Polypropylene production using a propylene stream containing propane

Abstract

Embodiments disclosed herein relate to the production of polypropylene. A propylene stream (17) containing propane is fed to a gas-phase polymerization zone (2). The propylene is reacted in the gas-phase polymerization zone (2) to produce a polymerization product (21). A recycle gas stream (9) from the gas-phase polymerization zone (2) is fed to a heat exchanger (7) to remove heat of the polymerization reaction, forming a first cooled gas stream (4). The first cooled gas stream (4) is recycled to the gas-phase polymerization zone (2), and a second cooled gas stream (24) from the heat exchanger (7) is fed to a first separation system (10) to form a propane-rich stream (12) and a second propylene stream (13). The first separation system (10) is operated by a heat pump (27) configured to compress the second propylene stream (13) to form an at least partially condensed propylene stream (13b). The liquid portion (13d) and optionally the vapor portion (13c) of the at least partially condensed propylene stream (13b) are fed to the gas-phase polymerization zone (2).

Description

[Technical Field]

[0001] The present disclosure relates to processes and systems for producing polypropylene from a propylene stream containing propane, such as at least about 0.5 wt. % propane, based on the total weight of the propylene stream. [Background technology]

[0002] The process for producing polypropylene conventionally involves adding propylene, a polypropylene-forming catalyst, and other optional compounds, including, for example, hydrogen as a chain transfer agent and a light gas such as ethylene as a comonomer, to a polymerization reactor to produce the polypropylene product. The product exiting the reactor contains various contaminants, including unreacted propylene, which generally must be removed to obtain an acceptable polymer.

[0003] These processes traditionally involve high purity propylene as a feedstock, commonly referred to as polymer-grade propylene, which contains propylene and propane as the primary diluent. In this disclosure, "polymer-grade" is intended to designate a propylene feed having a purity of at least about 99.5 wt.%, based on the total weight of the propylene feed.

[0004] An example of the use of high-purity propylene as a feedstock for a polypropylene reaction process is described in U.S. Patent No. 6,271,319 B1. According to this document, the presence of propane can be problematic in polypropylene reaction systems, and its removal is particularly difficult. Therefore, U.S. Patent No. 6,271,319 B1 describes a method for removing propane from propylene in a reactor vent stream using a gas separation membrane. Other documents disclosing the use of membranes to remove propane from propylene streams include U.S. Patent No. 6,963,018 B2, U.S. Patent No. 9,120,882 B2, and U.S. Patent No. 2006 / 0266213 A1.

[0005] The difficulty in removing propane from a propylene stream lies in their similar volatilities and boiling points. Therefore, obtaining polymer-grade propylene may require specialized, large-scale separation equipment, such as a distillation column requiring a large number of theoretical trays and a high reflux ratio. Alternative separation equipment may include membrane systems, which promise lower energy consumption but are largely unproven on an industrial scale. Therefore, prior art separation equipment may be large and / or unsuitable or unreliable for industrial production, and may also be expensive, significantly increasing the cost of polymer-grade propylene.

[0006] Therefore, it may be beneficial to use lower grade propylene feedstocks that are effective in producing polypropylene. It may also be beneficial to recover unreacted contaminants, such as propylene, from the polymer product.

[0007] In this disclosure, "intermediate-grade" propylene streams, which are intended to designate streams containing less than 99.5 wt.% propylene, e.g., less than 99 wt.% propylene, based on the total weight of the stream, can also be used as feedstocks. Intermediate-grade streams contain more propane than conventional polymer-grade propylene streams, but can still provide high-quality polypropylene products. One example of a process for making polypropylene includes contacting an oxygen-containing stream with an olefin-forming catalyst to form an olefin stream and separating an intermediate-grade propylene stream from the olefin stream, the intermediate-grade propylene stream containing less than 99.5 wt.% propylene, based on the total weight of the stream, and is disclosed in EP 1 723 183 A1.

[0008] In other examples, "chemical-grade" propylene streams (which in this disclosure are intended to designate streams containing less than 95 wt% propylene based on the total weight of the stream) may be used as feedstock. For example, WO 2022 / 129063 A1 describes the use of membrane gas separators to separate olefins and paraffins downstream of a polymerization reactor. This document describes the production of polyolefins from non-polymer-grade olefin monomers, particularly chemical-grade olefins having a purity of about 92-96%.

[0009] Whether the feed is made from an intermediate-grade propylene stream or a chemical-grade propylene stream, the conventional approach to propane removal is to fractionate the intermediate-grade or chemical-grade feed to remove propane before feeding the feed of polymer-grade purity to the polymerization zone. However, as discussed above, this fractionation arrangement requires distillation columns containing many trays and can consume excessive amounts of energy.

[0010] Additionally, in a typical polymerization process, the raw effluent from the polymerization reactor is continuously transferred to a flash tank, from which a raw polymer stream is withdrawn for further purification. A top gas stream containing unreacted monomer is also withdrawn from the flash tank and either recycled to the reactor or sent to a recovery unit. Thus, the propylene feed to the reactor can be a combination of fresh propylene, propylene recycled in the reactor / flash process loop, or recovered propylene. Even if only a small proportion of propane is introduced into the reactor loop supplying fresh polymer-grade propylene, the circulating amount quickly accumulates, reducing catalyst activity and reactor productivity. Propane accumulation and steady-state propane concentration in the reactor are essentially determined by the amount of reactor gas withdrawn from the polymerization section, the recycle rate of the withdrawn gas, and whether the withdrawn gas undergoes purification before being recycled to the polymerization zone. Propane accumulation is typically controlled to maintain a steady-state propane content in the recycle loop in the range of approximately 2-30%.

