Ind. Eng. Chem. Res. 1998, 37, 2099-2106
2099
Hydrogenation of Diethyl Adipate in a Catalytic Fixed-Bed Reactor Rengaswamy Jaganathan, Sopan T. Chaudhari, Chandrashekhar V. Rode, and Raghunath V. Chaudhari*,† Chemical Engineering Division, National Chemical Laboratory, Pune 411008, India
Patrick L. Mills*,‡ DuPont Central Research and Development, Experimental Station, E304/A204, Wilmington, Delaware 19880-0304
The vapor phase hydrogenation of diethyl adipate (DEA) was investigated using a copper chromite catalyst in a fixed-bed reactor over the temperature range of 523 to 563 K. The various products obtained during the hydrogenation reaction were identified by gas chromatography/mass spectroscopy (GC/MS) and quantified by GC. Under the conditions of the present work, the major products identified were 1,6-hexanediol, oxepane, and ω-hydroxyethyl caproate. The combined effects of reaction temperature, hydrogen pressure, and contact time on both the conversion of DEA and the selectivity to the main reaction products and various byproducts were also studied. The intrinsic kinetics of this hydrogenation were also investigated, and a suitable reaction rate equation based on a single-site Langmuir-Hinshelwood mechanism was used to interpret the integral reactor performance data. The kinetic rate equation constants were determined by nonlinear parameter estimation using the integral reactor data as the basis. The predicted concentrations of various components agreed with the experimental data within (7% over a reasonable range of experimental reaction conditions. Introduction Catalytic hydrogenation of the esters derived from C3 to C6 dicarboxylic acids is widely practiced in industry for the manufacture of fine and speciality chemicals because they are key chemical intermediates in the manufacture of various consumer products (Mullin, 1994). In these hydrogenations, diols are formed as the major products and various other important chemical intermediate products, such as monohydroxy acids or the condensation products of these hydroxy acids to yield lactones, can be produced. Further hydrogenation of these intermediates can yield cyclic ethers, such as caprolactone. Under more severe reaction conditions, the diols can be further hydrogenated to yield alcohols and hydrocarbons. Hydrogenation of diethyl adipate (DEA) is one such example for the production of 1,6hexanediol, which is an intermediate in the synthesis of polyesters and polyurethanes (Schossig, 1985). One of the other intermediates formed in the hydrogenation of DEA is -caprolactone, which can be converted to caprolactam. This latter intermediate is an important raw material for the manufacture of Nylon 6 (Ritz et al., 1986; Schossig, 1985; Weissermel, 1978). In the hydrogenation of DEA, it is desirable to achieve very high selectivity to 1,6-hexanediol (HDO) without further hydrogenation to n-hexanol and hydrocarbons. The degree of hydrogenation obviously depends on the type of catalyst and the reaction conditions used. A variety of catalysts have been proposed for the hydrogenation of dicarboxylic acids and their esters. It has been reported that cobalt metal alone, or cobalt in * To whom correspondence should be addressed. † Fax: (+91) 212 333941. E-mail:
[email protected]. ‡ Fax: (+1)302 695 3501. E-mail: Patrick.L.Mills@ usa.dupont.com.
