profile for this type of scrubber is shown in FIG. 2 with gas 
velocities being in feet per second. 
The main disadvantage of the prior art system is the poor gas distribution 
in the spray section. The entering gas leaves the bottom 28 of the 
downcomer at a velocity in excess of 50 to 90 feet per second making an 
unguided 180.degree. turn. The combined effect of the high gas velocity, 
the turn, the location of the outlet, and the lack of guiding devices, 
such as gas turning vanes, at the bottom of the outer annulus forces the 
gas towards the outer walls of the annulus and in the direction of the 
outlet. Poor gas distribution leads to excessive localized velocity zones 
near the outer walls, gas recirculation, poor gas liquid contact, and poor 
removal efficiency for a given scrubbing liquid flow rate. The poor gas 
distribution and the localized high velocity pockets are passed on to the 
mist elimination devices leading to failure in their performance and/or 
uneven solids deposits on the surface of the mist eliminator 30. This 
uneven flow through the mist eliminator 30, leads to pluggage of the low 
liquid flow zones which leads to even higher gas velocity, and increased 
potential for faster failure and liquid droplets carryover to the stack. 
The addition of turning vanes or other gas distribution devices in the 
outer annulus at the bottom of the downcomer, such as guiding, or turning 
vanes may correct the gas distribution problem, however, the use of 
scrubbing slurry and contamination of the slurry with the removed ash 
presents a potential for partial solid build-up on the vane and eventual 
total pluggage of the gas flow area. Such a solution to this problem is 
viable only if the scrubbing solution is clear, and the products of the 
scrubbing process are highly soluble in the scrubbing liquor to prevent 
solids deposition. 
The use of pie shaped trays to correct gas maldistribution has the 
potential to aggravate the problem because the pie shaped tray sections 
are wide near the outside of the annulus where gas flow area need to be 
restricted, and narrow near the downcomer where a bigger flow area is 
necessary to promote greater gas flow. Also, each pie shaped tray section 
requires two supports spanning the radius of the annulus from the outer 
wall to the downcomer. The proximity of the supports near the downcomer 
also contribute to the restriction of the flow area near the downcomer. 
Another option is employed in the present invention. This is to place a 
perforated tray at the bottom of the downcomer to affect gas distribution 
as the gas turns at the bottom of the annulus, and improve pollutants 
removal by the following methods: 
Providing even and optimum gas/liquid proportions at and above the 
perforated tray; 
Allowing longer contact time between the gas and the liquid, because the 
perforated tray acts as a contacting surface, as well as a gas 
distribution device; 
Acting as a storage bin for the liquid draining from the tray to the tank 
underneath, therefore, promoting cleaning of the structures under the 
tray; 
Straightening the gas flow upstream of the spray banks and providing 
maximum utilization of the scrubbing reagent; 
Locating the perforated tray at a proper distance from the lowest spray 
bank to avoid impingement of the spray on the tray and minimize erosion of 
the tray material; 
Locating the perforated tray at a proper distance from the lowest spray 
bank to insure that the spray is well developed before it is interrupted 
by any structures; and 
Building the tray sections of rectangular or non pie shaped geometric 
structures to maximize the flow area near the downcomer and minimize the 
tray section supports. 
SUMMARY OF THE INVENTION 
To correct the existing problem of gas maldistribution shown in FIG. 2, the 
gas turning at the bottom of the downcomer is evenly distributed across 
the outer annulus flow area as the gas makes the turn. Proper 
redistribution of the gas before it reaches the sprayed slurry leads to an 
even liquid-to-gas ratio, and proper contact between the gas and liquid. 
The objective of this invention is to provide means to retrofit center 
inlet scrubbers as described above or of similar design with a perforated 
plate or tray without excessive rework of the existing vessel, without 
excessive obstruction of the critical flow area with supports, and with 
the potential to manipulate the resistance near the downcomer walls. 
According to the present invention a perforated tray is located at the 
bottom end of the annulus at or near the bottom of the downcomer. The 
downcomer may need to be extended to avoid impingement of the spray on the 
tray which causes erosion of the tray material, or causes the spray to 
collapse before it is fully developed. If the distance between the lower 
spray header and the bottom of the downcomer is adequate, then the 
extension is not necessary and the tray supports and sections are attached 
directly to the downcomer or downcomer bottom end. The distance between 
the lowest spray bank and the tray, or tray baffles, varies from about 18 
inches to as much as about 5 feet, depending on the type of nozzles, the 
spray pattern in the scrubber, and the direction of the sprayed slurry. 
