Determination of Naphthenic Acid Number in Petroleum Crude Oils

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Determination of Naphthenic Acid Number in Petroleum Crude Oils and Their Fractions by Mid-FTIR Spectroscopy Ramachandra Chakravarthy, Ganesh N. Naik, Anilkumar Savalia, Unnikrishnan Sridharan, Chandra Saravanan, Asit Kumar Das, and Kalagouda B. Gudasi Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.6b01766 • Publication Date (Web): 08 Sep 2016 Downloaded from http://pubs.acs.org on September 22, 2016

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Graphical Abstract Title: Determination of Naphthenic Acid Number in Petroleum Crude Oils and Their Fractions by Mid-FTIR Spectroscopy Authors: Ramachandra Chakravarthy, Ganesh N. Naik, Anilkumar Savalia, Unnikrishnan Sridharan, Chandra Saravanan, Asit Kumar Das, and Kalagouda B. Gudasi*

Abstract: A quick and efficient spectroscopic method was developed for the measurement of total naphthenic acid number using Mid-FTIR technique coupled with variable path length liquid cell with calcium fluoride windows. The method is quick, accurate, robust, repeatable, reproducible, and applicable to wide range of samples. This article also presents the effect of solvents for the measurement of NAN, hydrogen bonding, formation of monomer and dimer etc.

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Determination of Naphthenic Acid Number in Petroleum Crude Oils and Their Fractions

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by Mid-FTIR Spectroscopy

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Ramachandra Chakravarthy, † ‡ Ganesh N. Naik, † Anilkumar Savalia, † Unnikrishnan Sridharan, †

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Chandra Saravanan, †Asit Kumar Das, † and Kalagouda B. Gudasi‡*

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† Reliance Industries Limited, Reliance Corporate Park, Thane - Belapur Road, Ghansoli-

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400701, Maharashtra, India

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‡Department of Chemistry, Karnatak University, Pavate Nagar, Dharwad- 580 003, Karnataka,

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India

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* Corresponding author E-Mail: [email protected] , Phone No. +91-(836) 2215377

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Abstract

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Naphthenic acid is a generic name used for all the organic acids present in crude oils. The

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quantitative determination of Naphthenic Acid Number (NAN) is an essential parameter for

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petroleum refineries to evaluate corrosive properties of crude oils prior to their processing.

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Currently, most of the refineries are using total acid number (TAN) as a measure of corrossivity

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of crudes during their selection, valuation and processing. Some of the organic molecules are

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being used as corrosion inhibitors to reduce corrosion in refinery process units and the dosage of

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the same depends on the total acid number as it has been understood from the studies that acid

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inhibitors form a protective layer on the surface of the pipes and thus reduces the corrosion due

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to acids present in crude oil. TAN measurement by titration over-estimates the acid number as

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each and every molecule like thiols and phenols etc. that are titratable by alkali are also included

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in the calculation and that causes the improper estimation of the addition of corrosion inhibitors.

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To get a better refinery margin in the present economic scenario, optimization of the addition of

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corrosion inhibitors is very much essential and thus accurate measurement of NAN is a primary 1 ACS Paragon Plus Environment

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concern. Hence, we present a quick and efficient Mid-FTIR spectroscopic method for the

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determination of NAN using a variable path length liquid cell with calcium fluoride windows.

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Two distinct photon absorption bands in the region of 1680 to 1800 cm-1 were observed during

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the spectral measurement and are due to the formation of monomeric and dimeric forms of

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carbonyl (C=O) group of carboxylic acids, and hence both are considered for the quantification.

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The method is applicable even to highly volatile crude oils that are not measurable by normal

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ATR-FTIR technique. This article also presents the effect of solvents, hydrogen bonding,

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formation of monomer and dimer etc. Currently, this method is being applied for the

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determination of NAN for crude oils and straight run VGO samples as they contain either

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negligible or no carbonyl compounds other than carboxylic acids that interfere in the region of

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interest.

