Introduction in Membrane Technologies

Chapter 1

Introduction in Membrane Technologies Maria Norberta de Pinho1,2 and Miguel Minhalma2,3 1

Chemical Engineering Department, Instituto Superior Te´cnico, Universidade de Lisboa, Portugal, 2Center of Physics and Engineering of Advanced Materials, CeFEMA, Instituto Superior Te´cnico, Universidade de Lisboa, Portugal, 3Chemical Engineering Department, Instituto Superior de Engenharia de Lisboa, Instituto Polite´cnico de Lisboa, Portugal

Chapter Outline 1.1 Introduction 1.2 Materials and Structures of Membranes 1.2.1 Materials 1.2.2 Structures 1.3 Classification of Membrane Processes 1.4 Membrane Modules 1.4.1 Plate-and-Frame Modules 1.4.2 Tubular Modules 1.4.3 Hollow Fiber Modules 1.4.4 Spiral Wound Modules 1.5 Pressure-Driven Membrane Processes

1 3 3 3 6 8 8 8 9 10 12

1.5.1 Membrane Characterization 12 1.5.2 Mass Transfer at the Fluid Phase Circulating Tangentially to the Membrane: Concentration Polarization 15 1.5.3 Operation Modes 17 1.5.4 Case Study Applications 19 1.6 Electrodialysis 21 1.6.1 Characterization of IonExchange Membranes 22 1.6.2 Process Operation 23 1.7 Pervaporation 24 1.8 Conclusion 27 References 27

1.1 INTRODUCTION Membrane processes occupy an important place among separation techniques nowadays. In fact, in the last few decades, the conventional separations or chemical engineering unit operations like distillation, solvent extraction, etc. have been in many situations substituted or complemented by membrane separation processes. This is of particular relevance in the food industry, which constitutes the second membrane market after that of water, including wastewater and desalination. In the early 1960s, the development by Loeb and Sourirajan of cellulose acetate asymmetric membranes for sea water Separation of Functional Molecules in Food by Membrane Technology. DOI: https://doi.org/10.1016/B978-0-12-815056-6.00001-2 © 2019 Elsevier Inc. All rights reserved.

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Separation of Functional Molecules in Food by Membrane Technology

desalination made it possible to envisage their use in reverse osmosis (RO) at a large scale and as an alternative to the energy-intensive thermal processes. The cellulose acetate asymmetric membranes display the singular feature of combining high transmembrane fluxes with high rejection coefficients to NaCl, whereas they are synthesized by the phase-inversion technique. The versatility of this technique opened the way for the production of membranes from many other polymers and with a wide range of structures. The extension beyond polymers to inorganic and hybrid materials together with the recourse of advanced preparation techniques gave rise to the production of a wider range of membrane structures as described in Section 1.2. The main role of synthetic membranes as agents of selective transport or chemical species separation is the result of the synergy between a membrane structure and a given driving force. For example, the RO, nanofiltration (NF), or ultrafiltration (UF) processes utilize asymmetric microporous membrane structures and hydrostatic pressure differences as the driving force and electrodialysis, which utilizes ion-exchange membranes, requires an electrical potential difference as the driving force for the separation of charged species. These combinations lead to a number of membrane separation processes that are classified in Section 1.3 upon membrane structure, driving force, separation mechanism, and range of application. The industrial scale-up of these processes is very much linked to the development in the 1970s of compact membrane modules, such as the spiral wound or the hollow fiber, that can accommodate hundreds of square meters of membrane surface area per cubic meter of volume. The performance of the pressure-driven processes of microfiltration (MF), UF, NF, and RO depends not only on the membrane characteristics but is also highly dependent on the mode and conditions of operation. The hydraulic permeability is the parameter that assesses the membrane capability to water permeation. It is the water permeation rate per unit of membrane surface area and unit of transmembrane pressure and on the average decreases an order of magnitude from UF, NF to RO. The selective permeation to solutes is assessed through rejection coefficients that evaluate the fraction of the solute in the feed that does not permeate through the membrane. Their values depend on the membrane characteristics and on the operating conditions, namely the feed cross flow velocity and the transmembrane pressure. In fact, a membrane that is a selective barrier is always associated to the phenomenon of concentration polarization that occurs as a consequence of the accumulation of solute material rejected by the membrane, and leads to the development of a concentration profile from a higher value at the membrane/feed interface to a lower value in the feed bulk. Concentration polarization acts as an additional mass transfer resistance that leads to the decrease of the permeate fluxes and therefore is essential to be taken into account in the design of membrane systems. In addition, it is a precursor of membrane fouling, as the material accumulated at the membrane/feed

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interface may be adsorbed and modify the membrane surface porous structure. In the long term, this leads to reversible (removed by membrane cleaning) or even to irreversible fouling, meaning the decrease of membrane lifetime. The electrically driven process electrodialysis has been traditionally used for the desalination of brackish water and is now the best available technique for many applications in the food industry, for example, in the desalination of cheese whey and tartaric stabilization of wine. In general, electrodialysis represents a strong asset in the removal of salts and acids in food processing with preservation of the product’s organoleptic properties. In contrast with the two previously mentioned processes, pressure-driven and electrically driven processes, pervaporation still has a very small worldwide market, for economic reasons. This concentration-driven process is a technical alternative solution to distillation for the situations of azeotropic mixtures and of mixtures with components having very close boiling points. A strong asset of pervaporation in the food industry is the processing of temperature-sensitive mixtures, as it can be run at room temperature. The increase of the feed temperature within a range compatible with the thermolabile products leads to the enhancement of the productivity, therefore making it economically feasible. In fact, the economic limitation of pervaporation is mainly due to the low permeate fluxes of the commercial pervaporation membranes in the present market.

