Final Report:
Analysis for the Conversion of Sludge to Ethanol
PSE 487
Senior Capstone Design
Spring 2009
University of Washington
Group # 3
Laura Rickman
Chris Thralls
Solomon Tibebu
Anthony Woen
Eric Zhao
June 11, 2009
TABLE OF CONTENTS
Executive Summary 4
Introduction & Objective 6
Results 8
Design/Approach & Project Planning 8
Approach on hydrolysis 8
Approach on fermentation 9
Approach on distillation 10
Process flow diagram 10
Critical Assumptions 11
Material and Energy Balance 13
Issues with unit operation 14
Key uncertainties 15
Mill Integration 15
Economic Analysis 16
Environmental and safety 20
Conclusion 21
References: 23
Appendix 25
Appendix A: Physical Properties 25
Appendix B: Specification Sheet 27
Appendix B: Specification Sheet 27
Appendix C: 32
Appendix D: Base case flow sheet 32
Appendix E: Final Wingem design 34
Appendix F: Design of storage 37
TABLE OF CONTENTS
Table 1: WSU Sulfite Pulping Mill #11 Data 7
Table 2: Compositional Analysis of Sludge from KC Everett Mill 7
Table 3: Assumptions for Production 12
Table 4: General Mass Balance 13
Table 5: EnthalpyBalance 13
Table 6: Total capital investments from scaled data 17
Table 7: NREL overall cost estimations 17
Table 8: Capital cost per gallon of ethanol 18
Table 9: Fixed Operating Cost Estimation 18
Table 10: Variable Operating Cost Estimation 19
Table 11: Economic Summary 19
TABLE OF fIGURES
Figure 1: Overlay of the unit operations under consideration 8
Figure 2: Total Carbon Dioxide Emitted in Washington State 20
Executive Summary
June 11, 2009
Professor Rick Gustafson
Professor, Forest Resources & Chemical Engineering
University of Washington
Seattle, WA 98105
Dear Professor Gustafson,
Please find enclosed the technical report “Analysis for the Conversion of Sludge to Ethanol”. This technical report is a summary of our group’s findings for our work completed the weeks of March 30th – June 11th to investigate the use of sludge from the Kimberly Clark Everett Mill as a source of lignocellulosic biomass for the conversion into ethanol.
Currently, the Kimberly Clark Everett Mill produces 46 oven dry tons of sludge each day, which entails an expensive disposal process in addition to environmental waste concerns. After performing a Klason Lignin Analysis on the sludge sample provided by the mill, it was determined that the sludge is approximately 76% glucose. From this, we strived to design an efficient and economical process that could ferment these sugars to produce ethanol. Our group considered several options before we achieved our final design. To tackle this challenging, but open-ended, design project our group relied on previous research done at NREL and the UW Bioenergy lab to help guide our design process.
After a brief introduction of sludge, the remainder of this report will walk through our groups design approach. Next, an in-depth economic analysis is provided to outline the costs involved for the incorporation of the equipment into the existing mill. The economic analysis also outlines the revenue that the KC Mill could generate from turning an expensive waste material into ethanol. As our enclosed report outlines, the completion of our final design yielded the following results.
Even though our findings yielded an initial design with negative profitability, the design project was an amazing experience which allowed us to use the knowledge acquired from several other courses, such as heat transfer, raw materials chemistry, and process and controls, to design the Biorefinery addition.
If you have any questions and/or concerns with the research we are presenting, please do not hesitate to contact us at your earliest convenience.
Thank you for support and mentoring throughout this project.
Respectfully,
Laura Rickman, Chris Thralls, Solomon Tibebu, Anthony Woen, and Eric Zhao
Introduction & Objective
One of the largest tasks present within the paper industry is maximizing profit while reducing waste generation. Reducing waste would not only reduce the expenses related to disposing of unwanted or hazardous materials, but also open new feed streams for products. Currently, Paper sludge is the largest solid waste stream produced by the pulp and paper industry[1], Sludge from paper mills consists of residues from the wood pulping and papermaking operations and inorganic additives (pulping chemicals, etc). The waste attributed to sludge has been documented to be approximately 1-6% of the mills product raw material capacity. The disposal of sludge is usually a problem for the paper industry due to its significant costs and the environmental issues related to the disposal process. However, due to its rich carbohydrate content, sludge from paper mills has the potential for bioconversion into valuable products.[2] Pulp and paper sludge has several advantages when compared to other value added product feed stocks. Sludge is: a low cost/waste material, it is available in large quantities, it is already highly processed, can reduce the mill’s dewatering capacity needs and is already produced on site. For our case study, we will be looking at the Kimberly Clark Everett Mill, an ammonium sulfite pulping mill, which currently produces tissue and toweling and the possible conversion of sludge into ethanol.
