Shear localized filtration system

A filtration apparatus is provided, of the type that includes a stack of rapidly rotating membrane packs (32, FIG. 2) and a stack of stationary separator elements (36A, 36B) interleaved with the membrane packs to leave thin gaps (40A, 40B) between them, which obtains the advantages of both series-connected and parallel-connected systems. A feed conduit (82) connects the radially outer ends (110) of the gaps, to carry feed fluid into and out of each gap. The rapidly rotating membrane packs cause radially outward flow (94, FIG. 5) near their surfaces, which causes radially inward flow (102) near the surfaces of the stationary elements, to cause fluid flow radially inwardly and then outwardly through each gap. The system is operated so particles (111, FIG. 6) of the feed fluid can build up on the rotating pack surfaces (92) only to a thickness that is much less than one-half the thickness (116) of the gap, before commencing any procedure to clean the membrane pack surfaces. The stationary elements have apertures to equalize the pressure on opposite sides thereof and to promote fluid shear at the membrane surfaces. Permeate migrating into the membrane packs flows radially inwardly to a hollow shaft (50, FIG. 2), with the permeate flowing in opposite axial directions (A, B) along the shaft. An accumulator (140, FIG. 1) is coupled to the feed inlet (12) to maintain feed fluid pressure during an abnormal loss of feed fluid pressure, for the time required to stop rotation of the stack of membrane packs.

BACKGROUND OF THE INVENTION: 
A feed fluid such as waste water, can be separated into permeate, such as 
pure water which passes through a membrane, and concentrate which includes 
water with a high concentration of particles. Such separation can be 
accomplished by the use of a stack of membrane packs lying within a 
container. Fouling of the membrane packs, by the buildup of particles at 
the surface which block pores of the membranes, can be reduced by rapidly 
rotating the membrane packs, as described in U.S. Pat. No. 4,025,425 by 
Croopnick. Fouling can be further reduced by placing stationary separator 
elements between pairs of membrane packs, to create turbulence in the gap 
between the rotating surface of the membrane pack and the stationary 
surface of the separating element. It is noted that when a membrane pack 
has large pores (many microns wide), it may be referred to as a filter 
pack, but applicant uses the term membrane pack herein for both. 
The turbulence-enhancing separator elements lying between membrane packs 
should be relatively thin to take up little space for a high density of 
filtration or membrane pack surface. However, the separator elements must 
not touch the rapidly rotating membrane packs or they will destroy them. 
It would be desirable if the separator elements could be designed for 
maximum strength against deflection while minimizing conditions that could 
cause their deflection. 
A common filtration construction directs the feed fluid in series through 
the gaps. For example, if there is a stack of fifty membrane packs and 
corresponding stationary elements to produce one hundred gaps, the fluid 
may flow in a series serpentine path through the one hundred gaps. Such 
serial flow has the advantage that the feed fluid moves along a long path 
in contact with the surfaces of the membrane packs, to remove a 
considerable portion of the filtrate. However, such serial flow has a 
disadvantage that the feed fluid is not homogeneous, in that the 
concentration of particles in the feed fluid may increase by many times 
between the upstream and downstream ends of the feed fluid path. Also, 
there can be a large pressure drop along the long path, due to friction 
applied to the moving feed fluid, especially for more viscous liquids. 
Such large pressure drop can result in the feed fluid pressure being 
optimum (for maximum permeate flow through the membranes while minimizing 
fouling) at only a small portion of the total feed fluid path. The flow of 
the feed fluid in parallel through all of the gaps is seldom used, because 
the short path length requires repeated return of the fluid for reflow, 
resulting in large pressure losses during flow near the center of the 
rotor. A filtration system which allowed the feed fluid to flow along a 
long path in contact with the membrane surfaces while maintaining the feed 
fluid largely homogeneous in pressure and particle concentration, would be 
of value in the filtration of a wide variety of fluids. 
