Alternatives for Bio-Butanol Production

Transcription

1 Alternatives for Bio-Butanol Production Feasibility study presented to Statoil 12/ Tutors Sixten Dahlbom Hans T. Karlsson Hanna Landgren Christian Hulteberg Peter Fransson Industry advisor/statoil representatives Børre Tore Børresen Per Nygård I

2 Abstract Title: Authors: Advisors: Course: Aim: Methodology: Theory: Data collection: Results: Key words: Alternatives for Bio-Butanol Production Sixten Dahlbom, Hanna Landgren and Peter Fransson Hans T. Karlsson, Christian Hulteberg, Per Nygård and Børre Tore Børresen KET050 (Projektering) The aim of this project is to improve the production of butanol through comparison with a butanol process investigated in 2008, and some other alternative butanol processes. The processes consist of: Identical butanol production plant as in the report from 2008, but with other purification processes. As well as using a similar ethanol plant as in the report from 2008 but with a chemical conversion of ethanol into butanol. The latter can be performed in a single- or multi-step procedure. For these alternatives the group will consider energy balances as well as investments and production costs. The group will first perform a literature survey and then simulate the best alternative for production of butanol. When the simulation is finished the group will calculate the costs of production and investment. ASPEN PLUS is used for the simulations and the Ulrich method is utilized for economical calculations. Mostly secondary data will be used; electronic sources and books. Also some primary data will be used; help from the advisors are used in this project. The results point at obvious flaws in the production of butanol through indirect synthesis using a condensation reactions, these drawbacks are mostly consisting of the poor productivity and large production of side products. None of the studied alternatives seems to be profitable, although out of the five compared alternatives the two perstraction methods are the most promising. The two perstraction methods have pay-off times of 15.2 years compared to the process using HAP as a catalyst which has a pay-off time of 40.1 years, with the current cost and revenue situation. Vacuum Distillation, Butanol Production, Separation Methods, Investment Costs, Operating Costs I

3 Table of contents 1 Introduction Company Presentation Aim Method The Simulation Program Aspen plus Economic Assessment The Feedstock The Bacteria, Clostridiaceae Background Risk Aspects in the Butanol Process Separation Processes to the ABE Fermentation Gas Stripping Membrane Separation Methods Perstraction Simulation Results for Perstraction Adsorption Liquid-Liquid Extraction Vacuum Distillation Simulation Results for Vacuum Distillation Direct and Indirect Butanol Synthesis from Ethanol Direct Synthesis Hydroxyapatite (HAP) as Catalyst Magnesium Oxide (MgO) as Catalyst Zeolites as Catalyst Process Flow Diagram, Direct synthesis Simulation Results for Direct Synthesis Further Investigations and Improvements Indirect Butanol Synthesis Ethanol Acetaldehyde Acetaldehyde Crotonaldehyde Crotonaldehyde Butanol Process Flow Diagram, Indirect Synthesis Financial Analysis Investment Expenditure for the five Alternatives Operating Costs for the five Alternatives Revenue for the five Alternatives Production cost for the five Alternatives Net Present Value for the five Alternatives Pay-Off Time for the five Alternatives Sensitivity Analysis for the Five Alternatives Financial Comparison of the five Alternatives Recommendations Synthesis of Butanol from Ethanol Separation Processes Economy References II

4 1 Introduction For 20 weeks, a group consisting of four students from Lund University in Sweden will work together with Statoil ASA, Norway. The group will continue the study made by Larsson, E. et al. [1] : A feasibility study on conversion of an ethanol plant to a butanol plant. More in detail, the group will investigate different separation methods for purification of the products from the butanol fermentation described by Larsson, E. et al. [1]. The group will also study whether it is economically sustainable to convert ethanol to butanol. The ethanol will be produced by the existing plant described by Larsson, E. et al. To assist the group, tutors from both Lund University (Hans T. Karlsson and Christian Hulteberg) and from Statoil ASA (Børre Tore Børresen and Per Nygård) are involved. 1.1 Company Presentation Statoil is an international company with activity in 34 countries and with the headquarters in Norway. The company is large, with around employees worldwide and is publicly traded on the New York stock exchange. Statoil wants to grow even more and operate in more countries than they do today. [2] Figure 1 below shows in which countries Statoil has offices today. [3] Figure 1: Shows which countries Statoil operates in. [3] Statoil produces electricity, oil, methanol as well as ethanol and has a large focus on natural recourses. Because of the fact that Statoil believes that natural gas will be more and more important in the future, they put large effort on finding supplies and harvesting it. Even though Statoil is currently coupled to negative environmental impact they try to compensate for it by paying respect to issues that can be improved in order to decrease their impact on the environment. [2] 1.2 Aim In order to reduce the dependence of fossil fuels, production of butanol as a gasoline substitute, diesel substitute or additive is an interesting alternative. Butanol is historically mainly produced by fermentation, but the toxicity of butanol to the organism producing it leads to low production, the concentration in the fermenter will be at most 1-2wt% butanol. However, today almost all butanol is produced from propane using the oxo process. Another problem with the fermentation route is that the butanol has to be separated from the 1

