Side-stream complex columns

Variations of the basic McCabe-Thiele method, such as incorporating tray efficiencies, nonconstant molal overflow, side streams, or a partial condenser, are often outlined in standard texts. These modifications usually are not justified modern computer programs can calculate complex column arrangements and nonideal systems considerably faster and more accurately than use of any hand-drawn diagram can. 

The atmospheric distillation column in a refinery is highly complex system because of the interactions between the main column with different side strippers and draw streams where the study of this complex system will be more difficult. However, the decomposition of the complex column into a series of simple columns ease and simplify its study. There are a number of advantages of decomposing a complex tower, namely ... 

The dynamic model proposed proved to represent well the separation in a DWC of a ternary hydrocarbon mixture. The values of internal flows and temperature distributions along the trays reached at steady state were in good agreement with the simulations obtained in the frame of commercial simulators. The use as control variables the reflux ratio or the side-stream flowrate proved to enable a reduction of the startup time with about 70 % compared with classical startup procedures. The complex technique developed can be a useful tool in studying dynamic behavior and startup optimization for complex columns and can be easily extended to various mixtures. 

The total material balances are formulated in a manner analogous to that demonstrated for complex columns. The returning vapor streams from the side-strippers to plates 5, 10,15, and 22 give rise to elements which lie above the three diagonals of the matrix R of Eq. (4-10). 

Simple Distillation. In this category, we include the separation of ideal or slightly non-ideal mixtures that do not form azeotropes, based on the differences in the relative volatilities of components. A simple column designates a device that separates one or several feeds in only two products top distillate and bottoms. Complex columns are can deliver more than two products. In this category we include columns with side-streams, columns equipped with auxiliary devices, as prefractionators, side-strippers and side-rectifiers, as well as thermally integrated columns. 

In the second part of this problem, we will examine the possibility to separate the ternary mixture benzene/toluene/diphenyl in a complex column. The relative volatility of the components is approximately 2.5/1/0.3. For the composition from Table 7.28 we obtain 5/<1.6, and A<0.72, B <0.26, C-0.02. Consequently, a side-stream draw of toluene below the feed position is recommended. This design ensures high purity of benzene, but only moderate purity of the toluene to be recycled, which in addition may tolerate small amounts of diphenyl. 

Example 8.7 A complex distillation column, equipped with a partial reboiler and total condenser and operating at steady state with a saturated liquid feed, has a liquid side stream draw-of in the enriching section. Making the usual simplifying assumptions ... 

Complex columns are in a broad sense all columns that are not simple columns, like the column shown in Figure 1.1. Complex columns may have multiple feeds, side product streams, stream transfers between two column units (thermally coupled columns), simultaneous chemical reaction(s) within the column body, hybrid membrane-distillation columns, and so on. Each of these columns present unique opportunities for cost saving. Typical complex columns are shown in Figure 1.3a d. 

Examples of such complex distillation structures are thus columns that have more than one feed point and/or more than two product streams, like distributed material addition/removal columns, and thermally coupled columns. Obviously, as the complexity of the distillation structure increases, so does the design itself thereof. This chapter will, as an introduction to complex column design, treat the design of elementary complex columns such as distributed feed and sidestream withdrawal columns, and side rectifiers, and strippers, before discussing more intricate complex columns like fully thermally coupled columns (sometimes referred to as the Petlyuk and Kaibel columns) in the subsequent chapter. Despite... 

The matrix approach is easily adapted to partial condensers and to columns with side streams (see Problems 6.C2 and 6.C1). The approach will converge for normal distillation problems. Extension to more complex problems such as azeotropic and extractive distillation or very wide boiling feeds is beyond the scope of this book however, these problems will be solved with a process simulator. 

One must take care in determining the number of steady-state and dynamic degrees of freedom for more complex columns. Tyreus [4] describes the determination of the degrees of freedom for an extractive distillation system and for an azeotropic column with an entrainer. In the case of an extractive distillation system, recycle streams reduce the dynamic degrees of freedom through an increase in the steady-state degrees of freedom if the recycle contains a component that neither enters nor leaves the process. Also, if it is important to control the inventory of a trapped component, such as an entrainer for azeotropic distillation, then it is necessary to provide extra control valves to account for the loss of degrees of freedom. The loss comes from the addition of a side stream. 

The lower part of the column is covered by stepping off stages in a fashion similar to that in the upper part of the column, and the final conni of theoretical stages is then determined. The Ponchon-Savarit method may be used for many situations more complex lhan the simple one just described mixed vapor-liquid distillate product, side draw streams, multiple feeds, and so on. Standard unit operations textbooks should be consulted for more dentils on this methnd. As mentioned, it suffers from a need for enthalpy-concentration data, but even a crude approximation based on linear variation of enthalpy with concentration can be better than the McCabe-Thiele approach if there is a very large difference in the latent heats of vaporization of the iwo components being distillnd. 

---