Purification of Lactic Acid via Esterification of Lactic Acid Using a

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Purification of Lactic Acid via Esterification of Lactic Acid Using a Packed Column, Followed by Hydrolysis of Methyl Lactate Using Three Continuously Stirred Tank Reactors (CSTRs) in Series: A Continuous Pilot Plant Study Sanjay P. Kamble,† Prashant P. Barve,† Jyeshtharaj B. Joshi,‡,§ Imran Rahman,† and Bhaskar D. Kulkarni*,† †

Chemical Engineering and Process Development Division, National Chemical Laboratory (NCL, CSIR), Dr. Homi Bhabha Road, Pune-411008, India ‡ Department of Chemical Engineering, Institute of Chemical Technology (ICT), Matunga, Mumbai-400019, India § Homi Bhabha National Institute (HBNI), Anushaktinagar, Mumbai-400094, India

bS Supporting Information ABSTRACT: The world market of lactic acid is growing every year, mainly as a solvent and precursor to poly(lactic acid) (PLA). The cost of renewable biomass-derived PLA will have to compete with other synthetic polymers, if it is to grab a significant and sustainable fraction of the market share. It is thus necessary to have efficient and cost-effective technology for the production of puregrade lactic acid (LA). In this article, a novel cost-effective, eco-friendly continuous process for the production of high-quality lactic acid at pilot plant scale has been demonstrated. The novelty of this process is that, for the first time, we report and use the concept of inverse reactive distillation for the esterification of crude concentrated LA in a continuous countercurrent packed column mode. This allows us to operate the column at higher temperatures, improving the kinetic rate process and leading to shorter columns. This is followed by the hydrolysis of methyl lactate (MLA) in a series of three continuously stirred tank reactors (CSTRs), where LA itself acts as a catalyst. The LA obtained in the pilot plant process shows 99.81% purity (by weight) on water-free basis and has an optical purity of 99.9%. The pilot scale experimental results pertaining to the autocatalytic esterification of LA and hydrolysis of MLA have been compared and validated, with respect to simulated results. The innovations reported here can make the process economically viable for commercial use.

1. INTRODUCTION Polymers and co-polymers of lactic acid are known to be environmentally friendly, because of their biodegradability into harmless products. Hence, recently, the production of poly(lactic acid) (PLA) from renewable biomass and its transformation through fermentation to L-lactic acid has acquired importance.1
4 Different techniques, such as electrodialysis, adsorption, solvent extraction, membrane separation, and reactive distillation, have been investigated for the separation of lactic acid from fermentation froth.5
7 Each technique has some advantages and disadvantages, but reactive distillation was found to be quite promising at the industrial scale, with some cost advantages.8
10 In order to minimize the overall cost of purification of lactic acid, a novel concept of autocatalytic countercurrent reactive distillation of lactic acid (LA) with methanol (MeOH), using a packed column, followed by the hydrolysis of methyl lactate (MLA) with LA catalyst using three continuously stirred tank reactors (CSTRs) in series, has been developed for the purification of LA at pilot plant scale. The advantage of autocatalytic reaction is the absence of a separation step and regeneration of the catalyst. Furthermore, the overall cost of the process is low, compared to that of a heterogenous catalyst.11,12 The main motive of this article is the demonstration of autocatalytic concept for the purification of LA at pilot plant scale and to compare and validate the experimental results of esterification r 2011 American Chemical Society

and hydrolysis with respect to simulated results. The reactive distillation simulation model consists of chemical reaction, vapor
liquid equilibria, and mass and heat balances.

2. EXPERIMENTAL SECTION 2.1. Materials. Lactic acid (LA) (90% w/w) was obtained from M/s Malladi Specialties Ltd., Chennai, India. Crude concentrated LA (74.8%
76.1%, w/w) and methyl lactate (MLA) (>99% purity) were obtained from M/s Godavari Bio-Refineries Ltd., Sameerwadi (Karanataka, India). Commercial grade methanol (99.5%) was purchased from M/s V. P. Chemicals (Pune, India). Demineralized water (DM) water was obtained from an in-house DM water plant. An agitated thin film dryer (ATFD) was made from Hastelloy C material and procured from M/s Technoforce (Nashik, India). 2.2. Analysis. Analysis of methanol and MLA was performed on a gas chromatograph (Shimadzu, Model GC-14 B) that was equipped with a flame ionization detector. A 30-m column Special Issue: Nigam Issue Received: March 30, 2011 Accepted: September 15, 2011 Revised: July 4, 2011 Published: September 15, 2011 1506

