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Simultaneous separation of impurities, concentration and
solvent exchange of nanolignin particle suspensions using
ultrafiltration
Sofia Faria Capelo
Thesis to obtain the Master of Science Degree in
Chemical Engineering
Supervisors:
Professor Anton Friedl (TU Wien)
Professor Maria Norberta Correia de Pinho (IST)
Examination Committee
Chairperson: Professor João Carlos Bordado
Supervisor: Professor Anton Friedl
Member of the Committee: Luís Miguel Minhalma
June 2019
i
Acknowledgements This master thesis has been performed at the Institute of Chemical, Environmental and
Bioscience Engineering, TU Wien, in Vienna.
I would like to express my gratitude to Professor Maria Norberta de Pinho, who gave me the
opportunity to carry out this work abroad. To professors Anton Friedl and Michael Harasek, as
supervisors, who received me and provided a good working space and a good environment. Also, to Dr
Martin Miltner and Stefan Beisl that supported and conducted me throughout all the work and for all the
dedication and precious advises provided.
I would also like to give a special thanks to Ruben Santos, Péter Adorján, Anja Dakic, Katarina
Knežević, Stefan Beisl and Rita Alves. People who made my time in Vienna wonderful, whom I have to
thank for making me feel at home, and which somehow helped me in the elaboration of this work.
To the ones who stayed in Portugal, specially to my family and friends that enabled me this experience.
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Abstract
Lignocellulosic biomass emerged as an alternative to non-renewable resources, since they are
scarce and highly polluting. The biomass is subdivided into several fractions such as lignin,
hemicellulose and cellulose, which defines the concept of biorefinery by the production of several
products.
The goal of the work was to evaluate the performance, the decline in performance and the
potential for regeneration of membrane performance during the ultrafiltration operation of nanolignin
suspension in diafiltration mode. The suspension used was produced from wheat straw using the
Organosolv pre-treatment, where the membrane used for its filtration had a MCWO of 30 kDa. This
membrane was used to concentrate particulate nanolignin in the retentate and to exchange solvent and
remove impurities when operated in diafiltration mode with distilled water. The regeneration of the
membrane was performed at certain points of the filtration by washing with organic solvent, since there
is fouling during ultrafiltration that will reduce the transmembrane flux.
The membranes used showed a removal efficiency for dissolved components of 93.6% and
85.2%, for the experiment using three and two membranes in series, respectively. The ethanol and
impurities were also reduced as intended.
The study of the flux in function of the concentration showed that the flux is affected by the
increase of the concentration of nanolignin particles, therefore the fouling is also affected.
The membrane regeneration revealed to be a good option to improve the performance of the
membrane, the regeneration after the concentration mode revealed to be more effective than
regeneration after the diafiltration step.
Keywords: Biorefinery, Organosolv, lignin, ultrafiltration, diafiltration, membranes.
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Resumo A biomassa lignocelulósica surge como uma alternativa aos recursos não renováveis, uma vez
que estes são escassos e altamente poluentes. A biomassa é subdividida em várias frações, como
lenhina, hemicelulose e celulose, que define o conceito de biorrefinaria pela produção de vários
produtos.
O objetivo do trabalho foi avaliar o desempenho das membranas, o seu decrescimento e o seu
potencial de regeneração durante a operação de ultrafiltração da suspensão de nanolenhina em modo
de diafiltração. A suspensão utilizada foi produzida a partir de palha de trigo utilizando como pré-
tratamento o Organosolv, no qual a membrana utilizada para a sua filtração apresentava um peso
molecular de corte de 30 kDa. Esta membrana foi utilizada para concentrar nanolenhina em partículas
no concentrado e para alterar o solvente e a remoção de impurezas quando operado em modo de
diafiltração. A regeneração da membrana foi realizada em determinados pontos da filtração por
lavagem com solvente orgânico, uma vez que há acumulação de partículas durante a ultrafiltração, o
que consequentemente reduzirá o fluxo transmembranar.
As membranas utilizadas mostraram uma eficiência de remoção para componentes dissolvidos
de 93,6% e 85,2%, para o procedimento utilizando três e duas membranas em série, respetivamente.
O etanol e as impurezas também foram reduzidos tal como pretendido.
O estudo do fluxo em função da concentração mostrou que o fluxo é afetado pelo aumento da
concentração de partículas de nanolenhina, portanto o fouling também é afetado.
A regeneração da membrana revelou-se uma boa opção para melhorar o desempenho da
membrana, a regeneração após a ultrafiltração revelou-se mais eficaz quando comparado com a
regeneração após a diafiltração.
Palavras-chave: Biorrefinaria, Organosolv, lenhina, ultrafiltração, diafiltração, membranas.
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Table of contents Acknowledgements .................................................................................................................................. i
Abstract .................................................................................................................................................... iii
Resumo ................................................................................................................................................... v
List of Acronyms and Nomenclature ...................................................................................................... ix
Introduction .............................................................................................................................................. 1
Literature Review ..................................................................................................................................... 3
Biorefinery ............................................................................................................................................ 3
Biomass ............................................................................................................................................... 3
Cellulose .......................................................................................................................................... 4
Hemicellulose .................................................................................................................................. 5
Lignin ............................................................................................................................................... 5
Nanolignin Particles Production........................................................................................................... 6
Pretreatment of Lignocellulosic material ............................................................................................. 7
Organosolv Pretreatment ................................................................................................................ 9
Membranes ........................................................................................................................................ 10
Membranes Technology ................................................................................................................ 10
Membranes Classification ............................................................................................................. 10
Membranes Material ...................................................................................................................... 11
Membranes Processes .................................................................................................................. 11
Ultrafiltration ................................................................................................................................... 13
Membranes Characterization ........................................................................................................ 14
Diafiltration ..................................................................................................................................... 17
Aim of the thesis ................................................................................................................................ 17
Material and Methods ............................................................................................................................ 19
Experimental Procedure for Nanolignin particles production ............................................................ 19
Extract Production ......................................................................................................................... 19
Precipitation ................................................................................................................................... 20
Membrane Filtration ........................................................................................................................... 20
Ultrafiltration Process Setup .......................................................................................................... 20
Membrane Instructions .................................................................................................................. 22
viii
Membrane Selection ...................................................................................................................... 23
Membrane Stability ........................................................................................................................ 23
Ultrafiltration and Diafiltration of Nanolignin Particles Suspension ............................................... 23
Membrane Regeneration ............................................................................................................... 24
Membrane Fouling ......................................................................................................................... 25
Analytics ........................................................................................................................................ 26
Results and Discussion ......................................................................................................................... 31
Membrane Filtration – 2 Membranes in Series ................................................................................. 31
Membrane stability......................................................................................................................... 31
Transmembrane Flux .................................................................................................................... 32
Ultrafiltration/Diafiltration of Nanolignin Particles Suspension ...................................................... 32
Membrane Fouling ......................................................................................................................... 35
Analytics ........................................................................................................................................ 36
Membrane Filtration – 3 Membranes in Series ................................................................................. 43
Membrane Filtration – Flux and Concentration Experiment .......................................................... 43
Membrane stability......................................................................................................................... 46
Ultrafiltration/Diafiltration of Nanolignin Particles Suspension ...................................................... 48
Membrane Fouling ......................................................................................................................... 51
Analytics ........................................................................................................................................ 53
Membrane Regeneration ................................................................................................................... 58
Conclusions ........................................................................................................................................... 62
References ............................................................................................................................................ 64
Appendix ................................................................................................................................................ 68
ix
List of Acronyms and Nomenclature Acronyms
KL − Kraft Lignin
MF − Microfiltration
MWCO − Molecular weight cut − off
NF − Nanofiltration
OL − Organosolv Lignin
OP − Organosolv Process
PES − Polyethersulphone
PESH − Hydrophilic Polyethersulphone
PSU − Polysulphone
PVDF − Poly(vinylidene fluoride)
RC − Regenerated Cellulose
RO − Reverse osmosis
TF − Transmembrane Flux
UF − Ultrafiltration
DF − Diafiltração
Nomenculature
Am − Membrane Active Area
CAa − Solute Concentration in the feed
CAp − Solute Concentration in the permeate
DM − Dry Matter
DMbefore centrifuge − DM content before centrifuge
DMafter centrifuge − DM content of supernatant
DMPermeates − DM amount in each permeate
DMInitial Suspension − DM amount in the supernatan of initial suspension
DMsamples − DM amount in retentate sample
DMRxW − DM of the final concentrate remaining inside the tank
Lp − Hydraulic Permeability
minitial − mass of sample before oven
mdry sample − mass of sample after the oven
fA − Retention Coefficient
PXMX − Permeate X for Membrane X
RE − Removal Efficiency
TL − Total nanolignin particles in the sample
vp − Permeation Flux
∆π − Average Osmotic Pressure
x
Qa − Feed Flow
Qp − Permeate Flow
Qr − Retentate Flow
xi
List of figures Figure 1 - Structure of lignocellulosic biomass with cellulose, hemicellulose, and basic elements of lignin
represented (Alonso, Wettstein, & Dumesic, 2012). ............................................................................... 4
Figure 2 - Structure of a cellulose molecule (Matsutani, Harada, Ozaki, & Takaoka, 1993). ................. 4
Figure 3 - Hemicellulose backbone of arborescent plants (Matsutani et al., 1993). ............................... 5
Figure 4 - Lignin/Phenolics-carbohydrate complex in wheat straw(Buranov & Mazza, 2008). ............... 6
Figure 5 - Potential and investigated applications of lignin from micro- to nanosize (S Beisl, et al., 2017).
................................................................................................................................................................. 7
Figure 6 - Schematic of goals of pretreatment on lignocellulosic material. ............................................. 8
Figure 7 - Ligno-cellulosic feedstock biorefinery (Gavrilescu, 2014)....................................................... 8
Figure 8 - Schematic diagram of the organosolv fractionation process (Nitsos, et al., 2018). ................ 9
Figure 9 - Symmetrical membranes: (a) Isotropic microporous, (b) Nonporous dense, (c) Electrically
charged. (Rautenbach, R. & Albert, 1989). ........................................................................................... 10
Figure 10 - Anisotropic membranes: (a) Loeb-Sourirajan, (b) Thin-film composite, (c) Supported liquid.
(Rautenbach, R. & Albert, 1989). .......................................................................................................... 11
Figure 11 - Pressure-driven membrane processes. (Gaspar, 2018.).................................................... 12
Figure 12 - Classification of membrane processes based on pore size. (Ultrafiltration, 2018) ............. 13
Figure 13 - Schematic representation of ultrafiltration. 𝐶𝐴𝑎 and 𝐶𝐴𝑝 represent the solute concentration
in the feed and permeate. 𝑄𝑎, 𝑄𝑝 and 𝑄𝑟 represent the flow of the feed, permeate and retentate...... 14
Figure 14 - Gel layer of colloidal material on the surface of an ultrafiltration membrane (Rautenbach et
al., 1989). ............................................................................................................................................... 15
Figure 15 - Permeate flux as a function of time with membrane cleaning (Rautenbach et al., 1989). . 16
Figure 16 - (a) Example of extract, (b) Autoclave in operation, (c) Autoclave ...................................... 20
Figure 17 - Precipitation Setup: (a) T-fitting and static mixer, (b) Syringe Pumps. ............................... 20
Figure 18 - MEMCELL plant for membrane flush, from OSMO Membrane Systems. .......................... 21
Figure 19 - MEMCELL plant for membrane filtration, from OSMO Membrane Systems. ..................... 22
Figure 20 - Calibration curve for lignin content in UV. ........................................................................... 25
Figure 21 - Analysis methodology for different samples. ...................................................................... 26
Figure 22 - Calibration curve for Acetic Acid content. ........................................................................... 28
Figure 23 - Calibration curve for HMF and Furfural content. ................................................................. 28
Figure 24 - Calibration curve for Ethanol content. ................................................................................. 28
Figure 25 – Initial transmembrane flux (g/min) over time (min) of each membrane (1.2 L/min at 4 bar).
............................................................................................................................................................... 31
Figure 26 - Ultrafiltration and diafiltration process for membrane 1 (1.2 L/min at 4 bar). ...................... 33
Figure 27 - Ultrafiltration and diafiltration process for other membranes, 2, 3, 4 and 5 (1.2 L/min at 4
bar). ....................................................................................................................................................... 33
Figure 28 - Transmembrane Flux (g/min) over time (min) for the final flush of each membrane (1.2 L/min
at 4 bar). ................................................................................................................................................ 35
Figure 29 - Ethanol content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.
