Patent Application
20140296587
This invention relates to a process for the production of butadiene. In particular, this is a process for the integration of a butadiene production process into a petrochemical plant.
The use of plastics and rubbers are widespread in today's world. The production of these plastics and rubbers are from the polymerization of monomers which are generally produced from petroleum. The monomers are generated by the breakdown of larger molecules to smaller molecules which can be modified. The monomers are then reacted to generate larger molecules comprising chains of the monomers. An important example of these monomers is light olefins, including ethylene and propylene, which represent a large portion of the worldwide demand in the petrochemical industry. Light olefins, and other monomers, are used in the production of numerous chemical products via polymerization, oligomerization, alkylation and other well-known chemical reactions. Producing large quantities of light olefin material in an economical manner, therefore, is a focus in the petrochemical industry. These monomers are essential building blocks for the modern petrochemical and chemical industries. The main source for these materials in present day refining is the steam cracking of petroleum feeds.
Another important monomer is butadiene. Butadiene is a basic chemical component for the production of a range of synthetic rubbers and polymers, as well as the production of precursor chemicals for the production of other polymers. Examples include homopolymerized products such as polybutadiene rubber (PBR), or copolymerized butadiene with other monomers, such as styrene and acrylonitrile. Butadiene is also used in the production of resins such as acrylonitrile butadiene styrene.
Butadiene is typically recovered as a byproduct from the cracking process, wherein the cracking process produces light olefins such as ethylene and propylene. With the increase in demand for rubbers and polymers having the desired properties of these rubbers, an aim to improving butadiene yields from materials in a petrochemical plant will improve the plant economics.
The present invention is a process for increasing the butadiene yields from a crude C4 stream. The crude C4 stream is generated by a cracking unit where C4s are a by-product. The process includes a first separation to generate a first byproduct C4 stream. The byproduct C4 stream is passed to a butadiene extraction unit to generate a purified 1,3 butadiene stream and an isobutylene containing C4 stream. The isobutylene containing C4 stream is passed to an MTBE reactor to remove isobutylene, while generating an MTBE product stream, and a second byproduct C4 stream. The second by product C4 stream is passed to a dehydrogenation unit to convert C4s to butadiene in a dehydrogenation process stream. The dehydrogenation process stream is recycled to the butadiene extraction unit.
In another embodiment, the invention comprises passing a raffinate stream from an isobutylene removal unit to generate a process stream comprising n-butane and n-butenes. The process stream is passed to a fractionation unit to generate a bottoms stream comprising n-butane and 2-butene while generating an overhead stream comprising 1-butene. The bottoms stream is passed to an oxidative dehydrogenation unit to produce 1,3 butadiene, and recycling the dehydrogenation process stream to a butadiene extraction unit.
Other objects, advantages and applications of the present invention will become apparent to those skilled in the art from the following detailed description and drawings.
The demand for plastics such as polyethylene and polypropylene has increased substantially, and will continue to increase in the foreseeable future. Due to the increase demand, the increase in demand for the monomers, ethylene and propylene or light olefins, has also increased. This increase in demand has led to improvements in the processes for the production of light olefins. The improvements increase yields from traditional sources, such as naphtha cracking, and from other sources by diverting other hydrocarbon streams for the production of light olefins. For purposes of the present invention, the reference to steam cracking, as used hereinafter, is intended to include any cracking unit, which can be a catalytic cracker, a stream cracker, or a cracking unit for hydrocarbon sources other than naphtha. With increased availability of ethane from associated gas of crude oil production, as well as, from increased recovery of natural gas liquids (NGL) which contains large amounts of ethane, use of ethane as a feed to steam crackers has increased. When ethane is used as a feedstock to a steam cracker, the byproduct C4 stream is significantly reduced. As a result of these changes, the production of byproducts C4s from the cracking process has decreased. An important part of this byproduct is the recovery of butadiene, and through changes in feedstocks for light olefins production, butadiene recovery has been reduced, or is not keeping up with increased demand.
The present invention provides for increasing the recovery of butadiene through an integrated process that generates a butadiene stream through the processing of a byproduct crude C4 stream generated from a naphtha cracker. The present invention provides for the integration with units that can be present in a refinery. For example, the present invention can be used to enhance the production of 1,3-butadiene by integrating with an on-purpose process for the production of 1,3-butadiene. The on-purpose butadiene process stream is combined with the crude C4 stream, which is then passed to the 1,3-butadiene recovery unit. The naphtha cracker can be an existing unit, or a new unit, with this invention added on to increase the recovery of 1,3 butadiene. The present invention is a process including passing a process stream from a cracking unit to a first separation unit to generate a first process C4 stream. The C4 stream is passed to a butadiene extraction unit to generate a 1,3 butadiene stream, and an isobutylene containing C4 stream, also known as Raffinate-I. The extraction unit can also separate out 1,2 butadiene from the first process C4 stream. The isobutylene containing C4 stream is passed to a methyl tertiary butyl ether (MTBE) process unit to generate an MTBE stream and a second process C4 stream, also known as Raffinate-II. The MTBE process unit comprises several components, which are known in the industry, and includes multiple reactors, a reactive distillation column, and other columns for separation of components. The second process C4 stream is passed to a dehydrogenation unit to generate a dehydrogenation process stream that includes 1,3 butadiene. The dehydrogenation process stream is passed to the butadiene extraction unit for an increase in the yield of 1,3 butadiene.
