Article pubs.acs.org/EF
Deactivation and in Situ Regeneration of Anion Exchange Resin in the Continuous Transesterification for Biodiesel Production Benqiao He,*,† Yanbiao Ren,† Yu Cheng,† and Jianxin Li*,†,‡ †
The State Key Lab of Hollow Fiber Membrane Materials and Processes, School of Materials Science and Engineering, Tianjin Polytechnic University, Tianjin 300160, P.R. China ‡ The Laboratory of Membrane Materials and Separation Engineering Technology, Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201203, P.R. China ABSTRACT: The deactivation and in situ regeneration of anion exchange resin (D261) used as a catalyst in the continuous transesterification of soybean oil with methanol for biodiesel production were studied in a fixed-bed reactor. The chemical and physical structures of the resins were investigated by means of X-ray photoelectron spectrometer (XPS), N2 adsorption/ desorption isotherms, and scanning electron microscope (SEM). The results showed that biodiesel conversion was over 90% within the run time of 4 h but rapidly declined to 23.7% after 8 h. The fouling of organic substances (triglyceride and glycerol) covered on the resin was the main reason leading to the decrease in the biodiesel conversion. The fouling extent on the resin was related to the resin position in the fixed bed. The largest activity loss of the resin occurred at the bottom of the fixed bed. The leakage of OH− groups from the resins, independent of the resin height in the bed, also resulted to the decline of the resin activity. An in situ regeneration method was put forward. The resin regenerated can be restored to the original catalytic activity to perform continuous transesterification for biodiesel production.
1. INTRODUCTION Biodiesel has attracted more and more attention as an alternative fuel resource because of its advantages of renewability and low emissions.1,2 Biodiesel is traditionally prepared through esterification of long-chain fatty acids or transesterification of triglycerides with alcohol, catalyzed by homogeneous acids or bases, respectively.3−5 Though the homogeneous catalysts could effectively catalyze the reaction under mild reaction conditions, the purification of the reaction products (methyl ester and glycerol) to separate the homogeneous catalysts by water washing at the end of the reaction was ineluctable. As a result, a large amount of waste water was produced to pollute the environment.6−9 Furthermore, it is difficult for homogeneous catalysts to carry on continuous production of biodiesel. Ion-exchange resin, as one kind of heterogeneous catalysts, which can overcome the shortcomings of the homogeneous catalysts,10 is suitable for continuous production biodiesel in a fixed-bed reactor in an experimental or industrial scale.11−14 The continuous esterification of long-chain fatty acids with alcohol for biodiesel production under cation-exchange resin as catalysts was widely investigated. Liu et al.15 found that the resin D002 retained about 91.3% of its original activity after 10 cycles of successive reuse. Tesser et al.9 investigated the continuous process to produce biodiesel for 150 h with a conversion of over 95%. Feng et al.14 reported a biodiesel production with a conversion of over 98.0% within a continuous running of 500 h. To further improve the catalytic stability of the cation-exchange resin, the deactivation of the resin catalyst was also investigated. Dixit et al.16 found that the deactivation of the resin (Amberlyst-15) resulted from the pore structural alteration or the deposition of styrene oligomers within the resin pores. Berhard et al.17 found that hydrogen © 2012 American Chemical Society
ions of the resin were exchanged with metallic cations, such as Na+, K+, Mg2+, and Ca2+ in the oil feedstock, leading to a fast catalyst deactivation. A water layer that formed on the hydrophilic surface of the resins could prevent the access of the relatively hydrophobic substrates, slowing down the reaction rate .12,15 For the transesterification of triglyceride, anion-exchange resin containing quaternary ammonium active groups were often adopted as heterogeneous base catalysts to catalyze the reaction. Shibasaki-Kitakawa et al.18 investigated a continuous transesterification of crude triolein with ethanol in a fixed-bed reactor packed with anion-exchange resin (PA360s) with a biodiesel conversion of 80%. The regeneration of the resin for reuse was studied in batch mode. Long et al.19 prepared Nmethylimidazole functionalized anion exchange resin (R+− OH−), containing NaOH and applied for transesterification of soybean oil, with a biodiesel conversion of 97.25% at a reaction time of over 10 h in a batch mode. R+OH− (Na) can be reused at least five times with a slow loss of activity by a direct ion exchange reaction of hydroxyl ions with the oleic acid groups. The deactivation and regeneration of anion-exchange resin catalysts in a continuous transesterification process have been rarely reported up to now. Our previous work reported the continuous transesterification of soybean oil with methanol in a fixed-bed reactor packed with anion-exchange resin D261.10 A fatty acid methyl ester (FAME) conversion of more than 90% within a run time of 4 h was obtained. However, the activity of the resin quickly declined after 4 h of run time. Therefore, the aim of this work was to investigate the deactivation cause of the Received: March 27, 2012 Revised: May 28, 2012 Published: May 30, 2012 3897
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Figure 1. Scheme of fixed-bed reactor system used for the continuous transesterification.
