Reboiler heat input to feed ratio

The dynamics can be greatly improved by using a feed-forward ratio scheme to anticipates that reboiler duty must be changed when feed flow rate changes occurs. The control structure is similar to the reflux-to-feed ratio discussed above. However, there are two differences. 

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CONVERTING FROM STEADY-STATE TO DYNAMIC SIMULATION 

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CONVERTING FROM STEADY-STATE TO DYNAMIC SIMULATION TC control 20% feed disturbances 

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Setting up the multiplier for the reboiler heat input to feed ratio requires the use of metric units flows in kmol/h and heat in GJ/h. Thus, the steady-state value of the second input to the multiplier, which is the temperature-controller output signal, is calculated... 

The use of a steam-to-feed ratio greatly improves this response, as shown in Figure 10.13b. The base-level controller output signal resets the reboiler heat-input-to-feed ratio. The steady-state ratio is 0.038689 (in the required Aspen Dynamic units of GJ/h per kmol/h ). The output range of the level controller is changed from 0 to 0.1, and the TC50 controller is retuned Kc = 3.2 and Ti = 29 min). The deviation in bottoms purity is greatly reduced, and saturation of the bottoms valve is also avoided. 

There are many situations when the flowrate of one stream needs to be changed when the flowrate of another stream changes. The blending of two or more streams is a common example. In distillation column control, it is often desirable to control a reflux ratio or to control a reflux-to-feed ratio. We may also want to control a reboiler heat input-to-feed ratio. 

The temperature on Stage 15 in the extractive column is controlled by manipulating the reboiler heat input-to-feed ratio. 

Therefore, we assume that an analyzer is available to measure sidestream MeOH composition. A 3 min dead time is used in this loop. Stage 17 temperature is controlled (the location of the steepest part of the temperature profile) by reboiler heat input through a heat-input-to-feed ratio (see Fig. 10.32). The temperature control loop is tuned first (Kc — 0.159 and Tj = 7.9 min) with the composition loop on manual. Then the composition loop is tuned with the temperature loop on automatic. Note that the flow controller on the sidestream flow is on cascade with its setpoint coming from the composition controller. There is also a reflux-to-feed ratio used. 

Obviously, all three of these control structures could be improved by using feedforward control. In CS1 and CS3, the reboiler heat input could be ratioed to the feed flow rate (with the ratio reset by the temperature controller), and in CS3, the condenser heat removal could be ratioed to the feed flow rate. Figure 8.16 illustrates that the improvement of the QR-to-F ratio provides for CS3 with the large distillate case. 

This problem can be greatly unproved by using a feedforward ratio control structure in which the reboiler heat input to the high-pressure column is ratioed to the feed flowrate, with the ratio adjusted by the temperature controller. The dashed lines in Figure 6.13 shows the responses with this control structure. The transient disturbance in xbi is drastically reduced to about 20 ppm instead of 200 ppm. 

Of these, the feed mixture may or may not vary, but is generally taken as given. The column pressure and the degree of subcooling are normally fairly constant. The main operational variables are the reflux ratio R and the heat input to the reboiler QR and once these are set, the amount of product withdrawal at the bottom or at the top will also be given by the product specifications. An optimum exists for the reflux ratio in terms of operating costs, and normally a number of ratios are tested, and the economics of each scenario is investigated, before a decision is reached. 

The design of a distillation column involves many parameters product compositions, product flow rates, operating pressure, total number of trays, feed-tray location, reflux ratio, reboiler heat input, condenser heat removal, column diameter, and column height. Not all of these variables are independent, so a degrees of freedom analysis is useful in pinning down exactly how many independent variables can (and must) be specified to completely define the system. 

Initially, the total feed is split equally between the two columns. This is achieved in the Splitter labeled Tl on the flowsheet shown in Figure 5.33. Two Design SpecA ary are set up in each column to adjust distillate flow rate and reflux ratio to attain the 99.9 mol% product purities of all foiu streams. The optimum feed tray location is determined by finding the feed stage that minimizes reboiler heat input. In column Cl, it is Stage 19. In column C2, it is Stage 18. 

The job of the column is to achieve the desired product purities in the face of disturbances. Recall that the design specifications are 1 mol% zC4 in the distillate and 0.5 mol% C3 in the bottoms. We are using single-end control with a temperature on a tray in the column controlled by manipulating reboiler heat input. The other degree of freedom is held constant and depends on the control structure. In CSl, it is the reflux-to-feed ratio, and in CS3, it is the RR. In control structure CS2, the other degree of freedom is a fixed condenser heat removal. 

Reflux Ratio Structure. If we ignore the results of the analysis that shows that the R/F structure can handle feed-composition disturbances better than the RR structure, we can set up another conventional control structure in which reflux-drum level is controlled by reflux flow rate, Stage 3 temperature is controlled by reboiler heat input, and distillate flow rate is ratioed to reflux flow rate. This structure is commonly used in many distillation systems and would be expected to provide stable regulatory control but result in more deviation of product purities for feed-composition disturbances. 

Dynamically, reflux flow rate controls reflux-dmm level, and Stage 3 temperature is controlled by reboiler heat input. The signal from the feed flow transmitter is sent to a multiplier whose constant is the desired reflux-to-feed ratio. The output signal from the multiplier is the desired reflux flow rate and is the set point signal of a VPC controller, which is basically a reflux flow controller. The output signal of this controller positions the control valve on the distillate line. 

