Patent Application
20240343839
The present invention relates to the field of gas phase polymerization, in particular to the field of polyolefin gas phase polymerization. Particularly, the present invention is directed towards improved gas recycling in said gas phase polymerization process.
Generally, in a gas phase polymerization process, polymers are polymerized from the gas phase, i.e. the monomers are provided in the polymerization reactor in gaseous form. Typically, not all of the monomers are consumed during operation, but a part of them leaves the reactor again. This is in particular true for fluidized bed gas phase reactors, in which the polymerization reaction and therefore the quality of the product benefits from the adjusted monomer gas stream in the reactor.
In gas phase polymerization processes known in the prior art, the gases leaving the reactor are usually collected, purified and at least partially reintroduced into the reactor for further polymerization. This reduces waste material, environmental impact reduction (no venting off), and energy efficiency (reduced temperature and pressure losses). This recycling is usually called circulation.
In the prior art, the circulation of gases back to the gas phase polymerization rector is usually carried out using a single compressor. The gas flow through the compressor, which is required to fluidize the fluidization bed in the gas phase polymerization reactor, is typically in the range of 1100 to 1700 t/h.
Nevertheless, gas phase polymerization processes often require that not all of the gases leaving the polymerization reactor are reintroduced again. There can be the need for flush gases, which are used in various parts of the process, in particular the gas phase reactor of the polymerization process, and which can provide energy to said parts. These flush gases are usually branched off the circulation loop back to the gas phase polymerization reactor (cf.
Flush gases need to overcome pressure losses and are therefore usually provided at higher pressure levels in comparison to the pressure in the circulation loop back to the gas phase polymerization reactor. To provide a pressure level high enough for the flush gasses, the circulation gas flow is artificially choked with a flow restriction element to generate sufficient pressure margin for the flush gasses. Outcome is the higher power consumption of the compressor running the overall circulation.
Furthermore, gas phase polymerization processes also often require that parts of the circulation gas are not directly recycled in the reactor, but are lead to a recovery area (cf.
The removal equipment in such recovery sections usually causes significant drops in pressure. Thus, typically, the gases reused from the recovery station in the gas phase reactor have to be pressurized again by a vent gas compressor before reentering the circulation loop (cf.
Therefore, one problem of the present invention is that pressure and thus energy losses are introduced in the circulation loop of a gas phase polymerization reactor by the need of branching off flush gases therefrom.
A further problem of the present invention is that in case at least parts of the circulation gases need to be adjusted in their composition, i.e. purified, before being fed back into the circulation loop, the pressure of these parts needs to be increased again leading to higher energy consumption of the process.
It is therefore an object of the present invention to provide a gas phase polymerization process, in particular a gas phase polyolefin polymerization process, utilizing a reaction gas circulation loop comprising a flush gas branching and/or a purification/gas composition modification section being more energy efficient.
It now has been surprisingly found that above-mentioned problem is solved and object is achieved by a process for gas phase polymerization of a polymer, comprising the steps of
Furthermore, it has now been surprisingly found out that above-mentioned problem is solved and object is achieved by a polymerization assembly comprising
The term ‘unreacted gas’ as used herein generally determines the top output of a gas phase polymerization reactor. Hence, a stream withdrawn from the top of a gas phase polymerization reactor comprises mainly unreacted monomers and optionally comonomers, but also hydrogen, fluidization gas and also parts of reacted molecules, i.e. entrained polymers or side products.
In the following two embodiments of the present invention are discussed in particular in comparison to respective embodiments known from the prior art (i.e.
Preferably, the polymerization reactor is a gas phase polymerization reactor, more preferably a fluidized bed gas phase polymerization reactor. Therefore, preferably, the reactor has a substantially cylindrical shape, whereas the reactor is placed such that the symmetry axis of the cylindrical shape is parallel to the direction of the gravitational force. The polymerization reactor (1) has an upper half and a lower half, whereas the upper half is the part of the reactor above an area perpendicular to the symmetry axis of the cylinder and located at the half of the length of the cylinder, whereas ‘above’ has to be understood with respect to the gravitational force. Likewise, whereas the lower half is the part of the reactor below an area perpendicular to the symmetry axis of the cylinder and located at the half of the length of the cylinder, whereas ‘below’ has to be understood with respect to the gravitational force.
Furthermore, the polymerization reactor (1) includes a bottom zone, a middle zone and a top zone. The bottom zone forms the lower part of the reactor in which typically the base of the fluidized bed is formed. Above the bottom zone and in direct contact with it is the middle zone. The middle zone and the upper part of the bottom zone contain the fluidized bed. Preferably, the base of the bed forms in the bottom zone with no fluidization grid, or gas distribution plate, being present. Because there is preferably no fluidization grid, a free exchange of gas and particles between the different regions within the bottom zone and between the bottom zone and the middle zone can occur. Finally, above the middle zone and in direct contact therewith is the top zone. Preferably, the top zone and the bottom zone have a conical shape.
