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Hydrodynamic cavitation as a novel approach for delignification of wheat straw for paper manufacturing
Ultrasonics Sonochemistry 21 (2014) 162–168
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Hydrodynamic cavitation as a novel approach for delignification of wheat straw for paper manufacturing Mandar P. Badve a, Parag R. Gogate a, Aniruddha B. Pandit a, Levente Csoka b,⇑ a b
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India University of West Hungary, Institute of Wood and Paper Technology, 9400 Sopron, Hungary
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 11 July 2013 Accepted 16 July 2013 Available online 31 July 2013 Keywords: Hydrodynamic cavitation Wheat straw Delignification Tensile index Electrical energy
a b s t r a c t The present work deals with application of hydrodynamic cavitation for intensification of delignification of wheat straw as an essential step in the paper manufacturing process. Wheat straw was first treated with potassium hydroxide (KOH) for 48 h and subsequently alkali treated wheat straw was subjected to hydrodynamic cavitation. Hydrodynamic cavitation reactor used in the work is basically a stator and rotor assembly, where the rotor is provided with indentations and cavitational events are expected to occur on the surface of rotor as well as within the indentations. It has been observed that treatment of alkali treated wheat straw in hydrodynamic cavitation reactor for 10–15 min increases the tensile index of the synthesized paper sheets to about 50–55%, which is sufficient for paper board manufacture. The final mechanical properties of the paper can be effectively managed by controlling the processing parameters as well as the cavitational parameters. It has also been established that hydrodynamic cavitation proves to be an effective method over other standard digestion techniques of delignification in terms of electrical energy requirements as well as the required time for processing. Overall, the work is first of its kind application of hydrodynamic cavitation for enhancing the effectiveness of delignification and presents novel results of significant interest to the paper and pulp industry opening an entirely new area of application of cavitational reactors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Pulp and paper industry is one of the continuously evolving industries due to the market competition, high energy costs associated with processing making it important to look for ways of intensification with lesser energy requirements and raw material scarcity. The current main challenges of the pulp and paper industry are to produce best quality of pulp by using available sustainable raw material and at the same time to preserve environment by using less quantity of water and energy, natural raw materials and minimizing the pollution resulting from effluents (black liquor) generated by the digestion of pulp. A constant increase in the demand for paper production and limited availability of conventional wood resources such as bamboo, kenaf and jute have directed researchers to look for non wood resources such as bagasse, rice straw, corn straw, wheat straw etc. which can also be essentially considered as sustainable and where the natural fibers are known to be present in abundance. Due to the fact that cellulose is the main component of such fibers, these fibers are also called as cellulosic fibers. Making paper from trees produces twice the ⇑ Corresponding author. Tel.: +36 99 518 305; fax: +36 99 518 302. E-mail address: [email protected] (L. Csoka). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.07.006
ecological footprint as compared to producing from straw [1] and hence efficient technologies based on straw must be developed as an alternative to the natural wood based processes. Currently, only 8–10% of world paper is made from agricultural-fibers such as wheat straw, corn straw and rice straw. India and China produce 20% of their paper from wheat straw, rice straw and sugar cane stocks such as bagasse. Wheat straw is one of the most important agricultural residues, especially considering the production capacities in Hungary. Properties of fibers essential for papermaking from wood or from annual crops can be influenced by both the synthesis conditions and genetic manipulation. For example, the wood morphology and chemical composition vary with location, genetics, and growth conditions. Fibrous raw materials such as wood or agricultural residues consist of three major polymers viz. cellulose, hemicelluloses and lignin. In general, wheat straw contains about 35–45% of cellulose, 20–30% hemicelluloses, 8–15% lignin and around 10% ash [2]. Plant cell walls usually consist of rigid cellulosic microfibrils embedded in soft hemicelluloses and lignin matrix. The structural material of the fibrous cells is usually with high levels of strength and stiffness. Cellulose has a straight carbohydrate polymer chain consisting of b-(1–4)-linked glucopyranose units and a degree of polymerization of about 10,000 [3]. Hydroxyl (–OH) groups in
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cellulose structures play a major role in governing its reactivity and physical properties. The relative density of lignin is only slightly less than that of cellulose. In pulp and paper manufacture, paper strength depends on the amount of lignin and cellulose in the raw material derived from the plants. Lignin is an undesirable polymer and its removal during pulping is essential and requires high amounts of energy and chemicals. Chemical pulping is one of the common methods for pulping of wheat straw. The chemical pulping of wheat straw involves separation of cellulose from cellulose-hemicellulose and lignin matrix. It is done by dissolving the lignin and hemicelluloses based on the cleavage of covalent linkages which holds the cellulose fibers together. Kraft process is one of the common and most widely used chemical processes for the production of cellulosic pulp from various lignocellulosic materials. In a typical delignification step of Kraft process, alkaline hydrolysis of wood chips (lignocellulosic material) is carried out in a continuous digester at a pressure of about 6–10 atm and a temperature of about 170–175 °C for the time period of about 2–5 h [4], though the actual value of these operating parameters vary with the type of raw material to be treated. Considering the high energy and water requirement of pulping industry, different methods have been explored for the delignification/cooking of wheat straw in the last decades with an objective of possibly reducing the energy and water requirements in the processing. These methods include oxidative delignification in aqueous organic solvents, biological treatments and pretreatment by steam (steam explosion) etc. It is extremely difficult to grasp the energy consumption pattern in the entire pulp and paper industry. Since the pulp process differs considerably depending on production items and composition of the material woods, it is very difficult to define the representative standard or efficient pulping process [5]. Other methods of cooking/digestion of pulp includes methods such as organosolv delignification, ammonia fiber explosion (AFEX), CO2 explosion, acid pretreatment and biological treatment. In organosolv fractionation/delignification methods of wheat straw, the pulp is treated with organic solvents like carboxylic acid or alcohols as cooking chemicals at temperatures in the range of 150–200 °C for the time period of around 60–120 min in presence of H2SO4 as a catalyst [6–8]. In the steam explosion method, the biomass is heated using high pressure steam (20–50 bar, 160–290 °C) for a few minutes and the reaction is then stopped by sudden decompression to atmospheric pressure. The liquid hot water method uses compressed hot liquid water (at pressure above saturation point) for delignification. The treatment generally occurs at temperatures of 170–230 °C and pressures above 5 MPa for 20 min [9,10]. Ammonia fiber explosion (AFEX) is an alkaline pretreatment process involving liquid ammonia and steam explosion. Moderate temperatures of 60–100 °C are used and residence time may vary from low (5–10 min) to intermediate (30 min) depending on the degree of saturation of the biomass. Ammonia loading is done at 1 kg ammonia per kg of dry substrates [2,11]. CO2 explosion technique acts in a similar manner to steam and ammonia fiber explosion technique [2]. In biological pretreatment, degradation of the lignocellulosic complex to separate cellulose can be brought about with the help of microorganisms like brown rot, white rot and soft rot fungi. Biological pretreatment renders the degradation of lignin and hemicelluloses [2,11] and white rot fungi seem to be the most effective microorganism. Brown rot attacks cellulose while white and soft rots attack both cellulose and lignin. A critical analysis of these pretreatment methods indicates that most of the methods require conditions of very high temperature and pressure making them energy intensive operations. In the case of biological methods, the processing times are expected to be significantly higher and hence it is important to develop a new treatment approach for effective delignification under opti-
