Polymerization in the Gas Phase

For the polymerization of ethylene and propylene large-scale gas-phase processes are well established. The implementation of gas-phase technology to the production of sticky polymers such as the ethylene/propylene-based rubbers EPM and EPDM was pioneered by UCC [519]. In a series of patents, UCC describes various approaches to overcome the inherent stickiness of rubber granules in the gas-phase polymerization. These approaches include the use of anti-agglomerants such as carbon black, silica, inorganic salts or appropriate catalyst supports and antistatic voltage etc. [520-535]. The addition of fluidization or anti-agglomeration aids is described by Zollner et al., silica is used in particular [536,537]. 

Contrary to the production of EP-rubbers, from a statistical point of view the formation of gel is much more pronounced in BD polymerization. This as- 

In 1992/1993 Bayer recognized the potential of Nd catalysis for the gas-phase polymerization of BD [538,539]. The first patent in this field claims the use of supported Nd catalyst systems for the gas-phase polymerization of dienes. Soon after, gas-phase polymerization of BD is mentioned in a patent filed by UCC [540,541]. Finally, Goodyear [542,543] and Ube [544,545] joined Bayer and UCC in their efforts to establish gas-phase technology for BR production. 

Owing to the heat capacity of the catalyst support (this holds especially true for inorganic supports) overheating is avoided particularly in the early stages of the polymerization after the supported catalyst is introduced to the gas-phase reactor. Another approach is taken by UCC who describe the use of catalyst solutions rather than supported catalysts. In the UCC-approach catalyst solutions are injected into the gas-phase reactor and the polymerization heat is removed by evaporation of the solvent [560-563]. 

As in Nd-catalyzed solution processes in gas-phase polymerization of BD regulation of molar mass is a serious problem as there are no agents for the control of molar mass readily available. Vinyl chloride and toluene are no viable options. Vinyl chloride is ruled out due to ecological reasons and toluene is not applicable due to low transfer efficiencies and the required low concentrations if applied in a gas-phase process. For the control of molar mass and MMD in the polymerization of dienes a combination of different methods is recommended [457,458] (1) temperature of polymerization, (2) partial pressure of BD, (3) concentration of cocatalyst (or molar ratio of Al/MNd) (4) type of cocatalyst, (5) residence time of the rare earth catalyst in the polymerization reactor. 

---

Polymerization in the Gas Phase. Many polymerization catalysts can be adapted for use in the gas phase. A gas-phase reactor contains a bed of small PE particles that is agitated either by a mechanical stirrer or by employing the fluidized-bed technique. These processes are economical because they do not requite solvent tecitculation streams. 

Gaseous monomers can polymerize in the gas phase in the presence of a fluidized catalyst bed. As polymer forms, hot gas forces the newly made material out of the reactor to a collector. Figure 2.15 shows a simplified schematic diagram of a generic polymerization reactor. 

The properties of the molecule are accounted for by this structure. The extra energy of the three-electron bond stabilises the molecule relative to structure I to such extent that the heat of the reaction 2NO — > NaO is small,8 and the substance does not polymerize in the gas phase. 

A variety of technological processes arc used for polyethylene manufacture. They include polymerization in supercritical ethylene at a high ethylene pressure and temperature above the PE melting point (110-140°C), polymerization in solution at 120-150°C or in slurry, and polymerization in the gas phase... 

Initiation must occur exclusively on the surface, to eliminate polymerization in the gas phase,... 

Kig. 13. Microphotograph of polypropylene growing on crystal edges. Polymerization in the gas-phase 36). By permission of Hiithig Wepf Verlag... 

There followed a very important series of papers coocenung the emulsifier-free system (Machi et al., 1978, I9 9a-d)- The equipment used was a modification of that used eailier, but the tetrafluoroethylene pressure was continuously recorded with the use of a strain gauge. In the first paper of the series (Machi er cl., 1978) the rate of polymerization was shown to be proportional to the 1.0 and 1.3 powers of the dose rate and the initial pressure, respectively. The activation energies were 0.8 above and —5.2 kcal/mol below 70°C. There was a maximum in the molecular weights at about the same temperature. This behavior is reminiscent of the behavior of ethylene and was again attributed to the increased mobility of the growing chains above the maximum temperature. The very low mobility would also account for the first-order dependence of the rate on the dose rate below 70 C. As before, n-hexadecane proved to be an excellent inhibitor of polymerization in the gas phase. Particle sizes in the range of 0.1-0.2 microns were obtained. 

