Patent Application
20180334623
The present invention relates to a process for the production of light olefins from a naphtha feed stream and the production of gasoline components from the naphtha feed. This invention also relates to an improved flow process for increasing the yields of light olefins or shifting operations to increase gasoline production.
Ethylene and propylene, light olefin hydrocarbons with two or three atoms per molecule, respectively, are important chemicals for use in the production of other useful materials, such as polyethylene and polypropylene. Polyethylene and polypropylene are two of the most common plastics found in use today and have a wide variety of uses for both as a material fabrication and as a material for packaging. Other uses for ethylene and propylene include the production of vinyl chloride, ethylene oxide, ethylbenzene and alcohol. Steam cracking or pyrolysis of hydrocarbons produces essentially all of the ethylene and propylene. Hydrocarbons used as feedstock for light olefin production include natural gas, petroleum liquids, and carbonaceous materials including coal, recycled plastics or any organic material.
An ethylene plant is a very complex combination of reaction and gas recovery systems. The feedstock is charged to a cracking zone in the presence of steam at effective thermal conditions to produce a pyrolysis reactor effluent gas mixture. The pyrolysis reactor effluent gas mixture is stabilized and separated into purified components through a sequence of cryogenic and conventional fractionation steps. A typical ethylene separation section of an ethylene plant containing both cryogenic and conventional fractionation steps to recover an ethylene product with a purity exceeding 99.5% ethylene is described in an article by V. Kaiser and M. Picciotti, entitled, “Better Ethylene Separation Unit.” The article appeared in HYDROCARBON PROCESSING MAGAZINE, November 1988, pages 57-61 and is hereby incorporated by reference.
Methods are known for increasing the conversion of portions of the products of the ethylene production from a zeolitic cracking process to produce more ethylene and propylene by a disproportionation or metathesis of olefins. Such processes are disclosed in U.S. Pat. Nos. 5,026,935 and 5,026,936 wherein a metathesis reaction step is employed in combination with a catalytic cracking step to produce more ethylene and propylene by the metathesis of C4 and heavier molecules. The catalytic cracking step employs a zeolitic catalyst to convert a hydrocarbon stream having 4 or more carbon atoms per molecule to produce olefins having fewer carbon atoms per molecule. The hydrocarbon feedstream to the zeolitic catalyst typically contains a mixture of 40 to 95 wt-% paraffins having 4 or more carbon atoms per molecule and 5 to 60 wt-% olefins having 4 or more carbon atoms per molecule. In U.S. Pat. No. 5,043,522, it is disclosed that the preferred catalyst for such a zeolitic cracking process is an acid zeolite, examples includes several of the ZSM-type zeolites or the borosilicates. Of the ZSM-type zeolites, ZSM-5 was preferred. It was disclosed that other zeolites containing materials which could be used in the cracking process to produce ethylene and propylene included zeolite A, zeolite X, zeolite Y, zeolite ZK-5, zeolite ZK-4, synthetic mordenite, dealuminized mordenite, as well as naturally occurring zeolites including chabazite, faujasite, mordenite, and the like. Zeolites which were ion-exchanged to replace alkali metal present in the zeolite were preferred. Preferred cation exchange cations were hydrogen, ammonium, rare earth metals and mixtures thereof.
