MB Control with the Fig. 19.6c Configuration

Note that the above only applies to scheme 16.4a with a fast- 

When vapor flow up the column is small compared to the side draw rate, configuration 19.6c (with any of the Fig. 16.4 control schemes) can induce large swings in column vapor traffic above the side draw, and a severe interference with the pressure controller. This is analogous to the problem described with configuration 19.66. The vapor swings are most pronounced with schemes 16.46 and d, because the top section temperature is more sensitive to the feed than to the changes in the small vapor flow. 

Two techniques can alleviate the above problems and render configuration 19.6c compatible with schemes 16.4a, 6, and d. 

Controlling the internal vapor flow to the section above the side draw Reboiler heat duty is measured and divided by the latent heat of the boiling mixture the measured side product flow is subtracted from the quotient to give the internal vapor rate in the section above the side draw. In a steam (or condensing vapor) reboiler, the internal vapor rate is computed as a constant times the measured steam rate less the measured side product flow, with the constant equal to the ratio of the latent heat of steam to that of the boiling mixture. An internal vapor controller (IVC) uses this computed internal vapor to manipulate product flow (Fig. 19.76). A limitation of this technique is that internal vapor is computed as a small difference between two large numbers and can therefore be in error. The error escalates as the internal vapor rate becomes a smaller fraction of the total vapor traffic below the side draw. 

The internal vapor rate controller can be substituted by a differential pressure controller measuring pressure drop in the section above the side draw (260, 263, 362). This option, however, is less satisfactory (260, 263) because of the shortcomings inherent in pressure-drop controllers (Section 19.4). 

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