US3291573A

US3291573A - Apparatus for cracking hydrocarbons

Info

Publication number: US3291573A
Application number: US348988A
Authority: US
Inventors: Leonard O Frescoln
Original assignee: Hercules LLC
Current assignee: Hercules LLC
Priority date: 1964-03-03
Filing date: 1964-03-03
Publication date: 1966-12-13
Anticipated expiration: 1983-12-13
Also published as: BE653468A; NL6409295A; ES304056A1; GB1024005A

Description

Dec. 13, 1966 o. FRESCOLN 3,291,573

APPARATUS FOR CRACKING HYDROCARBONS Filed March 5, 1964 2 Sheets-Sheet 1 Process Steam Burners I l le Naphtha 4 Steam Superheafer 1 Boiler Q c 3 l6 Boiler 22 U Feed Water 8 8 1 23 g a:

4- (V) Naphrha 2 g Vaporizer Steam T 1 6 I aph'rha 29 Nophtho Preheater l 3 Cracked Products LEONARD O. FRESCOLN l 6 INVENTOR Flue Gases K 1 C VJLQ/QL AGENT Dec. 13, 1966 APPARATUS FOR Filed March 5, 1964 L. O. FRESCOLN Crude Oil Flue Guses Condenser Prim 1ry Stripping BoHoms Preheat Liquid CRACKING HYDROCARBONS Recfificotion Secondary Stripping Asphulf Residuum Soaking Drum 2 Sheets-Sheet 2 LEONARD O. FRESCOLN a. '7 I (Auk/1st: 11/

INVENTOR.

AG ENT United States Patent 3,291,573 APPARATUS FGR CRACKING HYDROCARBGNS Leonard 0. Frescoln, Wilmington, DeL, assignor to Hercules Incorporated, a corporation of Delaware Filed Mar. 3, 1964, Ser. No. 348,988 9 Claims. (Cl. 23-284) This invention relates to the pyrolysis of hydrocarbons and more particularly to apparatus and method for the thermal cracking of normally liquid, as well as normally gaseous, hydrocarbons in tubular furnaces.

The .art of high temperature cracking in tubular furnaces has reached a high degree of proficiency in more recent years. This is particularly evident in that the utilization of tubular cracking furnaces has provided practical flexible processes for the production of hydrogen, olefins, and other valuable gaseous and liquid products. Furthermore, the versatility of these furnaces is most readily appreciated in considering the variety of charging stocks which may be used including normally gaseous as Well as normally liquid hydrocarbons and the excellent reaction control possible to obtain predetermined, desired end products. Still further, advances in the art of tubular cracking have afforded yields of cracked products and throughput of charging stock which is attractive to large-scale handling of hydrocarbon stock such as is en countered in the refining of petroleum and other largescale operations. These advances in the art may be more explicity appreciated with reference to, for example, Patent No. 2,914,386, which discloses a tubular reaction furnace fired at a plurality of spaced points to give flexible and high tempera-ture control of the reacting fluids and extended tube life; Patent No. 2,525,276, which discloses a process for the cracking of hydrocarbon oils with a minimum of carbon deposition for the substantial production of olefins and including injection means for normally gaseous as well as normally liquid hydrocarbon feedstock; and Patent No. 2,904,502, which discloses a process of converting hydrocarbons to obtain high capacity throughput in a tubular furnace with a high yield of liquid products.

Although the above patents are exemplary of the proficiency presently existing in the tubular furnace art, still further advances are being sought and particularly in respect to capital investment, processing variables, continuity of operation, and the quantity and quality of desired end products.

Accordingly, a principal object of this invention is to provide fundamental improvements in the art which pertain to the factors being sought as heretofore set forth. The accomplishment of this object and others will appear hereinafter, the novel features and combinations being set forth in the appended claims.

Generally described, the present invention provides apparatus for the thermal cracking of normally liquid, as well as normally gaseous, hydrocarbons in a tubular furnace in which the improvement comprises: a tubular cracking furnace having at least one vertical elongated cracking tube disposed therein substantially extending from the bottom to the top of said furnace; a core disposed within the cracking tube. substantially coextensive therewith and forming a cracking annulus therebetween; a flute conduit in communication with the furnace having a plurality of communicating heat exchangers disposed therein adapted to be heated by the passage therethrough of flue gas from said furnace; means associated with the heat exchangers for passing hydrocarbon and steam therein and'therethrough to effect a vaporized and superheated mixture; and means associated with the heat exchangers for passing the vaporized and superheated mixture therefrom and into the cracking annulus of the furnace. The

'ice

invention further provides an improved method for operation of the apparatus, which comprises passing a vaporized and superheated hydrocarbon-steam mixture through a vertical, elongated annular cracking zone substantially extending from the bottom to the top of a multiple fired tubular furnace; 'andthe hydrocarbon-steam mixture having been vaporized by heat transfer with flue gas emanating from the furnace and said vaporized mixture having been super-heated to a temperature from about 800 to about 1200 F. by heat transfer with said flue gas.

