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Solvent effect in gas–liquid hydrotreatment reactions
Applied Catalysis A: General 253 (2003) 515–526
Solvent effect in gas–liquid hydrotreatment reactions Alfredo Guevara, Robert Bacaud∗ , Michel Vrinat Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received 21 May 2003; received in revised form 3 July 2003; accepted 4 July 2003
Abstract The evaluation and ranking of hydrotreatment catalysts at laboratory scale is generally performed with model compounds, thiophene or dibenzothiophene (DBT), either in the gaseous or in the liquid-phase. According to the considered reactant and the chosen conditions, confusing data have been published. The presence of an inert solvent has been claimed to affect the measured activity but no comprehensive experimentation supports this assertion. Some of the possible causes for the observed discrepancies include the inhibiting effect of hydrogen sulfide, the distribution of the reactants in the gaseous and liquid and the control of transport phenomena. The rate of tetralin hydrogen was determined in a micro up-flow three-phase reactor, the reactant being either in the gaseous or the liquid-phase, as a pure compound or in the presence of inert solvents. Expressing the reaction rate as a simplified form of the Langmuir–Hinshelwood formalism, the reaction rate constant and the relative tetralin-to-solvent adsorption constant ratio were determined. The rate constant is independent of the reaction conditions (gas- or liquid-phase) and of the nature of the solvent, indicating that the intrinsic activity of the active phase does not depend on the experimental conditions. The values of the relative adsorption constant indicate that the competitive effect is more pronounced as the molecular weight of the solvent increases. © 2003 Elsevier B.V. All rights reserved. Keywords: Liquid-phase; Gas-phase; Hydrotreatment; HDS; Solvent; Sulfide
1. Introduction Hydrotreatment of petroleum fractions is industrially carried out in fixed bed reactors fed with liquid reactants and gaseous hydrogen. Owing to the decisive importance of hydrotreatment processes—more specifically hydrodesulfurization (HDS)—in the general framework of increasingly severe environmental regulations, the development of catalytic formulations for HDS is the subject of intensive research effort. The assessment of catalytic activity at laboratory scale is generally performed with model compounds, ∗ Corresponding author. Fax: +33-472-445-399. E-mail address: [email protected] (R. Bacaud).
thiophene or dibenzothiophene (DBT), either in the gaseous or in the liquid-phase. The pertinent information for the evaluation of new catalytic formulations is a relative ranking of a given family of catalysts. Obviously, the significance of the measured activity, i.e. the independence of the ranking with respect to the composition of the reactant, is of paramount importance and is a prerequisite for the usefulness of activity measurements. Published data concerning the comparison of HDS activity of various catalysts, using either thiophene or DBT, in the gaseous or liquid-phase, lead to confusing conclusions. Thiophene HDS and DBT HDS have been performed by Ledoux et al. [1] over two series of Ni-Mo and Co-Mo/Al2 O3 catalysts. With one
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00550-7
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exception, the respective ranking for the two reactants is identical. A comparison of thiophene, DBT and gas oil HDS was conducted by Tanaka et al. [2] over various Co-Mo/TiO2 -Al2 O3 catalysts. A positive effect of titanium-based supports upon activity was evidenced with these three reactants. In contrast, Bacaud et al. [3] drew an opposite conclusion and demonstrated that the positive effect of titanium-containing supports observed in DBT HDS was not confirmed in the HDS of a diesel cut. Conflictive results were also observed by van Veen et al. [4] and by Reinhoudt et al. [5] in the comparison of gas-phase versus liquid-phase DBT HDS performed over Ni-Mo or Ni-W supported catalysts prepared or sulfided under variable conditions. In most of the published data, the ranking of catalysts is generally based upon a single activity measurement, performed under arbitrarily chosen conditions. Several points that may have a contrasted effect upon catalytic activity must be addressed before definitive conclusions relative to the actual value of reaction rate are drawn. 1. Does the measured rate for the reactant conversion reflects the chemical process or is it limited by transport phenomena? This point is seldom considered and the significance of activity measurements performed at high conversion level, such as the data published by Qusro and Massoth [6], is questionable. 2. Thiophene HDS is representative of hydrotreating of light fractions performed in the gaseous phase. DBT HDS is chosen as a model for the elimination of sulfur in mid range distillates and may be performed either in the gaseous phase or in liquid-phase, as a pure compound or in the presence of a solvent. An evaluation of the distribution of the reactants in the phases is rarely presented in published data and, under some experimental conditions, there exist some doubt concerning the physical state of the reactant. This point is related with the evaluation of transport phenomena, which are described by different models according to the presence, or the absence of a liquid-phase under the conditions of reaction. Although this point seems obvious, it must be realized that the quantity of liquid required for a complete wetting of the catalyst surface is small, relative to the flow of reactants.
Therefore the determination of the volatilization of the liquid flow must be performed with a good precision. Unfortunately, the result of such a calculation depends on the selected model, and significant variations can be evidenced. 3. The presence of a solvent has been shown to substantially modify the activity. Some tentative interpretations have been proposed to justify this so-called solvent effect, which remain unelucidated. The most evident influence is related with the partial volatilization of the solvent causing a variation of the concentration of the reactants. Although this effect seems quite obvious and has been quantified for DBT HDS by Letourneur et al. [7], it is not accounted for in some of the published data dealing with the solvent effect [8–10]. Similarly, the reciprocal solubility of the present species can change according to their respective concentrations, and depends in turn on the level of conversion. An appropriate model for the evaluation of the distribution of the reactants and products in the reacting medium could account for these complex interactions. 4. The inhibiting effect of hydrogen sulfide upon HDS reactions is well documented; furthermore, the sensitivity of catalysts to this inhibition depends on the nature of the active phase, the activity of NiMo catalysts being more affected than CoMo [11]. Since H2 S is a product of the reaction, a precise control of its partial pressure requires activity measurement to be performed at the same level of conversion. This condition is hardly fulfilled in published data where conversion varies from less than 1 to nearly 90% [1]. The possibility of an inhibiting effect in the desulfurization of diesel oil, related with the strong adsorption of hydroaromatic compounds, has been evoked by Schulz et al. [12]. This competitive adsorption, causing a partial blockage of the active surface, could explain the observed matrix effect. Reinhoudt et al. [13] discussed the difference between gas- and liquid-phase hydrotreatment reactions. As an interpretation to the so-called gas–liquid controversy, they stated that a specific surface morphology of the active phase could favor some interaction of a condensed phase. However, this interpretation looks quite speculative since the experimental facts it relies on may be explained by
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several of the possible causes of divergence enumerated in the introductory paragraph. There is no room for a supposed controversy when the facts (activity ranking) are established in dissimilar conditions. We attempted in the present work to identify the origins of the discrepancies observed between gas- and liquid-phase hydrotreatment reactions and to explain the influence of inert solvents upon the observed reaction rate. In order to achieve this goal, the preceding points were addressed through a critical selection of experimental conditions. The choice of the reactant and the reaction was dictated by the following criteria. • The reactant must be representative of hydrotreatment reactions as for the pressure and temperature range required for a controllable conversion level (1–5 MPa, 550–630 K). • The reaction must reflect one of the steps involved in hydrotreatment scheme. • It must be able to be present either in the gaseous or the liquid-phase under the considered range of experimental conditions. • For a positive control of the inhibiting effect of hydrogen sulfide upon catalytic activity, the transformation of the reactant must not produce H2 S. The hydrogenation of 1,2,3,4-tetrahydronaphthalene (tetralin) over an industrial Ni-Mo-S/Al2 O3 catalyst was selected as the model reaction. This reaction fulfills the preceding conditions and it is representative of HDS since the initial step of sulfur elimination in refractory sulfur compounds involves a hydrogenation of aromatic structures. The rate of tetralin hydrogenation was determined in a micro up-flow three-phase reactor, the reactant being either in the gaseous or the liquid-phase, as a pure compound or in the presence of an inert solvent. Our first concern was to characterize the flow conditions in the reactor and to evaluate the transport rate of reactants to the catalyst surface in order to establish the conditions of reaction rate measurement. The influence of hydrogen sulfide upon conversion was evaluated and further experiments were performed under constant H2 S partial pressure in the gaseous phase or at constant H2 S concentration in the liquid-phase for experiments conducted in biphasic flow conditions.
