Vaporisation Process
January 28th, 1976
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:
- liquid (adipic acid) atomisation and
- 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.