**4. Specific application: hydrocarbon leak sensors. The thermomechanics principles**

oxygen molecules through a reducing reaction which causes the release of electrons and allows that a current flows freely through the sensor. Consequently, the gas concentration is detected by the resistance change of MOS [4]. This kind of sensor seems to be the most important because the huge amount of investigation regarding to them. Pt nanoparticles-modified Al-doped ZnO (AZO) porous macro/mesoporous nanosheets prepared by solution combustion method were assessed for butane gas sensor at low temperature. The large surface area of 50.17 m<sup>2</sup>

the broad pore size distribution between 3 and 110 nm calculated by BJH method provided a good contacting interface between sensing material and gas molecules allowing a maximum response of 56–3000 ppm of butane. The gas sensitivity is related to the electron flow through the interface from AZO to Pt. An oxidation-reduction reaction happened when Pt nanoparticles-modified AZO nanosheets were exposed to reducing gas. A large amount of electrons are released which back to the conduction band of AZO leading a decrease of resistance [39]. Hierarchical flower-like structure composite formed by combination of metal oxides with high surface area and porous structure have shown improved gas sensing performance in comparison with pure metal oxide. NiO:CuO nanocomposites (molar ratio 1:1) showed 2 s response

formed at the interface between NiO and CuO could accelerate the speed response. O<sup>2</sup>

these gas molecules are adsorbed onto the surface by extracting electron from conduction band.

uated thought output voltage measurements and the sensing response was defined by the ratio

The existence of p-n junction in composite sensor which induced a new potential barrier when

to 450°C. Sensor response (S) was determined following the equation S = Rair/Rgas, where Rair and Rgas are the sensor resistances recorded in presence of test gas and dry air, respectively. The ini-

ing, leading 85 kΩ at 250°C and 18 kΩ at 450°C in ethanol sensing test. In addition, the sensor response increased from 1.77 to 3.29 with increasing ethanol concentration from 50 to 500 ppm

for ethanol detection [42]. In order to improve its ethanol sensing capability at the temperature range between 25 until 125°C, other researchers have been introduced Au nanoparticles into ZnO nanostructures by sputtering technique. The sensing mechanism is based on the surface electron density changes of the semiconductor. The oxygen molecules in air react to the surface electrons of ZnO forming oxygen species determined by the constant reaction koxy as follows:

O3

 particles gave benefits to exhibit superior gas sensing performance. The reduction of operating temperature was attributed to lower energy required for electron transition derived from the decrease of band gap of p- and n-type semiconductor [41]. In another work, porous hol-

ecules are absorbed on the surface of sensor when it is exposed to air. These O<sup>2</sup>

with n-type SnO<sup>2</sup>

ture electrons from conduction band of sensing material forming ions O<sup>2</sup>

O5

method and were evaluated for sensing ethanol, CO, and NH<sup>3</sup>

perature (100°C) and high response (800–5 ppm NO<sup>2</sup>

at room temperature and relative humidity of 42%. The heterojunction

, the resistance is decreased [40].

in composite sensor for NO<sup>2</sup>

and air atmospheres. Relevant results indicate low operation tem-

particles, large surface area and porous structure of Sb<sup>2</sup>

hollow balls sensor decreasing when operating temperature increas-

time to 100 ppm NO<sup>2</sup>

58 Recent Advances in Porous Ceramics

Because electrons are transferred to NO<sup>2</sup>

low balls formed by self-assembly of α-Fe<sup>2</sup>

O3

at 400°C. Low sensor response to CO and NH<sup>3</sup>

The combination of p-type Sb<sup>2</sup>

electrons transfer between SnO<sup>2</sup>

of sensor resistance in NO<sup>2</sup>

tial resistance of α-Fe<sup>2</sup>

SnO<sup>2</sup>

/g and

mol-

molecules cap-

have also eval-

O5 and

atmosphere,

− . At NO<sup>2</sup>

) at short time (5 s) of composite sensor.

