Copolymers, production reactor requirements

For impact copolymer production, a second reactor (4) in series is required. A reliable and effective gas-lock system (3) transfers powder from the first (homopolymer) reactor to the second (copolymer) reactor, and prevents cross contamination of reactants between reactors. This is critically important when producing the highest quality impact copolymer. In most respects, the operation of the second reactor system is similar to that of the first, except that ethylene in addition to propylene is fed to the second reactor. Powder from the reactor is transferred and depressurized in a gas/powder separation system (5) and into a purge column (6) for catalyst deactivation. The deactivated powder is then pelletized (7) with additives into the final products. 

In the process, homopolymer and random copolymer polymerization occurs in the loop-type reactor (or vessel-type reactor) (1). For impact copolymer production, copolymerization is performed in a gas-phase reactor (2) after homopolymerization. The polymer is discharged from a gas-phase reactor and transferred to the separator (3). Unreacted gas accompanying the polymer is removed by the separator and recycled to the reactor system. The polymer powder is then transferred to the dryer system (4) where remaining propylene is removed and recovered. The dry powder is pelletized by the pelletizing system (5) along with required stabilizers. 

Copolymer properties are known to be a function of the molecular weight distribution (MWD), the copolymer composition distribution (CCD) and in some cases the sequence length distribution (SLD). The optimal design, operation and control of reactors to produce high quality copolymers with efficient production rates requires ... 

One might suppose that the lower temperatures required to make copolymers would also lower the MW of the polymer, making control more difficult. Fortunately, the incorporation of comonomer also tends to accelerate termination (raise the MI), which usually compensates for the required lower temperature. Consequently, when Cr/silica is the catalyst there is not usually a substantial difference in MI potential in homopolymer- versus copolymer production. However, variations to this general rule are possible, because some catalysts can be more sensitive to comonomer than others. For example, when 1-hexene is added to the reactor with a Cr/silica-titania catalyst, the polymer MI tends to rise more than with Cr/silica. 

There are many parallels between polypropylene and polyethylene manufacturing processes. The reactor configurations are similar, but, due to the different requirements of the polymer, it will be seen that there are significant differences between the processes as well. While propylene homopolymer can be produced in reactors of various configurations, for impact copolymer production, gas phase is the reactor of choice because of the stickiness of the polymer and the solubility of the copolymer in the monomer and diluent. 

The dominant process in this market segment is the Spheripol process by Basell. Similar to the dominance achieved by the Phillips process in HOPE, roughly one-third of the world s polypropylene is produced using the Spheripol process. The Spheripol process uses loop reactors. A small loop reactor is used to prepolymerize the catalyst the main polymerization, for homopolymer or random copolymer, takes place in one or two loop reactors. For impact copolymer production, a gas-phase reactor is required after the loop reactor because of the limited solubility of ethylene in liquid propylene. A typical flow diagram of the Spheripol process is shown in Figure 2.40. 

The Unipol process, initially developed for polyethylene production, was later extended to polypropylene manufacture. The process consists of a large fluidized-bed gas-phase reactor for homopolymer and random copolymer production, and a second smaller reactor for impact copolymer production. The second reactor is smaller than the first one because only 20% of the production comes from the second reactor. This reactor typically has a lower pressure rating as copolymerization is usually carried out at lower temperatures and pressures. Condensed mode operation is used in the homopolymer reactor but an inert diluent is not required because propylene is partially fed as a liquid. The copolymerization reactor is operated purely in the gas phase. The Unipol process has a unique and complex product discharge system that allows for very efficient recovery of unreacted monomer, but this does add complexity and capital cost to the process. 

In ordinary batch copolymerization there is usually a considerable drift in monomer composition because of different reactivities of the two monomers (based on the values of the reactivity ratios). This leads to a copolymer with a broad chemical composition distribution (CCD). In many cases (depending on the specific final product application) a composition drift as low as 3-5% cannot be tolerated, for example, copolymers for optical applications on the other hand, during production of GRIN (gradient index) lenses, a controlled traj ectory of copolymer composition is required. This is partly circumvented in semibatch operation where the composition drift can be minimized (i.e., copolymer composition can be kept constant ) by feeding a mixture of the monomers to the reactor with the same rate by which each of them is consumed in the reactor. 

The enthalpy of the copolymerization of trioxane is such that bulk polymerization is feasible. For production, molten trioxane, initiator, and comonomer are fed to the reactor a chain-transfer agent is in eluded if desired. Polymerization proceeds in bulk with precipitation of polymer and the reactor must supply enough shearing to continually break up the polymer bed, reduce particle size, and provide good heat transfer. The mixing requirements for the bulk polymerization of trioxane have been reviewed (22). Raw copolymer is obtained as fine emmb or flake containing imbibed formaldehyde and trioxane which are substantially removed in subsequent treatments which may be combined with removal of unstable end groups. 

The main advantages for the high-pressure process compared to other PE processes are short residence time and the ability to switch from homopolymers to copolymers incorporating polar comonomers in the same reactor. The high-pressure process produces long-chain, branched products from ethylene without expensive comonomers that are required by other processes to reduce product density. Also, the high-pressure process allows fast and efficient transition for a broad range of polymers. 

The production of copolymers leads to some additional constraints to reactor design beyond what is required for homopolymer. The most important of these is composition drift. The reactivity ratios of a monomer mixture define the composition of a copolymer that is instantaneously produced from a given monomer mixture. This is true in a plug flow reactor or a backmixed reactor. However, in the plug flow reactor, the copolymer composition drifts from that produced from the initial monomer composition to that produced by the monomer composition at the end of the polymerization. In contrast, in the backmixed reactor, all copolymer produced is of the same composition, which... 

In contrast to the requirements for homopolymerisation processes, the parameters needed to fully describe copolymerisation processes are more numerous. Molecular features such as the copolymer composition, composition distribution and chain sequence structure and their variation with conversion are compounded with those of copolymer MW and MWD. To understand copolymerisation processes, it is desirable to decouple as many of these molecular parameters as possible and study the influence of polymerisation reactor conditions on each. As yet there have been relatively few reports on the detailed behaviour of copolymerisation reactors (6-9 ). This work forms part of a wider range of investigations which are being carried out in our laboratories of control methods for the production of speciality polymers. 

---