Intensified solvent extraction of L-tryptophan in small channels using D2EHPA

Highlights

• Intensified continuous extraction of amino acids in small channels.

• High extraction efficiency and mass transfer coefficients at short residence times.

• Extraction efficiency increases with residence time at constant mixture velocity.

• Extraction efficiency increases with mixture velocity at constant residence time.

Abstract

The continuous extraction of an amino acid, L-tryptophan, from aqueous solutions was studied in small channels with diameters of 0.5 and 2 mm. L-tryptophan was separated from nitric acid solutions into hexane using Di(2-ethylhexyl)phosphoric acid (D2EHPA) as extractant. It was found that the plug flow dominated in the flow pattern map at mixture velocities below 0.1 m/s and all organic phase volume fractions in the 0.5 mm channel, while it appeared at mixture velocities below 0.02 m/s and organic phase volume fractions below 60% in the 2 mm channel. During plug flow, the specific interfacial area increased with mixture velocity and acquired maximum values of 4432 and 1456 m²/m³ in the 0.5 and 2 mm channels, respectively. Extraction efficiencies and percentage of up to 95% and 50% were reached in the 0.5 mm channel for residence times less 45 s, while both values were about 5–10% less in the 2 mm channel. Mass transfer coefficients of up to 0.16 and 0.13 s⁻¹ were reached in the 0.5 mm and 2 mm channels, respectively.

Introduction

Liquid-liquid, or solvent, extraction (LLE) is one of the most important and efficient separation methods widely used in many industries. It is employed broadly in various processes including separation of metals, retrieval of biological compounds, such as proteins, enzymes, and amino acids, recovery and purification of biomolecules from fermented broth, industrial wastewater treatment, and reprocessing of spent nuclear fuel [1], [2], [3], [4], [5], [6], [7], [8], [9], [10]. The separation is based on the solubilities of the compounds of interest in two immiscible liquids, an aqueous phase and an organic solvent. In particular, amino acids are an important class of compounds that have a variety of biological, industrial, and environmental applications, including food, animal feed additives, and in agriculture as plant nutrients [11], [12], [13], [14]. The separation of amino acids plays an important role in many scientific and industrial fields. Amino acids contain an amino (-NH₃⁺) and a carboxylate group (-COO⁻), which result in their existence in acidic, basic, or zwitterion form depending on the pH value of the solution. These features make amino acids hydrophilic at all values of pH and therefore difficult to remove from aqueous solutions in conventional solvent extraction. To enhance the extractive potential of organic solvents, certain extractants, such as crown ether based extractants (18-crown–6, benzo-18-crown–6, dibenzo-18-crown–6, dicyclohexyl-18-crown–6) [15], [16], [17], [18], [19], [20], cationic extractants (Di(2-ethylhexyl)phosphoric acid (D2EHPA)) [11,[21], [22], [23], [24], [25], [26]] and metal complexes (such as BINAP-metal, MeO-BIPHEP-metal, and SDP-metal) [27], [28], [29], [30], [31], are added in the organic phase. Crown ether based extractants and D2EHPA form stable hydrophobic complexes with the amino acids via hydrogen bonding and have long been used in their liquid-liquid extraction. For the extraction of amino acids, crown ethers are generally dissolved in chlorinated hydrocarbons or chloroform, while D2EHPA is used with hydrocarbons such as hexane, heptane and octane. Metal complexes have also been tested in the liquid-liquid extraction process of amino acids very recently as chiral selectors. Similar to crown ether based extractants, metal complexes are dissolved in chlorinated hydrocarbons or chloroform.

The conventional equipment used in liquid-liquid extraction, such as mixer-settlers, packed, spray, pulsed and rotating disk columns, has many disadvantages, such as use of large solvent volumes, multistage extractions and long extraction times [32], [33], [34], [35], [36]. Moreover, the interfacial area related to the drop size in the conventional equipment, cannot be easily controlled. The drop size distributions are often broad and result in a wide range of times needed to complete the extraction. Non-uniform drop sizes result in varying extraction rates in the equipment. The extraction rate increases with decreasing drop size because the interfacial area is increased. However, with further decrease of the drop size the extraction rate decreases because the small drops behave as rigid spheres where molecular diffusion governs the mass transport [37,38].

To overcome the drawbacks of the conventional equipment, process intensification (PI) which often involves small scale devices, such as small channels, can be applied in liquid-liquid extraction. Studies have shown that during multiphase flows in small channels, bubble/drop sizes are small and have narrow size distribution; large interfacial areas can thus be achieved [39], [40], [41]. In small channels, the interfacial and viscous forces dominate over gravity and result in regular patterns, such as plug flow, where elongated drops of one liquid (plugs), with diameter larger than the channel diameter, are separated by the continuous phase (slugs), while a thin film separates the plugs from the channel wall. Plug flows have been associated with improved mixing due to the circulations within the plugs and slugs [42], [43], [44], short diffusion distances because of the thin films [45] and large interfacial areas (ranging from 2760 to 8500 m²/m³ in channels with internal diameter smaller than 1 mm) [8,46,47], which increase mass transfer rates. Previous studies have shown that the efficienciesand overall mass transfer coefficients of metal extractions, such as Eu(III), Ti, Pt(IV), and U(VI), in small channels could reach above 90% and 0.06 s⁻¹, respectively [8,[48], [49], [50], [51]].

There are currently no studies on the continuous extraction of amino acids in small channels with conventional organic solvents, despite the potentially significant improvements on mass transfer rates. In this paper, we report for the first time the continuous separation in small channels of L-tryptophan from an aqueous solution into an organic phase using D2EHPA as extractant. We present detailed flow pattern maps which indicate the range of flowrates where plug flow occurs, that enhances mass transfer.  The effects of the flowrates of the two phases, the residence time, the channel size and the concentrations of the amino acid and the extractant on the extraction percentage and the mass transfer coefficients are investigated. The findings can be implemented for the design of intensified solvent extractions in small channels of other biomolecules.