5 fermentation broth, which costs a lot of money and energy. To improve the process of butanol production, the group will compare a process studied in 2008 [1] with some other processes. These are: Identical butanol plant as in the report from 2008 [1], but with other purification processes. Using a similar ethanol plant as in the report from 2008 [1] and afterwards convert ethanol to butanol. This can be performed in a single- or multi-step procedure. For the three alternatives listed above the group will consider: Energy balances Investment costs Production costs 1.3 Method The project will start by making a literature survey where facts will be gathered from primarily peer-reviewed articles, but also from books. The literature study will be used as a basis for performing calculations on parameters such as investment and production costs, as well as energy efficiencies. Simulations of the process will be performed in the computer program Aspen. Data that might be of interest can be viewed in appendix A The Simulation Program Aspen plus Aspen plus is a flow sheeting software, that lets the user draw large and advanced industrial processes. It also simulates and calculates mass and energy balances. The software includes physical properties for a numerous chemicals, and features a vast variety of models for phase equilibrium calculations. Aspen will be used in the project for simulation of the overall process, along with the occasional integration of MATLAB code. 1.4 Economic Assessment The production economy will be calculated using the Ulrich method where the results contain both operation and investment costs. 1.5 The Feedstock The feedstock that will be used to produce butanol is lignocellulose, which is a combination of cellulose, hemicellulose and lignin. Lignocellulose is the largest of the world s carbon based renewable natural resources and exists in plants and trees. [4] The lignocellulose to be used in this project consists of 40-50% cellulose 15-25% hemicellulose and 15-30% lignin. Cellulose is a polysaccharide consisting of multiple glucoses on a chain, hemicelluloses is similar but consists of several different sugars, not only glucose. Lignin is a large macromolecule which acts as a glue binding together all the polysaccharides. These three substances are the main components of trees and the lignocellulose to be fermented. Since the sugars are bound in polysaccharides they will need to be broken down before the butanol producing cells can utilize them as feed. The composition of spruce and its bark is seen in table 1 below. [1] 2

6 Table 1: The raw material composition. [1] Component Spruce Bark Glucan Xylan Galaktan Arabinan Mannan Lignin Ash - - Other The Bacteria, Clostridiaceae To produce butanol a bacteria family called Clostridiaceae is used. These bacteria produce solvents (butanol, ethanol and acetone, therefore the fermentation is called ABEfermentation), organic acids (acetate and butyrate) and gases (carbon dioxide and hydrogen). Depending on which bacteria that is used, different parameters are relevant. One possible bacterium is Clostridium saccharoperbutylaccetonicum which during anaerobic immobilized cell batch systems can reach productivities of 0.36gABE L -1 h -1 at PH 6.0 and a temperature of 30 C. C. Saccharoperbutylaccetonicum is a hyper butanol producing strain and it is somewhat resistant to the product inhibition, therefore it can produce a higher concentration of butanol than the commonly used bacteria. In large scale production it might be best to use immobilized microbial cells since they have many advantages, they are e.g. easier to separate from the product, they can reach a higher cell density and the productivity is greatly improved. At most these bacteria can grow up to a total solvent concentration of 20gABE L -1. The results are based on cultures growing in a medium containing a pure carbon source, in our case a complex medium is used consisting of lignocellulose as carbon source which will most likely lower the production rate. [5] 1.7 Background The purity of butanol shall be 99.5 wt%. The amount of ingoing dry spruce is tons per year. In 2008 Larsson, E. et al. [1] made a feasibility study on converting an ethanol plant to a butanol plant. A flow diagram describing the existing ethanol plant can be viewed in figure 2. To convert the ethanol plant to a butanol plant the following changes were suggested: The yeast in the fermentation step shall be changed to the bacteria Clostridium Acetobutylicum. The total fermenter volume is to be increased. The distillation part of the plant has to be expanded, this because much more water is included in the butanol processes. As with the distillation, the evaporation part is to be expanded. For a detailed description of the existing ethanol plant or the suggested modifications please review the report by Larsson, E. et al. [1]. A simplified flow diagram describing the butanol process is shown in figure 3. 3

7 Figure 2: Simplified flow diagram over the existing ethanol plant [1] Figure 3: Simplified flow diagram over the, by Larsson et al., suggested butanol process. 1.9 Risk Aspects in the Butanol Process In the butanol process there are only a few risks concerning the extra steps with regards to producing butanol instead of ethanol. The bacteria cultures used (Clostridium) can cause stomach problems if ingested [6]. Of course there are some risks coupled to the equipment but the risks are not larger than for the existing production of ethanol. 4

8 2 Separation Processes to the ABE Fermentation The high cost of distillation in the current ABE process [1] is a consequence of product inhibition. Due to the inhibitory effect, butanol can only be produced as a diluted component. In order to lower the cost for product purification different methods have been studied, these are: Gas stripping Perstraction Pervaporation Adsorption Liquid-liquid extraction Vacuum distillation A comparison between different refining methods was published [24], the results are shown in table 2 and 3. Table 2: A comparison between different refining methods [24] Stripping Adsorption Extraction Pervaporation Capacity Moderate Low High Moderate Selectivity Low Low High Moderate Fouling Low High Moderate Low Operational High Low Low High Table 3: A comparison between different refining methods, A-acetone, B-butanol, and E-ethanol [24] Recovery method Product Estimated total heat of recovery (MJ/kg ABE) Stripping B and AE mixture 21 Adsorption B and AE mixture 33 Extraction and perstraction ABE mixture 14 Pervaporation B and AE mixture Gas Stripping Gas stripping is a simple way to separate butanol because it does not require expensive equipment and it lowers the butanol concentration. Inert gas will be sparged through the fermentation broth during fermentation and volatile butanol will vaporize and go out with the gas stream in the top of the reactor. Inert gases are appropriate to use in gas stripping, examples of such can be N 2 and CO 2 (care will have to be taken with the ph-decrease associated with CO 2 dissolving into the liquid). Gas stripping is a better technique than membrane filtration and adsorption since it does not remove the reaction intermediates from the broth. [17] Gas stripping is also viewed as one of the most economic techniques. [18] One disadvantage is that the butanol has the highest boiling point of the solvents and therefore is disfavored when the stream takes up the volatile substances from the fermentation broth. The concentration of ABE will be high in the gas stream and therefore contain much butanol [17]. N. Qureshi and H.P. Blaschek found that by using gas stripping as shown in figure 4 the productivity can be increased by 41% and the bacteria will use 50% more of the lactose than normal. The concentration in the recovery stream can be as high as 75.9 g ABE l -1 which is much better than in a batch reactor. Of course these results are from lab scale experiments so it is not sure that the same numbers will be reached in a full scale. [17] The effect on the butanol 5

