Hydrodynamic-Acoustic-Cavitation for Biodiesel Synthesis

Marcus Franke 1 +

, Bernd Ondruschka 1, Patrick Braeutigam

1

1 Institute for Technical Chemistry and Environmental Chemistry, Friedrich-Schiller-University Jena,

Lessingstr. 12, 07743 Jena, Germany

Abstract. In the present work, a new and pioneering hybrid technology, called Hydrodynamic-Acoustic-

Cavitation (HAC), was established and investigated to proof the feasibility for the transesterification of

rapeseed oil with methanol. The reaction takes place in a circular system, powered by a special multiphase

pump.. An orifice plate specific parameter (β0) was investigated and an optimal value of 8.4% was found. For

other parameters, like ultrasonic power, catalyst concentration, methanol to oil ratio and temperature, design

of experiments (DoE) was used and the effects of these parameters were evaluated. Under optimal conditions

(β0-8.4%, methanol to oil ratio of 4:1, 125 µm ultrasonic amplitude, 0.5 wt% catalyst and 30 or 45°C) a

FAME yield more than 96.5% was obtained in a single-step reaction with a hydraulic residence time in the

cavitation reactor of only one second, which is the minimum in the European standard. Furthermore, the

synthesized FAME was analyzed and compared with other important properties (viscosity, cold filter

plugging point, stability and density), which are regulated by European standards. The HAC process shows a

great potential for biodiesel synthesis with good scale-up possibilities and low investments costs.

Keywords: biodiesel synthesis, hydrodynamic cavitation, acoustic cavitation, process intensification

1. Introduction

Alternative fuels derived from biomass are gaining more and more importance and socio-economic

interest, due to the depleting fossil fuel resources. These so-called biofuels show a great potential to ensure

the mobility of humankind in the next decades or even centuries. For example, biodiesel (FAME – fatty acid

methyl ester) is a suitable alternative for fossil diesel. It is a safe, renewable, sustainable, non-toxic,

biodegradable and less sulphur containing fuel. Other advantages are the reduction of different emissions like

carbon monoxide, carbon dioxide, particular matter, sulphur oxides, heavy metals, and volatile organic

compounds [1]-[5].

FAME are produced by the transesterification of animal fats or vegetable oils with methanol or ethanol.

Usually, a catalyst (basic or acidic / homogeneous or heterogeneous) is involved in the reaction. Typical

feedstocks are palm, rapeseed, and sunflower oil and basic homogeneous catalysts like sodium hydroxide

(NaOH) or potassium hydroxide (KOH) are used in industrial biodiesel plants [6]-[9]. Alternative feedstocks

like oil from algae, waste and non-edible oils are promising for future application to reduce costs and

minimize the socio-economic-conflict with food (“food vs. fuel”) [10]-[13].

The transesterification of fats or oils with an alcohol has three reaction steps, where the first one is

limited by mass transport. Triglycerides and alcohols form two immiscible phases and intensive mixing of

the reactants is necessary to perform the reaction. Usually, extensive stirring, high temperatures and

pressures, long reaction times and high excess of alcohol are used in industrial processes to achieve more

than 95% FAME.

To overcome the rising demand of biodiesel and ensure the mobility of humankind, it is necessary to find

effective and sustainable technologies for biodiesel production. In the past, different reactor types and

Corresponding author. Tel.: +493641948458; fax: +493641948402.

E-mail address: marcus.franke@uni-jena.de.

2014 3rd International Conference on Environment, Chemistry and Biology IPCBEE vol.78 (2014) © (2014) IACSIT Press, Singapore

DOI: 10.7763/IPCBEE. 2014. V78. 6

23

reaction conditions were investigated to improve the transesterification of oils and fats [14]-[16]. Another

suitable and promising technology, which is able to enhance the mass transport of the reaction, is cavitation.

It describes the formation, growth and implosive collapse of bubbles in liquids [17]. Ultrasound or

hydrodynamic setups are used to generate pressure differences in a liquid medium and as a result

multibubble cavitation [18]. The physical effects of cavitation i.e. shear forces and micro jets, are used as an

alternative and efficient energy input to enhance the mass transport of the immiscible phases and to form fine

emulsions [19], [20].

