Method of forming tubesheet for hollow fibers

Hollow fiber membranes can change dimensions upon drying or other processing creating leaks in tubesheets formed on fiber bundles. An adherent layer is formed in contact with an inner surface of each tubesheet and the fiber membranes therein in such a manner as to fill gaps formed between the hollow fiber membranes and each tubesheet during the drying or processing of the hollow fiber membranes and tubesheets. Preferably, the adherent layer is cast from a diglycidyl ether of bisphenol A containing an acrylo-nitrile-modified polyamine curing agent and the hollow fibers are cellulose ester.

BACKGROUND OF THE INVENTION 
This invention relates to an improved tubesheet composition and method of 
forming the tubesheet. In one particular embodiment, an improved method of 
preparing a dry cellulose ester hollow fiber membrane device is disclosed. 
A variety of techniques for drying water-wet cellulose ester membranes are 
known in the prior art. It is essential that the drying be conducted in a 
manner which does not deleteriously affect the pore structure and 
consequently the membrane characteristics of the hollow fiber. U.S. Pat. 
No. 4,430,807 and a number of references cited therein describe the 
displacement of water from cellulose ester membranes using organic 
solvents. U.S. Pat. No. 4,127,625 describes the treatment of the membrane 
with a salt solution followed by freeze drying. 
It has been found convenient to assemble water-wet, cellulose ester hollow 
fiber membranes into a bundle complete with one or more tubesheets and 
then to dry the bundle. U.S. Pat. Nos. 3,228,876, 3,422,008, 3,455,460 and 
3,755,034 are illustrative of some of the methods described in the art for 
fabrication of hollow fiber membranes devices. Techniques for the 
formation of tubesheets on hollow fiber membrane devices are also 
disclosed in U.S. Pat. Nos. 3,339,341, 3,423,491, 3,619,459, 3,722,695, 
3,728,425 and 4,138,460. U.S. Pat. No. 4,323,454 suggests at column 4, 
lines 19-31 that a tubesheet can be cast in successive castings. 
Cellulose ester hollow fiber membranes shrink significantly in 
cross-section upon drying. It has now been discovered that this shrinkage 
can produce unrelieved tensions, distortions, delaminations and cracks in 
the tubesheet. A convenient method of repairing the tubesheet is 
desirable. 
SUMMARY OF THE INVENTION 
A method of preparing a hollow fiber membrane device is described. In this 
method first at least one tubesheet is formed on a plurality of hollow 
fibers. Next the fibers are subjected to a processing step which results 
in a change in the dimensions or porosity but does not deleteriously 
affect the membrane characteristics of the hollow fibers. Then, at least 
one adherent layer is formed in intimate contact with each of the 
tubesheets and with the fibers embedded in the tubesheet. 
A novel membrane device comprising a plurality of hollow fiber membranes, 
an epoxy resin tubesheet and an adherent layer consisting of an epoxy 
resin cured with cyanoethylated polyamines is also claimed. Optionally, 
the membranes in this device can be any conventional hollow fiber 
membrane, as enumerated in U.S. Pat. No. 3,228,876.

DETAILED DESCRIPTION OF THE INVENTION 
Hollow Fiber 
The hollow fiber membranes described herein are of the type which undergo a 
change in dimensions in a post-spinning treatment step. A variety of 
polymeric hollow fibers known in the prior art undergo such changes in 
dimensions. In particular, organic fibers which contain a significant 
percentage of solvents or other components undergo a change in dimensions 
when these solvents or other components are evaporated or leached from the 
hollow fiber. In one preferred embodiment of this invention 
water-containing cellulose esters are employed. 
