Excellent CO2 bulk and trace removal from (bio)gas with membranes in the iLNG process scheme

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.

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

However, complex amine gas treating is usually the only option in LNG schemes when there is no outlet (eg pipeline) for the high flow of tail gas from a Pressure Swing Adsorption unit.

At the same time, membrane gas separation is a standard and low-cost technology to remove the bulk of CO2 from biogas. Yet, membrane gas separation is generally considered not suitable for CO2 trace removal, which is essential in case the biogas is being liquefied.

Osomo have developed and tested their proprietary iLNG scheme. The performance of membrane separation is extremely improved by sweeping the low pressure permeate side of the membrane with clean flash gas, originating from the cryogenic end-flash of LNG.

It is common knowledge that sweepgas improves membrane separation performance. But the challenge is to find the right source, composition and quantity of sweep gas. And where to go with the permeate gas, which is diluted with sweep gas? Osomo has solved these issues in the ZR-iLNG scheme.

The patented “Flash-2-Sweep”® [1] results in a lower CO2 bulk concentration on the permeate side as well as a reduction of the concentration gradient near the membrane wall due to a higher gas flow and more turbulence. It can also increase the extent of counter-current flow in the membrane module. Thereby the driving force for permeation through the membrane is significantly increased. This is comparable to the crucial counter-current flow principle in efficient heat-exchange and distillation processes.

An order of magnitude lower CO2 concentration in the retentate is achieved with “Flash-2-Sweep”, for the same membrane area, as illustrated in table below.

The effect is even much more pronounced with the ongoing development of highly selective membrane materials. For example, a small flow of “Flash-2-Sweep”® gas reduces the water concentration in the retentate by a huge factor of 70, in case of the water separation from gas with a proven membrane selectivity of H2O/CH4 = 200.

Table; Improvement of membrane separation performance; retentate outlet concentration in ppm mol for a given/fixed membrane area, feed (retentate) pressure of 18 bar, permeate pressure of 1.2 bara and feed concentration of 2 mol% (20,000 ppm) CO2 and 500 ppm H2O .

Conclusion;

The separation performance of standard membranes is boosted by the ‘Flash-2-Sweep’ concept. It enables the unique use of membranes in the LNG scheme for both bulk and trace removal of contaminants like CO2 and water. It is by far the simplest gas treating method.

Gas separation with new, highly selective membranes will be increasingly ‘pressure ratio limited’. ‘Flash-2-Sweep’® will solve this limitation and fully utilize the potential of new membrane materials.

Smart process integration has been key to the success, resulting in Osomo’s ZR-iLNG [1][2]. For licensing please contact Osomo at info@osomo.nl or +31 641237658

References:

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

[2]    Patent 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]    http://www.iLNG.nl

Paper edited and published September 2017 by Michiel van Aken, Technical Director Osomo, all right reserved