titration solvent is added and the sample aliquot is reheated to 65 °C to ensure the dissolution of the paraffin content. The warm sample may be analyzed immediately without adversely affecting endpoint resolution, further supporting the method ruggedness of D8045.
insuffi cient dissolution of crudes and fractions in the titration solvent can pose a problem: asphaltic, paraffi nic (waxy), and bitumen materials are not all readily dissolved in the titration solvent that the D664 method stipulates. When the sample is not fully dissolved, the titrant cannot react with all of the acid contained in the sample. Also, the undissolved sample forms a coating on the glass membrane of the electrode in the titration cell, reducing its ability to accurately sense voltage changes during the titration and subsequently leading to imprecision and poor accuracy.
A colorimetric titration method, namely ASTM D974, has been used to measure some clear refi nery fractions. This test method cannot be applied to measure crudes and front-end refi nery fractions due to the intense dark color they exhibit when dissolved in the titration solvent.
A new method for determining the acid number
Recommended sample weights Crudes and distillation fractions with an expected acid num- ber less than 1 mg KOH/g should be analyzed using a sample mass of 10 to 20 g. If the acid number is greater than 1 mg KOH/g, the analyst should use 5 g of sample for the measure- ment. The amount used can be adjusted to accommodate for solubility limitations. For unknowns, it is advised to start with 5 g and adjust the sample size as needed for subsequent mea- surements. The volume of titrant consumed must be at least 0.15 mL. A titrant volume smaller than 0.15 mL indicates that additional sample is required. A titrant volume greater than 5.0 mL suggests that less sample is needed. Recommended sample weights according to the expected value of the acid number are presented in Table 1.
Sample preparation
The heterogeneous nature of crude oil can affect determinations, particularly when measuring relatively small sample sizes of 3–5 g. To improve the precision of the test protocol, it is necessary to homogenize the crude before analysis using a shear mixer. Studies by the work group have shown that this improves the precision of the new test method.
Table 1. Recommended sample weights
Since 2008, Metrohm USA has been working with industry partners to overcome the sample and method challenges and has developed an improved, reliable method to measure acidity in the challenging matrices of crude and refi nery fractions. The new titration method is based on thermometric endpoint detection, and uses the Metrohm thermometric titrator 859 Titrotherm (Figure 1).
Thermometric titration: Measuring principle
Previous acidity measurements of crudes and fractions according to ASTM D664 used a potentiometric titration in which a pH electrode detects the reaction between the titrant and naphthenic acid. During the titration, the sensor often becomes coated with heavier fractions which are diffi cult to dissolve in the titration solvent stipulated by the standard, leading to measurement imprecision.
The new Titrotherm method, ASTM D8045, uses a thermometric sensor which overcomes this issue in two ways: fi rstly there is no glass membrane to coat, and secondly the analyst can vary the solvent composition to aid the dissolution of heavier crudes such as bitumen.
04
The thermometric sensor developed by Metrohm performs well in non-aqueous titration of acidity in crude. It employs a thermistor for precise temperature measurements in the titration vessel. The neutralization reaction of naphthenic acid is exothermic, therefore, the temperature increases in the course of the reaction. To obtain a sharp discontinuity in the temperature curve at the endpoint, a thermometric indicator that reacts endothermally with excess hydroxide after the endpoint is added to the sample solution. The thermometric titration gives an inverted V-shaped curve as shown in Figure 2. Evaluation of the titration endpoint is handled by the tiamo™ software.
Expected acid number [mg KOH/g]
Metrohm White Paper High paraffi n content:
Crudes and refi nery fractions that are liquid at room temperature are weighed directly into a beaker; 30 mL of titration solvent containing the thermometric indicator is added to dissolve the sample which is then titrated with 0.1 M potassium hydroxide in 2-propanol. Samples that are not liquid at room temperature, such as asphalt and high-paraffi n-content fractions, require sample preparation
Recommended sample mass [g] ± 10%
10–20 5 1
0.05–0.99 1.00–4.99 5.00–15.00
Particularly challenging samples
The analyst may encounter crude oils with high paraffi nic content which are referred to as «waxy crudes». These samples can be challenging because the paraffi ns are often solid at room temperature. Samples with high paraffi nic content should be fl uidized and homogenized by heating them to 80 °C so that a representative aliquot is obtained and analyzed. The warm sample is weighed into the titration beaker and 10 mL of solvent is added (toluene or xylene). The majority of crudes analyzed, including bitumen samples, did not require this fi rst dissolution step. Approximately 30 mL of the xylene–2-propanol titration solvent is added and the sample aliquot is reheated to 65 °C to ensure the dissolution of the paraffi n content. The warm sample may be analyzed immediately without adversely affecting endpoint resolution, further supporting the method ruggedness of D8045.
