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Chapter 2
Twin-Screw Extrusion: A Key Technology for the Biorefinery P. Evon, V. Vandenbossche, L. Candy, P.-Y. Pontalier, and A. Rouilly* Laboratoire de Chimie Agro-industrielle, LCA, Université de Toulouse, INRA, Toulouse, France 31030 *E-mail:
[email protected].
For more than 30 years, the Laboratory of Agro-industrial Chemistry (LCA) develops an ambitious and multi-scale research topic on the use of twin-screw extrusion (TSE) for the processing of biomass for non-food applications. This chapter will give an overview of past and present projects, discussing specific operating conditions and their consequences on biopolymer native organization. For the production of agro-materials, compounding processes have been designed and in some cases industrialized integrating specific targeted actions such as the plasticization of primary cell-walls (sugar beet, tobacco), the “fusion” of storage polymers (starch, oilseed proteins) and/or the destructuring of secondary cell-walls (lignocellulosic fibers). For the pretreatment of lignocellulosic fibers, the conjugated use of chemicals is also discussed. Those processes have also been coupled with biodegradable polyester blending (involving compatibilization with acid citric) and compounding. In integrated biorefining processes, TSE may also be used simultaneously as a continuous liquid-solid extractor through mechanical pressing or solvent extraction, for extracting oil, polysaccharides, proteins, polyphenols or hydroxycinnamic acids and as a pre-treatment of the fibrous raffinate. This is especially efficient for the processing of oilseed crops and the production of binderless fiberboards or to prepare technical fibers for composite applications. This has been widely demonstrated on sunflower, jatropha or more recently coriander. Finally, in the bioenergy field, a specific pretreatment process for the production of bioethanol from © 2018 American Chemical Society Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
lignocellulosic feedstock has been developed and is actually in the up-scaling phase. Integrating the use of enzymes in a one-step TSE, this process has been called “bioextrusion”.
Introduction Since the end of the 19th century with the apparition of the first counterrotating twin-screw extruders, this highly versatile technology never ceased to improve and to widen its scope of applications. Created for the rubber and plastic industry, extruders have then been used successively in the food sector (extrusioncooking), the paper industry (continuous liquid/solid extractor) and is even used nowadays in care and cosmetic applications. Continuous technology allowing to adjust temperature and mechanical energy while using chemicals, it can be considered as a thermo-mechano-chemical reactor for heterogeneous treatment involving solid/liquid transformation. It is then totally adapted for the processing of biomass and as it can also involve some liquid/solid separation to its biorefinery. That is the way chosen 30 years ago by Dr Luc Rigal, a French State Research Engineer, that started a specific research topic on this technology in the Laboratory of Agro-industrial Chemistry (LCA) and to whom this chapter is dedicated. Five twin-screw extruders later, after many research projects (local, national and international), PhDs, industrial contracts, patents and publications, this chapter intend to describe the various ways of using such an amazing tool for the treatment of agro-industrial by-products for non-food application. Three subtopics will be addressed: destructuring processes leading to thermoplastic materials, continuous liquid/solid extractions of biopolymers and biomolecules and pre-treatment of lignocellulosic materials.
Destructuring/Compounding Processes of Agricultural Products and Agro-Industrial By-Products The first type of process concerns the disassembling of native natural structures to produce bio-based thermoplastic materials that can be used for material applications (i.e. disposable objects) but as densification and forming process for other applications as well. Extrusion-cooking of starch is the first and well-known example (1) and according to the moisture content during the process it can be adapted to produce thermoplastic starch (TPS) from starch-containing agricultural product (2). It is a perfect example of the thermo-mechanical transformation of native starch granule organization into a thermoplastic amorphous phase. In the literature twin-screw is mostly applied to pure starch while it is possible to process complex starch-based cereal products, to adapt this type of process to globular protein and eventually to destructure and plasticize more complex structure than storage corpuscles such as primary cell walls.
