Integration of low-grade heat from exhaust gases into energy system of the enterprise

The paper presents a Process Integration application for waste heat utilisation from exhaust gas streams with partial condensation. It is based on the hot composite curve construction representing the gaseous mixture cooling with accounting for the condensable vapour part's gas–liquid equilibrium. With cold composite curve for streams requiring heating, the Pinch Point is determined. On this basis, the heat exchanger network (HEN) structure for utilised heat integration into the factory's energy system is developed. It accounts for the possible splitting of two-phase flow on gas and liquid streams and plate heat exchanger (PHE) type selection for specific positions in HEN. The method is illustrated by a case study of heat utilisation from exhaust gases after superheated steam tobacco drying and flue gases from natural gas-fired boiler. Heat transfer areas of PHEs in HEN are optimised with the total annualised cost as an objective function. The received solution's payback period is less than four months, with a substantial saving of energy, up to 10.9 TJ/y. It also leads to the reduction of CO₂ emissions up to 600 t/y. About 3830 t/y of steam is not discharged to the atmosphere and as the water returned to the production process.