Effects of Dehydration Rate on the Yield of Ethyl Lactate in a

Jun 16, 2015 - Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin Universit...
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Effects of Dehydration Rate on the Yield of Ethyl Lactate in a Pervaporation-Assisted Esterification Process Minhua Zhang,†,§ Lihang Chen,†,§ Zhongyi Jiang,‡,§ and Jing Ma*,†,§ †

Key Laboratory for Green Chemical Technology of Ministry of Education, R&D Center for Petrochemical Technology, Tianjin University, Tianjin 300072, China ‡ Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China § Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China ABSTRACT: Esterification of lactic acid and ethanol is typically a reversible reaction. The yield of ethyl lactate is low due to the limitation of chemical equilibrium. In this study, pervaporation composite membranes (glutaral crosslinked chitosan (GCCS)/ carbomer (CP)/polyacrylonitrile (PAN), glutaral crosslinked gelatin (GCGE)/PAN, and glutaral crosslinked hyaluronic acid (GCHA)/hydrolysis modification (HM)-PAN) inspired by the bioadhesion phenomena were fabricated and used for the synthesis of ethyl lactate, which could promote the reaction by removing the water perferentially. The influence of dehydration rate on chemical equilibrium shift in different operating conditions was investigated. Pervaporation-assisted esterification results suggested that the yield of ethyl lactate was determined by the match between reaction rate and dehydration rate. The process is controlled by the reaction rate initially and the dehydration rate as time goes by. The incorporation of the pervaporation process could break the limit of initial chemical balance, and the yield of ethyl lactate was substantially enhanced by 28.2%. Furthermore, the optimum technological parameter for the coupling process was confirmed, and the yield of ethyl lactate reached 94.9% after 8 h.

1. INTRODUCTION Ethyl lactate, also known as α-hydroxyl propionate, is widely used in chemical, food, medicine, and other industries due to its nontoxic and biodegradable properties. Ethyl lactate is synthesized by lactic acid and ethanol under a strong acid catalyst. The chemical reaction equation is

Generally, there are two types of esterification−pervaporation coupling processes. One is split coupling, utilizing the membranes only with the function of separation.7,8 The other is integral coupling, containing the membranes with a catalytic function.9−11 The latter can simplify the subsequent processing, but it is difficult to prepare a membrane with both good catalytic and dehydration properties. Technically, split coupling is more suitable for the industrial application. Sert and Atalay12 studied the esterification reaction of acrylic acid and n-butanol to produce n-butyl acrylate using the pervaporation− esterification hybrid process. A Pervap 2201 polymeric membrane was used to separate water and shift the equilibrium. Guo et al.13 employed a membrane reactor with the SPES/ PES/NWF CCM as a heterogeneous catalyst coupled with a PVA pervaporation membrane to produce ethyl acetate from acetic acid and ethanol. Li et al.14 proposed vapor permeation using a NaA zeolite membrane to intensify the esterification of acetic acid and n-propanol. Benedict et al.15 applied GFT-1005 and T1-b into the esterification of acetic acid and succinic acid with ethanol coupled with pervaporation. Lauterbach and Kreis16 used PERVAP 2201 to study the esterification of propionic acid and 1-propyl alcohol coupled through pervaporation and vapor permeation. Liu and co-workers17,18 investigated the esterification−pervaporation process of acetic acid and n-butyl alcohol under the composite membrane of PVA and ceramic. Aminabhavi and co-workers19 prepared

It is a reversible reaction, and the equilibrium conversion rate is low. Traditionally, benzene and toluene are used as dehydrant to remove the water from the reaction system, but the quality of the products is affected due to the existence of toxic dehydrant like benzene and toluene. Thus, the pervaporation technology is introduced to the esterification reaction to overcome the deficiency caused by dehydrant and improve the ethyl lactate yield. Pervaporation technology is unlimited to the vapor−liquid equilibrium conditions between the various components of the feed and has a high single separation rate and low energy consumption. The flux and selectivity of the components are important properties of the membrane. A variety of research has been completed to enhance the flux and selectivity of water for the dehydration process. Phosphotungstic acid has been incorporated with sodium alginate and other inorganic composites to enhance the flux and selectivity of water for dehydration of isopropanol and ethanol.1−6 Esterification coupled with pervaporation can break the equilibrium limitation of the esterification reaction and achieve a higher ester yield by continuously removing the water in the system. © 2015 American Chemical Society

Received: Revised: Accepted: Published: 6669

March 31, 2015 June 11, 2015 June 16, 2015 June 16, 2015 DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