The unreacted gas removed from the at least one outlet (3) is led to the inlet of a circulation gas compression unit (5) via the unreacted gas line (4). In the circulation gas compression unit (5), the unreacted gas is pressurized. From the outlet of the circulation gas compression unit (5) the pressurized unreacted gas is withdrawn via the pressurized unreacted gas line (7), which is split up in a flush gas line (9) and the circulation gas line (8), whereas the circulation gas line (8) is returned into the polymerization reactor (1) via the at least one inlet (2). In the reactor assembly of the prior art, the pressure in the circulation gas line (8) is too high to be introduced in the polymerization reactor (1). The polymerization reactor needs to have a certain gas flow characteristic, in particular if the polymerization reactor is a fluidized bed polymerization reactor. Unfavorable gas flow characteristics can lead to polymerization material agglomerating at the reactor walls and other parts of the reactor leading to increased temperature control. Furthermore, the height of the fluidized bed is influenced by the gas flow characteristic. Lower fluidized beds lead to lower production efficiency, higher fluidized beds lead to polymer entrainment into the circulation gas line leading to fouling of the downstream equipment. Therefore, the pressure and thus the gas flow velocity at the inlet (2) of the polymerization reactor (1) need to be accurately adjusted. On the other hand, the flush gas as withdrawn from the flash gas line (9) also needs to have a certain pressure when being used as an energy supply for other means in the process. Usually, the pressure needed in the flash gas is higher than the pressure needed at the inlet (2) of the polymerization reactor (1). Therefore, in the reactor assembly of the prior art, a pressure drop is usually realized via a choking valve in the circulation line (8) before entering the polymerization reactor (1). As a result, the system loses energy and also small amounts of material rendering the process less efficient.
In the present invention according to
Preferably, the circulation gas compression unit (5) comprises, preferably consists of, one or more compressors, fans or set of fans suitable for compressing hydrocarbons. Likewise, preferably, the flush gas compression unit (6) comprises, preferably consists of, one or more compressors, fans or set of fans suitable for compressing hydrocarbons.
In a first more preferred embodiment, the circulation gas compression unit (5) and/or the flush gas compression unit (6) comprise, preferably consist of, API 617 Chapter 3 integrally geared compressors, which are energy efficient and result in smaller foot prints. Furthermore, upgrades in the reactor assembly can be achieved more easily.
In a second more preferred embodiment of the invention, the circulation gas compression unit (5) and the flush gas compression unit (6) are mechanically connected to the same source of rotational energy, such as a motor. Preferably, the circulation gas compression unit (5) and the flush gas compression unit (6) are overhung compressor stages on opposite sides of the motor, preferably the electrical motor.
Preferably, the flush gases are cooled after being compressed by the flush gas compression unit (5) resulting in a higher power saving benefit. However, for some systems, such as certain polyethylene polymerization systems, cooling down should be avoided to prevent the formation of a two-phase system after cooling down. Thus, preferably, the polymerization assembly comprises a cooler with an inlet and an outlet, wherein the inlet is fluidly connected to the outlet of the flush gas compression unit (6) and the outlet is fluidly connected to the flush gas withdrawal line (9).
Usually, such means require the pressure to be let down before or during performing respective purification/separation tasks. This can be done, for example, by a low pressure flash separator, which at the same time lowers the pressure of the introduced gas stream and performs a separation into a vapour and a condensed phase. However, such pressure let down usually requires the circulation gas stream to be re-pressurized before being reintroduced into the polymerization reactor (1). Reason is the same as explained above, i.e. that the reactor conditions usually require a defined pressure of the introduced gas streams.
The preferred embodiment of the present invention as depicted in
In comparison to the process known from the prior art and depicted in
Accordingly to the description of the polymerization assembly, the polymerization process is herewith also described. Generally, the process of the present invention relates to the gas phase polymerization of a polymer. Preferably, the process relates to the polymerization of polyolefins, more preferably polyolefin homo- or copolymers. In an especially preferred case, the present invention relates to a process for the polymerization of polyethylenes.
