Balanced pressure tubular molecular filtration system

A balanced pressure tubular molecular filtration (Reverse Osmosis or Ultra Filtration) system in which semipermeable membranes are cast on or inserted into internal passages of a semiporous tubular substrate, and may also be cast on or affixed to the external surface of said semiporous tubular substrate, said tubular substrate also having one or more low pressure passages for collecting permeate water passing through said semiporous membranes, said tubular substrate being installed in a pressure vessel and operated in such a way that its external surface and all of its internal membrane coated passages are exposed to operating pressure, so that mechanical forces are in balance, thereby overcoming hoop stress and burst strength problems common to internal pressure tubular molecular filtration designs.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the fields of (I) pollution control, (II) water 
supply and (III) product separation and recovery. 
2. Description of the Prior Art 
I. In the field of pollution control, it more particularly relates to the 
treatment of mixed industrial waste waters, segregated industrial waste 
streams, packing house wastes, fish processing wastes, chemical and 
petrochemical wastes, mining wastes, metal finishing wastes, water base 
paint wastes, nuclear wastes, photographic film processing wastes, and 
general sewage wastes. 
Specific examples from the field of Pollution control include the 
following: 
1. In the case of segregated industrial waste streams, metal finishing and 
paint wastes, this invention makes it possible to reconcentrate these 
segregated wastes in such a way that they may be returned to the 
industrial process stream, thereby recovering valuable materials. Examples 
of this type of application include the recovery of metal phosphates from 
corrosion proofing processes (e.g. Parkerizing) chromic acid, nickel 
sulfamate, nickel fluoborate, copper pyrophosphate, zinc chloride and 
similar substances from plating rinse solutions, (R-18, C-9, C-15, C-17, 
C-18, C-19) and the recovery of latex, emulsion and electro-deposited 
paint residues from paint rinse and spray booth waters. (C-9, C-15, C-17 & 
C-18) 
(All references are to literature listed in the BIBLIOGRAPHY OF KNOWN PRIOR 
ART hereinbelow.) 
2. In the field of pulp and paper, it permits the recovery and reuse of 
processing waters, the removal of color and BOD causing constituents, the 
recovery and concentration of by-products such as polysaccharides and 
lignosulfonates. (R-1, R-3, R-17, R-21, C-3, C-15, C-17, C-18) 
3. In the field of nuclear wastes, it permits the recovery and 
concentration of dissolved radioactive substances from laundry water, 
floor washing solutions, boiler blow-down water and any other solutions 
containing dissolved radioactive substances. (C-15, C-17, C-18) It also 
permits the recycling of high quality water for further uses, such as in 
laundry facilities at nuclear installations. 
4. In the case of the treatment of general sewage wastes, it permits the 
production of high quality, essentially bacteria and virus free water, 
with a low concentration of total dissolved solids (TDS) and virtually no 
suspended solids (SS), suitable for virtually any type of re-use. (R-13, 
R-23) 
II. In the field of water supply, it relates to the production of potable 
water from sea water, brackish water and industrial wastes. It has also 
been used to produce high quality industrial water for specialized 
purposes such as boiler make-up, semiconductor manufacture, use in nuclear 
reactor test and operation, pharmaceutical manufacture and other 
applications requiring very low levels of suspended solids and total 
dissolved solids. It has also been employed in the re-use and recycling of 
industrial process waters, permitting "closed drain" operations. (R-1, 
R-2, R-3, R-4, R-9, R-10, R-13, R-14, R-15, R-19, R-20, R-21, R-22, C-1, 
C-2, C-3, C-5, C-6, C-7, C-8, C-12, C-14, C-15, C-16, C-17, C-18) 
III. In addition to recovery of substances from segregated industrial 
wastes, mentioned under I, above, specific examples of product separation, 
concentration and recovery include the following: 
1. Chemical product separation and recovery. (R-17) 
2. Fermentation product separation and recovery. (R-17) 
3. Treatment of whey from cheese and cottage cheese manufacture, permitting 
the recovery, separation and purification of proteins, amino acids, lactic 
acid and sugars. (R-3, R-17, C-4, C-11) 
4. Extraction of protein from soybean and other vegetable protein products. 
(R-17, C-4) 
5. Concentration of skimmed milk. (R-17, C-4) 
6. Concentration of citrus, pineapple and other juices. (R-17) 
7. Treatment of soft and alcoholic beverage streams. (R-17) 
8. The recovery of water soluble oils, emulsions and synthetic coolants 
from metal working waste waters. (C-5, C-10, C-15, C-16, C-17, C-18) 
GENERAL BACKGROUND OF THE INVENTION 
This invention relates more particularly to the field of Molecular 
Filtration. Molecular filtration is frequently subdivided into two fields, 
Reverse Osmosis (RO) and Ultra Filtration (UF). Hereinafter "Molecular 
Filtration" will be referred to simply as "RO", and no separate reference 
made to Ultra Filtration unless specifically applicable. 
In RO, an aqueous liquid is divided into two separate streams, a 
"permeate," which is substantially free of the dissolved substance 
(solute) to be controlled or recovered, and a "concentrate," which 
contains the majority of said substance. (R-1, R-2, R-3, R-4, R-5, R-9, 
R-10, R-13, R-14, R-15, R-16, R-23) 
This type of separation is accomplished by use of a semi-permeable 
membrane. In all practical methods for employing said membrane, the fluid 
to be treated is pressurized to a pressure substantially above the osmotic 
pressure of the feed solution and passed across a substantial area of said 
membrane. During its transport across said membrane, water molecules 
preferentially pass through the membrane, with a small, limited amount of 
the dissolved substances also passing through. 
The amount of dissolved substances passing through said membrane is 
dependent upon numerous factors including (1) the nature of the membrane 
and its pretreatment, (2) the pressure and temperature, (3) pH, (4) the 
size and charge of the ions or molecules in solution and (5) the 
turbulence of the solution adjacent to the membrane. A large amount of 
specific data on the preferential passage or rejection of certain ionic or 
molecular species is known to those skilled in the art. (R-1, R-2, R-3) 
As said transport continues, the concentration of the dissolved solids in 
the feed solution increases until the residual solution passes through a 
pressure controlling valve and emerges from the RO machine. This residual 
solution is called "final concentrate" or, simply, "concentrate". 
The limit of concentration of the final concentrate is dependent upon 
numerous factors, the most important of which relate to the maximum 
achievable concentration of the substances in solution, (saturation 
concentration) and the nature of solids once formed. However, in most 
commercial RO devices, the practical limit is considerably less than the 
saturation level, due to the fact that a preferential increase in the 
concentration of dissolved solids occurs at the membrane surface. This 
preferential concentration increase is often referred to as "Concentration 
Polarization". It results from the fact that water from the stagnant 
boundary layer passes though the membrane, increasing the concentration of 
solids in the residual liquid. Concentration Polarization is worst under 
conditions of laminar flow. It can be minimized by increasing the linear 
velocity or turbulence in the immediate vicinity of the membrane surface. 
It has been found that, at linear velocities of 0.38 M/sec or above, or at 
Reynold's numbers of 5,000 or more, the thickness of the stagnant boundary 
layer in contact with the membrane substantially decreases, thereby 
providing a major reduction in concentration polarization. For 
particularly intractible solutions, further improvement can be achieved by 
increasing the linear velocity to 1.5 M/sec or above or the Reynold's 
number to 20,000 or more. By such a reduction of boundary layer, solutions 
can be enriched to a final concentration closely approaching the 
saturation level of the least stable substances in solution, while, at the 
same time, minimizing membrane fouling, scaling and similar deleterious 
phenomena, with attendant loss of productivity and potential membrane 
compaction, (which results in permanent loss of productivity or total 
destruction of the membrane.) (R-1, R-2, R-3, R-4, R-5, R-6, R-7, R-8, 
R-9, R-10, R-11, R-12, R-14, R-15) 
The most commonly used RO membranes are manufactured from selected 
cellulose acetate resins. Other membranes include ethyl cellulose, 
polysulfone and composite membranes with, for example, ethyl cellulose 
overlying polysulfone. 
In the classical method for the fabrication of cellulose acetate membranes, 
a solution of the resin in one or more water soluble organic solvents is 
spread on a flat surface, such as a glass plate, and a doctor blade is 
drawn over the surface, thereby producing a layer of resin solution with 
uniform thickness. After several minutes of evaporation in the air, the 
plate is then lowered into ice water and left there until the resin gels 
and the water soluble organic solvents are leached from the membrane. 
As produced, these membranes have a very low tendency to reject substances 
in solution. To achieve increased rejection, the membrane is next placed 
in a bath of hot water for a prescribed period of time. Considerable 
literature is available on the temperature-time relationship and its 
effect upon the solute rejection characteristics of the heat treated 
membrane. (R-1, R-2, R-3, R-16) Casting techniques are also taught by Loeb 
(PJ-1, PA-1, 2, 3, 4 & 5), Mahon (JP-2) and Merten (JP-3). 
It is known that any flexing, bending or embossing of the membrane after 
casting, causes it to lose productivity or "flux", normally expressed as 
gallons per square foot per day, or tons (M.sup.3) per square meter per 
day. In his early work, Dr. Sidney Loeb (PA-1) discovered that, if he 
mounted his cellulose acetate membranes over high quality laboratory 
filter paper in his test holder, they would, nonetheless, lose flux due to 
embossment over the fibers in the paper; if, on the other hand, he mounted 
them over smooth Millipose Type HA filter membranes, their productivity 
was preserved. Therefore, it is beneficial to cast membranes directly on a 
rigid substrate, and to employ them without removal therefrom. It is also 
important to protect them from stretching due to the expansion of the 
surface on which they are mounted. (R-2) 
There are five basic types of commercial RO devices in use at this time. 
They may be classified as follows: 
1. Flat sheet devices 
a. Single sheet devices, largely for laboratory applications, as taught by 
Loeb (PA-1). 
b. Plate and frame or multiplate devices as taught by Loeb (PA-1), Huggins 
(PJ-4), Cahn (PJ-5), Strand (PJ-6), Hanzawa (PJ-7), Donokos (PJ-8), and 
Conners (PJ-9). 
2. Hollow Fiber, as taught by Mahon (PJ-10) and Geory (PJ-11) and practiced 
by Du Pont. (C-2) (R-19, R-20) 
3. Spiral Module devices in which flat membranes, with the required 
separators and spacers, are rolled into a cylindrical form, as taught by 
Merten (PJ-12, and 13, PA-6), Michaels (PJ-14), Westmoreland (PA-7), Bray 
(PA-8 and PJ-15) and Shirokawa (PJ-16), and as practiced by Universal Oil 
Products, Eastman Chemical and Envirogenycs Div. of Aerojet General Corp. 
(R-3, R-20, R-21) (C-1) 
4. Internal pressure tubular designs, as taught by Signa (PJ-17) and Loeb 
(PA-9), and as practiced by Universal Oil Products (formerly Havens 
Int'1.,) Abcor, Patterson-Candy, Westinghouse, Union Carbide, Universal 
Water Corporation, Philco-Ford, Aerojet-General. (C-3, C-4, C-5, C-6, C-7, 
C-8, C-9, C-10, C-11, C-12, R-9, R-14, R-3, R-21, R-23) 
5. External Pressure Tubular designs as taught by Shippey (PA-10), Block 
(PA-11, PJ-18 & 19), Saito (PJ-20) and Baldon (PJ-21), and as practiced by 
Rev-O-Pak, Inc., Subsidiary of Raypak, Inc. and Sumitomo Heavy Industries. 
(R-22, C-13, C-14, C-15, C-16, C-17, C-18, C-19, C-21) 
Comparative characteristics of devices described above: 
Regarding No. 1, plate and frame equipment is limited in size due to the 
fact that the high operating pressures over even moderate cross sections, 
require extremely large bolts and tensioning members. It is also difficult 
to control external leakage. The membrane is, however, maintained in its 
original flat condition, though the support medium often causes embossing. 
In radioactive applications, flat sheets of membrane may be disposed of 
with ease. However, the small treating capacity of these devices 
eliminates them from consideration for the treatment of most nuclear 
facility wastes. 
Regarding No. 2, in the hollow fiber technique, minute capillaries of a 
semipermeable substance are mounted in a fiber wound pressure vessel, with 
the open ends of the capillaries penetrating through a header of epoxy or 
other encapsulating resin. When the solution is pumped through the 
pressure vessel, portions of the fluid penetrate the walls of the 
capillaries and pass down the internal passages to the permeate chamber. 
With a reasonable flow rate through the capillaries, a large back pressure 
develops. This back pressure may be as much as 200 psi (13.3 kg/cm.sup.2) 
at the midpoints of the fibers. Inasmuch as the maximum working pressure 
for the glass fiber pressure vessels used in hollow fiber devices is 600 
psi (40 kg/cm.sup.2), this phenomenon results in a 33% loss in working 
pressure at these points, or a net working pressure of only 400 psi (26.7 
kg/cm.sup.2). This phenomenon is known as "parisitic pressure loss". Since 
the osmotic pressure of many solutions exceeds 400 psi (26.7 kg/cm.sup.2), 
the applications for this technique are reduced. In addition, the flow of 
feed solution through the pressure vessel is rather slow and largely 
laminar. As a result, these devices are very sensitive to fouling by 
suspended solids and by scale forming substances. Feed solutions must be 
extensively prefiltered, and scale forming minerals (calcium, magnesium, 
iron) removed prior to treating with hollow fiber RO devices. In potable 
water service, broken fibers can permit microorganisms to enter the water 
supply. 
Due to its sensitivity to scale forming substances (calcium and magnesium), 
and their limited pressure capability, hollow fiber devices are not suited 
to single pass desalination of sea water. They may, however, be employed 
as a second stage, following some other RO device. 
Large systems require complex arrays of modules, arranged in 
parallel-series configurations, with costly and complex high pressure 
manifolding. 
With respect to the problem of treating radioactive wastes, the hollow 
fiber devices suffer from the fact that the glass fiber reinforcement in 
the pressure vessels yields a high inorganic ash, thereby increasing the 
volume of waste which must be disposed of. 
Regarding No. 3, the spiral module technique employs membranes which are 
cast as flat sheets but are subsequently rolled, causing disruption of 
some of the membrane structure. Also, in service they become embossed upon 
the supporting layers of fabrics and screen, further reducing their 
desirable characteristics. There are also stagnation zones between the 
leaves of membranes, in which concentration polarization occurs. Further, 
suspended solids tend to build up on membrane surfaces, especially on the 
leading edge of the leaves of the spirals. In order to minimize the 
effects of suspended and dissolved solids, frequent reverse flow cleaning 
cycles are required. Costly valving is required in order to provide the 
reverse flow cleaning capability. 
