APPLICATION OF NANOFILTRATION FOR CONDENSING MODEL SUGAR JUICES

APPLICATION OF NANOFILTRATION FOR CONDENSING MODEL SUGAR JUICES

Aplikace nanofiltrace pro model kondenzce cukerních šťáv

P. Regiec

Agricultural University of Wrocław, Departament of Food Storage and Technology

Summary: In the sugar industry the condensation of thin juice in the evaporator is a very energy-consuming process. The application of nanofiltration may enable us to separate part of the water before evaporating, and thus save energy. The aim of the study was to determine the effect of pressure on the content of some substances in permeates and retentates when filtrating solutions containing sucrose. Condensation of model sucrose solutions in a nanofiltration station was carried out in the experiment. The results presented allow to state that nanofiltration can be applied as a process that partially substitutes juice condensation in evaporator. Condensation of thin juice from 15 to 25% will enable the solution mass to decrease from 100 to 60 kg, and thus obtain a decrease of the water evaporated by 40 kg per 100 kg juice, which should markedly diminish the energy consumption of production. An additional, positive effect is the partial removal of non-sugars which results from decreased losses of sugar in molasses. As a disadvantage of the process should be seen the high costs of installation and, for the moment, the hard to estimate membrane life.

Key words: nanofiltration, evaporation, thin juice

Souhrn: Kondenzace lehké šťávy vodparce cukrovaru je energeticky velice náročný proces. Aplikace nanofiltrace nám umožní separovat část vody před odpařováním a tak šetřit energii. Cílem výzkumu bylo určit účinnost tlaku na obsah některých látekprostupujících a zadržovaných, jestliže filtrační roztoky obsahují sacharózu. Vpokusu byl uskutečněn model kondenzace cukerních šťáv vnanofiltrační stanici. Předložené výsledky dovolují konstatovat, že nanofiltrace může být aplikována jako proces, který částečně nahrazuje kondenzaci šťávy vodparce. Kondenzace lehké šťávy z 15 na 25 % umožní pokles množství roztoku z 100 na 60 kg, a tak získat pokles odpařené vody o 40 kg na 100 kg šťávy, jež by mohla markantně snížit spotřebu produkční energie. Navíc pozitivním efektem je částečné odstranění necukrů, které rezultují zpoklesu ztrát cukru vmelase. Jako nevýhoda procesu by mohla být vidět vysoká cena instalace, vpřípadě přísného odhadu životnosti membrány.

Klíčová slova: nanofiltrace, odpařování, lehká šťáva

Introduction

In the food industry, the membrane techniques offer alternative methods to traditional filtration and condensation by water evaporation. The most often used are the pressure processes: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO).

Nanofiltration belongs to the most modern membrane separation techniques. In a classification based on dimensions of the molecules separated, it occupies the middle position between ultrafiltration and reversed osmosis. The nanofiltration membranes exhibit features which are partly typical of ultrafiltration membranes, i.e. the sieve effect, but also typical of the reversed osmosis membranes, i.e. the ability to separate molecules of dimensions smaller than the membrane pores [8]. Molecules of molecular mass in the range of 200-500 are stopped, which corresponds to the theoretical size of pores of 1 nm, hence the name of the process [10]. Because of similar transport mechanisms and smaller pressure needed for obtaining fluxes similar to those in reversed osmosis, nanofiltration is also called “low pressure reverse osmosis” [11].

In the food industry nanofiltration finds application, among others, for removing alcohol from wine, whey desalination, egg protein condensation [12,13]. Its main advantage, compared with reversed osmosis, is the considerably increased fluxes.

The factor that limits nanofiltration velocity may be the osmotic pressure or viscosity. Rautenbach [11] has found that in the case of solutions containing sodium chloride, copper sulfate, glucose or sucrose the deciding factor is the osmotic pressure. Viscosity of the solutions has great importance when nanofiltrating such substances as milk, whey or protein.

In order to insure proper concentration of the retentate, pressure must be applied that exceeds the osmotic pressure of the solution, which in turn increases with increasing concentration of the substance.

In the sugar industry the condensation of juice in the evaporator is a very energy-consuming process. The application of nanofiltration may enable us to separate part of the water before evaporating, and thus save energy.

The aim of the study was to determine the effect of pressure on the content of some substances in permeates and retentates when filtrating solutions containing sucrose.

Material and methods

Condensation of model solutions in a nanofiltration station was carried out in the experiment. The following solutions were used in the study:

A 15% sucrose solution

B 15% sucrose solution + 2% invert + 0.66% NaCl (acidic hydrolysis of sucrose)

C 15% sucrose solution + 0.4% molasses

In the investigation was used an apparatus which enables to apply pressures of 3 MPa, temperature of 40°C. The Romembre SU610 membrane was used.

Membrane parameters:

Material type: crosslinked polyamide composite

Retention of NaCl: 55%

Retention of MgSO4: 99%

Roll-type

Range of pH: 2-10

Area: 10 m2

The process was conducted at constant flux of retentate 2.5 l/h/m2 at 30°C.

