Enhancement of LNG plant propane cycle through waste heat powered absorption cooling

Abstract

In liquefied natural gas (LNG) plants utilizing sea water for process cooling, both the efficiency and production capacity of the propane cycle decrease with increasing sea water temperature. To address this issue, several propane cycle enhancement approaches are investigated in this study, which require minimal modification of the existing plant configuration. These approaches rely on the use of gas turbine waste heat powered water/lithium bromide absorption cooling to either (i) subcool propane after the propane cycle condenser, or (ii) reduce propane cycle condensing pressure through pre-cooling of condenser cooling water. In the second approach, two alternative methods of pre-cooling condenser cooling water are considered, which consist of an open sea water loop, and a closed fresh water loop. In addition for all cases, three candidate absorption chiller configurations are evaluated, namely single-effect, double-effect, and cascaded double- and single-effect chillers. The thermodynamic performance of each propane cycle enhancement scheme, integrated in an actual LNG plant in the Persian Gulf, is evaluated using actual plant operating data. Subcooling propane after the propane cycle condenser is found to improve propane cycle total coefficient of performance (COP_T) and cooling capacity by 13% and 23%, respectively. The necessary cooling load could be provided by either a single-effect, double-effect or cascaded and single- and double-effect absorption refrigeration cycle recovering waste heat from a single gas turbine operated at full load. Reducing propane condensing pressure using a closed fresh water condenser cooling loop is found result in propane cycle COP_T and cooling capacity enhancements of 63% and 22%, respectively, but would require substantially higher capital investment than for propane subcooling, due to higher cooling load and thus higher waste heat requirements. Considering the present trend of short process enhancement payback periods in the natural gas industry, subcooling propane after the propane cycle condenser is recommended as the preferred option to boost propane cycle performance.

Introduction

The liquefaction of natural gas (NG) serves to reduce NG volume for economic transportation. Liquefaction is achieved by cooling NG below −160 °C [1], which requires a considerable amount of energy.¹ Enhancing the energy efficiency of the liquefaction process could therefore significantly improve the efficiency of liquefied natural gas (LNG) plants, and reduce both fuel consumption and associated carbon emissions. This is critical to LNG plants located in the Persian Gulf, whose refrigeration capacities are constrained by high yearly ambient temperatures.

Today, the majority of base-load LNG plants employ the propane pre-cooled mixed refrigerant (APCI) cycle [3]. The plant considered in this study is a major LNG facility in the Persian Gulf, whose liquefaction process is based on APCI cycle. In this process, which is illustrated in Fig. 1, the feed gas is passed through a gas sweetening section for removal of H₂S, CO₂, H₂O and Hg. As it passes through the precooler and cold box, its temperature decreases to approximately −30 °C, resulting in condensation of certain components. The remaining gas and condensate are separated in the separator. The condensate is sent to the fractionation unit, where it is separated into propane, butane, pentane, and heavier hydrocarbons. The remaining gas is further cooled in the cryogenic column to below −160 °C and liquefied. Its pressure is then reduced to atmospheric pressure by passing through the LNG expansion valve. Two refrigeration cycles are involved in the overall process shown in Fig. 1, which are the propane cycle and the multi component refrigerant (MCR) cycle. The first cycle provides the required cooling to the precooler, cold box and fractionation unit. The second cycle supplies the cooling demand of the cryogenic column. Ambient temperature significantly impacts plant efficiency and production capacity. In Middle East/Persian Gulf facilities, the condensers and desuperheaters of both refrigeration cycles are typically cooled by sea water, average daily surface temperatures of which range from 15 °C to 32 °C, with maxima at 35 °C [4].

Approaches previously investigated in the literature to enhance the efficiency of the liquefaction cycle have included optimization of refrigerant composition, mass flow rate and operating pressure [5], [6], [7], [8], [9], improvement of compressor and driver cycle efficiency [10], and improvement of cycle components [11], [12], [13]. Component improvements include replacement of Joule Thomson expansion valves with turbine expanders [11], replacement of LNG and mixed refrigerant expansion valves with liquid turbines [12], and replacement of expansion valves with liquid turbines and two-phase expanders [13]. However, in many instances such strategies require significant modification of existing licensed processes, which effectively prevents their integration into existing facilities.

Waste heat utilization is an alternative approach of enhancing LNG plant efficiency, which can be implemented with minimal modification of existing processes. Waste heat utilization applications pertinent to LNG plants previously investigated in the petroleum industry include waste heat powered absorption refrigeration for liquefied petroleum gas (LPG) recovery using waste heat from a reactor effluent in an oil refinery [14], and gas turbine (GT) inlet air cooling using a waste heat powered ejector-refrigerator [15].

This study investigates the use of waste heat powered absorption refrigeration to improve the efficiency of the liquefaction cycle through novel propane cycle enhancement approaches, focusing on the needs of LNG facilities in the Middle East or exposed to elevated ambient temperatures. Absorption refrigeration systems (ARSs) offer advantages over vapor compression systems, including lower energy consumption and thus lower operating costs [16], and their capability to utilize waste heat [17]. Fig. 2 provides an overview of waste heat sources, recovery approaches and applications suitable for waste heat powered absorption chillers. GTs, which are typically employed in LNG plants for power generation, are a significant source of high-grade waste heat, which could be used to power absorption refrigeration systems. A limited number of studies [18], [19] have investigated the use of GT waste heat-powered absorption refrigeration in LNG plants. Kalinowski et al. [18] used a GT waste heat powered ARS to replace the propane cycle in an LNG plant. Mortazavi et al. [19] investigated the energy efficiency improvements obtained in the APCI (Air Products and Chemicals Incorporation) liquefaction process with various options of utilizing GT waste heat powered absorption chillers. The absorption chillers were used for either GT inlet air cooling, replacing propane cycle evaporators, subcooling propane, cooling the condenser of the propane cycle, or inter-cooling the compressor of the mixed refrigerant cycle with absorption chillers. Alternatively, a vapor compression cycle (VCC) may also be enhanced using waste heat recovered from a power generation cycle, which is typically a GT cycle. Hwang [20] proposed a generic scheme (i.e., not industry specific) based on this concept, in which waste heat from a microturbine was employed for subcooling the conventional vapor compression refrigeration system after the condenser. Using a thermodynamic model implemented in Engineering Equation Solver, Hwang [20] estimated that this scheme would reduce the annual energy consumption of the VCC by 12%. However, the model was not experimentally validated.

Although the above efforts [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [18], [19], [20] have provided valuable insight into the potential of waste heat utilization to enhance LNG plant energy efficiency, either i) major changes in the design of the LNG plant were required that may be considered prohibitive by plant operators due to installation time, undiscovered issues with the new cycle design, or payback period of the required capital investment, or ii) the thermodynamic model employed, hence the predicted plant performance improvements, were either not experimentally validated, or iii) not constructed based on actual plant operational data. In this context, the main objective of this study is to investigate several waste heat utilization-based propane cycle enhancement strategies in a major APCI LNG plant in the Persian Gulf, to enhance plant energy efficiency and production capacity, without significant modification to the plant configuration. Since the propane cycle cools the MCR cycle, any enhancement in the energy efficiency and production capacity of the liquefaction process firstly requires improvements to the propane cycle. As a first step, this study therefore focuses on enhancing the propane cycle. This is achieved using an experimentally validated thermodynamic model, which is constructed based on actual plant operating data.