Process enhancement of supercritical methanol biodiesel production by packing beds

Process enhancement of supercritical methanol biodiesel production by packing beds

Accepted Manuscript Process enhancement of supercritical methanol biodiesel production by packing beds Bao-Quan Qiao, Dan Zhou, Gen Li, Jian-Zhong Yin...

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Accepted Manuscript Process enhancement of supercritical methanol biodiesel production by packing beds Bao-Quan Qiao, Dan Zhou, Gen Li, Jian-Zhong Yin, Song Xue, Jiao Liu PII: DOI: Reference:

S0960-8524(16)31763-1 http://dx.doi.org/10.1016/j.biortech.2016.12.085 BITE 17461

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

27 October 2016 21 December 2016 22 December 2016

Please cite this article as: Qiao, B-Q., Zhou, D., Li, G., Yin, J-Z., Xue, S., Liu, J., Process enhancement of supercritical methanol biodiesel production by packing beds, Bioresource Technology (2016), doi: http://dx.doi.org/10.1016/ j.biortech.2016.12.085

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Process enhancement of supercritical methanol biodiesel production by packing beds Bao-Quan Qiaoa

Dan Zhoua

Gen Lia Jian-Zhong Yin a,* Song Xueb Jiao Liub

a

State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, China b

Marine Bioengineering Group, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China (Corresponding author.E-mail address: [email protected] (J.-Z. Yin).)

Abstract Continuous fixed bed reactors filled by three kinds of packing which were glass bead, glass spring and Dixon rings were investigated. The effect of temperature, pressure, the molar ratio of methanol to oil, flow rate, the size and shape of the packing were researched. The highest yield 90.84% of FAME was obtained by filling Dixon rings as packing with the condition of the temperature was 350°C, the pressure was 22MPa, the molar ratio of methanol to oil was 42:1. In addition, the reusability of Dixon rings was perfect. Numerical simulation was researched to provide theoretical basis for experimental results, besides the kinetics and thermodynamics behavior were investigated to explore the reaction mechanism. Keywords: biodiesel; supercritical; fixed bed reactor; packing; kinetics

1. Introduction The dwindling petroleum reserves and increasingly serious environmental problems demand a substitute for petroleum. Among all the potential alternatives,

1

biodiesel is an excellent green renewable fuel because of its good biodegradability, non-toxicity, high cetane number, sulfur-free properties, and similar properties to petrodiesel fuel(Atabani et al., 2012; Hoekman et al., 2012), which make it a promising alternative for fossil fuels. At present, the production cost of biodiesel is still high and many methods are carried out to improve the yield and reduce cost, which include using cheap raw materials(Boey et al., 2011; Koh & Ghazi, 2011; Lam et al., 2010; Mata et al., 2010), efficient catalysts(Baskar & Aiswarya, 2015; Caldas et al., 2016; Sani et al., 2014), new technologies and equipment(Barekati-Goudarzi et al., 2016; Farobie et al., 2015; Xu et al., 2016; Xu et al., 2015). Designing suitable reaction equipment should be first considered. Reactor is the core equipment for biodiesel production. Well designed reactors can strengthen mass transfer and break thermodynamic equilibrium of transesterification so as to improve unit space-time yield and reduce production cost. Batch reactors are widely employed in the early research of biodiesel production (Antolin et al., 2002; Lopez Granados et al., 2007; Shu et al., 2010; Yin et al., 2008). While they suffer some obvious disadvantages such as large reactor volume, less efficient, unreliable production quality and high labor costs(Darnoko & Cheryan, 2000; He et al., 2007). Compared with batch reactors, continuous reactors can reduce production cost, provide uniform quality of the final product and are more conducive to large scale industrial production (Leevijit et al., 2008; Monbaliu et al., 2011). Consequently, various types of continuous reactors were designed and employed including continuous stirred tank reactor(Darnoko & Cheryan, 2000), tube reactor(Xu 2

