Removal of Acetic Acid from Fuel Ethanol Using Ion-Exchange Resin

Nov 6, 2012 - Using sorption or ion exchange resins in aqueous media is an .... 1.0 mL/min solution of formic acid and methanol (9:1, v/v) as mobile p...
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Removal of Acetic Acid from Fuel Ethanol Using Ion-Exchange Resin Huisheng Lv,† Yanpeng Sun,† Minhua Zhang,*,† Zhongfeng Geng,† and Miaomiao Ren† †

Key Laboratory for Green Chemical Technology of Ministry of Education, Tianjin University R&D Center for Petrochemical Technology, Tianjin 300072, China ABSTRACT: The amount of trace organic acids must be controlled in the fuel ethanol product in order to reduce the chance to corrode automotive cylinder. Ion-exchange resin was investigated to remove acids from fuel ethanol in this paper. Industrial resins, D301R, 330, 201×7, and D201, were selected as candidates, and a series of experiments were carried out to determine which one is the best. Acetic acid was employed as a simulated compound in these experiments for it is the main residual acid in fuel ethanol product. The results showed that the 330 resin was the most effective one to remove acid from fuel ethanol, and then, both static and dynamic experiments were carried out to evaluate the performance of the 330 resin. It was found that equilibrium data can be well described by Langmuir isotherm during the temperature range from 25 to 35 °C. The kinetic data fitted well with the pseudo-second-order kinetic model. Furthermore, a bench scale fixed bed was set up to determine the optimal adsorption and regeneration conditions. When the initial concentration of acetic acid solution was 200 mg/L, the optimum operating conditions were as follows: A flux of 6.37 BV/h at a temperature of 30 °C. The optimum regeneration conditions were determined as follows: A 4% solution of sodium hydroxide, flux was 3.18 BV/h, and the temperature was 30 °C. A refined product with acidity under 56 mg/L was obtained under optimal operating conditions. At last, industrial fuel ethanol was used to test the selected resin and the established process conditions. No obvious difference was observed after five adsorption and regeneration cycles. Therefore, it can be concluded that the ion-exchange method would be a successful industrial process to remove acids from fuel ethanol.

1. INTRODUCTION Fuel ethanol from biomass is a renewable energy source which is cleaner than regular gasoline as it produces less carbon dioxide, carbon monoxide, and oxides of nitrogen.1,2 It is of great significance as it can help reduce the dependency on fossil fuels, lower environmental pollution, and promote grain transformation.3−7 Production of fuel ethanol is similar to the production technology of edible alcohol. Contrary to edible alcohol, fuel ethanol permits more impurities but less water. In order to minimize the energy lost during the production process, thermal coupling and precise rectification technologies were introduced. Utilization of such methods can reduce the manufacturing costs. However, when using such technologies the resulting fuel ethanol product is of high acidity, especially in light of the fact that recently nongrain cassava is used as a raw material which results in a higher concentration of acidic residues in the final product than other substrates. As these acidic residues are corrosive to automotive component materials, actions must be taken to remove the acid from the fuel ethanol. Acetic acid is the main component affecting acidity. According to the international standard, the acidity should be limited at 56 mg/L as acetic acid.8 At present, the main method to resolve the problem is by adding alkali during the rectification process or to final product directly. There are some drawbacks of using such a method. For one thing, severe evaporation causes crystallization of sodium hydroxide on each tray and in the reboiler which results in blockage of the column tray in the precise rectification tower and scab of heat exchanger. Periodic cleaning of tower and heat exchanger will increase cost. Second, the addition of sodium hydroxide causes the wastewater to become alkaline which © 2012 American Chemical Society

would increase treating cost. Last but not least, residual alkali metal ion in the final fuel ethanol product would accelerate the corrosion to engine cylinder. Continuous addition of sodium hydroxide, periodic cleaning of the distillate column and the heat exchanger, and more difficult treatment of wastewater are all accompanied by increased financial cost during ethanol production. Therefore, the financial cost of using a sacrifice reagent (NaOH) is high, and it is necessary to develop a new method to remove the acid from fuel ethanol more economically. Using sorption or ion exchange resins in aqueous media is an accepted method for ionic species as well as for ionizable organic molecules, particularly for acids.9−13 Experimental and theoretical works have been reported for a wide variety of organic substances. Unlike that in aqueous conditions, the literature on adsorption from nonaqueous phases on ion-exchange resins is limited.14 Cren published a few papers on the determination of adsorption isotherms and breakthrough curves for oleic acid and linoleic acid removal from ethanolic solutions.15,16 The work indicated the feasibility of recovering fatty acids from ethanolic solutions by means of ion exchange in a fixed resin bed. The reported efficiency of oleic acid removal was always close to 98%. Gaikar published a few papers on the adsorption of acetic acid, naphthenic acid, and alkylphenols from nonaqueous phases using ion-exchange resins, demonstrating very high capacity and selectivity.17,18 Received: February 16, 2012 Revised: November 5, 2012 Published: November 6, 2012 7299

