Gas-phase process

Gas phase polymerization is flexible and can produce a wider range of polymer grades than the earlier processes. Capital and operating costs are relatively low and the process has been very successful. Modem process developments include the injection of liquid hydrocarbon directly into the fluid bed, rather than to the recycle gas. 

This makes the process more economic by allowing better temperature control and increased conversion. This has become known as the condensation mode and can considerably increase the capacity of existing units. All types of active catalysts can be used. 

The Exxon catalyst is a derivative of cyclopentadiene, indene, or fluorene which may be bridged, and which may also be substituted with up to five different atoms or radicals. This organic group is bonded to a metal atom from Group IV b of the Periodic Table, titanium, zirconium or hafnium, which has been activated by addition of a strong Lewis acid, to give a cationic structure. 

Features - particles of growing polymer form in fluidized bed - catalyst residence time 2-4 hours - morphology and psd of catalyst are important - previously (pre-1990s) restricted in range of comonomer that could be used because of emergence of condensed mode operation, a wide range of comonomers may now be used 

In the 1990s, an improvement for the gas phase process was developed called condensed mode operation of Unipol reactors (12). This technique greatly expanded capacity of gas phase reactors and product capability by making it more practical to use higher alpha-olefin comonomers such as octene-1. 

---

Almost all aspects of the field of chemistry involve tire flow of energy eitlier witliin or between molecules. Indeed, tire occurrence of a chemical reaction between two species implies tire availability of some minimum amount of energy in tire reacting system. The study of energy transfer processes is tluis a topic of fundamental importance in chemistry. Energy transfer in gases is of particular interest partly because very sophisticated methods have been developed to study such events and partly because gas phase processes lend tliemselves to very complete and detailed tlieoretical analysis. 

Adsorption. In the design of the adsorption step of gas-phase processes, two phenomena must be considered, equiUbrium and mass transfer. Sometimes adsorption equiUbrium can be regarded as that of a single component, but mote often several components and their interactions must be accounted for. Design techniques for each phenomenon exist as well as some combined models for dynamic performance. 

Bromine Trifluoride. Bromine trifluoride is produced commercially by the reaction of fluorine with bromine ia a continuous gas-phase process where the ratio of fluorine to bromine is maintained close to 3 1. It is also produced ia a Hquid-phase batch reaction where fluorine is added to Hquid bromine at a temperature below the boiling poiat of bromine trifluoride. 

Eig. 1. Melting curves (dsc) of two ethylene—1-hexene copolymers produced in a gas-phase process one with a uniform branching distribution (1-hexene content 2.5 mol %) and another with a nonuniform branching distribution (1-hexene content 2.8 mol %). 

Eluidized-bed reactors are highly versatile and can accommodate many types of polymerization catalysts. Most of the catalysts used for LLDPE production are heterogeneous Ziegler catalysts, in both supported and unsupported forms. The gas-phase process can also accommodate supported metallocene catalysts that produce compositionaHy uniform LLDPE resins (49—51). 

In addition to the Hquid-phase -butyl nitrite (BN) process, UBE Industries has estabHshed an industrial gas-phase process using methyl nitrite (50—52). The oudine of the process is described in Eigure 4 (52). This gas-phase process is operated under lower reaction pressure (at atmospheric pressure up to 490 kPa = 71 psi) and is more economical than the Hquid-phase process because of the foUowing reasons owing to the low pressure operation, the consumption of electricity is largely reduced (—60%) dimethyl oxalate (DMO) formation and the methyl nitrite (MN) regeneration reaction are mn... 

Polypropylene. PP is a versatile polymer, use of which continues to grow rapidly because of its excellent performance characteristics and improvements in its production economics, eg, through new high efficiency catalysts for gas-phase processes. New PP-blend formulations exhibit improved toughness, particularly at low temperatures. PP has been blended mechanically with various elastomers from a time early in its commercialisation to reduce low temperature brittleness. 

Nitrile Process. Fatty nitriles are readily prepared via batch, Hquid-phase, or continuous gas-phase processes from fatty acids and ammonia. Nitrile formation is carried out at an elevated temperature (usually >250° C) with catalyst. An ammonia soap which initially forms, readily dehydrates at temperatures above 150°C to form an amide. In the presence of catalyst, zinc (ZnO) for batch and bauxite for continuous processes, and temperatures >250° C, dehydration of the amide occurs to produce nitrile. Removal of water drives the reaction to completion. 

A Hquid-phase variation of the direct hydration was developed by Tokuyama Soda (78). The disadvantages of the gas-phase processes are largely avoided by employing a weakly acidic aqueous catalyst solution of a siHcotungstate (82). Preheated propylene, water, and recycled aqueous catalyst solution are pressurized and fed into a reaction chamber where they react in the Hquid state at 270°C and 20.3 MPa (200 atm) and form aqueous isopropyl alcohol. Propylene conversions of 60—70% per pass are obtained, and selectivity to isopropyl alcohol is 98—99 mol % of converted propylene. The catalyst is recycled and requites Htde replenishment compared to other processes. Corrosion and environmental problems are also minimized because the catalyst is a weak acid and because the system is completely closed. On account of the low gas recycle ratio, regular commercial propylene of 95% purity can be used as feedstock. 