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Industrial & Engineering Chemistry Research (Model DB-1, J and W Scientific USA) was used with nitrogen as a carrier gas at a flow rate of 1.5 mL/min. The injector and detector temperatures were maintained at 493 and 573 K, respectively. The oven temperature was varied over a range of 393
513 K with the ramp rate of 277 K/min. High-performance liquid chromatography (HPLC) analysis for free LA was performed on a C18 Hypersil BDS column, using a Dionex HPLC system comprised of a Dionex Summit P 680A pump and a Dionex ASI 100 autosampler. The mobile phase consisted of 0.01 M potassium dihydrogen phosphase (pH 2.5) with 7% acetonitrile as organic modifier. The chromatographic peaks were monitored with UV detector at detection wavelength of 205 nm. The moisture content in the reaction mixture was determined by Karl Fischer titration method. The optical purity of LA was measured using an enzyme kit. 2.3. Experimental Setup. The continuous countercurrent esterification of crude concentrated LA was performed in a packed column. The crude MLA was recovered using ATFD and subsequently purified by fractional distillation. The hydrolysis of pure MLA was performed in three CSTRs in series, in order to get highly pure LA. The details of the experimental setup are given as follows: 2.3.1. Continuous Countercurrent Esterification of Crude Concentrated Lactic Acid in a Packed Column. The continuous esterification of crude concentrated LA can be broadly divided into three sections, viz, (i) reaction section to convert crude LA to crude MLA; (ii) isolation of MLA by flash distillation, using an ATFD; followed by (iii) purification of flashed MLA, using a fractional distillation column. Figure 1 shows the schematic diagram for esterification of crude concentrated LA. The packed column consists of five sections (labeled as C-1 to C-5). Each section was 50 mm in diameter and 1000 mm in length. At the bottom of column C-1, an overflow loop was created as liquid seal and the material overflowing from column C-1 was cooled using a heat exchanger (identified as HE-01). The columns were packed with glass Raschig rings (inner diameter (id) of 4/5 mm, 5 mm in length) and have a dry surface area of 525 m2/m3 and a packed volume of 9.5 L. The complete experimental setup was procured from M/s Silicaware Ltd., Mumbai, India. The sample points were provided across the packed column at a length of 1.5, 2.5, 3.5, and 4.5 m from the top of column for measurement of the local concentration of reaction mixture. The Pt(100) sensors were employed across all the sample points for measurement of temperature profile across the packed column. Methanol was fed at the bottom of column C-1 through a preheater (HE-200), using a metering pump (P-200). The methanol vessel (V-200) was placed on the weighing balance (WT-200). The molar ratio of methanol to concentrated crude LA was adjusted by varying the methanol flow rate through the metering pump (P-200). The crude concentrated LA vessel (V-210) was placed on weighing scale (WT-210). The flow rate of crude lactic acid was controlled with the help of a metering pump (P-210). The desired LA feed temperature was maintained by passing through the preheater (HE-220). The entire process of the continuous esterification of crude LA using methanol was controlled using a supervisory control and data acquisition (SCADA) system that was procured from M/s Texol Engineering, Pvt, Ltd. (Pune, India). ATFD was used for the separation of MLA from LA, water, dilactide, etc. The ATFD consisted of a residue receiver (2-L capacity), a condenser (shell and tube type), and a distillate

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receiver (20-L capacity). The condenser provides a heat-transfer area of 0.12 m2. The ATFD vessel was equipped with a feed tank with a 20-L capacity and a metering pump with a rating of 0.1
10 LPH. The metering pump was connected to the ATFD through a back-pressure regulator arrangement. The ATFD stirrer has a mechanical seal arrangement, and the assembly was able to withstand a vacuum of 1 mbar. The entire ATFD vessel was operated under vacuum. The MLA obtained in the ATFD vessel is further purified by fractional distillation. The fractional distillation column was equipped with a stirred jacketed mild steel glass line (MSGL) vessel having a capacity of 18 L. The distillation column has 30 stages, and it was packed with Pro-Pak (316ss, 0.16 in.2, specific surface area = 1890 m2/m3). The distillation facility was equipped with twin distillate receivers (each with a capacity of 5 L), reflux distributor/timer, vacuum pump, etc. Each condenser was equipped with a chiller capable of achieving a condenser temperature of +279 K. 2.3.2. Hydrolysis of Methyl Lactate to Highly Pure Lactic Acid Using Three CSTRs in Series. Figure 2 shows a schematic diagram for hydrolysis of MLA using three CSTRs in series. The CSTR setup consisted of three conical glass reactors (R-101, R-102, and R-103), each having a capacity of 7 L. All three CSTRs were equipped with thermo-siphon-type reboilers (electrical heaters embedded in a quartz casing). The heat was supplied to three CSTRs via electrical heaters (H-101, H-102, and H-103) having an electrical duty of 2.5 kW for each heater and was controlled using the SCADA system. The CSTR identified as R-101 was equipped with a glass fractional distillation column (C-101, id = 25 mm, length = 2000 mm), packed with glass Raschig rings (20 stages) supplied by M/s Silicaware, Pvt., Ltd., Mumbai, India. Distillation column C-101 was equipped with a distillate receiver with a water-cooled glass condenser (HE-101). The top of column C-101 was also equipped with a reflux splitter/timer to adjust the reflux ratio in such a fashion that the column C-101 top temperature remains at the boiling temperature of methanol. To avoid heat losses to the surroundings, the column was insulated. Sensors (Pt-100) were provided at the top of column C-101 and CSTR R-101 to measure the temperature. Similarly, the CSTR identified as R-102 was equipped with a glass fractional distillation column (C-102, id = 25 mm, length = 1000 mm) packed with glass Raschig rings (10 stages). The distillation column was equipped with a distillate receiver with a water-cooled glass condenser (HE-102). The overflow of CSTR R-101 was connected to CSTR R-102 with an overflow tube and liquid seal arrangement. The distillate received from column C-102 was fed to the middle of column C-101, using a tube with a rotameter and liquid seal arrangement by gravity flow. The CSTR identified as R-103 was also equipped with a glass stripper column (C-103, id = 25 mm, length = 2000 mm) packed with glass dumped packing. The overflow of CSTR R-102 was connected at the middle of stripper column C-103. Column C-103 was equipped with a glass condenser (HE-103). The distillate collected at the top of column C-103 was continuously fed to reactor R-101 through a rotameter and dip tube arrangement. Two metering pumps (identified as P-301 and P-302) (SSI model, USA) were used to charge the MLA and DM water to CSTR R-101 and CSTR R-103, respectively. MLA and DM water feed were taken in vessel V-101 and vessel V-104, respectively. F5 is the flow rate of MLA to CSTR R-101, while 1507

