Accepted Manuscript Title: Biodiesel production from waste cooking oils through esterification: Catalyst screening, chemical equilibrium and reaction kinetics Author: Kolja Neumann Kathrin Werth Alejandro Mart´ın Andrzej G´orak PII: DOI: Reference:
S0263-8762(15)00459-1 http://dx.doi.org/doi:10.1016/j.cherd.2015.11.008 CHERD 2091
To appear in: Received date: Revised date: Accepted date:
22-6-2015 12-11-2015 18-11-2015
Please cite this article as: Neumann, K., Werth, K., Mart´in, A., G´orak, A.,Biodiesel production from waste cooking oils through esterification: Catalyst screening, chemical equilibrium and reaction kinetics, Chemical Engineering Research and Design (2015), http://dx.doi.org/10.1016/j.cherd.2015.11.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Biodiesel production from waste cooking oils through esterification: Catalyst screening, chemical
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equilibrium and reaction kinetics
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Kolja Neumann a,*, Kathrin Werth a, Alejandro Martín a, Andrzej Górak a,f
TU Dortmund University, Department of Biochemical and Chemical Engineering,
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Laboratory of Fluid Separations, Emil-Figge-Straße 70, 44227 Dortmund, Germany Lodz University of Technology, Faculty of Process and Environmental Engineering,
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Department of Environmental Engineering, Wólczañska 213, 90-924 Lódz, Poland
* Corresponding author. Tel.: +49 231 755 2357; Fax: +49 231 755 3035.
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E-mail addresses:
[email protected],
[email protected],
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[email protected] 2 Page 1 of 38
KEYWORDS Process intensification; free fatty acids; homogeneous catalysis; heterogeneous catalysis;
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biofuels; esterification
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HIGHLIGHTS
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Acid catalysed esterification enables biodiesel production from waste cooking oils. Sulphuric acid identified as promising homogeneous catalyst for investigated system.
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Chemical equilibrium determined experimentally and modelled between 348 and 393 K.
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ABSTRACT
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Reaction kinetics determined based on experimental reaction data.
Fatty esters provide new building-blocks for sustainable chemical and biochemical processes
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or can be used as biofuels. For a greener and more economic production of these esters waste cooking oils are a promising feedstock, but a pre-treatment step is required. In this step the high content of free fatty acids is reduced by an acid catalysed esterification. To further enhance the overall process efficiency, reactive distillation is favourable for the pre-treatment. In order to enable a comprehensive analysis of an industrial production process, a step-by-step procedure considering lab-scale experiments and determination of important model parameters is provided. In this study the reaction kinetics of the esterification of oleic acid with ethanol forming the ester ethyl oleate is determined. Prior, a suitable catalyst matching the operating window of a reactive distillation process is identified in an experimental 3 Page 2 of 38
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screening.
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1 INTRODUCTION
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The application of renewable feedstocks and their efficient processing are crucial in the development of sustainable processes. Fatty esters, which are formed in the transesterification
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of vegetable oils, provide new building-blocks for sustainable chemical and biochemical processes. However, nowadays fatty esters are mainly used as biodiesel, which is a promising
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alternative fuel based on renewable resources (Demirbas 2008; Sims et al. 2008; Martinez-
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Hernandez et al. 2014). Biodiesel offers various environmental benefits like a drastic reduction of carbon dioxide emissions compared to fossil fuels (van Gerpen 2005); it is
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biodegradable and non-toxic. Furthermore, the combustion properties are similar to petroleum based diesel, and the integration into the existing infrastructure of fossil fuels is easy (Lotero
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et al. 2005; Sadhukhan und Ng 2011).
In common industrial biodiesel production processes the transesterification of vegetable oils
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is performed batch-wise using short-chain alcohols, mostly methanol, and homogeneous
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alkaline catalysts (Ahn et al. 1995; Kiss 2014; Santacesaria et al. 2012). Nevertheless, there are several disadvantages of the alkaline catalysed transesterification. Oil feedstocks with a high purity are required; especially containing a low amount of free fatty acids to avoid losses of raw material by saponification, which occurs in the presence of the alkaline catalyst and water that is inevitably in the mixture. Furthermore, the soap formation would lead to difficulties in the following purification steps of biodiesel and glycerine (Freedman et al. 1984). To avoid saponification and to overcome the aforementioned limitations meso- and macro-porous heterogeneous catalysts are investigated as an alternative process concept (Davison et al. 2013; Kapil et al. 2011).