Preferred embodiments of the invention have been chosen for the purpose of illustration and description of plant operation and are shown in the accompanying drawings wherein reference symbols refer to like parts wherever they occur and wherein:

FIG. 1 is a diagrammatic flow chart depicting process and apparatus for cracking a light hydrocarbon feedstock such as naphtha; and

FIG. 2 is a diagrammatic flow chart depicting process and apparatus for cracking a heavy hydrocarbon feedstock such as crude oil, which may contain undesirable, high boiling point components.

In the operation hereinafter described for the cracking of naphtha, the following conditions are typical:

Naphtha feedl7,000 lb./hr. at 30 F. or higher temperature.

Primary steam feed to naphtha vaporizer15,000 to 17,000 lb./hr. at 360 F.

Steam feed to core (optional)0 to 2,000 lb./hr. at

1200 F. to 1500 F.

Boiler water feed-25 ,200 1b./hr. at 220 F.

Process steam generated23,900 lb./hr. at 422 F.

Boiler blowdownl,300 lb. water/hr.

Furnace, 10 tubes-Reaction tube Incoloy alloy 800, 8 in. I.D., wall thickness 0.25 in., and 45 feet long inside the radiant section of the firebox.

Core tube Incoloy, alloy 800, 6.5 in. O.D., wall thickness 0.25 in., and 47 feet long. Annulus 0.75 in. wide in cracking zone.

Burners, in rows at various heights up the side of the furnace, most of the burners burning fuel gas, but some being fired, as required, with fuel oil. Referring now particularly to FIG. 1, the naphtha is passed through a line 1 into a pump 2 from which it is passed by a valved line 3 into a naphtha preheater 4.

The preheater 4 is a first heat exchanger disposed in an elongated flue conduit 5 with flue gas passing therethrough at an ingress temperature of about 570 F. and an egress temperature of about 450 F. The flue gas emanating from the egress side of the preheater 4 is passed through an induced draft fan to a plant chimney (not shown) through an exit conduit 6. The preheated naphtha leaves preheater 4 at a temperature of about 320 F. through line 7 and is passed into a naphtha vaporizer 8. The primary steam is passed through a line 9 into a valved line 10 into the naphtha vaporizer 8. The vaporizer 8 is a second heat exchanger disposed in the elongated flue conduit 5 with flue ga passing therethrough at an ingress temperature of about 720 F. with the egress end thereof in direct communication With preheater 4 and thusly, possessing approximately its ingress temperature. The naphtha is thoroughly mixed with the steam and vaporized by virtue of passing through the vaporizer 8, and the mixture is passed from the vaporizer through line 11 at a temperature of about 400 F. into a naphtha-steam superheater 12. The superhe-ater 12 is a third heat exchanger disposed in the elongated flue conduit 5 with flue gas passing therethrough at an ingress temperature of about 2200 F. and an egress temperature of about 1680 F. The superheated naphtha-steam mixture is passed from the superheater 12 through line 13 at a temperature of about 1100 F. and pressure of about 70 p.s.i.g into the plurality of reaction tubes represented by 14, which are suspended vertically in the furnace 15, which has a plurality of burners 16 located at various heights up the side Walls of the furnace. The core 17 is concentrically disposed Within each of the reaction tubes 14, which extend throughout the height of the furnace. Similarly, the core extends substantially the length of the reaction tube, thereby defining a final preheat and reaction annulus or cracking zone 18 which extends substantially throughout the height of the furnace.

