Rapid, Robust and Reliable - The New TET Way To Determine Acid Number In Crude Oils And Refinery Intermediates And Products

Good agreement with result obtained by conventional titrimetric method
Many crude oils contain a range of acidic substances which can cause corrosion damage to refineries during processing. These acidic substances can be categorized as “napthenic acids” which have been defined as an “unspecific mixture of several cyclopentyl and cyclohexyl carboxylic acids with molecular weight of 120 to well over 700 atomic mass units”. Sulfur-containing acids may also be present, and can contribute to corrosion problems. The bulk acid content of an oil is defined by its AN (Acid Number) value, defined as mg KOH/g of oil. The unit price of a crude oil can be discounted significantly according to its AN, so it is important to be able to analyze the AN value with accuracy and precision.

To date, the conventional method to determine the AN value of crude oils has been a potentiometric titration based on ASTM method D664. The success of this method is dependent on the type of crude oil analyzed, and the skill of the analyst in identifying problems which may affect the result. Further, the performance of the pH probe is affected by dehydration and fouling of the sensor glass membrane, as well as its reference junction. Potentiometric pH probes may require cleaning and regeneration (rehydration) of the sensor after only a few titrations. In contrast to the potentiometric method, the thermometric probe requires no conditioning and minimal maintenance. It may be stored dry between titrations, and is always ready for service. It is thus very suitable for use in remote locations with minimal laboratory facilities.

In thermometric endpoint titrations (TET), the endpoint is determined by the rate of change of temperature of the titration solution as the titrant is added. In the case of non-aqueous titrations of weakly acidic species, the temperature inflection at the endpoint is too low to be detected reliably. In this case, a thermometric indicator (paraformaldehyde) is employed. At the endpoint, the first trace of excess KOH titrant catalyzes the endothermic decomposition of the paraformaldehyde. This provides a reliable marker for the endpoint. The titration is carried out in an anhydrous mixture of xylene and 2-propanol. There is no need to add water to the solvent, as is the case with potentiometric titrations.

A sample of a very dark, highly viscous crude oil African origin (Doba, Chad) was provided for evaluation of the procedure.

Reagents:

1. Titrant: c(KOH) = 0.1 mol/L potassium hydroxide in 2-propanol

2. Solvent: 1:1 mixture of xylene with 2-propanol

3. Thermometric indicator: paraformaldehyde (e.g., Sigma Aldrich cat. no. 158127)

4. Standard: c(C₇H₆O₂) = 0.1 mol/L benzoic acid in 2-propanol
Method Outline:

Approximately 3 g of crude oil is weighed into a titration vessel, and 30 mL of solvent mixture added. Approximately 0.5 g of paraformaldehyde is then added. The titration is then commenced, and stopped after appearance of the endpoint inflection.

In the case of the semi-automated titrations reported on here, the paraformaldehyde is added as a powder. A level 1/8^th kitchen teaspoon measure was used to dispense the paraformaldehyde. In the case of fully automated titrations employing a sample changer, the paraformaldehyde is slurried with the solvent mixture, and delivered by peristaltic pump as part of the titration program. The titration can be stopped automatically after the endpoint appears.
Results:

Due to the small amount of oil provided, the average AN was computed from a limited number of determinations, with sample masses ranging from approximately 0.5 to 4 g.. Individual results were 4.1, 4.3, 4.2, 4.2, 4.2 and 4.3 with an average of 4.2±0.09 mg KOH/g. The result obtained appears to be insensitive to the amount of sample mass, at least within the range tested.

The customer recorded a value of 4.4 mg/100g KOH by a procedure similar to that specified in ASTM D664.

TET plot of acids in crude oil in 1:1 xylene/2-propanol, using c(KOH) = 0.1 mol/L as titrant.  

Rapid automated analysis of nitrating acid mixtures by TET

Mixtures of concentrated nitric and sulfuric acids are used in the preparation of explosives and propellants for military and civilian use. Tight control of the amount of the individual acids is necessary in order to obtain products of consistent composition and performance. Rapid feedback to the production department of analyses is essential to permit corrective action to out of specification nitrating mixtures without losing valuable time. Traditional manual analytical procedures may not be capable of delivering results within a timely manner, and so automated analytical procedures may be considered. Further, traditional manual methods may be subject to analyst error, and a high degree of training and supervision may be required in order to obtain results of sufficient accuracy and precision.

A new automated TET analytical protocol for the analysis of sulfuric, nitric and nitrous acids has been developed, employing the Metrohm 859 Titrotherm thermometric titration system. Nitrous acid is a by-product of the nitration process, and its presence has to be compensated, in order to obtain an accurate value for the nitric acid content. The protocol requires two separate titration sequences. In the first titration, the HNO2 content is determined by a direct TET, employing 0.1 mol/L KMnO4 as titrant. The result is saved by the software. In the second sequence, the sulfuric acid content of the sample is determined by TET with 1 mol/L BaCl2, and the result saved. The second titration of the sequence then starts. This is the determination of the Total Acid content, by TET with 2 mol/L NaOH. In the calculation at the end of the second titration, the HNO3 content is determined by subtracting the H2SO4 and HNO2 contents, after converting the results to an HNO3 equivalent.


A complete H2SO4-HNO3-HNO2 analysis can be completed in less than 7 minutes. For higher analytical productivity, a sample changer can be used.  The analyst only needs to weigh the samples into the titration vessels, and place them in the rack of the sample changer. All other steps are fully automated. The procedure is also suitable for completely automated on-line analysis.

Typical TET titration plots are illustrated in Figs. 1, 2 and 3. The solution temperature as a function of titrant delivered is shown as a red curve. The endpoint is accurately located from the inflection in the second derivative curve.

Fig.1. TET plot - titration of H2SO4 in nitrating mixture with 1 mol/L BaCl2

Fig. 2. TET plot - titration of Total Acids (H2SO4+HNO3+HNO2) with 2 mol/L NaOH

Fig. 3. TET plot - titration of HNO2 with 0.1 mol/L KMnO4

It should be emphasized that these three different titrations were all accomplished with the same sensor. This is simply a highly sensitive, rapidly responding electronic thermometer. It requires no calibration and minimal maintenance, and is thus ideally suited for routine process and quality control applications.