Hollow fiber cartridge

A cartridge containing a plurality of hollow fiber membranes is disclosed. The cartridge comprises a plurality of fibers arranged in a bundle and at least one end of the bundle embedded in a tubesheet. The tubesheets are fitted with end caps to provide a chamber for the permeate. A feed tube extends longitudinally through the bundle and a permeate discharge tube is housed, preferably concentrically, within a feed tube. The cartridge does not require a high pressure seal, such as an O-ring seal, against the inner wall of the pressure vessel. The cartridge is configured as a single unit adapted for simple drop-in installation into a pressure vessel. Multiple cartridges may readily be inserted into a pressure vessel, and arranged so as to operate in series or in parallel. The hollow fiber membrane cartridge is adapted for industrial performance with high volumetric efficiency and high solute rejection. In addition, a process for installing the cartridges in a pressure vessel previously using spiral wound elements is disclosed.

FIELD OF THE INVENTION 
The present invention relates to improvements in fluid purification 
equipment, particularly of the reverse osmosis or ultrafiltration type. In 
particular, it relates to a fluid separation cartridge, particularly to a 
fluid separation cartridge using hollow fiber membranes having a selective 
permeability to fluid, particularly water. More particularly, it relates 
to a compact, unitary cartridge of hollow fiber membranes which readily 
enables multiple cartridges to be installed in a single pressure vessel, 
and may easily be installed in a pressure vessel, including a pressure 
vessel previously using spiral wound membranes. The cartridge is adapted 
for simplified installation and replacement on an as-needed periodic basic 
and is particularly useful for the purification and/or desalination of 
seawater, brackish water and wastewater. 
BACKGROUND OF THE INVENTION 
In this specification, the term "hollow fiber" refers to fibers of a 
generally tubular shape having a continuous passageway (or lumen) disposed 
substantially along the axial center line of the fiber. The term 
"membrane" refers to a porous or microporous material, typically a 
polymeric material, which may be in the shape of a hollow fiber or a film. 
The term "spiral" or "spiral wound" refers to membranes or membrane 
separation devices wherein the membranes are in the form of an asymmetric 
film or thin film composite material which is wrapped compactly around a 
central support. 
Hollow fiber bundles comprise a plurality of hollow, porous fibers which 
can be arranged in the shell or vessel. The hollow fiber membranes provide 
a large overall surface area for contact with the feed water. 
A fluid separation apparatus comprises a pressure vessel which houses one 
or more bundles of membranes. The fluid separation apparatus separates 
fluids by using a membrane having a selective permeability and may be 
applied to various techniques such as gas permeation, liquid permeation, 
dialysis, softening, ultrafiltration, reverse osmosis, or the like. 
Recently, attention has been particularly given to the reverse osmosis 
which is especially effective for desalination and purification of sea 
water or brackish water, for recovering useful or harmful components from 
waste water or for reuse of water. 
Fluid separation apparatuses are generally classified into flat membrane 
type, tubular type, spiral type and hollow fiber type according to the 
shape and form of the semi-permeable membrane used therein. Among these, 
the hollow fiber type and the spiral type membranes are especially well 
known in the art, particularly for reverse osmosis separations, such as 
desalination of sea water or brackish water and purification of 
wastewater. In fluid separation apparatuses which incorporate spiral wound 
membranes, the membranes have a disadvantageously large spacing between 
the membranes, often ten to twenty times greater than the spacing for 
comparable hollow fiber membranes. Moreover, the pressure vessels used for 
spiral wound membranes tend to be very long in order to operate with 
desirably high feed flow rates, at high conversion and prevent 
disadvantageous concentration polarization. Pressure vessels for spiral 
wound membranes are known to be more than twenty feet long. The flow path 
for the fluid flowing on the reject side of the membrane and through the 
pressure vessel is, therefore, also very long, which requires the large 
spacing between membranes, so as to enable reasonable pressure drops 
across the length of the pressure vessel. 
Hollow fiber fluid separation apparatuses solve most of these problems 
associated with spiral wound fluid separation apparatuses. For example, 
the flow paths for the fluid on the reject side of the membrane are 
relatively short, the radial flow path is short and the pressure drop 
across the bundle and down the annulus along the length of the device is 
not excessive. In addition, the spacing between hollow fiber membranes is 
typically small (generally about 25 microns vs. 25 mils for spiral wound 
membranes). As a result, a hollow fiber type apparatus has very high 
membrane separation efficiency per unit volume of the apparatus. Hollow 
fiber membranes are particularly suited for reverse osmosis separations. 
The use of the reverse osmosis membrane inherently requires appropriate 
membrane housing and associated plumbing connections to handle three 
separate water flows. Specifically, the membrane housing and plumbing 
connections must accommodate connection of the feed fluid to the membrane, 
as well as flow of the purified (permeate) and reject (residue) fluids 
from the membrane. In the past, spiral reverse osmosis membranes have been 
provided in a cartridge form with a view toward facilitated cartridge 
replacement on a periodic basis, but prior reverse osmosis systems have 
not provided any optimally simplified cartridges, especially hollow fiber 
cartridges, for quickly and easily installing and removing the cartridge 
in a substantially fail-safe, error-free manner. 
