Chemistry of hydrocarbon compounds – Saturated compound synthesis – By condensation of a paraffin molecule with an olefin-acting...
Reexamination Certificate
1999-10-04
2001-02-13
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Saturated compound synthesis
By condensation of a paraffin molecule with an olefin-acting...
C585S710000, C585S720000
Reexamination Certificate
active
06187986
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the art of catalytic alkylation. More specifically, the invention relates to a method for adding fresh or inventoried liquid alkylation catalyst, to an alkylation unit.
BACKGROUND OF THE INVENTION
Alkylation is a reaction in which an alkyl group is added to an organic molecule. For example, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. In petroleum refining, the process reacts a C
2
to C
5
olefin with isobutane in the presence of an acidic catalyst to produce an upgraded product stream referred to as alkylate. This alkylate is a valuable blending component in the manufacture of gasoline due not only to its high octane rating but because it is free of aromatic components.
Industrial alkylation processes have historically used concentrated hydrofluoric or sulfuric acid catalysts under relatively low temperature conditions. Acid strength is preferably maintained at 88 to 94 weight percent by the continuous addition of fresh acid and the continuous withdrawal of spent acid. As used herein, the term “concentrated hydrofluoric acid” refers to an essentially anhydrous liquid containing at least about 85 weight percent HF.
Hydrofluoric and sulfuric acid alkylation processes share inherent drawbacks including environmental and safety concerns, acid consumption, and sludge disposal. For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et al., “Alkylation of Isobutane with C
4
Olefins”, 27
Ind. Eng. Chem. Res
., 381-397, (1988). For a survey of hydrofluoric acid catalyzed alkylation, see 1
Handbook of petroleum Refining Processes
23-28 (R. A. Meyers, ed., 1986).
Hydrogen fluoride, or hydrofluoric acid (HF) is highly toxic and corrosive. However, it is used as a catalyst in isomerization, condensation, polymerization and hydrolysis reactions. The petroleum industry uses anhydrous hydrogen fluoride primarily as a liquid catalyst for alkylation of olefinic hydrocarbons to produce alkylate for increasing the octane number of gasoline. Years of experience in its manufacture and use have shown that HF can be handled safely, provided the hazards are recognized and precautions taken. Though many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape creating a vapor cloud that can be spread for some distance. Previous workers in this field approached this problem from the standpoint of containing or neutralizing the HF cloud after its release.
U.S. Pat. Nos. 4,938,935 and 4,985,220 to Audeh and Greco, as well as U.S. Pat. No. 4,938,936 to Yan teach various methods for containing and/or neutralizing HF acid clouds following accidental releases.
In addition to these efforts directed at making the HF acid circulating in a plant safer by reducing its cloud forming tendencies, there is concern about making every part of the process safer. One significant hazard, which has previously been overlooked by others, is adding fresh HF acid to the plant. A similar problem is re-inventory, that is, the return of the HF acid inventory to the plant after a plant shutdown to permit repair or inspection of equipment.
Now this is done by rotating equipment, typically pumps with seals. Such equipment can and does leak or fail. Standard practice is to use tandem seals which should prevent a catastrophic leak, however failures do happen and leaks can occur.
Usually the fresh acid, added to replenish acid consumed or lost during processing, is relatively pure acid. Fresh acid does not contain sulfolane or other agents which are present in the HF acid circulating in acid inventory of the plant, so it represents a potential threat to the environment even when sulfolane or the like is present in the acid inventory.
Some efforts have been made to reduce the amount of rotating pumps used in HF alkylation units, which are reviewed briefly hereafter.
TRANSFER OF HF ACID WITHIN AN ALKYLATION PLANT
U.S. Pat. No. 5,334,788, Baucom et al, taught fluorine gas reactions in an eductor.
U.S. Pat. No. 5,322,673, a Division of U.S. Pat. No. 5,220,094 taught use of an alkylation recontactor with an internal mixer.
FIG. 2
showed an eductor moving acid inventory and mixing it with hydrocarbon.
U.S. Pat. No. 5,304,522, Jalkian et al, taught use of an eductor to process a slurry of solid sorbent with an alkylate rich stream and a sulfolane-enriched stream, which had previously been stripped of most HF acid in an HF stripper. The sulfolane was added as a vapor suppressant, but the process is still basically an HF alkylation unit with an acid inventory less likely to form a large vapor cloud if accidentally released.
U.S. Pat. No. 5,185,487, Love et al, taught use of an eductor to mix organic fluoride-containing alkylate with HF acid to release the HF and make more alkylate.
U.S. Pat. No. 4,349,931, Mikulicz, taught use of eduction to move an HF containing stream from a reactor to a stripping column. The hydrocarbon stream used as a “pumping means, comprises about 92 mole % isobutane, 2 mole % propane and 6 mole % n-butane.”
U.S. Pat. No. 4,199,409, Skraba, taught use of an eductor to recycle an HF rich acid soluble oil (ASO) from the bottoms to various intermediate levels of an HF acid rerun column. The motive fluid was isobutane vapor.
U.S. Pat. No. 4,046,516 Burton et al, taught use of an eductor to transfer catalyst from one reaction zone in the HF alkylation unit to another.
U.S. Pat. No. 4,014,953, Brown taught use of an eductor in the base of the acid regenerator associated with an HF alkylation unit. Isoparaffin was the drive fluid, and stripping fluid, for a liquid ASO stream containing HF.
U.S. Pat. No. 3,910,771, Chapman, taught use of something similar to an eductor in a vessel designed to convert alkyl fluorides in HF alkylate into alkylate.
U.S. Pat. No. 2,894,999, Lawson, taught use of an eductor in an HF alkylation plant, using relatively hot, high pressure vapor from the deisobutanzier to educt vapor from, and provide evaporative cooling of, an HF alkylation reactor.
These patents were directed to “internal” eductors, that is, eductors moving an HF containing liquid from one place in the relatively high pressure HF alkylation plant to another place within the plant. The relatively modest pressure differentials, and the fact that the liquid streams being educted were under relatively high pressure made eductors safe and easy to use.
FRESH ACID TRANSFER
Despite the widespread use of eductors for internal liquid transfer in HF units, they have never been used, so far as is known, to add either fresh or inventoried HF acid back into the unit. Acid eggs have probably been used. One approach, reviewed below, avoided mechanical equipment, but called for a sophisticated device with a significant number of extra valves and pressure gages, all potential “weak spots” in an HF alkylation plant.
U.S. Pat. No. 4,982,036, Hachmuth et al, taught use of compressed gas to transfer acid catalyst from a transport vehicle to the alkylation process. The approach is similar to use of an “acid egg”, wherein acid is added to a vessel, following which the vessel is pressurized with a gas to provide the “head” needed to transfer the acid to the desired location. While a useful approach, it involves some capital expense for vessels and creates volumes of inert gas—the drive fluid used to energize the “acid egg”—which must eventually be processed.
Thus while many improvements have been made in the HF alkylation process, such as the above mentioned attempts to reduce the use of mechanical pumps in the process, there were still some problems.
One problem area was getting fresh acid into the plant to periodically make up for acid losses. Now mechanical pumps are used to do this and these can leak or fail. Use of a pump with shut off valves at a remote tank leaves a significant amount of concentrated HF acid in the pump and the transfer line to the plant. While concentrated HF acid is not especially co
Griffin Walter D.
Marathon Ashland Petroleum LLC
Stone Richard D.
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