US3480689A - Cracking of hydrocarbons - Google Patents

Description

Nov. 25, 1969 B. B. BOHRER 3,480,689

CRACKING OF HYDROCARBONS Filed May 10, 1967 I 5 I v l j i I i r: I l j ZONEA I r I T i INVENTOR. BYRON B. BOH'RER EMMK ATTOR Y.

United States Patent Int. Cl. C07c 3/30 US. Cl. 260-683 10 Claims ABSTRACT OF THE DISCLOSURE A hydrocarbon cracking process which provides heat to the cracking zone and mitigates carbon deposition on structure surfaces thereof by providing for the coating of cracking zone structure surfaces with flowing molten metal. The molten metal coating is accomplished by providing the walls of the structure with grooves as a flow path for the molten metal. Grooves are provided by machine rifling the walls or by compressing a helical coil of wire until it engages the cracking zone wall and affixing it in that position. A preferred manner of grooving the walls and for providing heat to the reaction zone involves the use of a double-wire helical coil member aflixed to the cracking zone wall and the upper end of the double-wire coil terminates in a molten metal bath whereby molten metal is fed down the double-wire member by capillary action. In a modification such double-wire helical coils are suspended within the cracking ZOne in lieu of or in addition to affixing same to the cracking zone wall. Cracking conditions are conventional, in the temperature range of about 1200 to 1900 F. and about 1 to 3 atmospheres of pressure.

This invention relates to a process for pyrolysis or thermal cracking of hydrocarbons to produce lower olefins and other unsaturated compounds. More particularly, this invention relates to an improved cracking process and related necessary apparatus for carrying out the invention.

It is well known that when gaseous or vaporized hydrocarbons illustrated by methane, ethane, propane, butane, pentane, hexane, and naphtha are heated to high temperatures, they undergo pyrolysis, whereby it is possible to obtain hydrogen, lower olefins and other unsaturated compounds such as acetylene, ethylene, and butadiene which are raw materials for the petro-chemical industry.

As one type of apparatus for such thermal cracking, there is a type known as a moving or fluid layer type, wherein circulation, that is movement, takes place between a reaction section where pyrolysis action occurs and a regeneration section where heat replenishment for the reaction is accomplished. Heretofore, such apparatuses wherein sand, carbon, pebbles, coke, and other materials are used as heating media and are circulated between regeneration sections, where they are heated by combustion gases, and reaction sections, where they are caused to flow counter to the flow of the hydrocarbons and to undergo cracking reaction, have been proposed. More recently it has been proposed to spray a molten metal through a nozzle directly into the gas to be cracked in counterflow direction. While each of the prior art techniques have met with varying degrees of limited success, the heating media proposed heretofore, that is, such solid particles as sand, pebbles, and carbon particles, have a high rate of consumption of the particles themselves because of such destructive action during the moving process as wear due to mutual abrasion of the particles and wear due to collision and abrasion be- 3,480,689 Patented Nov. 25, 1969 tween the particles and the furnace wall and other structure surfaces. This wear causes the disadvantage of difficulty in maintaining a suitable particle size and, moreover, creates the possibility of clogging of the circulation path by fine particles produced by these abrasive actions not to mention the need and cost to replace the worn equipment. To a lesser extent the same is true of the more recent sprayed or droplets of molten metal technique. The individual fine droplets produced by egress through a nozzle under pressure can be easily solidified in the cooler end of the cracking reactor from whence the hydrocarbon cracking stock is charged. Furthermore, many droplets are so fine that they are carried away with the cracked hydrocarbon. The fine droplets carried away with the gaseous product are deposited on cool surfaces of the subsequent equipment with eventual plugging requiring shutdown. Still further some of the fine droplets remain in the gaseous product as a substantial contamination of same and must be removed therefrom. The prior art methods generally do not provide for or admit to facile removal of coke formed in the cracking reaction. A process and related equipment for alleviating or even substantially mitigating the foregoing problems and particularly the more serious ones is to be highly commended.

It is accordingly highly desirous and a principal object of the present invention to provide a cracking process and necessary suitable equipment wherein the consumption of material employed as heating medium is low, regulation of the furnace interior temperature is easy, and the efficiency of heat transfer is high.

It is a further object of this invention to provide such a method and apparatus as above-stated wherein heating of the heat medium can be easily accomplished.

It is another object of the invention to provide a process and apparatus which minimizes contamination of the product with the heat exchange medium.

It is still a further object to provide a method of carrying out a cracking operation wherein substantial amounts of coke formed are continuously removed and in a facile manner.

Other objects will become apparent from reading the application in entirety.

