Hollow fiber permeators in industrial waste stream separations

Desalination - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands

HOLLOW FIBER PERMEATORS IN INDUSTRIAL WASTE STREAM SEPARATIONS* WILLIAM P. COOKE "Permasep" Products - Organic Chemicals Department, E. I. rht Pont de Nemours Company, Wilmington, Delaware (U.S A .) (Received April 29, 1969)

SUMMARY

Use and advantages of hollow fibers as semi-permeable membranes are reviewed . Commercial permeators are described . Test data on separations involving organic and inorganic compounds are provided . Process description for separations via these permeators is outlined . SYMBOLS

P PF C CF CP T Y SPE (SPE)Ms

-

pressure feed pressure solute concentration concentration of solute in feed solution concentration of solute in permeate temperature - conversion [(permeate rate/feed rate) x 100] - solute passage, experimental (CP/CF x 100) - solute passage (experimental) using mixed sulfates

Unit conversions ft 2 gals. psi ft 2 /ft3 in .

x 0.0929 = m 2 x 0.00379-- m' x 0.07 = kg/cm2 x 3 .27 = m2/m3 x 0.0254 - m

Presented at PIJRA UA - U.S. Exhibition and International Conference on Water Puri-

fication and Desalination,

held

in Rome, Feb. 17-23, 1969. Desalination, 7 (1969)70) 31-46

32 ft. kWh/1000 gals. ppm

WILLIAM P. COOKE

x 0.3048 = m x 264 = kWh/1000 m" = mg/l x 1 .0

INTRODUCTION

Process separations achievable through the use of semi-permeable membranes have been of interest to chemists, biochemists and biologists for many years, and an abundance of technical information on the subject has appeared . Much of this information has concerned itself with the theoretical aspects of membrane performance and various procedures for preparing membranes with a desired combination of properties . It is only in recent years that commercial-sized equipment utilizing this principle of separation has become available . In the United States, much of this technology growth has resulted from substantial infusion of funds from the federally operated Office of Saline Water. However, this new technology is paralleled by an awakening public recognition of the world's rapidly dwindling supply of potable water, and actions of governmental authorities in establishing quality standard for rivers, streams and other receiving bodies of water. As a result, there has been a rapid increase in interest and activity in connection with semi-permeable membrane devices (reverse osmosis processes) . DU PONT HOLLOW FIBERS

There are several advantages of hollow fibers as a configuration for semipermeable membranes compared with those reverse osmosis devices using supported membranes (e.g. tubular, jelly roll, plate-and-frame) . The most significant of these advantages is shown by the following permeation rate equation which expresses the rate of a solvent such as water through the membrane : Water flow =

KA (AP-Air)

--

where K = permeability coefficient of membrane for water, A = membrane area, AP = pressure differential across membrane, Air = osmotic pressure differential across membrane, t = membrane thickness . This shows that the rate of permeation, usually referred to as flux in the case of aqueous separations, is directly proportional to the permeability coefficient of the membrane at a given temperature . directly proportional also to the membrane surface area and to the pressure differential across the membrane, less the osmotic pressure differential, and inversely proportional to the membrane thickness . It follows that a major objective in designing a device to achieve separation via semipermeable membranes is to obtain the highest possible membrane surface area per unit of thickness, consistent with the strength required to sustain the necessary Desalination, 7 (1969/70) 31-46



33

HOLLOW FIBER PERMEATORS

pressure differential . A second major objective is to pack as much membrane surface area as possible into a given size of container or vessel . Thin membrane films, of course, require some means of support to withstand the necessary pressure differentials, and the support structure severely restricts the amount of membrane surface area which can be contained in a vessel of a given size . hollow fibers, which can be thought of as heavy walled plastic pipe of very small size, as the photomicrograph in Fig . I reveals, require no support. As fibers are made smaller

Fig . 1 . Photomicrograph of hollow fibers .