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Key Words: Naphthenic Acid Number, FTIR spectroscopy, Liquid cell, Total Acid Number

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1. Introduction

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Crude oil extraction, transportation and its processing is a great challenge to petroleum

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refineries in terms of economic costs and benefits1. Currently, many refining industries purchase

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“opportunity crudes” also referred to as “poor quality” crudes to increase their profit margin.

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However, in spite of the fact that these acidic crudes are less expensive, their processing in

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refineries raises a concern about the potential corrosion problems2. Several studies have been

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conducted for the corrosive molecules speciation and proved that, carboxylic acids and sulfur

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species are the major responsible molecules for the corrosion

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name used for all the organic acids present in crude oils. From the literature survey, it is learnt

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that, correlation of corrosion rate with TAN/NAN and total sulfur is complex in nature, as all the

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sulfur and organic acid species present in the sample do not contribute to the corrosion. Several

1-4

. Naphthenic acid is a generic

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methods are available for TAN/sulfur quantification in crude oils and their fractions3-6, whereas

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no quick and efficient methods are available for NAN measurement. Currently, most of the

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refineries use total acid number (TAN)7-8 as a measure of corrossivity of crudes during their

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selection, valuation and processing. To reduce corrosion in processing units, most of the

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refineries use corrosion inhibitors and the dosage of the same depends on the total acid number.

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TAN measurement by titration over estimates the acid number, as each and every molecules such

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as thiols and phenols etc. that are titratable by alkali are also included in the calculation. This

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causes the improper estimation of the addition of corrosion inhibitors. To get a better refinery

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margin in the present economic scenario, optimization of the addition of corrosion inhibitors is

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very much essential and thus accurate measurement of NAN is a primary concern. A

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considerable amount of work has been done in the past to quantify the naphthenic acids in crude

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oils and their fractions using various analytical techniques such as GC-MS, LC-MS, GC X GC,

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FTIR and FTICR techniques.

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The common analytical techniques such as GC-MS and LC-MS that are being used

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regularly in several refineries for the speciation of naphthenic acids present in the crude oils uses

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a tedious procedure for sample preparation and analysis. In these techniques, usually naphthenic

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acids were separated from the crude oil samples using various extraction techniques followed by

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detection using GC-MS and/or LC-MS. Since the total acidic components in the petroleum

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samples are usually very large in number, in maximum cases, there will be a merging of several

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components and makes the proper speciation analysis difficult 9-36.

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Some other techniques such as HPLC, NMR and UV that are being used in some of the

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analytical laboratories for the measurement of naphthenic acids are unable to provide detailed

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composition analysis37-39. Separation and quantification of all the components in each case using


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the above mentioned techniques is very tedious process, and most of the times, there will not be

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a clear separation due to co-elution of components. To address the separation issues, new

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inventions were made by several scientists using “Fourier Transform Ion Cyclotron Resonance

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Spectrometer” (FTICR). Due to its very high resolving power, this technique is robust enough

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and identifies almost all the components present in petroleum samples but it requires a huge

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capital investment due to its high cost40-46.

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The recent developments for the measurement of naphthenic acids present in the

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petroleum samples uses “Fourier Transform Infrared Spectroscopy” (FTIR)39, 47-49. Infrared (IR)

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spectroscopy in combination with multivariate calibration is the powerful analytical technique

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that has received much attention during the past decade50-52 because of its enormous applications

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both in qualitative and quantitative determinations. FTIR spectral region includes namely far

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infrared (FIR, 20-400 cm-1) mid-infrared (MIR, 400-4000 cm-1) and near infrared (NIR, 4000-

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12800 cm-1). Out of three regions, Mid-IR region is the most specific for the measurement of the

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carboxylic acid functional groups for the quantification as the absorption bands are specific,

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sharp and highly sensitive. Even though several methods have been reported9 for the

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quantification of several functional groups using this technique, no method has been reported for

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the measurement of NAN in volatile crude oils using liquid cell.

