olsson moa, lignin, biodiesel, oxidativ...
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Karlstads universitet 651 88 Karlstad Tfn 054-700 10 00 Fax 054-700 14 60
[email protected] www.kau.se
Fakulteten för hälsa, natur- och teknikvetenskap Miljö- and energisystem
Moa Olsson
Preparation of Lignin Diesel
Experimental and Statistical Study of the Biodiesel
Preparation Process from a Pulp- and Paper Industry Residual Product
Framställning av Lignindiesel
Experimentell och Statistisk Studie av Framställningsprocessen av Biodiesel från en Restprodukt från Pappers- och Massaindustrin
Examensarbete 30 hp
Civilingenjörsprogrammet Energi- och Miljöteknik
Juni 2015 Handledare: Lars Nilsson Examinator: Roger Renström
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Summary The use of fossil fuels is depleting the petroleum resources and the emissions exhausted during the use is contributing to the planets temperature rise, glaciers reciding and rised sea level etc. In a global perspective, the liquid petroleum fuels are dominating the fuel market. In the coming ten years, the use of liquid fuels is expected to grow. In this work a method of preparing a biodiesel microemulsion between petroleum diesel and kraft lignin has been examined. Lignin is a renewable by-‐product from the pulp-‐ and paper industry, extracted from black liquor. In its natural appearance, lignin is not soluble in water and has to be modified to work as the hydrophilic phase in the microemulsion. The modification is achieved in a oxidative ammonolysis process. As an indication of how well the modification is performing, the amount of dissolved lignin in water were measured. The influence by the reaction time, pH-‐value and water content on the amount of dissolved lignin were examined in a statistical model in the software MODDE. A screening examination was performed to find the most influential factors. The MODDE model was optimized and could thereafter be used as a predictive tool and predict the outcome of responses within the experimental range. Ultrasonication was used to create the microemulsion. A stabilization test was performed by observing the created lignin diesel samples during three weeks. The operational cost of producing lignin diesel was calculated based on the chemical cost and the cost of electricity consumed during the production process. A microemulsion was not created between diesel and modified lignin, rather an emulsion was achieved. The highest amount of dissolved lignin in the oxidative ammonolysis process were 99.77 %. The most influential factor was the pH-‐value in the oxidative ammonolysis process. The water content also affected the amount of dissolved lignin, while the reaction time factor within its range did not affect the amount of dissolved lignin. The statistical model design, execution and predictive ability were evaluated in MODDE and given a satisfying grade. In the stability test, a separation in the bottom of the samples were observed after 0.5 h time. After one week, there was a small colour gradient in the top of one of the samples. After two weeks, the same colour gradient were observed in all of the samples. In none of the samples, a total phase separation was observed under the three weeks.
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Sammanfattning Användningen av fossila bränslen utarmar jordens petroleum resurser och under användning utsöndras emissioner som bland annat bidrar till den globala uppvärmningen, smältande glaciärer och höjda havsnivåer. Globalt sätt dominerar de flytande petroleum bränslena bränslemarknaden och dess användning förväntas inom de närmsta tio åren öka. I detta examensarbete undersöks och testas en metod för framställning av lignindiesel. Lignindieseln består av petroleumdiesel och lignin, vilka hålls ihop med hjälp av en mikroemulsion. Lignin är en förnybar restprodukt från pappers-‐ och massaindustrin som utvinns från svartlut. I naturligt utförande är lignin inte blandbart med vatten och behöver därför modifieras för att kunna agera som hydrofil fas i mikroemulsionen. Modifieringen görs genom en oxidativ ammonolysprocess. Som indikation på hur modifieringen verkade på ligninet mättes mängden löst lignin i vatten. Påverkan av faktorerna reaktionstid, pH-‐värde och vatteninnehåll på ligninets löslighet i vatten undersöktes i en statistisk modell som gjordes i programvaran MODDE. Den statistiska modellens design, utförande och predikteringskapacitet utvärderades. En screeningundersökning utfördes för att identifiera hur de olika faktorerna påverkade lignets löslighet i vatten. Modellen i MODDE optimerades och kunde därefter användas som en predikterande modell inom undersökningens omfattning. Ultraljudssonikering användes för att skapa mikroemulsionen. Ett stabiliseringstest gjordes genom att de olika lignindieslarna placerades i provrör som observerades under tre veckors tid. Driftkostnaden i form av kemikaliekostnad och kostnad för konsumerad elektricitet under produktionen beräknades. En mikroemulsion kunde inte framställas. Dock skapades en emulsion mellan diesel och modifierat lignin. Den högsta halten av löst lignin i vatten var 99.77 %. pH-‐värdet under reaktionen var den faktor som påverkade ligninets löslighet mest. Vatteninnehållet i det modifierade ligninet påverkade också lösligheten samtidigt som reaktionstiden inte påverkade lösligheten nämnvärt inom det givna spannet. Den statistiska modellens design och utförande var tillfredställande och den prediktiva kapaciteten var mycket bra. Stabilitetstestet visade att en separation observerades i botten ett av lignindieselproverna efter 0.5 h. Efter en vecka observerades en liten färggradient i toppen av ett av provrören. Efter två veckor syntes samma sorts färggradient i alla lignindieselproverna. Inget av lignindieselproverna undergick fullständig fasseparation under the tre veckornas separationstest.
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Foreword This article represents the result of my master thesis in Energy-‐ and Environmental Technology at Karlstad University. The work were executed at COWI Sweden’s office in Karlstad and the experimental work were performed at Karlstad University. There are several people and institutions that have been contributing to this thesis. I would therefore like to thank: Alina Hagelqvist, my mentor at COWI, for her commitment and excellent support. I would also like to thank all the employees at COWI’s office for the warm welcome and the opportunity to execute my master thesis there. Lars Nilsson, my mentor at Karlstad University, for his big engagement and help during the work. Pia Eriksson, Mikael Andersén and Gunnar Henriksson at the chemical engineering department at Karlstad University for being able to work in their laboratory, information and help during the process. Mats Andreasson and Urban Jonsson at BYCOSIN, for their help with material and information. Christopher Lindgren at Cleanflow Black, for providing material. Niklas Berglin and Per Tomani at LignoBoost, for their contribution with material and the visit at Bäckhammar Mill. Julia Botström for excellent team work and cooperation. At last, I would like to thank my family for their support. The thesis has been presented to an audience with knowledge within the subject and has later been discussed in a seminar. The author has actively been acting as an opponent to another student’s thesis. Moa Olsson
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Table of Content SUMMARY ......................................................................................................................................................... 2 SAMMANFATTNING ....................................................................................................................................... 3 FOREWORD ...................................................................................................................................................... 4 TABLE OF CONTENT ...................................................................................................................................... 5 LIST OF FIGURES ............................................................................................................................................ 6 LIST OF TABLES .............................................................................................................................................. 7 1. INTRODUCTION ...................................................................................................................................... 8 1.1. DIESEL AND BIODIESEL .......................................................................................................................................... 9 1.2. LIGNIN ................................................................................................................................................................... 10 1.2.1. Alternative lignin applications .............................................................................................................. 12
1.3. OXIDATIVE AMMONOLYSIS ................................................................................................................................. 12 1.4. MICROEMULSION ................................................................................................................................................. 12 1.4.1. Ultrasonification .......................................................................................................................................... 15
1.5. STATISTICAL MODEL ........................................................................................................................................... 15 1.5.1. Model design .................................................................................................................................................. 15 1.5.2. Evaluation of raw data ............................................................................................................................. 15 1.5.3. Regression analysis and model interpretation ............................................................................... 16
1.6. AIMS AND OBJECTIVES ........................................................................................................................................ 18 2. METHOD ................................................................................................................................................. 18 2.1. EXPERIMENTS ...................................................................................................................................................... 18 2.1.1. Chemicals ........................................................................................................................................................ 18 2.1.2. Equipment ...................................................................................................................................................... 18 2.1.3. Laboratory method ..................................................................................................................................... 19
2.2. DESIGN OF EXPERIMENTS .................................................................................................................................. 21 2.2.1. Factor and response design .................................................................................................................... 21 2.2.2. Experimental plan ....................................................................................................................................... 22 2.2.3. Optimization .................................................................................................................................................. 22
2.3. CALCULATION OF OPERATIONAL COST ............................................................................................................ 23 3. RESULTS ................................................................................................................................................. 24 3.1. SOLUBILITY ........................................................................................................................................................... 24 3.2. REGRESSION MODEL ........................................................................................................................................... 24 3.2.1. Evaluation of raw data ............................................................................................................................. 24 3.2.2. Optimization .................................................................................................................................................. 27 3.2.3. Use of model ................................................................................................................................................... 30
3.3. STABILIZATION TEST OF LIGNIN DIESEL .......................................................................................................... 31 3.4. OPERATIONAL COST ............................................................................................................................................ 33