The reverse osmosis is usually carried out by treating a fluid under 
pressure higher than the osmotic pressure of the fluid, by which the 
components of fluid are separated via a membrane having a selective 
permeability. The feed pressure may vary with the kinds of fluids to be 
treated, the properties of the selectively permeable membranes, or the 
like, but is usually in the range from 10 to 1000 psig for spiral wound 
membranes and from 40 to 2,000 psig for hollow fiber membranes. 
The prior art describes hollow fiber membrane type fluid separation 
apparatuses where at least one pair of hollow fiber bundles are contained 
within a vessel; however, fluid separation apparatuses housing multiple 
hollow fiber bundles, especially more than two bundles, are not well known 
in practice. These devices may require complicated hardware and the fluid 
to be treated is separated by bundles typically in series, thereby 
disadvantageously reducing the volumetric efficiency of the apparatus. The 
hardware typically includes at least two O-rings at each end of the bundle 
of hollow fiber membranes. The O-rings form a seal between the outside of 
the bundle and the inner surface of the pressure vessel. The O-rings serve 
to hold the bundle in place within the pressure vessel and to seal various 
fluid streams from each other during operation of the device. The O-ring 
seals and the complicated hardware makes it difficult to install or 
replace the hollow fiber bundles in the pressure vessel. 
It is also known that failure of the hollow fiber membranes may necessitate 
prompt replacement of a bundle. For example, the fibers are fragile and 
may be easily damaged during transport, handling, assembly and operation 
of the fluid separation apparatus; as soon as fibers break or develop a 
fault, it may be necessary to replace the bundle of hollow fibers. 
Moreover, repair of damaged fiber is not economic. Therefore, it is highly 
advantageous that the hollow fiber bundles be readily installed and 
replaceable within the pressure vessel. However, as noted above, typically 
hollow fiber bundles use O-rings at the vessel inner diameter to seal the 
low pressure permeate from the high pressure feed fluid. This is achieved 
with a series of O-rings at each end of the bundle. The O-ring/bundle 
assembly must then be inserted with considerable mechanical force into the 
pressure vessel. Fiber bundle inspection and replacement is, as a 
consequence, difficult. 
Existing designs have many other disadvantages. For example, in many 
applications (such as shipboard or portable use) the maximum possible area 
of membrane must be contained in the smallest possible volume and it is 
desirable to have greater flow per unit volume of the device. Pressure 
vessels which house only one bundle of hollow fiber membranes require 
excessive external piping which is costly to install and takes up space. 
There is also a need for compact transportable equipment for mobile or 
military use. There is also a need that such equipment be at least 
partially assembled during transport and that it be easy to complete the 
assembly for rapid use in the field. 
Furthermore, it is desirable to have a range of sizes and dimensions of 
hollow fiber bundles made available for different applications. Varying 
feedstocks to be treated contain different amounts of impurities and for 
economy, those with few impurities should be treated at high flux rates 
through the membrane. Long cartridges containing fine hollow fibers are 
not able to provide a high velocity of drawoff of permeate because of the 
hydraulic pressure drop of flow in the narrow lumens of the fibers and 
hence short cartridges are required. Conversely, some feeds require longer 
cartridges where the lower membrane flux rates present no problems of 
lumen pressure drop. 
Another problem with prior art designs arises because different types and 
batches of fibers have different quality in terms of initial defects or 
service failure rates per unit of fiber surface area. There is also a risk 
of construction defects. It is, therefore, desirable to periodically test 
or inspect the membranes, which necessitates installing and removing 
numerous bundles from pressure vessels. 
The testing of bundles for defects also presents problems. Bundles, 
especially spiral wound bundles, are typically tested for failure by means 
of a bubble pressure test. Existing hollow fiber bundles must be installed 
into a pressure vessel for such testing. During testing, when water 
occupies all of the pores in the membrane, a certain pressure, known as 
the bubble point of the membrane, has to be exceeded to overcome the 
interfacial tension of the water in the pores. In the bubble pressure 
test, air is forced back into the lumens of the wetted fiber. Failed 
fibers allow air to pass through the fiber walls at a pressure lower than 
the bubble point of the membrane. The opacity of prior art pressure 
vessels does not allow visual detection of a failed hollow fiber. This 
problem further magnifies the need for a hollow fiber bundle cartridge 
which may be readily installed into and removed from the pressure vessel 
and a hollow fiber cartridge which may be tested without installing it 
into a pressure vessel. 
It is also desirable to have a hollow fiber membrane cartridge which may be 
readily installed in a pressure vessel which previously housed hollow 
fiber bundles or spiral wound elements. Unfortunately, in typical spiral 
wound devices the permeate fluid is discharged from a center tube. In 
typical hollow fiber devices the feed fluid is introduced through a center 
tube and the permeate fluid is discharged through another opening in the 
pressure vessel. Therefore, to retrofit an existing spiral wound bundle 
with a prior art hollow fiber bundle, it is necessary to use complicated 
pipes or fittings to "reverse" the flows of the feed fluid and the 
permeate fluid from the respective ports. 
