RU2742851C1 - Method of removing oxygen from liquid fuel - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to methods for removing oxygen dissolved in liquid fuel in order to reduce coke formation and can be used in systems for supplying liquid fuel to the combustion chamber of aircraft engines. The method for removing oxygen from liquid fuel consists in the fact that liquid fuel is preliminarily exposed to ultrasound for a predetermined time to form cavitation bubbles in the fuel, and then the fuel is fed into the combustion chamber. The time of exposure to ultrasound is selected from the range of 0.1-1 s, frequency and amplitude of sound pressure of ultrasound is selected from given ratios, and radius of the cavitation bubble is chosen to be larger than free path of the most active radical in gas mixture of the bubble, which limits the chain process of oxidation of fuel vapor by oxygen in it. In the process of exposure to ultrasound, degree of solubility of oxygen in liquid fuel is controlled, and with a constant degree of solubility, fuel is additionally influenced by an electric field with a strength selected from a given condition.

EFFECT: decrease in the concentration of oxygen in the fuel by means of exposure to ultrasound while reducing harmful effect of ultrasound on fuel.

1 cl, 5 dwg

Description

The invention relates to methods for removing oxygen dissolved in liquid fuel in order to reduce coke formation and can be used in systems for supplying liquid fuel to the combustion chamber of aircraft engines during their operation, fuel lines of oil refining complexes and other fuel systems in which there is a problem of coke formation during heating fuel [1-3].

It is known from the prior art that liquid-phase coke deposits are caused by the oxidation of liquid fuels by oxygen dissolved in them. Fuels in equilibrium with air, under normal conditions, typically contain approximately 70 ppm of dissolved oxygen [4], which, when heated, initiates chemical autooxidation reactions in the fuel [5]. As a result of autooxidation in fuel, a variety of oxygenated compounds are formed, such as hydroperoxides, alcohols, ketones, aldehydes, acids and esters, many of which play an important role in liquid phase coke deposition. The problem with coke formation is that it can reduce the cooling life of the fuel used to cool the combustion chamber, or can lead to blockage, an increase in hydrodynamic resistance in the narrowing of the fuel lines (jets, injectors).

Known methods for removing oxygen from liquid fuel (US 7041154, 2006 and US 7465336, 2008), in which the liquid fuel is exposed to ultrasound with the formation of cavitation bubbles in the fuel, and then the fuel is fed into the combustion chamber through a device for separating oxygen from the fuel: a membrane or catalytic plate.

A common disadvantage of the known technical solutions is that the devices used in them for separating oxygen from fuel have a high hydrodynamic resistance and strongly inhibit the fuel flow. In order to provide acceptable levels of fuel consumption, a large area and a complex shape of the device, a three-dimensional structure of the pores, are required. The device itself can fail quite quickly. small pores are easily clogged with various sediments, including coke and paraffins, therefore, forced cleaning of the device or a special coating that prevents the sorption of impurities is required.

The rate of oxygen evolution through the membrane depends on the diffusion coefficient and pressure gradient. During cavitation, bubbles appear, inside of which the diffusion coefficient in the gas phase is two to three orders of magnitude higher than in the liquid. As a result, at some point, the near-surface fuel layer near the membrane becomes oxygen-depleted, which reduces the efficiency of its further separation. Ultrasonic waves effectively mix the liquid, which contributes to the enrichment of the near-surface layer with oxygen from the deep layers of the fuel.

In addition, the descriptions of known technical solutions do not justify the choice of modes of exposure to ultrasound on fuel and do not touch upon the problem of the harmful effects of ultrasound on fuel from the point of view of the destruction of the latter and the production of precursors of coke formation, namely, cyclic hydrocarbons and hydroperoxides.

The closest analogue of the claimed invention is a method for removing oxygen from liquid fuel (US 10527011, 2020), characterized in that for a given time the liquid fuel is exposed to ultrasound with the formation of cavitation bubbles in the fuel, after which fuel is fed into the combustion chamber through a membrane or catalytic a plate designed to separate oxygen from the fuel by sorption.

