Phase behaviour studies related to biodiesel production using supercritical methanol
Creators: name: Al-Habsi, Sultan
Publisher: name: Imperial College London
Year: 2019

description: Biodiesel is a promising renewable and sustainable fuel that can replace fossil fuels. Among the different techniques used to produce biodiesel, the transesterification process is currently the preferred method. The conventional transesterification process is based on acid-base catalysis, but this technique has many drawbacks including a requirement for high-purity feedstocks, and costly pre-treatment and downstream processes. A recent alternative process, using a supercritical alcohol (preferably methanol) without a catalyst, may offer some advantages. This process can utilise a wide range of potential feedstocks (especially wastes), shows high production efficiency, and requires only simple post-processing. However, this technique requires conditions of high temperature and high pressure which increase the utility costs and may restrict the economic feasibility and sustainability of the process. In order to fully explore these issues and to optimise the process conditions, better understanding of the phase behaviour of the mixtures involved in the biodiesel process is required. The components of interest include fatty acids, esters alcohols and co-solvent such as carbon dioxide and the conditions include high pressures and wide ranges of temperature. Phase equilibrium studies on systems relevant to biodiesel production with supercritical methanol available in the literature are very limited. The principal focus of this project is the experimental investigation of the phase behaviour of representative mixtures with small molecular chains, which exist during biodiesel production, over wide ranges of temperatures and pressures. In addition to the experimental work, the research will include both modelling works on the mixtures of interest supported by a simulation for the process using gPROMS, a simulation tool developed by Process Systems Enterprise (PSE) company. In this project, new fluid-phase equilibrium measurements have been carried out on two relevant representative binary systems: (methyl propanoate + carbon dioxide) and (butanoic acid + carbon dioxide) using a high-pressure quasi-static analytical apparatus with compositional analysis using a gas chromatography. The measurements for the (methyl propanoate + carbon dioxide) mixture were made along six isotherms at temperatures from (298.15 to 423.15) K and at pressures up to near the mixture critical pressure at each temperature while for the mixture (butanoic acid + carbon dioxide) the measurements were made along eight isotherms at temperatures from (323.13 to 423.2) K and pressures up to the mixture critical pressures. Vapour-liquid equilibrium (VLE) data obtained for the mixtures have been compared with the predictions of SAFT- Mie model, a group-contribution version of the Statistical Associating Fluid Theory (SAFT). The group interaction parameters in SAFT- Mie reported in literature have been revised by fitting to the new experimental VLE data. After parameters optimisation, the model was found to be in a good agreement with the measured VLE data for both bubble and dew points. The experimental data were also compared with the description of Peng Robinson equation of state (PR EoS) combined with the classical one-fluid mixing rules integrating one temperature-independent binary interaction parameter for (methyl propanoate + carbon dioxide) system and two temperature-independent binary interaction parameters for (butanoic acid + carbon dioxide) system. The results after tuning show that the PR EoS can also predict well the system measured data, except in the critical regions in which PR EoS shows overprediction. Furthermore, the phase equilibria of (methyl propanoate + propionic acid + carbon dioxide), (tert-butanol + water + carbon dioxide) and (toluene + water + carbon dioxide) ternary systems were studied by the means of the high-pressure quasi-static analytical apparatus. Compositions of present phases coexisting in vapour-liquid equilibrium (VLE) for (methyl propanoate + propionic acid + carbon dioxide) mixture were measured along six isotherms at temperatures from (323.12 to 423.11) K and pressures from (1 to 20) MPa at equal feed molar ratio of (methyl propanoate + propionic acid). Phase behaviour measurements were also collected at different compositions of the mixture (methyl propanoate + propionic acid) at fixed temperatures and pressures. Compositions of coexisting phases of the ternary system (tert-butanol + carbon dioxide + water) have been obtained along five isotherms at temperatures of (283.2, 298.18, 323.13, 373.10 and 423.17) K and at pressures of (4.0, 8.0, 12.0 and 18.0) MPa with different known feed compositions of (tert-butanol + water) while the phase behaviour of the system (toluene + water + carbon dioxide) was investigated along four isotherms at temperatures from (338.15 to 413.15) K and pressures up to the upper critical end point (UCEP). The data obtained for the ternary mixtures have been compared with the descriptions of SAFT- Mie and PR equation of states. Other cross interactions available in biodiesel systems such as (COOH - CH3OH), (OH_Gl - CH3OH), (CO2 - CH=), (CH3OH - CH=), (COOH - CH=) and (H2O - CH=) were estimated in this work by regression to fluid-phase behaviour data published in literature. The comparison between the predictions of SAFT- Mie reported in literature and those of SAFT- Mie after refining the parameters were shown. Preliminary designs of one-step process (transesterification) and two-step processes (hydrolysis and esterification) for biodiesel production under supercritical conditions were suggested and simulated using gPROMS ProcessBuilder software. The CO2 co-solvent effect on the one-step process based on literature data was also examined by a process flowsheet. The research including new phase behaviour measurements, modelling and gPROMS simulation is expected to contribute to optimisation of biodiesel production processes.