26 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
1. Compounding of Complex Starch-Based Cereal Products Our laboratory has a long experience on the compounding of raw cereal products (i.e. whole corn plant, wheat grain, wheat flour) to produce thermoplastic materials suitable for an injection-molding final forming. This research work started with the design of a twin-screw extrusion (TSE) process to plasticize whole corn plant (3) giving birth in the same time to a company that is still specialized in the compounding and injection-molding of biodegradable starch-based materials: VEGEPLAST©. For this process, many operating units are involved: mixing with water and/ or an organic plasticizer, starch fusion, stem defibration, additives incorporation, homogenization and forming in the die (Figure 1). This utilization of all parts of the plant presents many advantages: increase of the mechanical properties of the injected material, retrogradation in a lower extent and obviously decrease of the price.
Figure 1. Schematic twin-screw extrusion process for the compounding of starch-based thermoplastic from whole corn plant. Following this, the process has been improved and scaled up. One of the major improvement has been found by addition of polylactic acid (PLA) in the mixture. Using citric acid as compatibilizing agent it has been shown that a small amount of PLA (25% w/w) allowed to increase the mechanical (4) and barrier (5) properties of the composite thanks to a better repartition of the PLA in the starchy phase. In this case, during the TSE process, an online vacuum pump is necessary to dehydrate the compound before the melting of the polyester and the die. As final improvement, the controlled incorporation of natural fibers from miscanthus and bamboo inside poly(lactic acid) based biocomposites was also considered through twin-screw extrusion to improve their mechanical and thermal properties (6).
2. Compounding of Sunflower Oil-Cake Local crop, sunflower has been widely studied in our laboratory, and has obviously been processed by TSE. Liquid/solid extractions of oil and proteic fraction have been tested and patented but that will be discussed later in this chapter. The oil cake has also been processed into a thermoplastic composite and 27 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
formed either directly at the die into pellets as soil enrichment (7) or through injection-molding into biodegradable planting-out flower pots (8). Industrial sunflower oil extraction produces a by-product, the sunflower oilcake (SFOC), that does not have much applications in cattle feeding like other oil cakes because of a deficit in some essential amino acids. Schematically SFOC can be considered as a complex material made of three main fractions: some lignocellulosic fibers from the husk, some storage proteins (11S globulins) and some aggregates made of other proteins and polyphenolic compounds (9). Or when extracted, sunflower proteins are known for their “thermoplastic” behavior (10) and interesting mechanical properties when thermopressed (11, 12). The idea was then to set a one-step TSE process to compound SFOC to (1) denature the storage proteins (and/or the aggregates) to form an amorphous phase and (2) defibrate the lignocellulosic fibers from the husk. That was done with a very simple screw profile: one crushing/mixing zone equipped with monolobe paddles and one high shear zone equipped with a reverse screw to perform both denaturation and defibration, at 100°C and 25% w/w of moisture content (8). In a first step, using only water as plasticizer, the use of a die was impossible because of too high fiber to matrix ratio (7/3) and because of the presence of disulfide bridges in the proteic matrix disturbing the disentanglement of the polypeptides. The use of a reducing agent was then necessary to improve the rheology of the proteic matrix (8, 10) and to allow the forming of pellets at the end of the extruder (7). Protein-based agro-materials are interesting when compared to starch-based materials because of the amphiphilic nature of proteins that gives better moisture resistance and encapsulating properties (13) to the material and a high nitrogen content that is favorable for soil enrichment during biodegradation. 3. TSE Destructuring Process of Sugar Beet Pulp Sugar beet pulp (SBP) is the main by-products of beet sugar industry. Composed of primary cells emptied from their content during the hot water extraction, its components are mainly those of the cell-walls, i.e. polysaccharides: cellulose (≈25% w/w), hemicelluloses (≈25% w/w) and pectins (≈25% w/w). As it is a cheap raw material without many applications, we proposed a complete study for its use as thermoplastic matrix of biodegradable materials. To get thermoplastic properties, the native organization of the cell walls has to be destructured to free the biopolymers from their interactions and obtain a mixture of amorphous polysaccharides that can then be processed through conventional technologies such as injection-molding and extrusion. In a first paper, the complete extrusion study was described (14). First, various screw elements were tested: mono- or bi-lobes mixing paddles, double-thread or single-thread reverse screws to select the most performing element. The variable chosen to evaluate was the water soluble content after TSE process. Then for one chosen screw configuration, the liquid/solid ratio was decreased to evaluate the changes in the destructuring process and it is particularly obvious in this case. The best properties were obtained for L/S=0.31, leading to a maximum temperature of 123°C but involving a very high specific mechanical energy (the mechanical energy supplied by the extruder to the material) of 745.3 Wh/Kg. The complete 28 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
transformation of SBP into a thermoplastic material was then possible in one pass but in the upper limit of the extruder used. To improve that and decrease the SME by 43% it is possible to use a specific single-thread grooved screw element just in front of the reverse screw that allows to increase the residence time by 40%. Following this study, the plasticized SBP has been totally characterized for its thermoplastic properties (15) and film produced by extrusion/calendering with various plasticizers were finally prepared and tested (16).
