polymeric membranes

Author’s Accepted Manuscript ADVANCES IN WATER BREAKTHROUGH MEASUREMENT AT HIGH TEMPERATURE IN MACROPOROUS HYDROPHOBIC CERAMIC/POLYMERIC MEMBRANES Felipe Varela-Corredor, Serena Bandini www.elsevier.com/locate/memsci

PII: DOI: Reference:

S0376-7388(18)30374-0 https://doi.org/10.1016/j.memsci.2018.08.005 MEMSCI16379

To appear in: Journal of Membrane Science Received date: 8 February 2018 Revised date: 3 August 2018 Accepted date: 4 August 2018 Cite this article as: Felipe Varela-Corredor and Serena Bandini, ADVANCES IN WATER BREAKTHROUGH MEASUREMENT AT HIGH TEMPERATURE IN MACROPOROUS HYDROPHOBIC CERAMIC/POLYMERIC M E M B R A N E S , Journal of Membrane Science, https://doi.org/10.1016/j.memsci.2018.08.005 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ADVANCES IN WATER BREAKTHROUGH MEASUREMENT AT HIGH TEMPERATURE IN MACROPOROUS HYDROPHOBIC CERAMIC/POLYMERIC MEMBRANES.

Felipe Varela-Corredor and Serena Bandini *

Department of Civil, Chemical, Environmental and Materials Engineering- DICAM Alma Mater Studiorum – University of Bologna – School of Engineering and Architecture Via U. Terracini, 28, I-40131, Bologna, Italy

(*) corresponding author e-mail : [email protected] tel +39 051 2090231 fax +39 0512090247

Key Words: Liquid Entry Pressure; Liquid Entry Temperature; normalized volume flux; macroporous hydrophobic membranes; tubular ceramic membranes

ABSTRACT

A systematic method of measurement of the liquid breakthrough of macroporous membranes at high temperatures is introduced. The method is based on the construction of the initial part of the “flooding curve”, performed by experiments carried out in a proper apparatus described in detail. Protocols of measurements

1

have been defined in order to get a measure of the volume flux across the flooded pores, at temperatures higher than the normal boiling point of the non-wetting liquid. To the best of our knowledge, the basis of the Liquid Entry Pressure measurement at high temperatures is presented for the first time and experimental results are documented in the temperature range from 20 to 140 °C. The new concept of Liquid Entry Temperature is also introduced. A new theoretical-based criterion of data elaboration has been developed. The concept of “normalized volume flux” has been introduced to compare results at different temperatures and to allow an improved definition of the minimum Liquid Entry Pressure. An overall characterization of the process applicability of hydrophobic membranes has been made possible. Prototype macroporous hydrophobized membranes supported on multilayer ceramic (titania) mono-channels were tested. The protocol of measurement has been validated by a commercial PTFE membrane also.

1. INTRODUCTION In the last two decades ceramic membranes gained the interest of industry for all the applications requiring high mechanical, thermal and chemical stability. Treatment of food products (milk, wine, beverages), clarification of fermentation broths in bio-industry [1], environmental applications such as waste-water treatment and oil/water separations [2] and membrane reactors [3] are some of the most interesting examples. Membranes are typically asymmetric, homogeneous or composite, in which layers of different porosity are deposited to achieve the desired cut-off;  – Al2O3 as well as TiO2 and ZrO2 are commonly used to manufacture macroporous and/or mesoporous membranes, as monochannels or monoliths, useful for microfiltration, ultrafiltration and fine ultrafiltration [4,5]. Microporous TiO2 and ZrO2 membranes have been proposed for Nanofiltration applications also, both as mono-channels and in capillary geometries [6–8] 2

More recently, studies have been devoted to the modification of the selective layer in order to perform separations requiring hydrophobic surfaces. Pervaporation of ethanol/water mixtures and Organic Solvent Nanofiltration of organic isomers were proposed by a secondary growth of a zeolite layer [9–11] and by Carbon incorporation [12] in the top-layer of the lumen of a tubular TiO2 membrane. Carbon-based alumina membranes were also developed for gas separation [13,14]. Owing to their high thermal stability, ceramic membranes have been also proposed to be used as Membrane Contactors (MC) for mass transfer operations at high temperature. Membranes are functionalized by grafting polymers such as fluoroalkylsilanes onto the membrane surface to make the macroporous top-layer hydrophobic, with the aim of performing the typical Membrane Distillation (MD) operations [15–27]. Working at higher temperatures than in case of traditional polymeric membranes (TF200, PVDF, polypropylene) is remarkably advantageous in that higher transmembrane fluxes can be obtained [19,27,28]. The quality and the extent of hydrophobicity of the functionalized membranes have been well documented by the typical characterization methods in which the contact angle on flat samples and/or the minimum Liquid Entry Pressure (LEPmin) are often measured with water at room temperature, sometimes in addition to thermogravimetric analyses and SEM pictures. However, membrane characteristics remarkably depend on the functionalization method, as well as on the procedure followed to deposit the ceramic layers which determines the pore radius distribution, the porosity and tortuosity of each layer (which are partially claimed as confidential by authors). Generally, it is not possible to understand by these characterization techniques if the sample can remain un-flooded along a wide temperature range; only the measurement of the LEPmin as a function of temperature can give a correct univocal information about the applicability of the membrane as a hydrophobic membrane contactor operating at high temperatures. Contemporary to the development of hydrophobized ceramic membranes, new high temperature resistant polymeric membranes were proposed [29–33]. Sirkar and coworkers [29] reported the use of PTFE flat sheets in Direct Contact MD (DCMD) for brine desalination at temperatures up to 124 °C; although the transmembrane pressure differences were not explicitly reported, the description of methods allows to understand that experimentation was performed only at very low transmembrane pressure values, quite close to zero. Conversely, theoretical studies [34] 3

proposed that raising the feed temperature from 80 to 180 °C for DCMD of desalination, by keeping 10-15 °C of temperature difference across the membrane, might increase the flux up to nine times, doubling the thermal efficiency; however the study was performed assuming that the LEPmin might be a constant value with temperature. The question of how the LEPmin can depend on temperature is neither a trivial problem nor an academic topic only. In membrane contactor processes, such as MD, an efficient operation requires that the pressure difference across the membrane is kept lower than a threshold value, the LEPmin, so that a liquidvapour interface is immobilized at the feed-membrane side; the liquid does not enter the membrane pores and diffusion of vapors occurs across a gas phase located inside the membrane. When the pressure difference overcomes LEPmin, the larger pores of the membranes become flooded and the separation efficiency of the membrane decreases, since a part of the liquid flows across the membrane. Pore wetting was remarkably put in evidence in [35] as one of the most important problems in a stripping step for carbon-dioxide removal in which the liquid temperature was increased up to 60 °C. In that case, severe performance losses were observed by using polypropylene 270 nm pore diameter hollow fibers, owing to a liquid flow across the membrane. Although in [29] the operation with polymeric membranes is documented at high temperatures, the pressure difference across the membrane was kept very close to zero. Operating with an industrial module, it is very difficult to accept an application constrained to keep pressure differences across the membrane very close to zero since the pressure drops in the feed side and in the permeate side can reverse the pressure difference value across the membrane along the module. In an industrial module it is then necessary to keep operative conditions which can allow LEPmin values higher than a minimum threshold value which should be set as the most convenient for design purposes. Basing on the Laplace –Young equation, it is well-known that the LEP depends on membrane material and on the membrane pore morphology, through the contact angle and the pore radius distribution, as well as on the thermodynamic properties of the fluid, through the surface tension. Owing to a non-regular pore geometry and pore radius distribution, as well as to the effect of the surface heterogeneity which affects the contact angle, the minimum LEP has to be experimentally measured. Typically, two basic techniques are followed: the visual method 4