SUMMARY OF THE INVENTION 
In accordance with one embodiment of the present invention, a filtration 
system is provided, of the type wherein a stack of rotatable membrane 
packs is spaced by separator elements to leave gaps through which feed 
fluid moves, which produces enhanced filtration. A filtration system is 
operated so the feed fluid is flowed into a conduit that connects the 
radially outer edges of the gaps that separate the rotating membrane packs 
from the substantially stationary separator elements. Portions of the feed 
fluid flow largely radially inwardly along inward paths that lie adjacent 
to the stationary elements, and flow largely radially outwardly along 
outward paths that lie adjacent to the rotating membrane packs, to produce 
a largely circulating flow along each gap. Portions of the fluid that have 
moved radially inwardly and outwardly along a path, pass into the feed 
conduit to move to another gap, while other portions move back into the 
same gap along the inward paths. The feed fluid moves along a long path in 
moving into and out of each of the gaps and flowing radially inwardly and 
outwardly along each gap, and yet a substantially homogenous feed fluid is 
maintained because fluid in each gap is constantly mixed with fluid from 
other gaps by way of the feed conduit. 
As the feed fluid moves through the gaps, permeate of the feed fluid passes 
through membranes of the membrane packs and moves out of the apparatus. 
The centrifugal force and large shear (difference in fluid velocity near 
the pack surfaces) minimizes the buildup of particles at the membrane 
surface which would clog its pores. The stationary separator elements have 
apertures to leave spokes, which helps create shear and which equalize 
pressure on opposite faces of the elements. The membrane packs are cleaned 
before the buildup of particles is more than one-tenth the thickness of 
the gap, and usually before the membrane surface is completely coated with 
particles to a depth of more than an average of one particle thickness. An 
accumulator is preferably connected to the feed conduit, as at the feed 
inlet. Such accumulator assures that the feed fluid pressure can only 
gradually decrease, to prevent blowout of the membranes as the membrane 
packs stop rotating in the event of loss of feed fluid pressure. 
The novel features of the invention are set forth with particularity in the 
appended claims. The invention will be best understood from the following 
description when read in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 illustrates a rotary filtration apparatus 10 which has a feed fluid 
inlet 12 for receiving feed fluid. The feed fluid generally includes a 
liquid, and particles in the liquid which are of micron size (average 
diameter less than ten microns) or submicron size. The apparatus separates 
the feed fluid into filtrate, or permeate which flows out of permeate 
outlets 14, 16, and concentrate which flows out of an outlet 20 (or out of 
an inlet 12 in the case of batch processing). Concentrate comprises liquid 
with a high concentration of particles, and is left after much of the 
permeate in the original feed fluid has been removed. The apparatus 
includes a rotor 22 which lies within a sealed container 24. A motor 26 is 
coupled to the rotor to rapidly rotate it about an axis 28. 
As shown in FIG. 2, the rotor 22 includes a stack 30 of axially-spaced (in 
direction X that is parallel to the axis) membrane packs 32 that lie 
within the container. The apparatus also includes a stack 34 of plate-like 
separator elements 36. The separator elements 36 are stationary, and 
positioned so a separator element 36 lies between each pair of membrane 
packs 32. This leaves gaps 40 between each surface of the membrane pack 
and adjacent surfaces of separator elements. The gaps extend radially 
(parallel to radial directions Y), in that they have large radial 
dimensions Y and short axial dimensions X. In particular, FIG. 2 shows 
first and second membrane packs 32A, 32B and shows separator elements 36A 
and 36B and shows first and second gaps 40A, 40B on opposite sides of the 
first membrane pack 32A. 
FIG. 2 shows that the membrane packs 32 are mounted at their axial middles 
on a shaft 50 which is hollow to form a permeate conduit 52. The permeate 
conduit extends through the entire length of the shaft, to form the 
permeate outlets 14, 16 at the shaft opposite ends. The shaft is rotatably 
mounted on bearing 54, 56, 58, with a stand indicated at 59 to support the 
lower bearings, and with the upper bearing supported on the container 24. 
It is possible to rotatably support the rotor on a bearing assembly that 
includes a single bearing. 
The membrane packs 32 have radially inner and outer ends 60, 62. The inner 
ends 60 are mounted on the shaft, while the outer ends 62 are free and 
therefore unsupported. The separator elements 36 have radially inner and 
outer ends 64, 66. The outer ends 66 are mounted on a group of tie rods 
70, and are spaced apart by spacers 72. The radially inner ends 64 of the 
separator element are free and therefore unsupported. 
