(694e) New Application of Ionic Liquids As a Sweep FLUID in Methanol Synthesis in a Membrane Reactor

Authors: 
Zebarjad, F. S., University of Southern California
Hu, S., University of Southern California
Tsotsis, T., University of Southern California
Methanol (MeOH) finds broad use as a transportation fuel and as an important raw material used in the production of other fuels and chemicals. There are several commercial methanol synthesis (MeS) processes (e.g., Lurgi and Haldor-Topsoe), but they are all challenged by low per-pass conversion that necessitates the recycling of the unreacted syngas. This challenge is especially problematic for MeOH production from renewable biomass, where the use of oxygen-blown gasifiers is not economic, and the available syngas has a large nitrogen content. One way to overcome the thermodynamic limitations of the MeS reaction, and thus increase the per-pass conversion is to utilize a membrane reactor (MR). In this study, a novel high-pressure MR is used, in which a membrane with the desired characteristics serves as an interface contactor between the MeS environment in the shell-side and a sweep liquid solvent flow in the membrane permeate-side. Regarding the sweep liquid, the application of an ionic liquid (IL) is investigated in the MR in order to separate methanol during MeS with higher efficiency. The general idea is to increase the rate of conversion of the syngas into MeOH via its in-situ separation through a circulating IL solvent. Specifically, the MeOH synthesized from dilute syngas, typical of that produced from air-blown biomass gasifiers, is selectively removed in-situ via membrane separation, using an IL solvent purge (sweep).

First, in this work, the solubilities of the various components involved during the methanol synthesis process in an IL are measured. These include the so-called syngas components (CO, N2, CO2, and H2) and the condensable methanol and water vapors in order to determine their ideal and real separation selectivities under different temperature and high-pressure conditions relevant to the MeS process. Second, the efficiency of two different sweep liquids have been compared in the MR experiments: a high boiling point (B.P.) petroleum-derived solvent, tetraethylene glycol dimethyl ether (TGDE) and an ionic liquid, 1-Ethyl-3-methylimidazolium tetrafluoroborate ([Emim][BF4]). The choice of the TGDE and the IL as sweep liquids is because MeOH has high solubility in both of them, while the permanent gases like H2 and CO do not. Compared to TGDE, the IL has higher decomposition temperature (400 oC), which implies less demanding downstream processing requirements for the separation of the MeOH from the solvent; its lower vapor pressure means lower solvent loss, and its higher thermal capacity affords a broader range of MeS operating conditions. The TGDE, on the other hand, is at present more readily available and less costly In this study, the conversion and yield of the MeS reaction in a MR using IL as the sweep liquid was measured at different conditions and the results were compared with the MR system employing TGDE as the sweep liquid.

The experimental set-up consists of the syngas delivery system, the MR, the liquid injection system employing a high-pressure HPLC pump, and the gas and liquid analysis sections utilizing GC/TCD and GC/FID instruments. A commercial mesoporous alumina membranes, whose surface is rendered hydrophobic via the application of an appropriate modifying agent, is installed in the reactor. During operation, the syngas mixture is fed into the membrane shell-side that operates under high pressure and temperature, where it contacts the MeS Cu-ZnO-Al2O3 catalyst to convert into MeOH. The sweep liquid is pumped through the membrane tube-side via a HPLC pump to continuously remove in situ the produced MeOH, and thus help increase the conversion and yield of the MeS reaction. Experiments were conducted in a broad range of pressures and temperatures, sweep liquid flow rates and for two different sweep liquids, TGDE and IL. The lab-scale system operates also as a conventional packed-bed reactor (PBR) when the inlet and outlet of the membrane tube-side are closed, so as to compare its behavior to that of the MR.

The experiments in the project, which will be reported at a greater detail at the meeting, investigate the performance of the MR under different pressures, sweep liquid flow rates and syngas feed compositions (expressed in terms of the stoichiometric number SN = (FH2-FCO2)/(FCO+ FCO2) and the carbon factor CF = FCO/(FCO+ FCO2)) and compare the carbon conversion of the MeS reaction in the MR with that of the PBR under the same conditions. For the whole range of pressures and flow rate studied, the IL attains higher conversions than the TGDE, which can be related to its higher methanol solubility. This result is in good agreement with the results of solubility tests which have revealed that methanol has a higher solubility in the IL compared to TGDE.

Constrains with the size of the lab-scale system limit the gains in conversion afforded by the MR system. Companion modeling studies, however, indicate that conversions >90% are readily attainable with the proposed process. Further, economic evaluations indicate that such conversions would allow the proposed process to operate on biomass-derived syngas in an one-pass configuration without recycle of unreacted syngas needed.