Polypropylene production using propylene stream comprising propane

Abstract

Embodiments disclosed herein relate to the production of polypropylene. A propylene stream (17) comprising propane is fed to the gas phase polymerization zone (2). 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, thereby forming a first cooled gas stream (4). The first cooled gas stream (4) is recycled to the gas-phase polymerization zone (2) and the second cooled gas stream (24) from the heat exchanger (7) is fed to a first separation system (10) forming 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). A liquid portion (13d) and optionally a vapour portion (13c) of the at least partially condensed propylene stream (13b) is fed to the gas phase polymerization zone (2).

Description

Technical Field

[0001] This disclosure relates to a method and system for producing polypropylene from a propylene stream containing propane (such as, for example, at least about 0.5% by weight of propane based on the total weight of the propylene stream). Background Technology

[0002] Methods for producing polypropylene typically involve adding propylene, a polypropylene-forming catalyst, and other optional compounds, including light gases (such as hydrogen as a chain transfer agent and ethylene as a comonomer), to a polymerization reactor, and producing a polypropylene product. The product exiting the reactor contains various contaminants, including unreacted propylene, which must typically be removed to obtain an acceptable polymer.

[0003] These methods typically involve high-purity propylene as a feedstock, often referred to as polymer-grade propylene, which contains propylene and propane as the primary diluent. In this disclosure, "polymer-grade" is intended to specify a propylene feedstock having a purity of at least about 99.5% by weight based on the total weight of the propylene feedstock.

[0004] US 6,271,319 B1 describes an example of using high-purity propylene as a feedstock in a polypropylene reaction process. According to this document, the presence of propane can be a problem in polypropylene reaction systems, and its removal is particularly difficult. Therefore, US 6,271,319 B1 describes a method for removing propane from propylene in a reactor discharge stream using a gas separation membrane. Additional references disclosing the use of membranes for removing propane from propylene streams are US 6,963,018 B2, US 9,120,882 B2, and US 2006 / 0266213 A1.

[0005] The difficulty in removing propane from a propylene stream lies in their similar volatility and boiling point. Therefore, specific and extensive separation equipment may be required, such as distillation columns requiring large theoretical trays and high reflux ratios to obtain polymer-grade propylene. Alternative separation equipment may include membrane systems, which, while promising to reduce energy consumption, are largely unproven on an industrial scale. Therefore, existing 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 will be effective in the manufacture of polypropylene. It may also be beneficial to recover unreacted contaminants, such as propylene, from the polymer product.

[0007] "Intermediate" propylene streams can also be used as feedstocks, which in this disclosure are intended to refer to streams containing less than 99.5% by weight of propylene based on the total weight of the stream, for example, less than 99% by weight of propylene. Although intermediate streams contain a higher amount of propane than conventional polymer-grade propylene streams, they can still provide high-quality polypropylene products. Examples of methods for preparing polypropylene are disclosed in EP 1 723 183 A1, which includes contacting an oxygen-containing compound stream with an olefin-forming catalyst to form an olefin stream and separating an intermediate propylene stream from the olefin stream, wherein the intermediate propylene stream contains less than 99.5% by weight of propylene based on the total weight of the stream.

[0008] In other instances, a “chemical-grade” propylene stream can be used as a feedstock, which in this disclosure is intended to refer to a stream containing less than 95% by weight of propylene based on its total weight. For example, WO 2022 / 129063 A1 describes the use of a membrane gas separator for separating olefins and alkanes downstream of a polymerization reactor. This document describes the production of polyolefins from non-polymer-grade olefin monomers, and particularly chemical-grade olefins with a purity of about 92% to 96%.

[0009] For feedstocks made from either intermediate or chemical-grade propylene streams, conventional propane removal methods involve fractionating the intermediate or chemical-grade feedstock to remove propane before feeding it into the polymerization zone with polymer-grade purity. However, as discussed above, such fractionation arrangements may require distillation columns with a large number of trays and consume excessive energy.

