Wide-boiling feeds

Convergence to the final solution is rapid with the TG method for narrowboiling feeds but may be slow for wide-boiling feeds. Generally, at least five column iterations are required. Convergence is obtained when successive sets of tear variables are identical to approximately four significant digits. This is accompaniedby 0 = 1.0, a. = normahzed x, and nearly identical successive values of ft as well as Q,.. 

The procedure for wide-boiling feeds is shown in Figure 2-13. Note that the energy balance is used last. This is standard procedure since accurate values of and y are available to calculate enthalpies for the... 

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. 

Absorbers, like flash distillation, are equivalent to very wide boiling feeds. Thus, in contrast with distillation, a wide-boiling feed (sum rates) flowchart such as Figure 2-13 should be used. The flow rate loop is now solved first, since flow rates are never constant in absorbers. The energy balance, which requires the most information, is used to calculate new temperatures, since this is done last. Figure 12-13 shows the sum-rates flow diagram for absorbers and strippers when K = K (T, p). If K = Kj (T, p, Xj, X2,. .. Xj,) a concentration correction loop is added. The initial steps are very similar to those for distillation, and usually the same physical properties package is used. 

The Lewis-Matheson method does not work well for wide-boiling mixtures because of the required component distribution. There is also the difficulty of deciding what is a good match at the feed stage. 

The SR method is suitable for modeling absorbers and strippers. For some extremely wide boiling systems, especially those with noncondensables, it is the best method. It has been found to work very well for the side strippers of a refinery fractionator. Absorbers typically have a rich gas bottom stage feed and a lean oil top stage feed. The equations of the SR method do not allow its direct use for reboiled absorbers, absorbers with condensers, or distillation columns. For these columns, other methods like that of Tomich (Sec. 4.2.8) or Russell Sec. 4.2.10) can be used. 

The feed stream is a wide-boiling mixture with the following composition ... 

The Puertollano plant illustrates a mode of operation for a hydrocracking unit in that each feedstock yields several waxy base stocks after vacuum fractionation. This requires feeds with wide boiling ranges. In such cases, when severity is increased to obtain higher Vis, it is still possible to produce overhead and bottoms base stocks of the same viscosity, but cut point adjustments are required. This will affect base stock yields, particularly of the heaviest base stocks. 

The feed enthalpy is normally inferred from a temperature measurement of the feed leaving the preheater, and preheat is manipulated to control this temperature. This is satisfactory when the feed is a single-phase fluid, and often also with partially vaporized wide-boiling mixtures at superatmospheric pressures, but not with partially vaporized narrow-boiling mixtures. In the latter case, fractional... 

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