Patent Application
20240010765
The present invention concerns a polymerization plant suitable for extra large-scale propylene polymerization. The present invention further concerns a process for polymerization of propylene using such plant.
Coupling of loop and gas phase reactor is known for more than two decades under the trademark Borstar® and has found its way into practically any textbook in the field of polyolefins. The basic process layout is for example described in WO9858975A1 dealing with the preparation of propylene homopolymers and copolymers, which comprises polymerizing propylene optionally with comonomers in the presence of a catalyst at elevated temperature and pressure in at least one slurry reactor and at least one gas phase reactor, the polymerization product of at least one slurry reactor, containing unreacted monomers, being directly conducted to a first gas phase reactor essentially without recycling of the unreacted monomers to the slurry reactor.
One important aspect of loop—gas-phase reactor configurations is once-through conversion. For example, early WO9925741 describes a method of introducing polymer slurry obtained from a slurry reactor into a gas phase reactor containing a fluidized bed (C, D) with a first fluidized zone (C) on top of a second fluidized zone (D) with said fluidized zones having different flow patterns, in order to optimize the once-through conversion of the process. Unfortunately medium size polypropylene production plants have significant higher operational costs and fixed costs per ton of produced product when compared to extra-large size polypropylene plants. However, by increasing the scale of the production, quality issues and operational issues can do rise. Increasing capacity of loop reactors can be done by adding further legs to the reactor. In industry, loop reactors have been scaled up from two legs to six legs, and even 8 legs loop reactors are not uncommon. However, the single circulation time using an 8-leg loop reactor will be roughly doubled when compared to a 4-legs loop reactor. Unfortunately, such modifications may result in drawbacks; e.g., significant concentration gradients can occur. For example, in polypropylene homo-polymerization the hydrogen to propylene ratio can vary over the length of the loop reactor, and especially in random propylene ethylene co-polymerization the ratio of ethylene to propylene can cause a significant gradient in the loop reactor. It goes without saying, when the C2/C3 ratio drops over the length of the loop reactor, the randomness will be affected. Thus, when producing for example random propylene ethylene copolymer workarounds such as local fed of ethylene or multiple feed points to the reactor become a must. To sum up, upscaling of loop reactors is extremely cumbersome.
Another challenge for polymerization processes at very high throughput and especially for very large scale gas phase reactors is to control the temperature in a wide operating range, e.g. also at turndown ratios of 50%, and to establish a broad operation window, e.g. broad temperature ranges. Turndown ratio characterizes the ability to run a plant at reduced throughput. Turndown ratio is usually defined as [1−(minimum capacity/design capacity)]. In case of turndown, the cooling water return (CWR) flow temperature can increase to high temperatures outside the normal operating window of cooling water systems. Reduced efficiency and corrosion cracking problems in the context of stainless steel equipment can be expected. Avoiding unnecessary high temperatures in the cooling water return flow is particularly important as chlorine is added to cool water circuits and/or towers for preventing biological growth.
Thus, there remains still the need for a process and a plant allowing upscaling to extra-large scale avoiding at least in part these and related problems.
The present invention provides a plant for preparing propylene homopolymers or propylene copolymers, comprising
wherein the circulation gas cooler (16) is a heat exchanger within a closed loop cooling water system (300) comprising a cooling water pump (301), a secondary heat exchanger (302), expansion vessel (303) and a by-pass (304) over a secondary heat exchanger.
The Present Invention Further Provides
a process for preparing propylene homopolymers and copolymers, comprising
The plant according to the present invention, in contrast to conventional Borstar plants, contains a recovery feed line (29) allowing to redirect non-condensed propylene back to the gas phase reactor (13). This setup surprisingly turned out to be beneficial with respect to the overall conversion and even further with respect to the monomer factor. For example, when producing random polypropylene copolymer huge savings are observed.