[0010] Furthermore, in typical polymerization methods, the crude effluent from the polymerization reactor can be continuously transferred to a flash tank, from which a crude polymer stream is withdrawn for further purification. The overhead gas stream containing unreacted monomers is also withdrawn from the flash tank and recycled back to the reactor or sent to a recovery unit. Therefore, 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 when the proportion of propane introduced into the reactor loop along with the fresh feed of polymer-grade propylene is small, the amount recycled can accumulate rapidly, thereby reducing catalyst activity and reactor productivity. Propane accumulation and steady-state propane concentration in the reactor are essentially limited by the amount of reactor gas withdrawn from the polymerization section, the recirculation rate of the withdrawn gas, and whether the withdrawn gas undergoes purification before being recirculated to the polymerization section. Propane accumulation is typically controlled so that the steady-state propane content in the recirculation loop is in the range of approximately 2% to 30%.

[0011] The amount of gas entrained in the polymer product depends on the polymerization method, and varies, for example, depending on the operating conditions of the polymerization reactor and the arrangement of the product extraction. The composition of the reactor gas phase will change accordingly. However, the polymer product will always contain a minimal amount of gas entrainment. The use of lower purity propylene requires a higher amount of purging from the polymerization reactor to limit the accumulation of propane in the reactor gas.

[0012] Recovering propylene from the effluent is expensive in terms of both capital costs and energy consumption. Therefore, polymerization processes typically attempt to reduce the amount of gas discharged from the polymerization reactor. Consequently, it is also necessary to limit the amount of propane fed with the feed stream.

[0013] EP 0 887 359 A1 describes a method for propylene polymerization, comprising: feeding propylene containing not less than 0.1% by weight and not more than 20% by weight of propane into a polymerization reactor; distilling at least a portion of unreacted propylene discharged from the reactor to remove propane contained in the unreacted propylene; and recycling the purified propylene back into the polymerization reactor, thereby extracting propane from the polymerization reaction system. The method for propylene polymerization is carried out as a liquid-phase polymerization method or a gas-phase polymerization method. According to one embodiment of EP 0 887 359 A1, propylene is polymerized in the gas phase in the polymerization reactor. The resulting propylene polymer is continuously or intermittently extracted from a polymer recovery outlet. The unreacted propylene discharged from the polymerization reactor contains propane introduced along with the starting material. The unreacted propylene is typically cooled by a heat exchanger and subsequently separated into condensate and a gas phase by a separator (gas-liquid separator). The gas phase is recycled to the polymerization reactor from a first line via a compressor, while the condensate is recycled to the polymerization reactor from a second line. At least a portion of the condensate (unreacted propylene) in the second pipeline is directed to a distillation unit 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 recycled back to the polymerization reactor.

[0014] In recent decades, heat pump fractionation columns have been used for mixtures with near-close boiling points due to their low energy consumption. Fractionation columns typically operate at low or medium pressures, where a heat pump compressor increases the pressure of the top product to allow it to condense in the reboiler at the bottom. Heat pump fractionation columns are described, for example, in US 3,568,457 A, US 4,783,667 A, and US 7,842,847 B2. To achieve complete condensation of the top product, conventional heat pump fractionation columns require a small feed of light gases, as these gases accumulate and are trapped in the top section of the column.

[0015] Therefore, in the context of methods and systems for producing polypropylene, there is a need to limit energy consumption and increase energy efficiency without increasing the size or complexity of the system, while obtaining high-quality polymers. Summary of the Invention

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

[0017] The embodiments disclosed herein enable efficient energy use even in the presence of light gases such as hydrogen and ethylene, reducing energy consumption for propane removal and decreasing the size of the separation system used to separate propane from propylene. For example, when the separation system includes a propylene fractionator, its height and / or diameter can be reduced compared to conventional methods and systems.

[0018] In addition, the embodiments disclosed herein can enable the enhancement and simple recovery of unreacted contaminants (including propylene) from the polymerization products.

[0019] In one aspect, embodiments of this document relate to a method for producing polypropylene, the method comprising: feeding a first propylene stream containing propane into a gas-phase polymerization zone, wherein propylene reacts to produce a polymerization product and a carrier gas is optionally recovered; feeding a recirculated gas stream from the polymerization zone into a heat exchanger to remove heat from the polymerization reaction, thereby forming a first cooled gas stream and optionally a gas vapor stream; recirculating the first cooled gas stream into the polymerization zone; feeding a second cooled gas stream from the heat exchanger into a first separation system, thereby forming a propane-rich stream and a second propylene stream; operating the first separation system by a heat pump configured to compress the second propylene stream to form at least partially condensed propylene stream; and feeding the liquid portion and optionally the vapor portion of the at least partially condensed propylene stream into the gas-phase polymerization zone.

