Fluid-membrane separation

Details Fluid-membrane separation Fluid-membrane separation

1997-03-25

2003-11-18

Fortuna, Ana (Department: 1723)

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

C210S644000, C210S640000, C585S818000, C585S819000, C095S043000, C095S045000

Reexamination Certificate

active

06649062

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a combination of fluid extraction and membrane technologies in which unequal rates of transport through a nonporous membrane is achieved by solution and diffusion mechanisms driven by a difference in free energy of the preselected species across the membrane. More specifically this invention uses a supercritical fluid on at least one side of a nonporous membrane and at least one fluid that is fresh to the process on at least one side of the membrane after which the fluid is either recycled to the opposite side of the membrane or removed from the separation process. Fluids and membranes, including modified membranes, are provided to facilitate passage of one or more components of a mixture through the membrane while rejecting other components of the mixture.
2. Background
Reverse osmosis, as a commercial separation technique, has been available for over twenty-five years. In reverse osmosis, the osmotic pressure normally present between two solutions with different solute concentrations is reversed by using an externally applied pressure in the presence of a semi-permeable membrane. The membrane, placed between solutions of different concentrations, preferentially passes one component of the more concentrated solution into the more dilute solution, e.g., water from a saline solution to the pure water side. Much of the work in reverse osmosis has involved the development of new membrane materials and structures. Most membranes currently available are based on cellulose acetate, aromatic polyamide, and polyethyleneimine/polysulfone polymers. Variations in substituted polymer end groups are used to affect the flux and selectivity of a membrane for a specific application. The structure of the membrane is affected by the method of membrane deposition, the amount of cross-linkage, the temperature at which it is formed, and the speed at which the formed membrane is cured. Because of the number of chemical and structural variables available, membrane development has remained an empirical and costly art.
Because of the costly empirical nature of developing new membranes and the large number of membranes currently available, the current direction in membrane research is toward developing novel methods for the use of existing membranes. The approach is particularly evident in the field of biotechnology where conventional membranes are used in cell/protein and protein/protein separations.
Pervaporation is a membrane separation process where a liquid mixture is contacted with the membrane. A component of the mixture diffuses through the membrane and evaporates from the downstream surface of the membrane. Characteristically, a pervaporation system is operated by passing the liquid feed (at atmospheric pressure) over the surface of the membrane and removing the permeate vapor under vacuum on the downstream side of the membrane. The process is useful, in terms of effective separations at reasonable flux rates, only if the partial pressure of the permeating component(s) in the downstream gas phase is small relative to the upstream liquid vapor pressure.
The process suffers from two serious operational difficulties. First permeate vapor delivered at low pressure will have a low mass density and will require high volumetric fluxes in order to operate as an economical process. The high vapor flux requirements necessitate large trans-membrane pressure drop that increase capital and operating costs for a single-stage process to uneconomical levels. These costs can be reduced somewhat by staging the process. Second, the energy required to evaporate the permeate is not available from the sensible energy of the systems and must be supplied from external sources. Construction of the membrane module from thermally conductive materials, e.g., copper or aluminum, can provide the needed heat but makes the cost of the modules unacceptably high. The problem can be avoided by the addition of steam to the vapor side of the membrane. However, the steam temperature must be held below the boiling temperature of the upstream liquid mixture to minimize upstream vapor formation. Also, the stream-to-permeate ratio must be kept large enough to keep the partial pressure of the permeate in the stream low relative to the vapor pressure of the liquid mixture. This ensures the separation of the mixture components.
Perstraction is a membrane process whereby the permeate is removed from the downstream side of the membrane by a receiving liquid. The receiving fluid, if properly selected, has the effect of lowering the chemical potential on the downstream side of the membrane. Selection of the receiving liquid is critical to the concept. The receiving liquid must be 1) essentially incapable of permeation through the semi-permeable membrane, 2) lower in volatility than the permeating components, and 3) capable of dissolving a substantial quantity of the permeate. Examples of such materials include high-boiling, high molecular weight hydrocarbon oils, hydrotropes (e.g., sodium toluene sulfonate), concentrated aqueous solution of soap or micellizing surfactant, and concentrated, stable polymer lattices. It is noted that is unnecessary to recover all the permeate from the receiving liquid. It may be more economical to remove only a portion of the permeate from the receiving liquid and recycle the receiving liquid back to the membrane module. The ratio of receiving liquid to permeate can be as high as 20:1. The perstraction technique offers several advantages over other membrane separation processes. First, perstraction offers lower heat and power costs over competing process. Second, the permeation process can operate near atmospheric pressure without imposing significant stress on the membrane. Third, unlike pervaporation, the permeate is in a liquid rather than a vapor state; a fact which reduces the amount of downstream processing needed.
Although some effort has been made in using supercritical solvents with nonporous membranes (Schuker, U.S. Pat. No. 5,430,224), such efforts have focused on the use and recycling of the supercritical solvents on both sides of the membrane. Such recycling can result in the build-up of unwanted materials in the recycled solvent especially when either the permeate (components passing through the membrane) or the retenate (components not passed through the membrane) or both are not readily separated from the supercritical solvent. Further the energy used in separating and recycling the solvent often is not cost effective. And finally the energy costs associated with providing a supercritical solvent to both sides of the membrane are often not justified in many separation processes.
Accordingly, it is an object of the present invention to provide a membrane separation process that 1) extends the range of possible separations to a wider range of components, 2) improves the selectivity for passing or retaining various mixture components with respect to a nonporous membrane, and 3) renders the entire process more cost effective.
SUMMARY OF THE INVENTION
The process of the instant invention consists of first providing a nonporous (dense) membrane having two sides, a first side and a second side. The membrane may be in a flat sheet, spiral wound, a hollow fiber, or other suitably provided membrane configuration. Generally the hollow fiber configuration is preferred since it allows manifolding of fluids to the first side or second side or both sides of the nonporous membrane. The membrane is selected or otherwise treated to reject or pass various components from the mixture to be separated. For example, a membrane with hydrophobic properties or treated to have hydrophobic properties can be used to reject water and water-like mixture components from passing through the membrane while a membrane with hydrophilic properties or treated to have hydrophilic properties can be used to facilitate passage of water and water-like mixture components through the membrane.
The next step in the process is to provide a mixture for separation

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