Ind. Eng. Chem. Res. 1994,33.1645-1851
1845
Net-Type Spacers: Effect of Configuration on Fluid Flow Path and Ultrafiltration Flux Andre R Da Costat and Anthony G. Fane' UNESCO Centre for Membrane Science and Technology and School of Chemical En&eeriw and Industrial Chemistry, University of New South Wales, Sydney, NSW, Australia 2053 Ultrafiltration of dextran T-300and T-500solutions waa performed in a flat channel fitted with various net-type spacers to examine the effect of spacer configuration on the fluid flow path and fluxandtoidentifyoptimalspacercodigurations.Itwaa found thatthesizeandlocationoffilaments positioned transverse to flow is as important to flux enhancement as angle of flow diversion induced by the spacers. Furthermore, it has been established that spacers which induce no directional flow changes (0 = Oo) and with transverse filaments perpendicular to flow, adjacent to the membrane, perform better than spacers characterized by zigzag flow path (with the comparison based on the same voidage). COWcIrnTE
Introduction
Successful application of membrane technologyrequirea not only high-performance membranes, but also modules which provide good fluid management a t the membrane surface. Fluid management determines performance because it induces mass transport, controls concentration polarization, and controls fouling. The two most popular module concepts for ultrafiltration (UF) and reverse osmosis (RO)are the spiral-wound and the hollow fiber modules. This paper is concernsd with aspects of the spiral-wound module. Net-type spacers are a key feature in the spiral-wound module (depicted in Figure 1). The spacers have the dual function of keeping adjacent membranes apart to form a feed channel and of promoting turbulence, i.e., facilitating themixingbetweenthebulkoffluidandthefluidadjacent to the membrane surface thus reducing the thickness of the boundary layer and increasing local shear and enhancing flux. In previous work we have deseribed the effect of spacer configuration and linked its orientation and geometric characteristics to UF performance (Da Costa et al., 1991, 1993, 1994). Other authors Glatzel and Tomaz, 1966; Hicks, 1967;Farkova, 1991;etc.) who studied the effect of net-type spacers on pressure drop and heat and mass transfer phenomena also observed that changing the orientation of the spacer in the channel caused a marked effect on the results. However, they did not explain or predict this higher performance in terms of the spacer geometry. On the basis of flow visualization to determine the flow path in a spacer-filled channel, we introduced (Da Costa et al, 1991) an important spacer charaderistic-the hydrodynamicangle B (see Figure 3c),which describes the change in direction of the fluid as it flows in the channel (Lagrangian approach). Correlations were developed that allow the characterization and design of net-type spacers in any combination of the following geometric characteristics: angle, mesh size, thickness, filament size and voidage. Mass transfer and pressure drop models that account for spacer hydrodynamic angle and voidage were also proposed (Da Costa et al., 1994). Another factor that is important to m w transfer is the size and locationof filamentspositionedtransverseto flow, and this is the subject of this paper. Flow past transverse 'Present addrese: I n c h Ltd.,P.O.Box SO, Meyfield NSW 2304. Australia.
08885885/94/2633-1&15%04.50/0
7
Y
L PEIYUTE
Figure 1. The spiral-wound module.
fiiaments generatea wakes that caw local disturbanees of the concentration boundary layer affecting local transfer rates. Thomas (1965)and Watson and Thomas (1967) showed that mass transfer is substantially increased directly beneath cylinders placed within the boundary layerandthatafurtherincreaseisohtainedbythepresence of a second cylinder. Paul Feron (1991)showed that rods attached to the transfer surface perform better than centered orsuspendedrods. These latter rodsgavesimilar mass transfer rates to the attached ones only when rods were very close to the wall (1 mm) and the inter-rod distance was the smallest considered (6 mm).