9 production varies with the gas recycle rate, acetone and ethanol concentrations in the broth and gas bubble size. [19] Figure 4: Flow sheet for gas stripping. 2.2 Membrane Separation Methods Two methods involving a membrane for purification have been investigated by the group. These two methods are pervaporation and perstraction. [20,21,22] Perstraction The substance to be separated diffuses through a membrane into a solvent on the other side. This way of separating liquids does not differentiate much from the method known as extraction. However there is at least one benefit; the bacteria is separated from the solvent. This enables the use of a solvent harmful to the bacteria. It is said that the diffusion of butanol through the membrane is the rate controlling step [20]. However as long as the production rate in the reactor is even lower, this should not be a problem. Qureshi, N. and Maddox, I.S. have studied the reduction in butanol inhibition using perstraction. They used a silica tubing as membrane and oleyl alcohol as solvent. As substrate they used lactose/whey permeate. Their result was satisfying; a lactose concentration of 227gL -1 could be used (compared with 28.6gL -1 normally), at this lactose concentration the productivity was 0.07gL -1. Using perstraction ca. 99 gl -1 h -1 (fermentation broth volume) ABE was produced. The experiment was run for 391 hours and the concentration of ABE in the oleyl alcohol was at maximum 9.75gL -1. Figure 5 shows butanol, ethanol and butyric acid concentrations in oleyl alcohol at various levels of these chemicals in aqueous phase during the experiment. According to Qureshi, N. and Maddox, I.S no acetone can be found in the organic solvent. 6

10 Figure 5: Butanol, ethanol and butyric acid concentrations in oleyl alcohol at various levels of these chemicals in aqueous phase during fermentation perstraction experiment. [20] In situ product separation in butanol fermentation by membrane-assisted extraction has been studied by Jeon, Y.J.; Lee, Y.Y. [21]. They used the same membrane and the same organic solvent as Qureshi, N. and Maddox, I.S. The results of Jeon and Lee are depicted in table 4. Table 4: Overall performance of membrane-extractive butanol fermentation. [21] In figure 6 (below), a flow diagram that shows where the perstraction shall be implemented in the ABE process. 7

11 Figure 6: Simplified flow diagram over the ABE process using perstraction Simulation Results for Perstraction First of all it is important to mention that it is not sure that any suitable membrane exists, therefore these results are speculative. However, as long as the solvent is not harmful to the bacteria, about the same result can be achieved using extraction. Since the bacteria are separated from the solvent by the membrane, a wide range of solvents can be used. The group has mainly focused on two different solvents, mainly due to limited time. The focus was on a 50/50 wt% decane/oleyl alcohol mixture and on mesitylene. The reasons: the oleyl/decane mixture seems to be the most commonly used in experimental reports and mesitylene was suggested by a research group to lower the energy demand in the case of separating the products using extraction [35]. According to this group s results as low as 5.7 MJ/kg butanol was needed using mesitylene (compared to 15 MJ/kg butanol using pure oleyl alcohol). Since the solubility of mesitylene in water is extremely low they suggested that the toxicity towards the bacteria might be negligible. If not, this can be solved using perstraction, as long as a membrane with sufficient properties exists. The fact that acetone, ethanol and butanol are removed continuously allows the process to be run as a fed-batch process. Therefore the project group estimated that 20 fermenters are needed (compared to originally 23), the retention time is 60 hours. The volume of each fermenter shall be m 3 using mesitylene as solvent (the fermenter volume is 1000 m 3, the extra volume is reserved for the solvent) and m 3 using oleyl alcohol/decane as solvent. Inside every fermenter a membrane shall be placed, figure 7. The effective fermentation volume is 800 m 3. 8