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Figure 1. Schematic diagram for esterification of lactic acid (LA) using a packed column.

F6 is the DM water flow rate to CSTR R103. D1 is the distillate from column C-101, consisting of methanol and water. B3 is the product, consisting of LA and a negligible amount of water that was withdrawn from CSTR R-103. The temperatures of CSTRs R-101, R-102, and R-103 are denoted as TB1, TB2, and TB3, respectively, and the temperatures of columns C-101, C-102, and C-103 are denoted as TT1, TT2, and TT3, respectively. The distillate (aqueous methanol) was withdrawn from column C-101 and collected in vessel

V-102. The overflow of CSTR R-103 was collected in V-103 and placed on the weighing scale identified as WT-240 through product cooler HE-104. Each condenser was equipped with a chiller capable of achieving a condenser temperature of +279 K. 2.4. Experimental Procedure. 2.4.1. Continuous Counter Current Esterification of Crude Concentrated Lactic Acid in Packed Column Using Autocatalytic Approach. The experimental setup for continuous countercurrent esterification of crude 1508

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Figure 2. Schematic diagram for hydrolysis of methyl lactate (MLA) using three continuously stirred tank reactors (CSTRs) in series.

concentrated LA has been shown in Figure 1. The stepwise experimental procedure is as follows: (1) Initially pure methanol was charged to methanol feed tank V-200, and crude concentrated LA was charged to acid feed tank V-210. (2) Methanol preheater HE-200 and lactic acid preheater HE-210 were heated to the desired temperature by circulating hot oil through the jacket of both preheaters. (3) Superheated methanol was fed though pump P-200 at the bottom of packed column continuously. Simultaneously preheated crude concentrated LA was continuously fed from the top of the packed column through feed pump P-210. The flow rate of LA and methanol was adjusted, as per requirements, by changing the pump flow rates of P-200 and P-210, respectively.

(4) The hot crude LA trickles down continuously across the packed column and reacts with superheated rising methanol vapors in countercurrent mode across the column. During the esterification reaction, the temperature across the packed column was maintained at ∼373 K. (5) The overflow of packed column (i.e., product) was continuously collected from the bottom of the packed column C-1 into a receiver (V-01) after cooling through inline cooler HE-01. (6) The distillate of packed column (water and excess methanol) was fed into another column (C-6 and C-7), where methanol was removed at the top column (C-6) and was recycled to reaction zone via weighing scale WT-240, whereas the water was collected in the receiver and placed on weighing scale WT-230. 1509

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Table 1. Continuous Countercurrent Autocatalytic Esterification of Crude Concentrated Lactic Acid Using a Packed Column Feed Rate (kg/h)

Output (kg/h)

Temperature (K) rate of formation

run no.

for crude LA, F2

for MeOH, F1

for crude MLA, F3

for distillate [ MeOH and water], F4

for crude LA, T8

for MeOH, T1

for crude MLA, T2

for distillate, T9

of MLA @ 373 K (kg/(m2 h))

