IL23108A - Flame cracking of hydrocarbons - Google Patents

Description

"nan1? o'aa'sns mss« "FLAME CRACKING OF HYDROCARBONS" This Invention relates to a process and apparatus for the thermal cracking of a hydrocarbon, or mixture of hydrocarbons, such as a petroleum fraction, by contact with a flame, to produce fuel gas.

It is known to crack hydrocarbons by contacting the hydrocarbon feed stock with a heated refractory surface, such as of checker-brick or ceramic particles. Such surfaces are, however, rapidly cooled by the endothermic cracking reaction, and passage, of the feed stock must be discontinued whilst a flame is passed over the surface to reheat it. Such cyclic alternation of heating and cooling results in low efficiency of the process.

Continuous processes are known in which a refractory chamber is continuously heated by an external heat source, whilst the hydrocarbon is passed through the chamber. The refractory material is, however, a poor conductor of heat, and such processes are also inefficient, and the apparatus is costly, In both the cyclic and continuous processes, there is, furthermore, considerable deposition of carbon deposits on the heated surfaces, which deposits must be periodically removed, and the refractory surfaces rapidly become eroded by the flame and need frequent replacement. Furthermore, the gases produced are of varying quality, and contain considerable amounts of impurities such as drip oil, tars and smoke, which must be removed by expensive after-treatment.

Prior apparatus have heretofore been used primarily in peak shaving . operation, for instance during short periods in the winter, and it is hence especially It is therefore an object of this invention to provide a process and apparatus for Inexpensively effecting continuous cracking of hydrocarbon, to produce a high yield of combustible gas which can be utilized directly with a minimum of cleaning or af er-treatment,, The present invention comprises flame-cracking a hydrocarbon or. mixture of hydrocarbons by direct contact with a burner flame, a process which may be referred to as "flame-cracking". It is believed that the feed stock injected (which may be at a temperature, for example, of 700°P. (370°C.) cools the surface of the tip of the flame from about 3000°Po (l650°C. ) to 80O°-l60O°F. ( 25°-870°C), and this temperature permits the desired degree of cracking with the production of the minimum of by-products.

The feed stock should impinge on the flame at a point where the combustion in the flame is substantially complete, and the flame consists substantially only of the hot combustion products. To ensure this, a burner should be used which produces a steady, elongated flame so that the position of the tip of the. flame remains predictably constant; this is suitably achieved by providing a narrow, elongated combustion chamber around the base of the burner, and permitting the tip of the flame to extend into a cracking chamber.

The process according to the invention thus comprises introducing a hydrocarbon or mixture of hydrocarbons as feed stock in finely distributed form to Impinge on the flame of a burner, firing into one end of a cracking zone, in such manner that the impingement zone temperature is 800°-l600 F. (425°- ° The apparatus according to the invention comprises a chamber defining a cracking zone, a burner positioned to direct a flame into the zone, and orifice means for feeding said hydrocarbon as feed stock in finely distributed form into the chamber so as to impinge on the flame.

The feed stock may be injected as a vapor, a gas or an atomized liquid, and the term "finely distributed form" embraces all these forms.

The feed stock is preferably injected from a plural-ity of locations, eeg., three nozzles, around the burner. It is advantageous for the feed stock to be directed at an acute angle with the axis of the flame and toward the flame, to increase the contact time of the hydrocarbon with the flame„ The cracking chamber is preferably a somewhat elongated tube which provides an elongated flow path for the combustion and cracked product gases away from the burner, and a constricted exit from the cracking chamber may be provided to improve the mixing of the cracking gases.

A heat exchanger is advantageously provided in the path of the hot gases from the cracking zone, so as to preheat and if desired vaporize the feed stock.

It is especially advantageous to utilize the same hydrocarbon as the cracking feed stock and as the burner fuel-. A wide variety of hydrocarbon feed stocks, especially petroleum fractions, can be cracked.

