CHEMICAL ENGINEERING LABORATORY CHEG 237W

Transcription

1 HEMIAL ENGINEERING LABRATRY HEG 237W BIDIESEL PREPARATIN LAB BAKGRUND: Global warming will become one of the most challenging tasks for man to overcome over the next century. As with any task, when viewed in the proper perspective it can become an avenue of enormous opportunity. As future chemical engineers, you will have the skills to place you at the forefront of the worldwide effort to develop a solution to this problem. This lab experience will draw from all aspects of your education over the last four years. It is representative of the type of project you will be exposed to as you begin your professional career. All energy utilized on the Earth today with the exception of nuclear energy ultimately is derived from solar radiation impinging upon the Earth. Approximately 30 % of this thermal radiation is reflected back into space. The Sun, as it is in a period of relatively steady state energy output is therefore essentially a constant source of energy for the Earth. The problem that arises comes from the fact that arbon Dioxide in the high atmosphere re-reflects a portion of the thermal energy that was to be radiated into deep space back down to the Earth. This increases the net energy content of the Earth, thus increasing its average temperature. There is essentially a fixed mass of arbon on the planet. The Element arbon is the basis for all life here on the Earth. ne of the reasons life probably developed based on arbon is that there is a vast difference if the Gibbs free energy of arbon in its most reduced forms relative to its oxidized forms. There is a constant cycle of arbon being incorporated into life from the environment only to be returned at the death of the organism. This leads to a relatively stable balance of arbon that is free in the environment and that which is sequestered. The large energy difference between arbon s reduced and oxidized states has lead to its use as an energy source for society. As energy usage has exploded exponentially over the past century, we have disrupted the balance of arbon thus increasing the amount of oxidized arbon in the biosphere. This fact makes reduced arbon based compounds an ideal energy storage media for societies insatiable appetite for energy. There is no other readily available energy storage media that can match the energy storage density of arbon based fuels. So, until better energy storage media is developed, we will continue to use arbon based fuels to survive. The best option at present is therefore to change the way we treat arbon from an expendable commodity as it is presently being utilized into a working fuel. An analogy to help explain this process is to imagine arbon assuming the role of Water in a steam power plant. In the boiler, energy is added to the water molecules giving them the potential to perform work. In the arbon cycle, oxidized arbon is extracted from the atmosphere by photosynthetic organisms and converted utilizing solar radiation into an energy dense storage media, triglyceride molecules. When we burn the triglyceride products for energy we obtain energy in a similar fashion to steam being run through a turbine to produce work. xidized arbon is released into the atmosphere to be recycled

2 again. The advantage to this process is that it is closed cycle contributing no net increase of green house gasses, unlike the open cycle system utilized today. BJETIVE: The objectives of this lab includes the following: 1. Introduce the concept of Biodiesel production as one of the methods that will be employed to solve the crisis of global warming. 2. Introduce the process of scaling up reactions from the bench top to full scale. 3. Associated with the increase in the scale of the process, is the development of strategies to compensate for imperfect mixing and mass transfer limitations. THERY: In this lab, we center our attention on processing raw fuel, triglyceride into a more refined product, fatty-acid- methyl-esters, which are better suited to the currently available energy conversion technologies available today. The reaction pictured above in Figure 1 is carried out by mixing KH with Methanol to form the strong organic base Methoxide. Methoxide solution is added to Triglyceride with heating and stirring to complete the reaction. The reaction proceeds via the following nucleophillic substitution mechanism.

3 Reaction Mechanism Partial Positive harge Fatty Acid linked via Ester linkage H 3 - H 3 - Full Negative harge Methoxide Bond Broken Electrons to Glycerol H H 3 H H H H 3 - Methyl Ester Glycerol Nucleophillic Attack by the Methoxide on the arbonyl Group Unfortunately, life is not that simple. All chemical reactions are theoretically reversible and this one lends a significant reality to that theory. The reverse reaction is possible via a similar nucleophillic mechanism. Reverse Reaction Mechanism Methyl Ester H H 3 H H H H 3 Methanol H H Glycerol H Mono-Glyceride H