[0011] The amount of gas entrained in the polymer product varies depending on the polymerization process, e.g., the operating conditions of the polymerization reactor and the sequence of product extraction. The composition of the reactor gas phase varies accordingly. However, a minimum amount of gas is always entrained in the polymer product. When low-purity propylene is used, a higher purge rate from the polymerization reactor is required to suppress the accumulation of propane in the reactor gas.

[0012] Recovery of propylene from vent streams is costly in terms of both capital costs and energy consumption. Therefore, polymerization processes generally seek to reduce the amount of gas released from the polymerization reactor. This requires that the amount of propane fed with the feed stream also be limited.

[0013] EP 0 887 359 A1 describes a propylene polymerization process that includes feeding propylene containing 0.1 wt. % to 20 wt. % propane to a polymerization reactor, distilling at least a portion of the unreacted propylene discharged from the reactor to remove the propane contained in the unreacted propylene, and circulating the purified propylene to the polymerization reactor, thereby withdrawing propane from the polymerization reaction system. The propylene polymerization process can be carried out as a liquid-phase or gas-phase polymerization process. According to one embodiment of EP 0 887 359 A1, propylene is polymerized in the gas phase in the polymerization reactor. The resulting propylene-based polymer is continuously or intermittently withdrawn from a polymer recovery port. The unreacted propylene discharged from the polymerization reactor contains propane introduced with the starting material. The unreacted propylene is typically cooled in a heat exchanger and then separated into a condensate and a gas phase using a separator (gas-liquid separator). The gas phase is circulated from the first line through a compressor to the polymerization reactor, and the condensate is circulated from the second line to the polymerization reactor. At least a portion of the condensate (unreacted propylene) from the second line is introduced into a distillation apparatus and separated into propylene and propane by distillation. That is, the unreacted propylene is purified by distillation, the propane is discharged outside the reaction system, and the purified propylene is circulated to the polymerization reactor.

[0014] Heat pump fractionation columns have been used for close boiling mixtures for decades due to their low energy consumption. Fractionation columns are traditionally operated at low or medium pressure, with a heat pump compressor increasing the overhead product pressure and allowing them to condense in a bottom product reboiler. Heat pump fractionation columns are described, for example, in U.S. Pat. Nos. 3,568,457 A, 4,783,667 A, and 7,842,847 B2. In conventional heat pump fractionation columns, a small supply of light gases is required to fully condense the overhead product, as these gases accumulate and are trapped in the top section of the column.

[0015] Thus, within the context of processes and systems for producing polypropylene, there is a need to reduce energy consumption and increase energy efficiency while obtaining high quality polymers, without increasing the size or complexity of the system. Summary of the Invention

[0016] Embodiments disclosed herein provide an energy-efficient process and system for producing polypropylene from a propylene stream containing propane as a diluent or primary diluent, e.g., at least about 0.5 wt.% or more propane, based on the total weight of the propylene stream. The propane can be purged from a polymerization zone where the propylene reacts to produce a polymerization product. According to one or more embodiments, the process is a gas-phase process.

[0017] Among other things, embodiments disclosed herein may result in efficient use of energy even in the presence of light gases such as hydrogen and ethylene, reduced energy consumption for propane removal, and reduced size of separation systems used to separate propane from propylene. For example, if the separation system comprises a propylene fractionation column, its height and / or diameter may be reduced compared to conventional processes and systems.

[0018] Additionally, embodiments disclosed herein may facilitate and facilitate the recovery of unreacted contaminants, including propylene, from the polymerization product.

[0019] In one aspect, embodiments herein relate to a process for producing polypropylene, the process comprising: supplying a first propylene stream comprising propane to a gas-phase polymerization zone, wherein the propylene is reacted to produce a polymerization product and optionally recovering a carrier gas; supplying a recycle gas stream from the polymerization zone to a heat exchanger to remove heat of the polymerization reaction and form a first cooled gas stream and an optional gas vapor stream; recycling the first cooled gas stream to the polymerization zone; supplying a second cooled gas stream from the heat exchanger to a first separation system to form a propane-rich stream and a second propylene stream; operating the first separation system with a heat pump configured to compress the second propylene stream to form an at least partially condensed propylene stream; and supplying a liquid portion and optionally a vapor portion of the at least partially condensed propylene stream to the gas-phase polymerization zone.

[0020] In a further aspect, embodiments disclosed herein relate to a system for producing polypropylene, the system comprising: a gas-phase polymerization zone configured to convert a first propylene stream comprising propane to polypropylene and produce a polymerization product; a recycle gas cooling loop including a heat exchanger configured to remove heat of the polymerization reaction with a recycle gas stream from the polymerization zone and form a first cooled gas stream and an optional gas vapor stream that are recycled to the polymerization zone; a first separation system configured to separate the second cooled gas stream from the heat exchanger to form a propane-rich stream and a second propylene stream; a heat pump configured to compress the second propylene stream to power the first separation system and form an at least partially condensed propylene stream; a flow line for supplying a liquid portion of the at least partially condensed propylene stream to the gas-phase polymerization zone; and an optional flow line for supplying a vapor portion of the at least partially condensed propylene stream to the gas-phase polymerization zone.

[0021] By arranging the first separation system and heat pump in the polymerization zone recycle gas cooling loop downstream of the heat exchanger relative to the direction of the recycle gas flow recovered from the polymerization zone, efficient energy use and improved separation are obtained without compromising system simplicity and size.

[0022] Other aspects and advantages will become apparent from the following description and appended claims, which define further embodiments of the process and system. [Brief explanation of the drawings]

[0023] [Figure 1] 1 shows a simplified diagram of a polypropylene production system according to one or more embodiments.

