US3400920A - High temperature transfer line heater - Google Patents

Description

Sept. 10, 1968 B. V. MOLSTEDT E HIGH TEMPERATURE TRANSFER LINE HEATER Filed June 22, 1967 FIG.- 8

FIG-3 S R O T N E V m n n 0 am f h! n m ms oL V n M W 0 BR PATENT ATTORNEY FIG-4 United States Patent 3,400,920 HIGH TEMPERATURE TRANSFER LINE HEATER Byron Victor Molstedt, East Baton Rouge, La., and Robert L. Scheuermann, Florham Park, N.J., assignors to Esso Research and Engineering Company, a corporation of Delaware Continuation-impart of application Ser. No. 298,916, July 31, 1963. This application June 22, 1967, Ser. No. 652,391

8 Claims. (Cl. 263-21) ABSTRACT OF THE DISCLOSURE A transfer line heater is described for heating solids particles by first radiating heat from burning gases in full view of the solids and then mixing the hot combustion products of the burning gases with the solids particles. The invention is especially useful in high temperature coking process.

This is a continuation-in-part of Ser. No. 298,916 filed July 31, 1963, now abandoned.

This invention relates to transfer line heaters for heating solids in a system wherein solids are heated and used to transfer heat from a heating zone to a reaction zone. More particularly, the invention relates to an improved gas-solids inlet design for high temperature transfer line heaters.

In the high temperature converison of hydrocarbons to produce hydrogen and coke it is necessary to provide for rapid transfer of heat from hot gases to the coke to be heated. The coke produced is of high quality when a relatively pure hydrocarbon feed is used and therefore is a valuable product. In fluid bed coking processes, some of the coke produced is withdrawn from the conversion zone and partially burned to heat the rest of the coke particles in a transfer line heater. The heated coke particles are returned to the conversion zone to supply the heat of conversion. Where the coke product is valuable, it is preferred to burn an extraneous fuel such as fuel gas in preference to the coke.

According to the present invention a hydrocarbon feed such as naphtha, gas oil, diesel oil, natural gas, residuums, or the like, is cracked or converted in a thermal cracking zone at a temperature between about 900 F. and 2400 F. in a dense fluidized turbulent bed of coke particles to produce coke and either a range of gas-liquid products at low temperature or hydrogen at high temperature. The hot converted gaseous products are taken overhead and further treated to recover hydrogen. Coke particles are withdrawn from the thermal cracking zone and passed at a temperature between about 100 F. and 2500 F. to a transfer line heating zone to heat the coke particles to a temperature between about 1500 F. and 2800 F., preferably between about 1800 F. and 2500" F. The heated coke particles are then recycled to the cracking zone to supply the heat of cracking.

Where the hydrocarbon feed is a relatively pure distillate stock, the coke produced is of premium quality and is especially adapted for use in the manufacture of electrodes for aluminum manufacture. Because of the premium quality of the coke produced, it is preferred to burn extraneous fuel rather than the coke to heat the circulating coke solids for recycle to the cracking zone in an extremely short time to a high temperature with substantially no secondary chemical reactions occurring.

According to the present invention, fuel and gas are premixed in a multiplicity of gas nozzles or inlets which surround a central solids inlet. This design integrates the pre-mixing nozzles in close-coupled relationship to the main heater and the solids inlet so that it eliminates "Ice dangerous flashback which could occur into a more remote mixer and provides stable flames with minimum burning of circulating coke solids. Extraneous fuel is burned with air or oxygen in preference: to the circulating coke solids in an extremely short time. The residence time and holdup of solids in the transfer line heater are reduced or cut down by selecting relatively short transfer line burners. The burning fuel from each inlet is directed upwardly, either vertically, or at a slight angle toward the center of the vertically arranged transfer line heater to provide a multiplicity of streams of flames or hot gases radially disposed around the solids inlet to counteract the normal tendency for the solids to concentrate near the wall of the transfer line heater. As a result, the solids are more evenly distributed in the transfer line heater and rapid radiant heat transfer from the flames to the solids is eifected. Combustion of the fuel occurs such that the flames are in view of the central core or stream of coke and radiate heat thereto, but the flames do not physically contact the coke particles to any appreciable extent. This minimizes coke burnup without lowering heat efliciency. Thus, after combustion is completed, the resulting hot combustion gases mix with the coke stream to heat the coke still further.

