Vessel reactor, feature

Table 9 Comparison of Pipe and Vessel Reactor Features...

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. 

An important feature of the vessel reactor is the ability to incorporate a corrosion-resistant but replaceable Uner, which can also be combined with a thermally insulating sleeve. In addition to allowing operation with highly corrosive feeds, the liner assembly allows the pressure shell to be fabricated of a lower-alloy material and to operate at temperatures much tower than that of the process. This feature reduces reactor cost and facilitates the fabrication of large vessels. 

When a three phase system seems to be the best (or the sole) solution for a specific application, there remains the difficult task of selecting the most suitable reactor type among the numerous possibilities of contacting a gas and a liquid in the presence of a solid catalyst. Several papers have been devoted to this problem (see for example references 2,3, and 5) Fundamental characteristics such as residence time distribution are as important as technological aspects such as tightness of pressure vessels. Main features on which can be based a comparison between the two broad classes of three phase reactors - slurry and fixed bed-have been collected in Tables 2 and 3. Of course, such a general comparison is very rough and each mentioned item has to be discussed for every specific case. 

This high-pressure reactor system is designed for fhe use of interchangeable steel pressure vessels. Safety features guarantee safe reactions under pressure. 

The key feature of the pressurized water reactor is that the reactor vessel is maintained above the saturation pressure for water and thus the coolant-moderator does not bod. At a vessel pressure of 15.5 MPa (2250 psia), high water temperatures averaging above 300°C can be achieved, leading to acceptable thermal efficiencies of approximately 0.33. 

To reduce the risk of container failure, the pressure vessels are equipped with several safety features. These can include an effective self-venting system where unforeseen overpressure is released by a quick open-resealing step, or the use of safety disks which rupture when their pressure limit is reached. The small vials (0.2-20 mL) of some monomode reactors are protected by the pressure limit (20 bar) of the caps used, which is significantly lower than the operating limit of the vials themselves (40-50 bar). 

Table 1 reports a wide spectrum of typologies of biofilm reactor upflow anaerobic sludge bed (UASB), fluidized bed, airlift, fixed bed with and without recycle, mechanically agitated vessel, rotating drum and rotating biological contactor. Each reactor is characterized by positive features and drawbacks. 

In a laminar flow reactor (LFR), we assume that one-dimensional laminar flow (LF) prevails there is no mixing in the (axial) direction of flow (a characteristic of tubular flow) and also no mixing in the radial direction in a cylindrical vessel. We assume LF exists between the inlet and outlet of such a vessel, which is otherwise a closed vessel (Section 13.2.4). These and other features of LF are described in Section 2.5, and illustrated in Figure 2.5. The residence-time distribution functions E(B) and F(B) for LF are derived in Section 13.4.3, and the results are summarized in Table 13.2. 

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