Cellulose ester hollow fiber membranes are well-known in the prior art. As 
manufactured, these membranes contain up to 70 weight percent water. Dry 
cellulose ester membranes are useful for separation of gases. Evaporating 
the water directly from the membrane deleteriously affects membrane 
characteristics by greatly reducing permeability. Physical integrity of 
the membrane can be adversely affected by drying. Consequently, a number 
of techniques have been developed in the prior art to dry cellulose ester 
membranes without adversely affecting membrane and physical 
characteristics. Cellulose ester membranes and processes for drying these 
membranes are described in U.S. Pat. No. 4,430,807, which is incorporated 
herein by reference. 
In a preferred embodiment of the instant process, cellulose acetate hollow 
fiber membranes, more preferably cellulose diacetate or triacetate, are 
employed. While these hollow fibers can be homogeneous or asymmetric, 
asymmetric cellulose triacetate containing from about 40 to about 44 
weight percent acetate is preferred. 
First Tubesheet 
It has been found to be advantageous to assemble the hollow fibers into a 
bundle and fabricate one or more tubesheets for the bundle prior to drying 
the entire fibers. Fabrication of the water-wet fibers into a bundle 
minimizes manipulation of the fibers once dry. Fabrication of tubesheets 
prior to drying keeps the fibers in a stable configuration during drying 
and can facilitate drying the fibers in certain embodiments of this 
invention. 
The tubesheet can take a variety of shapes and forms. U.S. Pat. No. 
3,228,876 and 3,422,008 describe fibers arranged in a parallel, as a helix 
or bias-wrap fashion around a core or axis with a tubesheet at each end. 
The hollow fibers are embedded in and have lumens communicating through 
both sides of the tubesheet. The hollow fibers can also be looped so that 
both ends of the fiber extend through a single tubesheet, as in FIG. 1 of 
U.S. Pat. No. 3,422,008. Alternatively, one end of the fibers can 
terminate in an endsheet with the opposite ends communicating through a 
single tubesheet. U.S. Pat. No. 4,080,296 describes a membrane device 
wherein the fiber lumens communicate through a drilled tubesheet at a 
point removed from the ends or return loops of the hollow fibers. U.S. 
Pat. No. 3,455,460 discloses one or two longitudinal tubesheets parallel 
to the axis of the fiber bundle. 
The tubesheets referred to herein can take any of the configurations and be 
present in the number suggested in the aforementioned prior art 
references, which are incorporated herein by reference. This invention is 
also intended to encompass modifications and alternative configurations of 
the tubesheets, fibers and optionally cores, which would be obvious to one 
of ordinary skill in the art. Preferably, the hollow fibers are arranged 
in a bias wrap or parallel fashion about a perforated core with a 
tubesheet at each end of the bundle. 
The tubesheets can be fabricated from any thermoplastic or thermosetting 
material which bonds to and does not deleteriously affect the hollow fiber 
membrane. See, e.g., U.S. Pat. No. 4,323,454, which is incorporated herein 
by reference. Because an adherent layer will later be added to augment the 
tubesheet, the tubesheet fabricated prior to drying need not meet all the 
specifications of the tubesheet required when the membrane device is put 
into operation. All that is necessary is that the tubesheet tolerate the 
environment to which it is exposed during drying of the fiber. Artificial 
and natural rubbers, phenolaldehydes, acrylic resins and epoxy resins are 
illustrative of the types of materials which can be used as tubesheets. 
Epoxy resins, e.g., the diglycidyl ether of bisphenol A, reacted with 
amines or other curing agents and optionally reactive diluents, fillers 
and other modifiers are preferred compositions for the fabrication of 
tubesheets. 
The tubesheet can be formed and cured by any one of a number of techniques 
known in the prior art. The tubesheet can be formed concurrent with the 
assembly of the fiber bundle by the application of the tubesheet forming 
material to the desired area of the fiber. Alternatively, the tubesheet 
can be cast or molded in the desired region after the fiber bundle is 
assembled. Centrifugal casting, as described in U.S. Pat. No. 3,339,341, 
is also operable. 