12 Since the process is run as a fed-batch, fewer fermenters are needed, however they will probably have to be washed and the lignin in the feed has to be removed somehow, emptying the tank seems to be the easiest way of doing it. The outlet from the fermenter consists mostly of water and lignin. When the fermenter is emptied, the outlet contains the same mass of lignin as the mass of lignin retained from separation (only) by distillation, based on the same time. The outlet is passed to an evaporator line. The design of the evaporation line depends on how often the fermenters are to be emptied (energy demand and sizing), this will be left to later studies. Due to time limitations, the group will consider the energy and the size to be the same as in separation by distillation. The same goes for the pretreatment (size and energy). Figure 7: A fermenter fitted with a membrane which on the other side has the solvent flowing Depending on the membrane s properties, different answers will be returned. The group has made two assumptions, these are: Only acetone, butanol, ethanol and water can diffuse through the membrane. The mass transfer limitations are small enough to be neglected (since the production rate is low). If not, a larger membrane area is needed to obtain the same result. Whether water can diffuse through the membrane or not, as well as if the limitations in mass transfer can be neglected, can be discussed Decane/Oleyl Alcohol A suggested flow sheet is depicted in figure 8. Five distillation columns are needed, these will operate under different pressures and energy is to be added to two of them (table 5). A total of 16.1 MW is needed for separation and kg pure butanol will be produced every hour. This means that 17.0 MJ/kg butanol is needed. Important streams are shown in table 6. Table 5: Specifications for the distillation columns needed in the Decane/Oleyl alcohol alternative Col-1 Col-2 Col-3 Col-4 Col-5 Pressure [bar] Reboiler temp. [ºC] Energy demand [MW] Table 6: Specifications of the streams for the Decane/Oleyl alcohol alternative Mass fraction [wt/wt] Flow Total flow [kg/h] Water Butanol Ethanol Acetone Oleyl alcohol Decane Temp. [ºC] Feed , e-4 3.6e Butanol , e e-3 40 Et/Bu/Wa , e-4 75 Acetone e e Solvent ,

13 Figure 8: Flow sheet for Decane/oleyl alcohol alternative Mesitylene A suggested flow sheet can be viewed in figure 9. For the separation of the products only three distillation columns are needed, pressure, reboiler temperature and energy demand for each one of them are shown in table 7. Energy is only needed in the first column (12.6MW), kg pure butanol will be produced every hour. This means that 14.2 MJ/kg butanol is needed. Important streams are shown in table 8. The experimental distribution coefficients are reported [35] to differ a lot from the coefficients generated by the UNIFAC model. The distribution coefficients are (in the same report) also reported to be quite a lot higher at 80ºC than at 25ºC, therefore the perstraction will be performed at 80ºC. In order to keep the temperature in the fermenter at 37ºC, it need be cooled instead of heated. Table 7: Specifications for the distillation columns needed in the Mesitylene alternative Col-1 Col-2 Col-3 Pressure [bar] Reboiler temp. [ºC] Energy demand [MW] Table 8: Specifications of the streams for the Mesitylene alternative Mass fraction [wt/wt] Flow Total flow [kg/h] Water Butanol Ethanol Acetone Mesitylene Temp. [ºC] Feed e e-4 4.6e Butanol Et/Bu/Wa e Acetone e e Solvent

14 Figure 9: Flow sheet for the Mesitylene alternative Pervaporation Pervaporation and different membranes for this application has been studied by numerous researchers. The group has focused on one report (due to limited time) and therefore it cannot be guaranteed that this setup is the best, but it is experimentally verified. The report describes ABE recovery by pervaporation using different silicate/silicone composite membranes from a fed-batch reactor [22]. Qureshi, N. et al. made their own membranes, the procedure is well described in their report. The different membranes were characterized for flux and selectivity (defined as in equation 1) at 78 C using model ABE solution and actual fermentation broth, the results are shown in table 9. Selectivity = (y/(1 y))/(x/(1 x)) [Eq. 1] y = weight fraction of component in permeate sample x = weight fraction of component in retentate sample In their further studies they choose the membrane with the highest selectivity towards butanol. Their experimental setup is shown in figure 10. A UF-membrane is used between the reactor and the pervaporation, this since the reactor temperature is 35 C, but the pervaporation temperature is 78 C. 11

15 Table 9: Flux and selectivity of a silicate-silicone and silicone membranes using model solution and fermentation broth at 78 C. [22] Figure 10: A schematic diagram of ABE production in fed-batch reactor and recovery by pervaporation (a) fermentation reactor; (b) ultra filtration membrane unit; (c) buffer tank; (d) pervaporation membrane unit (e) cold traps. (1.1) cell culture stream; (1.2) ultra filtration membrane permeate; (1.3) pervaporation membrane retentate recycle; (1.4) cell free fermentation broth recycle stream through pervaporation membrane; (1.5) pervaporation membrane permeate stream. [22] Qureshi, N. et al. showed that the butanol selectivity not was affected by the broth. The experiment lasted for 120 hours. They also showed that ethanol and acetic acid not diffused through the membrane at concentrations lower than 0.04gL -1. In total the fed batch reactor was operated for 870 hours and 155 gl -1 solvent was produced. The solvent yield (g solvents/ g glucose utilized) was Adsorption A stream of fermentation broth from a butanol producing fermenter can be run through a membrane filter or a centrifuge to separate the biomass from the water, butanol, ethanol and acetone. The solids can then be recirculated to the fermenter while the liquids are run through adsorption columns. The columns can be filled with hydrophobic adsorbents of almost any kind (often silica). In the case of butanol production, the substances which will bond to the column are butanol, ethanol, acetone and some water. The components not adsorbed will be taken back to the fermenter. Thus the fermentation process can be run continuously, see figure

16 Figure 11: Shows the process for absorption of butanol [25]. After the packing in columns have adsorbed butanol it can be desorbed by increasing the temperature to around 200 C. This has been suggested to greatly decrease the energy costs as ordinary distillation would require 73.3 MJ/kg butanol, while adsorption only would need 8.2 MJ/kg. Two columns should be used to ensure continuous production, then the two columns can alternate between being loaded (adsorption) and unloaded (desorption). [1] Research has shown that some zeolites can favor adsorption of butanol over both ethanol and acetone resulting in highly concentrated desorbed solutions. Studies have been made on the adsorption of the ABE-system by zeolites where the feed consists of either filtered or nonfiltered fermentation broth. The results here also showed that butanol is favorably adsorbed on the solid phase adsorbent; results can be seen in table 10. The table shows that about 100g butanol/kg adsorbent can be reached which is a good result, but can be improved further if used on industrial scale as adsorption material and conditions could be optimized further. The desorbed solution will contain over 90% of butanol which is relatively high; the rest will mostly contain acetone and some water. [23] The butanol concentration can then be increased further by distillation [1]. Table 10: Shows the feed concentrations and the adsorbed concentration of two filtered and two non-filtered fermentation broths. [24] 13

