US3362902A - Process and apparatus for the utilization of low heat value fuels - Google Patents

Description

Jan. 9, 1968 J. A. KIVLEN ET AL 3,362,902

PROCESS AND APPARATUS FOR THE UTILIZATION OF LOW HEAT VALUE FUELS 2 Sheets-Sheet 1 Filed July 7, 1965 FIGURE I Catalytic Cracking Cu'mlysi Regeherutor 3 E R U m F INVENTORS JOHN A. KIVLEN JAMES F. HARRIS IRVING D. CRANE BY 4 Mi.

PATEN T ATTOR NEY J. A, KIVLEN ET AL PROCESS AND APPARATUS FOR THE UTILIZATION Jan. 9, 1968 OF LOW HEAT VALUE FUELS 2 Sheets-Sheet 2 Filed July 7, 1965 26 cozwanEoo mm cozomm 8522;

cozuww cocu coo m INVENTORS JOHN A. KIVLEN JAMES E HARRIS IRVING D. CRANE BY WMMYQ.

PATENT ATTORNEY United States Patent ice 3,362,902 PROCESS AND APPARATUS FOR THE UTI- LIZATION 0F LSW HEAT VALUE FUELS John A. Kivlen and James F. Harris, Sparta, and Irving D. Crane, Morristown, N..I., assignors to Esso Research and Engineering Company, a corporation of'Delaware Filed July 7, 1965, Ser. No. 470,017 6 Claims. (Cl. 208113) This invention relates to a process and apparatus for the utilization of low heat value fuels. More particularly, the invention relates to a process and apparatus for utilizing the heating value of a CO containing gas in a CO furnace.

Catalysts employed in the processing of hydrocarbon oils are regenerated by burning off carbonaceous matter and the operation produces a regenerator flue gas. This gas contains from about to about volume percent carbon monoxide; the remainder consisting of inert gases including carbon dioxide, nitrogen and water vapor.

CO has a heating value of about 4347 B.t.u./lb., and

under some circumstances it is considered worthwhile to burn it to recover the heat of combustion. The principal problem lies in raising the temperature of the CO containing gas, sometimes hereinafter referred to as the low heat value fuel, to the temperature at which the CO will ignite and burn completely. The ignition temperature of CO is about 1130 F., and since regenerator off gas is sometimes available at relatively low temperatures ranging from 400-1100 F. it is necessary to heat it with a high heat value fuel to burn the CO. For purposes of this description low heat value fuels are those with a heating value of less than 10,000 B.t.u./lb. and high heat value fuels are those with a heating value of more than 10,000 B.t.u./lb. The object of this invention is to provide a process and apparatus in which a low heat value fuel containing CO is heated and completely burned employing a minimum amount of expensive high heat value fuel. Another object of this invention is to use the products of CO combustion to heat petroleum process streams and to. make steam. A more specific object of this invention is to provide a CO furnace for use in conjunction with a catalytic cracking unit.

These objects and additional objects and advantages of the invention will be apparent from the description which follows.

We have found that by burning the CO in two distinct combustion stages and by contacting the fluids to be heated with the'hot combustion gases in a particular manner, an eificient CO utilization system is achieved.

The invention will be more fully described with reference to the attached drawings in which:

FIGURE 1 is a schematic view of the CO furnace showing its relationship to the catalytic cracking unit and the regenerator.

FIGURE 2 is a side view of the CO furnace, particularly the combustor, the transition section and the heat transfer section.

FIGURE 3 is a cross-sectional view taken along line 3,3 of FIGURE 2.

Referring to FIGURE 1, reference numeral 1 denotes a catalyst regenerator. Details of the regenerator and the regeneration procss will be omitted since they are not pertinent to the description of the invention. Regenerator off gas is carried in line 2 to Y 3 where the gas stream 3,362,902 Patented Jan. 9, 1968 is divided into two streams which are usually of approximately equal volume. Regenerator off gas contains from about 5 to about 15 volume percent CO. Analysis of one regenerator off gas revealed the following composition:

Component Percent by Volume Percent by Weight The temperature of the gas in line 2 ranges from 400- 1100 F. depending on regenerator conditions and heat losses. It the gas is passed through a conventional heat exchange facility such as a waste heat boiler after it leaves the regenerator, the gas temperature can drop to a range of 400 700 F. Conventional CO furnaces are not suited to these cold gas feeds because of the exorbitant amounts of high heat value supplementary fuel required to obtain complete combustion of the CO. The invention is not limited to regenerator off gases as low heat value fuel. Any waste gas containing from 5-15 volume percent CO or any other combustible low heat value fuel can be burned in the process and apparatus described herein.

