L Extraction Membrane

Article pubs.acs.org/IECR

On the Design of a 4-End Spiral-Wound L/L Extraction Membrane Module Christoph Bayer,† Michael Follmann,† Hans Breisig,† Ingrid M. Wienk,‡ F. Petrus Cuperus,‡ Matthias Wessling,*,† and Thomas Melin† †

Chemical Process Engineering − AVT.CVT, RWTH Aachen University, Turmstraße 46, 52056 Aachen, Germany SolSep BV, St Eustatius 65, 7333NW Apeldoorn, The Netherlands

‡

ABSTRACT: The present work describes the design and application of 4-end spiral-wound modules, mainly for extraction applications. Membrane spacers, a key part of spiral-wound modules, are tested for pressure drop, mass transfer, and mixing behavior. Further, different permeate channel designs are studied by means of a transport-limited surface reaction (copper dissolution method). The findings led to the design of a 4-end spiral-wound membrane module with reduced propensity to dead zones and maldistribution and finally to its application to the reactive extraction of phenol. use of the solvent or low solute recoveries. Even though flat-sheet contactors are rarely covered in the literature,1,14−18 they might offer distinct advantages for liquid−liquid applications. The flow channels adjacent to the membranes are usually defined by spacers, which provide a very regular filament pattern along a mostly rectangular flow channel. This should yield a very regular flow field accompanied by mixing parallel to the membrane. The emerging flow pattern is therefore expected to resemble plug flow with dispersion as found with residence time techniques by Van Gauwbergen and Baeyens19 in reverse osmosis spiral-wound modules. Hence, flat-sheet contactors may provide hydraulic conditions similar to those found in the lumen of hollow fibers, but for the feed side and also for the strippingfluid channel. Flat-sheet membranes are deployed either as membrane stacks or as spiral-wound modules. Stacks of membranes with spacers in between are used for few applications because of their costly construction. They have been used for VOC recovery (3-end),20 as 4-end module for the separation of ethene/ ethane15 or gas upgrading.16 The 4-end designs used to date as gas−liquid contactors suffer from insufficient flow control of the solvent. Proper design of inlets and outlets remains challenging. One of the most important applications of such 4-end flat-sheet stacks remains electrodialysis.21 Other module designs generally do not allow the buildup of homogeneous electric fields. Spiralwound modules are widely applied for high-pressure filtration applications like desalination or natural gas upgrading (3-end). The design is robust and compact; the construction of the inlets/outlets is straightforward. In 4-end spiral-wound modules, the stripping solvent is fed to the permeate tube on one side and enters the membrane pocket. The fluid spreads such that the overall pressure drop remains minimal, which leads to a fluid

1. INTRODUCTION The primary objective of most mass transfer unit operations is efficient contacting of two immiscible phases to facilitate solute exchange. The interfacial area is usually formed by dispersing one phase in the other (e.g., as droplets). Substantially different are membrane contactors, which provide phase boundaries either with porous or dense membranes. Such nondispersive contacting not only extends the range of attainable hydraulic operating conditions, but also offers increased efficiency because of much larger volumetric mass transfer coefficients,1,2 which is mainly due to the large volumetric contact area.3 As a consequence, membrane contactors have drawn a lot of attention in research especially for gas−liquid contacting applications and are under investigation even for a few large scale operations, that is, carbon capture and storage.4 So far, the focus has been mainly on the development of polymeric materials with improved properties and the determination of mass transfer properties for specific applications. In contrast, only few works have been devoted to the development of a generalized perspective onto module design, scale-up, and operation.5−7 Hollow-fiber contactors can be considered state of the art. They are particularly used in blood oxygenation.8 Corresponding modules offer a large interfacial volumetric surface area and well-defined lumen side flow. The longitudinal fluid flow in the module shell is, however, ill-defined, prone to maldistribution, channeling, and dead zones, as the large uncertainty in Sherwood-correlations shows3,7,9,10 This reduces in effect the available membrane area especially in the case of liquids because of the low coefficient of solute diffusion. Better results can be obtained with transversal flow module configurations, which make use of the fibers to enhance mixing.11 This is the reason why the LiquiCel configuration has become the standard module design,2 in particular for but not limited to gas−liquid contactors. In gas−liquid contacting, the liquid solvent usually flows through the fiber lumen, mainly because of the favorable hydraulic conditions. In the case of liquid−liquid extraction, the placement of the streams either into the fiber lumen or into the module shell is not obvious. The phase flowing at the shell side is usually prone to back-mixing,12,13 which results in inefficient © 2012 American Chemical Society