[0024] [Figure 2] 1 shows the energy consumption of a propylene fractionator of a system for producing polypropylene according to one or more embodiments with different concentrations of propane in the feed to the propylene fractionator. DETAILED DESCRIPTION OF THE INVENTION

[0025] In one aspect, embodiments herein relate to a process for producing polypropylene, the process comprising: supplying a first propylene stream comprising propane to a gas-phase polymerization zone, wherein the propylene is reacted to produce a polymerization product and optionally recovering a carrier gas; supplying a recycle gas stream from the polymerization zone to a heat exchanger to remove heat of the polymerization reaction and form a first cooled gas stream and an optional gas vapor stream; recycling the first cooled gas stream to the polymerization zone; supplying a second cooled gas stream from the heat exchanger to a first separation system to form a propane-rich stream and a second propylene stream; operating the first separation system with a heat pump configured to compress the second propylene stream to form an at least partially condensed propylene stream; and supplying a liquid portion and optionally a vapor portion of the at least partially condensed propylene stream to the polymerization zone.

[0026] According to one or more embodiments, the first separation system is driven by a heat pump, thereby achieving efficient use of energy, which is independent of the nature of the feed to the polymerization reactor, which in addition to propylene may also include light gases as described above.

[0027] According to one or more embodiments, the heat pump may be configured to compress the second propylene stream within a range of about 15 barg to about 40 barg, such as about 20 barg to about 35 barg.

[0028] According to one or more embodiments, a process for producing polypropylene includes supplying a first propylene stream comprising propane to a gas-phase polymerization zone, where the propylene is reacted to produce a polymerization product and optionally recovering a carrier gas; supplying a recycle gas stream from the polymerization zone to a heat exchanger, such as a condenser, to at least partially condense the recycle gas stream and form a first gas condensate stream and a gas vapor stream; recycling the first gas condensate stream to the polymerization zone; supplying the second gas condensate stream from the heat exchanger to a first separation system to form a propane-rich stream and a second propylene stream; operating the first separation system with a heat pump configured to compress the second propylene stream to form an at least partially condensed propylene stream; and supplying the liquid and optionally vapor portions of the at least partially condensed propylene stream to the polymerization zone.

[0029] In another aspect, embodiments disclosed herein relate to a system for producing polypropylene, the system comprising: a gas-phase polymerization zone configured to convert a first propylene stream comprising propane to polypropylene and produce a polymerization product; a recycle gas cooling loop including a heat exchanger for removing heat of the polymerization reaction with a recycle gas stream from the polymerization zone to form a first cooled gas stream and an optional gas vapor stream that are recycled to the polymerization zone; a first separation system configured to separate a second cooled gas stream from the heat exchanger to form a propane-rich stream and a second propylene stream; a heat pump configured to compress the second propylene stream to power the first separation system to form an at least partially condensed propylene stream; a flow line for supplying a liquid portion of the at least partially condensed propylene stream to the polymerization zone; and an optional flow line for supplying a vapor portion of the at least partially condensed propylene stream to the gas-phase polymerization zone.

[0030] According to one or more embodiments, a system for producing polypropylene includes: a gas-phase polymerization zone configured to convert a first propylene stream comprising propane to polypropylene and produce a polymerization product; a recycle gas cooling loop including a heat exchanger, such as a condenser, for removing heat of the polymerization reaction with a recycle gas stream from the polymerization zone and forming a first gas condensate stream and a gas vapor stream that are recycled to the polymerization zone; a first separation system configured to separate the second gas condensate stream from the heat exchanger and form a propane-rich stream and a second propylene stream; a heat pump configured to compress the second propylene stream to power the first separation system and form an at least partially condensed propylene stream; a flow line for supplying a liquid portion of the at least partially condensed propylene stream to the polymerization zone; and an optional flow line for supplying a vapor portion of the at least partially condensed propylene stream to the gas-phase polymerization zone.

[0031] The following describes embodiments that are applicable to both processes and systems.

[0032] According to one or more embodiments, the first propylene stream comprising propane fed to the polymerization zone comprises at least about 0.5 wt.% propane, e.g., 0.5 wt.% to 20 wt.% propane, 1 wt.% to 15 wt.% propane, 2 wt.% to 12.5 wt.% propane, and 3 wt.% to 10 wt.% propane, based on the total weight of the stream.

[0033] According to one or more embodiments, the second propylene stream fed to the polymerization zone comprises at least about 0.5 wt.% propane, based on the total weight of the stream, such as from 0.5 wt.% to 20 wt.% propane, from 1 wt.% to 15 wt.% propane, from 2.5 wt.% to 12.5 wt.% propane, and from 5 wt.% to 10 wt.% propane.

[0034] According to one or more embodiments, each of the first and second propylene streams may comprise or consist of an intermediate grade propylene stream or a chemical grade propylene stream.

[0035] According to one or more embodiments, the process may further include providing any catalyst suitable for the polymerization of propylene, such as any catalyst selected from the group including Ziegler-Natta catalysts and single-site catalysts, such as metallocene catalysts.

[0036] According to one or more embodiments, the catalyst may comprise a Ziegler-Natta catalyst. The Ziegler-Natta catalyst comprises a magnesium / titanium / electron donor complex, optionally supported on a suitable support comprising, for example, silica or MgCl, in combination with an organoaluminum cocatalyst and an external selectivity control agent, such as, for example, an aromatic carboxylic acid ester or an alkoxysilane compound. Suitable Ziegler-Natta catalysts may include titanium-based catalysts such as those described in U.S. Pat. No. 4,376,062 A, U.S. Pat. No. 4,379,758 A, U.S. Pat. No. 5,066,737 A, and U.S. Pat. No. 9,522,968 B2.