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Table 1. Sources and Specification of Ion-Exchange Resins item

type

matrix structure

functional group

exchange capacity (mmol/g (dry))

moisture holding capacity

harmonic mean size

201×7 330 D201 D301R

strongly base weakly base strongly base weakly base

styrene-DVB epoxy styrene-DVB styrene-DVB

-N+(CH3)3 =NH, -NH, ≡N,=N+= -N+(CH3)3 -N(CH3)2

≥3.6 ≥9.0 ≥3.7 ≥4.8

40−50% 55−65% 60−70% 50−60%

0.3−1.2 mm not available 0.3−1.2 mm 0.3−1.2 mm

Table 2. Composition of Industrial Fuel Ethanol methanol, volume %

ethanol, volume %

0.037

99.7

fusel oil mg/L water content, volume % 85.4

0.24

2. MATERIAL AND METHODS 2.1. Material. Anhydrous ethanol, acetic acid, and sodium hydroxide were purchased from Tianjin Chemical Company (China). All reagents used in the present study were of analytical grade. Acetic acid solutions of different concentrations were prepared by adding acetic acid to anhydrous ethanol. Adsorbents used in this study were commercial weakly basic resins D301R and 330, while 201×7 and D201 were commercial strongly basic resins. D301R, 201×7, and D201 resins were purchased from Nan Kai chemical plant (China), while 330 resins were purchased from Sanxing Co., Ltd. (China). Some properties of the resin are given in Table 1. Industrial fuel ethanol was supplied by Hua Run Ltd. in Heilongjiang province of China. The composition of industrial fuel ethanol is shown in Table 2. 2.2. Conditioning of the Resin. The resin was first stirred in the 1.0 mol/L HCl solution and soaked for 2 h followed by thoroughly washing with large volume of deionized water to ensure the resin completely free of acid. Then the resin was stirred in the 1.0 mol/L sodium hydroxide solution and soaked for 4 h also followed by thoroughly washing with large volume of deionized water. This procedure was repeated for three times. Then the resin was dried in vacuum drying oven at 60 °C overnight and subsequently cooled to 30 °C in desiccator. 2.3. Static Adsorption Experiments. Either 0.5 g, 1.0 g, 2.0 g, 4.0 g, and 8.0 g of resins was put into a 250 mL flask with 20 mL of anhydrous ethanol and then sealed with lids in order to induce swelling. According to the static adsorption experiments, the solution concentration does not change any more after 10 h. So we chose swelling time as 12 h which should be long enough for swelling. After 12 h swelling, 150 mL of a stock solution acetic acid was added into the flask, and then the flask was placed in a thermostat controlled shaking assembly. Stock solutions had been prepared in the following concentrations, 150 mg/L, 200 mg/L, and 250 mg/L. The solutions were stirred continuously at 25 °C, 30 °C, and 35 °C for over 10 h until equilibration was achieved. The experiments were repeated three times, and average values were reported. The concentration of acetic acid solution after adsorption was analyzed via basic acid−base titration with NaOH solution, using phenolphthalein as the indicator. The equilibrium adsorption capacity was calculated according to eq 1

(c0 × v1 − c × v2) w

acidity (as acetic acid) (mg/L)

pH

appearance

0.792

200

5.7

clear and bright

2.4. Bench Scale Fixed Bed Experiments. Column studies were conducted using a glass column with its inner diameter as 2.0 cm and its length as 80.0 cm with circulation heating system to control the temperature. Bench scale fixed bed was prepared using the slurry packing technique in order to ensure the uniform packing of resin. The break point is defined as a concentration. After this point, the quality of product is not measured up to the standard and the adsorption affinity decreased, or not-detectable, and an obvious increase in concentration of acetic acid is observed. Different experiments have different break points. In order to obtain qualified fuel ethanol, 56 mg/ L is defined as the break point in this study according to the ASTMD4806. It is important to set up the breakthrough curve, and the dynamic experiments would be stopped at this point. Each experiment was repeated three times, and average values were reported. Each presented data point represents the mean fraction. Error bars represent the 95% confidence intervals. The solid lines demonstrate the trend but are not a fit of the data. 2.4.1. Dynamic Adsorption Experiments. 50 g of resins was packed in the fixed bed column. Acetic acid solution of 150 mg/L, 200 mg/L, or 250 mg/L was passed through the column. The samples were taken every 2 min, 5 min, 1 h, or 2 h. Then concentrations of the acetic acid in the effluent were analyzed as stated in section 2.3. The adsorption capacity was calculated according to eq 2:

Removal of acids from aqueous media and organic media using ion-exchange resins via adsorption or ion exchange was found to be an efficient and economic process. A favorable process of acid removal from fuel ethanol was possible using ion-exchange resin. However, to the best of our knowledge, the use of ion-exchange resin for removal of acetic acid from fuel ethanol has not been reported in the literature.

q=

density (g/cm3)

v

q=

∫0 i (c0 − ce)dv (2)

m

The removal ratio of acid was calculated according to eq 3

θ=

c0v − ∑ cevi c0v

(3)

where q is the equilibrium adsorption capacity (mmol/g(dry resin)), c0 and ce are the initial and efflux concentration (mmol/L) of acetic acid in solution, respectively, m is the weight of dry resin and θ is the removal ratio of the acid, v is the volume of the acetic acid solution through the fixed bed (L), and vi is the volume of efflux (L). 2.4.2. Dynamic Regeneration Experiments. The regeneration of a used resin was also investigated. Acid adsorbed on the resin was eluted by 130 mL of deionized water in the flow rate of 4.78 BV/h (BV: Bed Volume) and then eluted by a 700 mL NaOH solution of 2%, 3%, 4%, 5%, and 6% in the flow rate of 2.39, 3.18, 4.77, and 6.37 BV/h at temperatures 25 °C, 30 °C, 35 °C, and 40 °C. All eluent was collected, and the concentration of NaAc was determined using high performance liquid chromatography (HPLC). The instrument was Agilent 1100 purchased from Agilent corporation. The HPLC conditions were as follows: Zorbax C18 column (250 × 4.6 mm) as stationary phase, 1.0 mL/min solution of formic acid and methanol (9:1, v/v) as mobile phase. Detection wavelength was set at 213.8 nm in the ultraviolet detector. The regeneration ratio of the resin was calculated according to eq 4 c uvs η= c0v − ∑ cevi (4)

(1)

where q is the equilibrium adsorption capacity (mmol/g (dry resin)), c0 and c are the initial and equilibrium concentration (mmol/L) of acetic acid solutions, respectively, v1 and v2 are the initial and equilibrium volume of solution (L), respectively, and w is the weight of dry resin (g).

where η is the regeneration ratio of the resin, and cu and ce are the average acetic acid concentration of eluant and efflux concentration 7300

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(mmol/L) of acetic acid in solution, respectively. vs is the volume of eluant (L), and vi is the volume of efflux (L).

the resin polymeric structure cannot be ruled out particularly when the solvent can penetrate and swell the resin structure. The difference between the sorption behaviors of the resins from organic solvents is probably due to solvation of acetic acid in the solution. Its interaction may be ion-exchange force or weak hydrogen bonded complexes between the amino groups on the resin matrix and acidic hydrogen of the acetic acid. In addition, styrene-divinyl benzene copolymer matrix for resins 201×7 and D201 is harder for solute to penetrate than epoxy matrix of resin 330. The regeneration ratio of the 201×7 resin was the lowest. 201×7 resin was strongly anion exchanger, so the interaction between the 201×7 resin and the acid maybe stronger and the regeneration ratio of the resin was low. 330 resin is the optimal, so all the following studies were conducted on the 330 resin. 3.2. Adsorption Isotherm. The adsorption equilibrium of acid on resin was studied as a function of concentration at different temperatures. A proper equilibrium model was explored for engineering purposes. To evaluate the efficiency of the 330 resin for the removal of acetic acid, 0.5 g, 1.0 g, 2.0 g, 4.0 g, and 8.0 g of resins were treated with same concentration stock solutions according to the static adsorption method mentioned in 2.3 at 25 °C, 30 °C, and 35 °C, respectively, to obtain the final concentration and equilibrium adsorption capacity. The adsorption isotherms are presented in Figure 1.