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An economic production of biodiesel is challenging, because of the high raw material costs for the refined virgin vegetable oils, which account for up to 75 % of the total production costs (Lim und Teong 2010; Phan und Phan 2008). Therefore, the use of cheaper low-quality oils, such as waste cooking oil, non-refined non-edible oil (e.g. Jatropha curcas oil, castor oil)
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or animal fat, have a great potential to reduce the production costs. The raw material costs for waste cooking oils could be about one third of the price of virgin vegetable oils (Phan und
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Phan 2008). Additionally, competition with the food procurement in developing countries is
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avoided by applying non-edible feedstocks to produce a new generation of biodiesel, also known as second generation biodiesel (Carriquiry et al. 2011). But the main drawback of the
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low-quality oils is the high content of free fatty acids and water that prevent the direct application in the conventional biodiesel production process. Non-refined Jatropha curcas oil
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contains around 14 wt% of free fatty acids (Kumar Tiwari et al. 2007), whereas waste cooking oils could contain up to 34 wt% (Liu et al. 2010). Using a non-refined oil feedstock in the
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transesterification an decreased ester yield was observed by Feedman et al. (1984). Under the alkaline conditions in the transesterification and in the presence of water, the free fatty acids
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tend to hydrolyse. Furthermore, neutralisation reaction of the alkaline catalyst and the free
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fatty acids would lead to losses of catalyst and free fatty acids (Kumar Tiwari et al. 2007). In order to avoid this undesired side reactions and material losses a pre-treatment of the lowquality oil is necessary prior to the transesterification, in which the amount of free fatty acids is reduced below 3 wt% (Meher et al. 2006). In the pre-treatment the free fatty acid content of the low quality oil is reduced by an acid catalysed esterification of the free fatty acids with an alcohol. Though the acid catalysed esterification is more time consuming than, for example, a neutralisation, it results in a higher recovery of biodiesel due to lower losses of feedstock (Bhosle und Subramanian 2005). In state of the art processes the equilibrium limited esterification reaction is carried out in a
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batch reactor using an excess of alcohol to shift the equilibrium towards the products. To fulfil the product specifications of less than 3 wt% of free fatty acids in the reaction product, a high amount of catalyst and alcohol is necessary to reach the desired conversion in a reasonable reaction time that could result in costly processes. Furthermore, the reaction rate of
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the esterification, and thus the time for one batch cycle, is dependent on the initial content of free fatty acids (Marchetti et al. 2007). For high initial contents of free fatty acid the product
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specifications might not be fulfilled in the conventional pre-esterification step anymore,
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because a high amount of water is formed, which limits the conversion. In this case high pressure esterification, the so called hydrotreating, is an option (Bezergianni et al. 2010),
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though it might lead to higher operating costs compared to the low pressure process. Generally several approaches of process intensification to improve the esterification of fatty
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acids can be found in literature such as the use of simulated moving beds (Kapil et al. 2010) or cavitational reactors (Kelkar et al. 2008). Qiu et al. (2010) and Kiss (2014) published a
field of biodiesel production.
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comprehensive review about several process intensification technology investigated in the
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A further opportunity to intensify the esterification of free fatty acids is the application of
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reactive distillation. Reactive distillation integrates reaction and separation into one apparatus, thus, leading to increased conversion by overcoming equilibrium limitations (Harmsen 2007; Górak und Stankiewicz 2011). Furthermore, reactive distillation would enable a continuous process and might be more flexible regarding fluctuating feedstock composition. Reactive distillation could lead to reduced capital costs and energy consumption, which may increase the profitability of the biodiesel production process. To enhance the reaction rates homogeneous and heterogeneous catalysts could be applied in the reactive section of the distillation column to achieve the required conversion. Homogeneous catalysts are suitable for most reactive distillation processes, since the reaction rate can easily be adjusted by the
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amount of catalyst used, which enables more flexible processes. However, using heterogeneous catalysts is advantageous because no separation of catalyst from the product is necessary (Keller 2014). In the biodiesel production process the acid catalysed esterification of the low-quality oils would be followed by an alkaline catalysed transesterification;
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therefore, the application of a heterogeneous catalyst in the reactive distillation is favourable, since no additional separation step is necessary. Both catalyst concepts have already been
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discussed in literature for the esterification of fatty acids. The homogeneous catalysed
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esterification of myristic acid with isopropanol was investigated by Bock et al. (1997) in a column setup with alcohol recovery system. Steinigeweg and Gmeling (2003) showed the
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feasibility of heterogeneously catalysed reactive distillation for the esterification of decanoic acid with methanol, obtaining almost full acid conversion. Bhatia et al. (2006) reported the
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production of isopropyl palmitate in a reactive distillation column using a custom-made zinc acetate catalyst.
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Though, the feasibility of reactive distillation for the esterification of short-chain fatty acids has already been shown in literature, the investigation of the esterification of fatty acids with a
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chain length of 18 carbon atoms like oleic acid or linoleic acid is rare. Since these long-chain
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fatty acids are the main components in most vegetable and low-quality oils, we investigate the esterification of oleic acid with ethanol to form ethyl oleate and water in order to determine the potential of reactive distillation for the pre-treatment of waste cooking oils. There are limited studies available that deal with the transformation of a lab-scale process into a process design activity for the integration of process intensification technologies into large scale processes. Therefore, we develop a step-by-step procedure to enable a model-based process design of an industrial process and its economic and ecologic evaluation, incorporating the determination of all essential experimental data for the model development. For the design of reactive distillation processes knowledge about the reaction kinetics is crucial. In a first step
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heterogeneous and homogeneous catalysts are screened to identify a suitable catalyst to integrate the esterification into a reactive distillation column. For the best-suited catalyst a systematic experimental investigation of the chemical reaction equilibrium and the reaction rate is performed to determine the reaction kinetics. The effects of the main process variables,
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such as reaction temperature, amount of catalyst and ethanol to oleic acid ratio, on the reaction rate are investigated. Based on these results the chemical equilibrium constant and
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the reaction kinetics are determined. The kinetic parameters will be used in following studies
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biodiesel production process based on waste cooking oils.