To the accomplishment of the foregoing and related ends the material to be cracked is passed in gaseous or vapor form through a substantially vertical cracking reactor at cracking conditions, said reactor comprising a cracking zone constructed to provide for closely spaced helical grooves on the inner walls of said cracking zone and/ or at least one wire member aligned with the longitudinal axis of the cracking zone, and to provide for a flow of molten metal down said helical grooves in said wall and/or down said wire members, whereby carbonaceous matter formed by the cracking which occurs, is deposited on tthe flowing metal and is carried out of the cracking zone, the carbonaceous matter .is then separated, the metal is then reheated and recycled to the cracking zone.

FIGURE 1 is a diagrammatic view of the apparatus for carrying out the invention and is described in more detail in the illustrative example.

FIGURE 2 is an enlarged cut-away longitudinal crosssectional view of one cracking tube with a helical coil member aflixed as a liner on the tube and further described herein.

Apparatus that fits the foregoing broad description of the pyrolysis chamber and related structure that will be found suitable for the practice of this invention is described in detail and claimed in US. Patents 2,981,665 and 3,015,340 to Hans J. Kloss et al. Others will also be described herein. In the interim the important operative principles or modus operandi there, and in some of the embodiments here also are to be set forth herein. It is, of course, to be readily appreciated by those skilled in the art that the different objects of the present invention necessarily begets certain minor but significant modifications of the apparatus there described at least in some embodiments of this invention. For purposes of fullness of disclosure certain of the important features and operative principles of that apparatus which are of some importance here will not only be mentioned but will be discussed in some detail.

In one embodiment material to be cracked is passed through a pyrolysis chamber cracking vessel equipped with a plurality of double wire helical spiral members (having a striking similarity in appearance to a long, thin spring), positioned and aligned with each other and the longitudinal axis of the vessel. Each of said double wire members (and similar somewhat equivalent members defined in the Kloss patents) being fitted at the top with t a liquid holding heat medium feed tray for the heat medium. The molten metal heat medium in the tray is fed onto the double wire helical spiral by capillary action, or other suitable means where the capillary action will not occur with the particular metals involved, and continues down the spiral under gravitational force. The molten metal travels the full length of the spiral and coke deposited on the molten metal is carried with the flowing metal to a collecting pool where the coke is separated, for example, by skimming it from the surface of the molten heat medium. The surface area covered by the molten metal is, of course, protected against coke deposition. For that reason it is desirable to maximize the surface area of the pyrolysis chamber coated or covered with the flowing molten metal. Thus, certain modifications to the foregoing will be desired in most cases to achieve minimal coke deposition on the surfaces of the equipment by maximizing the surface covered with the flowing heat medium.

Examples of such modifications in structure which will also operate to provide for surface area of suspended heat related elements in the pyrolysis vessel to be covered and protected from coke deposition thereon is to pack the pyrolysis chamber with a plurality of substan tially vertically disposed single wire or wire-like members positioned with respect to a liquid hold-up molten metal feed tray so as to feed molten metal onto each of said wire members whereby the liquid will wet the entire surface of the wire and flow down around it in an annular moving film. The feed of the molten metal onto the wire can be conveniently accomplished by providing a plurality of small holes in the holdup tray and positioning one wire immediately below each hole sufficiently close to contact and break the protruding miniscus of the molten metal. This is a less preferred embodiment of the equipment.

Another modification is to arrange the adjacent pair of wires so that they are, generally speaking, horizontally disposed with respect to each other rather than vertically disposed. The two wires will in effect form a small trough as a path for the molten metal. In such a structure the feed means can also be simplified to provide a direct feed to the trough without the need for reliance on capillary action which may be difficult to obtain with the particular materials preferred for construction and the molten metal or alloy preferred as the heat medium in some cases. This modification likewise is a less preferred embodiment.

In all cases the walls of the pyrolysis tubes can be protected against coke deposition, and additional heat supplied as well, by flowing molten metal down the inside of the tubes or even in some cases by cascading the molten metal on or near the vessel walls. Preferably, because much less molten metal will be handled enabling better heat control and because of economies of energy due to a reduction in the amount of the molten metal to be handled, the latter method of cascading is not employed and instead the tube walls are adapted to provide for a flowing thin layer of molten metal. This can be accomplished in several ways. The interior of the tube wall can be closely rifled by machining or a closely wound spring can be inserted in the tube, compressed against the interior walls and welded in place. Either of the foregoing will provide grooves for distributing flow of the molten he'at medium down the vessel wall, even when using materials such as carbon steels which do not wet easily.

The pyrolysis zone is preferably constructed of the same general design as a direct externally fired radiantheated tubular thermal cracker comprising a plurality of cracking tubes. Preferably this is simply lined with a helical coil member as above described but a plurality or bundle of helical coil members can be placed within each of said tubes in the cracker or pyrolysis zone. The tubes are direct fired in conventional fashion.