Spiral modules also experience parisitic pressure drop. Further, internal 
leakage paths in the seals between individual modules in the same pressure 
vessel result in the leakage of concentrate into the permeate stream. 
Due to the limited upper pressure capability and the high osmotic pressure 
of sea water, spiral module devices cannot ordinarily produce potable 
water (500 ppm TDS maximum) from sea water in a single pass. Normally it 
is necessary to employ two separate systems, each with its own booster 
pump, high pressure pump, cell bank, back pressure regulator, back wash 
system and instrumentation, for each stage of a sea water treating spiral 
module system. In many cases, it is also necessary to remove hardness 
minerals by use of an ion exchange resin prior to the first stage of 
spiral module treatment. 
With respect to the problem of treating radioactive wastes, the spiral 
module devices suffer from the fact that they contain metalic screens, 
spacers and other parts which add to the bulk of the final ash. Glass 
fiber fabrics, employed in some designs, further add to the ash which must 
be disposed of. 
Large systems require complex arrays of modules arranged in parallel-series 
configurations, with costly and complex high pressure manifolding. This 
requirement results from boundary layer phenomena and hydraulic 
considerations, which impose a narrow range of suitable flow rates for 
spiral modules. 
Regarding No. 4, in internal pressure tubular RO devices a membrane is cast 
on or inserted into the inner surface of a porous tube. The tubing is 
subjected to internal pressure, with resultant hoop stress, causing the 
membrane to be stretched, permanently degrading its performance. These 
tubes can also rupture, causing catastrophic failure of the entire RO 
system, an intolerable condition for potable water and sewage application. 
As practiced commercially, several of these tubes are installed in 
parallel, pressed tightly between two headers, so the finished assembly 
resembles a heat exchanger tube bundle. These headers are maintained in 
place against the high fluid pressure by installing a tension rod between 
them, transmitting an objectionable compressional load to the membrane 
tubes in the "at rest" condition. However, under operating pressure, the 
load balance changes, resulting in a stress change on the membranes and 
tubes. Those stress changes induce fatiguing of the membranes and of the 
porous tubes. Further, there are high stress concentrations in the 
immediate area of the header, which affect the life of the tubes and the 
membrane. Pressures in most internal pressure designs are limited to 800 
psi (53.3 kg/cm.sup.2). 
As mentioned under spiral modules, due to the limited maximum pressure of 
internal pressure tubular RO, it is usually necessary to employ two stages 
for sea water desalination. However, these devices are considerably less 
sensitive to hardness minerals than spiral module devices. 
With respect to the problem of treating nuclear wastes, most internal 
pressure devices present a serious disposal problem. The fiberglas 
employed in the tubes and the outer shell, the metal headers, tensioning 
rods, pipe fittings and other mineral and metalic accessories, create a 
particularly difficult disposal problem. Certain devices also employ large 
quantities of sand between the tubes, further complicating disposal 
problems. 
In order to minimize some of these deficiencies of internal pressure 
devices, Patterson-Candy has employed costly perforated stainless steel 
supports around their membranes, which membranes are inserted therein in 
the form of a "soda straw", with a membrane film on the inner surface 
thereof. Maximum pressures are limited to 1,200 psi (80 kg/cm.sup.2). This 
design also employs large amounts of costly stainless steel in its headers 
and end pieces, greatly escalating the manufacturing costs. 
As mentioned under hollow fiber and spiral module, above, large systems 
require complex and costly high pressure manifolding to establish the 
proper flow rates in the several stages of series-parallel systems. 
Regarding No. 5, in the external pressure tubular design the membrane is 
cast on the outer surface of an essentially incompressible tubular, 
porous, ceramic cores. The permeate passes through the external membrane, 
on through the porous substrate, and into the internal permeate duct. 
One, seven or nineteen of these cores are installed in a 1, 2, 21/2 or 4 
inch pressure vessel, and the permeate is ducted out of one end of the 
vessel. In most cases, a plastic covered wire or spring is wound around 
the tubular core to increase the tubulence. Since the core is pressurized 
uniformly and does not yield to this external pressure, the membrane is 
not subjected to stresses, as in the case of internal pressure tubular 
designs. Operating pressures to 1,500 psi (100 kg/cm.sup.2) have been 
achieved in these systems. 
In external pressure tubular designs, a hydraulic imbalance is created by 
the fact that one end of the core is subjected to system pressure but the 
other end communicates with the atmosphere. (The core is connected to the 
system at only one end rather than two, as in internal pressure and spiral 
module designs.) This imbalance has the beneficial effect of holding the 
string of cores in tight contact one with the other, while at operating 
pressure, thereby improving seal efficiency and minimizing internal 
leakage. However, this force does subject the core to a longitudinal 
compressional force. The thickness of the ceramic walls must be sufficient 
to withstand radial and axial compressional forces without breaking. When 
cores are fabricated with less rigid porous substrates, such as sintered 
polyethylene, this longitudinal compressional force causes axial 
compression and radial enlargement of the cores, creating objectional 
tensional forces in the membrane skin and, in some cases, even causing the 
membrane to separate from the surface of the tube. It would be highly 
desirable to be able to employ these less rigid substrates, since the 
ceramic cores are fragile and require considerable care to prevent damage 
during handling and shipping. In some cases, the ceramic cores break in 
service, permitting large volumes of concentrate to contaminate the 
permeate. 
External pressure tubular RO devices can accept high inlet pressures and 
have a much lower sensitivity to hardness minerals than hollow fiber or 
spiral module devices, permitting single pass desalination of sea water. 
However, the possibility of broken cores or separated seals greatly limits 
the use of external pressure tubular devices in potable water supplies and 
makes them unsuitable for use in sewage systems. 
The external pressure design permits operation at very high turbulence and 
high linear flow rates. While the above mentioned longitudinal 
compressional force tends to maintain a proper seal between individual 
cores in a series string, this beneficial force is occasionally overcome 
by the high viscous drag experienced when operating at high linear flow 
rates. As presently manufactured, these viscous forces oppose the 
longitudinal compressional forces in one half of the pressure vessels. 
Recently, in several cases, these viscous forces have caused the 
connectors between cores in a series string to open, permitting 
concentrate to enter the permeate duct. 
Also, during system start up, prior to establishing system pressure, the 
hydraulic imbalance is not yet established and viscous forces may 
occasionally separate cores, causing serious internal leakage. 
With respect to the problem of treating radioactive wastes, the ceramic 
cores cannot be ashed, and, therefore, result in a very high volume of 
solid waste, increasing the nuclear waste disposal problem. Further, 
conventional turbulators are made of plastic coated wire. The residual 
wire further complicates disposal problems. 
Practical considerations limit external pressure tubular devices to two 
commercial configurations, 7-core and 19-core. As a matter of fact, 
19-core bundles present such severe assembly, installation and maintenance 
problems that their applications are very limited. However, assuming that, 
with care, they could be used, they cannot accept feed rates in excess of 
250 tons per day. Systems with larger flow rates require complex and 
costly high pressure manifolding to permit series-parallel operations, so 
that each cell may operate within established hydraulic limits. 
SUMMARY OF THE INVENTION 
I have invented a totally new RO design in which I have preserved the 
beneficial effects of internal and external pressure RO devices, while, at 
the same time, overcoming most of the deficiencies associated therewith. 
This new device is defined and described as the "balanced pressure 
tubular" design. It employs a tube or core composed of a porous substrate, 
containing internal, tubular, membrane coated surfaces. The external 
tubular surface may also be coated with membrane, though the external 
surface is more subject to damage; further, the external surface only 
makes a significant contribution to the total membrane area in the smaller 
core sizes. 
In order to understand the various phenomena with respect to my device, it 
is necessary to realize that it contains two separate, though related, 
force or pressure systems. One is a mechanical system consisting of a 
heterogeneous, porous solid. The other is a hydraulic system consisting of 
wter or an aqueous mixture of solutes in water. Under dynamic, operating 
conditions, aqueous media surround the tubular core and fill the internal 
tubular passages, plus the cavities within the heterogenous, porous solid. 
In my device, the hydraulic pressures within adjacent internal tubes, and 
the hydraulic pressure on the outer surface, oppose one another, so that 
the internal tubes are not subjected to hoop stress. These fluid forces 
impose a static, mechanical, compressional load on the granules within the 
porous substrate. Being tightly packed in the space between tubular 
surfaces, the granules carry this compressional load by point-to-point 
contact between pressurized surfaces. Compressional loads are thus 
balanced against one another, excepting for the zone within the central 
circle of tubular surfaces, as shall be explained later in this 
specification. 
Some of the deficiencies of internal pressure RO so rectified are as 
follows: 
1. Tensile stress on membrane due to hoop stress on supporting tubing. 
2. Stress concentration at header ends of tube bundles. 
3. Catastrophic system failure due to rupture of pressure tubing. 
4. Relatively low operating pressures due to limited burst strength of 
tubes. 
5. Costly internal tensioning rod, which also causes stress concentrations. 
6. Relatively low "packing density" (square meters of membrane per cubic 
meter of cell bank.) 
7. Replacement or factory overhaul of entire module due to failure of one 
tube (required in most designs.) 
8. Inability to effect single pass desalination of sea water. 
9. High residue on combustion of spent tubes used in nuclear applications. 
10. Costly high pressure series-parallel manifolding for high volume 
systems. 
Some of the deficiencies of external pressure RO devices corrected by my 
design are as follows: 
1. Brittle small diameter ceramic cores often break during shipping, 
handling, installation and, occasionally, in service. 
2. Large number of seals increases the chance of seal failue. [There are 
133 separate core seals in a standard 7-core, 18.5 foot (5.64 meter) 
pressure vessel.] 
3. Standard design with 7 or 19 cores in parallel and six strings in series 
makes assembly nd installation difficult, requires 4 men to insert or 
remove a bundle of cores. 
4. Large longitudinal and radial compression forces increase required 
thickness of core substrate, increasing weight, with resultant increase in 
material and transportation costs. 
5. Hand wound turbulator wires or springs used on cores increase assembly 
costs. 
6. "Dead space" between cores reduces hydraulic efficiency. 
7. "Dead space" also increases the hold up volume, limiting the "degree of 
concentration" or "concentration ratio" in many applications 
EQU (Concentration Ratio=Volume of feed/Volume of Final Concentrate) 
8. Relatively low "packing density" of membrane. 
9. Large clear working space required opposite "service end" of machine. 
Clear space must be approximately as large as the length (and width) of 
the cell module, resulting in inefficiencies in facility lay-out and 
utilization of plant space. 
10. Ceramic cores with uniformly circular cross sections are an absolute 
necessity or the core will not pass through the ring die. In cases of 
moderate "elipticity" of the core, a non-uniform layer of membrane 
results. 
11. Many small parts, connectors, turbulators and seals are required on 
cores, increasing complexity and chance of failure. 
12. Seal failure due to high viscous drag separates "slip joint" 
connectors, resulting in system failure while in service. 
13. High volume of inorganic residue creates disposal problem in nuclear 
applications. 
14. Leakage occasionally occurs around the complex permeate collector due 
to imperfections in the seals or to defects or cracks in the permeate 
collector. 
15. Costly investment castings required on ends of pressure vessels. 
16. Costly high pressure series-parallel manifolding required for high 
volume systems. 
The objective of my invention is to improve upon the deficiencies listed 
above, but the scope of my invention is not necessarily limited to the 
improvement of these deficiencies, and those skilled in the art will 
understand from the description in the specification the object and the 
working effect of my invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
My invention is described by a series of drawings. FIG. 1 shows a cross 
section and FIG. 2 an isometric rendering of the simplest embodiment of my 
invention. Core 1 is a porous tube with two tubular internal passages. 
Item 2 is a relatively large tube for internal passage of feed solution, 
here shown as a cylindrical surface, though there is no intrinsic 
limitation dictating that it be such. Similarly, item 3 is a relatively 
small tube for internal passage of permeate, also shown as cylindrical, 
but not herein limited to said contour. The outer surface of core 1 is 
also shown as cylindrical, but again is not intrinsically limited thereto. 
Item 4 is a membrane on the inner surface, 2, where it is either cast in 
place or inserted after fabrication. Item 5 is an optional membrane, cast 
on or attached to the outer surface of core 1. 
Passage 3 is the permeate duct for this RO core. In service, it 
communicates with the outside of the machine and is maintained at, or 
close to, atmospheric pressure. The tube itself is installed in a pressure 
vessel, item 7, and the membrane coated inner surface and the outer 
surface of the core 1 are exposed to balanced system pressure. The 
membrane surfaces resist the intrusion of ions and molecules of dissolved 
solids (solutes), but permit water molecules to pass with relative ease. 
Once the water molecules have passed through the membrane skin, they 
proceed into the relatively low density substrate, item 6. This substrate 
may be composed of ceramic materials, sintered glass, sintered metal, 
sintered or foamed plastics such as polyethylene, polyvinylidene fluoride, 
polyvinyl chloride, acetal, polystyrene or polyurethane, granulated or 
foamed rubber, fused, consolidated, resin treated or otherwise fixed sand, 
silica, feldspar, clay, diatomaceous earth or other inorganic mineral or 
fossil substance, treated wood, wood fiber or wood powder, consolidated 
coal, asphaltite, gilsonite or other bitumen powder, or any one of a 
number of other porous solid substances. (The use of plastic, coal, 
asphaltite, gilsonite or other bitumen, rubber, wood based or other 
organic substrates is particularly beneficial for use in treating 
radioactive wastes, since they may be converted to an ash or chemically 
decomposed to minimize the volume of radioactive waste.) 
The water molecules can pass through the porous substrate to tube 3, which 
is the permeate duct. The highest flux through membranes seldom exceed 
0.014 ml/cm.sup.2 /minute. Consequently, the internal flow rates are very 
low. Many available porous substrates can accept these flows without 
creating objectionable internal parisitic pressure drops. (This matter is 
further analyzed in a subsequent section.) 
Item 8 shows the end of the core, which may be sealed by fusing, glazing, 
encapsulating or other technique. In the absence of membrane 5, the outer 
surface of core 1 would also be sealed against fluid intrusion, or it may 
be surrounded with a thin plastic or metal shell, as shown in a subsequent 
drawing. 