[0020] In a further aspect, the 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 containing propane into polypropylene and produce a polymerization product; a recirculating gas cooling circuit including a heat exchanger configured to remove heat from the polymerization reaction by means of the recirculating gas stream from the polymerization zone, thereby forming a first cooled gas stream and optionally a gas vapor stream recirculated to the polymerization zone; a first separation system configured to separate a second cooled gas 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 operate the first separation system and form at least partially condensed propylene stream; a flow line for feeding a liquid portion of the at least partially condensed propylene stream into the gas-phase polymerization zone; and an optional flow line for feeding a vapor portion of the at least partially condensed propylene stream into the gas-phase polymerization zone.

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

[0022] Other aspects and advantages will become apparent from the following description and the appended claims, which define further embodiments of the method and system. Attached Figure Description

[0023] Figure 1 A simplified diagram of a system for producing polypropylene according to one or more embodiments is shown.

[0024] Figure 2 The energy consumption of a propylene fractionator in a system for producing polypropylene according to one or more embodiments is shown for different concentrations of propane in the feed to the propylene fractionator. Detailed Implementation

[0025] In one aspect, embodiments of this document relate to a method for producing polypropylene, the method comprising: feeding a first propylene stream containing propane into a gas-phase polymerization zone, wherein propylene reacts to produce a polymerization product and a carrier gas is optionally recovered; feeding a recirculated gas stream from the polymerization zone into a heat exchanger to remove heat from the polymerization reaction, thereby forming a first cooled gas stream and optionally a gas vapor stream; recirculating the first cooled gas stream into the polymerization zone; feeding a second cooled gas stream from the heat exchanger into a first separation system, thereby forming a propane-rich stream and a second propylene stream; operating the first separation system by a heat pump configured to compress the second propylene stream to form at least partially condensed propylene stream; and feeding the liquid portion and optionally the vapor portion of the at least partially condensed propylene stream into the polymerization zone.

[0026] Since the first separation system is driven by a heat pump according to one or more embodiments, energy efficiency is achieved, and this is independent of the nature of the feed to the polymerization reactor, which may include light gases as described above in addition to propylene.

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

[0028] According to one or more embodiments, a method for producing polypropylene includes: feeding a first propylene stream containing propane into a gas-phase polymerization zone, wherein propylene reacts to produce a polymerization product and a carrier gas is optionally recovered; feeding a recirculated gas stream from the polymerization zone into a heat exchanger, such as a condenser, to at least partially condense the recirculated gas stream, thereby forming a first gas condensate stream and a gas vapor stream; recycling the first gas condensate stream back into the polymerization zone; feeding a second gas condensate stream from the heat exchanger into a first separation system, thereby forming a propane-rich stream and a second propylene stream; operating the first separation system by a heat pump configured to compress the second propylene stream to form a propylene stream that is at least partially condensed; and feeding the liquid portion and optionally the vapor portion of the at least partially condensed propylene stream into the polymerization zone.

[0029] In another aspect, the 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 containing propane into polypropylene and produce a polymerization product; a recirculating gas cooling circuit including a heat exchanger for removing heat from the polymerization reaction by means of the recirculating gas stream from the polymerization zone, thereby forming a first cooled gas stream and optionally a gas vapor stream recirculated to the polymerization zone; a first separation system configured to separate a second cooled gas 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 operate the first separation system and form at least partially condensed propylene stream; a flow line for feeding a liquid portion of the at least partially condensed propylene stream into the polymerization zone; and an optional flow line for feeding a vapor portion of the at least partially condensed propylene stream into 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 containing propane into polypropylene and produce a polymerization product; a recirculating gas cooling circuit including a heat exchanger such as a condenser for removing heat from the polymerization reaction by means of the recirculating gas stream from the polymerization zone, thereby forming a first gaseous condensate stream and a gaseous vapor stream recirculated to the polymerization zone; a first separation system configured to separate a second gaseous 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 operate the first separation system and form at least partially condensed propylene stream; a flow line for feeding a liquid portion of the at least partially condensed propylene stream into the polymerization zone; and an optional flow line for feeding a vapor portion of the at least partially condensed propylene stream into the gas-phase polymerization zone.

