1. Aids Characteristics

The granulometry of the filter aids is an essential parameter because this parameter plays an important part in the relationship of tortuosity, filterability, retention capacities and porosity. Particles size distribution was determined by laser diffraction using a Mastersizer (model 3000, Malvern Instruments) in a range between 10 nm and 3500 µm. The measurements are based on the theory of diffraction by Fraunhofer, which states that the intensity of scattered light by a particle is directly proportional to the particle size.

The measurements were done in dry mode using an Aero S dry powder dispersion accessory. Particles disperse and travel through the detection cell by means of airflow with settings of 0.5 bar pressure, 3 mm of hopper gap and a feed rate of 20 % using a vibrational feeder. For each measurement, size distributions (mean diameter in volume) were provided (calculated from equation 1) and statistical volume diameters, D₁₀, D₅₀ and D₉₀ were given (D_x indicates a particle size for which x % of the particles are below that size).

$\left(1\right) The volume mean diameter=D\left[4.3\right]=\frac{\sum _i{N}_i{D}_i^4}{\sum _i{N}_i{D}_i^3}$

where N_i is the number of drops with a diameter D_i.

2. Initial permeability

Classical aids used in this study were Diatomyl® P00 and were provided by Laffort Oenologie. These aids are commonly used to filter the most charged liquids (wine lees) and represents appreciatively 80 % of the total production. The tested polyamide particles (Rilsan®) were D80 (80 µm) and D100 (100 µm), which were provided by Arkema. The filtration capacity and the characterisation of the powders (permeability before filtration and after regeneration) will be measured using the same device. The experimental permeameter was used to evaluate water permeability, filtration capacity and regenerable possibilities of the precoat layer expressed in the Darcy unit. The laboratory pilot used is presented in Figure 1.

Figure 1. Experimental permeameter.

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The first step consists of creating the first precoat with about 1 kg.m^-² of filtration aids on the sintered stainless-steel sheet (22.4 × 10^-4 cm²-40 µm pore size). The filtration aids were suspended and placed in the liquid chamber. Diatomyl® P00 is suspended in water, and Rilsan® in a mixture of water with 12 % of ethanol because Rilsan® are hydrophobic and require ethanol to be suspended. Agitation in body feed is maintained to ensure the mixing of filtration aids (200 rpm) and the device is pressurised to 0.4 bars. First, 200 ml of water is passed through the filter cell to form the cake. In a second step, the filtration time of 200 ml of water through the cake is measured. The cell is dismantled to measure the thickness of the cake with a calliper. The definition of the permeability for a filter aid results from Darcy’s Law.

Permeability is the volume of a fluid passing through a unit cross-section of the medium in unit time under the action of a unit pressure gradient. These data were used to calculate the permeability according to Equation 2:

$\left(2\right) Q= \frac{B\Delta P}{\mu e}$

With Q, the flow rate expressed in m³/s per filtration surface unit (m²), B the permeability in m², µ the dynamic viscosity in Pa.s, ∆P the pressure gradient in Pa and e the thickness of the cake in m. The result is expressed in Darcy.

A Darcy is defined as 1 mL/(sec.cm²) of a liquid with 1 centipoise viscosity (water) passing through a 1 cm thick cake at a 1-atmosphere pressure differential. In the practice of wine filtration, the permeability of the layer depends on the permeability of the filter aid, but also the retention of suspended particles. To recover the initial characteristics of polyamide aids, the initial permeability must be recovered after cleaning.

Filtration curves express the variations in the filtered volume (V) as a function of time (t). These curves are not linear, as the flow rate decreases over time, while the cake of filtration increases in thickness due to the continuous alluvium. They are therefore not extrapolable. However, it is possible to obtain linear filtration curves, per extrapolating the results using a logarithmic form, expressing the variations of log V as a function of log t (Dubourdieu et al., 1976). The curves become straight lines that can be extended to predict the volumes that could be filtrated over time. In this case, the slope is closed to 0.5. We can demonstrate, from Darcy's law, this linearity of the filter curves in logarithmic coordinates and the value 0.5 of their slopes when there is no clogging phenomenon.