Interposed between the naphtha vaporizer 8 and the naphtha-steam superheater 12 is a heater 19 and a boiler 20, the heater and boiler both being disposed in the elongated flue conduit superjaoent and subjacent respectively, to the vaporizer and superheater. The boiler feed water is passed through a line 21 into and through a pump 22 from which it is passed by a valved line 23 into the heater 19. The heater 19 is a fourth heat exchanger with flue gas passing therethrough at an ingress temperature of about 900 F. with the egress end thereof in direct communication with vaporizer 8 and thusly, approximately its ingress temperature. The heated feed water is passed from the heater 19 through line 24 at a temperature of about 380 F. into the boiler 20. The boiler 20 is a fifth heat exchanger with the temperature of the flue gas passing therethrough corresponding to the egress conditions of superheater 12 and ingress conditions of heater 19 since the boiler is interposed between these units in direct communication therewith. Steam and unvaporized water from the boiler 20 are passed through a line 25 at a temperature of about 425 F. into a steam drum 26, thus providing excess process steam for other operation of the plant as desired as well as importantly providing means for substantially reducing the flue gas temperature to an optimum range prior to its entry into the vaporizer 8. Unvaporized water from the steam drum is recirculated back through the boiler 20 through line 26a and pump 26b. Core steam is passed from line 9 through a valved line 27 into the top section of superheater 12 and is passed therefrom through a line 28 at a temperature of about 1300 F into the lower portion of core 17, which is provided with a top opening to permit efliux of the core steam.

The furnace 15 is operated under the following approximate range of conditions for the cracking of naphtha:

Reaction tube external temperatures1630 F. to 1950 Refractory wall temperatures-2000 F. to 2250 F.

Core temperaturesl330 F. to 1640 F.

Temperature of the reactants leaving the cracking tube- 1450 F. to 1500 F.

Pressure of cracked vapors leaving the cracking tube to pounds per square inch gage.

Steam dilutionl lb./ lb. of naphtha.

The cracked products emanating from the reaction tubes 14 of the furnace 15 pass through exit conduits 29 and are then passed into a quench tank or suitable heat recovery system (not shown) as desired. The feedstock properties and the products of the cracking operation for the naphtha process are as follows:

Feedstock properties Designation Kuwait naphtha.

Specific gravity, 60/ 60 F. 0.7168. Hydrogen/ carbon ratio, -atom/atom 2.14.

PONA analysis:

Parafrins, liquid vol. percent 80.0 Olefins, liquid vol. percent 0.3 Naphthenes, liquid vol. percent 12.0

Aromatics, liquid vol. percent 7.7

4 ASTM distillation:

Initial boiling point F 116 30% overhead by volume F 199 50% overhead by volume F 237 70% overhead by volume F 271 overhead by volume F 304 End point (97.5 vol. percent) F 335 Residue, vol. percent 1.5 Loss, vol. percent 0.7 Wt. percent sulfur=0.046

Productivilies 1 H 0.93 CH 14.7 CZHZ (3 H; (ethylene) 30.7 C 11 2.9 C 11 0.50 C H (propylene) 11.9

1-1 0.39 0 1-1 0.09 1,3-butadiene 3.60 1,2-butadiene 0.40 C H 3.00 C H 0.10 CO 0.25

CO 0.19 H 8 0 06 Total, C; and lighter 70.0 C to 400 F. B. P. liquid 23.0 400 F.+material 7.0

Total 100.0

1 Lb./'100 lb. naphtha feed, no loss basis.

It is acknowledged, with reference to the foregoing, that the cracking of a hydrocarbon such as light naphtha normally presents minimal problems with respect to carbon and tar formation in a cracking tube and its feed and discharge lines an auxiliaries, and presents minimal or no problems due to H S corrosion. It will be appreciated, however, that an important objective of the present invention is to retain the desirable processing features known to the art and at the same time, contribute improvements which increase furnace capacity, minimize furnace cost per unit of productive capacity, and obtain a highly thermally eflicient furnace without generating an undue amount of excess steam. In this respect, the naphtha is preheated and flashed at low temperature and in conjunction with part or all of the diluent steam is highly preheated by heat exchange with the hot flue gases flowing from the radiant section of the furnace. Thus, the temperature of the steamnaphtha mixture is, at a high heat value, in the range of about 800 to 1200 F. and preferably in the range of about 900 to 1100 F., as it enters the cracking tubes. Moreover, the final heating and cracking zone extends substantially the entire height of the furnace, all of which contributes to improved thermal efficiency, lower investment cost per given furnace capacity, and operation with a minimum of lost production time for periodic decoking operations, for a wide range of hydrocarbon feedstocks.

