Cavitation Reactor for Pretreatment of Liquid Agricultural Waste

Abstract

One of the most well-known methods of intensifying the process of anaerobic digestion is the pretreatment of raw materials. For the first time, the use of a jet-driven Helmholtz oscillator for biomass pretreatment is proposed. The design of the device is optimal for creating hydraulic cavitation; however, in this case, acoustic oscillations are generated in the system and resonance occurs. In this study, the optimal design of this device was determined for the subsequent design of a cavitation reactor. The diameter of the resonant chamber was varied in the range from 28.3 to 47.5 mm, and its length from 6 to 14 mm; in addition, the diameter of the outlet was changed from 6.1 to 6.3 mm. Based on the experimental data obtained, it was found that the optimal ratio of the length of the resonator chamber to the diameter of the inlet nozzle is 1.73, and the inner diameter of the resonator chamber to the diameter of the inlet nozzle corresponds to 5.5. Improving the technology of agricultural waste disposal will ensure their maximum involvement in economic circulation, reduce the consumption of traditional fuel and energy resources, and improve the technological and machine-building base, which makes it possible to produce competitive cavitation reactors.

1. Introduction

Anaerobic digestion (AD) technology is widely throughout the world. The unique advantage of this technology is the solution of both environmental and energy problems. Environmental protection and energy conservation are among the most important issues at present [1]. The efficient use of biomass resources, including agricultural waste, will increase the world’s reserves of renewable energy resources and help achieve climate change mitigation goals [2]. It should be noted that 772.6 million tons of waste with a content of 228 million tons of dry matter is generated annually in the agro-industrial complex of Russia [3], and this is a constant and large-tonnage source of energy.

To increase the productivity of AD technology, it is necessary to pretreat the feedstock entering the reactor [4]. This allows the preparation of substrates for biological decomposition by microorganisms involved in the fermentation process, as well as increasing the yield of biogas [5]. All methods of pretreatment can be divided into the following categories: physical, thermal, chemical, biological, and a combination of these. A separate group is represented by more innovative methods [6]. Figure 1 shows a number of methods used to prepare raw materials. Innovative pretreatment technologies are based on extreme and non-classical processes, such as ultrasound [6], gamma rays [7], electron beam irradiation [8], pulsed electric field [9], high hydrostatic pressure [10], and high-pressure homogenization [11].

Of particular interest among innovative methods is the generation of cavitation, when vapor microbubbles in a liquid medium collapse due to sharp drops in local pressure, and a large amount of energy is released, as well as intense local heating (about 5000 °C) and high pressure (about 50 MPa) [12]. If the pressure decrease occurs due to high local velocities in the flow of a moving droplet liquid, then the cavitation is called hydrodynamic, and if it is due to the passage of acoustic waves it is known as acoustic. Under the action of directed and controlled cavitation in biomass, the destruction of complex fibrous bonds of organic substances at the molecular level occurs, the particle sizes decrease to 0.1–8 µm, as a result, the biogas yield increases by 30–50% [13].

The main advantages of integrating ultrasonic technologies that create acoustic cavitation into agricultural biogas plants (full-scale application) [14] are:

• the intensification of the AD process;
• increase in biogas output;
• increasing the content of methane in the resulting gas;
• the economy of the substrate;
• decrease in viscosity;
• reduction in the energy consumption of agitators and pumps.

Cavitation reactors have shown significant promise for application in biogas technologies due to advantages such as short processing times and higher degradation rates [15]. In practice, the energy consumption of high-intensity ultrasonic systems can be maintained at a fairly low level (<5 kWh/m³) [14]. Thus, the use of high frequency sonic waves is an attractive pretreatment method, but experimental evaluation is still needed to weigh the potential benefits of sonication against additional energy requirements [16].

