Patent Application
20160220960
The present invention pertains to hollow fibre membrane modules which can be used in various separation applications. In particular, it pertains to improved hollow fibre membrane modules for osmotic membrane distillation.
Hollow fibre membrane modules or contactors are devices used for separating a variety of components from numerous types of fluids or fluid mixtures. They can also be used as absorbers for various species and/or for heat exchange. A corresponding wide variety of hollow fibre membrane modules are available commercially for various separation and other applications. They generally comprise a plurality of hollow fibre membranes or lumens arranged suitably in a housing. In separation applications, separation is accomplished by a membrane separation process, and the membrane materials and operating conditions (e.g. temperature, pressure) are chosen in accordance with the membrane separation process and species involved. Numerous membrane separation processes are possible, including reverse osmosis, ultrafiltration, nanofiltration, microfiltration, vapour permeation, pervaporation, membrane distillation, and so on. Osmotic membrane distillation (OMD) is another candidate separation process that is receiving increased attention for industrial uses. For instance, WO2005/118114 teaches the benefits of using OMD for various industrial uses and particularly for use in electrolytic haloalkali production plants where reduced capital, operating re-concentration, and dilution costs can be achieved.
The membrane materials employed in such modules typically comprise one or more polymeric materials or composites. Further, the membranes may be microporous and/or chemically treated depending on the process and species to be separated. The hollow fibre membranes or lumens are arranged in the module with suitable seals provided so that fluid communication between the inside of the lumens and the outside of the lumens is prevented. The fluid or fluid mixture to be treated can then be directed over either the outer or inner surface of the lumens whereupon the species to be separated passes through the membranes and is collected and/or disposed from the other side of the lumens. Commonly the fluid is directed over the outside of the lumens in a configuration known as the shell-side configuration (because the fluid is on the same side of the lumens as, and in common contact with, the shell of the housing).
To make the most effective use of the membrane in shell-side configuration modules, it is desirable to bathe the entire external surface of the lumens as uniformly, and with as much of the supplied fluid as possible while the fluid traverses the module. Various designs and configurations have thus been considered in the art in order to improve the fluid flow distribution in this regard. For instance, U.S. Pat. No. 5,264,171 discloses various designs for spiral-type hollow fibre membrane modules containing flow-directing baffles which are axially positioned to promote improved radial flow of fluids throughout the hollow fibre bundle. A common present-day commercial hollow fibre membrane module employs a similar such shell-side configuration design. The common commercial module comprises a hollow fibre bundle, a hollow internal core in the centre of the bundle, and a central baffle in the bundle. The internal core is perforated and has a plug located at its centre. When fluid is supplied to an inlet of the module, it is first directed to an end of the internal core. The presence of the plug forces all the fluid to flow radially outward through the perforations in the internal core and over the lumen surfaces in the inlet side of the module. The fluid then flows around the central baffle, over the lumen surfaces in the outlet side of the module, and back through the perforations into the internal core. Thus, the presence of these features establishes a cross-flow pattern in the hollow fibre bundle. Thereafter, fluids exits the module through an outlet connected to the other end of the internal core. (This common commercial module is illustrated in
There still remains a need however to improve fluid flow distribution within such modules in order to increase efficiency and performance and to maximize utilisation of the often expensive membrane surface available inside. Preferably this is done in as compact and simple a manner as possible. The present invention addresses these needs and provides other benefits as disclosed below.
Surprisingly it has been discovered that incorporating appropriate flow distributors in the hollow internal core of hollow fibre membrane modules can result in a worthy improvement in fluid flow distribution over the fibre bundle without unacceptable adverse consequences, and particularly without an unacceptable increase in pressure drop. The use of such flow distributors does not increase module size and is a relatively simple and effective modification to the module fabrication and assembly.
In a basic embodiment, the hollow fibre membrane module has a shell-side configuration and comprises a hollow fibre bundle comprising a plurality of hollow fibre membranes, a perforated hollow internal core inside the bundle (e.g. in the centre), and at least one bundle baffle within the hollow fibre bundle. There is at least one plug located part way within the internal core and there is at least one perforation on either side of the plug. The module further comprises a housing comprising a shell, shell ends, a shell inlet for a shell fluid (e.g. fluid supplied to the module), a shell outlet for the shell fluid, and a lumen outlet for a lumen fluid (e.g. fluid separated from the shell fluid by the membranes). The housing contains the hollow fibre bundle and internal core. The shell inlet is fluidly connected to one end of the internal core, and the shell outlet is fluidly connected to the other end of the internal core. The lumen outlet is fluidly connected to an end of each hollow fibre membrane. Suitable seals are provided within the housing and prevent fluid communication between the shell fluid and the lumen fluid. The bundle baffle within the hollow fibre bundle is located between at least one perforation on the shell inlet side of the internal core and at least one perforation on the shell outlet side of the internal core. And in accordance with the invention, the internal core comprises at least one flow distributor.