Referring now particularly to FIG. 2, the procedure for cracking a heavy hydrocarbon feedstock, which contains undesirable high boiling components, such as crude oil, will be described with the furnace, the quantity of hydrocarbon feed to the furnace, the firing and flue arrangements remaining much the same as that described for FIG. 1. The crude oil is passed through a line 30 into and through a pump 31 from which it is passed by a valved line 32 into a crude oil vaporizer 33. Part of the dilution steam is added to the oil at an intermediate point in this vaporizer. The vaporizer 33 is a first heat exchanger disposed in the elongated flue conduit 5 with flue gas passing therethro-ugh. The flue gas emanating from the egress side of the vaporizer 33 is passed through the exit conduit 6 by induced draft to other heat recovery units (not shown) such as a combustion air preheater, or directly to the plant chimney (not shown). The totally or partially vaporized crude oil is passed from the vaporizer 33 at a temperature from about 650 to 750 F. through a line 34 and is passed into a column, or rectification apparatus, 35 at a point above its primary stripping section 36. Steam is passed through a line 37 into a steam preheater 38. The preheater 38 is .a second heat exchanger disposed in the elongated flue conduit 5 with flue gas passing therethrough. A portion of the preheated steam is passed from the preheater 38 at a temperature from about 750 to 1100 F. through a line 39 and is passed into the rectification column 35 at a point above its bottoms preheat section 40. The other portion of the preheated steam is passed from the preheater 38 at the temperature from about 750 to 1100 F. through a valved line 41 and is passed into a high temperature heater 42. The heater 42 is a third heat exchanger disposed in the elongated flue conduit 5 in juxtaposition to the furnace with the very hot flue gas from the furnace passing directly therethrough. The highly heated steam is passed from the heater 42 at a temperature from about 1000 to 1500 F. through a line 43 and is passed into the rectification column 35 at a point below the secondary stripping section 44. Soaking means, such as a soaking drum 45 and/or soaking coils (not shown) is positioned between the bottoms preheat section 40 and the secondary stripping section 44. These units are provided with supplemental heating facilities (not shown) when so required. A valved line 46 passes liquid products from the soaking drum 45 into secondary stripping section 44, the combination of which gives liquid phase cracking with the highly heated overhead gaseous products passing through a line 47 beneath the bottoms preheat section 40. A selective rectification section 48 followed by a condenser 49 operated with liquid reflux are positioned at the top of the rectification column 35. The products from the top of the column 35 are passed through a line 50 into an oil-steam superheater 51. The superheater 51 is a fourth heat exchanger disposed in the elongated flue conduit 5 between the high temperature heater 42 and the steam preheater 38 and similar to the other heaters, has the flue gas passing therethrough. The superheated and vaporized oil-steam mixture, substantially freed of deleterious products, is passed from the superheater 51 at a temperature of about 800 to 1100 F. through a line 52 into the plurality of reaction tubes represented by 14. As previously described, that part of the dilution steam which is added to the oil at an intermediate point in vaporizer 33 is fed thereto through a valved line 53. In all instances, the heavy bottoms are ultimately removed from the rectification column 35 through a line 54 to residuum recovery (not shown).

Since the oil cracking process and furnacing from hereon are similar to the naphtha cracking process, they will not be further described. Sutfice to say, if core steam is desired for the oil process, it may be obtained by the method previously described, that is, for example, by using an appropriate section of the heater 42. Similarly, if the generation of process steam is desired in the oil process, it can be generated in the same manner employed in the naphtha cracking process, it being understood, of course, that the heat requirements for converting and fractionating the crude oil to a suitable vapor phase feed product to the cracking tubes must first be essentially provided. The cracking conditions, feedstock properties and productivities for the crude oil process are as follows:

Cracking conditions Steam dilution 1.8 lb./ lb. of crude oil. Cracking tube exit pressure g3 p.s;i.g. Cracked gas temperature leaving cracking tube F. 1460.

Feedstock properties Designation Gulf Coast crude oil. Specific gravity, 60/60 F 0.9104.

Hydrogen/ carbon ratio, atom/atom 1.75. Comradson carbon, wt. percent 0.94. Bromine No. 2.26. Ash, Wt. percent 0.006.