............................................................................................................................................................... 38
xii
Figure 30 - Acetic Acid, HMF and furfural content (mg/L) of the retentate samples for the experiment 2
Membranes in Series. ............................................................................................................................ 38
Figure 31 - Particle size for all retentate samples for the experiment 2 Membranes in Series. ............ 40
Figure 32 - DryMatter content (%) for all retentate samples, before and after centrifuge for the experiment
2 Membranes in Series. ......................................................................................................................... 41
Figure 33 – Dry Matter content (%) for all permeate samples for each membrane for the experiment 2
Membranes in Series. ............................................................................................................................ 42
Figure 34 - Experiment 1 of Flux and Concentration Experiment, using membrane 1, 6 and 7 (1.2 L/min
at 4 bar). ................................................................................................................................................ 44
Figure 35 - Experiment 2 of Flux and Concentration Experiment, using membrane 1, 6 and 8 (1.2 L/min
at 4 bar). ................................................................................................................................................ 44
Figure 36 - Experiment 3 of Flux and Concentration Experiment, using membrane 1, 6 and 9 (1.2 L/min
at 4 bar). ................................................................................................................................................ 45
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). ...................................................................... 46
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). ...................................................................... 47
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). ............................................................................................ 48
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). ........................................................... 49
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). ............................................................................................ 49
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). ...................................................................... 51
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). ...................................................................... 52
Figure 44 - Ethanol content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.
............................................................................................................................................................... 54
Figure 45 - Acetic Acid content (mg/L) for all retentate samples for the experiment 3 Membranes in
Series. .................................................................................................................................................... 55
Figure 46 - Particle Size measurements for all samples for the experiment 3 Membranes in Series. . 56
Figure 47 - Dry Matter content (%) for all retentate samples, before and after centrifuge for the
experiment 3 Membranes in Series. ...................................................................................................... 56
Figure 48 - DryMatter content (%) for all permeate samples for the experiment 3 Membranes in Series.
............................................................................................................................................................... 58
Figure 49 - Mean transmembrane flux for each regeneration step with 15 %wt ethanol solution. ....... 59
Figure 50 - Initial TF and TF before and after regeneration of membrane 1. ........................................ 60
Figure 51 - Product temperature (°C) and pressure (bar) for one extract production experiment. ....... 68
xiii
Figure 52 - Membranes used in the experiment 2 Membranes in Series: (a) Membrane 1; (b) Membrane
2; (c) Membrane 3; (d) Membrane 4; (e) Membrane 5. ......................................................................... 69
Figure 53 – (a) Membrane 1 (regenerated) at the end, after being used in all the experiments; (b)
Membrane 6 (without regeneration) at the end, after being used in experiments Flux and Concentration
and 3 Membranes in Series ................................................................................................................... 69
Figure 54 - Membranes used in the experiment Flux and Concentration: (a) Membrane 3; (b) Membrane
4; (c) Membrane 5. ................................................................................................................................ 70
Figure 55 - Membranes used in the experiment 3 Membranes in Series: (a) Membrane 6; (b) Membrane
7; (c) Membrane 8; (d) Membrane 9. .................................................................................................... 71
xiv
List of tables Table 1 -Technical data of the MEMCELL plants. ................................................................................. 21
Table 2 - Properties and application areas of NADIR® PESH membrane. .......................................... 23
Table 3 - Initial transmembrane flux for each membrane used in the experiment 2 Membranes in Series.
............................................................................................................................................................... 32
Table 4 - Mean transmembrane fluxes for UF/DF for membrane 1 and other membranes, 2,3, 4 and 5.
............................................................................................................................................................... 34
Table 5 - Mean transmembrane fluxes for each membrane. ................................................................ 34
Table 6 - Initial and final transmembrane flux each membrane and the respective flux decline for the
experiment 2 Membranes in Series. ...................................................................................................... 36
Table 7 - Samples labeling code for the experiment 2 Membranes in Series. ...................................... 37
Table 8 - Degradation products characterization of straw at 180ºC. ..................................................... 39
Table 9 - DM amount (g) of different samples used for the mass balance of the filtration system for the
experiment 2 Membranes in Series. ...................................................................................................... 43
Table 10 - Transmembrane flux for all membranes for each experiment. ............................................ 45
Table 11 - Initial TF for each membrane for the experiment 3 Membranes in Series. .......................... 47
Table 12 - Mean TF for each filtration step for the experiment 3 Membranes in Series. ...................... 50
Table 13 - Mean TF of each membrane used in the filtration for the experiment 3 Membranes in Series.
............................................................................................................................................................... 50
Table 14 - Initial and Final transmembrane flux for each membrane for the experiment 3 Membranes in
Series. .................................................................................................................................................... 52
Table 15 - Sample labeling for the experiment 3 Membranes in Series. .............................................. 53
Table 16 - DM amount (g) of different samples used for the mass balance of the filtration system for the
experiment 3 Membranes in Series. ...................................................................................................... 57
Table 17 - Regeneration steps for membrane 1.................................................................................... 58
Table 18 - Lignin removed from membrane 1 in each step. .................................................................. 59
Table 19 - Membrane recovery (%) for each regeneration step. .......................................................... 60
Table 20 - Characteristics of membrane 1. ........................................................................................... 61
Table 21 - Characteristics of membranes used in experiment 2 Membranes in Series........................ 61
Table 22 - Characteristics of membranes used in experiment 3 Membranes in Series........................ 61
xvi
1
Introduction
Lignocellulosic biomass is the most abundant renewable resource in the world and has been
considered with the potential to produce chemicals and biomaterials. The main contents of biomass are
cellulose, hemicellulose, and lignin that is the second most abundant biopolymer in the world (Weinwurm
et al., 2016).
Nowadays there are several industrial sectors that produce waste that can be used as a source
for other processes and the waste produced from the pulp and paper industry has a lot of lignin in it.
Almost half of the lignin retrieved from processes is incinerated and then used for producing energy, the
other half can be transformed (S Beisl et al., 2017). Therefore, the valorisation through transformation
of lignin into high-prize products has gained more interest over the past years to improve the economic
performance of lignocellulosic biorefinery concepts. So, in the past few years, the main goal was to
retrieve as much lignin as possible and the research regarding processes of extraction have gained
interest improving through the years.
Lignin is a highly irregularly branched polyphenolic polyether, consisting of the primary
monolignols, p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol, which are connected via
aromatic and aliphatic ether bonds as well as non-aromatic C-C bonds (Stefan Beisl, Miltner, et al.,
2017). The lignin can be separated into three major types: softwood lignins, hardwood lignins, and grass
lignins. Softwood lignins are mainly composed of coniferyl alcohol, hardwood lignins contain coniferyl
and sinapyl alcohol and the grass lignins contain all three types of lignols (S Beisl et al., 2017).
Lignin is present in the biomass and to retrieve the lignin it’s necessary to fractionize the biomass
with the help of different treatments, like cooking it at high temperatures with elevated temperatures
(160-240ºC) under pressure or cooked with diluted acid or in alkaline conditions. The organosolv
treatment (60 wt% ethanol solution and a mass ratio of straw to solvent of 1:11) is widely used and was
developed as an alternative to conventional pulping processes and is quite promising in regards of
achieving high delignification of the biomass, with relatively high purity lignin (Weinwurm et al., 2016).
After the organosolv treatment, a step of precipitation is performed that results in a diluted
nanolignin suspension with some impurities. Therefore, it’s necessary to purify this mixture with the goal
of increasing the concentration of nanolignin particles and exchange the alcoholic solvent used in the
organosolv process with water. Through membrane separation, ultrafiltration, it is possible to achieve
separation with several benefits like, for example, the possibility of withdrawing the solvent at any
position and the possibility to obtain lignin with defined properties (Stefan Beisl, Loidolt, Miltner, Harasek,
& Friedl, 2018).
In the present work, the focus is the ultrafiltration step in diafiltration mode giving high
importance to the membranes used in the process, in order to concentrate the nanoparticles suspension
and cleaning the suspension from impurities (sugars, degradation products and dissolved lignin) without
changing the properties of the lignin particles. The membranes used should be mechanically strong
enough to withstand the pressure applied without rupture or distortion. It’s important that they don’t react
with the mixture passed through them and that they are isoporous preventing that some free occasional
pores be larger than the average. The size of the pores in the membranes should be chosen in a way
2
that they can be large enough to let the components desired in the filtrate pass and small enough to
retain the components desired in the residue (Ferry, 1936).
Each membrane used in the following work has been subjected to different conditions to find
out which conditions are better to improve the filtration step obtaining a product with a high concentration
of nanolignin particles.
3
Literature Review
Biorefinery
The demand for crude oil has been increasing in the past years and the population is starting to
be conscious of the finite nature of the world’s oil supplies. This consciousness has led to an increase
in the price and the fear of the potential use of crude oil as a political weapon (Prasad, Singh, & Joshi,
2007). The raw material used is neither environmentally friendly nor sustainable (Gavrilescu, 2014).
Consequently, there is a need to develop and implement new technologies based on alternative energy
platforms, wind, water, sun, nuclear fission and fusion, and biomass (FitzPatrick et al. , 2010).
“New technologies are being developed that use biomass to make not only low-value products
such as fuels but also high-value materials such as polymers. It is also important to look back at what
happened in the past when crude oil prices surged.” (Aresta et al., 2012)
There are three biorefinery systems being researched and developed (Gavrilescu, 2014) (Kamm
& Kamm, 2004): “The whole crop Biorefinery”, that uses cereals as raw material or maize with the aim
of producing straw and corn. “The Green Biorefinery” that uses biomasses such green grass. The third
and last system is “The Lignocellulose Feedstock (LCF) Biorefinery” which uses straw, reed, grass,
wood, and others as raw material.
Although the biorefinery is a key technology for a more sustainable world, the high needs of
capital for this type of industry leads to a lack of adherence. Previous studies show that biorefineries
should focus on high-value chemicals/materials and use residual waste for energy integration, producing
energy and fuels (Aresta et al., 2012).
Biomass
The carbon neutrality and renewable characteristics of the biomass made it a respectable
alternative as a fundamental resource in the sustainable society, for energy and material resources.
Biomass is a renewable material derived from living or recently living organisms. Biomass is a plant
material derived from the photosynthetic process, where the carbon dioxide that comes from the
atmosphere and water from the plant roots are combined. This reaction produces carbohydrates that
are converted in biomass. This process is a cycle, the existing carbon in biomass combined with the
oxygen from the atmosphere (during biomass combustion) leads to the production of water and carbon
dioxide, being then available again for biomass production (Parmar, 2017). This reverse reaction is due
to the sunlight that is stored in the chemical bonds of the structural components of biomass (McKendry,
2002).
The lignocellulosic biomass’s structure is composed of three main polymeric components, which
are cellulose, lignin, and hemicellulose. Biomass can be transformed into fibers and chemicals when
plants or animal matter are used as a raw material. Besides this, biomass can also be used to produce
energy by using residues and waste matter (Jaya Shankar Tumuluru, et al. 2013). They store chemical
energy, however, the quantity of energy is dependent on the type of plant, and also their proportions
determine the optimum energy conversion (McKendry, 2002).
4
The three components that constitute biomass have different chemical composition and
structure which results in different chemical reactivities. Besides this, carbon, hydrogen, and oxygen in
biomass molecules exhibit complications in the catalytic conversion of biomass to fuels and chemicals
due to their inert chemical structure and compositional ratio (Kohli, Prajapati, & Sharma, 2019). Figure
1 represents the structure of a lignocellulosic biomass, where basic elements of lignin, cellulose and
hemicellulose are represented.
Cellulose
Main component of biomass, it represents half of the organic carbon in the biosphere (Kohli et
al., 2019) and is the main constituent of the plant cell wall, which confer structural support. Cellulose
has linear chains of (1,4)-D-glucopyranose units, where they are connected to 1-4 in β-configuration
(McKendry, 2002). These chains are grouped to form microfibrils thereby forming cellulose fibers. The
microfibrils are connected by covalent bonds, by hydrogen bonding and Van der Waals forces, which
determine the straightness of the chain being able to present crystalline or amorphous structure in the
final form (Agbor, et al., 2011).
