Scrubber having fixed throat venturi and adjustable plug

Apparatus is disclosed for scurbbing contaminants from a gas steam. The apparatus includes a vertically extending cylinrical vessel having an upper gas steam inlet and a lower gas steam outlet. Means are provided for forming an annular venturi gas flow passage fixed within the vessel, and a central opening. A vertically movable plug is supported within the central opening. The plug has a variable cross section and is movable between a position in which the plug closes the central opening and other positions in which portions of the gas stream flow around the plug and through a secondary annular gas flow passage formed within the central opening. A scrubbing liquid supply means is provided above the plug at the center line of the cylindrical vessel. The scrubbing apparatus combines the high efficiency of a fixed-throat venturi scrubber with the compactness, simple construction and low-cost of a plug-type scrubber.

BACKGROUND OF THE INVENTION 
This invention relates to an improved apparatus for the scrubbing of a gas 
stream to remove entrained solid particles and other solid and condensable 
gaseous contaminants such as mist, or noxious gaseous compounds, and 
sulphur dioxide. 
Industrial or commercial facilities often generate hot waste gas streams 
which are discharged to the atmosphere. These streams are generally 
ladened with entrained solids, mist, or gaseous contaminants such as 
sulphur dioxide, which must be removed prior to discharge into the 
atmosphere in order to reduce or prevent air pollution or to recover 
valuable entrained components. In recent years, more stringent air 
pollution regulations have been enacted in many communities, which have 
necessitated the provision of more efficient and effective facilities and 
apparatus for treating waste gases to remove entrained and contained 
contaminants and prevent or substantially reduce air pollution. 
Generally, there have been two basic types of methods of extracting solid 
particles from gas streams, the dry process and the wet process. The dry 
process typically contemplates the use of electric precipitators or some 
type of filter or screening means, such as passing the gas through a dust 
bag. In many instances, the installation of conventional dry process 
equipment such as bag filters, is usually not warranted due to the cost of 
such facilities. Another dry process includes the use of cyclonic type 
separators. Dry processes are usually satisfactory if the entrained solid 
particles are relatively large, but these processes are not very 
successful when the entrained solid products are small, particularly when 
the solids are in submicron range. Electric precipitators are successful 
in extracting small particles, but these precipitators are usually 
prohibitively expensive when large gas volumes are involved. 
Separators or scrubbers using a wet process are usually more effective in 
extracting solid particles of small dimension. Known wet scrubbers include 
means for passing gas through apparatus which include areas in which the 
gas is contacted by liquids. Water is the usual scrubbing liquid. This 
apparatus provides an intimate mixture of the scrubbing liquid and the gas 
to be treated. The apparatus is designed so that as the gas flows through 
the apparatus, it separates the liquid into extremely small liquid 
particles which contact the small solid particles entrained in the gas. 
Thereafter, the physical mixture of gas and entrained particles of 
scrubbing liquid containing the trapped solid particles and materials 
dissolved from the gas by the liquid is passed to an apparatus that 
separates the liquid particles from the gas. This apparatus may be, for 
example, a conventional moisture eliminator. 
Conventional wet scrubbing apparatus is usually built with a specific 
combination of dimensions that are designed for a particular optimum rate 
of gas flow within a narrow range of flow rates. In order to secure 
maximum efficiency, each such scrubber must be designed and constructed 
for a specific flow rate. Any change in the flow rate or other perimeter 
of the scrubbing system, such as changes in gas or liquid pressures, size, 
weight and distribution of entrained particles, results in the apparatus 
functioning at less than its maximum designed effectiveness and 
efficiency. The construction of gas scrubbers has heretofore required 
specially designed apparatus having specifically dimensioned components 
and such scrubbers do not operate efficiently when the gas flow is below 
or exceeds the limited range for which the scrubber is designed. The prior 
art scrubbers generally fail to adequately compensate for variations in 
gas flow rate, such as is encountered in periodic cyclic or batch type 
processes which generate or discharge gas streams at varying or 
intermittent flow rates. 
Various scrubbing apparatus have been designed which include vertically 
adjustable plugs which vary the throat of an adjustable angular venturi 
passage and thereby adjust the gas flow and pressure drop through the 
scrubber. Such scrubbers are shown in U.S. Pat. No. 3,544,086, U.S. Pat. 
No. 3,601,374, U.S. Pat. No. 3,690,044, U.S. Pat. No. 3,767,174, U.S. Pat. 
No. 3,199,267, U.S. Pat. No. 3,854,908, U.S. Pat. No. 3,844,744, and U.S. 
Pat. No. 3,976,454. All of these patents relate to generally similar 
scrubber designs in which a centrally located plug can be moved vertically 
to change the throat dimensions of an outer annular venturi passage by 
making the venturi passage adjustable, however, these scrubber designs do 
not provide the highly efficient particle remover capabilities of a 
fixed-throat venturi passage. 
SUMMARY OF THE INVENTION 
The present invention maintains a fixed-throat venturi design while adding 
the adjustability of a plug-type scrubber in order to accommodate larger 
gas volumes and minimum pressure drop requirements. 
