GB2036604A - Separation of finely divided solids from gases - Google Patents

Abstract

Gas containing finely divided solids is filtered through a moving mass of particulate material (2), which has disposed therein one or more electrically conductive members or electrodes (9) to which a high electrical potential is applied. Preferably the particulate material falls through a chamber (1) having louvred walls to permit the flow of gas thereacross. A cylindrical arrangement has gas flowing radially through an annular mass of particulate material containing a ring of electrodes. <IMAGE>

Description

SPECIFICATION Separation of finely divided solids from gases This invention relates to a process and apparatus for separating finely divided solids from a gas, particularly with the use of a particulate material through which the gas is filtered.

It has been known for some time that finely divided 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 divided solids from gas by passing a gas feed containing finely divided solids through a downwardly moving mass of granular, solid material are described in U.S. Patent No.4017 287 and its British counterpart, U.K. Patent No. 1 515 407. The method and apparatus described in this patent are now in commercial use in a number of operating plants.

The recent and current interest in air quality control has resulted in the imposition of increasingly stringent requirements with respect to the solids content of gaseous effluents from commercial plants.

While the method and apparatus described in U.S. Patent No.4,017278 have been satisfactory for removing solids from gaseous plant effluents, it has been found that if the gas feeds have a substantial proportion of very finely divided solids present in the feed, e.g. below 1-2 microns, then the efficiency of the apparatus expressed in terms of percent of total solids removed from the gas feed tends to fall. This decrease in efficiency is due to the fact that the very finely divided solid particles are more difficult to remove and a larger proportion of them manages to pass through the granular bed.Recourse to the use of more finely divided granular material in the beds, or the use of thicker beds of the coarser material, will increase the efficiency and may accomplish the desired reduction in the amount of finely divided solids contained in the effluent from the units, but recourse to either of these methods for achieving this result, results in a higher pressure drop as the gas feed passed through the bed and therefore more energy is required to move the feed gas through the unit. If thicker beds of granular material are used, not only does the pressure drop increase, but also the equipment must be of larger size in order to accommodate the 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 charging 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 electrically 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 voltageihigh 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 the present invention, a process for separating finely divided solids material from a gaseous feed stream comprises contacting the stream with a moving mass of substantially electrically resistive solid particles which has disposed therein an electrically conductive member, and applying a substantial voltage to the member during such contacting.

The invention also provides apparatus for removing finely divided solids from a gas comprising a housing defining a chamber and means for moving a mass of solid particles therethrough, at least a part of the chamber walls being perforated to permit passage of a stream of gas thereacross; inlet and outlet ducting defining a flow path for such a gas stream into and from the chamber; an elongate electrically conductive member disposed within the chamber; and means for applying an electrical potential between the member and the housing.

The electrically conductive member is so constructed, sized and formed that there is no significant impediment to either the movement of the bed of solid material or the flow of the feed gas through the material. This can be the case even when a plurality of members are employed. Movement of the bed of granular solid material normally along the length of the conductive member or members, exerts a continuous cleansing action on the surface of the member or members so that it continues throughout the onstream period to operate at essentially the efficiency of a completely clean conductor. Electric consumption is very low and all of the solid material removed from the gas feed leaves the system with the solid contact material from which it is then removed before the solid contact material is re-circulated.

The invention will now be described by way of example and with reference to the accompanying drawings, in which: Figure 1 is a side view of a granular bed separator embodying the invention; Figure 2 is a top plan view of the separator shown in Figure 1; Figure 3 is a side view of a granular bed separator corresponding in character to the granular bed separator described in British Patent No. 1 515 407, the disclosure of which is incorporated herein by reference, and having an electrically conductive member disposed within the granular bed; Figure 4 is a section taken on the line 4-4 of Figure 3; and Figure 5 illustrates an arrangement of the electrically conductive members which are disposed in the granular bed of the separator shown in Figures 3 and 4.

