With the scarcity of crude oil supplies these days which reflects itself as the sharp increase in gas prices, it is increasingly important to do better quality control on the existing crude oil supplies. The composition of crude oil varies with natural origin, transport and storage conditions (Reference 1). Physio-chemical properties of crude which are of importance are specific gravity (also called API gravity), true boiling point curve and viscosity (Reference 1). API gravity is short for American Petroleum Institute gravity which is an inverse measure the weight of petroleum liquids in comparison to water. If a liquid has API gravity of more than 10, it will be considered as a light oil that floats on water.
Analytical methods of quality control for crude oil are very time consuming, expensive and require sample preparation. Near IR Spectroscopy (NIRS) is a rapid, inexpensive method that does not require any sample preparation and can be used for in-situ measurements of crude oil’s physio-chemical properties onsite. Bonds such as CH, NH and SH are target absorption bonds which NIRS can identify. Figure 1 shows the NIRS spectrum of crude oil in a wide range. Allied Scientific Pro offers T1 and T11 portable near IR spectrometers with cuvette which can measure part of the NIR spectrum as shown in figure 1.
Figure 2: Sulfhydryl group (SH bond) can be detected using the T11 spectrometer at 1750
Partial Least Square modelling was used in a study to model 153 crude oil samples (Reference 1). First derivative and Savitzky Golay (SG) filter were applied to the collected and prediction were made on 30 spectra in a testing set which were subjected to first derivative and the same SG filter. The API range for the samples were 20-43° and the sulfur content was 0.3-3.8%. nm. The PLS regression model had an accuracy of 2.73% for API gravity and 4.46% for sulfur content which are reasonable accuracies.
In another study that was conducted at Allied Scientific Pro, several samples of crude oil were supplied for testing with the T1 spectrometer which has a range between 900-1700 nm. The samples were all sealed and apparently had varying viscosity. Because the crude oil samples are generally very viscos and have dark colors a nominal 1 cm cuvette for a transmittance spectrometer is not suitable to measure their transmission. For this reason, a 1 mm thick cuvette was provided. Figure 3 shows the crude oil sample in a 1 mm thick cuvette.
Figure 3: Crude oil sample in a 1 mm thick cuvette
The cuvette was placed in the transmittance spectrometer as shown in figure 2 and the transmission of it over the range 900-1700 was measured.
Figure 4: 1 mm cuvette filled with crude oil was placed inside the cuvette holder of T1 spectrometer
Several samples were tested and their absorption plot is shown in figure 5.
1- Rapid crude oil analysis using near-infrared reflectance spectroscopy, J Long et.al, Petrolium science and technology, vol 37, no 3, 2019.
2- Sulfur content. The science of Victorian brown coal, science direct 1991.