Cellulose typically comprises 40-50% of lignocellulosic biomass feedstock and has gained
interest as a source of fuels and chemicals via catalytic processing. Its hydrolyzation degree influences
how soluble it is and the cellulose does not melt at high temperatures, however, it starts to decompose
at 180ºC (Harmsen et al., 2010). The structure of a molecule of cellulose is represented in Figure 2.
Figure 1 - Structure of lignocellulosic biomass with cellulose, hemicellulose, and basic elements of lignin represented (Alonso, Wettstein, & Dumesic, 2012).
Figure 2 - Structure of a cellulose molecule (Matsutani, Harada, Ozaki, & Takaoka, 1993).
5
Hemicellulose
Hemicellulose is also one of the most abundant polymers and represents 20-50% of
lignocellulosic biomass feedstock. It is a low molecular weight heterogeneous polymer composed by
pentoses, hexoses and acetylated sugars. The composition of hemicellulose depends on the type of
biomass. Hemicelluloses from straw and grasses are mainly composed of xylan, whereas hemicellulose
from softwood is mainly composed by glucomannan. Hemicellulose at low temperatures is not soluble,
has a random and amorphous structure when compared with cellulose, this structure makes it less
resistant against hydrolysis. Compared to cellulose, the hydrolysis is conducted at lower temperatures
which makes it soluble at higher temperatures (Matsutani et al., 1993). In Figure 3 is represented the
chemical structure of hemicellulose.
Lignin
Lignin is also amongst the most abundant biopolymers, being the most complex natural
polymer. This polymer waterproofs the cell wall, which improves the transport of solutes and water
through the vascular system. It is decisive in the integrity of the cell wall structure and stiffness, and
strength of the steam. Lignin is an amorphous three-dimensional polymer with phenylpropane units as
the predominant building blocks (Matsutani et al., 1993). It is composed of three main kinds of
monolignols (p-coumaryl, coniferyl and sinapyl alcohols). These monomers are synthesized in the
cytoplasm during lignin deposition, and posteriorly polymerized into lignin (Li & Chapple, 2010).
There are three major groups of lignin, most of the lignin from softwoods is constituted of
coniferyl alcohol and the remaining of p-coumaryl alcohol units. The lignin of hardwoods is composed
by coniferyl and sinapyl alcohol in different ratio variations. The other main group is lignin from grasses.
(Matsutani et al., 1993).
Figure 3 - Hemicellulose backbone of arborescent plants (Matsutani et al., 1993).
6
A new biorefinery focus is to use agricultural residues as raw material, due to the amount of
waste that is produced annually, as reported in (Agricultural, et al., 2016). After harvested, the biomass
must be subjected to pretreatments to convert it into chemical compounds or fuels. Afterwards, post-
treatments can be required for purification or stabilization of the final product (Hu, et al., 2018).
The characteristics of the product depend on the type of straw collected and on its habitat.
Although this change oscillates on a larger scale, the elemental composition on moisture and ash free
basis does not differ much (Tröger, et al., 2013).
Nanolignin Particles Production Due to the structural complexity of the products obtained from lignin it is necessary to obtain
lignin with superior properties, therefore producing nanolignin particles. From the point of view of the
particle size distribution, there are three parameters (pH, temperature and ratio of lignin/solvent) that
can be varied in order to obtain the best condition (Gilca et al., 2015). In comparation with molecules
with larger dimensions, the structure of the nanoparticles (especially in the 1-100 nm range) present
distinct properties due to the increase of the surface area (S Beisl et al., 2018).
By an OP, the wheat straw structure fractures at high temperatures and low pressure. By
changing the operational conditions (temperature and pressure) and the organic solvent, the final extract
will acquire different characteristics. There are different ways to produce nanolignin particles from wheat
straw, however most of them have a very high solvent consumption. To reduce the consumption of
solvent, the most appropriate method is the direct precipitation of lignin nanoparticles. This method uses
an Organosolv pretreatment with a 60wt% ethanol solution at 180ºC (S Beisl et al., 2018).
Lignin Nanomaterials Applications
For several years, researchers have been studying practical applications for the lignin that is
obtained from biomass or pulping liquors. The lignin structure depends on the source of the biomass
and the isolation method used, therefore the applications will depend on the type of lignin used. Lignin
may be subjected to chemical treatments in order to make it usable in a particular application, that
treatment can improve the reactivity or even its efficacy (Calvo-Flores, et al., 2015). This is an
Figure 4 - Lignin/Phenolics-carbohydrate complex in wheat straw(Buranov & Mazza, 2008).
7
opportunity to adapt the features of the final product, optimizing the entire process chain. Lignin is known
to have different branches of applications, such as resistance to decay and biological attacks, UV
absorption, the capability to retard and inhibit oxidation reactions and high stiffness. Nanostructured
materials have different properties than molecules of larger dimension (same composition), and their
applications field starts in simple polymer blends with upgraded mechanical properties to capable drug
carriers. In Figure 5 is shown the published and potential applications of lignin micro/nanomaterial
(Stefan Beisl, et al., 2017).
Pretreatment of Lignocellulosic material
Figure 5 - Potential and investigated applications of lignin from micro- to nanosize (S Beisl, et al., 2017).
8
Lignocellulosic biomass is a promise in the renewable sources of carbon since it is available
around the world with low cost, however, the major adversity is the complex chemical composition of
the lignocellulosic biomass. In order to access more easily the components, present in the lignocellulosic
material, it is necessary to alter the structure with a pretreatment. The goals of pretreatment on
lignocellulosic material are represented in Figure 6:
The optimization of pretreatments steps is crucial for an economically reliable biorefinery. The
aim of a pretreatment is to extract the lignin in its natural form and to prepare the materials for enzymatic
degradation since the properties of the natural lignocellulosic turn the material more resistant. To choose
the best pretreatment it is necessary to consider the type of lignocellulose feedstock. The main factors
are the degree of polymerization and degree of acetylation. Figure 7 shows the general lignocellulosic
feedstock biorefinery.
Many pretreatment methods have been studied and are still in development, they can be divided
into four different categories (Agbor et al., 2011):
Figure 7 - Ligno-cellulosic feedstock biorefinery (Gavrilescu, 2014).
Figure 6 - Schematic of goals of pretreatment on lignocellulosic material.
9
a) Physical pretreatment: Based on the principle of particle size reduction by mechanical
stress;
b) Biological pretreatment: Non-energy intensive processes, using fungi and
actinomycetes. Some microbes used in this process consume part of the carbohydrates
in the biomass which will affect the sugar yield. Another negative aspect is that this
process requires longer residence time, which is a limitation in a larger scale biorefinery;
c) Chemical pretreatment: Use of different chemicals to study the effect on the native
structure of lignocellulosic biomass;
d) Physicochemical pretreatment: This pretreatment includes treatments such as steam
explosion, ammonia pretreatments, liquid hot water pretreatment, wet oxidation, among
others. These methods have the advantage of affecting both the physical and chemical
properties of biomass.
Organosolv Pretreatment The Organosolv process (OP) is integrated within the chemical pre-treatments. Comprises the
cooking of lignocellulosic biomass in a mixture of an organic solvent with water that leads to the
deconstruction of lignin and hemicellulose and its dissolution in the liquor. It produces three different
streams, a cellulose-rich pulp, a lignin rich solid precipitate as well as a hemicellulose rich liquid.
Moreover, the solvent can be recovered from the liquid stream by distillation (Nitsos, et al., 2018). Figure
8 represents the process diagram of an ethanol organosolv fractionation.
Initially, this pretreatment was operated in the pulp and paper industry. Even though this process
has minor consequences to the environment, it does not achieve the necessary degree of delignification
when compared to the kraft process.
Seen as a promising alternative to remove practically pure lignin from the biomass with sugars
available for conversion is the Organosolv pretreatment. Moreover, Organosolv pretreatment is the only
pretreatment capable of isolating each component of the biomass, which can then be possibly sold as
a by-product or even transformed into a higher value product. For this pretreatment, it is necessary to
have a recyclable and efficient solvent. From previous studies, a solvent recovery unit is required in
Figure 8 - Schematic diagram of the organosolv fractionation process (Nitsos, et al., 2018).
10
order to make it economically viable in an operation sequence of distillation, neutralization, and
evaporation or membrane processes like nanofiltration and reverse osmosis. (da Silva, Errico, & Rong,
2018)
The Organosolv process (OP), extracts low molecular weight lignin from the biomass, nearly
pure, which presents the minimum amount of carbohydrates and impurities, thus being possible to
convert it into final products of higher value rather than heat and power generation (S. Beisl, et al., 2017).
In this pretreatment, there are some side reaction’s products that inhibit microorganism’s
fermentation. Moreover, the use of volatile organic liquid at high temperatures leads to special care due
to higher pressure and because it also uses high-value chemicals (Agbor et al., 2011).
Membranes
Membranes Technology
Since the 1960s the Loeb-Sourirajan development of reverse osmosis asymmetric cellulose
acetate membranes stands as a milestone on the use of pressure-driven membrane technology at the
industrial large scale. In the last years, membrane separation processes have been used in
pharmaceutical, biological, chemical and food industry. The focus of research has been the fractionation
of spent cooking liquor in the kraft chemical pulping process (Wallberg, Jönsson, & Wimmerstedt, 2003).
With the development of new membranes that present upgraded transport properties and are chemically
and thermally stable, new applications were then identified to these membranes (Strathmann, 2001).
Membranes Classification There is a vast diversity of membranes, depending on the materials and structures. In Figure 9
and Figure 10, the principal types of membranes, symmetric and asymmetric are shown. (Rautenbach
et al., 1989).
Symmetric membrane has identical structural morphology at all positions within it. A
microporous membrane (a), is rigid with interconnected pores distributed randomly, these pores have a
diameter in the order of 0.01-10µm. The separation of solutes is a function of molecular size and pore
size distribution. Membrane type (b) is nonporous and dense, where the permeants are transported by
diffusion under the driving force of pressure, electrical potential gradient or concentration. Membrane
Figure 9 - Symmetrical membranes: (a) Isotropic microporous, (b) Nonporous dense, (c) Electrically charged. (Rautenbach, R. & Albert, 1989).
(a) (b) (c)
11
type (c), is a charged membrane, it works by the exclusion of ions of the same charge as the ions present
on the membrane structure.
An asymmetric membrane is a composite of two or more structural planes of divergent
morphologies. Anisotropic membranes have different permeabilities and structures for each membrane
layer supported on a thicker porous substructure. The Loeb-Sourirajan membrane, type (a), consists of
a single membrane material, yet the pores size and the porosity differ depending on the membrane
layer. Membrane (b), has an ultra-thin top layer which is responsible for separation selectivity, where a
microporous sublayer supports this top layer (Wu, 2015). Membrane type (c), liquid membrane, consists
of a thin film which separates two phases, gas or aqueous solutions mixtures. The porous structure
provides the mechanical strength of the membrane, whereas the selective separation barrier is provided
by the liquid-filled pores (Drioli et al., 2010). The membrane structure for ultrafiltration is an asymmetric
membrane made following the Loeb-Sourirajan method, allowing high permeation fluxes and selectivity.
Membranes Material
Membranes should combine high permeability and high selectivity with enough mechanical
stability. Usually, organic polymers are the most used in pressure-driven membrane processes, the most
common polymers used in membranes include polyethersulfone (PES), regenerated cellulose (RC) and
poly(vinylidene fluoride) (PVDF) (Rheingaustr et al., 2018).
PES membranes are used peculiarly in the pharmaceutical industry and sterilizing filtration.
They are resistant at high temperatures and their performance decreases with the fouling as a
consequence of the hydrophobic character (Wavhal & Fisher, 2002). Materials from regenerated
cellulose are better for the environment since they are a non-toxic material with low cost. These
membranes have an important role in seawater desalination, filtering methanol, ethanol, and urea
(Bhongsuwan & Bhongsuwan, 2008). PVDF membranes also present a high level of hydrophobicity and
are thermodynamically compatibility with other polymers. Poly(vinylidene fluoride) membranes show a
chemical resistance and a high mechanical strength which presents a good option for wastewater
treatment (Liu et al., 2011).