The flux of permeate was measured during condensation.

In the feed, permeates and retentates the contents of the following was determined:

Apparent dry substance by refractometric method [4]

Sucrose by polarimetric method [4] and liquid chromatography * (solutions B and C)

Glucose and fructose by liquid chromatography * (solution B)

Reducing substances by the reduction method [9] (solution B)

Ash by conductometric method [4] (solutions B and C)

Colouring by colorimetric method [4] (solution C)

* Chromatographic analysis was carried out using a HPLC ProStar Varian, HPX-87c column, at temperature 85°C and pressure 37 atm.

Based on the analyses performed, the retention coefficients (R) were calculated using the formula:

R=1- Cp/Cf

where: Cp substance concentration in the permeate

Cf substance concentration in the substrate

After nanofiltration of solution C the membrane fouling was determined by measuring the filtration efficiency of distilled water.

Results

Figures 1, 2 and 3 show the dependence of permeate efficiency on retentate concentration for condensation of solutions A, B and C. The curves are described by second order polynomials. From the data presented by the plots it follows that the permeate output decreased with rising dry matter content in the concentrate, the course of the curves being more even for higher pressures. Similar dependences between the fluxes were found by Bichsel [3]. The maximum concentrations of the concentrates obtained for the respective pressures were as follows: 2 MPa 19.5 19.8%, 2.5 Mpa - 22.722.9 %, and 3.0 Mpa - 25.125.5 %. From literature data [5] it follows that the osmotic pressure of 20% sucrose solution is 2.08 Mpa, and the concentrate concentration obtained under the same conditions was 19.5-19.8%, meaning that almost the maximum possible condensation of the solution has been attained.

From the data presented in tab. 1 it follows that the applied membrane retained almost 90% of sucrose during nanofiltration of solutions A, B and C. Its content in the permeate was ca. 1.8 2.0 %. Almost identical results were obtained by Koekoek [7], who used different membranes at temperature 70°C. Hence, it appears that, for now, it is very difficult to find a membrane that would insure practically full retention of sucrose, at fairly large fluxes and lowest retention coefficients of non-sugars. In the industrial practice the presence of sucrose in the permeate would mean losses of sugar. This problem can be solved by using permeate as the feed liquid for the diffuser which would enable turning the sucrose back to the process. Another proposed solution is a repeated condensation of the permeate to ca. 15% [7], which is however a rather dubious solution both from the energetic and economical viewpoint.

Table 1: Retention coefficients of sucrose and non-sugars at various pressures of nanofiltration for solutions A, B and C

When filtrating solution B (sucrose and invert) it was found that the retention coefficients of simple sugars and sodium chloride depended on the pressure applied, the higher the pressure the greater the coefficients. The effect of pressure on retention coefficients was also found by Bhattacharaya [1].

Solution C was prepared adding such an amount of molasses that the solution obtained had a colouring close to the thin juice in sugar factory. That colouring measured 1380 ICUMSA units, proving that the membrane applied retains the colour compounds in 100%. Slightly different results were obtained by Gyura [6], using a membrane of MWCO = 5kDa at 30°C, and finding about 22% of the initial amount of colour substances in the permeate. The reason for that might be differences in the structure of membranes, or the shear rate applied [2].

The ash components in solution C (sucrose + molasses) permeated the membrane to a lesser degree than NaCl present in solution B. That is due to their differentiated chemical composition, i.e. a fairly large amount of sulfates and other two- and multi-valence salts, of considerably smaller retention coefficients. The producers specification gives 99% for retention coefficient of pure MgSO4 solution.

Figure 4 presents the nanofiltration efficiency of water before and after condensation of solution C. In this way the membrane fouling was determined. At the pressure of 0.5 Mpa the efficiency was practically the same. With increasing pressure the difference between efficiency before and after nanofiltration increased, and at 3 Mpa it was 8.9 l/h/m2, i.e. about 9%. This phenomenon was, however, reversible, as on applying the standard washing procedure the process efficiencies returned to former values. That effect was, however, considerably dependent on the duration of process between the membrane washing cycles.

Conclusion

The results presented allow to state that nanofiltration can be applied as a process that partially substitutes juice condensation in evaporator. Condensation of thin juice from 15 to 25% will enable the solution mass to decrease from 100 to 60 kg, and thus obtain a decrease of the water evaporated by 40 kg per 100 kg juice, which should markedly diminish the energy consumption of production. An additional, positive effect is the partial removal of non-sugars which results from decreased losses of sugar in molasses. As a disadvantage of the process should be seen the high costs of installation and, for the moment, the hard to estimate membrane life.

References

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13. WITROWA-RAJCHERT D. : Procesy membranowe w technologii żywności. Przemysł Spożywczy 2001, 55, 52-55.

Adresa autora

This research work was supported by the Polish State Committee for Scientific Research (Grant No 6P06H03620, year 2001-2003)"