Experimental Seetion Ultrafiltration experiments were performed in a tlat channel test cell with channel dimensions of 25 mm wide and 285 mm long. A photograph of the cell is shown in Figure 2. It has a stainless steel base plate housing the membrane support, with four outlet ports for permeate. The body of the cell was machined from perspex (polymerized methy1methacrylate)togivetaperedinletandoutlet regions, and to define the channel width. The perspex section was sandwiched between the base plate and a stainless steel cover plate with a viewing slot. Silicon rubber tubes were attached to the outlet permeate ports located on the stainless steel base plate. Within the base plate was a porous steel plate supporting the membrane against the pressure differential. Its area was equal to the active membrane area. Between the edge of the base plate and the support plate was an o-ring to seal the cell and avoid leakage. Within the perspex body the spacer can be seen fitted in the grooved channel. To avoid bypassing (channeling)at the edges, the spacers were cut accurately to fit in the channel. A good check for channeling was provided by flow visualization with dye. The channel was incorporated in a flow rig which has been fully described elsewhere (Da Costa et al., 1991). 0 1994 American Chemical Society
1846 Ind. Eng. Chem.
Res., Vol. 33, No. 7,1994
Figure 2. Spacer-filledchannel.
The membrane used was a HFK-131 polysulfone membranewithanominalmolecularweightcutoffof 5000 Da, supplied by Koch Membranes Inc., and several commercially available netlike feed spacers of the vexar type were used. Extra configurations were obtained by changing the orientationof thespacer in the channel. Some of the spacers used in this work are depicted in Figure 3a, and Table 1summarizes the configurationof the spacers. The spacers are made of polypropylene or polyethylene, and some samples were supplied by Nalle Plastics Inc. and Conwed Plastics. An electronic digital caliper (NSK
MAX-CAL) was used to measure the spacer dimensions. The measured and calculated geometric characteristics of the spacers are shown in Table 2. After the spacer was inserted in the channel and after the membrane was placed on the porous plate, the perspex block and the stainless steel plates were held together by tightening 16 screws with a torque wrench at 20 N m. In the case of spacers thinner than the channel, aluminum plateswereplaced betweenthechannelroofand thespacer tofill thechannel. Thegapsbetween thealuminum plates and the channel side walls were filled with silicon rubber to eliminate channeling of fluid. Before each run the membrane was cleaned to remove glycerol from the pores by simply soaking it in a water/ ethanolmixture for about 20 min. Experimentalsolutions were made up using dextran T-300 and T-500 (Sigma Co.) and Milli-Q water, to give concentrations in the range of 1-10 kg/m3. In a typical run about 8L of dextran solution was circulated at the required flow rate and pressure for 5-10 min to allow stabilization of temperature at 25 "C. A series of experiments consisted of measuring permeate flux and pressure drop across the spacer-filled channel in duplicate for a particular spacer at various flow rates and desired transmembrane pressure. Flow rates ranged from 0.4 to 3.0L/min (equivalentRe = 23&1661 in the unfilled channel), and pressure was 300 Wa. A Shimadzu Libror electronic balance with attached EP-50 printer and timer were used to record permeate flux, and a new membrane was used for each spacer tested.
FiKure 3. (a. top) Feed spacen, (from left to right: CONWED-1, CONWED-2, NALTEX-124. NALTEX-51-1,NALTEX-51-2, UFI. UF2, UF3. and UF4). (b,bottom left) Definition of dn, de. I,,, and 1.9. (e, bottom right) Definition of hydrodynamic angle 8.
