Pretreatment of Lignocellulosic Biomass Using Vortex-Based Devices for Cavitation: Influence on Biomethane Potential

Hydrodynamic cavitation mediated Spirulina valorisation with insights into phycocyanin extraction and biogas production

Commercial phycocyanin extraction is energy-intensive and lacks scalability. Alternatively, this study reports the systematic investigation of hydrodynamic cavitation for intensified phycocyanin extraction from Spirulina. Additionally, biomethane potential of the residual biomass, obtained after phycocyanin extraction was also investigated. The biomethane generation rate decreased with an increasing number of passes while the biomethane potential remained unaffected. To reliably compare phycocyanin extraction across systems, dimensionless normalised yields were defined. A normalised phycocyanin yield of 4.3 (52 mg phycocyanin g-1) at an inlet pressure of 150 kPa and 90 passes was identified (optimum cavitation). Lowest specific energy input (0.06kWh kg-1) was calculated for processing 100 g L-1 Spirulina, which is one to two orders of magnitude lower than current state-of-the-art. Furthermore, a net energy gain of 600-2497kWh kg-1 obtained from biomethane generation showcased a viable Spirulina biorefinery, intensified via hydrodynamic cavitation. This work provides a route for phycocyanin extraction with significantly reduced energy input and potential for wider bioproduct extraction and biorefining from a range of biomasses via hydrodynamic cavitation.

Enhancement of biomethane potential of brown sludge by pre-treatment using vortex based hydrodynamic cavitation

Novel, non-thermal and economically benign pre-treatment process was developed for enhancing valorisation potential of brown sludge generated by dairy industry wastewater treatment plant (WWTP). Vortex-based hydrodynamic cavitation (HC) device was used to quantify influence of pretreatment by measuring biomethane potential (BMP) of untreated and treated brown sludge. Pre-treatment parameters, primarily, pressure drop and number of passes through the cavitation device were varied to quantify influence on BMP. BMP tests were performed at 39 °C containing 5% of total solids in each reactors using an automatic BMP measurement system containing 15 reactors with each volume of 500 mL fitted with overhead stirrer. HC treatment increased the soluble chemical oxygen demand (sCOD) by more than 25% which increased the BMP. HC treatment was able to push the BMP of treated sludge to more than 80% of the theoretical BMP. Volatile solids (VS) removal was more than 65%. Highest methane yield was 376 mL/g-VS of sludge. The methodology and results presented here show significant potential to valorise brown dairy sludge via vortex based hydrodynamic cavitation.

Pretreatment of herbal waste using sonication

The effect of hydrodynamic cavitation (HC) and the manner it affects the biodegradability of herbal waste suspended on municipal wastewater subjected to mechanical pre-treatment was examined in this paper. The HC was performed at an optimal inlet pressure equal to 3.5 bar and with the cavitation number of 0.11; the number of recirculation passes through the cavitation zone amounted to 30.5. The BOD₅/COD ratio was enhanced by more than 70% between the 5th and 10th minute of the process, indicating the enhanced biodegradability of herbal waste shortly. Fiber component analysis, FT-IR/ATR, TGA and SEM analysis were conducted to check the findings and to demonstrate changes in the chemical and morphological structure of herbal waste. It confirmed that hydrodynamic cavitation visibly influenced the herbal composition and their structural morphology, decreased hemicellulose, cellulose and lignin content, but did not form the by-products affecting the subsequent biological treatment of herbal waste.

Intensifying extraction of biomolecules from macroalgae using vortex based hydrodynamic cavitation device

Macroalgae have a tremendous potential to become an important renewable resource for valuable biomolecules and chemicals. New and improved ways of cell disruption and of enhancing rate as well as yield of extraction of valuable products from macroalgae are needed to fully realise this potential. In this work, hydrodynamic cavitation (HC) was used for intensifying rate and yield of extraction of phycoerythrin, proteins and carbohydrates from marine macroalgae Palmaria palmata. We use vortex-based HC devices which do not use small restrictions like orifice-based HC devices or moving parts like rotor-stator based HC devices. A bench scale setup with a nominal slurry flow rate of 20 LPM was established. Dried and powdered macroalgae was used. Influence of key operating parameters like pressure drop and number of passes on extraction performance (the rate and yield) was measured. A simple, yet effective model was developed and used for interpreting and describing experimental data. The results indicate that there exists an optimum pressure drop across the device at which extraction performance is maximum. The extraction performance with HC was found to be significantly better than the stirred vessels. HC has resulted in 2 to 20 times improvement in the rate of extraction of phycoerythrin (R-PE), proteins and carbohydrates. Based on the results obtained in this work, pressure drop of 200 kPa and number of passes through the HC devices of about 100 were found to be most effective for HC-assisted intensified extraction from macroalgae. The presented results and model will be useful for harnessing vortex-based HC devices for intensifying the extraction of valuable products from macroalgae.

