Alkylation using HF is a widely used commercial refining and petrochemical process. Generally, alkylation is the addition of an alkyl group to another hydrocarbon. Commercially, HF is used to alkylate isobutane with propylene, butylene, and amylene isomers to produce high octane gasoline blending components, as well as to alkylate benzene in the manufacture of detergents.
The commercial process for alkylating isobutane with propylene is typical of many commercial HF alkylation process. Briefly, propylene and a stoichiometric excess of isobutane contact HF in a reactor where alkylation reactions produce alkylate. The reactor effluent passes to a settler in which the effluent separates into an acid phase containing HF and a hydrocarbon phase containing unreacted isobutane and alkylate. The acid phase is recycled to the reactor. The hydrocarbon phase passes to a fractionation column, commonly called an isostripper or a deisobutanizer, from which the isobutane is also recycled to the reactor and alkylate is recovered as product.
In addition to producing alkylate, HF alkylation processes also produce a by-product called acid-soluble oil, or ASO, which is soluble in HF. ASO is a recognized term in the art of alkylation, and is sometimes referred to as HF-soluble oil, polymer, conjunct polymer, or polymer by-product. Because ASO is soluble in HF, ASO in the settler concentrates not in the hydrocarbon phase but in the acid phase, and consequently the ASO is recycled to the reactor. In the reactor, ASO generally decreases the catalytic activity of the HF. While this effect can be beneficial at relatively low concentrations of ASO, if allowed to accumulate to relatively high concentrations ASO has a detrimental effect on the process. For this reason, ASO is typically removed at least periodically from a slip stream of the acid phase by a process that is commonly called regeneration.
Regeneration is typically performed in two ways, internal regeneration and external regeneration. In internal regeneration, a slip stream of the acid phase, which is hereinafter referred to as a by-product stream, remixes with the hydrocarbon phase and passes to the isostripper. The ASO, being a heavy polymerized by-product, tends to pass downward in the isostripper and is rejected from the bottom of the isostripper and out of the process. The HF that enters the isostripper in internal regeneration tends to pass upward in the isostripper, is recovered from the overhead of the isostripper, and is recycled to the reactor. External regeneration, on the other hand, uses a stripping column commonly called an external regenerator to remove ASO from a by-product stream of the acid phase. In an external regenerator, either heat or isobutane or both strip HF overhead, leaving a residue of mostly ASO which is rejected from the bottom of the external regenerator and out of the process. The overhead of the external regenerator either is recycled as a condensed liquid to the reactor or is passed as a vapor to the isostripper where it may be used to assist in fractionation.
A growing and recent trend in HF alkylation processes is the use of complexing agents to minimize the volatility of the acid phase. Because HF is volatile and the environmental risks arising from an accidental release of HF to the atmosphere are more and more undesirable, complexing agents that reduce the vapor pressure of HF and the tendency of HF to form an aerosol are now in greater use than ever before. Complexing agents typically contain at least one Lewis base site, which commonly comprises a Group 5A element such as nitrogen. Certain nitrogen-containing compounds, such as pyridine, picolines, quinoline, trimethylamine, and triethylamine, are known to form complexes with HF to reduce its volatility. For example, see Japanese Patent Disclosure No. 57(1982)-92502 (Oda et al). U.S. Pat. No. 5,073,674 (Olah) discloses that mixtures of HF and preferred nitrogen-containing compounds (complexing agents) such as ammonia, methylamines, ethylamines, propylamines, butylamines, pentylamines, pyridine, picolines, melamine, and hexamethylene-tetramine remained effective catalysts in alkylation of alkanes by alkenes. Mixtures of Olah's preferred complexing agents with HF are hereinafter referred to as HF-amine complexes.
A problem that arises with the use of some complexing agents in HF alkylation processes is that both internal and external regeneration tend to reject not only ASO but also the HF-agent complex from the process. As a solution to this problem, a two-step method for removing ASO from a by-product stream containing HF, ASO, and HF-agent complex, where the complexing agent contains at least one Lewis base site containing a Group 5A element and the by-product stream has a molar ratio of HF per Lewis base site substantially above 5:1, is practiced in the art. First, a portion of the HF in the by-product stream is selectively removed to produce an HF-depleted stream having a molar ratio of HF per Lewis base site of 3:1 to 5:1. Then, the HF-depleted stream is separated into a hydrocarbon phase enriched in ASO and an acid phase depleted in ASO that contains a substantial portion of the HF-agent complex. The selectively removed HF and the acid phase are both recycled to the hydrocarbon alkylation step, and the hydrocarbon phase is rejected from the process. Thus, this method rejects ASO but not HF-agent complex from the process.
One of the problems with the method described in the previous paragraph is that the HF-enriched stream that contains the selectively removed HF also contains 10 to 15 wt-% ASO. The ASO in the HF-enriched stream tends to be lighter than the heavier ASO that remains in the HF-depleted stream and which is ultimately rejected from the process in the hydrocarbon phase. Nevertheless, like the heavy ASO, this light ASO which is recycled to the alkylation reactor has a detrimental effect on the alkylation reactions. Thus, a method is sought for rejecting ASO, in particular light ASO, from an HF alkylation process that uses an HF-agent complex, wherein the complexing agent contains at least one Lewis base site containing a Group 5A element.
U.S. Pat. No. 5,073,674 (Olah) discloses a process of alkylating aliphatic hydrocarbons with alkenyl hydrocarbons in the presence of liquid HF-ammonia or HF-amine complexes.
U.S. Pat. No. 5,191,150 (Child et al.) discloses a method of separating a stream containing ASO, HF, and sulfolane by first separating out HF and then gravitationally separating the HF-depleted stream into a polymer-rich stream and a sulfolane-rich stream.
U.S. Pat. No. 5,382,746 (Child et al.) discloses an HF alkylation process comprising passing a reactor effluent settler acid phase containing sulfolane, ASO, and HF to an external regenerator; passing the external regenerator overhead stream containing HF and light ASO to an isostripper; recycling an isostripper overhead stream containing HF to the alkylation reactor; and recovering an isostrpper bottom stream containing alkylate and light ASO.
In motor fuel alkylation processes, the passage of the overhead vapor stream of an external regenerator to an isostripper, wherein the overhead vapor stream enters the isostripper at an intermediate point that is below the feed point of the isostripper, is practiced commercially.
The regeneration of HF used as catalyst is also practiced in the production of detergents by the reaction of C.sub.8 -plus normal olefinic hydrocarbons and benzene. The use in such a process of an external regenerator for regenerating HF and of another stripping column, which is called an HF stripper, for separating HF from the hydrocarbon-containing reactor effluent phase is shown in U.S. Pat. Nos. 4,237,327 (Winter, III) and 4,237,328 (Winter, III). U.S. Pat. No. 4,237,327 teaches a process in which an overhead vapor stream having a low concentration of ASO exits an external regenerator and enters an HF stripper at an upper intermediate point that is below the feed point of the HF stripper. U.S. Pat. No. 4,237,328 teaches passing the overhead vapor stream of an external regenerator and the overhead vapor stream of an HF stripper to a common single overhead condenser.