Recycle loop, control with

Time-Delay Compensation Time delays are a common occurrence in the process industries because of the presence of recycle loops, fluid-flow distance lags, and dead time in composition measurements resulting from use of chromatographic analysis. The presence of a time delay in a process severely hmits the performance of a conventional PID control system, reducing the stability margin of the closed-loop control system. Consequently, the controller gain must be reduced below that which could be used for a process without delay. Thus, the response of the closed-loop system will be sluggish compared to that of the system with no time delay. 

The unit was built in a loop because the needed 85 standard m /hour gas exceeded the laboratory capabilities. In addition, by controlling the recycle loop-to-makeup ratio, various quantities of product could be fed for the experiments. The adiabatic reactor was a 1.8 m long, 7.5 cm diameter stainless steel pipe (3 sch. 40 pipe) with thermocouples at every 5 centimeter distance. After a SS was reached at the desired condition, the bypass valve around the preheater was suddenly closed, forcing all the gas through the preheater. This generated a step change increase in the feed temperature that started the runaway. The 20 thermocouples were displayed on an oscilloscope to see the transient changes. This was also recorded on a videotape to play back later for detailed observation. 

The output of the recycle block is added to the original input to the process u, and the sum of these two signals enters the forward block Gp(s>. It is important to note that the recycle loop in this process features positive feedback, not negative feedback that we are used to dealing with in feedback control. Most recycles produce this positive feedback behavior, which means that an increase in the recycle flowrate causes an increase in the flowrates through the process. 

Once we have fixed a flow in each recycle loop, we then determine what valve should be used to control each inventory variable. This is the material balance step in the Buckley procedure. Inventories include all liquid levels (except for surge volume in certain liquid recycle streams) and gas pressures. An inventory variable should typically be controlled with the manipulated variable that has the largest effect on it within that unit (Richardson rule). Because we have fixed a flow in each recycle loop, our choice of available valves has been reduced for inventory control in some units. Sometimes this actually eliminates the obvious choice for inventory control for that unit. This constraint forces us to look outside the immediate vicinity of the holdup we are considering. 

Case 1 is desirable from the standpoint that it eliminates interactions from the rest of the plant. In other words, it is transparent to the reactor whether it is an isolated unit or part of a process with recycles. The economic objectives of the process are satisfied through partial control by adjusting the setpoints of the feedback loops. We can argue that the vinyl acetate reactor discussed in Chap. 11 falls in this category. The dominant variables are reactor exit temperature and oxygen inlet concentration. Both of these variables are controlled at the unit, making the reactor resilient against disturbances from the separation system. 

Here we have dealt with the control of chemical reactors. We covered some of the fundamentals about kinetics, reactor types, reactor models, and open-loop behavior. In particular we have shown that reactors with recycle or backmixing can exhibit multiple steady states, some of which are unstable. Nonlinearities in reactor systems also frequently give rise to open-loop parametric sensitivity. 

Step 7. Methane is purged from the gas recycle loop to prevent it from accumulating, and its composition can be controlled with purge flow. Diphenyl is removed in the bottoms stream from the recycle column, where steam flow controls base level. Here we control composition (or temperature with the bottoms flow. The inventory of benzene is accounted for via temperature and overhead receiver level control in the product column. Toluene inventory is accounted for via level control in the recycle column overhead receiver. Purge flow and gas-loop pressure control account for hydrogen inventory. 

The most direct way to control the remaining levels would be with the exit valves from the vessels. However, if we do this we see that all of the flows around the liquid recycle loop would be set on the basis of levels, which would lead to undesirable propagation of disturbances. Instead we should control a flow somewhere in this loop. Acetic acid is the main component in the liquid recycle loop. Recycle and fresh acetic acid feed determine the component s composition in the reactor feed. A reasonable choice at this point is to control the total acetic acid feed stream flow into the vaporizer. This means that we can use the fresh acetic acid feed stream to control column base level, since this is an indication of the acetic acid inventory in the process. Vaporizer level is then controlled with the vaporizer steam flow and separator and absorber levels can be controlled with the liquid exit valves from the units. 

Step 7. Ethane is an inert component that enters with the ethylene feed. It can be removed from the process only via the gas purge stream, so purge flow is used to control ethane composition. Carbon dioxide is an unwanted by-product that leaves in the C02 removal system. As long as the amount of carbon dioxide removed is proportional in some way to the C02 removal system feed, we can use this valve to control carbon dioxide composition. Oxygen inventory is accounted for via composition control with fresh oxygen feed. Inventory of ethylene can be controlled to maintain gas loop pressure, since ethylene composes the bulk of the gas recycle. 

Once fixed a flow in each recycle loop, one can determine what valve should be used to control each inventory variable. Inventory may be controlled with fresh reactant make-up streams but also with streams leaving the units. Liquid streams may be added to a location where the level varies with the amount of that component in the process. Similarly, gas fresh feed streams may be added to a location where the pressure gives a measure of the amount of that material in the process. 

We stress that design for controllability can either aim at reducing control bandwidth limitations, imposed by fundamental process properties, or at reducing the control requirements imposed by disturbance sensitivities. Based on results from linear systems theory we have presented simple model based tools, based on the decomposed models above, which can be used to improve stability, non-minimum phase behavior and disturbance sensitivities in plants with recycle. One important conclusion of the presented results is that the phase-lag properties of the individual process units play a crucial role for the disturbance sensitivity of an integrated plant. In particular, by a careful design of the recycle loop phase lag, it is possible to tailor the effect of process interactions such that they serve to effectively dampen the effect of disturbances in the most critical frequency region, that is, around the bandwidth of the control system. 

In the above series, an important paper of Tyreus and Luyben [5] deals with second-order reactions in recycle systems. Two cases are considered complete one-pass conversion of a component (one recycle), and incomplete conversion of both reactants (two recycles). As general heuristic, they found that fixing the flow in the recycle might prevent snowballing. In the first case, the completely converted component could be fed on flow control, while the recycled component added somewhere in the recycle loop. In the second case, the situation is more complicated. Four reactant feed control alternatives are proposed, but only two workable. This is the case when both reactants are added on level control in recycles (CSl), or when the reactant is added on composition control combined with fixed reactor outlet (CS4). As disadvantage, the production rate can be manipulated only indirectly. Other control structures - with one reactant on flow control the other being on composition (CS2) or level control (CS3) - do not work. The last structure can be made workable if the recycle flow rates are used to infer reactant composition in the reactor. This study reinforces the rule that the flow rate of one stream in a liquid recycle must be fixed in order to prevent snowballing. 

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