European Patent No. 109,059B1 discloses a process for the conversion of a feedstream containing olefins having 4 to 12 carbon atoms per molecule into propylene by contacting the feedstream with a ZSM-5 or a ZSM-11 zeolite having a silica to alumina atomic ratio less than or equal to 300 at a temperature from 400 to 600° C. The ZSM-5 or ZSM-11 zeolite is exchanged with a hydrogen or an ammonium cation. The reference also discloses that, although the conversion to propylene is enhanced by the recycle of any olefins with less than 4 carbon atoms per molecule, paraffins which do not react tend to build up in the recycle stream. The reference provides an additional oligomerization step wherein the olefins having 4 carbon atoms are oligomerized to facilitate the removal of paraffins such as butane and particularly isobutane which are difficult to separate from C4 olefins by conventional fractionation. In a related European Patent 109060B1, a process is disclosed for the conversion of butenes to propylene. The process comprises contacting butenes with a zeolitic compound selected from the group consisting of silicalites, boralites, chromosilicates and those zeolites ZSM-5 and ZSM-11 in which the mole ratio of silica to alumina is greater than or equal to 350. The conversion is carried out at a temperature from 500 to 600° C. and at a space velocity of from 5 to 200 kg/hr of butenes per kg of pure zeolitic compound. The European Patent 109060B1 discloses the use of silicalite-1 in an ion-exchanged, impregnated, or co-precipitated form with a modifying element selected from the group consisting of chromium, magnesium, calcium, strontium and barium.
Generally, the heavier olefins having six or more carbon atoms per molecule which are produced in commercial ethylene plants are useful for the production of aromatic hydrocarbons. Portions of the olefin product include olefins with four carbon atoms per molecule. This portion includes both mono-olefins and di-olefins and some paraffins, including butane and iso-butane. Because the portion with four carbon atoms per molecule is generally less valuable and requires significant processing to separate di-olefins from the mono-olefins, processes are sought to improve the utilization of this portion of the ethylene plant product and enhancing the overall yield of ethylene and propylene.
Likewise, a portion of gasolines are generated from a naphtha feedstream, and fuel quality demands and environmental concerns have led to the widespread removal of antiknock additives containing lead, and to the subsequent reformulation of gasoline. Because of the demands of modern internal-combustion engines, refiners have had to modify processes and install new processes to produce gasoline feedstocks that contribute to increasing the “octane,” or autoignition resistance. Premature autoignition causes the “knock” in internal combustion engines. Refiners have used a variety of processes to upgrade the gasoline feedstocks, including higher fluid catalytic cracking (FCC), isomerization of light naphtha, higher severity catalytic reforming, and the use of oxygenated compounds. Some of these processes produce higher octane gasoline feedstocks by increasing the aromatics content of the gasoline at the expense of reducing the content low-octane paraffins. Gasolines generally have aromatics contents of about 30% or more.
Faced with tightening automotive emission standards, refiners are having to supply reformulated gasoline to meet the stricter standards. Requirements for the reformulated gasoline include lower vapor pressure, lower final boiling point, increased oxygenate content, and lower content of olefins and aromatics. Aromatics, in particular benzene and toluene, have been the principal source of increasing the octane of gasoline with the removal of lead compounds, but now the aromatics content may eventually be reduced to less than 25% in major urban areas and to even lower ranges, such as less than 15%, in areas having severe pollution problems.
Alternate formulations for gasolines have been comprising aliphatic-rich compositions in order to maintain the octane ratings, as refiners have worked to reduce the aromatic and olefin content of gasolines. Currently, the processes for increasing the aliphatic content of gasolines include the isomerization of light naphtha, isomerization of paraffins, upgrading of cyclic naphthas, and increased blending of oxygenates. However, oxygenates are also becoming an issue as the use of methyl tertiary-butyl ether (MTBE) is being phased out, and ethanol has become the primary oxygenate for use with gasoline.
It is difficult in naphtha cracking to obtain high selectivity to ethylene and propylene, while maintaining high conversion. Improvements in catalysts and processes that accomplish this are therefore desirable.
A process is presented for the flexible operation of the conversion of a naphtha feedstream to higher value products from either a cracking unit, or the generation of a stream for blending with a gasoline pool.
