Combined sonic agglomerator/cross flow filtration apparatus and process for solid particle and/or liquid droplet removal from gas streams

A gas filtration apparatus and process is disclosed combining heating to maintain undesired material in the form of liquid droplets, sonic agglomeration and porous cross flow filtration. The apparatus and process are particularly suited to gas streams comprising particulates and/or liquid droplets of under 10 microns in diameter, such as the effluent gas stream of coal gasifiers, which are at a high temperature and high pressure. Liquid droplets in the gas stream are agglomerated by sonic agglomeration, and a portion of the gas stream is then passed through a porous cross flow filter element for separation of the agglomerates resulting in a clean gas stream by a continuous, low pressure drop and self-cleaning filtration process. The gas stream may be seeded to enhance agglomeration and/or to induce chemical reaction of undesired gaseous components for their removal as liquids.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a gas filtration apparatus and process, and more 
particularly to a gas filtration apparatus and process which combines 
heating with sonic agglomeration and porous cross flow filtration for 
liquid droplet removal. 
Gas streams comprising small solid particles and/or liquid droplets are 
produced by many chemical processes and by combustion processes, such as 
in the effluent gas stream of coal gasifiers. Such particulate matter, 
liquid droplets and frequently, undesired gases, should be removed before 
the gas stream is passed to downstream equipment or processes or is 
released to the atmosphere. It has been especially difficult to remove 
such particulate matter from high temperature and high pressure gas 
streams frequently encountered. Many of the conventional methods, such as 
cyclones and electrostatic precipitation, fail under high pressure and 
temperature conditions. Particles and droplets which are over 10 microns 
in diameter may be removed from gas streams by conventional porous filters 
and the like, but smaller particles and droplets are difficult to remove 
because the porosity of the filter must be so small that it creates a 
substantial pressure drop across the filter. This is undesirable because 
the higher pressure drop requires greater energy and may inhibit the 
combustion process. Thus, porous flow filters do not remove small 
particles or liquid droplets from gas streams efficiently. Liquid droplets 
may be more efficiently filtered by a cross flow filter than solid 
particles of corresponding size. 
Small droplets in a turbulent gas stream collide with each other and may 
agglomerate with other droplets on impact to form larger agglomerates. The 
number of collisions may be increased by confining the stream in a tube or 
flue, and subjecting the droplets in the flue to a sonic field. Sonic 
agglomeration has been used for many years to agglomerate small solid 
particles or liquid droplets into larger agglomerates. Liquid droplets are 
easier to agglomerate by sonic agglomeration than solid particles of 
corresponding size. 
DESCRIPTION OF THE PRIOR ART 
Sonic agglomerators have been used in combination with inertial separators, 
such as cyclones, as described in U.S. Pat. No. 2,935,375. Cyclones spin 
the gas stream, exerting centrifugal force on the particles in the stream. 
This centrifugal force propels the particles against the outer wall of the 
cyclone, from which they may be removed. The agglomerates may break up 
into many small particles on impact with the outside wall, however, which 
negates the beneficial effects of the sonic agglomerator. U.S. Pat. No. 
3,172,744 teaches placement of an ultrasonic agglomerator in the discharge 
of a cyclone separator. Also, small particles may follow the air stream 
through the clean gas exit rather than be removed by the device. U.S. Pat. 
Nos. 2,720,939 and 3,681,009 teach introducing secondary particles of 
water vapor into a particulate contaminated gas stream and then exposing 
the gas stream to a sonic field causing agglomeration followed by removal 
of the agglomerates from the gas stream in a cyclone separator. Inertial 
agglomeration such as taught by U.S. Pat. No. 4,139,351 has been suggested 
as a means for producing larger solid particles prior to filtration 
through a felt filter. 
Non-inertial capture systems, such as electrostatic precipitation, do not 
suffer the limitations of cyclones, but are difficult to utilize at 
elevated temperatures and pressures due to problems such as electric arc 
breakdown. 
Water vapor has been introduced into a gas stream to trap and remove 
particles which have been subjected to sonic agglomeration, as taught by 
U.S. Pat. No. 3,390,869, and water may be passed over a screen filter to 
remove agglomerated particles, as taught by U.S. Pat. No. 3,763,634. These 
systems are impractical in applications with high temperature gas streams. 
U.S. Pat. No. 3,834,123 teaches agglomeration of dust particles by 
ultrasonics, which is stated to be unreliable and requires too high an 
expenditure of energy for use in combination with pocket or bag textile 
filters and suggests recycling dust released from the filter to the 
contaminated gas stream. This patent teaches the necessity of reverse flow 
cleaning cycles when using a textile filter. U.S. Pat. No. 2,769,506 
teaches vibration of bag filters by sound waves to free collected aerosols 
from the external surfaces of the bags. Bag filters, of course, cannot be 
used at high temperatures. 