et al., 2016), coiled tubular reactor(Ngamprasertsith et al., 2014), fixed bed reactor (Furuta et al., 2004), spiral reactor(Delavari et al., 2014; Farobie et al., 2015), bubble column reactor(Stacy et al., 2014), microchannel reactor(Yamsub et al., 2014), ultrasonic reactor(Choedkiatsakul et al., 2014), membrane reactor(Xu et al., 2014), spinning disk reactor(Chen et al, 2014), stirring packed-bed reactor(Li et al., 2013), static mixers tubular reactor(Likozar et al., 2016), pulsed loop reactor(Syam et al., 2012) and liquid-liquid film reactor(Narváez et al., 2009). The well-recognized transesterification process including reaction kinetic, chemical equilibrium and mass transfer can explain the mechanisms well in different continuous reactors(G. Vicente, 2006; O.S. Stamenkovi ć, 2008). Ultrasound enhances both mass transfer and chemical reactions(H.D. Hanh, 2008), while it obviously increases the production cost and the industry application is questionable. The collapsed bubbles disrupts the liquid-liquid boundary layers and provides energy which benefits to the mass transfer and chemical reaction(J.A. Colucci, 2005). Cavitation strengthens the mass transfer rates, reaction kinetic and equilibrium in the later reaction stages, while it increases the energy cost(H.D. Hanh, 2008). Mechanical mixing such as static mixers only can increase the mass transfer rate but it needs low investment/operating costs(Likozar & Levec, 2014; Likozar et al., 2016). The fixed reactor filled by catalysts can increase the mass transfer rate due to the packing, besides the catalysts can strengthen the chemical reaction and equilibrium(Furuta et al., 2006; Furuta et al., 2004). As for industrial production of biodiesel, high yield is just one aspect to be 3

considered when choosing reactor type. Other factors including simple structure, cheap manufacture and energy cost, easy operation and maintenance are also important. Among all reactors, fixed bed reactors are the most commonly employed reactors for biodiesel industrial production. The packing filled in fixed bed reactors includes catalytic solid materials such as solid acid catalysts(Furuta et al., 2004), solid base catalysts(Buasri et al., 2012; Furuta et al., 2006; Xiao et al., 2012), enzymes(Chen et al., 2009), ion exchange resins(Feng et al., 2011; Ren et al., 2012) and non-catalytic solid materials(Silva et al., 2014). Catalyst beds can improve the efficiency of the reactors, while they are easy inactivation and need regular replacement which increases labor intensity. So non-catalytic fixed bed reactors are more promising for industrial application. Non-catalytic packing can reduce back mixing, increase the reaction area and improve the yield, while few articles have reported how to intensify the process with non-catalytic packing and what effects do the packing have on the reactions. So the main objective of this work is to investigate the effect of different packing on the yield of the continuous transesterification in a fixed bed reactor of soybean oil under supercritical methanol conditions. For this purpose, different packing including glass bead, glass spring and Dixon rings were discussed respectively and the numerical simulation, kinetic, as well as the thermodynamics behavior were also studied.

2. Materials and Methods 2.1 Materials. 4

Soybean oil (C16H32O2 11.5%, C18H36O2 4.6%, C18H34O2 29.1%, C18H32O2 54.0%, C18H30O2 0.8%) was purchased from Shanghai Fulinmen Food Co.,Ltd. Both methanol (>99.5%) and n-hexane (>95%) with the purification of analytical grade were obtained from Beijing Chemical Works. N2 (99.999%, 14MPa) was supplied by Dalian Guangming Special Type Gas Co., Ltd. Glass beads(diameters are 0.5mm, 3mm, 6mm respectively),glass spring(Φ3×3mm) and Dixon rings(Φ3×3mm) (the basic drawings are showed in Fig. A.1 in Appendix A) were purchased from Jiangxi Haichuan Chemical Packing Equipment Co., Ltd, Zhejiang Kaiwei Chemical Co., Ltd and Dongguan SUTU Technology Co., Ltd, respectively. 2.2 Apparatus and Experimental Procedure The continuous supercritical biodiesel production system is shown in Fig. 1. The fixed bed reactor was a straight tube reactor (Φ18×5mm, 250mm) filled by three kinds of packing respectively. Soybean oil 1 and methanol 2 were preheated by preheater 7 after fed by pumps 3, 4 respectively, then mixed by mixer 9 and flowed into reactor 10. The temperature of the preheater and the reactor were controlled by temperature controllers 8, 12 respectively. The products were obtained in the collector 19 every 30min after cooled by condenser 16 and depressurization by back pressure valve 17 when the pressure and temperature reached what we needed. The timer started when the first oil was observed. The samples’ analysis after the post-treatment were reported in our previous work (Xu et al., 2016). 2.3 Analysis The FAME content of the samples was determined by an Agilent 6890N gas 5

