CA1136067A

CA1136067A - Method and apparatus for removing finely divided solids from gas

Info

Publication number: CA1136067A
Application number: CA000338531A
Authority: CA
Inventors: Richard G. Reese
Original assignee: Combustion Power Co Inc
Current assignee: Babcock and Wilcox Co
Priority date: 1978-10-30
Filing date: 1979-10-26
Publication date: 1982-11-23
Also published as: AU5193079A; AU537853B2; IT7950682A0; DE2942223A1; FR2440221B3; CH642869A5; GB2036604B; ZA795571B; FR2440221A1; IT1148211B; JPS5559820A; GB2036604A; SE7908972L

Abstract

ABSTRACT OF THE INVENTION

An improvement in the method of removing finely divided solids from gas by passing a feed gas containing finely divided solids transversely through a moving bed of material of low electrical conductivity, which consists in disposing within the moving bed an electrically conductive member to which a voltage in the range about 2000 to 50,000 volts is applied, said member, being so sized and formed that it does not significantly impede either the movement of the bed or the flow of the feed gas through the bed.

Description

~1360i;7 !
. BACKGROUND OF THE INVENTION-l It has been known for some time that finely divided i solids suspended in gas may be removed from the gas by passing . the gas through a bed of granular, solid material, as shown in , Perry's Chemical Engineers' Handbook, 4th Edition, McGraw-Hill, . at 20-74 and 20-75. A method and apparatus for removing finely vided solids trom gas by passing a gas Eeed contoin ng Ei~e~ ¦

,i /.' ~ :113~ 67 . . .
1 ¦ divided solids through a downwardly moving mass of granular,

2 1 solid material are described in U.S. Patent No. 4,017,278.

3 ¦ The method and apparatus described in this patent are now in ¦ commercial use in a number of operating plants.
5 ¦ The recent and current interest in air quality control 6 has resulted in the imposition of increasingly stringent require-7 I ments with respect to the solids content of gaseous effluents 8 ¦ from commercial plants.
g I While the method and apparatus described in U.S. Patent 10 ¦ No. 4,017,27~ have been satisfactory for removing solids from 11 ¦ gaseous plant effluents, it has been found that if the gas feeds 12 I have a substantial proportion of very finely divided solids 13 1 present in the feed, e.g., below 1-2 microns, then efficiency 14 1 of the apparatus expressed in terms of percent of total 15 ¦ solids removed from the gas feed tends to go down. This de-16 ¦ crease in efficiency is due to the fact that the very finely 17 ¦ divided solid particles are more difficult to remove and a 18 ¦ larger proportion of them manages to pass through the granular 19 1 bed without being removed. Recourse to the use of more finelv 20 1 divided granular material in the beds, or the use of thicker 21 ¦ beds of the coarser material, will increase the efficiency and 22 ¦ may accomplish the desired reduction in the amount of finely di-23 ¦ vided solids contained i~L the effluent from the units, but 24 recourse to either of these methods for achieving this result, 25 ¦ results in a higher pressure drop as the gas feed passes through 26 1 the bed and therefore more energy is required to move the feed 27 ¦ gas through the unit. If thicker beds of granular material are 28 used, not only does the pressure drop increase, but also the 29 1 equipment must be of larger size in order to accomodate the . .

. ` ~,, 1, increased loading of granular material.
Consideration has been given to charging the finely divided solids in the feed gas by means of a high voltage charg-ing device such as that used in electrical precipitation, as described in Perry's Chemical Engineering Handbooks at 20-82 et seq., passing the feed gas containing the now-charged finely divided entrained solids through the granular solids bed. The metallic structure enclosing the granular solids bed is electri-cally grounded and will capture the charged particles. This concept will indeed reduce the residual solids content of the gas to sufficiently low levels to meet the rigid maximum solids requirements but this result is achieved by using only the inlet face of the granular solids bed not taking advantage of the deep bed filtration and also at the very considerable cost for the addition of high voltage/high energy electrical precipitator type charging device in the process flow line and the past and present difficulties in operation of such charging devices which arise out of the buildup of heavy solids accumulation on the conductor surfaces.
According to one aspect of the present invention there is provided a process for separating finely divided solid materials contained in a gaseous feed stream by a contact in the stream with a downwardly moving mass of finely divided solid particles of low electrical conductivity. An electrical conductive member to which a voltage in the range of 2,000 to 50,000 volts is applied is disposed in the mass of solid particles.
According to another aspect of the present invention there is provided an apparatus for re~oving finely divided solids from a gas stream, the apparatus having a oenerally cylindrical vessel wlth pc/ ~

