US3414632A

US3414632A - Process and apparatus for cracking hydrocarbons

Info

Publication number: US3414632A
Application number: US455705A
Authority: US
Inventors: Buschmann Karl; Meyer Hermann; Schweitzer Armin
Original assignee: BASF SE
Current assignee: BASF SE
Priority date: 1964-05-16
Filing date: 1965-05-14
Publication date: 1968-12-03
Anticipated expiration: 1985-12-03
Also published as: BE663878A; NL6506199A; DE1443521A1; GB1099904A

Description

Dec. 3, 1968 K. BUSCHMANN ET 3,414,632

PROCESS AND APPARATUS FOR CRACKING HYDROCARBONS Filed May 14, 1965 4 Shegts-Sheet 1 FIG] FIG '0 INVENTORSI' s R .Y M H CH A N AZ N T RN M E EE YNW NH @WM 0 C U BNT A U N LMM RRLM AEER K H H A Dec. 3, 1968 BUSCHMANN ET AL 3,414,632

PROCESS AND APPARATUS FOR CRACKING HYDROCARBONS Filed May 14, 1965 4 Sheets-Sheet 2 FIG. 2

FIG. 20

INVENTORSI KARL BUSCHMANN HERMANN MEYER HELMUT NONNENMACHER ARMIN SCHWEITZER ATT'YS K. BUSCHMANN ETAL 3,414,632

PROCESS AND APPARATUS FOR CRACKING HYDROCARBONS Dec. 3, 1968 4 Sheets-Sheet 3 Filed May 1-4, 1965 FIG?) R o N E V m o 3 5 f 0 L11 2 I D .l I ""fllll i I. I C l'"l"ll" b Q I Q I Q n Q m M HR M N Z RWWK I MYNWQ NH MOC SNNS M U i B T LMW RRL AEER KHHAM 5 W B ATT'YS Dec. 3, 1968 K, BUSCHMANN ET AL 3,414,632

PROCESS AND APPARATUS FOR CRACKING HYDROCARBONS Filed May 14, 1965 4 Sheets-Sheet 4 FIG.

FIG. 40

INVENTORS: KARL BUSCHMANN H ER MAN N MEYE R HELMUT NONNENMACHER ARMIN SCHWEITZER (Mafia ATT'YS United States Patent Office 3,414,632 Patented Dec. 3, 1968 76,8 19 Claims. (Cl. 260-683) ABSTRACT OF THE DISCLOSURE Process and apparatus for thermally cracking hydrocarbons to produce olefines in a cracking tube and for quenching the hot gases in an expanded quench zone in which the connecting member between the cracking tube and the quench zone has a circular cross-section and in which steam is passed tangentially into said connecting member.

The present invention relates to a process and apparatus for cracking gaseous and vaporizable liquid hydrocarbons at high temperatures using short residence times. More particularly, the subject invention is directed to a process and apparatus for cracking such hydrocarbons wherein the formation of coke deposits at critical points in the apparatus is substantially reduced.

It is known that olefinic hydrocarbons can be produced by cracking gaseous and liquid vaporizable hydrocarbons in metal tubes at temperatures above 750 C. In the usual process, a vaporizable liquid hydrocarbon of the gasoline range boiling between 35 and 170 C. is cracked in a tube heated by external means using extremely short residence times. This process is often referred to as high severity cracking. Cracked gas rich in olefines such as ethylene and propylene is thus obtained, which gas also contains higher olefines and diolefines and other cracked products. To avoid secondary reactions it is necessary to cool this very reactive gas mixture. This cooling, or quenching, is effected in industry either by direct injection of coolant, for example liquid hydrocarbons, into the reaction material, or by indirect cooling, for example by means of water, in a quench boiler.

Indirect cooling is generally preferred in order to improve heat recovery. It is a disadvantage of indirect cooling, however, that deposits of coke and other cracked products often form. These deposits occur particularly in the connecting pipe between the cracking tube and the quench boiler and also in the individual cooling tubes of the quench boiler. The pressure difference in the system is increased by these deposits and the uniform distribution of the hot cracked gas to the cooling surfaces is disturbed. The plant must then be stopped at more or less short intervals of time and the deposits must be removed.

