journal of MEMBRANE SCIENCE ELSEVIER Journal of Membrane Science 137 (1997) 17-29 Spiral wound, hollow fiber membrane modules: A new approach to higher mass transfer efficiency Mark L. Crowder, Charles H. Gooding* Department of Chemical Engineering. Clemson Uniw, rsity, Clemson, SC 29634-0909. USA Received 31 March 1997: received in revised fnrm 8 July 1997: accepted 8 July 1997 Abstract A new type of transverse flow, hollow fiber module was evaluated for membrane separation. First, small sections of fabric were woven using silicone rubber hollow fiber membranes and monofilament nylon. Water deoxygenation experiments were conducted on each fabric in a flat cell, yielding mass transfer coefficient (k) values. An optimal fabric construction was identified based on k values exceeding 0.01 cm/s at moderate velocities, low pressure drop, and high membrane packing density. Spiral wound prototype modules were made and tested, each with a fabric wrapped in layers around a central permeate tube. For 02 removal from water, k values for the prototypes were slightly below those observed in the flat cell tests. Wrapping the fabric tightly around the permeate tube was less effective than looser wrappings. Pervaporation experiments with trichloroethylene were attempted, but the performance was reduced, apparently due to leaching of petroleum .jelly used in module construction and redeposition on the membrane. The k values observed for oxygen transfer were at least 20% higher than those achieved with traditional spiral wound modules, and the membrane packing densities achieved in the prototypes were 300 to 400% higher. This module design could prove to be practical and advantageous for membrane separation processes in which the mass transfer coefficient on the feed side of the membrane limits flux. ~{') 1997 Elsevier Science B.V. Kevwords. Concentration polarization; Fiber membranes; Modules 1. Introduction itself. A prime example of this is the removal of dilute amounts of sparingly soluble volatile organic com- As membrane science develops, better membrane pounds (VOCs) from water by pervaporation. This materials are being formulated to accomplish a wide particular separation is useful for the treatment of variety of separations. In some cases, currently avail- groundwater and industrial wastewater [1-3], and able membranes are so effective that the separation is for the recovery of flavors, aromas, and other organics limited by the rate of mass transfer to the feed- from process streams [4]. If the VOC has a relatively membrane interface rather than through the membrane high Henry's law constant, pervaporation through a thin film of silicone rubber (poly(dimethylsiloxane) or *Corresponding author. Fax: +1 864 656 0784: e-mail: PDMS) yields high selectivities that are limited by [email protected], boundary layer effects rather than the membrane 0376-7388/97/$17.00 1997 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00 1 74-9
Transcript
journal of MEMBRANE
SCIENCE ELSEVIER Journal of Membrane Science 137 (1997) 17-29
Spiral wound, hollow fiber membrane modules: A new approach to higher mass transfer efficiency
Mark L. Crowder, Charles H. Gooding*
Department of Chemical Engineering. Clemson Uniw, rsity, Clemson, SC 29634-0909. USA
Received 31 March 1997: received in revised fnrm 8 July 1997: accepted 8 July 1997
Abstract
A new type of transverse flow, hollow fiber module was evaluated for membrane separation. First, small sections of fabric were woven using silicone rubber hollow fiber membranes and monofilament nylon. Water deoxygenation experiments were conducted on each fabric in a flat cell, yielding mass transfer coefficient (k) values. An optimal fabric construction was identified based on k values exceeding 0.01 cm/s at moderate velocities, low pressure drop, and high membrane packing density.
Spiral wound prototype modules were made and tested, each with a fabric wrapped in layers around a central permeate tube. For 02 removal from water, k values for the prototypes were slightly below those observed in the flat cell tests. Wrapping the fabric tightly around the permeate tube was less effective than looser wrappings. Pervaporation experiments with trichloroethylene were attempted, but the performance was reduced, apparently due to leaching of petroleum .jelly used in module construction and redeposition on the membrane.
The k values observed for oxygen transfer were at least 20% higher than those achieved with traditional spiral wound modules, and the membrane packing densities achieved in the prototypes were 300 to 400% higher. This module design could prove to be practical and advantageous for membrane separation processes in which the mass transfer coefficient on the feed side of the membrane limits flux. ~{') 1997 Elsevier Science B.V.