In the process according to the present invention, column (28) is operated at a pressure higher than the pressure of the gas phase reactor 13). This enables to recycle part of the hydrocarbons via recovery lines (29, 29′) back to the gas phase reactor. It surprisingly turned out, that high hydrogen concentrations in the presence of ethylene are possible and the process is characterized by a high once-through conversion. As another surprising aspect, high amount of hydrogen recovery was observed. This is particularly beneficial for high melt flow rate grades: the required fresh hydrogen for the gas phase reactor can be lowered.
In a further aspect, nitrogen, steam and propylene as obtained are separated into
The plant according to the present invention is configured such that the circulation gas cooler (16) is a heat exchanger within a closed loop cooling water system (300) comprising a cooling water pump (301), a secondary heat exchanger (302), expansion vessel (303) and a by-pass (304) over a secondary heat exchanger. In the inventive plant and process, the gas circulation flow is cooled by a heat exchanger in order to have a very efficient temperature control in the gas phase reactor, usually and preferably a fluidized bed reactor. The polymerization heat is preferably transferred to a closed loop cooling water system (300), comprising a cooling water pump (301), a secondary heat exchanger (302), expansion vessel (303) and a by-pass over the secondary heat exchanger (304). The polymerization heat is further transferred via the closed loop cooling water system heat exchanger to a common site cooling water system, which can be for example cooling water towers. The main advantage is that the cooling water flow through the primary heat exchanger can be kept constant at a temperature above the dew point of the circulation gas and offering a wide operating window on production rates in the gas phase reactor. Turndown ratios of 50% or more become possible. Turndown ratio characterizes the ability to run a plant at reduced throughput. Turndown ratio is defined as [1−(minimum capacity/design capacity)]. In case of using a direct cooling water system cool water return temperature would rise to high values above typical designed operating temperature of the common site cooling water system, i.e. the operating window had to be limited.
The plant according to the present invention preferably comprises one or more of the following:
Such units increase the safety of the inventive plant.
In the process according to the present invention, the circulation gas cooler (16) is a heat exchanger and the heat is transferred to a closed loop cooling water system, comprising a cooling water pump, a secondary heat exchanger, expansion vessel and a by-pass over the secondary heat exchanger. More preferably, the polymerization heat is transferred via the closed loop cooling water system to a common site cooling water system, such as a cooling water tower, for enabling a constant cooling water flow through the heat exchanger within the gas circulation at a temperature above the dew point of the circulation gas.
In a further aspect, the process according to the present invention is characterized by possible turn down ratios of more than 50%. This high turn down allows really high flexibility with respect to varying products and demands.
The split, i.e. the ratio of the amounts of material produced in loop and gas phase reactors respectively ranges from 40-60 to 60-40.
In yet a further aspect, the polymerization temperature in the first and/or the second loop reactor, preferably in both loop reactors is below 72° C., more preferably below 70° C.
In the following, the invention shall be described with respect to the figures.
Reference Numbering for
Reference Numbering for
Reference numbering for
The plant according to the present invention shall be further described with respect to
The plant according to the present invention for preparing propylene homopolymers and copolymers comprises feed tank(s) for catalyst (1), optional co-catalyst (2), optional activator and/or optional external donor (3). Optionally there is a pre-contacting unit (4) for catalyst mixing being connected by feed lines (5, 5′, 5″) with the feed tank(s). In various embodiments and for various catalyst systems, a pre-contacting tank is not necessary.
The plant according to the present invention also includes a prepolymerization reactor (6) connected with the feed tank(s) (1, 2, 3) or the pre-contacting unit (4). Such prepolymerization is known in the art. The plant also includes a propylene feed tank (7), a first loop reactor (8) connected with the prepolymerization reactor, and a second loop reactor (9) connected with the first loop reactor via a loop reactors connecting line (10) as well as means for feeding comonomer (11) and hydrogen (12) to one or more of first loop reactor (8), second loop reactor (9), and/or loop reactors connecting line (10) between the loop reactors.
