Ind. Eng. Chem. Res. 1998, 37, 759-769
759
Liquid Phase Hydrogenation of Maleic Anhydride to 1,4-Butanediol in a Packed Bubble Column Reactor Uwe Herrmann and Gerhard Emig* Lehrstuhl fu¨ r Technische Chemie I der Universita¨ t Erlangen-Nu¨ rnberg, Egerlandstrasse 3, D-91058 Erlangen, Germany
The liquid phase hydrogenation of maleic anhydride was investigated in a packed bubble column reactor using different copper-based catalysts. A copper-zinc catalyst was found to be active in the formation of 1,4-butanediol, whereas on zinc-free copper catalysts, mainly succinic anhydride and γ-butyrolactone were formed. At suitable reaction conditions, maleic anhydride hydrogenation over a copper-zinc catalyst gave valuable products with high yield and selectivity whereas succinic anhydride was absent in the reactor outlet. Based on a three-phase reactor model and a kinetic model of the reaction mechanism, the influence of reaction conditions on reactor performance was determined. It was observed that the use of large particles and a high axial dispersion of liquid phase is a necessary condition for the feasibility of a “one-step-hydrogenation” of highly concentrated maleic anhydride feed solutions because of a significant decrease of succinic anhydride formation rate. Introduction Increasing demand for polymers with different properties and compositions led to a rise in the production of suitable monomers in the past. An important example is 1,4-butanediol, which is used as a feedstock for the synthesis of polyesters, polyurethanes, and polyethers (Weissermel and Arpe, 1994). A major 1,4butanediol-based polymer is polybutyleneterephthalate, which is a starting material for the synthesis of films, fibers, adhesives, and engineering plastics (Harris and Tuck, 1990; Weissermel and Arpe, 1994). Other relevant 1,4-butanediol applications are the formation of tetrahydrofuran and γ-butyrolactone. Both chemicals are important solvents and additionally serve as a feedstock for organic synthesis (Harris and Tuck, 1990). Currently, most 1,4-butanediol is produced by condensation of formaldehyde and acetylene and a subsequent hydrogenation of the condensation product 1,4-butynediol (Brownstein, 1991). Several disadvantages within this Reppe process led to the development of other 1,4-butanediol synthesis routes (Brownstein and List, 1977). A promising alternative to the Reppe process is the liquid phase hydrogenation of maleic anhydride to 1,4-butanediol. This multistep reaction is illustrated in Figure 1. Reaction intermediates are succinic anhydride and γ-butyrolactone. Depending on the reaction conditions, the dehydration of 1,4-butanediol to tetrahydrofuran is observed. Any stage of the multistep hydrogenation of maleic anhydride to 1,4butanediol is known to be catalyzed by copper-based catalysts (Castiglioni et al., 1993; Herrmann and Emig, 1997; Messori and Vaccari, 1994). Whereas γ-butyrolactone, 1,4-butanediol, and tetrahydrofuran are valuable products, succinic anhydride is an undesirable intermediate. Succinic anhydride decreases 1,4-butanediol selectivity on maleic anhydride hydrogenation due to competitive adsorption with γ-butyrolactone on the active copper surface. It was observed that the adsorption of γ-butyrolactone will be inhibited if the succinic * Corresponding author. E-mail:
[email protected].
Figure 1. Reaction scheme of maleic anhydride hydrogenation to 1,4-butanediol/tetrahydrofuran.