1. I n t r o d u c t i o n itself. A pr ime example o f this is the removal of dilute
amounts of sparingly soluble volat i le organic com-
As membrane sc ience develops , better membrane pounds (VOCs) f rom water by pervaporat ion. This
materials are being formula ted to accompl i sh a wide part icular separat ion is useful for the t reatment of
variety o f separations. In some cases, current ly avail- g roundwater and industrial wastewater [1-3], and
able membranes are so ef fec t ive that the separat ion is for the recovery of flavors, aromas, and other organics
l imi ted by the rate of mass transfer to the f e e d - f rom process streams [4]. I f the V O C has a relat ively
membrane interface rather than through the m e m b r a n e high Henry ' s law constant, pervaporat ion through a
thin film of s i l icone rubber (po ly(d imethyls i loxane) or
*Corresponding author. Fax: +1 864 656 0784: e-mail: P D M S ) yields high select ivi t ies that are l imited by [email protected], boundary layer effects rather than the membrane
0376-7388/97/$17.00 1997 Elsevier Science B.V. All rights reserved. PI I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 1 74-9
[1-6]. Concentration polarization effects are also cer- ~00o tainly known to exist in reverse osmosis, ultrafiltra- staggered b a n k / tion, and even gas separation applications. Clearly ~ ~00
there is an incentive to improve the mass transfer ~ ~ e ~ ~ ' ~ e r ! ~.~.-----'-/spira 1 efficiency of current membrane modules and to develop new module designs that can accommodate ~ ~0
inside tube increasing membrane performance capabilities. ~ _ i
l . . . . . . . . l . . . . . . . . i . . . . . . . . 10 100 1000
2 . Background Reynold . . . . bet
Fig. 1. Mass transfer correlations for five module configurations. 2.1. Mass transfer in common module configurations
The most common types of membrane modules are ments on gas desorption from and absorption into tubular, plate-and-frame, hollow fiber, and spiral water, using microporous polypropylene fibers. The wound. Of these, the latter two have proven to be inside tube line is for laminar feed flow inside hollow the most economical to construct. They are used fibers. The equation used to plot the line widely, especially in reverse osmosis and gas permea- tion. Some membrane process applications require Sh = 1.64(Re Sc d /L) °33 (2)
robust modules to handle high pressures, high tem- is nearly identical to the classic Leveque correlation peratures, or severe chemical conditions. Other appli- [8]. An inside diameter to length ratio of 10 3 was cations, such as pervaporative water decontamination, assumed for this illustration, which shows that laminar are conducted under milder conditions, and the flow in a tube yields relatively low Sherwood numbers economic efficiency of the module design may be a and thus low mass transfer coefficients. primary concern, particularly when there is little or Yang and Cussler [7] obtained more attractive no recovery of valuable products to offset costs. To results with flow outside of and perpendicular to reduce the capital expenditure per unit volume of closely packed hollow fibers, shown as the 'bundle' feed treated, it is important to maximize both the line in Fig. 1. The equation of this line is overall mass transfer coefficient (or permeability), which may be dominated by the feed-side boundary Sh 1.38Re°34Sc°33 (3) layer, and the membrane surface area per unit volume of module. Fig. 1 shows that this correlation is similar to the mass
Generally mass transfer correlations are expressed transfer analog of a well established heat transfer in a dimensionless form such as correlation for flow across or transverse to a 'staggered
bank' of tubes [9], Sh = a RebSc C (1)
Sh 0.575Re)556Sc°33 (4) The Sherwood number can be viewed as the ratio of the characteristic dimension of the flow path to the If the bundle correlation is extrapolated, a mass trans- boundary layer thickness. In laminar flow, some cor- fer coefficient of 10 2 cm/s would be achieved at a relations contain an additional factor involving the Reynolds number of about 50, assuming an organic characteristic dimension divided by the length of the diffusivity of 10 5 cm2/s, which is in the range of flow path. most VOCs, and an outside fiber diameter of 0.05 cm.
Five correlations obtained from prior work are For this case, the transverse flow velocity required is summarized in Fig. 1. To plot each line, the Schmidt only 10 cm/s, which should result in a relatively low number was assumed to be 1000, a reasonable value feed-side pressure drop. Yang and Cussler also tested for dilute, low molecular weight organics in water, configurations with flow outside of and parallel to The correlations labeled 'inside tube' and 'bundle' bundled hollow fibers, but their results (not shown in were obtained by Yang and Cussler [7] from experi- Fig. 1) were generally inferior to transverse flow.
The line labeled 'transverse' is from data reported at a reasonable velocity (< 2 m/s) and pressure drop. by Lipski et al. [10]. Their work involved the perva- Assuming the same Schmidt number and diffusivity as poration of dilute aqueous solutions of toluene before, the mass transfer coefficient should be through an array of silicone rubber coated hollow l0 2 cm/s. Unfortunately, fibers with such a large fibers mounted transverse to the liquid flow. The ID yield relatively low surface area to volume ratios. results are similar to the bundle and staggered bank To summarize this analysis, three module config- correlations, but somewhat lower, urations have the potential to provide a mass transfer
The final line in Fig. I, labeled 'spiral', is for a flat coefficient near 10 2 cm/s: teed flow transverse to the channel containing a spiral wound feed spacer and was outside of hollow fibers, a spiral wound module with obtained by Hickey and Gooding [11,12]. This line an effective feed-side spacer/turbulence promoter, represents the best spacer or turbulence promoter that and a large hollow fiber with turbulent feed flow inside Hickey found from twenty tested, based on high mass the capillaries. Of these, the transverse flow arrange- transfer coefficients and low pressure drop. The equa- ment is likely to be the most attractive because high tion for the line is mass transfer coefficients can be achieved at lower
Reynolds numbers, which should correspond to lower Sh = ().648Re°379Sc °33 (5) feed pump energy costs.