Membranes Processes The separation of molecular and particulate mixtures in membrane reactors, artificial organs,
energy storage, and conversion systems, and also the controlled discharge of active agents are the four
Figure 10 - Anisotropic membranes: (a) Loeb-Sourirajan, (b) Thin-film composite, (c) Supported liquid. (Rautenbach, R. & Albert, 1989).
(a) (b) (c)
12
main areas where membranes are used (Strathmann, 2001). The membrane process is determined
according to the driving force used. The most common and relevant are the pressure-driven processes,
based on the pressure difference between the permeate and the feed. Operations that use this type of
membrane process are reverse osmosis, nano-, ultra-, and microfiltration. Another driving force for the
process is concentration-gradient, being used essentially by processes such as dialysis. Moreover,
partial-pressure-driven processes such as pervaporation and gas permeation, and electrical-potential-
driven processes such as electrodialysis and electrolysis are known (Strathmann, 2001). In this work,
the focus is in the pressure-driven membrane processes, which include reverse osmosis, nano-, ultra-
and microfiltration, the following Figure 11 shows the characteristics for each membrane process.
Membrane Process Membrane Type Transmembrane Pressure
Mechanism
Reverse Osmosis (RO)
Asymmetric composite with homogeneous skin
High (20-100 bar) Solution-diffusion
Nanofiltration Asymmetric composite with homogeneous skin
High (10-40 bar) Solution-diffusion and electrostatic interactions
Ultrafiltration (UF) Asymmetric microporous Low (0.5-8 bar) Size exclusion
Microfiltration (MF) Symmetric and asymmetric microporous
Low (0.1-1 bar) Size exclusion
Figure 11 - Pressure-driven membrane processes. (Gaspar, 2018.)
Membranes can be defined as a barrier that split two phases and have a selection process in
relation to the transport of different components. Membranes can be flat sheets, hollow fiber, tubes or
capillaries installed in a suitable device (membrane module). The nature of the membrane used depends
on the intended application (Strathmann, 2001). The key properties for a good membrane performance
are high selectivity and fluxes, and also the need for thermal, chemical and mechanical stability.
The biggest difference between the processes described, is the average pore diameter of the
membrane applied, where the separation process is distinguished by the size of the particles or
molecules that is possible to retain, as can be seen in Figure 12 (Satyanarayana, Bhattacharya, & De,
2000).
13
Ultrafiltration Ultrafiltration has been used for a long time as a separation method which relies on molecular
size exclusion, however, the need to present a low mechanical and chemical resistance led to the
development of new membranes (Strathmann, 2001). Which happened with the appearance of the
anisotropic cellulose acetate membrane by Loeb-Sourirajan for reverse osmosis, that made this process
a potentially practical method of desalting water. It consisted of an ultrathin and selective surface film
on a thicker but much more permeable microporous support, that granted fluxes much higher than any
membrane available at the time (Baker, 2004).
The separation process used in this work was ultrafiltration, which was chosen based on the
particle size of the nanolignin particles in suspension and on the type of material being processed
(anisotropic membranes). This separation method is usually used for the concentration, clarification,
diafiltration, and fractionation of macromolecules. UF membranes represent almost 40% of the food and
biotechnological industry, however, this is a high-cost market and there is a need to develop cost-
effective and scalable purification processes. Ultrafiltration features high throughput of product, easy to
clean and to sanitize equipment, and it can be easily scaled-up (Ghosh, 2003).
The fractionation of lignin from the black liquor resulting from pulping processes was previously
studied comparing two different methods, selective precipitation, and ultrafiltration. Both methods were
effective although ultrafiltration showed better results since the lignin obtained was less contaminated
with hemicellulose (Toledano, et al., 2010). Furthermore, the final average molecular weight of the
product is controlled by the MCWO of the membrane. Ultrafiltration also showed the advantage of not
needing temperature or pH adjustment, as well as the fact that the concentration of the liquor not being
Figure 12 - Classification of membrane processes based on pore size.
(Ultrafiltration, 2018)
14
crucial, making it possible to improve the pulp quality of bleachability by decreasing the lignin
concentration from the cooking liquor (Jönsson & Wallberg, 2009).
However, the method to extract the lignin has to be chosen according to the desired application,
since the selective precipitation shows less energy consuming which reflects in lower operating costs
(Toledano, et al., 2010).
The module designs depend on the application desired, the most common designs are
distinguished from each other by the hydraulic diameter and the package density. Tubular modules such
as polymeric or ceramic element, plate modules, spiral wound modules, follow fiber modules and
capillary modules are the most common module designs in ultrafiltration (OSMO Membrane Systems,
2019).
Membranes Characterization
Molecular Weight Cut-off The choice of MCWO is as important as any other decisions in a filtration process, this
parameter specifies the size of the molecules that will go through the membrane. The research on the
effects of different MCWO membranes on the permeate flux and membrane rejection was conducted in
a stirred cell module. For pure water, the higher the MCWO of the membrane the higher the flow, since
the pores are bigger (Toledano, et al., 2010). Moreover, if the pore size has increased and the amount
of solute that passes through the membrane increases, that also means that the rejection values will be
lower. It is necessary to evaluate specifically each experimental procedure and select a suitable
membrane, that would be ideal for each situation, depending on the type of suspension used.
Ultrafiltration membranes are usually in the range of 1-500 kDa. In this work the MCWO chosen was
30kDa, because from previous studies it was the one that showed the most promising results to separate
nanolignin particles from the impurities. This membrane MCWO retained the least quantity of dissolved
components, being more efficient at separating and purifying the nanolignin particles (Gaspar, 2018).
Hydraulic Permeability
The feed circulates tangentially to the surface of the membrane, this procedure has as a result
two output currents. The coefficient of hydraulic permeability, 𝐿𝑝, is the capacity of water permeation,
which is obtained by calculating the slope of the variation of the water permeation flux, 𝑣𝑝, with pressure.
Figure 13 - Schematic representation of ultrafiltration. 𝐶𝐴𝑎 and 𝐶𝐴𝑝 represent the solute concentration
in the feed and permeate. 𝑄𝑎, 𝑄𝑝 and 𝑄𝑟 represent the flow of the feed, permeate and retentate.
15
The variation of the permeation flux with the coefficient of hydraulic permeability is described in the
equation 1.
𝑣𝑝 = 𝐿𝑝. ∆𝑃 (1)
If the components get rejected and retained on the membrane surface, it will increase the
resistance to mass transfer. The permeation flux variations are represented by equation 2 due to
difference osmotic pressures between the permeate and the feed.
𝑣𝑝 = 𝐿𝑝. (∆𝑃 − 𝜋) (2)
To characterize the membranes performance the rejection coefficient is used, which indicates
the quantity of solute retained by the membrane. This coefficient is based on equation 3.
𝑓𝐴 =𝐶𝐴𝑎−𝐶𝐴𝑝
𝐶𝐴𝑎 (3)
Concentration Polarization and Fouling During the ultrafiltration process the membrane begins to retain material, creating a layer on the
surface of the membrane. The major problem in this membrane separation process is the decrease of
the permeate flux over time. Initially, the flux reduction is due to the build-up of osmotic pressure of the
solution, however, the gradual decline is caused by some consolidation on the membrane surface and
some in the membrane pores, formed by concentration polarization (Satyanarayana et al., 2000).
In ultrafiltration, the suspension is carried in the direction of the membrane surface by the
solution permeating the membrane, there is an accumulation of the larger molecules while the solvent
molecules permeate the membrane. In the course of the filtration, the solutes retained on the membrane
surface get so concentrated that they form a gel layer which is characterized as a second layer of the
membrane. Figure 14 illustrates a model of the gel layer (Rautenbach et al., 1989).
Once a gel layer is formed, the increase of the pressure will not increase the flux, however, the
gel thickness increases (Bhattacharjee et al., 1992). There are three factors that affect concentration
polarization which are boundary layer thickness, volume flow and diffusion coefficient.
Figure 14 - Gel layer of colloidal material on the surface of an ultrafiltration membrane (Rautenbach et al., 1989).
16
The boundary layer thickness can be minimized with the increase of the turbulence at the
membrane surface, through the increase of the flow velocity of the fluid, the use of membrane modules
or pulsing the feed fluid flow through the membrane. However, the turbulence must be increased
cautiously due to the energy needed for this achievement.
The total volume flow is also an aspect to consider, when the volume increases also the
concentration polarization increases. This is an aspect that can be improved by changing the operation
conditions. The consequences of high fluxes are one of the critical aspects in ultrafiltration.
The third factor is the diffusion coefficient that affects the concentration polarization, in
comparison with reverse osmosis, ultrafiltration filters colloids and macromolecules which have diffusion
coefficients about 100 times smaller than solutes in reverse osmosis. This is an important factor for
concentration polarization which explains why the size of the solute diffusion coefficient is such important
in ultrafiltration (Rautenbach et al., 1989).
Membrane Cleaning
To remove or decrease the layer created on the surface of the membrane it is necessary to
have repeated membrane cleaning, allowing the restoring of the membrane capacity. Figure 15 shows
the effects of membrane cleaning.
A different approach to control membrane fouling was studied, applying a pretreatment with
chlorination (Yu, et al., 2014). One of the major contributors to membrane fouling in typical ultrafiltration
processes are microorganisms called biological fouling. From laboratory tests it was concluded that
when conducting this type of pretreatment, the membrane fouling decreases substantially, which was
subsequently confirmed on a pilot scale. The addition of chlorination compounds resulted also in a lower
production of substances that cause fouling, proteins and polysaccharides, resulting ultimately in a
thinner cake layer.
Figure 15 - Permeate flux as a function of time with
membrane cleaning (Rautenbach et al., 1989).
17
Diafiltration Diafiltration is a well-recognized technique used in membrane separation process, with many
applications in the biotechnological, pharmaceutical, and food industries (Kovács et al., 2008). In this
process, the retentate is diluted with a solvent and further ultrafiltered in order to obtain selective removal
of lower molecular weight components. While working with kraft black liquor, the goal is not only to
obtain pure lignin but also to obtain the chemicals present in the permeate, which represent a significant
value. The addition of deionized water during diafiltration affects the viscosity. The reduction of the
viscosity increases the flux (inversely proportional) and reduces the thickness of the layer as a result of
the increase of Reynolds number in the tubes, which results in a decrease of the mass transfer
resistance (flux increase). (Wallberg et al., 2003)
Diafiltration can be performed in batch or continuous mode, the mode used is usually
determined by the rest of the process. For the same volume reduction before diafiltration, the final
product present higher purity in the continuous system (Wallberg et al., 2003).
Aim of the thesis
The aim of the work is to contribute to the development of the state of the art on the separation
and purification of wheat straw nanolignin particles from its impurities. To achieve that an ultrafiltration
membrane process ultrafiltration of nanolignin suspensions in diafiltration mode is used, recurring to a
commercial UF membrane with a MWCO of 30kDa will be used which has been analyzed in preliminary
works (Gaspar, 2018).
The following steps were based on previous experiments (Beisl et al., 2018):
1. Organosolv-Extraction of wheat straw at 180°C for 1h to prepare extracts for further usage (this
is the standard extract);
2. Filtering and centrifugation of the extract to remove particulate matter;
3. Precipitation of nanolignin with given operational conditions to produce nanolignin suspensions;
4. Ultrafiltration of nanolignin suspensions:
a) Pre-condition fresh membranes by flushing a given time;
b) Increase the concentration of the suspension in UF mode;
c) Remove impurities and ethanol in DF mode;
d) Trace the membrane performance decline as a function of permeate mass and stop
experiment at a given time;
e) Regenerate used membranes by flushing or backflushing;
f) Repeat these experiments several times with the regenerated membrane and compare
the performance with a completely fresh membrane to elaborate a ‘long-term’ stability
of the regenerated membrane;
5. Determine particle size of nanolignin suspension from time to time to check for particle size
stability;
6. Analyze important parameters in final nanolignin suspension.
18
19
Material and Methods
Experimental Procedure for Nanolignin particles production
There are 2 essential steps to obtain the nanolignin suspension used in current work. Firstly,
the wheat straw is subjected to a Pretreatment/Extraction in order to separate the lignin, which is then
followed by a Precipitation. The extract production, the choice of the antisolvent and setup was based
on previous experiments (Stefan Beisl et al., 2018).
Extract Production
The wheat straw used was harvested in 2015 in Margarethen am Moos, region in Lower Austria.