Hydrodynamic cavitation devices can be broadly classified into two categories [17]:

• those with moving parts, such as a rotor–stator [18,19,20,21];
• without moving parts, for example, a vortex-based device [22,23,24,25], swirling jet [26], venturi tube [27,28], hydrosonic pump [12], and orifice plate [29,30].

The hydrodynamic cavitation reactor is based on a stator and rotor and has been used for pretreatment of wheat straw, which made it possible to increase the yield of biogas by two times [18]. Hydrodynamic cavitation occurs in a flow system with a constriction, such as a venturi tube [27]. The efficiency of the pretreatment of lignocellulosic biomass using a combination of hydrodynamic cavitation and the introduction of sodium percarbonate was studied in [27]. It has been proven that the efficiency of the process can be increased by changing the geometry of the constriction, i.e., to achieve this, it is necessary to use a narrower neck. A hydrodynamic cavitation reactor, including a stator–rotor assembly, was used for the pretreatment of wheat straw, which allowed a two-fold increase in the biogas yield at AD [18]. The synergistic effect of the combined pretreatment can also be achieved by maximizing the release of extracellular polymeric substances from the biomass [31]. In a number of studies, the influence of temperature on hydrodynamic cavitation flows was determined in order to find optimal conditions that increase the intensity of cavitation treatment [32,33,34,35,36]. It should be noted that the effect of temperature on hydrodynamic cavitation characteristics was significant for lignocellulosic raw materials, since pretreatment at 70 °C resulted in greater enzymatic digestibility than at 40 °C [37].

In ref. [12], acoustic and hydrodynamic cavitation was compared during pretreatment. The same effect of these two methods on the anaerobic fermentation of cattle manure and slurry mixed with wheat straw was experimentally established. Thus, any pretreatment that generates cavitation is a promising solution due to low capital investment and ease of operation [38].

Acoustic cavitation in raw materials pre-treatment systems before anaerobic digestion is based on the use of an ultrasonic transducer, which converts high-frequency voltage into ultrasonic frequency mechanical vibrations. Hydrodynamic cavitation is artificially created by constriction in the channel where the liquid moves. A feature of the jet-driven Helmholtz oscillator (JDHO) is the presence of a constriction, as a result of which acoustic oscillations are generated in the system. The JDHO, which can continuously generate a spatial jet stream entirely based on its internal geometry without any moving parts [39]. The JDHO creates an oscillating jet with different frequencies without the influence of any external forces [40]. Thus, this cavitation device is very efficient [41]. The authors of [42] present the results of a study of the cavitation behavior of an oscillating jet flowing through a JDHO. They showed that such a design creates cavitation clouds in the liquid, which propagate over long distances. In the study [43], it was also demonstrated that such a cavitation device design is more efficient than the classical “organ pipe nozzle”. A greater effect is achieved due to the resonant amplification of self-oscillations of the submerged jet [44]. The fluidic oscillators are widely used in the physical and chemical effects of acoustic cavitation, which are utilized in biotechnology [45,46], oil refining [47], and the oil industry [48], in particular, in drilling wells and intensifying oil production [49], green chemistry [50], the food industry [51], agriculture [52], etc. [53]. The JDHO is currently used in the oil industry and has never before been used for the pre-treatment of raw materials before anaerobic digestion. There is no description in the literature of studies on the use of a jet oscillator in the process of the pretreatment of biomass for the subsequent production of biogas. The aim is to determine the optimal design of the JDHO for the design of a cavitation reactor that allows the pretreatment of liquid agricultural waste before AD.

2. Object and Method of the Research

2.1. Jet-Driven Helmholtz Oscillator

The design of the considered device is based on the JDHO [41,44]. It is presented as a dual device that combines two independent elements in one housing: a jet generator of pressure oscillations and an acoustic resonator. The acoustic resonator consists of a cylindrical resonant chamber with two parallel covers (Figure 2a).

