Polymerization pipe reactor

A continuous bulk polymerization process with three reaction zones in series has been developed. The degree of polymerization increases from the first reactor to the third reactor. Examples of suitable reactors include continuous stirred tank reactors, stirred tower reactors, axially segregated horizontal reactors, and pipe reactors with static mixers. The continuous stirred tank reactor type is advantageous, because it allows for precise independent control of the residence time in a given reactor by adjusting the level in a given reactor. Thus, the residence time of the polymer mixtures can be independently adjusted and optimized in each of the reactors in series (8). 

A continuous process for polymerization of nylon 6,6 in which a fluidized bed solid state polymerization reactor is used as the high polymerizer is represented schematically in Figure 3 (26). In this process the low molecular weight polymer is produced in a filled pipe reactor located just upstream of the spray drier. The liquid product of this step is then sprayed into a hot inert gas atmosphere where the water is flashed off and a fine powder is produced. This powder is fed into an opposed-flow, fluidized bed reactor at 200 °C where the high molecular weight polymer powder is generated at temperatures well below the 255 °C melting point of nylon 6,6. The powder is then melted in the extruder and converted into fiber or chip. 

The bulk of the styrene is to be heated to 85°C before being charged. This is done in a vertical double-pipe heat exchanger, which is directly above the reactor. To prevent polymerization from occurring in the heat exchanger or piping system, there are to be no obstructions between this heat exchanger and the reactors. 

One of these will be needed for each reactor, because they must be positioned vertically above the reactors. This is to prevent any hot styrene from remaining in the exchanger or the piping, where it might polymerize. 

Explosion phenomena have occurred in all types of confined and unconfined units reactors, separation and storage units, filter systems, pipe lines, and so forth. Typical reactions that may cause explosions are oxidations, decompositions, nitrations, and polymerizations. Examples of chemical and processing system characteristics that increase the potential for an explosion are the following ... 

The monomers are piped from the tank farm to the caustic soda scmbbers where the inhibitors are removed. Soap solution, catalysts, and modifiers are added to produce a feed emulsion which is fed to the reactor train. Fewer reactors are normally used than the number required for a cmmb product line. When polymerization is complete, the latex is sent to a holding tank where stabilizers are added. 

The question of choosing a PFTR or a CSTR will occur throughout this book. From the preceding arguments it is clear that the PFTR usuaUy requires a smaller reactor volume for a given conversion, but even here the CSTR may be preferred because it may have lower material cost (pipe is more expensive than a pot). We will later see other situations where a CSTR is clearly preferred, for example, in some situations to maximize reaction selectivity, in most nonisothermal reactors, and in polymerization processes where plugging a tube with overpolymerized solid polymer could be disastrous. 

Many billions of pounds of polymer are extruded annually into products ranging from household plumbing pipe to thin film grocery sacks, textile fibers to weather stripping. In fact, almost all polymer passes through an extruder at some point, since many polymerization reactors feed extruders in order to include additives and to pelletize. Furthermore, most primary processing... 

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). 

The heat of polymerization can be removed by a number of techniques. Cooled reactor jackets are most common, but internal cooling surfaces in the form of coils or pipe baffles are also... 

Emulsion polymerization as a continuous operation has been described in the patent literature an has found industrial application. One technical process mentioned uses a coiled pipe of such dimensions that at a certain temperature the monomer emulsion polymerizes completely during its pa sage. If necessaiy, a pipe system can be built that is heated to different temperatures at its various sections. Other continuous processes can be carried out in polymerization towers if the monomer has a lower density than water and the polymer has a higher one. The monomer emulsion is added at the top of the tower and agitated by stirring devices, with turbulence limited to the upper parts of the tower. As the polymerization proceeds and the polymer sinks to the bottom of the tower, new monomer is introduced at the top and latex removed at the base. Instead of one tower, a battery of interconnected reactors can be advantageously used in a similar procedure. 

The polymerization reactor is made of large diameter pipes assembled in a long vertical loop configuration. Early designs consisted of four- or six-leg loops. The reactor walls are made of carbon steel and are equipped with water jackets to control temperature in the isothermal reactor. Ethylene is polymerized under a total pressure of about 25 bars to 40 bars and at a temperature between 75°C and 110°C (167°F and 230°F). The recycle diluent, fresh monomer, comonomer, hydrogen, catalyst and co-catalyst (when required), are fed to the reactor. Polyethylene is formed as discrete particles in a rapidly circulating diluent-polymer slurry. On average, 98% of the ethylene is polymerized. 

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