CA1114307A

CA1114307A - Crimped hollow fibers for fluid separations and bundles containing the hollow fibers

Info

Publication number: CA1114307A
Application number: CA317,048A
Authority: CA
Inventors: Richard L. Leonard
Original assignee: Monsanto Co
Current assignee: Monsanto Co
Priority date: 1977-11-30
Filing date: 1978-11-29
Publication date: 1981-12-15
Also published as: DE2851687A1; FR2410494A1; IL56081A; JPS583047B2; SE7812302L; GB2009034A; ZA786707B; TR20777A; IT7830319A0; IL56081A0; NO784013L; LU80600A1; NO146625C; NL7811649A; JPS5488317A; DD140000A5; AU510106B2; GB2009034B; DE2851687C2; SU1022650A3

Abstract

CRIMPED HOLLOW FIBERS FOR FLUID SEPARATIONS AND BUNDLES CONTAINING THE HOLLOW FIBERS ABSTRACT OF THE DISCLOSURE Hollow, semi-permeable fibers which are intended for use in fluid separations axe provided with relatively low amplitude waves, or crimps. The relatively low amplitude crimps enable the hollow fibers to be assembled in a bundle comprising substantially parallelly-oriented hollow fibers wherein the bundle has a relatively high packing factor, i.e., the percentage of the cross-sectional area of the bundle occupied by hollow fibers, such that unduly large volumes of separation apparatus are not required for a given amount of hollow fiber surface area. The relatively low amplitude crimps enable good fluid dispersion through a bundle comprising substantially parallelly-oriented hollow fibers such that shell-side (i.e., the exterior sides of the hollow fibers) feed of the fluid mixture to be treated can be attractive, even when the flow of the feed is predominantly axial to the hollow fibers. Shell-side feed of the fluid mixture to be treated is often particularly advantageous since the fluid mixture can frequently be recovered from the fluid separation with a relatively low pressure drop.

Description

~4~'7 This invention relates to hollow semi-permeable fibers which are intended for use i~ fluid separations;
to aggregates (bundles) of the hollow fibers; and to separation apparatus containing the aggregates of the hollow Eibers.
By this i~vention there are provided hollow fibers which are particularly advantageous for use in fluid ~eparation apparatus in which the fluid separation is efected by selective permeation through the hollow fibers. The hollow fibers of this inventlon enable desirable membrane surface area to be provided per unit ~olume of separation apparatus and can provide enhanced separation efficiencies.
Many proposals have been made Eor the use of semi-permeable membranes to eff~ct the separation by selective permea~ion of at least one flu~d ~rom a fluid mixture containing a~ least one other fluid. The commercial adoption of semi-permeable membranes for 1uid separations, however, has been limited, and presently larOe scale commercial use o~ semi-permeable membranes iQ primarily only for water desalination. ~hile semi-permeable mem~ra~es ha~e been developed which exhiblt suitable select~vit~ of separation or many operations, a fundamental difficul~y which has been obser~ed is the relatively low flux which can be obtained through the semi-permeable membranes, Accordingly, large surface areas of the semi-permeable membranes must be provided in order to obtain desirable quanti~ies of permeated produc t .
The configuration of the semi-permeable membranes significantly influences ~he Emount of active membra~e surface area which can be o~tained in a given volume o~ a semi-permeable membrane-containing separation apparatus. A
highly desirable membrane configuration Eor providing high ~ 3~ ~ 07-0005 ratios of active surface area per ~mit volume o~ separation apparatus is a hollow fiber, or hol.low filament~ For instance, Mahon in United States Patent No. 3~228,877 states a~ column 2, lines 35 et. seq., ~hat with the semi-permeable membranes in hollow fiber form ten thousand square feet or more of active surface area can be provided per cubic ~oot of volume of separa~ion apparatus. To accomplish such high ratios Qf membrane surace area per unit volume of separation apparatus, Mahon teaches that the hollow fibers should have relatively small outside diameters and that the more advantageous range of outside diameters of the hollow fibers is between 10 to 15 microns. Others ha~e similarly found that hollow fibers having relatively small outside diEmeters are ad~antageous for utilization in separation apparatus in order to provide suficiently large membrane surace areas. For Example, Maxwell et al, disclose in United States Patent No. 3,339,341 that hollow fibers ha~i~g outside diæmeters between 20~and 250 microns are especially preferred.
In fact, the pate~tees state a~ column 14, line 63, that pilot plant work was done utilizing hollow fibers having ou~side diameters of 29.2 microns.
The use of relatively small diameter hollow fibers , as semi-permeable membranes provides advantages in addition to the ability to achieve high ratios of active surface . 2'5 areas per unit volume of a separation apparatu~. As noted -~ by Mahon at column 10, lines 57, ~ g., the amount oE
: pressure diferential which can be withstood by a hollow fiber is directly related to ~he ratio of the thickness of : the wall of the fiber to the inside diameter of the fiber.
Since in many separation opera~ions the higher the pressure

-2-:

`7 differential across the membrane the greater the flux which can be obtained, the provision of hollow fibers which can withstand high pressure differential is desirable. Mahon thus concludes that the smaller the diameter of the fiber, the smaller the corresponding wall ~hickness chat is necessary to withstand a gi~en pressure drop, and walls of lesser thicknesses advan~ageously exhibit less resistance to permeate 10w; and hence provide greater fluxes, than exhibited by walls of greater thicknesses.
Although previous proposals have been made to provide relatively large active membrane surace area per unit volume of separation apparatus, such apparatus may not peror~
adequately for a 1u~d separation in the environment in which the fluid separation must be conducted for th~ apparatus to be advantageous on economic and processing bases. For instance, in assembling the hollow fibers in a separation apparatus, several problems can occur which reduce ~he .
efectiveness of the separation apparatus. Firs~, hollow fibers can, and essentially always do, contact other fibers i~ the assembled separation apparatus. The resulting area -- ~ of contact is unavailable ~o effect the desired separation, and thus the ~lux and efficiency which can be obtained axe reduced. Second, the con~act of the hollow fibers hinders , .,;~
the ~low of fluid around and ~e~een the hollow ibers thereby resulting in a non-uni~ormity of flow within the . apparatus and even in localized pockets of fluid. These pockets of fluid, when in contact with the e~teriors of the fibers, contain an increased concentration of a less permeable fluid of the feed. This greater concer.tration of the less permeable fluid of the feed ~ 3 . ...~ ..-.'~ . ' .
' ' _~ 07-0005 causes an increased permeation of the less permeabLe fluid through the membranes and hence reduces the selectivity of separation. In extreme cases iIl which the feed is a liquid, the pockets of fluid may become so saturated wi~h the less permeable fluid that the :Less permeab~e fluid precipitates or separates between the hollow fibers.
Third, since the genexally hollow fibers have relatively small outside diameters and thin walls, they are very flexibLe. Thus, even if the hollow fibers are assembled in the separation apparatu~ in a manner which minimi~es the contact between the hollow fibers, the hollow fibers may come i~ increased contact with one and another and form uneven distribution channels due to the ease of movement o~ the highl~ flexible hollow ~ibers during operation of the separation apparatus.
Another important factor in~olved in the consideration of whether or not a separation apparatus is advantageous on economic and processing bases is the effect o~ the separation apparatus on the energy of the fluid stream being processed. Proposals have been made ~o empLoy semi-permeable membrane-containing separation apparatus to selectively separate one or more fluids from a fluid mixture con~aining at lea~t one additional fluid wherein the fluld mixture (retentate) is subje~tet to processing subseque~t to the - 25 separa~ion operatian. If the separa~ion apparatus provides significant resistance to the flow of the fluid mixture, substantial energy expenditures may be required to recompress the fluid mixture (retenta~e) to d~sired pressures for the subsequent processing. The pressure drops to the fluid mixture caused by the fluid flow resistance ,.
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~43~ 7 Q7-0005 of the sep~ration apparatus are frequently subs~antial when the fluid mixture is fed to the bores of the ~ibers.
For example, Gardner, et al, disclose, inter alia, in L~G~ .3~Y~a~C~ - L3~ - C~ Oct~ber 1977, pages 76 to 78, the usa of hollow fiber membrane-cantaining separation apparatus for removing hydrogen ~rom a hydrogen and carbon monoxide feed stream in an Oxo alcohol synthesis plant.
The feed str~-~am~ is at a pressure of 350 pounds per square inch gauge (psig), compressed to a pressure o~ 600 psig, passed through the bores of the hollow fibers in the separation apparatus, a~d recovered as a retentate stre&m rom the separation apparatus a~ 330 psig. Clearly, the compression of the feed stre~-lm is expenslve :in terms of the capital expenditure for the compressor and the operating costs fcr the compressor. Although a substantial pressure drop is i~curred due to the bore-side feed of thé ~eed itream to the separation apparatus, the bore-~ide ~eed is apparently necessarv due to the lack of distribution and loss of e~iciency i~ the feed stream~ were fed to the exterior (shell) side of the hollow fibers.
S~ell side feed to hollow fiber-containing separation apparatus can pro~ide oth~r ad~antages. For in3tance, a greater surface area for ef~e~ting the separatio~ is provided ~ ~ at the exter1Or surface~ of the hollsw fibers than at ~he j 25 interior surfaces of the hollow fibers. Moreover, hollow fibers may be able to withstand higher pressure differ~entials when the higher pressure is at the exterior as opposed to the interior of the fibers c~lnce generally materials exhibit greater compressive than tensile strengths.
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~j 07-0005 Effor~s have been expended to provlde hollow fiber ~embrane-containing separation appara~us having improved fluid distribution between the hollow Ei.bers. Rosenblatt in United States Patent No. 3,616,92~ di.scloses the use of highly crimped hollow fibers for use as the semi-permeable membranes. The crimped hollow fibers are adhesively bonded to one and another at a plurality of the abu~ing areas in order to maintain the spatial relationship between the hollow fi~ers. One disclosed separation apparatus is prov:ided with means to introduce the feed at the periphery of the assembly of hollow fibers such that the feed flows radial~y inward through the hollow fibers. The patentee provides no general irldication of the proportion of the cross-sectional area of the se~aration apparatu~ which is occupied by the hollow fibers (i~.e., packing factor or packing density), however, this proportion appears to be relatively low~ e.g., about 16 percent in Example 4, as compared to con~entional s~paration apparatus in which the feed is introduced into ~ -the bores of the hollow filaments (of~en abou~ 45 to 60 or more percent as illustrated by Maæwell, et al, in United States Patent No. 3,339,341 at column 5, lines 10 to ~5).
The use of the low packing fac~ors as appa~ently suggested ; by Rosenblatt is directly contrary to the desire for minimizing the size of the separa~ion apparatus, Moreover, the essential adhesive bonding of ~he hollow fibers to maintain their spa~ial relatio~shlps requires an additional processing step, and the presence of ~he adhesive rPduces the available membrane surface area for effe~ting the fluid separation.
In accordan~e with this invention hollow, semi-permeable fiber~ which are intended for use in fluid ..
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~epara~ions have relatively low amplitude waves, or crimps.
These hollow fibers are particularly advantageous for assembling bundles of hollow fiber~; which are oriented substantially parallel to one and another. The bundles can S be assembled to provide desirably high packing factors to enable separation appara~us having a large membrane surface area to be advantageously compact ln volume, Moreover, even though high packing ~actors are provid d, good f}uid distri-bu~ion throughout the bundle can be obtained. Thus the fluid mixture containing at least one fluid to be permeated can be fed to exteriors of the hollow fibers with the permeate be~ng facilely remo~ed from the interiors of the hollow fibers. Moreover, assembly o~ the hollow fibers to form a b1mdle can be non-complex and not re~uire spacing means or special techniques to provide a packing factor which permits fluid dispersion through the bundle. Advantageously, the:
hollow fibers in the bundle need not be fixed in a relationship to other hollow fibers by the use of an adhesive ma~erial in order to maintain a de~irable packing factor thro~ghout - 20 the bundle when employed in a fluid separation operation.
sir.ce good fluid dispersion can be obtained through ~: :. bundles compri~ed of hollow fibers of this invention, a ~: fluid mixture containing at least one fluid can be permeated through the membrane (permeat~ng fluid~ ca~ be introduced to ~ 25 the exterior sides of the fibers (i.e., ~he shell-side of the :- ~ bundle), and attractive 1uid sepaxation efficienc1es ca~ be ~ obtained. Accordingl~, the fluid mixture can be processed -~ ~ in and recovered frcm the shell-side of a fluid separation pparatus containing a bundle of hollow iber membranes in - : 30 aecordance with this invention, and the recovered fluid mixture may be at substantially the same psessure as the 1uid ; ,;~ ' ~, ~ 7-~ 3'~

