Phase equilibria for biomass conversion processes in subcritical and supercritical water

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Abstract

The description of phase equilibria for two biomass conversion processes, the hydrothermal upgrading (HTU) process and supercritical water gasification (SCWG) process, has been carried out. The HTU process is a liquefaction process under subcritical water conditions, the product contains biocrude, organic compounds, gases, and water. In the SCWG process, the product is fuel gas containing more than 50% hydrogen on a mole basis. Biocrude is the target product in HTU, and hydrogen in SCWG. The description of phase equilibria indicates the possible routes and operating conditions for separating the target product from the product mixture. For the HTU process, the task has been accomplished by properly characterizing biocrude and the application of the Statistical Associating Fluid Theory (SAFT) equation of state. The calculated result for biocrude separation is in good agreement with the experimental data. In the SCWG process, for the removal of CO2 from gas product to produce higher purity hydrogen, four equations of state of PSRK, PR, SRK, and SAFT have been applied to calculate the phase equilibria. Water and 1-hexanol are the solvents for dissolving CO2. The amounts of solvent required have been indicated for achieving certain hydrogen purity in the vapor phase. The predicted comparison results show that 1-hexanol is a better solvent than water. Using the weight amount of one-tenth of water, 1-hexanol can make higher or comparable hydrogen purity in the vapor phase and less hydrogen dissolved in the liquid phase.

Introduction

The biomass fuels are potential substitutes for fossil fuels. Biomass conversion processes, pyrolysis, gasification, and liquefaction, are under development. In this work, we focus on the phase behavior and phase equilibria for hydrothermal upgrading (HTU) [1], [2] and supercritical water gasification (SCWG) [3], [4] processes.

The HTU process is a promising liquefaction process, it can be used for the conversion of a broad range of biomass feedstocks, which is an advantageous characteristic. The process is especially designed for wet materials, the drying of feedstock is not necessary. Treated in water in a temperature range of 300–350 °C and a pressure range of 120–180 bar, biomass is depolymerized to a hydrophobic liquid product so-called “biocrude”. Gases are also produced, consisting of CO2, H2, methane and CO. Other product includes water and organic compounds.

In SCWG, the reaction generally takes place at the temperature over 600 °C and a pressure higher than the critical point of water. With temperature higher than 600 °C, water becomes a strong oxidant, and oxygen in water can be transferred to the carbon atoms of the biomass. As a result of the high density, carbon is preferentially oxidized into CO2 but also low concentrations of CO are formed. The hydrogen atoms of water and of the biomass are set free and form H2. The gas product consists of hydrogen more than 50% on a mole basis, CO2 in second quantity of about 33%, and others including CH4 and CO.

In both the processes, the target products of biocrude and hydrogen need to be separated from the product mixture. Thermodynamic models of Statistical Associating Fluid Theory (SAFT) [5], [6], PSRK [7], PR [8], and SRK [9] have been applied to model the phase equilibria for the reactor and separation units.

Section snippets

The HTU process

Goudriaan et al. [1], [2], [10] gave detailed descriptions of the HTU processes for the conversion of biomass to biocrude. The conversion reaction generally takes place in aqueous environment in the temperature range of 300–350 °C and a pressure range of 120–180 bar. The HTU process mainly consists of pre-treatment, reactor, high-pressure separator, low-pressure separator, and upgrading units, as shown in Fig. 1. In this work, the reaction temperature and pressure are specified as 330 °C and 180 

SCWG process

By treatment of biomass in supercritical water, biomass can be converted into fuel gases, which are very rich in hydrogen. As shown in Fig. 9, the pilot plant with a capacity of up to 10 kg feedstock per hour at the University of Twente (Netherlands), is taken as the reference for the scheme of the supercritical water gasification (SCWG) process. The SCWG process consists of main units as feed pumping, heat exchanger (HE), reactor, high-pressure gas–liquid separator (S1), low-pressure gas–liquid

Conclusions

The description of phase equilibria for two biomass conversion processes has been carried out. For the HTU process, by properly characterizing biocrude produced and the application of the SAFT equation of state, the described phase behavior is in accordance with the real case, and the result of simulated biocrude separation is in good agreement with the data of a pilot plant.

For the SCWG process, four models have been applied to predicate the results of phase equilibria for the removal CO2 by

Acknowledgements

We acknowledge financial support of the Netherlands Science Foundation (NWO) within the scientific cooperation project between The Netherlands and Japan on biomass conversion, Dr. Frans Goudriaan for providing information of the HTU process. WF thanks Dr. Th.W. de Loos for the help of the calculation programme.

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