The composition of the straw was 16.1 %(w/w) of lignin and 63.1 %(w/w) carbohydrates, which consists
in arabinose, glucose, mannose, xylose and galactose (S Beisl et al., 2018) .
A stirred autoclave of 1L (Zirbus, HAD 9/16, Bad Grund, Germany) was used for the Organosolv
extraction, in order to obtain approximately 400mL of extract in each extraction. The necessary steps
to conduct the experiment are shown below:
1. Measure the humidity of the straw in order to determine the amount of water required.
2. An aqueous solution of 60wt% aqueous ethanol mixture was mixed with 8.3 %(w/w) of wheat
straw inside the reactor.
3. The reactor was set to a mantle and product temperature of 210°C and 180°C respectively, the
temperature and pressure are continuously recorded in the program LabVIEW.
4. When the product reaches the desired temperature, the mantle temperature is changed to
190°C, so that the product temperature does not exceed the set temperature.
5. After 1h of extraction, the cooling system is started so that the mixture is cooled down and
reaches room temperature after approximately 1h.
6. To take all the liquid from the solid part, the mixture was placed in the hydraulic press (Hapa,
HPH 2.5, Achern, Germany) at 200bar.
7. The solid part is stored in the freezer for future analyses, sugars, degradation products, and
lignin, and the liquid part is centrifugated, Sigma 4K15, Germany at 24000g for 20min.
To achieve the goal of the thesis, approximately 3L of extract were produced for that 8
extractions were conducted. The following Figure 16, shows a portion of the extract produced at 180°C
and the equipment used to obtain the extract.
20
Precipitation
For the precipitation step an antisolvent composed of pure water at 25°C was used
(Jääskeläinen, Liitiä, Mikkelson, & Tamminen, 2017) to dilute the amount of ethanol in the mixture. This
procedure results in the reduction of the solubility of lignin in the solution, forcing it to precipitate. The
precipitation occurs in a T-fitting followed by a static mixer, using 2 Syringe Pumps, Figure 17, with a
volume ratio of extract to antisolvent of 1:5. The use of a T-fitting followed by a static mixer was based
on (Stefan Beisl et al., 2018), this setup showed a faster mixing and a smaller particle size when
compared to other setups.
Membrane Filtration
Ultrafiltration Process Setup
Figure 16 - (a) Example of extract, (b) Autoclave in operation, (c) Autoclave
(a) (b) (c)
(a)
Figure 17 - Precipitation Setup: (a) T-fitting and static mixer, (b) Syringe Pumps.
(b)
21
Two MEMCELL plants, from OSMO Membrane Systems, were used to perform the membrane
separation experiments. The systems present a flat-membrane, where the ultrafiltration is operated in
cross-flow.
Table 1 -Technical data of the MEMCELL plants.
A smaller plant, Figure 18, was used to flush the membranes and regenerate them. In this
montage, the concentrate stream returns to the feed tank, which is connected to a gear pump from
Liquiflo.
For the filtration experiments another setup was used, Figure 19, this plant was adapted for a
5L tank, with 2 gear pumps in parallel, from Liquiflo, connected to the feed tank. For the last filtration
experiment, the two pumps used had to be replaced by another pump, Liquiflo, due to technical issues.
A stirrer was used in the feed tank to ensure good homogenization of the suspension and to avoid
aggregation of the particles. In both montages, global valves are installed to control the flow rate
additionally a pressure valve.
Technical Data
Working Pressure (bar) 64 (standard)
Material Stainless steel (1.4571 standard)
Feed Tank Volume (L) 0.5-2
Active Membrane Area (cm2) 80
Figure 18 - MEMCELL plant for membrane flush, from OSMO Membrane Systems.
22
Both systems consist of flat modules with an active membrane area of 80 cm2 each, the first
setup with only one membrane module while the second setup has 4 membranes modules. The
modules can be operated in parallel or in series and the suspension flows tangentially to the membrane
surface area. Particles smaller than the pores of the membrane penetrates the membrane, whereas
particles larger than the pores are retained and flow along the membrane surface. Thus, there are two
outflows streams, a permeate stream that flows through the membrane, and a concentrated stream that
passes along the membrane surface and returns to the feed tank. It was used the program RsCom
which was connected to a scale to record the permeate mass over time, this program was used during
the filtration of nanolignin particles suspension and membranes flush.
Membrane Instructions
When handling membranes, some precautions have to be taken into account. The main
concerns are:
1. Store the membrane in distilled water when not in use, between 5-50°C.
2. Before installation, flush the system with water to remove any existing residue. Flush the
membrane for 15 minutes to remove the preservative.
3. For a good filtration performance, the permeate must be drained without pressure on the system
and the valves should be opened gradually.
4. For a system shutdown of more than 24 hours, the plant must be flushed and cleaned.
Figure 19 - MEMCELL plant for membrane filtration, from OSMO Membrane Systems.
23
Membrane Selection
In ultrafiltration, membranes separate lignin based on MCWO that restrict passage based in
molecular size. The membrane material chosen was polyerthersulfone (PESH), a chemical compatible
with ethanol, since the suspension used had an ethanol content of 15wt%.
Table 2 shows the properties and application areas of NADIR® PESH membrane. The
membrane chosen was operated with a MCWO of 30kDa since it was the membrane that retained less
quantity of dissolved lignin at 4 bar and 1.2 L/min (Miltner et al., 2019).
Table 2 - Properties and application areas of NADIR® PESH membrane.
Membrane Stability
Fresh membranes need to be conditioned by flushing them with 15 %w/w hydroalcoholic
solution for a certain time, so the membrane maintains its stability for longer periods. From the program
data, it is possible to trace the membrane performance decline as a function of permeate mass per unit
of membrane surface area and time, thus obtaining the transmembrane flux from the slope of the linear
regression. The definition of transmembrane flux and the calculation of this factor is given in the next
equation 6:
𝑇𝑟𝑎𝑛𝑠𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒 𝐹𝑙𝑢𝑥 (𝑔
𝑐𝑚2.𝑚𝑖𝑛) =
𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑚𝑎𝑠𝑠 (𝑔)
𝐴𝑚𝑒𝑚𝑏𝑟𝑎𝑛𝑒(𝑐𝑚2)×𝑇𝑖𝑚𝑒 (min) (6)
Ultrafiltration and Diafiltration of Nanolignin Particles Suspension
The goal of this thesis is, firstly to increase the concentration of the suspension in ultrafiltration
mode and remove impurities and ethanol in diafiltration mode. Several experiments were done. In the
first experiment two membranes in series were used, where the first membrane is regenerated, and the
second membrane is repeatedly changed for a new one. On the second filtration experiment, three
membranes were used, the first membrane was the same as in the first experiment, with regeneration,
a second membrane that is neither changed nor regenerated and a third membrane that is repeatedly
swapped. The experimental work of the membrane filtration follows the following steps:
Material Properties Range of
pH
Max.
Temperature
Line of business, industrial
sector
Permanently hydrophilic
polyethersulfone
(PESH)
Hydrophilic (low
fouling potential)
High chemical
stability
0 – 14
95°C
Environmental protection
Metal processing
Textile manufacture
Paper manufacture
Food industry/dairy
Pharma/biotechnology
Chemical industry
24
1. Initial membrane flush with a 15wt%ethanol solution for 2h15 for all membranes that will be
used in the procedure;
2. Lay the membranes in series and start filtration. Set the pressure and flow rate to 4 bar and 1.2
L / min, respectively.
3. The number of output currents depends on the number of membranes used since the
experiments were conducted using two membranes and three membranes in different runs,
therefore at the most 3 output streams were obtained and collected separately in beakers placed
on scales. When the first beaker reached 400g the filtration is stopped and a sample of all
permeates and the retentate was taken for future analyses. The beakers were changed, and
filtration is restarted;
4. When the suspension volume is reduced to approximately half, for the first experiment (2
membranes in series) the membrane 1 is regenerated and the second membrane is changed
for a new one. For the second experiment (3 membranes in series), membrane 1 is regenerated,
membrane 2 is held equal and the third membrane is replaced by a new one;
5. After the membranes have passed the required procedure written in point 4, the filtration is
continued in order to obtain in the end between 5 and 10% of the initial volume of suspension
in the feed tank.
6. The membranes were again subjected to regeneration/alteration as explained in step 4. After
this, the tank was filled with water to the initial volume, but the initial volume was reduced by the
sample amount taken previously;
7. Restart the filtration, in diafiltration mode, and the suspension was again filtered until
approximately half of the initial volume;
8. The membranes were again subjected to regeneration/alteration as explained in step 4, and
filtration was restarted and done until the 5 or 10% of the initial volume of suspension is reached.
9. In the end, membranes were flushed with 15wt%ethanol for 2h15 in order to obtain the final flux
of the membranes.
It is necessary to check that all the pressures stay constant during all experiments.
After each experiment, the setup needs to be cleaned with solutions of 50wt% acetone and
15wt% ethanol.
Membrane Regeneration
The experimental work of the membrane regeneration follows the following steps:
1. Membrane 1 is flushed for 1h with a solution of 15wt%ethanol without pressure with a flow rate
of 1.2L/min;
2. After 1 h a sample of concentrate is withdrawn and analyzed in UV-spectrometer. After 30 more
minutes, repeat the analyses and if the value is constant finish the flush, if not, continue flushing
for another 30minutes and repeat until the value is constant, that is, until the membrane can no
longer be cleaned. (for the analyses of the samples in the UV it is necessary to add ethanol until
the mixture has 60wt% of ethanol);
25
3. After this, membrane 1 is again flushed with a 15wt%ethanol solution for 2h15 at 4 bar and
1.2L/min in order to understand how the regeneration, step 2, improved the membrane
performance.
For the calculation of the amount of lignin removed from the membrane and for the membrane
recovery, a calibration curve was prepared using different values of lignin concentration and the
respective absorbances for a wavelength of 280 nm, shown below in Figure 20. These experiments
were repeated to elaborate long-term stability of the regenerated membrane.
Membrane Fouling
After ultrafiltration/diafiltration of nanolignin particles suspension, the membranes are also
flushed to understand how the fouling affects the membranes. The last membrane flush is made with
the same type of solution as used initially, 15wt% ethanol solution. The final transmembrane flux was
then compared with the initial TF to understand how the performance decreases with nanolignin particles
suspension, and whether all membranes behave similarly.
Although the initial TF calculations were obtained with the linear regression slope, for the final
flush, the transmembrane flux was determined based on the first and last 10% of the derivative of the
curve because it does not show linear behavior.
Figure 20 - Calibration curve for lignin content in UV.
26
Analytics
As described before, samples were taken when the first permeate outlet stream reached 400g.
In every system shutdown, samples of the permeate and the concentrate were taken for future analyses.
The diagram shown below, Figure 21, illustrates which analyses were done on each sample.
Dry Matter Content
Dry matter refers to material remaining after removal of liquid. This is a simple method which
allows to compare membranes and how efficient they are. To realize this method, a certain amount (at
least 10g) of sample is collected in a container that is left in an oven at 110ºC for at least 24 hours, being
weighed at the end in order to determine the quantity of solid components in the sample. This is done
for all the permeate samples and for the retentate samples, before centrifugation (dissolved components
with nanolignin particles) and after centrifugation (dissolved components), and the difference gives lignin
particles. Therefore, there is the possibility of having a mass balance of the dissolved components. The
drymatter concentration is calculated based on equation 7:
%𝐷𝑀 =𝑚𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒
𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙
(7)
%𝐷𝑀 − % 𝑜𝑓 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡,𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝑚𝑖𝑛𝑖𝑡𝑖𝑎𝑙 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑏𝑒𝑓𝑜𝑟𝑒 𝑜𝑣𝑒𝑛, 𝑔
𝑚𝑑𝑟𝑦 𝑠𝑎𝑚𝑝𝑙𝑒 − 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑎𝑚𝑝𝑙𝑒 𝑎𝑓𝑡𝑒𝑟 𝑡ℎ𝑒 𝑜𝑣𝑒𝑛, 𝑔
𝑇𝐿 = 𝐷𝑀𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 (8)
Figure 21 - Analysis methodology for different samples.
