Analysis and Design of LowTemperature Processes withFocus on LNG: Developing new Thermodynamics based Tools and introducing Exergy in Design Methodologies
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This thesis focuses on the development of new tools and methods for the analysis and design of highly efficient Low Temperature Processes (LTPs), in particular for those operating at sub−ambient conditions. Exergy Analysis (EA) and a combination of EA with Pinch Analysis (PA) are taken as basis in the development of the suggested tools and design methodology. The consumed energy by sub−ambient processes can be very large at low temperatures; the lower the heat surplus inlet temperature is, the larger the amount of energy (power or electricity) is consumed. Therefore, there is a general interest to reduce of the energy consumption in these processes. A Coefficient of Exergy Performance (CEP) named Exergy Transfer Effectiveness (ETE) is proposed by following the source−sink philosophy used in Process Synthesis. It is also shown that by recognizing the true exergy sources and exergy sinks for all unit operations as well as for overall processes, it is possible to evaluate the actual exergy transfer rate (i.e. measuring the thermodynamic perfection of the processes). An important aspect in the identification of exergy sources and sinks, especially in LTPs, is the decomposition of the thermo−mechanical exergy of process streams. The thermo−mechanical exergy should be divided into the temperature based and pressure based exergy components. Four previously defined CEPs as well as the ETE are applied in typical LTP unit operations at different different operating conditions. Differences in the formulation of the CEPs and ETE are evident when the unit operations are operating below and across ambient temperature. Small differences between the results of the old CEPs and ETE are observed while evaluating heat exchangers operating across ambient temperature. The discrepancies are less than 1% point when the stream pressures as well as the pressure drop are low. However, the differences are close to 6% points for high values of stream pressures and pressure drop. This indicates that not all CEPs evaluate the change in pressure based exergy due to pressure drop in the same manner. Three novel representations for exergy transfer are introduced in the thesis. One is a new exergetic composite curves diagram, and the other two are exergy cascades. The exergy diagram and the exergy cascades use a new energy quality parameter referred to as the exergetic temperature. In the new exergy diagram, the exergy targets are easy to quantify. In addition, given that the exergetic temperatures have a linear relationship with the temperature based exergy of the process streams, the construction of the exergetic composite curves is straightforward. These representations of exergy transfer are integrated into a methodology for the design of LTPs. The key feature of this methodology is the utilization of pressure based exergy of process streams to reduce the energy (i.e. heat and work) requirement in an Heat Recovery System (HRS). Three case studies are used to demonstrate the application of the graphical representations and proposed design methodology. The first HRS operates above ambient temperature, the second is a generic LTPs and the third is the Reverse Brayton process. In addition, the PRICO process for liquefaction of natural gas is used to study the different Coefficient of Exergy Performance (CEPs).