Diafiltration
Ultrafiltration
Figure 29 shows the ethanol content whereas, Figure 30 shows the HMF, Furfural, and Acetic Acid content. It is expected that at the end only 10-15% of the concentration of the initial suspension remain. The concentration of the components was not expected to vary greatly for each filtration step.
During volume reduction the ethanol concentration is constant and only drops when refilling with water.
The ethanol content in sample R4 is an error, as samples R5 and R6 show high and relatively similar ethanol contents like samples R1, R2 and R3.
After the addition of water, the concentration of ethanol decreases as expected since for the same concentration of ethanol a large volume of water was added, resulting in a decrease in ethanol content. In the first three samples there are no major changes except for the ethanol content not being constant, which is explained by the fact that the samples that are being taken are not being considered, also possible evaporation of ethanol may occur and even errors associated with the analysis.
The ethanol content decreased significantly in sample R10W when compared to R9W, the difference between them was when the samples were measured, since the HPLC has a limited space sample.
The same way as the ethanol content, the other degradation products content drops after the addition of water. However, for sample R7W only acetic acid remains in the samples which disappears after sample R10W, where there is a membrane change (membrane 4 to membrane 5). Even though the membrane changes this is not a reason for such decline, since the content should maintain constant during the filtration step. However, the last samples where measured in a different batch which may explain the nonexistence of content in the last samples due to evaporation.
To conclude, it’s necessary to make sure how much of the degradation products are being removed from the nanoparticle’s suspension. From previous experiments made in the laboratory, it was concluded the amount of each component that was present in straw at 180ºC, the values are presented in Table 8.
Table 8 - Degradation products characterization of straw at 180ºC.
Component Quantity (𝒎𝒈 86.8%. As explained before, after sample R9W there are some errors associated and the decrease of the content is not anticipated, which can explain such high removal percentage for the ethanol. If the last samples were ignored, and for this calculation was used the last content value credible (R9W), the removal of ethanol would be 97% (closer to the assumption value).
Particle Sizing
One of the most important parts of the work was to ensure that there was no particle agglomeration, this process would only be feasible if this did not happen. For this purpose, particle size measurement was performed on all samples using ZetaPals from Brookhaven Instruments Corporation.
In Figure 31 is shown the different measurements made during the filtration process for each sample. Each sample was measured twice, one was diluted and the other one was concentrated. For the diluted sample, the properties were already defined while for the concentrated samples it was necessary to measure the density and refractive index of each sample, since the ethanol content after water addition is lower and therefore, the viscosity changes.
To evaluate the particle size, it is more reliable to analyze the values of the diluted samples, since the viscosity and refractive index of water are well known, while for the concentrate it is necessary to resort to correlations. However, it is possible to determine that the particle size is approximately constant over time.
Dry Matter Content
The Dry Matter method was used for all samples despite being a method with many errors. The method was applied for all samples, retentate and permeate. For the retentate samples, the method was applied before and after centrifugation, and the results are shown in figure 32.
Figure 31 - Particle size for all retentate samples for the experiment 2 Membranes in Series.
Samples before the centrifuge (dissolved components with lignin particulates), should always have higher values of DM than the samples after centrifugation (dissolved components), since the nanolignin particles are removed, leaving only the supernatant. The difference between them gives particulate lignin. Therefore, sample R7W must be discarded, the error was probably caused by the fact that a small sample was used for the method.
Samples after centrifugation show that the dissolved components content is approximately constant during UF mode, which means there is no retention of dissolved components, while in diafiltration, the content of dissolved components is rising slightly, which means there is retention of dissolved components.
Samples before centrifugation show an increase of lignin particles during each of the filtration steps and the drop after adding water.
Removal efficiency for Dry Matter method
To understand how efficient the membranes were, the Dry Matter method was also executed for the permeate samples since the biggest error of this method is due to the small amount used from the retentate samples, as explained in Material and Methods. For this method it is needed at least 10g of sample to obtain credible values.
