CFD SIMULATION OF A MEMBRANE DISTILLATION MODULE CHANNEL

Transcription

1 CFD SIMULATION OF A MEMBRANE DISTILLATION MODULE CHANNEL A. Cipollina *, A. Di Miceli *, J. Koschikowski, G. Micale *, L. Rizzuti * * Dipartiento di Ingegneria Chiica dei Processi e dei Materiali, Università di Palero, Viale delle Scienze, Ed.6, Palero (Italy). E-ail: cipollina@dicp.unipa.it. Fraunhofer ISE, Heidenhofstr.2,D Freiburg (Gerany). Abstract The interest towards the use of Mebrane Distillation (MD) processes for seawater desalination has been rising recently due to the ease of coupling MD with waste and/or solar theral energy. Notwithstanding the flexibility of the process and its potential for further developents in ebrane perforances, one of the ain drawbacks is the theral efficiency reduction caused by teperature polarization. Because of such phenoenon, only a sall aount of the driving force potentially available for the separation process, i.e. the teperature difference between evaporating and condensing fluids, is actually used for the separation. In order to reduce teperature polarization a study on the effects of spacer and channel geoetry has been perfored using Coputational Fluid Dynaics (CFD) techniques. A siple reference geoetry has been built to siulate the flow and teperature fields of a portion of a Spiral Wound MD odule channel. Results show how spacers can significantly affect teperature gradients within the channel, therefore odifying the effective driving force between the faces of the ebrane. The ain features, which an optial spacer should possess, have been thus indicated. Keywords: Mebrane Distillation, Coputational Fluid Dynaics, teperature polarization, spiral wound channel, spacer. Introduction and literature review Recent developents of desalination industry have been related to the rapid growth of ebrane technology. To this regard, while the first ost growing process is presently the Reverse Ososis, accounting for about 50% of the world desalinated water production, Mebrane Distillation is finding a space aong the so called hybrid processes. In fact the Mebrane Distillation process cobines the use of theral energy as energy input for heating the feed and a hydrophobic ebrane for obtaining the separation of the vapour fro the concentrated brine. It is worth noting that the interest towards MD technology has been steadily increasing in the last years due to the ease to couple this siple technology with nonconventional theral energy sources (i.e. solar theral energy or waste heat), particularly for sall scale units. Moreover, iproveents in ebrane technology have generated a draatic decrease in production costs, which, along with the fast increase in energy and etallic aterials costs, is further enhancing the potential advantage of MD process over conventional theral processes. The MD process, well established in several separation applications of the cheical industry, consists in contacting a hot liquid ixture with a icroporous ebrane. The ebrane acts as a filter for liquid olecules, while it allows for the passage of vapours, thus separating the coponents according to their different vapour pressures and ass transfer rates across the ebrane itself. When MD is used for desalination, in practice, only water fors vapour, which passes through the ebrane and, thus, separates fro the salt solution. 1