combination with a noble metal, gives better activity for the hydrogenation of dicarboxylic acids, whereas copper chromite catalysts are suitable for the hydrogenation of ester derivatives (Fujita et al., 1973; Ishimoto et al., 1972; Corry, 1994). Use of a supported cobalt catalyst has also been reported for the hydrogenation of adipic acid at 7 MPa pressure and 493 K to produce hydroxycaproic acid in high yields (Fujita et al. 1974 a; 1974 b). Bimetallic catalysts gave hexanediol yields as high as 90% in the hydrogenation of adipic acid (Kanetaka and Mori, 1980). Most of this previous work is patented and has been focused on evaluation of different catalysts for the hydrogenation of adipic acid. There are very few reports on the hydrogenation of diethyl adipate, and no published reports are available on the intrinsic reaction kinetics, reaction engineering analysis, and prediction of reactor performance of this industrially important reaction. Also, previous work has mainly been carried out in a slurry reactor under conditions of very high pressure (30 MPa) and elevated temperature (573 K). Because a number of important processes involve heterogeneous catalyzed hydrogenation processes and fundamental data on the kinetics and catalysis are lacking, the current study was undertaken. The particular objectives of this study were: (1) to investigate the activity and selectivity of a copper chromite catalyst for the hydrogenation of diethyl adipate in a fixed-bed reactor between the pressure range of ∼2-6 MPa; (2) to identify the major and minor reaction products and to study the selectivity behavior over a reasonable range of reaction conditions; (3) to study the intrinsic kinetics of this reaction using experimental data obtained in a laboratory-scale, continuous-flow, high-pressure, fixedbed reactor system; and (4) to develop a fundamental intrinsic kinetics model that provides reasonably accurate predictions of the experimental integral reactor performance data.
S0888-5885(97)00631-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/28/1998
2100 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 1. Synthetic steps involved in the copper chromite catalyst preparation.
Experimental Section a. Catalyst and Materials. The hydrogenation experiments were carried out using a heterogeneous copper chromite catalyst that was synthesized by following the steps illustrated in Figure 1. Referring to this figure, the catalyst was prepared by a coprecipitation method in which an aqueous ammonium chromate solution was added to an aqueous copper nitrate solution to give an yellow precipitate of copper chromate. In some cases, modified catalysts were prepared by the addition of promoters, such as Al and Zn, in the form of their nitrate salts. This precipitate was then filtered, dried, and calcined over the temperature range 623723 K. The resulting material was activated as described later and then used for the hydrogenation experiments. The resulting catalyst had the following physical properties: surface area (m2/kg), 3.88 × 104; solid-phase density (kg/m3), 4.2 × 103; and porosity, 0.53. The hydrogen gas was supplied by M/s Indian Oxygen Ltd., Mumbai, India, with a purity in excess of 99.98%, and was used directly from the cylinder. The reactant diethyl adipate was procured from M/s Aldrich Chemicals, Milwaukee, WI. All chemicals were used as received without further purification. b. Fixed-Bed Reactor System. All of the experiments were performed in a laboratory-scale, highpressure, fixed-bed reactor manufactured by M/s Geomechanique, Paris, France. A schematic of the system is shown in Figure 2. It contained a fixed-bed reactor and various supporting subsystems for introducing hydrogen gas and a liquid feed at controlled flow rates, condensing the higher boiling fraction of the product vapor, separating the condensed product effluent into gas and liquid streams, sampling these streams for offline analysis, controlling the reactor pressure, and measuring the off-gas flow rate. The reactor (R1) consisted of a 15 mm i.d. × 30 cm long tube that was heated by two tubular furnaces whose zones (TIC1 and TIC2) were independently
controlled at the desired bed temperature. The liquid reactant was introduced from a feed buret using a micrometering pump. The hydrogen gas was supplied to the top of the reactor from a high-pressure cylinder through an electronic mass flow controller. Upon entering the reactor inlet, the liquid reactant and hydrogen gas were mixed in a short section of inert packing where the liquid was vaporized to form a homogeneous vapor mixture with the hydrogen gas. This mixture was contacted in the catalytic zone and then entered a second zone of inert packing before exiting the reactor. The resulting reaction product then entered a heat exchanger (E1) to condense the higher-boiling reaction products. This two-phase gas-liquid mixture then passed into a gas-liquid separator (V1) from which the condensed liquid products were periodically drained. The off-gas that exited the separator flowed through an electronic pressure control valve, which controlled the reactor back-pressure. The flow rate of the off-gas was measured by a wet gas-flow meter (FT1) before it was vented to the laboratory hood at atmospheric pressure. The reactor system could be operated at a maximum temperature of 623 K, a maximum pressure of 9.58 MPa, liquid flow rates between 1 × 10-6 and 3 × 10-4 m3/h, and gas flow rates between 1 × 10-2 and 10 m3/h (at STP). c. Experimental Procedure and Analytical. In a typical hydrogenation experiment, a known weight of catalyst was packed in the central portion of the reactor tube to avoid any temperature gradients, while the top and bottom portions were filled with inert material, such as carborundum. The catalyst was activated at 473 K using a continuous flow of H2 gas (1 × 10-2 m3/h) and a H2 pressure of 0.5 MPa for 72 h. After the catalyst was activated, the temperature, pressure, and hydrogen flow rates were set at the desired values. Once the system was stabilized at these reaction conditions, the liquid reactant (DEA) feed rate was introduced to the reactor from the feed buret. The product vapors leaving the reactor were cooled to ∼5 °C and collected in the gasliquid separator. At this temperature, there was no
Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998 2101
Figure 2. High-pressure fixed-bed reactor system used for the DEA hydrogenation experiments.