Of course it can be readily appreciated once attention has been called to the need, that the pitch of the helical coil members, riflings or the like must be sufiicient to insure an adequate downward gravitational flow of the molten metal. Further discussion is unnecessary for those skilled in the art.

Examples of the heat medium that can be employed are lead, tin, magnesium, lithium, sodium, boron oxide, aluminum, and alloys such as babbit metal, solder and woods metal. Water is to be avoided in the case of lithium and sodium. The preferred heating mediums have high boiling points and low melting points exemplified by the materials above.

The cracking vessel or pyrolysis chamber and other structural materials within same can be constructed of austenitic stainless steel (i.e., 18% Ni, 8% chrome), or specialty steels, such as those containing 25% chrome and 25% nickel. Usually because of cost considerations the heat medium is lead, and the vessel is constructed of carbon steel. High temperature strengths can be obtained by cladding a carbon steel liner within an outer layer of alloy steel containing 25 Ni and 25 chrome such as used in pyrolysis tubes and mentioned above. Lest a reader think the foregoing is an inadvertent error it is pointed out that this is the reverse order of cladding such layers. Where wetting of the apparatus by the molten metal is desired combinations of the foregoing are selected to achieve this. Examples of such combinations are tin as the heat medium and apparatus constructed of austenitic stainless steel. Generally liquid metals will readily wet any alloy steel. There is some dissolution of the alloying components and this should be checked from time to time after extended use of the equipment.

The materials that may be cracked in the present invention include ethane, propane, butane, butene, pentane, pentene, octane, dodecane, dodecenes, eicosane, cyclopentane, cyclohexane, alkyl benzenes of a total of about 8 to 30 carbon atoms, naphthalene and alkyl naphthalenes of about 12 to 30 carbon atoms and mixtures of the foregoing, such as naphtha, and similar materials.

The cracking conditions are the same as those used in the prior art processes which are well known to those skilled in the art and accordingly will not be discussed at great length. Briefly, however, they involve a temperature range of about 1200 F. to 1900 F. and pressures of about 1 to 3 atmospheres or similar to the known high severity processes. The upper temperature limit being determined by metallurgical limits. Within these ranges the preferred conditions vary with the particular hydrocarbon being cracked but roughly the temperature correlates directly with the ethylene yield desired and inversely with propylene yield. Pressure favors the formation of saturates and accordingly pressure is to be minimized. Incidentally, the ethylene yield is increased by a higher heat transfer at a given residence time and thus this invention enables higher ethylene yield at a given metal wall temperature, all else the same, than in the prior art.

The molten metal heat medium, which has passed through the pyrolysis chamber, may conveniently be returned to its feed tray by mechanically driven ladles, or by a conduit using a pump, to a reservoir above the tray and poured therein where it can be reheated. The coke on the surface may be skimmed oil? before being returned to the tray or burned off while in the ladle or elsewhere.

In the prior art steam has been charged to the cracking zone to reduce the coke deposition, this may be employed here but lesser amounts if any will be found adequate. The throughput of hydrocarbon is thereby increased.

To facilitate the understanding of the invention, certain details and illustrative embodiments will not be set forth; however, of course, it is to be fully understood and appreciated that the invention is not limited to the specific conditions or details set forth in these examples, since the process is capable of many modifications and variations and conditions, such modifications and variations being aided, suggested or indicated by the discussion of the process as found herein and the discussions of the trends of the effect of the various factors.

ILLUSTRATIV E EXAMPLE The discussion of the following description and reference to the drawing in connection therewith is in all cases to be related to FIGURE 1 unless otherwise expressly indicated.

To a pyrolysis vessel constructed of two zones, A & B, the upper one A is constructed of high alloy steel with a special carbon steel lining (as shown in the enlarged cross-sectional view of a fired reaction tube 3 and a coil member 3 aflixed therein as said liner in FIGURE 2 which is discussed below) and of a conventional radiantheated direct-fired multiple tube configuration and B a lower portion constructed of austenitic stainless steel, naphtha preheated to about 800 F. in the amount of about 40,365 pounds per hour (and about 3,330 pounds per hour of ethane from product mixed therewith) is charged to the vessel feed inlet 2 in the lower zone. The radiant-heated fired tubular zone is fitted with a helical coil liner 3 in each tube 3 (as shown in FIGURE 2) and each wire liner 3 of each tube is equipped at the top with the same molten-metal feed tray 4 adapted for feeding a molten metal heat medium by capillary action from the feed tray down the wire members which at the top extend up through apertures in the molten metal feed tray which.

