US2753061A - Control of flow of finely divided solids - Google Patents

Description

July 3, 1956 R. P. TRAINER CONTROL OF FLOW OF FINELY DIVIDED SOLIDS 2 Sheets-Sheet 1 Filed May 5, 1952 Reqenerafor Side Valve Fig. I

July 3, 1956 R. P. TRAINER 2,753,061

CONTROL OF FLOW OF FINELY DIVIDED SOLIDS 2 Sheets-Sheet 2 Filed May 5, 1952 \nvenror Richard P .Tmmer' Ham Hie Horn 21:;

CONTROL OF FLOW F FINELY DIVIDED SOLIDS Richard P. Trainer, Alton, Ill., assignor to Shell Development Company, Emeryville, Calif., a corporation of Delaware Application May 5, 1952, Serial No. 286,217 4 Claims. (Cl. 214-152) This invention relates to an improvement in the controlled flow of fluidized solids. More particularly the invention provides an improved method and apparatus for the controlled feeding of a finely divided solid through a standpipe. A particular aspect of the invention relates to the controlled feeding of a fluidized catalyst powder through a standpipe in a fluidized catalyst reaction system, e. g., in the catalytic cracking of hydrocarbon oils.

The problem of feeding a particulate solid at a controlled rate is frequently encountered in various industries. In some cases, especially where the flow rate is low, various mechanical feeding devices can be used. How ever, since such devices are subject to considerable wear and also usually require power for their operation, it is preferable, where possible, to cause the powder to flow like a liquid and control the rate by a valve means. In order to control the flow by a valve means it is essential that there be an appreciable pressure differential across the valve means. This pressure differential is best obtained by providing a hydrostatic head. Thus, in a typical case the hopper or other vessel containing the finely divided solid is provided at or near its bottom with a standpipe, i. e., a discharge conduit of some length provided with a valve or other flow control means located at a sufiiciently low elevation to provide the desired head.

In order to retain the finely divided solid in a freeflowing condition it is the practice to inject a suitable aeration gas into the standpipe. In general several injection points are provided and the amount of aeration gas supplied is regulated to maintain the finely divided solid in a free-flowing so-called fluidized 0r pseudo liquid state.

While this method of controlling the flow of finely divided solids is in wide use it is not without shortcomings. In practice it is found that the control is very sensitive to slight changes in the conditions and is frequently erratic and sometimes undependable. In the case of the catalytic cracking of hydrocarbon oils, for example, where this system is in wide use, failure to maintain the proper static head for even a moment would allow the possi' bility of backup and place the plant and personnel in jeopardy. To guard against this situation it has been the practice to make the standpipe (in this case frequently called a catastat) extra long and also to provide an automatic means for closing the control valve whenever the pressure differential across the valve falls below a safe limit. This overdesign is to be avoided if possible since it requires supporting the plant at a higher elevation to accommodate the increased length of the standpipe.

A study of the conditions obtaining in a conventional standpipe under normal operating conditions has now shown that under these conditions where the finely divided solid is flowing there is a tendency for the aeration gas to become trapped and accumulate in the standpipe, thereby creating sections or zones of low density. In tests made on standpipes (18" and 30" diameter and about 50 feet in length between the top and the control valve) of a fluidized catalyst catalytic cracking plant operating at a constant normal flow of powdered cracking catalyst,

nited States atent O Patented July 3, 1956 zones having a density as low as about 5 pounds per cubic foot were found. Also, zones of high density and partial plugging were observed in some instances. The formation and escape of such bodies of trapped gas are largely responsible for the erratic behavior.

It is an object of the present invention to provide a method and apparatus whereby the control of the flow of finely divided solids through a standpipe may be improved by avoiding or repressing the formation of zones of abnormal density caused by the trapping of gases in the standpipe. It is also an object of the invention to provide a method and apparatus which allow an overall greater average density to be maintained in the standpipe and thereby allow shorter standpipes to be safely employed. A further object of the invention is to provide a method and apparatus for the controlled flow of catalyst through a catastat with decreased occlusion of aeration gas with the effluent catalyst.

The foregoing and related objects may be accomplished by withdrawing aeration gas from the column of flowing finely divided solid by means of suitable filters located in the standpipe. Thus, in the method and apparatus of the invention the free-flowing character of the column of finely divided solid in the standpipe is maintained by injecting a suitable aeration gas at suitable points, but at least part of this aeration gas is simultaneously withdrawn from the standpipe through a suitable filter.

