Biodiesel is the result of the alcoholysis of oil with alcohol in the presence of an acid or alkaline catalyst [8]. The main oils used in the production of biodiesel [21-22] are sunflower oil, soybean, palm, castor oil, and high oleic sunflower, animal fats, used deep-frying oils, and microalgae. Among the alcohols, methanol and ethanol are used, highlighting methanol for its physicochemical advantages, reaction with triglycerides at low temperature and natural dissolution of alkaline catalysts [21]. In the alcoholysis, catalysts are commonly used: basic homogeneous (NaOH, KOH, NaHCO₃), homogeneous acids (sulfuric acid, sulfonic), heterogeneous (ZrO₂, ZnO, CaO, SO² ₄ ^-/SnO₂, SO² ₄ ^-/ZrO₂, KNO₃/KI, zeolitas), and enzymatic catalysis (lipases) [23].

The process for obtaining small scale biodiesel (Fig. 1) [12], consists of the following equipment: ultrasonic actuator, ultrasonic device with positive pressure module, probe and flow (power of 500 - 1,000 W with operating frequency of 20 kHz); electric power and energy meter; processing tank (80 L capacity); heating element (1 to 2 kW); pre-mixed catalyst tank (capacity 10 L); catalyst premix agitator; centrifugal pump (10-20 L / min with pressure from 1 to 3 bar); back pressure valve to adjust the pressure in the flow cell; manometer for feeding pressure. The processing and pre-mixing tanks are made of high-density polyethylene.

Figure 1 Source: The Authors.

In the analysis of the technical specifications of the existing equipment in the market for transesterification processes, there are similarities and some differences in the design of their equipment. These range from the actuator used to the different functionalities and modes of operation by the user. For the analysis of this equipment, the following characteristics are taken into account: actuator power, processing capacity and operating frequency (Table 1).

Table 1

Source: The Authors.

The methodology used to reach the investigation’s goal is defined in the following steps:

Establishing functional requirements of the reactor.

Based on the existing equipment in the market, a work plan structured in a modular way is established for the construction of an ultrasonic reactor for transesterification, it derives from the established functional requirements of the analysis of the aforementioned operation characteristics (Table 1).

The operation of the electronic subsystem is subject to the interaction of three blocks designed in such a way that the signal that is delivered to the actuator is free of noise, with stable values of voltage, current, and frequency. The detailed study of the electronic subsystem involves describing three blocks:

Block of integrated circuits composed of the following modules: frequency generator, resonator, remote connection, temperature monitor, alarm, and controller.

Where the block of integrated circuits is the most complex since it is organized in six modules. There is an interaction of the three blocks present (Fig. 6), where the power block will be responsible for supplying specific voltage levels to each of the modules present, both in the block of integrated circuits and in the HMI block. The block of integrated circuits provides the necessary resources to the actuator by means of its modules and the HMI block, allows a local configuration of the system.

Figure 6 Source: The Authors.

It is important to keep in mind that in the design of ultrasound equipment both the actuator and the electronic system form a single piece, and the entire system is designed based on the requirements of the actuator, that is why each module has components with special features that provide a high degree of confidence in the performance of the entire equipment.

This block has been organized in six modules, which provide the necessary resources for the operation of the chosen actuator: frequency generator module, resonator, temperature monitor, remote connection, alarm and controller (Fig. 7).

Figure 7 Source: The Authors.

The ultrasound systems must be stable and the key parameter for its operation is the operating frequency, which must be in the ranges preset by the manufacturer of the actuator to be used. The inputs and outputs of this module (Fig. 8) are: external clock reference, time base to obtain the desired frequency; configuration allows the interaction of the module with the controller module, in order to set working parameters; 5 Vdc power supply, is the energizing voltage of the devices present, as the output signal is the operating frequency of 20 kHz.

Fig. 8 Source: The Authors.

Taking as reference the frequency of work of the ultrasonic actuator (20kHz), it is decided to generate this signal with an electronic device of easy configuration, the integrated circuit reference AD9833, which has a wide range of configurable frequencies by means of the SPI communication protocol (serial peripheral interface) [38].

The electronic subsystem for its operation and coupling of signals, needs different levels of voltage, the power block has 120 Vac - 60 Hz as its input voltage. From this voltage level, the internal component power supply output voltages of 5, 12 and 45 Vdc are obtained. The 5 Vdc power supply is responsible for energizing the following modules: frequency generator module, remote connection module, alarm module, controller module. The 45 Vdc power supplies the resonator module. The 12 Vdc feeds the fans for heat extraction inside the equipment. For the generation of 5 and 12 Vdc, switched sources are used, due to their efficiency, size and functionality. The 45 Vdc are obtained from a TR-17 reference transformer, then a diode bridge and later capacitors for the elimination of noise.