[0031] The following describes embodiments applicable to both the method and the system.

[0032] According to one or more embodiments, based on the total weight of the stream, the first propylene stream containing propane fed into the polymerization zone contains at least about 0.5% by weight of propane, such as, for example, 0.5% to 20% by weight of propane, 1% to 15% by weight of propane, 2% to 12.5% ​​by weight of propane, and 3% to 10% by weight of propane.

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

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

[0035] According to one or more embodiments, the method may further include feeding any catalyst suitable for propylene polymerization, such as any catalyst selected, for example, from the group comprising Ziegler-Natta catalysts and unit point catalysts (such as metallocene catalysts).

[0036] According to one or more embodiments, the catalyst may include a Ziegler-Natta catalyst. The Ziegler-Natta catalyst comprises a magnesium / titanium / electron donor complex, optionally supported on a suitable support (e.g., including silica or MgCl2), used in conjunction with an organoaluminum co-catalyst 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 US 4376 062 A, US 4379 758 A, US 5066 737 A, and US 9522 968 B2.

[0037] According to one or more embodiments, the catalyst may include a metallocene catalyst. The 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 US 7,169,864 B2.

[0038] According to one or more embodiments, the method may further include feeding additional optional compounds into the polymerization zone. The additional optional compounds may comprise, for example, one or more comonomers (such as ethylene and / or butene) to produce a corresponding copolymer or terpolymer of propylene; and / or hydrogen to control the molecular weight of the polymerized product; and / or inert compounds (such as, for example, nitrogen or argon), which may be used for other purposes to control pressure and monomer concentration; and / or polymerization additives, such as, for example, electrostatic control agents or activity limiting agents (e.g., ATMER 163).

[0039] According to one or more embodiments, depending on the required capacity and range of polymerization products, the polymerization zone may include at least one reactor or multiple reactors. For example, the at least one reactor may include 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 includes at least two reactors, such as any type of reactor indicated above. According to one or more embodiments, the two reactors may be arranged in series or in parallel. For example, two reactors may be arranged in series to produce propylene impact copolymers. For example, two reactors may be arranged in parallel to achieve higher capacity and / or produce bimodal resins. If the polymerization zone includes more than one reactor, each reactor may include a corresponding recirculating gas cooling loop, which includes a heat exchanger to remove the heat generated during polymerization. For example, the heat exchanger may include a condenser.

[0041] According to one or more embodiments, the method further includes discharging a polymerization product, for example, in the form of polymer powder, from a polymerization zone (e.g., from a reactor as defined in any of the embodiments indicated above) 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 includes a polymer separator for separating the polymer from reduced-pressure propylene. The polymer separator may operate, for example, at pressures from 1 bar to 7 bar, such as, for example, 1 bar to 4 bar, or 2 bar to 4 bar.

[0042] According to one or more embodiments, the polymer can be further degassed, for example, by countercurrent purging with nitrogen or argon or other purging gases in a purge silo. According to one or more embodiments, the purge exhaust gas can be sent to a purge gas recovery system. According to one or more embodiments, the powder can then be converted into granules incorporating well-dispersible 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, and for example, about 20°C to about 40°C.

[0044] According to one or more embodiments, in addition to a major amount of propane, the propane-rich stream may also contain propylene. According to one or more embodiments, in addition to a major amount of propane, the propane-rich stream may also contain propylene and butene, as well as a small amount of butane (such as less than 1 vol% butane). According to one or more embodiments, the propane-rich stream may contain at least 50 vol% propane and less than 50 vol% propylene, such as, for example, 40 vol%, 30 vol%, 20 vol%, 10 vol%, or 5 vol% to less than 50 vol% propylene.

[0045] According to one or more embodiments, the carrier gas may include propylene and propane, as well as 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 fed into the polymerization zone during the polymerization process, for example as comonomers, or as fluidizing gases or purge gases, or to remove light hydrocarbon feed from the polymerization product.

