Reactors post reactor processing

Melt-fed SSEs are used in post-reactor processing. This involves three stages ... 

Applications for the diskpack are specialty polymer processing operations, such as polymerization, post-reactor processing (devolatilization), continuous compounding, etc. As such, the diskpack competes mostly with twin screw extruders. Presently, twin screw extruders are usually the first choice when it comes to specialty polymer processing operations. 

Failure of the pressure containment system (piping, SCWO reactor, post-reactor air cooler, or pressure let-down system) could result in rapid depressurization and dispersal of hot fluids and debris at high velocities. Similarly, failure of the pressure let-down system could result in a large pressure surge that could rupture equipment downstream.. .. The pressure let-down system may be the weak Unk in the full-scale SCWO process chain (NRC, 1998b). 

Figure 8.32. Proper replication of the catalyst particles is essential for stable reactor operation and also for the handling of the polymer particles in post-reactor processes. Reactor residence time distribution in CSTRs may have an important effect on the replication phenomenon the references at the end of the chapter provide more details on this subject [46-50].

Borzenski, F. J., An approach to the use of rheology in post reactor processing. Plastics Compounding, 25-28 (Sept./Oct. 1978). 

PVDE is manufactured using radical initiated batch polymerization processes in aqueous emulsion or suspension operating pressures may range from 1 to 20 MPa (10—200 atm) and temperatures from 10 to 130°C. Polymerization method, temperature, pressure, recipe ingredients, the manner in which they are added to the reactor, the reactor design, and post-reactor processing are variables that influence product characteristics and quaUty. 

Polymerization in Hquid monomer was pioneered by RexaH Dmg and Chemical and Phillips Petroleum (United States). In the RexaH process, Hquid propylene is polymerized in a stirred reactor to form a polymer slurry. This suspension is transferred to a cyclone to separate the polymer from gaseous monomer under atmospheric pressure. The gaseous monomer is then compressed, condensed, and recycled to the polymerizer (123). In the Phillips process, polymerization occurs in loop reactors, increasing the ratio of available heat-transfer surface to reactor volume (124). In both of these processes, high catalyst residues necessitate post-reactor treatment of the polymer. 

Gas-phase polymerization of propylene was pioneered by BASF, who developed the Novolen process which uses stirred-bed reactors (Fig. 8) (125). Unreacted monomer is condensed and recycled to the polymerizer, providing additional removal of the heat of reaction. As in the early Hquid-phase systems, post-reactor treatment of the polymer is required to remove catalyst residues (126). The high content of atactic polymer in the final product limits its usefiilness in many markets. 

In the 1970s, Solvay iatroduced an advanced TiCl catalyst with high activity and stereoregulahty (6). When this catalyst was utilized ia Hquid monomer processes, the level of atactic polymer was sufftciendy low so that its removal from the product was not required. Catalyst residues were also reduced so that simplified systems for post-reactor treatment were acceptable. Sumitomo has developed a Hquid monomer process, used by Exxon (United States), ia which polymer slurry is washed ia a countercurrent column with fresh monomer and alcohol to provide highly purified polymer (128). 

There is an interior optimum. For this particular numerical example, it occurs when 40% of the reactor volume is in the initial CSTR and 60% is in the downstream PFR. The model reaction is chemically unrealistic but illustrates behavior that can arise with real reactions. An excellent process for the bulk polymerization of styrene consists of a CSTR followed by a tubular post-reactor. The model reaction also demonstrates a phenomenon known as washout which is important in continuous cell culture. If kt is too small, a steady-state reaction cannot be sustained even with initial spiking of component B. A continuous fermentation process will have a maximum flow rate beyond which the initial inoculum of cells will be washed out of the system. At lower flow rates, the cells reproduce fast enough to achieve and hold a steady state. 

Moderate Reactor Productivity. The rhodium catalyst is continuously recycled, but the catalyst is inherently unstable at low CO partial pressures, for example in the post-reactor flash tank. Under these conditions the catalyst may lose CO and eventually form insoluble Rhl3 resulting in an unacceptable loss of expensive catalyst. This reaction is also more likely to occur at low water concentrations, hence in order to run the process satisfactorily catalyst concentrations are kept low and water concentrations relatively high. Hence through a combination of lower than optimum reaction rate (because of low catalyst concentrations) and water taking up valuable reactor volume the overall reactor utilization is less than optimum. 

