GB2169301A - Water-free hydrocarbon separation membrane and process - Google Patents

Abstract

Water-free membranes suitable for separating aliphatically unsaturated hydrocarbons from saturated hydrocarbons, particularly ethylene from ethane, comprise a separation barrier material of metal ions dissolved in one or more polyhydric alcohols incorporated into the pores or on the surface of a porous support. The water-free membrane provides for facilitated transport of the unsaturated hydrocarbons across the membrane by virtue of the metal ions in the separation barrier material interacting selectively and reversibly with the unsaturated species.

Description

SPECIFICATION Water4ree hydrocarbon separation membrane and process Background of the invention This invention relates to water-free separation membranes for separating unsaturated hydrocarbons from saturated hydrocarbons. In another aspect, the invention relates to a process for utilizing a waterfree membrane comprised of a separation barrier material of metal ions dissolved in polyhydric alcohols incorporated into the pores or on the surface of a porous support to separate aliphatically unsaturated hydrocarbons from fluid hydrocarbon mixtures. The invention is especially useful for separating ethylene from gaseous mixtures containing ethylene and other hydrocarbons, for example, ethane, methane and hydrogen in the absence of water.

Considerable commercial interest exists for separating various aliphatically unsaturated hydrocarbons from mixtures of hydrocarbons. Aliphatically unsaturated hydrocarbons are reactive materials that serve various roles in the chemical and hydrocarbon industry, generally as intermediates in chemical synthesis.

A number of unsaturated hydrocarbons are employed as monomers in the formation of polymers and in this regard, olefins such as ethylene, propylene, butadiene and isoprene are well known. Aliphatic unsaturated hydrocarbons are most frequently available on a commercial basis in admixtures with other chemical compounds. These unsaturated hydrocarbon-containing streams are usually by-products of chemical synthesis or separation processes. When the hydrocarbon streams are liquid under normal conditions or can readily be liquified, ordinary distillation techniques are used to separate the hydrocarbon components provided they have sufficiently different boiling points for the process to be economically feasible.However, distillation may not be an attractive separation procedure when the hydrocarbon mixtures contain materials having close boiling points which is often the case with hydrocarbons of the same number of carbon atoms or having a difference of only one carbon atom. In these cases, different separation processes must be used which are frequently costly and involve operations such as solvent extraction, extraction distillation, cryogenics and the like.

When the aliphatically-unsaturated hydrocarbons and mixtures thereof with other similar boiling point hydrocarbons are in essentially the gaseous state at ambient temperatures and pressures, separation of the desired component from the mixture may be even more troublesome. In these situations, cryogenic processes may be used, however, frequently expense is prohibitive. These difficulties have created the need for use of semipermeable membranes for separating unsaturated hydrocarbons from saturated hydrocarbon mixtures.

Facilitated transport is a membrane process in which one species of a feed mixture is transported across a membrane preferentially by virtue of a component of the membrane interacting specifically and reversibly with that species. The facilitated aliphatically-unsaturated hydrocarbon transport process most thoroughly studied has been the separation of ethylene from its mixtures with saturated hydrocarbons.

Unsaturated hydrocarbons have been known to interact with silver ions. The kinetics and equilibria of this reaction are such that it can be utilized to facilitate membrane transport. Use of metal ions, for example, silver ions has been taught for facilitated transport of unsaturated hydrocarbons; however, in all these teachings, the systems require water to both impregnate the membrane and to saturate the feed gas in order to prevent drying of the membrane. It is thought that the use of aqueous saturation is an absolute requirement for the success of the facilitated transport process, for example, the separation of ethylene from ethane. It is assumed that the reason for this need for the presence of water is that water acts as a solvent for the silver salts and/or provides the necessary electronic environment for the silver ion.The high volatility of water makes this requirement a major limitation to the process. Accidental drying of the membrane irreversibly destroys its separation capacity.

