Material balance control column overhead

Adjust the benzene product flow rate. This is the overhead product of the benzene distillation column and depends on the value of the feed flow rate. In the next section we will see how to adjust the overhead flow rate in order to maintain the desired benzene purity and fractional recovery by a material balance controller which manipulates the distillate to feed ratio (D/F control). 

To illustrate mathematical modeling for column material-balance control, let us first lose the conventional column of Figure 14.1. The feed, Wp, is split by the column into two parts top product, Wp, and bottom product, Wp. It is assumed that vent losses overhead are negligible. It is further assumed that the heat-transfer dynamics of both the condenser and the reboiler are negligible this will be true for most columns. Let us start at the column base and work up. For convenience the equations are written in Laplace transform notation. 

Scheme 3 (Figure 3.17) indirectly adjusts the material balance through the two level loops. If the steam flow is increased, then the sump level controller decreases the bottom flow. As the additional vapors go overhead and condense, the reflux accumulator level control increases the distillate flow a like amount. The separation is held constant by manually setting the reflux flow to maintain a relatively constant energy per unit feed. This scheme is recommended for columns with a small energy per unit feed (VfF < 2). This scheme also offers the fastest dynamics. 

For example, in steady state simulation setting the purity of overhead product of a distillation column is usual, but in dynamic mode this specification may lead to higher-than-one index problem. The physical reason is that a conflict could occur in initialisation because of the input-output material balance. A better solution is to let free product purity, and set other variable that may have an influence on it, as for example the reflux. If a given purity must be achieved, then the best way is to implement a controller that can manipulate the reflux or the distillate rate. 

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