Preparation method of polypropylene

Abstract

The invention relates to a preparation method of polypropylene. The method comprises the following steps: cooling first liquid-phase propylene, mixing the cooled first liquid-phase propylene with a polypropylene catalyst, and carrying out low-temperature prepolymerization reaction, feeding an obtained slurry containing the low-molecular-weight polypropylene prepolymer into a double-loop reactor, carrying out liquid-phase bulk slurry polymerization on the slurry and supplemented second liquid-phase propylene, hydrogen and ethylene monomers, and feeding part or all of polypropylene slurry flowing out of the first loop reactor into a second loop reactor, carrying out directflash evaporation on the polypropylene slurry flowing out of the second loop reactor to remove non-condensable gas and most of propylene monomers, and carrying out purification and granulationan obtained polypropylene intermediate to obtain the product; or carrying out copolymerization reaction on the polypropylene intermediate, and then carrying out powder purification and granulation to obtain the product. According to the preparation method disclosed by the invention, high-melt-index / high-rigidity and high-rubber-content impact-resistant co-polypropylene can be produced through flexible switching of devices, and meanwhile, high-isotacticity homo-polypropylene and random co-polypropylene can also be produced.

Description

technical field [0001] The invention belongs to the field of olefin polymerization in petrochemical industry, and in particular relates to a preparation method of polypropylene obtained by multi-stage polymerization of propylene. Background technique [0002] Polypropylene has excellent comprehensive properties, rich sources of raw materials, easy processing and application, and has become one of the main forces in the plastics industry. With the continuous increase of polypropylene production, the polypropylene market competition is becoming increasingly fierce. How to use a set of equipment to produce more grades of polypropylene products with higher performance is a problem that major manufacturers are currently studying and solving. [0003] The current industrialized polypropylene production process in the world basically adopts liquid-phase bulk method, gas-phase method or a combination of the two processes, and different process technologies use different types of rea...

AI Technical Summary

Problems solved by technology

[0004] In the above-mentioned existing double-loop or multi-tank series process technology, when using a liquid-phase loop reactor or a liquid-phase stirred tank reactor in series with a gas-phase fluidized bed reactor to produce impact copolymer products, it is usually the second loop reactor Or all the materials in the second stirred tank reactor enter the gas phase reactor for gas phase copolymerization, and the materials in the first loop reactor or the first stirred tank reactor only enter the second loop reactor or the second stirred tank reactor. The liquid phase homopolymerization or random copolymerization reaction is carried out in the tank reactor, and the material in the first loop reactor or the first stirred tank reactor has not been directly entered into the gas phase reactor for gas phase copolymerization reaction, thus causing the catalyst to be in the gas phase. The attenuation of the activity during the reaction eventually reduces the control range of the rubber phase content and the molecular weight distribution of the impact copolymer

Generally speaking, the above existing technology can only produce traditional impact polypropylene products with a maximum melt index of about 15g / 10min and a maximum rubber phase content of about 15% when a gas phase fluidized bed reactor is connected in series

Moreover, when producing impact copolymer products, although most of the light components in the material generated by the liquid phase reactor will be separated through the flash process, the powder entering the gas phase fluidized bed reactor will still be entrained Part of the hydrogen, which brings difficulties to the increase of molecular weight and the regulation of molecular weight distribution in the gas phase reaction process

In addition, the problem of wall sticking and caking in the gas phase fluidized bed poses serious challenges to the long-term operation in the production process of high rubber content impact copolymer products, thereby limiting the content of rubber phase in impact products

[0005] For the gas-phase polymerization process, the catalyst is usually directly added to the gas-phase polymerization reactor, but the initial activity of the catalyst is relatively high. On the one hand, the catalyst particles are easily broken in the reactor, and the fine powder content in the reaction system is too high. The rapid release of the reactor is easy to cause local overheating in the reactor, so that the reaction process is easy to generate lumps and cause wall sticking, which ultimately affects the quality of the polymer and the long-term operation of the device

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