Polyethylene tubular reactors

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

Mathematical Model of Low Density Polyethylene Tubular Reactor... 

Figure 1. High pressure polyethylene tubular reactor.

Agrawal, S. and C.D. Han. Analysis of the High Pressure Polyethylene Tubular Reactor with Axial Mixing, AIChE J., 21 (1975), 449-465. 

Donati, G., L. Marini, G. Marziano, C. Mazzaferri, M. Spampinato and E. Langianni. Mathematical Model of Low Polyethylene Tubular Reactor Chemical Reaction Engineering (ACS Symposium Series 196, J. Wei and C. Georgakis, eds.), American Chemical Society, Washington, D.C., (1981), 579-590. 

Yoon, B.J. and H. Rhee. A Study of the High Pressure Polyethylene Tubular Reactor, Chem. Eng. Commun., 34 (1985), 253-265. 

Mavridis H. and C. Kiparissides. Optimization of a High-Pressure Polyethylene Tubular Reactor, Polymer Process Eng., 3 (1985), 263-290. 

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. 

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.

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. 

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. 

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. 

Exxon and Phillips manufacture polypropylene in tubular reactors where the monomer is in the liquid form (see Section 6.8.2). One of the manufacturing processes for polyethylene involves the use of a loop reactor that has a recycle configuration. Here, under elevated pressure and temperature, a mixture of the catalyst, comonomer, hydrogen, and a solvent are introduced from one end of the reactor. The product and the unreacted starting materials are collected at the other end, and recycled back into the reactor. 

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). 

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