Flowrate,control

A preliminary experimental error analysis indicated that the flowrate control and, to a lesser degree, the temperature control would be critical. It is necessary to change the off-the-shelf flow controllers for commercial chromatographs and desirable to change the temperature controller. 

For the two commercial FIFFF models, symmetric and asymmetric, it is possible to modulate the applied field strength during a run by programming the cross-flow rate however, there are few examples of programmed analyses since, in FIFFF, the whole flowrate control is complex because of the presence of two flow axes, one for channel flow and the other for cross-flow. The back pressures on the channel and cross-flow outlets have to be adjusted, so that each pair of entering and... 

The simulation of a gas distribution system having a tree structure with one supplier may be reduced to a system of equations which has as many dependent variables as there are sinks or consumers. At each sink, the pressure (pressure control loop termination) or the mass flowrate (mass flowrate control loop termination) must be specified. The purpose of this short paper is to demonstrate some of the most commonly used mathematical models for the simulation of a gas net with a tree structure and point out some of the problons which arise if the net has an overload, a severe leakage or a bleeding at one point or another leading to choked flow. 

Branches with specified flowrate (flowrate control). Source pressure 9.7 bar. 

This process adds additional degrees of freedom, which we may want to utilize. The control scheme shown in Fig. 6.23c is simple and practical. Reflux flowrate in the first column is flow-controlled and heat input prevents A from dropping out the bottom of the first column. Heat input to the second column controls the impurity of B in the bottoms product. Sidestream flowrate controls the purity of the sidestream product. Reflux flowrate in the second column controls the impurity of B in the distillate product. 

Now there are only three product streams, so the control problem is 3 X 3. In the control system shown, reflux flowrates control the distillate purities in each column. Bottoms purity in the high-pressure column is controlled by manipulating the heat input to the reboiler. 

So what can we do in this case If column operation requires that we stick to this control structure, feedforward control will help to reduce the swings in distillate flowrate. However, better plantwide performance can be achieved if we can switch the control structure to one in which the vapor distillate is not used to control pressure. One possible alternative is shown in Fig. 6.27. Condenser cooling is used to control pressure, reflux flowrate controls reflux drum level (with P-only control), and the flowrate of the vapor distillate is ratioed to reflux flowrate. With this structure we allow the disturbances that the column energy... 

The startup time for optimal reflux control is 120 min. and for side-stream flowrate control is 130 min. These results (for each case obtained after 44805 numerical integrations of the system (l)-(3) with 200 differential equations) are remarkable in comparison with operating at constant control parameters where the startup time is 380 min. For all these regimes the final stationary state corresponds to an average value of the normalized derivatives less than 10 (maximum integration step is 1 s.). 

In figures 1 and 2 are presented the optimal reflux control and the optimal side-stream flowrate control (subscript s indicates final, stationary value). 

Figure 2. Optimal side-stream flowrate control...

In figures 3 and 4 are presented the evolutions of components mole fractions in the top and bottom products. In figure 3 the evolution of the benzene concentration in the top product at constant control parameters and at optimal side-flowrate control overlaps (for the entire time domain of optimal side-flowrate control, respectively 130 min.). 

Figure 4. Evolution of ethylbenzene concentration in the bottom product for startup at constant control parameters (—), at optimal reflux control (—) and at optimal side-flowrate control (- -) ...

The control algorithm consist in two parts the first part is dedicated to temperature balancing for parallel multiple passes and the second part is for total flowrate control. 

The total flowrate M, processed in furnace is computed as the sum of the measured flowrates mi and m2, M= j + 2, and controlled with the two flowrate control... 

In this section, two load disturbances are introduced to test the proposed column configuration. The overall control strategy, shown in Figure 9.26, is also implemented here. The only difference is a side stream at Stage 10 with the flowrate controlled at 50 kg/h. The two load disturbances considered are feed F3 water molar composition +10% changes and feed F3 m-xylene component flowrate +20% changes. 

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