2 The driving force for the separation is in fact the partial pressure difference between the two sides of the ebrane, which in turn depends on the teperature difference across the ebrane and on the feed and distillate fluids teperatures. Several possible configurations have been presented so far [1], however two ain configurations are adopted for seawater desalination: Direct Contact MD (DCMD) and Air Gap MD (AGMD). In Direct Contact MD, the vapour, after passing across the ebrane, encounters a cold liquid and condenses by direct ixing with it. The cold liquid is usually fresh water and this configuration has the advantage of being very siple, with high ass transfer coefficients for the vapour passing across the ebrane. In Air Gap MD, an air gap is aintained between the ebrane and a cold surface (cold liquid or solid surface) where the vapour condenses. This configuration presents additional resistance to the transport of vapour fro one side to the other, but increases the theral efficiency by reducing significantly the heat losses due to conduction fro the hot to the cold side of the ebrane. Moreover it allows for the design of very copact spiral wound systes with direct heat recovery fro the condensing vapour flow to a pre-heating feed strea [2]. In any cases, where a spiral wound geoetry odule is adopted, the hot liquid (feed) flows inside a channel, obtained by positioning two ebrane layers separated by a spacer. This last acts as a echanical support for the ebranes but also as a turbulence prooter for the flow inside the channel. In order to highlight the ain phenoenological features of a Mebrane Distillation process a sketch of a Direct Contact Mebrane Distillation odule is shown in Fig.1. The process is characterised by the presence of a ass flux and a heat flux. The ass flux of vapour across the ebrane is regulated by three diffusion echaniss [3]: 1) Knudsen diffusion through the ebrane pores; 2) Poiseuille flow through the ebrane icro channels; 3) olecular diffusion through the air trapped inside the pores. For the sake of brevity the atheatical description of the three echaniss is not reported here, and only the final overall expression for the ass flux is given: N v ( ) = C P 1 P 2 (1) where C is to be regarded as a ass transfer coefficient and (P 1 -P 2 ) is the vapour pressure difference at the two sides of the ebrane. Considering the equilibriu relationship between T and P, and neglecting the effects of salt on such equilibriu eq.1 can be then rewritten as: where dp dt T N v = C dp dt T ' ( T T ) = C ( T ) 1 2 T 1 T2 is the slope of the vapour-liquid equilibriu curve at an average teperature T and C T a new ass transfer coefficient, which can be considered constant for sall ranges dp of operating pressures, where.can be considered constant. dt The heat flux is related to the passage of heat fro the hot liquid to the cold pereate, and is characterised by three resistances in series: 1) resistance fro the hot bulk to the ebrane surface, due to convective transport; 2) resistance across the ebrane, related to the transport of latent heat due to the vapour flux N v and the conductive transport due to the theral conductivity of the ebrane and the fluid filling the pores (k ); 3) resistance fro (2) 2

3 the ebrane surface to the cold bulk, due to convective transport. The relevant expressions are: with Q Q h ( T ) = h (3) h h T 1 ' ( T T ) + C ( T ) = Q + λ N = k λ (4) k ev v Q c c 1 2 T ev 1 T2 ( T T ) c = h 2 (5) Q = Q = Q (6) h The presence of a teperature gradient (T h -T 1 ) between the bulk of the liquid and the liquid in contact with the ebrane surface is defined as Teperature Polarization effect. It is an iportant paraeter as it causes a reduction of the overall teperature gradient (T h -T c ) to a saller effective gradient (T 1 -T 2 ) available for the ass flux across the ebrane. Thus one of the ais of the current research is to find MD odules geoetries which iniizes the Teperature Polarization effect. c Q h T h δt h Hot liquid (feed) N v T 1 T 2 Q k Q c Cold liquid T c Fig.1 Transfer Phenoena in Direct Contact Mebrane Distillation Coputational Fluid Dynaics has been recently adopted for the odelling of a wide range of systes for which experiental investigation is hapered, for exaple due to particular geoetrical features. This is the case of the study of thero-fluid dynaics in channels adopted for ebrane separation processes, in which the sallness of characteristic lengths does not allow for the introduction of probes and for the experiental analysis of the syste. Several studies have been perfored for the analysis of RO and NF channels in which only the fluid dynaics was investigated, highlighting the differences in spacers geoetry [4, 5]. In ost of these works the focus was on the analysis of pressure drops along the channel and the turbulence level generated by the spacers. Soe works also analysed the ass transfer relating it to the values of the local wall shear stress at the ebrane surface [6]. Fewer studies were found regarding the Mebrane Distillation process, where ainly experiental works on the physical properties of the ebrane and on the odule geoetries are presented [7, 8], while, to authors knowledge, no works have been yet presented on the study of the thero-fluid dynaics of MD odules. In the present work soe preliinary CFD results are presented for the case of a feed MD channel. The focus is ainly on velocities, pressure and teperature distributions inside the channel, with particular regard to the effect of the spacer geoetry on the distribution of such variables. 3