Finally, the HDPE-slurry from the second reactor is sent to the post reactor (3) to reduce dissolved monomer. The process total conversion is up to 99.5%. In the decanter (4), the polymer is separated from dispersing medium. The polymer containing the remaining hexane is dried in the fluid bed dryer (5) and then pelletized in the granulation section. The separated and collected dispersing medium of the fluid separation step (6) with the dissolved cocatalyst and comonomer is recycled to the polymerization reactors. A small part of the dispersing medium is distilled to maintain the composition of the diluent. 

Bimodal molecular weight distribution may be achieved by several techniques. The simplest method is post-reactor blending of polyethylene with different melt indices. Two other methods involve in-reactor production of polyethylene. One approach involves use of mixed catalyst systems that polymerize ethylene in different ways to produce polyethylene with different molecular weights. The latter requires that the catalysts are compatible. Another technique employs use of reactors in series operated under different conditions (see section 7.6 in Chapter 7). Figure 1.9 illustrates polyethylene with a bimodal molecular weight distribution produced with a single site catalyst system in a Unipol gas-phase process. 

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). 

Except for the reactor zones, autoclave and tubular processes are very similar (3, 4). Peripherals in both cases are designed pre-reactor to ramp pressures and temperatures to very high levels and post-reactor to reduce temperatures and lower pressures to near ambient conditions to enable product isolation. Simplified process flow diagrams for the autoclave and tubular processes are shown in Figures 7.1 and 7.2, respectively. 

One of the first success of zeolites as catalysts, and the first commercial molecular shape selective catalytic process, was the use of erionite in a post-reforming process named selectoforming (39). Ihis 8 MR zeolite was able, based on the principle of size exclusion, to selectively crack the short chain n-parafiins to produce LPG. To avoid the deactivation by coke NiS was deposited on the zeolite. The erionite based catalyst is generally located at the bottom of the last reactor of the reformer unit and operates then at the reformer pressure, and at the temperature of the last reformer reactor. When more flexibility was to be achieved from the selectoforming, the catalyst is introduced... 

The Dranco process (Dry anaerobic composting) (De Baere and Six, 1988) also utilises the segregated organic fraction of MSW. This is mixed with recycled effluent, and fed to the reactor. The retention time is 18 days, and after post-fermentation processing, dewatering, and drying, the final product is marketed... 

Polypropylene made with modern catalysts, on the other hand, has an insignificant amount of catalyst residues because of their very high activity and practically no atactic content. For this reason modern processes do not require post-reactor purification. Some catalysts, such as the ones used in the Spheripol process (Section 2.5), will even produce large spherical polypropylene particles that do not necessarily require pelletization. As a consequence of these many advances, modern polypropylene (and polyethylene) manufacturing processes have very few units basically one or more reactors in series, compressors, recovery systems for diluent (for some processes) and unreacted monomer, and an extruder for making pellets. 

Currently, REX is an important post-reactor technology to functionalize nonpolar polymers, or to adjust the functionality of polar polymers to specific applications and properties. In the field of polymer blends, functionalized polymers are currently employed to improve the compatibility and adhesion between immiscible polymers by a process called reactive blending. 

Kenics-type static mixers have been used as inserts in tubular reactors. Compared to an open tube operated at the same pressure drop, the static mixer gives about 40% more heat transfer. Stand-alone mixer reactors of the Koch or Sultzer SMR type have been used as post-reactors and devolatilization preheaters. The polymer flows through the shell side of the SMR and the heat transfer fluid flows inside tubes that have been strategically placed to promote radial mixing of the polymer. One bulk polystyrene process used the SMR as in a recycle loop as the first reactor, but the capital cost is high compared to alternatives such as a boiling CSTR or a proprietary stirred-tube reactor. 

Again in view of their reduced pressure drop, it has been recognized that monolith structures also hold a good potential for applications as pre- and post-reactors of selective oxidation processes the related concepts and the existing conunercial examples are reviewed in Section 28.3. 

Several polymerization processes use only one reactor, but two or more reactors can also be operated in series (tandem reactor technology) to produce polyolefins with more complex microstructures [5]. Each reactor in the series is maintained under different operating conditions to produce products that are sometimes called reactor blends . Although, in principle, the post-reactor blending of different resins could lead to the same product, in reactor blends the chains are mixed on the molecular scale, permitting better contact between the polymer chains made in different reactors at a lower energy cost. 

---