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to enable a model-based analysis and design of reactive distillation and an evaluation of the
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2 REACTION SYSTEM
In this study the esterification of oleic acid with ethanol is investigated. Oleic acid is chosen,
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because it represents one of the mostly present fatty acids in non-edible and waste cooking oils (Atabani et al., 2013; Issariyakul et al., 2007). The use of ethanol, instead of methanol
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like in most common processes, provides an interesting opportunity to further increase the
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ecological benefits of the process, since it can be produced from renewable resources by fermentation. Scheme 1 shows the reaction equation of the esterification of oleic acid (OAc) with ethanol (EtOH) to produce the desired product ethyl oleate (EtO). The reaction is equilibrium-limited and water is formed as a by-product. Most often acidic catalysts are used to enhance the reaction rate. The boiling temperatures of the pure components are listed in Table 1. The reaction mixture shows non-ideal thermodynamic behaviour with a homogeneous light-boiling azeotrope being formed between ethanol and water (see Table 2).
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Scheme 1. Esterification of oleic acid with ethanol to form ethyl oleate and water.
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Table 1: Molecular formula and pure-component boiling points Tboil at atmospheric pressure.
Molecular formula
Tboil / K
Reference
Ethanol
C2H6O
351.55
(Stull 1947)
Oleic acid
C18H34O2
633.15
(Stull 1947)
Ethyl oleate
C20H38O2
608.36
(García Santander, Carlos M. et al. 2012)
Water
H2O
373.17
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Component
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(NIST 2015)
Table 2: Azeotropic data for ethanol and water at atmospheric pressure (Lide 2009).
Ethanol - water
351.25
xi / mol mol-1
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Taz / K
0.897
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Binary system (i - j)
3 MATERIALS AND METHODS
3.1 CATALYSTS
Ethanol and oleic acid react in the liquid phase without the use of a catalyst, but very low reaction rates are reported (Pinnarat und Savage 2010). Since relatively fast reaction rates are required to carry out the esterification of oleic acid with ethanol in a reactive distillation column, catalysts have to be used in order to enhance the reaction rates. For other esterification reactions, which have already been realized in reactive distillation processes, 10 Page 9 of 38
several acidic homogeneous and heterogeneous catalysts are reported in literature (Brehelin et al. 2007; Buchaly et al. 2012; Calvar et al. 2007). To identify a suitable catalyst for this reaction system an experimental screening of a homogeneous and a heterogeneous catalyst is performed.
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3.1.1 Homogeneous catalyst Sulphuric acid is the most common homogenous catalyst used for esterification (Berrios et al.
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2007), further investigated acids can be found in the review of Lotero et al. (2005). Since
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sulphuric acid shows high reaction rates and good economics, it is chosen to represent the homogeneous catalyst in this study.
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3.1.2 Heterogeneous catalyst
Steinigeweg and Gmehling (2003) used the strong acidic ion-exchange resin Amberlyst® 15
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to investigate conversion of short-chain fatty acids with methanol in reactive distillation.
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Several ion-exchange resins were used as catalyst for esterification of long-chain fatty acids with alcohols in batch reactors (Marchetti et al. 2007; Park et al. 2008); (Özbay et al. 2008)
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(Park et al. 2010). Again, the review of Lotero et al. (2005) provides a good overview of the available literature. Amberlyst® 15 is chosen for the comparison with sulfuric acid in this
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study, since this catalyst has already been successfully tested for different reactive distillation applications (Steinigeweg und Gmehling 2003). Due to the presence of water in the reaction system, which might lead to catalyst deactivation, the modified water-resistant Amberlyst® 15 hydrogen form wet is used.
3.2 CHEMICALS Oleic acid pure Ph. Eur. was purchased from the Company AppliChem with a purity of 65-88 %. The impurities are other fatty acids such as myristic acid (C14:0, max. 5 %), palmitic acid (C16:1 max. 16 %), palmitoleic acid (C16:1 max. 8 %) and stearic acid (C18, 11 Page 10 of 38
max. 6 %). Ethanol was purchased by VWR Prolabor with a purity of ≥ 99.8 %. Sulphuric acid with a purity of ≥ 98 % was supplied by Merck KGaA. Amberlyst® 15, hydrogen form wet, was delivered by Sigma Aldrich. Ethyl dodecanoate, the internal standard for the GC
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analysis, was purchased by Alfa Aesar with a purity of ≥ 98 %.
3.3 EXPERIMENTAL SETUP
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The chosen reactor allows the investigation of the whole range of expected reaction
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conditions in the reactive distillation column according to the changing boiling temperature along the column height. To ensure that the reaction takes place in the liquid phase, the
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pressure in the experiments has to be adjusted in order to prevent an increased evaporation of the light boilers, here ethanol and water, yielding in erroneous results. Therefore, the
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different batch reactors (see Figure 1).