4. Summary Almost all vegetable structures can be destructured by TSE. According to the type of structure, the nature of the biopolymers involved and the purpose of the process (extraction or compounding), operating conditions and energy involved can change (Table 1). The “melting” of storage biopolymers is achieved with low moisture content, medium temperature and requires a low mechanical energy. For cell-walls, the primary ones necessitate low moisture content, average temperature and a high mechanical energy while for the secondary ones, higher liquid/solid ratio and temperature have to be used and even in these conditions if no chemicals are used the mechanical energy is still high (around 400-500 Wh/Kg). Cellulose fibers themselves can be processed into nanofibrillated cellulose fibers (NCF) by TSE (17) but that necessitates a lot of water and a lot of energy (up to 7 successive passes). Nevertheless, from all known processing technologies, TSE is the less energy consuming to produce NCF.
Table 1. Summary of the operating conditions (Type of extruder, Liquid/solid ratio (L/S), Temperature and Specific mechanical energy (SME)) for the destructuring of vegetable structures. Values for NFC production according to ref. (17). Process
Extruder
L/S
T(°C)
SME (Wh/Kg)
Sugar beet pulp
Destructuring Compounding
BC45
0.3
125
745 (421 C1FR)
Tobacco leaves
Destructuring Compounding
EV53
0.2
90
170
Destructuring
BC45
0.6
120
380-500
Destructuring Chemical (NaOH)
BC45
0.7
150
250
Primary cell walls
Secondary cell walls Wheat straw
Storage polymer fusion Continued on next page.
29 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Table 1. (Continued). Summary of the operating conditions (Type of extruder, Liquid/solid ratio (L/S), Temperature and Specific mechanical energy (SME)) for the destructuring of vegetable structures. Values for NFC production according to ref. (17). Process
Extruder
L/S
T(°C)
SME (Wh/Kg)
Sunflower oil cake
Melting Destructuring
BC45
0.3
100
277
Wheat flour
Melting Compounding Glycerol
EV53
0.1
125
250
Wheat flour
Melting Compounding Water
EV53
0.23
110
180
Paper pulp
Destructuring
Haake 7passes
5
10
10000
Paper pulp (enzymatic pre-treatment)
Destructuring
Haake 7passes
5
10
5500
Cellulose
Twin-Screw Extruder Used as a Continuous Liquid/Solid Extractor Recently, TSE technology has seen its scope expanded to the field of thermo-mechano-chemical fractionation of plant material (18, 19). The use of an equipment operating continuously and capable of several elementary operations into a single step allows the intensification of fractionation processes as well as a better co-valuation of the obtained fractions. Conducted for nearly twenty-five years in the LCA, this new concept has allowed the development of real reactors capable of transforming or fractionating physically and chemically the plant material in a single step (20). Due to the combination of mechanical (adjustment of screw profiles), thermal (thermal regulation of the barrel and viscous dissipation) and eventually chemical (injection points of liquid reagents) actions in a single step, the twin-screw extruder is considered as a thermo-mechano-chemical (TMC) reactor. It operates continuously and, when being provided with one or more filtration modules, it allows the individual collection of a liquid extract and a solid raffinate (6, 7) (Figure 2).
