Patent GB-810340-A (GB)
Title: Dehydrogenation of organic compounds
IPC: C07C5/56
Filed: 1957-06-27
Abstract: In the dehydrogenation of iodine-reactive hydrocarbons, hydrogen iodide free oxygen and water are injected into an iodine recovery zone comprising at least a first reaction zone where hydrogen iodide is oxidized to water and iodine, the resultant vapour is passed to a second reaction zone where it is reacted with an iodine-reactive compound while adding free oxygen to convert at least part of the hydrogen iodide formed to iodine and water, the vapour effluent is contacted within a maximum of five minutes in a solvent contact zone with a liquid aqueous solvent stream comprising water and hydrogen iodide, the resultant mixture is separated into an organic phase substantially free of hydrogen iodide and iodine and a liquid aqueous phase substantially free of organic compounds, the desired product is separated from the organic phase and the aqueous phase is divided into at least two portions, one of which is returned to the iodine recovery zone while the other is cooled and returned to the solvent contacting zone.  In Fig. 2 the zone A represents the first zone wherein hydrogen iodide is oxidized to iodine and water, the effluent is mixed with the organic feed which is dehydrogenated in zone B, the effluent vapours contacted with water containing hydrogen iodide and iodine in zone C and the liquid and vapour phases separated in zone D.  Part of the liquid is recycled to zone A and part cooled and recycled to zone C.  In a preferred embodiment a minor proportion is also passed to zone K where iodine is recovered and recycled to zone A and/or B, aqueous hydrogen iodide which is used for scrubbing the product vapours in zone F after which it is returned to zone K and water which is partly discarded to compensate for the water formed in zones A and B and partly used to scrub further the product vapours in zone G.  The product vapours may, if desired, be further caustic washed and water washed in zones H and I before passing to fractionation or other recovery system.  Alternatively the scrubbing liquid in zone F may be taken direct from the liquid phase separated in zone D.  In Fig. 3 the iodine recovery zone includes zones A and K, the latter being placed after zone F in the flow whereby iodine is separated from the effluent from zone A and passed to zone B, aqueous hydrogen iodide is separated and passed to zone F and/or recycled to zone A and water is discarded and/or passed to zone G.  The hydrogen iodide solution used in zone C, preferably contains at least 40 per cent by weight hydrogen iodide to permit it to absorb the iodine from the reaction effluent without exceeding an iodine to hydrogen iodide mol. ratio of 1 in the resulting fat solvent. Preferably, however, there is sufficient hydrogen iodide, e.g. the amount of hydrogen iodide is within 2 per cent by weight of the constant boiling mixture, in the solvent to ensure that the resultant solution contains at least 2 per cent by weight more hydrogen iodide than the constant boiling mixture and a vapour phase is then stripped from the solution containing more than 65 per cent by weight hydrogen iodide which is passed to zone A leaving approximately a constant boiling mixture which is recycled to zone C.  This is illustrated in Fig. 4 in an embodiment employing two solvent contacting and separation zones C and D, the liquid effluent from the second preferably being passed to stripper K although part from zone C may also be included. Similarly, the liquid effluent from zone C is preferably passed to zones E and L with the liquid from zone K although some from zone D may be included.  In zone L the liquid is treated to separate iodine and in zone M the aqueous hydrogen iodide is partially converted to iodine and water to break the constant boiling mixture so that water may be removed from the system.  A system in which only one solvent contact zone is used and in which aqueous hydrogen iodide recovered from zones L-M is recycled into the solvent stream is illustrated in Fig. 5.  Reaction zones A and B may be constructed as an integral unit employing similar conditions in each zone, e.g. 300-800 DEG  C. and 1-3 atmospheres. The amount of oxygen used should be such that in the effluent from zone A and at any point of injection in zone B there is present no more than 1 molecular weight of oxygen for each atomic weight of iodine present in all iodine species and is preferably between 0.025 and 0.50.  Zone A may contain inert packing such as quartz chips or a catalyst such <PICT:0810340/IV (b)/1> <PICT:0810340/IV (b)/2> <PICT:0810340/IV (b)/3> <PICT:0810340/IV (b)/4> as calcined diatomaceous earth or activated silica-alumina. The vessel must be constructed of material resistant to corrosion such as silicon carbide, fused kaolin, platinum, palladium or tungsten. The nominal residence time in zone B, i.e. that calculated by adjusting the flow for temperature and pressure conditions without adjusting for change in the number of molecules, may be up to 5 minutes but is generally 0.1-10 seconds.  A minimum of 0.1 mol. of iodine per mol. of organic feed is maintained to suppress side reactions.  Zones A and B may be adiabatically balanced, the heat supplied by oxidation of hydrogen iodide being equal to the heat required for dehydroganation of the feed and heat losses; extraneous fuel may be added if the hydrogen iodide is supplied as a liquid solution or when starting up.  The reactor effluent vapours should be passed to the solvent contact zone within as short a time as practicable, e.g. less than 1 second and without the temperature dropping below 300 DEG  C., this being particularly important when the product contains non-aromatic unsaturation.  The solvent contact zone may be constructed with fused alumina or graphite or ceramic lining or of glass on steel or acidproof brick over a plastic membrane on steel.  The temperature of the solvent introduced is preferably such that organic materials remain in the vapour phase while iodine and hydrogen iodide are removed in the liquid aqueous phase; the organic effluent should contain less than 10 weight per cent (3 mol. per cent) of hydrogen iodide and iodine.  The separation of iodine and water when necessary may be effected by distillation or by chilling and filtering.  The remaining parts of the apparatus may be constructed of platinum, palladium, tungsten, titanium, "Hastelloy" (Registered Trade Mark) C or other suitable corrosion resistant materials.  The invention may be applied to the dehydrogenation including aromatization of hydrocarbons such as described in Specifications 793,135, 793,214 and 793,215 including ethane to ethylene and acetylene; ethylene to acetylene; propane to propylene and methylacetylene or allene; n-butane to 1- and 2-butenes and 1,3-butadiene; n-butenes to 1,3-butadiene; n-pentane to 1- and 2-pentenes and 1,3-pentadiene; n-pentenes to 1,3-pentadiene; isopentane to 3-methyl-1-butene, 3-methyl-2-butene, 2-methyl-1-butene and isoprene; n-hexane to benzene; n-heptenes to toluene; methyl-hexanes to toluene; n-octane to ethyl benzene and o - xylene; methylheptanes or -heptenes to xylenes; 5-methyl nonane to n-butylbenzene, propyltoluene, 1,4-dimethyl - 2 - ethylbenzene, m - ethyltoluene, p-xylene and toluene; cyclopentane to cyclopentene and cyclopentadiene; cyclohexane to benzene; methylcyclohexane to toluene; 1,4-dimethylcyclohexane to p-xylene; ethylbenzene to styrene and n-propyl benzene to b -methylstyrene.  Specification 798,214, [Group XXIX], also is referred to.