Feeding responses, control

The efficiency of the Claus Process, long the means of conversion of H2S to sulfur, has been increased through improvements in reaction furnace, catalyst bed and computerized feed composition control leading to recovery efficiencies in excess of 98%. Recent development of a Claus Process under pressure may yield further important improvements. Add on tail gas clean-up processes have further reduced plant effluent in response to environmental protection requirements. 

Forcing function is a term given to any disturbance which is externally applied to a system. A number of simple functions are of considerable use in both the theoretical and experimental analysis of control systems and their components. Note that the response to a forcing function of a system or component without feedback is called the open-loop response. This should not be confused with the term open-loop control which is frequently used to describe feed-forward control. The response of a system incorporating feedback is referred to as the closed-loop response. Only three of the more useful forcing functions will be described here. 

We conclude this discussion by showing the closed-loop response to two different disturbances. In Fig. 5.26 we reduce the setpoint of the reactor feed temperature controller. In Fig. 5.27 we reduce the amount of toluene fed to the reactor. The changes shown were the largest that could be handled by the system with the small furnace and the large exchanger without a bypass. The design with the bypass (CS2) and the... 

Long time response (2) long time response and increasing complexity (3) V/F control is not desirable in general (4) column bottom level control by steam flow is desirable. (Comments are due toT. Umeda). b Direct M.B. = direct material balance control Indirect M.B. indirect material balance control V/F = vapor to feed ratio control... 

Figure 8.50 gives responses to 20% increases and decreases in the set point of the feed flow controller. The solid lines are increases and the dashed lines are decreases. Stable control is obtained with transients settling out in about an hour. Stage 3 temperature is tightly controlled by manipulating distillate flow rate. Bottoms water composition remains quite close to the desired 12 mol% specification. Distillate water purity remains quite close to the desired 99.5 mol% specification. 

Figure 18.17 gives the responses of the process to the 30% disturbances in the setpoint of the feed flow controller for this system with no anti-reset-windup protection. The column pressure drop increases when feed flow rate increases as the temperature controller calls for a higher reboiler duty. When the pressure drop gets to 0.5 bar, the... 

For applications which demand a high NOx-reduction but which have slow changes in exhaust temperature and massflow, a feed-back control based on NOx-measurement downstream of the SCR is additionally used. Depending on the volume of catalyst installed and on the operating temperature, the response time of... 

Figure 6.13 shows the response of the system to a 5% step increase in the setpoint of the feed flow controller at time equals 0.2 h. The solid lines are for the case with the control structure discussed above and shown in Figure 6.12. Both product flowrates (Bj and B2) and the distillate flowrates (Dj and D2) increase. The temperature on Stage 14 in the low-pressure column drops quickly from 79°C to about 73°C, but the controller eventually...

Figures 12.13 and 12.14 give responses for feed flowrate and feed composition disturbances. Stable regulatory control is shown with product purities held close to their specifications for all of these large disturbances. Figure 12.13 gives the responses of the system for step changes in the setpoint of the feed flow controller at 0.2 h. The solid lines are for a 10% increase, the dashed lines are for a 20% increase, and the dotted lines are for a 20% decrease. Increasing the feed results in increases in both distillate product streams, as expected. The two temperatures in the extractive column are held at their setpoint values after a short transient period, as is the average temperature in the solvent recovery column. The purities of the two products are held quite close to their desired values. The largest offset in the acetone product purity occurs for the 20% decrease in feed flowrate where it drops to 99.4 mol%. The system comes to a new steady state in less than 2 h.

The two start-up feed water control valves and the two start-up feed water isolation valves must close automatically in response to potential over-cooling of the reactor coolant system or potential over-filling of a steam generator. 

The bottom half of the figure shows the material balance control scheme in which the top quality is controlled by the distillate draw-off However, the draw-off does not affect the top quality but rather the level. The level in turn affects the reflux flow, which subsequently affects the top quality. This means that there is a severe degree of mutual interaction between the control loops. It was found that the control structure of Fig. 34.8b would result in an oscillatory behavior of the quality control loop for feed flow changes of +10%. Only addition of a feedforward loop from distillate flow to reflux would stabilize the quality control loop. In that case, the distillate flow changes were subtracted from the reflux flow changes calculated by the level controller. After addition of feed-forward control, the response of the top and bottom qualities were similar to the responses of the energy balance control scheme for this situation with virtually no deviation from setpoint of the top quality and a bottom quality response similar to Fig. 34.7. 

The matrixes Y, C, D, and v are the output states, the output matrix, the feed forward control force matrix, and the noise matrix, respectively. In the case where the output variables are the same with the states of the system and there is no application of the control forces to the output variables, the matrixes C, D are the identity and zero matrix, respectively. The noise matrix depends oti the sensor that is used to measure the response of the system. The above equation can be solved by any numerical technique for differential equations, like an explicit Rtmge-Kutta formula, the Dormand-Prince pair, Bogacki-Shampine, and Adams-Bashforth-Moulton PECE solver. 

Figure 8.15 illustrates the second common choice, indirect feed spht control, where the distillate flow is increased indirectly by increasing the steam flow. The compositions are controlled by a ten5)erature controller that manipulates the steam flow. This alternative has two advantages. One is that the temperature loop has faster closed-loop response (i.e. shorter natural period) and, therefore, provides better disturbance rejection. The second is that, because the reflux drum level sets the distillate flow, the reflux dmm can be used to smooth flow disturbances to other... 

Disturbances in both throughput and feed composition were used to evaluate the effectiveness of this alternative control stmcture. Figure 15.11 shows the responses to 20% step changes in the setpoint of the methanol feed flow controller. Solid lines are increases, and dashed lines are decreases. The increase is handled qirite well. It is surprising that the decrease is not handled well. There is a significant transient drop in MTBE purity in the bottoms product (middle left graph. Fig. 15.11), whieh lasts for several hours. In addition and of more importance, there is a very large increase in the isobutoie lost in the distillate. 

Thus, they concluded that ATP-sulfurylase was subject to feed-forward stimulation by a reaction product of nitrate reduction and that nitrate reductase was subject to an analogous feed forward by a product of sulfate, thus coordinating the first step of each of the two pathways. Some support for this mechanism has come fi om experiments with cell cultures of Ipomea (Zink, 1984) and from measurements of ATP-sulfurylase and nitrate reductase in the leaves of whole tobacco plants (Barney and Bush, 1985), though in the latter study the response of ATP-sulfurylase to nitrogen stress was far less pronounced than the response of nitrate reductase to sulfur stress. However, as discussed below, APS sulfo-transferase rather than ATP-sulfurylase appears to be the principal determining site for the feed-forward control of sulfate assimilation by nitrogen assimilation products in Lemna. 

The size of the leakage was determined as follows. Radiation detectors were mounted on the inlet of the first side, and the outlet of the second side of the heat exchanger and at additional locations for control. A short pulse of Kr-85 tracer (<0.1 sec) was injected into the feed stream, which gave rise to detector responses shown in figure 3. 

Control of secretion of anterior pituitary hormones also includes inhibition by hormones produced by target organs. For example, CRH stimulates the anterior pituitary to secrete ACTH, which in turn stimulates the adrenal cortex to secrete corticosteroids. Corticosteroids then feed back to inhibit the secretion of ACTH. Feedback mechanisms are important for the control of most hormones. For example, insulin (qv) secretion from the pancreas increases in response to increased blood glucose resulting from ingestion of a meal. Insulin increases tissue uptake and metaboHsm of glucose, which lowers blood glucose and in turn reduces insulin secretion. 

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