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The current literature methodology39 involves the separation of the naphthenic acids from

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the matrix and quantification using FTIR using a suitable calibration plot. To simplify this

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methodology, we have made an attempt to quickly quantify the naphthenic acids by directly

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dissolving the petroleum samples in various solvents and quantified using a suitable calibration

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standards and converted analytical measurements in terms of mg KOH/g (which was not

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measured by any of the present methodologies), as it is an essential parameter to optimize the


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addition of corrosion inhibitors during the processing of crude oils. This NAN measurement is

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also useful for the determination of relative abundance of naphthenic acid species measured by

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FTICR or LC-TOF techniques. Considering this aspect and its vast applicability for the

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refineries, we have developed a FTIR method for the quantification of naphthenic acids in crude

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oil and other fractions such as VGO.

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1.1. Background of corrosion in refineries.

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Processing acidic crudes in petroleum refineries is always associated with high corrosion

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rates especially in atmospheric and vacuum distillation columns, side strippers, furnaces, piping,

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and overhead systems. Naphthenic acid corrosion products also produce fouling in heat-

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exchangers, atmospheric and vacuum units and poison the catalytic conversion units causing

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their breakdown. Among the most typical corrosive agents in oil, naphthenic acids were proved

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to be one of the most aggressive and as a consequence numerous studies have been focused on

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investigating their corrosive effects. Several literature reports suggest that the refinery corrosion

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is mainly depends on the concentration of naphthenic acid and sulfur content. Based on the

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previous practical observations and experiments, it is proved that sulfur containing compounds

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mitigate corrosion by forming iron sulfide scale on the metal surface53-56. The general reaction

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for the naphthenic acid with metals, usually iron, can be represented as follows.

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Fe + 2RCOOH → Fe (RCOO)2 + H2

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Iron carboxylate once forms react with hydrogen sulfide and regenerates naphthenic acids and

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simultaneously iron gets converted to iron sulfide which is corrosive product. The reaction

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proceeds as follows.

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Fe (RCOO) 2 + H2S → FeS + 2RCOOH


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Iron sulfide further reacts with naphthenic acids and forms iron carboxylates. The general

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reactions can be given as follows. FeS + 2RCOOH→ Fe (RCOO) 2 + H2S

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Thus, the corrosion reaction becomes chain reaction and causes lot of damages and accidents in

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refineries. Hence, exact measurement of carboxylic acid concentration is very essential in

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refineries to control the corrosion by adding appropriate quantity of acid neutralizing additives.

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2. Experimental Section

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2.1. Materials and Methods

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Petroleum samples (crude oils and VGOs) were collected from ‘Jamnagar Refinery

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Units’, Gujarat, India. Hexanoic acid (99.9%), a standard compound, used for the calibration was

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procured from Aldrich chemicals. All the solvents were of HPLC grade with 99.9% purity

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obtained from Merck chemicals and used without further purification. MIR spectra of petroleum

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samples were collected using a Perkin Elmer Fourier transform infrared (FTIR) Spectrum 100

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spectrometer equipped with deuterated triglycine sulphate detectors (DTGS) and liquid cell with

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calcium fluoride (CaF2) windows accessory procured from Specac Ltd (Part No. GS07502). Data

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analysis was carried out using “Spectrum” software provided by Perkin Elmer. Samples were

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weighed accurately to the nearest milli gram and dissolved in known volume of particular

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solvent and mixed well to dissolve completely prior to FTIR measurement. Few samples were

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highly viscous and were partially soluble, and such samples were sonicated to get the clear

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solution before measurement. All samples and solutions were stored in airtight bottles to avoid

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evaporation.

136 137


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2.2. Acquisition of Mid-FTIR spectra

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All the spectra were collected in the Mid-FTIR spectrometer over the 4000 to 650 cm−1

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spectral region. The resolution of collected spectra was 4 cm−1, with the scan rate of 32 scans per

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min. Background spectra of solvent was collected before collection of the sample spectrum. The

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liquid cell of path length of 0.25 µM was fixed throughout the experiment for the analyses of

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standards as well as samples. Crude oil samples were stored in air tight containers at 20 0C to

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avoid evaporation of volatile components.

