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Please suggest an effective catalytic route for removing carbon dioxide from our SMR hydrogen.

Mar-2021

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Mel Larson, Becht, mlarson@becht.com

This may be more of an issue of the engine than the fuel. The historical marine engine was built and tuned for CCAI values of 850. In order to meet the lower sulphur VLSFO specification, less residua with less carbon content is being delivered as well as different viscosity. Each component in the bunker fuel blend should be assessed for compatibility as well. Globally, there has been an increase in light sweet crude consumption to meet the VLSFO specification. Light sweet and unconventional crudes tend to be more paraffinic in composition. The blending of highly paraffinic and conventional oils may result in precipitation either in the tank or throughout a system lacking a homogenous fuel to the engine.

Apr-2021
Chris Claesen, Nalco Water, Chris.Claesen@ecolab.com

Blending different fuels to make IMO 2020-compliant VLSFO blends can create unstable mixtures that cause sedimentation and sludge. Sedimentation can occur in these blends because paraffins are an antisolvent (or non-solvent) for highly aromatic asphaltenes. Sometimes the sedimentation is immediate, particularly if the blend components are highly incompatible, but often the sedimentation is much slower and may only be observed after a prolonged period of time or after exposure to external stress such as elevated temperatures.

These challenges can be addressed by treating with Nalco Water additives that stabilise the asphaltenes in the final blend.

The principal function of the Nalco Water stability additives is to prevent flocculation and/or maintain the asphaltenes in a dispersed state in the fuel blend. By stabilising the asphaltenes in the blend, the additives will ultimately reduce the amount of sediment formed from otherwise incompatible or unstable VLSFO blends.

The stability of a blend and the potential need for a stability additive can be predicted with a proprietary Nalco Water blending model.

Apr-2021
Jari Marci, Petrogenium, jari.marci@petrogenium.com

I assume that the performance issue is related to unstable blends of fuel oil, and thus to a total sediment potential exceeding the allowable limit of 0.1 wt%.

A very common approach to producing residual low sulphur fuel oil (LSFO), since the advent of IMO 2020, is to blend low sulphur vacuum residue with desulphurised gasoil. Such fuels are common on the market and are characterised by good stability (ASTM D7112) in terms of their P-value (P-value approx. 2.0-3.0). The P-value is a measure of the blend stability and a ratio of two parameters: P0 and FRMax.

P0 is the available aromaticity of the non-asphalthene phase, and FRMax is the required aromaticity of the asphalthene phase — so long as P0 > FRMax, the blend is stable and all asphalthenes are kept in solution. The individual components of P0 and FRMax could be, for example, 46 and 20 respectively. Furthermore, these oils have a low asphalthene content.

In the past and in some cases today, marine residual fuel is produced by processing of residue in a visbreaker. Visbreaking is a mild thermal conversion process by which the viscosity of a residue is substantially reduced. Producing fuel oil from thermally cracked residue still requires external diluent, but much less (-50%). One effect of the thermal conversion is that the P0 of the non-asphalthene phase is reduced, while the FRMax of the asphalthene phase is strongly increased.

When blending to fuel oil viscosity specification, the residual stability of such a blend can be as low as 1.1 with e.g. a P0=68 and a FRMax=62. Furthermore, these blends are high in asphalthene content. When blending LSFOs from straight run origin with LSFO made with thermally cracked material, the largely different FRMax of the individual components leads to instability.

By contrast, the FRMax is dominated by the cracked material, and the P0 is an intermediate value between the straight run and cracked component. Even with minor additions of cracked material, the blend becomes unstable and asphalthenes are rejected from the oil.

Fundamentally, there is no cure for this problem other than to identify the source and quality of different fuel oils and to keep them separate. Limiting the quantity of cracked base material to low levels (10-15%) may help and result in stable blends. This depends on the respective LSFOs properties. Dedicated blending tests or calculation (pending availability of stream stability data) can be conducted to determine stability limits and maximum allowed fraction of thermally cracked component.

Apr-2021
Tom Ventham, Unicat B. V. / G. W. Aru, LLC, tom.ventham@gwaru.com

Hydrogen production is a complex, multi-unit process that must be optimised through the design phases to operation, to ensure it fulfils the objectives set out. Carbon dioxide is a by-product of SMR reforming reactions. Carbon dioxide is also generated through the water gas shift reaction taking place in HTS, MTS, or LTS reactors to increase hydrogen yield.

Post-processing of the SMR CO2 rich stream will either happen using a PSA system to purify the hydrogen product and release CO2 as purge gas typically routed to the reformer furnace or fuel gas network, or a methanator to catalytically react CO and CO2 to methane using a nickel or ruthenium based catalyst. In ammonia plants and other larger syngas production units, a Benfield system can be found to remove CO2 from the syngas stream. In terms of a purely catalytic route, the methanator catalysts Unicat can offer as part of the MC-range are an attractive solution to refiners looking to remove carbon dioxide from the SMR hydrogen stream.

Mar-2021