Andreas
Toth
*,
Susanne
Lux
,
Daniela
Painer
and
Matthäus
Siebenhofer
Graz University of Technology, Institute of Chemical Engineering and Environmental Technology, NAWI Graz Central Lab Biobased Products, Inffeldgasse 25c/III, 8010 Graz, Austria. E-mail: andreas.toth@tugraz.at; Tel: +43 (0)316 873 7460
First published on 8th October 2018
A concept for isolation of low molecular weight carboxylic acids (e.g., acetic acid) from dilute aqueous streams was developed. This concept of combining chemical conversion of the carboxylic acids with in situ liquid–liquid extraction enhanced by catalysis and emulsification was proven applicable for carboxylic acid concentration of 1 mol l−1. Chemical conversion was achieved by esterification with 1-octanol, catalysed by the surfactant 4-dodecylbenzenesulfonic acid. Emulsification induced by the catalyst was confirmed to be essential for high conversion and separation efficiency. Investigations were supported and evaluated by design of experiments and yielded conversions beyond 54.3% and separation efficiencies beyond 57.5% for acetic acid. Evaluation of process parameters yielded a quadratic model for prediction of process performance. Applicability of the concept for formic acid, propionic acid and butyric acid isolation from aqueous feed was confirmed.
A compilation of separation technologies for isolation of carboxylic acids from dilute aqueous streams is given by Talnikar et al.,4 including reactive separations. The latter have shown great potential for dealing with dilute aqueous streams e.g., reactive distillation for isolation of acetic acid or formic acid.2,5,6 While these processes work well for a concentration of 30 wt% carboxylic acid in the feed, for lower concentrations (6 wt% and below) the only industrial applied technology is reactive extraction.7 This technology is based on adduct formation of the carboxylic acid with a reactive extractant and requires complex solvent regeneration. Sustainability and application range of such processes are critically lowered due to the preferred use of organophosphorous compounds and various aliphatic amines as reactive extractants.4,7 Some of the reactive extraction processes reviewed by Talnikar et al.4 require residence times of up to 24 h, hindering an industrial implementation.
In order to provide a simple, yet efficient alternative technology for isolation of low molecular weight carboxylic acids, the present work presents the concept of combining the two main benefits of reactive distillation and reactive extraction. These are the (catalysed) chemical conversion of reactive distillation and the moderate, easy-to-implement process of liquid–liquid extraction. In this context, chemical conversion does not refer to adduct formation in the sense of conventional reactive extraction, but rather to the transformation of the constituent to a product (e.g., esterification of carboxylic acids). For esterification, low concentration of reactants and a high excess of water inhibit the reaction rate and shift the reaction equilibrium composition to the reactant side. Nevertheless, high conversion rates may be achieved by utilizing surfactant catalysts like 4-dodecylbenzenesulfonic acid (4DBSA). The catalyst activity and its influence on the reaction was stated by Manabe et al.,8 who achieved high yields in dehydrative esterification of lauric acid with 3-phenyl-1-propanol in water. Emulsification induced by surfactant catalysts like 4DBSA has been reported to be beneficial for etherification reactions in water9 and various organic reactions in aqueous and biphasic media.8,10–12 Although the benefits of emulsification are highlighted e.g., for dehydration reactions,8 the work of Hohl et al.12 indicates the complexity of emulsified multiphase systems. Thorough evaluation of process parameter influence on emulsification and process performance is essential for applicability and modeling of the concept.
The proposed concept of combining extraction with chemical conversion targets the limitation of the state of the art separation processes by using a reactive, catalytically active solvent phase. For this purpose, the solvent phase (e.g., n-undecane) contains a higher aliphatic alcohol for esterification (e.g., 1-octanol) and a surfactant catalyst (e.g., 4-dodecylbenzenesulfonic acid) for enhancing reaction rate and mass transfer. While esterification of higher molecular weight carboxylic acids is investigated more frequently, less effort has so far been spent on low molecular weight carboxylic acids, which occur more frequently in biobased effluents. The concept was thus applied for acetic acid isolation (1 mol L−1) and extended to other low molecular weight carboxylic acids. In order to meet requirements of efficient and environmentally sound separation processes, the proposed concept aims at meeting the “24 principles of green engineering and green chemistry” summarised by Tang et al.13 and the CHEM21 selection guide for solvents by Prat et al.14
Water was filled into the batch reactor together with the solvent phase and heated up to the desired operation temperature under constant stirring. The solvent phase consisted of 1-octanol as reactant, 4-dodecylbenzenesulfonic acid (4DBSA) as catalyst and n-undecane as diluent. After reaching the reaction temperature, admixing the carboxylic acid initiated the reaction. The experiments were stopped after an arbitrarily chosen reaction time of 3 h. At this time, the stirrer was stopped and after primary phase separation of 3 min samples of 10 ml were taken from the top and the bottom of the reaction mixture with a syringe. These samples were centrifuged for 10 min to generate enough clear sample for GC analysis to avoid distorting process performance by uncontrolled reaction progress. For the mass balance, the remaining reaction mass was split by centrifugation for 30 min to achieve adequate phase separation with minimum entrainment of the two phases into each other. A Heraeus Labofuge 400 benchtop centrifuge was used for emulsion splitting at rcf 2383.