mixture introduced into the separatlon apparatus. Therefore, expensive fluid compression apparatus may not be required to recompress the recovered fluid ~o su~ioient pressures for subsequent processing, and7 even if recompression is reiquired, the size of the compression apparatus and the amount of compression required may be significantly less than the size of the appara~us and amount of compression required if the fluid mixture were fed to the bore side of the bundle.
The advantages provided by the hollow fiber membranes having the low amplitude crimp of this i~vention can particularly be observed when the fluid mixture i~s fed to the shell-side o the bundle as compared to when the fluid mi~ture i~ fed to the shell-side of a bundle of hollow fibers which have an essential absence of cri~ps~ These advantages can be observed when ~he fluid mixture is fed radially, i.e., the fluid mi~ture is introduced in a mid-por~ion of the bundle and 10ws substantiall~ perpendicularly to the orienta~ion of the hollow ibers, or predominan~ly axially, i.e., the fluid mixture is introdueed at a~ outside portion of ~he ~0 bundle, flows generally in the same tirection as the orienta~ion of the fibers, and exi~s at another portion of the bundle. While often radial feed i9 consldered to pr~vide bet~er fluid separation efficieneies, advantRgeous ~luid ; separation efficiencies can be obtained employing axial feed - 25 to bundles comprising the hollow fiber m~mbranes of this ~- invention. Axial feed may ~e desirable since the separation appara~us may be less complex in design than radially-fed separation apparatus, and sinoe no radial feed conduit need be positioned within the bundle, ~he bundles employed for axial flow may comprise a greater ratio available memibrane . ., ~ surface area per given volume of separation apparatus than .... " , ~;
the ratio of available membrane surace area per given ~o~ume of radially-fsd separatlo~ appara~us. While shel.l-side feed ~: 07-0005 to the bundle is generally desirable, there may be separation operations in which bore-side feed may be dasirable. For instance, when a fluid mixture for processing by membrane separation need not be main~ained at high pre~sures for furth~r processing bore-feed may be attractive to recover ~he permeated fluid at the shell-side with little pressure drop subsequent to the permeation.
The hollow, semi-per~eable fibers o~ this invention ha~e crimps, or waves, of low amplitude. Surprisingly, it has been found that desirable dispersion o~ fluid mixtures ~hrough bundles of the hollow ~ibers can be obtained eve~
though the bundles have a relatively high packlng factor.
The amplitude of the crimps is no~ so large that an undue amount of cross-over occurs when the hollow fibers are assembled in a close-packed sub~9tantially parallelly-oriented fiber bundle. The term "cross-over" as employed herein refers to situations in which the crimp has such a large amplitude that th~ crimp protrudes sufficiently far from the axis of the hollow fiber that it bec~es positioned between ~wo or more adiacent hollow fibers and separates the adjacent hollow fibers by at least the diameter of the fiber. Such .
cross-overs tend to prevent ~he obtaining of bundles having desirably high pac~ing factors.
:~ The amplituda of the crimp~ as referred to herei~
: 25 is one-hal the lateral distance betwe~n the midpoint o~ the hollow fiber at one apex ~o the midpoint of the hollow fiber at the next adjacent9 diametrically-opposed ape~. When no : adjacent, diametrically-opposed apex e~ists, the æmplitude is .
the lateral distance between the midpoint of the hollow fiber ` 30 at the apex to the midpoint of the hollow fiber which is not ;.~.. ~. crimped. Ad~antage~usly, the amplitude of ~he crimps is l~ss ~ ' _g_ ~ .