Item 8 is a connector for connecting the core to the outside of the 
machine, or for coupling the core to another core of the same type in a 
series string. FIG. 3 shows the opposite end of the same core, on which 
there is installed a similar, but mating connector, item 10. If this core 
was to be used alone, or was to be the last in a string of cores in 
series, a blind plug, such as item 12, and seal 13 would be inserted in 
connector 10. A turbulator, item 11, surrounds core 1 as an optional 
accessory. 
This design has the considerable advantage that membrane coated surfaces, 4 
and 5, and supporting structures, 6, are exposed to balanced pressures. 
The membrane coated internal tubular surface, 2, is not exposed to hoop 
stress, as in the conventional internal pressure design. The outer surface 
is not exposed to high compressional forces. The granules of heterogenous 
substrate below the membranes carry these forces. There are radial 
compressional forces in the area of the permeate duct, but the large 
amount of solid substrate in that zone provides adequate support against 
them. The bulk of the end faces, 8, are exposed to balanced axial 
compressional forces. A small area near the permeate duct, 3, physically 
carries the hydraulic load imposed by the fact that one end, 12, is 
exposed to system pressure while the other communicates with the 
atmosphere. 
The determination of the pressure drop within the substrate will be given 
later, by use of Darcy's equation. The significance of this internal 
pressure drop is limited to the more dense substrates, very large tubes, 
ultrafiltration with its high fluxes at relatively low feed pressures and 
combinations of these effects. 
FIG. 4 shows a further embodiment of my invention, in which a perforated 
stainless steel liner, item 20, is inserted into the permeate duct, 3. 
This extra reinforcement is particularly suitable where high operating 
pressures or thin wall sections are to be employed. It is also an 
important component of the configuration of my device recommended for 
producing potable water and for treating sewage wastes. This internal 
liner 20 may be extended beyond the ends of the porous tube, 1, 
facilitating the inter-connection of a series string of cores, and the 
delivery of the permeate. For potable water, food product and sewage 
applications, suitable tubing connectors, pipe couplers or sanitary 
fittings may be employed to provide highly reliable inter-core 
connections, as further described below. 
FIGS. 5 and 6 show cross sections of similar tubes in which two permeate 
ducts are employed and in which either the outer or inner profile is an 
elipse or other contour which permits the use of two permeate ducts, and 
maximizes the amount of membrane surface. 
FIG. 7 represents a standard 7-core external pressure tubular RO design and 
FIG. 8 shows how a similar pressure vessel could accommodate seven 
balanced pressure cores, designed in accordance with the principles 
described above. In this modification, FIG. 8, the packing density of 
membrane has been substantially increased by providing membrane on both 
inner, 2, and outer surfaces of the porous cores. This design yields 
substantially increased permeate when compared to the conventional design, 
FIG. 7. 
FIGS. 9, 10 and 10A show ways in which the strength of the boss and the 
diameter of the permeate duct may be increased. However, for casting 
efficiency it has been found to be preferable to maintain a circular cross 
section for the membrane coated inner surface or surfaces of a core. 
FIGS. 11 and 12 show designs intended to be used in the same pressure 
vessels as those used for a standard external pressure, 7-core RO system. 
Morphologically, these cores are the same as shown in FIGS. 1 and 2. 
However, the outer surfaces have been contoured so that the individual 
cores nest together, eliminating the "dead space" shown in the standard 
external pressure design, FIG. 7, item 21. In these designs, FIGS. 11 and 
12, turbulators (FIG. 2, item 11), must be installed on at least half of 
the cores to keep the adjacent surfaces separated and to permit fluid 
flow. The design shown in FIG. 12 provides 2.5 times as much membrane 
surface as the standard 21/2 inch, 7-core, external pressure tubular 
design, FIG. 7, for a 150% increase in membrane area. This increased 
membrane area provides a comparable increase in the amount of permeate 
produced by a given section of pressure vessel, thereby substantially 
reducing the number of costly pressure vessels, manifolds, etc. in a cell 
bank. Progressing from the 5-core design, FIG. 11, to the 8-core design, 
FIG. 12, the strength of the permeate duct may be progressively increased. 
FIGS. 13 and 14 expand further upon the concept introduced in FIGS. 11 and 
12. In these designs, two internal tubular surfaces, 2, are coated with 
membrane and a third, 3, is employed as a permeate duct. In FIG. 14, the 
membrane surface is 2.6 times that of the standard external pressure, 21/2 
inch design, FIG. 7, for a 160% increase of membrane area. 
For ease of installation, maintenance and service, it is beneficial to 
reduce the number of permeate connections. Therefore, in FIGS. 15 and 16 I 
show one of the preferred embodiments of this invention. The central tube, 
32, is the permeate duct, and all other internal tubular surfaces, 31, are 
coated with membrane, 4. The external surface may also be coated with the 
optional membrane 5. This design offers numerous advantages over my other 
balanced pressure tubular designs. Except for the internal tubular 
passages, item numbers on the figures have the same meanings as on 
previous figures. The principle difference is in the use of many 
cylindrical, membrane-coated, internal tubular surfaces, 31; one 
cylindrical external surface with optional membrane, 5; and one central, 
uncoated permeate duct, 32, equipped with connectors. 
In this design, the permeate duct, 32, is not necessarily smaller or larger 
than the internal feed flow ducts, 31. 
FIGS. 15 and 16 illustrate two of the optional ways in which this concept 
could be used in pipe sizes from 2 inches to 3 inches. 
FIGS. 17 and 18 illustrate two possible designs suitable for 31/2 inch and 
4 inch pipe sizes. They are given as illustrations only, and do not 
indicate any limitation upon the upper limits of pressure vessel sizes, or 
upon the diameters of the feed or permeate ducts or the number of rows of 
internal tubular passages. 
FIG. 19 is a cross section of two cores designed in accordance with FIG. 
15, showing a means for interconnecting individual cores. 
FIG. 20 shows an assembly of pressure vessels containing cores similar to 
those illustrated in FIGS. 15 through 19. The internal passages are not 
shown. In addition to the other standard designations, 41 is the permeate 
collector, 42 is the permeate portion of the header casting, 43 is the 
feed return portion of the header casting, 44 is a pipe coupler, 45 is a 
retainer flange placed at the permeate end of the pressure vessel, 46 is a 
gasket for pipe coupler 44, and 47 is a collar for pipe coupler 44. While 
I have found this configuration to be an improvement over designs in which 
one end of the machine has only type 42 headers and the other end only 
type 43 headers, my core design can be employed in either way, provided 
that a non-slip connector design is employed, e.g. bayonette or threaded 
connector, pipe or tubing fitting. 
This embodiment of my invention overcomes many of the disadvantages of 
other tubular designs and introduces new advantages not anticipated during 
the development effort. 
With respect to internal pressure tubular designs, my invention treats the 
aforementioned deficiencies as follows: 
1. There is no problem of membrane being put under tension due to hoop 
stress, since the internal and optional external membrane coated surfaces 
of the core are in static mechanical balance. 
2. There are no headers with fluid pressure at opposite ends of the tubes, 
resulting in stress concentration at the ends of the tubular surfaces. 
3. Since the tubular surfaces are not subjected to unbalanced hoop 
stresses, tube rupture does not occur. 
4. Due to the lack of unbalanced hoop stresses, working pressure limits can 
be increased from the usual 800 psi (53.3 kg/cm.sup.2) to over 2,000 psi 
(133 kg/cm.sup.2), should the application so require. 
5. Costly tensioning rods are eliminated. `6. Packing densities of 
considerably higher magnitude can be achieved. 
7. Catastrophic failures of tubing are eliminated. In the case of minor 
membrane defects, membranes supported on a semi-porous substrate undergo a 
"self-healing" phenomenon when minor membrane defects occur. Suspended 
solids in feed solutions usually plug the passages in the substrate, 
eliminating such leakage paths. When required, this process can be 
accellerated by introducing a latex or a vegetable gum into the feed. If a 
defect does not yield to this treatment quickly enough, a single segment 
of a series string of cores may be replaced in the field. Since the 
pressure carrying vessel, item 5, is a piece of conventional pipe, it is 
unaffected by such a defect. Only the core requires repair or replacement. 
In a like manner, my invention treats the aforementioned deficiencies of 
external pressure tubular designs as follows: 
1. The cores shown in FIGS. 15 through 19, even if fabricated with 
ceramics, have a much larger overall cross section (diameter to length 
ratio) than conventional external pressurized cores, increasing strength 
and minimizing the problems of breakage. Further, a much broader range of 
engineering materials may be employed in their construction. For example, 
sintered plastics, such as sintered polyethylene, are not fragile and can 
be employed in my invention. 
2. Instead of seven pairs of couplers per joint (0.91 meters), as in the 
21/2 inch 7-core external pressure design, in my invention there is only 
one pair. In addition, 2 meter cores may be made, whereas conventional 
external pressure designs are limited to 0.91 meters, due to the brittle 
nature of the small diameter ceramic cores. This factor permits a further 
55% reduction in the number of joints. 
3. Cores may be individually connected as they are inserted by a lone 
operator, thereby eliminating the need for a four man team. 
4. Lower density porous substrates can be employed, reducing weight. 
5. Only one turbulator is required per joint, instead of the conventional 7 
or 19, as in the 21/2 inch or 4 inch external pressure tubular designs. In 
the absence of the optional external membrane, it can be totally 
eliminated. 
6. The diameter of the internal passages and the annular clearance between 
the core and the pressure vessel can be controlled and designed to provide 
optimum hydraulic efficiency and flow distribution (internal and 
external), thereby eliminating the problem of dead spaces between cores 
and the poor flow distribution of conventional designs. When the external 
membrane, 5, is omitted, a close fitting core will minimize external flow. 
Or, a single "O" ring or similar seal or gasket may be employed to prevent 
external flow without sacrificing the balanced pressure concept. 
7. The elimination of "dead spaces" also reduces hold up volume, thereby 
permitting a higher Concentration Ratio. 
8. Higher packing densities are achieved, providing 147% more membrane than 
achieved with conventional 21/2 inch 7-core external pressure tubular 
designs. 
9. Since individual joints can be connected at the machine as they are 
inserted, the clear working space can be limited to the length of one core 
segment; for example, one meter will suffice for 1 meter cores, instead of 
the usual 6 meters required for 7-core external pressure tubular designs. 
10. Sintered or otherwise consolidated cores are more nearly true to the 
circular cross section of their mandrel than are ceramics. (Considerable 
effort must be expended in order to prevent "slumping" or flattening of 
ceramics during firing.) Circular cross sections are essential for 
external pressure tubular membrane casting efficiency. 
11. Only one set of connectors is required per joint, instead of the usual 
7 or 19. Further, the connectors can be molded directly into plastic 
substrates, as shown in FIG. 19, improving the seal efficiency and 
limiting the required elastomeric seals to one per joint, instead of the 
usual three. In the case of 19 core designs, this savings permits the 
elimination of 38 elastomeric seals per joint. Then, as mentioned in item 
2, above, the joints may be made 2 meters in length instead of 0.91 
meters, for the equivalent savings of 84 seals per segment. 
12. Due to the smaller servicing space required, the cell module may be 
serviced from both ends. In conventional external pressure designs, one 
set of castings is required for the service end and a different set is 
employed on the other. With my invention, the same casting may be employed 
on both ends. 
13. Seal failures due to viscous drag are eliminated by use of the castings 
described in item 12, above, and shown in FIG. 20. Cores may be installed 
in such a way that the forces of viscous drag are always in the same 
direction as the longitudinal compressional forces, rather than opposing 
them in half of the cells, thereby improving connector reliability. 
In addition to the above, my invention makes possible the following 
advantages not anticipated by prior experience: 
1. Concept lends itself to a much broader range of pressure vessel sizes 
rather than being limited to 2 inch, 21/2 inch and 4 inch pipe sizes. This 
phenomenon further reduces pressure vessel costs and reduces operating 
costs (pumping costs). 
2. The end faces of individual joints of cores need not be equipped with 
seals, since they may be fused, encapsulated or otherwise rendered 
impervious without the use of additional elastomeric seals. This 
innovation is particularly beneficial for preventing impingement from the 
suspended solids present in some feed solutions. 
3. Cores as shown in FIGS. 15, 16, and 19 may be used to retrofit external 
pressure tubular 7-core systems, thereby achieving increased performance 
at the same operating pressure or the same product water flow at much 
lower pressures, and substantially reducing operating costs. Similarly, 
the 4 inch design shown in FIG. 18 can be used to retrofit external 
pressure 19-core systems. 
4. A further unexpected operational advantage has been realized due to the 
space provided between cores in series. In internal and external pressure 
tubular designs, high concentrations of solutes and stagnant boundary 
layers develop in the immediate vicinity of the membrane surface, and 
spread down stream as the feed solution progresses through or around the 
tubes. However, in my design the fluid passes into the 1 to 15 cm. space 
between joints, where the feed from the internal tubular passages and the 
optional external annular passage are thoroughly mixed under turbulent 
conditions, prior to entering the next segment of balanced pressure 
tubular RO core. 
5. Because of the balanced pressure design, large wall sections between the 
outer tubular surface and internal tubes are not required, nor are large 
separations required between adjacent internal tubular surfaces. These 
characteristics facilitate maximizing of packing densities. If the walls 
are sufficiently strong to prevent rupture during manufacturing, shipping 
and installation, and if they permit passage of the permeate, they will 
not fail in normal service. The coefficient of fluid resistance of many 
available porous substrates is much higher than that of membranes, 
resulting in relatively low internal flow rates. Further, the geometric 
considerations relating to tubing layouts provide adequate cross sections 
for the transmission of permeate from the membrane surfaces to the 
permeate duct, so that parasitic losses can be controlled. This matter is 
discussed extensively, below. 
6. When core materials are selected from one of the classes of porous 
organic substrates (e.g. plastic, bitumen, etc.), and when the 
turbulators, connectors and seals are also composed of organic substances, 
the resultant device is particularly suitable for use in nuclear 
applications. It may be decomposed by thermal or chemical means, yielding 
the smallest possible volume of solid waste. 
7. Due to the broad latitude in the diameter of pressure vessels and cores, 
complex, high pressure, series--parallel manifolding is not required, as 
further delineated below. 
FIG. 21 shows another configuration based upon the design shown in FIG. 4, 
in which a perforated liner, 20, is employed in the porous core. In this 
case, the perforated liner extends beyond the end of core (perforations 
only within the core) and terminates in a pipe coupler, 50, facilitating 
interconnecting of cores. The threaded end may then couple directly into 
the retainer flange, 45, without the need for an unreliable permeate 
collector, item 41, FIG. 20. The terminal core is then plugged with a 
conventional pipe cap, 51. 