in a temperature range from 250

nanoparticles were synthesized by hydrothermal

was detected, thus, this sensor is suggested only

Diverse fluids used in the petrochemicals processes such as propane, liquid petroleum gas (LPG), butane, propylene; and other organics and inorganics fluids are widely stored, transported, or used in a pressure-liquefied state. Therefore, these fluids are very important in the industry when leak sensing technology is needed. Specifically, pressure-liquefied gas (PLG have a high probability of ending in a fatal accidental leak due to its own fluid properties and behavior [48, 49]. So, sensing technology needed must cover a range of fluids; however, there are some clear examples where the widely and frequently used of some fluids highlight some extra needs for specially design sensors or sensing technology.

#### **4.1. Theoretical basis of the problem: phenomenon involved**

Flashing depends on the initial parameter values of the fluid as pressure and temperature as well as the type of fluid. A particular combination of those variables can create, for some cases, a complete breaking of the liquid core into droplets at the same time that it is going out of container like unstable two phase jet or liquid jet. The major difficulty in the understanding of this flashing phenomenon and the parameters interactions within it belongs to the existence of a compromise between the physical and thermodynamics mechanism that acts on the released fluid [50]. Specific behavior and characteristics of these liquid-gas mixtures and the potential for the formation of vapor-liquid aerosols during a superheated liquid release due to the breaking of the metastable state can significantly affect the hazard zone and the mitigation steps that can be taken to minimize the release impact for the hydrocarbon industry [49, 51, 52].

Geometrical characteristics as diameter of the nozzle, length of the nozzle in the experimental system have influence on the dynamics behavior of the system, driving the velocity profile as well as the temperature profile. Evaporation and convection processes are also involved and their relevance along the leaking fluid properties as well as the characteristics of the jet is not well determined yet. Even so, in more recent works, some authors applied the jump condition analysis to the shock waves in the discharge of a superheated liquid [59]. Due to the metastable liquids supply the energy stored within them via the latent heat of vaporization,

Porous Ceramic Sensors: Hydrocarbon Gas Leaks Detection

http://dx.doi.org/10.5772/intechopen.72315

61

the evaporation wave was assumed as an adiabatic phase transition.

**based on the concept**

**4.2. Identification of key parameters for sensing technology: viability for sensing** 

As direct consequence of the understanding of the process, it is possible to evaluate the potential of different variables or parameters to detect in the best possible way any leak, keeping in mind that the primary purpose of leak detection systems is to assist pipeline operators in detecting and locating leaks. The first aspect to consider is the magnitudes of the scale of mass discharge, the critical explosion limits, change in temperature of the surrounding, the time scale of the process, or any other parameters that be used to catch this phenomenon. Mass discharge directly seems to be no a convenient parameter to be measured directly but based on the amount of mass going out concentration of the leaked fluid in the surrounding will change. As described, this variable will depend on several physical parameters such as pressure difference, temperature, and fluid properties that will determine the discharge velocity. Time scale of the leak is related with the jet velocity; however, the thermodynamics process of phase change are under the influence of another parameter as the fluid properties, which can be described based on the temperature variation along the centerline of the yet. Initially, there is a time lag of the initiation of flashing, followed by a drop of temperature driven by a phenomenon of sudden change of phase and finally an increment of the temperature to the ambient condition driven mainly by the mechanics mechanism of energy exchange. Characteristics of centerline droplets temperature of a R134a flashing jet by using and exponential function, which started with an almost exponential decay with the fastest drop in the temperature taking place near the nozzle exit, explained by the presence of rapid evaporation of the droplets and the insufficiency of the convective heat transfer from the surrounding [54]. This exponential decay of droplets average temperature can be described by an exponential function followed by less rapid temperature decay. Meanwhile, experimental data have pointed out that for different substances that there is a visible minimum in the temperature profile that can be related with the factor the mechanics mechanisms take over the thermodynamics mechanics of energy exchange [60]. Mentioned time scales are not commonly reported, however, it is possible to be calculated using the equilibrium model [61]. A different approach described that the flashing process can be detected based on the fact that rapid vaporization or phase change of superheated fluid produced an acoustic pulse that can detect by an acoustic sensor [62, 63]. Nucleation of vapor bubble requires a minimum amount of energy related to the vibrating media that will be traduced in pressure waves (noises). As mentioned in Section 3, the ceramics porous materials (e.g., catalytic and MOS sensors) are other available options for sensing hydrocarbon vapors and this will be discuss in details on the next section.

