Liquid purification or separation – Casing divided by membrane into sections having inlet
Reexamination Certificate
2001-12-19
2003-11-11
Drodge, Joseph (Department: 1723)
Liquid purification or separation
Casing divided by membrane into sections having inlet
C210S321750, C210S321840, C210S640000, C210S180000, C210S231000, C210S340000, C055S423000
Reexamination Certificate
active
06645380
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a membrane separation apparatus and to its use for the separation of fluid mixtures by, for example, pervaporation, vapor permeation and vacuum membrane distillation.
BACKGROUND OF THE INVENTION
Pervaporation, vapor permeation, and vacuum membrane distillation (VMD), are all well known processes for separating fluid mixtures. In all of these three separation processes, an initial feed stream is separated into two streams: i) the retentate stream, which is rich in non-permeating components; and ii) the permeate stream, which is rich in the components which are able to pass through a selectively permeable membrane used in the separating process.
In pervaporation, the membrane acts as a selective barrier between the initial feed in liquid phase and the permeate in vapor phase. Thus, the membrane only allows the desired component(s) of the initial feed stream to transfer through the membrane by vaporization. The separation process is governed by the chemical affinity of the separating component(s) and the membrane, and not by the volatility difference of the separating components or the vapor liquid equilibrium. The driving force for the transfer of the permeating components(s) of the initial mixture is the partial pressure gradient of the selective or better permeating component(s) of the mixture across the membrane. Vapor permeation utilizes the same type of membrane. The only difference is that the initial feed stream, instead of being in the liquid phase as with pervaporation, is in contact with the membrane in the vapor phase.
Liquid mixtures with components having different volatilities and marked different boiling points are usually separated by distillation. However, in many cases during distillation, commonly used solvents, including aqueous mixtures, such as ethanol-water, isopropanol-water, etc. reach a limit point at which the concentration of the components in the vapor phases is similar to the concentration of the components in the liquid phase. This point is called the azeotropic point. Beyond the azeotropic point, simple distillation cannot perform any further separation. This problem is traditionally overcome by the addition of a third compound (solvent) that selectively associates with the more polar key components in the initial feed mixture, and can significantly increase the relative volatility of close-boiling-point components, thus making further separation by distillation beyond the azeotropic point technically possible. Although this method is widely used in industry, it suffers the drawback of contaminating the products by the third component, which can be especially detrimental in food or pharmaceutical applications.
Pervaporation and vapor permeation each offer an alternative solution to distillation in this regard. In these two processes, membranes are exposed to the initial mixed feed stream on one side of the membrane, and a vacuum is applied on the opposite other side thereof. One of the components of the initial feed stream is preferably absorbed by the membrane and diffuses through the membrane to be removed as vapor from the other side. The component of the initial feed stream that passes through the membrane is called the permeate. Since the separation is not dependent on the vapor liquid equilibrium, a proper selection of the membrane will allow separation of fluid mixtures both below and above the azeotropic point.
The VMD process is very similar to pervaporation. The main difference with pervaporation is that the membranes used therein, through which one (or more) of the components diffuse, are non-porous. In contrast, in VMD, separation takes place by evaporation through porous hydrophobic (water repellant) membranes. The hydrophobic nature of the membrane prevents the flow of water through its pores. As long as pressure on the vacuum side is maintained below the minimum required for liquid to penetrate the pores, a liquid-vapor interface exists at the pores of the membrane. Separation is thus governed by the vapor-liquid equilibrium.
Due to the porous nature of the membrane used in VMD, the permeate flux is significantly higher than achieved by pervaporation membranes Accordingly, VMD is particularly useful where the volatility of the components to be separated is quite different: eg. the removal of volatile organic compounds from water; evaporating liquid from a salt solution, for example, desalination of sea or brackish water for making ultra-pure water.
The flat sheet membranes used in all of the three processes discussed above can be housed in a module having the same general design. These modules should have reliable sealing between the feed side of the module and the permeate side of the module. They must also have high resistance to harsh operating conditions. To make these separation processes economically feasible, these modules should house a significantly large membrane surface area.
One way of reduction in overall size of the separation device is by the utilization of plate and frame type modules, each housing a plurality of membrane layers stacked with respect to one another in close parallel relation, and operatively connected for exposure to the fluid feed mixture to be separated. An example of such a prior art device is described in U.S. Pat. Nos. 5,437,796 (Bruschke et al.) and 4,769,140 (van Dijk et al.), which patents teach a plurality of feed plates, gaskets and membranes stacked upon one another.
However, the plate and frame module of Bruschke et al. and van Dijk et al. suffer from undue complexity of design utilizing a very large number of layers of components for securing and sealing the membranes, resulting in unduly high costs of production, installation and maintenance.
A further drawback of the separation devices of the prior art is the general way in which their membrane separation modules are housed for applying a vacuum thereacross. The known practice is to arrange the plurality of membrane separation modules within a single large vessel or housing (similar to a giant bell jar) which is kept under vacuum during operation. This greatly complicates maintenance of the plurality of separation modules within the vacuum vessel, as, over a period of time, the layers of gaskets in the separation modules begin to loosen, so as to require periodic tightening of the bolts or of the tide rods that hold the gaskets in sealing relation with the membranes. Failure to do this tightening maintenance on a regular basis may result in the gaskets leaking, with consequential loss of separating efficiency in the respective module. Following current practice, it is not possible to access a particular separation modules for tightening or other maintenance without first removing the common vacuum vessel. Such removal cannot be accomplished online, and requires shutting down the entire separation plant, at significant downtime cost. As a result, preventative maintenance, of the type just discussed is not carried out on a routine basis, but is typically left until the lack thereof causes a general plant shut down, with consequent removal of the vacuum vessel.
It should also be appreciated that actual lifting of the vacuum vessel typically requires a heavy duty crane, and, in some instances, such as where the separation plant is located inside of a building, removal of a portion of the ceiling or roof of the building. Furthermore, if any one of the membrane separation modules fails, or otherwise requires maintenance, the entire vacuum vessel containing the plurality of separation modules must be shut down to identify, and service, the particular malfunctioning module. This is both inconvenient and costly.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome, inter alia, the shortcomings of the prior art described above by providing a separation apparatus that is suitable for use for pervaporation, vapor permeation and vacuum membrane distillation that does not suffer from unduly high production, installation or maintenance costs and undue complexity of assem
Al-Hassani Aiser
Baig Fakhir U.
Kazi Abdul M.
Drodge Joseph
Hofbauer Patrick J.
Menon Krishnan S
Petro Sep International Ltd.
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