[0037] According to one or more embodiments, the catalyst may comprise a metallocene catalyst. A metallocene catalyst comprises an organometallic coordination complex of one or more ligands associated with a metal atom. Examples of suitable metallocene catalysts are described, for example, in U.S. Pat. No. 7,169,864 B2.

[0038] According to one or more embodiments, the process may further include feeding additional optional compounds to the polymerization zone. The additional optional compounds may include, for example, one or more comonomers, such as ethylene and / or butene, to produce respective copolymers or terpolymers of propylene, and / or hydrogen to control the molecular weight of the polymerization product, and / or inert compounds, such as nitrogen or argon, that may be used for other purposes, such as controlling pressure and monomer concentration, and / or polymerization additives, such as, for example, static control agents or activity limiters (e.g., ATMER 163).

[0039] According to one or more embodiments, depending on the required capacity and polymerization product range, the polymerization zone may comprise at least one reactor or multiple reactors. For example, at least one reactor may comprise a gas-phase stirred-bed reactor, such as, for example, a vertical gas-phase stirred-bed reactor.

[0040] According to one or more embodiments, the polymerization zone comprises at least two reactors, for example, of any of the types set forth above. According to one or more embodiments, the two reactors may be arranged in series or in parallel. For example, to produce a propylene impact copolymer, two reactors may be arranged in series. For example, to achieve higher capacity and / or to produce a bimodal resin, two reactors may be arranged in parallel. When the polymerization zone comprises two or more reactors, each reactor may be equipped with a corresponding recycle gas cooling loop including a heat exchanger for removing heat of polymerization therefrom. For example, the heat exchanger may comprise a condenser.

[0041] According to one or more embodiments, the process further includes discharging the polymerization product from the polymerization zone (e.g., from the reactor(s) as defined in any one of the above-described embodiments), e.g., in the form of a polymer powder, and separating the polymerization product from the carrier gas and any unreacted contaminants in a polymer separation system. According to one or more embodiments, the polymer separation system comprises a polymer separator for separating the polymer from the reduced pressure propylene. The polymer separator may, for example, operate at a pressure ranging from 1 bar to 7 bar, e.g., from 1 bar to 4 bar, or from 2 bar to 4 bar, etc.

[0042] According to one or more embodiments, the polymer may be further degassed, for example, by countercurrent flushing with nitrogen or argon or other flushing gas in a purge silo. According to one or more embodiments, the purge off-gas may be sent to a purge gas recovery system. According to one or more embodiments, the powder may then be converted into pellets, which may incorporate well-dispersed additives.

[0043] According to one or more embodiments, the second cooled gas stream may have a temperature of about 5°C to about 50°C, such as, for example, about 15°C to about 45°C, e.g., about 20°C to about 40°C.

[0044] According to one or more embodiments, the propane-rich stream may contain propylene in addition to a predominant amount of propane. According to one or more embodiments, the propane-rich stream may contain propylene and butenes, as well as minor amounts of butane (e.g., less than 1% v / v butane), in addition to a predominant amount of propane. According to one or more embodiments, the propane-rich stream may contain at least 50% v / v propane and less than 50% v / v propylene, e.g., 40% v / v, 30% v / v, 20% v / v, 10% v / v, or 5% v / v to less than 50% v / v propylene.

[0045] According to one or more embodiments, the carrier gas may include propylene and propane, and one or more additional gases, which may include any one of nitrogen, argon, ethane, ethylene, butene, butane, and hydrogen. According to one or more embodiments, the one or more additional gases may be supplied to the polymerization zone during the polymerization process, for example, as a comonomer, as a fluidizing gas, as a purge gas, or to remove light hydrocarbons from the polymerization product.

[0046] According to one or more embodiments, the carrier gas may be recycled to the polymerization zone. According to one or more embodiments, the carrier gas may be sent to a recovery system to separate the propylene, which may then be recycled to the polymerization zone. According to one or more embodiments, the carrier gas may be sent to an external recovery system or user.

[0047] According to one or more embodiments, the heat exchanger comprises a condenser. In this (these) embodiments, the recycle gas stream is at least partially condensed, thereby forming a gas condensate stream and a gas vapor stream. According to one or more embodiments, the gas vapor stream may be fed to the polymerization zone or to the gas condensate stream that is recycled to the polymerization zone. According to one or more embodiments, the condenser may comprise a shell-and-tube heat exchanger, such as, for example, a vertical shell-and-tube heat exchanger.

[0048] According to one or more embodiments, the second gas condensate stream fed to the first separation system can be taken from the liquefied recycle gas of a single reactor and / or any liquefied recycle gas of a cascade reactor system or a parallel reactor system.

[0049] According to one or more embodiments, the first separation system in which propane is separated from propylene may comprise a propylene fractionation column. According to one or more embodiments, the first separation system in which propane is separated from propylene may comprise a distillation column. According to one or more embodiments, the first separation system may comprise a medium pressure fractionation column, such as a medium pressure distillation column. According to one or more embodiments, the medium pressure fractionation column may operate at a pressure of from about 8 barg to about 25 barg, for example, from 10 barg to about 22 barg.

[0050] According to one or more embodiments, the process further includes heat exchanging the compressed propylene stream from the heat pump with a bottoms product from the first separation system to form an at least partially condensed propylene stream. According to one or more embodiments, the process further includes returning a portion of the heated bottoms product and / or the condensed propylene stream to the first separation system, e.g., to the bottom and top of the distillation column, respectively. According to one or more embodiments, such heat exchange may be performed in a reboiler, e.g., a bottoms product reboiler. In this manner, the heat consumed by the reboiler is provided by the condensation of the second propylene stream.