30 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
Figure 2. Schematic representation of a twin-screw extruder used as a liquid/solid extractor and separator. Several LCA’s studies have highlighted the multiple possibilities of the TSE technology for both fractionation and valorization of agricultural resources: -
-
The mechanical pressing and/or the solvent extraction of different vegetable oils (21), especially from sunflower (22–27), neem (28), jatropha (29) and coriander (30) oleaginous seeds, The extraction of hemicelluloses from lignocellulosic plant materials (31–34), The extraction of pectic substances from the pith of sunflower stalk (35) or sugar beet pulp (36), The extraction of proteic isolate from sunflower meal (37) or proteins from alfalfa (38, 39), The extraction of polyphenolic extracts with antioxidant activity (40) or hydroxycinnamic acids (41).
1. Mechanical Pressing of Oleaginous Seeds Looking more specifically at the recovery of vegetable oil from oleaginous raw materials, the latest research consisted in using the twin-screw reactor for both the expression (26, 28–30) and/or the extraction (25, 26, 28, 42, 43) of oil, in the same apparatus (26). For extraction, the solvents chosen were mainly water, ethanol or water/ethanol mixtures rather than apolar liquids (25, 26, 28, 42, 43). The raw materials were oilseeds of various origins (29, 30) or even whole plants (43). Four different oilseeds were used for the expression of oils by mechanical pressing, that is to say without adding solvent: sunflower (Helianthus annus L.) (26), neem (Azadirachta indica A. Jussi) (28), jatropha (Jatropha curcas) (29) and coriander (Coriandrum sativum) (30). The oil expression requires the use of an adapted screw profile defined by four successive functional areas: -
A feeding zone: with trapezoidal (T2F) and then conjugated (C2F) double-thread conveying screws. A crushing zone: the oilseed crushing is performed by an assembly of monolobe (BL20 or BL02) and/or bilobe (BL22) paddle-screws, which both provide strong shearing and mixing effects on the material. The raw material heating is also carried out in this functional area. 31
Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
-
-
A filtration zone: the filter is most often placed at the level of the penultimate module. The elements in that zone are conveying screws having a decreasing pitch, to ensure an axial pressure of the solid from the pressing zone to the filtration one. The oil is thus expressed and flows freely by gravity at the filter. The filtering sieves are permanently unclogged thanks to the rotation of the screws that scrape their surface. A pressing zone: the pressing takes place in the last module where the liquid/solid separation occurs. For that, double-thread (CF2C) or simplethread (CF1C) reverse screws are used. This ensures the formation of a dynamic plug of biomass against which the conveyed material comes to compress, and the oily liquid can then express freely. The residual solid material or meal is extruded through the reversed screws and conveyed towards the outlet. The oil flow is responsible for the entrainment of a solid particles fraction through the holes of the filter. These particles are separated by centrifugation and named “filtrate’s foot”
The conditions of pressing were optimized to reach the highest efficiency, and simultaneously to provide oils of good quality. Various parameters have been studied. The first one was an adaptation of the screw profile, comprising two separate working areas, i.e. the crushing zone and then the pressing one. For the crushing, the objective was to reduce as much as possible the size of the solid particles, in order to facilitate the cell lysis and thus the oil release but without compromising the subsequent liquid/solid separation. Two successive levels of destructuring occur. The cotyledon cell walls are firstly broken thus promoting the release of the cytoplasmic components, in particular the lipid droplets within which the oil is located. Then, it is the lipid droplet membranes that break, allowing the oily liquid to flow out of the solid. The choice of the most suitable grinding zone has been studied for jatropha seeds (29). In addition to a first series of monolobe paddle-screws, one or two additional series of bilobe paddles were used. When two successive series of BL22 paddles were used, the reduction in the size of solid particles was so high that it led to a large amount of filtrate’s foot (56-65%). Conversely, the foot content was significantly lowered with just one serie of bilobe paddles (up to 31%), and this resulted in an important increase in the oil yield (up to 71%). Optimizing the pressing area corresponds to optimizing the quality of the liquid/solid separation. The positioning of the reverse screws with respect to the filtering grid is particularly important. -
-
When reverse screws are too close to the filtration module, more solid particles are driven through the filter. The foot is then more difficult to eliminate by centrifugation. A reduction in the diameter of holes of the filtering grid will limit this drawback to some extent. Conversely, when the distance between the end of the filter screen and the beginning of reverse screws is too high, the oil already expressed but which accumulates in this zone cannot flow freely, leading to an oil yield reduction (30). 32
Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