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mized conditions of energy requirement and the processing times. The present work deals with application of hydrodynamic cavitation for treatment of wheat straw pulp for the first time and hence is expected to open an entirely new field of research as well as possibly establish treatment methodologies of commercial importance. One of the major advantages of hydrodynamic cavitation is that the treatment can be carried out at ambient conditions of temperature and pressure. Cavitation is the formation, growth and violent collapse of bubbles in the liquid medium. The collapse of the bubbles, generates localized ‘‘hot spots’’ with transient temperature of the order of 10,000 K and pressure of about 1000 atm [12], which can induce chemical and physical transformations. The chemical effects of cavitation are due to the formation of free radicals, like hydroxyl radicals, formed due to the decomposition of water molecules. Shock waves are generated during violent collapse of cavities which are responsible of pyrolysis/molecular breakdown of organic molecules trapped inside cavities or molecules which are in the vicinity of cavities. Hydrodynamic cavitation is one of the emerging process and its effectiveness in applications such as waste water treatment, water disinfection etc. is already proven [13–16], though the application for delignification has not been investigated till now. The present work hence provides a novel report of first application of hydrodynamic cavitation reactors for delignification of wheat straw and also illustrates the energy consumption analysis in comparison with the other more generally used processes. 2. Materials and methods 2.1. Preparation of wheat straw Wheat straw (Triticum aestivium) was used as the raw material and was chopped to 5 mm length using standard straw chopper. This chopped wheat straw (1 kg) was then retted in alkaline solution of 0.3 M potassium hydroxide (KOH) for 48 h for pre-softening of lignin cellulose complex at room temperature. Disintegration of this retted pulp was performed every 12 h prior to hydrodynamic cavitation treatment. During the actual process, the pulp is disintegrated 4 times (each disintegration step lasts for 2.5 min) for obtaining the well mixed conditions. The disintegrator liberates the retted fibers from each other. A standard laboratory disintegrator has been used in the present work similar to that used for the pulp preparation. The disintegrator consists of a container (capacity of 3 L) with spiral baffles and a rotating blunt propeller at a speed of 2500 rotation/min. Disintegration also helps in further softening of retted wheat straw pulp. The original lignin content of the straw was 17%. 2.2. Experimental setup The device used for the generation of cavitation in this work is a stator and rotor assembly. A rotor rotates at very high speed in a confined annular space and liquid is passed through the gap between the stator and the rotor. Due to high speed of rotation, very high surface velocities are generated. Some indentations are also provided on the surface of rotor. Liquid at such velocities enters the indentation, due to rotary action of the cylinder and when liquid comes out of the indentation due to centrifugal flow, a low pressure region/vacuum is created near the upper surface of the indentations resulting into cavitation. Pressure drop across the component is the main driving force for cavitation. At such high surface velocities of the liquid on the surface of the cylinder, liquid pressure drop across the surface of the cylinder and indentation is sufficient enough for cavitation to occur. Fig. 1 gives the schematic of the experimental setup, which basically contains a storage tank
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Fig. 1. Schematic of the hydrodynamic cavitation setup.
of capacity 15 L. Liquid from the tank is pumped into the cavitating device by using an open impeller type pump with 3 blades and flow capacity of 5–15 L/min with a pressure head of 4.5 bar. Heat exchanger was added between the cavitating device and storage tank to cool the liquid coming out of the cavitating device. Fig. 2 shows schematic details of the actual cavitating device i.e., a stator and rotor assembly. Rotor is attached to a gear assembly, which is connected to a variable frequency drive (VFD: YASKAWA J1000, type: CIMR-JC 4A0011BAA). With the help of VFD, rotor can be rotated at different speeds of rotation. Rotor is a solid cylinder which has indentations (like a golf ball) on its surface. The diameter of the rotor is 19 cm with ratio of diameter to height of the rotor as 1.9 and this rotor is contained in a cylinder with diameter of 21 cm. Gap between the stator and the rotor is 10 mm and it is fixed. There are a total 204 indentations equidistant from each other. Each indentation is 12 mm in diameter, 20 mm deep and the schematic of an indentation has been given in Fig. 3, which also shows a likely liquid circulation pattern inside a single indentation. Due to high speed of rotation of the rotor, very high surface velocities are created at the surface of the cylinder. Such high velocities (typically 18–20 m/s) are responsible for the creation of high turbulence. This high turbulence/high shear is responsible for breakage/lysis of lignin, cellulose and hemicellulose complex. The hydroxyl radicals formed during cavitation are also responsible for hydrolysis/ionization of lignin molecule which makes it more hydrophilic and easy to remove from pulp. 2.3. Experimental and analytical procedure All the experiments were performed with wheat straw retted with 0.3 M KOH. Experiments were performed at a rotor rotational speed of 2200 rpm and 2700 rpm. The rotation speed was optimized using the Weissler reaction which quantifies the iodine liberation due to the cavitational effects and is a true measure of the
Fig. 3. Schematic of the single indentation, showing liquid flow pattern.
cavitational intensity [17]. At lower rotational speeds, it was observed that the velocities generated are not sufficient to create conditions of cavitation and are hence definitely not sufficient for enhancing the delignification based on the cavitational effects. The preliminary studies indicated that maximum cavitational activity as measured by the Weissler reaction is obtained at 2200 rpm. In addition the shear forces generated due to the rotational effects will be maximum at 2700 rpm (maximum speed that could be obtained based on the equipment) and hence additional experiments were also conducted at 2700 rpm. The wheat straw to water ratio (consistency) was varied from 1% to 7% wt/wt (1%, 3%, 5% and 7%). All the experiments were performed with tap water at room temperature. Total treatment time for all the experiments was 15 min. Samples were drawn after each 5 min for further analysis of the partially digested wheat straw. Hand-sheets of cavitation treated pulps were made on the basis weight of 100 g/m2 by using SHAAGE D-4330, systems laboratory sheet former according to DIN EN ISO 5269-2. After drying at temperature of 90 °C for 5 min, all the samples were conditioned at 50% relative humidity and a temperature of 23 °C. 3. Results and discussions 3.1. Reaction mechanism of delignification with hydrodynamic cavitation The mechanism of delignification in the case of conventional processes such as organosolv pulping, KOH and NH4OH pulping, oxygen delignification and peroxide delignification [18–20] have been well investigated. Also, few work have reported the effect of ultrasound operating parameters on the delignification [21] but the mechanistic details for the effect of cavitation on the delignification has been lacking in the open literature. Considering these aspects, we have tried to propose a delignification mechanism under the cavitating conditions for the treatment of wheat straw using hydrodynamic cavitation in the alkaline conditions. Important reactions of lignin, a polymer network of three major C6–C3 phenylpropane skeletons with many C–C and ether linkages, are initiated when phenolic hydroxyl group in lignin dissociates under alkaline conditions to form a resonance stabilized phenoxy radical. R
H + OH -
+ H2 O
O OH Fig. 2. Schematic of the cavitating device (stator and rotor assembly).
