Low density polyethylene tubular

Fig. 21. A low density polyethylene tubular reactor used by Phillips Petroleum (85).

Mathematical Model of Low Density Polyethylene Tubular Reactor... 

Buchelli, A., Call, M., Brown, A., Bokis, C Ramanathan, S., and Franjione, J. (2002) Physical properties, reactor modebng and polymerization kinetics in the low-density polyethylene tubular reactor process. Industrial S. Engineering Chemistry Research, 41 (5), 1017-1030. 

An independent development of a high pressure polymerization technology has led to the use of molten polymer as a medium for catalytic ethylene polymerization. Some reactors previously used for free-radical ethylene polymerization at a high pressure (see Olefin polymers, low density polyethylene) have been converted to accommodate catalytic polymerization, both stirred-tank and tubular autoclaves operating at 30—200 MPa (4,500—30,000 psig) and 170—350°C (57,83,84). CdF Chimie uses a three-zone high pressure autoclave at zone temperatures of 215, 250, and 260°C (85). Residence times in all these reactors are short, typically less than one minute. 

Low density polyethylene is made at high pressures in one of two types of continuous reactor. Autoclave reactors are large stirred pressure vessels, which rely on chilled incoming monomer to remove the heat of polymerization. Tubular reactors consist of long tubes with diameters of approximately 2.5 cm and lengths of up to 600 m. Tubular reactors have a very high surface-to-volume ratio, which permits external cooling to remove the heat of polymerization. 

Van Vliet et al. (2004, 2006) investigated the formation of hot spots and reactor efficiency in various geometrical configurations of a tubular reactor for manufacturing Low-Density Polyethylene (LDPE) by means of the above... 

Table V compares M, M and M values for two polyethylenes analyzed by SEC in TCB solution at l45°C. Sample C is a linear low density material listed in Table 1. NBS 1 476 is low density polyethylene which is stated to be a low conversion tubular reactor product with density 0.931 gem and melt index 1.2 (11). IL and are little af fected by the existence of aggregates in these two samples but values are more severely influenced.

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. 

The advantages known from the production of low-density polyethylene (LDPE) become obvious also when metallocene catalysts are used under high-pressure conditions. The compressed monomer can dissolve the polymer which is formed during polymerization, which means that no additional solvent is required for the polymer. The high-pressure polymerization proceeds with a high rate, which requires a short residence time and small reactor volume. Established technology, with stirred autoclaves as well as tubular reactors, can be applied. 

Film blowing. A tubular 50 pm thick low density polyethylene film is blown with a draw ratio of 5 at a flow rate of 50 g/s. The annular die has a diameter of 15 mm and a die gap of 1 mm. Calculate the required pressure inside the bubble and draw force to pull the bubble. Assume a Newtonian viscosity of 800 Pa-s, a density of 920 kg/m3 and a freeze line at 300 mm. 

A mathematical model was developed, able to predict monomer conversion and temperature profiles of industrial tubular reactors for the production of low-density polyethylene, in different operating conditions. 

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. 

A similar technique is applied to low-density polyethylene reactors. Some of these systems operate in cooled tubular reactors at a very high pressure. Since the reactor has a thick tube wall, the temperature response to changes in the coolant is slow. Instead, the reaction rate (and thereby temperature.) is controlled by injecting initiator at select places along the length of the reactor tube (see Fig. 4.28). 

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

Tseng, H.-S. Lloyd, D. R. Ward, T. C., "Solubility of Nonpolar and Slightly Polar Organic Compounds in Low-Density Polyethylene by Inverse Gas Chromatography with Open Tubular Column," J. Appl. Polym. Sci., 30, 1815 (1985). 

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. 

The emphasis to this point has been on viscous behavior in shearing modes of deformation. However, any operation which reduces the thickness of a polymeric liquid must do so through deformations that are partly extensional and partly shear. In many cases polymers respond very differently to shear and to extension. A prime industrial example involves low density and linear low density polyethylenes, i.e., LDPE and LLDPE, respectively (Section 9.5.3). LDPE grades intended for extrusion into packaging film have relatively low shear viscosities and high elonga-tional viscosities. As a result, extrusion of tubular film involves reasonable power... 

White, J.L., and H. Yamane, "A collaborative study of the stability of extrusion, melt spinning and tubular film extrusion of some high-, low- and linear-low density polyethylene samples," Pure Appl. Chem. 59, No. 2,193-216 (1987). 

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,... 

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.,...

Kolhapure and Fox, R. (1999), CFD analysis of micromixing effects on polymerization in tubular low-density polyethylene reactors, Chem. Eng. Sci., 54, 3233-3242. 

Continuous Continuous stirred tank reactor loop reactor, stirred tank reactors, fluidized reactors, tubular reactors Polyvinyl acetate, styrene-butadiene, PVC (E), polystyrene (S), low-density polyethylene (B)... 

---