PROCESS AND SYSTEM FOR VAPOR PHASE POLYMERIZATION OF OLEFIN MONOMERS

Abstract

The present invention relates to a continuous olefin polymerization process comprising polymerization of at least one olefin monomer in at least two serial vapor phase polymerization reactors containing an agitated bed of forming polymer particles, comprising a polymer particles transfer step wherein forming polymer particles are transferred from an upstream reactor to a downstream reactor comprising in a repeating sequence the steps of discharging at least one charge of polymer powder and reactive gases from the upstream reactor into a gas-solid separator, collecting the polymer powder separated in the gas-solid separator in a pressure transfer chamber; increasing the pressure in the pressure transfer chamber with a pressurizing gas to a pressure that is higher than the operating pressure of the downstream reactor, and discharging the polymer powder from the pressure transfer chamber into the downstream reactor, wherein said process reduces the carry-over of reactive gases from the upstream reactor to the downstream reactor. The present invention further relates to a system suitable for the present continuous vapor phase olefin polymerization process. The present invention further relates to the use of the present process and system for producing heterophasic polypropylene copolymers.

Claims

1. A continuous olefin polymerization process comprising polymerization of at least one olefin monomer in at least two serial vapor phase polymerization reactors containing an agitated bed of forming polymer particles, comprising a polymer particles transfer step wherein forming polymer particles are transferred from an upstream reactor to a downstream reactor, wherein the forming polymer particles transport step comprises in a repeating sequence the steps of: (a) discharging at least one charge of forming polymer powder and reactive gases from the upstream reactor into a gas-solid separator wherein the polymer powder is separated from the reactive gases; (b) collecting the polymer powder separated in the gas-solid separator in a pressure transfer chamber; (c) closing a valve between the gas-solid separator and the pressure transfer chamber and increasing the pressure in the pressure transfer chamber with a pressurizing gas to a pressure that is higher than the operating pressure of the downstream reactor; (d) discharging the polymer powder from the pressure transfer chamber into the downstream reactor by opening and subsequently closing a valve between the pressure transfer chamber and the downstream reactor; and (e) opening the valve between the gas-solid separator and the pressure transfer chamber, wherein the pressure in the pressure transfer chamber before said discharging at least one charge of forming polymer powder and reactive gases from the upstream reactor into the gas-solid separator is 10-700 kPaa.

2. The process according to claim 1, wherein the gas-solid separator is in open gas communication with the inlet of an offgas gas compressor during steps (a)-(e).

3. The process according to claim 1, the offgas gas compressor is a multistage offgas gas compressor.

4. The process according to claim 1, wherein the gas stream from the gas-solid separator to the inlet of the offgas gas compressor is subjected to a second gas-solid separator.

5. The process according to claim 1, wherein the polymer particles transfer step is performed in two alternating repeating sequences.

6. The process according to claim 1, wherein each repeating sequence takes 60-600 seconds, preferably 100-300 seconds, most preferably 120-240 seconds.

7. The process according to claim 1, wherein the pressurizing gas is selected from the group consisting of one or more selected from the group consisting of nitrogen, fuel gas, methane, ethane, propane, ethylene and propylene.

8. The process according to an claim 1, wherein the upstream reactor is a horizontal stirred reactor containing multiple reaction zones, each reaction zone having at least one inlet for a gaseous stream and optionally additionally an inlet for a liquid stream and, wherein one reaction zone comprised in the upstream reactor vessel is a polymer discharge reaction zone from which forming polymer particles are discharged and subsequently transported to the downstream reactor and wherein the ratio of the hydrogen concentration to the olefin monomer concentration in the polymer discharge reaction zone ([H.sub.2].sub.discharge zone/[Olefin monomer].sub.discharge zone) is reduced compared to the ratio of the hydrogen concentration to the olefin monomer concentration in the preceding reactor zone ([H.sub.2].sub.preceding zone/[Olefin monomer].sub.preceding zone).

