Article pubs.acs.org/IECR
Vapor−Liquid Equilibrium of Hydrogen, Vacuum Gas Oil, and Middle Distillate Fractions Barbara Browning,*,† Reynald Henry,† Pavel Afanasiev,† Gregory Lapisardi,‡ Gerhard Pirngruber,‡ and Melaz Tayakout-Fayolle† †
Institut de Recherches sur la Catalyse et l’Environnement de Lyon (IRCELyon), CNRS UMR 5256, Université Claude Bernard Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne cedex, France ‡ IFP Energies Nouvelles, Rond-point de l’échangeur de Solaize, BP3, 69360 Solaize, France ABSTRACT: Vapor−liquid equilibrium (VLE) data for hydrogen/hydrocarbon mixtures based on real feedstocks at high temperature and pressure is scarce and is needed to model hydrocracking systems accurately. Experiments with a vacuum gas oil have been used to generate VLE data for real hydrogen/hydrocarbon mixtures under hydrocracking conditions in a semibatch reactor with a range of compositions. Flash calculations were performed using a process simulation program and compared to the experimental results. The data was used to verify the existence of equilibrium conditions in the reactor and to show that the Grayson Streed activity coefficient model, combined with Peng−Robinson equation of state, provides a good estimate of liquid phase and vapor phase compositions and individual component volatility.
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INTRODUCTION Hydrocracking is used in industry to convert heavy oil fractions and residues to more useful mixtures, for example, the upgrading of vacuum gas oil (boiling range 370−510 °C) to middle distillate (boiling range 150−370 °C). The traditional method of upgrading these fractions is by catalytic cracking in fluidized beds. Despite being more costly, due to the requirement for high pressure hydrogen, hydrocracking offers much more flexibility as heavier feedstock compositions can be processed. This has become increasingly important as the crude oils now being sent to refineries tend to comprise substantial fractions of heavy material, while the demands of the market are for increasing quantities of lighter fuels such as gas oil and kerosene.1,2 In industry, a typical hydrocracking process is usually carried out in a trickle bed reactor operating between 300 and 450 °C and at pressures of 80−180 bar. A bifunctional catalyst is used to reduce carbon chain lengths and simultaneously hydrogenate the hydrocarbons produced.3 Due to its importance for the oil industry, much laboratory research, using various reactor types, has been given to understanding hydrocracking.4 Real oil fractions are complex mixtures of many different hydrocarbons, and so, for simplicity, experimental studies into the kinetics and performance of bifunctional catalysts for hydrocracking are often based on model molecules.5−9 One drawback of this is that extrapolating results from a model molecule to real reaction mixtures does not allow for interactions between the components. Also, traces of sulfur and nitrogen, present in real mixtures, may affect reaction kinetics. Finally, the feedstocks for hydrocracking tend to be mostly composed of larger molecules than those generally used as model hydrocarbons. Work with real feedstocks allows these aspects to be taken into account and actual process plant conditions to be replicated and explored so as to provide better data for design and development of new units. The VLE data is important in hydrocracking. In a typical trickle bed reactor for hydrocracking, the hydrogen is mostly in © 2014 American Chemical Society
the vapor phase with some dissolved in the liquid, and the heavy hydrocarbons are mainly in the liquid phase which is in contact with the catalyst. However, experimental studies have shown that there is sufficient vaporisation of the hydrocarbons to impact the observed reaction kinetics.10−12 Modeling studies have confirmed this,13,14 and some authors have accounted for the VLE in their kinetic models for hydrocracking.15−17 A problem caused by the lack of reliable VLE data is that in laboratory scale hydrocracking studies, accurate composition analysis can only usually be made by stopping the reaction and cooling the reactor. Accurate VLE data would allow liquid and vapor compositions to be determined from samples taken under operating conditions, and therefore much more information to be derived from a single experiment. So, it is clear that knowledge of the VLE data at the reaction conditions is important for accurate study and modeling of hydrocracking. For process modeling or design applications, it is conventional to use a process simulator to find VLE data by performing flash calculations. An appropriate thermodynamic model is selected based on the fluid composition and the process conditions. When a single equation of state (EoS) is used to represent the whole mixture this is known as a homogeneous thermodynamic model. Of these, Valderrama18 recommends either the Soave−Redlich−Kwong (SRK) or Peng−Robinson (PR)19 EoS for modeling reservoir fluids, which would include the vacuum gas oil used in this study. Alternatively, a heterogeneous thermodynamic model can be used, with the EoS applied only to the gas phase and the liquid phase represented by an activity coefficient model, such as those of Chao−Seader20 or Grayson Streed (GS).21 Experimentally derived binary interaction parameters for all the Received: Revised: Accepted: Published: 8311
March 4, 2014 April 8, 2014 April 22, 2014 April 22, 2014 dx.doi.org/10.1021/ie500930n | Ind. Eng. Chem. Res. 2014, 53, 8311−8320
