The effect of agitation on reverse osmosis desalination

Desalination, 79 (1990) 261-269 Elsevier Science Publishers B.V., Amsterdam

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The Effect of Agitation on Reverse Osmosis Desalination J . LORA and J.M. ARNAL Department of Chemical and Nuclear Engineering, Politechnical University of Valencia (Spain)

(Received October 29, 1989 ; in revised form November 11, 1990) SUMMARY

Chemical and physical phenomena such as precipitation fouling and concentration polarisation are of notorious influence on the performance and lifetime of reverse osmosis (RO) membranes and other similar ones . In analogy to other mass transfer operations we have considered agitation as a means of improving the RO process and have designed a patented selfstirred module, suitable for plate-and-frame operation . The plate acts as a membrane support, and the space accorded by the frame serves as agitation chamber . A flat-blade turbine with appropriate baffles rotates with the liquid flow, its proper construction preventing damage to the membranes . No external access for any axis is required, thus avoiding complication of the setup . The rotation speed of the turbine is a linear function of inlet flow . RO separation of water from dissolved salts with this self-stirred module has been investigated . Both brackish (1,000 ppm) and seawater (35,000 ppm) have been tested . The improved appearance of the surface, clean and free of fouling, reflected the marked improvement in fluxes, about 90% for brackish water and some 140% for seawater, when compared with unstirred operation . Also, with stirring, some increased salt rejection was noted . Clearly this improvement would apply to other similar membrane processes as well . INTRODUCTION

One of the most important problems that appears during the use of RO membranes is the precipitation fouling on their active layer . As RO increasingly becomes the separation method of choice in many applications, the problem of controlling membrane fouling assumes even greater importance [1,2] . 0011-9164/90/$03 .50

© 1990 Elsevier Science Publishers B .V.

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Precipitation at membranes makes manifest the concentration polarisation of the precipitating salts . This is also manifested by the flux decline accompanying the increased hydraulic resistance of growing deposits . Concentration polarisation can be reduced by enhancement of the mass transfer between the membrane-saltwater interface and the bulk of the feed stream . The usual way of making it is to increase flow and turbulence [3]. Other techniques include physical wiping and the use of buoyant force [4] . In our preliminary experiments we have tried to increase the mass transfer in a plate RO module by using agitation. As is known, this technique which is analogous to methods used in other separation processes causes substantial variations in pressure drop, heat exchange and mass transfer in fluid mixing. We assumed that the agitation would promote turbulence, preferentially near the membrane surface, causing less precipitation fouling . The aim of our study has been to examine the effects of agitation on RO membrane performance . Desalination of water was selected because they have usually been used in RO.

EXPERIMENTAL

The self-stirred module In order to examine the effects of agitation on RO tests, we used a proper design of plate module as is shown in Fig . 1 . Within the plate module the agitation is caused by using a flat-blade turbine (4) settled in this module in a way that would not damage the membrane (2) . This impeller is moved by the own inlet flow in the agitation chamber of the plate module (3) . Further details of this element can be found in Arnal's patent [5] . On the other hand, we have referred to the membrane installed into the plate module as a stirred membrane . RO tests were carried out in an experimental plant that can be seen in Fig. 2 . Membrane modules, low pressure and high pressure pumps and some measuring and control instruments are represented in this diagram . The feed solution (A) was pressurized by a dosing pump (D) with variable flow (max. 1000 1/h) and pressure (max . 100 atm) . Two plate modules designed in our laboratory with an effective area of 184 cm 2 were connected in series with the high pressure pump . Each module was equipped with a flat Osmonics Inc. membrane type SEPA 92 . As Fig. 1 shows the flat-blade turbine was only settled in module 2 .



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1-PLATE SUPPORT OF MEMBRANE, 2-STIRRED MEMBRANE, 3-AGITATION CHAMBER, 4-FLAT-BLADE TURBINE, 5-UNSTIRRED MEMBRANE, 6-INLET FLOW, T-OUTLET FLOW

Fig . 1 . The self-stirred module .

E

A • C • • F • • I J • L M

Fig. 2. Flow diagram of the experimental plant .

FEED SOLUTION TANK AUXILIARY PUMP FILTERS DOSING PUMP PLATE MODULE MANOMETER REGULATION VALVE RELIEF VALVE TERMOMETER FLOWMETER BRINE RECIRCULATION LINE COUNTER ELECTRONIC DEVICE PERMEATE RETURN LINE



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Enriched brine exited from plate module 1 through the regulation valve (G) and rotameter (J) to feed solution tank (A) . The desalted water that was analysed was also recirculated (M) at this tank. Thus, it could be supposed that the concentration of each test was constant. The test solutions were tap water and NaCl at concentration of 1000 ppm and 40,000 ppm. Conductivity measurements indicated salt concentration . The agitation speed was estimated by using a counter electronic device (L) . In this way the influence of agitation on the performance of membranes could be observed directly. The behaviour of membranes was evaluated by the water flux Jw (1/m2 day) and salt rejection R (%) that is defined as : (CF - Cp) 100/CF where CF and Cp are feed and product solution concentrations, respectively . Data were plotted as the above parameters, both as a function of RO operation time. In order to examine the state of membranes that were used, after each RO test a series of photographs was made .

