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Experimental study and economic analysis of heavy oil partial upgrading by solvent deasphalting-hydrotreating Gabriel Díaz-Boffelli, Jorge Ancheyta, José A. D. Muñoz, and Guillermo Centeno Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b02442 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 25, 2017
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Experimental study and economic analysis of heavy oil partial upgrading by solvent deasphalting-hydrotreating Gabriel Díaz-Boffelli, Jorge Ancheyta*, José A.D. Muñoz, Guillermo Centeno Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas Norte 152, Col. San Bartolo Atepehuacan, México City 07730, México. Corresponding author: E-mail:
[email protected] ABSTRACT Technical and economical evaluation was carried out for upgrading of heavy crude oil by a combined scheme of deasphalting and hydrotreating. Deasphalted oil was obtained at 60°C of temperature, 25 kgf/cm2 of pressure, 5:1 solvent/oil ratio and 1 hour of residence time, using n-heptane as solvent. Deasphalted oil product was upgraded by means of hydrotreating at low severity reaction conditions: 360°C of temperature, 60 kgf/cm2 of pressure and 4 hours of reaction time. The technical and economic analysis demonstrated that the upgrading scheme is an appropriate option for producing transportable oil. Keywords: Heavy oil, upgrading, deasphalting, hydrotreating 1. INTRODUCTION It has been recognized that the combination of more than one conversion process can be an excellent choice for upgrading of heavy oils. The advantages of each process can be put together in an integrated scheme that may yield higher benefits than the use of single processes. Since asphaltenes are the main responsible for catalyst deactivation, it is highly
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recommended that they are removed from the heavy feed before it enters to other catalytic processes.1 There are some successful cases reported in the literature that corroborate the advantages of combining upgrading processes, such as: 2-4 •
Solvent deasphalting (SDA) plus gasification. The combined process showed 1.33 USD/Bbl of net realization versus 1.10 USD/Bbl for the solvent deasphalting alone for the Canadian Cold Lake crude
•
Solvent deasphalting plus delayed coking (DC). A representative combined scheme of this option is the ASCOT process
•
Solvent deasphalting plus delayed coking plus gasification. The benefits of SDA/DC are improved by addition of gasification that can supply advantages such as: power for site and export, hydrogen for further upgrading, among others
•
Solvent deasphalting plus hydrocracking in ebullated-bed. Important reductions in impurities content are observed as well as significant increase in API gravity and reduction of viscosity
All of the process combinations have been focused on maximizing the conversion of the residue fraction, and no much attention has been put on partial upgrading to accomplish the properties of the oil only for transportation purposes. However, one important aspect of the aforementioned process combinations is that all of them use SDA as a process for pretreating the feed to another upgrading process. On the other hand, the transportation of heavy crude oil with high viscosity through pipelines presents the following problems:
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•
Increase of operating pressure and required pumping capacity
•
Increase in energy, environmental and economic costs
•
Increase of risks of ruptures and failures due to overpressure (the pipelines tend to lose strength)
•
High
concentration
of
asphaltenes
and
the
associated
problems
of
incompatibility during blending of crude oils and instability during storage Despite the risks and potential problems of transportation of heavy crude oil by pipelines, it remains the most effective method in terms of safety in order to accomplish the following required specifications5: •
The reported values of viscosity for transportation of heavy and extra-heavy crude oil range between 350 cSt at 7°C and 150 cSt at 50°C
•
The most common maximum specification of viscosity for transportation of heavy and extra-heavy crude oil is 250-400 cSt at 37.8ºC
When the heavy oil is deasphalted, most of the refractory components, i.e. asphaltenes, is removed, and the produced feed becomes lighter. This lighter feed can be hydrotreated to increase more its API gravity and reduce viscosity at the desired values for transportation, as well as in parallel to reduce the impurities content. Hydrotreating a deasphalted oil will require less severe reaction conditions and in consequence, the investment and operating costs of the hydrotreating plant will be lower. This is the reason why in the present study, a series of experiments was carried out to evaluate, from technical and economic points of view, the effect of combining solvent deasphalting and hydrotreating to upgrade the flow properties of a heavy crude oil. 2. EXPERIMENTAL 3 ACS Paragon Plus Environment
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2.1. Solvent deasphalting The solvent deasphalting was carried out in a batch reactor model Parr 1370 HC2-T316, provided by a heating jacket, stirring system, manometer and thermocouple which send the signals to a controller. The solvent-heavy oil mixture was loaded to the reactor and the asphaltenes precipitation was carried out at 60°C temperature, 25 kgf/cm2 pressure, 5:1 solvent/oil ratio and 1 hour of residence time at 1000 rpm of stirring rate.6 High purity nitrogen was used for pressurizing the system, n-heptane of technical grade was the paraffinic solvent used due to its solvation power, which allows for the precipitation of high molecular weight asphaltenes and gives consistent experimental repeatability. 6 The heavy crude oil used in this study was recovered from an offshore facility located in the Gulf of Mexico and its properties are reported in Table 1. After the precipitation, the product was filtered in a vacuum system for separating the deasphalted oil and solvent from the asphaltenes. Finally, the solvent was removed from the deasphalted oil by distillation resulting the pure deasphalted oil (DAO). A simplified scheme of these operations is shown in Figure 1. 2.2. Hydrotreating The DAO was used as feed for hydrotreating experiments, which were carried out in a stirred batch reactor model Parr 4575 series 30996, with a basket for loading the catalyst, heating jacket, manometer, stirring system, and thermocouple which send the signals to a controller.
