ABSTRACT

The world petroleum industry shows a decreasing in the oil reserves, specially the light kind. For this reason is very important to implement process schemes that give the possibility to improve the recuperation of valuable products of heavy oil. In this case the residue processing in each one of the petroleum refining stages earns great importance with the purpose of maximizing the quantity of fuel by barrel of feedstock processed. Therefore, it has been proposed the modification of the currently vacuum residues process scheme in the Ecopetrol's Barrancabermeja refinery (DEMEX-Visbreaker-Hydroprocessing). That modification consists in the incorporation of an additional Visbreaker stage, previous at DEMEX extraction stage.

This investigation was developed with plant pilot tests combined with statistical models that predict the yield and the quality of the products obtained in the industrial plants. These models were developed by the Instituto Colombiano del Petróleo (ICP).

The modified scheme Visbreaker I-DEMEX- Visbreaker II- Hydroprocessing, gives the possibility to increase the yield of middle distillates. Besides decrease the quantity of demetalized oil produced in DEMEX stage. This reduction is very favorable since environmental point of view, because it allows have a percentage of free capacity in the Hydroprocessing unit in order to removed sulfur of valuable products like Diesel and in this way to respect the environment law to this kind of fuel.

Keywords: visbreaking, Demex, hydrotreating, vacuum residues, light gas oil.

El panorama mundial de la industria del petróleo muestra una preocupante escasez de las reservas de crudo, en especial de las del tipo liviano. Por esta razón se hace imperante la necesidad de implementar esquemas de procesamiento que permitan aumentar la recuperación de productos valiosos de los crudos pesados. Ante esta situación el procesamiento de los fondos de las diferentes etapas de la refinación del petróleo cobra gran importancia con el fin de maximizar la cantidad de los combustibles recuperados por barril de crudo procesado. En este sentido se plantea la modificación del esquema actual de procesamiento de fondos de vacío de la refinería de Ecopetrol en Barrancabermeja (Desasfaltado con solvente (DEMEX)- Viscorreducción- Hidrotratamiento); dicha modificación consiste en la incorporación de una etapa de viscorreducción adicional, previa al proceso de Extracción DEMEX.

La investigación se realizó mediante pruebas a nivel piloto combinadas con modelos estadísticos que predicen el rendimiento y la calidad de los productos obtenidos en las plantas industriales. Estos modelos fueron desarrollados por el instituto Colombiano del Petróleo (ICP).

El esquema de procesamiento modificado Viscorreducción I-DEMEX- Viscorreducción II- Hidrotratamiento, brinda la posibilidad de incrementar el rendimiento de destilados medios, y a su vez permite disminuir la cantidad de aceite desmetalizado producido en el proceso DEMEX. Esto último es muy favorable desde el punto de vista medioambiental, pues permite tener un porcentaje de capacidad libre en la etapa de hidrotratamiento, la cual puede ser aprovechada para desulfurizar productos valiosos tales como el combustible Diesel, y de esta manera cumplir la legislación ambiental para este tipo de productos.

Palabras Clave: viscorreducción, Demex, hidrotratamiento, fondos de vacío, destilados medios.

1. INTRODUCTION

Currently, faced with the scarcity of light crudes, the oil refining industry is concerned with increasing the yield of its products, granting an important role to the residue treatment processes as a means to achieving this goal.

The vacuum residue processing design, (Figure 1), solvent deasphalting (SDA) - Visbreaker (VBK) -hydrotreatment (HDT) that is currently being used in the Barrancabermeja refinery, it is not very effective in respect to middle distillate yield and in addition has an association to high fuel oil production. These weak points can be attributed to the fact that the potential of chemical conversion that visbreaker offers is applied to a feedstock actually quite poor in valuable distillates. The feed to the visbreaker process has been previously subject to a solvent deasphalting process which has had demetallized oil (DMO) removed, and which has a high potential to be chemically transformed using thermal treatment.