The temperature in the impingement zone should preferably not exceed 1450°P. (790°C;), to ensure production of a uniform product relatively free from by-products. The cracking A detailed description of the invention follows, taken in conjunction with the accompanying drawings, wherein: Fig, 1 is a side elevation, partly in section, of a preferred embodiment of a gas preparation apparatus according to the invention; Pig. 2 is a vertical section, on an enlarged scale, of the burner end of the cracking tube of the apparatus; Fig. 3 is a section taken along line 3-3 of Fig. 2, showing the location and disposition of the injection orifices in the cracking tube; and Fig, 4 is a somewhat schematic sectional view of the burner end of the cracking tube, with a representation of a flow path.

Referring to Fig. 1, the apparatus includes an elon-gated cracking tube indicated generally as 1, and consisting of a cracking and mixing chamber 20 into which three injection conduits 6 feed, an intermediate tube portion 31 (shown broken away In Fig. l), and a heat exchange portion 3. At the left-hand end of the tube is a burner 5 communicating with the cham-ter 20 via a combustion chamber 18, and the right-hand end of the tube communicates with the bottom of a scrubbing tower 2.

A storage tank 7 supplies liquid hydrocarbon via a pump 9t valve 17 and conduit 8 and coupling 14 to the burner * and also via a valve 59* and conduit 60 to a heat exchanger 57 within the tube 31 and thence via a conduit 61, manifold 29, valves 30 and feedpipes 25 to the injection conduits 6. The conduits 25 are coupled to the manifold 29 by flanges 28.

Referring to Fig. 2, the cracking chamber is shown velocity oil burner in which liquid fuel is vaporized before it is burned, the heat for. the vaporization being taken from the combustion chamber of the burner; the burning of the vapor as compared to a liquid gives a high heat release rate, which should preferably be at least 1 to 10 million BTU (British Thermal Units) per hour per cubic foot of combustion space (1 kllo-cal per cubic meter s 8. 92 BTU per cubic foot at atmospheric pressure and 15i 5°C.)e The burner has an inlet 15 for •air under pressure (from a blower, not shown) connected to a coupling 16, and as mentioned, a coupling 14 for fuel intake.

A metal pipe 11 (Pigs. 2 and 3 ) provides structural support for the burner end of the cracking tube, and has an annular end plate 10 to which a mounting plate 12 of the burner 5 is attached by bolts 13.

A refractory combustion chamber 18 (Fig, 2 ) extends outwardly from the end plate 12, and comprises a wider portion 31 adjacent the burner and a narrow portion 33* the two portions being connected by a shoulder 3 .

The cracking chamber has a refractory lining 20 and a collar member 21 which forms a constriction at its downstream end. Three apertures 24 are formed radially through the wall of the cracking tube to receive the conduits 6; these apertures are spaced equidistantly round the tube diameter (see Fig. 3 ) and debouch at a point near the tip of the flame 19 of the burner (see Fig. 2) . Through each conduit 6 passes a coaxial injection pipe 25, which is cut off terminally at a 45° angle so that the cut end faces toward the burner. To each cut end is secured a plate 26 having a small aperture 27 (best seen in to pipe 11 by bolts 37 passing, through flanges 35* 36., and this section 3 1 is in turn Joined at flanges 5 , 55 (Fig. 1 ) to the heat-exchanger portion 3. The tubes 11, 3 * and 3 are in line with each other, and provides a straight gas flow path. The tubes are shown as of circular cross section, but other cross sections can be utilized,- Heat insulatory material surrounds this flow path, namely a body of refractory material 22 within the pipe 11 and surrounding the chambers 18 and 20, a double jacket 23 of insu-lation around the pipe 11, a jacket 4 ' around the pipe 3 ' and a jacket 4 around the pipe A helical metal heat-exchanger coil 7 (Pig. l) is supported within the pipe ^ An elongated hollow baffle 58 having' its upstream end closed is supported within the coil. Two or more coils 57 could alternatively be used, connected together in series. The coil 57 is connected to the injection manifold 29, as already described.

