Equilibrium stage extractor

EQEX - Simple Equilibrium Stage Extractor System... 

Point T, however, will separate into two liquid phases. If the extractor consists of one true equilibrium stage, this mixture (point T) separates into two phases represented by points R and E. 

Now notice how small an equilibrium stage step was made for the third step in Fig. 7.6. It is very obvious that the next stage step will be even smaller, yet will require as much of the extractor column physical size as the first equilibrium stage step required. Thus, for a critical specification of 1.0% or less as stated here, a fourth stage is required. This fourth stage ensures that the specification will be met. 

Extraction involves the transfer of components between two liquid phases, much as absorption or stripping involves the transfer of components from liquid to vapor phase or vice versa. As in vapor-liquid multistage separation processes, the device employed to carry out liquid-liquid extraction is usually a counterflow column that performs the function of a number of equilibrium stages interconnected in counterflow configuration. In each stage, two inlet liquid streams mix, reach equilibrium, and separate into two outlet liquid streams. As in vapor-liquid columns, the lack of complete equilibrium in liquid-liquid extractors is accounted for by some form of tray efficiency. Liquid-liquid extraction may also be carried out in a cascade of mixing vessels connected in series in counterflow. 

FIGURE 11.3 Equilibrium. stages in a liquid-liquid extractor. 

FIGURE 11.4 Graphical construction of extractor equilibrium stages. 

Extractor equipment considerations are discussed here in the context of their effect on the process performance and not for the purpose of describing detailed design. The main parameter of interest at this point is the number of equilibrium stages that represent the process. Liquid-liquid extraction requires thorough mixing of two liquid phases to achieve thermodynamic equilibrium, followed by complete separation of the phases. The particular equipment selected for a given process is determined, in part, by the mixing and separation characteristics of the phases. 

The efficiency of extractors and the equivalent number of equilibrium stages depends on internal design and other factors. Detailed description of extractor equipment is provided by Najim (1989) and is outside the scope of this book. 

A pure solvent 5 is used to remove component E from a binary liquid solution containing 30% mole E and 70% mole R. In a single-stage extractor how many kmol solvent per 100 kmol feed are required to bring the concentration of E in the raffinate to 3% mole What are the resulting rates and compositions of the raffinate and extract Use liquid-liquid equilibrium data from Figure 11.2. 

It is required to recover 93% of component E from a binary mixture of components E and R by treating with a solvent in a single-stage extractor. What is the required solvent rate per 100 kmol of feed, and what are the product rates and compositions Assume the liquid-liquid equilibrium... 

Using rectilinear representation of the operating lines and equilibrium data, determine the required number of equilibrium stages. Use the horizontal coordinate for and the vertical coordinate for X. The equilibrium data at the extractor temperature of 25°C are given below. 

A multi-component countercurrent extractor model may be delined for the shortcut column section method in a manner very similar to the vapor-liquid model. The extractor model is developed for liquid-liquid equilibrium stages leading to component flow relationships comparable to the Kremser equation ... 

One hundred kilogram-moles per hour of an equimolar mixture of benzene B, toluene T, n-hexane C6, and n-heptane C7 is to be extracted at 150°C by 300 kgmole/hr of diethylene glycol (DEG) in a countercurrent liquid-liquid extractor having five equilibrium stages. Estimate the flow rates and compositions of the extract and raffinate streams by the group method. In mole fraction units, the distribution coefficients for the hydrocarbon can be assumed essentially constant at the following values. 

The holdups of the two phases are very small, which is advantageous in certain applications. As the phases are in the co-current flow, they can attain equilibrium. Therefore, the extractor is at best one equilibrium stage. However, multiple units can be configured to form a multi-stage countercurrent extraction unit. There is a need to evaluate the relative merits of the multiple units configuration vis vis the Podbielniak extractor, which offers 4 to 6 stages in a single unit. 

In Fig. 6.3-4 typical performance data of ten different extractor designs are presented (Stichlmair 1980). All extractors have been operated with the system tolu-ene/acetone/water with toluene as dispersed phase and acetone as transfer component. The initial concentration of acetone in water was approximately 5 wt.%. The mass transfer took place from the continuous water phase into the dispersed toluene phase. The ratio of toluene to water has been chosen so that operating and equilibrium lines are parallel in the y/x diagram. Such operation conditions avoid pinches and, in turn, facilitate the calculation of the equilibrium stages from measured concentrations. The data presented in Fig. 6.3-4 are valid for small units only, e.g., with diameters from 50 to 100 mm. 

The Graesser contactor takes an extreme position in the diagram. It has the highest separation efficiency (10 stages per meter) but the lowest capacity (1-2 m/h). The other extremum takes the static sieve tray extractor with only one equilibrium stage per meter and up to 50 m/h capacity. The capacity of a pulsed sieve tray column is as high as 30 m/h with a separation efficiency of 5 to 6 stages per meter. 

Since it is quite expensive to have a large number of equilibrium stages in a commercial extractor, the ratios in Eq. ri3-14bl should be significantly different. 

D8. We are extracting acetic acid from benzene (diluent) into water (solvent) at 25°C and 1.0 atm. 100.0 kg h of a feed that is 0.00092 wt frac acetic acid and 0.99908 wt frac benzene is fed to a column. The inlet water (solvent) is pure and flows at 25.0 kg h. We have an extractor that operates with 2 equilibrium stages. 

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