4 Models and ethods Siulations were perfored using ANSYS-CFX11. The doain geoetry and coputational grid were generated using Gabit. The doain represents a sall portion of a spiral wound MD channel with a double layer filaents spacer (Figs. 2 and 3). Doain sizes are 25x25x3 respectively in the x, y, z directions (see Fig. 2). The spacer was odelled overlapping two layers of longitudinal and transversal (at an angle of 45 ) filaents with a diaeter of The grid encopasses about 2,050,000 tetrahedral cells. Cell size was set in order to be coparable with the Kologorov length scale, to perfor a Direct Nuerical Siulation. To this regard, the flow regie had to be deterined, by estiating the Reynolds nuber using an equivalent diaeter De, which gives the following results: 4 A = 1.86 Pw v D Re = av e = 91 De = ν water λk = lz Re3 / 4 = 0.1 (7) (8) (9) where, A is the cross sectional area of the channel and Pw is the wetted perieter (considering the ebrane and the filaents), vav is the average liquid velocity, νwater is the liquid kineatic viscosity, lz is the characteristic length along the z-direction and λk is the Kologorov length. Consequently, the cell characteristic diensions were chosen to be 0.05 along the z-direction, and 0.15 along x- and y-directions. A transient siulation was then perfored setting a tie step of s in order to aintain the Courant nuber below 1. Also a Steady-State siulation was perfored for the sae geoetry, setting a nuber of iterations equal to Fig. 2 Siulated doain Fig. 3 Meshed geoetry An alternative coarse grid was generated with about 406,000 cells in order to assess grid dependency effects of the CFD results. In this case the tie step was set to s. Lateral periodicity was set in all cases, while inlet and outlet conditions were iposed by setting the inlet ean velocity (Dirichlet boundary conditions) and the outlet pressure 4

5 (Dirichlet boundary conditions on pressure, and Neuann boundary conditions on velocities) respectively (Fig.3). Syste features were chosen on the base of an MD spiral wound odule already operating and tested [2]. Inlet velocity was 0.05 /s, inlet Teperature 70 C. The syste consisted of a single phase, considering only water flowing in the channel. The evaporation on both sides of the channel at the ebrane surfaces was indirectly odelled by iposing an outgoing heat flux through the ebrane, thus allowing the investigation of the teperature polarization effect in the various zones of the doain. The iposed heat flux was of 11,250 kj/2h, corresponding to a ass (vapour) flux across the ebrane of 5 kg/2h. Siulations results Results for the standard case siulation are reported in Figs. 4, 5 and 6, as colour aps of velocity, pressure and teperature distributions respectively. Colour aps have generated with reference to the planes highlighted in Fig.2, i.e. z = (longitudinal filaents cut, Figs. 4a, 4b, 4, z = (transversal filaents cut, Figs. 4d, 4e, 4f) and x = 6.25 (vertical aps, Figs. 4g, 4h, 4i), at three different ties (0 sec, 1 sec, and 5 se. a) b) t = 0s d) t = 1s t = 0s 1s g) i) e) f) h) Fig.4. Siulated velocity aps at 0 sec, 1 sec and 5 sec on the three planes shown in Fig.2. 5

6 a) b) t = 0s t = 1s e) d) t = 0s 1s f) g) i) h) Fig. 5. Siulated Pressure aps at 0, 1 and 5s on the three planes shown in Fig.2. a) b) t = 0s d) t = 1s e) t = 0s 1s f) g) h) i) Fig. 6. Siulated Teperature aps at 0, 1 and 5s on the three planes shown in Fig.2. Line C Line B Line A Line C Line B Line A Line C Line B Line A Fig.4 shows that velocity field already stabilizes after about 1 s. As expected, velocities are uch higher aong longitudinal filaents, whereas are lower on the other plane. This is further confired by the presence of cal zones behind each transversal filaent as shown in the vertical profiles. Such velocity distribution affects in turn the teperature distribution (Fig.6), resulting in colder zones behind transversal filaents, with lower values of the heat transfer coefficient. Moreover the transient behaviour lasts longer for the teperature field, as indicated by the differences (although sall) between aps at 1sec and 5sec. Finally, pressure aps (Fig.5) indicate a pressure drop of about 10 Pa for a length of 25, which corresponds to about 400 Pa/ of channel. Fig.7 Location of horizontal profiles 6

