Design spec/vary

Now that we have some reasonable guesses for the values of the recycle streams, the Design Spec/Vary capability can be used to drive the compositions of the two product streams to their desired values. The key feature in the first column is to keep enough benzene in the column to entrain out the water so the bottoms is high-purity ethanol. On the other hand, if too much benzene reflux is fed to the column, it will go out the bottom and drive the bottoms off-specification. A Design Spec/Vary is set up to maintain the benzene composition of the bottoms at 0.5 mol% by manipulating the REFLUX stream, which is consider a Feed rate on the list of choices given in the Vary, Specifications, Adjusted variable, and Type. 

The initial guessed value of the reflux ratio in the second column was 2. The bottoms purity was very high. The reflux ratio was reduced to about RR = 0.2 without affecting the bottoms purity significantly. A second Design Spec/Vary is set to maintain the ethanol composition of the bottoms of the second column at 0.1 mol% by varying the bottoms flow rate B2. 

In the steady-state design using Aspen Plus, two Design Spec/Vary functions are typically used to manipulate the flow rates of reflux and distillate to achieve the specified purities (or impurities) of the two products. These calculations are done at the design feed composition and the design feed flow rate. 

Set up the steady-state simulation at design feed composition with the two Design Spec/Vary functions active. Record the required reflux ratio RR and reflux-to-feed... 

It is important to note that all of these methods use only steady-state information, so steady-state process simulators such as Aspen Plus can be easily used to perform the calculations. The methods all require that various variables are held constant, while other variables change. For example, two product compositions can be held constant, or a tray temperature and reflux flow rate may be held constant. The Design Spec/Vary feature in Aspen Plus is used to achieve the fixing of the desired independent variables and the calculation of all the remaining dependent variables. 

StBady-State Design. The feed conditions, the pressure, the total stages, and the feed stage are specified in the steady-state design using Aspen Plus (version 7.3). Two Aspen Design Spec/Vary functions are set up to achieve the desired product specifications of 0.001 mol fraction water in the liquid distillate and 0.001 mol fraction methanol in the bottoms. 

The economic design objective is to minimize total annual cost, given feed conditions, and desired product specifications. The two product specifications are 99.5 mol% water in the distillate and 12 mol% water in the bottoms. The Aspen Design Spec/Vary feature is used to attain these specifications by varying the distillate flow rate and the reflux ratio. The Aspen Tray Sizing feature is used to determine the diameter of the column. A tray spacing of 2 ft is used to determine column height. 

The presence of a sidestream provides an additional degree of freedom. Three purities can be set. We use three Design Spec/Vary functions to achieve the following... 

The steady-state simulation is converged by first setting the RR equal to 0.5. Then two Design Spec/Vary functions were used the first adjusted distillate flow rate to achieve 0.01 mol% MeOH in the distillate, and the second adjusted sidestream flow rate to achieve 0.01 mol% MeOH in the bottoms. 

The first three Design Spec/Vary setups are straightforward. The fourth is somewhat different than the conventional. Figure 12.11 shows how the benzene impurity in the sidestream is specified. Figure 12.12 shows how the fourth Vary is defined to be the vapor... 

Using Design Spec/Vary Function in RadFrac... 

The second Design Spec/Vary is set up in a similar way. Clicking Design Spec opens a window in which you specify a new design spec ( 2 ). Then the mole fraction ratio of IPA to the sum of IPA and water in the bottoms stream is set to be 0.001. The reason we set the IPA... 

Once a converged simulation result is achieved, the Design Spec j Vary function can be used to achieve the exact bottom composition. Notice that only one Design Spec/Vary can be set up with a particular flowrate of the makeup stream. The optimum makeup flowrate is usually set by minimizing the reboiler duty while achieving a specific bottom composition specification. 

The compositions of the two bottoms products are fixed at 99.9 mol% purity of their respective component. The design spec/vary feature of Aspen Plus is used to hold these... 

The separation in the extractive column depends on the amount of solvent circulating around the system. Figure 12.4 shows that high solvent flowrates reduce the impiuity of chloroform in the distillate acetone product. For each solvent flowrate, there is a nonmonotonic effect of reflux ratio. To achieve the desired distillate purity of 99.5 mol% acetone, the minimum solvent flowrate is 145kmol/h (solvent-to-feed ratio of 1.45). These results are obtained with the impurity of acetone in the bottoms held at 0.1 mol% acetone using the design spec/vary feature of Aspen Plus and manipulating distillate flowrate. 

The optimum solvent flowrate is found by determining the minimum total energy required in the reboilers of the two columns Qja and Qr2), using four design spec/vary specifications. In the extractive column, the distillate impurity is held at 0.5 mol% chloroform and the bottoms impurity is held at 0.1 mol% acetone by varying distillate flowrate Di and reflux ratio RRi. In the solvent recovery column, the distillate impurity is held at... 

In order to obtain the same basic operation while design parameters are changed, two design spec/vary functions are set up in column block Cl. 

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