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1847 Table 1. Summary of Spacer Configurations. angle configuration (deg) symmetry CONWED-1 90 rhomboid mesh: filaments are 45O to channel axis CONWED-2 0 no change of d&ection;filaments perpendicular to each other NALTEX-56 56 rhomboid mesh; filaments are 28' to channel axis NALTEX-124 124 filaments are 62O to channel axis UF1 0 parallelogram mesh; thick filaments are parallel to channel axis 0 parallelogram mesh; thick fiiaments are 40° to channel axis UF2 135 parallelogram mesh; thick filments are 60° to channel axis UF3 UF4 45 parallelogram mesh; thick filamenta are 20° to channel axis NALTEX-51-1 51 parallelogram mesh; thick f i i e n t s are 30° to channel axis; thin filaments are 22O to channel axis NALTEX-51-2 0 no change of direction; thick filamenta are 51' to channel axis; thin filaments are parallel to channel axis NALTEX-51-3 0 no change of direction; thin f i i e n t a are 51O to channel axis; thick filaments are parallel to channel asis NALTEX-129 129 thick fiiaments are 52O to channel axis; thin f i i e n t s are 77O to channel axis UF Spacer Configurations UF1 spacer with thin filaments adjacent to membrane UF1-N 0 0 UF1 spacer with thick filaments adjacent to membrane UF1-CH forming parallel fluid channels on its surface 0 UF2 spacer with thin filaments adjacent to membrane UF2-CH forming parallel fluid channels on ita surface 135 UF3 spacer with thick filaments adjacent to membrane UF3-K 135 UF3 spacer with thin filaments adjacent to membrane UF3-N
comments Figure 3a suDDlied bv Conwed Plasticsb 1 C6NWEDL1 spacer rotated by 45Ob 2 supplied by Nalle Plastics, Inc. NALTEX-56 spacer rotated by 9 0 O 3 UF3 spacer rotated by 60° 6 UF3 spacer rotated by 100' 7 8 UF3 spacer rotated by 80° 9 supplied by Nalle Plastics, Inc. 4 NALTEX-51-1 spacer rotated by 160' NALTEXdl-1 spacer rotated by 30' NALTEX-61-1 spacer rotated by 82O
5
obtained by turning spacer UF1-N upside down
obtained by turning spacer UF3-K upside down
0 Unless otherwise mentioned, all symmetric spacers (i.e., with thin and thick filamenta) were wed with the thick filaments adjacent to the membrane. Part no. XN 4510.
Table 2. Geometric Characteristics of Spacers Used CONWED-1 (2)' NALTEX-56 (124)' NALTEX-51-1 (129)' NALTEX-51-2 (51-3)' UF-3 (4)'
2.01 1.11
1.17 1.17 1.68
1.03 0.55 0.5 (0.7)f 0.6 (0.711 0.76 (1.07)f
2.17 4.3 2.89 (5.37)s 2.89 (5.37)s 4.06 (5.318
0.618 0.880
3883 7273 6827 6827 4344
0.846 0.846
0.763
0.997 1.316 1.226 1.226 1.375
90 (0) 56 (124) 51 (129) 0 (0) 135 (45)
*
Mesh length is defined in Figure 3b. Voidage defiied by Da Costa et al. (1993b): t = 1- [(*(dn21m2+ d&m1))/(41&mlh~p sin e)]. Specific surface of the spacer defined by Schock and Miquel(1987): Svsp = 4/df. Hydraulic diameter defined by Schock and Miquel(l987): d h = 4c/[(2/hsp) + (1- c)Svsp. e Means CONWED-2 and NALTEX-51-3 have angle of Oo, NALTEX-124 has an angle of 124O, NALTEX-129 has angle of 129O, UF4 has angle of 45O. f Thin and thick filament sizes. 8 Short and long mesh sizes. 0
The concentrations of dextran solutions were measured by UV spectrophotometry at a wavelength of 490 nm according to procedure of Dubois et al. (1956).
*0°
I THIN CHAN-G
Results and Discussion 1. Effect of Spacer Configuration on Flux. 1.1. Hydrodynamic Angle. If the flux values for a feed rate of 3 L/min and for spacers with similar voidage are plotted (Figure 4)against hydrodynamic angle (where the 0 (no change of direction)), then, SLIT has an angle of ' as previously reported (Da Costa et al., 19941,a maximal flux occurs at about 90'. Very similarly shaped curves are obtained at lower flow rates. Since the size of f i i e n t a and velocities in the channel are approximately the same, it can be concluded that this is a genuine effect of angle. The reason for the maximum a t 90' can be explained by the proposed mass transfer mechanism (Da Costa et al., 1994), namely developing velocity and concentration profiles for laminar flow with the entrance region equal to half the mesh size. The entrance region reaches a minimum at 90°, causing mass transfer to be maximized (Da Costa et al., 1994). This is a good illustration of the effect of hydrodynamic angle and elucidates the mass transfer mechanisms in zigzag flow spacer-filled channels. 1.2. Transverse to Flow Filaments. Another category of spacers that differ from the zigzag flow does not induce directional flow changes. To compare the performance of these spacers, which are characterized by straight
c
THICK CHAN-
d.846
O
!
0
.
I
20
.
,
40
.
,
60
.
,
80
.
I
100
.
I
120
.
