Influencing factors and control measures of high-density polyethylene reaction

Keywords: high-density polyethylene; Influencing factors; Control Measure 1 Device Introduction: This device adopts the Hostalen low-pressure slurry process from Basell, Germany, which can produce both single peak and double peak high-density polyethylene (HDPE) products. Its double peak HDPE technology is world leading, and the double peak products can perfectly combine mechanical and processing properties, meeting the needs of the global market. This production process adopts the slurry method for suspension polymerization, with hexane as a dispersant to uniformly disperse ethylene, butene-1, catalyst, and polyethylene particles. Ziegler Natta catalyst is used, and the polymerization reactor is equipped with (4+1) stage stirring blades. Ethylene, comonomer, hydrogen, catalyst, activator, hexane, and recovered mother liquor continuously enter the reactor from the bottom. The polymerization reaction occurs rapidly, with a high total conversion rate (including the post reactor) in one pass, and does not require a recycling step for ethylene. The catalyst for the Hostalen process can generally be self configured using THT/THE/THB technology. In addition, Basel's newly developed high-efficiency catalyst Z501 has achieved the production of PE100 grade pipes, opening up a new market.

The characteristics of the Hostalen low-pressure slurry process: using butene as a comonomer to produce high-performance HDPE products; The unique reactor mode is designed to produce high-performance pipe and film products, and the external circulation cooling system can accurately and conveniently achieve efficient heat transfer in the polymerization system. The catalyst is specially configured to produce different grades of products, and the fluidized bed drying system effectively removes volatile components.
The polymerization reaction occurs in two reactors, which can be in series or parallel mode. The reaction temperature is 78-85 ℃ and the reaction pressure is between 0.3-1.0 MPa (G). All reactors are equipped with (4+1) stage agitator blades with a speed of approximately 120 r/min.
The polymerization reaction releases intense heat, therefore a strong cooling system (880-900 kcal/kg ethylene) is required. The reactor is equipped with a coiled jacket and each reactor has two external coolers, which can take away 80% of the reaction heat. HDPE suspension accounts for 90% to 95% of the reactor volume, and the liquid level control is mainly measured using radioactive methods.
Under the action of polymerization pressure and slurry pump, the polyethylene suspension leaves the reactor controlled by liquid level and is sent to the post reactor. The suspension enters the post reactor, where the remaining ethylene is reacted and then pumped to the centrifuge system to achieve separation of powder and mother liquor. The powder enters the fluidized bed to remove hydrocarbon substances.
2 Optimization Operation 2.1 Control of Impurities in Reaction Raw Materials Polyethylene process and catalyst are sensitive to certain impurities. When these impurities reach a sufficient concentration, they can participate in reactions or alter the activity of the catalyst lattice, leading to a decrease in catalyst activity and/or a change in the properties of the resin.

The main impurities in raw materials include polar organic compounds such as methanol, ethanol, propanol, etc; Polar inorganic substances such as water, CO, etc; Non-polar organic compounds, such as alkanes (trace amounts); Non polar inorganic substances such as CO2, SO2, H2S, O2, etc.
The possible effects of impurities: (1) deactivation of the catalyst, covering the polymerization active centers, reacting with the catalyst active centers, resulting in incorrect catalyst mixing ratios; (2) Changing resin properties, preventing the incorporation of comonomers, and altering polymer chain length and branching (3) can increase the amount of required reactants, requiring more alkyl aluminum or catalysts. (4) Changing the reaction operating conditions, some gases that do not participate in the reaction can only remove impurities through emissions, which may lower the temperature to ensure product performance and may reduce the reaction rate.
2.2 Catalyst Stability Adjustment The catalyst is prepared according to the formula provided by the patent holder, using titanium tetrachloride and magnesium ethoxylate as raw materials, and is prepared on-site. The problems that arise during the preparation process mainly focus on the following aspects: (1) The activity of the prepared catalyst varies greatly from batch to batch, resulting in fluctuations in the reaction when switching between each batch of catalyst, and the differences in activity are difficult to effectively control during the preparation process. The self prepared catalyst is an unstable semi-finished catalyst, which is greatly affected by factors such as impurities and active aluminum concentration. Therefore, it is necessary to strictly follow the operating procedures and accurately feed according to the fixed ratio to reduce the adverse effects of activity differences. (2) The particle size of the prepared catalyst is uneven. If there are too many small catalyst particles, the reactor system is prone to wall hanging and pipeline blockage. When the blockage is severe, the equipment needs to be stopped for cooking.

(3) The fluctuation of the catalyst pump causes instability in the reactor. The catalyst pump adopts a diaphragm metering pump, which is safe, reliable, and has high accuracy. However, occasional issues with insufficient quantity may occur, resulting in fluctuations in reactor pressure and temperature. This is mainly due to the solid particles of the catalyst, which can cause valve chamber blockage or valve ball blockage, or damage to the return valve or built-in safety valve. When this situation occurs, rinse the pump in a timely manner to prevent serious blockage from affecting normal production.
By visiting similar devices and communicating with Basell in Germany, the use of finished catalysts can effectively prevent activity fluctuations and uneven particle size in the preparation procedures. In addition, with the research and development of catalysts by relevant domestic manufacturers, catalysts with stable activity and reasonable prices have been successfully prepared. When used, it only needs to be diluted with hexane, which has a long production cycle, saves a lot of labor costs, and is more environmentally friendly. Currently, multiple companies have started using domestically produced catalysts. We can directly purchase imported or domestically produced catalysts based on the actual situation of the device and production brand.
2.3 Reactor Fluctuations Operation Under normal production conditions, reactor fluctuations are mostly caused by insufficient catalyst pumping or impurities added to the raw materials, resulting in a decrease in catalyst activity. This phenomenon is characterized by an increase in reactor pressure, a decrease in reaction temperature, an increase in external circulation temperature to maintain reaction temperature, and a decrease in reactor liquid level. When fluctuations are detected, slowly increase the catalyst amount. During the operation, it is strictly prohibited to increase too much catalyst quickly, in large quantities, or at once. Observe the reactor pressure. When the pressure drops, adjust the reaction temperature in a timely manner to prevent overheating. Quickly reduce the catalyst to the corresponding value of the current production and observe the reactor pressure. If the pressure still rises within one liquid level control cycle, repeat the above operation. Stable control can only be achieved when the catalyst is given to a value equal to or slightly higher than the stable value and the reactor pressure is basically stable.
During the transportation process, it is common to encounter blockages in the check valve at the inlet of the catalyst pump, which can cause fluctuations in the catalyst feed and affect the stability of the device. It has been found that some catalysts have large particles that can cause adhesion during transportation and easily block pipelines. After adding heat tracing to the catalyst storage tank, the catalyst is softened at a controlled temperature of 60 ℃, and the bonded parts are easily dispersed by the agitator to reduce the frequency of blockage. In addition, filters should be added to the storage tank and pump inlet respectively to filter out large catalyst particles and stabilize the catalyst feed. Summary: (1) The content of impurities in raw materials should be strictly controlled: CO[2] Zhan Hairong, Yang Xue, et al Development History and Trends of Ziegler Natta Catalysts [J]. Chemical Technology Market, 2008, 31 (12): 6-9.
[3] Xu Baocheng Analysis and Prevention of Explosive Aggregation in HDPE Reactor by Slurry Method [J]. Synthetic Resin and Plastic, 1991, 2:4-9