Processes for producing propylene from paraffins

Embodiments of processes for producing propylene from paraffins are provided. The process comprises the steps of combining an effluent that comprises propylene and propane from a paraffin dehydrogenation reactor with an offgas stream that comprises propane to form a combined effluent stream. The combined effluent stream is separated into a propylene product stream and a propane-rich recycle stream. The propane-rich recycle stream is introduced to the paraffin dehydrogenation reactor operating at dehydrogenation conditions to convert propane in the propane-rich recycle stream to propylene.

FIELD OF THE INVENTION

The present invention relates generally to processes for producing light olefins from paraffins, and more particularly relates to processes for producing propylene from the catalytic dehydrogenation of paraffins such as propane.

BACKGROUND OF THE INVENTION

Catalytic dehydrogenation processes are commonly used for the production of light olefins by conversion from their corresponding paraffins. One specific application of this technology produces propylene from the conversion of propane. Propylene is one of the world's largest produced petrochemical commodities and is used, for example, in the production of polypropylene, acrylonitrile, acrylic acid, acrolein, propylene oxide, glycols, plasticizers, oxo alcohols, cumene, isopropyl alcohol, and acetone.

The growth in propylene production is primarily driven by the industry demand for polypropylene. Polypropylene is used in such everyday products as packaging materials and outdoor clothing. The growth rate of polypropylene is expected to be about 5% per year for the near future. To meet this growing demand, producers of propylene are looking for ways to increase their propylene production preferably with minimal additional equipment and cost.

Accordingly, it is desirable to provide processes for increasing production of propylene from paraffins. Moreover, it is desirable to provide processes for increasing production of propylene from paraffins with minimal additional equipment and cost. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description of the Invention and the appended Claims, when taken in conjunction with the accompanying drawings and this Background of the Invention.

SUMMARY OF THE INVENTION

Processes for producing propylene from paraffins are provided herein. In accordance with an exemplary embodiment, a process for producing propylene from paraffins comprises the steps of combining an effluent that comprises propylene and propane from a paraffin dehydrogenation reactor with an offgas stream that comprises propane to form a combined effluent stream. The combined effluent stream is separated into a propylene product stream and a propane-rich recycle stream. The propane-rich recycle stream is introduced to the paraffin dehydrogenation reactor operating at dehydrogenation conditions to convert propane in the propane-rich recycle stream to propylene.

In accordance with another exemplary embodiment, a process for producing propylene from paraffins is provided. The process comprises the steps of feeding n-butane to a butane isomerization process to form an iso-butane product stream and an offgas stream that comprises propane. Propane is fed to a paraffin dehydrogenation reactor that is operating at dehydrogenation conditions to produce an effluent that comprises propylene and propane. The effluent is combined with the offgas stream to form a combined effluent stream. The combined effluent stream is separated into a propylene product stream and a propane-rich recycle stream. The propane-rich recycle stream is introduced to the paraffin dehydrogenation reactor to convert propane in the propane-rich recycle stream to propylene.

DETAILED DESCRIPTION

Various embodiments contemplated herein relate to processes for producing propylene from paraffins such as propane. A fresh propane feed is introduced to a paraffin dehydrogenation reactor that contains dehydrogenation catalyst and is operating at dehydrogenation conditions to produce an effluent. The effluent contains propylene, unconverted propane, hydrogen, and various other hydrocarbons such as C2−, C4+, and some dienes and alkynes. As used herein, Cxmeans hydrocarbon molecules that have “X” number of carbon atoms, Cx+ means hydrocarbon molecules that have “X” and/or more than “X” number of carbon atoms, and Cx− means hydrocarbon molecules that have “X” and/or less than “X” number of carbon atoms. An offgas stream containing at least some propane is combined with the effluent to form a combined effluent stream that contains the additional propane from the offgas stream. Preferably, the offgas stream, e.g., a waste or tail gas stream for burning and the like, is formed from another process in the plant that is being used to produce, for example, a valuable product stream.

In an exemplary embodiment, the offgas stream is produced from a paraffin isomerization process, such as, for example, a butane isomerization process that converts n-butane to iso-butane. The butane isomerization process includes an isomerization reactor that converts an n-butane feed to an iso-butane-rich stream that is separated via a distillation column or the like into an iso-butane product stream and an offgas stream. The offgas stream contains propane, hydrogen, some n-butane and/or iso-butane, and other various light hydrocarbons, such as, for example, C2−, C5+, and the like.

The effluent from the paraffin dehydrogenation reactor is compressed to a predetermined high pressure prior to being combined with the offgas stream that is preferably at a similar or greater pressure than the compressed effluent. The offgas stream and the effluent are combined to form a combined effluent stream that is subsequently cooled and separated to form a propylene product stream and a propane-rich recycle stream. The propane-rich recycle stream is then introduced to the paraffin dehydrogenation reactor along with additional fresh propane feed. Because the propane-rich recycle stream contains additional propane from the paraffin isomerization process, more propane is available in the paraffin dehydrogenation reactor for conversion to propylene compared to conventional processes, thereby increasing the amount of propylene product produced. Moreover, minimal additional equipment and/or cost are needed to fluidly couple the paraffin isomerization process to the effluent section of the paraffin dehydrogenation process.