The present invention provides a hollow fiber membrane cartridge which 
keeps the advantages of the prior art and multiple cartridges may readily 
be installed in and removed from a pressure vessel. The cartridge is 
particularly suitable for use in pressure vessels that previously housed 
spiral wound elements. The inventive cartridge is a simple, economical 
device. The flow paths, are designed so that the permeate discharges 
through a center tube, which facilitates the installation of the inventive 
cartridges in a pressure vessel that previously housed spiral wound 
elements. Each cartridge is equipped with a tubesheet having individual 
pressure end caps which may be connected to appropriate ports or fittings 
to deliver the permeate fluid or the residue fluid and/or for making 
external fluid connections to other cartridges. These cartridges can be 
manufactured so that economy, convenience and utility can be optimized by 
varying the number of fibers per cartridge. These objectives, as well as 
other objects, features and advantages of the present invention, will 
become apparent to those skilled in the art from the following 
description. 
SUMMARY OF THE INVENTION 
The present invention is a simple, efficient, low-cost cartridge which 
contains a plurality of hollow fiber membranes. Multiple cartridges may 
readily be installed in a single pressure vessel and during assembly the 
cartridges are readily installed in and removed from a pressure vessel; 
the cartridge is configured as a single unit adapted for simple drop-in 
installation into an open-ended pressure vessel. The term cartridge refers 
to a device comprising a bundle of hollow fiber membranes, having at least 
one tubesheet, end caps and internal piping, all as more fully described 
herein. The center of each cartridge has a feed tube extending 
substantially from one end to the other. A permeate discharge tube is 
housed, preferably concentrically housed, within the feed tube. A 
tubesheet is provided at, at least one end of each hollow fiber bundle, 
the lumens of the hollow fibers being opened into the tubesheet. Each 
tubesheet has pressure end caps sealed thereto. The end cap and the 
adjacent tubesheet define a space for collection of the permeate fluid. 
The space is in fluid communication with the internally housed permeate 
discharge tube. The end caps are securely engaged with the tubesheet so as 
to confine the low pressure permeate fluid within the cartridge. The end 
caps are equipped with fittings and/or ports to deliver the permeate fluid 
or the residue fluid. Such fittings and/or ports also permit the cartridge 
to be connected, in series or in parallel, to other cartridges. The 
cartridges optionally have seals attached thereto, such as lip seals at 
the feed end of the bundle, so as to facilitate installation of the 
cartridge in a pressure vessel. One or more cartridges may be installed in 
a single pressure vessel. The pressure vessel is typically cylindrical 
with end plates that are sealed to form a container. 
Another embodiment of the present invention is that multiple fluid 
separation apparatuses, each containing a plurality of bundles, may 
readily be connected in series to increase conversion at a given of feed 
flow. Multiple cartridges may be removably arranged longitudinally 
end-to-end in series by connecting the concentrically housed permeate 
discharge tubes for each adjoining cartridge, allowing the collection of 
permeate fluid from each subsequent bundle to be discharged in the tube 
that is concentrically housed in the feed tube. In addition, the annular 
spaces of each hollow fiber bundle are preferably in fluid communication 
with the central feed tube in the next successive cartridge. Optionally, 
some of the feed fluid may bypass a cartridge by having an orifice in, for 
example, the end cap, which permits some of the feed fluid to bypass each 
cartridge. If pressure drop across a bundle is too large, then the orifice 
may be adjusted to regulate the pressure drop and to optimize flow 
characteristics and conversion in the bundle. The bypass of feed fluid is 
especially useful when multiple hollow fiber bundles are stacked in large 
pressure vessels. The residue fluid from the last cartridge is ultimately 
collected and discharged through a common port. 
The flow of the fluids through the apparatus is typically so-called 
"inside-out" flow (although "outside-in" flow is also possible). The feed 
fluid passes into the fluid separation apparatus containing one or more 
cartridges through a central feed tube. The feed fluid is distributed to 
the first cartridge through distributing holes or perforations in the feed 
tube. The hollow fiber membranes are selective to one or more of the 
fluids, so such fluids will pass into the permeable fibers much more 
quickly than the other fluids. The permeate flows through the bores of the 
hollow fibers to a tubesheet and is collected at one or more ends of the 
fiber bundles, and then flows into a tube which is housed within the feed 
tube where it may be combined with permeate from other bundles, further 
treated or is discharged from the apparatus at either end. In summary, 
feed fluid generally travels through all bundles in the same pattern; 
i.e., radially outward from the feed tube into the hollow fibers, 
selectively permeating the hollow fibers. The permeate then exits the 
hollow fibers at open ends. The permeate from all of the cartridges in the 
pressure vessel is then removed from the fluid separation apparatus at one 
or both ends of the apparatus. The amount of permeate removed is a 
function of the properties of the hollow fiber, feed temperature, the 
composition of the feed fluid, the feed to permeate pressure differential 
and the flow rate of the feed fluid. 