anion exchange resin so as to develop an in situ regeneration method for continuous transesterification to produce biodiesel.
Ah =
Xh Xh/t → 0
× 100%
(1)
Where Xh is the conversion obtained at the bed height z = h at some time. Xh/t→0 is the conversion corresponding to the initial time when the catalyst is assumed with full activity. The surface chemistries of the resin were analyzed by using an X-ray photoelectron spectrometer (XPS, Theromfisher Kα). Survey and high-resolution spectra were collected under 200 and 20 eV pass energy, respectively. For evaluating the catalytic performance of the resin used, ion exchange capacity (IEC), the number of activity sites of the resin, was determined by acid−base titration methods. 1 g of resin was mixed with 200 mL of 0.1 mol/L HCl solution with 5 wt % NaCl and allowed to stand overnight at room temperature. 50 mL of the supernatant liquid withdrawn was subsequently titrated with 0.1 mol/ L NaOH, using phenolphthalein as an indicator to determine residual amount of acid. IEC (Q, mmol/g) was calculated using the following equation:21
2. MATERIALS AND METHODS 2.1. Materials. The soybean oil was purchased from China Oil Food Corporation and distilled to remove water before use. Methanol and n-hexane as a cosolvent were purchased from Tianjin Kermel Chemical Reagent Ltd., Co., China. The unactivated ion-exchange resin D261 with macroporous structure in a quaternary ammonium chloride form was purchased from the Chemical Plant of NanKai University, China. The resin was activated according to the following procedures. The resin was first pretreated by water washing to remove some impurities. Then, D261 was immersed into the solution of 5.0 wt % KOH in methanol for 12 h to transform from chloride form into hydroxyl form (activated process of D261). Finally, D261 was washed with deionized water to pH∼7 and washed with methanol to swell the resins and displace water remaining in resins. The activated D261 was stored under airtight condition before use. 2.2. Continuous Transesterification in a Fixed-Bed Reactor. The continuous transesterification was performed in a fixed-bed reactor, as shown in Figure 1. The reactor is a water-jacketed stainless steel column with an internal diameter of 30 mm and a height of 450 mm, which is composed of one inlet at the bottom, one outlet at the top, and three side outlets at different heights (Z) (No. 1, 2, 3, corresponding to heights Z = 110, 220, 330 mm, respectively). The column was packed with the activated D261 resin. Methanol, soybean oil, and n-hexane were mixed and preheated in a feedstock tank and fed to the inlet of reactor using a peristaltic pump. The reaction temperature with an error of ±0.5 K was controlled by a thermostat water bath. To investigate the activity of the resin in the fixed bed, 10 mL of reaction mixture were taken at each hour of run time from the outlets at different positions for conversion measurement. 2.3. Measurement of FAME Conversion. The sample taken from the outlets was purified by reduced pressure distillation to remove the excess methanol and n-hexane. The methanol and nhexane that were distilled out were reclaimed. The left sample was collected for the conversion measurement by 1H nuclear magnetic resonance (1H NMR, Bruck Co, 300 MHz). For the detailed procedure, the reader is referred to the literature.10,20 2.4. Chemical and Physical Properties of Resin D261. The activity retention (Ah) values of the resin located at different sections in the fixed bed were expressed in the following eq 1:9
Q=
V1 × C1 − nV2 × C 2 M
(2)
In eq 2, V1 (ml) is the volume of the HCl solution for immersion of the resins; V2 (ml) is the volume of NaOH used for titration of the remaining HCl; n is the ratio of V1 to V2; C and C2 (mol/L) are concentrations of the HCl originally added and of NaOH used for titration, respectively; and M (g) is the weight of the resin taken for analysis. Specific surface area, volume of pores, and pore size of the resins were evaluated from N2 adsorption/desorption isotherms obtained at 77 k using a Micromeritics ASAP 2020 instrument. The samples were outgassed in a vacuum at 473.15 k for 2 h. Specific surface areas were calculated from the adsorption branches according to the Brunauer− Emmett−Teller (BET) method. The average pore size and pore volume were obtained from the desorption branches according to the Barret−Joyner−Halenda (BJH) method. Resin morphologies were observed on Field Emission Scanning Electron Micrograph (FESEM, Hitachi S-4800) after sputtering golden particles. 2.5. In Situ Regeneration of Resin D261. To maintain the activity of resin D261 in a fixed-bed reactor, the resin needs to regenerate when the catalytic efficiency dropped. An in situ 3898
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regeneration method was put forward. Basically, the method consisted of the following steps: (1) Flushing from inlet to outlet with n-hexane at a flow rate 10 mL/min for at least 20 min to remove the organic substances adsorbed onto the resin particles. (2) Washing from inlet to outlet with the solution of 5 wt % KOH in methanol at a flow rate of 1.5 mL/min for 90 min to restore the activity of the resin. (3) Washing from inlet to outlet with methanol to pH∼7. The regenerated resin was available for the next run.