Sonic agglomeration has been used in combination with cross flow filtration 
to remove small solid particles, under 10 microns diameter, on a 
continuous basis from a high pressure gas stream, as described in U.S. 
Pat. No. 4,319,891. While this combination has improved solid particle 
removal, I have found that solid particle removal may be further improved 
by melting the solid particles and maintaining them in a liquid state 
during the filtration process, and preferably during agglomeration. Liquid 
droplets are easier to agglomerate in a sonic agglomerator, and are not 
susceptible to forming a cake on a rigid cross flow filter element, as are 
solid particles. 
Accordingly, an object of this invention is to provide apparatus and 
process for removal of solid micron and sub-micron sized particles from a 
gas stream utilizing the combination of heating to render the particles 
liquid droplets, sonic agglomeration and cross flow filtration. 
It is another object of this invention to provide apparatus and process for 
removal of undesired solid materials from gas streams at temperatures 
above the solidifying temperature of the liquid droplets. 
It is yet another object of this invention to provide apparatus and process 
for removal of solid particulates and liquid droplets from gas streams at 
elevated temperatures. 
It is yet another object of this invention to provide an apparatus and 
process for continuous removal of liquid droplets from high temperature 
gas streams while incurring a relatively small pressure drop across the 
apparatus. 
It is still a further object of this invention to provide an apparatus and 
process for maintaining the temperature of undesired solid particulate 
matter in a gas stream above the solidifying temperature of the liquid 
droplets thereof while agglomerating the liquid droplets into larger 
agglomerates, and removing the agglomerates from the gas stream through a 
cross flow filter. 
SUMMARY OF THE INVENTION 
In keeping with one aspect of this invention, an apparatus for removing 
solid particles and/or liquid droplets, especially those having diameters 
of 10 microns and less, from a gas stream which may have an elevated 
temperature and/or pressure, comprises heating means for maintaining the 
temperature of the gas stream above the solidifying temperatures of the 
solid particles to maintain them in the liquid state; in combination with 
a sonic agglomerator, the liquid state enhancing the agglomeration of the 
droplets to form agglomerates of 10 microns and greater in diameter; and a 
porous cross flow filter. The filter comprises an inclined rigid porous 
filter element having pore diameters suitable to retain the liquid 
agglomerates. The liquid agglomerates continuously drip off the filter 
element, which prevents a filter cake from forming in the filter. A rigid, 
porous filter element is maintained in a filter housing having an input 
port in communication with the exit of the sonic agglomerator and the 
input side of the element. A clean gas port in the filter housing is in 
communication with the filter element output side for that portion of the 
gas stream which flows through the filter element. An exit port in the 
filter housing is in communication with the filter element input side 
through which a portion of the gas stream passes without flowing through 
the filter element. The liquid agglomerates are continuously removed from 
the filter as they drip off the filter element and pass through the exit 
port, and separated from the exit gas stream by flowing across a run-off 
collector and by the gas stream passing through a conventional quenching 
and scrubbing apparatus. Additional liquid droplets or solid particles may 
be injected into the gas stream, if desired, to remove pollutant gases by 
reaction therewith and to increase removal efficiency by inducing 
agglomeration when the gas comprises particles and/or droplets at low 
concentrations and/or small diameters.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
As shown in FIG. 1, a gas stream comprising small liquid and/or solid 
particles produced in combustion chamber or reactor 12, such as a coal 
gasifier or similar reaction device, pass through gas conduit 14 in 
communication with sonic agglomerators 16 to agglomerate the undesired 
material and then through porous cross flow filter 18 to separate a clean 
gas stream from the agglomerates. Heating means 17 surrounds or passes 
through agglomerators 16 and filter 18. Heating means 17 may comprise any 
known heating means capable of maintaining the temperature of the gas 
stream above the solidifying temperatures of liquid droplets of undesired 
material. The apparatus can operate at any elevated temperature limited 
only by the materials of construction, usually the cross flow filter, 
ceramic filters limiting the maximum temperature to about 1400.degree. C. 
The clean gas stream may be used for any desired purpose, such as passed 
from clean gas port 24 to catalyst beds for gas upgrading, to turbines for 
power recovery, or may be released into the atmosphere. The gas stream 
from the combustion chamber entering the agglomerators comprises 
combustion products which may include pollutant gases, fine solid 
particulates and small liquid droplets which are desirably removed before 
the gas is used or released to the atmosphere. If the particles or 
droplets are not removed, catalyst beds may be plugged, and the turbines 
may be eroded. Pollutant removal is also desirable to preserve the 
environment. 