originally introduced by Franken [36] and the flooding curve method presented by McGuire et al. [37] and followed by others [17,38,40,42]. Franken and coworkers [36] gave the first guidelines to measure the LEPmin , defined as the pressure at which the first liquid drop of a non-wetting liquid can be visually evidenced in the permeate side of the membrane, kept at atmospheric pressure; the measurement is generally performed at room temperature. The procedure has been reproduced by several authors [18,27,43–47] with minor changes and it is well documented in literature [48]; it is important to observe that this kind of measurement is rather subjective and the accuracy of the value depends on the operator ability. McGuire et al. [37], aiming to develop a method to evaluate the pore size distribution of microultrafiltration membranes with a non-wetting liquid, introduced a procedure according to which the flux of the non-wetting liquid was measured as a function of the applied transmembrane pressure, obtaining a sort of “flooding curve”, basing on the work of Grabar and Nikitine [49]. The method was followed later by García-Payo et al. [40], who performed the flooding curve for various hydrophobic polymeric membrane samples, using alcohol-water mixtures. From their accurate results, it was evident that even at very low pressure values a liquid flux occurred across the membrane; as a consequence, they defined the LEPmin as the transmembrane pressure at which the flux overcame a value of 2.7 dm3/(hm2), assumed as an arbitrary appropriate value. The liquid permeation technique procedure has been used by other authors [17,38,39,41,42]. In [39,41] the main objective was obtaining a pore size distribution; in [17,38,42] authors defined their own LEPmin arbitrary criterion, either according to a “visual” method [17,38] or according to an arbitrary flux definition, strictly dependent upon the flow measurement capacity of the instrumentation [42]. Finally, it is important to observe that, to the best of our knowledge, minimum LEP values were typically measured at room temperature. Only Saffarini et al. [44] tested PTFE samples up to 70 °C, obtaining a remarkable reduction of the LEPmin down to 40 % with respect to the values measured at room temperature. Aim of this work is the introduction of a systematic method of measurement of the minimum Liquid Entry Pressure at high temperatures, which is expected to decrease with temperature, based on the flooding curve method, to obtain the water breakthrough at temperatures up to 140 °C. The experimental apparatus and the operative procedure are described in detail. The same 5

apparatus is used also to perform experiments at a constant pressure difference across the membrane as a function of temperature of the non-wetting liquid; the new concept of the Liquid Entry Temperature is defined straightforwardly as a further characterization parameter. In addition, an advanced criterion for data elaboration is also presented, which allows to obtain a value of the LEPmin, which is less dependent of the operator ability and/or of the definition of the “minimum appropriate flux” with respect to the conventional techniques. The same criterion can be also used to compare results obtained at different temperatures and/or pressures. Experimentation is above all carried out with prototype ceramic membranes, coated with different types of hydrophobic layers, with a twofold objective: to develop the method of LEP measurement at high temperatures and to document that the method is a tool to test, by using only one unique characterization technique, the “process applicability” of a membrane as potential element of a membrane contactor. Commercial polymeric TF200 membranes are also tested in order to support the validity of the method, since their characteristics are well known to the scientific community working on membrane distillation [48].

2. THEORETICAL PREMISES

The new method for the minimum Liquid Entry Pressure measurement here proposed, as well as the criterion for data elaboration, rely on the following theoretical premises. The discussion is presented with reference to hydrophobic membranes characterized by macro-pores with diameter greater than 20-50 nm typically used in membrane contactors; water is the nonwetting liquid.

2.1

the “normalized volume flux”

For a cylindrical rigid hydrophobic single macropore of radius rp, the capillary equilibrium of pressure forces between a liquid phase (PL) and a gas phase (PG) is described by the LaplaceYoung equation, represented by Eq. (1), in which  is the contact angle of a water drop with the solid pore wall and LG is the water surface tension at the liquid-gas interface. It is well known 6

that liquid water cannot enter the pore as far as the pressure difference across the interface (P= PL-PG) is kept lower than the equilibrium value which represents the liquid entry pressure (LEP). Since the surface tension is a decreasing function with temperature and the contact angle typically decreases with temperature [50,51], the LEP is expected to be a decreasing function of temperature.

equilibrium at L-G interface

PL  PG 

2 LG (T ) cos  (T ) rp

 LEP(T )

(1)

When the pressure difference P overcomes the LEP value, at a given temperature value, the liquid water flows across the macropore following typically a laminar regime; the liquid volumetric flow rate (Q) can be then described by Eq. (2), in the case in which  represents the pore length and w is the water viscosity inside the pore, according to a Hagen-Poiseuille flow.

 rp4 P at T=T0, for any P  LEP(T0 ) then Q  8  w (T0 )

(2)

Equations (1) and (2) can be extended to the case of an hypothetical membrane with a total number of cylindrical pores (Np,tot) accounting of a pore size distribution. In such a case, the minimum value of LEP (LEPmin) can be defined in correspondence of the maximum pore radius as a function of temperature, as described by Eq. (3).

LEPmin (T ) 

2 LG (T ) cos  (T ) rp,max

(3)

For an un-wetted membrane, at a constant temperature value (T0), by increasing the pressure difference P above the corresponding LEPmin, the number of flooded pores increases, as a function of P. Assuming that rp(P,T0) represents the generic pore which becomes flooded at the corresponding pressure P, the interfacial equilibrium is represented by Eq. (4). We can observe that, at the same pressure value P, there is a number of pores with a pore radius in the range from rp(P,T0) to rp,max which has been already flooded; the cumulative volumetric flow rate of liquid (Qp) , occurring across all the flooded pores, depends on the pore size 7

distribution, and it can be expressed as reported in Equation (5). The pore size distribution is described by a function (f(x)), in which (x) represents a dummy variable of integration related to the radius of the cylindrical pores. at T=T0 , at P  PL  PG  LEPmin (T0 ) rp (P, T0 ) 

Qp  

2  LG (T0 ) cos  (T0 )

rp ,max

rp ( P ,T0 )

P

N p ,tot

 x 4 P f ( x)dx 8  w (T0 )

(4) (5)

Eq.(5) is the Grabar and Nikitine [49] relationship, as it is reported and used by McGuire et al. [37] to determine the pore size distribution in hydrophobic microfiltration membranes from liquid permeation measurements. Eq.(5) can be re-elaborated accounting of the membrane total surface (A) and of the surface porosity () [48] as shown in Eq.(6), in which the surface porosity and the membrane thickness are assumed as constant values with temperature and pressure. Qp 

r  A P  x 2 f ( x)dx  Qp , H (P, T0 )  Qp , E (P, T0 ) 8  w (T0 ) r ( P ,T ) p ,max

p

(6)

0

Apparently, the liquid volumetric flow rate (Qp) is due to the contribution of two terms: -

the hydraulic component (Qp,H) which depends on the geometry and on the dimension of the pore (which in turn determines the flow regime), on the pressure difference (P) and on temperature, which is included in the water viscosity (w);

-

the equilibrium component (Qp,E), represented by the integral part, which depends on the membrane material and on the fluid type, through rp(P,T0), according to Eq.(4), as well as on the pore geometry; only this term contains information about the number of flooded pores.

By accounting Eq.(4), it is apparent that the number of flooded pores can change both with temperature and/or with the pressure difference: at constant temperature, by increasing

P, smaller pores can become flooded; conversely, at constant P, some smaller pores can become flooded by increasing temperature also. As a consequence, Qp,E can increase both with temperature and/or with P.

If all the pores are flooded, when the membrane 8

undergoes an increase of the pressure difference P and/or of the temperature, Qp,E keeps as a constant value, whereas Qp,H increases. Conversely, if none of the pores is penetrated, both the hydraulic and the equilibrium terms vanish to zero. Equations (3) to (6) can be easily generalized for the case of a “real membrane” as it is summarized in Table 1, in which equations are written accounting of the following assumptions: -

the pore size distribution is described by a function (f(x)), in which (x) represents the radius of hypothetical cylindrical pores;

-

the complex pore morphology can be described by using the average pore tortuosity () and the geometrical parameter ( accounting of the shape of the pore;

-

the parameter B, introduced by Franken [36], is used to account of all the irregularities of the pores which can affect the contact angle with respect to the value existing on a smooth flat surface;

-

a laminar motion occurs inside the flooded macropores of the membrane according to a Hagen-Poiseuille flow (Eq.(2) or Eq.(6));

-

all the defects in the membrane are collected as the sum of hypothetical cylindrical holes with an average radius rD,i and an average length D,I;

-

the volume flow rate across the defects (Qp,D) is expressed according to a laminar motion.