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

[0047] According to one or more embodiments, the heat exchanger includes a condenser. In these embodiments, the recirculated gas stream can be at least partially condensed, resulting in a condensate gas stream and a vapor gas stream. According to one or more embodiments, the vapor gas stream can be fed into the polymerization zone or recirculated to the condensate gas stream in the polymerization zone. According to one or more embodiments, the condenser can include 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 into the first separation system may be taken from the liquefied recirculated gas of a single reactor and / or any liquefied recirculated gas of a cascade reactor system or a parallel reactor system.

[0049] According to one or more embodiments, a first separation system for separating propane and propylene may include a propylene fractionator. According to one or more embodiments, a first separation system for separating propane and propylene may include a distillation column. According to one or more embodiments, the first separation system may include a medium-pressure fractionator, such as a medium-pressure distillation column. According to one or more embodiments, the medium-pressure fractionator can operate at pressures from about 8 barg to about 25 barg, such as, for example, from 10 barg to about 22 barg.

[0050] According to one or more embodiments, the method further includes exchanging heat between a compressed propylene stream from a heat pump and bottoms from a first separation system to form at least partially condensed propylene stream. According to one or more embodiments, the method further includes returning a portion of the heated bottoms and / or condensed propylene stream to the first separation system, and for example, to the bottom and top of a distillation column, respectively. According to one or more embodiments, such heat exchange can take place in a reboiler (e.g., a bottom reboiler). In this way, the heat consumed by the reboiler is provided by the condensation of a second propylene stream.

[0051] According to one or more embodiments, the method further includes discharging at least partially condensed or completely condensed propylene stream from the reboiler. According to one or more embodiments, the method 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 a first separation system, such as, for example, returned to the top of a distillation column. According to one or more embodiments, the separation can be performed using a gas-liquid separator.

[0052] According to one or more embodiments, the first separation system may include a fractionator and an optional external recovery unit. The fractionator includes a reflux drum from which the discharge stream can be removed to improve the efficiency of the fractionator's operation. According to one or more embodiments, the discharge stream may be sent to the external recovery unit or returned to the polymerization zone. According to one or more embodiments, the fractionator is a medium-pressure fractionator 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 way, the flow rate of the discharge stream can be minimized and / or the recirculation of the discharge stream to the polymerization zone can be facilitated. For example, the discharge pressure can be selected such that the discharge stream from the reflux drum is directed in a gas vapor stream to the suction side of the compressor or directly to the polymerization zone.

[0053] According to one or more embodiments, separation in the first separation system can be carried out at temperatures ranging from about 0°C to about 70°C, such as 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 method may further include feeding a propylene feedstock containing propane into a second separation system to form a first propylene stream and a light hydrocarbon stream that may contain ethane. According to one or more embodiments, in addition to ethane, the light hydrocarbon stream may further contain one or more of methane, propylene, propane, hydrogen, nitrogen, ethylene, methane, oxygen, CO, CO2, and acetylene.

[0055] According to one or more embodiments, based on the total weight of the feedstock, the propylene feedstock fed into the second separation system contains at least 0.5% by weight of propane, such as, for example, 0.5% to 20% by weight, 1% to 15% by weight, 2% to 12.5% ​​by weight, or about 3% to 10% by weight of propane.

[0056] According to one or more embodiments, a second separation system for separating the light hydrocarbon stream from propylene may include, for example, a stripping tower or a deethanizer, which includes, for example, a discharge tower for recovering ethylene from the overhead product of the deethanizer, thereby forming a propylene stream fed into the polymerization zone. In these embodiments, the propylene stream is therefore a purified propylene stream.

[0057] According to one or more embodiments, the method may further include filtering any incompletely formed polymer from the product stream from the polymerization zone. According to one or more embodiments, the method 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 method may further include compressing these unreacted components before feeding them to the second separation system. Correspondingly, the system according to one or more embodiments may include a compressor for compressing such unreacted components, the compressor being arranged in a 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 comonomers, hydrogen, solvents, and other components used in the method.

[0058] According to one or more embodiments, the method may further include feeding a portion of a pre-compressible unreacted component into a heat exchanger.

