Influence of processing conditions on hydrogen Sonoproduction from methanol sono-conversion: A numerical investigation with a validated model

HIGHLIGHTS

• Hydrogen production by methanol sono-pyrolysis was computationally analyzed.

• An accurate model was developed for H₂ production from methanol sono-conversion.

• Hydrogen yields were substantially increased with methanol addition at low dosage.

• Hydrogen yielding and CH₃OH conversion decreased with frequency increase.

• The H₂ yield is ideal at an acoustic intensity of 1 W/cm² and liquid temp. of 20-30 °C.

Abstract

A comprehensive numerical investigation was conducted to analyze the effect of ultrasound frequency (213-1000 kHz), ultrasonic intensity (0.7-1.5 W/cm²) and medium temperature (293.15-333.15 K) on the production of hydrogen from methanol decomposition inside acoustic bubbles (methanol sono-conversion). The adopted model was firstly compared with some reference data. It was found that in the absence of methanol (in the presence of methanol), the bubble temperature, hydrogen production (and methanol conversion) are decreased monotonously with an increase in frequency. The maximal bubble temperature is slightly impacted by the presence of methanol under ultrasonic frequencies that are equal to or greater than 515 kHz. In addition, the yield of H₂ is larger in the presence of methanol, regardless of the utilized frequency. Between 213 to 355 kHz, methanol conversion and hydrogen production are most efficient. Both H₂ production and CH₃OH degradation are accelerated under an ideal acoustic intensity of 1 W/cm². At a liquid temperature of 303.15 K, a turning point in bubble temperature may be noticed in the absence of methanol, and at a liquid temperature of 323.15 K, the highest hydrogen production can be achieved. Hydrogen production and methanol conversion are most efficient at liquid temperatures between 293.15 and 303.15 K.

Introduction

In comparison to other traditional energy sources (methane, diesel and gasoline), hydrogen has numerous advantages, including higher heating value (141.9 kJ g⁻¹) [1], ability of storage in different forms (liquid, gas, or in combination with metal hybrid), availability, and the fact that it is a clean fuel with really no CO₂ emissions, as well as being able to be utilized in fuel cells to generate electricity [2], [3], [4]. Fossil fuels are now the primary source of hydrogen production, which can be accomplished by a variety of techniques like gasification, vapor reforming and partial oxidation [1,5,6]. Alternative methods for hydrogen production, such as water electrolysis, biological photosynthesis and photocatalytic processes have been developed [7], [8], [9]. Once produced, hydrogen is a clean synthetic fuel: when burnt with oxygen, the only exhaust gas is water vapor, but when burnt with air, lean mixtures have to be used to avoid the formation of nitrogen oxides [10]. The hydrogen as a fuel is a great source of energy that could be used in different applications including space industry, transportation, fuel cells, rockets, pumping and heating and more [10,11].

The sonochemical production of hydrogen has gained the interest of many researchers as a clean, non-toxic and low-cost technique [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22]. However, lower hydrogen yields has been reported from pure water sonolysis (<40 µM/min) [12]. To overcome this inconvenient, many different additives, either in the saturating gas or in the sonolyzed water, can be used to increase the efficiency of the sonolytic yielding of hydrogen. For example, the inclusion of hydrocarbons such as methane and ethane results in a weighty increase in the production of hydrogen [23]. In the hot gas phase of the bubble, methane and ethane can penetrate, and their pyrolysis results in excessive amounts of hydrogen [23]. Alcohols such as conventional and bio-methanol, on the other hand, serve the same role as CH₄ within the bubble, following the same pyrolytic pathway [24,25]. The concentration of these additives, on the other hand, should be carefully controlled because an excessive concentration could reduce (or suppress) the yield of hydrogen production. On the other hand, hybrid technologies, such as sonocatalysis, photosonolysis and sonophotocatalysis are being developed to improve the efficiency of the sono-generated hydrogen. An interesting review tracing the different studies on hydrogen production by ultrasound has been provided by Merouani and Hamdaoui [12].