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In order to evaluate the role played by solvents, various n-alkanes were selected as the solvents, according to their range of vapor pressure under the conditions of hydrotreatment. n-Heptane is totally vaporized and was used for an evaluation of a possible effect of competitive adsorption under gaseous flow conditions. n-Dodecane and n-hexadecane were selected as, respectively, representative of light and medium feeds. The determination of the distribution of the reactants in the liquid and gaseous phases was performed using different models and the reaction rate was estimated as a function of the actual tetralin concentration, either in the gas or the liquid-phase. 2. Experimental 2.1. Catalyst The catalyst selected for activity measurements was an industrial Ni-Mo/␥-Al2 O3 containing 3 wt.% NiO and 14 wt.% MoO3 , BET surface area: 250 m2 g−1 ; pore volume: 0.48 cm3 g−1 . The calcined solid was crushed and the fraction 80–125 m was selected. Gas-phase activity measurements were performed in a flow microreactor. The catalyst (200 mg) was sulfided under the reaction pressure (3 MPa) with a mixture containing 5% H2 S in hydrogen at a total flow rate 3.6 × 10−4 mol s−1 , at 673 K for 4 h (heating rate 5 K min−1 ). Tetralin was introduced in the flow of reactants through a saturator maintained at controlled temperature. An automated injection valve allows periodical gaseous samples to be analyzed by gas chromatography (HP 5890) equipped with a HP5 column (30 m long, 0.53 mm i.d.) and a FID. Gas–liquid-phase activity measurements were carried out in an automated unit, which allows consecutive experiments to be performed under variable process conditions [14]. The following parameters can be programmed: pressure, temperature, flow of gaseous and liquid reactants, selection of liquid reactant, sample collection. The reactor is a stainless steel tube (120 mm long, 6 mm i.d.) packed with 0.1–2 cm3 of catalyst located between two layers of inert alumina. Following the reactor, an automated gas–liquid-phase separator (4 cm3 internal volume) and a sample collector allows periodical sampling of
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the liquid effluent (no appreciable amounts of solvent and tetralin are eluted in the gas-phase). Non-impregnated ␣-alumina of the same particle size was used for the layers of inert packing. A run consists of a series of automated operations. First, the catalyst is sulfided by a solution of dimethyldisulfide (1 wt.%) in n-dodecane at 20 h−1 liquid hourly space velocity (LHSV), under 3 MPa total pressure, in the presence of hydrogen. The hydrogen-to-liquid flow ratio is 300 (l/l) mol mol−1 . The temperature is maintained at 573 K for 3 h (heating rate 5 K min−1 ). The next part of the run is composed of programmed sequences defined by the temperature, total pressure, gas and liquid flow rates, and number of samples analyzed per experiments. The LHSV can be varied in the range 2–40 h−1 . The experiments reported here were performed at 573 K under 3 MPa total pressure. Liquid samples were analyzed by GC in the same conditions as for gas-phase experiments. The amount of naphthalene in the products does not exceed 0.5% of the sum of the concentrations of cis- and trans-decalin. The conversion X is defined as a function of initial and final tetralin concentrations (C0 , C): X=
C0 − C C0
For a molar flow rate F, and a catalyst amount w, the reaction rate is: FX −r = w
Two preliminary tests were performed in order to check the inertness of the reactor filling and the thermal homogeneity. Using non-impregnated ␥-alumina instead of the catalyst, it was first checked that alumina was actually inert with respect to both tetralin and the solvents. Second, since the solvent partially vaporizes at the reactor inlet, identical experiments were repeated using a preheating section made of a tube (200 mm long) packed with inert alumina. For given operating conditions these experiments gave identical results, and it was thus concluded that liquid–vapor equilibrium is reached at the catalyst bed inlet. Further experiments over the whole range of operating conditions lead to the same conclusions. The reproducibility of reaction rate measurement was evaluated form three consecutive determinations, including the complete experimental steps: reactor filling, catalyst sulfidation and activity measurement at 573 K, 3 MPa total pressure. The relative standard deviation was 3%. 2.2. Evaluation of the distribution of the reactants between the liquid and the gaseous phase The calculations were performed with Aspen Plus® . Several models are proposed for the determination of the composition of the liquid and the gaseous phases in the range of pressure and temperature currently used in refining processes. They either rely upon a state equation or a virial expression. The choice of the most
Fig. 1. Variation of hydrogen molar fraction in n-hexadecane as a function of pressure at 623 K. Comparison of experimental and calculated values.
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appropriate model was carried out by comparison of calculated values for the solubility of hydrogen in hexadecane with experimental data published by Lin et al. [15], reported in Fig. 1. The following models were evaluated. • Peng–Robinson–Boston–Matias (PR–BM). • Redlich–Kwong–Soave–Boston–Matias (RKS–BM). • Benedict–Weeb–Rubin–Lee–Starling (BWR–LS). The PR–BM model was selected for subsequent calculations.
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fect the reaction rate or the catalyst performance was performed, after it was established that the measured rate was not limited by transport phenomena. The examined factors include the following. • H2 S partial pressure: the inhibiting effect is strongly dependent upon H2 S partial pressure. • Catalyst sulfidation: are the surface composition, and consequently the activity, affected by a difference in the sulfidation protocol? • What is the role of the presence of n-dodecane in liquid-phase reaction? 3.2. Flow regime and mass-transfer in the three-phase reactor
3. Results and discussion 3.1. The experimental basis for a controversy An evaluation of the rate of tetralin hydrogenation was performed either in the gaseous phase or dissolved in n-dodecane in the liquid-phase. The respective experimental conditions are summarized below (Table 1). The measured rate for gas-phase hydrogenation exceeds the liquid-phase reaction rate by an eight-fold factor. This observation is consistent with data published by Reinhoudt et al. [13] for DBT HDS, which is the basis for the so-called gas–liquid-phase controversy. Table 1 gives a list of the reaction parameters among which one may expect to find the cause(s) of this discrepancy. So, a critical examination of the influence of all of the reaction parameters that may af-
Table 1 Comparative conditions for tetralin hydrogenation in gas- and liquid-phase Parameter
Gas-phase reaction
Liquid-phase reaction
Temperature (K) Total pressure (MPa) Total flow rate (mol s−1 )
573 3 1.1 × 10−4
573 3 4.5 × 10−5
Feed composition (mol%) Tetralin H2 S Hydrogen n-Dodecane
0.21 0.03 99.7 –
0.27 0.03 73.4 26.3
Sulfidation Rate (mol s−1 g−1 )
H2 S/H2 41 × 10−8
DMDS/n-dodecane 5.2 × 10−8
3.2.1. Intraparticle diffusion Assuming a homogeneous particle size distribution and uniform porosity, an evaluation of the effective diffusivity (De ) and Thiele modulus (φ) is achievable. The effectiveness factor (η), representative of the ratio of the observed rate to the maximum rate of reaction in the catalyst particles for a first-order reaction, is defined as: 1 1 η= coth 3φ − φ 3φ The values of De , φ, and η, reported in Table 2, indicate that the measured reaction rate will not be affected by mass transfer limitations under the considered range of experimental conditions. 3.2.2. Flow regime and external transfer in the reactor An evaluation of the respective values of the gasand liquid-phase Reynolds number (Re) indicated that, for the considered range of space velocities, deviation from plug-flow regime could be neglected. An experimental confirmation of the absence of external transfer limitation was performed, for both reactants. Firstly, the flow of tetralin was varied from 8 Table 2 Evaluation of the effectiveness factor Transfer Tetralin/n-dodecane Hydrogen/n-dodecane
De (m2 h−1 ) 10−5
9× 1 × 10−4
φ
η 10−4
4× 3 × 10−4
r = 4.1 × 10−8 kmol s−1 kg, LHSV 19 h−1 , 573 K, 3 MPa.
