Distillation Block Comparison

This problem not only compares the performance of property methods used in ASPEN Plus, but also the performance of different blocks that represent the same unit operation: the RadFrac block and the DISTWU block. The DISTWU block is intended to act as an estimation of required conditions of distillation in order to achieve a specified result using ideal assumptions, which the RadFrac block is a more in-depth block that accounts for non-ideal conditions/mixtures in order to model a distillation process more realistically. Each block, using both IDEAL and NRTL property methods, is used to solve the following system:

A mixture of 50 mol/hr of benzene, 50 mol/hr of toluene, and 50 mol/hr of p-xylene is to be distilled at 5 bar. The feed is saturated liquid. The column is to operate at a reflux ratio of 2.8 and a 1-psi pressure drop across the column, and the column has a kettle reboiler and a partial-vapor condenser. The specifications for this unit operation are that 95% of the benzene and no more than 5% of the p-xylene should be taken overhead.

The images below show the utilization of ASPEN's different blocks and property methods to solve this problem: the first 2 images utilize the DISTWU block and generate a table of required reflux ratio vs theoretical number of stages that produce the desired results given in the problem, with the 1st images displaying results from the IDEAL property method and the second image displays that of the NRTL property method.

Since the IDEAL property method considers more ideal conditions as opposed to the NRTL property method, and the mixture to be separated is not considered to be an ideal separation, the results of the NRTL property method are likely more realistic. The next 2 images display the solution of the system using the RadFrac block, which is considered to be more realistic as it considers more non-ideal conditions such as the existence of azeotropes. The RadFrac block is utilized by displaying the final results of the outlet compositions from this separation based on the following process conditions:

12 theoretical stages with feed on stage 6. 50mol/hr of distillate is required.

The first image displays the outlet results using the IDEAL property method, and the second image displays the outlet results using the NRTL property method:

As seen, both property methods produce similar trends to the data but slightly different values. The NRTL property method results are likely more realistic for reasons given earlier. It is also clear that the RadFrac blocks do not achieve the desired outcome of the problem as the benzene mole fractions are not sufficient. This is different from the DISTWU blocks as the DISTWU blocks assume the required conditions and output the most ideal process conditions under ideal circumstances, while RadFrac blocks output process results assuming the inputted process conditions. These functions can work together as DISTWU can be used to produce inputs for the RadFrac block, which would provide a realistic performance of the model of the unit operation's performance. Other notes associated with the comparison of the two blocks and property methods are given below: