LDPE autoclave process

LDPE is produced either in a tubular reactor (PFTR, LDPE tubular process) or in a high-pressure, continuous stirred tank reactor (CSTR, LDPE autoclave process). In both process designs the very high pressures require very special, thick-walled equipment and fatigue is a major design aspect for the applied pumps and compressors. 

In the LDPE autoclave process, the general process design (e.g., ethylene compression, high and low pressure product separation and ethylene recycling) is very similar to the LDPE tubular process. Instead of the jacketed tube reactor, however, the autoclave process employs a high-pressure stirred tank reactor. The reactor is designed to allow a residence time of 30-60 s (typical reactor volumes are around 1 m for large plants). The autoclave is usually operated in an adiabatic manner the reaction heat is removed by the fresh ethylene... 

In all technical PE production processes, the cost for the ethylene feedstock accounts by far for the largest share, typically 70% (for the LDPE autoclave process) to 78% (for the HOPE fluid bed process). Therefore, PE production costs depend heavily on the ethene price, which means, today, on the crude oil price. In the light of this dependency, attempts to produce PE from bioethanol-derived or for coal-derived ethene become understandable. 

A review is presented of the nitrogen autoclave process for the manufacture of crosslinked polyolefin foams. Process and product developments over the last few years are summarised and future possibilities are described. Process developments include use of higher temperatures and pressures to produce foams having densities as low as 10 kg/cub.m. Product developments include foams based on HDPE/LDPE blends, propylene copolymers and metallocene-catalysed ethylene copolymers. The structure and properties of these foams are compared with those of foams produced by alternative processes. 5 refs. 

Compared to the single-autoclave process, the cascade arrangement gives more flexibility to adjust the properties of LDPE resins. 

Sumitomo Chemical Co.,Ltd. Polyethylene, LDPE-EVA Ethylene and vinyl acetate Tubular and autoclave processes with EVA up to 41% 8 1994... 

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). 

Table 7.2 Typical operating features of autoclave processes for LDPE.

LyondellBasell Polyethylene, LDPE, autoclave Ethylene High-pressure Lupotech A autoclave reactor process produces low-density polyethylene (LDPE) homopolymers, EVA and various acrylic-type copolymers NA NA... 

The product mix of autoclave and tubular reactors are similar in terms of LDPE homopolymers (0.910-0.935 g/cc) and some specialty grades of polyethylene such as ethylene/vinyl acetate copolymers up to about 30 wt% vinyl acetate (VA). However, the autoclave process provides higher levels of vinyl acetate (40 wt%) in ethylene/VA copolymers and additional specialty grades of polyethylene such as ethylene/methyl acrylate, ethylene/acrylic acid and ethylene/n-butyl acrylate. Polyethylene molecular weight can be varied over a wide range with the high-pressure process, with Melt Index values (I ranging from 0.15 to 40. 

Two different high-pressure processes using autoclave or tubular reactors are applied for LDPE production. The autoclave process was developed by ICI, whereas the tubular reactor process was developed by BASF Aktiengesellschaft (predecessor of LyondellBasell s Lupotech T process). Monomer conversion rates of the adiabatic autoclave process can reach 25% compared to values up to 40% for tubular reactors, where the heat of polymerization can be partly removed through the jacketed reactor tubes via circulating cooling water. 

LDPE is produced in either a stirred autoclave or a tubular reactor total domestic production, divided between the two systems at 45% for tubular and 55% for autoclave, is estimated to be 3.4 million metric tons per year (5). Neither process has gained a clear advantage over the other, although all new or added capacity production in the 1990s has been through the autoclave. 

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. 

LDPE was occasionally found in 1933 by R.O. Gibson and E.W. Fawcett, when they tried to perform reactions with ethylene [1]. Based on their invention. Imperial Chemicals Ltd (ICI), Great Britain, developed a process with a stirred autoclave in which ethylene was radically polymerized under high pressure [2], Later, BASF AG in Germany designed a tubular reactor to produce LDPE under similar high-pressure conditions [3]. 

Single stirred-tank reactors were run in the first industrial scale processes to manufacture LDPE. Today they are used only for plants having lower capacities. The design of a single autoclave is shown in Fig. 5.1-6. It consists of a thick-walled forged-, or two-layer shrunk mantle. The ratio of inside length to inner diameter is typically in the range of one to two, and the volume is 1 - 2 m3. 

Application To produce low-density polyethylene (LDPE) homopolymers and EVA copolymers using the high-pressure free radical process. Large-scale tubular reactors with a capacity in the range of 130-400 Mtpy, as well as stirred autoclave reactors with capacity around 100 Mtpy can be used. 

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

EniChem Polyethylene, LDPE-EVA Ethylene and vinyl acetate monomer Autoclave or tubular process designs with EVA content of VA up to 40% 15 1995... 

ExxonMobil LDPE, EVA high pressure autoclave and tubular (separate processes) organic peroxides Autoclave operates about 1600 bar, tubular about 2800 bar... 

High-Pressure LDPE. The original ICI (and, later, Du Pont) process uses steel autoclave reactors, operating at pressures ranging from... 

PBE with both short and long branches are prepared from radical polymerization in the traditional high pressure process. These PE are called low density polyethylene (LDPE). A molecular model of a conformation of a typical LDPE is shown in Eig. 3.3. LDPE with different proportions of short and long branches are formed using autoclave or tubular reactors. There are many short branches with average... 

Chemically, LLDPE can be described as linear polyethylene copolymers with alpha-olefin comonomers in the ethylene chain. They are produced primarily at low pressures and temperatures by the copolymerization of ethylene with various alpha-olefins such as butene, hexane, octane, etc., in the presence of suitable catalysts. Either gas-phase fluidized-bed reactors or liquid-phase solution-process reactors are used. (In contrast, LDPE is produced at very high pressures and temperatures either in autoclaves or tubular reactors.)... 

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