110.000

a 11,000

1100 5

50 500 FIBER O.0. (MICRON)

a a vs. fiber diameter (at 50% pack dens ty) . Desati

,

1-46

34

WILLIAM P . COOKE

and smaller, the amount of surface area which can be packed into a given size vessel gets larger and larger, as Fig . 2 discloses . It wilt be noted that with a fiber outer diameter of 50 microns and a packing density of 50%, a hollow fiber device can achieve a surface area of 9,000 ft 2/ft3 of vessel volume, many times that achievable with a supported film device . Because of this advantage, hollow fiber reverse osmosis devices can utilize polymers whose permeability coefficients would be far too low fo.- use in the supported film type of device, and this greatly increases the probability of finding materials with high service life expectation . The first hollow fiber ready for permeator commercialization in the field of aqueous separations is a specially developed nylon . Nylon tends itself well to such applications because of its exceptional strength, and its long history of durability in saline solutions . "`PERMASEP"" PERMEATOR DESIGN AND SPECIFICATIONS

These hollow fibers are utilized in a de ice similar in design to a single-end heat exchanger, with the fibers encapsulated in an epoxy resin which forms the tube sheet. A cutaway diagram of the permeator is shown in Fig . 3. The solution

Fig. 3. Cutaway diagram of the pernieator . to be processed is introduced into the shell at the side at a sufficiently high pressure . Water which permeates through the walls of the hollow fibers in the bundle travels up the fibers' bores, countercurrent to the feed solution, passing into the head on leaving the tube sheet area . That part of the feed solution which fails to permeate through the hollow fiber walls is continuously discharged from the far end of the permeator. Although permeators can be and have been made in various shapes and sizes, the commercial size product consists of a pressure shell fabricated from 14 inch schedule 40 steel pipe . Corrosion protection is provided by a baked-on epoxy coating. Fig. 4 is an engineering drawing of such a permeator, showing an overall Desalination, 7 (1969/70) 31-46



35

HOLLOW FIBER PERMEATORS

SUPPORT LUG

2 NOZZLES 130 • APART

Fig . 4. Permascptt permeator.

length of 10 ft. 31 in. or 3 .14 meters . Specifications for this permeator are based upon a computer-derived optimization for this particular nylon fiber : Fiber outside diameter 45 microns 24 microns Fiber inside diameter Hollow fiber void fraction 28.5 Fiber packing density 50 Number of fibers 28 million Permeation area 84,500 ft 2 (7850.3 m 7. ft3 (0.198 m3 ) Shell-side vessel volume Using 0.15 % aqueous soCapacity 6 gpm (22.7 1/min .) Selectivity 15 % solute passage lution of mixed sodium and magnesium sulfates at 600 psig (42kg/cm2) feed pressure and 30'C, and at 50% conversion of feed to product water) 1500 lbs. (680 kg) Weight PERMEATOR PERFORMANCE PARAME I ERS

Operation of many permeators of various sizes, under a variety of conditions, has served to establish the effects of various operating parameters, as follows :

1 . Pressure Recalling the equation discussed earlier, increasing feed solution pressure to the permeator should increase product flow rate, and this has been found to be so . However, increasing permeator capacity does not keep pace with increased pressure ; consequently, overall economics may not be thereby benefited . However, separation efficiency is usually increased with increased pressure since solute flux, unlike solvent flux, is unaffected by pressure . A 600 psig feed pressure appears to be a good compromise, and standard permeator vessels are code-rated at 650 psig . Desalination. 7 (1969/77) 31-46



36

WILLIAM P . COOKE

2. Temperature In keeping with chemical processes, temperature has a substantial effect on the flux rate of hollow fiber membranes, as shown in Fig . 5, in which the product water capacity of the permeator previously described is plotted against the feed to

T i

I to TING

t I } 2 30 40 so so TEMP . K

Fig. 5. Effect of temperature on permeator capacity .

water temperature. In numerical terms . the membrane flux changes 3 % for every I °C of temperature change above and below 30 °C . However, tests do not show that product water quality is affected by changes in feed water temperature, all other conditions being equal. A practical upper limit of 40 °C, at 600 psig feed pressure, is dictated by the epoxy tube sheet . 3. Undissolved solids It will be obvious that the bundle of hollow fibers in a permeator functions as a rather efficient filter for undissolved solids . Such solids build up on the outside of the bundle, increasing the pressure drop through the unit, therefore decreasing the net driving force, and, consequently the permeator capacity . Ultimately, it would be necessary to shut down the permeator to flush the solids from the surface . As a consequence. when using these permeators, feed solutions must be treated to reduce the suspended solids concentration to an acceptable level . Studies are in progress to define this parameter . 4. Conversion Conversion, or recovery as it is often called, is simply the volume of permeate expressed as a percentage of the feed solution volume . Conversion is controlled by varying the output of the feed pump, at a fixed solution pressure, or by altering the back pressure on the reject solution line . In the purification of water supplies, Desalination, 7 (1969/70) 31-46

