US3743593A

US3743593A - Catalytic cracking process with maximum feed vaporization

Info

Publication number: US3743593A
Application number: US00093755A
Authority: US
Inventors: A Saxton
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1970-11-30
Filing date: 1970-11-30
Publication date: 1973-07-03
Anticipated expiration: 1990-07-03

Abstract

PETROLEUM HYDROCARBON FEEDSTOCKS IN THE GAS OIL BOILING RANGE ARE CRACKED IN A FLUIDIZED CATALYTIC REACTOR FOLLOWING ADMIXING SAID FEEDSTOCK WITH COKER EFFLUENT TO VAPORIZE THE FEEDSTOCK PRIOR TO ITS INTRODUCTION INTO THE REACTOR.

Description

July 3, 1973 A. SAXTON CATALYTIC CRACKING PROCESS WITH MAXIMUM FEED VAPORIZATION Filed Nov. 30, 1970 KN 95.3.55 f: ...o ...nim-WW X j Prinz United States Patent O U.S. Cl. 208-55 14 Claims ABSTRACT F THE DISCLOSURE Petroleum hydrocarbon feedstocks in the gas oil boiling range are cracked in a lluidized catalytic reactor following admixing said feedstock with coker effluent to vaporize the feedstock prior to its introduction into the reactor.

Optimum performance for the uid catalytic cracking reactors would be realized with an all vapor feed. If the catalyst is initially wetted by liquid high boiling feed components, any portions of the feed which do no't vaporize quickly may crack thermally rather than catalytically, thus yielding an undesirable product distribution and a coke laden catalyst.

This initial wetting is very diflicult to avoid in current typical cat cracking operations using conventional furnaces to preheat the feed. Preheat furnace tube coking considerations require limiting furnace outlet temperatures to about 800 F., at which temperature most typical cat feeds are vaporized to a very small degree, if at all.

I have developed a processing technique in which catalytic cracking feed is vaporized and catalytically cracked under optimized mixing, heat transport and heat exchange conditions. The process avoids coking problems encountered when liquid cracking feed is preheated and/ or vaporized by conventional heating in tube furnaces and heat exchangers.

Brieliy stated, the process of the linvention comprises preheating liquid cat cracker feeds with heat supplied by hot moving solids.

In the general case the heating system comprises two vessels with solids circulating continuously between the vessels in either a uidized or non-fluidized state. The circulating solids are heated in one of these vessels by direct combustion of fuel with air. Catalytic cracking feed is heated and vaporized in the second vessel by direct `contact with the circulating hot solids. Typical temperatures in these two vessels are in the range of l000 1200 F. in the combustion vessel and 800-1000 F. in the feed vaporizer/preheater vessel. The temperatures can be controlled by conventional means at any level which is required to accomplish vaporization of the catalytic cracking feed. Any coke which is formed at high temperatures (above about 800), which may be required to accomplish complete vaporization, will deposit on the circulating solids and will provide a portion of the fuel required in the combustion vessel. In conventional catalytic cracking systems this coke would be deposited either (1) in preheat furnace tubes resulting in early tube failure due to overheating or (2) on the cracking catalyst thus effectively blinding some of the active catalyst surface.

It is convenient with this improved catalytic cracking feed system to add a diluent such as steam or light hydrocarbons to the feed preheater vessel which will reduce the partial pressure of the catalytic cracking feedthereby reducing the temperature required for complete vaporization, and minimizing the amount of thermal cracking and coke formation during the feed vaporization step.

3,743,593 Patented July 3, 1973 ICC In a preferred embodiment which incorporates the above general features and accomplishes other improvements relative to present normal practice, a iluid coker system is used as a catalytic cracking feed preheater. This combined system is illustrated in FIG. 1. The total feed to this system includes heavy petroleum residuum from a vacuum pipestill and virgin gas oils produced in the primary crude distillation unit. The total catalytic cracking feed, which is vaporized in the fluid coker system, includes the above mentioned virgin gas oils, the gas oil and lighter fractions produced by thermal cracking of the vacuum residuum in the fluid coker, and recycled gas oil from the catalytic cracking unit. A light hydrocarbon gas can also be recycled from the catalytic cracking unit to serve as a diluent which will increase the yield of vaporized catalytic cracking feed at any given temperature from the coker system.