The connecting member between the cracking reactor and the quench boiler supplies the cracked gas to the cooling tubes and is designed so that its diameter tapers out from the diameter of the cracking tube to the diameter of the base of the quench boiler. In many cases it has proven to be advantageous to give this tapered portion a conical or hemispherical shape. When the base of the quench boiler has a large diameter, a relatively large space is formed in the connecting member which further promotes the formation of the deposits and decreases the yield of olefin due to the long residence time of the cracked gas in this space.

It is an object of the present invention to provide improved apparatus for cracking hydrocarbons at high temperatures to form olefins which apparatus can be operated substantially without the formation of coke deposits in the connecting zone between the cracking tube and the quench boiler of the apparatus.

Another object of the invention is to provide a process for cracking hydrocarbons at high temperatures and short residence times to form olefins which process does not cause the formation of substantial deposits in the zone connecting the cracking tube with the quenching means. Another object of the invention is to provide a high temperature process for cracking hydrocarbons which increases the yield of olefins.

Other objects will become apparent to those skilled in the art from the following detailed description of the invention.

In general, the present invention is based on the discovery that the formation of coke deposits can be markedly reduced in high severity cracking processes by introducing steam tangentially and at high velocity into the connecting member between the cracking tube and the quench boiler. The subject invention specifically applies to the thermal cracking of gaseous and/or vaporizable liquid hydrocarbons at temperatures of more than 750 0., preferably of from about 800 C. to 850 C., wherein the cracked gas is cooled in a quench boiler to a temperature of less than 400 C., and preferably less than 350 C.

The apparatus and process can best be seen by reference to the attached drawings in which:

FIGS. 1 and la illustrate one embodiment of the invention in which FIG. 1 is a sectional view taken along lines 1--1 of FIG. la; and

FIG. 1a is a sectional view taken along lines 1a--1a of FIG. 1;

FIGS. 2 and 2a illustrate a second embodiment of the invention in which:

FIG. 2 is a sectional view taken along lines 22 of FIG. 2a; and

FIG. 2a is a sectional view taken along lines 2a2a of FIG. 2;

FIGS. 3 and 3a represent a still further embodiment of the invention in which FIG. 3 is a sectional view taken along lines 3-3 oi FIG. 3a; and

FIG. 3a is a sectional view taken along lines 3a3a of FIG. 3; and

FIGS. 4 and 4a represent a still further embodiment of the invention wherein- FIG. 4 is a sectional view taken along lines 4-4 of FIG. 4a; and

FIG. 4a is a sectional view taken along lines 4a4a of FIG. 4.

In the embodiment shown in FIGS. 1 and 1a, enlarged cone-shaped connecting member 2 is arranged between pipe 1 and quench boiler 3. FIG. 4 is a steam inlet line. As can best be seen from FIG. 1a, the steam is introduced through line 4 tangentially to the inner surface of connecting member 2.

In the embodiment shown in FIGS. 2 and 2a, connecting member 2 includes a cylindrical section 5 above the conical section 2. In this embodiment, the steam is passed through line 4 tangentially to the cylindrical section 5 as in a cyclone.

In FIGS. 3 and 3a, a still further embodiment of the invention is disclosed. In this embodiment, steam is supplied tangentially to cylindrical section 5 through line 4, line 4 being located adjacent to the entrance to quench boiler 3. Additional steam is supplied tangentially through line 6 to conical section or member 2.

FIGS. 4 and 40 set forth a highly advantageous embodiment of the invention. In addition to the steam that is introduced tangentially into cylindrical member 2 through line 4, additional steam is introduced through a plurality of pipes 7 at the bottom of connecting member 2. The number of such pipes can vary from one up to as many as five or more. In the embodiment shown herein, three such pipes are included. The steam which is passed upwardly through the pipes preferably is deflected laterally by bafiles 8.

The problems outlined above with respect to the high severity cracking of vaporizable liquid hydrocarbons can be overcome by utilizing the subject process and apparatus. It has been found, for example, that coke deposits can be avoided even during prolonged periods of operation. As is pointed out above, the cracking temperature of the hydrocarbons generally is greater than 750 C. and most often is from about 800 to 850 C. In the cooling process taking place in the quench boiler, the temperature of the cracked gas is lowered to at least 400 C., and preferably is lowered to 350 C.

