(178e) Use of Molecular Sieves to Remove H2S and CO2 from Landfill Gas, Producing a High-Energy Content Methane Stream

Pressure swing adsorption experiments were carried out to evaluate the potential of the process in separating landfill gas towards a high-energy content methane stream and a carbon dioxide stream of increased H2S concentration. The solid adsorbents used for this purpose were 4A, 5A, and 13X molecular sieves. The adsorption pressure and H2S feed concentration were varied in each run to determine their effect on the PSA process efficiency. The experimental results indicate that molecular sieves 13X and 5A are capable of producing high purity methane (98% or more) of zero or nearly zero H2S concentration, for short periods of time. This is a very promising result that can be utilized in rapid-PSA system design for commercial application of the separation process. The average H2S concentration in the accumulated desorption product obtained with zeolite 4A is found higher than that in the adsorption feed by one order of magnitude (2.9 % vs. 0.3%). This is another promising result, as it supports the potential of a multi-stage PSA system in providing the FSEC photolytic hydrogen production process with the required high concentration H2S feed stream.

High methane recovery rates were achieved in most adsorption experiments, averaging at 60-70% for all sieves, and topped by 100% in certain 13X runs. Similarly, high H2S recovery rates were typically achieved in desorption experiments, averaging at 72% with sieve 4A, and reaching 100% in some 13X and 4A runs. The true utilized sieve capacity was of the order of 17-24% for CO2 and 0.1-0.5% for H2S, indicating significant deviation from equilibrium, as is typical in PSA systems. No significant pressure effects were encountered, as the pressure was found to affect mostly secondary performance measures. All observed thermal effects were in accordance with theoretical predictions and earlier experience, as were also the effects of feed flowrate and cycle time on adsorption product concentration.