RN-01

1.850

2.282

2.056

2.019

369.1

394.1

371.8

353.5

0.261

RN-02

1.915

2.410

2.141

2.130

373.1

408.5

372.2

354.6

0.270

RN-03

1.890

1.890

1.960

1.805

350.8

406.8

374.4

349.8

0.257

RN-04

1.616

1.408

1.496

1.517

371.1

454.8

377.4

352.1

0.209

RN-05

1.744

1.472

1.645

1.558

370.2

455.4

379.1

350.6

0.229

(7) Periodic samples were withdrawn from various sample points across the packed column, as well as from the bottom (overflow of packed column) and top (distillate) for its analysis. (8) The crude MLA collected as the bottom product of the packed column was subjected to purification by flash distillation over ATFD and then sent to a fractional distillation column. ATFD is used for the removal of all acidic impurities, nonvolatile impurities, high boilers, etc. from MLA. (9) The flashed MLA from ATFD was taken for the purification of MLA by fractional distillation. The fractional distillation of mixture (MLA, methanol, and water) was done by monitoring operating pressure and temperature so that the excess methanol, water, and pure MLA was separated as first, second, and third (main) cuts, respectively. The composition of various distillate cuts and residue were further analyzed using high-performance liquid chromatography (HPLC) analysis. 2.4.2. Hydrolysis of Methyl Lactate (MLA) to Highly Pure Lactic Acid (LA) Using Three CSTRs in Series. The continuous reactive hydrolysis of MLA was performed, in pilot scale, in three CSTRs in series, as shown in Figure 2, to produce 4
6 kg/h pure LA as per the following reaction scheme: methyl lactate þ water h lactic acid þ methanol

ð1Þ

The stepwise procedure for the hydrolysis experiment was as follows: (1) Initially, 3.5 kg of pure MLA was charged in CSTR R-101, along with an equal quantity of DM water. Pure LA (90% solution in water) was charged into this mixture at 5% of the total MLA charged. (2) Similarly, a 5-kg mixture of LA, MLA, and DM water—in a ratio of 50%, 15%, and 35%, respectively—was charged to CSTR R-102. CSTR R-103 was also charged with a 5-kg mixture of LA and DM water in a 50:50 ratio. (3) All the heaters of CSTRs R-101, R-102, and R-103 were started and the contents of each reactor were brought to its boiling point by maintaining the temperature around 373 K. Initially column C-101 was put on total reflux. (4) Once the above-mentioned condition was achieved, then the desired flow rate of MLA (F5) and DM water (F6) was started using pumps P-301 and P-302, respectively. (5) The distillate of column C-102 was recycled (recycle stream 1) to the middle of column C-101. The distillate obtained from column C-103 was recycled (recycle stream 2) to CSTR R-101.

(6) The overflow of CSTR R-102 was fed to the middle of stripper column C-103, where the contents of R-102 were treated in countercurrent mode with the rising steam from the reboiler of CSTR R-103. (7) The distillate (D1) from column C-101 was collected in vessel V-102 through HE-101 and the overflow of CSTR R-103 was collected in vessel V-103 through HE-104. (8) Periodic samples were collected from column C-101, C-102, and C-103, as well as from an overflow of CSTRs R-101, R-102, and R-103, respectively, for their analysis.

3. RESULTS AND DISCUSSION 3.1. Continuous Esterification of Crude Concentrated Lactic Acid (LA) in a Packed Column. The continuous ester-

ification of crude concentrated LA was performed in a packed column under different experimental conditions, such as residence time, molar ratio (LA to MeOH), etc. The countercurrent esterification of LA with methanol was performed in reactive distillation mode in a packed column, using an autocatalyst (using LA as a catalyst). The operating parameters were finalized after conducting several pilot plant trials. The experimental data obtained during the pilot plant trials at steady state for five different runs are given in Table 1. Typically, each run takes ∼0.5
2 h to attain the steady state. The flow rate of LA was varied from 1.6 kg/h to 1.9 kg/h, while the methanol flow rate was varied from 1.4 kg/h to 2.4 kg/h. Table 1 also shows the effect of molar ratio on the esterification reaction. It was found that, with an increase in the molar ratio of MeOH:LA, the conversion of LA was found to increase. Table 2 shows the analysis of feed, distillate, and bottom product (column overflow) during the continuous countercurrent esterification of crude concentrated LA. It was observed that only 0.81%
15% unreacted LA was found in the overflow of column (product), while 66%
85% of MLA was formed at different molar ratios of reactants. From the distillate analysis, it was found that ∼1.3%
6.88% MLA comes along with methanol and water. This is due to the contribution of MLA vapor pressure at the boiling point of water. Generally, in the conventional co-current process, crude LA, methanol, and a small amount of sulfuric acid are added in the reactor/reboiler.12 The temperature of the reboiler is maintained at ∼398
423 K and the product (MLA, water, and excess methanol) is taken out of reactive distillation reactor by maintaining the still at higher temperature. This leads to the accumulation of the acidity in the reactor and gives rise to the undesired byproduct, such as hydroxyl methyl furfural, 2-pentene-1-ol, pentyl lactate, etc. It is known that, in the product mixture of 1510

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Table 2. Analysis of Various Streams Generated during Continuous Counter Current Autocatalytic Esterification of Crude Concentrated Lactic Acid

Table 3. Analysis of Reaction Mixture at Various Locations Across the Packed Column for Run RN-01 Component Analysis (wt %)