In the front cover (1), in the direction of flow, there is an inlet nozzle through which a working agent (gas, liquid) is fed into the resonator. The inlet nozzle forms a jet of the working agent. In the rear cover (2), there is an outlet with sharp edges through which the working agent is removed from the resonator. The inlet nozzle and the outlet are located on the axis of the resonant chamber (3). The configuration “nozzle–jet–hole” is a jet generator. It generates the hole tone. The liquid volume contained in the resonant chamber (3), the inlet nozzle in the front cover, and the outlet in the power housing is responsible for amplifying the harmonic of the hole tone at frequencies close to the natural frequency of the resonator, the value of which can be determined by the formula [49]:

$$f_0=\frac{c_0}{2\pi D_C}\sqrt{\frac{1}{L_C}\left(\frac{d_1^2}{{\mathcal{l}}_1+0.25\pi d_1}+\frac{d_2^2}{{\mathcal{l}}_2+0.25\pi d_2}\right)}$$

(1)

where c₀ is the speed of sound in the working medium, (m/s); D_C is the inner diameter of the resonator chamber (m); L_C is the length of the resonator chamber (m); d₁ and d₂ are the diameters of the inlet nozzle and outlet (m); and ℓ₁ and ℓ₂ are the effective lengths of the inlet nozzle and outlet (m).

In this work, the object of study was the JDHO with the following geometric characteristics: an inlet nozzle with a diameter of d₁ = 5.5 mm and a length of ℓ₁ = 1 mm; outlet with diameters d₂ = 6.3, 6.2, and 6.1 mm and length ℓ₂ = 1 mm; and the diameter of the resonant chamber D_C varied in the range from 28.3 to 47.5 mm, and its length L_C varied from 6 to 14 mm. The thickness of the covers in which the inlet nozzle and the outlet are placed is 4 mm.

2.2. Experimental Setup

The working agent in this work was air. The results of experiments presented as the dependences of dimensionless similarity numbers (Strouhal number, Mach number) can be extended to other liquids, including incompressible ones.

As any oscillatory fluid–flow phenomena with fixed-geometry boundaries, jet-driven oscillators are governed by the Strouhal number, defined as [54]:

$${\mathrm{Sh}}_{\mathrm{d}}=\frac{f\cdot d}{W}$$

(2)

where f is the frequency of the oscillations (Hz); d is inlet nozzle diameter (m); and W bulk velocity in the exit of the inlet nozzle (m/s).

The definition employed for the Mach number was [55]:

$$M=\frac{W}{c_0}$$

(3)

To conduct experimental studies, a stand was developed, the scheme of which is shown in Figure 3.

As seen in Figure 3, the emitter (1) under study was located in the cover of the vacuum chamber. The formation of a jet in the resonant chamber of the device was carried out by sucking air from the vacuum chamber with the help of a vacuum pump (5) through a hose (4). Air was supplied to the inlet of the device from the surrounding space. With the help of voltage regulator (6), the suction pressure and, accordingly, the jet velocity were varied. The change in air pressure in the vacuum chamber and the generated pressure fluctuations were recorded on a computer through an external analog-to-digital converter (ADC) board with a sampling rate of 10 kHz. To measure these parameters, a measuring microphone and a pressure transducer were used.

The experimental data recorded on a computer were processed and the change in the pressure drop across the emitter (in time), the root-mean-square amplitude p, and the characteristic frequency of the generated oscillations f were determined. The pressure drop was used to calculate the jet velocity W in the resonant chamber of the device. According to the data obtained, the change in the mean square oscillation amplitude p/q reduced to the velocity head (q = ρ · W²/2) on the jet velocity was studied, similarly to [56].

An experimental study of the frequency of natural oscillations of the resonator was determined by the method of sounding the device with “white noise”. For this, two measuring microphones were used: one of which recorded the incident sound wave and the second one recorded that which passed through the resonator. The ratio of the spectra of the transmitted signal to the incident signal makes it possible to obtain the resonant curve of the device.

