Znd. Eng. Chem. Res. 1995,34, 413-415
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Simple Apparatus for the Gravimetric Adsorption of Liquid Vapors on Solid CatalystslAdsorbents Vasant R. Choudhary,* S. Mayadevi, and Anand Pal Singh Chemical Engineering Division, National Chemical Laboratory, Pune 41 1 008, India
A simple apparatus for the measurement of single component adsorption of liquid vapors on solid adsorbentdcatalysts is described. The intracrystalline sorption capacity for liquid n-hexane of silicalite-I measured using the apparatus is in good agreement with that reported earlier. Adsorption isotherm data for the sorption of n-hexane on silicalite and water on NaM were collected using the apparatus. This apparatus provides reliable and accurate adsorption isotherm data for the sorption of liquid vapors on a large and hence representative sample of solid catalysts and absorbents by gravimetric method, and it does not require a high-vacuum system.
Introduction Methods for the measurement of adsorption may be broadly divided into dynamic and static methods. Dynamic methods are exemplified by chromatographic determination of isotherms whereas static measurements are normally made by volumetric or gravimetric techniques. Gravimetric methods are superior to volumetric methods due to inherent advantages like the simplicity and accuracy with which adsorption data could be obtained, particularly for adsorption of liquid vapors. Earlier work on the gravimetric adsorption measurement is based on the McBain sorption balance (McBain and Britton, 1930). There have been several modifications of this balance, and analytical balances have also been modified for gravimetric adsorption measurements (Boehlen and Guyer, 1964; Zuech et al., 1983; Hayhurst and Lee, 1983). In this paper we present a simple apparatus which we have developed for the gravimetric measurement of adsorption isotherms of liquid vapors on solid catalysts and adsorbents.
Experimental Section Apparatus. The schematic representation of the apparatus is given in Figure 1. The liquid adsorbate is taken in a jacketed glass vessel with a rotaflow stopcock. The vapor pressure of the liquid, which is the adsorption pressure, is adjusted by controlling the temperature of the water that is circulated through the jacket, and the liquid is magnetically stirred to facilitate mass transfer. The solid adsorbentkatalyst is taken in an adsorption bulb or cell (made of quartz) of capacity 4 cm3. The adsorption cell is kept inside a movable glass jacket; water/oil is circulated through the jacket. The temperature of adsorption is maintained by controlling the temperature of the liquid in the jacket using a thermostatlcryostat. The temperature of the adsorption cell is measured using a thermometer, as shown in Figure 1. The adsorption cell is connected to the vapor generator, pressure/vacuum gauge (gauge pressure range: -760 Torr to 760 Torr), and the vacuum pump using a manifold (valves FMV-1, BV-1 to BV-41, the arrangement of which is schematically presented in Figure 1. The pressure/vacuum gauge and the valves are made of stainless steel. FMV-1 is a fine metering valve for controlling the vacuum during evacuation. BV-1 to
* To whom correspondence
should be addressed.
