J.M. Winterbottom

Studies on the hydrogenation of cinnamaldehyde over Pd/C catalysts†

The hydrogenation of cinnamaldehyde was investigated using a 5% Pd/C catalyst in a 250 cm3 stirred tank reactor and 500 cm3 autoclave. The experiments were carried out at 273–343 K and 0·1–1·1 MPa. Non-polar solvents, e.g. toluene, decane, methylcyclohexane, decalin, ether and heptane, and polar solvents such as methanol, ethanol, propan-1-o1, propan-2-ol, butan-1-ol and butan-2-ol were used to study the selectivity with respect to hydrocinnamaldehyde formation, the reaction kinetics and mass transfer. The additives, such as potassium acetate, ferrous chloride, ferrous sulphate and quinoline were incorporated into the catalyst in order to improve the catalyst selectivity, which was observed especially in the case of potassium acetate.

Selective catalytic hydrogenation of 2-butyne-1,4-diol to cis-2-butene-1,4-diol in mass transfer efficient slurry reactors

The hydrogenation of 2-butyne-1,4-diol over a commercial 5% Pd/charcoal and a 5% Pd/TiO2 catalyst prepared in-house was investigated and the selectivity towards cis-2-butene-1,4-diol under various reaction conditions is reported. The experiments were carried out in a glass stirred tank reactor (STR) and a cocurrent downflow contactor reactor (CDCR). It was found that besides cis-2-butene-1,4-diol and 1,4-butanediol, some side products such as cis- and trans-crotyl alcohol, n-butanol, and n-butyraldehyde were also observed. A reaction scheme is proposed. In the STR, the effects of catalyst loading, temperature, base, poisons and metal additives on the selectivity were studied. The results indicate that triethyl phosphite and lead acetate led to inhibition of further hydrogenation. The use of lead poisoned Pd/charcoal catalyst gave partial hydrogenation with selectivity towards cis-2-butene-1,4-diol of 97.4-98.8%. The use of Pd/TiO2 catalyst exhibited higher selectivity, more than 99%. In the reaction with Pd/charcoal, CDCR has shown its potential giving increased selectivity (up to 100%) to cis-2-butene-1,4-diol, at the end of first hydrogenation step, even with no addition of poison and with much higher reaction rates.

Determination of bed voidage using water substitution and 3D magnetic resonance imaging, bed density and pressure drop in packed-bed reactors

Abstract Using a water substitution method to determine bed voidage, an independent relationship between bed height and bed voidage was observed for the trilobe (virgin and crushed) and cylindrical alumina supports. Typical bed voidage values of 0.49–0.51 (virgin trilobe). 0.46–0.52 (crushed trilobe) and 0.28–0.31 (cylindrical) were observed within 0.1 and 0.19 m i.d. columns. However, bed voidage values were approximately 6% larger in the 0.05 m i.d. column and could be attributed to a greater extent of wall zone voidage. Dense packing of the columns in all cases resulted in a decrease in bed voidage which had significant effects on bed density and column pressure drop. In addition to the water substitution measurements of bed voidage, three-dimensional magnetic resonance imaging (MRI) data were used in conjunction with digital image analysis techniques to obtain one-dimensional radial profiles of voidage from comparable alumina catalyst support material. Similar results and trends in voidage values between the water substitution method and those obtained from MRI data are evident. In all cases, the analysis of the MRI data yields voidage values that are consistently higher than those obtained from water substitution measurements.

Operating and hydrodynamic characteristics of a cocurrent downflow bubble column reactor

Abstract The hydrodynamic stability of the gas—liquid two-phase dispersion in a cocurrent downflow column (CDC) depends critically on the column configuration and such operating parameters as the inlet liquid velocity, column liquid velocity and bubble dispersion height. The hydrodynamic characteristics of the CDC were investigated using a column of 0.076 m (3in) i.d. with liquids of different surface tension, viscosity and coalescence properties. The two-phase flow patterns, bubble formation, stability and entrainment were observed for oxygen with water and aqueous solutions of 1-propanol, glycerol and sodium alginate. The minimum inlet liquid velocity required to maintain a stable gas—liquid dispersion was determined, as was the maximum column liquid velocity which did not give bubble entrainment in the outlet. An empirical correlation defining stable column operating conditions was proposed. Strongly coalescing liquids, such as water and water containing low concentrations of 1-propanol (≪0.15 wt%) or glycerol (≪5wt%), gave large diameter bubbles (3-5mm) and good gas disengagement which allowed the use of relatively high column liquid velocities (up to 0.14 m/s). Coalescence-inhibiting liquids such as aqueous 1-propanol (>0.015 wt%) and aqueous glycerol (>5 wt%) gave rise to very small bubbles ( The CDC compared with other types of downflow bubble columns was found to be capable of greater operational flexibility. No excess gas input or recycling was required in the CDC due to its ability to achieve 100% gas utilization. The CDC would be particularly suited to applications where high ratios of liquid-to-gas throughputs were required, such as in waste water or effluent treatment.