The largest single portion (about 40%) of the energy used in the United States is petroleum-derived fuel, which makes up more than half of the imported energy (U.S. Department of Energy, 1995). One low cost petroleum replacement is ethanol produced from lignocellulosic biomass resources-conveniently referred to as bioethanol. (Lynd et al., 1991). Ethanol is already an important element of transportation fuel production. About 12% of U.S. gasoline sold contains 8% to 10% ethanol as a fuel additive to boost octane and reduce carbon monoxide and other toxic air emissions. About 25% of U.S. gasoline now has a petroleum-derived additive that it would be advantageous to phase out because of water pollution concerns. Because ethanol can directly replace this additive, ethanol can also be used as an alternative fuel (typically in an 85% blend) to reduce the U.S.’s dependence on foreign oil (which currently supplies more than half the nation’s petroleum supply). Ethanol can be produced from inexpensive and abundant lignocellulosic biomass such as wastepaper, municipal solid waste, energy crops grown as feedstocks, and paper sludge. However, even though lignocellulosic biomass provides a low-cost resource, lignocellulosic material is very resistant to enzymatic breakdown. Thus, the biological conversion of ethanol from paper sludge, or other lignocellulosic biomass can be achieved by pretreatment, enzymatic hydrolysis and fermentation.
To get the metaphorical ball rolling on converting waste streams into sellable products we consulted a study performed by researchers at Washington State University (WSU). While they looked at the conversion of sludge to ethanol in 13 different mills, the mill of interest to us was Mill 11, a sulfite pulping mill. In the study conducted by WSU, sludge from the sulfite pulping process was 4% of the total product from the mill. The sludge, which was approximately 30% solids, had the following chemical characteristics.
Table 1: WSU Sulfite Pulping Mill #11 Data
Compound / % CompositionAsh / 5.50%
Glucose / 80.00%
Xylose / 3.00%
Mannose / 0.30%
Acid Soluble Lignin / >1%
Acid Insoluble Lignin / 11.00%
Our initial process flow diagram and base stream were modeled off of the above table; however, after receiving a sludge sample from the KC Everett Mill, we performed our own Klason Analysis on the biomass, and found that the compositional analysis is that represented below in Table 2
Table 2: Compositional Analysis of Sludge from KC Everett Mill
Compound / % CompositionArabinose / 0.12%
Galactose / 0.21%
Glucose / 75.56%
Xylose / 3.04%
Mannose / 3.54%
Acid Soluble Lignin / 2.12%
Acid Insoluble Lignin / 15.42%
As more research is conducted on alternative energy, biomass is viewed as the only low cost, abundant resource for organic fuels. In addition, sludge is an unavoidable byproduct of the paper mill, and thus a sustainable source of biomass. A stand alone lignocellulosic bioethanol facility could prove to be prohibitively costly. A bioethanol facility integrated into a paper mill could prove to have many synergistic effects, positively influencing both processes. The most significant would be the reducing capital expenditures for the ethanol side.
Based on our preliminary research and findings, the following unit operations schematic, Figure 1, was devised to initially guide us in developing our process diagram. Please note that after several revisions and further feedback and research, an updated version of our flow process can be seen in Figure 1. As it will be addressed later in the report, given the composition of the sludge we decided to eliminate the hemicellulose hydrolysis of 5-C sugars and focus on reducing costs and yeast requirements by strictly focusing on the 6-C sugars.
Figure 1: Overlay of the unit operations under consideration
Results
Design/Approach & Project Planning
In order to approach the process of ethanol production from sludge, many critical issues need to be considered. Our design is trying to achieve as economical, environmentally friendly, and socially responsible as possible. There are three major unit operations involved in the process. They are hydrolysis, fermentation, and distillation. The general project plan regarding to these processes will be covered in the following.
Approach on hydrolysis
Enzymatic hydrolysis
The enzyme complex known as cellulase is usually applied in enzymatic hydrolysis. There are three types of activities occur in breaking the long 1-4 glycosidic bonds.
