Control double differential temperature

Average Temperature Control Including double differential temperature control. For sharp splits and azeotropic dlstiiiation break points. 

Sharp splits. Boyd (58, 59) developed a double differential temperature control scheme, which is essentially an average temperature control scheme that compensates for pressure and differential pressure variations. The scheme overcame the problem described in the previous section and was demonstrated to maintain tight control on both... 

It can be argued that the use of a double differential temperature is superfluous in many situations and that it would suffice to control by an average between a control temperature in the bottom section (e.g., tray 30) and the one in the top section (e.g., tray 15). Since the temperatures of trays 5 and 45 are practically unaltered in all of Boyd s plots (Fig. 18.5a), they can he treated as constant at a given pressure and differential pressure. Simple arithmetic will then show that con-... 

Flgure 18.6 Boyd s double differential temperature control, (a) Control scheme. (Porta from "Fractionation Column Control, D. M. Boyd, Chemical Engineering Progr, vol. 71, no. 6, p.55 (June 1975). Reproduced by permission of the American Institute of Chemical Engineers.)... 

Figure 18.6 (Continued) (b) Relationship between double differential temperature and composition compared to relationship between average control temperature and composition (based on data in Fig. 18.5 a). (Part b is based on Boyd s data from same reference as part a.)...

In Boyd s example, both pressure and dififerential pressure variations had a significant effect on the control temperature (59). To compensate for these variations, it was necessary to use the double differential temperature (see Sec. 18.8). If pressing and differential pressure variations have little effect on the control temperatures, it would suffice to use the average between the tray 15 and tray 30 temperatures. 

Boyd (58, 59) applied a double differential temperature control to a column producing high-purity products. Although the main purpose of this system was to optimize the location of the control tray, it was also effective in compensating for both pressure and differential pressure variations. This system is described in detail in Sec. 18.5. 

Luyben (261) analytically studied the application of a double differential temperature control to a deisobutanizer. His study indicated that this technique not only effectively compensates for pressure and differential pressure variation, but can also move the location of the maximum in Fig. 18.86 and c to a composition where it would not be troublesome. This, however, was achieved at the expense of having to control a very small differential temperature range (about 3°F). Others (53) also found the very small range to be a handicap with this technique. 

Double differential temperature ccHitrol gave stable control of top and bottom product purities. Both product purities were in the parts per million range. A conventional temperature control was uitable to aooampiish this. 

Luyben, W. L., "Feedback Control of Distillation Columns by Double Differential Temperature Control, Ind. Eng. Chem. Fund. 8(4), 1969, p. 739. 

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