The disadvantage of the known technical solution is also the need to use a device that complicates the implementation of the method for removing oxygen from liquid fuel, increasing the time of fuel supply to the combustion chamber, weight and size parameters and energy losses of the propulsion system - membrane or catalytic plate.

The technical problem solved by the claimed invention is to implement a method for removing oxygen from liquid fuel using ultrasound without the use of mechanical (on the membrane) or chemical (on the catalytic plate) separation of cavitation bubbles to separate oxygen from the fuel.

The technical result achieved with the implementation of the invention consists in reducing the oxygen concentration in the fuel by acting on it with ultrasound while reducing the harmful effects of ultrasound on the fuel.

The technical result is achieved due to the fact that when implementing the method for removing oxygen from liquid fuel, the liquid fuel is preliminarily influenced by ultrasound for a given time to form cavitation bubbles in the fuel, and then the fuel is fed into the combustion chamber, and the time of exposure to ultrasound is selected from the range 0.1- 1 s, and the frequency ν and the amplitude p _{m of the} sound pressure of ultrasound are selected from the following ratios:

Where:

R is the radius of the cavitation bubble;

ρ is the density of liquid fuel;

p ₀ - static pressure of liquid fuel;

τ _in is the time of autoignition of the gas mixture in the bubble,

the radius of the cavitation bubble is chosen to be larger than the free path of the most active radical in the gas mixture of the bubble, which limits the chain process of oxidation of fuel vapors by oxygen in it, while in the process of exposure to ultrasound, the degree of solubility of oxygen in liquid fuel is controlled, and with a constant degree of solubility, the fuel is additionally affected by electric field with strength E, selected from the condition:

E> E _d (ε + 2) / 26 (ε-1),

Where:

E _d is the breakdown strength of the gas mixture in the bubble;

ε - dielectric constant of liquid fuel.

These essential features provide a solution to the technical problem posed with the achievement of the claimed technical result, since only the totality of all actions and operations that make up the invention eliminates the disadvantages inherent in the known methods and provides the advantages of the claimed invention.

Additionally, unlike the prior art, a more efficient and energy-efficient reduction of the oxygen concentration in liquid fuel is provided, the harmful effect of ultrasound on the fuel is reduced - its destruction and the formation of coke precursors in it, which is achieved by reducing the required power and the time of exposure to ultrasound on the fuel due to the high the rate of chain processes of chemical binding of oxygen in collapsing bubbles in comparison with the required power of ultrasound and the time of oxygen sorption on a membrane or catalytic plate.

The present invention is illustrated by illustrations, where:

figure 1 shows a schematic diagram of a system for implementing a method for removing oxygen from liquid fuel;

figure 2 shows the profiles of pressure, temperature and volume of the gas mixture in the collapsing cavitation bubble;

figure 3 shows the profiles of the molar fractions of O ₂ , O, H ₂ O in a collapsing cavitation bubble;

figure 4 shows the profiles of molar fractions of CO, CO ₂ , H ₂ O ₂ in a collapsing cavitation bubble;

Fig. 5 shows the profiles of molar fractions of C ₆ H ₅ , C ₆ H ₅ O, C ₆ H ₅ OH in a collapsing cavitation bubble.

Figure 1 shows a schematic diagram of a system for implementing a method for removing oxygen from liquid fuel. The system consists of a tank 1 with fuel, a pump 2, a pipeline 3, a cuvette 4, an ultrasonic generator 5, a control unit 6 for an ultrasonic generator 5, a high pressure pump 7, a combustion chamber 8, an electric field generator 9, a control unit 10 for an electric field generator 9, sensor 11 solubility of oxygen in fuel and control system 12.

Aviation or rocket kerosene can be used as liquid fuel, for example, grades T1, TS-1, RT (according to GOST 10227-2013), foreign fuel grades JP-1, Jet A-1, TS-1, motor gasolines, biofuel.

An acoustic emission sensor [6] can be used as a sensor 11 to control the degree of oxygen solubility in real time.

The method for removing oxygen from liquid fuel is carried out as follows.