i~36~67 one gas flow opening in a top portion thereof and another gas flow opening in a side wall thereof with a solids outlet centrally disposed in a bottom portion thereof and a solids inlet opening disposed in its top portion and disposed laterally from the gas flow opening. A first generally cylindrical wall member has a diameter less than the ~ide wall of the vessel and is disposed in the vessel to provide an elongated annular space between the first wall member and the side wall of the vessel and in sealing engagement with the top portion of the vessel, the annular space between the first wall member.and the side wall of the vessel being in open communication with the solids outlet at the bottom of the vessel. A second generally cylindrical wall nlember is provided which has a diameter less than the first wall member disposed in the first wall member to provide an elongated annular space between the two wall members, the space enclosed by the second wall member being in open communication with the gas flow in the top portion of the vessel and with the solids outlet at the bottom portion of the vessel. A mass of particulate solid contact material fills the annular space between the two cylindrical wall members from the upper end of the second cylindrical member to the lower portion of the vessel, the mass being in open communication with the solids outlet opening. The surface of the first cylindrical wall member is a louvered surface formed by perforating the wall to form outwardly extending louver vanes inclinded to the vertical, the first cylindrical wall member being in open communication throughout the entire louvered surface thereof with the gas flow opening in the side wall of the vessel, the surface of the second cylindrical wall being louvered similarly to the first cylindrical wall member but having inwardly extending - 3a -pc/, ~

~136067 louver vanes. Means is provided for continuously withdrawing particulate solid material through the solids outlet at the bottom of the vessel and means is provided for continuously introducing particulate solid materials into the top of the annular space between the two cylindrical wall members. An electrically conductive member is supported by and insulated from the vessel and extends downwardly into the mass of solid contact material.
A feature of a specific embodiment of the invention is that the electrically conductive member is so constructed, sized and formed that there is no significant impediment to either the - 3b -pc/~

ll 1 ~ m veme~t of the ~cd of solid materi~l or tlle ~low o~ tlll~ Fec~l gas 2 through the material. The downward movement of the bed of 3 ¦ granular solid material exerts a continuous cleansing action

4 on the surface of the electrically conductive member so that it continues throughout the onstream period to operate at essen-6 tially the efficiency of a completel.y clean con~.ucto~. Electric 7 ¦ consumption is very low and all of the solid material removed 8 1 from the gas feed leaves the system with the solid contact 9 material from which it is then removed before the solid contact material is re-circulated.
11 l 12 ¦ DETAILED DESCRIPTION OF THE INVENTION
13 In the appended drawings, Figure l is a side view of 14 a very simple form of a granular bed separator, having an electrically conauctive member disposed in the bed of granular 16 material.
17 Figure 2 is a top plan view partially broken away of the ¦
18 separator shown in Figure l.
19 Figure 3 is a side view of a granular bed separator corresponding in character to the granular bed separator des-21 1 cribed in U.S. Patent No. 4,017,278, and having an electrically 22 conductive member disposed within the granular bed.

24 Figure 4 is asection at 4-4 of the granular bed separator shown in Figure 3.
26 Figure 5 illustrates an arrangement of the electrically 27 conductive member which is disposed in the granular bed 28 shown in Figures 3 and 4.

