LNG PRE-TREATMENT-LOWER LIFE CYCLE COSTS WITH UOP SEPARSIV ADSORBENT PROCESS FOR HEAVY HYDROCARBON REMOVAL FROM LEAN NATURAL GAS

摘要

Lean natural gas feeds are becoming common for newer North American LNG plants as many of these facilities process feed from unconventional sources. These new gas resources have more methane and less NGL's than conventional feeds. However, many lean feed gas sources contain disproportionately high amounts of heavy hydrocarbons (HHCs), which can be similar in magnitude to those found in the conventional feed gas. The amount of these heavy components can exceed the maximum amount allowed to avoid freezing in the liquefaction process. Components such as octane, nonane, benzene, and ethyl benzene are particularly challenging as even small amounts of these components can freeze at LNG temperatures. To prevent freezing of the heavies in LNG production, one of the requirements in the pretreatment step is the removal of heavy hydrocarbons (C6+). Traditionally, this is done using a distillation approach by a scrub column. It is difficult for a traditional distillation column to operate stably and efficiently on lean feed gas because there are not enough NGL components present to provide sufficient reflux and downward liquid traffic in the column. Possible solutions for providing the needed cold liquid include providing it from the downstream cold box or/and feed gas expansion. However, both options are very energy inefficient. If not properly handled, freezing of HHCs can become so severe that it is necessary to shut down and defrost the plant, leading to loss of LNG production and lost revenue. UOP has developed an alternative process for heavy hydrocarbon removal from lean natural gas, called SeparSIV. This adsorption-based process is built on commercially proven technology for water and heavy hydrocarbon removal to meet pipeline gas specifications. To further meet the more stringent specification for LNG, UOP's SeparSIV adsorption system uses a mix of adsorbents capable of removing C5+ to less than 0.1 mol% and BTEX to less than 1 ppmv from lean gas. This system operates at high pressure with a low pressure drop and does not require the gas expansion and recompression commonly found in an NGL extraction unit. The adsorption system is also very selective for heavy hydrocarbons without removing lighter ones. Also, UOP's SeparSIV can be designed to remove HHCs and water simultaneously in a single process unit. Other benefits of this new process will be discussed in later sections of this paper.