High paraffin content: Particularly challenging samples The analyst may encounter crude oils with high paraffinic con- tent which are referred to as «waxy crudes». These samples can be challenging because the paraffins are often solid at room temperature. Samples with high paraffinic content should be fluidized and homogenized by heating them to 80 °C so that a representative aliquot is obtained and analyzed. The warm sample is weighed into the titration beaker and 10 mL of sol- vent is added (toluene or xylene). The majority of crudes ana- lyzed, including bitumen samples, did not require this first dis- solution step. Approximately 30 mL of the xylene–2-propanol titration solvent is added and the sample aliquot is reheated to 65 °C to ensure the dissolution of the paraffin content. The warm sample may be analyzed immediately without adversely affecting endpoint resolution, further supporting the method ruggedness of D8045.
Recommended sample weights
Crudes and distillation fractions with an expected acid number less than 1 mg KOH/g should be analyzed using a sample mass of 10 to 20 g. If the acid number is greater than 1 mg KOH/g, the analyst should use 5 g of sample for the measurement.
The amount used can be adjusted to accommodate for solubility limitations. For unknowns, it is advised to start with 5 g and adjust the sample size as needed for subsequent measurements.
The volume of titrant consumed must be at least 0.15 mL. A titrant volume smaller than 0.15 mL indicates that additional sample is required. A titrant volume greater than 5.0 mL suggests that less sample is needed. Recommended sample weights according to the expected value of the acid number are presented in Table 1.
Recommended sample weights Crudes and distillation fractions with an expected acid num- ber less than 1 mg KOH/g should be analyzed using a sample mass of 10 to 20 g. If the acid number is greater than 1 mg KOH/g, the analyst should use 5 g of sample for the measure- ment. The amount used can be adjusted to accommodate for solubility limitations. For unknowns, it is advised to start with 5 g and adjust the sample size as needed for subsequent mea- surements. The volume of titrant consumed must be at least 0.15 mL. A titrant volume smaller than 0.15 mL indicates that additional sample is required. A titrant volume greater than 5.0 mL suggests that less sample is needed. Recommended sample weights according to the expected value of the acid number are presented in Table 1.
Table 1. Recommended sample weights
Expected acid number [mg KOH/g]
Figure 2. As titrant is added, an exothermic reaction is measured, i.e., the temperature in the titration vessel increases. After reaching the neutralization or endpoint, excess titrant endothermally reacts with the thermometric indicator present in the solvent, resulting in an abrupt temperature decrease.
The measuring response time of the thermistor is less than 0.003 seconds and is much faster than that of a pH glass membrane. This means that thermometric titration by ASTM D8045 can be carried out much faster than the titration according to potentiometric methods which use pH indication, without sacrifi cing precision or accuracy. The thermometric sensor also allows the use of nonpolar solvents such as xylene, which improves the solubility of many oils, including crude.
Thermometric titration does not require an insulated reaction chamber because a relative temperature change is monitored to indicate completion of the reaction.
04
0.05–0.99 1.00–4.99 5.00–15.00
Blank determination
It is important to determine a blank periodically. This should consume less than 0.1 mL titrant, in particular when measuring samples with acid numbers less than 1.0 mg KOH/g. To ensure that the blank value is less than 0.1 mL, only ACS Reagent grade solvents should be used. To determine the blank, a stable sample with a known acid number is measured three or more times, using a different sample mass each time. The largest sample size must not use a titrant volume greater than the volume of the burette. The example below shows a blank calculation for a crude oil sample with an acid number of approximately 0.9 mg KOH/g. In a plot of the titrant volume consumed until the endpoint against the sample mass, the blank value is equal to the value of the
Table 1. Recommended sample weights
Recommended sample mass [g] ± 10%
10–20 5 1
To optimize the solvent system in ASTM D8045, a solvent study was conducted. It was found that a mixture of xylene and 2-propanol (also called isopropyl alcohol or IPA) in the ratio of 75:25 by volume worked best to dissolve the variety of crudes and refi nery fractions. The xylene–2-propanol titration solvent is effi cient enough that only 30 to 40 mL of solvent is required compared to 120 mL for ASTM Method D664. The reduction in total solvent volume and waste disposal saves substantial operating costs.
proximately 0.9 mg KOH/g. In a plot of the titrant volume consumed until the endpoint against the sample mass, the blank value is equal to the value of the titrant volume y when- the sample value x is set to zero (Figure 3). In this particular example, it equals 0.039 mL. tiamo™ titration software can be configured to fit the data and calculate the slope automati- cally.