Article

Industrial & Engineering Chemistry Research zeolite hybrid membrane filled with sodium alginate and applied it into the esterification of acetic acid and ethanol. All of the above research indicated that the esterification−pervaporation coupling process could improve the ester yield. Esterification of lactic acid and ethanol is typically a reversible reaction, limited by chemical equilibrium, resulting in a low yield of ethyl lactate. Coupling esterification with pervaporation in a membrane reactor can break the limitation of the original reaction equilibrium, achieving higher conversion and yield of the ester by removing the water from the reaction system. Budd et al.20 investigated the influence of the zeolite/ chitosan (CS)/poly(sodium-styrennesulfonate) (PSS) polyelectrolyte membrane on the ethyl lactate yield. In the reaction process, the p-toluenesulfonic acid was used as catalyst; the results indicated that the conversion rate of the lactate was 80% after the reaction was performed for 6 h. There are also a few works regarding the hybrid process of pervaporation-assisted esterification using Amberlyst catalysts for lactic acid and ethanol, as showed in Table 1.

(assuming that the pervaporation membranes only permit water through), the content of the water is lower. Therefore, the permeation rate JW could be obtained by this equation JW = PW C W

JW and PW, respectively, represent the pervaporation flux (mol/ (m2·s)) and the flow rate (m/s) of water. The esterification− pervaporation coupling model is dC W /d t = dC E/dt − JW (S /V )

catalysts XN-1010 15 15 15

membranes

reference

GFT-1005 PERVAP 2201 CS-TEOS PERVAP 2216 and PERVAP 2201

3 4 5 6

S and V, respectively, represent the area (m ) and volume (m3) of the membrane. It is obvious that the key point of the coupling is whether the esterification rate matches the dehydration rate.26 In this study, pervaporation composite membranes with high hydrophilicity and structural stability inspired by the bioadhesion phenomena were fabricated and used for the synthesis of ethyl lactate, with lactate and ethanol catalyzed by the Amberlyst 15 in the split coupling membrane reactor. The effects of dehydration rate on the yield of ethyl lactate in the pervaporation-assisted esterification process were investigated by changing the catalyst loading amount, initial molar ratio of the ethanol and lactate, reaction temperature, and dehydration performance of the membranes.

2. EXPERIMENTAL SECTION 2.1. Materials. Carbomer (CP) was purchased from Lubrizol Company in America; gelatin (GE) was purchased from the Sigma Company, and the hyaluronic acid (HA) was purchased from the Shandong Qufu Haixin Company (Shandong, China). Chitosan (CS, the degree of deacetylation was 90.2%) was purchased from Jinan Haidebei Marine Bioengineering Co. Ltd. (Jinan, China). Lactic acid (88 wt % lactic acid−water solution), ethyl lactate, and anhydrous ethanol were received from Tianjin Jiangtian Chemicals Ltd. (Tianjin, China). Amberlyst 15 catalyst was supplied by Nankai University Chemical Factory (Tianjin, China). The catalyst was repeatedly washed with ethanol and water prior to use for removing impurities and dried at 353.15 K under vacuum until the mass remained unchanged. 2.2. Membrane Preparation. Inspired by the bioadhesion phenomena and principle, three kinds of composite membranes (GCCS/CP/PAN, GCGE/PAN, and GCHA/H-PAN) were facilely fabricated by introducing bioadhesive carbopol (CP), gelatin (GE), and hyaluronic acid (HA), respectively. GCCS/ CP/PAN composite membrane was prepared through layer-bylayer casting onto the PAN substrate with CP adhesive followed

The formation rate of esters is determined by esterification reaction dynamics. Futhermore, the dehydration rate is related to the property of pervaporation membranes. Plenty of kinetic models have been studied to describe the coupling of esterification with pervaporation, including the common idea as follows.25 Esterification has the reversible reaction K1

A+B⇄E+W K2

(1)

A, B, E, and W, respectively, represent acids, alcohols, esters, and water. The rate of the reversible reaction is expressed as follows dC E/dt = k1C AC B − k 2C EC W

(4) 2

Table 1. Investigation of Pervaporation-Assisted Esterification of Ethyl Lactate Amberlyst Amberlyst Amberlyst Amberlyst

(3)

(2)

CA, CB, CE, and CW represent the concentrations of acid, alcohols, esters, and water (mol/L) and k1 and k2 represent the kinetic constants of the esterification (L/(mol·s)). When the pervaporation is used in the esterification to remove the water