In forming the tubesheet, it is desirable that the viscosity of the 
material forming the tubesheet be low enough that good penetration between 
the fibers is attained, but not so low that excessive "wicking" into other 
regions of the fiber bundle occurs. The preferred viscosity will depend on 
the size of the hollow fibers, the packing factor for the bundle, the 
method used to form the tubesheet and other factors. The best viscosity 
for the tubesheet resin can readily be determined empirically for a given 
set of conditions. 
The tubesheet resin advantageously does not exotherm excessively upon 
curing. Localized regions of high temperature during curing can 
deleteriously affect tubesheet strength and/or fiber properties. 
It is preferable that the surface of the hollow fiber membranes be dry in 
the region to be embedded in the tubesheet to promote best adhesion. 
Surface dryness is conveniently achieved by blowing warm air over the 
hollow fiber in the region to be potted. Of course, premature drying of 
the membrane surface to be utilized subsequently in membrane separation 
processes should be avoided. 
Access to the hollow fibers bores in the tubesheet can be achieved by one 
of several prior art techniques. The tubesheet can be drilled to open the 
fiber bores as in U.S. Pat. No. 4,080,296. Loops of hollow fiber 
protruding from one side of the tubesheet can be cut. A portion of the 
tubesheet itself can be cut. Alternative methods are described in U.S. 
Pat. Nos. 3,422,008, 4,183,890 and 4,369,605. 
Membrane Processing 
After assembly into a bundle and formation of one or more tubesheets, the 
hollow fibers can be processed by any one of the techniques described in 
the prior art which results in a change in fiber dimensions. For example, 
the fibers can be subjected to heat to anneal them or soaked in a solvent 
bath to remove certain components. In a preferred embodiment of the 
invention, water is removed from cellulose ester hollow fibers. U.S. Pat. 
Nos. 3,842,515, 4,068,387, 4,080,743, 4,080,744, 4,120,098 and 4,430,807 
are illustrative of drying techniques and are incorporated herein by 
reference. The method disclosed in U.S. Pat. No. 4,430,807 is preferred. 
Where it is desirable to maintain different fluids or conditions in the 
bores than on the external surface of the fibers, the tubesheets can be 
sealingly engaged with one or more conduits for introduction and removal 
of fluids from the fiber bores as appropriate. The fiber bundle can be 
subjected to the desired conditions external to the fibers by immersion in 
a bath or establishing the desired conditions in a vessel containing the 
bundle. 
In drying or other processing of the hollow fiber membrane, in general, 
some intermingling of fluids in the fiber bore and those external to the 
fiber can be tolerated. Consequently, it is not necessary prior to the 
drying step that the tubesheets be machined and adapted to make a 
leak-free seal. Moreover, the tubesheet has frequently been observed to 
change shape as the fibers dry. Accordingly, the effort made to 
meticulously machine the tubesheet is frequently wasted. 
In one preferred embodiment of the invention an inflatable, deformable seal 
is used about the tubesheet while the hollow fibers are dried in 
accordance with U.S. Pat. No. 4,430,807. Optionally, a shallow circular 
groove is machined in the circumference of the tubesheet and an O-ring is 
inserted in the groove to provide a seal. 
Adherent Layer 
As noted previously, as the hollow fibers change dimensions leaks develop 
in the tubesheet. The addition of an adherent layer to the tubesheet 
present during drying or other processing can plug these leaks and 
strengthen the tubesheet structure. 
In embodiments of the invention in which open fiber ends protrude some 
distance through the tubesheet, it is operable to cast the adherent layer 
on this side of the tubesheet so long as the bores of substantially all 
the fibers remain open. More commonly, the adherent layer will be formed 
on the side or sides of the tubesheet from which the active membrane 
surfaces protrude. In the case of tubesheets as in U.S. Pat. No. 4,080,296 
wherein active membranes protrude from two sides of the tubesheet, it is 
desirable to form an adherent layer on both sides of the tubesheet. In any 
event, all tubesheets present during processing of the fibers are 
advantageously covered with an adherent layer. Endsheets in which fibers 
terminate generally do not require an adherent layer unless leaks are 
observed. 
