Base level control

As a final example, suppose we are controlling the base level in a distillation column with the bottoms product flow rate. The valve would be AO because we want it to fail shut (we don t want to lose base level in an emergency). The level transmitter signal increases if the level increases. If the level goes up, we want the bottoms flow rate to increase. Therefore the base level controller should be increase-increase (direct acting). 

We are then left with controlling base level in the DIB column. The only remaining valve is the fresh n C4 feed flowrate into the column. The feed is liquid and there are only 20 trays between the lower feed point and the column base, so base level control using feed should be possible. This base level is also an indication of the nC4 inventory wdthin the process. 

Had we started to assign the DIB column base level control first, we would have ended up with the same inventory control structure. The reason is as follows. Assume we had chosen the DIB column base valve to control base level. After resolving the purge column inventory loops, we would have found that we needed to control the purge column base or reflux drum level with the fresh feed flow to the DIB column. The dynamic lags associated with these loops would have forced us back to the control strategy as described above. 

White, W. B., and White, E., 1974, Base-level control of underground drainage in the Potomac River Basin, in Proceedings of the 4th Conference on Karst Geology and Hydrology H. W. Rauch and E.Werner, eds., West Virginia Geological Survey, Charleston, pp. 41-53. 

This paper presents an on-line model based level control of a batch reactor with reaction rate uncertainties. The analyzed chemical batch process is catalyzed by a catalyst which decomposes in the reactor therefore it is fed several times during the batch. The chemical reaction produces a vapour phase by-product which causes level change in the system. The on-line control method is based on the shrinking horizon optimal control methodology based on the detailed model of the process. The results demonstrate that the on-line optimization based control strategy provides good control performance despite the disturbances. 

The action of the controller should be Direct because if the level increases, the signal to the valve should increase (PVt. OP]) to remove more bottoms. In some columns, base level is controlled by manipulating a valve in the feed to the column. In that control structure, the base level controller action should be Reverse. 

Controller Tuning. The reflux drum and column base are sized to provide 5 min of holdup when at 50% level. The base level control is proportional only with Kc = 2. The column pressure controller used Aspen default tuning of Xc = 20 and Ti= 12 min. 

The use of a steam-to-feed ratio greatly improves this response, as shown in Figure 10.13b. The base-level controller output signal resets the reboiler heat-input-to-feed ratio. The steady-state ratio is 0.038689 (in the required Aspen Dynamic units of GJ/h per kmol/h ). The output range of the level controller is changed from 0 to 0.1, and the TC50 controller is retuned Kc = 3.2 and Ti = 29 min). The deviation in bottoms purity is greatly reduced, and saturation of the bottoms valve is also avoided. 

Flow controllers are added to the two cmde feeds and the stripping steam. A base-level controller is added that manipulates bottoms flow rate. A temperature controller is added that holds the furnace outlet temperature by manipulating furnace heat input. 

Figure 14.11a gives results for a 20% step increase in the flow rate of feed gas. The solvent-to-feed ratio immediately increases the lean solvent flow rate to the absorber, which rapidly drops the level in the stripper base from 2.4 m down to a minimum level of 1.2 m before coming back up to a steady-state level of 1.8 m. Remember this level controller is proportional only. Makeup water is increased by the base level controller, lining out at a higher flow rate since more makeup water is required at the higher feed flow... 

A less common variation of the distillate scheme is used on the occasion where the first column in a distillation train is for removing light components, and the bottoms flow rate needs to be very steady as the flow rate to a large diameter fractionator as the second tower. In this case, the bottoms flow rate is set manually for the controller that manipulates the bottoms valve. The column-base level controller in the first-column manipulates the feed rate to the first column. 

In the reflux scheme, a column temperature controller manipulates a control valve in the reflux line. The reflux drum level controller manipulates a valve in the distillate line. The column base level controller manipulates a valve in the bottoms line. The feed and reboiler steam are each on flow rate control. In some cases, there is a controller for the pressure drop across the trays that manipulates the valve in the reboiler steam line. However, it is preferred to use a steam flow rate controller and simply monitor the tower pressure drop. With this scheme the separation power base is derived from the ratio of steam/feed. The distillate/feed material balance split is maintained by the MRT point controller. 

The manipulated bottoms flow rate scheme is not used very frequently because of problems with the column base level control using steam. When a thermosiphon reboller Is used and the steam flow Is Increased, there Is usually a reversal in the column base level response. The column level first rises and then falls. The usual case for using this scheme is when the feed concentration is 95% light key or more. In other words, most of the feed is distilled overhead from a few percent heavies or tars. The MRT point is usually in the reboiler. 

A variation on the scheme is to put the steam on flow rate control and let the column base level controller manipulate the feed rate to the column, similar to the way a maple syrup evaporator is run. 

Reflux drum or column base level controller, 10 to 30 min... 

All base level controllers in e train should have proportional-only control. In some plants, these controllers are difficult to find because they have been considered inferior. Proportional control is the best, if you can permit the measurement to move around the set point. Surge capacity is frequently provided in the design of accumulators, but then negated by specifying proportional-plut-reset controllers. Proportional-only control cannot be achieved with most instruments by turning the reset adjustment to the maximum reset time. A slow cycle with high amplitude will result. 

To provide automatic control of this sort, we make extensive use of variable configuration controls that are usually implemented by overrides. If, for instance, base composition is normally controlled by steam flow that can be taken over or overriden by high column AP, this is a variable configuration. If base level control is normally achieved by a PI controller that can be overridden by high or low base level proportional-only controls, we call that variable strucmre. Multivariable control may involve both variable configuration and variable structure controls. Hardware permits us to automate this kind of control with a speed, precision, and reliabilin that are completely beyond the capabilities of human operators. 

Base-Level Control via Bottom-Product Withdrawal... 

Base-Level Control via Feed-Flow Manipulation... 

Here should be 20 minutes or more and base-level control should be cascaded to feed-flow control. A mathematical analysis is given in Chapter 16. 

Base-Level Control via Steam-Flow Manipuiation... 

Column base level control by steam flow manipulation, cascade arrangement... 

Bottom-Product Demand Base-Level Control Via Feed... 

Since there is a dead time involved—the time for liquid to flow fi-om the feed tray to the column base—the base level controller settings should be determined by the method of Chapter 16, Section 6. This section also gives a complete design, including overrides for bottom product and steam flows. The reflux drum level controller settings (if a PI controller is used) should be determined by the method of Chapter 16, Section 2. 

Bottom product demand, overhead level control via reflux, base level control via feed... 

Base level controller settings may also be determined by the method of Chapter 16, Section 6. Overhead composition control may be accomplished by trimming the top-product/bottom-product ratio. Base composition may be controlled by trimming the reflux/bottom-product ratio. Because of interactions between composition controls, this column control scheme is not desirable, although probably not impossible. 

The calculation of reflux drum level controller settings follows the method of Chapter 16, Section 4 the calculation of base level controller settings fi)Uows that of Chapter 16, Section 6. 

As indicated earlier, reflux drum level control via boilup is difficult unless a lot of holdup is available. Overhead composition may be controlled by trimming the reflux/distillate ratio base composition may be controlled by trimming the bottom-product/distillate ratio. Note that if either distillate or bottom product (or side product) is a demand flow, either reflux drum or base level control must manipulate feed rate. 

Reflux Drum Level Control Via Distillate Base Level Control Via Bottom Product (Figure 6.6)... 

---