Feed composition sensitivity analysis

In single-end control structures, only one composition or one temperature is controlled. The remaining control degree of freedom is selected to provide the least amount of product quality variability. For example, a constant reflux ratio RR can be maintained or the reflux-to-feed ratio R/F can be fixed. The control engineer must find out whether this more simple approach will provide effective control of the compositions of both product streams. One approach to this problem is to use steady-state simulations to see how much the reflux ratio and the reflux flow rate must change to maintain the specified impurity levels in both product streams (heavy-key impurity in the distillate X/>(hk) and light-key impurity in the bottoms Xb(lk)) when changes in feed composition occur. The procedure is call feed composition sensitivity analysis. ... 

The justification for choosing constant reflux as opposed to constant reflux ratio is given in the feed composition sensitivity analysis shown in the top three rows of Table 6.2. As the results in Table 6.2 clearly show, the required changes in reflux flow rate are much smaller than the required changes in the reflux ratio. 

The obvious way to implement this ratio is to simply use a multiplier block whose first input signal is feed flow rate, whose second input is the desire R/F ratio and whose output set the reflux flow rate. However, Aspen Dynamics has the strange limitation that the reflux flow rate sent to the column block is a mass flow rate. However, the R/F ratio determined in the feed composition sensitivity analysis is a molar flow rate ratio. 

Complications arise when the column has a large RR and, at the same time, the feed-composition sensitivity analysis suggests the use of a reflux-to-feed ratio. Distillation control wisdom suggests that reflux flow rate should be used to control reflux-drum level, but this is in conflict with the desire to use the R/F structure. In this section, we suggest a control structure that handles this situation. 

When a large change is made in Zqb,b (to 0.90), the new steady-state values of the manipulated variables were only slightly different from the base-case values. The makeup flow rates of fresh feed change Fqa increases 10 percent and Fqb decreases 10 percent. Production rates of D2 and B3 stay the same, as do other flow rates and compositions throughout the process. Thus, the steady-state sensitivity analysis suggests that this structure should handle disturbances easily. Dynamic simulations confirm that this control structure works quite well. 

A small change in the composition of the fresh feed of component A from lQA,A = 1 to Zqa.a = 0-99 and Zqa.b = 0.0 1 produces 15 to 20 percent changes in the recycle flow rates Vi and D3. Therefore, the steady-state sensitivity analysis predicts that this control structure will not be able to handle large disturbances. 

Use the sensitivity analysis tool to evaluate the effect of the composition of the feed on the energy consumed. 

The determination of the temperature control point inside the column is from a combination of closed-loop sensitivity analysis and verification by open-loop sensitivity analysis. For the closed-loop sensitivity analysis, closed-loop simulation runs are made in which both the top and bottom product purities are held at their specifications for different feed compositions. This is to mimic ideal (although not practical) situation with two online composition measurements and two composition control loops setting aqueous reflux flow and reboiler duty. The tray temperature with the least amount of variation for changes in feed composition is selected as the temperature control point. The specific feed composition changes are feed F3 water molar composition +10% changes while keeping the total molar flowrate of F3 the same by adjusting the acetic acid molar composition. 

A mathematical analy.sis of optimum operating conditions for the catalytic removal of acetylene from ethylene with a palladium catalyst has been presented by Huang (1979). Parameters investigated include temperature, space velocity, and feed gas composition. The results of the analysis show that, for the case studied, temperature is the most sensitive parameter. Optimum operating conditions, yielding the lowest acetylene concentration (I ppmv) and the lowest ethylene loss, include a reactor temperature of 240°F and a space velocity of 7,000 (vol)/(volXhr). 

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