Apparatus for the reduction of gasoline benzene content by alkylation with dilute ethylene (UOP LLC)

PATENT
Apparatus for the reduction of gasoline benzene content by alkylation with dilute ethylene (UOP LLC)
Publication number
US20130230437 A1
Application number
US 13/858,547
Publication date
Sep 5, 2013
US8414851, US20110305602
Inventors
Christopher P. Nicholas, Alakananda Bhattacharyya
Original Assignee
UOP LLC
Abstract
The apparatus converts ethylene in a dilute ethylene stream and dilute benzene in an aromatic containing stream via alkylation to heavier hydrocarbons. The catalyst may be a zeolite such as UZM-8. The catalyst is resistant to feed impurities such as hydrogen sulfide, carbon oxides, and hydrogen and selectively converts benzene. At least 40 wt-% of the ethylene in the dilute ethylene stream and at least 20 wt-% of the benzene in the dilute benzene stream can be converted to heavier hydrocarbons.
BACKGROUND OF THE INVENTION
The field of the invention is an apparatus for alkylating benzene in a naphtha stream with dilute ethylene. The alkylated product may be used as motor fuel.
Dry gas is the common name for the off-gas stream from a fluid catalytic cracking unit that contains all the gases with boiling points of ethane and lower. The off-gas stream is compressed to remove as much of the C3 and C4 gases as possible. Sulfur is also largely absorbed from the off-gas stream in a scrubber that utilizes an amine absorbent. The remaining stream is known as the FCC dry gas. A typical dry gas stream contains 5 to 50 wt-% ethylene, 10 to 20 wt-% ethane, 5 to 20 wt-% hydrogen, 5 to 20 wt-% nitrogen, about 0.05 to about 5.0 wt-% of carbon monoxide, 0.1 to about 5.0 wt-% of carbon dioxide and less than 0.01 wt-% hydrogen sulfide and ammonia with the balance being methane.
Currently, the FCC dry gas stream is sent to a burner as fuel gas. An FCC unit that processes 7,949 kiloliters (50,000 barrels) per day will burn about 181,000 kg (200 tons) of dry gas containing, for example, about 36,000 kg (40 tons) of ethylene as fuel per day. Because a large price difference exists between fuel gas and motor fuel products or pure ethylene it would appear economically advantageous to attempt to recover this ethylene. However, the dry gas stream contains impurities that can poison catalysts and is so dilute that ethylene recovery is not economically justified by gas recovery systems.
There is need for utilization of dilute ethylene in refinery streams.
Catalytic reforming is a well-established hydrocarbon conversion process employed in the petroleum refining industry for improving the octane quality of hydrocarbon feedstocks, the primary product of reforming being motor gasoline. In catalytic reforming, a naphtha feedstock is admixed with a recycle stream comprising hydrogen and contacted with catalyst in a reaction zone at temperatures of around 493° to 510° C. (920° to 950° F.) and moderate pressure around 1379 to 3792 kPa (200 to 550 psig). The usual feedstock for catalytic reforming is a petroleum fraction known as naphtha and having an initial boiling point of about 46° C. (115° F.) and an end boiling point of about 204° C. (400° F.).
The catalytic reforming process is particularly applicable to the treatment of straight run gasoline comprised of relatively large concentrations of naphthenic and substantially straight chain paraffinic hydrocarbons, which are subject to aromatization through dehydrogenation and/or cyclization reactions. The catalyst “reforms” the molecular structures of the hydrocarbons contained in the raw naphtha by removing hydrogen and rearranging the structure of the molecules so as to improve the octane number of the naphtha. However, the increase in octane number also reduces the liquid volume of the naphtha as the specific gravity is increased. Because of the multiplicity of the compounds in the raw naphtha, the actual reactions which occur in catalytic reforming are numerous. However, some of the many resulting products are aryl or aromatic compounds, all of which exhibit high octane numbers. The aryl compounds produced depend upon the starting materials which in a refinery are controlled by the boiling range of the naphtha used and the crude oil source. The “reformed” product from a catalytic reforming process is commonly called reformate and is often separated into two fractions by conventional distillations, a light reformate having a boiling range of about 46° to 121° C. (115° to 250° F.) and a heavy reformate having a boiling range of about 121° to 204° C. (250° to 400° F.). The aryl compounds in each fraction are thus dependent upon their boiling points. The lower boiling or lighter aryl compounds, e.g., benzene, toluene and xylenes, are contained in the light reformate, and higher boiling aryl compounds are contained in the heavy reformate.