CH 3
O O
-
CH 3
ð1Þ
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This phenoxy radical is nothing but oxidized form of lignin which is more hydrophilic in nature and can be easily removed from pulp. Wheat straw contains all the three main lignin units, namely guayacyl, syringyl and coniferyl and hence there is abundant occurrence of phenolic hydroxyl group. When cavitation takes place, water molecule dissociates into OH and H due to the conditions of high pressure and temperature. Cavitation
H2 O ! OH þH
ð2Þ
These OH radicals are highly reactive and could be responsible for the oxidation of lignin molecule to some extent. But under the alkaline conditions, oxidation potential (reactivity) of the hydroxyl radicals decreases and recombination reaction between hydroxyl radical takes place to form hydrogen peroxide (H2O2).
OH þOH H2 O2
ð3Þ
However, under alkaline conditions (alkaline pH), H2O2 dissociates to form the hydroperoxy radical as per the reaction given below:
H2 O2 HOO þH
ð4Þ
HOO reacts with the residual H2O2 again to form highly reactive OH and superoxide (O2)
H2 O2 þ HOO ! OH þO2 þ H2 O
ð5Þ
The resonance stabilized lignin intermediates then undergo reactions with themselves which is also called as lignin condensation or with the hydroxyl (OH), hydroperoxy (HOO) radicals which are formed during cavitation to form organic acids, carbon dioxide and other low molecular weight organic products through the side chain elimination, ring opening and demethoxylation process (reaction (6)) as illustrated in the following reaction mechanisms [22]. R
R
O R
O
CH3
O
-
O
-
CH3
Lignin Condensation O O
-
CH3
HOO.
R
R
OOH
+
OOH O
O O
-
CH3
Side Chain Elimination
O
-
CH3
nents are present that can react with OH and/or O2, observed yield of O2 will reduce below that predicted theoretically. It has been reported that when H2O2 decomposes under alkaline conditions in presence of lignin containing substrates, less than the expected theoretical amount of O2 is evolved, indicating that at least some of the OH and/or O2 reacts with lignin [20]. The final acid insoluble lignin content was 4–5%. Once the lignin polymer is degraded the connector linkage between lignin and hemicellulose and celluloses are also significantly cleaved during the cavitation attack and cellulosic fibers can be liberated. At higher rotation speed the mechanistic effect of lignin degradation by cavitation helps the defibration of individual cellulosic fibers resulting into increased access to surface functional groups to participate in the bonding mechanism during sheet formation. 3.2. Effect of hydrodynamic cavitation on mechanical properties of paper Hand-sheets of cavitation treated pulps were made using laboratory sheet former according to DIN EN ISO 5269-2. After drying, all the samples were conditioned at 50% relative humidity and at a temperature of 23 °C before testing for its mechanical properties. The main objective of the experiments was to study the effect of hydrodynamic cavitation on the quality of pulp produced quantified in terms of the mechanical properties. Tensile strength measures the maximum force per unit width that a paper can resist before breaking when applying the load in a direction, parallel to the length of the strip (machine direction) [23]. Considering the fact that the weight of the each analyzed strip is different, for effective comparison of results, tensile index must be calculated by dividing the tensile force required to break the sheet by grammage (g/m2) of the sheet used. Grammage is nothing but weight of the one hand sheet per unit area. Fig. 4 shows the effect of the rotation speed on tensile strength of the paper sheet made from alkali treated wheat straw of different consistency (1%, 3%, 5%, and 7%) which is treated in hydrodynamic cavitation device for 15 min. It can be clearly seen that as the rotation speed increases from 2200 to 2700 rpm, tensile strength of treated fiber increases by almost 45% at 7% consistency after 15 min of treatment. The value of tensile strength as 5465 N/ m after 15 min of treatment is sufficient for types of paper like newsprint paper and/or paper board manufacture according to standard values for such type of paper [24]. Fig. 5 shows the effect of rotation speed on tensile index of the paper made from alkali treated wheat straw of different consistency (1%, 3%, 5%, and 7%) which is treated in hydrodynamic cavitation device for 15 min. Similar to tensile strength, tensile index of
Ring opening Demethoxylation Lignin Degradation
ð6Þ Use of only KOH is incapable of delignification of wheat straw in such a short span of time. Combining delignification of wheat straw in alkaline conditions with hydrodynamic cavitation provides additional hydroxyl and hydroperoxy radicals required for delignification reaction thereby intensifying the overall rates of reactions. In the absence of other reactants, hydroxyl and superoxide radicals can react with each other as illustrated below:
OH þO2 þ H ! O2 þ H2 O
ð7Þ
Combining reaction ((5)) and ((7))
H2 O2 þ HOO þH ! O2 þ 2H2 O
ð8Þ
It can be said that maximum amount of O2 that can be evolved from H2O2 is equal to half the molar amount of water which is formed during the reaction illustrated as Eq. (8). If other compo-
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Fig. 4. Effect of rotation speed on tensile strength of the paper.
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Fig. 5. Effect of rotation speed on tensile index of the paper.
hand sheet prepared after the hydrodynamic cavitation treatment also increased with an increase in the rotation speed. This increase in tensile strength and tensile index of the hand sheets with rotation speed of rotor can be explained by the fact that, as the speed of rotation increases surface velocities generated at the surface of rotor increases. These high surface velocities are responsible for generation of cavitating conditions in this type of device [25]. Also, with an increase in the surface velocity due to an increase in the rotation speed of rotor, cavitational intensity also increases which results into more number of reactive hydroxyl radicals (OH) and mechanical effects in terms of turbulence and shear, which in turn increases delignification of the wheat straw. The extent of beneficial effects obtained by the increasing speeds of rotation of rotor must be weighed against the increase in the energy required for the rotation of rotor as it increases with an exponent of 3 over the speed of rotation. Fig. 6 shows the change in tensile index of the paper made from the treated pulp with respect to the treatment time of hydrodynamic cavitation. It can be clearly seen that as the treatment time increases (data at time equal to zero indicates the control data without the use of hydrodynamic cavitation), tensile index of the paper increases linearly. Also when pulp is treated by hydrodynamic cavitation there is an improvement in the tensile index of
Fig. 6. Effect of rotation speed on change in tensile index of the paper with time.
the paper. At 7% consistency and rotation speed of 2200 rpm, tensile index increases by approximately 45% after 15 min of hydrodynamic cavitation treatment. Similarly, at 7% consistency and rotation speed of 2700 rpm, tensile index increases by approximately 66% (from 30 to 50 N-m/g) after 15 min of hydrodynamic cavitation treatment. This can be explained by the fact that as treatment time increases, exposure time of lignin-cellulose complex to cavitation also increases, which in turn increases the overall delignification. As the lignin is removed from the cellulose-lignin complex, strength of the paper fiber increases. In the present work the lignin removal has not been quantified but credence to the hypothesis can be obtained from earlier reported work [21,26]. It has been also reported that shock waves created due to cavitation are responsible for the breakage of chemical bonds in complex organic structure [16]. When shear generated by cavity collapse exceeds the strength of the chemical bond, breakage of bond takes place. In the present case also, shock waves are responsible for the breakage of some chemical bonds between lignin cellulose complexes. Another significant mechanical property of the paper is its breaking length. Breaking length is the limiting length of a strip of uniform width, beyond which, if such a strip were suspended by one end, it would break of its own weight. Breaking length is usually expressed in km. Fig. 7 shows the effect of rotation speed on the breaking length of the hand sheet made from the treated pulp. It can be clearly seen that as the rotation speed of the rotor increases, breaking length also increases. This can be explained on the basis of the fact that as the rotation speed of the rotor increases, cavitational intensity is more and hence there is a significant improvement in mechanical properties of paper. It should be noted here that there is an improvement in the mechanical properties of the hand sheets made from alkaline treated wheat straw after the hydrodynamic cavitation treatment. Depending on the properties of the final paper required it is easily possible to optimize process parameters such as solid to liquid ratio (consistency), concentration of alkali, hydrodynamic cavitation parameters such as rotation speed, time of operation and different types of cavitaing devices (orifice or venturi). The tensile strength of the paper depends on several factors that include: (1) the average load-bearing ability of the individual fibres, (2) the number of fibres at any given cross-section available for load transfer, and (3) the uniformity of the load transfer that is allowed by the network structure [27]. On the other hand, non-homogeneity is a fundamental feature of the paper network, meaning that non-uniformity of the load transfer cannot be avoided. It is also obvious that improving fiber bonding (removing lignin and hemicellulose) can improve the ability of network to
Fig. 7. Effect of rotation speed on breaking length of the paper.