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Sample size was fixed throughout the experiment in the range of 1.00 to 1.50 gm in 10

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mL of solvent. The spectrum collected at lower than this concentration, has shown wavy baseline

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and higher concentration affects the liquid cell, as the resultant solution was highly viscous.

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Hence, transfer and cleaning the cell becomes difficult. It is also observed that quality of the

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spectrum depends on the concentration of the carboxylic acids present in the sample. After the

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spectral acquisition of the sample, the solution left in the liquid cell was removed completely and

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washed several times with solvent and ensured that it was cleaned completely prior to the next

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sample analysis.

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FTIR spectroscopy also provides a very good information regarding monomer and dimer

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formation and their equilibrium state in different solvents. Polar and non-polar solvents

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enormously vary the spectral width and intensity during analyses. Hence, in the current study,

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the effect of solvents and the formation of hydrogen bonding in three different solvents Viz.,

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dichloromethane, cyclohexane and toluene have been discussed.

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2.3. Calibration, Validation and determination of the NAN

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The calibration model was developed based on Beer-Lambert’s law using hexanoic acid

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as a standard. The calibration graph was drawn using concentration (mg KOH/g) of the solution


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Vs area obtained by FTIR and is presented in Figure 1a and 1b. All the measurements were

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made in triplicate and the results presented as the average of three values. The standard stock

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solution was prepared by dissolving the known quantity of hexanoic acid in known volume of

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the solvent. This solution was further used for the series of dilutions to prepare the standard

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solutions of various concentrations. The concentrations of the hexanoic acid were converted into

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mg KOH/g for the calculation of NAN and the conversion equation is presented below. ( / ) =

ℎ ℎ ℎ ℎ

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The calibration graph was drawn using concentrations (mg KOH/g) of hexanoic acid

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solutions against area obtained by FTIR measurements. For the purpose of accurate measurement

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of low and high NAN value, two calibration plots were drawn ranging from 0.020-0.318 and

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0.357-1.986 mg KOH/g for lower and higher concentration samples respectively. The straight

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line equation obtained by the graph was used to calculate the naphthenic acid number of the

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VGO and crude samples. Further, dilution factor was applied to all samples and the total

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naphthenic acid concentration was calculated using following equation. , / =

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!" #ℎ $ ℎ ℎ #,

2.4. Determination of TAN

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The total acid number of hexanoic acid was determined by traditional potentiometric

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titration method7-8. In the present determination, normality of potassium hydroxide solution

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determined was 0.07343 and is the average of five replicates. This value was used to calculate

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the TAN of hexanoic acid. Approximately 80 to 110 mg of the standard hexanoic acid was

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dissolved in 125 mL of dichloromethane and titrated against alcoholic KOH and the volume

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(burette reading) of KOH consumed for the neutralization of hexanoic acid was noted. The TAN


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of hexanoic acid was calculated using following formula where V1 and V2 are the volume of

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potassium hydroxide solution consumed for sample and the blank solution respectively ℎ , / =

(%1 − %2) ) 56.1 ) " $ ℎ ℎ #,

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The TAN was measured in triplicate and the average value was taken for the calculation

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of naphthenic acid number. The TAN data were compared with NAN value obtained by FTIR

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method. The percentage of NAN of the sample with respect to TAN was calculated using the

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following formula. - =

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3. Results and Discussion

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3.1. Determination of NAN

#ℎℎ ! ) 100 !

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Carboxylic acid functional groups are composed of two specific groups such as (–OH)

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and (–C=O) and both are specific, where C-OH stretching appears in the spectral region of 2500-

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3300 cm-1 and C=O stretching appears in the spectral region of 1690 to 1760 cm-1. Due to the

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merging of –OH group of carboxylic acid with other classes of OH frequency, it is not

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considered for the quantification whereas, the absorption of carbonyl group of carboxylic acids is

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highly specific, sharp, and is exclusive, hence carbonyl group was considered for quantification.

