A Step toward Industrial Plant of Continuous Biodiesel Production Using Reactive Distillation Process
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Copyright © 2012 by Modern Scientific Press Company, Florida, USA
International Journal of Environment and Bioenergy
Journal homepage: www.ModernScientificPress.com/Journals/IJEE.aspx
ISSN: 2165-8951 Florida, USA Article
A Step toward Industrial Plant of Continuous Biodiesel Production Using Reactive Distillation Process
Arief Budiman *
Frontier Research Center for Smart Energy and Eco-efficiency (ForSEE), Faculty of Engineering, Gadjah Mada University, Yogyakarta 55281, Indonesia
* Author to whom correspondence should be addressed; E-Mail: [email protected]; Tel.:
+62-274-902171; Fax: +62-274-902170.
Article history: Received 12 June 2012, Received in revised form 7 August 2012, Accepted 8 August 2012, Published 22 August 2012.
Abstract: Biodiesel has become more attractive because of its environmental benefit and the fact that it is a better fuel than fossil-based diesel in terms of engine performance, emissions reduction, lubricity, and environmental benefits. Biodiesel can be made from transesterification of vegetables oils with methanol in the presence of a catalyst. The process of biodiesel production divided into three main sections: pre-treatment for degumming & reducing free fatty acid, main process for transesterification reaction and post-treatment for biodiesel washing & glycerol refining. Technologies for the first and last sections are already established. While, the conventional processes for second sections take place in batch reactor and distillation column, respectively. In fact, they can carry out simultaneously using a reactive distillation column. The most important benefit of this configuration is reduction in capital investment, plant operating cost and energy demand by heat integration. In this paper a novel of reactive distillation process was developed for biodiesel production. Two different continuous reactive distillation capacities for the process of 15 and 150 liter/day were studied. Then, to assess the commercial feasibilities of the proposed process, preliminary design of its industrial plant was carried out.
Keywords: biodiesel; transesterificaton; reactive distillation; palm oil.
1. Introduction
At present, the governments, research communities, and private organizations around the world are looking for alternative and renewable sources of energy due to the depletion of fossil fuel
resources, increasing in energy demands, unpredictability of fossil oil production, and increasing concerns of rising greenhouse gas emissions. These renewable energy resources should be technically feasible, competitive in price compared with the existing source of energy, environmentally friendly, abundantly available, and having sustainability character. One promising renewable energy source is biodiesel, a green fuel that holds various advantages compared with fossil fuel in terms of renewability, non-toxicity and biodegradability characteristic. Biodiesel also has high cetane number and low sulfur content, hence it will prolong the machine’s lifetime (Mittelbach and Remschmidt, 2008).
Biodiesel fuel can be produced by transesterificating oils where the process involves a catalyst and an alcohol to be reacted in a batch reactor. The most common catalyst for biodiesel production is homogeneous liquid catalyst, such as NaOH, KOH etc. General equation for transesterification of triglycerides with methanol for producing biodiesel is:
(1)
Previous studies have revealed that transesterication reaction consists of a number of consecutive and reversible reaction (Om Tapanes et al., 2008; Sharma et al., 2008). Mechanism and kinetics of biodiesel production have been studied by many researchers (Freedman et al., 1986;
Noureddini and Zhu, 1997; Joelianingsih et al., 2008). They concluded that reaction with methanol, triglyceride (TG) is converted stepwise to diglyceride (DG) and subsequently monoglyceride (MG).