trace of glycerol in the effluent was tested by NMR in the reaction system with methanol/oil molar ratio of 9:1. It suggests that glycerol produced was adsorbed on the resin.10 Too much glycerol adsorbed on the resin retarded the catalytic activity of the resin, resulting to a lower conversion beyond 6 h of run time compared to the system with methanol/oil molar ratio of 28:1. It implies that glycerol fouling should be one of the reasons resulting in the deactivation of D261. 3.2. Activity Retention of the Resins at Different Heights in the Fixed Bed. To investigate the activity retention of the resins in fixed bed, the catalytic activity of the resins located at different bed heights were investigated with run time as shown in Figure 3.
3. RESULTS AND DISCUSSION 3.1. Catalytic Stability of the Resins with Run Time. Catalytic stability of the resins is one of the important parameters for a fixed-bed reactor, which is affected by chemical and physical stability of the resin. Figure 2 shows the catalytic
Figure 3. Activity retention of the resins at various reactor sections with run time.
Figure 2. D261 catalytic stability within 8 h of continuous transesterification at different reaction conditions. (A) Reaction temperature of 313.15 K and methanol/oil ratio 9:1. (B) Reaction temperature of 323.15 K and methanol/oil ratio 9:1. (C) Reaction temperature of 323.15 K and methanol/oil ratio 28:1.
It was found that the resin activity was lost with run time. However, the rate of activity loss was different for the resin at different bed heights. For the resin located at side outlet 1, the activity retention quickly declined from 100% to 44% within 1 h of run time and then slowly decreased to 30% within 4 h of run time. For the resins located at side outlets 2 and 3, the activity retention declined to 52% and 72% within 2 h of run time, respectively, while almost no activity loss occurred for the resins located at the upper outside within 3 h of run time. From these results, one can find that the rate of activity loss of the resins in the fixed bed is related to the height at which the resin was located. The activity loss of the resin in the bottom of the fixed bed was fastest; and activity retention was best at the top. The phenomena may be related to fouling of reaction mixtures on the resins. During the catalysis of the transesterification, triglyceride and methanol can be first adsorbed on the resin surface to react.18 Unreacted triglyceride would cover the active sites to hold back methanol access due to the hydrophobilty of triglyceride, leading to a low reaction rate. The concentration of triglyceride is the highest at the bottom, so serious fouling may occur at this section. With the reactant mixture flowing from the bottom to top of the fixed bed, the concentration of triglyceride becomes less and less, which abates the fouling of triglyceride to resins, leading to higher activity retention at higher bed positions. Meanwhile, the glycerol produced can be also adsorbed by the resin to decrease the resin catalytic activity, as mentioned previously. In addition, the glyceroxide anions produced may poison the active sites of the resin.23,24 3.3. Adsorption of Organic Substances on the Resins. XPS was used to survey atomic species and their relative atomic concentrations on the surface and cross section of resins in different states as shown in Tables 1 and 2, respectively. From
activity of D261 resins with run time under different temperatures and methanol/oil molar ratios. Other reaction conditions are D261 of 80 g, n-hexane to oil weight ratio of 1:2, feed flow rate of 1.2 mL/min. It can be seen from Figure 2 that a conversion of greater than 90% was obtained at reaction temperature of 313.15 K and methanol/oil ratio 9:1 (Line A in Figure 2) with 4 h of run time in the fixed bed, and a quickly decreased FAME conversion from about 90% to 30% occurred with the run time from 4 to 6 h. The conversion obtained further declined to 23.7% after 8 h. The same trend was observed at reaction temperature of 323.15 K and methanol/oil ratio 9:1 (Line B in Figure 2). It suggested that the applied temperature in the range 313.15−323.15 K has no obvious effect on the activity of the resin. Further increasing the methanol/oil ratio from 9:1 to 28:1, a lower FAME conversion (above 80%) was obtained within the run time of 4 h. After the run time of 6 h, a conversion of over 40% was achieved at the methanol/oil molar ratio of 28:1, higher than that at the methanol/oil