Determination of H 2 S in Crude Oil

Analytical Methods in Environmental Chemistry Journal Vol 2 (2019) 37-44 38 Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019) specially in the presence of mercaptan, chlorine and some basic additives and scavengers [10-17]. Furthermore, there is demand for a method which comprises requirements of a good method. These requirements including easy operation in field or laboratory, low health and safety problem, operator independent, low cost, repeatable, reproducible and providing very low detection limit as well as wide linear dynamic range. Among the methods of H2S determination, each of them has its own limitation [18-20]. The health and safety hazards related to H2S are summarized in Table 2 [21]. Furthermore, among the properties of the crude oil reported for the sale and refinery designing, H2S may be the most unstable parameter, even in comparison with Reid vapor pressure and specific gravity. So far, to the best of our knowledge, there is not an appropriate analytical method for determination of H2S in crude oil for use in both oil field and laboratory test. In the present work, a rapid method was developed and investigated using factorial experimental design for the determination of H2S of crude oil which can be used in laboratory or field test and led to the repeatable and accurate results. 2. Materials and Methods All chemicals were purchased from Merck and used as received. In a typical determination, 50 mL of crude oil and predetermined amount of toluene as Diluting ratio were poured into a decanting funnel and shaken for a about 5 minutes. Then 25 mL of 5 w/w% aqueous caustic solution was used as extracting agent and was shaken for a predetermined time. The mixture was aged for 10 minutes to separate oil and aqueous solution. Afterwards, 10 mL of aqueous solution was decanted on a paper filter, and the filtrate was collected in a titration beaker. After addition of 2 mL on concentrated solution of ammonia, the filtrate was titrated with 0.05 N of silver nitrate solution. The first equivalent point at about -600 mV is related to the H2S. Titration was carried out Table 1. Standard methods of H2S determination and their properties (Nadkarni 2000). Method Disadvantage Scope of work Dynamic range ASTM D5623 Narrow dynamic range, High cost, Limited sample boiling range GC of Liquid distillates with FBP < 230 °C 0.1 100 ppm UOP 212 Limited sample boiling range, High cost Potentiometric titration, ethane to such gasoline 1 to several thousand ppmw of H2S ASTM D5705 Only applicable to vapor phase, Poor repeatability H2S in the vapor phase (equilibrium) of a residual fuel oil for field test using gas detection tubes and Can test 5 ppm v/v to 4000 ppm v/v UOP 163 Not applicable to crude oil, Complex data interpretation H2S in Liquid hydrocarbons by potentiometric titration 1 up to 100 ppmw. ASTM D7621 IP 570 ISO 8217 Not applicable to crude oil, mercaptan interference H2S in Fuel Oils by Rapid Liquid Phase Extraction 1 up to 50 ppmw UOP 41 ASTM D4952 Doctor Test Only for qualitative analysis, Using of poisonous metal qualitative test of H2S in gasoline, jet fuel, kerosine and similar petroleum products and solvents ASTM 6021 skilled operator and complex calculations, High cost H2S in Residual Fuels by Multiple Headspace Extraction and Sulfur Specific Detection 0.01 to 100 ppmw IP 399 Complex procedure and pure materials needed. Oxidation and absorption may occur. H2S in Residual Fuels by sprctrophotometric determination. 0.50 to 32 ppmw 39 Determination of H2S in Crude Oil Amir Vahid using a Metrohm titrando 880 according to the UOP 209. For the optimization of the condition of determination, a factorial design was applied for the investigation of three main effects, including time required for extraction, diluting ratio ration and crude type according to its API. 12 experimental runs were designed and carried out. Calculation and modeling of results were done using Design Expert 7. Recovery is defined as the ratio of the concentration of the H2S obtained in the designed test to H2S obtained at 2 hours and 60 C with diluting ratio of 5. 3. Results and Discussion The condition of statistically designed runs and their corresponding results are given in Table 3. Analysis of variance of obtained results is calculated and give in Table 4. The obtained Equation for model is given as Eq. 1 in terms of coded factors. Recovery = +91.75 (1.25 * A[1]) + (0.50 * A[2]) +(4.75 * B) + (1.75 * C) + (0.42 * BC) (Eqn. 1) As seen in Figure 1, and according to the Equation 1 and ANOVA (Table 4), it can be said that Time has very little effect on the recovery of H2S. It is a very good property for an analytical method to carry out in short time. Figure 2 displays the effect of Diluting ratio which is the most effective factor among all. Moreover, it is generally known that crude oil has very broad range of properties in term of density and viscosity. H2S is trapped in the complex matrix Table 2. Exposure limit and its related hazards. Concentration mg/kg Health & Hazard Effect < 0.02 Odor Detection Limit 10 8 Hours Exposure Limit 15 15 Min. STEL 100 Common Ship Headspace Spec. 300 Considered Immediately Hazardous 713 LC50 Concentration 1000 Common Tank, Ship Headspace Concentration Table 3. Designed test runs for H2S determination. Std. run order A: Time (min) B: Diluting ratio C: Crude type (API) Recovery % 1 2 1 22 84 2 6 1 22 86 3 10 1 22 87 4 2 2 22 93 5 6 2 22 95 6 10 2 22 95 7 2 1 36 87 8 6 1 36 89 9 10 1 36 89 10 2 2 36 98 11 6 2 36 99 12 10 2 36 99 40 Analytical Methods in Environmental Chemistry Journal; Vol. 2 (2019) Table 4. ANOVA of obtained results of H2S analysis. Source Sum of Squares df Mean Square F Value P-value Prob > F Model 319.1 5 63.8 328.2 < 0.0001 A-Time 9.5 2 4.8 24.4 0.0013 B-Diluting ratio 270.8 1 270.8 1392.4 < 0.0001 C-Crude type 36.8 1 36.8 189.0 < 0.0001 BC 2.1 1 2.1 10.7 0.0170 Residual 1.2 6 0.2 Corr. Total 320.3 11 Design-Expert® Software