17 2.4 Liquid-Liquid Extraction Liquid-liquid extraction is considered to be suitable for butanol recovery. The unit operation can be placed inside as well as outside the fermentation tank [24]. To choose an appropriate organic solvent in the extraction setup, some considerations have to be made [25] : The solvent has to be compatible with the bacteria used for fermentation. The solvent has to have a high capacity for the fermentation products. This to minimize the amount of solvent needed and the product recovery cost. If the products are recovered from the solution by distillation, the solvent should be less volatile than the products. However it is important that the solvent is not so nonvolatile that high pressure steam is needed in the reboiler. The solvent has to be barely soluble in water, this to minimize solvent losses. A solvent with more or less these properties has been reported by Roffler, S. et al., namely oleyl alcohol diluted to 50 wt% in decane [25]. One can probably think of some other solvents, but to the groups knowledge no other has been reported. It is imported to mention that oleyl alcohol is toxic to the culture when exposed over a long period of time [26]. Data for pure oleyl alcohol and for the oleyl alcohol/decane mixture can be seen in table 11. Table 11: Data for pure oleyl alcohol and for the oleyl alcohol/decane. [26] Extractant Oleyl alcohol Reference Oleyl alcohol /decane (50 wt%) Reference Distribution coefficient (g/l butanol in solvent)/ (g/l butanol in broth) Selectivity Density [kg/m 3 ] Viscosity [cp] Butanol diffusion coefficient [m 2 /s] 1.1* * Specific heating capacity [kj/kg] N.N. Solubility in water [mg/l] N.N Roffler, S. et al. have studied the economic savings that can be made using a fed-batch extraction process for the production of butanol. They came up with a flow sheet describing the process, this can be seen in figure 12. They also came up with, for this feasibility study, some relevant remarks. Based on their calculations the production cost of butanol can be reduced by ca. 20% (year 1987). They also said that the productivity increased from 0.58 g/(l*hr) in batch culture to 1.5 g/(l*hr) in fed-batch culture. When the productivity is increased the numbers of fermenters are reduced. They also conclude that concentrated feedstocks can be fermented and stillage treatment costs are significantly reduced. 14

18 Figure 12: Process flow diagram of a fed-batch fermentation employing oleyl alcohol/decane as Extractant. [25] 15

19 2.5 Vacuum Distillation A decreased pressure leads to a lower boiling point. The reduction of pressure means that less energy is needed to boil any components, however, more energy is then needed to sustain the low pressure. A diagram for the boiling points of butanol, ethanol and acetone at different pressures can be seen in figure 13. One has to consider that these boiling points are for the pure elements and not for the ternary mixture. The vacuum distillation set up does not need to be very different from the regular distillation set up. The pressure in the column has somehow to be controlled. In figure 14 the difference between the two processes is shown. Figure 13: A diagram for the boiling points for butanol (dots), ethanol (triangles) and acetone (crosses) at different pressures [30],[31],[32] Figure 14: Vacuum distillation (left), regular distillation (right) [29] In figure 15 (below), a flow diagram that shows where the vacuum distillation shall be implemented in the ABE process. 16

20 Figure 15: Simplified flow diagram over the ABE process using vacuum distillation. 17

21 2.5.1 Simulation Results for Vacuum Distillation The main idea behind the vacuum distillation process is to lower the pressure in the distillation columns, see distillation step in butanol process specification [1], and determine the changes in the process, e.g. the overall power demand. Figure 16: Flow sheet over vacuum distillation step The overall process diagram (figure 16) does not differ from the original butanol process [1] and in the calculation the project group assumed that process specifications (e.g. energy demand, size of apparatus and others) are the same as in the old process, except for the distillation step. Pressure reduction was mainly carried out in the stripper part of the distillation process, due to the fact that the main energy demand lies here. Two conditions were taken into accounts when determine the pressure in the stripper columns. 1) The temperature in the high-pressure columns should be high enough to run the reboiler in the low-pressure columns, a pressure differential of 0.9 bar was chosen. 2) The temperature in the top of the low-pressure column should be high enough to use cooling water, which is assumed to have a temperature of 17 C, for liquefaction of the top stream. With the above mentioned conditions, a pressure of 1 bar in the high-pressure columns and 0,1 bar in the low-pressure ones was chosen. Power requirement for the high pressure columns where calculated to 22.7 MW (11.3MW each).the pressure in the decanter is set to 3.5 bar and a temperature of 115 C, for a good separation. The pressure in the three remaining distillation towers kept at atmospheric pressure, except for rectifier number one, which is set to a pressure of 0.5 bar. The reason for this decision is because condense from rectifier number 2 should be able to heat the reboiler in distillation column 1 and 3. No changes were made in the second decanter. 18