-4- j 1136~67 Referring now to Figure 1 of the appended dra~ings, elongated, hollow column 1, having a generally square or rec-tangular cross section is filled with a mass of particulate solid contact material 2. Solid contact material is introduced into the column through solids inlet opening 3 and withdrawn from the column through solids outlet opening 4. The solid contact material moves downwardly through the column by gravity flow at a linear rate in the range 0.5 to 40 feet per hour. The downward movement of the solid contact material may be made intermittent if desired. The rate of flow of the solid contact material is controlled by the rate at which solid contact material is withdrawn through solids outlet opening 4 and the actual rate of movement will vary with the quantity of finely divided solid materials contained in the feed entering the column, being greater when the amount of contained finely divided solid material is high.
The feed gas, which contains finely divided solid material which it is desired to remove, is introduced through gas inlet 5 which communicates with enlarged, generally rectangular gas inlet housing 6. A portion of the side wall of column 1, which communicates with enlarged inlet housing 6, is louvered, the louver vanes being steeply inclined so that gas may enter through the louvers, but escape of solid contact material through them is prevented. The entering gas contacts the particulate solid contact material 2 and moves across the mass of solid contact material to reach the opposite wall of column 1. At the opposite wall, an enlarged gas outlet housing 7 pc/ ~

606~ ~

1 of generally rectan~ular cross section is in contact with the 2 outer wall of column 1 and the portion of the wall of column 1 3 which communicates with the gas outlet hous~ng is steeply louvered 4 to permit the escape of the gas, but to prevent escape of solid contact material through the louvers. Gas almost completely 6 free of suspended solid particulate material is withdrawn through 7 gas outlet line 8 which communicates with enlarged gas outlet 8 housing 7. The configuration of the electrically conductive member, 9 shown, is a conductor composed of a series of elongated electri-cally conductive rods or pipes, 9, which are in welded or 11 screwed connection with metal rod 10, which is supported and 12 insulated from the walls of column 1 by insulators 11 through 13 which an electric current is introduced at high voltage. Column 14 1 and all of the interior metal portions including the louvered portions of the inlet and outlet wall of column l are electrically 16 grounded. As the high voltage is applied to the electrically 17 conductive member, a potential is established between the 18 conductive member and the inlet and outlet louvers of column l.
19 This potential creates an electrical field which drives the finely divided solids contained in the feed gas towards and on 21 to the moving bed of electrically low conductive material 2.
22 Some portion of the finely divided solids will be attracted to 23 and retained by the elec~rically conductive member. The 24 continuous downward movement of the solid material in contact 26 with the ele~trically conductive pipes 9, continuously moves 1 ¦ ny accumulation of solid matter from the surface ~f the con-2 ductive member preventing any buildup of an electrically resistan 3 ¦ layer on the surface of the conductive member that consequentiall ¦ would reduce and finally eliminate the creation of the electrical

5 ¦ field from the electrically conductive member.

6 ¦ The path of least resistance to the feed gas is the

7 ¦ path between enlarged inlet housing 6 and enlarged outlet housing 7.