1 0
ANGLE
Figure 4. Flux versus hydrodynamic angle (AP= 300 kPa, C = 10 kg/m3 dextran T-500, Q = 3 L/min).
flow past transverse filaments, with those characterized by changes in flow direction, a CONWED spacer was used in different orientations. The CONWED spacer consisted of superimposed f i i e n t a crossed a t 90'. In confiiation 1 (CONWED-l), one of the mesh diagonals was parallel to channel axis, forming a rhombus (diamond), and fluid was forced to change direction by 90° (see Figure 3a). Fluid flows in zigzag fashion past an alternate row of suspended and attached filaments. In configuration 2 (CONWED-2 (seeFigure 3a)) a set of filamentawas parallel to the channel
1948 Ind. Eng. Chem. Res.,Vol. 33, No. 7,1994
300
1 SPACERS
200
S
CONWEDBCH
N
E
1
CONWED1
2
2
U
100
0 0.6
1
1.6
2
2.6
3
FLOW RATE, llmin
Figure 6. Effect of transverse filament location and angle to channel axis on flux (Conwed spacers, AP = 300 P a . C = 6.6 ks/ma dextran T-300). 300
SPACERS UF3-k S
200
UF3-N
N
E >
UF4
X‘
UFl-CH
3 -I
U
100
0
UF1-N
H
UF2-CH UF2-k
0 1
llmin
3 llmin
FLOW RATE
Figure 8. Effect of transverse filament size and location on flux (UF spacers, AP = 300 P a . C = 1k g / d dextran T-500).
axis and the transverse filaments were on the membrane surface. This is similar to the cavity-type spacer used by KangandChang (1982) andtoFeron’sattachedrodswith the exception that in the present case the upper flow is divided in parallel channels. The third configuration (CONWED-2CH)was the inverse of configuration 2; the parallel channels were facing the membrane and the transverse filaments were in contact with the roof (not a transfer surface). This is similar to Thomas’ and Feron’s suspended cylinders. The results are depicted in Figure 5 for flow rates between 0.6 and 3 L/min. Configuration CONWED-2CH gives fluxes slightly higher (up to 10%)than CONWED-1 despite having all filaments suspended as opposed to the alternating suspended-attached filaments in the later configuration. However it was mentioned earlier that results from the literature show higher rates for attached cylinders. This discrepancy can be explained by the fact that the frontal velocity to the filaments is smaller in configuration 1and this produces proportionately smaller wakes and local mass transfer. In configuration 1 the filaments are 45’ to the channel axis and up= u, sin 0/2 = u, sin 90/2 = 0 . 7 0 7 ~(where ~ up and usare the velocity
components perpendicular (frontal) to the filaments and parallel to the x-axis, respectively (Da Costa et al., 1994)). In comparison for configuration 2CH the filaments are perpendicular to the channel axis and up= u, sin 90 = u,. These higher local velocities generate wakes big enough to affect mass transfer on the opposite wall, beneath the filamentstothesamelevelasspacer CONWED-I in which the efficiency of attached filaments in breaking up the boundary layer is reduced by lower local velocities and alternate suspended filaments. Configuration 2 (CONWED-2) gives 66% higher flux than configuration 1 at 0.6 L/min and 40% higher at 3 L/min. This is expected because the filaments are perpendicular to the flow direction and hence maximum local velocities are achieved, and because the filaments are in contact with the membrane, the spacer is more effective in breaking up the boundary layer. 1.3. Filament Size and Angle of Transverse Filaments to Flow Direction. Figure 6 shows the effect of filament size and angle of transverse filaments to channel axis in the example of UF spacers at 1and 3 L/min. The UF spacers are asymmetric consisting of parallelogram meshes and thicker filaments overlaying thinner ones (see
Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 1849 Figure 3a,b). In configuration UF3 the thicker filaments are 60' to channel axis and the mesh is oriented with the longer diagonal perpendicular to flow. Fluid moves in a zigzag changing direction by 135'. Configuration UF4 was obtained by rotating UF3 by 80' and fluid is forced to zigzag by 45'. Configuration UF1 is derived from UF3 rotated by 60'. Thicker filaments are parallel to the channel axis, and thinner filaments are transverse at 40'. Most of the fluid flows parallel to the x-axis (or channel axis); a small proportion might be diverted especially at low velocities. Finally, if UF3 is rotated by looo, configuration UF2 is obtained with the thinner filaments parallel to the channel axis and thicker filaments transverse at 45'. When UF3 is placed in the channel with the thicker filamentson the membrane (UF3-K,Table 11,flux is 6-9 % higher than when thinner filaments are on the membrane (UF3-N) at the same local velocity up = ux sin 135/2 = 0 . 9 2 4 ~because ~ bigger wakes are formed with thicker filaments. Spacer UF4 gives 18%less flux at 1L/min and 5 % less at 3 L/min than spacer UF3-K. This is due to lower local velocities (frontal) at the filaments (up= uxsin 45/2 = 0 . 3 8 3 ~ ~and ) in addition to lower local velocities between parallel filaments (or velocity vector transverse to the filaments at the same angle as the hydrodynamic angle), ug = ux/cos 45/2 = 1.08uxcompared with ug = u,/ cos 135/2 = 2 . 6 1 ~ for~ spacer UF3-K. At high flow rates the differences in flux are small because a limit is reached to the rate of increase in mass transfer and the greater part of the kinetic losses does not contribute to enhancement. However, the differencein kinetic losses is enormous with pressure drops differing by up to a factor of 10 (Da Costa et al., 1994). Configuration UF1-N, with the thinner filaments transverse to flow at 40' on the membrane, gives 23-30% higher fluxes than when the thicker filaments parallel to flow are on the membrane forming a series of parallel channels. In this latter case because the transverse filaments in contact with the channel roof (not a transfer surface) are thin, the wakes formed are not big enough to affect the opposite wall (membrane). A similar comparison applies between UF2-K and UF2-CH. However in this case the transverse filaments are the thick ones so the wakes affect more substantially the opposite wall and channeling reduces flux only by 10% at high flow rates and 20% at low rates. The effect of filament size is such that even in the presence of channeling on the membrane surface (suspended thick filaments, UF2-CH) flux is 8% higher at 3 L/min than when thin filaments are attached to the membrane (UF1N). When thick filaments are attached (UF2-K) flux is always higher, except if compared with UF3 and UF4 spacers,which give 10and 5.5 % higher fluxes, respectively. This is because ug = ux (no change in direction) and up = uxsin 45 = 0.707,, whereas for UF4 ug = 1.08uxand for UF3 ug = 2 . 6 1 ~ ~ . It should be noted that although the above trends were reproduciblethe absolute values have a likely experimental error of about h5 5%. In spiral-wound membrane elements channels are limited by membranes on both sides. Therefore, results obtained for symmetric spacers are the same for both surfaces, whereas for asymmetric ones flux will be given by an average. It should be noted that although the experiments reported in this paper were performed with a membrane at the bottom of the channel, similar performance would be expected for a membrane located above the channel. For example in a recent study of
Table 3. Effect of Angle of Transverse Filamenta to Flow Direction on Flux Cdt 1 nun) N
spacer name
angle of filamenta to flow
up
UF4
22.5
0.383~~
CONWED-1
45
0.707~~
CONWED-2
90
ux
uF3
67.5
0.924~~
flux (L/(m*h)) 43.W 95.40 68.4 163.8 114.0 228.6 76.8 165.6
a Values for low and high flow rates (0.6 and 3.0 L/min) (applies to all data).