[0051] According to one or more embodiments, the process further includes discharging the at least partially condensed or fully condensed propylene stream from the reboiler. According to one or more embodiments, the process further includes separating the at least partially condensed propylene stream discharged from the reboiler into a liquid portion and a vapor portion. In these embodiments, the liquid portion is at least partially returned to the first separation system, such as, for example, to the top of the distillation column. According to one or more embodiments, the separation may be performed by a vapor-liquid separator.

[0052] According to one or more embodiments, the first separation system may include a fractionation column including a reflux drum from which a vent stream can be removed to operate the fractionation column more efficiently, and an optional external recovery unit. According to one or more embodiments, the vent stream can be sent to the external recovery unit or returned to the polymerization zone. According to one or more embodiments, the fractionation column is a medium-pressure fractionation column and includes a heat pump. According to one or more embodiments, the heat pump may include a compressor whose discharge pressure can be selected within a predetermined range. In this manner, the flow rate of the vent stream can be minimized and / or recycling of the vent stream to the polymerization zone can be promoted. For example, the discharge pressure can be selected so that the vent stream from the reflux drum is directed to the suction side of the gas vapor compressor or directly to the polymerization zone.

[0053] According to one or more embodiments, the separation in the first separation system may be carried out at a temperature ranging from about 0°C to about 70°C, for example, from about 10°C to about 60°C, for example, from about 30°C to about 50°C.

[0054] According to one or more embodiments, the process may further include feeding the propylene feedstock comprising propane to a second separation system to form a first propylene stream and a light hydrocarbon stream that may comprise ethane. According to one or more embodiments, the light hydrocarbon stream may further comprise, in addition to ethane, one or more of methane, propylene, propane, hydrogen, nitrogen, ethylene, methane, oxygen, CO, CO2, and acetylene.

[0055] According to one or more embodiments, the propylene feedstock supplied to the second separation system comprises at least 0.5 wt.% propane, e.g., 0.5 wt.% to 20 wt.%, 1 wt.% to 15 wt.%, 2 wt.% to 12.5 wt.%, or about 3 wt.% to 10 wt.% propane, based on the total weight of the feedstock.

[0056] According to one or more embodiments, the second separation system in which the light hydrocarbon stream is separated from the propylene may comprise, for example, a stripper or a deethanizer, the deethanizer comprising, for example, a vent column for recovering ethylene from the overhead product of the deethanizer, to form the propylene stream that is fed to the polymerization zone. In this (these) embodiment, the propylene stream is therefore a purified propylene stream.

[0057] According to one or more embodiments, the process may further include filtering any incompletely formed polymer from the product stream from the polymerization zone. According to one or more embodiments, the process may further include feeding any unreacted components that may be found in the product stream from the polymerization zone to a second separation system. According to one or more embodiments, the process may further include compressing such unreacted components before feeding them to the second separation system. In a corresponding manner, the system according to one or more embodiments may include a compressor that compresses such unreacted components, the compressor being arranged in line for feeding the unreacted components to the second separation system. According to one or more embodiments, the unreacted components may include unreacted propylene, propane, unreacted comonomer, hydrogen, solvent, and other components used in the process.

[0058] According to one or more embodiments, the process may further include feeding a portion of the unreacted components to a heat exchanger, where they may be compressed first.

[0059] According to one or more embodiments, the process may further include supplying a gas vapor stream to the polymerization zone.

[0060] Referring now to Figure 1, a polypropylene production system according to one or more embodiments is generally designated by the reference numeral 1. In Figure 1, the same reference number may designate both a flow line and a flow stream therein.

[0061] In the embodiment shown in this figure, the polypropylene production system 1 comprises a gas-phase polymerization zone 2, a polymer separation system 20, a recycle gas cooling loop including a heat exchanger 7, a heat pump 27, a first separation system 10, and a second separation system 14.

[0062] Polymerization zone 2 may comprise a gas reactor such as a vertical gas-phase stirred-bed reactor 3. While FIG. 1 depicts this type of reactor, other types of reactors may be used according to one or more embodiments. Suitable reactors include any gas-phase reactor suitable for the polymerization of propylene, whether batch, semi-continuous, or continuous, including fluidized-bed reactors, horizontal or vertical stirred powder-bed reactors, and the like. Furthermore, although only one reactor is depicted in FIG. 1, polymerization zone 2 may comprise two or more reactors, e.g., at least two reactors arranged in series or parallel, in one or more embodiments.

[0063] The gas-phase stirred-bed reactor 3 converts a first propylene stream 17 containing propane into polypropylene. The first propylene stream 17 may be supplied to the gas-phase stirred-bed reactor 3 by a corresponding line that is in fluid communication with the second separation system 14 in the embodiment shown in FIG. 1 . However, according to the present disclosure, the system can achieve, among other things, efficient energy use, energy savings, simplified construction, and compactness even without the presence of the optional second separation system 14. Indeed, these technical effects are achieved by removing propane from the polymerization zone 2 in an energy-efficient manner, as will be described in further detail below.

[0064] The first propylene stream 17 may contain at least about 0.5 wt% propane, e.g., between 0.5 wt% and 20 wt% propane, between 1 wt% and 15 wt% propane, between 2 wt% and 12.5 wt% propane, between 3 wt% and 10 wt% propane, based on the total weight of the stream.