2.3. Goal and Scope

The goal of this study was to determine the optimal design of the JDHO for the design of a cavitation reactor that allows the pre-treatment of organic waste. Based on the results of experimental studies, the following are determined: (a) the range of experimental data at which the amplitude of pressure pulsations is maximum, (b) the optimal ratio of the length of the resonator chamber to the diameter of the inlet pipe and the ratio of the inner diameter of the resonator chamber to the diameter of the inlet pipe, and (c) the optimal location of the JDHO in the design of the cavitation reactor depending on the Mach number and a dimensionless parameter that takes into account the atmospheric pressure of the medium and the operating pressure.

2.4. Assumptions and Limitations of the Study

When conducting research, the main assumptions and limitations should be noted:

• To determine the optimal design of the JDHO, which provides the maximum amplitude of pressure pulsations, the parameters d₂, D_C, and L_C were varied.
• During the operation of the JDHO, two cavitation mechanisms operate, namely: hydrodynamic and acoustic.
• The hydrodynamic mechanism is based on the fact that a local pressure drop occurs in the fluid flow when flowing through the nozzle. If the pressure in this area becomes lower than the pressure of saturated vapors or dissolved gases, then microbubbles are formed. Then, with an increase in local pressure and the collapse of microbubbles, cavitation occurs. This hydrodynamic mechanism also works during the development of vortex structures in the resonant chamber, since when the flow swirls, a region of low pressure is created in the center of the vortex.
• The acoustic mechanism is caused by the fact that pressure fluctuations created by the oscillator propagate in the environment and create elastic waves. During the passage of an elastic wave (in the half-cycle of the lower half-wave), a reduced pressure is created, which is lower than the pressure of the saturated vapors of a liquid or dissolved gases. This creates conditions for the formation of cavitation bubbles, which, when the pressure rises (in the half-cycle of the upper half-wave), collapse and create a cavitation effect.
• The shape of the cavitation bubbles, their size, and many other factors influence the collapse pressure. However, these issues are beyond the scope of this work and will be the subjects of future research.
• The JDHO is fundamentally new and has never before been used for pretreatment in AD technology.

3. Results of Experimental Studies

The study of natural oscillations of the emitter made it possible to obtain its resonance curve (Figure 4) and to determine the frequency of natural oscillations f₀. The obtained values with an error of less than 2% coincide with the theoretical value calculated by Equation (1).

An increase in the pressure drop across the emitter leads to the formation of a jet, the speed of which is determined by the properties of the working agent (in our case, air) and the value of the pressure drop, which is measured by a strain gauge pressure difference sensor. At a certain value of the jet velocity, the generation of the tone of the hole begins. Hole tone harmonics are amplified through resonance when their frequency coincides with the natural frequency of the radiator. The resonant amplification band is determined by the quality factor of the resonator (the width of the resonant curve).

The mechanism of generation of pressure oscillations by a JDHO is based on the resonant amplification of pressure fluctuations in a liquid (gas) flow passing through the emitter. The frequency of generated oscillations depends on the jet velocity and, at a certain liquid flow rate, coincides with the frequency of the natural oscillations of the resonator. From the point of view of the efficiency of its application, it is important that the operating mode of the device be resonant, which is why it is so important to determine the natural frequency of the emitter.

The mechanism observed in this work, due to the presence of the JDHO cavity, differs from the theoretical model proposed for the wedge tone and hole tone due to the presence of a closed (resonant) volume inside the chamber. The following sequence of events is assumed:

(1)

The flow in the jet contains a low-frequency ordered axisymmetric variable component (and periodic volume flow fluctuations). When this component enters the outlet and the jet encounters various resistances in the outlet plane of the oscillator, periodic pressure pulses arise inside the chamber.

(2)

These pulses are selectively amplified by the Helmholtz resonance mechanism and a pulsating pressure field is installed in the chamber.