4. DISCUSSION ........................................................................................................................................... 34 5. CONCLUSION ......................................................................................................................................... 37 6. REFERENCES .......................................................................................................................................... 38
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List of Figures FIGURE 1. LIQUID FUEL CONSUMPTION IN THE TRANSPORT SECTOR IN SWEDEN 2013. ................................................................. 8 FIGURE 2. AN EXAMPLE OF A LIGNIN MOLECULE FROM SOFTWOOD (HENRIKSSON 2010). .......................................................... 10 FIGURE 3. THE DIFFERENT MONOMERS THAT TOGETHER FORMS THE LIGNIN POLYMER(HENRIKSSON 2010). ....................... 11 FIGURE 4. EMULSIFIER WITH ITS HYDROPHOBIC AND HYDROPHILIC PARTS. .................................................................................... 13 FIGURE 5. SURFACTANT FORMATION AROUND A WATER DROPLET SURROUNDED WITH OIL. ........................................................ 14 FIGURE 6. CCF STRUCTURE. ..................................................................................................................................................................... 15 FIGURE 7. SYMMETRICAL AND UNSYMMETRICAL MODEL. ................................................................................................................... 16 FIGURE 8. PROCESS FLOW CHART, STEP 1 AND 2. ................................................................................................................................. 19 FIGURE 9. REPLICATE PLOT. ..................................................................................................................................................................... 25 FIGURE 10. SCATTERPLOT, DISSOLVED LIGNIN DEPENDING ON RUN ORDER. ................................................................................... 25 FIGURE 11. SCATTER PLOT, DISSOLVED LIGNIN DEPENDING ON REACTION TIME. ........................................................................... 25 FIGURE 12. SCATTER PLOT, DISSOLVED LIGNIN DEPENDING ON PH-‐VALUE. .................................................................................... 26 FIGURE 13. SCATTER PLOT, DISSOLVED LIGNIN DEPENDING ON WATER CONTENT. ....................................................................... 26 FIGURE 14. HISTOGRAM OF DISSOLVED LIGNIN. ................................................................................................................................... 26 FIGURE 15. BOX-‐WHISKER PLOT OF DISSOLVED LIGNIN. .................................................................................................................... 27 FIGURE 16. R2/Q2 PLOT. .......................................................................................................................................................................... 27 FIGURE 17. COEFFICIENT PLOT OF FACTORS. ........................................................................................................................................ 28 FIGURE 18. COEFFICIENT PLOT, EXCLUDED VERSION. .......................................................................................................................... 28 FIGURE 19. NORMAL PROBABILITY PLOT OF RESIDUALS. .................................................................................................................... 29 FIGURE 20. R2/Q2 PLOT AFTER POINT EXCLUSION. ............................................................................................................................. 30 FIGURE 21. RESPONSE CONTOUR PLOT. ................................................................................................................................................. 30 FIGURE 22. STABILIZATION TEST AND COMPARISON BETWEEN THE SONIFIED LIGNIN DIESEL SAMPLES AT TIME: A) 0 H, B)
0.5 H, C) 18 H, D) 48 H, E) 1 WEEK, F) 2 WEEKS, G) 3 WEEKS. .............................................................................................. 31 FIGURE 23. BOTTOM SEPARATION IN THE SONIFIED LIGNIN DIESEL SAMPLES, TIME 18 H. ........................................................... 32 FIGURE 24. BOTTOM SEPARATION IN THE SONIFIED LIGNIN DIESEL SAMPLES, TIME 1 WEEK. ...................................................... 32 FIGURE 25. DISTURBED LIGNIN DIESEL SAMPLES AT TIME 0.5 H. ...................................................................................................... 33 FIGURE 26. DIESEL PRICE VARIATION DURING THE YEARS 1981 -‐ 2014. ....................................................................................... 34
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List of Tables TABLE 1. CHEMICALS, STEP 1. ................................................................................................................................................................. 20 TABLE 2. CHEMICALS, STEP 2. ................................................................................................................................................................. 21 TABLE 3. EXPERIMENTAL PLAN. .............................................................................................................................................................. 22 TABLE 4. CHEMICAL COST PER LITRE. ..................................................................................................................................................... 23 TABLE 5. EXPERIMENTS AMOUNT OF DISSOLVED PERCENTAGE OF LIGNIN. ...................................................................................... 24 TABLE 6. ANOVA TABLE, DISSOLVED LIGNIN. ...................................................................................................................................... 29
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1. Introduction Today, the use of fossil fuels is exhausting the petroleum resources on earth, while the use itself leads to green house gas emissions. This leads to many negative effects including climate change, receding of glaciers and raised sea level etc. Fossil fuels are limited in the access to crude oil, which is a non-‐renewable and therefore limited resource. According to (Shafiee & Topal 2009), the depletion time of crude oil was earlier calculated incorrectly. The depletion time is estimated as 35 years in their report, which is a shorter time than previous calculations. In a global perspective, the market of liquid fuels is dominated by the fossil fuels. Even though the use of natural gas is expected to grow, the use of oil and coal is expected to grow continuously. In 2035, the use of fossil fuels is estimated to represent 75 % of the total use of liquid fuel in the world. Renewable fuels are expected to increase their market share to 8 % of the total fuel use until year 2035. In 2013, the market share for renewable fuels was 3 % of the total fuel use. In Sweden during 2013, 5.4 million m³ diesel fuel were delivered to consumers. A large part of that amount of fuel was used in the transport sector, where the diesel and biodiesel use were 53 % respective 3 %, which is presented in Figure 1. (Svenska Petroleum & Biodrivmedel Institutet 2014)
Figure 1. Liquid fuel consumption in the transport sector in Sweden 2013.
In Sweden, the total energy use within the transport sector in 2013 reached 120 TWh, which is the lowest amount during the period 2005 – 2013. In 2013, the use of petroleum diesel was 45.03 TWh, which corresponds to 4.5 million m3 diesel and 37.5 % of the total amount of energy used that year. In the same year, the used amount of biodiesel was 5.42 TWh, which correspond to 0.55 million m3 biodiesel and 4.5 % of the total amount. (Energimyndigheten 2014) Non-‐renewable fuels cause emissions during use. The earth now faces big challenges with the increased earth temperature and the depletion of natural resources. Finding alternative fuels to replace the petroleum based fuels or making the non-‐renewable fuels more effective are two possible solutions of the problem. Different techniques have been developed and examined for producing biodiesel. Either to use directly in an engine or as diluted into petroleum diesel. (Sun et al. 2014) have developed a method for diluting kraft lignin into fossil diesel using microemulsion. Kraft lignin is a residue from the pulp-‐ and paper industry. Black-‐liquor is normally combusted in order to create combined heat and power in the recovery boiler. Technologies have been
52%
1% 3%
7%
37%
Diesel fuel
Natural gas
Biodiesel HVO
Other renewable fuels
Gasoline
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developed to separate kraft lignin from the black-‐liquor. (Laurichesse & Avérous 2014; Doherty et al. 2011) writes that since there is a surplus of kraft lignin today, research is performed to find alternative usages for the material. In cases where the recovery boiler is a bottleneck in the pulp production, lignin can be extracted for increasing the pulp production. The Swedish energy politics is highly affected of the decisions that are taken in the European Union. The European Union commission directive regulates the quality of petrol and diesel fuels (Directive 2011/63/EU). In Sweden the law of engine fuels regulates which engine fuel qualities that can be sold. It is difficult to interpret the actual laws in in this area. It is not clear what applies in this work. There is an opportunity that there could be a tax relief for the renewable part in the lignin diesel, which would be tax free. But according to the Swedish tax agency, this is not certain. An alternative way is to produce and sell the modified lignin In packages in the size of one litre or smaller, since this would be a tax free product. This would be a problem though, since the emulsifiers are not included in the product and complex equipment would be necessary to create the microemulsion. (Swedish Tax Agency 2015)
1.1. Diesel and biodiesel Diesel oil is a fossil and lipophilic fuel that is produced from crude oil. The product properties depend on its composition of hydrocarbons. Diesel has higher density than gasoline and is used in diesel-‐engines where the ignition happens through compression which separates it from the gasoline engine where the ignition is done through a spark. Emissions such as CO, CO2, NOx, hydrocarbons, SOx, N2O and particles are exhausted when diesel is used in a diesel engine. (Arnäs 1997) Biodiesel is used in a diesel engine in the same way as regular diesel. The difference between these fuels is that biodiesel is completely or partially renewable. Biodiesel has similar properties as diesel when tested in a diesel engine. The emissions are primarily CO, CO2, NOx, SOx and particles. Emissions of non-‐combusted hydrocarbons and NOx tend to be higher from biodiesel than regular diesel. (Basha et al. 2009; Sun et al. 2014) Biodiesel is a secondary biofuel, which means that the biomass raw material has to be processed before use. This differs from the primary biofuels which are used in their original form and not processed. Biodiesels can be divided into three different groups depending on the raw material and process used. The groups are the first, second and third generation of biodiesels. The first generation contains biodiesel produced from substrates like seeds, grains or sugars. The second generation of substrates are lignocellulosic biomass and the third generation is processed from algae and sea weeds. In this work the production of a second generation biodiesel is examined. These substrates does not compete with food production, which the substrates from the first generation would do. There is still though, a competition between the food production regarding plantage area. The most commonly tested biodiesel is vegetable oil, which is produced from the first generation of substrates . Vegetable oil can be used as a biodiesel directly or blended into petroleum diesel. This creates a competition with the production of food, which increases the prices on both food and vegetable oil biodiesel. The use of vegetable oil in an engine is a problem due to the oils high viscosity. To avoid the high viscosity biodiesel is mainly processed by four techniques; microemulsion, pyrolysis, catalytic cracking and transesterification. (Nigam & Singh 2011)
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1.2. Lignin Lignin is a polymer with hydrophobic properties that glues the celluloses microfibers together with the hemicellulose and gives the cell walls their wooden properties. Lignin strengthens the stem in the plant and gives the cell walls its water-‐resistant properties, which is used to transport water inside the plant. The tight structure of lignin on the plant is protecting the tree from bacterial digestion. (Chabannes et al. 2001; Henriksson 2010; Sarkanen & Ludwig 1971)
Figure 2. An example of a lignin molecule from softwood (Henriksson 2010).
