Permeability Properties of Cellulose Triacetate Hollow-Fiber

15 Permeability Properties of Cellulose Triacetate Hollow-Fiber Membranes for One-Pass Seawater Desalination Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch015

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KAORU FURUKAWA , MASAAKI SEKINO, HIROSHI MATSUMOTO, KAZUTO HAMADA, TETSUO UKAI, and HIROHITO MATSUI Research Center, Toyobo Co., Ltd., 1300-1 Honkatata-cho, Otsu Shiga, 520-02 Japan

RO process for the desalination of seawater was proposed for the first time by Reid in 1953, but no significant advancement was observed until the invention of asymmetric membrane with high water flux by Loeb and Sourirajan in 1960. Since that time, RO process showed remarkable progresses in practical applications in the field of desalination of brackish water for potable and pure water. On the other hand, the development of seawater desalination, which was the original target of RO process, was delayed because of the insufficient performance of membrane under operating conditions of high pressure and high salt concentration. For this reason, two pass seawater desalination process have been necessarily employed till quite recently, and the results obtained have been satisfactory to some extent with regard to water quality and practical operation. However, one pass process has advantages over two pass process for simple and compact plant, simple operation, easy maintenance and lower energy consumption. Although several one pass RO systems have been developed so far, the membrane performances, especially salt rejections have not been satisfactory and were sometimes not stable in long term operation (1) (2). In the use of membranes having insufficient salt rejection, the product water recovery of module is limited to much lower than 30%. Hence two pass process is sometimes employed for high salinity seawater instead of one pass process for high salinity seawater ( 40,000 ppm TDS) (3). Moreover, with high salinity seawater, water productivity is comparatively low because of its high osmotic pressure. In this case high pressure operation should be advantageous from the stand point of water productivity and salt rejection. However, the conventionally available modules can not be operated under such high 1

Current address: 2-8, Dojimahama 2-chome Kita-ku, Osaka, 530 Japan. 0097-6156/81/0153-0223$05.00/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.

SYNTHETIC

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224

MEMBRANES:

DESALINATION

pressure as 75 Kg/cm G i n a p r a c t i c a l use because of t h e i r i n s u f f i c i e n t high pressure t o l e r a n c e . This may be the reason why there has been l i t t l e i n v e s t i g a t i o n about high pressure d e s a l i n a t i o n . We have developed one pass seawater d e s a l i n a t i o n module so c a l l e d , "Hollosep-High Rejection Type" with e x c e l l e n t s a l t r e j e c t i o n i n 1978. Long term experiments of one pass seawater d e s a l i n a t i o n using our module have been c a r r i e d out at Chigasaki Laboratory of Water Reuse Promotion Center under the s u p e r v i s i o n of M i n i s t r y of I n t e r n a t i o n a l Trade and Industry, Japan (4). A continuous long term t e s t f o r 12,000 hours was s u c c e s s f u l l y conducted with the m-value of 0.02. Another demonstration p l a n t with an 800 m3/D c a p a c i t y has a l s o been operating over 3,000 hours at the recovery r a t i o of k0% using the feed water of F.I. value of about 4. Hollosep High R e j e c t i o n Type i s c h a r a c t e r i z e d by C e l l u l o s e T r i Acetate (CTA) hollow f i b e r with dense membrane s t r u c t u r e and high s a l t r e j e c t i o n , and a l s o by the module c o n f i g u r a t i o n f a v o r able f o r uniform flow of feed water through hollow f i b e r l a y e r s (5). These f e a t u r e s suggest that Hollosep may be operated under the c o n d i t i o n s of higher recovery r a t i o compared to conventional conditions. The purpose of the present work i s to evaluate the a p p l i c a b i l i t y and merit of high pressure d e s a l i n a t i o n process by the use of Hollosep-High Rejection Type. I am going to speak of an experimental study of the membrane p e r m e a b i l i t y under high pressure of the CTA hollow f i b e r f o r one pass seawater d e s a l i n a t i o n , "Hollosep-High Rejection Type". The module performance has been simulated f o r high pressure operating range by s i m p l i f i e d module model based on the data of the hollow f i b e r , and examined the agreement with the a c t u a l module performance. Furthermore, we w i l l d i s c u s s l a t e r on the r e s u l t of operating cost study under high pressure operation. Experimental 1) Preparation of Hollow F i b e r Membrane. CTA ( C e l l u l o s e T r i - A c e t a t e ) hollow f i b e r membranes were prepared by apinning a dope s o l u t i o n of CTA followed by soaking and a n e a l i n g . 2) Module F a b r i c a t i o n . The bundle of s e v e r a l thousands of hollow f i b e r s i s f a b r i c a t e d i n t o an element and assembled to a module. Hollow f i b e r s i n an element are arranged i n a mutually crossed c o n f i g u r a t i o n without any kind of supporting m a t e r i a l s between hollow f i b e r l a y e r s . This module c o n f i g u r a t i o n cont r i b u t e s to uniform flow of feed water, small pressure drop, minimizing concentration p o l a r i z a t i o n and extending the allowance of f o u l i n g index of feed water, up to F.I. = 4. Tube sheet part of the element i s f a b r i c a t e d by the use of improved epoxy r e s i n with r e s i s t a n c e against high pressure and high temperature.