The principles of this invention are also applicable in cases where the 
spray headers are spraying co-current to the upwardly rising gas flow in 
the outer annulus. 
Perforated trays are supported from rings attached to the outer wall of the 
annulus and the downcomer, or from the bottom of the downcomer extension. 
One distinction of the invention is that the perforated tray sections are 
preferably rectangular or other geometric shape other than pie shaped. 
This approach was adopted to minimize the support steel material 
associated with the pie shaped tray sections, and to provide maximum open 
flow area near the outside of the downcomer where gas flow is lacking and 
is mostly needed in this area of the scrubbing zone. 
Another advantage to this structure is minimum waste of the perforated tray 
material, in manufacturing, since the tray sections are rectangular rather 
than pie shaped. The cut off sections from the outer perimeter tray 
sections can also be used to fill the uneven sections of the downcomer's 
outer perimeter. 
To minimize the effect of the scrubber's side outlet on gas distribution, 
the tray support is oriented to compensate for the resulting low 
resistance to the gas flow. Baffles located on top of the tray segment 
form the tray sections into compartments and restrict the side to side 
motion of the liquid accumulating on top of the tray sections. This helps 
to retain a liquid layer in each section and provides controlled 
resistance to the gas flow particularly when various open area tray 
sections are used. Baffles or vanes located under the perforated tray also 
help with support orientation in guiding the upward flow of the gas. 
The gas redistribution by the perforated tray may also be assisted by 
varying the open area of the tray in different locations of each section, 
or in an entire section, to force the gas to follow the path of least 
resistance. The location of the variable area sections of the tray are 
determined by modeling, physical or numerical, or with best engineering 
judgement. In other cases, a uniform open area tray may be adequate. This 
decision is made after proper modeling of the scrubber. 
While one embodiment of the invention has two main support beams under the 
perforated tray, on similar or larger diameter scrubbers two or more 
additional beams may be located perpendicular to these beams in other 
embodiments. 
In cases where tighter control over gas distribution is required, the 
support may be designed to act as turning vanes extending under the 
perforated tray or additional turning vanes may be added under the tray 
between the support beams. The extensive and evenly flowing liquid 
drainage from the perforated tray keeps the support beams/gas turning 
vanes clean and free of solids build up. 
Two stages of horizontal gas flow mist eliminators are located in the 
outlet flue. In addition to the retrofit described above, a set of guiding 
plates or turning vanes may be located above the top spray header to help 
even the gas distribution through the turn to the side outlet. The 
position and shape of these plates are a function of the operating gas 
velocity in the outer annulus and the absorber outlet location. 
At a lower gas velocity, the carry up from the spray bank is minimal. 
Continuous or intermittent wash from the make up water system may be 
required to keep the upper gas turning devices clean. At a gas velocity 
equal to or greater than about 12 feet per second, the liquid carry up 
from the sprays below is heavy enough to wash any deposited solids and 
keep the plates clean. 
The various features of novelty which characterize the invention are 
pointed out with particularity in the claims annexed to and forming a part 
of the disclosure. For a better understanding of the invention, its 
operating advantages and specific objects attained by its uses, reference 
is made to the accompanying drawings and descriptive matter in which a 
preferred embodiment of the invention is illustrated.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the drawing and first to FIGS. 1 and 2 in particular, the 
invention is an improvement over the known scrubber structure of FIG. 1. 
It improves the prior art velocity profile across the outer annulus of the 
scrubber. The velocity profile is shown in FIG. 2 where the numerals 
superimposed on the annulus flow area pie shaped represent the gas 
velocity measured in the annulus in feet per second. 
Throughout the drawings, the same reference numerals are used to designate 
the same, or functionally similar parts. 
