This specification relates to filtration using semipermeable separation elements, for example, spiral wound membranes used in reverse osmosis, nanofiltration, ultrafiltration and microfiltration processes.
The following background discussion is not an admission that anything discussed below is citable as prior art or common general knowledge.
U.S. Pat. No. 5,851,267 describes a separation module that uses a series of separation elements with interconnecting hardware that reduces the time necessary for assembly of interconnected elements and the machining or preparation of an extended part of the module inside diameter for acceptance of the elements. The elements use an interconnection between the modules that provides a sliding seal for first engaging adjacent modules and allowing alignment while a secondary seal is brought into contact and locked to provide a rigid axial attachment between the separation elements.
U.S. Pat. No. 6,632,356 describes a separation end cap adapted for connecting adjacent separation elements. The end cap can be located at the distal ends of a separation element and is adapted for connection with a permeate tube located within the separation element. In one embodiment the end cap includes an inner hub for receiving an O-ring to seal against an inner hub of an end cap on an adjacent separation element.
U.S. Pat. No. 7,387,731 describes a coupler for a spiral membrane filtration element having a spiral membrane enclosed within a rigid outerwrap includes a center support, a plurality of spokes extending outwardly from the center support, a circular rim coupled with the spokes, with the face of the rim being perpendicular to the axis of the overwrap. The rim includes a channel on its face for receiving a compressible seal, and a plurality of receptacles around its outer surface for joining two face-to-face adjacent couplers when a pair of aligned keepers is place in each receptacle.
The following discussion is intended to introduce the reader to the more detailed discussion to follow, and not to limit or define any claim.
Reverse osmosis and nanofiltration are filtration methods that can be used to create potable water from seawater. Simple reverse osmosis systems, such as single stage desalination systems, can use multiple separation elements placed in line in a common pressure vessel. Each of the separation elements can include a permeate conduit for collection of the filtered permeate solution. The permeate conduits can be connected in series using interconnectors. In such configurations, permeate solution can be forced through a series of contractions and expansions as it flows through the permeate conduits and the interconnectors, which can cause significant pressure losses. Pressure loss can be mitigated, for example, by enlarging the inner diameter of the permeate conduits, or by using interconnectors having an inner diameter that is larger than the permeate conduits. Another approach is to eliminate the use of interconnectors and provide another mechanism of sealing the permeate from the feed, for example, the interlocking end-cap described in U.S. Pat. No. 6,632,356.
Described herein is an apparatus in which an interconnector includes a diverging section of increasing cross sectional area exiting into the permeate conduit. The interconnector can further include a converging section of decreasing cross sectional area. With the arrangement of converging and diverging sections, the interconnector resembles a Venturi design. The diverging section can provide a more gradual divergence of the permeate solution exiting the interconnector, which reduces flow separation from the permeate conduit and thus can reduce pressure losses. Combined converging and diverging sections can result in even lower pressure losses. The reduced pressure loss in the permeate conduits can raise the net driving pressure for flow across the separation elements, as well as increasing the flow of permeate solution per element, thereby improving the energy efficiency of the filtration process. Higher permeate flows per element, at same solute rejections, can translate to more compact filtration plants with lower capital expenditure.
For simplicity and clarity of illustration, where considered appropriate, reference numerals may be repeated among the drawings to indicate corresponding or analogous elements.
The apparatus 10 includes a plurality of separation elements or modules 24 arranged in series within the chamber 22. For clarity of illustration, only a few modules are shown, although an apparatus of this type can in practice be sized to hold six to eight or more separation elements 24.
In the example illustrated, peripheral seals 34 extend around the outer side of each of the separation elements 24, and seal against the inner wall of the chamber 22 to ensure that the feed solution proceeds downstream from the first end 14 to the second end 18 within the chamber 22, in series sequentially through each of the separation elements 24.
Each of the separation elements 24 includes a permeate conduit 26 for collecting filtered permeate solution therein. In the example illustrated, the permeate conduits 26 are axially arranged along a central axis A of the chamber 22. The permeate conduits 26 are connected to each other via interconnectors 28, so that permeate solution can flow axially between the permeate conduits 26 of adjacent ones of the separation elements 24.