Combinations of liquid barrier permeation involving metal complexing techniques have been utilized in separating aliphatically-unsaturated hydrocarbons from mixtures of other hydrocarbon materials. These systems provide for a liquid barrier as a continuous, distinct or separate liquid phase adjacent to and in contact with a semipermeable film membrane which is relatively non-selective with respect to the passage of the components of the hydrocarbon feed mixture. In addition, systems are known which are directed to methods for separating various materials from mixtures of aliphatically-unsaturated hydrocarbons and other hydrocarbons involving the combined use of liquid barrier permeation and metal complexing techniques which exhibit selectivity for the unsaturated hydrocarbons.In these processes, the liquid barrier is an aqueous solution having dissolved therein metal ions which will complex with the component to be separated. The liquid barriers are employed in contact with semipermeable membranes which are essentially impermeable to the passage of the separation barrier. The selectivity and separation ability of these aqueous barrier, metal ion containing systems can be rendered ineffective over prolonged use due to the loss of water from the barrier membrane system. The selectivity and separation ability of these aqueous, metal ion membranes decreases upon drying, thus the need for addition of water, generally through mixture with the feedstream.

Summary of the invention An object of the present invention is to provide a membrane and a process for separating aliphatically unsaturated hydrocarbons from hydrocarbon mixtures wherein the membrane is water-free.

Another object of the present invention is to provide a permeable membrane system which is simpler to operate and less prone to failure due to the lack of dependence on water.

These objects are obtainable according to the present invention by dissolving metal ions in polyhydric alcohols in high concentrations and incorporating these solutions into the pores or on the surface of a porous support. The resulting separation membrane is suitable for selectively permeating unsaturated hydrocarbons such as ethylene through the barrier material which is comprised of metal ions dissolved in polyhydric alcohols. For purposes of describing the invention, polyhydric alcohols are defined as compounds containing more than one hydroxyl group, inclusive of monomeric and polymeric materials.The barrier material can form the membrane on the porous support or an effective membrane incorporated into the pores of the porous support wherein the polymers or materials of the porous support are substantially neutral as to influencing the selectivity or permeability of the aliphatically-unsaturated hydrocarbon permeate.

The present invention provides a simple means to obviate the need for use of water in the separation membranes or the feedstreams wherein aliphatically-unsaturated hydrocarbons are separated from saturated hydrocarbons. The separating barrier materials of metal ions dissolved in polyhydric alcohols or polymers plasticized by the polyhydric alcohols as incorporated into the pores or on the surface of the porous support can be in the form of a liquid or a substantially solid barrier membrane. These barrier membranes, due to surface tension phenomena, can be formed on the surface of the porous support material when the material is hydrophobic in nature thus preventing the penetration of the barrier material into the pores of the porous support.On the other hand when hydrophilic type materials are used in the structure of the porous support the barrier materials can penetrate into the pores of the porous support thereby forming a barrier membrane of the metal ions and the polyhydric alcohols in the pores of the porous support.

The novel features which are considered characteristic for the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specifc embodiments.

Description of the preferred embodiments Metal ion containing membranes, both liquid and solid, can be operated as water-free systems when comprised of the metal ions in solution with polyhydric alcohols or in polymer polyhydric alcohol mixtures, the barrier separation membranes being incorporated into the pores or on the surface of the porous support materials. These metal ion containing polyhydric alcohol membranes provide for the separation of aliphatically-unsaturated hydrocarbons from mixtures of saturated hydrocarbons wherein the barrier membrane containg the metal ions provides for chemical selectivity for the unsaturated hydrocarbon permeation. Metal salts, for example, silver salts such as nitrate or trifluoromethanesulfonate can be dissolved in polyhydric alcohols such as glycerol to high concentrations.These solutions can be incorporated into the pores or on the surface of the porous support as either thin membranes or barrier membranes incorporated into the pores. If the porous support materials are hydrophobic in nature, surface tension phenomena prevent the polyhydric alcohol metal ion barrier materials from penetrating the pores, thus generally forming a thin barrier membrane on the surface of the porous support when the polyhydric alcohol is polymeric. This surface tension relationship results in the formation of a coating barrier film or membrane which are reasonably thin and provide for acceptable permeability rates as well as the desired selectivity.The water-free separation barrier materials as solutions can be incorporated into the porous support thus forming separation barrier with the barrier material remaining either as a liquid or semi-solid or solid, and when fed by a dry, ethylene-ethane mixture, exhibits selectivities of from about 50 to 300 and permeabilities of from about 3 to about 10 x 10-10 cm3-cm cm2-sec-cmHg The low volatility of these polyhydric alcohols prevents their loss from the membrane and performance is generally stable over extended periods of time.

