Theoretical separation stage

Thermodynamic design the ratio of extract to raffinate flow rate, the phase ratio, and the number of theoretical separation stages necessary for the separation task can be worked out from mass balances in connection with the solution equilibria (see Chapters 2-8). 

In general, distillation columns should be operated at a low pressure. For example, Fig. 3.3 shows an isobutane-normal butane stripper. This fractionator is performing poorly. A computer simulation of the column has been built. The column has 50 actual trays. But in order to force the computer model to match existing operating parameters (reflux rate, product compositions), 10 theoretical separation stages (i.e., 10 trays, each 100 percent efficient) must be used in the model. This means that the trays are developing an actual tray efficiency of only 20 percent. 

The once-through reboiler functions as the bottom theoretical separation stage of the tower. The circulating reboiler does not, because a portion of its effluent backmixes to its feed inlet. This hackmixing ruins the separation that can otherwise be achieved in reboilers. 

Also from Fig. 5.2a, the value ofyin, corresponding to the use of four theoretical separation stages is 0.36, so Ract = 1.64. Therefore, for this particular separation,... 

Structured packing. A layer of structured packing can sometimes be used to support a bed of random packings. This technique can add a fraction or even a full theoretical separation stage to the bed. The structured packing may also improve vapor distribution. 

Phases taking place in mass and heat transfer are guided in cocurrent flow through the separation apparatus. The maximum efficiency of this separation apparatus is the same as that for a single theoretical separation stage. 

With stage as a theoretical separation stage, the leaving phases are in equilibrium and no further mass or heat transfer is possible. Therefore, y and x are phase equilibrium concentrations. 

Method to Determine the Number of Theoretical Separation Stages for a Countercurrent Coiumn... 

A discussion of a method of determining the number of theoretical separation stages required to separate a mixture in a counterflow column follows. 

Theoretical separation stage number, indice for the vapor and reflux mole ratio of the low-boiling component EC Equilibrium curve BL Balance or operating line y Molar fraction of the low-boiling... 

Fig. 1-57. Schematic of a countercurrent flow column with mass transfer from phase II to phase I (a) and an operating diagram to determine graphically the number of theoretical separation stages (b). EC Equilibrium curve...

For a theoretical separation stage j for each component /, the relationship between the phase equilibria of the leaving phases is... 

A mass balance for each individual component i in each theoretical separation stage j (see Fig. 1-61) gives... 

When a > 1 the released vapor generated under pressure contains a higher concentration of low-boiling components than the mixture introduced into the separator. In flash operation processes, therefore, a partial separation of the feed occurs in which the separator can at best only act as one theoretical separation stage. 

The number of theoretical separation stages A/, required for a rectification problem can be determined by... 

The required number of theoretical separation stages N, is obtained by the construction of steps between the equilibrium curve and the operating lines of the stripping and enrichment zones, as described in Chapter 1.9.2. The example in Fig. 2-44 contains 4 theoretical steps, including the reboiler in the stripping section, and 4 theoretical steps in the enrichment zone. In this case, a feed of composition Xp is separated into an overhead product of composition Xp and a bottom product of composition x, of the lower-boiling key component. The stage with a state point on the... 

The method of Ponchon and Savarit [2.61] is a graphical determination of the number of theoretical separation stages in counterflow columns. It is particularly applicable to binary mixtures in rectification processes. Exact results are obtained when the enthalpy-concentration diagram of Pon-... 

This method is based on the assumption of constant flow rates, R and G, constant relative volatility O] 2 and an infinite reflux ratio V. According to Fenske and Underwood [2.64] the required minimum number of theoretical separation stages for binary rectification is... 

Figure 2-49 shows an empirical relationship between the number of theoretical separation stages N, and the reflux ratio v introduced by Gilliland [2.64]. 

Regular packing type Characterization Design Material Favored application Porosity (-) Specific surface (mVm ) Gas load factor Liquid flow rate (mV (m h)) No. of theoretical separation stages per m Specific stage pressure drop (mbar/m) ... 

HETS Height of packing equivalent to a theoretical separation stage... 

The required packing height Z for mass and heat transfer between counterflow phases may be calculated in two different ways as discussed in Chapter 1.9.2. After determination of the desired number of theoretical separation stages N, by numerical or graphical means, and the separation efficiency experimentally under conditions close to the operating conditions and suitable for scale up, the packing height is... 

Partial load behavior, separation efficiency (no. of theoretical separation stages per m)... 

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