0.99 0.99
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Fig. 2. Influence of external tetralin transfer upon tetralin hydrogenation (573 K, 3 MPa).
to 24 × 10−8 mol s−1 . The observed reaction rate was constant as illustrated in Fig. 2. Secondly, in order to identify a possible restriction of hydrogen availability in the liquid-phase, the hydrogen molar fraction was varied in the most demanding conditions with respect to hydrogen consumption: highest concentration of tetralin in the heaviest solvent (hexadecane). Fig. 3 indicates that the measured reaction rate was independent of hydrogen molar fraction. Thus, from the estimates of flow regime and experimental confirmation, it can be concluded that chemical regime prevails in the reactor in the considered
range of experimental conditions. The measured rates of tetralin transformation will therefore be representative of adsorption and reaction, independently of any limitation due to transport phenomena. 3.3. Gas-phase versus liquid-phase sulfidation The initial sulfidation of the catalyst in hydrotreating reactors is generally performed in the liquid-phase, contacting the oxidic form of the catalyst with a sulfiding reagent dissolved in the feed. Substantial activity variations have been reported depending upon the nature of the sulfiding reactant [16]. This process causes
60 Reaction rate 10-8mol s-1 g-1
50
Hydrogen molar fraction (%) 40 50
60
70
80
Fig. 3. Influence of external hydrogen transfer upon tetralin hydrogenation (20% tetralin in hexadecane, LHSV 19 h−1 , 573 K, 3 MPa).
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a carbonaceous substance to cover the solid surface in considerable amount (several wt.%), but further contact of the catalyst with the feed does not produce a subsequent increase of the carbon content [17], indicating that a competitive process of carbon deposition and elimination takes place. Hence, the definition of a “freshly sulfided” catalyst is ambiguous since the initial contact of the surface with hydrocarbons generates a “coked” solid. A liquid-phase sulfidation cannot easily be practiced in the case of gas-phase experiments. As a consequence, the catalyst was initially treated under a gas flow containing H2 S. The resulting sulfided solid is obviously carbon-free and some consequence in terms of activity may be expected, owing to the difference in surface composition of catalysts, respectively, sulfided in the liquid- or the gas-phase. In order to evaluate the impact of the sulfidation method as a possible cause for the gas–liquid controversy, the activity for tetralin hydrogenation of the same catalyst (Ni-Mo/␥-Al2 O3 ) was measured in the liquid-phase after it was sulfided either in the gas- or in the liquid-phase. The experimental conditions of sulfidation and reaction are presented in Table 3 with the Table 3 Influence of the conditions of sulfidation upon the liquid-phase reaction rate of tetralin hydrogenation Parameters
Liquid-phase sulfidation
Gas-phase sulfidation
Sulfidation flow rate (mol s−1 )
6.6 × 10−4
3.6 × 10−4
Mole fractions (%) H2 H2 S n-Dodecane DMDS
0.73 0.007 0.26 0.003
0.95 0.05 – –
Total pressure (MPa) Temperature (K), time (h) LHSV (h−1 )
3 573, 3 19.5
3 673, 4 –
Reaction flow rate (mol s−1 )
4.5 × 10−5
4.5 × 10−5
Mole fractions (%) H2 H2 S n-Dodecane Tetralin
73.4 0.03 26.3 0.27
73.4 0.03 26.3 0.27
Total pressure (MPa) Temperature (K)
3 573
3 573
Reaction rate (10−8 mol s−1 g−1 )
5.2
5.1
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respective values of the measured activity. No appreciable change in the reaction rate is observed. It may thus be inferred that, after the initial sulfidation stage, the surface composition of the catalyst under the reaction conditions is predominantly dictated by the contact with the reacting mixture. As a consequence, the observed reaction rate is determined by this interaction during the early stage of activity measurement. 3.4. Influence of H2 S partial pressure The presence of H2 S is required for maintaining sulfide catalysts in a sulfided state when a reaction is performed in a reducing atmosphere. As mentioned earlier, the inhibiting effect of H2 S in hydrogenation and HDS reactions is well established. Since the hydrogenation of tetralin does not produce H2 S, the partial pressure of this reactant can be controlled, independently of the level of conversion. In order to evaluate the effect of H2 S partial pressure as a possible cause of the observed discrepancy between gas- and liquid-phase experiments, the rate of tetralin hydrogenation was measured in the liquid-phase, in the presence of n-dodecane. Fig. 4 presents the reaction rate as a function of H2 S partial pressure. As expected, a decline of the reaction rate is observed. But even at the lowest partial pressure, the measured rate is five times lower than the rate observed in the gas-phase under the same H2 S partial pressure. Further rate measurements were performed in the presence of a constant H2 S concentration fixed at 0.03 mol% of the total feed, either for gas- or liquid-phase hydrogenation. 3.5. The “solvent effect” The discrepancy between gas-phase versus liquidphase hydrogenation may originate from the competitive adsorption of the solvent. Although an inert hydrocarbon does not participate to the reaction process, it may compete with the reactants for the adsorption on active sites on the catalyst surface. Such an effect must be evidenced under both gas- and liquid-phase conditions of reaction. A comparison of the rate of tetralin hydrogenation, in the gas-phase, either pure or diluted with n-heptane, clearly evidences the inhibitor effect caused by the presence of an inert alkane (Fig. 5).
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5
H2S (mol %) 0 0
0.2
0.4
Fig. 4. Rate of tetralin hydrogenation vs. H2 S partial pressure (feed: 0.26 mol% tetralin in dodecane, 0.12 × 10−3 mol h−1 hydrogen, LHSV 19 h−1 , 573 K, 3 MPa).
Assuming a single-site mechanism, the rate of the surface reaction can be described by the Langmuir– Hinshelwood model: KT KH2 CT CH2 r=k (1) 1 + KT CT + KH2 S CH2 S + KH2 CH2 where k is the reaction rate constant, K are the adsorption constants and C the concentrations, with the respective superscripts (tetralin, H2 S, H2 ). This model assumes that the rate is proportional to the concentration of the adsorbed species, irrespective of whether adsorption occurs from the gas- or the liquid-phase. Thus, if the reaction proceeds in the presence of an inert liquid, the rate constant must be the same. This implies that the adsorption equilibrium constant of the
solvent must be small compared with the adsorption constant of tetralin. However, in the presence of large solvent-to-tetralin ratios, the competitive adsorption may significantly affect the reaction rate. The introduction of the adsorption terms of the solvent (superscript S) in Eq. (1) yields: r=k
KT KH2 CT CH2 1 + KT CT + KS CS + KH2 S CH2 S + KH2 CH2 (2)
It must be observed that: • the reaction is performed under 3 MPa total pressure, thus the fraction of vacant adsorption sites is probably low;
50
Reaction rate
10-8 mol.sec-1.g-1
0.21 mol %Tetralin
0.27 mol %Tetralin 37 mol % n-heptane
0
Fig. 5. Influence of n-heptane upon the rate of gas-phase hydrogenation of tetralin.
