Equipment And Methods Of Laser Rapid Analysis Of Petroleum Products
To assess the quality of petroleum products sufficiently large number of methods and devices has been developed [1–3]. However, most recognized and certified methods of quality control of fuel are laboratory installations, not adapted for mobile and operational control directly near the operating engine or the fuel storage. Existing methods of express-control determine typically no more than one or two characteristics . Often the implementation of these methods requires expensive equipment, highly trained staff and long time to analyze, from a few hours to several days. These problems can be solved with the development of a rapid method and the establishment of mobile portable equipment for analysis, a measuring device having a light weight, high performance, satisfactory accuracy, low cost, ease of maintenance and the ability to measure multiple characteristics of petroleum products.
We have developed one of these techniques and created the appropriate equipment. The method relates to the field of optical diagnostics of the state of liquid media technology. It was named "Laser phase analysis’ (LPA) [5,6]. LPA method consists in analyzing liquids by laser optical control of phase transformations occurring in a turbid liquid medium while being cooled until solidification and subsequent heating. It is well known that the nature of the interaction of laser radiation with a substance undergoing phase transformations is unique not only for any substance, but also for the different concentrations of its components. This fact has been used by us to determine the concentration of toxic substances.
The existing methods of photometry based on radioscopy of the medium with laser have several disadvantages: 1) after solidification, the petroleum becomes strongly turbid and it requires fairly powerful radiographic sources; 2) such petroleum products as engine oil, are generally opaque and are not transparent to laser.
To improve the accuracy and correctness of the results, it was proposed to use the specular reflection principle (Figure 1). In this case, the laser radiation falls on the radiation detector without passing through the medium, and is reflected from its surface.
The physical essence of the method is as follows. Low-temperature properties of petroleum products are mainly determined by the content of paraffin fractions. Their content greatly increases the viscosity of the fuel and its freezing point. The main reason for this problem is not even in a mass fraction of paraffin, but in the property of paraffin crystals to significantly increase in volume as the temperature decreases. If in normal conditions, paraffin crystals form the colloidal or true solutions with the bulk, then with cooling gel, sol is formed or similar conditions occur. The depressants in the composition of additives for fuels and lubricants are used in the technology to eliminate this process. Such depressants prevent the growth of paraffin crystals, increasing the number of nucleation centers. The study of the mechanism of their action and the use of conditions is described in a number of researches , but full clarity is not achieved due to a large number of obstinate factors that affect the efficiency of depressant action. The oil itself contains natural depressant such as resin and asphaltene, and therefore adding of synthetic depressants in the oil does not often lead to the desired results.
The developed technique allows controlling the growth of paraffin crystals and based on this fact we can create a methodology for assessing the various properties of petroleum products.
To implement this technique, we have developed and manufactured automated rapid analyzer of liquid petroleum fluids named "ACC-Express’ (Figure 2). This device allows receiving information about the temperature of solidification, water content, amount of solids, concentration of foreign matter, as well as pouring rate of liquid substances such as fuel and motor oil, water and oil, as a matter of minutes.
The device comprises a cooling element, a lamp, a measuring tool, a detector of phase transformations. Furthermore, the lamp is designed as a laser radiation source with a wavelength of 650 nm and a focusing system, the measuring means is a cylindrical cell, the bottom of which strongly scatters laser light, and the detector of substance surface reflected laser consists of a receiver of the reflected beam and an associated computer.
Using the laser source allows us to increase accuracy of the method, since for the monochromatic radiation blurring of variations in the substance reflectance during phase transitions is reduced. As a result of the phase transformations occurring in the substance as it cools to the freezing temperature by the action of the cooling element, there is a change in reflectance of the surface substance. Dependence of change in intensity of specular reflection from the substance temperature during its cooling is unique for each substance.
The software allows defining a quantitative assessment of changes occurring in the substance, which, in turn, serves as a basis for determining the required substance properties.
Thermostatic unit (Figure 3) consists of a cylindrical cell, the bottom of which is arranged with a high degree roughness, a laser source and a receiver of the reflected laser radiation. The source and the receiver of laser radiation are arranged at angles mutually inverse for optimal data recording. The cell with the test substance is located on the Peltier element, which serves to convert electrical energy into heat. By varying the flow of electric current through the Peltier element it is possible to perform both cooling and heating of the substance in the cell. Between the cell with the substance and the Peltier element there is a thermally conductive plate, the purpose of which is to more efficiently transfer the heat from the Peltier element to the substance. The plate, in turn, is mounted on the Peltier element using a thin layer of thermal grease, the use of which has the same aim of improving the heat transfer. The signal from the photodetector is transmitted to the control computer for further processing.
