Mineral oils: processes and products – Chemical conversion of hydrocarbons – Cracking
Reexamination Certificate
2000-11-08
2002-08-20
Dang, Thuan D. (Department: 1764)
Mineral oils: processes and products
Chemical conversion of hydrocarbons
Cracking
C208S107000, C208S142000, C208S143000, C208S144000, C208S145000, C208S153000, C208S157000, C208S209000, C208S25100H, C208S25400R
Reexamination Certificate
active
06436279
ABSTRACT:
BACKGROUND OF THE INVENTION
Hydrocarbon compounds are useful for a number of purposes. In particular, hydrocarbon compounds are useful, inter alia, as fuels, solvents, degreasers, cleaning agents, and polymer precursors. The most important source of hydrocarbon compounds is petroleum crude oil. Refining of crude oil into separate hydrocarbon compound fractions is a well-known processing technique.
Generally speaking, a refinery receives the incoming crude oil and produces a variety of different hydrocarbon products in the following manner. The crude product is initially introduced to a crude tower, where it is separated into a variety of different components including naphtha, diesel, and atmospheric bottoms (those that boil above 650° F.).
The atmospheric bottoms from the crude tower is thereafter sent for further processing to a vacuum still, where it is further separated into a heavy vacuum residue stream (e.g. boiling above 1000° F.) and vacuum gas oil (VGO) stream (boiling between 650° F. and 1000° F.). At this point the heavy vacuum residue product can be further treated to remove unwanted impurities or converted into useful hydrocarbon products.
Likewise, the VGO stream is further processed in order to yield a usable hydrocarbon product. This further processing may comprise some conversion of the VGO feedstock to diesel (boiling between 400° F. and 650° F.) as well as some cleaning hydrotreatment prior to its final processing in a Fluid Catalytic Cracker (“FCC”) Unit, where it is converted into gasoline and diesel fuels.
It is at this point in the overall refinery, the hydrotreatment/hydrocracking of the VGO stream, which is the subject of the invention. As mentioned above, hydroprocessing or hydrotreatment to remove undesirable components from hydrocarbon feed streams is a well-known method of catalytically treating such heavy hydrocarbons to increase their commercial value.
More particularly, the aim of such treatment of these hydrocarbon feedstocks, particularly petroleum vacuum gas oil, may include hydrodesulfurization (HDS), carbon residue reduction (CRR), nitrogen removal (HDN), and specific gravity reduction. Additionally, such hydrocarbon streams may be hydrocracked to convert the feedstream into other lighter valuable products.
“Heavy” hydrocarbon liquid streams, and particularly heavy vacuum gas oils and deasphalted oils (DAO), generally contain product contaminants, such as sulfur, and/or nitrogen, metals and organometallic compounds which tend to deactivate catalyst particles during contact by the feedstream and hydrogen under hydroprocessing conditions. Such hydroprocessing conditions are normally in the temperature range of between 212° F. to 1200° F. (100° to 650° C.) and at pressures of from 20 to 300 atmospheres.
Generally such hydroprocessing is conducted in the presence of a catalyst containing group VI or VIII metals such as platinum, molybdenum, tungsten, nickel, cobalt, etc., in combination with various other porous particles of alumina, silica, magnesia and so forth having a high surface to volume ratio. More specifically, catalyst utilized for hydrodemetallation, hydrodesulfurirzation, hydrodenitrification, hydrocracking etc., of heavy vacuum gas oils and the like are generally made up of a carrier or base material; such as alumina, silica, silica-alumina, or possibly, crystalline aluminosilicate, with one more promoter(s) or catalytically active metal(s) (or compound(s) plus trace materials. Typical catalytically active metals utilized are cobalt, molybdenum, nickel and tungsten; however, other metals or compounds could be selected dependent on the application.
Additionally, in a modern petroleum refinery, the down-time for replacement or renewal of catalyst must be as short as possible. Further, the economics of the process will generally depend upon the versatility of the system to handle feed streams of varying amounts of contaminants such as sulfur, nitrogen, metals and/or organometallic compounds, such as those found in a vacuum gas oils and DAO's.
Hydrogenating processes treat the charge in the presence of hydrogen and suitable catalysts. The commercial hydroconversion technologies presently on the market use fixed-bed or ebullated-bed reactors with catalysts generally consisting of one or more transition metals (Mo, W, Ni, Co, etc.) supported on alumina (or equivalent material).
The decision to utilize a fixed-bed or ebullated-bed reactor design is based on a number of criteria including type of feedstock, desired conversion percentage, flexibility, run length, product quality, etc. From a general standpoint, the ebullated-bed reactor was invented to overcome the plugging problems with fixed-bed reactors as the feedstock becomes heavier and the conversion (of vacuum residue) increases. In the ebullated-bed reactor, the catalyst is fluid, meaning that it will not plug-up as is possible in a fixed-bed. The fluid nature of the catalyst in an ebullated-bed reactor also allows for on-line catalyst replacement of a small portion of the bed. This results in a high net bed activity, which does not vary with time.
More specifically, fixed-bed technologies have considerable problems in treating particularly heavy charges containing high percentages of heteroatoms, metals and asphaltenes, as these contaminants cause the rapid deactivation of the catalyst and subsequent plugging of the reactor. One could utilize numerous fixed-bed reactors connected in series to achieve a relatively high conversion of such heavy vacuum gas oil or DAO feedstocks, but such designs would be costly and, for certain feedstocks, commercially impractical.
Therefore, as mentioned above, to treat these charges, ebullated-bed technologies have been developed and sold, which have numerous advantages in performance and efficiency, particularly with heavy crudes. This process is generally described in U.S. Pat. No. Re 25,770 to Johanson, incorporated herein by reference.
The ebullated-bed process comprises the passing of concurrently flowing streams of liquids or slurries of liquids and solids and gas through a vertically cylindrical vessel containing catalyst. The catalyst is placed in motion in the liquid and has a gross volume dispersed through the liquid medium greater than the volume of the mass when stationary. This technology is utilized in the upgrading of heavy liquid hydrocarbons or converting coal to synthetic oils.
A mixture of hydrocarbon liquid and hydrogen is passed upwardly through a bed of catalyst particles at a rate such that the particles are forced into motion as the liquid and gas pass upwardly through the bed. The catalyst bed level is controlled by a recycle liquid flow so that at steady state, the bulk of the catalyst does not rise above a definable level in the reactor. Vapors, along with the liquid which is being hydrogenated, pass through the upper level of catalyst particles into a substantially catalyst-free zone and are removed at the upper portion of the reactor.
In an ebullated-bed process, the substantial amounts of hydrogen gas and light hydrocarbon vapors present rise through the reaction zone into the catalyst-free zone. Liquid is both recycled to the bottom of the reactor and removed from the reactor as net product from this catalyst-free zone. Vapor is separated from the liquid recycle stream before being passed through the recycle conduit to the recycle pump suction. The recycle pump (ebullating pump) maintains the expansion (ebullation) of the catalyst at a constant and stable level. Gases or vapors present in the recycled liquid materially decrease the capacity of the recycle pump as well as reduce the liquid residence time in the reactor and limit hydrogen partial pressure.
Reactors employed in a catalytic hydrogenation process with an ebullated-bed of catalyst particles are designed with a central vertical recycle conduit which serves as the downcomer for recycling liquid from the catalyst-free zone above the ebullated catalyst bed to the suction of a recycle pump to recirculate the liquid through the catalytic reaction zone. Alte
Axens North America, Inc.
Dang Thuan D.
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