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support load. Less lignin helps the development of interfiber bond between accessible OH group on the surface of cellulosic fibers. 3.3. Energy calculations (electrical energy per order EEO) In this section, the total electrical energy consumed by the hydrodynamic cavitation is compared with the electrical energy consumed by conventional Kraft pulping and steam explosion. As defined by Bolton et al. [28] electrical energy per order can be defined as the electric energy in kilowatt hours [kWh] required to degrade/react a contaminant/reactant by one order of magnitude in a unit volume [e.g., 1 m3 (1000 L)] of contaminated water or air. For the present work,EEO values [kWh/m3/order] can be calculated using the following formulas:
EEO ¼
EEO ¼
P t 1000 Batch operation
v log
ci cf
P t 1000 Continuous operation F log cfci
where, P is the rated power [kW] of the system, V is the volume [L] of water or air treated in the time t [hr], F is the water or air volume flow rate [m3/hr], ci, cf are the initial (or influent) and final (or effluent) concentrations [M or mol/L or g/L], and the factor of 1000 converts g to kg. Higher EEO values correspond to lower removal efficiencies. EEO of hydrodynamic cavitation is calculated based on the operating parameters used in the present work. For Kraft pulping and steam explosion EEO is calculated by using data illustrated in the earlier work [3,4]. EEO for hydrodynamic cavitation, Kraft pulping and for steam explosion was found to be 1785 kWh/m3/order, 3942 kWh/m3/order and 3482 kWh/m3/order, respectively. It can be established that electrical energy per order (EEO) required for hydrodynamic cavitation is considerably less (nearly 50%) than that required for conventional Kraft pulping or the steam explosion. The main advantage of hydrodynamic cavitation is that even when the conditions of high temperature and high pressure zones are generated locally which are required for delignification to take place, the overall conditions are always ambient avoiding the requirement of any heat duties. In the other pulping techniques, more energy is consumed in creation of such conditions of high temperature and pressure which is mainly required in the case of Kraft pulping and steam explosion. Another advantage of hydrodynamic cavitation is the reduced requirement of the treatment time compared to conventional processes, which also could be responsible for reduced energy consumption.
4. Conclusions and path foward The present work has clearly established the utility of hydrodynamic cavitation for the effective treatment of wheat straw in alkaline conditions. The investigations related to the effect of operating parameters such as rotation speed and straw to water ratio revealed that rotation speed and the straw to water ratio has a significant effect on final properties of the paper hand sheet made out of the treated pulp. Following important conclusions can be drawn from the present work: 1. As rotation speed of the rotor was increased from 2200 rpm to 2700 rpm, extent of delignification increases which in turn increases the tensile properties of the hand sheet. 2. As water consistency (straw to water ratio) increases, mechanical properties of paper also increases. When consistency was increased from 5% to 7%, tensile index increases by 50%. The
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consistency should be selected based on the desired properties of the final paper and availability of water and the present work has clearly illustrated the correspondence between the mechanical properties and the water consistency. 3. Calculations of electrical energy required per order per unit volume confirmed that hydrodynamic cavitation requires significantly less energy as compared to other conventional techniques. Overall, it can be said that the major advantage of hydrodynamic cavitation treatment of wheat straw for the production of paper or paper board is less treatment time, less energy consumption compared to other conventional techniques and operation at near ambient conditions. Hydrodynamic cavitation reactors are relatively easy to scale up and various operating parameters such as rotation speed of the rotor, consistency of the straw can be easily optimized depending upon the required final properties of the paper or paper board. It is also important to note that hydrodynamic cavitation can be easily scaled up for industrial size plants as compared to the use of sonochemical reactors. Depending on the final qualities of the paper required and optimized results of the lab scale study, cavitating conditions can be finalized. Cavitating device can be designed accordingly depending on the volume to be treated and treatment time required. To achieve this objective of application at commercial scale installations, investigations would be required to check the effectiveness of the cavitation over conventional processes of cellulose digestion in terms of energy and time requirement, for a wider range of starting raw materials and also operating parameters such as alkali concentration and temperature.
Acknowledgements One of the authors, PRG, would like to acknowledge the funding from University Grants Commission, India and Hungarian Scholarship Board, Balassi Institute for funding the visit to University of West Hungary, Sopron under the Indo-Hungarian Educational Exchange Program 2012.
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[12] V.K. Saharan, M.P. Badve, A.B. Pandit, Degradation of Reactive Red 120 dye using hydrodynamic cavitation, Chem. Eng. J. 178 (2011) 100–107. [13] A.G. Chakinala, D.H. Bremner, P.R. Gogate, K.C. Namkung, A.E. Burgess, Multivariate analysis of phenol mineralisation by combined hydrodynamic cavitation and heterogeneous advanced fenton processing, Appl. Catal. B 78 (2008) 11–18. [14] G. Loraine, G. Chahine, C. Hsiao, J. Choi, P. Aley, Disinfection of gram-negative and gram-positive bacteria using DYNAJETS hydrodynamic cavitating jets, Ultrason. Sonochem. 19 (2012) 710–717. [15] B. Kompare, M. Zupanc, T. Kosjek, M. Petkovšek, E. Heath, B. Širok, Removal of pharmaceuticals from wastewater by biological processes, hydrodynamic cavitation and UV treatment, Ultrason. Sonochem. 20 (2013) 1104–1112. [16] V.K. Saharan, M.A. Rizwani, A.A. Malani, A.B. Pandit, Effect of geometry of hydrodynamically cavitating device on degradation of orange-G, Ultrason. Sonochem. 20 (2013) 345–353. [17] P.R. Gogate, I.Z. Shirgaonkar, M. Sivakumar, P. Senthilkumar, N.P. Vichare, A.B. Pandit, Cavitation reactors: efficiency assessment using a model reaction, AIChE J. 47 (2001) 2526–2538. [18] J. Zhang, M. Yao, Clean Production Of Corn Stover Pulp Using Koh + Nh 4 Oh Solution And Its Kinetics During Delignification, Chem. Ind. Chem. Eng. Q. 18 (2012) 137–145. [19] R. Sun, J. Tomkinson, Comparative study of lignins isolated by alkali and ultrasound-assisted alkali extractions from wheat straw, Ultrason. Sonochem. 9 (2002) 85–93.
