Control distillate flow

Figure 17.56 illustrates the flooded reflux drum method. Here the level control of the reflux drum is eliminated. The reflux drum runs full of liquid, and sometimes the reflux drum itself can be omitted. The pressure controller directly controls distillate flow. Due to the tight pressure control usually required, distillate flow controlled by this method is likely to fluctuate. These fluctuations may cause instability in downstream units. Distillate control by this method should be avoided (77, 301, 362) unless the product goes to storage. 

Whenever distillate flow is set to hold the material balanee, the bot-toms-level controller must manipulate bottoms flow. This allows the heat input to be set independently to establish the separation eapability of the tower. For this reason, whenever eomparison of the produet flow rate does not overwhelmingly favor manipulation of bottoms flow for composition control, distillate flow should be seleeted. 

Amplitude of controlled variable Output amplitude limits Cross sectional area of valve Cross sectional area of tank Controller output bias Bottoms flow rate Limit on control Controlled variable Concentration of A Discharge coefficient Inlet concentration Limit on control move Specific heat of liquid Integration constant Heat capacity of reactants Valve flow coefficient Distillate flow rate Limit on output Decoupler transfer function Error... 

Pyrex jacket. The test fluid, distilled water, flowed vertically upwards through the annulus, while inside the heated tube a control fluid flowed which was either water or nitrogen gas, depending on the tube temperature required. 

While open channels are not used frequently for the flow of material other than cooling water between plant units, a weir is frequently installed for controlling the flow within the unit itself, for instance in a distillation column or reactor. 

The compositions are controlled by regulating reflux flow and boil-up. The column overall material balance must also be controlled distillation columns have little surge capacity (hold-up) and the flow of distillate and bottom product (and side-streams) must match the feed flows. 

Another example is sketched in Fig. 8.3i>. A hot oil stream is used to reboil a distillation column. Controlling the flow rate of the hot oil does not guarantee a fixed heat input because the inlet oil temperature can vary and the AT requirements in the reboiler can change. The heat input Q can be computed from the flow rate and the inlet and outlet temperatures, and this Q can then be controlled. 

Example The location of the best temperature-control tray in a distillation column is a popular subject in the process-control literature. Ideally, the best location for controlling distillate composition xa with reflux flow by using a tray temperature would be at the top of the column for a binary system. See Fig. 8.9o. This is desirable dynamically because it keeps the measurement lags as small as possible. It is also desirable from a steadystate standpoint because it keeps the distillate composition constant at steadystate in a constant pressure, binary system. Holding a temperature on a tray farther down in the column does not guarantee that x will be constant, particularly when feed composition changes occur. 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

The reactor effluent is separated in a distillation column. The overhead is mostly excess reactant A which is recycled back to the reactor. The bottoms from the column is mostly product C. The reaction occurs in the liquid phase so the reactor feed streams are liquid. Reactant B is added directly to the reactor on flow control. The flow rate of the recycle stream is ratioed to the flow rate of the B feed stream. The composition of A in the column base sets heat input. The composition of C in the column overhead sets reflux. 

For example, in a distillation column the manipulated variables could be the flow rates of reflux and vapor boilup R — V) to control distillate and bottoms compositions. This choice gives one possible control stmcture. Alternatively we could have chosen to manipulate the flow rates of distillate and vapor boilup D V). This yields another control structure for the same basic distillation process. 

As a minimum, a distillation assembly consists of a tower, reboiler, condenser, and overhead accumulator. The bottom of the tower serves as accumulator for the bottoms product. The assembly must be controlled as a whole. Almost invariably, the pressure at either the top or bottom is maintained constant at the top at such a value that the necessary reflux can be condensed with the available coolant at the bottom in order to keep the boiling temperature low enough to prevent product degradation or low enough for the available HTM, and definitely well below the critical pressure of the bottom composition. There still remain a relatively large number of variables so that care must be taken to avoid overspecifying the number and kinds of controls. For instance, it is not possihle to control the flow rates of the feed and the top and bottom products under perturbed conditions without upsetting holdup in the system. 

Cover temperature is another variable which controls distillation rate and efficiency. All of the heat transferred to the underside of the cover from the basin, plus the small solar absorption in it, must be dissipated by convection to the surrounding air and by radiation to the sky. Ambient temperature, wind velocity, and atmospheric clarity all influence the temperature driving force necessary to attain the equilibrium heat transfer rate. Cover temperature, in turn, affects basin temperature, so that an over-all equality in heat flows prevails. The primary variable remains, of course, the solar energy input rate, its most important effect being the temperature level in the salt water basin. 

Controlling Quality of Two Products Where the two products have similar values, or where heating and cooling costs are comparable to product losses, the compositions of both products should be controlled. This introduces the possibility of strong interaction between the two composition loops, as they tend to have similar speeds of response. Interaction in most columns can be minimized by controlling distillate composition with reflux ratio and bottom composition with boil-up, or preferably boil-up/bottom flow ratio. These loops are insensitive to variations in feed rate, eliminating the need for feedforward control, and they also reject heat balance upsets quite effectively. 