Such setup is frequently found in existing plants.
Apart from the loop reactors the plant according to the present invention also comprises a gas-phase reactor (13) equipped with a gas circulation line (14), a circulation gas compressor (15) and a circulation gas cooler (16), the gas-phase reactor being coupled to the at least second loop reactor by a direct feed line (17). means for feeding monomer (44) and/or comonomer (18) and/or hydrogen (19) to the gas-phase reactor (13). In addition to the plant optionally includes a product discharge vessel (20) connected with the gas-phase reactor. Such product discharge vessel contributes to the operational stability. Optionally a product outlet heater (21) is present. Usually a product outlet heater will be several units. The plant according to the present invention also includes a product receiver tank (22) connected with the optional product discharge vessel (20) or with the gas-phase reactor (13), at least one purge bin (23). In addition to that, the plant also includes at least one propylene nitrogen recovery unit (24) with a column supply line (241) for feeding a hydrocarbon stream to a column (28), a nitrogen re-feed line (243) for re-feeding a nitrogen rich stream to the purge bin (23), optionally a thermal oxidizer unit (43) and an exhaust line (242) for discharge of an exhaust stream optionally to the optional thermal oxidizer (43). Optionally there can be further feed lines such as a nitrogen re-feed line (243), an external nitrogen feed line (245), a feed line for catalyst deactivating agents, i.e. usually low pressure steam. Preferably, there is also an outlet for oligomers (249).
The plant according to the present invention also includes means for propylene homopolymer or propylene copolymer recovery (25) said means (25) optionally including means for homogenization, additivation, and pelletization, a recovery gas treating unit (26) comprising at least one compressor (27), said column (28) and a reflux feed vessel (28a), the reflux feed vessel (28a) being connected via a recovery line (29) with the gas circulation line (14) of the gas-phase reactor (13).
In addition to that, there is a cooling circuit (30) for the circulation gas cooler (16). Such circulation gas cooler contributes to the broad operational window.
Apart therefrom there is also a blow down unit (31) comprising a high pressure blow down bin (32), a low pressure blow down bin (33), the blow down unit (31) being optionally connected via connecting line (34) with the product receiver tank (22).
The plant according to the present invention further includes a recovery feed line (35) connecting recovery gas treating unit (26) with the propylene feed tank (7). This important recovery feed line (35) allows refeed of propylene also to the loop reactors, i.e. results in an integrated recovery system.
The cooling setup according to the present invention shall be described with reference to
For properly adjusting temperature and keeping the flow through the primary heat exchanger (300) constant, there is also a by-pass (304) over the secondary heat exchanger in the closed loop cooling water system. In this closed loop cooling water system, heat is transferred from gas to water in the gas phase reactor circulation gas cooler (300). As known in the art there is also a expansion vessel of the closed loop cooling water system (303).
A direct heat exchanger on site cooling water system with recirculation pump is shown in
The plant and the process according to the invention have been exemplified in the following examples. These examples are included for illustrative purposes and do not limit the invention.
In REF1 and REF2, no gas circulation cooler was used and there was only one loop reactor. In example IE1 a gas phase reactor and a gas circulation cooler was used. In example IE2, the recirculation was lowered.
It can be seen that the inventive plant and the inventive process simultaneously allowed high production in the gas phase reactor and high once-through conversion.
In a second experimental evaluation, the catalyst mileage (IE1 versus REF2) was assessed.
Surprisingly a high catalyst mileage was obtained for the inventive process:
In yet a further experimental evaluation using the inventive plant and the inventive process, the cooling water temperatures were monitored when producing random ethylene propylene copolymer. The inventive used a closed loop cooling water system as shown in
The results are shown in Table 3 provided below.
| Number | Date | Country | Kind |
|---|---|---|---|
| 20209881.0 | Nov 2020 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/082469 | 11/22/2021 | WO |