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• the reaction is performed in the presence of a large excess of hydrogen, and the concentration of hydrogen can be considered as constant; and • provided the concentration of H2 S and the solvent are kept constant, the corresponding terms can be eliminated in the rate expression. The simplified expression of the rate is given as: r=k
KT CT KT C T + K S C S
(3)
A determination of the reaction rate as a function of tetralin concentration allows the tetralin-to-solvent adsorption constant ratio (KT /KS ) to be calculated from Eq. (3), assuming that the rate of gas–liquid equilibrium is faster than the reaction rate. Tetralin hydrogenation was therefore performed in the presence of various alkanes, selected for their respective vapor pressure under the conditions of reaction (573 K, 3 MPa): • n-heptane is completely vaporized; • n-dodecane is partly vaporized; and • n-hexadecane is predominantly in the liquid-phase. The distribution of the reactants in the phases under the reactor conditions (Fig. 6) was determined by a flash calculation with Aspen Plus® , using the Peng–Robinson model. All experiments were performed under constant total flow rate, at the same temperature (573 K), under the same total pressure (3 MPa). The variation of the reaction rate as a function of tetralin concentration (expressed as tetralin-to-solvent ratio in the feed) is presented in Fig. 7. From the corresponding rate data, the values of the reaction rate constant (k) and the tetralin-to-solvent adsorption constant ratios (KT /KS ) are presented in Table 4. The mean values of the liquid holdup for reactions performed in the presence of n-dodecane and n-hexadecane are mostly sufficient for a full coverage
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of the catalyst by the liquid-phase, thus indicating that the reaction effectively proceeds in the liquid-phase. The reaction rate constant is independent of the nature of the solvent, whether the reaction is performed in the gaseous or the liquid-phase. The order of magnitude of the relative adsorption constant indicates that, although tetralin is by far more strongly adsorbed than the considered solvents, the competitive effect arising from the presence of these non-reactive partners justifies the decrease of reaction rate observed at low tetralin concentration. It can be observed that the competitive adsorption is more severe as the molecular weight of the solvent increases. The significance of these two parameters can be underlined noting that the simplified expression of the reaction rate derived from the Langmuir–Hinshelwood formalism (Eq. (3)), includes two different series of terms: • the rate constant (k), which characterize the chemical reaction, and as such, is directly related with the intrinsic activity of the catalytic active phase; and • the second series of terms describe the interaction of the reactants with the catalyst surface. Since all of the reaction rate measurements were performed in the presence of the same catalyst, the numerical values of these parameters are representative of this specific formulation. The composition and the catalytic properties of the active phase are partly determined by the interaction of the catalyst surface with the reacting medium. As evidenced by the uniform value of the reaction rate constant, the intrinsic activity of the surface is not related with the conditions under which the reaction is performed (liquid- or gas-phase), nor is it affected by the molecular weight of the solvent. A cause of alteration could be a change in the behavior of the catalyst with respect to deactivation as a function of time of stream that could be induced by modifications of the support. The increase
Table 4 Reaction rate constants and relative adsorption constants for tetralin hydrogenation in the presence of various solvents (573 K, 3 MPa) Solvent/phase
Rate constant, k (mol s−1 g−1 )
Relative adsorption constant, KT /KS
Liquid holdup, εL (%)
n-Heptane–gas n-Dodecane–liquid n-Hexadecane–liquid
5 × 10−7 5 × 10−7 5 × 10−7
26.4 17 15.3
– 19 22
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Fig. 6. Distribution of the reactants in the gaseous and liquid-phase at reaction conditions, for the considered solvents (heptane, dodecane, hexadecane).
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525
80 C7 g-1
C12
Reaction rate 10-8 mol
s-1
C16
Tetralin/Alkane 0 0
0,1
0,2
0,3
0,4
Fig. 7. Rate of tetralin hydrogenation in various solvents as a function of tetralin-to-alkane ratio.
of the relative adsorption constant with the molecular weight of the solvent is consistent with the increased hindering of heavier alkanes over the catalyst surface. However, the measured values are representative of a given catalyst, and it may be expected that different values would be observed if the composition of the catalyst is modified.
4. Conclusions The gas–liquid controversy essentially relies upon partial, hardly comparable experimental data. As a consequence, the derived notion of solvent effect has not yet received satisfactory explanations. The present work was undertaken in order to establish the experimental basis for a pertinent evaluation of the effect of inert solvent addition upon the rate of transformation of a single compound under invariable conditions of temperature and total pressure. The reactant (tetralin) and the reaction (hydrogenation) were chosen as representative of hydrotreatment reactions and for the possibility of controlling the inhibitor effect of hydrogen sulfide upon the activity of metallic sulfides catalysts. It was established that when the reaction is performed in the gaseous phase, the addition of a gaseous alkane (n-heptane) in the feed produces a decrease of the reaction rate. The same phenomenon is observed if the reactants are in the liquid-phase (n-dodecane or n-hexadecane). The possible causes of perturbation of activity measurements were inves-
tigated. The flow regime in the reactor, internal and external transport phenomena were analyzed and it was concluded that the measured reaction rate under biphasic flow conditions was not affected by diffusion. The inhibiting effect of hydrogen sulfide was established and controlled, as well as the influence of the protocol of sulfidation. We therefore postulated that the experimental evidence of the solvent effect could be interpreted as a competition between the reactant and the solvent for the adsorption on the catalyst surface. Expressing the reaction rate as a simplified form of the Langmuir–Hinshelwood formalism, the reaction rate constant and the relative reactant-to-solvent adsorption constant ratio were determined. The rate constant is independent of the reaction conditions (gas- or liquid-phase) and of the nature of the solvent, indicating that the intrinsic activity of the active phase does not depend on the experimental conditions. The values of the relative adsorption constant indicate that the competitive effect is more pronounced as the molecular weight of the solvent increases. The present results and the proposed interpretation relate to a single catalyst. They do not explain some of the observed discrepancies in catalysts ranking. However, we must emphasize the fact that a pertinent ranking of different catalytic formulations would require an evaluation of the respective terms of intrinsic activity and adsorption constant. The first term is effectively representative of the surface composition of the active phase under the actual reaction conditions, whilst the second term which reflects the interactions
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of the reactants with the surface may be affected by changes in the support composition. Although the Langmuir–Hinshelwood formalism is probably not sensitive to those parameters reflecting weak interactions (e.g. affinity of some components for the catalyst surface due to van Der Waals force) it may be expected that a modification of the composition of the catalyst would affect the adsorption constant. Work is presently in progress in order to evaluate the effect of support composition upon the solvent effect. References [1] M.J. Ledoux, C.P. Huu, Y. Segura, F. Luck, J. Catal. 121 (1990) 70. [2] H. Tanaka, M. Boulinguiez, M. Vrinat, Catal. Today 29 (1996) 209. [3] R. Bacaud, D. Letourneur, J.J. Lecomte, M. Vrinat, in: A. Centeno, S.A. Giraldo, E.A. Páez Mozo (Eds.), Actas del XVI Simposio Iberoamericano de Catalisis, Cartagena, Colombia, 1998, p. 67. [4] J.A.R. van Veen, H.A. Colijn, P.A.J.M. Hendriks, A.J. Van Welsenes, Fuel Proc. Technol. 35 (1993) 137.
[5] H.R. Reinhoudt, R. Troost, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A. Moulijn, J. Catal. 203 (2001) 509. [6] Q. Qusro, F.E. Massoth, Appl. Catal. 29 (1987) 375. [7] D. Letourneur, R. Bacaud, M. Vrinat, D. Schweich, I. Pitault, Ind. Eng. Chem. Res. 37 (1998) 2662. [8] A. Ishihara, T. Itoh, T. Hino, M. Nomura, P. Qi, T. Kabe, J. Catal. 140 (1993) 184. [9] A. Ishihara, T. Kabe, Ind. Eng. Chem. Res. 32 (1993) 753. [10] P.T. Vasudevan, Catal. Rev. Sci. Eng. 38 (1996) 161. [11] S. Kasahara, T. Shimizu, M. Yamada, Catal. Today 35 (1997) 59. [12] H. Schulz, W. Böhringer, P. Waller, F. Ousmanov, Catal. Today 49 (1999) 87. [13] H.R. Reinhoudt, C.H.M. Boons, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A. Moulijn, Appl. Catal. A 207 (2001) 25. [14] R. Bacaud, D. Letourneur, J.J. Lecomte, M. Vrinat, in: A. Centeno, S.A. Giraldo, E.A. Páez Mozo (Eds.), Actas del XVI Simposio Iberoamericano de Catalisis, Cartagena, Colombia, 1998, p. 2249. [15] H.M. Lin, H.M. Sebastien, C. Kwang-Chu, J. Chem. Eng. Data 25 (1980) 252. [16] R. Prada-Silvy, P. Grange, F. Delannay, B. Delmon, Appl. Catal. 46 (1989). [17] S. Gamez, R. Bacaud, M. Vrinat, J.R. Bernard, in: J.M. Orfao, J.L. Faria, J.L. Figueiredo (Eds.), Actas do XVII Simposio Iberoamericano de Catalise, Porto, Portugal, 2000, p. 827.
