Polyethylene vessel reactor

The Effect of Mixing on Steady-State and Stability Characteristics of Low Density Polyethylene Vessel Reactors... 

The effect of mixing on the steady state and stability characteristics of low density polyethylene vessel reactors is examined by the use of two reactor models. 

The polymerization of ethylene to yield low density polyethylene (LDPE) is performed in vessel reactors at high pressures (1000 2500 kg/cm2) and in the temperature range between 150 and 300°C. Two main features characterize these type of reactors a) the very high power input per unit volume required to maintain good mixing conditions in the reaction zone and b) the absence of appreciable heat exchange, so that the reactor can be considered practically adiabatic. 

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Low density polyethylene is made at high pressures in one of two types of continuous reactor. Autoclave reactors are large stirred pressure vessels, which rely on chilled incoming monomer to remove the heat of polymerization. Tubular reactors consist of long tubes with diameters of approximately 2.5 cm and lengths of up to 600 m. Tubular reactors have a very high surface-to-volume ratio, which permits external cooling to remove the heat of polymerization. 

The reaction is maintained at isothermal conditions. The effluent from the reactor is allowed to pass to a separatory vessel in which unconverted ethylene is removed for recycling. The molten polyethylene gets chilled below its crystalline Melting point. 

The rate experiments were carried out in 3.5 liter glass jars covered with a plexiglass lid. Two baffles of 2 cm width extended from the lid to 1 cm from the bottom of the reactor. Agitation was provided by a stirring motor with polyethylene stirrers, and the speed of the motor was controlled with a variac. Three liters of solution without the adsorbent particles was agitated, and allowed to reach equilibrium with the vessel before withdrawing the initial sample for analysis. At time t - 0, a measured quantity of activated carbon was introduced into the reactor and agitated. Samples, 10 ml each were withdrawn periodically, until equilibrium was reached, and analyzed for the solute concentration in the liquid phase. The numerical procedure developed earlier was adjusted to allow for the volume reduction due to the withdrawal of samples at discrete time Intervals. 

The ethylene and polyethylene leave the reactor and pass into a primary separation vessel which operates at a much lower pressure than the reactor itself. Most of the ethylene (and any comonomer) is flashed off in this unit and recycled through compressors to the tube inlet. Conversion per pass is of the order of 30% with ethylene flow rates about 40,000 kg/h. 

Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high temperatures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7). 

In this way the subtleties of the normal commercial polyethylene grades are reproduced faithfully, but without the expense of purchasing, purifying, and storing a-olefin comonomer. Continuous addition of the organochromium compound directly into the reactor, or with the chromium oxide catalyst to a pre-contacting vessel which flows to the reactor, provides precise and instantaneous control of the resin density (i.e. level of branching). 

Reactors are stationary vessels that are classified as batch, semi-batch, or continuous. Some reactors use mixers to blend the individual components. Reactor design depends on the type of service the reactor will be used in. Some of the reactor processes (among many others) include alkylation, catcracking, hydrodesulfurization, hydrocracking, fluid coking, reforming, polyethylene, and mixed-xylene. Figure 7-14 shows the standard symbols for reactors. 

One of the earliest reports of a fluidized-bed operation for the production of polyethylene [30] was provided in a British patent issued in 1958. The process was initiated by adding 100 ml of hexane to the polymerization reactor and then adding 3 ml of triethylaluminum and 1 ml of TiCl to the hexane, which produces a solid catalyst particle. Ethylene is continuously recycled to the reactor and the hexane is gradually removed by distillation as the heat of polymerization increases the reactor temperature to 65 C, which yields about 50-100 g of polyethylene before the hexane has been completely removed. Polymerization is continued at 85 C by continued ethylene feed through the vessel to maintain a fluidized-bed process so that after 8 hours, 460 g of polyethylene has formed. 

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