Chemistry of hydrocarbon compounds – Saturated compound synthesis – By condensation of a paraffin molecule with an olefin-acting...
Reexamination Certificate
1999-12-30
2002-05-21
Dang, Thuan D. (Department: 1764)
Chemistry of hydrocarbon compounds
Saturated compound synthesis
By condensation of a paraffin molecule with an olefin-acting...
C585S712000, C585S713000, C585S727000
Reexamination Certificate
active
06392114
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to the alkylation of hydrocarbons to produce useful chemicals and motor fuel. This invention specifically relates to a process for producing motor fuel blending components by alkylating paraffins with olefins using a solid catalyst, which is regenerated in the presence of hydrogen.
BACKGROUND OF THE INVENTION
Hydrocarbon alkylation is widely used in the petroleum refining and petrochemical industries to produce a variety of useful acyclic and cyclic hydrocarbon products that are consumed in motor fuel, plastics, detergent precursors, and petrochemical feedstocks. Alkylation comprises reacting an alkylation substrate feedstock such as isobutane and benzene with an alkylation agent feedstock such as C
2
-C
22
olefins. For example, large amounts of paraffins for high-octane gasoline are produced by the alkylation of isobutane with butenes. In addition, valuable aromatic hydrocarbons including cumene, ethylbenzene, and C
16
-C
22
linear alkylaromatics are produced in large amounts by alkylating benzene with olefins of the appropriate carbon number. The variety of feedstock alkylation substrates and alkylation agents and the passage of time has led to the development of a number of effective alkylation technologies which are employed in large scale commercial facilities. Much of the installed base of alkylation capacity uses liquid phase hydrofluoric acid, generally referred to as HF, as the catalyst.
FIGS. 1.4.3 and 1.4.4 of the book entitled
Handbook of Petroleum Refining Processes
, edited by Robert A. Meyers, Second Edition, McGraw-Hill, New York, 1997, show process flow diagrams of HF alkylation processes, including the product recovery facilities for recovering the hydrocarbons in the alkylation reactor effluent. Referring to these figures, the hydrocarbon phase, which contains alkylate, isobutane, some propane, and dissolved HF, flows from the acid settler, is preheated, and passes to a fractionation column, which is commonly called an “isostripper.” The hydrocarbon phase effluent from the reactor section enters at a feed tray near the top of the isostripper so that the isostripper consists mostly of a stripping section, except for a small rectification section on the top of the isostripper. The stripping section strips the more volatile HF, propane, and isobutane from the descending liquid alkylate, and product alkylate is recovered from the bottom of the isostripper. A bottom reboiler and one or more side reboilers add heat to the isostripper. When applicable, saturate field butane feed comprising isobutane and normal butane is fed to the stripping section of the isostripper at a tray below the reactor effluent feed tray, and any normal butane that may have entered the process is withdrawn from a sidedraw tray located below the field butane feed tray. Unreacted recycle isobutane is also withdrawn as a sidedraw, via a tray located between the reactor effluent and field butane feed trays. The rectification section reduces the concentration of the less volatile alkylate in the overhead vapor stream and thereby provides for efficient rejection of propane from the process. The overhead stream, which contains isobutane, propane, and HF, is condensed in an overhead condenser and collects in an overhead receiver. A drag stream of condensed overhead material undergoes further processing and separation in order to prevent an accumulation of propane in the process and to recycle isobutane and HF.
The use of HF in these motor fuel and detergent processes has a long record of highly dependable and safe operation. However, the potential damage from an unintentional release of any sizable quantity of HF and the need to safely dispose of some byproducts produced in the process has led to an increasing demand for alkylation process technology which does not employ liquid phase HF as the catalyst. U.S. Pat. No. 5,672,798, for example, discloses alkylating paraffinic hydrocarbons such as isobutane with olefinic hydrocarbons such as propylene or butenes in a fluidized riser-reactor using a solid catalyst. The effluent of the riser-reactor comprises the desired alkylate product, byproducts of the alkylation reaction, unreacted isobutane, and solid catalyst. The solid catalyst is separated and the remainder of the riser-reactor effluent passes to product recovery facilities.
Numerous solid alkylation catalysts have been described in the open literature. The previously cited U.S. Pat. No. 5,672,798 teaches a number of suitable solid catalysts that contain or have been treated with a Lewis acid, such as a large pore zeolite and a Lewis acid such as boron trifluoride and aluminum chloride, a large pore crystalline molecular sieve and a gaseous Lewis acid, a crystalline transition alumina treated with a Lewis acid, an acid washed silica treated with antimony pentafluroride, and a refractory inorganic oxide impregnated with a monovalent cation whose bound surface hydroxyl groups have been at least partially reacted with a Friedel-Crafts metal fluoride, chloride, or bromide.
These catalysts appear to suffer from slight but significant halogen loss rates when used at commercially useful alkylation reactor conditions. While some catalysts have a sufficiently useful halogen retention to allow the performance of alkylation, the gradual depletion of halogen results in a change in product composition and also requires the occasional replenishing of the halogen content of the catalyst. Some of the halogen loss is believed to be caused by the stripping of halogen from catalytically active sites of the catalyst by isobutane and also by the deposition on the catalytically active sites of heavy compounds. As used herein, the term “heavy compounds” means molecules that have at least one carbon atom more than the number of carbon atoms than the highest number of carbon atoms of those molecules that are desired to be in the alkylate.
However, in addition to exhibiting halogen loss, these catalysts also seem to suffer from unacceptably high deactivation rates when employed at commercially feasible conditions. While some catalysts have a sufficiently useful lifetime to allow the performance of alkylation, the rapid change in activity results in a change in product composition and requires the periodic regeneration of the catalyst. Such periodic regeneration is typically accomplished by removing deactivated catalyst from the reaction zone, reactivating the catalyst in a separate zone, and returning the reactivated catalyst to the reaction zone. Some of the deactivation is believed to be caused by the deposition of heavy compounds on the catalytically active sites of the catalyst.
Continuous processes for alkylation that are not subject to periodic reaction zone stoppages or variation in the product stream composition are desirable, and the previously mentioned U.S. Pat. No. 5,672,798 describes such a process. In order to remove the heavy hydrocarbon deposits and at least partially restore the activity of the catalyst, U.S. Pat. No. 5,672,798 teaches contacting the catalyst within the process with hydrogen in two separate and simultaneous modes of regeneration: a mild liquid-phase washing and a hot vapor-phase stripping.
The hot vapor-phase stripping which is disclosed in U.S. Pat. No. 5,672,798 consists of contacting the catalyst with a vapor-phase gas stream at a temperature that is typically greater than that employed in the alkylation zone. Because the gas stream uses hydrogen and the contacting occurs at an elevated temperature, hot vapor-phase stripping, which is also referred to in U.S. Pat. No. 5,672,798 as “hydrogen stripping” or “severe regeneration.” U.S. Pat. No. 5,627,798 teaches that the presence of some isobutane in the gas stream is desirable to increase the heat capacity of the gas and thereby to increase the catalyst heat-up rates. This hot hydrogen-isobutane stripping removes liquid phase hydrocarbons and deposits of heavy compounds from the catalyst and produces a vapor phase regeneration zone effluent stream. U.S. Pat. No. 5,672,798 teaches that th
Sechrist Paul A.
Shields Dale J.
Dang Thuan D.
Moore Michael A.
Spears, Jr, John F.
Tolemei John G.
UOP LLC
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