4 Thus, there are three linked equilibrium relationships being modeled. This leads to six Arrhenius dependent kinetic parameters as one would expect from the Reaction Kinetics classes. Using experimental data to determine the kinetic parameters the following reaction model can be derived. kf 1 kr1 Tri Glycerides + Methanol Di Glycerides + Methyl Ester kf 2 kr 2 Di Glycerides + Methanol Mono Glycerides + Methyl Ester kf 3 kr3 Mono Glycerides + Methanol Glycerol + Methyl Ester This model will give an idealized optimal reaction time given perfect mixing, the first assumption on which modeling assumption are based upon. The trans-esterification reaction is mildly exothermic; an isothermal approach to the kinetics is a valid assumption. PRELIMINARY PREPARATINS: Using a basis of 500 ml oil, the first assignment to be completed before pre-lab is, given the following physical properties, determine the amount of Methanol and KH that need to be mixed for a stoichiometric conversion. The density of Methanol and vegetable oil are 0.79 g/ml and 0.9 g/ml respectively. Use 885 g/mole as an average molecular weight for the vegetable oil. From your stoichiometric calculations, we are going to double the amount of Methanol, and be prepared to explain why we re going to do this. Second, we re going to double the amount of base as well due to the fact that some of the triglyceride breaks down naturally leaving free fatty acids in the oil. This free acid will use up your Methoxide and not leave enough for adequate conversion of the Triglyceride into Methyl-Ester. High fatty acid content in used oil leads to other problems as well. Base catalysis cannot convert free fatty acids into Methyl-Esters. To convert free fatty acids, an immobilized Sulfuric acid catalyst system can be used. SAFETY PREAUTINS: The mixing of Methanol and KH is highly exothermic and must be carried out in the hood without heating. The KH will dissolve with stirring. Those little bubbles you see if you mix too much too fast are methanol vapor that is just as hazardous as the liquid.

5 Appropriate eye protection and chemical resistant gloves are required at all times. Look up safety sheets for all chemicals that will be used. EQUIPMENT: See Diagram PREDURE: To introduce the experiment a small batch reaction is first carried out. Under the hood in the lab a 500 ml sample of vegetable oil is heated to 50 in a water bath. While the oil is heating up, KH is dissolved in Methanol. In large scale operation, it is advisable to pretest the oil s acid content prior to reaction to ensure enough catalyst is utilized. The mixing of Methanol and KH is highly exothermic and must be carried out in the hood without heating!! The KH will dissolve with stirring. Those little bubbles you see if you mix too much too fast are methanol vapor that is just as hazardous as is the liquid. Appropriate eye protection and chemical resistant gloves are required at all times when handling Methanol and KH. The Methoxide, once mixed, can be poured into the flask containing the oil. Upon pouring the Methoxide into

6 the oil you will see two distinct phases. A polar phase will be seen on top of a lipid phase. With stirring, a homogeneous phase will be observed, but in reality if one was to look on a sufficiently small scale one would see a dispersed phase in which small spheres (why spheres?) of the minority component in a sea of the main constituent. The size and quantity of the spheres are inversely related and will follow a rectangular hyperbola in shape as the Reynolds number is increased. Since the reaction can only occur at the interface of the two phases it makes sense and has been shown experimentally that the kinetic rate constants follow the same rectangular hyperbola as does the total surface area. The reaction is run for an hour at 50 with the magnetic bar spinning at least 300 rpm. At the end of the reaction the heating and stirring are stopped, and within 5-10 minutes two phases will be seen to develop. What reaction components will be found in each phase? Where is your product? For the next lab period the large reactor (water heater) will be utilized. The basis for your calculations will be 50 gallons of used vegetable oil from various UNN food establishments. We will maintain the same methanol ratio as the small batch reaction. Since there is variability in the acid content of used oil you will first titrate a small representative sample to determine its acid content. First, dissolve gm of KH in 1 liter of distilled water (0.1% w/v KH solution, weight-to-volume). In a smaller beaker, dissolve 1 ml of the oil in 10 ml of pure isopropyl alcohol. Warm the beaker gently by standing it in some hot water, stir until all the oil dissolves in the alcohol and turns clear. Add 2 drops of phenolphthalein solution. Using a graduated syringe or a pipette, add 0.1% KH solution drop by drop to the oil-alcohol-phenolphthalein mixture, stirring all the time. It might turn a bit cloudy, keep stirring. Keep on carefully adding the KH solution until the mixture starts to turn pink (magenta) and stays that way for 15 seconds. Take the number of milliliters of 0.1% KH solution you used and add (the basic amount of KH needed for virgin oil). This is the number of grams of KH you'll need per liter of the oil you titrated. Scale the quantity of KH needed up to 50 gallons of oil. The quantity of Methoxide that needs to be mixed cannot be made at once due to the exothermic nature of the reaction. Methanol is obtained from the 55 gallon drum in the flammable liquid locker; it is transported to the hood in a 20 liter poly-propylene carboy. Methoxide is mixed in a six liter Erlenmeyer flask in the hood. 5.5 liters of Methanol is added to the flask which is then placed on the stirrer. About 600 grams of KH will be added to the flask at a maximum of 100 g at a time to allow the heat of dissolution to dissipate. About minutes per flask will be the quickest you will be able to complete this procedure. nce mixed, remove the stirrer and carry the flask over to the Methoxide tank and pour it in. replace the cap on the tank to prevent Methanol vapors from escaping into the room. Repeat this procedure for as many times as needed then dilute to the desired amount with pure Methanol. The reactor is an 80 gallon hot water heater with the upper heating element disconnected (why?). il is pumped into the reactor via the fitting in the circulating loop. There is a vent line that must be opened at all times when the reactor is being filled or emptied (why?). The heating element is never turned on without the tank at least half full and the circulating pump on. Mixing in the reactor is accomplished only via the circulating pump, and the valves must be configured accordingly to allow this. nce the

