Method for improving the cooling capacity of a gas solids olefin polymerization reactor

Abstract

The present invention relates to a method for improving the cooling capacity of a gas solids olefin polymerization reactor by splitting the fluidization gas and returning part of the fluidization gas to the reactor into the bottom zone of the reactor and another part of the fluidization gas directly into the dense phase formed by particles of a polymer of the at least one olefin suspended in an upwards flowing stream of the fluidization gas in the middle zone of the reactor.

Claims

1. A method for improving the cooling capacity of a gas solids olefin polymerization reactor comprising: a top zone; a middle zone, which comprises a top end in direct contact with said top zone and which is located below said top zone, the middle zone having a generally cylindrical shape; and a bottom zone, which is in direct contact with a bottom end of the middle zone and which is located below the middle zone; comprising the following steps: a) introducing a first stream of fluidization gas into the bottom zone; b) polymerizing olefin monomer(s) in the presence of a polymerization catalyst in a dense phase formed by particles of a polymer of the olefin monomer(s) suspended in an upwards flowing stream of the fluidization gas in the middle zone; c) withdrawing a second stream comprising the fluidization gas from the top zone; d) introducing the second stream into a cooler; e) withdrawing the cooled second stream from the cooler; and f) splitting the cooled second stream into a cooled third stream and the first stream; g) introducing the cooled third stream through one or more feeding ports in a feeding area of the middle zone at the dense phase in the middle zone of the gas solids olefin polymerization reactor; wherein the cooled third stream comprises from 1 to 30 wt % condensed fluidization gas, and wherein a bulk density of the dense phase during polymerization is in the range of from 100 to 500 kg/m.sup.3.

2. The method according to claim 1, wherein the feeding area of the middle zone is located on the surface of the middle zone between the top end and 50% of the total height of the middle zone, whereas the bottom end corresponds to 0% and the top end corresponds to 100% of the total height of the middle zone.

3. The method according to claim 1, wherein difference of the maximum temperature and the minimum temperature, ΔT, of the dense phase during polymerization is not higher than 10° C.

4. The method according to claim 1, wherein the cooled third stream is introduced through the one or more feeding ports into the dense phase in the middle zone of the gas solids olefin polymerization reactor in an angle of 5° to 75°, wherein the introduction angle is the angle between the projection of the direction of the cooled third stream after introduction into the reactor on a projection plane, which crosses a tangent plane of the generally cylindrical shape of the middle zone at the location of the one or more feeding ports and along an intersection line between the tangent plane and the generally cylindrical surface of the middle zone, whereas the projection plane is located perpendicular to the tangent plane, and a perpendicular line, which crosses the generally cylindrical surface of the middle zone at the location of the one or more feeding ports, is located parallel to the projection plane, and is perpendicular to the tangent plane.

5. The method according to claim 1, wherein number of feeding ports for introducing the cooled third stream is in the range of 1 to 15.

6. The method according to claim 1, wherein the feeding ports are distributed across the middle zone of the gas solids olefin polymerization reactor in axial and/or radial direction with the proviso that the cooled third stream is introduced into the dense phase.

7. The method according to claim 1, wherein the cooled second stream is split into the cooled third stream and the first stream at a ratio of 5:95 (v/v) to 75:25 (v/v).

8. The method according to claim 1, further comprising the steps of introducing the second stream into a compressor; withdrawing the compressed second stream from the compressor and introducing the compressed second stream into the cooler.

9. The method according to claim 8 wherein the pressure difference between the cooled third stream and the polymerization pressure in the gas solids polymerization reactor, ΔP, is at least 0.1 bar.

10. The method according to claim 1, wherein the superficial gas velocity of the upwards flowing stream of the fluidization gas in the middle zone is from 0.3 to 1.2 m/s.

11. The method according to claim 10 wherein the superficial gas velocity of the first stream of fluidization gas introduced into the bottom zone is lower than the superficial gas velocity of the upwards flowing stream of the fluidization gas in the middle zone and is in the range of from 0.1 to 1.3 m/s.

12. The method according to claim 1, wherein the gas solids olefin polymerization reactor is a fluidized bed reactor comprising a fluidization grid.

13. The method according to claim 1, wherein the gas solids olefin polymerization reactor is a fluidized bed reactor comprising a top zone having a generally conical shape, a middle zone, in direct contact with said top zone and located below said top zone, having a generally cylindrical shape, a bottom zone, in direct contact with said middle zone and located below said middle zone, having a generally conical shape, the gas solids olefin polymerization reactor not containing a fluidization grid.

