2. Materials and Methods
Two separate simulations were set up as base case scenarios: One using batch enzymatic hydrolysis and one using fed-batch enzymatic hydrolysis. The simulations differed only in their hydrolysis operation; they were exactly the same for every other process operation. Simulation results from these base cases were compared to identify the techno-economic effects of using a fed-batch operation instead of a batch operation.
This study used the SuperPro Designer (SPD) simulation software (Version 9.5, Intelligen, Scotch Plains, NJ, USA, 2015) [13
], because it was designed specifically to model bioprocesses. SPD also has built-in economics calculations, which was a key component of this study. It is important to note there are three levels of complexity in an SPD simulation. “The simplest physico-chemical transformation step that can be modeled by SuperPro Designer [is a unit operation]. Operations are strung together to form a unit procedure and unit procedures are put together to make up a process (or a recipe)” [14
]. An operation may be as simple as ‘Charge’ or ‘Mix,’ or it may be more complex, e.g., ‘Distill’ or ‘React.’ A procedure is “a sequence of actions representing the most elementary physico-chemical transformations supported by the software all assumed to take place within the same equipment resource” [14
]. This paper uses the same naming convention for these steps.
SPD comes with an example process flow sheet for converting corn stover to ethanol. This process flow sheet was modified to fit the needs of this study. Appendix A
provides a sample flowsheet from this study, for reference. The operating parameters for the simulation can be found in Table 1
The plant in this study is assumed to be located in Ravenna, Nebraska due to the availability of corn stover, as this is a high corn-producing region of the state. The plant capacity is set to match the processing capacity of plant designs in other techno-economic analyses, such as the National Renewable Energy Laboratory (NREL) standard [15
] and others [12
Corn stover is the biomass in this study because it has shown promise as a lignocellulosic ethanol feedstock, and it is readily available in Nebraska. The study assumed the corn stover biomass would be transported 50 kilometers (km), and each shipment contained 20 metric tons (MT). Our overall operation would require nearly 66,000 shipments/year. The composition of the corn stover was assumed to be as follows (mass percentages given): 5.2% ash, 37.4% cellulose, 21.1% hemicellulose, 18% lignin, and 18.3% other solids [15
]. After the feedstock arrives at the plant facility, it is first washed and ground to reduce particle size. The feedstock mixture for the ethanol production process consisted of 50% corn stover, 50% water (mass percentages given).
For pretreatment, our design uses thermal hydrolysis (hot steam). The thermal hydrolysis pretreatment will degrade the structure of the biomass and leave the cellulose more accessible to the enzyme in the upcoming enzymatic hydrolysis operation. Hot, high-pressure steam is fed into the reactor at a rate of 30 metric tons per hour, temperature of 200 degrees Celsius (°C), and pressure of 10 bar. The feedstock slurry enters the reactor at 215 metric tons per hour, 88 °C, and 10 bar. Within the reactor, the contents sit at 180 °C and 10 bar. The residence time is 30 minutes. During this time, some cellulose is broken down into glucose, and the majority of the hemicellulose is broken down into xylose. The conversion of cellulose to glucose is set to 10%. The conversion of hemicellulose to xylose is set to 70%. The pretreatment reaction is assumed to be adiabatic.
Entering the pretreatment reactor, the feedstock slurry has the following composition (approximate mass percentages given): 15% cellulose, 9% hemicellulose, 48% water, 13% lignin, 3% glucose, 1% xylose, 11% other. Leaving the pretreatment reactor, the slurry composition changes to become (approximate mass percentages given) 12% cellulose, 2% hemicellulose, 53% water, 11% lignin, 4% glucose, 7% xylose, 11% other.
After the thermal hydrolysis, the slurry is flash cooled. Some excess water is removed and some of the xylose is filtered out of the slurry. After cooling and filtration, the slurry has the following composition (mass percentages given): 16% cellulose, 3% hemicellulose, 45% water, 15% lignin, 3% glucose, 6% xylose, 12% other.
Next, the hydrolase enzyme is mixed into the slurry for the enzymatic hydrolysis operation at a rate of 13 metric tons per hour, 25 °C, and 1 bar. After mixing the enzyme into the slurry stream, the hydrolase comprises just 0.2% mass composition of the stream. This study assumed the hydrolase is purchased from an external source at $11.40/kg protein. This price factors out to about $0.50/gallon (gal) of ethanol produced.