3. RESULTS AND DISCUSSION 3.1. Selection of Resin. Both static and dynamic methods were employed to select the optimum resin among the four candidates. 3.1.1. Static Experiments. 0.5 g of each of the four kinds of pretreated resins was treated according to the static adsorption method mentioned in 2.3 using a stock solution of 200 mg/L at 35 °C. The comparison of effective adsorption capability of acetic acid among the four kinds of resins measured by bench static method is presented in Table 3. Table 3. Static Adsorption Capability between the Four Kinds of Resins item

symbol

units

201×7

330

D201

D301R

static adsorption capability

qs

mmol/g (dry resin)

0.56

0.53

0.44

0.11

It could be seen that the D301R resin had the lowest static adsorption capability (0.11 mmol/g (dry resin)), so the D301R resin was ruled out. The dynamic selection experiments were done among the other three kinds of resins. 3.1.2. Dynamic Experiments. 50 g of the 330, 201×7, and D201 resins were packed in the fixed bed column, respectively. A 200 mg/L acetic acid solution was passed through the column at a flow rate of 4.78 BV/h (1 BV = 130 mL) at 35 °C. Acid adsorbed on the resin was eluted by 130 mL of deionized water in the flow rate of 4.78 BV/h and then eluted by 700 mL of a 4% NaOH solution in the flow rate of 2.39 BV/h. The dynamic adsorption capability, removal ratio of acetic acid, and regeneration ratio of resin obtained by dynamic method mentioned in 2.4 for the other three kinds of resins were also shown in Table 4. Table 4. Dynamic Results between the Three Kinds of Resins item

dynamic adsorption capability

dynamic removal ratio of acetic acid

regeneration ratio of resin

symbol units 330 201×7 D201

qd mmol/g(dry resin) 0.67 0.46 0.32

θ % 88.5 88.0 83.4

η % 97.8 90.3 90.7

Figure 1. Adsorption isotherms of acetic acid on 330 resins.

The adsorption capacity showed that the adsorption was decreased with the increase of temperature, indicating that the process was apparently exothermic. The adsorption data were fitted to the Langmuir equation20 5 q Kbc q = max 1 + Kbc (5)

It could be seen from Table 4 that the 330 resin had highest adsorption capability than the other two kinds, so the optimal resin for the removal of acetic acid from acetic acid solution is 330 resin. The 330 resin is a weakly base ion-exchange resin. According to studies of V. G. Gaikar, the adsorption is due to the formation of weak hydrogen bonded complexes between the amino groups on the resin matrix and acidic hydrogen of the acetic acid.19 The interaction between the lone pair of electrons on nitrogen of secondary and tertiary amino groups and the acidic hydrogen can be treated as a Lewis acid−base interaction. The resins 201×7 and D201 were strongly basic resins with quaternary amino functional groups on a styrenedivinyl benzene copolymer matrix. Because a large amount of organic solvent is in contact with the resin, its interaction with

where q is the equilibrium adsorption capacity of dry resin (mmol/g), Kb is the Langmuir equilibrium constant, and c is the equilibrium concentration of acetic acid in solution (mmol/ L). The linear form of eq 5 can be expressed as eq 6: 1 1 1 = + q qmKbc qm (6) 7301

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Figure 2 showed a plot of 1/q vs dimensionless c0/c. The values of Kb and qm can be obtained from the intercept and slope. Fitted values of equilibrium equation are presented in Table 5.

adsorbent resin surface was likely to be chemical mechanisms rather than physical mechanisms. The value of ΔG is −3.58, −1.46, and −0.34 kJ/mol at the temperatures 25 °C, 30 °C, and 35 °C, respectively. The negative values of ΔG (from −3.58 to −0.34 kJ/mol) indicated the spontaneous nature of the process in the temperature range from 25 to 35 °C. There was an increasing trend for ΔG values from −3.58 to −0.34 kJ/mol as the temperature increased from 25 to 35 °C. This trend indicated that a higher temperature had an adverse effect on the adsorption of acetic acid onto the resin. The negative values of ΔS (−324.5 J/mol) showed the decreased randomness at the solid surface after adsorption of the acetic acid onto the resin. It also indicated that the adsorbed acetic acid molecules onto the 330 resin surface were organized more regularly compared to the acetic acid molecules in the fuel ethanol molecules. 3.3. Adsorption Kinetics. Adsorption kinetics is significant as it provides valuable insights into the reaction pathways and into the mechanism of sorption reactions. It describes the solute uptake rate which in turn governs the residence time of an adsorption reaction. It is an important character in defining the efficiency of an adsorption process. The mechanism of adsorption depends on the physical and/or chemical characters of the adsorbent as well as on the mass transport process. In order to analyze the adsorption of acetic acid on the weakly basic anion exchange resin, 1.25 g of resins was treated according to the static adsorption method mentioned in 2.3 at 30 °C. The agitation speed is 200 rpm which is fast enough to eliminate the effect of external diffusion. The samples were taken every two hours, and concentrations of the acetic acid in the solution after adsorption were analyzed by titration according to the static adsorption method mentioned in 2.3. The pseudo-first-order and pseudo-second-order as well as Weber and Morris intraparticle diffusion kinetic models were used to fit the data.22−25The pseudo-first-order kinetic equation was intuitively associated with the model of one-site occupancy adsorption kinetics governed by the rate of surface reaction. More recently, its generalization for the two-sites-occupancy adsorption was proposed and called the pseudo-second-order kinetic equation.26 3.3.1. Pseudo-First-Order Model. The pseudo-first-order eq 9 is given as