37

HOLLOW FIBER PERMEATORS

or in the concentration of industrial waste streams, it is economically desirable to operate at as high a conversion as possible . For a given feed solution, as conversion is increased, the average solute concentration in the permeator shell will increase . Since solute flux rate is a function of the solute concentration differential across the membrane, increasing conversion will be accompanied by increasing concentration of the solute in the permeate . At high conversions, i.e., 75 or above, solution velocities in the permeator shell become quite low, resulting in rather sharp increase in solute concentration of permeate stream . These relationships are illustrated in Fig . 6, showing solute passage* conversion vsn for a permeator similar to that previously described on the standard mixed sulfate solution referred to earlier.

50 40

a W

30

*nn 20

t0

0 t0

20

30

40 SO 60 CONVERSION . %

TO

80

90

Fig. 6. Solute passage vs . conversion . There is one consideration which may limit conversion to a point below that at which shellside flow distribution becomes a problem . That is the case where the least soluble solutes begin to precipitate, as a result of the concentration which takes place in the permeator shell . Fig. 7 is a graph of solute concentration in the reject stream plotted against conversion, at three different feed solution concentrations, assuming zero salt passage. Calcium carbonate and calcium sulfate are two solutes of low solubilities which limit the conversion which can be employed with water supplies containing the ions which combine to form these salts. Use of sequestering agents, such as sodium tripolyphosphate and sodium hexametaphosphate, to inhibit precipitation of these calcium salts, permits the operation of permeators at higher conversions than would otherwise be possible .

5. pH With nylon hollow fibers, pH has a marked effect on the separation which

* Solute passage as we use the term is the concentration of the solute in the permeate, expressed as a percentage of its concentration in the feed . Desalination, 7 (1969/70) 31-46



WILLIAM P. COOKE

38

30 .000 zE z ° s

20.E

4 _¢ U F

W40"S< z ¢

vw o tat f_ '-U

m om to ¢

.000 10 8,000 6,006 3,000 4,000

3,000 2,000

Fig. 7 . Concentration of solutes as a function of conversion !assuming no solute passage into permeate stream) . can be accomplished on certain solutes . With some inorganic and organic salts, permeability through our nylon hollow fibers is associated with the ion form which exists at a particular pH . For instance . Fig . 8 shows solute passage of boron plotted against pH, showing lower and lower passage at increasingly alkaline

r

PF-600 PSIG m T- 30•C Y - 30% d m 40- CF 140 ppm H3503 60-

tSPEIMS ; 20-'

12% 23% 1 S

6

Fig . 8 . Effect of pH on boron passage (upper curve, 12% ; lower curve 23%) .

pH values . It is known that at pH values below 7, essentially all of the boron in aqueous solutions would be present as undissociated boric acid . As the pH increases above 7, H 2B03 ions begin to form, then HBO 3 2 ions, and then B0 3 1 ions at pH's above 10 . It would appear that the greater the charge on the ion, the greater its rejection by our nylon hollow fiber membranes. A similar relationship is seen in Fig. 9 dealing with sodium chromate . It can be noted that the decreasing solute passage of chromium with increased pH parallels the increasing Desalination, 7 (1969/70) 31-46



39

HOLLOW FIBER PERMEATORS i

30

1

1

I

20 v

0

G W a a

PF - 600 PSIG T-30'C

t0

V-50%

CF - 1500 PPM

"*Per 04

tSPEIMS -16Y. 0 1 1 t 1 1 E 1 4 5 6 7 it 6 9 to PH

Fig. 9. Effect of pH on chromate passage .

dissociation of sodium chromate . Table l shows the marked difference in solute passage of 5 organic acids at pH 2 .1 vs. that of the sodium salts at pH 8 .5. This behavior, also, correlates with the degree of dissociation which is quite low for the acids themselves, and much higher for their sodium salts .

TABLE 1 EFFECT OF PH ON SOLUM PASSAGE

Compound Acetic acid Succinic acid Adipic acid Citric acid Gluconic acid

'

Solute passage, pH 2.1

% Solute passage. pH 8.5'

60

100

24

118 146 192 196

100 100 66 12

3 3 9 7

xfol. Wi.

