Recycling of HDPE

Continuous polymerization process in slurry tank reactor

In this process, the polymerization solvent is n-hexane, the catalyst is a highly active z-N catalyst, ethylene and hydrogen are mixed and enter the first reactor, and the polymerization reaction occurs when mixed with the catalyst. The polymer in the reactor is suspended in hexane in the form of slurry, the polymerization temperature is about 80°C, and the polymerization pressure is less than 10 bar. This process can produce products with a density range of 0.942~0.965 g/cm3, a melt index range of 0.2~80, and comonomers are propylene and butene-1. It produces traditional HDPE and bimodal HDPE. The high-density pipe has excellent performance and is suitable for making pressure pipes, reaching PE100+.

The characteristics of the continuous polymerization process of the slurry reactor are: low operating pressure and operating temperature; the double reactor reactor can produce single-peak and double-peak products by adopting different forms of parallel and series connection; the process operation is highly flexible, the product brand conversion is fast, and the raw material purity requirement is not high; the copolymer monomers use propylene and 1-butene; hexane is used as the solvent, and the recovery unit is simple. The characteristic of this process is that the polymerization is carried out in an inert hydrocarbon diluent.

The process flow is: the polymerized ethylene (ethylene 99.9%, ethane 0.1%) is sent to the dryer and then enters the reactor together with the circulating diluent composed of n-hexane. The catalyst uses a brand containing titanium and manganese on the carrier and triethyl aluminum as a co-catalyst. A small amount of hydrogen is added to control the molecular weight. The polymerization reaction forms polyethylene particles. The reaction temperature is 90°C and the pressure is 1.8MPa. The reaction can be carried out in two steps in two polymerization reactors. The concentration of the produced slurry solid is 34% (mass fraction), and the monomer conversion rate can reach 97%. The polymer flows out of the second reactor and flashes to a pressure of 0.14 MPa. The unreacted ethylene, ethane in the discharge, and 2% of the cyclohexane diluent are compressed and cooled to 2.5 MPa twice. The degassing tower recovers the ethane for recycling. The slurry left after the flash is centrifuged to recover most of the diluent, and the solid filter cake is sent to the dryer to reduce the volatile component content to about 5% (mass fraction). The dryer is operated in a closed cycle with nitrogen protection. The dried polymer powder is sent to a fluidized bed dryer to remove all hydrocarbon diluents. The dried polymer particles are sent to the mixing section to add various additives and then granulated.

Loop reactor process

Typical representatives of the loop reactor process are Phillips' Phillips process and INEOS' Innovene S process. The Phillips process uses isobutane as a diluent and a chromium catalyst. The catalyst must be activated before use. The activated catalyst powder forms a catalyst slurry with high-purity isobutane under nitrogen protection, and then enters the loop reactor. The raw material ethylene monomer is refined, pre-mixed with hydrogen and comonomer hexene-1, and then injected into the loop reactor. Ethylene generates polyethylene under the action of the catalyst. The axial flow pump maintains the high-speed flow and very uniform mixing of the materials in the reactor, and the reaction heat is evenly withdrawn by the jacket cooling water.

The production MI range of this process is 0.15-100, and the density is 0.936-0.972 g/cm3. The characteristics of the loop reactor process are: less equipment, short process, and low investment cost; no wax and oligomers are produced, and no wall sticking; the powder has a good shape and is easy to transport; the reaction heat is removed by the reactor jacket cooling water, which is easy to remove heat and convenient to adjust; the raw material requirements are high and need to be purified; the comonomer is hexene; isobutane is used as a solvent, which is easy to remove residual solvent.

The process flow is as follows: fresh polymer-grade ethylene is dried, mixed with molecular weight regulator hydrogen, antifreeze and circulating diluent isobutane, and then sent to a multi-loop continuous process reactor, and the catalyst supplement isobutane is filled into the reactor. The reaction temperature is 106.7 degrees Celsius and the pressure is 3.9MPa. The polymer and diluent slurry pass through the loop reactor at a speed of 6m/s with the help of an axial flow pump. The water cooling in the reactor jacket controls the reaction temperature, and the polymer solids are discharged from the vertical settling port in the loop reactor. As a result, the slurry concentration can reach 55% and the conversion rate is 98%-99%. After the polymer is discharged, it is flashed to discharge isobutane and residual monomers into the diluent recovery device. Other solid polymers are mixed with additives and granulated.

Gas phase polymerization method

The typical representative of gas phase polymerization method (gas phase fluidized bed method) is the univation technology of DOW Chemical Company and the Innovene technology of INNOS Company. The univation technology process uses a low-pressure gas phase fluidized bed reactor, z/n catalyst and chromium catalyst, and the purified raw materials are injected into the reactor. Under the action of the catalyst storage, a polymerization reaction occurs. The reaction is carried out at 85~110. The pressure is 2.41 MPa, and the ethylene single-pass conversion rate is about 1%~2%. The removal of reaction heat is mainly through the cooling of the circulating flow. The MI range of the production product is 0.01~150, and the density range is 0.915~0.970 g/cm3. The characteristics of the gas phase fluidized bed polymerization process are: low operating pressure and low temperature; can produce full-density polyethylene; catalyst system includes titanium and chromium; metallocene catalyst; high requirements for raw material purity, all raw materials must be refined; no solvent is required, low energy consumption, low maintenance and operation costs. The production process is: dry monomers are added to the reactor system together with hydrogen, and the raw materials are added to a large circulating steam flow loop and enter the bottom of the large fluidized bed reactor through gas distribution. According to the design, the reactor raw materials are 69.57% ethylene (ethylene content is 99.9%, 0.1% is ethane), 10.43% hydrogen, 7.56% ethane and 12.44% nitrogen. The product produced by this raw gas composition has a melt index of 8g/10min and a density of 0.964g/cm3. The catalyst is a mixture of magnesium dioxide with titanium trichloride and tetrahydrofuran as promoters, and the co-catalyst is triethylaluminum.

The catalyst enters the reactor in solid form together with nitrogen from the reactor part. The operating temperature is 105℃, and the specific temperature is determined according to the product brand. The operating pressure of the reactor is 2.0MPa. The reaction gas comes out from the top of the reaction and is separated from the catalyst containing solid matter by a cyclone separator and sent back to the reactor. Then the gas coming out of the cyclone separator is compressed and circulated to the bottom of the reactor through a circulating cooler.

The reactor discharges the product particles to the material tank intermittently through an airlock system. Part of the gas entering the discharge tank enters the compressor circulation system through the upper buffer tank, filter, gas cooler, and separation tank. The polymer comes out from the lower part of the discharge tank and enters the purge tank and post-processing system. The post-processing system includes adding various additives to the polymer, melting, granulation and packaging.