Remove the heavy load

Feed gas analysis and decision factors

The analysis starts with a study of the feed gas composition and determination of the lean-to-rich range of the design. The amount of natural gas liquids (NGL) and potentially freezing components, typically marked by benzene concentration, are prima-ry factors. But even for a set composition, other factors, such as the ability to sell NGL product(s) if there is a fuel sink to blend in the removed heavies and the pipeline pressure, favour different configurations.

The range of compositions to consider is key to making design decisions. Gas from a single production source may vary but is typically more consistent in composition and can have a narrower, targeted process. LNG plants on a large pipeline system – like the US Gulf Coast with many different producers – can see significant variations, so flexibility to meet removal specifications is warranted but comes at a higher cost. For these systems, feed gas analysis should be conducted throughout the project lifetime to account for changes that may alter feed gas composition by the time the plant starts up.

Feed gas pressure is also key to design selections. Lower pressure directly results in lower liquefaction efficiency, so options that feed the main heat exchanger at higher, super-critical pressures exhibit higher efficiencies. However, many LNG plants do not receive gas at super-critical pressure, particularly those on pipeline systems and small scale units, and therefore may be less restricted by configurations that favour lower pressure for separation.

Regardless of the configuration chosen, a use or disposal method for the heavy hydrocarbons must be found as this can be a constraining factor. The heavies stream can be blended to fuel with a turbine drive system if it meets the quality and volume needed. However, sometimes the blend exceeds the consumption rate (particularly when the facility is motor driven) or specification and requires further processing to remove the light ends. This results in an LPG/NGL/condensate product that needs to be stored and moved offsite.

Heavies removal configurations

Sorting through available heavy hydrocarbon removal options from those proven in the LNG industry can be a daunting task. This article will discuss the merits and drawbacks for the three most common general flowsheets: partial condensation integrated with the liquefaction process, expander-driven units either integrated with or upstream of liquefaction, and adsorption technology.

Partial condensation

Partial condensation configurations are the simplest in both flowsheet and operation. However, phase separation cannot be achieved unless the stream is below the critical pressure, and sometimes considerably so in the case of a simple flash to en-sure freezing components are adequately removed. Therefore, partial condensation configurations are most favourable at lower feed gas pressures since the gas is already below the critical point. High feed gas pressures require significant pressure reduction for partial condensation to be effective, reducing overall efficiency.

Simple flash

A stalwart for small scale applications, the simple flash has a proven history and is still a fit for many applications. The separation temperature can be customised to the feed gas conditions to properly balance efficiency and product retention with the need to ‘over-condense’ a significant portion of methane to ensure heavy components stay with the liquid phase. The heavies liquid stream is usually vaporised, either through a separate heater or as another pass in the main exchanger, and can be used for fuel or re-injected back to the feed pipeline. A variation by Black & Veatch uses a simple flash as the first removal step then fractionates the heavies stream to effectively recover C4 and lighter components for reliquefaction in a separate pass of the main exchanger, while also producing a stabilised condensate product or a blend into fuel.

While the simplicity and low cost is advantageous, this configuration carries the lowest efficiency of all options and could be limited by heavies product disposition. Being reliant on just a single-stage flash renders application to lean feed gas difficult as not enough liquid can condense to be effective in removing freezing components. It also lacks flexibility to significant changes in feed gas composition as there is limited control over the separation conditions. These disadvantages can typically be over-come for small scale and intermittent applications which favour a low CAPEX approach, but larger baseload units may take a more sophisticated approach for better overall economics.

Scrub column

Applying a scrub column with reflux enables heavies removal at higher pressure, usually with less loss to the liquid stream compared to a simple flash. Reflux can be sourced from multiple locations depending on simplicity and efficiency desired; an intermediate pass in the main exchanger could be employed to limit the refrigerant duty consumed or it can be pulled directly from the LNG product stream. Higher flexibility is also gained from reflux, though it is still limited by the liquids which can be produced and may struggle on leaner feed gas. While better heavies removal certainty is achieved with this design, the need for sub-critical pressure and lack of gas machinery still leaves this configuration on the lower end of efficiency. Another variation on this concept where reflux is not required is to strip the liquid flowing down through the column to manage product loss.

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