Inventory loop

These same notions can be extended to an entire plant in which several unit operations are connected together. The HDA process for hydrodealkylation of toluene to form benzene is a good example of where an eigenstructure can be found that provides a more easily and simply controlled plant. See Fig. 8.15. Assuming that the toluene feed rate to the unit is fixed, this plant has 22 valves that must be set. There are 11 inventory loops (levels and pressures), so they require 11 valves. One possible conventional control structure is shown in Fig. 8.15. 

Figure 2.2a shows the situation where the fresh feed stream is flow-controlled into the process. The inventory loops (liquid levels) in each unit are controlled by manipulating flows leaving that unit. All disturbances propagate from unit to unit down the series configuration. The only disturbances that each unit sees are changes in its feed conditions. 

Figure 2.26 shows the on-demand situation where the flowrate of product C leaving the bottom of the second column is set by the requirements of a downstream unit. Now some of the inventory loops (the base of both columns) are controlled by manipulating the feed into each column. 

Then the inventory loops are revisited. The liquid holdups in surge volumes are calculated so that the time constants of the liquid level loops (using proportional-only controllers) are a factor of 10 larger than the product-quality time constants. This separation in time constants permits independent tuning of the material-balance loops and the prod-... 

Had we started to assign the DIB column base level control first, we would have ended up with the same inventory control structure. The reason is as follows. Assume we had chosen the DIB column base valve to control base level. After resolving the purge column inventory loops, we would have found that we needed to control the purge column base or reflux drum level with the fresh feed flow to the DIB column. The dynamic lags associated with these loops would have forced us back to the control strategy as described above. 

In pressure driven simulation the flow rate is related with the pressure drop. Consequently, valves must be added for both inlet and outlet streams. For example, the inlet flow rate can be calculated from the pressure drop between feed supply and vessel. Similarly, the product flow rates can be calculated from the pressure drop between vessel and outlet lines. As before, controllers are added automatically for the basic inventory loops, as levels and pressures. 

Then produet quality loops are closed on eaeh of the individual units. These loops typically use fast proportional-integral eontrollers to hold product streams as close to speciheation values as possible. Sinee these loops are typically quite a bit faster than the slow inventory loops, interaction between the two is often not a problem. Also, sinee the manipulated variables used to hold produet qualities are quite often streams internal to each individual unit, changes in these manipulated... 

Subtract the steady-state degrees of freedom from the overall degrees of freedom to determine how many inventory loops can be closed with available control valves. 

Then, product quality control loops are closed on each of the individual units. These loops typically use fast PI controllers to hold product streams as close as possible to specification values. Since these loops are considerably faster than the slow inventory loops, interaction between the two is generally not a probleia Also, since the manipulated variables used to hold product qualities are often streams that are internal to each individual unit, changes in these manipulated variables have little effect on the downstream process. The manipulated variables frequently are utility streams that are provided by the plant utility system, i.e. cooling water, steam, refrigerant, etc. Thus, the boiler house will be disturbed, but the other process units in the plant will not see disturbances coming from upstream process units. Of course, this is only tme when the plant utilities systems have effective control systems that can respond quickly to the many disturbances that they see coming in from units all over the plant. 

This brief overview of separate treatment techniques is not complete, however it can be used for a first inventory and identification of treatment steps which may be considered as part of a complete closed loop water system. It has to be taken in mind that the above-mentioned overview of separate treatment techniques is primarily based on one type of pollutant and one physical state of that pollutant. It will be clear that very often the same treatment step can be applied to remove different types of pollutants. It is also evident that a large percentage of the separate treatment steps mentioned will result in a concentrate containing the pollutants. This concentrate has to be treated subsequently. 

The starting point in the development and designing of a closed water loop system is an inventory of the amounts and the quality of the process and transport water flows which are needed for the various steps in the production process. Each production step where process or transport water is involved causes a certain amount of wastewater. The pollution of this water is strongly dependent on the process step. The selection of separate treatment steps which, together, comprise a closed loop water system is complex. As already mentioned, various complete treatment scenarios can be developed and designed to satisfy the requirements set for process and transport water and treatment of wastewater. A technical and economic evaluation, in combination with environmental sustainability assessment, is necessary to determine the treatment system which is most appropriate. 