15.

FURUKAWA E T AL.

One-Pass Seawater

Desalination

225


The c o n s t r u c t i o n of Hollosep i s shown i n Figure 1. The s p e c i f i c a t i o n s of the modules are shown i n Table 1. 3) Measurements of RO Performance. RO performances were measured by the simple apparatus, as shown i n F i g u r e 2. The water f l u x and s a l t r e j e c t i o n of the hollow f i b e r membranes under operating pressure i n the range of 50 to 120 Kg/cm2G were determined u s i n g the feed water of 3.5? NaCl, at 25°C and at product water recovery r a t i o of l e s s than 1? , a f t e r an elapsed time of 2 h r s . The RO performance of module was evaluated under operating c o n d i t i o n s of pressures i n the range of 40 to 75 Kg/cm G, NaCl concentration of feed water i n the range of 3.5 to 5.0? and product water recovery r a t i o of 30 to 60?. 2

4) Simulation of Module Performance. The module performance over the wide range of operating c o n d i t i o n s was c a l c u l a t e d by a s i m p l i f i e d module model on the b a s i s of the data of the hollow fibers. The s i m p l i f i e d module model and the c a l c u l a t i o n scheme w i l l be shown l a t e r i n Figure 7 to 8 and Table 3. Results and Discussions 1) C h a r a c t e r i s t i c s of Hollow F i b e r Membrane. A microscopic view of hollow f i b e r membrane of Hollosep i s shown i n Figure 3. C h a r a c t e r i s t i c s of the hollow f i b e r membrane i s shown i n Table 2. The outer diameter and w a l l thickness of t h i s hollow f i b e r membrane i s f a i r l y t h i c k compared with those of other hollow f i b e r s f o r seawater d e s a l i n a t i o n . S a l t r e j e c t i o n of hollow f i b e r membrane i s high enough to be a p p l i e d to one pass seawater d e s a l i n a t i o n . Resistance of the hollow f i b e r membrane against high pressure was evaluated by measuring water f l u x r a t e and s a l t r e j e c t i o n under operating pressure of up to 120 Kg/cm^G i n 3.5? NaCl feed water. The data obtained were analyzed i n terms of pure water p e r m e a b i l i t y A and s o l u t e t r a n s p o r t parameter given by Kimura-Sourirajan*s equation (6). The r e s u l t s are shown i n Figure 5. Membrane performance remained almost unchanged up to the pressure of 100 Kg/em2G. The r e s u l t may suggest that t h i s hollow f i b e r membrane i s w e l l r e s i s t a n t against high pressure and can be p r a c t i c a l l y operated under appreciably high pressure. 2) C h a r a c t e r i s t i c s of Module, "Hollosep-High Rejection Type" RO performance of the module Hollosep-High R e j e c t i o n Type i s compared with various kinds of modules reported so f a r i n terms of A and D^/K^ value. The r e s u l t s are shown i n Figure 5. The A value of Hollosep i s lower by a f a c t e r of l e s s than 10 compared to that of f l a t sheet membranes, whereas the D^/K^ value of Hollosep i s q u i t e low by a f a c t o r of 100 than that of


226

SYNTHETIC

Vessel


O-ring

MEMBRANES:

DESALINATION

Resin layer

Hollow fiber layer

Figure 1.

Construction of hollosep

Figure 2. Schematic view of reverse osmosis test loop: (1) hollow fiber membrane; (2) pressure vessel; (3) feed water; (4) filter; (5) pressure pump; (6) relief valve. 5. 4

Figure 3.

3. Concentrate

Microscopic view of hollow fiber of hollosep—high-rejection type


15.

FURUKAWA E TA L .

One-Pass

Seawater

Desalination


TABLE 1. SPECIFICATION OF HOLLOSEP MODEL

HR5350S

HR8350

HR8650

MM MM

lao 1220

305 1330

305 2640

M/D I

>3.5 99.2

>10 99.2

>20 99.2

SIZE DIAMETER LENGTH PRODUCT FLUX SALT REJECTION

3

TEST CONDITION FEED WATER PPM PRESSURE KG/CMG TEMPERATURE °C RECOVERY Z

35000 55 25 30

2

35000 55 25 30

35000 55 25 30

TABLE 2. CHARACTERISTICS OF THE HOLLOW FIBER OF HOLLOSEP-HIGH REJECTION TYPE Hollow Fiber Dimension : OD. ID.

165 M 70 v

Reverse Osmosis Performance of Hollow Fiber Flux Rate

50 t/m Day

Salt Rejection

99.7 X

2

(Test Conditions) Feed Water

35,000 ppm NaCl

Pressure

55 Kg/cnr^G

Temperature Recovery

25 °C

Permeability Properties of Cellulose Triacetate Hollow-Fiber

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