chromatograph which equipped with a HP-5 capillary column (30m× ϕ0.32mm× 0.25 µm). The column temperature program was set at a starting temperature of 160 °C followed by an initial rate of 3 °C/min to 225 °C, held at 225 °C for 0.5 min, and then it was ramped at 15 °C/min to 270 °C , held at this temperature for 4min. Nitrogen was used as carrier gas. The inlet pressure was 43.4 kPa. The split ratio was 40:1. The injector and detector temperatures were set at 270 °C and 290 °C, respectively. The FAME content of the products was determined by external standard method and the standard sample was prepared using potassium hydroxide catalysis method which yield is nearly 100%. 3 Results and Discussion 3.1 The Operating Conditions for Continuous Supercrtical Transesterification 3.1.1 Effect of Reaction Temperature Experiment was carried out in the fixed bed reactor filled by glass beads and so as the following experiments about the research of the operating conditions. As shown in Fig. 2a, with the increase of temperature, the yield increased, and the growth of yield from 250 °C to 300 °C was larger than the region above 300°C. According to the calculation results of the critical parameters for the mixture of oil and methanol (Appendix B), the critical temperature was 298.25 °C when the molar ratio of methanol to oil was 42:1, and it just crossed the critical temperature when the temperature rose from 250 °C to 300 °C, considering the speciality of critical points, it seems to be the reason of the yield elevated. High temperature was in favor of transesterification reaction, while up to 350 ºC, the fatty acid methyl esters became 6

unstable and slightly decomposed(Imahara et al., 2008). So this work only investigated the temperature ranged from 250 to 350 °C, and 350 °C was the optimal temperature. 3.1.2 Effect of Reaction Pressure Pressure has great influence on supercritical transesterification and Fig. 2a shows that with the increase of pressure, the yield increased. In order to explain the influence mechanism of pressure on yield, the solubility parameters difference was calculated according to Appendix C, and the result was shown in Fig. C.1. The solubility parameters difference between oil and methanol decreased with the increase of pressure at the same temperature, especially when the pressure was beyond the critical pressure, which means the oil and methanol were mixed better. As a result, the yield increased along with the pressure. Therefore, increasing the pressure appropriately was beneficial to transesterification and the optimal pressure was 22MPa under experimental conditions. 3.1.3 Effect of Methanol to Oil Molar Ratio As is depicted in Fig. 2b, the yield grew with the increase of the molar ratio of methanol to oil until the ratio reached to 42:1, which was because of the reaction equilibrium can be promoted to the side of production by improving the concentration of reactants for reversible transesterification. While when the ratio was above 42:1, the yield had a little decrease. It may due to the excess methanol amount was too much which diluted the reaction system. So the ratio of 42:1 was selected. 3.1.4 Effect of Flow Rate 7