1.2 MATERIALS AND STRUCTURES OF MEMBRANES 1.2.1 Materials The development of Loeb and Sourirajan cellulose acetate asymmetric membranes (Loeb and Sourirajan, 1963) led to the application of membranes in large-scale processes and created the need of membrane production from a wide variety of synthetic polymers, inorganic materials, and hybrid polymer/ inorganic materials. Table 1.1 presents the most common materials used in membrane synthesis.

1.2.2 Structures In parallel with the developments in materials science, the advancement in membrane preparation techniques leads to the production of membranes with a myriad of physical structures that can be classified in four major groups: 1. 2. 3. 4.

Microporous Homogeneous Homogeneous ion exchange Asymmetric: integral and composite

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TABLE 1.1 Most Common Materials Used in the Manufacturing of Synthetic Membranes (Strathmann, 1986) Material Type

Material

Inorganic

Glass Ceramic Metallic Zeolitic

Natural based polymers

Cellulose diacetate and triacetate (CA) Cellulose acetate propionate (CAP) Cellulose acetate butyrate (CAB) Cellulose acetate methacrylate (CAM)

Synthetic polymers

Polyamide (PA) Polyether sulfone (PES) Polyimide (PI) Polyacrylonitrile (PAN) Polysulfone (PS) Polyvinylchloride (PVC) Polyvinylidene fluoride (PVDF) Polypropylene (PP)

1.2.2.1 Microporous Membranes With the manufacturing process, microporous membranes may acquire a variety of structures as shown in Table 1.2. They are applied in MF, which is an operation widely used in the food industry for beverage clarification and in general for the removal of suspended solids and colloidal matter. 1.2.2.2 Homogeneous Membranes One type of homogeneous membrane is constituted by dense films that allow the separation of molecules with similar dimensions due to the fact that they present different solubilities and diffusivities in the membrane matrix. The most important applications for these membranes are gas permeation and pervaporation. Another type of homogeneous membranes is the ionexchange membrane, which is mainly used in electrodialysis. Table 1.3 presents the most common materials and manufacturing processes for these membranes.

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TABLE 1.2 Most Common Microporous Membranes (Strathmann, 1986) Material

Pore Dimension (μm)

Manufacturing Process

Ceramic, metal, polymer powder

1
20

Pressing and sintering

Homogeneous polymer sheets

0.5
10

Stretching

Homogeneous polymer sheets

0.02
10

Track-etching

Polymer solution

0.01
5

Phase inversion

TABLE 1.3 Most Common Homogeneous Membranes Material

Manufacturing Processes

Polymer

Extrusion of films and casting of polymer solutions

Polymer ion-exchange resin

Pressing and casting of solutions

0.1-1 μm 100-200 μm

FIGURE 1.1 Scheme and cross-sectional diagram of an asymmetric membrane.

1.2.2.3 Asymmetric Membranes Asymmetric membranes have a central place in membrane separation technology due to the fact that they combine high permeation fluxes and high selectivity. In fact, that was a milestone in promoting from the 1960s onward the industrial scale-up of the pressure-driven membrane processes. The feature responsible for the high permeation/selectivity is the asymmetric crosssectional structure of a very thin dense layer whose thickness can vary from 0.1 to 1 μm and a subjacent porous layer with thicknesses varying from 100 to 200 μm. This is schematically illustrated in Fig. 1.1. The thin dense layer is responsible for the membrane selectivity, often called the active layer, and the porous layer gives essentially mechanical strength to the membrane. In contrast with the symmetric membranes, which act as traditional filters and retain particles in their pores with subsequent fouling and decline of permeation fluxes, the asymmetric membranes act as surface filters retaining the rejected material at the membrane surface and not inside the pores and therefore avoiding pore blocking/fouling, as shown in Fig. 1.2.

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200 μm

FIGURE 1.2 Scheme and cross-sectional diagram of a symmetric membrane.

1.2.2.3.1 Integral Asymmetric Membranes These membranes are prepared by the phase inversion method (Loeb and Sourirajan, 1963; Strathmann, 1986; Kesting, 1985), in which a starting polymeric casting solution, containing a polymer and a solvent system, is cast into a film and after some evaporation time the film is quenched in a nonsolvent, which for most cases is water. The versatility of this process both in terms of casting solution composition and in terms of the casting parameters (evaporation time, quenching media type, and temperature) allows the tailoring of membranes with different structures and permeation characteristics.

1.2.2.3.2

Composite Asymmetric Membranes

These membranes are the result of a deposition of a dense selective layer onto a porous substrate of a different polymeric material. Commercial composite asymmetric membranes may be prepared by several methods, such as (Kesting, 1985; Cadotte et al., 1981): 1. Preparation of a dense thin layer, which is then mechanically laminated on a porous support layer. 2. Preparation of a dense film directly over the microporous substrate layer by dip coating. 3. Plasma polymerization at the surface of the porous membrane. 4. Polycondensation or interfacial polymerization over the surface of a porous support layer.

1.3 CLASSIFICATION OF MEMBRANE PROCESSES The selective permeation of membranes with different structures and/or physicochemical properties can be attributed to different mechanisms associated to sieving, steric hindrances, membrane/solvent/solute(s) interactions, solution/diffusion characteristics, and electrical migration. The external action of pressure, electrical, and concentration driving forces together with different transport mechanisms leads to the membrane separation processes described in Table 1.4.