anhydride concentration raises above a threshold value. This threshold value is higher for copper-zinc catalysts than for the zinc-free copper catalyst, for which the threshold value is lower than the detection sensitivity of the gas chromatograph (Herrmann and Emig, 1998), In a previous paper it was concluded that succinic anhydride formation rate is strongly influenced by maleic anhydride concentration at the active sites of the catalyst (Herrmann and Emig, 1998). A high maleic anhydride concentration favors succinic anhydride formation and thus leads to a undesirable high succinic anhydride concentration on the active catalyst surface. If other reaction conditions are kept constant, maleic anhydride concentration at the active sites of the catalyst will be influenced by two major effects: (i) backmixing of the liquid phase in the reactor and (ii) size of the catalyst particles. Backmixing results in a decrease of the substrate concentration in the bulk liquid in low axial distances of the catalyst bed. Maleic anhydride concentration at the active sites of the catalyst may be decreased by pore diffusion resistance and thus by the size of the catalyst particles. The continuous stirred tank reactor that was used in the
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760 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 Table 1. Characteristics of Catalysts Used on Maleic Anhydride Hydrogenation type
supplier
composition [wt %]
composition [at. %]
carrier
R 3-11 G 13
BASF AG Su¨d-Chemie AG BASF AG
100 Cu 61 Cu 39 Cr 50 Cu 50 Zn
Mg2SiO4 (spinel-structure)
R 3-12
30 Cu 40 Cu 26 Cr 32 Cu 32 Zn
hydrogenation of maleic anhydride in a previous study (Herrmann and Emig, 1997) provides a complete backmixing of the liquid phase and thus fulfills one of the aforementioned demands. Major disadvantages are the complex setup and the dynamic sealing that may cause problems when performing high-pressure experiments. In contrast, tubular reactors usually provide a simple setup and, therefore, are suitable for a large-scale industrial production even under severe reaction conditions. The most used three-phase tubular reactors are three-phase bubble columns, trickle bed reactors, and packed bubble columns. Both, three-phase bubble columns and trickle bed reactors do not meet the aforementioned requirements. Three-phase bubble columns are usually not operated with large particles. Furthermore, a pronounced backmixing of the liquid phase is not present in trickle bed reactors (Shah, 1979; Froment and Bischoff, 1979; Ramachandran and Chaudhari, 1983). In contrast, packed bubble column reactors fulfill both demands. A cocurrent upward flow of gas and liquid phase causes a backmixing of the liquid phase. The extent of backmixing is influenced by the flow rate of both phases (Hofmann, 1982). Furthermore, packed bubble column reactors are usually operated with large particles in the 1-5-mm range (Baerns et al., 1987). In the present paper the investigation of maleic anhydride hydrogenation in a packed bubble column reactor using different copper-based catalysts is described. A major aim of this study was to gain insight into the influence of various reaction conditions on succinic anhydride formation rate and thus on 1,4butanediol synthesis. Because no references or patent literature are so far available, another concern was to evaluate the feasibility of the maleic anhydride hydrogenation to 1,4-butanediol under technically relevant feed concentrations in a packed bubble column reactor. For this purpose, a three-phase reactor model was developed, and the influence of various reaction parameters on the reactor performance was determined quantitatively. Experimental Section Catalysts and Reaction Procedure. The characteristics of catalysts used in the maleic anhydride hydrogenation are listed in Table 1. All catalysts are commercial and were supplied by Su¨d-Chemie AG, and BASF AG, Germany. Prior to use, the catalysts were crushed and sieved to obtain the desired fraction of 1.03.0 mm. The catalyst was fixed in the packed bubble column reactor between two packings of glass spheres. Drying was carried out at 423 K and 100 kPa with pure nitrogen for 1 h. Catalyst reduction was performed subsequently by replacing nitrogen with hydrogen at a concentration of 6 vol % hydrogen at 423 K for 1.5 h and finally 12 vol % hydrogen for 15 h at 473 K. After this procedure, temperature and pressure as well as feed rates of gas and liquid were adjusted to the desired levels.