Analytical Instrumentation
1.6 1.4 1.2 1.0
0.8 0.6 0.4 0.2 0
39
y = 0.156x + 0.039 R2
= 0.9988
0 24 68 10 Sample mass [g]
Figure 3. The blank value is determined from three or more determinations of the same sample, each of which is done using a different sample mass. The titrant volume required in these determinations is plotted against the respective sample mass. After a linear fit is applied, the blank value, equal to the titrant volume when the sample mass is 0 g, is extrapolated.
Figure 3. The blank value is determined from three or more determinations of the same sample, each of which is done using a different sample mass. The titrant volume required in these determinations is plotted against the respective sample mass. After a linear fi t is applied, the blank value, equal to the titrant volume when the sample mass is 0 g, is extrapolated.
titrant volume y when the sample value x is set to zero (Figure 3). In this particular example, it equals 0.039 mL. tiamo™ titration software can be confi gured to fi t the data and calculate the slope automatically
Blank determination
Correlation of thermometric and potentiometric methods
It is important to determine a blank periodically. This should consume less than 0.1 mL titrant, in particular when measur- ing samples with acid numbers less than 1.0 mg KOH/g. To ensure that the blank value is less than 0.1 mL, only ACS Re- agent grade solvents should be used. To determine the blank, a stable sample with a known acid number is measured three or more times, using a different sample mass each time. The largest sample size must not use a titrant volume greater than the volume of the burette. The example below shows a blank calculation for a crude oil sample with an acid number of ap- proximately 0.9 mg KOH/g. In a plot of the titrant volume consumed until the endpoint against the sample mass, the blank value is equal to the value of the titrant volume y when- the sample value x is set to zero (Figure 3). In this particular example, it equals 0.039 mL. tiamo™ titration software can be configured to fit the data and calculate the slope automati- cally.
Repeatability
The acid number repeatability of the thermometric method was studied for low-TAN samples in a single laboratory. A crude oil, a mineral oil, and a refi nery fraction were analyzed.
1.6 1.4 1.2 1.0
The results are shown in Table 3, Table 4, and Table 5 on the next page. ASTM D8045 demonstrates excellent method precision for low TAN values.
A variety of crude types and refi nery fractions have been analyzed using the new thermometric ASTM D8045 titration standard.
A study comparing this method to the ASTM D664 potentiometric method shows good correlation (Table 2).
In a three-laboratory study, 89 samples were analyzed using potentiometric and thermometric methods to compare the method results. The results showed that the new thermometric ASTM Method D8045 produces equivalent results to ASTM Method D664 as shown in Figure 4.
0.8 0.6 0.4 0.2 0
y = 0.156x + 0.039 R2
= 0.9988
0 24 68 10 Sample mass [g]
Figure 3. The blank value is determined from three or more determinations of the same sample, each of which is done using a different sample mass. The titrant volume required in these determinations is plotted against the respective sample mass. After a linear fit is applied, the blank value, equal to the titrant volume when the sample mass is 0 g, is extrapolated.
Precision
The single-laboratory precision as well as the precision between multiple laboratories have been studied in the development of the ASTM D8045 thermometric acid number standard.
This was done in a ten-laboratory study of twelve crudes and refi nery fractions. Both the single-laboratory precision, or repeatability, and the precision between multiple laboratories, or reproducibility, proved to be much better than that of method D664 when measuring crudes and refi nery fractions.
These improvements are attributed to an improved solvent system that makes the sample fully accessible for reaction to the titrant as well as the use of a precise, enthalpy-detecting sensor that is unaffected by diffi cult samples or harsh solvents.
WWW.PETRO-ONLINE.COM
Figure 4. Correlation of the results of the thermometric TAN determination and the potentiometric TAN determination according to ASTM D664
Titrant volume [mL]
Titrant volume [mL]
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52
