Gas separation device

Details Gas separation device Gas separation device

2001-11-13

2003-10-21

Spitzer, Robert H. (Department: 1724)

Gas separation: apparatus

Apparatus for selective diffusion of gases

C095S052000, C096S012000, C096S014000

Reexamination Certificate

active

06635104

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to membranes for separations of gas mixtures and more particularly, but not necessarily exclusively, to porous mechanical supports filled with a water transfer material for water vapor separations from gas mixtures as well as to porous mechanical supports filled with a gas transfer material for acid and basic gas separations.
2. Description of the Related Art
Water vapor removal from air or other gases is an important process in a variety of industries including chemical, electric, electronic, and food industries as well as for the moisture control of air for air conditioning in buildings. The use of membranes for removal of water vapor from gases has many advantages over other conventional methods such as compression, cooling, or adsorption, including lower operating and energy costs and continuous operation.
There are a number of patents that deal with membrane-based gas dehydration processes and membrane modules as well as with membrane materials. Membranes applied in gas dehydration processes can be divided into two general groups, i.e., one containing dense homogeneous membranes or dense separating layers and one containing porous membranes often carrying a humectant in the pores.
The homogenous membranes provide a high separation ratio but have a disadvantage in that the permeation rate is low. A typical homogeneous membrane used for separation of water from hydrocarbons and chlorinated hydrocarbons is disclosed in U.S. Pat. No. 4,857,081 [1] and consists of hollow fiber made of cuproammonium cellulose. The permeability of this membrane to water is extremely low, amounting to less than 20 ml of water per hour per mmHg per square metre.
The transport of gas or vapor through a dense membrane is described by the solution-diffusion mechanism, i.e., the permeability of gas through the membrane is the product of the gas solubility in the membrane material and its diffusivity in the membrane [2].
The diffusivity is a kinetic parameter which reflects the rate with which the penetrant is transported through the membrane. The parameter is dependent on the geometry (size) of the penetrant. Generally, the diffusion coefficient decreases with an increase in the molecular size of the penetrant. However, in the strongly interacting systems where the penetrant has an ability to swell the membrane material, even large molecules of organic vapors can have large diffusion coefficients.
Solubility is a thermodynamic parameter that gives a measure of the amount of the component sorbed by the membrane under equilibrium conditions. The solubility of an ideal gas is described by the well known Henry law which states that the concentration of gas in the polymer is proportional to the applied pressure. When strong interactions occur between the penetrant molecules and the polymer, the sorption isotherms show large positive deviations from Henry's law.
Polyelectrolytes are generally very hydrophilic materials interacting strongly with water and thus providing high values of the solubility parameter. However, most polyelectrolytes are water soluble materials with poor film-forming properties. As such they cannot be used to form dense gas separation membranes. These problems can be overcome to a certain extent by using ionomer-type polyelectrolytes which contain small amounts of ionic groups on a hydrophobic chain or on pendents of the main hydrophobic chain. Such polymers are insoluble in water and have typically good film-forming properties. Salemme in U.S. Pat. No. 3,735,559 [3] discloses a permeselective membrane for water vapor transport made from dense films of partially sulfonated polyxylylene oxide (ionomer-type polyelectrolyte insoluble in water) in various ionic forms. The disclosed membranes have, however, some problems such as the need to pre-shrink them to avoid rupturing. They are also unstable in the acid form resulting in formation of detrimental uncontrolled cross-linking. Moreover, hydrolysis in the presence of water can lead to the liberation of sulfuric acid. Changing the ionic form of the polymer from the acid form to the salt form makes the membranes more stable but they are then prone to densification in the presence of water.
In another example of use of a dense film ionomer type of membrane disclosed in U.S. Pat. No. 5,160,511 [4], small diameter, thin-walled tubing of perfluoroethylene sulfonic acid obtained from E. I. DuPont de Nemours under their trade name “Nafion” was immersed in lithium hydroxide for several hours before being washed and dried. The sulphonic acid groups of the original tubing were converted to lithium salts thus increasing thermal stability of the material without substantially reducing the water vapour permeability. The tubing was used as a membrane in a gas dehydration process.
The major deterrent in using the fluorocarbon-based membranes is their cost. The perfluorinated materials are very chemically stable but very expensive materials. They are used predominantly in extremely aggressive environments such as are faced, for example, by membrane separators in chlor-alkali industry [5].
Since the permeability of molecules through dense polymers is generally very low, it is generally accepted that the efficiency of membranes can be improved by making them very thin. Ultra thin membranes are not mechanically strong and to overcome this drawback, composite membranes are made in which a very thin separating layer is superimposed on an anisotropic, non-selective porous support of high permeability. This construct allows for a reasonably high permeation rate combined with mechanical strength. The essential function of the porous support is to provide mechanical support for the thin separating layer.
There are two types of these “thin film” membranes that are suitable for gas or vapour separation, namely, asymmetric membranes and composite membranes. In the asymmetric membranes, the thin separating layer is made from the same material as the microporous supporting layer. This density gradient across the membrane thickness is typically achieved by casting a membrane from a polymer solution, letting the solvent evaporate partially from one surface of the cast followed by immersion precipitation in a non-solvent bath. A classical example of an asymmetric membrane for gas separation is the Loeb-Sourirajan type membrane made of acetyl cellulose [6]. In composite membranes, the very thin selective layer is deposited on a non-selective sublayer by coating, interfacial polymerization, or plasma polymerization. Examples of gas separation membranes having thin separating layers superimposed on a porous support are provided by Klass et al., U.S. Pat. No. 3,616,607 [7], Stancell et al., U.S. Pat. No. 3,657,113 [8] and by Kikukawa et al., U.S. Pat. No. 4,875,908 [9]. In the latest example, a material, described as fluororesin-type copolymer containing hydrophilic sulfonic or sulfonate groups is used to form the separating layer. The material is cast as a dense film from a solution preferably onto a support layer. The membrane has a thickness of about 0.1 to about 50 micrometers and shows excellent water vapor permeability combined with high selectivity.
Examples of asymmetric gas dehydration membranes with controlled porosity and graded-density skin are disclosed in U.S. Pat. No. 4,783,201 [10]. The membranes have water vapour permeance in the range of 3-15×10
−4
cm
3
/cm
2
·s·cmHg and a separation factor for water vapour over slow gas components of the feed stream of about 10 to about 50. This water vapour permeance is up to 3000 times larger that the permeance of a comparable dense membrane as described in U.S. Pat. No. 4,857,081.
A major drawback of both asymmetric and composite gas separation membranes is the existence of minute defects caused by gas bubbles, dust particles, etc. These are very difficult to eliminate. Such defects do not significantly affect the membra

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