Given the need to manipulate variables such as frequency during the set-up of equipment and time, it is necessary to use components that allow the entry and display of system information, as an input device there is keyboard, responsible for increasing or decreasing the numerical value of the variables as time, in addition to an emergency stop button with which the entire process is disabled. As an output, a 4 line LCD screen by 20 characters (4x20 LCD) is used, where the configuration information and process status is displayed, this module is powered by 5 Vdc.

The electrical resistance applied requires AC voltage for its operation, the design of this equipment is made in several modules. The zero crossing module, phase control and controller all stand out, and in their continuous interaction offer the electrical resistance the signal necessary for its activation. The zero crossing signal provides the controller with a reference to activate an SCR device by allowing the flow of part of the mains voltage, by keyboard the instruction is given to the controller of increase or decrease of the percentage of mains voltage that must pass to the electrical resistance.

As a result of the mechanical and electronic development fulfilling the functional requirements, two main modules were obtained: the physical one, where all the mechanical components necessary for the operation of the reactor are located (Fig. 12.a). and the processing, consisting of the electrical and electronic components used in the control, manipulation and communication of the equipment (Fig. 12.b), the final device is the ultrasonic reactor (Fig. 12.c).

Figure 12 Source: The Authors.

The ultrasonic reactor has specific operating characteristics (Table 2), which determine its operating time, power consumption, processing capacity and sonic radiation power.

Table 2

Source: The Authors.

To evaluate the operation and performance of the ultrasonic reactor for transesterification, experimental biodiesel will be obtained from a mixture composed of 130 mL of castor oil, 31 mL of methanol and 3 g of potassium hydroxide (KOH) [42]. When mixing the substances, use a glass beaker and manual stirring using a glass rod for one minute before subjecting it to the experimental conditions.

The quantities of raw material are calculated so as not to overflow the capacity of the transesterification tank (162 cm³). The stoichiometric calculations start from the number of moles of the oil used; there is 125 g of castor oil (approximate molar mass of 936.38 g/mol) the number of moles of oils is 0.13. The oil / alcohol molar ratio is 1/6, then 0.78 mole of alcohol, represented in 25 g of methanol (molar mass 32.04 g/mol), is required. The glycerin formation (expected molar mass 92 g/mol) is 11.96 g. The catalyst is 2% of the total mixture [42] (125 g of oil + 25 g of alcohol = 150 g of mixture) in this case is 3 g. For the preparation of the catalyst, the potassium hydroxide was completely dissolved in the methanol with the aid of a mechanical stirrer. The products of the reaction are biodiesel and glycerin; its stoichiometry gives the reference value for the process of 9.5 mL of glycerin, which will be used in the calculation of the efficiency of the reaction.

In the research process carried out, the glycerin formation data are recorded until reaching a stable state in each test, comparing the efficiency in each experiment. As a standard curve, the reaction is obtained without any external catalyst at room temperature 22 ° C, together with response curves at 42 ° C, with the use of ultrasound, and the combination of ultrasound and temperature (yellow curve). The application of the temperature is for 6 minutes and that of the ultrasound is constant; the reaction was observed for a period of 2 hours.

In the experimental behavior (Fig. 13), the influence of the change in temperature on the reaction rate is observed, since for the first 20 minutes the slope of the curve of 42 °C is greater than the curve of ambient temperature. A similar effect occurs between the ultrasound curve (US) and the combined process curve (C), since the latter reaches a stabilization value of glycerin production at around 40 minutes, while the ultrasound curve takes 80 minutes in attaining a volume of glycerin produced almost constant (Table 3). Therefore, where a positive change in temperature occurs, the reaction stabilizes in less time.

Figure 13 Source: The Authors.

Table 3

Source: The Authors.

The temperature also influences the rate of change of the glycerin concentration, since the curve of 42 °C has a maximum slope of 0.7 mL / min, the process with ultrasound, and the combined, have a value of 0.5 mL / min, and the ambient temperature curve of 0.22 mL / min. However, it does not have the same effect on the production efficiency of glycerin compared to the reference value (9.5 mL); because the ambient temperature curve reaches a final glycerin volume of 7.1 mL (75% efficiency), and the curve of 42 °C a value of 6.7 mL (71% efficiency). The effect with the greatest impact on the reaction efficiency is ultrasound with a final value of 9.2 mL (97% efficiency); even above combining the ultrasound and a temperature of 42 °C, in which it ends with a volume of glycerin of 9 mL (95% efficiency).