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Introduction to Pinch Technology Downloaded from orbit.dtu.dk on: Sep 25, 2021 Introduction to Pinch Technology Rokni, Masoud Publication date: 2016 Document Version Peer reviewed version Link back to DTU Orbit Citation (APA): Rokni, M. (2016). Introduction to Pinch Technology. Technical University of Denmark. General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. Users may download and print one copy of any publication from the public portal for the purpose of private study or research. You may not further distribute the material or use it for any profit-making activity or commercial gain You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Introduction to Pinch Technology Division of Energy Section Masoud Rokni Technical University of Denmark [email protected] DTU Building 402, 1st Floor DK-2800 KGS, Lyngby July 2006 Revised April 2016 1 Introduction to Pinch Technology 1. Introduction The demand for effective usage of energy processes is increasing and nowadays every process engineer is facing the challenge to seek answers to the questions related to their process energy patterns. Below the questions that frequently may be asked are listed: Is the existing process as energy efficient as it should be? How can new projects be evaluated with respect to their energy requirements? What changes can be made to increase the energy efficiency without incurring any cost? What investments can be made to improve energy efficiency? What is the most appropriate utility mix for the process? How to put energy efficiency and other targets like reducing emissions, increasing plant capacities, improve product qualities etc, into a one coherent strategic plan for the overall site? All these questions and more can be answered with a full understanding of Pinch Technology and an awareness of the available tools for applying it in a practical way. The aim here is to provide the basic knowledge of pinch technology concept and how it can be applied across a wide range of process industries. The pinch technology was proposed firstly for optimization of heat exchangers and therefore it is introduced / described below for such devises. Heat exchange equipments encounter in many industries for at least two reasons; a) it is often necessary as part of the process to change the thermal condition and b) it is the ambition to minimize the energy consumption of the given process. With other words the idea is to maximize the energy recovery within the process or to minimize the use of external energy sources. Figs 1 and 2 illustrate this meaning. HEX HEX HEX HEX HEX HEX HEX HEX HEX HEX a) b) Figure 1. a) Process with only external energy sources and b) Process with internal and external energy sources. 2 In Fig. 1a heat is added or removed only with external sources (heaters and coolers) while the same process can be improved by using internal heat exchanges under the condition of an overlap in the temperature intervals, see Fig 1b. The need of external energy sources can be reduced by using internal heat exchanges between cold and hot streams and a more energy efficient process can thus be achieved. However, it does not mean that the total cost is reduced since the use of internal heat exchange results in increased capital cost through bigger or more heat exchanger units. Fig. 2 shows how heat can be recovered in s simple process scheme by using internal heat exchange. Steam Feed Product Reactor 1 2 200C 80C 90C Cold Water a) Steam Feed Product Reactor 1 200C 2 80C 90C Cold Water 3 b) Figure 2. A simple process a) with external energy sources only and b) with internal and external energy sources. The application of such optimization method is referred as “Pinch Technology” that guarantee minimum energy levels in design of heat exchanger network in a process. 