9. A system suitable for a continuous vapor phase olefin polymerization process according to claim 1, comprising at least two serial polymerization reactors containing an agitated bed of forming polymer particles, a means to transfer forming polymer particles from an upstream reactor to a downstream reactor, wherein the means to transfer forming polymer particles comprises a gas-solid separator that is connected through a valve with a pressure transfer chamber and a means to maintain the pressure in the pressure transfer chamber at 10-700 kPaa before discharging the polymer powder from the upstream reactor to the gas-solid separator.

10. The system according to claim 9, wherein the means to maintain the pressure in the gas-solid separator is an offgas gas compressor and wherein the inlet of said offgas gas compressor is in continuous open gas communication with the gas-solid separator.

11. The system according to claim 9, wherein the means to transfer forming polymer particles from an upstream reactor to a downstream reactor comprises two parallel gas-solid separators, two parallel pressure transfer chambers, and one offgas gas compressor.

12. The system according to claim 9, wherein the offgas gas compressor is a multi-stage offgas gas compressor.

13. The system according to claim 9, wherein the offgas from the gas-solid separator is subjected to a second sequential gas-solid separator, wherein said second sequential gas-solid separator.

14. The system according to claim 9, wherein the upstream reactor contains multiple reaction zones, each reaction zone having at least one inlet for a liquid stream and at least one inlet for a gaseous stream, said reactor comprising multiple inlets for a gaseous stream to set different hydrogen to olefin ratios and wherein one reaction zone comprised in the upstream reactor is a polymer discharge reaction zone comprising an outlet for forming polymer particles to the means to transfer the forming polymer particles to the downstream reactor.

15. (canceled)

16. The process according to claim 1, wherein the pressure in the pressure transfer chamber before said discharging at least one charge of forming polymer powder and reactive gases from the upstream reactor into the gas-solid separator is 90-500 kPaa, wherein the gas-solid separator is in open gas communication with the inlet of an offgas gas compressor during steps (a)-(e). the offgas gas compressor is a multistage offgas gas compressor, the gas stream from the gas-solid separator to the inlet of the offgas gas compressor is subjected to a cyclone, wherein each repeating sequence takes 100-300 seconds, wherein the pressurizing gas is selected from the group consisting of one or more selected from the group consisting of nitrogen, fuel gas, methane, ethane, propane, ethylene and propylene, and wherein the upstream reactor is a horizontal stirred reactor containing multiple reaction zones, each reaction zone having at least one inlet for a gaseous stream and additionally an inlet for a liquid stream.

17. The process according to claim 16, wherein one reaction zone comprised in the upstream reactor vessel is a polymer discharge reaction zone from which forming polymer particles are discharged and subsequently transported to the downstream reactor and wherein the ratio of the hydrogen concentration to the olefin monomer concentration in the polymer discharge reaction zone ([H.sub.2].sub.discharge zone/[Olefin monomer].sub.discharge zone) is reduced compared to the ratio of the hydrogen concentration to the olefin monomer concentration in the preceding reactor zone ([H.sub.2].sub.preceding zone/[Olefin monomer].sub.preceding zone).

Description

EXAMPLES

[0092] The development of a detailed and dynamic model of the gas-solid separator/pressure transfer chamber system enabled the understanding of the thermodynamic and transport phenomena occurring when PP powder is present inside the gas-solid separator/pressure transfer chamber system. This model was developed in Mobatec Modeller employing correlations derived from literature (for the mass transport phenomena) and from in-house developed models (namely thermodynamic models, based on PC-SAFT) and it was fitted with plant data. With this approach, it was possible to develop a model able to mimic the dynamic and sequential operation of the real system.

[0093] The sorption of the different gas components in the amorphous PP was estimated by developing simplified empirical correlations able to mimic the behaviour predicted by PC-SAFT. The detailed description of the transport phenomena occurring between the different gas systems showed that the non-instantaneous equilibrium between phases and the combined flow mechanisms of convection and diffusion between intra and interparticle gas phase are the main aspects affecting the gas distribution.

