Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By contact with solid sorbent
Patent
1998-10-21
2000-08-29
Griffin, Walter D.
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By contact with solid sorbent
585820, 585823, 585826, 208263, 208299, 208305, 208307, C07C 712, C10G 2505, C10G 2512, C10G 2500
Patent
active
061111627
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process for selectively removing oxygenated contaminants from hydrocarbon streams.
The presence of oxygenated impurities in these streams is generally extremely harmful, even at a level of tens part per million, especially when these streams must be sent to other reaction steps.
Olefinic cuts with four and five carbon atoms are very often subjected to these problems. In fact, for example, it is well known that iso-olefins react with R--OH alcohols (preferably methanol) to give the corresponding methyl teralkyl ethers (MTBE, TAME). After separation of the oxygenated products, the exhausted streams without iso-olefins (Refined products) can be sent to alkylation, if the oxygenated products are present in quantities of less than 10 ppm, to avoid abnormal consumption of the catalyst. The oxygenated products present in these cases are the corresponding ter-alkyl alcohols, obtained by the acid-catalyzed addition of water to the iso-olefin and alkyl-teralkyl ethers, generally deriving from impurities in the charge--for example MTBE in C.sub.5 cuts as it is extremely costly to obtain a C.sub.5 olefin stream without isobutene and also the boiling point of MTBE is very close to that of C.sub.5 hydrocarbons.
Another case in which oxygenated products are harmful is in the polymerization of iso-olefins, preferably isobutene with a high purity obtained by the decomposition of the corresponding alkylether, i.e. MTBE. Also in this case the total oxygenated products (methanol, dimethylether, water) must be less than 10 ppm.
Oxygenated products, on the level of impurities, are generally harmful in processes using zeolites owing to their great affinity. Competitive adsorptions can in fact arise which reduce the overall efficiency of the process.
The art discloses various methods for removing these oxygenated products. In particular EP-504980 can be mentioned wherein the teralkyl-alkyl ethers and corresponding alcohols (MTBE, TAME, TBA, TAA) are removed from C.sub.5 streams in the synthesis of TAME by catalytic cracking on suitable material based on silica with small quantities cc alumina, at temperatures of between 200 and 250.degree. C. In this case iso-olefins are obtained and the corresponding oxygenated product, methanol or water, which must then in turn be removed. It is evident that this system can only be applied when there is the possibility of the selective breaking of a C--O bond to give well-defined chemical species. Dimethylether and methanol for example do not belong to this group.
A process has been surprisingly found using a material which combines a high adsorbing capacity (molecules retained per unit of the adsorbent mass under conditions of equilibrium) for oxygenated compounds and a high adsorption rate of these molecules (molecules adsorbed per unit of time), also allowing said material to be easily and completely regenerated. This latter aspect, although not indicated in the art cited above, is of fundamental importance in applying the method on an industrial scale.
The process for selectively removing oxygenated contaminants from streams prevalently containing hydrocarbons with from 3 to 8 carbon atoms, of the present invention, is characterized in that it comprises an adsorption step wherein said oxygenated compounds are adsorbed with an adsorbent essentially consisting of silica gel, carried out at a temperature of between 0 and 150.degree. C. and a pressure of between 1 and 20 atms, and a regeneration step for removing the substances adsorbed by means of thermal treatment in a stream of inert gas, carried out at a temperature of between 100 and 200.degree. C., with the proviso that the oxygenated contaminant is not water.
The inert gas used in the thermal treatment can be selected from gases normally used for carrying out regenerations, such as nitrogen, helium, steam, flue gas, air, etc.
The silica gel used can have a surface area preferably higher than 300 m.sup.2 /g, more preferably higher than 400 m.sup.2 /g, and a porous volume preferably of between 0.38 and 1.
REFERENCES:
patent: 2653959 (1953-09-01), Moore et al.
patent: 2719206 (1955-09-01), Gilmore
patent: 4404118 (1983-09-01), Herskovits
patent: 5245107 (1993-09-01), Yon et al.
patent: 5466364 (1995-11-01), Kaul et al.
Piccoli Valerio
Rossini Stefano
Griffin Walter D.
Nguyen Tam M.
Snamprogetti S.p.A.
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