Reactive extraction and recovery of levulinic acid, formic acid and
furfural from aqueous solutions containing sulphuric acid
Thomas Brouwer
a, Marek Blahusiak
a, Katarina Babic
b, Boelo Schuur
a,⇑a
University of Twente, Faculty of Science and Technology, Sustainable Process Technology Group, PO Box 217, 7500AE Enschede, The Netherlands
b
DSM ACES Ahead R&D, P.O. Box 1066, 6160 BB Geleen, The Netherlands
a r t i c l e i n f o
Article history:
Received 11 November 2016 Received in revised form 17 May 2017 Accepted 20 May 2017
Available online 23 May 2017
Keywords: Levulinic acid Sulphuric acid Furfural Formic acid Solvent screening Liquid-liquid extraction
Temperature swing back extraction
a b s t r a c t
Levulinic acid (LA) can be produced from lignocellulosic materials via hydroxylation followed by an acid-catalyzed conversion of hexoses. Inorganic homogeneous catalysts are mostly used, in particular sulphuric acid, yielding a mixture of LA with sulphuric acid, formic acid (FA) and furfural. Significant attention has been paid to optimization of the yield, but purification of the LA is a challenge too. This work focuses on the separation of LA from the complex aqueous mixtures by liquid-liquid extraction. Two aqueous product feeds were considered, reflecting two different processes. One aqueous product stream contains sulphuric acid and LA, while the second product stream also contains formic FA and furfural. Furfural could be removed selectively via liquid-liquid extraction with toluene. For selective extraction of LA and FA without co-extracting sulphuric acid, 30 wt.% of trioctylphosphine oxide (TOPO) in methylisobutylketone (MIBK) was found most suitable, showing a high selectivity over sulphuric acid, and a high equilibrium partitioning of LA. When instead of MIBK, 1-octanol was applied as diluent, the co-extraction of FA was enhanced, while hexanoic acid suppressed the acid extraction. To obtain the LA pure, eventually a distillation is required, and the potential of temperature swing back extraction (TSBE) at 90°C to pre-concentrate the acid solutions was evaluated for 30 wt.% TOPO in MIBK. This pre-concentration step increased the concentrations of LA and FA by a factor of 2.45 and 2.45 respectively, reducing the distillation reboiler duty from roughly 31.5 to 11.3 GJ per ton LA, at a cost of roughly 4.5 GJ heating duty per ton produced LA.
Ó 2017 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).
1. Introduction
Levulinic acid (LA) is widely described as one of the high poten-tial platform chemicals which can be derived from lignocellulosic materials[1]. The predominant conversion route in production of LA starts with hydrolysis of the polymeric lignocellulosic materials into the hexose- and pentose monomers under acidic conditions. LA is formed via an acid-catalyzed reaction of the hexoses via the intermediate 5-hydroxymethylfurfural (5-HMF). Formic acid (FA) is obtained as an unavoidable side product. In parallel, the pen-toses are converted into a second side product, furfural. The total yield of useful products is reduced by the formation of black insol-uble particles, called humins, via a polymerization reaction [2].
This unwanted by-product can be removed together with the unre-acted lignocellulosic materials by in situ filtration of the reaction mixture[3]. Various acidic materials have been reported to facili-tate the reaction to LA. Usually homogeneous acid catalysts, such as inorganic acids[4–7], enzymes[8], acidic functionalized ionic liquids[9], organic acids[5]and metal salts of such acids[8]are considered. Besides those, heterogeneous acid catalysts i.e. ion exchange resins, were mentioned too[8,10]. Despite purification complications, inorganic acids are most often used in the conver-sion to LA, because very long reaction times are required to obtain similar yield when using a heterogeneous catalyst[11].
The homogeneous mineral acids hydrochloric acid and sul-phuric acid can be used both, with sulsul-phuric acid as preferred acid [12], due to the potential risk of releasing chlorine in the biosphere when using hydrochloric acid[11].