The residue fluid is extracted simultaneously from all bundles by flowing 
radially outward through the bundles; the residue fluid does not readily 
permeate the hollow fiber membranes. The residue fluid is then collected 
in the annular space in between the hollow fiber bundle and the pressure 
vessel. The residue may then be combined with the bypass feed of the 
upstream cartridge. This combined stream may then become the feed for the 
next cartridge. The annular spaces around the respective cartridges may be 
also in fluid communication with each other. The residue fluid is 
ultimately discharged from the fluid separation apparatus. 
The inside-out flow scheme of the cartridge reduces the likelihood of 
nestling of the hollow fiber membranes. When the feed fluid is introduced 
to the outside of the hollow fiber bundle, flowing inward to the center, 
there is a tendency for the hollow fibers to nestle. This effect results 
in increased bundle pressure drop with accompanying reduction in permeate 
flow. The likelihood of fouling is also increased as fibers press more 
closely together. 
The cartridge is adapted for quick and easy drop-in or slide-in 
installation into a pressure vessel. In particular, the cartridges may 
readily be installed in a pressure vessel that previously housed spiral 
wound elements or hollow fiber bundles. The spiral wound elements or 
hollow fiber bundles may, for example, be replaced using the following 
method: 
1. Open one or both of the end plates of the pressure vessel. 
2. Remove the spiral wound elements. This may be accomplished by any method 
known in the art. In practice, the spiral wound elements or hollow fiber 
bundles are pushed out of one end of the pressure vessel by pushing the 
replacement cartridge or cartridges into the other end. Because prior art 
hollow fiber bundles form a pressure-tight seal with the inside wall of 
the pressure vessel, removal of such bundles may not be trivial. 
3. One or more hollow fiber membrane cartridges are then inserted into the 
pressure vessel. The cartridges may be specifically configured to fit 
within the pressure vessel. Alternately, so-called spacers may be added to 
ensure that the cartridges fit securely longitudinally within the length 
of the pressure vessel and/or circumferentially within the inner 
circumference of the pressure vessel. 
4. The cartridges are connected by appropriate fittings. The feed fluid is 
in communication with the center feed tube. The permeate is discharged 
through a port, typically a port located in the center of one or both of 
the end plates of the pressure vessel, which is in fluid communication 
with the concentrically housed discharge tube. The second end of the last 
cartridge is fitted such that the residue fluid port is in communication 
with the annular spaces around the cartridge. It is not necessary to use 
specialized piping or fittings in order to "reverse" the flow of the feed 
fluid and the residual fluid. 
The inventive cartridge facilitates the objects of the present invention. 
The fluid-separation apparatus may use any number of cartridges aligned in 
series or in parallel in a single pressure vessel. The cartridges may 
readily be installed into and removed from most conventional pressure 
vessels. The piping and fittings required to assemble multiple cartridges 
in a pressure vessel is minimal. External piping and porting around the 
apparatus are minimal resulting in the use of less space, greater 
durability and lower installation costs for the end user. More important, 
existing pressure vessels for traditional spiral wound permeators may 
readily be retrofitted with the inventive cartridge, thereby reducing the 
cost of replacing spiral wound bundles with hollow fiber bundles. 
Moreover, the cartridge of the present invention retains the advantageous 
features of the prior art.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows the hollow fiber cartridge of the present invention within a 
pressure vessel. In the Figures, "F" denotes the feed fluid, "P" denotes 
the permeate fluid and "R" denotes the residue or reject fluid. The 
cartridge is inserted into a pressure vessel, preferably an elongated 
cylindrical pressure vessel which may be enclosed by end caps (not shown). 
The inner walls of the vessel are designated 10. There is a tubesheet 30, 
at one end of the bundle of hollow fibers 20, where the fibers are joined 
or sealed with epoxy 34 and are cut back to expose the bores of the hollow 
fibers to fluid communication through the bundles. The cross-sectional 
configuration, (i.e., the configurations lying in a plane perpendicular to 
the longitudinal orientation of the hollow fiber membranes) of tubesheet 
is usually generally circular. It is also apparent that the 
cross-sectional configuration may be in any other form such as triangular, 
tribal, square, rectangular, trapezoidal, pentagonal, hexagonal, free 
form, or the like. The maximum cross-sectional dimension of the tubesheet 
may also vary substantially. The face of the tubesheet may be any suitable 
configuration and is generally substantially the same configuration as the 
cross-sectional configuration of the tubesheet. The face may be 
substantially flat or may be curved or irregular in surface contour. The 
tubesheet may contain one or more bundles of hollow fiber membranes, 
preferably one bundle. 
The other end of the bundle is sealed with an epoxy 32 so as to create a 
fluid-tight seal. 
The center of the cartridge contains a feed tube, 36, extending 
substantially from one end of the bundle to the other. Feed tube, 36, has 
holes or perforations bored in the wall of the feed tube so as to allow 
the feed fluid to radially flow into the bundle of hollow fiber membranes. 
The size, location and configuration of the openings is not critical; 
however, the openings preferably should facilitate uniform radial flow of 
the feed fluid into the bundle. The openings are preferably circular holes 
with a diameter of 0.1 cm to 2.0 cm. 