High complexity level of the whole process in combination with the need of more information based on experimental, analytical, or numerical models is the main difficulty to be overcome in order top developed new sensing technology. Different authors have concentrated efforts of the jet characterization. The developed information about the jet can help to understand the parameters that can be used as key indicators of accidental release of hydrocarbons.

Flashing phenomenon complexity required calculation of the velocity discharge, void fraction, and mass flow of a flashing jet together with the estimation of the temperature. Due to the nature of the nucleation process, the assumptions of adiabatic flow with non-reversible work for the surface tension forces are made. Those considerations are found to be more realistic that the isentropic condition used until now by different authors. Dynamics conditions usually considered include the mixture velocity after flashing as critical conditions. Frequently numerical modeling techniques are only applied after the flashing jet is formed. No droplets generation or vapor generation are included in the modeling. Droplets are imposed as part of the boundary condition of a gas jet. Droplets transport mechanics and their interaction and momentum exchange with the gas current is made using droplet interaction models as for example Disperse Model (DDM). Geometrical aspects as nozzle dimension, as well as, turbulent model used have a large impact on the core region length of the velocity profile. The numerical results are compared based on the centerline or cross section velocity profiles [53].

In general, centerline, the temperature profile presents an initial decay from the exit of the nozzle until a certain distance, where a minimum value is achieved presumably connected with the location of cessation of boiling and completion of nucleation as main interaction mechanics of energy exchange. After that point, mechanical and evaporation mechanisms become the main driving mechanisms for energy exchange instead. The position at which the minimum temperature occurs is known as Minimum Temperature Distance (MTD). Previous works have not report major observations on this particular parameter. However, other related concepts as Cold Spray Distance (CSD), which refers to the spray distance where the spray maintains a specific temperature considered "cold" and the Spray Thermal Length (STL) refers to a total spray length where liquid droplets exist [54]. Similar behavior of the temperature profiles at the centerline has been observed several experimental settings [55–58]. Geometrical characteristics as diameter of the nozzle, length of the nozzle in the experimental system have influence on the dynamics behavior of the system, driving the velocity profile as well as the temperature profile. Evaporation and convection processes are also involved and their relevance along the leaking fluid properties as well as the characteristics of the jet is not well determined yet. Even so, in more recent works, some authors applied the jump condition analysis to the shock waves in the discharge of a superheated liquid [59]. Due to the metastable liquids supply the energy stored within them via the latent heat of vaporization, the evaporation wave was assumed as an adiabatic phase transition.

#### **4.2. Identification of key parameters for sensing technology: viability for sensing based on the concept**

**4.1. Theoretical basis of the problem: phenomenon involved**

industry [49, 51, 52].

60 Recent Advances in Porous Ceramics

velocity profiles [53].

Flashing depends on the initial parameter values of the fluid as pressure and temperature as well as the type of fluid. A particular combination of those variables can create, for some cases, a complete breaking of the liquid core into droplets at the same time that it is going out of container like unstable two phase jet or liquid jet. The major difficulty in the understanding of this flashing phenomenon and the parameters interactions within it belongs to the existence of a compromise between the physical and thermodynamics mechanism that acts on the released fluid [50]. Specific behavior and characteristics of these liquid-gas mixtures and the potential for the formation of vapor-liquid aerosols during a superheated liquid release due to the breaking of the metastable state can significantly affect the hazard zone and the mitigation steps that can be taken to minimize the release impact for the hydrocarbon

High complexity level of the whole process in combination with the need of more information based on experimental, analytical, or numerical models is the main difficulty to be overcome in order top developed new sensing technology. Different authors have concentrated efforts of the jet characterization. The developed information about the jet can help to understand the