These activities consist of:
1. Endoglucanase, which randomly attacks the cellulose chain to produce polysaccharides of shorter length.
2. Exoglucanase, which attaches to the non-reducing ends of these shorter chains and removes cellobiose moietie.
3. Glucosidase, which hydrolyzes cellobiose and other oligosaccharides to glucose.
The approach we are using for our design was initially a two step acid hydrolysis. The reason is because enzymes do not react and break down hemi cellulose and acid can break down both as well as more economical. However, from our research we found out that sludge is only composed of a small fraction of hemicelluloses. Taking that into account, it would be more economical to get rid of the acid hydrolysis stage and treat the sludge directly with enzymes in an SSF fermentation stage. This way we can save some equipment and cost for the company.
Approach on fermentation
Two factors are concerned in fermentation stage. One is the type of microorganism to use and two is the type of fermentation techniques.
Bacteria kinetics
The microorganism we initially thought about using was Z. Mobilis due to its benefits of shorter fermentation time and higher ethanol yield than yeast and its ability to ferment both five and six carbon sugars. It has a metabolic yield of 0.48 g of ethanol produced per g of sugar consumed. The fermenter conditions for the bacteria is at pH of 6, 30 degree Celsius, and time taken to complete fermentation is 48 hours. However, after the Klason Analysis, it was decided that since there were minimum amounts of five carbon sugars, it would be much more economical to just focus on using Saccharomyces Cerevisiae, or brewer’s yeast. S. Cerevisiae is a generally recognized as safe (g.r.a.s.) organism that has ethanol yield and productivity, is easy to culture, and is resistant to inhibitors and has a high ethanol tolerance. Further in-depth information on S. Cerevisiae is provided with the process flow diagram.
SSF vs. SHF
The difference of SHF and SSF is that SHF is separate hydrolysis and fermentation where enzymatic hydrolysis and fermentation is performed sequentially in separate unit operations. On the other hand, SSF means Simultaneous Saccharification and Fermentation, which combines cellulose hydrolysis and fermentation in one step. Because the glucose produced by the hydrolysis process is immediately consumed by the microorganism, only very low levels of cellobiose and glucose are observed in the reactor. This reduces cellulase inhibition, which in turn increases sugar production rates, concentrations, and yields, and decreases enzyme loading requirements.
With the advantages of SSF in sugar production yield and less enzyme loading requirements, we think this technique is ideal for our fermentation design. Therefore, the paper sludge will be treated simultaneously with enzymes and bacteria in the fermentation stage to break down cellulose chains and produce ethanol.
Approach on distillation
Once fermentation is complete, all contents will be sent to the distillation stage to separate out ethanol. First is applying azeotrope separation to achieve 95% ethanol content. Second is to remove the remaining 5% water by using either molecular sieves or benzene. The description of each separation is covering in the following.
Azeotrope separation
The fermented mash is pumped into a multi-column distillation system where additional heat is added. The columns utilize the differences in the boiling points of ethanol and water to boil off and separate the ethanol. By the time the product stream is ready to leave the distillation columns, it contains about 95% ethanol by volume. At this point, the mixture reaches azeotrope and further boiling is ineffective. The residue from this process, called stillage, contains non-fermentable solids and water and is pumped out from the bottom of the columns into the centrifuges or water treatment.
Molecular sieves separation or separation agent-benzene
To remove the remaining 5% water, ethanol can ran through a molecular sieve to physically trap water molecules and yielding 99-100% ethanol. Also, a material separation agent such as benzene can change the molecular interactions and eliminates azeotrope. However, another separation is needed to remove the benzene. Therefore we would like to apply dehydration (molecular sieve) into our distillation design.
Process flow diagram
The complete process flow diagram is available in the excel sheet. Please refer to the excel sheet PFD_3 tab for detailed information about the process.
Critical Assumptions
There are quite few critical assumptions that we considered in this design project.
First of all, the sludge is composed of 5C and 6C sugars, and experimental analysis indicated that the concentration of 6C sugar is much higher than that of 5C sugar. So we assumed that the contribution of 5C sugar to the overall production of alcohol is negligible compared to 6C sugars. We also assumed that all the 6C sugars are derived from the hydrolysis of cellulose with appropriate enzymes (cellulase). Second, we haven’t taken in to account the possible effect of inhibitors during fermentation process. Since a ~4g/l of inhibitors is not a threat to the fermentation process, we simply decided to ignore the impact of inhibitors. Another reason to avoid the impact of inhibitors is that we don’t have complete information about the types of inhibitors that exist in the sludge and the corresponding concentration. At this point we considered that the impact of inhibitors in the fermentation process is very low, so we avoided it.