Apr-2023

Intervening when feed gas is too lean: Heavies removal challenges for LNG projects

UOP and Kiewit, based on previous work experience, have found that one size does not fit all when it comes to gas treating for LNG plants.

Dipanjan Bhattacharya and Bhargav Sharma
Honeywell UOP

Article Summary

While many gas treatment process configurations are sound designs based on expected feed gas composition ranges, the actual operational experience seen in North America is that feed gas compositions have been extremely lean — leaner than ever anticipated. Whenever plant configurations are consistently operating outside of their design boundaries, operational challenges will result. This paper was developed in order to promote natural gas treatment alternatives that may be suitable when feed gas is “too lean”. Additionally, these schemes can result in an economic design with minimal operational intervention.

While many gas treatment process configurations are sound designs based on expected feed gas composition ranges, the actual operational experience seen in North America is that feed gas compositions have been extremely lean — leaner than ever anticipated. Whenever plant configurations are consistently operating outside of their design boundaries, operational challenges will result. This paper was developed in order to promote natural gas treatment alternatives that may be suitable when feed gas is “too lean”. Additionally, these schemes can result in an economic design with minimal operational intervention.

Gas processing block flow scheme for LNG
Figure 1 shows a typical block flow diagram for North American LNG Plants. Some variations like the preference for locating the Hâ‚‚S removal unit (upstream vs. inside acid gas removal) and mercury removal unit (upstream of Amine unit vs. downstream of Molecular Sieve unit) is observed. These sensitivities in the overall block flow for gas processing is omitted for this paper as it does not impact the subject matter of dealing with lean feed gas compositions.

Options for heavy hydrocarbon removal
Option 1. Conventional heavies removal design

The LNG industry has been using conventional heavies removal technology for the last 40-50 years. Figure 2 shows a schematic of a conventional turboexpander-based process. Most North American LNG projects have adapted a design that is similar in concept to the design as shown in Figure 2. There are other similar cryogenic schemes that have been successfully used as well. The type of system used depends on the inlet conditions of the gas stream and the desired recovery levels of hydrocarbon. Process schemes range from a simple high pressure wash column utilising external refrigeration to the turbo-expander plant as shown in Figure 2.

Such cryogenic flowschemes have worked well for reasonably rich feed gas with relatively high amount of C₃+ in the feed. For the lean feed gas as is currently observed in North America, it is difficult to create the necessary reflux streams to adequately absorb the heavies from the predominantly methane stream in a cryogenic process. As the gas becomes leaner, heavies removal becomes even more difficult to the extent that it may require injection of mid-level components such as propane or butane to help absorb the heavy tail. Without intervention, this condition could result in continuous heavies slip which can end with solids formation in the liquefaction system cryogenic heat exchangers and even in downstream equipment resulting in blockage. Blockages can lead to excessive downtime over the life of the plant. Moreover, lack of adequate quantities of these mid-level hydrocarbon components may result in operational difficulties, not only in the Demethaniser, but also in downstream fractionation columns. As a result, these operational difficulties can possibly result in highly operator intensive “batch operations” in the debutanisers/deethanisers.

Option 2. Adsorption-based heavies removal designs
Adsorption systems are commercially proven for removal of water and heavy hydrocarbons to protect equipment and meet pipeline specifications. However, the typical hydrocarbon removal unit using a single adsorbent, such as silica gel, cannot efficiently remove heavy hydrocarbons and water to the levels required for LNG pretreatment. Efficiently removing hydrocarbons and water to tight LNG specification levels requires a combination of different types of high performance adsorbents. In order to overcome these challenges, the SeparSIVTM Process is proposed for two schemes outlined below. The first system is the most compact of the two options to show the advantages of using SeparSIV for these heavies removal challenges.

Option 2a. Adsorption-based heavies removal design combined with dehydration unit
This adsorption-based configuration combines dehydration and heavies removal in the same unit using the SeparSIV process. Figure 3 shows the block flow of the combined heavies removal system for the pre-treatment section.

The SeparSIV process utilises Thermal Swing Adsorption technology (TSA) with a multi-layer system of adsorbents to target and optimise adsorption of C₅+ to less than 0.1 mole%, BTEX and C₈+ to < 1ppmv, and water down to <0.1ppmv. Figure 4 shows the loading capacity of C₅ and C8 as a function of partial pressure of these components on different layers of adsorbent (SS-3 and SS-7). The idea is to use the right adsorbent to surgically remove the targeted impurity in the right order. The technology can be combined with an advanced control system that enables the process to flexibly adjust to varying feed gas compositions.

With adsorption-based technology, the higher the carbon number, the easier it is to adsorb. SeparSIV can remove water and C₆ plus hydrocarbon fractions while leaving the C₄ and lighter hydrocarbon fractions in the LNG product. This works well with overall LNG plant needs as heavy hydrocarbon removal is desirable to prevent freezing but leaving light hydrocarbons in the LNG product can command better value for LNG when traded on a thermal (BTU) basis. This process can also be adjusted to “surgically” remove additional contaminants like mercaptan sulphur and mercury when suitable treatment of regeneration gas is implemented.

Combining water and heavy hydrocarbon removal in a single unit requires a water and hydrocarbon management strategy. Since water and hydrocarbons are condensed in the same regeneration drum, an efficient effluent separation system needs to be designed along with the use of a three-phase separator; this strategy will then allow for bulk separation of water and heavy hydrocarbons. Water saturated liquid hydrocarbons can then go to a separation system for efficient water removal. The hydrocarbons can be further stabilised to meet the required condensate Reid Vapour pressure.

We have considered the combined water and heavy hydrocarbon removal adsorption-based scheme (Option 2a, Figure 3) as the preferred option to compare with conventional heavies removal scheme (Option 1). A combined adsorption system will minimise the equipment count and result in greater CAPEX and OPEX savings. However, it is worthwhile to consider a decoupled process strategy when reviewing a range of configurations and options.