[0065] Additional components (not shown) may be fed to polymerization zone 2. For example, any catalyst suitable for polymerizing propylene, such as a Ziegler-Natta catalyst, may be fed to polymerization zone 2. The additional components may include, for example, hydrogen to control the molecular weight of the polymerization product, and / or optional comonomer(s) such as ethylene and / or butene, and / or an organoaluminum cocatalyst, and / or an external selectivity control agent, such as, for example, an aromatic carboxylic acid ester or an alkoxysilane compound.

[0066] Gas phase stirred bed reactor 3 produces a polymerization product 21. Polymerization product 21 can be a homopolymer or a copolymer, depending on whether a comonomer is fed to gas phase stirred bed reactor 3.

[0067] Generally, the polymerization can be carried out at a temperature ranging from about 40°C to about 150°C, e.g., from about 60°C to about 90°C, and / or at a pressure ranging from about 10 bar to 50 bar, e.g., from about 20 bar to about 35 bar.

[0068] In a cascade reactor configuration, the second polymerization zone downstream of the gas-phase stirred-bed reactor 3 may be operated at a pressure ranging from about 10 bar to about 30 bar and / or a temperature ranging from about 30°C to about 90°C.

[0069] The reaction time may depend on the reaction conditions selected. According to one or more embodiments, the reaction time may be from about 0.2 hours to about 5 hours, for example, from about 0.5 hours to about 2 hours.

[0070] In the embodiment shown, product stream 19 exits gas-phase stirred-bed reactor 3. Product stream 19 may contain, in addition to polymerization product 21, various components 22 including unreacted propylene, propane, unreacted comonomer, hydrogen, incompletely formed polymer, and any other components used in the process. These components may be removed from polymerization product 21 using polymer separation system 20.

[0071] According to one or more embodiments, polymer separation system 20 may include any one of a centrifuge, a cyclone separator, a flashing separator, a cooling separator, a distillation separator, an absorption separator, or a combination thereof. The separation system used may depend on the type of polymerization reactor and the feed components. FIG. 1 illustrates polymer separation system 20 in which polymerization product 21 (e.g., polypropylene) is recovered, but components 22 removed from the polypropylene may be recycled to second separation system 14. However, according to one or more embodiments, instead of recycling components 22 to second separation system 14, these components may be withdrawn via stream 26.

[0072] In this manner, according to one or more embodiments, light gases such as nitrogen may be removed from the polymerization zone. In the case of transitions between different polymerization products, ethylene and hydrogen may be removed from the polymerization zone to shorten the transition time between different polymerization products. In the case of a cascade reactor system in which only or primarily ethylene is fed to the second reactor, ethylene may be removed from the carrier gas, thereby allowing the recovered propylene to be fed to the first reactor without ethylene contamination. Additionally or alternatively, a portion 25 of component 22 may be returned to polymerization zone 2, for example, by recycling portion 25 to heat exchanger 7. In this manner, the monomer is re-fed to the polymerization zone without incurring additional processing costs.

[0073] Among the advantages of the processes and systems of the present disclosure, the propane concentration in the polymerization zone can be controlled independently of the amount of direct recycle because the propane can be purged independently of the recycle stream and polymer separation system.

[0074] In accordance with the present disclosure, recycle gas stream 9 is withdrawn from polymerization zone 2 and at least cooled, or partially or completely condensed, in heat exchanger 7 arranged downstream of gas-phase stirred-bed reactor 3 relative to the direction of recycle gas stream 9.

[0075] According to one or more embodiments, the heat exchanger 7 of the recycle gas cooling loop comprises a condenser 8 configured to at least partially condense the recycle gas or to fully condense the recycle gas. In the embodiment shown in Figure 1, a first gas condensate stream 4 and a gas vapor stream 23 are formed. The first gas condensate stream 4 and the gas vapor stream 23 are recycled to the gas-phase stirred-bed reactor 3, thus closing the recycle gas cooling loop. Optionally, as shown in the figure, the gas vapor stream 23 may be introduced into the first gas condensate stream 4. In this way, a more uniform gas composition of the reactor feed is obtained.

[0076] System 1 further includes a first separation system 10. First separation system 10 is configured to separate second condensate gas stream 24 from heat exchanger 7 to form a propane-rich stream 12 as a bottoms product and a second propylene stream 13 as an overhead product. Accordingly, first separation system 10 is arranged downstream of heat exchanger 7 relative to the direction of second condensate gas stream 24. For example, if about 90% or more of the recycle gas is condensed, the recycle gas flow rate can be about six to about nine times the amount of polypropylene produced. However, according to one or more embodiments, heat can be removed without necessarily reaching 90% condensation, provided the recycle gas cooling loop flow rate is sufficient. In any case, arranging first separation system 10 in the recycle cooling loop can significantly increase the flow rate.

[0077] The separation in the first separation system 10 may be carried out in a medium pressure separation column which may be operated, for example, at a temperature in the range of 0°C to 70°C, for example 10°C to 60°C, for example 30°C to 50°C, and / or at a pressure in the range of 8 barg to 25 barg, for example 10 barg to 22 barg, for example about 12 barg to about 18 barg.

[0078] The first separation system 10 may include, for example, a distillation column 11, e.g., a propylene fractionator, that produces a propylene-rich distillation product. According to one or more embodiments, the distillation column 11 may include trays. The first separation system 10 is operated via a heat pump 27, e.g., at a pressure within any of the ranges described above. According to one or more embodiments, the heat pump 27 may be configured to compress the second propylene stream to a pressure range of about 15 barg to about 40 barg, such as about 20 barg to about 35 barg.

[0079] The propane stream 12 recovered by the first separation system 10 may also contain heavier components such as butanes and butenes. The second propylene stream 13 is returned to the polymerization zone 2.