The cocurrent downflow contactor (CDC) reactor : chemically enhanced mass transfer & reaction studies for slurry & fixed bed catalytic hydrogenation

Abstract The CDC has been shown to be a mass transfer device of high efficiency (100% gas utilization, 97% approach to equilibrium) giving overall mass transfer coefficient (k L a) which are very high (0.2–1.5 s −1 for O 2 /H 2 O) and when used as a slurry reactor for the catalytic hydrogenation of various unsaturated compounds, the reactions were shown to be mostly surface reaction rate controlled. The larger values of k L a are due mainly to be high interfacial area generated (1000–6000 m 2 .m −3 fluid for 50% gas hold-up). These results, which were obtained using physical measurements have been confirmed using the SO 3 2− oxidation method. The CDC has been employed as a slurry reactor with and without added tangentail flow (swirl flow) for (i) itaconic acid hydrogenation (ii) rape seed oil hydrogenation and (iii) as a fixed bed reactor for itaconic acid and soybean oil hydrogenated using palladium and nickel catalysts. Due to the large values of k L a, all the reactions were operated under largely surface reaction controlled conditions and hydrogenation of triglycerides was observed to occur with greater selectivity than in stirred reactors. Use of swirl flow further enhanced mass transfer and reaction rate.

Explaining mass transfer observations in multiphase stirred reactors: particle-liquid slip velocity measurements using PEPT

Abstract Solid–liquid mass transfer coefficients were determined using the technique of dissolving a sparingly soluble solid, salicylic acid loaded onto silica, in water. Mass transfer was found to be dependent on various particle characteristics. Of particular interest is the influence of the particle-liquid density difference. It is suggested that the change in mass transfer coefficient with these parameters is related, to some extent, with the particle-liquid slip velocity. The technique of positron emission particle tracking (PEPT) has been used in parallel with the mass transfer measurements in order to study the effect of different operating conditions on the liquid flow patterns and the particle-liquid slip velocities. Using PEPT, time-averaged slip velocities were determined by simple subtraction of the data from a neutrally buoyant particle. In this way, important information about solid–liquid behaviour and zones of poor mass transfer in stirred vessels is revealed.

A comparison of triglyceride oil hydrogenation in a downflow bubble column using slurry and fixed bed catalysts

The hydrogenation of the triglyceride oil, soya bean oil, has been studied in the temperature range 130–160 °C and in the pressure range 100–600 kPa using (i) a 5% w/w Pd/C slurry catalyst and (ii) a 3% w/w Pd/Al2O3 Raschig ring catalyst in a cocurrent downflow contactor (CDC) reactor. Separate studies of residence time distribution (RTD) were carried out in a modified CDC device in order to determine dispersion numbers and dispersion coefficients. The RTD measurements indicated that the overall flow was a mixture of well-mixed and plug flow for the unpacked CDC, so that the entry section (0–30 cm from entrance) was perfectly mixed and the remainder of the column (30–130 cm) gave predominantly plug flow behaviour. The introduction of random packing in the form of 13 mm Raschig rings gave rise to increased back mixing in the lower part of the CDC and the overall dispersion number increased due to liquid and gas circulation around the packing elements. Kinetic studies revealed an initial rate reaction order of 1.24–1.26 with respect to hydrogen concentration both in slurry and fixed bed CDC reactors and is interpreted as a combination of a parallel pair of first and second order reactions during the initial stages of reaction. Mass transfer coefficients for gas absorption (kLa) and liquid–solid mass transport (ks) were determined for both types of reactor. The kLa values lay in the range 1.0–3.33 s−1 and the liquid–solid transport resistances (XLS) were all <1%, so that the reaction was almost totally surface reaction rate controlled. Apparent energy of activation measurements gave values of EA = 49 ± 6 kJ mol−1, which is strongly indicative of surface reaction rate control involving the hydrogenation of an olefinic double bond. The selectivity in respect of linolenate (three double bonds) removal and linoleate (two double bonds) retention was high with, for palladium, relatively low trans-isomer production (<30%). The overall selectivity was slightly, but significantly, better for the fixed bed CDC reactor and this is attributed to the greater degree of plug flow behaviour in the latter, despite the bed causing an increase in dispersion number. However, there is no reaction in the well-mixed section of the fixed bed CDC reactor as there is in the slurry CDC reactor and this is likely to improve selectivity in a consecutive reaction sequence. © 2000 Society of Chemical Industry

Effect of gassing rate on solid-liquid mass transfer coefficients and particle slip velocities in stirred tank reactors

