CRACKING FURNACE

Abstract: A cracking furnace for thermal cracking of hydrocarbon feedstocks, capable fo reducing its length, site area, wall surface area, and the number of quenching heat exchangers installed, thereby making the apparatus compact and lightweight and reducing damage of parts due to high temperatures, which cracking furnace comprises a central burner (13) provided vertically at the centre of the ceiling of a combusion chamber (1); side burners (12) provided vertically at both sides of the central burner (13); reaction tubes (7) vertically arranged between the central burner (13) and respective side burners (12), forming arch bends at the lower part of the chamber (1), and arranged in rows along the longitudinal direction of the chamber (1), a combustion gas-inducing duct (18) provided at the bottom of the chamber (1); a quenching heat exchanger (10) provided at the upper part of the chamber (1); and a reaction tube exit heder (9) connecting a plurality of exits of the reaction tubes (7) to the quenching heat exchanger (10).

Description: This invention relates to a cracking furnace and more particularly it relates to a tubular furnace for thermal cracking of hydrocarbons and like organic feedstocks, especially petroleum hydrocarbons into ethylene, propylene, butadiene, etc. Fig. 4 of the accompanying drawings shows a crosssectional view of a cracking furnace according to the prior art, Fig. 5 shows a crosssectional view in the direction of arrow mark along the line V-V of Fig. 4, and Fig. 6 shows a crosssectional view of a quenching heat exchanger connected by the medium of a header to the exits of the reaction tubes of Fig. 4. The above apparatus comprises a plurality of reaction tubes 7 provided in a row along the longitudinal direction and at the central part of combustion chamber 1, the reaction tube forming a meander having a bend 8; inlet 21 of an organic feedstock provided at one end of the reaction tube 7; exit 22 of the reaction tube 7 provided at the other end thereof; quenching heat-exchanger 10 united by the medium of header 9 (Fig. 6) to the exit 22 of the reaction tube; reaction tube-suspending fittings 16 suspended from the ceiling of combustion chamber 1 and connected to counterweight 17; convection heat transfer tubes 11 provided at convection heat transfer part 6 communicated to the upper part of the combustion chamber 1 by the medium of connecting duct 5; hearth burners 20 provided vertically at the bottom part of the combustion chamber 1; and wall burners 19 provided on the wall surface of the combustion chamber 1. In such an apparatus, an organic feedstock is usually mixed with steam; preheated at convection heat transfer part 6; thereafter introduced via reaction inlets into reaction tubes 7; heated by radiation from hearth burners 20 and wall burners 19 to cause pyrolytic reaction; introduced via header 9 into quenching heat exchanger 10; and quenched so that cracked products are obtained without causing excess pyrolytic reaction of polymerization reaction. In the cracking furnace, when ethylene is produced on the base of 30,000 t/year from naphtha (specific gravity: 0.70), it is necessary to make the length of the cracking furnace around 15 m, and in the case of 50,000 ton/year, 20 m or longer of the length is required; thus the site area per ton of naphtha becomes broader. Further, there occurs a drawback that the heat loss from the furnace wall becomes large. Further, when combustion gas 23 in combustion chamber 1 moves toward convection heat transfer part 6, it is necessary for the gas to traverse reaction tube-suspending fittings 16 so that they are liable to be damaged due to high temperature gas. Further, since connecting duct 5 connecting combustion chamber 1 to convection heat transfer tube 11 is so short that when combustion gas having non-uniform temperatures is not yet sufficiently mixed, the gas is introduced into convection heat transfer tubes 11 which usually consist of 4 to 12 passes so that the convection heat transfer tubes 11 corresponding to respective passes are non-uniformly heated, and as a result, the temperatures of the respective passes at the exits of the convection heat transfer tubes are different. Some large difference amounts to 20 to 50°C. Ideal reaction tubes are those which are of a small pressure-loss type; can rise up to a definite temperature within a definite retention time and carry out the maximum treatment of a feedstock per one pass; and can make as short as possible, the connection length of the exits of the reaction tubes to the inlet of the quenching heat-exchanger by way of header 9 of exits of reaction tubes. Further, it is desirable from an economical point of view to use a small number of quenching heat exchangers each having a large capacity. According to the above cracking furnace, however, in order to reduce the number of quenching heat exchangers 10 installed, reaction tubes having a complicated bend or a bend of a small bending radius, Y type piece for collecting tubes, etc. as shown by the configurations of various reaction tubes of Fig. 8 and Fig. 9 are used. Thus, the smooth flow of the fluid in the furnace is hindered to increase the pressure loss and causing coking. Further, when the configuration of reaction tubes is complicated, there is a drawback that the reaction tubes and bending parts are liable to be damaged due to occurrence of extraordinary thermal stress under high temperature condition (750° to 1,100°C). The configuration of the reaction tube of Fig. 5 is very simple, but the quantity of feedstock per one pass is so small that there is a drawback that it is necessary to use a large number of quenching heat exchanger having a small capacity. For example, in the case where ethylene is produced on the base of 30,000 ton/year from naphtha, it is necessary to provide 16 or more quenching heat exchangers. An object of the present invention is to provide