Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1997-01-22
2001-10-30
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S640000, C210S649000, C095S043000
Reexamination Certificate
active
06309550
ABSTRACT:
The present invention relates to a method for achieving mass transfer, i.e. transfer of one or more substances, from a flow of a first fluid to a flow of a second fluid, at least one of the fluids being a gas phase (where the term gas includes vapour), the other fluid being a liquid phase. More specifically, the invention relates to a method for mass transfer between such first and second fluids, comprising contacting the first fluid with the outer surface of one or more hollow tubular member(s) at least a part of which comprising one or more porous membrane(s) and contacting the second fluid with the inner surface of the hollow tubular member(s).
The invention also relates to an apparatus suitable for achieving mass transfer from a first fluid to a second fluid.
BACKGROUND
A hollow fibre membrane is a capillary whose wall functions as a semipermeable membrane. Hollow fibre membranes cover a wide range of separation problems with a specific membrane, of suitable membrane characteristics, being most suitable for each problem. The membrane characteristics need to be established to ascertain the optimal membrane for specific application. Such characterisation methods may be different depending upon the mechanism of specific separation, as will become clear from the description provided below.
The membranes for use in achieving mass transfer from a first fluid to a second fluid are unusual in membrane processes, in that the membranes are essentially nonselective. The selectivity for the species transferred is achieved by careful selection of the first fluid and the second fluid. Moreover, transport (mass transfer) of a species or component across the membrane are generally diffusion driven by an imposed concentration gradient (partial pressure gradient) of the component between the inside and the outside of the hollow fibre, and/or diffusion driven by chemical absorption.
The production of hollow fibres, used for conventional membrane processes such as reverse osmosis, ultrafiltration, microfiltration, dialysis, gas separation, pervaporation etc., are well known in the prior art (e.g. U.S. Pat. Nos. 3,899,309, 4,293,418, 4,430,219, 4,690,873, 4,717,394, 4,758,341, 4,781,834, 4,911,846, 4,961,760, 5,026,479, EP-A 0446947, Cabasso, I. et al. “Hollow Fibre membranes”, Kirk-Othmer: Encycl. of Chem. Tech., Vol 12, 3. ed., pp 492-517 (1980)).
In the production of hollow fibre membranes, the critical physical parameters are generally considered to be the diameter of the hollow fibre, the wall thickness, the pore size, and the porosity of the membrane.
The basic wall morphology of the hollow fibres has been selected to obtain desired hydraulic permeability and desired mechanical properties. The key concerns for membrane characteristics for pressure driven processes such as reverse osmosis, ultrafiltration, microfiltration, gas separation etc. have been hydraulic permeability (i.e. convective permeability) and rejection characteristics of the membranes. Characterization of suitable hollow fibre membranes for such applications is well described in the prior art (e.g. Kesting, R. E., “Synthetic Polymeric membranes”, McGraw-Hill Book Company, 1971; Mulder, M., “Basic principles of membrane technology”, Kluwer Academic Publ., Dordrect, 1991). Generally, structural parameters as thickness, pore size and porosity of the membrane are combined in a hydraulic permeability constant, which is measured directly by a suitable method. Such measurement of hydraulic permeability, when used in combination with retention characteristics, such as the molecule cut off value, specifies the transport characteristics of the membrane.
When dealing with mass transfer through hollow fiber membranes with gas-containing pores, the same concerns have guided the selection of suitable membranes. Thus, e.g., EP 0374873 discloses the use of hollow fibre membranes for absorption/desorption where the hydraulic permeability has been specified for selecting the membrane.
BRIEF DESCRIPTION OF THE INVENTION
According to the present invention, it has been found that the above-mentioned parameter set is not the most relevant parameter set with respect to characterizing the suitability of a membrane for achieving mass transfer between two fluid flows; this is particularly true when dealing with high mass transfer rate diffusional transport between a gas and a liquid through a membrane with gas-containing pores where the controlling resistance is in the gas phase and/or the membrane. According to the invention, it has been found that membranes with gas-containing pores which show a surprisingly high mass transfer rate with respect to mass transfer between a gas and another fluid are membranes which have a low tortuosity factor and, in particular, membranes which come close to the tortuosity/porosity relation of &tgr;=1/&egr;. As will appear from the detailed description herein, this discovery gives rise to a new understanding of the decisive criteria for designing hollow fibre membrane mass transfer processes and apparatus and thereby to new kinds of such processes and apparatus with much higher efficiencies than hitherto.
Thus, in one aspect, the invention relates to a method for transferring mass between a flow of a first fluid and a flow of a second fluid, at least one of the fluids being a gas phase, comprising contacting the first fluid with the outer surface of one or more porous membranes in the form of one or more hollow fibres the pores of which membranes are gas-containing and contacting the second fluid with the inner surface of said membranes, the maximum pore size of said membranes being such as to prevent direct mixing of the two fluids, the membranes having a porosity (&egr;) of at least 0.50, the mass transfer coefficient of the membranes is at least 3 cm/s, and the tortuosity factor, as defined herein, of the membranes is at the most 1.4/&egr; when the porosity &egr; is lower than 0.80 and at the most 1.3/&egr; when the porosity &egr; is 0.80 or higher.
The mass transfer coefficient of the membranes with respect to the mass transfer in question is indicative of the capability of the membranes to transfer mass in the process in question; this mass transfer coefficient is explained and defined in detail below. The mass transfer film coefficient with respect to the mass transfer in question is indicative of the mass transfer which would take place between the two fluids in question; as will be understood, one of the fluids will have a lower mass transfer film coefficient than the other fluid and will thus be the limiting factor in the mass transfer. What is expressed above is that the membranes are so selected or adapted that where the membranes will not be the limiting factor (that is, where the mass transfer coefficient of the membrane is equal to or higher than the mass transfer film coefficient of the limiting fluid) or will be limiting only to a moderate extent, the preferred situation, now made possible and realistic through the present invention, being, of course, the situation where the membrane has little limiting effect or no limiting effect at all. As will appear from the following description of utilizations of the invention, this possibility of designing mass transfer processes and apparatus where the hollow fibre membranes have little or no limiting effect give rise to extremely efficient and fast mass transfer rate processes of types which would previously not have seemed economically realistic.
The key to the extremely high efficiencies obtainable using the principles of the present invention is the special tortuosity “rule” expressed by the tortuosity factor, &tgr;, being close to 1/&egr;. The tortuosity factor of a porous material will be explained in detail in the following. The tortuosity factor depends on the geometric structure of the material and is dependent, inter alia, on the manufacturing process of the material. As will appear from the comparative examples which follow, selection of a hollow fibre membrane based on the tortuosity factor considerations above will result in the select
Bhatia Vinay Kumar
Dam-Johansen Kim
Iversen Steen Brummerstedt
Jonsson Gunnar
FLS Miljo A/S
Fortuna Ana
LandOfFree
Mass transfer method and apparatus does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Mass transfer method and apparatus, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Mass transfer method and apparatus will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2583329