Chemical apparatus and process disinfecting – deodorizing – preser – Chemical reactor – Waste gas purifier
Reexamination Certificate
2000-08-07
2003-05-13
Tran, Hien (Department: 1764)
Chemical apparatus and process disinfecting, deodorizing, preser
Chemical reactor
Waste gas purifier
C422S168000, C422S169000, C422S170000, C096S271000, C096S361000
Reexamination Certificate
active
06562304
ABSTRACT:
The present invention relates generally to a novel and most efficient scrubber for the treatment of flue gases from power generation plants or from chemical plants, in order to absorb objectionable impurities, such as SO
2
, SO
3
, HCl, nitrous oxides, into, a water-based solution, in which solids are not substantially precipitated.
BACKGROUND OF THE INVENTION
Flue gases from power generation plants and boiler houses contain variable amounts of SO
2
, SO
3
, nitrous oxides or acids and similar impurities of acidic nature, deriving from the fuel burned Similarly, exit gases from many chemical plants contain such compounds or similar impurities. These impurities are known to be hazardous to the environment and it is generally required by statutory regulations to treat these gases before their discharge to the atmosphere, in order to reduce, as far as possible, the content of these objectionable impurities. In some cases, CO
2
is also included in this category.
There are many processes and methods actually used, generally grouped under the name of wet Flue Gases Desulfurization (FGD), all of which consisting of contacting the flue gases with an aqueous solution or slurry, having a basic reaction, in order to absorb the objectionable impurities. These processes and methods could be subdivided into:
a) alkaline slurry containing limestone or burned lime, generally resulting in a slurry of calcium sulfite and/or sulfate crystals and a contaminated aqueous solution bleed
b) alkaline solution of hydroxides of either alkali (potassium, sodium), alkaline earth (calcium, magnesium) or ammonium, resulting in solutions of sulfite and/or sulfate salts in which solids are not substantially precipitated, and
c) dilute saline solutions, such as sea water, used on a once-through basis.
An intimate contact between the flue gases and the slurry or solution is required in order to absorb the acidic impurities into the aqueous solutions and react them with the basic-reacting component. Such contact is carried-out in specially-designed equipment, termed contactor, absorber or scrubber. The term scrubber will be used in this application, although the other terms are included in the scope of the present invention.
Generally, the industrial scrubbers used consist of vertical columns, in which the slurry or solution is flowing downwards and the gases may flow either downwards (that is co-current contact) or upwards (that is counter-current contact). In smaller operations in the past, packed columns and/or tray columns were used but counter-current spray-columns are now used more extensively. Although the packed or tray columns give a more efficient contact than spray columns, they cannot operate with concentrated slurry, are much more expensive to install and create a larger pressure drop on the gas flow.
The design of industrial scrubbers must accommodate contradictory requirements; on one hand to adjust the liquid/gas volumetric flow-rates to a workable range, by a significant internal liquid recycle, and on the other hand to maintain a very small forwards net liquid flow, needed to provide a concentrated solution for further processing.
A counter-current spray column generally consists (see illustration in
FIG. 1
) in an empty vertical cylindrical chamber with the following process steps:
a) A solution or slurry is pumped under pressure and sprayed in the upper part, by means of a large number of spray nozzles, organized in a manifold system covering the whole horizontal cross section of the column, at one or several different heights.
b) The resulting drops flow downwards and are collected into a sump at the lower end of the column; a large part of the drops impinge with one another in their flight and coalesces into larger drops.
c) The gases are introduced from a horizontal duct at one side of the column, and their flow-lines must perform a 90° C. turn, before they can flow vertically upwards, against the flow of drops; as a result, uneven velocities and dead-zones are created.
d) The vertical contact results in both partial absorption and temperature equilibration, as the gases can be cooled by water evaporation; sometimes small drops may be entrained upwards by the upraising gas flow.
e) Above the upper row of spray nozzles, the gases are usually demisted from entrained droplets before being discharged.
f) The largest part of the solution or slurry from the sump is pumped back to the sprayed nozzles and fresh solution or slurry is added to this stream; as a result, a bleed stream of solution or slurry is removed continuously from the sump and sent to a further processing step, which is an integral and necessary part of any FGD process.
This generally accepted configuration of spray-column scrubber has been used in a large number of FGD plants producing calcium sulfite or sulfate slurries, since it is simple to conceive and construct, and can be operated with more or less concentrated slurries. It has however a number of inherent deficiencies which impair on its effective applications in other processes typical examples are:
a) The scrubber operates as one single equilibrium stage, at most, due to the facts that the large liquid recirculation rate is from end-to-start and that the concentration changes in the liquid cycle are relatively small. Any process requiring more than one equilibrium stage (in order to obtain a lower residual concentration of objectionable impurities in the exit gases) cannot be performed in a single spray column scrubber. The term counter-current is misleading here, since it refers only to the hydrodynamic flows but not to the process results. In addition, in order to approach a one-stage equilibrium and to provide for the necessary mass transfer driving force, the circulation load must be increased considerably and excess reactants must be maintained, leading to waste of unused reactant in the bleed stream.
b) Relatively low gas flow vertical velocities, generally less than 1 to 3 meters/second, are used to limit back-mixing and entrainment of liquid/slurry drops, which would be counterproductive to the absorption process. The lower gas velocities, in connection with the large volumetric flows of flue gases, generally result in industrial columns with very large diameters.
c) These large diameters are combined with the significant heights required for the different duties, (i.e. collecting sump, gas turning section, contacting section, separation and demisting) result into very bulky columns with costly construction and foundations problems.
d) These significant heights also increase the pumping energy consumed for slurry recirculation, using more expensive high pressure pumps.
The object of the present invention is generally to provide a novel and more efficient scrubber, which avoids, or at least reduces significantly, the above mentioned inherent deficiencies of the spray column scrubber and is particularly suited for FGD processes aiming at high elimination efficiency and using a water-based solution in which solids are not substantially precipitated.
Within this class of FGD processes, those using ammonia as the basic reactant are of special interest, since these produce a concentrated solution of ammonium sulfate which can be processed profitably into a variety of fertilizers.
SUMMARY OF THE INVENTION
The novel scrubber comprises (see illustration in
FIG. 2
) a horizontal chamber with a rectangular cross section, divided along the horizontal axis into a number of compartments by demisters. The gases enter through a duct at one end and exit through a duct at the other end. The “picket-fence” demisters between the compartments assure that gas flow lines are spread evenly over the whole cross section area and that entrainment of drops between compartments is kept to a minimum. If needed for layout constraints or convenience, the horizontal axis can be bent or turned in any direction, or even arranged as a horse-shoe complete turn-around.
In each compartment, there is a liquid-collecting sump and a centrifugal pump (except possibly for the last one) which distribut
Clue AS
Fulbright & Jaworski LLP
Tran Hien
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