The major processes for polyolefins production using Ziegler-Natta catalysts involve polymerization in the gas phase or in slurry, including bulk liquid monomer in the case of propylene. LLDPE is also produced via a solution process operating at temperatures in the range 130-250 °C. 

It is clear that further reaction of the hydrocarbons produced in reaction (3) will produce a more complicated series of aldehydes and nitriles via reactions (1) and (2). In the presence of liquid water, a still more complicated reaction sequence becomes possible. Because of the limitations of space, I shall confine the discussion to the possible production of the components of proteins and nucleic acids. The principle pathways of interest here are the synthesis of carbohydrates from formaldehyde and the formation of amino acids, purines and pyrimidines by reactions involving aqueous solutions of HCN. [Although there have been suggestions that HCN can undergo direct polymerization in the gas phase (Matthews and Moser, 1966 Matthews et al., 1977), this reaction has never been directly observed.]... 

The first gas-phase polymerization was first commercialized in Wesseling, Germany by ROW Co. in a joint venture with BASF and Shell companies in 1969. This facility employed the Novolen process for propylene polymerization in the gas phase. UCC and Sumitomo companies later developed fluidized-bed processes for the gas-phase polymerization of propylene. The advantages of this process are its high-effidency catalysis, elimination of residual removal, and elimination of evaporation or centrifugal separation. Its polymer product can be used in almost all applications [12,13,71,72]. 

As discussed above, there have been major improvements in catalyst efficiency and selectivity. The amount of atactic polymer has been reduced, and the number of pounds of polymer produced per pound of catalyst has been increased from five- to tenfold and more. Polymerization in the gas phase has been improved [73] in order that resin with low atactic content can be produced without solvent removal or polymer washing. The new processes [74] have reduced and, in some cases, eliminated the purification and solvent recovery steps, thereby simplifying the polymerization process and reducing the cost of a polymer plant. In addition, the new processes yield polymers with reduced catalyst residues and fewer gels, resulting in better filterability and improved pack life during fiber melt spinning. 

Gas phase polymerization. Compared to the slurry process, polymerization in the gas phase has the advantage that no diluent is used which simplifies the process [74-76]. A fluidized bed that can be stirred is used with supported catalysts. The polymerization is carried out at 2 to 2.5 MPa and 85 to 100 °C. The ethene monomer eireulates, thus removing the heat of polymerization and fluidizing the bed. To keep the temperature at values below 100 °C, gas conversion is maintained at 2 to 3 per pass. The polymer is withdrawn periodically from the reaetor. 

Bulk polymerization of tetrafluoroethene (TFE) by radiation was studied in the gas, liquid, and solid phase over a wide range of temperatures from —196 to 90 °C by a number of methods (e.g. NMR and FTIR spectroscopy). Volkova et al. [710] studied the radiation-induced polymerization in the gas phase from 12 to 90 °C. Different activation energies were found below and above 70 °C. Enslin et al. [711] reported that the rate of polymerization in the gas phase was a zero-order function of the monomer pressure. However, the rate of polymerization was profoundly influenced by the initial monomer pressure (4.6-order dependence) and on the radiation intensity (0.36-order dependence). 

Rabeony and Weiss investigated theoretically nonterminated polymerization in the gas phase without convection 54),... 

One example of polymerization in the gas phase is found in the work of Jones and Melville [83]. These investigators initiated the polymerization of vinyl chloride is the gaseous state with methyl radicals from the photolysis of acetone. The monomer vapor was also polymerized by ultraviolet radiation generated from a zinc-spark source. This spark source emits radiation in the range of 2100 A. Overall the quantum yield of polymer was found to be low. 

In Union Carbide processes, ethylene is polymerized in the gas phase. Pressures of 0.7-2 MPa (7-20 atmospheres) and temperatures of about 100°C are used. Polymerization is effected by proprietary catalysts, which consist of supported organochromium compounds such as chromacene ((C5H5)2Cr). The processes use a fluidized bed and are inherently simple in that ethylene serves as the ffuidizing gas (as well as reactant) and polyethylene is the bed material. The polymer is produced as granules (from which catalyst is not removed), which can be used directly. Since no solvent is involved, gas phase processes are simpler to operate and have lower energy consumption than other processes for high density polyethylene. 

---