As seen m the figure, mass transfer in a spiral wound 2.2. Transverse .flow alternatives
module is generally not as effective as the staggered bank or bundle geometries, but Hickey was able to
Many possibilities exist for the arrangemen! of demonstrate mass transfer coefficients near
fibers and flows to produce a transverse flow hollow 10 2 c m / s using a few of the spacers he tested.
fiber module. Historically, the most important of these Surprisingly, the mass transfer coefficients for most
has been the radial flow module used in reverse spacers varied less than one order of magnitude over a osmosis. In the unit shown in Fig. v feed water enters wide range of flow rates. So the 'spiral' line in Fig. I is
the module through a central tube, and holes in this characteristic of the mass transfer performance of
tube distribute the flow radially outward across a several good turbulence promoters. This line has a
bundle of hollow fibers. The fibers are normally low velocity dependence when compared to the cor- relation reported by Schock and Miquel [13], who packed tightly,. The retentate collects in a space
between the fiber bundle and the outer shell of the tested three common RO feed spacers, but they appar- ently forced a Reynolds number exponent of 0.875 module and leaves through one or more exit ports.
The well-studied DuPont B-9 Permeator, used in based on heat transfer correlations. The largest expo- nent observed by Hickey using regression techniques reverse osmosis, is of the radial flow type. Published
analyses of this separator [15,16] neglect concentra- was 0.51@ and the average exponent of 15 different tion polarization in the teed because the membrane spacers was 0.440. The wide range of pressure drops dominates mass transfer resistance in RO. Neverthe- observed by Hickey for flow through various feed
less, radial flow modules have good potential in spacers also demonstrates clearly the need to choose the most appropriate spacer for a given application applications where the boundary layer dominates [141. because the fibers are mounted transverse to the feed
flow. One other possible configuration, which is out of the
range of Fig. 1, is the mass transfer analog to the well- known Sieder-Tate equation, an empirical heat trans- fer relation for turbulent flow inside tubes. The corre- Epoxy Seal Central Feeder I.]p~,xy Seal
P /
Typical F[~'r ( 'oncent ra lc Using hollow fibers with inside diameters of 0.2 cm, m o d e s t l y t u r b u l e n t f low (Re = 4 0 0 0 ) c a n b e a c h i e v e d Fig. 2. Schematic of a radial flow. hollow fiber membrane module.
alternate plates are rotated 90 ° so that the fibers in adjoining plates are perpendicular to one another. Preliminary results reported by Zenon are shown as the 'transverse' mass transfer correlation in Fig. 1. Zenon has a pervaporation system available for com- mercial use, but details of the system cost and mass transfer performance under field conditions have yet to
Fig. 3. Essential portion of Strand's patented module, be reported.
Hoechst-Celanese has commercialized a radial flow 3. Objectives hollow fiber device [17] that is similar in concept to the module shown in Fig. 2. In this case, however, The overall objective of this study was to evaluate Celgard "~: microporous polypropylene fibers are the performance potential of a new type of membrane knitted into a fabric using smaller multifilament module based on a combination of hollow fiber and polyester as cross-strands, and the fabric is wound spiral wound features. The new design would consist around the central tube. The small, solid cross-strands of a woven, hollow fiber fabric rolled into a compact hold the hollow fibers apart and in place. These unit much like the membrane sheet and spacers of a modules are suitable for a variety of applications, spiral wound module. This design has the potential to including blood oxygenation and liquid-liquid extrac- provide high mass transfer coefficients (via transverse tion. A similar module made with silicone rubber feed flow) at low pressure drops and high packing fibers might prove to be a step forward in pervapora- densities. Four steps were necessary to accomplish the tion efficiency, objective.
Other transverse flow hollow fiber modules have 1. Determine experimentally the permeability of been patented and/or developed for commercial oxygen through silicone rubber fibers so that use. One of the oldest ideas was patented by oxygen permeation through these fibers could be Strand in 1967 [18]. As shown in Fig. 3, Strand used as the basis for mass transfer studies. used a fabric in which both warp and weft were hollow 2. Determine whether the silicone rubber hollow fiber membranes. Liquid flowed through the mat, fibers could be woven with solid cross strands to which was held in a square holder, and on to the next produce a fabric. mat. The permeate flowed inside the fibers to exit 3. Construct and evaluate various fabric types. Deter- channels at the periphery. The packing density (mem- mine the boundary layer mass transfer coefficient brane area per unit of module volume) of these plate- (k) resulting from transverse flow across the hollow and-frame units would be rather low as compared to fibers, the pressure drop per unit length of fabric more typical hollow fiber and even spiral wound (AP/L), and the membrane packing density. Test designs, different fabrics in terms of spacing between fibers,
A similar approach is described in a patent by size of and spacing between cross strands, and type Nichols [19] in 1989. Nichols' module was made of weave. Vary channel height and evaluate its of a series of circular wafers with each wafer effect. consisting of fibers mounted chord-wise and parallel 4. Construct prototypes of the most promising designs to one another. Nichols mentions a woven cross for full-scale testing. Evaluate k (again using water fiber as one means for keeping the hollow fibers deoxygenation) and AP/L as functions of flow parallel, rate, and compare them to the correlations devel-
Zenon Environmental of Canada has developed oped for small-scale fabrics. Test the prototype in modules similar to the Strand patent for pervaporation the pervaporative mode with trichloroethylene [20]. These consist of many square plates, each of (TCE), a model VOC. Operate the prototype both which contains hollow fibers mounted parallel to one with and without an inert plastic sheet to separate another. The fibers are transverse to the liquid flow and fabric layers.