[0094] In the example tables, the results on the amount of hydrogen still present in the powder at the inlet of the downstream reactor are expressed as hydrogen Take-over, which is the excess of hydrogen in the downstream reactor. In other words, hydrogen Take-over is hydrogen carry-over from pressure chamber to downstream reactor minus hydrogen consumption in downstream reactor. This means the hydrogen Take-over could also be a negative value and additional hydrogen would need to be fed to the downstream reactor. This is a preferred situation with respect to the hydrogen/olefin control in the downstream reactor.

[0095] In the Examples the reactor is divided into four imaginary zones, each being or 25% of the reactor volume. The discharge zone is the last zone and comprises or 25% of the total reactor volume.

[0096] In order to show that the purge step does not have a significant impact in the removal of the hydrogen from the polymer bed, the following model results are shown (the hydrogen take-over in the base case is approximately 416 g/h). For the case with a longer purge, the overall sequence duration was kept constant but the powder settling step was completely replaced by the purge to show the longest purge duration.

TABLE-US-00001 TABLE 1 Summary of the model results regarding the purge conditions: duration and flow. F.sub.purge. base is below the entrainment flow of polymer particles and is selected in such a manner as to prevent entrainment of the polymer particles. The entrainment flow can be calculated by a skilled person based on e.g. particle size. Operating conditions in gas-solid separator/pressure transfer chamber Deviation H.sub.2 Take- from base Manipulated variable Variation over (g/h) case (%) Purge flow (F.sub.purge) 0.5 F.sub.purge, base 430 +3.4 2.5 F.sub.purge, base 406 2.4 Purge duration No purge 457 +10 50% longer purge 409 1.7

[0097] Regarding the parameters that directly affect the invention, the effect of opening the offgas valve on top of the gas-solid separator and the effect of changing the pressure at the beginning of each gas-solid separator/pressure transfer chamber cycle (keeping the sequence that is currently used in the plant) are presented separately in Table 2 and 3.

TABLE-US-00002 TABLE 2 Summary of the model results regarding the set-up of the offgas valve on top of the gas-solid separator or the starting pressure for the gas-solid separator/pressure transfer chamber (g-ss/ptc) cycle. For P.sub.base values were selected which are typical plant operation data, i.e. from the prior art. P.sub.base is lower than the pressure of the reactor, preferably between 25 to 50% of the pressure of the reactor; the higher the reactor pressure is, the lower this percentage. For example, for a low pressure P.sub.base is 50% of said pressure and for a high pressure, P.sub.base is 25% of said pressure. In an embodiment, P.sub.base is equal to or higher to 7.5 barg. Operating conditions in gas-solid separator/pressure transfer chamber Deviation H.sub.2 Take- from base Manipulated variable Variation over (g/h) case (%) Offgas valve set-up Open during purge 350 16 Open during intake 290 30 (2nd powder shot) Always open 278 33 0.8 P.sub.base 323 22 Starting pressure of each 0.6 P.sub.base 240 42 g-ss/ptc cycle 0.2 P.sub.base 84 80

[0098] Setting the top valve to be opened for the whole sequence, it allows to keep a constant pressure inside the gas-solid separator/pressure transfer chamber system during the powder intake and purge, which improves the gas flow from intra to interparticle volume. This effect was combined with different starting pressures. The results for the combined effect are shown hereafter in Table 3.

TABLE-US-00003 TABLE 3 Summary of the model results regarding the combined effect between keeping the top valve open for the whole sequence and successive lower pressures P.sub.base values were the same as indicated for Table 2. Operating conditions in gas-solid separator/pressure transfer chamber Deviation H.sub.2 Take- from base Manipulated variable Variation over (g/h) case (%) Constant pressure during 0.8 P.sub.base 189 55 powder intake and purge 0.6 P.sub.base 54 87 0.2 P.sub.base 54* 113 *The hydrogen entering in downstream reactor (R2) via gas-solid separator/pressure transfer chamber is lower than the required for the reaction, resulting in no excess (and no take-over)

[0099] As a side-remark, the process of the present invention also surprisingly decreases not only the amount of hydrogen but also the amount of propane entering downstream reactor (R2) by 72% in the above example. The amount of propylene increases by 1.7%.