Composite for filtering hot gas and method of its manufacture

A composite filter element for filtering hot gasses is made up of a porous ceramic core to which a porous ceramic, thin outer layer is integrally bonded. The outer layer has a mean pore size which is at least several times smaller than the mean pore size of the substrate and the substrate is at least several times thicker than is the outer layer. The outer layer and the substrate are preferably formed of the same material or of two different materials having essentially the same coefficient of thermal expansion.

The present invention relates in general to rigid, porous filter elements 
and it relates in particular to ceramic filters of the type used to remove 
fine particulates from hot gas streams exiting from, for example, low rad 
waste incinerators, pressurized fluidized bed combustors, coal gasifiers 
and the like and from certain liquid streams although the preferred 
embodiment which is hereinafter described is described in connection with 
the filtering of hot gas. 
This is a continuation-in-part application of pending prior application 
Serial No. 06/801,286 filed on Nov. 25, 1985 by Henry Schmidt, Jr., James 
F. Zievers, and Paul Eggerstedt for a Composite Filter Element and Method 
of its Manufacture. 
BACKGROUND OF THE INVENTION 
In a typical installation utilizing ceramic filter elements of this general 
type, the particulate-laden gas stream is directed through one or more 
inertial separating devices such, for example, as cyclones, which remove 
the bulk of the particulate matter. The particle size distribution of 
solid material leaving the inertial separators is generally in the range 
of about 0.2 to 15 microns with the most commonly occurring size being 
about 4 microns. Such gas streams commonly have a temperature in excess of 
1,600 degrees F., which accounts for the fact that porous ceramic material 
is currently the most durable filter material used for these applications. 
A major problem associated with ceramic filters as well as with metal or 
plastic filters has been plugging of the filters. Other problems have been 
constructional in nature, i.e., the filter element itself must be 
resistant to thermal shock and must also be sufficiently strong to 
withstand rough handling during shipment, installation and overall filter 
maintenance. 
As is known in the art, the denser the filter, i.e., the lower the mean 
pore size, the more effective is the filter in removing small size 
particles. It should be understood that the term "grade" of a porous 
ceramic element is commonly defined as the quantity of air (SCFM) that 
will pass through a 1 square foot surface area of the ceramic material 
with a thickness of one inch and a differential pressure of two inches of 
water. It may thus be seen that as the grade of the element decreases 
numerically so does the mean pore size of the element and so does the air 
flow through the filter element at a constant pressure differential. If, 
however, the grade of the ceramic filter section is increased, the 
effectiveness of the filter element to remove microscopic particles from 
the gas being filtered is decreased due to the increase in mean pore size. 
On the other hand as the pore size or "grade" of the porous ceramic 
element is decreased the resistance to gas flow through the filter element 
is increased. In addition, the resistance to gas flow through the filter 
element increases proportionately to the thickness of the filter element. 
It might appear that optimum filtration performance could be achieved by 
the use of a very thin and very tight filter element. However, such a 
filter element would inherently be too fragile for use in practical 
applications As a consequence, the filter elements of the prior art, 
whether designed for gas or liquid filtration, have been a design 
compromise between optimum pore size for efficient filtration and optimum 
thickness for strength and durability. 
SUMMARY OF THE INVENTION 
Briefly, there is provided in accordance with one aspect of the present 
invention a ceramic filter element having a relatively thick and 
relatively porous substrate to one surface of which a thin filter layer of 
a substantially finer grade of porous ceramic is integrally bonded. 
Preferably, the thin filter ceramic layer and the substantially thicker 
ceramic substrate are simultaneously fired to assure that the portions of 
different porosity are integrally united into a unified filter element 
which is virtually free of internal stresses when initially manufactured 
and when subjected to high temperatures and significant temperature shocks 
during use. The ceramic materials forming the substrate and the filter 
layer must have essentially the same coefficient of thermal expansion, and 
this characteristic may be assured by using the same ceramic material for 
both the substrate and the filter layer, only the pore sizes and wall 
thicknesses being different. 