In FIG. 3, the scrubber of the present invention comprises a cylindrical 
enclosure 34 enclosing an upper center inlet 16 with a venturi section 18, 
which is only shown schematically. A center inlet downcomer 20, extends 
downwardly from the venturi section 18 and ends at the bottom end 28, 
which may be the bottom of the unextended downcomer 20, or may be the 
bottom end of the downcomer extension 29. The bottom end of the downcomer, 
or the bottom of the extension 29 brings the lower end 28 of downcomer 20, 
an adequate distance below the lowest spray header 12 in the annulus 26. A 
perforated tray 40, the subject of this invention, is located at the lower 
end of the downcomer 28, or the bottom of the extension 29. The perforated 
tray 40 consists of baffled sections 36 and extends around the bottom of 
the annulus and serves to even the gas distribution. The gas flows under 
the lower end 28, passes through the perforated tray 40, and begins to 
ascend up the annulus in the direction of the curved arrow. Extension 29 
is selected to extend the lower end of the downcomer 20 by a distance of 
about 18 inches to about 5 feet below the lowest spray bank 12 in the 
annulus 26. Supports for the perforated tray 40 are attached to the outer 
wall 34 and are also supported off the walls of the downcomer 20, or by 
the lower end 28 of the downcomer 20, or the downcomer extension 29, if an 
extension is required to satisfy the 18 inches to 5 feet design criteria. 
As a result of the even gas distribution ascending in the annulus, the 
liquid to gas ratio is optimized and the scrubbing efficiency of the 
system is improved. 
FIG. 4 shows the support structure of the perforated tray. A ring bar 38 
attached to the outer walls of the downcomer 20, or to the lower end of 
the downcomer 28, or to the extension to the downcomer 29, supports the 
inner ends of the tray sections 36 located between the main support beams 
63. An outer ring 39 attached to the outer wall of the annulus 34 supports 
the other end of the tray sections 36. Support beams 63 are attached to 
the outer annulus wall 34 and are also supported off the downcomer 20, or 
the bottom of the downcomer 28, or attached to the bottom of the downcomer 
extension 29. Main support beams 63 are also connected to the trays 
sections 36 by way of either being mounted above, or preferably below the 
trays. Support bars 65 typically located between the support 63 and the 
downcomer 20, or the downcomer extension 29 act as span breakers and 
provide added support for the long tray sections 36a shown in FIG. 6. The 
added support provide added protection for personnel and equipment using 
the tray 40 as a platform for inspection during maintenance. FIG. 5 shows 
an alternate support system arrangement in which support beams 78 are 
positioned perpendicular to the outlet flue in addition to support beams 
63 for better control of gas partition and distribution. 
Turning next to FIG. 6, the perforated tray 40 is preferably made up of 
multiple rectangular, or other geometric shaped sections 36, other than 
pie shaped sections. Portions which are cut off from the rectangular tray 
sections fitted against the outer walls of the annulus 34 can be used to 
fill in the inner perimeter of the scrubber cross section around the 
outside walls of the downcomer. FIG. 6 also shows the location of the main 
support beams 63 spanning across the outer annulus 34 and attached to the 
downcomer 20, the bottom of the downcomer 28, or the bottom of the 
extension 29. Support bars 65 span between the main support 63 and the 
downcomer wall to break the span of the long tray sections 36a. The tray 
sections 36 are compartmented by baffles 37 to control the liquid level 
and lateral movement in the various tray sections. 
FIG. 7 shows the attachment of extension 29 to the original downcomer 20 
using internal gussets 32. The main support beam 63 is attached to the 
bottom 28 of the downcomer extension 29. The tray sections 36 are laid on 
top of the support beam 63. The tray sections may be welded to each other 
for support or may be bolted in place. A combination of both attachment 
methods is also possible to provide inspection access to the structures 
under the tray. 
In FIG. 6, the porosity of the tray sections 36 is uniform across the 
annulus. The uniform porosity of the tray sections 36 could vary from 
about 15 to about 80% open surface area depending on the gas velocity 
leaving the downcomer, the extent of gas maldistribution, and the general 
operating conditions of the scrubber. 
FIGS. 8, 9, 10, 11 and 12 show alternate methods of attaching the tray 
support structures to the existing structure of the scrubber. 
FIG. 13 shows a typical attachment of the tray section baffles 37 to the 
tray sections 36. 
FIG. 14 shows the support of the tray section 36 and the baffle plate 37 
off the outer support ring 39 and the support beam 63 attached to the 
downcomer 20. 
FIG. 15 shows the support of the tray sections 36 off the inner and outer 
support rings 38 and 39. 