Vacuum residue, although a residue flow, offers the possibility of still having valuable products extracted from it. The incorporation of a chemical transformation stage via thermal conversion, prior to the SDA process (Figure 2), offers the possibility of a larger yield of valuable products within the vacuum residue processing design. The above-mentioned is possible, since in the thermal conversion processes there are a series of large-molecule breakdown reactions, hydrogen redistribution, and also condensation and polymerization reactions.

Figure 3 shows a diagram of blocks and flows of the current residue processing design of the Barrancabermeja refinery that were calculated with the simulation model developed by the ICP.

Table 1 presents the main features of the flows of this processing design.

As can be seen in Table 1, in the solvent deasphalting stage, the reduction of the metals and sulfur content that is present in DMO, and the increase in Conradson Carbon content in asphaltene stand out. As for the visbreaker stage, the aspect that is worthwhile mentioning is the reduction in residue viscosity in respect to the feedstock at this stage (residue of the solvent deasphalting stage DMXR) and its high sulfur content. The topics previously mentioned are evidence that the model predicts well the feedstock behavior in each of the studied processes.

Vacuum Residue Modified Processing Design

Visbreaker Stage I (VBK I)

In this project-level pilot runs were not carried out in this first stage due to operating limitations, since it implied a work of several months in the pilot plant due to the amount of product necessary for the research; in addition, in the Cartagena refinery the vacuum residue are submitted to this process. This situation was favorable to the interests of the project, since we counted on samples of visbroken vacuum residue. This consideration was validated through the simulation model of the visbreaker unit of the Barrancabermeja refinery, developed by Ecopetrol-ICP. In said validation, a satisfactory reproduction of the yield and quality of the products was obtained.

This extrapolation was carried out considering as subject of comparison the sum of the yield of the naphtha and kerosene fraction, considering them as a single flow, and the sulfur content of these flows was predicted using the model. After feeding the model with the physical and chemical characteristics of the residue (VBK I stage feed) the following aspects stand out:

• The naphtha and kerosene yield difference between the one provided by the model and the operating datum of the industrial plant was of 6,57% in volume (5,4% of the model in comparison to the actual 5,78%).
• The sulfur content of the naphtha resulting from the model was of 0,9% mass and the datum of the naphtha characterization was of 0,912% mass with a difference of 1,31%.

As can be seen in the Table 2, the vacuum residue have a high content of aromatic compounds; therefore, based on the studies of Raseev (2003) on the impact of feedstock characteristics on thermal cracking processes, we can predicted that due to the high percentage of aromatic type species in the feedstock during the thermal conversion, high fraction yields are obtained; both light and heavy; and also a lesser conversion toward middle distillates and gas oils (Di Carlo & Janis, 1992; Raseev, (2003).

After the visbreaker process there is a recovery of 7,2% of middle distillates, 25,8% of diesel oils, in addition to a conversion of gasses of 4,56%.

The increase in the production of valuable distillates, in the modified processing design, according to Salazar and Meyers (1986), is attributed to the action of the thermal cracking on the feedstock. In this cracking, the breakdown of some aliphatic type chains that are bound to the complex aromatic structures present in the feedstock, allows the attainment of a larger conversion percentage. This result concurs with the data reported by Salazar and Meyers (1986), in which he shows that a significant yield of diesel oils is obtained in the vacuum visbreaker process.

The visbroken vacuum residue, when compared to the feedstock, show an increase in the content of Conradson Carbon and of the nickel and vanadium content. Fainberg, Podorozhasky, Hetsroni, Brauch, & Kalchouck (1996) found that these pollutants are strongly bound to complex aromatic structures and in face of the difficulty of being removed by the action of the thermal rupture, said compounds concentrate on the residue product.

Solvent Deasphalting of the Visbroken vacuum Residue

Table 3 presents the yield and quality of the demetallized oil and of the asphaltenes obtained in this processing stage.

The most important control variable of the visbreaker process is visbroken residue stability, since unstable visbroken residue generate sediments and therefore storage and transport problems. This stability is measured through the merit test. Therefore a series of preliminary experiments were performed in order to determine the maximum allowable severity in the visbreaker process. Table 4 summarizes the yield and merit test of the visbroken residue of these tests.