Research papers and patents focus mainly on the description of the pre-fractionation[4,5,13–15]or conversion[6,7,16–22]of var-ious types of feedstocks to LA. In acid-catalyzed reactions of ligno-cellulosic materials to LA, two approaches are commonly applied http://dx.doi.org/10.1016/j.seppur.2017.05.036
1383-5866/Ó 2017 The Authors. Published by Elsevier B.V.
This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Abbreviations: 5-HMF, 5-hydroxymethylfurfural; HPLC, High Pressure Liquid
Chromatography; KF, Karl Fischer; DIPE, diisopropylether; TOA, trioctylamine; TOPO, trioctylphosphine oxide; TB, 4-tert-butylbenzenediol; TSBE, temperature swing back extraction; GJ, gigajoule.
⇑Corresponding author.
E-mail address:b.schuur@utwente.nl(B. Schuur).
Contents lists available atScienceDirect
Separation and Purification Technology
using strong inorganic acids, i.e. using high acid concentrations at mild temperatures (55–60 wt.% hydrochloric acid or 31–70 wt.% sulphuric acid at 20–50°C), or diluted acids (<10 wt.%) at high tem-peratures of 170–240°C [3,12,16,20,23–32]. The high reaction temperature and diluted catalyst option is most often applied [6,7,12,16,20,22,29,32]. This prevents the use of expensive equip-ment which can withstand the high acid concentration, which is considered beneficial over the advantage of high acid concentra-tions that according to Mullen et al.[6]Reunanen et al.[12]and Cuzens et al.[24]result in less char formation, a higher LA yield and fewer by-products at lower temperatures. .
After the conversion step, producing the LA, a complex aqueous product mixture is obtained, which needs further fractionation. Although the LA concentration in the mixture is strongly depen-dent on the type of lignocellulosic material used and the reaction conditions applied, often 3–8 wt.% has been found, for both FA
and furfural a concentration of 1–5 wt.% was found
[7,20,22,24,31,32].
For dilute high boilers such as LA, due to the low concentration of products in the aqueous stream, direct distillation is economi-cally less favorable, and liquid-liquid extraction has been men-tioned as potential technique for fractionating the product mixture[7,32].
Traditionally high molecular weight aliphatic amines [21,33– 36]and organophosphorus extractants[12,37]have been used to extract carboxylic acids from diluted aqueous streams. In addition a large variety of physical solvents have been suggested such as alkylphenols [6,17], ketones [6], alcohols [6], fatty acids [6,12], esters[12], ethers[6,20], and halogenated hydrocarbons[6]. Very recently, a study has been reported using octanol, MIBK and fur-fural as physical solvents, and the heat duty to recover the LA from these solvents was compared[38]. Recently new extractants have been designed to further improve the extraction of carboxylic acids, such as aromatic amines[39]and ionic liquids[40–43]. Rey-hanitash et al.[44]recently showed that both aliphatic amines and ionic liquids cause significant co-extraction of sulphuric acid, mak-ing them less suited for selective extraction of LA due to the pres-ence of sulphuric acid as catalyst. Since the raffinate of the acid extraction is recycled back to the reaction step, and the product should not contain sulphuric acid, any sulphuric acid co-extraction results in the necessity of an additional catalyst recov-ery step.
It is thus important to find a solvent that shows a high selectiv-ity for LA over sulphuric acid, and here, we report a solvent screen-ing study focusscreen-ing on the acid fractionation, which is a key step in the process allowing product isolation without loss of catalyst. Pos-sibly, the acid fractionation is preceded by a furfural extraction sec-tion (but this might also be done afterwards). In the complete fractionation scheme, as shown in Fig. 1, these are the second and third sections. After the physical solvent screening for furfural removal, the composite solvent screening study was performed in two stages, in the first stage only sulphuric acid and LA were pre-sent in the mixtures, while in the second stage the more complex mixtures were applied that also contain FA and furfural.
Next to selectivity and distribution in the extraction process, any economically feasible process should exhibit good recyclability of the solvent. Next to the solvent screening, also recyclability by back-extraction at elevated temperature was investigated.
2. Experimental 2.1. Chemicals
All chemicals were purchased by Sigma Aldrich and used as received without further treatment, i.e. levulinic acid (98%),
1-octanol (99%), formic acid (95%), heptane (99%), 4-tert-butylcatechol (98.0%), 4-methyl-2-pentanone (98.5%), dode-cane (99%), sulphuric acid (95.0–98.0%), toluene (99.9%), diisopropyl ether (98.5%), furfural (99%), 1-pentanol (99%), 1-butanol (99.7%), 1-hexanol (99%), hexanoic acid (98.0%), trioctylamine (99.6%) and trioctylphosphine oxide (99%).
2.2. Liquid – liquid extraction experiments
Liquid-liquid extraction experiments were conducted in 10 mL glass vials. An analytical balance was used to weigh all compounds in both phases with an accuracy of 0.5 mg. The biphasic systems in the vials was shaken rigorously using a mechanical mixer and con-secutively shaken in a shaking bath for at least 20 h at a constant temperature. All the experiments have been performed at 25 ± 0.02°C, with the exception of experiments where the temper-ature is indicated otherwise. The mass of the aqueous phase was kept constant at 4 g with a solvent to feed ratio of 1 (mass based). The aqueous phase contained 8 wt.% LA and 10 wt.% sulphuric acid, and in case the more complicated feed was used, also 5 wt.% FA and 5 wt.% furfural was added to the aqueous phase. These concentra-tions reflect achievable concentraconcentra-tions for the acid catalyzed con-version in various processes with various lignocellulosic feedstocks.
2.3. Analytical procedures
2.3.1. High Pressure Liquid Chromatography (HPLC)
The aqueous phase concentrations of LA, FA, furfural and sul-phuric acid were determined with HPLC using an Agilent 1200 ser-ies apparatus, equipped with a Hi-Plex-H column (300 7.7 mm) and a refractive index detector (RID). 5 mM aqueous sulphuric acid solution was applied as eluent at a flow rate of 0.6 mL/min. The injection volume was 10
l
L and the column oven was isothermally operated at 65°C.2.3.2. Karl Fischer Titration (KFT)
The water content of all the organic phases was analyzed with Karl Fischer Titration using a 787 KF Titrino 730 TiStand of Metrohm, each sample was analyzed in duplo. A methanol/dichloromethane (volumetric ratio of 3:1) solution and HYDRANALÒwere used as titrant. A standard deviation over all measurements was deter-mined to be 0.19 vol.%.
2.4. Definitions
The reported distribution coefficients are defined on molar basis: KD;i¼½i½io aq mole=L mole=L ð1Þ
where i represents LA, FA, sulphuric acid or furfural. The organic phase concentrations were determined using the mass balance, and taking into account the mass of water leached to the organic phase and the mass of solvent leached to the aqueous phase. 3. Results and discussion
Eight physical solvents were evaluated, both as monomolecular solvent, and as diluent when in a composite solvent with an extrac-tant. In addition the process side product, furfural, was evaluated as described by Nhien et al.[38]. Though in presence of sulphuric acid, furfural was found to be unstable and formed humins.
3.1. Physical solvent evaluation
A series of 8 different physical solvents was evaluated for extracting the complex mixture containing all four components LA, FA, furfural and sulphuric acid. The obtained distribution coef-ficients for LA, FA, furfural and sulphuric acid are presented in
Fig. 2, whereas the mutual solubility’s of these physical solvents and water are given inFig. 3.
As can be seen fromFig. 2, dodecane and toluene show only affinity towards furfural, although the distribution in dodecane is low. In addition furfural has the highest distribution coefficient with respect to the other compounds in all physical solvents. This
Fig. 2. The distribution coefficient of LA, FA, furfural and sulphuric acid at 25°C for
8 different physical solvents. Fig. 3. Mutual solubility of water and organic solvents at 25°C. Fig. 1. The conceptual design of the process which separates LA, FA and furfural from an acidic aqueous solution. In the acid section, LA and FA are extracted, and sulphuric acid (the catalyst), is fully recovered and recycled to the reaction section.
can be attributed to the high activity coefficient of furfural in water [45], making the extraction of furfural from the aqueous phase into an apolar phase highly energetically favorable. For the alcohols, it was found that the extraction of LA and FA increases as the molec-ular size of the alcohol decreases. This trend can be assigned to the increasing relative amount of functional hydroxyl groups in the organic phase with decreasing molecular size of the alcohols. The alcohols are thus less suitable for selective removal of furfural from the mixture. Sulphuric acid extraction is negligible in all investigated physical solvents, which is due to the unfavorable solvation of the dissociated sulphuric acid in the hydrophobic environments.
Next to toluene, also MIBK showed good furfural extraction capacity, however, as follows fromFig. 3, MIBK leaches signifi-cantly. Therefore the toluene may be considered most suitable for the furfural removal prior to acid fractionation. Toluene was also described in the patent by De Jong et al. for furfural removal [46].
3.2. Composite solvent evaluation
The extraction capacity of composite solvents can benefit from complexation interactions between the solute and the extractant. In order to investigate whether a beneficial interaction for LA is possible, without too strong co-extraction of sulphuric acid, three different extractants, trioctylamine (TOA), trioctylphosphine oxide (TOPO) and 4-tert-butylbenzenediol (TB), have been evaluated in combination with several diluents. TOA and TOPO are well known extractants for acid extractions[12,33,37], while also alkylphenols are known to extract LA [17]. The LA distribution into TB is expected to benefit from TB’s two hydroxyl groups.
3.2.1. Simple mixtures with only LA and sulphuric acid
The experimental results concerning the extraction of LA and sulphuric acid with TOA, TOPO and TB in various diluents are pre-sented inFig. 4.
As can be seen in Fig. 4a and b, the extraction of LA and sul-phuric acid increases as function of the TOA concentration, except when hexanoic acid or 1-octanol are used as diluents. At 30 wt.% of TOA, the molar distribution coefficient of TOA for sulphuric acid was observed to be 0.77–0.84 for all diluents, except for hexanoic acid, for which the capacity at 30 wt.% was only 0.47, indicating hexanoic acid is counteracting the complexation of TOA with sul-phuric acid. 1-Octanol as diluent also causes a slight decrease in the distribution coefficient of LA as function of the TOA concentra-tion, though the molar capacity is not decreased relative to the other diluents.
Due to the low pH of the aqueous phase, LA is present in its undissociated form. Therefore it is likely that the extraction is done via H-bonding and solvation[47]. Senol et al.[47]also explain the high distribution coefficient of LA when using toluene as diluent by
p
-system–carboxylic group stabilization interaction during the complexation stage. In the case of hexanoic acid as diluent, adducts are potentially formed between the hexanoic acid and TOA, which can decrease the capability for interactions with the sulphuric acid, as is supported by the experimental observations.The significant extraction of dissociated sulphuric acid can be explained by ion pair formation with TOA. The highest selectivity of 4.4 was obtained with hexanoic acid and 10 wt.% of TOA. At 30 wt.% of TOA independent of the diluent used, the selectivity drops to approximately 0.5.
A negligible amount of sulphuric acid was observed in the extract when using TOPO as an extractant, as can be seen inFig. 4d. This was also observed by Shuyun et al.[48] when using TOPO diluted in heptane. The lack of sulphuric acid extraction may be explained by the fact that TOPO is a solvatizing carrier[49], instead
of an anion-active carrier such as high aliphatic amines. Solvatizing carriers form non-stoichiometric complexes with neutral solutes, thereby making sulphuric acid extraction unlikely. The solvation mechanism, extraction without the formation of complexes, will also be present.
InFig. 4f, it can be seen that also a negligible amount of sul-phuric acid was found in the extract when TB was used as an extractant, which is comparable to the extraction with TOPO in Fig. 4d. In contrary to the results for TOPO and TOA, hexanoic acid as diluent results in the highest distribution coefficient for LA, as can be seen inFig. 4e. This may be explained by the absence of a complexation interaction and solely the solvation mechanism was predominant. The dimer formation between hexanoic acid, LA and TB were not disrupting complexation interactions, but were possibly enhancing the solvation mechanism.
Due to the low selectivity arising from the preferential tion of sulphuric acid. TOA was considered an unsuitable extrac-tant to isolate LA from an acidic aqueous mixture. For this reason, TOA was not evaluated in the following experiments where additionally FA and furfural was added to the complex acidic aque-ous mixture.
3.2.2. Complex mixtures containing LA, FA, furfural and sulphuric acid 3.2.2.1. Trioctylphosphine oxide (TOPO). The experimental results concerning the extraction of LA, FA, furfural and sulphuric acid with TOPO in various diluents are presented inFig. 5.
In Fig. 5a an increasing distribution coefficient of LA with increasing TOPO concentration can be observed for MIBK and toluene, whereas for hexanoic acid the LA distribution coefficient drops with increasing TOPO concentration, and for 1-octanol as diluent the distribution coefficient is constant. In Fig. 5b, it can be seen that with 1-octanol as a diluent also the formic acid distri-bution coefficient is more or less stable, at values just over 2. With hexanoic acid as diluent, FA extraction was not very significant, while for toluene and MIBK an increasing distribution with increasing TOPO concentration was measured, similar to LA. In Fig. 5c significant distribution coefficients for furfural can be observed, which coincides with statements made during the phys-ical solvent screening. It was however observed that the selectivity between FA and LA can be significantly influenced through selec-tion of the appropriate diluent. Sulfuric acid extracselec-tion is negligible for all diluents with TOPO.
3.2.2.2. 4-tert-butylbenzenediol (TB). The experimental results con-cerning the extraction of LA, FA, furfural and sulphuric acid with TB in various diluents are presented inFig. 6. There were no exper-iments done using toluene as diluent, because the solubility of TB in toluene was found to be very low.
It follows from Fig. 6a that the distribution coefficient of LA increases with hexanoic acid as diluent up to a value of about 1.2, i.e. higher than the highest value observed with TOPO. How-ever, FA is not extracted very well using hexanoic acid as diluent, as follows fromFig. 6b. In all cases, just as with TOPO, the furfural has a higher distribution coefficient than the acids, and will be co-extracted when it is not removed prior. Based on the extraction results at 25°C, TOPO with toluene and with MIBK, as well as TB in hexanoic acid were selected for studies on temperature swing back extraction.
3.3. Temperature swing back extraction equilibrium
The solvent screening in the previous sections confirmed the possibility of selectively removing furfural from an aqueous mix-ture containing LA, FA and sulphuric acid with toluene, followed by selective extraction of the carboxylic acids FA and LA with
posite solvents containing TB and TOPO as extractant without the co-extraction of sulphuric acid.
In order to obtain a pure LA product, the carboxylic acids need to be recovered from the solvent, and fractionated. For this solvent regeneration and acid fractionation, either a direct distillation from
the solvent could be applied, or a back-extraction followed with a distillation from an aqueous stream. For the last option, tempera-ture swing back extraction (TSBE) was attempted. Back extraction experiments at 50°C, 75 °C and 90 °C, seen in Fig. 7, showed a decreasing distribution coefficient of LA with increasing
Fig. 5. The distribution coefficient of LA (A), FA (B), furfural (C) and sulphuric acid (D) at 25°C for 4 different diluents containing TOPO.