Both,
The process of the present invention according to
In step a) monomers may be polymerized in the presence of a polymerization catalyst. Monomers which can thus be polymerized include olefins, diolefins and other polyenes. Thus, in step a), ethylene, propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-heptene, 1-octene, 1-decene, styrene, norbornene, vinyl norbornene, vinylcyclohexane, butadiene, 1,4-hexadiene, 4-methyl-1,7-octadiene, 1,9-decadiene and their mixtures may be polymerized. Preferably, in step a) of the process of the present invention, ethylene and propylene and their mixtures are polymerized, optionally together with other alpha-olefin comonomers having from 4 to 12 carbon atoms.
In addition to the monomers different co-reactants, adjuvants, activators, catalysts and inert components may be introduced in step a).
Any polymerization catalyst may be used to initiate and maintain the polymerization. Such catalysts are well known in the art. Especially the catalyst should be in the form of a particulate solid on which the polymerization takes place. Suitable catalysts for olefin polymerization are, for instance, Ziegler-Natta catalysts, chromium catalysts, metallocene catalysts and late transition metal catalysts. Also different combinations of two or more such catalysts, often referred to as dual site catalysts, may be used.
Examples of suitable Ziegler-Natta catalysts and components used in such catalysts are given, for instance, in WO-A-87/07620, WO-A-92/21705, WO-A-93/11165, WO-A-93/11166, WO-A-93/19100, WO-A-97/36939, WO-A-98/12234, WO-A-99/33842, WO-A-03/000756, WO-A-03/000757, WO-A-03/000754, WO-A-03/000755, WO-A-2004/029112, WO-A-92/19659, WO-A-92/19653, WO-A-92/19658, U.S. Pat. Nos. 4,382,019, 4,435,550, 4,465,782, 4,473,660, US-A-4560671, U.S. Pat. Nos. 5,539,067, 5,618,771, EP-A-45975, EP-A-45976, EP-A-45977, WO-A-95/32994, U.S. Pat. Nos. 4,107,414, 4,186,107, 4,226,963, 4,347,160, 4,472,524, 4,522,930, 4,530,912, 4,532,313, US-A-4657882, U.S. Pat. Nos. 4,581,342, 4,657,882, EP-A-688794, WO-A-99/51646, WO-A-01/55230, WO-A-2005/118655, EP-A-810235 and WO-A-2003/106510.
Examples of suitable metallocene catalysts are shown in WO-A-95/12622, WO-A-96/32423, WO-A-97/28170, WO-A-98/32776, WO-A-99/61489, WO-A-03/010208, WO-A-03/051934, WO-A-03/051514, WO-A-2004/085499, EP-A-1752462, EP-A-1739103, EP-A-629631, EP-A-629632, WO-A-00/26266, WO-A-02/002576, WO-A-02/002575, WO-A-99/12943, WO-A-98/40331, EP-A-776913, EP-A-1074557 and WO-A-99/42497,
The catalysts are typically used with different activators. Such activators are generally organic aluminium or boron compounds, typically aluminium trialkyls, alkylaluminium halides, alumoxanes In addition different modifiers, such as ethers, alkoxysilanes, and esters and like may be used.
Further, different coreactants, may be used. They include chain transfer agents, such as hydrogen and polymerization inhibitors, such as carbon monoxide or water. In addition, an inert component is suitably used. Such inert component may be, for instance, nitrogen or an alkane having from 1 to 10 carbon atoms, such as methane, ethane, propane, n-butane, isobutane, n-pentane, isopentane, n-hexane or like. Also mixtures of different inert gases may be used.
The polymerization is conducted at a temperature and pressure where the fluidization/circulation gas essentially remains in vapour or gas phase. For olefin polymerization the temperature is suitably within the range of from 30 to 110° C., preferably from 50 to 100° C. The pressure is suitably within the range of from 1 to 50 bar, preferably from 5 to 35 bar.
In accordance to the explanations given in the description of the gas phase polymerization assembly, the difference of the most general embodiment of the process of the present invention according to
In the process of the prior art according to
According to the second, more preferred embodiment of the present invention, the gas phase polymerization process can be further modified. Such a process includes the steps of
It should be understood that this process does not necessarily need the reintroduction of circulation gas into step a). Hence, in an alternatively preferred embodiment of the present invention, the gas phase polymerization process does not comprise the step e). In such a process, all of the circulation gas is lead into step g), e.g. into the purification/gas composition modification section. This is in particular useful, if no circulation should be carried out but the gas for the flush gas should have a certain composition/purity grade.
Apart from such modification, the preferred process of the present invention involves all advantages known from the most general embodiment of the present invention. Furthermore, the process is also more energy efficient than the process known from the prior art (i.e.
According to
Both embodiments of the process according to the present invention preferably comprise the step of
| Number | Date | Country | Kind |
|---|---|---|---|
| 21194405.3 | Sep 2021 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/074089 | 8/30/2022 | WO |