As shown in FIG. 3, each of the separator elements 36 has through apertures 
131-138 that leave spokes 141-148. The apertures extend radially (away 
from axis 28) further than the peripheries 150 of the membrane packs. This 
leaves spaces 152 at the radially outer portions of the apertures, along 
which feed fluid can move. Also, the radially outer ends 66 of the 
separator elements are radially spaced from the sidewalls 80 of the 
container 24. This leaves additional space 154 along which feed fluid can 
move. The spaces 152, 154 form a feed conduit 82, the conduit 82 being of 
largely toroidal shape (with the peripheries of the spacer element lying 
in it). 
As shown in FIG. 4, the cross sectional area of the feed conduit 82, as 
seen in the sectional view of FIG. 4, is much greater than that of any of 
the gaps 40. As a result, the feed fluid tends to be substantially 
homogenous throughout the filtration apparatus, in that there is a 
substantially uniform pressure of the feed fluid and a substantially 
uniform concentration of solids in the liquid of the feed fluid. It is 
noted that the radially inner ends 60 of the membrane packs are spaced 
apart by seal spacers 84. Permeate flows radially inwardly along each 
membrane pack, and through holes 86 in the rotor shaft and along the 
permeate conduit 52 of the shaft. 
FIG. 5 illustrates the flow of fluids in the gaps such as 40A and 40B and 
along the feed conduit 82. Before the membrane packs such as 32A begin to 
rotate, feed fluid 90 fills the feed conduit 82 and the gaps such as 40A, 
40B. When the membrane pack 32A is rapidly rotated, fluid lying adjacent 
to the membrane pack surface 92 rotates with the membrane pack. Such 
rotation results in centrifugal force which causes feed fluid lying 
adjacent to the surface 92 to move radially outwardly along an outward 
path 94 (which also includes circumferential components in the direction 
of membrane pack rotation). The radially outward flow along outward paths 
94 results in a lower pressure at the radially inward end 100 of the gap, 
and this causes a radially inward flow of feed fluid along inward paths 
102. The result is that there is a circulating flow of feed fluid along 
each gap. This circulating flow causes fresh feed fluid from the feed 
conduit 82 to repeatedly flow across the membrane pack surface 92. 
Permeate of the feed fluid moves along membrane-crossing paths 104 through 
a membrane 105 or other filtering element into porous backup sheets 106 of 
the membrane packs, and in inward directions at 108 to the centers of the 
pack from which the permeate is removed. 
It should be noted that the membrane or filtering element may be a 
polymeric membrane, a screen (woven or matt or etched), a porous ceramic, 
a sintered metal, or other construction that passes only small particles 
or molecules. Applicant uses the term "membrane pack" for all of such 
elements. Common processes include dialysis, electrodialysis, reverse 
osmosis, and various size filtration. Applicant's system is especially 
useful for microfiltration and is useful for ultrafiltration, although it 
possibly can be used for nanofiltration and even possibly for reverse 
osmosis. 
At the radially outer ends 110 of the gaps, fluid moving along the outer 
paths 94 is mixed with the feed fluid, and some of the fluid (at least one 
percent) that has moved along the outer paths 94 is returned as indicated 
by path 112, while some of it (at least one percent) is moved into the 
feed conduit as indicated by path 114. The fluid that is not recirculated 
within the same gap 40A, can move along the feed conduit 82 and into 
another gap such as 40B. At the gap inner end 100, much of the fluid 
passes in a loop indicated at 120 back along the gap. Some of the fluid 
passes in paths 122 between adjacent gaps such as 40A and 40C, but since 
there is substantially the same pressure at the radially inner ends of 
both gaps 40A, 40C, there is little flow in the directions 122. 