[20] J.M. Gould, Studies on the Mechanism of alkaline peroxide delignification of Agricultural Residues, Biotechnol. Bioeng. 27 (1984) 225–231. [21] A. Iskalieva, B. Mbouyem, P.R. Gogate, M. Horvath, P.G. Horvath, L. Csoka, Cavitation assisted delignification of wheat straw: a review, Ultrason. Sonochem. 19 (2012) 984–993. [22] P.W. Hart, A.W. Rudie, The Bleaching of Pulp, 5th ed., TAPPI Press, 2012. [23] D. I. Standard, Paper and board — Determination of tensile properties, Draft International Standard ISO/DIS 1924–2, 2007. [24] D.F. Caulfield, D.E. Gunderson, Proceedings of the paper preservation, Symposium 66 (1988) 31–40. [25] M. Badve, P. Gogate, A. Pandit, L. Csoka, Hydrodynamic cavitation as a novel approach for wastewater treatment in wood finishing industry, Sep. Purif. Technol. 106 (2013) 15–21. [26] L. Csoka, A. Lorincz, A. Winkler, Sonochemically modified wheat straw for pulp and papermaking to increase its economical performance and reduce environmental issues, Bioresources 3 (2008) 91–97. [27] W.F. Cowan, Explaining handsheet tensile and tear in terms of fiber quality numbers, Tappi J. 78 (1995) 133–137. [28] J.R. Bolton, K.G. Bircher, W. Tumas, C.A. Tolman, Fig.s-of-merit for the technical development and application of advanced oxidation technologies for both electric- and solar-driven systems, Pure Appl. Chem. 73 (2001) 627– 637.
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Ultrasonics Sonochemistry journal homepage: www.elsevier.com/locate/ultson
Hydrodynamic cavitation as a novel approach for delignification of wheat straw for paper manufacturing Mandar P. Badve a, Parag R. Gogate a, Aniruddha B. Pandit a, Levente Csoka b,⇑ a b
Department of Chemical Engineering, Institute of Chemical Technology, Mumbai 400 019, India University of West Hungary, Institute of Wood and Paper Technology, 9400 Sopron, Hungary
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 11 July 2013 Accepted 16 July 2013 Available online 31 July 2013 Keywords: Hydrodynamic cavitation Wheat straw Delignification Tensile index Electrical energy
a b s t r a c t The present work deals with application of hydrodynamic cavitation for intensification of delignification of wheat straw as an essential step in the paper manufacturing process. Wheat straw was first treated with potassium hydroxide (KOH) for 48 h and subsequently alkali treated wheat straw was subjected to hydrodynamic cavitation. Hydrodynamic cavitation reactor used in the work is basically a stator and rotor assembly, where the rotor is provided with indentations and cavitational events are expected to occur on the surface of rotor as well as within the indentations. It has been observed that treatment of alkali treated wheat straw in hydrodynamic cavitation reactor for 10–15 min increases the tensile index of the synthesized paper sheets to about 50–55%, which is sufficient for paper board manufacture. The final mechanical properties of the paper can be effectively managed by controlling the processing parameters as well as the cavitational parameters. It has also been established that hydrodynamic cavitation proves to be an effective method over other standard digestion techniques of delignification in terms of electrical energy requirements as well as the required time for processing. Overall, the work is first of its kind application of hydrodynamic cavitation for enhancing the effectiveness of delignification and presents novel results of significant interest to the paper and pulp industry opening an entirely new area of application of cavitational reactors. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Pulp and paper industry is one of the continuously evolving industries due to the market competition, high energy costs associated with processing making it important to look for ways of intensification with lesser energy requirements and raw material scarcity. The current main challenges of the pulp and paper industry are to produce best quality of pulp by using available sustainable raw material and at the same time to preserve environment by using less quantity of water and energy, natural raw materials and minimizing the pollution resulting from effluents (black liquor) generated by the digestion of pulp. A constant increase in the demand for paper production and limited availability of conventional wood resources such as bamboo, kenaf and jute have directed researchers to look for non wood resources such as bagasse, rice straw, corn straw, wheat straw etc. which can also be essentially considered as sustainable and where the natural fibers are known to be present in abundance. Due to the fact that cellulose is the main component of such fibers, these fibers are also called as cellulosic fibers. Making paper from trees produces twice the ⇑ Corresponding author. Tel.: +36 99 518 305; fax: +36 99 518 302. E-mail address: [email protected] (L. Csoka). 1350-4177/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ultsonch.2013.07.006
ecological footprint as compared to producing from straw [1] and hence efficient technologies based on straw must be developed as an alternative to the natural wood based processes. Currently, only 8–10% of world paper is made from agricultural-fibers such as wheat straw, corn straw and rice straw. India and China produce 20% of their paper from wheat straw, rice straw and sugar cane stocks such as bagasse. Wheat straw is one of the most important agricultural residues, especially considering the production capacities in Hungary. Properties of fibers essential for papermaking from wood or from annual crops can be influenced by both the synthesis conditions and genetic manipulation. For example, the wood morphology and chemical composition vary with location, genetics, and growth conditions. Fibrous raw materials such as wood or agricultural residues consist of three major polymers viz. cellulose, hemicelluloses and lignin. In general, wheat straw contains about 35–45% of cellulose, 20–30% hemicelluloses, 8–15% lignin and around 10% ash [2]. Plant cell walls usually consist of rigid cellulosic microfibrils embedded in soft hemicelluloses and lignin matrix. The structural material of the fibrous cells is usually with high levels of strength and stiffness. Cellulose has a straight carbohydrate polymer chain consisting of b-(1–4)-linked glucopyranose units and a degree of polymerization of about 10,000 [3]. Hydroxyl (–OH) groups in
M.P. Badve et al. / Ultrasonics Sonochemistry 21 (2014) 162–168
cellulose structures play a major role in governing its reactivity and physical properties. The relative density of lignin is only slightly less than that of cellulose. In pulp and paper manufacture, paper strength depends on the amount of lignin and cellulose in the raw material derived from the plants. Lignin is an undesirable polymer and its removal during pulping is essential and requires high amounts of energy and chemicals. Chemical pulping is one of the common methods for pulping of wheat straw. The chemical pulping of wheat straw involves separation of cellulose from cellulose-hemicellulose and lignin matrix. It is done by dissolving the lignin and hemicelluloses based on the cleavage of covalent linkages which holds the cellulose fibers together. Kraft process is one of the common and most widely used chemical processes for the production of cellulosic pulp from various lignocellulosic materials. In a typical delignification step of Kraft process, alkaline hydrolysis of wood chips (lignocellulosic material) is carried out in a continuous digester at a pressure of about 6–10 atm and a temperature of about 170–175 °C for the time period of about 2–5 h [4], though the actual value of these operating parameters vary with the type of raw material to be treated. Considering the high energy and water requirement of pulping industry, different methods have been explored for the delignification/cooking of wheat straw in the last decades with an objective of possibly reducing the energy and water requirements in the processing. These methods include oxidative delignification in aqueous organic solvents, biological treatments and pretreatment by steam (steam explosion) etc. It is extremely difficult to grasp the energy consumption pattern in the entire pulp and paper industry. Since the pulp process differs considerably