Samples neutralized with sodium hydroxide

FLEXIBILITY IN HOLLOW FIBER PERMEATION CHARACTERISTICS

By variations in its manufacturing process, Du Pont is able to produce hollow fibers with a range of permeation characteristics . In this process, increased water permeability of the fiber, and therefore increased permeator capacity, is gained at the expense of selectivity, or ability to reject solutes dissolved in the water_ Similarly, improvements in selectivity can be realized at the expense of permeator capacity . Within the limits of fiber dimension previously disclosed, Desalination . 7 (1969/70) 31-46



WIWAM P. COME

that is. O.D . of 45 microns and I .D . of 24 microns, permeators show the flux to solute passage relationship illustrated in Fig. 10. For instance . nylon hollow fiber so produced as ,to have a flux rate for water of 0 .09 gal ./day/ft2 of membrane surface to 0.s of 0.4

02 S

a J C O

0-1 .0e

W H t

.06

i C W b

04

03 .02

t .01

l t I t 1 fill t I 1 t 1 I 4 5 6 7 0 910 20 so 40 506070 2 + SOLUTE PASSAGE . '/.

Fig. 10. Flux-solute passage relationship .

will have a solute passage of 10% . using standard mixed sulfate solution under the conditions of pressure, temperature, and conversion previously disclosed, whereas . the solute passage is with a fiber so produced as to have a flux of 0 .25 gal/day/ft' 38 %. For a given application . selection of the optimum fiber selectivity, or solute passage, the yardstick of selectivity, is dictated by the composition of the feed solution and the separation which is desired . This yardstick of selectivity, that is the solute passage using standard mixed sulfate solution, does not necessarily correlate meaningfully where separations are carried out with other salts or with organic compounds . For example, in tests carried out using four permeators of low, medium, high and very high selectivity respectively, on 0 .1 % solutions of polyethylene glycols of molecular weights of 1540, 6000, and 20,000, respectively, the relationship between flux and solute passage as shown in Fig . 1 I was realized . In this case, solute passage refers to the passage of the polyethylene glycol . It can be seen that a normal and expected relationship was experienced below the 8-10% solute passage range, but disproportionate increases in flux, compared with solute passage, are experienced above this range . Desalination. 7 (1969/70) 31-46



41

HOLLOW FIBER PERMEATORS 50 1 40 s0 20

PF • 600 PSIG T • 30•C . CF • 1000 PPM fSPE1Mg • VARIABLE

Fig, It . Flux-solute passage relationship for polyethylene glycols of several molecular weights.

This data does illustrate that separations based on molecular weight are feasible . and that the degree of separation achievable will vary with the selectivity of the fiberINDUSTRIAL WASTE STREAM TEST RESULTS WITH "PERMASEP" PERMEATORS

A substantial number of prospective applications have been proposed for "Permasep" permeators . Some of these are clearly beyond the capability of hollow nylon fibers, such as concentrated salt solutions where the osmotic pressure is too high, solutions with a high percentage of suspended, undissolved solids and solutions containing chemicals injurious to nylon . However, many of the proposed applications have merited laboratory tests or field tests in some instances . The following three illustrate the variety of applications for which permeators may prove useful : 1 . Whey processing Whey. the fluid portion of milk obtained by coagulation of casein during manufacture of cheese or casein, contains about half of the milk solids, including of1 A the protein and most of the water soluble vitamins most of the lactose, about and minerals . Many cheese factories dispose of whey to the nearest river, but it is an objectionable pollutant and such a means of disposal is now prohibited in many areas . Large cheese factories will process whey by spray or drum drying for use in animal feeds. The high salt content of such whey essentially prevents Desalination, 7 (1969/70) 31-46

WILLIAM P. COOKE

42

its use as a human food supplement- Obviously, a less costly means of dewatering wh c , combined with reducing its salt content should be of interest . Preliminary data developed in tests thus far, using a 4 inch by 7 ft permeator . indEcate the above objectives are realizable . Permeator performance is being measured by analysis of feed, permeate and reject streams for COD (chemical oxygen demand), as a means of the organic compounds present, and conductivity, as a measure of the salt content . Tests will be carried out using permeators of different selectivities, and at various conversions . 2. Pulp trill waste stream processing In the multistage bleaching of kraft paper pulp, the pulp is first chlorinated so as to chemically change the residual lignin content still present in the wood fibers . In the next stage, the pulp is treated with caustic soda solution which dissolves the chlorinated lignin formed in the first stage . The dark brown filtrate which results when the pulp is dewatered after this stage is often discharged to the nearest river . However, it too is a pollutant and such means of disposal is being increasingly prohibited by the appropriate governmental agencies . It would be advantageous to be able to separate most of the dilute caustic from the dark brown sodium chlorolignate so that the former may be reused in the mill . The sodium chlorolignate could be added to other pulp mill waste streams which are burned to recover chemical value.