Each generating method has different advantages and disadvantages. For example, hydrodynamic

cavitation (HC) is easier to scale up, shows a low investment volume and provides a higher bubble density

than ultrasonic induced cavitation. In contrast, acoustic cavitation (AC) generates conditions that are more

drastic (high temperature/high pressure) [21]-[23]. Therefore, a combination of both methods, called

Hydrodynamic-Acoustic-Cavitation (HAC) is discussed, where the positive effects of each method were

used and the negative effects were neglected [24]. Some work was already performed for wastewater

treatment applications with promising results [25], [26]. One reason for these outstanding results is the high

bubble density generated by HC and the extreme collapse conditions induced by AC.

In this work, the potential of the HAC-System on the transesterification of oils and fats with methanol

and different basic homogeneous catalysts is investigated.

2. Material and Methods

Rapeseed oil (BKK Biodiesel GmbH, Rudolstadt-Schwarza, Germany), methanol (tech. grade, VWR),

potassium hydroxide (>85%, VWR), toluene (99.5, VWR), potassium methanolate (30% solution in

methanol, Merck), glycerin (Sigma), acetic acid (99.7%, Sigma-Aldrich), methyl heptadecanoate(>99%,

Fluka) and methyl nonadecanoate (>98%, Fluka) were used as received without any pretreatment or

purification.

The setup of the HAC-device is shown in Fig. 1. The loop system consists of an oil reservoir on the

suction side of the pump (PBU201E10, multiphase pump, 1.5 kW, 2900 rpm, Edur Pumpenfabrik GmbH &

Co. KG, Kiel, Germany), equipped with an inlet, sampling port and heating/cooling coil, which is connected

to a thermostat. Another reservoir for the methanol catalyst solution was installed on the suction side of the

pump. The HAC-reactor on the pressure side of the pump consists of variable orifice plates (Tab. 1) and a

sonotrode, connected to an ultrasound generator (UP200S, Hielscher Ultrasonics GmbH, Teltow, Germany,

24 kHz, 200 W, sonotrode: S14L2D, tip diameter: 14 mm, max. amplitude: 125 µm). The flow of the system

is measured (DRS-9250I4L4420, Kobold Messring GmbH, Hofheim, Germany, 0-40 L min-1

, accuracy:

1.5%) and the pressures are measured with the help of manometers (PGI series, Swagelok Germany,

Markranstädt, Germany, accuracy: 0.5-2.5%). The temperature was measured in the oil reservoir (GTH 1150,

Greisinger electronic GmbH, Regenstauf, Germany).

Fig. 1: Setup for creating Hydrodynamic-Acoustic-Cavitation

(1 – pump, 2 – manometer, 3 – orifice plate, 4 – HAC-reactor, 5 – ultrasonic generator, 6 – temperature, 7 – reservoir

(oil) with cooling coil, 8 – flow meter, 9 – reservoir (methanol), 10 – valve (methanol)

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Table 1: Overview on the orifice plates used and characteristic parameters

Plate

number

Diameter of the hole

[mm]

Area of the hole

[mm2] β0 [%]

1 2.8 6.16 7.8

2 2.9 6.61 8.4

3 3.0 7.07 9.0

4 3.2 8.04 10.2

4 3.5 9.62 12.2

5 5.0 19.63 25.0

6 10.0 78.54 100.0

For a typical procedure, the rapeseed oil was filled in the reservoir. The methanol catalyst mixture was

transferred in the methanol reservoir. The pump and the ultrasound generator (for HAC-experiments) were

switched on and the oil was pumped in the loop system, until a desired reaction temperature was reached.

Then, the methanol reservoir was opened and the solutions were combined in the HAC device. After

complete addition of the methanol catalyst mixture (2.5-5 s), the device run for another specific time (10 s).