FIG. 6 indicates the velocity profile, in a circumferential direction, (the 
radial velocity component is not shown) of feed fluid passing along a gap 
40, by the length of the arrows. The circumferential direction is 
perpendicular to the radial direction and is parallel to the membrane pack 
surface motion. The path at 93A is very close to the membrane pack surface 
92, and the fluid moves at almost the same speed as the rotating surface 
92. The velocity at path 93 is much less than the velocity at 93A, and the 
difference component 93D tends to sweep away particles 111 from the 
surface 92 of the membrane pack. This phenomenon can be referred to as the 
localized shear that tends to sweep particles from the surface. The 
magnitude of such shear, for a given velocity of the surface 92 with 
respect to the stationary separator element surface 113 of the separator 
element, depends upon the thickness 116 of the gap. The smaller the 
thickness 116, the greater the localized shear effect, or ratio of 
velocity difference 93D with distance 118 along the gap. Applicant prefers 
to make the gap 116 as small as possible, but must prevent the surfaces 
92, 113 from touching since this could cause damage to the membrane pack. 
Applicant is able to obtain a gap thickness 116 of about two millimeters 
without causing membrane pack damage in a stack of many tens of membrane 
packs. In addition to the velocity differential per unit distance, the 
small gap results in large turbulence at the surfaces, and such turbulence 
near surface 92 also tends to sweep away particles that might otherwise 
block the pores of the membrane. It is noted that the large 
circumferential fluid movement at 93A and 93 results in fluid near the 
membrane pack flowing radially outwardly. 
Thus, by applicant connecting a feed conduit to the radially outer ends of 
the gaps between membrane packs and separator elements, applicant causes a 
recirculating flow through each of the gaps, wherein feed fluid moves 
radially inwardly near the surface of the separator member and radially 
outwardly near the surface of the filter packs. Feed fluid moving largely 
radially outwardly near the outward ends of the gaps, flows into the feed 
conduit and also recirculates. With fluid circulating, perhaps several 
times, through many gaps, applicant obtains the advantages of a serial 
connection of the gaps of having each quantity of fluid move along many 
membrane pack surfaces to remove a large proportion of the filtrate from 
the feed fluid. Applicant avoids disadvantages of a serial connection, 
because the present system results in a substantially homogenous fluid 
throughout the apparatus, in that the pressure is about the same 
everywhere and the mixing of fluid near the outer ends of the gaps results 
in the feed fluid everywhere having about the same concentration of 
solids. As discussed above, the substantially uniform pressure allows 
applicant to apply an optimum pressure to the feed fluid. For example, in 
one situation, a pressure of 40 psi will produce a high flow of permeate 
into the membrane packs while obtaining minimal fouling of the membrane 
packs, while a pressure of 80 psi could cause fouling and a pressure of 20 
psi could result in a low permeate flow rate. The optimum pressure depends 
upon the concentration of solids. Applicant can adjust the rate at which 
concentrate is drawn off, based on the permeate flow, to vary the 
concentration of solids so as to minimize membrane fouling while still 
obtaining a considerable permeate flow. 
As shown in FIG. 3, applicant prefers to construct each separator element 
36 with apertures 131-138, to leave spokes 141-148 that form wall portions 
on circumferentially (perpendicular to a radial line) opposite sides of 
each aperture. One advantage of the apertures is that they result in the 
same pressure on opposite faces 160, 162 of the element. Applicant prefers 
that the membrane packs have a small thickness such as 8 mm, and that the 
spacer elements have a small thickness such as 6 mm, with the gaps each 
having a thickness such as 3 mm, for a system using membrane packs of 16 
inches (41 cm) diameter. The small thickness of a spacer element would 
allow locations on it to be axially deflected, when there was a small 
pressure difference between its opposite faces. Any such axial deflection 
which results in direct contact of a spacer element with a rapidly 
rotating membrane pack, would cause destruction of that membrane pack. By 
providing at least one aperture in each imaginary 90.degree. sector 164 of 
a separator element, and with the apertures occupying at least 10% of the 
area of each spacer element and of each sector and separator walls (e.g. 
spokes) lying in each sector, applicant avoids such a pressure difference. 
When the membrane packs rotate so portions shown in FIG. 7 move in the 
circumferential directions of arrows 170, feed fluid at 172 lying adjacent 
to the pack face 92 also moves circumferentially. Applicant constructs 
each spoke such as 144, 145 with a leading edge 174 that is designed to 
interfere with circumferential fluid movement. The result is a large 
change in fluid velocity over a short distance near the membrane pack 
face, which helps sweep away particles. 