An end cap 28 is shown in a sliding engagement with the tubesheet 30. The 
end cap 28 may be securely sealed to the adjacent tubesheet 30 by any 
number of means known in the art. In practice, the end cap is sealed to 
the tubesheet and/or the bundle with epoxy. The end cap may be fabricated 
from any suitable material, but it is preferably made from plastic so that 
it may be molded into the desired configuration. The desired openings and 
orifices may be cut or bored into the end cap before or after attaching it 
to the tubesheet and/or bundle. The tubesheet and end cap may also be 
engaged in a threaded connection or any other suitable connection, instead 
of the sliding one shown. In addition, the end cap may be sealed to the 
circumference of the tubesheet by epoxy or any suitable means, so as to 
define a permeate collection space 38 which is at relatively low pressure 
during operation. The permeate collection space 38 is in fluid 
communication with the permeate discharge tube 40 which is housed inside 
the feed tube 36. The permeate discharge tube 40 may then be in fluid 
communication with adjacent cartridges or a discharge port through 
suitable sections, 41 and 42. The cartridge is fitted within the pressure 
vessel 10 so as to form space 60. The pressure vessel is not a part of the 
inventive cartridge, but is illustrated simply to show the relationship of 
the cartridge and the respective fluid streams during operation. The 
residue or reject fluid is collected in space 60, which is typically an 
annular space. 
The permeate collection space 38 is at low pressure compared to the 
pressure of the feed fluid. The end cap and the seal between the end cap 
and the adjacent tubesheet and/or bundle must be sufficiently strong to 
resist the pressure differentials within the cartridge. For example, the 
pressure of the feed fluid may be about 1200 psig. The pressure of the 
residue fluid in the annular space 60 surrounding the bundle may be about 
1180 psig. The pressure of the permeate fluid in the permeate collection 
space 38 may be about 20 psig. 
The inventive cartridge with the end cap/permeate collection 
space/internally housed permeate discharge tube configuration, therefore, 
eliminates the need for a tubesheet O-ring or other similar seal which 
seals against the inner wall of the pressure vessel and seals the high 
pressure residue from the low pressure permeate. 
The end cap may also be configured to maintain the appropriate spacing in 
the annular space; i.e., between the outer diameter of the bundle and the 
inner diameter of the bundle. In FIG. 1, the end cap 28 has a spacer 53, 
which may be of any suitable design or configuration. The spacer 53 firmly 
holds the cartridge, preferably concentrically, within the pressure 
vessel, but permits residue fluid to flow from the annular space 60 to the 
next cartridge and/or be discharged from the fluid separation apparatus. 
Feed pipe 36 and permeate discharge pipe 40 as well as fittings 41 and 42 
may be made of plastic material such as nylon, glass-reinforced plastic 
polyvinylchloride, fiber-reinforced epoxy resin; or metals such as 
stainless steel; carbon steel, or titanium. The dimensions thereof such as 
thickness, diameter and length are not specified and may be altered to 
achieve the desired function. The flexibility in choice of construction 
allows the selection of a corrosion resistant manufacturing material for 
the cartridge for a particular application. The materials of constructions 
may, for example, be chosen to resist high temperatures. 
In an alternate version, the end cap 28 may be shaped to facilitate 
stacking of the cartridges one on the other. The end cap at feed end of 
bundle may also incorporate an orifice 48 or a flow control valve that can 
restrict the fluid communication between cartridges. When cartridges are 
operating in series, such an orifice or a valve may be used to restrict 
flow out of one cartridge and, therefore to bypass flow to the fiber bores 
of the adjoining cartridge. This makes possible trimming of the flow 
through individual fiber bundles of a multi-bundle permeator assembly so 
the purity of each bundle cartridge in an assembly can be balanced with 
other cartridges. 
A support block may optionally be situated adjacent to the face of the 
tubesheet. Seal 47 serves to prevent leaks between different compartments 
of the permeator by isolating the water flows from each other. Seals, such 
as O-rings are not preferred because the O-rings make the cartridge 
extremely difficult to install into the pressure vessel. When the 
cartridge is installed into the pressure vessel, the seals are preferably 
so-called lip seals which utilize the upstream fluid pressure to force the 
seal against the wall. Thus, when the pressure is relieved the seal fits 
loosely against the wall of the vessel and the cartridge is easily 
removed. The seal 47 serves to seal the annular chamber 60 so that high 
pressure feed fluid does not leak and mix with the residue. As the bundles 
expand and contract or shift slightly with different operating conditions, 
the seal 47 can slide axially while maintaining a seal. The lip seals 
permit simple slide-in installation of the cartridge into the pressure 
vessel. In the present invention, the lip seal is used at the feed end of 
the cartridge. 
Lip seals are generally known for use with spiral wound elements, primarily 
because the pressure differentials of the fluids in a spiral wound element 
are relatively small. The lip seal typically will withstand pressure 
differentials of up to 50 psig, which is satisfactory for most spiral 
wound elements and not satisfactory for most hollow fiber cartridges. The 
configuration of the present hollow fiber cartridge eliminates the need 
for a seal which must withstand a large pressure differential. 
