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Ind. Eng. Chem. Res. 2000, 39, 3108-3111
NiCl2 on γ-Alumina as Selective Adsorbents for Acetylene over Ethylene Adriaan J. Kodde,† Joel Padin,‡ Peter J. van der Meer,† Marjo C. Mittelmeijer-Hazeleger,† Alfred Bliek,*,† and Ralph T. Yang‡ Department of Chemical Engineering, University of Amsterdam, Nieuwe Achtergracht 166, NL-1018 WV Amsterdam, The Netherlands, and Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136
It has been found that previous results on selective adsorption of acetylene over ethylene and ethane reported by Yang et al. were influenced to a significant extent by acetone impurities (in acetylene). This work reports corrected adsorption isotherm results with acetone carefully removed on γ-alumina and monolayer NiCl2/γ-alumina. For both sorbents, adsorption strengths followed the rank order acetylene > ethylene. The selectivity towards acetylene of NiCl2/γalumina is higher compared to the bare γ-alumina. Introduction Separation of acetylene from other low molecular weight hydrocarbons (both olefins and paraffins) is of industrial importance. Hence, it is of interest to search for a selective sorbent that selectively forms a reversible bond with acetylene but not with olefins and paraffins. Yang and Foldes1 first reported the preparation of selective adsorbents for acetylene by spreading a monolayer of NiCl2 on γ-alumina. Padin and Yang2 further extended this work to CoCl2 and FeCl2 supported on various substrates with high surface areas. Unfortunately, however, acetylene contained acetone, and the acetone contamination was not removed in the previous work.1,2 The “high-purity” grade of acetylene that is available from all industrial gas suppliers is labeled 99.8%. For safety reasons, however, acetylene is stored in acetone as a solvent. For a new cylinder (at “250 psi”), the actual concentration in the gas phase of acetone is approximately 5000 ppm, and the concentration increases steeply while the pressure of the cylinder decreases, to as high as 25% as the pressure reaches 15 psig.3 It is clear, therefore, that the previous results of Yang et al. needed to be reexamined. Experimental Section Preparation of the Sorbents. The sorbents described in this paper contained Ni2+ cations dispersed over a high surface area substrate. This dispersion was accomplished by using the technique of spontaneous (thermal) monolayer dispersion. This technique has been described in detail in the literature.4 Spontaneous monolayer dispersion has been used to successfully prepare various sorbents capable of π-complexation.5,6 Thermal monolayer dispersion involves heating finely divided powders of a metal salt and substrate at a temperature between the Tammann temperature7 and the melting point of the salt. However, the temperature should not be too high to cause the metal salt to oxidize * To whom correspondence should be addressed. E-mail:
[email protected]. Fax: +31 20 525 5604. Tel: +31 20 525 5265. † University of Amsterdam. ‡ The University of Michigan.
or react with the substrate. The sorbent described in the present work was prepared by the dispersion of NiCl2 over γ-Al2O3. It was shown by Xie and Tang4 that NiCl2 can be dispersed to form a monolayer on γ-Al2O3 simply by heating it at 70 °C for 78 h. Two sorbents were prepared, and their characteristics are given in Table 1. Sorbent I represents an optimized sorbent, whereas sorbent II is prepared from an off-theshelf precursor. The synthesis procedure was as follows: The alumina (100-200 mesh) was outgassed for several hours at 200 °C and 300 mesh), and the resulting mixture was thoroughly stirred. The mixture was placed in an open crucible in an oven at 70 °C for 80 h with occasional stirring. Equilibrium Isotherm Measurement. Equilibrium isotherms on sorbent I and its precursor were measured gravimetrically, employing a Cahn system 113 recording microbalance with temperature control. Approximately 10 mg of the sample was loaded into the microbalance and heated to 110 °C in dry helium to remove water from the sample. Any water present in the system would severely limit the performance of the sorbent. The total gas flow rate of the system was kept constant at 170 cm3/min, while the composition of the gas phase was varied using calibrated rotameters. Hydrocarbon/He mixtures were alternated with pure dry helium to check for reversibility of the adsorption. The hydrocarbons used were ethylene (CP, Matheson, minimum purity 99.5%) and acetylene (Matheson, minimum purity 99.6%). The helium gas used was of highpurity grade obtained from Cryogenic Gases with a minimum purity of 99.995%. A 3A zeolite column was used to dry all of the gases used. Except for the acetone isotherm, the isotherms measured were fully reversible within 5% of the maximum adsorbed amounts. Helium was used as the carrier gas and for sorbent regeneration. In the measurement of isotherms, upon each step change in concentration, buoyancy and friction forces were carefully calibrated prior to each measurement.8 Adsorption equilibria on sorbent II and its precursor were measured volumetrically on a Sorptomatic 1990 (Carlo Erba Instruments). The chemisorption version
10.1021/ie000080f CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000
Ind. Eng. Chem. Res., Vol. 39, No. 8, 2000 3109 Table 1. Comparison of Synthesis Parameters in the NiCl2/Al2O3 Sorbents unit place of synthesis γ-alumina manufacturer BET area pore size distribution salt mixing ratio NiCl2/Al2O3 a
m2/g
g/g
sorbent I (data from refs 1 and 2)
sorbent II
The University of Michigan PSD-350 Alcoa Separations Technology Inc. 340 trimodal with a significant percentage of mesopores (>2 nm) NiCl2 (Strem Chemical Inc.) 0.44a
University of Amsterdam CK300 Ketjen (Akzo Nobel) 207 no micropores, mesopores only (max at 5 nm) NiCl2‚6H2O (Aldrich) 0.12
The optimum ratio of salt over substrate was determined by maximizing acetylene over ethylene selectivity.
of the Sorptomatic 1990 contains stainless steel tubing only and a controlled vent from the vacuum system. Therefore, all proper precautions for acetylene handling could be observed. The sample holder was thermostated with a Haake DC 10 thermostat. Around 2 g of the sample was used for the adsorption measurement. To prepare an acetylene source free of acetone for volumetric experiments, acetylene (Praxair, purity > 99.6%) was led over a fully regenerated silica trap at low gas space velocity while the composition of the outgoing gas was monitored by on-line mass spectrometry (MS). Once the regeneration gas was completely removed from the tubing and well before the detection of any acetone, the gas was used to fill a 1.5 L lecture bottle up to 2 bar. Ethylene (Praxair, purity > 99.7%) was used directly from the bottle. Reversibility of the adsorption on the sorbent was checked separately in a thermogravimetric analysis (TGA) setup.