TABLE 1.4 Membrane Separation Processes: Operating Principles and Applications Separation Process

Membrane Type

Driving Force

Separation Mechanism

Range of Application

Microfiltration (MF)

Microporous 0.1
10 μm

Pressure 0.1
1 bar

Sieving

Sterilization

Asymmetric

Pressure 0.5
8 bar

Ultrafiltration (UF)

Clarification Sieving

Separation of macromolecular solutes

Membrane/solvent/solute interactions Nanofiltration (NF)

Asymmetric “skin type”

Pressure 10
40 bar

Reverse osmosis (RO)

Asymmetric “skin type”

Pressure 20
100 bar

Solution/diffusion

Separation of salts and microsolutes

Dialysis

Microporous 0.1
10 μm

Concentration gradient

Diffusion

Separation of salts and microsolutes from macromolecular solutions

Electrodialysis (ED)

Ion exchange

Electrical potential gradient

Electrical migration

Desalting of ionic solutions

Gas permeation (GP)

Homogeneous

Pressure and concentration gradient

Solution/diffusion

Separation of gas mixtures

Concentration gradient

Solution/diffusion

Concentration and separation of small organic solutes

Symmetric and asymmetric Pervaporation (PV)

Homogeneous Symmetric and asymmetric

Sieving Membrane/solvent/solute interactions Solution/diffusion

Separation of salts and small organic solutes

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1.4 MEMBRANE MODULES Although the development of asymmetric membranes, which are highly selective and have high permeation fluxes, has significantly contributed to the industrial development of membrane processes, this had to be accompanied by the development of a membrane support system, where membranes are arranged in different configurations characterized by high membrane surface area per unit of volume and with the capability to process fluids at the right hydrodynamic conditions and operating transmembrane pressures. Besides those characteristics, this support system should also be easily cleaned and maintained. In membrane separation processes the system where the membranes are inserted is called the membrane module. Membrane modules have the functions of supporting the membranes, separating physically the feed solution from the permeate solution, promoting a good mass transfer in the different compartments, and allowing the establishment of the process driving force. Additional constraints related to technical and economical parameters have to be taken into consideration. The modules should accommodate membranes with a very high packing degree and the pumping system for solutions distribution should present a reasonable cost. There are several modules capable of satisfying the above requisites and they should be carefully chosen considering the physicochemical characteristics of the feed to be processed. Industrial membrane modules may be classified into four types: plate-and-frame, tubular, hollow fibers, and spiral wound.

1.4.1 Plate-and-Frame Modules In the plate-and-frame modules the feed flows in a narrow channel with a rectangular cross section where the channel walls may be one or two membranes. Due to the very small channel thickness, which restrains the feed flow rate, the normal flow conditions in industrial modules are limited to laminar flow. Although laminar flow is in place, the mass transfer is kept at high values because the channel length is only of a few centimeters and therefore the growth of the concentration boundary layer is controlled. The easy cleaning and the low internal volume make these modules adequate in the cases where periodical disinfections are necessary. In the plate-and-frame modules represented in Fig. 1.3 the membrane support plates have a membrane sheet in each side and they are stacked alternately with spacers. The feed stream, which flows tangentially to the membrane surface (active layer), follows the path described in Fig. 1.3. The pressurized fluid that permeates through the membrane is collected in the permeate channels at atmospheric pressure.

1.4.2 Tubular Modules In tubular modules, represented in Fig. 1.4, tubes of a porous material with adequate chemical, thermal, and mechanical resistances are coated with a

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FIGURE 1.3 Representation of a plate-and-frame module.

FIGURE 1.4 Representation of a tubular module. Figure courtesy of Koch Membrane system, Inc. (www.kochmembrane.com) and used by the publisher with permission.

film that constitutes the membrane. The feed solution circulates in the inner part of the tubes in contact with the active layer, while the permeate passes through the tubes. There are monochannel or multichannel configurations for these membranes and they are mounted in outer shells that may accommodate several membrane units.

1.4.3 Hollow Fiber Modules The hollow fibers module, shown in Fig. 1.5, is composed by several polymeric capillary fibers that are introduced inside an outer shell. The feed solution may flow on the inside of the fibers and the permeate is collected in the

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FIGURE 1.5 Representation of a hollow fiber module. Figure courtesy of Koch Membrane system, Inc. (www.kochmembrane.com) and used by the publisher with permission.

FIGURE 1.6 Representation of a spiral wound module. Figure courtesy of Koch Membrane system, Inc. (www.kochmembrane.com) and used by the publisher with permission.

space between the fibers and the outer shell. In this situation the membrane active layer is inside the fibers. One other situation is when the feed solution flows on the outside of the fibers, the active layer is on the outer fiber face, and the permeate is collected inside the fibers. The choice between these two situations depends mainly on the application purpose and on the feed solution characteristics.