2. Problem Statement A typical industrial process may consist of several numbers of hot and cold process streams which may demand cooling and heating respectively. Heat exchangers can be used to recover some of the heat demand while external heaters and coolers can be used to achieve the temperature demand of the process streams. Suppose an industrial plant with hot and cold process 3 streams as shown in table 1. Heat capacity rate is defined as mass flow rate times heat capacity mC p . Table 1. Process streams in an industrial plant. Process stream Inlet Temp. Outlet Temp. Heat capacity rate Q [kW] Nr / Type [C] [C] [kW/K] 1. cold 90 420 10 3300 2. cold 170 350 32 5760 3. cold 200 390 29 5510 4. hot 440 140 27 8100 5. hot 510 300 24 5040 The task is to find the optimal network of heat exchangers, external coolers and external heaters with respect to the capital and annual operating cost. The maximum heat that can be transferred in a heat exchanger is limited by the minimum allowed temperature difference between hot and cold streams, called as Tmin. The temperature level at which Tmin is observed is called as “pinch point” and the analysis to find this temperature with respect to the laws of thermodynamic is called as “Pinch Analysis” or “Pinch Technology”. Capital costs depend mainly on the number of heaters, coolers, heat exchangers and their sizes (area) while operating cost is mainly dominated by the need for external energy supplied such as heating and cooling. Thus the main objective of pinch analysis is to achieve financial savings by better process heat integration (maximizing the process heat recovery and reducing the external utility loads). In table 1, the heating effect and cooling effect under steady state condition (constant heat capacities and temperatures) are Qheat = 10 x (420 – 90) + 32 x (350 – 170) + 29 x (390 – 200) = 14570 kW Qcool = 27 x (510 – 300) + 2.0 x (440 – 140) = 13140 kW which must be supplied by external heater and cooler such as steam water and cold water. The idea is now to find the lowest possible Qheat for this particular problem. 3. The Pinch Method The pinch method consists of two steps. In the first step the maximum energy recovery in the system should be calculated which in turn reduces the external energy needed to a minimum level. Consider the process streams and it temperatures in table 1. Figure 3 shows the process streams and the temperature intervals for the plant. The inlet and outlet temperatures can now be divided into temperature intervals where the first temperature interval is between the largest temperature and the second largest temperature. The next interval is between the second largest temperature and the third largest temperature, etc. Such temperature intervals results are shown in table 2. For a given interval, one may calculated the amount of heat to be supplied to the plant (called as Qheat) and how much how much heat must be taken from the plant (called as Qcool). The relationship between these external cooling and heating can be written as Q Qcool Qheat . (1) 4 mC p 5 24 4 27 3 29 2 32 1 10 0 100 200 300 400 500 600 T [C] Figure 3. Temperatures of the process streams. Table 2. Temperature interval and external heating and cooling effects. Interval Temperature Stream Q Q Q Number interval [C] Numbers cool heat [kW] [kW] [kW] 1 510 – 440 5 1680 0 1680 2 440 – 420 4 + 5 1020 0 1020 3 420 – 390 1 + 4 + 5 1530 300 1230 4 390 – 350 1 + 3 + 4 + 5 2040 1560 480 5 350 – 300 1 + 2 + 3 + 4 +5 2550 3550 - 1000 6 300 – 200 1 + 2 + 3 + 4 2700 7100 - 4400 7 200 – 170 1 + 2 + 4 810 1260 - 450 8 170 – 140 1 + 4 810 300 510 9 140 – 90 1 0 500 - 500 For example in interval 4 where the temperature interval is between 390C to 350C, the cooling and heating demands are: Qheat = 10 x (390 – 350) + 29 x (390 – 350) = 1560 kW Qcool = 27 x (390 – 350) + 24 x (390 – 350) = 2040 kW Q = Qcool – Qheat = 2040 – 1560 = 480 kW It means that a cooling effect of 480 kW must be supplied to the interval 4 so that the process stream can deliver the lowest needed temperature which is 350C. 5 Another important conclusion from table 2 is that in interval 6 the need for external heating is – 4400 kW which is the largest heating demand and must be supplied by external heating to the process. In pinch method this external heating must be first supplied to the system at the respective temperature interval and then all other temperature intervals must be updated according to this heating demand. There are two major methods to calculate the pinch point, table method and composite curve method. The table method will be discussed next. However, in between the importance of Tmin is going to be discussed first. 3.1 The meaning of Tmin The interval and calculated heat transfer in table 2 requires an ideal heat transfer within a heat exchanger that cool the hot stream down to the minimum temperature difference of Tmin = 0. It means that the heat exchanger area (size) is infinite, Tmin 0 heat exchanger size and price . This of course is not possible in practical applications and Tmin ≠ 0 is always valid.
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