The hollow fibers used throughout this work were ~ ~ . . . . . . _ ~ _ _. ~)___>
, .JIIIIl[ I I.x3 prefilter vacuum Dow Coming Silastic ~ brand silicone rubber with a teml~p~o~-?31111.~ ~ ' pump 305 gm ID and 635 pm OD. To determine the perme- ability of oxygen through these fibers a m i n i - m o d u l e o,~ electro.de " _ - ~ 1 " - ' ~ _ ~ J,[
was used as depicted in Figs. 4 and 5. Thirteen fibers reservoir ~-~--,=~----~) ~ tJov~mete r were potted into a tee and tube arrangement. The stirrer [ feed pump 0.5 cm of fiber surrounded by potting material I
(Dow Coming 3140RTV) allowed the development Fi~_. 5. Test loop for mass transfer experiments.
of laminar flow in the fibers before the beginning of the mass transfer zone. The water in the batch flow loop was first aerated and then all bubbles were versus flow rate to the -1 / ' 3 power for several runs carefully eliminated. Flow was started and vacuum and extrapolating to infinite velocity to get the mem- was applied to the shell side of the small module, brahe resistance to 02 transfer. This method is less Constant temperature (20 ± I-C) was maintained by a susceptible to experimental error than a single run water bath (not shown) surrounding the feed tank, and determination, and it does not require exact confor- the circulation flow rate was determined by a call- mity to the Leveque correlation. brated rotameter. The oxygen content of the water was For the small cell tests, 8 by 10 cm fabric sections monitored continuously by an Orion Research model were woven using a handloom with monofilament 9708 electrode connected to a digital readout, nylon as the warp (cross strand) and PDMS hollow
An oxygen balance on this batch deoxygenation fibers as the weft. Seven different fabrics were system yields woven and tested. Table 1 summarizes important
ln/C,\ K,,At characteristics of the fabrics. Fabrics 1 through 6 V (7) were plain weaves; Fabric 7 was a twill. Compared
' to Fabric 1, Fabrics 2 and 3 tested the effect of The overall mass transfer coefficient can be deter- reducing the space between the PDMS hollow fibers. mined from a semilog plot of the concentration ratio Fabric 4 was similar to Fabric 3 in spacing of the versus time. In principle the membrane permeability hollow fibers, but contained only half as many solid can then be obtained from a single run by accounting cross strands. Fabric 5 incorporated cross strands of for the resistance to oxygen transport of the lumen- alternating size, and Fabric 6 used the larger cross side boundary layer, which may be calculated from the strands throughout. Fig. 6 is an SEM photomicro- Leveque correlation [81. In practice we used the graph of Fabric 6. Wilson plot technique instead [21], plotting l/Ko Each of the fabrics was potted on each side with
epoxy so that it could be mounted in the mass transfer and pressure drop cells illustrated in Figs. 7 and 8. respectively. Each Plexiglas R test cell had a flow channel of 38 by 100 by 2 mm at maximum height. The working height was adjusted to provide a snug fit for each fabric by gluing thin Mylar sheets to the top H
piiiiii~oL---~ ~ , n g ~ half of the channel. The pressure drop was measured using either an inverted water-filled U-tube or a
to standard mercury-filled U-tube flooded with water. The overall mass transfer coefficient was determined
potting hollow fibers for each fabric at various flow rates by incorporating
the mass transfer test cell in the apparatus shown in feed Fig. 5 and using the batch deoxygenation method
Fig. 4. Pervaporation module with liquid flow inside fibers, described earlier.
a Salient fabric feature. b Fabric 7 was a twill weave. All others were plain weave.
outlet
permeate hollow fiber port
fabric -- --
stagnant z o n e
caulk seal
Fig. 7. Top view of mass transfer test cell.