At the VBK II stage we only took into account as interesting analysis of characterization the viscosity of the visbroken residue and the simulated distillation of naphtha and visbroken residue. These are shown in Figures 5 and 6.

After having concluded the operating stage, we proceeded to the characterization of the Hydrogenated Demetallized Oil DMOH and the gasses obtained in each of the tests. The characterization results corresponding to these products are presented in Tables 5 and 6, and in addition in Figure 7.

After comparison of the results contained in Tables 3 and 6 we highlight the behavior of the basic nitrogen content, since it is notable that in the DMOH processed at 330°C there was no decrease of this element in respect to the content present in the DMO; however, at temperatures of 350 and 370°C we did achieve a reduction of basic nitrogen; based on the studies of Sumbogo, Yang, Choi, Korai, & Mochida (2003) this can be attributed to the fact that at lower temperatures there is removal of nitrogen in species with basic nitrogen and simultaneously with the fact that some compounds of non-basic nitrogen are hydrogenated to compounds of basic nitrogen, before eliminating the nitrogen molecule from the compound.

The DMOH obtained in the run at 370°C presents a decrease in the sum of nickel and vanadium present in the sample since according to the results this reduction was of approximately of 98,5%. This indicates that these metals have been deposited as metal sulfides on the surface of the catalyst, poisoning the active sites and obstructing the catalytic pores, leading to the irreversible deactivation of the catalyst.

Additionally, it is worthwhile to highlight that despite it being a feedstock where more compounds of the olefinic type are present than there are in a typical feedstock, no flow problems occurred during the operation. Proof of this was the behavior of the catalyst in respect to sulfur removal, since the percentage of the removed pollutant is within the range that is usually obtained with products of lesser olefin content.

In respect to the bromine number, it is remarkable that in the three hydrotreatment tests its value in the products decreased 45% on average in respect to the value of the feedstock. The latter is proof of the reduction of the quantity of olefin present in the products after the hydrotreatment process. According to Cabrera and Meyers (1986) this phenomenon is due to the rupture of the double bonds in unsaturated compounds, due to hydrogenation reactions.

According to what was observed in the gas chromatography (Table 6) increasing the operating temperature implies an increase in the quantities of methane, ethane, propane, n-pentane, and hexanes in the sample. On the other hand, the analysis shows that an increase in the operating temperature causes a decrease of the quantities of H₂S, CO, and ethylene. In the particular case of H₂S reduction, the results obtained are far from the expected results, since due to the increase of the operating temperature, the quantity of sulfur removed from the liquid product is increased, and this removed sulfur should be transformed to H₂S in the gaseous flow of the process. This could be attributed to the fact that before the directly proportional relationship of the solubility with temperature, the compound in question presents separation problems after the reaction, since it is solubilized in greater proportion within the liquid sample.

Global Yields of the Modified Design of Visbreaker I- Solvent Deasphalting - Visbreaker II -Hydrotreatment

Figure 8 presents the experimental volumetric yield of the modified processing design.

After comparing the volumetric yields of the products in each of the processing designs shown in Figures 3 and 8, the main aspects of the modified processing design that outperform the current design are evident.

• The distillate yield (naphtha, kerosene, and diesel) passes from 3,1%, in the current residue processing design, to 5,3% in the modified processing design.
• The quantity of recovered distillates with the current design is of 0,5 KBPD; small quantity, if we considered that with the modified processing design the recovery of these products reaches 0,6 KBPD and that additionally, 6,45 KBPD of gas oils is recovered in the visbreaker stage.
• The fuel oil production is reduced by 11%, since its main feedstock material, the visbroken tar, decreases by that same percentage.
• In the modified processing design, the feedstock to the hydrotreatment stage decreases, since DMO production drops by 27%. This situation is favorable since it provides the option of using the surplus capacity to hydrogenate other types of products.

Table 7 presents the volumetric balance, in barrels, of both the current and proposed residue processing designs.

Table 8 presents the cash flow, in dollars, of the two refining designs.

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