The following are exemplary principal dimensions of the apparatus described: inches centimetres ppiippee 1111 outside dia. 24 61 combustion chamber 18: total length 11 28 larger portion 31 internal dia. 5. 25 13.4 smaller " 33 4 10 cracking chamber 20 inside dia, 14 35.5 outside dia. 15.5 39.5 inside dia. at collar 21 27 ta th inches centimetres orifice 27 dia. 0. 25 0.6 distance from burner end of chamber 20 9.5 24 pipe 3 » dia, 14 35.5 length ca. 96* 255 pipe 3 length ca. 60 1 0 baffle 53 dia. 7.5 1-9.8 ♦this length may be considerably shorter.

Such apparatus has a capacity of 0.3-1 million cubic feet (8,400-28, 000 cubic meters) of product gas per day.

The scrubbing tower 40 is not essential to the invention and its construction and operation are described later. The operation, according to the process of the invention, of the cracking apparatus described is as follows.

The hydrocarbon to be cracked is stored in tank 7, and fed to the burner * which is also supplied via conduit 15 with an oxygen-containing gas under pressure, Air is generally used, but oxygen or oxygen-enriched air may be used and will give a higher calorific value to the product gas which will contain less or no nitrogen. The air hydrocarbon ratio is 60 to 100$ of stoichiometric. The resultant flame fills the combustion chamber 18, and extends a short distance (usually less than 4 inches (10 cm)) axially into the cracking chamber 18, the flame preferably having a conical shape as shown at 19 in an orifice for the flame to prevent the feed stock being drawn back into the flame. The chamber 18 stabilizes the flame so that the combustion is substantially complete within the burner, and no carbon deposit should be formed on the walls of the combustion chamber or cracking tube.

According to this embodiment,, about 3-15 of the hydrocarbon is used as the burner fuel, and the remainder as feed stock, which is pumped from the tank 7 through the heat-exchanger coil 57> and is thereby heated by the hot gases pass-ing down the tube 1, which gases are diverted by the baffle 58, If the heat exchanger is not present or is by-passed, the feed can be used directly without preheating, but it will be necessary to use more fuel to obtain the desired cracking temperature, and there will be a higher proportion of burner combustion gases in the mixed gaseous products leaving the cracking tube, and hence the calorific value of the product gas will be lower.

In a typical example, the calorific value of the product gas obtained is 600 BTU with preheating of the feed stock to 700°P. (370°C), but only 4 0 BTU without preheating. If the feed stock is normally liquid, it will be vaporized by passage through the heat exchanger.

The feed stock is injected (as a liquid, vapor or gas) from the orifices 27 into the cracking chamber 20, in the form of streams preferably directed at an acute angle (about 45°) toward the flame 19> and impinges on the flame; each stream then reverses its direction and is swept down the cracking tube by the combustion gases from the flame; the resultant flow path of a feed stock stream is shown in Fig. 4 at 53. This reversal in direction, due to the angle of impingement, increases the Impingement zone is thereby cooled from about 3000°F. (l650°C. ) to about 8000-i6d00F. (425°-870°C, J, and preferably to below 1450°F. (790°C.). The flame may thereby lose its luminosity and become gray-gas„ The feed stock is quickly and efficiently cracked in the chamber 20; it is believed that the cracking takes place essentially at the boundary of the flame 1 * which here consists of the gaseous combustion products formed by the combustion In the chamber 18„ There is a short residence time of the feed stock in this cracking zone.

The cracked products and combustion gases pass from the flame down the chamber 20, and are constricted at the collar member 21, which causes the gases to be further mixed together and improves the cracking efficiency. This collar member thus provides a restricted orifice from the cracking chamber. The degree of mixing can be further controlled by alteration of the number of injection conduits and their distance from the flame.

The gases then pass, substantially unchanged in composition, down the intermediate tube 3'* and have a temperature of about 1250°P. t (675°C, ) at the downstream end thereof. The gases pass the heat exchanger 57» where they loose heat to the feed stock and are cooled to about 900°P. (480°C. ) when they leave the heat exchanger and enter the scrubbing tower 2.

Referring again to Pig. 1, the scrubbing tower 2 shown there is of a conventional spray type, and comprises a vertical, cylindrical casing 38 having at the top thereof spray heads 40 connected by conduits 41 to a manifold 42 which is connected via a control valve 44 and a conduit 43 to a source - - provides a labyrinth passage for the flow of gases from the cracking tube 1 against the counter flow of condensing oil or water from the spray heads 40.