7 In order to quantitatively analyze the CFD results, v- and T-profiles (Figs. 8 and 9) have been extracted along horizontal lines, lying in the plane y-z at x = 6.25 as shown in Fig.7, with an increasing distance fro the transversal filaents. Lines A, B, C were positioned after the first transversal filaent, A, B, C after the third filaent and A, B, C after the fourth filaent, therefore being in the sae relative position within the spacer eleentary cell (i.e. any region of the coputational doain liited by two subsequent longitudinal and transversal filaents), but at different distances fro the inlet (Fig.7). In this case, results refer to the standard case (fine grid) siulated under both transient and Steady-State conditions (Fig. 8a and 8b) and also to the siulation perfored with the coarse grid (Fig. 8. As it can be seen fro Figs. 8 and 9, the three cases are very siilar each other, thus indicating that 5 seconds are enough for the achieveent of steady-state conditions and that results are practically grid independent (Figs 8c and 9. a) b) Fig. 8. Velocity profiles along the z-direction. a) standard case, transient siulation (5s); b) standard case, Steady-State siulation; coarse grid, transient siulation (5s). 7

8 a) b) Fig. 9. Teperature profiles along the z- direction. a) standard case, transient siulation (5s); b) standard case, Steady-State siulation; coarse grid, transient siulation (5s). Moreover the figures clearly show that the presence of the inlet affects velocity and teperature profiles in the first spacer eleentary cell (i.e. the one containing the inlet), while in the third and forth spacer eleentary cells both velocity and teperature profiles are quite siilar, although in the case of teperatures the cooling process of the fluid generate a translation of profiles towards lower teperatures as they ove away fro the inlet. The above considerations highlight the attainent of fully developed flow after the third spacer eleentary cell, thus indicating the possibility of iposing a full periodicity at all 4 boundary planes of the coputational doain. This would justify for the use of a sall portion of the ebrane to analyse the behaviour of the whole unit. With regards to the teperature polarization phenoenon, Fig. 9 interestingly shows how the presence of tranversal filaents significantly increases the ΔT between the bulk of the liquid and the liquid in contact with the erbane surface due to the presense of cal zones, as already observed in Fig.4. Such findings provide an iportant preliinary inforation for the design of novel spacers geotry, which will be part of further research activities to follow. Conclusions Siulations of a typical MD channel geoetry have been perfored using the coercial CFD code ANSYS-CFX11. Preliinary results have shown that, under the assued operating conditions and geoetry, teperature polarization is significantly increased by the presence of transversal filaents, which are however necessary for the echanical role of the spacer. On the basis of these preliinary findings, novel spacer configurations can be designed and tested using the developed CFD ethods. 8

9 Acknowledgents This work has been carried out with the financial support of the MEDIRAS project within the EU-FP7 research prograe (contract nuber TREN/FP7EN/218938). References 1) K.W. Lawson, D.L. Lloyd, Mebrane Distillation, J. Mebr. Sci., 124 (1997), ) J. Koschikowski, M. Wieghaus, M. Roel, Solar theral-driven desalination plants based on ebrane distillation, Desalination, 156 (2003), ) R.W.Schofield, A.G. Fane and C.J.D Fell, Heat and Mass transfer in Mebrane Distillation, J. Mebr. Sci. 33 (1987), ) V. V. Ranade, A. Kuar, Fluid dynaics of spacer filled rectangular and curvilinear channels, J. Mebr. Sci. 271 (2006) ) C.P. Koutsou, S.G. Yiantsios, A.J. Karabelas, Direct nuerical siulation of flow in spacer-filled channels: Effect of spacer geoetrical characteristics, J. Mebr. Sci., 291 (2007) ) J.L.C. Santos, V. Geraldes, S. Verlizarov, J.G. Crespo, Investigation of flow patterns and ass transfer in ebrane odule channels filled with flow-aligned spacer using coputational fluid dynaics (CFD), J. Mebr. Sci., 305 (2007) ) A. Criscuoli, M.C. Carnevale, E. Drioli, Evaluation of energy requireents in ebrane distillation, Cheical Engineering and Processing, 47, 2008, pp ) S. Srisurichana, R. Jiraratananona, A. G. Fane, Mass transfer echaniss and transport resistances in direct contact ebrane distillation process, J. Mebr. Sci., 277 (2006)