1.4.4 Spiral Wound Modules The configuration of the spiral wound modules is presented in Fig. 1.6. These modules have several membrane sheets folded and connected to a perforated central tube. The feed solution flows tangentially to the membrane

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surface (active layer side) and the permeate gets through the membranes and is collected in the central tube. The comparison of the membrane modules in terms of properties and applications is shown in Table 1.5. The 4th column is related to the control degree of the concentration polarization phenomenon, which is explained in the Section 1.4.2. The dimensions of tubular membranes are compatible with high feed flow rates and turbulent flows. Table 1.6 provides the most adequate modules for each of the membrane processes. TABLE 1.5 Membrane Modules: Properties and Applications Module Type

Compaction (m2/m3)

Price

Concentration Polarization Control

Application

Plate-andframe

400
600

High

Acceptable

MF, UF, NF, RO, PV, and GP

Tubular

20
30

Very high

Very good

MF, UF (tolerance of feed with high solids content)

Hollow fiber

600
1200

Very low

Poor

MF, UF, NF, RO, PV, and GP

Spiral wound

800
1000

Low

Acceptable

UF, NF, RO, PV, and GP

TABLE 1.6 Membrane Processes and Modules Separation Process

Plate-andFrame Module

Tubular Module

Hollow Fiber Module

Spiral Wound Module

Microfiltration

1

11

1

2

Ultrafiltration

11

11

1

1

Reverse osmosis

1

1

11

11

Pervaporation

11

2

1

1

Gas permeation

2

2

11

11

Electrodialysis

11

2

2

2

11, Very adequate; 1 , adequate; 2 , not adequate.

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TABLE 1.7 Modules Comparison in Reverse Osmosis Module

Packing Density

Fouling Control

Cleaning Performance

Plate-and-frame

Moderate

Fair

Good

Tubular

Low

Very good

Very good

Hollow fiber

Very high

Poor

Poor

Spiral wound

High

Fair/poor

Fair/poor

The selection of the membrane modules has to take into account the physicochemical feed characteristics and the possible pretreatments required. Given the industrial importance of RO, the Table 1.7 compares the different RO modules’ characteristics.

1.5 PRESSURE-DRIVEN MEMBRANE PROCESSES The pressure-driven processes, MF, UF, NF, and RO, and their main characteristics and applications are summarized in Table 1.4 and in Fig. 1.7. Fig. 1.8 represents schematically the operation mode in any of the membrane processes. The feed stream flows tangentially to the membrane surface and generates two other streams: the concentrate stream enriched in the solutes that are rejected by the membrane and the permeate stream that passes through the membrane. The efficiency of the UF, NF, and RO processes depends greatly on the concentration polarization phenomenon that occurs when a concentration profile is developed at the membrane surface, due to the rejected solutes accumulation. The membrane characteristics and the operating conditions are important factors in the development of this phenomenon and its quantification is crucial for the membrane processes’ design. From this perspective, membrane characterization is presented below.

1.5.1 Membrane Characterization The hydraulic permeability, LP, is the characterization parameter that quantifies the membrane capacity to permeate pure water. This parameter can be determined by representing the permeate fluxes to pure water, JP, in function of the transmembrane pressure applied. The slope of the straight line obtained is the hydraulic permeability, LP, as depicted in Fig. 1.9. The variation of the permeate volume per unit of time, membrane surface area, and transmembrane pressure is the hydraulic permeability, LP, according to Eq. (1.1): JP;pure water 5 LP ΔP

ð1:1Þ

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Particles and colloids

Microfiltration (MF) Proteins, polysaccharides, polyphenols, and other macromolecules

Ultrafiltration (UF) Glucose, fructose, amino acids and small organic solutes, and bivalent salts

Nanofiltration (NF) Salts and small organic solutes

Reverse Osmosis (RO)

FIGURE 1.7 Applications of pressure-driven membrane processes.

Feed Q0, CA0

Concentrate QC, CAC

Permeate QP, CAP

FIGURE 1.8 Schematic representation of membrane process. QA0, QAP, QAC—Flow rate of feed, permeate and concentrate streams, respectively; CA0, CAP, CAC—concentration of solute A in the feed, permeate and concentrate, respectively.

For an aqueous solution of a given solute A, that is totally or partially rejected by the membrane, the permeation flux, JP, is given by the ratio of the permeate flow rate, QP, divided by the membrane surface area. For the same transmembrane pressure, the JP value is generally lower than the one for pure water (JP, pure water), and is given by Eq. (1.2). JP 5 LP ðΔP 2 ΔπÞ

ð1:2Þ

where Δπ 5 π0
πP, is the average osmotic pressure difference between the feed side (π0) and the permeate side (πP). The membrane separation performance for a given solute A is given by the apparent rejection coefficient, fA, defined as: fA 5

CA0 2 CAP CA0

ð1:3Þ

where CA0 and CAP are the solute A concentration in the feed and permeate streams, respectively. The accumulation of solute A at the membrane surface leads to an increase of its concentration, CAM, and therefore a new intrinsic rejection coefficient, f0 A, is defined as: f 0A 5

CAM 2 CAP CAM

ð1:4Þ

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Separation of Functional Molecules in Food by Membrane Technology 120.0

J P (kg/h/m2)

100.0 80.0 60.0 40.0 Lp

20.0 0.0 0

0.5

1

1.5

2

2.5

3

ΔP (bar)

FIGURE 1.9 Variation of pure water permeate flux, JP, with the transmembrane pressure applied, ΔP.

100% 90% 80%

fA (%)

70% 60% 50% 40% 30% 20% 10% 0% 0

10

20

30

40

50

Neutral solutes, A, molecular weight (kDa)

FIGURE 1.10 Example of representation of the profile of rejection coefficients to model solutes, A.

One other important characterization parameter is the molecular weight cutoff (MWCO), which is defined as the molecular weight (MW) of a solute A that is rejected by the membrane in the range from 0.9 to 0.99, depending on the author/manufacturer criteria. The rejection coefficient profile, fA versus MW, obtained for a serious of solutes of increasing MW is shown in Fig. 1.10. The upper part of this figure is nearly a plateau, making it very difficult to have an accurate intersection point of the curve, fA versus MW, with the horizontal line set at the fA value selected by the author between 0.9 and 0.99. In order to overcome this problem, the higher range of fA values is linearized through the representation of log(fA/(1
fA)) versus MW. This results in a well-defined intersection point for MWCO determination. As an

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example, this is shown in Fig. 1.11, for setting the value of fA equal to 0.91 and log(fA/(1
fA)) 5 1.