8 ¦ The mass of contact material lying above the upper level of

9 outlet hous.ing 7 and below the bottom of outlet housing 7 is of

10 ¦ sufficient length to prevent any flow of gas through solids

11 ¦ inlet opening 3 or through solids outlet opening 4. A portion

12 ¦ of the gas may take a curved path which rises somewhat above

13 ¦ and somewhat below the extremities of outlet housing 7 during its

14 ¦ traverse of the mass of contact material, but the gas following

15 ¦ the path of least resistance finds its way to and out of outlet

16 ¦ housing 7 through outlet line 8,

17 ¦ Referring now to Figure 2 of the appended drawings, a

18 top plan view of the column is shown. The electrically conduc-

19 i 21 , /
22 / ._ za ~

' _7_ il36067 1 ¦ tive member is shown made up of a plurality o pi~-e electro~cs 9 2 which are in electrical contact wlth electrically con~uctive 3 I rod 10, at spaced intervals usualiv about two to twelve inches.
4 Sufficient space is left between the electrodes 9 closest to the S I adjacent walls of column l to assure minimum voltage leakage 6 between the end electrodes and the walls of column l. The 7 electrically conductive rod lO is supported by insulated 8 engagement with the lateral walls of column l.
9 Figure 3 of the appended drawings shows an apparatus for separating finely divided solids from gas,which corresponds ll generally to the apparatus described in U.5. Patent No. 4,017,27~.
12 Cylindrical vessel20, usually having a flat or frusto-conical 13 top and a tapered frusto-conical bottom, has a solids outlet 14 line -21 at its bottom and at least one solids inlet opening 22 at its top. Gas flow openings 23 and 24 are at the top and 16 side of vessel 20, respectively. The gas feed containing sus-17 pended solid materials may be introduced through opening 24 18 and the clean gas withdrawn through opening 23, or vice versa.
19 A first cylindrical wall member 25, having a louvered surface and a diameter smaller than that of vessel 20, is concentrically 21 ¦ disposed in vessel 20 to leave an annular space 26 between the 22 side wall of vessel 20 and the wall member 25. Cylindrical wall 23 member 25 is sealed at 28, at its upper end, to the top of 24 vessel 20, to close off annular space 26 at its top. Annular space 26 is open at its bottom, communicating with the frusto-26 conical bottom of vessel 20. A second cylindrical wall member 27 2g, having a diameter smaller than that of first cylindrical wall 28 member 25, and having a louvered surface, is concentrically dis-2 posed in the first cylindrical wall member 25, to leave an 1136~i7 annular space between the two cylindrical wall members which extends from the top to the bottom of vessel 20. Second cylin-drical wall member 29 communicates with gas flow opening 23 at the top of the vessel and generally extends beyond the top of vessel 20 as either a chimney from which treated gas leaves the vessel or as an opening through which gas to be treated is introduced. The lower end of cylindrical wall 29 communicates with the mass of contact material which is disposed in the frusto-conical ~ottom portion of vessel 20. A mass of particulate solid contact material 30 fills the annular space between cylin-drical wall memhers 25 and 29, and the mass of solid contact material is in open communication with solids outlet 21 at the bottom of the vessel. Solid contact material leaving the annular space between walls 25 and 29, through solids outlet opening 21, enters a solids separator 31, capable of separating finely divided solids from the particulate of solid contact material.
Suitable solids separators include oscillating screen separators which may be either reciprocating or gyratory screens having screens with openings sized to permit passage of the very finely divided materials separated from the gas under treatment from the particulate solid contact material which circulates through the system. The finely divided material which has been collected from the feed gas and which is shaken away from the surface of the contact material in solid separator 31, is withdrawn through opening 32. Solid contact material leaving the solids separator 31 is transported by elevator 33 to the top of vessel 20 and reintroduced into the annular space between cylindrical walls 25 and 29 via the solids inlet line 22. In the present invention, an electrically conducting grid consisting of a plurality of g pc/ /1- .

~i36067 electrodes 34 is disposed in the mass of solid contact material so as to extend vertieally downward in the mass of contact material and lie between eylindrieal walls 25 and 29. Electric current is introduced by a line 35 through lnsulator 36 which insulates the power line from the vessel, into contact with electrodes 34.
Figure 4 is a eross seetion of Figure 1 along line 4-4. A plurality of electrodes 3~ are attached to eonduetive ring 37 by welding or screw attachment and extend downwardly ~rom the ring through the mass of solid cor.tact material.
Figure 5 is a grid arrangement showing a plurality of electrodes 34 which are welded or screwed to attach conductive ring ~7 at the top and welded or screwed to attach ring 3~ at the bottom. Attachment of the electrodes to the two rings hold them in fixed position relative to the mass of contact material in which they are embedded. Support rods 39 extend from insul-ators 36 down ring 37 to which they are connected. Insulators 36 are attaehed to the support rods 39 and to the top o~ the vessel and provide support for support rods 39 and the ring and eleetrodes attached.
Direet current at a voltage in the range about 2000 to 50,000 volts is applied to the eleetrodes. The material constituting the mass of particulate solid contact material through whieh the feed gas passes should be electrically non-eonduetive or of low eonduetivity and temperature resistant at the temperature of the feed gas, preferably has rounded rather than angular surfaces to facilitate flow and prevent bridging and the partieles should ~ave reasonable uniformity in size.
Direet or alternating current at a voltage in the `~3 cg/ ~' - 10 -1~3~