membrane orientation (Youm et al., 1994)using the same simulator no effect of orientation on ultrafiltration of proteins has been observed provided the channel is spacerfilled and the flow rate exceeds about 0.3 L/min. The outcome of this analysis indicates that thicker filaments have a high impact on mass transfer at the wall they are attached to and are more likely to promote mass transfer on the opposite wall than thinner filaments. Also, as shown in Table 3, the maximum effect occurs when the transverse filaments are perpendicular to flow. This configuration proved more effective than the case that involves changes in direction, because it generates a bigger wake. For instance, the average flux for configurations CONWED-2 and CONWED-PCH is 94.5 L/(m2 h) at 0.6 L/min and 200 L/(M2h) at 3 L/min whereas CONWED-1 (same flux for both walls) gives respectively 68.4 and 163.8 L/(m2 h). In the next section it will be shown which of these configurations is more economical. 2. Spacer Optimization. We have shown (Da Costa et ai., 1994)that flux enhancementwith spacers is achieved at the expense of high pressure losses. Clearly, there is a trade-off between flux improvement and energy input. Does the gain outweight the losses? The real test is to solve the optimization problem which compares total processing costs for the different spacers, including equipment costs (inversely proportional to flux) and operating (energy) costs (proportional to pressure loss). The treatment cost assumes a recovery - bermeatelfeed rate) of 10% and is given by processing cost ($/m3)= [capital cost ($/m3)]+ [operating cost ($/m3)1 where capital cost = [area (m2/(m3h-'))I X [(equipment + membrane) cost ($/m2)l X [capital factor (yr-')1/8000 (h/yr) operating cost = [energy (kW h/m3)l [cost ($/kW h)] with (equipment + membranes)cost = 1250 ($/m2) energy cost = 0.15 ($/kW h) capital factor (for amortization, interest, maintenance) = 0.4 (yr-')
1850 Ind. Eng. Chem. Res., Vol. 33, No. 7, 1994 ...
1.0-
O WED-1
-
0.9
COMNEDXH
A
-
0.8
0.6
-
0.5
-
COWED-2
0.7
0.4
0
1
I
I
1
2
3
I 4
FLOW RATE, llmin
Figure 7. Effect of transverse filament location and angle to channel axis on costa (Conwed spacers, AP = 300 P a , C = 5.5 kg/ms dextran
T-300).
energy = 10“ APQI(3.WA,)
(4)
The assumed equipment and membrane costs and capital factor are equivalent to $1000/m2for installed equipment and $200/m2 for replacement membranes (with a 2-year life). In other words “annual costs” for equipment and membranes (per m2) equal 0.4 X 1250 = 0.4 X 1000 (equipment) + [200/23 (membranes). The cost data are within the range quoted by Kulkarni et al. (19921, Le., “installed capital costs are generally in the range of $600 to $1200/m2ofmembrane area. ...the costs for membranes in the spiral-wound form usually lie between $100 to $2001 m2 ...n. For each spacer the calculation is based on experimental fluxes (section 1)and pressure losses reported elsewhere (Da Costa et al., 1994). 2.1. Effect of Transverse to Flow Filaments. The effect of transverse filament location and angle to channel axis on costs is shown in Figure 7. Configuration CONWED-PCH, which generates flow channeling on the membrane surface, gives costs similar to CONWED-1, which induces a zigzag flow path with alternate attached and suspended filaments. This indicates that the wakes due to suspended transverse filaments at 90° to channel axis (CONWED-2CH)generate higher mass transfer rates per unit pressure loss than frictional losses due to changes in flow direction (CONWED-1). Configuration CONWED-2 (the inverse of CONWED2CH) with attached transverse filaments gives superior performance. Since in spiral-wound modules feed channels are bound by membranes on both sides, the costs to consider are the average between those given by CONWED-2 and CONWED-2CH. This average is 20% smaller than for CONWED-1 at the optimal velocity (minimum costs) of 0.86 m/s (Q = 1.5 L/min), thus indicating that spacers characterized by no directional changes and which have attached transverse filaments at 90° to channel axis are economically best. 2.2. Effect of Transverse Filament Size. Figure 8 shows the effect of transverse filament size. Both NALTEX-51-2 and -51-3generate channeling on the membrane surface, but have different transverse filament size a t 51O to the channel axis. The results show that suspended
I
I.-
1.4
0
NALTEX-51-1
1.21.0-
-
0.8 0.6
-
0.4
0
1
2
3
FLOW RATE, I/min
Figure 8. Effect of transverse filament size and location on costa (Naltex spacers, AP = 300 @a, C = 10 kg/ma dextran T-500).