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The objective of this study is to produce upgraded oil with suitable properties for its transportation by pipeline (kinematic viscosity lower than 250 cSt at 100°F and API gravity higher than 16), so that the reaction conditions of the hydrotreating step must be of low severity. In addition, the produced DAO would possess reduced amounts of metals and sulfur, hence the operating conditions chosen were: 360°C temperature, 60 kgf/cm2 pressure, 1, 2 and 4 hours of reaction time, and 750 rpm of stirring rate. A commercial supported catalyst (NiMo/Al2O3), which properties are shown in Table 2, was previously ex situ activated at the following conditions: dried with 30 ml/min nitrogen flux at 120°C and atmospheric pressure for 2 hours. Then, it was soaked by circulating 55 ml/min of hydrogen through carbon disulfide at 400°C and atmospheric pressure for 3 hours.7-8 The activated catalyst was loaded to the basket in nitrogen atmosphere so that contact with air was avoided. After closing the reactor, it was purged several times with hydrogen to assure there was no air left inside the reactor. Then hydrogen was fed until the initial pressure was reached. The initial pressure was first calculated with equation 1. Where P1 means the initial pressure, T1 the ambient temperature, P2 the reaction pressure, and T2 the reaction temperature. =
(1)
Heating was then started from room temperature to 360°C. The reaction began until all conditions were established and stirring rate was initiated. For each reaction time, fresh sample of sulfided catalyst was employed.
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After reaction, the reactor was depressurized and hydrotreated products were characterized by elemental analysis (ASTM D-5291), specific gravity (ASTM D70), kinematic viscosity (ASTM D7042) and metal content (ASTM D5863). 3. RESULTS AND DISCUSSION 3.1. Solvent deasphalting During deasphalting the product separation resulted in 74.988 wt% of deasphalted oil and 25.012 wt% of asphaltenes. Due to asphaltenes fraction concentrates most of all the impurities (sulfur, nitrogen, metals) in their structures, when separating them from the maltenes fraction, the produced DAO reduces the content of these impurities as can be seen in Table 3. In addition, hydrogenation and partial hydrocracking of asphaltenes cause increase in the H/C atomic ratio and consequently an increase in API gravity and reduction in viscosity.9 Metal content in DAO decreased more than 50%, which is quite normal since metal porphyrins and non-porphyrins type of structures are concentrated in the asphaltene fraction, for this reason only a minimum amount of these metallic compounds are kept in DAO fraction, which is mostly composed by maltenes.9- 10 H/C atomic ratio resulted to be higher in DAO as compared with heavy oil. Since H/C atomic ratio indicates the polarity or aromaticity and solubility of the sample, when its value increases, the molecule is transformed into saturate and paraffinic compounds diminishing its solubility, therefore the hydrogen requirement for subsequent hydrotreating will decrease.9
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The reduction of kinematic viscosity of the DAO is linked to the amount of asphaltene fraction separated that allow for obtaining additional benefits in the transportation properties as compared with the heavy crude oil.9 From these results, it is observed that deasphalting is a beneficial process for removing heteroatoms and heavy metals contained in the crude oil that allows for obtaining a lighter deasphalted oil without asphaltenes. The produced DAO has lower aromaticity and would facilitate hydrotreating reactions of heavier molecules into lighter ones.11 This is the reason why DAO is ranked as good raw material to be fed in a subsequent hydrotreating process. Despite these clear advantages, when examining the fluid properties of the DAO (kinematic viscosity and API gravity), they are not sufficient to assure the transportation of the oil. In other words, DAO needs further processing to accomplish with the required properties, which can be achieved by means of hydrotreating. 3.2. Hydrotreating DAO was processed in a hydrotreating batch reactor. The properties of the hydrotreated products are shown in Table 4, which were treated at 360°C and 60 kgf/cm2 respectively. HDAO-1, HDAO-2 and HDAO-4 represent hydrotreated DAO at 1, 2, and 4 hours of reaction time respectively. As expected, longer reaction time (HDAO-4) causes better quality oil, i.e. with reduced amount of impurities, lower kinematic viscosity, higher H/C atomic ratio and enhanced API gravity as compared with the other samples, that is due to a rupture in the molecules enhancing the hydrotreating performance. 12-13