Sonic agglomerates 16 comprises gas conduit 14 and at least one sound 
source 20 which may be provided at either end of gas conduit 14 and 
generate a sonic field within the gas conduit for agglomerating liquid 
droplets into liquid agglomerates as the gas stream passes through conduit 
14. Suitable generators of sonic fields for use in this invention are well 
known to one skilled in the art. Suitable frequencies for agglomeration in 
accordance with this invention are about 3 to about 15 kHz, about 8 to 
about 12 kHz being preferred, at intensities of about 100 to about 170 dB, 
about 130 to about 160 dB being preferred. Residence time of a liquid 
droplet in the sonic field may be about 1 to about 20 seconds, about 2 to 
about 10 seconds being preferred. 
The length of gas conduit 14 may vary according to the requirements of the 
system. Lengths in the order of about 8 to 16 feet are suitable, about 12 
feet long may result in about 98 percent particulate removal from the gas 
stream under most desired conditions. Two or more sections of gas conduit 
14 may be used as shown in FIG. 1 with the same or different sonic field 
properties for enhanced agglomeration. An important aspect of this 
invention is the more efficient agglomeration of liquid droplets as 
compared with solid particles. It is more effective to melt solid 
particles in the gas stream to provide liquid droplets throughout the 
system. Heating means 17 may be used, as shown in FIG. 1, to maintain 
undesired normally solid materials in the liquid state preferably 
throughout the apparatus. 
The exhaust gas in gas conduit 14 may be at ambient pressure or at elevated 
pressures, such as effluent gas streams of combustion devices or chemical 
reactors having gas stream pressures of about 300 to 2000 psig. The 
temperature is maintained above the solidifying temperature of the 
material to be removed. For most purposes temperatures of about 
200.degree. to 1400.degree. C. are suitable to prevent solidification and 
to minimize vaporization. It is preferred that the temperature be 
maintained below the boiling point of the undesired material to be 
removed. In the case of coal combustion, solidification of the fly ash 
produced can be prevented by maintaining temperatures between about 
1000.degree. C. and about 1100.degree. C. Any suitable heating means 
either external to or within conduit 14 and cross flow filter 18 may be 
used. External heating means 17 as shown in FIG. 1, is preferred to avoid 
interference with the gas flow in conduit 14. Heating means 17 may be 
provided by means known in the art, the heat being provided by 
electricity, process heat or combustion heat. 
Agglomeration of liquid droplets is most effective above loadings of about 
1 gram/cubic meter. When agglomeration efficiency is lowered due to low 
loadings or very small droplet size, agglomeration efficiency may be 
increased by providing solid particle or liquid droplets to the gas 
stream. Seed hopper 30 with control means shown as valve 31 may be 
provided in conduit 14 as shown in FIG. 1. Solid or liquid seeds in hopper 
30 may be released into conduit 14 at a rate which may be controlled by 
the valve 31. Introduction of seed particles or droplets should be 
effected far enough upstream from filter 18 and at least one of the 
agglomerators so that solid particles, if used, will melt, and so that the 
molten droplets will travel a sufficient distance in the sonic 
agglomerator to promote agglomeration of smaller droplets. Thus the liquid 
or solid seeds act as initiators of agglomeration of the small droplets 
when the concentration of the contaminant is so low that agglomeration 
does not proceed as efficiently as desired and when the mean contaminant 
droplet size is small and the droplets do not agglomerate to desired 
agglomerate size for efficient filtration. Addition of particles or drops 
of a specific chemical material may be added to the gas stream to cause 
chemical reaction of an undesired component with the added particle or 
drop to form a liquid reaction product for removal of the undesired 
gaseous component from the gas stream. For example, fine particles of 
alumina might be added to remove sodium and potassium vapors from the gas 
stream. The liquid reaction product is then removed by the cross flow 
filter. While FIG. 1 shows introduction of solid or liquid seeds between 
two agglomerators, they may be introduced upstream of the first 
agglomerator only, or different seeds may be introduced just upstream of 
each agglomerator. Likewise, any number of agglomerators may be used to 
achieve desired agglomeration of different materials or undesired gaseous 
removal from gas streams. For most gas streams, it is preferred to have 1 
to about 4 agglomerators in series. 
Cross flow filter 18 comprises a generally cylindrical housing 19 housing 
element 22 having an input side 32 and an output side 34, housing 19 has 
input port 23 in communication with the exit of conduit 14 and with filter 
element input side 32. Clean gas port 24 in housing 19 is in communication 
with filter element output side 34. Exit port 26 in housing 19 is in 
communication with filter element input side 32. Filter elements with a 
high surface area to volume ratio, such as filter blocks or other 
geometric shapes and configurations with respect to flow through gas 
supply may be used in place of the cylindrical shape as long as a low 
pressure drop gas flow through structure is provided to which input gas 
may be supplied to one side and clean gas removed after flowing through 
the filter from the other side with a portion of the input gas passing 
along the input side to promote cleaning of the filter input side and 
removal of agglomerates from the filter through an exit port in 
communication with the filter input side. 