B t~1-- 50 50 mm--~"
Fig. 6. SEM micrograph of Fabric 6, top-view. brace brace
hollow fiber fabric
Prototype modules were made using larger sections Fig. 8. Side view of pressure drop test cell.
of Fabric 6, measuring about 24 x 35 cm. Fibers on one of the 24 cm sides of the fabric were cut open; the open ends were inserted into a 1/2-inch slotted perme- between layers. However, a plastic sheet was wrapped ate tube; and the slot was sealed with silicone rubber, around the outside layer. For all cases, o-rings were which was then cured. For Prototypes IA and 1B, a placed around the wrapped layers to keep the fabric in thin plastic sheet was taped to the permeate tube and position and to provide a snug fit of the spiral assembly rolled with the fabric to form four concentric layers within the glass module shell. Prototype 2 contained separated by an inert sheet. Prototypes 1C and 2 were thinner o-rings, allowing more room for the fabric nearly identical, but they did not contain a plastic sheet layers. A prototype is shown in the unrolled and rolled
I I Results of a typical batch experiment are shown in Fig. 11. Throughout this study the linearity of the data was excellent in every case, as the linear correlation coefficient (R 2) was usually 0.999 or 1.000. The slope of the line in these plots was used to calculate the overall mass transfer coefficient, K,,. lor the particular experiment according to Eq. (7).
For laminar liquid flow inside tubes, the mass transfer results were used to determine the membrane resistance via the Wilson plot method [21]. The Leveque equation [8] can be expanded to yield
( D / ) I 3 ( L ) P 1 '3 k = 1.62 - (8)
Fig. 10. Fabric wrapped around permeate tube and held with
plastic sheet and o-rings. Applying t he r e s i s t a n c e - i n - s e r i e s approach c o m -
m o n l y used in heat and mass transfer studies, a plot of l/Ko versus (L/v) 1/3 should yield a straight line
states in Figs. 9 and 10, respectively. Water was with a y-intercept equal to the membrane resistance. pumped axially through the spiral wound module in The data and linear regression are shown in Fig. 12. the conventional manner, flowing transversely across The intercept, 62.2 s/cm, translates into an oxygen the hollow fibers. The inside of the permeate tube was permeability through the silicone rubber hollow fiber connected to a vacuum pump. walls of 2.5 < 10 s mol / (m s atm), which is 30%
Prototype mass transfer experiments were con- higher than the frequently quoted value reported by ducted using the batch deoxygenation method Robb 1221. However, Robb's value is within the con- described earlier. The system volume was 20.1 1 and fidence interval of our data unless we reduce the the membrane area was approximately 2500 cm 2 in confidence level to only 35e~:. Our experimental value each prototype. Experiments involving the removal of tor the membrane resistance was used in the fabric trichloroethylene (TCE) from water were also con- mass transfer evaluations since it is conservative, ducted with organic concentration determined using a yielding lower fabric k values than would Robb's gas chromatograph (HP5480A) with an FID. determination.
400. - / " by having larger cross-strands, yielded the highest
mass transfer coefficients. The larger cross strands 300- [] [] opened the fabric more to allow water to flow above,
E below, and ' through' the fabric. Once again, though,
2 0 0 " there seems to be a limit. Fabric 5 was quite open, having cross-strands that alternated in size, but it
~0o- apparently allowed flow channeling away from the 2,, = 6 2 . 2 + 323x R̂ 2 = 0.93 surface of the hollow fibers. Hence, Fabric 5 had lower
• i . , . i * J , o o 0.2 0.4 06 08 ~.0 k values. Relatively open flow channels were also (L/v)̂ 0.33 recommended by Hickey [11] in his analysis of tur-
bulence promoters used in spiral wound modules. Fig. 12. Wilson plot determination of membrane resistance to Fabric 7, the twill, performed poorly. oxygen permeation through silicone rubber.
For all fabrics, mass transfer increased with velo- city, as expected. Quantitatively,
l k = av b (9) [] Fab r i c I o F a b r i c 5
• Fab r i c 2 [] F a b r i c 6
III F a b r i c 3 " Fabri~ ~ where v is the superficial velocity, and values for a and o Fab r i c 4
I:1 ° t l .0~ ~ ~, ~ , ~ ~ , ~ a ~ b determined by linear regression are given in Table 2. • • ~ ~ ,b (The pressure drop regression coefficients are dis-
• D ~ u ~ "In o a • ~ ~ ~[]o ~ o o ~ cussed in Section 5.3.) The number of digits reported • • for the regression coefficients exaggerates the statis-
.0o~. " tical significance that can be justified from the modest • number of data points obtained with each fabric, but
the correlations are useful for preliminary calculations and comparisons.
. 0 0 0 1 i
0~ • ~ In dimensionless form the mass transfer correlation wJoci~y, m/~ for Fabric 6 is
Fig. 13. Compilation of mass transfer results in small cell tests. Sh = 0.62Re°42Sc°'33 (10)
with the superficial velocity and outside fiber diameter used as characteristic values, and the standard Schmidt
