Polymerization control process

Details Polymerization control process Polymerization control process

2001-05-24

2003-07-01

Wu, David W. (Department: 1713)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

Polymers from only ethylenic monomers or processes of...

C526S068000, C526S060000

Reexamination Certificate

active

06586539

ABSTRACT:

The present invention relates to a continuous process for controlling the gas-phase co-polymerisation of olefins in a fluidised bed reactor.
Processes for the co-polymerisation of olefins in the gas phase are well known in the art. Such processes can be conducted for example by introducing the gaseous monomer and comonomer into a stirred and/or gas fluidised bed comprising polyolefin and a catalyst for the polymerisation.
In the gas fluidised bed polymerisation of olefins, the polymerisation is conducted in a fluidised bed reactor wherein a bed of polymer particles is maintained in a fluidised state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of such a polymerisation generally employs a bed of polymer particles similar to the polymer which it is desired to manufacture. During the course of polymerisation, fresh polymer is generated by the catalytic polymerisation of the monomer, and polymer product is withdrawn to maintain the bed at more or less constant volume. An industrially favoured process employs a fluidisation grid to distribute the fluidising gas to the bed, and to act as a support for the bed when the supply of gas is cut off. The polymer produced is generally withdrawn from the reactor via a discharge conduit arranged in the lower portion of the reactor, near the fluidisation grid. The fluidised bed consists in a bed of growing polymer particles. This bed is maintained in a fluidised condition by the continuous upward flow from the base of the reactor of a fluidising gas.
The polymerisation of olefins is an exothermic reaction and it is therefore necessary to provide means to cool the bed to remove the heat of polymerisation. In the absence of such cooling the bed would increase in temperature and, for example, the catalyst may become inactive or the bed commence to fuse. In the fluidised bed polymerisation of olefins, the preferred method for removing the heat of polymerisation is by supplying to the polymerisation reactor a gas, the fluidising gas, which is at a temperature lower than the desired polymerisation temperature, passing the gas through the fluidised bed to conduct away the heat of polymerisation, removing the gas from the reactor and cooling it by passage through an external heat exchanger, and recycling it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidised bed at the desired polymerisation temperature. In this method of polymerising alpha olefins, the recycle gas generally comprises the monomer and comonomer olefins, optionally together with, for example, an inert diluent gas such as nitrogen or a gaseous chain transfer agent such as hydrogen. Thus, the recycle gas serves to supply the monomer to the bed, to fluidise the bed, and to maintain the bed at the desired temperature. Monomers consumed by the polymerisation reaction are normally replaced by adding make up gas or liquid to the polymerisation zone or reaction loop.
A gas fluidised bed polymerisation reactor is typically controlled to achieve a desired melt index and density for the polymer at an optimum production. Conditions within the polymerisation reactor have to be carefully controlled to reduce the risk of agglomerate and/or sheet formation which may ultimately lead to bed instabilities and a need to terminate the reaction and shut down the reactor. This is the reason why commercial scale reactors are designed to operate well within proven stable operating zones and why the reactors are used in a carefully circumscribed fashion.
Even within the constraints of conventional, safe operation, control is complex adding further difficulty and uncertainty if one wishes to find new and improved operating conditions.
There is no generally accepted view as to what causes agglomerates or sheeting. Agglomerates or sheets can, for example, form when the polymerisation temperature is too close to the polymer sintering temperature or when the polymer particles become excessively sticky. Highly active fine particles can, for example, concentrate in the upper elevations of the polymerisation zone, towards the top of the fluidised bed and in the powder disengagement zone above the bed thus leading to local hot spots and potential agglomeration and/or sheeting.
It is known that the powder disengagement and velocity reduction zones of the reactor are particularly vulnerable to sheet formation and there have been many attempts to mitigate these effects.
EP-0692495 discloses a method for polymerising olefins in a gas phase reactor having an expanded section wherein a tangential flow of gas is introduced into the expanded section in order to reduce fines entrainment and to reduce solid particle build-up on the interior surfaces of the expanded section.
EP-0695313 discloses a continuous process for the gas phase polymerisation of olefins in a fluidised bed reactor comprising a polymerisation zone and a gas velocity reduction zone situated above the bed wherein the make-up monomers are directly sent to the fluidised bed reactor in one or more points above the fluidised bed. This process is presented as a general solution to the fouling problems occurring in the reactor system.
BP patent EP-0 855 411 discloses a process for continuous gas phase polymerisation of olefin(s) in a reactor containing a fluidised bed, consisting of a cylinder with a vertical side wall and of a desurging or disengagement chamber (
3
) mounted above the said cylinder, characterised in that the fluidised bed occupies at least all of the cylinder with a vertical side wall of the reactor. This process not only allows to increase the output efficiency of industrial plants but also to reduce the fouling phenomenon experienced in the past.
WO 94/25495 describes a method of determining stable operating conditions for a fluidised bed polymerisation process which comprises: (a) observing fluidised bulk density changes in the reactor associated with changes in the composition of the fluidising medium; and (b) increasing the cooling capacity of the recycle stream by changing the composition without exceeding the level at which a reduction in the fluidised bulk density or a parameter indicative thereof becomes irreversible. The aim of this invention is to control the stability of operation of the fluidised bed by monitoring and controlling conditions within the fluidised bed itself.
Existing continuous gas fluidised bed processes have demonstrated that high space time yield polymerisations can be reached. One of the major problems encountered with these high space time yield polymerisation processes is to ensure a good control of the operating conditions leading to a safer use of the process;
The present invention provides means to monitor and control stability of the entire polymerisation zone, not just that within the fluidised bed which consists essentially in the well mixed region. It is desirable to provide a method of defining stable operating conditions to minimise potential for sheet formation especially outside of the well mixed region in the fluidised bed, particularly for high space time yield polymerisation processes.
It is therefore an objective of the present invention to provide criteria to determine a stable operating envelope for a gas phase polymerisation process of two or more reactants and to run the process safely with low risk of malfunction, for example agglomeration or sheeting or off-specification polymer, particularly at high space time yields.
Although it is known that a fluidised bed ensures good solids mixing and good heat transfer, it has been found that control of variations in reactant gaseous concentrations within the polymerisation zone is key to the prevention of agglomerate or sheet formation and optimisation of stable and safe operating envelope, including uniform product properties, particularly at high space time yields. This control also enables safe and stable increase in the conversion of reactants per pass of the recycle stream even at lower space time yields.
The ability to operate safely at higher con

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