Solvent effect in gas–liquid hydrotreatment reactions Alfredo Guevara, Robert Bacaud∗ , Michel Vrinat Institut de Recherches sur la Catalyse, CNRS, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France Received 21 May 2003; received in revised form 3 July 2003; accepted 4 July 2003
Abstract The evaluation and ranking of hydrotreatment catalysts at laboratory scale is generally performed with model compounds, thiophene or dibenzothiophene (DBT), either in the gaseous or in the liquid-phase. According to the considered reactant and the chosen conditions, confusing data have been published. The presence of an inert solvent has been claimed to affect the measured activity but no comprehensive experimentation supports this assertion. Some of the possible causes for the observed discrepancies include the inhibiting effect of hydrogen sulfide, the distribution of the reactants in the gaseous and liquid and the control of transport phenomena. The rate of tetralin hydrogen was determined in a micro up-flow three-phase reactor, the reactant being either in the gaseous or the liquid-phase, as a pure compound or in the presence of inert solvents. Expressing the reaction rate as a simplified form of the Langmuir–Hinshelwood formalism, the reaction rate constant and the relative tetralin-to-solvent adsorption constant ratio were determined. The rate constant is independent of the reaction conditions (gas- or liquid-phase) and of the nature of the solvent, indicating that the intrinsic activity of the active phase does not depend on the experimental conditions. The values of the relative adsorption constant indicate that the competitive effect is more pronounced as the molecular weight of the solvent increases. © 2003 Elsevier B.V. All rights reserved. Keywords: Liquid-phase; Gas-phase; Hydrotreatment; HDS; Solvent; Sulfide
1. Introduction Hydrotreatment of petroleum fractions is industrially carried out in fixed bed reactors fed with liquid reactants and gaseous hydrogen. Owing to the decisive importance of hydrotreatment processes—more specifically hydrodesulfurization (HDS)—in the general framework of increasingly severe environmental regulations, the development of catalytic formulations for HDS is the subject of intensive research effort. The assessment of catalytic activity at laboratory scale is generally performed with model compounds, ∗ Corresponding author. Fax: +33-472-445-399. E-mail address: [email protected] (R. Bacaud).
thiophene or dibenzothiophene (DBT), either in the gaseous or in the liquid-phase. The pertinent information for the evaluation of new catalytic formulations is a relative ranking of a given family of catalysts. Obviously, the significance of the measured activity, i.e. the independence of the ranking with respect to the composition of the reactant, is of paramount importance and is a prerequisite for the usefulness of activity measurements. Published data concerning the comparison of HDS activity of various catalysts, using either thiophene or DBT, in the gaseous or liquid-phase, lead to confusing conclusions. Thiophene HDS and DBT HDS have been performed by Ledoux et al. [1] over two series of Ni-Mo and Co-Mo/Al2 O3 catalysts. With one
0926-860X/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-860X(03)00550-7
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exception, the respective ranking for the two reactants is identical. A comparison of thiophene, DBT and gas oil HDS was conducted by Tanaka et al. [2] over various Co-Mo/TiO2 -Al2 O3 catalysts. A positive effect of titanium-based supports upon activity was evidenced with these three reactants. In contrast, Bacaud et al. [3] drew an opposite conclusion and demonstrated that the positive effect of titanium-containing supports observed in DBT HDS was not confirmed in the HDS of a diesel cut. Conflictive results were also observed by van Veen et al. [4] and by Reinhoudt et al. [5] in the comparison of gas-phase versus liquid-phase DBT HDS performed over Ni-Mo or Ni-W supported catalysts prepared or sulfided under variable conditions. In most of the published data, the ranking of catalysts is generally based upon a single activity measurement, performed under arbitrarily chosen conditions. Several points that may have a contrasted effect upon catalytic activity must be addressed before definitive conclusions relative to the actual value of reaction rate are drawn. 1. Does the measured rate for the reactant conversion reflects the chemical process or is it limited by transport phenomena? This point is seldom considered and the significance of activity measurements performed at high conversion level, such as the data published by Qusro and Massoth [6], is questionable. 2. Thiophene HDS is representative of hydrotreating of light fractions performed in the gaseous phase. DBT HDS is chosen as a model for the elimination of sulfur in mid range distillates and may be performed either in the gaseous phase or in liquid-phase, as a pure compound or in the presence of a solvent. An evaluation of the distribution of the reactants in the phases is rarely presented in published data and, under some experimental conditions, there exist some doubt concerning the physical state of the reactant. This point is related with the evaluation of transport phenomena, which are described by different models according to the presence, or the absence of a liquid-phase under the conditions of reaction. Although this point seems obvious, it must be realized that the quantity of liquid required for a complete wetting of the catalyst surface is small, relative to the flow of reactants.
Therefore the determination of the volatilization of the liquid flow must be performed with a good precision. Unfortunately, the result of such a calculation depends on the selected model, and significant variations can be evidenced. 3. The presence of a solvent has been shown to substantially modify the activity. Some tentative interpretations have been proposed to justify this so-called solvent effect, which remain unelucidated. The most evident influence is related with the partial volatilization of the solvent causing a variation of the concentration of the reactants. Although this effect seems quite obvious and has been quantified for DBT HDS by Letourneur et al. [7], it is not accounted for in some of the published data dealing with the solvent effect [8–10]. Similarly, the reciprocal solubility of the present species can change according to their respective concentrations, and depends in turn on the level of conversion. An appropriate model for the evaluation of the distribution of the reactants and products in the reacting medium could account for these complex interactions. 4. The inhibiting effect of hydrogen sulfide upon HDS reactions is well documented; furthermore, the sensitivity of catalysts to this inhibition depends on the nature of the active phase, the activity of NiMo catalysts being more affected than CoMo [11]. Since H2 S is a product of the reaction, a precise control of its partial pressure requires activity measurement to be performed at the same level of conversion. This condition is hardly fulfilled in published data where conversion varies from less than 1 to nearly 90% [1]. The possibility of an inhibiting effect in the desulfurization of diesel oil, related with the strong adsorption of hydroaromatic compounds, has been evoked by Schulz et al. [12]. This competitive adsorption, causing a partial blockage of the active surface, could explain the observed matrix effect. Reinhoudt et al. [13] discussed the difference between gas- and liquid-phase hydrotreatment reactions. As an interpretation to the so-called gas–liquid controversy, they stated that a specific surface morphology of the active phase could favor some interaction of a condensed phase. However, this interpretation looks quite speculative since the experimental facts it relies on may be explained by
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several of the possible causes of divergence enumerated in the introductory paragraph. There is no room for a supposed controversy when the facts (activity ranking) are established in dissimilar conditions. We attempted in the present work to identify the origins of the discrepancies observed between gas- and liquid-phase hydrotreatment reactions and to explain the influence of inert solvents upon the observed reaction rate. In order to achieve this goal, the preceding points were addressed through a critical selection of experimental conditions. The choice of the reactant and the reaction was dictated by the following criteria. • The reactant must be representative of hydrotreatment reactions as for the pressure and temperature range required for a controllable conversion level (1–5 MPa, 550–630 K). • The reaction must reflect one of the steps involved in hydrotreatment scheme. • It must be able to be present either in the gaseous or the liquid-phase under the considered range of experimental conditions. • For a positive control of the inhibiting effect of hydrogen sulfide upon catalytic activity, the transformation of the reactant must not produce H2 S. The hydrogenation of 1,2,3,4-tetrahydronaphthalene (tetralin) over an industrial Ni-Mo-S/Al2 O3 catalyst was selected as the model reaction. This reaction fulfills the preceding conditions and it is representative of HDS since the initial step of sulfur elimination in refractory sulfur compounds involves a hydrogenation of aromatic structures. The rate of tetralin hydrogenation was determined in a micro up-flow three-phase reactor, the reactant being either in the gaseous or the liquid-phase, as a pure compound or in the presence of an inert solvent. Our first concern was to characterize the flow conditions in the reactor and to evaluate the transport rate of reactants to the catalyst surface in order to establish the conditions of reaction rate measurement. The influence of hydrogen sulfide upon conversion was evaluated and further experiments were performed under constant H2 S partial pressure in the gaseous phase or at constant H2 S concentration in the liquid-phase for experiments conducted in biphasic flow conditions.