Special Issue: Baker Festschrift Received: Revised: Accepted: Published: 1004

November 11, 2011 April 3, 2012 April 4, 2012 April 4, 2012 dx.doi.org/10.1021/ie202594h | Ind. Eng. Chem. Res. 2013, 52, 1004−1014

Industrial & Engineering Chemistry Research

Article

distribution of the stripping fluid must be tight to achieve equal loading. However, the uptake capacity of such fluids is generally large relative to the rate of species accumulation. For this reason, the flow velocities in membrane modules are low, which may adversely affect boundary-layer mass transport and fluid distribution. Mixing in spacer filled channels has already been investigated by Da Costa et al.31 By injecting dye at Re = 200, they found that mixing is more efficient with high-angle spacers. From the movement of bubbles, they concluded that large portions of the fluid changes direction at each mesh. Chen et al.32 present similar observations. Spacer mass transfer coefficients are also widely documented.22,33,34 However, literature lacks data on each of the aforementioned properties for flow conditions relevant for the target application (Re ≪ 100). Nonetheless, low cross-flow velocities should yield low mass transfer coefficients. Besides, mixing is expected to behave differently, because viscous forces dominate. Therefore, this section provides • mass transfer coefficients from limiting current measurements, • insights with respect to fluid distribution from mixing experiments, and, for completeness, • pressure drop measurements. This section also includes mass transfer data obtained with the copper dissolution technique to calibrate this measurement approach against the limiting current method. 2.1. Materials and Methods. 2.1. Spacers. Available for testing were three spacer nets, one woven and two extruded (Figures 1 and 2). Key parameters are summarized in Table 1. Start-up Procedure. To avoid entrapment of air, the setup is subsequently flushed with CO2, aqueous caustic soda, deionized water, and finally with the respective test solutions. It is of paramount importance to avoid contact of sodium hydroxide with the electrolyte solution used for the limiting current method. Otherwise, unwanted reaction products form.

short-cut to the permeate tube outlet. Consequently, baffles (i.e., glue lines) on the permeate/stripping side must be incorporated to ensure proper solvent distribution.18 However, detailed information regarding performance, design, and construction remain unavailable. Therefore, the objective of this Article is to extend the understanding of 4-end spiral-wound modules. Since flow in spacer-filled channels and spiral-wound modules at high Reynolds numbers has been readily investigated,22 the study focuses on liquid flow at low Reynolds numbers. This encompasses spacer experiments on pressure drop, mass transfer, and fluid distribution (dispersion). On the basis of these findings, recommendations are given for proper use of spacers. Thereafter, spiral-wound modules equipped with copper foil (instead of membranes) are used for studying the overall mass transfer performance of different permeate channel designs. At the copper surface a reaction takes place whose rate is limited by the transport of reactants from the fluid bulk toward the copper surface.23,24 A module equipped with a silicone-type membrane is finally designed and built according to previous findings and applied to the liquid−liquid reactive extraction of phenol25−30 because of its inherent operational complexity.

2. PROPERTIES OF SPACERS AT LOW REYNOLDS NUMBERS The viability of spiral-wound modules for extraction applications relies on • proper distribution of fresh stripping solvent as well as • high boundary-layer mass transfer coefficients. Mass transfer coefficients represent the degree of mixing normal to the membrane. In other words, better mixing promotes transport of species between the fluid bulk and the surface of the membrane. To ensure efficient extraction, new stripping solvent must be distributed evenly in a module. In particular, the residence time

Figure 1. Scans of the examined spacers (magnification approximately 2).

