Process Optimization blocks inside the aromatic complex. The column overhead vapor is used to re-boil the column bottoms of Unit B (xylene fractionation), Unit C (benzene-toluene fractionation block) and Unit D (solvent extraction), whereas the side stream is used as heating medium in other columns inside Unit A (paraxylene separation block) and Unit E (transalkylation block). Unit B has an internal heat integration network shown as a TABLE 1. Relief load from individual units during GEPF Unit name Unit A (paraxylene separation block) Unit B (xylene fractionation unit) Unit C (benzene toluene fractionation block) Unit D (solvent extraction block) Unit E (transalkylation block) Total GEPF relief load X 1 Unit C Integrated enthalpy in (overhead) Integrated enthalpy out (overhead) Integrated enthalpy in (sidedraw) Integrated enthalpy out (sidedraw) Integrated other heat in/out Integrated mass in/out Fired heat in PSV Relief valve tag PSV 1 PSV 2 PSV 3 PSV 4 PSV 5 PSV 6 GEPF load 700 t/hr 100 t/hr 200 t/hr 120 t/hr 60 t/hr 20 t/hr 1,200 t/hr sub-block of Unit B in FIG. 2 and detailed in FIG. 3. The same concept of using the column overhead vapor inside the unit is used to re-boil the bottoms of the other columns of Unit B. For a common mode of failure (general power failure is considered for the case study), the contribution of the relief load from each unit (Units A-E in FIG. 2) to the overall relief load of the complex is discussed in the sections below-from this, a realistic overall relief load is evaluated for the entire complex. The UBH method is applied for evaluating relief loads. CASE STUDY By modifiying the enthalpy balance envelope, a more optimized common mode failure load can be calculated, even with a simplified steady-state calculation for a heat integrated system. Approach 1: Individual unit enthalpy balancing method. During a global electrical power failure (GEPF), individual units (Units A-E) of the complex (mainly the columns) began relieving. The unbalanced heat and enthalpy across each column of the units were responsible for the resulting relief loads. 3 6 Internal HEN of Unit B Unit B 4 2 Unit C 5 Unit A Part of Unit A Unit D FIG. 2. Heat integration network inside the complex. 46 AUGUST 2021 | HydrocarbonProcessing.comhttp://www.HydrocarbonProcessing.com