Fractionator control scheme

To illustrate how the control function requires extra capacity of process equipment, let us use a typical fractionation system, as shown in Figure 1. This sample illustrates the point being made rather than recommending any particular fractionation control scheme. The best... 

A good understanding of control schemes is essential for understanding fractionation systems. It is not possible... 

A variety of control schemes are shown separately in Figures 3.14 and 3.15 for the lower and upper sections of fractionators. To some extent, these sections are controllable independently but not entirely so because the flows of mass and heat are interrelated by the conservation laws. In many of the schemes shown, the top reflux rate and the flow of HTM to the reboiler are on flow controls. These quantities are not arbitrary, of course, but are found by calculation from material and energy balances. Moreover, neither the data nor the calculation method are entirely exact, so that some adjustments of these flow rates must be made in the field until the best possible performance is obtained from the equipment. In modern large or especially sensitive operations, the fine tuning is done by computer. 

Because the first committed step of gene expression is the transcription of the gene, a large fraction of genetic control takes place at this step, especially through the initiation of transcription. Genetic control schemes may be classified in several ways. 

In Section 8.3, we introduce a coherent control scheme for enhancing the fraction of one desired enantiomer, given a racemic mixture of molecules. This scheme, called laser distillation, is of great practical interest because the effect obtained is substantial, even in the presence of decoherence. An alternative scheme, due to Fujimura and co-workers [263, 264] is also briefly described. 

A good understanding of control schemes is essential for understanding fractionation systems. It is not possible to discuss all possible control hookups. The following discussions will therefore include many of the major... 

When the condenser contains excess heat transfer area, the fraction of condenser area that is flooded can be manipulated to vary the rate of condensation. This principle is used in "flooded condenser control schemes (Sec. 17.2.2). 

Flgure 18.6 Boyd s double differential temperature control, (a) Control scheme. (Porta from "Fractionation Column Control, D. M. Boyd, Chemical Engineering Progr, vol. 71, no. 6, p.55 (June 1975). Reproduced by permission of the American Institute of Chemical Engineers.)... 

Finally, and still in the process control subject, it is important to provide the technical and supervisory staff with suitable and convenient office space and related work areas. In all chemical processing, regardless of the control scheme being used, the eyeball contact with the process operations is important. This is the case, most of all, for the bulk drug pilot plant, as a great fraction of the activities are being done for the first time and repetitive processing is so infrequent. 

It has also been suggested that super-real-time simulation may be used for trajectexy planning within advanced control schemes [32]. That is, if the simulation of a robotic system could be accomplished in a fraction of the time needed for the determination of the correct control inputs, the effects of alternate trajectories could be previewed and optimized as a part of the control algorithm itself. Such a capability would naturally improve the overall performance of the entire system. 

A less common variation on this control scheme is used on the occasion where the first column in a distillation train is for removing light components, and the bottoms flow rate needs to be very steady as the flow rate to a large diameter fractionator as the second column. In this case the bottoms flow rate from the first colunrn is the manually adjusted setpoint for the controller that manipulates the bottoms valve. The column-base level controller for the first column manipulates the feed rate to the first colunrn. 

The control schemes are developed for the case of dual composition control. Also other material and energy balance control schemes are possible. The main difference between the control schemes is that in the case of energy balance control the reflux and vapor flow affect the outpoint (distillate-bottom-ratio) as well as the fractionation, whereas in the case of material balance control outpoint and fractionation control are separated. Either the reflux ratio or the vapor flow is manipulated to control the fractionation. As can be seen, the developed control scheme of Fig. 34.5 is similar to the energy balance control scheme. 

The outpoint remains therefore the same and only the fractionation changes slightly. As a result, changes in both product qualities are small. Hence, a material balance control scheme will possess a certain degree of self-regulation for the internal reflux, this contributes to a minor effect on both product qualities. 

Feed split means the fraction of the feed removed as either distillate or bottom product. The D/F and B/F ratios can be manipulated either direcdy (as proposed by Shinskey in his material balance control scheme) or indirecdy. The steady-state effectiveness of both the direct and indirect schemes is identical. In either case feed split has a very strong effect on product composition. A slight change in feed split can change product compositions very drastically, particularly when produa purities are high. 

The blending system in Fig. 13.3 has a bypass stream that allows a fraction / of inlet stream W2 to bypass the stirred tank. It is proposed that product composition x be controlled by adjusting/via the control valve. Analyze the feasibility of this control scheme by considering its steady-state and dynamic characteristics. In your analysis, assume that xi is the principal disturbance variable and that X2, wi, and W2 are constant. Variations in liquid volume V can be neglected because W2 wi. 

A control scheme is to be developed for the evaporator shown in Fig. E18.20. The feed and product streams are mixtures of a solute and a solvent, while the vapor stream is pure solvent. The liquid level is tightly controlled by manipulating the feed flow rate, wp. The product composition, JCp, and the feed flow rate, wp, are to be controlled by manipulating the product flow, Wp, and the steam flow rate, The evaporator economy is approximately constant, because E kg of solvent are evaporated for each kg of steam. The flow rates have units of kg/min, while the compositions are expressed in weight fraction of solute. 

The feed split is simply the amount of feed that leaves as distillate versus the amount that leaves as bottoms. The other variable, fractionation, is the amount of separation that occurs per stage. The overall column fractionation depends on the number of stages, the energy input, and the difficulty of separation. A typical control scheme for this column is shown in Figure 8.3. 

Select a candidate control scheme. The literature abounds with alternative control configurations. Consider as an example a typical column that has feed as the disturbance stream. As was pointed out in the previous section, for such a column, only two degrees of freedom remain, i.e. feed split and fractionation. The resulting best feed-split control schemes are shown in Figures 8.14 and 8.15. 

Simulations allow essentially aU types of variable to be used as controlled variables. However, a control scheme must be implementable in an operating plant. This requires that the variable being controlled can be accurately measured to provide feedback in a control loop. Examples of variables that can be easily measured include flows (especially liquid flows), temperature, and pressures. Some compositions, mainly mass and volumetric fractions, can be measured using analysers, but these instmments are generally expensive and often introduce considerable dead time to processes. Consequently, they are excluded from many control systems. 

There is significant previous work that addresses the issues of process dynamics and control for the integrated FCC unit We particularly note the efforts by Arbel et al. [2] and McFarlane et al. [3] in this regard. Subsequent authors [4, 5] use similar techniques and models to identify control schemes and yield behavior. However, most of the earlier work uses a very simplified reaction chemistry (yield model) to represent the process kinetics. In addition, prior work in the literamre (to our knowledge) does not connect the integrated FCC model with the complex FCC fractionation system. This work fills the gap between the development of a rigorous kinetic model and industrial apphcation in a large-scale refinery. 

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