Indeed, in the case in which the membrane is heterogeneous and asymmetric, formed by various hydrophilic intermediate layers of different porosities, as it is the case of ceramic membrane contactors, as an example, some defects of different nature might exist in the membrane, especially in the hydrophobic coating. We can account as defects some pores in the top layer which have not been successfully coated by the hydrophobic layer, as well as a tailed pore size distribution in the ceramic top layer on which the coating is deposited, and/or microfractures (pinholes) of the top layer, and/or leakages across the final end caps of the membrane, and so on. Those “defects” can give rise to an additional volumetric flow rate (QP,D) which cannot be ascribed to any penetrated hydrophobic pores, but it is an additional flow rate which occurs also when no hydrophobic pore is flooded. Basing on Eq.(1.3)-Table1, the “normalized volume flux” can be finally defined, as reported in Eq.(7).

9

"normalized volume flux "  N J v  w Q p  w (T ) 1 defects    P A P A i 1



CD

J v w P rp ,max  rD4,i    x 2 f ( x)dx r (  P , T ) p 8 D ,i  

+

(7)

CM  FE (P, T )

The “normalized flux” is very close to the classical definition of the hydraulic permeability of an un-fouled hydrophilic microfiltration membrane (as well as of a nanofiltration membrane [52]). Apparently, it accounts of three contributions. CD is a sort of “hydraulic permeability” associated to the whole defects, which is independent of temperature and pressure conditions; CM is a coefficient containing information on the morphological characteristics of the membrane; FE is the quantity containing all the information related to the hydrophobic character of the membrane. FE remains as zero-value, as far as none of the hydrophobized pores is penetrated by the liquid; as a consequence, the P value and/or the temperature value at which FE increases correspond exactly to the condition at which the membrane becomes to be penetrated by the non-wetting liquid, also in case of “defects”. In the case in which the morphological parameters (CM) can be assumed independent of temperature and pressure, as well as on the operating fluid (that is the case in which swelling phenomena can be neglected), Eq. (7) becomes a good basis to establish a criterion for data elaboration. Indeed it can allow the comparison among flooding curves data obtained at different temperatures and for different membrane samples, as documented in the following section.

2.2

Liquid Entry Pressure and Liquid Entry Temperature

The most representative behaviors of flooding curves are reported in Figure 1a), as they are typically presented by academics along undergraduate courses [37, 40, 48, 53, 54]. In the case of hydrophilic membranes, a linear behavior can be observed of the volume flux (Jv) as a function of pressure difference (P) at a constant temperature, since in macroporous membranes a laminar motion occurs following the Hagen-Poiseuille flow. In the case of a hydrophobic “ideal membrane” with a unique pore size, Jv is zero until the LEP is overcome (according to Eq.(1)); at P greater than LEP, the flooding curve matches up with the 10

hydrophilic case, since all the pores are flooded. Finally, in the case of a “no-defects real membrane” with a pore size distribution, Jv is zero until the LEPmin is overcome (Eq. (1.1)-Table 1); at P greater than LEPmin, the flooding curve increases as the pressure difference increases, assuming different slopes depending on the pore size distribution; only when all the pores are wetted, the flooding curve matches up with the hydrophilic case. However, in the case in which the membrane is a “real” membrane with “defects”, the volume flux Jv can slightly increase with the pressure difference at low P values and only at higher P values can change sharply its slope when the hydrophobized pores are becoming flooded. When a case like that occurs, it might be difficult to detect the exact value of the LEPmin, which can correspond to the knee-zone of the solid line in Fig.1a). The problem can be by-passed if the data are elaborated according to the “normalized flux method”, as defined in Eq. (7); in Fig.1b) elaborations are shown of the corresponding cases reported in Fig.1a). At low P, in case of defects, the volume flux can appear linear with the driving force and, correspondingly, the “normalized volume flux” can appear as a constant value (represented by the CD contribution in Eq.(7)). Although it is not possible to discriminate among the defects type (if they are leakages or some non-hydrophobized pores, for instance), the magnitude of the “normalized flux” gives an indication of the whole entity of the membrane defects and a sort of “minimum permeability” can be related to the minimum extent of defects which can be acceptable to design a membrane contactor.

The absolute value of this minimum

permeability is an important indication about the morphological quality of the membrane, since it puts in evidence that a liquid flux occurs across the membrane, which cannot be avoided by keeping the operative conditions lower than the LEPmin: the lower is the “minimum permeability” value, the better is the membrane for MC operations. The LEPmin of a hydrophobic membrane can be then defined as the lowest differential pressure value at which the normalized volume flux increases at constant temperature, as it is indicated in Fig. 1b). At P values lower than the LEPmin, the membrane can be considered unflooded, according to the usual meaning of this term. Remarkably, the “normalized flux” method allows the evaluation of the LEPmin without using any arbitrary flux value as reference, as performed by all the previous authors [17, 40, 42]. In addition, it allows also the definition of the maximum value of the “minimum permeability” which can be assumed as an acceptable value for a proper design of a membrane distillation 11

operation. The “flooding curve” and the “normalized flux” method are therefore a tool to obtain an overall characterization of the quality of the coating techniques, both with regard to the process applicability of the membrane (through LEPmin) and with regard to the coating procedure and/or to the membrane manufacturing technique (through the quantification of the “minimum permeability”). No additional characterizations, such as morphological characterization, contact angle measurements, porosimetry, pore size distribution, and so on, are strictly necessary to draw a final conclusion about the potentialities of the membrane as a membrane contactor. An example of experimental flooding curve at 25 °C is reported in Figure 2 (see section 2 “materials and methods” for details). In Fig. 2a) a typical trend is obtained of the initial part of the flooding curve, in which all the phenomena described previously on discussing Eq.(7) can be observed. A graphical representation of the LEPmin calculation is reported in Fig.2b), by using the “normalized flux method”, accounting of the number of experimental data and of the instrumentation precision (=0.1 bar). With reference to the points A and B in Fig.2b), The LEPmin and the corresponding flux are obtained as indicated by the relationships (8).

LEPmin  PB  (  );   PB  PA J v at LEP  J v at P min

(8)

B

The shape of the initial flooding curve reported in Fig. 2b), although it represents only a part of the overall curve, can give partial information about the pore size distribution, when it is compared with the theoretical curve reported in Fig.1b): in this case, a very steep increase of the curve after the LEPmin value indicates a narrow pore size distribution, at least with regard to the larger pores, whereas a gentler slope would have indicated a wider pore size distribution. As a general conclusion, it can be observed that the initial part of a flooding curve of the type reported in Fig. 1, or in Fig.2, can be obtained from experimental measurements of Jv vs. P, when operating at constant temperature, or from experimental measurements of Jv vs. the temperature of the non-wetting liquid, when operating at a constant value of P across the membrane.