In the pulping process the wood fibers are separated from each other when the lignin is removed in a mechanical or chemical process. In the chemical process, most of the lignin is removed through adding chemicals which induce degradation of the lignin molecules. Hydrogen sulfide and hydroxide-‐ions make the lignin water soluble and then it can easily be washed away. (Henriksson 2010) Based on its complex structure (Figure 2) of aromatic and aliphatic hydrocarbons, the lignin is divided into different groups. The aromatic parts contain benzene rings and the aliphatic parts contain hydrocarbon chains. Lignin is mainly polymerized from three different monolignols; p-‐coumaryl alcohol, coniferyl and sinapyl alcohol. The different monolignols are showed in Figure 3. The monolignols differ from each other in the amount of methoxy groups that are attached to the benzene rings. (Henriksson 2010)
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Figure 3. The different monomers that together forms the lignin polymer(Henriksson 2010).
Generally hardwood consists principally of coniferyl and sinapyl units with just a small amount of p-‐coumaryl units. Softwood mainly consists of coniferyl alcohol, smaller amounts of p-‐coumaryl alcohol and almost no sinapyl. Lignin from hardwood tend to have straighter and less branched molecules than lignin from softwood. This results in a simplified production of kraft pulp from hardwood. Another factor that differs between lignin from softwood and hardwood, is the methoxy groups. The methoxy groups are present in a smaller amount in softwood. (Boerjan et al. 2003; Sjöström 1993; Henriksson 2010) (Sjöström 1993) writes that lignin is poorly soluble in most solvents, which causes trouble when the macromolecular properties of lignin are investigated. Therefore, few attempts have been made to characterize pure lignin and more research has been made on the soluble reaction products of lignin. The reaction products generally have low viscosity, which imply a compact and spherical structure among the soluble lignin molecules. According to (Henriksson 2015), lignin is not soluble in diesel oil. This makes the preparation of lignin diesel complex. One method for extracting the kraft lignin from the black-‐liquor is called Lignoboost. 50 -‐ 70 % of the kraft lignin can be extracted trough this method. The black-‐liquor is taken from the evaporation plant. The kraft lignin is deposited by gradually lowering the black-‐liquor’s pH by adding CO₂. Filtration is used to drain the kraft lignin. The kraft lignin is dissolved in recycled water and acid which results in a slurry. Cakes of almost pure kraft lignin are formed when the slurry is drained and washed with acidic wash water. The remaining black-‐liquor is returned to the black-‐liquor cycle. (Valmet 2014) Another method for extracting lignin from black liquor is the method from the company Cleanflow Black. The difference from the Lignoboost process is that a ceramic pipe with small holes in is used. The black liquor flows through the pipe and with low pressure inside the pipe, the lignin is extracted as small molecules through the small holes. This gives a finer lignin powder with smaller molecules. (Henriksson 2015) Today kraft lignin is burned in the recovery boiler to create combined power and heat to the mill. (Eriksson & Harvey 2004) study the possibility to use biomass fuels from pulp and paper mills to produce energy. The surplus of biofuel is calculated using a model based on the average Swedish market. The calculations show that 178.4 MW surplus of biomass fuels exist in a pulp and paper mill. (Olsson et al. 2006) examine how kraft lignin separation influences the combined heat and power production in the recovery
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boiler. When kraft lignin is separated, the amount of produced combined heat and power energy is reduced. Two models were designed to describe a typical Scandinavian pulp mill. The difference between the models were the amount of process water used in the mills. The produced quantity of heat and power decreased with 30 % per year when the amount of separated kraft lignin was as largest, which was when 36000 ton lignin per year was taken out of the production. This amount corresponds to an energy content of 500 GWh per year.
1.2.1. Alternative lignin applications Because of its beneficial properties, lignin is a current research subject. (Berghel et al. 2013) used kraft lignin as an additive in preparation of pellets, which increase the products mechanical durability and length. (Herreros et al. 2014) examine how cyklohexanol extracted from kraft lignin affects the emissions when mixed into diesel and used in a diesel engine. This was proved to decrease the amounts of particles in the emissions but increase the amount of nitrogen oxides and carbonmonoxide. (Baumlin et al. 2006) and (Osada et al. 2006) use different methods to successfully produce hydrogen and synthesis gas respective carbon dioxide and methane gas out of kraft lignin. (El Mansouri et al. 2011) use kraft lignin in the production of phenol formaldehyde, Bakelite. Kraft lignin was proved to be a good alternative material. (Pan & Saddler 2013) add kraft lignin in the production of polyurethane foam in order to replace some of the petroleum material that otherwise was used. The polyurethane foam with a kraft lignin content of 19 – 23 % showed satisfying properties. (Li et al. 2014) produce active carbon from kraft lignin which is favourable since it is an economically and environmentally beneficial product. (Dallmeyer et al. 2013) produce a non-‐woven fabric with lignin content. The material has shape-‐memory and changes shape when exposed to moisture. When the exposure stops, the material return to its original form. This can be useful in different applications.
1.3. Oxidative ammonolysis (Sun et al. 2014) use hydrogen peroxide and ammonium hydroxide in order to modify kraft lignin. Their main reason is to make the lignin water soluble, hydrophilic, in order to be able to use it in a water-‐in-‐oil microemulsion together with diesel. The functional groups that are bound to the aromatic rings are replaced. This decreases the number of ether-‐ and alcoholic groups while the carbon-‐oxygen, carbon-‐nitrogen and carbon-‐nitrogen-‐hydrogen groups increase. According to (Sun et al. 2014), this could increase the hydrophilic properties of the kraft lignin. (Capanema et al. 2001a) write that the reaction mechanism of the oxidative ammonolysis of lignin has not been established by scientists because of its complexity. The oxidative ammonolysis can be used in order to break the molecular bonds inside the big and complex lignin molecule to make smaller molecules and then adding some functional groups. This would be beneficial when using the lignin diesel in an engine (Holby 2015). (Capanema et al. 2006; Capanema et al. 2001a) means that an increased pH-‐value and a longer reaction time dissolves more lignin into water during the oxidative ammonolysis reaction.