In certain embodiments, it can be desirable to allow lumen fluid to exit from both ends of the hollow fibre lumens. Alternatively, it can be desirable to provide an external flow of fluid through the hollow fibre lumens in order to absorb and/or carry away species separated from the shell fluid. The housing in such embodiments therefore comprises a lumen inlet for the lumen fluid and the lumen inlet is fluidly connected to the other end of each hollow fibre membrane.
The plug and the bundle baffle need not be aligned and neither need be centrally located in the module. However, in certain embodiments this can be preferred. In other embodiments, more than one plug may be employed (with appropriate additional perforations in the internal core) and more than one bundle baffle may also be employed.
More than one flow distributor may be employed in the internal core. For instance, the internal core can comprise at least one flow distributor on each side of the plug. This can provide for improved flow distribution on either side of the plug. Further, the internal core can comprise more than one flow distributor in series on the shell inlet side of the plug. These flow distributors in series can provide an increasing restriction to flow in the direction from the shell inlet to the plug. When properly arranged, these flow distributors (restrictors) advantageously improve fluid flow distribution by directing flow into the bundle more evenly. In a like manner, the internal core can comprise more than one flow distributor in series on the shell outlet side of the plug. And as above, the flow modification provided by these series flow distributors can provide an increasing restriction to flow in the direction from the plug to the shell outlet and thereby advantageously improve fluid flow distribution in the bundle.
Several different types of flow distributors may be considered. For instance, the flow distributor can comprise a core baffle which obstructs flow. Such a flow distributor can further comprise locating tabs attached to the periphery of the core baffle in order to locate the core baffle centrally with respect to the walls of the internal core. A particularly suitable core baffle is double cone shaped in which the axes of the cones are aligned along the axis of the internal core. The angles of suitable double cones with respect to the cone axes can be about 30 degrees. Other types of flow distributors which may be employed include an orifice plate or plates located against the inner wall of the internal core, or helical-shaped flow distributors.
The flow distributor or distributors are located in the internal core to provide effective improved fluid flow distribution in the fibre bundle. While the preferred location or locations vary from case to case, an effective location in the Examples below was at about ⅙ of the distance between the shell inlet and the plug in the internal core. A tie rod or other similar mechanical device may be used to connect the flow distributor to the plug and thereby serve to locate the flow distributor as desired.
In combination with the present invention, the configuration of the perforations in the internal core can also be varied if desired to improve the fluid flow distribution. For instance, the perforations in the internal core can be asymmetric on opposite sides of the plug, and the perforations need not be uniform on the same side of the plug (i.e. can be of different sizes or shapes). And if desired, the packing density of the hollow membrane fibres can be made to be lower at the centre of the bundle than at the perimeter of the bundle.
The hollow fibre membrane modules of the invention can be used for various industrial and other applications. An exemplary application is for the osmotic membrane distillation of a fluid in which use of a flow distributor might be expected to lead to improvement given the flow rates, pressures, and other operating conditions typically encountered in such a separation process.
Unless the context requires otherwise, throughout this specification and claims, the words “comprise”, “comprising” and the like are to be construed in an open, inclusive sense. The words “a”, “an”, and the like are to be considered as meaning at least one and are not limited to just one.
In a numerical context, the word “about” is to be construed as meaning plus or minus 10%.
Herein, a “flow distributor” refers to a structure which modifies fluid flow but does not completely block it. The structures can comprise various baffles, plates, orifices, helical-shapes, and the like or combinations thereof.
In reference to flow distributor shapes, “double cone shaped” refers to shapes described by two cones joined at common bases (and is not that of the rigorous mathematics definition with two cones joined at a common apex).
An “orifice plate” is a plate having an orifice of some kind in it.