Correspondingly, the definition of a minimum value of the Liquid Entry

Temperature (LETmin) can be derived straightforwardly, by using the “normalized flux method” according to the meaning of FE contribution in Eq.(7). Eqs. (8) can be used for a graphical

12

representation of LETmin also, by replacing the pressure term with temperature (as indicated in Fig.1b). Definitions of LEPmin and LETmin are indeed a consequence of Eq.(7): as already observed, the “normalized volume flux” can increase with differential pressure and/or with temperature only, when the operative conditions lead to an increase of the parameter FE. The membrane can be then flooded both at constant temperature, by increasing the differential pressure, and at constant pressure difference, by increasing temperature. Obviously, LET and LEP are related to each other: if the LEPmin is available at a temperature value of T0 we have also the LETmin=T0 at a pressure difference across the membrane corresponding to the same LEPmin value. The knowledge of LETmin becomes important and useful for design purposes, since it defines the maximum temperature at which the membrane contactor can operate, after the minimum pressure difference which can be kept across the membrane in an industrial equipment has been set. To explain the concept, we can refer as an example to the case of a stripping operation with a Membrane Contactor, performing the so-called Sweeping Gas Membrane Distillation (SGMD) [48,53], in which a gaseous stream is used downstream the membrane to extract volatile compounds from a liquid stream, without phase mixing. Independently of the knowledge of the membrane morphology, it is a matter of fact that the SGMD process is possible if the liquid does not penetrate into the membrane pores. That requirement is fulfilled by operating at proper conditions which can be selected accounting of the following constrains, for any section of the module: a) the liquid pressure should be higher than the gas pressure in order to avoid gas bubbling into the liquid stream; in addition, the pressure difference across the membrane should be lower than the LEPmin, which can be estimated at the liquid bulk temperature (TL), according to Eq. (9.1);

requirement n.1)

PL  PG and P  PL  PG  LEPmin (TL )

(9.1)

b) the liquid stream should typically leave the module at a pressure not lower than the atmospheric value; at the same time, the liquid pressure should be higher than its 13

boiling pressure (PBP) and should account the pressure drops in the liquid side (Pdrop,L), according to Eq.(9.2);

requirement n.2)

PL  Patm  Pdrop, L and PL  PBP (TL )  Pdrop, L

(9.2)

In order to fulfill all the requirements (Eqs. (9.1)-(9.2)), it is necessary to set the liquid temperature TL and the P value across the membrane at appropriate values. However, we can recognize that these quantities are interdependent each other. In Figure 3, two different equivalent criterions are presented to perform a correct selection of operative conditions. According to flow sheet a) of Figure 3, it is necessary to set the liquid temperature TL, in order to determine LEPmin and to calculate the minimum value of the liquid pressure (PL,min); at the same time, the acceptable range of P across the membrane is defined straightforwardly. The operating liquid pressure PL can be then selected and the minimum value of gas pressure can be defined; finally, after the selection of the P value, the operating gas pressure PG is obtained. Conversely, according to the flow sheet b) of Figure 3, the design chart can start with the selection of a reasonable value of P across the membrane, which should be consistent with the equipment constraints; correspondingly, the maximum operating temperature of the liquid stream is obtained as the LETmin at the selected P. The liquid temperature TL can be then selected, and the minimum value of the liquid pressure (PL,min) is obtained straightforwardly. Finally, the operating liquid pressure PL can be selected and the corresponding value of the gas pressure is obtained. It is self-evident that the sequence reported in flow sheet b) is more convenient for design purposes.

3. MATERIALS AND METHODS

3.1 Membranes Ceramic and polymeric membranes were tested. 14

Polymeric membranes were mainly used to validate the method of measurement; to that purpose the commercial flat TF200 membrane was selected, since it was one of the most widely investigated membranes for membrane distillation applications [48]. TF200 membrane is a 60 m thickness PTFE film with an average pore diameter of 200 nm and a surface porosity of 80 % [19,48], supported on a polypropylene mesh, manufactured and distributed by Pall Inc. Ceramic membranes, on the contrary, were prototype membranes manufactured by Fraunhofer Institute for Ceramic Technologies and Systems (IKTS, Hermsdorf, Germany) as macroporous tubular multilayer titania membranes (7 mm inner diameter, 10 mm outer diameter, total effective length of 224 mm, excluding the end caps length). Membranes were coated in the top layer by the manufacturer with different polymeric materials (claimed as confidential) to provide hydrophobicity; in the following, the samples tested are tagged as “silane”, as “F” (=Fluoroalkylsilane) or as “P+F” (a combination of polyester and Fluoroalkylsilane on carbon-based titania membranes [12]) to identify the different coatings. Tubular multilayer titania membranes with top layer of 100 or 250 nm average pore diameter were used. In all the samples the top layer was located in the lumen-side, the pore size of each intermediate layer increased from the inner to the outer diameter of the channel with average pore diameters of 250, 800, 4500 nm, respectively. Morphological characteristics of each ceramic layer, with the exception of the 100 nm top layer, have been widely documented in [9]. Impermeable glass end-caps have been designed by the manufacturer to prevent leakages of liquid from the lumen side towards the outer side. Membranes are accommodated in a housing containing only one sample; the inner membrane area of 49.2 cm2 was used as reference area for data elaboration. A detail of the membrane-vessel assembling is reported in Figure 4c) in which the membrane coating, the glass end caps and the viton sealing are sketched; the correct path of the liquid during the “flooding” operation is also indicated by dashed lines and arrows. Proper tests were performed to check the correct sealing of the viton o-ring. One sample of an uncoated titania membrane with a 100 nm top layer, endowed by the same kind of glass end caps, was also tested in the same housing. The choice of testing ceramic samples was planned with various purposes. First of all, there was the request of performing the characterization of prototype membranes with the aim of testing their applicability as potential elements of membrane 15

contactors to be used at high temperatures. To that purpose, it has been put in evidence in the previous sections that the LEPmin measurements, as well as /or the LETmin measurements, are the most proper characterization techniques to obtain an overall process characterization of a membrane, without the need of measuring any other supplementary parameters. Secondly, ceramic materials were considered very interesting since they allowed experimentation at higher temperatures than the TF200 samples, which could not be used over 70-80°C, owing to the PP support. In the temperature range investigated in this work, ceramic membranes were also expected to be not affected by swelling phenomena and then the geometrical and morphological parameters of the membrane could be assumed as constant values. As a consequence, those membranes were the perfect tool to develop and test the method of LEP measurement at high temperatures here proposed.

3.2 Experimental apparatus The flow sheet of the experimental setup designed to perform the measurements is shown in Figure 4a); rough schemes of the tubular membrane housing (Fig.4c)) as well as of the cell containing a flat sample (Fig.4b)) are also reported. The flat cell is a circular cell, in which the feed enters the center and flows out radially; one sample of 10.8 cm2 can be tested. Detailed description has been reported in [52]. Pictures of the overall apparatus as well as of the open flat cell are reported in the supplementary material. The set-up was designed to be used with any fluid compatible with AISI 316 H, up to a maximum gauge pressure of 10 bar; the following description is made with reference to water. The housing is immersed in a thermostatic oil bath, operating from 30 to 150 °C; the bath temperature (T1) is controlled, whereas temperature of stream (4) is measured by a thermoresistance (T2), located immediately downstream the bath. Streams (1) and (2) are the inlet and outlet streams of the feed side, respectively, which is the lumen of the tubular membrane or the PTFE layer of the flat membrane; correspondingly, streams (3) and (4) represent the permeate side.

16

The tank is filled with demineralized water (3-14 S/cm) at room temperature and kept pressurized by a compressed air line (V-1, V-3); another compressed air line is used (V-2, V-4), when necessary, to pressurize the permeate side also. All the instruments are detailed in the caption of Figure 4a), noticeable is the back-pressure micro-valve (V-5) with a flow coefficient of 0.004, located on the stream (4) before the flow measuring system (F1). Valves (V-3) and (V-4) are used in association with the corresponding pressure reducers (V-1) and (V-2) for a fine regulation of pressure (P1) and (P3). The volume flow measurement on stream (5) is based on two micro-pipettes of different capacity and it is performed at atmospheric pressure and at temperature (T3). The volume flow measurement could be replaced by a mass flow meter also. During all the experimental runs, (T2) was recorded as coincident with (T1). The same apparatus is used to perform flooding curve measurements at constant temperature or at constant differential pressure across the membrane in a “dead-end” configuration (V-10 is always kept closed). Details of the experimental procedure are reported in the following section. The set-up was also used to measure the hydraulic permeability of uncoated samples at room temperature, in a “dead-end” operation mode. 3.3 Experimental Procedure In case of hydrophobized ceramic membranes, the following protocol was defined, accounting that high LEPmin values were expected at low temperatures. a) Drying of the fresh virgin sample in a convection oven at 60°C, 4 hours long; b) Sample conditioning in demineralized water (3-14 S/cm), at room temperature, at least for 12 hours; c) Filling up the apparatus (streams (1) to (4)) with demineralized water at atmospheric pressure and at room temperature, then closing valves (V-6, V-8 , V-11); d) Bath temperature stabilization, typically 1 hour long; e) measurement of the initial part of the “flooding curve” according to one of the Procedures; f) Shut down procedure: the equipment is kept at the last pressure value investigated until the temperature decreases down to 70 °C; the pressure in the permeate side is decreased to the atmospheric value and the overall system is switched off; g) Sample drying in oven at 60°C (4 hours long) and at 130 °C (6 hours long). 17