ASTM distillation; F.:

Initial boiling point 248 50% by volume 645 90-95% by volume 1000 Distillate analyses:

Sulfur, Wt. percent 0.055 Paraffins, vol. percent 79.9 Aromatic, vol. percent 16.3 Sp. Gr., 60/60 F. 0.866

Residue analysis:

Sulfur, wt. percent 0.29 Hydrogen/carbon ratio, atom/atom 1.67

Productivities 1 H 1.1 CH 9.2 C l-I 0.70 C H (ethylene) 17.4 1-1,,- 1.5 0 1-1 0.76 C H (propylene) 7.9

1,3-butadiene 3.6 1,2-butadiene 0.5 C H 3.1 C4H10 CO 0.6 CO 2.8 H 8 0.0

Total, C, and lighter 49.7 55 C to 400 F. B.P. liquid 20.1 400 F.+material 32.6

Total 102.4

1 Lb./1001b. crude oil feed, no loss basis.

With reference to the foregoing, it will be appreciated that when whole crude oils are cracked which have substantial amounts of high boiling aromatic components such as asphalt or asphaltines, tar and carbon formation can be a problem. In some cases, carbon is deposited in the cracking tube itself, and in many cases, the condensation of high boiling temperature tars in lines and equipment beyond the cracking tube creates problems. This especially applies if heat is extracted from the vapors containing the cracked gases either under controlled conditions for efliciency or subsequent processing or under uncontrolled conditions, such as imposed by severe climatic variation. Moreover, crude oil feedstocks which contain a relatively large amount of sulfur can present serious corrosion problems within the cracking tube or in lines 7 and auxiliary equipment on the feed or discharge end of the cracking tube. The combination of utilizing flue gas heat for operation of the rectification column and passage of highly vaporized and rectified products therefrom into the cracking tube efficiently alleviates these problems.

From the foregoing embodiments, it is evident that there are several factors which will influence conditions for most satisfactory operation of the invention. This will be readily appreciated in light of reference to certain features involved in practicing the invention and the attendant advantages.

When the top portion of a cracking tube is used to preheat relatively low temperature steam and vaporize the hydrocarbon feedstock, part of the potential cracking capacity of the vertical tube must be sacrificed. Furthermore, in order to obtain good thermal efficiency from such a furnace without sacrificing unduly the capacity of the cracking tubes, it is sometimes necessary to generate a relatively large amount of excess steam from the heat available in the flue gases leaving the radiant section of the furnace.

In accordance with the present invention, it will be seen that the capacity of the vertical tube is not sacrificed and that the fine gas temperature is utilized to elfect a diminishing temperature gradient by heat exchange with the essential constituents required for the cracking reaction. Therefore, the generation of excess steam is minimized and additionally utilized by heat exchange to obtain optimum temperature conditions for feeding the essential constituents into the cracking system. This situation obtains for the thermal cracking of normally liquid, as Well as normally gaseous, hydrocarbons.

The most complicated situation involves the cracking of a hydrocarbon crude oil which has a substantial amount of high boiling aromatic or asphaltic residues in it, and a relatively high concentration of sulfur. With this type of oil, carbon formation on surfaces within the tube is frequently a problem. The condensation of high boiling tarlike components on the inside of lines, vessels, heat exchange surfaces and the like leading from the cracking tube is also frequently a problem. Corrosion within the tube and/or in inlet and discharge lines and equipment due to the presence of sulfur-containing compounds, such as .H S, or compounds containing other harmful inorganic constituents can also create severe problems.

These problems are minimized or eliminated in accordance with the present invention by preheating and vaporizing the crude oil, heating the diluent steam and superheating the oil-steam mixture by heat exchange with the flue gas and then feeding the vaporized oil-steam mixture to the cracking tubes via the rectification column. When so required, the top of the rectification column is operated with some added liquid reflux condensate to further reduce the higher boiling coke and tar-forming components and components containing harmful inorganic constituents in the vapor feed to the cracking tubes.

In most naturally occurring hydrocarbon feedstocks, the sulfur content of the higher boiling portion of the oil is much higher than that in the lighter ends. Consequently, much of the sulfur present in the raw hydrocarbon feed can be concentrated in these bottoms in the rectification column, and thereby kept out of the cracktube and other downstream lines and auxiliaries.

The higher boiling components leaving the bottoms sections of the rectification column contain both aromatic components with essentially no hydrocarbon side chains, aromatics with short and relatively long side chains and/or naphthenic rings, naphthenes, straight and branched chain al'kanes, alkenes, diolefins, etc. The aromatic rings cannot be cracked to produce any substantial amount of ole-fins. However, the non-aromatic side chains, the naphthenes, alkanes, alkenes, diolefins, etc. can be cracked to produce substantial yields of the desired olefins. Thus, a yield loss is incurred in these refiractory bottoms unless some means is employed for recovering the desirable components.