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

[0060] Now for reference Figure 1 A system for producing polypropylene according to one or more embodiments is generally indicated by reference numeral 1 in the accompanying drawings. Figure 1 In this context, the same reference numerals can represent both the flow pipeline and the flow within it.

[0061] In the embodiment shown in the accompanying drawings, the system 1 for producing polypropylene includes a gas-phase polymerization zone 2, a polymer separation system 20, a recirculating gas cooling loop including a heat exchanger 7, a heat pump 27, a first separation system 10, and a second separation system 14.

[0062] The polymerization zone 2 may include a gas reactor, such as a vertical gas-phase stirred bed reactor 3. Although Figure 1 This type of reactor is shown, but other types of reactors may be used according to one or more embodiments. Suitable reactors may include any gas-phase reactor suitable for propylene polymerization, whether batch, semi-continuous, or continuous, such as fluidized bed reactors or horizontal or vertical stirred powder bed reactors. Furthermore, although... Figure 1 Only one reactor is shown, but in one or more embodiments, polymerization zone 2 may include more than one reactor, such as, for example, at least two reactors arranged in series or in parallel.

[0063] The gas-phase stirred bed reactor 3 converts the first propylene stream 17, containing propane, into polypropylene. The first propylene stream 17 can be fed into the gas-phase stirred bed reactor 3 through a corresponding pipeline. Figure 1In the illustrated embodiment, the pipeline is in fluid communication with the second separation system 14. However, according to this disclosure, the system can also achieve energy efficiency, energy savings, structural simplification, and size reduction without the second separation system 14, which is optional. In fact, these technical effects are achieved by removing propane from polymerization zone 2 in an energy-efficient manner, as described in more detail below.

[0064] Based on the total weight of the stream, the first propylene stream 17 may contain at least about 0.5% by weight of propane, such as, for example, 0.5% to 20% by weight of propane, 1% to 15% by weight of propane, 2% to 12.5% ​​by weight of propane, or 3% to 10% by weight of propane.

[0065] Additional components (not shown) may be fed into polymerization zone 2. Any catalyst suitable for propylene polymerization may be fed into polymerization zone 2, such as, for example, a Ziegler-Natta catalyst. Additional components may include, for example, hydrogen to control the molecular weight of the polymerization product, and / or optional comonomers, such as ethylene and / or butene, and / or organoaluminum cocatalysts and / or external selectivity control agents, such as, for example, aromatic carboxylic esters or alkoxysilane compounds.

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

[0067] Typically, polymerization can be carried out at temperatures ranging from about 40°C to about 150°C, for example from about 60°C to about 90°C, and / or at pressures ranging from about 10 bar to about 50 bar, for example 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 can operate at pressures ranging from about 10 bar to about 30 bar and / or at temperatures ranging from about 30°C to about 90°C.

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

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

[0071] According to one or more embodiments, the polymer separation system 20 may include any one of a centrifuge, cyclone separator, flash separator, cooling separator, distillation separator, absorption separator, or a combination thereof. The separation system used may depend on the type of polymerization reactor and the feed components. Figure 1 A polymer separation system 20 is shown that recovers polymer product 21 (e.g., polypropylene) from it, while component 22 removed from the polypropylene can be recycled to a second separation system 14. However, according to one or more embodiments, instead of recycling component 22 to the second separation system 14, these components can be taken out via stream 26.

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

[0073] Among the benefits of the methods and systems disclosed herein is that the propane concentration in the polymerization zone can be controlled independently of the amount of direct recycle, since propane can be purged independently of the recycle stream and the polymer separation system.

[0074] According to this disclosure, the recirculated gas stream 9 is drawn from the polymerization zone 2 to be at least cooled or partially or completely condensed in a heat exchanger 7, which is arranged downstream of the gas-phase stirred bed reactor 3 relative to the direction of the recirculated gas stream 9.

