Distillation composition control

Figure 2.2 Three different systems for the distillate composition control of a simple distillation column (a) feedback (b) feedforward (c) inferential.

Steam to the turbine driving the compressor is flow controlled, reset by a speed controller, which is reset by a distillate composition controller. 

Reflux-to-distillnte ratio was 70 1. Boilup was controlled fay bottom composition, distillate by aocumulatw level, and r lux by distillate composition. Control was unstable and extreme slow. System was modified to control reflux ly aocumulatcn level, boilup by steam flow, and distillate fay set flow ac usted manuaUy for distillate composition. 

These figures illustrate some of the issues that our control design will later have to address. Firstiy if both MVs affect both product compositions, which one do we select for distillate composition control and which for bottoms Secondly, if we adjust one MV to correct off-spec production, how do we deal with the problem that this will put the other product off grade ... 

We have seen that the distillate composition controller on the preferred material balance scheme relies on the level controller. If this is switched to manual then the composition control should be automatically disabled and not permitted to be recommissioned until the level is back on automatic. Similarly, on the less preferred version of the material balance scheme, bottoms composition control should not be permitted if the column level controller is switched to manual. Without this precaution the composition controller will ramp its output until it saturates. 

We have seen that the Ryskamp scheme largely breaks the interaction in one direction so that corrections made to the bottoms composition have little impact on the distillate. Although the converse is not true an adjustment to the reflux ratio will affect the bottoms composition, but when its controller takes corrective action it will not light the distillate composition controller. 

Fortunately the remedy is simple. The dynamic element in the heat-input loop, g (t), should be a lag, adjusted to favor bottoms-composition regulation the dynamic element in the distillate loop, gD(t), must also be a lag, adjusted for distillate-composition control after the other is set. This arrangement is favored even when reflux is manipulated from accumulator level the steam loop acts as a formidable accelerating agent. 

A more efficient way to operate a batch still is on the basis of constant distillate composition. Because bottoms composition continually changes, separation must also change if constant distillate composition is to be maintained. Consequently, D/V will be high at the beginning of a batch and will gradually be reduced to zero by the distillate-composition controller, when all of the recoverable product has been withdrawn. A control system operating on this basis appears in Fig. 11.26. The temperature controller would require reset action, to maintain constant quality with changing distillate rate. 

With all feed conditions and the column configuration specified (number of trays in each section, tray holdup in the reactive section, feed tray locations, pressure, and desired conversion), there is only one remaining degree of freedom. The reflux flowrate is selected. It is manipulated by a distillate composition controller to drive the distillate composition to 95 mol% C. The vapor boilup is manipulated to control the liquid level in the base. Note that the distillate and bottoms flowrates are known and fixed as the dynamic model is converged to the steady state that gives a distillate composition of 95 mol% C. The composition of the bottoms will be forced by the overall component balance to be 95 mol% D. 

Avoid attempts to recover simultaneously both high and low boiling nodes in high purity from mixtures of >3 components, particularly in columns that reflux compositions different from the distillate composition, ie, reflux of one phase from a decanter, as such operations may be difficult to control. 

RGA Example In order to illustrate use of the RGA method, consider the following steady-state version of a transfer function model for a pilot-scale, methanol-water distillation column (Wood and Berry, Terminal Composition Control of a Binaiy Distillation Column, Chem. Eng. Sci, 28, 1707, 1973) Ku = 12.8, K = -18.9, K. i = 6.6, and Koo = —19.4. It follows that A = 2 and... 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at periodic intervals, a sample of the overhead product is analyzed in the laboratory and the information provided to the process operator. Should this analysis be outside acceptable limits, the operator would adjust the set point for the temperature. This procedure effectively compensates for an inaccurate temperature measurement however, the success of this approach requires good repeatability from the temperature measurement. 

Parameter Estimation Relational and physical models require adjustable parameters to match the predicted output (e.g., distillate composition, tower profiles, and reactor conversions) to the operating specifications (e.g., distillation material and energy balance) and the unit input, feed compositions, conditions, and flows. The physical-model adjustable parameters bear a loose tie to theory with the limitations discussed in previous sections. The relational models have no tie to theory or the internal equipment processes. The purpose of this interpretation procedure is to develop estimates for these parameters. It is these parameters hnked with the model that provide a mathematical representation of the unit that can be used in fault detection, control, and design. 

Devise a control scheme, such that the column maintains a distillate composition at xq > 0.9. 

The primary objective of distillation column control is to maintain the specified composition of the top and bottom products, and any side streams correcting for the effects of disturbances in ... 

Operation at constant reflux ratio is better than operation with constant distillate composition for high-yield batch separations. However, operation with constant distillate composition might be necessary if high product purity is required. In fact, it is not necessary to operate in one of these two special cases of constant reflux ratio or constant distillate composition. Given the appropriate control scheme, the reflux ratio can be varied through the batch... 

Automatic control of distillate composition (xD) may also be affected by control of the reflux ratio, for example to maintain the distillate composition at constant set point (xDset). 

Batch distillation with continuous control of distillate composition via the regulation of reflux ratio is illustrated in the simulation example BSTILL. In this an initial total reflux condition, required to establish the initial concentration profile with the column, is represented in the simulation by a high initial value of R, which then changes to the controller equation for conditions of distillate removal. 

The process is as described in Section 3.3.3.2 and consists of a distillation column containing seven theoretical plates, reboiler and reflux drum. Distillation is carried out initially at total reflux in order to first establish the column concentration profile. Distillate removal then commences at the required distillate composition under proportional control of reflux ratio. This model is based on that of Luyben (1973, 1990). 

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. 

To illustrate the concept, consider a single distillation column with distillate and bottoms products. To produce these products while using the minimum amount of energy, the compositions of both products should be controlled at their specifications. Figure 8.13u shows a dual composition control system. The disadvantages of this structure arc (1) two composition analyzers are required, (2) the instrumentation is more complex, and (3) there may be dynamic interaction problems since the two loops are interacting. This system may be difficult to design and to tune. 

Figure 11.3d shows a process where the manipulated variable affects the two controlled variables and in parallel. An important example is in distilla tion column control where reflux flow aSecte both distillate composition and a tray temperature. The process has a parallel structure and this leads to a parallel cascade control system. 

Notice that the pairing assumes distillate composition jtp is controlled by reflux R and bottoms composition Xg is controlled by vapor boilup V. 

Distillate composition is measured by a chromatograph with a deadtime equal to the sampling period. If a proptortional sampled-data controller is used with a zero-order hold, calculate the ultimate gain for T, = 2 and 10. 

Figure 13.10. Batch distillation McCabe-Thiele constructions and control modes, (a) Construction for constant overhead composition with continuously adjusted reflux rate, (b) Construction at constant reflux at a series of overhead compositions with an objective of specified average overhead composition, (c) Instrumentation for constant vaporization rate and constant overhead composition. For constant reflux rate, the temperature or composition controller is replaced by a flow controller.

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. 

In the use of temperature measurement for control of the separation in a distillation column, repeatability is crucial but accuracy is not. Composition control for the overhead product would be based on a measurement of the temperature on one of the trays in the rectifying section. A target would be provided for this temperature. However, at... 

In this example, the five manipulated variables are so assigned to the five controlled variables that the heat input at the reboiler and the distillate composition are fixed and therefore the bottoms flow and composition are allowed to change with the variations in feed flow or composition. 

Figure 2.83 illustrates a possible end result of calculating the relative gain (RG) values. Here, it was concluded that fixing the production rate (heat input to the column [QB]) and controlling only the distillate composition, while... 

Distillate composition can be controlled by a cascade temperature master on the upper part of the column, which manipulates the reflux flow L (left). Similarly, the bottoms composition can be controlled by a cascade temperature master located on the lower half of the column, throttling the reboiler heat input (right). 

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 ... 

Interaction is unavoidable between the material and energy balances in a distillation column. The severity of this interaction is a function of feed composition, product specification, and the pairing of the selected manipulated and controlled variables. It has been found that the composition controller for the component with the shorter residence time should adjust vapor flow, and the composition controller for the component with the longer residence time should adjust the liquid-to-vapor ratio, because severe interaction is likely to occur when the composition controllers of both products are configured to manipulate the energy balance of the column and thereby "fight" each other. 

Kerkhof and Vissers showed that for difficult separations an optimal reflux control policy yields up to 5% more distillate, corresponding to 20-40% higher profit, than either constant distillate composition or constant reflux ratio policies. 

Let us consider a CSTR/separator/recycle system, where the first-order reaction A —> P takes place. Figure 4.3(a) presents the conventional control of the plant. The fresh feed flow rate is kept constant at the value F0. The reactor holdup V is controlled by the effluent. The reaction takes place at a constant temperature, which is achieved by manipulating the utility streams. Dual-composition control of the distillation column ensures the purities of the recycle and product streams. 

The control structure discussed in this section is presented in Figure 4.4(a). The reactor-inlet flow rate is fixed at the value l. Reactor effluent controls the reactor holdup V, while the coolant flow rate controls the reactor temperature. Dual composition control is used for the distillation column. The reactant is fed on level control. For illustration purposes, a buffer vessel was considered. This increases the equipment cost and might be unacceptable due to safety or environmental concerns. An alternative is to feed the reactant in the condenser dram of the distillation column. This strategy achieves the regulation of reactant inventory, because any imbalance is reflected by a change of the holdup. 

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