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The exact position of this band mainly depends on whether the carboxylic acid is saturated or

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unsaturated, dimerized, or has internal hydrogen bonding. We have observed well distinguished

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two specific signals in the region of 1680 to 1800 cm-1, during analyses of standard as well as

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sample, and are due to the formation of monomer and dimer

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for the quantification. Aldehydes, ketones and esters present in crude oil and straight run VGO

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samples may interfere in FTIR analysis of NAN measurement. However, literature suggests that

52

, and hence both are considered


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there is either no or negligible amount of aldehydes, ketones, and esters are present in crude

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oils57 and straight run VGO samples and hence are not considered for calculation.

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The analyses summary and the comparison data of NAN with TAN suggests that, total

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carboxylic acid (NAN) content determined by FTIR technique is always less or equal to TAN

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value determined by potentiometric method. It reveals that, the TAN value determined by

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potentiometric method does not distinguish organic and inorganic acids, as it predicts each and

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every molecule including phenols, thiols etc. that is titratable by base in the petroleum samples.

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The typical FTIR spectra for sample 8 at five different concentrations is displayed in Figure 2.

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The spectra depicts the presence of monomer and dimer at all five measured concentrations. The

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similar behaviour was observed in other tested samples also. The ratio of monomer and dimer are

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not proportional to the dilution, hence area under any one of the band was not recommended for

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the NAN measurement. Hence whole area under both the bands were taken for the determination

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of NAN.

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In general, increase in the percentage of naphthenic acid number, significantly causes

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corrosion in the refinery units; hence, the present FTIR method for the naphthenic acid

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measurement is more precise for the prediction of corrosion rate as compared to the one

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predicted by TAN measurement 39.

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3.2. Detection Limits: Suitability for Trace Analysis

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The detection limit was measured by taking the hexanoic acid solution of various

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concentrations ranging from 0.004 to 0.3971 mg KOH/g. The results obtained shows that the

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detection limit of the present method was found to be 0.004 mg KOH/g. But, this concentration

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was not used for the quantification as the spectrum baseline was wavy. The lowest possible

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concentration for the quantification was found to be 0.02 mg KOH/g and above this


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concentration, the calibration graph is linear and NAN calculation was also found to be accurate.

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The Figure 3 represents the detection and quantification limits of the present method.

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3.3. Effect of solvents on the measurement of NAN

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The NAN measurement studies in different solvents suggest that dichloromethane (DCM)

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is the best solvent than toluene and cyclohexane for the naphthenic acid quantification. The

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reason for this is due to absence of any absorption band in the carboxylic acid measurement

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region (1680 to 1800 cm-1), whereas, the other two solvents have distinguishable band that

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interferes in the same region, hence quantification became difficult. An interesting feature was

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observed during the analysis of the hexanoic acid in dichloromethane that the breakdown of

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hydrogen bond and formation of the monomer upon dilution39. At lower concentrations, usually

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carbonyl stretch of a carboxylic acid was sharp and appears as single band and it can be assigned

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as monomer. Similar way, at higher concentrations, well distinguishable two photon absorption

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bands were observed and identified them as monomer and dimer forms of carboxylic acid.

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To study the existence of monomer and dimer in solution state, we have conducted

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experiment using different solvents in various concentrations. This effect is very clearly

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observed when the solution was studied in DCM, toluene and cyclohexane. The toluene and

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cyclohexane are non-polar solvents, hence changes in the concentration of monomer and dimer

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are not observed. Whereas DCM being a polar solvent, it can breakdown the hydrogen bond and

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favors the formation of the monomer upon dilution, hence formation of monomer is more

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favored than the formation of the dimer. This study increases the specificity and sensitivity of the

244

analytical technique. As presented in Figure 4, it was clearly observed that, the concentration of

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monomer and dimers were varied upon dilution in polar solvent like DCM due to breakdown of

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the hydrogen bond present between two carboxylic acid units. Similarly, the solvent effect


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studies in cyclohexane, shows that, the concentration of monomer and dimer were not varied

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significantly (as presented in Figure 5), due to non-reactivity of nonpolar solvent towards

249

hydrogen bonding. Hence these studies may be useful in the functional group focused

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quantification studies in various refinery complex reactions.