Description

FIGURES

(1) FIG. 1 shows an embodiment of the polymerization process according to the present invention in a fluidized bed reactor with a fluidization grid.

(2) FIG. 2 shows the experimental set-up of the temperature measurement in the experimental part.

(3) FIG. 3 shows the definition of the introduction angle of the cooled third stream.

REFERENCE SIGNS FOR FIG. 1

(4) 1 fluidized bed reactor 2 bottom zone 3 middle zone 4 disengaging zone (top zone) 5 fluidized bed (dense zone) 6 first stream of fluidized gas 7 second stream of fluidized gas 8 compressor 9 compressed second stream of fluidized gas 10 cooler 11 cooled second stream of fluidized gas 12 cooled third stream of fluidized gas 13 feeding ports for the cooled third stream of fluidized gas 14 feeding port for polymerization catalyst 15 polymer withdrawal 16 fluidization grid

(5) Description of FIG. 1

(6) FIG. 1 shows an embodiment of the gas solids olefin polymerization reactor system according to the present invention. The fluidized bed reactor (1) comprises a bottom zone (2), a middle zone (3) and a disengaging zone as top zone (4). The middle zone (3) and the bottom zone (2) are separated by the fluidization grid (16). The first stream of fluidized gas (6) enters the fluidized bed reactor (1) through the bottom zone (2) and flows upwards, thereby passing the fluidization grid (16) and entering the middle zone (3). Due to the substantially cylindrical shape of the middle zone (3) the gas velocity is constant so that above the fluidization grid (16) the fluidized bed (5) is established in the middle zone (3). Due to the conical shape of the disengaging zone (4) the gas entering the disengaging zone (4) expands so that the gas disengages from the polyolefin product of the polymerization reaction so that the fluidized bed (5) is confined in the middle zone (3) and the lower part of the disengaging zone (4). The polymerization catalyst is introduced into the fluidized bed reactor (1) through feeding port (14) directly into the fluidized bed (5). The polyolefin product of the polymerization process is withdrawn from the fluidized bed reactor through outlet (15).

(7) The fluidized gas is withdrawn from the disengaging zone (4) as second stream of fluidization gas (7) and introduced into a compressor (8). The compressed second stream (9) is withdrawn from the compressor (8) and introduced into a cooler (10). The cooled second stream (11) is withdrawn from the cooler (10) and split into a third cooled stream (12) and the first stream (6). The cooled third stream (12) is introduced into fluidized bed (5) of the fluidized bed reactor (1) through one or more feeding ports (13) as such that the fluidized gas of the cooled third stream (12) is directed into the fluidized bed (5).

REFERENCE SIGNS FOR FIG. 3

(8) a projection of the direction of the cooled third stream b perpendicular line c projection plane d tangent plane e location of the feeding port f intersection line g generally cylindrical surface of the middle zone α introduction angle γ angle between planes (c) and (d)

(9) Description of FIG. 3

(10) FIG. 3 demonstrates the definition of the introduction angel α of the cooled third stream. Said introduction angle (α) is the angle between a projection (a) and a perpendicular line (b), whereas the projection (a) is the projection of the direction of the cooled third stream after introduction into the reactor on a projection plane (c), which crosses the tangent plane (d) of the generally cylindrical shape (g) of the middle zone at the location of the one or more feeding ports (e) and along an intersection line (f) between the tangent plane (d) and the generally cylindrical surface (g) of the middle zone, whereas the projection plane (c) is located perpendicular to the tangent plane (d) (γ=90°) and whereas the perpendicular line (b) crosses the generally cylindrical surface (g) of the middle zone at the location of the feeding port (e), is parallel to projection plane (c) and is perpendicular to tangent plane (d).

EXAMPLES

(11) FIG. 2 shows the reactor set-up used for the example 1 of the present invention. Thereby, the dimensions of the reactor set-up are given in FIG. 2. The reactor set-up comprises a fluidized bed reactor comprising a fluidization grid (or distributor plate) into which before the start of the experiments HDPE powder was filled.