Figure 1. Experimental apparatus for pervaporation-assisted esterification. 1, heating bath; 2, condenser; 3, mixing blade; 4, thermometer; 5, esterification reactor; 6, feed pump; 7, membrane cell; 8, membrane; 9, permeate collection tube; 10, liquid nitrogen trap; 11, vacuum pump. 6670

DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

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Industrial & Engineering Chemistry Research by glutaral (GA) cross-linked CS solution. GCGE/PAN composite membrane was facilely fabricated through casting GA cross-linked GE solution onto the PAN support layer. GCHA/H-PAN composite membrane was facilely fabricated through casting GA cross-linked HA solution onto the modified PAN support layer. The detailed preparation methods for the three kinds of membranes, which exhibited desirable structural stability and hydrophily in the separation process for aqueous ethanol solution, were shown in the published articles of our group.27−29 The three membranes include: (1) GCCS/CP/PAN composite membrane, the content of the CP is 0.5 wt % and the molar ratio of the CS repeating unit to GA is 60, labeled as GCCS(60)/CP(0.5)/PAN; (2) GCGE/PAN composite membrane, the content of the GE is 2 wt % and the mass ratio of the GA to GE is 2.5, labeled as GC(2.5)GE(2)/PAN; (3) GCHA/ HM-PAN membrane, the content of the HA is 0.8 wt % and the cross-linking degree is 0.3, labeled as GC(0.3)HA(0.8)/ HM-PAN. 2.3. Pervaporation-Assisted Esterification Experiments. The pervaporation-assisted esterification experiment was performed in a batch operation mode. Experimental equipment is shown in Figure 1. Ethanol together with Amberlyst 15 catalyst was added to the reactor and heated to the reaction temperature. Lactate acid was heated to the reaction temperature separately and then added to the reactor. Meanwhile, the reaction mixture was pumped continuously through the pervaporation unit at 60 L/h. This moment was taken as the starting time for the experiments. The reaction temperature was kept constant within ±0.5 °C by using a water heating bath. In the experiments, concentration polarization is assumed to be of minor importance.21−24 The ethyl lactate yield was calculated by eq 5. m the yield of ethyl lactate(%) = e × 100 mc (5)

Table 2. Pervaporation Performance of Composite Membranes for the Couple Process membrane materials

separation factor

total flux (kg/m2h)

water flux (kg/m2h)

GCCS(60)/CP(0.5)/PAN GC(2.5)GE(2)/PAN GC(0.3)HA(0.8)/HM-PAN

256 298 233

1.247 1.080 1.634

1.205 1.048 1.546

Figure 2. Effect of pervaporation on the yield of ethyl lactate.

where me is the mass of ethyl lactate obtained in the experiment and mc is the mass of ethyl lactate calculated from the whole conversion of lactic acid in the feed. 2.4. Analytical Procedures. About 1 mL of the reaction mixture was taken out at each sampling and immediately cooled in an ice/water mixture to stop the reaction. Samples from both the reactor and the permeate membrane side were analyzed by gas chromatography using Agilen 4890, equipped with a thermal conductivity detector (TCD) and a column (length = 4 m, o.d.= 3 mm) packed with GDX103 (Tianjin Chemical Reagent Co., China). The oven was operated at variableprogrammed temperature from 180 to 240 °C at a rate of 20 °C/min. The injector and detector were at 220 and 250 °C, respectively. Cyclohexanone was used as the internal standard.

Figure 3. Effect of pervaporation on water concentration in the reaction mixture.

the water feed concentration is 0.1. It can be speculated that the permeation flux of PERVAP 2201 was lower than that of the composite membranes prepared in this work, even if they had the same feed rate. Compared with the dehydration property of the membranes, the results demonstrate that the order of the dehydration efficiency was GC(2.5)GE(2)/PAN < GCCS(60)/ CP(0.5)/PAN < GC(0.3)HA(0.8)/HM-PAN. 3.2. Pervaporation-Assisted Esterification Reaction Process. 3.2.1. Effect of Pervaporation on the Ethyl Lactate Yield. The effect of pervaporation on the chemical equilibrium shift was investigated by comparing the yield of ethyl lactate with the reaction time. Esterification and the pervaporationassisted esterification process were studied at the same conditions (the reaction temperature was 80 °C, the molar ratio of ethanol to lactate was 2:1, and the quality of catalyst