5.2. Mass transfer in fabr ic - f i l l ed channels number dependence assumed. Wickramasinghe et al.
[23] suggest the following design equation for trans- A compilat ion of the mass transfer results for small verse liquid flow through knitted fabrics made of
fabrics is shown in Fig. 13. Four fabrics exhibited k values over 0.01 cm/s , confirming expectations for transverse flow across tubes. Fabrics in which the
Table 2 hollow fibers were touching, such as Fabrics 4 and
Mass transfer and pressure drop correlations for small fabrics. (See 6 yield the highest values. This phenomenon was also Eqs. (9) and (12)) observed by Yang and Cussler [7], but for fibers that were 'a lmost touching' . There is l ikely a limit, how- Fabric a x 103 b c x 10 -~ d
(cm/s) / (m/s) h (Pa/m)/ (m/s) a ever, as seen in the low k values for Fabric 3. Fibers in
1 13.8 0.397 0.132 1.55 this fabric were not only touching but were pressed 2 12.9 0.290 0.142 1.45 against one another. 3 7.35 0.239 3.82 1.59
Another noteworthy observation is that openness in 4 16.0 0.306 2.62 1.71 the flow channels gave higher k values. Fabric 4 which 5 6.28 0.217 1.05 1.51 had fewer cross strands than Fabric 3, performed 6 24.3 0.422 1.00 1.56
7 2.34 0.450 1.84 1.51 better. Fabric 6, which differed from Fabric 4 only
The two correlations are quite similar, which is not z ; • [ = I 35 f11[11
5 D I = i 6 ~ [ n [ ] l surprising since both were obtained for transverse flow ~ across hollow fibers of similar size and spacing.
The idea of combining hollow fiber membrane as the spacer between flat sheet membranes was also , . /7 / ' ,
y = 6 0 1 1 + 1 .46x R ^ 2
tested in a modified small cell test. Batch experiments 4 ' ' ' 1 4 - 1 2 - 1 0 0 . 8 0 6 ,P
yielded an overall oxygen removal rate. From this, the ~og ~v. r,/~ removal rate of oxygen using only the fabric was Fig. 14. Pressure drop in Fabric 3 at diflerent channel heighls. subtracted. The remaining flat sheet removal rate was disappointingly small, the maximum k value being 0.00250 cm/s. For this reason, and because city, the range of exponential dependence being 1.45 construction of modules involving both flat sheet to 1.71. Hickey and Gooding [14] found similar results and hollow fiber fabric membranes would be more in their work with thin channels containing convert- complex than the fabric alone, the feasibility of this tional spiral wound spacers. For Eq. (12), an exponent arrangement was not pursued further, of 1.0 would indicate laminar flow, while the exponent
The effect of channel height on fabric performance for fully developed turbulent flow is approximately was also investigated. For the closely packed Fabric 3, 1.75 [24]. All combinations of fabric-filled channels increasing channel height from 1.35 to 1.65 mm had tested in this work appeared to be on the high end of virtually no effect on the observed k at five different the transitional regime between laminar and turbulent velocities. Thus, within reasonable limits, it would flow. appear that wrapping the fabric more loosely or more As indicated in Table 2, the pressure drop for flow tightly around the central permeate tube of a spiral through the seven fabrics varied significantly. The wound module would have little effect on the mass magnitude of the pressure drop generally decreased transfer coefficient achieved. Tighter wrapping would as void fraction increased. Various methods were tried be expected to increase the frictional pressure drop, to develop unifying pressure drop and mass transfer but it would also increase the amount of membrane correlations, including the use of average or maximum surface area in the module, velocity in lieu of superficial velocity, but none was
successful [25 [. 5.3. Pressure drop
5.4. Fabric comparison .for scaleup As expected and shown in Fig. 14, pressure drop
increased with velocity and decreased with channel A rigorous comparison of alternative fabric designs height for the same fabric. Ultimately, the tradeoff and of the fabric concept to conventional modules will between reduced pressure drop and reduced mem- have to take into account all capital and operating brahe area with looser fabric packing (taller channel costs for a specific application, but a preliminary height~ will have to be addressed for specific applica- evaluation can be made by considering two important tions, characteristics: the mass transfer coefficient and the
The pressure loss in the fabric-filled channels can be pressure drop [26]. Higher mass transfer coefficients described with an equation analogous to Eq. (9). reduce capital costs and lower pressure drops reduce
operating costs. A "module efficiency" defined as A P / L = cv J (12) k / ( A P / L ) was calculated from the regression equa-
Correlation values for c and d are reported in Table 2 tions for each fabric at a single superficial velocity of for each fabric with v again defined as the superficial 0.16 m/s. The membrane area per module was calcu- velocity. In contrast to the mass transfer coefficient lated for a nominal 4-inch module, actually 1 m long correlation, pressure drop increased sharply with velo- by 10 cm in diameter. The results are shown in
Table 3. This analysis indicates that Fabric 6 is the Table 4 best design of the fabrics tested in this work. At Prototype pressure drop correlations according to Eq. (12):
A p = c~ t reasonable pressure drops, mass transfer coefficients for this fabric exceeded 0,01 cm/s, which is 20% Set-up Channel height c x 10 6 d
higher than the maximum value reported by Hickey (mm) (Pa/m)/(m/s) d