In the present embodiment, the permeate discharge tube 40 is concentrically 
housed with the feed tube; however, the size, configuration and location 
of the discharge tube 40, are not important as long as they can 
accommodate the flow of permeate fluid. This assembly allows simple 
assembly of multiple cartridges by simply inserting section 41 in between 
the facing ends of another cartridge so as to effect fluid communication 
between the permeate discharge tube of the first cartridge and the feed 
tube of the second cartridge. 
Port 51 provides means for external fluid communication for the feed fluid. 
Sections 41 and 42 provide external fluid communication for the permeate 
or product fluid. Sections 41 and 42 are preferably removable, so that 
they may be readily replaced with substitute fittings that are suitable in 
size, shape and configuration for the particular pressure vessel in which 
the cartridge will be installed. 
The cartridge optionally has a channel or an orifice 48 or a flow control 
which permits bypass, preferably controlled bypass, of feed fluid to space 
60. The orifice 48 may be situated in the end cap or any other convenient 
location, so as to permit fluid communication between some of the feed 
fluid and the residue fluid. For example, as shown in FIG. 2, an orifice 
or channel 48A may extend through the end cap so as to permit fluid 
communication between the feed fluid and the residue fluid outside of the 
end cap. Bypass of some of the feed fluid is particularly useful when 
multiple cartridges are operated in series. The bypass of feed fluid 
prevents excessive pressure differentials between the feed fluid and the 
residue fluid, which is a particular concern for the first several 
cartridges in a multicartridge system. Accordingly, the use of the orifice 
to bypass feed fluid facilitates the use of the lip seal. Moreover, the 
bypass of feed fluid may facilitate optimization of the flow 
characteristics and conversion of the successive cartridges. 
The embodiment of the invention shown in FIG. 1 is particularly suitable 
for the purification of sea water, brackish water or waste water. The flow 
of the respective fluids through the fluid separation apparatus can 
readily be demonstrated by describing the purification of salt water, as 
follows: Still referring to FIG. 1, salt water is fed into port 51, into 
the central feed tube 36 where it is simultaneously radially distributed 
through the openings in the feed tube to the membranes 20 of the hollow 
fiber bundle. The hollow fiber membranes are selective to one or more of 
the fluids, so such fluid will pass the permeable fibers more quickly than 
the other fluids. In this case, the hollow fiber membranes are selectively 
permeable to fresh water. The product water, or the permeate, flows 
through the center of each hollow fiber and is collected at the tubesheet 
30 end of the hollow fiber bundle in the permeate collection space 38, 
which is defined by the end plate 28 and the face of tubesheet 30. In 
summary, feed fluid generally travels through all bundles of hollow fiber 
membranes in the same pattern; i.e., radially from the feed tube into the 
hollow fibers, selectively permeating the hollow fibers. The permeate then 
exits the hollow fibers at the open ends adjacent to both end plates. The 
permeate is then fed to the adjoining bundle of hollow fibers. The amount 
and purity of permeate removed is a function of the properties of the 
hollow fiber, the feed temperature, the composition of the feed fluid, the 
feed to permeate pressure differential, and the flow rate of the feed 
fluid. 
The residual salt water flows radially outward through the bundle of hollow 
fiber membranes. The residual salt water is then collected in the annular 
space 60 adjacent to the wall of the pressure vessel 10 and thereafter 
flows longitudinally to the tubesheet end of the bundle and around the end 
cap where it may be collected. The residue fluid can be combined with the 
bypass from the first cartridge and then fed into the next cartridge or 
discharged from the pressure vessel. 
The present invention facilitates inside-out flow of the fluid to be 
treated, although the cartridge may also be used for so-called 
"outside-in" flow. The "inside-out" flow has certain advantages. In 
particular, the fluid separation is usually operated at high pressure, 
ranging from 10 to 2,000 psig. When the fluid flows at high pressure from 
the outside of the bundle inward, the fibers tend to nestle together, 
thereby resulting in non-uniform flow of the feed fluid radially through 
the bundle and an increase in the pressure drop radially through the 
bundle, along with an accompanying reduction in permeate flow. In addition 
to the increased pressure drop, it is believed that the feed fluid cannot 
uniformly access the outer surface of each hollow fiber, thereby reducing 
the overall effectiveness of the bundle and increasing the possibility of 
fouling. Inside-out flow of the fluid to be treated significantly reduces 
the likelihood of nestling of fibers. 
FIG. 2 is a cross-sectional view of a cartridge with a so-called 
double-ended bundle, which has tubesheets on both ends of the hollow fiber 
bundle. 
This embodiment comprises tubesheets 30 and 31, at both ends of the bundle 
of hollow fibers 20, where the fibers are joined or sealed with epoxy 34 
and are cut back to expose the bores of the hollow fibers to fluid 
communication through the bundles. 
The center of the cartridge contains a feed tube 36 extending substantially 
from one end of the bundle to the other. Feed tube 36 has holes or 
perforations bored in the wall of the feed tube so as to allow the feed 
fluid to radially flow into the bundle of hollow fiber membranes. 