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experimental investigations are performed at atmospheric and elevated pressure using two
Figure 1: Experimental setup. Left: conventional continuous stirred batch reactor; right: high-pressure continuous stirred batch reactor.
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The first series of experiments are performed in a 250 ml three-necked stirred glass flask placed in a tempered oil-bath, heated by a thermostatic bath, to maintain a constant reaction temperature. Reaction mixture and oil bath are agitated by a magnetic stirrer at approximately 360 rpm. A thermometer is installed through the first neck of the flask to measure the
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temperature in the reaction mixture. At the second neck in the middle a water-cooled total condenser is placed to prevent a loss of volatile components. The third neck is equipped with
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a septum to serve as sampling point and to insert the catalyst to start the reaction.
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Esterification of oleic acid with ethanol at 10 bars is performed in a high-pressure reactor, from the Company Parr. At this operating pressure the evaporation of the volatile components
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is minimized in the investigated temperature range. The high-pressure reactor consists of a 300 ml stainless steel vessel with a flange with a seal in between. Several valves are provided
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at the flange to have an access to the reaction mixture. One valve incorporates an inlet for the helium supply, which provides the desired operating pressure. Helium is used since it acts as
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an inert and does not take part in the reaction. Once pressurized, two valves are used to feed chemicals and to withdraw samples, respectively. Additionally to insert the homogeneous
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catalyst, a stainless-steel cannula was installed at the top flange. Furthermore, a manometer to
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observe the pressure and a PT-100 thermocouple to measure the temperature in the reactor are installed. The heating system consisted of a thermostatic bath which is controlled by the measured temperature and the programmed set-point. To ensure a good mixing a magnetic stir bar is used.
3.4 EXPERIMENTAL PROCEDURE To run an experiment at ambient pressure with the homogeneous catalyst sulphuric acid, oleic acid is added to the flask and heated up to the reaction temperature. The second reactant ethanol is mixed with the sulphuric acid separately to avoid a start of the reaction before
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reaching the required temperature. After reaching this temperature, ethanol and catalyst mixture are filled into the flask and the time measurement is started. In order to obtain reliable data to determine reaction kinetics a sufficient number of samples during the kinetic regime are required. Subsequently, the intervals between sample times have to be large enough to
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evaluate the reaction rate at the beginning of the experiment. The time intervals between two samplings are increased with progress of the reaction since those are intended to verify the
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setting of the chemical equilibrium. Each sample is withdrawn from the reactor using a
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syringe and had a volume of 2 ml. The sulphuric acid in the samples is neutralized by adding diluted alkaline solution, and the samples are stored subcooled in order to stop the reaction.
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The experimental procedure to determine reaction kinetics at elevated pressures is similar to the procedure described above. At the beginning reagents are preheated separately and are
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mixed afterwards to start the reaction. Oleic acid is preheated directly in the reactor while sulphuric acid and ethanol are mixed, preheated and stored in a cannula, which is connected
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to one valve at the top flange. To start the reaction an overpressure is applied to the cannula, thus the mixture of ethanol and sulphuric acid enters the reactor. A suction line connected to
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the sampling port is used to take samples from the reaction mixture during the experiments.
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Prior to take each sample, the suction line was flushed by discarding approximately 1 mL of liquid from the line.
The experimental procedure using Amberlyst® 15 as heterogeneous catalysts only differs slightly form the procedure using sulphuric acid. In the experiments with Amberlyst® 15, oleic acid and ethanol are mixed in the reactor and heated up, because the non-catalytic reaction is considered to be negligible. After reaching the reaction temperature a sample is taken to verify the assumption that the acid index in the mixture at room temperature do not change after reaching the reaction temperature. To start the reaction the heterogeneous
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catalyst was added and this point was defined at time 0. Samples are taken over the whole reaction time to observe the progress of the reaction.
3.5 ANALYTICAL METHODS
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Different offline analytical methods are applied in order to analyse the reaction mixture compositions. A gas chromatograph from Shimadzu (type GC-14B), equipped with a flame-
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ionization detector with a temperature of 543.15 K, is used to analyse the content of ethyl
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oleate and ethanol. A non-polar column HP5 from the company Agilent with a length of 30 m, an inner diameter of 0.32 mm and film thickness of 0.25 μm is applied, and helium is used
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as carrier gas at a pressure of 50 kPa. Single component calibration curves are established with an overall accuracy of 2.02 % using ethyl dodecanoate as internal standard to calculate
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the mass fractions in the samples. Each sample is analysed three times, and the average mass fractions are used for evaluation. All data are recorded and evaluated using the software
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LabSolutions®. The developed method takes in total 38 minutes and starts at a temperature of 333.15 K. With a sampling rate of 100 ms the sample is injected into the column with a
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volume of 1 µL and a temperature of 543.15 K. After starting GC analysis the temperature is
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increased to 393.15 K with a heating rate of 10 K/min followed by a further increase to 503.15 with a heating rate of 7 K/min. This temperature is maintained for 5 minutes and with a heating rate of 7 K/min the temperature is increased up to 534.15 K. After reaching the 543.15K the heating rate is changed to 9 K/min and the column is heated till the final temperature of 573.15 K is reached which is maintained constant for additional 2 minutes. Typical retention times were 1.5 min for Ethanol, 24.1 min for oleic acid and 24.4 min for ethyl oleate. The mass fraction of oleic acid is determined by acid-base titration. The ASTM D 664/D 974 and EN 14104 are standardised methods to determine the acid index, and a detailed
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description can be found in literature (Knothe 2006). In the present study the procedure of the EN 14104 is applied incorporating the titration of the samples with 0.055 molar alcoholic potassium hydroxide solution and phenolphthalein as indicator. The mass fraction of fatty acid
is calculated according to Eq. 3.1 with VKOH as the volume of potassium solution
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added by titration, cKOH as the molar concentration of the potassium solution and msample as the
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weight of the sample analysed.
