US3725242A

US3725242A - Cracking hydrocarbon residua to coke and aromatic gas oil

Info

Publication number: US3725242A
Application number: US00106278A
Authority: US
Inventors: G Hamner; R Mason
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1971-01-13
Filing date: 1971-01-13
Publication date: 1973-04-03
Anticipated expiration: 1990-04-03

Abstract

COKE AND A GAS OIL FRACTION BOILING BETWEEN 400 AND 10500*F. ARE PRODUCED BY HYDRODEPOLYMERIZING A HYDROCARBON RESIDUA. IN PARTICULAR A STEAM CRACKED TAR HAVING A BOILING POINT ABOVE 6500*F., WITH A SUBSTANTIAL AMOUNT OF THE TAR HAVING A BOILING POINT ABOVE 10050*F., IS MIXED IN SPECIFIC PROPORTIONS WITH A GAS OIL HAVING A BOILING POINT BETWEEN 400 AND 650*F. AND AN ACYCLIC HYDROCARBON MODIFIER IN A REACTION ZONE OPERATING AT A TEMPERATURE BETWEEN 775 AND 900*F. AND AT A PRESSURE BETWEEN 50 AND 1000 P.S.I.G. TO MAXIMIZE THE AMOUNT OF COKE PRODUCED.

Description

United States Patent No Drawing. Continuation-impart of application Ser. No.

840,814, July 10, 1969. This application Jan. 13, 1971, Ser. No. 106,278

Int. Cl. Cg 37/06 US. Cl. 208-46 Claims ABSTRACT OF THE DISCLOSURE Coke and a gas oil fraction boiling between 400 and 1050 F. are produced by hydrodepolymerizing a hydrocarbon residua. In particular a steam cracked tar having a boiling point above 650 F., with a substantial amount of the tar having a boiling point above 1050 F., is mixed in specific proportions with a gas oil having a boiling point between 400 and '650" F. and an acyclic hydrocarbon modifier in a reaction zone operating at a temperature between 775 and 900 F. and at a pressure between 50 and 1000 p.s.i.g. to maximize the amount of coke produced.

RELATED APPIIICATIONS This is a continuation-in-part of Ser. No. 840,814, filed July 10, 196-9, now abandoned.

BAOKGROU ND OF THE INVENTION This invention relates to hydrodepolymerizing a hydrocarbon residua to produce coke and a gas oil fraction boiling between 400 and 1050 F.

It is expected that steam cracking facilities will expand in the future, particularly in Europe. This will require means for easily disposing of the considerable amounts of tar which are a concomitant part of the steam cracking process. One obvious method is to up grade these tars as well as other residuas by thermally treating the tars with a hydrogen donor diluent material. The donor diluent is a hydrogen-containing material, aromaticnaphthenic in nature, that has the ability to take up hydrogen in a hydrogenation zone and readily release it to a hydrogen-deficient oil in a thermal cracking zone. Unfortunately, however, there is often undesired coke deposition and creation of hot spots in preheater zones, leading to plugging of the equipment.

In pending application Ser. No. 29,629, filed Apr. 17, 1970 for Ralph B. Mason and Glen P. Hamner there is described and claimed a process for treating hydrocarbon residua, a particular steam cracked tar, to produce primarily an aromatic gas oil. However, under the process of this invention very little coke is produced.

It is an object of this invention to produce a low sulfur content coke product by hydrodepolymerizing a hydrocarbon residua such as steam cracked tar.

SUMMARY OF THE INVENTION It has now been found that coke yield can be maximized when desired by mixing in a reaction zone 50 to 80 weight percent of a hydrocarbon material having a boiling point above 650 F. (heavy gas oil+hydrocarbon residua fraction) with to 50 weight percent of a hydro carbon material having a boiling point between 400 and 650 F. (light gas oil) together with 1 to weight percent of an acyclic hydrocarbon modifier wherein the reaction zone is operated at a temperature between 775 and 900 F., and a pressure between 50 and 1000 p.s.i.g.

In a preferred embodiment of this invention the noncoke product produced in the above process is separated into a hydrocarbon fraction having a boiling point be- 7 3,725,242 Patented Apr. 3, 1973 tween 400 and 650 F., a hydrocarbon fraction having a boiling point between 650 and 1050 F. and a hydrocarbon fraction having a boiling point above 1050 F. so that the hydrocarbon fraction having a boiling point above 1050 F. may be recycled to the reaction zone along with that portion of the hydrocarbon fraction having a boiling point between 400 and 650 F. necessary to maintain the 20 to 50 weight percent in the reaction zone, and either recovering or recycling the hydrocarbon fraction having a boiling point between 650 and 1050 F.