22 Table 12: Distillation tower specifications HP 1 and 2 LP 1 and 2 R-1 R-2 R-3 Pressure [bar] Reboiler temp. [ºC] Energy demand [MW] Table 14: Flow specification for vacuum distillation. Mass fraction [wt/wt] Flow Total flow [kg/h] Water Butanol Ethanol Acetone Temp. [ºC] Feed Acetone Water/ethanol , Butanol

23 3 Direct and Indirect Butanol Synthesis from Ethanol There is an existing desire to find an alternative to the well-known ABE-fermentation for industrial production of butanol. One possible alternative is to use a catalyst for a single step synthesis of butanol from ethanol; another is to convert ethanol to acetaldehyde, acetaldehyde to crotonaldehyde and crotonaldehyde to butanol. 3.1 Direct Synthesis As for now, there are no existing industrial size plants in operation which uses this type of reaction. However, there exist numerous reports of experiments using different types of catalysts. The most common once are hydroxyapatit (HAP) [7], magnesium oxide (MgO) [8] and Zeolites. [9] Hydroxyapatite (HAP) as Catalyst Hydroxyapatite has been reported as a useful catalyst for condensation of ethanol to butanol by Tsuchida et al. al [7]. Non-stoichiometric hydroxyapatite can be written as: Ca (10-Z) (HPO 4 ) Z (PO 4 ) (6-Z) *nh 2 O, where 0<Z<1 and n=0 2.5 The reaction mechanism as suggested by Tsuchida et al. is as follows: Ethanol adsorbs on the HAP-complex and a C-C bond is formed between a β-carbon in the ethanol molecule and an α-carbon in an n-c n H 2n+1 OH molecule. The products are n-c n H 2n+1 CH 2 CH 2 OH and water. Further it was suggested that a part of the formed n-c n H 2n+1 CH 2 CH 2 OH molecule will be adsorbed and form a C-C bond between a β-carbon, in the C n H 2n+1 CH 2 CH 2 OH molecule and an α-carbon in an n-alcohol, and a branched alcohol is produced. As shown above, besides n-butanol the reaction mechanism leads to numerous n-alcohols and branched alcohols as byproducts. These include aromatics, acetaldehyde, ethylene and ether, just to name a few. Takashi et al. determined the optimal condition for the butanol production by varying the ratio between calcium- and phosphorous atoms in the HAP-complex, the temperature and the contact time. The experiments were performed at atmospheric pressure. The highest butanol yield was obtained at Ca/P ratio of 1.64 [7]. At this ratio the ethanol conversion is about 15%. Further it was establish that a temperature of 300 C [7] and a contact time of 1.72 s [7] is best suited to gain as high butanol selectivity as possible; which in this case, according to the report, is 76% [1]. The effects of different contact times and the temperature can be seen in table

24 Table 15: Effect of temperature and contact time on selectivity, taken from Tsuchida et al. [1] As mentioned above, the experiments were carried out at atmospheric pressure, which is not well suited for an industrial process. A process at higher pressures is more suitable. To investigate how an increase in pressure will affect yield and conversion, a MATLAB simulation of the reaction rates were carried out. The reaction rate constants for the main reaction (S1) and the unwanted reactions (S2-S13) are given in table 16 together with activation energy. Table 16: Reaction rates, taken from Tsuchida et al. report [7] The result of the simulation was satisfying, plots showing yield, conversion and selectivity as functions of time can be seen in figures 17 to 19. With an increase in pressure the optimal contact time decreases, the ethanol conversion and selectivity, at optimal contact time, will decrease respectively increase. The butanol yield will however increase. 21

25 Figure 17: Butanol yield and optimal contact time at different pressures. Figure 18: Ethanol Conversion at different pressures. 22

26 Figure 19: Ethanol Selectivity at different pressures. As of now, HAP catalytic condensation of ethanol looks promising. However, the largest disadvantage is numerous byproducts which compete with butanol Magnesium Oxide (MgO) as Catalyst A.S. Ndou et al. studies of magnesium-oxide catalyzed synthesis of butanol from ethanol show a maximum butanol yield of 20%, byproducts/intermediates include acetaldehyde, crotonaldehyde, crotylalcohol and butanal The experiments were carried out at atmospheric pressure and around 400 o C [8]. A mechanism was proposed by Ndou et al. This is: butanol is derived from two different reaction mechanisms. In the first mechanism ethanol dehydrogenates to acetaldehyde and further acetaldehyde is converted to crotonaldehyde via an aldol condensation, and in the last step crotonaldehyde is hydrogenated to butanol. The second mechanism is the same as the one proposed by Tsuchida et al. (mentioned above) Zeolites as Catalyst The mechanism behind the condensation reaction of ethanol, using zeolites as catalyst, has been studied [9]. Yang et al. [9] proposed two thinkable mechanisms for reaction, of which both has already been mention above. After analysis of the results from the experiments Yang et al. [9] came to the conclusion that the mechanism proposed by Tsuchida et al. [7] is the most likely one have taken place. The conclusion made by Yang et al. [7] seems to fit the other above mentioned catalysts Process Flow Diagram, Direct synthesis As a first approach, the project group suggests a process for single step synthesis of butanol from ethanol, based on the existing ethanol process [1]. 23

27 Figure 20: Simple process flow diagram over single step synthesis of butanol. The suggested process is the same as the existing ethanol plant, except for some changes after the distillation step (see figure 20). In the original process the ethanol/water stream from the distillation would be directed to a dewatering step to produce pure ethanol. Instead the ethanol/water stream will be sent to a tubular reactor packed with a catalyst. The outlet gas from the reactor will be distillated in a number of steps, to separate butanol and ethanol from byproducts. The separated ethanol will be recirculated to the reactor. 24