A first embodiment of the invention is a process for increasing light olefin yields from naphtha, comprising passing a naphtha feedstream to a naphtha splitter to generate a light naphtha stream and a heavy naphtha stream, wherein the naphtha splitter is operated to split the naphtha stream around the boiling point of methylcyclopentane; passing the heavy naphtha stream to a reforming unit to generate a reformer effluent comprising aromatics; passing the light naphtha stream to a separation unit to generate an extract stream comprising normal hydrocarbons and a raffinate stream comprising non-normal hydrocarbons; and passing the extract stream to a cracking unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the naphtha splitter is operated increasing the temperature to split the naphtha stream around the boiling point of cyclohexane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the naphtha splitter is operated increasing the temperature to split the naphtha stream around the boiling point of dimethylpentane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the reformer effluent to an aromatics recovery unit to generate an aromatics stream and a recovery unit raffinate stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the recovery unit raffinate stream to the separation unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the separation unit is an adsorption separation unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the adsorption separation unit uses a light desorbent. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph wherein the light desorbent is n-butane or n-pentane. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the raffinate stream to a gasoline blending pool. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the aromatics raffinate stream to the cracking unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the aromatics raffinate stream to the separation unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the first embodiment in this paragraph further comprising passing the aromatics stream to an aromatics complex.
A second embodiment of the invention is a process for increasing gasoline blending stock from naphtha, comprising passing a naphtha feedstream to a naphtha splitter to generate a light naphtha stream and a heavy naphtha stream, wherein the naphtha splitter is operated to split the naphtha stream around the boiling point of methylcyclopentane; passing the heavy naphtha stream to a reforming unit to generate a reformer effluent comprising aromatics; passing the light naphtha stream to a separation unit to generate an extract stream comprising normal hydrocarbons and a raffinate stream comprising non-normal hydrocarbons; passing the raffinate to a raffinate splitter to generate a raffinate splitter bottoms stream and a raffinate splitter overhead stream; and passing the extract stream to a cracking unit. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the raffinate splitter includes a side draw stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the raffinate splitter is operated to send iC5 and iC4 into the overhead stream, further comprising passing the raffinate splitter overhead stream to the reformer to isomerize the iC5 and iC4 components to nC5 and nC4. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the reformer effluent stream to an aromatics recovery unit to generate an aromatics stream and an aromatics raffinate stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the aromatics raffinate stream to the separation unit to recover normal paraffins. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the adsorption separation unit uses a light desorbent. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph wherein the raffinate splitter is operated to send iC5, cyclopentane, iC6 and methylcyclopentane into the side draw stream. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the side draw stream to a gasoline blending stock. An embodiment of the invention is one, any or all of prior embodiments in this paragraph up through the second embodiment in this paragraph further comprising passing the raffinate splitter bottom stream to the reforming unit to convert methylcyclopentane and heavier components to aromatics.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawing.
The present invention is a process that improves flexibility in the operation of a refinery, and enables the refiner to readily shift product with minimal additional equipment. The operation of the new and existing equipment allows for shifting of production to increase higher value products as the market shifts. Market shifts include the price of raw materials, or oil, and the demand for different products such as precursor materials to plastics, such as light olefins or aromatics, or increased production of gasoline blending streams. Traditional processing of a naphtha feedstream is to only use the naphtha splitter, which segregates according to boiling point ranges. However, there are significant overlaps of boiling points of different hydrocarbons, wherein the operation of a cracking unit works most efficiently with normal hydrocarbons, and the reforming unit performs more efficiently on heavier hydrocarbons and aromatic precursors such as methylcyclohexane, cyclohexane and higher naphthenes. It is also advantageous to remove normal C5 and C6 hydrocarbons from the feed to the reforming unit.
It is desirable to have a naphtha complex that is flexible to maximize light olefin yields, or to have flexible production of high octane light naphtha to meet changing market demands, or to have a flexible aromatics production to control the generation of benzene and xylenes.
The present invention, as shown in
In one embodiment, the separation unit 20 is an adsorption separation unit, and uses a light desorbent to displace the adsorbed normal paraffins. The light desorbent is selected from hydrocarbons that have a boiling point lower than the components in the light naphtha stream 12. The light desorbent can comprise a light normal paraffin, such as n-butane, or n-pentane.