Referring toFIG. 1, a schematic depiction of an apparatus10for producing propylene from paraffins in accordance with an exemplary embodiment is provided. The apparatus10comprises a paraffin isomerization section12and a paraffin dehydrogenation section14. The paraffin isomerization section12is configured as a butane isomerization process for converting n-butane to iso-butane. The paraffin dehydrogenation section14is configured as a catalytic dehydrogenation process for converting propane to propylene. The paraffin isomerization section12comprises a reactor section16and a stabilizers section18.

FIG. 2is a detailed illustration of paraffin isomerization section12. The reactor section16and the stabilizer section18of paraffin isomerization section12include a reactor17and a distillation column19, respectively, that are in fluid communication. As illustrated, a feed stream20, which is preferably rich in n-butane and may also contain relatively small amounts of iso-butane, pentanes, and heavier materials, is passed through a dryer22for removing water. A hydrogen feed24is passed through a dryer26for removing water and is combined with the dried n-butane rich stream to form a combined stream21. The combined stream21is passed through a heat exchanger28and a heater30, and is introduced to the reactor17.

In an exemplary embodiment, the reactor17is a fixed-bed catalytic reactor operating at a temperature of about 90 to about 150° C. and containing a high-activity chloride-promoted catalyst to isomerize n-butane to iso-butane to produce an iso-butane-rich stream32. The iso-butane-rich stream32is passed through the heat exchanger28and is introduced to the distillation column19that separates the iso-butane-rich stream32into an iso-butane product stream34as, for example, a bottom stream and a LPG (liquefied petroleum gas) stream36as, for example, an overhead stream. The LPG stream36is passed through a cooler38and is introduced to a vent drum40. A liquid stream42is removed from the bottom of the vent drum40and is passed back to the distillation column19for reflux.

Light volatiles are removed from the vent drum40and form an offgas stream44. The offgas stream44contains propane, hydrogen, some n-butane and/or iso-butane, and other various light hydrocarbons, such as, for example, C2−, C5+, and the like. In an exemplary embodiment, the offgas stream44is vented from the vent drum40at a relatively high pressure of from about 1,000 to about 2,000 kPa. Referring back toFIG. 1, the offgas stream44may optionally be passed through a scrubber46to remove any chloride containing compounds, such as, for example, hydrogen chloride that may have been formed during interaction with the high-activity chloride-promoted catalyst in the reactor17. Alternatively, any chloride containing compounds may be removed from the offgas stream44downstream in the paraffin dehydrogenation section14as will be discussed in further detail below.

A fresh propane feed48is passed through a feed guard bed (not shown) and a feed dryer (not shown) before being directed to a depropanizer unit50(e.g. single or multiple column) for C4+ removal. The feed guard bed removes organic metal compounds and the feed dryer removes nitrogen compounds and water from the fresh propane feed48to protect downstream catalyst and reactor performance and to form a treated propane feed52.

The treated propane feed52is introduced to a dehydrogenation reactor and effluent section56.FIG. 3is a detailed illustration of the dehydrogenation reactor and effluent section56in accordance with an exemplary embodiment. The dehydrogenation reactor and effluent section56comprises a reactor section58in fluid communication with an effluent section60. As illustrated, the reactor section58comprises four fired heaters62,64,66, and68and four moving bed reactors70,72,74, and76alternately connected in series. The four moving bed reactors70,72,74, and76each contain dehydrogenation catalyst, e.g., platinum-containing catalyst and the like, and are configured to advance the dehydrogenation catalyst as moving beds between the reactors70,72,74, and76as is well known in the art. Other reactor arrangements for dehydrogenation of paraffins, such as, for example, swing bed reactor arrangements and the like may also be used.

The treated propane feed52is passed through a heat exchanger78to the first fired heater62that provides the necessary thermal energy for converting a first portion of the propane in the treated propane feed52to propylene via a one-step endothermic dehydrogenation reaction in the first reactor70to form a partially reacted stream80. In an exemplary embodiment, the four moving bed reactors70,72,74, and76are operating at a temperature of from about 575 to about 675° C. and at a relatively low pressure of within about a few 100 kPa of atmospheric pressure. The partially reacted stream80is passed along from the first reactor70to the second fired heater64and reactor72for further conversion of propane to propylene to form a second partially reacted stream82. The second partially reacted stream82is passed along to the third fired heater66and third reactor74for further conversion of propane to propylene to form a third partially reacted stream84that is then passed along to the fourth fired heater68and fourth reactor76for further conversion of propane to propylene to form an effluent86.

