Control in the Direction Opposite to Flow

Figure 7-5 Material balance control in the direction opposite to flow.

As shown elsewhere, the size and location of tanks and the concept of overall material-balance control used can have a great influence on plant investment and process control. If the design engineer uses the concept of control in the direction opposite to flow, tanks may be smaller and plant fixed investment and working capital can be lower than if control in the direction of flow is used. The meaning of these expressions is illustrated, for simple tanks with level controllers, in Figures 1.1 and 1.2. 

When more than one tank is involved, other advantages of control in the direction opposite to flow are (1) less difficulty with stability problems, and (2) reduced internal turndown requirements. Turndown, as used here, is the ratio of maximum required flow rate to minimum required flow rate. In this instance the meaning is that, in response to a given change in demand flow, the required change in the manipulated flows will be smaller in one case than in the other. 

Material-balance control, in the direction opposite to flow, can lead to many interesting level-control and flow-ratio options. These arc discussed in detail in Chapter 6. 

We will begin with combinations of level control and feedforward compensation for applications where material-balance control is in the direction opposite to flow. Then we will consider schemes in which material-balance control is in the direction of flow. Unfavorable schemes—those that are hard to design or to make work—will be pointed out their use should be avoided unless no suitable option is available. 

Normally the pneumatically controlled pulse feeder operates with limited pressure difference between the hopper and the stop plate. When the pressure difference is in the direction opposite to that of the downward solids flow, the downcomer needs to be lengthened in order to seal the solids, or else other modes of downcomer design must be resorted to. 

Generally speaking the direction of material-balance control is determined by the demand stream. In recycle S5retems we may find some material-balance controls in the direction of flow while others are in the direction opposite to... 

Lay out a logical control scheme to handle all the liquid levels and pressure loops throughout the plant so that the flows from one unit to the next are as smooth as possible. Buckley called these the materiaUbalatice loops. If the feed rate is set into the front of the process, the material-balance loops should be set up in the direction of flow i.e., the flow out of each unit is set by a liquid level or pressure in the unit. If the product flow rate out of the plant is set, the material-balance loops should be in the direction opposite flow i.e., the flow into each unit is set by a liquid level or pressure in the unit. 

What is material balance control for a complete plant In what sense is it different from the set point control we have seen in earlier chapters Discuss the two approaches for the implementation of material balance control. Which one is preferable, implementation in the direction of process flow or opposite to it Elaborate on your answer. Would you use steady-state or dynamic models for material balance control, and why (Note You will find Chapter 13 of Ref. 16 very useful in answering these questions.)... 

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. 

For the last tank in a series (final product storage), the demand flow is always the shipments to a customer. As shown by Figures 1.3 and 1.4, the choice in control strategy is between adjusting the flow into the last tank (control in direction opposite to flow) or adjusting flow into the first process step (control in direction of flow). In the first case, we can easily use single automatic controls. In the second case, it is more common to have an operator make the adjustment. 

The effect of the electroosmotic flow on the resolution is also evident from Eq. (16). A high electroosmotic flow in the direction of the moving ions can significantly diminish resolution. Theoretically, infinite resolution of two peaks could be reached when jlEOF is equal but opposite to the average mobility m.Av. In this case one of the solutes would migrate in the direction of the detector and the other one in the opposite direction. In other words, the separation run would be infinitely long. Thus, for a practical separation the electroosmotic flow should be controlled in a way to achieve baseline resolution (R = 1) at minimal separation time. 

Eyring et al. (226) examine the entanglement problem from a somewhat different point of view, the activated complex theory of liquid viscosity. In a monomeric liquid the molecules move by random jumps from one equilibrium position to another. The jump frequency is controlled by an activation barrier between neighboring sites. According to activated complex theory, a shear stress lowers the barrier in the direction of the stress and raises it in the opposite direction, producing a bias in jump frequency and a net flow of molecules in the stress direction. For low stresses, the expression for viscosity in a monomeric system is ... 

Note that in this structure levels are controlled in the opposite direction from flows. There is now a direct handle on production rate. When the organic product flowrate is changed, the plant responds by sequentially and gradually changing the flowrates back through the process. The last flowrates to change are the fresh feeds. 

Since the layer is in steady flow with no acceleration, by the momentum principle the sum of all forces on the control volume is zero. The possible forces acting on the control volume in a direction parallel to the flow are the pressure forces on the ends, the shear forces on the upper and lower faces, and the component of the force of gravity in the direction of flow. Since the pressure on the outer surface is atmospheric, the pressures on the control volume at the ends of the volume are equal and oppositely directed. They therefore vanish. Also, by assumption, the shear on the upper surface of the element is neglected. The two forces remaining are therefore the shear force on the lower surface of the control volume and the component of gravity in the direction of flow. Then... 

An inlet vane control system (Fig. 29) comprises a static guide-vane unit which is installed in front of the fan s impeller and whose radial vanes can be swivelled by means of a control device so as to vary the inlet air flow conditions. These vanes deflect the inflowing air in the direction of rotation or in the opposite direction. As a result of this preliminary guidance, the entry losses are substantially less than those associated with ordinary damper control. 

It should be noted that the inventory controls can be in the direction of the flow, i.e. products come off due to level control, or in the opposite direction, i.e. feed is brought in on level control. The same design procedure applies. 

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