3. RESULTS AND DISCUSSION 3.1. Dehydration Property of the Composite Membrane. The dehydration property of the composite membrane was investigated by separating 90 wt % ethanol solution. When the operating temperature was 80 °C, the flow rate of the feed was 60 L/h and the downstream pressure was 0.3 kPa; the pervaporation property is shown in Table 2. The commercial membrane PERVAP 2201 was also utilized to investigate the pervaporation properties during the esterification of lactic acid with ethanol.24 With the 36 L/min feed rate, the total permeation flux for the quaternary feed mixture at 327.15 and 348.15 K is about 0.1 kg/(m2·h) and 0.3 kg/(m2·h) when 6671

DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

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Industrial & Engineering Chemistry Research

Figure 4. Effect of mass ratio of catalyst to lactic acid on yield of ethyl lactate in the pervaporation-assisted esterification process (a) GC(2.5)GE(2)/ PAN membrane, (b) GCCS(60)/CP(0.5)/PAN membrane, and (c) GC(0.3)HA(0.8)/HM-PAN membrane (REthanol/RLactic acid = 2, T = 80 °C).

3.2.2. Effect of Pervaporation on the Water Content in the Reaction Mixture. Water in the reaction mixture is the medium in the coupling of esterification and pervaporation, and the change of the water content in the reaction mixture reflects the relation between the esterification rate and dehydration rate. The water in the reaction mixture comes from two parts: one is from the 88 wt % lactate feed (a higher concentration of lactate will generate polylactic acid). The other is the water generated in the reaction process of lactate and ethanol. The influence of pervaporation on the water content in the reaction mixture is shown in Figure 3. As presented, the water content kept increasing in the process without pervaporation although the increasing rate slowed down after 4 h. It is because the reaction rate of lactate and ethanol is initially rapid, with water generated continuously. After 4 h, the reaction rate became slow due to the chemical equilibrium. However, during the coupling process of pervaporation and esterification, the water content increased in the first 2 h but decreased after 2 h. It is because the water was produced quickly at the beginning and could not be removed immediately. With the reaction proceeding, the esterification rate decreased, and the permeation flux of the pervaporation membranes increased with the increasing water content. Thus, the dehydration rate increased and the water content decreased. Furthermore, it can be seen from the

was 2 wt % lactate). As the results show in Figure 2, for the esterification reaction without pervaporation coupling, the reaction rate began to slow down after 4 h, and the ethyl lactate yield changed slowly, indicating that the esterification tended to be in equilibrium. The ethyl lactate yield was 55.5% at 8 h. When pervaporation was used in the esterification reaction, although the reaction rate was slow at the initial stage, the reaction rate kept increasing after 4 h, which indicates that pervaporation broke the original equilibrium and promoted the reaction going to the positive direction. By comparing the influence of three kinds of membranes (GC(2.5)GE(2)/PAN, GCCS(60)/CP(0.5)/PAN, and GC(0.3)HA(0.8)/HM-PAN) on the ethyl lactate yield, it is observed that the yields at 8 h are 67.7%, 71.6%, and 83.7%. The rank of the influence by the pervaporation membrane is GC(2.5)GE(2)/PAN < GCCS(60)/CP(0.5)/PAN < GC(0.3)HA(0.8)/HM-PAN. It is the same as the order of the dehydration efficiency, which illustrated that, at the reaction time of 8 h, the reaction rate is higher than or equal to the dehydration rate, and the ester yield is controlled by the dehydration rate. Besides, it was observed that the esterification−pervaporation coupling process could make the ethyl lactate yield increase by 28.2% compared to the esterification reaction. 6672

DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

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Industrial & Engineering Chemistry Research

Figure 5. Effect of initial molar ratio of ethanol to lactic acid on the yield of ethyl lactate in the pervaporation-assisted esterification process (T = 80 °C, mCat./mLactic acid = 3 wt %). (a) GC(2.5)GE(2)/PAN membrane, (b) GCCS(60)/CP(0.5)/PAN membrane, and (c) GC(0.3)HA(0.8)/HM-PAN membrane.

