US3156538A - Cooling of bed reactors - Google Patents

Description

Nov. 10, 1964 Filed Oct- 26, 1960 R. A. SCHNEIDER 3,156,538

COOLING 0F BED REACTORS 3 Sheets-Sheet 1 FIG.

$ INVENTORI RICHARD A. SCHNEIDER BY: MZ

HIS ATTORNEY Nov. 10, 1964 R. A. SCHNEIDER COOLING OF BED REACTORS 3 Sheets-Sheet 2 Filed Oct. 26, 1960 FIG. 6

INVENTORI RICHARD A. SCHNEIDER M/ /MW HIS ATTORNEY Nov. 10, 1964 R. A. SCHNEIDER 3,156,538

COOLING OF BED REACTQRS Filed Oct. 26, 1960 3 Sheets-Sheet 3 34a 3| 4o 24 32 h FIG. 3

I 2 lo /lOu 5 3e 3 I, I 43 34 II [I I FIG. 5

INVENTORZ RICHARD A. SCHNEIDER HIS ATTORNEY United States Patent 3,156,538 COOLING 0F BED REACTORS Richard A. Schneider, St. Louis, Mo., assignor to Shell Oil Company, New York, N.Y., a corporation of Delaware Filed Oct. 26, 1960, Ser. No. 65,098 4 Claims. (Cl. 23-288) The invention relates in general to cooling beds of contact material in which an exothermal chemical reaction is carried out by heat exchange with a cooling device, and is, more particularly, concerned with an improved method and apparatus for abstracting heat from such a bed by partially vaporizing a liquid coolant within each of a plurality of vaporizing tubes embedded in said material.

It is known to remove heat from fixed or fluidized beds of contact material by coils or heat exchange tubes embedded therein, but such devices have been subject to frequent failure, due to stresses and thermal cycling occurring in the coils or other limitations or hazards as explained below. For example, it is common practice to mount a heat exchange coil within the fluidized bed of a catalyst regenerator, wherein carbonaceous deposits on finely-divided cracking catalyst is burned in air, which is supplied as oxidant and fluidizing gas. It is recognized that vaporization of the liquid coolant within the heat exchange coils leads to the highest heat transfer rates. However, vaporizing coolers have heretofore not been successful and resulted in frequent failure.

Investigation of the cases of such failures indicated that a major cause is thermal cycling. Thus, if the internal coil surface is alternately wet and dry the temperature fluctuates, producing a stress cycle in the metal. This is because the internal heat transfer film coefrcient varies considerably depending upon whether the internal surface is wet or dry; the metal temperature also varies under these two conditions. 'With each temperature cycle there is a stress cycle. Eventually the metal cracks. If the internal surface is always dry, the metal temperature is high, the heat transfer rate is low, and steam corrosion of the metal results unless high-quality and costly alloys are used. On the other hand, if vaporization is suppressed by pressurizing the coolant-usually waterhigh pressures, determined by the bed temperature and often of the order of 700 p.s.i.g. (lbs. per sq. in. gage) would be required and the temperature of the coolant would be increased, resulting in reduced cooling capacity, and the necessity of using higher-strength alloys, heavier tubing and increased pumping costs, and leading to increased hazard-in the event of a tube rupture. The increased hazard would be due both to the higher pressure and to the increased volume of liquid within the coil which, if it escaped to thecontact bed, would vaporize to many times the liquid volume; this could rupture "the shell. 1

Cooling coils forcontact beds have been mounted horizontally in the form of hair-pins, bayonets'or single pipes extending uniformly around the confining vessel. These presented difiiculties with surging of coolant. Vertical coils, when used, were used either with dry internal surfaces or under pressure to suppress vaporization. Vertical coils wherein liquid is vaporized, however, would present advantages if thermal cycling could be avoided.

3,156,538 Patented Nov. 10, 1964 Advantages include high heat-transfer rates, lower internal pressure and increased safety.

It is the object of this invention to provide an improved method and apparatus for cooling beds of contacting rnaterial wherein an exothermal chemical reaction is carried out wherein vaporization of coolant is effected in a plurality of upright vaporizing heat-exchange tubes supplied from a common header and wherein thermal cycling is suppressed.

A further object is to provide an improved heat exchanger for cooling beds of contacting material having an increased life. Moreparticularly, it is an object to provide an improved cooling device having an improved mounting and piping whereby stresses due to thermal expansion and contraction are reduced.

