Permeate flux prediction in apple juice concentration by reverse osmosis

E L S E V I E R Journal of Membrane Science 127 (1997) 25-34

journal o f MEMBRANE

SCIENCE

Permeate flux prediction in apple juice concentration by reverse osmosis

V. Alva rez , S. A lva rez , E A . Riera*, R. A l v a r e z

Department Chemical Engineering, University of Oviedo, (2/Julidn Claver[a s/n, 33071 Oviedo, Spain

Received 12 August 1996; received in revised form 30 September 1996; accepted 2 October 1996

Abstract

This work studies apple juice concentration by reverse osmosis (RO) using polyamide tubular membranes at different operating conditions. Permeate flux has been predicted by using the solution-diffusion model combined with the film model. Good agreement was found between the experimental values and the ones calculated by using both models. Pressure was found to be the most important variable controlling the process. Physico-chemical analyses were made in order to evaluate the quality of concentrated apple juice.

Keywords: Reverse osmosis; Apple juice; Concentration; Modelization

I. Introduction

The concentration of apple juice, like any other food product, involves removal of water in order to reduce package, storage and transport costs. Reverse osmosis (RO) is a membrane process becoming of increasing interest in the food industry, which presents several advantages with respect to evaporation and freezing, the other two techniques most widely used for this purpose. As the process is carried out at low temperatures, minimum thermal damage is caused. Other advantages are the lower energy consumption and the lower capital equipment costs [1,2].

Several research works [2-5] using RO for apple juice concentration have been made in the last few

*Corresponding author.

years. These studies pay special attention to the aroma compounds and sugars retention by using different types of membranes (different composition, salt rejection and configuration) at different operating conditions. The first work published on the concentration of juices by RO was by Merson and Morgan [6], in which they studied orange and apple juice concentration by means of cellulose acetate (CA) membranes. Many papers have followed the first one [7,19]. Thus, Chua et al. studied permeate flux and flavour retention by CA and polyamide (PA) membranes [5]. PA membranes were found to be better in terms of both flux and flavour retention. Sheu and Wiley [2] used different CA and high resistance (HR) membranes to concentrate apple juice. They reported permeate fluxes between 15 and 26.9 1/h m 2 and apple flavour retention from 16.9 to 87% [2]. Nevertheless, there are not many studies in which solvent and solutes permea-

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26 V Alvarez et al./Journal of Membrane Science 127 (1997) 25-34

tion have been predicted at different operating conditions using theoretical transport models.

The performance of crossflow filtration systems is limited by membrane fouling and by the concentrations polarization layer. In the case of RO, concentration polarization takes on special importance due to the increase in osmotic pressure as the concentration at the membrane surface increases. The effect of the concentration polarization phenomenon on the permeate flux in RO can be described by the solution- diffusion model. Some authors used this model to predict permeate flux through RO membranes [8- 13]. In most of the cases this model has been applied only to solutions containing one component. Sourir- ajan et al. [10] have studied the applicability of this model to a mixture of o-glucose and D,e-malic acid and Kimura et al. [12] to a mixture of sucrose and glucose trying to simulate the components of fruit juice. In both cases a good agreement was obtained between experimental RO data and those calculated on the basis of the solution-diffusion model. There is, however, little research to predict permeate flux during RO of multisolute solutions, because there is little information about the properties of such solutions [121.

There are several correlations in the literature to estimate solution properties. Osmotic pressure was traditionally calculated using the van't Hoff relationship. However, this equation differs greatly from experimental data at high concentration [14]. Kimura et al. proposed an equation that expresses the osmotic pressure, / / , of glucose and sucrose solutions in terms of solute concentration [12]. The mass transfer coefficient, k, when the flow is turbulent is generally calculated by the correlation of Deissler, valid for pipes [15]. Gladden and Dole studied the diffusion coefficients, D, of binary solutions containing sucrose and glucose [16].

The aim of this work, was to develop a model to predict the permeate flux in apple juice concentration by RO in order to serve as a tool for the design of reverse osmosis plants in juice concentration. For this purpose the solution-diffusion model and the film model (to take into account the higher concentration of rejected solutes at the membrane surface) have been used. Furthermore, physico-chemical analysis have been made to evaluate the characteristics and quality of concentrated apple juice.