During engine operation, fuel from tank 1 is supplied by pump 2 through pipeline 3 to cuvette 4 with a given pressure p ₀ and then by means of high-pressure pump 7 to combustion chamber 8.

The control system 12 controls the ultrasonic generator 5 in such a way that it acts on the liquid fuel in the cuvette 4 with ultrasound for a predetermined time to form cavitation bubbles in the fuel, before feeding it into the combustion chamber 8.

For this, using the control system 12, a signal is supplied to the control unit 6 of the ultrasonic generator 5, which sets the exposure time, frequency ν and amplitude p _m of the ultrasonic sound pressure.

In this case, the time of exposure to ultrasound is selected from the range of 0.1-1 s, and the frequency ν and the amplitude p _{m of the} sound pressure of ultrasound are selected from the following ratios:

Where:

R is the radius of the cavitation bubble;

ρ is the density of liquid fuel;

p ₀ - static pressure of liquid fuel;

τ _in is the time of self-ignition of the gas mixture in the bubble.

The radius of the cavitation bubble is chosen to be larger than the mean free path of the most active radical in the gas mixture of the bubble, which limits the chain process of oxidation of fuel vapors with oxygen in it, so that chemical reactions occur in the volume of the bubble, and the radical is not quenched on the walls of the bubble.

The formation of cavitation bubbles in the fuel is due to the fact that ultrasound (US), re-reflected from the walls of the cell, forms three-dimensional standing waves. In the region of the maximum ultrasonic gradient at the nodes of standing waves on impurities, including on individual molecules of dissolved oxygen [7-9], bubble nuclei are formed. In accordance with the theory of cavitation, the phase of nucleation of bubbles begins when the local absolute pressure in a completely degassed fuel drops to the value of the pressure p _{g2 of} its saturated vapors [10]. In particular, for kerosene TS-1 after degassing, the minimum pressure p _g2 is 4.3 kPa [10]. In the rarefaction phase, the embryos grow rapidly and merge into a single bubble of radius R at the node of the standing wave, which then collapses or forms a conglomeration, depending on the frequency of ultrasound and the size of the bubble, given by the amplitude of sound pressure.

Since the concentration n of oxygen in kerosene is about 1.48 · 10 ²⁴ m ^-3 with its solubility X _O2 about 0.079 kg / m ³ [4], the average distance l between molecules of dissolved oxygen O ₂ is:

l ~ n ^1/3 = 10 ^-8 m

Bubble nuclei formed on individual impurity molecules merge into one bubble if their radius becomes greater than l . Further, the bubble grows in the region of the maximum gradient of the ultrasonic wave to the resonance values of its radius R. The oxygen concentration in the nucleus and the growing bubble is also about 10 ²⁴ m ^-3 , as in kerosene before the ultrasonic action, since all the oxygen that previously enters the bubble nuclei. was in this region of kerosene, and the partial pressure p _{g1 of} oxygen in the bubble at 20 ° C is 6 · 10 ^-2 atm. In addition to oxygen, the bubble contains saturated kerosene vapors, which enter the cavity by evaporation through the bubble walls. For jet fuels, the pressure p _{g2 of} saturated vapor at a temperature T equal to 20 ° C is from 3 · 10 ^-3 to 6 · 10 ^-2 atm. [four]. In this case:

p _g0 = p _g1 + p _g2 = 0.066 atm, where:

p _g0 is the total pressure of the gas displacement in the bubble.

The gas composition in the bubbles at the initial stage of collapse at room temperature is taken in the calculations as follows: the vapor concentration of the surrogate is 10 ²³ m ^-3 , the oxygen concentration is 10 ²⁴ m ^-3 (10% of the surrogate with respect to oxygen).

The choice of the time of exposure to ultrasound from the range of 0.1-1 s is due to the need to limit the formation and decomposition of hydroperoxides, which play a major role in liquid-phase coke deposition, during sound-chemical reactions occurring in the fuel when exposed to ultrasound [5], [7].

