Method for improving a polymerization reaction by taking out and analysing a sample

Abstract

The invention relates to a device for taking out and analysing a sample from a polymerisation reactor comprising one or more sample conduits (2), for taking a sample out of said reactor and for conducting said sample to one or more sample flash tanks (3), whereby said conduits each are in communication with said reactor (19) and each are provided with at least two sampling valves (4, 5); comprising one or more sample flash tanks (3), for separating said solid particles and evaporated gas, whereby said sample flash tanks are connected to said conduits (2) and provided with means for analysing said evaporated gas (7), and comprising one or more sample receivers (6), for purifying said solid particles, whereby said receivers are connected to said sample flash tanks (3) and provided with means (8) for analysing said solid particles. The invention further relates to a method for improving a polymerisation reaction in a polymerisation reactor.

Description

FIELD OF THE INVENTION [0001] This invention relates to process control. In a first aspect, the invention relates to a device for taking out and analysing a sample from a polymerisation reactor, in particular a polymerisation reactor suitable for polymerising ethylene. In another aspect the invention relates to a method for improving a polymerisation reaction in a polymerisation reactor during a process for preparing bimodal polyethylene. BACKGROUND OF THE INVENTION [0002] In a typical polymerisation reaction, monomer, diluent, catalyst, co-catalyst and optionally co-monomer and hydrogen are fed to a reactor where the monomer is polymerised. The diluent does not react but is typically utilised to control solids concentration and also to provide a convenient mechanism for introducing the catalyst into the reactor. The reactor effluent a mature of polymer, diluent, unreacted (co-)monomer and hydrogen, is removed from the reactor and fed to a flash tank where the polymer is separated f...

AI Technical Summary

Benefits of technology

[0019] In another aspect the invention relates to the use of a sampling device according to the invention for taking a sample out of a polymerisation reactor and for analysing said sample. Chemical and physical analysis of samples obtained from the reactor by use of a sampling device according to the present invention provide accurate and representative information of the reaction conditions inside the reactor as well as the properties of the composition of the gas phase and the solid particles in the reactor. Because of the negligible residence time in the sampling device, samples taken by means of the present sampling device give an accurate and representative picture of the conditions inside the reactor at sampling time.

[0021] In addition, the sampling device according to the invention can also be used for improving a polymerisation reaction in a polymerisation reactor that consists of two reactors that are connected to each other, preferably in series. Such reactor configuration advantageously allows applying different operational conditions in the different reactors, which allows playing on the properties of the final product. The problem in such reactor configuration however, consists of correctly determining the suitable moment on which a reaction product has certain desired properties and is suitable for being transferred from a first to a second reactor and from the second reactor to means for further processing. Transfer of a reaction product having sub-optimal properties from such second reactor to further processing means considerably reduces product quality. Using the sampling device according to the present invention in such configuration allows frequent analysis and follow-up of the operational reaction conditions in both reactors. Thus, by frequently taking and analysing samples from a first and a second reactor the present invention allows to adapt the reaction conditions in the first as well as in the second polymerization reactor. Moreover, the sampling device according to the invention can also be used for improving the polymerisation reaction conditions in a second slurry loop polymerisation reactor, by taking a sample out of a first slurry loop polymerisation reactor which is connected thereto, and by analysing said sample. The suitable moment for transferring the reaction product from the second reactor to further processing can be correctly determined, and a final reaction product having optimal properties is supplied from the second reactor to further processing.

[0022] In another aspect, the present invention relates to methods for improving a polymerisation reaction in a polymerisation reactor. The term “improving a polymerisation reaction in a polymerisation reactor” as used herein relates to the following up of a polymerisation reaction and the fine-tuning—if required—of operational reaction conditions thereof in an individual reactor, in order to improve the efficiency of the polymerisation reaction and / or the product quality in this individual polymerisation reactor. This term also refers to the following up of a polymerisation reaction and the fine-tuning—if required—of operational reaction conditions thereof in two or more reactors which are connected to each other, preferably in series, such that the efficiency of the polymerisation reaction is ensured and that a final reaction product resulting from the polymerisation reaction in said reactors is fed at a suitable time and having optimal product quality to further processing means.

Problems solved by technology

However, the slurry in said standpipe is generally not in equilibrium with the reactants, and hence it is almost entirely impossible to obtain a representative sample.

In polymerisation plants using flash tanks which are connected to a reactor by means of flash lines and settling legs, the settling legs themselves can present problems.

Also, there is a lag in time between recovery of slurry in the settling legs and further processing of the slurry to the flash tank.

Analysis of a gas sample taken from the flash tank does not provide updated information on the reaction conditions in the polymerisation reactor and will result in an inaccurate analysis of the gas composition in the polymerisation reactor.

However, the described apparatus and method do not allow to take solid particles out of the reactor and to analyse these.

However, the described apparatus and method do not allow to take solid particles out of the reactor and to analyse these.

However, the sample taken out of the loop reactor does not contain solid particles.

Analysis of said sample is therefore not fully representative for the reaction conditions in the loop reactor.

Thus, analysis of a sample taken from such transfer pipe does not provide fully updated information on the reaction conditions in the polymerisation reactor.

A drawback in the above-described devices and methods is that they do not allow the control of several different variables of the polymerisation process, such as e.g. monomer, co-monomer and hydrogen in the gas phase and proper ties of the polymerisation product such as the melt flow index and density, in response with the analysis of the sample.

In addition, the above described methods and devices are not suitable for controlling the polymerisation reaction in system wherein bimodal polyethylene is prepared, i.e. in system comprising two interconnected polymerization reactors.

In particular, the above-described methods and devices do not provide for a representative sampling of a first polymerisation reactor in such bimodal system.

The problem in such reactor configuration however, consists of correctly determining the suitable moment on which a reaction product has certain desired properties and is suitable for being transferred from a first to a second reactor and from the second reactor to means for further processing.

Transfer of a reaction product having sub-optimal properties from such second reactor to further processing means considerably reduces product quality.

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