TABLE It PERMEATION TESTS WITH CAUSTIC EXTRACTION EILTRATE

PF = 600 psig ; TF = 30°C; Y - 83%: (SPe).us = 9 .6%.

Feed Permeate Reject

Color (PPM)

COD (PPM)

BOD (PPM)

TDS (PPm)

NaCl (PPM)

PH

20,000 175 25.000

4,350 345 10.500

320 175 1 .200

7,100 2,300 15,600

1,790 1,700 2.050

9.6 7.2 9.7

Table II presents data obtained on representative caustic extraction filtrate using a research size "Permasep" permeator whose hollow fiber selectivity was 9 .6 % SPE on standard mixed sulfate solution . In these tests a very high conversion, 83 %, was utilized . The content of organic compounds, sodium chlorolignate and associated resins and resin degradation products, is indicated by color, COD and BOD, which stands for biological oxygen demand . It will be noted that this caustic extraction filtrate was concentrated about four times and that the permeate dissolved solids content was mainly sodium chloride . This permeate actually Desalination, 7 (1969/70) 31-46

HOLLOW FIBER PERMEATORS

43

was a very light amber color compared to the very dark brown color of the feed solution. The quality of this permeate is such that it could be reused in the pulp mill bleach plant . Tests were also run with a more selective permeator (SPE = 2 .3 %), and the permeate was of somewhat better quality than that shown, as would be expected . Tests on several other pulp mill waste streams are planned .

3 . Sugar refinery molasses pr, :;cessing After processing of sugar cane or sugar beet extract via multiple-effect evaporation and crystallization of the desired sucrose, there remains what is known as molasses . This molasses still contains a substantial amount of the desired sucrose, but the high concentration of accompanying non-sugars (amino acids, betaine, oxalic acid, sodium and potassium chloride and sulfate, etc) . makes it very difficult to crystallize and recover this sucrose . This molasses is sold mostly to farmers for cattle feed for prices averaging 10/lb . in the United States, whereas it contains over 70 worth of sucrose if the latter could be recovered as such . If the molasses could be processed in some way so as to decrease the percentage of non-sugars, the molasses could be returned to evaporators and crystallizers for further sugar recovery . A substantial number of tests have been carried out with sugar beet molasses under a variety of conditions, and testing continues ; however, for the sake of simplification, only one set of data is presented in Table III . In this test, the molasses was first diluted with water to about 5 % dissolved solids content .

TABLE III PERMEATION TESTS WITH DILUTE MOLASSES PP = 600 prig ;

Feed Permeate Reject

TP = 40"C ; Y = 67% ; (SPe)us = 4 .9%. Sugars ( % by n7.)

Non-sugars

Ratio

(% by wt .)

sugarsjnon-sugars

2.82 0.32 8.38

1 .92 0.45 4.15

1 .47 0.71 2.01

A 4 inch diameter by 7 ft perteator with high selectivity was chosen (i.e., a SPE of 4.9 % as measured with standard mixed sulfate solution) . The diluted solution, which was a very dark brown color, was introduced into the permeator at 600 lbs . per sq . in. pressure and 40 °C. In this particular run, a conversion of 67% was utilized. Although the analysis of the permeate and the reject streams are both shown, attention should be directed to the reject stream, which is the stream which could be returned to the evaporators for further processing . It is obvious that the non-sugars have passed through the hollow fiber membrane faster than the Desalination, 7 (1969/70) 31-46



WILLIAM P. COOKE

sugars, so that the ratio of sugar to non-sugar in the reject stream is higher than that of the feed solution by a factor of 1 .36. Not reflected in these data is the fact that :he permeate stream in these tests was a light yellow color, indicating that most of the color bodies in the feed solution were rejected by the hollow fiber membrane . Color bodies are known to be high molecular weight compounds . Performance of this permeator suggests the possibility of introducing the reject stream into a second permeation stage which would utilize a much less selective fiber, so that most of the desired sucrose would pass through into the permeate stream . leaving behind the color bodies and undesired raffinose, another high molecular weight compound . SEPARATION PROCESS DESIGN USING HOLLOW FIBER PERMEATORS