Example 1

(12) The experimental set up mentioned above was employed to assess the effect of the spit fluidized gas concept on the cooling capability and the thermal homogeneity in the fluidized bed reactor. Through the gas inlet at the bottom of the fluidized bed reactor cold fluidization gas was introduced at a volumetric feeding rate equal to 137 m.sup.3/h to establish a superficial gas velocity just above the fluidization grid of 0.54 m/s so that a fluidized bed of HDPE with a height of 86 cm was established in the middle zone of the reactor above the fluidization grid. The superficial gas velocity at the end of the fluidized reactor horizontal part (i.e., end of dense phase) was kept constant at 0.54 m/s throughout the whole experiment.

(13) The temperature in the fluidized bed reactor is measured throughout the whole experiment at three measuring points T.sub.1, located at a point of 5 cm above the distribution plate, T.sub.2, located at a point of 80 cm above the distribution plate and T.sub.3, located at a point of 136 cm above the distribution plate. Thereby, at measuring points T.sub.1 and T.sub.2 the temperature of the dense phase of the fluidized bed is measured whereas at measuring point T.sub.3 a mixed temperature of gas and solid in the lean phase of the fluidized bed above the dense phase of the fluidized bed is measured.

(14) 2.5 min after starting to introduce cold fluidization gas the heating of the fluidization gas was switched on and the fluidization gas was controlled to have a temperature of 100° C. at the entry at the bottom end of the fluidized bed. The hot fluidization gas feed was kept at a constant flow of 137 m.sup.3/h. The HDPE powder in the fluidized bed was heated by the hot fluidization gas until thermal equilibrium was reached after about 70 min after starting to introduce hot fluidization gas. The temperature measurement points T.sub.1 and T.sub.3 were deviating from each other by approximately 3° C. showing that the gas-solid mixing conditions in the bed is not ideal (i.e., T.sub.1=73° C. and T.sub.3=70° C.).

(15) 72 min after starting to introduce hot fluidization gas from the bottom of the reactor, its volumetric flow rate was reduced from 137 m.sup.3/h to 91 m.sup.3/h and at the same time cooled fluidization gas circulation stream (i.e., at temperature equal to 25° C.) was reintroduced into the fluidized bed reactor through a injection point in the middle zone of the fluidized bed reactor into the dense zone of the HDPE powder in a downwards direction in an angle of 20°, determined from the general cylindrical shape of the middle zone. The cooled fluidization gas circulation stream had a constant flow of 46 m.sup.3/h and a pressure difference between that injection point and the fluidized bed reactor was equal to 3 bar (i.e., ΔP=3 bar). With the constant flow of the hot fluidization gas of 91 m.sup.3/h, the split of the cooled fluidization gas circulation stream (JG) and the hot fluidization gas stream (FG) was 33.5:66.5 (v/v).

(16) After introducing the cooled fluidization gas circulation stream at t=72 min the temperature at all three measuring points drops by about 10° C. until again an equilibrium is obtained.

(17) It has to be highlighted that during the heating phase of the HDPE powder in the fluidized bed at t=2.5 min to t=72 min the temperatures of the dense phase of the fluidized bed T.sub.1 and T.sub.3 were deviating each other by 3° C. After introducing the cooled fluidization gas circulation stream all the three measuring points were exactly the same (T.sub.1=T.sub.2=T.sub.3=60° C.).

(18) The contact of the cooled fluidization gas circulation stream and the HDPE powder in the fluidized bed leads to an improved mixing of the polymer powder in the fluidized bed resulting in an efficient heat exchange and a decreasing fluidized bed temperature. From the same temperature profile at the measuring points of the dense phase of the fluidized bed T.sub.1 and T.sub.2 and the measuring point of the mixed temperature of gas and solid in the lean phase of the fluidized bed T.sub.3 it can be concluded that the cooled fluidization gas circulation stream contributes to sufficient heat removal from the fluidized bed.

Examples 2-4

(19) In Examples 2-4, the same reactor set-up used for the Example 1 was employed with the only difference being that T.sub.1 was located at the middle of the dense zone of the fluidized bed reactor, T.sub.2 was located at the inlet pipe of the cooled fluidization gas circulation stream and T.sub.3 was located at the top gas pipe exit (see FIG. 3).

Example 2 (Comparative)

(20) Through the gas inlet at the bottom of the fluidized bed reactor hot fluidization gas (FG) was introduced at a feeding rate equal to 150 m.sup.3/h to establish a superficial gas velocity just above the fluidization grid of 0.60 m/s. The superficial gas velocity at the end of the fluidized reactor horizontal part (i.e., end of dense phase) was kept constant at 0.60 m/s throughout the whole experiment.