Afterwards, the solution was filled in a special glass container and samples (4 mL) were taken at different

separation times. Each sample was quenched with glycerin 2 mL (1% acidic acid, v/v) to stop the reaction

and residual methanol was evaporated (10 mbar, 1 h) from the upper FAME phase. The FAME was analyzed

in correspondence with the European Standard EN 14103. A gas chromatography with a carbowax 20M

column (50 m, 0.32 mm, 0.25 µm) and methyl heptadecanoate or methyl nonadecanoate as internal standard

was used.

3. Results and Discussion

3.1. Characterization of rapeseed oil

Different parameters of the rapeseed oil, used as starting material, were analyzed in order to validate the

quality (Tab. 2).

Table 2: Properties of rapeseed oil

Parameter Rapeseed oil (mean) DIN standards

Water content [%]

DIN EN 51605 0.075 0.075

Density g mL-1

DIN EN 51605 0.91 0.9 – 0.93

Viscosity [mm2 s

-1]

DIN 51606 35.5 36

As can be seen, all parameter accomplish the standard values. Furthermore, a complete transesterificated

sample was analyzed with GC (Fig. 2).

Fig. 2: Chromatogramm of a complete transesterificated rapeseed oil sample

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(C 16:0 – palmitic acid methyl ester, C 17:0 (IS) – heptadecane acid methyl ester, C 18:0 – stearic acid methyl ester,

C 18:1 – oleic acid methyl ester, C 18:2 – linoleic acid methyl ester, C 18:3 – linolenic acid methyl ester, C 19:0 (IS) –

nonadecane acid methyl ester, C 20:0 – arachidic acid methyl ester, C 20:1 – Eicosenoic acid methyl ester)

The peaks were identified by standard solutions of FAME. A typical composition of rapeseed oil was

observed and a mean molar weight of 881.4 g mol-1

for the applied oil was be calculated.

3.2. Optimization of the HAC process

The orifice specific parameter β0 is defined by the ratio of the orifice hole area to the cross-sectional pipe

area and is given in % [27]. This important parameter affects the hydrodynamic generated cavitation and it

was investigated in the hydrodynamic setup (HC) without ultrasound as well as in the Hydrodynamic-

Acoustic-Cavitation setup (HAC) with ultrasound (Fig. 3).

Fig. 3: Effect of β0 on the FAME yield

(700 mL oil (0.72 mol), methanol:oil ratio 3.5:1, 0.5 wt% KOH (3.2g), 60 °C, reaction time 10 and 60 s, 0 min

separation time, a and b: 0 µm amplitude, c: 125 µm amplitude, 24 kHz)

A considerably optimum exists at β0 values of around 8.4% (2.9 mm hole diameter) regardless to the

additional application of ultrasound. The same optimum was found for sodium hydroxide, sodium methoxide

and potassium methoxide. Without any applied orifice plate (β0 = 100%) and no additional acoustic

cavitation the FAME yield reaches 60% (potassium hydroxide as catalyst). This clarifies the effective mixing

effect of the special multiphase pump. At the optimal β0 value of 8.4%, the FAME yield increases up to 73%

for the hydrodynamic cavitation setup. With the HAC setup the FAME yield decrease to 69% due to the fine

emulsion which was not separated completely (no separation time was applied).

For any further investigation the optimal orifice plate (β0 – 8.4%) was used in case of HC and HAC and

the orifice plate with a β0 of 100% was used for AC applications.

In order to validate the influence of different parameters (variables) on the transesterification process in

the HAC system and to find effective working conditions a design of experiments (DoE) was developed (Tab.

3). Potassium methoxide (MeOK) was used as catalyst in this investigation.

In Fig. 4, the catalyst concentration shows the highest effect on the transesterification of rapeseed oil in

the HAC system.

26

Table 3: Variables and levels for the DoE (central composite plan)

Variables

Levels

Ultrasound ampl.