Finally, monoglyceride forms methyl ester (biodiesel) and glycerol (GL). A mole of ester is released at each step, hence three moles of methyl ester are yielded from one mole of triglyceride. However, excess methanol is used to shift the equilibrium to the product side. The three stages of the transesterification reaction of vegetable oil (TG) with alcohol (M) to esters and glycerol (GL) are pointed out on the following equation:
k1
TG + M ⇔ DG + Ester k2
k3
DG + M ⇔ MG + Ester (2)
k4 k5
MG + M ⇔ GL + Ester k6
Transesterification reaction commonly proceeds in batch reactor with the retention time of approximately 2 h. After being settled within several hours, the reaction products will clearly form two layers. The upper layer is biodiesel (methyl ester) and the lower layer is glycerol. These two layers are then separated by using decanter to obtain high purity biodiesel. Batch reactor for biodiesel production is simple, but it is only effective for the production capacity of 500 – 10,000 tons biodiesel/year. For the capacity over 30,000 ton biodiesel/year, it will be more economical if the preparation conducted via a continuous process.
Several types of continuous-flow processes for biodiesel preparation have been introduced (Noureddini et al., 1998; Peterson et al., 2002; Joelianingsih et al., 2012). However, most existing continuous processes still employ conventional configuration in which reaction and product separation occurs separately. This conventional configuration is not economical since it requires high capital as well as operation and energy cost. Hence, to acquire a more effective and efficient process system, development of a novel configuration of biodiesel production which enables the integration of the chemical reaction and product purification in a single equipment is necessary. The candidate for this technology is a continuous transesterification process for biodiesel production using reactive distillation technique.
Reactive distillation (RD) is a chemical unit operation which integrates the function of chemical reactions and separation simultaneously in one unit. The configuration brings about numerous advantages. It will abridge the process flow sheet by reducing separation steps (Kiss et al., 2006). RD technique has also exhibited potential in reduction of capital investment and operating cost through the reduction of equipments (pumps, piping, etc) and possibility in overcoming the equilibrium thermodynamic of the reaction. In terms of energy consumption, Lee and Westerberg (2000) suggest that RD is considered efficient since the heat of reaction can reduce the heat load of a condenser or reboiler. Some papers reports that RD process is effective for the reactions comprising several consecutive and reversible reactions, including transesterification reaction (Bisowarno et al., 2004; Tang et al., 2005; He et al., 2006). Kiss et al. (2006) state that combining reaction and separation into a single unit can shift the reaction equilibrium towards the desired products by continuous removal of the products. Therefore, a higher reaction conversion and selectivity can be achieved. Hence, reactive distillation is an attractive alternative to the classical batch reactor for biodiesel production.
In this work, an experimental investigation of biodiesel production from palm oil using reactive distillation system has been conducted. Two experimental studies on the capacity of 15 and 150 liter/day oil were performed. Effects of main parameters, including column temperature, catalyst loading, and molar ratio of methanol to triglyceride were examined to determine the optimal operation
condition resulting best reaction conversion. Then, preliminary design of its industrial plant was carried out.
2. Experimental Research on the Capacity of 15 Liter/Day
The authors have succeed to study a laboratory-scale continuous RD for the biodiesel synthesis in the presence of NaOH catalyst with the capacity of 15 liter/day (Budiman et al., 2009;
Kusumaningtyas et al., 2009 a & b; Sutijan et al., 2009). The configuration of the equipment illustrated in Fig. 1.
We concluded that reactive distillation system was feasible for the biodiesel production. The experimental investigation showed the effects of main parameters including column temperature, ratio molar methanol to triglyceride, and catalyst loading on the oil conversion. It was found for palm oil feed stock that the best conversion achieved was 94.61%, obtained with the operating parameter of 65 ºC column temperature, 1% (w/w) NaOH catalyst/oil, 8:1 metanol:palm oil (molar ratio), and retention time of 5 min. Biodiesel produced through this process condition held methyl ester content of 98.85%, which met the national specification.
Furthermore, tests on its physical characteristic showed that the values of specific gravity at 60/
60 ºF, kinematic viscosity at 40 ºC, flash point PMCC, pour point, copperstrip corrosion (3 h/50 ºC), water content, cloud point, 90% distillation temperature, and methyl ester purity were 0.8776, 4.4363 mm2/s, 192 ºC, 12 ºC, 1a, trace (% volume), 15 ºC, 345 ºC, and 98.85%, respectively, which met the standard of ASTM D 6751 and Indonesian National Standar on biodiesel product. For the comparison, the reaction performed via batch process at operating condition of 65 ºC, 0.5% (w/w) NaOH/oil, and 1:6 metanol:palm oil (molar ratio) resulted in a reaction conversion of 87.82% in 2 h reaction time.
