Method for selectively separating hydrocarbons

Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Treating polymer containing material or treating a solid...

Reexamination Certificate

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C528S480000, C521S027000, C210S654000, C210S653000, C210S650000

Reexamination Certificate

active

06252038

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for separating certain hydrocarbons from a mixtures of hydrocarbons. More specifically, the present invention relates to a method for separating and concentrating specific hydrocarbons from a mixture of hydrocarbons. This is a problem commonly found in the petroleum refining industry or petrochemical industry.
2. Description of the Prior Art
In the petroleum refining industry and the petrochemical industry, methods for separating certain hydrocarbons from mixtures of hydrocarbons by using a separating membrane have been under scientific and economical development for many years. To this date, many approaches have been published. For example, U.S. Pat. No. 2,958,656 proposes a method wherein a mixture of hydrocarbons (that is, naphtha) is supplied to a non-porous cellulose ether membrane, through which one portion of the hydrocarbons is permeated. Subsequently, the permeated hydrocarbons are removed using a washing gas or a washing liquid, thereby separating unsaturated compounds, saturated compounds and aromatic compounds. U.S. Pat. No. 2,930,754 proposes a method wherein one portion of the mixture, having a distillate temperature approximately equal to the boiling point of gasoline, is selectively permeated through a non-porous cellulose ether membrane. The permeated hydrocarbons are then removed using a washing gas or a washing liquid, thereby separating unsaturated hydrocarbons and other hydrocarbons such as aromatic compounds. Many fluorine-containing polyimides are known to be materials for membrane separation with excellent heat resistance and gas permeatability. Aromatic polyimides containing fluorine are disclosed, for example, in Publication of Unexamined Japanese Patent Application (Tokkai) No. Hei 5-7749, U.S. Pat. No. 3,822,202, U.S. Pat. No. 3,899,309, U.S. Pat. No. 4,532,041, U.S. Pat. No. 4,645,824, U.S. Pat. No. 4,705,540, U.S. Pat. No. 4,717,393, U.S. Pat. No. 4,717,394, U.S. Pat. No. 4,838,900, U.S. Pat. No. 4,897,092, U.S. Pat. No. 4,932,982, U.S. Pat. No. 4,929,405, U.S. Pat. No. 4,981,497, and U.S. Pat. No. 5,042,992.
However, these conventional methods have an insufficient resistance of most of the separating membranes against aromatic hydrocarbons, unsaturated hydrocarbons, saturated hydrocarbons, etc. and have difficulties isolating specific hydrocarbons. Therefore, their use on an industrial scale is severely limited because of poor performance and high costs.
SUMMARY OF THE INVENTION
It is an object of the present invention to solve these problems, and to provide a method for selectively separating certain hydrocarbons from a mixture of hydrocarbons with a membrane that is highly resistant against hydrocarbons, and has excellent ability to separate specific hydrocarbons etc. from a mixture of hydrocarbons. Furthermore, the membrane should be of low cost and easy to implement.
In order to achieve these objects, a method for selectively separating hydrocarbons in accordance with the present invention comprises contacting a mixture including a hydrocarbon with one surface of a separating membrane having as its main component a fluorine-containing polyimide resin whose fractional free volume is in the range of 0.130-0.175; and passing the mixture through the separating membrane, so that a particular hydrocarbon permeates selectively and is thereby separated.
In the present invention, the fractional free volume (FFV) of the fluorine-containing polyimide resin is determined from
FFV=(V
298
−V
0
)/V
298
  Equation 1
wherein V
298
is the molar volume of the polyimide resin at 25° C. This is determined by dividing the molecular weight of the unit structure of the polyimide resin by the density of the polyimide resin at 25° C. V
0
is the molar volume of the molecule at 0 K (zero Kelvin) as determined by the Bondi formula (see Equation 1) and is equal to the van-der-Waals volume of the polyimide resin multiplied by 0.130. If the fractionl free volume of the polyimide resin is less than 0.130, the molecular chain packing becomes excessively strong, and there is the danger that the permeability for hydrocarbons becomes too small, which is not preferable. Furthermore, if the fractional free volume of the polyimide resin is higher than 0.175, the plasticization due to the hydrocarbons becomes considerable, which has the effect of decreasing the ability to separate decreases, which is also not preferable.
A separating membrane whose main component is a fluorine-containing polyimide resin with a fractional free volume of 0.130-0.175 can be prepared as explained in the examples below.
In the above-noted method, it is preferable that the certain hydrocarbon, which is permeated selectively and separated, includes at least one hydrocarbon selected from the group consisting of unsaturated hydrocarbons and aromatic hydrocarbons. In this method, it is also preferable that a structural unit of the repeating molecular structure constituting the fluorine-containing polyimide resin comprises at least one —CF
3
group. Furthermore, in this method, it is preferable that a main component of the fluorine-containing polyimide resin is a repeating molecular structure that is substantially expressed by
wherein
A
1
and A
2
are tetravalent organic groups comprising an aromatic, alicyclic or aliphatic hydrocarbon,
R
1
and R
2
are divalent aromatic, alicyclic or aliphatic hydrocarbon groups, or divalent organic groups wherein these hydrocarbon groups is bonded with a divalent organic group,
at least one of the organic groups A
1
and A
2
comprises at least three fluorine atoms;
m and n indicate the polymerization units;
0<m<1, 0<n<1, and m+n=1; and
the main component is either a random copolymer or a block copolymer.
It is preferable that the weight-average molecular weight of the polymer in Formula 1 is in the range of 30,000-1,000,000.
The fluorine-containing polyimide resin used in the present invention contributes to the ability to separate specific hydrocarbons, and is characterized in that its fractional free volume is in the range of 0.130-0.175. The permeability of a gas through a homogenous polymer depends on the process of dissolving the gas into the polymer and the process of diffusing it throughout the polymer. In particular, permeability is commonly expressed by the product of the solubility coefficient of the gas into the polymer and the diffusion coefficient of the gas in the polymer. Therefore, utilizing the differences in solubility or diffusivity, a gas mixture can be separated into several components. When permeating a hydrocarbon with a carbon number of three (C
3
) or more, the macromolecules of the membrane material usually plasticize due to the permeated components, and as a result, the free volume of the macromolecules increases, thereby increasing the diffusivity. In the case of glassy macromolecules such as polyimide, excessive plasticization leads to a large increase in free volume, and as a result, the separation ability, i.e. the ability to sieve using the difference in the molecule size of certain permeation components, deteriorates. On the other hand, when there is no excessive plasticization due to the hydrocarbons to be separated, the permeability becomes too small, which is not desirable. The inventors of the present invention have concentrated on this aspect, and came to the result that if a fluorine-containing polyimide resin with a fractional free volume of 0.130-0.175 is used for the membrane material, extreme plasticization due to hydrocarbons can be averted and a separating membrane with high separation ability with regard to certain hydrocarbons can be obtained.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the present invention, there is no requirement that the tetravalent organic groups have at least three fluorine atoms as shown in the above-noted general formula (1), as long as the protons of the tetravalent organic groups A
1
or A
2
are substituted with fluorine atoms or groups including a fluor

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