The autoignition time τ _in is calculated in advance by means of computer simulation of the reactions of oxidation of fuel vapors with oxygen in a bubble, for example, in the commercial Chemkin software package, and kerosene vapors can be replaced by a chemical surrogate, which contains 80% C ₁₀ H ₂₂ (n-decane) and 20 % С ₆ Н ₆ (benzene), according to the method [11]. Based on the value of the autoignition time, the ultrasound parameters are chosen so that the oxygen in the bubble has time to chemically bond with the fuel vapor into water and carbon dioxide vapor during the bubble existence (relation (1)). In addition, for self-ignition of fuel vapors in the bubble, high temperatures and pressure of the latter are required, which can be achieved with a rapid collapse of the bubble.

In order for the cavitation bubble not to start growing indefinitely under the action of ultrasound, it must collapse in half the ultrasound period at the high pressure stage. This requires the fulfillment of relation (2).

In the process of exposure to ultrasound, the degree of solubility of oxygen in liquid fuel is monitored, for this, the sensor 11 transmits data on the degree of solubility of oxygen in liquid fuel to the control system 12, which checks for a decrease in the degree of solubility, and with a constant degree of solubility sends a signal to the control unit 10 to turn on generator 9 of the electric field, which additionally affects the fuel in the cell 4 with an electric field with an intensity E, selected from the condition:

Ε> E _d (ε + 2) / 26 (ε-1),

Where:

E _d is the breakdown strength of the gas mixture in the bubble;

ε - dielectric constant of liquid fuel.

Bubbles formed in fuel in an ultrasonic field can deform and become unstable under the action of an external electric field. Capillary waves appear on the surfaces of the bubbles, locally amplifying the external electric field. Thus, conditions are created for the occurrence of a corona discharge, which leads to the appearance of free electrons and the development of plasma-chemical reactions in the bubbles, which accelerate the chemical binding of oxygen. Under specially selected conditions (according to the reduced field strength near the antinode of the capillary wave on the inner surface of the cavitation bubble [12]), singlet oxygen O ₂ _a1 can begin to be generated, the reactivity of which is higher than that of unexcited O ₂ . In this case, oxygen quickly binds into chemical compounds and affects the composition of the gas mixture in the bubble. It is known that for humid air under normal conditions the _{breakdown strength E d} is about 3 · 10 ⁶ V / m. However, at reduced total pressure p _0g gas mixture in a collapsing bubble equal to 0.066 atm, which corresponds to the start time of the bubble collapse, E _d breakdown strength is reduced to about 10 ⁴ V / m (the length of the interelectrode gap of about 0.1 m) [4]. Considering the antinode as a conducting ellipsoid (for a well-conducting liquid, for example, water), proceeding from the known fact of the field amplification in its vicinity [13]:

- это эксцентриситет эллипсоидаWhere

is the eccentricity of the ellipsoid

Then:

E _d ≈25Е,

where e≈0.98,

and the limiting values of the amplitude A of oscillations and the radius r of curvature of the top of the antinode were taken as the semiaxes of the ellipsoid [14]:

Α = 16r / π = 16λ / π ²

r = λ / π.

λ is the capillary wavelength on the bubble surface.

Since kerosene is a liquid dielectric with a dielectric constant ε in a static mode equal to 2 [4], it is necessary to make a correction for the incomplete (partial) polarization of the dielectric according to the Clausis-Mossotti formula for a dielectric sphere in an external field:

, где:

where:

E _dd is the intensity in the vicinity of the antinode of the capillary wave on the surface of the bubble in a liquid dielectric.

In accordance with this, for the discharge to appear in the bubble at the antinode of the capillary wave, it is necessary to place it in an external field of intensity Ε of the order of 10 ³ V / m.

For the excitation reaction

O ₂ + e = O ₂ _a1 + e

at a concentration of N _O2 oxygen in kerosene of about 1.48 · 10 ²⁴ m ^{-3, the} cross section σ _{max of the} interaction of an electron (e) with a neutral oxygen molecule is about 10 ^-21 m ² [12], and the energy W of excitation of O ₂ to the singlet state is about 7 eV , which set the value of the _{electric field strength E d} required to accelerate free electrons until the kinetic energy W is reached:

E _d ≈Wσ _max N _O2 ~ 10 ⁴ B / m

Thus, the breakdown voltage in an atmosphere of bladder and tension required for the generation of singlet oxygen free electron same order of magnitude E _d, equal to 10 ⁴ V / m, and to achieve these conditions in an ultrasonic cavitation mode of the external electric field can be reduced to tension Ε about 10 ³ V / m.