Modeling of Hydrodynamic Cavitation Reactors: Reflections on Present Status and Path Forward

Hydrodynamic cavitation (HC) is finding ever increasing applications in water, energy, chemicals, and materials sectors. HC generates intense shear, localized hot spots, and hydroxyl radicals, which are harnessed for realizing desired physicochemical transformations. Despite identification of HC as one of the most promising technology platforms, its potential is not yet adequately translated in practice. Lack of appropriate models for design, optimization, and scale-up of HC reactors is one of the primary reasons for this. In this work, the current status of modeling of HC reactors is presented. Various prevailing approaches covering empirical, phenomenological, and multiscale models are critically reviewed in light of personal experience of their application. Use of these approaches for different applications such as biomass pretreatment and wastewater treatment is briefly discussed. Some comments on extending these models for other applications like emulsions and crystallization are included. The presented models and discussion will be useful for practicing engineers and scientists interested in applying HC for a variety of applications. Some thoughts on further advances in modeling of HC reactors and outlook are shared, which may stimulate further research on improving the fidelity of computational models of HC reactors.

Pretreatment of milled and unchopped sugarcane bagasse with vortex based hydrodynamic cavitation for enhanced biogas production

Anaerobic digestion can potentially valorise sugarcane bagasse to biogas and fertiliser. Pretreatment is however required to overcome recalcitrance and enhance the biogas yields. Literature reporting the investigation of various biomass pretreatments often use milled biomass as substrate rather than as-received fibrous biomass. This does not establish the true influence of the pretreatment type on biogas generation. Additionally, milling energy is also ignored when calculating net energy gains from enhanced biogas yields and are thus misleading. In this work, a vortex-based hydrodynamic cavitation device was used to enhance the biomethane yields from fibrous as-received biomass for the first time. Clear justification on why milled biomass must not be used as substrates for demonstrating the effect of pretreatment on biogas production is also discussed. The net energy gain from milled hydrodynamic cavitation pre-treated bagasse can be similar to as-received bagasse only when the specific milling energy is ≤700 kWh/ton.

Intensification of Acidogenic Fermentation for the Production of Biohydrogen and Volatile Fatty Acids—A Perspective

Utilising ‘wastes’ as ‘resources’ is key to a circular economy. While there are multiple routes to waste valorisation, anaerobic digestion (AD)—a biochemical means to breakdown organic wastes in the absence of oxygen—is favoured due to its capacity to handle a variety of feedstocks. Traditional AD focuses on the production of biogas and fertiliser as products; however, such low-value products combined with longer residence times and slow kinetics have paved the way to explore alternative product platforms. The intermediate steps in conventional AD—acidogenesis and acetogenesis—have the capability to produce biohydrogen and volatile fatty acids (VFA) which are gaining increased attention due to the higher energy density (than biogas) and higher market value, respectively. This review hence focusses specifically on the production of biohydrogen and VFAs from organic wastes. With the revived interest in these products, a critical analysis of recent literature is needed to establish the current status. Therefore, intensification strategies in this area involving three main streams: substrate pre-treatment, digestion parameters and product recovery are discussed in detail based on literature reported in the last decade. The techno-economic aspects and future pointers are clearly highlighted to drive research forward in relevant areas.

