Article pubs.acs.org/IECR
Liquid-Phase Hydrogenation of Nitrobenzene in a Tubular Reactor: Parametric Study of the Operating Conditions Influence Clara Sá Couto,† Luis M. Madeira,*,‡ Clemente Pedro Nunes,§ and Paulo Araújo† †
CUFQuímicos Industriais, S.A., Quinta da Indústria, 3860-680 Estarreja, Portugal LEPABE, Departamento de Engenharia Química, Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias s/n, 4200-465, Porto, Portugal § CERENA, Departamento de Engenharia Química, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal ‡
S Supporting Information *
ABSTRACT: Fixed-bed reactors for nitrobenzene (NB) hydrogenation into aniline (ANL) in the liquid phase have not experienced extensive industrial-scale development, mostly due to difficulties in removing the reaction heat generated. In this work, NB hydrogenation was performed in a tubular fixed-bed reactor using a Pd/Al2O3 commercial catalyst. The influence of some operating conditions was analyzed by assessing catalyst performance in NB conversion and selectivity toward ANL and secondary products. It was found that catalyst time of usage is extremely importantwhile NB conversion remains stable, selectivity toward the products formed changes (ANL selectivity increases with the catalyst use but the formation of secondary products decreases until almost disappearing). It was also found that temperature and pressure are the most important parameters in the ranges studied and that NH3, which is a reaction product with potential influence in gas composition, does not affect NB conversion but helps decreasing secondary products formation.
1. INTRODUCTION Aniline (ANL) is a major chemical product that can be produced by many routes. However, catalytic hydrogenation of nitrobenzene (NB) is the one that dominates and gives the highest selectivity.1 Most of the processes are carried out in the gas-phase in catalytic fixed-bed reactors while the liquid-phase involves suspended, highly active metal−supported catalysts (slurry reactors).2 Therefore, new reactor configurations for ANL production through NB hydrogenation in the liquid-phase have been studied. For instance, in 1995 Peureux et al.3 tried this reaction in a membrane reactor, using the catalytic membrane as an active contactor between gaseous and liquid reactants. A microstructured falling film reactor with Pd catalyst deposited as films or particles was also tested for the hydrogenation of NB to ANL in ethanol and proved to be feasible, although deactivation has been detected mainly caused by the formation of organic compounds on the catalyst surface and due to Pd loss.4 Because these technologies are still far from commercialization, other solutions are required, like the more conventional fixed-bed reactors. Fixed-bed reactors have not experienced an extensive industrial-scale development in the case of NB hydrogenation due to the difficulty in removing the heat generated during the reaction and that can lead to runaway situations, hot-spots and a decrease in process performance. Nevertheless, this type of reactor is the most © XXXX American Chemical Society
appropriate to use when the objective is to minimize the dimension of reaction units while using more active catalysts that afterward do not need to be separated from the reaction mixture.5 Another advantage of this configuration is related to the well-specified residence time with minimum back-mixing.6 Actually, it is one of the most common reactor configurations used in other hydrogenation reactions, such as of cumene hydroperoxide,7 1,3 butadiene at high conversions,8 or acetylene in large-scale, which is generally conducted in a series of two adiabatic, fixed-bed reactors to minimize the temperature increase through the bed.9 Fixed-bed reactors are also appropriate for benzene hydrogenation, although some authors verified that mass transfer limitations appeared to have a considerable impact on the reactor performance.10 For the hydrogenation of nitro compounds in tubular fixed-bed reactors, the most used catalysts are typically palladium, platinum, and nickel, supported or not.2,11,12 The use of tubular reactors in liquid-phase ANL production has also been studied, but the number of reports found is quite limited, and in most of them with conditions still far from the Received: Revised: Accepted: Published: A
January 30, 2017 March 3, 2017 March 8, 2017 March 8, 2017 DOI: 10.1021/acs.iecr.7b00403 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Scheme of the setup used for the catalytic tests: (1) H2 cylinder, (2) N2 cylinder, (3) feed tank, (4) high-pressure pump, (5) gas−liquid mixture head, (6) tubular fixed-bed reactor, (7) heat-exchanger, (8) gas−liquid separator, (9) gas-stream outlet, and (10) product tank.
