US3303828A - Heat exchanger - Google Patents

Description

HEAT EXCHANGER 2 Sheets5heet 1 Original Filed Nov. 16, 1961 IN VEN TORS. OLIVER E CAMPBELL NORMAN E4 PENNELS A TIORNEYS.

1967 o. F. CAMPBELL ETAL 3,

HEAT EXCHANGER Original Filed Nov. 16. 1961 2 Sheets-Sheet 2 INVENTORS. OLIVER F. CAMPBELL NORMAN E. PENNELS A TTORNE V5.

United States Patent C) 3,303,828 HEAT EXCHANGER Oliver F. Campbell, Chandler, Ariz., and Norman E. Pennels, Olympia Fields, 111., assignors to Sinclair Research, Inc., Wilmington, Del., a corporation of Delaware Origi 2] application Nov. 16, 1961, Ser. No. 152,733, now Patent No. 3,256,924, dated June 21, 1966. Divided and this application Sept. 24, 1965, Ser. No. 505,216

2 Claims. (Cl. 122--352) This application is a division of application Serial No. 152,733, filed November 16, 1961, now Patent No. 3,256,924.

This invention relates to a novel relatively small package apparatus for recovering heating values from waste flue gases, particularly from flue gases produced by the regenerator of a hydrocarbon catalytic cracking operat1on.

In petroleum catalytic conversion processes the catalyst employed is continuously regenerated by burning off carbonaceous matter that deposits on the catalyst. Since catalysts are deactivated by sintering at high temperatures, the regenerator temperatures are controlled and limited to usually a maximum temperature of about 11001200 F. Low temperatures coupled with limited quantities of air fed to the regenerator result in incomplete combustion of the coke and produces so-called regenerator gas or regenerator flue gas, which contains catalyst fines and, in addition to inert materials such as nitrogen, water vapor and carbon dioxide, small amounts of hydrocarbons and some carbon monoxide, e.g. about 1 to 15% by volume with about 47% being average.

The sensible heat of the flue gases and the latent heat represented by the carbon monoxide and small amounts of hydrogen and hydrocarbons which burn to carbon dioxide and water constitute a potentially valuable source of heating values. The heating values are commonly recovered by use of what is known in the art as CO boilers. Conventional CO boilers contain a combustion furnace and a steam generator which is usually a fluid heat exchanger comprising a lower water drum and a larger steam drum placed vertically above the water drum and connected by a plurality or bank of tubes and pro viding communication between the interiors of the drums and tubes. Water is supplied to the steam drum and flows down some of the tubes and perhaps special downcomer tubes. Steam is produced by allowing the combustion gases resulting from the burning of regenerator flue gases in the furnace section to pass, generally across the tubes between the drums and on the outside of the tubes in indirect heat exchange relationship with the water in the plurality of tubes.

Unlike normal steam generators, i.e. where ordinary fuel (i.e. high Btu. fuel) is burned and Where the combustion value of the combustion gases is on the order of about 1000 to 2000 or more Btu/cu. ft., the heat value of the regenerator flue gas (i.e. low B.t.u. fuel) is on the order of only about 15 to 35 B.t.u./cu. ft. To attain steam production from regenerator flue gases, which have a heat value about 100 times less than that found in the ordinary fuel burned in ordinary steam boilers, and the resulting combustion gases extremely large amounts of gases must pass through the heat exchanger. To handle these flue gases in quantities economically justifying a flue gas recovery operation, the fluid heat exchanger units are necessarily of high capacity and necessarily of a very large size and surface area. The actual size of the CO 'boiler to utilize the regeneration gases produced varies with the size of the fluid cracking units. There are many large fluid cracking units in existence which produce volumes of regeneration gases from which can be obtained on the order of 200,000 to 450,000 lbs. of steam per hour. The

Patented F eb. 14, 1967 field construction of CO boilers of a size large enough to yield this quantity of steam is generally not only justified by the amount of steam produced but by the fact that it is actually relatively cheaper to field construct a large CO boiler than one of smaller size. However, there are many smaller fluid cracking units whose steam-producing poten tial is only about 50,000 to 100,000 lbs/hr. but to field construct CO boilers for them is in most cases too expensive. There remains, therefore, a need for a CO boiler to accommodate these smaller fluid cracking units.