T he des gn of a process to carry out a desired separation using "hollow fiber" permeators will vary with the composition of the solution to be processed, the quality of permeate and/or reject streams desired, minimum yield necessary, etc . Determination of optimum hollow fiber selectivity would be based on actual laboratory tests with the feed solution in question, using research size permeators . Hollow fiber selectivity . coupled with the viscosity and osmotic pressure of the feed solution, collectively determine the permeate stream capacity for a permeator such as that previously specified . Selection of the conversion to be utilized involves a compromise between yield and the composition or purity of the desired product stream, which will be the permeate stream in some instances, and the reject stream in others . Fig. 12 is a process flow diagram showing the major elements and necessary steps for effecting a desired separation using "hollow fiber" permeators . The particular configuration shown is a two-stage separation in which the reject stream - - - - - - - - - - - - - - - - - - - - - - - - - -

r

<

HYDRAULIC ACCUMULATOR

Cb

1 E

FEED

LINE NO .

=©© 4

~650CS2o® FLOW-AS % OF 100 75 FEED FLOW

50 0

15

1MCM

Fig. 12 . Process flow diagram for 2-stage separation with "Permasep" permeators . Desalination. 7 (1969/70) 31-46

45

HOLLOW FIBER PERMEATORS

from the first stage is processed in a second stage ; however, many separations would consist of one stage only . This process flow diagram shows provision for pretreatment, such as acid or alkali for pH adjustment or polyphosphate to inhibit precipitation of calcium or other polyvalent cations, an in-line cartridge type filter to remove undissolved solids of over 10 microns in diameter, a high pressure reciprocating or centrifugal feed pump, with either variable speed drive or a recycle loop, a hydraulic accumulator to dampen out pressure surges in the pump discharge, the necessary number of permeators piped up in parallel, or, in the case shown, both parallel and in series, and a back pressure regulator to control the conversion of the system to the desired value . The dotted line shows provision for bypassing the separations plant in the event of necessary shutdown or repair of the latter . The particular process shown calls for 4 essentially identical permeators insofar as size, flux and selectivity are concerned . three permeators comprising the first stage, and one permeator the second . Each stage is operated at 75 % conversion, such that the overall conversion of the two stages combined is 91 % . Such an arrangement would provide for more efficient separations than the same four permeators operated as a single-stage with 91 % conversion, because of the

Fig. 13 . Test installation involving twenty 14 inch diameter permeators .

Desalination, 7 (1969/70) 31-46

WILLIAM P . COOKE

46

distribution problem described earlier in this paper . Shellside pressure drop with our permeators as currently designed is small enough that the feed pressure to the second stage permeator is not greatly less than that to the first stage permeators ; accordingly . the permeate stream capacity of the former would not be significantly different than that of each of the latter . If the particulars of the desired separation dictated using a second stage with different hollow fiber selectivity, then the ratio of second stage permeators to first stage permeators would be something other than the i : 3 shown. The Du Pont Company has constructed and operated a var test installations involving one or more 4 in . I.D. x 7 ft. and 14 in . I .D. x 10 ft . permeators, and the associated equipment . Fig. 13 is a picture of an installation involving twenty 14 in . diameter permeators . The vertical mounting shown requires the least area ; however, horizontal mounting could be employed equally as well . While this installation is processing a municipal brackish water supply, a plant to process cheese whey . pulp mill waste streams, molasses or other industrial waste streams might look very much the same . The elements shown in the process flow diagram of the previous figure would be more or less common to each . This particular plant is presently producing 100 .000 gals ./day of potable permeate, but can produce 145 .000 gals ./day with all permeators "on stream". In summary. "Permasep" permeators constitute a new unit process with exciting possibilities in industrial water and waste stream purification and separation and possibly in some rather complicated chemical separations . The nylon hollow fibers of the current generation of permeators appear very promising for a variety of applications, and the future holds promise for fibers spun from more sophisticated polymers which could effect separations of which the nylon hollow fibers are incapable. Included among the latter is the desalination of brackish chloride water supplies, and, ultimately, the single stage desalination of sea water . REFERENCES

W. T. RosiNsos AND R. J. MArrsoN. A new product concept for improvement or industrial waters, J. Water Pollution Control Federation . 40 (3) (March 1968) . 2 . R. J. MArrsos AND V. J.Tonsic, Improved water quality. Chem. Dig. Progr., 65 (1) (January 1969).

Desalination, 7 (1969/70) 31-46