leading to a change in catalyst activity and/or in the ANL and secondary products formation. Therefore, in the present work, the main goal is to study and evaluate NB liquid-phase hydrogenation in a tubular fixed-bed reactor using an active and selective catalyst that was chosen in a previous work and that proved to be feasible to use under moderate conditions of temperature and pressure.17 Another important issue is related with assessing catalyst performance along the catalytic tests, to determine if there is any deactivation. Moreover, a parametric study will be done aiming to determine the impact of the operating conditions on both NB conversion and aromatic compounds production (selectivity toward ANL and secondary products). The effect of a H2 stream containing NH3 as a contaminant will be evaluated as well. The better comprehension of all these aspects is crucial for subsequently optimizing the process and maximize ANL production.
possibility of up-scaling. For instance, a specific tubular catalytic apparatus in which the heat release and heat-exchange surfaces are not spatially separated was presented by Kirillov et al.13 and was applied to NB hydrogenation (coolant temperature, 200 °C; feed reagent ratio, 0.86/0.14; and gas velocity, 1 m/s) obtaining conversions of 98.2%; they also concluded that higher NB concentrations could be used under compatible conditions. Du et al.14 worked on carbon nanofiber-coated monoliths for three-phase NB hydrogenation. Reaction with a monolith catalyst was performed in a continuous flow reactor, and its performance was evaluated. For testing a catalyst based on ruthenium, Bombos et al.15 used a continuous fixed-bed catalytic reactor in the total pressure range of 10−40 bar, temperature range of 45−75 °C, in cocurrent with downward flow of reagents, having concluded that the rise in temperature favors the increase of the NB conversion and the yield in both total aromatic compounds and ANL. On the other hand, the knowledge of the main gas contaminants nature and the understanding of how they affect the hydrogenation reaction or the catalyst performance are key aspects to be evaluated, missing in most literature reports. In the case of the NB hydrogenation reaction, it was found that NH3 is formed during the reaction.16 Its influence might be important, since it can influence the performance of the reactor,
2. MATERIAL AND METHODS 2.1. Catalyst Characterization. The commercial Pd/ Al2O3 catalyst was characterized by H2 temperature-programmed reduction (H2-TPR), by X-ray diffraction (XRD), by high resolution transmission electron microscopy (HRTEM), by nitrogen adsorption for BET surface area determination and by inductively coupled plasma mass B
DOI: 10.1021/acs.iecr.7b00403 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research Table 1. Operating Conditions Used in the Catalytic Tests Performed in the Tubular Reactor
*
Highlighted blue tests correspond to tests performed at the same operating conditions of section 3.1.
The XRD patterns were obtained on a Rigaku diffractometer, Geigerf lex, in the angular range of 10° to 100° (2θ) with a scan rate of 3°/min. HRTEM was performed on a JEOL 2200FS apparatus to determine the Pd particle size distribution in the support. Nitrogen adsorption and desorption measurements were carried out at 77 K with an automatic Micromeritics ASAP 2000 apparatus. Prior to analysis, the samples were pretreated at 448 K under vacuum for 6 h. The BET surface area (Sext), the total pore volume (Vtotal), calculated from the adsorbed volume of nitrogen for a relative pressure P/P0 of 0.99, and the average pore diameter (Daverage) were estimated. The determination of elements by ICP-MS was performed on an ICP-MS Thermo X Series apparatus. The sample to be analyzed was rigorously weighed (ca. 0.05 g), and to it was added 1 mL of HNO3 + 3 mL of HCl + 1 mL of HF. Then the
spectrometry (ICP-MS). Two samples were characterized: without suffering any reaction (fresh sample) and after the catalytic tests (used sample), except for H2-TPR, where only the fresh material was analyzed because the used sample has some organic compounds, which interfere with the thermal conductivity detector (TCD) signal. H2-TPR experiments were performed on a Micromeritics AutoChem II 2920 apparatus, using 130 mg of catalyst. No pretreatment was realized because Pd particles are reduced in the presence of argon. H2-TPR was carried out under a mixture of 5% H2/argon with a flow rate of 30 mL/min (NTP), from room temperature up to 900 °C at a heating rate of 10 °C/min. Hydrogen consumption was measured with a TCD; water formed during the reduction processes was trapped in a dry ice trap. C