The adherent layers, in general, can be fabricated from the same classes of 
materials as the tubesheets recited hereinbefore. The tubesheet and 
adherent layer can be fabricated from the same or different materials. It 
is usually advantageous that the tubesheet and adherent layer have good 
adhesion. However, in some embodiments of the invention, the tubesheet 
present during fiber processing will subsequently be removed leaving the 
adherent layer to function as a tubesheet. In those embodiments in which 
the original tubesheet is to be removed, good adhesion between the first 
tubesheet and adherent layer is not essential. 
In embodiments of the invention wherein the tubesheets present during fiber 
processing are to serve as structural supports engaging directly or 
indirectly a pressure case of the membrane device, the adherent layer can 
be comparatively thin. Preferably, the adherent layer is as thin as 
possible while still providing the desired sealing and integrity to the 
tubesheet. Where the original tubesheet is to be removed, the adherent 
layer should have the dimensions generally specified for tubesheets in 
membrane devices of the type being fabricated. 
The adherent layer is conveniently formed by casting. Centrifugal casting 
can be employed to minimize the reduction in effective membrane area. 
Alternatively, the tubesheet can be enclosed in a mold and positioned so 
that the resin used to make the adherent layer will settle and cure on the 
tubesheet in the desired region. If desired, the mold is sized and shaped 
to also cast an adherent layer on peripheral surfaces of the tubesheet, 
which will be machined and adapted to engage a pressure case in the 
finished device. 
Of course, the resin cast to form the adherent layer is advantageously a 
liquid having a viscosity low enough to permit penetration between the 
fibers in the bundle but not prone to migrate far from the tubesheet. In 
one preferred embodiment of the invention, the resin consists by weight of 
from about 80 to about 100 parts of polyepoxide, such as a diglycidyl 
ether of a bisphenol, from about 0 to about 20 parts of a reactive diluent 
and an effective amount of a curing agent. Illustrative polyepoxides, 
reactive diluents and curing agents are disclosed in U.S. Pat. No. 
3,728,425. Diglycidyl ethers of bisphenol A are preferred as polyepoxides. 
Ethyl hexyl glycidyl ether is preferred as a reactive diluent. 
A cyanoethylated polyamine, e.g., ethylene diamine or other aliphatic 
polyamines modified with acrylonitrile, is particularly preferred as a 
curing agent. Lee and Neville, Handbook of Epoxy Resins, pp. 7-22 to 7-24 
(1967), describes such curing agents. Particularly preferred are the 
modified polyamines sold by Pacific Anchor Chemical Corporation under the 
designation ANCAMINE.RTM. 1636 and ANCAMINE.RTM. 1942. The amount of 
curing agent required depends on its equivalent weight and other factors 
and can readily be determined empirically. The curing agent ANCAMINE.RTM. 
1942, for example, is advantageously present in from about 15 to about 50, 
preferably about 30 to about 45, parts per hundred resin by weight. As in 
the tubesheets, additives, fillers and modifiers may be advantageous in 
some embodiments. 
The time required for the adherent layer to solidify will vary dependent on 
temperature, the resin composition, curing agent, if any, size and other 
factors. The time needed for the adherent layer to form a solid can 
readily be determined empirically. 
Assembled Membrane Device 
Once an adherent layer has been formed on each tubesheet, the tubesheet and 
adherent layers can be machined as necessary to fit the associated 
pressure case. In one embodiment of the invention, as noted hereinbefore 
the original tubesheet can be cut off or otherwise removed leaving what 
was the adherent layer to serve as a tubesheet. In a preferred embodiment 
of the invention, the tubesheet and adherent layer are machined to accept 
an O-ring and sealingly engaged in a pressure vessel. 