Spray medium and condensate collect in the bottom of the casing 38 at 46. A drain line 47 controlled by a valve 48 is provided to remove spray medium and condensate collected at 46 at the bottom of the casing. This liquid 46 forms a seal preventing gases from escaping through the drain line 47. To maintain this seal at a level sufficient to cover the drain line 47 but below the entrance 39 of the cracking tube 1 into casing 38, a level control unit 49 is provided. This latter unit is coupled as at 50 to valve 48, and operates this valve in a well-known manner to control the flow of liquid from the casing 38.

An exit duct 51 is connected above the spray heads 40, and includes a demister 52 within the duct 51 to remove from the gases any traces of liquid remaining in mist form.

The gaseous products pass from the cracking tube 1 into the base of the tower 2, wherein the cool water or oil spray, passing through the perforated trays 45 countercurrently to the flow of gases, cools the gases, causing condensation of the consensable ingredients upon the cool surfaces of the trays 45. The gaseous product then passes upwardly through the demister 5 and the duct 51 to suitable storage and distribution facilities. The water or oil spray medium, and the condensate, collect in the bottom of the casing 38, wherein the level control unit 49 maintains a proper fluid level to provide a liquid seal, preventing escape of the gaseous products. The collected the gaseous products in an oil or water scrubber. The gas with the condensates removed is of a quality suitable for direct commercial distribution. The burning quality of gas produced in a particular cracking operation is a function of the compo-sition of the feed stock.

Numerous operating factors can be varied to achieve optimum performance. These include type and capacity of burner, burner fuel and air feed rates, feed stock rate, and type and capacity of injection orifices. These variables and others can be coordinated in accordance with known engineering practice to obtain efficiency from any size unit. The number of orifices used depends on the size and throughput of the apparatus, but it has been found that three orifices, as described, give best yield.

The apparatus can be modified for pressure operation by conventional techniques, such as back pressure devices.

Superatmospherlc cracking zone pressures can be used, preferably in the range of »05 to 100 p.s.l.g.

If desired, any number of cracking units can be oper-ated from a single source of feed stock, by proper manifolding. The units can be operated in parallel.

The feed stock employed in the process of the invention is preferably a normally liquid petroleum hydrocarbon fraction, although other materials, such as crude petroleum, petroleum gas, or a pure hydrocarbon, such as hexane, heptane or di-isobutylene, can be used. If a hydrocarbon fraction is used, it will preferably have a final boiling point up to about 750°P. ( 00°C.) and a 95$ boiling point ranging up to about 00°F 0°C exam les are kerosene No 1 fuel oil No 2 It is preferred that the feed stock contain a majority, e.g., 70#-100#, of saturated hydrocarbons having from to 10 carbon atoms per molecule. ' This includes straight and branched chain paraffins and alkyl-substituted naphthenes. It is desirable to use feed stocks having a fairly low content of olefins and aromatics.

Petroleum1 hydrocarbon fractions having a boiling range of 80°F.-750°F. (27°- 00°CA ■) and containing 70#-lQ0# Vol saturated hydrocarbons, and less than 30 preferably less than 15$, aromatic hydrocarbons and olefins, are the preferred cracking stocks. The fraction can contain small amounts of sulfur compounds/ nitrogen compounds, and other contaminants usually found in hydrocarbon fractions.

Hydrocarbon mixtures of the following compositions give good results: Vol., % paraffinlc. hydrocarbons 75-100 branched chain paraffinlc hydrocarbons 25-40 naphthene hydrocarbons 1 i preferably 12 aromatic hydrocarbons <15, preferably 10 unsaturated hydrocarbons <25, preferably -^10 olefin hydrocarbons <10, preferably < 5 A preferred paraffinlc light petroleum hydrocarbon fraction has the following characteristics: API gravity 70 initial boiling point 100°F. (37.8°C.) end point 300°F. (l49°C ) A particularly desirable cracking feed for the process is petroleum naphtha, i.e., refined, partly refined, or unrefined petroleum products and gas liquids not less than 95$ of which distill below 464 F. (240°C ) . A preferred tnate-rial is that known as "Bajo Grande light naphtha", which is a virgin straight-run gasoline having an API gravity of 71°.. a Reid vapor pressure of 12 lbs. and an end point of 300°F. (149°C. ) .