The apparatus of the invention comprises a conventional standpipe or catastat having the control valve means near the bottom and communicating at the top with a hopper or other vessel, and in combination therewith a plurality of filter elements in the standpipe, each of said filter elements being connected by a pipe through which gases may be introduced into, or withdrawn from, the standpipe. The piping and filter elements. are preferably arranged so that gas may be injected into the standpipe by one set of one or more filter elements and simultaneously withdrawn from the standpipe by a second set of one or more filter elements. The piping is also preferably arranged so that the direction of flow through the filters may be periodically reversed.

One suitable arrangement of apparatus is illustrated semi-diagrammatically in the accompanying drawing wherein Figure I is an elevational view, partly in section, of part of a supply vessel with a standpipe, filter elements, and control valve, and Figure 11 is a sectional plan view taken through the plane lI-II and illustrating the positions of the filter elements in the standpipe. Referring to the drawing, the powdered solid (in this case a silicaalumina-cracking catalyst in the form of a powder passing a mesh U. S. standard sieve) is in the supply vessel 1 (in this case the regenerator of a fluidized catalyst catalytic cracking plant) and it is desired to feed the powder at a controlled rate of, for example, 25 tons per minute into the line 2 wherein it is picked up by a stream of vapor and carried to a reactor vessel (not shown). In a typical case, the pressure in line 2 is 18 pounds per square inch. In such a case it is desired that the pressure just above valve 3 be above 18 pounds per square inch by a small safe margin, e. g., l2 pounds per square inch. The desired pressure at this point, e. g., 20 pounds per square inch, is obtained with the aid of the head provided by the standpipe 4. This standpipe, by way of example, may be 30 inches in diameter and 40 feet in length. The flow rate is controlled by the valve 3 which may be regulated by suitable control instruments not illustrated. In the case illustrated valve 3 is a hydraulically operated slide valve. It is to be understood however that any other suitable valve means may be used.

In operation the standpipe is filled with the flowing fluidized powder. In order to withdraw gas from this powder at least one filter element is required.

If only one such element is provided it is preferably located near the bottom of the standpipe above the control valve, as filter element a. In such a case it would be necessary to supply aeration gas to the standpipe in the usual way with suitable aeration inlets such as aeration inlet lines 6a and 6b. It is preferred, however, to provide at least two filter elements, and preferably more, located at spaced points throughout the standpipe, somewhat as illustrated. If desired, the filter elements may be used only for withdrawing gas from within the fluidized powder, in which case it is necessary to supply aeration gas by separate inlets such as aeration lines 6a and 612. It is preferred, however, to supply at least part of the aeration gas through the filter elements. Thus, for example, in the case illustrated aeration gas may be supplied .by lines 6a and 6b and by filter elements 5a, 5c and 5.2 while gas is withdrawn through filter elements 5b, 5d and 5 At convenient intervals the flow through the filter elements may be reversed. Thus, filter elements 5a, 50 and 5e may be connected to one branch manifold line (not shown), and filter elements 511, 5d and 5 may be connected to a second branch manifold line, and the two manifold lines may lead to a four-port, two-position valve (not shown) which is connected to a vent and to a source of compressed air. In some cases it may be necessary or desirable to apply a vacuum instead of merely venting to the atmosphere.

The filter elements may be of any desired shape and of any suitable size and may be fabricated of most any material capable of withstanding the conditions of temperature, pressure, etc., in the standpipe in the operation contemplated. Filter cylinders of porous ceramic, porous metal, wire screen, fiber glass, cloth, or the like, may be used. The filter elements must be sufliciently fine, how ever, to prevent the solid material from passing. In the case illustrated the filter elements 5a5f are bayonet type porous stainless steel filters manufactured by the Micro Metallic Corporation of Brooklyn, New York, which filter elements consist of a hollow bayonet-shaped body closed at the ends except for the inlet pipe connection; each is 54 inches long by 3 inches wide by inch thick and has a filter area of about 2.19 square feet; each filter has a capacity of about 2000 cubic feet per minute at 5 pounds per square inch differential pressure. The filter elements may hang down from their supporting pipes as illustrated, or they may be supported in the standpipe by any suitable brackets or braces and in any other desired position. Since the powder is flowing through the standpipe there is little tendency for the solid to form a cake on the filter surface.

By simultaneously introducing and withdrawing aeration gas from the standpipe the flowing fluidized powder may be maintained in the desired free flowing condition While avoiding or repressing the formation of gas pockets. Since at least some of the supplied aeration gas is removed through one or more of the filter elements, the average density of the powder flowing through the standpipe is increased. Since the head produced by the standpipe depends upon the density of pseudo liquid powder in the standpipe, it is, therefore, possible to obtain the desired head with a shorter standpipe.