Figure 2. Schematic drawing of woven (left) and extruded spacers (right). 1005

dx.doi.org/10.1021/ie202594h | Ind. Eng. Chem. Res. 2013, 52, 1004−1014


Article

Table 1. Examined Spacers spacer

height [mm]

mesh size [mm2]

mesh angle

porosity

type

Conwed X04410 Naltex NO1329_90pa Filmoor PBT

1.0 0.5 0.4

2×2 1.4 × 1.4 0.2 × 0.2

90° 90° 90°

0.75 0.80 0.35

extruded extruded woven

Pressure Drop Measurement. The pressure drop Δp across the spacer mesh is determined (with the water density ρ and the gravitational acceleration g) via the difference in hydrostatic level Δh: Δp = ρg ·Δh

(1)

Figure 3. Schematic drawing of the dispersion test cell. Inlets are fed separately by a tubing pump, and a digital camera fixed on top of the cell captures the color-change boundary.

Limiting Current Technique. Mass transfer coefficients from the bulk toward channel walls are determined with the limiting current technique. It relies on a redox reaction induced by a voltage difference between two electrodes. With increasing potential difference, the rate of reaction increases until reaching a limit. The rate limiting step is diffusive mass transfer of reactants between the electrodes. According to eq 2, the limiting current Ilim defines the boundary-layer mass transfer coefficient k: I k = lim nFcA

of mass transfer between a channel wall and fluid bulk. To this end, a solution of 1 mol/L sulfuric acid containing potassium dichromate contacts a copper surface. Cr4+ ions oxidize the copper while being reduced to Cr3+: 3Cu + K 2Cr2O7 + 7H 2SO4 → 3CuSO4 + Cr2(SO4 )3 + 7H 2O

(2)

4+

The decline in Cr transfer coefficient k: c V̇ k = ·ln in A cout

where n corresponds to the number of electrons participating in the reaction on the measuring electrode, F is Faraday’s constant, c represents the concentration of [Fe(CN6)]3− in the fluid bulk, and A is the area of the measurement electrode. Li et al.33 describe the method in detail. The experimental setup consists of the measuring cell for spacer samples of size 200 × 80 mm, a 1 L feed container continuously stirred, temperature controlled and stripped with N2 as well as a calibrated tubing pump (Ismatec MCP Process equipped with 12 roller pump head). The platinum electrodes are connected to the direct current (DC) power source Agilent E3640A. The current is tracked with the ampere meter Agilent 34401A. The cathode as measurement electrode is smaller than the anode and 70 mm × 40 mm in size. Flow rates were randomly chosen to avoid experimental bias. For a given flow rate the order of applied voltages was also randomized. Steady state was assumed when the electrical current was stable within 0.1 mA. The setup was shielded from sunlight thus eliminating UV-light triggered degradation of cyanide-ions. Optical Investigation of Dispersion. To investigate lateral mixing, a test cell is designed in which hydrochloric acid is contacted with sodium hydroxide.35 The spacer (260 × 150 mm) is confined between two plates tightly bolted together and to the sides by a silicone sealing. This way, any bypassing over and under the spacer is avoided. The hydrochloric acid and the sodium hydroxyde are fed with a tubing pump (Ismatec MCP Process). Both streams flow first in parallel separated by an impermeable barrier (Figure 3). As the barrier ends, they start to mix. Both streams are marked with a pH indicator, which changes its color at pH 7 (30 mg/L of each neutral red and methylene blue). The points in space of neutral pH can be observed as a color-change boundary, which shifts its location with different acid to alkaline concentration ratios. To avoid experimental bias, the set of flow rates was randomized. Finally, the images were taken with conventional digital cameras placed inside a housing with indirect lighting. Copper Dissolution Technique. Gregory and Riddiford23 proposed the copper dissolution technique to measure the rate

(3)

concentration yields the mean mass

(4)

