Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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T~'~ Ig: Multi-Stage Filtration and Softening Module and Reduced Scaling
Operation
Field ofthe Invention
This invention relates to a multi-stage filtration module and to
processes for using such a module to filter water and to remove harness,
particularly in small scale systems.
Background ofthe Invention
Hollow fibre semi-permeable membranes are useful for filtering
solids rich fluids. Membranes in the ultrafiltration, nanofiltration and
reverse osmosis range are also useful for separating salts. For example,
U.S. Patent No. 5,152,901 describes a nanofiltration membrane material
capable of filtering out suspended solids and large organic molecules
and generally rejecting calcium salts while generally permeating sodium
salts. Such a membrane, and others with similar characteristics, are
useful for filtering and softening a potable or domestic water supply.
Membranes as described above may be used in the form of hollow
fibres operated in an inside-out flow mode. The hollow fibres are
suspended between a pair of opposed tube sheets or headers. The
headers fluidly separate the lumens of the membranes from their outer
surfaces. Thus, pressurized feed water can be supplied to the lumens of
one end of the membranes, permeate can be collected as it leaves the
outer surface of the membranes, and a concentrate or retentate can be
extracted from the lumens at the other end of the membranes.
Various characteristics of hollow fibre membranes, however, make
them difficult to use in such an inside-out flow mode. For example, the
inner diameter of the hollow fibre is small which results in significant
pressure and flux reductions towards the outlet end of long hollow fibres.
This often results in significantly reduced flux through the fibres near their
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outlet ends. The problem is most significant when the feed pressure is
low, for example when treating water obtained from an ordinary municipal
water distribution network without using pumps to pressurize the feed
water.
U.S. Patent No. 5,013,437 describes one method of attempting to
correct the problem of pressure and flux loss in long fibres. In an
embodiment of that patent, an inside-out hollow fibre filtration module is
split into two stages. The retentate from the first stage becomes the feed
for the second stage. The ratio of the surface areas of the first to the
second stages is preferably about 1.5 : 1 to 2.25 : 1. This helps to
increase the pressure and velocity of the retentate from the first stage as it
becomes the feed to the second stage such that both stages have more
nearly equal pressure drops. The stages are arranged concentrically,
however, and permeate, particularly from the second stage, must flow
along the outside of the fibres to reach an outlet port. With a reasonable
packing density of hollow fibre membranes, the head loss in the permeate
flow is substantial. Thus the transmembrane pressure differential across
the membranes of the second stage is significantly reduced resulting in
decreased production.
A similar principle has also been used in large scale systems
using spiral wound membranes. A large number of membrane modules
are arranged in stages. Each successive stage has fewer modules than
the preceding stage and the retentate from preceding stages becomes the
feed of the succeeding stages. The number of modules in succeeding
stages is chosen to maintain an approximately constant feed or retentate
velocity through the system. Such a system is both large and complex and
not suited to residential or small commercial systems.
Another characteristic of hollow fibre membranes, is that their pores
become fouled over time, for example, because of carbonate scaling. In
large scale systems, carbonate scaling may be addressed by partially
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softening the feed water using resin exchange beds or by adding an acid
or an anti-scalant to the feed water. Such techniques are generally not
practicable in small scale systems.
Makers of small scale membrane filtration systems typically try to
address the problems above by using a single stage filtration module and
recirculating the retentate to the feed inlet. This technique requires a high
rejection, high permeability membrane which must be operated at a very
low per pass recovery. This leads to rapid fouling and either frequent
cleaning or replacement of the membranes. Energy costs are also high
because of the high rate of recirculation.
Summary of the Invention
It is an object of the invention to improve on the prior art. It is
another object of the invention to provide a filtration module, particularly
one that is useful for small scale filtration of water. It is another object
of
the invention to provide a process to reduce scaling of such a module
used to soften water and to provide a module that may be used with that
process.
In various aspects, the invention provides a filtration module having
a plurality of preceding or succeeding stages (some stages being both
preceding and succeeding) of hollow fibre membranes suspended
between opposed headers. The lumens of the hollow fibre membranes
are open at first and second ends of the stages. A module feed inlet is
connected in fluid communication with the first end of a first stage. The
remaining stages are connected in series behind the first stage with fluid
connections between the second end of each preceding stages and the
first end of each directly succeeding stage. A module outlet is connected
in fluid communication with the second end of a last stage. A permeate
collection plenum surrounds the stages and is in fluid communication
with each stage. Permeate from each stage may flow to the permeate
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plenum without flowing through other stages. The surface area of the
membranes of each preceding stage is between 1 and 2.5 times the
surface area of the membranes of a directly succeeding stage and the
surface area of the stages decreases from the first stage to the last stage.