[µm]

Catalyst

concentration

[wt%]

Molar

ratio

MeOH:Oil

Temperature

[°C]

-α 0 0.15 3 20

- 0 0.25 4 30

0 50 0.375 5 37.5

+ 100 0.5 6 45

+α 100 0.6 7 55

Fig. 4: Effect of catalyst concentration (MeOK), ultrasound amplitude and molar MeOH:oil ratio on the FAME yield at

30 °C

(Overall volume 800 mL, MeOK 0.15 – 0.5 wt%, β0 – 8.4%, 30 °C, 60 min separation time, 0 – 125 µm amplitude, 24

kHz)

In Fig. 5, the same graphs with 45 °C reaction temperature are shown.

As can be seen, the FAME yield increase at low catalyst concentrations and low ultrasound amplitudes in

comparison to the results shown in Fig. 4. This is due to the lower viscosity and the enhanced mass transport

at high temperatures, which accelerate the reaction. In case of high ultrasound amplitudes, the FAME yields

decrease. At high temperatures, the HAC process forms a fine micro emulsion of methanol/catalyst mixture

and oil and after the transesterification to FAME and glycerin, the phase separation needs more time than at

low temperatures. Therefore, with longer separation times the results of the HAC process should show higher

FAME yields.

In Fig. 6 a comparison between two different methanol to oil ratios and two temperatures after 240 min

separation time can be seen.

In consequence to the long separation time there are two points in the experimental set, which show a

higher FAME yield as requested in the European standard (>96.5%). With a methanol to oil ratio of 4:1, 125

µm amplitude, 0.5 wt% catalyst and 30 or 45°C, 96.8% and 96.6 FAME yield were achieved. Furthermore,

the negative effect of acoustic cavitation at high temperatures and short separation time (60 min) is inverted

(MeOH:oil 4:1) or less significant (MeOH:oil 6:1).

27

3.3. Quality control of the synthesized FAME

In accordance with the European standards, the synthesized FAME was analyzed with respect to the

viscosity, cold filter plugging point (CFPP), oxidation stability, water content and density. The two FAME

samples (> 96.5%, see Fig. 4) meet all requirements and the HAC process has no negative effects on the

synthesized FAME.

Fig. 5: Effect of catalyst concentration (MeOK), ultrasound amplitude and molar MeOH:oil ratio on the FAME yield at

45 °C

(Overall volume 800 mL, MeOK 0.15 – 0.5 wt%, β0 – 8.4%, 45 °C, 60 min separation time, 0 – 125 µm amplitude, 24

kHz)

Fig. 6: Effect of catalyst concentration (MeOK), ultrasound amplitude, molar MeOH:oil ratio and temperature on the

FAME yield

28

(Overall volume 800 mL, MeOK 0.15 – 0.5 wt%, β0 – 8.4%, 240 min separation time, 0 – 125 µm amplitude, 24 kHz)

4. Conclusions and Summary

In this work, a new hybrid cavitation technology for the biodiesel synthesis was investigated. An optimal

orifice with a diameter of 2.9 mm (β0=8.4%) was found for the hydrodynamic setup. A DoE was planned and

realized with four different parameters (ultrasound amplitude, catalyst concentration, molar ratio methanol to

oil, temperature). The catalyst concentration and the ultrasonic amplitude show the highest effects. Under

optimal conditions, it was possible to achieve FAME yields of more than 96.5% in 10 s reaction time (1 s

residence time in the HAC reactor) and 240 min separation time in a single-step reaction. In comparison with

other continuous transesterification processes [28]-[31] the HAC-device shows high FAME yields with low

methanol to oil ratios (4:1), short reaction times (10 to 60 s), and moderate separation times (60 to 240 min).

Furthermore, the quality of the synthesized FAME was analyzed. The FAME fulfill all the tested

requirements and the HAC process shows no negative effects on the product. Therefore, it is an effective

technology to substitute the common batch reactors and shows a great potential in decentralized small-scale

biodiesel plants.

5. Acknowledgement

The authors thank the German Federal Ministry of Food and Agriculture for the financial support (FKZ

22403511). Moreover, the authors thank Mr. Wolfgang Faehndrich from the Institute of Physical Chemistry

(Friedrich-Schiller-University Jena) for constructing different parts for the HAC-device.

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