251 252 253

3.4. Recovery and Repeatability studies

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The recovery of an analyte is the detector response obtained from an amount of the

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analyte added and extracted from the matrix, compared to the detector response for the true

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concentration of the pure authentic standard. The recovery studies were conducted by spiking the

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known concentrations (C, 2C and C/2, where C is the concentration) of the standard solution to

258

the sample solution. In all the cases, the recovery was found to be more than 90 %. The recovery

259

results of one of the samples in there different concentrations was presented in Table 2. The

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percentage recovery (R) results were calculated using the following formula R, % =

Experimental value from graph (D) − Actual amount present in sample(A) x100 Theoretical value(C) − Actual amount present in sample(A)

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The repeatability of NAN measurement for four different samples were carried out in five

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replicates and details are presented in Table 3. The results suggest that, all are within the

263

acceptable range.

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4. Conclusion

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A quick and efficient analytical method was developed for the quantification of total

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Naphthenic acid number using Mid-FTIR spectroscopic technique equipped with liquid cell. The

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technique allows for the quick quantification of naphthenic acid number by directly dissolving

268

the petroleum samples in dichloromethane. The method converts the analytical measurements of 12 ACS Paragon Plus Environment

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naphthenic acid in terms of mg KOH/g (which is not being measured by any of the present

270

methodologies using FTIR), as it is an essential parameter to optimize the addition of corrosion

271

inhibitors during the processing of crude oils. Several crude oils and VGO samples were tested

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for the NAN measurement and results were compared with total acid number (TAN) determined

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by ASTM D-664 method. Hexanoic acid was used as a standard for the present study and based

274

on its response, calibration curve was plotted and used for the calculation of NAN. The current

275

method is rapid, accurate and repeatable with recovery of more than 90 %. It has also been

276

observed that NAN and TAN ratios are varying in several crude and VGO samples and this

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information will be very useful for understanding the refinery corrosion studies. The accurate

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NAN values is useful for the optimization of the addition of exact quantity of acid inhibitors that

279

are being added to neutralize the acid species present in crude oils. The NAN measurement is

280

also useful for the determination of relative abundance of naphthenic acid species measured by

281

FTICR or LC-TOF techniques. Thus, this method should be promising to petroleum refineries

282

for routine analyses of NAN.

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Acknowledgment

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We are very thankful to Petroleum Refinery Units, Reliance Industries Limited, Jamnagar, India

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for providing the crude oil and VGO samples.

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References

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1.

Marcel D. Inc., New York, NY. 2001; pp 1-456.

288

289 290

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54. Turnbull, A.; Slavcheva, E.; Shone, B. Factors controlling naphthenic acid corrosion, Corr. 1998, 54 (11), 922-930. 55. Slavcheva, E.; Shone, B.; Turnbull, A. Review of naphthenic acid corrosion in oil refining. Br. Corr. J. 1999, 34 (2), 125-131 56. Babaian-Kibala, E.; Craig H.L.; Rusk, G.L.; Quinter R.C.; Summers M.A. Naphthenic acid corrosion in refinery settings. Mater. Perform. 1993, 50-55.

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443

444

445

446

447

448


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Table 1. NAN and TAN results presented in mg KOH / g for crude oils and VGO samples and the percentage of NAN w.r.t TAN (Each NAN and TAN values are the average of three replicates) Sample ID

NAN by FTIR

SD

RSD, %

(mg KOH/g)

