(198e) Integrated Thermochemical Process Design for Co-Producing Liquid Fuels and Propylene

Biorenewable value-added products are regarded as promising near-term solutions to increasing environmental requirements and strong dependence on imported petroleum. During the past two decades, biochemical, thermochemical, and catalytic routes for biomass conversion have gained significant attention. For the thermochemical routes, gasification and fast pyrolysis are two potential technologies that offer high efficiencies for the production of liquid transportation fuels and chemicals [1,2]. The gasification method first converts biomass to syngas which will be further upgraded towards liquid fuels/chemicals via Fischer-Tropsch synthesis or methanol synthesis, while the fast pyrolysis method converts biomass to bio-oils which are then converted to gasoline/diesel through hydrotreating and hydrocracking.

In this presentation we systematically design and optimize an integrated gasification-fast pyrolysis based biorefinery process. In addition to the production of liquid transportation fuels including gasoline and diesel, propylene is also considered as a co-product. In the designed process, the heat for the gasifier and fast pyrolysis reactor will be provided by bio-char combustion.

The hydrocracking unit is replaced by a catalytic cracking unit [3] which will process the long-chain hydrocarbons from the bio-oil hydrotreating unit and wax from the Fischer-Tropsch product fractionator. C₅₊ gasoline, distillate, propylene and coke are the main products of the catalytic cracking unit. Hydrogen for the bio-oil hydrotreater, naphtha hydrotreater, distillate hydrotreater, and C₄ isomerizer can be provided through pressure swing adsorption of synthesis gas from the gasification unit or provided by the reforming of water soluble pyrolysis oil. All the generated CO₂ will be captured for re-use or sequestration. The developed process superstructure, which considers multiple catalysts and reactor types for certain processing units, is formulated as a mixed integer nonlinear programming model. To reduce the computational complexity of the MINLP, simple surrogate models for the gasifier are introduced. The bilinear, trilinear, quadrilinear, and concave terms are reformulated for enhancing the MINLP model to be solved to global optimality. The economics associated with the designed integrated process and the stand-alone gasification-FT /fast-pyrolysis are compared. Trade-offs between process economics and hydrogen source selection are considered. The optimal topology and the corresponding optimal operating conditions are evaluated under different scenarios, for example, maximum propylene production and maximum liquid fuels production. The designed integrated process exhibits high flexibility to cope with uncertainties on market demands and feedstock sources.

Reference:

[1]. Wright MW et al. Techno-economic analysis of biomass fast pyrolysis to transportation fuels. Fuel. 2010; 89: S2-S10.

[2]. Swanson RM et al. Techno-economic analysis of biomass-to-liquids production based on gasification. Fuel. 2010; 89: S11-S19.

[3]. Komvokis VG et al. Upgrading of Fischer-Tropsch synthesis bio-waxes via catalytic cracking: Effect of acidity, porosity and metal modification of zeolitic and mesoporous aluminosilicate catalysts. Catalyssis Today. 2012; 196: 42-55.