Although a definite need exists for a CO boiler small enough to be transported for assembly at the desired site, yet of suflicient capacity to produce the desired volumes of steam, prefabricated, portable CO boilers, unlike ordinary steam boilers, are unheard of. Sound reasons exist for factory production of such boilers rather than construction at the sites. It should be noted, for example, that conventional steam generators are welded together. This requires inspection, such as X-ray inspection, of the separate and multitudin-ous welds as well as, in many cases, preheating of the segments to be welded, and heat treatment of the welds. Such operations can be carried on most eflectively as shop operations where the weld inspection apparatus and the heat treatment equipment may be most effectively maintained and operated. Moreover, the operations are conducted more cheaply in shops, i.e. indoors, wherein weather interruptions are eliminated and special tools, and better-trained personnel are available. However, a CO boiler of ordinary design, or even a heat exchanger alone built using conventional principles and having a large enough capacity to generate the required amount of steam for low heat value gases, is too big to be transported from the factory to the site on currently available carriers. Consequently what CO boilers exist have been built at the sites of their use and where such expensive construction is not possible, due to lack of the technical personnel needed, or due to cost factors, the regenerator off gases are wasted. It has been proposed that a heat exchanger of smaller size yet capable of incorporation in a CO boiler to produce the desired quantities of steamrnight be obtained by increasing the number of tubes connecting the drums of the fluid heat exchanger. This solution, however, presents a problem, for increasing the number of tubes in a conventional flow path for the gases increases the velocity of the combustion gases passmg in heat exchange relationship with the tubes. Since regenerator flue gases invariably contain catalyst fines, increasing the velocity of the gases causes undue erosion and damage to the tubes by the catalyst fines. Furthermore, there is a limitation as to how many tubes may be rolled into each boiler drum since a certain area in ligament must be left in the drums for maximum strength and steam production.

The present invention provides apparatus for recovering heat values from large volumes of waste flue gases, rnost advantageously from catalytic regenerator flue gases which apparatus is made up of several components, of such design that the assembled boiler is capable of absorbing the necessary heat, that is, having a maximum capacity of up to about 100,000 lb./hr. of steam at 600 p.s.i.g. and 750 F. and yet each component may be made small enough in size to be shop prefabricated and the major components, boiler and furnace, shipped to any part of the United States for connection together at the site. For example, current size limitations in every part of the United States for shipping an item by railway are ordinarily: maximum width, 10 feet 8 inches and maximum height, 12 feet 3 inches. However, since furnaces are more easily made than the heat exchanger and readily supplied locally, the furnace, in cases where cross-country transportation is unnecessary or not desired, can be of larger size.

The apparatus of the present invention includes a specifically designed combustion chamber or furnace and boiler. The flow path for combustion gases in the heat exchanger is generally normal to the heat exchanger tubes rather than parallel. Such a flow path enables two or more prefabricated heat exchanger units to be connected to provide a serial path for the combustion gas through the number of prefabricated heat exchange units desired for full utilization of the heat values of the combustion gas.

The dimensions of the heat exchanger are reduced to transportable proportions by providing a plurality of steam and water drums and by providing heat exchange tubes generally parallel to each other throughout the hotgas contacting passage. Also, the steam and water drums are generally excluded from the insulated chamber through which the burner gases flow, since their capacity for heat exchange by heat gain from hot gases or heat loss to the atmosphere is comparatively low. The heat exchanger is generally lower in height and the contacting passage is longer than in heat exchangers known in the art. The provision of a plurality of steam drums at the top and water or mud drums at the bottom of the heat exchanger accomplishes several desirable purposes. First, it reduces the height of the heat exchanger while providing for the same capacity of steam or water. Secondly, it maintains the strong cylindrical shape of the drums, rather than give them a flattened and therefore weaker design. Thirdly it provides a more extensive total surface for the drums, so that more heat exchanger tubes may be provided without decreasing the amount of ligament below what is required for the inetgrity of each drum. Overall, this feature of the present invention makes best use of the available room, i.e. the size limitations for shipping (10'8" width by 125 height, maximum). It provides maximum flow area for gases while at the same time keeping the velocity down.