FIG. 22 shows another configuration similar to FIG. 21, but employing 
conventional tubing fittings. In this case, AN or MS fittings are shown, 
but other commercial types of tubing fittings such as "Swagelok" or 
"Eastman" could be used with no change in the basic concept. In addition 
to items previously identified, 60 is a "B" nut, AN-818 or MS-20818; 61 is 
a sleeve, AN-819 or MS-20819; 62 is a tubing union, AN-815 or MS-24392; 63 
is a tubing plug, AN-806 or MS-24404 serving as a blind plug, similar to 
FIG. 3, item 10; item 64 is a pipe to tubing union, AN-816, or MS-20816, 
coupled directly into the retainer flange, 45, without a permeate 
collector, item 41, FIG. 20, a known source of incipient leakage; item 65 
is an "O" ring (this "O" ring has been found particularly suitable for 
preventing leakage from the pressure chamber into the permeate duct, even 
though it is known that this "O" ring design is of limited utility in 
preventing leakage in hydraulic applications, for which it was originally 
designed); 66 is an optional swivel joint; and 67 is an optional impeller. 
This design has been found to be one of the most adaptable, since it 
greatly simplifies installation of membranes, minimizes leakage, and 
yields the highest reliability in potable water and sewer applications. 
The high pressure capability, combined with high reliability, permit this 
design to be used in single pass desalination of sea water. The optional 
swivel joint permits the string of cores to rotate slightly, especially on 
start up, driven by the torque created by the turbulator 11. This 
innovation minimizes the propensity for fouling and suspended solid (SS) 
buildup on the under side of the optional external membrane. For larger 
sizes of cores, the relatively smaller volume of feed solution passing 
through the annulus is not sufficient to cause rotation of the string of 
cores on start up, necessitating the use of the optional impeller, item 
67. 
FIG. 23 shows a design similar to FIG. 22, except that qualified and 
approved sanitary fittings have been employed in order to comply with the 
"clean in place" requirements of the food industry, 70 is the retainer 
nut; 71 is the flange; 72 is the threaded coupling; 73 is the sanitary 
gasket; and 74 is a sanitary "blank" or end plug. 
With the designs shown in FIGS. 21, 22 and 23, a rotation of the cores may 
be effected intermittently without the use of the swivel joint. During 
regular maintenance periods, the pipe couplings, 44, may be loosened and 
the retainer flange, 45, manually rotated 60.degree. to 90.degree., 
thereby rotating the entire string of cores. 
The designs shown in FIGS. 21, and 22 also permit retrofit and upgrading of 
conventional external pressure tubular designs. The backside of the 
existing retainer flange, 45, can be tapped to accept the appropriate 
coupler. In the case of FIG. 23, a piece of sanitary tubing is welded 
through the retainer flange. The unreliable permeate collector, 41, is 
thereby eliminated. 
With the designs shown in FIGS. 21, 22 and 23, the problem of seals and 
slip fittings being pulled apart due to viscous drag is totally 
eliminated. Accordingly, no special attention need be given to maintaining 
flow toward the permeate end of the pressure vessel. In the case in which 
a swivel joint is employed to cause core rotation, it is preferable to 
have flow away from the permeate end of the pressure vessel, the reverse 
of what is shown in FIG. 20. 
External pressure tubular RO designs employ specially molded or machined 
plastic couplers and elastomeric seals, resulting in unreliable connectors 
with frequent failures due to leakage. These special use, single source 
components also increase costs and create difficult logistic problems. 
However, in the configurations illustrated in FIGS. 21, 22 and 23, I 
employ highly reliable, time tested, commercially available components. 
Most of these connectors may be procurred in the field from multiple 
sources, thereby further reducing costs and logistics problems. Further, 
these connectors totally eliminate the problem of separation of joints due 
to viscous drag. 
FIGS. 24 and 25 show further embodiments in my invention in which I have 
totally eliminated the use of the investment cast permeate header, 42, 
return header, 43, and retainer flange, 45. The tooling for, and 
production of, these components have constituted major elements in the 
cost of fabrication conventional external pressure tubular RO systems. I 
have found that I can replace them with conventional, commercially 
available 180.degree. tubing returns, 80. To these standard tubing 
returns, I add one or two permeate delivery tubes, 81, centrally 
positioned so that the tube or tubes will emerge from the tubing return on 
the axis or axes of the mating pressure vessels. Two collars, 82, are also 
added to permit connection by use of a pipe coupler, item 44. Feed 
solution may then enter the cell bank and concentrate may leave directly, 
or through conventional tubing elbows, 83, also fit with collars for pipe 
couplers, 44. 
FIGS. 24 and 25 are essentially the same except that, in FIG. 24, all 
permeate connections are on the same end of the machine, and in FIG. 25 
they alternate in order to permit all flow to be either toward or away 
from the permeate end of the cell, according to the special needs 
described above. Accordingly, in FIG. 25 it is necessary to have one 
permeate tube, 84, in one of the tubing elbows, 85. Flow may either be 
toward or away from the permeate end, depending upon whether this elbow, 
85, is the feed inlet or concentrate outlet. If the elbow 85 is to be the 
concentrate outlet, flow would be toward the permeate end of the cells. If 
the elbow 85 is to be the feed inlet, flow would be away from the permeate 
end of the cells. 
With hollow fiber, spiral module and internal and external pressure tubular 
RO, economical considerations have dictated that pressure vessels and 
modules be limited to a maximum of two or three sizes. These limitations 
result from numerous factors, such as the use of highly specialized 
castings, various manufacturing tools and fixtures, geometric 
considerations, etc. However, with the elimination of these constraints, 
new opportunities for improved designs were created. 
In order to accommodate large flows with the limited number of available 
module sizes, it has been common to make use of series-parallel arrays of 
pressure vessels and modules. However, the piping for such series-parallel 
systems requires complex and costly high pressure manifolding. These 
manifolds become particularly complicated when high concentration ratios 
are to be achieved. For a concentration ratio of 10:1, they may start out 
with 12 to 20 pressure vessels in series-parallel and stage progressively 
down to a number of vessels in series only. 
Starting with the design which employs commercially available 180.degree. 
tubing returns (FIGS. 24 & 25), I have eliminated the need for 
series-parallel manifolding by adding standard tubing reducers, as shown 
in FIG. 26. This design is essentially the same as shown in FIG. 24, 
except that reducers, item 86, have been employed to permit a number of 
different sizes of pressure vessels to be used in series, thereby 
maintaining high turbulence in spite of the fact that feed flow decreases 
due to the removal of quantities of permeate at each stage. In this 
figure, an eccentric reducer has been shown, though there is no reason 
that a concentric reducer should not be used, provided space and alignment 
considerations so permit. 
In order to illustrate some of the possible core designs, Table I shows the 
characteristics of many of the combinations and permutations 
TABLE I 
__________________________________________________________________________ 
Core sizes, numbers of internal tubes, sizes of internal tubes 
membrane areas, crossections and linear velocities 
Approx. 
Approx. Total High Flow rate at 
Nominal Number 
Approx. 
Max. max. Area 
membrane 
pressure 
0.38 M/sec or 
pipe Pipe 
of I.D. outer internal 
Ratio 
inner and 
cross- 
1.26 ft/sec. 
Entry 
Size, 
Pipe I.D., 
Internal 
of tubes, 
membranes 
membrane 
inner/ 
outer section, 
liters/ 
M.sup.3 / 
No. inches 
Schedule 
(cm) 
tubes 
(cm) (M.sup.2 /M) 
(M.sup.2 /M) 
outer 
(M.sup.2 /M) 
(cm.sup.2) 
min day 
__________________________________________________________________________ 
1 11/2" 
5S 4.50 
6 1.2 0.13 0.23 1.8 0.36 9.48 21.0 
31.1 
2 2" 5S 5.70 
18 1.0 0.17 0.57 3.4 0.79 17.6 40.0 
57.8 
3 21/2" 
5S 6.88 
18 1.2 0.20 0.68 3.3 0.88 25.7 58.6 
84.4 
4 3" 5S 8.47 
18 1.5 0.25 0.84 3.3 1.09 37.0 84.4 
122 
5 3" 5S 8.47 
36 1.05 0.25 1.19 4.7 1.44 36.4 82.9 
119 
6 3" 40 7.79 
36 1.0 0.24 1.13 4.7 1.36 33.0 75.3 
109 
7 31/2" 
10S 9.01 
36 1.2 0.29 1.36 4.6 1.65 46.6 106 153 
8 31/2 40 9.01 
36 1.2 0.27 1.36 5.0 1.63 46.2 105 152 
9 4" 40 10.23 
36 1.4 0.31 1.58 5.1 1.89 61.7 140 203 
10 4" 40 10.23 
60 1.0 0.31 1.89 6.1 2.19 53.4 122 175 
11 4" 40 10.23 
60 1.0 -- 1.89 -- 1.89 47.1 107 155 
12 5" 40 12.82 
60 1.3 0.39 2.45 6.3 2.84 87.6 200 288 
13 5" 40 12.82 
90 1.0 0.39 2.83 7.3 3.22 83.3 190 273 
14 5" 40 90 1.05 -- 2.97 -- 2.97 77.9 178 256 
15 6" 40 15.41 
90 1.2 0.47 3.39 7.2 3.86 111 254 366 
16 6" 40 15.41 
126 1.05 -- 4.16 -- 4.16 109 249 358 
17 8" 40 20.27 
168 1.2 0.63 6.33 10.1 
6.96 203 461 665 
18 8" 40 20.27 
162 1.2 -- 6.10 -- 6.10 183 417 602 
19 8" 40 20.27 
210 1.05 -- 6.93 -- 6.93 181 415 597 
20 8" 40 20.27 
264 0.93 -- 8.29 -- 8.29 179 409 589 
21 8" 40 20.27 
324 0.83 -- 8.45 -- 8.45 175 400 576 
22 8" 40 20.27 
390 0.75 -- 9.19 -- 9.19 172 395 566 
23 8" 40 20.27 
462 0.68 -- 9.87 -- 9.87 167 383 551 
24 8" 40 20.27 
540 0.60 -- 10.18 -- 10.2 153 348 501 
25 8" 40 20.27 
624 0.55 -- 10.78 -- 10.8 148 338 487 
26 8" 40 20.27 
714 0.50 -- 11.21 -- 11.2 140 320 460 
27 10" 40 25.5 
390 1.0 12.25 -- 12.3 306 698 1,006 
28 12" 40 30.3 
540 1.0 -- 16.96 -- 17.0 424 967 1,392 
29 12" 40 30.3 
624 0.93 -- 18.23 -- 18.2 424 966 1,392 
30 12" 40 30.3 
714 0.86 -- 19.29 -- 19.3 415 946 1,362 
31 14" 40 33.3 
624 1.0 -- 19.60 -- 19.6 490 1,117 
1,609 
32 14" 40 33.3 
714 0.95 -- 21.31 -- 21.3 506 1,154 
1,661 
33 16" 40 38.1 
912 0.98 -- 28.08 -- 28.1 688 1,568 
2,258 
34 18" 40 42.9 
1134 1.0 -- 35.62 -- 35.6 891 2,031 
2,924 
35 20" 40 47.6 
1380 1.0 -- 43.35 -- 43.4 1084 2,471 
3,558 
36 22" 20 54.0 
1794 1.0 -- 56.36 -- 56.4 1409 3,213 
4,626 
37 22" 60 51.4 
1650 1.0 -- 51.84 -- 51.8 1296 2,955 
4,255 
38 24" 40 57.5 
1944 1.0 -- 61.07 -- 61.1 1527 3,481 
5,013 
39 28" 30 67.9 
2784 1.0 -- 87.46 -- 87.5 2187 4,985 
7,179 
40 32" 40 77.8 
3564 1.0 -- 112 -- 112 2799 6,382 
9,190 
41 36" 40 87.6 
4674 1.0 -- 147 -- 147 3670 8,370 
12,052 
42 42" 80S 104.1 
6480 1.0 -- 204 -- 204 5089 11,600 
16,700 
43 48" 80S 119.4 
8580 1.0 -- 270 -- 270 6739 15,400 
22,100 
44 21/2" 
5S 6.88 
7 1.600 
0.356 -- 0.356 23.3 53.2 
76.6 
45 21/2" 
40 6.27 
7 1.600 
0.356 -- 0.356 17.0 38.9 
56.0 
46 19 1.34 -- 0.76 0.76 25.4 57.9 
83.3 
__________________________________________________________________________ 
Entries 44 and 45 represent the standard ROpak 7 core design. 
Entry 46 represents the standard PattersonCandy design. 
of cores which could be manufactured under this general principle. The pipe 
sizes shown range from 11/2" to 48". At the bottom of this table are shown 
the characteristics of one commercially available internal and one 
commercially available external pressure tubular design. 
Under 3" I have shown two different wall thicknesses, schedule 5S and 40, 
and two different numbers of internal tubes, 18 and 36. 
Table I shows that, as the diameters of the pressure vessels increase, the 
ratio of the internal to external membrane surface increases. The external 
surface varies as the first power of the O.D., whereas the internal area 
varies approximately as the square of the O.D. In addition, as the size 
increases, the possiblity of damaging the external membrane increases, and 
dimensional considerations make it very difficult to cast a good membrane. 
Membranes are normally only 75 to 150 microns thick. While, for short 
cores, dip casting has been used (R-22), superior membranes are achieved 
by use of "extrusion" through rigid ring dies. With these dies, core 
elipticity must be carefully controlled to 25 to 50 microns maximum. It is 
every difficult to fabricate large porous cores with this degree of 
precision in their outside surfaces. 
However, due to handling problems, and since the external membrane provides 
only a small portion of the total membrane on a large core, it is 
beneficial to omit the optional membrane (5) on the external surface of 
larger cores. 
The right half of FIG. 27 shows a cross section of an 8" core in which 
there would be 210 internal tubular membrane coated surfaces and no outer 
membrane coated surface. For simplicity, the internal membranes, 4, have 
not been shown in this and subsequent figures. The outer surface may be 
sealed as in item 7, FIG. 1, or a thin metallic or plastic shell, 90, may 
surround the core. 
Referring again to Table I, under the 8" entry I have shown one design with 
external membrane and eight different designs without external membrane. 
