Reactors for Polyolefins

Because of the nonuniform temperature and complex flow pattern in the reactors, it is not possible to model the process in order to predict all the distributions required to describe the structure. In addition, it is not possible to characterize the structure of LDPE in any detail using analytical methods because of its complex and irreprodudble structure. Finally, different batches of polymer made in the same reactor often vary in their structure. These variations may not be detectable using standard test methods while stiU affecting the processing behavior of the melt [115]. 

While it was at one time thought that LLDPE would replace LDPE in most film production, LDPE continues to be important commercially, especially for the production of film and the coating of paper and wires. While the global production of LLDPE has grown much more rapidly than that of LDPE, one third of all commercial polyethylene is still LDPE, and new plants continue to be built. The latest plants are built around very large tubular reactors. 

The first reactors used to make HOPE were based on a solution process. This had the advante es of small reactor size, short residence time, and the possibility of removing catalyst particles from the product by filtration or centrifugation. Dupont Canada later took advantage of the high operating temperature, 150 °C, for the incorporation of octene comonomer to make Sclair LLDPE. 

The slurry reactor was developed by Hochst to make polyethylene using Ziegler catalysts. The reaction medium, called a diluent , is a hydrocarbon that is a solvent for the monomer but not for the polymer. The product is thus formed as a suspended powder. Bimodal products, i.e., products that are, in effect, blends of two polymers having distinctly different molecular weight distributions, can be made using a cascade of two reactors in which the reaction conditions are substantially different [116]. Phillips Petroleum later developed a pipe-loop slurry reactor for use with its chromium oxide catalyst, which required moderately high temperatures and pressures to accommodate the isobutane diluent used. 

A recent development in this technology is the multizone circulating reactor (MZCR) developed by Basell ( Spherizone process), in which a given polymer particle flows back and forth between two reaction zones of a loop reactor, in each of which the conditions are different 

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Another important feature of a modem fluidized-bed reactor for polyolefin production is the disengaging zone the diameter of the reactor is increased at its topmost part to form the disengaging zone. This expanded cross section of the reactor slows down the upward movement of the polymer particles and prevents or minimizes them from being carried out by the fluidizing gas. Using this design procedure, the circulating gas velocity can be increased for better heat transfer. 

Today, large polyolefin extruders are built as ZSK MEGAcompounders, mainly as 320, 350, and 380 Me sizes. The reason for this is clear from Fig. 14.8 which shows the same relationship as Figure 14.7, but specifically for polyolefin applications, here with the ZSK Me limit curve for maximum throughput. Normally, the torque limit is only reached with LLDPE and FfDPE if pressure is also built up by the ZSK and the powder from the polymerization reactor is coarse enough to prevent intake problems. With PP or melt-charged LDPE or two-stage machines for bimodal polyethylene, the torque required is well below the Me limit curve. 

The PetroFCC process changes an FCC unit from a gasoline-producing device to a source of petrochemical feedstocks. It can provide both aromatic naphtha to an aromatics complex and light olefins to a polyolefins complex. Feedstock utilization can be further improved by recycling the raffinate by-product from BTX solvent extraction back to the PetroFCC reactor for conversion to light olefins. 

Ohkita et al.43 have used a two-zone reactor for the catalytic degradation of PE at 400 °C over amorphous silica-alumina and ZSM-5 zeolite. The polyolefin... 

CORl s are the prime machines for polymer blending and reactive extrusion [Brown, 1992]. They have been used as reactors for the addition polymerization (polyacrylates, SAN, S-MMA, PA-6, POM, or TPU) and for the polycondensation (PA-66, polyarylates, PEST, PEI). Polymer grafting (polyolefin + silane, maleic anhydride, acetic anhydride, etc.) as well as mechanical and chemical degradation of polypropylene have also been carried out. 

A complete set of differential equations has now been derived that describe how the MWD of a polyolefin varies in a batch reactor. For easy reference, these equations are summarized in Table 2.5, where the lumped constant Ktr is introduced to account for the frequency of all transfer reactions taking place in the reactor ... 

Semibatch and continuous stirred-tank reactors (CSTRs) are much more commonly found in polyolefin production. Semibatch reactors are the standard choice for laboratory-scale polymerizations, while CSTRs dominate industrial production, as will be seen in Section 2.5. The equations derived above are easily translated into semibatch and CSTR operation mode by simply adding terms for the inflow and outflow streams in the reactor. For instance, consider Equation 2.49 for the zeroth moment of dead chains. The molar flow rate [mol s ] leaving the reactor is given by... 

This variety of reactor configurations is unique for polyolefins among all commodity and specialty polymers. 

The simplest reactor is the stirred autoclave reactor. In polyolefin production, this reactor is operated as a CSTR and is used in slurry, bulk and solution processes. The main advantages of this configuration are that the reactor is easy to build and to run, and provides a relatively uniform reaction medium with proper stirring. Its principal disadvantage is that the heat transfer area-to-volume ratio is relatively low and heat removal is, therefore, limited. This limitation is especially difficult to overcome for new plants with increasing production capacity. 

Union Carbide, now part of Dow Chemical, was the first company to commercialize the technology for polyolefin production using fluidized-bed gas-phase reactors. Since polymerization occurs in the gas phase, separation of the unreacted monomer from the polymer product is achieved simply by flashing off the monomer. Any low molecular weight polymer formed remains in the polymer particles and no further separation is necessary. The process only requires a fluidized-bed gas-phase reactor, a product discharge system to get... 

However, there are still a number of important polymer properties that can only be measured by laborious and time-consuming off-line analyses. In this category one can include the MWD (especially in dispersed systems and/or for polyolefins), branching and crosslinking density, the gel content, the PSD, among other properties. (Despite the several examples reported in the scientific literature, at present no commercial equipment can ensure the fast and robust on-hne measurement of the entire size distribution of polymer particles in industrial reactors.)... 

Fluidized bed reactors (FBRs) are chemical reactors in which (catalytic) particles interact with a gas stream that is fed from the bottom, such that the mixture (emulsion phase) behaves as a fluid. This type of reactors is often used in the chemical and process industries, where they have gained their popularity due to their excellent heat and mass transfer characteristics. FBRs are used for instance for gas-phase polymerization reactions for polyolefin production (polyethylene, polypropylene), chemical looping combustion or reforming processes, and gas-phase Fischer—Tropsch synthesis. 

In summary, therefore, continued investment in the understanding of ziegler-Natta catalysis has developed an ideal catalyst for polyolefins. Polypropylene now stands at a crossroads with many more new opportunities possible. The production of the final polymer resin in the reactor without the need for subsequent remelting and pelletizing has expanded the potential for modifying this most versatile polymer to develop unique combinations of properties that enable it to compete economically with a wide range of existing... 

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