Figure 32 - DryMatter content (%) for all retentate samples, before and after centrifuge for the experiment 2 Membranes in Series.
To calculate the removal efficiency of dissolved components following equation 10, the DM of the permeates were used, Figure 33, and also the DM of the initial suspension, after centrifugation.
All the membranes show an increase in DM during UF, but during DF this is not the case. It is possible to conclude that the retention of DM is decreasing during UF, maybe due to an increase in DM content in the retentate samples. However, in DF the DM content in the retentate samples is approximately constant.
In Figure 33 is shown the variation of Dry Matter content (%) for the different membranes permeate.
Where,
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 = ∑ (𝐷𝑀𝑖 𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖× 𝑚𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖)
𝐷𝑀𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛= 𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛× 𝑚𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛
The DM amount was based on the sample amount multiplied per the percentage of DM content (𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛 ). The membranes removed 85% of the dissolved components from the nanolignin particle suspension, a result very close to the one intended (90-95%). From the dry matter, it is also possible to perform a mass balance to the filtration system, based on the equation 9.
𝑅𝐸(%) = ∑26𝑖=1𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛
= 85,2%
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛=
∑
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠26
+
∑
𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠+ 𝐷𝑀𝑅13𝑊12
Figure 33 – Dry Matter content (%) for all permeate samples for each membrane for the experiment 2 Membranes in Series.
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠− 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛− 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.
𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠− 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑟𝑒𝑡𝑒𝑛𝑡𝑎𝑡𝑒 𝑠𝑎𝑚𝑝𝑙𝑒, 𝑔.
𝐷𝑀𝑅13𝑊− 𝐷𝑀 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑛𝑎𝑙 𝑠𝑎𝑚𝑝𝑙𝑒 𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑖𝑛𝑠𝑖𝑑𝑒 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘, 𝑔.
The mass balance is made to the dissolved components, as such, the values used are relative to those of the supernatant after centrifugation. The values obtained for each part of the equation 9 are represented in Table 9.
Table 9 - DM amount (g) of different samples used for the mass balance of the filtration system for the experiment 2 Membranes in Series.
Since the mass balance to the dissolved components does not close, this means that there are losses in the system, possibly deposition on the membranes.
Membrane Filtration – 3 Membranes in Series
Membrane Filtration – Flux and Concentration Experiment
The next step was membrane filtration with 3 membranes in series. When compared with the previous experiment with 2 membranes in series, the difference relies in the addition of a membrane that is never altered or regenerated. However, prior to filtration, there was the need to understand if the flow was decreasing due to increased concentration. As in the first experiment the transmembrane flux was decreasing, and so the question of whether it was decreasing because of the increase in scale or increase in concentration remained.
Three experiments (experiment 1, 2 and 3) were conducted with an initial suspension of 5L, which were filtered to 50% of the initial volume, approximately 2.5L. This was repeated three times, with refilling of the permeate into the feed tank so that the lignin concentration did not change. The results obtained are shown below, from Figure 34 to Figure 36, together with the mean transmembrane flux
values for each membrane, in each experiment, Table 10. Membrane 1 is regenerated, membrane 6 is held equal and the third membrane is repeatedly replaced by a new one.
Experiment 1 of the flux and concentration experiments, there is a gap in membrane 6 due to some complications with the scale used. The scale stopped recording at 74g and restarted at 510g, that is why there is a difference in the time of this membrane compared to the others. The TF was calculated based on the permeate mass and time after the scale restarted, the first values were ignored.
Figure 34 - Experiment 1 of Flux and Concentration Experiment, using membrane 1, 6 and 7 (1.2 L/min at 4 bar).
Experiment 1
Experiment 2
In experiment 2 the program crushed, and the setup was immediately stopped. The peaks are a result of stopping the pumps and turning them on again. In Figure 36 is represented experiment 3, where the same problem occurred.