~ `7 than about S0 percen~ of the diameter of the hollow fiber, and generally, the amplitudes of the cr~mps are within the range o about 1 to 30 percent of the diameter of the hollow fiber. Fiber crimp amplitudes of above about 50 percent of S the diameter can also be employed; however, g~nerally after the bundle of fibers is assembled, the bundle must be compressed to obtain a desirably high packi~g factor. Some compression of the bundle serves to maintain the hollow fibers in a substantially fixed relativnship to each other, and thus the tendency of the hollow fibers to move such that lateral flow channels, which flow channels decrease the efficiency o separation, is generally abated. The c~mpressio~ should not unduly affect localized regions o~ hollow fi~ers 9UC~ that fluid penetration is inhibited in those region~ or that such non-uniform loading of the hollow ~ibers is provided that the hollow fibers may collapse. Each of the crim~s in a hollo~ fiber or 2mong the hollow fibers employed to fo~m the bundle may have the same or diferent amplitude than ; other crimps, and the ~mplitudes o the crimps may vary over a range to assist in breaking any register between hollow fibers. Moreover, the bundle may conta.in hollow fibers having substantially no crimps, which fibers are i~terspersed with hollow fibers ha~in~ crlmps. For example, hollow fib~r~
ha~ing a distribution of crimp amplitudes of from about 10 to about 30 percent of the diameter of the hollow fiber have been assembled into a bundle having a packing factor of abo~t 50 percent which bundle exhibits good fluid dispersion when axially fed.
~- Hollow fiber diameters may be selected over a wide range, however7 th~ hollow fiber should have sufficient wall thickness such that the crimp is maintained. Frequently, ~` the outside diame~er of the hollow fibers is a~ least about . -. -;:....

'~L$~L4L3f~7 50, say, at least about lOO,microns, and the same or different outside diameter fibers may be contained in ~ bundle. Often, the outside diameters are up to about 800 or 1000 microns.
Although larger outside diameter hollow fibers can be S employed, they are less preferred due to the low ratios of hollow fiber surface area per unit volume of fluid separation apparatus which are provided. Preferably, the outside diameter of the hollow fibers is about lS0 or 350 to 800 microns.
Thus, the amplitude of the crimps is often in the range of about from 10 to 400 microns, say, about 10 to 300 micro~s, with an average crimp amplitu~e of about 15 to 250 microns.
It has been found that the crimps need not be conti~uous o~er the length of a ho~low ~iber in order to provlde desirable hollow iber membranes for assembly into a ~undle. Thus, ~he crimps may be intermittently spaced ovPr the leng~h of the hollow fiber, and the freque~cy of ~he crimps may be irregular. Moreover, as stated above, fibers with a distribution of crimp frequency ca~ be emp1oyed.
Ge~erally, at least about 50 percent, preferably at least about 75 percent, of the fibers in a bundle are cr~mped. The hollow fibers which are crimped frequently have an average of at least o~e crimp per each five ce~timeters of fiber length. The average frequency of crimps over the length of a hollow fiber is often about 0~2 to 10 or more, say, about 0.25 ~o 5, per centimeter.
If the frequency of the crimps in the hollow fiber i irregular, the crimps generally range in frequency from about 1 to about 50 crLmps per five centimeters, e.g., from about I ~o about 30 crimps per five centimeters, of hollow fiber length.
The period of the crimps, i.e., ~he lellgth of each crimp, is desirably sufficiently short that the crimp maintains .. ~
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its conflguration and substantial changes in amplitude oE
the crimp does not occur when the hollow fibers are assembled into a bundle. For instance, if the period of the crimp is too long and gradually ascends to its apex, then even minor mechanical forces may tend to straighten out the hollow fiber.
In order to obtain the advantages provided by this invention, the period of the crimp can be relatively short, e.g., less than about 5 cen~imeters. The shortness of the crimps is generally llmited by the dimensions of the hollow fiber, i.e., with smaller diameter hollow fibers generally smaller crimp periods can ~e obtained. Frequently, the average crimp period is about 0.05 to 5, e.g., about 0.1 to 2 centimeters. The ratio of the average crimp period to the average frequency of crimps may vary widely, for instance, from about 0.05:1 to 1:1, often about 0.1:1 to 1:1.
The amplitude, frequency of crimps, and crimp period are factors which relate to the configuration of the hollow fibers. A useful aid which encompasses these factors for describing the configuration of the hollow fibers i5 the ratio of the actual length of the crimped hollow fibers to the length of the hollow fibers if they were straightened.
Op~ical analytical tools are available for such determinations, such as image analyzers available from Quantimet, Monsey, N.Y., which do not require physical straightening of the hollow fibers.
In view of the small differen~es in crimped and uncrimped length, ~ -a convenient procedure is to report the differences in percent of length change due to crimping. The percent of length change is frequently in the range of about 0.01 to 10, e.g~, about 0.05 to 5.
The hollow fibers of this invention can be assembled to form bundles of any suitable configuration. Advantageously, the hollow fibers are substantially parallel:Ly oriented.
The cross-section of the bundle may be any suitable shape .4L3'r~7' 07-0005 for use in fluid separation apparatus, e.g., circular, oval, etc. The packing factor of the bundle is influenced by ~he ampli~ude of the crimps, the frequency o the crimps, the period of the ~rimps, and the compression o~ the bundle.
Generally, the packing factor of the bundle is at least about 40 percent and may be up to 65 peroent or more. Often, the packing factor of the bundle is about 45 to 6S percent. For axially-fed separation apparatus, ~he packi~g factor of the bu~dle is frequently about 45 to 55 percent. S~l~e the packing factor can be maintained due to the con~iguration of the hollow fibers, spacing means to provide pack:ing factors in the desired range need not be employed. For bundles havlng substantially circular cross-sectio~s, the diameter~
of the bundles may ~Jary widely, e.g., oten ~rom at Least about 0.02 up to 1 or more me~ers. ~he~ the separation apparatus is radially ~ed, the diameter o~ the bundle may be greater than 1 meter with ade~uate fluid dispersion throughout the bundle without ~esulting in undue pressure drops. On the other hand, when the separation apparatus is axially fed, it has been found that enhanced fluid dispersions through the bundle are obtained with higher space veloci~ies.
Accordingly, sm~tller bundle diameters are often preferred, e.g., about 0.02 or a . os to 0.5 meter. The efecti~e length -- o~ the hollo~ fibers in ~he bundle may also vary widely, for inst nce, from about 0.2 to 15 or 20 meters, e.g., about 1 to 10 meters, The bundle may be encased (or potted) proximate at least one end to prevent fluid commu~ication between the e~teriors and interiors of the hollow fibers except through the semi-permeable walls of the fibers. Any suitable ;~ ~; method for embeddi~g the fibers in the potting material can be employed, e.g.~ by casting a pot~ing material around .