1.4. Microemulsion Microemulsions are homogenous compounds consisting of water, oil and emulsifier. A microemulsion is a dispersed system where the small particles are in the size from nano-‐ to micrometer scale. The small particles do not catch light and therefore, the
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microemulsion appears transparent of bluish (Myers 2006). The small particles are not soluble in the other phase. At the microscopic level, the compound consists of different domains of oil and water kept together with an amphiphilic molecular membrane. (Holmberg & John Wiley & Sons 2003) (Holmberg & John Wiley & Sons 2003) writes that within a specific temperature interval the microemulsion is thermodynamically stable, depending on temperature and the content of surface active substances. Outside this interval, the phases occur separated as one water-‐repellent and one water-‐soluble phase. When the conditions return to the stable range, the microemulsion will regenerate. A microemulsion is spontaneously formed when the two liquid phases are combined with one or more surface active substances. The surface active species consists of two parts, one hydrophilic part and one hydrophobic part. This is presented in Figure 4. The hydrophilic part attracts water and liquids with water-‐like properties. The hydrophobic part is water repellent and therefore poorly soluble in water. The hydrophilic and hydrophobic part turns to the water respectively oil phase in the compound, which creates the stable structure shown in Figure 5. The surface active compounds have both hydrophilic and hydrophobic properties and is therefore called emulsifier or surfactant. (Larsson 2008)
Figure 4. Emulsifier with its hydrophobic and hydrophilic parts.
According to (Holmberg & John Wiley & Sons 2003), the driving force that makes the emulsifiers willing to adsorb to a surface is the released energy by the interface. In most of the cases an extra emulsifier is needed to make microemulsions form spontaneously, a co-‐emulsifier. Different structures are formed in the microemulsion depending on the water-‐oil ratio and the choice of emulsifiers. One method for systematic choose of emulsifier is by the HBL-‐value. HBL stands for hydrophilic-‐ lipophilic balance and describes at which degree an emulsifier is hydrophilic or hydrophobic (Griffin 1949). The HBL-‐value is calculated according to Equation (1) (Griffin 1954).
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𝐻𝐵𝐿 = 20 1− !
! (1)
In Equation (1), S stands for the saponification value and A stands for the acidity constant, which measures how much base that is needed to saponify one unit of fat respectively neutralize one unit chemical substance. The HBL-‐value in a water-‐in-‐oil (W/O) microemulsion should be between 4 – 6 units. (Griffin 1949)
Figure 5. Surfactant formation around a water droplet surrounded with oil.
Diesel consists of many different aliphatic and aromatic hydrocarbons. These hydrocarbons complicate the formation of W/O microemulsions when low concentrations of one emulsifier is used. Therefore combinations of emulsifier and co-‐emulsifiers are preferably used in these situations to make the microemulsion more stable. Midrange carbon chains can be used as co-‐emulsifiers to facilitate the formation process. (Holmberg & John Wiley & Sons 2003) For practical use as part in fuels with the purpose of supporting a sustainable development, the emulsifier should easily burn without emitting smoke. It should neither contain sulfur or nitrogen. This is limiting the choice of emulsifiers since it only can contain carbon, hydrogen and oxygen to fulfill the demands. The emulsifiers are classified depending on their hydrophilic group. With these criteria non-‐ionic-‐, polyol-‐ and sugar emulsifiers by different kinds are usable. (Kayali et al. 2015) (Lif & Holmberg 2006) writes that the large amount of surfactant that is used to create the microemulsion outweigh the benefit with thermodynamic stability because of its high costs. In another publication (Kayali et al. 2015) show that a microemulsion by the W/O type reduces emissions such as NOx, soot and CO₂ when used in a diesel engine. When the engine was running at low speed, the emissions of CO were also reduced. (Ahmad & Gollahalli 1994) writes that water blended into diesel through microemulsion reduces the emissions by NOx and CO during use in a diesel engine. (Kayali et al. 2015) also write that the amount of emulsifier that is used to create the microemulsion is reasonable and a good alternative when developing alternative fuels because of its potential of reducing the mainly air pollutants that is emitted during use in a diesel engine.
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1.4.1. Ultrasonification Ultrasonification technique has recently been used with the purpose of creating stable emulsions by decomposition of the chemical species (Schramm 2005). The ultrasonification creates an ultrasound wave propagation in the liquid. This makes the liquid flow in the same direction as the propagation, which is called acoustic streaming. The high power ultrasonic preparation of a solution creates cavitation bubbles and the collapse of these bubbles. During the collapse of the bubbles, high speed microjets , microstreaming and shockwaves are produced. The effects of these activities is that the preparation of oil-‐in-‐water emulsions is more effective according to (Imazu & Kojima 2013). (Sargolzaei et al. 2011) writes that when the bubbles collapse near the interface of the two immiscible liquids, this helps the phases to disrupt into each other.
1.5. Statistical model MODDE is a software that is used to create experimental plans, screen the influential factors and predict the outcome of future experiments within the same variable span. The process for creating a predictive model can be described in mainly three steps; value the raw data, regressions analysis, interpretation of the model and finally regression model application. (Eriksson 2008)
1.5.1. Model design In this work, a response surface model is created in the software MODDE. This model enables screening investigations and determines the yield of the factors significance after optimization. The model can be used as a predictive tool for further investigations within same range of factors. The design used in this work were the CCF design, which is a central composite face-‐centred design. This means that there is one middle point. For understanding how the factors affects the middle point, each factor has three levels. which gives a graphic picture of a cube as shown in Figure 6. The practical consequence of this is that each factor will vary with three different values. (Eriksson 2008)
Figure 6. CCF structure.
1.5.2. Evaluation of raw data The raw data was evaluated using the plot of replications in MODDE. The plot of replication is a graphic tool that shows the response value for each experiment. Observing this plot can reveal any deviating values, which if not excluded can make the
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model useless. If the variance between the values is big the model can also be impracticable. (Eriksson 2008) The condition number describes how well the design performance of the experiment is executed. The conditions described as the ratio between the longest and shortest design diagonal values of the experiments, as shown in Figure 7. A symmetrical model is preferred for achieving a good spectrum of the experiments. There are some guide lines when it comes to the condition number. For achieving a good optimization design, the condition number should be lower than eight. The geometry of the model is shown in the scatter plot, which also is a valuable tool after finding the condition number. The scatter plot shows whether the design is symmetrical or not. Even though the geometry is skewed, the design can be used if the condition number is low. The scatter plot is more useful in investigations with few factors and responses. It is even an impractical tool in situations where there are more that 4 factors and/or 4 responses. (Eriksson 2008)
Figure 7. Symmetrical and unsymmetrical model.
The histogram is a tool that describes the statistical properties of the raw data. It is beneficial to have normally distributed data. Together with the descriptive statistics of response, a lot of information can be gained from the statistic information of the model. (Eriksson 2008)
1.5.3. Regression analysis and model interpretation If the plot of replications does not show any deviating values, the next step in the process is the regression analysis and model interpretation step. One important tool for this step is the R2/Q2 tool. This tool consists of two parameters, R2 and Q2. R2 is called the goodness of fit and is a value used for measuring how good the regression model can be made to fit the raw data. This parameter has values between 0 – 1 where the value 1 corresponds to a perfect model. A disadvantage with the R2 parameter is that it can be arbitrarily modified to the value 1 by including more terms into the model. The parameter Q, which is called the goodness of prediction, is an indicator of the models ability to predict outcomes of experiments. This is a better validity of a regression model
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than the R2 value because it estimates the final goal of modelling, prediction. The Q2 value lies between –∞ and 1, where 1 indicates a perfect model. To determine whether a regression model is useful or not, the combination of the R2 and Q2 values is examined. These values should both be close to the value 1 and the difference between them should maximum be 0,2 – 0,3 units. Generally the value of Q2>0.5 is seen as good and Q2>0.9 indicates that an excellent model has been created.(Eriksson 2008) The coefficient plot is a tool that is used for model interpretation and cleaning among the raw data. When the R2/Q2 analysis is satisfied, the coefficient plot is scrutinized. In this plot the small value factors are seen as insignificant. Those factors have small influence on the response factors and can be eliminated in order to get better prediction ability. The large value bars have big influence on the response factor and those are supposed to be used for getting the wanted result. (Eriksson 2008) Multi linear regression (MLR) is a statistic model that describes a process or a condition. The model examines if there is a statistical relation between the response variable y and the independent variables x1, x2, …, xn. This relation is described in Equation (2). 𝑦 = 𝛽! + 𝛽! ∗ 𝑥!…+ 𝛽! ∗ 𝑥! + 𝜀 (2) In Equation (2) βn represents the regression coefficient and ε is the random error between the expected and the observed y-‐value. Classically the error term is assumed to follow the normal distribution E(ε)=0 and the constant variance Var(ε)=σ2. The purpose of using MLR is creating a temporary relation between the response variable y and the independent variables x1, x2, …, xn. Furthermore the opportunity to predict how the y-‐value varies with different combinations of x1, x2, …, xn. The model enables evaluating which independent variables that are more important than others. This means that the response variable can be explained more effectively and precisely. (Yan & Su 2009) The regression analysis in this work is examined by two methods, ANOVA and normal probability plot of residuals. ANOVA is short for analysis of variance. ANOVA is used for analyzing variances through statistical methods. When a hypothesis is established, its significance can be tested through the use of random samples and standard deviation.(Hassmén, Peter & Koivula, Nathalie 1996) In MODDE, ANOVA does two tests that are calculating different types of variability in the response data and the probability value, p. The first test estimates the significance of the regression model. The p value for this test is satisfying when p < 0.05. The other test is measuring the lack of fit value, which describes the model error and replicate error. The lack of fit test can only be performed when replicate experiments have been performed. (Eriksson 2008) The normal probability plot, N-‐plot, is used for finding deviating experiments and responses. In the plot, the horizontal axis corresponds to the numerical response values divided by the standard deviation. The vertical axis corresponds to the normal probability in the distribution of the residuals. A straight line that has to go through the point (0, 0.50) is drawn by eye through the majority of the values. Points in the plot that do not lie close to that line can be suspected as deviating values. On the horizontal axis the areas lower than -‐4 and higher than 4 are exclusion areas. This means that values
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that are in this area should be excluded from the model because these values are considered as statistically significant deviating values. The values that are within the ±4 area and outside the ±3 area on the horizontal axis have to be carefully considered, since these values can be considered as extreme values. Deviating values that are found can be excluded from the model. In order to have a useful regression model, the amounts of points in the N-‐plot have to be at least 12 – 15 points. (Eriksson 2008)
1.6. Aims and objectives The aim of the study is to prepare a lignin diesel by creating a microemulsion between petroleum diesel and modified kraft lignin. The lignin diesel has earlier been prepared by (Sun et al. 2014) and the method’s feasibility is tested. To achieve the aim of the study, the following objectives should be met:
• Evaluate if oxidative ammonolysis can be used for dissolving lignin into water. • Examine if a microemulsion can be created between modified lignin and
petroleum diesel. • Analyse how the amount of dissolved in water is affected by variations in pH-‐
value, reactions time and water content. • Create a statistical model which describes how the lignin’s solubility in water is
affected by variations during the reaction in pH-‐value, reaction time and water content.