BV-4 are a set of four ball valves connected to one another through a 1/8 in. 0.d. stainless steel cross. All the valves have 118 in. Swagelok end connections, and all the connections are made using 118 in. 0.d. stainless steel tubing. BV1, BV2, BV3, BV4, and FMVl are fured on an aluminum panel and enclosed in a metallic casing which is heated to avoid condensation of vapors in the lines. The connecting lines are also heated using heating tapes to a temperature about 30 deg above that used for the evaporation of the liquid adsorbate sample. The liquid vessel is connected to BV-3 using a stainless steel ultra-torr (1/4 in.) Swagelock (1/8 in.) reducing union. The liquid adsorbate can be introduced into the liquid vessel by removing the Teflon plunger of the rotaflow stopcock. The adsorption cell is connected t o BV-4 using a stainless steel ultra-torr (1/4 in.) to Swagelok (118 in.) reducing union and Swagelok double end shutoff quick-connect system. The connection through quick connect permits the easy attachment/ detachment of the adsorbent sample cell to the unit and the cell is automatically sealed off when detached from the unit. Materials. Commercial grade synthetic sodium mordenite (NaM, Si/Al = 5.5) of particle size 20-50 mesh was obtained from Norton Co., USA. Pentasil silicalite (silicalite-I) was synthesized from a gel composition of SiOz:H20:TPAOH (tetrapropylammonium hydroxide) = 6.4:76:1 by hydrothermal treatment under autogeneous pressure at 443 K for 10 days. The crystals were filtered, washed throughly with distilled-deionized water, dried in an air oven at 393 K or 24 h, calcined at 813 K for 12 h in a flow of nitrogen, and characterized by X-ray diffraction (XRD). It was further treated with 0.5M HC1, washed and heated at 813 K for 12 hours before use. n-Hexane of high purity (> 99%) was obtained from High Purity Chemicals Pvt. Ltd., India. Procedure. The adsorption measurement using the apparatus is carried out as follows. About 2 g of the adsorbent sample is accurately weighed into the adsorption cell, and it is attached to the apparatus. The valve BV3 is closed and the entire unit up to BV3 is evacuated, by gradually opening FMVl so as to avoid the carry-over of solid particles. The adsorbent in the adsorption cell is pretreated i n situ at 673 K for 2 h under vacuum (0.005 Torr) using a movable furnace in place of the glass jacket. When the pretreatment is completed, BV-1, BV-4, and FMV-1 are closed, and the adsorbent cell is cooled, it is detached from the unit, and its weight is taken. The difference between this
0888-5885/95/2634-0413~09.~0l0 0 1995 American Chemical Society
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Ind. Eng. Chem. Res., Vol. 34, No. 1, 1995
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Sample liquid -Refrigerant cryostat
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Figure 1. Experimental setup for the measurement of adsorption isotherms of vapors (BV,ball valve; FMV, fine metering valve; and RFS, rotaflow stopcock).
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in the cell are carried away along with the liquid vapors during evacuation even a t the low vacuum (0.005 Torr) used in the present work, thus avoiding the necessity of a high vacuum generally required for adsorption studies. For the collection of adsorption data, the adsorption pressure at each point is decided by the vapor pressure of the liquid in the liquid vessel and is changed by changing the temperature of the refrigerent mixture circulating through the outer jacket of the vessel. The refrigerant mixture of the required temperature is circulated through the outer jacket of the liquid vessel, the stopcock RFS and valve BV-2 are opened, and the vapor pressure of the liquid is read on the pressure gauge. BV-4 is then opened and the adsorbent is exposed to the vapor for a period sufficient to establish the adsorption equilibrium. BV-4 is then closed. The cell is allowed to attain room temperature and then detached from the unit for direct weighing. The increase in weight, after correcting for the pressure of adsorbate in the bulk vapor phase, gives the amount of vapor adsorbed at the adsorption pressure and temperature. The experiment is then repeated for collecting isotherm data at different pressures, after changing the temperature of the refrigerant mixture circulating through the jacket of the liquid vessel. The weight of the adsorption cell was measured using an electronic analytical balance with a sensitivity of 0.0001 g.
Results and Discussion As a test for the validity of the data collected using this apparatus, the intracrystalline sorption capacity for liquid n-hexane of silicalite-I (crystal size 2-3 pm, Si/ Al ratio 10 000 at 305 K and PIP0 = 0.4 was measured. It was found to be 0.195 cm3g-l(1.49 mmol g-l), which is in very good agreement with that (0.199 cm3 g-l) reported earlier (Olson et al., 1980). Adsorption isotherm data were collected for the sorption of n-hexane on silicalite-I and n-hexane and water
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similar purpose because of the direct weighing of the adsorption cell. The data collection is not time-consuming, but for the time required for the initial pretreatment of the adsorbent and the evacuation of the lines. For those systems which equilibrate quickly, each data point can be measured within a period of 1 h. The accuracy of the data collected depends on the volume of vapor space in the cell and the amount adsorbed. It is advisable to reduce the vapor space in the cell so that the amount of adsorbate in the space above the adsorbent is negligibly small compared to the amount adsorbed a t the pressure and temperature of measurement. The accuracy of adsorption measurement is also related to the accuracy of weighing and change in weight due to adsorption. Since a large representative sample is used for making the measurements, accurate adsorption data (with
