Inventory Control Loops

The inventory control loops are designed as follows. For the heterogeneous azeotropic column with the decanter, the aqueous phase level is controlled by manipulating the AO flow, the column bottom level is controlled by manipulating the bottom IPA product flow, and the column top pressure is controlled by manipulating a control valve in the overhead vapor line. 

One important loop pairing from past experience (see Chien et al. ) is that the organic phase level should be controlled by the entrainer makeup flow, not by an internal recycling flow of OR, so that the snowbaUing effect can be avoided. For the combined preconcentrator/recovery column, the reflux dmm level is controlled by the distillate flow (feed to the heterogeneous azeotropic column) the column bottom level is controlled by the bottom water product flow and the column pressure is controlled by the condenser duty. 

The remaining manipulated variables for the heterogeneous azeotropic column are the OR flow and the reboiler duty, and the remaining manipulated variables for the preconcentrator/recovery column are the reflux flow and the reboiler duty. We will investigate the simplest overall control strategy first with only one tray temperature control loop in each column. 

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Before performing a controllability analysis, ensure the stability of the plant. The first step is to close all inventory control loops, by means of level and pressure controllers. Then, check the stability, by dynamic simulation. If the plant is unstable, it will drift away from the nominal operating point. Eventually, the dynamic simulator will report variables exceeding bounds, or will fail due to numerical errors. Try to Identify the reasons and add stabilizing control loops. Often a simple explanation can be found in uncontrolled inventories. In other situations the origin is subtler. Some units are inherently unstable, as with CSTR s or the heat-integrated reactors. The special case when the instability has a plantwide origin will be discussed in Chapter 13. 

An important inventory control loop in this overall process is the bottom level of the entrainer recovery column. The control of this level was suggested by Grassi ° and Luyben to be held by the entrainer makeup flow. However, because this flow is very small, the bottom level essentially floats as changes in the entrainer flowrate occur. With this control pairing, the entrainer feed to the first column is flow-controlled in Grassi. ° We adapted this control pairing for the overall control strategy in our study. 

Each group has its own distinctive arrangement, of flow and inventory-control loops. 

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. 

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. 

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. 

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. 

In this section we explore two basic effects of recycle (1) Recycle has an impact on the dynamics of the process. Th e overall time constant can be much different than the sum of the time constants of the individual units. (2) Recycle leads to the snowball effect. This has two manifestations. one steady state and one dynamic. A small change in throughput or feed composition can lead to a large change in steady-state recycle stream flowrates. These disturbances can lead to even larger dynamic changes in flows, w hich propagate around the recycle loop. Both effects have implications for the inventory control of components. 

We call this high sensitivity of the recycle flowrates to small disturbances the snowball effect. We illustrate its occurrence in the simple example below, It is important to note that this is not a dynamic effect it is a steady-state phenomenon. But it does have dynamic implications for disturbance propagation and for inventory control. It has nothing to do with closed-loop stability. However, this does not imply that it is independent of the plant s control structure. On the contrary, the extent of the snowball effect is very strongly dependent upon the control structure used. 

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. 

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. 

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

In the first approach, we may consider a classical standalone control structures, as displayed in Fig. 13.5. Reactor feed is on flow control (PI), and outlet stream on level control (PI). For the distillation column a classical inventory control is column pressure with condenser cooling (PI), base level with bottom product (P), and reflux drum level with distillate (P). Quality control loops are top composition with reflux and bottom composition with reboiler duty, both as PI controllers. 

Let s take a look at the control loops. Now the make-up stream of reactant B is fed on level controller LCf of the buffer tank. The flow rate of the exit stream (Recycle) is set at a constant value by a simple specification on the stream s script. There are also two level controllers LCt and LCb for the top and bottom inventories of the distillation column, which manipulate the distillate and bottom products, respectively. Besides, the top column pressure is kept constant by means of the condenser duty. Quality control is implemented only for the bottom product, the reflux being fixed. We considered a composition measure with a first-order lag transmitted to a controller that manipulates the reboiler duty. 

Select the best valves to control each of the product-quality, safety, and environmental variables. The above quantities must be tightly controlled for economic and operational reasons. The selection of the manipulated variables has to ensure good dynamic characteristics with respect to the controlled variables small time-constants and dead-times, as well as large steady state gains. Considering quality control loops before overall material balance inventory is a distinctive feature of this plantwide control... 

Fix a flow in every recycle loop and then select the best manipulated variables to control inventories. A simple and effective way to prevent large changes in recycle flows (snowball) is fixing a flow in every control loop. Whenever level controllers set all flows in a recycle loop, wide excursions can occur in these flows because the total system inventory is not regulated. 

Secondly, candidates for manipulated inputs can be found. When they are flow rates connecting BFS s, the choice affect the control of both upstream and downstream. As an example, it was proposed to keep reactant recycle on flow control and to change the setpoint of this loop when production changes are required. This implies that the inventory control of the upstream unit is in direction opposite to flow, while the inventory control of the downstream unit is in the direction of flow. Manipulated flows should be chosen with care to avoid over-specification with respect to plant mass balance. The set-points of the BFS s form another category of plantwide manipulated variables. Examples are reaction conversion, separation performance, etc. 

The inventory of impurities is a plantwide control problem, because it involves both the reaction and separation subsystems through recycles. Ideally, the inventory of each component should be traced from the source to its final destination. Recent systematic studies on the dynamics and control of the recycle systems have been started, as described in the Chapter 13. Luyben and Tyreus (1998) proposed a ten steps plantwide control design procedure (section 13.7). The step 7 consists of Checking component balances, identify how chemical components enter, leave, and are generated or consumed in the process. At this stage it is necessary to find the specific mechanism or control loop to guarantee that there will be no uncontrollable build-up of any chemical component within the process . 

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