Figure 1. Configuration of reactive distillation column for biodiesel production.
3. Study on a Pilot Plant with the Capacity of 150 Liter/Day
We have developed small scale of RD of 15 liter/day for biodiesel production. Before going to the commercial application for industrial scale that representing an increase around 10,000 to 1,000,000 as compared to small one, we have developed medium scale of 150 liter/day reactive distillation and tested on overall process parameters as illustrated in Fig. 2. The main apparatus was a single 200 cm length glass-column with outer diameter of 10 cm. This column was filled up by glass raschig ring packing with diameter of 1.0 cm and 1.5 cm in length. This RD column was equipped with condensor, reflux control, reboiler, feed tanks of palm oil and methanol-NaOH, heater, and thermometer.
Figure 2. Pilot plant of biodiesel using RD, capacity 150 liter/day.
3.1. Material
The main material for this experiment was refined palm oil that’s comercially available in Indonesia. Composition of fatty acid in the oil is given in Table 1. Chemical analysis showed the acid value, density at room temperature, and kinematic viscosity (ASTM D 445 Method) of this refined palm oil were 0.4489 mg KOH/g oil, 908 kg/m3, and 40.2748 mm2/s, respectively. The total acid value has satisfied the ASTM D 6751 requirement, which stipulates that the total acid number should be 0.50 mg KOH/g max. Technical grade methanol of 99% purity was obtained from PT Kaltim Methanol Indonesia (Bontang, Indonesia). Certified NaOH of 99% purity was obtained from Merck (Darmstad, Germany). Other analytical reagents and standard chemicals were all analytical grade and purchased from Merck (Germany).
Table 1. Fatty acid composition of refined palm oil
Fatty acid Formula Systemic name Molecular weight wt %
Lauric C12H24O2 Dodecanoic 200.32 0.1
Myristic C14H28O2 Tetradecanoic 228.37 1
Palmitic C16H32O2 Hexadecanoic 256.43 42.8
Stearic C18H36O2 Octadecanoic 284.48 4.5
Oleic C18H34O2 cis-9-octadecenoic 282.46 40.5
Linoleic C18H30O2 cis-9,cis-12-octadecedianoic 280.45 10.1 Linolenic C14H28O2 cis-6,cis-9,cis-12-octadecatrienoic 278.44 0.2
3.2. Experimental Procedure
Palm oil, methanol, and NaOH catalyst had to undergo pretreatment step prior to the biodiesel synthesis process. Palm oil was primarily preheated at moderate temperature for an hour before each experiment in order to remove water and other impurities. Methanol and NaOH solution were then prepared with the intention that the corresponding amount of catalyst would provide the desired methanol-to-triglyceride molar ratio once mixed with the palm oil. In this work, the influence of catalyst loading was investigated at 0.5, 1.0, 1.5 and 2.0%.
After the preparation, both the reactants were poured into the feed tanks of the reactive distillation reactor system. Column and reboiler were heated by electrical heating and mantel, respectively, and maintained at the desired temperature. Column temperature was varied from 55 to 65 ºC and considered as the reaction temperature. The methanol & catalyst premix and palm oil were fed into the mixer and subsequently entering the column. Flowrates of the feed were set-up based on the desired molar ratio of triglyceride and methanol (3:1, 4:1, 5:1, and 7:1). Each segment of the reaction zone in the column functioned as a reactor. Hence, both methanol and triglyceride gradually flowed to
the bottom and reacted in the liquid phase to produce ester and glycerol, with the residence time of 20 min.