The apparatus of the present invention for scrubbing contaminants from a 
gas stream with a scrubbing liquid comprises a vertically extending 
cylindrical vessel having an upper gas stream inlet and the lower gas 
stream outlet. An annular venturi assembly is fixed within the vessel. The 
assembly forms an annular venturi passage and forms a central opening. A 
vertically movable plug is supported within the central opening. The plug 
has a variable cross section and is movable between the position in which 
the plug closes the central opening and other positions in which portions 
of the gas stream flow around the plug and through a secondary annular gas 
flow passage in the central opening. A scrubbing liquid supply means is 
provided above the plug at the center line of the cylindrical vessel to 
supply scrubbing liquid to the apparatus. 
The annular fixed-throat venturi passage provides high efficiency of 
particulate removal at a minimum design gas flow rate. When the gas flow 
rate exceeds the design rate, the plug may be raised to increase the 
scrubbing capabilities of the apparatus by increasing the gas flow passage 
cross section, and cleaning the remaining portion of the increased gas 
flow, thereby increasing the maximum design gas volume. Similarly, for the 
maximum pressure drop, the total gas flow is directed into the annular 
venturi passage. For a lower pressure drop, the plug is raised and a 
portion of the gas passes through the annular secondary passage created 
around the plug in the lifted position. 
Among the advantages of the scrubber of the present invention is that it 
combines the high efficiency of particulate removal inherent in a 
fixed-throat venturi scrubber with the compactness, simple construction, 
and low cost inherent in a plug-type scrubber. The scrubber of the present 
design also results in approximately up to a 50% savings in energy over 
that required to operate a plug-type wet scrubber obtaining the same gas 
cleanliness. Furthermore, the present invention provides the capability of 
cleaning up to three times the gas volume practically possible with either 
a plug or venturi scrubber of the same size when operating at the same gas 
pressure across the scrubber.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring more particularly to the drawings, FIG. 1 shows a vessel 10 which 
is generally cylindrical and extends vertically. The vessel 10 has a 
central cylindrical portion 11 with an upper cylindrical vertically 
extending gas inlet 12 and a lower cylindrical horizontally extending gas 
outlet 13. The gas inlet 12 is connected to a conduit 14 by means of an 
expansion joint 15. Contaminated gas is supplied through the conduit 14 
from a source as previously described. The contaminated gas enters through 
the inlet 12 and flows generally downwardly through the vessel 10 where it 
is cleaned. Clean gas is discharged to a conventional moisture eliminator 
through a conduit 16 which is attached to the gas outlet 13. 
The vessel 10 includes a horizontally extending ledge 18 which is located 
directly below the cylindrical gas inlet 12 and which extends radially 
outwardly beyond the cylindrical inlet 12. A radially inwardly converging 
portion 19 extends downwardly from the ledge 18 to the central cylindrical 
portion 11 of the vessel 10. The gas outlet 13 extends horizontally from 
the central cylindrical vessel portion 11. Below the central cylindrical 
portion 11 the vessel 10 has a conical bottom portion 20. 
Scrubbing liquid is provided to the vessel 10 through a liquid supply line 
22. The supply line 22 is connected to a central tube 23 which extends 
into the vessel 10, curving downwardly to supply a vertical flow of 
scrubbing liquid within the vessel. The liquid is discharged from the 
central tube 23 at the center line of the cylindrical vessel through a 
downwardly projecting outlet 24. The liquid supply line 22 is also 
connected to a plurality of lines 25 and 26 which supply scrubbing liquid 
to a plurality of circumferentially spaced tube 27 located around the 
inside periphery of the vessel 10 directly above the ledge 18. The 
discharge of the scrubbing liquid from the peripheral tube 27 is 
horizontal and tangential to the sidewalls of the cylindrical vessel 10. 
The tubes 27 discharge a portion of the scrubbing liquid onto the ledge 
18, so that the liquid flows along the ledge in a substantially circular 
flow pattern. The scrubbing liquid flows off the ledge 18 and flows around 
and down the interior side walls of the converging portion 19 of the 
vessel. 
As the clean gas passes horizontally outwardly through the gas outlet 13, 
the heavier liquid flows downwardly and is collected in the conical bottom 
portion 20 where it is withdrawn through a liquid outlet line 29. The 
scrubbing liquid removed through the line 29 is passed to a suitable means 
for disposal or treatment. The liquid may be physically or chemically 
processed for regeneration and recycled back to the supply line 22. 
The scrubbing liquid is intermixed with the contaminated gas in the upper 
part of the cylindrical center vessel portion 11 by means which includes a 
central movable plug assembly 31 surrounded by an outer fixed annular 
venturi assembly 32. The plug assembly 31 includes a generally 
frusto-conical vertically movable plug 33 supported on the top of a 
vertically extending rod 34. The bottom of the rod 34 is attached to 
suitable actuating means (not shown) for moving the rod vertically. The 
plug 33 is centrally located within the vessel 10 so that scrubbing liquid 
from the central tube 23 is discharged through the liquid outlet 24 
directly onto the center of the upper surface 35 of the plug 33. The upper 
surface of the plug 33 slopes downwardly as it extends radially outwardly 
from a central peak so that liquid discharged from the central tube 23 
flows outwardly and evenly over the plug 33. 