After step g), the sample can undergo further measurements. On presenting the results, trials are sequentially numbered in chronological order. In case of polymeric membranes, since only one trial was performed by using a virgin sample, the protocol was followed from step b) to f). 3.3.1 the measurement of the initial part of the “flooding curve” Three different procedures were developed, which can be applied for any non-wetting liquid. Each of them should start with equal pressure values between the liquid feed side and the liquid permeate side. Procedure 1 is the advanced protocol developed in this work for measurements at high temperature: it is followed to perform the LEPmin measurement at liquid temperatures close/above the normal boiling point of the non-wetting liquid. It requires to keep the permeate side at a pressure value higher than the boiling pressure of the liquid, at the given temperature under investigation, so that the non-wetting fluid remains liquid along all the experimentation. It basically consists in creating an increasing pressure difference across the membrane, at constant temperature, operated by keeping the feed side pressure at the highest desired value and by decreasing the permeate side pressure along the experimentation. The method is recommended at temperatures higher than 80 °C, in case of water as a non-wetting liquid. Procedure 2 is exactly the usual protocol accepted by the scientific community and also carried out by other authors [17,37,38,40,42,48] by keeping the permeate side at atmospheric pressure and by increasing the feed liquid pressure at constant temperature; in case of water, it is recommended at temperatures lower than 80°C. Procedure 3 is a modification of Procedure 1 operated in order to perform the LETmin measurement, by keeping the pressure difference across the membrane as a constant value and by increasing the temperature of the non-wetting liquid; the procedure is applicable in any range of temperature; in this work it was applied in the whole range of temperature from 40 to 140 °C. A comparison between the results obtained with the same sample tested both according to Procedure 1 and to Procedure 2 is reported in Figure 5 (operative pressures are reported in detail in the caption, for both trials). The validity of the new method is self-evident, since 18

results obtained according to Procedure 1 are aligned with those obtained according to Procedure 2, leading to a unique flooding curve.

Procedure 1 In this case, the operative pressure values should be set by accounting that (P2) should be higher than (P3), which is constrained to be higher than the water vapor pressure at the desired temperature T1, to avoid evaporation in the permeate side. It is convenient to select the set point of (P2) as corresponding to the higher value desired in the whole trial. The bath stabilization at the desired temperature T1 (step (d)) is carried out by keeping valves (V-5, V-9 to V-11) closed; (V-6) and (V-8) are initially closed. Before the heating procedure, pressures (P2) and (P3) are increased up to the set-point of (P2), by keeping P3=P2. The procedure of pressure stabilization is carried out by using the compressed air line connected to the tank S1 and the compressed air line connected to the stream (3), by opening valves (V-6) and (V-8). This operation should be performed carefully. Firstly, valves (V-3) and (V-4) are opened to equilibrate the pressure in the two lines upstream the valves (V-8) and (V-6), respectively. Secondly, valves (V-6) and (V-8) are opened contemporary and carefully, so that a gradual increase of pressures (P2) and (P3) is obtained; during this operation, it is recommended to avoid transmembrane pressure differences (P2-P3) greater than 0.2 bar. Pressures equilibration should be checked along the heating procedure. After the temperature and pressure stabilization, the measurement procedure can start. After opening valve (V-9), the pressure of the permeate side (P3) can be decreased by opening the micro needle valve (V-5), so that the desired transmembrane pressure P=P2-P3 = P1-P4 can be set. For each P value, a minimum of 60 minutes stabilization time is required before taking flux measurements; at least three subsequent measurements should be performed, repeated every five minutes; the arithmetic mean of measurements is reported as the final value. Since the “flooding curve” is performed only in its initial part, very low volume flow rates are typically measured. An estimation of residence time in the whole feed line gives values widely higher than 3 hours, in correspondence with the higher flow rates measured along the whole experimentation.

19

The procedure can be repeated by increasing the P value, which is typically operated by keeping (P2) at the original set point and by decreasing the pressure (P3) as desired or required, accounting that (P3) should be always higher than the water vapor pressure at T1, along the whole measurement.

Procedure 2 After the bath stabilization occurred at the desired temperature T1 ((step (d)), the valve (V-10) is closed, whereas valves (V-5), (V-7) and (V-9) are completely open, in order to keep the permeate side at atmospheric pressure. The value of transmembrane pressure P is regulated by increasing the pressure (P1), which is equal to (P2), by using the compressed air line connected to the tank S1. The same protocol as in Procedure 1 is followed to perform the flux measurements.

Procedure 3 In this case, in order to define the operative pressure values, first of all the desired maximum temperature (T1max) of the specific trial should be fixed, since (P3) should be kept higher than the water vapor pressure at the maximum temperature ( P3  PBP (T1max ) ) to avoid evaporation in the permeate side along the whole trial. Then, it is possible to select the value of (P3), and, after setting the desired value of P across the membrane, also the (P2) value is defined straightforwardly as the set point. The same protocols as in Procedure 1 can be then followed for temperature and pressure stabilization. The measurement procedure can start after the temperature T1 has been stabilized at the desired value and pressures (P2) and (P3) have been equilibrated as P3=P2, in which P2 is the value defined at the previous step. The same protocol as in Procedure 1 can be then followed to create the desired transmembrane value P, as well as the protocol for flux measurements. As the temperature T1 is increased up to another set point value, the liquid viscosity decreases and the volume flux can increase; at the same time, owing to an increased temperature, some additional pores can become flooded (as suggested by Eq. (1.2)-Table1) and a further increase in the volume flux can occur. As a consequence, pressure (P3) may increase; valve (V-5) can be

20

then regulated to re-establish the desired transmembrane value P. After a stabilization time, the system is ready for the flux measurements according to the same protocol as in Procedure 1.

4. RESULTS AND DISCUSSION

Figures 6 to 10 show the results obtained on the membrane characterization according to the “flooding curve” method, both of coated ceramic membranes and of TF200 polymeric membranes. All the experimental volume fluxes ( Jv ) are reported at room temperature (typically 25 °C) as they were measured. Data were finally elaborated according to the “normalized volume flux” method introduced by Eq.(7), accounting the corresponding value of temperature inside the membrane, as documented in Eq. (10) (see Figure 4a) for symbols reference), to calculate the actual volume flow rate at each condition. J v w Qp (T1) w (T1) Qp (T3)   (T3)  w (T1)   P A P  (T1) A P

(10)

The evaluation of LEPmin as well as of LETmin values for each sample is performed according to the relationships (8); results are summarized in Table 2. Eq. (10) is used also to elaborate the results of dead-end microfiltration with water performed with one un-coated titania sample with 100 nm top layer, as documented in Figure 6. The “normalized flux” of the uncoated membrane is very high and it is constant in a wide range of pressure driving forces, even at very low pressure differences, as it was expected, since titania membranes are hydrophilic. That value corresponds to an hydraulic permeability of 1.2*103 (dm3/(h m2 bar)), which is lower but rather aligned with the corresponding values of polymeric membranes [40]. It is important to observe that such a “normalized flux” would correspond to the “completely flooded” condition of a hydrophobized membrane, as documented in Fig. 1b), in the case in which the coating procedure does not alter the pore size nor the surface porosity, nor the pore size distribution. Notwithstanding in this work only the initial part of the flooding curve is measured, the result of Figure 6 is however the first term of comparison to understand if the so-called “minimum permeability” (the CD 21