a cracking furnace which will shorten the length of cracking furance, reduce the site area and the surface area of the wall of cracking furnace, and save the number of quenching heat exchangers, thereby making the apparatus compact and lightweight and reducing the damage of the parts of the furance. The present invention provides a cracking furance for thermal cracking of organic feedstocks comprising; a hollow furnace body; a combustion chamber provided in said furance body; a ceiling provided at the upper part of said combustion chamber; a central burner provided vertically at the center of the ceiling of said combustion chamber; side burners provided vertically at both sides of said central burner, respectively; reaction tubes in rows vertically arranged between said central burner and respective side burners, and forming arch bends at the lower part of said combustion chamber; a means for feeding the organic feedstocks into said reaction tubes; a quenching heat exchanger provided at the upper part of said combustion furnace; a reaction tube exit header connecting a plurality of exits of said reaction tubes to said quenching heat exchanger; and a combustion gas-inducing duct, provided at the bottom part of the combustion chamber and connected to an exit duct of the combustion gas. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 shows a crosssectional view of the cracking furnace as an embodiment of the present invention. Fig. 2 shows a crosssectional view in the arrow mark direction along II-II line of Fig. 1. Fig. 3 shows a configulation of a reaction tube exit header of Fig. 1 Fig. 4 shows a crosssectional view of a cracking furnace according to the prior art. Fig. 5 shows a crosssectional view in the arrow mark direction along V-V line of Fig. 4 (Prior Art). Fig. 6 shows a crosssectional view of a quenching heat exchanger-connecting part at the exits of reaction tubes in Fig. 4 (Prior Art). Figs. 7, 8 and 9 show configuration of various reaction tubes according to the prior art. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS The reaction tubes in the present invention are provided vertically on both sides of a central burner, united by the medium of an arch-form bend at the lower part of the combustion chamber, and arranged in a row in the longitudinal direction of the combustion chamber; hence a header connecting the exits of a pluratity of reaction tubes to a quenching heat exchanger can be a short connecting tube having a simple shape (Fig. 3). Thus it is possible to inhibit undesirable reactions of active components produced inside the reaction tubes. Further, it is possible to reduce the number of the quenching heat exchangers installed, and also it is possible to ensure a structural stability due to the reasonable connection of the reaction tubes to the quenching heat exchangers. Further, in the present invention, since the burners are provided vertically at the ceiling of the combustion chamber, and the connecting duct and the convection duct are provided by the medium of a combustion gas-inducing duct at the bottom of the combustion chamber, the combustion gas and the burner flames do not contact with the reaction tubes-suspending fittings and the reaction tube exit header. Thus, damage of the above-mentioned parts due to high temperature gas is prevented. Further, the above-mentioned combustion gas-inducing duct is provided at the central bottom part of the combustion chamber, preferably forming a V-letter form in crosssection, thereby paths of combustion gas are formed between the combustion gas-inducing duct and the arch bends of the reaction tubes. Thus, most of the combustion gas flows forming a parallel flow along the wall surface and the reaction tubes so that it is possible to uniformly heat the reaction tubes, while a portion of the combustion gas passes downwardly between the bends of the reaction tubes. Further, since the combustion-inducing duct and the connecting duct are provided between the combustion chamber and the exit duct including the convection heat transfer tubes, combustion gas having uneven temperatures is sufficiently mixed before it is introduced into the exit duct so that it is possible to uniformly heat the respective passes of the convection heat transfer tubes. The present invention will be described in more detail by way of an example. Example Figs. 1, 2 and 3 illustrate an embodiment of the cracking furnace and parts thereof. This apparatus comprises a hollow furnace body 3; a combustion chamber 1 provided in the furnace body 3; a central burner 13 provided vertically at the center of the ceiling 1A of combustion chamber 1; side burners 12 provided at both sides of the central burner 13; respectively; reaction tubes 7 vertically disposed between said central burner 13 and respective side burners 12, forming arch bends 8 at the lower part of said combustion chamber, and arranged in rows along the longitudinal direction A (Fig. 2) of the furnace body 3 inlet part 7′ of the reaction tube 7 connected to inlet pipe 7A being aligned with the row of the reaction tubes 7 by means of an ordinal bend; reaction tubes-suspending fittings 16 attached to the ceiling 1A of the combustion chamber for suspending the reaction tubes 7 therefrom; a quenching heat exchanger 10 provided at the upper part of the combustion chamber 1; a reaction tube exit header 9 connecting the exits of reaction tubes 7 (four tubes in this case) to the quenching heat exchanger 10; a combustion gas-inducing duct 18 of V-letter form provided at the central bottom part of the combustion chamber 1, paths of the combustion gas being formed between the combustion gas-inducing duct 18 of V-letter form and the arch bends 8 of the reaction tubes 7; a vertical inducing duct 4 connected to the combustion gas inducing duct 18; and convection heat transfer tubes 11 provided at an exit duct 6 connected to the vertical inducing duct 4 by the medium