Approximately equal volumes of gas are passed by lines 4 and 5 to gas distributors 6 and 7 respectively. High heat value fuel is fed by line 8 to a burner shown generally by reference numeral 9. Primary air is supplied to the burner by line 10. Air for combustion of CO in the first stage is also supplied by line 10. Following two-stage combustion in horizontal combustion zone 11, the com bustion products pass through transition section 12 into and through convection section 13. Spent gases are vented to the atmosphere through stack 14. In the convection section, steam is generated by passing feed water into the section by line 15 andrecovering steam through line 16. A hydrocarbon oil to be cracked is preheated by passing it through line 17, crossover line 18 and line 19, and then to catalytic cracking unit 20. Details of the catalytic cracking unit will be omitted since the function of the unit is not a part of the invention.

A more detailed description of the CO furnace will be provided with reference to FIGURES 2 and 3. Like reference numbers in FIGURES 1, 2 and 3 are used to designate like elements.

Referring to FIGURE 2, the combustion section or combustor of the furnace shown generally by reference numeral 11 is disposed horizontally. It is provided with an end wall 21 containing one or more openings 22 for one or more burners 9. The Wall or walls 23 of the combustion section are circular or rectangular as desired. The downstream end of the combustor is unobstructed except for a downwardly hanging wall or baflie wall 24. When the combustor is circular in cross-section, the wall is a segment of a circle. When the combustor is rectangular in cross-section, the wall extends across the top. The wall occupies from 10-35 percent of the cross-sectional area at the downstream end of the combustor. The function of the hanging or bafile wall will be discussed subsequently. The combustor as well as the transition section and the convection section of the furnace comprise a metal shell lined with a suitable insulating refractory shown generally by reference numeral 25.

As stated previously, the CO containing gas is preferably divided into two streams of approximately equal volume. However, from 30-70 volume percent can be fed to the first stage if desired. The streams pass through pipes 4 and 5 into distribution ducts 6 and 7 respectively. In the preferred embodiment, the combustor is circular in cross-section and the distribution ducts are annular. Referring to FIGURE 3, which is a cross-sectional view taken along line 3-3 of FIGURE 2, the distribution duct comprises an outer shell 26, an air plenum 27, and a CO plenum 28. The external surface of the combustor 23 and the insulation layer 25 are pierced by a plurality of spaced inlets shown generally by reference numeral 29. While FIGURE 3 shows eight inlets spaced 45 of arc apart, any other desired number of inlets can be used. Dampers 30 and 31 are set to cause the CO containing gas to flow into and around plenum 28 so that the gas distribution between plenum 28 and the first stage plenum is approximately equal, i.e. the dampers in ducts 4 and 5 are used to control gas fiow at Y 3 of FIGURE 1. The internal configuration of distribution ducts 6 and 7 is the same except that duct 6 does not have an air plenum. Air for the second stage of C combustion is supplied by pipe 32 to duct 7. While we have disclosed two stages of CO combustion and two distribution ducts, we wish to include the embodiment of three or more stages of combustion and three or more distribution ducts.

Burner 9 can be any conventional type of burner designed for burning gas, oil or both types of high heat value fuel. The burner is operated with sufficient excess air to provide combustion air for the first stage of CO combustion. A range of 60l20 percent excess air is satisfactory. In addition, the burner is operated to provide a heat output suificient to heat the first stage CO combustion area to a temperature of 1l001400 F., preferably 1150-4200 F. We have found that temperatures in this range are sufiicient to heat, ignite and completely burn the CO in a gas containing from -15 percent CO entering the combustor at a temperature ranging from 400- 1100 F. The first CO distribution duct is located at a point downstream of the burner at which combustion of the high heat value fuel is 40-100 percent complete. Of course, location of the ducts depends on the cross-sectional area of the cornbustor, number and type of burners, type of high heat value fuel, etc. Conventional heat engineering techniques and calculations can be used to locate the CO ducts. Generally speaking, the minimum distance between the burner end of the combustor and the inlet to the first CO burning stage will be .02 /A to /A where A is the cross-sectional area of the combustor.

