Ind. Eng. Chem. Res. 1987,26, 928-933
928
Modern Version of Volumetric Apparatus for Measuring Gas-Solid Equilibrium Data Bal K. Kaul Exron Research and Engineering Company, Florham Park, New Jersey 07932
This paper describes a modern version of the volumetric apparatus which was used for measuring gas-solid equilibrium data. It also presents adsorption data for methane, ethane, ethylene, and propane a t 38 and 149 “C on Kureha beads (GP Grade), an attrition-resistant activated carbon. The new apparatus was validated with literature data for the ethane-ethylene system on a molecular sieve (13X), and excellent agreement was obtained. The vacancy solution model was used for correlating pure-component isotherms, and excellent agreement was obtained. This paper describes a modern version of a volumetric apparatus for measuring adsorption equilibrium data for pure or mixed gases. The new volumetric apparatus was validated with the literature data (Danner and Choi, 1978) for the ethane-ethylene system on a molecular sieve (13X). This paper also presents new adsorption data for light hydrocarbon gases on Kureha beads. Kureha beads, an attrition-resistant activated carbon (supplied by Taiyo Kaken Co., Japan), are spherical carbon beads with an average size of 0.45 mm and surface area of 800-1200 m2/g as reported by the manufacturer. The gas-phase (GP) grade of Kureha beads has been used for the measurements reported in this paper. The vacancy solution model (Suwanayuen and Danner, 1980) was used for correlating pure-component isotherms. Some of the novel features of the apparatus are its direct measurement of pressure and its wide range of operating conditions (pressures to 1000 psia and temperatures up to 1100 OF). Because of these novel features, fast and accurate measurements can be obtained from the newly built apparatus as compared to the several previous generations of the volumetric apparatus. For pure-gas isotherm measurements, two data points can be obtained in 1h. For mixtures, the data rate at 150 “C with the ethane-ethylene system is 5 data points in 8 h. Adsorption equilibrium data provide the capacity and the selectivity of an adsorbent needed for the selection and the design of adsorption processes. Most adsorption processes are cyclic and require the desorption or regeneration of the adsorbent by temperature or pressure swings or by replacement by another species. Calculations for the time required (cycle time) for adsorption and desorption modes of the process use adsorption thermodynamic equations obtained from the equilibrium data, in addition to rate data. Development of these thermodynamic equations also allows correlation and prediction of mixture data which minimizes the number of experiments needed to define the gas-solid equilibrium (Kaul, 1984). Before we describe our apparatus, a brief description of different methods for measuring adsorption equilibrium data is presented. Volumetric Method Providing Reliable Gas-Solid Equilibrium Measurements There are three types of experimental techniques used for measuring gas-solid equilibrium data: gravimetric, volumetric, and dynamic. The volumetric and gravimetric apparatus are static types; that is, the adsorbate equilibrates with the adsorbent in a closed system. The number of moles adsorbed is found either by weight measurements (gravimetric) or by pressure, volume, and temperature measurements (volumetric). In the dynamic type of experiment, a gas chromatograph is used for measuring 0SSS-~SS5/S7/2626-0928$01.50/0
equilibrium data either by frontal analysis or by perturbation analysis. In the frontal analysis, the slopes of the frontal portion of a GC peak are analyzed and the adsorption isotherm is calculated from them. In the perturbation analysis, an elution peak is obtained for data analysis either by perturbing the concentration of a component or by introducing a radioactive tracer component. The volumetric method is most reliable for measuring gas-solid equilibrium data for pure and mixed gases. In fact, most of the data reported in the literature have been obtained by volumetric methods. A gravimetric method is fast, accurate, and can be easily used over a wide range of pressures and temperatures for pure gases. However, this method is not adequate for gas mixtures since the composition of the adsorbed phase cannot be determined from weight measurements alone. A chromatographic method is also fast, but interpretation