• Evaluate if the statistical model can be used for prediction of the outcome of experiments within the same range.
• Calculate the operational cost for producing the lignin diesel, including costs of chemicals and the electricity consumption during the production.
2. Method In this section, the methods in this work are described in the following sections. The sections are divided into three main parts; the experimental part of the work, the creation of the statistical model and the theoretical operational cost estimation.
2.1. Experiments In the following section the experimental method are described divided into the sections; chemicals, equipment and laboratory method.
2.1.1. Chemicals In the experiments all of the chemicals except for diesel and lignin were taken from Karlstad University department of chemical engineering. The chemicals that were used were hydrogen peroxide with concentration 30 %, ammonium hydroxide with concentration 25 % and sodium hydroxide with concentration 1 mole/litre. The surfactants that were used were SPAN-‐80 and n-‐butanol. Distilled water was obtained from the university’s distilled water production. The diesel that was used was tax-‐free diesel received from BYCOSIN in Karlstad. Lignin in powder form was received from Cleanflow Black AB’s production.
2.1.2. Equipment In the experiments, a water bath, OBN 28, from Heto together with a temperature controller device, HMT 200, from Heto were used. Stirring devices, IKA RW 20 DZM.n from Buch & Holm were used. The reaction vessel was a three-‐necked flask with
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rounded bottom. The reaction vessel has a volume of 800 ml. A pH-‐meter (model 3320) from Jenway, equipped with one pH-‐meter electrode and one temperature electrode was used to measure the pH-‐value and temperature. The filter paper that is used in the experiments is MUNKTELL Analytical Filter Papers of quality 5, which is a qualitative filter paper with weight 130 g/m2. The filter paper has a filtration speed of 1000 ml/min through a 100 cm2 area. The combined drying and heating chamber used was of the brand Binder. The sonication device used was Branson Sonifier 450 with a 3 mm tapered microtip from Branson Ultrasonics Corporation.
2.1.3. Laboratory method Figure 8 presents a flow chart that describes the process of creating the lignin diesel. This process was divided into two steps. The first step was the process in which the lignin powder was modified through oxidative ammonolysis. The second step was creating the microemulsion with diesel, modified lignin and emulsifiers.
Figure 8. Process flow chart, step 1 and 2.
In the oxidative ammonolysis, three batches were made. According to the experimental plan, each batch had different pH-‐value. The batches were made with the same amounts of hydrogen peroxide, ammonium hydroxide, distilled water and lignin powder. This is presented in Table 1.
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Table 1. Chemicals, step 1.
Chemical Portion (weight unit) Lignin 1 Distilled water 10 Ammonium hydroxide 1,8 Hydrogen peroxide 0,15 First, the water bath was set to 65°C and the reaction vessel was placed in the water bath. Lignin powder was placed in the reaction vessel. Distilled water was poured into the closed vessel through the neck during agitation. While the lignin-‐mixture was tempered, ammonium hydroxide was mixed together with hydrogen peroxide in a round flask using a magnetic stirrer. When the lignin-‐mixture had reached the temperature of 65°C, parts of the mixed ammonium hydroxide and hydrogen peroxide were poured into the reaction vessel every five minutes. All of the ammonium hydroxide-‐hydrogen peroxide-‐mixture was carefully added into the reaction vessel after 20 minutes. (Sun et al. 2014) The pH-‐value varied in the batches and the different values were 9.7, 10.85 and 12 pH-‐units. Without any pH modification, all three of the batches had the pH-‐value 9.7. In the batches where the pH-‐value was supposed to be 10.85 and 12, sodium hydroxide was added until the desired pH-‐value was reached. The vessel was kept in the water bath with agitation during varying reaction time. The different reaction times were given the discrete values of 18, 21 and 24 hours. At these different times, samples were taken out of the reaction vessel. The last parameter to vary was the amount of water in the modified lignin. Evaporation was used to obtain the desired amounts of water content. The water content was calculated in comparison with the original amount of water in the modified lignin when the samples were taken out of the vessel and the water content were 100 %, 74.5 % and 49 %. The water content was calculated based to the sample weight and the amounts of added lignin, water and in some samples sodium hydroxide. To predict the time at which the water content should contain the right amount of water, the evaporation rate was calculated. The modified lignin samples containing the water content 100 % were placed in a fume hood during one hour in order for the ammonia gas to take off. The modified samples were placed in small reservoirs with lids. The samples were then filtrated with a pre-‐weighed filter paper. The filter papers were after filtration placed in an oven at 105°C during 4 h for drying. After 4 h, the filter paper was weighed and then the amount of dissolved lignin was calculated using the added amount of lignin powder in the beginning of step one. In the second step of the process, the amounts of chemicals that were used are described in Table 2 (Sun et al. 2014). Not all of the samples from step 1 were used in step 2. Three samples were selected to be used in step 2. The samples from step one with the highest dissolution ratio in each batch were selected.
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Table 2. Chemicals, step 2.
Chemical Vol % Diesel 82,4 SPAN-‐80 6 n-‐butanol 1,6 Modified Lignin 10 In step 2, diesel was poured in a vessel without lid. SPAN-‐80 and n-‐butanol were added during agitation (Schramm 2005). When the mixture was evenly mixed, the modified lignin was slowly dropped into the vessel using a pipette. After one hour with vigorous agitation, one sample was taken out of each batch and placed in a sampling tube. (Sargolzaei et al. 2011) After one hour, the batches were placed in a Styrofoam container filled with ice in order to prevent the temperature from rising during the sonication process (Branson Ultrasonics Corporation 2011). The sonifier tip was placed one cm under the microemulsion surface in the vessel. The vessel had a diameter of 10 cm and the total volume of lignin diesel in the vessel was 500 ml. The sonication device was set to a continuous pulse with the power of 100 W. The sample was sonified during 10 minutes (Imazu & Kojima 2013). Samples were taken out and put in sampling tubes, which were placed in a rack. The stabilization test was performed by observation of the samples during three weeks time. Photos were taken of the sampling tubes at different time intervals. The photos were observed in order to see any changes in the samples over time.(Panapisal et al. 2012)
2.2. Design of experiments The experimental plan was developed in the software MODDE 7. The process of model creation is described below in the subsections; parameters, experimental plan, statistical plan.