Fig. 6. The distribution coefficient of LA, FA, furfural and sulphuric acid at 25°C for 3 different diluents containing TB.
ture for 30 wt.% of TOPO in MIBK and toluene, however no signifi-cant temperature dependency was observed using 30 wt.% of TB in hexanoic acid.
The absence of a temperature dependency in the mixture con-taining TB limits the applicability of TSBE to zero, and this solvent can thus not be used for this application. The composite solvent 30 wt.% TOPO in MIBK was found to obtain the highest distribution coefficient for LA and in addition showed a strong temperature dependency, it was therefore chosen as most appropriate solvent for this recovery strategy, and the TSBE was studied in more detail for this solvent.
The molar concentrations of LA and FA are related to the molar ratio units which are used in the McCabe-Thiele method[50];
XLA¼ LA ½ aq 1 LA½ aq ; XFA¼ FA ½ aq 1 FA½ aq YLA¼ LA ½ o 1 LA½ o ; YFA¼ FA ½ o 1 FA½ o ð2Þ
where X is the molar ratio in the aqueous phase, Y is the molar ratio in the organic phase and the concentrations of the acids are in weight fractions. Because the acid concentrations are relatively low, in Eq.(2)only the acids themselves and the solvent are consid-ered. The equilibrium isotherms of LA and FA have been determined at 25°C and 90 °C and are shown in Fig. 8. Three regions of LA extraction behavior were observed, as can be seen inFig. 8a. Region I is at low concentrations of LA, where a significant fraction of LA is likely in the dissociated form, and at increasing acid concentration, more of the weak acid will be in the undissociated form. This buffer-ing equilibrium causes the line to flatten off in the region II, where the undissociated LA becomes predominant. In region III an over-load of LA in the organic phase is seen. The shape of the LA iso-therms are consistent with a type II adsorption isotherm described by Brunauer[51]and is associated with multimolecular BET adsorption. [52] Due to the complex nature of the BET isotherms and the relative small amount of data points. The isotherms are fitted via a 5th degree polynomial function (see Tables 1 and 2):
YLA¼ a5X5LAþ a4X4LAþ a3X3LAþ a2X2LAþ a1XLAþ b ð3Þ
Tamada et al.[53]also observed Type II isotherms of carboxylic acids in amine extractants and assigned the overload to higher order stoichiometric complexations between the carboxylic acid and amine extractant. TOPO most likely forms neutral
non-stoichiometric complexes with LA. Hence, a specific stoi-chiometry could not be coupled to these isotherms. The FA behav-ior could be described by a Freundlich isotherm[54], as can be seen inFig. 8b: YFA¼ K X 1 n FA ð4Þ 3.4. Process design
A schematic overview of a Temperature Swing Bach Extraction (TSBE) is given inFig. 9. Two Counter-Current Extraction column are used, where a solvent is circulated between both columns. The feed of the process was defined to contain; 8.4 wt.% LA, 5.3 wt.% FA and 10.5 wt.% sulphuric acid in water. It has been assumed that all furfural is already removed via extraction with toluene, seeFig. 1. As can be seen inFig. 9, there are four operation points that can be distinguished, A1, A2, B1and B2.
Under the condition of minimal solvent to feed ratio, point A1
describes the equilibrium of the aqueous feed stream with the exit-ing organic stream, the extract. To really reflect a minimum solvent to feed ratio, A2for the exiting aqueous stream (the raffinate) is
also drawn on the equilibrium line. The raffinate is in equilibrium with the organic stream coming from the back extraction column. Both equilibria are in the forward extraction column at 25°C. The Point B1 represents the point where the loaded organic stream
enters the back-extraction. The organic phase composition is thus equal to that of A1, and the aqueous phase composition is selected
such that the operating line of the back extractor does not cross the equilibrium line at 90°C. At minimal Wash to Extract ratio, point B1indicates equilibrium between the water wash stream leaving
the back extraction column and the organic stream entering. The organic phase composition of point B2 in the back-extractor is
linked to the composition of the raffinate in the extractor, since the solvent is cycled back to the extraction with that composition. The maximum driving force for washing the acids out of the sol-vents is obtained when clean wash liquor is entering the back-extractor, hence, X(B1) = 0.
The following procedure is applied to design the TSBE opera-tion, seeFig. 10;
(1) Point A1is set on the 25°C equilibrium line, at the known
acid feed concentration.
(2) A2is an operator choice which is on the 25°C equilibrium
line. The choice of A2 may be limited by the 90°C back
extraction equilibrium. By choosing A2, also the B2operation
point will be defined. Any acids left in the raffinate will not be lost, but may be recycled back in the process, seeFig. 1, thus depending on the back-extraction considerations, the choice for a relatively high raffinate concentration that may be necessary for technical feasibility is not necessarily detrimental for the process feasibility.
(3) Point B1is again an operator choice. An aqueous
concentra-tion as high as possible was chosen here, so the highest obtainable acid concentration in the product stream was obtained.
(4) From point B1to B2the operation line of the back extraction
will hit the equilibrium line of 90°C at a minimum wash to extract ratio, (W/E)min.
The solvent to feed ratio, (S/F), and the wash to extract ratio, (W/E), can be determined from operational lines resp. A1-A2and
B1-B2by the following equation[50]; Yout¼ F S Xoutþ Yin F S Xin ð5Þ Fig. 7. The temperature dependency of three composite solvents; TOPO (30 wt.%) in
MIBK, TOPO (30 wt.%) in toluene and TB (30 wt.%) in hexanoic acid. The deviation in the distribution coefficient is ± 10%.
From the operating equation, see Eq. (4), the solvent to feed ratio, or wash to extract ratio, can be determined by taking the inverse of the slope. If the operating line simulate a limited case, than the minimum solvent to feed ratio, (S/F)min, or minimum
wash to extract ratio, (W/E)min, can be obtained.
Fig. 10simulated a TSBE operation wherein a minimum solvent to feed ratio, (S/F)min= 0.78, and a minimum wash to extract ratio,
(W/E)min= 0.39, is used. Though each isotherm has been measured
independently, the co-extraction of both acids is simulated by
coupling both McCabe-Thiele approaches seen in Fig. 10a and b. The coupling is done by keeping the slopes of the operation lines between A1and A2, and B1and B2, equal for both acids. By keeping
the slopes equal in both approaches, the same extraction operation is simulated simultaneously, thereby reflecting the co-extraction of LA and FA. Resulting from the TSBE operation, two aqueous raffi-nate streams are obtained starting with the feed concentration, F. R1is the recycle stream which is lean on LA and FA, while R2 is
the aqueous stream where LA and FA are concentrated. The extrac-tion of LA was observed to be the limiting factor, due to the fact that during the forward extraction FA cannot achieve equilibrium at 25°C due to the equilibrium line of the back extraction at 90°C. The result of the TSBE operation is that LA and FA are con-centrated with a factor of resp. 1.69 and 1.58.
An increase in solvent to feed ratio or wash to extract ratio, causes the operational lines to become less steep. If the acid feed concentration is set and both operational lines become less steep,
Fig. 8. The isotherm extraction profiles of LA and FA at 25°C and 90 °C between water and the composite solvent TOPO (30 wt.%) in MIBK.
Table 1
The fitting parameters of the LA isotherms at 25°C and 90 °C for a 5th degree polynomial function. The error in YLAis given as twice the standard deviation, e.g. a 95% confidence
interval.
a5 a4 a3 a2 a1 b error
25°C 389.6 365.3 134.8 24.00 2.297 7.992 103 3.001 103
90°C 207.1 174.5 60.43 11.14 1.382 4.181 103 3.292 104
Table 2
The fitting parameters of the FA Freundlich isotherms at 25°C and 90 °C. The error in YFAis given as twice the standard deviation, e.g. a 95% confidence interval.