Al2O3
Table 2. Range of Experimental Operation Conditions on Maleic Anhydride Hydrogenation condition
range
temperature pressure catalyst mass hydrogen feed rate liquid feed rate size of catalyst pellets maleic anhydride feed concentration
473-518 K 5.0-9.0 MPa 30 g 1 NL/min 2-6 mL/min 1.0-3.0 mm 0.5-2.0 mol/L
1,4-Dioxane was used as a solvent for maleic anhydride, and n-tetradecane was used as internal standard. Both solvent and internal standard were inert under the reaction conditions applied. Nitrogen and hydrogen were delivered by Linde AG, Germany, with >99.99% purity. Maleic anhydride and n-tetradecane were supplied by Fluka AG, Germany, with >99% purity. 1,4Dioxane was delivered by Merck, Germany, with >99% purity. The range of reaction conditions applied in hydrogenation experiments is listed in Table 2. (Particular reaction conditions of any experiment are outlined in the legend of the corresponding figure). Samples were drawn by a tube above the catalyst bed in stationary state of reactor operation. Concentrations were calculated by the internal standard method (Schomburg, 1987). Assuming a constant reaction volume, conversion, yield, and selectivity were calculated as follows:
conversion of substrate i:
Xi )
ci,0 - ci ci,0
(1)
yield of product k referred to substrate i: ck - ck,0 νi | | (2) Yk,i ) ci,0 νk (νMA ) -1, νSA, νγ-BL, ν1,4-BD, νTHF, νn-BuOH ) +1) selectivity of product k referred to substrate i: Yk,i (3) Sk,i ) Xi In eqs 1-3 ci,0 and ck,0 are feed concentrations and νi and νk are stoichiometric coefficients of species i and k, respectively. Apparatus. Hydrogenation of maleic anhydride was performed in a stainless steel (SS-316) tubular reactor. The internal diameter and height of the reactor tube were 30 and 600 mm, respectively. To control isothermal reactor operation, four thermocouples were installed with an axial distance of 120 mm in the reactor wall. Heating of the reactor was performed with two heating units that were operated independently. Gas and liquid phase were introduced into the bottom of the tubular reactor with thermal mass flow controllers for the gas phase (Brooks 5850 E) and a HPLC pump (Kontron 422 M) for the liquid phase. After passing the reactor, gas and liquid were fed into a pressure vessel and were finally stored in a tank. The pressure vessel regulates
Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998 761
Figure 2. Flow chart of the apparatus: (FIC) flow indicator and controller; (LC) level controller; (PC) pressure controller; (PI) pressure indicator; (PIC) pressure indicator and controller; (TI) temperature indicator; (TIC) temperature indicator and controller; (WI) weight indicator.
the reactor pressure with a dip tube that is linked to an overflow valve (Herrmann and Emig, 1997). The setup of the apparatus is shown in Figure 2. Analytical Setup. Analysis of the liquid phase was performed with a Hewlett Packard (5890 Series) gas chromatograph with both a HP-5 and a HP-FFAP column. The analytical setup is described in detail in Herrmann and Emig (1997). Results and Discussion The main objective of the present study was to investigate the following items: (i) catalyst screening on the liquid-phase hydrogenation of maleic anhydride in a packed bubble column reactor, (ii) evaluation of the feasibility of maleic anhydride hydrogenation to 1,4butanediol by experimental investigation of the influence of reaction conditions on reactor performance, and (iii) prediction of suitable reaction conditions based on a three-phase reactor model and a kinetic model of the reacting system. Catalyst Screening. Prior to catalyst screening, the absence of homogeneous reactions of reactants, solvent, and internal standard in the presence and absence of hydrogen was proven experimentally. Catalyst screening was performed at 493 K and 7.0 MPa, with a catalyst mass of 30 g. Maleic anhydride feed concentration was 0.5 mol/L. Feed rates of hydrogen and liquid phase were 1 NL/min and 6 mL/min, respectively. Figure 3 shows the yields of hydrogenation products referred to maleic anhydride. On zinc-free copper catalysts G 13 (CuCrOx) and R 3-11 (Cu/Mg2SiO4), hydrogenation of maleic anhydride mainly leads to succinic anhydride and γ-butyrolactone; by-products are tetrahydrofuran and n-butanol in case of R 3-11. 1,4Butanediol is not formed on G 13 nor on R 3-11. Because of the small succinic anhydride hydrogenation rate, the yield of succinic anhydride greatly exceeds the yield of γ-butyrolactone. Similar results as on G 13 were obtained on other zinc-free copper chromite cata-
Figure 3. Product yields on maleic anhydride hydrogenation in a packed bubble column reactor over copper-based catalysts (T ) 493 K; p ) 7.0 MPa; mCat. ) 30 g; hydrogen gas feed rate, 1 NL/ min; liquid feed rate, 6 mL/min; cMA,0 ) 0.5 mol/L; dp ) 3 mm).
lysts promoted with Mn and Ba as well as over different noble metal catalysts based on Pd, Re, and Ru. The formation of 1,4-butanediol was observed on the copperzinc catalyst R 3-12 (CuZnOx/Al2O3). On this catalyst, the yield of succinic anhydride is low compared with that on zinc-free copper catalysts G 13 and R 3-11. Whereas the formation of 1,4-butanediol is a consequence of a low succinic anhydride concentration at the active sites of the catalyst, the low succinic anhydride yield is a result of the complex reaction mechanism on copper-zinc catalysts (Herrmann and Emig, 1998; Turek et al., 1994). It was observed that zinc oxide promotes succinic anhydride hydrogenation, thus leading to a small succinic anhydride concentration and finally to 1,4-butanediol formation. Byproducts on R 3-12 are tetrahydrofuran and n-butanol. A comparison of these results with the hydrogenation in a continuous stirred tank reactor (Herrmann and Emig, 1997) reveals a similar tendency of catalyst activity. 1,4-Butanediol is formed exclusively on copper-zinc catalysts, whereas the formation of mainly succinic anhydride and γ-butyrolactone is observed on zinc-free copper catalysts.