The content of naphthenes in the feed stock can be as high as 10-40 Vol.

The following Example illustrates the process of the invention.

Example An apparatus similar to that shown in the drawings was employed to flame-crack the preferred paraffinic light petroleum hydrocarbon fraction, having the characteristics previously set forth. The fuel-air ratio employed in the burner was 93$ of stoichiometric. The temperature at a point approximately 8 feet (2„44 metres) downstream of vapor injec^ tion was steady at about 1299°F. (704°C ) . Heating fuel was fed to the burner at a rate of 11. 2 gallons (42.3 liters) per hour, while the cracking feed stock rate was 109 gallons (412 liters) per hour. The temperature of the vaporized cracking feed stock, as injected into the cracking tube, was 695°P. (369°C ) .

An average sample of the fuel gas produced had a specific gravity of 0.97, and its gross heating value, in BTU per standard cubic foot (measured by a calorimeter), was total feed stock (including the portion used as burner fuel) was 8l 9%, The gas composition was as follows: Vol. : 8.5 7. 8 13i 3 3.9 2. 8 0.6 1.0 51. 8 3.9 6.4 It was significant that the total drip oil, tar, and carbon deposit remaining in the apparatus was minimal regardless of the duration of operation. It was also significant that the hydrogen content of the product gas was relatively low: a low hydrogen content, e.g;, less than 30$, is necessary in the product gas so that carbon deposits may be kept minimal. It was very significant that substantially all of the CgS in the product gas were ethylene; no appreciable amount of the undesirable product acetylene was formed in the cracking reaction, as is the case in prior art cracking processes; The process of the invention provides a satisfactory fuel gas having a heating value of 200-800 BTU per cubic foot. Many industrial users burn fuel gases in the 300-700 BTU range, and these users can burn directly the gas produced by the pro-cess of the invention. Alternatively, the gas can be mixed as a supplement with propane, natural gas, or other gas having a higher heating value. The capacity of a propane-air plant can thus be greatly increased, at low cost, by installation of apparatus according to the invention, the mixed gas having a high heating value, over 1000 BTU per cubic foot.

The gas produced according to the invention is also an excellent source of ethylene, which is present in high concentration and with almost no admixture of acetylene, the ethylene * In one embodiment of the invention, a two-step process is carried out in which the flame oraoking as previously described is preoeded by a partial thermal craoking of the hydrocarbon feed stock in a fired heater, that is, a heater in which the feed stook is heated by indireot heat exchange with hot gases suoh as oombustion gases. In this two-step prooess, the first step is perormed by introducing the feed stook into a fired heater whioh oan be separate from the flame-oraoki^g apparatus.

In this fired heater, the feed stock is vaporized and partially thermally cracked to oonvert the less refractory oomponenets of the feed stook into oraoked products. The temperature of this oraoking is usually in the range from 750 to 1250 though other temperatures oan be used in some oases. The mixture of cracked produots and unoraeked, more refractory oomponenets of the feed stook is then introduced into the. second step whioh consists of flame-cracking, in the manner previously desoribed* In the flame-oraoking step, further thermal oraoking of the hydrooarbon mixture takes plaoe, to convert more refractory oomponenets of the original feed stock to oraoked products.

The conditions i the partial thermal oraoking step are such s to minimize carbon formation* This two-step prooess is advantageous in some instances in that it enables a gas of higher heating value and lower specific gravity to be obtained than in flame -oraoking as previously desoribed, or in flame-oraoking with previous vaporization under substantially non-cracking conditions, also as previously desoribed. The properties of the produot gas oan be varied by varying the amount of oraoking whioh takes plage In the first step* A relatively high degree of craokLng in the first step makes it possible to reduce the amount o oxygen^bontaining gas which is used in the flame-cracking-' step.f relative to the amount of hydrocarbon which is braoked, and this results in higher heating value and lower speoifio gravity in the produot gas* In some oommeroial installations, the latter oharaoteristios may be important* The produot gas characteristics may be varied between 0*75 and 1*10 speoifio gravity for example, and between 350 and 1000 BTXT per cubic oot heating value.