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In addition, all samples presented a kinematic viscosity lower than 250 cSt and API gravity higher than 16°API, as a result, all of them satisfied the specifications for transporting the oil through pipelines.14 Variations in API gravity and sulfur content in the hydrotreated samples are shown in Figure 2. The hydrotreated crude oil quality is directly related to the hydrocracking and hydrogenation reaction rates. It is worth mentioning that the hydrocracking reactions are influenced by catalyst properties since it promotes the reduction of molecule sizes, which causes increase of API gravity and decrease of viscosity of heavy crude oil. Hence, the best quality of HDAO-4 sample is a consequence of the longer contact time and the catalyst properties used during hydrotreating. Metal removal is reported in Figure 3. It is observed that both vanadium and nickel contents decrease and then reach lowest value at 4 hours of reaction time. Since part of these metals are contained in structures associated to resins, the other part of metals remain in the asphaltene fraction.6 In this case it is acceptable that deasphalted oil contains lower metal content because asphaltenes are separated in previous deasphalting.11 Similarly to DAO, but on the opposite site, the hydrotreated products have lower viscosity and higher API gravity than required. This allows for defining a process scheme in which part of the heavy crude oil can be processed, another part can be by-passed, and the two streams be blended to produce a transportable oil. 3.3. Economic analysis The economic analysis was carried out considering the global benefit, which is determined as the income minus the investment cost (CAPEX) minus operating costs (OPEX). The 8 ACS Paragon Plus Environment
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revenues were calculated with the flowrates according to the mass balance with data obtained experimentally, and costs of heavy crude oil, hydrogen and products (upgraded crude and asphaltenes). The investments of the process plants were taken from the literature, and scaled-up and indexed with the six-tenths equation and the Nelson-Farrar indices respectively. The resulting investment cost was annualized considering a 20-year horizon. The operating costs were determined with the cost of utilities, catalysts, heavy crude oil and hydrogen. The costs of the heavy crude oil and upgraded oil were determined with the methodology of Wang. In the studied scheme, the heavy crude oil is split in two streams, one is directed to the deasphalting and hydrotreating steps, and the deasphalted-hydrotreated stream is then mixed with the other stream previously separated to achieve the fluid properties of minimum 16°API and kinematic viscosity lower than 250 cSt. Based on experimental results discussed above, the technical feasibility of the combination of deasphalting plus hydrotreating is demonstrated. The next step is to evaluate the possible economic benefits for such a process combination. To do so, firstly some properties of the blend need to be calculated. API gravity and other properties depending on the mass can be easily estimated by linear blending rules. However, viscosity does not depend on the mass. It must be calculated based on blending rules. To estimate upgraded oil viscosity a Chevron blending rule was used.15 It utilizes individual blending indexes for each component: heavy oil and deasphalted hydrotreated oil (equation 2), thus a blending index of the mixture is determined by equation 3, whereby equation 4 can be used to predict the viscosity of the mixture.
( )
= ∗ 1000 ( )
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(2)
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( ) = ( ) = 10
∗ ( ) ! ( )
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(3)
(4)
Apart from the properties of the upgraded oil to be estimated, some other parameters that need to be defined. For instance, the amount of catalyst that is needed for hydrotreating was estimated by assuming a continuous experiment instead of batch system, that is the liquidhourly space velocity, defined as a volumetric flowrate of liquid divided by the volume of catalyst, is assumed to be the inverse of the reaction time in batch system. Knowing the catalyst bulk density, the amount of catalyst needed for hydrotreating can be estimated. From data provided by the licensor of catalyst, it is known that the catalyst has a 30 % of metals retention capacity and its cost is 9.54 USD/kg. So that, life of catalyst was calculated with this information and with the difference between metal contents in the feed and in the product. Table 5 present the investment cost taken from the literature for the SDA and HDT processes studied and Farrar indexes for respective year of reference. ROSE process (resid oil super critical extraction) was considered as the representative deasphalting technology which can handle a variety of feeds such as: vacuum residue, atmospheric residue and heavy oil.16 Chevron Lummus technology was chosen for hydrotreating process due to its capability of handling feeds such as: middle distillates, and vacuum gas oil. 17 The information of the Farrar indexes at the reported year and at 2016 are also included in Table 5. Investment costs were updated with the six tenth relationship and Farrar indexes as follows.14
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