(3)

The pulsating pressure field causes flow rate pulsations at the chamber inlet, which leads to an effective amplification of the jet oscillations at the frequency of the ordered component.

(4)

The viscous jet displacement layer (the expansion of the jet from its exit from the inlet nozzle to the start of collision with the outlet nozzle) responds to the amplification of jet oscillations in the range of its own acoustic frequencies and amplifies them. As a result, the ordered motion inside the jet is enhanced, vortex rings appear, and the circuit closes.

As the jet velocity increases, the amplitude of oscillations increases and, having reached the maximum value (it has its own for each version of the device geometry), begins to decrease. This is accompanied by an increase in the frequency of the generated oscillations. This pattern of changing the parameters of the generated oscillations is known as the mode (Mode). Each mode has its own Strouhal numbers. The first mode (Mode 1) is characterized by high jet velocities, and a further increase in velocity does not lead to the generation of harmonic oscillations at the resonant frequency. Figure 5 shows the dependences of the Strouhal numbers on the dimensionless length of the resonant chamber. The frequency of the generated oscillations and the jet velocity corresponded to the maximum value of the oscillation amplitude within each mode. These dependencies were approximated via the following linear function:

$${\mathrm{Sh}}_{\mathrm{I}}=0.417-0.08\frac{L_C}{d_1}$$

(4)

$${\mathrm{Sh}}_{\mathrm{II}}=0.609-0.087\frac{L_C}{d_1}$$

(5)

Within each mode, the characteristic Strouhal number practically does not change with increasing jet velocity. As the jet velocity increases, a jump from one mode to another (from Mode 2 to Mode 1) can occur. From the point of view of process hydrodynamics, the mode number represents the number of simultaneously moving vortex rings in the resonant chamber around the jet [41]. Inside the vortex ring, there is a zone of low pressure due to the circulation of the flow, and favorable conditions are created in this zone for the formation of cavitation clouds. In the study [42], photographs were taken using a high-speed camera, showing that the cavitation cloud is able to stretch over a length of 1 d₁ to 10 d₁, and at this distance, the cloud is continuous. The cavitation cloud grows in the radial direction until it reaches the maximum distance at which it collapses [57].

Figure 6 shows the dependences of the maximum value of the root-mean-square amplitude of pressure fluctuations in the resonant chamber reduced to the velocity head on the length of the chamber. It can be seen that although the oscillations in the first mode are larger in absolute values than the oscillations in other modes, in relative terms, the amplitude is inferior to the second mode, which operates at lower jet velocities.

Figure 7 shows the dependences of the relative root-mean-square amplitude of the generated oscillations on the jet velocity (Mach number) for various geometry options. So, for a chamber (D_C/d₁ = 5.5, L_C/d₁ = 1.5), the second mode is the strongest, while an increase in the resonant chamber diameter leads to the first mode becoming strong (red and blue lines). Another experiment with L_C/d₁ = 1.5 showed (Figure 7b) that a decrease in the resonant chamber diameter leads to a decrease in the oscillation generation efficiency—the relative amplitude of the second mode is two times lower. Moreover, the maximum value is shifted towards higher values of speed. There is a completely logical explanation for this, since a resonator with a smaller diameter of the resonant chamber has a higher frequency of natural oscillations and, in order to achieve the required value of the Strouhal number, large jet velocities are required. The hatching on the graphs means that in these ranges of Mach numbers (jet velocity), the emitter operates not only in the hydrodynamic cavitation mode but also in acoustic cavitation due to the generation of intense acoustic waves.

Figure 6c shows a comparison of the curves for emitters with the same chamber diameter but different lengths. The oscillation amplitude in the second mode is much larger for a resonator with a larger chamber length, although resonance sets in at high velocities. Obviously, the length of the chamber determines the frequency of the hole-tone generation, and this is confirmed by the data in Figure 5.