In the design with the external membrane, I have shown 216 internal 
tubular membrane coated surfaces, whereas the first of the designs without 
an external membrane has only 210 internal tubular surfaces. The reason 
for this difference is that, with increasing volume of permeate, it is 
necessary to increase the diameter of the permeate duct. In order to 
accommodate the larger duct, it was necessary to omit the inner row of 
tubular surfaces. 
Another problem developed as the size of the cores increased. This problem 
resulted from the geometrical limitation on the clearance between tubes in 
the first row. As an hypothetical example, if the diameter of the tubes in 
this inner row were equal to their distance from the center line, they 
would be tangent to one another and no flow of permeate could pass from 
the outer rows to the permeate duct. For example, if the first row of 
tubes were to be located on a circle 1 centimeter from the central axis, 
and if the tubes were 1 cm in diameter, placed every 60.degree., they 
would touch, and there would be no space for porous substrate between 
them. On the other hand, if the centers of the tubes on the second row of 
said core were to be on a circle 2 centimeters in diameter, and one 
centimeter tubes were to be placed every 30.degree. around the circle, a 
small clearance would exist between tubes, as shown in FIG. 28. The 
center-to-center distance between these tubes is 4 sin 15.degree.=1.0353. 
Since the radius of the cores is only 0.5 centimeters, a clearance of 
2(0.518-0.500)=0.036 would exist. Similarly, in the third row, 18 tubes 
would be placed 20.degree. apart and the distance between centers would be 
6 sin 10.degree.=1.0419 cm. The clearance between adjacent one centimeter 
tubes would be 2(0.521-0.500)=0.042 cm. Therefore, it is seen that, for 
rows beyond the first row, there is a significantly larger clearance 
available for permeate to flow toward the permeate duct. This phenomenon 
is best seen in the difference between the arcs and the chords, as 
follows: 
##EQU1## 
where A is the length of the arc between centers of adjacent tubes, 
n is the row number, 
m is the number of tubes in row n, and 
R.sub.1 is the radius of row n=1. 
Assuming R.sub.1 =1.0 cm, 
EQU A=.pi./3=1.047198 
EQU C=2n sin .theta./2, (Eq. 2) 
where 
C is the length of the chord between the centers of adjacent tubes, and 
.theta. is the angle subtended by the lines connecting the centers of the 
tubes with the center of the circle on whose circumference the centers of 
the tubes are located. 
Calculating the length of the chords for various values of n, with m=6, we 
find the following relationships: 
__________________________________________________________________________ 
n = 1; 
C = 2 .times. 1 .times. sin 60/2 = 2 .times. 0.5 = 1.00000; 
A-C = 0.04720 
n = 2; 
C = 2 .times. 2 .times. sin 30/2 = 4 .times. 0.25882 
A-C = 0.01192 
n = 3; 
C = 2 .times. 3 .times. sin 20/2 = 6 .times. 0.17365 
A-C = 0.00531 
n = 4; 
C = 2 .times. 4 .times. sin 15/2 = 8 .times. 0.130528 
A-C = 0.00299 
n = 5; 
C = 2 .times. 5 .times. sin 12/2 = 10 .times. 0.104528 
A-C = 0.00192 
n = 10; 
C = 2 .times. 10 .times. sin 6/2 = 20 .times. 0.052336 
A-C = 0.00047 
n = 20; 
C = 2 .times. 20 .times. sin 3/2 = 40 .times. 0.026177 
A-C = 0.00012 
__________________________________________________________________________ 
It is thus demonstrated that, as the relative distance from the center 
increases, the length of the chord approaches that of the arc. 
One solution to this problem is to have the tubes in the first row slightly 
smaller than those in rows 2 and above. However, the flow rate in the 
smaller tube would be lower, resulting in poorer rejection. In marginal 
cases, it would result in preferential fouling of the membranes in the 
smaller tubes. I therefore prefer to have all tubes the same diameter. For 
Ultra Filtration, where the flux is particularly high, I have found that, 
even with the smaller sizes of cores, such as the 3", Schedule 5S, 36 tube 
design shown on Table I (entry 5), it is frequently beneficial to omit one 
of the tubes in the central row, reducing the total number of tubes to 35; 
the area of internal membrane is thereby decreased from 1.19 to 1.16 
M.sup.2 /M and the total membrane decreased from 1.44 to 1.41 M.sup.2 /M, 
a loss of only 2%. 
The simplest design is based upon six tubes in the first row and six more 
tubes in each subsequent row of a core. In order to show the effect of a 
larger or smaller increment, FIG. 29 is presented. In this figure, a 6" 
pipe is illustrated. The symbol "n" is again used to represent the row 
number, and "m", the number of tubes in each row. In the smallest circle 
shown, n=2; Row n=1 has been omitted. In section A, m=6. In sections C and 
D, m=7 and in rows E and F, m=5. 
The diameter of the tubes in sections A, B and F is the same. In section F 
(where m=5), it is apparent that there is much lost space between tubes in 
the same row, resulting in a lower packing density than in section A. 
(There are some potential benefits from this increased space, especially 
in the case of Ultra Filtration, where flux rates are very high.) In 
Section E, in order to take advantage of the available space in row n=2, 
the diameter of the tubes in row n=2 have been increased. However, it was 
then necessary to increase the diameter of the circle n=3. This increase 
made it possible to further increase the diameter of tubes in row n=3. 
Accordingly, the diameter of the circle n=4 had to be increased, making 
possible a further increase in the diameter of the tubes in row n=4. 
Ultimately, the diameter of the tubes in row n=5 grew to 1.6 times the 
diameter of the standard tubes in sections A and F, and the circle n=6 was 
lost. Since, as previously noted, it is undesirable to have more than one 
size of tube in a core, the design shown in Section E would not be 
desirable for RO. 
Next, referring to Section C, with the increased number of tubes in each 
row (m-7), it was necessary to decrease the diameter of the tubes to 
compensate for the loss of clearance. It is therefore seen that there is 
increased clearance between rows in section C. Such an increase provides 
no benefit. 
In Section D, in order to reduce the unused space between rows, the 
diameter of the circle n=3 has been reduced. However, as a result, the 
diameter of the tubes in row n=3 had to be reduced to preserve the 
clearance between adjacent tubes. In a manner similar to that demonstrated 
in section E, the diameter of circles 4 through 6 were further reduced 
requiring additional and progressive reductions in the diameter of the 
tubes. An extra row, n=7, was added. It is thereby demonstrated that, when 
m=7, an undesirable RO design results. Either space is lost between rows 
or multiple tube sizes result. 
In section B is shown an alternate design in which, for rows n=2 and n=3, 
m=5. This concept is beneficial for very large cores, especially when 
there is limited porosity of the substrate. For rows n=4, 5 and 6, of 
section B, m=6. 
The number of tubes in designs of this type is given by the following 
equation: 
EQU X=m(n.sup.2 +n/2), (Eq. 3) 
where n is the total number of rows of tubes, m is the number of tubes in 
each row and X is the total number of tubes. 
As has been shown in FIG. 28, there is restricted space in row 1. 
Therefore, for the maximum packing density (with uniform tubing sizes) if 
row 1 is to be used, it is often desirable to employ only 5 tubes in this 
row. Therefore, if m=5 when n=1, the total number of tubes is X-1. If row 
1 is omitted, the total number of tubes is X-6. 
In order to maximize the internal clearance between tubes, it is beneficial 
to displace the centers of the tubes in adjacent rows. In so doing, I 
prefer to place the center of the first tubes in odd numbered rows at 
0.degree., displacing the center of the first tubes in the even numbered 
rows by half the angle between tubes in those rows. Table II shows the 
progression of the number of tubes and their arrangement. 
For good packing density, the sizes of the progressive circles is given by 
the following equation: 
EQU R.sub.n =nR.sub.1, (Eq. 4) 
where 
R.sub.1 is the radius of the circle for row n=1, 
n is the row number and R.sub.n is the radius of row n. 
Referring again to FIG. 28, the clearance in row n=x can be found by the 
equation 
EQU (R.sub.t +S.sub.x /2/R.sub.x)=Sin .theta./2, (Eq. 5) 
where 
R.sub.t is the radius of the tubes, S.sub.x is the clearance between 
adjacent tubes in the row in which n=x, R.sub.x is the radius of the row 
n=x and .theta. is the angular spacing between tubes. For row n=4, 
.theta.=15.degree.. 
By rearranging and solving for S.sub.x, the following equation results: 
EQU S.sub.x =2(R.sub.x sin .theta./2-R.sub.t) 
EQU S.sub.4 =2(0.1305R.sub.4 -R.sub.t) (Eq. 6) 
I prefer to maintain R.sub.t between 0.5 and 2.0 cm and S between 0.1 and 
0.6 cm, depending upon core size, substrate permeability and membrane 
TABLE II 
______________________________________ 
Number of internal tubular surfaces per row and per core, 
angular spacing and location of first tube in each row., upon 
condition in which m = 6.* 
Angular 
Number of Total number 
Location of 
spacing of 
Row tubes in of tubes in 
first tube 
tubes in 
number, (n) 
row, (6n) core, (X) in row n. 
row n, (Y) 
______________________________________ 
1 6 6 0.degree. 
60.degree. 
2 12 18 15.degree. 
30.degree. 
3 18 36 0.degree. 
20.degree. 
4 24 60 7.5.degree. 
15.degree. 
5 30 90 0.degree. 
12.degree. 
6 36 126 5.degree. 
10.degree. 
7 42 168 0.degree. 
8.57.degree. 
8 48 216 3.75.degree. 
7.5.degree. 
9 54 270 0.degree. 
6.67.degree. 
10 60 330 3.degree. 
6.degree. 
11 66 396 0.degree. 
5.45.degree. 
12 72 468 2.5.degree. 
5.degree. 
13 78 546 0.degree. 
4.62.degree. 
14 84 630 2.14.degree. 
4.29.degree. 
15 90 720 0.degree. 
4.degree. 
16 96 816 1.88.degree. 
3.75.degree. 
17 102 918 0.degree. 
3.53.degree. 
18 108 1026 1.67.degree. 
3.33.degree. 
19 114 1140 0.degree. 
3.16.degree. 
20 120 1260 1.5.degree. 
3.degree. 
21 126 1386 0.degree. 
2.86.degree. 
22 132 1518 1.36.degree. 
2.73.degree. 
23 138 1656 0.degree. 
2.61.degree. 
24 144 1800 1.25.degree. 
2.5.degree. 
25 150 1950 0.degree. 
2.4.degree. 
30 180 2790 1..degree. 
2.degree. 
34 204 3570 0.88.degree. 
1.76.degree. 
35 210 3780 0.degree. 
1.71.degree. 
39 234 4680 0.degree. 
1.54.degree. 
40 240 4920 0.75.degree. 
1.5.degree. 
45 270 6210 0.degree. 
1.33.degree. 
46 276 6486 0.65.degree. 
1.30.degree. 
50 300 7650 0.6.degree. 
1.2.degree. 
______________________________________ 
*number of tubes in row n is 6n since, for this table, m = 6. 
Equation for total number of tubes; 
##STR1## 
Equation for spacing of tubes; 
##STR2## 
Location of first tube on even numbered rows = Y/2 
In practive, it is prefereble to place only 5 tubes in row n = 1. In such 
cases the angular spacing is 72.degree. and the total number of cores is 
- 1. In other cases, the row n = 1 is omitted, and the total number of 
tubes is X - 6. When row n = 1 is omitted and m = 5 for row n = 2, the 
angular displacement for row n = 2 is 36.degree. and the total number of 
tubes is X - 8. 
flux. Within these constraints, the maximum value of R.sub.t is calculated 
using Equation 5. 
The simplest way to lay out a tube array is on polar coordinate graph 
paper. To convert the design to a form suitable for manufacturing, the 
locations of tube centers may be mathematically expressed in polar 
coordinates, with templates being cut on an N/C vertical boring mill, 
using a computer transformation of polar to rectangular coordinates. 
As will be shown below, unbalanced compressional forces are restricted to 
the space within the center line for the innermost circle of tubes. For 
cores up to 2", it is best to use the more rigid of the available porous 
substrates. However, for larger cores, it is possible to make use of a 
composite structure in which a substrate with lower compressional strength 
and greater flexibility (larger pore size, higher void volume and lower 
elastic modulus) is built up on a precast 2" or 3" core. For example, for 
nuclear applications, a 2" or 3" sintered polyethylene core can form the 
central portion of a 12" core in which the outer portion is composed of a 
semi rigid plastic foam. Such a core possesses superior characteristics 
for shipping, handling and installation. Similarly, for potable water 
applications, a 2" or 3" sintered silica core can support a similar 
plastic foam core of larger diameter. Such a design is illustrated in FIG. 
30, in which, for the central section, m=5, and n=2 and 3. The more dense 
central substrate is shown as item 91. Or, a central section of substrate 
up to 3 cm in diameter, with no tubes except the permeate duct, can be 
used to carry a portion of the higher unbalanced force within the 
innermost circle of tubes. Such a central section is shown as item 92, 
FIG. 27. As previously noted, for larger sizes no tubes are used in the 
row n=1, so that it is only required that the central core be small enough 
so that it would not conflict with the tubes in the second row. The 3 cm 
dimension satisfies this requirement for the designs shown in FIG. 27. By 
use of this composite core design, the unbalanced internal mechanical 
forces are carried by the central substrates, which are best able to 
withstand them. 
A decrease in manufacturing costs also results from the use of these 
composite cores. Tooling costs, fabricating costs and energy consumption 
are much lower for some of the softer or less dense materials employed in 
the outer portion of the larger sizes. Core weight, and the attendant 
handling difficulties, are also substantially reduced. 
In FIG. 27, vectors "P" illustrate the way in which system pressure is 
applied uniformly to the internal and external surfaces of core 1. The 
force on the internal surface, for a one centimeter length of core, may be 
expressed by the equation for hoop stress, as follows: 
EQU F.sub.i =2R.sub.i .DELTA.P cm., (Eq. 7) 
F.sub.i is the internal force, 
R.sub.i the radius of the internal surface, and 
.DELTA.P the pressure differential. 
The manner in which the external force is carried is empherical, and 
relates to such variables as the void volume, pore size and elastic 
modulus of the substrate 6. With closely packed, high elastic modulus 
substrates, very little yielding takes place in the substrate. A thin 
tangential layer of the external surface carries the unbalanced 
compressional force. In the absence of yielding, these forces are not 
propogated into the internal portion of the substrate. 
