Chemistry of hydrocarbon compounds – Purification – separation – or recovery – By plural serial diverse separations
Reexamination Certificate
2000-12-12
2002-01-15
Griffin, Walter D. (Department: 1764)
Chemistry of hydrocarbon compounds
Purification, separation, or recovery
By plural serial diverse separations
C585S843000, C585S845000, C585S849000, C585S850000, C585S848000, C208S219000, C208S223000
Reexamination Certificate
active
06339182
ABSTRACT:
FIELD OF THE INVENTION
The present invention is in the field of organic separations, in particular, separation of olefins from non-olefins such as paraffins, oxygenates and aromatics.
BACKGROUND OF THE INVENTION
There are many product streams, particularly in the field of petroleum chemistry, which include olefins and non-olefins. For example, ethane crackers tend to produce a mixture of ethane and ethylene. The ethylene is typically separated from the ethane via distillation. The boiling points of ethylene and ethane are relatively close to one another, and the distillation is typically done at very low temperatures and/or high pressures. This makes the separation relatively expensive. The same problems are observed when separating propane from propylene in dehydrogenation facilities.
Fischer-Tropsch chemistry tends to provide a variety of gaseous and liquid products, including unreacted synthesis gas, methane, and C
2-4
hydrocarbons (a mixture of olefins and paraffins). These gases are typically separated from the liquid products. The methane and other light paraffins can be recycled through an upstream synthesis gas generator. However, the light olefins tend to coke the catalysts, and need to be removed before the recycle gas is sent to the synthesis gas generator. The olefins are typically removed via cryogenic separation.
Typically, about 75% of the C
2-8
products from Fischer-Tropsch synthesis are normal alpha-olefins (NAOs). Separation may be accomplished using conventional distillation. However, there is only a small difference in relative volatility between an olefin and the corresponding paraffin with the same number of carbons. Therefore, distillation requires a large number of stages and/or high reflux ratios. While the olefins are extremely commercially valuable, the commercially available method for separating them is expensive.
Silver and copper salts have been dissolved in aqueous solutions and used to complex olefins. This technology has been proposed for use in separating olefins from paraffins (See, for example, U.S. Pat. No. 5,859,304 to Barchas et al., the contents of which are hereby incorporated by reference). However, there are a number of disadvantages to this approach. Since water has a significant vapor pressure, some water will go into the olefinic product during the regeneration step. The water must therefore be removed from the olefins product. This effect is exasperated since thermal regeneration is one of the preferred methods of regeneration. The high vapor pressure of water also limits the maximum regeneration temperature that can be used. Sweep gases are also sometimes used in the regeneration process, and these will further increase the evaporation of water. Also, water lost in the regeneration step must be replaced and the proper balance of water to complexing agent must be maintained.
It would be advantageous to provide new methods for separating olefins from paraffins. The present invention provides such methods.
SUMMARY OF THE INVENTION
Methods for separating olefins from non-olefins, such as paraffins, including cycloparaffins, oxygenates and aromatics, are provided. The methods use metal salts to complex the olefins, allowing the paraffins to be separated by a variety of methods, including decantation and distillation, preferably extractive distillation. The metal salts are incorporated, e.g., dissolved, in ionic liquids, which tend to have virtually no vapor pressure, and which poorly solubilize paraffins and other non-olefins. Accordingly, the non-olefins phase separate well, and can be decanted or distilled without carrying over any of the ionic liquid.
The olefins can be recovered from the ionic liquids by a number of regeneration options, including any combination of thermal regeneration (increasing the solution temperature to reverse the complexation) and pressure swing regeneration (reducing the pressure to reverse the complexation). Sweep gases may also be used in the regeneration step. Preferred olefin-complexing metal salts are Group IB salts, more preferably silver salts. References to the Periodic Table follow the 1975 rules of the International Union of Pure and Applied Chemistry. A preferred silver salt is silver tetrafluoroborate. Preferred ionic liquids are those which form stable solutions or dispersions of the metal salts, and which do not dissolve unwanted non-olefins. Further, if the olefins are subject to isomerization, and this is undesired, the ionic liquid is preferably relatively non-acidic.
The methods involve forming a solution or dispersion of a suitable olefin-complexing salt in an appropriate ionic liquid. An olefin-containing mixture is contacted with the ionic liquid/salt solution or dispersion, and the olefins are adsorbed. After the non-olefins are removed, the olefins can be isolated by desorption. The olefin-containing mixture can be in the gas phase or in the liquid phase. The flow of olefin-containing mixtures over/through the ionic liquid can be for example, co-current, counter-current, or staged in stirred tanks. Countercurrent is preferred as it is the most efficient.
Silver complexes can be poisoned by various compounds, including sulfur compounds, cyanides and acetylenes. Silver acetylides also pose potential risk of explosion. Accordingly, these compounds should be removed before the mixtures are brought into contact with the ionic liquids. Methods for removing such contaminants are well known to those of skill in the art. One method for removing acetylenes involves complexing the acetylenes with a nickel salt. The resulting nickel acetylides are not explosive. While nickel also complexes olefins, it does so to a significantly lesser degree. Accordingly, a nickel salt can be added to the ionic liquid to form a solutions, suspension or dispersion, acetylides formed, and the solution regenerated in the same ways described above. Alternatively, selective hydrogenation can be used to convert the acetylenes to olefins.
Nickel has been shown to form reversible complexes with acetylene. Unlike silver, Ni does not form the explosive acetylide compound. Dissolving Ni into ionic liquids produces a contacting solution that can be used to safely remove acetylene from olefin streams. While Ni will also complex olefins, it does so to a smaller degree. The solution can be regenerated in the same ways described above.
The methods described herein can be optimized using combinatorial chemistry.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to compositions and methods for separating olefins from non-olefins. The compositions include solutions or dispersions of metal salts, which form complexes with olefins, in ionic liquids. The methods involve contacting a mixture including olefins and non-olefins with a solution of an olefin-complexing metal salt in an ionic liquid. After the non-olefins are separated, the olefins can be removed/desorbed from the ionic liquid.
Olefins
The olefins are preferably C
2-50
mono-olefins, more preferably C
2-20
mono-olefins. They may include other functional groups, such as hydroxy groups, carboxylic acid groups, heteroatoms, and the like, provided that such groups do not react with either the olefin-complexing salt or the ionic liquid.
These mono-olefinic compounds can be used commercially to form a wide variety of petrochemicals. For example, these compounds can be used to form polyethylene, polypropylenes, polyisobutylene and other polymers, alcohols, vinyl chloride monomer, acrylonitrile, methyl tertiary butyl ether and other petrochemicals, and a variety of rubbers such as butyl rubber.
Sources of Olefins
The olefins can be derived from a variety of sources. A major source is Fischer-Tropsch synthesis. In Fischer-Tropsch synthesis, the product includes olefins, paraffins and oxygenates. The oxygenates are mostly alcohols, which can be converted to olefins via dehydration over a suitable catalyst. Other commercial sources include wax thermal cracking, the Paragons™ process, ethylene and propylene derived from ethane, ethane/propane (EP cracker) an
Boudreau Laura C.
Driver Michael S.
Munson Curtis L.
Schinski William L.
Chevron U.S.A. Inc.
Griffin Walter D.
Nguyen Tam M.
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