The intensification of hydrogen sono-production (from water sonolysis) using alcohols dilute aqueous solution has been proven experimentally. Buettner et al. [26] have studied the sonolysis of water-methanol mixtures at 1 MHz under argon atmosphere. In deionized water, H₂ was generated at 22 µM/min. When methanol is added to the solution, the H₂ rate increased to 100 µM/min at 5% (v/v) of methanol and 150 µM/min at 10% (v/v) of methanol, but as methanol higher concentrations rise, the molar yield of H₂ progressively goes down until no H₂ was detected at roughly 80% (v/v) of methanol. When using 38 kHz ultrasound in water and water/ethanol (20% v/v) under argon saturation, Penconi et al. [27] observed a 1.4-fold increase in the rate of H₂ generation. Rassokhin et al. [24] have noted that the generation rates of the sonolysis products (e.g. H₂, CH₄ and CO) of methanol increase (H₂: from 33.3 to 167.0 µM/min) with the increase of its concentration from 0.001 to 0.50 M in the sonicated solutions, where the molar yield of these species is as: H₂>CO>CH₄. Rassokhin et al. [24] have shown that depending on the mole fraction of methanol in the bulk solution, an optimum medium temperature is retrieved for the maximal hydrogen production ([H₂]_max = 200 µM/min at ∼30°C and x_MeOH= 0.018), where the increase of methanol concentration decreases the temperature at which the maximal H₂ formation rate is reached.

Several numerical studies have focused on hydrogen synthesis using sonochemistry [12,14,[28], [29], [30]]. These studies have shown promising outcomes of clean and efficient hydrogen generation. Merouani et al. [12] and Kerboua et al. [17] have recently published fascinating reviews on hydrogen generation using sonochemistry. Kerboua et al. [19] studied numerically the influence of the saturating atmosphere (O₂ and Ar) on the sono-pyrolysis of CH₃OH, finding that regardless of the quantity of argon, a maximum molar yield was obtained for a solution comprising 40% of CH₃OH. Kerboua et al. [19] also reported that a 40% methanol concentration and a 40% molar fraction for argon provide the maximum molar output. Kerboua et al. [29] determined that the optimal combination for H₂ generation and CH₃OH conversion is achieved for a solution with 20% methanol (v/v) saturated with 70% molar of argon, thereby 99.9% of the methanol is removed and about 60% of the methanol is converted to hydrogen. Dehane et al. [31] used a comprehensive mathematical model for single bubble sono-pyrolysis of methanol to investigate the effect of methanol dosage (in the bulk phase) on the maximum sonochemical performance for hydrogen production, CH₃OH consumption and the range of active bubbles. They discovered that the aqueous phase methanol dosage (0-100 % (v/v)) has little impact on the spectrum of active bubble sizes for CH₃OH consumption. In the case of hydrogen generation, however, for methanol concentrations more than 20%, the active bubble ranges gradually diminish. At 80% argon (in the initial gas composition) and a methanol content of 7 to 20%, the maximum efficacy for CH₃OH conversion, hydrogen production and the width of active bubble sizes (for H₂ generation and CH₃OH decomposition) is reached. Nevertheless, the Dehan's investigation [31] is not complete as all results were done for single matrix of operational conditions (frequency: 355 kHz, In = 1 W/cm² and T_liq = 20°C). According to the available theoretical studies [19,29,31] focusing on methanol sono-decomposition, the effect of operational conditions (ultrasound frequency, acoustic intensity and the bulk temperature) have not covered yet.

In the present paper, the sono-production of hydrogen from methanol sono-conversion in a single bubble is analyzed at varying ultrasound frequencies (213-1000 kHz), acoustic intensities (1 and 2 W/cm²) and liquid temperatures (293.15-333.15 K). The model developed early [31] by our research group is adopted for this task. To the best of our knowledge, the effects of these operational conditions have not been treated previously. For this purpose, a reaction mechanism for methanol combustion within the cavity has been adopted. The model was firstly validated by some available experimental data before assessing the parameters’ influence. It should be stressed here that a deep analysis of the effects of these operating conditions (wave frequency, acoustic power, and liquid temperature) enables us to better understand their impacts on the formation of H₂ from CH₃OH sono-conversion as well as to determine the appropriate conditions (of ultrasound frequency, acoustic intensity and liquid temperature) for the maximal production (intensification) of H₂ (from methanol sono-combustion).