Biodiesel production in a batch reactor

Transcription

1 Biodiesel production in a batch reactor Biodiesel is obtained through transesterification reaction of soybean oil by methanol, using sodium hydroxide as a catalyst. The reaction is taking place in a batch reactor. In order to record the progress of the transesterification reaction the fatty acid methyl esters of biodiesel are analysed by gas chromatography (GC). The samples that collected in intervals are firstly separated in a centrifuge to glycerine and biodiesel and subsequently analysed by the GC. The conversion should be monitored and plotted for all the collected samples as a function of time. The selectivity of the catalyst for at least one of the products should be investigated as well. Finally the second order of the reaction has to be confirmed using the integral method. 1. Theory Biodiesel production is gaining increasing attention since in principle can reduce more CO 2 emissions significantly. It has also many other environmental advantages [1]. The most common way to obtain biodiesel is the transesterification reaction of vegetable oils in the presence of a low molecular weight alcohol and a catalyst. The transesterification reaction involves the exchange of organic groups R1, R2, R3 belonging to a glyceride with the organic group of an alcohol R, as is shown in figure 1. Fig. 1: The transesterification reaction. R1, R2, R3 is a mixture of various fatty acid chains. The alcohol used for producing biodiesel is usually methanol (R = CH3) [2]. The overall process is normally a sequence of three consecutive steps, which are reversible reactions. In the first step, diglyceride is obtained from triglycerides, from diglyceride monoglyceride is produced and in the last step from monoglycerides glycerine is formed (figure 2). In all these reactions fatty acid methyl esters (FAME) are produced. The stoichiometric relation between alcohol and the oil is 3:1. However, an excess of alcohol is usually more appropriate to improve the reaction towards the desired product: - 1 -

2 Fig. 2: The three consecutive and reversible steps of the transesterification reaction [2]. The reaction product is an immiscible two phases mixture of biodiesel including excess methanol and glycerine. During separation (e.g. centrifuge) the hydrophilic and denser glycerine migrates to the bottom of the mixture creating a separate layer, while the less dense biodiesel stays on top together with the unconverted oil. 2. Reaction mechanism The mechanism of the base-catalyzed transesterification of vegetable oils is shown in figure 3. The first step is the reaction of the base with the alcohol, producing an alkoxide and the protonated catalyst. The nucleophilic attack of the alkoxide at the carbonyl group of the triglyceride generates a tetrahedral intermediate, from which the alkyl ester and the corresponding anion of the diglyceride are formed. The latter deprotonates the catalyst and reacts with a second molecule of alcohol starting a new catalytic cycle. Diglycerides and monoglycerides are converted by the same mechanism to a mixture of alkyl esters and glycerine. Fig. 3: Mechanism of the alkali-catalyzed transesterification of vegetable oils (B:base)[3]

3 Alkaline-catalyzed transesterifications proceed at considerably higher rates than acidcatalyzed transesterifications. Due to this fact and also because alkaline catalysts are less corrosive to industrial equipment, most commercial transesterifications are conducted with alkaline catalysts. 3. Kinetics To determine the reaction rate constant the following equation (1) can be used in combination with the integral method (see Scott Fogler, Elements of Chemical Reaction Engineering): C i ra t There are different factors, which influence the reaction rate, such as the influence of the temperature and the catalyst. (1) 4. Set up The experiments will be performed in a batch reactor, as shown in figure 4. The following equipment will be used during the experiments: 1. Five-necked batch reactor (500 ml) 2. Electric stirrer for batch reactor 3. Reflux 4. Thermocouple 5. Water bath with electric stirrer and temperature indication 6. Magnetic stirrer plate and magnet 7. Erlenmeyer flask (500 ml) 8. Stop watch 9. Ice bath 10. Graduated pipette (5 ml) 11. Graduated pipette (50 ml) 12. Graduated cylinder (100 ml) 13. Pipette balloon 14. Funnel 15. Centrifuge tubes 16. Centrifuge apparatus 17. Vials 10ml Fig. 4: Batch reactor. Materials: 1. Soybean oil (SBO) 2. Methanol 99% 3. Sodium hydroxide (NaOH) in pellets (the catalyst) 4. Methyl heptadecanoate (Internal standard IS) in heptane solution, with concentration 5mg/mL - 3 -

4 5. Procedure The following steps are given in order to perform the reaction and subsequently the analysis of the products using gas chromatography (GC). A. Reaction 1. Take with volume the SBO amount, place it in the reactor and start heating the water bath to the desired temperature and start the stirring at the desired speed. 2. Take with volume the MeOH amount, weight the NaOH amount and place both in an Erlenmeyer flask (500 ml) and cover with parafilm. Stir vigorously using a magnetic plate and stirrer bare until the NaOH pellets are completely dissolved and the methoxide solution is formed. 3. When the reactor reaches the set-point temperature, add the catalyst solution using the funnel, while simultaneously starts measuring the time using the stop watch. 4. Start collecting samples after 3 min (the 1 st sample in the 3 rd min). In total you should collect 12 samples with a 5 ml pipette in the indicated intervals (7 samples each 1 min, 3 every 3min and 2 every 5 min). Try to be accurate in the recording of the time while collecting the samples. (Take note of the average temperature while you collect each sample, shown in the thermocouple indication). 5. Place the sample into numbered centrifuge tubes cooled in the ice bath. (SHAKE slightly the tube to stop the reaction and then let it settle). 6. Place the centrifuge tubes in the centrifuge apparatus using 400rpm for 10min. (Note the maximum centrifuge speed for the rotor is 400rpm). Note that your glassware should be clean and DRY. When water is present, deesterification takes place via hydrolysis (and forms soap which causes problems such as plugging, gel formation, an increased viscosity that may hamper the product separation), which should be avoided. Note that sodium methoxide solution id is a strong base and should be handle with care. B. Analysis The next steps are for the preparation of the GC sample that contains BIOD and unconverted SBO. 1. Weight 250mg of sample in 10 ml vials using PASTEUR pipette (TAKE the sample form the upper layer only!). 2. Adjust 5ml of the IS methyl heptadecanoate in heptane solution (Use the beaker only for the IS solution). 3. Collect 1 L and inject it into the GC (Wash ten times the syringe using the sample each time before collecting the sample amount). 4. Press START in the chemstation software, make the injection in the back injection port and press START button in the GC (Try to repeat the injections following the same timing between injection and pressing of the START button in order to get repeatable GC retention times)

