ARTICLE pubs.acs.org/IECR
Propionic Acid Production from Raw Glycerol Using Commercial and Engineered Strains John A. Posada and Carlos A. Cardona* Departamento de Ingeniería Química, Universidad Nacional de Colombia Sede Manizales, Cra 27 No. 64-60, Manizales, Colombia ABSTRACT: Five technological schemes for propionic acid production from raw glycerol were designed, simulated, and economically assessed. Fermentative scenarios considered two different qualities of glycerol: 88 and 98 wt % with concentrations in the fermentation media from 20 to 50 g/L. Raw glycerol (60 wt %) was considered as the feedstock feeding the production process in all cases; then a purification process of raw glycerol up to the required quality was used. Simulation processes were carried out using Aspen Plus, while economic assessments were performed using Aspen Icarus Process Evaluator. Propionic acid recovery and purification processes were based on reactiveextraction with tri-n-octylamine using ethyl acetate as extractant agent. This technology results in energy-intensive processes with total energy consumption ranging from 120.4 to 138.9 MJ/kg. However, further significant improvements could be obtained using energy integration techniques. On the other hand, it was found that the production costs are directly related to the fermentation stage and decrease as follows: (i) increasing the glycerol concentration, (ii) decreasing the glycerol purity, and (iii) increasing the glycerol consumption.
’ INTRODUCTION Propionic acid is a naturally occurring carboxylic acid which, in the pure state, is a colorless, corrosive liquid with an unpleasant odor. Propionic acid is used in the manufacture of herbicides, chemical intermediates in the synthesis of cellulose fibers, herbicides, perfumes and pharmaceuticals, artificial fruit flavors, and preservatives for food, animal feed, and grain.1,2 Nowadays, propionic acid production is around 440 million pounds with an annual growth rate of 1.8%; and a sale price of USD $0.510.54 per pound. Commercial production of propionic acid is mostly done by petrochemical routes; however, it could also be produced by fermentative processes through groups of bacteria belonging to the genera Propionibacterium, Veillonella, Selenomonas, Clostridium, Fusobacterium, etc., via the dicarboxylic acid pathway with acetic acid and succinic acid as byproducts.37 Although propionic acid production by fermentation has not been economically competitive compared to chemical processes, the increasing cost of oil has recently brought attention to biological routes of transformation employing renewable feedstocks and industrial wastes, which could lead to a reduction of waste disposals. Besides increasing oil prices, the high demand of propionic acid for use as a natural preservative has stimulated developments of new fermentation processes to achieve improved propionic acid production from low-cost carbon sources. While most studies on propionic acid production by Propionibacterium acidipropionici have focused on glucose and whey lactose,812 also some studies have explored glycerol as the carbon source6,13 and it was observed that glycerol might be advantageous since less acetic acid was produced during consumption of glycerol.3,13 Raw glycerol, as the main byproduct of biodiesel production, is a promising, renewable, and low-cost raw material, which can be used as a unique carbon and energy source by many bacteria species to produce several metabolites including propionic acid. Also, the growing market of biodiesel has generated a glycerol r 2011 American Chemical Society
oversupply leading to low prices of glycerol in the market. Since glycerol sales have represented an important profitability for the biodiesel industry, it is reasonable that low prices of glycerol could impact the economy of biodiesel producers negatively. For that reason, correct exploitation of glycerol as a raw material should be focused on its transformation to added value products. Thus, use of glycerol is a high-priority topic for managers and researchers related to biodiesel production. In this sense, establishment of glycerol’s biorefineries able to cogenerate added value products is an excellent opportunity not only to raise the profitability but also to produce other chemicals from a biobased raw material. However, conventional fermentative processes for propionic acid production undergo from low values of: propionic acid yield, final propionic acid concentration, and propionic acid productivity caused by a strong inhibition of the final products. Thus, the low yield and productivity due to inhibition of propionic acid on cell growth and propionic acid synthesis has been identified as the main process problem.5,14 In order to overcome these drawbacks related to inhibition of propionic acid on microbial growth and propionic acid synthesis, extractive propionic acid fermentation10,1517 and propionic acid production with propionic acidtolerant bacteria obtained via adaptive evolution have been developed.5,17,18 However, currently, the most promissory bacterial strains for glycerol fermentation to propionic acid are P. acidipropionici19 and the engineered P. acidipropionici ACK-Tet.6 In order to make economically attractive the fermentative production of propionic acid, development and analysis of new fermentation and downstream processes are required. In this sense, the processes design, processes simulation, and Received: June 17, 2011 Accepted: December 20, 2011 Revised: December 13, 2011 Published: December 20, 2011 2354
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economical assessment of propionic acid production from raw glycerol were here performed considering the above-mentioned two strains and five fermentative scenarios. Simulations were performed using Aspen Plus, while economic analysis was carried out using Aspen Icarus Process Evaluator. Metabolic Pathways of Glycerol Fermentation. Propionibacteria are, in general, Gram-positive, nonmotile, catalase-positive, nonspore-forming, rod-shaped bacteria, facultative anaerobes, capable of utilizing a broad range of carbon sources as substrate such as glucose, lactose, xylose, sucrose, glycerol, and lactate.3 Propionate is produced mainly by the dicarboxylic acid pathway, and its synthesis is usually accompanied by formation of acetate and carbon dioxide. The dicarboxylic acid pathway from glycerol is shown in Figure 1. Main Stages for Propionic Acid Production Process from Raw Glycerol. The whole technological scheme for propionic acid production was divided in three main stages. These stages are (i) glycerol purification, (ii) glycerol fermentation, and (iii) propionic acid recovery and purification. Glycerol Purification. The raw glycerol obtained as byproduct during biodiesel production by means of the transesterification process of triglycerides with methanol commonly contains high quantities of water, methanol, soaps, and salts with a 4088 wt % of glycerol.20 Thus, to remove these impurities, raw glycerol must be refined using traditional separation processes such as filtration, chemical additions, and fractional vacuum distillation. Purification of this mixture is industrially performed through filtration processes, followed by mixing with chemical additives leading to precipitation of salts, and finally, vacuum fractional distillation is used to produce different qualities of commercial glycerol. On the other hand, this layer can also be refined by a less energy-intensive method based on filtration through a series of
Figure 1. Dicarboxylic pathway to propionate.
ion-exchange resins21 or by means of a commercial purification process called Ambersep BD50, which was recently and jointly developed by Rohm and Haas and Novasep Process.20 Fermentation of Glycerol. For glycerol fermentation to propionic acid two strains were identified as the most promissory bacteria available from the literature. The first one is a commercial P. acidipropionici which consumes pure glycerol,19 and the second one is the engineered P. acidipropionici ACK-Tet,6 which is able to consume both pure and crude glycerol as the only source of carbon and energy. In this sense, five different scenarios were considered for glycerol fermentation which involve use of pure and crude glycerol and immobilized cells. Also, in all cases glycerol was completely consumed. The specific conditions for glycerol fermentation at each scenario are shown in Table 1, where the fermentation temperature for scenarios 1 and 2 is 30 °C while for scenarios 35 it is 32 °C. Propionic Acid Recovery and Purification. With respect to the recovery and purification methods of organic acids several alternatives have been evaluated. Some examples are liquid extraction,22 reverse osmosis,23 electrodialysis,24 liquid surfactant membrane extraction,25 anion exchange,26 precipitation and adsorption,27 and reactive liquidliquid extraction.28 Otherwise, Keshav et al.2943 studied widely the reactive extraction of propionic acid from a fermentation broth, and different diluent (e.g., benzene, toluene, hexane, n-heptane, n-octane, n-dodecane, ethyl acetate, butyl acetate, 1-octanol, 2-octanol, 1-decanol, 1-dodecanol, petroleum ether, paraffin liquid, MIBK, oleyl alcohol, sunflower oil) and extractant (e.g., tri-n-butylphosphate, tri-n-octylamine, Aliquat 336, and tri-noctylphosphine oxide) agents have been analyzed. Besides the final extraction performance, the kinetic behavior has also been studied.33 On the basis of the results reported by Keshav et al.2943 for the reactive extraction of propionic acid from the fermentation broth, it was noticed that the best configuration for this process requires the use of tri-n-octyl amine (TOA) as extractant agent while the diluent agent must be ethyl acetate. The concentration that leads to the best performance is 0.686 kmol/m3 and the extraction temperature is 32 °C according to Keshav et al.38 Process Description. Since two quatities of glycerol could be used as substrate for fermentative production of propionic acid (98 wt % for pure glycerol and 88 wt % for crude glycerol, according to Table 1), a unique raw glycerol was considered as the raw material. The typical composition of a raw glycerol stream in a biodiesel production process is methanol 32.59 wt %, glycerol 60.05 wt %, NaOCH3 2.62 wt %, fats 1.94 wt %, and ash 2.8 wt %.44 This stream must be purified up to the specified concentration established in Table 1 for glycerol fermentation. Thus, the purification process previously reported20 was here applied, and a brief description is as follows: the feed mixture is
Table 1. Basis Information for the Glycerol Fermentation to Propionic Acid6,9 scenario
strain
glycerol
glycerol purity for
fermentation
molar yield
conc. (g/L)
fermentation, wt %
time (h)
to S.A.
1 2
P. acidipropionici P. acidipropionici
20 50
98 98
120 150
0.79 0.57
3
P. acidipropionici ACK-Tet
46
98
280
0.54
4
P. acidipropionici ACK-Tet
17
88
160
0.71
5
immobilized cells of
41
98
104
0.59
P. acidipropionic ACK-Tet 2355
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Table 2. Stoichiometry of the Fermentation Process for Each Scenarioa
a
scenario, MW
glycerol 92.09
propionic acid 74.08
acetic acid 60.05
succinic acid 118.09
biomass 24.73
ΔHo, kJ/mol
1
1
0.9821
0.0925
0.0905
0.3481
10.79
2
1
0.7086
0.0785
0.0660
0.2820
111.83
3
1
0.6713
0.0368
0.0507
0.3442
151.80
4
1
0.8826
0.0537
0.0546
0.1903
5
1
0.7334
0.0414
0.0569
NA
65.26 122.98
Adapted from refs 6 and 9.
evaporated, and 90% of methanol contained in the fed stream is recovered at 99 wt % of purity. Then, the produced stream is neutralized with an acid solution. Further, ashes are retired by centrifugation, and then this stream is washed with water. In a later evaporation process more than 90% of the water content and the remainder of the methanol are removed. As a result, a glycerol stream at 80 wt % is obtained. Finally, glycerol quality is achieved through a distillation column. Then, the fermentative process is carried out according to the five scenarios considered (see Table 1), which take place according to the fermentation stoichiometry shown in Table 2. Scenario 1 uses a commercial P. acidipropionici strain and glycerol diluted at 20 g/L, while in scenario 2 the fermentation media contains the same strain and glycerol diluted at 50 g/L. For these two scenarios the fermentation process is carried out at 30 °C using pure glycerol.19 For scenarios 35 the engineered strain P. acidipropionici ACK-Tet is considered, and pure glycerol at 46 g/L is required in scenario 3. While crude glycerol at 17 g/L is used in scenario 4, scenario 5 considers a completely different configuration for the fermentation process. It is a fibrous-bed bioreactor packed with immobilized cells and fed with pure glycerol at 41 g/L. For these three scenarios the fermentation process is carried out at 32 °C.6 The downstream process for propionic acid recovery and purification from the fermentation broth is based on a reactive extractive process because of its good process characteristics, such as low toxicity, low cost, low boiling point, extraction yield, and recovery yield. Tri-n-octylamine and ethyl acetate are used as extractant and active diluent, respectively. A mixture of tri-noctylamine diluted in ethyl acetate at 0.686 kmol/m3 was considered for this process. The back-extraction process is carried out by a combined effect of changing the extractant concentration (Regeneration Swing Concentration Process) and changing the temperature profile (Regeneration Swing Temperature Process), which is reached by a distillation process under vacuum conditions in order to obtain a highly pure propionic acid. Simulation Procedure. Simulations were performed using Aspen Plus (Aspen Technologies Inc., USA), and the total production process of propionic acid from glycerol was considered as the sum of three different stages: glycerol purification, glycerol fermentation to propionic acid, and propionic acid recovery and purification. The glycerol purification process from raw glycerol (60 wt %) to the required purity, 80, 90, or 98 wt %, was previously reported and analyzed.45,46 The fermentation processes were simulated using a yielding approach where glycerol is consumed and products including cell mass are formed according to Table 2. On the other hand, products formed during the reactive extractive process (i.e., the complexes for each carboxylic acid) were created in Aspen Plus using the UNIFAC method. The
Table 3. Costs and Prices Used in the Economic Assessment costs
value
units
operatives
2.14
UDS$/h
supervisors
4.29
UDS$/h
electricity
0.03044
UDS$/kwh
water low pressure vapor
1.252 8.18
UDS$/m3 UDS$/Ton
raw glycerol
132.45
UDS$/Ton
crude glycerol (85 wt %)
540.84
UDS$/Ton
succinic acid
2492.2
UDS$/Ton
trioctylamine
2550
UDS$/Ton
methanol
290
UDS$/Ton
1-octanol
1835
UDS$/Ton
reactive extraction process of carboxylic acids with tertiary amine extractants is composed of three sequential steps: dissociation of carboxylic acid, proton transfer to the amine, and recombination of ammonium salt. The following reaction describes the overall process, but its stoichiometry varies with several factors, such as the property and concentration of amine, acid, and diluent. R 3 N þ HA T R 3 NHA
ð1Þ
Also, design of the distillation columns required definition of preliminary specifications using the DSTWU short-cut method included in Aspen Plus. This method provides initial estimates of the minimum number of theoretical stages, minimum reflux ratio, localization of the feed stage, and products split. Then, the RadFrac module (based on the MESH equations) was used to perform rigorous calculation of the distillation columns. In order to study the effect of the main operation variables (e.g., reflux ratio, feed temperature, number of stages, etc.) on the product composition, a sequential design procedure including sensitivity analysis was performed. Thereafter, to reach the desired purity, a specific design was applied. In addition, calculation of energy consumption was based on the thermal energy required by heat exchangers, reboilers, and flash drier units and the power supply required by the pumps. Economic analysis was performed using the Aspen Icarus Process Evaluator (Aspen Technology, Inc., USA) package. This analysis was estimated in U.S. dollars for a 10-year period at an annual interest rate of 16% considering the straight line depreciation method and a 33% income tax. All costs and prices of raw material and products are shown in Table 3.20,45,46 The abovementioned software estimates the capital costs of process units as well as the operating costs, among other valuable data, utilizing the design information provided by Aspen Plus and the data introduced by the user for specific conditions such as project location among others. 2356
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Figure 2. Simplified flow sheet for propionic acid production from raw glycerol: E, evaporator; R, reactor; C, centrifuge; Dec, decanter; DC, distillation column; M, mixer; F, fermentator; RE, reactor extractor; BE, back extraction column.
Additionally, in order to compare the performance of each scenario different operational parameters, such as extraction efficiency, distribution coefficient, loading, global propionic acid recovery, tri-n-octylamine recovery, ethyl acetate recovery, and global yield from glycerol to propionic acid, were determined.
’ RESULTS AND DISCUSSIONS Since the analyzed scenarios differ mainly on the fermentation stage according to Table 2, which shows the stoichiometry of the fermentation process for each scenario, different operational conditions, material and energy requirements, and equipment sizes are required for each scenario. Also, due to the lack of experimental data related to energy consumption (generation) during fermentation, the energy balances were based only on the overall stoichiometry of glycerol fermentation, and the specific energy used for bacterium growth was not considered. This approach is supported by the fact that energy requirements from the fermentation process are usually much lower compared to the total energy requirements for the whole process. However, more precise calculations should be addressed in the future for scaling up the process. The problem lies in the fact that the fermentations are usually exothermic and operate at low temperatures, which implies use of a large amount of cooling water, increasing water consumption of the plant. Thus, water consumption is directly related to the real energy generated at the fermentor. The simplified flow sheet for propionic acid production is shown in Figure 2. First, raw glycerol is purified up to the required quality in order to perform the fermentation process. Then, fermentation takes place according to Table 2, and the fermentation broth is clarified throughout a centrifugation process. This clarified stream is mixed with two organic streams. These streams contain fresh and recycled mixtures of TOA and ethyl acetate, and the resulting mixture is TOA in ethyl acetate at 0.686 kmol/m3. Thus, the reactive extraction process is carried out at 32 °C. The aqueous phase, containing high quantities of ethyl acetate, is distilled in order to recover it. Otherwise, the organic phase is subjected to a distillation process in which the contained ethyl acetate is recovered and mixed with this coming from the aqueous phase. Then, the resulting organic phase is
subjected to the back extraction process, and TOA is recovered by bottoms and recycled to the reactive extraction process. Finally, the resulting stream is distilled under vacuum conditions, and propionic acid at high quality is obtained. A summary of the main simulation results for each scenario is given in Table 4. In this case and due to glycerol being completely consumed during the fermentation process in all scenarios, final production of propionic acid is only related to the fermentation yield. Thus, the decreasing order for propionic acid production is scenario 1 > scenario 4 > scenario 5 > scenario 2 > scenario 3 (see Table 4). This obtained order agrees with the fermentation yield (see Table 2). Finally, the behavior of the reactive extraction process was analyzed based on its extraction efficiency, the obtained distribution coefficient, and the used loading, but no large differences were found for the extraction efficiency, while the distribution coefficient varied from 3.8 to 15. Also, it was observed that the global recovery of propionic acid from the fermentation broth is directly related to the extraction efficiency in the reactive extractive process as shown in Table 5. Besides of the high recovery levels for propionic acid, the extractant and diluent agents were almost completely recovered during the downstream process. Thus, low quantities of both agents must be fed as a fresh stream. As it could be expected, the global yield for the downstream process agreed with the order found for the fermentation yield and propionic acid production (i.e, scenario 1 > scenario 4 > scenario 5 > scenario 2 > scenario 3) with a relative difference between scenario 1 and scenario 3 