RESULTS AND DISCUSSION

At first we verified that pressure drop through the plate module can be considered negligible . We have also confirmed that speed agitation into the plate module is a linear function of inlet module flow, as Fig . 3 shows. In this way, when an inlet flow is fixed, the impeller must be rotated with a constant speed. On the other hand, this was a method to verify the normal running of the impeller within the plate module .

Low salinity water tests Then RO tests with low salinity water were carried out at room temperature during 500 hours, under the following conditions :

Effective pressure

Feed solution

Feed solution rate

Agitation speedmodule 2

30 atm

1000 ppm NaCl

250 1/h

200 rpm



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Q (L/h)

0

50

100

150 200 Agitation speed (rpm)

300

250

Fig . 3 . Correlation of agitation with inlet module flow Q .

J (L/m2 day)

R (%) -100

2800 2400

96

2000 -9 1600

∎

1200

Stirred membrane

-*- Unatirred membrane

m

I

90

85

800 I

400 0

100

i

200 300 RO operation lime (h)

400

80 600

Fig. 4 . Effects of agitation on RO desalination of low salinity water in a large duration test .



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100 A J (x) 80

80

40

20

0 0

100

200 300 RO operation time (h)

400

Fig. 5. Water flux increase for the stirred system as a function of time Js : stirred membrane flux; Jn : unstirred membrane flux .

Figs. 4 and 5 show the effects of agitation on RO desalination of low salinity water . So for stirred membranes both water flux and salt rejection were higher than those characteristics for unstirred membranes . Fig. 6 also shows that there is a difference in the surface of the above membranes . It can be seen that agitation tends to improve circulation near the membrane surface; for the stirred system the amount of particulate fouling is reduced . Thus, the fully developed fouling layer in unstirred membrane surfaces can cause this membrane to lose up to 85% of their water flux . Once RO tests of low salinity water were finished, membranes were cleaned by pumping a 1 wt.% citric acid solution during 24 h and fresh water for the next 4 h at 20 atm .

High salinity water tests

Finally a feed solution of NaCl at a concentration of 40,000 ppm in tap water was prepared . The RO tests of high salinity water were carried out at room temperature during 720 h under the following conditions : Effective pressure

Feed solution

Feed solution rate

Agitation speedmodule 2

50 atm

40,000 ppm NaCl

300 1/h

250 rpm



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Fig. 6 . State of membrane surface after 500 h of RO test by using (a) stirred membrane and (b) unstirred one .

J (L/m2 day) 2800 2400 2000 1600 1200 800 400 0 0

200

400 RO operation time (h)

600

0 800

Fig. 7. Effects of agitation on RO desalination of high salinity water in a large duration test .



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aJ

300

300

250

250

200

200

150

150

100

100

(%)

50

50

0 0

200

400 RO operation time (h)

800

0 800

Fig. 8 . Water flux increase for the stirred system as a function of time . Js : stirred membrane flux; Jn : unstirred membrane flux .

Fig. 9 . State of membrane surface after 720 h of RO test by using (a) stirred membrane and (b) unstirred one .

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In Figs . 7 and 8 it can be observed that both flux and selectivity of stirred membranes are improved in relation to unstirred membranes, having water flux increases of 140% for those membranes . Therefore, it is confirmed that the fouling caused by precipitation is more important when high salinity is used. Fig. 9 shows the same effect on membrane surface than Fig . 6, as a consequence of agitation in stirred membranes .

CONCLUSIONS

From the preliminary experiments the following conclusions can be drawn:

• The tests of RO desalination by using stirred membranes have demonstrated that agitation can play a significant role in reducing concentration polarization and particulate fouling, or specifically, in increasing mass transfer from the membrane to the bulk fluid . In this way the said stirred membranes can be used in order to make the RO processes more effective. • The effect of the agitation on membrane transport properties shows a large water flux increase at almost the same or even slightly higher retentions in relation to unstirred membrane systems. On the other hand, the time taken to obtain a specific water flux decline is greater when agitation is used. We wish to point out that stirred membranes can be used in other processes also, such as ultra and microfiltration . The investigation will be continued in our laboratory .

ACKNOWLEDGMENT

This work was supported by the Comisi6n Asesora para la Investigaci6n de la Ciencia y la Tecnologia . Proy. No. 915/84. The aid and advice of D . Ashboren are also appreciated .

REFERENCES 1 2 3 4 5

J. Gi ron and D . Hasson, Desalination, 60 (1986) 9 . I.G . Rficz, J. Groot Wassink and R. Klaassen, Desalination, 60 (1986) 213 . A. Chiolle, G . Gianotti, M . Gramondo and G . Parrini, Desalination, 26 (1978) 3 . IC. Eid and G.B. Andeen, Desalination, 47 (1983) 191 . J.M. Arnal and J. Lora, Spanish Patent No . 280084/5 .