Article pubs.acs.org/IECR
Liquid−Liquid Extraction for Recovering Low Margin Chemicals: Thinking beyond the Partition Ratio Vishesh H. Shah,† Viet Pham,‡ Paul Larsen,* Sanjib Biswas,§ and Timothy Frank Engineering and Process Sciences Laboratory, The Dow Chemical Company, Midland, Michigan 48640, United States S Supporting Information *
ABSTRACT: Recovery of carboxylic acids from fermentation broths is an active area of research due to ongoing interest in utilizing renewable feedstock for chemical production. Several recent studies have focused on recovery via liquid−liquid extraction using reactive extraction solvents such as high molecular weight amines because they yield significantly higher partition ratios. However, these solvents tend to be more expensive than conventional physical extraction solvents. We have measured the liquid−liquid phase equilibrium behavior for extracting propionic acid from aqueous solutions at 26−91 °C using 1-butanol (physical extraction) and a blend of trioctylamine and 1-octanol (reactive extraction). As expected, the amine-based solvent system is more effective at extracting propionic acid. Additional analysis shows, however, that the 1-butanol process is still preferred in spite of its lower partitioning for propionic acid due to the high cost of the amine solvent relative to the product (propionic acid). Our study therefore shows that solvents must be evaluated based not only on the partition ratio but also on solvent cost, product cost, mutual solubilities, thermal stability, and ease of recovery.
1. INTRODUCTION Chemical engineers have developed several technologies over the past decades for recovering chemicals of interest from mixtures on a commercial scale. Even though distillation remains the most common among these separation technologies, it is not always the most attractive. Distillation exploits differences in the volatilities of components in a mixture and can involve repeated boiling and condensation. Distillation may thus be unsuitable if the relative volatilities of key components are too low, or if the components are thermally sensitive, or if the energy demand is too high. In such cases, other options such as liquid−liquid extraction, crystallization and adsorption can be considered. Liquid−liquid extraction exploits differences in the polarity (hydrophobicity/hydrophilicity) of components. A liquid− liquid extraction process involves adding a solvent to create two liquid phases to preferentially extract one or more components of a feed mixture. Liquid−liquid extraction processes typically have two effluent streams. The extract stream contains solvent enriched with extracted components, whereas the raffinate stream contains mostly less-extracted components of the feed mixture. An additional unit operation is typically required to separate the extracted components from the solvent in the extract stream. Moreover, because the raffinate stream also often contains dissolved solvent, another separation unit operation is frequently used for solvent recovery. In most cases, liquid−liquid extraction requires additional major unit operations. Solvent selection plays a key role in the economic feasibility of a liquid−liquid extraction process. Although general guidelines are available for solvent selection based on activity coefficient and functional group analysis,1,2 selecting the best solvent for commercial implementation requires thorough experimental and economic evaluation. It is not sufficient to rely only on rules of thumb or intuition because liquid−liquid © XXXX American Chemical Society
phase equilibrium behavior can be significantly affected by several factors including even trace components. 1.1. Extraction-based Recovery of Carboxylic Acids from Fermentation Broth. One area where extraction processes have received significant attention is for the recovery of fermentation products, especially as an alternative to the practice of recovering carboxylic acids by precipitating their salts from aqueous broths. A specific example of an extraction process used for recovering a fermentation product involves citric acid separation using trialkylamine solvents.2 Research activity in this field of biochemicals is rapidly increasing as countries around the world are seeking fuel and hydrocarbon raw material security as well as addressing climate change concerns. Fermentation broths typically exhibit the following challenging features: (i) the presence of a large amount of water, (ii) dilute concentration of the component(s) of interest, (iii) the presence of several unwanted byproducts, and (iv) the presence of biomass sludge. The desired component(s) are often higher boiling relative to water. On the other hand, sometimes they form minimum boiling azeotropes but these azeotropes have high water content. Therefore, the use of distillation to recover the component(s) of interest can be quite energy intensive and unattractive. In such circumstances, liquid−liquid extraction can be a promising alternative. Although treatment of the extract and raffinate streams may require distillation, these subsequent distillations usually do not involve the challenges associated with distillation of the feed mixture directly. The extraction of carboxylic acids from fermentation broths has been studied for a long time. In the 1980s, Kertes and King Received: October 17, 2015 Revised: December 23, 2015 Accepted: January 25, 2016
