Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
1998-02-11
2002-12-03
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S172000, C422S176000, C261S109000
Reexamination Certificate
active
06488899
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates, in general, to flue gas desulfurization absorbers, and in particular, to a new and useful absorber arrangement having a gas inlet at the transition between a lower, large diameter tank section which contains a liquid slurry level, and an upper small diameter absorber section.
2. Description of the Related Art
Commercialization and development of high velocity absorbers is pursued because of the economic advantages they offer such as lower capital cost, less real estate requirements, shorter more compact absorbers, and improved SO
2
removal efficiency. On the other hand, high velocity has some disadvantages such as increased resistance to gas flow and increased sensitivity of the system to changes in the hydraulic behavior of the gas and liquid phases. Physical model studies show that the gas velocity through the inlet of the absorber affects gas distribution in the absorber and affects the performance and behavior of the absorption zone and mist eliminator.
Regardless of the physical shape of the absorber, the resistance to gas flow is categorized as either useful resistance or parasitic resistance. Useful resistance is converted directly and entirely into scrubbing efficiency and participates in gas redistribution such as the absorption zone pressure drop. Parasitic resistance is expended to conduct the gas through the absorber confines without effective participation in the chemical process. The inlet and outlet resistances are good examples of this type of resistance. The use of turning vanes or other gas distribution devices is a simple solution to outlet resistance. However, the inlet pressure drop is not easy to reduce because it is a combination of the gas and the scrubbing liquid interaction throughout the absorber.
Traditional absorber inlets vary in shape and size but the shape of the inlets is basically the same.
FIG. 1
shows the commonly offered inlet design (without protective awning). In this design, the liquid flowing off the absorber walls
12
or sprayed by nearby spray headers, falls into the inlet
14
and forms a solid growth at the wet/dry interface causing maldistribution and higher resistance. To overcome this problem and as shown in
FIG. 2
, protective intrusive awnings
16
were placed on top of the inlet
14
(see U.S. Pat. No. 5,281,402). The awning diverted the contact point (between the hot gas and the liquid curtain flow) from the vicinity of the inlet to the center of the absorber. Solids deposition is averted because gas humidification occurs in an area where there is minimum contact between the hot gas and the inlet flue surfaces. This design has been proven functional at the traditional gas velocities when the spray zone resistance is large enough to affect even distribution before the gas reached the mist eliminator further up in wall
12
. As the gas velocity increases, however, the curtain resistance adds significantly to the overall system parasitic pressure drop and distortions to gas flow pattern becomes more critical.
While the liquid curtain is needed to humidify and help gas redistribution, it has two significant drawbacks. It significantly increases the inlet pressure drop compared to an inlet without an awning, and it distorts the flow pattern as the gas rises through the absorber.
In a new generation of high velocity absorbers, gas velocity is set between about 15 to about 20 feet per second. Minor distortion in the gas flow pattern results in localized high gas velocities approaching or exceeding the critical velocity of the mist eliminator and may result in functional failure of the mist elimination device.
To overcome the negative effects of high gas velocity in the inlet, one could increase the inlet's flow area and limit the gas velocity to the conventional 3,000 feet per minute. This solution, while simple and practical, will result in a larger inlet aspect ratio (height to width) and increases the absorber height and inlet resistance. An increase in absorber height minimizes the advantages gained by high velocity scrubbing. Other options include advanced low pressure drop gas inlets for the new generation of high velocity absorbers, or the use of available means within the system to redistribute the gas flow without significant increase in inlet resistance.
The current industry inlet design uses the installation of the protective awning
16
on top of the inlet
14
to deflect the slurry away from the hot flue gas flow and prevents the deposition of solids at the wet/dry interface. However, at high absorber gas velocity, obstruction of the gas path by the high density liquid curtain deflects the gas to the sides causing an increase in gas velocity and possible distortion.
Model studies and operating experience teaches that at an absorber velocity between about 1 to about 12.5 feet per second, the current inlet designs provide good gas distribution across the absorber at or below about 3,000 feet per minute inlet velocity. The good gas distribution is provided partially by the resistance of the liquid curtain, falling off the awning to the entering gas. The primary function of the awning is to provide protection against inlet wetness and solids deposition and to provide ample resistance to slow down the entering gas, thus allowing the gas adequate time to redistribute itself across the absorber flow area. At an absorber gas velocity below 12.5 feet per second reasonable gas distortion in the absorber will not approach the critical failure velocity of the mist eliminator.
As the gas velocity increases above about 12.5 feet per second and approaches about 20 feet per second or more, the resistance of the liquid curtain falling off the awning becomes significant and magnifies the effects of gas flow distortion.
Several attempts were made to reduce the resistance of the awning first by introducing a new generation of non intrusive awnings. In these designs, the awning is removed from the inlet's gas stream and placed above the inlet. See U.S. Pat. No. 5,403,523; 5,558,818 and 5,648,022. Each of these developments contributed to the reduction of the inlet's parasitic pressure drop caused by the intrusion of the original awning into the gas flow path. These designs, however, added 3 feet to the height of the absorber and none of them totally eliminated the effect of the heavy density liquid curtain.
These prior efforts were steps in the right direction to reduce the inlet's parasitic resistance, however, in every case the curtain resistance remained the same. Considering that one inch (water) of pressure drop is evaluated at approximately $1 million over the life of the power plant, reduction of the parasitic resistance of the absorber provides a significant competitive edge. Table 1 compares the pressure drop of an inlet with and without awning.
TABLE 1
Comparison of Inlet Pressure Drop
for No Awning and Awning Designs*
Non-Intrusive
Description
Intrusive Awning
Awning
No Awning
Inlet Pressure
4.59
3.50
2.50
Drop (Inch Water)
*Inlet velocity about 3,600 feet per minute, liquid flux about 60 gpm per square feet, absorber velocity about 15 feet per second.
SUMMARY OF THE INVENTION
The present invention provides a combination of an advanced gas inlet with the sought protection against wet/dry zone solids deposits and reduces the liquid curtain to the same density observed in the inlet design without an awning. The new inlet does not promote the formation of thick high density liquid curtains in the gas path, thus reducing the parasitic pressure drop experienced with awning equipped inlets. The new design places the inlet in the transition between the large diameter tank and the small diameter absorption zone.
Successful application of this invention could possibly result in an evaluated cost of about $1,000,000 less than the current designs and over about $2,000,000 in savings over the original intrusive awning design.
The present invention is suitable for absorbers with flared tanks. The inlet is located in the transition b
Gohara Wadie F.
Hall William H.
Piaskowski Edward J.
Marich Eric
The Babcock & Wilcox Company
Tran Hien
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