the end of the bundle such as disclosed in United States Patent Nos. 3,339,341 (Maxwell et all ancl 3,442,389 (McLain) or by impregnating the ends of the fîbers with potting material while assembling the hollow fibers to form the bundle such as disclosed in United States Patent Nos.

3,45$,460 (Mahon) and 3,69 a, 465 (McGinnis et al~
In assembling the bundles/ it is desired that the crimps in the hollow fibers do not fall in register. The substantial avoidance of register can be achieved in various manners. For instance, the fibers can be aligned such that the crimps, e.g., of regularly crimped hollow ~ ~
fibers do not match. This procedure may be unduly complex. ~ ~;
Advantageously, at least some of the hollow fibers vary in at least one of crimp frequencies, crimp period, and crimp `
amplitudes such that with a random assembly of the hollow fibers, the probability of obtaining an undue amount of fibers in register is minimal.
The hollow fibers may be fabricated from any suitable synthetic or natural materîal suitable for fluid separations 2Q or as supports for materials which effect the fluid separations. The selection of the material for the holiow fiber may be based on the ~est resistance, chemical resistance~
and/or mechanical strength of the hollow fiber as well as other factors dictated by the intended fluid separation in which it will be used and the operating conditions to which it will be subjected. In order to maintain the de6ired fiher crimp of this invention, the fiber should exhibit appropriate mechanical properties such that -the crimps do not unduly dissipate with time or during the separation operation. With hollow fibers fabricated from materials having lesser strengths, it may be necessaxy to employ larger fib~r diameters and wall thicknesqes to impar~ sufficient strength to the hollow fiber crimps that they substantially retain their configurations. Often, the wall thickness of the hollow fi.bers is at least about 5 microns, and in so~e hollow fibers, the wall thickness may be up to about 200 or 300 microns, say, about 50 to 200 microns. In many instances the mat:erial of the llollow fiber exhibits a relatively high tensile modulus, i.e,, modulus of elasticity or Young's modulus, that the crimps can be retained even under longitudinal and lateral stressing.
Often the tensile modulus (ASTM D638) is at least about 15 kilograms per square millimeter (~g/mm2)l e.g., at least about 40 kg/mm2, and for some metals and allo~s, the te~sile modulus is up to about 3000 or more kg/mm2. Most frequently, pol~meric materials which are to be employed are selected from those poly~ers which exhibit a tensile modulus of about 60 ~o SOO kg/mm2.
In order to provide desirable fluxes through the hollow fibers, particularly using those hollow fibers having walls at least about 50 microns in thick~ess, the hollow fibers may have a substantial void volume. Voids are regions within the walls of the hollow fibers which are vacant of the material of the hollow fibers, Thus, when voids are present, the density of the hollow fibar is less than the density of the bulk material of the hollow fiber.
-- Often, when voids are desired, thç void volu~e of the holl.ow fibers is up to about 90, say, about 10 to 80, and sometimes about 20 or 30 to 70, percent based on the superficial volume, i.e., the volume contained within the gross dimPnsions, of ~he hollow fibers. The density of the hollow fiber can be essentially the ~ame throughout its wall thiekness, i.e., ':