Table 10 - Transmembrane flux for all membranes for each experiment.
The three filtration sets lasted approximately 20 hours where 2.1L of volume filtrated in each filtration step. From the results, it was verified that the transmembrane flux decreases with the increase of the suspension’s concentration, since there is no decrease in the transmembrane flux while the lignin concentration stays constant. In fact, the fouling of the membrane did not result in flux decline since none of the membranes reveals a large change in the transmembrane flux from experiment to experiment. In fact, membrane 6 has a gradual but not very significant increase while membrane 1 has Experiment 1 Mean TF ( 𝑳
𝒎𝟐.𝒉) Experiment 2 Mean TF ( 𝑳
𝒎𝟐.𝒉) Experiment 3 Mean TF ( 𝑳
𝒎𝟐.𝒉)
Membrane 1 9.8 Membrane 1 8.8 Membrane 1 11.9
Membrane 6 14.1 Membrane 6 15.2 Membrane 6 18.0
Membrane 7 20.4 Membrane 8 22.7 Membrane 9 29.1
Figure 36 - Experiment 3 of Flux and Concentration Experiment, using membrane 1, 6 and 9 (1.2 L/min at 4 bar).
Experiment 3
an oscillating flow value and the third membrane module (membranes 7, 8 and 9) have different values.
However, the third module is not relevant because it is known that two membranes under the same conditions do not behave in the same way.
After these experiments to confirm the reason for the decrease of the flux, ultrafiltration and diafiltration were again applied, to concentrate the suspension of nanoparticles and remove the impurities and ethanol. For this, three membranes were used in series. A first membrane, membrane 1, that was the same as the one used in the previous experiments (regenerated membrane). Membrane 6, which was initially a new membrane but was then used throughout the procedure without any change and a third membrane that is replaced by a new one whenever 50% of the suspension is filtered.
Membrane stability
As was done for the other procedure, the membranes used were subjected to an initial flush with a solution of 15wt% ethanol. For better understanding, the membranes curves were divided in two different diagrams, Figure 37 and Figure 38. These figures show the curves obtained for each membrane and Table 11 shows the mean initial transmembrane flux for each membrane.
Figure 37 - Initial Transmembrane Flux (g/min) over time (min) for membranes 1, 6 and 10 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
The behavior of membranes 6, 10, 11 and 12 is physically unexpected, since the operating conditions of these membranes were identical when compared to the others. They were measurement errors, maybe due to not having constant pressure or to accumulation of permeate in the permeate hose connected to the scales. For these membranes, the initial TF was calculated based on the average over the whole time. However, the TF of membrane 1 and 13 was calculated based on the last few minutes of flushing. Table 10 shows the linear regression of each membrane and the respective initial TF within an uncertainty range of 10%.
Table 11 - Initial TF for each membrane for the experiment 3 Membranes in Series.
Membrane Initial TF (L/(m2.h))
Membrane 1 14.9
Membrane 6 225.8
Membrane 10 335.3
Membrane 11 358.6
Membrane 12 418.6
Membrane 13 134.7
The new membranes used, 10, 11, 12 and 13 were from a different pack of membranes, which may explain this significant difference of the initial flux to the membranes used before. However, this deviation is critical for future work since membranes may exhibit such dissimilar capacities. The experiment should be repeated several times with different membrane samples in order to be possible
Figure 38 - Initial Transmembrane Flux (g/min) over time (min) for membranes 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
to calculate mean values to reach statistically relevant conclusions, since the membrane performance parameters are highly fluctuant.
Membrane 1 presents a higher value of initial TF than the mean value obtained in experiment 3 of the flux and concentration experiment. This is due to the fact that that membrane has been regenerated between experiments.