Thus, with the crude oil cracking aspect of the present invention, the liquid bottoms are subjected to an elevated temperature cracking/stripping operation. It is well known that alkanes, alkenes, diolefins, and even na-phthenes crack much more readily than do the arc matic ring nuclei. Consequently, as the former [types of units crack into lower molecular weight fragments, their boiling points drop, and they are readily stripped out of the remaining bottoms to be carried into the vapor which eventually enters the cracking tube, by a relatively small amount of superheated stripping steam. The partial flash vaporization, rectification, liquid phase cracking, etc., can be performed in an in-line manner with the cracking furnace, or .as separate off-line operations. The latter may include external means for the heating and additional cracking of the heavy bottoms from the stripping section, external means for stripping the lower molecular Weight cracked products dirom the heavy cracked bottoms by vacuum, or steam stripping, or a combination of both, and means for returning the cracked products to the rectification apparatus below the rectification section but above (the stripping section. The preferred embodiment is the in-line scheme utilizing flue gas heat exchange since this leads to optimum economies and better process control.

From :the foregoing, it will be apparent that the process and apparatus herein disclosed for diagrammatic presentation of the invention are susceptible to numerous other possible combinations and arrangements.

The cores :may be made of heat resistant stainless steel alloys such as stabilized or unstabilized type 310 stainless steel, of other heat resistant alloys such as Incoloy 800 or Inconel, of ceramic materials, or,- when advantageous to do so, of materials which act as catalysts for the cracking reaction. The cores usually have centering spacer lugs on the outside of them, and they can be either plain cylindrical tubes on the outside, or have deflector vanes or scroll work on the outside to improve convective heat transfer rates. The core sizes and designs must in all respects be compatible with the interior size of the cracking tube and the process requirements. In general, the cores are designed so :high heat transfer rates and low carbon formation rates are obtained. However, adequate open cross sectional area must also be provided in the annular section between the tube and core to provide the optimum residence time for the cracking reaction to proceed, and to prevent the pressure drop in the reaction zone from exceeding tolerable limits. Annulus widths employed have ranged from 0.25 to 1.5 inches.

In accordance with this invention, the feedstock may range from normally gaseous hydrocarbons such as ethane, propane, butane, etc. to relatively high boiling liquid such as crude oil. The portion of the hydrocarbon feedstock which is to be' fed to the cored cracking tubes is substantially totally vaporized before being fed to the cracking tubes, being usually vaporized in contact with part 011' all of the cracking dilution steam. It is then introduced into the top of the cracking tubes, either before or after the balance of the cracking dilution steam has been added to it. The ratio of steam to hydrocarbon employed depends primarily upon the difiioulty encountered in the vaporization of the feedstock, and the coke or tar formation problems encountered in the cracking tubes and subsequent quench and heat recovery units. Normally, the ratio of steam to hydrocarbon feed. employed will range from 0.5 to 3.0 pounds of steam per pound of hydrocarbon. With high boiling feedstooks, the in-line method of fractionation utilizes the cracking dilution steam to reduce the partial pres sure of the hydrocarbon feed, and thereby makes it possible to fractionate or partially vaporize these materials 9 without the use of vacuum stills, or large amounts of other auxiliary steam. Additionally, the steam reduces the partial pressure of reactants and products Within the cracking tube, and thereby suppresses undesirable secondary reactions which lead to losses of the desired olefinic products such as ethylene, propylene, butadienes, and butylenes. In general, the external temperature of the cracking tube will range from about 1630 F. to 1950 F. The corresponding furnace refractory temperatures will range from about 2000 F. to 2250 F.

The cores in relationship to the annuli and size of the tubes are a process variable. They are designed to effeet a balance between the following process requirements: (1) high heat transfer rates and low carbon deposition rates; (2) adequate but minimum residence time for the cracking reaction to be accomplished; and (3) minimum pressure drop consistent with requirement 1) above. The preferred vertical tube arrangement disclosed herein using high temperature alloys is conducive to cleanliness and a minimum of operating problems. The tubes may be varied considerably as to size, being for example, 20 to 80 feet in length and 3 to 16 inches in internal diameter.