To insure that the combustion occurs in full view of the coke, the fuel gases must be injected through the nozzles at velocities at least above the flame velocity of the combustible fuel-oxygen mixture, about 1 to 10 feet/sec. for most fuels. Otherwise, burning will occur inside the nozzles, thus wasting radiant heat and possibly damaging the nozzles. Generally, nozzle gas velocities must be above about 40 ft./ sec. to insure that no coke particles fall into the nozzles. More preferably, to achieve optimum thermal and operating efficiency, the nozzle velocity is maintained at about to about ft./sec.

In order to insure that the flames do not contact the coke to a significant extent, it is important to provide a sufficient volume of space in the burner around the central core of coke to allow substantially complete utilization of oxygen by the fuel before the combustion gases contact the coke. It has been determined that this volume of combustion space should be sufliciently great to keep the heat release density, i.e., the heat generated per unit volume of combustion space, below about 10 million B.t.u. hr./ft. preferably below about 5 million B.t.u. hr./ft.

In the drawing:

FIG. 1 represents diagrammatically a unit for cracking hydrocarbons;

FIG. 2 represents an enlarged vertical cross section taken through the transfer line burner or heater diagrammatically shown in FIG. 1; r

FIG. 3 represents a horizontal cross section taken substantially on line 3-3 of FIG. 2; and

FIG. 4 represents an enlarged detail. of the fuel-air nozzles or inlets for the transfer line heater.

Referring now to the drawing, the reference character 10 designates a fluid bed reaction vessel having an inlet 12 for the hydrocarbon to be cracked. While a fluid bed reaction vessel is shown and described, the invention is not to be restricted thereto as other forms of reaction vessels may be used. The hydrocarbon feed to be added through line 12 is preferably preheated and is preferably a distillate stock such as naphtha or gas oils, but other lighter or heavier feeds, such as methane, residual oils, reduced crude oils, and the like, may be used. The vaporous and gaseous cracked products are taken overhead through line 14, cooled and recovered. Where an extremely high temperature above about 2000 F. is used, the products are primarily hydrogen and coke and the hydrogen is recovered from the products passing overhead through line 14.

Where a lower temperature is used, such as one below about'1700" F., lower boiling hydrocarbons including aromatic hydrocarbons and gaseous hydrocarbons are produced. In fluid coking of residual hydrocarbon oils or fluid cracking at a lower temperature below about 1200 F., gas oil, naphtha and gas are produced. In all these cracking processes, coke is produced which is deposited on the coke solids in the reaction vessel. The coke solids in the fluid bed in reactor 10 are substantially of a size between about 40 and 600 microns.

The coke solids are withdrawn from the fluid bed in the reactor 10 and passed through line 16 into the bottom portion of transfer line heater 18. Air or other oxygen-containing gas is introduced through one or more lines 20 into transfer line heater 18. Air, hydrogen, or recycled flue gas is preferably introduced into line 16 through line 22 to aid in transferring coke solids from reactor 10 to heater 18. The coke solids and hot combustion gases are passed as a suspension of coke solids in gases upwardly through transfer line heater 18 at a velocity between about 40 and 150 feet/second and the suspension has a density between about 0.1 and 2.0 pounds/cubic foot. The suspension has a residence time of between about 0.05 and about 1.0 second, generally less than 0.5 second and preferably between about 0.3 and 0.5 second in the transfer line heater between the inlet adjacent line 20 and the cyclone separator 24 into which the suspension passes from the top of the trans fer line heater 18 through line 26. Where high temperature cracking is to be used the temperature of the coke solids in the transfer line heater 18 is usually between about 2200 F. and 2500 F.