2.2.1. Factor and response design A response surface model was chosen. After some initial experiments, three different factors were determined to have great influence in the oxidative ammonolysis process. Those factors were reaction time, pH-‐value and water content after the oxidative ammonolysis process and these were added into the factor spread sheet in MODDE. All factors were quantitative factors and therefore, a value range was set. The factors were also controllable by changing the experimental settings. For the reaction time, the lower boundary value was set at 18 h, which was based on logistical reasons. The upper boundary was set at 24 h, which was based on the investigations made by (Capanema et al. 2001a) where reaction activity was examined during 1455 minutes. The pH-‐value was measured in the standard oxidative ammonolysis without addition of sodium hydroxide, and this value was set to the lower limit in the pH-‐factor. (Capanema et al. 2006) tested oxidative ammonolysis at the pH-‐value 12.7. To be sure that the pH-‐value was reachable, the upper level was set to a pH-‐value of 12 units. The water content’s upper limit was set to 100 % of the original amount of water in the test. (Sun et al. 2014) write that the modified lignin was poured into the microemulsion preparation.
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This information was used to define the lower limit of the water content. An experiment was performed to examine by which amount of water content the modified lignin was in fluid form. The experiment showed that the modified lignin could be poured at the water content 49 % of the original amount, which was set as the lower limit in MODDE. Since the solubility of lignin in water affects the ability of creating a microemulsion between the modified lignin and diesel, this was set as the response factor in MODDE. The measurement of the dissolved lignin is described in the laboratory method section.
2.2.2. Experimental plan When these parameters were added into the MODDE software an experimental plan was presented. The plan described which combination of these factors that was to be used during the laboratory work. The amount of design runs were set as 15 experiments, which are presented in Table 3. No replicate experiments were performed. Table 3. Experimental plan.
Experiment number
Experiment name
Reaction time (h)
pH-‐value Water content (%)
1 N1 18 9.7 49 2 N2 24 9.7 49 3 N3 18 12 49 4 N4 24 12 49 5 N5 18 9.7 100 6 N6 24 9.7 100 7 N7 18 12 100 8 N8 24 12 100 9 N9 18 10.85 74.5 10 N10 24 10.85 74.5 11 N11 21 9.7 74.5 12 N12 21 12 74.5 13 N13 21 10.85 49 14 N14 21 10.85 100 15 N15 21 10.85 74.5 MODDE also suggested a run order for the experiments. This run order was not followed because of the simplification when three batches of different pH-‐values could be produced instead of 15 different batches.
2.2.3. Optimization When the amount of dissolved lignin was calculated after the laboratory work, the values were added into the response column in MODDE. The following process of creating a useful model for prediction is described below. The raw data were entered and examined in MODDE software. First, the replicate plot was examined for finding possible extreme values. If those were to be found in this plot, their exclusion was considered. Second, the condition number in the first design was examined. If it was lower than the boundary value eight and no changes were performed. The scatter plot’s symmetries were examined and evaluated. The histogram
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of response was used for evaluating the statistical information of the raw data. The histogram was compared to a normal distributed curve and evaluated. When the evaluation of raw data was finished, the model interpretation and regression analysis were the next steps in the process of creating a predictive model. The R2/Q2 plot was examined. If the values were not sufficient, exclusion of values was considered. The coefficient plot was examined in order to reveal any factors that either affect the response factor or do not affect the response factor. The factors with small bars were excluded. For evaluating the models predictive ability, the ANOVA table and its reference values were analysed. The normal probability plot of residuals and its warning boundary values were analysed. Possible factors in the region outside the -‐4 < x < 4, were to be immediately excluded while the points in the region between ±3 and ±4 were carefully considered before evaluating.
2.3. Calculation of operational cost The operational costs were calculated depending on the material cost of chemicals. The prices were obtained from (Sigma-‐Aldrich 2015), which are presented in Table 4. The quality of the distilled water is assumed be chemically purified water, its price being assumed to be the same as Swedish tap water. No cost data for kraft lignin were obtained because of confidentiality. The price of lignin powder was calculated using the assumption that 1 MWh outtaken amount of lignin in the recovery boiler is replaced with 1 MWh of forest by-‐products. The price of forest by-‐products (Skogsstyrelsen 2014) was used to calculate the corresponding lignin price. The operational cost of 1 litre lignin diesel was calculated using the lignin and chemical prices and the electrical cost. The lignin diesel operational cost when ordered in bulk volume were calculated based on the assumption that ordering bulk volume reduces the price to a level of 10 % of the original price per litre (Andersén 2015). This does not include the distilled water and lignin powder since those prices are assumed not to vary in bulk order. The amount of electricity used in the production of lignin diesel were obtained from (Botström 2015). The calculated amount of electricity was 0.008265 kWh per produced litre lignin diesel. The electricity price was obtained from (Statistics Sweden 2015) and describes the average price for industrial consumers during the period July – December 2014. Depending on the annual consumption, the price for electricity varies between 0.43 – 1.23 SEK/kWh. The electricity cost was added to the calculated chemical cost to get the operational cost. The cost variation in historical diesel prices were examined and compared with the lignin diesel operational cost. Table 4. Chemical cost per litre.
Chemical Amount per litre lignin diesel (l/l)
Price (SEK/l)
Lignin Powder 0.0105 1.56 Distilled Water 0.0738 0.025 Ammonium Hydroxide 0.0148 365.04 Hydrogen Peroxide 0.0009 600.00 SPAN-‐80 0.0600 1650.83 n-‐butanol 0.0160 548.55 Diesel 0.8240 13.60
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3. Results In this section, the results of the work are presented. The first subsection is the solubility test continue with the statistical analysis and stabilization test of the produced lignin diesel. The operational costs are presented in the end of this section.
3.1. Solubility The amount of dissolved lignin in the different samples from step 1 in the process is presented in Table 5. Table 5. Experiments amount of dissolved percentage of lignin.
Experiment Reaction time (h) pH-‐value Water content (%) Dissolved lignin (%)
N1 18 9.7 49 48.21 N2 24 9.7 49 50.62 N3 18 12 49 99.37 N4 24 12 49 98.49 N5 18 9.7 100 62.84 N6 24 9.7 100 80.03 N7 18 12 100 98.79 N8 24 12 100 99.77 N9 18 10.85 74.5 92.68 N10 24 10.85 74.5 95.67 N11 21 9.7 74.5 74.08 N12 21 12 74.5 99.20 N13 21 10.85 49 83.69 N14 21 10.85 100 97.16 N15 21 10.85 74.5 94.08 It is clear that the samples where the pH-‐value was higher, had the highest solubility of lignin. The amount of dissolved lignin varies from almost 100 % to about half of that percentage. The values that are closest to 100 % in Table 5, are the experiments from the batches with the higher pH-‐values. The lowest dissolved values is represented as the batch with the pH-‐value of 9.7 units. This is the most apparent trend at this point of the model evaluation. The samples used in step 2, which were the best values from each batch in step 1, were sample N6, N8 and N14.
3.2. Regression model The statistical analysis is divided into three parts; evaluation of raw data, model optimization and use of model. The results from all of these steps are presented in the following subsections.
3.2.1. Evaluation of raw data The replicate plot is showed in Figure 9.
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Figure 9. Replicate plot.
In Figure 9, there are not extreme values and the software does not declare any warning messages about the values. Therefore, no changes were made among the raw data. The condition number without any changes in the raw data is 4.3973. Which indicates a good model since it is lower than eight. The scatter plots is shown in Figure 10 -‐ 13.
Figure 10. Scatterplot, dissolved lignin depending on run order.
Figure 11. Scatter plot, dissolved lignin depending on reaction time.
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Figure 12. Scatter plot, dissolved lignin depending on pH-‐value.
Figure 13. Scatter plot, dissolved lignin depending on water content.
In Figure 10 -‐ 13, the values are not perfectly symmetrical in their first performance. Figure 9 has mostly spread values. Figure 11 and 13 has a very symmetrical look. Figure 12 is not that symmetrical but still the values are collected into three areas of the plot. The overall evaluation indicates is a good symmetry. In combination with the low condition number this indicates that the model is well-‐conditioned.
Figure 14. Histogram of dissolved lignin.
The histogram that describes the statistical properties of the model is shown in Figure 14. The data and bars in Figure 13 is normal distributed but skewed to the right. This is also visible is Figure 15, which presents the statistical data in a Box-‐Whisker plot.
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Figure 15. Box-‐Whisker plot of dissolved lignin.
Figure 15 is telling us the same information as Figure 14, just with a different graphical tool. In fig 14 and 15 the data is skewed. This means that the response values in the allowed range has values in the higher part of the range.