Figure 2. The plot of 1/q vs c0/c according to Langmuir isotherms.

Table 5. Fitted Values of Equilibrium Equation on Adsorption T (K)

qmax /(mg/g(dry))

Kb

R2

298.15 303.15 308.15

137.17 251.26 403.23

4.237 1.785 1.144

0.9999 0.9945 0.9947

From Table 5, we can see that the adsorption isotherms were in accordance with the Langmuir model with correlation coefficients over 0.99. The equilibrium data were well described by the Langmuir isotherm model. Thermodynamic parameters such as Gibbs free energy (ΔG), enthalpic change (ΔH), and entropic change (ΔS) for the adsorption of acetic acid onto adsorbent resin were calculated in the following manner. ΔG was calculated using eq217 ΔG = −RT ln Kb

(7)

dqt

where T is the absolute temperature, and R is the universal gas constant. The relation between Kb and the thermodynamic parameters ΔH and ΔS can be described by van’t Hoff eq 8:

dt

ΔS ΔH − (8) R RT Then, ΔH and ΔS were calculated from the slope and intercept of the van’t Hoff plot, respectively. The calculated constants are presented in Table 6. The negative value of ΔH showed that the adsorption process was exothermic. In addition, a ΔH value of −100.18 kJ/ mol also indicated that the adsorption of acetic acid onto

log(qe − qt) = log qe −

dq t

ΔG (kJ/mol)

ΔH (kJ/mol)

ΔS (J/mol K)

R

298.15 303.15 308.15

4.237 1.785 1.144

−3.58 −1.46 −0.34

−100.18

−324.5

0.94

kf t 2.303

(10)

The data of kf and qe could be obtained from the plot in Figure 3, as shown in Table 7. 3.3.2. Pseudo-Second-Order Model. The pseudo-secondorder model is as eq 11

Table 6. Thermodynamic Parameters for the Adsorption Process Kb

(9)

qt is the amount of acetic acid adsorbed at moment t (mg/g); qe is the adsorption capacity at equilibrium; kf is the pseudo-firstorder rate coefficient (min−1); and t is the contact time (min). The integration of eq 9 can be described as eq 10 with the initial condition of qt = 0 at t = 0.

ln K b =

T (K)

= k f (qe − qt)

dt

2

= ks(qe − qt)2

(11)

In this equation, ks is the pseudo-second-order rate coefficient (mg/g·min). If eq 11 is integrated, a linear equation can be obtained, and the linear equation is shown as eq 12. 7302

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that the general use and wide applicability of the pseudo-firstorder model and the pseudo-second-order model equations during more than one century have not resulted in a corresponding fundamental search for their theoretical origin.26 The adsorption of several kinds of substances on peat and bottom ash or activated carbon was analyzed in Ho’s study.27 Ho drew a conclusion that the highest correlation coefficients were obtained for the pseudo-second-order kinetic model which may be chemical adsorption or chemisorptions. The adsorption of acetic acid on 330 resins was well fitted to the pseudo-second-order kinetic model in this study. In section 3.2 of this paper, the ΔH value of −100.18 kJ/mol also indicated that the adsorption of acetic acid onto adsorbent resin surface was likely to be chemical mechanisms rather than physical mechanisms. Therefore, it suggested that the overall rate of the adsorption process was controlled by chemisorptions which involved valence forces through sharing or exchange of electrons between the sorbent and sorbate. 3.3.3. Intraparticle Diffusion Model. Intraparticle diffusion model based on the theory proposed by Weber and Morris was tested to identify the diffusion mechanism. The model is as equation28 13:

Figure 3. The plot based on the pseudo-first-order model for the adsorption of acetic acids on 330 resins.

