Polymer processing post-reactor

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). 

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. 

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. 

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. 

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. 

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].

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. 

Some slurry processes use continuous stirred tank reactors and relatively heavy solvents (57) these ate employed by such companies as Hoechst, Montedison, Mitsubishi, Dow, and Nissan. In the Hoechst process (Eig. 4), hexane is used as the diluent. Reactors usually operate at 80—90°C and a total pressure of 1—3 MPa (10—30 psi). The solvent, ethylene, catalyst components, and hydrogen are all continuously fed into the reactor. The residence time of catalyst particles in the reactor is two to three hours. The polymer slurry may be transferred into a smaller reactor for post-polymerization. In most cases, molecular weight of polymer is controlled by the addition of hydrogen to both reactors. After the slurry exits the second reactor, the total charge is separated by a centrifuge into a Hquid stream and soHd polymer. The solvent is then steam-stripped from wet polymer, purified, and returned to the main reactor the wet polymer is dried and pelletized. Variations of this process are widely used throughout the world. 

One of the drawbacks associated especially with slurry and solution CSTR processes is the necessity of removing the solvent or diluent in a post-production step. In a gas phase reactor the polymerisation takes place in a fluidised bed of polymer particles. Inert gas or gas mixture is used for fluidisation. The gas flow is circulated through the polymer bed and a heat-ex-changer in order to remove the polymerisation heat. Gaseous ethylene and comonomer are fed into the fluidisation gas line of the reactor, and a supported catalyst is added directly to the fluidised bed (Fig. 7). Polymerisation occurs at a pressure of about 20-25 bar and a temperature of about 75-110 °C. The polymer is recovered as a solid powder which is, however, usually pelletised. Due to the limited cooling capacity of the fluidising gas, reactor... 

For most polsrmerizations starting from monomer, tubular reactors have been avoided because of the various stability problems. They can he used in recycle loops where the per-pass conversion is low, in solution polymerizations with large amounts of solvent, and in post- or finishing reactors intended to drive a polymerization to completion. Shell-and-tube designs with as many as 1000 tubes are used in polystyrene processes where they also serve as devolatilization preheaters. The entering polymer solution has a concentration of about 70%, and its viscosity is high enough to avoid tube-to-tube instabilities. 

In the Hoechst process, for example, hexane is used as the diluent (108,109). Hexane, ethylene, alpha-olefin, catalyst components, and hydrogen are continuously fed into a stirred reactor for polymerization. The slurry is then transferred into a smaller reactor for post-polymerization, after which the total charge is separated by a centrifuge into a liquid stream (which is returned to the initial reactor) and solid polymer. The wet polymer is steam-stripped from the solvent, dried, and pelletized. The stripped hexane is purified and recycled. Although stirred tanks are most common, loops can also be used in this fashion. In some schemes, a portion of the recycle diluent from the centrifuge is returned to the reactor, and a portion is fed to recycle purification for wax removal. This step removes some of the lowest molecular weight pol5mier, which dissolves in the diluent. 

Droplet transformation into particles can be achieved in a different way. For example, both emulsification of a monomer mixture and polymerization can be achieved in the continuous mode in a MF reactor (or in situ), as it is done for the photoinitiated synthesis of polymer beads. Typically, this polymerization mode is used for fast polymerization process, for example, for redox-initiated or photoinitiated polymerizations. Alternatively, following emulsification and partial polymerization of the monomer mixture in the MF reactor, the droplet particles are transferred into a batch reactor and post-polymerized using, for example, thermoinitiated polymerization. This polymerization process can be used for the synthesis of, for example, polystyrene particles. 

Hydrophobically modified hydroxyethyl cellulose (HEC) has also been synthesized by post-polymerization reaction. HEC is reacted with low levels of certain reactive hydrophobes e.g. long chain alkyl epoxides, alkyl halides, acyl halides, isocyanates or anhydrides [1, 2, 25]. (Scheme 1.1 [3]). To handle the incompatibility of the hydrophobic reagent and hydrophilic polymer, the reaction is run in solution with a common solvent or in slurry ipedia. Slurry methods are preferred because of their lower reactor viscosities. A typical slurry process involves swelling HEC with isopropanol, adding NaOH in H2O, followed by addition of a long chain epoxide such as 1,2-epoxydecane, 1,2-epoxydodecane. 

Reaction rates at high conversions ultimately will slow down so drastically that it generally is impractical to complete the polymerization in the reactor. Although post-curing may be carried out at some point downstream in the manufacturing process, unreacted monomers most often are removed by devolatilization. The devolatilized exiting polymer melt then is directly extruded to form pellets for subsequent fabrication steps. In some instances the melt may be fabricated directly into the final product. 

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