1.5.2 Mass Transfer at the Fluid Phase Circulating Tangentially to the Membrane: Concentration Polarization

log (fA/(1-fA))

The tangential circulation of a solute A solution at a given operating pressure results in a convective flow perpendicular to the membrane surface. Depending on the membrane selective characteristics the solvent permeates preferentially through the membrane and the solute A is totally or partially rejected by the membrane. The rejected solute accumulates near the membrane surface and in steady state, the convective flux toward the membrane is balanced by the solute flux through the membrane and the diffusive flux from the membrane surface to the bulk solution. The development of a concentration profile in the liquid boundary layer, as schematically shown in Fig. 1.12, is designated by concentration polarization. 2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0

y = 0.0552x + 0.1826 R2 = 0.9606

MWCO = 14.8 kDa 0

10

20

30

40

50

Neutral solutes molecular weight (kDa)

FIGURE 1.11 Example of graphic representation for membrane MWCO determination.

x Feed

CA0

δ

Δx

CA CAm

vp.CA

–DAB.

Membrane Permeate, vp vp.CAp

FIGURE 1.12 Concentration polarization, concentration profile, CA, in steady state.

∂CA ∂x

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The quantification of the concentration polarization phenomenon is carried out through a differential mass balance to solute A in a volume with differential thickness, Δx, in the fluid boundary layer adjacent to the membrane. The result is the differential Eq. (1.5): vp CA 2 DAW

dCA 5 vp CAp dx

ð1:5Þ

where vp is the permeate flux (m/s) and DAw is the diffusivity of solute A in the solvent. For a boundary layer of thickness δ the boundary conditions are: x 5 0; CA 5 CAM

ð1:6Þ

x 5 δ; CA 5 CA0

ð1:7Þ

The integration of Eq. (1.5) with the boundary conditions (Eqs. 1.6 and 1.7) results in: vpδ CAM 2 CAp 5 eDAW CA0 2 CAp

ð1:8Þ

Using the film theory (Bird et al., 2002), a mass transfer coefficient k, is introduced as k 5 DAw/δ, and the concentration polarization is quantified by Eq. (1.9): vp CAM 2 CAp 5ek CA0 2 CAp

ð1:9Þ

The mass transfer coefficient is obtained by empirical correlations for Sherwood number, Sh 5 Sh(Re, Sc). These correlations are obtained in the literature for different geometries and flow regimes. For example, for a circular geometry the most common correlations are: Sh 5

kDe 5 0:04Re 0:75 Sc 0:33 DAW

Turbulent regimen

ð1:10Þ

and Sh 5


kDe 5 Re Sc De =L DAW

Laminar regimen

ð1:11Þ

where Sh, Re, and Sc are Sherwood, Reynolds, and Schmidt numbers, with De and L being the diameter and length of a cylindrical channel, respectively. As previously mentioned, two different rejection coefficients can be defined as membrane characterization parameters, an apparent rejection coefficient, fA, and an intrinsic rejection coefficient, f0 A, based on the solute concentration in the bulk feed solution, CA0, and in the fluid/membrane interface, CAM, respectively. These two different coefficients are the result

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of the occurrence of the concentration polarization phenomenon, and they can be related to each other using Eqs. (1.3), (1.4), and (1.9). Eqs. (1.12) and (1.13) express these interrelationships.
fA exp vp =k 0 
 fA 5 ð1:12Þ 1 2 fA 1 2 exp vp =k and


exp vp =k CAM

5 0 0 CA0 fA 1 1 2 fA exp vp =k

ð1:13Þ

Simplifying the concentration polarization module, described in Eq. (1.9), for the particular case of total rejection of solute A, CAP 5 0, one gets Eq. (1.13). The CAM/CA0 ratio varies with the operating parameters and membrane characteristics (f0 A). The operating parameters of transmembrane pressure and feed circulation velocity (or Re) are directly determining vp and k, respectively. The formation of concentration polarization acts as an additional resistance to mass transfer and leads to permeate flux decline (Macedo et al., 2011). The apparent rejection coefficient, fA, is not only dependent on the membrane characteristics but is also strongly dependent on the process operating conditions, namely feed flow velocity and transmembrane pressure.

1.5.3 Operation Modes The pressure-driven membrane processes, MF, UF, NF, and RO, can be operated in batch or in continuous mode.

1.5.3.1 Batch Operation The batch operation can be run in total recirculation of the feed/concentrate stream and continuous withdraw of the permeate stream, Fig. 1.13A. A recirculation loop may be introduced, in order to better control the hydrodynamic conditions inside the membrane module, Fig. 1.13B (Kulkarni et al., 1992). (A)

(B)

Feed tank

Feed Pump Permeate

Feed tank

Feed pump Recirculation pump Permeate

FIGURE 1.13 Batch operation (B) with and (A) without recirculation loop.