of the chromatographic data can be difficult. Considerable disagreement between the data taken by the static and the chromatographic methods has been reported (Al-Ameeri, 1979). Limited mixture data reported by using radioactive tracer components show good agreement with the static methods, but the unavailability of the radioactive isotopes limits this method to only a few systems (Hyan and Danner, 1982). The use of the volumetric method for measuring gassolid equilibrium data has been reported in the past by several authors (Ray and Box, 1950; Danner and Wenzel, 1969; Reich et al., 1980), but measurements were tedious and time-consuming, especially for mixtures. With the availability of microprocessor-based products such as pressure transducers, a new type of the volumetric apparatus has been built. There are two novel features of this “modern” version of the volumetric apparatus. Direct measurement of pressure results in fast data acquisition. In 8 h, either 15 equilibrium data points can be obtained for pure components or 5 data points for mixtures. A wide range of operating conditions (pressures to 1000 psia and temperatures to 1100 OF) provides data at useful process conditions (Kaul and Sweed, 1983). Description of the New Volumetric Apparatus Figure 1shows different components of the new volumetric apparatus. These fall into five major categories. The detailed description for each category is as follows. Figure 2 shows the flow diagram of the apparatus, depicting the use of each of these components. I. Adsorption Cell and Temperature Measurement and Control. The adsorption cell, a l-in.-0.d. X 8-in. pipe nipple with retainer screens to support the adsorbent, is kept in a constant temperature sand bath. The sand is fluidized with air. The sand bath has a peripheral extractor which prevents the escape of the dust particles into the air. The bath can be used from 50 to 1100 OF, and the 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 5,1987 929
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Figure 1. Components of the new volumetric apparatus for singlecomponent and multicomponent isotherm measurements.
Table I. Temperature and Pressure Limits on Various Items of the Volumetric Apparatus item temp and pressure limits high-temp sand bath 50-1100 O F digital temp gauge 0-2000 O F vacuum pump thermocouple vacuum gauge 250 OF digital pressure transducers, 0-2000 and 0-50 psia ranges Heise pressure gauges 15-150 OF transmitter (0-1000 psia) transmitter (0-25 psia) receiver volumetric micrometer -65 to 180 OF 0-1000 psia thermal conductivity cell 392 OF, 1000 psia recirculating pump -60 to 350 O F 0-12000 psia V1EYYnl~
Y
:w
Figure 2. Flow diagram of the volumetric apparatus.
Figure 3. Schematic of the volumetric apparatus.
uniformity and the stability of temperature within the bath is f l O F . A thermocouple-actuated digital indicator is used for measuring temperature. 11. Vacuum System. The vacuum system of the apparatus is used whiIe regenerating the adsorbent and for isotherm measurements under subatmospheric conditions. The main components of the vacuum system are a vacuum pump with an ultimate pressure of 0.1 mmHg and a thermocouple vacuum gauge. The gauge provides a readout of vacuum in the system and is also used for checking leaks in the system. 111. Pressure Measurement. The components of the pressure measurement system are pressure transducers and digital Heise gauges. The pressure readout accuracy of the transducers is 0.05% of the full scale range. The transducers are used for direct measurement of pressure in the system. The calibration of the transducers is checked periodically against the digital Heise gauges. IV. Volumetric Micrometer. The micrometer (supplied by Volumetrics Company, Paso Robles, CA) allows precise volumetric changes to be made in the gas volume. The micrometer can displace a total volume of 12.756 in3; each turn of the micrometer provides a displacement volume of 0.3774 in2. The volumetric micrometer in the apparatus serves three important functions. First, with an inert gas such as helium, the volume and pressure of the gas in the system can be varied without changing the number of moles, and the dead volume of the system can be calculated as outlined below. Second, while taking measurements with a pure gas, the pressure can be changed rapidly without introducing any additional gas
