ABSTRACT

The Instituto Colombiano del Petróleo (ICP), has implemented a methodology for separating vacuum residue fractions using the technique of supercritical fluid extraction at the pilot scale. The present study evaluates the efficiency of extraction of fractions of a typical vacuum residue in the Barrancabermeja refinery. The extraction test was carried out with n-hexane under supercritical conditions of temperature and pressure of the 265°C in the range from 450 to 1250 psi, respectively. Finally, each of the fractions were analyzed for their density, viscosity, sulfur content, Conradson Carbon Residue (CCR) content, SARA compositional analysis, and metals.

Key words: supercritical extraction, vacuum residues, physical properties.

El Instituto Colombiano del Petróleo (ICP) ha implementando una metodología de separación de fracciones de fondos de vacío mediante la técnica de extracción con fluidos supercríticos a escala piloto. El presente estudio evalúa el rendimiento de extracción de fracciones de un fondo de vacío típico de la refinería de Barrancabermeja. La prueba de extracción se realizó con n-hexano bajo condiciones supercríticas de temperatura y presión de 265°C y 450 a 1250 psi, respectivamente. Finalmente, cada una de las fracciones se les determinó su densidad, viscosidad, contenido de azufre, residuo Carbón Conradson, análisis composicional SARA y metales.

Palabras Clave: extracción supercrítica, fondos de vacio, propiedades físicoquímicas.

1. INTRODUCTION

A supercritical fluid is defined as a pure component found in the region with temperature and pressure greater than or equal to the critical conditions, T_r ≥ 1,0, P_r ≥ 10, where liquid and vapor states are not differentiable. Supercritical fluids (SF) have properties similar to the liquid form (i.e. density) and properties similar to the gaseous form (i.e. diffusivity, viscosity, and zero surface tension). These properties vary with changes in temperature and pressure, which substantially affect the mass transfer, (Han, Yang, Ke, Mao, & Yan, 1998; Gallego & Cardona, 2004).

The solvent power of supercritical fluids depends on its density. These fluids are able to dissolve weakly volatile substances and easily penetrate solid matrices (respective properties of traditional liquid solvents and gases when they reach values close to those of a liquid under supercritical conditions. Thus, supercritical fluids are considered to be effective solvents.

The advantages of supercritical fluids compared with those of other solvents are: low power consumption, easy separation of the solvent and the extracted fraction, high selectivity, and low resistance to mass transfer. The supercritical extraction technique began in the XIX century with the research done by Hannay and Hogarth (1897) in the study of solubility of inorganic salts (KI, KBr, and CaCl₂) in supercritical ethanol, methane, ethane and carbon dioxide, Hannay and Hogarth (1897). With the release in 1930 of vapor-liquid diagrams (VLE) of hydrocarbons at high pressures, it was possible to develop the first industrial application of solvent deasphalting of crude in 1943, (Messmore, 1943; Northup & Sloan, 1986).

The supercritical fluid extraction technique has impacted several sectors in industry since the 1970's and 1980's. Many authors have evaluated the effect of variations in temperature, pressure, time, flow and nature of the solvent in the extraction of natural essences, such as pesticides from solid waste and refined oils, (Gallego & Cardona, 2004 and Esquivel & Vargas, 2007).

Subramanian and Hanson (1998) proposed a method for supercritical fluid extraction in oil sands using propane as a solvent. Along the same lines, Masaka, Motonobu, & Tsutomu (1997) and Suoqui, Zhiming, Chunming, Keng, & Chung (2005) applied the extraction technique to waste oils, in order to get free fractions of asphaltene fractions using n-pentane as solvent. Sheng- Li, Sheng-Sheng, Yun-Hua, & Suo-Qi (1984); Tie-Pan, Yun-Xiang, Tong, & Ren-An (1997) and Guangua and Ren-An (1999) characterized the fractions obtained from heavy crude oil, and vacuum residue with Propane, Butane and Pentane in supercritical conditions. Swanson, Dollimore, & Diehl (1985) evaluated the conversion efficiency and extraction conditions of light fractions present in coal by supercritical extraction with solvents such as pentane, hexane, heptane, octane and cyclohexane.