PREFERRED EMBODIMENTS OF THE INVENTION In accordance with the present. invention a hydrocarbon residua having a boiling point above 650 F. with a substantial amount, preferably more than 30 weight percent, of the residua having a boiling point above 1050" F. is mixed with a gas oil, then introduced into a reaction or depolymerizing zone where the mixture is contacted with an acyclic hydrocarbon modifier under specified conditions. Alternatively, all of the above could be mixed together at the same time in the reaction or depolymerizing zone instead of as above.

Suitable hydrocarbon residua would include thermal tar from steam cracking, catalytic clarified oil, reduced crude, shale oil residua, liquefied coal fractions and other similar materials, but most preferably steam cracked tar, catalytic clarified oil or other hydrocarbon residua which have an aromatic content greater than about 70 weight percent since these residua are highly hydrogen deficient. Because these materials are generally too viscous to be handled easily, it is necessary to add a solvent such as gas oil to reduce the viscosity. This permits easy pumping at moderate temperatures and prevents coking at hot spots in the system. It is preferred that a light gas oil (i.e., a gas oil having a boiling point between 400 and 650 F.) be used as the solvent, but heavy gas oils (i.e., gas oil having a boiling point between 650 and 1050 F.) may also be used, provided that there is 20 to 50 weight percent, based on the feed to the reaction zone, light gas oil in the mixture.

Since it is desired to maximize the coke yield, it is preferred that only enough light gas oil is needed as a solvent be used since the higher boiling range fractions contain more Conradson carbon or coke precursors. Therefore, 20 to 35 weight percent of light gas oil is preferred.

This mixture of gas oil and hydrocarbon residua is then maintained in a liquid phase in a reaction zone operating at a temperature between 775 to 900 F., preferably 780 to 800 F., and at a pressure between 50 and 1000 p.s.i.g. An acyclic hydrocarbon modifier is jetted, sprayed or otherwise thoroughly mixed and passed through the liquid in the reaction done.

Suitable acyclic hydrocarbon modifiers would include parafiins of 4 to 10 carbon atoms per molecule or their isomers which have boiling points less than 350 F. Preferred examples of such modifiers are heptane and its isomers; hexane and its isomers; n-pentane; and 2,4,4- trimethyl pentane because of the valuable by-products produced when they are used. When n-heptane is the modifier n-butane, n-pentaneand n-hexane are the valuable by-products formed. On the other hand, if isoheptane is the modifier iso-butane, iso-pentane and isohexane are formed. If n-hexane is mixed in the reaction zone n-pentane and n-b-utane result. Likewise if isohexane is the modifier iso-pentane and iso-butane is formed. When n-pentane is used n-b-utane is formed. The

use of 2,2,4-trimethyl pentane unexpectedly results in the production of the important blending agent, triptane.

Other suitable modifiers are olefins and diolefins of similar skeletal configuration, heavy alkylates and the 210-400 F. fraction made by polymerizing propylene and butylene with H PO on kieselguhr [Hydrocarbon Processing, v. 47:170 (September 1968)], and the like.

Without intending to limit the invention to any area of what occurs, it is believed that the mechanism is one in which the modifier is being consumed with accompanying hydrogen exchange, demethanation, alkylation, isomerization, aromatic disproportionation and probably every known hydrocarbon reaction. The most plausible explanation is a free radical mechanism in which the condensed ring aromatic compounds of the tar depolymerize with the formation of free radicals which attach themselves to the modifier as a sink. In doing so the modifier in turn forms free radicals involving stepwise degradation and rearrangement reactions leading to gaseous products and coke.

It has been found that to maximize coke yield these modifiers must be added in amounts of about 1 to 20 weight percent based on the hydrocarbon residua, preferably between about 5 and weight percent. Under the operating conditions the hydrocarbon modifier remains in the vapor phase and has a residence time of between 5 and 60 minutes on through the liquid in the reaction zone. During the time the modifier remains in the reaction zone a portion of it is consumed and is the major source of the coke formed which remains suspended in the liquid. Thus increasing the residence time aids in this conversion to coke. For this reason a temperature of about 780 F. and a residence time of about 60 minutes is preferred. If the temperature is increased, then the residence time must be decreased in order to maintain maximum coke yield. If the residence time is increased, the temperature is decreased.

