02 key concepts of pinch analysis

Lecture 2: Key concepts of Pinch Analysis Do Xuan Truong truong.doxuan@hust.edu.vn Heat recovery and Heat exchange Basic concepts of heat exchange: The feed, which starts cold and needs to be heated up, is known as a cold stream Conversely, the hot product which must be cooled down is called a hot stream eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange Basic concepts of heat exchange: eat integration course _ Hanoi University of science and technology 2017 Clearly, we will need to supply 180kW of steam heating and 180kW of water cooling to operate the process Heat recovery and Heat exchange Basic concepts of heat exchange: Can we reduce energy consumption? Yes; if we can recover some heat from the hot stream and use it to heat the cold stream in a heat exchanger, we will need less steam and water to satisfy the remaining duties eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange Basic concepts of heat exchange: The temperature–enthalpy diagram eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The temperature–enthalpy diagram Basic concepts of heat exchange: eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The temperature–enthalpy diagram Basic concepts of heat exchange: eat integration course _ Hanoi University of science and technology 2017 exchange The temperature–enthalpy diagram eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The temperature–enthalpy diagram eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The temperature–enthalpy diagram The resulting T/H plot is a single curve representing all the hot streams, known as the hot composite curve A similar procedure gives a cold composite curve of all the cold streams in a problem eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange A targeting procedure: the “Problem Table” (Linnhoff and Flower 1978) any heat available in interval i is hot enough to supply any duty in interval i +1 eat integration course _ Hanoi University of science and technology 2017 Assuming that no heat is supplied to the hottest interval from hot utility, then the surplus of 60kW from interval is cascaded into interval Heat recovery and Heat exchange A targeting procedure: the “Problem Table” (Linnhoff and Flower 1978) There it joins the 2.5kW surplus from interval 2, making 62.5kW to cascade into interval Interval has a 82.5kW deficit, hence after accepting the 62.5kW it can be regarded as passing on a 20kW deficit to interval Interval has a 75kW surplus and so passes on a 55kW surplus to interval Finally, the 15kW deficit in interval means that 40kW is the final cascaded energy to cold utility eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The pinch and its significance eat integration course _ Hanoi University of science and technology 2017 Golden rules: ● Don’t transfer heat across the pinch ● Don’t use cold utilities above the pinch ● Don’t use hot utilities below the pinch Heat recovery and Heat exchange The pinch and its significance five simple and effective concepts: ● Targets: Once the composite curves and Problem Table are known, we know exactly how much external heating is unavoidably required ● The Pinch: Above the pinch the process needs external heating and below the pinch it needs external cooling This tells us where to place furnaces, steam heaters, coolers, etc It also tells us what site steam services should be used and how we should recover heat from the exhaust of steam and gas turbines ● More in, more out: An off-target process requires more than the minimum external heating and therefore more than the minimum external cooling ● Freedom of choice: The “heat sink” and the “heat source” are separate ● Trade-offs: A simple relationship exists between the number of streams (process streams plus utilities) in a problem and the minimum number of heat eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange Heat exchanger network design eat integration course _ Hanoi University of science and technology 2017 ... Basic concepts of heat exchange: eat integration course _ Hanoi University of science and technology 2017 Heat recovery and Heat exchange The temperature–enthalpy diagram Basic concepts of heat... across the pinch ● Don’t use cold utilities above the pinch ● Don’t use hot utilities below the pinch Heat recovery and Heat exchange The pinch and its significance five simple and effective concepts: ... University of science and technology 2017 Heat recovery and Heat exchange Basic concepts of heat exchange: The temperature–enthalpy diagram eat integration course _ Hanoi University of science