Separation of waste-derived volatile fatty acids from fermented wastewater
Ehsan Reyhanitash is a PhD Student in the research group Sustainable Process Technology (SPT). His supervisor is Professor Sascha Kersten from the Faculty of Science and Technology.
The aim of this thesis is to provide a practice to enable separation of volatile fatty acids (VFAs) from fermented wastewater. Two affinity separation techniques, namely liquid-liquid extraction (LLX) and adsorption, were initially proposed as the candidates which were to be assessed and compared. A sample of an actual fermented wastewater was represented by a few model solutions containing various VFAs and salts.
Part 1. Liquid-liquid extraction
The first candidate to examine was LLX. The practice of LLX can be seen as migration of a VFA from its home solution into a solvent, followed by removing it from the solvent to obtain a high purity VFA stream and reuse the solvent. This simple concept is the core of all elaborate LLX-based processes designed to meet various demands imposed by the nature of the carboxylic acid and its home solution.
To cover a wide range of basic and modified LLX-based practices involving carboxylic acids, Chapter 2 provides an overview on a large collection of studies reported as journal articles or patents. A comprehensive set of energy demand calculations provided a tool to assess economic feasibility of a selection of practices. To meet the selection criterion, a LLX-based practice had to be a complete carboxylic acid recovery process introducing a remarkable novelty to facilitate either extraction or solvent regeneration. The initial carboxylic acid concentration of the selected processes was then lowered to 1 wt% to see whether they can separate carboxylic acids from fermented wastewater in an economically desired fashion or not. A final carboxylic acid concentration of 99.7 wt% was set as the target. Most of the processes that were designed for the higher side of the concentration range ceased to be feasible when applied to a feed as dilute as fermented wastewater, due to the energy penalty imposed by evaporation of the extra water. Another outcome of this chapter was that a combined application of a low-boiling organic solvent and a CO2-switchable solvent in a LLX-based practice can efficiently recover carboxylic acids from their dilute aqueous streams.
The ionic liquid [P666,14][Phos] is proven promising for extraction of carboxylic acids in the literature. In Chapter 3, this ionic liquid was used to extract lactic, acetic, propionic and butyric acids from a complex model solution prepared to represent fermented wastewater. The extraction mechanism induced by [P666,14][Phos] proved very similar to that induced by the benchmark molecular solvent trioctylamine (TOA) in n-octanol. Since, for this mechanism to proceed, the existence of molecular carboxylic acids is necessary, lowering the pH of the aqueous solution can significantly improve extraction. Co-extraction of the mineral anions together with the H+ originating from the VFAs as a charge neutral acidic unit reduced the selectivity of the solvents for the VFAs. [P666,14][Phos] exhibited a higher extraction capacity and selectivity than that of the benchmark molecular solvent.
In Chapter 4, acetic acid (HAc), representing VFAs, was extracted from a fermented wastewater model solution. [P666,14][Phos] was superior to the benchmark molecular solvent in terms of extraction capacity and selectivity. Extraction of carboxylic acids is facilitated in the presence of CO2 as a result of acidification of the aqueous solution. The ionic liquid further outperformed the molecular solvent when VFA extraction was boosted by pressurized CO2. The improvement in the extraction capacity of the ionic liquid was higher than what had been predicted suggesting that CO2 had most likely altered some of the physical properties of the ionic liquid. This implies that pressurized CO2 may improve extraction of other compounds by this ionic liquid too.
Chapter 5 explored various techniques to regenerate [P666,14][Phos] and recover the extracted VFAs. HAc represented VFAs for this chapter. Unfortunately, it turned out that stripping with an inert gas at a reduced pressure or reactive regeneration by esterification were not capable of achieving a complete regeneration. Back-extraction of the VFAs from [P666,14][Phos] with a KOH solution was so successful that, for this thesis, it has been the standard laboratory method for determination of the ionic liquid’s VFA loading. The drawback of this method is production of potassium carboxylate salts rather than free VFAs. To maintain a successful back-extraction and obtain free VFAs, the KOH solution was replaced with an aqueous solution of a volatile alkali, namely ammonia or trimethylamine (TMA). Both of these alternatives were capable of performing a successful back-extraction. However, the complexes of ammonia with propionic and butyric acids are irreversibly dehydrated to propionamide and butyramide at the temperatures needed for their decomposition. This made TMA the ultimate choice of volatile alkali for regeneration of the ionic liquid. Since the product stream of decomposition of TMA-VFA complexes is a gaseous mixture of TMA and VFAs, an entrainer has to be introduced to capture the VFAs with no interaction with TMA. This necessitates an extra distillation column to separate the VFAs from the entrainer.
Chapters 3 to 5 describe the effort put to improve extraction of VFAs with an ionic liquid and regenerate the resulting loaded ionic liquid. A remarkable improvement was achieved in extraction especially in the presence of pressurized CO2. However, regeneration of the ionic liquid was challenging, and only a complex procedure of back-extraction with a TMA solution and decomposition of the forming complexes was successful. This procedure adds the following extra unit operations to the rather simple two-column operation.
An extra extraction column for back-extraction
A distillation column for recovering TMA from the ionic liquid
A distillation column for separation of the entrainer from the VFAs
The process is so complicated that it might be practically unfavorable for recovery of VFAs from fermented wastewater. On a bigger picture, it seems that the approach taken for this thesis to investigate the use of an ionic liquid for separation can be improved to enable a future researcher to simultaneously study extraction and solvent regeneration. Chapter 6 provides an alternative approach aiming at a substantial improvement.
Chapter 6 evaluates the experimental approach taken for chapters 3 to 5. An alternative approach for an experimental work involving extraction with an ionic liquid is given. A mathematical demonstration of practicality/impracticality of such an experimental work is provided too. Since an ionic liquid does not have any vapor pressure, regenerating it in a distillation column to achieve a deep recovery of the solute it carries is impractical. Any attempt to perform a deep recovery will express itself as a drastic increase in the temperature of the ionic liquid in the reboiler, and eventually degrade it. The provided mathematical method describes this impracticality as an unreasonably high reboiler temperature, higher than the degradation temperature of the ionic liquid to be regenerated.
Part 2. Adsorption
Adsorption was the second candidate to study. The affinity needed to adsorb VFAs from an aqueous solution is induced by a solid, namely an adsorbent, often immobilized in a column. The following step, referred to as desorption, removes the adsorbed VFAs from the adsorbent and prepares it to be reused. To avoid transportation of the adsorbent, an adsorption-based operation is intrinsically semi-continuous, and performed with multiple columns undergoing adsorption or desorption at the same time.
Chapter 7 explores the principles of adsorption of the VFAs from the complex model solution containing lactic, acetic, propionic and butyric acids and salts. Four adsorbents including a primary, secondary or tertiary amine supported on a crosslinked polystyrene support, and the support itself were tested for their adsorption capacity and selectivity. The support, referred to as the non-functionalized adsorbent, outperformed the others by having no capacity for mineral acid units, which in turn gave it an excellent selectivity for acetic, propionic and butyric acids. The mineral acid units formed as pairs of the salt-originating anions and the VFA-originating H+. Further adsorption and desorption experiments in a column proved that the non-functionalized adsorbent was very stable. Temperature-profiled desorption resulted in formation of three condensate fractions:
Fraction 1. A mixture of water and acetic acid to be returned to fermentation for acetic acid to undergo chain elongation to propionic and butyric acids
Fraction 2. A mixture of propionic and butyric acids to be distilled to obtain two pure streams
Fraction 3. A pure stream of butyric acid to merge with that obtained with distillation
Chapter 7 includes an appendix providing a brief energy-ordinated evaluation of the proposed adsorption-desorption scheme. The energy demand of evaporation of water when a heat exchanger recovers a fraction of its latent heat of vaporization is about 15 MJ/kgVFAs mixture costing under 50 $/tonVFAs mixture.