Another suitable arrangement of apparatus is illustrated semi-diagrammatically in Figures III and IV of the draw ing. Figure III is an elevational view, partly in section, of part of a supply vessel with a standpipe provided with annular filter elements. Figure IV is a sectional view taken through the plane III--III. Referring to Figures III and IV the supply vessel (regenerator), standpipe and control valve are essentially as described in connection with Figures I and II. However, instead of providing bayonet type filter elements suspended within the inside diameter of the standpipe, a number of annular filter elements are provided. Thus, a section of the standpipe 10, which is preferably fabricated separately and is connected to the remaining part of the standpipe by flanges 11 and 12, is provided with slots or holes 13 which are covered on the outside by a suitable filter such as a fine screen 14 or porous metal filter plugs. Sealed to the outside of the section 10 and covering the area of the slots is a jacket 15. As shown in Figure IV the annular space between the pipe section and the jacket is divided into sections by vertical partitions 16a-d. The separate sections are provided with one or more pipes 17a-d and 18ad communicating through the wall of the jacket and arranged to allow gas to be withdrawn through one or more of the annular sections while gas is introduced into the other one or more sections for blowing the filter elements.

In some cases when a very finely divided solid is being used some of the finest particles may pass the filter element. This is particularly the case if a relatively coarse filter is used in order to increase the amount of gas withdrawn. If any appreciable amount of the powder passes the filter elements, this powder may be returned separately to the main body of the powder. One suitable method is, for example, to return the powder to the supply vessel, preferably above the fluidized bed level. The powder may be returned by means of an ejector and gas lift pipe through which any suitable transport gas may be passed.

The apparatus and method of the invention described above are particularly advantageous in the treatment of hydrocarbon oils using the fluidized catalyst technique. In the normal operation of a conventional standpipe where aeration gas is not withdrawn through filters, the bulk, if not all, of the aeration gas supplied passes downward with the flowing powder through the control valve. In the catalytic treatment of hydrocarbon oils where air is used as the aeration gas in the regenerator standpipe, the air carried with the catalyst in a typical plant causes the combustion of about 50 barrels of oil per day. This not only represents an appreciable loss of valuable oil but it is disadvantageous also in other ways. Thus, this combustion considerably increases the already excessive heat which must be removed by suitable coolers, and the gases produced by the combustion are carried with the product and must then be compressed in the following absorption system. The only known solution to this difficulty is to use steam as the aeration medium in the standpipe but this solution is not practical since the addition of steam to the hot regenerated catalyst causes excessive damage to the catalyst. While the method and apparatus of the present invention are not a complete solution to this problem, they materially reduce the amount of aeration air carried with the regenerated catalyst through the regenerator standpipe.

I claim as my invention:

1. In the controlled feeding of powder from a supply source at a low pressure to a zone of higher pressure through the static head created by a confined column of the pseudo liquid powder above the throttling control point, the improvement which comprises injecting gas into said confined column at a plurality of points in an amount only suflicient to retain the powder in the pseudo liquid condition and simultaneously withdrawing aeration gas substantially free of powder at a plurality of separate points from the flowing column of pseudo liquid powder.

2. The process according to claim 1 further characterized in that the flow of aeration gas through the said separate pluralities of points is periodically reversed.

3. In the feeding of a controlled stream of fluidized powder from a vessel at a given pressure to a zone of higher pressure through the use of a. standpipe in which aeration gas is injected in an amount only suflicient to retain the downwardly flowing powder in the standpipe in the pseudo liquid condition, the improvement which comprises simultaneously withdrawing gaseous aeration medium from a point below the point of said gas injection in the flowing fluidized powder through a filter means of sufficient fineness to retain the powder in the standpipe.

4. Process according to claim 1 further characterized in that aeration gas is withdrawn from the flowing column of pseudo liquid powder near the bottom of said column just above the said throttling control point.

References Cited in the file of this patent UNITED STATES PATENTS McCord Feb. 4, 1913 Jewell Dec. 15, 1942 Campbell et a1 Oct. 19, 1948 England Nov. 27, 1951 Palmer Aug. 18, 1953

Claims (1)

1. IN THE CONTROLLED FEEDING OF POWDER FROM A SUPPLY SOURCE AT A LOW PRESSURE TO A ZONE OF HIGHER PRESSURE THROUGH THE STATIC HEAD CREATED BY A CONFINED COLUMN OF THE PSEUDO LIQUID POWDER ABOVE THE THROTTLING CONTROL POINT, THE IMPROVEMENT WHICH COMPRISES INJECTING GAS INTO SAID CONFINED COLUMN AT A PLURALITY OF POINTS IN AN AMOUNT ONLY SUFFICIENT TO RETAIN THE POWDER IN THE PSEUDO LIQUID

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