A common present-day commercial hollow fibre membrane module is shown in the schematic side cross-section view of
Module 1 comprises a housing 7 which includes shell 8, shell ends 9a, 9b, shell inlet 10, and shell outlet 11. Shell inlet 10 and shell outlet 11 are fluidly connected to internal core ends 4a and 4b respectively. Housing 7 also has lumen inlet 12 and lumen outlet 13 which are fluidly connected to lumen ends 3a and 3b respectively. Various seals 14 are used within housing 7 to prevent fluid communication between those fluids inside hollow membrane fibres 3 and those fluids outside hollow membrane fibres 3 and flowing through internal core 4.
Module 1 also comprises bundle baffle 15 which is located in the middle of fibre bundle 2 outside internal core 4 but is aligned with plug 5 in internal core 4. Bundle baffle 15 appears outside hollow fibre membranes 3 but not inside hollow fibre membranes 3. (Baffle structures like this can conveniently be created during assembly for instance if spirally wound sheets of flat-sheet microporous membranes are used by appropriate application of potting compound to the centre of the sheets before winding.)
In separation applications then, the fluid or fluid mixture to be treated is supplied as shell fluid at shell inlet 10. The shell fluid flows through the shell inlet side of internal core 4 and is distributed via perforations 6a over fibre bundle 2. Because the shell fluid cannot pass through plug 4 nor through bundle baffle 15, all the supplied shell fluid must flow over fibre bundle 2 on the shell inlet side and into the space between fibre bundle 2 and shell 8 before being directed around the edge of baffle bundle 15 to the shell outlet side of module 1. The arrows shown in
In a like manner, shell fluid on the shell outlet side of module 1 is directed over fibre bundle 2 and through perforations 6b into internal core 4 whereupon it exits shell outlet 11. Again, arrows are provided to give a qualitative indication of the direction and distribution of the shell fluid flow in this side of module 1.
As the shell fluid bathes the surfaces of hollow fibre membranes 3, membrane separation of the desired species from the shell fluid occurs, and these species pass through and into hollow fibre membranes or lumens 3. As shown, a lumen fluid is provided to absorb and/or carry away these separated species. Lumen fluid is provided at lumen inlet 12, is directed through hollow fibre membranes (lumens) 3, and then out through lumen outlet 13. (In other embodiments not shown here, a lumen fluid may not be needed and thus only a lumen outlet may be required in the module to remove the separated species therefrom. Alternatively, the separated species may be removed from both ends of the lumens, thus effectively using both lumen inlet and outlet 12, 13 in
In the present invention, the hollow fibre membrane module is similar to that shown in
While the extent to which the flow distribution can be improved will be specific to a given embodiment and operating conditions, various other core baffle profiles are expected to be suitable for this purpose.
Instead of core baffles, an orifice plate or plates may be used as the flow distributor.
Further still, helical shaped baffles may be used instead of core baffles or orifice plates as are exemplified in
Selecting an appropriate flow distributor or series of flow distributors to improve fluid flow distribution in a given hollow fibre membrane module requires consideration of many factors, including the dimensions and component types in the module along with fluid properties and the operating conditions to be encountered. And depending on circumstances and if opportunity allows, it may be preferred to modify a module dimension (e.g. inner diameter of internal core) or operating condition (flow rate or pressure of the supplied shell fluid) because the combination of incorporated flow distributor and modified module or modified operating condition leads to greater improvement. While somewhat complex, those in the art are capable of such consideration and can be expected to arrive at appropriate selections using general engineering principles (such as heat and mass transfer, and fluid dynamics) and modeling (for instance as illustrated below).
The following Examples have been included to illustrate certain aspects of the invention but should not be construed as limiting in any way.
In a first study, computation fluid dynamics (CFD) modeling was done on a conventional hollow fibre membrane module design as shown in
In all cases, the fibre bundle was about 100 cm long and 18 cm in diameter and contained about 4800 PTFE fibres or lumens with an average outer diameter of 1.9 mm. The fibres were spirally wound around an internal core with an outside diameter of about 6.4 cm. The distance from each end of the internal core to the plug was about 50 cm. The internal core was perforated with staggered circular holes of 0.4 cm diameter such that 40% of the internal core surface was open. And this fibre bundle was housed in a shell with an inside diameter of 20 cm.
To simplify the calculations, the model was only applied to the shell inlet side of the module (i.e. the half of the module from shell inlet 10 up to plug 5 and bundle baffle 15). A porous media model was employed to further simplify the bundle simulation, although the inner rows of lumens were maintained with their actual dimensions in order to predict core-to-bundle flow with better accuracy. The shell side fluid was assumed to be 30% caustic solution at 80° C., which might typically be treated by osmotic membrane distillation in industrial applications. With 10 m3/hour caustic feed assumed in the simulations, the flow was treated as turbulent in both the internal core and bundle regions.
