Distillate flow rate

Amplitude of controlled variable Output amplitude limits Cross sectional area of valve Cross sectional area of tank Controller output bias Bottoms flow rate Limit on control Controlled variable Concentration of A Discharge coefficient Inlet concentration Limit on control move Specific heat of liquid Integration constant Heat capacity of reactants Valve flow coefficient Distillate flow rate Limit on output Decoupler transfer function Error... 

Develop a mathematical model for the three-column train of distillation columns sketched below. The feed to the first column is 400 kg mol/h and contains four components (1, 2, 3, and 4), each at 25 mol %. Most of the lightest component is removed in the distillate of the first column, most of the next lightest in the second column distillate and the final column separates the final two heavy components. Assume constant relative volatilities throughout the system ai, CI2, and a3. The condensers are total condensers and the reboilers are partial. Trays, column bases, and reflux drums are perfectly mixed. Distillate flow rates are set by reflux drum... 

AFTER TOTAL REFLUX STARTUP, DISTILLATE FLOW RATE IS FIXED ASSUMPTIONS CONSTANT RELATIVE VOLATILITIES (TERNARY) BQUIMOLAL OVERFLOW, IDEAL TRAYS... 

Avoid saturation of a manipulated variable. A good example of saturation is the level control of a reflux drum in a distillation column that has a very high reflux ratio. Suppose the reflux ratio (R/D) is 20, as shown in Fig. 8.10. Scheme A uses distillate flow rate D to control reflux drum level. If the vapor boilup dropped ouly 5 percent, the distillate flow would go to zero. Any bigger drop in vapor boilup would cause the drum to run dry (unless a low-level override controller were used to pinch back on the reflux valve). Scheme B is preferable for this high reflux-ratio case. 

At steady state, the presence of a large molar liquid recycle R implies an equally large molar vapor boilup V. On the other hand, the feed flow rate F, the distillate flow rate D, and the bottom-product flow rate B are of the same order of magnitude and much smaller than the flow rates of the internal streams. Therefore, we can define e = (Fs/Rs) -C 1 and k = Vs/Rs = 0(1), where the subscript s refers to the nominal steady state. Let us also define the scaled vapor and reflux flow... 

Subsequently, we used Aspen Dynamics for time-domain simulations. A basic control system was implemented with the sole purpose of stabilizing the (open-loop unstable) column dynamics. Specifically, the liquid levels in the reboiler and condenser are controlled using, respectively, the bottoms product flow rate and the distillate flow rate and two proportional controllers, while the total pressure in the column is controlled with the condenser heat duty and a PI controller (Figure 7.4). A controller for product purity was not implemented. 

The simulation of the columns C201 and C202 needs careful analysis of specifications. The design of HC1 column must ensure both high recovery and purity of HC1 in order to prevent accumulation in recycle. About 30 stages and reflux flow rate at 1000 kg/h are convenient. The distillate flow rate should ensure complete HC1 recovery, but with minimum losses in VCM. From the simulation viewpoint, making use of a design specification is the best way to finely adjust... 

The three quality specifications regarding the impurities in EDC, available by direct concentration measurements, such as by IR spectroscopy or online chromatography, are the outputs of the plantwide control problem. The degrees of freedom indicate as first choice manipulated variables belonging to the large column S2 D2-distillate flow rate, SS2-side-stream flow rate, and Q2-reboiler duty. We may also consider manipulated variables belonging to the small column... 

S4, connected to S2 by a recycle, but dynamically faster. Thus, supplementary inputs are D4-distillate flow rate, and Q4-reboiler duty. A major disturbance of the material balance can be simulated by a step variation in an external EDC feed. A second significant disturbance is the amount of impurity I3 introduced in the plant. 

Example 3.9 A depropanizer normally operates as described in Example 2.4. The column is computer-controlled, using the Jafarey et al. algorithm. The algorithm manipulates reflux flow to control top product purity. Distillate flow rate must remain fixed, but bottom purity is allowed to vary. The top product purity spec is temporarily relaxed from 0.5 mole percant to 0.9 mole percent. What would the controller set the reflux flow at ... 

Diffusion coefficient for key component, fta/s Distillate flow rate, ]b-mole/h Outer diameter of packing particle, in Fractional height element, ft... 

MB = bottoms flow rate, mol/h Md = distillate flow rate, mol/h... 

Operation Total exergy losses (kJ/h) Distillate flow rate (kg/h) Distillate composition (%)... 

Number of chemical species Distillate flow rate Diffusion coefficient Efficiency Energy flux Energy transfer rate Eeed flow rate Column height Enthalpy Liquid holdup Height of a transfer unit Vapor-liquid equilibrium ratio (K value)... 

Figure 13-40 shows the tradeoff in product purities when we change the specified distillate flow rate, maintaining all other specifications at the values... 

Distillate flow rate Acetic acid Temperature Pressure Density... 

The experiments were conducted in the temperature range 35°C-80°C at feed inlet, and 5°C-30°C at distillate inlet, and with feed and distillate flow rates up to 1500 dm /h. Under these conditions permeate stream was 10-50 dm /h (60-300 dm /m day). During the experiment run the activity of the distillate was stable on the level of natural background radioactivity and the concentrating of radioactive compounds took place in retentate. Retention of radioactive ions in retentate was almost complete (decontamination factors oo. Table 30.13). Most of radionuclides were not detected in distillate only trace amounts of Co-60 and Cs-137 were present. Also retention coefficients of non-active ions were high (Table 30.14). 

The results from the rigorous model with the inputs specified as above show a flow rate of 1084.5 kg/h of n-hexane in the distillate product. This exceeds the requirements calculated from the problem statement (1065.5 kg/h). The simplest way to get back to the required specification is to use it directly as a specification for the column. From the Design tab on the column window, we can select Monitor and then Add spec to add a specification on the distillate flow rate of n-hexane, as shown in Figure 4.50. This specification can then be made active, and the bottoms flow rate specification can be relaxed. When the simulation is reconverged, the bottoms flow rate increases to 19,350 kg/h, and the n-hexane in the distillate meets the specification flow rate of 1065.5 kg/h. 

D = Distillate flow rate, mols/hr B = Bottoms flow rate, mols/hr X = Mole fraction of the light key component in the liquid... 

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