To construct the connections between the stages, the outer
surfaces of the membranes are sealed to the headers while their lumens
are made open at the distal faces of the headers. A first cap covers the
distal face of one header and a second cap covers the distal face of the
other header. The permeate plenum includes the space between the
proximal faces of the headers and an outer shell. Dividers within one or
both of the caps collect groups of the membranes into the stages while
leaving open fluid connections between the second end of each preceding
stage and the first end of each directly succeeding stage. The module
inlet and module retentate outlet are provided in the caps so as to be in
fluid communication with the first end of the first stage and the second end
of the last stage respectively. Thus feed water enters the first end of the
first stage and the portion not permeated exits the second end of the first
stage. From there, the second end cap directs it to the first end of the
second stage. The water not permeated in the second stage arrives at the
first cap. In a two stage device, the water not permeated then leaves the
module. In a module with more stages, the first cap redirects the water to
the first end of another stage and the water not permeated flows to the
second cap and so on until the second end of the last stage is reached.
The stages are arranged so that each is adjacent the perimeter of the
module and interstage flows are generally parallel to the periphery of the
module. Further, all permeate can flow directly through the path of
minimum head loss to the permeate plenum which includes space within
the group of membranes and, optionally, around the perimeter of the
module. For example, the stages may be configured as sectors of a
cylinder.
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The dividers are fitted with valves and arranged such that when feed
water flows into the module in a reverse direction, entering through the
module retentate outlet, the dividers re-collect the groups of membranes
into second preceding and second succeeding stages having first and
5 second ends. The dividers leave open fluid connections generally parallel
to the periphery of the module between the second end of each second
preceding stage and the first end of each second succeeding stage. In
the re-collection of the membranes, the surface area of the membranes of
each second preceding stage is between 1 and 2.5 times the surface area
of the membranes of a second directly succeeding stage and the surface
area of the stages decreases from the first stage to the last stage. This is
achieved by using one way valves opening in a direction such that the
grouping and re-grouping of membranes is performed by the action of
liquid flowing through the module, ie. opening valves where the pressure
differential is in the direction that the valve opens and closing valves where
the pressure differential is opposite the direction that the valves open.
Such a module is used to filter water and can be used to remove
hardness when optionally fitted with hollow fibre membranes adapted to
selectively reject hardness causing salts. Water to be filtered flows
through the stages in series while a filtered and optionally softened
permeate is collected from the outer surfaces of the membranes. When
the module is used to provide a softened permeate, carbonate scale may
form in the membranes. To control scaling, carbon dioxide or other
suitable acids may be added to the feed water before the feed water
enters the lumens of the hollow fibre membranes. The carbon dioxide
may be added continuously to the feed water in amounts such that the
Langelier Index is zero or slightly negative. Alternatively, the carbon
dioxide may added to the feed water from time to time, preferably when the
need for permeate is low and either no permeate is produced or the
permeate produced during these times is discarded. Further, the direction
of flow through the module can be reversed while carbon dioxide is being
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added to apply the maximum concentration of acid to the most heavily
scaled stage.
Brief Description of Drawings
Embodiments of the invention will be described below with
reference to the following figures.
Figure 1 shows a partially cut away elevation of a membrane
module which may be used as a four stage module.
Figure 2 shows a plan view of the module of Figure 1 with top cap
removed.
Figures 3 and 4 show the forward and reverse flow respectively
through a three stage module.
Figures 5 and 6 show the forward and reverse flow respectively
through a four stage module.
Description of Embodiments
Figures 1 and 2 show a filtration module 10. The module 10 has a
plurality of filtering hollow fibre membranes 12 suspended between
opposed headers 14. The membranes are typically in the reverse
osmosis, nanofiltration or ultrafiltration range, preferably in the
nanofiltration range and more preferably able to selectively retain
hardness causing salts and permeate softened water. The ends 16 of the
membranes 12 are potted in a closely spaced relationship in the headers
14 such that their outer surfaces are sealed to the headers 14 and the
lumens of the membranes 12 are open at the distal faces of the headers
14. A first cap 20 and a second cap 22 cover the distal faces of the
headers 14 and are sealed to the headers 14. The membranes 12 are
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arranged into groups 24 each group separated by an area of the headers
14 having no membranes 12 potted in it. The membranes 12 may be
maintained in groups 24 during potting by wrapping ends of groups in an
expandable plastic mesh. Dividers 26 within one or both of the caps 20,
22 (and optionally formed as part of the caps 20, 22) extend from the distal
surface of the caps 20, 22 to sealingly contact some or all of the areas of
the headers 14 having no membranes 12. Optionally, the dividers 26 may
be inserted into the headers 14 during potting in which case the dividers
26 help separate groups 14 of membranes 12 and the dividers 26
become bonded to the headers 14. Some or all of the dividers 26 may
have openings which may include one or more one way valves 28, typically
flap valves, located within them.