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In order to evaluate the role played by solvents, various n-alkanes were selected as the solvents, according to their range of vapor pressure under the conditions of hydrotreatment. n-Heptane is totally vaporized and was used for an evaluation of a possible effect of competitive adsorption under gaseous flow conditions. n-Dodecane and n-hexadecane were selected as, respectively, representative of light and medium feeds. The determination of the distribution of the reactants in the liquid and gaseous phases was performed using different models and the reaction rate was estimated as a function of the actual tetralin concentration, either in the gas or the liquid-phase. 2. Experimental 2.1. Catalyst The catalyst selected for activity measurements was an industrial Ni-Mo/␥-Al2 O3 containing 3 wt.% NiO and 14 wt.% MoO3 , BET surface area: 250 m2 g−1 ; pore volume: 0.48 cm3 g−1 . The calcined solid was crushed and the fraction 80–125 m was selected. Gas-phase activity measurements were performed in a flow microreactor. The catalyst (200 mg) was sulfided under the reaction pressure (3 MPa) with a mixture containing 5% H2 S in hydrogen at a total flow rate 3.6 × 10−4 mol s−1 , at 673 K for 4 h (heating rate 5 K min−1 ). Tetralin was introduced in the flow of reactants through a saturator maintained at controlled temperature. An automated injection valve allows periodical gaseous samples to be analyzed by gas chromatography (HP 5890) equipped with a HP5 column (30 m long, 0.53 mm i.d.) and a FID. Gas–liquid-phase activity measurements were carried out in an automated unit, which allows consecutive experiments to be performed under variable process conditions [14]. The following parameters can be programmed: pressure, temperature, flow of gaseous and liquid reactants, selection of liquid reactant, sample collection. The reactor is a stainless steel tube (120 mm long, 6 mm i.d.) packed with 0.1–2 cm3 of catalyst located between two layers of inert alumina. Following the reactor, an automated gas–liquid-phase separator (4 cm3 internal volume) and a sample collector allows periodical sampling of
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the liquid effluent (no appreciable amounts of solvent and tetralin are eluted in the gas-phase). Non-impregnated ␣-alumina of the same particle size was used for the layers of inert packing. A run consists of a series of automated operations. First, the catalyst is sulfided by a solution of dimethyldisulfide (1 wt.%) in n-dodecane at 20 h−1 liquid hourly space velocity (LHSV), under 3 MPa total pressure, in the presence of hydrogen. The hydrogen-to-liquid flow ratio is 300 (l/l) mol mol−1 . The temperature is maintained at 573 K for 3 h (heating rate 5 K min−1 ). The next part of the run is composed of programmed sequences defined by the temperature, total pressure, gas and liquid flow rates, and number of samples analyzed per experiments. The LHSV can be varied in the range 2–40 h−1 . The experiments reported here were performed at 573 K under 3 MPa total pressure. Liquid samples were analyzed by GC in the same conditions as for gas-phase experiments. The amount of naphthalene in the products does not exceed 0.5% of the sum of the concentrations of cis- and trans-decalin. The conversion X is defined as a function of initial and final tetralin concentrations (C0 , C): X=
C0 − C C0
For a molar flow rate F, and a catalyst amount w, the reaction rate is: FX −r = w
Two preliminary tests were performed in order to check the inertness of the reactor filling and the thermal homogeneity. Using non-impregnated ␥-alumina instead of the catalyst, it was first checked that alumina was actually inert with respect to both tetralin and the solvents. Second, since the solvent partially vaporizes at the reactor inlet, identical experiments were repeated using a preheating section made of a tube (200 mm long) packed with inert alumina. For given operating conditions these experiments gave identical results, and it was thus concluded that liquid–vapor equilibrium is reached at the catalyst bed inlet. Further experiments over the whole range of operating conditions lead to the same conclusions. The reproducibility of reaction rate measurement was evaluated form three consecutive determinations, including the complete experimental steps: reactor filling, catalyst sulfidation and activity measurement at 573 K, 3 MPa total pressure. The relative standard deviation was 3%. 2.2. Evaluation of the distribution of the reactants between the liquid and the gaseous phase The calculations were performed with Aspen Plus® . Several models are proposed for the determination of the composition of the liquid and the gaseous phases in the range of pressure and temperature currently used in refining processes. They either rely upon a state equation or a virial expression. The choice of the most
Fig. 1. Variation of hydrogen molar fraction in n-hexadecane as a function of pressure at 623 K. Comparison of experimental and calculated values.
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appropriate model was carried out by comparison of calculated values for the solubility of hydrogen in hexadecane with experimental data published by Lin et al. [15], reported in Fig. 1. The following models were evaluated. • Peng–Robinson–Boston–Matias (PR–BM). • Redlich–Kwong–Soave–Boston–Matias (RKS–BM). • Benedict–Weeb–Rubin–Lee–Starling (BWR–LS). The PR–BM model was selected for subsequent calculations.
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fect the reaction rate or the catalyst performance was performed, after it was established that the measured rate was not limited by transport phenomena. The examined factors include the following. • H2 S partial pressure: the inhibiting effect is strongly dependent upon H2 S partial pressure. • Catalyst sulfidation: are the surface composition, and consequently the activity, affected by a difference in the sulfidation protocol? • What is the role of the presence of n-dodecane in liquid-phase reaction? 3.2. Flow regime and mass-transfer in the three-phase reactor
3. Results and discussion 3.1. The experimental basis for a controversy An evaluation of the rate of tetralin hydrogenation was performed either in the gaseous phase or dissolved in n-dodecane in the liquid-phase. The respective experimental conditions are summarized below (Table 1). The measured rate for gas-phase hydrogenation exceeds the liquid-phase reaction rate by an eight-fold factor. This observation is consistent with data published by Reinhoudt et al. [13] for DBT HDS, which is the basis for the so-called gas–liquid-phase controversy. Table 1 gives a list of the reaction parameters among which one may expect to find the cause(s) of this discrepancy. So, a critical examination of the influence of all of the reaction parameters that may af-
Table 1 Comparative conditions for tetralin hydrogenation in gas- and liquid-phase Parameter
Gas-phase reaction
Liquid-phase reaction
Temperature (K) Total pressure (MPa) Total flow rate (mol s−1 )
573 3 1.1 × 10−4
573 3 4.5 × 10−5
Feed composition (mol%) Tetralin H2 S Hydrogen n-Dodecane
0.21 0.03 99.7 –
0.27 0.03 73.4 26.3
Sulfidation Rate (mol s−1 g−1 )
H2 S/H2 41 × 10−8
DMDS/n-dodecane 5.2 × 10−8
3.2.1. Intraparticle diffusion Assuming a homogeneous particle size distribution and uniform porosity, an evaluation of the effective diffusivity (De ) and Thiele modulus (φ) is achievable. The effectiveness factor (η), representative of the ratio of the observed rate to the maximum rate of reaction in the catalyst particles for a first-order reaction, is defined as: 1 1 η= coth 3φ − φ 3φ The values of De , φ, and η, reported in Table 2, indicate that the measured reaction rate will not be affected by mass transfer limitations under the considered range of experimental conditions. 3.2.2. Flow regime and external transfer in the reactor An evaluation of the respective values of the gasand liquid-phase Reynolds number (Re) indicated that, for the considered range of space velocities, deviation from plug-flow regime could be neglected. An experimental confirmation of the absence of external transfer limitation was performed, for both reactants. Firstly, the flow of tetralin was varied from 8 Table 2 Evaluation of the effectiveness factor Transfer Tetralin/n-dodecane Hydrogen/n-dodecane
De (m2 h−1 ) 10−5
9× 1 × 10−4
φ
η 10−4
4× 3 × 10−4
r = 4.1 × 10−8 kmol s−1 kg, LHSV 19 h−1 , 573 K, 3 MPa.