contribution as defined in Eq. (7)) is really a low value, as it is expected when the coating procedure is successfully. Basing on the elements contained in Eq.(7)), the discussion of the results is performed accounting of the CD contribution, related to “minimum permeability” of the defects, of the LEPmin and/or LETmin values, and of the shape of the initial part of the “normalized flux” curve which is related to the morphology of the coating. In Figure 7 all the results obtained with the same coated sample, tagged as S1921, are reported. It can be observed that behaviors obtained in Figures 7a) and 7b) are aligned with the theoretical trends reported in Fig. 1a) and 1b), respectively. In this case, the LEPmin value decreased from 8.7 bar to 4.9 bar, as the temperature increased from 23 to 60 °C, thus indicating a good extent of membrane applicability in that range. Correspondingly, the “minimum permeability” value, that is the CD constant in Eq. (7), ranges from 0.42 to 0.80 *10-13 dm3/m2 , which can be considered as a low value. The trend of the “normalized flux” at 100 °C (Fig.7b)) does not allow to draw a well-defined conclusion. The LEPmin at 100°C is affected by uncertainty: it seems to be located in a short range from 0.5 to 0.9 bar, in which only two flux measurements were available. In addition, the shape of the normalized flux-pressure curve is rather atypical. The doubts about that measurement can be dispelled by the results reported in Figure 7c) and 7d). Following the initial part of the flooding curve at differential pressure of 1 bar (Figure 7c)), we can observe that the membrane can become flooded by increasing the liquid temperature, in agreement with the theoretical premises. By using the “normalized flux” method (Figure 7d)), a well-defined LETmin value can be obtained as 90°C, as reported in Table 2, according to the graphical representation introduced in Eqs. (8), by a proper change of symbols. The LETmin of 90°C at P=1 bar means also that the LEPmin at 90 °C corresponds to 1 bar; therefore this value is highly consistent with the results obtained from the LEPmin measurements from 23 to 100 °C of Figure 7b) and it is a confirmation of what observed at 100 °C, that is the LEPmin is lower than 1 bar and probably located in the range from 0.5 to 0.9 bar. Remarkably, the LET measurement seems to be a more explicative and representative method of characterization above all at high temperatures (that is at low differential pressures), since it is easier to get a sufficient number of data to draw a univocal characterization. 22

Correspondingly, it is very important to observe that the “minimum permeability” value obtained in Fig. 7d) is significantly constant in the range from 40 to 90 °C and close to 1.2*10-13 dm3/m2; that value is also consistent with those measured in Figure 7b) and also in agreement with the corresponding results obtained for different samples of the same kind, which are reported in Fig.8. Making reference to the notation of Eq. (7), it is apparent that the defects (contained in the CD contribution) are rather constant in the temperature range investigated; that behavior suggests that there is a high reproducibility in membrane manufacturing and in the coating procedure. Aa a first conclusion, we can put in evidence that the results reported in Figure 7 are an intrinsic confirmation of the validity of the characterization technique here proposed; the criterion for data elaboration, introduced and discussed in the previous sections, seems to be adequate. Although some detailed morphological characteristics of the membrane coating are unknown, however some clear conclusions can be drawn with regard to the applicability of this sample as element of a membrane contactors only trusting on the results of the initial part of a flooding curve. In the case of sample S1921, the coating procedure was rather successful since a rather good LEPmin was measured up to 100 °C, together with a low value of the “minimum permeability”, thus indicating few defects along the overall manufacturing procedure. A further intrinsic validation of the method here proposed is also documented by the results reported in Figure 8, in which the behaviors of different samples of the same kind are compared. Noticeably the initial flooding curves at 1 bar differential pressure for samples S1921 and S1919 are rather coincident in the wide temperature range from 40 to 140 °C (Figure 8a)): the LETmin values are calculated as 90 and 95 °C, correspondingly (Table 2), and the minimum value of the “normalized flux” was quite similar for both the samples. Those results can be joined with the “normalized fluxes” measured at 60 °C reported in Figure 8b), in which five samples are compared.

Notwithstanding the results can be

considered scattered at a first glance, the shapes of the “flooding curves” are rather similar and sometimes superimposed, as it can be observed with samples S1919 and S2044, as well as with samples S2043 and S1919. Correspondingly, the LEPmin values are in the range from 4.8 bar to 5.9 bar (Table 2) with “minimum permeability” values remarkably constant and lower that 0.8*10-13 dm3/m2, which suggests the same extent of defects. Since the 23

characterization is performed at the same temperature, basing on Eq.(1.1) of Table 1, the spread in LEPmin values should be ascribed to different values of the maximum pore radius which can depend both on the coating procedure and on the manufacturing technique of the ceramic top-layer. Conversely, sample S1920 might appear a little bit worse than the others, since LEPmin is only 4.0 bar in correspondence with a little bit higher minimum permeability value. The increasing part of the curves of S1920 seems to be rather different from the samples S1921 or S2043, both in the values of the normalized fluxes and in the slopes. Basing on the meaning of Eq.(7), the increasing part of the curve is to be ascribed to a different value of the product ( CM  FE ), which suggests a different morphology and/or a different pore radius distribution of the coated samples. That hypothesis is in principle correct, however the whole flooding curve would be necessary to draw a final conclusion. However, if the results reported in Figures 7 and 8 are compared with those reported in Figures 2 and 9, a quite clear and univocal conclusion can be drawn accounting both the order of magnitude of the normalized fluxes and/or the shape of them. Since the values of the normalized fluxes in the range of pressure differences up to 6 bar are greatly lower than those measured for samples reported in Figures 2 and 9, and the shapes of the initial part of flooding curves are rather regular, we can easily conclude that most of the 100 nm P+F type membranes tested can be classified as accepted for a further development of modules, since they showed a rather good reproducibility of the results. On the contrary, results reported in Figures 2 and 9 put in evidence that those kinds of samples should be classified as rejected for membrane distillation purposes. Sample S1925 (Fig.2), in which the 250 nm top-layer is coated with the P+F polymers, presents a very high “minimum permeability” value which is 50 times greater than the corresponding values measured in Figures 7 and 8, in spite of an interesting LEPmin value of 5.2 bar (Table 2). In the case of the F-coated sample S1900, reported in Figure 9, both the shape and the values of the “normalized fluxes” clearly document that the coating technique was rather unsuccessful. If the values of the “normalized flux” are compared with the corresponding values obtained with the un-coated sample (Fig. 6), we can conclude that the top-layer was certainly coated, since “normalized fluxes” were 5-7 times lower than the corresponding un-coated sample. 24

However, the defects were so many that the membrane was to be considered as improper for membrane distillation purposes. Although the aim of this work is not devoted to the development of new membranes, it is self-evident that the method here presented is a good tool to obtain an overall process characterization of hydrophobic membranes to test if the membrane can be proper for membrane distillation applications. Finally, Commercial polymeric TF200 membranes were also tested in order to support the validity of the method, since their characteristics are well known to the scientific community working on membrane distillation [48]. The results are reported in Figure 10 and in Table 2. In Figure 10a), the “normalized flux” values measured at room temperature, following “Procedure 2” of section 3, are reported as a function of the differential pressure, ranging from 0.5 to 6.5 bar. Accounting of the whole set of data, since the normalized fluxes are rather high, it can be recognized that the LEPmin might be located in the range from 2.5 to 3.0 bar. By applying the criterion introduced by Garcìa-Payo et al. [40], which corresponds in this case to a normalized flux of 35*10-13 dm3/m2, an LEPmin of 2.8 bar would have been calculated, which remarkably corresponds to the value typically declared by the manufacturer [19,48]. However, by zooming in the pressure range from 0.5 to 3.0 bar, as reported in the detail inside Figure 10 a), the criterion for data elaboration presented and discussed along the whole paper allows to calculate a LEPmin value of 2.45 bar (see also Table 2). It is self-evident that the final result is affected by the criterion used to elaborate experimental data; the method here proposed allows to obtain a more precise result, less dependent on the operator ability and on the selection of a “minimum appropriate flux”. In passing, it should be remind that the reference value by [40] would be remarkably too high to be used for ceramic membranes, such as the P+F type samples reported in Figures 7 and 8 of this work. Finally, LET measurements performed with TF200 at a differential pressure of 2 bar, according to “Procedure 3” of section 3 (Fig.10b)), document a LETmin of 70 °C (that is a LEPmin of 2 bar at 70 °C), and clearly confirm the decreasing trend of LEPmin with temperature, in agreement with the results presented by other authors for similar polymers [44].

