Column temperature control is perhaps the most popular way of controlling product compositions. In this case, the control temperature is used as a substitute to product composition analysis.
Ideally, both top and bottom compositions should be controlled to maintain each within its specifications. See the Figure below.
In practice, simultaneous composition control of both products suffer from serious "coupling" (interaction) between the 2 controllers, resulting in column instability. In the system shown, suppose that there are concentration changes in the feed conditions that result in lower column temperature. The top and bottom temperature controllers will respond by decreasing reflux and increasing boil-up respectively.
If the actions of the 2 controllers are perfectly matched, and response is instantaneous, both control temperatures will return to their set points without interaction.
However, the 2 actions are rarely perfectly matched, and their dynamics are dissimilar - usually the boil-up response is faster. The reflux and boil-up will "cycle" as shown in the Figure above.
The interaction can be avoided
by controlling only 1 of the 2 product compositions.
On-line analyser can be used together with temperature control to control product composition. The principal control action is rapidly performed by the temperature controller, while the analyser slowly adjusts the temperature set point to prevent off-specification product purity. A set up is shown in the Figure below.
In the above set-up, delayed analyser response is acceptable, as its time lags become a secondary consideration. The fast temperature controller action renders this control method less sensitive to upsets and step changes in an analyser-only control system.
Another advantage is that, should
the analyser become inoperative, the temperature controller will maintain automatic
control of the process.
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