[0080] According to the present disclosure, the heat pump 27 is configured to compress the second propylene stream 13 to power the first separation system 10 and form an at least partially condensed propylene stream 13b. The liquid portion 13d and optionally the vapor portion 13c of the at least partially condensed propylene stream 13b are fed to the polymerization zone 2.

[0081] 1, system 1 further comprises a reboiler 28 configured to heat exchange compressed propylene stream 13a from heat pump 27 with a bottoms product 31 from first separation system 10. As a result of this heat exchange, a heated bottoms product is obtained from bottoms product 31 and a condensed propylene stream is obtained from compressed propylene stream 13a.

[0082] In the embodiment shown in FIG. 1, flow line 32 returns the heated bottoms product to the bottom of first separation system 10, and flow line 13e returns a portion of the condensed propylene stream as reflux to the top of first separation system 10.

[0083] According to the embodiment of FIG. 1, flow line 13e provides reflux to first separation system 10 from a gas-liquid separator described below.

[0084] System 1 may further comprise an additional heat exchanger 30 downstream of heat pump 27 to close the heat balance, for example, by at least partially removing the energy input of heat pump 27. According to one or more embodiments, heat exchanger 30 may be arranged downstream of heat pump 27 but in parallel with reboiler 28.

[0085] 1, system 1 may further comprise a gas-liquid separator 29 for separating the at least partially condensed propylene stream 13b discharged from reboiler 28 into a liquid portion 13d and a vapor portion 13c, and for collecting the reflux in first separation system 10. In this embodiment, liquid portion 13d is fed to the flow line feeding first gas condensate stream 4 to gas-phase polymerization zone 2, and vapor portion 13c is fed to the flow line feeding gas vapor stream 23 to gas-phase polymerization zone 2. However, according to other embodiments, liquid portion 13d and vapor portion 13c may be fed directly to gas-phase polymerization zone 2.

[0086] 1, system 1 further comprises both a flow line for feeding the at least partially condensed liquid portion 13d of propylene stream 13b to gas-phase polymerization zone 2 and a flow line for feeding the at least partially condensed vapor portion 13c of propylene stream 13b to gas-phase polymerization zone 2. However, according to other embodiments of the system, only liquid portion 13d may be fed to gas-phase polymerization zone 2.

[0087] According to one or more embodiments, the system does not include a gas-liquid separator. According to one or more embodiments, the system may include a vessel, such as a reflux drum or accumulator, from which reflux may be provided to the first separation system 10.

[0088] According to one or more embodiments, system 1 further comprises a second separation system 14. Second separation system 14 is configured to separate propylene feed 18, which includes propane, to form a first propylene stream 17 as a bottoms product and a light hydrocarbon stream 16, which may contain C1-C2 hydrocarbons and / or other impurities, as an overhead product. The presence of separation system 14 may be beneficial, for example, when light impurities are present in propylene feed 18 or when venting light gases from stream 22 before recycling C3s and C4s to polymerization zone 2.

[0089] Propylene feedstock 18 may comprise or consist of an intermediate grade propylene stream or a chemical grade propylene stream. For example, propylene feedstock 18 may include at least about 0.5 wt% propane, e.g., 3 wt% to 10 wt% propane, based on the total weight of the feedstock.

[0090] The light hydrocarbon stream 16 may comprise ethane and other light components such as ethylene, methane, acetylene, nitrogen, hydrogen, oxygen, and incompletely separated propylene and propane.

[0091] The separation in the second separation system 14 may be carried out in a high-pressure separation column, which may be operated at a temperature in the range of -20°C to 80°C and / or at a pressure in the range of 30 barg to 40 barg, for example about 35 barg.

[0092] The second separation system 14 may include a distillation column 15, e.g., a deethanizer, that produces a top product rich in methane, ethane, and ethylene. According to one or more embodiments, the distillation column 15 may include trays. For example, the deethanizer may operate at a pressure between 10 barg and 40 barg, e.g., about 35 barg.

[0093] According to one or more embodiments, the propane concentration in polymerization zone 2 is between 2 wt. % and 30 wt. % based on the total weight of polymerization zone gas. According to one or more embodiments, the propane concentration in first propylene stream 17 fed to polymerization zone 2 comprises between 0.5 wt. % and 20 wt. % propane based on the total weight of polymerization zone gas. Because the propane concentration in polymerization zone 2 is higher than the feed to polymerization zone 2, separation of propane and propylene is facilitated and more convenient.

[0094] According to one or more embodiments, the first propylene stream 17 may also be fed downstream where stream 24 is withdrawn from heat exchanger 7 .

[0095] FIG. 2 shows the energy consumption of a propylene fractionator of a polypropylene production system according to one or more embodiments for different propane concentrations in the feed to the propylene fractionator. Specifically, the results are for a heat-pumped propylene fractionator and were generated using flowsheet simulations using simulation software Aspen Plus. The propylene content in the bottom of the propylene fractionator is set to 5 wt% in each case. The feed flow rate is set to purge the same amount of propane, while the recycle rate is minimized to reduce energy consumption. The results show that increasing the propane content in the feed to the propylene fractionator from 5 wt% to 10 wt% reduces the energy consumption per unit of purged propane by more than 50%.

[0096] As described above, embodiments herein provide a process and system for producing polypropylene, which can advantageously reduce the energy requirements for propane removal, and therefore the overall energy requirements of the process and system, and use energy in an efficient manner. In addition, the system may be small in size. In addition, the process and system can produce a high-quality polymerization product that is substantially indistinguishable from a polymerization product produced using a polymer-grade propylene feedstock, despite the presence of propane in the propylene feedstock.

