Polypropylene resin is one of the four general-purpose thermoplastic resins (polyethylene, polyvinyl chloride, polypropylene, polystyrene). It is produced by polymerization using propylene as raw material and ethylene as a comonomer.
The process methods used in the production of polypropylene in the world are divided into the following categories according to categories: solvent method, solution method, liquid phase bulk method (including combined liquid phase and gas phase) and gas phase method. The characteristics of each process are as follows:
Solvent polymerization
Solvent method (also known as slurry method or slurry method, slurry method) is the earliest polypropylene production process. However, due to the deashing and solvent recovery procedures, the process is long and complicated, etc., with the progress of catalyst research technology, Since the 1980s, the solvent method has tended to stagnate and has gradually been replaced by the liquid phase bulk method.
Process characteristics: (1) Propylene monomer is dissolved in an inert liquid phase solvent (such as hexane), and the solvent polymerization is carried out under the action of a catalyst. The polymer is suspended in the solvent in the state of solid pellets, and a tank-type stirred reactor is adopted; (2) There are deashing and solvent recovery procedures, the process is long, more complicated, the equipment investment is large, and the energy consumption is high. However, the production is easy to control, and the product quality is good; (3) The polypropylene particles are separated by centrifugal filtration and then subjected to airflow boiling drying and extrusion granulation.
Solution polymerization
Process characteristics: (1) Use high boiling point straight-chain hydrocarbons as solvents, and operate at a temperature higher than the melting point of polypropylene, and the obtained polymers are all dissolved in the solvent in a homogeneous distribution; (2) High-temperature gas stripping method evaporation to remove the solvent Molten polypropylene is obtained, and then extruded and pelletized to obtain pellets; (3) The only manufacturer is Kodak, USA. [10]
Liquid phase bulk method
The production process of polypropylene containing liquid phase and gas phase combined, liquid phase bulk method is a new process developed in the middle and late stages of polypropylene production. The production process came out seven years after the industrial production of polypropylene 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 solvents in the reaction system to carry out the liquid phase bulk polymerization of propylene. The polymer continuously precipitates from the liquid phase propylene and is suspended in the liquid phase propylene as 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 by flash evaporation to obtain a powdered polypropylene product. This is a relatively simple and advanced industrial production method of polypropylene. The liquid phase bulk method represents the new technology and level of international polypropylene production in the 1980s.
Process characteristics: (1) No solvent is added in the system, propylene monomer is polymerized in liquid phase in a tank reactor in liquid phase, and ethylene and propylene are copolymerized in a fluidized bed reactor in gas phase; (2) The process is simple, Less equipment, less investment, low power consumption and production cost; (3) Homopolymerization adopts tank-type stirred reactor (Hypol process) or loop reactor (Spheripol process), and both random copolymerization and block copolymerization are in agitated type In a fluidized bed.
The typical representative of the liquid phase bulk method is the Spherizone liquid phase bulk method of BASELL. Spherizone is a gas-phase circulation technology that uses Ziegler-Natta catalysts to produce polymers that maintain toughness and processability while having high crystallinity, rigidity and more uniformity. It can produce highly uniform multi-monomer resins or bimodal homopolymers in a single reactor. The Spherizone cycle reaction has two interconnected zones, and different zones play the role of gas and liquid loop reactors of other processes. These two regions can produce resins with different relative molecular mass or monomer composition distribution, which expands 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 two parts: riser and downcomer. The polymer in the riser is blown up by the reaction gas to form fluidization, and is sent to the upper part of the downcomer after passing through the cyclone separator, and 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 a circulator cooler on the external circulation pipeline. The reactor product is discharged through a valve installed at the lower part of the downcomer. After the discharged powder is degassed at high pressure and low pressure, it is directly steamed and dried when producing homopolymers and random copolymers to obtain powder products. When producing impact-resistant products, the high-pressure degassed powder 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 at the top and bottom, and a fluidized bed at the bottom. The main body is made of stainless steel and the inner surface is polished.
The current single-line maximum production capacity of this process has reached 450,000 tons/year. The ethylene content of MZCR (multi-zone circulating reactor) impact copolymer products can be as high as 22% (rubber content is greater than 40%), and ternary copolymer products containing ethylene and 1-butene can also be produced.
Vapor phase bulk method
Process characteristics: (1) The system does not introduce solvent, and the propylene monomer is polymerized in the gas phase in the reactor in the gas phase bulk; (2) The process is short, the equipment is small, the production is safe, and the production cost is low; (3) The polymerization reactor has flow Chemical bed (Union Carbon/Shell UNIPoi process), vertical stirred bed (BASF Novolen process) and horizontal stirred bed (Amoco/El Paso process). [10]
The typical representative of the gas phase bulk method is the Unipol gas phase process of DOW Chemical Company. The Unipol gas-phase polypropylene process is a gas-phase fluidized bed polypropylene process developed by U.S. United Carbon Corporation (UCCP) and Shell in the 1980s. It is a process that transplants the fluidized bed process used in polyethylene production to Polypropylene is in production and has been successful. The process uses a high-efficiency catalyst system, the main catalyst is a high-efficiency carrier catalyst, and the co-catalyst is triethyl aluminum and an electron donor.
The UNIPOL process has the characteristics of simplicity, flexibility, economy and safety; the process can produce a full range of products including homopolymers, random copolymers and impact copolymers with very little equipment. Adjust the operating conditions within the operating range to maintain uniform product performance. Because the number of equipment used is small, the maintenance workload is small and the reliability of the device is improved. Due to the limitation of the fluidized bed reaction kinetics itself and the low operating pressure reduces the storage of materials in the system, the process is safer than other processes, and there is no danger of equipment overpressure when the accident is out of control.
This process has no discharge of liquid waste and very few hydrocarbons discharged into the atmosphere, so the impact on the environment is very small. Compared with other processes, this process is easier to meet various strict specifications of environmental protection, health and safety. Another notable feature of this process is that it can be operated in a super-condensed state, the so-called super-condensed gas phase fluidized bed process (SCM). This technology can increase the existing production capacity by 200% by increasing the proportion of the liquid phase in the reactor to 45%. Since the liquid content is not the basic factor of fluidized bed instability and formation of polymer agglomeration, the key operating variables of this technology are the density of the expanded bed and the ratio of expanded bulk density to sedimentation bulk density. Since the super-condensed state operation can most effectively remove the reaction heat, it can increase the production capacity of the reactor by more than 2 times without increasing the volume, and the investment saving is very considerable. Impact copolymer products with ethylene content up to 17% (rubber content greater than 30%).
The core equipment of the process is a gas-phase fluidized bed reactor, a circulating gas compressor, a circulating gas cooler and an extrusion pelletizing unit. The fluidized bed reactor is a hollow vessel with an enlarged section at the top and a sparger at the bottom. The operating pressure of the first reactor is 3.5MPaG and the temperature is 67℃. The operating pressure of the second reactor is 2.1MPaG and the temperature is 70℃. ; The circulating gas compressor is a single-stage, constant-speed, centrifugal compressor.