(21) The temperature in the fluidized bed reactor after 60 min of operation reached a steady state value (thermal equilibrium) of 60° C. measured at three measuring points T.sub.1 (located at the middle of the dense reactor zone), T.sub.2 (located at the cooled fluidization gas circulation stream) and T.sub.3 (located at the top gas pipe exit). The hot fluidization gas feed was kept at a constant flow of 150 m.sup.3/h.

(22) 62 min after starting to introduce hot fluidization gas into the bottom zone of the fluidized bed reactor, the fluidization gas withdrawn from the top zone of the fluidized bed reactor was directed through a compressor/cooler unit in order to cool it down to a temperature equal to 25° C. before re-introducing the cooled fluidization gas (FG) stream into the bottom zone of the fluidized bed reactor. The volumetric gas flow rate of the cooled fluidization gas was not changed and it was equal to 150 m.sup.3/h. No cooled fluidization gas circulation (jet gas (JG)) stream was used and the split between the jet gas (JG) stream and the fluidization gas (FG) stream was 0.0:100.0 (v/v).

(23) The temperature in the dense phase of the fluidized bed captured by the measurement point T.sub.1 was equal to 60° C. at the steady state operation (introduction of hot FG stream). After that the fluidized bed was cooled by using only cooled fluidization gas (FG) for 30 min.

(24) The temperature decrease rate in the fluidized bed reactor (measured at measure point T.sub.1) after 10 min, 20 min and 30 min (ΔT.sub.10, ΔT.sub.20 and ΔT.sub.30) was equal to 15° C./10 min, 20.5° C./20 min and 25° C./30 min, respectively.

(25) The main conditions and results of this experiment are summarized in Table 1.

(26) TABLE-US-00001 TABLE 1 Conditions and main results of Example 2. Conditions Values JG Pressure drop [bar] 0 JG Flow [m.sup.3/h] (% Split (v/v))  0 (0% split) FG Flow [m.sup.3/h] (% Split (v/v)) 150.0 (100% Split) Overall Gas Feed [m.sup.3/h] 150.0 SGV.sub.FG [m/s] 0.60 SGV.sub.total [m/s] 0.60 Final bulk density (Kg/m.sup.3) 250 ΔT.sub.10 (° C./10 min) 15 ΔT.sub.20 (° C./20 min) 20.5 ΔT.sub.30 (° C./30 min) 25

Example 3 (Inventive)

(27) The Example 2 was repeated. Heating of the bed fluidization bed reactor was performed following the procedure described in Example 2. The temperature in the fluidized bed reactor after 60 mins of operation reached a steady state value (thermal equilibrium) of 60° C. measured at three measuring points T.sub.1 (located at the middle of the dense reactor zone), T.sub.2 (located at the cooled fluidization gas circulation stream) and T.sub.3 (located at the top gas pipe exit). The hot fluidization gas feed was kept at a constant flow of 150 m.sup.3/h.

(28) 62 min after starting to introduce hot fluidization gas from the bottom of the fluidized bed reactor, the fluidization gas withdrawn from the top zone of the fluidized bed reactor was directed through a compressor/cooler unit in order to cool it down to a temperature equal to 25° C. The volumetric gas flow rate of the cooled fluidization gas (FG) stream re-introduced into the bottom zone of the fluidized bed reactor was reduced from 150 m.sup.3/h to 110 m.sup.3/h. In this experiment cooled fluidization gas circulation (jet gas (JG)) stream having a temperature of 25° C. was introduced into the fluidized bed reactor through an injection point in the middle zone of the fluidized bed reactor into the dense zone of the HDPE powder in a downwards direction in an angle of 20°, determined from the general cylindrical shape of the middle zone (ΔP for injecting the JG was 5.0 bar, see Table 2) and the split between the fluidization gas circulation stream (JG) and the fluidization gas stream (FG) was 26.7:73.3 (v/v).

(29) The temperature in the dense phase of the fluidized bed captured by the measurement point T.sub.1 was equal to 60° C. at the steady state operation (introduction of hot FG stream). After that, the fluidized bed reactor was cooled by using both FG and JG for 30 min.

(30) The temperature decrease rate in the fluidized bed reactor ((measured at measure point T.sub.1)) after 10 min, 20 min and 30 min (ΔT.sub.10, ΔT.sub.20 and ΔT.sub.30) was equal to 17° C./10 min, 24.5° C./20 min and 28° C./30 min, respectively.