into the system. Third, change of the system pressure with a micrometer aids in maintaining constant pressure for taking mixture data, thereby making data analysis easier. V. Additional Items for Measuring Mixture Isotherms. The additional items of the apparatus used for measuring mixture isotherms are a thermal conductivity cell, a recirculating pump, and a gas chromatograph. The thermal conductivity cell, which has a pressure rating of 1000 psia, monitors the composition of the gas going into and coming from the adsorption cell. Equilibrium of the gas mixture with the adsorbent is indicated when the thermal conductivity cell shows a null reading on a recorder. A Hewlett-Packard gas chromatograph (Model HP-5880A) is used to analyze the gas mixture in the system before and after equilibrium has been attained. Pressure and Temperature Limitations of the Apparatus Table I provides the pressure and temperature limitations on various items of the apparatus. The volumetric micrometer and the thermal conductivity cell have a pressure rating of 1000 psia, which limits the use of apparatus to less than 1000 psia. As mentioned earlier, the high temperature limit of the sand bath containing the adporption cell is 1100 OF. The high temperature limit for the recirculation loop, which is outside the sand bath, is 180 O F . The unit therefore has a limitation that the gases should not condense at the temperature of the recirculation loop. Experimental Procedure The experimental procedure, explained with the simplified schematics of the apparatus (Figure 3), for meas-
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Table 11. Adsorption Isotherm Data for Ethylene on Molecular Sieve (13X) at 50 O C Vad~d,~ P," V a d ~ d , ~ P," P," Vadsd,b Dsia cm3 (STP)/K Dsia cm3 (STP)/p Dsia cm3 (STP)/p 0.04 0.04 0.15 0.64 1.49 1.76 4.07 4.66 5.27
2.55 2.57 5.51 15.71 29.40 29.98 41.76 43.47 44.37
P = pressure.
6.29 7.98 16.01 18.97 21.28 22.98 26.35 28.55 32.72
47.57 49.59 55.91 57.36 58.88 58.94 60.04 59.23 61.18
35.22 37.7 40.06 43.61 50.2 59.1 59.6 67.7 79.3
61.37 61.54 62.68 62.55 63.40 65.71 64.32 65.64 67.11
* V adsd = volume adsorbed.
50
Pressure, psia
Figure 4. Adsorption isotherms for ethylene on molecular sieve (13X) at different temperatures.
Table 111. Adsorption Isotherms for Ethylene on Molecular Sieve (13X) at 100 and 150 OC P," V a d ~ d , ~ P," Vad~d,~ V a d ~ d , ~ P," psia cm3 (STP)/g psia cm3 (STP)/g psia cm3 (STP)/g Temperature = 100 "C ~
uring adsorption isotherm of pure and mixed gases is as follows. Adsorbent of a known weight (moisture- and contaminant-free basis) is charged into the adsorption cell and is activated at high temperature and under high vacuum. The adsorption cell is then isolated from the rest of the system. Before any adsorption measurements, the volumes of the recirculation loop and the adsorption cell loop are obtained by using helium. These volumes are used in calculating the adsorption isotherms. To find the volume of the recirculation loop, helium gas is introduced into the recirculation loop and its pressure, P,, and the reading of the volumetric micrometer, V,, are noted. Then the volume of helium in the recirculation loop is changed by using the volumetric micrometer. The pressure, P2, and the micrometer reading, V2, are again read. The volume, VD, of the recirculation loop, excluding the micrometer, is calculated by
A similar procedure is used for finding the total volume of the system, recirculation loop plus the adsorption cell loop, excluding the micrometer. For single-component measurements, pure gas is introduced into the recirculation loop and its pressure and temperature are measured. These measurements provide the initial number of moles of the gas in the loop. The gas is then allowed to contact the adsorbent by opening the two three-way valves. The pressure and temperature are measured after equilibrium has been achieved, and the number of moles remaining in the gas phase is calculated. The number of moles adsorbed is then obtained by difference. A similar procedure is used for gas mixtures, but in addition, the final equilibrium composition of the gas phase is obtained with an on-line gas chromatograph. Results
To validate the unit, isotherms were measured for ethane, ethylene, and their mixtures on Union Carbide's molecular sieve 13X adsorbent (1/16-in.pellets). Figure 4 is a plot of the volume of ethylene a t STP adsorbed per gram of adsorbent vs. pressure. The figure shows excellent agreement (