When hydrophobic materials are selected for the porous supports for the barrier membrane of metal ions and hydrophilic alcohols, i.e. the coating barrier is hydrophilic and can be formed on or adjacent to the porous support surface of the hydrophobic support, thus forming a hydrophilic surface in contact with the hydrophobic support material. The hydrophilic coating, barrier membrane may be formed on the surface of the hydrophobic porous support by, for instance, coating the hydrophilic barrier film on the hydrophobic porous substrate. The separation of aliphatically unsaturated hydrocarbons from, for example, gaseous hydrocarbon feed which contains saturated hydrocarbons mixed with the unsaturated hydrocarbons, results primarily from the function of the polyhydric alcohol barrier membrane which contains the complex forming metal ions.

The hydrophobic film support materials employed according to the invention are of the essentially solid, water insoluble, semipermeable type. The support material is not adequately selective with respect to passage of or permeation by the aliphatically-unsaturated hydrocarbon to perform the desired separation of the components in the feedstock. However, by having the support materials in contact with sufficient liquid or solid barrier materials which are formed into a thin film separation membrane, the physical passage of gas through the barrier membrane is reduced or prevented, and the components of the feedstream must therefore traverse the barrier membrane separation zone primarily by becoming part of and then being separated from the polyhydric alcohol, metal ion complex barrier membrane. Preferably, the polyhydric alcohol used to form the thin membrane is polymeric.Thus, in the absence of complexing metal ion in the polyhydric alcohol mediu, there could be a slight or no separation of the hydrocarbons affected by the hydrophobic porous support materials. According to the present invention, however, the selectivity of the separation of aliphatically-unsaturated hydrocarbons is greatly increased due to the presence of the complexing metal ions in the polyhydric alcohol barrier membrane.

In the operations of the invention, the polyhydric alcohol, metal ion barrier membrane is useful for gaseous, liquid or mixed fluid feedstocks. The thin polymeric, polyhydric alcohol membrane can also include other polymeric materials which add strength to the thin membrane without interfering with the membrane separation function. The barrier membrane is sufficiently thin that the unsaturated hydrocarbons pass through the barrier membrane at reasonable permeability rates which are thickness dependent while the selectivity is controlled in part by the metal ion concentrations.

The barrier membrane contains sufficient alcohol soluble metal ions to form a suitable complex with at least one aliphatically-unsaturated hydrocarbon component of the fluid feed. The metal ions readily form the complex upon contact with the fluid feed and, in addition, the complex disassociates back to metal ion and an aliphatically-unsaturated hydrocarbon component of the complex under the conditions which exist at the discharge side of the barrier membrane. The released aliphatically-unsaturated hydrocarbons exit the discharge side of the barrier membrane and flow freely through the porous support. The aliphatically-unsaturated hydrocarbons can be removed from the vicinity of the uncoated porous support surface as by a sweep fluid or through the effect of vacuum and/or other means.Thus, the unsaturated hydrocarbon forms a metal complex and is decomposed upon its travel through the metal ion containing polyhydric alcohol barrier membrane, and as a result, the material passing through the barrier membrane is more concentrated with respect to at least one aliphatically-unsaturated hydrocarbon component than is present in the feedstream.

The feed fluid need only contain a small amount of aliphatically-unsaturated hydrocarbon, as long as the amount is sufficient so that the unsaturated material to be separated selectively reacts with the metal complex ions to a significant extent, thus at least one of the components of feed is less reactive or nonreactive with complex forming metal ions. The aliphatically-unsaturated materials of most interest with regard to separation by the method of the present invention and by the membranes of the present invention have from 2 to about 9 carbon atoms, preferably 2 to about 4 carbon atoms per molecule. The separation of ethylene or propylene from mixtures of other normally gaseous materials, such as one or more of ethane, methane and propane and hydrogen is of particular importance.Frequently these feed mixtures contain from about 1 to 50 weight percent ethylene, about 0 to 50 weight percent ethane and about 0 to 50 weight percent weight methane.