The flow rate of reactants has an effect on the residence time, thus affect the yield. Fig. 2c showed that the yield increased with the flow rate decrease. It was because when the flow rate decreased, the residence time of reactants in reactor would increase which helped reactants to react sufficiently. But when the flow rate of oil decreased from 0.067mL/min to 0.033mL/min , the improvement of yield was not significant. Besides, the amount of FAME also should be considered. The amount of FAME decreased when the flow rate decreased. Considering the yield and the amount of FAME comprehensively, 0.067mL/min for oil and 0.12mL/min for methanol were chosen respectively. 3.2 Transesterification in Fixed Bed Reactor Filled by Different Packing 3.2.1 Effect of the Size for Packing (1) Experimental research The effect of the packing size on transesterification is shown in Fig. 3a and the experiments were controlled in the same mean residence time. It was obvious that the yield in fixed bed reactors was higher than the tube reactor and with the decrease of the packing size, the yield increased. There are two possible reasons can explain the result. First, for the non-porous packing with the same material, shape and weight, the smaller the particle size, the larger the surface area which can provide larger reaction area for methanol and oil. Second, with the decrease of the packing size, the void fraction increases and the voids in reactor are more homogeneous which can prevent obvious wall effects. As a result, the flow was more homogeneous and the yield increased. 8

(2)

Numerical simulation research

In order to verify the influence discipline of packing size on the yield, as well as provide theoretical basis for experiment results, Fluent simulation was used to research the effect of the packing size on the flow field of the reactor. The model and the velocity contour graphics were shown in Fig. D.1 and Fig. D.2 respectively. The partial graphics was shown in Fig. 4. Comparing the tube reactor (Fig. 4a) with the fixed bed reactors (Fig. 4b-d), it was obvious that the flow field was not homogeneous and the flow rate varied widely in the tube reactor, which caused terrible back-mixing. While in fixed bed reactors, the flow fields and rates were more homogeneous which was beneficial to keep the molar rate and residence time fixed and ensured the effective reaction of methanol and oil. So the theoretical simulation agreed with the experiment result that the yield in fixed bed reactors were higher than tube reactors. Comparing the fixed bed reactors filled by different sizes of packing (Fig. 4b-d), when the diameter was 6mm as shown in (Fig. 4b), the flow was heterogeneous in partial areas and the wall effect occured near the wall. With the decrease of the packing size, the flow fields were more homogeneous , and when the packing was 0.5mm (Fig. 4d) , the flow was nearly plug flow and the molar ratio and residence time was almost constant in all areas. Therefore, with the decrease of the packing sizes, the state of flow field was more favorable to the reaction of methanol and oil, which agreed with the experimental results. 3.2.2 Effect of The Shape of The Packing 9

The shape of the packing is another important factor for the bed structure of reactor and three shapes of packing were tested under the same mean residence time. As is shown in Fig. 3b, the yield obtained by filling three different packings in the following order: Dixon rings>glass beads>glass spring. The channels formed by the voids between the packing were curved and intertwined, and the channel number, cross-section, length and distribution were different with the change of the packing shapes. The channels could influence the residence time , mixing, as well as the reaction, so different yields were obtained by using different packing. Dixon rings made of stainless steel mesh material, are a kind of porous packing offer greater void fraction and surface area and more channels which were more homogeneous. All of these characters of the Dixon rings making them ideal for use in fixed bed reactor. However, for the glass spring, the channels were not homogeneous which had great wall effect, lead to the lowest yields. Besides the yield, the amount of FAME is another essential aspect when producing biodiesel. As is shown in Table 1, the amount of FAME sorted in the following order: Dixon rings>no packing>glass spring>glass beads. Though the yield by using glass beads was higher than glass springs, the amount of FAME was on the contrary. Due to the void fraction of glass beads was much lower than the glass spring, the total volume of the glass beads was much bigger which makes the effective volume of the reaction decreased, as a result, the production capacity decreased in the same residence time. When took the yield and the amount of FAME into consideration, only the fixed bed reactor filled by Dixon rings was better than 10

tube reactor. So the best packing for fixed bed was Dixon rings. 3.2.3 The Reusability of Dixon rings Since the Dixon rings are a kind of inert packing, the replacement frequency can be decreased, the stable product quality can be ensured and the investment of material and labor input was diminished consequently, which is superior to catalyst packing. The reusability of Dixon rings was investigated in the fixed bed reactor and the yields were stable even if the packing was reused up to 60 hours. It showed that the reusability of packing was pretty excellent and would not inactive under the harsh reaction condition. 3.3 Kinetics of Soybean Oil to Methyl Esters In order to correlate experimental data, the experimental results were analyzed further in terms of the kinetics of soybean oil to methyl esters. The experimental data was obtained in the fixed bed reactor filled by Dixon rings when the molar ratio of methanol to oil was 42:1, the pressure was 22MPa. The transesterification of vegetable oil and methanol in supercritical condition can be divided into three steps which can be seen from Eq.1. One molecule of FAME can be obtained in every reaction step. Because every step of the transesterification is reversible, the whole reaction contains six different rate constants. K1