It has also been found that small amounts of oxygen added to the modifier as it enters the reaction zone results in an increase in the amount of coke produced. The amount of oxygen added is very small, generally on the order of less than 100 parts per million of fresh tar feed and preferably less than 50 parts per million. At the preferred temperature of about 780 F. 25 parts per million of oxygen should be added.

In a preferred embodiment of the invention the products produced in the reaction zone are removed after an appropriate time and introduced into a fractionator where the product is separated into a fraction containing the modifier, a hydrocarbon fraction having a boiling point between 400 and 650 F., a hydrocarbon fraction having a boiling point between 650 and 1050 F., hydrocarbon fraction having a boiling point above 1050 F. and coke.

The modifier and the hydrocarbon fraction boiling above 1050 F. are recycled to the reaction zone along with that portion of the hydrocarbon fraction boiling between 400 and 650 F. necessary to maintain the to 50 weight percent of such fraction going into the reaction zone. The remaining portion of the hydrocarbon fraction having a boiling point between 400 and 650 F., along with the coke and light products boiling below 400 F. may be recovered. The hydrocarbon fraction having a boiling point between 650 and 1050 F. may be either recycled to the reaction zone or recovered as desired.

The following example is included to illustrate the effectiveness of the instant process for the depolymerization of tars, however, without limiting the same.

EXAMPLE 1 A steam-cracked tar consisting of 35.7% material, boiling 430-650 F., 34.3% boiling 650-1050 F., and 30% boiling 1050 F.+ was subjected to two different cycles of hydrodepolymerization for four hours each at different temperatures, one at 765F., and the other at 775 F.

Tar feed, grams 456. 2 394. 3 Wt. percent tar 32. 8 32. 8 Wt. percent, 650 F.recycle- 34. 0 34. 0 Wt. percent 650F.+recycle 33. 2 33. 2

n-Heptaue added to charge, grams 50. 1 50. 1

Operating conditions:

Avg. temp., F 765 775 Hours of run 1 2 4 Pressure, p.s

Maximum 1, 400 2, 000

Recoveries, grams:

n-Ileptanc 35. 4 30. 1 Liquid and gas ex. n-heptane:

C -gas l2. 7 23. 5 C 22l F. cut. 2. 5 3. 4 221650 F. cut. 2 260. 1 2 213.5 650 F.+p0rti0n 186. 1 125.5 Coke 14. 4 33. 0

1 Autoclave was pressured to 1,000 p.s.i.g. with nitrogen prior to run.

2 Value does not contain small correction for possibly entrained 430-650 F. vacuum flashing of tetrallne. Hence recoveries are not considered complete and material balance not calculated.

The above data show that increasing the temperature increases the coke make.

The present invention having thus been fully set forth and illustrated and specific examples of the same given, what is claimed as new, useful and unobvious is:

1. A non-catalytic thermal process for producing coke and aromatic gas oil which comprises:

(a) introducing into a reaction zone 50 to wt. percent of a steam cracked aromatic tar, 20 to 50 wt. percent of a gas oil hydrocarbon fraction having a boiling point between 400 and 650 F., and 1 to 25 wt. percent of an acyclic hydrocarbon modifier boiling below said gas oil hydrocarbon fraction;

(b) operating the reaction zone at a temperature between 775 and 900 F., and a pressure between 50 and 1000 p.s.i.g. to produce coke and aromatic gas oil, and

(c) recovering the coke and the aromatic gas oil.

2. The process of claim 1, wherein less than p.p.m. of oxygen based on said aromatic tar is added to the modifier before introducing in the reaction zone.

3. The process of claim 1, wherein a non-oxidizing gas is present in said reaction zone.

4. The process of claim 3, wherein said non-oxidizing gas is hydrogen.

5. The process of claim 1, wherein said modifier remains in the vapor phase in said reaction zone for a residence time of about 5 to 60 minutes.

6. The process of claim 1, wherein the modifier is a paraflin or isoparaffin hydrocarbon of 4 to 10 carbon atoms per molecule and wherein 5 to 20 wt. percent of the modifier is introduced into the reaction zone.

