Hollow fiber composite membrane modules for reverse osmosis

Desalination, 22 (1977) 221-227 @Elkvia ScientiEcPublishingCompany,

HOLLON FIBER COMPOSITE R_ B, DWIS,

MEC!B'RA?4E WDULES

inYi%e Nethticls

FOR REVERSE OSWSIS

R. D. BLKHESKI', FI.J. COPIA3

FRL - an Albany International Company, Dedhzn,

Ebssachusetts U. S.

A.

The development of hollow fiber composite reverse osciosismodules capable of delivering 100 - 300 gpd permeate is describd.

Such modules are in fieId

trial against brackish and seahater. T,r! modules employ hollow fibers of 25011or SOOv 0. D. and wall Tbesi Quantro thickncssof7Su or 150~.

First generation FRL composite membranes are composed

of a porous support of commercial polysulfone and a furan-based rejection barrier having cation exchange properties, The continuing developnent of a reliable pilot plant facility is described with attention to fiber spinning, coating, winding, and potting. kdule

design

is briefly discussed_ Present fiber properties in laboratory tests include collapse pressures in excess of 1500 psi against 3,000 ppm aquwus

sodium chloride. At 800 psi, flux/

rejection combinations of 10 gfd/99.7% and ,35gfd/99,2% haye been generatedChemical modification of the rejection barrier produced fiber delivering 4 gfd and 99.00;rejection of seawater at 12110psi with excellent rejection stability. No effort was mde

to avoid iron fouling.

Such fiber is nde

reliably and predictably in pilot plant qwntities.

The module design incorporates features of superior brine flow to minimize concentration polarization and module fouling.

It also employs a superior pot

design to avoid creep and shear failure.

A previous paper described initial resesrch

at FRL on composite

Eeabranes

and results of the successful adaptation to a hollow fiber configuration(I). Tbls paper will describe our present process for fiber production, including refinmortts and controls developed since publication. It will nlso describe the development of a module of sufficient productivity to enable broad scope industrial testing against seawater. brackish water, and various industrial waste

We will

waters. laboratory

HOLLOW

FT3ER

The substrate

The Non-solvent

ZSdC, but

of polysulfone

to 100°C at high

all

subsequent

incompatible

of reducing

the glass

processing

with

polysulfone

transition

Non-solvent

concentrations.

coagulation.

has

at

temperature

limited

solu-

in DMF.

Dope tion.

is prepared

At ambient

components, 13,000

but

at a temperature

temperatures,

is a polyphasis

whose

shear history,

and

and can cause

be controlled

by chzmgc

typically

of the dope.

surface

less

agita-

viscosity

crystalline

of all

8,000

-

oligomers

of

on the temperature,

Typically,

they comprise

10~ X 2~~

They

than

imperfections

solvents

efficient

true solution

of bulk of

and sizes are dependent

in size are

of

is not a simple,

consisting

and rate of cooling

1% of the solids

be filtered

system,

concentrations

of 100°C with

in excess

the dope

A nuisancephaseexists

poises,

polysulfone

prior

is

from

Non-solvent

during

structure

to influence

Non-solvent

capable

RO membrane is is prepared

(DMF), and a proprietary

of the wall

found

preparation.

fiber

A spin dope

spinning.

dinethylformamide

to the development

is a plasticizer

hollow

for the FRL composite

of dry jet/wet

has also been

through membrane

than

structure

polysulfone,

is critical

bility

by

as disclosed

membrane

of OUT

PREPARATIOX

by the process

coizncrcial which

the limitations

tests.

SUBSTRATE

prepared

define

further

in the as-spun

or by use of a specific

cannot

fiber.

less

easily They

temperature

can

prorocol

to spinning. In our process,

hoIe possessing formation. in which

the dope

a centered

Extrudate plasticized

The

6-filament

of semi-infinite

passes polymer

bundle

is extruded

needle

a spinneret

through

of six hcles,

of gas necessary

an air gap into an aqueous

each

to lunen

coagulation

bath

precipitates.

passes

continuous

through

for introduction

over msh

filament,

rolls

Critical

and

is wound

up as a package

to the ultimate

per-

nembaxme

formance and substrate uniformity in this operation are the temperature of the

spinneret, temperature and humidity of the air gap, composition, agitation of the coagulation

bath,

path

length in the bath, and the efficacy of CMF removal.