FIG. 16 illustrates an alternate embodiment where different sections 36 of 
the perforated tray 40 have different porosity. For example, an inner set 
of tray sections 44 has a 35% opening or porosity while outer tray 
sections 46 have a 30 or 33% porosity. Inner tray sections 48 which is on 
the side of the outlet 14 may have a porosity of 33% while the 
symmetrically oriented tray sections 50 opposite from the outlet 14 has a 
greater porosity, for example 35%. In likewise fashion, the outer tray 
sections 52 away from the outlet 14 has a higher porosity, for example 
33%, than the outer tray sections 54, which are located on the outlet side 
and has a porosity of about 28%. In FIG. 6, the porosity of the tray 
sections 36 is uniform across the annulus. The uniform porosity of the 
tray sections 36 could vary from about 15 to about 80% open area depending 
on the gas velocity leaving the downcomer, the extent of gas 
maldistribution, and the general operating conditions of the scrubber. The 
same section, for example tray section 52, may also have different 
porosities as shown in sections 52 and 52a of FIG. 16. 
The asymmetry can also be from side to side with the section 46 on the 
downstream end of the annulus with respect to the rotation of the gases 
having a porosity of 30% while the upstream end 47 has a porosity of 33%. 
The porosity of the each tray section 36 could vary from about 15 to about 
80% open area depending on the extent of gas maldistribution leaving the 
downcomer and the general operating conditions of the scrubber. 
In this way, the porosity of the tray sections can be tailored to provide a 
custom velocity profile of the upwardly moving gases to avoid the 
differences in velocity and thus, differences in scrubbing effect which 
was experienced in the prior art as illustrated in FIG. 2. 
The porosity of the tray could be varied by any one of the following means: 
Use a one size hole in all tray segments and vary the number of holes per 
segment, thus changing the porosity of the various segments; 
Use different sized holes in the various sections while maintaining the 
same number of holes per section of the tray to effect the desired 
variation in the tray section porosity; or 
Use a combination of the above two options, namely, using a different 
number and size of holes simultaneously. 
The desirable range for tray section holes varies from about 1/4 inch to as 
large as about 3 inches. 
FIG. 17 illustrates blocked off areas in cross hatching which are 
distributed around the rectangular trays and tray sections. Outer blocked 
out areas are shown at 60 and inner blocked area are shown at 62. Each 
blocked area contains one or two holes for proper drainage of the falling 
liquid. FIG. 17 illustrates the location of the support channels 63, a 
typical baffle location 64 and the support bars 65. Support bars 65 act as 
span breakers to support the long tray sections. One or more support bars 
65 may be located depending on the structural support requirements of the 
tray 40 and tray sections 36. All trays are advantageously the same width. 
The width of the tray section depends on the span between the support 
beams 63 and the width of the overall sector facing the outlet. The width 
of the tray section could vary from about 2'0" to 4'10", with a typical 
width of about 2'5". The unit length is about 7'11" made up of one full 
rectangular tray and at least one tray baffle 37. 
FIG. 18 illustrates the location of turning vanes 70 which are located 
below the perforated tray and the baffles 37 which are above the tray. The 
baffles 37 could be located in line with the vanes 70 or offset from vanes 
70. Vanes 70 are located between the support beams 63 and are bent 
inwardly to conform more closely to the flow of gases as they curve around 
the lower end of the downcomer. 
FIG. 19 illustrates the placement of the tray baffles 37 over a perforated 
tray and turning vanes 70 under the tray relative to the location of the 
main supports 63. In FIGS. 18 and 19 the vanes are located in line with 
the turning vanes 70. In other cases, the orientation of the baffles 
(above tray vanes) 37 may not line up with the under tray supports or 
vanes 70. 
FIGS. 20 and 21 show another embodiment of the invention using sloped 
turning vanes 80 at the upper annular section of the scrubber to provide 
even gas distribution into the outlet flue and the mist elimination device 
30. 
One or more perforated tray 40 may be located under one or more spray 
header 12 in a single annulus enclosure 34 as needed to correct the gas 
distribution within the annulus and as dictated by the operating 
conditions. FIG. 22 is an illustration of locating multiple trays in a 
single annulus. 
While specific embodiments of the invention have been shown and described 
in detail to illustrate the application of the principles of the 
invention, it will be understood that the invention may be embodied 
otherwise without departing from such principles.