28 3.1.5 Simulation Results for Direct Synthesis The overall process is based on the ethanol plant [1] with an added HAP step, which includes a reactor for conversion of ethanol to butanol and five distillation columns for purification of butanol and ethanol recovery. The simulations and process description (figure 21, tables 17 to 19) below focus on the HAP step, for more information on the ethanol plant see the rapport from 2008 [1]. Figure 21: Flow sheet over HAP step The outlet ethanol stream from the existing ethanol plant, 4 721kg/h ethanol, 217 kg/h water will be vaporized and preheated up to 107 C. Next step is to compress the gas stream in a compressor from 1 bar to 10 bar. Further the gas stream needs to be preheated up to the desired temperature of 300 C and with this the pretreatment of inlet stream to the tube reactor concludes. The reactor is supposed to run isotherm at 300 C and because the overall net reaction is exothermic the reactor needs to be equipped with a cooling jacket The results from the simulation states that 750 kw need to be removed to keep the reactor isothermal, this gives an opportunity to preheat other streams in the process. The optimal size of the reactor were estimated with the help of a MATLAB simulation, the volumetric flow of the inlet stream were estimated to 897 m 3 /h (300 C, 10 bar) and the reactor volume was calculated to 0.24m 3 (the reactor is packed with hydroxyapatite and was assumed to have a void of 0.5, this corresponds to 385 kg catalyst). The outlet stream consist of kg/h water, kg/h ethanol, kg/h butanol, kg/h higher alcohols (170 kg/h hexanol, 687 kg/h 2-etyl- 1butanol, 766 kg/h 2-etyl-1butanol and others), 39 kg/h alkenes (13.3 kg/h 1,3-butadien, 9.7 kg/h hexen and others) and 0.75 kg/h hydrogen. The Outlet stream from the reactor will be distilled in the first distillation tower. Here ethanol will be separated from the butanol, where ethanol will go out from the top of the tower and the butanol in the bottom. The alkenes from 25

29 the inlet stream will pass through the distillation tower with the ethanol over the top and the other alcohols will follow the butanol. A small part of the top stream will be non-condensable, due to the hydrogen in the stream, and will pass over the top out from the process. The energy that needs to be removed in order to condensate the rest of the top stream is large enough to run the reboiler for distillation tower 5 and heat exchanger HX2, for vaporization and preheating of the ethanol feed. In order to achieve the above mentioned energy savings the temperature in the top of the column has to be high enough i.e. the pressure in the distillation column needs to be high enough. A pressure of 7 bar satisfies these conditions. The function of the second distillation tower is to separate butanol and water from higher order alcohols. The higher alcohols will leave in the bottom of the column and butanol/water leaves in the top. The energy received from condensing the top stream is enough to cover the power demand of distillation tower 4. In order to get pure butanol, the top stream from the second distillation column will be separated, in a decanter, into a water phase and an organic phase. The stream has a butanol weight percentage over the butanol/water azeotrope and now the organic phase can be distilled in distillation column 5. Pure butanol will pass through the column in the bottom, kg/h, and the top water/butanol stream, 516 kg/h water and 706 kg/h butanol, will be recirculated to the decanter. Distillation tower number 4 separates ethanol and water from alkenes. Alkenes, 7 kg/h water, 133 kg/h ethanol, 4 kg/h butene, 9 kg/h hexene, 5 kg/h acetaldehyde, 12 kg/h 1,3-butadiene, leaves in over the top of the column and ethanol/water, 348 kg/h water, 2919 kg/h butanol, in the bottom. In the last distillation column water, 159 kg/h water, 4 kg/h butanol, pass through the bottom and ethanol/water stream, 188 kg/h water, kg/h butanol, at the azeotrope, passes over the top of the column and mixes with the ethanol feed before the pretreatment step. Table 17: Distillation tower specifications for the HAP catalysis alternative Distillation Tower 1 Distillation Tower 2 Distillation Tower 3 Distillation Tower 4 Distillation Tower 5 Pressure [bar] Reboiler temp. [ºC] Energy demand [kw]

30 Table 18: Flow specifications for HAP catalysis Stream, mass fraction (wt/wt) Component Feed Inlet Outlet Gas1 Dest1 Bot1 Dest2 Bot2 Water Ethanol e Butanol e Hexanol Etyl-1Butanol Octanol e e Etyl-1Hexanol Ethene e-4 9.7e-3 1.9e Butene e-4 5.6e-3 1.2e Hexene e e Acetaldehyde e-4 8.6e-3 1.4e Hydrogen gas e e Octene e e-3 1.9e-3 0 1,3-Butadiene e e Total Flow (kg/h) Table 19; Flow specifications for HAP catalysis Stream, mass fraction (wt/wt) Component Organ Wat Dest3 Bot3 Gas2 Dest4 Bot4 Dest5 Bot5 Water e Ethanol 1.0e-3 5.0e-4 2.4e e-2 Butanol e e-3 Hexanol Ethyl- 1Butanol Octanol Ethyl- 1Hexanol Ethene e Butene Hexene Acetaldehyde Hydrogen gas Octene 1.0e-3 4.0e-3 2.7e Butadiene Total Flow (kg/h)