Ultrafiltration/Diafiltration of Nanolignin Particles Suspension
The initial suspension for this experiment was the same as used in the flux and concentration experiment. The initial volume was 5L as in the first experiment with 2 membranes in series, and the goal was to increase the concentration in ultrafiltration mode and remove ethanol and impurities in diafiltration mode. The procedure was the same, filter up to 10% volume of suspension with membrane change and membrane regeneration every time the volume is reduced by 50%.
The difference now is the number of membranes used, it was used 3 membranes in series, a first membrane (membrane 1) with regeneration. A second membrane (membrane 6) that is never changed and a third one that it is changed every time membrane 1 is regenerated. The graphs below, Figure 39 to Figure 40 show the performance of the membranes for each step, ultrafiltration (UF) and diafiltration (DF). Figure 39 represents the fitration of membrane 1, and the vertical line (MR) represents when the membrane was regenerated. This membrane regeneration occurs at the same time as the membrane is changed in the third module, which is represented in Figure 40. Figure 41 represents membrane 6 which is never changed or regenerated.
Figure 39 - Transmembrane Flux (g/min) over time (min) for membrane 1 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
Figure 40 - Transmembrane Flux (g/min) over time (min) for the other membranes, 10, 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
Figure 41 - Transmembrane Flux (g/min) over time (min) for membrane 6 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
The pressure was always being controlled, however there was a certain fluctuation specially after the system was restarted, which results in peaks from turning on the pump after temporary shutdown for sampling. After some time, the membrane returns to its initial behaviour, before the withdrawn sample due to pressure peaks during pump startup.
From the graphs it is possible to perceive that when there is membrane change there is also the regeneration of membrane 1. It is possible to see the effects of the regeneration, where the first regeneration in this filtration happens at minute 400, having as a consequence the increase of the transmembrane flux almost till the initial flux. Table 12 shows the means TF for each filtration step.
Table 12 - Mean TF for each filtration step for the experiment 3 Membranes in Series.
UF/DF Step Mean Transmembrane
Ultrafiltration Other Membranes (Membranes 10 and 11) 27.3 Diafiltration Other Membranes (Membranes 12 and 13) 70.5
For this filtration, the flow varies the same way as in the first filtration with 2 membranes in series.
After the water addition, the transmembrane fluxes increase for all membranes which can be explained by the dissolved components being reduced in DF.
The graph represents the way the filtration worked, a first step, where 3 membranes were used in series. Membranes 1, 6 and 10 were used up to minute 400. Between minute 400 and approximately minute 700, membrane 1 (after regeneration), 6 and 11 were used. Membrane 1 (after regeneration), 6 and 12 were handled from minute 700 to 900, and until the end membrane 1, 6 and 13 were used.
Table 13 - Mean TF of each membrane used in the filtration for the experiment 3 Membranes in Series.
UF/DF Step Mean Transmembrane Flux ( 𝑳
𝒎𝟐.𝒉)
In Table 13 is shown the transmembrane flux of all membranes used in this experiment.
Membranes 10, 11, 12 and 13 have the highest fluxes because they are new.
When comparing all the membranes used, membrane 6 is the one without regeneration and has a higher mean TF than membrane 1, as can be seen in Table 13. So, this filtration process is not enough to create a thick layer as dense as the one on the surface of membrane 1. Although membrane 6 is not regenerated, it had better fluxes right from the start. From Figure 40 and Table 13 membrane 1 had a TF of 90 (L/m2.h) while membrane 6 had a TF of 345 (L/m2.h).
During the filtration in UF mode (membranes 1, 6, 10 and 11) it is possible to see that while the filtration occurs the TF reduce, since the concentration of nanolignin particles is increasing and because of membrane fouling. When the filtration changes from UF mode to DF (membranes 1, 6, 12 and 13), the flux increases considerably due to the lower concentration of dissolved components.
Membrane Fouling
At the end of the filtration, the membranes were again subjected to a flush with a 15wt% ethanol solution. The TF of the membranes was divided in two graphs for a better viewing.
