Flow-rate upsets

One approach which has resulted in experimental implementation is that of Randolph and co-workers f88-92 >. Using a simulation (21) Randolph and Beckman demonstrated that in a complex RTD crystallizer, the estimation of nuclei density could be used to eliminate cycling or reduce transients in the CSD. Randolph and Low (gg) experimentally attempted feedback control by manipulation of the fines dissolver flow rate and temperature in response to the estimated nuclei density. They found that manipulation of fines flow rate upset the fines measurement indicating that changes in the manipulated variable disturbed the measured variable. Partial fines dissolution resulting from manipulation of the fines dissolver temperature appeared to reduce CSD transients which were imposed upsets in the nucleation rate. In a continuation of this work Randolph et. al. < 921 used proportional control of inferred nuclei density to control an 18 liter KCl crystallizer. 

The high viscosity of some high MW samples is known to cause flow rate upsets as the sample passes through the SEC column frits. Such flow rate upsets often occur at the time of elution of the sample. While the flow rate upsets like this are likely to cause viscosity detection errors in most other SEC viscosity detectors, the signal of the present viscosity detector, however, will remain true, cind unaffected by the high sample viscosity problem. 

Volumetric heat generation increases with temperature as a single or multiple S-shaped curves, whereas surface heat removal increases linearly. The shapes of these heat-generation curves and the slopes of the heat-removal lines depend on reaction kinetics, activation energies, reactant concentrations, flow rates, and the initial temperatures of reactants and coolants (70). The intersections of the heat-generation curves and heat-removal lines represent possible steady-state operations called stationary states (Fig. 15). Multiple stationary states are possible. Control is introduced to estabHsh the desired steady-state operation, produce products at targeted rates, and provide safe start-up and shutdown. Control methods can affect overall performance by their way of adjusting temperature and concentration variations and upsets, and by the closeness to which critical variables are operated near their limits. 

All the previous material balance examples have been steady-state balances. The accumulation term was taken as zero, and the stream flow-rates and compositions did not vary with time. If these conditions are not met the calculations are more complex. Steady-state calculations are usually sufficient for the calculations of the process flow-sheet (Chapter 4). The unsteady-state behaviour of a process is important when considering the process start-up and shut-down, and the response to process upsets. 

In some situations it is very important to be able to increase the flow rate above the design conditions (for example, the cooling water to an exothermic reactor may have to be doubled or tripled to handle dynamic upsets). In other cases this is not as important (for example, the feed flow rate to a unit). Therefore it is logical to base the design of the control valve and the pump on having a process that can attain both the maximum and the minimum flow conditions. The design flow conditions are only used to get the pressure drop over the heat exchanger (or fixed resistance part of the process). 

For example, it is important to have large enough holdups in surge vessels, reflux drums, column bases, etc., to provide effective damping of disturbances (a much-used rule of thumb is 5 to 10 minutes). A sufficient excess of heat transfer area must be available in reboilers, condensers, cooling Jackets, etc., to be able to handle the dynamic changes and upsets during operation. The same is true of flow rates of manipulated variables. Measurements and sensors should be located so that they can be used for eflcctive control. 

It is also necessary to sample influent over a period of time to arrive at the average influent quality It should account for variations in daily operations caused by fluctuations in flow rates, shortterm variations in brine charactensucs, and equipment upsets... 

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. 

Nevertheless, excessive noise at the point of discharge may be generated aero-dynamically by a full lift SRV discharging to atmosphere at a maximum emergency flow rate during an occasional upset of the process. As just described, it should be noted that the noise level is usually of short duration. 

Some typical disturbance patterns and control difficulties are summarized here. A detailed discussion is made in (1). Hydraulic disturbances are significant in amplitude. Diurnal variations as well as shock loads from rain storms or melting snow may cause major upsets. Significant disturbances also appear from internal sources like primary pumps, back-washing of deep bed filters or return sludge flow rate changes. The amplitudes are such, that quasi-stationary of linear control methods are seldom adequate. 

In any reaction system operating at steady state, there will always be upsets to the system—some large, some small—that may cause it to operate ineflfi-ciently, shut down, or perhaps explode. Examples of upsets that m occur are variations of the feed temperature, composition, and/or flow rate, the cooling jacket temperature, the reactor temperature, or some other variable. To correct for these upsets, a control system is usually added to make adjustments to the reaction system that will minimize or eliminate the effects of the upset. The material that follows gives at most a thumbnail sketch of how controllers help cause the reactor system to respond to unwanted upsets. 

We now consider what can happen to a CSTR operating at an upper steady state when an upset occurs in either the ambient temperature, the entering femperature, the flow rate, reactor temperature, or some other variable. To illustrate, let s reconsider the production of propylene glycol in a CSTR. 

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