The flow near the membrane packs is usually turbulent, and the separators 
can be referred to as "turbulators". Of course, in the absence of a 
separator, the fluid between a pair of membrane packs would soon rotate 
with the packs. With the separators, most of the fluid is static or only 
slowly rotating, which results in a rapid change in velocity near the 
membrane packs. The cross section of each spoke such as 144 in FIG. 7 (in 
which the thickness is exaggerated) is selected so the spoke is 
self-centering. That is, if the spoke approaches the face 93 of one 
membrane pack, the reaction of the spoke with fluid moving in the 
circumferential direction 170 is to move the spoke away from the membrane 
pack surface 93. 
Applicant has experimented with spacer elements having different numbers of 
spokes. It was found that an element such as shown in FIG. 3, which has 
eight spokes, was best in the tests. An element with four spokes operated 
almost, but not quite, as well. The number of spokes is preferably at 
least four but not more than sixteen. The radially inner ends of the 
spokes are tied together by a 360.degree. continuous hub 180. The radially 
outer ends of the spokes are preferably tied together by a continuous rim 
182, although this is not absolutely necessary. The apertures and spokes 
can be angled from a radial direction. 
The membrane packs are rotated rapidly enough that large centrifugal forces 
and large shear is created to avoid membrane fouling. Membrane packs of 16 
inch diameter, are rotated at at least 200 rpm, and usually at about 760 
rpm to 1000 rpm. At 200 rpm, the average surface velocity (at a point four 
inches from the axis) is about 2 meters per second. Thus, the system 
operates with an average membrane pack velocity of at least 2 meters per 
second, and a velocity at the pack periphery of at least 4 meters per 
second. The most effective rotational speeds for membrane packs usually 
create centrifugal forces that increase pressure by at least 20 psi. 
The rotary filtration apparatus 10 (FIG. 1) can be operated in a batch or 
continuous process, or in a combination. In a batch process, feed fluid 
with a predetermined concentration of solids, such as 200 ppm (parts per 
million) is pumped into the container. Inlet and outlet valves 220, 221 
are closed. A pump 222 may be connected to a recirculation conduit 230 to 
maintain a more uniform concentration of particles, although this can be 
accomplished within the container 24 (e.g. by dividing the toroidal feed 
conduit 82 of FIG. 3 into two parts and pumping fluid up in one part and 
down in the other). The inlet and outlet 12, 20 form axially spaced 
locations of the feed conduit, and the recirculation conduit 230 lies 
outside of the feed conduit. A sensor 224 is connected to the container to 
sense the concentration of particles. The motor 26 is energized so the 
rotor 22 rotates at a predetermined speed, and permeate is constantly 
drawn from the feed fluid, while the concentration of particles in the 
feed fluid increases. The sensor 224 senses this, and can control the pump 
222 to change the pressure of the feed fluid, and also control the motor 
26 to change the speed of rotation of the rotor, for optimum conditions 
(large permeate flow into the membrane pack and low fouling of the 
membrane pack surfaces). In a continuous process, feed fluid can flow from 
a source 232 continually, though at a slow rate, into the feed fluid inlet 
12. Concentrate flows out of the outlet 20, for use or for further 
processing. 
It is noted that in some prior art filtration systems, particles of 
concentrate were allowed to mechanically build up to a large thickness (a 
plurality of particles thick) on the filter or membrane surface. In the 
present invention only "fouling" which is a chemical phenomenon, rather 
than "buildup" which is a mechanical phenomenon occurs. In fouling, 
solutes (particles) are absorped or adsorped to the membrane by chemical 
attraction, to the point where permeate flow through the membrane is 
significantly reduced. Fouling occurs both at the surface and below it. 
For example, very small particles can cling to the walls of membrane pores 
until the particles close the pores, causing permeate flow to stop and 
fouling to occur. The thickness of particles fouling a membrane will be 
less than one-tenth the gap width, when cleaning must start. 
The present invention can prevent any build up, and also reduces the rate 
of fouling. Selection of the proper membrane material for a particular 
application is of great importance in reducing the rate of fouling. Once 
fouling occurs, the membrane can be cleaned as by the use of chemicals to 
dissolve or loosen particles that have adhered to the interstices of the 
membrane. Measurement of the permeate flow rate enables a determination of 
the degree of fouling and indicates when cleaning is needed. 
