Evaluation of different NGL-Extraction Processes in LNG Plants
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The world s energy demand is expected to rise in the future, and fossil fuels are expected to account for a major portion of this increase. Demand for natural gas is expected to rise with 40% until 2030 due to its clean burning characteristics and low carbon emissions. As a result of the more easily available gas fields with lean gas being depleted, more remote gas fields with richer gas will be extracted in the future. Transportation over long distances favors the natural gas to be transported in its liquid form as LNG, the need for NGL-extraction is expected to increase accordingly. In order to liquefy the natural gas, extraction of heavy hydrocarbons are vital to prevent freeze-out due to cryogenic temperatures in the liquefaction process and to meet heating value specifications for the sales gas. NGL and condensate offer greater revenue potential if sold separately instead of as a part of the LNG. A number of different concepts for NGL-extraction, both employed in the industry and novel extraction concepts, have been presented in the literature survey in this thesis. Use of the novel energy and cost effective distillation solution of a dividing wall column, based on the idea of Petlyuk in the 1950s, has been investigated and compared to other extraction technologies. These technologies include both simple scrubber columns and more technologically advanced processes utilizing compander solutions to reduce the effects of pressure changes. Five schemes for NGL-extraction, including that of the dividing wall column, were modeled using Aspen HYSYS with two gas feeds and two heating values for sake of comparison. Figures for energy consumptions per unit of LNG product, kWh/ton LNG, were obtained for the cases modeled. The various extraction schemes were compared with respect to energy consumption and equipment requirements. For the most energy efficient extraction scheme modeled, an integrated version of the Linde Recontactor-Deethanizer Process, energy consumptions ranged from 241,1-268,8 kWh/ton LNG for the least and most energy intensive combination of feed gas and heating value requirement. The other models resulted in energy consumptions a few percent higher than that of the best case. The dividing wall column generally yielded the highest energy consumptions, ranging form 280,6-258,5 kWh/ton LNG. The dividing wall column performed best relative to the other processes when a strict heating value requirement was desired from a gas rich in heavy hydrocarbons, an indication on the ability of the dividing wall to perform sharp separations. In order to fully classify the possible advantages of the dividing wall column, modeling of the downstream processing has to be included in the simulation models. Opportunities for process integration have been investigated, and general methods within the field of pinch analysis have been presented. In conclusion of this thesis, proposals for further work within the field of NGL-extraction have been discussed, especially to better quantify the possible energy savings obtainable with a dividing wall column.