If S is the filtering surface, the volume filtered at the end of time (t) is written:

$\left(3\right) V=S{\int }_0^{t}q \left(t\right) dt$
    

On the other hand, S the filtration surface (m²), the thickness (e) of the filter layer at time (t) is proportional to the filtered volume (V) and the dose the alluvium per unit of surface C (in g) and δ as a constant; therefore

$\left(4\right) e=\delta \frac{C}{S}V$
    

Within these equations, we could consider that:

$\left(5\right) Q=\frac{1}{\sqrt{2Lt}}$
    

with

$\left(6\right) L=\frac{n \delta C}{B ∆P}=constant$
    

so,

$\left(7\right) V=S{\int }_0^t\frac{1}{\sqrt{2Lt}}dt=S\sqrt{\frac{2t}{L}}$
    

then,

$\left(8\right) V=S\sqrt{\frac{2B ∆P}{n C \delta } x \sqrt{t}}$
    

this equation could be written as follows

$\left(9\right) \log V=0.5\log t+\log S\sqrt{\frac{2B ∆P}{n C \delta } }$
        

$\left(10\right) logV=0.5logt+constant$
        

We can notice that the liquid viscosity differential pressure also affected the ordinate at the origin of the straight line, but not its slope. The filtration of coarse liquid containing high content of suspended particles is tough; however, it can be interpreted from the previous simple model. Clogging results in a decreased value of this slope. Hence, obtaining a linear filtration curve in logarithmic coordinates is a condition to maintain good filtration. The prediction of the volume filtered after 8 h will be represented by a specific graph. Thus, thanks to the directing coefficient of the lines (noted k), it is possible to estimate the volume filtered after 8 h. With these results, we can therefore forecast the possibilities for an industrial filtration system.

3. Filtration test

The first step is to form the precoat with water in the experimental setup presented in Figure 1, which ensure initial clarity. Then, a mix is made with wine lees and filter aid to reproduce the wine filtration step in continuous alluvium mode. In both cases, 1 kg.m^-² of filtration aids will be used for creating the first precoat on the sintered stainless-steel sheet (22.4 × 10^-4 cm² - 40 µm pore size). Once the filter cake is created, the water in the feed flask is replaced by 2 L of the liquid to be filtered (in our case the wine lees). The addition of filtration aids into the liquid to be filtered is realised and called body feed. The wine lees are mixed with P00, D80 and D100 with a content equal to 2 kg/hL. Agitation in body feed is maintained to ensure the mixing of wine lees and aids (200 rpm). If the alluvium is enough and well-adapted, no rapid clogging will appear and the filtration will be subjected to the law of cake filtration.

The experiments are carried out at constant pressure (2 bars) supplied by a compressed nitrogen source. We note the volumes of filtrate collected during the time. The filtration test was carried out at 20 °C with the experimental permeameter for the three aids studied: P00 (as reference), D80 and D100.

4. Wine lees – dried matters

Real lees with a dry matter content of 10.8 % and 20.2 % have been used to compare Rilsan® D80, D100 and Diatomyl® P00. Dried matters were estimated by weighing after removing the water by heating a sample. The sample was dried for 48 h in an oven at 100 °C and weighed after membrane filtration (membrane Whatman 0,45 µm). The dry matter (DM) was expressed in g/100 g of products.

5. Physico-chemical analyses

Filtration efficiency was estimated by measurement turbidity with a nephelometer (HACH 2100Q with an accuracy of 0.2 NTU). A pH meter (Labomat H128 brand, precision 0.1) was used. The chromatic characteristics of the wines were measured using a Perkin Elmer LAMBDA 850 spectrophotometer. The absorbance of the wine sample was measured by placing an aliquot of each sample in a quartz cuvette with 1 cm light path at a wavelength of 420 nm detects yellow tones (Optical Density OD₄₂₀), 520 nm detects red tones (OD₅₂₀) and 620 nm detects purple tones (OD₆₂₀). The colour index (CI) was obtained from the sum of the absorbance of the samples for the three wavelengths. The Total Polyphenols Index (TPI) or OD₂₈₀ were determined according to the OIV methods (2018). Current analyses in oenology (tannins, anthocyanin, total acidity, volatile acidity) were carried out using a sequential enzymatic analyser, Y15 from BioSystems. The Y15 equipment was calibrated with the external standards that are provided in every kit by BioSystems.

6. Regeneration, cleaning in place

The two regeneration procedures consisted of an immersion of the filter cake in 1 % of an alkaline detergent chlorinated (Divos 120CL) during 3 min or in 12 % of Divos 120CL for 3 weeks followed by at least three rinses with water (up to a neutral pH).

7. Statistical analyses

All the analyses were realised in triplicate. Statistical analyses were performed using the R software and the Kruskal–Wallis test (a = 0.05).

