Dedication. 20(1977)155-162 0 ElsevierScientificPublishiigCompany,Am&rd~-PPrintedin TheNetherhds

155

AN ALTERNATIVE DESIGN CONCEPT IN REVERSE OSMOSIS DESALINATION

K.W.BGddeker, W.Hilgendorff, J.Kaschemekat

GKSS, 2054 Geesthacht-Tesperhude. Germany

Summary

A highly adaptable plate system for reverse osmosis and ultrafiltra-

tion with easily accessible flat membranes is introduced, employing

a straight-channel construction of plastic components, designed to

tolerate comparatively bold operation conditions at the calculated

expense of membrane service life. pilot installations are illustrated.

Introduction

Conceptually, pressure-driven membrane separations are of almosrr

universal application, defined but by the performance characteristic

of the membranes available. For practical purposes, however, appli-

cation as well as performance are subject to the engineering design

of the process which has to cope with the single most important

limiting cause of membrane separations, viz. fouling. The various

module configurations imply different process concepts in that they

invariably affect the balance between fouling defense and membrane

performance.

In the desalination of natural brackish waters and seawater by reverse

osmosis emphasis is placed on extending the useful life of the mem-

branes while maintaining constant production rates of the individual

prefabricated modules /)I. This is achieved by employing extensive

pretreatment measures in combination with relatively low operating

pressures, to the effect that the membranes are not used to their po-

tential capacity.

The design concept described below represents an attempt to gain flexi-

bility and a less stringent dependence on fouling conditions by de-

emphasizing the relative weight of the membrane service life within the

array of cost-contributing process variables. Conditions for a success-

ful reapportionment along this line are: (a) an open module design with

. accessibly mounted membranes (preferably flat), capable of in-place

156 K.W. BODDEKEB ET AL

assembly and membrane exchange with a minimum of tooling; (b) a con-

struction permitting high pressure operation without a restricting

pressure vessel; (c) a non-corrosive construction of few different

plastic components adaptable to mass production; (d) an unobstructed

feed water passage, possibly in the laminar flow regime, to avoid pre-

cipitation sites and minimize pressure loss.

Description of the system

The reverse osmosis system under consideration comprises a novel plate

configuration, an improved pressure regulator, and. partly for reasons

of commercial unavailability. sheet membranes of cellulosic stock.

The plate system is shown schematically in Fig. 1, while Fig. 2 provides

a perspective view of the prototype version 121. A module consists of

a non-specified number of identical plastic support plates (e.g.,Noryl)

with interspaced porous membrane backing plates (sintered polyethylene)

in vertical alignment between steel flanges, closed resp. opened along

tie rods by manually operating a single external jack-screw. The assem-

bly of plates, wherein each support plate is displaced against the pre-

ceding one by 180°. defines a continuous feed water pathway of uniform

Fig. I. Schematic view of

3 the plate assembly (not to scale).

cross sectional area throughout, requiring two ports at the far ends

regardless of module size (number of plates) while leaving the flow

direction at liberty. The hydraulic diameter alternates regularly be-

tween circular windows in the diametral CU~IS (part;tioned by an essen-

tially staggered arrangement of the membrane backings) and narrow rec-

tangular flow channels inbetween, two such channels lined by two mem-

branes of neighboring backing plates being traversed in parallel after

DESIGN IN SO DESALINATION 157

each turn (cf. Fig. I). The polygonal membrane backings carrying flat

membrane cuts of corresponding shape in loose-leaf fashion are thus

exposed to a countercurrent of feed water of minute pressure difference

j Fig. 2. Perspective view

showing shape and alignment of plates.

each, i.e. they do not have to be pressure supporting. Sealing is pro-

vided by O-rings which symmetrically frame the membrane backings along

their circumference, thereby defining the effective membrane area (which

includes the tapered turning partitions). Since neither backing plates

nor membranes are impaired by perforations or additional seals, they do

not require particular attention on installation or membrane replace-

ment. The permeate entering the porous backings from both sides drains

off their open rim and is collected in a trough underneath the module.

The prototype specifications of the plate system are as follows.

Cross section of feed flow passage: 200 mm2

Hydraulic diameter, in circular turn: 1.6 cm in rectangular channel: 0.1 cm

Cross section of rectangular channel: 190 x 0.5 rml

Effective membrane area per backing (2 membranes): 600 cm2

Thickness of backing plate: 3mm

Length of stack per m2 of membrane area: 20 cm

Typical module size for pilot plants, membrane area: 5 m* no. of plates: 83

Packing density. based on plate dimensions: 120 m2/m3 of complete 5 m2 module: ca. 60 m2/m3

Weight of a complete 5 m2 module: ca. 200 kg

Current maximum operating pressure: 90 bar

A complete module includes steel tie rods and steel end flanges, the

dimensions of which depend on the assigned operating pressure_ It is

158 K.W. BODDEKER ET AL

anticipated chat the allowable operating pressure will be raised as the

injection molding of the plates develops.

The pressure drop across a 5 m2 module as a function of feed flow rate

at a pressure level of 60 bar is shown in the following graph (Fig. 3)

along with the feed flow dependence of the flux (1.7 X NaCl = Baltic

Sea) which indicates a minimum inlet feed flow rate of SC0 l/h under

these conditions.