A 20,000 mL reboiler was fitted to the bottom of the column to drive methanol off the product mixture before discharging to the decanter. On the other hand, unreacted methanol vapor that rose to the top part of the column were condensed by a water-cooled condenser and refluxing back to the column directly from the top. The product mixture was withdrawn from the reboiler to the decanter, where glycerol and ester were separated by gravity. Each experiment was carried out for a period of 1- 3 h to assure steady state operation. During the continuous operation, samples were periodically taken at the reboiler. Samples were collected only after steady state operation had been reached.
3.3. Chemical Analysis
The completeness of the reaction (reaction conversion) was measured by the wet-chemical American Oil Chemists’ Society (AOCS) method for determining glycerol, i.e. AOCS official method Ca 14-56 entitled “Total, Free, and Combined Glycerol Iodometric-Periodic Acid Method” (Canakci and Van Gerpen, 1999; Van Gerpen et al., 2004). This difference was stated as a percentage of the original total glycerin amount. Biodiesel yielded on the optimum condition was also analyzed using gas chromatography (HP 5890 gas chromatograph series II) and GC-MS (Shimadzu QP2010S with Rtx-5MS column) for the superior information obtained, including chemicals composition and methyl ester content.
3.4. Results and Discussion
Fig. 3 shows effect of residence time on triglyceride conversion. From this figure, one can find that the longer residence time results in an increase in the conversion, but after a maximum value the conversion then decreases. The highest conversion is reached at 20 min of residence time. This can be understood that as long as the feed in contact along the column, the reaction is going well, so the 20 min will be enough to convert triglycerides to biodiesel. The elongation of residence time leads to the decrease in the conversion because of unexpected further reactions. Thus, other experiments to produce the biodiesel using reactive distillation were performed for the residence time of 20 min.
Fig. 4 shows correlation between triglyceride conversion and residence time at various temperature. The concentration of KOH catalyst is 1% by weight of the feed oil. Judging from the boiling point of the feed, methanol boils at around 64 ºC so that the reaction temperature is not expected to be much higher than such the boiling point. If the reaction temperature is higher than 64 ºC, methanol will entirely evaporates as distillate and does not react with the oil palm as feed.
Therefore, the reaction temperature variation starts from 55 to 65 ºC with an interval of 5 ºC.
The conversion shows a tendency of increase with the temperature, as shown in Fig. 4.
Considering that the transesterification reaction is endothermic, in which the increase in temperature will shift the equilibrium towards the endothermic reaction, in this case to product. However, the highest conversion is obtained at a reaction temperature of 60 ºC. The plausible reason is that when the temperature rises above the boiling point of methanol, methanol becomes possible to evaporate more and more drawn as the distillate so that contact between the reactants becomes loss, and thus the conversion obtained at a temperature of 65 ºC decreases down as given by the conversion as a function of residence time used in this experiment.
Figure 3. Effect of residence time on conversion. Figure 4. Effect of temperature on conversion.
Percentage of the catalyst in the mixture that gives the optimum value was obtained by varying weight percent (w/w) of the catalyst with respect to the vegetable oil. Fig. 5 shows correlation between triglyceride conversion and residence time at various catalyst concentration. From this figure we may see that conversion tends to increase with the increase of percentage of catalyst, and the optimum conversion was achieved when the percentage was about 1%. The biodiesel conversion then decreased in higher percentage of the catalyst. This finding is consistent with the theoretical prediction that if the excessive amount of KOH it will cause a decrease in conversion of biodiesel due to the saponification reaction between the fatty acid with KOH.
Feed mole ratio is one of the variables that greatly affect the conversion rate. In the stoichiometric ratio of moles of the feed methanol to oil is 3:1 to get 3 moles of alkyl ester and 1 mole of glycerol. However, excess of this ratio sometimes is needed to complete the reaction. Fig. 6 shows effect of methanol to oil ratio on triglyceride conversion. This figure reveals that more methanol is added, the conversion of biodiesel will increase. This suggests that the number of the collisions
between the reactants with the catalyst is increased by adding more methanol to oil ratio in the mixture. For the mol feed ratio of 5:1, the highest ratio in this study, the conversion was achieved to be more than 98%.