significant loss of volatile species. Excess hydrogen gas was vented out to the atmosphere after depressurization and then passed through a flame arrester. Liquid product samples were periodically withdrawn from the gas-liquid separator and were analyzed by two independent gas chromatography (GC) systems. Routine sample analyses were performed using an HP 5840 gas chromatograph equipped with a 2.5 m long × 1/8 in. i.d. stainless steel packed column containing 10% OV-17 on Chromosorb W packing. The GC conditions were: FID temperature ) 573 K; injector temperature ) 523 K; column temperature ) 423 K; and nitrogen carrier gas flow rate ) 2 × 10-5 m3/min. Identification of the reaction products was also performed using a Hewlett-Packard 5890 Series II gas chromatograph that was equipped with a Hewlett-Packard 5972 mass selective detector. A 0.53 mm i.d. by 30 m J&W DB-FFAP megabore column with a film thickness of 5 µm resolved nearly all of the main and secondary reaction byproducts. The following chromatographic conditions were used: sample injection volume ) 0.5 µL; helium carrier gas flow rate ) 10 sccm; split ratio ) 40:1; injector temperature ) 250 °C, detector temperature ) 250 °C; temperature program ) 35 °C for 3.75 min, 10 °C/min to 75 °C, then 20 °C/min to 165 °C, and then hold for 10 min. Results and Discussion The main objectives of the present work were twofold: (1) to determine the product identification, distribution, and selectivity behavior for the hydrogenation of DEA using a copper chromite catalyst in a fixed bed reactor; and (2) to study the intrinsic kinetics of the complex multistep reactions. For this purpose, the effect of temperature, pressure, and contact time of the vapor phase on the conversion of DEA and product formation was investigated over the temperature range
Table 1. Range of Reaction Conditions parameter
range
temperature H2 pressure W/F
523-563 K 1.98-5.96 MPa 0.29-0.96 h
of 523 to 563 K. The range of reaction conditions is given in Table 1. In all experiments, the concentrations of the reactant and products in the reactor effluent were determined from which the conversion and selectivity were evaluated. The results are discussed in the following sections. a. Product Distribution and Selectivity. In the initial experiments, the hydrogenation reaction products were identified as a function of the vapor phase contact time W/F. A typical capillary column GC analysis of the reaction products obtained at a reaction pressure of 40 bar and a reaction temperature of 523 K for a particular W/F is shown in Figure 3. For these particular conditions, the major reaction products in this sample are ethanol [retention time (RT) ) 4.07 min], oxepane (RT ) 6.28 min), 1-hexanol (RT ) 11.06 min), diethyl ester hexanedioic acid or diethyl adipate (RT ) 17.08 min), and 1,6-hexandiol (RT ) 25.20 min). In Table 2, all of the compounds shown in Figure 3 that were positively identified by GC/mass spectroscopy (MS) and their retention times are tabulated according to elution order. One major peak was obtained at a retention time of 14.82 min, but this could not be positively identified. The reaction byproducts include straight-chain and cyclic aliphatic alcohols, ester derivatives of aliphatic carboxylic acids, and several ketones. A GC analysis of the liquid feed is shown in Figure 4 and indicates that trace quantities of acetic acid, ethyl ester (RT ) 2.67 min), 1-butanol (RT ) 8.04 min), acetic acid (RT ) 11.68 min), and butenedioic acid diethyl ester (RT ) 14.97 min) are present, in addition to the diethyl adipate (RT ) 17.36 min). Hence, most of the various compounds in the reaction product shown in Figure 3