1. Aids characteristics

The complexity to understand the phenomenon that occurs inside filtration aids are involved in limited information such as packing porosity, particle shape and tortuosity. Filters are composed of a complex network of interconnected pores each with tortuous paths. For either porous material, high permeability is dependent on low tortuosity. It is important to note that tortuosity will increase during filtration and the path travelled by the fluid will be longer as the pores will be clogged. Aids particles may be plates or needle-shaped, with small circular perforations. For a large distribution of particles, tortuosity is low and this packing modifies permeability and retention capacities. A compromise has to be identified between all these parameters but we first verify that there is a potential difference in granulometry between our polyamide particles (D80 and D100). Figure 2 indicates the particle size distributions of the two grades of Rilsan® and Table 1 summarise statistical diameters and mean diameter in volume.

Figure 2. Particle size distribution for Rilsan® D80 and D100.

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This is a monomodal distribution with a positive skew. The D₅₀ referring to the median value is 91.8 μm for D80 Rilsan® and 111 μm for D100. The mode value that is the peak frequency distribution is 92.0 μm for D80 and 118.8 μm for D100. The volume mean diameter D[4.3] for D80 is equal to 97.7 μm. This diameter differs from those measured for the D100 equal to 119.0 µm. We observe a D[4.3] larger than D₅₀ for both Rilsan® particles. These results are fairly typical because D[4.3] emphasise the detection of large particles. The particle sizes vary from 8 µm to 290 µm for the D80, and from 8 µm to 425 µm for the D100. We observe a positive skew in the particle distribution where the mean, the median and the mode are three different values. Concerning D80 and D100, the particles are also far from spherical and the diameter based on the real volume of the particles cannot give sufficient information. However, the long path those particles must take through the filter leads to an increase in the possibility that a particle may be trapped. The large distribution size with the smallest particles started at 8 µm could be locked in the inner space formed by the largest particles in both cases of Rilsan® (> 200 µm). This improves the probability of a particle being removed by the filter aids rather than passing through.

2. Initial permeability 

Considering the dispersion of the potential particles, tortuosity will be modified and will impact permeability (Griffiths et al., 2020). This is the reason why permeability must be characterised and represents a very important characteristic to consider in our work. Table 2 indicates the initial permeability measured for P00, D80 and D100. To realise these experiments 6 g for P00 and 10 g for Rilsan® were added to form the precoat cake thickness (1 cm) and flux were measured for 20 min. The thickness of the cake layer must be the same in both cases (1 cm) and the weight is different because the initial density of Diatomyl® and Rilsan® are 2.3 ± 0.1 g.cm-3 and 1.24 ± 0.1 g.cm-3, respectively.

First, we can notice that our system permits having reproducible values (low dispersion). These values will be used as a reference and cleaning protocols must ensure to recover the initial permeability.

Diatomyl® P00, which has permeability closed to 4 Darcy and these aids are usually used for coarse filtration in wineries. We can notice that these values are very close to D100 (3.93 Darcy). These results obtained confirm the potential utilisation of polyamide particles especially D100 for coarse filtration.

3. Filtration

To assess filtration capacities in the case of coarse wine, two matrices (10.8 and 20.2 % DM) were studied. The results obtained using filter P00, D80 and D100 aids were compared. The filterability was evaluated thanks to filtrate accumulation curves. Figure 3 represents respectively the evolution of the permeability along the time for a solution at 10.8% DM (case a) and 20.2 % DM (case b).

Figure 3. Evolution of the permeability (Lp) along the time for a solution at 10.8 % DM (case a) and 20.2 % DM (case b) for Diatomyl® P00, Rilsan® D80 and Rilsan® D100.

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The performance of each filter aids can be compared and results tend to prove that Rilsan® D80, D100 and Diatomyl® P00 have similar filtration capacities in case of 10.8 % of DM (Figure 3, case a). Concerning the experiments realised with high concentration for dried matters with 20.2 % (Figure 3, case b), D100 permeability is still 30 % higher after 45 min of filtration (146 L.h^-1.m^-2.bar^-1 for D100 compared to 110 L.h^-1.m^-2.bar^-1 and 113 L.h^-1.m^-2.bar^-1 for P00). In that way, D100 are more suitable to perform wine lees filtration when DM is around 20.2 % compared to D80 and P00.

This difference could be caused by a higher dense structure of the filter cake during the filtration stage for D80 and P00. The compressibility is higher for aids when pressure and tortuosity increase. Considering D80 and D100, polyamide is not impacted within this pressure and cake structure is conserved during filtration. As shown above in Figure 2, the particle size distribution implies that D80 is composed of the finest particles, in comparison with the other D100. It means that internal fouling can occur and limit the flux for matrices that have a high content of dried matter. Concerning D100, the inner flow is facilitated per the low tortuosity and higher fluxes are maintained. Within these values, we could also if alluvium is well realised and if a rapid clogging occurs by plotting log V and log t (Figure 4a and 4b).

