Method for production of polymer

Abstract

Method for controlling a process for the production of a polymer by polymerization of a monomer and a comonomer. The process includes maintaining a substantially constant effective flow ratio (EFR), the effective flow ratio being defined as EFR=(Q.sub.comoL.sub.como)/(Q.sub.monoL.sub.mono), Q.sub.como and Q.sub.mono being, respectively, flow rates of comonomer and monomer to the reactor, L.sub.como and L.sub.mono being, respectively, losses of comonomer and monomer.

Claims

1. A method for controlling a process for the production of a polymer by polymerisation of a monomer and a comonomer which process comprises: maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.comoL.sub.como)/(Q.sub.monoL.sub.mono) Q.sub.como and Q.sub.mono being, respectively, flow rates of comonomer and monomer to the reactor, L.sub.como and L.sub.mono being, respectively, losses of comonomer and monomer.

2. A method according to claim 1 wherein Q.sub.mono and Q.sub.como are flow rates of fresh monomer and fresh comonomer to the reactor.

3. A method according to claim 1 wherein L.sub.mono and L.sub.como are losses of monomer and comonomer to purges.

4. A method for controlling a process for the production of a polymer by polymerisation of a monomer which process comprises: maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.hydL.sub.hyd)/(Q.sub.monoL.sub.mono) Q.sub.hyd and Q.sub.mono being, respectively, flow rates of hydrogen and monomer to the reactor, L.sub.hyd and L.sub.mono being, respectively, losses of hydrogen and monomer.

5. A method according to claim 4 wherein Qmono and Qhyd are flow rates of fresh monomer and fresh hydrogen to the reactor.

6. A method according to claim 4 wherein Lmono and Lhyd are losses of monomer and hydrogen to purges.

7. A method according to claim 1 which is applied under both steady-state and non-steady-state conditions.

8. A method according to claim 4 which is applied under both steady-state and non-steady-state conditions.

9. A method according to claim 1 which is a method for the start-up or transition of a process for the production of a polymer by polymerisation of a monomer and which process comprises during the start-up or transition: I. maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.comoL.sub.como)/(Q.sub.monoL.sub.mono) Q.sub.como and Q.sub.mono being, respectively, flow rates of comonomer and monomer to the reactor, L.sub.como and L.sub.mono being, respectively, losses of comonomer and monomer, and optionally II. maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.hydL.sub.hyd)/(Q.sub.monoL.sub.mono) Q.sub.hyd and Q.sub.mono being, respectively, flow rates of hydrogen and monomer to the reactor, L.sub.hyd and L.sub.mono being, respectively, losses of hydrogen and monomer.

10. A method according to claim 4 which is a method for the start-up or transition of a process for the production of a polymer by polymerisation of a monomer and which process comprises during the start-up or transition: I. maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.hydL.sub.hyd)/(Q.sub.monoL.sub.mono) Q.sub.hyd and Q.sub.mono being, respectively, flow rates of hydrogen and monomer to the reactor, L.sub.hyd and L.sub.mono being, respectively, losses of hydrogen and monomer, and optionally, II. maintaining a substantially constant effective flow ratio (EFR), said effective flow ratio being defined as:
EFR=(Q.sub.comoL.sub.como)/(Q.sub.monoL.sub.mono) Q.sub.como and Q.sub.mono being, respectively, flow rates of comonomer and monomer to the reactor, L.sub.como and L.sub.mono being, respectively, losses of comonomer and monomer.

11. A method according to claim 9 which comprises the above control during a start-up and subsequent steady-state operation.

12. A method according to claim 10 which comprises the above control during a start-up and subsequent steady-state operation.

13. A method according to claim 9 which comprises the above control during a transition and in preceding and subsequent steady-state operations.

14. A method according to claim 10 which comprises the above control during a transition and in preceding and subsequent steady-state operations.

15. A method according to claim 1 which is carried out in a gas phase fluidized-bed reactor.

16. A method according to claim 4 which is carried out in a gas phase fluidized-bed reactor.

17. A method according to claim 1 wherein the monomer is ethylene or propylene.

18. A method according to claim 4 wherein the monomer is ethylene or propylene.

19. A method according to claim 17 wherein the comonomer is an olefin having 4 to 8 carbon atoms, or ethylene where propylene is the monomer, or an olefin having 4 to 8 carbon atoms, or propylene where ethylene is the monomer.

20. A method according to claim 1 where the polymerisation process uses a Ziegler-Natta type catalyst, optionally in conjunction with a cocatalyst which is an organometallic compound of a metal from Groups I to III of the Periodic Classification of the Elements.

21. A method according to claim 4 where the polymerisation process uses a Ziegler-Natta type catalyst, optionally in conjunction with a cocatalyst which is an organometallic compound of a metal from Groups I to III of the Periodic Classification of the Elements.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is further described with reference to the accompanying drawings, in which:

(2) FIG. 1 shows the feed comonomer to ethylene mass flow ratio, which is constant, and the effect that the changes in reactor composition has on the instantaneous density of the polymer produced in the reactor;

(3) FIG. 2 shows the oscillations in reactor pressure;

(4) FIG. 3 shows that the adjustment of comonomer feed to compensate for the increased loss of ethylene reduces the variation of comonomer to monomer in the reactor, which in turn reduces the oscillations in instantaneous density of the polymer.