In Figure 1 elongate, hollow column 1, having a generally square or rectangular 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 the 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 port 6. A portion of the sidewall of column 1, which communicates with enlarged inlet port 6, is louvred, the louvre vanes being steeply inclined so that gas may enter through the louvres, 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 outlet port of generally rectangular cross section is in contact with the outer wall of column 1 and the portion of the wall of column 1 which communicates with the gas outlet port is steeply louvered to permit the escape of the gas, but to prevent escape of solid contact material through the louvres. Gas almost completely free of suspended solid particulate material is withdrawn through gas outlet line 8 which communicates with enlarged gas outlet port 7. The configuration of the electrically conductive member, shown, is a conductor composed of a series of elongated electrically conductive rods or pipes 9, which are in welded or screwed connection with metal rod 10, which is supported and insulated from the walls of column 1 by insulators 11 through which an electric current is introduced at high voltage.Column 1 and all of the interior metal portions including the louvered portions of the inlet and outlet wall of column 1 are electrically grounded. As the high voltage is applied to the electrically conductive member, a potential is established between the conductive member and the inlet and outlet louvres of column 1. This potential creates an electrical field which drives the finely divided solids contained in the feed gas towards and onto the moving bed of electrically low conductive material 2. Some portion of the finely divided solids will be attracted to and retained by the electrically conductive member.The continuous downward movement of the solid material in contact with the electrically conductive pipes 9, continuously moves any accumulation of solid matter from the surface of the conductive member preventing any buildup of an electrically resistant layer on the surface of the conductive member that consequentially would reduce and finally eliminate the creation of the electrical field from the electrically conductive member.

The path of least resistance to the feed gas is the path between enlarged inlet port 6 and enlarged outlet port 7. The mass of contact material lying above the upper level of outlet port 7 and below the bottom of outlet port 7 is of sufficient length to prevent any flow of gas through solids inlet opening 3 or through solids outlet opening 4. A portion of the gas may take a curved path which rises somewhat above and somewhat below the extremities of outlet port 7 during its traverse of the mass of contact material, but the gas following the path of least resistance finds its way to and out of outlet port 7 through outlet line 8.

Referring now to Figure 2 of the appended drawings, a top plan view of the column is shown. The electrically conductive member is shown made up of a plurality of pipe electrodes 9 which are in electrical contact with electrically conductive rod 10, at spaced intervals usually about two to twelve inches. Sufficient space is left between the electrodes 9 closest to the adjacent walls of column 1 to assure minimum voltage leakage between the end electrodes and the walls of column 1. The electrically conductive rod 10 is supported by insulated engagement with the lateral walls of column 1.

Figure 3 of the appended drawings shows an apparatus for separating finely divided solids from gas, which corresponds generally to the apparatus described in British Patent No. 1 515 407. Cylindrical vessel 20, usually having a flat or frusto-conical top and a tapered frusto-conical bottom, has a solids outlet opening 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 side of vessel 20, respectively. The gas feed containing suspended solid materials may be introduced through opening 24 and the clean gas withdrawn through opening 23, or vice versa. A first cylindrical wall member 25, having a louvered surface and a diameter smaller than that of vessel 20, is concentrically disposed in vessel 20 to leave an annular space 26 between the side wall of vessel 20 and the wall member 25.Cylindrical wall member 25 is sealed at 28, at its upper end, to the top of vessel 20, to close off annular space 26 at its top. Annular space 26 is open at its bottom, communicating with the frusto-conical bottom of vessel 20. A second cylindrical wall member 29, having a diameter smaller than that of first cylindrical wall member 25, and having a louvered surface, is concentrically disposed in the first cylindrical wall member 25, to leave an annular space between the two cylindrical wall members which extends from the top of the bottom of vessel 20. Second cylindrical 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 bottom portion of vessel 20. A mass of particulate solid contact material 30 fills the annular space between cylindrical wall members 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 solid 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 the vessel 20 and reintroduced into the annular space between cylindrical walls 25 and 29 via the solids inlet line 22.An electrically conducting grid consisting of a plurality of electrodes 34 is disposed in the mass of solid contact material so as to extend vertically downward in the mass of contact material and lie between cylindrical walls 25 and 29. Electric current is introduced by a line 35 through insulator 36 which insulated the power line from the vessel, into contact with electrodes 34.