Additional advantages and details of the invention will be apparent from the following detailed description. In this most preferred embodiment, the hot solids are circulated in a uid coking unit and the heat required to produce a vaporized catalytic cracking feedstock is provided by burning coke produced from thermal cracking of the vacuum residuum. Referring to the drawing, reference numeral 1 denotes the heater vessel of a fluid coking unit and 2 denotes the coker reactor. Reference numeral 3 denotes a fluidized transferline catalytic cracking zone and 4 denotes the dense bed catalytic cracking zone. Reference numeral 5 denotes the fractionator used to separate the products of catalytic cracking.

In this embodiment a substantial proportion of the cracking feed is coker gas oil; another substantial proportion of the cracking feed is virgin gas oil. In most cases cycle oil from the cat fractionator is also included in the cracking feed.

The operation of the fluid coker is conventional except that the entire coker overhead is recovered as vaporized cat cracking feed. A typical fluid coker operation is disclosed in U.S. Pat. No. 2,881,130, issued Apr. 7, 1959, to Pfeiffer et al. A whole crude or petroleum residuum fraction is fed by line 6 to iluid coker 2. Suitable liquid coking feeds boiling in the range above about 900 F. include heavy or reduced crudes or vacuum bottoms or other heavy hydrocarbons containing a substantial amount of constituents which cannot be vaporized without decomposition. The feed is introduced into a dense turbulent uidized petroleum coke bed within the coking vessel. It is important that the liquid coking feed be quickly dispersed and distributed over the coke bed. It is a feature of this invention that recycle gas from line 7 can be mixed with the coker feed to assist in dispersing the feed into tine particles. In performing this function, the gas can replace steam required for this purpose in normal fluid coker practice. The gas also acts as a diluent and vaporizing aid throughout the preparation of the various gas oil fractions as vaporized cat cracking feed. In the fluid bed the feed is converted to solid coke and hydrocarbon vapor. The coking operation is usually carried out at relatively low pressure, such as from 0 to 50 p.s.i.g. Coking temperature may range from 850 to 1200 F., preferably 900 to ll00 F.

A stream of solids is continuously removed from the coking vessel by line 8 for transfer to coke heater 1. Air is supplied to the heater by line 9 and a portion of the coke is continuously burned to supply hot coke which is returned by line 10 to the reactor. The heater or burner vessel is operated at a temperature of 1000 to 1500 F., usually 200 to 300 F. above coking temperature. Coked materials are separated from coke in cyclone 11 and the vapor passes into scrubbing section 12. In this section, entrained ne coke particles are scrubbed from the coker eiuent vapors by direct contact with a recirculated heavy liquid hydrocarbon stream, and the heaviest portion of the coker eluent is condensed for recycling back to the Coker reactor. In a conventional coking operation, the latent heat of this recycle coker feed is recovered by pumping the above mentioned recirculated liquid stream through external heat exchangers. In this invention, this heat is recovered by vaporizing gas oil which is to be cat cracked. This gas oil feed may be introduced through line 14 as a portion of atmospheric pipestill reduced crude, or it may be gas oil which had been separated from crude oil in a prior fractionation step. It may also include cycle gas oil from cat cracking fractionator 5 passed by line 13 and introduced into line 14. In any event, according to the present invention, this vaporized gas oil, along with the total vapor products from the coking reactor (excluding the condensed coker recycle feed) is passed directly by line 15 to cat cracking transferline cracking zone 3.

Hot regenerated catalyst is fed from the catalyst return line, shown generally by reference numeral 16, into the lower most section of the transferline. This catalyst is at a temperature of ll to 1300 F. T'he feed vapors and the catalyst mix together rapidly as they move upwardly through the transferline. Although the residence time is relatively short, carbon-to-carbon bonds are broken producing such fractions as C2-C4 hydrocarbon gases, gasoline, kerosene, heating oil and the like. In the present embodiment, the transferline feeds into a dense bed cracking zone 17. Cracking is completed in this zone. Cracking temperatures in both zones range from 700 to 1000" F. and the pressure ranges from to 40 p.s.i.g. In many operations, particularly those using zeolite catalyst, the total desired conversion can be obtained in the transferline zone and the dense bed catalyst zone can be omitted. There are numerous structural arrangements for transferline cracking with or without the dense bed. Typical arrangements are shown in U.S. Pat. No. 2,902,- 432, U.S. Pat. No. 2,906,703, U.S. Pat. No. 3,123,547, U.S. Pat. No. 3,158,562 and U.'S. Pat. No. 3,355,380. Additional arrangements are disclosed in the Oil & Gas Journal, vol. 68, No. 20, May 18, 1970, at p. 84. A typical transferline lluidized cracking zone is characterized by a length to diameter ratio in the range of from about 4 to about 50.