2. Materials and methods

2. l. Membrane filtration equipment

A reverse osmosis unit supplied by PCI (Paterson Candy International, England) was used in this work. This was equipped with 18 AFC99 tubular polyamide membranes (99% NaCI rejection), with 0.0125 m inner diameter and 1.2 m length. The total effective surface area was 0.9 m 2.

The apple juice was pumped tangentially over the membrane by using a CAT piston pump (model 311, 5.5 HE USA), with a range of velocities between 0 and 2.95 m/s and transmembrane pressure of up to 5.50 MPa. Transmembrane pressure was computed as the mean value of the inlet and outlet membrane module pressure. The flow rate in the retentate stream was measured using a Rosemount magnetic flowmeter (model 8711, USA). Velocity and pressure in the retentate stream were independently controlled by means of the pump velocity regulation and a needle valve placed at the outlet of the membrane module.

All the experiments were carried out with retentate recycling. Permeate was recirculated in experiments in which concentration was kept constant and removed from the equipment in those in which concentration was increased. Membranes were cleaned in place by pumping 0.2 mass % NaOH solution at 2 m/s and 313K.

2.2. Apple juice characterization

The apple juice used in this work was supplied by the IEPA (Instituto de Experimentaci6n y Promoci6n Agraria, Villaviciosa, Spain) and clarified as described in a previous work [17]. Malic acid was measured as total acidity by titration with 0.1 mol/1 NaOH. The indirect analysis of total solids was made with an Anton Paar digital densimeter (DMA 35, Vienna, Austria). Soluble solids were determined by using an Abbe refractometer (Standard type, Shibuya Opti- cal, Tokoyo, Japan) and expressed as °Brix. The pH was measured with a pH-meter (Crison 2001, Barce- lona, Spain). Turbidity and color were determined by spectrophotometry (Philips PU 8700, Cambridge, England) at 650 and 420 nm respectively. Phenolic substances were analyzed following the Folin-Cio-

V Alvarez et al./Journal of Membrane Science 127 (1997) 25-34 27

calteu method [18]. Viscosity was measured with an Afora Cannon-Fenske viscosimeter (Lugo, Spain).

3. Model

To predict permeate flux, the solution--diffusion model has been used. This model assumes that the membrane is a nonporous diffusive barrier in which all components, solvents and solutes, dissolve in accor- dance with phase equilibrium considerations and dif- fuse in an uncoupled manner following the same mechanisms that govern diffusion through liquids and solids [ 19]. According to this model, the permeate flux can be described by the expression:

J = Lp(AP - A I I ) (1)

where J is the membrane flux, Ap the applied pressure difference across the membrane, Lp a membrane constant that can be calculated as the water perme- ability of the membrane and A_H the osmotic pressure difference between the retentate and permeate sides of the membrane, defined as

A / / = / / ( C m ) - / / ( C p ) (2)

where Cm and Cp are the solute concentrations at both sides of the membrane (see Fig. 1).

Due to the polarization concentration phenomenon the concentration at the membrane surface (Cm) is higher than the concentration in the bulk solution (Cb), as illustrated in Fig. 1. This phenomenon can be described under steady state conditions by the

J C <

Ds dC/dZ

j

Cp

> Z

Cb

Fig. 1. Concentration polarization phenomenon.

following equation:

J = k In Cm - Cp (3) G - Cp

where k is the mass transfer coefficient defined as D/d, D being the solute diffusion coefficient and d the concentration polarization layer thickness. In some reverse osmosis membranes, Cp is very low due to their high solutes rejection (99% NaC1 in case of the membrane used in this work). In that case, the term Cp can he neglected in Eq. (3). Then Eqs. (1) and (3) can be written as:

J = Lp[AP - H(Cm)] (4)

Cm = Cb e x p ( J / k ) (5)

By knowing Lp and the dependence of the osmotic pressure on the solute concentration at the membrane surface, the value of the permeate flux can be obtained using Eqs. (4) and (5). In the case of a solution containing i solutes the process can be described with the following i+1 equation system:

J = Lp[Ap - H(Cmi)] (6)

Cmi = Cbi exp(J /k i ) (7)

where the subscript " i" includes the species considered in the model. From Eq. (1), Lp can be easily calculated by measuring the permeate water flux through the membrane using distilled water as feed, which implies that H(Cm)=0, then

Lp = J/Ap (8)

The Lp dependence with temperature is shown in Fig. 2.

The mass transfer coefficient, ki c a n be estimated using Deissler's correlation, Eq. (9), which is generally accepted for turbulent flow in pipes.