QP I/h

AP bar

Fig.

7- 6- 5- 4- 3- 2- 1

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3. Pressure drop and flux across a 5 m2 plate module as function of inlet feed flow rate at 60 bar.

The pressure regulator used with the present module system is depicted

in Fig. 4 137. Employing a floating piston principle with deliberately

weak balancing compression coil, it is capable of adjusting the system

pressure unattended to a preset value over a wide range of feed flow

rates (e.g., at unstable energy supply). The system pressure may be re-

leased intermittendly without affecting the initial pressure setting

by retracting the valve needle (thereby raisin8 the piston up to a

stop) at a fixed adjustment of the coil support.

The membranes, according to the design concept, are considered part of

the running expenditures to be used by weighing the gain of bold process

conditions against the constraint of flux decline_ As such they should

be inexpensive but need not be of utmost durability_ In principle, any

membrane may be used as long as it is available in sheet form.

DESIGN IN RO DESALINATION 159

Fig. 4. Design principle of pressure regulating valve.

RO system

brtne cd

The membranes under investigation for desalting purposes are recent va-

rieties of cellulose acetate base, adapted to mechanical production on

steel belts. They include ammonia-modified Loeb-type membranes for mode-

rate pressure applications 141, brackish water membranes with improved

flux performance obtained by incorporation of partially gelled hydro- ,

philic bentonites 151. and an as yet unpublished anisotropic seawater

membrane in which the desirable effects of high acetylation are de-

veloped by judicious solvent-nonsolvent manipulation.

Pilot installations

The plate design, being variable with respect to module size. membrane

specification, and, to a considerable degree, operating pressure is

applicable to any conceivable reverse osmosis and ultrafiltration task

at the user5 disposition. The pilot plants are part of a continuing

program to explore modes of operation under actual production condi-

tions, study scale-up conditions and, ultimately, establish costs.

With accessibly mounted flat membranes the plate module is a test in-

strument in itself, both for membranes and candidate raw water samples.

This is illustrated in Fig. 5 by a laboratory size module containing

an effective membrane area of 2 m* which , in a current test series,

reduces a 0.1 X MgS04 solution at 20 bar, 20° C from 1100 to 6 @/cm

at a yield of 1600 L/d.

160 K.W. BODDEKER ET Ah

A complete seawater test facility for modules and membranes exists on

board the nuclear research vessel NS Otto Kahn /6/, operating on natu-

ral seawater as encountered along the routes of the ship. but fitted

also for closed-loop operation as indicated in the flow diagram of

Fig- 6. Through an extensive testing routine with model plate

arrangements, using cartridge filtration as the only pretreatment, the

facility has aided in the development of the present system. The exper-

iments suggest that shipboard seawater desalinatian by reverse osmosis

may under certain conditions be feasible.

Fig. 6. Flow diagram of seawater test facility on board NS Otto Hahn,

A self-contained high pressure reverse osmosis unit with a total of

10 m2 of effective membrane area in two parallel plate modules, served

by a three-piston pump of 1800 Ifh capacity, is shown in Fig. 7. It is

used at various desalination sites in preparation for a mobile con-

tainer plant.

DESIGN IN RO DESAl.TNATION 161

An experimental plant coverLng most aspects of applied membrane desa-

lination on a reference scale is under construction on the Baltic Sea

(17000 ppm TDS). Fig. 9 gives the lay-out of the plant, vhich is to be

augmented by an electrodialysis unit later. The 24 modules of the re-

verse osmosis installation, amounting to a membrane area of 120 m2,

are shelved individually on 3 mobile racks depicted in Fig. 8. Keeping

two modules at redundance, the rated capacity at an operating pressure 3

of 60 bar is 60 m /d. The station is equipped with a dual intake system

of 20 m3/h (above and below the sea floor), and a bypass grid Co study

corrosion and barnacle growth. Two high pressure feed water pumps with

10 m3/h capacity each are selected to enable a comparison of displace-

ment pump vs. centrifugal pump. A third pump with integral hydraulic

energy recovery will be added when completed. Pretreatment so far con-

sists of mechanical filtration (fabric with a provision for back flush-

ing,_ followed by cartridge filters) and voluntary acidification.

Pig. a. Module arrangement for Baltic Sea desalting plant.

162 K.W. BODDEKER ET AL

Fig. 9, Lay-out and flow diagram of the experimental RO plant on the Baltic Sea

References

/I/ K.C.Channsbasappa, Desalination 13 <t9?5) 31. -

[2/ K.W.Bdddeker, W.Hilgendorff, J.Kaschemekat, Chem. Ing. Techn.

48 (1976) 641. -

131 H,Tianu, GKSS TechnicaL Contribution, 1976.

141 K.W.BBddeker, J_K%sehemekat. H.Wofdmann, Proc. 4th fne. Symp.

Fresh Water from the Sea 5 (1973) 65.

151 K.W,BGddeker, J.Kaschemekat, M.Willamowski, 169th ACS Meeting,

Philadelphia, 1975.

161 K.W.B6ddeker, W.Hilgendorff, S.Kaschemekat.

Desalination, in press.