TAN by titration

SD

RSD, %

NAN, %

mg KOH/g

VGO 1

0.5140

0.0092

1.7899

0.5980

0.0020

0.3302

85.95

VGO 2

0.8690

0.0009

0.1013

3.2840

0.0072

0.2178

26.46

VGO 3

1.6860

0.0040

0.2366

2.2960

0.0055

0.2414

73.43

VGO 4

2.7076

0.0180

0.6661

2.8000

0.0134

0.4794

96.69

VGO 5

4.7319

0.0064

0.1357

5.9130

0.0074

0.1249

80.02

VGO 6

13.8050

0.0135

0.0978

14.7200

0.0086

0.0582

93.78

Crude 7

0.9743

0.0019

0.1970

1.6000

0.0014

0.0889

60.89

Crude 8

1.2660

0.1741

1.3761

2.3600

0.0114

0.9535

81.53

Crude 9

2.4669

0.0417

1.6885

4.4100

0.0063

0.1437

55.93

Crude 10

7.3231

0.0578

0.7890

9.8251

0.0140

0.1425

74.53


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Page 24 of 31

Table 2. The recovery study results of VGO sample for naphthenic acid measurement in mg KOH/g. Sl.

Weight of sample taken,

Actual amount of NAN

Amount

Theoretical

Experimental Value

Recovery,

No

g in 10 ml of DCM

present in sample(A) in

of acid

value,

obtained from

%

10 ml DCM

spiked(B)

C=(A+B)

calibration graph (D)

1

1.0139

1.3902

1.3929

2.7831

2.6684

91.77

2

1.0145

1.4218

2.7803

4.2021

3.9727

91.75

3

1.002

1.3949

0.6951

2.09

2.0299

91.35

Average recovery

91.62


Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Energy & Fuels

Table 3. Repeatability studies of VGO and Crude samples for NAN measurement Sample No.

VGO 7

VGO 8

Crude 11

Crude 12

Weight of

NAN

Weight of

NAN,

Weight of

NAN

Weight of

NAN

sample (g)

mg KOH/g

sample (g)

mg KOH/g

sample (g)

mg KOH/g

sample (g)

mg KOH/g

Rep 1

1.0132

1.6383

1.0745

1.4141

0.326

8.8000

0.3038

1.0700

Rep 2

1.2232

1.5796

1.2693

1.4390

0.6738

8.7338

0.6038

1.0250

Rep 3

1.5331

1.6270

1.5667

1.4071

0.9684

8.7344

0.9038

1.0560

Rep 4

1.7656

1.5968

1.8056

1.3717

0.9652

8.6723

1.2228

1.0414

Rep 5

2.0030

1.5786

2.0076

1.3781

1.6002

8.5847

1.5211

1.0277

Average

1.6041

1.4020

8.7050

1.0440

SD

0.0273

0.0276

0.0752

0.0141

RSD (%)

1.7067

1.9685

0.8639

1.3511


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Page 26 of 31

Figure Captions Figure 1. Calibration graph derived from hexanoic acid using Mid-FTIR (a) Calibration graph in the range of 0.020 to 0.318 mg KOH/g (b) Calibration graph in the range of 0.357 to 1.986 mg KOH/g Figure 2. Typical overlaid FTIR spectra for Sample 8 at five different concentrations in dichloromethane Figure 3. FTIR spectra showing the detection and quantification limits in dichloromethane for hexanoic acid. It is also observed that, in very dilute solutions only monomeric form of acid is dominant. (Intext: Linearity graph) Figure 4. Typical FTIR spectra showing the breakdown of the hydrogen bond and formation of monomer upon dilution in dichloromethane for hexanoic acid at different concentrations Figure 5. Spectra showing minute/no changes in the concentration of dimer upon dilution in cyclohexane for hexanoic acid standard (Intext: Linearity graph)


Page 27 of 31

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Energy & Fuels

(a)

(b) Figure 1.


Energy & Fuels

0.073 0.070 0.065 0.060 0.055 0.050 0.045 0.040 0.035

A

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

0.030 0.025 0.020 0.015 0.010 0.005 -0.000 -0.002 1800

1780

1760

1740

1720

1700

1680

cm-1 Name

Description

Figure 2.


Page 29 of 31

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Figure 3.


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Page 30 of 31

Figure 4.


Page 31 of 31

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Figure 5.


Determination of Naphthenic Acid Number in Petroleum Crude Oils

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