Analysis (wt%) composition of streams

RN-01 RN-02 RN-03 RN-04 RN-05

Crude LA (feed) LA DiLA moisture MeOH (Feed) moisture

76.1

76.1

74.8

74.8

76.1

8.3

8.3

9.7

9.7

8.3

11.5

11.5

10.3

10.3

11.5

0.52

0.52

0.52

0.56

0.56

Overflow of column C-1 (product) LA MLA

3.1

0.81

6.5

11.51 14.56

83.5

68.13 66.4

81.2

85.8

moisture

1.5

1.1

MeOH

9.8

9.58 10.2

2.28

3.79

3.3

10.01

9.18

Distillate MeOH MLA

72.5 3.6

73.1 3.8

71.3 4.3

79.9 85.07 6.88 1.3

moisture

21.3

22.3

22.8

26.56 21.20

methanol, water, and MLA obtained from the co-current reactive distillation, MLA forms an azeotrope with water and also reacts with water. The isolation of MLA in pure anhydrous form from the mixture of methanol, water, and MLA is difficult. If the MLA is not further purified by fractional distillation, then the LA generated in the hydrolysis cycle of MLA gets contaminated with other impurities, such as hydroxyl methyl furfural, 2-pentene-1-ol, and pentyl lactate.12 In the present process, a countercurrent mode of operation was employed for esterification of crude LA, where crude MLA is produced in anhydrous form, which is further purified separately, using ATFD, followed by a fractional distillation column. The major advantages of countercurrent reactive distillation are that the acidity buildup in the bottom of the reactive distillation column is absent and the temperature of reaction mixture across the reactive distillation column is maintained constant at 373 K, which prevents the formation of side products. Table 3 shows analysis of the reaction mixture at various locations across the packed column for run RN-001. It can be seen from Table 3 that only 20% LA was found at a column length of 1.5 m (from top of the column) which indicates that most of the crude LA reacts with MeOH in the upper section of column (∼1.5 m, 33% of the reactor length). This is because, at the upper section of the column, the stripping of the water is highly efficient and concentration of LA as a catalyst also is high. The separation of nonvolatile impurities from MLA was attempted using ATFD. This is essential in order to obtain the maximum yield and a high quality of MLA later in the fractional distillation operation. The crude MLA gets separated from all acidic impurities and other high boilers. It was found that >94% recovery of MLA was achieved during this operation. The flashed MLA from ATFD was further purified via fractional distillation. The material balance of MLA purification across the fractional distillation column is shown in Table 4. The first cut mainly contains MeOH and a small amount of moisture, whereas the second cut is a mixture of both methanol and MLA. The third cut contains only MLA (99.93%). This shows that the MLA can be separated from methanol and water

sample location

LA

MLA

MeOH

moisture

1.5 m below LA feed point

20.08

43.82

25.70

10.57

2.5 m below LA feed point

15.81

48.61

29.67

10.50

3.5 m below LA feed point 4.5 m below LA feed point

13.71 6.51

59.72 66.40

25.70 14.50

8.98 3.50

3.10

81.20

9.80

1.50

bottom

using a fractional distillation column that has 30 theoretical stages. The number of theoretical stages of the present fractional distillation column was estimated by distillation of the mixture of ethylbenzene and chlorobenzene. 3.2. Hydrolysis of Methyl Lactate (MLA) to Highly Pure Lactic Acid (LA) Using Three Continuously Stirred Tank Reactors (CSTRs) in Series. The hydrolysis of MLA was performed under different experimental conditions, such as varying residence time, molar ratio (MLA to water), etc. The operating parameters were obtained after conducting several pilot plant trials. The experimental data obtained during the pilot plant trials at steady state for five different runs are given in Table 5. Table 5 also shows the effect of molar ratio on the hydrolysis of MLA and the rate of formation of LA, with respect to molar ratio, was observed to be almost constant. The analytical details of different runs (Run Nos. RN-01 through RN-05) are given in Table 6, which shows the concentration (in units of wt %) of MLA in feed, the overflow of CSTR R-103, and the distillate of column C-101. From the analysis of the overflow of CSTR R-103, it was found that the complete conversion of MLA is possible and, in most of the experiments, the residual concentration of MLA was found to be practically zero. The purity of synthesized LA was determined to be 99.81% (by weight, on a water-free basis). The distillate of column C-101 primarily contained methanol and a small amount of water. This methanol is dehydrated using a fractional distillation column and recycled to the esterification section. The optical purity of LA is one of its important properties; it directly affects the physiochemical properties of poly(lactic acid) (PLA), such as mechanical strength, thermal plasticity, fabricability, etc. Hence, the optical purity of LA prepared in the pilot plant and commercial LA purchased from Malladi Specialties, Ltd. (Tamilnadu, India) was measured and found to be 99.9% and 100%, respectively. It indicates that, during the purification of LA via the esterification of LA and the hydrolysis of MLA, the optical purity remains practically the same. The cost of purifying 1000 kg of LA (>99%) was calculated for both autocatalytic and ion-exchange resin processes, based on operating cost. It was found that the cost of purifying LA, using autocatalysis, is 1178 US $/ton, whereas for the ion-exchange resin process, the cost is ∼1223 US $/ton. This clearly indicates that the final cost of purifying LA using the current process is less (by 45 US $/ton), compared to a conventional heterogeneous catalyst.

