Polyethylene continuous stirred-tank reactor

A solution to this problem is the enzyme membrane reactor (Figure 10.8). This is a kind of CSTR (continuous stirred tank reactor), with retains the enzyme and the cofactor using an ultrafiltration membrane. This membrane has a cut-off of about 10000. Enzymes usually have a molecular mass of 25000-250000, but the molecular mass of NAD(H) is much too low for retention. Therefore it is first derivatized with polyethylene glycol (PEG 20000). The reactivity of NAD(H) is hardly affected by the derivatization with this soluble polymer. Alanine can now be produced continuously by high concentrations of both enzymes and of NAD (H) in this reactor. 

The manufacture of linear low-density polyethylene (LLDPE) by slurry polymerization in hexane (see Sections 6.2 and 6.8) is carried out by Hoechst, Mitsui, and a number of other chemical manufacturers in a series of continuous stirred tank reactors. The manufacture of butyraldehyde from CO, H2, and propylene using a soluble rhodium phosphine complex (see Sections 5.2 and 5.5) is also carried out in a continuous stirred tank reactor. 

The Unipol process employs a fluidized bed reactor (see Section 3.1.2) for the preparation of polyethylene and polypropylene. A gas-liquid fluid solid reactor, where both liquid and gas fluidize the solids, is used for Ziegler-Natta catalyzed ethylene polymerization. Hoechst, Mitsui, Montedison, Solvay et Cie, and a number of other producers use a Ziegler-type catalyst for the manufacture of LLDPE by slurry polymerization in hexane solvent (Fig. 6.11). The system consists of a series of continuous stirred tank reactors to achieve the desired residence time. 1-Butene is used a comonomer, and hydrogen is used for controlling molecular weight. The polymer beads are separated from the liquid by centrifugation followed by steam stripping. 

Reactors used in ethylene polymerizations range from simple autoclaves and steel piping to continuous stirred tank reactors (CSTR) and vertical fluidized beds. Since the 1990s, a trend has emerged wherein combinations of processes are used with transition metal catalysts. These combinations allow manufacturers to produce polyethylene with bimodal or broadened molecular weight distributions (see section 7.6). 

Continuous Continuous stirred tank reactor loop reactor, stirred tank reactors, fluidized reactors, tubular reactors Polyvinyl acetate, styrene-butadiene, PVC (E), polystyrene (S), low-density polyethylene (B)... 

In addition to the semi-batch slurry experiments, 9/MAO was used in solution in a continuous stirred tank reactor (CSTR) to further investigate the influence of [ethylene]/[macromonomer] ratio on LCB. Figure 7 shows a quantitative analysis of the C-NMR-based LCB content in polyethylene as a function of the [ethylene]/ [macromonomer] ratio [85]. The LCB content was the highest at low ratios and rapidly decreased with an increase in the [ethylene]/[macromonomer] ratio. This is in line with LCB formation via the copolymerization reaction. 

LCB affects the properties of LDPE, low density polyethylene made by free-radical polymerization see Section 10. The continuous polymerization is carried out in stirred reactors or in tubes several authors (5, 90, 92) have considered the effects of LCB on MWD in perfectly-stirred tank reactors. 

Slurry-phase processes may involve either an inert diluent such as iso-butane or heptane, or condensed monomer such as propylene. In either case the catalyst particles are suspended and well mixed in the liquid medium. Monomer concentrations are high and the liquid provides good removal of the heat produced by the polymerization of the polymer particles. The two main reactors for slurry-phase olefin polymerization are the loop reactor and continuous-stirred tank. Slurry-phase processes are very attractive for high crystalline homopolymer products such as polypropylene and polyethylene. 

In the solution mode process illustrated in Figure 5.9, the solvent utilized must be an excellent solvent for polyethylene, as the polymer produced is completely dissolved in the solvent. Consequently, the first commercial plant employed cyclohexane as the solvent. Temperature of polymerization was 125-175°C at reactor pressures of 400-500 psig. The cyclohexane solvent is continuously fed to the reactor along with the silica-supported powder catalyst, ethylene and 1-butene. Polymer solution is continuously removed from the stirred autoclave to a holding tank where unreacted ethylene monomer and 1 -butene are easily flashed off and recycled to the reactor. The solid catalyst is removed from the solution by filtering and the polyethylene is isolated after the solvent has been removed by steam stripping. 

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