FIG. 1 illustrates in section an assembled membrane device. The fluid to be 
separated is introduced into the inlet 1 of the feed pipe 2. The feed pipe 
2 passes through a first tubesheet 3 and an adherent layer 15 in contact 
with the tubesheet 3. The feed pipe also passes through a second adherent 
layer 16 in contact with a second tubesheet 4, in which the feed pipe 2 
terminates. The section of the feed pipe 2 between the two tubesheets 3 
and 4 contains a plurality of perforations 5 through which the feed passes 
to contact the external surfaces of a number of hollow fibers 6 arranged 
in a generally longitudinal fashion about the perforated feed pipe. 
Wrapped around the hollow fiber bundle is a porous or woven polymer outer 
wrap 9, which helps to prevent shifting of fibers in the bundle. Some of 
the feed passes axially and radially through the hollow fiber bundle to an 
outlet 7 in the pressure case surrounding the bundle. The remainder of the 
feed permeates through the walls of hollow fibers. The bores of the hollow 
fibers communicate at each end through the tubesheet with a head space 11 
and 12 on the far side of both tubesheets. The fluid which has permeated 
through the hollow fibers and collected in the header space can be removed 
through outlets 13 and 14 in the pressure case which each communicate with 
one of the header spaces. Optionally, a sweep fluid could be introduced 
into one header space and removed through the outlet in the other header 
space to assist in the collection of the permeate. 
Where dry asymmetric hollow fibers serve as the membrane, the resulting 
membrane device is especially well-suited for the separation of a gas 
mixture. The gaseous feed is preferably introduced external to the hollow 
fibers, but operably can be introduced into the bores of the fibers. These 
membrane devices are particularly useful in the separation of carbon 
dioxide from methane. 
The following example is presented to illustrate the invention. All parts 
and percentages are by weight unless otherwise indicated. 
EXAMPLE 
A hollow fiber bundle of about 50,000 cellulose triacetate fibers is 
assembled on a perforated core, fitted with a tubesheet at each end and 
dried in the manner of Example 1 in U.S. Pat. No. 4,430,807. Each 
tubesheet was roughly cylindrical with a diameter of about 5 inches and a 
thickness of about 3.5 inches. The gas separation of the resulting bundle 
was tested at 20.degree. C. and a feed pressure of 50 pounds per square 
inch guage. The carbon dioxide permeation flux in units of cm.sup.3 
/(second cm.sup.2 cm of Hg) was 7.4.times.10.sup.-5. The calculated 
CO.sub.2 /CH.sub.4 separation factor was 18.6. 
The fiber bores in the tubesheets were temporarily sealed with a polymeric 
foam and the membrane device placed sitting on one tubesheet upright in a 
mold. The mold was poly(tetrafluoroethylene) coated steel in the shape of 
an inverted, truncated cone five inches deep. The mold had a diameter at 
the bottom of 5.5 inches and a diameter of 6.25 inches at the top. 
A mixture of 90 parts of the diglycidyl ether of bisphenol A, 10 parts 
ethyl hexyl glycidyl ether and 40 parts of an acrylonitrile-modified 
polyamine (ANCAMINE.RTM. 1942) was prepared. The epoxy resin mixture was 
poured around the tubesheet in the mold covering the tubesheet to a depth 
of 3 inches. A 5.5-inch diameter, 6-inch high cardboard tube was fitted 
concentrically around the hollow fiber membrane bundle in contact with the 
surface of the liquid epoxy in the mold. 
When the resin in the mold has cured sufficiently to seal the bottom of the 
tube, a mixture of the same resin as used in the mold was poured into the 
tube at 28.degree. C. to a depth of 5 inches. After 2 hours the level of 
resin in the tube has declined to 3 inches above the resin in the mold. 
After an additional 4 hours the resin has gelled and curing is completed 
by heating the resin to 65.degree. C. 
The carbon dioxide permeation of the finished unit in the same units as 
hereinbefore was 4.94.times.10.sup.-5. The calculated CO.sub.2 /CH.sub.4 
separation factor was 36.6.