of a connection duct 5 and provided at side of the combustion chamber 1. The inside wall of the combustion chamber 1 is covered by a fire resistant wall 2. Reaction tubes-suspending fittings 16 are connected to a counterweight 17. Further, the exits of four reaction tubes 7 are connected to a large capacity quenching heat exchanger 10 by a reaction tube exit header 9, as shown in Fig. 3. In such an apparatus, a feedstock like naphtha is usually mixed with steam; preheated passing through heat transfer tubes 11 at the convection duct 6; fed to reaction tubes 7 and 7′ from an inlet pipe 7A provided at the bottom part of the combustion chamber 1; heated by radiant heats of central burner 13 at the ceiling of combustion chamber 1 and side burners 12 to cause pyrolytic reaction of the feedstock; and introduced from reaction tube exit header 9 into quenching heat transfer 10 where the resulting pyrolytic gas is quenched. Combustion gas 23 from central burner 13 and side a burners 12 flows along the wall surface in a parallel flow to reaction tubes 7, without traversing reaction tubes-suspending fittings 16 and reaction tube exit heater 9, and passes between tubes of arch bend 8 downwardly or flows along the paths formed between the arch bend 8 and V-letter form inducing duct 18 at the bottom part of the combustion chamber 1; is led to vertical duct 4; and introduced into exit duct 6 provided with convection transfer tubes 11 via connecting duct 5. Thus, the reaction tubes-suspending fittings 16 and the reaction tube exit header 9 do not contact with the combustion gas 23, and the burners 12 and 13 are arranged at the ceiling; hence the fittings and the header do not contact with the respective flames 14 and 15 so that damage of parts due to high temperature is prevented. Further, since the arch bend 8 is of a simple shape of a large radius as compared to the ordinal bend, the pressure loss at the reaction tubes 7 is small and coking is reduced. Further, while the combustion gas 23 is passed through inducing duct 18, vertical duct 4 and connecting duct 5, the gas is mixed to have a uniform temperature so that uniform heating of feedstock flowing along the respective paths of the convection heat transfer tubes 11 is possible. Furthermore, since the reaction tubes 7 are arranged in the longitudinal direction A of combustion chamber 1 in a row, it is possible to connect the reaction tube exit heater 9 to the quenching heat exchanger 10 in a reasonable and simple form, and also since the connecting tube of header 9 is shortened, it is possible to inhibit undesirable reactions of active components formed inside the reaction tubes and also to ensure the structural stability of the tube. Table 1 shows comparison of the apparatus of the present invention (Example 1) with conventional apparatus of the same productivity. (Comparative example 1) in the aspect of cracking furnace length, cracking furnace site area, cracking furnace wall surface area and cracking furnace weight. As seen from the Table, the apparatus of the present invention is able to make the cracking furnace more compact and lightweight by 30% or more as compared with the conventional apparatus. According to the present invention, it is possible to reduce the length of cracking furnace to thereby reduce the site area of cracking furnace and the wall surface area thereof and also to reasonably reduce the number of quenching heat exchangers so that it is possible to make the apparatus compact and lightweight. Further, since the burners are provided vertically at the ceiling of the furnace and a V-letter form inducing duct is provided at the bottom of the furnace, it is possible to uniformly heat the reaction tubes, in cooperation with reasonable arrangement of burners and reaction tubes. Further, since the arch bend of reaction tubes is of a simple shape, it is possible to reduce the pressure loss and also reduce coking trouble at the bend part.

Claims: 1. A cracking furnace for thermal cracking of organic feedstocks comprising a hollow furnace body; a combustion chamber provided in said furnace body; a ceiling provided at the upper part of said combustion chamber; a central burner provided vertically at the center of the ceiling of said combustion chamber; side burners provided vertically at both sides of said central burner, respectively; reaction tubes vertically disposed between said central burner and respective side burners, forming arch bends at the lower part of said combustion chamber, and arranged in rows along the longitudinal direction of the furnace body; a means for feeding the organic feedstocks into said reaction tubes; a quenching heat exchanger provided at the upper part of said combustion furnace; a reaction tube exit header connecting a plurality of exits of said reaction tubes to said quenching heat exchanger, and a combustion gas-inducing duct provided at the bottom part of the combustion chamber and connected to an exit duct of the combustion gas. 2. A cracking furnace according to Claim 1, wherein said combustion gas-inducing duct is in the form of a V in crosssection. 3. A cracking furnace according to Claim 1 or Claim 2, wherein said exit duct of the combustion gas is provided with convection heat transfer tubes for preheating the organic feedstocks to the fed to said reaction tubes. 4. A cracking furnace according to any one of Claims 1, 2 or 3, wherein said reaction tubes are suspended by fittings attached to the ceiling of the combustion chamber. 5. A cracking furnace according to any one of the preceding claims, wherein plural sets of said central burners, side burners and reaction tubes are arranged in rows along the longitudinal direction of the furnace body.

Inventors: HOKKEDO, MAMORU, KITAYAMA, YUTAKA, NAKASE, NORIAKI, YAMAMOTO, ETSUJI, YOSHIDA, MASAHIKO
Applicants: MITSUI PETROCHEMICAL INDUSTRIES, LTD.
Application year: 1989
Main classification: C10G9
IPC codes: C10G9