The perimeter of the module 10 is surrounded by a casing 30. The
volume inside of the casing 30 between the proximal faces of the headers
14 and not occupied by membranes 12 forms a permeate plenum 32.
The permeate plenum 32 includes the space around the membranes 12
and may also include an open space adjacent the periphery of the module
10 in direct fluid communication with each of the groups of membranes
12. A permeate outlet 34 in fluid communication with the permeate
plenum 32 allows permeate to be removed from the module 10. Thus
water permeated through the membranes 12 in a group of membranes 12
can flow directly to the permeate outlet 34 through the path of least
resistance and is not required to flow through the groups 24 in a selected
path. This can be achieved by arranging the groups of membranes 12 as
sectors of a cylinder as shown. Other configurations are also possible.
For example, in a square or rectangular module groups of membranes 12
of various sizes can be located on either side of a centre line of the
module. A module feed inlet 36 admits feed water into one of the caps
20, 22. Retentate leaving the module 10 flows out of a module feed outlet
38 also located in one of the caps 20, 22.
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Referring to Figures 3 and 4, a three stage module 110 is divided
into four groups 124a,b,c,d of membranes (not shown). The arrangement
and potting of membranes in groups 124 allows the dividers 26 to isolate
groups 124 with no membranes improperly crossing over divisions
between stages. The size of the groups are 1/6, 1/6, 1/3 and 1/3
respectively of the size of the entire amount of membranes. Dividers 26
comprise solid dividers 40 and one way dividers 42 in the locations
shown. The one way dividers 42 open to allow flow in the direction
shown. The dividers 26, 40, 42 divide the groups 124 into stages I, II and
III depending on the direction of feed flow.
In Figure 3, feed flows first into group 124a through the module feed
inlet 36 in the first cap 20. The feed may flow into group 124d and groups
124a and 124d form stage I. Feed/retentate flows in stage I to the second
cap 22 where it flows over to group 124c which forms stage II.
Feed/retentate in stage II flows back to the first cap 20 where it flows over
to group 124b which forms stage III. Feed/retentate is prevented from
flowing back into stage I through the one way divider 42 by the greater
pressure in stage I, a pre-requisite for having flow from stage I to stage II.
Feed/retentate flowing in stage III flows to the second cap 22 where it
leaves the module 110 through the module retentate outlet 38. Through
all stages, permeate flows from each stage directly to the permeate
plenum (not shown) and out through the permeate outlet (not shown).
Stages I, II and III thus involve 1/2, 1/3 and 1/6 of the total amount of
membranes respectively.
In Figure 4, the feed and retentate flows are reversed. Feed flows
first into group 124b through the module retentate outlet 38 in second cap
22. The feed may flow into group 124c and groups 124b and 124c form
stage I. Feed/retentate flows in stage I to the first cap 20 where it flows
over to group 124d which forms stage II. Feed/retentate in stage II flows
back to the second cap 22 where it flows over to group 124a which forms
stage III. Feed/retentate flowing in stage III flows to the first cap 20 where
it
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leaves the module 110 through the module feed inlet 36. Stages I, II and
III thus still involve 1/2, 1/3 and 1/6 of the total amount of membranes
respectively. As above, in some places undesired flow through the one
way dividers 42 is prevented by the pressure gradient between phases I, II
and III.
Figures 5 and 6 show a four stage module 210 of similar operation.
Other modules with up to six stages can also be created. Modules with
even more stages might also be possible, but the complexity of such a
module would be a concern. The four stage module has five groups
224a,b,c,d,e of membranes (not shown). The size of the groups are 1/8,
1/4, 1/4, 1/4 and 1/8 respectively of the size of the entire amount of
membranes. Dividers 26, 40, 42 as discussed above divide the groups
224 into stages I, II and III depending on the direction of feed flow.