0.99 0.99
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Fig. 2. Influence of external tetralin transfer upon tetralin hydrogenation (573 K, 3 MPa).
to 24 × 10−8 mol s−1 . The observed reaction rate was constant as illustrated in Fig. 2. Secondly, in order to identify a possible restriction of hydrogen availability in the liquid-phase, the hydrogen molar fraction was varied in the most demanding conditions with respect to hydrogen consumption: highest concentration of tetralin in the heaviest solvent (hexadecane). Fig. 3 indicates that the measured reaction rate was independent of hydrogen molar fraction. Thus, from the estimates of flow regime and experimental confirmation, it can be concluded that chemical regime prevails in the reactor in the considered
range of experimental conditions. The measured rates of tetralin transformation will therefore be representative of adsorption and reaction, independently of any limitation due to transport phenomena. 3.3. Gas-phase versus liquid-phase sulfidation The initial sulfidation of the catalyst in hydrotreating reactors is generally performed in the liquid-phase, contacting the oxidic form of the catalyst with a sulfiding reagent dissolved in the feed. Substantial activity variations have been reported depending upon the nature of the sulfiding reactant [16]. This process causes
60 Reaction rate 10-8mol s-1 g-1
50
Hydrogen molar fraction (%) 40 50
60
70
80
Fig. 3. Influence of external hydrogen transfer upon tetralin hydrogenation (20% tetralin in hexadecane, LHSV 19 h−1 , 573 K, 3 MPa).
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a carbonaceous substance to cover the solid surface in considerable amount (several wt.%), but further contact of the catalyst with the feed does not produce a subsequent increase of the carbon content [17], indicating that a competitive process of carbon deposition and elimination takes place. Hence, the definition of a “freshly sulfided” catalyst is ambiguous since the initial contact of the surface with hydrocarbons generates a “coked” solid. A liquid-phase sulfidation cannot easily be practiced in the case of gas-phase experiments. As a consequence, the catalyst was initially treated under a gas flow containing H2 S. The resulting sulfided solid is obviously carbon-free and some consequence in terms of activity may be expected, owing to the difference in surface composition of catalysts, respectively, sulfided in the liquid- or the gas-phase. In order to evaluate the impact of the sulfidation method as a possible cause for the gas–liquid controversy, the activity for tetralin hydrogenation of the same catalyst (Ni-Mo/␥-Al2 O3 ) was measured in the liquid-phase after it was sulfided either in the gas- or in the liquid-phase. The experimental conditions of sulfidation and reaction are presented in Table 3 with the Table 3 Influence of the conditions of sulfidation upon the liquid-phase reaction rate of tetralin hydrogenation Parameters
Liquid-phase sulfidation
Gas-phase sulfidation
Sulfidation flow rate (mol s−1 )
6.6 × 10−4
3.6 × 10−4
Mole fractions (%) H2 H2 S n-Dodecane DMDS
0.73 0.007 0.26 0.003
0.95 0.05 – –
Total pressure (MPa) Temperature (K), time (h) LHSV (h−1 )
3 573, 3 19.5
3 673, 4 –
Reaction flow rate (mol s−1 )
4.5 × 10−5
4.5 × 10−5
Mole fractions (%) H2 H2 S n-Dodecane Tetralin
73.4 0.03 26.3 0.27
73.4 0.03 26.3 0.27
Total pressure (MPa) Temperature (K)
3 573
3 573
Reaction rate (10−8 mol s−1 g−1 )
5.2
5.1
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respective values of the measured activity. No appreciable change in the reaction rate is observed. It may thus be inferred that, after the initial sulfidation stage, the surface composition of the catalyst under the reaction conditions is predominantly dictated by the contact with the reacting mixture. As a consequence, the observed reaction rate is determined by this interaction during the early stage of activity measurement. 3.4. Influence of H2 S partial pressure The presence of H2 S is required for maintaining sulfide catalysts in a sulfided state when a reaction is performed in a reducing atmosphere. As mentioned earlier, the inhibiting effect of H2 S in hydrogenation and HDS reactions is well established. Since the hydrogenation of tetralin does not produce H2 S, the partial pressure of this reactant can be controlled, independently of the level of conversion. In order to evaluate the effect of H2 S partial pressure as a possible cause of the observed discrepancy between gas- and liquid-phase experiments, the rate of tetralin hydrogenation was measured in the liquid-phase, in the presence of n-dodecane. Fig. 4 presents the reaction rate as a function of H2 S partial pressure. As expected, a decline of the reaction rate is observed. But even at the lowest partial pressure, the measured rate is five times lower than the rate observed in the gas-phase under the same H2 S partial pressure. Further rate measurements were performed in the presence of a constant H2 S concentration fixed at 0.03 mol% of the total feed, either for gas- or liquid-phase hydrogenation. 3.5. The “solvent effect” The discrepancy between gas-phase versus liquidphase hydrogenation may originate from the competitive adsorption of the solvent. Although an inert hydrocarbon does not participate to the reaction process, it may compete with the reactants for the adsorption on active sites on the catalyst surface. Such an effect must be evidenced under both gas- and liquid-phase conditions of reaction. A comparison of the rate of tetralin hydrogenation, in the gas-phase, either pure or diluted with n-heptane, clearly evidences the inhibitor effect caused by the presence of an inert alkane (Fig. 5).
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5
H2S (mol %) 0 0
0.2
0.4
Fig. 4. Rate of tetralin hydrogenation vs. H2 S partial pressure (feed: 0.26 mol% tetralin in dodecane, 0.12 × 10−3 mol h−1 hydrogen, LHSV 19 h−1 , 573 K, 3 MPa).
Assuming a single-site mechanism, the rate of the surface reaction can be described by the Langmuir– Hinshelwood model: KT KH2 CT CH2 r=k (1) 1 + KT CT + KH2 S CH2 S + KH2 CH2 where k is the reaction rate constant, K are the adsorption constants and C the concentrations, with the respective superscripts (tetralin, H2 S, H2 ). This model assumes that the rate is proportional to the concentration of the adsorbed species, irrespective of whether adsorption occurs from the gas- or the liquid-phase. Thus, if the reaction proceeds in the presence of an inert liquid, the rate constant must be the same. This implies that the adsorption equilibrium constant of the
solvent must be small compared with the adsorption constant of tetralin. However, in the presence of large solvent-to-tetralin ratios, the competitive adsorption may significantly affect the reaction rate. The introduction of the adsorption terms of the solvent (superscript S) in Eq. (1) yields: r=k
KT KH2 CT CH2 1 + KT CT + KS CS + KH2 S CH2 S + KH2 CH2 (2)
It must be observed that: • the reaction is performed under 3 MPa total pressure, thus the fraction of vacant adsorption sites is probably low;
50
Reaction rate
10-8 mol.sec-1.g-1
0.21 mol %Tetralin
0.27 mol %Tetralin 37 mol % n-heptane
0
Fig. 5. Influence of n-heptane upon the rate of gas-phase hydrogenation of tetralin.