It has been found that surprisingly small amounts of steam introduced in accordance with the present invention are sufficient to suppress the formation of coke deposits on the surfaces of the connecting member. The amount of steam can, of course, vary widely. It has been found, however, that from 0.02 to 1.0 metric ton per metric ton of dry cracked gas is satisfactory for most purposes. It has further been found to be advantageous to use from 0.05 to 0.2 metric ton of steam per metric ton of dry cracked gas.

The temperature of the steam supplied should not be higher than the temperature of the cracked gas upon entry into the quench boiler. It is advantageous to choose a temperature of from 110 C. to 400 C.

The velocity of the steam supplied should be high in order to avoid the formation of coke deposits. It should advantageously be less than the velocity of sound but at least 10 meters per second. A velocity of from to 300 m./sec., has proven to be of special advantage.

If W is the velocity of the cracked gas upon entry into the tapering connecting member to the quench boiler, 'y is the density of the cracked gas at this point, W3 is the velocity of the steam introduced and 72 is its density, the ratio of the back pressure lies as a rule in a particularly preferred range of from 0.01 to 2.0, particularly 0.1 to 1.0.

The steam is in general introduced in the neighborhood of the entrance of the cracked gas into the quench boiler. It may, however, also be introduced wholly or partly at other points in the connecting member. Particularly in the case of cracked gases which have been produced by indirect heating of tubes having a tube wall temperature of from more than 900 C., it is recommended that the steam supply be subdivided to two, three, or more places, for example, by introducing at least half in the vicinity of the entry of the cracked gas into the quench boiler and the remainder in the vicinity of the entry of the cracked gas into the expanded connecting member as is shown in FIGS. 3 and 3a of the drawings.

The raw material for the cracking may be, for example, saturated hydrocarbons, such as ethane, propane, butane and the like, or mixtures of liquid hydrocarbons, such as light naphtha, kerosene and the like. As was pointed out above, however, the process has special applicability for cracking vaporizable liquid hydrocarbons of the gasoline range boiling between C. and 170 C., particularly light naphtha stocks.

Cracking of these raw materials is effected in cracking tubes which are heated by burning gases in a furnace. The cracking process is carried out at about atmospheric pressure or slightly above, for example, at 14.2 to 28.4

p.s.i. and particularly about 24.0 to 28.4 p.s.i. As has been pointed out, the cracking temperature is generally above 750 C., for example, 800 to 850 C. The process according to the present invention is of particular importance for socalled short-time or high severity cracking in which cracking temperatures of from 800 C. to 850 C. and a residence time in the cracking tube of less than 0.4 second, for example, preferably from 0.3 second to 0.1 second with reference to the state of the reaction mixture at the exit of the gas from the cracking tube, are used. The connecting member between the cracking tube and the cooling tubes of the quench boiler should be kept as short as possible, from 0.3 to 2.0 meters, so that the cracked gas is distributed in the connecting member to the cooling tubes after the manner of a diffuser and free from turbulence.

The invention is further illustrated by the following example.

EXAMPLE 3,700 kg./h. of light naphtha having a boiling range of 35 C. to 170 C. is mixed with 1,850 kg./h. of steam, heated up to 600 C. and thermally cracked at temperatures which rise to 820 C. in a tube heated externally by gas flames. The residence time of the hot reaction mixture is about 0.3 second from the beginning of the cracking reaction to the exit of the cracked gas from the cracking tube.

Distribution of the cracked gas to the cooling tubes of the quench boiler is effected in a connecting member as shown in FIG. 3. The distance from the point where the gas leaves the cracking tube to the entry into the cooling tubes is 0.5 m.

5,550 kg./h. of cracked gas is obtained by the reaction. The velocity of the cracked gas at the entry into the connecting member is 300 m./sec. and the density at this point is 0.57 kg./m. 350 kg. per hour of steam at a temperature of 200 C., a velocity of m./sec. and a density of 1.0 kg./m. is supplied to these cracked gases at the points indicated in FIG. 3, 200 kg. per hour being introduced through pipe 4, and kg. per hour through pipe 6. In accordance with the formula given in the description the ratio of the back pressures may be calculated as 0.2.

