Hydrate-based removal of carbon dioxide and hydrogen sulphide from biogas mixtures: Experimental investigation and energy evaluations

Abstract This paper presents an experimental study on the application of gas hydrate technology to biogas upgrading. Since CH 4 , CO 2 and H 2 S form hydrates at quite different thermodynamic conditions, the capture of CO 2 and H 2 S by means of gas hydrate crystallization appears to be a viable technological alternative for their removal from biogas streams. Nevertheless, hydrate-based biogas upgrading has been poorly investigated. Works found in literature are mainly at a laboratory scale and concern with thermodynamic and kinetic fundamental studies. The experimental campaign was carried out with an up-scaled apparatus, in which hydrates are produced in a rapid manner, with hydrate formation times of few minutes. Two types of mixtures were used: a CH 4 /CO 2 mixture and a CH 4 /CO 2 /H 2 S mixture. The objective of the investigation is to evaluate the selectivity and the separation efficiency of the process and the role of hydrogen sulphide in the hydrate equilibrium. Results show that H 2 S can be captured along with CO 2 in the same process. The maximum value of the separation factor, defined as the ratio between the number of moles of CO 2 and the number of moles of CH 4 removed from the gas phase, is 11. In the gas phase, a reduction of CO 2 of 24.5% in volume is achievable in 30 min. Energy costs of a real 30-min separation process, carried out in the experimental campaign, are evaluated and compared with those obtained from theoretical calculations. Some aspects for technology improvement are discussed.