The invention relates to a process for converting light olefins, such as ethylene and propylene, to gasoline and/or distillates. In particular, the invention relates to a process for converting light olefins, such as ethylene and propylene, to gasoline and/or distillates using a catalyst that contains an aluminosilicate clay molecular sieve.
World demand for alkylate, the monomolecular distillation product of isobutane and methanol, is growing rapidly. In fact, about 80% of current alkylate demand is for gasoline. Light olefins are known to co-produce with alkylate a wide variety of petrochemicals such as high octane gasoline, polymer gasoline, light olefins, benzene, aromatics and light hydrocarbons. In addition, light olefins are also valuable in petrochemical feedstock for their diene content. Thus, light olefins, such as ethylene and propylene, can be co-produced with gasoline and/or distillates from olefins by the selective conversion of the ethylene and propylene to higher olefins (C4+). Moreover, with the advent of new technologies, propylene production is rising quickly. For example, Fischer-Tropsch (FT) production of the olefins is expected to rise to 3 billion lbs by the year 2005, from 1 billion lbs. in 1999.
The light olefins can be prepared by cracking hydrocarbon feeds, such as ethane, propane and naphtha in the presence of medium pore zeolites or by conversion of methanol over ZSM-5 and ZSM-11. The olefins produced by either of these processes may then be reacted further in the presence of medium pore zeolites or by conversion of methanol over ZSM-5 and ZSM-11.
Although zeolites and related materials are known to catalyze a number of important hydrocarbon conversion reactions, including the conversion of aliphatic and aromatic hydrocarbons to lower olefins, aromatics, hydrogen and carbon monoxide and the conversion of oxygenates to lower olefins and aromatics, they are not efficient for the conversion of light olefins, such as ethylene and propylene, to higher olefins, such as C4+ olefins. Accordingly, a need exists for an improved catalyst.
U.S. Pat. No. 5,043,522 discloses a two-stage process for converting an oxygenate to lower olefins in which oxygenates are converted in a first stage to medium to lower-range olefins using a ZSM-5 type of zeolite catalyst, followed by a second stage in which said olefins are converted to higher-range olefins, mainly benzene and xylene, using conventional light-weight paraffin dehydrogenation catalyst. It is recognized in the art that dehydrogenation of lower-range olefins formed in the first stage, is critical to provide a high selectivity for C4+ olefins. Moreover, conversion of the lower-range olefins to higher-range olefins decreases selectivity to the C4+ olefins.
The use of ZSM-5 catalyst is known to improve the yield of light olefins as compared with non-zeolite catalysts. However, zeolite catalysts are relatively expensive and have been difficult to make economically.
It is also known in the art that the product distribution and yield of olefins can be improved by reducing the relative residence time of the feed within the catalyst bed. It is further known in the art that longer residence times are associated with lower yields of C4 olefins. The art discloses a number of processes for converting olefins over zeolite catalysts having reduced catalyst bed length as described below.
U.S. Pat. No. 5,045,582 discloses a process for converting an oxygenate feedstock to olefins. The process comprises contacting the feedstock with a first zeolite catalyst in a first reaction zone operated at a first temperature effective to convert about 80 weight percent of the feed to a product stream containing lower olefins, the product stream being withdrawn from said first reaction zone; contacting the effluent from the first reaction zone with a second zeolite catalyst at a second temperature effective to convert about 20 weight percent of the lower olefins in the product stream to a second product stream containing olefins, the second product stream being withdrawn from said second reaction zone; and returning a portion of the product stream to the first reaction zone. The ""582 patent teaches that this two-stage process is more economic than a one-stage process in which all the olefins are converted in the presence of only one of the zeolites.
U.S. Pat. No. 5,160,497 discloses a process for converting an oxygenate feedstock to olefins. The process comprises: (a) passing the feedstock through a fluidized bed comprising ZSM-5 having a crystal size not greater than 4 micrometers under conditions effective to convert about 50 weight percent of the feed to a lower olefin product stream containing ethylene and propylene; (b) passing the effluent from the reactor of (a) to a fluidized bed of ZSM-5 having a crystal size not greater than 4 micrometers under conditions effective to convert about 20 weight percent of the lower olefins in said lower olefin product stream to C4 olefins, and to increase the ethylene content of the C4 olefin product stream; and (c) withdrawing from the reactor of (b) a higher olefin stream containing at least 50 weight percent C4 olefins, and (d) recovering C4 olefins from the effluent from step (c). The ""497 patent discloses that conversion of olefins to light olefins (C4 to C10) improves selectivity to C4+ olefins and improves the yield of light olefins.
U.S. Pat. No. 5,126,308 discloses a process for converting oxygenate to olefins. The process comprises: (a) passing an oxygenate feedstock to a reaction zone containing a first medium pore zeolite catalyst having a constraint index of from 0 to 12, the catalyst having a silica to alumina ratio of at least 300 to less than about 10,000; and (b) withdrawing from the reaction zone a first product stream comprising lower olefins; (c) withdrawing from the reaction zone a second product stream comprising lower olefins; (d) separating at least a portion of the first product stream into a light olefin stream containing ethylene and propylene, and a heavy olefin stream; (e) separating at least a portion of the second product stream into a light olefin stream and a heavy olefin stream; (f) passing at least a portion of the light olefin stream and at least a portion of the heavy olefin stream to a second reaction zone containing a second medium pore zeolite catalyst having a constraint index of from 4 to 12, the catalyst having a silica to alumina ratio of at least about 10,000 to about 600,000, and separating at least a portion of each stream into a product stream comprising ethylene and propylene and a recycle olefin stream; and (g) recovering ethylene and propylene from the product stream.
A further advance in the art was the introduction of MCM-49. U.S. Pat. No. 5,962,772 discloses an improved process for producing olefins by contacting an oxygenate feedstock with a medium pore zeolite catalyst. The catalyst used in the reaction is a medium pore zeolite having a pore size diameter of from about 3 angstroms to about 10 angstroms. The catalyst can be ion exchanged with rare earth ions, and the zeolite is maintained during the exchange in a steamed form. Further, the catalyst is maintained during the ion exchange in a steamed and calcined form. Preferably, the steaming and calcining is conducted for at least about 5 hours. In one embodiment, the catalyst is steamed and calcined for at least about 10 hours. The catalyst has a pore size diameter of from about 5 angstroms to about 10 angstroms, has a YO2/WI3 ratio of at least about 2.5 to less than about 10.
Typically, a reactor operating under vacuum conditions is employed to produce olefins by converting the feedstock, that is a mixture of light olefins and light paraffins, to olefins and paraffins over a layered metal sieve catalyst. One such reactor is shown in U.S. Pat. No. 4,910,344 in which a moving bed reactor is used to produce olefins.
Accordingly, it is an object of the invention to provide an improved process for the production of olefins and paraffins.
It is a further object of the invention to provide a catalyst for use in the conversion of light olefins to heavier olefins and paraffins.
It is another object of the invention to provide a process for the conversion of light olefins to paraffins and olefins by contacting light olefins in the presence of a catalyst selected from an aluminosilicate clay molecular sieve and an alumina binder.
It is a still further object of the invention to provide a process for converting light olefins to olefins and paraffins by contacting light olefins in the presence of a catalyst containing an aluminosilicate clay molecular sieve.
These and other objects of the invention are provided by a process for converting an oxygenate feedstock