The process further includes passing the reformer effluent stream 42 to an aromatics recovery unit 50 to generate an aromatics stream 52 and an aromatics recovery unit raffinate stream 54. The aromatics stream 52 is passed to an aromatics complex 60. The recovery unit raffinate stream 54 comprises iso and normal hydrocarbons, and in one embodiment, is passed to the cracking unit 30. In an alternate embodiment, the recovery unit raffinate stream 54 is passed to the separation unit 20. When the raffinate stream 54 is passed to the separation unit 20, it may be first hydrotreated, or passed to a hydrotreatment unit 80, before being passed to the separation unit 20. The hydrotreatment unit 80 can be used to hydrogenate olefins, or facilitate the removal of any sulfur that is obtained from the aromatics recovery unit 50. The separation unit 20 separates the normal paraffins for passage to the cracking unit, and the non-normal hydrocarbons, and in particular the isobutane, isopentanes and isohexanes, are passed to the reforming unit 40. The reforming catalyst will isomerize the iso-paraffins, and will increase the amount of normal paraffins that can be sent to the cracking unit 20.
In one embodiment, the invention is a process for increasing the content for a gasoline blending pool from the naphtha feedstream. In one embodiment, the naphtha splitter 10 is operated to increase the temperature to split the naphtha stream around the normal boiling point of cyclohexane, or around 81° C. The naphtha splitter 10 can also be operated at conditions to have the split around the normal boiling point of dimethylpentanes, or a temperature greater than 80° C., or in the range of 80° C. to 90° C.
The present invention includes the processing of a naphtha feedstream to improve the feed to a steam cracker, or to improve the feed to a reforming unit, and to improve the composition of a gasoline blending stock. The present invention is presented in the form wherein the temperatures are temperatures for a fractionation process operated at, or near, atmospheric pressures. The invention is intended to include appropriate shifts in temperatures for changes in operating pressures. One of ordinary skill in the art working with hydrocarbons would readily understand and know the temperature shifts for fractionation at either higher or lower pressures. For example, the boiling points are expected to rise for operations with increased pressures, and to drop for operations with decreased pressures, relative to atmospheric pressure.
The raffinate stream 24 from the separation unit 20 is passed to a second fractionation column 70 to generate a second fractionation overhead stream 72, and a second fractionation bottoms stream 76. In one embodiment, the second fractionation column 70 can have a side draw stream 74. The second fractionation column 70 can be a divided wall column, or is intended to include a two column system for producing the three streams.
In one embodiment, the raffinate stream 24 can be passed to the gasoline blending pool. In another embodiment, either the second overhead stream 72 or the side draw stream 74 comprise gasoline components for passing to the gasoline pool. The second bottoms stream 76 is passed to the reformer 40, to aromatize cyclohexane, and to isomerize iso-paraffins.
In another embodiment, the process, as shown in
The reformer effluent stream 42 is passed to an aromatics recovery unit 50 to generate an aromatics stream 52 and a recovery unit raffinate stream 54. The aromatics stream 52 is passed to an aromatics complex 60. The recovery raffinate stream 54 is passed to the separation unit 20. The reformer 40 isomerizes a portion of the reformer feed 14 that is not aromatized. The separation unit 20 further separates the aromatics raffinate stream 54 to direct normal hydrocarbons to the extract stream 22 and non-normal hydrocarbons to the raffinate stream 24. The recovered non-normal hydrocarbons are directed to a gasoline blending pool.
While the invention has been described with what are presently considered the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but it is intended to cover various modifications and equivalent arrangements included within the scope of the appended claims.
This application is a Continuation of copending International Application No. PCT/US2017/028099 filed Apr. 18, 2017, which application claims priority from U.S. Provisional Application No. 62/334,938 filed May 11, 2016, now expired, the contents of which cited applications are hereby incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62334938 | May 2016 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2017/028099 | Apr 2017 | US |
| Child | 16051208 | US |
