Feed split control

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. 

A useful way to think about the remaining two degrees of freedom that can be utilized for composition control is to classify them as two fundamental manipulated variables fractionation and feed split. 

Feed split means the fraction of the feed that leaves in one product stream, e.g., the DIF ratio. Feed split can be set directly and explicitly by using either D or B to control one composition. Or it can be set indirectly and implicitly by using reflux or vapor boilup to control one composition and removing D or B to hold reflux drum or base level. 

The new reactor configuration is also much more flexible than the conventional one from a control viewpoint. Rather than having to manipulate emulsifier and initiator feeds to control the particle properties and conversion, the major manipulated variables are physical ones, namely the split of feed between the first and second reactor, or the residence time of the small first reactor. While small variations in the initiator flow might also... 

There is only one reboiler duty to manipulate, and only one variable can be controlled by this input. As discussed later, we will use a control structure in which a tray temperature in the high-pressure column is controlled by manipulating the heat input to the reboiler in the high-pressure column (the only steam-heated reboiler). Then, we will vary the feed split (the fraction of the total feed fed to the low-pressure column) to control a temperature on a tray in the low-pressure column. 

No matter what manipulated variables are chosen to control what controlled variables, there are basically two fundamental manipulated variables that affect compositions. These are feed split and fractionation. ... 

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. 

A third arrangement, which is used in some systems, involves splitting feed between two columns that make the same separation (Figure 8.5). The supply column, however, runs at a higher pressure than the load column. The feed split is controlled to maintain a heat balance. 

Once the inventory variables are controlled there are two degrees of freedom left in the case of the column shown in Figure 8.1. One degree of freedom should be used to control the feed split and the last available degree of freedom controls fractionation. Feed split has a much more significant effect on the product compositions than fractionation. Therefore, after the inventory and capacity variables have been paired, the primary controlled variable is normally used to set the feed split and the secondary controlled variable is used to set fractionation. 

For the simple distillation column in Figure 8.1 there are five degrees of freedom, which translates into five independent valves from a control point of view. In this 5x5 system, there are 120 possible SISO control combinations of controlled and manipulated variables. Fortunately, most of these combinations are not useable due to various constraints, such as economics. From a steady-state degree of freedom analysis there are only two degrees of freedom, since a total condenser is assumed. If the column had a partial condenser then there would, of course, be three degrees of freedom instead of two. Inventories that must be controlled are the reflux drum level Hr, level in column base or reboiler Hr, and the column pressure (vapour holdup). The remaining two variables are used to control the feed split and the fractionation. 

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. 

In the stabilizer described above, we will iiutially use the bottoms composition to set the feed split and the energy input to set the fractionation. As yet, we have not paired these controlled variables to control valves (maiupulated variables). 

After the control variables have been established to control the feed-split and column fractionation, set the other variables to control inventory and capacity of the column. 

The LV control configuration is often described as an energy balance configuration and the DV and LB configurations are material balance configurations. This is because the DV and LB configurations manipulate the feed-split or material balance directly... 

The automatic controls, which split the feed and recycle to the two reactors so that the temperature reached in each stage does not exceed 482°C, were operated for a considerable portion of the total on-stream time of the methanation section. They performed quite satisfactorily. 

Example 1.3. Our third example illustrates a typical control scheme for an entire simple chemical plant. Figure 1.5 gives a simple schematic sketch of the process configuration and its control system. Two liquid feeds are pumped into a reactor in which they react to form products. The reaction is exothermic, and therefore heat must be removed from the reactor. This is accomplished by adding cooling water to a jacket surrounding the reactor. Reactor elHuent is pumped through a preheater into a distillation column that splits it into two product streams. 

One way to utilize a stabilizer Is illustrated In Figure 5, which is simply the Figure 4 process with the liquids from K and 4 diverted to a stabilizer. The stabilizer could be either refluxed or cold-feed, as a further variation. This process reduces the recycle load significantly in the two lower compression stages, as compared to the previous processes. This process also provides an additional control for the crude oil vapor pressure which can be independently varied, since the fractionator split can be controlled and the fractionator bottom product is blended with the crude stream. It may be desirable to blend this stream into separator 1... 

Through the temperature profile, which is controlled by the initiator feed and the split of fresh ethylene, the properties of the polymers can be varied in a wide range. Different peroxides, e.g., f-butylperoxy neodecanoate together with t-butylperoxy 2-ethylhexanoate, or t-butylperoxy perbenzoate, or di(t-butyl)peroxidc are required when the zones are run at different temperatures. 

I well remember one pentane-hexane splitter in Toronto. The tower simply could not make a decent split, regardless of the feed or reflux rate selected. The tower-top pressure was swinging between 12 and 20 psig. The flooded condenser pressure control valve, shown in Fig. 3.1, was operating between 5 and 15 percent open, and hence it was responding in a nonlinear fashion (most control valves work properly only at 20 to 75 percent open). The problem may be explained as follows. 

All of these systems have some common control loops. The system pressure is controlled by manipulating the fresh feed of A (F0A). The concentration controller with ratio control is used to control reactor inlet gas composition by manipulating the fresh feed of B (F0B). Bypassing (Fhy) around the FEHE is used to control gas mixture temperature Tmix. Reactor inlet temperature (Tin or T ) is controlled by manipulating the furnace heat input QF. The setpoints of these two temperature controllers are the same, and the controller output signals are split-ranged so that bypassing and furnace heat input cannot occur simultaneously. 

---