As indicated above, about one-half of the cold low heat value fuel, i.e. the CO containing waste gas, is mixed with air and the hot combustion gas from combustion of the high heat value fuel, i.e. the supplementary fuel and the CO in the mixture is burned in the first stage. The hot combustion gases from the first stage pass to the sec- 0nd stage. In the second stage, the other portion of the relatively cold CO containing gas is introduced into the combustion zone along with combustion air and the second stage of combustion takes place. The second stage of combustion differs from the first stage in that the high temperature gas mass from the first stage rather than supplementary firing is used to heat the second half of the CO to ignition temperature. The distance between the first stage CO inlet and the second stage CO inlet depends on the characteristics of combustion in the first stage. It is desirable that the first stage combustion be 40-100 percent complete. The distance will be from 1.5 /A to 0.5 /A where A is the area of a cross-section through the combustor at the CO inlet. The quantity of air added in the second stage will range from 2.5-4.5 times the quantity of CO introduced into the second stage.

The temperature of the combustion gases following combustion of the CO will depend on combustor condi- 4 tions, etc. Generally, the temperature will range from 16002200 F.

The CO furnace of our invention is unique in that it provides a transition section in which the flow of the combustion gases is turned through of are from horizontal flow to vertical fiow. The transition section features a hanging wall 24 at the inlet and a sloped floor 33. The hanging wall or bafiie wall and the sloped floor cooperate to turn and to spread the rapidly moving gases to provide an even flow of hot gas over the surfaces of all the tubes in the convection section.

The first set of tubes 34 in the convection or heat transfer section are steam tubes. By first we means to convey that these tubes are contacted first with the combustion gases. Water enters by line 15 and steam is removed by line 16. Any suitable number of tube passes can be provided for the steam making operation.

The hot but partially spent combustion gases are next used to heat a petroleum process stream and in a preferred embodiment the feed to a catalytic cracking unit. The second set of tubes shown generally by reference numeral 35 includes an oil inlet line 17, a set of tubes 36 in which the liquid progresses downwardly through the heat transfer section countercurrent to combustion gas flow, a crossover line 18 which moves the partially heated liquid to the bottom tube of a set of tubes 37 in which the flow of fluid is concurrent with the flow of the hot combustion gases and an oil outlet line 19. We have found that this combination of countercurrent flow in the relatively cooler area and concurrent flow in the relatively hotter area of the convection zone insures acceptable heat transfer rates to the oil in the critical tubes. This design permits heating of the catalytic cracking feed to temperatures in the range of 750850 F. without cracking of the lighter components of the feed in the convection section of the furnace.

The following example will serve to illustrate the invention.

A CO furnace was designed to recover 300MM B.t.u./ hr. of available heat by combusting the CO in regenerator gas from the regenerator of a catalytic cracking unit. The hot products of combustion are to be used to provide 300MM B.t.u./hr. of catalytic cracker feed and to generate p.s.i.g. steam. The regenerator gas temperature is 450 F and the gas flow rate is 780,000 lb./hr. Employing a high heat release fuel, i.e. refinery fuel gas, having a higher heating value of 2l,00024,000 B.t.u./ 1b., it was found that a conventional single stage combustor would require 340MM B.t.u./hr. of supplementary firing to burn the CO compared to 220MM B.t.u./hr. required for a two-stage combustor. Due to the higher percentage of supplementary firing the conventional combustor operates with a higher exit temperature than the dual stage combustor (1975 F. v. 1825 F.). However, this is a detriment rather than an advantage because more expensive construction materials must be used in the combustor and the convection section.

Thus, the multi-stage combustion of low heating value fuels is a means for significantly reducing the amount of high heating value fuel required for complete combustion of the low heating value fuel. The multi-stage technique provides combustor flexibility and control as well. By manipulating the dampers on the ducts which distribute the gases in each stage, gas flow to each stage can be regulated and distributed to provide optimum combustion for various firing conditions.

The description of the invention given above in connection with the drawings is for illustrative purposes only and is not to be considered a limitation on the process and apparatus of the invention. Obvious modifications which would occur to those skilled in the art are intended to be included in the scope of the specification and the claims.

What is claimed is:

1. A process for the combustion of low heating value gaseous fuels having a temperature of from about 400 to about 700 F. containing from about -15 volume percent CO and from about 80 to about 95 volume percent inert gases comprising sequentially burning a high heating value fuel to produce hot combustion gases, mixing the hot combustion gases with a portion of the low heating value fuel and burning the CO in the low heating value fuel in a first stage, mixing the hot combustion gases from the first stage with the remainder of the low heating value fuel and burning the CO in the loW heating value fuel in a second stage whereby a reduced quantity of high heating value fuel is required.