Equations for expressing this phenomenon quantitatively are not available 
for heterogeneous substances. The magnitude of this component must equal 
the difference between the two forces as follows: 
EQU F.sub.t =2.DELTA.P(R.sub.e -R.sub.i) cm., (Eq. 8) 
F.sub.t is the tangential component and R.sub.e is the radius of the 
external surface. 
The balance of the internal and external forces is transmitted directly 
from one surface to the other, through point-to-point contacts within the 
substrate. 
In analyzing the variables in balanced pressure, tubular RO designs, it is 
necessary to differentiate between the loading within the granules of the 
porous substrate, as explained above, and the fluid pressure differentials 
within the porous channels between granules. While the mechanical system 
may be in static balance, the fluid within the passages in the substrate 
constitutes a separate, dynamic hydraulic system, subject to analysis by 
established techniques. 
In order to evaluate the phenomena which occur within the static mechanical 
system, without the complications introduced by the hydraulic system, it 
is valuable to consider a hypothetical core in which all surfaces exposed 
to system pressure are sealed against fluid passage, i.e., a core without 
membrane and with no fluid flow within the porous substrate. Next, we 
assume that the permeate duct communicates with the atmosphere, so that 
the pressures within the permeate duct and in the voids within the porous 
substrate, are Zero (gauge). 
Under such circumstances, the compressional loads are seen to be in balance 
in all parts of the core, with one exception. Within the center line 
circle on which the centers of the innermost row of tubes is located, a 
compressional force imbalance exists. For example, referring to the right 
side of FIG. 27, a mechanical pressure imbalance exists in the substrate 
located within the innermost circle of tubes. With substrates possessing 
high elastic moduli, low void volumes and small particle sizes, this 
mechanical load is dissipated within the first few layers of the substrate 
granules on the convoluted profile, within the center line circle (n=2). 
In other words, along the inner half of the circumferences of the 
innermost row of tubes, and in the zone where the centerline circle passes 
from tube to tube. 
These compressional forces do not propagate toward the permeate duct unless 
the substrate is composed of low elastic modulus, loosely compacted, high 
void volume or large particle size substances, or a combination of these 
effects, referred to herein as "less dense substrates". In cases where 
these less dense substrates are employed, the compressional forces would 
not penetrate past the more dense, composite, inner substrate shown as 
item 92 of FIG. 27. 
Having thus analyzed the mechanical forces within the substrate, additional 
analyses may be made of the flow of a fluid through the membrane and, 
thereafter, through the interstices between the solid substances of which 
the porous substrate is composed. These phenomena are described in a later 
portion of this specification. 
It is thus seen that the core constitutes a system in which three separate 
phases exist, namely, (1) a continuous solid phase through which (2) a 
continuous aqueous phase passes, and, (3) the membrane. In some cases 
there is a discontinuity in the liquid system at the point at which a 
portion of the pressurized feed liquid passes into the semipermeable 
membrane, as will be explained in the next section. If such a 
discontinuity exists for high rejection membranes, such is not the case 
for ultrafiltration membranes. Therefore, the fluid system may be 
considered to consist of (1) a pressurized aqueous mixture, (2) a membrane 
acting, in some ways, like an orifice plate, (3) a series of labyrinth 
passages and, (4) a low pressure collecting duct for the aqueous phase 
passing through the system. 
Having thus segregated the four different zones of the system, we next turn 
to a description of the nature of membranes and the passage of water 
through them. Membranes normally consist of a skin measuring approximately 
0.25 microns in depth, supported by a spongy layer approximately 100 
microns in depth. The flow velocity or flux through a membrane in my 
device is given by the following equation: 
EQU v=k.sub.m (P.sub.f -P.sub.p -.DELTA..pi.-.DELTA.P.sub.s), (Eq. 9) 
v=linear velocity (cm/sec), or flux (ml/cm.sup.2 /sec) 
P.sub.f =Feed pressure (kg/cm.sup.2) 
P.sub.p =Permeate pressure 
.DELTA..pi.=Differential osmotic pressure, feed solution minus permeate 
(kg/cm.sup.2) 
.DELTA.P.sub.s =pressure drop within the porous substrate. 
k.sub.m =membrane constant (cm.sup.3 /gm/sec) 
For my device, the value of k.sub.m will vary from 1 to 5.times.10.sup.-8 
cm.sup.3 /gm/sec, for membranes with a nominal rejection between 98% and 
80%. 
In reverse osmosis (as contrasted with ultrafiltration), a widely held 
theory suggests that the process of the passage of water through the skin 
of a cellulose acetate membrane involves molecular phenomena in which 
water molecules associate with acetate groups, and then migrate 
progressively from one acetate group to another, driven by the net 
pressure differential (v/km), until they emerge into the open, spongy 
layer beneath the skin. 
Accordingly, unless the value of .DELTA.P.sub.s is significant when 
compared with P.sub.f -P.sub.p -.DELTA..pi., the entire pressure drop may 
be thought of as occurring within the membrane skin. 
In order to estimate the magnitude of the internal pressure drop within 
cores of various compositions and fluxes, the following analysis is 
offered: 
The most critical internal flow rate is that which occurs between adjacent 
tubes in either the row closest to the permeate duct or the first row in 
which m=6. To determine the magnitude of this flow rate, it is first 
necessary to estimate the area of membrane outboard of the point of 
closest approach between adjacent tubes. For a 1 meter length of core, 
this value may be calculated as follows: 
##STR3## 
R.sub.t is the radius of tubes (cm) n.sub.x is the value of n in the row 
for which the inter-tube flow velocity is being estimated. 
m.sub.x is the value of m in that row. 
n.sub.x+1 and m.sub.x+1 are the values of m and n in the subsequent row. 
m.sub.n and n.sub.n are the values of m and n in the largest row. 
A.sub.e is the area of membrane in a meter of core outboard of the circle 
n=x, expressed M.sup.2 
The flow rate is given by the following expression: 
EQU v=(FA.sub.e /8.64 m.sub.x n.sub.x S.sub.x), (Eq. 11) 
where 
S.sub.x =the space between the tubes in row where n=x, expressed in cm. 
F=flux in M.sup.3 /M.sup.2 /day 
v=flow rate (ml/cm.sup.2 /sec), (or cm/sec) 
The value of S.sub.x may be calculated as follows (see FIG. 28): 
EQU (R.sub.t +S.sub.x /2)/R.sub.x =sin (360/2m.sub.x n.sub.x) (from Eq. 5) (Eq. 
12) 
##EQU2## 
For example, take the core represented in the right side of FIG. 27. In 
this case, the row closest to the center is n=2, in which m=6. There are 
198 tubes in rows 3 through 8. Add to this m.sub.2 n.sub.2 /2 or 
6.times.2/2=6 extra tubes, making 204 tubes outboard of the circle n=2. 
Assuming R.sub.t =0.5 and R.sub.1 =1.15, R.sub.2 =2.times.1.15, 
##EQU3## 
EQU S.sub.2 =-2(0.50-2.3 sin 
15.degree.)=-2(0.51-2.3.times.0.2588)=-2(0.50-0.59528) 
EQU S.sub.2 =-2(-0.09528)=0.19056 
Assume a flux of 1 M.sup.3 /M.sup.2 /day, 
EQU A.sub.e =(2.pi./100)R.sub.t .times.204=6.41 M.sup.2 
Now, 
##EQU4## 
For the left side of FIG. 27, in row n=2, m=5, the number of tubes outboard 
the circle n=2 would be as follows: 
Row n=2, 5/2; row n=3, 15; rows n=4 through 8, 180; the total is therefore 
197.5. 
##EQU5## 
EQU S.sub.2 =-2(0.50-2.3.times.0.3090)=-2(0.50-0.710)=-2(-0.211)=0.421 
EQU A.sub.e =(2.pi.R.sub.t /100).times.197.5=6.20 M.sup.2 
EQU v=(6.20/8.64.times.5.times.2.times.0.421)=0.170 ml/cm.sup.2 /sec 
For either side of FIG. 27, row n=4, the following is a calculation of the 
velocity of flow: 
The number of tubes in row n=4 is 24/2; for rows n=5 through n=8, there are 
156 tubes; total outboard is 168. 
##EQU6## 
EQU S.sub.4 =-2(0.50-4.6.times.0.1305)=-2(0.50-0.600)=2.times.0.100=0.20 
EQU A.sub.e =(2.pi.R.sub.t /100).times.168=5.28 M.sup.2 
EQU v=(5.28/8.64.times.6.times.4.times.0.20)=0.127 ml/cm.sup.2 /sec 
In a similar manner, the outboard areas, intertube spacing and intertube 
flow rates were calculated for the other rows of the core illustrated in 
FIG. 27. The results of these calculations are summarized in Table III. 
Having thus established the flow rate at the point of nearest approach, it 
is next necessary to make use of Darcy's equation in order to determine 
the pressure drop. 
EQU dP/dt=.alpha..mu.v/g.sub.c, (Eq. 14) 
where 
.alpha.=viscous resistance coefficient for a porous medium (cm.sup.-2) 
.mu.=viscosity of fluid, in this case, water, at 0.010 poise=0.010 gm 
sec.sup.-1 cm.sup.-1 
v=superficial velocity of fluid (cm/sec or ml cm.sup.-2 sec.sup.-1) 
g.sub.c =gravitational constant (981 cm/sec.sup.2) 
Nickelson, et al (R-22) measured the viscous resistance for three highly 
porous candidate materials for external pressure RO cores. 
These values of .alpha. are as follows: 
sintered polyvinylidene fluoride, 25 micron pore size, 
.alpha.=2.7.times.10.sup.5 cm.sup.-2 
sintered polyethylene, 10 micron pore size, .alpha.=1.2.times.10.sup.6 
cm.sup.-2 
ceramic, 1 micron pore size, .alpha.=3.6.times.10.sup.6 cm.sup.-2 
Using these values for .alpha., the value of dP was integrated for the 
passage of water between adjacent tubes of radius R.sub.t, in row n=2, 
separated by the distance S.sub.n. These values are also shown in Table 
III. As can be seen, for an 8 inch core with a flux of 1 M.sup.3 /M.sup.2 
/day, with these substrates, very little internal pressure drop would 
occur, even in ultrafiltration applications with twice the flux used in 
this analysis. 
In the same manner, the viscous resistance coefficient was calculated for 
the ceramic material used for external pressure cores, and for a nominally 
rated 2 micron, sintered polyethylene filter cartridge. 
They were as follows: 
sintered polyethylene from 2 micron nominal filter, 
.alpha.=6.2.times.10.sup.8 cm.sup.-2 
external tubular RO ceramic core, (0.1 to 0.5 microns) 
.alpha.=2.1.times.10.sup.12 cm.sup.-2. 
TABLE III 
__________________________________________________________________________ 
Internal pressure drops for 8" cores, n = 2 to 8; m = 5 or 6. 
##STR4## 
out board membrane 
V (ml/cm.sup.2 /sec 
polyvinylidine 
polyethylene, 
ceramic 
polyethylene, 
ceramic 
n m area (M.sup.2 /M) 
S.sub.n (cm) 
or cm/sec) 
fluoride, 25.mu. 
10.mu. 1.mu. 
2.mu. nominal 
0.1-0.5.mu. 
__________________________________________________________________________ 
2 6 6.41 0.191 
0.323 0.00052 0.0023 0.0069 
1.19 4,000 
3 6 5.94 0.198 
0.193 0.00032 0.0014 0.0043 
0.73 2,500 
2 5 6.20 0.421 
0.170 0.00034 0.0015 0.0045 
0.78 2,600 
3 5 5.89 0.434 
0.052 0.00011 0.00049 
0.0015 
0.25 847 
4 6 5.28 0.200 
0.127 0.00021 0.00095 
0.0028 
0.49 1,600 
5 6 4.43 0.202 
0.085 
6 6 3.39 0.203 
0.054 
7 6 2.17 0.203 
0.030 
8 6 0.75 0.203 
0.0009 
__________________________________________________________________________ 
The latter ceramic material is far too dense to be considered for even 
moderate sized, balanced pressure tubular RO cores. However, using the 2 
micron nominal polyethylene and m=5 for n=2, a moderate pressure drop of 
0.78 kg/cm.sup.2 was realized for the permeate flowing through the 
inter-tube zone, as seen in Table III. 
Next, this same analysis was performed for row n=2 of larger cores, using 
m=5 and m=6. The results are shown in Table IV. 
In the above analyses, the velocity of the fluid at the point of nearest 
approach of adjacent tubes is correct. There is a small increase in the 
total fluid flow after the point of nearest approach, and a slightly 
smaller total flow approaching this point, due to the contribution of the 
row of tubes being analyzed. However, these effects are opposite and 
virtually compensate for one another. Therefore, the use of the average 
flow volume to estimate the pressure drop by integrating dP across the gap 
R.sub.t to -R.sub.t is justified. 
With respect to higher or lower flux rates, the anticipated flow rates and 
pressure drops can be estimated by multiplying the values shown in Tables 
III and IV by the flux in M.sup.3 /M.sup.2 /day. 
In order to demonstrate the low relative pressure drop in substrates as 
compared to membranes, it is valuable to compare equations 9 and 14. 
EQU dP/dt=(.alpha..mu.v/g.sub.c) (Eq. 14) 
EQU v=k.sub.m (P.sub.f -P.sub.p -.DELTA..pi.-.DELTA.P.sub.s) (Eq. 9) 
Rearranging equation 9 we find, 
EQU P.sub.f -P.sub.p -.DELTA..pi.-.DELTA.P.sub.s =v/k.sub.m 
Replacing P.sub.f -P.sub.p -.DELTA..pi.-.DELTA.P.sub.s with .DELTA.P.sub.n, 
the net driving pressure, 
EQU .DELTA.P.sub.n =v/k.sub.m 
Comparing this equation with equation 14, it is possible to convert 
equation 9 
TABLE IV 
__________________________________________________________________________ 
Internal pressure drops for various core sizes, n = 2; m = 5 or 6 
Internal 
##STR5## 
flow rate, V, 
polyvinylidine 
Nominal Pipe 
Outboard Membrane 
(ml/cm.sup.2 /sec 
fluoride 
polyethylene 
ceramic 
polyethylene 
Size, inches 
m n Area (M.sup.2 /M) 
S.sub.n (cm) 
or cm/sec) 
25.mu. 10.mu. 1.mu. 