The experiments performed show that the maximum value of the oscillation amplitude depends on the consistency of the regime parameters and the geometric dimensions of the emitter channel. Based on the experimental studies carried out, it can be argued that the optimal, from the point of view of the efficiency of converting the jet energy into oscillation energy, geometric relationships are L_C/d₁ = 1.73 and D_C/d₁ = 5.5.

4. Discussion and Future Research

The maximum values of the oscillation amplitude within each mode are reached at a frequency whose value is slightly (about 7%) higher than the frequency of natural oscillations of the resonator. This experimental fact was also observed by other researchers [41]. The obtained data on the values of the Strouhal number (Equations (4) and (5)) and the optimal ratios of the geometric parameters make it possible to calculate the required jet velocity, including for liquid, to achieve efficient generation. As an example, consider the calculation for the second mode. According to Equation (5) we obtain:

$${\mathrm{Sh}}_{\mathrm{II}}=0.609-0.087\frac{L_C}{d_1}=0.609-0.087\cdot 1.73=0.458$$

on the other hand,

$${\mathrm{Sh}}_{\mathrm{II}}=\frac{f\cdot d_1}{W}=\frac{1.07\cdot f_0\cdot d_1}{W}$$

(6)

Expression (1) for the frequency of natural oscillations can be rewritten as

$$\begin{array}{l}{f}_0=\frac{c_0}{2\pi D_C}\sqrt{\frac{1}{L_C}\left(\frac{d_1^2}{{\mathcal{l}}_1+0.25\pi d_1}+\frac{d_2^2}{{\mathcal{l}}_2+0.25\pi d_2}\right)}=\\ =\frac{c_0}{2\pi \left(D_C/d_1\right)\sqrt{L_C}}\sqrt{\frac{1}{{\mathcal{l}}_1+0.25\pi d_1}+\frac{{\left(d_2/d_1\right)}^2}{{\mathcal{l}}_2+0.25\pi d_2}}\end{array}$$

(7)

Substituting Expression (7) into (6) we obtain

$$\begin{array}{l}{\mathrm{Sh}}_{\mathrm{II}}=\frac{1.07}{\left(W/c_0\right)}\frac{d_1}{2\pi \left(D_C/d_1\right)\sqrt{L_C}}\sqrt{\frac{1}{{\mathcal{l}}_1+0.25\pi d_1}+\frac{{\left(d_2/d_1\right)}^2}{{\mathcal{l}}_2+0.25\pi d_2}}=\\ =\frac{1.07}{\left(W/c_0\right)}\frac{\sqrt{d_1}}{2\pi \left(D_C/d_1\right)\sqrt{L_C/d_1}}\sqrt{\frac{1}{{\mathcal{l}}_1+0.25\pi d_1}+\frac{{\left(d_2/d_1\right)}^2}{{\mathcal{l}}_2+0.25\pi d_2}}=\\ =\frac{1.07}{2\pi \cdot M\cdot \left(D_C/d_1\right)\sqrt{L_C/d_1}}\sqrt{\frac{1}{\frac{{\mathcal{l}}_1}{d_1}+0.25\pi }+\frac{{\left(d_2/d_1\right)}^2}{\frac{{\mathcal{l}}_2}{d_1}+\frac{0.25\pi d_2}{d_1}}}\end{array}$$

(8)

Substituting into the resulting Expression (8) the optimal ratios of geometric parameters for the second mode, we obtain

$$M_{\mathrm{II}}=\frac{0.035}{{\mathrm{Sh}}_{\mathrm{II}}}=0.076$$

(9)

The value was calculated similarly for the first mode and obtained $M_{\mathrm{I}}=0.126$. It should be noted that the values obtained fall within the range of experimental data (Figure 8), for which the values of the amplitude of pressure fluctuations should be maximum.