Reports in the literature indicate favorable results porous polysblfone polysulfone

with

by means of other

polymers,

of work concurrent with tkt substam'ally or effects, reduces strate

different This before

e.

latter Non-solvent,

coating.

g.,

of

about

in the preparation of

a solution

of

a nixture

polyvinylpyrrolidone (2. 3),

cited in these references,

Non-solvent

the Tg of polysulfone fiber

the co-extrusiorl

system

which

results

uhen present 100°C.

we have

As a result

discovered

in substantially

in effective

It nust be extracted

of

a superi-

concentrations, fron OUT sub-

The type of non-solvent and the zxth&

of

extrac-

223

tion sigrificantly affect ultimate composite most

favorable

membrane

we have experienced

conditions,

values

properties.

Under

the

of 25 gfd flux at rejec-

tions of 99.2";of 2,000 ppm aqueous sodium chloride. RO CONPOSITE

PlE3IBRME FORHATXOS

Inherent able

in the composite

flexibility

possible

of tailoring

by varying

approach

to RO membrane

the rejection

the composition

bers pass, as well as by varying

barrier

preparation

is the dcsir-

to an applitzation,

This

is

through which the substrate

of the bath

the conditions

of the curing

fi-

operations.

Typically, the coating bath comprises an aqueous based sclvent containing reactive monomers, such as furan derivatives, and vinyl. co-reactants, sulfuric and catalysts. At the time of the earlier

acid,

the bath was an

wblication,

aqueous alcoholic solution of furfuryl alcohol, sulfuric acid, and certain additional ingredients affecting flux and rejection. Vuch of the effort since report

has been directed

seawater.

tim

hhile

initial

were excellent,

testing.

toward

the application

flux and rejection

rejection

of our composite

values

of the then

By varying the fomulation

formulation

standard

that against

formla-

began to decrease after 500 hours of seawater in the coating bath and conditions in the

curing operation, it has been possible to stabilize present

membrane

is an ~tqueous solution

high rejection

of furfuryl

Our

values.

sulfuric acid,

alcohol,

and a co-catalyst used to impart a higher degree of cross-link density. The coating bath is made up continuously

by independently

metered

a mixing chamber, from which it passes into a thermo5tatet.i conduit filament

bundle.

Bath age, composition.

Each of these variable The fibers

parameters

cquilibratc

and teqerature

influences

membrane

with the bath within

curing tower saturated with the bath solution. length,

current with the fiber bundle. speeds

are thus controlled. properties.

The curing

tower

is 10 feet in

forced hot air flows con-

Air temperatures in the tower are typically

of 30 feet/minute.

Fibers

to

seconds, and pass into the

with heated insulated nzzlls. Thermostat&

with running

flows

t
150°C

exit the toker in a dry state

and are hound on a package for subsequent use in module fabrication. "ZERO-LENGTH"

MM3FUNE

FRL @anmoW

PROPERTIES

fibers PJX typically tested as 20-30 cm samples against 2000

ppn aqueous NaCl at 800 psi and 2S*C in a recirculating test loop.

SUrfaCe

Such conditions give properties of the menfluid velocities are 2-4 ft/sec. brane independent of concentration polarization, and influences due to bore

flow pressure

drop.

224 Consistent

with

others

working

with

hollow

fiber

RO membranes,

we

have

formd

for a given sample that with increased length both flux and rejection drop off, due to bore flow pressure drop (4, 5). Tn the following table are found the results of experiments

in

which two

given samples were tested under conditions defined a'bove,but with variable length. These fibers were each 250~ 0. D. kith a 7511 I. D.

Each was an FRL

composite in which the rejection barrier was furan based.

TABLE 1 Influence of Fiber Length Sample

Length

(cm)

Flux

Rejection

(gfd]

:

;:

13-6 13.1

98.3 98.8

A

183

9.1

93.6

B B

22 89 183

26.3

99.2

19.3

98.9

19.0

98.4

B

(%I

These data are consistent with those predicted by assuming the node1 described in Reference 4.

As the fiber length increases, or as membrane permeability

increases, the pressure drop dohslthe bore of

the

fiber

This

increases.

pressure reduced the effective transmembrane pressure drop.

Flux

lumen

is linearly

dependent on transmembrane pressure, and therefore falls as lumen pressure increases. Salt passage is probably a constant over the entire length. Thus, as fiber length increases and water flux falls, so does rejection. It is consistent that the fiber lrBrt above of very high "zero-length" flux, 26 gfd, shows the drop at shorter length of 89 cm.