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• the reaction is performed in the presence of a large excess of hydrogen, and the concentration of hydrogen can be considered as constant; and • provided the concentration of H2 S and the solvent are kept constant, the corresponding terms can be eliminated in the rate expression. The simplified expression of the rate is given as: r=k
KT CT KT C T + K S C S
(3)
A determination of the reaction rate as a function of tetralin concentration allows the tetralin-to-solvent adsorption constant ratio (KT /KS ) to be calculated from Eq. (3), assuming that the rate of gas–liquid equilibrium is faster than the reaction rate. Tetralin hydrogenation was therefore performed in the presence of various alkanes, selected for their respective vapor pressure under the conditions of reaction (573 K, 3 MPa): • n-heptane is completely vaporized; • n-dodecane is partly vaporized; and • n-hexadecane is predominantly in the liquid-phase. The distribution of the reactants in the phases under the reactor conditions (Fig. 6) was determined by a flash calculation with Aspen Plus® , using the Peng–Robinson model. All experiments were performed under constant total flow rate, at the same temperature (573 K), under the same total pressure (3 MPa). The variation of the reaction rate as a function of tetralin concentration (expressed as tetralin-to-solvent ratio in the feed) is presented in Fig. 7. From the corresponding rate data, the values of the reaction rate constant (k) and the tetralin-to-solvent adsorption constant ratios (KT /KS ) are presented in Table 4. The mean values of the liquid holdup for reactions performed in the presence of n-dodecane and n-hexadecane are mostly sufficient for a full coverage
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of the catalyst by the liquid-phase, thus indicating that the reaction effectively proceeds in the liquid-phase. The reaction rate constant is independent of the nature of the solvent, whether the reaction is performed in the gaseous or the liquid-phase. The order of magnitude of the relative adsorption constant indicates that, although tetralin is by far more strongly adsorbed than the considered solvents, the competitive effect arising from the presence of these non-reactive partners justifies the decrease of reaction rate observed at low tetralin concentration. It can be observed that the competitive adsorption is more severe as the molecular weight of the solvent increases. The significance of these two parameters can be underlined noting that the simplified expression of the reaction rate derived from the Langmuir–Hinshelwood formalism (Eq. (3)), includes two different series of terms: • the rate constant (k), which characterize the chemical reaction, and as such, is directly related with the intrinsic activity of the catalytic active phase; and • the second series of terms describe the interaction of the reactants with the catalyst surface. Since all of the reaction rate measurements were performed in the presence of the same catalyst, the numerical values of these parameters are representative of this specific formulation. The composition and the catalytic properties of the active phase are partly determined by the interaction of the catalyst surface with the reacting medium. As evidenced by the uniform value of the reaction rate constant, the intrinsic activity of the surface is not related with the conditions under which the reaction is performed (liquid- or gas-phase), nor is it affected by the molecular weight of the solvent. A cause of alteration could be a change in the behavior of the catalyst with respect to deactivation as a function of time of stream that could be induced by modifications of the support. The increase
Table 4 Reaction rate constants and relative adsorption constants for tetralin hydrogenation in the presence of various solvents (573 K, 3 MPa) Solvent/phase
Rate constant, k (mol s−1 g−1 )
Relative adsorption constant, KT /KS
Liquid holdup, εL (%)
n-Heptane–gas n-Dodecane–liquid n-Hexadecane–liquid
5 × 10−7 5 × 10−7 5 × 10−7
26.4 17 15.3
– 19 22
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Fig. 6. Distribution of the reactants in the gaseous and liquid-phase at reaction conditions, for the considered solvents (heptane, dodecane, hexadecane).
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525
80 C7 g-1
C12
Reaction rate 10-8 mol
s-1
C16
Tetralin/Alkane 0 0
0,1
0,2
0,3
0,4
Fig. 7. Rate of tetralin hydrogenation in various solvents as a function of tetralin-to-alkane ratio.
of the relative adsorption constant with the molecular weight of the solvent is consistent with the increased hindering of heavier alkanes over the catalyst surface. However, the measured values are representative of a given catalyst, and it may be expected that different values would be observed if the composition of the catalyst is modified.
4. Conclusions The gas–liquid controversy essentially relies upon partial, hardly comparable experimental data. As a consequence, the derived notion of solvent effect has not yet received satisfactory explanations. The present work was undertaken in order to establish the experimental basis for a pertinent evaluation of the effect of inert solvent addition upon the rate of transformation of a single compound under invariable conditions of temperature and total pressure. The reactant (tetralin) and the reaction (hydrogenation) were chosen as representative of hydrotreatment reactions and for the possibility of controlling the inhibitor effect of hydrogen sulfide upon the activity of metallic sulfides catalysts. It was established that when the reaction is performed in the gaseous phase, the addition of a gaseous alkane (n-heptane) in the feed produces a decrease of the reaction rate. The same phenomenon is observed if the reactants are in the liquid-phase (n-dodecane or n-hexadecane). The possible causes of perturbation of activity measurements were inves-
tigated. The flow regime in the reactor, internal and external transport phenomena were analyzed and it was concluded that the measured reaction rate under biphasic flow conditions was not affected by diffusion. The inhibiting effect of hydrogen sulfide was established and controlled, as well as the influence of the protocol of sulfidation. We therefore postulated that the experimental evidence of the solvent effect could be interpreted as a competition between the reactant and the solvent for the adsorption on the catalyst surface. Expressing the reaction rate as a simplified form of the Langmuir–Hinshelwood formalism, the reaction rate constant and the relative reactant-to-solvent adsorption constant ratio were determined. The rate constant is independent of the reaction conditions (gas- or liquid-phase) and of the nature of the solvent, indicating that the intrinsic activity of the active phase does not depend on the experimental conditions. The values of the relative adsorption constant indicate that the competitive effect is more pronounced as the molecular weight of the solvent increases. The present results and the proposed interpretation relate to a single catalyst. They do not explain some of the observed discrepancies in catalysts ranking. However, we must emphasize the fact that a pertinent ranking of different catalytic formulations would require an evaluation of the respective terms of intrinsic activity and adsorption constant. The first term is effectively representative of the surface composition of the active phase under the actual reaction conditions, whilst the second term which reflects the interactions
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of the reactants with the surface may be affected by changes in the support composition. Although the Langmuir–Hinshelwood formalism is probably not sensitive to those parameters reflecting weak interactions (e.g. affinity of some components for the catalyst surface due to van Der Waals force) it may be expected that a modification of the composition of the catalyst would affect the adsorption constant. Work is presently in progress in order to evaluate the effect of support composition upon the solvent effect. References [1] M.J. Ledoux, C.P. Huu, Y. Segura, F. Luck, J. Catal. 121 (1990) 70. [2] H. Tanaka, M. Boulinguiez, M. Vrinat, Catal. Today 29 (1996) 209. [3] R. Bacaud, D. Letourneur, J.J. Lecomte, M. Vrinat, in: A. Centeno, S.A. Giraldo, E.A. Páez Mozo (Eds.), Actas del XVI Simposio Iberoamericano de Catalisis, Cartagena, Colombia, 1998, p. 67. [4] J.A.R. van Veen, H.A. Colijn, P.A.J.M. Hendriks, A.J. Van Welsenes, Fuel Proc. Technol. 35 (1993) 137.
[5] H.R. Reinhoudt, R. Troost, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A. Moulijn, J. Catal. 203 (2001) 509. [6] Q. Qusro, F.E. Massoth, Appl. Catal. 29 (1987) 375. [7] D. Letourneur, R. Bacaud, M. Vrinat, D. Schweich, I. Pitault, Ind. Eng. Chem. Res. 37 (1998) 2662. [8] A. Ishihara, T. Itoh, T. Hino, M. Nomura, P. Qi, T. Kabe, J. Catal. 140 (1993) 184. [9] A. Ishihara, T. Kabe, Ind. Eng. Chem. Res. 32 (1993) 753. [10] P.T. Vasudevan, Catal. Rev. Sci. Eng. 38 (1996) 161. [11] S. Kasahara, T. Shimizu, M. Yamada, Catal. Today 35 (1997) 59. [12] H. Schulz, W. Böhringer, P. Waller, F. Ousmanov, Catal. Today 49 (1999) 87. [13] H.R. Reinhoudt, C.H.M. Boons, A.D. van Langeveld, J.A.R. van Veen, S.T. Sie, J.A. Moulijn, Appl. Catal. A 207 (2001) 25. [14] R. Bacaud, D. Letourneur, J.J. Lecomte, M. Vrinat, in: A. Centeno, S.A. Giraldo, E.A. Páez Mozo (Eds.), Actas del XVI Simposio Iberoamericano de Catalisis, Cartagena, Colombia, 1998, p. 2249. [15] H.M. Lin, H.M. Sebastien, C. Kwang-Chu, J. Chem. Eng. Data 25 (1980) 252. [16] R. Prada-Silvy, P. Grange, F. Delannay, B. Delmon, Appl. Catal. 46 (1989). [17] S. Gamez, R. Bacaud, M. Vrinat, J.R. Bernard, in: J.M. Orfao, J.L. Faria, J.L. Figueiredo (Eds.), Actas do XVII Simposio Iberoamericano de Catalise, Porto, Portugal, 2000, p. 827.