Solvent-assisted cavitation for enhanced removal of organic pollutants - Degradation of 4-aminophenol

A new approach of solvent-assisted cavitation process was proposed for degradation of organic pollutants. The process envisages the use of suitable solvent as an additive, (1-5% v/V), in the conventional cavitation process to enhance the pollutant removal efficiency. A proof of concept was provided for the removal of ammoniacal nitrogen with significantly improved efficiency using solvent-assisted hydrodynamic cavitation (HC) compared to conventional HC. The efficacy of the process was studied on a pilot plant scale (1 m3/h) and using vortex flow based vortex diode as a cavitating device. Degradation studies were carried out using a model pollutant, 4-aminophenol and four different solvents as additives, 1-octanol, cyclohexanol, 1-octane and toluene. Relatively polar solvents were found to increase the efficiency of the pollutant removal (>65%) and also increase the rates to an extent of more than 200%, compared to only HC. A very high removal of ammoniacal nitrogen, more than 90%, was obtained for solvents 1-octanol and cyclohexanol, indicating the importance of the selection of solvent. Per-pass degradation model showed 3 to 4 times increase in the per pass degradation for polar solvents compared to cavitation alone. The results confirm no role of conventional solvent extraction and no specific contamination of wastewater due to the use of solvent as an additive in the process. Further, the cost was 2-3 times lower as compared to the conventional HC. The interesting observations in the proposed process can fuel further research to provide possible improvements in existing methodologies of wastewater treatment, in general, and for removal of ammoniacal nitrogen, in particular.

Recent advances in hydrodynamic cavitation-based pretreatments of lignocellulosic biomass for valorization

Recently, the hydrodynamic cavitation (HC)-based pretreatment has shown high effectiveness in laboratories and even in industrial productions for conversion of lignocellulosic biomass (LCB) into value-added products. The pretreatment capability derives from the extraordinary conditions of pressures at ∼500 bar, local hotspots with ∼5000 K, and oxidation (hydroxyl radicals) created by HC at room conditions. To promote this emerging technology, the present review summarizes the recent advances in the HC-based pretreatment of LCB. The principle of HC including the sonochemical effect and hydrodynamic cavitation reactor is introduced. The effectiveness of HC on the delignification of LCB as well as subsequent fermentation, paper production, and other applications is evaluated. Several key operational factors (i.e., reaction environment, duration, and feedstock characteristics) in HC pretreatments are discussed. The enhancement mechanism of HC including physical and chemical effects is analyzed. Finally, the perspectives on future research on the HC-based pretreatment technology are highlighted.

A simplified model for simulating anaerobic digesters: Application to valorisation of bagasse and distillery spent wash

Current anaerobic digestion (AD) design methods rely on crude empirical models or sophisticated anaerobic digestion models (like ADM1) requiring a large number of parameters which are difficult to obtain experimentally. A simplified model for simulating AD was developed in this work. The model requires knowledge of CH₄/CO₂ ratio in biogas or indigestible fraction in substrate and batch biomethane potential (BMP) data for estimating three kinetic parameters (maximum specific growth rate, half velocity constant and cell death rate). Reported lab scale BMP data of sugarcane bagasse and spent wash were used to first estimate the kinetics and then to simulate corresponding largescale AD. Simulated results of specific methane yield and digester performance were consistent with available largescale AD data. The potential of the model to simulate single and multi-stage AD were illustrated. The presented approach and model will be useful for effectively valorising a variety of complex biomass substrates to biogas.

Estimation of chemical and physical effects of cavitation by analysis of cavitating single bubble dynamics

Cavitation events create extreme conditions in a localized 'bubble collapse' region, leading to the formation of hydroxyl radicals, shockwaves and microscopic high-speed jets, which are useful for many chemical and physical transformation processes. Single bubble dynamics equations have been used previously to investigate the chemical and physical effects of cavitation. In the present study, the state of the art of the single bubble dynamics equations was reviewed and certain noteworthy modifications were implemented. Simulations reaffirmed previously reported collapse temperatures of the order ~5,000 K and collapse pressures well over ~1,000 bar under varying operating conditions. The chemical effects were assessed in terms of the hydroxyl radical generation rate (OHG), calculated by applying the minimization of the Gibb's Free Energy method using simulated collapse conditions. OHG values as high as 1x1012OH molecules per collapse event were found under certain operating conditions. A new equation was proposed to assess the physical effects, in terms of the impact pressure of the water jet - termed as the jet hammer pressure (JHP), formed due to the asymmetrical collapse of a bubble near a wall. The predicted JHP were found to be within a range of ~100 to 1000 bar under varying operating conditions. Important issues such as the onset of cavitation and chaotic solutions, for a cavitating single bubble dynamics were discussed. The Blake threshold pressure was found to be a sufficient criterion to capture the onset of cavitation. The impact of key operating parameters on the chemical and physical effects of cavitation were investigated exhaustively through simulations, over the parameter ranges relevant to acoustic and hydrodynamic cavitation processes. Presented methodology and results will be useful for optimisation and further investigations of a broad range of acoustic and hydrodynamic cavitation-based applications.