In the example illustrated, the permeate conduit 26 of the tail separation element 24 connects to a permeate outlet 30 via an end connector 32, which is shown extending out of an end wall at the second end 18 of the housing 12, adjacent to the outlet port 20. The apparatus 10 can further include an end connector and a permeate outlet (not shown) at the first end 14 of the housing 12, allowing permeate to flow from the permeate conduit 26 of the lead element 24 out of an end wall at the first end 14 the housing 12.
The separation elements 24 can comprise semi-permeable membranes that allow some components in a liquid solution to pass through while stopping other components. For example, each of the separation elements 24 can comprise spiral-wound membranes. Such separation elements include sheet membranes wrapped around its respective permeate conduit 26 to form an envelope that is spiral-wound with one or more feed spacers into a cylinder-shaped cartridge, with the permeable spacer in fluid communication with the respective permeate conduit 26. Each of the separation elements 24 can include an end cap or plate (not shown) to provide shape and structural rigidity, which can aid in assuring a generally open fluid path for the feed solution to optimally reach exposed surfaces of the outside membranes of the separation elements 24, and which can also help resist telescoping or deformation under high pressure flows within the chamber 22.
When a plurality of separation elements 24 are used in series, as in the apparatus 10 shown in
Referring to
Permeate solution flows in direction of flow f from the permeate conduit 26a, through the narrowed cross sectional area defined by an inner surface 36 of the interconnector 28, and exits into the permeate conduit 26b. The inner surface 36 can have a generally constant inner diameter across its length (but it is possible for the inner surface 36 to include a relatively small draft angle of, for example, less than 1 degree, to aid in the manufacturing of the interconnector 28 by injection molding). The permeate conduit 26b has an inner wall 38 that can have a significantly larger inner diameter than that of the inner surface 36 of the interconnector 28.
In such configurations, the permeate solution is forced through a series of contractions and expansions as it flows through the permeate conduits 26 from the first end 14 to the second end 18 of the apparatus 10 (
An analysis of the flow in these contraction-expansion geometries reveals that, in some examples, as much as half of the pressure can be lost in the section of relatively abrupt expansion in cross-sectional area where the permeate solution exits from the interconnector 28 and flows into the permeate conduit 26b. The inventors have determined that pressure losses at the contraction in cross-sectional area where the permeate solution enters the interconnector 28 from the permeate conduit 26a tend to be much lower than in the exit section. The relatively sharp increase in diameter along the direction of flow f between the inner surface 36 and the inner wall 38 can cause the permeate solution to be separated from the inner wall 38 of the permeate conduits 26, forming regions where the permeate solution recirculates in vortices. Larger and more intense vortices can irreversibly dissipate greater energy and pressure. The inventors propose to reduce these pressure losses by providing a more gradual divergence of the permeate solution at the exit of the interconnector, reducing flow separation and its associated pressure losses.
Referring to
However, it should be appreciated that the teachings herein are not necessarily restricted to gradually diverging/converging geometries and other inner surface profiles of the interconnectors can be utilized to reduce flow separation and its associated pressure losses.
The converging and diverging geometries can be optimized to reduce pressure losses over range of flows for a given filtration apparatus. Fluid dynamics theory suggests that converging and diverging angles in range 1 to 10 degrees can be suitable to significantly reduce pressure losses in some conventional filtration apparatuses.
By way of example, Table 1, with reference to
Case A resembles the interconnector 28 shown in
For Cases A and B, a computational fluid dynamics simulation was conducted to simulate the performance of the geometries in use.
Since pressure of the feed solution is limited by material and energy considerations, a lower pressure drop of the permeate solution across the pressure vessel raises the available pressure drop to drive the flow of permeate solution through the separation elements. The flow through the separation elements can vary directly with the applied pressure across the separation elements, and hence any reduction in pressure losses on the permeate side can enhance throughput of permeate solution.
Referring to
Referring to
Referring to
Referring to
Referring to
Referring to
Referring to
The interconnectors described herein can be manufactured by extrusion or injection molding, or by machining, or by a combination thereof. Materials such as engineering plastics and composite materials can be used to reduce dimensions of the interconnectors generally without sacrificing strength and the amount of membrane area that can be accommodated in a spiral-wound separation element.
Referring back to