LEVEL LOOPS. Most liquid levels represent material inventory used as surge capacity. In these cases it is relatively unimportant where the level is, as long as it is between some maximum and minimum levels. Therefore, proportional controllers arc often used on level loops to give smooth changes in flow rates and to filter out fluctuations in flow rates to downstream units. 

The constraint of the pressure drop across the downcomer is graphically illustrated in Fig. 10.8. For a given solids inventory in the downcomer and given gas and solids flow rates, the pressures at the bottom of the riser and the downcomer can be determined at steady state. Under normal operating conditions (point A in the figure), the pressure drop across the riser is balanced by the pressure drop across the recirculation loop. If a small reduction in gas velocity takes place, the flow in the riser responds by moving upward along the pressure drop curve of the riser to point B. On point B of line AB, the decrease in the gas velocity causes the pressure drop across the riser to rise by SPt, which has to be balanced by the... 

Helium loop pressures are shown in Figure 7. Pressure is to a first order proportional to hydrogen production rate, a consequence of inventory control. The production and consumption of power by major system components is shown in Figure 8. Essentially all the thermal power produced by the NGNP is consumed by thermal loads in the HTE plant and in generating electricity to power electrical loads which include the electrolyser and pumps and compressors. 

Control objectives related to the operation of the process units and the process itself (production rate, product quality, unit-level, and total inventory) should be addressed in the fast time scale. For instance, when a multi-loop linear control strategy is considered, the reset time for the controllers should be of the order of magnitude of the time constants of the individual process units. 

We say that the inventory is self-regulating. Similarly, the plantwide control can fix the flow rate of reactant at the plant inlet. When the reactant accumulates, the consumption rate increases until it balances the feed rate. This strategy is based on a self-regulation property. The second strategy is based on feedback control of the inventory. This consists of measuring the component inventory and implementing a feedback control loop, as in Fig. 4.2(b). Thus, the increase or decrease of the reactant inventory is compensated by less or more reactant being added into the process. 

The plantwide control deals, mainly, with the mass balance of the species involved in the process. The species inventory can be maintained based on two different principles, namely self-regulation and feedback control. Control structures based on self-regulation set the flow rates of fresh reactants at values determined by the production rate and stoichiometry. Control of inventory by feedback consists of fixing one flow rate in each recycle loop, evaluating the inventory by means of concentration or level measurements, and reducing the deviations from the setpoint by change of the feed rate of fresh reactants. 

Summing up, if the inventory of the main components can be handled by local control loops, the inventory of impurities has essentially a plantwide character. The rates of generation, mainly in chemical reactors, and of depletion (exit streams and chemical conversion), as well as the accumulation (liquid-phase reactors, distillation columns and reservoirs) can be balanced by the effect of recycles in order to achieve an acceptable equilibrium state. Interactions through recycles can be exploited to create plantwide control structures that are not possible from a standalone unit viewpoint. 

The conclusion of this exercise is that the plant cannot be operated by keeping constant feed rates F0, zA 0, and FB 0, reactor volume V and separation performance PI( pP. Bildea and Dimian [13] recommend to keep the reactor inlets on flow control and to supply the fresh reactant in any inventory device from the recycle loop. When the reactants are recycled together (as in our plant), the recommended strategy is to design the plant for high conversion of one reactant (butene), set its... 

As recommended in Chapter 4, the inventory of reactants in the plant is maintained by fixing the reactor-inlet flows. Acetic acid is taken with constant rate from a storage tank, and the fresh feed is added on level control. The gas rate going to the evaporator is a good estimation of the ethylene inventory. Therefore, this flow is kept constant by adjusting the fresh ethylene feed. The fresh oxygen rate is manipulated by a concentration control loop, as previously explained. 

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