Chemistry of hydrocarbon compounds – Purification – separation – or recovery
Reexamination Certificate
2001-05-03
2002-11-19
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
C585S802000, C585S807000, C585S809000, C585S820000, C585S833000, C585S500000
Reexamination Certificate
active
06483000
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds.
BACKGROUND TO THE INVENTION
In a published concise rendition of a lecture: C L Render, Z Denga “Sasol Alpha Olefins” from the lecture series “New processes in chemical technology” at the ACHEMA Congress 1994, a process is described which permits the recovery of alpha-olefins and more particularly 1-pentene and 1-hexene in relatively pure form from the liquid fraction produced in the Fischer-Tropsch-Synthesis. Since a large number of other compounds of various types boil in the immediate vicinity of the boiling temperature of the desired alpha-olefins, of which some even form azeotropic mixtures with the desired alpha-olefins, and although the distillative separation of interfering contaminants represents an important process step, it is nevertheless not possible with acceptable effort to attain the desired purity of the alpha-olefin product solely by distillation.
Accordingly, tertiary olefins are first converted with methanol into ethers in a pentene or hexene fraction pre-concentrated by distillation. This etherification step is followed in a so-called superfractionation by a sharp distillative separation of all the components which are either lower or higher boiling than the desired alpha-olefin. Jointly with the lower boiling components excess methanol derived from the etherification step is also separated. From this low boiling fraction the methanol is extracted with water and is recovered from the aqueous extract by a distillative methanol/water separation.
Jointly with the higher boiling components the ethers formed in the etherification step are likewise separated.
The alpha-olefin crude product of superfractionation has a typical purity of about 90%. It now only contains components which boil very close to the desired alpha-olefin and which, therefore, can be separated by normal distillation only with extremely great efforts. However, all these interfering components differ from desired alpha-olefins by their polarity and may, therefore, be separated relatively easily in the polar medium of the extractive distillation succeeding the superfractionation. The extractive distillation is performed with aqueous NMP (N-methyl-2-pyrrolidon) as a solvent. Accordingly, the alpha-olefin final product must still be dried. This is done by a distillative separation of the water.
An improvement of this known process for the recovery of alpha-olefins was filed as a patent application at the German Patent Office under file number 197 23 049. An important integer of this improved process resides in the provision of the etherification reactor downstream of the distillative fine separation of all lower boiling components (lower boiling than the desired alpha-olefin).
Due to a reaction equilibrium arising in the etherification reactor, it is not possible to attain a complete conversion of the tertiary olefins into the corresponding ethers. Depending on the original content of tertiary olefins there accordingly arises a maximum attainable purity of the alpha-olefin product, hardly exceeding 99%—even after practically complete removal of all other impurities. (Here and in what follows the purity is stated in percentages by mass). This applies to the process according to C L Render and Z Denga as well as to the improved process (197 23 049.0).
The process described by C L Render and Z Denga is well suited to recover from the fractions obtained in the coal liquefaction according to Fischer-Tropsch, e.g. 1-hexene with a purity of 98.5%. Higher purities can only be attained with progressively increasing yield losses. The reason therefor resides in the tertiary olefins accompanying the feedstock and the boiling points of which are close to 1-hexene and which, therefore, can virtually not be separated off by conventional separating methods. These tertiary olefins must be converted in the reactor to separable compounds. In a singe-stage reactor the degree of conversion is limited, however, by the chemical equilibrium so that the maximum attainable purity of the 1-hexene product is ultimately limited by the reaction equilibrium.
In order to overcome this limitation regarding attainable purity to about 99% a process is proposed in DE 198 25 295 A1.
From a mixture containing predominantly hydrocarbon compounds, as obtained in Fischer-Tropsch-Synthesis, alpha-olefins are recovered after an at least crude separation of components boiling higher and/or lower than the alpha-olefin. For that purpose tertiary olefins of the mixture boiling close to the alpha-olefin to be recovered are subjected to a catalytic etherification after a superstoichiometric addition of a low alcohol. A stream of ethers and other high boiling reaction products recovered from the etherification together with the alpha-olefin is forwarded to a distillative separation of components boiling higher than the olefin.
The etherification is performed in multiple stages, at least in two stages. After each etherification step etherification products are separated as residue streams, where applicable jointly with other higher boiling components.
In addition, it is proposed to separate the low boiling alcohol as an azeotropic mixture after the etherification and separation of the etherification products in a further distillation step and to subject the olefin fraction so obtained when desired or required to further separation steps, e.g. an extractive distillation and/or an adsorption.
The process disclosed in DE 198 25 295 A1 is directed at recovering by the same method a variety of alpha-olefins, in particular, however, 1-hexene or alternatively 1-pentene having a purity in excess of 99%.
Although the problems of purifying e.g. 1-hexene or 1-pentene are similar, they are not identical. Accordingly, the process according to the state of the art is not equally suitable for the recover of whatever alpha-olefin.
Accordingly, a need exists for a more simple process than that in the prior art for the recovery of 1-pentene and alpha-olefins boiling lower than 1-pentene.
SUMMARY OF THE INVENTION
The invention provides a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds, such as is obtained by Fischer-Tropsch-synthesis after an at least crude separation of components boiling higher and/or lower than the alpha-olefin, wherein tertiary olefins of the mixture, after super-stoichiometrical addition of a low alcohol, are subjected to catalytic etherification and a stream derived by etherification is fed jointly with the alpha-olefin and the ethers produced and other high boiling reaction products to a distillative separation of components boiling higher than the olefin.
Thus, according to a first aspect of the invention, there is provided a process for the recovery as a feedstock of an alpha-olefin from a mixture containing mainly hydrocarbon compounds, such as is obtained by Fischer-Tropsch-synthesis after an at least crude separation of components boiling higher and/or lower than the alpha-olefin, wherein tertiary olefins of the mixture, after super-stoichiometrical addition of a low alcohol, are subjected to catalytic etherification and a stream derived by etherification is fed jointly with the alpha-olefin and the ethers produced and other high boiling reaction products to a distillative separation of components boiling higher than the olefin, in which from the mixture, 1-pentene or an alpha-olefin lower boiling than 1-pentene is recovered, wherein
after a distillative fine separation of substances lower boiling than the alpha-olefin to be recovered from the mixture the etherification is performed with excess methanol in a single or a plurality of stages;
during the distillative separation of the components higher boiling than the alpha-olefin from the stream derived by the etherification a by-product is separated, comprising ethers and other components higher boiling than the alpha-olefin and a stream comprising the alpha-o
Griffin Walter D.
Knobbe Martens Olson & Bear LLP
Nguyen Tam N.
Sasol Technology (Pty) LTD
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