The furnace, as mentioned above, is a separate component from the heat exchange boiler and also may be made in a size small enough for rail transportation anywhere in the United States. It is designed to handle the extremely large amounts of off-gas produced by the ordinary catalyst regenerator and to bring these gases in a confined space to the ignition temperature of the carbon monoxide and to burn the carbon monoxide with a minimum amount of an oxygen-containing gas, such as air, and supplemental fuel to avoid a substantial increase in the already large amount of gas which is handled by the heat exchanger. An increase in the already large volume of gas increases the problem of keeping the velocity down in the boiler. These functions are performed by employing a refractory-lined furnace and by arranging for spin entry of gases to the furnace chamber for the creation of an area of thorough turbulence to mix the heated off-gas with air and the products of combustion of the auxiliary fuel burner. Louvers and angular slots are used to spin the gases at their entry to the furnace to provide a zone of primary mixing and a very simple, rugged device is provided to create further turbulence or a secondary mixing zone, near enough to the area of off-gas heating to insure complete heating to ignition temperature and mixing to bring carbon monoxide into contact with oxygen, yet far enough from the flame source of heat to prevent unstable flame conditions characterized by pulsation or blowing out. The turbulence is caused by a simple ring in the flow-path of the gases which is made of a refractory material and which can be manufactured or repaired by ordinary masonry techniques. The importance of employing a refractory-lined furnace resides in the fact that it also lowers the quantity of air and fuel required since it prevents loss of heat to the boiler tubes until the CO gas is burned. Employment of furnaces characterized by high heat loss, as for instance, waterwall furnaces, are unsuitable in that more fuel is needed in these furnaces to obtain the necessary combustion temperature. The use of more fuel, as aforementioned,

means an increase in the already large volume of gas which in turn increases the problem of keeping the velocity down.

A preferred form of the apparatus is illustrated in the accompanying drawings. It is to be understood that the apparatus as illustrated may be modified in certain particulars as will be evident to those skilled in the art. The invention will be described with reference to the accompanying drawings in which:

FIGURE 1 is an advantageous compact arrangement of the apparatus in operation;

FIGURE 2 is a horizontal cross-section, in part, of the furnace of the present invention along line 2-2 of FIG- URE 1, and

FIGURE 3 is a verticalcross-section of the boiler 'of the present invention along the line 3-3 of FIGURE 1.

Referring to the drawings, the apparatus includes a furnace 1, desirably cylindrical and preferably having a circular cross-section; The furnace is lined with a suitable refractory material 3 and can be of a size that permits transportation. A preferred refractory material is hightemperature insulating brick. This brick is made of alumina-silica mixed with sawdust so that on burning during its preparation, the sawdust decomposes to leave air pockets throughout the brick. The air pockets formed contribute to the production of a brick of low heat conductivity and at the same time a refractory material of relatively light weight, a feature highly desirable in that it decreases the shipping and handling weight of the prefabricated CO boiler of the present invention.

At one end of the furnace there is provided burner means 6, preferably centrally located within a conduit 9 containing an oxygen-containing gas inlet 12 and fuel line 15 which conducts fuel from a source (not shown) to the burner means where it is burned to provide the flame and heat necessary for recovering the heat values of carbon monoxide. The conduit 9 of the burner means extends across an annular chamber 18 provided with a plurality of louvers or registers 21 circumferentially disposed in the annular chamber about conduit 9 of the burner means. An annular conduit 24 surrounds and communicates with the annular chamber 18 and provides the means through which air is introduced into the annular chamber from the source 25. The air introduced is forced past the circumferentially disposed registers 21 set at an angle so as to impart a spin to it. Burner means '6 opens into an intermediate annular chamber 27, having the frustoconical sides 30. A plurality of angular slots 31 through which the flue gas is introduced are provided in the wall 32 which is disposed circumferentially about the end of the chamber 27 defined by the sides 30 and communicate with a second annular conduit 33 which is provided with regenerator off-gas from the source 34.