It will be understood that this invention is not limited to the normally liquid feedstocks cited herein by way of example. Other normally liquid and gaseous feedstocks fall within the intent and scope of the invention, and regardless of whether the feedstock is normally liquid or gaseous, the feedstock or portion of it which enters the annular cracking tubes is intended to be essentially completely vaporized or gaseous, as effectively as p-ractially possible within the objectives of the invention. Butane, propane, ethane, and gaseous mixtures containing these compounds are examples of feedstocks differing considerably from naphtha or higher boiling oils.

It will be seen from the foregoing that the advantages of this invention are multifold and include the following: the production of high yields of olefin from a wide variety of basic hydrocarbon feedstocks; the production of cracked gases containing high concentrations of the desired olefinic components; the reduction of energy and investment requirements for the production of olefins; and the minimization of operating problems resulting, for example, from the formation of carbonaceous deposits or from corrosion in the cracking tubes or subsequent cracked gas processing units.

It will be evident, therefore, that this invention may be carried out by the use of various modifications and changes without departing from its spirit and scope.

What I claim and desire to protect by Letters Patent 1. In apparatus for the thermal cracking of normally liquid, as well as normally gaseous, hydrocarbons in the presence of steam, the improvement which comprises:

(a) a tubular cracking furnace having at least one vertical elongate-d cracking tube disposed therein substantially extending from the bottom to the top of said furnace;

(b) a core disposed within the cracking tube substan- -tially coextensive therewith and forming a cracking annulus therebetween;

(c) a flue conduit in communication with the furnace having a plurality of communicating heat exchangers disposed therein adapted to be heated by the passage therethrough of flue gas from said furnace;

(d) means connected with the heat exchangers for passing hydrocarbon and steam therein and the-rethrough to effect a vaporized and superheated mixture comprising a first heat exchanger for preheating said hydrocarbon, means for passing said steam and preheated hydrocarbon into a second heat exchanger for vaporizing the said hydrocarbon in the presence of said steam, and means for passing the reit) sultin-g vapor into a third heat exchanger for superheating the vaporized hydrocarbon-steam mixture;

and (e) means connected with the third heat exchanger for 5 passing the superheated vaporized hydrocarbonsteam mixture therefrom and into the cracking annulus of the furnace.

2. In apparatus for the thermal cracking of normally liquid, as well as normally gaseous, hydrocarbons in the presence of steam, the improvement which comprises:

(a) a tubular cracking furnace having at least one vertical elongated cracking tube disposed therein substantially extending from the bottom to the top of said furnace;

(b) a core disposed within the cracking tube substantially coextensive therewith and forming a cracking annulus therebetween;

((1) means connected with the heat exchangers for passing hydrocarbon and steam therein and therethrough to elfect a vaporized and superheated mixture comprising a first heat exchanger for preheating said hydrocarbon, means for passing said steam and preheated hydrocarbon into a second heat exchanger for vaporizing the said hydrocarbon in the presence of said steam, and means for passing the resulting vapor into a third heat exchanger for superheating the vaporized hydrocarbon-steam mixture, said third heat exchanger having an isolated section thereof with means for passing steam thereinto and therefrom into the bottom portion of the core as superheated steam; and

(e) means connected with the third heat exchanger for passing the superheated vaporized hydrocarbon steam mixture therefrom and into the cracking annulus of the furnace.

3. The heat apparatus according to claim 2 in which (a) heat exchange apparatus is interposed in the flue conduit between the second and the third heat exchanger; and

('b) means connected with the heat exchange apparatus for passing water therethrough to reduce the flue gas temperature and simultaneously generate steam.

4. In apparatus for the thermal cracking of liquid hydrocarbons in the presence of steam, the improvement which comprises:

(c) a flue conduit in communication with furnace having a plurality of communicating heat exchangers disposed therein adapted to be heated by the passage therethrough of flue gas from said furnace;

(d) means connected with the heat exchangers for passing liquid hydrocarbon and steam therein and therethrough into rectification apparatus to effect removal of high boiling components from the hydrocarbon and to effect a vaporized hydrocarbon-steam mixture;

(e) means connected with the rectification apparatus for passing the vaporized hydrocarbon-steam mixture therefrom and into a superheat exchanger disposed in the flue conduit; and

(f) means connected with the superheat exchanger for passing the superheated mixture therefrom and into the cracking annulus of the furnace.

5. The apparatus according to claim 4 in which the rectification apparatus consists essentially of:

(a) a rectification section having a small amount of reflux to remove most of the high point boiling components from the hydrocarbon steam vapors.