The radial flux distribution of the supplied shell fluid for this conventional hollow fibre membrane module, Comparative Example 1, was determined and was compared to the ideal condition in which the Radial flow is uniform at the same radial distance as fluid spreads out over the fibre bundle.
The other module examples which were modeled included:
Out of all the module variations considered here, the results suggest that incorporating two double cone shaped core baffles in series (Inventive Example 2) should provide the best flow distribution since it has the lowest total deficiency area Atotal. The module with the inverted arrow head core baffle (Inventive Example 3) had the lowest Alower value and thus the fewest lumens with insufficient shell fluid flow. Modifying the perforation distribution could also significantly reduce the total deficiency area (comparing Comparative Example 2 to Comparative Example 1) and this suggests it may be advantageous to employ a combination of flow distributors and perforation modifications for the best possible flow distribution.
All the inventive examples showed an improvement in fluid flow distribution compared to the conventional module. In the embodiments employing the double cone shaped core baffles, a very modest increase in pressure drop is expected in the modules. Use of the inverted arrow head shape core baffle resulted in the greatest increase in pressure drop.
In a second study, an actual hollow fibre membrane module which might typically be used in a filtration application was built. The module had a similar layout to that shown in
In both cases, the fibre bundle was about 108 cm long and 19.4 cm in diameter. Again, PTFE fibres having an average outer diameter of 1.9 mm were used. However, in this second study, the module contained about 3240 fibres compared to about 4800 used in the first study. The fibres were spirally wound around an internal core with an outside diameter of about 5.0 cm. The distance from each end of the internal core to the plug and bundle baffle was about 51.3 cm. The internal core was perforated with staggered circular holes of 0.4 cm diameter such that 50% of the internal core surface was open. And this fibre bundle was housed in a shell with an inside diameter of 19.6 cm.
The comparative module here, i.e. Comparative Example 3, had no flow distributors in the internal core. The inventive module here, i.e. Inventive Example 4, included a flow distributor in the internal core comprising three double truncated cone shaped core baffles in series. Here, each of the truncated double cone shaped core baffles comprised a short cylindrical section in its centre. Specifically, these core baffles were assumed to be about 6.5 cm long. The central cylindrical section was 1.7 cm long. And the angles of the double cones with respect to the cone axis (and internal core axis) were again 30 degrees. The first core baffle was centred about 5.2 cm from the end of the internal core. The gap between the subsequent core baffles in series was about 7.5 cm.
As before, to simplify the calculations, the model was only applied to the shell inlet side of the module. And a porous media model was employed to further simplify the bundle simulation, although again the inner rows of lumens were maintained with their actual dimensions in order to predict core-to-bundle flow with better accuracy. The shell side fluid was again assumed to be 30% caustic solution at 80° C. and the flow was treated as turbulent in both the internal core and bundle regions. Here however, 12 m3/hour caustic feed was assumed in the simulations.
Again, normalized radial flux was determined in each case and
Inventive Example 4 showed more balanced shell side flow than Comparative Example 3. Thus again, the inventive module showed an improvement in fluid flow distribution compared to the conventional module. However, the inventive module also showed a significant increase in pressure drop.
Actual hollow fibre membrane modules were then fabricated in a like manner to the modeled Comparative Example 3 and Inventive Example 4. And these modules were tested in a pilot facility under similar conditions to those assumed in the modeling (e.g. 12 m3/hour feed). In preliminary testing to date, the pressure drop observed in these modules closely matched that predicted by the CFD modeling. And consequently, this provides a high level of confidence in the other CFD predictions. Table 3 below shows the pressure drops measured from the shell inlet to the shell outlet for the actual modules and compares them to those predicted by CFD. (Note that Table 2 shows the expected pressure drops over ½ the length of a module. Thus, the expected pressure drops in Table 3 are twice those presented in Table 2.)
All of the above U.S. patents, U.S. patent applications, foreign patents, foreign patent applications and non-patent publications referred to in this specification, are incorporated herein by reference in their entirety.
While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto since modifications may be made by those skilled in the art without departing from the spirit and scope of the present disclosure, particularly in light of the foregoing teachings. Such modifications are to be considered within the purview and scope of the claims appended hereto.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/US2014/055538 | 9/13/2014 | WO | 00 |
| Number | Date | Country | |
|---|---|---|---|
| 61879291 | Sep 2013 | US |
