The issue
It is commonplace that chemical gas cargoes which may potentially be off specification could result in expensive cargo claims as well as additional costs and delays with ensuing disputes between the owners and the charterers. Obtaining a representative sample and applying the most appropriate analytical test method could, therefore, be crucial. We have recently seen some cases where this has come into sharp focus during loading on LPG vessels. One of these cases concerned Vinyl Chloride Monomer (VCM) cargo and how to sample and test its iron content on board a LPG vessel.
The cargo
Vinyl Chloride Monomer (VCM) or chloroethylene is amongst the largest traded commodities by volume with a world production at over 30 million tonnes per year. VCM is chiefly used for the production of polyvinylchloride (PVC) from which many bottles, plastic cards and construction pipes are made. VCM is a colourless gas with a distinctive sweet odour, characterised by high toxicity, flammability and carcinogenic properties. VCM is generally produced through either direct chlorination or oxychlorination processes, using copper/iron-based catalysts and is usually shipped in medium/small parcels on board semi/fully-pressurised LPG vessels.
In addition to specification parameters typically encountered in the chemical gases trade (e.g. dew point, water content, various gaseous impurities), VCM cargoes demand a very strict limit on the amount of dissolved iron which is usually stipulated as ≤ 0.5 ppm. The reason underlying the strict quality control for dissolved iron is borne out of the fact that it plays a crucial role in determining VCM monomer suitability for PVC production, which remains its main market outlet.
'Dissolved' vs 'particulate' iron
There is a significant difference between 'dissolved' and 'particulate' iron. Dissolved iron is present in VCM cargoes as soluble species in the form of Fe(II)/Fe(III) irons whereas particulate iron will be present as insoluble iron oxide particulates (Fe2O3), commonly referred to as 'rust'.
Particulate iron can originate from both ashore as well as vessel's cargo tanks as either can be constructed from uncoated mild steel. It is especially common for cargo tanks on board semi/fully pressurised gas tankers to be constructed from uncoated mild steel containing a nickel component which will over time form a thin layer of adherent rust. The presence of particulate iron/rust in either ship and/or shore side tanks, if of uncoated mild steel construction, is therefore unsurprising.
Particulate iron, due to it's comparatively high density and non-homogeneous nature, has a tendency to settle rapidly towards the bottom-most reaches of the tanks, both ashore and on board gas carriers and therefore does not form part of the bulk liquid cargo quality.
Dissolved iron, on the other hand, will not settle from the cargo bulk and is considered a key quality parameter as excessive amounts have detrimental effects upon onward polymerisation process. The presence of dissolved iron is predominantly due to carry-over of residual iron-based chlorination catalysts used during manufacture of VCM and as a result of this, tends to be a quality parameter not readily affected by the vessel. Increase in dissolved iron content from contact with vessel's tanks is negligible.
Sampling
Drawing representative samples during quality determination of the product bulk is of crucial importance. Three main options exist on board semi/fully-pressurised vessels:
- During cargo recirculation operations (semi pressure or fully pressurised);
- 'Slip tube' sampling points located on the tank dome[1] (fully pressurised ships);
- Sample tubes (semi-pressurised ships).
Since the VCM cargo is sensitive to dissolved iron, but less so to particulate iron, consideration should be given to a sampling method that does not unnecessarily include high contents of particulate iron. Sampling of gas cargo can also be relatively expensive requiring quite specific equipment which is not always readily available.
Whilst all sampling methods can provide efficient means of sampling the cargo, the main difference between the different methods is that, when sampled via 'slip tubes' and sample tubes, the cargo is static and therefore tends to contain lower concentrations of particulate matter. If, on the other hand, samples were taken during recirculation operations, the possibility of particulate iron inclusion in the sample cannot be eliminated as the act of recirculation serves to distribute some of the settled, non-homogeneous components present on the tank top into the cargo column.
Testing
The three most common means of determining dissolved iron are:
- 'Phenanthroline – Red' method (ASTM E 394)
- Atomic absorption spectroscopy (AAS)
- Inductively coupled plasma spectroscopy (ICP)
The classical 'Phenanthroline – Red' method selectively determines 'dissolved' [Fe(II)] iron following complexing with ortho-phenanthroline which yields a coloured complex which can be measured at a specific wavelength and subsequently converted into a concentration value for Fe(II). Due to the method of detection, it will not measure particulate iron.
The AAS and ICP techniques measure iron concentration spectroscopically, using the characteristic wavelengths of light emitted (ICP) or absorbed (AAS) by excited gaseous iron species.
However, and importantly, the main difference between AAS/ICP and the 'Phenanthroline – Red' test method is that prior analysis via AAS/ICP, the samples need to undergo an acid digestion step, a process which will convert particulate iron into dissolved iron.
AAS and/or ICP are therefore not capable of selectively measuring 'dissolved' iron species and will erroneously overstate and instead measure 'total iron' content if particulate iron is present on basis that any particulate iron will be converted into dissolved iron during sample preparation and thus potentially overstate the true dissolved iron content.
It is therefore recommended that due care be taken during sample preparation to ensure that all particulate iron is filtered out prior to an analysis using AAS/ICP. If all the particulate iron is not removed prior to analysis via AAS/ICP, an erroneous overstatement of dissolved iron content will result. Better still is to agree and carry out testing according to the 'Phenanthroline – Red' test method depending on availability and resources available in or around any given port.
We have seen this issue on some cases and where the sample and test method has not been agreed beforehand. This has resulted in less preferred samples and test methods being applied, with delays and significant costs as a result, and where the risks for this have been allocated in the charter party.
Conclusions
Dissolved iron is an important and quality critical parameter for VCM cargoes and due care is required to ensure that the appropriate steps are taken to minimise the potential for overstatement of this parameter during testing.
In this regard, sampling of the static cargo column via the 'slip tubes' or fixed sample tubes is preferred, rather than by pump recirculation.
Dissolved iron determination via 'Phenanthroline – Red' method is to be preferred over AAS/ICP due to non-requirement of an acid digestion stage in the former test method. Should it not be possible to test the samples by the 'Phenanthroline – Red' method, due care should be taken to ensure that all particulate matter has been removed by filtration prior to AAS/ICP testing. Failing to do so, will result in the total iron content being reported and consequent overestimation of dissolved iron content.
In addition to be mindful of these loss prevention measures we also, where possible, recommend that sample and test method are agreed beforehand with the risks allocated appropriately in the charter party.
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[1] For sampling via the slip and sample tubes to be effective, sufficient pressure is required in the cargo tank.
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The Association is grateful to Pierre de Jager of CWA for contributing to this article.