Process flow sheeting is very basic steps in designing the chemical plant. It is giving you idea about the flows and eergy required i.e. heat and mass balance. Flow direction, major unit operations and equipments required can be known by PFD. You can see the major and minor controls and control logics as well as all the details inside the plant by refering the PID / PFD. So it is very important to have PFD before started working on any project.