5 6. Reaction conditions The reaction conditions will be given from the supervisor for each group in the 1 st meeting, as is shown in table 1. Table 1: Reaction conditions regarding reactor temperature, NaOH catalyst weight percentage and mole fraction MeOH/SBO. Reaction condition Temperature [ C] Stirring speed [rpm] Mole fraction MeOH/SBO NaOH %w/w V tot= ~ 350 ml Values A B C D 7. GC data calculations In the GC plot the peaks belongs to heptane (C:7), IS methyl heptadecanoate (C17:0) and the fatty acid methyl esters (FAME) mixture corresponding to the biodiesel products (BIOD) can be identified, as shown in the example in figure 5. Fig. 5: GC plot (example) of the FAME. It is assumed that response factors (RF) of the FAME equals one. This is valid since the IS methyl heptadecanoate (C17:0), is very similar to the methyl esters that we want to detect. So finally from the GC report you will get the weigh percentage (% w/w) of each peak

6 The total amount of the BIOD detected from the GC can be derived from the summary of the FAME %w/w equation (2). The range of the products includes the FAME of the palmitic (C16:0), the stearic (C18:0), the oleic (C18:1), the linoleic (C18:2) and the linolenic (C18:3) acid. % w / w % w / w % w / w... % w / w (2) BIOD C 16:0 C 18:0 C 18:3 The peak of heptane it is not included in the integration made from the GC software and it doesn t appears in the GC report. Also you should remember that the sample that you prepare and insert in the GC (look 5.B.) contains BIOD, IS in heptane and also a portion of unconverted soy bean oil (SBO UNC ). BUT the amount of SBO UNC can not be detected by the GC column. So the real composition of the sample injected to the GC is given by the equation (3). % w / w % w / w % w / w 100 (3) BIOD IS SBOunc Although from the GC report only the ratio of the %w/w BIOD to the %w/w IS can be calculated, as is shown in the equation (4). But as the real %w/w IS it is a known (calculated value), using equation (4) simultaneously with the correction factor a, which has been just calculated just before, the %w/w BIOD can derive (In total the equation 4 is used 2 times). Consequently the real %w/w SBO UNC can be obtained from equation (3). % w / w % w / w BIOD IS a (4) To calculate the % conversion of BIOD the formula given in equation (5) should be used. moles moles SBO SBO % Conversion UNC 100 (5) moles All the calculations about the conversion are referred to the sample that you prepare for the GC (look 5.B.). Based on that it is assumed that the initial moles of the SBO is those existing in the 250mg of the sample that we would weight when the conversion was equal to zero. To calculate the % selectivity of on product the formula given in the equation (6) has to be used: SBO moles FAME i % Selectivity FAME 100 i moles FAME i (6) - 6 -

7 Things you MUST do Before the Lab: (to be approved by me before you get access to the lab. Not done = no lab) 1. Fill out chemical cards, apparatus card and get an overview over HSE. Make a copy of the first page of the HSE booklet, sign it and give it to me. 2. Look at the reaction procedure. Try to understand what is happening. 3. Calculate amounts of reactants and catalyst from the given reaction conditions in the table 1 and make sure that are correct. 4. Calculate the %w/w of the IS in the sample injected in the GC (look 5.B.). 5. Prepare a plan showing how you plan to carry out the experiments. (The work plan has to be approved from the supervisor in order to be able to proceed the experiment). After the Lab: (to be included in the report) 1. Calculate the mass in grams of the SBO unconv, BIOD and IS contained in the sample that you prepare for the GC. 2. Calculate the concentrations of the SBO, BIOD and SBO UNC and use the integral method to validate the second order of the reaction. Justify your results. Average molecular weight (MW) of SBO and BIOD, as well their densities as well are given in the appendix Calculate the % conversion for each sample according to the formula (5) and plot the progression of the conversion as a function of the time. 4. Calculate the selectivity for all of the methyl esters for your entire number of samples and justify the possible preference of the catalyst for producing one specific methyl ester compound more than the others. You should deliver in appendix the detailed calculations for one of your samples and then tables contained all the calculated values for all of your samples and the different tasks. References [1] The Biodiesel handbook / editors: Gerhard Knothe, Jon Van Gerpen, Jürgen Krahl,(2005)AOCS Press. [2] J.M. Marchetti et al. / Renewable and Sustainable Energy Reviews, ARTICLE IN PRESS [3] A. Demirbas / Progress in Energy and Combustion Science 31 (2005) [4] H. Noureddini, D. Zhu/ JAOCS, vol.74, no. 11(1997) Appendix 1. Name AMW Density g/mol g/ml SBO 875,1 0,913 BIOD 291,5 0,891 IS 270,45 0,853 Methyl esters C16:0 C18:0 C18:1 C18:2 C18:3 MW 270,46 298,51 296,50 294,48 292,46-7 -

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