[0097] While the present disclosure includes a limited number of embodiments, those skilled in the art having the benefit of this disclosure will appreciate that other embodiments may be devised that do not depart from the scope of the present disclosure, which scope should therefore be limited only by the appended claims.

Claims

1. 1. A process for producing polypropylene, comprising: feeding a first propylene stream (17) containing propane to a gas-phase polymerization zone (2) where the propylene is reacted to form a polymerization product (21) and a carrier gas is recovered; feeding a recycle gas stream (9) from said gas phase polymerization zone (2) to a heat exchanger (7) to remove heat of said polymerization reaction and form a first cooled gas stream (4) and an optional gas vapor stream (23); recycling said first cooled gas stream (4) to said gas phase polymerization zone (2); feeding the second cooled gas stream (24) from said heat exchanger (7) to a first separation system (10) to form a propane-rich stream (12) and a second propylene stream (13); operating the first separation system (10) with a heat pump (27) configured to compress the second propylene stream (13) to form an at least partially condensed propylene stream (13b); feeding the liquid portion (13d) and optionally the vapor portion (13c) of said at least partially condensed propylene stream (13b) to said gas-phase polymerization zone (2).

2. exchanging heat between a compressed propylene stream (13a) from the heat pump (27) and a bottoms product (31) from the first separation system (10) to form the at least partially condensed propylene stream (13b); 10. The process of claim 1, further comprising returning a portion of the heated bottoms product (32) and / or the condensed propylene stream (13e) to the first separation system (10).

3. 3. The process of claim 1 or claim 2, further comprising separating the at least partially condensed propylene stream (13b) into the liquid portion (13d) and the vapor portion (13c).

4. The process of any one of claims 1 to 3, wherein the first propylene stream (17) comprises at least 0.5 wt% propane, based on the total weight of the first propylene stream (17).

5. 5. The process of any one of claims 1 to 4, wherein the second propylene stream (13) comprises at least 0.5 wt% propane, based on the total weight of the second propylene stream (13).

6. 6. The process of any one of claims 1 to 5, wherein the carrier gas comprises propylene, propane, and one or more of nitrogen, ethane, ethylene, butane, butene, and hydrogen.

7. The process according to any one of claims 1 to 6, wherein the heat exchanger (7) comprises a vertical shell-and-tube condenser (8).

8. The process according to any one of claims 1 to 7, wherein the first separation system (10) comprises a distillation column (11).

9. 9. The process of any one of claims 1 to 8, further comprising feeding a propylene feedstock (18) comprising propane to a second separation system (14) to form the first propylene stream (17) and a hydrocarbon stream (16) comprising C1-C2 hydrocarbons.

10. 10. The process of claim 9, wherein the propylene feedstock (18) comprises at least 0.5 wt% propane, based on the total weight of the propylene feedstock (18).

11. 11. The process of claim 9 or claim 10, further comprising feeding any unreacted components in the product stream (19) from the gas-phase polymerization zone (2) to the second separation system (14), and optionally feeding a portion (25) of the unreacted components to the heat exchanger (7).

12. The process of any one of claims 1 to 11, further comprising feeding said gas vapor stream (23) to said gas phase polymerization zone (2).

13. 1. A system for producing polypropylene, comprising: a gas-phase polymerization zone (2) configured to convert a first propylene stream (17) comprising propane to polypropylene and produce a polymerization product (21); a recycle gas cooling loop including a heat exchanger (7) configured to remove heat of the polymerization reaction with a recycle gas stream (9) from the gas-phase polymerization zone (2), forming a first cooled gas stream (4) recycled to the gas-phase polymerization zone (2) and an optional gas vapor stream (23); a first separation system (10) configured to separate the second cooled gas stream (24) from said heat exchanger (7) to form a propane-rich stream (12) and a second propylene stream (13); a heat pump (27) configured to compress the second propylene stream (13) to power the first separation system (10) and form an at least partially condensed propylene stream (13b); a flow line (13d) for feeding the liquid portion of said at least partially condensed propylene stream (13b) to said gas-phase polymerization zone (2); an optional flow line (13c) for supplying a vapor portion of said at least partially condensed propylene stream (13b) to said gas-phase polymerization zone (2).

14. 14. The system of claim 13, further comprising a reboiler (28) configured to heat exchange a compressed propylene stream (13a) from the heat pump (27) with a bottom product (31) from the first separation system (10), a flow line (32) for returning the heated bottom product to the first separation system (10), and a flow line (13e) for returning a portion of the condensed propylene stream to the first separation system (10).

15. 15. The system of claim 14, further comprising a liquid-vapor separator (29) for separating the at least partially condensed propylene stream (13b) discharged from the reboiler (28) into the liquid portion (13d) and the vapor portion (13c).

16. The system according to any one of claims 13 to 15, wherein the gas phase polymerization zone (2) comprises at least one gas phase stirred bed reactor (3).

17. The system according to any one of claims 13 to 16, wherein the heat exchanger (7) comprises a vertical shell-and-tube condenser (8).

18. The system of any one of claims 13 to 17, wherein the first separation system (10) comprises a medium pressure fractionator (11).

19. 19. The system of any one of claims 13 to 18, further comprising a second separation system (14) configured to separate a propylene feedstock (18) comprising propane to form the first propylene stream (17) and a light hydrocarbon stream (16) comprising C1-C2 hydrocarbons.

20. 20. The system of claim 19, further comprising a polymer separation system (20) for removing unreacted components from the polymerization product (21).

21. 21. The system of claim 20, further comprising a flow line for supplying the unreacted components to the second separation system (14).

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