BIODIESEL PRODUCTION IN A BATCH REACTOR 1. THEORY

Transcription

1 BIODIESEL PRODUCTION IN A BATCH REACTOR Date: September-November, 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. In this exercise biodiesel is produced by transesterification of soybean oil (SBO) by methanol, with sodium hydroxide used as the catalyst. The reaction is carried out in a batch reactor. The progress of the reaction is monitored by collecting samples in intervals, seperating out the product in a centrifuge, and analyzing the fatty acid methyl esters (FAME) by gas chromatography. The conversion should be monitored for all the samples, and plotted. Whether the catalyst is selective towards a specific product should be investigated. The reaction order should be investigated with 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 Fig. 1. FIGURE 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 (Fig. 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. Therefore a ratio of 6:1, as used by Noureddini et al. [3] will be used in this exercise. The transesterification reaction at this ratio has been described by a second-order mechanism [3, 4]. FIGURE 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. Date: September-November,

2 2 BIODIESEL PRODUCTION IN A BATCH REACTOR 2. REACTION MECHANISM The mechanism of the base-catalyzed transeste rification of vegetable oils is shown in Fig. 3. The first step is the reaction of t he 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. Alkaline-catalyzed transesterifications proceed at considerably higher rates than acid-catalyzed FIGURE 3. Mechanism of the alkali-catalyzed transesterification of vegetable oils (B=base) [5]. transesterifications. Because of this, 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 (3.1) can be used in combination with the integral method (see Scott Fogler, Elements of Chemical Reaction Engineering, section 5.2.2): (3.1) dc dt A r A where C A is the concentration of A, and r A is the reaction rate. There are different factors, which influence the reaction rate, such as the influence of the temperature and the catalyst.

3 BIODIESEL PRODUCTION IN A BATCH REACTOR 3 BEFORE THE LAB The reaction conditions will be given from the supervisor for each group before the lab. See Table 4. TABLE 1. Reaction conditions regarding reactor temperature, NaOH catalyst weight percentage and mole fraction MeOH/SBO. Reaction condition Values Temperature ( C) A Stirring speed (rpm) B Mole fraction MeOH/SBO C NaOH ( %w w ) D A workplan must be handed in to the supervisor two days before the lab. This workplan must be approved in order to gain access to the lab. 1. Read through the HSE form for the experiment. Write a brief summary of the potential hazards associated with the chemicals used in the lab, and how to handle them. 2. Write a plan for the experiment. For all groups: V tot 350mL 3. Calculate the amounts of reactants and catalyst from the given reaction conditions. 4. Calculate the weight percentage ( %w w ) of internal standard (IST) that is injected in the GC if the sample weight is 250 mg. The concentration of the IST is 5mg/mL. 4. EXPERIMENTAL SET UP The production of biodiesel will be performed in a batch reactor, as shown in Figure Five-necked batch reactor (500 ml) 2. Electric stirrer for the batch reactor 3. Reflux 4. Thermocouple 5. Water bath 6. Erlenmeyer flask 7. Stop watch 8. Automatic pipette plastic pipette tubes (one for each sample) 10. Funnel Centrifuge tubes (one for each sample) Glass vials (10 ml) (one for each sample) 13. Magnetic stirrer + magnet 14. Electronic analytical balance scale Materials: 1. Soybean oil (SBO) 2. Methanol 99% 3. Sodium hydroxide (NaOH) in pellets 4. Methyl heptadecanoate (internal standard, IST) in heptane solution, concentration 5 mg/ml FIGURE 4. Setup

4 4 BIODIESEL PRODUCTION IN A BATCH REACTOR 5. PROCEDURE 5.1. Preparing biodiesel. 1. Measure the amount of SBO and place it in the batch reactor. It can be advisable to add the SBO before fastening the lid of the reactor to avoid oil coating the neck of the reactor. 2. Measure the amount of MeOH and NaOH and place the reactants in the erlenmeyer flask. Cover immediately with parafilm. Stir the solution with a magnetic plate and stirrer until the NaOH pellets have dissolved. 3. Once the SBO in the reactor reaches the set point temperature, add the methoxide solution using a funnel and simultaneously start the time. 4. Start extracting samples (5 ml) with the automatic pipette after 3 minutes. See Tab. 2 in the appendix for an overview over when to extract the samples. Remember to change the pipette tube for each sample! Monitor and record the temperature as well as time for each collected sample. 5. Place each sample in a numbered centrifuge tube (kept in an ice bath), and shake the cooled tubes slightly to stop the reaction and let it settle. 6. Wipe off the centrifuge tubes and place them in the centrifuge. Let the centrifuge run for 10 min at 4000 rpm. (Note the maximum centrifuge speed for the rotor is 4000rpm). Note that your glassware should be clean and DRY. When water is present, de-esterification 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 is a strong base and should be handle with care Analysis of product. The following steps are for analysing the product in the GC. The lab supervisor will assist with this step. 1. Weigh 250 mg of each sample in 10 ml vials using a pasteur pipette. Be careful to only extract liquid from the upper layer. Remember to write down the weight for each sample. 2. Add 5 ml of IST to each sample. Do not extract IST directly from the volumetric flask to avoid contamination of the standard solution! 3. Transfer the sample solution to the GC vials. 4. Make sure the hand held gas detector is switched on, and placed near the detector. 5. Place the GC vials in the sample carousel of the automatic sampler. 6. Make a sequence table. 7. Run sequence. 6. AFTER THE LAB: (TO BE INCLUDED IN THE REPORT) 1. Calculate the mass in grams of the SBO unc, BIOD and IST contained in the sample that you prepared for the GC. 2. Calculate the concentrations of the initial amount of soybean oil in the reactor (SBO ini), BIOD and SBO unc. 3. Use the integral to investigate the order of the reaction. Justify your results. Average molecular weight (MW) of SBO and BIOD and their densities are given in the appendix. 4. Calculate the % conversion for each sample and plot the progression of the conversion as a function of time. 5. 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. Note Detailed calculations for one of the samples must be included in the appendix. Tables containing all the calculated values for each item in the above list must be included in the appendix for all the samples (even if data is presented as a figure in the report, then a table containing the values must still be included in the appendix). The calculations from the workplan should also be included in the appendix. BUT remember: derivations first, then the result.

