Analysis of Trace Metal Elements in Residues for Catalytic Cracking

In recent years, many refineries have used part of the residue as feedstock for catalytic cracking in order to increase economic efficiency. The residue contains trace metals such as iron, nickel, copper, vanadium, and calcium. Metal impurities deposit on the surface of the catalyst, reducing the activity and selectivity of the catalyst and affecting the yield of the product. With the application of molecular sieve catalysts, the control of vanadium content is more stringent. Therefore, it is of great practical significance to grasp the changes in trace metal content in raw material oils.

Similar atomic absorption measurements of metal elements in the system generally include metal organic compound standards and organic solvent system 111, emulsification technology 121, and extraction of metal elements in the oil phase into the aqueous phase 131. However, since the residue sample contains a large amount of carbon black, the carbon black has a strong adsorption effect and can adsorb metal elements. Therefore, the above method is not suitable for determining the metal elements in the residue. In this test, dry ashing method was used to treat the samples, and the trace metal elements in the residue were determined by atomic absorption spectrometry in the aqueous phase. The experimental results show that this method has high accuracy and can continuously measure the iron, nickel, sodium, copper, vanadium, and calcium metal elements in the residue at the same time. It is a fast, reliable and ideal analytical method.

1 Test part 1 Instrument and its working conditions US PERKIN-ELMER703 atomic absorption spectrophotometer, light source is iron, nickel, sodium, calcium, copper, vanadium hollow cathode lamp.

Its working conditions are listed in Table 1.

Table 1 Instrument working conditions are shown in Table 1. Elemental Wavelength 'nm Lamp Current/mA Slit' nm Flame Type Bxmin1 Burner Height/mm Iron, Nickel, Copper, Vanadium Standard Solution Prepared from Pure Metal; Sodium, Calcium Standard The solutions were prepared from superior grade pure sodium chloride and calcium carbonate.

(Volume ratio) Sulfuric acid solution: Formulated with high-grade pure sulfuric acid; 1:1 (volume ratio) sulfuric acid monophosphate mixed acid: mixed by superior grade pure sulfuric acid and superior grade pure phosphoric acid; 1:1 (volume ratio) hydrochloric acid solution: by Excellent grade pure hydrochloric acid.

11: Liu Ye (1968―) Female, Suining Sichuan Engineer, is currently engaged in atomic absorption spectroscopy.

Other solutions / L hydrochloric acid solution; 0% potassium nitrate solution.

1.3 Test method Weigh about 30g of oil sample in 1 (1) mL quartz dish, heat to smoke on the electric furnace, carbonize the residue with ignited ashless quantitative filter paper, put it into the muffle furnace, at 500 °C temperature Next, the mixture was calcined for 7 hours to make it ashed. After cooling, 3 mL of hydrochloric acid was added and dissolved by heating. The solution was collected in a 50 mL volumetric flask, diluted to the mark with deionized water, and subjected to atomic absorption spectrometry.

2 Results and discussion 2.1 Selection of ashing conditions The ashing conditions have a great influence on the accuracy of the atomic absorption analysis results. For example, the ashing temperature is too low to decompose completely, and the remaining small carbon particles are prone to adsorb metal elements and are difficult to use. Acid dissolves. If the ashing temperature is too high, it is easy to volatilize the metal elements and increase the phenomenon of dirt on the dish. For this purpose, the ashing time and ashing temperature conditions were tested, and the results are shown in Table 2 and Table 3, respectively. Table 2 Ashing time test results Ashing time/ashing temperature 3. Table 3 Ashing temperature test results Gray Temperature/ashing temperature elemental determination value/g As shown in Table 2, at 450°C, the sample burning time was extended to 9 h, and the sample could not be completely ashed at 500°C, 550°C, and 650°C. The time required for the complete ashing of the samples was 7h, 5h, and 4h, respectively. From Table 3, it can be seen that as the burning temperature increases, the measured values ​​of each element decrease. This is because the ashing temperature is too high, causing elements The volatilization and disc wall retention 51. Therefore, we choose to burn for 7h at 500 °C for ashing conditions.

2.2 Selection of acid and its concentration In the atomic absorption spectrometry analysis, in order to quickly determine and avoid causing interference, the acid dissolution method is generally used to decompose the sample. The absorbance of each element was measured with sulfuric acid, sulfuric acid monophosphoric acid, and hydrochloric acid at different concentrations, and the test results were as shown.

The relationship between the absorbance of each photon and the concentration of monophosphoric acid can be seen from the above figures. In the sulfuric acid medium, the absorbance of Ni, Ca, V, and Na decreases as the acid concentration increases; In the medium, the absorbance of Fe, Ni, Na, Ca, and V decreases with increasing acid concentration; in hydrochloric acid, only the absorbance of Ca decreases with increasing acid concentration. Therefore, HCl was selected as the acid medium, and the content of each element was determined using the standard solution and the acid concentration of the test solution. Because the acid concentration is too high, the instrument is susceptible to corrosion. Therefore, under the premise of ensuring that all the samples are dissolved, it is appropriate to select a smaller concentration of acid. Although 0.1 M hydrochloric acid dissolved the sample, it took a long time. Therefore, 0.2 M hydrochloric acid was selected as the solvent.

2.3 Coexistence element interference test A mixed solution of metallic elements was prepared using 0.2M HCl as an acid-dissolving medium. The concentration of each element was as follows: Fe was 10.96m/L. Table 4 Interference between coexisting elements Test results Sample absorbance Pure solution Mixed solution from Table 4 It can be seen that in the coexistence of six elements, Fe, Ni, Na, and Cu are not substantially disturbed, while V and Ca are greatly interfered.

2.4 Elimination of interference test reference. Beijing: Editorial Department of Spectroscopy and Spectral Analysis, 1982. Changsha: Hunan Science and Technology Press,