TEMA 11. EXTRACCIÓN | 11.3. EXTRACIÓN EN UNA ETAPA EN CORRIENTE CRUZADA

Single-Stage Extraction (Basis of Cross-Flow) In process engineering, single-stage extraction is the simplest operating unit and the fundamental building block upon which complex series configurations, such as cross-flow systems, are designed. In this scheme, contact between the immiscible liquid phases occurs discretely and independently within each operating unit. Stage Operating Mechanism The single-stage process unfolds sequentially through the following physical steps: Feed and Contact: The original liquid mixture (composed of the solute of interest and the carrier liquid or diluent) is introduced into a mixing tank along with a stream of fresh solvent (pure or regenerated). Mechanical Mixing: Through intense agitation, the solvent is fragmented into millions of tiny droplets dispersed within the feed. This maximizes the interfacial area, reduces film resistance, and allows the solute to diffuse into the solvent due to its greater chemical affinity. Reaching Equilibrium: The phases are retained for the time necessary for the system to reach thermodynamic equilibrium, at which point the compositions of both streams stabilize and net mass transfer ceases. Decantation and Outlet: The emulsion flows to a settling chamber where, acting solely on density differences, the liquids separate by gravity into two transparent phases: The Extract: The solute-enriched stream, composed mainly of the extraction solvent. The Raffinate: The residue of the original mixture, consisting of the solute-depleted diluent. Connection to the Multistage Cross-Flow: When thermodynamic equilibrium limitations prevent a single stage from achieving the required purity, multiple individual units are connected in series to create a cross-flow configuration: The raffinate leaving the first stage is transferred directly to become the feed stream for the second stage. The defining characteristic of cross-current distillation is that each individual stage receives its own independent injection of fresh solvent, rather than reusing internal streams from other stages. At the end of the process, the individual extracts from each stage are collected together in a single overall stream for further purification, while a single, highly purified raffinate is withdrawn from the final stage. Advantages and Disadvantages of the Model Advantages: It offers enormous operational flexibility and ease of process control. It allows for independent variation of the quantity and type of solvent introduced at each stage and is easily adaptable to batch production or laboratory and pilot plant scales. Disadvantages: From a resource optimization perspective, it exhibits low efficiency. Requiring fresh solvent in each unit, it consumes a very high total volume of solvent compared to countercurrent arrangements, resulting in very dilute overall extracts and driving up the thermal energy costs needed for subsequent solvent recovery and recirculation.