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components to be modeled are needed. For oil fractions it is impossible to identify all the components, and so, interaction parameters are usually found by generating pseudocomponents to represent long chain molecules not already included in the simulator database. A hydrocracking system is a combination of hydrogen and hydrocarbons at high temperature and pressure. The 1963 GS model was developed specifically for this type of system.22 However, because the behavior of hydrogen/hydrocarbon (H2/ HC) systems is difficult to predict, authors have continued to develop thermodynamic models for these mixtures to gain better accuracy over a wide range of temperature and pressures.23−30 All this work has been based on experiments with binary systems and not the complex mixtures found in a real hydrocracker. Both Riazi and Roomi31 and Torres et al.32 did test their methods for prediction of hydrogen solubility in hydrocarbons on data drawn from real mixtures at high temperature and pressure but these did not extend to hydrocracking conditions. There are so few studies of thermodynamic models for real hydrocracking mixtures and conditions reported in the literature that Alvarez and Ancheyta33 chose to neglect vaporisation in their dynamic model of hydrodemetallization of residues at 360−420 °C and 98 bar, because of absence of reliable thermodynamic data. One reason for the lack of VLE data for hydrocracking conditions is that, in a trickle bed reactor, conditions vary along the reactor length. So, laboratory scale trickle bed reactors, which are usually used for the study of the hydrocracking reaction, are not useful for gathering the VLE data. In the few studies that are reported, alternative methods are used. Gauthier et al.34 carried out hydrocracking of a vacuum residue in an ebulliated bed reactor. They analyzed VLE compositions under hydrocracking conditions (410−440 °C, 160 bar) and at different conversions (35−70%). Their experimental results were compared against flash calculations using both the PR and GS thermodynamic models, and they found the GS model to systematically underestimate vaporization by 10% while the PR model matched the experimental findings very well. Chen et al.35 used a continuous flow VLE cell with a flash calculation program to predict VLEs for middle distillates and heavy gas oils under hydrotreatment conditions up to 430 °C and 110 bar. They used the PR EoS and determined interaction coefficients between the hydrocarbons and hydrogen to fit the experimental data for the middle distillate. However, the data for the heavy gas oil was less precise as higher temperatures are needed for hydrocracking heavier mixtures. With conditions set to represent these more severe temperatures, the mixture was found to undergo thermocracking in the VLE cell.36 The objectives of this particular study are to provide experimental VLE data for hydrocracking of a real vacuum gas oil under typical operating conditions and to test a suitable thermodynamic model to represent these results. The underlying aim is to find intrinsic reaction parameters for hydrocracking. Dissociating the VLE effects from other factors affecting the reaction rate is a necessary part of this work. We obtain VLE data by analysis of liquid and vapor samples, taken from a batch reactor for hydrocracking a vacuum gas oil at reaction conditions and different conversions. The results are then compared to predictions by using a process simulation program.
Article
MATERIALS AND METHODS
Hydrocracking experiments were conducted on a vacuum gas oil at 400 °C and 120 bar in a 300 mL Parr 4842 bench scale pressure reactor equipped with a Robinson-Mahoney stationary catalyst basket. The vacuum gas oil used had a boiling range of 370−510 °C, the density was 0.8589 g·cm−3 at 20 °C, and carbon numbers ranged from 22 to 42. For each experiment, 10 g of catalyst were loaded to the reactor for 120 g of vacuum gas oil. At operating conditions, this corresponds to an initial liquid volume of about 195 mL. The reactor pressure was maintained at 120 bar by hydrogen injection. Hydrocracking produces volatiles, which increase the oil partial pressure as the reaction advances, so the hydrogen feed rate is slightly below the consumption rate and the partial pressure of hydrogen in the reactor is decreasing slowly during the experiments. The reaction was carried out over several bifunctional catalysts containing supported Ni−Mo and zeolite with different ratios. For details and discussion of the experimental method, the synthesis and characterization of the catalysts, and also the analysis of the initial feed and the reaction products, refer to the work of Henry et al.37 Increasing the agitation rate in this reactor had no influence on conversion or selectivity, so vapor−liquid mass transfer limitations appear to have negligible effect. However, it will be seen later that the vapor−liquid equilibrium does take some time to become established. For each experiment, samples are taken from the hot reactor at the end of the reaction period, after shutting off the hydrogen supply. The hot reactor is then plunged into an ice bath to ensure rapid cooling and stop the hydrocracking reaction. When the reactor reaches ambient temperature a final analysis of the reaction products is made. Recovery of the reaction products from the cooled reactor was straightforward. The residual pressure after cooling was noted and a gas sample taken before the reactor was depressurised. The reactor was then weighed before the remaining liquid and the catalyst were collected separately for analysis. Typically, the final mass balance was accurate to 97.5% with a small amount of material lost as volatiles