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isotropic, or the hollow fiber can be charac~erized by having at least one relatively den~e region within its wall thickness in barrier relationship to fluid flow through the wall of the hollow fiber, i.e., the hollow iber is anisotropic.
Generally, a relatively dense region of anisotropic hollow ~ibers is substantially at the exterior of the hollow fiber.
The material for ~orming the hollow fibers may be inorganic, organic or mixed inorganic and organic. Typical ~ inorganic materials include glasses, ceramics, cermets, metals and the like. The organic materials are ~sually polymers.
In the case of polymers, both addition and condensation pol~mer~
which can be fabricated in any suitable manner to provide hollow fibers are included. Gensrally organic and some times organic polymers mixed with inorganics (e.g., filler~) are used to prepare the hollow fibers. Typical polymers can be substltuted or unsubstitu~ed polymers and may be selected from polysulfones; poly(styrenes), i~cluding styrene-containing copolymers such as acrylonitrile-styrene copoly~ers, styrene-butadiene copolymers and styrene-vinylbenzylhalide copolymers;
polycarbonates; cellulosic poly~ers, such as cellulose acetate; cellulose-acetate-bu~yrate, cellulose propionate, ethyl cellulose, methyl cellulose, nitrocellulose, etc.;
polyamides and polyimideq, including aryl polyamides a~d ..,, .,.~ .
aryl polyimides; polyet~ers; poly(arylene oxides) such as poly(phenylene oxide) and poly(xylylene oxlde); poly(ester~
amide-diisocyanate); polyurethanes; polyesters (includi~g polyary}ate~), such as poly(ethylene lerephthalate), poly(alkyl methacrylates), poly(alkyl acrylates), poly ~phenylene terephthalate~, etc.; polysulfides; polymers from monomers having alpharolefinic unsaturation other than . ~ mentioned above such as poly(ethylene), poly(propylene), poly(butene-l), poly(4-methyl pentene-l), poly~inyls, e.g., paly(vinyl chloride), paly(vinyl fluoride), poly(vinylide~e chloride), poly(vinylidene fluoride), poly(vinyl alcohol), poly(vinyl esters) such as poly(vinyl ace~ate) and poly(vinyl propionate), poly(vinyl pyridines), poly(vinyl pyrrolidones), poly(vinyl ethers), poly(vinyl ketones), poly(vinyl aldehydes) such as poly(vinyl formal) and poly(vinyl butyral)) poly (vi~yl amides), poly(vinyl amines), poly(vinyl urethanes), poly(vinyl ureas), poly(vinyl phosphates), and poly(vinyl sulfates); polyallyls; poly(benzobenz~midazcle3; polyhydrazides;
polyoxa~iazoles; polytriazoles; poly(benzimidazole)i polycarbotiimides; polyphosphazinesi etc., and interpolymers, including bloc~ terpolymers containlng repeating units from the above such as terpolymers o~ acrylonitrile-vinyl bromide-sodium salt of para-sulfophenylmethallyl ethers; and grats and blends contai~ing any of the foregoing. Typical substituents providing substituted polymers include halogens such as fluorine, chlorine and bromine; hydroxyl groups; lower al~yl groups; lower alkoxy groups; monocyclic aryl; lower acyl groups and the like.
The crimps m.ay be i~duced into the ~ollow fîbers i~
any suitable manner. For instance, straight hollow fibers . may be sof~ened with solvent or plas~iciæer for the material o the hollow fiber, mechanically deformed to ~mpart the crimp configurat~on, and then treated, e.g., by drying, to ~ r~move the solvent or plasticizer such that the hollow fiber : 25 regai~s the desired -.oigidity. Alternatively, or in addition, the ~aterial of the hollow fiber may be softened by the application of heat to the hollow fiber. In any event, the softening is sufficient such that the bore remains substantially u~restricted upon application o mechanical force to provide the crimp. A co~venient method for :
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~7-0005 3~ 7 providing crimps in coagulation spun hollow fibers, i.e., hollow fibers which are spun from a solvent solu~ion of the material iIltO a non-solvent for the material, is by winding the spun hollow fiber onto a bobbi,n while wet. Upon loss o solvent, and drying if the hollow fiber is dried on the bobbin, hollow fibers tend to shrink and ehus put an increased pressure on the underlying fibers. This pressure provides the mechanical force required to impart the desired crimps, and the solvent loss enabLes the fibers to gain additional rigidity such that the crimp is retained. Slnce the forces exerted on the hollow fibers often vary with the depth of the hollow fiber within the bundle, irregular crimp patterns occur with the hollow fibers from the outer portions of the bundle tending to have fewer, ~ore widely-spaced crlmps than the hollow fibers closer to the center of the bundle.
The following examples are provided ~o ~urther illustrate the invention. All parts and perce~tages of liquids and solids are by weight, and all parts and percentages of gases are by volume, unless otherwise indicated.