An unconventional control structure has been shown to provide effective control in the situation where reflux ratio is large and a reflux-to-feed ratio is preferred in a single-end control structure. The reflux-drum level is controlled by reboiler heat input. The condenser heat-removal loop controlling pressure must be on automatic for this structure to work because reflux-drum level is not directly affected by reboiler heat input. 

The development of a control structure for this complex system turned out to be more difficult than for the stripper flowsheet. The initial control scheme evaluated is shown in Figure 10.21. It is a logical extension of the control structure used for the vapor sidestream column in which the vapor sidestream is ratioed to the reboiler heat input. As we will demonstrate below, this structure worked well for some disturbances, but it could not handle decreases in the composition of MeOH in the feed, resulting in a shutdown of the unit. 

The binary separation of methanol and water is used as an example column. A feed of 82 mol% methanol and 18 mol% water is fed to a column with 40 trays (42 stages in Aspen terminology with feed on Stage 27 and the condenser labeled as Stage 1). Condenser pressure is 1 bar, condenser pressure drop is 0.1 bar, and tray pressiue drop is 0.01 bar per tray (giving a base pressure of 1.5 bar). Product purities are 99.9 mol% methanol in the distillate and 99.9 mol% water in the bottoms. The required reflux ratio is 0.8569. Column diameter is 5.61 m. Reboiler heat input is 64.1 MW. Condenser heat removal is 60.0MW. The NRTL physical property package is used. 

Figure 15.11 gives results for the two-temperature control structure. When feed flow rate is dropped to 3000kmol/h, temperature rises up to 105 °C, and the TCI controller decreases reboiler heat input. At the same time, the R/F ratio controller produces an immediate drop in reflux flow rate. However, the rise in temperature causes the TC2 controller to increase reflux temporarily until the temperature is returned to the TCI set point of 101 °C. 

The distillation column used as a numerical example is shown in Figme 4.47. A binary mixture of methanol and water are separated in a 16-stage column operating a 14.7 psia in the reflux drum. The feed is 40 mol% methanol. Distillate purity is 99 mol% methanol. Bottoms purity is 99.5 mol% water. A reflux ratio of 1.27 is required to achieve these specifications. Reboiler heat input is 152 x 10 Btu/h. Column diameter is 0.474 ft. The reflux drum and column base are sized for 5 min holdup when half full. Valve pressure drops are 30 psi. 

Adding a ratio block between feed and heat input to the low-pressure column improves the response of the system. Figure 6.23 shows one way to implement this structure using an additive feedforward scheme. The output signal of the multiplier block ratioQtot/F is the calculated total heat input to the reboiler of the low-pressure column. This is fed... 

The control structure for the chlorobenzene system is given in Figure 11.26. Two features are different from the other control structures. They involve the use of steam-to-feed ratios. In both columns, the reboiler heat input is ratioed to the feed to the column. These are added to improve the load response of the system that was found to be inferior to those found in the other solvent systems. The feed flow is measured and sent to a multiplier block. The other input to the multiplier is the output signal from the temperature controller. The output of the multiplier sets the reboiler heat input. So the temperature controller is looking at temperature and outputting a ratio signal. Controller parameters are given in Table 11.6. Notice that the Qr/F ratios must be in units of GJ per kmol in the Aspen Dynamic convention. 

Reboiler heat input in the solvent recovery column is ratioed to the feed to the column (stream Bl). 

D2. A distillation column separating ethanol from water is shown. Pressure is 1 kg cm. Instead of having a reboiler, steam (pure water vapor) is injected directly into the bottom of the column to provide heat. The injected steam is a saturated vapor. The feed is 30 wt % ethanol and is at 20 °C. Feed flow rate is 100 kg min. Reflux is a saturated liquid. We desire a distillate concentration of 60 wt % ethanol and a bottoms product that is 5 wt % ethanol. The steam is input at 100 kg min. What is the external reflux ratio, L7D ... 

We wish to separate ethanol from water in a distillation column with a total condenser and a partial reboiler. We have 200 kmol/h of feed 1, which is 30 mol% ethanol and is saturated vapor. We also have 300 kmol/h of feed 2, which is 40 mol% ethanol. Feed 2 is a subcooled liquid. One mole of vapor must condense inside the column to heat up 4 moles of feed 2 to its boiling point. We desire a bottoms product that is 2 mol% ethanol and a distillate product that is 72 mol% ethanol. External reflux ratio is Lq/D = 1.0. The reflux is a saturated liquid. Column pressure is 101.3 kPa, and the column is well insulated. The feeds are to be input at their optimum feed locations. Find the optimum feed locations (reported as stages above the reboiler) and the total number of equilibrium stages required. 

For the partial condenser, Figure 5.21, a reflux ratio specification is used instead of the energy balance, since the condenser duty is unknown and depends upon the state of the overhead product (vapor, liquid, or liquid below bubble point). The feed tray, Figure 5.23, naturally has the feed stream included as an input term. Otherwise it is similar to the other trays. The reboiler s heat balance, Figure 5.25, reflects the heat duty of the reboiler, Qh... 

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