4. COMPARISON OF EXPERIMENTAL AND SIMULATED RESULTS Simulation of the reactive distillation column involves the simultaneous solution of material and energy balances and 1511

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Table 4. Material Balance for the Purification of Methyl Lactate (MLA) Using Fractional Distillation Column Composition Methyl Lactate (MLA) stream

total weight (kg)

(wt %)

weight (kg)

feed

57.35

87.95

50.44

1st cut

6.28

2nd cut

3.26

23.67

3rd Cut

41.68

99.93

residue

6.10

81.81

Methanol (MeOH) (wt %)

0.772

Moisture

weight (kg)

(wt %)

weight (kg)

9.52

5.56

1.28

0.734

97.13

6.1

2.86

0.18

27.26

0.889

49.37

1.6

41.65 4.990

0.07

0.029

1.87

0.114

Table 5. Hydrolysis of MLA to Highly Pure LA Using Three CSTRs in Series Output Rate

Recycled

(kg/h)

Stream (kg/h)

Feed Rate (kg/h)

for

Temperature (K)

recycle recycle

formation of

run for MLA, for water, for LA, MeOH, stream stream for R-101, for C-101, for R-102, for C-102, for R-103, for C-103,

lactic acid @

no.

F5

F6

B3

D1

R1

R2

TB1

TT1

TB2

TT2

TB3

TT3

373 K (kg/(L h))

RN-01

3.12

2.67

4.81

0.97

1.10

3.85

367.2

339.3

371.5

361.8

374.2

369.5

0.128

RN-02

3.66

2.66

5.14

1.13

1.20

3.90

367

339.8

371.8

361.3

375

369.3

0.150

RN 03

3.89

3.37

6.05

1.21

1.25

3.90

367

341.4

371.7

361

374.5

369.2

0.155

RN-04

3.74

3.11

5.67

1.15

1.25

3.95

369.6

339.5

371.6

361.1

375.4

369.3

0.153

RN-05

3.24

2.59

4.79

0.99

1.20

3.90

369.2

338.9

371.8

361.9

377.8

370.1

0.133

VN
1 KN
1, i xi, N
1
LN xi, N
Di þ vi ε N ¼ 0 i ¼ 1; :::; C

Table 6. Analysis of Various Streams Generated during Hydrolysis of MLA Using Three CSTRs in Series Composition (wt %)

C

RN-01

RN-02

RN-03

RN-04

RN-05

99.86

99.86

99.86

99.86

99.86

0.1

0.1

0.1

0.1

0.1

56.10 43.60

61.50 37.92

53.5 43.7

57.3 41.9

58.5 41.2

∑ xik
1 ¼ 0 i¼1

k ¼ 1; :::; N

ð4Þ ð5Þ

MLA MLA moisture

C

∑ Kikxik
1 ¼ 0 i¼1

Overflow of CSTR R-103 LA moisture MLA

nil

nil

2.1

nil

nil

MeOH

nil

nil

nil

nil

nil

MeOH

98.1

97.6

94.8

98.20

98.2

moisture

1.8

2.3

5.0

1.4

1.6

Kik ¼

fi1 þ L2 xi2
V1 Ki, 1 xi, 1
Bi þ vi ε1 ¼ 0 i ¼ 1; :::; C

ð2Þ

fik þ Vk
1 Ki, k
1 xi, k
1 þ Lkþ1 xi, kþ1
Lk xi, k
Vk Ki, k xi, k þ vi εk ¼ 0 k ¼ 2; :::; N
1, i ¼ 1; :::; C

ð3Þ

Psik αik P

ð6Þ ð7Þ

fk Hfk þ Vk
1 HVk
1 þ Lkþ1 HLkþ1
Lk HLk
Vk HVk þ εk ΔHk ¼ 0

Distillate of C-101

stoichiometric relationships.13 It is assumed that the chemical reaction occurs over several stages. The material balance, equilibrium, summation of mole fraction, and heat balance equations are as follows:

k ¼ 1; :::; N

ð8Þ

The above equations were solved simultaneously using the Newton
Raphson method for obtaining the component mole fraction on each stage.13 The vapor flow rate was calculated by solving the energy balance equation. The temperature on each stage was computed by the Newton
Raphson method. Convergence of the column model is dependent on starting guesses for the composition profile. The assumptions made for the modeling of reactive distillation column are as follows: (i) the vapor and liquid phases are in equilibrium on each tray; (ii) no reaction occurs in the vapor phase; (iii) the liquid phase is always homogeneous; (iv) the heat of reaction is considered negligible. The autocatalytic kinetics rate expression of LA with MeOH and the vapor
liquid equilibrium (VLE) data using the UNIQUAC model to describe the chemical and phase equilibria are presented in sections I and II in the Supporting Information.14,15 1512

dx.doi.org/10.1021/ie200642j |Ind. Eng. Chem. Res. 2012, 51, 1506–1514

Industrial & Engineering Chemistry Research

ARTICLE

Table 7. Experimental and Simulation Details for Esterification of Crude Concentrated Lactic Acid for Run RN-002 Feed