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1.5.3.2 Continuous Operation The continuous operation of MF/UF/NF/RO is used for large-scale production systems. In these systems the solution to be processed is fed into a set of modules, arranged in stages, with a large membrane area, and after a single passage on each module two streams is obtained, a permeate stream and a concentrate stream. Due to the large surface area in each module, which can reach 30 m2/module in the case of the spiral wound modules, the longitudinal average circulation velocity decreases and the solute concentration increases are very significant, and these variations have to be taken into account in membrane process design. In the case of spiral wound modules that have small feed channels that accommodate low feed flow rates, characteristic of laminar flow and low Reynolds numbers, the introduction of spacers in the feed channels promotes mixing, which leads to the minimization of the concentration polarization phenomenon. Furthermore, the hydrodynamic conditions needed to achieve a good mass transfer can be reached though the recirculation of the concentrate stream. On the other hand, for UF with tubular modules, the module’s dimensions allow the operation with higher feed circulation flow rates, characteristic of turbulent regimen, and therefore a good feed mixing can be achieved. Additionally, feed recirculation loops can be introduced in each stage to maintain adequate hydrodynamic and mass transfer conditions. This continuous operation mode with recirculation loops in each stage is shown in Fig. 1.14 (Kulkarni et al., 1992). The increase of solute concentration along the module leads to the axial decrease of the permeate flux. For the same total membrane area the increase of the number of stages in series leads to the increase of the productivity in the first stages due to smoother axial concentration profiles, and therefore to an increase of the overall operation efficiency. However, the technoeconomic analysis shows that for a number of stages above four the increase of the overall efficiency does not compensate the additional costs introduced by the recirculation loops. To assure the hydrodynamic conditions recommended by the manufacturers the number of modules in parallel in each stage should decrease in successive stages. This is commonly designated by the Christmas tree configuration.

Feed tank

Stage 2

Stage 1 Q1 Feed pump

Recirculation pump Permeate 1

Q2

Recirculation pump

Q3 Permeate 2

FIGURE 1.14 Continuous operation with recirculation loops in each stage.

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1.5.4 Case Study Applications Several applications of pressure-driven processes in the dairy industry are schematically presented in Figs. 1.15
1.17. In the first example, UF is used to concentrate the sheep milk proteins. This enrichment of milk in proteins is the basis for the yield increase in cheese manufacturing (Catarino et al., 2013).

1.5.4.1 Technoeconomical Analysis In batch mode, the processing of 1000 L/day of sheep milk is performed with an operation daily time of 4 hours, using UF membranes characterized by total rejection to the milk proteins and by permeate fluxes of 50 L/m2/h at the working transmembrane pressure. For a volumetric concentration factor (VCF) of 4 (from 1000 to 250 L) a membrane surface area of 3.75 m2 is required. A preliminary technoeconomic analysis is carried out in Table 1.8. A very common industrial use of UF is the fractionation of cheese whey into a concentrate stream enriched in cheese whey proteins and a permeate enriched in lactose and salts. Sheep milk

Vi = 1000 l Vf = 250 l Feed pump

Permeate with lactose and salts Jp = 50 L/m2/h

FIGURE 1.15 Batch sheep milk concentration by ultrafiltration.

Cheese whey Spray dryer

Cheese whey powder Feed pump Cheese whey concentrated for higher yield production of whey cheeses Permeate rich in lactose and salts

FIGURE 1.16 Concentration of cheese whey by ultrafiltration for subproducts recovery.

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Separation of Functional Molecules in Food by Membrane Technology Sheep cheese whey

Feed pump

Cheese whey concentrated for further use in food products

UF

Permeate rich in lactose

NF

RO

Concentrate

Water for reuse

FIGURE 1.17 Fractionation of sheep cheese whey by an integrated sequence of UF/NF/RO for water and subproducts recovery.

TABLE 1.8 Technoeconomic Analysis of UF Sheep Milk Concentration Capacity

1000 L/day

Daily operation time

4h

Operation days per year

330

Average permeate flux

50 l/m2/h

Concentration factor

4

Membrane surface area

3.75 m2

Investment (2000 h/m2)

7500 m2

Lifetime

10 years

Annual maintenance cost (5% of investment)

380 h

Capital costs

2.3 h/m3

Energy costs

0.5 h/m3

Maintenance costs

1.1 h/m3

Total costs

3.9 h/m3

As seen in Fig. 1.16 the concentrate can be further dried for cheese whey powder production or used for the higher yield production of whey cheeses (Macedo et al., 2015). An integrated process of UF/NF is presented in Fig. 1.17 as an example of processing sheep cheese whey for byproduct recovery and effluent minimization. The UF yields a concentrate that is protein enriched and a

Introduction in Membrane Technologies Chapter | 1

21

permeate that is further processed by NF for the production of a concentrate rich in lactose and a permeate that can be desalinated by RO to produce water to be used in the factory (Minhalma et al., 2007; Magueijo et al., 2006). The very abundant literature on the use of MF, UF, NF, and RO in the must and wine industries goes from the concentration and rectification of grape must (Santos et al., 2008; Catarino et al., 2008; de Pinho et al., 2015), wine clarification (Gonc¸alves et al., 2001; de Pinho, 2010), to recovery of polysaccharides and polyphenols from wine and winery wastes (Resende et al., 2013; Giacobbo et al., 2013a,b, 2015, 2017). The wide range of MWCO of UF and NF membranes is a strong asset in the fractionation of saccharide mixtures (Catarino et al., 2008; Minhalma et al., 2006).

