There are a variety of hydrocarbon conversion processes, and these processes utilize different catalysts.
Alkylation is typically used to combine light olefins, for example mixtures of alkenes such as propylene and butylene, with isobutane to produce a relatively high-octane branched-chain paraffinic hydrocarbon fuel, including isoheptane and isooctane. Similarly, an alkylation reaction can be performed using an aromatic compound such as benzene in place of the isobutane. When using benzene, the product resulting from the alkylation reaction is an alkylbenzene (e.g. ethylbenzene, cumene, dodecylbenzene, etc.).
The alkylation of paraffins with olefins for the production of alkylate for gasoline can use a variety of catalysts. The choice of catalyst depends on the end product a producer desires. Typical alkylation catalysts include concentrated sulfuric acid or hydrofluoric acid. However, sulfuric acid and hydrofluoric acid are hazardous and corrosive, and their use in industrial processes requires a variety of environmental controls.
Solid catalysts are also used for alkylation. However, solid catalysts are very water sensitive and are generally rapidly deactivated by oligomerization of feed olefins to coke, which may block the pores, leading to short active life and the need for expensive regeneration processes.
Acidic ionic liquids can be used as an alternative to the commonly used strong acid catalysts in alkylation processes. Ionic liquids are salts comprised of cations and anions which typically melt below about 100° C. Ionic liquids are described in U.S. Pat. Nos. 4,764,440, 5,104,840, and 5,824,832. The properties vary extensively for different ionic liquids, and the use of ionic liquids depends on the properties of a given ionic liquid. Depending on the organic cation of the ionic liquid and the anion, the ionic liquid can have very different properties.
Ionic liquids provide advantages over other catalysts, including being less corrosive than catalysts like HF, and being non-volatile.
Although ionic liquid catalysts can be very active, alkylation reactions need to be run at low temperatures, typically between −10° C. to 30° C., to maximize the alkylate quality. This requires cooling the reactor and reactor feeds, which adds substantial cost to an alkylation process utilizing ionic liquids in the form of additional equipment and energy. The most common ionic liquid catalysts for alkylation include imidazolium, or pyridinium-based cations coupled with the chloroaluminate anion (Al2Cl7−).
In addition, the use of ionic liquids for alkylation requires the use of HCl or an HCl precursor as a co-activator/co-catalyst. The HCl can be introduced into the process in several ways, such as the direct vapor phase injection of HCl to the ionic liquid using anhydrous HCl, or the introduction of a liquid organic chloride. Although the vapor phase method is effective, stringent governmental regulations covering the use of HCl gas may make it impractical. The liquid phase method is less effective because the organic chloride does not readily breakdown to HCl and paraffin in the absence of a catalyst.
Products from the alkylation reactor may contain excess HCl and organic chloride byproducts formed by the reaction of HCl with olefins in the feed, which must be removed from the products to meet specifications and avoid downstream corrosion.
A number of processes have been developed to handle the HCl and organic chlorides in alkylation processes. For example, U.S. Pat. No. 7,538,256 describes an alkylation process using an acidic ionic liquid catalyst and an organic halide promoter. The alkylate formed in the reaction is contacted with a hydrotreating catalyst and hydrogen to reduce the concentration of the organic halide.
U.S. Pat. No. 8,237,004 discusses a process in which the alkylation reactor effluent is sent to a stripper to separate it into a first fraction having an increased amount of hydrogen halide and a bottoms stream having less than 25 ppm hydrogen halide. The first fraction can be recycled to the reactor. The bottoms stream is then sent to a distillation column to be separated into one or more product streams such as an alkylate product stream and isoparaffin streams which can be recycled to the alkylation reactor. The equipment used for recovery is made from materials having poor corrosion resistance to HCl which is said to reduce equipment costs and upstream removal of HCl helps to protect this equipment.
Therefore, there is a need for an integrated system of subprocesses and equipment for more complete handling and control of organic chlorides and HCl in ionic liquid alkylation systems.