762 Ind. Eng. Chem. Res., Vol. 37, No. 3, 1998
Figure 4. Maleic anhydride conversion and product yields versus reaction temperature on maleic anhydride hydrogenation in a packed bubble column reactor over R 3-12 (p ) 9.0 MPa; mCat. ) 30 g; hydrogen gas feed rate, 1 NL/min; liquid feed rate, 6 mL/ min; cMA,0 ) 0.5 mol/L; dp ) 1.0-1.25 mm).
Figure 3 illustrates that the hydrogenation of maleic anhydride to 1,4-butanediol will be feasible in principle in a packed bubble column reactor if a copper-zinc catalyst is used. The undesirable presence of succinic anhydride and the poor 1,4-butanediol yield of ∼3.5% gave reason for a more detailed investigation. For this purpose, the influence of reaction conditions on the reactor performance was studied using the copper-zinc catalyst R 3-12. Influence of Reaction Conditions. Figure 4 shows the influence of temperature on maleic anhydride conversion and product yields between 473 and 518 K. Particle size was reduced from ∼3 mm used in screening experiments to 1.0-1.25 mm. This reduction resulted in an increase of catalyst activity and did not significantly influence succinic anhydride yield. Figure 4 illustrates that applying a reaction temperature >500 K results in a succinic anhydride yield of zero. This effect is a consequence of the reaction kinetics on copperzinc catalysts. A temperature increase raises the ratio of succinic anhydride consumption and formation rate and finally leads to a smaller succinic anhydride concentration (Herrmann and Emig, 1998). Due to an increase of catalyst activity, maleic anhydride conversion and yields of γ-butyrolactone and 1,4-butanediol raise with increasing temperature. The ratio of yields of 1,4-butanediol and γ-butyrolactone is diminished if temperature is increased due to exothermicity of the reversible γ-butyrolactone hydrogenation. The yields of tetrahydrofuran and n-butanol are only slightly influenced by temperature. To investigate the influence of hydrogen concentration in the liquid phase, pressure was varied in the 5.0 to 9.0 MPa range. The influence of reaction pressure on the maleic anhydride conversion and yield of succinic anhydride and byproducts was small. In contrast, the ratio of yields of γ-butyrolactone and 1,4-butanediol was shifted to 1,4-butanediol while the pressure was increased. Figure 5 shows the influence of the maleic anhydride feed concentration on maleic anhydride conversion and product yields. In view of a technical application of the process, a high maleic anhydride feed concentration is desirable because the amount of solvent that has to be recovered in a separation unit decreases with increasing feed concentration. Prior to a change of maleic anhydride feed concentration, the feed rate of the liquid phase was decreased from 6 mL/min that was used in
Figure 5. Maleic anhydride conversion and product yields versus maleic anhydride feed concentration on maleic anhydride hydrogenation in a packed bubble column reactor over R 3-12 (T ) 503 K; p ) 9.0 MPa; mCat. ) 30 g; hydrogen gas feed rate, 1 NL/min; liquid feed rate, 2 mL/min; dp ) 1.0-1.25mm).
Figure 6. Influence of reaction temperature on succinic anhydride yield for two different maleic anhydride feed concentrations on maleic anhydride hydrogenation in a packed bubble column reactor over R 3-12 (p ) 9.0 MPa; mCat. ) 30 g; hydrogen gas feed rate, 1 NL/min; liquid feed rate, 2 mL/min; dp ) 1.0-1.25 mm).
the investigation of temperature influence to 2 mL/min. This decrease resulted in an increase of maleic anhydride conversion from ∼75% (see Figure 4, T ) 503 K) to ∼100% (see Figure 5, cMA,0 ) 0.5 mol/L). Figure 5 shows that using feed concentrations