In cyclone 18 catalyst is separated from the total cracked eluent. Spent catalyst is passed by line 19 to a regenerator, not shown. Cracked effluent passes via line 20 to fractionator 5. From the fractionator a fraction boiling below about 350 F. is recovered overhead by line 21. The fraction is cooled in condenser 22 to a temperature of about 100 F. and condensate and gas pass into separator 23. Liquid reflux is returned to the fractionator by line 24. The remainder of the liquid passes by line 25 to absorber 26. By means of a pumparound system, not shown, the liquid is used to absorb additional liquid from the gas phase. Liquid is recovered by line 27 for further treatment. Gas from separator 23 passes via line 28, compressor 29 and line 30 to absorber 26. The dry gas characterized as C2 minus gas is recovered from the absorber by line 31. Part of this gas is recovered by line 32 for use as fuel or for further treatment. The remained of the gas passes via line 7 to the coker for utilization as described herein.

Returning to the cat fractionator, a high octane gasoline fraction is recovered by line 33 and a heating oil fraction is recovered by line 34. Qfcle oil boiling in the range of from about 600 to 800 F. is passed via line 13 to the coker for vaporization in the coker.

The integrated iluid coker/ cat cracker combination disclosed herein provides an economical process for cracking gas oil boiling range feedstocks in vaporized form. The vaporized feedstock comprises virgin gas oil, cycle oil and coker overhead as a balanced cracking feed. Large savings in both onsite process equipment and offsites, and

other benefits are obtained by integration as described below.

The normal coker product fractionation and gas oompression system, and the normal cat cracking feed preheat exchangers and furnace are completely eliminated. The steam normally premixed with residuum feed to the coker and with gas oil feed to cat cracking is eliminated. The olefin and diolen content of coker products are reduced to the much lower level characteristic of cat cracked products by polymerization and hydrogen-transfer reactions in the cat cracking reactor; thus the subsequent hydrogenation required on products from the integrated coking/cat cracking process is much less than is required in separate hydrogenation of coker and cat cracker products from present non-integrated operations. The net C2 minus gas yield from the integrated process and the yield of coke from cat cracking are reduced. The reduction in cat cracking coke results in a smaller air requirement and reduced equipment size for regeneration of the catalyst (by coke burnoif). This reduction in catalytic cracking coke yield does not result in a shortage of heat for the cat cracking reaction because of the increased total heat content in the all-vapor cat cracking feed. On an overall heat balance basis, heat released from combustion of low value product coke in the coker is supplying a substantial portion of the total cat cracking process heat requirement. The coking operation can be readily adjusted to regulate the amount of heat available from combustion of product coke, as may be required by changes in the amount of virgin gas oils or of cat recycle gas oil to the ooker scrubber. In an extreme case as, for example, if the residuum feed to the coker does not yield suiiicient coke to supply the necessary heat for the coking reaction and for vaporization of extraneous cat cracker feedstocks, auxiliary liquid or gaseous fuel can be burned in the coke burner.

The foregoing discussion concerned the preferred embodiment of my invention-an integrated fluid coking and cat cracking process. The primary objective of the invention, to provide an all vapor feed to cat cracking, can be accomplished without the residuum coking step. In such a case, the total feeds available to cat cracking, including recycle from the cat fractionator, would be injected, along with recycle cat product gas, into a fluidized bed vessel corresponding to vessel 2 in the drawing. The fluidized solids-which could be coke, sand, limestone, or any other chemically and physically stable solidwould circulate through heater vessel 1, where suflicient fuel would be burned to supply the heat required to vaporize the cat cracking feed in .vessel 2. Depending on the nature of the cat cracking feed and the amount of recycle gas included with the cat feed to vessel 2 (recycle gas reduces the temperature required to vaporize the cat feed), some coke may be deposited on the solids in vessel 2. This coke would supply a portion of the fuel required in heater 1, the balance of the fuel can be any available hydrocarbon fuel. In this general case, the scrubber section 12 is not required.

By proper choice of the solids which circulate as a heat transfer medium between vessels 1 and 2, some improvement in cat feed quality can be obtained. For example, some desulfurization can be obtained by using a sulfur-acceptor such as limestone or iron oxide.