and Gooding for spiral wound spacers evaluated at Prototype 1A 1.36 5.98 1.56 comparable (and even higher) velocities [11]. The Prototype 1B 1.40 1.95 1.30
Prototype 1C 1.42 1.35 1.28 corresponding 'optimum SW spacer' values were Prototype 2 1.57 0.93 1.57 determined by Hickey and Gooding [1 1] following Fabric 6 1.67 1.00 1.56 their analysis of over 20 inert spacers that are used or could be used as conventional spiral wound turbulence promoters. The fabric concept competes well with an Though Prototype 1B resembled Fabric 6 in that a flat, optimized spiral wound module in terms of mass impermeable sheet separated the fabric layers, this transfer coefficient and friction loss, and it provides prototype caused almost twice the pressure drop per more than three times the membrane area per module, unit length as Fabric 6. This is most likely due to the
thinner channel available for flow. Removing the 5.5. Prototype evaluation plastic sheet reduced the observed pressure drop, as
shown by the results for Prototype IC. Prototypes made with larger versions of Fabric 6 Both 1B and I C had lower pressure drop depen-
were tested in four variations. The pressure drop dencies on velocity than all small-scale fabrics. This results for all four prototypes are summarized in could be a sign of flow channeling between the outer Table 4. The calculated channel heights for each layer plastic wrap and the transparent shell, but the magni- of wrapping, as well as the results for Fabric 6 from tude of the pressure drop, as well as experimental small-scale tests are also included. Prototype 1A gave observations of flow, seem to discount this cause. a huge pressure drop, probably because the fabric was Rather the lower pressure drop dependencies were wrapped tightly. In addition, the fabric/permeate tube probably caused by minor channeling between fabric seal had a bulge in it at one end where a leak had been layers. The fabric layers in Prototypes 1B and 1C were sealed with additional silicone sealant. Since this was not perfectly concentric because the fabric/permeate not accounted for in the channel height determination, tube seal, though free of bulges, was still thick. Thus, the flow through this section of Prototype 1A was even small gaps between fabric layers existed at certain more constricted than it was elsewhere, points. At increased velocity, it is likely that an
Prototype 1B was formed by cutting the fabric in increased portion of the water flowed through these Prototype 1A and discarding the portion sealed into gaps, thereby limiting the increase in pressure drop. the permeate tube. The remaining fabric portion was Prototype 2 exhibited pressure drop results very simi- then resealed into the tube with no bulges present, lar to those of Fabric 6. In Prototype 2, the improved
ILI)I 2 - • sheet between fabric layers in Prototype I C yielded ,,,,l,, ~ mass transfer performance at least as good as Proto-
o
,o,oO,. 8o.O type 1B and pressure drop performance that was I11')OS - • P r o t o t y p e 2
~ o Prototype LC clearly preferred. The two data points obtained with • P r o t o t y p e LB [] P r o t o t y p e I , ~ X Prototype 2 indicated mass transfer performance that
,~., was even better than Fabric 6. o []
t) c~ 0 1 o12 0.3 5 . 6 . Protot)'pe testing on pervaporation ~f T C E ~ e [ o c i t v , i~1/~
Fig.. 15. Summary of prototype mass transfer results for oxygen After the mass transfer characteristics were evalu- rem,~val ated via the deoxygenation of water, several experi-
ments were conducted on Prototype 2 in which TCE was removed instead of oxygen. The k values for TCE
fabric/permeate tube seal was thin and free of bulges, were expected to be lower than those with oxygen which allowed for even more concentric wrapping because the diffusivity of TCE in water is half that of than Prototype 1. In addition, the fabric was wrapped oxygen. If the Sherwood number varies with the more loosely, yielding a larger channel height and cube root of the Schmidt number as normally assumed lower pressure drops. [7, 27], k varies with D to the 2/3 power. Thus, the k
Prototype mass transfer results for oxygen removal values for TCE removal were expected to be about are shown in Fig. 15. Prototype 1Ayielded poor mass 37% less than those involving oxygen. transfer performance. This was attributed to the tight- Results for TCE removal experiments are shown in hess of the fabric wrap and the presence of the inert Table 6. Obviously, the k values were about half as plastic sheet between layers, both of which decreased large as expected, and they appeared to decrease with the amount of membrane surface area in contact with time. The lower values are thought to have occurred the feed. The observed mass transfer coefficients for for two reasons. First, the overall mass transfer rate for Prototypes 1B and 1C were quite similar and were TCE was hindered by the small back-pressure on the 30% lower than those of Fabric 6 throughout the permeate side of the membrane. The driving force for velocity range tested. The lower k values may be pervaporation [1 I] can be expressed as (.r~ - yoP/H). due to the tightness of the wrap or to internal flow The back-pressure term was assumed to be negligible channeling, as discussed above. Though only two in the calculations used to determine the k wtlues trials were conducted for oxygen removal in Prototype reported in Table 6. but TCE has a Henry's law 2, the one at the higher velocity showed excellent mass constant about 100 times lower than that of oxygen. transfer, with k > 0.01 cm/s. The one at lower velo- With the permeate pressures achieved in this study. city had a k value similar to Prototypes 1B and LC. typically 3 to4 mm Hg, this caused a flux reduction of
