Date of Defense
Date of Graduation
Chemical and Paper Engineering
Bioethanol provides a great alternative to regular ethanol sources because it uses secondary substrates that are byproducts of other industries, rather than substrates that compete with food products. The technology is available for industrial bioethanol production, but the production costs just aren’t competitive enough with traditional ethanol production. The goal of this project was not to prepare a proposal for a bio-ethanol plant with all of the associated costs and financial calculations, but rather to provide a direct comparison of two pretreatment methods to highlight which factors could make one method less costly than the other. Pretreatment is a critical step in bioethanol production because it breaks up the tough structure of the plants and allows better chemical penetration to break down the biomass into sugars for fermentation. Numerous pretreatment methods exist, including dilute alkali, dilute acid, steam explosion, and hot water. This report focuses on the two most common, which are dilute acid pretreatment and hot water pretreatment. The overall production cost for each process is largely agreed to be about even, though some conditions may give the advantage to one pretreatment method over the other.
The general process for each pretreatment method is the same. First, biomass is mixed with steam and water, and in the case of the dilute acid method, sulfuric acid. The mixture reacts in the pretreatment reactor for a predetermined dwell time, where the structure of the biomass is broken down, partially into sugars, before it’s pumped through a series of tanks where further reactions occur and vapor is removed. The mixture then goes to an ammonia addition tank where the pH is raised before the mixture goes on to the hydrolysis stage. For comparison purposes, the same raw material costs were used for both methods, and a shared basis of 100,000 kg/hr of biomass was used. In the dilute acid pretreatment method, variable costs were found to be $26,600/hr and fixed costs were found to be $26,231,000. In the hot water pretreatment method, variable costs were calculated at $28,327/hr and fixed costs at $886,000. In each case, the driving variable cost is the biomass, which accounts for over 90% of the cost.
For the dilute acid pretreatment method, variable costs were slightly lower, but fixed costs were over 25 times higher. This gap in fixed cost comes solely from the fact that the pretreatment reactor for dilute acid is more advanced than that of the hot water method, and it requires numerous stages and acid resistant materials, whereas the hot water pretreatment reactor is essentially just a large tank. It’s important to understand though that the fixed costs for the pretreatment section alone are not a strong representation of costs for the overall process, as the gap is made up downstream in the hot water processes. This is because the hot water process does not break up the biomass as effectively, and because its streams are so dilute, downstream equipment is required to be significantly larger. Still though, the pretreatment equipment costs provide insight into the fixed cost behavior of the processes and how they react to changes in scale. It was found that hot water is more advantageous at lower production rates (less than 50,000 kg/hr) and that dilute acid fixed costs scale better with higher production (more than 200,000 kg/hr). It was also found that the hot water pretreatment’s variable costs are more sensitive to utility and energy prices, so if energy costs are high, dilute acid would be the preferred method. Other variables were not found to have a significant impact on overall cost. In terms of safety, the processes are about even - dilute acid requires the presence of H2SO4, and hot water requires higher temperatures and pressures, but both risks can be reasonably managed.
Eaton, Michael, "Comparison of Pretreatment Methods for Cellulosic Ethanol" (2020). Honors Theses. 3248.
Honors Thesis-Open Access