range about 2000 to 50,000 volts is applied to the electrodes.
Particle sizes preferably range from about 2 m~l . diameter to 12.5 mm. diameter. A mass of particles in which the largest particles present in substantial quantity have diameters not more than 3 to 4 times the diameter of the smallest particles present in substantial quantity is considered a reasonably uniform mass and exhibits good flow properties in the system. Coarse beach sand or finely divided gravel are cheap, readily available and constitute excellent contact masses. A San Simeon sand containing 8% U.S. sieve size No. 6, 62% U.S. sieve size No. 7, and 30% U.S.
sieve size No. 8 is satisfactory coarse beach sand. Fine gravel consisting of 66% U.S. seive size No. 4 particles, 26~ U.S. siev~
size No. 5 particles, and the remainder only slightly larger than No. 4 and slightly smaller than No. 6 is a suitable fine gravel for use in the process. In the even~ that gas at a very high temperature is to be treated then ceramic or quartz beads and similar materials which are low-conductive material and more resistant to temperature fracture than sand or gravel should be used as the solid contact material.
The downward flow rate of a solid contact material may lie in the range of about one-half foot to forty feet per hour, but is preferably in the range of about three to ten feet per hour. The downward movement continuously scrubs the surface of the electrically conductive member.
The applied voltage may be in the range 2000 to 50,000 volts. More complete removal of the very fine particles contained in the feed gas is achieved at higher voltages. For instance, where no voltage whatsoever is applied in a commercial-sized unit corresponding to that shown in Figure 3, particles of one-half pc/~,j~', ~i3~0~7 micron in size are 65% removed, when a voltage of 10,000 volts is applied to the electrodes inserted in contact material, removal of the small particles rises to 75% and at 20,000 volts, the removal is 95% of total contained half-micron material.
The configuration of the electrically conductive member may be varied but the controlling characteristic of its size and shape is that it must not significantly affect the downward movement of the solid contact material, nor the transverse move-ment of the gas feed through the mass of contact material. Rods or pipes, one-half to an inch in diameter, are suitable; also, flat bars one quarter inch in thickness and up to two inches in width may be used. Also, a coarse cylindrical screen may be used instead of the rod in the form of apparatus shown in Figure 3 of the drawings.
As indicated, the above described separator, as generally described in U.S. Patent No. 4,017,278, was modified by inserting an electrically conductive member. The separating unit had a design capacity of 40,000 actual cubic feet per minute and was used to process stack gases from a power house boiler fired with hog fuel. The particulate solid material used was No. 4 - No. 5 U.S.
sieve size. The annular mass of particulate solid material had a thickness of 18 inches and a height of about 16 feet. The rate of flow of the mass of particulate solid material downwardly through the annulus between cylindrical wall members 25 and 29 of Figure 3 was approximately three feet , .
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. 1' 1 ¦ per hour. Forty-two electrodes which were one-inch pipes, were 2 attached to a ring such as ring 37 of Figure 5 of the drawings, on 3 ¦ six-inch centers and the resulting grid arrangement was disposed in the annular mass of particulate material. A number of runs 5 ¦ were then made using the apparatus as modified which was used to 6 remove finely divided solid material from stack gases from a 7 I boiler fired with hog fuel.

8 ¦ The following table presents the data obtained in a g ¦ series of runs in which the grid voltage applied was varied from 10 ¦ 0 to 20,000 volts. The several headings in the table taken in 11 order are: gas flow, actual cubic feet per mi,nute at,the outlet 12 from the unit; pressure drop through the unit measured in inches 13 ¦ of water; grid voltage in kilovolts; grid current in milliamperes;
14 I temperature of the entering gas in degrees Fahrenheit; tempera-15 ¦ ture of the gas leaving the unit in degrees Fahrenheit; front half 16 ¦ content of finely divided solids in the feed gas measured in 17 grains per standard dry cubic foot and content of the outlet gas 18 I in grains per standard dry cubic foot corrected to give the gas 19 ¦ effluent a uniform CO2 content of 12% (these measurement.s were made pursuant to the EPA Method No. 5 for determining total 21 ¦ solids in gas); opacity recorded in percent and obtained by 22 ¦ subjecting the effluent gas to tests in a Lear Siegler opacity 23 meter; efficiency of solid material removal expressed in percent 26 ¦ o total colltai~ed ln feed. ¦

