Extractor column

In the extraction process, the LPG from the prewash tower enters the bottom of an extractor column. The extractor is a liquid/liquid contactor in which the LPG is counter-currently contacted by a caustic solution. Another option is the use of a fiber film contacting device. The mercaptans dissolve in the caustic (Equation 1-14). The treated LPG leaves the top of the extractor and goes on to a settler, where entrained caustic is separated. 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Sec. 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the... 

The modelling approach to multistage countercurrent equilibrium extraction cascades, based on a mass transfer rate term as shown in Section 1.4, can therefore usefully be applied to such types of extractor column. The magnitude of the mass transfer capacity coefficient term, now used in the model equations, must however be a realistic value corresponding to the hydrodynamic conditions, actually existing within the column and, of course, will be substantially less than that leading to an equilibrium condition. 

Yi stream denotes the weight percentage of the solute in the solvent entering the extractor column. 

Thus the extractor column raffinate outlet rate and the solvent inlet rate are approximately equal. This is indeed the minimum solvent rate allowed, since a lower rate will overload the solvent, referencing the plait point. This rate will also set the required minimum extractor column diameter. For some refinery-type extraction operations, such as lube oil extractors, where relatively much larger solvent-raffinate rates apply, this method for determining minimum solvent rate is very economical and desirable. 

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. 

An extractor column is generally a tall, vertical packed tower that has two or more bed sections. Each packed bed section is typically limited to no more than 8 ft tall, making the overall tower height about 40 to 80 ft. Tower diameter depends fully upon liquid rates, but is usually in the range of 2 to 6 ft. Liquid-liquid extractors may also have tray-type column internals, usually composed of sieve-type trays without downcomers. These tray-type columns are similar to duoflow-type vapor-liquid separation, but here serve as contact surface area for two separate liquid phases. The packed-type internals are more common by far and are the type of extractor medium considered the standard. Any deviation from packed-type columns is compared to packing. 

A last note about the continuous phase is the fact that it must completely immerse the packing section where the mixing of the two phases takes place. The inner phase between the two liquid phases is therefore to be near the extractor column s dispersed-phase outlet. The extract stream, having gained the transferred solute, exits the column at the opposite end from where the raffinate stream exits. The raffinate stream is the inlet feed stream containing the extracted solute. 

Determination of theoretical stage number and stage efficiency, or more simply HETS, has been established for any extraction process. The next item of order is the packed extractor column flooding limit. Just as fractionation columns must be sized for vapor and liquid, liquid-liquid extraction columns must be sized for flood limits. 

Nemunatis [8] observed the same flooding conditions below the saturation top curve (the Crawford-Wilke curve). In that instance, flood was noted at 20% of this saturation curve. Strigle [6] suggests that design not exceed a 12% value of the shown family of 100% flood curves. Note again that, whatever the ratio of Vc to VD, the curve for that specific ratio in Fig. 7.11 is indeed the 100% flood value for that particular extractor column condition. Therefore it is wise to place a loading 12% below flood levels before the extractor column is flooded. 

Finally, the solved Y value for the extractor is used in Eq. (7.19) to solve for VCVD2 and subsequently the Vc value for the extractor, Eq. (7.20). Note that this Vc value is the 100% flood rate (ft/h) for the continuous phase in the extractor column. Please also note that the VD value is the 100% flood rate (ft/h) for the dispersed phase. 

Once you calculate Vc in Eq. (7.20), you can easily find the dispersed-phase velocity using Eq. (7.23). Note that you now have the 100% flood values determined. This means you can now calculate the actual liquid-phase velocities from Eqs. (7.21) and (7.22) and ratio them to these 100% flood velocities. Thus this ratio times 100 is the flood condition of your extractor column. Please see Eqs. (7.24) and (7.25), where this is done. 

DIA, s = required extractor column internal diameter for solvent phase, ft... 

Equations (7.26) and (7.27) are based on 80% extractor column flood. (Note the 0.8 factor in both equations.) Please note that Vc and VD are the superficial full-column internal diameter velocities. Superficial simply means that you calculate these velocities as though the respective liquid phase were the only material in the full-column cross-sectional area... 