Figure 4. Evolution in logarithmic coordinates of the volume as a function of time of the filtration realised with Diatomyl® P00, Rilsan® D80 and Rilsan® D100 using the matrices 10.3 % DM (case a) 20.2 % DM (case b).

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First, the linearity of these filter curves in logarithmic coordinates and the slope value close to 0.5 indicate there is no clogging and the model cake law filtration without clogging is respected (equation 9). It means that the filter does not contain particles that can change the permeability of the porous layer (case of pure water). We can then consider that the flow is only managed by the aid’s tortuosity and, consequently, it is constant throughout the filtration. Rilsan® D100 could also realise good filtration for solutions with higher suspended matters (20.2 % DM). Within this representation and by considering the slope equation, it is possible to evaluate the volume that could be filtered after 8 h. In the case of 20.2 % DM filtration, this means that in the case of a 10 m² filter press, the volume of lees filtered after an 8 h of filtration cycle would be 74 hL, 66 hL and 102 hL, respectively, for P00, D80 and D100. In the case of D100, the filtered volume after 8 h is 1.37 higher in comparison to P00. Considering D80, this volume is 1.12 lower in comparison to P00. The retention capacities must also be considered for this filtration and physical-chemical characterisation must be done.

4. Physico-chemical characterisation of the wine after filtration

To determine if the new filter aids modified wine characteristics after filtration, chemical and usual oenological analyses were performed before and after filtration. These analyses were supplemented by considering turbidity (Table 3). These analyses were realised in the case of filtration of wine lees with 20.2 % DM.

These turbidities are comparable and represent a significative diminution. Before filtration, turbidity cannot be determined (> 2000 NTU). These values are consistent and in line with the expectations of winemakers. At this stage, it is necessary to reduce the turbidity of the wines; however, the objective is not to obtain a brilliant wine or a wine with low levels of micro-organisms. Several other physicochemical parameters were measured to ensure that the main components of the wines are not modified (Table 4).

No difference is remarkable concerning pH, total acidity, volatile acidity, polyphenols and anthocyanins after filtration for each aid. Total tannins and anthocyanin contents are not impacted which means that the polyphenolic compounds are not retained by the porous media by steric retention or specific adhesion phenomena to polyamide particles. Chromatic characteristics were also measured to validate these results (Table 5).

The absorbance was measured at 280 nm for the TPI and the sum of OD 420, 520, and 620 for the colour index. These results do not indicate significant differences. These outcomes correlate with the previous ones and we can consider that only the particulate fraction is stopped by the filter media. However, it should be noted that D100 allowed faster filtration than P00. Besides, analysis results showed that the polyamide particles did not affect wine characteristics.

5. Cleaning-in-place

The last part of this work was to evaluate if Rilsan® could be reused after filtration. Cleaning-in-place systems were developed using the circulation of chemicals and water. To ensure the viability of this CIP, it seemed essential to focus primarily on classical cleaning products already existing in the winery.

Regarding the particles retained inside the filter mainly composed of organic matters and we decide to retain a chlorinated alkaline detergent (Divos 120CL). The second concern was to validate that the polyamide was not impacted by this product after a long contact time. We have decided to leave the polyamide immersed in a concentrated solution (10 %) of Divos 120CL for a long time (3 weeks) then to check that the permeability remained unchanged. In the literature, polyamide membranes are known to swell in alkaline conditions, because the dissociation of carboxylic groups in polymer chains leads to a negative charge of the chains and they repel each other (Huang et al., 2021). Swelling expands the membrane matrix, increases the average pore size of the membrane and changes the membrane surface morphology (Dalwani et al., 2011). In our case, we have powder but the modification of permeability must be considered by measuring the permeability after various CIP. The results for D100 are presented in Figure 5.

Figure 5. Permeabilities for Rilsan® D100 after various CIP (Divos 1 %-5 min and Divos 10 %/3 weeks).

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In this case, the solid fraction retained inside the porous material is mostly a combination of yeast, organic acids (mainly tartaric acid), insoluble carbohydrates (such as cellulosic or hemicellulosic materials), inorganic salts, lignin, proteins, phenolic compounds, pulp and other parts of the grape. The caustics act through the solubilisation, swelling, and degradation of organic matters and detergent improve wetting and degradation. Within this CIP, initial permeability is at the same level after filtration and after a long exposition to the alkaline agent. The initial permeability is regained after cleaning and prolonged contact does not modify its characteristics. Rilsan® could be employed in the case of wine lees filtration and could be regenerable.