For both simulations (batch and fed-batch), the hydrolase enzyme is mixed into the stream before the slurry enters the hydrolysis reactor. The batch enzymatic hydrolysis reaction uses 2123 metric tons of hydrolase enzyme per year, which comes to 0.268 metric tons per hour. The batch enzymatic hydrolysis reaction was assumed to be adiabatic. The contents of the reactor were recorded at about 45 °C and a pressure of about 10 bar. The cellulose to glucose reaction was assumed to run to 90% completion, and the hemicellulose to xylose reaction was assumed to run to 70% completion. When the batch enzymatic hydrolysis is complete, the slurry stream composition is as follows (approximate mass composition percentages given): 2% cellulose, 1% hemicellulose, 48% water, 14% lignin, 18% glucose, 7% xylose, 10% other.
The simulated fed-batch enzymatic hydrolysis reaction uses 2091 metric tons of hydrolase enzyme per year, which comes to 0.264 metric tons per hour. The fed-batch enzymatic hydrolysis was also assumed to be adiabatic. The contents of the reactor were recorded at about 45 °C and a pressure of about 10 bar. The reaction was assumed to run to full completion due to the nature of a fed-batch operation within a continuous process. When the fed-batch enzymatic hydrolysis is complete, the slurry stream composition is as follows (approximate mass composition percentages given): 0% cellulose, 3% hemicellulose, 47% water, 14% lignin, 19% glucose, 6% xylose, and 11% other.
After hydrolysis, the hydrolysate slurry is filtered. The stream containing mostly glucose and water is sent on to fermentation. The stream containing mostly lignin, ash, and water is further processed. Most of the lignin is sent to be burned in the utilities section of the plant to generate power.
In the fermentation section, some of the slurry is used in seed fermentation tanks to grow the yeast cells. The whole slurry is fermented into ethanol. Our process used four 2220 m3 fermentation tanks with a temperature of 37 °C and a cycle time of 48 h. The slurry stream then enters a storage holding tank until it can be distilled to a higher percentage of ethanol.
The slurry stream leaves the storage holding tank and enters a heat exchanger to facilitate the distillation process. Leaving the heat exchanger, the stream has a temperature of 47 °C. The stream is now 9% ethanol and 80% water (approximate mass percentages given) when it begins the distillation process. The distillation columns operate at a temperature of 106 °C. Leaving distillation, the stream is 90% ethanol, 9% water (approximate mass percentages given). Next, an adsorption operation further dehydrates the stream, removing the little water remaining, such that the ethanol product reaches 99.9% purity.
The utilities section burns lignin obtained from hydrolysis to generate power. The generated power is sold back to the grid; it is not used within the plant. Selling the power generates additional profit for the production plant facility. The utilities section also recycles water for continued use within the plant.
Using the simulation, the sensitivity of the ethanol production cost to different process parameters can be tested. A review of the literature yielded differing values for basic parameters, so each parameter value was altered while the other parameters were held constant. Only one parameter was altered at a time, and the change in ethanol production cost relative to the base case scenario was monitored.
gives an economic summary of some basic economic parameters for our simulated SPD base cases. Comparing the cash flow analyses, the batch process had higher capital investments, lower gross profits, greater depreciation, and lower net cash flows. Hence, the batch process had a lower net present value and lower internal rate of return (IRR) compared to the fed-batch process. All economic values for this study are reported in US 2013 dollars.
Information for utility costs was obtained from the website for the Nebraska Energy Office. The average industrial rate for electricity cost in the Dawson Public Power District (service provider for Ravenna, NE) in November 2016 was $
0.12 per kilowatt-hour (kW-h) [21
This study assumed all labor workers in the plant were standard operators receiving the same wages of $
25 per hour. Information for operator salaries comes from the NREL studies [15
] and from the website for the United States Bureau of Labor and Statistics information for chemical plant and systems operators [22
It is important to note that the ethanol production cost accounts for all operations within the production process. Pretreatment, fermentation, distillation, and the utilities operations of the simulation all impact the ethanol production cost; it is not affected only by the enzymatic hydrolysis operation, even though this study aims to observe the impact of the enzymatic hydrolysis operation.