(3.1)
Following this procedure the conversion of oleic acid XOAc is determined by calculating the
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relative difference between the initial mass fraction of fatty acid
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equation:
according to the following
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mass fraction of a sample taken at a certain reaction time
(t0 = 0 min) and the
(3.2)
The water content is determined likewise by titration using a compact volumetric Karl Fischer titrator (type V30) from the company Mettler Toledo. Each sample is analysed by triplicate to determine the standard deviation of measurement. 3.6 DESIGN OF EXPERIMENTS At the beginning preliminary experiments are performed to identify a suitable homogenous or heterogeneous catalyst. Subsequently a first series of experiments is performed to determine 16 Page 15 of 38
the required operating conditions. To assess, whether the esterification reaction of the chosen chemical reaction system is appropriate for the application of reactive distillation, decisive parameters influencing the reaction rate and conversion are investigated. For esterification of long-chain fatty acids the following parameters are identified; reaction temperature, molar
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ratio of alcohol to fatty acid and mass fraction of catalyst (Hassan and Vinjamur, 2013; Lucena et al., 2011). The chosen parameters and their ranges are listed in Table 3. In this acid
-1
to 0.045 gcatalyst goleic
acid
-1
; and
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study catalyst mass fractions ωcat of 0.005 gcatalyst goleic
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ethanol to oleic acid ratios χEtOH/OAc between 1:1 and 6:1 are evaluated.
1
2
0.015
0.015
Treac / K
347
347
χEtOH/OAc / mol mol-1
1:1
-1
4
5
6
0.015
0.005
0.015
0.045
347
347
347
347
3:1
6:1
3:1
3:1
3:1
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ωcat / gcatalyst goleic acid
3
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Conditions
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Table 3: Investigated reaction operating parameters in the first series of experiments.
Once the operating conditions are set in the first series of experiments (see Section 4), in a
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second series of experiments the experimental data to determine the thermodynamic
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equilibrium and the reaction kinetics are generated. Reaction temperatures between 348.15 K and 393.15 K, which are in a relevant range for the application of the esterification in a reactive distillation column, are chosen. The upper temperature limit was set to 393.15 K to minimize etherification of ethanol, which is enhanced at elevated temperatures above 413.15 K (Kiviranta-Pääkkönen et al. 1998; Varisli et al. 2007).
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4 EXPERIMENTAL RESULTS AND DISCUSSION
4.1
CATALYSTSCREENING
A suitable catalyst for the application in reactive distillation has to satisfy different criteria,
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such as fast reaction kinetics in the expected temperature range and being commercially available and low priced. The experimental results for the homogenous catalyst sulphuric acid
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and the heterogeneous catalyst Amberlyst® 15 are presented in the following. The
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experiments are performed at a temperature of 347 K according to the experimental procedure described in section 3. Different mass fractions of the homogenous (ωcat = 0.015 gcatalyst goleic -1
) and the heterogeneous catalyst (ωcat = 0.285 gcatalyst goleic acid-1) are added to the reaction
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acid
mixture, since preliminary experiments revealed a big difference in the activity of the
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homogenous and the heterogeneous catalyst. Furthermore, the molar ratio of ethanol to oleic acid was increased in the heterogeneously catalysed experiments to enhance the reaction rate.
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Figure 2 shows that for the homogenous catalyst sulphuric acid a fast reaction rate is observed and the equilibrium conversion is reached after less than 120 min reaction time for the applied
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reaction conditions. The heterogeneous catalyst Amberlyst® 15 also shows a catalytic activity
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for the investigated esterification reaction. Nevertheless, the reaction rate is significantly lower compared to the homogeneously catalysed esterification. Thus, sulphuric acid is identified as suitable catalyst for the application in a reactive distillation column to achieve considerably high conversion in the respective residence time.
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Figure 2: Conversion of oleic acid at 347 K. Cycles: heterogeneous catalyst Amberlyst® 15 and stoichiometric ratio χEtOH/OAc = 25:1; triangles: homogeneous catalyst sulphuric acid and stoichiometric
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ratio χEtOH/OAc = 6:1.