In the cyclone separator the heated coke solids are separated from the hot gases and the heated coke solids are returned to the reactor through line 28. The separated hot gases pass overhead through line 32 and may be passed through a waste heat boiler or heat exchanger to recover heat therefrom.

Referring now to FIGS. 2, 3, and 4, the transfer line heater and fuel-gas-solids inlet design will be described. The transfer line heater 18 has an inner central passageway formed by refractory tube 36 which is vertically arranged and is surrounded by insulating refractory 38 which forms a heat insulating cylinder around the tube 36. An outer steel shell 42 encases the cylinder and the transfer line heater. Flange means 44 are provided near the lower end of the transfer line heater 18 to provide for disassembling the structure. The inner tube 36, refractory 38 and shell 42 are preferably made in sections as shown to permit disassembly of the heater when desired by releasing the flange means 44.

The upper portion of heater 18 has an outlet line 46 made of refractory and extends out at a right angle to the outlet top end of inner refractory tube 36 to lead the suspension to the cyclone separator 24 shown in FIG. 1. The upper end of tube 36 is closed by a removable and replaceable refractory plug 48. The heat insulating refractory 38 extends up over the top of the heater 18 and the plug 48 as at 52. A removable cover 54 is shown at the top of the heater 18. Outlet line 46 is also surrounded by refractory and has flange means 56 for disassembling the heater from the unit.

The lower end of the shell 42 of heater 18 below flange means 44 and below the bottom portion of tube 36 is tapered inwardly as at 62 to provide a tubular portion 64 of smaller diameter than steel shell 42 and extending downwardly from steel shell 42. Insulating refractory 38 of a smaller diameter extends down into tubular extension 64 and surrounds vertically arranged dense refractory tube or pipe 66 which is of a smaller diameter and which leads and opens into the bottom of inner refractory tube 36 in the heater 18 for conducting coke solids from reactor 10 and line 16 into the heater tube 36 in the transfer line heater 18.

As shown in the drawing, smaller inner tube 66 and larger inner tube 36 are of integral one-piece construc- 4 t r tion, although they can be fabricated in segments, and the tube 36 is shown as having bottom or floor portion 68 through which tube 66 passes at the center of the floor portion 68. The lower end of tube 36 tapers inwarly as at 72 to form a thickened or enlarged portion 74 at the bottom of tube 36 and at the region where the smaller refractory tube 66 discharges into the bottom of inner larger refractory tube 36. e

The lower end of larger tube 36 above the floor por tion 68 is cut away or extended outwardly as at 76 to form an enlarged chamber 78 in the lower portion of tube 36 to provide a chamber or space where substantial- 1y complete combustion of the fuel-oxygen mixture occurs before coke or other solids from tube or pipe 66 are mixed with the hot combustion gases. Nozzles 82 open into the enlarged chamber 78 and discharge a combustible mixture thereinto. The configuration of chamber 78 may be altered to suit the geometric requirements of a specific system but the transition to tube 36 should be streamlined. The wall 76 should form an angle with the vertical less than about 30 and preferably less than about 15. In the specific design shown in the drawing there are six nozzles provided and they are arranged equidistantly in a circle as shown in FIG. 3 surrounding the outlet of tube 66, but more or fewer nozzles may be used. Also, where a larger diameter tube 36 is used, concentric rows of nozzles 82 may be used. The total volume of space in the enlarged chamber 78 for combustion of fuel and air around the central stream of coke before mixing depends on the relative feed rates, nozzles types, etc., but must be sufiicient to maintain the heat release density below 10 million, preferably below 5 million B.t.u./hr./ft.

The nozzles comprise pipes or tubular elements which extend through the thickened portion 74 at the bottom of tube 36 and through smaller tubular metal shell portion 64 and are arranged at a slight angle to the vertical to direct the hot combustion gases from the nozzles upward and slightly toward the center of the inner tube 36 to promote quick pickup of solids with hot combustion gas products and for rapid admixture with the coke solids discharged from smaller central tube 66 to distribute the solids and to effect rapid heat transfer by the mixing. The streams of hot combustion gases are arranged and spaced to discharge upwardly around the centrally located solids inlet 66 so as to counteract the normal tendency for gas to preferentially flow up the center of the transfer line heater while solids concentrate near the wall.