2102 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 3. Gas chromatography analysis of a typical DEA hydrogenation reaction product sample. Reaction conditions: total pressure ) 40 bar, reaction temperature ) 523 K. Chromatographic conditions: J&W DB-FFAP megabore capillary column (0.53 mm i.d. × 30 m); film thickness ) 5 µm; 0.5 µL sample injection; split ratio ) 40:1; 10 sccm helium carrier gas; injector temperature ) 250 °C; detector temperature ) 250 °C; temperature program ) 35 °C for 3.75 min, 10 °C/min to 75 °C, then 20 °C/min to 165 °C, then hold for 10 min.
Figure 4. Gas chromatography analysis of a typical DEA hydrogenation reaction feed sample. The conditions are defined in Figure 3.
Table 2. Reaction Products from Diethyl Adipate Hydrogenation elution order
species name
retention time, min
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
acetaldehyde acetone tetrahydrofuran acetic acid, ethyl ester ethanol oxepane 1-butanol cyclopentanone, 2-methyl 1-pentanol cyclopentanol cyclopentanol, 2-methyl, trans cyclopentanol, 2-methyl, cis 1-hexanol cyclopentane methanol 2(3H)-furanone, dihydro unknown hexanedioic acid, diethyl ester 2-oxepanone hexanoic acid, ethyl ester 1,6-hexanediol
1.51 2.16 2.60 3.01 4.07 6.28 8.44 9.27 9.96 10.58 10.70 10.96 11.06 11.96 13.92 14.82 17.08 17.65 20.31 25.20
are the result of either heterogeneous or homogeneous chemical transformations versus impurities in the feed. GC analysis of the overhead gas from the gas-liquid separator was also performed and confirmed that possible volatile reaction byproducts from cracking of higher molecular weight species, such as light hydrocarbons and other low-boiling organic compounds, were either absent or present at such low concentrations that they could not be detected. The dimensionless concentrations of the diethyl adipate reactant and major reaction products are shown in Figure 5 as a function of the W/F ratio at a total pressure of 5.8 MPa, a reaction temperature of 523 K, and a hydrogen flow rate of 60 L/h (NTP). Under these conditions, the main reaction products formed from the hydrogenation of diethyl adipate were 1,6-hexanediol, the ethyl ester of hydroxycaproic acid (referred to as caproate in the figure), -caprolactone (oxepanone), and oxepane. The concentration of 1-hexanol was 95% material balance for the products formed in hydrogenation of DEA as per the reaction scheme shown in Figure 6. c. Kinetic Modeling. To study the intrinsic kinetics of the hydrogenation of DEA using the copper chromite catalyst, experimental tests were first performed to show that the DEA conversion versus contact time data were obtained under kinetically controlled conditions. A few experiments, in which the catalyst particle size was varied between 400 and 800 µm, indicated that the
2104 Ind. Eng. Chem. Res., Vol. 37, No. 6, 1998
Figure 10. Effect of DEA conversion on the selectivity to the minor hydrogenation reaction products. Reaction conditions: reaction temperature ) 523 K, reaction pressure ) 3.9 MPa, hydrogen flow rate ) 60 L/h (STP).
rates of reaction were within the errors associated with their experimental determination. These results indicated that for particle sizes of