K n error
25°C 0.1329 3.202 5.997 103
90°C 0.1105 2.775 3.425 103
Fig. 9. A temperature swing back extraction (TSBE) Operation where the forward extraction is performed with 30 wt.% TOPO in MIBK at 25°C and the back extraction is performed with water at 90°C.
the raffinate, A1, must be become richer in acid. Also the product,
B1, must become leaner in acid in order to keep the operational
lines within the border of the equilibrium lines. For this reason it would be most beneficial to operate the TSBE operation as to the (S/F)minand (W/E)minas possible.
Until now, a single TSBE operation was simulated. This method can be repeated to concentrate the acid to a further extent, hence a double TSBE is obtained inFig. 11. The aqueous concentration of R2
from the initial TSBE operation is equal to the aqueous feed con-centration of the second TSBE operation.
By applying a second TSBE operation, LA and FA are additionally concentrated with a factor of resp. 1.45 and 1.55. Combining both TSBE operations the LA and FA concentration are concentrated with a factor of resp. 2.45 and 2.45. During both TSBE operations an aqueous raffinate stream was obtained lean on LA and FA. Though, both raffinate streams decreases the total amount of LA and FA in the product stream, these raffinate streams may be recy-cled to the reaction section, seeFig. 1, preventing any loss of LA and FA.
A rough estimation of the heater duty for pre-dehydrating the product stream via a double TSBE is made in AspenPlus. Heating the organic phase and the water wash stream to 90°C will be the sources of the heater duty. In addition a heat exchanger is present to minimize the energy requirement. A minimum temperature dif-ference of 10°C over the heat exchanger over the cold outlet and
hot inlet stream was assumed. Roughly a heater duty of 4.5 GJ per produced tons of LA was necessary to concentrate LA and FA with a factor of resp. 2.45 and 2.45. Still, distillation has to be done to isolate LA from the aqueous FA mixture. Via the heat of vapor-ization of FA and water, this evaporation operation was estimated to require roughly 11.3 GJ per tons of produced LA on heater duty. Without the double TSBE operation, this evaporation would have required roughly 31.5 GJ per tons of produced LA on heater duty. 4. Conclusions
From an exploratory solvent study selective extraction of LA from an aqueous solution containing solely sulphuric acid was observed using polar solvents. In addition TOA was deemed unsuit-able due to the significant extraction of sulphuric acid. The possi-bility of selective removal of furfural using apolar solvents, in particular toluene was likewise observed. MIBK may also be used to extract furfural, though a lower selectivity to furfural is observed. Both TB as TOPO were suitable extractants for the selec-tive extraction of both LA and FA. It was observed that 1-octanol can enhance the FA extraction, while hexanoic acid suppresses this extraction. A double TSBE was performed to concentrate LA and FA with a factor of resp. 2.45 and 2.45 using a heater duty of 4.5 GJ per produced tons of LA. The heater duty of the final distillation col-umn drops due to the TSBE operations from approximately 31.5
Fig. 11. McCable-Thiele graphic approach to simulate a second temperature swing back extraction between 25°C and 90 °C of (a) LA and (b) FA. From the operational lines follow a (S/F)minand (W/E)minof resp. 0.48 and 0.28.
Fig. 10. McCable-Thiele graphic approach to simulate a temperature swing back extraction between 25°C and 90 °C of (a) LA and (b) FA. From the operational lines follow a (S/F)minand (W/E)minof resp. 0.78 and 0.39.
to 11.3 GJ per produced tons of LA. These rough calculations show the potential energy savings by pre-dehydrating an aqueous feed via TSBE operations.
Raising the environmental and toxicity issue of using toluene for the extraction of furfural. An alternative multistage fractional extraction column could also be designed to fractionate LA, FA and furfural from an aqueous solution containing sulphuric acid by the same composite solvent as in the LA and FA extraction, 30 wt.% TOPO in MIBK.
Acknowledgements
This has been an ISPT project. ISPT is Institute for Sustainable
Process Technology, headquartered in Amersfoort, the
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