depending on production items and composition of the material woods, it is very difficult to define the representative standard or efficient pulping process [5]. Other methods of cooking/digestion of pulp includes methods such as organosolv delignification, ammonia fiber explosion (AFEX), CO2 explosion, acid pretreatment and biological treatment. In organosolv fractionation/delignification methods of wheat straw, the pulp is treated with organic solvents like carboxylic acid or alcohols as cooking chemicals at temperatures in the range of 150–200 °C for the time period of around 60–120 min in presence of H2SO4 as a catalyst [6–8]. In the steam explosion method, the biomass is heated using high pressure steam (20–50 bar, 160–290 °C) for a few minutes and the reaction is then stopped by sudden decompression to atmospheric pressure. The liquid hot water method uses compressed hot liquid water (at pressure above saturation point) for delignification. The treatment generally occurs at temperatures of 170–230 °C and pressures above 5 MPa for 20 min [9,10]. Ammonia fiber explosion (AFEX) is an alkaline pretreatment process involving liquid ammonia and steam explosion. Moderate temperatures of 60–100 °C are used and residence time may vary from low (5–10 min) to intermediate (30 min) depending on the degree of saturation of the biomass. Ammonia loading is done at 1 kg ammonia per kg of dry substrates [2,11]. CO2 explosion technique acts in a similar manner to steam and ammonia fiber explosion technique [2]. In biological pretreatment, degradation of the lignocellulosic complex to separate cellulose can be brought about with the help of microorganisms like brown rot, white rot and soft rot fungi. Biological pretreatment renders the degradation of lignin and hemicelluloses [2,11] and white rot fungi seem to be the most effective microorganism. Brown rot attacks cellulose while white and soft rots attack both cellulose and lignin. A critical analysis of these pretreatment methods indicates that most of the methods require conditions of very high temperature and pressure making them energy intensive operations. In the case of biological methods, the processing times are expected to be significantly higher and hence it is important to develop a new treatment approach for effective delignification under opti-
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mized conditions of energy requirement and the processing times. The present work deals with application of hydrodynamic cavitation for treatment of wheat straw pulp for the first time and hence is expected to open an entirely new field of research as well as possibly establish treatment methodologies of commercial importance. One of the major advantages of hydrodynamic cavitation is that the treatment can be carried out at ambient conditions of temperature and pressure. Cavitation is the formation, growth and violent collapse of bubbles in the liquid medium. The collapse of the bubbles, generates localized ‘‘hot spots’’ with transient temperature of the order of 10,000 K and pressure of about 1000 atm [12], which can induce chemical and physical transformations. The chemical effects of cavitation are due to the formation of free radicals, like hydroxyl radicals, formed due to the decomposition of water molecules. Shock waves are generated during violent collapse of cavities which are responsible of pyrolysis/molecular breakdown of organic molecules trapped inside cavities or molecules which are in the vicinity of cavities. Hydrodynamic cavitation is one of the emerging process and its effectiveness in applications such as waste water treatment, water disinfection etc. is already proven [13–16], though the application for delignification has not been investigated till now. The present work hence provides a novel report of first application of hydrodynamic cavitation reactors for delignification of wheat straw and also illustrates the energy consumption analysis in comparison with the other more generally used processes. 2. Materials and methods 2.1. Preparation of wheat straw Wheat straw (Triticum aestivium) was used as the raw material and was chopped to 5 mm length using standard straw chopper. This chopped wheat straw (1 kg) was then retted in alkaline solution of 0.3 M potassium hydroxide (KOH) for 48 h for pre-softening of lignin cellulose complex at room temperature. Disintegration of this retted pulp was performed every 12 h prior to hydrodynamic cavitation treatment. During the actual process, the pulp is disintegrated 4 times (each disintegration step lasts for 2.5 min) for obtaining the well mixed conditions. The disintegrator liberates the retted fibers from each other. A standard laboratory disintegrator has been used in the present work similar to that used for the pulp preparation. The disintegrator consists of a container (capacity of 3 L) with spiral baffles and a rotating blunt propeller at a speed of 2500 rotation/min. Disintegration also helps in further softening of retted wheat straw pulp. The original lignin content of the straw was 17%. 2.2. Experimental setup The device used for the generation of cavitation in this work is a stator and rotor assembly. A rotor rotates at very high speed in a confined annular space and liquid is passed through the gap between the stator and the rotor. Due to high speed of rotation, very high surface velocities are generated. Some indentations are also provided on the surface of rotor. Liquid at such velocities enters the indentation, due to rotary action of the cylinder and when liquid comes out of the indentation due to centrifugal flow, a low pressure region/vacuum is created near the upper surface of the indentations resulting into cavitation. Pressure drop across the component is the main driving force for cavitation. At such high surface velocities of the liquid on the surface of the cylinder, liquid pressure drop across the surface of the cylinder and indentation is sufficient enough for cavitation to occur. Fig. 1 gives the schematic of the experimental setup, which basically contains a storage tank
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Fig. 1. Schematic of the hydrodynamic cavitation setup.
of capacity 15 L. Liquid from the tank is pumped into the cavitating device by using an open impeller type pump with 3 blades and flow capacity of 5–15 L/min with a pressure head of 4.5 bar. Heat exchanger was added between the cavitating device and storage tank to cool the liquid coming out of the cavitating device. Fig. 2 shows schematic details of the actual cavitating device i.e., a stator and rotor assembly. Rotor is attached to a gear assembly, which is connected to a variable frequency drive (VFD: YASKAWA J1000, type: CIMR-JC 4A0011BAA). With the help of VFD, rotor can be rotated at different speeds of rotation. Rotor is a solid cylinder which has indentations (like a golf ball) on its surface. The diameter of the rotor is 19 cm with ratio of diameter to height of the rotor as 1.9 and this rotor is contained in a cylinder with diameter of 21 cm. Gap between the stator and the rotor is 10 mm and it is fixed. There are a total 204 indentations equidistant from each other. Each indentation is 12 mm in diameter, 20 mm deep and the schematic of an indentation has been given in Fig. 3, which also shows a likely liquid circulation pattern inside a single indentation. Due to high speed of rotation of the rotor, very high surface velocities are created at the surface of the cylinder. Such high velocities (typically 18–20 m/s) are responsible for the creation of high turbulence. This high turbulence/high shear is responsible for breakage/lysis of lignin, cellulose and hemicellulose complex. The hydroxyl radicals formed during cavitation are also responsible for hydrolysis/ionization of lignin molecule which makes it more hydrophilic and easy to remove from pulp. 2.3. Experimental and analytical procedure All the experiments were performed with wheat straw retted with 0.3 M KOH. Experiments were performed at a rotor rotational speed of 2200 rpm and 2700 rpm. The rotation speed was optimized using the Weissler reaction which quantifies the iodine liberation due to the cavitational effects and is a true measure of the
Fig. 3. Schematic of the single indentation, showing liquid flow pattern.