The butadiene extraction is performed using a solvent extraction. An appropriate solvent is a solvent comprising a polar nitrogen compound, or a mixture of polar compounds. Examples of solvents, though not limited to these, include N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethyl acetamide, and acetonitrile (ACN). A more widely used extractive solvent is NMP.
In the MTBE process unit the isobutylene in the C4 Raffinate-I is reacted with a methanol stream in the MTBE reaction zone or zones to form MTBE. The C4 Raffinate-II stream recovered from the MTBE unit preferably comprises more than 50% n-butenes by weight.
A first embodiment of this process is shown in
The overhead stream 62 can first be passed to the selective hydrogenation unit 20 to convert acetylenic compounds. The overhead stream 62 can be passed to an oxygenate removal unit 90 if sufficient oxygenate impurities are not removed prior to the C4 splitter fractionation unit 60. In this case, the overhead stream 62 is passed through 90 via 66 to have oxygenate impurities removed. The oxygenate removal can include washing, fractionation, or an adsorbent unit.
The dehydrogenation unit 50 is preferably an oxidative dehydrogenation unit, and includes passing an oxygen rich stream 54 and a steam stream 56 to the unit 50. Oxidative dehydrogenation has the benefit of generating a relatively high concentration of 1,3 butadiene, while not generating isobutylene or isobutane. The oxidative dehydrogenation process does generate water and oxygenates. The water and a portion of the oxygenates can be removed with a dewatering unit 70 to generate a water rich stream 72 extracted from the dehydrogenation process stream 52. The dehydrogenation process stream 52 can also be passed through a light ends fractionation unit 80 to remove light gases 82, including light hydrocarbons and hydrogen.
The selective hydrogenation unit 20 further includes receiving a hydrogen stream for the hydrogenation of the carbon-carbon triple bonds. The butadiene extraction unit 30 can separate a heavies stream 38, including 1,2 butadiene.
A second embodiment comprises the conversion of a C4 Raffinate-II stream from an MTBE process unit. The C4 Raffinate-II stream comprises C4 compounds that can be converted to 1,3 butadiene. The process is shown in
Another embodiment is shown in
One example of the isobutylene removal reaction unit 240 can include an MTBE process unit in order to reach a high enough conversion of isobutylene that will enable the high purity recovery of 1-butene after fractionation. In this case, the isobutylene reaction unit 240 can comprise multiple MTBE reactors in series with inter-coolers, or can include one or more MTBE reactors followed by a reactive distillation zone which is integral to the fractionation of C4s and MTBE. The reactive distillation zone overcomes equilibrium constraints found in a single MTBE reactor. The isobutylene reaction unit 240 can include other reactor considerations and configurations for the removal of isobutylene from the second stream 234. Other options can include ethyl tertiary butyl ether (ETBE) reactors, or tertiary butyl alcohol (TBA) reactors. The use of the term isobutylene reaction unit is not intended to be limited to an MTBE reactor, but to a reaction system for the reaction and removal of isobutylenes.
The raffinate stream 246 from an MTBE unit can consist of n-butane and n-butenes with a butene content of nearly 70% by weight. This highly n-butene rich stream stream can be further processed in a dehydrogenation unit to convert the n-butene to butadiene. The converted butadiene stream can be recycled into the butadiene recovery process to increase the butadiene yields.
The raffinate stream 246 is passed to a selective hydrogenation unit 250 to generate a hydrogenated raffinate stream 252. The selective hydrogenation converts residual diolefins, and can perform some isomerization of 1-butene to 2-butene. The amount of 1-butene isomerization can be used as a trade-off of 1-butene production vs. 1,3-butadiene production, giving flexibility to a plant for choice of products. The selective hydrogenation unit 250 converts the residual 1,3-butadiene remaining in the C4 Raffinate-I stream to olefins. The residual amount of 1,3-butadiene in the C4 Raffinate-I stream is typically from 0.2 to 0.5 wt %. The operating conditions and catalyst are chosen to minimize olefin conversion to paraffins. The hydrogenation raffinate stream 252 is a butene rich stream. The hydrogenated raffinate stream 252 is passed to a separation unit 260 to generate a stream 262 rich in 2-butenes. The separation unit 260 also separates out 1-butene as a high value product stream 264. The separation unit 260 can comprise multiple fractionation columns for separating individual components, such as the 1-butene from the hydrogenated raffinate stream 252.
The butenes rich stream 262 is passed to a dehydrogenation unit 270 to generate a dehydrogenation stream 272 rich in 1,3-butadiene. A preferred dehydrogenation unit is an oxidative dehydrogenation unit to limit the production of isobutylenes. The dehydrogenation stream 272 is passed to a C4 splitter 280, after passing through a dewatering unit and a light ends removal unit, to generate an overhead stream 282 comprising butadienes and a bottoms stream 284 rich in unconverted 2-butenes and can be recycled to the dehydrogenation reactor 270. The overhead stream 282 is passed to the feed to the selective hydrogenation unit 220. In an alternative, the overhead stream 282 is passed to an oxygenate removal unit 300 to generate an overhead stream 302 free of oxygenates. The oxygenate free overhead stream 302 is then passed to the selective hydrogenation unit 220.
The oxidative dehydrogenation process generates water and oxygenates, as well as some light gases. The process can include a dewatering unit and degassing unit 290 for removal of water with some oxygenates 292 and removal of light gases 294 comprising C3 and lighter hydrocarbons, and lighter gases.
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.