We have found that when a newly fired filter element is initially used, a 
substantial number of particles having a size about one-third the mean 
pore size of the filter layer pass through the filter element. However, as 
the filter element is subjected to repeated filter and cleaning cycles the 
number and size of particles which pass through the filter decreases until 
an equilibrium state is reached where further cycling of the filter 
element has no effect on the number or size of particles which will pass 
through the filter element at a given face velocity of gas through the 
filter. Examination of the filter elements has shown that when the filter 
element is first used particles having a size approximating one-third the 
mean pore size of the filter layer become trapped within the upstream 
portion of the filter layer, and during subsequent cycles smaller and 
smaller particles become trapped until the equilibrium state is reached 
and the effective mean pore size of the filter layer remains constant 
irrespective of the number of further cycles of the filter. 
We have also found that after a filter element has reached the state of 
equilibrium the trapped particles have penetrated the filter layer to a 
maximum distance proportional to the mean pore size of the layer. 
Consequently, if the filter layer is any thicker than the maximum distance 
of particle penetration, no improved filtration results but there 0 is a 
greater pressure differential across the filter element and a reduced face 
velocity with a consequent reduction in operating efficiency. In addition, 
cleaning of the filter is also adversely affected if the filter layer is 
thicker than necessary. The optimum thickness of the filter layer is 
described in detail hereinafter. 
In the composite filter element of this invention the thin filter layer 
does not become plugged as does the thicker-walled ceramic filters of the 
prior art, and in addition, the porous substrate functions as a flow 
distributor to provide uniform backflow through the filter layer during 
reverse flow cleaning of the filter. 
In a preferred embodiment of the present invention the filter element is 
ceramic and of the candle type wherein the substrate constitutes a hollow 
cylindrical core portion and the filter layer constitutes a thin-walled 
hollow cylindrical outer layer which is integrally bonded to the core 
throughout the entire adjacent surface areas of the outer layer and the 
substrate. Indeed, there is a thin intermediate layer between the 
substrate and the thin outer layer wherein the smaller particles which 
make up the outer layer are intermixed with the larger particles which 
make up the substrate thereby to form a thin boundary layer which 
maintains the unitary composite nature of the entire filter element. 
In the candle construction, an external flange portion at one end and a 
plug portion at the other end, preferably formed of a nonporous ceramic to 
provide adequate structural strength, are used in those two areas where 
the stresses are greatest, and these portions are also simultaneously 
fired with the remainder of the filter element. 
In an alternative embodiment of the invention the filter element may 
consist of a porous plastic substrate to which a finer grade of porous 
plastic is integrally bonded to form a unitary filter element. Such 
plastic materials as polyvinylchloride, polycarbonate, polypropylene and 
polystryene are suitable.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now particularly to FIG. 1 there is shown a candle filter 10 
having an elongate tubular filter section 12, an external mounting and 
sealing flange section 14 at the top and a plug section 16 which extends 
completely across the lower end of the tubular filter section 12. The 
entire filter element 10 is a unitary composite ceramic or plastic member 
which for high temperature operation has the same coefficient of thermal 
expansion throughout its entirety. As is explained in greater detail 
hereinafter the tubular filter section 12 is porous to permit gas to pass 
through the filter section to cause the entrained particulate matter to be 
deposited at the upstream side of the filter section. Ordinarily, a gas to 
be filtered is passed from the outside to the inside of a candle filter 
and, therefore, the external surface of the filter section 12 constitutes 
the upstream side of the filter. 
The flange section 14 at the upper end of the filter element 10 need not be 
porous inasmuch as the gas to be filtered is not passed through the flange 
section. Rather, the flange section 14 is used only to mount the filter to 
a tube sheet and to provide a hermetic seal between the upper end of the 
filter element 10 and the tube sheet to which it is mounted. Therefore, 
the flange section 14 must be capable of withstanding high mechanical 
stresses as occur when the filter element 10 is tightly locked in place in 
a mounting aperture in the tube sheet and when the entire system is 
subjected to rapid thermal changes which thermally stress the filter 
element. 