cavitational intensity [17]. At lower rotational speeds, it was observed that the velocities generated are not sufficient to create conditions of cavitation and are hence definitely not sufficient for enhancing the delignification based on the cavitational effects. The preliminary studies indicated that maximum cavitational activity as measured by the Weissler reaction is obtained at 2200 rpm. In addition the shear forces generated due to the rotational effects will be maximum at 2700 rpm (maximum speed that could be obtained based on the equipment) and hence additional experiments were also conducted at 2700 rpm. The wheat straw to water ratio (consistency) was varied from 1% to 7% wt/wt (1%, 3%, 5% and 7%). All the experiments were performed with tap water at room temperature. Total treatment time for all the experiments was 15 min. Samples were drawn after each 5 min for further analysis of the partially digested wheat straw. Hand-sheets of cavitation treated pulps were made on the basis weight of 100 g/m2 by using SHAAGE D-4330, systems laboratory sheet former according to DIN EN ISO 5269-2. After drying at temperature of 90 °C for 5 min, all the samples were conditioned at 50% relative humidity and a temperature of 23 °C. 3. Results and discussions 3.1. Reaction mechanism of delignification with hydrodynamic cavitation The mechanism of delignification in the case of conventional processes such as organosolv pulping, KOH and NH4OH pulping, oxygen delignification and peroxide delignification [18–20] have been well investigated. Also, few work have reported the effect of ultrasound operating parameters on the delignification [21] but the mechanistic details for the effect of cavitation on the delignification has been lacking in the open literature. Considering these aspects, we have tried to propose a delignification mechanism under the cavitating conditions for the treatment of wheat straw using hydrodynamic cavitation in the alkaline conditions. Important reactions of lignin, a polymer network of three major C6–C3 phenylpropane skeletons with many C–C and ether linkages, are initiated when phenolic hydroxyl group in lignin dissociates under alkaline conditions to form a resonance stabilized phenoxy radical. R
H + OH -
+ H2 O
O OH Fig. 2. Schematic of the cavitating device (stator and rotor assembly).
CH 3
O O
-
CH 3
ð1Þ
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This phenoxy radical is nothing but oxidized form of lignin which is more hydrophilic in nature and can be easily removed from pulp. Wheat straw contains all the three main lignin units, namely guayacyl, syringyl and coniferyl and hence there is abundant occurrence of phenolic hydroxyl group. When cavitation takes place, water molecule dissociates into OH and H due to the conditions of high pressure and temperature. Cavitation
H2 O ! OH þH
ð2Þ
These OH radicals are highly reactive and could be responsible for the oxidation of lignin molecule to some extent. But under the alkaline conditions, oxidation potential (reactivity) of the hydroxyl radicals decreases and recombination reaction between hydroxyl radical takes place to form hydrogen peroxide (H2O2).
OH þOH H2 O2
ð3Þ
However, under alkaline conditions (alkaline pH), H2O2 dissociates to form the hydroperoxy radical as per the reaction given below:
H2 O2 HOO þH
ð4Þ
HOO reacts with the residual H2O2 again to form highly reactive OH and superoxide (O2)
H2 O2 þ HOO ! OH þO2 þ H2 O
ð5Þ
The resonance stabilized lignin intermediates then undergo reactions with themselves which is also called as lignin condensation or with the hydroxyl (OH), hydroperoxy (HOO) radicals which are formed during cavitation to form organic acids, carbon dioxide and other low molecular weight organic products through the side chain elimination, ring opening and demethoxylation process (reaction (6)) as illustrated in the following reaction mechanisms [22]. R
R
O R
O
CH3
O
-
O
-
CH3
Lignin Condensation O O
-
CH3
HOO.
R
R
OOH
+
OOH O
O O
-
CH3
Side Chain Elimination
O
-
CH3
nents are present that can react with OH and/or O2, observed yield of O2 will reduce below that predicted theoretically. It has been reported that when H2O2 decomposes under alkaline conditions in presence of lignin containing substrates, less than the expected theoretical amount of O2 is evolved, indicating that at least some of the OH and/or O2 reacts with lignin [20]. The final acid insoluble lignin content was 4–5%. Once the lignin polymer is degraded the connector linkage between lignin and hemicellulose and celluloses are also significantly cleaved during the cavitation attack and cellulosic fibers can be liberated. At higher rotation speed the mechanistic effect of lignin degradation by cavitation helps the defibration of individual cellulosic fibers resulting into increased access to surface functional groups to participate in the bonding mechanism during sheet formation. 3.2. Effect of hydrodynamic cavitation on mechanical properties of paper Hand-sheets of cavitation treated pulps were made using laboratory sheet former according to DIN EN ISO 5269-2. After drying, all the samples were conditioned at 50% relative humidity and at a temperature of 23 °C before testing for its mechanical properties. The main objective of the experiments was to study the effect of hydrodynamic cavitation on the quality of pulp produced quantified in terms of the mechanical properties. Tensile strength measures the maximum force per unit width that a paper can resist before breaking when applying the load in a direction, parallel to the length of the strip (machine direction) [23]. Considering the fact that the weight of the each analyzed strip is different, for effective comparison of results, tensile index must be calculated by dividing the tensile force required to break the sheet by grammage (g/m2) of the sheet used. Grammage is nothing but weight of the one hand sheet per unit area. Fig. 4 shows the effect of the rotation speed on tensile strength of the paper sheet made from alkali treated wheat straw of different consistency (1%, 3%, 5%, and 7%) which is treated in hydrodynamic cavitation device for 15 min. It can be clearly seen that as the rotation speed increases from 2200 to 2700 rpm, tensile strength of treated fiber increases by almost 45% at 7% consistency after 15 min of treatment. The value of tensile strength as 5465 N/ m after 15 min of treatment is sufficient for types of paper like newsprint paper and/or paper board manufacture according to standard values for such type of paper [24]. Fig. 5 shows the effect of rotation speed on tensile index of the paper made from alkali treated wheat straw of different consistency (1%, 3%, 5%, and 7%) which is treated in hydrodynamic cavitation device for 15 min. Similar to tensile strength, tensile index of
Ring opening Demethoxylation Lignin Degradation
ð6Þ Use of only KOH is incapable of delignification of wheat straw in such a short span of time. Combining delignification of wheat straw in alkaline conditions with hydrodynamic cavitation provides additional hydroxyl and hydroperoxy radicals required for delignification reaction thereby intensifying the overall rates of reactions. In the absence of other reactants, hydroxyl and superoxide radicals can react with each other as illustrated below:
OH þO2 þ H ! O2 þ H2 O
ð7Þ
Combining reaction ((5)) and ((7))
H2 O2 þ HOO þH ! O2 þ 2H2 O
ð8Þ
It can be said that maximum amount of O2 that can be evolved from H2O2 is equal to half the molar amount of water which is formed during the reaction illustrated as Eq. (8). If other compo-
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Fig. 4. Effect of rotation speed on tensile strength of the paper.