The concentration of benzene in gasoline is now being regulated by the American government. The Mobil Source Air Toxics regulation (MSAT II) requires that the average benzene level in gasoline produced by a refiner be lower than 0.62 vol-% with a maximum of 1.3 vol-% in gasoline produced at any one refinery. Benzene is commonly produced at levels higher than this by reforming processes and FCC processes. As reformate and the naphtha streams from the FCC unit are two of the largest sources of gasoline in a refinery, benzene reduction strategies have to be used.
Currently, benzene is commonly sent to a saturation unit to reduce benzene to cyclohexane. However, this process utilizes at least three moles of hydrogen for every mole of benzene converted and there is an octane loss associated with the conversion of benzene to cyclohexane. Methods for the reduction of benzene in gasoline without loss of octane or use of hydrogen are necessary.
The alkylation of concentrated benzene streams with concentrated ethylene streams is known. Alkylation typically involves the use of clean ethylene streams because alkylation catalysts are susceptible to feed impurities. Additionally, dilute ethylene is little used as an oligomerization feedstock because of its much lower reactivity relative to higher olefins. Benzene streams fed to alkylation reactors are also concentrated because of concern that heavier aromatics will preferentially alkylate, thereby requiring the use of a large excess of ethylene before reducing the benzene concentration and producing undesirable polyalkylated benzene.
SUMMARY OF THE INVENTION
We have found that dilute benzene in aromatic containing streams such as reformate or FCC light naphtha can be alkylated over zeolitic catalysts with ethylene in dilute ethylene streams, such as an FCC dry gas stream. The heavier hydrocarbons can be separated and blended in the gasoline and diesel pools. We have found most zeolitic catalysts that are suitable for alkylation of benzene with light olefins quickly deactivate in dilute ethylene streams. Neither the dilute nature of the ethylene, nor the impurities present substantially affect a catalyst comprising UZM-8. Additionally, conversion of benzene by alkylation with ethylene is as high as toluene and greater than heavier aromatics with UZM-8 catalyst. Consequently, dilute benzene in a naphtha stream such as reformate can be alkylated with dilute ethylene in an FCC dry gas stream to provide a liquid fuel product which is reduced in benzene concentration and easy to separate from the unconverted gas stream. The unconverted gas can then be burned as fuel gas, but with the more valuable ethylene recovered as gasoline range hydrocarbons.
Advantageously, the apparatus can enable utilization of dilute ethylene in a stream and in the presence of feed impurities that can be catalyst poisons.
Advantageously, the apparatus can enable reduction of the concentration of dilute benzene in a naphtha stream without utilizing hydrogen or reducing the liquid volume or the octane value of the naphtha stream even in the presence of heavier aromatic hydrocarbons.
In an embodiment, the invention comprises an apparatus for alkylating benzene with ethylene comprising a fluid catalytic cracking reactor for contacting cracking catalyst with a hydrocarbon feed stream to crack the hydrocarbon feed to cracked products having lower molecular weight and deposit coke on the cracking catalyst to provide coked cracking catalyst. The apparatus also comprises a regenerator for combusting coke from the coked cracking catalyst by contact with oxygen and a separator in communication with the fluid catalytic cracking reactor for separating C3 hydrocarbons from C2 hydrocarbons to provide a dilute ethylene stream. A reforming reactor for contacting a naphtha stream with reforming catalyst to produce a reformate stream is also comprised in the apparatus. Lastly, the apparatus comprises an alkylation reactor in communication with the separator and the reforming reactor for alkylating benzene in the reformate stream with ethylene in the dilute ethylene stream over a fixed bed of alkylation catalyst to heavier alkyl benzene hydrocarbons. No fractionation column is in communication between the reforming reactor and the alkylation reactor. In an alternative embodiment, no fractionation column is in communication between the separator and the alkylation reactor. In a still further embodiment, an absorber in communication with the product outlet of the FCC reactor provides an off-gas stream comprising a dilute ethylene stream and no fractionation column is in communication between the reforming reactor and the alkylation reactor and between the absorber and the alkylation reactor.
Additional features and advantages of the invention will be apparent from the description of the invention, the drawings and claims provided herein.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic drawing of an FCC unit, a reforming unit and an alkylation unit.
FIG. 2 is a graph of conversion and temperature over time.
FIG. 3 is a graph of conversion over time.
Free Full Text Source: http://www.google.com/patents/US20130230437