where cin and cout stand for the inlet and outlet concentrations of Cr4+. A represents the copper surface area and V̇ is the volumetric flow rate. A thorough description of the method is given by Gruber and Melin.24 Gruber and Melin24 propose a dichromate concentration of 0.3 mmol/L mainly to avoid convection due to density gradients. In contrast, Gregory and Riddiford23 used 30 mmol/L for convection-dominated transport. Apparently, for systems unaffected by natural convection, the concentration level can be safely chosen within these limits. A test cell with the same dimensions and setup of the limiting current method is used with the addition of one copper plate (80 × 40 mm) instead of electrodes. To ensure steady state, time series of samples in terms of the Cr4+ concentration were analyzed. Concentrations were measured with a UV photospectrometer of type Varian Cary 50 equipped with a Hellma 115-QS cuvette at a wavelength of 350 nm. 2.2. Results. 2.2. Pressure Drop Measurements. For identical flow velocities and consequently identical residence times in the channel, the thinnest spacer shows the largest pressure drop and the other way around (Figure 4). Interestingly, in comparison to a 45° attack angle (the attack angle defines the orientation of the spacer filaments toward the main direction of flow), 90° orientation causes a larger pressure drop for the woven spacer of type Filmoor PBT. This contrasts with the extruded spacers, which yield lower pressure drop in this case. In terms of volumetric flow rate (Figure 5), the spacer with the largest cross section available for flow-through exhibits the lowest pressure drop and vice versa. Limiting Current Measurements. Figure 6 depicts mass transfer coefficients as a function of flow velocity. The Conwed spacer stands out in terms of mixing normal to the membrane, whereas the Naltex and the Filmoor spacer perform similarly. 1006



Article

Generally, a 45° attack angle favors mass transfer toward the membrane. In terms of volumetric flow rate (Figure 7), the Naltex spacer is less effective relative to other spacers. Its mass transfer coefficients are generally lower.

Figure 4. Pressure drop measurements as function of flow velocity (volumetric flow rate divided by the open area).

Figure 7. Mass transfer coefficient data obtained with the limiting current method.

Lateral Mixing. Maldistribution stands for the uneven spatial distribution of liquid, so that the residence time distribution of the liquid in a membrane module is significantly broadened. Broad residence time distributions lead to uneven and thus inefficient use of the stripping solvent and might even reduce the effective membrane area available for mass transport. As mentioned before, studies about mixing/distribution of liquid (in a plane parallel to the membrane) are available for high Reynolds number flow conditions. Since the flow velocity and as a consequence the Reynolds number is comparatively low in this study, the distribution mechanisms should be fundamentally different. This is the main reason why the following images are presented. The upper left-hand image of Figure 8 shows fluid flowing perpendicular and parallel (90° attack angle) to the filaments of the Conwed spacer. The flow follows closely the filaments; it does not significantly spread up- or downward independent of flow velocity (range tested: 0.3−2 cm/s). The image to the right depicts the Conwed spacer arranged at 45° attack angle contacted at a low flow velocity. Spreading of the tracer can be clearly observed. In the bottom row the images show the Conwed spacer at 45° attack angle, but with the liquids flowing fast. In these cases, the tracer spreads more widely in comparison to the upper right-hand image, which depicts slow fluid flow. The images in the bottom row are taken under similar operating conditions. However, if the channel width is restricted as on the right side, the tracer is reflected at the upper boundary. It appears as though two layers are flipped over each other, explaning the dark shade (upper right-hand corner of the righthand image). Even though having a woven geometry, the Conwed spacer channels the fluid without actually mixing it. Hence, the flow distribution process is convection-dominated and none that predominantly leads to equilibration of

Figure 5. Pressure drop measurements as function of volumetric flow.

Figure 6. Mass transfer coefficient data obtained with the limiting current method. 1007



Article

Figure 8. Dispersion in spacer-filled channels with (extruded) Conwed spacers (Difference in color responds to lighting conditions).

Figure 9. Dispersion in spacer-filled channels with (extruded) Naltex spacers.

Copper Dissolution Measurements. The copper dissolution technique is of particular interest for the investigation of permeate channel flow patterns inside spiral-wound modules. It yields boundary-layer mass transfer data as does the limiting current technique. For the purpose of calibration, measurement results for extruded spacers are presented in the following. It is important to point out that only one surface of the test cell is covered with copper. Since the transport of Cr4+ toward the copper surface limits the rate of reaction, significant differences especially in case of channeling as for the Conwed spacer are to be expected. To distinguish the effect of channeling, the experiment was performed twice for attack angles of 90°, once

concentrations. Owing to the well-defined channels of extruded spacers, noticeable differences between an attack angle of 45° and 90° for the Naltex spacer at a flow velocity of 2 cm/s (Figure 9) were also found. But, tracer spreading is negligible at low flow rates. Yet in comparison to the Conwed spacer, the flow through the Naltex spacer at similar flow velocities results in less pronounced spreading (Figures 8 and 9). This outcome is due to thinner channels, which render viscous forces more significant. The woven spacer of Figure 10 does neither provide well-defined microchannels nor large openings to flow through. Indeed, the Filmoor spacer does not lead to spreading at any tested flow velocity and attack angle. 1008



Article

Figure 10. Dispersion in spacer-filled channels with (woven) Filmoor spacers.

with the filaments in contact with the copper surface oriented parallel (A) to the main direction of flow and once oriented perpendicular (B). Because of symmetry, one set of experiments at 45° attack angle is sufficient (Figures 11 and 12).

