Trace Adsorption of Light Hydrocarbons at Low Temperatures: Influence of Carrier Gas Coadsorption

Abstract

During the adsorptive separation of light hydrocarbons in the trace range, significant coadsorption of the carrier gas may occur due to low temperatures and much higher concentrations of the carrier gas compared to the adsorptive. This results in a reduction of the adsorbent's capacity, which leads to problems in industrial adsorber design and an incorrect interpretation of adsorption mechanisms. So far, there are only few studies on the carrier gas influence, and no data exist for low temperatures. In this work, we present experimental adsorption isotherms of methane and ethane from nitrogen and helium at temperatures between −80 and +20 °C on activated carbon and zeolite 13X. A comparison of the adsorption isotherms and the isosteric adsorption enthalpy reveals that coadsorption increases strongly with low temperatures, low partial pressures, and weak interactions between adsorptive and adsorbent. Finally, coadsorption is shown to be stronger on activated carbon than on zeolite 13X.

Adsorption of Light Alkanes and Alkenes on Activated Carbon and Zeolite 13X at Low Temperatures

“…Lowering the temperature below ambient temperature by integrating energy into the process can significantly increase the efficiency of an adsorber bed. This is especially beneficial for short-chain adsorptives, as shown in the authors’ preliminary work. − …”

“…Interfering effects resulting from coadsorption of the carrier gas nitrogen can be excluded for the adsorptives studied in this work. It was shown in a previous work of the authors that the carrier gas has no effect on the adsorption of hydrocarbon molecules with a chain length of C 2 and larger . Thus, coadsorption of the carrier gas is not relevant here.…”

“…It was shown in a previous work of the authors that the carrier gas has no effect on the adsorption of hydrocarbon molecules with a chain length of C 2 and larger. 6 Thus, coadsorption of the carrier gas is not relevant here.…”

“…The adsorbents were already characterized in previous studies. − Table lists the results of nitrogen sorption measurements at 77 K. The Brunauer–Emmett–Teller (BET) method (DIN ISO 9277) was used for calculating the inner surface area. The pore and micropore volumes were determined according to the Gurvich rule at p / p 0 = 0.98 and Dubinin–Radushkevich rule (DIN 66135-3).…”

“…Scheme displays a schematic flow diagram of the experimental plant, which has already been described in detail in previous publications. − In a gas mixing chamber (1) a defined gas mixture of carrier gas and adsorptive is produced with mass flow controllers 2–4, which are located in a tempered box. The process temperature is adjusted with a cryogenic circulation thermostat (2).…”