Behaviour of styrene studied at Cedre
Dissolution
In order to obtain a better understanding of issues related to solubility analysis in the laboratory, for which the literature only provides data for dissolution in freshwater, comparative tests were performed in Cedre’s laboratory with seawater and distilled water.
These tests showed that styrene dissolution was two times higher in freshwater than in seawater (40 mg/L compared to 20 mg/L in the test conditions), which should lead us to reconsider the reference value of 200 mg/L.
Evaporation
Outdoor experiments were conducted to determine the behaviour of styrene when spilt at the seawater surface (release of 10 litres of styrene at the surface of a 10 m2 water body with an agitator).
A particular focus was placed on studying the evaporation and dissolution kinetics, as well as the possibility of polymerisation.
In terms of explosion risks due to evaporation, the values measured remained below the detection threshold of the detectors (two explosimeters of different brands) throughout the experiment. The tests conducted by the Brest fire brigade on the gases emitted did not significantly indicate the presence of styrene in gas state (use of Dräger tubes). We note however that a strong styrene odour was emitted from the test tank.
Dissolution
The samples taken to assess the dissolution process revealed the evolution of styrene concentrations in the water column. This curve reached its maximum after 18 hours. By the end of the test, no styrene was left at the water surface while dissolution was no longer possible at this time.
The maximum value of 6 mg/l is far lower than the reference values available in the literature (around 200 mg/l).
The distribution of the styrene at the end of the experiment was as follows: 99.4% in the air and 0.6% in the water column.
Polymerisation
The capacity of styrene to polymerise in marine conditions was studied. It appears that after around 2 hours of contact time, styrene can form micro-emulsions. The appearance of the product in this new form was close to that of a polymer but the tests showed that it was still the initial styrene monomer.
In these test conditions which were designed to be as realistic as possible, the principal behaviour of the product was therefore evaporation. The dissolution process was very limited (maximum value of 6 mg/l) and in all cases was far lower than the reference value in the literature (dissolution of up to 200 mg/l).
No explosion risk was detected or values in the atmosphere in excess of 10 ppm. However, given the difference in scale with a large-scale spill at sea, these values cannot be presumed to be representative of those reached in the case of a real spill. Finally, no polymerisation phenomenon was detected; the formation of micro-emulsions due to wave action could explain the particular colouring of the styrene and could lead operators to believe it is a different product.
What is described here is only applicable to the conditions and duration of the tests conducted. These results are therefore only intended to provide an indication. The product may behave very differently after being under pressure, possibly in partial contact with water, then rising to the surface where it will be exposed to real sea conditions. It is essential to take samples on site to dispel doubts.
In addition to these outdoor tests, laboratory testing was also conducted to confirm the composition of the styrene, which we were informed by Shell was 99.9% pure with:
- 12.7 mg/kg of stabiliser (PTBC),
- 0.9 mg/kg of polystyrene,
- 36 mg/kg of aldehyde,
- 3 mg/kg of peroxide,
- less than 0.2 mg/kg of chlorine,
- less than 0.2 mg/kg of sulphur,
- 75 mg/kg of water.
The message received from SHELL on 1st November 2000 specified that all these components were typical of styrene under the ASTM standards in force.
Additional information on styrene: according to CEFAS (UK Centre for Environment, Fisheries and Aquaculture Science), styrene should not polymerise in less than:
- 170 days at 15.6°C
- 1 year at 4-5°C
This information therefore gives a window of at least 7 months without polymerisation in the temperature conditions expected on site.
Results of the tests conducted for the polymerisation study in marine conditions, some of which were conducted in an outdoor test tank and the others in the laboratory.
Protocol
Outdoors:
160- and 200-litre stainless steel tanks were immersed in an outdoor test tank filled with seawater to detect any initial signs of polymerisation of the product, with:
- stabilised styrene in a completely air-tight tank (Pf),
- stabilised styrene in a tank left slightly open via a small drainage outlet (Po),
- stabilised styrene in a completely open tank (Go),
- non-stabilised styrene in a completely open tank (Go ns).
The tanks were immersed on Friday 22nd December 2000. Analysis was conducted based on the protocols provided by Shell Chimie.
In the laboratory:
A reduced scale test was carried out in the laboratory at 30°C, in two stainless steel beakers:
- one with stabilised styrene and water circulation,
- the other with stabilised styrene without water circulation.
Results
Outdoors:
In the outdoor test tank, only the styrene without a polymerisation inhibitor, in large-scale contact with the water, showed the beginnings of polymerisation, although still very minor (1%) (Figure 1).
As concerns the styrene with an added polymerisation inhibitor (pTBC), no polymerisation was detected. However, it is important to note that the pTBC concentration dropped considerably for the styrene in the open tanks (Figure 2).
In the laboratory:
After 10 days of immersion in water at 30°C (temperature at which polymerisation initiates faster), the styrene in the beakers had still not polymerised, whether with or without agitation.
After 70 days, the two volumes of styrene showed polymer contents slightly in excess of 1%.