3.2.2. Optimization
Figure 16. R2/Q2 plot.
Figure 16 shows the R2/Q2 plot in the original performance without any changes. The R2 bar is close to the value 1. This indicates an excellent model in the factor of goodness of fit. As expected, the Q2 is lower than the R2 bar. Since the Q2 value is higher that 0.5 units
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and this is a good value of the goodness of prediction. The difference in values between the bars is approximately 0.23. The value is lower than 0.3, which is preferable.
Figure 17. Coefficient plot of factors.
Figure 17 shows the coefficient plot. The small bars indicate that the corresponding factors do not affect the response factor. These factors were excluded from the model and the refined coefficient plot is presented in Figure 18. The exclusion decreased the difference between R2 and Q2 bars, and lowered the condition number to 3.9193. The reaction time alone does not affect the response value. Neither in pair with itself, the pH-‐value factor or the water content factor, the reaction time factor had influence on the response factor. The pH-‐value has, according to Figure 18, the highest influence on the response factor. The water content factor also affects the response factor, but not as much as the pH-‐value factor.
Figure 18. Coefficient plot, excluded version.
Table 6 presents the ANOVA table based on the response values. The important number to observe in this table is the probability value, p. p was calculated as 0.000 in the ANOVA analysis, this is a very good value. The lack of fit test could not be performed since no replicate runs were included in the model.
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Table 6. ANOVA table, dissolved lignin.
Dissolved Lignin
DF SS MS (variance) F p SD
Total 14 10.8943 0.778163 Constant 1 10.4894 10.4894 Total correction
13 0.404869 0.0311438 0.176476
Regression 6 0.400695 0.0667825 111.993 0.000 0.258423 Residual 7 0.00417417 0.000596309 0.0244194 Lack of fit -‐-‐ -‐-‐ -‐-‐ -‐-‐ -‐-‐ -‐-‐ (Model Error)
Pure Error -‐-‐ -‐-‐ -‐-‐ -‐-‐ (Replicate Error)
N = 15 Q2 = 0.950 Cond.
No. = 3.7607
DF = 8 R2 = 0.990 Y-‐miss = 0 R2 Adj. = 0.981 RSD = 0.0244 Figure 19 presents the normal probability plot of residuals. All of the points are inside the ±3 area except from point 5 which is outside the ±4 area on the horizontal scale. This point was excluded, which lowered the condition number to 3.7607. This changed the R2/Q2 even more, which is presented in Figure 20. The Q2 bar is above the 0.9 limit.
Figure 19. Normal probability plot of residuals.
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Figure 20. R2/Q2 plot after point exclusion.
3.2.3. Use of model In Figure 21 the response contour plot is presented.
Figure 21. Response contour plot.
Figure 21 shows the correlation and influence the factors have on the response value. The water content and the pH-‐value factors are presented at the vertical respective the horizontal axis. The three pictures represent the different reaction times. The small boxes attached to the lines are the amount of dissolved lignin at that line. The scale indicates the highest amount of dissolved lignin in the red area and the smallest values are coloured blue. In all of the three plots, the highest amount of dissolved lignin is in the upper right corner. There is a small curve in the upper right corner, which indicates that there is a small interaction between the two factors. The solution grade of lignin in water in the experimental range can be described by Equation (3).
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𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛 𝑔𝑟𝑎𝑑𝑒 = 0.943555+ 0.00494926 ∗ 𝑅𝑒𝑎𝑐𝑡𝑖𝑜𝑛 𝑡𝑖𝑚𝑒 + 0.162092 ∗ 𝑝𝐻 +0.0759666 ∗𝑊𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 − 0.0803827 ∗ 𝑝𝐻! − 0.0425337 ∗𝑊𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡! −0.0763526 ∗ 𝑝𝐻 ∗𝑊𝑎𝑡𝑒𝑟 𝑐𝑜𝑛𝑡𝑒𝑛𝑡 (3) In Equation (3) the parameters reaction time, pH-‐value and water content is varying within the same range as used as boundary values in MODDE.
3.3. Stabilization test of lignin diesel The lignin diesel samples that were taken out to the stability test are presented in Figure 22 with different time intervals.
Figure 22. Stabilization test and comparison between the sonified lignin diesel samples at time: a) 0 h, b) 0.5 h, c) 18 h, d) 48 h, e) 1 week, f) 2 weeks, g) 3 weeks.
In Figure 22, picture a), the first picture of all three samples is presented. The picture is taken at the time which is referred to as time zero. Because of some process time, the samples are 4 h, 2 h and 0 h old, from left to right in the picture. The samples are taken from different batches and the pH-‐values are 12, 10.85 and 9.7 respectively, left to right. In picture a), there is a difference in the tone between the samples, where the sample to the left has the lightest colour. The colours in each test are even in the vertical direction. No separation occurs in the samples. Picture b) shows the three tests at the time 0.5 h. The samples have more tone between them compared to time zero but there is still a difference between their colours. In each test, the colour in vertical direction is even. The samples have some separation in the bottom of the samples. Picture c) presents the tests 18 hours after time zero. The colour in the samples have a more even tone between the different samples, compared to time zero. There is a separation at the bottom of the sample, which is presented more closely in Figure 23. It is hard to decide whether the separated phase is solid particles or viscous fluid. The colour in vertical direction is even in every sample.
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Figure 23. Bottom separation in the sonified lignin diesel samples, time 18 h.
Picture d) in Figure 22 is taken at the time 48 h. The colour is even between the tests though there are a separation in the bottom of the samples. The difference from the earlier stabilization pictures is that there are small colour gradients at the top of the samples. The gradient is more apparent at upper part of the sample to the right, which has the pH-‐value of 9.7 pH-‐units. After one week, picture e) is taken. The small gradient in the sample to the right has increased to an even more apparent colour. There is still a separation in the bottom of the sample, which compared with the time zero samples have not changed in size. The samples have an even tone in comparison to each other. A closer picture at the separated bottom part after one week is shown in Figure 24.
Figure 24. Bottom separation in the sonified lignin diesel samples, time 1 week.
After two weeks, picture f) in Figure 22 is taken. There are separations at the top and bottom in all three of the samples. Colour gradients are also visible in all three samples. The sample to the right has the most distinct colour gradient. In picture g), the samples after three weeks of stabilization is presented. The vertical direction colour gradients in the samples have been more apparent compared to picture f). There are separations in the top and bottom of the samples, which have not grown in size. All of the samples have a muddy appearance which is an indication that a microemulsion has not been created (Myers 2006). The unstable samples are also a proof that a microemulsion has not been created, rather an emulsion.
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Figure 25. Disturbed lignin diesel samples at time 0.5 h.
As a comparison to the sonified samples in Figure 22, Figure 25 presents samples from the same batches that are not sonified, only stirred. The time in Figure 25 is 0.5 h, which can be compared to the samples in Figure 22 picture b). All of the samples are separated with an apparent colour gradient in the vertical direction and a separated phase in the bottom of the samples. The samples also have different tones in comparison to each other, where the left sample is much lighter than the other two. The differences between the signified samples and the only agitated samples are significant.
3.4. Operational cost The operational cost per litre produced lignin diesel when ordering the chemicals in litre size including electricity was 124,96 SEK. If the lignin diesel is produced in large scale and the chemicals therefore can be ordered in bulk size, the price per litre including electricity cost would be minimum as small as to 12,51 SEK. Figure 26 presents the variation in diesel price from 1981 to 2014. The prices refers to stock sales by tanker directly to large consumers’ facility. In the total diesel price value added tax, tax, product cost and gross margin are included. In Figure 26, the gross margin acts as a regulate factor for obtaining the clean fossil fuel cost since almost all diesel fuels contain 5 % low interspersion. (Svenska Petroleum & Biodrivmedel Institutet, SPBI)
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Figure 26. Diesel price variation during the years 1981 -‐ 2014.