Shi = 0.023 Re °875 Sc? "25 (9)

where Shi is the Sherwood number (krdh]Oi), Re is the Reynolds number (dh'V'p/#) and Sc i is the Schmidt number (#/p.Di). ki being the mass transfer coefficient for the species i; dh the equivalent hydraulic diameter; v the tangential flow velocity; p the solution density and/z the solution viscosity. In this case p and # are the apple juice density and viscosity, respectively.

Regarding their high concentration and low molecular weight, sugars and organic acids are the major

28

24 Lp = - 102.01 + 0 .38 T R^2 = 0 . 9 7 9

" 20

12

8

28 V. Alvarez et al./Journal of Membrane Science 127 (1997) 25-34

4

0

2 8 0 1 I

2 9 0 3 0 0

T(K)

Fig. 2. Membrane ~ e ~ i l i t y vs. temperature.

!

3 1 0 3 2 0

constituents of apple juice contributing to osmotic pressure [6]. The concentration and molecular weight of these compounds in the clarified apple juice used in this work [17] are, respectively, fructose (66 g / l - 180.16 g/mol), glucose (19 g/1 - 180.16 g/mol), sucrose (16 g/1 - 342.3 g/mol), malic acid (4.6 g/l - 134.1 g/mol) and sorbitol (4 g/1 - 182.18 g/tool).

To establish the dependence of the osmotic pressure on the solutes concentration at the membrane surface, Eq. (10) has been used:

[I] [II] ['lO00-Cms -Cm~ _4Cs 2Cg 7

/~(Cms,mg,ma) RT In / Mw M, M~/_L R T ~ v~ U~-Cm~ c~L3c~__Cg l "

L Mw Ms Mg I

(it)

The first term [I] of the Eq. (10) has been experi- mentally obtained using a membrane osmometer for solutions containing different concentrations of sucrose and glucose and it was used by some authors to predict flux in RO of binary mixtures of these two types of sugars [12,20]. In this work, it has been supposed, taking into account their similar chemical composition and molecular weight, that fructose and sorbitol behave in the same way as glucose, regarding their contribution to the osmotic pressure. The second term [II] in Eq. (10) is the van't Hoff osmotic pressure relationship, added to take into account the contribu-

tion of malic acid to the osmotic pressure. In spite of the fact that the van't Hoff equation implies accepting several simplifications and that this law differs greatly from experimental data at high concentrations, it can be employed for diluted solutions, concentrations less than 1%, as is the case of malic acid in apple juice [14].

Thus, osmotic pressure (II) can be estimated using Eq. (10), where R and Tare the universal gas constant and the temperature of the apple juice, respectively; Vw is molar volume of pure water (=18.07x 10-31] mol). Mw, M~, Mg and MR are the molecular weights of water, sucrose, glucose and malic acid, respectively. As it has been mentioned, fructose and sorbitol are supposed to behave like glucose and are referred to in subscripts as g. The concentrations of sucrose, glucose and malic acid at the membrane surface are expressed as Cms, Cmg, and CmR.

According to the previous assumptions, and using Eqs. (10) and (4) the permeate flux can be described by the following equations:

J = L p - [ A p -- H(Cms, Cmg, Cma)] (11)

Cmi = Cbiexp(J/ki) (12)

where ki is the mass transfer coefficient and subscript " i" represents the three compounds considered (sucrose, glucose and malic acid).

v. Alvarez et al./Journal of Membrane Science 127 (1997) 25-34 29

3.1. Physico-chemical properties o f apple ju ice

To estimate the mass transfer coefficient of each component it is compulsory to know the apple juice viscosity and density as a function of concentration and temperature as well as the diffusion coefficient of each component. In order to estimate the apple juice viscosity and density, Eqs. (13) and (14), reported by Constela et al. [21], were used. These equations were proven and the values predicted fitted well with the experimental measurements:

AX ln(#/#w) -- (100 - BX) (13)

with A=-0.258+817.110/T and B----1.891-3.021 10-3T,

p ----- 0.8278 + 0.34708 exp(0.01X) - 5.479 × 10-4T

(14)

# and #w are the apple juice and water viscosities (kg/ m s) respectively; p the apple juice density (kg/m3); X the apple juice concentration expressed in °Brix; A and B are two coefficients; T being the absolute temperature.