27
𝑇𝐿 − 𝑇𝑜𝑡𝑎𝑙 𝑛𝑎𝑛𝑜𝑙𝑖𝑔𝑛𝑖𝑛 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑎𝑚𝑝𝑙𝑒,𝑔𝑛𝑎𝑛𝑜𝑙𝑖𝑔𝑛𝑖𝑛 𝑝𝑎𝑟𝑡𝑖𝑐𝑙𝑒𝑠
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝐷𝑀𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑏𝑒𝑓𝑜𝑟𝑒 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒,𝑔𝐷𝑀
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
𝐷𝑀𝑎𝑓𝑡𝑒𝑟 𝑐𝑒𝑛𝑡𝑟𝑖𝑓𝑢𝑔𝑒 − 𝐷𝑀 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 𝑜𝑓 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡,𝑔𝐷𝑀
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛
Equation 8 is the result of the difference between the DM before and after centrifugation, is the
solid amount of lignin in the sample. The sample before centrifugation corresponds to the total
components in the suspension, and the sample after centrifugation, from which the solid part is
withdrawn, corresponds to the dissolved components.
From the DM of the permeates and the samples after centrifugation, it is possible to make a
mass balance of the dissolved components, calculated based on the equation 9:
(9) 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.
𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑟𝑒𝑡𝑒𝑛𝑡𝑎𝑡𝑒 𝑠𝑎𝑚𝑝𝑙𝑒, 𝑔.
𝐷𝑀𝑅13𝑊 − 𝐷𝑀 𝑜𝑓 𝑡ℎ𝑒 𝑓𝑖𝑛𝑎𝑙 𝑐𝑜𝑛𝑐𝑒𝑛𝑡𝑟𝑎𝑡𝑒 𝑟𝑒𝑚𝑎𝑖𝑛𝑖𝑛𝑔 𝑖𝑛𝑠𝑖𝑑𝑒 𝑡ℎ𝑒 𝑡𝑎𝑛𝑘, 𝑔.
Where:
𝐷𝑀𝑥 = ∑ (𝐷𝑀𝑥 × 𝑚𝑥 )𝑖
To find the removal efficiency of the dissolved components from the initial nanolignin particle
suspension, the ratio of DM amount from the permeates and DM amount of dissolved components in
the initial suspension was calculated, equation 10.
𝑅𝐸 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠
𝑥𝑖=1
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 (10)
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑒𝑎𝑐ℎ 𝑝𝑒𝑟𝑚𝑒𝑎𝑡𝑒, 𝑔.
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 − 𝐷𝑀 𝑎𝑚𝑜𝑢𝑛𝑡 𝑖𝑛 𝑡ℎ𝑒 𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛 𝑜𝑓 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑠𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛, 𝑔.
Degradation Product
The solutions of nanolignin particles over the experiments present different amounts of
degradation products, it is possible to find ethanol in a large amount and other components such as
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠
𝑥
𝑖=1
+ ∑ 𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 𝐷𝑀𝑙𝑎𝑠𝑡 𝑠𝑎𝑚𝑝𝑙𝑒
𝑦
𝑗=1
28
acetic acid, HMF and furfural. One of the goals of this work was to remove impurities and ethanol from
the initial nanolignin particles suspension. To access this information, the samples have been taken
during filtration, permeate and retentate were analyzed in an HPLC, which is capable of identifying
compounds present in any sample that can be dissolved in a liquid in very low concentrations. The
equipment used was from Shimadzu Corporation, that pumps a sample in a solvent (mobile phase) at
high pressure through a column with chromatographic packing material (stationary phase). The
components of the column used were Sugar-SH1011 (Shodex), Guard Column: Sugar SH-G (Shodex),
the detector was the refractive Index, using 0.005M of sulfuric acid as eluent with a flow rate of 0.6
mL/min. The calibration curves obtained are shown in the Figures 22, 23 and 24.
Figure 24 - Calibration curve for Ethanol content.
Figure 23 - Calibration curve for HMF and Furfural content. Figure 22 - Calibration curve for Acetic Acid content.
29
Particle Size
One of the main points of this work was to purify the main product without particle agglomeration.
The particle size measurement was done for all retentate samples and was immediately measured after
the withdrawn sample. The equipment used to measure the particle size was ZetaPals from Brookhaven
Instruments Corporation. Each sample was analyzed twice, one first analysis with the original sample,
and another one using a sample diluted in deionized water with a volume ratio of 1:100. The diluted
samples are more reliable since it is not necessary to resort to correlations, due to the fact that the
viscosity and the refractive index are well known.
30
31
Results and Discussion
As explained previously, the goal was to increase the concentration of the suspension of
nanolignin particles and remove ethanol and impurities using membrane filtration, ultrafiltration followed
by diafiltration. Also, to compare and characterize membranes behavior and understand if it is possible
to regenerate a membrane to the initial condition. Previously was made membrane filtrations with the
same type of membrane and setup with the difference of the initial amount of feed, 1.2 L, and the amount
to concentrate, 50% concentration. These experiments were made with an initial feed of 5 L with the
goal to concentrate to 5-10 % of the initial suspension.
This chapter is divided into two parts, a first one where the concentration is increased using two
membranes, one of those with regeneration. And the other part of the chapter shows the results using
three membranes in series, with one of the membranes being regenerated, a second membrane used
since the beginning and a third one changed every 50% concentration. Is important to note that
membrane 1 was used previously in another experiment before being used in the next filtrations.
Membrane Filtration – 2 Membranes in Series
Membrane stability
The procedure outlined in earlier section Material and Methods was adopted in this work, all
membranes need to be flushed for a certain time so they can be stable for longer periods. Membranes
are also flushed after being used to compare their performance. It was used solution with 15wt% ethanol
since the suspension of nanolignin particles has a maximum ethanol concentration of 15wt%.
All membranes were flushed for 2h15, three parts of 45 min with a refill of the permeate inside
the feed tank, at 4 bar and 1.2 L/min of flow-rate. Figure 25 represents the mass of permeate over time
for all the membranes used in the first experiment, flushed with different 15 %wt ethanol solutions.
Figure 25 – Initial transmembrane flux (g/min) over time (min) of each membrane (1.2 L/min at 4 bar).
32
Transmembrane Flux
Initially the membranes performance decreases and stabilizes after a certain time, maintaining
that stability for the rest of the operational time. The initial TF was calculated based on the last 20
minutes of the initial flushing within an uncertainty range of 10%. In Table 3, is shown the initial TF
obtained for membranes 1, 2, 3, 4 and 5.
Table 3 - Initial transmembrane flux for each membrane used in the experiment 2 Membranes in
Series.
Membrane Initial TF
(L/(m2.h))
Membrane 1 78.8
Membrane 2 82.3
Membrane 3 98.4
Membrane 4 98.3
Membrane 5 91.5
As expected, the transmembrane flux of membrane 1 is lower since this membrane was used
in a previous experiment. However, the other membranes were all new and yet membrane 2 has a
transmembrane flux lower than the other membranes, being closer to the TF of membrane 1. It’s
expected that membrane 2 has a lower performance compared to the other new membranes, since it
has a similar TF to membrane 1. However, this TF difference is not so significant.
Ultrafiltration/Diafiltration of Nanolignin Particles Suspension
Initially, there was 5L of nanolignin particles suspension inside the feed tank, the first step was
to increase the concentration in ultrafiltration mode followed by diafiltration, to remove impurities and
ethanol. For the diafiltration, the feed tank was filled with water to the initial volume, but the initial volume
was reduced by the amount of the sample so that the lignin concentration be identical to the initial one
in the suspension.
In both modes 2 membranes in series were used, membrane 1 was regenerated and the other
membrane was changed when the feed suspension had a volume reduction of approximately 50%. For
future analysis, it was taken, for every 400g of one of the permeates, samples of the retentate.
Figure 26 and Figure 27, show the transmembrane flux over time for all the membranes. Figure
26 represents the filtration of membrane 1 and the vertical lines represent when the membrane was
regenerated, MR. Figure 27 shows the TF for the other membranes and the vertical lines represent
when there was a change of membrane, MC. The blue color represents the concentration mode and the
orange the diafiltration mode.
33
Figure 26 - Ultrafiltration and diafiltration process for membrane 1 (1.2 L/min at 4 bar).
Figure 27 - Ultrafiltration and diafiltration process for other membranes, 2, 3, 4 and 5 (1.2 L/min at 4 bar).
34
From the graph it is possible to notice that the permeate flux does not happen constantly with
time, the more pronounced variations occur when the pumps are restarted and system pressure
variation occurs, taking some time for the system pressure to remain constant. The pressure increased
even though there was made precautions, like opening the valves slowly.
As explained before, the vertical lines in Figure 27 represent the change of membrane.
Membrane 2 was used since the beginning until 50 % concentration, about 2.5 L. Afterwards membrane
2 was changed for a new one, membrane 3, which was used until the end of the ultrafiltration when the
10% of the suspension was reached. Membranes 4 and 5 were used in diafiltration mode under the
same conditions as membranes 2 and 3, respectively. The membrane regeneration to membrane
occurred at the same time as membrane change, as can been seen in the figures above.
Table 4 - Mean transmembrane fluxes for UF/DF for membrane 1 and other membranes, 2,3, 4 and 5.
UF/DF Step Mean Transmembrane Flux (𝑳
𝒎𝟐.𝒉)
Ultrafiltration (Membrane 1) 39.7
Diafiltration (Membrane 1) 49.1
Ultrafiltration (Membranes 2 and 3) 32.5
Diafiltration (Membranes 4 and 5) 72.3
Diafiltration step for membrane 1 and other membranes, Figure 26 and Table 4, shows a linear
trend contrary to what happens in the concentration step. That means that the amount of dissolved
components that passes through the membrane are constant with the time unlike what happens in
ultrafiltration. For all membranes the transmembrane flux increases when water is added, which is
explained by the fact that when water is added the membrane is flushed, getting cleaner, decreasing
the fouling.
Except for membrane 1 all the other membranes used in the procedure were new but for the
concentration mode the new membranes show a lower transmembrane flux, the difference can be
explained by natural variations of the membrane samples. However, for the diafiltration step membranes
show a higher transmembrane flux, which means the membranes are more susceptible to fouling in UF
mode. In Table 5 is shown the mean TF for each membrane used in experiment with 2 membranes in
series.
Table 5 - Mean transmembrane fluxes for each membrane.
UF/DF Step Mean Transmembrane Flux (𝑳
𝒎𝟐.𝒉)
Membrane 1 38.5
Membrane 2 31.6
Membrane 3 33.3
Membrane 4 86.2
Membrane 5 65.5
35
Membrane 1 is the regenerated membrane which presents a higher TF value than the
membranes that are used for the first time (membranes 2 and 3). Even though the regeneration is to
improve the performance of a used membrane, it was expected to have a lower transmembrane flux
than a new membrane.
Membranes 2 and 3 were new despite having a lower TF when compared to membrane 1.
Nevertheless, when comparing the initial flush it was already expected that the performance of
membrane 2 should be similar to membrane 1. This membrane was probably damaged by the process
system.
In the diafiltration mode, membranes 4 and 5 show an increase of TF which indicates that the
membrane gets cleaner, decreasing the fouling. Also, when the suspension gets more concentrated,
the transmembrane flux decreases which means the concentration affects the membrane performance.
In the concentration mode it was not possible to understand if the concentration affects the
transmembrane flux.
Membrane Fouling
Fouling results in a decrease in permeate flux and an increase in hydraulic resistance that is
inversely proportional to the flux. As explained in Material and Methods, fouling can occur due to the
absorption of feed components, accumulation of particles in the pores of the membranes, formation of
a layer on the membrane surface due to the continuous deposition of particles and a gel layer formed
because of concentration polarization. As described, membranes were also flushed with 15wt% ethanol
after the experiments with the suspension in order to be able to evaluate the fouling effect.
Figure 28 - Transmembrane Flux (g/min) over time (min) for the final flush of each membrane (1.2 L/min at 4 bar).
36
It is perceptible from Figure 28 that membranes 4 and 5 have higher fluxes when compared with
the other two membranes (membranes 1 and 2). When compared with the other membranes, membrane
5 shows a different performance in the flushing, during the first and last part of the flush (approximately
the first and last 45 minutes) this membrane showed a higher increase of the permeate mass.
Contrary to what happens to the initial flush (during this flush), the membrane will change again,
particles or precipitated lignin might be washed away. The TF was calculated based on the first minutes
of the final flushing with ethanol/water. In Table 6 is shown different values obtained for the
transmembrane flux of each membrane, and the respective flux decline.