Figure 42 - Final Transmembrane Flux (g/min) over time (min) for membrane 1, 6 and 10 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
In Figure 42 is represented the final transmembrane flux of membrane 1, 6 and 10. As was previously said, membrane 1 is the one with regeneration, membrane 6 is never changed and a new membrane, membrane 10, used during UF. Figure 43 represent membrane 11, 12 and 13, while these last two were used in diafiltration, membrane 11 was used during UF.
Table 14 - Initial and Final transmembrane flux for each membrane for the experiment 3 Membranes in Series.
Membranes Initial TF ( 𝑳
𝒎𝟐.𝒉) Final TF ( 𝑳
𝒎𝟐.𝒉) TF decline (%)
Membrane 1 14.9 12.9 10.8
Membrane 6 225.8 16.7 95.1
Membrane 10 335.3 34.0 89.4
Membrane 11 358.6 70.6 84.6
Membrane 12 418.6 61.4 88.1
Membrane 13 134.7 46.6 82.5
Figure 43 - Final Transmembrane Flux (g/min) over time (min) for membrane 11, 12 and 13 for the experiment 3 Membranes in Series (1.2 L/min at 4 bar).
In Table 14 is shown the different transmembrane fluxes for each membrane and the respective flux reduction. The last 20 minutes of the final flushing with ethanol/water were used for the final TF calculation due to particles or precipitated lignin that might be washed away.
Except for membrane 11, the final flux is significantly similar when compared to the mean TF.
As what happened in the experiment using two membranes, the diafiltration mode has higher final transmembrane flux than membranes used in concentration mode. Membrane 1 no longer shows a significant decrease of the flow, what can mean that a membrane can be regenerated several times and after a certain point it will be stable, but this is only possible to prove if more experiments are done with the same membrane.
Analytics
Table 15 describes each sample and where each membrane is used. The code of each sample is similar, with the difference that for each round of samples there is one more permeate because there is one more membrane module.
Table 15 - Sample labeling for the experiment 3 Membranes in Series.
Code Name Sample Step of filtration
S Initial Suspension Suspension after precipitation
R1 1st retentate 1st sample after 816.8g filtred
P1M1 1st permeate 1st sample after 165.1g of permeate membrane 1 P1M6 1st permeate 1st sample after 251.4g of permeate membrane 6 P1M10 1st permeate 1st sample after 400.3g of permeate membrane 10
R2 2nd retentate 2nd sample after 816.9g filtred
P2M1 2nd permeate 2nd sample after 155.3g of permeate membrane 1 P2M6 2nd permeate 2nd sample after 261.5g of permeate membrane 6 P2M10 2nd permeate 2nd sample after 400.1g of permeate membrane 10
R3 3rd retentate 3rd sample after 784.6g filtred
P3M1 3rd permeate 3rd sample after 162.3g of permeate membrane 1 P3M6 3rd permeate 3rd sample after 261.5g of permeate membrane 6 P3M10 3rd permeate 3rd sample after 360.8g of permeate membrane 10
R4 4th retentate 4th sample after 769.2g filtred
Addition of 4797g of Water (Initial suspension mass reduced by the amount of samples)
R6W 6th retentate 6th sample after 641.3g filtred
P6M1W 6th permeate 6th sample after 107.7g of permeate membrane 1 P6M6W 6th permeate 6th sample after 133.2g of permeate membrane 6 P6M12W 6th permeate 6th sample after 400.4g of permeate membrane 12
R7W 7th retentate 7th sample after 808.3g filtred
P7M1W 7th permeate 7th sample after 178.7g of permeate membrane 1
Table 16 - (continuation) Sample labeling for the experiment 3 Membranes in Series.
P7M6W 7th permeate 7th sample after 222.9g of permeate membrane 6 P7M12W 7th permeate 7th sample after 406.7g of permeate membrane 12
R8W 8th retentate 8th sample after 852.6g filtred
R8W 8th retentate 8th sample after 852.6g filtred