Control of Liquid Inventory

Liquid inventory of the distillation process can be controlled by controlling the liquid levels: the base level in the column, hB, and the level in the reflux drum, hR. Often the levels serve the purpose of smoothing disturbances, hence for control of the reflux drum level the combination (hR, D) or (hR, R) is suitable, since W is already used for pressure control and B and S provide insufficient power of control.

For bottom-level control, D is unsuitable since the power of control is nil. Hence both (hB, S) and (hB, B) are suitable combinations, since they both have a large power of control.

Under normal circumstances the most logical combinations are to use the distillate flow for controlling the reflux drum level and the bottom flow for controlling the column base level.

In columns with a small distillate flow, D, and a large reflux flow, R, this scheme does not work so well. This problem can be partly eliminated by establishing a ratio controller between distillate and reflux flow. When the distillate is then increased, the ratio controller will increase the reflux flow accordingly. However, in many cases with a small distillate flow, the reflux is used for reflux drum-level control. A similar situation holds for the bottom of the column. If the bottom draw-off B is very small, level control using the liquid draw-off might not work well. In that case one could use the steam flow for base-level control. However, increasing the steam flow to the reboiler (and accompanying larger vapour flow in the column) might temporarily increase the bottom level in the column. In the long term, however, increased heat input will result in increased evaporation and consequently a lower bottom level (Figure 4). This response is called inverse response or nonminimum phase response and is not desirable for control purposes.

For tray columns this effect can easily be quantified using detailed column models. Assuming that any increase in vapour flow will propagate through the

Figure 4

flow.

Bottom-level response to a positive change in steam

Figure 4

flow.

therefore initially remain at equilibrium, resulting in no change of liquid content on the trays. However, for the top tray, N, where the same flow of liquid (reflux) is still entering, a mass balance combined with eqn [4] gives:

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Solar Panel Basics

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