Cross flow filter element 22 may be any suitable material, such as sintered 
stainless steel or porous ceramic, having pore diameters smaller than the 
diameters of the agglomerated droplets. Suitable pore diameters for most 
systems are about 5 to about 15 microns, about 8 to about 12 microns being 
preferred. It is preferred that the surface area of the filter is not 
wetted by the liquid agglomerator to promote removal of and non-clogging 
by the liquid agglomerates. 
Filter element 22 is maintained at an angle to the horizontal, and is 
preferably substantially vertical, so that liquid agglomerates will drip 
down the input side 32 and onto a runoff collector 36 in exit port 26. It 
is preferred that the surface area of runoff collector 36 is not wetted by 
the liquid agglomerates to prevent any build-up of agglomerates in the 
runoff collector. For example, the surfaces may be the same material as 
the cross flow filter element such as ceramic or stainless steel. A 
portion of the gas stream passes in contact with the input side of the 
filter element and out exit port 26 promoting removal of agglomerates 
through exit port 26. The agglomerates may then pass through conventional 
quenching and scrubbing apparatus 38 where they are removed or released 
through release port 40. 
Contaminated gas enters filter 18 through input port 23 and a portion of 
the gas passes through filter element 22 and leaves the filter as clean 
gas through clean gas port 24 while the remainder of the gas stream passes 
through central passage 27, exit port 26, scrubbing means 38, and release 
port 40. The flow of gas through release port 40 and consequently through 
exit port 26 may be controlled by valve 28, and the flow of clean gas 
through clean gas port 24 may be controlled by valve 25. Valve 28 may be 
located in exit port 26 prior to scrubbing means 38 and terminology used 
herein identifying valve 28 to be in exit port 26 includes both specific 
locations. Valves 25 or 28 may be adjusted so that about 5 to about 50 
percent of the gas stream entering filter 18 passes through central 
passage 27 and exit port 26, and about 50 to 95 percent of the air passes 
through filter element 22 and leaves filter 18 through clean air port 24. 
Liquid droplets smaller than the agglomerate size are retained in the gas 
stream passing through central passage 27 and leave the filter with the 
gas stream through exit port 26. Filter element 22 is self-cleaning 
providing continuous operation, not becoming clogged nor developing a 
filter cake and does not require reverse flow cleaning cycles. The 
agglomerator may also be located to aid in preventing clogging and 
enhancing the removal of liquid drops by vibrating the filter element 
slightly. The small quantity of dirty gas released through exit port 26 
and release port 40 may subsequently be cleaned by conventional methods or 
may be recycled to sonic agglomerator 16 by recycle conduit 41 and 
injected back into conduit 14 by control means 29. Control means 29 
includes suitable valve means and blower means for passage of the desired 
gas stream through recycle conduit 41 and into conduit 14 upstream from an 
agglomerator 16. 
When sonic agglomeration is combined with cross flow filtration for liquid 
droplet removal approximately the same removal efficiency may be achieved 
with a 10 micron pore diameter filter as is obtained with a 2 micron pore 
diameter filter without sonic agglomeration. The pressure drop across the 
10 micron filter is about one-half that of a 2 micron filter, which 
reduces the energy requirements for filtration. In addition, the power 
requirements of a sonic agglomerator and cross flow filter are estimated 
to be less than that required by a filter alone, and no more than that 
required by systems using a cyclone. 
The combustion products generally released from industrial combustion 
chambers or reactors have a melting temperature of about 900.degree. C. to 
about 1200.degree. C. The boiling temperature of such materials is 
substantially higher. Thus, it is necessary to cool the gas stream as it 
passes through the sonic agglomerator and cross flow filter. Heating 
apparatus 17, which is commercially available or may be easily designed 
for specific applications, maintains a minimum temperature of at least 
900.degree. C. to about 1400.degree. C. in the gas stream to insure that 
the particles are melted while still being below the boiling temperature. 
The apparatus of this invention may be constructed of materials and 
components apparent to one skilled in the art upon reading this 
disclosure. Likewise, the specific design and sizing parameters of 
specific installations will be apparent to one skilled in the art upon 
reading this disclosure. 
While in the foregoing specification this invention has been described in 
relation to certain preferred embodiments thereof, and many details have 
been set forth for purpose of illustration, it will be apparent to those 
skilled in the art that the invention is susceptible to additional 
embodiments and that certain of the details described herein can be varied 
considerably without departing from the basic principles of the invention.