Having calculated a flood value for the assumed diameter discussed earlier, it is now time to find the preferred diameter for the extractor loadings. This preferred diameter is, of course, an 80% extractor column diameter loading. 

The column operation is gravity induced, with the heavier liquid flowing downward and the lighter liquid flowing upward by buoyancy. For the process to be workable, it is therefore required that one liquid phase be of a distinctly higher density than the other. The internal construction of an extractor column could be of several types, including trayed columns, packed columns, or spray columns with or without agitation. 

Extractors with reflux at one or both ends of the column (or series of mixing vessels) may be used to enhance the purity of the products. An extractor column with both extract and raffinate reflux is shown in Figure 11.1. In this configuration, the feed is sent to an intermediate stage, and the extractor performs as a distillation column, separating two components in the feed. In the column section above the feed, the raffinate phase is stripped of the extract component (the solute), and in the lower section the extract phase is enriched in the extract component. 

Feed stream 2o> flowing at a rate of 1000 kg/h, contains 45 wt% acetone in solution with water. It is required to extract the acetone in an extractor column, using 1,1,2-trichloroethane as the solvent, at a rate of 350 kg/h. The raffinate, Qf, should contain 10 wt% acetone. 

At the start of the calculations the liquid flow rates in the column, Q and E in Equation 12.45, may be assumed equal to the inlet feed and solvent rates. The initial values for the activity coefficients may also be based on the inlet compositions and thermal conditions of the streams. The temperature and pressure variations in the extractor column are usually small, but the compositions will vary, and this may require recalculating the activity coefficients. The column calculations may be repeated with updated values of Q and E, taken as respective averages of each phase inlet and outlet stream flow rates calculated in the first trial. The activity coefficients can also be refined by recalculating them at the column top and bottom compositions for each phase. Averages of the top and bottom coefficients for each phase can be used in Equation 12.47 to calculate the new E-values. The extraction factors are then recalculated with the new values of Q, L, and E by Equation 12.45. The product component flow rates E v and Q i are Anally calculated by Equations 12.43 and 12.44. If large variations appear between the first and second trials, more trials may be considered. 

A four-stage extractor column is used to recover acetone from an acetone-chloroform solution using a water-acetic acid solution as the solvent. The feed and solvent are defined below, along with average k-values. Calculate the products component flow rates. 

In continuous differential extractors (columns) it has been convenient to think in terms of a height equivalent to a theoretical stage (HETS), and to correlate HETS as a function of system and equipment variables. Alternately, correlations may be obtained on the basis of the height of a transfer unit (HTU), which is more amenable to calculations which separately include the effects of backmixing.l H ... 

The integrated UV detector signal produced by each of the aromatic hydrocarbons was determined to be proportional to its concentration. Individual response factors were found for each compound by first replacing the extractor column with a calibrated sample loop and then injecting acetonitrile solutions with known concentrations of the individual hydrocarbons. Details concerning the loop calibration technique and preparation of the acetonitrile solutions are given in Appendices A-3 and A-4. 

The DCCLC system was also designed to circumvent problems due to adsorption. After preparation and filtration, saturated solutions are transferred and extracted by a process that minimizes PAH contact with surfaces. The volume between the generator column and the extractor column is approximately 6 fxL. The time required for transfer of the saturated solutions at a flow rate of 5 mL/min is less than 75 msec. Furthermore, the walls of the transfer lines are presaturated with the compound being studied during the column-conditioning process. This further reduces the possibility of adsorptive losses of the PAHs during the brief time that the saturated solutions remain in these lines. 

A-l. Extractor Column Extraction Efficiency. Extraction of the PAHs from the generated solutions was accomplished by pumping volumes varying between 5.0-25.0 mL through the extractor column. Over this range, extraction efficiencies are quantitative for 11 of the 12 compounds studied in this investigation. Benzene was not extracted efficiently by the extractor column. The solubility of benzene, therefore, was determined by direct injection of 23.2 /xL of the generated solutions via a sample loop. 

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