Table 7. Kinetic Parameters for the Removal of Acetic Acids by 330 Resins

qt = k it 1/2 + I

parameter pseudo-first-order model pseudo-second-order model intraparticle diffusion model

1 1 t = + t qt qe ksqe 2

kf 0.416 ks 0.023 K1 3.675

qe(mg/g) 24.53 qe(mg/g) 27.88 K2 0.176

R12

(13)

In eq 13, ki is the intraparticle diffusion rate coefficient, and I is constant. These values can be found by a plot of qt versus t1/2. The slope is ki, and the intercept is I. As seen in Figure 5, the multilinear result was obtained from the origin. Values of k1 and k2 were shown in Table 7.

0.94 R22 0.99 R12 0.97

(12)

Values of ks and qe for the pseudo-second-order kinetic model were obtained from the plot shown in Figure 4. ks and qe values were shown in Table 7. A comparison of the results with the correlation coefficients was shown in Table 7. The correlation coefficients for acetic acid were 0.99 and 0.94, respectively. The correlation coefficient of the pseudo-second-order model was higher than the pseudo-first-order model. Wladyslaw Rudzinski indicated

Figure 5. The plot based on the intraparticle diffusion model for the adsorption of acetic acids on 330 resins.

From Figure 5, it can be seen that sharper portion is the instantaneous adsorption or external surface adsorption. This portion is the gradual adsorption stage, where intraparticle diffusion is the rate-limiting step. The second portion is the final equilibrium stage, where intraparticle diffusion starts to slow down because of the extremely low adsorbate concentrations left in the solutions. The value R12 of the first portion obtained from the intraparticle diffusion model was 0.97. This deviation might be due to the difference of the rate of mass transfer in the initial and final stages of adsorption. Such deviation indicated that the pore diffusion is not the only rate controlling step.

Figure 4. The plot based on the pseudo-second-order model for the adsorption of acetic acids on 330 resins. 7303

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3.4. Determination of Optimal Operation Condition via Bench Scale Fixed Bed Experiments. Bench scale fixed bed column experiments were carried out to investigate the influence of concentration, temperature, and flow rate on the breakthrough curve. 3.4.1. Effect of Acetic Acid Concentration. Experiments were carried out at different initial acetic acid concentrations (150 mg/L, 200 mg/L, and 250 mg/L) at 35 °C and flow rates of 9.55 BV/h (1 BV = 95 mL). Results are presented in Figure 6.

Figure 7. Effect of flow rate on adsorption capability. The concentration of acetic acid solution: 200 mg/L. Temperature: 35 °C.

of 6.37 (1 BV = 95 mL) and temperatures of 25 °C, 30 °C, 35 °C, and 40 °C. Results are presented in Figure 8.

Figure 6. Effect of concentration of acetic acid solutions on adsorption capability. Temperature: 35 °C. Flow rate: 9.55 BV/h.

Figure 6 shows the effect of different concentrations of acetic acid solution on the breakthrough curve; it could be seen that the handling capacities on 330 resins were 23.5 BV, 47 BV, and 51 BV when the concentrations of acetic acid were 250 mg/L, 200 mg/L, and 150 mg/L, respectively. It also showed that the higher the concentration of acetic acid was, the smaller the processing capacity. With the increasing of the acetic acid concentration, the mass transfer force and mass transfer rate were increased and the adsorption equilibrium could be reached earlier. Usually, the concentration of acetic acid in industrial fuel ethanol is 200 mg/L. 3.4.2. Effect of Flow Rate. 200 mg/L acetic acid solution was passed through the fixed bed column at flow rates of 3.18, 6.37, and 9.55 BV/h (1 BV = 95 mL) at 35 °C, respectively. Results are presented in Figure 7. Figure 7 shows the effect of flow rate on the breakthrough curve; it could be seen that the handling capacities on 330 resins were 68 BV, 60 BV, 47 BV, and 25 BV at flow rates of 3.18 BV/h, 6.37 BV/h, 9.55 BV/h, and 12.74 BV/h, respectively. The results indicated that lower flow rate facilitated adsorption. We assumed that film diffusion and intraparticle diffusion are more sufficient at lower flow rate. Under high flow rate conditions, dynamic adsorption capacity decreased because of insufficient contact time for the acetic acid and the resin. However, lower flow rate resulted in a long operation time. Therefore, a medium flow rate of 6.37 BV/h was selected. 3.4.3. Effect of Temperature. The temperature affects the equilibrium adsorption capacity and the mass transfer rate of acetic acid in the liquid film and resin. 200 mg/L acetic acid solution was passed through the fixed bed column at flow rate

Figure 8. Effect of temperature on adsorption capability. The concentration of acetic acid solution: 200 mg/L. Flaw rate: 6.37 BV/h.