It is therefore necessary to adapt the position of reverse screws relative to the filter. Moreover, their length and pitch also influence the quality of the liquid/solid separation, the increase of the first and the decrease of the second both contributing to a better efficiency of the separation (22, 23, 26, 29, 30). However, for coriander, reverse screws with a too little pitch (-25 mm) were too restrictive. They led to the systematic clogging of the twin-screw machine, even for the lowest fillings (30). Therefore, it was necessary to adapt the pitch of these reverse screws to both twin-screw extruders used (engine torque limit) and the treated oleaginous material. Other parameters that should be considered are the filling of the twin-screw reactor, the pressing temperature and the oilseed moisture (29, 30). Moreover as physical (seed size, presence or absence of a hull, hardness, rigidity) and chemical (lipid content, lipid/fiber ratio) characteristics of oilseeds can be very different, it is necessary to optimize the parameters for each raw material. These operating conditions have been optimized for sunflower (26), jatropha (29) and coriander (30) seeds. Looking at the production cost of jatropha oil, it was only 314 W h/kg expressed oil. For comparison, from a single-screw press of equivalent capacity, it was significantly higher i.e. about 1.6 kW h/kg expressed oil (44). Compared to single-screw presses, TSE has many advantages in extracting vegetable oil from oleaginous seeds by mechanical pressing. It is possible to mention a significantly improved ability to transport the oleaginous material, a better aptitude for mixing and grinding, especially in the interpenetration zone of the two screws, leading to a much better mechanical cell lysis, and the production of an oil at a lower cost. For that study, the twin-screw technology enabled also the production of jatropha oil of sufficient quality for the subsequent production of biodiesel (29). The effects of both the increase in the extruder filling and the decrease in the pressing temperature on oil extraction were confirmed in a more recent study conducted from coriander fruits (30). In addition, a reduction of the fruit moisture content from 9.8 to 0.3% was also considered. This reduction had a positive effect on the oil release, the residual oil content of the cake decreasing progressively from 16.5% to 11.3%. This was due to the increased rigidity of the pre-dried fruit, which led to an intensification of the friction inside the barrel, especially in the pressing zone. The highest oil yield (58%) was obtained from a median moisture content. Indeed, because the filtrate’s foot content significantly increased as the seed moisture content was reduced (up to 22% for a 0.3% moisture content), the isolation of expressed oil became delicate for the lowest humidity values, part of it being entrapped between the solid particles constituting the centrifugation pellet. 2. Mechanical Pressing of Green Plants Forage plants are grown for their protein-rich vegetative parts and are used, directly or after heat treatment, mainly for animal feed. Alfalfa (Medicago sativa) is the most widely grown forage plant in the world, used by the green crop drying industry for the production of stabilized pellets. During pellet production, the green crop was usually chopped and pressed prior to drying. The juice was only regarded as fertilizer even if it was an effective source of highly valuable proteins. 33 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
A TSE process was studied as a new approach for the pre-dehydration of the plant while producing a green filtrate rich in proteins (38, 39). The best results were obtained with more than 50% of the global proteins recovery in the extract, and the production of a residue with less than 50% of water (with an initial dry matter of 24%). As the fibrous residue still contained more than 60% water, it can be further valorized for animal feeding after a second dehydration step. The most efficient profile led to two optimal conditions for plant dehydration. The first one was high temperature and low liquid/solid ratio and the second one low temperature and high liquid/solid ratio. When water is introduced with the alfalfa in the TSE, the cell lysis is more efficient because of an improvement in the transmission of the mechanical effect through a better shearing. Thus, the protein recovery is higher even if under such conditions the temperature is too high, the viscosity of the mixture too low and the compression becomes inefficient, leading to a higher water content in the residue. When the temperature is high it is necessary to extract a lower extent of protein that are lubricating the system. This condition is obtained without water addition in the TSE, where mainly the interstitial water is recovered from the plant. The optimum for protein recovery and plant dehydration was obtained at 23°C, with a liquid/solid ratio of 8, and led to a protein recovery of 59% and a residue with 54% DW (Figure 3). The structure of the plant is crucial for the treatment efficiency. Hence, the harvesting period has a significant effect, particularly for alfalfa because of a water content change but also because the fiber content is drastically modified. The treatment is then less efficient with the first cut, when the plant contains a small quantity of cellulose, than with the second one, but the TSE screw profile can be adapted to this change, and can produce an almost constant residue. This result indicates that TSE is an efficient tool for green plant fractionation, allowing the recovery of molecules from the plant cytoplasm while producing a cellulosic residue that can be used for cellulose, hemicelluloses and lignin recovery.
