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
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.
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.
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.
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
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
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
Qa− Feed Flow Qp− Permeate Flow Qr− Retentate Flow
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
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
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
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
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
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.
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).
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).
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).
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).
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).
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.
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).
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)
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)
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).
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)
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.
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).
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).
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.
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.
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)
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.
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.
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
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);
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.
