Polymerization reactor autoclave-type

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. 

A bnlk polymerization reactor can be as simple as a tube into which the reactants are fed and from which the polymer mixture emerges at the end it can be more of a traditional, continnons stirred-tank reactor (CSTR), or even a high-pressure autoclave-type reactor (see Figure 3.21). A bulk polymerization process need not be continuous, but it should not be confnsed with a batch reaction. There can be batch bnlk polymerizations jnst as there are continnons bulk polymerizations processes. 

Each type of support was evaluated in a 2.2-liter stainless steel autoclave fitted with a temperature control jacket and a marine stirrer. Each supported, finished catalyst was prepared in situ by sequentially adding to the polymerization reactor 10-100 mg of finished support, 2.0 ml of a toluene solution containing 0.5 wt% (n-butyCpj ZrCl, 0.6 liter of isobutane, 1.0 mmol triethylaluminum and a second addition of 0.6 liter of isobutane. The contents of the reactor were stirred at 400 rpm while the reactor was heated to 90°C, after which ethylene was added to the reactor system to maintain total pressure at 550 psig. After the polymerization experiment, the ethylene flow to the reactor was stopped, the reactor was cooled to ambient temperature and solvents were removed by evaporation, and the reactor was opened and the polymer was collected. Note that no reactor fouling was detected and the granular polyethylene had acceptable morphology important for a commercial process. 

A typieal polymerization reaction took place in a tubular or an autoclave type reactor. It was charged with 1,960 parts of terf-butyl alcohol and 40 parts of water while maintaining a blanket of nitrogen over the operation. One part ammonium persulfate was added before the reactor was closed and pressurized to 1.4 MPa of a gas mixture containing 88.5% by weight TFE and 11.5 % of Et. Agitation was started and the reac-... 

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

Polyethylene is the simplest addition polymer, and we will briefly describe its polymerization process. Polyethylene, as discussed previously. Is made by opening the double bond in the ethylene molecule, and chemically bonding the monomers together in a reactor. That reactor can involve an autoclave (stirred tank) process or a tubular process. It can be done at low pressure (about 300 psi) or at pressures as high as 50,000 psi. Temperatures are controlled at some elevated level such as 125 to 250°C, but the temperature needed is very specific to the type of polymer structure desired. 

The Slurry Polymerization Process with Super-Active Ziegler-Type Catalyst Systems From the 2 L Glass Autoclave to the 200 m Stirred Tank Reactor... 

In the older process, LDPE resin was produced under high pressure (15,000-22,5(X) psi at 1(X)°C-300°C) in stirred autoclave or tubular-type reactors, where the liquefied ethylene gas is polymerized via a free radical reaction initiated by peroxide or by oxygen. 

The copolymerizations were carried out under argon using a 1 L Biichi A6 Type I autoclave equipped with an additional external cooling system. For the standard experiments, the reactor was evacuated at 95 C for 1 h and subsequently charged with a solution of norbornene in toluene, 190 mL toluene solvent, 500 mg MAO in 10 mL toluene (from Witco/Crompton), and ethylene at different pressures. Norbornene was dried over triisobutylaluminum and subsequently distilled before use. Polymerizations were initiated by injection of a toluenic metallocene solution into the reaction vessel... 

Ihe tubular reactor provides LDPE with a broader SCB distribution than the LDPE produced in the autoclave reactor. However, the autoclave reactor has a significantly higher SCB content in the region, with 10-15 methyl branches/1000 carbons. Wild also provided data showing the significant differences in the SCB distribution of various LDPE samples prepared by each type of reactor xmder different polymerization conditions. 

Polyolefins are produced in practically all types of reactor configurations -autoclaves, tubular reactors, loop reactors, fiuidized-bed reactors - making them a prime choice for polymer reaction engineering studies. Polymerization may take place in either gas or liquid phases. For liquid-phase reactors, the monomers can be either liquid (as in the case of propylene and higher a-olefins) or dissolved in an inert diluent. Industrial catalysts for olefin polymerization are mainly heterogeneous, but some processes also use soluble catalysts. There are many different types of catalysts for olefin polymerization and they can be used to synthesize polymer chains with very different microstructures and properties. 

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