The Complex Topology of Chemical Plants
  We show that flowsheets of oil refineries can be associated to complex network topologies that are scale-free, display small-world effect and possess hierarchical organization. The emergence of these properties from such man-made networks is explained as a consequence of the currently used principles for process design, which include heuristics as well as algorithmic techniques. We expect these results to be valid for chemical plants of different types and capacities. 
J. S. Andrade Jr., D. M. Bezerra, J. Ribeiro Filho, and A. A. Moreira