7. The process of claim 6, wherein the modifier is nheptane or its isomers.

8. The process of claim 6, wherein the modifier is nhexane or its isomers.

9. The process of claim 6, wherein the modifier is n-pentane.

10. The process of claim 6, wherein the modifier is 2,2,4-trimethyl pentane.

11. The process of claim 1, wherein the modifier is an olefin or iso-olefin hydrocarbon of 2 to 10 carbon atoms per molecule and wherein 5 to 20 wt. percent of the modifier is introduced into the reaction zone.

12. A non-catalytic thermal process for producing coke and a gas oil hydrocarbon fraction boiling between 400 and 650 E, which comprises:

(a) introducing into a reaction zone 50 to 80 wt. percent of a steam cracked aromatic tar, 20 to 50 wt. percent of a gas oil hydrocarbon fraction boiling between 400 and 650 F., and 1 to 25 wt. percent of an acyclic hydrocarbon modifier boiling below said gas oil hydrocarbon fraction;

(b) operating the reaction zone at a temperature between 775 and 900 F., and a pressure between 50 and 1000 p.s.i.g. to obtain a reacted mixture;

(c) removing a portion of said reacted mixture from the reaction zone;

(d) separating said portion of reacted mixture to obtain coke, a gas oil hydrocarbon fraction having a boiling point between 400 and 650 R, an acyclic hydrocarbon modifier boiling below said gas oil hydrocarbon fraction, and a hydrocarbon fraction having a boiling point above 650 F.;

(e) recycling to the reaction zone the acyclic hydrocarbon modifier and the hydrocarbon fraction having a boiling point above 650 F. along with the necessary amount of the gas oil hydrocarbon fraction having a boiling point between 400 and 650 F. to insure the 20 to 50 wt. percent needed in the reaction zone; and

(f) recovering the coke and the remaining portion of the hydrocarbon fraction having a boiling point between 400 and 650 F.

13. A non-catalytic thermal process for producing coke, a light gas oil fraction having a boiling point between 400 and 650 F. and a heavy gas oil fraction having a boiling point between 650 and 1050" R, which comprises:

(a) introducing into a reaction zone 50 to 80 wt. percent of a steam cracked aromatic tar, 20 to 50 wt. percent of a light gas oil hydrocarbon fraction having a boiling point between 400 and 650 F., and 1 to 25 wt. percent of an acyclic hydrocarbon modifier boiling below said light gas oil hydrocarbon fraction;

(b) operating the reaction zone at a temperature between 775 and 900 F. and a pressure between 50 and 1000 p.s.i.g. to obtain a reacted mixture comprising coke, distillable hydrocarbon products, heavy residua-like hydrocarbon products and acyclic hydrocarbon modifier;

(c) removing a portion of said reacted mixture from the reaction zone;

(d) separating said portion of reacted mixture to obtain coke, a light gas oil fraction having a boiling point between 400 and 650 R, an acyclic hydro carbon modifier boiling below said light gas oil hydrocarbon fraction, a heavy gas oil hydrocarbon fraction having a boiling point between 650 and 1050 F. and a hydrocarbon fraction having a boiling point above 1050 F.;

(e) recycling to the reaction zone the hydrocarbon fraction having a boiling point above 1050" F. and the acyclic hydrocarbon modifier together with that portion of the hydrocarbon fraction having a boiling point between 400 and 650 F. necessary to maintain the 20 to Wt. percent in the reaction zone; and

(f) recovering the coke and the remainder of the hydrocarbon fraction having a boiling point between 400 and 650 F. and the hydrocarbon fraction having a boiling point between 650 and 1050 F.

14. The process of claim 13, wherein said modifier is a parafiin or isoparafiin hydrocarbon of 4 to 10 carbon atoms per molecule and wherein 5 to 20 wt. percent of the modifier is introduced into the reaction zone.

15. The process of claim 13, wherein said modifier is an olefin or iso-olefin hydrocarbon of 2 to 10 carbon atoms per molecule and wherein 5 to 20 wt. percent of the modifier is introduced into the reaction zone.

References Cited UNITED STATES PATENTS 2,717,865 9/1955 Kimberlin et al. 208- 3,310,484 3/1967 Mason et a1. 208125 3,352,776 11/1967 Mason et a1. 208-125 HERBERT LEVINE, Primary Examiner US. Cl. X.R. 208-50