Life cycle and economic assessment of sugarcane bagasse valorization to lactic acid

In this work, detailed life cycle assessment (LCA) and techno-economic analysis (TEA) of a novel lactic acid (LA) production process from sugarcane bagasse is performed, with the objective of identifying process improvement opportunities. Moreover, this is first such study in the Indian context. Experimental data generated at the Vasantdada Sugar Institute (VSI) for upstream processes is combined with ASPEN Plus simulation of the downstream steps for a commercial plant producing 104 tonnes per day of LA. Equipment sizing is performed and costing is done using standard approaches. OpenLCA is used to develop the LCA model and Ecoinvent database is used to quantify life cycle impacts for 1 kg of LA. Different scenarios for the LA plant are studied. Results showed that the pretreatment stage was crucial from both economic and environmental perspectives. The total life cycle climate change impact for production of 1 kg of lactic acid was 4.62 kg CO₂ eq. The product cost of LA was USD 2.9/kg, and a payback time of 6 years was achieved at a selling price of USD 3.21/kg. Scenario analysis has revealed that lactic acid plant annexed to a sugar mill led to significant environmental and economic benefits. Sensitivity analysis has identified opportunities to reduce the life cycle climate change impact to 2.29 kg CO₂ eq. and product cost to USD 1.42/kg through reduced alkali consumption, higher solid loading, and reduced enzyme loading.

ANN based modelling of hydrodynamic cavitation processes: Biomass pre-treatment and wastewater treatment

We have developed artificial neural network (ANN) based models for simulating two application examples of hydrodynamic cavitation (HC) namely, biomass pre-treatment to enhance biogas and degradation of organic pollutants in water. The first case reports data on influence of number of passes through HC reactor on bio-methane generation from bagasse. The second case reports data on influence of HC reactor scale on degradation of dichloroaniline (DCA). Similar to most of the HC based applications, the availability of experimental data for these two applications is rather limited. In this work a systematic methodology for developing ANN model is presented. The models were shown to describe the experimental data very well. The ANN models were then evaluated for their ability to interpolate and extrapolate. Despite the limited data, the ANN models were able to simulate and interpolate the data for two very different and complex HC applications very well. The extrapolated results of biomethane generation in terms of number of passes were consistent with the intuitive understanding. The extrapolated results in terms of elapsed time were however not consistent with the intuitive understanding. The ANN model was able to generate intuitively consistent extrapolated results for degradation of DCA in terms of number of passes as well as scale of HC reactor. The results will be useful for developing quantitative models of complex HC applications.

Scale-up of vortex based hydrodynamic cavitation devices: A case of degradation of di-chloro aniline in water

Hydrodynamic cavitation (HC) is being increasingly used in a wide range of applications. Unlike ultrasonic cavitation, HC is scalable and has been used at large scale industrial applications. However, no information about influence of scale on performance of HC is available in the open literature. In this work, we present for the first time, experimental data on use of HC for degradation of complex organic pollutants in water on four different scales (~200 times scale-up in terms of capacity). Vortex based HC devices offer various advantages like early inception, high cavitational yield and significantly lower propensity to clogging and erosion. We have used vortex based HC devices in this work. 2,4 dichloroaniline (DCA) - an aromatic compound with multiple functional groups was considered as a model pollutant. Degradation of DCA in water was performed using vortex-based HC devices with characteristic throat dimension, dt as 3, 6, 12 and 38 mm with scale-up of almost 200 time based on the flow rates (1.3 to 247 LPM). Considering the experimental constraints on operating the largest scale HC device, the experimental data is presented here at only one value of pressure drop across HC device (280 kPa). A previously used per-pass degradation model was extended to describe the experimental data for the pollutant used in this study and a generalised form is presented. The degradation performance was found to decrease with increase in the scale and then plateaus. Appropriate correlation was developed based on the experimental data. The developed approach and presented results provide a sound basis and a data set for further development of comprehensive multi-scale modelling of HC devices.