In the present invention, metal ions are defined as metals capable of forming metal ions in water-free solutions of polyhydric alcohols and capable of reversibly complexing with aliphatically unsaturated hydrocarbons. Metals may be used which serve in the form of metal cations to separate aliphatically-unsaturated hydrocarbons in the feed mixture through the formation of metal complexes of desired properties, and these metals include, for instance, the transition metals of the periodic chart of elements having atomic numbers above 20. Included in these metals are those of the first transition series having atomic numbers from 21 to 29 such as chromium, copper, especially the cuprous ion, and the iron group metals, e.g. nickel and iron.Other of the useful complex forming metals are in the second and third transition series, i.e. having atomic numbers from 39 to 47 or 57 to 79, as well as mercury, particularly as the mercurous ion. Thus, the noble metals such as gold, silver, and the platinum group among which are platinum, palladium, rhodium, ruthenium and osmium are suitable. The useful base metals of the second and third transition series include, for example, molybdenum, tungsten, rhenium and the like. Various combinations of these complex-forming metals may also be employed according to the invention, either in the presence or absence of non-metal or non-complexing metal cations.

Facilitated transport membranes frequently show a saturation phenomenon in which selectivity decreases sharply with increasing pressure. The solid barrier membranes according to the invention which are formed into a composite with the porous substrate can be operated at pressures of 689 kPa differential or higher and still result in reasonable selectivity for aliphatically-unsaturated hydrocarbons.

A number of membranes were prepared according to the invention using various silver salts in combination with polyhydric alcohols or mixtures of polyhydric alcohols as barrier separation membranes and then used for the separation of ethylene from ethane. The barrier membranes were supported on the surface of porous polypropylene and in the pores of porous nylon sheet materials. Permeabilities and separation factors for seven different examples illustrating the inventive membrane and use of said membrane in the separation of ethylene from ethane are included under the discussion of Examples 1 through 6.

Permeabilities are expressed as cubic centimeters (STP) per square centimeter of membrane area per second per differential partial pressure of 1.333 kPa across a membrane of 1 cm thickness cm3 x cm (STP)/cm2-sec-cmHg. Unless otherwise noted all permeabilities are reported at ambient temperatures and pressures of approximately 240C and 1 atmosphere respectively.

Another convenient relationship for expressing gas permeation characteristics of a membrane is separation factor. A separation factor aalb, for a membrane for a given pair of gases "a" and "b" is defined as a ratio of the permeability, (Pa, of a membrane for a gas "a" of a gas mixture, to the permeability, (Pb), of the same membrane to gas "b" of the mixture.

Example 1 Silver nitrate was dissolved to a concentration of 1M in glycerol by stirring with gentle warming. A 0.11l nylon filter was immersed into the resulting clear solution. After a 48 hour equilibration, the filter was removed, its surface wiped of excess solution, and tested with a 1:1 dry ethylene/ethane feed at 133.3 kPa pressure. The following performance was measured after 24 hours of use: a C2H4/C2H6 = 50.2 PC2H4 = 3.6 x 10-10 Example 2 A liquid membrane was prepared and tested as in Example 1, except that silver trifluoromethanesulfonate was used instead of silver nitrate.