are then bent or turned down into the molten bath. A reservoir 5 for molten lead as heat medium is connected by means of a conduit 6 to the lead feed tray 4 at the top part of the reactor. Sufficient lead is allowed to flow down the conduit 6 from the reservoir 5 to maintain an adequate supply of the molten lead in the feed tray 4 at all times so that there will be a continuous flow of the lead by capillary action to coat and protect the surfaces thereof. The molten lead drops from the bottom end of the helical coil wire liner members 3' to a molten metal collecting pool 7 at the bottom of the reactor where the lead is collected. Below the surface of the collection pool is a steam distributor 8 connected to a source of superheated (i.e., about 1100= F.) steam. Steam is charged through the distributor 8 through the lead pool 7 having a coke layer 9 to the bottom of the reaction zone and up through it in countercurrent flow to the heat medium at the rate of about 8,650 pounds per hour. At the liquid surface level in the bottom of the reactor is a conduit 10 for removal of the molten metal and coke therewith which is sent to a pump 11 and returned to the molten metal heat medium reservoir 5. There the coke is burned off as a part of heating the molten lead in the reservoir prior to charging it to the reactor. The direct-fired tubular reactor zone A is heated in conventional fashion so as to maintain a temperature of about 1450 F. With a residence time of the naphtha feed and steam in the reaction zone of about 0.7 second and a pressure in the cracking vessel of about 0.5 p.s.i.g., the product gas leaving the cracking zone taken off at the top of the reactor at outlet 12, has a typical product distribution as follows:

Lbs./hour Tail gas 6,910 Ethylene 12,500

Gasoline Fuel oil Ethane Loss Steam 8,500

The ethane can be conveniently and advantageously recycled as in the above illustrative embodiment.

Having now described the invention, many ramifications and modified embodiments will readily occur to those skilled in the art. In so far as such variations do not depart from the spirit and scope of the invention described in this application, they are intended to be embraced by the appended claims in their broadest construction.

The invention claimed is:

1. A cracking process comprising passing a hydrocarbon cracking stock at cracking conditions through a substantially vertical cracking reactor constructed to provide for closely spaced helical grooves on the inner walls of the cracking zone of same and to provide for a flow of molten metal down said helical grooves whereby heat is supplied to said cracking zone of said reactor and carbonaceous matter produced by cracking which occurs in said cracking zone is deposited on said flowing molten metal in said cracking zone and is carried out of said cracking zone and the carbonaceous matter is then removed from said molten metal and the molten metal is recycled to the cracking zone.

2. A process according to claim 1 wherein said helical grooves are formed by affixing a compressed double-wire helical coil member to the inner wall of said cracking zone and wherein said molten metal is fed down said groove formed by said double-wire helical coil member by capillary action provided for by extending the upper ends of the double-wire helical coil members into a molten metal bath.

3. A process according to claim 1 wherein said molten metal and carbonaceous matter from the cracking zone are collected and are charged to a reheater wherein the carbonaceous matter is removed from the surface of the molten metal by combustion and thereby said molten metal is simultaneously reheated prior to recharging to said cracking zone.

4. A process according to claim 1 wherein said molten metal and carbonaceous matter from said cracking zone are collected in a collecting pool and said carbonaceous matter is removed by skimming same from the surface of the pool of molten metal.

5. A process according to claim 1 wherein the cracking zone of said reactor comprises a plurality of directfired radiant heated cracking tubes and wherein the inner walls of each of said tubes contain helical grooves and molten metal flowing down same as aforedescribed.

6. A process according to claim 1 wherein said molten metal is a member of the group consisting of lead, tin, magnesium, lithium, sodium, boron oxide, aluminum, babbit metal, woods metal and solders.

7. A process according to claim 1 wherein the cracking zone of the cracking reactor is equipped with at least one helical coil member suspended within said zone and aligned with the longitudinal axis of said zone and wherein each of said helical coil members is adapted to provide a flow path for molten metal, is equipped with a molten metal feed tray and means for feeding molten metal down said helical coil members.

8. A process according to claim 1 wherein the cracking conditions employed are in the range of about 1200 to 1900 F. and about 1 to 3 atmospheres of pressure.

9. A process according to claim 1 wherein the cracking feed charged to said reactor and is cracked therein is naphtha.

10. A cracking process comprising passing a vaporized hydrocarbon cracking stock through a cracking reactor said cracking reactor containing a cracking zone with at least one wire member in the cracking zone of said reactor aligned with the longitudinal axis of said cracking zone said wire member is a double-wire helical coil member equipped at the top with a molten metal feed tray and adopted for the flow of molten metal onto and down said helical coil member and wherein at least part of the heat to said cracking zone of said cracking reactor is supplied by a molten metal flowing down said wire member and whereby carbonaceous matter formed in said zone is deposited on said flowing molten metal and is carried out of said cracking zone and the carbonaceous matter is then removed from said molten metal and said molten metal is recycled to the cracking zone.

References Cited DELBERT E. GANTZ, Primary Examiner J. NELSON, Assistant Examiner US. Cl. X.R.

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