The rapid analysis device operates as follows. A liquid sample placed in the cell is cooled at a fixed speed using the Peltier element to a predetermined cooling temperature. The cooling speed is set by the controlling computer. Laser radiation from the source falls on the sample surface and is reflected therefrom. Then the reflected radiation falls on the photodetector. By a signal from the photodetector, the reflection coefficient of the surface substance is determined. Since the reflectance is changed during the cooling period, the signal from the photodetector is used by the software to plot reflectance dependence on temperature, and the characteristics set by the operator are displayed.
To use the device "ACC-Express’ we have developed several techniques of the petroleum products analysis. The temperature measurement method of solidification of liquid petroleum products allows performing the measurements with an accuracy to within not less than 0,1 °C in accordance with GOST for the corresponding substances. For the measurements, a sample, such as diesel fuel in an amount of 4.3 g, is placed in a small cell of thermally conductive substance (aluminum) that is cooled using a thermoelectric element. The cooling speed is set in the range of 7 to 11 °C/min, which provides uniform solidification. In the process of solidification the surface reflection factor of the sample is determined by measuring the reflection of laser. According to the change of this factor over time, we can plot a cooling curve. In the next stage of measuring procedure, the function describing the variation of the reflection coefficient, is differentiated. An example of such curve for summer diesel fuel (GSO 8356–2003) is shown in Figure 4. Pour point is defined as the minimum differential cooling function. Temperature measurement accuracy is 0.1°C, the analysis time is less than 10 minutes.
Method for determining the water content in the petroleum products is based on a comparison of the values obtained for the sample, with the standard values. To do this, the cooling curves plotting procedure is carried out curves by the method discussed above, for the reference and the tested petroleum products. For example, the samples of diesel fuel with a water content of 1, 2 and 3% are prepared and the cooling curves are plotted for these samples and reference diesel as shown in Fig. 5.
In the next stage, a comparison of the data is performed using special automated computer programs. The difference in the values for the cooling curves of the reference sample and the watered samples is determined, then the corresponding dependency is plotted (Figure 6). The areas beneath these curves are calculated, and a calibration dependence by these values is plotted (Figure 7). Next, we plot a cooling curve and a value differential curve for the diesel fuel with an unknown concentration of water. Using the calibration curves, the program outputs the actual content of water in the tested diesel fuel. The accuracy of measurement of water content is 0.01%, the analysis time is 10–15 minutes.
An important indicator of the fuel and oil is a slump loss at low temperature operation. A special method of estimating the slump loss has been developed. Before measurement a sample, such as diesel fuel, is placed in the device cell and is cooled at a speed varying in the range of 7–11 °C/min. In the process of solidification the cell contents is rocked using additional mechanism while constantly measuring the reflectance of the sample surface. The reflected laser radiation is recorded by the photodetector and a signal thereof is transmitted to the computer. By changing this factor over time, a cooling curve is plotted. The cooling curve has an oscillatory character. The length of the section corresponding to the signal incidence on the photodetector corresponds to the degree of fluidity. The length of the sections at the beginning of the curve, when the temperature corresponds to the ambient temperature, is 100% of fluidity, the length of the remaining sections of signal reduction is translated proportionally into percentage value by the program. We accept the condition that no further reduction of the signal in time in the photodetector corresponds to the complete absence of flow after full solidification of the sample. Then, based on these data, the dependence of fluidity reduction of diesel fuel on the temperature is plotted using special software on the computer (Figure 8). At the same time the temperature of complete solidification of the fuel is determined which corresponds to the termination of the oscillating changes of the reflected signal.
The method of determining the percentage of particles in the oil and fuel has been developed to determine the contamination of oil solids. The method is based on comparison of the substance with the reference. In particular, for mineral oil the content of metal particles having an average diameter of 300nm has been determined. The liquid sample with particles was exposed to ultrasonic homogenization. Then the sample was cooled and the measurements of the laser radiation transmitted through the sample were taken. Figure 8 shows the relative dependence of the radiation transmitted through the sample from the sample temperature.
There is a divergence in the curves for the pure sample and a sample doped with metal particles. The difference becomes noticeable even at relatively high temperatures not reaching the phase transition temperature. This fact is important for practical use, since it is practicable in the application of fuel purity control without deep cooling, which reduces the amount of time and energy. Subsequently, the relative intensity of radiation transmitted directly through a pure sample decreases significantly slower than for the contaminated samples, apparently due to the smaller dissipation factor. Up to a temperature of 0°C, the curves for liquid samples with different concentrations of the particles are practically the same. With further cooling, the difference between the intensity values becomes more visible, thus allowing for high resolution and the ability of the method to determine the concentration of impurities with high precision (Figure 9). Thus the optimum temperatures for the temperature measurement are lower than –8 °C. Measuring accuracy of this method is 5 to 8%, the analysis time is 10–15 minutes. The error of measurement of the concentration values determined for different moments of time is about 2%.
This research has been performed supported by the Russian Science Foundation, grant No 14–19–01216.