Figure 3. Isoresponse curves of L/S ratio and temperature influence on the dry weight (DW) in the residue (left) and protein recovery yield (right) during Alfalfa pressing.
34 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
3. Solvent Extraction Because of their physico-chemical composition, not all biomasses are prone to be directly extracted by mechanical pressing. Additionally, the extraction often seeks to obtain a specific molecular fraction of the raw material. Introducing an appropriate liquid phase is then obvious to reach the desired extracts. In this context, twin-screw extrusion can then be forecast as a promising tool for the solid/liquid extraction of the biomass. Compared to the disassembling/ compounding that essentially aims at modifying the cell walls, the target of the solid/liquid extraction is, most of the time, the cellular content. From a general point of view, all solid/liquid extraction’s processes of biomass involve 3 steps: -
the solvent diffusion in the biomass matrix, the interaction between the molecules of the solvent and the molecules of the biomass that have an affinity for the chosen solvent, the diffusion of the solvent concentrated in extracted molecules from the biomass matrix to its outside.
The yields and selectivities of each extraction process are then subjected to kinetics and diffusional limitations. Let’s notice that in extrusion case, the extraction also strongly modifies the cell walls because of the mechanical impact of the screw profile. The modeling of the extraction tends to be more complicated. As all extraction processes, TSE is affected by several factors: thermal, mechanical and chemical. The temperature profile has of course a strong influence on the kinetics of extraction, the polarity of the solvent and its diffusivity in the solid biomass matrix. It also has an impact on the cell wall degradation and increases the thermal agitation of the molecules. The mechanical treatment (screw profile) is responsible for the cell wall denaturing and promotes the solvent diffusion. Finally, the chemical influence is represented by the affinity between the solvent and the molecules that should be extracted. The polarity and the possibility to form weak interactions are then essential to the extraction process. The following examples will illustrate our purpose. In addition to mechanical pressing, vegetable oil recovery could be also conducted using water as extracting solvent. This was optimized for sunflower oil from seeds (25), oily cakes (26) or the whole plant (43). The screw profile and the operating conditions: device’s filling, temperature and water/solid ratio were investigated. Using water as extracting solvent instead of apolar liquids led to a continuous eco-friendly process. Furthermore, water/solid ratios were particularly limited compared to conventional batch processes. Nevertheless, water addition led to a decrease in the consistency of the liquid/solid mixture in the pressing zone. The separation became delicate for seeds and oily cakes. To improve the compression and obtain a valuable extract, it was necessary to add fibers upstream from the filter (25, 26). Two agricultural residues were successfully tested i.e. wheat straw and sunflower depithed stalk. Conversely, for the whole plant, its fibrous fraction coming from the stalk and the head allowed 35 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
a satisfactory liquid/solid separation without adding any texturing additive. The TSE process was thereby facilitated. For seeds, the highest oil yield was 55% of the initial lipids, corresponding to a residual lipid content in the cake of 33% instead of 50% for seeds (25). The oil was extracted as a particularly stable oil-in-water emulsion. Its stability was ensured by co-extracted natural surfactants at the interface, i.e. phospholipids and proteins (especially albumins). Its demixing was feasible by alcohol extraction, ethanol having both a denaturing action on proteins and a solvation one on lipids. Extracted oil was then isolated, and proteins with promising emulsifying properties were also produced. The oil-in-water emulsion revealed also a potential interest for the cosmetics industry. Nonetheless, the effectiveness of the aqueous extraction of sunflower oil from seeds was limited, due to the insufficient cell lysis but also to an incomplete liquid/solid extraction. Because the oil recovery was more efficient by mechanical pressing (efficiency close to 70%), a two-stage process, i.e. mechanical pressing followed by aqueous extraction of the residual lipids was tested. It improved the overall oil extraction yield. These two stages were implemented in two successive devices or even in the same twin-screw reactor (26). In this study, the highest overall oil yield of 78% was obtained when using the same machine for the two