a C2H4/C2H6 = 22.3 PC2H4 = 2.8 x 10-10 Example 3 As in Example, 1, except that a 3M solution of ARNO, in glycerol was utilized. The performance observed was: a C2Ha/C2H6 = 69 PC2H4 = 11.5 x 10-10 When the membrane cell was heated to 650 C, permeability increased substantially while selectivity remained at a good level: a C2H4/C2H0 = 24 PC2H4 = 46 x 10-1 Example 4 As in Example 1, except that 1M ARNO, in 1,4 butanediol was used.Performance: a C2H4/C2H0 = 5.7 PC2H4 = 46.8 x 10-1 Example 5 A coating solution was prepared as follows: to a 3% (wt/vol) solution of polyvinyl alcohol in water was added 6% triglycerol, hydrogen peroxide to make a 1.5% solution and silver nitrate to make a 4M solution in the non volatile components (PVA and triglycerol). The solution was filtered and applied as a thin layer to the surface of porous polypropylene sheet (Celgard 2402). After drying in a vacuum at ambient temperature it was tested with a dry 1:1 mixture of ethylene and ethane. The measured ethylene over ethane selectivity was 182. Ethylene permeability~3.1 x 10-10. Testing was done during 48 hours in which no change in performance occurred.

Example 6 As in Example 5 except that 5% PVA in water was used and 1.6% glycerol was used instead of triglycarol.

a C2H4/C2H6 = 350 PC2H4 = 10.2 x 10-10 In all the preceding examples, Examples 1 through 6, the membrane was water-free and the ethylene/ ethane feed was also water-free.

The following Example 7 illustrates the necessity of polyhydric alcohols presence in the water-free hydrocarbon separation membranes according to the invention.

Example 7 A composite membrane was prepared from a 10% aqueous solution of polyvinyl alcohol containing ten molar silver nitrate (based on polymers). The solution was coated as a thin layer on the surface of polypropylene and dried. On testing with a dry ethylene/ethane feed, no selectivity was measured. On removal from the testing cell, the membrane was observed to be cracked. Only upon addition of glycerol or triglycerol to the polyvinyl alcohol barrier materials could good performance be obtained as described in the preceding examples.

Claims (15)

1. A water-free membrane suitable for separating aliphatically-unsaturated hydrocarbons from saturated hydrocarbons comprising a separation barrier material of metal ions dissolved in a polyhydric alcohol or mixtures of polyhydric alcohols incorporated on the surface of a porous support or in the pores of the porous support.

2. The water-free membrane according to Claim 1 wherein the metal ions are selected from noble metals of the periodic chart.

3. The water-free membrane according to Claim 2 wherein the metal ion is silver.

4. The water-free membrane according to Claim 1 wherein the aliphatically-unsaturated and saturated hydrocarbons have from one to about nine carbon atoms per molecule.

5. The water-free membrane according to Claim 4 wherein the aliphatically-unsaturated and saturated hydrocarbons have from one to about four carbon atoms per molecule.

6. The water-free membrane according to Claim 1 which is suitable for separating ethylene from ethane.

7. The water-free membrane according to claim 1 wherein the porous support is hydrophobic and the separation barrier material forms a membrane on the surface of the porous support.

8. The water-free membrane according to claim 1 wherein the porous support is hydrophilic and the separation barrier material is liquid and the membrane is formed in the pores of the porous support.

9. A process for separating aliphatically-unsaturated hydrocarbons from saturated hydrocarbons by contacting said hydrocarbons with a water-free membrane comprised of a separation barrier material of metal ions dissolved in a polyhydric alcohol or mixture of polyhdric alcohols incorporated on the surface of a porous support or in the pores of the porous support resulting in the selective permeation of the aliphatically-unsaturated hydrocarbons through the water-free membrane.

10. The process according to Claim 9 wherein the aliphatically-unsaturated hydrocarbons and saturated hydrocarbons are water-free.

11. The process according to Claim 10 wherein the aliphatically-unsaturated hydrocarbons and saturated hydrocarbons have from one to about nine carbon atoms per molecule.

12. The process according to Claim 9 wherein ethylene is separated from ethane.

Amendments to the claims have been filed, and have the following effect: (b) New or textually amended claims have been filed as follows: (c) Claims 9 to 12 above have been renumbered as 10 to

13 and their appendancies corrected.

9. A water-free membrane substantially as described in any of Examples 1 to 6.

14. A process for separating aliphatically-unsaturated hydrocarbons substantially as described in any of Examples 1 to 6.

15. An aliphatically-unsaturated hydrocarbon separated by the process of any of Claims 10 to 13.