Figure 12. Mass transfer coefficient data obtained with the copper dissolution technique (Conwed spacer); (1)−(6) indicate sample order; orientation of filaments in contact with the copper surface to the direction of flow: (A) parallel or (B) perpendicular.

channeling. Indeed, if channeling is predominant, then any fluid element traveling through the Conwed spacer net (arranged at 45° attack angle) comes into contact with only half of the copper surface. Therefore, whenever the effective copper surface area is reduced, this method significantly underestimates the true mass transfer coefficient. The mass transfer coefficients obtained for the Conwed spacer with the copper dissolution method at 90° attack angle are below the respective results of the limiting current method (Figure 13). Both the Conwed and the Naltex spacer performed better at 90° attack angle, when the filaments in contact with the copper surface lay perpendicular to the main direction of flow (Figures 11 and 12), which is counterintuitive. The flow range covered with the copper dissolution technique is shifted to higher values compared to the limiting

Figure 11. Mass transfer coefficient data obtained with the copper dissolution technique (Naltex spacer); (1)−(6) indicate sample order; orientation of filaments in contact with the copper surface to the direction of flow: (A) parallel or (B) perpendicular.

Figure 11 summarizes the results obtained with the copper dissolution technique for the Naltex spacer. The mass transfer coefficients tend to be higher as for the limiting current measurements (Figures 7 and 13). Arranged at 45°, the spacer shows the best performance, which is in line with the limiting current measurements. The results for the Conwed spacer (Figure 12) are in sharp contrast to this. The 45° arrangement clearly underperforms relative to the spacer arranged at 90° attack angle, which supports the findings of the tracer dispersion experiments regarding 1009



Article

fluid flips over from one microchannel to enter another. Thereby, the Conwed spacer does not contribute to the equilibration of concentration differences beyond mere diffusive transport. The extruded Naltex spacer also shows spreading through microchannels, but it is not as pronounced, because viscous forces seem to dominate. Finally, which of the portrayed spacers is the best choice for membrane-based extraction applications? It may suggested to use the Naltex spacer because it balances pressure drop with boundary layer mass transfer performance and channel hold-up with flow channeling. The latter may be exploited to avoid dead spots in corners.

3. PERMEATE CHANNEL DESIGN OF SPIRAL-WOUND EXTRACTION MODULES The stripping fluid’s uptake capacity is large relative to the rate of solute permeation. This translates to low cross-flow velocities if the stripping uptake capacity is to be fully utilized. In the end, the question arises how to achieve both the full usage of the stripping solvent as well as the efficient solute transport across the membrane. As previously stated, the solvent needs to be replaced in regular intervals at every spot inside the module and should be equally loaded with solute when leaving the module. Hence, the liquid should be distributed uniformly; maldistribution must be avoided. However, these characteristics are not sufficient for optimal module design because they do not account for spatial differences in the driving force. Besides the geometric dimensions of the module, performance may be affected by • the type of spacer used; • the orientation of the spacer relative to the main direction of flow, that is, by the flow attack angle; • and by baffles, which redirect flow.

Figure 13. Mass transfer coefficient data obtained from the limiting current method (abbrev: LCM) and the copper dissolution technique (abbr. Cu; orientation of the filaments in contact with the copper surface to the direction of flow: perpendicular).

current technique. The upper flow limit arises from the decreasing change in Cr4+ concentration with increasing flow rate, which renders results of even higher flow rates uncertain. The lower limit originates from the time it takes to obtain a stable signal. After all, the copper dissolution technique yields data similar, although not identical, to the ones obtained with the limiting current technique. 2.3. Discussion and Conclusions. The experiments reveal clearly for all tested spacers that • pressure drop is sufficiently low (

L Extraction Membrane

Recommend Documents