Distillate Composition

Bottom Composition

(kg/h)

(mass fraction)

(mass fraction)

Temperature (K)

F2

F1

LA

MeOH

MLA

water

LA

MeOH

MLA

water

bottom

sim

1.915

2.410

0

0.744

0.036

0.22

0.033

0.104

0.85

0.013

363.4

exp

1.915

2.410

0

0.745

0.036

0.22

0.032

0.102

0.85

0.015

372.2

column

bottom flow rate (kg/h)

distillate flow rate (kg/h)

343

2.13

2.10

354.6

2.14

2.13

top

1

Table 8. Experimental and Simulation Details for the Hydrolysis of MLA Distillate Composition (mass fraction)

Feed (kg/h)

column

F5

F6

LA

MeOH

MLA

water

Bottom Composition (mass fraction)

LA

MeOH

MLA

water

Temperature (K)

bottom

top

bottom flow

distillate

rate (kg/h)

flow rate (kg/h)

1 sim

3.120

0

0.974

0.0000

0.025

366.02

338.5

0.9800

exp

3.120

0

0.981

0.0000

0.0188

367.2

339.3

0.972

373

361.8

371.5

361.8

2 sim

0

exp 3 sim

2.673

exp

2.673

0

0.516

0.0163

0.0000

0.468

375.93

366.4

5.493

0.561

0.000

0.0000

0.436

374.2

369.5

4.808

4.1. Esterification of Crude Concentrated Lactic Acid. The esterification column was simulated for an experimental run (RN-02) mentioned in Table 1. The simulation results of the esterification of LA to produce 2.13 kg/h MLA for a specific crude LA feed, bottom, and distillate composition is shown in Table 7, which is consistent with experimental results. It may be mentioned at this stage that the LA is fed from the top, whereas MeOH vapor enters the column from the bottom. The elevated temperature of the distillate and bottom in the experimental run, compared to the simulation result, is due to the formation of dilactide during the esterification of LA. 4.2. Hydrolysis of Methyl Lactate to Highly Pure Lactic Acid. For complete hydrolysis of MLA to highly pure LA, a three-CSTR series with three distillation column configurations was used. Three hydrolysis columns were individually simulated for a specified feed, bottom, and distillate. The distillate from columns C-102 and C-103 is recycled to the middle of column C-101 and CSTR R-101, respectively. These two recycled streams were determined by material balance calculations. The overflow of CSTR R-101 is fed into CSTR R-102, and the overflow of CSTR R-102 was fed into the middle of stripper column C-103, where the contents of R-102 were treated in countercurrent mode with the rising steam from the reboiler of CSTR R-103. The experimental run from Table 5, RN-001 to produce 4.8 kg/h LA, is chosen for simulation. Table 8 shows the simulation results, which are in good agreement with the experimental results. These simulation results would be useful for designing a commercial plant under the operating conditions. The motive of modeling and simulation is to sufficiently represent and verify the plant and process at higher scales.

5. CONCLUSIONS Autocatalytic continuous countercurrent esterification of crude concentrated lactic acid (LA) was attempted using a packed column. It was found that ∼80% of the crude LA was converted to methyl lactate (MLA) at the upper section (1.5 m) from the top of the packed column. This indicates that countercurrent esterification of crude LA in a packed column is a very efficient process. The hydrolysis of MLA in a three-CSTR series using autocatalysis shows promising results and achieving a high-quality pure LA (∼99.81%). In most of the hydrolysis experimental run, the conversion of MLA was almost complete. The advantage of this process is that the complete hydrolysis of MLA is achieved without using a catalyst, thus avoiding contamination of the LA. The experimental results of continuous countercurrent esterification of crude concentrated LA and continuous hydrolysis of MLA were compared with simulated results and the simulated results were found to be consistent with the experimental results. This indicates that the model may sufficiently represent the process at higher scales. ’ ASSOCIATED CONTENT

bS

Supporting Information. Reaction and kinetics data (homogeneous autocatalyzed reaction) and vapor liquid equilibria (VLE) data are available as supplemental material. (PDF) This information is available free of charge via the Internet at http://pubs.acs.org/.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: 020 25902150. Fax: 020 25902612. E-mail: bd.kulkarni@ ncl.res.in. 1513

