(410f) Simulation of Reactive Distillation for Esterification of Pyrolysis Bio-Oil

Sustainable energy management is a major issue in the modern developed and developing societies and the main attractions of bio-fuel are that they are safe, renewable, non-toxic and biodegradable. More importantly, they can be produced by thermal pyrolysis or hydrothermal liquifraction of virtually every waste material including agricultural and animal waste. Biodiesel has an overall positive life cycle energy balance of 3.2 units of fuel product energy per unit of energy consumed compared to 0.83 of the petroleum fuels, and can be blended together with petroleum diesel to upgrade the fuel efficiency, resulting in reduced CO, CO₂and other undesirable emissions. Crude bio-oil has very similar properties compared to petroleum fuel but it is highly viscous, corrosive, and have high water content and high flash point. To minimize the viscosity, corossivity and water content, and improve overall fuel quality a further upgradation is needed. Pyrolysis bio-oil is a very complex mixture of different organic compounds mainly different fatty acids, water, phenols and alcohols. Transesterification or esterification reaction with an alcohol to make an ester in presence of acidic or alkaline catalyst is carried out for the upgradation of crude bio-oil but the conventional homogeneous catalytic reaction involves costly separation of products from the catalysts.  A complete reactive separation unit such as reactive distillation in presence solid catalysts, which also facilitate the separation of water presents in bio-oil in a single operation unit, is of immense interest. The Aspen simulation for reactive distillation considering all the components present in bio-oil is a complex task and the goal in our initial studies were to first carry out reactive distillation considering the esterification of n-butanol with only few simple acids at first for the simulation and then expand the components to incorporate other short chain and long chain fatty acids, phenols and alcohols with reduced n-butanol feed.

In this work, the binary and ternary interactions between the components were first studied as a simple guide step using Aspen Property PLUS. Then chemical and phase equilibrium (CPE) diagram was drawn to understand the liquid phase separation and the region in which a homogeneous region of mixture is present. The flow diagram in Aspen for reactive distillation was drawn by using aspen process flow diagram with the RADFRAC Module acting as the column performing both reaction and separation. In the RADFRAC besides conventional distillation input parameters the reaction input parameters can also be given and the reactive stages can be specified. For the simulation, UNIQUAC property method was used as the base method for property estimation. The reaction equilibrium was determined by specifying Aspen to calculate it by minimizing Gibbs free energy. The process was further optimized using the design specification and sensitivity analysis tools by varying several parameters to maximize the fractional conversion and separation efficiency. From the stream results the conversion was 70% to 95% for various simulated bio oil and all the water was separated from the ester product. But some ester was carried away with water due to the presence of azeotrope in binary water-ester mixture as well as the unreacted n-butanol with ester and unreacted acids split between two streams almost equally. The temperature and composition profile was drawn along the column for different feed mixtures. Ester flow rate and fractional conversion were drawn with the alcohol and water feed rate. We are currently studying all the distillation column parameters such as condenser and reboiler heat duty, number of stages, reflux ratio with different inlet conditions to develop a completely energy cost minimized RD unit. A detailed stage-by-stage mathematical modeling was carried out for one or two components in simulated bio-oil and the mathematical equations were solved by MATLAB using the known values of reaction constants and thermodynamic constants and the results was compared to the Aspen results. The results of all these studies will be presented.