In an exemplary embodiment, the effluent86contains propylene, unconverted propane, hydrogen, and various other hydrocarbons such as C2−, C4+, and some dienes and alkynes. As indicated by the dashed lines88, partially spent dehydrogenation catalyst is transferred from the first reactor70progressively to each of the next reactors72,74, and76in a moving bed fashion for further conversion of propane to propylene, and is then sent to a regeneration unit90for regeneration of the spent dehydrogenation catalyst. The regenerated dehydrogenation catalyst is then transferred from the regeneration unit90back to the first reactor70to replenish partially spent catalyst that is being removed from the first reactor70.

As illustrated, the effluent86is passed from the reactor section58to the effluent section60where the effluent86is partially cooled via a heat exchanger78. The effluent86is then passed to a compressor92that compresses the effluent86to a predetermined high pressure. In an exemplary embodiment, the effluent86is compressed to a predetermined high pressure of from about 1,000 to about 2,000 kPa.

The offgas stream44is combined with the effluent86in the effluent section60to form a combined effluent stream94. In an exemplary embodiment, the offgas stream44is at about the same pressure or greater than the pressure of the effluent86after being compressed to the predetermined high pressure. Preferably, the offgas stream44is combined with the effluent86downstream from the compressor92as illustrated. However, although the offgas stream44is shown as being combined with the effluent86downstream from the compressor92, it should be understood that the offgas stream44can alternatively be combined with the effluent86upstream from the compressor92.

The combined effluent stream94is dried and treated by removing any chloride containing compounds via a chloride treater and dryer unit96. The combined effluent stream94is then passed along to the cold box section98of the effluent section60. As illustrated, the cold box section98comprises a separator100, an expander102, and a heat exchanger104that are cooperatively configured to cryogenically cool the combined effluent stream94. In particular, hydrogen is removed from the combined effluent stream94, which is a mixed vapor-liquid phase, downstream in the separator100as, for example, an overhead stream and is expanded via the expander102to form a chilled hydrogen stream99. The upstream combined effluent stream94is cryogenically cooled via indirect heat exchange with the chilled hydrogen stream99in the heat exchanger104and is removed from the effluent section60as, for example, a liquid bottom stream from the separator100. In an exemplary embodiment, the combined effluent stream94is cooled to a temperature of from about −130 to about −150° C. As illustrated, the chilled hydrogen stream99is split downstream from the heat exchanger104into a hydrogen recycle stream120that is combined with the treated propane feed52to provide fuel for the first fired heater62, and a second hydrogen stream122that is removed from the dehydrogenation reactor and effluent section56.

Referring again toFIG. 1, the combined effluent stream94is passed along to a selective hydrogenation reactor106where methyl acetylene and other triple bond hydrocarbons and dienes contained in the combined effluent stream94are saturated to enhance the purity and to form a first treated combined effluent stream95. The first treated combined effluent stream95is then introduced to the deethanizer unit108for removal of C2− hydrocarbons as, for example, an overhead stream110and to form a second treated combined effluent stream97. The second treated combined effluent stream97is removed from the deethanizer unit108as, for example, a bottom stream and is passed along to the propylene-propane splitter unit112. The propylene-propane splitter unit112separates the second treated combined effluent stream97into a propylene product stream114as, for example, an overhead stream and a propane-rich recycle stream116as, for example, a bottom stream.

The propane-rich recycle stream116is advanced to the depropanizer unit50and is mixed with the fresh propane teed48. In an exemplary embodiment, the depropanizer unit50removes heavier hydrocarbons from the combined fresh propane feed48and the propane-rich recycle stream116. As illustrated, the heavier hydrocarbons are separated and removed as a C4hydrocarbon stream126and a C5+ hydrocarbon stream128. In a preferred embodiment, the C4hydrocarbon stream126is passed along to the reactor section16of the paraffin isomerization section12for conversion with the n-hutane feed20to increase the amount of iso-butane contained in the iso-butane product stream34.

Accordingly, processes for producing propylene from paraffins such as propane have been described. The various embodiments comprise feeding fresh propane to a paraffin dehydrogenation reactor that contains dehydrogenation catalyst and is operating at dehydrogenation conditions to produce an effluent. The effluent contains propylene, unconverted propane, hydrogen, and various other hydrocarbons such as C2−, C4+, and some dienes and alkynes. An offgas stream containing at least some propane is combined with the effluent to form a combined effluent stream that contains the additional propane from the offgas stream. The combined effluent stream is cooled and separated to form a propylene product stream and a propane-rich recycle stream. The propane-rich recycle stream is then introduced to the paraffin dehydrogenation reactor along with additional fresh propane feed. Because the propane-rich recycle stream contains additional propane from the paraffin isomerization process, more propane is available in the paraffin dehydrogenation reactor for conversion to propylene compared to conventional process, thereby increasing the amount of propylene product produced. Moreover, minimal additional equipment and/or cost are needed to fluidly couple the paraffin isomerization process to the effluent section of the paraffin dehydrogenation process.