DOI: 10.1021/acs.iecr.7b00403 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research sample was digested under microwave heating (180 °C) for 5 min. The sample was finally taken up to 100 mL with ultrapure water and analyzed. 2.2. Catalytic Reaction. Hydrogenation of NB, in the liquid phase, was carried out in a tubular reactor with 15 mm of internal diameter and 400 mm long (with a catalytic bed of 120 mm), in continuous down-flow mode (under H2 pressure). The catalyst used was a commercial 0.3 wt % Pd/Al2O3 material in extrudate form (1.2−2.5 mm). A known amount of Pd/Al2O3 catalyst was loaded into the reactor, and the material pretreated, in situ. Pretreatment of the catalyst was performed at 150 °C and under hydrogen pressure (20 barg), gas flow of 2 g/h, for 2 h. H2-TPR experiments performed with the fresh catalyst have shown that the most important peaks appeared at 80 °C (PdO species). This means that the catalyst used was fully reduced, under the pretreatment conditions employed. All the tests were performed with the same sample of catalyst, about 10 g. The catalytic bed, at the center of the reactor, was positioned between two layers of SiC (21 g each, granulometry of 1.68 mm) and glass beads. The upper layer of glass beads and SiC served both as a mixer for the reactant streams and as a preheater region, with the objective of equalizing the temperature along the reactor and to help avoid hot-spots. Temperature regulation is made by an oven from Termolab equipped with one thermocouple that regulates the reactor heating. Inside the reactor there is a cannula where three thermocouples were positioned. The thermocouple in the middle is the one that provides for the control of temperature in the reaction zone. Figure 1 shows a scheme of the setup used for the catalytic tests. H2 (ALPHAGAZ 1 purity 99.999%, Air Liquide) is supplied from a gas cylinder. The liquid feed and gas are mixed before entering the reactor, being fed by the top through a mixture head. Products leave the reactor at the base (down-flow regime). The liquid reactant is pumped with a HPLC pump, JASCO PU-2087, at a required flow rate, being all this stream heated (up to the mixture head) to decrease temperature gradients inside the reactor. The total pressure inside the reactor is kept constant along each run using a back-pressure regulator that guarantees a constant exit gas flow. The inlet gas flow is dependent on the H2 consumption in the unit, being supplied with an excess of about 90%. After leaving the bottom of the reactor, the reaction mixture is cooled in a heat exchanger. Between the heat exchanger and the gas−liquid separator there is a set of valves where the samples are collected to be analyzed. The sampling of the liquid phase was performed at selected time intervals and analyzed by gas chromatography, in an Agilent 6890A chromatograph equipped with two flame ionization detectors (FID). The column used was a HP-1 (100% dimethylpolysiloxane 30 m × 320 μm × 4 μm). The temperature in the injector and in the detector was 250 °C, the pressure in the column was 14 barg, and helium was used as carrier gas. The column oven was temperature-programmed with a 1 min initial hold at 120 °C followed by an increase until 230 °C (15 °C min−1 rate) and then kept at 230 °C for 9 min. All the compounds were previously identified using the external standard method. Calibration curves were plotted for all the analyzed compounds which were easily identified since their retention times are well-known. Several samples were injected and the standard deviation associated with this method was found to be below 10%.
As a common industrial practice, the reaction feed is composed of a mixture of NB and ANL.16 There are three main purposes for having ANL in the feed: to act as a solvent for the water that is produced during the reaction, avoiding the formation of two phases (organic and aqueous) that would lead to the interruption of the reaction, and also to avoid the almost irreversible strong NB adsorption on the catalyst surface; moreover, it helps dissipate the excess heat generated due to the high exothermicity of the hydrogenation reaction. The reference values for temperature, pressure, and nitrobenzene concentration during the parametric study are Tref = 120 °C, Pref = 14 barg and Cref = 1.2 wt % NB. The experiments performed and the conditions used are given in Table 1. The NB conversion (XNB) was calculated based on the data obtained from GC analysis: XNB =
NB0(ppm) − NBout, t(ppm) NB0(ppm)
(1)
where NB0(ppm) is the reactor feed NB concentration (ppm) and NBout,t(ppm) is the NB concentration (ppm) at the reactor outlet at a given time instant t. The liquid phase analysis confirmed the presence of the following compounds: NB, ANL, and the byproducts cyclohexylamine (CHA), cyclohexanol (CHOL), cyclohexanone (CHONA), N-cyclohexylaniline (CHANIL), dicyclohexylamine (DICHA), cyclohexyldeneaniline (CHENO), and benzene (Bz) (cf. Nomenclature section). Nonetheless, the secondary products will not be presented individually but in groups: light products, Bz, CHA, CHOL, and CHONA; and heavy products, DICHA, CHENO, and CHANIL. It is important to note that NB and ANL (used as solvent), are of industrial grade, Table 2, thus containing some byproducts. Table 2. NB and ANL of Industrial Grade Detailed Composition18 compound
amount
nitrobenzene benzene dinitrobenzene nitrophenols water aniline CHA CHOL CHONA+CHENO DICHA CHANIL
≥99.96%