2.mu. nominal 
__________________________________________________________________________ 
8" 6 2 6.41 0.191 
0.32 0.00052 0.00023 
0.0069 
1.19 
8" 5 2 6.20 0.421 
0.17 0.00034 0.00015 
0.00046 
0.73 
12" 6 2 16.8 0.191 
0.84 0.0014 0.0063 
0.019 
3.24 
12" 5 2 16.7 0.421 
0.46 0.00091 0.0041 
0.012 
2.10 
18" 6 2 34.4 0.191 
1.8 0.0029 0.013 0.039 
6.7 
18" 5 2 34.4 0.421 
0.97 0.0019 0.0086 
0.026 
4.4 
24" 6 2 60.9 0.191 
3.0 0.0049 0.022 0.066 
11 
24" 5 2 60.9 0.421 
1.7 0.0033 0.015 0.045 
7.5 
36" 6 2 146.6 0.191 
7.4 0.012 0.054 0.161 
27 
36" 5 2 146.6 0.421 
4.0 0.008 0.036 0.108 
18 
48" 6 2 269.3 0.191 
14. 0.0022 0.098 0.293 
50 
48" 5 2 269.3 0.421 
7.4 0.015 0.066 0.198 
34 
__________________________________________________________________________ 
to a form subject to analysis by Darcy's equation. The so-called "membrane 
constant" may be replaced with a factor which includes dt, g.sub.c, 
.alpha. and .mu.. 
##EQU7## 
or, 
EQU dP/dt=(.alpha..mu.v/g.sub.c) (Equation 14) 
Solving equation 15 for .alpha., we find, 
EQU .alpha.=(g.sub.c /k.sub.m .mu.dt) (Eq. 16) 
As previously noted, values of k.sub.m for my device range from 1 to 
5.times.10.sup.-8 cm.sup.3 /gm/sec. For a membrane with a flux of 1 
M.sup.3 /M.sup.2 /day, k.sub.m =3.4.times.10.sup.-8. It is believed that 
the thickness of the active layer or skin of the membrane is 0.25 microns 
or 2.5.times.10.sup.-5 cm. Entering this value for dt, we find, 
##EQU8## 
EQU .alpha.=1.1.times.10.sup.17 cm.sup.-2 
As noted above, the values of .alpha. for several available porous 
substrates range from 2.7.times.10.sup.5 to 6.2.times.10.sup.8 cm.sup.-2. 
The value of .alpha. for membranes is therefore substantially greater than 
that of core substrate, further confirming that the vast majority of 
pressure drop in a properly design balanced pressure tubular RO core is in 
the surface of the membrane. 
From the above analyses, the following effects can be shown: 
1. For large cores with high fluxes and low feed pressures, it is 
beneficial to omit row n=1, and to use m=5 for rows n=2 and n=3. 
2. There is no significant benefit from using m=5 in rows n=4 and beyond, 
provided that a suitable balance of internal flow rate and viscous 
resistance coefficient can be achieved. 
3. By a judicious balance of the several operational, material and design 
parameters, a range of conditions can be established in which neither (1) 
high parasitic pressure drops nor (2) high instantaneous pressure drops in 
the area of the membrane occur. 
4. For substrates with viscous resistance coefficients in excess of 
10.sup.9 or 10.sup.10 /cm.sup.2, excessive parisitic pressure drops may be 
anticipated. 
In order to demonstrate the practical benefit of the broad range of sizes 
of pressure vessels, Table V illustrates the treatment of an industrial 
waste stream of 1,000 cubic meters per day. The results are shown on Graph 
1, herein identified as FIG. 34. 
With feed solutions possessing low to medium fouling tendancies, it has 
been found beneficial to maintain a minimum linear velocity of 0.38 meters 
per second (1.26 ft/sec.) With more difficult mixtures, or as the 
concentration ratio increases, (depending upon the fouling characteristics 
of the stream being treated), the minimum linear velocity should be 
increased. 
Each of the first four entries in Table V represent two 8" pressure 
vessels, six meters long, each containing six cores of the type shown in 
Table I, entry 17. Each core is 0.97 meters long, plus couplers, so that 
each pressure vessel contains 5.8 meters of core, the remaining 20 cm. 
being taken up with connections and space for remixing the feed solution. 
In this example, the initial feed entered the first pressure vessel with a 
linear velocity of 0.61 Meters/sec. and left the eighth pressure vessel at 
0.33 Meters/sec. Then, using the principle shown in FIG. 26, the diameter 
of the pressure vessel was decreased from 8" to 6", using a reducer in 
conjunction with a 180.degree. return. The linear velocity of the feed was 
thereby 
TABLE V 
__________________________________________________________________________ 
Cell Considerations Cumu- 
En- Size lative 
try and M.sup.2 .times. Flux % Conc. 
Per- 
No. 
Q TDS 
V P .DELTA.P 
No. = Perm. 
TDS Q TDS 
V P .DELTA.P 
Perm. 
Ratio 
meate 
__________________________________________________________________________ 
1 2 3 4 5 6 7 8 
1,000 762 762 537 537 409 409 293 
500 647 647 902 902 1167 1167 1601 
0.61 0.47 0.47 0.33 0.62 0.47 0.47 0.41 
70 69.9 69.9 69.85 69.85 69.75 69.75 69.42 
.06 x .03 x .06 x .03 x 
##STR6## 881 881 647 647 473 473 347 
564 564 756 756 1017 1017 
1364 1364 
0.54 0.54 0.40 0.40 0.54 0.54 
.40 0.48 
69.94 69.94 69.87 69.87 
69.79 69.79 69.72 69.72 
x .04 x .02 x .04 x 
5.5 5.5 5.5 5.5 5.5 
5.5 5.6 
1.14 1.31 1.55 
1.86 2.11 2.44 
2.88 
119 238 353 
463 527 591 
653 
9 10 11 12 13 
310 310 236 236 236 
1518 1518 1966 1966 1966 
0.67 0.67 0.51 0.51 0.83 
69.12 69.12 69.01 69.01 69.01 
x .06 x .04 0.10 
##STR7## 347 273 273 200 213 
1364 1712 1712 2300 2167 
0.75 0.59 0.59 0.43 0.75 
69.72 69.06 69.06 68.97 
68.91 
.06 x .05 x x 
5.5 5.5 5.5 5.6 
3.23 3.66 4.24 
5.0 690 727 764 
800 
14 15 16 17 18 19 20 21 22 23 
154 154 108 108 90 90 68 68 52 52 
2950 2950 4129 4129 4905 4905 6390 6390 8249 8249 
0.54 0.54 0.71 1.32 1.10 1.10 0.83 0.83 0.64 0.64 
68.85 68.85 68.63 68.63 68.09 68.09 67.60 67.60 67.38 
x .04 x .20 x .26 x .15 x .05 
##STR8## 200 132 132 102 102 79 79 57 57 
47 
2300 3414 3414 4358 4358 5547 
5547 7567 7567 9070 
0.70 0.46 0.87 1.25 1.25 0.97 
.97 0.70 0.70 0.57 
68.97 68.81 68.81 68.43 
68.43 67.83 67.83 67.45 
67.45 67.33 
.12 x .18 x .34 x .23 
x .07 x 
5.4 5.6 5.7 5.7 5.8 
6.0 6.1 6.2 6.3 
6.49 7.58 9.26 
9.80 11.11 12.66 
14.71 17.54 19.23 
21.28 
846 868 892 
898 910 921 
932 943 948 
__________________________________________________________________________ 
953 
Q is Quantity of feed or concentrate in M.sup.3 /day: TDG is Total 
Disolved Solids in ppm: V is linear velocity of fluid in Meters per 
second: P is pressure in Kg/cm.sup.2 : .DELTA.P is pressure drop in 
kg/cm.sup.2 : Flux is given in M.sup.3 /M.sup.2 of membrane surface: Perm 
is quantity of permeate in M.sup.3 /day: % Perm. is percent permeation 
which is the same as 100% minus percent rejection: Conc. ratio is 
concentration ratio, or Volume of initial feed divided volume of 
concentrate at each stage of the calculations: Cumulative Permeate is the 
volume of permeate from a given plus the volume of permeate from all uppe 
entries. 
increased from 0.33 to 0.62 Meters/sec. 
The next three entries show the feed solution progressing through six 
pressure vessels of the 6" size, with the linear flow rate dropping from 
0.62 to 0.40 Meters/second. Entries 8 and 9 show alternate options for the 
next stage. With the 5" pressure vessel, the linear velocity was only 
increased to 0.48 Meters/second and, after passing through only two 5" 
vessels, it had dropped to 0.41 Meters per second. For reasons of 
standardization, it is beneficial to limit the total number of possible 
sizes of cores. Therefore, entry 9 shows a more suitable selection for the 
next stage. In this entry it is seen that the linear velocity increases 
from 0.40 Meters/second (from the last 6" vessel) to 0.75. After eight 
pressure vessels, it drops to only 0.43 Meters/second (entry 13). 
These two options can also be seen in Graph 1. Above the 653 Tons per day 
entry on the horizontal coordinate, there are two entries for velocity, 
one for the 5" pressure vessel and one for the 4". The calculated velocity 
after the two 5" cells is shown above the 707 ton per day entry. 
Again, at entries 12 and 13 a choice must be made between passing through 
another two 4" pressure vessels or dropping down to a 3" pressure vessel. 
The effect is shown on Graph 1 above the 800 and 823 ton per day permeate 
entries. In this case, the decision as to which size to use for the next 
stage did not turn on the question of the linear velocity of the next 
stage. It had emerged from the previous 4" vessel with a velocity of 0.51 
Meters/second and only dropped to 0.43 after passing through another two 
vessels. By changing to 3" it would have risen to 0.83 Meters/second and 
would have only dropped to 0.75 after two 3" vessels. Either course could 
have been beneficial, based upon the prior applications engineering data 
on the particular stream being treated. If it possessed a relatively high 
fouling tendency, it would have been proper to have changed to the 3" 
pressure vessels at this point, rather than waiting until entry 14, where 
the change was actually effected. 
Next, after passing through six 3" pressure vessels (entries 14 and 15), 
the pressure vessel size was further reduced to 2" (entry 16) for four 
pressure vessels. Finally (entry 17), the pressure vessel size was reduced 
to 11/2". 
After passing through two 11/2" pressure vessels, a concentration ratio of 
approximately 10:1 was achieved (entry 17). If a concentration ratio of 
15:1 is required, an additional twelve 11/2" vessels (entries 18, 19 and 
20) are required. To achieve a 20:1 concentration ratio with this feed, 
eight more 11/2" pressure vessels (entries 21, 22 and 23) are required. 
Having thus illustrated the fact that not all sizes of pressure vessels 
provide beneficial results, Table VI shows eleven of the entries 
previously given in Table I. In this case, I have shown the ratios of the 
membrane areas and the ratios of linear velocities of the adjacent 
entries. The minimum and maximum ratios of membrane areas are 1.61 and 
2.45 and the minimum and maximum ratios of velocities are 1.61 and 2.40. 
Ratios larger than these would introduce excessive gaps in capabilities 
for treating feed solutions and, as shown in the case of the 5" vessel 
(Table II, entry 8), smaller gaps cannot be justified because they do not 
sufficiently improve treating capacity. 
Tables I and VI are based upon designs in which m is always 6. However, as 
noted above, for higher fluxes and, especially, for large diameter cores, 
it is beneficial to use cores in which m=5 for the central rows. The left 
half of FIG. 27 shows a core in which there is no row n=1, and, for n=2 
and n=3, m=5. Nonetheless, the entries in Tables I and VI are 
substantially correct. The deletion of one to four tubes results in a very 
small decrease in relative membrane area. 
TABLE VI 
__________________________________________________________________________ 
Practical Sizes, Area Ratios and Velocity Ratios 
Flow Rate, 
Nominal Area Ratio, 
M.sup.3 /day at 
Velocity Ratio, 
Pipe Size Number of 
Approx. 
Membrane area, 
lower entry/ 
0.38 M/sec. or 
lower entry/ 
(inches) 
Pipe Schedule 
tubes I.D., cm. 
M.sup.2 /M 
upper entry 
1.25 ft/sec. 
upper entry 
__________________________________________________________________________ 
11/2" 
5S 6 1.2 0.36 31.1 1.86 
2" 5S 18 1.0 0.74 2.06 57.8 1.89 
3" 40 36 1.0 1.36 1.61 109 1.61 
4" 40 60 1.0 2.19 1.90 175 2.05 
6" 40 126 1.05 4.16 1.67 358 1.67 
8" 40 210 1.05 6.93 2.45 597 2.33 
12" 40 540 1.0 17.0 2.09 1,390 2.10 
18" 40 1134 1.0 35.6 1.72 2,920 1.71 
24" 40 1944 1.0 61.1 2.40 5,010 2.40 
36" 40 4674 1.0 147 1.84 12,000 1.83 
48" 80S 8580 1.0 270 22,100 
__________________________________________________________________________ 
It might appear to be impractical to consider the fabrication of cores as 
large as 48 inches. However, a comparison of the economical considerations 
will reveal that there is a substantial incentive in producing such a 
core. One conventional external pressure core has a membrane area of 0.046 
M.sup.2 and a commercial value of 3,450 or US$11.50. One meter of 7-core 
pressure vessel contains 0.356 M.sup.2 of membrane with a value of 26,416 
or US$88.05. One Meter of 48" core, as shown in Table I, entry 43, has 270 
M.sup.2 of membrane surface or 758 times as much membrane, and could 
replace conventional external pressure cores costing 20,035,000 or 
US$66,800. In addition, each pressure vessel of this size would have the 
effect of eliminating 758 smaller pressure vessels, and an equivalent 
reduction in crossover castings (42), return headers (43), retainer 
flanges (45), permeate collectors (41) and permeate delivery tubes, plus 
2,274 pipe couplers (44). A comparable savings in the frames and supports 
for the pressure vessels is also realized with the larger sizes of cores. 
Thus it is seen that cost reductions can be realized by this technique 
which may make reverse osmosis practical for sewage treatment and water 
reuse in a medium sized city. 
It is also significant to consider the cost effectiveness of the stainless 
steel employed in the fabrication of the pressure vessels. Table VII shows 
the comparative relationships of the weight of steel required to house one 
square meter of membrane surface in the various sizes of pressure vessels 
previously described in Table VI. For comparison, three different designs 
of conventional external pressure 7-core devices are shown at the bottom 
of Table VII. This table shows that, for a 21/2 inch schedule 40 
conventional external pressure design, 24.2 kg of pressure vessel is 
required for each meter of membrane, whereas only 2.86 kg is required for 
a 36 inch pressure vessel fabricated in accordance with the principles of 
my invention. 