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EXA~E 1 A hollow iber is preparet from dried polysulfone polymer having the repeating unit .
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. . .
. . ..:, where n, representlng the degree of polymerization, is about 50 to 80 and is available from Union Carbide under the designation P-3500. The polysulfone is admixed with dimethylacetamide to provide a dope containing about 27.5 weigh~ percent polymer, and the dop~ is coagulation spun in~o water at a te~perature of about 4C through a spinnerette which is immersed in the water. Thç spinnerette has an outer orifice diameter af 0.0559 centimeters, an inner pin of 0.0229 centime~ers, and an injection port of 0.0127 centi~eters through which water is in~roduced.
The dope is pumped and metered to the spinnerette at a rate of about 7.2 milliliters per minute and is drawn from the spinnerette a~ a hollow fibe~ at a rate of about 33 meters per minute. Ater the coagulation has substantially occurred, the hollow fiber ~s washed in room temperature.
The hollow fiber is wound substantially without tension on a 12 inch (approximately 25.4 centimeter~ between inside heads) bobbin with a bobbin winder, i.e., the hollow fiber i5 fed through an axially traversing guide (which reverses at each end of the bobbin) and is collec~ed on the surface of a rotati~g bobbin so that the hollow fiber is wound on the bobbin in sequential layers of helical coils. The bobbin i stored in an aqueous vat at room temperature during which time the fibers on the bundle shrink to impart crimps.
The hollow fi~ers are then wound on a skeiner having about a six meter circumerence. The hollow fibers are removed as three meter long hanks and are hung ~nd allowed to dry at ambient labora~ory temperature and humidity. The hollow fibers hzve an outside diameter of abou~ 540 microns a~d an - 30 inside tiameter o about 260 microns.
A random sample of hollow fibers are removed from the dry hanks and are analyzed for configuration charac-, I teristics on an image analyzer obtained from Quantimet .. i .
. . -, , .

of Mon~ey, New York. The random sample contained samples of fibers which were obtained at the inside, middle and outside portions of the bobbin. The results (approximate frequency oE occurrence) are presented in Table I.

Crimped, hollow fibers prepared by substantially the process set forth in Example 1 are assembled in the form of a bundle of substantially parallell~oriented hollow fibers. Approximately 1200 hollow fibers having a length of about 30 centimeters are employed, and the fibers are randomly selected from fibers obtained from all portions of the bobbin. The hollow fibers form a cylindrical bundle which is about 2.5 centimeters in diameter (about a 50 percent packing factor). Epoxy seals are fabricated at both ends of the bundle by sealing the ends of the bundles and then immersing the ends (tube sheet and plu~ ends) in ~iquid epoxy resin and allowing the epoxy to cure. After curing, a knife is used to open the bores of the hollow fibers at the tube sheet end.
The bundle is immersed in a solution of 5 weight percent of the product marketed under the trademark SYLGARD
184 in isopentane. SYLGARD 184 is a cross-linkable dimethyl-siloxane polymer which is availahle from Dow Corning and cures at ambient temperatures. The bores of the hollow fibers are in communication with a vacuum of about 600 to 700 millimeters of mercury. The immersion is for about 15 minutes, and the vacuum is con-tinued ;
for about another 15 minutes after the bundle is emerged .: . ~

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3'~7 from the SYLGARD* 184 solution. The coated bundle is cured at about 4 n to 50C for about 24 hours and installed in an axially-fed fluid separation apparatus. A gaseous feed comprising hydrogen and carbon monoxide is introduced into the shell side of the separation apparatus and the permeabilities of the permeate gases are determined. The permeabilities are determined using the log mean partial pressure drop across the hollow fibers. A separation factor is determined by dividing the permeability of hydrogen by the permeability of carbon monoxide. A separation efficiency is also determined b~ dividing the separation factor calculated for the gaseous feed comprising hydrogen and carbon monoxide by a reference separation factor deter-mined from the separation factor determined from the separate ;~
determination of the permeabilities of essentially pure hydrogen and essentiall~ pure carbon monoxide. ~ -Lower separation efficiencies are often indicativa of poor fluid dispersion in the bundle such that localized areas of high concentrations of the undesired component ~carbon monoxide) occur and thus increase the permeation of the undesired component and lower the separation factor.
The results are provided in Table II. ;
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U~ ~0 ~ C~ ~ ~ ~ ~ ~ ~ ~ C~
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~ 3~7 07-0005 E.YAMPLE 3 (Co~parative) The procedure of Example 2 is essentially repea~ed except that the hollow fibers employed are not wound on bobbins af~er spinning and do not have crimps. Since the hollow ibers have an essentiAl absence of crimps, the bundle tends to be smaller in diameter. In the runs, the gaseous feed comprise.~ 22 volume percent hydrogeIl and 78 volume percent carbon monoxide. I~ ~wo runs, ~he shell pressure is about 2~ ahmospheres absolute and the bore pressure is about 4.1 atmospheres absolu~e. At a eed rate of 12.3 liters per minute (STP) the hydrogen permeability i5 observed to be 37.7 x 10 6 cc/cm2-sec-cmHg, and the carbon monoxide permeability ls 1.60 x 10 6 cc/cm2-sec-~mHg.
The separation factor is calculated to be 23.6 which is at a separation efficiency of about 63 percent. At a higher ~eed rate, 21.2 liters per minute (STP), the hydrogen permeability is 41.4 x 10 6 cc/cm2-sec-cmHg, a~d the carbon ~:
monoxide permeability is 1.74 x 10 6 cc/cm2-sec-cmHg. The separation factor i~ calculated to be about 23.8 which is at a separation efficiency of 64 percent. In both of these runs, calculations based o~ flow rates and permeabilities only accounted for less than 95 percent of the hydxogen introduced to the separation apparatus in the feed. The procedure is again repeated at a shell pressure of 2~
atmospheres absolute, a bore pressure of 2.5 atmospheres absolute, and a feed rate of about 14.5 li~ers (STP) per minute of a blend gas consisting of 27.2 percent h~drogen and 72.8 percent carbon monoxide. The hydrogen permeability i~ about 31.6 x 10-6 cc/cm2-seo-cmHg, the carbon monoxide permeability i9 about 2.0 x 10 6 cc/cm2-sec- ~g, and the separa~io~ factor is 15 . 8 . The reference separation factor ~, : ....... .

