An examination of modified hierarchical design structure for chemical processes

The goal of this work was to examine a modified hierarchical procedure for the conceptual design of chemical processes with chemical reactions. This was achieved by examining several designs of hydrodealkylation (HDA) of toluene to benzene process (a benchmark case in teaching chemical process design), using the modified and the conventional design hierarchies.

The conventional design hierarchy as described by Douglas [1] has at least two drawbacks. First, the operation of the chemical reactor is not considered as a separate step of the design exercise, but rather as a part of the recycle structure. Second, the use of a recycle stream is purely an economic decision that should be considered in the unit integration step, after the basic structure of the process is designed. Mann and Hoo [10] proposed a modified hierarchical design structure tor chemical processes that overcomes these drawbacks.

Results of this study show that by applying the modified hierarchical design structure to design the HDA process, a more economical plant is designed than one obtained by conventional hierarchical design structure. Specifically, the plant designed by the modified hierarchy requires 70% lower capital investment (bare module cost) and provides an operating loss 50% smaller than that of the conventional design. These economic advantages are obtained mainly due to modifications in the reactor configurations and the elimination of the gas recycle stream. These results indicate that the modified hierarchical design structure for chemical processes with chemical reactions proposed by Mann and Hoo [10] deserves further examination. The results indicate the importance of considering the design of the chemical reactor as a distinct step in the design exercise. The results also indicate that the implementation of a recycle stream should be assessed on the basis of economic considerations, rather than being an inherent part of the design procedure. It is recommended to continue the investigation of the modified hierarchical design structure and expand the study to include the dynamic behavior of the process and the required plant-wide control structure. Also, it is recommended to examine the application of the modified hierarchical design structure to other chemical processes with chemical reactions.