of 29.69%. The five analyzed scenarios were economically assessed as shown in Tables 6 and 7, where the production cost results were discriminated by raw materials, services, operatives, depreciation, and products and coproduct sales. The values here obtained do not consider the transportation costs, since the propionic acid production process was assumed to be a biorefinery adjacent the biodiesel production process. Low raw material costs were obtained in all cases when raw glycerol was used, representing only around 5% of the total production costs. However, both utilities (between 45.8% and 58.9%) and capital (between 20.8% and 29.9%) costs represent the highest production cost which combined vary between 2357
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Table 4. Summary of the Main Simulation Results for the Propionic Acid Production Processa Dilutgly
FermBro
303.1
303.1
water
27 995.5
glycerol
578.9
cell mass propionic acid
Aqueous1
Organic1
Bottoms1
Prop acid
379.8
scenario 1 283.1
283.4
293.1
27 995.5
27 763.1
229.6
8.79
0.018
0
0
0
0
0
0
54.05
0
0
0
0
0
457.3
88.89
7.11
368.4
367.4
succinic acid
0
67.18
62.23
4.96
4.96
4.96
acetic acid tri-N
0 0
457.3 0
32.31 26.52
2.58 331.4
2.58 2056.3
0.54 0
complex
0
0
0
2075.7
0
0
ethyl
0
0
2754.03
8363.3
0
0
temp. (K)
303.1
303.1
scenario 2 283.1
283.3
293.1
380
water
11 007.5
11 007.5
10 724.5
281.9
6.34
0
glycerol cell mass
578.9 0
0 43.78
0 0
0 0
0 0
0 0
temp. (K) mass flow (kg/h)
mass flow (kg/h)
propionic acid
0
329.96
55.48
13.78
274.5
274.0
succinic acid
0
48.99
39.20
9.80
9.80
9.80
acetic acid
0
29.63
23.72
5.88
5.88
1.20
tri-N
0
0
53.76
784.4
2029.0
0
complex
0
0
0
1497.6
0
0
ethyl
0
0
1114.0
10 003.4
0
0
temp. (K) mass flow (kg/h)
305.1
305.1
scenario 3 283.1
283.3
293.1
380
water
11 987.5
11 987.5
11 708.6
277.8
6.00
0
glycerol
578.9
0
0
0
0
0
cell mass
0
53.44
0
0
0
propionic acid
0
312.6
53.56
12.07
259.1
258.3
succinic acid
0
37.63
30.70
6.97
6.97
6.97
acetic acid
0
13.89
11.35
2.58
2.58
0.540
tri-N complex
0 0
0 0
59.06 0
844.6 1418.9
2023.7 0
0 0
ethyl
0
0
1203.8
9913.6
0
0
temp. (K)
305.1
305.1
scenario 4 283.1
283.3
293.1
379.9
water
12 036.6
12 036.6
11 757.2
278.2
7.89
0.018
glycerol
578.9
0
0
0
0
0
cell mass propionic acid
0 0
29.55 411.0
0 70.45
0 15.85
0 340.47
0 339.5
mass flow (kg/h)
succinic acid
0
40.5
33.06
7.44
7.44
7.44
acetic acid
0
20.27
16.57
3.72
3.72
0.781
tri-N
0
0
34.66
498.0
2047.8
0
complex
0
0
0
1865.4
0
0
ethyl
0
0
1213.7
9903.7
0
0
temp. (K)
305.1
305.1
scenario 5 283.1
283.4
293.1
379.9
mass flow (kg/h) water
13 492.5
13 492.5
13 218.4
272.8
6.56
0
2358
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Table 4. Continued glycerol cell mass
a
Dilutgly
FermBro
Aqueous1
Organic1
578.9 0
0 53.44
0 0
0 0
Bottoms1 0 0
Prop acid 0 0
propionic acid
0
341.5
59.93
11.78
281.6
280.8
succinic acid
0
42.24
35.31
6.97
6.97
6.97
acetic acid
0
15.63
13.03
2.58
2.58
0.54
tri-N
0
0
53.05
741.66
2029.4
0
complex
0
0
0
1550.4
0
0
ethyl
0
0
1351.4
9766.0
0
0
Dilutgly, diluted glycerol; FermBro, fermentation broth; Aqueous1, aqueous phase; Organic1, organic phase.
Table 5. Data Representing the Behavior of the Downstream Process for Propionic Acid Productiona scenario 1
scenario 2
scenario 3
scenario 4
scenario 5
reactionextraction process extraction efficiency (%)
80.56
distribution coefficient loading (Z)
83.19
8.761 0.844
3.790 0.629
82.87
82.85
82.46
14.156 0.594
12.965 0.781
15.032 0.645
downstream process global P.A. recovery (%)
80.34
83.05
82.64
82.61
82.24
tri-n-octylamine recovery (%)
98.73
97.42
97.17
98.33
97.45
EtAc recovery (%)
99.53
99.45
98.51
98.49
98.34
global process global process yield from glycerol to PA (%) a
0.7605
0.5672
0.5347
0.7027
0.5814
PA: propionic acid. EtAc: ethyl acetate.