Reaction progress of the reference experiments with formic acid, propionic acid and butyric acid as well as 4DBSA and hydrochloric acid was determined with a SI Analytics TitroLine 7800 autotitrator. A potassium hydroxide solution (0.1 mol L−1) was used for acid/base titration. The solvent phase was dissolved in a mixture of 2-propanol and deionized water prior to titration.
Process parameter | Min | Max | Unit |
---|---|---|---|
Temperature (T) | 25 | 60 | °C |
Stoichiometric ratio (OH:H) | 0.8 | 1.5 | molOcOH molA,0−1 |
Phase ratio (S:A) | 0.5 | 2.0 | — |
Catalyst load (CAT) | 0.015 | 0.060 | eq. molA,0−1 |
Stirrer speed (n) | 300 | 450 | rpm |
Two characteristic numbers were defined as response factors to enable comparison and evaluation of the single experiments. The first is the overall conversion of carboxylic acid as defined in eqn (1), which allows assessment of the reaction kinetics. In this equation, XA is the overall conversion, mA,0 the initial mass of carboxylic acid and mA,sol and mA,aqu the mass of carboxylic acid at the end of the experiment in the solvent and in the aqueous phase, respectively.
(1) |
The second characteristic number is the separation efficiency SEA based on the carboxylic acid (defined in eqn (2)) providing information on total carboxylic acid removal from the aqueous phase. Eqn (2) represents the separation efficiency SEA, which is defined as the ratio of the mass of the carboxylic acid removed from the aqueous phase to the initial mass of carboxylic acid.
(2) |
The uncatalysed reference experiment yielded a conversion of about 4.6 ± 0.2% and a separation efficiency of 15.9 ± 0.2%. These results were confirmed to be the maxima for the given process conditions as a consequence of the phase and reaction equilibria. As shown in Fig. 1, catalysis with hydrochloric acid and Amberlite® IR120+ yielded 4 ± 0.7% and 7.7 ± 0.8% conversion and 15.6 ± 0.8% and 18.9 ± 1.6% separation efficiency, respectively. For these two catalysts, the slight increase of SEA can be explained by the change of extraction properties of the solvent phase due to reaction progress. This improved reaction progress increases the concentration of ester in the solvent phase. The slightly better performance of the HCl-catalysed experiment compared to the uncatalysed reference is negligible and explained by a higher deviation of the HCl-catalysed results. As a result, the catalysis with a homogeneous catalyst in the aqueous phase has no significant influence on process performance.
Comparing these results with the performance of 4DBSA confirms the positive impact of emulsification on the process. The induced emulsification highly increases the mass transfer area up to quasi-homogeneous state and thus enhances phase contact, overriding mass transfer limitation. As a result, an overall conversion of 54.3 ± 1.9% and a separation efficiency of 57.5 ± 2.2% was achieved with 4DBSA. As low molecular weight carboxylic acids like acetic acid are preferably present in the aqueous phase and the higher alcohol 1-octanol has a much higher solubility in the solvent phase, catalysis at the interphase or in the solvent phase is more efficient. Targeting this necessity much better than HCl and Amberlite® IR120+ (both mainly present in the aqueous phase), the surface active catalyst 4DBSA is assumed to accumulate at the interphase as well as in the solvent phase due to its hydrophobic dodecyl group.15
(3) |
Parameter | X HAc | SEHAc |
---|---|---|
β i | 29.13 ± 0.72 | 34.62 ± 0.82 |
β T | 11.50 ± 0.85 | 10.30 ± 0.96 |
β S:A | 0.50 ± 0.82 | 0.88 ± 0.94 |
β CAT | 4.04 ± 0.81 | 3.47 ± 0.92 |
β OH:H | 3.50 ± 0.88 | 5.06 ± 1.00 |
β n | −1.40 ± 0.82 | −1.43 ± 0.93 |
β T·OH:H | 3.80 ± 0.97 | 4.00 ± 1.08 |
β T·n | −1.18 ± 0.92 | −1.32 ± 1.04 |
β S:A·CAT | 0.91 ± 0.85 | 1.07 ± 0.97 |
β S:A·n | 1.49 ± 0.87 | 1.30 ± 0.99 |
β CAT·OH:H | 1.52 ± 0.92 | Not used |
β OH:H·n | 1.00 ± 0.94 | Not used |
Twelve parameters enable prediction of acetic acid conversion with R2 = 0.984, Q2 = 0.956 and a reproducibility of 0.993. While the high R2 indicates a good fit of the model for the experimental data, Q2 close to 1 confirms a good response prediction within the investigated parameter ranges. Experimental noise level is very low, which is confirmed by the high reproducibility. A graphical comparison of experimental data and predicted conversions and separation efficiencies is shown in Fig. 2 and 3.