TG+CH3OH ⇔ R1COOCH3+DG K2

K3

DG+CH3OH ⇔ R2COOCH3+MG K4

(1)

K5

MG+CH3OH ⇔ R3COOCH3+GL K6

Where TG is triglyceride, DG is diglyceride, MG is monoglyceride and GL is 11

glycerol. The expected products of the reaction are methyl esters instead of triglyceride, diglyceride and monoglyceride, so the three steps transesterification equations can be simplified into one step by ignoring the three intermediate reactions according to Kusdiana(Kusdiana & Saka, 2001). The equation is given by Eq.2. K7

TG+3CH3OH ⇔ 3RCOOCH3+GL

(2)

K8

The equation can be further simplified into Eq.3 based on the hypothesis as follows(He et al., 2007): (1) The concentration of methanol is invariant. The change of the concentration of methanol can be ignored because the molar ratio of methanol to oil is 42:1 in this experiment which is far higher than the 3:1 of the reaction equation. (2)The transesterification is irreversible. The high molar ratio of 42:1 helps the reaction equilibrium move to the side of production which makes the reverse reaction be ignored, and the reaction is assumed as a first order reaction. K

TG+3CH3OH → 3RCOOCH3+GL

(3)

Due to the complicated reaction mixture, it can be classified into four groups including unmethyl esterified compounds (uME), methanol, methyl esters (ME) and glycerin (GL). Among the four groups unmethyl esterified compounds including triglycerides, diglycerides, monoglycerides and unreacted free fatty acids(He et al., 2007; Kusdiana & Saka, 2001). The equation can be rewritten as Eq.4. K

uME+CH3OH → ME+GL

(4)

k is the rate constant of the reaction which can be expressed as Eq.5. −

d [ uME ]

dt

= k [ uME ]

12

(5)

After integrating, Eq.6 is obtained. Where [uME,0] means the initial concentration of raw material oil and [uME, t] is the concentration of uME at time t. k=

ln [ uME,t ] − ln [ uME,0] t

(6)

By fitting the experimental data between ln[uME,0]- ln[uME, t] and t, an obvious linear relation can be found as Fig. 5 and the rate constants at different temperature can be obtained as Table 2. It indicates that with the increase of temperature, the rate constants increases.

According to Arrhenius equation Eq.7, the activation energy can be calculated by using the rate constants obtained by the above fitting. k = A exp(

Ea ) RT

(7)

Where Ea is activation energy, R is ideal gas law constant , T is temperature and A is the pre-exponential factor.

The non-linear relation between 1/T and lnk is shown in Fig. 6. The activation energy is 55.0 kJ/mol according to the slope obtained by fitting, and the activation energy is close to 56.0 kJ/mol obtained by He(He et al., 2007) but smaller than 85.5 kJ/mol and 92.0 kJ/mol obtained by Tsai(Tsai et al., 2013) and Wang(Wang & Yang, 2007) respectively. This discrepancy may because of the different raw materials and reactors. 4 Conclusion

Continuous production of biodiesel by supercritical method in fixed bed reactors 13

filled by three kinds of non-catalytic packing which were glass beads, glass springs and Dixon rings were compared. With the decrease of the packing size, the yield increased, and the numerical simulation provided theoretical basis that smaller packing sizes will provide a better mixing degree and more homogeneous flow field, and hence, a higher conversion. Besides, by filling Dixon rings as packing, it could obtain both the highest yield and amount of FAME when 350°C, 22MPa and the molar ratio of methanol to oil was 42:1. The kinetics was researched and the total activation energy was 55.0 kJ/mol in fixed bed reactor filled by Dixon rings.