The lower end plug portion 16 may be in coacting cooperative relationship 
with a conventional spacer element (not shown) located at the bottom of 
the filter element 10. It may be seen that a central recess 18 is provided 
at the bottom of the plug section 16. Its purpose is to receive an 
upstanding pin or locator element from a spacer assembly located beneath 
the filter element 10 to space the filter element 10 from a number of 
other similar filter elements mounted in the same filter tank. The plug 
section 16 is also formed of a ceramic but because its primary function is 
to close off the lower end of the filter, it need not be porous to the 
extent that gas will pass therethrough during normal operation of the 
filter. The plug section 16, must, of course, be sufficiently imperforate 
so as not to permit the particulates in the gas to be filtered to bypass 
the filter section 12. 
The filter section 12 includes a porous ceramic substrate portion 20 to the 
upstream side of which a porous ceramic coating layer 22 is integrally 
bonded throughout the entire adjacent surface areas of the substrate and 
the coating layer. The mean pore size of the substrate section 20 is 
appreciably greater than the mean pore size of the coating layer 22 
wherefore the coating layer 22 functions as the filter media which 
collects the particulates as the gas flows through the filter section 12. 
The coating layer has a thickness which is sufficiently great to remove 
the entrained particles from the gas but which is sufficiently thin to 
provide a relatively low pressure differential thereacross during normal 
use of the filter. The relationship between the optimum thickness and the 
grade of the coating layer is described hereinafter in connection with 
FIG. 5. Where desired, the tubular substrate 20 may constitute the 
external surface of the filter tube with the filter layer 22 being coated 
onto the internal surface of the substrate. 
In FIG. 2 there is shown a plate-like filter element embodying the present 
invention. A thin filtering layer 22' is integrally bonded to a substrate 
20' in the same manner as the layer 22 is bonded to the substrate 20. The 
substrate 20' and the filtering layer 22' may both be formed of ceramic or 
of a suitable porous plastic material. 
Referring to the graph shown in FIG. 3, it may be seen that the pressure 
differential across a fifteen millimeter wall of a given ceramic material 
is substantially greater than the pressure differential across a ten 
millimeter wall of the same material. For both materials, as the face 
velocity is increased the pressure drop across the wall also increases. It 
may thus be seen that if the filter layer or wall 22 of the filter 10 were 
sufficiently thick to enable the element 10 to be sufficiently strong for 
practical applications the pressure drop across the filter element would 
be so great as to require relatively higher pressures to force the gas 
through the filter elements. 
Referring to FIG. 4, it may be seen that by increasing the mean pore size 
or grade of the filter section the pressure drop across the filter section 
is reduced. In accordance with the present invention the substrate 20 is 
formed of a relatively coarse grade of ceramic material and the filter 
layer 22 is formed of a substantially finer grade of ceramic material. In 
FIG. 4, the curve identified as "grade 50/20" shows the relationship 
between the face velocity and the pressure drop across a composite filter 
section wherein the coating layer is formed of grade 20 ceramic material 
and the substrate section 20 is formed of grade 50 ceramic material. In 
this composite filter element, the thin filter layer 22 functions to 
remove the fine particulates from the gas stream and the thick substrate 
section functions as the mechanical support for the coating layer 22 and 
the distributor during backwashing of the filter layer resulting in a 
particle retention approaching that of a grade 20 ceramic element without 
the corresponding pressure drop. 
The substrate section 20 and the coating layer 22 are preferably formed of 
the same ceramic material although two different ceramic materials having 
substantially the same coefficient of thermal expansion can be used. When 
the element 10 is manufactured, the green ceramic materials, i.e., ceramic 
powder and a binder used for the flange 14, the lower end section 16 and 
the filter section 12 are compressed together in a mold having a shape 
which is complementary to the shape of the desired filter element to 
compact the different parts together prior to firing of the entire piece. 
Also a preferred core could be coated with the outer ceramic layer and 
again fired. As a consequence, a very thin intermediate section is formed 
between the substrate 20 and the outer layer 22 wherein the smaller 
particles which make up the outer layer are intermixed with the larger 
particles which make up the substrate. When the entire filter element 10 
is thereafter fired, a unitary composite ceramic candle is provided and 
this candle is virtually free of internal stresses. 