[0075] According to one or more embodiments, the heat exchanger 7 of the recirculated gas cooling loop includes a condenser 8 configured to at least partially condense the recirculated gas or completely condense the recirculated gas. Figure 1 In the illustrated embodiment, 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, thereby shutting off the recirculated gas cooling loop. Optionally, as shown in the figures, 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. The first separation system 10 is configured to separate the second condensate gas stream 24 from the heat exchanger 7, forming a propane-rich stream 12 as the bottom product and a second propylene stream 13 as the top product. Therefore, the first separation system 10 is arranged downstream of the heat exchanger 7 relative to the direction of the second condensate gas stream 24. For example, when more than about 90% of the circulating gas is condensed, the circulating gas flow rate can be about 6 to about 9 times the amount of polypropylene produced. However, according to one or more embodiments, heat can still be removed without reaching 90% condensation, provided a sufficient flow rate is achieved in the recirculating gas cooling loop. In any case, arranging the first separation system 10 in the recirculating cooling loop allows for a significant increase in flow rate.

[0077] Separation in the first separation system 10 can occur in a medium-pressure separation tower, which can operate, 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 that produces a propylene-rich distillate product, such as a propylene fractionator. According to one or more embodiments, the distillation column 11 may include trays. The first separation system 10 operates via a heat pump 27, for example, at pressures 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 in the range of about 15 barg to about 40 barg, such as, for example, about 20 barg to about 35 barg.

[0079] The propane stream 12 recovered by the first separation system 10 may also include heavier components, such as, for example, butane and butene. The second propylene stream 13 is fed back to the polymerization zone 2.

[0080] According to this disclosure, heat pump 27 is configured to compress a second propylene stream 13 to operate the first separation system 10 and form 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 into polymerization zone 2.

[0081] according to Figure 1 In the illustrated embodiment, system 1 further includes a reboiler 28 configured to exchange heat between a compressed propylene stream 13a from heat pump 27 and bottoms 31 from first separation system 10. Due to this heat exchange, heated bottoms are obtained from bottoms 31, and condensed propylene stream is obtained from compressed propylene stream 13a.

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

[0083] according to Figure 1 In one embodiment, flow line 13e provides reflux from the gas-liquid separator described below to the first separation system 10.

[0084] System 1 may further include a further heat exchanger 30 located downstream of heat pump 27 for shutting off thermal equilibrium, 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 parallel to reboiler 28.

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

[0086] exist Figure 1 In this system, system 1 further includes both a flow line for feeding a liquid portion 13d of at least partially condensed propylene stream 13b into the gas-phase polymerization zone 2 and a flow line for feeding a vapor portion 13c of at least partially condensed propylene stream 13b into the gas-phase polymerization zone. However, according to other embodiments of the system, only the liquid portion 13d may be fed into the 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 container, such as, for example, a reflux drum or accumulator, from which reflux can be fed into the first separation system 10.

[0088] According to one or more embodiments, system 1 further includes a second separation system 14. The second separation system 14 is configured to separate the propylene feedstock 18 containing propane and form a first propylene stream 17 as the bottom product and a light hydrocarbon stream 16 as the top product, which may contain C1-C2 hydrocarbons and / or other impurities. The presence of the separation system 14 can be advantageous, for example, when light impurities are present in the propylene feedstock 18, or when light gases are discharged from stream 22 before C3 and C4 are recycled back 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, based on the total weight of the feedstock, propylene feedstock 18 may contain at least about 0.5% by weight of propane, such as, for example, 3% to 10% by weight of propane.

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

[0091] Separation in the second separation system 14 can occur in a high-pressure separation tower that can operate at temperatures ranging from -20°C to 80°C and / or at pressures ranging from 30 barg to 40 barg, for example, at about 35 barg.

[0092] The second separation system 14 may include a distillation column 15, such as a deethaner, that produces an overhead product rich in methane, ethane, and ethylene. According to one or more embodiments, the distillation column 15 may include trays. For example, the deethaner may operate at pressures from 10 barg to 40 barg, such as about 35 barg.

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

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

[0095] Figure 2 The energy consumption of a propylene fractionator in a system for producing polypropylene according to one or more embodiments is shown for different concentrations of propane in the feed to the propylene fractionator. Specifically, the results relate to a heat pump propylene fractionator and were generated via flow chart simulation using Aspen Plus simulation software. In all cases, the propylene content at the bottom of the propylene fractionator was set to 5% by weight. The feed flow rate was set such that the same amount of propane was purged while minimizing the recirculation rate to reduce energy consumption. The results show that when the propane content in the feed to the propylene fractionator is increased from 5% by weight to 10% by weight, the energy consumption per unit of purged propane is reduced by more than 50%.