Additional objects will become apparent from the following description.

In summary, according to the invention the liquid coolant is admitted from outside the vessel which contains the bed of contacting material to a bottom header or distributing zone of a cooling device which is embedded in said material and distributed uniformly among a plurality of upright vaporizing tubes, within each of which a part of the coolant is vaporized, and the resultant mixture of liquid and vapor is discharged from the upper ends of the tubes into a top header and discharged from the vessel. The coolant is maintained wholly in the liquid state within the supply pipe and within the bottom header, and it was found in practice that by this expedient it is possible to avoid surging and consequent thermal cycling.

It was feared by some, prior to testing the invention, that unstable or unbalanced flow of channelling of steam through one vaporizing tube would reduces heat transfer rate and cause early failure. However, contrary to such expectations, the invention has resulted in extended service life and solved a long-standing difficulty.

The coolant is maintained in the liquid stateby employing a bottom heater which has a small cross-sectional area in relation to that of the vertical tubespreferably no greater than that of a single one of the pipesand by connecting a separate upright tube at each extremity of the header, so as to avoid stagnant zones wherein vaporization of coolant can occur. The supply pipe by which the liquid is supplied to the bottom header is preferably also of small size, both to provide a small heat transfer area which limits the heating of the liquid and to restrict the amount of liquid which can enter the bed in the event o'fa tube rupture. it is desirable to provide thermal insulation about the supply pipe whenever vaporization of coolant would otherwise occur therein. Uniform distribution of liquid to the upright pipes is effected by a symmetrical arrangement of the header and by preventing vaporization of the liquid coolant prior to entry thereof into the vertical, tubes.

Two important points should be noted: (1) Vaporization of liquid begins in the vertical tubes or rises, to establish uniform distribution of coolant and thereby to prevent thermal cycling. (2) Sufiicient vapor is generated with each verticaltube to assure a continuously wetted internal surface.

It is also desirable to connect the tubes to the top header so that one tube is at each extremity thereof, the tubes at said extremities being connected preferably by rounded fittings such as elbows, to avoid steam pocketing and to insure that the top header is wetted.

A further feature of the invention is the coil mounting, which is supported by a suitable hanger from the top header. The liquid supply pipe to the bottom header is made flexible to permit movement of the bottom header incident to thermal expansion and contraction of the cooling device.

The invention will be further described with reference to the accompanying drawings forming a part of this specification and showing one preferred embodiment, wherein:

FIGURE 1 is a sectional elevation view of a vessel equipped with a cooling device according to the invention;

FIGURE 2 is a fragmentary section view, taken on the line 22 of FIGURE 1 but on an enlarged scale and partly diagrammatic;

FIGURE 3 is an elevation view of the cooling device;

FIGURE 4 is a fragmentary sectional view taken on the line 44 of FIGURE 3;

FIGURE 5 is a sectional view through the vessel wall showing the passage thereof by the liquid coolant supply pipe, still further enlarged; and

FIGURE 6 is a diagram of a piping system for controlling the pressure of the coolant supplied to the cooling device.

Referring to the drawings, the reactor includes a vessel 10 which may have a refractory lining 10a and is suitable for the regeneration of catalyst which was used for cracking petroleum in a fluidized bed and bears carbonaceous deposits, e.g., a silica-alumina catalyst passing a US. Standard IOO-mesh screen. The used catalyst is admitted through a riser 11 from a supply standpipe 12 and entrained by compressed air admitted at 13. Fluidizationcombustion air is admitted to a lower part of the vessel from a supply pipe 14 through a perforated aeration ring 15 to fluidize the catalyst C to the level L and effect combustion of the carbon on the catalyst to form carbon monoxide and carbon dioxide. Regenerated catalyst is discharged continuously via the outlet 16. The temperature within the fluidized bed can be controlled in part by regulation of the rate of air admission, as by a control valve 17, and in part by a plurality of cooling devices 18 which are according to the invention and are embedded in the fluidized bed. Other known expedients for temperature control may be employed in addition to or in lieu of control of the air rate, such as direct injection of water into the bed, cooling of the catalyst in an external boiler, or controlling the rate of catalyst admission or the temperature of the admitted catalyst. The gaseous combustion products are discharged from the top of the bed and freed from entrained solids in one or more separators, represented diagrammatically by the cyclone 19, from which the clean gas leaves through a gas outlet duct 20.