Figure 8 shows the effect of temperature on the breakthrough curve. It could be seen that the handling capacity of acetic acid on 330 resins was about 58 BV at different temperatures. The handling capacity did not change markedly with temperatures. Increasing the temperature increased the handling capacity slightly which also increases the operating costs. As such, the appropriate temperature for the adsorption was 30 °C. 3.5. Determination of the Optimal Regeneration Condition via Bench Scale Fixed Bed Experiments. Elution was necessary to regenerate the used resin. In this part, regeneration conditions were investigated in order to obtain the optimum parameters for the acetic acid concentration, temperature, and flow rate. In order to investigate the optimal regeneration condition, we should obtain the resin adsorbed with acetic acid under the optimal adsorption condition determined in section 3.4. The resin’s adsorption process conditions were as follows: volume 7304

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of resin in fixed bed column is 95 mL, acetic acid concentration is 200 mg/L, flow rate is 6.37 BV/h, and temperature is 30 °C. The adsorption experiment was stopped at a break point of 56 mg/L, and the resin was treated about 10 h. The final adsorption amount is about 0.43 mmol/g (dry resin). Regeneration was carried out under the following conditions: NaOH solution at different concentrations (2%, 3%, 4%, 5%, and 6%) and temperatures (25 °C, 30 °C, 35 °C, and 40 °C). The flow rate of eluant was 3.18, 4.77, and 6.37 BV/h (1 BV = 95 mL). 3.5.1. Effect of NaOH Concentration. Acetic acid adsorbed on 330 resins was eluted using an NaOH solution at different concentrations of 2%, 3%, 4%, 5%, and 6% at 30 °C, and the flow rate of eluant was 3.18 BV/h (1 BV = 95 mL). Figure 9 showed the effect of NaOH concentration on the elution curve. It was seen that a high concentration NaOH Figure 10. Effect of flow rates on the elution curve. The concentration of NaOH solution: 4%. Temperature: 30 °C.

in a long experimental period which was accompanied with a high operating cost, and when comparing 3.18 BV/h to 4.77 BV/h, the difference of operation time was slight and the lower flow rate facilitated the elution process. Therefore, a flow rate of 3.18 BV/h was selected as the appropriate regeneration experiments condition. 3.5.3. Effect of Temperature. Acid adsorbed on 330 resins was eluted using a 4% NaOH solution at 25 °C, 30 °C, 35 °C, and 40 °C, and the flow rate was 3.18 BV/h (1 BV = 95 mL). The effect of temperature on the regeneration curve is presented in Figure 11.

Figure 9. Effect of NaOH solution concentrations on the elution curve. Temperature: 30 °C. Flow rate: 3.18 BV/h.

solution was beneficial to regeneration efficiency. However, using a high concentration NaOH solution would reduce the useful life of resin. It could be seen there was almost a change of 4%−6%; the mass transfer and diffusion may be the main influencing factors in this concentration range. However, trailing phenomena appeared when the concentration of the NaOH solution was 2% and 3%. So the appropriate concentration was 4%. 3.5.2. Effect of Flow Rate. Acid adsorbed on 330 resins was eluted using a 4% NaOH solution at different flow rates of 1.59, 3.18, 4.77, and 6.37 BV/h (1 BV = 95 mL) at 30 °C. The effect of flow rates on the regeneration curve is presented in Figure 10. Figure 10 showed that the amount of eluent needed to complete the regeneration of resin was as twice volumes as the fixed bed. It can be seen from the figure that the time of complete desorption was 67, 32, 21, and 16 min as flow rate was 1.59, 3.18, 4.77, and 6.37 BV/h, respectively. These results also indicated that trailing phenomena appeared at a flow rate of 6.37 BV/h. Therefore lower flow rates facilitated the elution process. Under lower flow rate conditions, the eluent component had more time to transfer into the pore of resin. Dynamic elution efficiency was increased for sufficient contact time between the NaOH solution and the resin. When comparing 1.59 BV/h to 3.18 BV/h, a lower flow rate resulted

Figure 11. Effect of temperature on the elution curve. Flow rate: 3.18 BV/h. The concentration of NaOH solution: 4%.

