3-phase separator as an effective solution against blockage in LNG equipment.

Globally, there is an increasing number of small (bio)gas sources. This so-called ‘stranded gas’ is usually difficult to develop because of the small quantities and the absence of a pipeline within short distance. Local, small-scale LNG production can be a perfect solution to monetize (bio)gas and sell the LNG locally as a (renewable) fuel for heavy-duty trucks or ships.

Small-scale LNG requires a very simple, but efficient and robust process with minimum equipment count.

However, gas treating processes in the LNG scheme, like the removal of CO2, water and heavy hydrocarbons require complex unit operations like absorption, adsorption and distillation. It makes the efficient but complex traditional LNG scheme not suitable for downscaling. The removal of heavy hydrocarbons in scrub column or flash vessel also prevents further optimization of the process (for example a high liquefaction pressure).

Less demanding gas specifications and therefore ‘simpler gas treating’ in the LNG scheme are possible but requires a solution to prevent blockage of cryogenic equipment by solids.

This is the essence of the ‘Cryo-3-Flash®’ concept. Contaminants like CO2 and heavy hydrocarbons are removed in the solid phase from the LNG, instead of the traditional upstream trace removal in the gas phase. The bandwidth of allowable impurities is enlarged for both gas treatment and liquefaction.

Figure; Simplified sketch of the ‘Cryo-3-Flash®’ separator.

Polishing of the LNG product (post-liquefaction) combined with end-flash, results in cost reduction by a significant relaxation of gas specifications at the outlet of gas treating facilities. The job of the treating units becomes a lot easier and is even not required in specific cases. The purpose is to prevent blockage in the liquefaction exchanger, which is operated at a relatively high pressure and temperature in a gas expansion scheme.

Crystallisation and solids handling have operational challenges. For example; the risk of forming conglomerates and so-called Ostwald ripening; bigger crystals grow at the expense of smaller crystals. Osomo has studied the behavior of solid CO2 particles in LNG in their 3-phase test separator, with an endoscope capable withstanding -160ºC and 0-9 barg. The above-mentioned issues have been addressed and solved in the proprietary design of the internals of the 3-phase separator; ‘Cryo-3-Flash®’

So-called ‘process intensification’ and therefore reduction of equipment count is achieved by combining several functions in the ‘Cryo-3-Flash®’ vessel;

  • Vapor-liquid separation; commonly called ‘end-flash’ in the traditional LNG scheme. The vapor is generated by reducing the pressure across a JT valve or liquid expander.
  • Crystallisation; Super-saturation of the liquid is caused by the combination of temperature reduction and concentration increase due to the flash evaporation. Crystals can grow because of sufficient residence time.
  • Liquid-solid separation by gravity settling; the internals of the vessel cause the larger solid particles to sink to the bottom of the vessel and are removed batch-wise. Solid contaminants are removed: CO2 as well as heavy hydrocarbons, siloxanes, methanol, etc.
  • Pumping; the clear ‘LNG solution’ in the top section of the vessel, still containing tiny particles (fines) is pumped to storage conditions. As a result, the LNG solution becomes under-saturated in the storage tank because of the slightly higher pressure and corresponding boiling temperature.

Conclusion;

The ‘Cryo-3-Flash®’ separator results in reduced equipment count and less demanding (less tight) gas specifications of gas treating units. The band-with of allowable impurities is significantly increased.

Cryo-3-Flash®’ is ideal for small scale LNG where process simplification is essential.

The concept is fully compatible with standard LNG end-flash. ‘Cryo-3-Flash®’ can be an add-on for small scale facilities with challenging gas compositions.

Smart process integration, in-depth understanding and testing of unit-operations have been key for Osomo’s integrated ZR-iLNG process scheme [1][2]. For licensing or more information, please contact Osomo at info@osomo.nl or +31 641237658

References:

[1]    iLNG BV; WO2016126159 – “System and Method for processing a hydrocarbon comprising fluid”

[2]    Gasconsult LTD; GB2486036;“Process for liquefying natural gas using low-pressure feed stream

[3]    Publication Gastech News, “Cost effective gas treatment by membranes in Liquid Bio Methane (LBM) and LNG process schemes”, March 2017

[4]    www.ilng.nl

Edited, published by Michiel van Aken, Technical Director Osomo, all rights reserved, October 2017.