As expected, membranes 4 and 5 are the ones with higher final transmembrane flux, since they
were used only in the diafiltration mode. The other membranes have a similar final transmembrane flux,
which means that the filtration of the nanoparticle suspension similarly affects the membranes in
concentration mode.
It is expected that the membranes behave the same way since they present the same material
and MCWO. For each filtration mode (ultrafiltration and diafiltration) the membranes have the same
MCWO, so the amount of dissolved components crossing the membranes should be the same and the
polarization effect should affect them equally. For all membranes, the transmembrane flux is decreasing
over time mainly because of the increase of the layer thickness on the membrane’s surfaces. The
concentration of the particles gradually increases on the surface of the membrane, due to solute
accumulation from the convective flow.
DF leads to significantly lower flux decline. As particle concentrations are similar in UF and DF,
the flux decline is mainly influenced by dissolved components which are reduced in DF.
Analytics
Previously, it was said that samples of retentate and permeates were taken. It was decided that
when the first permeate stream reached approximately 400g, the membrane filtration was stopped, and
the samples were taken and labeled. The code of each sample is shown in Table 7.
Table 6 - Initial and final transmembrane flux each membrane and the respective flux decline for the
experiment 2 Membranes in Series.
Membranes
Initial TF (𝑳
𝒎𝟐.𝒉) Final TF (
𝑳
𝒎𝟐.𝒉) TF decline (%)
Membrane 1 78.8 53.2 32.5
Membrane 2 82.3 60.3 26.7
Membrane 3 98.4 59.4 39.6
Membrane 4 98.3 96.5 1.8
Membrane 5 91.5 73.1 20.1
37
Table 7 - Samples labeling code for the experiment 2 Membranes in Series.
Code Name Sample Step of filtration
S Initial Suspension Suspension after precipitation
R1 1st retentate 1st sample after 764.3g filtred
P1M1 1st permeate 1st sample after 400.1g of permeate membrane 1
P1M2 1st permeate 1st sample after 364.2g of permeate membrane 2
R2 2nd retentate 2nd sample after more 764.8g filtred
P2M1 2nd permeate 2nd sample after 399.9g of permeate membrane 1
P2M2 2nd permeate 2nd sample after 364.9g of permeate membrane 2
R3 3rd retentate 3rd sample after more 917.9g filtred
P3M1 3rd permeate 3rd sample after 489.4g of permeate membrane 1
P3M2 3rd permeate 3rd sample after 428.5g of permeate membrane 2
R4 4th retentate 4th sample after more 737.1g filtred
P4M1 4th permeate 4th sample after 336.9g of permeate membrane 1
P4M3 4th permeate 4th sample after 400.2g of permeate membrane 3
R5 5th retentate 5th sample after more 734.7g filtred
P5M1 5th permeate 5th sample after 333.9g of permeate membrane 1
P5M3 5th permeate 5th sample after 400.8g of permeate membrane 3
R6 6th retentate 6th sample after more 480g filtred
P6M1 6th permeate 6th sample after 245.7g of permeate membrane 1
P6M3 6th permeate 6th sample after 234.3g of permeate membrane 3
Addition of 4687g of Water (Initial suspension mass reduced by the amount of samples)
R7W 7th retentate 7th sample after 810.8g filtred after water addition
P7M1W 7th permeate 7th sample after 295.1g of permeate membrane 1
P7M4W 7th permeate 7th sample after 515.7g of permeate membrane 4
R8W 8th retentate 8th sample after more 621.3g filtred
P8M1W 8th permeate 8th sample after 220.4g of permeate membrane 1
P8M4W 8th permeate 8th sample after 400.9g of permeate membrane 4
R9W 9th retentate 9th sample after more 622.5g filtred
P9M1W 9th permeate 9th sample after 221.5g of permeate membrane 1
P9M4W 9th permeate 9th sample after 401g of permeate membrane 4
R10W 10th retentate 10th sample after more 346.3g filtred
P10M1W 10th permeate 10th sample after 123.7g of permeate membrane 1
P10M5W 10th permeate 10th sample after 222.6g of permeate membrane 4
R11W 11th retentate 11th sample after more 658.1g filtred
P11M1W 11th permeate 11th sample after 257.5g of permeate membrane 1
P11M5W 11th permeate 11th sample after 400.6g of permeate membrane 5
R12W 12th retentate 12th sample after more 673.1g filtred
P12M1W 12th permeate 12th sample after 269.1g of permeate membrane 1
P12M5W 12th permeate 12th sample after 404g of permeate membrane 5
R13W 13th retentate 13th sample after more 631.5g filtred
P13M1W 13th permeate 13th sample after 251.6g of permeate membrane 1
P13M5W 13th permeate 13th sample after 379.9g of permeate membrane 5
38
Degradation Products
Degradation components like ethanol, acetic acid, HMF, and furfural were analyzed since one
of the goals is to remove ethanol and impurities from the nanoparticle’s suspension. For these products’
analysis HPLC (High-Performance Liquid Chromatography) was used.
Figure 29 - Ethanol content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.
Figure 30 - Acetic Acid, HMF and furfural content (mg/L) of the retentate samples for the experiment 2 Membranes in Series.
Diafiltration Ultrafiltration
Diafiltration Ultrafiltration
39
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
𝑳)
Acid Acetic 1282.5
Ethanol 545250
HMF 4.4
2-Furaldehyde 34.1
For this experiment (2 Membranes in Series), it’s not possible to conclude how good is the
removal in relation to the HMF and furfural, since after the water addition these components do not
present any concentration value. Ethanol has a removal percentage of 99.8% and the acid acetic of
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).
40
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.
41
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.
42
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 = ∑ (𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖 × 𝑚𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒 𝑖)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(%) =∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠
26𝑖=1
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = 85,2%
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 = ∑ 𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠
26
𝑖=1
+ ∑ 𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 + 𝐷𝑀𝑅13𝑊
12
𝑗=1
Figure 33 – Dry Matter content (%) for all permeate samples for each membrane for the experiment 2 Membranes in Series.
43
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.
𝐷𝑀𝐼𝑛𝑖𝑡𝑖𝑎𝑙 𝑆𝑢𝑠𝑝𝑒𝑛𝑠𝑖𝑜𝑛 (𝒈) 7.39
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 (𝑔) 6.29
𝐷𝑀𝑠𝑎𝑚𝑝𝑙𝑒𝑠 (𝑔) 0.73
𝐷𝑀𝑅13𝑊 (𝑔) 0.12
𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 (𝑔) 0.25
𝑆𝑐𝑎𝑙𝑒 𝐸𝑟𝑟𝑜𝑟 (𝑔) 3.4
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
44
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
Figure 35 - Experiment 2 of Flux and Concentration Experiment, using membrane 1, 6 and 8 (1.2 L/min at 4 bar).
Experiment 2
45
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
46
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).
47
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).
48
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).
49
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).
50
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
Flux (𝑳
𝒎𝟐.𝒉)
Ultrafiltration Membrane 1 10.0
Diafiltration Membrane 1 18.1
Ultrafiltration Membrane 6 17.0
Diafiltration Membrane 6 24.3
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)
Membrane 1 14.3
Membrane 6 21.7
Membrane 10 35.5
Membrane 11 32.0
Membrane 12 85.9
Membrane 13 44.2
51
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).
52
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).
53
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
P4M1 4th permeate 4th sample after 157.5g of permeate membrane 1
P4M6 4th permeate 4th sample after 211.4g of permeate membrane 6
P4M11 4th permeate 4th sample after 400.3g of permeate membrane 11
R5W 5th retentate 5th sample after 1031.5g filtred
P5M1 5th permeate 5th sample after 223.6g of permeate membrane 1
P5M6 5th permeate 5th sample after 326.6g of permeate membrane 6
P5M11 5th permeate 5th sample after 481.3g of permeate membrane 11
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
54
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
P8M1W 8th permeate 8th sample after 178.7g of permeate membrane 1
P8M6W 8th permeate 8th sample after 222.9g of permeate membrane 6
P8M12W 8th permeate 8th sample after 451g of permeate membrane 12
R9W 9th retentate 9th sample after 781g filtred
P9M1W 9th permeate 9th sample after 181.5g of permeate membrane 1
P9M6W 9th permeate 9th sample after 199.4g of permeate membrane 6
P9M13W 9th permeate 9th sample after 400.1g of permeate membrane 13
R10W 10th retentate 10th sample after 972.9g filtred
P10M1W 10th permeate 10th sample after 224g of permeate membrane 1
P10M6W 10th permeate 10th sample after 278g of permeate membrane 6
P10M13W 10th permeate 10th sample after 470.9g of permeate membrane 13
Degradation Products
The method of High-Performance Liquid Chromatography was once more used for the
determination of the degradation products, like, ethanol, acetic acid, HMF, and furfural. Figure 44 and
Figure 45 show the results obtained for each product in each step of filtration.
Diafiltration Ultrafiltration
Figure 44 - Ethanol content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.
55
Figure 44 and Figure 45 show the acetic acid and ethanol content in the retentate samples. As
explained before, the goal is to reduce the amount of degrading compounds at the end. For all of them,
it is expected that at the end there is 10% of the initial concentration of the initial suspension. For each
filtration step it is not expected that the concentration of ethanol varies. During volume reduction the
ethanol concentration is constant and only drops when the water is added.
For permeate and retentate samples, it is expected that for all components the concentration
decreases after the water addition, which only occurs after sample number 5. As the total amount of
ethanol and acetic acid remain the same for a bigger volume, the ethanol concentration in the
suspension decreases. However, between sample 6 and 7 there is no difference in the system that can
explain such a high decrease of the ethanol content. One possible explanation is the evaporation of
ethanol and some errors associated with the analysis.
The retentate sample number R7W shows a sudden drop in the ethanol content. From that
sample on, the ethanol content is not measured except for samples R8W and R10W, which can be
explained by some errors in the equipment such as not flushing the samplings line sufficiently. Whereas
in the permeates samples only sample R9W has no value. However, the last measurement compared
with the ones before (R6W to R8W) presents a higher content of ethanol, which should be like the
samples before since its expected that the total amount of ethanol stays constant for each filtration step.
As for the experiment with 2 Membranes in Series, it is necessary to calculate the removal
percentage of the degradation products. Taking in account the same characteristics of the suspension
analyzed in the laboratory (Table 8), the values obtained for the ethanol and acetic acid are 97.9% and
87.7%. The deviation from the expected value (90-95%) can be answered by the errors that happened
during the measurements, as explained.
Diafiltration Ultrafiltration
Figure 45 - Acetic Acid content (mg/L) for all retentate samples for the experiment 3 Membranes in Series.
56
Particle Sizing
The particle size was measured again for every different retentate sample, the values obtained
for diluted samples vary between 138.2±3.3 nm and 212.7±11 nm. In this case, the difference between
the nanoparticle’s diameter is higher but is still reliable. This increase may be related to the longer time
interval between taking the sample and analyzing it.
Dry Matter Content
The Dry Matter method was repeated at the same conditions, and the results are shown in
Figure 47.
Figure 46 - Particle Size measurements for all samples for the experiment 3 Membranes in Series.
Figure 47 - Dry Matter content (%) for all retentate samples, before and after centrifuge for the experiment 3 Membranes in Series.
57
In this filtration, the method functioned without errors because the DM of dissolved components
present higher values than the DM of nanolignin particles. Considering the DM before centrifugation,
(nanolignin particles) an increase of the particles during each step of filtration is observed for UF and
DF. In the DF there is a drop in the DM of the nanolignin particles, which is expected due to the addition
of water. The dissolved components are show a slight increase during UF and DF, which means that
some of the dissolved components are being retained in the system.
Removal efficiency for Dry Matter method
To calculate the efficiency of the removal of dissolved components, the mass balance
calculation was repeated. The mass of the samples and the percentage of DM (𝑔𝑙𝑖𝑔𝑛𝑖𝑛+𝑑𝑖𝑠𝑠𝑜𝑙𝑣𝑒𝑑 𝑐𝑜𝑚𝑝𝑜𝑛𝑒𝑛𝑡𝑠
𝑔𝑠𝑜𝑙𝑢𝑡𝑖𝑜𝑛)
were once again used for those calculations. the mass balance was calculated based on equation 9,
with the slight difference that the number of samples is now higher, and the last remaining sample in the
tank is 𝐷𝑀𝑅10𝑊. In Table 17 is shown the value of each parcel of the equation for the mass balance
calculation.