What is claimed is:

1. An integrated process for cracking hydrocarbon fractions boiling in the gas oil boiling range comprising the steps of:

(a) admixing a liquid hydrocarbon fraction boiling in the range of about 600 to 1000 F. with eluent and/ or solids of a iluid coker;

(b) passing vapor phase coked effluent from said fluid coker to a scrubbing zone;

(c) contacting said eflluent in said scrubbing zone with a heavy liquid hydrocarbon stream whereby said hydrocarbon fraction becomes vaporized to provide a vaporized cracking feedstock;

(d) passing said vaporized cracking feedstock into a ttuidized cracking zone wherein said feedstock is catalytically cracked; and

(e) recovering cracked efuent.

2. The process according to claim 1 in which said hydrocarbon fraction is selected from the group consisting of a virgin gas oil fraction, a cat cycle oil fraction and mixtures thereof.

3. The process according to claim 1 in which said hydrocarbon fraction is a virgin gas oil.

4. The process according to claim 1 in which said hydrocarbon fraction is a mixture of virgin gas oil and cat cycle oil.

5. The process according to claim 1 in which catalytic cracking is eiected in the presence of a crystalline zeolite catalyst.

6. The process according to claim 1 in which said fluidized cracking `zone is a transferline fluidized cracking zone characterized by a length to diameter ratio in the range of from about 4 to about 50.

7. An integrated process for coking of a residual hydrocarbon fraction boiling above 900 E. and for catalytic cracking of a gas oil hydrocarbon fraction boiling in the range of about 600-1000 F. comprising the steps of:

(a) subjecting said residual hydrocarbon fraction to Huid coking in a fluid coking zone at temperatures in the range of 900 to =1200 F. and pressures in the range of to 50 p.s.i.g. in the presence of noncatalytic lluid coke particles;

(c) mixing said eluent in said scrubbing zone with said gas oil hydrocarbon fraction whereby said fraction is substantially vaporized to provide a substantially vaporized catalytic cracking feedstock;

(d) passing said vaporized cracking feedstock into a transferline iuidized cracking zone in admixture with crystalline zeolite iiuid cracking catalyst;

(e) cracking said feedstock at a temperature in the range of 700 to 1000 F. and a pressure in the range of 5 to 40 p.s.i.g.;

(f) recovering cracked efuent.

8. The process according to claim 7 in which said residual hydrocarbon fraction fed to step (a) is mixed with a gas recovered from the cracked effluent of step (f) `9. A11 integrated process for cracking hydrocarbon fractions boiling in the gas oil boiling range comprising the steps of:

(a) vaporizing a liquid hydrocarbon fraction boiling in the range of about 600 to 1000 F. by contacting it with a heated effluent and/or solids of a vaporizer zone of a heating system, the heating s'ystem cornprising a discrete combustion zone in which fuel is burned with air and a separate vaporized zone, with particulate solids circulating between the ltwo zones whereby heat is transferred from said combustion zone to said vaporizer zone to thereby vaporize the cracking feedstock;

(b) passing the vaporized cracking feedstock to a scrubbing zone;

(c) contacting said vaporized cracking feedstock in said scrubbing zone with a heavy liquid hydrocarbon stream whereby entrained fine particles are removed therefrom;

(d) passing said vaporized cracking feedstock into a uidized cracking zone whereby said feedstock is catalytically cracked; and

(e) recovering cracked effluent.

10. The process of claim 1 wherein the contacting of step (c) is conducted in the presence of a diluent gas selected from the group consisting of steam or a light hydrocarbon.

11. The process of claim 10 wherein said diluent gas is a light hydrocarbon.

12. YThe process of colaim 7 wherein said mixing of step (c) is conducted in the presence of a diluent gas selected from the group consisting of steam or a light hydrocarbon.

13. The process of claim 9 wherein the contacting of step (c) is conducted in the presence of a diluent gas selected from the group consisting of steam or a light hydrocarbon.

14. The process of claim 13 wherein said diluent gas is a light hydrocarbon.

References Cited UNITED STATES PATENTS 2,731,394 l/l956 Adams et al. 208-55 3,382,188 5/1968 Cornelius et a1. 208-120 2,923,678 2/ 1960 Schutte 208-155 2,655,465 10/1953 Martin 208-55 2,763,600 9/ 1956 (Adams et al. 208-55 2,393,636 1/1946 Johnson 208-55 JAMES E. POER, Primary Examiner A. P. DEMERS, Assistant Examiner U.S. Cl. X.R.