" The cracked gas leaves the quench boiler 3 at a tem perature of about 350 C. and, after it has been further cooled to about 200 C. by direct cooling with liquid hydrocarbons having a boiling range of 220 C. to 250 C., it is worked up by conventional separation methods. 1,080 kg./h. of pure ethylene is thus obtained in troublefree operation.

We claim:

1. Apparatus for thermally cracking hydrocarbons to produce olefines which comprises: a cracking tube, an expanded quench member, said member including indirect cooling means, a connecting member between said cracking tube and said quench member, said connecting member having a circular cross-section, and conduit means connected to said connecting member for passing steam tangentially into said connecting member.

2. Apparatus as in claim 1 wherein said connecting member is cone shaped.

3. Apparatus as in claim 1 wherein said connecting member is cylindrical.

4. Apparatus as in claim 1 wherein the portion of said connecting member attached to said cracking tube is cone shaped and the portion attached to said quench member is cylindrical.

5. Apparatus as in claim 1 wherein more than one conduit means is connected to said connecting member for passing steam tangentially into said connecting member.

6. Apparatus as in claim 3 wherein one conduit means is connected to said connecting member for passing steam tangentially into the connecting member at a point adjacent to the cracking tube and another conduit means is connected to said connecting member for passing steam tangentially into the connecting member at a point adjacent to said quench member.

7. Apparatus as in claim 3 wherein said connecting member also includes conduit means connected to said connecting member for passing steam upwardly into said member and baffle means for deflecting said steam within said member.

8. In a process for the production of olefines by the thermal cracking of hydrocarbons at temperatures of from 750 C. to 850 C. followed by cooling of the cracked gas in a quench' boiler to a temperature below 400 C., the improvement which comprises: introducing steam at a velocity of at least meters per second tangentially into a connecting member between the cracking reactor and the quench boiler, whereby the deposition of coke on the surfaces of said connecting member is prevented.

9. A process as in claim 8 wherein the steam is supplied at the rate of 0.02 to 1.0 metric ton per metric ton of dry cracked gas.

10. A process as in claim 8 wherein the steam is supplied at the rate of 0.05 to 0.2 metric ton per metric ton of dry cracked gas.

11. A process as in claim 8 wherein the steam passed tangentially into the connecting member has a temperature of from, 110 C. to 400 C.

12. A process as in claim 8 wherein the steam is supplied to the connecting member at a velocity of from 20 to 300 meters per second.

13. A process as in claim 8 wherein the steam is supplied to the connecting member at a velocity of from 40 to 200 meters per second.

14. A process as in claim 8 wherein said steam is introduced into the connecting member adjacent to the entry point of the cracked gas into the quench boiler.

15. A process as in claim 8 wherein the steam is introduced into the connecting member at a plurality of points.

16. A process as in claim 8 wherein at least half of the steam is introduced into the connecting member at a point adjacent to the point of entry of the cracked gas into the quench boiler and the remainder is introduced adjacent to the point of entry of the cracked gas into the connecting member.

17. In a process for the production of olefines by the thermal cracking of hydrocarbons in a cracking tube at temperatures of from 750 C. to 850 C. followed by cooling of the cracked gas in a quench boiler to a temperature below 400 C., the improvement which comprises: introducing steam at a velocity of at least 10 meters per second tangentially into a cylindrical connecting member connecting said cracking tube and said quench boiler, the point of entry of the tangentially directed steam being adjacent to the entry of the cracked gas into the quench boi'er, and at the same time passing steam in a direction concurrent with the entering stream of cracked gas through the base of said connecting member through at least one tube.

18. A process as in claim 17 wherein at least half of the steam is introduced tangentially into said cylindrical connecting member, and wherein the balance of the steam is introduced in a direction concurrent with the entering stream of cracked gas through a plurality of tubes pass ing through the base of said connecting member.

19. A process as in claim 18 wherein the steam issuing from said plurality of tubes in the base of said cylindrical connecting member is deflected outwardly by bafiles arranged over the outlets of said tubes.

References Cited UNITED STATES PATENTS 1,807,457 5/1931 Weis 95 2,791,549 5/1957 Jahnig 20848 3,045,978 7/1962 Waldhofer 165-95 3,060,116 10/1962 Hardin, et al. 260683 3,073,875 1/ 1963 Braconier, et al. 260679 DELBERT E. GANTZ, Primary Examiner.

C. E. SPRESSER, Assistant Examiner.