Table 17 - DM amount (g) of different samples used for the mass balance of the filtration system for the experiment 3 Membranes in Series.
𝑫𝑴𝑺𝒖𝒑𝒆𝒓𝒏𝒂𝒕𝒂𝒏𝒕 𝒐𝒇 𝒊𝒏𝒊𝒕𝒊𝒂𝒍 𝒔𝒖𝒔𝒑𝒆𝒏𝒔𝒊𝒐𝒏 (𝒈) 7.14
𝐷𝑀𝑃𝑒𝑟𝑚𝑒𝑎𝑡𝑒𝑠 (𝑔) 6.68
𝐷𝑀𝑠𝑢𝑝𝑒𝑟𝑛𝑎𝑡𝑎𝑛𝑡 𝑠𝑎𝑚𝑝𝑙𝑒𝑠 (𝑔) 0.47
𝐷𝑀𝑃13𝑊 (𝑔) 0.24
𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 (𝑔) -0.26
𝑀𝑎𝑠𝑠 𝐵𝑎𝑙𝑎𝑛𝑐𝑒 𝑒𝑟𝑟𝑜𝑟 (%) 3.7
Once more the mass balance is made for the dissolved components and it does not close. This
error is negative, which means more DM is leaving the system than contained in the initial suspension.
For the removal efficiency of dissolved components, equation 10, the DM of the permeates was used to
get more accurate values. In Figure 48 is shown how the Dry Matter content varies in the whole filtration
process.
58
All the membranes show an increase in DM during ultrafiltration. However, this increase is not
so pronounced during DF. The retention of DM is decreasing in both filtration modes, which can be due
to an increase of DM content in the retentate.
When using equation 10, the removal efficiency of dissolved components from the initial
nanolignin particle suspension was 93.6%. Comparing with the 85.2% obtained in the experiment with
2 membranes, the removal efficiency now is higher (using 3 membranes), which means it is even closer
to the goal.
Membrane Regeneration
Membrane 1 was regenerated ten times, and in Table 18 is shown in each step when the
regeneration happened.
Table 18 - Regeneration steps for membrane 1.
Regeneration Step of Regeneration
1st Middle of ultrafiltration step in the filtration with 2 membranes in series
2nd End of ultrafiltration step in the filtration with 2 membranes in series
3rd Middle of diafiltration step in the filtration with 2 membranes in series
4th After membrane filtration with 2 membranes in series
5th Between Experiment 1 and 2 of Flux and Concentration experiment
6th Between Experiment 2 and 3 of Flux and Concentration experiment
7th Before filtration with 3 membranes in series
8th Middle of ultrafiltration step in the filtration with 3 membranes in series
9th End of ultrafiltration step in the filtration with 3 membranes in series
10th Middle of diafiltration step in the filtration with 3 membranes in series
Figure 48 - DryMatter content (%) for all permeate samples for the experiment 3 Membranes in Series.
59
As explained in Material and Methods, after each experiment, membrane 1 was cleaned with a
15wt% ethanol solution to be regenerated. Samples were taken to analyze in the UV-spectrometer,
when the value was constant, the regeneration reached an end, which means that it is not possible to
remove more lignin from the membrane. The amount of lignin removed in each regeneration steps is
shown in Table 19.
Table 19 - Lignin removed from membrane 1 in each step.
Figure 49 shows the mean TF of each flush after membrane regeneration. From the figure, it is
perceived that until the fourth regeneration (referring to the first experiment with 2 membranes in series)
there is a decrease in the transmembrane flux. The flux and concentration experiment was the one that
revealed to have quite a significant change in membrane 1. After this, the transmembrane flux remains
Regeneration
Step
UV
Lignin removed from
the membrane (mg)
1st 0.041 4.3
2nd 0.054 5.5
3rd 0.028 3.1
4th 0.02 2.3
5th 0.032 3.5
6th 0.04 4.3
7th 0.029 3.2
8th 0.042 4.5
9th 0.029 3.2
10th 0.025 2.9
Figure 49 - Mean transmembrane flux for each regeneration step with 15 %wt ethanol solution.
60
approximately constant, so it is difficult to conclude whether the membrane was affected by this
intermediate experiment or if the membrane reached its stabilization and conditioning characteristics.
To better understand how each regeneration affects the membrane, the mean flux value after
regeneration was compared to the initial value of TF. The comparison is shown in Figure 50 and the
percentage of membrane recovery in Table 20.
Table 20 - Membrane recovery (%) for each regeneration step.
Regeneration
Step
1st 2nd 3rd 4th 5th 6th 7th 8th 9th 10th
% Membrane
Recovery
66.0 66.3 57.7 54.3 20.8 21.4 18.9 22.0 18.2 22.3
Comparing the average flow value of the last filtering hour before its regeneration (Before
Regeneration) with the mean flux value after regeneration (After Regeneration), the third, fourth and
tenth regenerations present the lowest cleaning efficacy. These three regenerations take place after the
diafiltration steps, where the flux decline is low and consequently the regeneration is less effective.
Therefore, it will be necessary to make a cost analysis in order to confirm if the regeneration of the
membranes in the diafiltration step is worthwhile.
Regarding the membrane recovery, the TF after each regeneration was compared to the initial
TF of membrane 1. The highest recovery obtained was approximately 66% of the initial capacity of the
membrane, and the lowest recovery was 18%. It is also significant to mention that the membrane
recovery is roughly constant, although after the fourth regeneration the recovery percentage decreases
significantly. This drop was mentioned previously, and it was due to an experiment that affected the
membrane or the membrane that reached its stabilization and conditioning characteristics. Finally, the
Figure 50 - Initial TF and TF before and after regeneration of membrane 1.
61
working time and degeneration of the membranes were calculated, these values are shown in Table 21,
21 and 22.
Table 21 - Characteristics of membrane 1.
Initial Flux (𝑳
𝒎𝟐.𝒉) 89.4
Final Flux (𝑳
𝒎𝟐.𝒉) 13.9
Work time (min) 3615.3
Work time (h) 60.3
% Reduction 84.4
Flux decrease per hour (𝑳
𝒎𝟐.𝒉𝟐) 1.25
Table 22 - Characteristics of membranes used in experiment 2 Membranes in Series.
Table 23 - Characteristics of membranes used in experiment 3 Membranes in Series.
Membrane 2 Membrane 3 Membrane 4 Membrane 5
Initial Flux (𝑳
𝒎𝟐.𝒉) 82.3 98.4 98.3 91.5
Final Flux (𝑳
𝒎𝟐.𝒉) 60.3 59.4 96.5 73.1
Work time (min) 320.2 253.5 141.3 146.8
Work time (h) 5.3 4.2 2.4 2.4
% Reduction 26.7 39.6 1.8 20.1
Flux decrease per
hour (𝑳
𝒎𝟐.𝒉𝟐)
4.1
9.2
0.8
7.5
Membrane 6 Membrane 10 Membrane 11 Membrane 12 Membrane 13
Initial Flux (𝑳
𝒎𝟐.𝒉) 225.8 335.3 368.6 418.6 134.7
Final Flux (𝑳
𝒎𝟐.𝒉) 16.7 34.0 70.6 61.4 46.6
Work time (min) 2022.7 368.8 302.2 194.0 158.9
Work time (h) 33.7 6.1 5.0 3.2 2.6
% Reduction 92.6 89.9 80.8 85.3 65.4
Flux decrease per
hour (𝑳
𝒎𝟐.𝒉𝟐)
6.2
49.0
59.2
110.5
33.3
62
63
Conclusions
The aim of this work was to evaluate the performance and the potential of regeneration of
membrane performance during ultrafiltration of nangolignin suspensions in diafiltration mode. For this
study only one type of straw was used (wheat straw), and as the literature explains, the results are
dependent on the type of feedstock, as well as the pretreatment used (ethanol Organosolv). The
precipitation method also influences the type of particles that are obtained, specific shapes and particle
sizes. For the precipitation, water was used as an anti-solvent at an exact volume ratio.
This study allowed the successful analysis of the variation of the flux over time, which allowed
the understanding of how the membranes are affected by the suspension used. To implement
successfully a membrane filtration setup, it was important to select optimal operation conditions (1.2
L/min and 4 bar) at room temperature and use an ideal solvent to clean the setup (50wt% acetone and
15wt% ethanol).
Regarding the viability of this procedure, it was imperative that the particle size remained
constant. The particle size for both experiments showed that the nanoparticles of lignin were
approximately constant over time, which confirms that there is no agglomeration of the particles through
the process. However, the time between the sample collection and the measurements should be shorter
in order to minimize the possibility of errors. An additional goal was to remove the ethanol and the
impurities from the nanoparticle’s suspension, which was analyzed using HPLC. The results showed a
removal of ethanol of approximately 97% and acetic acid of approximately 87%. For the HMF and
Furfural there were no valid values. Some errors in the measurements of the initial suspension occurred,
therefore the values used for the suspension correspond to another experiment made in the laboratory.
To evaluate the performance of the ultrafiltration it was necessary to appraise the percentage
of the solute that was retained by the membranes. The removal efficiency of dissolved components was
calculated based on the Dry Matter content (measured in the permeate, the feed/retentate is depleted
in these dissolved components). Both experiments showed a high value of efficiency, close to the
desired value (90-95%). However, the experiment where 3 membranes were used in series showed
higher efficiency (93.6%) than when using 2 membranes (85.2%), which means that the more
membranes used, the less the components are retained by the membranes.
Fouling is one of the main aspects when it comes to ultrafiltration, and a consequence of it, is
the decline of the membrane flux. On the other hand, the influence of the concentration of the suspension
of nanoparticles was also studied in order to understand if this was also a factor in the decrease of the
flux. The results showed that the flux decreases due to the increase of the concentration of the
suspension, which means that the flux decline occurs mainly because of the accumulation of the
particles deposited on the membrane surface and the increase of the suspension of nanoparticles
concentration. Concerning the fouling, one of the membranes (membrane 1) was chosen to be
regenerated, in order to elaborate a “long-term” stability and a performance comparation with new
membranes. The results showed that the regeneration after the diafiltration step is less effective when
compared to the values of regeneration after concentration mode. The regeneration not only removes
Nanolignin particles from the membrane surface, but also the dissolved components that could block
the membrane. These dissolved components are higher in UF and lower in DF steps.
64
For further work, membrane 1 should keep being used in other filtrations in order to understand
if the membrane reached its stabilization and conditioning characteristics, since after the Flux and
Concentration Experiment this membrane does not show a significant variance in its behavior. In
addition to that, due to the divergent capacities of the membranes, it is essential to repeat the flushing
quite a few times with different membrane materials, to reach statistically relevant values.
Being that fouling is one of the biggest problems when it comes to ultrafiltration, it’s important to
focus on its improvement. It is essential to find the best cleaning method to clear the cross-flow system,
ensuring that there are no particles left in the system. The membrane regeneration method should also
be optimized. Several experiments should be realized using different operating conditions in order to
determine the most promising results to regenerate a membrane. Another aspect that should be worked
in the future, is to check if a different technique to store the membrane will affect its capacity. Since the
membrane is regenerated using a solution of ethanol, it should be tested if this being stored in a solution
of water instead of being stored in ethanol affects the membrane.
65
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Appendix
The temperature of the product inside the reactor and the pressure was measured for all the
experiments. Figure 51 describes the temperature and pressure versus time of one of the extractions
done in this work.
Figure 51 - Product temperature (°C) and pressure (bar) for one extract production experiment.
Figure 52 - Membranes used in the experiment 2 Membranes in Series: (a) Membrane 1; (b) Membrane 2; (c) Membrane 3; (d) Membrane 4; (e) Membrane 5.
(a) (b) (c) (d) (e)
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Figure 53 – (a) Membrane 1 (regenerated) at the end, after being used in all the experiments; (b) Membrane 6 (without regeneration) at the end, after being used in experiments Flux and
Concentration and 3 Membranes in Series
Figure 54 - Membranes used in the experiment Flux and Concentration:
(a) Membrane 3; (b) Membrane 4; (c) Membrane 5.
(a) (b) (c)
(a) (b)
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Figure 55 - Membranes used in the experiment 3 Membranes in Series: (a) Membrane 6; (b) Membrane 7; (c) Membrane 8; (d) Membrane 9.
(a) (b) (c) (d)