To estimate the influence of the concentration on the diffusion coefficient, the following expression has been used:

Di ---- Doi " (#w/#) °45 (15)

This expression was reported by Gladden and Dole [16] for binary solutions containing sucrose and glucose, where Di and Doi are the diffusion coefficients (m2/s) for the species i in the mixture and in the dilute water solution, respectively, and #w/# is the ratio between the water viscosity and the binary solution viscosity. In this work, Eq. (15) has been considered valid for sucrose and glucose as well as for malic acid in the clarified apple juice.

The values used for the diffusion for the coefficients for sucrose, glucose and malic acid at 298 K have been previously estimated in other works [22]. Their values are: 6.75x 10 -l°, 5.24x 10 - l ° and 7.05x 10 - l ° m2/s, respectively.

In order to take into account the influence of the temperature, the previous values have been corrected by using the following expression [20]:

Dil = (Dio .770. T1)/(rll . To) (16)

where To corresponds to 298 K and T 1 is the temperature at which the diffusion coefficient is going to be estimated; Zlo and z/1 are the kinematic viscosities at To and T1, respectively, and Di~ and Dio are the diffusion coefficients for each component at these temperatures.

4. Results and discussion

Using the previous equations to estimate the mass transfer coefficient for each one of the components and solving the systems formed by Eqs. (10)-(12), the permeate flux can be calculated.

Two series of experiments were performed. The first one was carried out by fixing the operating conditions Ap, T and v and removing the permeate stream from the apparatus, thus concentrating the apple juice. In the second one, the apple juice concentration was kept constant by recirculating the permeate stream and the effect of the operating variables on permeate flux at constant concentration was studied.

The comparison between experimental and calculated data is shown in the following figures. The solid lines represent the values of permeate flux predicted by the model and the points the experimental values. Fig. 3 shows the permeate flux as a function of feed concentration at different tangential velocities. The permeate flux is higher at higher velocities, but this positive effect is reduced as the concentration process takes place due to the increase in osmotic pressure. Increasing the velocity and so increasing the efficiency of the process is possible to reduce the filtration area or the operating time, but not the maximum limit of apple juice concentration. The effect of tangential velocity is important at low velocities but it considerably diminishes for values higher than 1.5-2 m/s. The behavior of permeate flux as a function of tangential velocity is well predicted by the solution-diffusion model.

Figs. 4 and 5 show the effect of the transmembrane pressure in permeate flux. Values in Figs. 5 and 7 have been obtained by interpolation of data in Figs. 4 and 6, respectively. As A p increases, not only the permeate flux increase at a constant concentration, but so does the maximum concentration that can be obtained, the limit being 19, 23 and 27.5°Brix at 350, 450 and 550 kPa, respectively. Therefore, transmembrane pressure is the most important variable controlling

30 V,, Alvarez et al./Journal of Membrane Science 127 (1997) 25-34

¢o 0

E - . j .

• 11°Brix

m 15°Brix

• 19.5°Brix

o 22°Brix

• 25°Brix

0 1 2

v(m/s) Fig. 3. Influence of tangential velocity on permeate flux at constant concentration degree (298 K, 550 kPa).

E , - j

10

4

~ e e x ~ ~ • 350kPa

• [] 450 kPa ~ ~ • 550kPa

• ,

10 16 22 ° B r i x

28

Fig. 4. Permeate flux variation as a function of concentration at different transmembrane pressures (298 K, 2.2 m/s).

V. Alvarez et al./Journal of Membrane Science 127 (1997) 25-34 31

10

8 • 110Bdx [] 15°Bdx • 19.5*Brix J

¢D o • 6 O 22°Brix J

E 4

2 • [] •

0 50 250 350 450 550

&P(kPa)

Fig. 5. Influence of transmembrane pressure on permeate flux at constant concentration (298 K, 2.2 m/s).

1 6

L ° ~ • 293 K 1 2 ] - ~ ~ ~n 298K

• 305.5 K

i o 313 K 8

4

0 l o 28

°Brlx

Fig. 6. Permeate flux variation as a function of concentration at different temperatures (2.2 m/s, 550 kPa).

the RO concentration process. When representing J as a function of A p at constant concentration (Fig. 5) the curves lose their linearity as the pressure increases due to the exponential relationship between osmotic pressure and concentration.