In Figure 5, feed flows first into group 224a through the module feed
inlet 36 in the first cap 20. The feed may flow into group 224b and groups
224a and 224b form stage I. Feed/retentate flows in stage I to the second
cap 22 where it flows over to group 124c which forms stage II.
Feed/retentate in stage II flows back to the first cap 20 where it flows over
to group 224d which forms stage III. Feed/retentate flowing in stage III
flows to the second cap 22 where it flows over to group 224e which forms
stage IV. Feed/retentate flowing in stage IV flows to the first cap 20 where
it leaves the module 210 through the module retentate outlet 38. Through
all stages, permeate flows from each stage directly to the permeate
plenum (not shown) and out through the permeate outlet (not shown).
Stages I, II, III and IV thus involve 3/8, 1/4, 1/4 and 1/8 of the total
amount of
membranes respectively.
In Figure 6, the feed and retentate flows are reversed. Feed flows
first into group 224e through the module retentate outlet 38 in first cap 20.
The feed may flow into group 224d and groups 124d and 124e form stage
I. Feed/retentate flows in stage I to the second cap 22 where it flows over
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to group 124c which forms stage II. Feed/retentate in stage II flows back to
the first cap 20 where it flows over to group 124b which forms stage III.
Feed/retentate flowing in stage III flows to the second cap 22 where flows
over to group 124a which forms stage IV. Feed/retentate in stage IV flows
5 back to the first cap 20 where it leaves the module 210 through the module
feed inlet 36. Stages I, II and III thus still involve 3/8, 1/4, 1/4 and 1/8
of the
total amount of membranes respectively. As above, in some places in
both Figures 5 and 6, undesired flow through the one way dividers 42 is
prevented by the pressure gradient between phases I, II, III and IV.
In summary, the modules 10, 110, 210 are provided with a plurality
of preceding or succeeding stages (I, II, II etc.), some stages being both
preceding and succeeding stages. The module feed inlet 36 into one of
the caps 20, 22 is connected in fluid communication with the first end of a
first stage. The dividers 26, 40, 42 collect the groups 24 of membranes 12
into preceding and succeeding stages having first and second ends, the
lumens of the membranes open to the ends. The dividers 26, 40, 42 also
leave open fluid connections created by the caps 20, 22 between the
second end of each preceding stage and the first end of each directly
succeeding stage. The fluid connections between stages permit an
interstage flow of retentate/feed that is generally parallel to the periphery
of
the module 10, 110, 210. For example, with the pie shaped stages
illustrated, interstage flows flow around the centre of the module although
it is not necessary that the interstage flows be geometrically perfect
circles. The module retentate outlet 38 from one of the caps 20, 22 is in
fluid communication with the second end of a last stage. The surface area
of the membranes of each preceding stage is between 1 and 2.5 times
the surface area of the membranes of a directly succeeding stage and the
surface area of the stages decreases from the first stage to the last stage.
This exact sizes of the stages can be selected to provide a nearly uniform
velocity through the module 10 despite permeation with limited variation in
velocity between the stages.
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The dividers 26, 40, 42 are fitted with valves and arranged such that
when feed water flows into the module in a reverse direction, entering
through the module retentate outlet 38, the dividers 26, 40, 42 re-collect
groups 24 of membranes 12 into second preceding and second
succeeding stages having first and second ends, the lumens of the
membranes open to the ends. Under reverse flow the dividers 26, 40, 42
leave open fluid connections created by the caps 20, 22 between the
second end of each second preceding stages and the first end of each
second succeeding stage. The surface area of the membranes of each
second preceding stage is between 1 and 2.5 times the surface area of
the membranes of a second directly succeeding stage and the surface
area of the stages decreases from the first stage to the last stage. The
valves are one way valves 28 and, to the extent that they are required to
move, they open in a direction such that the grouping of and re-grouping of
the stages in fonrvard and reverse flow is performed by the action of liquid
flowing through the module 10.
To permit flow through the module 10 to change directions, feed
and retentate lines to and from the module 10 are provided with valves,
typically solenoid valves, that allow each line to be connected to either the
module feed inlet 36 or the module retentate outlet 38. The valves are
operated simultaneously by a PLC or timer such that both the feed and
retentate lines are not both connected to the same point on the module 10
at the same time.