6. The apparatus according to claim 4 in which the rectification apparatus consists essentially of:

(a) a rectification section having a small amount of reflux to remove most of the high boiling point components from the hydrocarbon steam vapors; and

(b) a primary stripping section to strip most of the residual lower boiling point components from the higher boiling liquid hydrocarbons.

7. The apparatus according to claim 4 in which the rectification apparatus consists essentially of:

(a) a rectification section having a small amount of reflux to remove most of the high boiling point components from the hydrocarbon-steam vapors;

(b) a primary stripping section to strip most of the residual lower boiling point components from the higher boiling point liquid hydrocarbons;

(c) a preheat section for high temperature heating of the unvaporized liquid hydrocarbon bottoms;

(d) soaking means for liquid phase cracking of the unvaporized liquid hydrocarbon bottoms; and

(e) a secondary stripping section to strip the cracked products and obtain asphaltic bottoms as the residuum product with the vaporized hydrocarbonsteam mixture as the overhead product.

8. The apparatus according to claim 4 in which the rectification apparatus consists essentially of:

(b) a primary stripping section to strip most of the residual lower Iboiling point components from the higher boiling point liquid hydrocarbon;

(c) external means for the heating and additional section;

12 (d) external means for stripping the lower molecular Weight cracked products from the heavy cracked bottoms; and

(e) means for returning the cracked products to the rectification apparatus below the rectification section but above the stripping section.

9. The apparatus according to claim 4 in which the rectification apparatus consists essentially of:

(b) a primary stripping section to strip most of the residual lower boiling point components from the higher boiling point liquid hydrocarbon;

(0) external means for the heating and additional cracking of the heavy bottoms from the stripping section;

(d) means for returning the cracked heavy bottoms to the rectification apparatus; and

(e) means for steam stripping the lower boiling hydrocarbon components from the cracked heavy bottoms and carrying these components up through the rectification apparatus to commingle with and join the vapors going to the cracking tu'bes.

References Cited by the Examiner UNITED STATES PATENTS 1,873,454 8/1932 Moore 203 2,230,254 2/1941 Loumiet et a1. 208-653 2,304,138 12/1942 Barnes et a1. 196134 2,676,156 4/1954 Bailey 48-196 2,904,502 9/1959 Shapleigh 208l30 DELBERT E. GANTZ, Primary Examiner.

HERBERT LEVINE, Examiner.

Claims

1. IN APPARATUS FOR THE THERMAL CRACKING OF NORMALLY LIQUID, AS WELL AS NORMALLY GASEOUS, HYDROCARBONS IN THE PRESENCE OF STEAM, THE IMPROVEMENT WHICH COMPRISES: (A) A TUBULAR CRACKING FURNACE HAVING AT LEAST ONE VERTICAL ELONGATED CRACKING TUBE DISPOSED THEREIN SUBSTANTIALLY EXTENDING FROM THE BOTTOM TO THE TOP OF SAID FURNACE; (B) A CORE DISPOSED WITHIN THE CRACKING TUBE SUBSTANTIALLY COEXTENSIVE THEREWITH AND FORMING A CRACKING ANNULUS THEREBETWEEN; (C) A FLUE CONDUIT IN COMMUNICATION WITH THE FURNACE HAVING A PLURALITY OF COMMUNICATING HEAT EXCHANGERS DISPOSED THERIN ADAPTED TO BE HEATED BY THE PASSAGE THERETHROUGH OF FLUE GAS FROM SAID FURNANCE; (D) MEANS CONNECTED WITH THE HEAT EXCHANGERS FOR PASSING HYDROCARBON AND STEAM THEREIN AND THERETHROUGH TO EFFECT A VAPORIZED AND SUPERHEATED MIXTURE COMPRISING A FIRST HEAT EXCHANGER FOR PREHEATING SAID HYDROCARBON, MEANS FOR PASSING SAID STEAM AND PREHEATED HYDROCARBON INTO A SECOND HEAT EXCHANGER FOR VAPORIZING THE SAID HYDROCARBON IN THE PRESENCE OF SAID STEAM, AND MEANS FOR PASSING THE RESULTING VAPOR INTO A THIRD HEAT EXCHANGER FOR SUPERHEATING THE VAPORIZED HYDROCARBON-STEAM MIXTURE; AND (E) MEANS CONNECTED WITH THE THIRD HEAT EXCHANGER FOR PASSING THE SUPERHEATED VAPORIZED HYDROCARBONSTEAM MIXTURE THEREFROM AND INTO THE CRACKING ANNULUS OF THE FURNACE.