INFLUENCE OF THE AMOUNT OF CATALYST AND THE REACTANT RATIO
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4.2
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Several experimental runs are performed to analyse the influence of important operational parameters on the reaction rate, namely the amount of catalyst and the molar ratio of the
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reactants. For these fast pre-screenings an investigation of the conversion of oleic acid is sufficient, though resulting in larger errors for increasing amounts of catalyst, which also has to be neutralized. A detailed analysis of all reactants is performed for experiments used for the determination of the reaction kinetics. Since the influence of the temperature on the reaction rate was investigated in detail for the determination of the reaction kinetics, the reaction temperature during the pre-screening is kept constant at 347 K. There is a certain trade-off between fast reaction kinetics and an economic operation of the reactive distillation process. High excesses of alcohol and large amounts of catalyst will lead to very fast reaction rates. In contrast to these requirements an economic process requires low 19 Page 18 of 38
catalyst amounts because the catalyst has to be neutralized in a subsequent process step. Additionally, high amounts of sulphuric acid lead to corrosion, and thus to high investment costs. Furthermore, the stoichiometric ratio of alcohol to acid should be low to reduce the amount of alcohol, which is directly linked with the separation and raw material costs.
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The influence of the amount of the catalyst sulphuric acid on the conversion of oleic acid is shown in Figure 3. In the diagram the conversions are normalised to the conversion of oleic
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acid of the experimental run with ωcat = 0.015 gcatalyst goleic acid-1.
Figure 3: Comparison of the normed conversion of oleic acid for different amounts of sulfuric acid (ωcat: 0.005, 0.015 and 0.045 gcatalyst goleic acid-1) after certain reaction times at a temperature of 347 K and a stoichiometric ratio χEtOH/OAc of 3:1. The conversions are related to the conversion of oleic acid of the experimental run with ωcat = 0.015 gcatalyst goleic acid-1.
As expected high catalyst amounts result in faster reaction rates. Taking the error bars into account, the conversion of fatty acid after 60 min with a catalyst amount of ωcat = 0.015 gcatalyst goleic acid-1 is comparable to ωcat = 0.045 gcatalyst goleic acid-1. In comparison, a catalyst amount of ωcat = 0.005 gcatalyst goleic
acid
-1
leads to a much lower conversion of oleic
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acid. Due to the limited residence times in a column, the reaction rate for the lowest amount of sulphuric acid is too slow to ensure a high conversion of oleic acid in a reactive distillation process. According to these results a catalyst content of ωcat = 0.015 gcatalyst goleic acid-1 is chosen to investigate the influence of the stoichiometric ratio and later for the determination of the
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reaction kinetics. In Figure 4 the results for the investigation of different oleic acid to ethanol ratios are
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displayed, whereby the conversion is normalised to the conversion of oleic acid of the
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experimental run with a stoichiometric ratio χEtOH/OAc of 3:1. Very low conversion of oleic acid for the stoichiometric ratio χEtOH/OAc of 1:1 is observed and sufficient conversion for
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χEtOH/OAc of 3:1. The conversion rate after 15 minutes is remarkably close for the stoichiometric ratios of 3:1 and 6:1. The higher conversion after 120 minutes can be explained
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by the different thermodynamic equilibrium compositions.
Figure 4: Comparison of the normed conversion after certain reaction times at a temperature of 347 K, a catalyst amount of ωcat = 0.015 gcatalyst goleic acid-1 and different stoichiometric ratios (χEtOH/OAc = 1:1, 3:1 and 6:1). The conversions are related to the conversion of the experimental run with χEtOH/OAc of 3:1.
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According to these experimental findings the catalyst mass fraction and the molar ratio of reactants were set to ωcat = 0.015 g g-1 and χEtOH/OAc = 3:1 for the determination of the reaction
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5 CHEMICAL EQUILIBRIUM CONSTANT
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kinetics described in the following chapters.
The conversion of oleic acid and ethanol in the esterification is limited by the chemical
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equilibrium. Equilibrium conditions are reached when the composition of the reaction mixture
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does not change any more in course of time. Several expressions are used to describe reaction equilibria, for example, the equilibrium ratios Kx or Kc basing on molar fractions or
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concentrations, respectively, or the chemical equilibrium constant Ka basing on activities. Generally the chemical equilibrium ratios, Kx(Treac, xi) and Kc(Treac, ci), change with
ed
temperature and composition of the reaction mixture, whereas the chemical equilibrium constant Ka(Treac) only depends on temperature but not on composition (Sandler 2006). For
pt
homogenously catalysed esterification reactions Richert et al. (2015) found out that Kx
Ac ce
strongly dependents on the amount of alcohol in the reaction mixture, while Ka remains constant. Considering the esterification in a reactive distillation column, where the composition profile changes significantly along the column height, the application of the chemical equilibrium constant Ka is favoured to describe the reaction kinetics. The chemical equilibrium constant Ka can be calculated according to Eq. (5.1) as the ratio of the activities aieq of the reactants and products in the equilibrium.