Any desired number of cooling devices 18, e.g., five to twenty can be mounted within the vessel. Each such device includes a bottom header 21, a top header 22, and a plurality, e.g., six, vertical vaporizing tubes 23 (sometimes called coils or risers) which interconnect the headers. In the embodiment shown each header is assembled from Ts 24, Us 25 and a central fitting 24a or 24.5, which are welded together and to the tubes 23. The fitting 24a of the bottom header is a T with a small side opening to facilitate connection of a small-diameter supply pipe, to be described, while the fitting 24b is a T like the Ts 24 and provides a discharge from the top header. The fittings 24a and 24b are advantageously situated at the centers of their respective headers, to produce a symmetrical arrangement which promotes uniform distribution of coolant to the several vaporizing tubes 23, providing that no significant vaporization occurs before the coolant enters the tubes. The horizontally elongated passages thus formed within the headers are of small crosssectional area in relation to the aggregate area of the tubes 23, thereby to insure a high linear flow velocity; this results in a shorter exposure of the coolant to the high bed temperature and minimizes the chance for vaporization of coolant in the bottom header. For example, the header passages may be the same as those in the tubes 23 and, more particularly, in the lower parts of these tubes, it being noted that though tubes 23 of uniform diameters from bottom to top are shown, this is not essential. It is, however, important that the several tubes have substantially the same flow and heat transfer characteristics; preferably they are identical in shape and size. It should be further noted that each L 25 is placed at an end of a header, to connect a tube 23 to each extremity of each header passage; this insures that there is no pocket in the headers wherein a stagnant zone might be formed, leading to vaporization of coolant. Such vaporization, particularly in the bottom header, would cause steam to enter one or more of the vaporizing tubes, leading to thermal cycling and steam corrosion. It further leads to mail-distribution of liquid coolant among the several tubes, thereby causing overheating of some in relation to others and producing stresses due to non-uniform expansion.

The mechanism of the all-important uniform distribution of liquid coolant may be further explained as follows: The pressure difference between the top and bottom of each tube 23 is determined primarily by the density of the two-phase fiuid within the tube. Pressure drop due to friction and acceleration is negligible. Since the several tubes have equal. heat transfer surfaces, vaporization is for the most part equal among the tubes. The said fluid density can be equal only if the tubes receive liquid at the same rate (assuming identical tubes). If vapor is generated in the inlet pipe or in the bottom header and enters one or more of the tubes, this tube will contain a fluid of lower density than the other tubes; this unbalance causes preferential liquid flow into the said one tube or tubes, which temporarily robs the other tubes of liquid and causes overheating thereof. This causes steam corrosion, reduced steam generation, and thermal cycling in the latter tubes.

Each cooler is suspended from suitable support means, such as a pair of brackets 42 and 42a which are fixed to the vessel wall. Each bracket carries a pair of rigid, spaced, parallel mounting plates 26 between which is mounted a hanger 27 secured to the plates by a pin 28. The upper header has rigidly fixed to each end a pair of parallel, spaced mounting plates 29, between which a hanger 27 is secured by a pin 30. The entire cooler is thereby suspended from its top header, and the lower header is free to move incident to elongation and contraction of the hangers and tubes due to temperature changes, while the lower ends of the hangers can spread upon elongation of the top header. The bottom header is restrained against swinging motion by suitable means, such as a guide plate 31 which is rigidly fixed thereto and is interposed in sliding engagement between a pair of spaced stationary guide plates 32 which are carried by a stationary part of the vessel, such as a lower bracket 33 carried by the vessel wall.

Liquid coolant is supplied to the bottom header by a supply pipe 34 having a diameter which is small in relation to the passages in the cooler and is connected to the fitting 24a. For example, when the tubes and fittings in the cooler have passages of 4 I.D., the pipe 34 may be 1 /2" in ID. This pipe has sufficient length in relation to its diameter to afford flexibility and permit the lower header to move incident to thermal changes with only a small strain, the pipe being then deformed by bending and torsion. The pipe is preferably of a single piece or of sections and pipe fittings which are welded together; threaded joints which can rotate upon movement of the lower header are not desired because they are likely to result in leakage. The supply pipe may include a vertical traverse 34a (FIGS. 3 and 4) and horizontal traverses 34b and 34c (FIG. 2),"but this is merely illustrative of one specific embodiment. The pipe extends out of the vessel as is shown in FIG. 5, i.e., by being welded to a drilled blind flange 35 which is bolted to a flange at the end of a bushing 36 which is, in turn, welded to the wall of the vessel 10. The pipe 34 has welded thereto, prior to assembly to the flange 35, a plurality of longitudinal wedges 43 with a shallow taper to engage the bushing 36 internally at the inner end. When coolant flows through the pipe 34 the pipe is cooled and contracted in relation to the bushing, thereby pulling the wedges into the bushing and effecting a tight fit which will not vibrate.