Feedforward control system that provides constant separation by manipulating the distillate flow (top). At the bottom, a variety of dynamic compensators are shown, which can be used to match the "dynamic personality" of the process. 

Whenever the feed composition is unpredictable, one must directly control the compositions of both products. The main benefit of dual composition control is minimized energy consumption. The main limitation is caused by the interactions between the two composition loops. On the left of Figure 2.93 an example of a feedforward dual composition control system is shown. In this configuration, the distillate flow is manipulated to control the distillate composition by maintaining the relationship ... 

Multiproduct fractionator controls, where, after dynamic correction, the boil-up, side-draw and distillate flows are ratioed to the feed flow (left). On the right, the true boiling points are controlled by throttling the product flows, while heat balance is controlled by manipulating the reflux flows. 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

The three quality specifications regarding the impurities in EDC, available by direct concentration measurements, such as by IR spectroscopy or online chromatography, are the outputs of the plantwide control problem. The degrees of freedom indicate as first choice manipulated variables belonging to the large column S2 D2-distillate flow rate, SS2-side-stream flow rate, and Q2-reboiler duty. We may also consider manipulated variables belonging to the small column... 

Example 3.9 A depropanizer normally operates as described in Example 2.4. The column is computer-controlled, using the Jafarey et al. algorithm. The algorithm manipulates reflux flow to control top product purity. Distillate flow rate must remain fixed, but bottom purity is allowed to vary. The top product purity spec is temporarily relaxed from 0.5 mole percant to 0.9 mole percent. What would the controller set the reflux flow at ... 

The optimal robust controller designed with one of the new synthesis techniques is generally not of a form that can be readily implemented. The main benefit of the new synthesis procedure is that it allows the designer to establish performance bounds that can be reached under ideal conditions. In practice, a decentralized (multiloop) control structure is preferred for ease of start-up, bumpless automatic to manual transfer, and fault tolerance in the event of actuator or sensor failures. Indeed, a practical design does not start with controller synthesis but with the selection of the variables that are to be manipulated and measured. It is well known that this choice can have more profound effects on the achievable control performance than the design of the controller itself. This was demonstrated in a distillation example [17, 18] in which a switch from reflux to distillate flow as the manipulated variable removes all robustness problems and makes the controller design trivial. 

For example, suppose that the distillate flowrate from a distillation column is large compared to the reflux. We normally would use distillate to control level in the reflux drum. But suppose the distillate recycles back to the reactor and so we want to control its flow. What manipulator should we use to control reflux drum level We could potentially use condenser cooling rate or reboiler heat input. Either choice would have implications on the control strategy for the column, which would ripple through the control strategy for the rest of the plant. This would lead to control schemes that would never be considered if one looked only at the unit operations in isolation. 

R-V Reflux flow controls distillate composition. Heat input controls bottoms composition. By default, the inventory controls use distillate flowrate to hold reflux drum level and bottoms flowrate to control base level. This control structure (in its single-end control version) is probably the most widely used. The liquid and vapor flowrates in the column are what really affect product compositions, so direct manipulation of these variables makes sense. One of the strengths of this system is that it usually handles feed composition changes quite well. It also permits the two products to be sent to downstream processes on proportional-only level control so that plantwide flow smoothing can be achieved. 

Figure 6.9 Common types of columns and controls, ia) Stabilizer ismall distillate flow) (b) superfractionator with distillate-bottoms control structure.

In the second case (manipulate distillate to control reflux ratio), the variability of the distillate flow would be greatly reduced. The reflux drum level controller, manipulating reflux flowrate, is made P-only to get slow changes in reflux flowrate, and this gives slow" changes in distillate flowrrate in the reflux-ratio control structure. 

Having made the choice to fix the purge column distillate flow, we are faced wdth the problem of how to control purge column reflux drum level. We have two primary choices reflux flow or heat input. We choose the latter because the flowrate of the purge column reflux is small relative to the vapor coming overhead from the top of the column. Remember the Richardson rule, which says we select the largest stream. 

Seven liquid levels are in the process separator and two (base and overhead receiver) in each column. The most direct way to control separator level is with the liquid flow to the stabilizer column. Then stabilizer column overhead receiver level is controlled with cooling water flow and base level is controlled with bottoms flow. In the product column, distillate flow controls overhead receiver level and bottoms flow controls base level. 

If we use distillate flow from the recycle column to control overhead receiver level, then we see that all of the flows around the liquid recycle loop are set on the basis of level. This violates our original statement to fix the toluene recycle flow. We are then left with the question how to control the overhead receiver level in the recycle column. We can use the fresh makeup toluene feed to control this level since it represents the toluene inventory in the process. Such a scheme limits large flowrate changes to the refining section and automatically ensures the component balance for toluene. 

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