Figure 4 is a cross-section of Figure 1 along line 4-4. A plurality of electrodes 34 are attached to conductive ring 37 by welding or screw attachment and extend downwardly from the ring through the mass of solid contact material.

Figure 5 is a grid arrangement showing a plurality of electrodes 34 which are welded or screwed to attach conductive ring 37 at the top and welded or screwed to attach conductive ring 38 at the bottom. Attachment of the electrodes to the two rings hold them in fixed positions relative to the mass of contact material in which they are embedded. Support rods 39 extend from insulators 36 down to ring 37 to which they are connected. Insulators 36 are attached to the top of the vessel and provide support for support rods 39 and the ring and electrodes attached.

Direct or alternating current at a voltage in the range about 2000 to 50,000 volts is applied to the electrodes.

The material constituting the mass of particulate solid contact material through which the feed gas passes should have low electrical conductivity and be temperature resistant at the temperature of the feed gas, preferably have rounded rather than angular surfaces to facilitate flow and prevent bridging and the particles should have reasonable uniformity in size. Particle sizes preferably range from about 2 mm 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. sieve size No. 4 particles, 26% U.S. sieve 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 event 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 contatined 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 micron 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 movement 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.

A solid gas separator of the type described in reference to Figure 3 of the drawings was modified to insert an electrically conductive member as shown in Figure 4. The separating unit has 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 per hour.Forty-two electrodes which were one-inch pipes, were attached to a ring such as ring 37 of Figure 5 of the drawings, on six-inch centres and the resulting grid arrangement was disposed in the annular mass of particulate material. A number of runs were then made using the apparatus as modified which was used to remove finely divided solid material from stack gases from a boiler fired with hog fuel.

The following table presents the data obtained in a series of runs in which the grid voltage applied was varied from 0 to 20,000 volts. The several headings in the Table taken in order are: gas flow, actual cubic feet per minute at the outlet from the unit; pressure drop through the unit measured in inches of water; grid voltage in kilovolts; grid current in milliamperes; temperature of the entering gas in degrees Fahrenheit; temperature of the gas leaving the unit in degrees Fahrenheit; front half content of finely divided solids in the feed gas measured in grains per standard dry cubic foot and content of the outlet gas in grains per standard dry cubic foot corrected to give the gas effluent a uniform C02 content of 12% (these measurements were made pursuant to the EPA METHOD No. 5 for determining total solids in gas); opacity recorded in percent and obtained by subjecting the effluent gas to tests in a Lear Siegler opacity metre; efficiency of solid material removal expressed in percent of total contained in feed.

Run Gas flow #P Grid Grid TIN TOUT Front haif Opacity Eficiency No. ACFM "H2O Voltage Current F F gr/sdcf at 12% CO2 % % outlet kV MA Inlet Outlet 1 26220 2.8 0 0 308 320 0.191 0.033 5-6 83 2 25000 2.8 10 3 310 295 0.164 0.018 2 90 3 27100 3.0 10 13 352 340 0.320 0.011 4 97 4 26100 2.7 10 4 315 285 0.158 0.014 2 90 5 26000 3.0 15 9 325 300 0.152 0.009 1 94 6 26600 3.2 15 14 308 280 0.141 0.008 1.2 97 7 26900 3.3 15 6.5 320 305 0.137 0.010 1.0 92 8 25900 3.2 20 16-30 345 325 0.300 0.014 1-5 95 9 24900 3.0 20 14 350 330 0.325 0.012 1-3 96 10 25100 - 0 0 390 350 0.244 0.057 12 77 Both direct current and alternating current have been applied to the electrodes in the voltage ranges above described. Direct current is equally effective whether positive or negative.Alternating current produces an appreciable improvement in solids removal but appears to be less effective than direct current.