Figure 11 showed that the effect of temperature on the elution curve was insignificant. As the system was not affected by temperature insignificantly in the experimental range, the operational temperature of 30 °C was selected based on economic reasons. 3.6. Removal of Acetic Acid from the Industrial Fuel Ethanol. The composition of industrial fuel ethanol was determined by gas chromatography−mass spectrum (GC-MS) (Agilent-6890-5973N, Agilent corporation) equipped with a 7305

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PEG column (30 m × 0.25 mm × 0.25 μm). The injection temperature was kept constant at 240 °C, and the column flow rate was set to 1.5 mL/min. The GC column temperature was programmed at 60 °C for 5 min, raised to 200 °C at a heating rate of 10 °C/min, and then kept at this temperature for 4 min. From the results, we confirmed that there are acetaldehyde, acetone, ethyl formate, ethyl acetate, n-propanol, isopropanol, and isoamylol in the industrial fuel ethanol. The acidity of industrial fuel ethanol was 200 mg/L. The industrial fuel ethanol was treated under the optimum operating conditions obtained in section 3.4. The comparison of adsorption efficiency on industrial fuel ethanol and the model acetic acid solution is shown in Figure 12.

acetic acid, and the regeneration ratio of resin. Therefore, resin 330 is stable and has the potential for use in a large-scale process to remove acetic acid from fuel ethanol.

4. CONCLUSION Ion-exchange resin was investigated to remove acids from fuel ethanol in this paper. Resin 330 was found to be an excellent type of adsorbent. Both static and dynamic experiments were carried out to evaluate the thermodynamic and dynamic properties of resin 330. The results showed that the thermodynamic property of resin 330 can be described well by the Langmuir isotherm during the temperature range from 25 to 35 °C, and the dynamic property can be described by the pseudo-second-order kinetic model. The optimum process conditions for operation and regeneration of the bench scale fixed bed of resin 330 to remove acetic acid from fuel ethanol was also obtained. When the initial concentration of the acetic acid solution was 200 mg/L, the optimum operating conditions were as follows: A flux of 6.37 BV/h at a temperature of 30 °C. The optimum regeneration conditions were determined as follows: A 4% solution of sodium hydroxide, flux was 3.18 BV/ h, and the temperature was 30 °C. Under such conditions, a refined product with acidity under 56 mg/L was obtained. At last, industrial fuel ethanol was used to test the selected resin and the established process conditions. No obvious difference was observed after five adsorption and regeneration cycles. Therefore, it can be concluded that the ion-exchange method is a promising method and would be a successful industrial process to remove acids from fuel ethanol. However, energy cost and pilot plant experiments need to be carried out before this process can be used in industrial production.

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Figure 12. Comparison of adsorption efficiency on fuel ethanol and acetic acid solution. Flow rate: 6.37 BV/h. Temperature: 30 °C.

*Phone/Fax: +86-22-27406119. E-mail: [email protected].

The handling capacities on the 330 resins were 22 BV and 58 BV for the fuel ethanol and the acetic acid solution, respectively. The reason for the difference may be that many kinds of acid impurities included in the industrial fuel ethanol would affect each other in the adsorption process. However, it could be seen that the process dealt well with the unpurified fuel ethanol. A purified fuel ethanol product with the acidity under 56 mg/L was obtained after the adsorption under the optimum operating condition. 3.7. Stability Test. Stability test results are shown in Table 8. The acidity of fuel ethanol was 200 mg/L. The resin was treated under the optimal adsorption and regeneration conditions obtained in sections 3.4 and 3.5. After five adsorption and regeneration cycles, no marked change was observed in dynamic adsorption capacity, the removal ratio of

Notes

The authors declare no competing financial interest.

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dynamic adsorption capability

dynamic removal ratio of acetic acid

regeneration ratio of resin

symbol units 1 2 3 4 5

qd m mol/g(dry resin) 0.43 0.43 0.42 0.42 0.42

θ % 88.0 88.0 86.9 86.9 86.9

η % 97.8 97.8 97.5 97.5 97.5

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Table 8. Stability Test Results after Five Adsorption and Regeneration Cycles on Resin 330 item

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dx.doi.org/10.1021/ef301461c | Energy Fuels 2012, 26, 7299−7307