Polypropylene Production Process

Polypropylene resin is one of the four general-purpose thermoplastic resins (polyethylene, polyvinyl chloride, polypropylene, polystyrene). It is produced through polymerization reaction with propylene as raw material and ethylene as comonomer.
The process methods used to produce polypropylene in the world mainly fall into the following categories according to categories: solvent method, solution method, liquid phase bulk method (including liquid phase gas phase combined type) and gas phase bulk method. The characteristics of each process are briefly introduced as follows:
solvent polymerization
Solvent method (also known as slurry method or mud method, slurry method) is the earliest polypropylene production process. However, due to the deashing and solvent recovery processes, the process is long and complex, and with the advancement of catalyst research technology, Since the 1980s, the solvent method has tended to stagnate and has been gradually replaced by the liquid phase bulk method.
Process features: (1) Propylene monomer is dissolved in an inert liquid solvent (such as hexane), and solvent polymerization is performed under the action of a catalyst. The polymer is suspended in the solvent in the form of solid particles, and a kettle-type stirring reactor is used; ( 2) There are deashing and solvent recovery processes. The process is long and complicated, with large equipment investment and high energy consumption. However, the production is easy to control and the product quality is good; (3) The polypropylene particles are separated by centrifugal filtration and then dried by air flow boiling and extrusion granulation.
solution polymerization
Process characteristics: (1) Use high-boiling point linear hydrocarbons as solvents and operate at a temperature higher than the melting point of polypropylene. The resulting polymers are all dissolved in the solvent and uniformly distributed; (2) High-temperature gas stripping method evaporates and removes the solvent. Molten polypropylene is obtained, and then extruded and granulated to obtain pellet products; (3) The only manufacturer is the American Kodak Company.
Liquid phase bulk method
The liquid phase and gas phase combined type, liquid phase bulk method polypropylene production process is a new process developed in the middle and late stages of polypropylene production. This production process was developed seven years after polypropylene began industrial production in 1957.
The liquid phase bulk method is used to produce polypropylene. The catalyst is directly dispersed in the liquid phase propylene without adding any other solvent to the reaction system to perform the liquid phase bulk polymerization of propylene. The polymer continuously precipitates from the liquid propylene and is suspended in the liquid propylene in the form of fine particles. As the reaction time increases, the concentration of polymer particles in liquid propylene increases. When the propylene conversion rate reaches a certain level, the unpolymerized propylene monomer is recovered through flash evaporation to obtain a powder polypropylene product. This is a relatively simple and advanced industrial production method of polypropylene. The liquid phase bulk method represents the new technology and new level of polypropylene production in the world in the 1980s.
Process features: (1) No solvent is added to the system, propylene monomer undergoes liquid phase bulk polymerization in a kettle reactor in a liquid phase state, and ethylene propylene undergoes gas phase copolymerization in a fluidized bed reactor; (2) The process is simple, Less equipment, less investment, low power consumption and production costs; (3) Homopolymerization adopts kettle stirred reactor (Hypol process) or loop reactor (Spheripol process), random copolymerization and block copolymerization are both in stirred type performed in a fluidized bed.
A typical representative of the liquid phase bulk method is BASELL's Spherizone liquid phase bulk method process. Spherizone is a gas-phase recycling technology using a Ziegler-Natta catalyst that produces polymers that maintain toughness and processability while being highly crystalline, stiff and more uniform. It produces highly uniform multimonomer resins or bimodal homopolymers in a single reactor. The Spherizone cycle reaction has two interconnected areas. Different areas play the role of gas phase and liquid phase loop reactors of other processes. These two regions can produce resins with different relative molecular weights or monomer composition distributions, expanding the performance range of polypropylene.
The core equipment of this process is the MZCR (multi-zone circulation reactor system) reactor R230 system. The reactor consists of a riser and a downcomer. In the riser, the polymer is blown upward by the reaction gas to form fluidization, and is sent to the upper part of the downcomer and passes through the cyclone separator. The powder is collected in the downcomer. The reaction gas is circulated by a centrifugal compressor through an external pipeline, and the reaction heat is removed by means of a circulator cooler on the external circulation pipeline. The reactor product is discharged through a valve installed in the lower part of the downcomer. After the discharged powder is degassed under high pressure and low pressure, it is directly steamed and dried when producing homopolymers and random copolymers to obtain powder products. When producing anti-impact products, the powder after high-pressure degassing is discharged into the gas-phase fluidized bed reactor. The reactor still uses the Spheripol II gas phase reactor system. The copolymerization reactor is a vertical cylindrical vessel with spherical heads on the upper and lower parts and an ebullating bed on the lower part. The main material is stainless steel and the inner surface is polished.
The current maximum production capacity of this process in a single line has reached 450,000 tons/year. The ethylene content of MZCR (multi-zone circulation reactor) impact copolymer products can be as high as 22% (rubber content is greater than 40%), and terpolymer products containing ethylene and 1-butene can also be produced.