[0096] As described above, the embodiments herein provide a method and system for producing polypropylene that advantageously reduces the energy requirement for propane removal, and thus also reduces the total energy requirement of the method and system, and uses energy in an efficient manner. Additionally, the system can have a reduced size. Furthermore, the method and system can produce high-quality polymerization products that are substantially indistinguishable from those produced using polymer-grade propylene feedstock, despite the presence of propane in the propylene feedstock.

[0097] While this disclosure includes a limited number of embodiments, those skilled in the art who benefit from it will understand that other embodiments can be devised without departing from the scope of this disclosure. Therefore, the scope should be limited only by the appended claims.

Claims

1. A method for producing polypropylene, the method comprising: A first propylene stream (17) containing propane is fed into a gas-phase polymerization zone (2), where propylene reacts to produce a polymerization product (21) and the carrier gas is recovered. The recirculated gas stream (9) from the gas-phase polymerization zone (2) is fed into the heat exchanger (7) to remove the heat of the polymerization reaction, thereby forming a first cooled gas stream (4) and an optional gas vapor stream (23). The first cooled gas stream (4) is recirculated to the gas-phase polymerization zone (2); The second cooled gas stream (24) is fed from the heat exchanger (7) into the 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), which is configured to compress the second propylene stream (13) to form at least partially condensed propylene stream (13b); and The liquid portion (13d) and optional vapor portion (13c) of the at least partially condensed propylene stream (13b) are fed into the gas-phase polymerization zone (2).

2. The method according to claim 1, further comprising: The compressed propylene stream (13a) from the heat pump (27) is heat-exchanged with the bottoms (31) from the first separation system (10) to form the at least partially condensed propylene stream (13b); and A portion of the heated bottoms (32) and / or the condensed propylene stream (13e) is returned to the first separation system (10).

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

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

6. The method according to 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 method according to claim 1, wherein the heat exchanger (7) comprises a vertical shell-and-tube condenser (8).

8. The method according to claim 1, wherein the first separation system (10) comprises a distillation column (11).

9. The method according to claim 1, further comprising feeding a propylene feedstock (18) containing propane into a second separation system (14) to form the first propylene stream (17) and a hydrocarbon stream (16) containing C1-C2 hydrocarbons.

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

11. The method of claim 9, 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 method according to claim 1, further comprising feeding the gas vapor (23) into the gas-phase polymerization zone (2).

13. A system for producing polypropylene, the system comprising: Gas-phase polymerization zone (2), which is configured to convert a first propylene stream (17) containing propane into polypropylene and produce a polymerization product (21); A recirculating gas cooling circuit includes a heat exchanger (7) configured to remove heat from the polymerization reaction by means of a recirculating gas stream (9) from the gas-phase polymerization zone (2), thereby forming a first cooled gas stream (4) and an optional gas vapor stream (23) recirculated to the gas-phase polymerization zone (2). A first separation system (10) is configured to separate a second cooled gas stream (24) from the heat exchanger (7) to form a propane-rich stream (12) and a second propylene stream (13); A heat pump (27) is configured to compress the second propylene stream (13) to operate the first separation system (10) and form at least partially condensed propylene stream (13b); A flow line (13d) is used to feed the liquid portion of the at least partially condensed propylene stream (13b) into the gas-phase polymerization zone (2); as well as An optional flow line (13c) is used to feed the vapor portion of the at least partially condensed propylene stream (13b) into the gas-phase polymerization zone (2).

14. The system of claim 13, further comprising: A reboiler (28) configured to exchange heat between a compressed propylene stream (13a) from the heat pump (27) and bottoms (31) from the first separation system (10); a flow line (32) for returning the heated bottoms 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. The system of claim 14, further comprising a gas-liquid 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 claim 13, wherein the gas-phase polymerization zone (2) comprises at least one gas-phase stirred bed reactor (3).

17. The system of claim 13, wherein the heat exchanger (7) comprises a vertical shell-and-tube condenser (8).

18. The system of claim 13, wherein the first separation system (10) comprises a medium-pressure fractionator (11).

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

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

21. The system of claim 20, further comprising a flow line for feeding the unreacted component into the second separation system (14).

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