molar ratio of 9:1, as shown Figure 2. These phenomena may be related to glycerol solubility in methanol and adsorption of glycerol on the resins.22 In fact, glycerol in the effluent was detected by NMR in the reaction system with methanol/oil molar ratio of 28:1. It suggests that glycerol produced was soluble in the reaction system, which restrained the forward transesterifcation, leading to a lower conversion at the beginning stage compared to the system with methanol/oil molar ratio of 9:1. The dilution effect of methanol at the methanol/oil molar ratio of 28:1 may also lead to a lower conversion compared with the reaction system with the methanol/oil molar ratio of 9:1.10 On the contrary, no 3899
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in ester bond (−COO−). After resin regeneration, the peak at 288.6 eV completely disappeared (Figure 4b). It suggested the esters on the resin were removed. At the same time, the peak at 286.2 eV was strengthened, corresponding to C−N structure in the quaternary ammonium due to its exposal after removing organic substances. From the XPS results, the fouling of organic substances on the resin was further demonstrated. 3.4. IEC Change of the Resin. Hydroxyl ions acting as the active sites play an important role in resin catalytic property, which may fall off from the resins during reaction.18 Therefore, IEC was used to characterize the catalytic stability of the resins. It was found from the experiment that IEC of the activated resins was 0.32 mmol/g. After 8 h of run time, the IECs of the resin at the top and bottom of the bed fell to 0.27 and 0.21 mmol/g, respectively. After the resin located in the bed was washed by the n-hexane, the IEC of the resin at the bottom increased to 0.28 from 0.21 mmol/g, equal to the value of the resins at the top of the bed. From these results, the fouling of the organic substances and leakage of OH− groups18 led to the IEC drop. It is also suggested that the fouling of organic substances on the resins is related to resin position in the bed, which is consistent with the results obtained in Figure.2, while the leakage of OH− groups on the resins was dependent on the resin position in the bed. Further regenerating the deactivated resin by the immersion of KOH solution, the IEC of the resin recovered to the original value of 0.32 mmol/g, suggesting no quaternary ammonium groups were decomposed during the operation. 3.5. Physical Properties of the Resins. The physical properties such as surface area, pore volume, and pore size are considered to be important parameters affecting the catalytic performance of the resins.16 The physical properties of activated, deactivated, washed by n-hexane after deactivation, and regenerated resins are listed in Table 3. It can be seen from
Table 1. Relative Atomic Concentrations on the Surface of Resins in Different States sample
C1s (%)
O1s (%)
N1s (%)
Cl2p (%)
other (%)
O/C
unactivated resin activated resin deactivated resin regeneration resin
61.5 60.0 82.6 62.7
18.5 30.4 12.0 31.1
1.5 1.3 1.9 2.6
0.7
17.8 8.3 3.5 3.6
0.30 0.50 0.14 0.50
Table 2. Relative Atomic Concentrations on Cross Sections of Resins in Different States sample unactivated resin activated resin deactivated resin regeneration resin
C1s (%)
O1s (%)
N1s (%)
Cl2p (%)
other (%)
75.7 83.3 84.9 82.6
10.2 8.4 8.4 10.4
4.0 4.2 3.3 4.3
4.9 3.8 3.3 2.7
5.2 0.3 0.1
O/C 0.13 0.10 0.10 0.12
Table 1, the unactivated resin contained carbon (C), oxygen (O), nitrogen (N), chlorine (Cl), and some other atoms (mainly metal atoms). After activation, the chlorine content decreased and the oxygen content increased because Cl− ions were replaced by hydroxyl groups (OH−) acting as active sites. However, for the deactivated resins, the oxygen content markedly decreased, with an O/C ratio of 0.14, which is near the O/C ratio (about 0.13) of triglyceride molecule. The result suggested that the triglyceride fouled the resin surface. For the regenerated resins, the O/C ratio almost was resumed to the value of activated resins, prefiguring a good catalytic activity. Table 2 shows that the cross sections of the resins in different states almost have the same C, O, N, and Cl contents. The resins in different states all contain a large amount of chlorine, and the O/C ratio of the cross section of resins in different states also did not change. It was suggested that most of Cl atoms in the resin interior were not exchanged with hydroxyl groups during activation. Triglyceride seldom accessed the resin interior, which is different from the phenomena reported by Dixit et al.16 It implies that the active sites are mainly distributed on the surface of resin, making it possible to quickly in situ activate the resin. Figure 4 shows C element scans of deactivated and regenerated resins by XPS. It was found from Figure 4a that there were three types of C1s peaks at 284.5, 286.4, and 288.6 eV. The peak at 284.5 eV corresponds to C−C or C−H structure. The peak at 286.4 eV is attributed to C−O or C−N single band, possibly from glycerol/ester or quaternary ammonium, respectively. The peak at 288.6 eV is C1s peak