Surrounding the plug 33 is the venturi assembly 32 which comprises an inner 
ring 37 and an outer ring 38. The top of the outer ring 38 is secured to 
the side wall of the upper part of the central cylindrical vessel portion 
11 by an annular upper support bracket 39 located directly below the 
converging vessel portion 19. At the bottom the outer ring 38 is secured 
to the vessel side wall by a plurality of lower radially extending support 
braces 40 (FIGS. 2 and 3) which are circumferentially spaced and attached 
to a circular flange 41 which extends radially outwardly from the outer 
ring near the bottom. The inner ring 37 is attached to the outer ring 38 
by means of a plurality of radially extending ribs 42 (FIGS. 2 and 3). 
The outer ring 38 (FIG. 3) comprises a downwardly converging portion 44, a 
generally vertical throat portion 45, and a downwardly diverging portion 
46. Similarly, the inner ring 37 comprises a downwardly converging portion 
47, a vertically extending throat portion 48, and a downwardly diverging 
portion 49. The outer and inner rings together thus form an annular 
venturi passage 50 defined by a downwardly converging portion 44 and 47, a 
vertically extending throat portion 45 and 47, and a downwardly diverging 
portion 46 and 49. The dimensions of the venturi assembly 47 are selected 
to provide the desired dimensions of the annular venturi passage 50. The 
dimensions of the venturi passage 50 are fixed and are adapted for a 
particular optimum gas flow rate and pressure drop. The ribs 42 are 
circumferentially spaced within the annular venturi passage 50. 
A central circular opening is formed within the inner ring 37 of the 
venturi assembly 32, and the plug 33 is located within this central 
opening. The greatest outer diameter of the generally frusto-conical plug 
33 is approximately the same as the inner diameter of the top of the inner 
ring 37 so that the plug 33 closes the central opening within the inner 
ring 37 in its lower position as indicated in broken lines in FIGS. 1 and 
3. The plug 33 may be raised by means of the rod 34 to allow gas to flow 
around the plug through a secondary annular passage 52 formed between the 
plug and the inner ring 37 of the venturi assembly. As a result of the 
generally frusto-conical shape of the plug 33, the cross section of the 
secondary gas flow passage 52 may be enlarged or reduced by moving the 
plug 33 up or down. 
In the operation of the scrubber of the present invention, contaminated gas 
from a source is supplied through the conduit 14 and enters the vessel 10 
through the gas inlet 12. A minimum design gas flow is directed entirely 
through the fixed venturi passage 50, and the plug 33 is maintained in its 
bottom position substantially closing the secondary gas flow passage 52. 
Scrubbing liquid is supplied from the central tube 23 and discharged 
through the liquid outlet 24 onto the plug 33 and along the walls of the 
inner ring 37 of the venturi assembly 32, and scrubbing liquid is supplied 
through the outer tubes 27 and along the interior wall of the converging 
portion 19 of the vessel 10 and along the wall of outer ring 38 of the 
venturi assembly. If the gas flow increases, the plug 33 is moved upwardly 
a certain distance, opening the secondary gas flow passage 52 a 
predetermined amount and permitting the additional portion of the gas to 
flow through the passage 52. Scrubbing liquid from the central tube 23 
flows around the outside of the plug and through the passage 52 where it 
is intermixed with the gas, cleaning the additional portion of the gas 
flow. 
A variable pressure drop is often required in a scrubber due to process 
requirements and pollutant characteristics. For a maximum pressure drop, 
the plug 33 is maintained in its lower position, closing the secondary 
passage 52, so that all of the gas is directed through the fixed venturi 
passage 50. If a smaller pressure drop is required, the plug 33 is raised 
to permit some of the gas to pass through the annular secondary passage 52 
around the plug. Due to the generally frusto-conical shape of the plug 33, 
the cross section of the secondary gas passage 52 is variable, and the 
desired variable pressure drop can be achieved by vertically moving the 
plug. 
Clean gas is passed outwardly through the gas outlet 13 and into the 
conduit 16 for discharge to a conventional moisture eliminator which 
separates the scrubbing liquid from the gas, while the remainder of the 
scrubbing liquid is collected in the conical bottom portion 20 of the 
vessel. The scrubbing liquid collected in the bottom portion 20 contains 
captured particulate and some gaseous compound absorbed from the gas 
stream. The collected liquid is discharged through the outlet line 29 for 
processing and eventual recirculation to the scrubber. 
Various modifications apparent to those skilled in the art may be made in 
the apparatus disclosed above, and changes may be made with respect to the 
features disclosed, provided that the elements set forth in any of the 
following claims or the equivalence of such be employed.