# Recent Advances in Muconic Acid Extraction Process
## 3 Reactive Extraction
The diffusional domain consists of global process rate limitation by the diffusion of the system components and corresponds to the continuous increase in the extraction degree or mass transfer rate with the mixing intensity of the phases (represented by rotation, the Reynolds criteria). Reaching a constant level of variation in the monitored parameter indicates the transition into the kinetic domain of reactive extraction, in which the process is controlled by the complex formation [41; 42]. This step, complex formation, is a reversible process, with the extractant being recovered from the complex by raising the temperature or adding sodium hydroxide once separation is accomplished. Figure 3: Representation of reactive extraction mechanism. To achieve the separation of a compound by liquid-liquid extraction, one or more solvents are used. The choice of the solvent for the extraction operation is a complex problem, and it requires taking several factors into account with reference to the mixture subjected to separation and to the performance of the extraction operation, the efficiency of the operation being mostly determined by the choice of solvent. The following properties of a potential solvent must be considered before its use in a liquid-liquid extraction process [12; 40; 41]:  
* distribution coefficient;
* separation factor;
* extraction capacity;
* solvent selectivity;
* density;
* surface tension;
* vapor pressure;
* mutual solubility with the mixture to be separated;
* degree of flammability;
* toxicity;
* price. The distribution coefficient (Equation (1)) represents the ratio between the concentration of the solute in the extract, \([MA]_{org}\), and its concentration in the raffinate, \([MA]_{ai}\), at equilibrium. The choice of a solvent based on this criterion is established with the help of equilibrium diagrams [43]. \[K=\frac{[MA]_{org}}{[MA]_{ai}}. \tag{1}\]  
Extraction efficiency (\(E\%\)) is defined as the ratio of the concentration of MA in the extract (\([MA]_{org}\)) to the initial MA concentration in the aqueous phase \([MA]_{in}\) and is provided by Equation (2):  
\[E=\frac{[MA]_{org}}{[MA]_{in}}\times 100\%. \tag{2}\]  
The extraction capacity is a measure of the volume of solvent required for the effective separation of a solute. Density is also a very important property: a large density difference between the solvent and the solution to be separated creates conditions for good dispersion of the phases and their rapid separation. High surface tension causes rapid coalescence and generally requires strong mechanical agitation to change into droplets. A low surface tension allows the formation of droplets at low agitation intensities but leads to low coalescence rates. Surface tension usually decreases with increasing solute concentration and solubility [41; 42]. For the reactive extraction of carboxylic acids, three different extracting agents (extractants) have been studied: high molecular weight aliphatic amines and amine salts, phosphorous bonded oxygen bearing extractants (more affordable and efficient at forming compounds with carboxylic acids), and oxygen-bearing hydrocarbon extractants (very unspecific and thus not applicable for a selective recovery) [24]. Tertiary amines (TOA, tri-octylamine, and Alamine 336, a commercial mixture of trialkyl amines) have been the state-of-the-art extractants for carboxylic acids for a long time, but the processes' economic viability has been restricted by the relatively low acid concentrations in the fermentation broth [24].