Methylene chloride recovery process, страница 2

Controller unit operation

The Controller unit operation simulates a feedback controller. You can specify a desired value for a flowsheet unit operation parameter or stream parameter (called the specification or specification variable) in the final converged solution. Another flowsheet unit or stream parameter (called the control variable) will be varied by PRO/II in order to match the desired value for the specification variable.

The control variable will be varied by the controller.  It cannot be changed by any other unit operation. The controller will observe the change of the specification variable.  That variable cannot be fixed by any other unit operation.  Instead of, it must be allowed to change.

Each controller requires one specification variable and one control variable. The "specification is met" or the "specification is satisfied" when PRO/II finds a converged solution for the simulation which matches the desired value for the specification variable.

Controller stream S2 in  300-600 lb-mol/hr range

The Table 6, shows the results of the simulation using a controller in the steam flow rate fed on the Flash Tower1. To achieve the value of the specified parameter (150 ppm of Methylene chloride in the stream S7) it is necessary to supply 594.1 lb-mol/hr of steam in the Flash Tower1. Being the total steam supplied in the process of 871.7 lb-mol/hr (594.1+277.6).

Table. Results of the simulation with controller stream S2.

Controller stream S4 in the 300-500 lb-mol/hr range.

When the controller is used varying the steam flow rate feed on the second tower (Table 7), to achieve the value of the specified parameter (150 ppm of Methylene chloride in the stream S7), it is necessary to supply 349.6 lb-mol/hr of steam in the first tower. The total vapor supplied in the process is 904.7 lb-mol/hr (555.1+349.6).

It is evident, that in this condition a bigger consumption of vapor exists in the process, for that, the variable of more impact in the process cost has the steam flow rate supplied on the Flash Tower1. Nevertheless, to know the minimum value of steam consumed in the process it is necessary to install an optimizer.

Table 7. Results of the simulation with controller stream S4.

Optimization

The Optimizer unit operation either maximizes or minimizes an objective function by varying one or more variables. The objective function can be an operational criterion, such as maximum recovery or minimum loss, or an economic criterion, such as maximum profit or minimum cost. Then, the Optimizer can be used to minimize or maximize the calculator result.

We installed an optimizer, defining as objective function to minimize the sum of the steam S2 and S4 stream, varying both flows and specifying the Methylene chloride concentration in 150 ppm in stream S7. The Table 8, shows the results of simulation. The total value of consumed vapor was of 823.2 lb-mol/hr. This value is significantly smaller those obtained until at the moment.

Table 8. Results of the simulation with Optimizer.

Conclusions

1. The best results were received after optimization – total steam supplied in the process is 823.2 lb-mol/hr (704.6 from the stream S2 and 118.6 from the stream S4) at the methylene chloride concentration of 150 ppm in the bottom of the Flash Tower2.
2. Before optimization of process it is expedient to carry out a sensitivity analysis and use controller unit operation.