**Theoretical and Economic Evaluation of Low-Cost Deep Eutectic Solvents for E**ff**ective Biogas Upgrading to Bio-Methane**

#### **Edyta Słupek, Patrycja Mako´s \* and Jacek G ˛ebicki**

Department of Process Engineering and Chemical Technology, Faculty of Chemistry, Gdansk University of Technology, G. Narutowicza St. 11/12, 80-233 Gdansk, Poland; edyta.slupek@pg.edu.pl (E.S.); jacek.gebicki@pg.edu.pl (J.G.)

**\*** Correspondence: patrycja.makos@pg.edu.pl; Tel.: +48-508997100

Received: 29 May 2020; Accepted: 28 June 2020; Published: 1 July 2020

**Abstract:** This paper presents the theoretical screening of 23 low-cost deep eutectic solvents (DESs) as absorbents for effective removal of the main impurities from biogas streams using a conductor-like screening model for real solvents (COSMO-RS). Based on thermodynamic parameters, i.e., the activity coefficient, excess enthalpy, and Henry's constant, two DESs composed of choline chloride: urea in a 1:2 molar ratio (ChCl:U 1:2), and choline chloride: oxalic acid in a 1:2 molar ratio (ChCl:OA 1:2) were selected as the most effective absorbents. The σ-profile and σ-potential were used in order to explain the mechanism of the absorptive removal of CO2, H2S, and siloxanes from a biogas stream. In addition, an economic analysis was prepared to demonstrate the competitiveness of new DESs in the sorbents market. The unit cost of 1 m<sup>3</sup> of pure bio-methane was estimated to be in the range of 0.35–0.37 EUR, which is comparable to currently used technologies.

**Keywords:** biogas; deep eutectic solvents; upgrading; absorption; COSMO-RS; economic analysis