1.6 ELECTRODIALYSIS Electrodialysis is a separation process that uses ion-exchange membranes under the action of an electric field (driving force) for the separation of ionic species from aqueous solutions or other neutral solutes. The ion-exchange membranes are one of the key elements of this unit operation and they are selectively permeable to ions. Depending on the membrane characteristics, cation exchanger or anion exchanger, a preferential cation or anion permeation through the membrane will take place, respectively. The basic functioning principle of an electrodialyzer is depicted in Fig. 1.18. The feed solution containing ions flows through several compartments that have a cation-exchange membrane on one side and an anionexchange membrane on the other. Under the action of an electric field that is established between the anode and the cathode, the cations migrate into the cathode, permeating selectively through the cation-exchange membrane, while the anions migrate toward the anode permeating selectively through the anion-exchange membrane. In this way, there is the transport of the salt from the feed compartment, designated also by the diluate compartment, to the concentrate under the action of the electric field, which is the driving force. The salt is retained in the adjacent compartments, also designated by concentrate compartment because the anion-exchange membranes retain the cations that are migrating towards the cathode under the electric field action while the cation-exchange membranes block the anion transport that is moving toward the anode. The ions that reach the electrodes suffer redox reactions and there is the formation of chemical compounds, which are removed by the electrodes’ cleaning solution. An electrodialysis cell has a diluate compartment and a concentrate compartment, that together with a pair of ion-exchange membranes, one anionic and other cationic, constitute the basic unit of this separation process. An

22

Separation of Functional Molecules in Food by Membrane Technology Electrode washing solution

Feed Concentrate + + + + + + + + +

Anode

+ + + + +

-

+ + n pairs of + cation and anion + transfer + membranes + + + + + Electric current + + +

+

+

+ + + + + + + + + + + + +

-

Cathode

Concentrate Diluate

FIGURE 1.18 Schematic diagram of an electrodialysis stack operation.

industrial electrodialyzer has a few hundred of these cells, and Fig. 1.18 represents schematically the membrane stack part of an electrodialysis unit.

1.6.1 Characterization of Ion-Exchange Membranes The ion-exchange commercial membranes are films of ion-exchange resins with thicknesses between 0.1 and 1.5 mm, with exchange capacities ranging from 1 to 3.5 eq-mol/kg of dry resin equilibrated with sodium and water content that varies between 30% and 50%. For conventional applications, the membranes are generally produced from reticulated polystyrene that is further sulfonated to produce (2SO2 3 ) groups or quaternized to produce (2NR1 3 ) groups in the polymeric matrix, yielding cation-exchange and anion-exchange membranes, respectively. Due to the low mechanical resistance of these membranes it is necessary to add a fabric support layer (Strathmann, 1992). Membrane characteristics like electric resistance (Ω cm2), burst strength (MPa), and thickness (mm) are provided by the ion-exchange membrane manufacturers.

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Electrodes washing wastewater

Power

Diluate

+

Feed

-

Microfilter "Make-up" Concentrate purge Feed tank

Concentrate tank

FIGURE 1.19 Flow diagram of an electrodialysis plant.

1.6.2 Process Operation Fig. 1.18 shows the principle of operation of an electrodialysis stack with “n” membrane pairs. At a large scale an electrodialysis unit can comprise several stacks and each one may have from 20 to 500 pairs of membranes per electrode pair, with surface areas ranging from 0.02 to 2 m2. The diluate and concentrate compartments are delimited by the membranes, which have in between spacers that prevent the membranes from contacting each other and at the same time promote mass transfer. The spacers have usually a thickness ranging from 0.5 to 2 mm depending on the application and on the manufacturer. An electrodialysis plant in addition to the membrane stacks comprises power supply, pumps, tanks, process control devices, feed solution pretreatment unit, etc. A typical flow diagram is presented in Fig. 1.19. After MF the feed solution is circulated into the electrodialysis stacks through the diluate channels. The deionized solution and the concentrated brine streams leaving the last stack are collected in storage tanks when the desired deionization is reached. In case of salt precipitation risk the pH in the concentrate tank may be adjusted. In order to prevent the formation of free chlorine the electrodes are often rinsed with a separate chloride-free solution. Besides the typical use of electrodialysis for the production of potable water from brackish water, the food, pharmaceutical, and the chemical industries also use this technology. In the food industry, desalination of cheese whey and the wine tartaric stabilization by electrodialysis are wellestablished processes with economic and product quality advantages. In fact, for wine tartaric stabilization the electrodialysis presents two strong assets over the traditional cold tartaric stabilization method: 1. Preservation of the wine organoleptic properties 2. Avoidance of the diatomaceous earth filtration (Gonc¸alves et al., 2001; 2003; Narciso et al., 2005; Soares et al., 2009).

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Separation of Functional Molecules in Food by Membrane Technology

Other applications with different levels of technology maturity are found in the dairy, juice, and sugar industries. Hybrid processes of electrodialysis with other membrane processes or conventional unit operations are the subject of intense research and development (de Pinho et al., 2015).

1.7 PERVAPORATION In pervaporation the feed as a liquid mixture, at atmospheric pressure, circulates tangentially to the membrane and due to a concentration driving force the permeate in the vapor form is enriched in the preferential permeating component(s). This operation differs from the other membrane separation processes because there is a change of physical state between the liquid feed and the gaseous permeate stream (Aptel, 1986; Rautenbach and Albrecht, 1989). The driving force that is responsible for the mass transfer across the membrane is a gradient of chemical potentials between the liquid phase and the gaseous phase. This can be achieved by lowering the activity of the permeating component(s) in the permeate side, by maintaining the partial pressure lower than the saturation vapor pressure. This can be carried out by applying vacuum or using an inert carrier gas in the permeate side, as presented in Figs. 1.20 and 1.21, respectively. In the vacuum method, the permeate is kept at low pressure through the use of a vacuum pump, and the permeate condensation is carried out under vacuum. The inert carrier gas method is not generally used at the industrial scale.