~ 7 07-0005 is determined to be 36. a to provide a separation efficiency of 42. Virtually all the hydrogen is accounted for by material balances.
Since the diameters of the bundles tested in S Exæmple 2 and 3 are relatively small, the differences in separation efficiencie~ are not as pronounced as they might be in a comparison of larger bundles in which greater penetration into the bundle is required, :
' . ., .:

~ . .

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A hollow, semi-permeable fiber intended for use in fluid separations having a plurality of crimps, said crimps having crimp amplitudes up to about 50 percent of the outside diameter of the hollow fiber and an average crimp period of less than about 5 centimeters, wherein said hollow, semi-permeable fiber exhibits sufficient rigidity to retain the plurality of crimps during fluid separations.

2. The fiber of claim 1 in which the outside diameter is about 150 to 800 microns, the wall thickness of the hollow fiber is about 50 to 200 microns, the average crimp amplitude is about 15 to 250 microns, and the material of the hollow, semi-permeable fiber exhibits a tensile modulus of at least about 40 kilograms per square millimeter.

3. The fiber of claim 2 in which the hollow fiber has a void volume of about 10 to 80 percent.

4. The fiber of claim 2 wherein said fiber has a length change, due to said crimps, within the range of 0.01 to 10 percent.

5. The fiber of claim 4 in which the average crimp ampli-tude is about 1 to 30 percent of the outside diameter of the hollow fiber, and the average crimp period is less than about 5 centimeters.

6. The fiber of claim 5 in which the ratio of average crimp period to average crimp frequency is about 0.1:1 to 1:1.

7. The fiber of claim 6 in which the hollow fiber has a void volume of about 10 to 80 percent.

8. The fiber of claim 6 in which the hollow fiber is anisotropic.

9. In a bundle of hollow, semi-permeable fibers intended for use in fluid separations, said bundle comprising a plurality of substantially parallelly-oriented hollow fibers and having a diameter of at least about 0.02 meter and a pack-ing factor of at least about 40 percent, the improvement wherein at least about 50 percent of the hollow fibers are the crimped hollow fibers of claim 1.

10. The bundle of claim 9 in which the hollow fibers have an outside diameter of about 150 to 800 microns, a wall thickness of about 50 to 200 microns, an average crimp amplitude of about 15 to 250 microns, and the material of the hollow, semi-permeable fiber exhibits a tensile modulus of at least about 40 kilograms per square millimeter.

11. The bundle of claim 10 in which the crimped hollow fibers vary in at least one of crimp frequencies, crimp periods, and crimp amplitudes.

12. In a bundle of hollow, semi-permeable fibers intended for use in fluid separations, said bundle comprising a plurality of substantially parallelly-oriented hollow fibers and having a diameter of at least about 0.02 meter and a pack-ing factor of at least about 40 percent, the improvement wherein at least 50 percent of the hollow fibers are crimped hollow fibers of claim 4.

13. The bundle of claim 12 in which the hollow fibers have an average crimp amplitude of about 1 to 30 percent of the outside diameter of the hollow fiber and an average crimp period of less than about 5 centimeters.

14. The bundle of claim 13 in which the hollow fibers have a ratio of average crimp period to average crimp frequency of about 0.1:1 to 1:1.

15. The bundle of claim 14 in which the hollow fiber has a void volume of about 10 to 80 percent.

16. The bundle of claim 15 in which the hollow fiber is anisotropic.

17. The bundle of claim 16 in which crimped hollow fibers vary in at least one of crimp frequencies, crimp periods, and crimp amplitudes.

18. The bundle of claim 17 in which at least about 75 percent of the hollow fibers are crimped.

19. The bundle of claim 18 in which the packing factor is about 45 to 65 percent.

20. In a fluid separation apparatus comprising a vessel;
a bundle of hollow, semi-permeable fibers for effecting the fluid separation in the vessel; an entry port in the vessel adapted to selectively supply a fluid mixture to be treated to one side of each of the hollow fibers; an exit port in the vessel adapted to selectively remove said fluid mixture from said one side of the hollow fibers; and a permeate port in the vessel adapted to selectively remove a permeate product from the opposite side of each of said hollow fibers, the improvement wherein the bundle of hollow, semi-permeable fibers is the bundle of claim 9.

21. The apparatus of claim 20 in which said one side of the hollow fibers is the shell side of the hollow fibers.

22. In a fluid separation apparatus comprising a vessel;
a bundle of hollow, semi-permeable fibers for effecting the fluid separation in the vessel; an entry port in the vessel adapted to selectively supply a fluid mixture to be treated to one side of each of -the hollow fibers; an exit port in the vessel adapted to selectively remove said fluid mixture from said one side of the hollow fibers; and a permeate port in the vessel adapted to selectively remove a permeate product from the opposite side of each of said hollow fibers, the improvement wherein the bundle of hollow, semi-permeable fibers is the bundle of claim 12.

23. The apparatus of claim 22 in which said one side of the hollow fibers is the shell side of the hollow fibers.

24. The apparatus of claim 23 in which the hollow fibers have an average crimp amplitude of about 1 to 33 percent of the outside diameter of the hollow fiber and an average crimp period of less than about 5 centimeters.

25. The apparatus of claim 24 in which the hollow fibers have a ratio of average crimp period to average crimp fre-quency of about 0.1:1 to 1:1.