Gas separation: processes – Filtering – Through particulate solids
Reexamination Certificate
2002-11-26
2004-08-31
Smith, Duane (Department: 1724)
Gas separation: processes
Filtering
Through particulate solids
C095S275000, C095S276000, C095S107000, C055S282000, C055S474000, C055S517000, C096S108000, C422S216000
Reexamination Certificate
active
06783572
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to the intimate contacting of a gas and a loose granular solid material for purpose of chemically or physically treating one or both of the gaseous and solid substances, for example: to filter fine particulate matter (“dust”)from the gas; to effect a chemical change in gas or solid; to remove a chemical constituent of the gas by absorption or adsorption; or to heat a cold gas by contact with a hot solid. The invention also relates to the countercurrent contacting of a gas and a granular solid material.
BACKGROUND OF THE INVENTION
An old idea is to treat a gas and a granular solid material by causing the gas to flow in the horizontal direction across a bed of the solid material disposed in a “panel” that has often been tall in comparison with its width in the direction of gas flow. Often, the “panel bed” has been held in place by louvered walls that resembled venetian blinds. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) cites early art and is incorporated by reference in the instant application.
Some designs called for continuous or intermittent motion of granular material downward through the panel, fresh material being supplied at the panel's top and “spent” material being withdrawn at its bottom. A representative recent proposal along this line is to be found in U.S. Pat. No. 5,527,514 (Jun. 18, 1996). U.S. Pat. No. 4,017,278 (Apr. 12, 1977) provided performance data for a gas-filtration device of this kind. The panel contained “gravel” 2 to 12 mm in size. Superficial velocity of dusty gas approaching the panel was 25 to 100 cm/s. Herein, superficial velocity=rate of gas flow divided by the projected vertical frontal area of the panel (panel height×panel width). Downward speed of the gravel mass was 30 cm/hr. Dust content of filtered gas ranged from ~25 to ~150 milligrams per normal cubic meter (mg/Nm
3
). Dust that was captured by filtration accumulated within the gravel bed, not upon the gas-entry surfaces that it presented. In other words, the device filtered the dust by what practitioners term a “deep-bed filtration mechanism.”
In other designs, granular material was stationary much of the time. These designs filtered dust from gas by accumulating a cake of the dust (a “filter cake”)upon gas-entry faces of granular material retained in a panel bed. Such designs are capable of providing filtered gas containing levels of residual dust comparable to that provided by fabric filtration. Both a panel bed of this type and a fabric filter employ a “surface filtration mechanism,” in which the filter cake is in fact the filtration medium; the primary function of either granular bed or fabric is to support the cake. Means are provided for intermittent renewal of gas-entry faces through removal of a moiety of the granular material from these faces together with accumulated dust. Means for outwardly tipping louvers that support gas-entry faces and for drawing plows horizontally along the faces have been proposed. U.S. Pat. No. 3,800,508 (Apr. 2, 1974) not only provided means for pivoting louvers but also employed a gas-entry velocity at gas-entry faces sufficient to support the faces at an angle steeper than the dynamic angle of repose of the granular material, momentary interruption of gas flow produced a spill of this material along with filter-cake.
My U.S. Pat. No. 3,296,775 (Jan. 10, 1967) disclosed a puff-back method for renewing gas-entry faces of a panel bed of the type wherein louvers support gas-entry portions of the bed. Puff-back entailed creation of a reverse transient surge flow to produce en masse displacement of granular material toward gas-entry faces of this material. My U.S. Pat. No. 4,006,533 (Feb. 8, 1977) specified a reverse transient surge flow of a more particular character, whose discovery made possible development of a practical panel-bed filter employing a surface filtration mechanism for cleaning a dusty gas (K. C. Lee, I. Rodon, M. S. Wu, R. Pfeffer, and A. M. Squires, The Panel Bed Filter, EPRI AF-560, Electric Power Research Institute, Palo Alto Calif., May 1977, I. Rodon, K. C. Lee, R. Pfeffer, and A. M. Squires, Panel Bed Filtration Data for Three Dusts at 150° C., paper 79-56.5 presented at meeting of Air Pollution Control Association, Cincinnati, Ohio, June 1979, A. M. Squires, K. C. Lee, and R. Pfeffer, The Panel Bed: A Fluid-Solid Contacting Device Exploiting a New Mode of Soil Failure, paper presented at POWTECH 81, Birmingham, England, March 1981). Operation of a panel-bed filter is cyclic, an interval of filtration alternating with a puff-back that removes both filter cakes and a moiety of sand lying directly beneath the cake, thereby renewing the bed's gas-entry faces. During an interval of filtration, paralleling the formation of the filter cake is an increase in pressure drop in the gas flowing across the bed. This pressure drop cannot be allowed to increase without limit, for two reasons: an unduly large pressure drop would increase cost for gas-compression beyond an economic limit and would impose a force upon the filter cake sufficient to break off chunks of the cake, driving these deep into the granular bed and harming filtering efficiency. Although the object of face-renewal is to present a new free face, it is inadvisable to employ such a strong puff-back as to yield an absolutely clean face. In operation of panel-bed filters, experience has taught that a clean face does not filter as well as a somewhat dirty face. In a subsequent filtering interval, I believe, a new filter cake forms quicker upon a dirty face than upon a strictly clean one.
Commercial-scale panel-bed-filter modules have now successfully cleaned hot gaseous products of combustion of both coal and wood waste, hot gas from cement production, and hot gas from electrometallurgical manufacture of ferrosilicon (this latter gas, containing a fine silica fume, is particularly difficult to clean). Typically, dust in the cleaned gas amounted to less than 5 mg/Nm
3
.
In the tests, ordinary sand served as the granular filtration material. Suitably, the sand was about 0.15-0.45 mm in size. In small-scale tests at elevated temperature (e.g., 150 to 500° C.), I found this size, for dusts studied so far, to be substantially the largest sand size upon which a filter cake of good integrity can accumulate. Use of sands of smaller sizes affords filtered gas at lower residual dust remaining, at cost of either lower throughput or higher pressure drop in the filtered gas. For a given size of sand, to allow a filter cake to form, there are limits upon the velocity of gas entering a free face of the sand. For the 0.15-0.45 mm sand, in tests filtering a number of dusts at about 150-200° C., preferred gas-entry velocities have ranged from ~16 to ~28 cm/s (superficial velocities, from ~8 to ~14 cm/s). Herein, gas-entry velocity=rate of gas flow divided by the nominal total area of the free sand faces upon which a filter cake can accumulate. Since the profile of a gas-entry face, seen in vertical cross-section, is poorly defined, it is convenient to define a nominal area, thus:
Gas-entry face area=(straight-line distance between the face's outer and inner edges in the direction perpendicular to the edges)×(the mean of the horizontal lengths of these edges).
(Notice that this definition applies, regardless whether the face's edges are straight or curved in the horizontal direction. In state-of-the-art designs, edges are straight, but louvers that are circular in plan may be useful in some applications.) In general, a lower gas-entry velocity is preferable the smaller the size of dust to be filtered or the less cohesive the dust.
Parenthetically, I note that both larger granular material sizes and higher gas-entry velocities may be specified for panel-bed applications wherein a clean gas is treated by a contact with the granular material.
In a test of a commercial-scale panel-bed filter module, wood ash was filtered from gas at 200° C. emitted by a wood-waste boiler (H. Risnes and O. K. Sønju, Evaluation
Creighton Wray James
Greene Jason M.
Narasimhan Meera P.
Smith Duane
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