Abstract While solid–liquid dispersion in mechanically agitated vessels has been widely investigated, the suspension of particles with simultaneous gas dispersion is, however, less well understood. A consideration of the gassing rate is of particular importance when designing “dead-end” batch reactors. Solid–liquid mass transfer coefficients were determined using the technique of dissolving a sparingly soluble solid, salicylic acid loaded onto silica gel, in water. Mass transfer was found to be dependent on a variety of geometric, physical and hydrodynamic properties; with the significant exception of agitation speed the influence of the latter properties was independent of gas dispersion. Flow visualisation with positron emission particle tracking has been used alongside the mass transfer measurements to study the effects of gas injection on the liquid flow patterns and the solid–liquid slip velocities. Time-averaged relative slip velocities were determined by simple subtraction of the data obtained using a neutrally buoyant particle. Gas dispersion was found to affect the particle–liquid slip velocity, explaining the mass transfer coefficient trends observed. While only a small diameter vessel has been used it does point to considerable non-uniformity of mass transfer in larger vessels.

Study of mass transfer characteristics of a cocurrent downflow bubble column reactor using hydrogenation of itaconic acid

The performance of a cocurrent downflow contactor (CDC) bubble column reactor has been examined using a first order reaction involving the palladium catalyzed hydrogenation of itaconic acid. The reaction was carried out at ambient temperature and in the pressure range 110-290 kPa using 5% and 10% w/w Pd/charcoal catalysts in two solvents (water and 2-propanol). The quantitative evaluation of the mass transfer and kinetic parameters was achieved using the classical film model for a first-order reaction. Because of the large specfic gas-liquid interfacial area generated in the CDC, k L a, the volumetric gas-liquid mass transfer coefficient, was large (0.4-6 s -1 ) and was comparable with that of a much smaller stirred reactor (∼1/10 in size) with only ∼10% of the latters energy consumption. Unlike conventional bubble columns where the gas-liquid mass transfer is often the rate limiting step for this type of reaction, the relative contribution of the gas-liquid mass transfer resistance in the CDC was low (1-50% of the total resistance) compared with the liquid-solid mass transfer and surface reaction resistances for the three-phase catalytic hydrogenation of itaconic acid. The liquid-solid mass transfer coefficient (k s ) and first-order reaction rate constant (k l ) in the CDC were similar to those obtained in the stirred reactor with both the liquid-solid mass transfer and surface reaction being important rate steps. However, the magnitude of k s and k l depends critically on the method of calculating k s and an independent evaluation of k s should be carried out to obtain more accurate k l values. These initial studies indicate that the CDC has good potential as a three phase catalytic reactor and unlike some bubble columns can be used for relatively fast reactions.

Catalytic hydrogenation of CO over the doped perovskite oxide YBa2Cu3O7-x catalysts

Perovskite oxide structured YBa2Cu3O7-x(YBCO) has been first prepared by carbonate precipitation and then modified with palladium or ruthenium by impregnation on the perovskite oxide, while cobalt was co-precipitated simultaneously in the same pH range with perovskite oxide. After characterization the catalysts were used in the temperature range 300–450°C, in the pressure range 1–9 atmospheres and for H2/CO ratios in the range 1–4 in a differential plug flow reactor for the hydrogenation of carbon monoxide to give hydrocarbons. The perovskite oxide (YBCO) 20% (w/w) and doped 2% (w/w) cobalt oxide catalyst were prepared by the wet chemical method from their nitrate solutions and oxidized at 950°C. Perovskite oxide (Dursun, G. & Winterbottom, J. M., J. Chem. Technol Biotechnol. 63 (1995) 113–16) was also doped with palladium and ruthenium metal by impregnation followed by oxidation at 250°C. The catalysts prepared were characterized by using TemperatureProgrammed Reduction (TPR) to observe the reduction temperature and also to measure total and metal surface area. The modified perovskite oxide on alumina, ruthenium- and cobalt-doped catalysts, has been shown to give a better conversion and also selectivity towards saturated hydrocarbons compared with palladium-doped catalyst. The temperature effect of these catalysts is more consistent, giving a steady increase of conversion with increasing temperature. Although increase of pressure increases the conversion, it causes very little change in product distribution. The activation energy of palladium- and ruthenium-doped, and cobalt co-precipitated catalysts for the reaction has been measured to be 55 kJ mol−1, 75 kJ mol−1 and 50 kJ mol−1 respectively. A general rate equation of the form r=k[H2]m[CO]n has been observed and found to be applicable at the pressures and temperatures used for the catalytic system studied and found to be m≌1·0 for palladium-doped, m≌1·2 for ruthenium-doped and m≌0·95 for cobalt co-precipitated catalysts as n becomes zero or negligibly less than zero. The mechanism of reaction to produce hydrocarbons from syngas has been deduced from the results. It appeared that the carbon monoxide insertion mechanism has been more evident for palladium-doped catalysts whereas the carbide mechanism plays the main role for the ruthenium-doped and cobalt co-precipitated catalysts.