With the steady depletion of reserves of light crude, refineries have been forced to rethink their processes with new feed mixtures, based on heavy crude oil with a projected in high increase atmospheric and vacuum residue. However, to adjust the refining process residue vacuum is essential to know their physicochemical properties, because this characterization allows to increase the efficiency of conversion processes.

To make a broad characterization and estimation of physicochemical properties such as density, viscosity and molecular weight among others, is not an easy task because to get significant amounts of these fractions involves a complicated task with traditional methods such as the SARA separation method.

Ecopetrol S.A. in the Instituto Colombiano del Petróleo (ICP), has built a pilot unit of supercritical fluid extraction and fractionation with two purposes: first, to extract significant quantities of a large number of fractions of a different nature from vacuum residue for further characterization. Second, deepening on the behavior of heavier petroleum fractions to improve and integrate new and future refining schemes.

Although there are a variety of works in the area of supercritical extraction solvent paraffinic from propane to pentane, this research was selected over other n-hexane solvent, due to the fact that the solubility in supercritical conditions on high molecular weight hydrocarbons tends to be higher, as indicated by Swanson et al. (1985).

The purpose of this study is to evaluate the methodology of supercritical extraction implemented by Ecopetrol S. A., in a vacuum residue typical refinery in Barrancabermeja using as solvent n-hexane under operating conditions of 265°C at pressure range between 550 and 1250 psi.

2. EXPERIMENTAL METHODOLOGY

Materials

Vacuum residue typical Barrancabermeja refinery, and n-hexane (density at 25°C = 0,655 g/ml, Pc = 436,9 psi, Tc = 234,7°C).

Description of the extraction process

The extraction unit consists of two high-pressure pumps for solvent and vacuum residue (2, 6), an extraction column (3), a pressure regulator (4), a container to decompress the solvent and collect the samples at each pressure condition (5), and a solvent drum to recover the solvent (1), as shown in Figure 1.

The characterization vacuum residue and the fractions obtained was performed according to ASTM methods described in Table 1.

Table 1 reports an extraction yield of 82,6% compared to the total of vacuum residue used in the test. However, if we consider the extraction yield in relation to the vacuum residue without the fraction of asphaltenes, is possible to extract high yields of species maltenes fraction distributed in a large number of fractions.

Table 2 shows the amounts of the fractions obtained by supercritical fluid extraction and fraction SFEF.

Comparing the results obtained by Simulated distillation of high temperature according to ASTM D6352-04 (Figure 2) with the results of the technique of supercritical extraction can be said that the technique of simulated distillation requires temperatures near 720°C to recover for 67% of vacuum residue, while the supercritical extraction technique is possible to extract about 92,6%.

The results show that the technique of supercritical extraction is a very interesting tool for obtaining high yields of deep fractions from vacuum residue of varying nature. In Figure 3, illustrates the appearance of the fractions obtained by supercritical extraction technique.

Figure 4, illustrates the physicochemical properties of the fractions obtained by supercritical extraction from vacuum residue typical of the Barrancabermeja Refinery. The results indicate that as the condition is increased pressure to extract fractions at constant temperature increases the density and viscosity, which indicates that the fractions become heavier.

The results illustrated in Figure 5 indicate that the saturated fraction is concentrated in the first fractions and then begins to decrease its concentration with increasing pressure at constant temperature. The resin content in the fractions shows an inverse trend compared with the saturated concentration. A possible reason for this is due to increased solvency power of the solvent. Aromatic content distribution has a tendency with little variation in the range of 50 to 55%. This observation is related to the high aromatic content of about 50% in the vacuum residue (Table 1). The cut of residue is mainly composed of asphaltenes and resins.

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