Most commercial processes produce polypropylene by a Hquid-phase slurry process. Hexane or heptane are the most commonly used diluents. However, there are a few examples in which Hquid propylene is used as the diluent. The leading companies involved in propylene processes are Amoco Chemicals (Standard OH, Indiana), El Paso (formerly Dart Industries), Exxon Chemical, Hercules, Hoechst, ICl, Mitsubishi Chemical Industries, Mitsubishi Petrochemical, Mitsui Petrochemical, Mitsui Toatsu, Montedison, Phillips Petroleum, SheU, Solvay, and Sumimoto Chemical. Eastman Kodak has developed and commercialized a Hquid-phase solution process. BASE has developed and commercialized a gas-phase process, and Amoco has developed a vapor-phase polymerization process that has been in commercial operation since early 1980. 

Examination of equation 42 shows that T is directly proportional to the average stage holdup of process material. Thus, in conjunction with the fact that hquid densities are on the order of a thousand times larger than gas densities at normal conditions, the reason for the widespread use of gas-phase processes in preference to hquid-phase processes in cascades for achieving difficult separations becomes clear. 

Typical methods are those of F. C. Zevnik and R. L. Buchanan [Chem. Eng. Progi , 59, 70-77 (Feb. 1963)] and J. H. Taylor Eng. 6-Proc. Econ., 2, 259-267, 1977). The former is mainly a graphical method of estimating the cost per functional unit (Cp) based on the capacity, the maximum pressure, the maximum temperature, and the materials of construction. The Taylor method requires the determination of the costliness index, which is dependent on the complexity of the process. A simpler method was suggested by S. R. Timms (M.Phil. thesis, Aston University, England, 1980) to give the battery hmits cost for gas phase processes only in U.S. dollars with a Marshall and Swift index of 1000. The simple equation is... 

Thermodynamic paths are necessary to evaluate the enthalpy (or internal energy) of the fluid phase and the internal energy of the stationary phase. For gas-phase processes at low and modest pressures, the enthalpy departure function for pressure changes can be ignored and a reference state for each pure component chosen to be ideal gas at temperature and a reference state for the stationarv phase (adsorbent plus adsorbate) chosen to be adsorbate-free solid at. Thus, for the gas phase we have... 

Polypropylenes produced by metallocene catalysis became available in the late 1990s. One such process adopts a standard gas phase process using a metallocene catalyst such as rac.-dimethylsilyleneto (2-methyl-l-benz(e)indenyl)zirconium dichloride in conjunction with methylaluminoxane (MAO) as cocatalyst. The exact choice of catalyst determines the direction by which the monomer approaches and attaches itself to the growing chain. Thus whereas the isotactic material is normally preferred, it is also possible to select catalysts which yield syndiotactic material. Yet another form is the so-called hemi-isotactic polypropylene in which an isotactic unit alternates with a random configuration. 

K. C. H. Yi, and N. J. Maraschin, Bimodal HOPE Via Gas Phase Process, A New Frontier 1990, Union Carbide Chemicals Plastics Technology Corporation./ Mod. Plast. Int. January 1991, p. 12. 

Either a liquid or a gas-phase process is used for the alkylation reaction. In the liquid-phase process, low temperatures and pressures (approximately 50°C and 5 atmospheres ) are used with sulfuric acid as a catalyst. 

Older methods use a liquid phase process (Figure 10-11). ° New gas-phase processes operate at higher temperatures with noble metal catalysts. Using high temperatures accelerates the reaction (faster rate). The hydrogenation of benzene to cyclohexane is characterized by a highly exothermic reaction and a significant decrease in the product volume... 

Polymerization reactions can occur in bulk (without solvent), in solution, in emulsion, in suspension, or in a gas-phase process. Interfacial polymerization is also used with reactive monomers, such as acid chlorides. 

Reaction conditions are generally mild, but they differ from one process to another. In the newer Unipol process (Eigure 12-1) used to produce both HDPE and LLDPE, the reaction occurs in the gas phase. Ethylene and the comonomers (propene, 1-butene, etc.) are fed to the reactor containing a fluidized bed of growing polymer particles. Operation temperature and pressure are approximately 100°C and 20 atmospheres. A single-stage centrifugal compressor circulates unreacted ethylene. The circulated gas fluidizes the bed and removes some of the exothermic reaction heat. The product from the reactor is mixed with additives and then pelletized. New modifications for gas-phase processes have been reviewed by Sinclair. ... 

Catalysts developed in the titanium-aluminum alkyl family are highly reactive and stereoselective. Very small amounts of the catalyst are needed to achieve polymerization (one gram catalyst/300,000 grams polymer). Consequently, the catalyst entrained in the polymer is very small, and the catalyst removal step is eliminated in many new processes. Amoco has introduced a new gas-phase process called absolute gas-phase in which polymerization of olefins (ethylene, propylene) occurs in the total absence of inert solvents such as liquefied propylene in the reactor. Titanium residues resulting from the catalyst are less than 1 ppm, and aluminum residues are less than those from previous catalysts used in this application. 

Figure 12-2. The Union Carbide gas-phase process for producing polypropylene " (1) reactor, (2) centrifugal compressor, (3) heat exchanger, (4) product discharge tank (unreacted gas separated from product), (5) impact reactor, (6) compressor, (7) heat exchanger, (8) discharge tank (copolymer separated from reacted gas).

All the above deals with gases and gas phase processes. We now turn to non-gaseous components of the system. There are many ways of expressing this. Probably the simplest is to consider an ideal solution of a solute in a solvent. If the solution is ideal, the vapour pressure of the solute is proportional to its concentration, and we may write p = kc, where c is the concentration and k is the proportionality constant. Similarly, = Arc , which expresses the fact that the standard pressure is related to a standard concentration. Thus we may write from equation 20.198 for a particular component... 

---