thicker filaments (NALTEX-51-2) give lower costs than the other configurations up to the optimal velocity of 0.67 m/s ( Q = 1 L/min). A t higher flow rates spacer NALTEX-51-1, which induces a zigzag flow path, performs better. This and the smaller differences in costs at low rates can be explained by the fact that the channel is only 1.1 mm thick and frictional losses due to changes in flow direction have a higher contribution to mass transfer than in the 2-mmthick channel (CONWED spacers). 2.3. Effect of Flow Rate on Ultrafiltration Processing Costs. Increasing flow rate causes energy costs to rise due to increase in pressure losses. Membrane costs decrease due to enhancement in flux. Typical of optimization problems, the optimum is determined by the sum of both costs. Figure 9 shows that for a 1.1-mm-thick channel the optimum for most of the spacers considered occurs at about 1 L/min (u = 0.68 m/s) and differs only by up to 3 7%. This indicates that in thinner channels there is greater disturbance of the boundary layer and high mass transfer rates can be achieved at lower velocities. For this reason spacer configuration has reduced effect at the optimal flow rate. It is worth noting that typical spiralwound elements for ultrafiltration have an upper limited
Ind. Eng. Chem. Res., Vol. 33, No. 7,1994 1851 ."
Koch Membrane Systems, Inc., Nalle Plastics, Inc., and Conwed Plastics for providing spacer samples.
1I Q
1.4 1.6
PI
E
1.2-
H
NALTEX-129
0
NALTEX-124
0
NALTEX-56
Nomenclature
2 1.0-
0.6 0.8
".- ,
I
0
1
;
d
FLOW RATE, llmin
Figure 9. Processing costa versus flow rate for various spacers (C = 10 kg/ma dextran T-500, h = 1.1 mm).
for crossflow velocity of about 0.6 m/s (i.e., 35 gpm feed to a 4-in.-diameter element, which has about 50 % available cross section for feed channel flow). This upper limit is largely dictated by the maximum allowable pressure losses across the element to avoid telescoping. At high flow rates, the high angles of flow diversion (124O and 129') cause a sharp increase in costs.
Conclusions It has been shown that spacers which do not induce directional flow changes but rather have adjacent to the membrane transverse filaments a t 90° to flow direction give about 50% higher fluxes than when flow changes direction by 90°. In the former configuration, fluid is forced to jump attached filaments in a straight (parallel to channel axis) flow path. The frontal velocity to the filaments, which is proportional to the wake formed and local mass transfer, reaches its maximum value in this configuration, making it more effective in disturbing the boundary layer. Filament size may also be important. For instance, flux increased by up to 9% by changing the size of filaments in contact with the membrane from df = 0.76 mm to df= 1.07 mm (spacer UF3). Channeling on the membrane (the other set of filaments is transverse to flow at an angle and is in contact with the channel roof) reduces flux by 1020% (spacer UF2). From this study, coupled with our earlier work (Da Costa et al., 1991), we conclude that mass transfer in spacerfilled channels is based on two mechanisms. One is due to viscous friction generated by the mixing of fluid streams crossing each other at an angle, and the other is due to viscous friction generated by the wakes of fluid formed past transverse filaments. Under similar conditions the second mechanism appears to be dominant. A comparison of processing costs reveals that spacers which induce no directional flow changes and with transverse filaments perpendicular to flow, close to the membrane, are also economically better than spacers characterized by zigzag flow paths.
Acknowledgment Financial support from The Australian Research Council through the UNESCO Centre for Membrane Science and Technology and The Australian Postgraduate Research Awards Scheme is acknowledged. The authors also thank
A, = membrane area (m2) dfl = diameter of thinner filaments (m) de = diameter of thicker filaments (m) d h = hydraulic diameter (m) hsp = spacer thickness (m) J = permeate flux (m/s) 11, = shorter mesh size (m) L2= longer mesh size (m) AP = pressure drop along the channel (Pa) Q = feed flow rate (m3/s) Svsp = specific surface of the spacer (m-9 up = velocity normal to a spacer filament (m/s) u, = average velocity in the x-direction (m/s) ug = velocity parallel to opposite filaments in a mesh (m/s) Greek Symbols e = voidage 0 = hydrodynamic angle (deg)
Literature Cited Da Costa, A. R.; Fane, A. G.; Fell, C. J. D.; Franken, A. C. M. Optimal Channel Spacer Design for ultrafiltration. J. Membr. Sci. 1991, 62, 275-291. Da Costa, A. R.; Fane, A. G.; Wiley, D. E. Ultrafiltration of whey protein solutions in spacer-fiied channels. J. Membr. Sci. 1999, 76,245-254.
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Received for reuiew December 3, 1993 Accepted April 6,1994. Abstract publishedin Advance ACSAbstracts, May 16,1994.
