Polyethylene High-Pressure Tubular Process

A primary compressor increases the pressure of the entering ethylene gas (and propylene gas, which is added as a molecular weight control agent) from between 5 and 15 bar to about 250 bar. The secondary compressor further increases the gas pressure from 250 bar to the desired reactor pressure (approximately 2500 bar). An initiator is added to the gas as it enters the reactor. The reactor is operated to ensure a per-pass conversion of 15%-35% and is a wall-cooled reactor where the cooling water can be used to produce steam. The reaction mixture then enters the HP separator (-250 bar), where the mixture is flashed to produce two distinct phases a PE-rich melt phase and an ethylene-rich gas phase. The separated gas then enters the recycle loop. The ethylene gas is cooled before entering the secondary compressor. The PE enters the low-pressure separator. This low-pressure separator, also referred to as a hopper, performs the final degassing step. The separated ethylene gas is cooled and some components are removed. This step takes place 

---

The second type of solution polymerization concept uses mixtures of supercritical ethylene and molten PE as the medium for ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization in supercritical ethylene at high pressure (see Olefin POLYMERS,LOW DENSITY polyethylene) were converted for the catalytic synthesis of LLDPE. Both stirred and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C can also be used for this purpose. Residence times in these reactors are short, from 1 to 5 minutes. Three types of catalysts are used in these processes. The first type includes pseudo-homogeneous Ziegler catalysts. In this case, all catalyst components are introduced into a reactor as hquids or solutions but form soHd catalysts when combined in the reactor. Examples of such catalysts include titanium tetrachloride as well as its mixtures with vanadium oxytrichloride and a trialkyl aluminum compound (53,54). The second type of catalysts are soHd Ziegler catalysts (55). Both of these catalysts produce compositionaHy nonuniform LLDPE resins. Exxon Chemical Company uses a third type of catalysts, metallocene catalysts, in a similar solution process to produce uniformly branched ethylene copolymers with 1-butene and 1-hexene called Exact resins (56). 

A number of processes have been developed to obtain products of different physical properties. The nature of the product is affected by the addition of diluents or other additives before carrying out the polymerization. Autoclaves or stirred-tank reactors, and tubular reactors, or their combinations have been developed for the industrial production of high-pressure polyethylene.206,440 Pressures up to 3500 atm and temperatures near 300°C are typically applied. 

The first section of this chapter describes the most important high pressure process run under homogeneous conditions to manufacture Low Density PolyEthylene (LDPE). The radical polymerization of ethylene to LDPE is carried out in tubular reactors or in stirred autoclaves. Tubular reactors exhibit higher capacities than stirred autoclaves. The latter are preferred to produce ethylene copolymers having a higher comonomer content. 

Application The high-pressure Lupotech TS or TM tubular reactor process is used to produce low-density polyethylene (LDPE) homopolymers and EVA copolymers. Single-train capacity of up to 400,000 tpy can be provided. 

Application To produce low density polyethylene (LDPE) and ethylene vinyl acetate (EVA) by the high-pressure, autoclave or tubular EniChem process. 

Theonly important current application of tubular reactors in polymer syntheses is in the production of high pressure, low density polyethylene. In tubular processes, the newer reactors typically have inside diameters about 2.5 cm and lengths of the order of I km. Ethylene, a free-radical initiator, and a chain transfer agent are injected at the tube inlet and sometimes downstream as well. The high heat of polymerization causes nonisothermal conditions with the temperature increasing towards the tube center and away from the inlet. A typical axial temperature profile peaks some distance down the tube where the bulk of the initiator has been consumed. The reactors are operated at 200-300°C and 2000-3000 atm pressure. 

In Chapter 1, it was mentioned that highly branched low density polyethylene and copolymers made with polar comonomers are produced only by free radical polymerization at very high pressure and temperature. (All other forms of commercially available polyethylene are produced with transition metal catalysts under much milder conditions see Chapters 3, 5 and 6.) In this chapter we will review how initiators achieve free radical polymerization of ethylene. Low density polyethylene and copolymers made with polar comonomers are produced in autoclave and tubular processes, to be discussed in Chapter 7,... 

Because of the extremely high pressures (15,000 to 45,000 psig), ethylene exists in the liquid phase and polymerization occurs in solution. Owing to high temperatures (typically >200 °C), polyethylene is also dissolved in monomer and the reaction system is homogeneous. LDPE precipitates only after the reaction mass is cooled in post-reactor separation vessels. Relative to other processes, reactor residence times are very short (<30 seconds for the autoclave process and <3 min for the tubular process) (7). 

Figure 7.2 Schematic process flow diagram for tubular high pressure process for production of low density polyethylene. (Reprinted with permission of John Wiley Sons, Inc., Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, Inc.,...

The removal of precipitated polyethylene from the wall is an interesting operation. About once every 2-3 sec the expansion valve is opened more fully than required for the expansion/precipitation function this results in a rapid decrease in pressure in the reactor of as much as 300-600 bar. The concomitant rapid increase in the velocity of the gas phase in the tubular reactor shears the walls and strips off any deposited polyethylene so that a reasonably steady state heat transfer situation exists. This description of the operation of the polymerization process, the polyethylene precipitation step, and the accentuated expansion, which maintains a clean wall and a high heat transfer coefficient, help to illustrate the interesting SCF solubility behavior and they also supply some information on the commercial reality of high-pressure processing in what we consider to be an extreme case. 

During the past four years, linear low-density polyethylene (LLDPE) has probably become the most important of the thermoplastic copolymers. In contrast to the customary practice of producing branched ethylene homopolymer in a high-pressure reaction, a system of copolymerizing ethylene with a-C g olefins at low pressure is used to make LLPDE copolymer. This random copolymerization is commercially carried out in gas-phase, slurry, and solution processes in the presence of a transition metal catalyst 1-butene, 1-hexene, 4-methyl-l-pentene, or 1-octene are choices of comonomer. In the face of plant overcapacity and idle equipment existing at this time, LLDPE can also be made in high-pressure autoclaves and tubular reactors. 

Application To produce low-density polyethylene (LDPE) homopolymers and ethylene vinyl acetate (EVA) copolymers using the high-pressure free radical process. Large-scale tubular reactors with a capacity in the range of 130,000 tpy-425,000 tpy, as well as stirred autoclave reactors with capacity around 125,000 tpy can be used. 

---