31 3.1.6 Further Investigations and Improvements Further improvements need to be done in minimizing the losses of ethanol and butanol, as for now there is a loss of 88kg/h butanol (46kg/h in the water-phase stream and 42kg/h in the bottom stream of the second distillation column) and 133kg/h ethanol, e.g. recycle the waterphase stream to distillation column number 2. Some improvements can be made in the energy demand and recovery, e.g. with the help of pinch analysis. Further studies on the reaction kinetics need to be carried out in order to determine if there are any restrictions in mass transport or diffusion in relation to the reaction kinetics. 3.2 Indirect Butanol Synthesis Butanol can be produced if ethanol is reacted to acetaldehyde, acetaldehyde to crotonaldehyde and finally crotonaldehyde to butanol [10], reaction (1-3). These three reactions are all well studied and described in the literature. A drawback using this method of producing butanol is the additional need of process equipment. In this chapter the chemical processes used for production of acetaldehyde, crotonaldehyde and butanol will be described. [reac.1a] [reac.1b] [reac.2] [reac.3] Ethanol Acetaldehyde Acetaldehyde can be produced from ethanol in two different processes; ethanol can either be dehydrogenated (reac.1a) or oxidized (reac.1b). The benefit with the dehydrogenation reaction is the simultaneous production of hydrogen. Hydrogen is needed in reaction three, so the hydrogen produced in reaction one can be used in the third reaction. On the other hand the catalyst life in the oxidation process is longer and the possibility of recovering energy is better [10] (due to the fact that dehydrogenation of ethanol is an endothermic reaction) Dehydrogenation of ethanol In the temperature range C ethanol vapor is passed over a catalyst. The catalyst is typically made of copper. However, some other catalysts have also been reported, for instance nickel supported by SnO 2, Al 2 O3 or SiO 2 [11], and palladium or platinum modified alumina [12]. The reason of using copper as a catalyst is that the lowest amount of decomposition products are reported using this catalyst [10]. A drawback using copper as a catalyst is that the catalyst is subjected to rapid deactivation, which results predominantly from sintering [12]. The reaction takes place in a tubular reactor and a conversion of 25 50% is obtained [10] per pass. Acetaldehyde selectivity up to 100% has been reported at 200 C using Ni/SnO 2, Ni/Al 2 O 3 or Ni/SiO 2 as catalyst, at higher temperatures the selectivity is decreasing [11]. In industrial processes the selectivity to acetaldehyde is 90 95% [13]. In order to separate ethanol and acetaldehyde from the exhaust gas (mainly hydrogen), the gas is washed with an ethanol/water mixture. The ethanol which has not reacted is recovered in a 28

32 distillation step and recirculated. The final acetaldehyde yield in industrial processes is ca. 90 % [10]. A simplified flow sheet describing this process can be seen in figure 22. Byproducts in the dehydrogenation process are butyracetate, crotonaldehyde, higher alcohols and ethylene [13]. Figure 22: Schematic flow sheet over the process for dehydrogenation of ethanol. A reactor, B/C heat exchangers, D/E purification steps [10] Oxidation of Ethanol One of the most known processes is the Veba-Chemie Process [10]. In this process ethanol vapor and air are passed over a silver catalyst at a pressure of 3 bars and a temperature in the interval C. The conversion of ethanol varies between % per pass with a selectivity of 85 95% [13]. The ethanol which has not reacted and the produced acetaldehyde are removed from the exhaust gas by washing with ethanol. Ethanol and acetaldehyde are separated by distillation. A simplified flow sheet describing the Veba-Chemie Process can be seen in figure 23. The byproducts in this process are acetic acid, formic acid, ethyl acetate, CO and CO 2. Figure 23: Schematic flow sheeting describing the oxidation of ethanol. A reactor, B heat exchanger, C purification [10] step/reactor, D purification step 29

33 3.2.2 Acetaldehyde Crotonaldehyde The common method to produce crotonaldehyde is the aldol reaction of acetaldehyde followed by dehydration of the acetaldol (reaction 4). [reac.4] In order to produce acetaldol, acetaldehyde is reacted in a tubular reactor at a temperature ranging from 20 up to 25 C. The residence time is several hours [13]. To catalyze the reaction an aqueous sodium hydroxide solution is used [14] (sodium hydroxide will at any time be found as an ion in water). The conversion of acetaldehyde is restricted to 50 60% to limit resin formation and secondary side reactions. The reaction is stopped and the reaction-mixture neutralized by adding acetic acid [13]. The selectivity to acetaldol is 85% and the main byproduct is crotonaldehyde. The dehydration of acetaldol occurs readily in the presence of acetic acid. In industrial processes the dehydration step is implemented in the purification steps. The reactor vessel (where the aldol condensation takes place) is followed by a stripping column. The acetaldehyde which has not reacted is recirculated back to the reactor. The acetaldol is fed to a distillation column, dehydrated and a crotonaldehyde/water mixture is distilled to the azeotrope and separated into water and an aqueous crotonaldehyde phase containing 10% water. Finally, the crotonaldehyde/water mixture is fed into a rectification column. A simplified flow sheet describing the process can be seen in figure 24. Figure 24: The acetaldehyde to crotonaldehyde process [14] Crotonaldehyde Butanol This reaction can be carried out in both liquid- and vapor-phase processes [15],[16]. If the reaction is to be carried out as liquid reaction, a solvent must be used. The solvent has to be separated from the product later on, this is not needed if the reaction instead is a gas phase reaction. Using a Cu/Al 2 O 2 catalyst, crotonaldehyde conversion up to 100% and butanol selectivity up to 100% can be achieved [16]. The temperature should be at least 150 C. If this catalyst is exposed to sulfur it will be poisoned and has to be replaced. 30