25

A final summary of the results is available in Table 2 in which all the samples are compared both with reference to the LEPmin and/or LETmin , and with reference to the corresponding fluxes as well as to the “normalized fluxes”. As already observed, P+F type ceramic samples seem to be very encouraging membranes, since they show higher LEPmin values with respect to the TF200 membranes, whereas very similar water fluxes are obtained under flooding conditions ( Jv at P ). B

As a last comment, it can be observed that none of the membranes tested, including the wellknown TF200, can be classified as a “no-defects” real membrane as indicated in Fig.1.

5. CONCLUSIONS

An advanced method of measurement of the liquid breakthrough of macroporous membranes at high temperatures has been presented. The method is based on the construction of the initial part of the “flooding curve”, performed by experiments carried out in a proper apparatus described in detail. Protocols of measurements have been defined in order to get a measure of the volume flux across the flooded pores at temperatures higher than the normal boiling point of the non-wetting liquid. To the best of our knowledge, this paper presents for the first time the basis of the Liquid Entry Pressure measurement at high temperatures and documents it with experimental results of “flooding curves” in the temperature range from 20 to 140 °C. The new concept of Liquid Entry Temperature is also introduced. The knowledge of LETmin becomes important and useful for design purposes, since it defines the maximum temperature at which the membrane contactor can operate after the minimum pressure difference has been set to a reasonable value which can be kept across the membrane in an industrial equipment. Experimentation has been possible by using prototype macroporous hydrophobized ceramic membranes considered as confident for operation at high temperatures. At present, the method has been validated by a comparison with the results obtained for a commercial polymeric membrane.

26

A new theoretical-based criterion of data elaboration has been developed and the concept of “normalized volume flux” has been introduced. Main advantages of this criterion are: i) the possibility to get an accurate value of LEPmin and/or of LETmin , which is rather independent of the operator subjectivity when compared with the usual methods; ii) the possibility to compare results at different temperatures, for different kind of membranes; iii) the possibility to characterize the membrane by separating the possible “defects” from the evaluation of the real extent of hydrofobicity of the membrane surface. As a consequence of the “normalized volume flux”, the concept of “maximum allowable permeability” derives straightforwardly, associated to the idea of “defects”. This concept should be developed and discussed in the scientific community in order to get a sort of absolute scale for its quantification. Generally speaking, basing on all the experimental results reported in this paper, we can conclude that the “flooding curve” method, performed according to any of the procedures proposed, can be considered a valid protocol to perform LEP or LET measurements and then to perform an overall characterization to test the process applicability of the membrane, to the best of our knowledge. Finally, it should be observed that the “flooding curve” method is a permeation technique using a non-wetting liquid, here presented in the case of hydrophobic membranes and performed with water. The principles and methods associated to this procedure can be certainly reversed for the application of the so-called wet/dry method, using a wetting liquid and a gas fluid as transport vector.

ACKNOWLEGMENTS This work was supported by Saipem-S.p.A (Milan, Italy) [grants n. 658283 (2012-2015) and n.1040403 (2015-2017)]. Authors gratefully thank Dr. Hannes Richter and Dr. Marcus Weyd (Fraunhofer Institute for Ceramic Technologies and Systems - IKTS, Hermsdorf, Germany) for their cooperation in the results discussion. Authors gratefully thank Dr. Eleonora Ricci and Dr. Riccardo Zauli for their cooperation in performing experiments. 27

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30

MEMSCI_16379 Captions list of Figures Fig.1. Typical behaviors of flooding curves by water permeation across hydrophobic macroporous membranes: the cases of different morphologies are compared with a hydrophilic or completely flooded membrane. a) volume flux b) “normalized volume flux” vs. operative parameters. Fig.2. Example of data vs. the pressure difference across the membrane. a) flooding curve b) “normalized volume flux” (Procedure 2). The graphical method for LEPmin calculation according to Eqs. (8) is shown. Fig.3 Design chart for a stripping operation with hydrophobic membranes in SGMD: definition of operative conditions basing on LEPmin data (a) or on LETmin data (b). Fig. 4. Experimental apparatuses for the LEPmin / LETmin measurement according to the “initial flooding curve” method. a) Set up ; b) scheme of the flat cell; c) scheme of the tubular membrane-housing assembling. Fig.5. Comparison between initial part of flooding curves performed with the same membrane sample according to Procedure 1 (trial 1; P2=7 bar) and Procedure 2 (trial 2) ((section 3.3.1)). Fig.6. Dead-end microfiltration of demineralized water across an uncoated titania sample with 100 nm top layer: “normalized volume flux” is reported along the pressure difference. Fig.7. Initial part of flooding curves of a P+F-coated membrane on 100 nm titania top-layer. Volume fluxes at 25 °C are reported along the differential pressure across the membrane at constant temperature (a) and along the temperature at 1 bar pressure difference (c); the corresponding elaborations according to the “normalized volume flux” method (Eq.7) and Eq.(10)) are reported in (b, d). Trials 2 and 3 = Procedure 2; Trial 4 = Procedure 1 (P2=7 bar); Trial 5 = Procedure 3 (P3=6 bar).

Fig.8. Comparison among initial part of flooding curves of P+F-coated membranes on 100 nm top layer. “Normalized volume fluxes” are reported along the temperature at 1 bar pressure difference across the membrane (a) and along the differential pressure at constant temperature (b). (a)=Procedure 3 (P3=6 bar); (b) = Procedure 2

Fig.9 F-coated membrane on 100 nm top-layer: “normalized volume flux” vs. differential pressure across the membrane at 25 °C. (Procedure 2) Fig.10. Comparison among initial part of flooding curves of TF200 samples according to the “normalized volume flux” method (Eq.(7)). (a) Data are reported along the differential pressure across the membrane at 23 °C; a zoom in the low pressure range is inserted. (Procedure 2) 31

(b) Data are reported along the temperature at 2 bar pressure difference across the membrane. (Procedure 3, P3= 1 bar)

Table 1. General equations for interface equilibrium and motion across pores of a hydropohobic macroporous “real” membrane penetrated by water minimum liquid entry pressure LEPmin (T ) 

2 B  LG (T ) cos  (T )

(1.1)

rp,max

at T=T0 , at P  PL  PG  LEPmin (T0 ) interface equilibrium of pressure forces rp (P, T0 ) 

2 B  LG (T0 ) cos  (T0 )

(1.2)

P

Hagen-Poiseuille flow across defects and penetrated hydrophobic pores Qp  Qp , D (P, T0 ) +Qp , H (P, T0 )  Q p , E (P, T0 ) 

P w (T0 )

N defects

 i 1

r  rD4,i  A P   x 2 f ( x)dx r (  P , T ) 8 D ,i    w (T0 )

(1.3)

p ,max

p

0

B =1 for “ideal”cylindrical pores, 0
[36]

=1 for straight pores 3    8 ,  =3 for slit-like pores, =8 for cylindrical pores

[52]

rD,i ,  D,i  average radius, average length of a defect i ; Ndefects=total number of

defects

32

Table 2. Summary of membrane characterization (properties are defined according to Eqs(8)) Jv P at P T LEPmin Jv at PB @25°C B Sample (°C) (bar) (dm3/(hm2)) *1013(dm3/m2) S1919(#) S1921(#) Ceramic membranes S1920 (#) S2043(#) S2044(#) S1925(*) TF200

S29 Sample

S1919(#) Ceramic membranes S1921(#) TF200

S34

60 23 60 100 60 60 60 25

>5.9 8.7 4.9 0.5-0.9 4.0 4.8 >5.7 5.2

0.13 0.27 0.10-0.22 0.35 0.19 8.15

0.42 0.80 0.90-1.0 1.15 0.63 46.60

23

2.45

0.25

3.2

Jv P at T ΔP LETmin Jv at TB @25°C B (bar) (°C) (dm3/(hm2)) *1013 (dm3/m2) 1.0 95 0.12 1.00 1.0 90 0.13 1.19 2.0

