(119d) Evolutionary Design of An LNG Process Using the ExPAnD Method and a New Graphical Representation of Exergy

This paper presents the design of a novel natural gas liquefaction process with removal of heavy hydrocarbons (C₃+). This process is designed using the Extended Pinch Analysis and Design (ExPAnD) method and a new representation of exergy and exergy exchange. The ExPAnD method combines the classical Pinch Analysis with Exergy Analysis and is applicable for the development of processes where both temperature and pressure are important design variables such as in below ambient processes. The design of processes operating below ambient temperature presents several challenges. First, the design must be customized to match the climate conditions (i.e. ambient temperature and relative humidity) of the location where the plant will be installed. Second, the plant size and location (i.e. offshore or onshore) constrain the number of units and equipment size, possibly reducing the process efficiency (i.e. offshore natural gas liquefaction plants have lower energy efficiency, fewer unit operations and smaller equipment size than onshore plants). Third, the number of degrees of freedom in the design may vary because of the manipulation of temperature and pressure. Forth, processes below ambient temperature have large energy consumption in the refrigeration cycles (low temperature level results in high energy consumption). For low temperature heat exchanger networks, the temperature difference between process streams should be small and the refrigeration loads minimized. In consequence even from the conceptual design phase, processes operating below ambient temperature should be as energy efficient design as possible.

One option to achieve low energy consumption is to reduce avoidable irreversibilities. Exergy Analysis is a powerful tool to identify and calculate such irreversibilites; however, because exergy tends to have rather complex relations (non-linear) with the process variables (such as, temperature, pressure and composition) the calculations can be quite tedious. In addition, some graphical representations of exergy against process variables or other energy quality parameters require multiple calculations for their construction (i.e. the Exergy Composite Curves and Exergy Grand Composite Curve) due to the non-linear relations. For these reasons, exergy analysis is commonly used as a post-design tool and not integrated in the design procedure. Exergy calculations however, may be simplified by assuming ideal gas conditions and constant heat capacity. These assumptions might not lead to very large deviations for processes where gases such as, methane and nitrogen are used. Moreover, the exergy content of a process stream can be decomposed into temperature based exergy, pressure based exergy and chemical exergy. The sum of the first two components is the so-called thermo-mechanical exergy. If both temperature and pressure based exergies can be calculated using the above described assumptions, then the decomposition of the thermo-mechanical exergy is sharp in the sense that temperature based exergy only depends on temperature and pressure based exergy is a function of pressure only. Moreover, pressure based exergy can be transformed into temperature based exergy and vice versa. For example, a process stream expanded below ambient temperature will produce work and reduce the stream temperature. This means that temperature based exergy actually increases, and this can be seen as a partial transformation of pressure based exergy to temperature based exergy. The rest of the pressure based exergy is transformed into work, if isentropic expansion is assumed.

The new representation of exergy and exergy exchange utilized in this paper incorporates these simplified relations of exergy. The simplifications allow having a linear relation between exergy and a new energy quality parameter referred to as exergetic temperature. The exergetic temperature for temperature based exergy is only a function of temperature and the exergetic temperature for pressure based exergy is only a function of pressure.  The change in temperature or pressure based exergy is then equal to the product of a constant heat capacity flow rate and a difference in the corresponding exergetic temperatures. These changes in the energy quality parameter and changes in exergy can be plotted in a composite manner for all the process streams. In the special case of pressure change, if the change is assumed to be isentropic or with a specified isentropic efficiency, there is a direct relation between the change of exergetic temperatures due to temperature change and the change of exergetic temperatures due to pressure change. This direct relation aids to the inclusion of pressure change effects in the graphical representation of exergy exchange.

By using the ExPAnD methodology and the new graphical representation of exergy, it is possible to develop an evolutionary process design. The reverse Brayton process for the liquefaction of natural gas is used as a case study. The reverse Brayton process liquefies natural gas utilizing only nitrogen as working fluid in the refrigeration cycle. The exergy efficiency of the reverse Brayton in the literature is close to 34% (defined as change in thermo-mechanical exergy of natural gas divided by net compressor work) with 6 compression stages in the refrigeration cycle and no heavy component removal. The alternative design presented in this paper has an exergy efficiency of 64% (using the same efficiency definition) with 6 compression stages and without including the increment in chemical exergy due to the LNG purification.