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Fig. 5. Effect of rotation speed on tensile index of the paper.
hand sheet prepared after the hydrodynamic cavitation treatment also increased with an increase in the rotation speed. This increase in tensile strength and tensile index of the hand sheets with rotation speed of rotor can be explained by the fact that, as the speed of rotation increases surface velocities generated at the surface of rotor increases. These high surface velocities are responsible for generation of cavitating conditions in this type of device [25]. Also, with an increase in the surface velocity due to an increase in the rotation speed of rotor, cavitational intensity also increases which results into more number of reactive hydroxyl radicals (OH) and mechanical effects in terms of turbulence and shear, which in turn increases delignification of the wheat straw. The extent of beneficial effects obtained by the increasing speeds of rotation of rotor must be weighed against the increase in the energy required for the rotation of rotor as it increases with an exponent of 3 over the speed of rotation. Fig. 6 shows the change in tensile index of the paper made from the treated pulp with respect to the treatment time of hydrodynamic cavitation. It can be clearly seen that as the treatment time increases (data at time equal to zero indicates the control data without the use of hydrodynamic cavitation), tensile index of the paper increases linearly. Also when pulp is treated by hydrodynamic cavitation there is an improvement in the tensile index of
Fig. 6. Effect of rotation speed on change in tensile index of the paper with time.
the paper. At 7% consistency and rotation speed of 2200 rpm, tensile index increases by approximately 45% after 15 min of hydrodynamic cavitation treatment. Similarly, at 7% consistency and rotation speed of 2700 rpm, tensile index increases by approximately 66% (from 30 to 50 N-m/g) after 15 min of hydrodynamic cavitation treatment. This can be explained by the fact that as treatment time increases, exposure time of lignin-cellulose complex to cavitation also increases, which in turn increases the overall delignification. As the lignin is removed from the cellulose-lignin complex, strength of the paper fiber increases. In the present work the lignin removal has not been quantified but credence to the hypothesis can be obtained from earlier reported work [21,26]. It has been also reported that shock waves created due to cavitation are responsible for the breakage of chemical bonds in complex organic structure [16]. When shear generated by cavity collapse exceeds the strength of the chemical bond, breakage of bond takes place. In the present case also, shock waves are responsible for the breakage of some chemical bonds between lignin cellulose complexes. Another significant mechanical property of the paper is its breaking length. Breaking length is the limiting length of a strip of uniform width, beyond which, if such a strip were suspended by one end, it would break of its own weight. Breaking length is usually expressed in km. Fig. 7 shows the effect of rotation speed on the breaking length of the hand sheet made from the treated pulp. It can be clearly seen that as the rotation speed of the rotor increases, breaking length also increases. This can be explained on the basis of the fact that as the rotation speed of the rotor increases, cavitational intensity is more and hence there is a significant improvement in mechanical properties of paper. It should be noted here that there is an improvement in the mechanical properties of the hand sheets made from alkaline treated wheat straw after the hydrodynamic cavitation treatment. Depending on the properties of the final paper required it is easily possible to optimize process parameters such as solid to liquid ratio (consistency), concentration of alkali, hydrodynamic cavitation parameters such as rotation speed, time of operation and different types of cavitaing devices (orifice or venturi). The tensile strength of the paper depends on several factors that include: (1) the average load-bearing ability of the individual fibres, (2) the number of fibres at any given cross-section available for load transfer, and (3) the uniformity of the load transfer that is allowed by the network structure [27]. On the other hand, non-homogeneity is a fundamental feature of the paper network, meaning that non-uniformity of the load transfer cannot be avoided. It is also obvious that improving fiber bonding (removing lignin and hemicellulose) can improve the ability of network to
Fig. 7. Effect of rotation speed on breaking length of the paper.
M.P. Badve et al. / Ultrasonics Sonochemistry 21 (2014) 162–168
support load. Less lignin helps the development of interfiber bond between accessible OH group on the surface of cellulosic fibers. 3.3. Energy calculations (electrical energy per order EEO) In this section, the total electrical energy consumed by the hydrodynamic cavitation is compared with the electrical energy consumed by conventional Kraft pulping and steam explosion. As defined by Bolton et al. [28] electrical energy per order can be defined as the electric energy in kilowatt hours [kWh] required to degrade/react a contaminant/reactant by one order of magnitude in a unit volume [e.g., 1 m3 (1000 L)] of contaminated water or air. For the present work,EEO values [kWh/m3/order] can be calculated using the following formulas:
EEO ¼
EEO ¼
P t 1000 Batch operation
v log
ci cf
P t 1000 Continuous operation F log cfci
where, P is the rated power [kW] of the system, V is the volume [L] of water or air treated in the time t [hr], F is the water or air volume flow rate [m3/hr], ci, cf are the initial (or influent) and final (or effluent) concentrations [M or mol/L or g/L], and the factor of 1000 converts g to kg. Higher EEO values correspond to lower removal efficiencies. EEO of hydrodynamic cavitation is calculated based on the operating parameters used in the present work. For Kraft pulping and steam explosion EEO is calculated by using data illustrated in the earlier work [3,4]. EEO for hydrodynamic cavitation, Kraft pulping and for steam explosion was found to be 1785 kWh/m3/order, 3942 kWh/m3/order and 3482 kWh/m3/order, respectively. It can be established that electrical energy per order (EEO) required for hydrodynamic cavitation is considerably less (nearly 50%) than that required for conventional Kraft pulping or the steam explosion. The main advantage of hydrodynamic cavitation is that even when the conditions of high temperature and high pressure zones are generated locally which are required for delignification to take place, the overall conditions are always ambient avoiding the requirement of any heat duties. In the other pulping techniques, more energy is consumed in creation of such conditions of high temperature and pressure which is mainly required in the case of Kraft pulping and steam explosion. Another advantage of hydrodynamic cavitation is the reduced requirement of the treatment time compared to conventional processes, which also could be responsible for reduced energy consumption.
4. Conclusions and path foward The present work has clearly established the utility of hydrodynamic cavitation for the effective treatment of wheat straw in alkaline conditions. The investigations related to the effect of operating parameters such as rotation speed and straw to water ratio revealed that rotation speed and the straw to water ratio has a significant effect on final properties of the paper hand sheet made out of the treated pulp. Following important conclusions can be drawn from the present work: 1. As rotation speed of the rotor was increased from 2200 rpm to 2700 rpm, extent of delignification increases which in turn increases the tensile properties of the hand sheet. 2. As water consistency (straw to water ratio) increases, mechanical properties of paper also increases. When consistency was increased from 5% to 7%, tensile index increases by 50%. The
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consistency should be selected based on the desired properties of the final paper and availability of water and the present work has clearly illustrated the correspondence between the mechanical properties and the water consistency. 3. Calculations of electrical energy required per order per unit volume confirmed that hydrodynamic cavitation requires significantly less energy as compared to other conventional techniques. Overall, it can be said that the major advantage of hydrodynamic cavitation treatment of wheat straw for the production of paper or paper board is less treatment time, less energy consumption compared to other conventional techniques and operation at near ambient conditions. Hydrodynamic cavitation reactors are relatively easy to scale up and various operating parameters such as rotation speed of the rotor, consistency of the straw can be easily optimized depending upon the required final properties of the paper or paper board. It is also important to note that hydrodynamic cavitation can be easily scaled up for industrial size plants as compared to the use of sonochemical reactors. Depending on the final qualities of the paper required and optimized results of the lab scale study, cavitating conditions can be finalized. Cavitating device can be designed accordingly depending on the volume to be treated and treatment time required. To achieve this objective of application at commercial scale installations, investigations would be required to check the effectiveness of the cavitation over conventional processes of cellulose digestion in terms of energy and time requirement, for a wider range of starting raw materials and also operating parameters such as alkali concentration and temperature.
Acknowledgements One of the authors, PRG, would like to acknowledge the funding from University Grants Commission, India and Hungarian Scholarship Board, Balassi Institute for funding the visit to University of West Hungary, Sopron under the Indo-Hungarian Educational Exchange Program 2012.
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