Flashing phenomenon complexity required calculation of the velocity discharge, void fraction, and mass flow of a flashing jet together with the estimation of the temperature. Due to the nature of the nucleation process, the assumptions of adiabatic flow with non-reversible work for the surface tension forces are made. Those considerations are found to be more realistic that the isentropic condition used until now by different authors. Dynamics conditions usually considered include the mixture velocity after flashing as critical conditions. Frequently numerical modeling techniques are only applied after the flashing jet is formed. No droplets generation or vapor generation are included in the modeling. Droplets are imposed as part of the boundary condition of a gas jet. Droplets transport mechanics and their interaction and momentum exchange with the gas current is made using droplet interaction models as for example Disperse Model (DDM). Geometrical aspects as nozzle dimension, as well as, turbulent model used have a large impact on the core region length of the velocity profile. The numerical results are compared based on the centerline or cross section

In general, centerline, the temperature profile presents an initial decay from the exit of the nozzle until a certain distance, where a minimum value is achieved presumably connected with the location of cessation of boiling and completion of nucleation as main interaction mechanics of energy exchange. After that point, mechanical and evaporation mechanisms become the main driving mechanisms for energy exchange instead. The position at which the minimum temperature occurs is known as Minimum Temperature Distance (MTD). Previous works have not report major observations on this particular parameter. However, other related concepts as Cold Spray Distance (CSD), which refers to the spray distance where the spray maintains a specific temperature considered "cold" and the Spray Thermal Length (STL) refers to a total spray length where liquid droplets exist [54]. Similar behavior of the temperature profiles at the centerline has been observed several experimental settings [55–58].

parameters that can be used as key indicators of accidental release of hydrocarbons.

As direct consequence of the understanding of the process, it is possible to evaluate the potential of different variables or parameters to detect in the best possible way any leak, keeping in mind that the primary purpose of leak detection systems is to assist pipeline operators in detecting and locating leaks. The first aspect to consider is the magnitudes of the scale of mass discharge, the critical explosion limits, change in temperature of the surrounding, the time scale of the process, or any other parameters that be used to catch this phenomenon. Mass discharge directly seems to be no a convenient parameter to be measured directly but based on the amount of mass going out concentration of the leaked fluid in the surrounding will change. As described, this variable will depend on several physical parameters such as pressure difference, temperature, and fluid properties that will determine the discharge velocity.

Time scale of the leak is related with the jet velocity; however, the thermodynamics process of phase change are under the influence of another parameter as the fluid properties, which can be described based on the temperature variation along the centerline of the yet. Initially, there is a time lag of the initiation of flashing, followed by a drop of temperature driven by a phenomenon of sudden change of phase and finally an increment of the temperature to the ambient condition driven mainly by the mechanics mechanism of energy exchange. Characteristics of centerline droplets temperature of a R134a flashing jet by using and exponential function, which started with an almost exponential decay with the fastest drop in the temperature taking place near the nozzle exit, explained by the presence of rapid evaporation of the droplets and the insufficiency of the convective heat transfer from the surrounding [54]. This exponential decay of droplets average temperature can be described by an exponential function followed by less rapid temperature decay. Meanwhile, experimental data have pointed out that for different substances that there is a visible minimum in the temperature profile that can be related with the factor the mechanics mechanisms take over the thermodynamics mechanics of energy exchange [60]. Mentioned time scales are not commonly reported, however, it is possible to be calculated using the equilibrium model [61].

A different approach described that the flashing process can be detected based on the fact that rapid vaporization or phase change of superheated fluid produced an acoustic pulse that can detect by an acoustic sensor [62, 63]. Nucleation of vapor bubble requires a minimum amount of energy related to the vibrating media that will be traduced in pressure waves (noises). As mentioned in Section 3, the ceramics porous materials (e.g., catalytic and MOS sensors) are other available options for sensing hydrocarbon vapors and this will be discuss in details on the next section.