Externally of the vessel each supply pipe is connected via a valve 37 to a circular header 38 to which a liquid coolant, e.g., water, is supplied under pressure by a coolant pump 39. The pipe 34 is optionally covered with thermal insulating material 40 to reduce heat transfer from the bed to the pipe and thereby reduce the tendency toward vaporization of the liquid coolant.

The top header is connected at its T 24b to a discharge pipe 41, which is advantageously of diameter at least as large as the tubes 23 and which extends from the vessel 10, by any suitable fitting, such as that shown in FIG. 5. Externally of the vessel these pipes are connected to any suitable receiver, not shown.

Any suitable means may be provided to maintain the liquid in the header 38 at the desired pressure level to cause vaporization of coolant in the tubes 23 while preventing vaporization in the pipe 34 and in the bottom header. For example, the pressure may be such as to efiect vaporization at a temperature of the order of 50 F. below the desired bed temperature. This pressure may be constant or variable. In the former case an auxiliary device or expedient, such as those previously noted, is used to control the bed temperature, or the bed temperature can be regulated by placing a variable number of the cooling devices 18 into operation. In the latter case the pressure may be varied so as tomaintain the desired bed temperature. However, it must be remembered that the pressure must be low enough to cause an adequate coolant fiow velocity which will insure wetted walls in the tubes 23 and in the top header.

By way of illustration, a variable-pressure control system is shown in FIG. 6, wherein the discharge pipe 41 is connected to a steam-water drum 44 from which steam is discharged through a pipe 45 under control of a pressure-regulating valve 46. The valve is operated through a control line 47 to vary the pressure Within the drum 44 in accordance with the bed temperature, as measured by a temperattire-responsive device 48. Feed water is sup plied to the drumvia a feed pipe 49 and water is supplied from the drum to the pump 39 via a pipe 50.

In operation, all or any desired number of the coolers 18 may be pieced into operation by means of the valves 37. The coolant, usually water, flows through the. pipes 34 to the lower headers 21 and is distributed thereby among the several vaporizing tubes 23 while the coolant is wholly in the liquid state. The coolant ascends these tubes and is partially vaporized therein. The rate of admission is controlled to insure incomplete vaporization, so that a mixture of liquid and vapor emerges from each of the vaporizing tubes into' the top header 22, from which the combined streamsare discharged through the discharge pipe 41.

Initially, in the lower portions of the tubes 23, bubble flow occurs, i.e., there are small amount of vapor distributed throughout the continuous liquid phase. At some distance above the bottom there occurs a sharp transition to annular flow, in which an annular stream of liquid is in engagement with the tube Wall and surrounds a core of steam. This transition occurs without the intermediate flow regime called stratified flow. A primary function of the upright, viz., vertical, tubes is the avoidarms of such.intermediatestratified flow. It is permissible to turn the mixed stream into horizontal flow, e.g., in the top header and pipe 41 after a sufiicient vapor velocity has been attained to assure continued annular flow.

By insuring only partial vaporization of the liquid coolant in the tubes 23 and sharp transition from bubble to annular flow the inner surfaces thereof are maintained wet and thermal cycling is prevented. In actual operation this cooling device operated successfully for over 11,360 hours, including 83 operating cycles of the regenerator. In contrast, other cooling coils, using the standard bayonet construction, were undependable and frequently failed after less than half of the stated operating period.

When the variable-pressure coolant supply system according to FIG. 6 is used the pressure within the drum 44 may, for example, vary between 200 and 250 p.s.i.g., as determined by the operation of the pressure-regulating valve 46. The pump 39 increases the pressure on the liquid only by the amount necessary to elfect the necessary flow; hence, the pump discharge pressure rises and falls with the drum pressure and the vaporizing temperature is controlled by the temperature-sensitive device 48.

The parallel vertical vaporizing tubes present very low pressure drops in relation to other arrangements, thereby reducing pump head requirements and the pressures in the supply pipes. The coolers are, moreover, compact and can be readily fabricated in a shop, designed to pass through manways in the vessel, and replaced by two field welds. The water flow rate is not critical as in horizontal coils, where low velocity results in thermal cycling and results in metal fatigue and failure.