4. Discussion The response contour plot shows that a higher pH-‐value with contribution of a higher water content are the most influential factors. The solubility study also reveals that the batch with the highest pH-‐value has the highest solubility. This is the same result that (Capanema et al. 2006) presents in their examination of the oxidative ammonolysis reaction. (Sun et al. 2014) do not mention any variation in the pH-‐value. The results in this study reveal that the method that (Sun et al. 2014) use has to be modified to dissolve a large amount of lignin into water. The three different plots in Figure 19 look similar. This indicates that the reaction time factor does not contribute, which differs from the result (Capanema et al. 2001b) presents. It is clear that the sonication gives a more stable lignin diesel compared to the lignin diesel that was only stirred. What is achieved is rather three emulsions than three microemulsions. The muddy colour, opacity and instability reveals that this is the case (Myers 2006). When comparing the stabilization pictures from this work with the ones presented by (Sun et al. 2014), the difference is significant. Lignindiesel with the same stabilization as obtained by (Sun et al. 2014) can not be obtained from the given information in the paper. Regarding the operational costs, the production cost in a small scale production is expensive and does not give any economical incitements. If the lignin diesel could be produced in bulk size, the price per litre would decrease significantly. If the lignin diesel price would be lower than the petroleum diesel price, there would be an economical incitement for buying the lignin diesel instead of the petroleum diesel. If the lignin diesel were tested in an engine and the lignin diesel resulted in less energy output than the regular diesel, this would at a certain level not be economically beneficial and the economical incitements would disappear. Since the diesel price has been varying during the past 15 years, the petroleum diesel price compared to the lignin diesel price is very unpredictable. The diesel price varies with the oil price. If the oil price is increasing, the lignin diesel would be more beneficial compared to the petroleum diesel. If the oil price was low, this would benefit the petroleum diesel. Governmental regulation could, in certain forms like tax deductions, subventions or allowances, lower the lignin diesel price. With the operational cost presented in this work, governmental regulations are necessary for making the lignin beneficial. The electricity cost does not affect the
0
5
10
15
2001-‐01
2001-‐11
2002-‐09
2003-‐07
2004-‐05
2005-‐03
2006-‐01
2006-‐11
2007-‐09
2008-‐07
2009-‐05
2010-‐03
2011-‐01
2011-‐11
2012-‐09
2013-‐07
2014-‐05
2015-‐03 Price (SEK/liter)
Historic diesel price data, 2001 -‐ 2014
Value Added Tax
Tax
Product Cost
Gross Margin
35
operational costs much and the variation in electricity price depending on annual electricity consumption does not influence the operational costs in this number of decimals, therefore only one price per production size were included in this work. The electricity costs variation does not affect the operational price significantly. One explanation of the muddy and unstable lignin diesel could be that during the agitation in step 2, too much air entrapment took place in the samples which gave the samples their muddy appearance (Schramm 2005). (Sun et al. 2014) used a emulsion pump instead of sonication, this could have facilitated the creation of a microemulsion. (Sun et al. 2014) used lignin from poplar, which structure differs from the lignin used in this work. The stabilization tests could be performed with a more precise testing method since the current measurements are hard to evaluate. The observation method does not reveal important information about the lignin diesel samples. As an example of this, the characteristics of the separated phase are not possible to determine from just observing the samples. Neither, an evaluating grading system of the samples stability has been established. The emulsion stability could be evaluated as in (Imazu & Kojima 2013; Hu et al. 2011). It would also be interesting, as earlier mentioned, to determine the achieved molecular size in the lignin diesel. Which size and which actions that could decrease the particle size further in order to create a microemulsion could be examined. Further research should be performed on finding different methods that could optimize the creation of a stable microemulsion. An example could be calculating the HBL-‐value for each sample, since this differs among the samples with the method described in (Griffin 1954). There has been no replicate experiments performed and included in the statistical model. Since one objective with using MODDE was to examine the repeatability of the experiment, the lack of replicate experiment is a significant error in the work. As another result of this, no Lack of fit test could be performed. The model error and replicate error could therefore not be examined. This is an important part in the purpose of evaluating the models credibility and get an indication of whether the model is well executed or not. It also evaluates the model’s predictive properties. Since the repeatability of the work was not examined, the results of the statistical study should be seen as indications of the outcome of the experiments rather than quantitative results. In further experiments, replicate experiments should be included in the model to gain this information about the model. (Eriksson 2008) The different values of solubility are presented in the replicate plot. The values in the plot are based on the measured amount of dissolved lignin in the filtrated samples. Since the statistical MODDE model is based on the response values, any error during the filtration and weighing is included in the statistical model. The original performance of the model, which is presented in the R2/Q2 plot, Figure 14, indicated a satisfying model with a good predictive ability. In the R2/Q2 plot where some factors and points were removed, Figure 18, the model is even better and has a better capacity to predict results that could describe the actual system satisfactorily. After the changes, the condition number is lower. This strengthens the reliability of the model. In the ANOVA table, Table 6, the regression, p, value is very low, which indicates that the
36
significance of the regression model is very satisfying. This indicates that the model is well performed and its predictive power is good. In the first scatter plot, Figure 8, where the dissolved lignin depends on the run order, the values are very spread out. This could be a result of the changed run order that was made. In Figure 9, where the amount of dissolved lignin depends on reaction time there is a symmetrical pattern. This indicates a good parameter setting of the reaction time factor or the probability that the reaction time factor did not influenced the response factor. The scatter plot where the dissolved amount of lignin depends on the pH-‐value, Figure 10, does not have a symmetrical geometry and this indicates that a higher pH-‐value gives a higher solubility. The higher and lower boundary values could be changed to get a better model in this case but since this is the pH-‐factor which is limited to a 14 units scale this probably would not affect the scatter plot and making it better. One solution could be including several response factors or changing the response factor. The scatter plot where the amount of dissolved lignin depends on the water content, Figure 11, is symmetrical which indicates a well executed boundary value choice of the water content factor. Eventually, this is an indication on the factors lower affection on the response variable. The geometry in the four scatter plots in combination with the low condition number, gives a well executed model design as well as good factor and response design. The water content in the samples was never measured and therefore no exact values are obtained. It is possible that the calculated water content is lower than the actual value due to the exhaustion of ammonia gas. To be more accurate in further studies, the water content should be measured. To get a better picture of the water content’s effect on the formation of the microemulsion, more samples from step 1 should have been tested in step 2. Now there are only samples with the water content of 100 %. In further research, different amounts of water content should be examined in the creation and stabilization of the microemulsion since the water content is not mentioned by (Sun et al. 2014). A different range setting in the reaction time factor could result in a more varied result. In further work, the range should have a lower lowest value and a higher highest value. Earlier initial tests show that the reaction is influential on the response factor, the current range does not indicate this, which indicated that a different boundary value setting should be preferred. The filtration method could be made in a more accepted and accurate way. Some of the modified lignin samples were very difficult to filtrate. When the samples with lowest pH value were filtrated, the high amount of undissolved lignin sealed the filter paper. This prevented the dissolved lignin liquid from passing through the filter. The same problem was observed with some of the samples with low water content. This could indicate that the water was saturated and the ability of the water to dissolve lignin were not enough in the lower water content. A different filter paper can in further research be used. If the particle size is examined, a filter paper that would be more adjusted to that size could be used. The particle size could be measured using the methods in (Sargolzaei et al. 2011; Imazu & Kojima 2013). The histogram and the Box-‐Whisker plot, Figure 12 and 13, present a skewed normal distribution. Since the majority of the values are in the upper part of the response variables range, this normal distribution is formed. To get a more normal distributed
37
model, the factors and its ranges could be evaluated. The response variable could be measured in another way or eventually, more response variables could be included. More bars in the coefficient plot could be excluded, but at a certain degree of factor exclusion, the models ability to describe the system is impaired. To avoid this, the changes in the condition number observed through the process of creating the statistical model. Since the condition number still is lowered through every change, the model with exclusions still has a good predictive ability. The fifth point in the N-‐plot is excluded from the model, which lowers the models condition number and therefore increases its predictive ability. When excluding the fifth point, MODDE suggested that one further point could be excluded. The point was inside the ±4 area on the horizontal axis. This exclusion increased the condition number and the point was therefore kept in the model. Further research could be made with the goal to finding the optimal pH-‐value for creating the optimal oxidative ammonolysis process. This could be made with FTIR analysis as used in (Sun et al. 2014; Brandén 2015) Further research should be made in order to examine how the increase in pH-‐value affects the use in an diesel engine. Another interesting question is how the addition of sodium hydroxide affects the formation of the microemulsion and the use in an diesel engine? If a microemulsion lignin diesel in another experiment is created, this could be tested is an diesel engine and evaluated as in (Sun et al. 2014). The characteristic properties of the diesel should be evaluated in order to fulfil the quality requirements in (Directive 2011/63/EU) to sell the lignin diesel in the European Union.
5. Conclusion In this work the solubility of lignin in water was examined. It is possible to dissolve 100 % of the lignin powder into water. To create a stable homogenous mixture of modified lignin and diesel, more further research is needed. The operational cost for producing lignin diesel is expensive and subventions are needed to create incitements for consumers to buy lignin diesel.
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