Two stage hydrocracking process

Details Two stage hydrocracking process Two stage hydrocracking process

1999-08-16

2001-04-17

Myers, Helane (Department: 1764)

Mineral oils: processes and products

Chemical conversion of hydrocarbons

Plural serial stages of chemical conversion

C208S058000, C208S100000

Reexamination Certificate

active

06217746

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a hydrocarbon conversion process referred to in the art as hydrocracking. The process is used commercially in petroleum refineries to reduce the average molecular weight of heavy or middle fractions of crude oil. The invention more directly relates to an integrated hydrotreating/hydrocracking process which has a specific makeup hydrogen flowpath.
BACKGROUND OF THE INVENTION
Large quantities of petroleum-derived hydrocarbons are converted into higher value hydrocarbon fractions used as motor fuel by a refining process referred to as hydrocracking. In this process the heavy feed is contacted with a fixed bed of a solid catalyst in the presence of hydrogen at conditions of high temperature and pressure which result in a substantial portion of the feed molecules being broken down into molecules of smaller size and greater volatility. The high economic value of petroleum fuels has led to extensive development of both hydrocracking catalysts and the related process technology.
Raw petroleum fractions contain significant amounts of organic sulfur and nitrogen. The sulfur and nitrogen must be removed to meet modern fuel specifications. Removal or reduction of the sulfur and nitrogen is also beneficial to the operation of a hydrocracking reactor. The sulfur and nitrogen is removed by a process referred to as hydrotreating in which the organic sulfur and nitrogen is converted to hydrogen/sulfide and ammonia. Due to the similarity of the process conditions employed in hydrotreating and hydrocracking the two processes are often integrated into a single overall process unit having separate sequential reactors dedicated to the two reactions and a common product recovery section.
RELATED ART
Both hydrotreating and hydrocracking are widely practiced commercial processes. The very significant economic utility of the hydrocracking process has resulted in a large effort devoted to the improvement of the process and to the development of better catalysts for use in the process. A general review and classification of different hydrocracking process flow schemes and a description of hydrocracking catalysts is provided at pages 174-183 of the book entitled,
Hydrocracking Science and Technology
authored by Julius Scherzer and A. J. Gruia published in 1996 by Marcel Dekker, Inc. FIGS. 10.2, 10.3 and 10.4 show hydrotreating reactors upstream of the hydrocracking reactor. As noted therein it is an established practice to first pass a hydrocracking unit feed stream into a hydrotreating reactor in order to reduce the level of sulfur and nitrogen tied up in the target petroleum molecules. Two hydrocracking reaction zones may be used in series with some form of intermediate separation between the hydrocracking zones to reduce the amount of hydrogen sulfide and product hydrocarbons carried over to the second hydrocracking zone with the hydrocarbon phase. This type of unit is normally referred to a two stage hydrocracking unit as shown by FIGS. 10.4 and 10.5.
The high pressures employed in hydrocracking have prompted efforts to conserve the pressure of any portion of the hydrocracking effluent which is to be recycled and to also employ reductions in pressure as a separation mechanism in the product recovery section of the process. The effluent of a high pressure reactor such as a hydrocracking reactor therefore typically flows into a vessel referred to as a high pressure separator (HPS), which operates at a pressure close to the outlet pressure of the reaction zone. The vapor stream recovered from the HPS is often the recycle gas or the precursor of the hydrogen-rich gas stream recycled to the reactors.
The normal practice in hydrocracking processes is to employ a multistage compressor or bank of compressors to pressurize the makeup hydrogen stream and another compressor to pressurize the recycle gas stream. This use of two different compressors is shown for instance in U.S. Pat. No. 4,197,184.
The art also includes the adsorptive treatment of liquid-phase hydrocarbon recycle streams in a hydrocracking process to remove polynuclear aromatic (PNA) compounds as shown by U.S. Pat. Nos. 4,447,315 and 5,190,633.
SUMMARY OF THE INVENTION
The invention is a two stage hydrocracking process characterized in part by a novel hydrogen flow. The entire makeup hydrogen stream enters the process via the second stage hydrocracking reactor, which is operated at a low enough pressure to employ gas from the second stage of a three stage makeup gas compressor. The vapor recovered from the second stage reactor is fed into the third stage of the compression zone. The low pressure in the second stage hydrocracking reaction zone has been found to aid cracking paraffinic hydrocarbons not cracked in the first stage hydrocracking reactor. Thus the preferred second stage operating conditions interact synergistically with the process flow.
A broad embodiment of the invention may be characterized as a two stage hydrocracking process, which process comprises passing hydrogen and a feed stream comprising hydrocarbons having boiling points above 700° F. into a first hydrocracking zone operated at hydrocracking conditions including a first pressure and containing a hydrocracking catalyst and producing a first hydrocracking zone effluent stream comprising hydrogen, hydrogen sulfide, unconverted feed components and product hydrocarbons; separating the first hydrocracking zone effluent to yield a recycle gas stream and a first liquid process stream, which liquid process stream is passed into a product fractionation zone producing a distillate product stream and a bottoms stream comprising unconverted feed components; passing the bottoms stream and a makeup hydrogen gas stream into a second hydrocracking zone operated at paraffin selective hydrocracking conditions which include a lower second pressure, and producing a second hydrocracking zone effluent stream; separating the second hydrocracking zone effluent stream into a vapor phase stream and a liquid phase stream and passing the liquid phase stream into the product fractionation zone; and compressing the vapor phase stream and passing the vapor phase stream into the hydrotreating reaction zone as a makeup gas stream. In this embodiment the first hydrocracking zone may contain hydrotreating catalyst as a separate bed or reactor and is preferably operated at a pressure at least 300 psi above the pressure in the second hydrocracking zone.


REFERENCES:
patent: 4197184 (1980-04-01), Munro et al.
patent: 4447315 (1984-05-01), Lamb et al.
patent: 5190633 (1993-03-01), Fetzer et al.
Scherzer, Julius et al. “Hydrocracking Science and Technology”, Marcel Dekker, Inc. (1996) pp. 174-183.

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