Itis clear from the data presented in the Table that disposing electrodes in the mass of solid contact material and applying a fairly high voltage to the electrodes results in a very marked increase in efficiency of total solids removal. The precise manner in which this arrangement of apparatus accomplishes the improved results is not entirely clear, but the result itself is completely clear and highly desirable.

As above indicated, applied voltages are generally in the range 2,000 to 50,000 volts, preferably in the range 10,000 to 25,000 volts. While these ranges are quantitatively expressed, it should be noted that in a quantitative sense it is only necessary that the applied voltage should be sufficient to cause an appreciable improvement in solids removal efficiency.

Claims (15)

1. A process for separating finely divided solid materials from a gaseous feed stream comprising contacting the stream with a moving mass of substantially electrically resistive solid particles which has disposed therein an electrically conductive member; and applying a substantial voltage to the member during such contacting.

2. A process according to Claim 1 wherein the voltage is in the range 2000 to 50,000 volts.

3. A process according to Claim 1 or Claim 2 wherein the mass of particles is moved vertically downwards while the stream of gas passes transversely thereacross.

4. A process according to any preceding Claim wherein the mass of particles is substantially confined between two walls between which are disposed a plurality of spaced elongate conductive members to which the voltage is applied.

5. A process according to Claim 4 wherein the walls are cylindrical and perforate, and wherein the gaseous stream passes radially inwardly through the walls and the mass of particles, the conductive members being circumferentially spaced within the cylindrical chamber between the walls.

6. A process for separating finely divided solid particles from a gaseous feed stream substantially as described herein with reference to and as illustrated by Figures 1 and 2 or Figures 3 to 5 of the accompanying drawings.

7. Apparatus for removing finely divided solids from a gas comprising a housing defining a chamber and means for moving a mass of solid particles therethrough, at least a part of the chamber walls being perforate to permit passage of a stream of gas thereacross; inlet and outlet ducting defining a flow path for such a gas stream to and from the chamber; and elongate electrically conductive members disposed within the chamber; and means for applying an electrical potential between the member and the housing.

8. Apparatus according to Claim 7 wherein the chamber is of rectangular cross-section; and wherein a plurality of electrically conductive members are disposed therein spaced between opposed sidewalls thereof.

9. Apparatus according to Claim 7 wherein the chamber is of annular cross-section; and wherein a plurality of electrically conductive members are circumferentially spaced therewithin.

10. Apparatus according to Claim 8 or Claim 9 wherein the gas stream flow path traverses the chamber between opposed walls thereof.

11. Apparatus according to Claim 9 or Claim 10 wherein the gas stream flow path is radially inwards.

12. Apparatus according to Claim 9 or Claim 11 wherein the housing comprises a substantially cylindrical vessel having a first gas flow opening at one end thereof, a second gas flow opening in its side wall, and an outlet for solid particles at the other end, the chamber being defined within the housing by two substantially coaxial cylindrical walls, the outer of which is in sealing engagement with said one end of the vessel and the cylindrical space between the outer wall and the vessel side wall being in open communication with the solids outlet at the other end, and the inner of which is in open communication with the first gas flow opening and the solids outlet, the apparatus including means for withdrawing solid particles from the solids outlet and for delivering solid particles to the space between the cylindrical walls at said one end of the vessel.

13. Apparatus according to any of Claims 7 to 12 wherein the perforations in the chamber walls comprise outwardly extending louvres forming louvre openings of sufficient size to permit the passage of solid particles therethrough.

14. Apparatus according to Claim 13 wherein the louvre vanes extend outwardly of the chamber in a direction opposite to the intended direction of flow of a said mass of solid particles and at an angle of 150 to 800 to the wall from which they extend.

15. Apparatus for removing finely divided solids from a gas substantially as described herein with reference to and as illustrated by Figures 1 and 2 or Figures 3 to 5 of the accompanying drawings.