Applicant prefers to connect an accumulator 240 (FIG. 1) to the feed 
conduit 80 at all times. The accumulator assures that the pressure of feed 
fluid will change only slowly, despite a possible interruption of supply 
or other phenomenon that might rapidly decrease feed fluid pressure. If 
the feed fluid pressure should suddenly fall, but the pressure of permeate 
in the membrane packs falls only slowly, then the larger pressure of 
permeate in the membrane packs could cause the thin membranes to burst. 
The accumulator 240 is of a common type, which include a membrane, 
bellows, or piston divider member 242 lying in a container 244, and 
separating the container into an air chamber 246 that contains air under 
pressure, and a liquid chamber 248 that contains liquid under the same 
pressure. Any other means for maintaining feed fluid pressure, such as a 
valve connected to a high pressure liquid source such as a city water 
main, and opened only when a rapid pressure drop is sensed, can be used. 
When the accumulator must pump fluid for more than a very short time, as 
when it is almost empty after being full, the motor is preferably 
automatically deenergized. The rotor will substantially stop from perhaps 
700 rpm, in a period such as eight seconds, and the fluid pressure should 
be maintained during perhaps six of those seconds. 
FIG. 2 shows that permeate exits the apparatus in opposite directions A, B 
through opposite ends of the shaft at 14 and 16. Where there is a large 
filtrate flow, this allows for the use of a smaller shaft and bearings 54, 
56, 58 and seals such as 59', which reduces cost. 
FIG. 4 indicates that holes 250 can be formed in the radially inner ends of 
the membrane packs to allow feed fluid to flow largely axially from one 
gap to another at their inner ends. However, there would be only small 
flow through such a hole, and providing such a hole can increase the cost 
of the membrane pack because of the need to seal against the migration of 
feed fluid into the permeate. 
FIG. 8 illustrates a portion of another rotary filtration apparatus 160 
which is similar to that of FIGS. 1-6, except that the separator elements 
162 which lie between adjacent rotating membrane pack 164A, 164B, also 
form membrane packs. That is, each separator member 162 has membrane 
sheets 166 and flow sheets 168 for removing permeate. Permeate passing 
through a membrane pack separator member 162 passes through a hole 170 in 
a hollow tying member 172 which is comparable to the tying members 70 of 
FIG. 2, to remove the permeate. 
In one system that applicant has designed, the membrane packs had diameters 
of 16 inches (40 cm), and the gaps 40 were 3 mm thick, with the other 
dimensions being relative to the diameter, as shown in FIG. 2. In one 
application, aluminum oxide particles (originally used for dyes) of a size 
of 0.5 microns and up, were to be removed from a waste stream of salt 
water and dissolved solids. The aluminum oxide particles were to be 
concentrated from four percent to twenty percent of the volume of the 
stream, by removing the salt water and dissolved solids which constitute 
the permeate of the waste stream. Larger particles had been previously 
removed by settling and screening, so the largest particles were no more 
than about 10 microns in diameter. The feed fluid is initially maintained 
at a pressure of 40 psi and the membrane packs are rotated at 800 rpm. As 
the concentration increases, the pressure can be maintained constant, or 
can be increased slightly. Thus, after the concentration increases, the 
speed is increased to 1,000 rpm and the pressure is increased to 50 psi. 
Thus, the invention provides a filtration system wherein feed fluid moves 
through axially thin gaps between membrane packs and separator elements. 
The system obtains advantages of a serial connection of the gaps, of a 
long flow path along the membrane surfaces, while avoiding disadvantages 
of widely varying pressure and particle concentration. The system includes 
a feed conduit that connects to the radially outer ends of the gaps, to 
allow fluid to flow in a loop through each gap, and to promote homogeneous 
fluid throughout the system. The separator elements preferably have 
through apertures to leave spokes. 
Although particular embodiments of the invention have been described and 
illustrated herein, it is recognized that modifications and variations may 
readily occur to those skilled in the art, and consequently, it is 
intended that the claims be interpreted to cover such modifications and 
equivalents.