To insure maximum conversion of the carbon monoxide to carbon dioxide and thereby the realization of maximum heat value the flue gas should be thoroughly mixed with the hot oxidizing gases produced by the burner. This is accomplished by providing (1) a primary mixing zone produced by the whirlpool effect given the gases by the angularity of the louvers 21 and the angular slots 31 and (2) a secondary mixing zone effected by creating turbulence in an area 35, of the furnace which is far enough removed from the burner tip 6 as to present unstable flame conditions. To assist in the creation of this turbulence an anular ring 36 usually at least about 3 inches in height is positioned Within the furnace to deflect gas flow from the sides of thefurnace and thus deflect the carbon monoxide-containing gas into a zone of turbulence with the stream of hot gases produced by the burner. At the end opposite the burner, the furnace leads to the conduit 45 which communicates'with heat recovery means and provides passage of the hot combustion gases thereto.

Referring to FIGURE 3 the heat recovery means comprises a chamber or conduit 48, preferably provided with insulated walls 50, into which the combustion gases are introduced and which provides a flow path for the gases. Ordinarily, the cross-sectional area of the flow chamber effects a maximum gas velocity of about 80 feet per second. This heat exchanger or steam generator is preferably provided with three lower water drums 51, 2 upper steam drums 54 and a plurality of tubular conduits 57 providing communication therebetween. The upper drums 54 are advantageously of a larger diameter than the lower drums 51. Two upper drums with a diameter of about 36 inches and three lower drums with a diameter of about 24 inches are suitable. The drums are cylindrical in structure and extend longitudinally a distance no greater than that dictated by transportation size specifications, usually a maximum of about 30 feet. The walls of the conduit or chamber 48 should be pressure tight for the system operates under positive pressure. Construction of the walls can follow conventional boiler practice for pressurized furnaces. For example, the walls may be a steel casing with refractory lining to protect them from heat and to prevent heat loss. Alternatively, the walls may be a steel casing protected from the rest of the drum by placing boiler tubes against the casing. In this case, the tubes must be tangent or if not tangent must have side studs with the area between the tubes and behind the studs filled with refractory material. With this design insulation is provided outside the casing to limit heat loss to the atmosphere.

Water can be supplied to one or both steam drums, as shown in the drawing, by line 55. FIGURE 1 shows an arrangement by which the flue gases, cooled by passage through the heat exchanger 2 may be conveyed away by conduit 60 and brought to the economizer 64. This economizer is a heat exchanger which allows downward passage of the flue gas in indirect countercurrent contact to water to be fed to steam generation. Thus, water may enter the economizer from the line 66 and be heated in the economizer almost to its boiling point before it enters the steam generator. The economizer may be of conventional type. Exhaust gas passes from the economizer to the stack 68 and a conduit (not shown) conducts the steam generated to other parts of the plant where it can be utilized.

The distance which the ring 36 is placed from the CO entry is dependent in part on the height of the ring. When a ring of small height is employed it is positioned closer to the CO entry; when of greater height it may be positioned farther from the entry. Generally the annular ring is placed just behind a zone of desired maximum turbulence. This zone is generally a paraboloid within the combustion chamber, the surface of which paraboloid may be conveniently defined as the loci around the axis of the furnace of the parabola formed by the intersecting curves obtained when arcs are drawn from the intersection of the wall 3 of the combustion chamber with the CO entry wall 32 to the axis of the furnace using the opposite intersection as the center of the arc and the diameter of the furnace as the radius of the arc.

Advantageously the minimum placement distance of the ring from the CO entry should be approximately at a point where a line extension of the frusto conical side of annular member 31 intersects the aforementioned arcs. Placement of the ring too close to the burner is undesirable since the turbulence created thereby tends to interfere with the flame of the burner. On the other hand the further the ring is placed from the flame the higher the ring is required to be in order to obtain the desired turbulence in the minimum space. A large height may be disadvantageous from the standpoint of the cost of materials that make up the ring and the greater tendency of larger rings to erode and crack.