TABLE VII 
______________________________________ 
Pipe Size 
(nominal) 
Schedule kg/M M.sup.2 /M 
kg/M.sup.2 
______________________________________ 
11/2 5S 1.90 0.36 5.78 
11/2 10S 3.11 0.35 8.89 
2 5S 2.39 0.74 3.23 
2 10S 3.93 0.72 5.46 
2 40 5.65 0.70 8.07 
3 40 11.3 1.36 8.31 
4 40 16.1 2.19 7.35 
6 40 28.3 4.16 6.80 
8 40 42.6 6.93 6.15 
12 40 79.8 17.0 4.69 
18 40 156.1 35.6 4.38 
24 40 255.1 61.1 4.18 
36 40 421.0 147 2.86 
48 80S* 378.2 270 1.40 
2* 10S 3.93 0.356 11.0 
21/2** 5S 3.69 0.356 10.4 
21/2** 40 8.63 0.356 24.2 
______________________________________ 
*Data not available on 48 inch pipe with equivalent pressure capability a 
for the 36 inch size. 
**Entries for conventional external pressure/reverse osmosis designs. 
It is also noteworthy that, with larger pressure vessel sizes, further 
economies may be effected by use of mild steel pipe with epoxy or a 
similar internal coating. While these coatings have proved somewhat 
unreliable with smaller pressure vessels, these vessels are large enough 
to permit an operator to enter the vessel to inspect for and correct 
defects. 
It is also possible to fabricate the pressure vessels with stainless clad 
(stainless lined) pipe, substantially reducing the weight of costly 
stainless steel, without losing its beneficial corrosion resistant 
properties. 
In those cases in which a balance of internal pressure drop cannot be 
established by core design and material selection, it is possible to 
introduce additional axial permeate ducts, 93, as shown in FIG. 31. 
In this figure, an 8" core is shown with three additional permeate ducts in 
row n=6, placed 120.degree. apart, and three optional ducts, 94, at 
intermediate angles in the same row. For very large sizes with high values 
of .alpha., additional permeate ducts may be provided in rows beyond that 
shown. It must be recognized, however, that this technique introduces 
unbalanced mechanical forces in the zone between these axial ducts and the 
adjacent tubes. 
On installation, the several axial permeate ducts may be coupled directly 
to the corresponding duct from the adjacent core; they may then be 
interconnected at the first core in the pressure vessel. Or, for cores in 
which space limitations would make it difficult to couple more than one 
duct, the several ducts may be interconnected at each face of each core, 
reducing to one the number of connections required at the time of 
installation. 
Another way in which a balance in pressure drop can be established is by 
placing small radial ducts, 95, in the porous substrate, as illustrated in 
FIG. 32. In such a case, the staggered placement of tubes shown in Table 
II cannot be used. These ducts may then be placed 60.degree. apart, 
provided, of course, that m=6 for the entire core. 
Again it is recognized that this technique introduces unbalanced mechanical 
forces between the radical ducts and the tubes between which they pass. 
These ducts can be made in several manners. In one method, small radial 
rods are placed in the core mold prior to casting the core, with one end 
extending outside the mold. These are removed after fabrication, and the 
resulting holes, 96, plugged. In another effective method, small rods of a 
water soluble organic substrate, such as high molecular weight 
polyethylene glycol, or an inorganic substance such as NaCl or Na.sub.2 
SO.sub.4, are placed in some of the perforations of the permeate duct 
prior to fabrication of the core. When placed in service, the water 
soluble substrate gradually leaches away, leaving the desired radial 
permeate duct. In a third method, suitable for low temperature core 
fabrication methods, a waxy or crystaline substance can form the duct and 
it may be melted out of the core after curing. In all of these cases, it 
is beneficial to secure the rods in the proper position by wiring or 
otherwise attaching them to the mandrels forming several of the tubes 
prior to casting the core. 
Since the principle objective of the radial ducts is to increase the ease 
with which permeate from the outer portions of a core reaches the central 
permeate duct, it is possible to use a small diameter metal tubing, with 
or without perforations, to assist in withstanding the unbalanced 
compressional forces generated by the placement of these ducts in the core 
substrate. Such a duct liner would have one or more orifices within the 
axial permeate duct to facilitate delivering permeate thereto. To prevent 
the open end of these tubes from becoming plugged during fabrication, a 
water soluble plug of organic or inorganic substance may be placed in the 
ends prior to casting. 
For cores requiring radial permeate ducts, the number of these ducts 
required per linear section of core increases with the increasing diameter 
of the core. It is not practical to employ radial permeate ducts at 
intermediate angles. However, this method may be combined with the use of 
axial permeate ducts, 93, at intermediate angles, as shown at 30.degree., 
90.degree., 150.degree., 210.degree., 270.degree. and 330.degree. of FIG. 
33. 
Finally, in those cases in which no external membrane, 5, is used, and when 
maximum pumping efficiency is desired, it is desirable to prevent flow of 
fluid over the external surface of the core. This objective is achieved by 
the installation of an "O" ring, item 97, or other gasket on the outer 
surface of the core, as illustrated in FIG. 33. In this manner, fluid 
pressure continues to be exerted on the outer surface of the core, while 
all of the fluid flow is directed through the internal tubes. The 
installation of the core is facilitated by the use of molybdenum disulfide 
powder, or a suitable amorphous lubricant such as a soft grade of 
petrolatum or silicone base grease. 
BIBLIOGRAPHY OF KNOWN PRIOR ART 
I. Patent References 
______________________________________ 
Patents, American Patents, Japanese 
______________________________________ 
1. Loeb 3,133,132 
1. Loeb 42-2818 
2. Loeb 3,133,137 
2. Mahon 44-14215 
3. Loeb 3,170,867 
3. Merten 46-19806 
4. Loeb 3,283,042 
4. Huggins 39-30143 
5. Loeb 3,364,288 
5. Cahn 44-9443 
6. Merten 3,386,583 
6. Strand 45-36724 
7. Westmoreland 
3,367,504 
7. Hanzawa 46-484 
8. Bray 3,367,505 
8. Donokos 46-21444 
9. Loeb 3,446,359 
9. Comers 46-38963 
10. Shippey 3,400,825 
10. Mahon 39-28625 
11. Block 3,768,660 
11. Geory 44-5526 
12. Merten 45-13933 
13. Merten 46-9804 
14. Michaels 
44-18730 
15. Bray 45-1174 
16. Shirokawa 
45-13935 
17. Signa 36-10866 
18. Block 48-96459 
19. Block 48-96460 
20. Saito 49-48074 
21. Baldon 51-64481 
______________________________________ 
II. References from Literature 
R-1. S. Sourirajan, "Reverse Osmosis", Academic Press, New York, (1970) 
R-2. R. E. Lacey & S. Loeb, "Industrial Processing with Membranes", 
Wiley-Interscience, a Div. of John Wiley & Sons, Inc., New York, (1972) 
R-3. S. Kinura, H. Ohya, S. Suzuki, "Reverse Osmosis Systems, Membrane 
Separation Technology", Shokuhin Kogyo Gijutsu Ohosakai, (Food Industry 
Technology Research Association of Tokyo) (1973) 
R-4. Joseph W. McCutchan and Douglas N. Bennion, "Saline Water 
Demineralization by Means of a Semipermeable Membrane" Saline Water 
Research Progress Summary, Jan. 1, 1970-Dec. 31, 1970, Water Resources 
Center Desalination Report No. 40, School of Engineering and Applied 
Science, University of California at Los Angeles, p.p. 1-10 
R-5. Dieter Landolt, "Interfacial Phenomena on Reverse Osmosis Membranes", 
ibid p.p. 23-25. 
R-6 J. W. McCutchan & J. Glater, "Scale Control Studies", ibid p.p. 27-35. 
R-7 K. S. Murdia, J. Glater & J. W. McCutchan, "Hemihydrate Scaling 
Threshold Enhancement by Magnesium Ion Augmentation" Water Resources 
Center Desalination Report No. 49, April 1972, School of Engineering and 
Applied Science, University of California at Los Angeles. 
R-8. J. M. Jackson & D. Landolt, "About the Mechanism of Formation of Iron 
Hydroxide Fouling Layers on Reverse Osmosis Membrane", Water Resources 
Center Desalination Report No. 50, September 1972, School of Engineering 
and Applied Science, University of California at Los Angeles. 
R-9 D. Antoniuk & J. W. McCutchan, "Desalting Irrigation Field Drainage 
Water by Reverse Osmosis, Firbaugh, Calif.", Water Resources Center 
Desalination Report No. 54, August 1973, School of Engineering and Applied 
Science, University of California at Los Angeles. 
R-10. J. W. McCutchan, D. Antoniuk, G. Chakrabarti, M. Chan, V. Goel, N. K. 
Patel & E. Selover, "Saline Water Demineralization by Means of a 
Semipermeable Membrane", Water Resources Center Desalination Report No. 
57, Progress Report, Jan. 1, 1973 to June 30, 1974, Department of Chemical 
Engineering, University of California, Berkley and School of Engineering 
and Applied Science, University of California, Los Angeles. p.p. 55-68 
R-11. J. W. McCutchan, J. Glater, R. Dooly & M. Adler, "Scale Control 
Studies", ibid p.p. 73-87 
R-12. M. S. Adler, J. Glater 6 J. W. McCutchan, "Gypsum Solubility and 
Scaling Limits in Saline Waters", Water Resources Center Desalination 
Report No. 59, January 1975, School of Engineering and Applied Science, 
University of California at Los Angeles. 
R-13. M. B. Kim-E & J. W. McCutchan, "Reclamation of Hyperion Secondary 
Effluent by Reverse Osmosis", Water Resources Center Desalination Report 
No. 60, June 1975, School of Engineering and Applied Science, University 
of California at Los Angeles. 
R-14. J. W. McCutchan, D. Antoniuk, V. Goel, M. Chan, M. B. Kim-E, R. Reddy 
& E. Selover, "Saline Water Demineralization by Means of a Semipermeable 
Membrane; Firebaugh Agricultural Wastewater Desalting", Water Resources 
Center Desalination Report No. 62, Progress Report, July 1, 1974-June 30, 
1975, Sea Water Conversion Laboratory, University of California, Berkeley 
and School of Engineering and Applied Science, University of California, 
Los Angeles, p.p. 25-34. 
R-15. "Colorado River Desalting at LaVerne, California", ibid p.p. 34-40 
R-16. "Further Research on Cellulose Acetate Membranes", ibid p.p. 40-47 
R-17. M. C. Porter, P. Schrafler and P. N. Rigopulos, "By-Product Recovery 
by Ultrafiltration", Industrial Water Engineering, Vol 8, Number 6, p.p. 
18-24, June/July 1971. 
R-18. D. Dean Spatz, "Reverse Osmosis Reclamation Systems for the Plater", 
Finishers Management, July 1971. 
R-19. Vincent T. Burns, Jr., "Reverse Osmosis Cuts Solids", Water & Wastes 
Engineering, 1974. 
R-20. Radovan Kohout, "Operating History of a 324,000 pgd RO plant", 
Industrial Water & Engineering Conference, Mar. 14-16, 1973. 
R-21. Doyle, Boen and L. Johansen "Reverse Osmosis of Treated and Untreated 
Secondary Sewage Effluent", National Environmental Research Center, Office 
of Research and Development, United States Environmental Protection 
Agency, Report EPA-670/2-74-077, September 1974. 
R-22. Nickelson, Birkhimer, Coverdell, Lai and Wang, "Membranes for Reverse 
Osmosis by Direct Casting on Porous Supports", U.S. Dept. of the Interior, 
Office of Saline Water, R & D Progress Report 520, March 1970. 
III. Reference from Commercial Literature 
C-1. "Reverse Osmosis Systems for Industrial Water Purification and Waste 
Treatment", Roga Division Universal Oil Products Company, Catalog, 1976. 
C-2. "Design Manual for Du Pont Permasep Reverse Osmosis Systems", Permasep 
Products Division, E. I. du Pont de Nemours & Co., Inc., 1974. 
C-3. "Pollution Control and By-Product Recovery for the Pulp and Paper 
Industry", Bulletin FS-3, Fluid Sciences Division, Universal Oil Products 
Company, 1975. 
C-4. "Membrane Separations for the Dairy Industry", Bulletin FS-4 Fluid 
Sciences Division, Universal Oil Products. 
C-5. "Reverse Osmosis and Ultrafiltration, an Emerging Technology for 
Liquid Separations", Bulletin FS-2, Fluid Sciences Division, Universal Oil 
Products. 
C-6. "Philco Ford Reverse Osmosis", Product Bulletin 101. 
C-7. "Reverse Osmosis Advanced Tubular Technology Offering Unparalleled 
Versatility in Processing Liquids", Liquid Process Products Division, 
Philco-Ford Corporation, Catalog B-101, August 1971. 
C-8. "Westinghouse Reverse Osmosis Systems . . . Pure Water for Industry", 
Catalog B-160, Heat Transfer Division, Lester Branch, Westinghouse 
Electric Corporation. 
C-9. "Ultrafiltration Westinghouse Membrane Systems . . . End Paint Loss 
and Water Pollution Problems", Bulletin SA 471-2, ibid. 
C-10. "Reverse Osmosis Westinghouse Membrane Systems Cut Soluble Oil 
Costs", Bulletin SA 471-3, ibid. 
C-11. "Reverse Osmosis & Ultrafiltration Westinghouse Membrane Systems 
recover Valuable Byproducts from Cheese Whey", Bulletin SA 471-4, ibid. 
C-12. "Reverse Osmosis Systems", Catalog 3190-1, Patterson Candy 
International Ltd., 1976. 
C-14. "Reverse Osmosis", Catalog No. 9000a, Raypak, Inc., 1971. 
C-15. "Reverse Osmosis & Ultrafiltration" Catalog No. 9001a, Rev-O-Pak, 
Inc., 1975. 
C-16. "Pollution Control and Waste Recovery of Water Soluble Oil and 
Synthetic Lubricant Coolants" Catalog No. 9300, Rev-O-Pak, Inc. 
C-17. "Sumitomo ROpak" Catalog "Sumiko 51.6 3T" Sumitomo Jukikai, 1976. 
C-18. "Sumitomo/ROpak Reverse Osmosis" Catalog "Sumiko 76.9," Sumitomo 
Heavy Industries, 1976. 
C-19. "Pollution Control at Electroplating Wastes by Reverse Osmosis" 
Catalog No. 9301, Rev-O-Pak, Inc.