(5.1)
22 Page 21 of 38
The activity ai of a component i, is defined as the product of the activity coefficient molar fraction
and the
of component i, which are determined experimentally (Eq. (5.2)). In the
present study the activity coefficients are calculated by the group contribution method
ip t
UNIFAC using Aspen Properties®.
cr
(5.2)
us
To describe the temperature dependency of the chemical equilibrium constant the van’t Hoff equation (Eq. (5.3)) is applied (Sandler 2006). With the integrated form of the van’t Hoff
an
equation (Eq. (5.4)) the chemical equilibrium constants at different reaction temperatures and the ideal gas
M
Ka(Treac) can be calculated basing on the enthalpy of reaction
constant R. The integration boundaries are the reference temperature Tref and a certain reaction
Ac ce
pt
ed
temperature Treac.
(5.3)
(5.4)
In order to determine the temperature dependency of chemical equilibrium constant, the equilibrium composition is measured experimentally in a temperature range of 348.15 K to 393.15 K, which is a relevant range for the reactive distillation process. To ensure that the chemical equilibrium is reached and to determine the chemical equilibrium constant, only measurements after long reaction times are taken into account. To obtain the unknown parameter in Eq. (5.4), e.g.
, a linear least square regression of the experimentally
23 Page 22 of 38
determined values to the van’t Hoff equation is performed. The value for the chemical equilibrium constant obtained at the highest reaction temperature, Tref = 393.15 K, is used as the reference value Ka(Tref). The logarithm of the measured Ka values is plotted against the reciprocal value of the corresponding reaction temperature to obtain the van’t Hoff plot (see
ip t
Figure 5). The plot shows that the chemical equilibrium constant increases with increasing temperature, thus the esterification is favoured by higher reaction temperatures. The shown
cr
absolute error bars are calculated according to error propagation based on the error of the
us
molar fractions; the error of the activity coefficients cannot be determined. The solid line is calculated by linear least square regression of the van’t Hoff equation taking just the grey
an
indicated experimental values into account. The resulting expression is as follows in Eq. (5.5). All black coloured Ka values in Figure 5 are experimentally determined and were not used for
M
regression but for validation. These values show a comparable trend to the values calculated
pt
ed
by Eq. (5.5).
= 61.69 ± 3.52 kJ mol-1 reveals an endothermic
Ac ce
The determined enthalpy of reaction
(5.5)
reaction. For the esterification of comparable reaction systems, also consisting of a fatty acid and an alcohol, reaction enthalpy values higher than 50 kJ mol-1 are reported (Moya-León et al. 2006). Hence the calculated value is in good agreement with literature data.
24 Page 23 of 38
ip t cr us
an
Figure 5: Van’t Hoff plot for the chemical equilibrium constant Ka. Circles: experimental results for the esterification of oleic acid and ethanol between 348.15 K and 393.15 K (grey: used for linear least
ed
6 REACTION KINETICS
M
square regression, black: used for validation); solid line: result of the linear least square regression.
For the modelling of a reactive distillation process not only the knowledge about the chemical
pt
equilibrium but also about the reaction rate, especially the initial reaction rate, is necessary to
Ac ce
describe a reaction in the column. Thus, the kinetic model approach used for the esterification of oleic acid with ethanol is presented in the following, and consequently a model validation is performed based on the experimental results.
6.1 KINETIC MODEL APPROACH Generally, the reaction rate ri describes the variation of the concentration ci of a component i with time (see Eq. (6.1)). The reaction rate is proportional to the stoichiometric coefficient υi of component i. For homogeneously catalysed reactions the reaction rate is dependent on the
25 Page 24 of 38
amount of catalyst applied ωcat (Baerns 2006), as shown in the experimental investigation (see Section 4). Furthermore, the reaction rate is dependent on the reaction temperature Treac.
(6.1)
ip t
For the investigated esterification the variation of the concentration ci of a component i with
cr
time is given by the empirical approach of a second-order reaction (see Eq. (6.2)). Wherein kf is the forward reaction rate constant and kb is the backward reaction rate constant, which are
an
us
both dependent on the temperature.
(6.2)
M
For the continuously stirred tank reactor a constant reaction volume is assumed, thus the total
(6.3)
Ac ce
pt
and integrated into Eq. (6.1):
ed
concentration ctot of the liquid mixture is constant. According to this Eq. (6.2) is rearranged
To account for the strongly non-ideal thermodynamic phase behaviour of the reaction system, activities should be used to describe the composition of the liquid phase. Therefore, the reaction rate constants are substituted by the following expression (Berry et al. 1980):
(6.4)
This theory assumes that the reaction rate constant k is dependent on the activity coefficients γj of the reactants i and the activity coefficient of the transition state γ#. Since the formation of
26 Page 25 of 38
reaction products in the homogeneously catalysed esterification reaction is assumed to be ideal the activity coefficient for the transition state γ# is set equal to one. Considering Eq. (6.4) and Eq. (5.2) the reaction rate of the investigated esterification of oleic acid with ethanol can be described as follows:
cr
ip t
(6.5)
an
forward and backward reaction rate constant is given:
us
With the definition of the chemical equilibrium constant the following relation between the
(6.6)
M
By integrating this relation (Eq. (6.6)) into the expression of the reaction rate (Eq. (6.5)) the
ed
number of adjustable parameters can be reduced to one, since the value of the equilibrium constant can be directly calculated based on the experimental data (see Eq. (6.7)). The
pt
forward reaction rate constant kf’ was fitted to the experimental kinetic data by a non-linear regression. The regression was performed in the simulation environment Aspen Custom
Ac ce
Modeler® by use of a non-linear least square algorithm.