End caps 28 and 29 are shown in a sliding engagement with the tubesheet. In 
practice, the end cap is sealed to the tubesheet and/or the bundle with 
epoxy. The end caps may be sealed to the circumference of the tubesheet by 
epoxy or any suitable means, so as to define low pressure permeate 
collection spaces 38 and 39. The permeate collection spaces 38 and 39 are 
in fluid communication with the permeate discharge tube 40 which is housed 
inside the feed tube 36. The permeate discharge tube 40 may then be in 
fluid communication with adjacent cartridges or a discharge port through 
suitable sections, 41 and 42. The cartridge is fitted within the pressure 
vessel (not shown) so as to form space 60. The residue or reject fluid is 
collected in space 60, which is typically an annular space. 
Port 51 provides means for external fluid communication for the feed fluid. 
Sections 41 and 42 provide external fluid communication for the permeate 
or product fluid. 
As shown in FIG. 3, a fluid separation apparatus may be adapted to hold a 
plurality of the cartridges of the present invention. The number of hollow 
fiber cartridges to be contained in a fluid-separation apparatus may be 
varied after taking into consideration the size of the pressure vessel, 
overall pressure loss in flow of fluid within the apparatus and 
performance of hollow fibers at a high flow speed in the assembly. The 
number of hollow fiber cartridges is usually in the range of two to ten, 
preferably two to four. 
FIG. 3 shows a fluid separation apparatus where three hollow fiber 
cartridges are used. The fluid separation apparatus has three hollow fiber 
bundles in one pressure vessel. The fluid separation apparatus has three 
hollow fiber cartridges designated as 60, 60, and 60" in a single pressure 
vessel 62. The cartridges may be the same or different. The fluid to be 
treated is supplied via port 64 into cavity 65 which is in fluid 
communication with center feed tube 66. The feed fluid is distributed to 
the first hollow fiber bundle 60 via openings in the feed tube 66. The 
fluid which permeates the hollow fiber membranes is collected in the space 
designated as 68 which is in fluid communication with permeated discharge 
robe 70. The residue fluid does not readily permeate the hollow fiber 
membranes and flows into the annular spaces 80, 80' and 80" in between 
each hollow fiber bundle and the pressure vessel 62. The residue or reject 
fluid is collected in annular space 80 which is in fluid communication 
with space 85 which is between the facing end caps of adjacent cartridges. 
The residue then flows to the second cartridge 60', where it becomes the 
feed fluid. The permeate fluid follows the same path through the second 
and third cartridges 60' and 60" and is discharged from the pressure 
vessel via port 81. Thereafter, the residue fluid from all of the 
cartridges is discharged from the pressure vessel via port 82. The 
cartridges optionally comprise an orifice, preferably in the end cap, 
which permits fluid communication between the feed fluid for each 
respective bundle and the residue fluid for the bundle. The orifice is 
shown in FIG. 3 as 87 and 87'. The bypass of feed fluid facilitates the 
balancing of the concentration of residue in the feed fluids for the 
respective cartridges. In effect, the multiple cartridges are operating 
partially in parallel and partially in series. 
To assemble the device, the fiber cartridges are first individually 
assembled, then the cartridges may be inserted one by one. As each 
cartridge is inserted into the pressure vessel it is connected to the next 
cartridge by suitable piping. Alternately, each bundle may first be 
connected to each other by suitable piping. When all cartridges are 
assembled and connected, end fitting 41 engages the port 81 at the far end 
of the pressure vessel casing. The length and configuration of fittings 41 
and/or 42 may be adjusted to ensure that the cartridges securely fit 
within the pressure vessel housing. For replacement of the fiber bundles 
or servicing, the fiber bundles can be removed by reversing the assembly 
procedure. 
The cartridges may also readily be used to replace spiral wound bundles in 
a pressure vessel. In operation, the spiral wound bundles typically employ 
straight plug flow of the fluid to be treated. In particular, the fluid to 
be treated enters the pressure vessel. The feed fluid flows straight 
through the bundle through the brine channels of each cartridge. The 
spiral wound membranes are selective to one or more fluids which permeate 
through the membrane and are ultimately collected in the product channel 
between the two sides of the flat membrane. The permeate flows spirally 
inward and communicates to a center product tube. The residue fluid does 
not permeate through the spiral wound membrane and flows through wide 
reject channels and out the opposite end of the permeator. 
As noted above, hollow fiber membranes perform more advantageously than 
spiral wound membranes for many applications. The spiral wound elements or 
existing hollow fiber bundles may be replaced with hollow fiber cartridges 
as follows: 
1. Open one or both of the end plates of the pressure vessel. 
2. Remove the spiral wound bundles. This may be accomplished by any method 
known in the art. In practice the spiral wound element or hollow fiber 
bundles are pushed out of one end of the pressure vessel by pushing the 
replacement cartridge or cartridges into the other end. Because prior art 
hollow fiber bundles form a pressure-tight seal with the inside wall of 
the pressure vessel, removal of such bundles may not be trivial. It is 
often necessary to use hydraulic equipment to force the hollow fiber 
bundle out of the pressure vessel. 
