Adsorption Of Normal Paraffins And Sulfur Compounds On Activated Carbon
Authors: Grant Richard J. , Manes Milton , Smith Stanton B. 
Published: December 2007

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Adsorption isotherms on activated carbon were determined gravimetrically for normal C1 and C3 - C6 paraffins and subsequently for methyl, ethyl, and n-propyl mercaptans, H2S, COS, and CS2, at pressures up to 1 atmosphere and from -23 to 100C. Orthobaric liquid densities at low temperatures of some of the sulfur gases were determined pycnometrically. The adsorption data, on correlation by the method of Lewis, Gilliland, Chertow, and Cadogan, gave two separate curves, one for the paraffins, one for the sulfur compounds. The results show the correlation to be useful for estimating the adsorptive capacity of chemically similar compounds from a minimum of experimental data.

Introduction

The purpose of this work was to determine in detail the isotherms of individual normal paraffins on a single granular activated carbon adsorbent and, if possible, to correlate the isotherms by the method of Lewis, et al. When isotherms of methane and n-butane were determined initially on a conventional volumetric adsorption apparatus, and the results correlated, the n-butane isotherms diverged in the direction of lower capacity, in a manner similar to that previously noted by Walters. The effect we observed could be explained by solubility of n-butane in the stopcock grease. A gravimetric adsorption apparatus using me McBain balance principle was therefore constructed; it gave consistent isotherms of methane and of propane to hexane. Subsequently, for operating convenience, a completely grease-free gravimetric system was constructed and used to determine the adsorption isotherms of a series of sulfur-containing compounds. The results were treated in the same fashion as the hydrocarbon data.

EXPERIMENTAL

Apparatus

A schematic diagram of the McBain adsorption balance finally adopted is shown in Figure 1. Stainless steel valves (G. W. Dahl, Inc.) with Teflon seats were interconnected by stainless steel tubing. (The original all-glass adsorption balance was similar except that stopcocks were used in place of stainless steel valves.) The Pyrex pipe adsorption chambers enclosed quartz helical springs and buckets (Worden Laboratories, Houston, Texas). Spring extensions and mercury manometer pressure readings were measured with a Gaertner cathetometer to 0.05 mm. Low pressures were measured by Pirani gauge and pressures above 1 mm. Hg by mercury manometer. Adsorption temperatures were controlled by constant-temperature baths surrounding the adsorption chambers.

Materials

Matheson Company C. P. grade methane, instrument grade propane and n- butane, and Phillips Petroleum Company pure grade n-pentane and n-hexane were used without further purification. Methane, n-pentane and n-hexane were 99.0% minimum purity; propane and n-butane were 99.9% minimum purity. Matheson Company purified grade hydrogen sulfide (99.5% minimum purity) carbonyl sulfide (96% minimum purity, liquid phase) and methyl mercaptan (99.0% minimum purity) were used. By gas chromatographic analysis, these gases were 97.1%, 96.8% and 96.9% pure respectively. Ethyl mercaptan and n-propyl mercaptan came from Eastman Organic Chemicals and analyzed 98.7% pure by gas chromatography. Carbon disulfide was Mallinckrodt A. R. grade. The gases were condensed at -78C. (dry ice-acetone) and evacuated to constant vapor pressure to remove non-condensables. The remaining liquid was evaporated into 5-liter gas reservoirs. The three liquids were also evacuated at dry ice-acetone temperature to constant vapor pressure and sampled directly.

Adsorbents

Some properties of the two Pittsburgh Chemical Company coal-base (4x10 mesh) activated carbons (designated BPL) are shown in Table I. They differed essentially only in ash content, the higher ash content carbon on which the hydrocarbon data were determined now being obsolete. Helium and mercury densities of the carbons were determined by displacement according to the method of Juhola.