(31) The main conditions and results of this experiment are summarized in Table 2.

(32) TABLE-US-00002 TABLE 2 Conditions and main results of Example 3. Conditions Values JG Pressure drop [bar] 5 JG Flow [m.sup.3/h] (% Split (v/v))  40.0 (26.7% split) FG Flow [m.sup.3/h] (% Split (v/v)) 110.0 (73.3% Split) Overall Gas Feed [m.sup.3/h] 150.0 SGV.sub.FG [m/s] 0.43 SGV.sub.total [m/s] 0.60 Final bulk density (Kg/m.sup.3) 330 ΔT.sub.10 (° C./10 min) 17 ΔT.sub.20 (° C./20 min) 24.5 ΔT.sub.30 (° C./30 min) 28

Example 4 (Inventive)

(33) The Example 2 was repeated. Heating of the bed fluidization bed reactor was performed following the procedure described in Example 2. The temperature in the fluidized bed reactor after 60 mins of operation reached a steady state value (thermal equilibrium) of 60° C. measured at three measuring points T.sub.1 (located at the middle of the dense reactor zone), T.sub.2 (located at the cooled fluidization gas circulation stream) and T.sub.3 (located at the top gas pipe exit). The hot fluidization gas feed was kept at a constant flow of 150 m.sup.3/h.

(34) 62 min after starting to introduce hot fluidization gas from the bottom of the fluidized bed reactor, the fluidization gas withdrawn from the top zone of the fluidized bed reactor was directed through a compressor/cooler unit in order to cool it down to a temperature equal to 25° C. The volumetric gas flow rate of the cooled fluidization gas (FG) stream re-introduced into the bottom zone of the fluidized bed reactor was reduced from 150 m.sup.3/h to 110 m.sup.3/h. In this experiment cooled fluidization gas circulation (jet gas (JG)) stream having a temperature of 25° C. was introduced into the fluidized bed reactor through an injection point in the middle zone of the fluidized bed reactor into the dense zone of the HDPE powder in a downwards direction in an angle of 20°, determined from the general cylindrical shape of the middle zone (ΔP for injecting the JG was 2.25 bar, see Table 3) and the split between the fluidization gas circulation stream (JG) and the fluidization gas stream (FG) was 26.7:73.3 (v/v).

(35) The temperature in the dense phase of the fluidized bed captured by the measurement point T.sub.1 was equal to 60° C. at the steady state operation (introduction of hot FG stream). After that, the fluidized bed reactor was cooled by using both FG and JG for 30 min.

(36) The temperature decrease rate in the fluidized bed reactor (measured at measure point T.sub.1) after 10 min, 20 min and 30 min (ΔT.sub.10, ΔT.sub.20 and ΔT.sub.30) was equal to 16° C./10 min, 23.5° C./20 min and 28° C./30 min, respectively.

(37) The main conditions and results of this experiment are summarized in Table 3.

(38) TABLE-US-00003 TABLE 3 Conditions and main results of Example 4. Conditions Values JG Pressure drop [bar] 2.25 JG Flow [m.sup.3/h] (% Split (v/v))  40.0 (67.7% split) FG Flow [m.sup.3/h] (% Split (v/v)) 110.0 (73.3% Split) Overall Gas Feed [m.sup.3/h] 150.0 SGV.sub.FG [m/s] 0.43 SGV.sub.total [m/s] 0.60 Final bulk density (Kg/m.sup.3) 325 ΔT.sub.10 (° C./10 min) 16 ΔT.sub.20 (° C./20 min) 23.5 ΔT.sub.30 (° C./30 min) 28

(39) By comparing the results depicted in Table 1 with those shown in Tables 2 and 3 it can be seen that the use of JG has a positive influence on the cooling effect/capacity in the fluidized bed reactor. More specifically, the cooling rate in the fluidized bed reactor, expressed by ΔT.sub.10, ΔT.sub.20 and ΔT.sub.30, is enhanced when JG is used. It is also apparent that even at a lower pressure drop ΔP for injecting the JG, the cooling effect of JG in the fluidized bed reactor is fully maintained and is better compared to comparative example 1 where all the fluidization gas was introduced from the bottom of the fluidized bed reactor. Similarly, the fluidized bulk density attains higher values (i.e., 330 kg/m.sup.3 and 325 kg/m.sup.3) compared to comparative example 1 (i.e., 250 kg/m.sup.3), which is a direct indication of solids carry reduction.