stages. However, the contribution of the aqueous extraction stage was limited, representing never more than 5% of the lipids from seeds. The oil-in-water emulsion thus generated had also a limited stability over time. Consequently, a new substrate, i.e. the whole sunflower plant, has then been tested in a single aqueous extraction stage (43). Naturally rich in fibers, the whole plant is a promising raw material for the aqueous extraction of sunflower lipids. During the TSE process, the separated production of a liquid extract and a solid raffinate was possible without adding any fibrous residue upstream from the filter (43). The influence of the screw profile on the liquid/solid separation was investigated and a modeling of the TSE filling has also been proposed. From the optimized conditions, oil and protein yields were 65% and 55%, respectively. The extract was then treated. Four distinct phases were generated (from the least to the most dense): the upper hydrophobic phase, the hydrophilic phase (dilute phase containing both proteins and pectic substances, to be recycled to the process), the lower hydrophobic phase and the filtrate’s foot (to be added to the cake). Using an optical microscope, both hydrophobic phases appeared as oil-in-water emulsions, with phospholipids and proteins acting as surfactants at the interface. The lower emulsion contained also pectic substances inside the continuous aqueous phase, making it more viscous and more stable over time. Both oil-in-water emulsions would be usable in the cosmetics industry. In particular, the lower one could be used as a coemulsifier for the formulation of cosmetic creams (43). Regarding the upper one, it would be usable for the waterproofing treatment of agro-materials or for the production of “white” tensioactive proteins after demixing. The cake was used for the prodution of binderless fiberboards using hot pressing, its residual proteins being used as natural binder. Thanks to the mechanical shear applied in the pressing zone, these amorphous polymers revealed a thermoplastic behavior (45). Thus, 36 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
thermopressing allowed to produce cohesive materials without adding glue (46, 47). When using high conditions (49 MPa pressure, 300 s time, and 204 °C temperature), boards largely complied with French standard NF EN 312, type P4 (i.e. load bearing boards for use in dry conditions) (46). On the contrary, with reduced conditions (14.7 MPa pressure, 40 s time, and 160 °C temperature), boards were less dense and more fragile. However, they could be positioned on walls and ceilings for thermal insulation of buildings (47). Another example concerns the water extraction of polyphenols from the byproducts of the wood sector (paper industry, building or furniture manufacturing): maritime pine knots and stumps, aspen knots and barks (40). The originality of this study was based on the development of subcritical water conditions for the extraction of a specific class of secondary metabolites. Indeed, the presence of polyphenols and antiradical compounds of flavonoid type characterizes these raw materials. At analytical scale, they are extracted with medium polarity solvents (ethanol, methanol). Water, which represents the best candidate as a green solvent, is not efficient to extract flavonoids because of its high polarity at liquid state. We aimed at developing a process able to induce the modification of the physico-chemical properties of water so as it can be used as solvent for polyphenols extraction. We demonstrated that TSE fulfills these requirements. The temperature and pressure conditions to which the liquid/solid mixture is subjected in the zones of mechanical stress are favorable for the passage of water in the subcritical state. Under these conditions, its viscosity, surface tension and polarity decrease and reach the physico-chemical properties of methanol or ethanol. The flavonoids are then solubilized and extracted. At a flow rate of 15 kg/h in wood by-product, the influence of temperature (50-150°C) and liquid/ratio (3-6) was studied. After a conveying zone of 2 modules, the first part of the solvent was introduced at the module 3 ahead from a mixing zone of bilobe paddle screws. It ensured intimate mixing of the solid with the first liquid fraction and favored the diffusion of the solvent. The other liquid fraction was injected at the module 5 level. It has a washing effect under mechanical and thermal effects. Modules 2-5 corresponded to the liquid/solid extraction zone whereas modules 6-7 were responsible for the solid/liquid separation. It was concluded that the liquid/solid ratio had a major influence. An increase in liquid/solid ratio led to a higher breakdown of the material and a better solid/liquid separation for all plant materials. For example, for maritime pine stumps, the respective