A
DOI: 10.1021/acs.iecr.5b03914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
filtration.8 Even though baffle tray extraction columns are capable of handling suspended solids9 and can be used in an extraction process with whole unfiltered broth, this approach is viable only if the solids exhibit sufficiently low wettability in the extraction solvent to ensure adequate mass transfer efficiency of the column.2 Solid−liquid separation is thus usually preferred. Furthermore, a high pH is maintained typically during fermentation for high PA yield, and is commonly achieved by adding a base such as ammonia. The PA formed during the fermentation thus exists as ammonium propionate and needs to be converted to its free form in order to be recovered by distillation or extraction. One way of achieving this is by adding a mineral acid such as H2SO4 resulting in the formation of ammonium sulfate. Although the presence of an inorganic salt such as ammonium sulfate is known to be beneficial for extraction by increasing the partition ratio and decreasing solvent−water mutual solubilities, this approach increases the effluent produced from the overall process. Alternative technologies for obtaining the free acid such as thermally splitting ammonium propionate are thus preferred.7 Thermal salt splitting not only reduces effluent but also provides ammonia in a form that can be recycled back to the process. Given the above considerations, the case study in this paper assumes a solid−liquid separation step followed by thermal salt splitting prior to physical extraction using either 1-butanol (BuOH) or reactive extraction using a blend of trioctylamine (TOA) and 1-octanol (OctOH). The first two steps are common to both extraction processes and are not considered in our comparative analysis. Postextraction, a back extraction step can be implemented by contacting the extract stream with clean water at a different temperature to obtain a relatively impurity-free stream containing mostly PA in water at a significantly higher concentration than in the feed broth. However, because esterification of PA is a possible next step to produce PrOH, the presence of water may not be preferred and a back extraction step may be unattractive. Therefore, we focus in this paper on forward extraction followed by distillation for extract and raffinate processing and estimate the contribution of these extraction and distillation steps to the manufacturing cost of PA. Subsequent downstream steps, such as converting PA to PrOH, are not included in the analysis. To provide a basis for the analysis, we first measure the liquid−liquid phase equilibrium behavior for extracting PA from an aqueous mixture at various temperatures using BuOH and a blend of TOA and OctOH. These measurements are used in process design and economic calculations by which we show that BuOH-based extraction is more attractive for PA recovery in spite of the partition ratio being significantly lower than that of the TOA−OctOH system. These results thus disprove the notion that reaction enhanced extractions or extractions using solvents with the highest partition ratio are always the best choice for liquid−liquid extraction.
systematically classified solvents for extracting carboxylic acids from fermentation broths into three categories: (i) carbonbonded oxygen-bearing solvents, (ii) phosphorus-bonded oxygen-bearing solvents, and (iii) high molecular weight amines.3 They concluded that extractions using the first category i.e. conventional water-immiscible oxygenated solvents (alcohols, ketones, ethers) were relatively inefficient for acid recovery. The efficiency or power of an extractant is quantified by the partition ratio (ratio of wt % solute in organic phase to wt % solute in aqueous phase). Kertes and King thus advocated the development of more powerful extractants such as those based on high molecular weight amines.3 The use of amine extractants generally requires a diluent (such as kerosene or octanol) to control the viscosity and the density of the organic phase. Among physical extraction solvents, alcohols are generally most effective for extracting carboxylic acids, followed by oxygenated diluents, aromatic hydrocarbons, and last, aliphatic hydrocarbons.4 Because reactive extractants usually result in higher partition ratios, several researchers have studied and are continuing to study the extraction of carboxylic acids using amines. In fact, according to a recent review,5 at least 15 studies have been published on this specific solute−solvent family pair alone between 2008 and 2012. Many of these studies consider extraction as an attractive alternative to lower the cost of downstream processing. However, this consideration is without any supporting basis because none of the articles have considered the effect of the cost of the solvent and the cost of the carboxylic acid. In this paper, we apply process design calculations and economic analysis to show the importance of considering factors beyond the partition ratio in selecting an appropriate extraction solvent. To illustrate the methodology, we focus on the specific case of recovering propionic acid (PA) from fermentation broth. 1.2. Case Study Background and Scope. A renewable route to PA is important because the increased petrochemical feedstock contribution from shale gas in recent times has resulted in a shift to lighter feeds for North American crackers, leading to a relative propylene deficit. The propylene deficit can be met by on-purpose propylene technologies such as propane dehydrogenation. Alternatively, PA derived from fermentation can be converted to propylene via esterification, hydrogenolysis and dehydration (with n-propanol as an intermediate). Fermentation with propionibacterium to make PA has been studied since at least the 1940s.6 Rodriguez et al. evaluated the feasibility of biopropylene and showed that it is not currently economical due to the high cost of sugars relative to the cost of propylene.7 However, they also showed that instead of conversion to propylene, biopropionic acid or biopropanol could be economically competitive with petroleum derived PA or 1-propanol (PrOH) even at current sugar prices if fermentation yields were improved. Thus, we have not included these subsequent conversion steps in our study and have focused only on the economic contributions of alternative processes for recovering bio-PA. A process for recovering PA from fermentation broth can be broken down into the following steps: (i) separation of insoluble materials such as cell debris and undissolved nutrients, (ii) isolation of PA from other organic compounds, and (iii) concentration of PA (removal of water) to meet the desired specification. Separation of insolubles can be achieved by a solid−liquid separation process such as centrifugation or
2. LIQUID−LIQUID PHASE EQUILIBRIUM MEASUREMENTS The following sections describe our measurements of the liquid−liquid phase equilibrium (LLE) in the PA−H2O− BuOH system and the PA−H2O−TOA−OctOH system. 2.1. Materials. The feed solution for these measurements was prepared by diluting PA (Fisher, certified, Lot # 001117A) with deionized water to 9.1 wt % PA. B
DOI: 10.1021/acs.iecr.5b03914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
separation was noted and the layers were sampled once adequate phase separation was achieved. The upper layer was sampled by inserting a syringe through a top port, and the lower layer was sampled using the bottom stopcock. Both layers were sampled twice (approximately 1 mL for gas chromatography analysis and approximately 1 mL for titration). 2.3. Analytical. Gas chromatography was used to quantify the concentration of BuOH, OctOH, and TOA in both the organic and the aqueous layers. Titration with NaOH was used to quantify the concentration of PA, and Karl Fischer titration was used to quantify the concentration of water. Detailed analytical procedures can be found in the Supporting Information for this article. 2.4. Experimental Results. The results of the liquid− liquid equilibrium experiments are provided in Table 1 for BuOH, and these measurements are compared with values obtained from previous work by Solimo et al.10 and Kim et al.11 in Table 2. In Table 1, the PA concentration in the organic phase at 27 °C was calculated by difference due to an analytical error. All the other PA concentrations were measured directly. The total analytical accountability for each of the samples was 99 ± 0.8%, and the mass accountability for PA after the measurements were completed (100 × PA out/PA in) was 95.9%. The parameter K reported in the tables is the partition ratio defined as
The TOA−OctOH blend was prepared by diluting TOA (Aldrich, 98%, Lot # MKBD7791 V) with OctOH (SigmaAldrich, ACS reagent, >99%, Lot # 73596KMV) to achieve a concentration of 20 wt % TOA. This mixture is referred to as TOA20 throughout this article. BuOH was obtained from Dow Chemical (>99.8% purity). Samples for gas chromatography analysis were prepared using tetrahydrofuran (Fisher, HPLC grade without preservatives) dried over molecular sieves (Sigma-Aldrich, 4A, Beads, 4−8 mesh) with dryness verified by Karl Fischer titration. 2.2. Procedure. The LLE measurements were obtained using a 200 mL jacketed extraction vessel as shown in the schematic in Figure 1. The vessel was equipped with a
K=
mass fraction species in organic phase mass fraction species in aqueous phase
(1)
The variable KBancroft is the partition ratio of PA in Bancroft coordinates and is defined as2 KBancroft = mass fraction PA in organic phase, water‐free basis mass fraction PA in aqueous phase, solvent‐free basis (2)
Figure 1. Schematic of apparatus used for measuring LLE behavior.
The results show a decrease in the PA partition ratio with increasing temperature that correlates with the decreasing concentration of BuOH in the organic phase. The partition ratio measured at 27 °C is high relative to the results reported in the literature. The Solimo measurement is probably more accurate because the PA concentration in each phase was measured directly, whereas our reported value for the organic phase PA concentration was computed by difference (for the 27 °C sample only). The liquid−liquid equilibrium results for the TOA/OctOH/ water/PA system are given in Table 3 and Table 4. The concentrations of OctOH and TOA in the aqueous phase are below the detection limit for the room temperature measurement. The “TOA20” label in Table 4 refers to the 20:80 mixture of TOA in OctOH. The error in the volumetric Karl Fischer-based water concentration measurement increases with increasing concentration, hence some of the reported water
condenser (to minimize material loss during higher temperature measurements) and the contents were mixed using a topentry agitator. The temperature was controlled using a recirculating bath with a 50 wt % mixture of propylene glycol in water. The temperature was monitored to an accuracy of ±1 °C using thermocouples in submerged thermowells and the contents were maintained under a nitrogen pad. Experiments were carried out by loading the vessel with approximately 40 mL of feed and approximately 40 mL of solvent. Operating conditions were chosen to ensure a close approach to equilibrium. Mixing intensity was adjusted for complete circulation of the top liquid layer into the bottom layer for thorough contacting of the two liquid phases, and mixing was conducted for 15 min. Temperatures were typical of commercial process conditions, and the results were consistent with literature data when available. The time required for phase Table 1. LLE Measurements for BuOH/Water/PA organic phase (wt %)
a
aqueous phase (wt %)
PA
ln(K)
T (°C)
PA
H2O
BuOH
PA
BuOH
H2O
K
KBancroft
PA
BuOH
H2O
27 59 89
8.04a 6.85 6.39
22.06 25.73 33.87
68.9 65.62 58.18
1.94 1.8 2.11
7.79 7.37 8.83
89.13 90.27 88.24
4.14 3.81 3.03
4.91 4.83 4.24
1.42 1.34 1.11
2.18 2.19 1.89
−1.40 −1.26 −0.96
Calculated by difference. C
DOI: 10.1021/acs.iecr.5b03914 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 2. Comparison of BuOH/Water/PA LLE Measurements with Previous Literature organic phase (wt %)
aqueous phase (wt %)
PA
T (°C)
PA
H2O
BuOH
PA
BuOH
H2O
K
reference
27 30 30 25
8.04 7.40 10.50 6.55
22.06 24.49 23.99 17.75
68.9 68.11 65.51 75.70
1.94 2.10 3.10 3.01
7.79 7.90 7.90 5.21
89.13 90.00 89.00 91.78
4.14 3.52 3.39 2.18
this work Solimo et al.10 Solimo et al.10 Kim et al.11
Table 3. LLE Measurements for TOA/OctOH/Water/PA organic phase (wt %)
aqueous phase (wt %)
T (°C)
PA
H2O
OctOH
TOA
PA
OctOH
TOA
H2O
26 62 91
7.35 7.14 6.86
4.54 4.98 5.49
68.58 68.07 67.98
19.12 18.89 18.19
1.13 1.17 1.7
BDL 0.99 0.50
BDL 0.30 0.00
101.09 96.55 99.93
temperature in the TOA recovery column, and this higher temperature could ultimately result in a net increase in TOA losses due to thermal degradation. Thus, finding the economic optimum requires detailed analysis beyond simply comparing partition ratios. Such detailed analysis is demonstrated in the following section. Another important consideration for design of extraction processes is the practicality of achieving phase separation. The phase separation was slower for TOA20 (∼15 min for dispersion band to collapse) than for BuOH (