Acknowledgments

The work was supported by the National Natural Science Foundation of China (No. 21506027,

U1662130)

and

Chinese

Postdoctoral

Science

Foundation

(No.2015M571307).

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Figure Captions 1)soybean oil container ;2)methanol container;3)、4)high-pressure constant flow pump;5) 、6) 、15) 、18)needle valves; 7)pre-heater;8) 、12)temperature controller;9)mixer;10)reactor;11)electric heating furnace;13)thermocouple; 14)pressure gauge;16)condenser;17)back pressure valve;19)collector Fig. 1. The equipment flow chart for continuously producing biodiesel by the method of supercritical fluid

Fig. 2. The effect of operation parameters on the yield of biodiesel (a) the effect of temperature (b) the effect of the molar ratio (c) the effect of the flow rate note:(a) oil/methanol were 0.067mL/min and 0.12 mL/min, 42:1 (b) 350 ºC, 22MPa (c) 350 ºC, 22MPa, 42:1

Fig 3. The effect of size and shape of the packing on the yield of biodiesel ((a) size (b) shape) (350ºC, 22MPa, 42:1)

Fig. 4. Velocity contour partial graphic((a) tube reactor (b) Φ6mm (c) Φ3mm (d) Φ0.5mm) (350ºC, 22MPa, 42:1)

Fig. 5. Plot of ln[uME,0]- ln[uME, t] value of oil to FAME against reaction time at different temperature (22MPa, 42:1) Fig. 6. Plot of lnk value of oil to FAME against 1/T (22MPa , 42:1) 18

Table Table 1. The comparison of the yield and the amount of FAME between different packing Packing

Yield (%)

The amount of FAME (g/h)

No packing

71.48

4.81

Glass spring

80.54

3.87

Glass bead

85.68

3.16

Dixon rings

90.27

5.20

19

Table 2. The reaction rate constant at different temperature (The pressure is 22MPa and the molar ratio of methanol to oil is 42:1) Temperature

k(min-1)

R2

250

0.02737

0.979

280

0.05026

0.998

300

0.09917

0.985

350

0.19735

0.996

20

Figures

1)soybean oil container ;2)methanol container;3)、4)high-pressure constant flow pump;5) 、6) 、15) 、18)needle valves; 7)pre-heater;8) 、12)temperature controller;9)mixer;10)reactor;11)electric heating furnace;13)thermocouple; 14)pressure gauge;16)condenser;17)back pressure valve;19)collector Fig. 1. The equipment flow chart for continuously producing biodiesel by the method of supercritical fluid

21

(a)

(b)

(c)

Fig. 2. The effect of operation parameters on the yield of biodiesel (a) the effect of temperature (b) the effect of the molar ratio (c) the effect of the flow rate note:(a) oil/methanol were 0.067mL/min and 0.12 mL/min, 42:1 (b) 350 ºC, 22MPa (c) 350 ºC, 22MPa, 42:1

22

(a)

(b) Fig 3. The effect of size and shape of the packing on the yield of biodiesel ((a) size (b) shape) (350ºC, 22MPa, 42:1)

23

Fig. 4. Velocity contour partial graphic((a) tube reactor (b) Φ6mm (c) Φ3mm (d) Φ0.5mm) (350ºC, 22MPa, 42:1)

24

Fig. 5. Plot of ln[uME,0]- ln[uME, t] value of oil to FAME against reaction time at different temperature (22MPa, 42:1)

25

Fig. 6. Plot of lnk value of oil to FAME against 1/T (22MPa , 42:1)

26

Highlights  The packing size and shape had great effect on the flow field and reaction.  Simulation was applied to research the effect of packing size on the flow field.  With the decrease of the size, the flow field was more favorable to the reaction.  Reducing the packing size was benefited to improving yield.  Dixon rings were perfect as packing and the highest yield 90.84% was obtained.

27