The graphs (see Figs. 2-5) show the profiles of the main parameters of the gas mixture in the collapsing bubble based on calculations for a model cavitation bubble with parameters:

ν = 22 kHz, p ₀ = p _m = 6 atm, p _0g = 0.066 atm, q = 0.0066, T _g0 = 298K, R ₀ = 2 mm, gas mixture O ₂ : kerosene = 10: 1, where:

q is the gas content of the fuel;

T _g0 - temperature at the initial moment of bubble collapse;

R ₀ - bubble radius at the initial moment of collapse.

Figure 2 shows the profiles of pressure P _g , temperature T _g and volume V _{g of the} gas mixture in a collapsing bubble, where the time is indicated on the abscissa, and the corresponding values are indicated on the ordinate; Figs. 3-5 are the profiles of the molar fractions of the components gas displacement, where the time is indicated on the abscissa, and the corresponding values of the molar fractions of the indicated components are indicated on the ordinate.

In this example, an overpressure typical for hydraulic fuel systems was selected. The bubble radius was chosen from the condition of its resonance with an ultrasonic wave in accordance with the Minaert formula, and it was greater than the free path of the most active radical in a given gas mixture. It can be seen from the profiles (Figs. 3-5) that during the collapse the model gas mixture has time to self-ignite (τ _in is equal to 4.764 · 10 ^-5 s), and the O ₂ molecules are almost completely bound (Fig. 3). Moreover, the initial amount of O ₂ , which is 91% of the mixture composition, is predominantly bound in H ₂ O, CO and CO ₂ , and their peak values at time τ _in are 39%, 34%, 14%, respectively (Fig. 4). About 4% O ₂ binds to more complex chemical compounds, among which the peak values of the molar fractions of cyclic hydrocarbons from the total composition of the mixture are: C ₆ H ₅ OO - 0.35%, C ₆ H ₅ OH - 0.32%, H ₂ O ₂ - 3 % (Fig. 5). As a percentage of the initial amount of kerosene vapor in the bubble, they are respectively: C ₆ H ₅ OO - 3.5%, C ₆ H ₅ OOH - 3.2%, H ₂ O ₂ - 30%. In terms of all liquid kerosene flowing through the cuvette, the molar fraction of H ₂ O _{2 is} negligible and amounts to about 10 ^-4 , and C ₆ H ₅ OO is about 10 ^-5 . Moreover, these peak values have time to decrease by orders of magnitude in a time of ~ 10 ^-8 s at the final stage of bubble collapse. If we assume that without ultrasonic exposure, all dissolved O ₂ goes to the formation of coke precursors, then under ultrasonic exposure the peak concentrations of coke precursors, in particular H ₂ O ₂ , decrease 30 times, which confirms the achievement of the technical result in terms of reducing their concentration in fuel.

The intensity I of ultrasound at a frequency of 22 kHz without taking into account the absorption of sound, at its pressure

p _m = 6 atm

in liquid fuel, necessary for the collapse of bubbles with

R ₀ = 2 mm,

is:

I = (p _m) ² / 2ρs ~ 2 × 10 ⁵ W / m ² where

с is the speed of sound in the fuel (see [7-9]).

For an ultrasound generator operating at a frequency of 22 kHz, the above value of I correlates with an acoustic intensity of 10 W / cm ² , which, according to experimental data [3], is sufficient to influence the chemical composition of kerosene fuels.

In the calculations, the results of which are presented in Figs. 2-5, one bubble with R ₀ equal to 2 mm falls on a kerosene cell formed by a three-dimensional standing wave with half the wavelength λ equal to 3 cm, and the cell volume is about 3 10 ³ times the bubble volume. To reduce the O ₂ solubility in the entire cell, about 3 × 10 ³ (K) collapse cycles are required (resonance bubbles should cover the entire volume of the antinode of a standing three-dimensional wave), i.e. time t _{K of} stay of the allocated volume of kerosene in the cavitation zone is:

t _K ~ K / ν = 0.14 s.