By mounting the coolers in spaced, non-juxtaposed relation about the periphery of the vessel there is no danger of eroding a neighboring cooler by a leak in another, a difficulty inherent in the usual horizontal coils.

Because the coolers with vertical vaporizing tubes are readily suspended, they are supported with the tubes in tension. Sagging of the tubes between supports, which tend to form pockets in which vapor can form and thereby lead to metal fatigue and/ or steam corrosion, is thereby eliminated. Expansion and contraction of the parts causes no such adverse elfects in the cooler of the invention.

Surging flow of coolant, which is common in horizontal tubes, has heretofore made excess-flow valves inoperative and made it necessary to employ throttling devices. These are obviated by the cooler of the instant invention.

I claim as my invention:

1. The combination with a reactor vessel containing a bed of contact material for effecting exothermic chemical reactions, of a cooling device embedded in said material, said device including vertically spaced, horizontally elongated bottom and top headers and a plurality of substantially vertical vaporizing tubes interconnecting said headers for partial vaporization of liquid coolant ascending therethrough, said tubes being connected to said headers at intervals along the lengths of the latter and said tubes and headers being externally in contact with said bed, support means for said upper header fixed to said vessel, said lower header being suspended from said upper header through said vaporizing tubes, a supply pipe for liquid coolant extending from outside said vessel to said lower header, said supply pipe having flexibility to permit vertical motion of said lower header upon thermal expansion and contraction of the cooling device, and a,

discharge pipe connected to said upper header and extending outside of the vessel.

2. The combination as defined in claim 1 wherein said supply pipe has a cross-sectional area which is smaller than that of the bottom header and the flow area of the latter is smaller than the sum of the flow areas of the lower parts of said vertical vaporizing tubes.

3.'The combination as defined in claim 1 which ineludes, additionally, guide means in sliding engagement with a part of said bottom header.

4. The combination as defined in claim 1 wherein said vessel contains a bushing extending through the Wall thereof, said coolant supply extending through said bushing in radially spaced relation thereto, annular closure means extending between said pipe and bushing at one end of the latter and securing the pipe against axial movement into the bushing, and tapered centering means acting between the other end of the bushing and the pipe and arranged to place center the supply pipe and stress the same in tension upon contraction of the supply pipe in relation to the bushing.

References Cited in the file of this patent UNITED STATES PATENTS Bransky June 1, Gunness Mar. 1, Keith May 2, Miller July 8, Martin June 16, Sprague et al Feb. 28, Saunders et a1 June 20,

FOREIGN PATENTS France Nov. 24,

Claims (1)

1. THE COMBINATION WITH A REACTOR VESSEL CONTAINING A BED OF CONTACT MATERIAL FOR EFFECTING EXOTHERMIC CHEMICAL REACTANTS, OF A COOLING DEVICE EMBEDDED IN SAID MATERIAL, SAID DEVICE INCLUDING VERTICALLY SPACED, HORIZONTALLY ELONGATED BOTTOM AND TOP HEADERS AND A PLURALITY OF SUBSTANTIALLY VERTICAL VAPORIZING TUBES INTERCONNECTING SAID HEADERS FOR PARTIAL VAPORIZATION OF LIQUID COOLANT ASCENDING THERETHROUGH, SAID TUBES BEING CONNECTED TO SAID HEADERS AT INTERVALS ALONG THE LENGTHS OF THE LATTER AND SAID TUBES AND HEADERS BEING EXTERNALLY IN CONTACT WITH SAID BED, SUPPORT MEANS FOR SAID UPPER HEADER FIXED TO SAID VESSEL, SAID LOWER HEADER BEING SUSPENDED FROM SAID UPPER HEADER THROUGH SAID VAPORIZING TUBES, A SUPPLY PIPE FOR LIQUID COOLANT EXTENDING FROM OUTSIDE SAID VESSEL TO SAID LOWER HEADER, SAID SUPPLY PIPE HAVING FLEXIBILITY TO PERMIT VERTICAL MOTION OF SAID LOWER HEADER UPON THERMAL EXPANSION AND CONTACTION OF THE COOLING DEVICE, AND A DISCHARGE PIPE CONNECTED TO SAID UPPER HEADER AND EXTENDING OUTSIDE OF THE VESSEL.

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