Modules can also be constructed to used with flow in one direction
only. Such modules are simpler to construct but may have a shorter
service life than a module with reversing flow. Nevertheless, for small
systems, a module designed for flow in one direction only may be more
cost efficient. Referring to Figure 3, a three stage module for flow in one
direction only is made by providing a solid divider in the first cap 20
between groups 124c and 124d and between groups 124b and 124a and
in the second cap 22 between groups 124c and 124b and between
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groups 124b and 124a. All other dividers 26, 40, 42 shown in Figure 3 are
omitted. Referring to Figure 5, a four stage module for flow in one
direction only is made by providing a solid divider in the first cap 20
between groups 224b and 224c, groups 224d and 224e and groups 224e
and 224a and in the second cap 22 between groups 224c and 224d and
groups 224e and 224a. All other dividers 26, 40, 42 shown in Figure 3 are
omitted.
Where the module 10 is used to soften water, the water to be
filtered flows into a first end of the lumens of the membranes 12 which are
chosen to selectively reject, ie. retain, hardness causing salts. A softened
permeate is collected from the outer surfaces of the membranes 12 and a
retentate is collected from the second end of the lumens of the
membranes 12 and either exits the module 10 or flows to the next stage.
Hardness causing salts thus build up in the lumens of the membranes
12, particularly in the last stage. Periodically reversing the direction of
feed
flow through the hollow fibre membranes, such that water to be filtered
flows into the second end of the lumens and retentate flows out of the first
end of the lumens, helps distribute this scaling more evenly and extend
the life of the module 10.
To further extend the life of the module, carbon dioxide is added to
the feed water before the feed water enters the lumens of the hollow fibre
membranes. Other acids may be used for the scale control, but carbon
dioxide is preferred. Carbon dioxide is non-hazardous and suitable for
human ingestion. Carbon dioxide is also self limiting for very hard waters
with buffering capability, that is excessive dosages do not result in very low
pH and potentially unsafe water quality.
In one method, a small amount of carbon dioxide is added
continuously to the feed water. The flux of carbon dioxide is selected so
that the Langelier Index is zero or slightly negative at which point the feed
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water is non-scaling but only minimally corrosive. At such doses,
however, scaling is still controlled for most feed waters.
In another method, carbon dioxide is added to the feed water only
from time to time, for example once a day, and preferably when the need
for permeate is low. During such descaling events, the permeate outlet 34
may be closed with a valve to prevent permeation or permeate produced
during the descaling discarded. In either case, higher fluxes of carbon
dioxide can be used for rapid or intensive cleaning. The two methods
above may also be combined, providing continuous carbon dioxide
addition to the feed and a once a day intensive descaling.
The two methods above may also be advantageously combined
with flow reversal as described further above. With carbon dioxide
continuously added to the feed, the supply of carbon dioxide switches
between the module feed inlet 36 and the module retentate outlet 38 along
with the feed water. Thus, the first and last stages of the module 10
alternate between relatively low hardness water with high carbon dioxide
concentration and relatively high hardness water with low carbon dioxide
concentration, the carbon dioxide concentration decreasing with travel
through the module 10. Thus the carbon dioxide is added to the feed flow
while the feed flows first into the most heavily scaled part of the module at
least during a period right after the flow is reversed.
With carbon dioxide added periodically, the flow reversal is also
done only periodically and timed to coincide with the addition of carbon
dioxide to the feed. Thus, for most of the day feed flows in the forward
direction and scale builds up in the last stage. During an off-peak period,
flow is reversed and carbon dioxide is added to the feed. Thus the carbon
dioxide is added to the feed flow while the feed flows first into the most
heavily scaled part of the module.
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Examples
A. 0.5 mm internal diameter coated nanofiltration membranes which
selectively reject (ie. retains) hardness causing salts were used in a
series of four tests. In the tests, the membranes were used to filter and
soften a very hard and scaling feed water with total hardness exceeding
3000 mg/L. After six hours of operation, flux through the membranes had
dropped noticeably to varying degrees. A carbon dioxide solution with a
pH of 6.3 was circulated through the membranes. Flux through the
membranes recovered completely in three of the tests.
B. Two Desal DL1812 spiral wound nanofiltration modules were
operated at 50% recovery and approximately 99 psi TMP. The feed was
scale forming in nature with a positive Ryznar index. Carbon dioxide was
injected continuously into the feed of one of the modules to reduce its pH
from 8.0 to 6.5. The flux of the module without carbon dioxide added to the
feed stabilized at 0.20 gfd/psi. The flux of the module with carbon dioxide
added to the feed stabilized at .26 gfd/psi, a 30% improvement.
The embodiments described above are subject to various
modifications within the scope of the invention which is defined by the
following claims.