EXAMPLES

(5) The Examples relate to simulated production of an LLDPE grade being produced on an industrial gas phase reactor of diameter 5 m and bed height 20 m at a target production rate of 34.2 te/hr using a metallocene catalyst and a reactor pressure of 24.2 barg. The reactor operates with a small purge to remove inerts which may otherwise build-up, but ethylene is also lost via the purge. In the present Examples comonomer (hexene) is recovered and hence is not lost to purge. At steady-state the purge flow rate and composition is approximately constant.

(6) Prior to simulating changes to the system, the reactor is running in steady-state with average bed density and instantaneous powder density of 916.9 kg/m3. In both Examples a perturbation is introduced by a reduction in catalyst flow of 5%. Such a change in an actual plant could be caused, for example, by partial blockage of a feed line, poor control of feeder speed, change in catalyst density and fill factor or loss of catalyst activity.

Comparative Example

(7) In the Comparative Example the ethylene flow to the reactor is controlled by the operator and fixed at the target production rate of 34.2 te/hr. This rate is maintained at steady state and throughout the perturbation.

(8) The powder density is controlled via adjustment of mass flow ratio wherein the hexene flow rate is controlled to maintain fixed mass flow ratio of hexene to ethylene to the reactor. Feedback from powder analysis allows the operator to adjust this ratio if necessary but because of large lags in the system short-term perturbations and changes to instantaneous powder properties cannot be controlled accurately. In the Comparative Example, the hexene to ethylene mass flow rates are maintained at a ratio of 0.105, at steady state and throughout the perturbation.

(9) At steady-state, the purge rate is approximately 200 kg/h, of which approximately 100 kg/h is ethylene monomer (the remainder is mainly nitrogen).

(10) The loss of catalyst flow results in a reduction of ethylene consumption in the reactor, and consequently an increase in ethylene partial pressure as well as total reactor pressure.

(11) The reactor total pressure controller compensates by increasing the purge rate from the reactor. The perturbations cause an oscillation in the reactor pressure and in the purge flow rate. The increase in purge flow rate in leads to an increased loss of ethylene.

(12) The results of this are shown in FIGS. 1 and 2. In particular, FIG. 2 shows the oscillations in reactor pressure. Compensating for this the purge flow rate (note shown) changes correspondingly, from a maximum of about 1500 kg/hr to a minimum of nearly zero.

(13) FIG. 1 shows the feed comonomer to ethylene mass flow ratio, which is constant, and the effect that the changes in reactor composition has on the instantaneous density of the polymer produced in the reactor. As well as the magnitude of the oscillations in density immediately following the perturbation it is noticeable that even once the reactor returns to a new steady-state there is an off-set in the density of the polymer compared to that prior to the perturbation.

(14) The reactor is not controlled on effective flow ratio, but the process oscillates between a value for the hexene:ethylene effective flow ratio of 0.105 (when the purge valve is closed, wherein the effective flow ratio is the same as the feed ratio) to a value of 0.1073 (with the purge flow at its maximum and hence when most ethylene is lost to purge).

(15) Even small changes in instantaneous density may have a damaging effect on reactor operation; usually reactor conditions (notably operating temperature) are selected to prevent formation of low density and low sintering temperature material, and such perturbations can give rise to agglomerates.

Example According to the Invention

(16) In the Example according to the present invention the reactor is controlled on the hexene:ethylene effective flow ratio.

(17) The steady-state conditions are as in the Comparative Example. Thus, the ethylene flow to the reactor is controlled by the operator and fixed at the target production rate of 34.2 te/hr. This rate is maintained at steady state and throughout the perturbation.

(18) At steady-state, the purge rate is approximately 200 kg/h, of which approximately 100 kg/h is ethylene monomer (the remainder is mainly nitrogen).

(19) In this example, however, the hexene flow rate is controlled to maintain a fixed effective flow ratio of hexene to ethylene. More particularly, the hexene to ethylene effective flow ratio is maintained at a ratio 0.1053. At steady-state this corresponds to the same mass flow ratio of 0.105 used in the Comparative Example.

(20) (At steady-state, the flow rate of ethylene is 34.2 te/hr, the flow rate of hexene is 3.59 te/hr, and the loss of ethylene to purge is 0.1 te/hr, so the feeds flow ratio is 3.59/34.2=0.105, whilst the effective flow ratio is 3.59/(34.10.1), which is 0.1053.)

(21) The simulated loss of catalyst flow results in a reduction of ethylene consumption in the reactor, and consequently an increase in ethylene partial pressure as well as total reactor pressure.

(22) The reactor total pressure controller compensates by increasing the purge rate from the reactor. The perturbations cause an oscillation in the reactor pressure and in the purge flow rate. The increase in purge flow rate in turn leads to an increased loss of ethylene. In this Example, however, the comonomer feed is varied to compensate for the additional loss of monomer, and thereby maintain the effective flow ratio at 0.1053.

(23) The reactor pressure oscillates in the same manner as the Comparative Example. However, the adjustment of comonomer feed to compensate for the increased loss of ethylene reduces the variation of comonomer to monomer in the reactor, which in turn reduces the oscillations in instantaneous density of the polymer. Both of these are shown in FIG. 3.

(24) As is clear from comparison of FIG. 3 with FIG. 1, the use of effective flow ratio control significantly reduces the changes in instantaneous density of the polymer being formed in the reactor. Further, the new steady-state is achieved more rapidly than in the Comparative Example, and there is a smaller change in the powder density between the new steady-state and that prior to the perturbation.