Mass transfer correlation parameters for Prototypes 12%. accounting for some of the difference between IB and IC and for Fabric 6 are given in Table 5. expected and observed k values using TCE. Prototype I B gave a lower k dependence on velocity than Prototype 1C. Though this difference may not be
Table 6 significant, one thing is clear. The absence of a plastic Mass transfer coefficients observed in Prototype 2 fi~r the removal
of TCE. listed chronologically
Table 5 Velocity k
Prototype mass transfer correlations according to [9] k ~ av ~' (m/s ) (cm/s)
0.130 0.0(I34 Set-up Channel height a × | 0 - 3 b 0.201 0.0035
The rest of the unexpected drop in k values was low as would be expected considering the difference in attributed to membrane fouling. When aqueous solu- diffusivity between the two compounds. The k values tions of TCE were tested, the membrane fibers in the for 02 were lower than those in previous evaluations prototype, which were originally a translucent, water with the same fabric by about 20%, probably due to white, became a dull, light green. The darkness of the the slightly larger channel height used for the latter discoloration continued with time and worsened after experiments. Most importantly, these experiments the fabric roll was removed for inspection and re- showed that TCE can be removed as effectively as inserted into the module shell. The probable cause of 02 using woven fabrics made of silicone rubber this discoloration was determined to be the deposition hollow fibers. of some component of petroleum jelly on the fibers. TCE, a well-known degreaser, apparently dissolved 6. Summary and Conclusions the petroleum jelly used to help in sliding the o-tings (outside the fabric roll) into the module shell. Some The oxygen permeability of Dow Coming Silastic !~' component of the petroleum jelly then deposited onto hollow fibers was determined to be or absorbed into the membrane when the TCE was 2.5 x 10 s mol/(m s atm), which is 30% higher than passing through the membrane. Robb's frequently cited value [22] for silicone rubber.
To check the plausibility of this explanation, dis- Several hollow fiber fabrics were woven and tested colored fibers were put in pure TCE. The fibers in small cells. The best results were obtained with swelled, which is typical for silicone rubber in Fabric 6, a plain weave with hollow fibers touching TCE, but they also became clear again while the each other and nylon cross strands 2/3 as large as the TCE liquid became colored. This does not prove hollow fibers. Boundary layer mass transfer coeffi- the speculation concerning the cause of flux reduction, cients exceeding 10 2 cm/s were observed at super- but it does support it. ficial velocities and pressure drops comparable to
It should be noted that several researchers have common spiral wound module operating conditions, successfully used silicone rubber membranes for the which yield lower k values. In addition, the Fabric 6 pervaporative removal of TCE from water [ 1,3,5,11 ]. design characteristics translate into about three times More care in material selection should eliminate the the membrane packing density achievable in a con- fouling problem observed in the prototype tests. In ventional spiral wound module. fact, some successful experiments involving TCE Prototype modules were built and tested using the removal and additional deoxygenation experiments Fabric 6 geometry. Mass transfer coefficients obtained were conducted using Fabric 4 in the small test cell from water deoxygenation ranged from 30% lower described earlier. The test cell apparatus contained no than the small cell tests to comparable values. TCE petroleum jelly, no metal feed lid, no rubber o-tings, pervaporation tests yielded lower mass transfer results and a chemically resistant pump impeller. Results for due to an apparent fouling problem. these evaluations are shown in Table 7. The k values Though further development is certainly needed, for both O2 and TCE increased with velocity as spiral wound, hollow fiber fabric modules show con- expected. Though the k values for TCE removal were siderable promise for membrane separation applica- lower than those for 02 removal, they were not even as tions that require high feedside mass transfer co-
efficients to reduce concentration polarization effects at reasonable capital and operating cost. The modules
Table 7 would likely be subject to the same pretreatment Mass transfer coefficients observed in Fabric 4 for removal of both requirements as conventional spiral wound modules. oxygen and TCE
Velocity k (02) k (TCE) 7. L i s t of symbols (m/s) (cm/s) (cm/s)
0.120 0.0056 0.0044 a, b, c empirical constants dependent on flow 0.201 0.0068 0.0054 geometry 0.317 0.0081 0.0055
C solute concentration: Co at t ime:0, C, at [9] F. Kreith, W.Z. Black. Basic Heat Transfer. Harper and Row,
time=t New York. 198(/. [10] C, Lipski, P. C8t6, H. Flen]ing, Trans,,erse feed flow for
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sionless Conference on Pervaporation Processes m the Chemical
D diffusivity of solute in w a t e r Industry, R. Bakish (Ed.), Bakish Materials Corp., Engle-
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L length of flow path wound pervaporation modules, (J. Membrane Sci., 92 i l994)
P permeate pressure 59~, .1. Membrane Sci. 98(1994) II995~ 293.
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Re R e y n o l d s n u m b e r = dvp/tz [14] p.J. Hickcy, C.H. Gooding, Friction loss in spiral wound
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