2. The process according to claim 1 in which the portions of low heating value fuel in each stage are approximately equal in volume.

3. A process for the combustion of a gas having a temperature of 400-700 F. and containing 515 volume percent CO and 8095 volume percent inert gases consisting essentially of the steps of sequentially burning a fuel having a heating value of 21,000 to 24,000 B.t.u./lb. to provide combustion gases having a temperature of ll50l200 F., mixing the hot combustion gases with approximately 50 volume percent of the stream CO containing gas and burning the CO in said gas in a first stage, mixing the first stage combustion gases with the other 50 volume percent of the CO containing gas and burning the CO in said gas in a second stage whereby a combustion gas having a temperature in the range of from about 1600 to about 2200" F. is produced.

4. A process for generating steam and for preheating hydrocarbon oil for catalytic cracking in a catalytic cracking unit comprising a cracking unit and a cracking catalyst regenerator unit comprising the steps of passing regenerator off gas containing 5-15 volume percent CO from the regenerator to a CO furnace at a temperature of 400700 F., mixing from 3070 percent of the off gas with the products of combustion of a high heat value fuel at a temperature of 1150-1200 F. in a first stage, burning the CO in the off gas, mixing the first stage combustion gases with from 30-70 percent of the off gas in a second stage whereby a combustion gas stream having a temperature of 16002200 F. is provided, contacting water and hydrocarbon oil with the combustion gas stream, converting the water to steam and heating the hydrocarbon oil to a temperature sufficient for use as catalytic cracking feed and passing the oil to the said catalytic cracking unit.

5. A furnace for the combustion of low heat value fuels comprising walls defining a horizontal combustion zone, a vertical heat transfer zone and a transition zone joining the two zones, said combustion zone having a plurality of low heat value fuel supply means spaced horizontally apart along the walls of the combustion zone defining a plurality of horizontally spaced low heat value fuel combustion stages and burner means positioned in the end of said combustion zone to supply hot combustion gases from the combustion of high heat value fuel to the combustion zone.

6. A furnace for preheating oil for catalytic cracking with CO containing catalyst regenerator olf gas comprising in combination walls defining a horizontal combustion zone, a vertical heat transfer zone and a transition zone joining these zones, burner means for introducing hot combustion products derived from the combustion of high heat value fuel into one end of the combustion zone, at least tWo horizontally spaced apart gas ducts for introducing CO containing gas into the combustion zone in a direction normal to the flow of the combustion products from the burner whereby at least two CO combustion stages are provided, a hanging wall between the combustion zone and the transition zone, an upwardly inclined floor in the transition zone, a section of horizontally spaced steam tubes in the heat transfer zone, a set of horizontally spaced oil tubes disposed above the steam tubes in the heat transfer zone, inlet means for supplying relatively cold CO containing gas from a cracking catalyst regenerator to the said gas ducts, and egress means adapted to supply hot oil from said oil tubes to a catalytic cracker.

References Cited UNITED STATES PATENTS 2/1948 Van Dornick 208146 3/1961 Downs 158-1

Claims (1)

1. 4. A PROCESS FOR GENERATING STEAM AND FOR PREHEATING HYDROCARBON OIL FOR CATALYTHIC CRACKING IN A CATALYTIC CRACKING UNIT COMPRISING A CRACKING UNIT AND A CRACKING CATALYST REGENERATOR UNIT COMPRISING THE STEPS OF PASSING REGENERATOR OFF GAS CONTAINING 5-15 VOLUME PERCENT CO FROM THE REGENERATOR TO A CO FURNACE AT A TEMPERATURE OF 400-700*F., MIXING FROM 30-70 PERCENT OF THE OFF GAS WITH PRODUCTS OF COMBUSTION OF A HIGH HEAT VALUE FUEL AT A TEMPERATURE OF 1150-1200*F. IN A FIRST STAGE, BURNING THE CO IN THE OFF GAS, MIXING THE FIRST STAGE COMBUSTION GASES WITH FROM 30-70 PERCENT OF THE OFF GAS IN A SECOND STAGE WHEREBY A COMBUSTION GAS STREAM HAVING A TEMPERATURE OF 1600-2200*F. IS PROVIDED, CONTACTING WATER AND HYDROCARBON OIL WITH THE COMBUSTION GAS

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