Tubular multiple feed reactor

Figure 5.1-5. Tubular reactor with multiple feed. E, fresh ethylene I, initiator.

High conversion, similar to that in a tubular reactor with multiple feed, is obtained in a multi-chamber or multi-zone autoclave reactor. This is an elongated cylindrical vessel with a ratio of length to inner diameter of 10 - 12. It is subdivided by baffles and the stirrer into two to five chambers (Fig. 5.1-8, left). 

The costs assume the production of pellets on 1997 German prices. Polymerization-grade ethylene is available at 5 MPa. The on-stream time is 8,000 h/a. The tubular reactors equipped with multiple feeds of ethylene and peroxide initiators are operated at 200 MPa. The initiators are a mix of dicyclohexyl peroxy dicarbonate, /-butylperoxy pivalate, /-butylperoxy 2-ethylhexanoate, and di(/-butyl)peroxide, which is fed in after the heating zone and at two further locations downstream. 

Lack of flexibility in controlling the reaction and product properties is a disadvantage of tubular reactors, especially compared to semi-batch processes. Multiple feed points along the tube length would need to be employed with potential mixing problems. Hence tubular reactors are not widely utilized for commercial production. 

Scheme 12 presents a schematic diagram of a tubular reactor with multiple feed points. The temperature profile along the reactor length is an important measurement. The rate of polymerization can double for every 10 °C rise in temperature. These are reactor safety concerns as well as polyethylene product quality concerns. Large heats of polymerization are common for chain growth polymerization, where carbon-carbon double bonds are converted to carbon-carbon single bonds. It has been observed that at temperature above 210 °C, molten polyethylene may tend to cross-link, while at temperature above 290 °C, chain scission may dominate. 

Scheme 12 (a) Process for production of high-pressure polyethylene and copolymers and (b) schematic diagram of tubular reactor with multiple feed points. ... 

The reaction section consists of the high pressure reactors filled with catalyst, and means to take away or dissipate the high heat of reaction (300-500 Btu/lb of olefin polymerized). In the tubular reactors, the catalyst is inside a multiplicity of tubes which are cooled by a steam-water condensate jacket. Thus, the heat of reaction is utilized to generate high pressure steam. In the chamber process, the catalyst is held in several beds in a drum-type reactor with feed or recycled product introduced as a quench between the individual beds. 

Figure 4.5 Tubular reactor multiple cold ethylene feed points.

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