dx.doi.org/10.1021/ie200642j |Ind. Eng. Chem. Res. 2012, 51, 1506–1514

Industrial & Engineering Chemistry Research

’ ACKNOWLEDGMENT Financial support from, NIMITLI, CSIR, Government of India for this work is gratefully acknowledged. The authors take this opportunity to thank to Mr. Gopal Chaphekar, Mr. M. Y. Gupte, Dr. V. B. Chavan, Mr. Prakash Bhujang, and Dr. S. N. Shintre for their kind help during experimentation on the pilot plant scale and analysis of the reaction samples. ’ NOMENCLATURE a = activity Bi = flow rate of component from bottom of the column (kmol/h) C = number of component Di = distillate flow rate (kmol/h) EA,1 = apparent activation energy (kJ/mol) fik = feed flow rate of component i on tray k (kmol/h) HLk = enthalpy of the liquid stream off tray k (J/kmol) HVk = enthalpy of the vapor stream off tray k (J/kmol) Hfk = enthalpy of the feed stream off tray k (J/kmol) ΔHk = heat of reaction for the reaction (J/kmol) Lk = liquid flow rate off tray k (kmol/h) N = number of stages k°1 = pre-exponential factor for the esterification reaction (mol/(g min)) Keq = equilibrium constant Kik = vapor
liquid equilibrium coefficient for component i on tray k Ps = vapor pressure (kPa) R = gas constant (kJ/(mol K)) T = temperature (K) Vk = vapor flow rate off tray k (kmol/h) Vi = stoichiometric coefficient of component i in the reaction xik = mole fraction of component i in liquid phase on tray k

ARTICLE

(7) Pal, P.; Sikder, J.; Roy, S.; Giorno, L. Process Intensification in lactic acid production: A review of membrane based processes. Chem. Eng. Process. 2009, 48, 1549–1559. (8) Seo, Y.; Hong, W. H.; Hong, T. H. Effects of operation variables on the recovery of lactic acid in a batch distillation process with chemical reactions. Korean J. Chem. Eng. 1999, 16, 556–561. (9) Choi, J.; Hong, W. H. Recovery of lactic acid by batch distillation with chemical reaction using ion exchange resin. J. Chem. Eng. Jpn. 1999, 32, 184–189. (10) Kumar, R.; Nanavati, H.; Noronha, S. B.; Mahajani, S. M. A continuous process for the recovery of lactic acid by reactive distillation. J. Chem. Technol. Biotechnol. 2006, 81, 1767–1777. (11) Barve, P. P.; Rahman, I.; Kulkarni, B. D. Pilot plant study of recovery of lactic acid from ethyl lactate. Org. Process Res. Dev. 2009, 13, 573–575. (12) Barve, P. P.; Kulkarni, B. D.; Shinde, R. W.; Gupte, M. Y.; Joshi, C. N.; Thite, G. A.; Chavan, V. B.; Deshpande, T. R. Process for preparing L(+) lactic acid. U.S. Patent No. 7,820,859 B2. (13) Suzuki, I.; Yagi, H.; Komatsu, H.; Hirata, M. J. Calculation of multicomponent distillation accompanied by a chemical reaction. J. Chem. Eng. Jpn. 1971, 4, 26–33. (14) Sanz, M. T.; Beltran, S.; Calvo, B.; Cabezas, J. L. Vapor Liquid Equilibria of the Mixtures involved in the esterification of lactic acid with methanol. J. Chem. Eng. Data 2003, 48, 1446–1452. (15) Sanz, M. T.; Muga, R.; Beltran, S.; Cabezas, J. L. Autocatalyzed and ion-exchange resin-catalyzed esterification kinetics of lactic acid with methanol. Ind. Eng. Chem. Res. 2002, 41, 512–517.

Greek Letters

αik = activity coefficient for component i on tray k εk = extent of reaction on tray k Subscripts

LA = lactic acid MeOH = methanol MLA = methyl lactate W = water

’ REFERENCES (1) Garlotta, D. A Literature Review of Poly(Lactic acid). J. Polym. Environ. 2001, 9, 63–84. (2) Agrawal, A. K.; Bhalla, R. Advances in the production of poly(lactic acid) fibers—A review. J. Macromol. Sci., Polym. Rev. 2003, 43, 479–503. (3) Mehta, R.; Kumar, V.; Bhunia, H.; Upadhyay, S. N. Synthesis of Poly(Lactic Acid): A Review. J. Macromol. Sci., Polym. Rev. 2005, 45, 325–349. (4) Datta, R.; Henry, M. Lactic acid: Recent advances in products, processes and technologies—A review. J. Chem. Technol. Biotechnol. 2006, 81, 1119–1129. (5) Joglekar, H. G.; Rahman, I.; Babu, S.; Kulkarni, B. D.; Joshi, A. Comparative assessment of downstream processing options for lactic acid. Sep. Purif. Technol. 2006, 52, 1–17. (6) Vijayakumar, J.; Aravindan, R.; Viruthagiri, T. Recent trends in the production, purification and application of lactic acid. Chem. Biochem. Eng. Q. 2008, 22, 245–264. 1514

dx.doi.org/10.1021/ie200642j |Ind. Eng. Chem. Res. 2012, 51, 1506–1514