3. One or more hollow fiber membrane cartridges are then inserted into the 
pressure vessel. The cartridges may be specifically configured to fit 
within the pressure vessel. Alternately, so-called spacers may be added to 
ensure that the cartridges fit securely longitudinally within the length 
of the pressure vessel and/or circumferentially within the inner 
circumference of the pressure vessel. 
4. The cartridges are connected by appropriate fittings. The feed fluid is 
in communication with the center feed tube. The permeate is discharged 
through a port, typically a port located in the center of one or both the 
end plates of the pressure vessel, which is in fluid communication with 
the concentrically housed discharge tube. The reject port of the pressure 
vessel is in communication with the annular spaces around the last 
cartridge. 
The hollow fiber membranes may be of any convenient configuration, e.g., 
circular, hexagonal, trilobal, or the like in cross-section and may have 
ridges, grooves, or the like extending inwardly or outwardly from the 
walls of the hollow fiber membranes. The hollow fiber membranes are useful 
in fluid separations, i.e., they may serve as the support for coating 
which provides selective separation or as the medium which affects the 
separation. The hollow fibers used in the present invention include all 
fibers having an inner diameter of about 20 microns to about 200 microns, 
preferably about 40 microns, and a hollow ratio (being the area of the 
fiber bore divided by the area of the total cross-section of the fiber) of 
about 10% to about 50% percent, preferably about 20%. The dimensions of 
the fibers and hollow ratio, as well as the dimensions of the pressure 
vessel, are dependent in part on the operating pressure. In general, the 
hollow fibers must have a thicker wall, resulting in a lower hollow ratio, 
for higher operating pressures. The membranes may be fabricated from 
various polymers such as cellulose, cellulose esters, cellulose ethers, 
polyamides, silicone resins, polyurethane resins, unsaturated polyester 
resins or the like, or ceramics. 
The potting material to form the tubesheet may be comprised of any suitable 
material. Preferably the potting material can be in liquid form when 
preparing the tubesheet and can thereafter be solidified, e.g., by 
cooling, curing, or the like. The solidified potting material should 
exhibit sufficient structural strength for providing a tubesheet and be 
relatively inert moieties to which it will be exposed during fluid 
separation operation. The potting material may be organic, preferably 
epoxy, or inorganic or organic containing inorganic material, and the 
potting material may be natural or synthetic. Typical inorganic materials 
include glasses, ceramics, cermets, metals and the like. 
The pressure vessel used in the present invention is preferably a 
cylindrical vessel having an inner diameter of 5 cm. to 50 cm., most 
preferably about 25 cm., but the shape of the pressure vessel is not 
necessarily restricted. The thickness of the wall of the pressure vessel 
must be adapted to the specific operation conditions, particularly to 
operate safely at the operating pressure. 
The rugged construction of the cartridges enables them to be transported 
over rough terrain or delivered by helicopter into difficult sites. The 
only pipework needed is at each end of the pressure vessel. The cartridge 
is lightweight and also provides versatility of shapes, dimensions and 
configurations. Cartridges of differing and variable dimensions can easily 
be interchanged. Thus adjustment of cartridges for different feedstock 
properties or to accommodate different pressure vessel is possible. 
Insertion of the cartridge into the pressure vessel is made easy because 
the cartridge comprises a sealing means, preferably a simple lip seal. 
It is advantageous that multiple cartridges may be installed in a single 
pressure vessel. Moreover, one basic cartridge size can be joined to other 
cartridges in as simple assembly to provide any size fluid separation 
apparatus. Only a single pressure vessel is needed for each assembly of 
cartridges thus reducing cost of assembly, the cost of multiple pressure 
vessels, the cost of external piping and the cost of racks to hold a 
multitude of fluid separation apparatuses, ultimately reducing the cost 
per volume of permeate. In the assemblies using cartridges with the shaped 
end caps, the piping may be further simplified for any number of 
cartridges in an assembly. 
The cartridges of the present invention may also be advantageously tested 
for leaks. The cartridge may be tested for leaks without installing it in 
a pressure vessel. In particular, a bubble pressure test may be conducted 
on the cartridge, wherein the permeate discharge tube is filled with air 
and the end is plugged. The hollow fibers are then immersed in water and 
the bores of the hollow fibers are exposed to slight air pressure. Failed 
fibers allow air to pass through the fiber walls at a pressure lower than 
the bubble point of the membrane. The cartridge is observed to ascertain 
whether air bubbles appear in the water. It is also possible to discern, 
from the location of the air bubbles, to distinguish whether the leak is 
from the tubesheet or the hollow fibers. The cartridges may also be 
readily removed from a pressure vessel for periodic bubble testing. 
The fluid separation apparatus of the present invention may be applied to 
desalination of sea water, desalination of brackish water, purification of 
various kinds of waste water, preparation of ultrapure water, 
ultrafiltration such as recovery of paint from waste water in 
electrodeposition painting, liquid permeation such as separation of 
para-xylene from a xylene mixture, gas permeation such as recovery of 
helium and purification of hydrogen, and the like. In any case, it is 
possible to carry out efficiently a large-scale treatment by using the 
apparatus of the present invention.