In a futher embodiment of the invention, a three-step process is carried out in whioh the feed stock is first vaporized by indirect heat exohange with produot gas, in the manner previously described herein, and the vaporized feed stock, at a temperature for example of 700°F. , is then introduced into a separate ired heate wherein it is. raised to thermal cracking temperature to produce partial thermal oraoking, as desoribed jus previously in oonneotion with the two-step process. She mixture obtained om the partial oraoking step is then introduced into a flame-bracking step as previously described, in order to obtain further bracking. This three-step process differs from the two-step process previously desoribed in that* in the three-step process, the heat for initial vaporization of the feed is provided by heat exohange with the product gas, rather than by heating in the separate ired heater. In both processes, the heat or partial thermal oraoking, however, is provided in the separate fired hiater· which is then itself separated from the other gases.

The invention thus, provides an inexpensive, simple and efficient apparatus and process for the flame cracking of hydrocarbons, which process is simple to control and provides a good yield of relative pure combustible product gas, HAVING NOW particularly described and ascertained the nature of my /our said invention and in what manner the same is to be performed, I/We declare that what 1/ We claim is .-

Claims (18)

SUN OIL COMPANY Stoops et al (l-A) British Claims 2-I8-65 (BCD)

1. Process for cracking hydrocarbons, which comprises: introducing a hydrocarbon or mixture of hydrocarbons as feed stock in finely distributed form to Impinge on the flame of a burner firing into one end of a cracking zone in such manner that the impingement zone temperature is 800-l600°Fe (425-870°C. ), thereby to produce a gaseous mixture of combustion products and products of cracking of the feed stock.

2. Process according to claim 1, wherein the feed stock is introduced into said zone at a plurality of locations around the periphery thereof.

3. Process according to claim 1 or 2, wherein the feed stock upon said introduction is directed at an acute angle with the axis of the flame and toward the flame.

4. Process according to claim 1, 2 or 3* wherein the feed stock is a liquid light petroleum hydrocarbon fraction, which is vaporized before its introductio into said region.

5. A process according to any of claims 1 to 4, in which the hydrocarbon is a petroleum fraction having a 95$ boiling point below about 700°P.

6. Process according to any preceding claim, in v/hich the burner flame is provided from a hydrocarbon fuel and

7. Process according to claim 6, in which the burner fuel and the hydrocarbon cracking feed are the same hydrocarbon fraction.

8. Process for cracking a hydrocarbon, as hereinbefore described with reference to the accompanying drawings.

9. Process for cracking a liquid light petroleum hydrocarbon fraction, as hereinbefore described with reference to the foregoing Example.

10. Combustible fuel gas, when produced by cracking a hydrocarbon by a process according to any preceding claim.

11. Combustible gas according to claim 10, having a calorific value of 200-800 BTU per cubic foot.

12. Combustible gas according to claim 10 or 11, having a hydrogen content below 30 Vol.

13. Apparatus for cracking a hydrocarbon or mixture of hydrocarbons, comprising a chamber defining a cracking zone, a burner positioned to direct a flame into the zone, and orifice means for feeding said hydrocarbon as feed stock in finely distributed form into the chamber so as to impinge on the flame.

14. Apparatus according to claim 13, wherein the orifice means comprises a plurality of injection nozzles around the periphery of the chamber.

15. Apparatus according to claim 14, in which the nozzles are arranged to direct the feed stock at an acute angle with the axis of the flame and toward the flame.

16. Apparatus according to claim 13, which includes a tube defining an elongated flow, path for passage of the products of cracking and combustion, and a constriction in said tube toward the burner end thereof to promote intimate mixing of the gases.

17. Apparatus according to claim 16, including also a heat exchanger mounted in said tube and arranged to utilize the hot gases therein to heat the feed stock prior to the feeding thereof into said chamber.

18. Apparatus for cracking hydrocarbons, substantially as hereinbefore described with reference to the accompanying drawings.