Table 3. Structure Parameters Calculated from N2 Adsorption Isotherms different resins
surface area (m2/g) pore vol. (cm3/g) pore size (nm)
activated
deactivated
washed by n-hexane
regenerated
64.9
54.4
61.6
64.8
0.36 14.56
0.41 16.31
0.42 17.83
0.28 4.78
Table 3 that there was a drastic reduction in the surface area, pore volume, and average pore size of deactivated resin compared with other resins. This is because the organic
Figure 4. C1s scan of surfaces of deactivated resin (a) and regenerated resin (b). 3900
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Figure 5. SEM images of the surfaces of the resins in different states activated (a), deactivated (b), washed by n-hexane after deactivation (c), and regeneration resins (d).
adsorbed on the active sites, related to the resin position in the bed, was the predominant reason for the resin deactivation. The largest activity loss of the resins occurred at the bottom of the fixed bed. The leakage of OH− on the resins also resulted from the activity loss, independent of the resin height in the bed. The resin can be regenerated in situ and restored to the original catalytic activity to become a truly continuous transesterification of soybean oil.
substances covered or plugged the pore of the deactivated resins.15 This would hold back access of hydrophilic substance (methanol), leading to a low catalytic activity.16 Further, Figure 5 presents the SEM photos of the surfaces of activated, deactivated, washed by n-hexane, and regenerated resins. For the activated, washed by n-hexane and regenerated resins, the fine structures of the resin surfaces could be distinguished, and the pore structure was very clear. However, for the deactivated resin, only some smooth particles can be seen, and no pore structure can be found among the particles (Figure 5b). These results suggest the organic substances covered the resin surface and plugged the pore structure.15 The inner structures of the four resins have no obvious difference (no shown here), which is consistent with the result of the XPS analysis. 3.6. Catalytic Stability of D261 after Regeneration. From above studies, the resin can keep excellent catalytic stability within the run time of 4 h. Beyond 4 h, regeneration is necessary, because of the fouling of organic substances and the loss of active sites. The regenerated D261 resin by a three-step method mentioned in the Materials and Methods section was available for the next run. The catalytic stability of D261 after regenerating every 4 h of continuous run was shown in Figure 6. It was found that the FAME conversion remained almost
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AUTHOR INFORMATION
Corresponding Author
*Tel/Fax: +86 22 83955055. E-mail:
[email protected] (B.Q.H.);
[email protected] (J.X.L.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support by National High Technology Research and Development P r o g r a m o f Ch i n a (“8 63 ” P r o g r a m , G r a n t N o . 2009AA03Z223 and National Natural Science Foundation of China (Grant No. 21174104).
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REFERENCES
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Figure 6. Catalytic stability of D261 after regeneration every 4 h of continuous run.
greater than 90% for five runs. Thus, the resin could be repeatedly used for the transesterification without obvious loss in the catalytic activity, showing a high conversion and operational stability.
4. CONCLUSION In continuous transesterification using anion-exchange resin D261 as catalyst in a fixed-bed reactor, the FAME conversion can stay above 90% within the run time of 4 h, but it goes down to 23.7% at the run time of 8 h. The applied temperature (in the range 313.15−323.15 K) of resin has no obvious effect on the activity of the resin. The fouling of organic substances 3901
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