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1 ~oth direct current and alternating current have 2 been applied to the electrodes in the voltage ranges above 3 described. Direct current is equally effective whether 4 positive or negative. Alternating current produces an S appreciable improvement in solids removal but appears to be 6 less effective than direct current.
7 It is clear from the data presented in the table that 8 disposing electrodes in the mass of solid contact material and 9 applying a fairly high voltage to the electrodes results in a very marked increase in efficiency of total solids removal.
11 The precise manner in which this arrangement of apparatus 12 accomplishes the improved results is not entirely clear, but 13 the result itself is completely clear and highly desirable.
14 As above indicated, applied voltages are generally in the range 2,000 to 50,000 volts, preferably in the range 16 l0,000 to 25,000 volts. While these ranges are quantitatively 17 expressed, it should be noted that in a quantitative sense it 18 is only necessary that the applied voltage should be sufficient 19 to cause an appreciable improvement in solids removal efficiency.

223 ._ 33~

.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a process for separating finely divided solid materials contained in a gaseous feed stream by contacting said stream with a downwardly moving mass of finely divided solid particles of low electrical conductivity, the improvement which comprises disposing in the mass of solid particles an electrically conductive member to which a voltage in the range of 2,000 to 50,000 volts is applied.

2. In a process for removing finely divided solids from a gas by passing a feed gas transversely through a down-wardly moving mass of finely divided solid particles of low electrical conductivity, the improvement which comprises disposing in the mass of solid particles an electrically conductive member to which a voltage in the range about 2,000 to 50,000 volts is applied.

3. An apparatus for removing finely divided solids from a gas stream, comprising a generally cylindrical vessel having one gas flow opening in a top portion thereof and another gas flow opening in a side wall thereof, a solids outlet centrally disposed in a bottom portion thereof and a solids inlet opening disposed in its top portion and disposed laterally from the gas flow opening, a first generally cylindrical wall member having a diameter less than the side wall of the vessel and disposed in the vessel to provide an elongated annular space between said first wall member and the side wall of the vessel and in sealing engagement with the top portion of the vessel, and the annular space between the first wall member and the side wall of the vessel being in open communication with the solids outlet at the bottom of the vessel, a second generally cylindrical wall member having a diameter less than said first wall member disposed in said first wall member to provide an elongated annular space between the two wall members, the space enclosed by said second wall member being in open communication with the gas flow opening in the top portion of the vessel and with the solids outlet at the bottom portion of the vessel, a mass of particulate solid contact material filling the annular space between the two cylindrical wall members from the upper end of the second cylindrical wall member to the lower portion of the vessel, said mass being in open communication with the solids outlet opening, the surface of the first cylindrical wall member being a louvered surface formed by perforating said wall to form outwardly extending louver vanes inclinded to the vertical, said first cylindrical wall member being in open communication throughout the entire louvered surface thereof with the gas flow opening in the side wall of the vessel, the surface of the second cylindrical wall being louvered similarly to said first cylindrical wall member but having inwardly extending louver vanes, means for continuously withdrawing particulate solid material through the solids outlet at the bottom of the vessel and means for continuously introducing particulate solid material into the top of the annular space between the two cylindrical wall members, and an electrically conductive member supported by and insulated from the vessel and extending downwardly into the mass of solid contact material.

4. The apparatus of claim 3 wherein said electrically conductive member comprises a grid having a conductive ring with a plurality of elongate electrode members secured at spaced intervals throughout, said electrode members aligned with the direction of flow of the particulate contact material from the top to the bottom of the vessel.