7 oil is at 120 F, Methoxide is added to the reactor when the circulating pump is on by closing the valve from the reactor to the pump and simultaneously opening the valve from the Methoxide tank to the pump. While Methoxide is being pumped into the reactor, the line from the vent is placed into the Methoxide tank so that if you did not calculate volumes correctly, your spill will be contained. nce all of the Methoxide is in the reactor close the reactor vent (why?). Your group needs to come up with an estimate as to how long to run the reaction. At the end of the reaction turn the heating element off. pen the vent and pump the contents of the reactor over to the cone tank by opening and closing the appropriate valves. Phase separation will occur over the next half hour. Your waste phase can be transferred to one of the 30 gallon drums in the plastic oval tank. Your product is now partially converted due to the equilibrium nature of the reaction, probably % converted based on your recipe utilized. To attain fuel of ASTM quality it must be reacted again with stoichiometric KH and Methanol via the same procedure. At the end of the second reaction, there will probably be only one phase as you will most likely not generate enough Glycerol to allow the formation of two phases. We will use water as a stripping agent in the cone tank to remove any water soluble components (what are they?) from the Methyl-Ester product. Using the sprinkler and water from the sink add an equivalent volume of water to the product. It will look like white rain falling through the Ester layer. nce enough water is added it is pumped off into the sink taking care not to pump product out of the cone tank. This process is repeated at least 3-4 times, until the water in the tube is clean enough to see through. At this point the fuel is clean, but wet. We now use compressed air to bubble air through the Ester to dry the fuel. This takes 1-2 days to complete. The more bulk water you remove prior to initiating the air, the sooner your fuel will be dry enough for use. As your fuel dries, a few possible outcomes are possible. Good starting material will yield good clean fuel. Fuel with high fatty acid content will precipitate fatty acids as the emulsion is broken by drying the fuel. This precipitate must be removed prior to use as it will clog the fuel filter. The second possible complication is that if the oil was thermally abused in the cooking process it may be fully saturated rather that partially unsaturated. This raises the cloud point of the fuel such that it may actually solidify in the tank. Both phenomena have been observed here with university oil. ANALYSIS: What type of reactor is the large reactor? What type of unit operation is the washing step? Why does air dry the fuel? Would the fuel dry without compressed air? Describe the mass transfer that is occurring in the reaction, the washing, and the drying operations? How does mass transfer affect the overall kinetics of the reaction? Is the final equilibrium conversion a function of mass transfer? Explain in terms of Gibbs energy why we see two phases?

8 What is the catalyst being used in the experiment? How does the presence of a catalyst affect the conversion? How does the presence of a catalyst affect the kinetics? What part of Methoxide, Methanol or KH, determines the equilibrium conversion? What part of Methoxide, Methanol or KH, is responsible for its catalytic effect? To make this process commercially viable you would need to recover excess Methanol from the product and discarded phases, how can you do this? To make this process commercially viable you would need to make this a continuous process. How could this be done? Sulfuric acid immobilized catalyst kinetics are slower and the equilibrium conversion is less than that of base catalysis. Its advantage is that it can catalyze free fatty acids. hoose the appropriate unit operation and describe the ratios of feed stock to Methanol relative to the base conditions utilized in this lab. an reaction and separation occur in the same unit operation? ould this be advantageous? This reaction can be run with ethanol but it must be 99% anhydrous. Ethanol has an azeotrope with water at 95%. What is an azeotrope? How can we obtain 99% pure Ethanol? In the analytical section of the Senior Lab Biodiesel experience, you will be introduced to gas chromatography as an analytical tool. You will be taught how to use gas chromatography to derive concentration vs. time plots like the following. Mass Fraction vs. Time Mass Fraction Methyl Esters Triolein Diolein Monolein Time(s)

9 Using the species concentration vs. time data utilized to develop graphs like this and the differential equations seen in the Polymath model, how can the kinetic parameters be determined? REPRT: Describe the design of your experiment and your results (including a discussion of their precision and accuracy). Provide thoughtful and quantitative discussion of results, explain trends using physical principles and relate your experimental results to accepted empirical values from literature or predicted from theory. Express any discrepancies between observed and expected results in terms of quantified experimental uncertainties or limitations in published values or theory. You may find it useful to answer the questions in the analysis section above to help complete your report. REFERENES: 1. G. Knothe et. al. The Biodiesel Handbook