This value is much more consistent with the requirement to prevent the formation of harmful hydroperoxides than the limiting sounding time of 10 s in the closest analogue of the invention. If it is required to provide a kerosene consumption of 600 g / s, which is typical for aircraft engines of passenger or transport aviation at cruise mode, then the dimensions l ^{3 of the} ultrasonic cell should be

l ₃ = 5 cm 5 cm 5 cm at t _K ~ 0.14 s,

and the speed and pumping of kerosene through it is 0.34 m / s. For the case under consideration, the ultrasound power in the cell is

P _a = IS≈500 W

(S is the area of the front of a plane ultrasonic wave). In the case of atmospheric pressure in kerosene, as is considered in most analogs, for the cavitation threshold at a frequency of 22 kHz, much larger dimensions l ³ and the weight of the cell are required:

l ³ = 50 cm 50 cm 50 cm, u = 0.001 m / s, Ρ _a ~ 3 kΒt.

Thus, the invention makes it possible to reduce the required acoustic power of ultrasound by a factor of 6 - from 3 kW to 0.5 kW and to ensure the achievement of a technical result in terms of reducing the time of fuel supply to the combustion chamber.

Earlier it was shown that an increase in the frequency of ultrasound promotes an increase in the rate of capture of oxygen into the bubble at normal pressure in the cell (1 atm). The choice of the frequency ν and pressure p is due to the above formulated requirements of inequalities (1) and (2), where the density and ignition time are practically independent of the frequency, however, the ignition time τ _in depends on p, T. According to simplified calculations, without taking into account the electrically excited molecules, no ignition occurs in the bubble at a frequency of 0.2 MHz (parameters of the bubble collapse mode:

R ₀ = 0.1 mm, p _g0 = 0.066 atm, p ₀ ~ p _m = 1 atm, p = 1.7 atm, q = 0.039, T _g0 = 298K).

Nevertheless, with an increase in the ultrasonic frequency, the chemical binding of O _{2 is} possible under the action of an electric field, if we take into account the electrical excitation of molecules [7], which can accelerate the chain oxidation mechanism.

Additionally, a reduction in the mass and size characteristics of the system is provided for implementing the method for removing oxygen from liquid fuel, which is due to a decrease in hydrodynamic resistance due to the absence of mechanical or chemical separation of cavitation bubbles on the membrane or on the catalytic plate.

List of sources

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3. Anufriev R.V. Influence of ultrasonic treatment on the structural and mechanical properties and composition of oil dispersed systems. Dissertation for the degree of candidate of chemical sciences, Tomsk, 2017.

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6. Kuznetsov D.M. et al. Study of degassing processes in liquids by the method of acoustic emission // Modern science-intensive technologies. 2018. No. 3. from. 74-79.

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Claims (13)

A method for removing oxygen from liquid fuel, which preliminarily affects the liquid fuel with ultrasound for a predetermined time with the formation of cavitation bubbles in the fuel, and then supplies fuel to the combustion chamber, characterized in that the time of exposure to ultrasound is selected from the range 0.1 -1 s, and the frequency ν and the amplitude p _{m of the} sound pressure of ultrasound are selected from the following ratios:

Where: R is the radius of the cavitation bubble; ρ is the density of liquid fuel; p ₀ - static pressure of liquid fuel; τ _in is the time of autoignition of the gas mixture in the bubble, the radius of the cavitation bubble is chosen to be larger than the free path of the most active radical in the gas mixture of the bubble, which limits the chain process of oxidation of fuel vapors by oxygen in it, while in the process of exposure to ultrasound, the degree of solubility of oxygen in liquid fuel is controlled, and with a constant degree of solubility, the fuel is additionally affected by electric field with strength E, selected from the condition: E> E _d (ε + 2) / 26 (ε-1), Where: E _d is the breakdown strength of the gas mixture in the bubble; ε - dielectric constant of liquid fuel.

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