Paper to the Mixing and Polymer Processing Conference of the European Federation of Chemical Engineering, Delft, Holland

S F Bush with E Jongen.

Introduction

A major problem in the manufacture of a number of important polymers such as polyethylene, polyester and nylon, is the occurrence of non uniformities which show up as streaks and blobs in film and cause breaks in filament. The major source of this problem lies in the fact that during manufacture a tiny fraction of the material is exposed to reaction conditions very significantly different from those applying to the bulk of the material, by virtue of the exceptional residence times developed at vessel walls. Such non uniformities are particularly likely to occur at cooling surfaces since the unavoidable tendency to stick there is enhanced by the increase in viscosity through the thermal boundary layer.

The present paper outlines a description of the reaction and flow of polyethylene and ethylene through tubes from which there is a substantial extraction of heat. The driving force is provided by pressure drop.

In the high pressure stirred autoclave process, the reaction in the tube is incidental to its main function of cooling the polymer and reducing pressure (by about 400 atmospheres) at the outlet from the autoclave. In the tubular process, the overall pressure drop through a tube is greater (around 2000 atmospheres), since the tube is itself the main reactor. The chemical kinetics and fluid mechanics equations have been set up to cover both cases, but the applications referred to in this paper arise from the autoclave process.

See also the section on Applications to Existing Products and Processes.

GB Patent 1,422,781 28th January 1976; US Patent 3,969,449 13th July 1976.

M J Shires and S F Bush

Abstract

A heat sensitive liquid is defined as a liquid which degrades, usually to carbon, at temperatures below its normal vaporisation temperature. This patent* describes an invention whereby a heat sensitive liquid is vaporised with exceptionally low thermal degradation.

Many such liquids, for example light naphthas, used to make olefines, are vaporised in tubes in which the liquid passes from pure liquid to pure vapour through a two-phase vapour core-liquid film flow regime (qv). Because the organic liquid droplets are always present in the vapour, they impinge on the hot tube walls downstream of the last of the liquid film, degrade to carbon and then by diffusion into the steel tubes making them brittle and appreciably shortening their lives, with all that implies in terms of furnace shutdown and repairs.

While hydrocarbon liquids do not degrade appreciably to carbon below cracking temperatures (800+ ^oC), hydroxyocarbon liquids degrade at much lower temperatures, typically 250 ^oC. An important example of this is the dicarboxylic adipic acid (CH₂)₄(CO OH)₂ which is needed to be mixed with ammonia NH₃ in 1:2 molar proportions over a solid catalyst to make adiponitrile (CH₂)₆(NH₂)₂ the last step before polymerisation with adipic acid to make nylon poly(hexamethylene amide) – in effect a synthetic protein with a single repeating acid group.  All these processes we carried out on the largest industrial of scales, typically 2-3 tonnes per hour.

Carbon and tarry residues formed at the adipic vaporisation stage have deleterious effects on the solid catalyst at the adiponitrile stage.  Shortening its life, requiring around 700 tubes to be recharged at often monthly intervals, causing excessive lost production time and cost of catalyst replacement.

The invention provides a general principle of design by which droplets of liquid do not impinge to measurable degree on heating surface above the degradation temperature in the adipic acid case 250 ^oC which is some 60 ^oC below the one atmosphere boiling temperature.  This is achieved by combining two technologies:

1. liquid (adipic acid) atomisation and
2. internal jet flows (qv) produced by ammonia gas needed for the next, synthesis of adiponitrile, stage.

The patent describes how the ammonia jet in a relatively compact vaporiser (about 1.3 metres in diameter for 2 tonnes per hour of adipic acid) creates a circulation rate of some 20-30 times the ammonia flow, such that unvaporised adipic acid droplets from their entry (atomisation) point at 180 ^oC are never in the vicinity of the heating surfaces (typically 450-480 ^oC).

This innovation demonstrates again the versatility of the jet principle not only in process engineering, but also in the design of hydrocarbon combustion proceses, especially internal combustion chambers which also face ever-tightening demands to reduce carbon and tarry particulate emissions (see papers on diesel engine combustion, by Bush, Chen and Whitehouse).

Footnote

* These two patents are basically the same although GB 1,422,781 describes two alternatives.

To see the US patent in full, go to US Patent Office. On the first page click on USPTO Patent Full-Text and Image Database (PatFT), then under the heading “Searching Full Text Patents (since 1976)”, click on Patent Number Search and enter the patent number (with or without commas) into the “Query” box, then click on “search”. To search for another US patent, click on Pat Num in the red display at the top of the page.

Group II Research Note, ICI Corporate Laboratory

S F Bush with P Dyer.

Summary

A short historical account of the CL-B continuous nylon polymeriser project is given to record (a) its achievements and (b) the lessons learnt in the conduct of such a project. The report is organised in sections each of which summarises a major result of decision. An annotated bibliography covering both CL-B and Divisional sources is given.

Group II Research Note, ICI Central Instrument Research Lab.

S F Bush

Summary

Experimental studies of the terylene polymerization reaction in thin films have shown that, although the back reaction is known to occur, the overall rate of reaction appears nonetheless to vary as the square of the polymer ends concentration and inversely as the film thickness (Ref 2).

This note explains this observation in analytical terms and determines the relative importance of liquid diffusion and vapour diffusion rates in the achieved rates of polymerization. The results appear to apply to both Terylene and Nylon 6.6 polymerizations.

References

[1] S F Bush, M J Shires, M A L Sita-Lumsden, CIRL Research Note, 5th October 1970, “Some results from a study of the possible applications of mathematical models to the continuous terylene processes”.

[2] B R Hefford, K Porter, 11th June 1968, “Terylene fast polymerization kinetics . . .”