Permeate Vacuum pump FIGURE 1.20 Schematic representation of pervaporation by vacuum.

Permeate Carrier gas FIGURE 1.21 Schematic representation of pervaporation with inert carrier gas.

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The heat required for the liquid/vapor phase change, from the feed to the permeate side, is supplied by the sensible heat from the liquid mixture, and therefore there is a decrease of the feed temperature along the membrane. In industrial applications the pervaporation units incorporate heat exchangers, in order to compensate the liquid mixture temperature decrease. The pervaporation performance depends greatly on the membrane/solute (s)/solvent interactions. The mass transfer is usually described by the solution
diffusion model, which assumes a three-step mechanism: (de Pinho et al., 1990). 1. Preferential sorption at the membrane/liquid feed interface 2. Diffusion through the membrane, described by Fick’s first law 3. Desorption on the membrane/permeate interface There are two main pervaporation types: the most common one uses membranes that permeate preferentially water, and the other one uses membranes that permeate preferentially organics (de Pinho et al., 1990; Cipriano et al., 1991; Neto and de Pinho, 2000). In fact, the first commercial application of pervaporation reports to the preferential water permeation in water/ ethanol systems as part of the downstream processing in the bioethanol production. The two main parameters used to access the pervaporation separation performance are the selectivity and the permeate flux. The membrane selectivity results from the preferential affinity of the membrane material for a given component of the liquid mixture, that can be quantified by two parameters, α or β, defined as: α5

Yi =Yj Y i ð 1 2 Xi Þ 5 Xi =Xj Xi ð1 2 Yi Þ

ð1:14Þ

Yi Xi

ð1:15Þ

β5

where Xi and Yi correspond to the feed and permeate mass fractions, respectively, for a component i (the species that permeates preferentially), and Xj and Yj correspond to the mass fractions of the other component in the feed and permeate, respectively. For a membrane that permeates preferentially water, this is very well illustrated in Fig. 1.22, for the water (i)/isopropanol (j) system. As it can be seen, the selectivity depends greatly on the water fraction in the feed. For very low feed water content the permeate is rich in isopropanol, but for increasing feed water content the permeate gets enriched in water with low amounts of isopropanol. In systems that have an azeotrope, as shown by the vapor/liquid equilibrium of the water/isopropanol system also represented in Fig. 1.22, pervaporation constitutes a viable technical alternative to distillation.

26

Separation of Functional Molecules in Food by Membrane Technology

Mole fraction of water in permeate

1 0.8 0.6 0.4 ELV 0.2 0 0

0.2 0.4 0.6 0.8 Mole fraction of water in feed

1

FIGURE 1.22 Comparison of the water/isopropanol separation by distillation and pervaporation with a membrane that permeates preferentially water.

Permeate flux (kg/h/m2)

1 Memb. A

0.8 0.6

Memb. B -70ºC

0.4 0.2

Memb. B -35ºC

0 0

0.2

0.4 0.6 0.8 Mole fraction of water in feed

1

FIGURE 1.23 Variation of permeate flux with feed water content and temperature for a water/ isopropanol system.

On the other hand, permeate flux quantifies the mass transport through the membrane and it is dependent on the composition and temperature of the feed, as depicted in Fig. 1.23. As the feed temperature and the preferential permeating solute concentration increases, the permeate flux increases. The application of pervaporation at industrial scale has not been as successful as in the cases of the pressure-driven and electrically driven membrane processes. This is mainly due to the higher energy requirements associated to the occurring phase change. However, pervaporation is still an important solution for processes where distillation is not technoeconomically viable, like the systems that have azeotropes or mixtures whose components have very close boiling points. One other important application is the processing of temperature-sensitive mixtures.

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The pervaporation application can be arranged into three main categories: 1. Dehydration of organic solvents with membranes permeating preferentially water. 2. Purification of aqueous streams contaminated with traces of organic compounds. Although membranes permeating preferentially organics exhibit very low permeation fluxes, this separation can be technoeconomically feasible because of the small organic quantities that have to be removed. 3. Organic
organic separation.

1.8 CONCLUSION The design of membrane processes requires not only deep knowledge of the feed solution to be processed and of the products to be obtained, but also careful selection of membranes, membrane modules, and pretreatments. This will allow production enhancement through the optimization of the operating conditions that lead to the minimization of the concentration polarization and fouling phenomena. The decision of the mode of operation (batch or continuous) depends mainly on the feed volume to be processed. Two main features of membrane processes should be emphasized in the development of new applications: (1) the modular character of the membrane technology, which allows to envisage its application in small, medium, and large scale installations; and (2) the versatility in the synthesis of hybrid processes involving membrane processes with other operation units either upstream as pretreatments, or downstream, as polishing stages for the recovery of valuable compounds from multicomponent mixtures.

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de Pinho, M.N., Geraldes, V., Catarino, I., Method for Simultaneous Concentration and Rectification of Grape Must Using Nanofiltration and Electrodialysis, US 8,945,645 B2, Feb. 3 2015. Giacobbo, A., Oliveira, M., Duarte, E., Mira, H.M.C., Bernardes, A.M., de Pinho, M.N., 2013. Ultrafiltration based process for the recovery of polysaccharides and polyphenols from winery effluents. Sep. Sci. Technol. 48, 445
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Soares, P.A.M.H., Geraldes, V., Fernandes, C., Cameira dos Santos, P., de Pinho, M.N., 2009. Wine tartaric stabilization by electrodialysis: prediction of required deionization degree. Am. J. Enol. Viticult. 60, 183
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