5 BIODIESEL PRODUCTION IN A BATCH REACTOR 5 RESULTS FROM GC In the GC plot the peaks belongs to heptane (C:7), IST 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 Fig. 5. It is assumed that response factors (RF) of the FAME equals one. This is valid since the IST 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. FIGURE 5. Example of GC plot of FAME. 1. Use the area of the peaks (pa*s). 2. Sum the values for C16:0, C17:0, C18:0, C18:1, C18:2, C18:3 to obtain the total area. 3. Get the percentages of each peak by Area C16:0 % w wc 16:0 AreaTotal 4. Get the weight percentage of biodiesel for each sample % w w % w w % w w % w w % w w % w w BIODGC C16:0 C18:0 C18:1 C18:2 C18:3 5. Use the relation below to find the actual weight percentage of BIOD (%w/w BIODR ) in the sample % w wbiod % w w GC % w w % w w BIODR C17:0GC C17:0 R where % w w C17:0 R is the calculated weight percentage of internal standard for each sample. 6. Obtain the amount of SBOunc in each sample (SBOunc is not detected by the GC) % w w % w w % w w 100 SBOunc BIODR C17:0R CONCENTRATIONS All calculations are based on the samples you weigh (approximately 250 mg) after centrifugation. 1. Calculate the mass of BIOD, unconverted soybean oil (SBO unc) from the weight percentages and known weight of the samples. 2. Find the volume of the samples from the calculated masses and the given densities. V sample = V SBOunc + V BIOD 3. Calculate the initial concentration of soybean oil (SBO ini) (imagine weighing a sample of pure SBO). 4. Plot 0th, 1st and 2nd order using the integral method.

6 6 BIODIESEL PRODUCTION IN A BATCH REACTOR nsbo CONVERSION Plot conversion as a function of time. is the amount of moles in the sample if the entire amount was SBO msample ( nsbo ). You will therefore have one value per sample. M SBO n %Conversion n SBO SBO unc n SBO SELECTIVITY Calculate the selectivity of each FAME in BIOD. Plot the selectivity (in this instance the mol percentage) as a function of time. n %Selectivity n FAME,i THE REPORT FAME,i X100 It is recommended to use BibTeX, Mendeley, EndNote, or other reference managers to handle the references. The report must include an abstract. Otherwise see Felles Lab: Short Report Guidelines by Heinz, A Preisig. REFERENCES [1] Gerhard Knothe, Jon Van Gerpen, and Jrgen Krahl. The Biodiesel handbook. AOCS Press, Champaign, Illinois, first edition edition, [2] J.M. Marchetti, V.U. Miguel, and A.F. Errazu. Possible methods for biodiesel production. Renewable and Sustainable Energy Reviews, 11(6): , [3] H. Noureddini and D. Zhu. Kinetics of transesterification of soybean oil. Journal of the American Oil Chemists Society, 74(11): , [4] Bernard Freedman, Royden O. Butterfield, and Everett H. Pryde. Transesterification kinetics of soybean oil 1. Journal of the American Oil Chemists Society, 63(10): , [5] Ayhan Demirbas. Biodiesel production from vegetable oils via catalytic and non-catalytic supercritical methanol transesterification methods. Progress in Energy and Combustion Science, 31(56): , 2005.

7 BIODIESEL PRODUCTION IN A BATCH REACTOR 7 7. APPENDIX TABLE 2. Overview of when to take samples from the batch reactor Time [min] Sample No TABLE 3. Molar weights and densities of the components injected into the GC. Component MW (g/mol) Density (g/ml) SBO 875,1 0,913 BIOD 291,5 0,891 IST 270,5 0,853 TABLE 4. Molar weights of the methyl esters. Methyl esters C16:0 C18:0 C18:1 C18:2 C18:3 MW (g/mol) 270,46 298,51 296,50 294,48 292,46