(6.7)
To describe the temperature dependency of the forward reaction rate constant an Arrhenius approach is applied (Eq. (6.8). The Arrhenius approach describes the forward reaction rate constant as a function of the pre-exponential factor k0, the activation energy EA and the ideal gas constant R.
27 Page 26 of 38
(6.8)
Figure 6 presents the Arrhenius plot for a temperature range of 348.15 K to 393.15 K. To determine the unknown parameters k0 and EA a least square regression of the fitted reaction constants
were
performed.
The
pre-exponential
factor
determined
as
Ac ce
pt
ed
M
an
us
cr
k0 = 4.72·102 m3 mol-1 s-1and the activation energy is EA = 36.62 kJ mol-1.
is
ip t
rate
Figure 6: Arrhenius plot for the reaction rate constant kf. Rhombus: fitted to experimental results for the esterification of oleic acid and ethanol between 348.15 K and 393.15 K (grey: used for linear least square regression, black: used for validation); solid line: result of the linear least square regression.
6.2 MODEL VALIDATION
To verify the accuracy of the determined kinetic parameters the simulated temporal concentration courses of the reaction are compared with experimental data. In Figure 7 the experimentally determined temporal course of the mass fraction of ethyl oleat for the reaction temperatures of 348.15 K and 378.15 K are compared with the simulated results. It can be
28 Page 27 of 38
shown that the experimental data well agree with the applied activity-based kinetic approach of second order. The simulated molar fractions of all components in comparison to the experimental values for the kinetic experiments between 348.15 K and 393.15 K can be found
ed
M
an
us
cr
ip t
in Figure 8.
Figure 7: Experimentally determined and simulated temporal course of the mass fraction of ethyl
pt
oleate in dependence of the reaction temperatures. Cycles: experimental data at 348.15 K, black line:
Ac ce
simulated data at 348.15 K; triangles: experimental data at 378.15 K, grey line: simulated data at 378.15 K.
29 Page 28 of 38
ip t cr us
an
Figure 8: Experimentally determined and simulated molar fractions of all components for reaction
M
temperatures between 348.15 K and 393.15 K, dashed lines: +/-30 % accuracy.
ed
7 CONCLUSION
In order to enable a comprehensive computer-aided process analysis of an industrial biodiesel
pt
production process based on waste cooking oils, a step-by-step procedure considering labscale experiments and determination of important model parameters is provided. In the first
Ac ce
series of experiments a catalyst screening is performed to identify a suitable catalyst for a reactive distillation process. It is found that the homogenous catalyst sulphuric acid outperform the heterogeneous catalyst Amberlyst® 15. To develop a reliable kinetic model the influence of the amount of sulphuric acid, the reactant ratio and the reaction temperature on the reaction rate and the chemical equilibrium are investigated experimentally. The results are used to fit the parameters of the kinetic model, which considers an activity based approach of second order. The developed kinetic model is validated against own experimental data and it is shown that the simulated data are in good agreement with the experimental results. Hence, the kinetic model can be used in the following works incorporating the development of a 30 Page 29 of 38
reactive distillation model, to enable process analysis and design of a biodiesel production process using waste cooking oils or other low-quality oils as raw material.
ip t
ACKNOWLEDGEMENTS The research leading to these results has received funding from the Ministry of Innovation,
cr
Science and Research of North Rhine-Westphalia in the frame of CLIB-Graduate Cluster
us
Industrial Biotechnology
an
We gratefully acknowledge the valuable scientific support of Katrin Kissing.
NOMENCLATURE
M
Latin letters Activity of component i (mol mol-1)
c
Molar concentration (mol m-3)
EA
Activation energy (kJ mol-1)
ed
ai
k0 K m ri
Reaction rate constant (m3 mol-1 s-1)
Ac ce
k
pt
Enthalpy of reaction (kJ mol-1)
Pre-exponential factor (m3 mol-1 s-1) Equilibrium ratio/constant mass (kg)
Reaction rate of component i (mol m-3 s-1)
R
Ideal gas constant (J mol-1 K-1)
t
Time (min)
T
Temperature (K)
V
Volume (m-3)
xi
Molar fraction of component i (mol mol-1)
31 Page 30 of 38
Xi
Conversion of component i (%)
Greek letters Activity coefficient of component i
υi
Stoichiometric coefficient of component i
χEtOH/OAc
Molar ratio of ethanol and oleic acid (mol mol-1)
ωcat
Mass fraction of catalyst (g g-1)
cr
ip t
γi
Subscripts Activity-based
az
Azeotrope
b
Backward
boil
Boiling point
c
Concentration-based
f
Forward
i
Component i
j
Reactant j
reac
Reaction
ref
Reference value
x #
an M
ed
pt
Ac ce
tot
us
a
Total
Molar-based
Transition state
Abbreviations AI
Acid index
CO2
Carbon dioxide
DOE
Design of experiments
EtOH
Ethanol
KOH
Potassium hydroxide
32 Page 31 of 38
OAc
Oleic acid
ip t
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