Figure 5. Effect of catalyst on conversion. Figure 6. Effect of methanol-oil ratio on conversion.
4. Prospect for Industrial Plant
A small scale of biodiesel production using reactive distillation has been realized to enable process development under capacity of 15 liter/day. Before going to the commercial technology, technology piloting to the capacity of 150 liter/day has been demonstrated. The advantage of this technology piloting is for a) validation of small scale results, b) demonstration of process scalability, scaling solution, d) effect of technology process integration, and e) commercial risk reduction.
To assess the commercial feasibilities of the proposed process, process simulation was first carried out for the capacity of 300,000 ton/year or 1,000 ton/day. There are three units for biodiesel production using reactive distillation that is a) pretreatment, b) reactive process, and c) post treatment, as shown in Fig. 7. Pretreatment process involves degumming and reducing high free fatty acid. Some crude vegetable oils contain phospholipids that need to be removed in a degumming step or pre- treatment. Phospholipids can produce lecithin, a commercial emulsifier (Van Gerpen and Dvorak, 2002) using several alternative methods, such as membrane filtration, hydration, acid micelles degumming, supercritical extraction, etc. Many methods for reducing the high free fatty acid content of the oils have been proposed, including steam distillation, extraction by alcohol, and esterification by acid-catalysis (Mittelbach and Remschmidt, 2008). The common pretreatment is esterification of the FFA with methanol in the presence of acidic catalysts (usually sulphuric acid). The catalysts can be homogeneous acid-catalysts or solid acid-catalysts (Di Serio et al., 2005).
Post-treatment refers to washing of the product for removing contaminants in products. Since glycerol and methanol are highly soluble in water, water washing is very effective for removing both contaminants. The primary material for water washing is distilled warm water or softened water. After washing several times, the water phase becomes clear, meaning that the contaminants have been completely removed. Then, the biodiesel and water phases are separated by a separation funnel or centrifuge (Predojevic, 2008).
The one may use ASPEN Plus to simulate the process. After flow-sheeting of proposed process, chemical component involved in the process such as triglyceride, methanol, catalyst etc.
should be defined. The next procedure is selecting thermodynamic model, choosing proper operating unit and setting up input conditions. After the input information and operating unit is set up, the process steady-state simulation can be executed by Aspen Plus Ver 11.1.
Figure 7. Flow diagram of biodiesel via RD from CPO.
5. Conclusions
Biodiesel production through transesterification of palm oil with methanol has been carried out using a novel continuous reactive distillation. It was revealed that this reactor system was feasible for the biodiesel production. The experimental investigation for both capacities of 15 and 150 liter/day showed the effects of main parameters including column temperature, molar ratio of methanol to triglyceride, and catalyst loading on the oil conversion. For the capacity of 15 liter/day, it was found that the best conversion achieved was 94.61%, obtained with the operating parameter of 65 ºC column temperature, 1% (w/w) NaOH catalyst/oil, 8:1 metanol:palm oil (molar ratio), and retention time of 5 min. For the capacity of 150 liter/day, the best conversion was 98 %, the operating parameter of 60 ºC column temperature, 1% (w/w) NaOH catalyst/ oil, 5:1 metanol to palm oil (molar ratio), and retention time of 20 min. Biodiesel plant using reactive distillation process can be divided into three units: a) pretreatment, b) reactive process, and c) post treatment. Feasibility study from technological aspect can be performed using simulation software such as Aspen Plus.
Acknowledgments
The author acknowledges the help provided by Kusmaningtyas, R. D., Arifta, T. I., Johan, S., Sawitri, D. R., Zahara, Z. F., and Narulita, N. The present work is financed by Directorate General Higher Education and Gadjah Mada University Indonesia.
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