There are other methods of manufacturing which we have found to provide a 
satisfactory composite filter element. For example, the thin outer layer 
may be applied to the substrate by brushing or spraying a slurry of the 
fine ceramic particulates onto the substrate and then firing the coated 
substrate. 
There are also several thermal techniques which may be used for applying 
the thin outer layer to the substrate. In one such system the coating 
media is heated just to the melt point by feeding it through a gun and 
heating it with oxy-acetylene or hydrogen flame and then propelling the 
molten softened particles with an aspirating gas onto the substrate. The 
nature of the coating will vary with the distance between the gun and the 
substrate and with the melting temperature, which is generally below 5,000 
degrees F. Sputter coating or plasma coating can also be used. In plasma 
coating an inert gas stream was excited by an electric arc and the ceramic 
particulates were fed into the plasma flame and blown onto the substrate. 
An advantage of the plasma system is that it is less expensive. 
Another system which has also been used successfully uses a binding 
material such as carboxy methyl cellulose which is coated onto the surface 
of a tubular substrate and permitted to dry until it becomes tacky. Then 
the coated tube is rolled in a mixture of the fine ceramic particles and 
carboxy methyl cellulose to apply a thin layer to the surface of the 
tubular substrate. The coated tube is then placed in a kiln and fired to 
cure the ceramic and to eliminate the binder. 
In still another system a homogeneous slurry of the fine particulates is 
passed under pressure through the substrate until a cake of the fine 
particles having the desired thickness has been deposited on the 
substrate. The coated substrate is then placed in a kiln and fired. 
It has been found that to achieve the advantages of the present invention 
the substrate should have a thickness of at least about three times the 
thickness of the coating layer and the substrate should have a mean pore 
size or grade which is at least two times the mean pore size or grade of 
the outer layer. 
In order to maximize the operating efficiency of a composite filter element 
it is important to optimize the thickness and pore size of the filter 
layer. Inasmuch as the filter layer is the functional part of the filter 
element its pore size must be sufficiently small to prevent all particles 
of a size greater than the desired predetermined size to pass through the 
filter layer. However, each time a composite filter element is cycled, 
i.e., used as a filter and then cleaned, its effective mean pore size is 
decreased until a state of equilibrium is reached whereupon the effective 
mean pore size becomes constant and appreciably less than the initial mean 
pore size of the filter layer. For example, where the initial mean pore 
size is about 30 microns (grade 20), no particles larger in size than 
about 10 microns will initially pass through the filter, but after 
repeated cycles the filter reaches equilibrium and no particles larger in 
size than about 0.3 micron will pass through the filter. This is because 
small particles trapped in the filter layer have reduced its effective 
mean pore size. We have found that the filter layer should have an initial 
mean pore size about 2.5 times the mean size of the dust particles to be 
removed from the gas passed through the filter element. 
We have found that the optimum thickness of the filter layer is dependent 
on the initial pore size of the filter layer and the relationship between 
initial grade and optimum thickness is shown in FIG. 5. While the 
thickness need not be precise, that being difficult and costly to achieve 
from a manufacturing standpoint, an economically satisfactory filter 
element is provided when the thickness of the filter layer is held within 
.+-.20 percent of the optimum thickness. The relationship between mean 
pore size and layer thickness shown in FIG. 5 can be expressed by the 
following equation: 
EQU Grade=7.58t 
wherein: 
Grade is the grade of the filter layer 
t is the thickness of the filter layer expressed in millimeters. 
While many different types of ceramic material may be used in a filter 
element embodying the present invention, the materials selected will 
depend upon the temperatures at which the element is designed to operate 
as well as the desired particle size retention, allowable pressure 
differential and the strength and weight characteristics which are 
required. Materials which may be used for hot gas filtration applications 
are silicon carbide, aluminum oxide and mullite. Where the operating 
temperature of the filter element will not exceed about 400.degree. C., 
the filter element may be formed of a quartz ceramic. 
While the present invention has been described in connection with 
particular embodiments thereof, it will be understood by those skilled in 
the art that many changes and modifications may be made without departing 
from the true spirit and scope of the present invention. Therefore, it is 
intended by the appended claims to cover all such changes and 
modifications which come within the true spirit and scope of this 
invention.