Table 6. Production Costs of Propionic Acid from Raw Glycerol (USD$/kg) scenario 1
scenario 2
scenario 3
scenario 4
scenario 5
raw materials
0.0851
0.0851
0.0851
0.0851
0.0851
utilities
1.1355
0.8192
0.8275
0.9709
0.9497
operating labor
0.0561
0.0624
0.0706
0.0745
0.0691
maintenance
0.0342
0.0651
0.0783
0.0841
0.0762
operating charges
0.0140
0.0156
0.0176
0.0186
0.0173
plant overhead G and A cost
0.0452 0.1625
0.0638 0.1161
0.0744 0.1171
0.0793 0.0636
0.0727 0.0618
depreciation of capital
0.4011
0.4614
0.5398
0.5717
0.5279
0.0055
0.0055
0.0055
0.0055
0.0055
gly. purify. + ferm.
0.3100
0.3108
0.3104
0.2959
0.3104
L.A. purification
1.618
1.372
1.495
1.647
1.544
total
1.928
1.683
1.805
1.942
1.854
sale price/production cost
0.985
1.129
1.053
0.978
1.025
coproducts sales
75.76% for scenario 3 to 79.69% for Scenarios 1 and 5. Also, it was found that the utilities costs depend mainly on the glycerol concentration in the fermentation media. In addition, the total production cost was divided in two processing sections: (i) glycerol purification plus glycerol fermentation and (ii) propionic acid recovery and purification, as shown in Tables 6 and 7. The glycerol purification and glycerol fermentation stages only represented between 15.23% and 18.46%, which indicates that most efforts are required for the downstream process since a high share of the total production costs (between 81.54% and 84.77%) is consumed for propionic acid purification.
Total production costs of propionic acid from raw glycerol range from 1.683 to 1.942 USD$/kg of P.A., which represents a relative difference of 15.4%. The increasing order for the total production costs is as follows: scenario 2 < scenario 3 < scenario 5 < scenario 1 < scenario 4. The ratio between the sale price to the production cost was calculated for the five analyzed scenarios, and this ratio was higher than unity only for the scenarios 2, 3, and 5. Thus, it is stated that this process could be profitable only when a high concentration of glycerol is used in the fermentation media and a high yield to propionic acid is obtained. 2359
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Table 7. Share of the Production Costs of Propionic Acid from Raw Glycerol (%) scenario
scenario
scenario
scenario
scenario
1
2
3
4
5
4.41 58.89
5.05 48.67
4.71 45.85
4.38 49.99
4.59 51.22
raw materials utilities operating labor
2.91
3.71
3.91
3.83
3.73
maintenance
1.78
3.87
4.34
4.33
4.11
operating charges
0.73
0.93
0.98
0.96
0.93
plant overhead
2.34
3.79
4.12
4.08
3.92
G and A cost
8.43
6.90
6.49
3.27
3.33
depreciation of
20.80
27.41
29.91
29.43
28.47
capital coproducts sales
0.28
0.33
0.30
0.28
0.30
’ CONCLUSIONS A preliminary techno-economic analysis of five different strains, including commercial, engineered, and immobilized, was made for producing propionic acid from raw glycerol. Although a unique technological scheme was designed, simulated, and economic evaluated, significant differences were noticed in the fermentation stage and in the downstream process for each scenario. These differences were lower when the total production costs were compared resulting in values very close to the commercial sale price of propionic acid. The highest ratio sale price to production cost was obtained in scenario 2 with a value of 1.129. This result suggests that this is a promissory alternative for an efficient use of raw glycerol due to further improvements in both process operation (e.g., heat integration) and strains (e.g., either higher yields or higher productivity) would represent an economic profit resulting in a positive income. Also, it can be noticed that the strain used in scenario 2 leads to the best performance because the fermentation is performed at the highest concentration with mid-fermentation times. Thus, when glycerol concentration increases in the fermentation stage, the production cost of propionic acid decreases. These results are encouraging in the sense that current available strains and technologies could be significantly improved by bioengineering and process analysis tools leading to a completely new and profitable process for production propionic acid from byproduced raw glycerol. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: +57 6 8879300 ext 50417. Fax: +57 6 8879300 ext 50199. E-mail:
[email protected].
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