Laboratório de EXtração, TeRmodinâmica Aplicada e Equilíbrio

The use of DSC in the determination of the vapor pressure of fatty acids

Rafael M. Matricarde Falleiro a, Luciana Y. Akisawa Silva a, Antonio J. A. Meirelles b, Maria A. Krähenbühl a, *

a Laboratory of Thermodynamic Properties, LPT, School of Chemical Engineering, University of Campinas, Brazil

b Laboratory of Extraction, Applied Thermodynamics and Equilibrium, ExTrAE, School of Food Engineering, University of Campinas, Brazil

*Corresponding author e-mail: mak@feq.unicamp.br

The present work aims to determine the vapor pressure of long-chain fatty acids by Differential Scanning Calorimetry (DSC). The boiling temperature of the fatty acids was measured as a function of previous established pressure, within a dynamically heated environment [1].

Palmitic, stearic and oleic acids are the major components of soybean triacylglycerols. These, through the transesterification reaction with ethanol, give rise to the principal constituents of soybean biodiesel [2]. All the studied fatty compounds are still poorly characterized, since data of their physicochemical properties are scarce in the literature. Therefore, these data are important in order to improve the biodiesel production processes.

INTRODUCTION

METHODOLOGY Palmitic and stearic acids with purity greater than 99% were acquired from Sigma, and oleic acid, with 99% purity, was acquired from Fluka.

The experimental apparatus (Figure 1) was especially projected based on the ASTM and 1782-03 guidelines [3], consisting of a Differential Scanning Calorimeter (DSC – Model 2920) with a vacuum system fitted to it. For the DSC analysis, the vapor pressure was set within the range from 1.33 to 9.33 kPa. Samples of 3 to 5 mg were used in the analysis, with a heating rate of 25 °C.min−1 and a small ball (Figure 2) placed over the pinhole (Figure 3), in order to avoid the pre-vaporization of the sample, since it behaves as an exhaust valve, releasing the vapor phase in a controlled manner [4].

tungsten ball

pinhole

Figure 2 – Small ball and the crucible with a pinhole

Figure 3 – Tungsten ball being placed on the pinhole

Figure 1 – (a) General view of experimental apparatus room; (b) Expanded perspective of the DSC furnace; (c) Perspective in more details of some accessories under of the bench.

(b)

(c)

(a)

According to this work, the methodology involving the DSC is an appropriate technique to obtain the vapor pressure data of fatty acids, since the data determined in this study showed a mean deviation of 0.70 ºC from the data of the literature [5, 6].

Figure 4 – Boiling endotherms measured between 1.33 and 9.33 kPa. (a) palmitic acid; (b) stearic acid; (c) oleic acid.

(a) (b)

(c)

7N,N1TT Cº/ deviation Mean

N

1 i

calculatedalexperiment

*

Fatty acids A* B* C* Validity range / ºC Mean deviation / ºCa

Palmitic acid 5.7743 1111.1435 22.6771 210.14 to 260.24 0.27

Stearic acid 9.7847 4289.055 260.1239 228.12 to 280.09 0.37

Oleic acid 10.822 5380.57 324.261 223.52 to 274.25 0.32

Table 1 – Antoine constants.

0.0035 0.0040 0.0045 0.00500.0

0.2

0.4

0.6

0.8

1.0

log 10

p /

kPa

T-1 / ºC-1

Figure 5 - Vapor pressure curves of the fatty acids. () palmitic acid; (●) stearic acid; (○) oleic acid and (−) Antoine.

The results proved that the DSC technique is reliable, since the data obtained showed precision similar to those obtained using the conventional techniques. Differential Scanning Calorimetry was shown to be capable of determining vapor pressure data with efficacy, in a short time and requiring low amount of chemicals.

CONCLUSION

RESULTS AND DISCUSSIONFor each established pressure, the boiling temperature was determined by the Differential Thermal Curves (Figure 4), and the experimental values obtained were compared to the literature data obtained by conventional techniques [5] and [6].

[1] R. J. Seyler, Thermochim. Acta, 17, 129 - 136 (1976).[2] G. Knothe, J. Van Gerpen, J. Krahl and P. L. Ramos, Biodiesel Guideline, 1st edition (2006).[3] ASTM (American Society for Testing and Materials) E 1782-03 (2003).[4] R. F. Farritor and L.C. Tao, Thermochim. Acta, 1, 297 (1970).[5] D. R. Stull, Ind. Eng.Chem, 39 (4), 517 – 540, (1947).[6] Texas A&M University, Thermodynamics Research Center, (1980). [7] DDBST - Dortmund Data Bank Software Package - Educational version - 2003, www.ddbst.de.

REFERÊNCIAS BIBLIOGRÁFICAS

From the nonlinear regression of data obtained in this study, the Antoine constants (Table 1) were determined using the DDB Software Package [7].

C C)(º TB -A (mmHg)p log

a