70

0.15

0.84

(#)P+F coating on 100 nm top layer ; (*)P+F coating on 250 nm top layer

HIGHLIGHTS



A systematic method for liquid breakthrough measurement is presented



The experimental apparatus is developed and the protocols are defined



Water Liquid Entry Pressure is obtained for macroporous membranes from 20 to 140 °C



The “normalized volume flux” and the Liquid Entry Temperature are introduced



The method is a tool to obtain an overall characterization of process applicability

33

Captions list of Figures Fig.1. Typical behaviors of flooding curves by water permeation across hydrophobic macroporous membranes: the cases of different morphologies are compared with a hydrophilic or completely flooded membrane. a) volume flux b) “normalized volume flux” vs. operative parameters. Fig.2. Example of data vs. the pressure difference across the membrane. a) flooding curve b) “normalized volume flux” (Procedure 2). The graphical method for LEPmin calculation according to Eqs. (8) is shown. Fig.3 Design chart for a stripping operation with hydrophobic membranes in SGMD: definition of operative conditions basing on LEPmin data (a) or on LETmin data (b). Fig. 4. Experimental apparatuses for the LEPmin / LETmin measurement according to the “initial flooding curve” method. a) Set up ; b) scheme of the flat cell; c) scheme of the tubular membrane-housing assembling. Fig.5. Comparison between initial part of flooding curves performed with the same membrane sample according to Procedure 1 (trial 1; P2=7 bar) and Procedure 2 (trial 2) ((section 3.3.1)). Fig.6. Dead-end microfiltration of demineralized water across an uncoated titania sample with 100 nm top layer: “normalized volume flux” is reported along the pressure difference. Fig.7. Initial part of flooding curves of a P+F-coated membrane on 100 nm titania top-layer. Volume fluxes at 25 °C are reported along the differential pressure across the membrane at constant temperature (a) and along the temperature at 1 bar pressure difference (c); the corresponding elaborations according to the “normalized volume flux” method (Eq.7) and Eq.(10)) are reported in (b, d). Trials 2 and 3 = Procedure 2; Trial 4 = Procedure 1 (P2=7 bar); Trial 5 = Procedure 3 (P3=6 bar).

Fig.8. Comparison among initial part of flooding curves of P+F-coated membranes on 100 nm top layer. “Normalized volume fluxes” are reported along the temperature at 1 bar pressure difference across the membrane (a) and along the differential pressure at constant temperature (b). (a)=Procedure 3 (P3=6 bar); (b) = Procedure 2

Fig.9 F-coated membrane on 100 nm top-layer: “normalized volume flux” vs. differential pressure across the membrane at 25 °C. (Procedure 2) Fig.10. Comparison among initial part of flooding curves of TF200 samples according to the “normalized volume flux” method (Eq.(7)). (a) Data are reported along the differential pressure across the membrane at 23 °C; a zoom in the low pressure range is inserted. (Procedure 2) (b) Data are reported along the temperature at 2 bar pressure difference across the membrane. (Procedure 3, P3= 1 bar)

Fig.1. Typical behaviors of flooding curves by water permeation across hydrophobic macroporous membranes: the cases of different morphologies are compared with a hydrophilic or completely flooded membrane. a) volume flux b) “normalized volume flux” vs. operative parameters.

Fig.2. Example of data vs. the pressure difference across the membrane. a) flooding curve b) “normalized volume flux” (Procedure 2). The graphical method for LEPmin calculation according to Eqs. (8) is shown.

set P

set TL PL ,min  PBP (TL )  Pdrop , L

TL ,max  LETmin @ P

LEPmin  LEPmin @ TL

set PL  PL ,min

set TL  TL ,max

P   0, LEPmin 

PL ,min  PBP (TL )  Pdrop , L

PG  PL  LEPmin

set P set PL  PL ,min PG  PL  P

a)

b)

PG  PL  P

Fig.3 Design chart for a stripping operation with hydrophobic membranes in SGMD: definition of operative conditions basing on LEPmin data (a) or on LETmin data (b).

S1 C1 TB V-1, V-2 V-3, V4

a)

Pressure tank Membrane Housing Thermostatic Bath Pressure reducer Needle Valve

V-5 V-6 – V-11 P T F

Micro Needle Valve Plug flow valve Analogic Manometer Electronic Thermometer Flowmeter

b)

C1: Flat Cell

Correct liquid path

Glass end-caps

Viton O-Ring

C1: Tubular Housing

Hydrophilic layers

Hydrophobic coating

c)

Fig. 4. Experimental apparatuses for the LEPmin / LETmin measurement according to the “initial flooding curve” method. a) Set-up ; b) scheme of the flat cell; c) scheme of the tubular membrane-housing assembling.

Fig.5. Comparison between initial part of flooding curves performed with the same membrane sample according to Procedure 1 (trial 1; P2=7 bar) and Procedure 2 (trial 2) ((section 3.3.1)).

tubular / un-coated sample 40000

Jvη/ΔP*1013 (dm3/m2)

35000 30000 25000 20000 15000 Trial 1, S751 - 100 nm, 22 °C

10000 5000 0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

ΔP (bar)

Fig.6. Dead-end microfiltration of demineralized water across an uncoated titania sample with 100 nm top layer: “normalized volume flux” is reported along the pressure difference.

1.0

2.0

3.0

5.0

ΔP (bar)

4.0

Trial 4, S1921 - 100 nm, 100 °C

Trial 3, S1921 - 100 nm, 23 °C

Trial 2, S1921 - 100 nm, 60 °C

tubular / P+F coating 8.0 7.0 6.0

5.0 4.0 3.0 2.0 1.0 0.0 0.0

Trials 2 and 3 = Procedure 2; Trial 4 = Procedure 1 (P2=7 bar); Trial 5 = Procedure 3 (P3=6 bar).

6.0

7.0

8.0

9.0

Fig.7. Initial part of flooding curves of a P+F-coated membrane on 100 nm titania top-layer. Volume fluxes at 25 °C are reported along the differential pressure across the membrane at constant temperature (a) and along the temperature at 1 bar pressure difference (c); the corresponding elaborations according to the “normalized volume flux” method (Eq.7) and Eq.(10)) are reported in (b, d).

b)

Jvη/ΔP*1013 (dm3/m2)

tubular / P+F coating 9.0 Trial 6, S1919 - 100 nm, 1 bar

Jvη/ΔP*1013 (dm3/m2)

8.0 Trial 5, S1921 - 100 nm, 1 bar 7.0 6.0 5.0 4.0 3.0 2.0

1.0 0.0 20

40

60

a)

80

100

120

140

160

5.0

6.0

7.0

T(°C) 7.0 Trial 3, S1919 - 100 nm, 60 °C

6.0

Jvη/ΔP*1013 (dm3/m2)

Trial 2, S1920 - 100 nm, 60 °C 5.0

Trial 2, S1921 - 100 nm, 60 °C Trial 1, S2043 - 100 nm, 60 °C

4.0

Trial 1, S2044 - 100 nm, 60 °C

3.0 2.0 1.0 0.0

0.0

b)

1.0

2.0

3.0

4.0

ΔP (bar)

Fig.8. Comparison among initial part of flooding curves of P+F-coated membranes on 100 nm top layer. “Normalized volume fluxes” are reported along the temperature at 1 bar pressure difference across the membrane (a) and along the differential pressure at constant temperature (b). (a)=Procedure 3 (P3=6 bar); (b) = Procedure 2

tubular /F - coating 2500

Jvη/ΔP*1013 (dm3/m2)

Trial 1, S1900 - 100 nm, 25 °C 2000

1500

1000

500

0 0.0

1.0

2.0

3.0

4.0

5.0

6.0

ΔP (bar)

Fig.9 F-coated membrane on 100 nm top-layer: “normalized volume flux” vs. differential pressure across the membrane at 25 °C. (Procedure 2)

Fig.10. Comparison among initial part of flooding curves of TF200 samples according to the “normalized volume flux” method. (a) Data are reported along the differential pressure across the membrane at 23 °C; a zoom in the low pressure range is inserted. (Procedure 2) (b) Data are reported along the temperature at 2 bar pressure difference across the membrane. (Procedure 3, P3= 1 bar)