Prospective of Upfront Nitrogen (N2) Removal in LNG Plants: Technical Communication

Abstract

Conventional natural gas (NG) liquefaction processes remove N2 near the tail of the plant, which limits production capacity and decreases energy efficiency and profit. Engineering calculations suggest that upfront N2 removal could have substantial economic benefits on large-scale liquefied natural gas (LNG) processes. This article provides an overview of the most promising technologies that can be employed for upfront N2 removal in the LNG process, focusing on the process selection and design considerations of all currently available upfront N2 removal technologies. The literature review revealed that although adsorption has proven to be a huge success in gas separation processes (efficiency ≥ 90%), most of the available adsorbents are CH4-selective at typical NG conditions. It would be more encouraging to find N2-selective adsorbents to apply in upfront N2 removal technology. Membrane gas separation has shown growing performance due to its flexible operation, small footprint, and reduced investment cost and energy consumption. However, the use of such technology as upfront N2 removal requires multi-stage membranes to reduce the nitrogen content and satisfy LNG specifications. The efficiency of such technology should be correlated with the cost of gas re-compression, product quality, and pressure. A hybrid system of adsorption/membrane processes was proposed to eliminate the disadvantages of both technologies and enhance productivity that required further investigation. Upfront N2 removal technology based on sequential high and low-pressure distillation was presented and showed interesting results. The distillation process, operated with at least 17.6% upfront N2 removal, reduced specific power requirements by 5% and increased the plant capacity by 16% in a 530 MMSCFD LNG plant. Lithium-cycle showed promising results as an upfront N2 chemical removal technology. Recent studies showed that this process could reduce the NG N2 content at ambient temperature and 80 bar from 10% to 0.5% N2, achieving the required LNG specifications. Gas hydrate could have the potential as upfront N2 removal technology if the is process modified to guarantee significant removals of low N2 concentration from a mixture of hydrocarbons. Retrofitting the proposed technologies into LNG plants, design alterations, removal limits, and cost analysis are challenges that are open for further exploration in the near future. The present review offers directions for different researchers to explore different alternatives for upfront N2 removal from NG.

A critical review on the practical use of lithium cycle as an upfront nitrogen removal technology from natural gas

“…21,24 While not the focus of this study, hydrogenation can be an alternative to hydrolysis, especially if H 2 gas is required. As demonstrated by Reaction (5), in the hydrolysis method the Li 3 N reacts with water in a violent exothermic reaction to produce lithium hydroxide and ammonia as a side product.…”

“…Lithium nitridation does not require multistage operation as membranes do; however, it must undergo sorbent regeneration like adsorption, with the latter having no positive aspect/economic side products. With research regarding adsorption centered around methane separation, and membrane technology showing decreased separation performance at higher pressures, 5 causing the need for costly pressure reduction and copression, the Li‐Cy is an option worth investigating for NG streams with low N 2 content in the hot section of the LNG plant, and Table 6 summarizes the comparison between the different separation technologies discussed in this subsection.…”

“…Removing the nitrogen almost entirely or partially before entering the cold section removes the dead volume that it occupies and increases the LNG capacity of the plant. For example Almomani et al, 5 reported preliminary engineering calculations that exemplified a removal of 70% of N 2 upfront at the hot section decreases the energy required for the propane/refrigerant cycle by 6.1%. According to the thermodynamic calculations, the required compression work (30% efficiency) for the upfront removal of nitrogen (70%) would require 55% of the energy.…”

“…The literature review outlines numerous processes for UNR, including (1) physical separation technologies, such as adsorption, 6–8 membrane separation, 9–11 hybrid processes, and distillations 12 ; (2) chemical separation technologies, 13–15 including absorption 15 and lithium‐based adsorption 14,16 ; and (3) gas hydrate technology 17–20 . A detailed discussion of all these processes can be seen in our previous publication 5 …”

Reducing Power Use in the Cold Section of LNG Plants

“…About 80% of the proven NG reserves are located in only 10 countries, hence it is of paramount importance that the transport of NG over long distances is both economical and efficient . Therefore, NG is normally transported in liquid form (LNG or Liquefied NG) at 113 K, which reduces its volume by a factor of 600 at ambient pressure . Processing raw NG from a gas field into LNG requires a series of upgrades to remove NGLs (NG Liquid) such as ethane and propane, inert like moisture, nitrogen, helium, and carbon dioxide, and contaminants such as mercury and hydrogen sulfide to meet LNG specifications before transport.…”