In operation, flue gases from a fluid catalyst regenerator are introduced by conduit 34 into annulus 33 which imparts a rotary motion to the gases as they enter into the furnace proper via CO entry openings 31. The gases are normally at temperatures of about 900 to 1100 F. and issue at high velocities. Auxiliary fuel which can be liquid or gaseous and preferably is natural gas, is fed to the burner means 6 by line 15 which is connected to a suitable fuel source. Air to support combustion of auxiliary fuel and the carbon monoxide content of the flue gases is fed to the furnace and burner means through members 12 and 24, respectively. Only a slight excess of air normally is employed to insure substantially complete combustion of the carbon monoxide. The air introduced through inlet 24 is forced past registers 21 which give the air a spin as it enters the furnace. The annular ring 36 deflects the flue gas flowing into the furnace thereby forcing it to mix with the hot gas and create the turbulence desired to combust the carbon monoxide. The combustion gases, after burning, exit from the furnace via conduit 45 and enter chamber 43 of the steam generator where they pass in heat exchange relationship with tubes 57 of the steam generator. A water supply means 58 provides to the steam drums 54, water which can be preheated by passage through an economizer 64. An economizer is utilized to cool the exhaust gases in order to increase efiiciency of operation. The outlet water temperature will depend upon (a) the initial water temperature, (b) the degree of cooling of the exhaust gases, and (c) the initial temperature of the exhaust gases. The economizer temperature of the water is up to about 50 F. of the boiling point. A conduit (not shown) conducts the steam generated from upper drums 54 to other parts of the plant where it can be utilized as desired. Exhausted gases passing through the steam generation area are passed via conduit 67 into a stack 68 where they can be released to the atmosphere.

We claim:

1. A heat exchanger comprising three water drums, two steam drums arranged above said water drums, a plurality of generally parallel heat exchange tubes providing interior communication between water and steam drums, said heat exchange tubes being enclosed within walls defining a chamber for flow of gases normal to the heat exchange tubes, a sufficient number of heat exchange tubes being provided to absorb enough heat from a gas having a heat value of about 15 to 35 B.t.u./ cubic ft., and containing catalyst fines, to provide about 50,000 to 100,000 pounds per hour of steam at 600 p.s.i.g. and 750 F. while not reducing the cross-sectional area of said flow chamber enough to cause a gas velocity of greater than about feet per second, said heat exchanger being no more than about 10 feet, 8 inches wide and about 12 feet, 3 inches high.

2. The heat exchanger of claim 1 wherein the three water drums and two steam drums are outside the walls enclosing the heat exchanger tubes.

References Cited by the Examiner UNITED STATES PATENTS 710,340 9/1902 Rust 122352 1,304,998 5/ 1919 Morrison 122352 1,839,125 12/1931 Smith 1227 2,421,074 5/ 1947 Kuhner 122347 3,115,120 12/1963 Durham 1227 CHARLES J. MYHRE, Primary Examiner.

Claims (1)

1. A HEAT EXCHANGER COMPRISING THREE WATER DRUMS, TWO STEAM DRUMS ARRANGED ABOVE SAID WATER DRUMS, A PLURALITY OF GENERALLY PARALLEL HEAT EXCHANGE TUBES PROVIDING INTERIOR COMMUNICATION BETWEEN WATER AND STEAM DRUMS, SAID HEAT EXCHANGE TUBES BEING ENCLOSED WITHIN WALLS DEFINING A CHAMBER FOR FLOW OF GASES NORMAL TO THE HEAT EXCHANGE TUBES, A SUFFICIENT NUMBER OF HEAT EXCHANGE TUBES BEING PROVIDED TO ABSORB ENOUGH HEAT FROM A GAS HAVING A HEAT VALUE OF ABOUT 15 TO 35 B.T.U./

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