Tubular process

Figure 10.13. Methods of producing polyethylene film (a) tubular process using air cooling (b) flat film process using water bath cooling (c) flat film process using chill...

In the tubular process a thin tube is extruded (usually in a vertically upward direction) and by blowing air through the die head the tube is inflated into a thin bubble. This is cooled, flattened out and wound up. The ratio of bubble diameter to die diameter is known as the blow-up ratio, the ratio of the haul-off rate to the natural extrusion rate is referred to as the draw-down ratio and the distance between the die and the frost line (when the extrudate becomes solidified and which can often be seen by the appearance of haziness), the freeze-line distance. 

One alternative approach to the two-stage steam moulding process is that in which impregnated beads are fed directly to an injection moulding machine or extruder so that expansion and consolidation occur simultaneously. This approach has been used to produce expanded polystyrene sheet and paper by a tubular process reminiscent of that used with polyethylene. Bubble nucleating... 

PROBOSCIS Any of the various tubular processes or prolongations of the head or oral region of animals. 

TubE-FEET The small tentacle-like, flexible, tubular processes of starfishes. 

The HP tubular process is, as stated earlier, a continuous process and has been in use for decades in the chemical process industries. A simplified version of a generic HP tubular process is given in Figure 11.2 [1]. 

The goal of this LCA study is to examine the environmental burden of using PE film (made with the high-pressure tubular process) for food packaging. The results will be used to improve the environmental performance of the production process by changing, where possible, process parameters. This LCA will not compare the PE film made by the high-pressure tubular process with that made with the high-pressure autoclave process. 

G. Flouret. 1982. Renal tubular processing of small peptide hormones./. Lab. Clin. Med. 100 1-14. 

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

Figure 2.16. Nephron function, a Filtration occurs in the glomerulus. The filtrate is fuimeled into the tubule, b Schemat-ie of the blood vessel wall stiueture in the glomeralus. Both the endothelium within and the podocytes outside the arterioles have sUts and fenestrations that are a few nanometers wide. As in the eapillaries elsewhere in the body, the basal membrane functions as the sieve, e In the tubule and the colleeting duct, the filtrate is extensively post-proeessed water, substrates and ions are reabsorbed but also aetively secreted and exchanged. Tubular processing is under hormonal control.

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). 

Except for the reactor zones, autoclave and tubular processes are very similar (3, 4). Peripherals in both cases are designed pre-reactor to ramp pressures and temperatures to very high levels and post-reactor to reduce temperatures and lower pressures to near ambient conditions to enable product isolation. Simplified process flow diagrams for the autoclave and tubular processes are shown in Figures 7.1 and 7.2, respectively. 

Features - continuous stirred tank reactors with agitators are used - multiple reactor zones typically used - polymerization takes place in "solution" - organic peroxides typically used as initiators - polymer has fewer branches but of somewhat longer length relative to tubular process... 

Table 7.3 Typical operating features of tubular processes for LDPE.

Figure 7.2 Process flow diagram for tubular process 90...

The nitrating acid in the Hercules Tubular Process is composed of 18-40% HNOs, 45-70% H5SO4 and 11-17% 0. The preferred acid to glycerine ratio... 

The renal clearance of a drug represents the net effect of glomerular filtration and the renal tubular processes that may be involved in the renal handling of the drug ... 

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