Figs. 6 and 7 show the effect of temperature on permeate flux. J increases proportionally to T due to the increase of Lp (see Fig. 2). But, as in the case of tangential velocity, it does not affect the maximum concentration that can be obtained.


16 "

12 • 19.5 Brix

4

0 I I

290 300 310 320

T(K)

Fig. 7. Influence o f temperature on permeate f lux at constant concentration (2.2 m/s, 550 icPa).

According to these results, it can be said that the solution-diffusion model describes reasonably well the effect of operating variables (Ap, T, v) on permeate flux at different concentration degrees. As this model has been deduced from the different components of apple juice it could be used to predict the permeate flux of other types of fruit juices when their composition is known.

4.1. Physico-chemical characteristics of concentrated apple juice

Analyses of physico-chemical properties (color, turbidity, pH, viscosity, density, total soluble solids, phenolic compounds, malic acid and sugar concentration) of the concentrated apple juice have been carried out for all the runs. Table 1 shows these characteristics for an apple juice concentrated up to 26°Brix and compared to those of the clarified apple juice without concentration. Color and turbidity, expressed as spec- trophotometric absorbance at 420 and 650 nm, respectively, increase with the concentration degree of the juice. No color and turbidity were detected in permeate streams due to the total retention of phenolic compounds by the RO membrane. Density and viscosity increase in the same way as predicted by Eqs. (13) and (14) with the apple juice concentration.

Table 1 Physcio-chemical characteristics of clarified and concentrated apple juice

Clarified Concentrated apple juice apple juice

Turbidity (Abs. 650 nm) 0.025 0.06 Color (Abs. 420 nm) 0.650 1.850 Density (kg/m 3) 1044 1100 °Brix (%) 11.0 26.0 Total soluble solids (g/l) 114.2 278 Total phenols (nag/l) 890 2127 Malic acid (g/l) 4.6 11 pH 3.52 3.30 Viscosity (kg/m s) x 103 1.42 2.35

The pH of concentration apple juice slightly diminishes, from 3.5 to 3.2 due to the increase in organic acid contents (partial retention of malic acid).

Sugar retention is around 100%, considering that no sugars were detected in the permeate stream and that the minimum detection level of the refractometer used was 0.2°Brix. Malic acid retention ranged between 96 and 98%. Sugar retention was higher than the results found by Sheu and Wiley [2,23] using cellulose acetate membranes (De Danske Sukkerfabrikker, Copenhagen, Denmark). This shows the good perfor-

v Alvarez et aL/Journal of Membrane Science 127 (1997) 25-34 33

mance of polyamide membranes, taking into account that sugar retention is considered as the main objective in fruit juice concentration and the maintenance of a constant sugar/acid ratio in the apple juice and deri- vatives is important to assure a final good flavour.

5. Conclusions

Apple juice concentration using AFC99 tubular polyamide membranes (99% NaC1 rejection) was carded out with high retention of sugar and malic acid. The solut ion-diffusion model combined with the film model have been successfully used to model ize the reverse osmosis process in apple juice concentration. The model represents with a high degree of accuracy the real performance of the reverse osmosis operation and can be considered as a useful tool for plant design in fruit juice concentration.

6.1. Subscr ip t s

a malic acid b bulk g glucose i species i m membrane o dilute solution p permeate s sucrose w water

Acknowledgements

The authors wish to thank FICYT (Fundaci6n para la Investigaci6n Cientlfica y la Technologla) and CICYT (Comisi6n Interministerial de Ciencia y Tech- nolog~a) for their financial support.

References

6. List of symbols

A empirical coefficient in Eq. (13) ( - ) B empirical coefficient in Eq. (13) ( - ) C solute concentration (g/l) D diffusion coefficient (mE/s) dh equivalent hydraulic diameter (m) J permeate flux (m/s) k mass transfer coefficient (m/s) Lp membrane permeabil i ty (m/s kPa) M molecular weight (g/mol) Re Reynolds number ( - ) Sc Schmidt number ( - ) Sh Sherwood number ( - ) R universal gas constant (kPa I/K mol) T absolute temperature (K) v tangential velocity (m/s) Vw water molar volume (1/mol) X °Brix (wt%) A p transmembrane pressure (kPa) d concentration polarization layer thickness (m) # apple juice viscosity (kg/m s)

kinematic viscosity (m2/s) H osmotic pressure (kPa) p apple juice density (kg/m 3)

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Permeate flux prediction in apple juice concentration by reverse osmosis