yields in total extracted molecules and polyphenols (g/kg dry raw material) increased from 19 and 0.9 at a ratio of 3 to 31 and 1.6 at a ratio of 6. The optimal extraction yield in polyphenols of 1.6 g/kg dry material represented 25% of the analytical polyphenolic content. The twin-screw extracts, thanks to their polyphenolic content and their high antiradical activity, can be used as mass products (preservatives for woody materials, antioxidant for paints) or as fine value-added substances (for food and health applications). Not all targeted molecules are free in the lignocellulosic matrix. They can be entrapped by covalent bonds. Their extraction then depends not only on their dissolution in the solvent but also on the preliminary breaking of the chemical bonds. Introducing a solvent in the TSE process is then insufficient to 37 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
reach appreciable yields. The efficiency is increased by introducing well-chosen catalysts and/or reagents to react on the chemical structure of the biomass and to free the biomolecules of interest. For example, using TSE process demonstrated advantages for the recovery of hydroxycinnamic acids from hemp by-products (41). Hemp hurds and hemp dust were studied as potential sources for the production of two high-value added hydroxycinnamic acids (HCA): ferulic (FA) and p-coumaric acids (p-CA). Prior to TSE, FA and p-CA analytical contents were evaluated to 0.3 and 3.5 g/kg dry matter (DM) for hemp hurds and 0.1 and 0.8 g/kg DM for hemp dust as potentials of reference. The continuous TSE pilot scale extraction was then studied. Mild conditions were developed: 50°C, alkaline aqueous or hydroalcoholic solvent (less than 0.5 M NaOH) and low liquid to solid ratios. The mechanical effect helps the diffusion of the solvent, promotes the hydrolysis of the ester and ether bonds and favors the extraction of HCA in a short time. Yields in p-CA and FA reached 50% and 33% of the free and bound contents for hemp hurds. For hemp dust, all of p-CA was extracted whereas 60% of FA was recovered. Extraction by extrusion could be seen as an alternative green processing technique as it is responsible for a reduction of extraction time and energy and a decrease in solvent and reagent consumption. 4. Summary Looking at all these different examples, it is possible to conclude that developing the twin-screw extruder as an extraction tool is of high interest. Apart from being a performant tool for pressing, it offers a large range of potentialities for solid/liquid extraction. To manage an extraction, the essential parameters are the screw profile, the temperature and the solvent (physico-chemical properties and liquid/solid ratio). Varying the solvent and/or the reagents influences the yield and selectivity of extraction. Most of the time, this continuous process can advantageously substitute batch extraction with lower liquid/solid ratios, shorter extraction times and an efficient solid/liquid in situ separation. As drawbacks, as residence times are short, it can be detrimental to the kinetics and consequently to the extraction yields. Fortunately, the mechanical effect compensates by intensifying the solvent’s diffusion in the biomass. Moreover, it is possible to design multistage extractions to reach a complete depletion of the raw material.
Pretreatment of Lignocellulosic Fibers for Bioethanol Production: Coupling Thermo-Mechano Chemical Action and Bioextrusion To consider the transformation of biomass into bioethanol two important steps are essential before achieving the fermentation of sugars into ethanol. The first step, called pretreatment, aims to deconstruct the organization of the cell wall and facilitates the access of hydrolitic enzymes to carbohydrates (48). Among the many pretreatments investigated in the past, thermo-mechano-chemical process by TSE presents the interest to be effective, continuous and easy to adapt 38 Ayoub and Lucia; Biomass Extrusion and Reaction Technologies: Principles to Practices and Future Potential ACS Symposium Series; American Chemical Society: Washington, DC, 2018.
at industrial scale. The flexibility of TSE technology also allows starting the saccharification, the second step of the process, by incorporation of the enzymes continuously in the extruder. With the intense mixing, the saccharification is initiated directly in the extruder at high consistency. This part of process has been called “bioextrusion”. This chapter provides an overview of the coupling thermo-mechano-chemical action for pretreatment and bioextrusion. 1. Feedstock Feedstocks mainly concerned by this type of processing must be rich in fiber and poor in cellular content. Priority should be given to biomasses with specific chemical composition most suitable for the production of 2nd generation ethanol: cellulose >34%, hemicelluloses