Figure 1. MS spectra of the off gas during regeneration of the SiO2 filter for acetylene purification. A time window of 11 min is shown here. Helium was used as the purge gas.
Results Purification of Acetylene. As mentioned before, acetylene stored in cylinders is commonly dissolved in acetone. Removal of traces of acetone has been described in the literature. It could be removed by distillation in vacuo9 or by scrubbing and subsequent drying.10 We have devoted lots of attention to acetone removal and will show that it can conveniently be removed by adsorption in a SiO2 bed. Also, as mentioned, the concentration of acetone exiting the cylinder changes depending on the pressure in the tank. A SiO2 bed was used in this work to purify the acetylene. In the TGA setup, the acetone concentrations before and after flowing through the silica gel bed were measured to determine the effectiveness of the SiO2 bed. A gas chromatograph equipped with a Porapack Q column with thermal conductivity and flame ionization detectors was used to measure the acetone concentration before and after the bed at 5000 (with a new cylinder at 250 psi) and 6 ppm, respectively. In a separate experiment, the out-flowing regeneration gas from a saturated SiO2 filter was analyzed by MS, and the results are shown in Figure 1. The figure clearly shows an initial release of acetylene (m/e 26) from the interparticle voidage and prolonged desorption of acetone (m/e 43, 58, and 15 in a ratio of 100:24:14). This proves that acetone was accumulating in the filter. To determine the effect of the small concentration of acetone (6 ppm) which could not be removed by the silica gel trap from the acetylene gas, a single component equilibrium isotherm for acetone on sorbent I was measured at 25 °C (Figure 2). As mentioned before, the acetone concentration in the acetylene source without the silica gel bed purification was measured at 5000 ppm (∼0.5 kPa). At this partial pressure, acetone
Figure 2. Single-component equilibrium isotherms for C3H6O (0), C2H2 (b), and C2H4 (2) on sorbent I at 25 °C. The line through the acetone data is a fit with the Langmuir equation.
adsorption would dominate and overwhelm any acetylene adsorption on this material. At this partial pressure, it can be observed from Figure 3 that acetone adsorption would not be significant. Also, because the acetylene uptake was 100% complete in less than 5 min, the cumulative effect of acetone adsorption was also minimized. From the experimental conditions, the maximum amount of acetone in the sorbent was estimated to be less than 2 × 10-6 mmol/g, by assuming that 10% of the acetone present in the gas flow through the TGA was cumulatively adsorbed by the sample over a period of 5 min. Therefore, it is clear that the adsorption data presented here are strictly due to acetylene adsorption on NiCl2/Al2O3. The preferential adsorption of acetone
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Figure 3. Single-component equilibrium isotherms for C2H2 and C2H4 on sorbent I and its Al2O3 precursor at 25 °C. C2H2: 4, Al2O3; 2, NiCl2/Al2O3. C2H4: O, Al2O3; b, NiCl2/Al2O3.
Figure 4. Single-component equilibrium isotherms for C2H2 and C2H4 on sorbent II and its Al2O3 precursor at 25 °C. C2H2: 4, Al2O3; 2, NiCl2/Al2O3. C2H4: O, Al2O3; b, NiCl2/Al2O3.
over acetylene by silica gel is also clearly shown in Figure 1. Adsorption Equilibria. Figures 3 and 4 compare the adsorption selectivity for NiCl2/Al2O3 with the bare Al2O3 support for sorbents I and II, respectively. For sorbent I equilibrium capacities for acetylene and ethylene on the Al2O3 support at 25 °C and 1 atm were established at 0.74 and 0.46 mmol/g, respectively, and on NiCl2/Al2O3 they were at 0.79 and 0.26 mmol/g, respectively. Thus, the pure-component selectivity ratio of adsorption for acetylene over ethylene increases from 1.6 to 3. For sorbent II the increase in selectivity is small. Equilibrium capacities for acetylene and ethylene on the Al2O3 support at 25 °C and 1.1 atm were measured at 0.42 mmol/g (P ) 111.3 kPa) and 0.30 (P ) 112.8 kPa) mmol/g, respectively, and on NiCl2/Al2O3 they were at 0.37 mmol/g (112.6 kPa) and 0.24 mmol/g (110.8 kPa), respectively. Thus, the pure-component selectivity ratio of adsorption for acetylene over ethylene increases from 1.39 to 1.54.
exceeds the bare support for sorbent I. In sorbent II it does not. For both sorbents the increase in selectivity arises mainly from the earlier flattening of the ethylene curve. Therefore, the selectivity rises with pressure in the pressure range studied. The presence of any adsorbed water severely reduced the acetylene adsorption on this sorbent. Acetylene adsorption at 0.2 atm and 25 °C on sorbent I with some adsorbed water and one fully dehydrated was measured at 0.15 and 0.36 mmol/g, respectively. Acetylene adsorption was reduced by about half by the presence of small amounts of surface water. Water is primarily removed during the outgas procedure. In a volumetric setup outgassing is always employed under vacuum of