Fig. 2 Comparison of experimental conversion XHAc,exp and predicted conversion XHAc,predict based on the DoE-model (R2 = 0.984; Q2 = 0.956; reproducibility = 0.993). |
Fig. 3 Comparison of experimental separation efficiency SEHAc,exp and predicted separation efficiency SEHAc,predict based on the DoE-model (R2 = 0.974; Q2 = 0.931; reproducibility = 0.993). |
Fig. 4 (left diagram) shows the influence of scaled and centered parameters on the conversion of acetic acid. Temperature (T) is the main influence parameter followed by catalyst load (CAT) and stoichiometric ratio (OH:H). Although the impact of the phase ratio (S:A) is not significant in itself, its interaction with catalyst load and stirrer speed (n) influences the overall conversion. The only parameters causing a retarding effect on the overall conversion are an increased stirrer speed and the interaction of stirrer speed and temperature. This may be explained by a shift of the drop size distribution towards smaller droplet diameters. The surface of small droplets becomes rigid due to the surfactant molecules and thus hinders mass transfer.15
(4) |
Similar results were found for the separation efficiency, which is influenced by 9 parameters as shown in eqn 4 and Table 2. In contrast to the overall conversion, the interaction between catalyst load and stoichiometric ratio (βCAT·OH:H) and stoichiometric ratio and stirrer speed (βOH:H·n) has no significant impact on SEA. Nevertheless, the model predicts the separation efficiency as displayed in the diagram in Fig. 3 very well. This is confirmed by a R2 of 0.974, a Q2 of 0.931 and a reproducibility of 0.993.
The right-hand side diagram in Fig. 4 gives an overview of the parameter influence on the separation efficiency including the 95%-confidence intervals. Reaction temperature is still the most important parameter, but compared to the overall conversion it has slightly less impact on separation efficiency. By contrast, the influence of the stoichiometric ratio is increased for the separation efficiency, as it corresponds with the composition and therefore the extraction capacity of the solvent phase. The limiting effect of an increased stirrer speed is observed in the same magnitude as for overall conversion.
A second effect is the enhanced mass transfer due to a high catalyst load which is best observed in plot C1 of Fig. 5. With a CATmax of 0.06 eq. molA,0−1 even at a stoichiometric ratio of 0.8 and nearly ambient temperature separation efficiencies in the range of 35% are possible at a stirrer speed of 475 rpm. The latter leads to the third effect: decreased separation efficiency for high stirrer speeds. This effect is assumed to be caused by too small droplets with highly rigid surfaces due to the surfactant (catalyst) molecules. Such rigid surfaces increasingly hinder mass transfer, hence limiting the separation efficiency and boosting unwanted emulsion stability.
Additionally enhancing the extraction would shift the equilibrium composition to the product side and eliminate mass transfer limitation. Thus, a multi-step process similar to a mixer–settler cascade in counter-current operation is proposed. The multi-stage process concept maximises extraction driving force and may give access to nearly complete conversion. Finding an optimal temperature range with respect to conversion and reaction time requires further investigation.
Carboxylic acid | logKOW16 | SEi |
---|---|---|
Formic acid | −0.54 | 61.4 ± 2.3% |
Acetic acid | −0.17 | 57.5 ± 2.2% |
Propionic acid | 0.33 | 70.5 ± 2.7% |
Butyric acid | 0.79 | 79.6 ± 0.4% |
A | Acid |
0 | Initial |
sol | Solvent phase |
aqu | Aqueous phase |
exp | Experimental |
predict | Predicted |
HAc | Acetic acid |
OcOH | 1-Octanol |
X | Conversion |
SE | Separation efficiency |
β | Parameter of the models derived by design of experiments |
DoE | Design of experiments |
4DBSA | 4-Dodecylbenzenesulfonic acid |
HCl | Hydrochloric acid |
R 2 | Coefficient of determination |
Q 2 | Percent of variation |
logKOW | n-Octanol/water partition coefficient |
This journal is © The Royal Society of Chemistry 2018 |