In effect, the bottom of tube 36 is partially closed and has opening 66 and nozzle inlet openings 82 arranged around opening 66. While the nozzles 82 are shown at an angle to the vertical, where a larger diameter tube 36 is utilized the nozzles may be arranged vertically at right angles to the horizontal floor of chamber 78 to inject the hot gas around the solids inlet from line 66 in an upward direction to avoid premature mixing and to minimize the normal tendency for gas to go up the center of the tube 36 and solids to concentrate near the inner wall of the tube 36.

The nozzles are provided for burning a mixture of fuel and oxygen or air. In the design shown in FIG. 4, each nozzle comprises a refractory tube 86 mounted as a slip fit on a metal tube 88 to allow for relative expansion. Tube 86 is made of high temperature resistant material such as fused alumina. Metal tube 88 is made of stainless steel, or the like, to which the fuel and air pipes or lines can be attached. Each nozzle 82 has a fuel inlet 92 connected to a fuel manifold 94 and an air inlet line 96 connected to an air manifold 98. The lower end of each nozzle has a closed end 102 having a central opening through which the fuel inlet 92 extends. The inner end 104 of each fuel inlet 92 is arranged on the axis of each nozzle tube 82 and has its discharge end adjacent the refractory tube 86 or it may extend into tube 86. Fuel inlet 92 terminates with any one of several devices which effectively promote pre-mixing of fuel and air. In FIG. 4 are shown nipples 105 set back from the closed end of fuel inlet 92 for injecting fuel outward into the air as a plu rality of streams. Other turbulence inducers or arrangements for imparting centrifugal motion to the air and fuel may be used. Also, the air can be brought in tangentially through line 96. The fuel and air are mixed and ignition initiated in zone 106 in each nozzle. The resulting hot combustion gases are directed as multiple high velocity flames upward and slightly toward the center of refractory tube 36. After combustion is essentially complete the gases pick up solids discharged upwardly from central smaller refractory tube 66 into chamber 78 provided at the bottom of tube 36. Flashback of flames into the distribution system or manifold is avoided by mixing fuel and air in the refractory inlet at, or very near, the point of entry to the heater and by maintaining the velocity of the combustion gases passing through each nozzle 82 between about 40 and 150 feet/ second.

In a specific example wherein about 35 barrels per day of naphtha having a boiling range of 117 F. to 310 F. were introduced into the reactor where the holdup of coke was about 1.7 tons. The temperature of cracking in reactor was 2000 F. and the vapor residence time was about 12 seconds to produce hydrogen and coke. About 223M s.c.f./ day of hydrogen were recovered and about 6200 pounds/ day of coke.

About 15,000 pounds/hour of coke were circulated from reactor 10 through line 16 by fluidized transport with hydrogen at a superficial gas velocity of about 30 feet/ second and the mixture passed through line or tube 66 into the lower portion, or mixing chamber 78, of refractory tube 36.

The specific transfer line heater 18 had a refractory inner tube 36 of 9 /2 inches internal diameter and a length from floor 68 to the center of outlet line 46 of about 16 feet. The tube 36 had a wall 3 inches thick surrounded by 6% inches of insulating refractory 38. Each of the six nozzles had a 1% inch inside diameter alumina tube 86 and a 1% inch stainless steel tube 88. The fuel inlet line 92 was 0.364 inch inside diameter steel pipe. The air inlet line 96 was 1%. inches schedule 80 steel pipe. The mixing-ignition chamber 106 was about 9 inches long from the outlet of gas line 92 and the floor 68 of tube 36 and had a diameter of about 1%. inches.

The temperature in heater tube 36 was 2400 F. To heat the coke particles about 16 s.c.f./hour of natural gas was added to each nozzle through line 92 and about 160 s.c.f. air/hour was passed through each line 96 so that about 176 s.c.f. of combustion gases per hour was added to enlarge mixing chamber 78 at the bottom of tube 36. The velocity of the fuel-air mixture passing through each nozzle 82 was about 80 feet/ second. The temperature of the coke solids introduced through line 66 into the bottom of heater tube 36 was raised from about 2000 F. to 2280 F. at outlet line 46 in about 0.24 second. The velocity of the suspension passing up through tube 36 was about 68 feet/ second. About onehalf the volume of chamber 78 was occupied by the coke solids stream and the remainder substantially by the flames of the fuel-air mixture surrounding the coke stream. This provided a heat release density of about 4 million B.t.u./hr./ft.

The amount of heated coke particles leaving heater tube 36 and being recycled to reactor 10 was about 14,970 pounds/hour.

While various specific examples have been included, the invention is not to be restricted thereto.

What is claimed is:

1. An apparatus including a coking reactor for converting hydrocarbons to fluidizable coke solids;

transfer line heating means adapted to receive said solids from the coking reactor, said transfer line heating means including a vertically arranged tube having an inlet and an outlet, means for mixing and burning fuel and air in a plurality of nozzles adjacent one end of said tube, each nozzle being arranged vertically to direct burning gases upwardly into one end of said tube, means for introducing a stream of said coke solids into said last mentioned end of said tube in the center thereof whereby multiple streams of high velocity burning gases from said nozzles radiate heat to the solids and pick up the solids after burning of the gases is substantially complete and mix and distribute the solids to effect rapid heat transfer from the gases to the solids;

means for separating the thus-heated solids from the gases; and

means for returning the separated solids to said coking reactor.

2. An apparatus according to claim 1 wherein the volume of space around said stream of solids for combustion prior to said mixing is sufiicient to maintain the heat release density of the burning gases below about 10 million B.t.u./hr./ft.

3. An apparatus according to claim ll wherein the end of each of said nozzles includes a centrally arranged fuel inlet line spaced within the walls of each of said nozzles, and a line for each nozzle for introducing air into the space between said fuel inlet line and said walls.

4. A process for heating finely divided coke particles without burning any substantial amount of the coke particles which comprises passing a gaseous suspension of coke particles at a temperature between about F. and 2500 F. into one end center portion of an elongated tubular heating zone for passage therethrough, passing a multiplicity of streams of hot combustion gases into said heating zone and disposed around the center of said tubular heating zone to effect rapid mixing of the coke solids with said hot combustion gases and rapid heating of said coke solids to between about 1500" F. to 2800 F. in less than 0.5 second and separating heated coke solids from said hot combustion gases.

5. A process for heating finely divided coke particles without burning any substantial amount of the coke particles which comprises passing a gaseous suspension of coke particles at a temperature between about 100 F. and 2500 F. into one end center portion of an elongated tubular heating zone for passage therethrough, passing a multiplicity of streams of burning gases into said heating zone disposed in full view around the central suspension of coke particles to radiate heat thereto, substantially completely combusting the burning gases and mixing the resulting combustion gases with the coke suspension thereby further heating the coke to a temperature between about 1500 F. and 2800 F. in a time ranging from about 0.05 to about 1.0 second and separating the heated coke from the gases.

6. The process of claim 5 wherein the burning gases are substantially completely combusted in a volume of space around the central coke suspension sufficiently large to maintain the heat release density therein less than about 10 million B.t.u./hr./ft.

7. The process of claim 6 wherein the heat release density is less than about 5 million B.t.u./hr./ft.

8. The process of claim 7 wherein said heating time ranges from about 0.3 to about 0.5 second.

References Cited UNITED STATES PATENTS 920,334 5/1909 Hughes. 2,300,042 10/ 1942 Caldwell. 2,431,884 12/ 1947 Neuschotz. 2,698,171 12/ 1954 Schoenmakers et al. 2,912,315 11/1959 Haney. 2,932,498 4/1960 Metcalfe et al.

JOHN J. CAMBY, A cling Primary Examiner.

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