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Efficiency of Liquid-liquid Extraction



Liquid-liquid extraction, or LLE, is a technique used to separate liquids that can not be separated with distillation due to temperature-sensitive components or similar solvent boiling points. Mass transfer in LLE is driven by the solubility difference of the solute in the immiscible or partially miscible feed and solvent streams. The feed stream containing the solute is mixed with the solvent stream, often using an agitator, allowing the solute to transfer from the feed to the solvent. The depleted feed, known as the raffinate, is separated from the extract, which is the solute-rich solvent phase. This video will illustrate an extraction of isopropanol from n-nonane, using pure water and study how operating variables affect the overall column efficiency.

LLE is typically performed in continuous stages using co-current or counter-current flow. Counter-current systems are generally preferred, as they tend to be more efficient. Usually, the stages are housed within a single unit. Counter-current extraction columns can be set up two ways. When the solvent is heavier than the feed liquid, or diluent, the solvent is introduced at the top of the column and the solute then exits at the bottom. When the solvent is lighter than the diluent, the solvent is introduced at the bottom of the column, and the solute will exit the column at the top. At steady-state, the material balance of the solute between the feed-end of the process and any stage denoted by N is as shown. Where X is the mole fraction of solute in the diluent, Y is the mole fraction of solute to the solvent. F is the molar flow-rate of feed diluent and S is the molar flow-rate of solvent. The analysis of theoretical plates is used to evaluate the efficiency of the separation process. These plates are hypothetical stages where two phases are in equilibrium with each other. If the two liquids are in equilibrium at a stage, meaning that there would be no change in concentration of either given longer mixing time, then the stage is considered to be a theoretical plate. The higher the number of theoretical plates, the more efficient the process. Operating variables such as temperature, pressure, flow rates and agitator speed affect efficiency and therefore theoretical plate analysis. The following experiment will examine an LLE process to extract the solute isopropanol from n-nonane using water as the solvent. This system, containing three liquids, is called a ternary system. Often, all three liquid components are miscible to some degree. Equilibrium behavior for these and other solvents can be found in the literature. Now that the basics of LLE and its operation have been explained, let's take a look at a separation process.

A York-Scheibel column will be used for this experiment. It is an agitated column with internal paddle-wheel impellers, connected to one vertical drive. Each level contains wire-mesh packing to enable phase-separation, and is separated by partitions to provide individual stages. First use the control knob and digital readout on the control panel to control the speed of the agitator. Use the rotameters on the feed and solvent inlets to measure the flow rate of feed and solvent. Use graduated cylinders and a watch to measure the flow rates of the extract and raffinate. Now start the mixer and keep agitator speed constant at 300 rpm. Open the ball valves for the solvent, feed, extract and raffinate. Start the flow of water into the column to obtain the desired solvent-to-feed molar ratio at a rate of 200 milliliters per minute. Observe whether an interface between solvent entrance and raffinate exit is present, and if not, let the dispersed phase rise to form the upper interface. Start the feed flow when the upper interface forms. Carefully adjust the height of the inverted U on the extract line from the bottom of the tower, to control the level of the upper interface between the two phases. This assures that the raffinate phase does not flow into the extract tank if adjusted too low, or that the extract is not flowing into the raffinate tank if interface is set too high.

Collect samples every 10 minutes at the raffinate sample point in four milliliter bottles. Use gas chromatography, or GC, to quantify the components and confirm that a steady-state has been reached. Next, using a clean graduated cylinder, collect 250 milliliters of the extract at the sample point, then measure the specific gravity or relative density of the sample to water, using a hydrometer. Interpolate the weight per cent of the isopropanol in the sample using the provided table, which displays extract stream composition versus specific gravity. Repeat the procedure for two other lower feed-rates. Make sure to keep both the solvent-to-feed ratio and the agitator speed constant. When finished, turn off the agitator and main power switch and close the feed and solvent ball valves, leaving the raffinate and extract ball valves open. Now let's evaluate the results.

First, let's take a look at the per cent recovery of isopropanol with varied feed-flow rate and varied agitation rate. With increased feed-flow rate, per cent recovery increases and levels off. This is typical of a system which is not near flooding. Increased agitation rate also increased per cent recovery. Stage efficiency, calculated using theoretical plate analysis via computer simulation is also affected by these parameters. As expected, both stage efficiency and per cent recovery increase with higher flow-rate and agitation. This is due to improved mixing, which results in smaller droplets and improved dispersion, thereby improving mass transfer, however, both relationships plateau at higher feed rates. Efficiency and per cent recovery level off and eventually decrease due to emulsification and flooding. The formation of an emulsion negatively impacts recovery and efficiency because the phases can no longer separate cleanly in order to move up or down to the next stage. This can be a problem in systems like the York-Scheibel unit, making mixer and settler vessels in series an appealing alternative.

Liquid-liquid extraction is a separation technique used in a wide range of separations and can be used in a variety of setups. In the case where the emulsification of phases is a challenge, mixer settler tanks in series can be used. This simple setup utilizes a tank where the two phases are mixed by an agitator. The two phases then coalesce in the settler tank, where the heavy phase eventually settles to the bottom and is removed through an outlet on the bottom of the tank. The light phase settles to the top and is removed via another outlet. Another separation technique that harnesses the solubility properties of a solute is solid-liquid extraction. In solid-liquid extraction, the solute present in a solid matrix is extracted into a liquid through vigorous mixing. This technique is used on a large scale for many applications, such as the removal of toxins like the herbicide Atrazine from soil.

You've just watched Jove's introduction to liquid-liquid extraction. You should now understand the basics of an LLE extraction using a York-Scheibel column and how process variables such as agitator and flow-rate can affect the solute recovery and efficiency of the column. Thanks for watching.

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