was the same. The increase of the catalyst loading amount could accelerate the esterification rate and shorten the reaction time to achieve a certain conversion and the maximum water content. Therefore, in the membrane reactor, the time to achieve a certain conversion could be regulated by controlling the catalyst loading amount and then provide a choice to optimize the production cost. As shown in Figure 4, the influence of catalyst loading amount on the ethyl lactate yield for the three kinds of membrane systems show the same trend. When the catalyst content increased from 2 to 3 wt %, the yield of ethyl lactate shows a significant increase, but when the catalyst loading amount increased to 4 wt %, it only presents a slight increase. Therefore, the catalyst content of 3 wt % was used in this study. 3.3.2. Effect of the Initial Molar Ratio of Ethanol to Lactic Acid. Regularly, excess ethanol is used in the esterification of the ethyl lactate to promote the reaction to the positive direction due to the lower price of ethanol, and the total volume of the lactic acid and ethanol solution remains constant.

influence of the three kinds of pervaporation membranes on water content that the water reducing degree is directly proportional to the dehydration properties. 3.3. Effect of the Reaction Conditions and Membrane Property on the Pervaporation-Assisted Esterification Process. Via the esterification experiment, the influence of external diffusion (such as mixing speed) and internal diffusion (such as particle size of the catalyst) was eliminated. The intrinsic kinetics of the esterification−pervaporation coupling process was investigated by changing ethyl lactate yield with time under different reaction conditions. 3.3.1. Effect of the Catalyst Loading Amount. The catalyst loading amount was referred to the mass of lactate in the feed. Figure 4 shows how catalyst loading amount (mCat./mLatic acid = 2, 3, 4 wt %) has an effect on the ethyl lactate yield in the same reaction conditions (REthanol/RLactic acid = 2, T = 80 °C) with the three kinds of membranes. It is shown that the initial reaction rate varied with different catalyst loading amounts. The tendency of the ethyl lactate yield change over reaction time 6673

DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

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Industrial & Engineering Chemistry Research

Figure 6. Effect of reacting temperature on yield of ethyl lactate in pervaporation-assisted esterification process (REthanol/RLactic acid = 3, mCat./ mLactic acid = 3 wt %). (a) GC(2.5)GE(2)/PAN membrane, (b) GCCS(60)/CP(0.5)/PAN membrane, and (c) GC(0.3)HA(0.8)/HM-PAN membrane.

Table 3. Relationship between Pervaporation Performance and Yield of Ethyl Lactate membrane material total flux (kg/ (m2h)) water flux (kg/ (m2h)) ethanol flux (kg/ (m2h)) separation factor PSI (kg/(m2h)) ethyl lactate yield (8 h, %)

GC(0.3) HA(0.8)/HMPAN

GC(0.1) HA(0.8)/HMPAN

GC(0.3) HA(0.4)/HMPAN

1.634

1.704

2.001

1.573

1.601

1.872

0.061

0.103

0.128

233 379 94.9

140 237 86.2

131 260 88.2

Figure 5 shows the changes of ethyl lactate yield along with reaction time at different initial molar ratios of ethanol to lactic acid (2:1, 3:1, and 4:1), with three kinds of membranes in the esterification−pervaporation coupling process. For the esterification reaction, when the catalyst loading amount is kept constant, the increase of the initial molar ratio of ethanol to

Figure 7. Effect of pervaporation performance on yield of ethyl lactate (REthanol/RLactic acid = 3, mCat./mLactic acid = 3 wt %, T = 80 °C).

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DOI: 10.1021/acs.iecr.5b01199 Ind. Eng. Chem. Res. 2015, 54, 6669−6676

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prepare GCHA/HM-PAN, investigating the relationship between the yield of ethyl lactate and the pervaporation property (total flux, water flux, ethanol flux, separation factor, separation index (PSI = J × (β − 1), where J presents the total pervaporation flux and β presents the separation factor, indicating the total properties of the membranes), as shown in Table 2. Figure 7 compares the yield of ethyl lactate with the composite membrane GCHA/HM-PAN in different conditions, illustrating that the yields of ethyl lactate perform the same trend as the time goes by, although the dehydration performance of the three kinds of membranes is different. The yield of ethyl lactate has a strong relation with the dehydration rate in the coupling process. However, as shown in Table 3, the yield of ethyl lactate is not in direct proportion to the water flux, because in the coupling process, the membranes allow not only water but also ethanol through. If the separation selectivity is low, the yield of ethyl lactate will be reduced because the ethanol flux is so large that the ethanol concentration in the feed is reduced, making the positive reaction rate slow down. Therefore, a high yield of ethyl lactate needs high water flux and separation selectivity of the pervaporation membranes. As shown in Table 3, the yield of ethyl lactate is in direct proportion to the separation index of pervaporation.

lactic acid could promote the reaction to the positive direction and increase the ester yield. However, in the initial stage of the pervaporation-assisted esterification process(