The cavitation intensity of a jet is affected by multiple factors, such as operating pressure, ambient pressure, and nozzle structure [42]. To evaluate the effect of operating pressure and ambient pressure, the dimensionless parameter P_r can be used.

$$P_r=\frac{P_{in}}{P_{out}}\hspace{1em}\to \hspace{1em}{P}_{in}-P_{out}=P_r{P}_{out}-P_{out}=P_{out}(P_r-1)$$

(10)

where P_out is the ambient pressure (pressure at the JDHO outlet, Pa) and P_in is the working pressure (pressure in front of the JDHO, Pa).

The jet velocity in the JDHO is determined by the pressure drop and can be written as:

$$\Delta P=P_{in}-P_{out}=\rho \frac{W^2}{2}=\rho \frac{M^2\hspace{0.17em}{c}_0^2}{2}$$

(11)

Based on Equations (3), (10) and (11), we can write an expression for the dimensionless parameter:

$$\rho \frac{M^2\hspace{0.17em}{c}_0^2}{2}=P_{out}(P_r-1)\hspace{1em}\to \hspace{1em}\rho \frac{M^2\hspace{0.17em}{c}_0^2}{2P_{out}}=P_r-1$$

(12)

$$P_r=\frac{\frac{M^2\hspace{0.17em}{c}_0^2}{2}+\frac{P_{atm}}{\rho }+gh}{\frac{P_{atm}}{\rho }+gh}$$

(13)

where P_atm is the atmospheric pressure (Pa), h is the JDHO location depth in the cavitation reactor of the anaerobic digestion system (m), g is the gravitational acceleration (9.81 m/s), and ρ is the fluid density (kg/m³).

Equation (13) relates to the jet velocity, characterized by the Mach number; the depth of the JDHO installation in the tank; and the dimensionless parameter P_r (Figure 9). In ref. [42], the optimal value of the pressure coefficient is considered to be 25. This value corresponds to our experimental data for the second mode. The resulting dependence (13) allows the determination of the optimal location when designing a cavitation reactor.

The choice of the substrate pretreatment method is a difficult step in the design of the AD process (Figure 10) [3]. It is important to rank the methods according to energy costs and the effect they give. The physical method of pretreatment of liquid agricultural waste is an important step for improving the efficiency of the bioconversion, compaction, and distribution of the particles, enzymatic availability, and overall conversion of feedstock to biogas without the formation of toxic by-products [58].

The advantages of the proposed cavitation reactor is the simplicity of its design and operation, easy scalability, the possibility of obtaining stable emulsions from fat-containing waste, and a high degree of the homogenization of liquid raw materials [59]. This is confirmed by the presence of a number of commercial cavitation reactors. For example, Cavitation Technologies, Inc., owns the patented CTi Nano Neutralization technology, which includes a multi-stage hydrodynamic cavitation device [59]. The reaction system is flexible in scale and can be applied in the field of edible oil refining, algae oil extraction, renewable fuel production, biodiesel production, etc. A number of studies have shown that with a decrease in the particle size of the raw material supplied to the digester, the yield of biogas increases [56,60,61,62,63]. An increase in the surface area of the organic material provides a larger contact area for microorganisms, which leads to an intensification of the gas formation process [63].

Currently, the design and development of a laboratory–analytical complex for studying the processes of anaerobic digestion on a semi-industrial scale with a hydraulic mixing system [64], as well as subsequent thermochemical conversion of the digestate [65], is underway. It is planned to use a cavitation reactor for the pre-treatment of agricultural waste.

5. Conclusions

Acoustic cavitation is a promising approach that can be applied to biomass pretreatment to improve AD efficiency. The JDHO are easy to scale, and this is an advantage of their use. Based on the experimental data, it was found that the optimal ratio of the length of the resonator chamber to the diameter of the inlet nozzle is 1.73 and the ratio of the inner diameter of the resonator chamber to the diameter of the inlet nozzle is 5.5. The results obtained make it possible to improve existing jet vibration emitters for the implementation of acoustic impact and to expand the scope of their application in the economy.