on opening the reactor and as liquid remaining on the reactor cover and agitator. Analysis of the initial vacuum gas oil and liquid samples was by two-dimensional gas chromatography (GC × D). This separates components by both boiling point and polarity. The apparatus used37,38 allowed mass fractions to be determined, according to hydrocarbon family and carbon number, from C5 and up. The hydrocarbon families considered were: n-paraffins, iso-paraffins, naphthenes, and aromatics. In the work of Henry et al.37 the naphthenes with a carbon number higher than 20 were not analyzed separately but included in the iso-paraffin and n-paraffin families, while in this work these naphtenes have been separated and considered. The gas samples taken from the condenser are analyzed by gas chromatography for hydrogen, hydrogen sulfide and light hydrocarbons including methane. It is possible to calculate the mass balance for the reactor after cooling very precisely. At ambient temperature, the components with carbon number greater than C6 can be said to be completely condensed in the liquid phase and all the hydrogen assumed to be in the gas phase. The only unknown quantities are for the gases and light hydrocarbons as some material may be dissolved in the liquid phase and therefore not measured. However, at low and moderate conversions, these constitute only a tiny fraction of the material in the reactor. 8312
dx.doi.org/10.1021/ie500930n | Ind. Eng. Chem. Res. 2014, 53, 8311−8320
Industrial & Engineering Chemistry Research
Article
Figure 1. Liquid phase hydrocarbon distributions based on analysis of a cold reactor by family and carbon number for typical experiments: (a and b) immediately after reaching reaction conditions, (c and d) at 61% conversion.
Figure 2. Process flow diagram of system as represented in simulation.
Figure 1a and b shows the GC × D results of the liquid composition analysis for a typical experiment where the reaction has been stopped immediately the reactor reached operating conditions. The initial feed contained only hydrocarbons of carbon number greater than 20,37 so a small quantity of product has been formed during heating but the composition remains largely unchanged. Figure 1c and d shows the analysis results for an experiment of longer duration, where many more hydrocracking products are present. It can be seen that certain components are produced more readily than others and some hydrocarbons are formed only in very small quantities, particularly the naphthenic molecules of size C17 to C20.
Also, the total quantities of hydrogen sulfide and methane are known from the initial amount of DMDS added to the reactor, so mass balances can be completed for these two components from the gas phase analysis only. The range of data available comes from use of the fed batch reactor setup to test eight catalysts for hydrocracking of the vacuum gas oil mentioned previously. The hydrocracking rate varied between the catalysts and experiments of different duration were conducted with conversions from very slight to approximately 75%. The results of these experiments therefore provide composition data for different H2/HC mixtures under realistic hydrocracking conditions. 8313
dx.doi.org/10.1021/ie500930n | Ind. Eng. Chem. Res. 2014, 53, 8311−8320
Industrial & Engineering Chemistry Research
Article
Figure 3. Comparison between vapor exit stream and vapor sample mole fraction against time for (a) hydrogen, (b) methane, and (c) H2S.
mode is used to describe a single instant of the fed batch reaction and that Figure 2 does not represent the actual experimental setup. The mass balance data from the cooled reactor was used as the process input and the reactor setup was described using a two phase flash to represent the reactor, plus a cooler and separator to represent the condenser on the reactor vapor sample line. This allows a direct comparison to be made between the predicted and measured vapor compositions. The PR EoS was tested with and without the GS activity coefficient model. The results for the PR EoS without an activity coefficient model were nonphysical, falsely calculating that no liquid phase existed. However, the GS activity coefficient model combined with the PR EoS appeared to give reasonable results and so, this model was used. For the simulation, the reaction system includes some molecules which can be considered individually, such as hydrogen, other gases, n-alkanes, and small hydrocarbons. The physical data for these can be found in the simulation program property data bank. The remaining components, grouped by carbon number and family, include multiple isomers with slightly different physical properties. Each group is represented by a single pseudocomponent, the properties of which are generated within the simulation program. The required input data is molecular weight, estimated from the molecular structures, and boiling point, estimated from the GC × D analysis and verified by checking the boiling points of typical compounds within the groups using the Nannoolal method.41
Taking representative samples of each phase from the hot reactor was more challenging, and a strict sampling method was developed. First, both the agitator and the hydrogen supply were shut down to prevent gas entering the liquid sample valve. For the liquid samples, the heated sample valve was flushed through twice prior to sampling, to avoid contamination. Also, the sample container was plunged into a cold bath at −78 °C to immediately condense and capture the most volatile components. As with the liquid sample from the cooled reactor, the small quantity of light hydrocarbons (