The severity of this problem is an inverse

function of the lumen radius. From various furan formulations and curing conditions, we have prepared fibers Khose r'zero-length'* flux/rejection values at 25OC against 2000 ppm aqueous ?;aClat 800 psi are 6 gfd/99.8%, 10 gfd/99.2%, 99.2";.

15 gfd/99.48,

25 gfd/

Fibers of 0. 9. of SOOu, lOOOu, 15OOl.1 have also been prepared with ob-

vious advantages of lower bore flow pressure drop for high flux membranes. The formulation chosen for optimum rejection stability against seawater exhibited the following "zero-length" perforance.

Against 2000 ppiaNaCl at

800 psi at 25"C, this formulation will deliver 6 gfd at 99.4% rejection.

Against

3400 ppn brackish water at 800 psi, it will deliver 5 gfd at 99.20,rejection, and at 300 psi, 2 gfd at 95% rejection. Against 35.000 ppm synthetic seawater 3t 1200 psi, it will deliver 2 gfd at 99% rejection. Typically, after 2000 hours against seawater, flux rerrainsunchanged and rejection has gradually

225

declined to 98.0%.

Throughout our testing no effort was o;zdeto avoid iron foul-

ing. Recent acid

tests

have

after a rejection

shorn

that

decline

post-treatments of the membrane with sulfuric

can res?ore

the rejection, suggesting that this

ion-exchange membrane nay have suffered some iron fouling

it uas degraded

or that

by seawater due to high salinity. The influence could be due to the degree of swelling of the nenbrane, or chenically altered due to hydrolysis or other reaction. Other reactive post-trcatoents have she-a promise for long-tern rejection stability or regeneration. NODULE DESIGN For the next phase of testing. Quantrof’;li Eodules are being prepared on a routine basis, while research on new formulations continues.

The objective

application for this phase of testing is seawater desalination. These nodules. therefore, utilize fiber forrmlations shokn to be rooststable against seawater and are sized to yield infomation concerning the efficacy of these fibers in a fiber bundle under 2S%-30? recovery conditions. These fiber bundle approximtely

test

modules

S cm in diameter and 20 cr;l in length.

utilize a

Depending upon

which forrzulationis used, the productivity of these ~~odulcsis 100-300 gpd. Y**f Quantro nodules incorporate fibers in a geometric array designed to provide a volumetric packing density of 50%-605 while ninimizing chas?neIing,stagnation, and particulate fouling.

Bundling techniques

have

been developed

at FRL

to produce such fiber bundles irith fibers cross-lapped in a helical wind to nect such design criteria. U. S. Patent 4,045,8Si

assigned to Albany International is practicd as one

approach to this der;und. Figure I shows such an array of fibers before potting.

The design of the pot and techniques for opening the fiber ends are proprietary. are avoid4

In general, pet failure in shear or creep and fiber end occlusion in use.

Attention has been paid to the iclplicationsof bore flow resistance on nodule design.

Fibers of approximtely

75~ 1. D. wil? not suffer sericus loss

of properties up to three feet in length for fibers of flux less than lo-15 gfd. For fiber species of higher flux and for nodules of greater productivity, bore flow resistance will have to be accanaodated by either employing larger bore fibers or shorter bundles. future aodules.

Both approaches are under development for

226

Figure

1.

Hollow

Fiber

Bundle.

FlJ+llJRE DIRECTION

It is expected

that trsting

of existing

fornuiations and fiber wiI1 con-

tinue with greater intensity_ Research on larger fibers and fomzulations of greater oxidative stabiliq- will continue in the near future.

REFEREXES 1.

3 -_

3. 4.

5.

A. E, ALLEGREZZA, JR., R. D. BURCIiESKY. C. GTZ, R. f3. DAVIS, V. J. COPLXX, Desalination, 20 (1977) 67. I. CAMSSO, E. KLEIN, XVD J. K. SNKTH, J. AppZ. Poly. Sci., 20 (1976) 2377. I. CAEMSSO, E. KLEIN, VJD J. K. SMITfi, J, Appl. Poly. Sci., 21 (19771 165. T. A. OROFINO, ivTechnolog)' of Ifollow Fiber Reverse Osmosis System," Reverse Osmosis and Synthetic Hembrancs, Ed. S. Sourirajan, National ReSearch Council of Canada Publications, Ottawa, Canada (1977) 313, C. CHEN iWD C. A, PETIY, Desalination, 12 (1973) 281,